NASA / TM--1999-209450

Chemical Gas Sensors for Aeronautics and

Space Applications III

G.W. Hunter and P.G. Neudeck

Glenn Research Center, Cleveland, Ohio

L.Y. Chen

AYT Corporation, Cleveland, Ohio

C.C. Liu, Q.H. Wu, M.S. Sawayda, Z. Jin, and J. Hammond

Case Western Reserve University, Cleveland, Ohio

D. Makel

Makel Engineering, Chico, California

M. Liu and W.A. Rauch

Georgia Institute of Technology, Altanta, Georgia

G. Hall

John F. Kennedy Space Center, Kennedy Space Center, Florida

Prepared for the

Sensors Exposition

sponsored by Sensors Magazine

Cleveland, Ohio, September 14-16, 1999

National Aeronautics and

Space Administration

Glenn Research Center

October 1999

https://ntrs.nasa.gov/search.jsp?R=20000000447 2018-05-14T14:06:40+00:00Z
Acknowledgments

The authors would like to acknowledge the contributions of Dr. Jih-Fen Lei, Dr. G. Fralick, Dr. Herbert Will,

Dr. David Larkin, Matthew Gray, F. DeAngelo, and Dr. Daniel L.P. Ng of NASA GRC, the technical

assistance of D. Androjna and C. Salupo of Cortez/NASA GRC, and Prof. A.D'Agostino of

Notre Dame College in Maryland.

This report is a formal draft or working

paper, intended to solicit comments and

ideas from a technical peer group.

NASA Center for Aerospace Information

7121 Standard Drive

Hanover, MD 21076

Price Code: A03

Available from

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22100

Price Code: A03
CHEMICALGASSENSORSFORAERONAUTICANDSPACEAPPLICATIONSIll

G.W.HunterandP.G.Neudeck

NationalAeronauticsandSpaceAdministration

GlennResearchCenter

Cleveland, Ohio44135

L.Y.Chen

AYTCorporation

GlennResearchCenter

Cleveland, Ohio44135

C.C.Liu,Q.H.Wu.M.S.Sawayda, Z.JinandJ.Hammond

CaseWesternReserveUniversity

ElectronicsDesignCenter

Cleveland. Ohio44106

D. Makel

MakelEngineering

Chico,California95973

M. LiuandW.A.Rauch

GeorgiaInstituteofTechnology

Atlanta,Georgia30332

G.Hall

KennedySpaceCenter

KennedySpaceCenter,Florida32899

ABSTRACT

Aeronauticandspaceapplicationsrequirethedevelopmentofchemical sensorswithcapabilitiesbeyondthoseof

commerciallyavailablesensors.Areasof interestinclude

launchvehiclesafetymonitoring,emissionmonitoring,

andfiredetection. Thispaperdiscussestheneedsof aeronauticandspaceapplicationsandthepoint-contact sensor

technologybeingdevelopedtoaddresstheseneeds.The

development ofhesesensorsibasedonprogressintwo

typesof technology: 1)Micromachiningandmicrofabricationtechnologyto fabricateminiaturizedsensors. 2)The

development of hightemperaturesemiconductors, especiallysiliconcarbide.Sensordevelopment foreachapplicationinvolvesitsownchallengesinthefieldsofmaterials

scienceandfabricationtechnology. Thenumberof dualusecommercialpplicationsof thismicrofabricated gas

sensortechnologymakethisareaofsensordevelopment a

fieldofsignificant interest.

INTRODUCTION

Aeronauticandspaceapplicationsrequirethedevelopmentof chemicalsensorswhichoperatein a numberof

environments. Threeareasof particularinterestarcsafcty

monitoring, emissionmonitoring, andfiredetection.Each

of theseareasarethesubjectof eflortthroughout NASA

to,forexample,improvesafetyanddecreasethecostof

spacetravel,significantlydecreasetheamountof emissionsproducedby aeronauticengines,andimprovethe

safetyofcommercial airlinetravel.Eachareaof applicationhasvastlydifferentproblemsassociatedwiththe

measurement of chemical species.However,thedevelopmentofacommonbasetechnologycanaddressthemeasurement needsofanumberofapplications.

