U.S. Department of Energy 

Office of FreedomCAR and Vehicle Technologies 

1000 Independence Avenue S.W. 

Washington, DC 20585-0121 

FY 2003 

Progress Report for Automotive Propulsion 

Materials Program 

Energy Efficiency and Renewable Energy 

Office of FreedomCAR and Vehicle Technologies 

Edward Wall Program Manager 

December 2003 
U.S. Department of Energy 

Office of FreedomCAR and Vehicle Technologies 

1000 Independence Avenue S.W. 

Washington, DC 20585-0121 

FY 2003 

Progress Report for Automotive Propulsion 

Materials Program 

Energy Efficiency and Renewable Energy 

Office of FreedomCAR and Vehicle Technologies 

Edward Wall Program Manager 

December 2003 

CONTENTS 


1. INTRODUCTION ........................................................................................................... 1


2. POWER ELECTRONICS................................................................................................................... 7 


A. Low-Cost, High-Energy-Product Permanent Magnets ................................................. 7


B. Development of Improved Powder for Bonded Permanent Magnets ......................... 13


C. Carbon Foam for Electronics Cooling.......................................................................... 23


D. Evaporative Cooling of Power Electronics ................................................................... 29


E. High-Temperature Film Capacitors .............................................................................. 35


F. Mechanical Reliability of Electronic Materials and Electronic Devices ...................... 41


3. CIDI ENGINES ............................................................................................................... 47 


A. 	Microwave-Regenerated Diesel Particulate Filter Durability Testing .......................... 47


B.	 Material Support for Nonthermal Plasma Diesel Engine 


Exhaust Emission Control ............................................................................................ 53


C. Fabrication of Small Fuel Injector Orifices................................................................... 57


D. Technology for Producing Small Holes in Advanced Materials .................................. 63


E.	 Electrochemical NO Sensor for Monitoring Diesel Emissions ................................... 69
 x

APPENDIX A: ACRONYMS AND ABBREVIATIONS.......................................................... 75


iii 

1. INTRODUCTION

Automotive Propulsion Materials R&D: 
 

Enabling Technologies to Meet FreedomCAR Program Goals 
 

The Department of Energy’s (DOE’s) Office of FreedomCAR and Vehicle Technologies 

(OFCVT) is pleased to introduce the FY 2003 Annual Progress Report for the Automotive 

Propulsion Materials Research and Development Program. Together with DOE national 

laboratories and in partnership with private industry and universities across the United States, 

the program continues to engage in research and development (R&D) that provides enabling 

materials technology for fuel-efficient and environmentally friendly light-duty vehicles. 

This introduction summarizes the objectives, progress, and highlights of the program in 

FY 2003. It also describes the technical barriers remaining and the future direction of the 

program. The FY 2003 annual progress reports on Combustion and Emission Control for 

Advanced CIDI Engines and Power Electronics provide additional information on OFCVT’s R&D 

activities that support the development of propulsion materials technology. 

In May of 2001, the President’s National Energy Policy 

Development Group published the National Energy 

Policy (NEP). This comprehensive energy policy 

specifically addresses the development of energyefficient vehicle technologies, including hybrid systems, 

advanced emission control technologies, fuel cells, and 

hydrogen-based systems. The NEP is a strong indicator 

of the continuing federal support for advanced 

automotive technologies and the materials work that 

supports them. 

The Automotive Propulsion Materials (APM) R&D 

Program has supported the FreedomCAR Program since 

its inception. FreedomCAR is not an automobile or 

prototype but rather a new approach to developing the 

technologies for vehicles of the future. In research areas 

where industry views the risks as too high and Report of the National Energy uncertain, the FreedomCAR Program conducts long- Policy Development Group term research, development, and demonstration 

activities that bring the technology to a stage of maturity such that industry can undertake 

the final commercialization stages. 

The APM Program is a partner with the OFCVT programs for Power Electronics and Electric 

Machines and for Combustion and Emissions Control for Advanced CIDI Engines. Projects 

within the APM Program address materials concerns that directly impact the critical technical 

barriers in each of these programs—barriers such as thermal management, emissions 

reduction, and reduced manufacturing costs. The program engages only the barriers that 

involve fundamental, high-risk materials issues. 

1 
 
Enabling Technologies 

The APM Program focuses on enabling 

materials technologies that are critical in 

removing barriers to the power electronics 

and compression-ignition, direct-injection 

(CIDI) engine and emissions control 

research programs. The program supports 

these two core technology areas by 

providing materials expertise, testing 

capabilities, and technical solutions for 

materials problems. The component 

development, materials processing, and 

characterization that the program provides The Propulsion Materials Program 

are enablers of the successful development focuses on two applications. 

of efficient electric drive systems and 

emissions-compliant CIDI engines. 

Thermal management is a crosscutting engineering issue that affects both the power 

electronics and CIDI programs. The components necessary for high-fuel-economy, lowemission hybrid electric and fuel cell vehicles require that power electronics be smaller and 

lighter and operate at higher temperatures than those for conventional vehicles. These 

requirements are being addressed by developing electronic materials (i.e., materials for lowcost dc bus capacitors) that operate at higher temperatures and by improving the capability to 

dissipate heat generated in electronic devices. The APM Program has been addressing electric 

drive system heat dissipation issues through the development of advanced carbon foam 

technology. 

Current CIDI engines must strike a delicate balance between high efficiency and low tailpipe 

emissions. CIDI engine and aftertreatment system development will greatly benefit from the 

Program’s efforts to develop improved engine components and subsystems. The APM Program 

featured two projects in FY 2003 to develop technology to produce very small (~50 micron) 

orifices for fuel injectors used in high-pressure common rail systems. The smaller orifices can 

enable better control of fuel atomization and can increase efficiency and reduce emissions. 

The Program is also working to reduce emissions through the development of advanced 

particulate filters and is collaborating with Pacific Northwest National Laboratory in the 

development of a nonthermal plasma aftertreatment device to reduce NOx emissions from 

diesel engines. This is the last year for nonthermal plasma aftertreatment system development 

within the Program. 

Collaboration and Cooperation 

As with other programs under FreedomCAR, collaboration and cooperation across 

organizations is a critical part of the APM Program. Throughout the FreedomCAR Program, 

scientists at the national laboratories are collaborating with manufacturers to identify and 

refine the materials characteristics necessary for meeting system performance requirements. 

Researchers at Lawrence Livermore National Laboratory are working with Ford Motor 

Company to develop low-cost, rapid-response NOx sensors that can be used in feedback 

control loops to monitor and minimize NOx emissions from diesel engines. There is also 

cooperation among national laboratories to take advantage of the expertise of each facility. 

2 
 
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✔ Kemet 

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Oak Ridge and Argonne National Laboratories (ORNL and ANL), for example, are 

collaborating in the development of higher-strength NdFeB permanent magnets that will 

enable significant reductions in the size, weight, and cost of electric motors used in hybrid 

vehicles. In another pro ect, Sandia National Laboratories (SNL) is fabricating smaller, highertemperature dc bus capacitors; and ORNL is developing techniques to characterize the 

~5-micron-thick polymer films developed by SNL and correlate their mechanical properties 

with their dielectric behavior. These electric drive system pro ects in turn are regularly 

reviewed by the EE Technical Team of the U.S. Council for Automotive Research to get 

feedback on research direction and progress. 

In addition to national Laboratory/contractor-industry collaboration 

laboratory and large Laboratory Industrial Partner 

industry participation, the 

FY 2003 APM Program 

included important R&D 

conducted by a small busi

ness. Industrial Ceramic 

Solutions, 

located in Oak Ridge, 

Tennessee, is developing a 

ceramic filter to reduce 

particulate emissions from 

diesel engines. As in the 

collaborative efforts of 

national laboratories with 

industry, researchers at ICS 

are working closely with 

representatives from 

DaimlerChrysler, Ford, 

General Motors, and ORNL 

to develop a filter that will 

Argonne 

National 

Ability Engineering Technology, Inc. 

Atlas Cylinders, Inc. 

Daimler-Chrysler Corporation 

Ford Motor Company 

Laboratory Bronson and Bratton, Inc. Lucas-Varity 

Cryomagentics, Inc. Purdue University 

CRUMAX UGIMAG, Inc. 

Oak Ridge 

National 

Bronson and Bratton, Inc. 

Caterpillar 

Industrial Ceramic Solutions 

Motorola 

Laboratory Conoco Corporation Murata 

Cryomagnetics, Inc. Peterbilt 

CRUMAX Poco Graphite 

Daimler-Chrysler Corporation UGIMAG, Inc. 

Dow Chemical Company University of Tennessee 

Ford Motor Company 

General Motors 

Sandia 

National 

Laboratories 

Daimler-Chrysler Corporation 

Ford Motor Company 

General Motors 

Degussa 

Ferro 

Materials Research Associates 

Murata 

Pennsylvania State University 

TAM 

Tokay 

TPC Ligne Puissance 

TPL, Inc. 

TRS Ceramics 

meet the emissions targets of the program. 

Accomplishments 

FY 2003 featured notable accomplishments in both materials program areas. This section 

highlights some major accomplishments during FY 2003. 

Power Electronics 

High-power electronic components such as power modules 

and inverters being developed for hybrid electric and fuel 

cell vehicles generate significantly more heat than 

conventional devices. Dissipation of the heat generated by 

these devices in hybrid electric vehicles requires an 

additional cooling loop and water-cooled heat sink to 

prevent overheating and failure of the devices. The 

increasing power requirements of electronic devices require 

that more-sophisticated heat sinks be developed to keep the Graphite foam material 

temperature of the electronics below about 120°C. High- developed at ORNL 

3 
 
thermal-conductivity, high-surface-area carbon foam is an innovative new material that offers 

great potential for advanced heat sinks and heat spreaders; however, the high cost of the 

material is a concern. 

An economic model was developed in FY 2003 to help quantify the cost factors in the process 

for fabricating high-thermal-conductivity carbon foam. More than 50 factors affecting the cost 

of the foam were identified in this extensive study. The Excel-based model showed that the 

pitch precursor and graphitization step (temperature, time, furnace size, etc.) had the most 

dramatic effect on the cost of the foam. The model also provided insight into the sensitivity of 

expected pitch price to all of the key process and business parameters and therefore served as a 

tool to guide further development, cost reduction, and commercialization. Research efforts 

have been redirected to evaluate less costly pitch precursors and simpler graphitization steps. 

A collaborative effort between the National Security Agency (NSA) and ORNL has effectively 

used high-conductivity carbon foam to dramatically increase the cooling capacity of a highpower-density chip system. The lightweight carbon foam has a ligament thermal conductivity 

comparable to that of diamond wafers but has a surface area that is more than two orders of 

magnitude greater. When the diamond spreader in the NSA chip assembly was replaced by 

ORNL’s graphite foam, a power density of greater than 110 W/cm2

 was attained without 

overheating the system. The same technology has been used to develop a low-cost passive 

heat sink for power modules and inverters. 

Advanced Combustion Engine and Emissions R&D 

ICS has developed a pleated ceramic fiber filter 

cartridge to remove hazardous carbon particulates 

from the exhaust of small diesel engines. This 

cartridge removes greater than 95% of the diesel soot 

particles at a fraction of the backpressure of 

conventional wall-flow filters. (Excess backpressure 

on a diesel engine is detrimental because it reduces 

the efficiency of the engine and increases the fuel 

consumption.) The backpressure of the ICS fiber filter 

is 1/10 that of a conventional extruded honeycomb 

filter product. In addition, the thermal mass of the 

ICS filter is 1/3 that of the honeycomb filter. Thus it Schematic of ICS particulate trap requires less auxiliary energy for filter cleaning, again 

reducing the fuel penalty for the emission control device. These advantages can reduce the 

fuel consumption penalty from 15% down to 1%. The final advantage is the cost of the 

product in a high-volume market. The ICS filter is a simple pleated design that is similar to 

millions of air and liquid filtration devices used around the world. It is manufactured by 

established processes on high-volume equipment. 

ICS is now entering product qualification programs with diesel engine suppliers in the United 

States and Europe. These suppliers require manufacturing capacity for millions of diesel 

particulate filter units per year. ICS is in final negotiations with a New York Stock Exchange 

U.S. filter products company, with international sales, to license the patented technology for 

volume production. If the qualification tests are successful, it is likely that the low-cost ICS 

filter will be incorporated into a variety of systems that enable automakers to meet the more4 
 
stringent emissions requirements in the United States in 2007 and in Japan and Europe in 

2008. 

Another method for reducing particulate emissions from diesel engines is to improve fuel 

atomization. This can be accomplished in a number of ways: by increasing the fuel injection 

pressure, by sophisticated control of the injection timing, and/or by reducing the diameter of 

the orifices through which fuel is injected into the cylinder. Increasing the injection pressure 

is problematic because of safety concerns and the increased risk of cracking the injectors. 

Control systems—particularly those requiring specialized sensors for monitoring the 

combustion process—can add to the complexity and expense of the engine. Research has 

shown that the use of injectors with orifice diameters of around 50 μm should result in 

significant reductions in particulate emissions; however, no processes are available for 

fabricating such fuel injectors. 

Work conducted at ANL in FY 2003 demonstrated the feasibility of 

using electroless nickel plating to reduce the injector orifice 

diameter of standard injectors from 200 μm to about 50 μm by 

depositing thick, uniform layers of nickel plate along the interior 

surfaces of the injector orifices. Electroless nickel plating is a 

mature technology, but it has not been used in this application 

before. It is highly scalable and appears to be an economical 

technique for fabricating small-orifice injector nozzles. In addition 

to achieving major reductions in orifice diameter, ANL research 

has shown that electroless nickel deposition reduces the surface 

roughness of the orifice interior. Changes in the plating bath 

chemistry have also been shown to vary the hardness of the nickel Typical diesel fuel plate, suggesting the possibility of tailoring the hardness so as to injector nozzles reduce cavitation and erosion. In addition, bench tests indicate 

that electroless nickel coatings are about 50% less susceptible to deposit formation than the 

base metal of the injector. 

Future Direction 

The APM Program will continue to work closely with FreedomCAR partners and industry to 

understand propulsion materials–related requirements. Building upon the recent advances in 

materials technologies, many of this year’s projects will be moved out of the laboratory and 

over to industry for testing. Strategic partnerships will be established in 2003 with exhaust 

system, engine, or vehicle manufacturers to move the particulate filter system from vehicle 

testing toward commercialization. The scale-up and commercialization of carbon foam at 

POCO Graphite for thermal management applications will continue, with the focus on cost 

reduction. Other projects will continue to refine manufacturing requirements and necessary 

characteristics to meet the challenges of the FreedomCAR program. 

As advanced automotive technology developments uncover new challenges, the APM Program 

will continue to provide breakthrough technology solutions through collaboration with 

industry, FreedomCAR partners, national laboratories, and small businesses. 

5 
 
Project Abstracts 

The remainder of this report communicates the progress achieved during FY 2003 under the 

APM Program. It consists of ten abstracts of national laboratory projects—six that address 

power electronics and four that address combustion and emission technologies. The abstracts 

provide an overview of the critical work being conducted to improve these systems, reduce 

overall cost, and maintain component performance. In addition, these abstracts provide 

insight into the challenges and opportunities associated with advanced materials for highefficiency automobiles. 

Rogelio Sullivan 

Team Leader, Materials Technologies 

Office of FreedomCAR and Vehicle Technologies 

Energy Efficiency and Renewable Energy 

6 
 
Propulsion Materials 	 FY 2003 Progress Report 

2. POWER ELECTRONICS 

A. Low-Cost, High-Energy-Product Permanent Magnets 

Y. S. Cha and John R. Hull 

Energy Technology Division, Bldg. 335 

Argonne National Laboratory, Argonne, IL 60439 

(630) 252-5899; fax: (630) 252-5568; e-mail: yscha@anl.gov 

E. Andrew Payzant 

Oak Ridge National Laboratory 

P.O. Box 2008, MS 6064 

Oak Ridge, TN 37831-6064 

(865) 574-6538; fax: (865) 574-3940; e-mail: payzanta@ornl.gov 

DOE Technology Development Manager: Susan Rogers 

(202) 586-3976; fax: (202) 586-9811; e-mail: susan.rogers@ee.doe.gov 

ORNL Technical Advisor: David Stinton 

(865) 574-4556; fax: (865) 241-0411; e-mail: stintondp@ornl.gov 

Contractor: Argonne National Laboratory, Argonne, Illinois 

Prime Contract No.: W-31-109-Eng-38 

Objective 

•	 Develop a low-cost process to fabricate anisotropic NdFeB permanent magnets (PMs) with an 

increase of up to 25% in energy product (a measure of the torque that a motor can produce for a 

given magnet mass). 

Approach 

•	 Use high fields of superconducting solenoids to improve magnetic grain alignment while 

pressing compacts for sintering, thus producing higher-performance magnets. 

•	 Develop a batch-mode press that enables alignment of the magnetic domains of NdFeB powders 

in the higher-strength magnetic fields created by a superconducting solenoid. 

•	 Characterize, compare, and correlate engineering and microscopic magnetic properties of 

magnets processed under varying conditions, including some in current production. 

•	 Use a reciprocating feed to automate the insertion of loose and compacted magnet powder into, 

and its removal from, the steady magnetic field of a superconducting solenoid. 

Accomplishments 

•	 Achieved energy products within 10% of the theoretical maximum by optimizing batch-mode 

alignment/pressing of cylindrical axial-die-pressed PMs in a 9-Tesla (T) superconducting 

solenoid. The magnetic properties are comparable to those of the more expensive magnets made 

by the transverse-die pressing technique. 

7 
 
FY 2003 Progress Report 	 Propulsion Materials 

•	 Achieved energy product improvements of 15% for thin near-final-shape magnets, production 

of which is the current cost-saving thrust in the PM industry. Additional PMs made with 

relatively small length-to-diameter ratios (L/D <0.5) support the previous findings. 

•	 Completed preliminary conceptual design of a vacuum die-filling and powder-pressing system 

to alleviate cracking that was observed in fabricating compacts with low L/D ratios (<0.25). The 

vacuum system will also speed up the powder-filling and pressing process, necessary for 

industrial acceptance of this new technology. 

•	 Carried out an electromagnetic modeling study of grain alignment in powder compacts. 

Analyses were carried out by assuming that all the magnetic particles are either initially oriented 

in the direction of the alignment field or initially randomly oriented. The results of both 

analyses show that the distortion of the alignment field (caused by the self-field of the compact) 

is inversely proportional to the magnitude of the alignment field. These results will help to 

provide scale-up design rules to industry. 

Future Direction 

•	 Complete the design and fabrication of the vacuum powder-filling and compact-pressing system 

and of the pressurized compact ejection system. Demonstrate that compacts can be made at 

rates acceptable to industry. 

•	 Conduct an economic study comparing the estimated cost of PM manufacturing in continuousmode operation using the superconducting magnet, with the cost of a conventional system 

currently used by industry. 

•	 Continue to collaborate with Magnequench UG on PM fabrication and property measurements, 

with Oak Ridge National Laboratory (ORNL) on microstructure characterization of the finished 

PM samples, and with Ames Laboratory on processing the NdFeB powder for bonded PMs. 

Introduction 

Approach 

The strength of sintered NdFeB magnets 

greatly depends on the method by which the 

compact is magnetically aligned and pressed. 

Large blocks can be made by cold-isostatic 

pressing that are within 5% of their 

theoretical maximum, but these must be 

sliced, diced, and ground to final shape, 

making the magnets very expensive. 

production for high-energy-product, nearfinal-shape PMs. The higher-performance 

magnets will replace ones made by 

traditional powder metallurgy processing 

and enable significant size and weight 

reductions in traction motors for hybrid 

vehicles. 

Magnets that are axial-die-pressed and 

sintered to near-final shape are the least 

expensive to make, but they have magnetic 

properties farthest from their theoretical 

maximums. The current industry goal is to 

fabricate higher-energy-product magnets by 

near-net-shape processing to avoid expensive 

machining operations. The major objectives 

of this project are to increase the energy 

product of the sintered PMs by 10 to 15% (to 

within ~10% of the theoretical maximum) 

and to develop low-cost methods of 

Our approach is to align the NdFeB 

powder in a superconducting magnet, which 

can generate magnetic fields much higher 

than those generated by the electromagnets 

presently used in industry. Alignment of the 

powder in these higher magnetic fields 

improves the properties of the PMs. To 

develop a low-cost mass-production method 

for high-energy-product PMs, we plan to 

design, fabricate, and demonstrate the 

reciprocating axial-die-press system for 

automation. 