In launchvehiclesafetyapplications, detectionof low

concentrations ofhydrogenatpotentiallyowtemperatures

is importantforapplicationsinvolvedwith,forexample,

operationof theSpaceShuttle.In 1990,theleaksonthc

SpaceShuttlewhileon the launchpad temporarily

groundedthefleetuntiltheleaksourcecouldbeidentified.

Themethodofleakdetectionusedwasamasspectrometer connectedto an arrayof samplingtubesplaced

throughout theregionof interest.Althoughabletodetect

hydrogenia varietyof ambientenvironments, themass

spectrometer hadadelaytimeassociatedwithitsdetection

of a leakandpinpointingtheexactlocationof theleak

wasproblematic.

NASA/TM--1999-209450 I
In responseto thehydrogenleakproblems,NASAendeavoredtoimprovepropellant leakdetectioncapabilities

duringassembly, pre-launchoperations, andflight. The

objectivehasbeento reducetheoperational costof assemblingandmaintaininghydrogendeliverysystemswith

automateddetectionsystems.In particular,effortswere

madeto developanautomatedhydrogenleakdetection

systemusingpoint-contact hydrogensensors.However.

nocommercial sensorsexistedthatoperatedsatisfactorily

inthisandotherspacerelatedapplications. Thereasonfor

thisistheconditionsinwhichthesensormustoperate.

Thehydrogensensormustbeabletodetecthydrogenfrom

low concentrationsthroughthe lowerexplosivelimit

(LEL)whichis4%:inair.Thesensormustbeabletosurvive exposureto 100%hydrogenwithoutdamageor

changeincalibration.Further,thesensormaybeexposed

to gasesemergingfromcryogenicsources.Thus,sensor

temperaturecontrolisnecessary. Operationiinertenvironmentsisnecessary sincethesensormayhavetooperate

inareaspurgedwithhelium.Abilitytooperateina vacuumisdesirable.Beingabletomultiplexthesignalfrom

anumberofsensorssoasto"visualize"themagnitudeand

locationofthehydrogenleakisalsodesired.If anumber

of sensorsareto beplacedinanarea,thensize,weight.

andpowerconsumptionforeachsensorbecomesanissue.

Commerciallyavailablesensors, whichoftenneededoxygento operateor dependeduponmoisture(1),did not

meetheneedsof thisapplicationandthusthedevelopmentofnewtypesofsensorswasnecessary (2).

Thedevelopment of anewclassofsensorsialsonecessaryforthemonitoringofemissionsfromaircraftengines.

Thecontrolof emissionsfromaircraftenginesis animportantcomponent ofthedevelopment ofhenextgenerationoftheseengines.Theabilitytomonitorthetypeand

quantityofemissionsbeinggeneratedbyanengineisimportantinnotonlycontrollingthoseemissionsbutalsoin

determiningthestatusof theengine.Ideally,anarrayof

sensorsplacedintheemissionsstreamclosetotheengine

couldprovideintormationonthegasesbeingemittedby

theengine.However, thereareveryfewsensorsavailable

commerciallywhichareabletomeasurethecomponents

oftheemissionsofanenginein-situ.Theharshconditions

andhightemperatures inherent nearthereactionchamber

oftheenginerendermostsensorsinoperable.

A notable exception to this limitation in sensor technology

is the commercially available oxygen sensor presently in

use in automobile engines (3). This sensor, which is based

on the changes in the properties of zirconium dioxide

(ZrO2) upon reaction with oxygen, has been instrumental

in decreasing automotive engine emissions. However,

comparable sensors for other components of the gas

stream do not exist: sensitive monitoring of emissions of

nitrogen oxides, hydrogen, and hydrocarbons is not presently possible in-situ with point-contact sensors placed

near the engine. Even the traditional ZrO2 based sensor

has sensitivity limits as well as size, weight, and power

consumption requirements which prevent its use in some

applications.

In emission monitoring applications, the sensor must operate at high temperatures with exposure to low concentrations of the gases to be measured. Although the measurement of nitrogen oxides is important in these applications,

the measurement of other gases present in the emission

stream such as hydrogen, hydrocarbons and oxygen is also

of interest. The measurement range depends on the gas

and the engine but generally the detection of nitrogen oxides, hydrogen, and hydrocarbons may be necessary at

sensitivities of less than 200 ppm with corresponding

measurements of oxygen from less than l C_ to near 20%.

The sensors should be small so as not to interfere with the

flow of gases in the engine or become significant projectiles if dislodged from their measuring site and emitted

into the engine.

In fire safety applications, fire detection equipment presently

used in the cargo holds of many commercial aircraft relies

on the detection of smoke. Although highly developed,

these sensors are subject to false alarms. These false alarms

may be caused by a number of sources including: changes in

humidity, condensation on the fire detector surface, contamination from animals, plants, or other contents of the

cargo bay (4). A second method of fire detection to complement existing techniques, such as the measurement of

chemical species indicative of a fire. will help reduce false

alarms and improve aircraft safety. Although, many chemicals species are indicative of a fire. two species of interest

are carbon monoxide and carbon dioxide (5).

In order to meet the needs of these applications, a new

generation of sensor technology must be developed. This

paper discusses point-contact sensor technology being

developed to address these needs. The development of

these sensors is based on progress in two types of technology: 1) Micromachining and microfabrication technology

to fabricate miniaturized sensors, and 2) The development

of high temperature semiconductors, especially silicon

carbide, to provide electrical components and sensors operable at the temperatures of interest. Using these technologies, point-contact sensors are being developed to

measure hydrogen (H2), hydrocarbons (C×Hy), nitrogen

oxides (NO0, carbon monoxide (CO), carbon dioxide

(CO_,) and oxygen (02). The detection of each type of gas

involves its own challenges in the fields of materials science and fabrication technology.

NASA/TM-- 1999-209450 2
This paperpresentsan overviewof microfabricated

chemicalsensortechnologyandhightemperatureelectronics.Thesetechnologies areused,whereappropriate,o

developH2,CxHy+ NOx, CO, CO2 and 02 sensors. A description is given of each sensor type and its present stage

of development. The silicon (Si) based hydrogen sensor is

at a relatively mature stage of development while the state

of development of the other sensors ranges from the proof

of concept level to prototype stage. The number of dualuse commercial applications make this general area of

sensor development a field of significant interest.

MICROFABRICATION AND MICROMACHINING

TECHNOLOGY

A significant number of silicon-based microfabrication

processes have been derived from the advancements in the

integrated circuit (IC) industry. Of the various processing

techniques, lithographic reduction, thin film metallization,

photoresist patterning, and chemical etching have found

extensive chemical and biological applications, particularly in sensor-related development. These processes allow the fabrication of very small sensor structures. The

ability to batch process these sensors using presently

available semiconductor processing techniques significantly decreases the fabrication costs per sensor. However, these processes produce mainly two-dimensional

planar structures, which have limited application. By

combining these processes with micromachining technof

ogy, three-dimensional structures can be formed which

have a wider range of application to chemical sensing

technology. Micromachining technology is generally defined as the means to produce three-dimensional structures

using both bulk and surface micromachining techniques.

The techniques used in micromachining fabrication include chemical anisotropic and dry etching, the

sacrificial layer method, and LIGA (lithographic, galvanoforming, absorbing).

Chemical anisotropic etching is an etching procedure that

depends on the crystalline orientation of the substrate. For

silicon etching, potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAH) solutions are most

commonly used as etching agents. Dry etching processes

include ion milling, plasma etching+ reactive ion etching,

and reactive ion beam etching. These dry etching processes are not dictated or limited by the crystalline structure. However, although not many chemicals are required.

the capital equipment cost of any of these dry etching processes is relatively high.

The sacrificial layer method employs a deposited underlayer that can be chemically removed. The sacrificial

layer method has been used to create cantilever type

structures for physical sensor and actuator applications.