8 
 
Propulsion Materials 

Previously, a 9-T superconducting 

solenoid was procured and made 

operational. Production-grade magnet 

powder was obtained from Magnequench 

UG. The 3- to 5-micron single-crystal grains 

of powder were aligned and compacted at 

Argonne National Laboratory (ANL). Then 

the anisotropic compacts, with their grains 

mechanically locked in place, were returned 

to Magnequench UG for sintering, 

annealing, machining, and measuring of 

engineering magnetic properties. Finally, 

selected PMs were sent to ORNL for 

measurement of microscopic properties and 

texture (the alignment of the crystals’ easy 

magnetic axes). 

Previously, we had demonstrated that 

significant improvement of energy product 

(10–15%) can be achieved by using higher 

alignment fields (>2 T).1

 Industry considers 

an improvement in energy product of as 

little as 3–5% compared with current PMs to 

be significant. The progress achieved so far is 

based on a batch process at ANL. Major 

issues remaining for acceptance of the 

technology by industry are (1) to 

demonstrate that PMs can be made at a 

much faster rate than in the batch process 

and (2) to demonstrate that near-final-shape 

PMs can be processed, resulting in greater 

cost reduction. 

Results 

In FY 2003, we continued the previous 

study with emphasis on a smaller L/D ratio 

(<0.25) where improvement in energy 

product is expected to be greater. The most 

effective use of the high alignment fields 

that can be provided by superconducting 

solenoids is in making near-final-shape 

magnets. Their finite and usually short 

length in the direction of magnetization 

makes alignment of the powder grains 

especially difficult. When subjected to a 

uniform alignment field, the powder in the 

die cavity develops a highly nonuniform 

self-field. Because grains align along the total 

field lines, unidirectional alignment can be 

FY 2003 Progress Report 

achieved only by increasing the strength of 

the applied alignment field until the effects 

of the self-field become negligible. Because 

the self-field distortion becomes greater for 

shorter magnets, there will always exist short 

magnets that conventional 2-T 

electromagnets cannot adequately align. 

A 1.125-in. die and punch system was 

used in the present study in order to achieve 

an L/D ratio smaller than 0.25. A large 

number of powdered compacts were 

processed under various conditions. 

However, all the samples were observed to 

have hairline cracks after ejection from the 

die and eventually broke into pieces. Figure 1 

shows one of the cracked samples. 

Figure 1. Photograph of a cracked compact after 

ejection from the die. 

A number of factors could have caused 

the powered compacts to crack (some were 

suggested by our industrial partner, 

Magnequench UG). They include 

•	 Maximum compact pressure and 

duration 

•	 Rate of pressure loading and unloading 

(trapped air) 

•	 Powder fill density 

•	 Lubricant (type and amount) 

•	 Magnetic field for alignment 

•	 Die/punch geometry (e.g., clearance, 

punch diameter, taper angle of die) 

•	 Oxygen content 

9 
•	 Powder homogeneity and filling 

procedure 

•	 Compact ejection pressure 

We have investigated all these factors 

and have concluded that the most likely 

causes for compact cracking are either air 

trapped during powder compaction and/or 

radial stress during compact ejection from 

the die. The production-grade magnet 

powder, received from Magnequench UG, is 

porous and contained a lot of air. If 

sufficient time and clearance are not 

provided during powder compaction, air will 

be trapped in the compact. The trapped air 

can subsequently cause the compact to crack 

when the pressure is released. This is a wellknown problem for this type of process. The 

other likely cause for compact cracking is the 

radial stress during compact ejection from 

the die. Figure 2 shows the stresses on the 

compact during pressing and during 

compact ejection for the current system. 

During compact ejection, the axial stress is 

released while the radial stress is still very 

large; this could cause the compact to crack. 

Figure 2. Schematic diagram showing probable 

cause of cracking during compact 

ejection from the die. 

The industrial practice is to eject the 

compact under axial pressure. In order to 

eject the compact under pressure, the current 

set-up must be upgraded. 

To address the issues of trapped air and 

compact ejection under pressure, we came 

up with an innovative method of filling the 

die and pressing the compact under vacuum. 

Using the vacuum system will reduce the 

trapped air in the powder significantly, 

which (1) may alleviate the cracking 

problem, (2) will increase the powderpressing speed, and (3) will speed up the 

powder-filling process. The last two 

improvements are also a necessary step 

toward demonstration of a process that can 

manufacture PMs at a much faster rate than 

the batch mode. By using the vacuumcompatible die, we also came up with a 

preliminary conceptual design for a system 

that can eject the compact under pressure, 

which may alleviate the cracking problem 

described previously. 

To understand the effect of the self-field 

of the magnet powder on the distortion of 

the alignment field, we carried out an 

electromagnetic study of grain alignment in 

powder compacts using the computer code 

Opera developed by Vector Fields, Inc. 

Previously, we conducted the analysis by 

assuming that all the magnetic particles are 

initially oriented in the direction of the 

alignment field. The results provided a lower 

bound on the distortion of the field lines. In 

the present analysis, we assume more 

realistically that the magnetic particles are 

initially randomly oriented. Figures 3 and 4 

show the calculated field lines of one-quarter 

of a PM for alignment fields of 1 and 8 T, 

respectively. These calculations were carried 

out using the same powder density (3.5 g/m3

) 

and the same L/D ratio (0.643). The field 

lines will all be parallel to the z-axis if there 

is no distortion. It is clear that distortion of 

the alignment field is substantial when the 

alignment field is 1 T (Figure 3). The 

distortion decreases with increasing 

alignment field. At an alignment field of 8 T, 

the distortion becomes very small (Figure 4). 

10 
 
Propulsion Materials FY 2003 Progress Report 

Figure 3. Field lines (one-quarter) for compact 

partially pressed to mechanical 

lockup. Alignment field H =1 T. 

Parameters: ρl

 = 3.5 g/m3, L/D = 

0.643, B0 = 0.65 T. 

Figure 4. Field lines (one-quarter) for 
 

compact partially pressed to 
 

mechanical lockup. Alignment 
 

field H = 8 T. Parameters: ρl

 = 3.5 
 

g/m3

, L/D = 0.643, B0 = 0.65 T. 
 

Figure 5 shows the comparison of the 

field line distortion between the previous 

analysis (assuming magnetic particles are 

initially oriented in the direction of the 

alignment field) and the current analysis 

(assuming magnetic particles are initially 

randomly oriented). B and B are defined as r z 

the radial and the axial components of the 

magnetic flux density B, respectively. The 

vertical axis represents the volume average of 

the ratio B /Bz

, which is a good measure of r 

the overall distortion of the field lines. As 

expected, Figure 5 shows that the distortion 

of the field lines is greater if the magnetic 

particles are initially randomly oriented, 

Figure 5. Comparison of field line distortion 

between previous and present analyses. 

compared with the case where the magnetic 

particles are initially oriented in the 

direction of the alignment field. Both 

analyses show that the distortion of the field 

lines decreases with increasing alignment 

field. 

Conclusions 

Previously, we demonstrated that 

significant improvement of energy product 

(10–15%) can be achieved by using higher 

alignment fields (>2 T). Industry considers 

an improvement of 3–5% in the energy 

product of current PMs to be significant. 

Major issues remaining for acceptance of the 

technology by industry are (1) to 

demonstrate that PMs can be made at a 

much faster rate than in the batch process, 

and (2) to demonstrate that near-final-shape 

PMs can be processed, resulting in greater 

cost reduction. In FY 2003, we continued to 

fabricate powdered compacts with an even 

smaller L/D ratio (<0.25) by using a larger 

(1.125–in.) die and punch set. However, all 

the samples were observed to have cracks 

after ejection from the die. With the help of 

industrial partner Magnequench UG, we 

identified two factors that probably caused 

the compacts to fail: trapped air in the 

11 
 
powdered compact during pressing and the 

radial stress during ejection of the compact 

from the die. 

To alleviate the trapped air problem, we 

decided to use a vacuum-compatible die. We 

have completed a preliminary conceptual 

design of a vacuum die/punch set. The 

system also allows the present system to be 

modified so that the compact can be ejected 

under pressure, which also may alleviate the 

cracking problem. We have completed a 

preliminary conceptual design for upgrading 

the existing system for just this purpose. 

To help us understand and quantify the 

distortion of alignment field due to the selffield of the magnet powder, we continued 

the electromagnetic study of grain alignment 

in powder compacts using the computer 

code Opera. Previously, we assumed that all 

the magnetic particles are initially oriented 

in the direction of the alignment field. In 

the present analysis, we assume more 

realistically that the magnetic particles are 

initially randomly oriented. As expected, the 

results show that the distortion of the field 

lines is greater if the magnetic particles are 

initially randomly oriented. Both analyses 

show that the distortion of the alignment 

field lines decreases with increasing 

alignment field. 

In FY 2004, we plan to complete the 

design, fabricate the vacuum die/punch set, 

and demonstrate that PMs can be 

manufactured at a much faster rate than 

with the batch process. We will also perform 

an economic study comparing the estimated 

cost of the present superconducting system 

with that of the conventional system used by 

industry. Our long-term and final goal is to 

demonstrate the feasibility of competitive 

factory operation by designing, fabricating, 

and operating a reciprocating press in 

continuous-mode operation. 

References 

1. 2002 Annual Progress Report, Automotive 

Propulsion Materials, U. S. Department of 

Energy, Office of Energy Efficiency and 

Renewable Energy, Office of FreedomCar and 

Vehicle Technologies, Washington, D.C. 

FY 2003 Publications/Presentations 

T. M. Mulcahy, J. R. Hull, E. Rozendaal, 

J. H. Wise, and L. R. Turner, “Improving 

Sintered NdFeB Permanent Magnets by 

Powder Compaction in a 9-T Superconducting Solenoid,” presented at the 47th 

Annual Conference on Magnetism and 

Magnetic Materials, Tampa, Florida, 

November 11–15, 2002 (to be included in 

the proceedings). 

T. M. Mulcahy, J. R. Hull, E. Rozendaal, 

J. H. Wise, and L. R. Turner, “Improving 

Sintered NdFeB Permanent Magnets by 

Powder Compaction in a 9-T Superconducting Solenoid,” J. App. Phys. 93(10), pt. 3, 

8680–82 (2003). 

Y. S. Cha and J. R. Hull, “Low-cost Highenergy-product Permanent Magnets,” 

presented at the Capacitor and Magnetic 

Materials, Power Electronics/Electric 

Machinery Program Review, U.S. Council on 

Automotive Research, Southfield, Michigan, 

May 13, 2003 (to be included in the 

proceedings). 

12 
 
Propulsion Materials	 FY 2003 Progress Report 

B. Development of Improved Powder for Bonded Permanent Magnets 

Iver E. Anderson, R. William McCallum, and Matthew J. Kramer 

Metallurgy and Ceramics Program 

Ames Laboratory, Iowa State University 

Ames, IA 50011 

(515) 294-9791; fax: (515) 294-8727; e-mail: andersoni@ameslab.gov 

DOE Technology Development Manager: Susan Rogers 

(202) 586-8997; fax: (202) 586-1600; e-mail: susan.rogers@ee.doe.gov 

Contractor: Ames Laboratory, Iowa State University, Ames, Iowa 

Prime Contract No.: W-7405-Eng-82 

Objectives 

•	 Increase the maximum operating temperature of permanent magnet (PM) electric drive motors 

from 120 to 200°C while maintaining sufficient motor operating characteristics. 

•	 Reduce the cost of isotropic PM material in electric drive motors through the use of bonded 

isotropic magnets and injection molding technologies for high-volume net-shape 

manufacturing. 

Approach 

•	 Develop innovative PM alloy design and processing technology for production of improved PM 

alloy powders for bonded isotropic magnets with a tolerance for high temperatures. 

•	 Investigate PM alloy design improvements through melt-spinning methods, with the specific 

goal of developing an improved spherical magnet alloy powder through gas atomization 

processing. 

•	 Develop an enhanced gas atomization process, along with a gas-phase powder surface reaction 

capability, for production and environmental protection of powder for isotropic bonded 

magnets. 

•	 Conduct experimental isotropic PM molding trials on as-atomized and annealed magnet 

powders to characterize isotropic bonded magnet properties and microstructures in 

collaboration with Magnequench International (MQI) and Oak Ridge National Laboratory 

(ORNL). 

Accomplishments 

•	 Filed a U.S. patent application for a new mixed rare earth (MRE)–Fe-B family of 2-14-1 

compounds with demonstrated high-temperature magnetic stability, better than that of 

commercial isotropic materials, for PM traction motors based on bonded magnets. 

•	 Identified a promising path forward for MRE–Fe-B alloy design to increase the remanence and 

energy product for isotropic PMs while maintaining superior temperature coefficient values. 

•	 Observed that post-atomization annealing may not be needed to improve the magnetic 

properties of spherical MRE–Fe-B powders, potentially a significant processing simplification 

that also can facilitate the fluoride coating process. 

13 
 
FY 2003 Progress Report	 Propulsion Materials 

•	 Performed direct comparative testing of bonded magnet samples molded from experimental 

gas-atomized fine spherical powders and commercial atomized powders, aided by interaction 

with MQI, a major commercial supplier. 

Future Direction 

•	 Pursue an alloy design strategy for MRE–Fe-B to increase the remanence and energy product for 

isotropic PM material, while maintaining superior temperature coefficient values to 200 ̊C, 

using a melt-spinning approach. Use commercial isotropic particulate properties and motor 

designer input as benchmarks. 

•	 Test enhanced MRE–Fe-B alloy designs in gas-atomized spherical powders to verify magnetic 

property improvements in as-solidified and annealed states. Use microstructural analysis to 

investigate the differences between rapidly solidified ribbons and powders with the goal of 

simplified production of optimum powder. 

•	 Refine the fluidized bed fluorination process for gas-atomized MRE–Fe-B powder to optimize the 

coating barrier properties for protection of PM powders during the magnet molding process and 

bonded magnet use. 

•	 Generate bonded magnet samples with increased (> 60%) loading for comparative magnetic 

testing from a polyphenyl sulfide binder blended with experimental or commercial PM 

powders. Develop (in collaboration with an industrial partner) a molding process for producing 

a prototypical motor part with a filled magnet cavity to investigate the effects on the molding 

process of powder/polymer blend characteristics, attempting to achieve maximum magnet 

loading within acceptable molding parameters. 

Introduction 

To meet the cost and performance 

objectives of advanced electric drive motors 

for automotive applications, it is essential to 

improve the alloy design and processing of 

PM powders. This work is critical to enable 

the widespread introduction of electric drive 

automobiles, generally referred to as hybrids 

or fuel cell vehicles, a key technology goal of 

the President of the United States and of 

DOE. Two primary objectives to be pursued 

for PM materials are (1) to increase the useful 

operating temperature for magnets to 200 ̊C 

and (2) to reduce the finished magnet cost to 

about 25% of its current level. Currently, 

high-performance anisotropic sintered 

magnet material can operate with high 

torque in an electric motor at temperatures 

ranging up to about 120 ̊C, and the finished 

cost of anisotropic sintered magnetic 

material is approximately $90/kg. Difficult 

motor assembly procedures and the need for 

corrosion protection (by surface coatings) 

also add to the cost for PM motors made 

from sintered anisotropic PMs. As an 

alternative PM material form, polymerbonded isotropic particulate magnets offer 

the benefit of greatly simplified 

manufacturing and improved (polymer 

encapsulation) corrosion protection. 

Compared with anisotropic sintered 

magnets, they provide a more moderate level 

of stored magnetic energy (25–50%), which 

is still compatible with advanced PM motor 

designs. However, to exploit the potential of 

bonded isotropic PM materials for such 

motors, it is necessary to develop a 

particulate magnet material with hightemperature properties that can be loaded to 

a high volume fraction in an advanced 

polymer binder. Improving materials and 

processing to enable increased operating 

temperatures and simplified production of 

net-shape bonded isotropic magnets by gas 

atomization and injection molding will be a 

significant advance toward mass production 

of advanced electric drive motors at reduced 

cost. 

14 
 
Propulsion Materials 

Approach 

Innovative PM alloy design and 

processing technology will be developed for 

production of improved PM alloy powders 

based on the Nd2Fe14B, or “2-14-1” phase for 

bonded isotropic PM magnets with a 

tolerance for high temperatures. Melt 

spinning will be used to select alloy 

modifications that boost coercivity, 

remanence, and energy product at elevated 

temperatures (up to 200°C) and improve 

alloy quenchability for optimum yield from a 

gas atomization process. 

This effort builds on an alloy concept 

patented by Ames Laboratory for improved 

quenchability of the neodymium (Nd)-based 

2-14-1 phase using, for example, TiC. A highpressure gas atomization process was 

modified for enhanced yields of spherical, 

rapidly solidified powder of improved PM 

alloys. This atomization work uses the 

patented Ames Laboratory technologies for 

bonded magnets from passivated fine 

spherical powders. A secondary gas-phase 

reaction coating process is being developed 

to increase environmental protection of the 

fine magnet alloy powder without degrading 

the magnetic properties or molding 

rheology. This work builds on recent studies 

of fluidized bed fluorination of similar rare 

earth (RE) alloy powders. A model polymer, 

polyphenyl sulfide (PPS), was selected and is 

being used to test injection molding 

characteristics and magnetic properties of 

high-temperature bonded isotropic magnets. 

The magnetic properties of bonded 

isotropic powder samples will be determined 

as a function of loading fraction, powder 

size, annealing schedule, coating treatment, 

and temperature, up to a maximum of 

200 ̊C, in collaboration with industrial 

partner MQI of Raleigh-Durham, North 

Carolina. Some of the atomized powder and 

bonded isotropic magnets also will be 

provided to ORNL for additional 

microstructural and magnetic 

characterization. To improve further the 

potential for successful technology transfer, 

FY 2003 Progress Report 

industrial interactions have been emphasized 

at Ames Laboratory. These include work with 

MQI on alloy design and rapid solidification 

processing through a DOE cooperative 

research and development agreement 

(CRADA) on advanced sensors and control of 

the melt-spinning process, and other CRADA 

projects and research collaborations with 

U.S. Council on Automotive Research 


partners. 


Results 

In FY 2003, the Iowa State University 

Research Foundation filed a U.S. patent 

application on November 18, 2002, for a 

“Permanent Magnet Alloy with Improved 

High Temperature Performance.” Even nonoptimized MRE–Fe-B PMs based on this 

invention have demonstrated exceptional 

resistance to high-temperature 

demagnetization, with useable magnetic 

properties up to 250 ̊C, beyond the target 

temperature range of this project. The 

scientific basis of the work reaches back to 

fundamental characterization of 2-14-1 

compounds made from the complete range 

of possible RE elements, along with Fe and B 

additions. It was recognized that a mixture 

(probably substitutional and random) of 

alternative RE elements in a 2-14-1 phase 

may have an average of the magnetic 

properties of the well-known Nd2Fe14B phase, 

as shown in Figure 1. Such MRE–Fe-B 

compounds appear to offer some new alloy 

design territory that can be exploited for 

raising the useful temperature range for high 

magnetic performance in this type of PM 

phase. The prototype of this new family of 

MRE–Fe-B magnet alloys is composed of an 

equi-atomic mixture of Y and Dy in the 

2-14-1 stoichiometric compound of 

YDyFe14B. A thorough search of the open 

literature verified that this type of compound 

is novel and thus suitable for patent filing. 

Melt spinning was used to generate a 

range of rapidly solidified ribbon samples 

with compositions within the range of the 

novel MRE–Fe-B PM alloy family. Wheel 

15 
 
FY 2003 Progress Report 

Figure 1. Illustration of the intermediate 

temperature dependence of the 

anisotropy of Nd-based 2-14-1 phase, 

showing the potential for hightemperature magnet alloy design with 

mixed REs, e.g., Y and Dy. 

speeds of 10, 16, and 22 m/s were used to 

simulate the range of gas atomization 

quench rates from argon to helium gas. 

Annealing temperatures were tested from 

650 to 850 ̊C to seek beneficial solidification 

and annealing conditions to use for 

subsequent spherical powder processing. 