This technique can be used to make a chamber electrode

structure to protect the integrity of the sensor element.

LIGA techniques have been used to produce high aspect

ratio multistructures. These microstructures can be used to

define volumes as well as for microanalytical elements

such as microcapillary structures.

For many applications, temperature control is necessary.

Incorporation of a heating element and a temperature detector allows fieedback control of the operating temperature. In

these microstructures, a small thermal mass is desirable in

order to minimize heat loss and heat energy consumption.

This is accomplished by selective removal of the silicon

substrate producing a suspended diaphragm structure.

This sensor processing is done using Si either as a semiconductor that is part ot+ an electrical circuit or as a substrate on which a structure is built. If Si is used as a substrate, the temperature range of the sensor can he rather

broad: from cryogenic temperatures to above 600 C.

However, if Si is to be used as pan of the electrical circuit,

the temperature range is limited to below 300 C. Thus, for

this and other applications+ high temperature electronics

must be developed. The most advanced high temperature

electronic material is silicon carbide (SIC). An overview

of SiC-based high temperature electronics is given in the

next section.

SiC-BASED HIGH TEMPERATURE ELECTRONICS

Silicon carbide based semiconductor electronic devices

and circuits are presently being developed for use in hightemperature, high-power, and/or high-radiation conditions

under which conventional semiconductors cannot adequately perform. Silicon carbide's ability to function under

such extreme conditions is expected to enable significant

improvements to a far ranging variety of applications and

systems. These range from greatly improved high-voltage

switching for energy savings in public electric power distribution and electric vehicles to more powerful microwave electronics for radar and communications to sensors

and controls for cleaner-burning more fuel-efficient jet

aircraft and automobile engines (6).

Silicon carbide occurs in many different crystal structures

Ccalled polytypes) with each crystal structure having its

own unique electrical and optical properties. The electrical properties of the more common SiC polytypes are

compared to the properties of silicon and GaAs in Table 1,

which was constructed from data in references 7-14. In

many device applications, SiC's exceptionally high breakdown field (> 5 times thai of Si), wide band gap energy (>

2 times that of Si), high carrier saturation velocity (> 2

times that of Si), and high thermal conductivity (> 3 times

NASAfI'M--1999-209450 3
thatof Si) could lead to substantial performance gains, in

spite of the low carrier mobility disadvantages.

Complex electronics and sensors are increasingly relied

upon to enhance the capabilities and efficiency of modern jet

aircraft. Many of these electronics and sensors monitor and

control vital engine components and aerosurfaces that operate at high temperatures. However, since today's siliconbased electronics technology cannot function at high temperatures, these electronics must reside in environmentally

controlled areas. This necessitates the use of long wire runs

between the sheltered electronics and the hot-area sensors

and controls or the fuel-cooling of the electronics and sensors located in high-temperature areas. Both of these lowtemperature-electronics approaches suffer from serious

drawbacks, as the wire runs add a substantial amount of

weight, fuel cooling has harmed aircraft fuel efficiency, and

both have negatively impacted aircraft reliability.

A family of high temperature silicon carbide electronics

and sensors that could function in hot areas of the aircraft

would alleviate the above-mentioned technical obstacles to

enable substantial aircraft-performance gains. Uncooled

operation of 300 - 600°C SiC electronics and sensors

mounted in the aircraft hot areas would save weight and

increase reliability by replacing hydraulic controls with

"smart" electromechanical controls. SiC-based distributed

control electronics would eliminate 90 % of the wiring and

connectors needed in conventional sheltered-electronic

aircraft control systems. This is extremely crucial given

the fact that wiring and connector problems are the most

frequent cause of propulsion maintenance activity and

downtime in commercial aircraft today. The U. S. Air

Force has estimated that advanced SiC control electronics

implemented on an F-16 fighter would allow the aircraft to

shed as much as 800 pounds of weight, operate with increased capabilities and fuel efficiency, and operate more

reliably with less maintenance and downtime (15).