X-ray diffraction (XRD) measurements 

indicated that the as-solidified ribbons 

contained a phase mixture of MRE2Fe17 phase, 

some 2-14-1 phase, and a minor amorphous 

phase fraction, without any reflections from 

bcc-Fe. Upon annealing, XRD data show that 

the magnetic microstructure transformed 

into essentially single-phase 2-14-1. The 

magnetic hysteresis loop of an annealed 

ribbon sample of the YDyFe14B compound is 

illustrated in Figure 2, which also shows the 

reduced effect of elevated temperature on the 

magnetic properties. 

To explore the trends suggested in 

Figure 1, some ribbon samples were made 

with a heavier balance of Y, compared with 

Dy. The expectation was a lower coercivity 

and energy product but a further-decreased 

temperature dependence of the magnetic 

properties compared with the equi-atomic 

Propulsion Materials 

Figure 2. SQUID magnetometer measurements of 

a melt-spun (@22 m/s) ribbon sample of 

YDyFe14B that was annealed at 700 ̊C for 

15 min and measured at 27 and 127 ̊C, 

where a 15% loss of energy product was 

observed for a 100 ̊C temperature rise. 

mixture of Y and Dy. As Figure 3 shows, the 

energy product was only slightly reduced and 

coercivity was slightly increased. Also, the 

temperature effect was slightly higher than 

expected. Apparently, this MRE compound 

has an increased range of tolerance for alloy 

adjustments, which needs further testing. 

Figure 3. SQUID magnetometer measurements of 

a melt-spun ribbon sample of 

Fe Y 14B (Y-rich) that was annealed 1.35Dy0.95 

at 650 ̊C for 15 min and measured at 27 

and 127 ̊C, where a 20% loss of energy 

product was observed for a temperature 

rise of 100 ̊C. 

16 
 
Propulsion Materials FY 2003 Progress Report 

In recent work, melt spinning was used to 

explore the effect on magnetic properties of a 

range of Nd additions, known to promote 

2-14-1 remanence, using an alloy 

formulation of [Ndx(YDy)1/2(1-x)]2.2Fe B. The 14 

intent was to seek an understanding of the 

magnetic property dependence of this 

extended MRE–Fe-B formulation, with the 

goal of improved remanence and energy 

product without adverse effects on the 

desirable temperature coefficients for 

coercivity and remanence. As expected from 

relative magnetic moment data, a rising 

trend of the ambient temperature energy 

product was observed with Nd additions to a 

level at × = 0.8 of about 11 MGOe, almost 

double the initial value of about 6 MGOe 

(Figure 4). The coercivity exhibited a falling 

trend over the same range of Nd content, but 

the lowest level of about 14 kOe is still 

appreciable for many magnet applications. 

The temperature coefficients displayed a 

more nonlinear behavior, but the ability to 

adjust these properties was also apparent. 

More study of similar alloy series will permit 

rapid identification of promising 

compositions for continued powder 

processing experiments using gas 

atomization. 

Following trials to determine processing 

corrections to the nominal composition, gasatomized powder of the prototype YDyFe14B 

composition of our novel magnet alloy was 

produced under high quenching conditions 

(helium atomization gas) and tested at 

ambient and elevated temperature as a 

function of powder size and annealing 

treatment. XRD measurements of the assolidified powder indicated that 2-14-1 

compound was essentially the only product 

phase without any detectable amorphous 

phase, even in the finest powder size 

fraction. 

In full agreement with the XRD results, it 

was discovered that the as-atomized 

YDyFe14B powder did not need annealing to 

enhance magnetic properties, contrary to the 

behavior of melt-spun ribbon of the same 

composition, as discussed earlier. As Figure 5 

illustrates, the only result of annealing 

appeared to be a slight reduction of 

coercivity that is typically associated with 

coarsening of the 2-14-1 grain size. In other 

words, the atomized droplet solidification 

process appears to produce directly a 

desirable magnetic microstructure, without 

the need for an additional high-vacuum 

Figure 4. Summary of the effect of Nd additions to an [Ndx(YDy)1/2(1-x)]2.2Fe14B alloy, using 

melt-spun ribbon samples, on (a) ambient temperature (27°C), coercivity, and 

energy product and (b) the temperature coefficients for coercivity and remanence, 

measured between 27 and 127°C. 

17 
 
FY 2003 Progress Report 

Figure 5. SQUID magnetometer results that show 

the effect of annealing on as-atomized 

YDyFe14B powder that had been sieved 

at 20 μm to exclude larger particles. 

heat-treatment procedure with a highly 

controlled dwell time. The implications of 

this observation, if verified for other alloy 

compositions in the MRE–Fe-B alloy family, 

can be significant for industrial powder 

processing simplification and thus for cost 

reduction of the resulting bonded magnets. 

In other characterization results for the 

helium-atomized YDyFe14B powder, 

Propulsion Materials 

unannealed powder was divided into a wide 

range of size fractions for magnetometer 

testing. As shown in Figure 6, while the 

finest powder had a high coercivity of 

13 kOe, even some of the larger particle size 

classes exhibited a coercivity greater than 

10 kOe. In fact, size distribution analysis of 

the full powder yield from this atomization 

trial indicates that 90% (by mass) of the asatomized powder yield has a diameter of 

<45μm and, as Figure 6 shows, a coercivity 

above 9.5 kOe. Thus only a simple screen 

cut, instead of pneumatic size classification, 

would be needed, presumably, to prepare 

this powder for coating and bonding. This 

type of observation, if verified for more 

optimized alloys, would have additional 

beneficial effects on bonded magnet cost 

reduction. 

One key indication of a promising PM 

alloy for high-temperature service is a 

reduction in the downward slope with 

increased temperature of coercivity and 

remanence, expressed as temperature 

coefficients ∆Hci and ∆Mr, respectively. All 

of the commercial materials listed in Table 1 

are offered for use in bonded magnets and 

can serve as a useful benchmark. Numerous 

experimental melt-spun and atomized 

Figure 6. Effect of particle size on the coercivity of as-atomized YDyFe14B powder. 

18 
Propulsion Materials FY 2003 Progress Report 

Material Mr Hci (BH) ∆Mr 

(%/

∆Hci 

(%/

7.2 9.5 10.0 –0.11a –0.4a 

7.6 9.4 11.5 –0.13a –0.4a 

8.2 14 14.0 –0.13a

 –0.4a 

8.95 10 15.8 –0.11a

 –0.4a 

14

13

Y Dy Fe

Nd2Fe14

14

6.1 

6.3 

5.89 

7.2 

4.7 

15 

12.3 

17 

10.8 

13 

8.1 

7.5 

7.6 

6.8 

4.4 

–0.07b 

–0.03b 

–0.09b 

–0.16b 

b

 –0.33b 

–0.33b 

–0.32b 

–0.38b 

–0.33b 

Table 1. Summary of magnetic properties and temperature coefficients for representative 

commercial (MQ prefix, from MQI) and experimental alloys (as listed) in both melt 

spun ribbon particulate form and atomized spherical powder form (underlined) 

(kG) (kOe) max  ̊C)  ̊C) 

MQP-S-9-8 

MQP-S-11-9 

MQP-O

MQP-B 

YDyFe B (X5-132B) 

YDyFe CoB (X5-201A) 

1.35 0.95 14.6B (MWF43A) 

B-TiC (BT-4-138) 

YDyFe B (BT-4-204) –0.085 a

Measured from 22 to 100°C. 

b

Measured from 27 to 127°C 

samples with a variety of compositions 

(examples provided in Table 1) were subject 

to ambient and elevated temperature SQUID 

magnetometer measurements, which 

determined a desirable reduction in ∆Hci 

values (from 27 to 127°C), compared with 

those values for commercial melt-spun and 

atomized particulate (from 22 to 100°C). 

Moreover, the ∆Mr values for all 

experimental MRE-based alloys were reduced 

considerably, compared with the commercial 

materials. The exception is the Nd-based 

atomized powder (with a composition 

similar to that of the commercial materials) 

that had a comparable ∆Mr and most other 

magnetic properties. Generally, the MRE 

experimental alloys exhibited increased 

coercivity, but improvements in remanence 

and energy product are needed to exceed the 

commercial-standard materials. Systematic 

modifications of the experimental alloys are 

being pursued to gain the understanding 

needed to enhance the full range of magnetic 

properties to enable superior hightemperature bonded magnets. 

An additional requirement for hightemperature bonded magnets is the ability to 

resist oxidation and corrosion in the 

environment of a polymer molding process 

and during service in a “semi-permeable” 

polymer matrix. This is especially important 

to enable bonding of fine atomized powders 

with a high surface-to-volume ratio without 

sacrificing active magnetic volume. The 

fluorination coating process under 

development is based on low-temperature 

(about 200 ̊C) reaction of magnet alloy 

powder in a fluidized bed with NF3 gas. This 

coating has promise for a wide range of 

environmental conditions, but air oxidation 

appeared to represent a sufficient screening 

test for the coated powders. Thermogravimetric analysis results on annealed and 

fluoride-coated Nd-based gas-atomized 

powders demonstrated a 50°C increase in air 

oxidation stability for powders with 

diameters of < 25μm. To ensure that only a 

minor fraction of the spherical powder 

volume was affected by the coating reaction, 

the powder surfaces were analyzed with 

scanning electron microscopy, as shown in 

Figure 7, and Auger electron spectroscopy. 

These results revealed no surface disruption 

or rumpling and a fluoride penetration depth 

of only 15 nm, well within the range of 

desirable surface film coverage. Initial 

fluorination processing experiments on 

unannealed YDyFe14B powders indicated a 

reduced reaction temperature, compared 

with the Nd-based powders; and the 

resulting coating exhibits similar increases in 

air oxidation resistance, compared with 

uncoated powders. Tests for fluoride coating 

thickness (Auger) are in progress. When final 

alloy design permits full development of the 

fluoride coating process, this capability is 

19 
 
FY 2003 Progress Report 

Figure 7. Representative results of scanning 

electron microscope analysis of 

fluorinated Nd2Fe14B-TiC powder 

showing no detectable surface damage 

from the coating process. 

likely to permit ambient (air) powder 

handling and processing and a marked 

increase in bonded magnet environmental 

stability in drive motor applications. 

To complete the series of technology 

development areas for improved bonded PM 

magnets, gas-atomized spherical magnet 

alloy powders have been subjected to 

bonding experiments using PPS as a hightemperature (melting point of about 300°C) 

polymer binder. Using desired test part 

dimensions and granulated PPS provided by 

MQI, a procedure was developed for dry 

blending and hot compression molding of 

cylindrical bonded magnets (diam = 9.5 mm, 

height = 9.5 mm). Magnet samples were 

molded from experimental Nd2Fe B-TiC 14 

powder that was annealed and coated, and 

from commercial MQP-S-11-9 powder in an 

as-received condition. Two levels of loading, 

50 and 60 vol%, were used, and the resulting 

bonded samples were sent to MQI for pulse 

magnetization and characterization to 

investigate the magnetic properties as a 

function of powder loading fraction. As 

Propulsion Materials 

shown in Figure 8, the data for the bonded 

MQI powder are a close match to the 

predicted curve, using a well-established 

value for the maximum energy product of 

the fully aligned and sintered magnet alloy. 

On the other hand, the data for the Ames 

experimental powder rest well above the 

predicted curve, perhaps reflecting 

uncertainty in the BHmax value of an optimum 

form of this alloy. Preliminary bonded 

magnet samples made from the experimental 

YDyFe14B powder were produced and sent for 

testing. Characterization results are not yet 

available. A polymer-assisted molding 

process with this new type of magnet alloy 

powder needs considerable development in 

collaboration with our industrial partner to 

extend the magnet powder loading fraction 

above 70% while maintaining suitable 

molding pressures for practical bonded 

magnet applications. 

Figure 8. Comparison of PPS bonded magnet 

energy product data points to loading 

fraction prediction curves calculated 

]

bonded = ƒ2

from [(BH)max (BH)max , where the 

MQI and Ames curves are based on 

(BH)max values of 11.1 and 6.8 MGOe, 

respectively. 

Conclusions 

Considerable progress on a novel alloy 

design approach for a PM compound with 

high-temperature magnetic stability has 

20 
 
Propulsion Materials 

produced a U.S. patent application. This new 

MRE–Fe-B family of 2-14-1 compounds has 

demonstrated increased coercivity with 

reduced temperature coefficients for 

remanence and coercivity, better for traction 

motor applications than the current 

commercial particulates or spherical powders 

for bonded magnets. A clear path forward for 

MRE–Fe-B alloy design was also identified to 

increase the remanence and energy product 

while maintaining superior temperature 

coefficient values. Initial gas atomization 

trials with MRE-Fe-B suggest that postatomization annealing may not be needed to 

improve the magnetic properties of the 

spherical powders, potentially a significant 

processing simplification. The fluidized bed 

fluoride coating process appears to enhance 

air oxidation stability of both Nd-based and 

MRE-based 2-14-1 fine powders, perhaps 

producing a beneficial effect on powder 

handling during polymer processing and 

adding to bonded magnet service life. 

Bonded magnet samples also have been 

molded from a high-temperature polymer 

and the gas-atomized powders, and their 

magnetic properties have been characterized 

through a relationship with a major 

commercial supplier in the bonded magnet 

industry. 

References 

R. W. McCallum, K. W. Dennis, B. K. 

Lograsso, and I. E. Anderson, “Method of 

Making Bonded or Sintered Permanent 

Magnets,” U.S. Patent 5,240,513, August 31, 

1993. 

I. E. Anderson, B. K. Lograsso, and R. L. 

Terpstra, “Environmentally Stable Reactive 

Alloy Powders and Method of Making Same,” 

U.S. Patent 5,372,629, December 13, 1994. 

R. W. McCallum, K. W. Dennis, B. K. 

Lograsso, and I. E. Anderson, “Method of 

Making Bonded or Sintered Permanent 

Magnets (continuation),” U.S. Patent 

5,470,401, November 28, 1995. 

I. E. Anderson and R. L. Terpstra, 

“Apparatus for Making Environmentally 

FY 2003 Progress Report 

Stable Reactive Alloy Powders,” U.S. Patent 

5,589,199, December 31, 1996. 

I. E. Anderson, B. K. Lograsso, and R. L. 

Terpstra, “Environmentally Stable Reactive 

Alloy Powders and Method of Making Same,” 

U.S. Patent 5,811,187, September 22, 1998. 

R. W. McCallum and D. J. Branagan, 

“Carbide/Nitride Grain Refined Rare EarthIron-Boron Permanent Magnet and Method 

of Making,” U.S. Patent 5,803,992, 

September 8, 1998. 

R. W. McCallum and D. J. Branagan, 

“Carbide/Nitride Grain Refined Rare EarthIron-Boron Permanent Magnet and Method 

of Making,” U.S. Patent 5,486,240, 

January 23, 1996. 

D. J. Branagan and R. W. McCallum, “The 

Effects of Ti, C, and TiC on the 

Crystallization of Amorphous Nd2Fe14B,” 

J. Alloys and Compds., 245 (1996). 

D. J. Branagan, T. A. Hyde, C. H. Sellers, 

and R. W. McCallum, “Developing Rare 

Earth Permanent Magnet Alloys for Gas 

Atomization,” J. Phys. D: Appl. Phys., 29, 

p. 2376 (1996). 

M. J. Kramer, C. P. Li, K. W. Dennis, 

R. W. McCallum, C. H. Sellers, D. J. 

Branagan, L. H. Lewis, and J. Y. Wang, 

“Effect of TiC Additions to the 

Microstructure and Magnetic Properties of 

B6 Nd Melt-spun Ribbons,” J. Appl. 9.5Fe84.5 

Phys., 83(11), pt. 2, p. 6631 (1998). 

D. J. Branagan and R. W. McCallum, 

“Changes in Glass Formation and Glass 

Forming Ability of Nd2Fe14B by the Addition 

of TiC,” J. Alloys and Compds., 244 (1–2), 

p. 40 (1996). 

FY 2003 Publications/Presentations 

M.J. Kramer, Y. Xu, K.W. Dennis, I. E. 

Anderson, and R.W. McCallum, 

“Development of Improved Powder for 

Bonded Permanent Magnets,” IEEE 

Transactions on Magnetics, (in press, 

publication in September 2003). 

I. E. Anderson, “Development of 

Improved Powder for Bonded Permanent 

21 
 
FY 2003 Progress Report Propulsion Materials 

Magnets,” presented at INTERMAG 2003, presented at PM2 TEC 2003, APMI/MPIF 

Boston, Massachusetts, April 2003. Annual Meeting, Las Vegas, Nevada, June 

I. E. Anderson, “Development of Powder 2003. 

Processing for Bonded Permanent Magnets,” 

22 
 
Propulsion Materials 	 FY 2003 Progress Report 

C. Carbon Foam for Electronics Cooling 

Nidia C. Gallego, Beth Armstrong, April D. McMillan, and J. W. Klett 

Oak Ridge National Laboratory 

P.O. Box 2008, MS 6087, Bldg. 4508 
 

Oak Ridge, TN 37831-6087 
 

(865) 241-9459; fax: (865) 576-8424; e-mail: gallegonc@ornl.gov 
 

B. E. Thompson and A. G. Straatman
 

University of Western Ontario,
 

London, Ontario, Canada 
 

DOE Technology Development Manager: Rogelio Sullivan 

(202) 586-8042; fax: (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

ORNL Technical Advisor: David P. Stinton 

(865) 574-4556; fax: (865) 241-0411; e-mail: stintondp@ornl.gov 

Contractor: Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Prime Contract No.: DE-AC05-00OR22725 

Objectives 

•	 Collaborate with automotive partner to develop carbon foam heat exchanger and heat sink 

designs that dissipate more than 30 W/cm2

 using standard cooling fluids. 

•	 Develop a strategy to reduce the high cost of carbon foam. 

•	 Determine the foam structure or morphology that results in optimum heat transfer for various 

thermal management applications. 

Approach 

•	 Study fundamental mechanisms of heat transfer in carbon foam and develop an engineering 

model that allows comparison of conventional and carbon foam heat exchangers. 

•	 Develop a cost model that can be used to guide process development efforts to reduce the high 

cost of carbon foam. 

•	 Build and operate a passive evaporative cooling system (PECS) to evaluate and compare the 

performance of carbon foam and diamond heat spreaders. 

Accomplishments 

•	 Developed a process cost model that indicates that the price of the pitch precursors and the 

time and temperature of the carbonization and graphitization cycles have the greatest influence 

on the cost of carbon foam. 

•	 Developed an engineering model of heat flow through carbon foam that indicates that carbon 

foam heat exchangers failed to perform as expected because cooling air flowing over the foam 

failed to access the extensive surface area. This would suggest that designs that force air through 

the foam and not over the foam would perform much better. 

23 
 
FY 2003 Progress Report 	 Propulsion Materials 

•	 Proved high-conductivity carbon foam to be an excellent heat spreader when evaluated in the 

PECS system; it dissipated more than three times the amount of heat dissipated using state-ofthe-art diamond heat spreaders. 

Future Direction 

•	 Conduct processing studies focused on finding alternative low-cost pitch precursor materials to 

replace the current raw materials and therefore reduce the cost of carbon foam. 

•	 Modify the structure or morphology of the carbon foam for optimum heat transfer. Modeling 

results indicate that more open, less dense structures with easily accessible porosity would 

increase the heat transfer. 

•	 Operate the PECS using carbon foam heat spreaders with more open, less dense structures. 

Introduction 

Many improved electronic components 

introduced in recent decades, such as higherpower computer chips and power converters, 

generate significantly more heat and require 

more efficient devices for dissipating that 

heat. Many techniques have been explored 

to improve the efficiency of heat transfer 

devices, such as micro-channels, heat pipes, 

and other exotic designs; however, none has 

been able to adequately cool the electronics. 

These devices must incorporate very effective 

heat spreaders into the design of the heat 

sink to prevent localized hot spots and 

ensure that the temperature of the 

electronics does not exceed 125oC. 

The high-conductivity carbon foam 

developed at Oak Ridge National Laboratory 

(ORNL) is an open-cell structure with highly 

aligned graphitic ligaments (see Figure 1); 

studies have shown the typical interlayer 

spacing (d002) to be 0.3356 nm, very near 

that of perfect graphite (0.3354 nm). As a 

result of its near-perfect structure, thermal 

conductivities along the ligament are 

calculated to be approximately 1700 W/m•K, 

with bulk conductivities = 180 W/m•K. 

Furthermore, the material exhibits low 

densities (0.25 to 0.6 g/cm3

) so that the 

specific thermal conductivity is 

approximately four to five times greater than 

that of copper. The high conductivity 

combined with the very high surface area 

(20,000 m2

/m3) results in overall heat transfer 

Figure 1. Graphite foam material developed at 

ORNL. 

coefficients for foam-based heat exchangers 

that are up to two orders of magnitude 

greater than those of conventional heat 

exchangers. As a result, foam-based heat 

exchangers or heat sinks could be 

dramatically smaller and lighter than 

conventional ones. 

Automotive and industry partners are 

very concerned about the high cost of highconductivity carbon foam. They are 

convinced that the cost of carbon foam must 

be reduced significantly before it can be used 

in most automotive applications. Work was 

24 
 
Propulsion Materials 

initiated during FY 2003 to determine what 

factors have the greatest impact on the cost 

of the foam. Once those factors have been 

identified, research efforts will be redirected 

to those areas. 

Results 

Cost Modeling of Carbon Foam 

A detailed process economic model was 

constructed of the ORNL high-thermalconductivity carbon foam manufacturing 

process.1

 The Excel-based model included the 

effects of 146 input variables on the 

expected sales price and manufacturing cost 

of this material. It also provides estimates of 

the capital investment, plant size, and 

employee level required. A set of values of 

the input variables was established as a 

baseline case, using data supplied by ORNL 

or obtained from industrial sources and 

potential suppliers. The results of the 

baseline case were analyzed to determine the 

most significant cost drivers (see Figure 2) 

FY 2003 Progress Report 

and the sensitivity of results to these cost 

drivers. A set of advanced model analysis 

tools was used to visualize the relative 

magnitude of these drivers and their 

potential effect on product price. Risk 

analysis showed the probability of deviation 

from the expected product price. Another 

analysis provided insight into the sensitivity 

of results to production volume. From these 

results and analyses, we can provide the 

following results, which are based on high 

production volumes. 

Assuming that major variables are close 

to the baseline values, the price of carbon 

foam at a production rate of 44,000 parts 

(2×3×1.5 in.) per week will be approximately 

$4.95/piece ($0.55/in.3

), with a 

manufacturing-only cost of $2.97/piece 

($0.33/ in.3

). 

This level of production will require a 

capital investment for equipment of 

approximately $9.25 million and will require 

about 8400 ft2

 of manufacturing space, 

29 technicians/operators, and 3 engineers. 

Figure 2. Carbon foam process cost factors. 

25 
FY 2003 Progress Report 	 

The density of the foam product has the 

greatest influence on product price, on the 

basis of price per cubic inch. This result is 

apparently caused by the strong influence of 

amount of pitch required per volume of 

foam and the high sensitivity of foam price 

to pitch price. 

Pitch price has the next greatest 

influence on foam price. Price could vary 

from $0.45/in.3

 to $0.68/in.3 as the pitch 

price varies from $3 to $8/lb. The pitch raw 

material accounts for 24% of the sale price of 

foam (see Figure 3). 

Figure 3. Correlation of the cost of the pitch 

precursor and the cost of carbon foam. 

After business costs such as profit and 

sales cost, the three furnace operations are 

the next most important cost drivers. The 

carbonization and graphitization times and 

temperatures need to be reduced to 

minimize cost. 

Modeling of Porous Carbon Foam Heat 

Exchangers 

An engineering model2 was developed 

that provides values for the thermal 

resistance and pressure drop in air-water heat 

exchangers with plate-fin and annular-fin 

configurations, based on a combination of 

the following: 

•	 measurements of bulk conductivity of 

porous carbon foam 

Propulsion Materials 

•	 well-established correlations for 


convective heat transfer from tubes, 


plates, and fins 


•	 measured thermal resistance at the 

interface between aluminum and porous 

carbon foam with a number of different 

joints 

•	 an engineering approximation for the 

effects of porosity on convective heat 

transfer 

The model was compared both with 

available measurements obtained with 

conventional aluminum heat exchangers 

and with copycat heat exchangers made 

from carbon foam. Agreements between 

calculated and measured values of the heat 

transfer coefficient and pressure drop were 

sensible and within about 15% and 10%, 

respectively, which is adequate for typical 

design needs in thermal engineering. 

The model suggests that finned carbon 

foam heat exchangers failed to perform as 

expected because the air flowing over the 

carbon foam failed to access the extensive 

surface area. If radiator designs could be 

developed that effectively use the surface 

area, 20 to 60% improvements over 

optimum aluminum-finned heat exchangers 

could be achieved. Note that the 

performance of a carbon foam heat 

exchanger is controlled more by the surface 

area accessed than by the high conductivity 

of the foam. This suggests that the porosity, 

pore density, dendrite structure, and 

strength of the carbon foam should be 

optimized simultaneously to enhance heat 

transfer with minimal increase in pressure 

losses. 

It may not be possible to access sufficient 

surface area of carbon foam in conventional 

finned heat exchanger designs. This would 

suggest that designs that force air through 

the foam, and not over the foam, may be 

required. The structure of the foam would 

act like the fins of conventional heat 

exchangers. These designs would use all of 

the available surface area because the 

structure of the foam would act as the “fins.” 

26 
 
Propulsion Materials 

In a cost model developed recently, the 

development of less-expensive pitch 

precursors was found to have the most 

dramatic effect on the cost of the carbon 

foam. Low-cost precursors have not been 

pursued in the past because we were 

concerned that the thermal conductivity 

would be negatively impacted. The 

engineering model was exercised to 

determine the significance of the thermal 

conductivity of the carbon foam for the 

performance of radiators. It was quite 

surprising to learn that a 25 to 50% 

reduction in thermal conductivity had only 

a minor effect (~5%) on the performance of 

the heat exchangers. 

Passive Evaporative Cooling 

A PECS was built in FY 2003 and 

operated extensively at ORNL. A simple 

schematic illustrating the concept is shown 

in Figure 4. A more thorough description of 

the operation of the system is included in 

article 2D, “Evaporative Cooling of Power 

Electronics” in this annual report. Similar 

systems developed by the National Security 

Agency that contained a limited-surface-area 

diamond heat spreader were able to achieve 

a power density of 28 W/cm2

 and were 

considered to be state-of-the-art technology. 

The system that was operated at ORNL with 

Figure 4. Schematic diagram 

of the evaporative 

cooling system. 

FY 2003 Progress Report 

a graphite foam heat spreader was able to 

achieve a power density of over 120 W/cm2

, 

more than four times the power density 

attained using the diamond heat spreader. 

The graphite foam has proved to be an 

excellent heat spreader because of the 

extensive surface area, the very high thermal 

conductivity, and the fact that the foam is 

easily wetted by the evaporative coolants. 

Conclusions 

A process cost model was developed that 

evaluated the effect of more than 100 

variables on the cost of carbon foam. The 

model indicates that the price of the pitch 

precursors and the time and temperature of 

the carbonization and graphitization cycles 

have the greatest influence on the cost of 

carbon foam. An engineering model of heat 

flow through carbon foam was also 

developed that indicates that carbon foam 

heat exchangers failed to perform as 

expected because cooling air flowing over 

the foam failed to access the extensive 

surface area. Additional research is needed to 

understand the relationship between 

material structure and heat transfer. Findings 

suggested that more-porous, less-dense foams 

would recruit additional surface area— 

provided, of course, that the carbon structure 

has adequate strength. The capability to 

control pore density (pores per inch) and to 

maintain an open pore structure (percentage 

of open pores) is important if the benefits of 

the porous surface structure are to be realized 

in practical applications. Finally, carbon 

foam proved to be a very effective heat 

spreader when used in an evaporative 

cooling system. The carbon foam heat 

spreader was able to dissipate more than four 

times as much heat as the previously used 

diamond heat spreader. 

References 

1. E. H. Kraft, Process Economic Model for 

High Thermal Conductivity Carbon Foam, Final 

Report, June 4, 2003. 

27 
 
FY 2003 Progress Report 

2. B. E. Thompson, A. G. Straatman, R. 

Culham, M. Yovanovich, A. G. Hunt, Q. Yu, 

and P. Oosthuisen, Modeling of PorousCarbon-Foam Heat Exchangers, February 28, 

2003. 

FY 2003 Publications/Presentations 

N. C. Gallego and J. W. Klett, “Carbon 

Foams for Thermal Management”, presented 

at the International Seminar: Advanced 

Applications for Carbon Materials, Jeju, 

Korea, September 12–13, 2002 (to be 

published in the proceedings). 

N. C. Gallego and J. W. Klett, “Carbon 

Foams for Thermal Management,” Carbon 41 

(2003) 1461–1466. 

N. C. Gallego, J. W. Klett, and A. D. 

McMillan, “Effect of Processing Conditions 

on Properties of Graphite Foams,” presented 

Propulsion Materials 

at Carbon ’02: An International Conference 

on Carbon, Beijing, China, September 15–19, 

2002 (to be published in the proceedings). 

N. C. Gallego, A. D. McMillan, and S. K. 

Stinton, “Effect of Heat Treatment on the 

Rheological Properties of Mesophase Pitch 

and Final Properties of Carbon Foam,” 

presented at Carbon ’03: An International 

Conference on Carbon, Oviedo, Spain, 

July 6–10, 2003 (to be published in the 

proceedings). 

N. C. Gallego, T. D. Burchell, and J. W. 

Klett, “Neutron Irradiation Damage in 

Graphite Foam and Its Effect on Properties,” 

presented at Carbon ’03: An International 

Conference on Carbon, Oviedo, Spain, 

July 6–10, 2003 (to be published in the 

proceedings). 

28 
 
Propulsion Materials 	 FY 2003 Progress Report 

D. Evaporative Cooling of Power Electronics 

R. D. Ott, B. L. Armstrong, A. D. McMillan, C. A. Walls 

Oak Ridge National Laboratory 

P.O. Box 2008, MS 6083, Bldg. 4508 

Oak Ridge, TN 37831-6083 

(865) 574-5172; fax: (865) 574-4357; e-mail: ottr@ornl.gov 

DOE Technology Development Manager: Nancy Garland 

(202) 586-5673; fax: (202) 586-9811; e-mail: nancy.garland@ee.doe.gov 

ORNL Technical Advisor: David Stinton 

(865) 574-4556; fax: (865) 574-6918; e-mail: stintondp@ornl.gov 

Contractor: Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Prime Contract No.: DE-AC05-00OR22725 

Objectives 

•	 Identify methods of utilizing carbon (graphite) foam for passive evaporative cooling of power 

electronics. 

•	 Study the durability of graphite foam under environmental conditions similar to those of 

various applications. 

•	 Design and build a prototype evaporative chamber for testing different methodologies for 

evaporative cooling. 

Approach 

•	 Construct test chamber with a hollow cavity for evaporative fluid and vapor. 

•	 Conduct tests with the unit, with either chilled water or air as a cooling medium, to regulate 

variables to determine the effectiveness of the cooling system. 

•	 Measure the durability of the graphite foam for thermal stability and compressive stress, 

corrosion, erosion, vibration, and salt fog resistance. Construct and conduct the appropriate 

tests for each of the six durability areas. 

Accomplishments 

•	 Determined that pool boiling and thin film evaporation are effective methodologies for use of 

graphite foam for evaporative cooling. 

•	 Demonstrated power dissipations of up to 1500 W at temperatures of up to 100°C. 

•	 Found no effect of thermal cycling over a ∆T of 60–75°C and 100,000 cycles on the durability of 

graphite foam. In addition, no decrease in compressive stress or modulus resulted after cycling. 

•	 Found that the foam structure itself is unaffected by exposure to a mixture of propylene glycol 

and water (ZerexTM) at room temperature, 60°C, and 100°C for up to 1000 hours. However, the 

mechanical integrity of the solder attaching the foam to the substrate is a valid concern. 

29 
 
FY 2003 Progress Report 	 Propulsion Materials 

Future Direction 

•	 Combine this research project in FY 2004 with the project “Carbon Foam for Electronics
 

Cooling.” 
 

Introduction 

A novel technique for creating pitchbased carbon foam was developed at Oak 

Ridge National Laboratory (ORNL)1,2 before 

1997. This technique uses mesophase pitch as 

a starting material but does not require the 

costly blowing or stabilization steps that 

typical carbon foams require.3

 The ORNL 

foam is an open-cell structure with highly 

aligned graphitic ligaments; studies have 

shown the typical interlayer spacing (d002) 

to be 0.3356 nm, very near that of perfect 

graphite (0.3354 nm). Because of its nearperfect structure, thermal conductivities 

along the ligament are calculated to be 

approximately 1700 W/m·K, with bulk 

conductivities 180 W/m·K. Furthermore, 

the material exhibits low densities (0.25– 

0.6 g/cm3

) such that the specific thermal 

conductivity is approximately four to five 

times greater than that of copper. The very 

high surface area (20,000 m2

/m3) combined 

with the high thermal conductivity suggests 

that graphite foam has significant potential 

for application as a thermal management 

material. 

One potential application of graphite 

foam as a thermal management material is in 

the area of cooling integrated circuit chips 

(ICCs). Current electronic ICCs, similar to 

power electronic integrated gate bipolar 

transistors used to control voltages in hybrid 

vehicles, are placed printed-side-up in a 

ceramic package with “over-the-top” wire 

bonds. Unfortunately, this design is not 

suited for significant heat dissipation through 

the top of the package to a standard heat 

sink, as there is an insulating air gap between 

the silicon chip and the ceramic package. A 

new concept being developed is the “flipchip” design, in which the silicon chip is 

inverted and the back of the printed chip is 

oriented toward the top of the package. In 

this case, it is simple to join a heat spreader 

directly to the silicon chip and immerse it in 

an evaporative cooling fluid (i.e., fluorocarbon). In the process, the latent heat of 

vaporization removes significantly more heat 

than the sensible heat change of the fluid 

and transfers it very efficiently to the fins of 

the heat sink. 

The limitations of this design are the 

surface area and thermal conductivity of the 

spreader mounted to the back of the chip. 

Current state-of-the-art spreaders are 

polycrystalline diamond wafers with thermal 

conductivities of up to 1600 W/m·K (more 

than four times that of copper). As a result of 

the limited surface area of the diamond 

spreader, the maximum power density 

achieved without overheating the system is 

28 W/cm2

. When the diamond spreader is 

replaced with graphite foam, a power density 

of 100 W/cm2 is attained without overheating 

the system. Theoretically, significantly higher 

power densities can be attained with proper 

design and fluid content. If this concept 

could be applied to power electronics, the 

benefit would be substantial in reducing 

temperatures, increasing reliability, and 

possibly reducing costs. 

One issue of importance with regard to 

the performance of the ORNL foam is its 

durability under typical operating conditions. 

It is hoped that once we understand how the 

foam behaves in environments 

approximating actual operating parameters, it 

can be modified as necessary to meet 

application-specific needs. To this end, 

several durability studies have been 

undertaken, the constraints directed by either 

the Energy Efficiency Tech Team or by 

industrial partners. The studies include 

thermal cycling, compression testing, 

corrosion, erosion, vibration, and salt spray 

(fog). 

30 
 
Propulsion Materials 

Approach 

The use of a passive evaporative cooling 

system (PECS) would be advantageous in ICC 

applications to minimize pressurized systems 

and limit the weight and size of existing 

spray cooling systems. Most PECSs are 

characterized by pool boiling or thin film 

evaporation modes. The high thermal 

conductivity and surface area of the graphite 

foam lends itself well to these applications, 

but demonstration of its feasibility was 

required. A PECS was constructed based on a 

hollow aluminum chamber with graphite 

foam mounted onto a heater plate. An 

evaporative fluid (evaporant) will be added to 

the system, and the heat flux will be 

measured as a function of heat added to the 

system. 

Six sets of durability experiments will be 

carried out in the areas of thermal stability 

and compressive stress, corrosion, erosion, 

vibration, and salt spray (fog) exposure. The 

thermal conductivity and compressive stress 

change will be measured as a result of 

thermal exposure (∆T of 60–75°C and 

100,000 cycles at 5 gal/min). Thermal 

conductivity and joint integrity will be 

measured and examined as a result of 1000­

hour exposures in 50% propylene glycol/50% 

water mixture at room temperature, 60°C, 

and 100°C. Mass loss will be measured after 

exposure to 1600 ft/min airflow. Thermal 

conductivity and structural integrity will be 

measured and examined after vibration 

conditions of 7 g and 10–20 Hz for 40 hours. 

Mass change and thermal conductivity will 

be measured after exposure to salt fog 

[modified standard Society of Automotive 

Engineers (SAE) J12111 and SAE J726]. 

Results 

The PECS was built and evaluated as an 

appropriate methodology for testing the heat 

dissipative capability of graphite foam in ICC 

applications. A simple schematic illustrating 

the concept of the PECS is shown in Figure 1. 

The PECS is hollow and contains an 

evaporator (graphite foam) and a working 

FY 2003 Progress Report 

Figure 1. Schematic diagram of the 

evaporative cooling system. 

fluid at a reduced pressure equal to the vapor 

pressure of the fluid at room temperature. 

Heat is added to the bottom of the PECS, 

passes through the metal base plate 

(Figure 1), and is then transferred to the 

graphite foam through a soldered or brazed 

joint. The extremely high ligament thermal 

conductivity of the foam (equivalent to that 

of diamond) rapidly transmits the heat to all 

the internal surfaces of the foam. 

Concurrently, capillary action wicks the 

working fluid into the foam and coats the 

internal surfaces. After all the permanent 

gases are removed (gases that do not 

condense, such as oxygen or nitrogen), any 

heat that is added to the system disturbs the 

equilibrium, and the working fluid 

evaporates (or boils). The vapor then 

condenses on a wall of the chamber that is 

cooled externally (typically with air). 

The advantage of the graphite foam is 

that its extremely high surface area results in 

dramatically higher heat fluxes that can be 

transmitted to the working fluid uniformly. 

This aspect of the project was conducted in 

collaboration with another project, 

“Advanced Cooling for Naval Radars,” which 

was funded by Office of Naval Research. The 

feasibility of pool boiling and of the PECS 

31 
 
FY 2003 Progress Report 

was demonstrated using a chip provided by 

the National Security Agency (NSA) as the 

heater core in a flip-chip design 

configuration. Power dissipations of up to 

1300 W at temperatures of up to 100°C were 

demonstrated (Figure 2). The feasibility of 

using graphite foam as a heat spreader to 

prevent hot spots on a chip or dissipate high 

heat fluxes was demonstrated. 

Figure 2. Heat flux results of a PECS test using an 

NSA chip and graphite foam as the heat 

spreader. 

Several durability tests were performed to 

determine the graphite foam’s ability to 

withstand corrosion, thermal cycling, 

vibration, erosion, and salt spray (fog). Each 

test was conducted following ASTM standards 

or a modification of a standard, and the work 

is still ongoing in order to establish a 

database of durability information. The 

durability tests were also conducted in 

collaboration with another DOE project, 

“Carbon Foam for Electronics Cooling.” A 

milestone report3

 entitled Durability of 

Graphite Foam: Effects of Thermal Cycling, 

Compression, Corrosion, Erosion, Vibration, and 

Salt Fog was completed in June 2003. The 

procedures and detailed results from the 

durability tests can be found in the milestone 

report. Some of the significant data and 

conclusions are described in the following 

paragraphs. 

Thermal cycling tests were performed in 

an infrared furnace that allowed the 

Propulsion Materials 

temperature to be rapidly cycled from room 

temperature to 110°C in a matter of minutes. 

After more than 42,000 thermal cycles, there 

was no statistically significant change in 

compressive stress4

 or compressive modulus 

(see Figure 3.) 

Figure 3. Graphite foam thermal cycling 

compression test results: maximum 

compression stress. 

To address the concern that the graphite 

foam might become corroded in applications 

where it is in direct contact with an 

evaporant or coolant, a study was completed 

to determine if the foam structure and 

conductivity, or the reliability of the braze or 

weld that joins the foam to the support 

material, was altered after exposure. Samples 

of graphite foam were exposed to a 50% 

propylene glycol/50% water mixture (Zerex) 

for up to 1000 hours at room temperature, 

60°C and 100°C.5

 The study samples 

consisted of a series of treated and untreated 

foam blocks bonded to aluminum, copper, 

brass, and stainless steel plates using 50/50 

(tin/lead) solder. From this initial study, it 

was evident that the foam structure itself is 

unaffected by exposure to Zerex at 

temperatures of up to 100°C after 1000 hours 

(Figure 4). However, the mechanical integrity 

of the solder attaching the substrate (metal 

plate) to the foam was affected, especially at 

the 100°C condition. 