Oandgap(eV)

BreakdownField

@1017cm"3(MVlcm)

ElectronMobility

@10_cm3 (cm2/V-s)

Saturated

ElectronDrift

Velocity(cmls}

Thermal

Conductivily(Wlcm-K'

HoleMoblltly

@10t6cm"3(cmqV-s)

SI GaAs 6H-SiC 4H-SiC 3C-SIC

1.1 1.42 3.0 3.2 2.3

0,6 0.6 3.2 3 > 1.5

1100 6000 370 800 750

I0 r 10r 2x107 2x107 2.5X10r

1.5 0.5 4.g 4.g 5.0

420 320 90 115 40

Table 1:Comparison of selected semiconductor room temperature physical properties.

SiC electronics and sensors offer similar improvements to

commercial jetliners, where increased fuel efficiency,

weight savings, and reduced pollution carry particularly

large economic and environmental payoffs over an aircraft's multi-decade operational lifetime. It has been

speculated that the economic savings value will be in the

millions of dollars per aircraft.

Therefore, silicon carbide electronics and sensors that

could function while mounted in hot engine and aerosurface areas of an aircraft would enable substantial weight

savings, increased jet engine performance, and increased

reliability. Use of SiC electronics and devices in other

applications such as in the fields of power distribution,

automobiles, and communications and radar could have

correspondingly significant effects on the operation of

these systems.

SENSOR DEVELOPMENT

The needs of aeronautic and space applications require the

development of new sensor technology to operate in environments in which conventional sensors are inoperable or

do not provide the required measurements. These applications require operation in a variety of conditions: from

cryogenic temperature to above 600 C, from inert environments to corrosive engine conditions, and from the detection of one gas over a wide concentration range to the detection of several gases over more narrow concentration

ranges. Combined with this is the desire to minimize size,

weight, and power consumption as well as decrease the

cost of the sensor. In order to meet these needs, a flexible

approach is necessary. Microfabrication and micromachining technology as well as the use of SiC semiconductor technology can address many of the needs of

aeronautics and space applications.

Microfabrication and micromachining allows the fabrication of a variety of structures. Size, weight, and power

consumption are minimized by microfabricating the

structure while micromachining allows complex shapes to

be incorporated into a small region. A given structure can

be tailor-made to measure different gases by changing, for

example, the gas sensitive element. Batch processing using Si semiconductor technology can decrease cost per

sensor and allow a large number of sensors to be produced

in one series of processing. Silicon can be used as substrate or part of the electrical circuit. However, Si is not

appropriate for sensor designs in which the substrate is an

electrical component of the sensor and the sensor must

operate at high temperature. Thus, the use of SiC is necessary in these applications.

NASA/TM-- 1999-209450 4
The following reviews the application of microfabrication

and micromachining technologies as well as SiC technof

ogy to develop sensors for the detection of a variety of

gases. This work is done at NASA Glenn Research Center

(GRC), Case Western Reserve University (CWRU), Makel

Engineering Inc. (MEI), and Georgia Institute of Technology (GIT). The sensor design and sensing approach depends strongly on the application.

Si-Based Hydrogen Sensor Technology

One component of the sensor development program at

NASA GRC and CWRU in conjunction with MEI involves

the development of palladium (Pd) alloy Schottky diodes

on silicon (Si) substrates. These sensors are designed to

detect hydrogen in space applications. This type of sensor

is based on metal-oxide-semiconductor (MOS) technology

such as that used in the semiconductor electronics industry. The gas sensing MOS structures are composed of a

hydrogen sensitive metal deposited on an oxide adherent

to a semiconductor. This forms a Schottky diode in the

case of a very thin layer of oxide. The most common

MOS structure used for hydrogen detection is the Pd-SiO2-

Si structure. Hydrogen disassociates on the Pd surface and

diffuses to the Pd-SiO: interface afl_cting the electronic

properties of the MOS system (I 6). The use of pure Pd at

near room temperatures as the hydrogen sensitive metal is

problematic for several reasons. The most serious of these

involves a phase change that occurs at high hydrogen concentrations which can lead to hysteresis or film damage.