Erosion tests were carried out on graphite 

foam samples (1-in. cube with a 0.25-in. hole 

machined through the center) to measure the 

32 
 
Propulsion Materials FY 2003 Progress Report 

Figure 4. Effects of corrosion on thermal conductivity after 1000 hours in Zerex solution 

at room temperature, 60°C, and 100°C. 

mass loss and mass loss rate at a given 

impinging air velocity. Air was blown onto 

the graphite foam cubes at a face velocity of 

approximately 1600 ft/min for 5 min. The 

mass loss rate of uncoated and coated carbon 

foam exceeded the design limits set by the 

industrial partners directing this test set. 

Thus the graphite foam would not be 

acceptable for applications subject to 

impinging airflow. 

Vibration testing was conducted to 

identify any degradation of the graphite 

foam or the interface of a foam mounted to a 

metallic substrate. Thermal conductivity 

before and after testing was used as the 

measure of degradation. None of the 18 foam 

samples tested showed any visual degradation 

or physical debonding from the substrates. 

Thermal conductivity measurements were 

identical before and after testing, indicating 

no degradation of either the foam structure 

or the bond between the foam and substrate. 

Conclusions 

A PECS was built and operated extensively 

in FY 2003. The performance of the system 

operated using a diamond heat spreader was 

compared with the performance of a graphite 

foam heat spreader. The system operating 

with the graphite foam heat spreader proved 

to be far superior and dissipated more than 

three times as much heat as the diamond 

heat spreader. The total heat transfer of 

approximately 1500 W at temperatures of less 

than 60°C on the substrate demonstrates that 

graphite foam is a viable material as a heat 

dissipater or heat spreader for power 

electronics applications. The capability to use 

a PECS will enable applications in automotive 

markets where weight is a concern. 

The durability tests showed that graphite 

foam could withstand several conditions, 

such as thermal cycling and salt fog exposure, 

without degradation of the structure or its 

properties. Since graphite foam has a very 

low compressive modulus (0.08–0.114 GPa) 

33 
 
FY 2003 Progress Report 

compared with other materials commonly 

used as heat sinks or spreaders, such as 

aluminum (70 GPa), it is anticipated that the 

foam will be highly shock resistant. The data 

from the thermal cycling and compression 

testing indicate the same conclusion. The 

corrosion tests revealed that the foam 

structure itself was unaffected by the 

exposure to the Zerex solution even at 100°C. 

However, issues related to the bonding of the 

graphite foam to metallic substrates were 

raised as a concern, indicating that the 

selection of the solder relative to the 

substrate (heat exchanger or base plate) will 

be important. The erosion tests revealed that 

the foam erosion rate was too high for 

applications subject to impinging airflow. 

There was no evidence that the vibration 

testing produced an effect on the thermal 

properties or the bonding interface that 

would limit its use in automotive 

applications. 

References 

1. J. Klett, “High Thermal Conductivity, 

Mesophase Pitch-Derived Carbon Foam,” in 

Proceedings of the 1998 43rd International 

SAMPE Symposium and Exhibition, Part 1 (of 2), 

Anaheim, California, 1998. 

2. J. Klett, “High Thermal Conductivity, 

Mesophase Pitch-Derived Carbon Foams,” 

Journal of Composites in Manufacturing, 15(4), 

1–7 (1999). 

Propulsion Materials 

3. B. L. Armstrong, A. McMillan, 

N. Gallego, R. Ott, M. Trammell, and 

C. Walls, Durability of Graphite Foam: Effects of 

Thermal Cycling, Compression, Corrosion, 

Erosion, Vibration, and Salt Fog, Letter Report, 

Oak Ridge National Laboratory, July 2003. 

4. American Society of Testing and 

Materials C695, Standard Test Method for 

Compressive Strength of Carbon and Graphite, 

2000. 

5. Society of Automotive Engineers 

J12111, Recommended Environment Practices for 

Electronic Equipment, 1978. 

FY 2003 Publications/Presentations 

N. C. Gallego and J. W. Klett, “Carbon 

Foams for Thermal Management,” Carbon 41 

(2003) 1461–1466. 

N. C. Gallego and J. W. Klett, “Carbon 

Foams for Thermal Management,” presented 

at International Seminar: Advanced 

Applications for Carbon Materials, Jeju, 

Korea, September 12–13, 2002 (to be 

published in the proceedings). 

N. C. Gallego, J. W. Klett and A. D. 

McMillan, “Effect of Processing Conditions 

on Properties of Graphite Foams,” presented 

at Carbon ’02: An International Conference 

on Carbon, Beijing, China, September 15–19, 

2002 (to be published in the proceedings). 

34 
 
Propulsion Materials	 FY 2003 Progress Report 

E. High-Temperature Film Capacitors 

B. A. Tuttle, G. Jamison, D. Wheeler, J. S. Wheeler, and D. Dimos 

Sandia National Laboratories 

P.O. Box 5800, MS 1411 

Albuquerque, NM 87185-1405 

(505) 845-8026, fax: (505) 844-2974 

DOE Program Managers: Susan Rogers 

(202) 586-3976; fax( (202) 586-9811; e-mail: susan.rogers@ee.doe.gov 

Rogelio Sullivan 

(202) 586-8042; fax (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

Graham Hagey 

(202) 256-2457; e-mail: graham.hagey@hq.doe.gov 

Contractor: Sandia National Laboratories, Albuquerque, New Mexico 

Prime Contract No.: 04-94AL85000 

Objectives 

•	 Develop a replacement technology for aluminum electrolytic dc bus capacitors currently used 

for fuel cell/electric hybrid vehicles. 

•	 Develop a high-temperature polymer dielectric film technology that has dielectric properties 

technically superior to those of aluminum electrolytic dc bus capacitors and that is of 

comparable or smaller size. 

•	 Scale up cost-competitive polymer film dielectric technology. 

•	 Develop high-volume production process for dielectric film roll fabrication. 

•	 Fabricate prototype capacitors. 

Approach 

•	 Contact automobile design and component engineers, dielectric powder and polymer film 

suppliers, and capacitor manufacturers to determine state-of-the-art capabilities and to define 

market-enabling technical goals. 

•	 Develop a project plan with automobile manufacturers and large and small capacitor companies 

to fabricate polymer dielectric sheets suitable for FreedomCAR capacitor manufacturing. 

•	 Synthesize unique conjugated polyaromatic chemical solution precursors that result in 

dielectric films with low dissipation factors (DFs) and excellent high-temperature dielectric 

properties. 

•	 Develop polymer film processes and technologies that will lower costs to permit competitive 

high-volume capacitor manufacturing. 

•	 Interact with Oak Ridge National Laboratory (ORNL) with regard to mechanical characterization 

of films. 

35 
 
FY 2003 Progress Report	 Propulsion Materials 

Accomplishments 

•	 Invented chemical synthesis procedures that resulted in polymer films with four times the 

energy density of commercial polyphenylene sulfide (PPS) at 110°C. Scaled-up hydroxylated 

polystyrene (PVOH) films exhibited dielectric constants of 4.7 and DFs of 0.009 at temperatures 

of up to 110EC, thus meeting technical requirements for commercialization. 

•	 Collaborated with TPL, Inc., and Steiner Corporation to adjust high-volume metallization 

procedures for Sandia National Laboratories (SNL) 10-in.-wide, 450-ft-long dielectric film. 

•	 Initiated a project to investigate the effects of three different cross-linker species and their 

concentrations on the dielectric and mechanical properties of SNL PVOH films. This effort 

would permit fabrication of wound capacitors by high-volume metallization processing. 

•	 Collaborated with ORNL to characterize mechanical properties of PVOH films. 

Future Direction 

•	 Perform extensive electric field and temperature characterization to determine that SNL 
 

polyfilm dielectrics will meet FreedomCAR requirements for breakdown field and DF for 
 

temperatures from –40 to 110oC. 
 

•	 Fabricate and characterize 1-μF wound capacitors using non-high-volume metallization 

techniques to determine if they will meet General Motors (GM) requirements. Supply capacitors 

to GM, Ford, and Chrysler for testing in inverter-type environments. 

•	 Have large-value (20- to 200-μF) capacitors fabricated by vendor(s) evaluated by GM and SNL in 

simulated electric hybrid vehicle environments. 

•	 Develop chemical synthesis procedures for high-performance, more cost-effective films. 

Results 

Strategy and Interactions 

SNL has actively interacted with a 

number of representatives from the 

automobile industry to obtain their 

perspective on what is needed for 2004 

automobiles. In FY 2003, SNL continued its 

emphasis on the development of polymer 

film dielectrics for two reasons. First, GM has 

been adamant about soft breakdown 

dielectric film technology—a phenomenon 

that bulk ceramic capacitors do not exhibit. 

Second, emphasis on polymer dielectrics 

provides greater balance of the DOE effort 

between polyfilm and ceramic technologies, 

as requested by the Electrical and Electronics 

Tech Team. SNL has developed a close 

working relationship with the Advanced 

Technology Vehicles Division of GM. Based 

on phone calls and e-mail exchanges with 

GM staff that outlined technical progress and 

component qualification procedures, SNL 

has modified project plans for the 

development of polymer film dielectrics for 

FreedomCAR applications. 

These interactions led us to the 

conclusion that the most viable replacement 

technology for electrolytic dc bus capacitors 

by 2004 is multilayer polymer film 

capacitors. Reducing the size of the polymer 

capacitors was most often cited by 

automobile design engineers and capacitor 

manufacturers as a needed technologyenabling breakthrough. In addition, it is 

necessary to improve high-temperature 

(110EC) performance while keeping the 

technology cost-competitive. For polymer 

film dielectrics, based on the GM 2004 

criteria for capacitance density of 2.0 μF/cm3

, 

the team agreed that if a polyfilm of K = 4.5 

could be developed that met the operating 

temperature requirements, then that film 

would be suitable for scale-up. 

36 
 
Propulsion Materials 

Based on the criteria from GM, an 

individual dielectric layer thickness of 

approximately 3 μm for polyfilm capacitors 

is projected. These thickness values are based 

on operating field strengths of 2 MV/cm for 

the newly developed polyfilm capacitors. 

Based on these assumptions and on 

measurement of presently available 

commercial capacitors, size comparisons and 

capacitance densities were obtained for 

500 μF dc bus capacitors for different 

technologies. Soft breakdown behavior and 

lower cost are assets for polymer film 

capacitors. The projected polymer film 

capacitor volume was calculated assuming 

that there is 40% non-active capacitor space 

and 200-nm thick electrodes are used. Note 

that the volumetric capacitance efficiency for 

a K = 4.5 polyfilm capacitor of 2.4 μF/cm3 

exceeds the near-term commercialization 

goal of 2 μF/cm3

. 

Polymer Film Dielectric Development 

SNL polymer film dielectric development 

has been based on the request from 

manufacturers that the new polyfilm 

dielectrics have voltage and temperature 

stability equivalent to that of present PPS 

technology. Thus a structural family of 

polymer dielectrics has been designed and 

synthesized to meet two of the most 

stringent FreedomCAR requirements: (1) low 

dielectric loss and (2) extremely good 

temperature stability. A patent disclosure has 

been initiated covering the design and 

synthesis techniques for this polymeric 

family. A further criterion for wound 

FY 2003 Progress Report 

capacitors is that PVOH films can be 

adequately released from underlying carriers 

without tearing. In FY 20O3, we investigated 

three different cross-linker species at four 

different concentration levels in an attempt 

to enhance the ductility of our PVOH-based 

polymer. Our baseline cross-linker species 

was VEctomer 4010 at the 30 wt% level. 

Figure 1 shows schematic diagrams of the 

three different types of cross-linkers 

evaluated, VEctomer 4010 and 5015 and 

tetraethylene glycol divinyl ether (TEGDE). 

Structurally, these cross-linkers varied in the 

amount of rigid benzene cores and the 

density of vinyl ether termini. 

PVOH films were synthesized using MEK 

solvent—similar to that used for casting at 

Brady Corporation of Milwaukee. 

Concentrations of 5, 15, 20, and 30 wt% 

were investigated. It is encouraging that we 

were able to synthesize films that had 

reasonable dielectric loss at low cross-link 

addition levels. Based on the work of 

Wereszczak, films with less than 30 wt% 

cross-linker have slightly lower hardness and 

elastic modulus. An example of low dielectric 

loss (low DF) with low cross-link density is 

shown in Figure 2 for VEctomer 5015 crosslinker. The dielectric constant and DF were 

measured at 1 kHz and 25°C in Figure 2. DFs 

of 0.005 were measured for the 5 wt% crosslinker addition at 110°C as well as for the 

25°C measurements shown. Breakdown 

strengths appeared quite reasonable, in the 

2.2 to 4.2 MV/cm range, for these spindeposited films of approximately 0.5 μm in 

thickness on aluminum-coated silicon 

wafers. These measurements were made 

Figure 1. Schematic diagrams of three different cross-linkers: (a) VEctomer 4010, 

(b) VEctomer 5015, and (c) TEGDE 

37 
 
FY 2003 Progress Report Propulsion Materials 

Figure 2. Dielectric constant and dissipation factor of SNL 

hydroxylated polystyrene film as a function of 

VEctomer 5015 cross-linker concentration. 

using 3-mm-diam gold electrodes. Based on 

mechanical property data, we will select one 

of the lower (less than 30 wt%) cross-link 

concentration films for scale-up at Brady 

Corporation. 

Figure 3 is a pictorial view of our PVOH 

film scale-up activities in FY 2003. 

Continuous slot-die-cast PVOH polymer film 

sheet is shown in Figure 3(a). The PVOH film 

sheets deposited on mylar carriers range from 

20 to 50 ft in length. The 10-in.-wide film 

roll is then slit to 2-in. width for capacitor 

winding. We worked with TPL of 

Albuquerque to fabricate these slit rolls. 

Development of technology that was more 

amenable to high-volume production 

required interaction with Brady Corporation. 

We were able to cast 450-ft lengths of 10-in.-

wide polymer film using the Brady prototype 

caster on 6-in.-diam mandrills. A highvolume electrode deposition process step is 

essential to manufacturing large-value 

capacitors with good volumetric efficiency. 

Steiner Corporation deposited aluminum 

electrodes on SNL films by a proprietary 

high-volume manufacturing process. The slit 

film roll electroded by Steiner is shown in 

Figure 3b. Because our polymer film tore 

during winding, the cure temperature of the 

PVOH film was decreased and the Steiner 

process was modified. Two wound polymer 

film capacitors of the modified process are 

shown in Figure 3c. Silver epoxy 

terminations were used after the zinc flame 

spray termination process was unsuccessful. 

Our dielectric film thickness is presently 4 to 

5 μm for these capacitors. 

Summary 

Critical economic and technical issues for 

improvement of dc bus capacitors for 

FreedomCAR vehicles were determined 

through discussions and visitations with 

automobile design engineers, chemical 

synthesis companies, and capacitor 

manufacturers. PVOH film dielectric 

development was emphasized in FY 2003, 

and we have developed a multi-step project 

38 
 
Propulsion Materials FY 2003 Progress Report 

Figure 3. SNL PVOH film scale-up activities. 

plan for large-scale commercialization of 

polymer film dc bus capacitors. In FY 2003, 

we developed, in conjunction with TPL, a 

continuous process for fabricating 450-ftlong rolls of SNL PVOH sheets. We 

investigated several different aspects of 

capacitor fabrication, including lowertemperature cures and adjustment of 

deposition parameters at Steiner Corporation 

to minimize the brittleness of SNL PVOH 

films. There are different termination 

procedures for wound polymer film 

capacitors. SNL polymer film wound 

capacitors of 1-μF capacitance were 

fabricated using two different termination 

techniques, metallized paper and highvolume electrodes, and with and without oil 

backfilling. These capacitors met the 

extended temperature requirements for fuel 

cell vehicles. In addition, we are supplying— 

on a continuous basis—dielectric films to 

Andrew Wereszczak of ORNL from our scaleup and development activities. Understanding mechanical property–process 

relationships is critical to developing film 

technology that exhibits adequate release 

from the underlying carrier. In FY 2004, we 

will concentrate our efforts on fabricating 

capacitors using aluminum foil rather than 

high-volume metallization processing. SNL 

PVOH film rolls fabricated at Brady Corporation will be used to fabricate a series of 1-μF 

and larger capacitors for evaluation by SNL, 

ORNL, GM, Ford, and Daimler Chrysler. 

Presentations and Publications 

1. D. Wheeler and G. Jamison, “Novel 

Polymer Film Synthesis Routes of Voltage 

and Temperature Stable Dielectrics,” 

October 5, 1999. 

2. B. A. Tuttle, D. Wheeler, G. Jamison 

and D. Dimos, “dc Buss Capacitors for PNGV 

Power Electronics,” Automotive Propulsion 

Materials 2002 Annual Progress Report, U.S. 

Department of Energy, pp. 27–32, October, 

2002. 

39 
 
FY 2003 Progress Report Propulsion Materials 

3. B. Tuttle, J. Wheeler, G. Jamison, Components for Pulsed Power Applications 

D. Williams, D. Wheeler, J. Cesarano and P. Workshop, Arlington, Virginia, June 2–3, 

G. Clem, “Dielectric Materials for Fuel Cell 2003 (invited), to be published in the 
 

Vehicles and Pulsed Discharge Applications,” proceedings.
 

presented at the Dielectric Materials and
 

40 
 
Propulsion Materials 	 FY 2003 Progress Report 

F. Mechanical Reliability of Electronic Materials and Electronic Devices 

A. A. Wereszczak, L. Riester, H. -T. Lin, and M. K. Ferber 

Ceramic Science and Technology Group 

Oak Ridge National Laboratory 

P.O. Box 2008, MS 6068, BLDG 4515 

Oak Ridge, TN 37831-6068 

(865) 576-1169; fax: (865) 574-6098; e-mail: wereszczakaa@ornl.gov 

DOE Technology Development Manager: Nancy Garland 

(202) 586-5673; fax: (202) 586-9811; e-mail: nancy.garland@ee.doe.gov 

ORNL Technical Advisor: David Stinton 

(865) 574-4556; fax: (865) 241-0411; e-mail: stintondp@ornl.gov 

Objectives 

•	 

•	 

Approach 

•	 

E H

•	 

•	 E H

E H

•	 

•	 

•	 

films. 

Contractor: Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Prime Contract No.: DE-AC05-00OR22725 

Identify and use mechanical test methods that accommodate the testing of hydroxylated 

polystyrene (PVOH) dielectric film specimens (and geometries thereof) provided by Sandia 

National Laboratories (SNL). 

Measure and interpret mechanical properties of PVOH films and identify which are the best 

candidates for future manufacturing scale-up based on mechanical robustness. 

Perform mechanical properties microprobing on a suite of PVOH-elastomer films to quantify 

the elastic modulus ( ) and hardness ( ) of each. 

Perform additional mechanical tests (e.g., scratch-testing, indentation creep) in an effort to 

further assess the PVOH-elastomer films’ propensity to crack or tear under a tensile strain. 

Accomplishments 

Measured the and of 15+ PVOH-elastomer films, finding that the use of 4010 elastomer in 

PVOH resulted in lower and than the use of 5015 and tetraethylene glycol divinyl ether 

(TEGDE) elastomers in PVOH. 

Conducted scratch testing (which exploits a film’s propensity to crack or spall) that showed 

PVOH-4010 films tend to be more spall resistant than PVOH-5015 and PVOH-TEGDE films. 

Future Direction 

Measure the tensile strength distribution of the two down-selected elastomer-PVOH dielectric 

films as a function of temperature and link the strengths to the population of strength-limiting 

flaws. 

Predict and verify allowable manufacturing and service strains of elastomer-PVOH dielectric 

41 
 
FY 2003 Progress Report 

Introduction 

A primary focus of the Power Electronics 

effort in the Automotive Propulsion 

Materials Program is to develop polymer 

capacitor technology that will replace 

current electrolytic, dc bus capacitors for 

power electronic modules in hybrid electric 

vehicles. The ultimate objective is to make 

the power modules more compact while 

maintaining tight voltage and temperature 

requirements and long service life without 

compromise caused by mechanical 

breakdown of the dielectric film. Toward 

that end, we collaborated with SNL in this 

project to mechanically evaluate a suite of 

SNL-manufactured PVOH dielectric polymers 

that have the potential to satisfy the 

objectives. The present effort has two 

objectives: (1) measure baseline mechanical 

properties of that suite of PVOH 

compositions and interpret their results so as 

to suggest which are most suitable for 

manufacturing scale-up, and (2) quantify the 

mechanical performance of those 

manufactured films so manufacturers and 

end-users of these dielectric films may use 

them without mechanical breakdown. 

Results 

A suite of PVOH-elastomer films were 

provided by SNL for mechanical 

characterization. The PVOH films were 

fabricated so that each one used one of three 

different elastomers designated as 4010, 

5015, and TEGDE in varied additive contents 

of up to 30 wt %. The PVOH-elastomer 

matrix is shown in Table 1. The films were 

spin-coated onto metallized (aluminum) 

silicon wafers; a representative cross-section 

of this three-material system is shown in 

Figure 1. The PVOH-elastomer film was 

nominally 600 nm thick and was well 

bonded to the aluminum metallization layer 

under it. Because of the small film thickness 

and strong bonding (i.e., film removal was 

prohibited), the mechanical characterization 

was limited to use of a mechanical property 

microprobe (MPM) and other techniques 

Propulsion Materials 

Table 1. Test matrix of elastic modulus and 

hardness tests 

Dielectric film Elastomer 

content Anneal (oC) 

PVOH 0% 220 and 250 

PVOH/4010 5% 220 and 250 

15% 220 and 250 

20% 220 and 250 

30% 220 and 250 

PVOH/5015 5% 250 

15% 250 

20% 250 

30% 250 

PVOH/TEGDE 5% 250 

15% 250 

20% 250 

30% 250 

Figure 1. Cross-section of PVOH dielectric film 

(top), aluminum electrode layer 

(middle), and silicon substrate (bottom). 

that could test the mechanical robustness of 

the PVOH-elastomer film in situ. 

The MPM is an automated instrument 

consisting of four primary components: an 

indenter (Berkovich indenter) whose vertical 

displacement (nanometer resolution) and 

applied load (microgram resolution) are 

controllable; an optical microscope (OM) 

42 
 
Propulsion Materials 

with several objective lenses; a precision X 

and Y stage that translates the 

metallographically prepared specimen 

between the OM and the indenter; and a 

computer that controls the OM’s lens turret 

(50–1500× magnification), stage movement, 

and indenter load and displacement. A 

schematic of the MPM is shown in Figure 2. 

Figure 2. Schematic representation of the 

mechanical properties microprobe. 

The computer also collects data on the 

indenter’s displacement and load and is 

interfaced with a TV monitor and camera 

that show the microscope’s field of view and 

allow inspection of each indentation. The 

MPM is housed inside an insulated cabinet 

that serves to minimize its susceptibility to 

laboratory room temperature fluctuations 

and vibrations (which are problematic when 

controlling displacement at the nanometer 

level). In a further effort to minimize the 

effects of vibration and thermal fluctuations, 

both the computer and the MPM-containing 

cabinet are housed in an insulated room 

within the laboratory. 

The load/displacement history generated 

during indentation can be interpreted with 

the aid of an appropriate model to calculate 

the E and H of the material, as shown in 

Figure 3. H is calculated by dividing the peak 

load by the residual indent area, while E is 

determined from the slope of the unloading 

curve. 

FY 2003 Progress Report 

Figure 3. Typical load-displacement curve 

generated during and unloading with 

mechanical properties microprobe. 

The measured E and H as a function of 

elastomer and elastomer concentration are 

shown in Figures 4 and 5, respectively. 

Sublayers under the top film can influence 

the apparent E and H of thin films if the 

indentation penetrates too far, so only E and 

H measurements from sufficiently shallow 

indentation depths (< ~10% of the PVOH 

film thickness) were considered. The E and H 

of the PVOH/4010 films tended to be of 

lower value than those of PVOH/5015 and 

PVOH/TEGDE films for a given additive 

content, particularly for additive content of 

5 and 15%. The E and H of PVOH/4010, 

PVOH/5015, and PVOH/TEGDE were 

arguably equivalent for 20 and 30% additive 

content. Regarding the effect of annealing 

temperature on PVOH/4010, it was observed 

that annealing at 250°C rather than 220°C 

tended to increase the E, though H was 

essentially unaffected by the warmer 

temperature. 

In an additional effort to distinguish 

differences among the PVOH films, 

indentation creep tests were performed on 

selected compositions (0%, 15% and 30% of 

each elastomer―all 250oC annealed) of the 

test matrix shown in Figure 1. A 5-mN load 

was applied, and the displacement of the 

deepening indenter penetration was 

43 
 
FY 2003 Progress Report 

Figure 4. Comparison of elastic modulus as a 

function of elastomer additive content. 

Figure 5. Comparison of hardness as a function 

of elastomer additive content. 

measured as a function of time to 

10 minutes. These results were found not to 

be a discriminator, as the measured creep 

responses of these seven films were 

equivalent, although the PVOH/30% TEGDE 

showed anomolously rapid creep 

deformation. 

A limitation of the described indentation 

testing is that the results are all a 

consequence of compressive loading, 

whereas past observed cracking or tearing in 

PVOH (and other) dielectric films fabricated 

at SNL was a consequence of tensile 

straining. Even though that limitation was 

recognized early, prior to the indentation, 

the indentation proceeded because of the 

Propulsion Materials 

nature of the supplied PVOH/aluminum/ 

silicon test coupons and because a tensile 

test could not be performed on that PVOH 

film. If differences in E or H or creep 

resistance among the PVOH films in the test 

matrix existed, then these compressionbased mechanical tests would have found 

them, and those differences potentially 

could have been used to infer tensile 

performance. As described, though, the 

differences in E or H or creep resistances were 

nonexistent or subtle. 

A series of scratch tests was performed on 

seven PVOH films (0% additive content and 

15% and 30% content of 4010, 5015, and 

TEGDE) using a diamond stylus, and 

differences were analyzed. The use of such a 

test was pursued because it subjected a 

component of tensile strain to the PVOH 

film, and the performance of the film under 

a positive sign of strain could be examined. 

OM images in Figures 6–8 of the 0%, 15%, 

and 30% content 5015, respectively, show 

the former two PVOH films exhibited little 

or no spallation (i.e., brittleness), whereas 

there was significant spalling in the 30% 

5015/PVOH film. Assuming that the material 

state of the films has not changed since the 

time of their deposition, the scratch test 

results suggest that the 4010/PVOH systems 

Figure 6. Scratch of PVOH (no elastomer). 

44 
Propulsion Materials FY 2003 Progress Report 

Figure 7. Scratch of PVOH 15% 5015/PVOH. 

Figure 8. Scratch of 30% 5015/PVOH. 

are the least brittle in that there was no 

significant evidence of spallation in the 

vicinity of the applied scratch for either the 

15 or 30% concentrations of 4010. The 30% 

5015 and 30% TEGDE films showed 

significant amounts of spallation around the 

scratch, indicating a greater degree of 

brittleness in those films. There was a small 

extent of lobing around the scratch in the 

15% TEGDE film, indicating a moderate 

level of brittleness was being exposed. 

Conclusions 

Spallation is believed to be an indicator 

of brittleness, and this brittleness is likely 

linked to a PVOH dielectric film’s propensity 

to tear or crack when strained in tension 

during manufacture or, ultimately, during 

service. The 0% PVOH, 15% 4010/PVOH, 

15% 5015/PVOH, and 30% 4010/PVOH films 

exhibited little or no brittleness and are 

therefore the best candidates for 

manufacturing scale-up. This lack of 

brittleness in the PVOH containing the 4010 

elastomer is perhaps consistent with its 

generally having lower E and H than the 

PVOH films that contain 5015 or TEGDE 

elastomers. If any of the examined films are 

apt to tear or crack when subjected to tensile 

straining, then those would be the 30% 

5015/PVOH and 30% TEGDE/PVOH films. 

45 
 

Propulsion Materials	 FY 2003 Progress Report 

3. CIDI ENGINES 

A. Microwave-Regenerated Diesel Particulate Filter Durability Testing 

Dick Nixdorf 

Industrial Ceramic Solutions, LLC 

1010 Commerce Park Drive, Suite I 

Oak Ridge, TN 37830 

(865) 482-7552; fax: (865) 482-7505; e-mail: nixdorfr@indceramicsolns.com 

DOE Technology Development Managers: Rogelio Sullivan 

(202) 586-8042; fax: (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

ORNL Technical Advisor: David Stinton 

(865) 574-4556; fax: (865) 241-0411; e-mail: stintondp@ornl.gov 

Contractor: Industrial Ceramic Solutions, Oak Ridge, Tennessee 

Prime Contract No.: 4000000723 

Subcontractors: Transportation Research Center, East Liberty, Ohio; Lydall Technical Papers, 

Rochester, N.H. 

Objectives 

•	 Develop, fabricate, and road-test a pleated ceramic fiber filter system that operates with reduced 

engine backpressure compared with conventional particulate filters, and investigate microwave 

regeneration at all engine operating conditions. 

•	 Develop a microwave regeneration system that will operate at all engine load conditions. 

•	 Conduct a road test of the pleated filter on the Ford F-250 diesel truck to test filter durability at 

high exhaust flow rates. 

Approach 

•	 Conduct air-flow and strength testing to establish an acceptable prototype pleated filter 

cartridge. 

•	 Validate the filter system’s particulate removal efficiency on a stationary diesel engine test cell. 

•	 Install and road-test the improved microwave filter system on a 7.3-L diesel vehicle to verify 

durability. 

Accomplishments 

•	 Developed a pleated ceramic fiber filter cartridge and demonstrated that it achieved 1/10 the 

exhaust backpressure of a conventional wall-flow particulate filter while withstanding the forces 

of the truck exhaust. 

•	 Completed efficiency testing on Oak Ridge National Laboratory’s (ORNL’s) 1.7-L Mercedes test 

cell to show greater than 96% removal of diesel particulate matter (PM) down to 10 

nanometers. 

47 
 
FY 2003 Progress Report	 Propulsion Materials 

•	 Designed, fabricated, and road-tested a pleated filter cartridge system and investigated 
 

microwave regeneration at engine idle condition on a 7.3-L Ford F-250 truck. 
 

•	 Conducted the road testing of the pleated filter cartridges on the Ford F-250 diesel truck at a full 

range of engine operating conditions. 

Future Direction 

•	 Enlist exhaust system, catalyst, engine, and vehicle manufacturers in a joint product 

development effort to move toward commercialization of the ceramic fiber pleated particulate 

filter in 2004. 

•	 Convert the rectangular pleated filter geometry to a round filter shape desired by the diesel 

industry to meet Environmental Protection Agency (EPA) 2007 Tier II requirements. 

•	 Continue on-road testing on the 7.3-L Ford truck with the round pleated filter for an additional 

10,000 miles, under high-load conditions, to extend the durability performance database. 

•	 Partner with a volume filter cartridge manufacturer to provide large quantities of the pleated 

filter cartridges necessary to the diesel industry over the next 10 years. 

Introduction In the FY 2003 work, a second product to 

control diesel particulates across all market As diesel engine manufacturers and diesel applications has emerged―the ceramic fiber vehicle original equipment manufacturers pleated filter cartridge itself. All of the (OEMs) move closer to the 2007 EPA approaches to PM control must use some deadline for effective emission controls, the form of a ceramic filter. To date, that filter industry’s requirements for the microwave- has been an extruded honeycomb “wall- regenerated diesel PM filter system have flow” product. Experiments conducted by become more defined. The alternative ICS and others under the FY 2003 program catalyst-regenerated systems, which require have demonstrated that the pleated ceramic high exhaust temperatures, seem to be fiber DPF places much less exhaust working reasonably well on heavy-duty backpressure on the diesel engine than do diesel trucks (>10 L engine sizes). These the wall-flow filters. This will improve engine engines account for about 25% of the diesel performance and reduce the fuel penalty engine market in the United States. However, imposed by the PM control device. The medium-duty trucks, buses, delivery vans, pleated ceramic fiber DPF, which weighs pickup trucks, and future diesel sport utility significantly less than the extruded wall-flow vehicles (SUVs) and off-road vehicles will filter, exhibits a lower thermal mass to need a self-heating particulate filter to achieve faster soot combustion temperatures operate under all conditions. This creates an and adds less weight to the vehicle. ICS is in opportunity in 75% of the diesel engine discussions with many diesel engine and market for a demonstrated microwave- exhaust system manufacturers to test the regenerated diesel particulate filter (DPF). pleated ceramic fiber filter cartridge in their There are number of actively heated DPFs 

under development, but none that have a existing regeneration technologies. 

proven record on an operating diesel vehicle. 

Industrial Ceramic Solution’s (ICS’s) Approach 

completion of the FY 2003 DOE Propulsion The first phase of the FY 2003 work was 

Materials Program vehicle road test will place the development of the pleated ceramic fiber 

the microwave-regenerated DPF high on the filter cartridge. Many experiments were 

list of viable active DPF systems. conducted to determine the optimum 

48 
 
Propulsion Materials 

geometry of the filter and the best means to 

fabricate this geometry in the laboratory. A 

pleating process, using a ceramic binder, was 

developed. A number of structural designs to 

enclose and support the pleated fiber media 

in the exhaust were investigated. The 

finished product is shown in Figure 1. This 

cartridge was tested for exhaust backpressure 

properties against a wall-flow filter. Those 

data are presented in Figure 2. Since the 

pleated cartridge is a significant change from 

the wall-flow design the ICS has used for the 

last several years, verification of the PM 

removal efficiency was needed. Therefore, 

pleated filter cartridges were canned and 

tested by Oak Ridge National Laboratory 

Figure 1. The new design of the ceramic fiber 

pleated filter cartridge. 

Figure 2. Exhaust backpressure comparison 

between the ICS pleated filter and a 

wall-flow filter. 

FY 2003 Progress Report 

(ORNL) on a 1.7-L Mercedes stationary test 

cell. The particulate removal efficiency from 

those tests is shown in Figure 3 for carbon 

mass and Figure 4 for diesel soot particle size 

distribution. 

Figure 3. ORNL 1.7-L Mercedes test cell 

measurement of PM removal efficiency 

of the pleated filter. 

Figure 4. ORNL 1.7-L Mercedes test cell 

measurement of particle size 

distribution before and after the pleated 

filter. 

The Oak Ridge testing, although highly 

successful in PM removal efficiency, showed 

a weakness in the filter cartridge strength; 

there were several ruptures in the filter media 

at high engine loadings. These results 

prompted ICS to spend another 6 months 

strengthening the media and the filter 

structure design. These improvements were 

tested on the exhaust system of the 7.3-L 

Ford truck and showed a 10× improvement 

in filter media strength. No media failures 

49 
 
FY 2003 Progress Report 

have been recorded with the improved filter 

cartridge on the truck exhaust tests. 

The microwave-regenerated DPF system, 

which will regenerate under any engine load 

condition, was designed and fabricated for 

the Ford F-250 7.3-L diesel truck. The system 

was fabricated and installed on the truck 

(Figure 5). It was installed in the bed of the 

Figure 5. Microwave-regenerated pleated 

cartridge DPF installed on a Ford 

7.3-L truck for testing. 

truck to accommodate the temperature and 

pressure sensors and the need to disassemble 

the can to examine the filter cartridges 

periodically. Installation of the system under 

the truck would have created an impossible 

hardship, since ICS does not have a 

hydraulic lift available for the truck. All 

microwave equipment, system controls, 

monitoring sensors and computer data 

acquisition are installed on the truck for the 

road and track testing. As testing on the 

truck began, a noticeable deficiency in 

microwave heating of the cartridges was 

observed. Further investigation revealed that 

ICS had received a defective lot of silicon 

Propulsion Materials 

carbide fibers from the fiber manufacturer. 

Upon review of the fiber manufacturer’s 

procedures, it was learned that a key 

processing parameter had been changed to 

achieve a higher product yield. This 

parameter was corrected, and a quality 

control procedure was implemented to 

prevent future occurrences of this problem. A 

proper lot of silicon carbide fibers were 

obtained. Converting these fibers to paper 

media, pleating the media, and fabricating it 

into filter cartridges delayed the truck testing 

by 6 months. All of the new cartridges have 

been fabricated, which will allow the track 

testing of the truck to begin in August 2003. 

Results 

The pleated fiber filter cartridge offers 

several advantages over the conventional 

extruded ceramic wall-flow filter. The 

exhaust backpressure on the engine created 

by the DPF is decreased by a factor of 10 to 

20 (Figure 2). The weight of the pleated 

cartridge will be approximately half that of 

the cordierite wall-flow product and onethird that of the silicon carbide wall-flow 

product. The PM removal efficiency of the 

pleated filter cartridge is 96% to 98% (Figures 

3 and 4), which equals that of the 

conventional DPF products. 

The microwave-regeneration system was 

tested on the Ford F-250 7.3-L diesel truck 

under high-idle exhaust flow conditions. 

Regeneration temperatures were not 

achieved because of a non-uniform 

microwave field. Road testing of the pleated 

filter on the 7.3-L diesel truck under a full 

range of engine operating conditions showed 

good durability and high potential for the 

use of the pleated DPF in commercial 

applications. 

Conclusions 

The pleated ceramic fiber filter cartridge 

has demonstrated lower backpressure and 

thermal mass properties that can make it a 

valuable addition to all types of PM 

regeneration systems. ICS is negotiating with 

50 
 
Propulsion Materials 

a major international filter manufacturer to 

license the use of the technology to supply 

the large volumes of filter cartridges that will 

be necessary to supply the worldwide diesel 

industry. Completion of the successful road 

testing of the Ford diesel truck will allow ICS 

to demonstrate the truck to several U.S. 

diesel engine and vehicle OEM suppliers. 

This exercise will convince one or more of 

these companies to investigate the 

incorporation of the round, pleated ceramic 

fiber filter in their total emissions control 

solution for light-duty vehicles. Support by 

diesel engine manufacturers and vehicle 

FY 2003 Progress Report 

OEMs is expected to lead to fleet testing and 

eventual commercial use. The pleated 

ceramic filter cartridge DPF can solve many 

problems that exist for companies seeking 

particulate control solutions for 2007 and 

beyond. 

FY 2003 Publications/Presentations 

“Microwave-Regenerated Particulate 

Filter,” presented at the DOE National 

Laboratory Advanced Combustion Engine 

R&D Merit Review and Peer Evaluation, 

Argonne National Laboratory, May 13–15, 

2003 

51 
 

Propulsion Materials	 FY 2003 Progress Report 

B. Material Support for Nonthermal Plasma Diesel Engine Exhaust Emission Control 

Stephen D. Nunn 

Materials Processing Group 

Oak Ridge National Laboratory, P.O. Box 2008, MS 6087 

Oak Ridge, TN 37831 

(865) 576-1668; fax: (865) 574-4357; e-mail: nunnsd@ornl.gov 

DOE Program Manager: Rogelio Sullivan 

(202) 586-8042; fax: (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

ORNL Technical Advisor: David Stinton 

(865) 574-4556; fax: (865) 574-6918; e-mail: stintondp@ornl.gov 

Contractor: Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Prime Contract No.: DE-AC05-00OR22725 

Objectives 

•	 Identify appropriate ceramic materials, develop processing methods, and fabricate complexshaped ceramic components that will be used in nonthermal plasma (NTP) reactors designed by 

Pacific Northwest National Laboratory (PNNL) for the treatment of diesel exhaust gases. 

•	 Fabricate and ship components to PNNL for testing and evaluation in prototype NTP reactors. 

•	 Develop a component design and establish a fabrication procedure that can be transitioned to a 

commercial supplier. 

•	 Assemble a prototype NTP reactor and test it on a laboratory diesel engine. 

Approach 

•	 Evaluate commercially viable forming methods to fabricate complex-shaped ceramic 

components that meet PNNL design specifications. 

•	 Modify processing as needed to accommodate material and design changes. 

•	 Evaluate bonding and sealing materials for preparing assemblies of the ceramic components. 

•	 Fabricate components and assemblies for testing and evaluation. 