Schottky diodes using Pd alloys as the hydrogen sensitive

metal are presently being fabricated. The first generation

of these sensors used palladium silver (PdAg). The use of

PdAg in hydrogen sensing applications was pioneered by

Hughes (17). Palladium silver has advantages over Pd.

Palladium silver is more resistant to damage from exposure to high hydrogen concentration than Pd. Furthermore, the alloy has faster response times than Pd.

The sensor structure is shown in Figure 1. The structure

includes a Pd alloy Schottky diode, a temperature detector,

and a heater all incorporated in the same chip. The surface

area of the Schottky diode is 6.1x10 -3 cm: and the sensor

dimensions are approximately 2.2 mm on a side. The response of the Schottky diodes was determined by measuring the diode's reverse current.

The properties of the PdAg sensor make it very useful for

applications where sensing small amounts of hydrogen is

necessary. The sensor responds in an inert environment (no

oxygen) to the presence of hydrogen. The presence of oxygen decreases the sensor response but the sensor is still sensitive to low concentrations of hydrogen ( I 8-19). The sensor

Pd-ALLOY SCHOTTKY

JDIODE CONNECTORS_

2.2 mm

tlEATER TEMPERATURE HEATER

CONNECTOR DETECTOR CONNECTOR

CONNECTOR

Figure 1. Schematic diagram of the Schottky Diode Hydrogen

Sensor. The Pd alloy Schottky diode resides symmetrically on

either side of a heater and temperature detector. The resistor has

been added to the PdCr based sensor design.

response is large, rapid, and repeatable. If quick recovery is

necessary, then the sensor should be operated in oxygen

containing gases. If detection of the presence of hydrogen is

required without rapid recovery, then this sensor can also be

used in inert environments. The sensor responds to hydrogen across a wide concentration range with a signal and response time that is temperature dependent. This sensor can

be used to monitor leaks in a multipoint leak detection

scheme involving a number of these sensors. Further, this

PdAg sensor has been shown to have a sensitivity and response comparable to that of a mass spectrometer (20).

2.0E-04 * T=100 C

_ APPLIED VOLTAGE = - 5 V _ POAg

1.8E-04 _ .. _ ' -,_ PdCr

mZI"4E'04 [ [ i | "I" _ i-'_---C_ • "

_1.2E-04- j * , ] [ : ' .-_

u_ 1.0E-04 - NZ_ .mi m N' I _N, ,NZl mp [m ] _z i; !

2.1E-0S _ik ="_ l" =_'. _ *-*- _J" i%

0 50 100 150 20O

TIME(MIN)

Figure 2. The response of PdAg and PdCr Schottky diode sensors at 10OC to repeated exposures of 9 minutes of air. I minute

of nitrogen. 10 minutes of 100% hydrogen, and 1 minute of nitrogen. The PdCr diode shows a more repeatable baseline and a

more stable response than the PdAg diode.

NASA/TM-- 1999-209450 5
Although the PdAg sensor showed excellent properties lbr

a number of applications, at higher temperatures and

higher hydrogen concentrations its calibration changed and

the sensor occasionally failed (18). Thus, the sensor behavior needed to be stabilized if it were to be exposed to

100c_ hydrogen as in the Shuttle application. This led to

the development of the next generation of sensor. This

sensor, which uses PdCr as the hydrogen sensitive alloy, is

in its later stages of development. A comparison of the

response to 100c_ hydrogen at 10ffC of the two types of

sensors is shown in Figure 2. The PdCr sensor is much

more stable than the PdAg sensor in these conditions. The

response of the PdAg sensors to 100% hydrogen decreases

with the number of exposures and its recovery to baseline

is increasingly less complete. In contrast, after an initial

break-in period, the PdCr diode response to 100% hydrogen is nearly consistent with an equally consistent return to

a common baseline.