Accomplishments 

•	 Fabricated more than 16 ceramic dielectric plates for assembly of full stacks (7 individual plates) 

to be used in the NTP reactor. 

•	 Conducted shear strength experiments to determine the strength of the sealing glass bond 

between joined ceramic components. 

•	 Fabricated a full-scale stainless steel sheet metal housing for the NTP reactor assembly. 

53 
FY 2003 Progress Report	 Propulsion Materials 

Future Direction 

•	 Complete the evaluation of sealing materials to bond and seal the various ceramic and metal 

components of the NTP reactor into a complete assembly for functional testing. 

•	 In collaboration with PNNL, finalize the NTP reactor design for assembly and testing to 
 

determine the durability and operational performance of the device. 
 

•	 Fabricate an NTP reactor assembly and conduct tests of the device on a 1.7-L diesel engine. 

Introduction 

Approach 

Results 

NTP reactors have been shown to be 

effective devices for reducing unwanted 

nitrogen oxides and particulate exhaust gas 

emissions from diesel engines. Researchers at 

PNNL are developing new, proprietary design 

configurations for NTP reactors that build on 

past experimental work. To improve 

effectiveness, these designs include ceramic 

dielectric components having complex 

configurations. Oak Ridge National 

Laboratory (ORNL) has extensive experience 

in the fabrication of complex ceramic shapes, 

primarily based on prior work related to 

developing ceramic components for gas 

turbine engines. The ORNL expertise is being 

utilized to support PNNL in its development 

of the new NTP reactor designs. 

A supply of ceramic dielectric plates was 

fabricated from Dupont 951AX Green Tape®. 

This tape-cast ceramic material can be 

formed and bonded together as described in 

previous reports. Figure 1 shows examples of 

Figure 1. Individual ceramic dielectric plates 

fabricated from Dupont 951AX Green 

Tape®. 

Collaborative discussions between ORNL 

and PNNL are used to establish new ceramic 

component designs for improved NTP 

reactors. Meeting NTP reactor design 

objectives is balanced with the limitations of 

ceramic manufacturability to arrive at a new 

component configuration. The ceramic 

processing facilities and expertise at ORNL 

are then used to establish fabrication 

capabilities and to produce components for 

testing at PNNL. This is an iterative process 

as both parties gain more knowledge about 

fabricating the components and about their 

performance in NTP reactor tests. The 

ultimate goal is to identify a design that 

performs well and that can be readily 

produced by a commercially viable process. 

the ceramic plates. The ridges on the plates 

are hollow, and the interior surface of each 

ridge is coated with a silver electrode that 

extends to the outer surface at one end of the 

plate. When the plates are stacked and a 

strong electric field applied, there is a field 

concentration along the opposing ridges of 

the plates that initiates a plasma discharge. 

In constructing the NTP reactor that is being 

planned for testing on a small diesel engine, 

a stack assembly of seven individual plates 

will be used (Figure 2). The plates are 

separated by ceramic spacers that define the 

exact spacing between the opposing ridges. 

Sealing glass is used to join the spacers and 

the plates and to provide a gas-tight seal. 

A test fixture was designed and fabricated 

to measure the shear strength of bonded 

samples of the ceramics used in the NTP 

54 
 
Propulsion Materials 

Figure 2. Example of a 7-layer ceramic dielectric 

plate stack that will be used in the 

experimental NTP reactor. 

reactor assembly. This fixture is shown in 

Figure 3, along with one of the test samples. 

Using standard testing procedures, the bond 

strength achieved using different sealing 

glass compositions was evaluated and 

compared. Each test sample consisted of a 

ceramic button bonded to a small ceramic 

plate with the sealing glass. Test specimens 

of the ceramic were bonded together and 

placed in the test fixture, and an increasing 

shear load was applied until failure occurred. 

Examples of two test samples that underwent 

the shear strength test are shown in Figure 4. 

The two principal sealing glass candidates are 

lead-free glass compositions manufactured by 

Ferro Corporation, designated as EG2964 and 

EG2998. Figure 5 shows a comparison of the 

shear strength of these two materials. The 

results of the tests will be used along with 

other factors, such as thermal cycling 

stability, to select the best sealing glass 

composition for making the NTP assemblies. 

In preparation for testing the NTP reactor on 

a laboratory diesel engine, a full-size model 

of the NTP housing has been made. The 

housing will hold the ceramic dielectric plate 

stack in position and connect to the engine 

exhaust system. Figure 6 shows the housing, 

along with the ceramic dielectric stack it will 

contain. The model is being used to check 

the fit of the dielectric stack and the 

FY 2003 Progress Report 

Figure 3. Shear test fixture assembly used to 

measure the shear strength of the 

sealing glass joint. 

Figure 4. Shear test specimens of glass 
 

compositions EG2964 (left) and
 

EG2998 (right) after testing. 
 

assembly and sealing procedures so any 

necessary design changes can be 

incorporated before the final housing is 

fabricated from stainless steel sheet material. 

The engine tests, currently planned for a 

1.7-L diesel engine, will evaluate the 

55 
 
FY 2003 Progress Report Propulsion Materials 

durability and function of the NTP assembly 

under engine operating conditions. 

Conclusions 

A supply of ceramic dielectric plates has been 

fabricated for the assembly of stacks to be 

used in an experimental NTP reactor that 

will be tested on a small diesel engine. A 

shear strength test apparatus was used to 

measure the bond strength of sealing glass 

compositions that are being evaluated for use 

in assembling the ceramic dielectric stacks. 

The two compositions under consideration, 

Ferro Corp. EG2964 and EG2998, had 

average shear strengths of 3034 psi and 

2476 psi, respectively. Figure 5. Results of the shear strength test A prototype NTP reactor housing was 
 measurements on glass 
 fabricated from stainless steel sheet metal. compositions EG2964 and EG2998. Using the prototype, the final dimensions 

required to accommodate the dielectric stack 

and to attach it to the exhaust system of a 

1.7-L laboratory diesel engine can be 

determined. 

Figure 6. Stainless steel sheet metal NTP 

reactor housing and ceramic 

dielectric stack. 

56 
 
Propulsion Materials 	 FY 2003 Progress Report 

C. Fabrication of Small Fuel Injector Orifices 

John B. Woodford, George R. Fenske 

Argonne National Laboratory 

9700 South Cass Avenue 

Argonne, IL 60439 

(630) 252-0910; fax (630) 252-4798; e-mail: gfenske@anl.gov 

DOE Technology Development Manager: Rogelio Sullivan 

(202) 586-8042; fax: (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

Objectives 

•	 μ

•	 

•	 

Approach 

•	 

•	 

•	 μ μm 

•	 

under various conditions. 

•	 

•	 

•	 

•	 

•	 

Contractor: Argonne National Laboratory, Argonne, Illinois 

Prime Contract No.: W-31-109-Eng-38 

Develop a methodology for reducing the diameter of fuel injector orifices to 50 m by applying 

material to the internal diameter (ID) of the orifice. 

Characterize the spray and combustion properties of the fuel injector systems so treated. 

Transfer the technology to an industrial partner. 

Evaluate the potential of electroless nickel (EN) plating for reducing orifice diameter, identify 

the critical barriers to commercialization, and resolve those barriers. 

Evaluate the potential of EN plating for improving surface finish and reducing deposit 

formation on injector nozzles. 

Accomplishments 

Successfully reduced the orifice diameter of diesel engine fuel injectors from 200 m to 50 

with EN plating. 

Compared the surface roughness of coatings deposited by EN plating at various thicknesses and 

Conducted initial testing that indicates that EN plating reduces by nearly twofold the mass of 

deposit formation on the injectors. 

Initiated contact with International Harvester/Navistar regarding possible transfer of the plating 

technology. 

Future Direction 

Characterize the spray characteristics of coated injectors. 

Prepare coated injector tips for flame spray characterization tests. 

Perform further tests on deposit formation using alloy coatings. 

57 

FY 2003 Progress Report 

Introduction 

Recent work on the control of particulate 

matter (PM) emissions and improvement of 

engine combustion efficiency in diesel 

engines has shown the effectiveness of 

reducing the diameter of injector orifices in 

promoting fuel atomization, leading to more 

complete combustion and reduction in soot 

formation.1,2 The current industry target is a 

50-μm diameter; however, because mean 

droplet size and soot incandescence both 

decrease steadily with orifice diameter, any 

reduction in orifice diameter should result in 

improved efficiency and reduced PM 

emissions. Advancements in the 

electrodischarge machining (EDM) 

technology currently used to fabricate 

orifices are not expected to achieve the 

50-μm target nor to alleviate the rough 

interiors produced by EDM. Currently, 

orifices produced by EDM must be polished 

using an abrasive slurry to achieve an 

acceptable surface finish of around 0.5 μm RA 

with a surface morphology characterized by 

sharp peaks and valleys. During the fuel 

injection process, this morphology leads to a 

highly turbulent layer adjacent to the wall, 

which reduces the effective fuel delivery area 

of the orifice. As the orifice diameter is 

pushed down toward 50 μm, the impact of 

surface finish is greatly magnified, thus 

demanding an even smoother finish. 

In addition to being a potential route to 

economical production of 50-μm diameter 

fuel injector orifices, EN plating offers 

potential solutions to the problems of soot 

formation and surface roughness. It is 

possible to deposit coatings with different 

surface chemistries, some of which may 

reduce the propensity of diesel fuel to form 

deposits on the injector nozzle. The 

morphology of EN-plated surfaces appears to 

be generally smoother, as well. 

Approach 

As documented in the 2002 annual 

report, EN plating was selected over a 

Propulsion Materials 

number of alternative coating processes 

because of its cost, maturity, and potential to 

coat interior surfaces. EN plating is a method 

of depositing nickel/phosphorus or 

nickel/boron alloys onto metallic surfaces 

from aqueous solution, although other 

metals may be deposited using the same 

principle. We plan to perform further 

evaluations of the suitability of EN plating 

for ID reduction in orifices, for mitigating 

deposit formation, and for changing the 

surface morphology of the underlying 

substrate material. 

Results 

As a result of interactions with Siemens’ 

injector supplier, we began to consider 

surface roughness as a critical parameter. We 

performed a number of trials using different 

deposition methods to determine the change 

in surface roughness with EN plating at 

various thicknesses and under various 

deposition conditions. Figure 1 contains a 

plot of RA values for orifice walls plated 

under different conditions, compared with 

the RA value of an unplated orifice wall. As 

part of this study, we also established that 

although ultrasonic agitation of the plating 

bath increases the deposition rate, it also 

tends to increase the surface roughness 

compared with the unplated surface. In 

general, the longer the plating time, the 

smoother the plate. 

Erosion due to cavitation is expected to 

be a major problem in fuel injectors as orifice 

diameter decreases and injection pressure 

increases. The mechanical properties of EN 

plating depend upon the amount of 

phosphorus in the plate, its microstructure, 

and its thermal history. In order to 

determine the effect of plating bath 

composition on the plate’s mechanical 

properties, we measured the Knoop hardness 

of the plate as a function of position (and 

thus bath depletion) (Figure 2). The first 

plate to be deposited is hardest; then, as the 

bath depletes, the plate hardness decreases. 

Thus we should be able to tailor the plating 

58 
 
Propulsion Materials FY 2003 Progress Report 

Figure 1. Surface roughness RA for different deposition conditions. 

Figure 2. Knoop hardness of the nickel plate as a function of position. 

hardness for maximum erosion resistance by 

maintaining the bath composition at a 

particular level. 

We designed, constructed, and tested a 

system for forced-circulation plating of 

multiple injectors (Figure 3). To avoid the 

previous difficulty with orifice clogging, the 

bath is filtered before being pumped into a 

multiple-injector manifold. Initial test results 

showed that forcing circulation at even 

modest pressures resulted in marked 

reduction in deposition rate on the orifice 

interior (Figure 4). However, the coating on 

the orifice interior was also quite smooth. 

Further work is ongoing. 

As described earlier, deposits are expected 

to be a significant problem for small orifices. 

Thus we have begun to characterize the 

relative vulnerability of EN-plated injectors 

to deposit formation. Initial tests involved 

heating plated and unplated injector surfaces 

to around 240oC, dropping diesel fuel onto 

them, and measuring the change in mass. 

We expected the nickel plate to be less 

59 
 
FY 2003 Progress Report 	 Propulsion Materials 

deposit mass over the unplated steel. We are 

planning further tests to characterize the 

deposits and their adhesion to the metal 

surface. 

Before the large forced-circulation plating 

system was completed, we attempted 

another method of forcing bath circulation 

through the injector. Spinning the injector 

in the plating bath resulted in a low, 

constant flow of plating bath through the 

injector. Using that method, we have 

successfully reached and surpassed the ID 

goal of 50 μm (Figures 6 and 7). 

Conclusions/Future Work 

The work done in the past year has 

Figure 3. Forced-circulation plating system. 	 demonstrated the promise of EN plating as a 

technique for meeting the current industry 

target for fuel injector orifice diameter. We 

have also shown its potential for reducing 

plugging due to deposit formation and for 

improving the surface finish of the orifice 

interior. 

Following the recommendations of a peer 

review panel, our future efforts will include 

further testing, including benchmarking the 

coated injectors vs uncoated injectors in 

terms of their surface hardness, discharge 

coefficient, spray form, hole-to-hole 

variation, and target accuracy. Following this 

evaluation, engine testing and/or 

commercial evaluation have been strongly 

encouraged. We have been advised to 

investigate the use of maskants and/or 

special fixturing to restrict plating to the 

Figure 4. Micrograph of the outer edge of the nozzle area. Another reviewer pointed out 

plated orifice, showing the difference that hole geometry is a critical design 

in thickness between the orifice parameter and said that we should 

interior and the injector exterior. determine orifice circularity. These are all 

excellent suggestions, and we intend to work 

susceptible to deposit formation than pure on all of them, on our own and in concert 

nickel (also tested for comparison) as a result with Argonne National Laboratory–Energy 

of the presence of phosphorus in the Systems personnel. We will also solicit 

coating. Figure 5 shows the results. The EN further assistance in testing from our 

plating affords a nearly twofold reduction in industrial contacts. 

60 
 
Propulsion Materials FY 2003 Progress Report 

Figure 5. Deposit mass on pure nickel, unplated injector segment, and 

plated injector segment. 

Figure 6. Plot of coating thickness as a function of position along the orifice. 

References 

1. John B. Heywood, Internal Combustion 

Engine Fundamentals, McGraw-Hill, 1988. 

2. Lyle M. Pickett and Dennis L. Siebers, 

Paper No. 2001-ICE-399, Internal 

Combustion Engines-Vol. 37-1, Proceedings of 

the 2001 Fall Technical Conference, American 

Society of Materials Engineers 2001, Ed. V. W. 

Wong, American Society of Materials 

Engineers, 2001. 

Figure 7. Micrograph, cross-section of coated 

injector tip. 

61 
 

Propulsion Materials	 FY 2003 Progress Report 

D. Technology for Producing Small Holes in Advanced Materials 

Stephen D. Nunn 

Materials Processing Group 

Oak Ridge National Laboratory, P.O. Box 2008, MS 6087 

Oak Ridge, TN 37831 

(865) 576-1668; fax: (865) 574-4357; e-mail: nunnsd@ornl.gov 

DOE Program Manager: Rogelio Sullivan 

(202) 586-8042; fax: (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

ORNL Technical Advisor: David Stinton 

(865) 574-4556; fax: (865) 574-6918; e-mail: stintondp@ornl.gov 

Contractor: Oak Ridge National Laboratory, Oak Ridge, Tennessee 

Prime Contract No.: DE-AC05-00OR22725 

Objectives 

•	 Explore methods for producing ultra-small holes (<50μm) in advanced materials for fuel injector 

nozzles. 

•	 Evaluate carbide-based cermets and high-temperature ceramics as candidate materials. 

•	 Devise methods to produce precisely a desired pattern of holes having a well-controlled size and 

shape. 

•	 Investigate methods to fabricate fuel injector shapes or inserts having ultra-fine holes. 

Approach 

•	 Form holes in situ during the sintering process by incorporating various pore formers in the 

green (unsintered) body while it is being shaped. 

•	 Use nickel alloy wires in nickel aluminide-bonded (Ni3Al-bonded) titanium carbide (TiC) to 

form holes during sintering. 

•	 Embed into gelcast ceramic materials combustible organic fibers that will vaporize during 

sintering at high temperatures. 

•	 Gelcast ceramic materials around metal wires that can be removed from the green body prior to 

sintering, leaving a hole in the part. 

•	 Evaluate thermalplastic coextrusion as a method to form a complex hole-forming pattern in 

ceramic materials. 

Accomplishments 

•	 Produced holes of 108, 76, 51, and 26 μm in diameter in a TiC-Ni3Al cermet composition. 

•	 Formed 8-hole arrays of ~50-μm-diam holes in gelcast alumina and zirconia ceramics. 

•	 Formed fuel injector nozzle shapes containing an array of fine holes in alumina and zirconia 

ceramics by gelcasting in a plastic mold with metal wire inserts. 

•	 Fabricated a 6-hole array of 30-μm-diam holes in alumina ceramic using thermalplastic 

coextrusion. 

63 
 
FY 2003 Progress Report Propulsion Materials 

Future Direction 

• Fabricate a precision mold to gelcast ceramic fuel injector nozzles. 

• Evaluate joining methods for producing hybrid steel-/ceramic-tip fuel injector nozzles. 

• Assess the durability of advanced ceramics in the diesel engine combustion environment. 

Introduction Approach

To meet the ever-growing demand for 

improved efficiency and reduced emissions, 

future fuel injection systems will require 

improvements in both design and materials. 

The objective of this project is to explore 

new methods for forming ultra-small 

(<50-μm) holes in advanced materials. 

Carbide-based cermets (ceramic/metallic 

composites) and high-temperature ceramics 

are candidate materials to be used in this 

study. They have the desirable properties of 

high strength, high modulus, and low 

density compared with steel. In addition, 

they are corrosion resistant and, because of 

their high hardness, are resistant to wear and 

erosion. 

Currently, fuel injector nozzle holes in 

steel are 150 tο 200 μm in diameter. This size 

is at the limit for machined holes. One of the 

major challenges to using advanced materials 

for improved nozzles is the lack of capability 

to manufacture very fine holes in the 

densified material by conventional 

machining processes. To overcome this 

limitation, this project will explore methods 

for the in situ fabrication of holes during the 

sintering step that is a normal part of the 

fabrication process for advanced materials. 

Various techniques for incorporating fugitive 

pore-formers will be investigated. These poreformers will be incorporated in the 

unsintered (green) body during the forming 

process and will be removed before or 

consumed during the sintering step to leave 

precisely-controlled pores (holes) in the 

densified material. If successful, this 

technique will not only allow the formation 

of simple holes but also add the potential to 

form holes having variable geometries. 

It has been observed in past studies of the 

cermet Ni3Al-bonded TiC that metal particles 

in the unsintered powder compact can leave 

voids in the sintered part because the metal 

is drawn into the carbide particle array by 

capillary action when the metal becomes 

molten. This tendency will be exploited in 

the present study to intentionally form holes 

in predetermined locations. Nickel alloy 

wires of the appropriate size will be located 

in the unfired material to form fine holes in 

the sintered part. 

Two ceramic materials were selected for 

investigation in this project, alumina (Al2O3) 

and zirconia (ZrO2). To form holes in the 

ceramics, fugitive pore formers will be 

incorporated in gelcast ceramic green bodies. 

Gelcasting is a method of forming powdered 

materials into complex shapes. Organic 

monomers are added to an aqueous slurry of 

the ceramic powder. When a polymerization 

reaction is initiated, the organic molecules 

and the slurry are transformed into a solid 

(gelled) body. By casting the slurry in a mold 

and then gelling it, a complex-shaped part 

can be formed. One approach to forming 

holes in the gelcast ceramic part is to place 

fibers in the mold that are trapped in desired 

locations in the gelled part. These fibers are 

burned away during the sintering process, 

leaving holes in the densified part. The fibers 

can be synthetic polymers (e.g., nylon, poly

ethylene), natural fibers (cotton, silk), or 

even graphite. 

The second approach to forming holes in 

gelcast ceramic parts is to locate fine metal 

wires within the casting mold. Like the 

fibers, the metal wires are trapped in the 

gelled part. But rather than being burned 

-


64 
 
Propulsion Materials 

out, the wires can be withdrawn from the 

green body prior to sintering. Very fine holes 

that are formed in the green part can be 

retained in the sintered ceramic. 

Another method being evaluated for 

forming holes in ceramic materials is the use 

of coextrusion to form complex arrays of 

holes on a very fine scale. The coextrusion 

process involves blending powder materials 

in a thermalplastic resin and then 

assembling segments of the filled resin to 

form a desired pattern. Each segment can 

contain a different filler material. In the 

present study, the segments contain either 

alumina ceramic or carbon powder, and the 

segments are arranged to form a hexagonal 

array of carbon-filled “holes” in an alumina 

matrix. The assembly is then extruded to 

reduce the cross-sectional area while 

retaining the segment pattern. The 

coextrusion can be repeated until the desired 

reduction of the segment pattern is achieved. 

Results 

A powder compact of Ni3Al and TiC 

containing small lengths of nickel wire was 

prepared; several different diameters of 

nickel wire were incorporated in the 

specimen. At the firing temperature used to 

sinter the compact, the nickel became 

molten and was drawn into the surrounding 

powder matrix by capillary action, leaving a 

void where the wire had been. After 

sintering, the sample was sectioned, 

polished, and examined by optical 

microscopy. Elongated holes were observed 

in the otherwise dense body. The holes had 

measured diameters of 108, 76, 51, or 26 μm, 

depending upon the initial size of the nickel 

wire embedded in the sample. One of the 

holes in the cermet is shown in the scanning 

electron microscopy (SEM) micrograph in 

Figure 1. 

Fine holes that were produced in a gelcast 

alumina ceramic sample by incorporating 

organic fibers were examined by SEM. An 

example is shown in Figure 2, where the hole 

is intersected at an oblique angle by the 

FY 2003 Progress Report 

Figure 1. SEM micrograph of a 76-μm hole 

formed in an Ni3Al–TiC cermet 

material. 

Figure 2. SEM micrograph showing the fine 

surface finish on a 30-μm hole 

formed by embedding an organic 

fiber in gelcast Al2O3. 

polished surface of the sample. The hole is 

approximately 30 μm in diameter. One 

interesting feature is that the surface 

roughness of the hole is at a scale defined by 

the grain boundary curvature of the 

individual crystallites that form the sintered 

ceramic. These crystallites average 2–3 μm in 

diameter. The curvature forms shallow 

grooves at the grain boundaries that are no 

more than 100 nm deep. This is an extremely 

fine surface finish that should minimize 

turbulence in fluid flowing in such a hole. 

65 
 
FY 2003 Progress Report Propulsion Materials 

A mold design was developed for 

gelcasting ceramic material in the shape of a 

fuel injector nozzle. The plastic mold was 

designed with removable wires in fixed 

locations to produce a controlled array of 

holes in the gelcast ceramic. An optical 

micrograph of the 8-hole array in the unfired 

part is shown in Figure 3. The sintered part is 

shown in Figure 4, along with a steel fuel 

injector nozzle that was used as the model 

for making the mold. 

Figure 3. Optical micrograph of an 8-hole 

array formed on the tip of a fuel 

injector nozzle shape by 

incorporating removable metal 

wires in gelcast ZrO2. 

Figure 4. Gelcast and sintered ZrO2 fuel 

injector nozzle shape (left) 

containing an array of eight holes 

in the tip. A steel nozzle is shown 

at right. 

A coextrusion process was used to 

fabricate rods of unfired alumina ceramic 

containing a hexagonal array of fugitive 

hole-formers. Figure 5a shows two extruded 

rods approximately 50 mm long and 5 mm 

in diameter. The hole-forming array runs 

through the entire length of the rods. A thin 

disk was cut from the length of one of the 

rods; the optical micrograph in Figure 5b 

shows the cross-sectional view of the holeforming array on the disk. Disks were fired at 

a high temperature to sinter the ceramic and 

burn away the fugitive material in the holes. 

After firing, the disks were about 4 mm in 

diameter as a result of sintering shrinkage, 

and the holes were about 32 μm in diameter. 

One of the holes is shown in the SEM 

micrograph in Figure 6. These disks could be 

used as inserts in a steel housing to form a 

hybrid fuel injector nozzle with ultrafine 

holes. 

Conclusions 

Fine holes of various sizes were produced 

in a Ni3Al–TiC cermet material by incorporating fine nickel wires in the powder 

compact prior to sintering. Holes as fine as 

26 μm in diameter were produced using this 

method. 

SEM examination of holes produced in 

gelcast Al2O3 by trapping organic fibers in the 

casting and then burning them away during 

the sintering process revealed an 

exceptionally smooth surface finish on the 

holes that were formed. 

A mold was designed to produce a 

controlled array of holes in gelcast ceramics 

having a fuel injector nozzle shape. A 

circular array of eight holes was formed in 

the tip of the nozzle shape. 

Coextrusion was shown to be a viable 

forming method for producing an array of 

fine holes in Al2O ceramic. 3 

66 
Propulsion Materials FY 2003 Progress Report 

Figure 5. (a) Coextruded alumina rods 50 mm × 5 mm containing a hexagonal array of 

hole-formers. (b) Cross-section of a coextruded alumina rod showing the holeforming array. 

Figure 6. SEM micrograph of a hole in the 

coextruded alumina after sintering. 

67 
 

Propulsion Materials	 FY 2003 Progress Report 

E. Electrochemical NO x Sensor for Monitoring Diesel Emissions 

L. Peter Martin and Robert S. Glass 

Lawrence Livermore National Laboratory 

P.O. Box 808, MS L-353 

Livermore, CA 94551-0808 

(925) 423-9831; fax: (925) 423-7040; e-mail: martin89@llnl.gov 

DOE Program Manager: Rogelio Sullivan 

(202) 586-8042; fax: (202) 586-1600; e-mail: rogelio.sullivan@ee.doe.gov 

Contractor: Lawrence Livermore National Laboratory, Livermore, California 

Prime Contract No.: W-7405-Eng-48 

Objectives 

•	 Develop a compact, rapid-response electrochemical nitrogen oxide (NO) or total nitrogen oxides 

(NOx) sensor for compression-ignition, direct-injection (CIDI) exhaust gas monitoring. 

•	 Demonstrate the commercialization potential of a new NOx sensor technology developed 

during FY 2002. 

•	 Collaborate with the Ford Research Center to optimize sensor materials, operating parameters, 

and performance. 

Approach 

•	 Use an ionic (O2−) conducting ceramic as a solid electrolyte and a catalytic metal oxide as a 

working electrode. 

•	 Apply a constant current-bias (through the electrolyte) between the working electrode and an 

“inert” counter electrode. 

•	 Correlate the NOx concentration with the potential associated with the applied current-bias. 

•	 Optimize sensor performance by identifying and resolving materials science issues associated 

with sensitivity, response speed, cross-sensitivity, and stability 

Accomplishments 

•	 Submitted a patent application and conference abstract on the current-biased NOx sensing 

technique. 

•	 Published one journal article and one conference proceeding on related potentiometric NOx 

sensor techniques. 

•	 Identified electrode materials exhibiting ~90 mV response to 500 ppm NO at 650oC with a 90% 

response time of t90 (baseline to 90% of equilibrium value) ~1.25 s. 

•	 Sent a first-generation laboratory prototype of the current-biased NO sensor to Ford Research 

Center for sensitivity/selectivity testing (data presented in this report). 

•	 Began cross-sensitivity testing for NO2, O2, H2O, CO, CO2, and NH3. 

69 
FY 2003 Progress Report Propulsion Materials 

Future Direction 

• Continue to explore materials issues related to sensitivity and stability. 

• Investigate sensing mechanism using electrochemical and analytical techniques. 

• Evaluate microstructural evolution during sensor aging. 

• Evaluate cross-sensitivity to important redox gases. 

• Demonstrate the commercialization potential of the current-biased sensor technique. 

• Transfer the technology to a commercial entity. 

Introduction x 

°

Approach 

New emissions regulations will increase 

the need for compact, inexpensive sensors 

for monitoring and control of automotive 

exhaust gas pollutants. Species of interest 

include hydrocarbons, carbon monoxide, 

collaborators) are sensitivity to the total NO

content in the range of 1 to 1000 ppm, 

operating temperature of >600 C, and a 

response time of 1 s. 

and NOx. The most promising NO sensors for 

exhaust gas monitoring are based on 

ionically conducting solid state electrochemical devices.1,2 These devices typically 

consist of a solid ceramic electrolyte onto 

which two or more metal or metal-oxide 

electrodes are deposited, and they can be 

operated in either potentiometric or 

amperometric modes. Significant progress 

has been made toward the development of 

deployable sensors using yttria-stabilized 

zirconia (YSZ) as the electrolyte and 

catalytically active metal oxides as the 

sensing electrodes.3

 However, improvements 

are still needed in sensitivity, response time, 

reliability, and cross-sensitivity. The current 

work is directed toward the development of 

fast, high-sensitivity electrochemical NOx 

sensors for automotive diesel applications. 

A NOx sensor with fast response time and 

high sensitivity to NO at 650oC has been 

developed. The sensor is based on a novel 

current-biased mode of operation where a 

fixed current is applied between the sensing 

and counter electrodes and the resultant 

potential is measured. This method is 

distinctly different from conventional 

amperometric sensor technology. The target 

operating parameters for the proposed sensor 

(based on discussion with industry 

The proposed approach is to develop a 

sensor using catalytic metal oxide electrodes 

on a solid ionic conductor electrolyte. 

Similar technology has been widely 

investigated for various gas sensing 

applications and has been successfully 

developed for use in exhaust gas oxygen 

sensors. The current effort approaches the 

problem by the application of novel 

materials and fabrication processes designed 

to optimize electrode microstructures. In 

addition, a novel mode of operation has 

been identified that yields enhanced NO 

response sensitivity and speed. 

Sensors are prepared by spray deposition 

of metal oxide working electrodes onto dense 

YSZ electrolyte substrates, followed by 

sintering at 900–1000°C. Working electrodes 

are composed of nanocrystalline n-type 

semiconducting oxides, usually tin-doped 

indium oxide (ITO), promoted with a small 

addition of Rh metal. Counter electrodes are 

composed of platinum or PdAg applied using 

screen printing paste. NOx sensing 

experiments are performed in a quartz tube 

inside a furnace using a standard gas 

handling apparatus. All tests were performed 

in the temperature range of 600–700°C. 

Sensors are typically first evaluated by the 

generation of current–voltage (I-V) curves in 

70 
 
Propulsion Materials FY 2003 Progress Report 

various atmospheres. These curves yield the 

current (I) as a function of applied voltage 

(V). The measured current is directly related 

to the amount of oxygen being “pumped” 

through the electrolyte by the reaction 

1⁄2O2 + V + 2e–

 = O2– (1) O 

where VO represents an oxygen vacancy and 

O2- represents an oxide ion in the electrolyte 

lattice. Because all tests are performed in 

high O2 (~10%) and low NOx (≤ 500ppm) 

concentrations, changes in the I-V behavior 

indicate relative changes in the reaction 

kinetics upon a change in the gas 

composition. Once the I-V behavior is 

determined, the operating bias condition can 

be selected based on the desired NO and NO2 

sensitivities. Sensor testing under a constant 

current bias is then performed to determine 

the sensitivity and stability characteristics of 

the sensor. 

Figure 1. I-V characteristic in 10% O2 and 

Results 10% O2 plus 500 ppm NO or NO2. 

Figure 1 shows the I-V characteristic of 

the sensor when operated at 650o

C in 10% 

oxygen and the balance nitrogen. Also 

shown are the data for 10% oxygen with 

500 ppm NO or NO2. It can be seen that the 

introduction of either NOx species shifts the 

I-V behavior significantly, indicating changes 

in the electrochemical rate constants of the 

reactions occurring at the electrodes. The 

sensor can be biased galvanostatically 

(constant current) or potentiostatically 

(constant voltage). The introduction of 

500 ppm NO, for example, is detected as a 

change in measured potential (galvanostatic 

mode) or current (potentiostatic mode), 

corresponding to shifting the I-V behavior 

from the 10% O2 curve to the NO curve in 

Figure 1. Potentiostatic biasing is equivalent 

to the traditional amperometric mode of 

operation. However, we have postulated that 

there may be some performance benefits to 

operating in the current-biased, galvanostatic 

mode. Figure 2. Sensor response to NO and NO2 in 

The data shown in Figure 2 were acquired 3(●), 10 (■), and 20% (♦) O2. 

during independent testing at Ford Research 

71 
 
FY 2003 Progress Report 

Center.4

 As shown in Figure 2, when the 

sensor is negatively biased galvanostatically, 

the measured potential (response) is positive 

for both NO and NO2. In comparison, 

traditional electrochemical potentiometric 

sensors have competing (opposite sign) 

responses to NO and NO2. That the NO and 

NO2 have the same sign response may 

suggest the possibility that both species are 

reduced during the negative bias, or possibly 

that the NO is being converted (nonelectrochemically) to NO2, which is then 

detected electrochemically. It is interesting to 

note that the effect of O2 concentration is 

much more pronounced for NO2 sensing 

than for NO. 

Figure 3 shows the fast response of the 

sensor to 500 ppm NO and NO2. The 

sampling interval in the figure is 0.25 s. Thus 

the 90% turn-on and turn-off times are ~1.25 

and 2.5 s, respectively, for the NO response. 

This fast response is attributed to the 

enhancement of the electrode kinetics 

caused by the biasing current. Ongoing 

investigations are being performed to 

Figure 3. Sensor response vs time for 

500 ppm NO and NO2 in 10% O2. 

Propulsion Materials 

elucidate the sensing mechanism. It has been 

observed during testing that there is a 

tendency for the sensor baseline to drift over 

time (~1-5 mV/hour). This drift is probably 

attributable to coarsening of the electrode 

microstructure during testing. Future efforts 

will involve identifying the mechanism for 

the baseline drift and evaluating means for 

stabilizing this effect. Possible techniques for 

accomplishing these tasks are modifying the 

electrode microstructure (for example, by 

incorporation of a more refractory phase), 

intentional aging of the electrodes, or 

selection of alternate electrode materials. 

Conclusions 

A novel current-biased mode of operation 

has been developed for NOx sensing in 

automotive diesel exhaust. Electrode 

materials have been identified that provide 

excellent sensitivity and response time for 

sensing NO at 650o

C in 10% O . This 2 

technique has generated significant interest 

from the automotive industry. Sensor 

stability and cross-sensitivity will be further 

addressed in future efforts. Long-term 

dynamometer testing is to be performed by 

Ford, and planning for ultimate technology 

transfer to a commercial entity has been 

initiated. 

References 

1. F. Menil, V. Coillard, and C. Lucat, 

“Critical Review of Nitrogen Monoxide 

Sensors for Exhaust Gases of Lean-Burn 

Engines,” Sensors and Actuators B, 67, 1–23 

(2000). 

2. N. Miura, G. Lu, and N. Yamazoe, 

“Progress in Mixed-Potential Type Devices 

Based on Solid Electrolyte for Sensing Redox 

Gases,” Solid State Ionics, 136–137, 533–542 

(2000). 

3. T. Ono, M. Hasei, A. Kunimoto, 

T. Yamamoto, and A. Noda, “Performance of 

the NOx Sensor Based on Mixed Potential for 

Automobiles in Exhaust Gases,” JSAE Rev., 

22, 49–55 (2001). 

72 
 
Propulsion Materials 

4. Rick Soltis and Dave Kubinski, Ford 

Research Center, Dearborn, Michigan. 

FY 2003 Publications/Presentations 

L. P. Martin, Q. Pham, and R. S. Glass, 

“Effect of Cr2O3 Electrode Morphology on the 

NO Response of a Stabilized Zirconia Sensor,” 

accepted for publication in Sensors and 

Actuators B (May 2003). 

L. P. Martin, Q. Pham, and R. S. Glass, 

“Electrochemical NOx Sensors for 

Automotive Diesel Exhaust,” presented at the 

2002 Materials Research Society Fall Meeting, 

Boston, Massachusetts, December 2–7, 2002. 

A paper with the same title was accepted for 

publication in the Proceedings of the 2002 

Materials Research Society Fall Meeting, (in 

press, 2003). 

L. P. Martin, D. Kubinski, R. Soltis, 

J. Visser, M. Parsons and R. S. Glass, “A 

FY 2003 Progress Report 

Current-Biased NOx Sensor for Automotive 

Diesel Exhaust,” submitted for presentation 

at The 204th Meeting of The Electrochemical 

Society, Orlando, Florida, October 12–17, 

2003. 

L. P. Martin, D. Kubinski, R. Soltis, 

J. Visser, M. Parsons and R. S. Glass, 

“Galvanostatic NOx Sensor Based on YttriaStabilized Zirconia and a Metal Oxide 

Working Electrode,” submitted to Solid State 

Ionics (August 2003). 

Patents 

Patent application: L. P. Martin and A.-Q. 

Pham, “Current-Biased Potentiometric NOx 

Sensor for Vehicle Emissions,” LLNL 

Docket #: IL-11022, submitted April 2003. 

73 
 

APPENDIX A: ACRONYMS AND ABBREVIATIONS 
 

AHE 

ANL 

ASTM 

CIDI

CO 

CRADA 

DF 

DOE 

DPF 

E

EDM 

EDX 

E/E 

EGR 

EN

EPA

F

FTP

GGFD 

GM

GTRI 

H

HA-ADF 

H+ 

H2

HDI

HEPA 

IC

ICC

ICS 

ID

LANL 

L/D

MEA 

MLCC 

MPM 

MQI

aerodynamic heat exchanger 

Argonne National Laboratory 

American Society for Testing and Materials 

 compression-ignition direct-injection 

carbon monoxide 

cooperative research and development agreement 

dissipation factor 

U.S. Department of Energy 

diesel particulate filter 

 elastic modulus 

electrodischarge machining 

energy-dispersive X-ray spectroscopy 

Electrical and Electronics 

exhaust gas recirculation 

 electroless nickel 

 Environmental Protection Agency 

 fluorine 

 Federal Test Protocol 

ground graphitized foam dust 

 General Motors 

Georgia Tech Research Institute 

 hardness 

high-angle annular dark-field 

hydrogen ion 

 hydrogen gas 

 high-density infrared 

high-efficiency particulate air (filter) 

 internal combustion 

 integrated circuit chip 

Industrial Ceramic Solutions 

 internal diameter 

Los Alamos National Laboratory 

 length-to-diameter 

membrane electrode assembly 

multi-layer ceramic composite 

mechanical properties microprobe 

 Magnequench International 

75 
 
MRE mixed rare earth 

MW-DPF microwave-regenerated diesel particular filter 

ND neodymium 

NEP National Energy Policy 

NFC near-frictionless carbon 

NIST National Institute for Standards and Testing 

nm nanometer, 10–9 meters 

NO nitrogen oxide 

NOx oxides of nitrogen 

NSA National Security Agency 

NTP nonthermal plasma 

OAAT Office of Advanced Automotive Technologies 

OEM original equipment manufacturer 

OHVT Office of Heavy Vehicle Technologies 

OM optical microscope 

ORNL Oak Ridge National Laboratory 

OTT Office of Transportation Technologies 

PECS passive evaporative cooling system 

PEM polymer electrolyte membrane 

PEMFC polymer electrolyte membrane fuel cell 

Pd palladium 

PM permanent magnet 

PNGV Partnership for a New Generation of Vehicles 

PNNL Pacific Northwest National Laboratory 

POEM porous oxide electrolyte membrane 

PPS polyphenylene sulfide 

Pt platinum 

PVD physical vapor deposition 

PVOH hydroxylated polystyrene 

PVP hydroxylated polystyrene 

R&D research and development 

RE rare earth 

SO 

S sulfur 

SAE Society of Automotive Engineers 

SEM scanning electron microscope 

SHE standard hydrogen electrode 

SNL Sandia National Laboratories 

3

2– sulfonate group 

STEM scanning transmission electron microscope 

T Tesla 

TEGDE tetraethylene glycol divinyl ether 

TEM transmission electron microscope 

TiN titanium nitride 

TiC titanium carbide 

TIVM toroidal intersecting vane machine 

76 
 
WC tungsten carbide 

XRD X-ray diffraction 

YSZ yttria-stabilized zirconium 

Z characterized by a high atomic number 

77