The PdCr sensor is presently under development for use

on the NASA experimental vehicle, the X-33, in its hydrogen leak detection system. A PdCr resistor, whose resislance is dependent on the hydrogen concentration, has

been incorporated into the sensor design to expand the

detection range of the sensor (Figure 1); a Schottky diode

provides sensitive detection of low concentrations of hydrogen while the resistor provides sensitivity up to 100%

hydrogen. Hardware and software ("Smart" electronics)

have also been included with the sensor to provide signal

conditioning and control.

This complete hydrogen detection system (two sensors on

a chip with "Smart" electronics) has flown on the STS 95

mission of the Space Shuttle (launched 10-98) and again

on STS 96 (launched 5-99). The hydrogen detection system was installed in the aft compartment of the Shuttle and

used to monitor the hydrogen concentration in that region.

Presently, a mass spectrometer monitors the hydrogen

concentration in the aft compartment before launch, while

after launch "grab" bottles are used. The inside of these

"grab" bottles are at vacuum. During flight, the "grab"

bottles are pyrotechnically opened for a brief period and

the gas in the aft compartment is captured in the bottle.

Several of these bottles are opened at different times during the takeoff and their contents used to determine the

time profile of the gases in the aft chamber. However, this

information is only available after the flight.

The data of the STS 95 mission has been analyzed. The

response of the hydrogen sensors was compared to that of

the mass spectrometer and those obtained by the "grab"

bottles. During ground monitoring, the hydrogen sensor

(Schottky diode) response paralleled that of the mass

spectrometer but with a quicker response time. The hydrogen sensor response during launch and in-flight is

shown in Figures 3. No sensor response is seen until the

cut-off of the main engine. Near this time, a spike in the

hydrogen concentration is observed which decreases with

time back to baseline levels. These results are qualitatively

consistent with the leakage of very small concentrations of

unburnt fuel from the engines into the aft compartment

alter engine cut-off. These observations also agree with

those derived from analyzing the contents of the grab bottles. Moreover, the advantage of this microsensor approach is that the monitoring of the aft compartment is

continuous and, in principle, could be used for real-time

health monitoring of the vehicle in flight.

600

_" 5OO

a.

_Z400

l.-

rr

300

I11

0

z

0 200

z

ILl

0 100

ha

>-

3:

0

STS-95 Launch

/

Main Engine Cut Off (MECO)

0 200 400 600 800

TIME(SEC)

Figure 3. The response of PdCr Schottky diode sensor during

the launch of the STS-95 Shuttle Mission.

High Temperature Hydrogen and Hydrocarbon Detection

The development of high temperature hydrogen and

hydrocarbon sensors for use in harsh environments has

centered on the development of a stable SiC-based Schottky

diode. A Schottky diode is composed of a metal in contact

with a semiconductor (MS) or a metal in contact with a very

thin insulator or oxide on a semiconductor (MIS or MOS).

For gas sensing applications, the metal is often a catalytic

film. The advantage of a Schottky diode sensing structure in

gas sensing applications is its high sensitivity. This is

especially useful in emission measuring applications where

the concentrations to be measured are low.

The detection mechanism for hydrogen, as discussed in the

previous section, involves the dissociation of hydrogen on

the surface of a catalytic metal. The atomic hydrogen

migrates to the interface of the metal and the insulator, or

the metal and the semiconductor, forming a dipole layer.

This dipole layer affects the barrier height of the diode

resulting in an exponential change in the current or a

quadratic change in the capacitance. The magnitude of

NASA/TM-- 1999-209450 6
thiseffectcanbecorrelatedwiththeamountof hydrogen

andothergasspecies(especiallyoxygen)presentin the

surroundingambientatmosphere. Thedetectionof gases

suchashydrocarbons is madepossibleif thesensoris

operatedat a highenoughtemperatureto dissociatehe

hydrocarbonadproduceatomichydrogen.Theresulting

atomichydrogenaffectsthesensoroutputinthesameway

asmolecularhydrogen(16,21-23).

TheSchottkydiodestructureunderdevelopment atNASA

GRCbeganwithPdon SiC MSstructures(Pd/SiC).

Directcontactbetweenthe catalyticmetaland the

semiconductor allowschangesin thecatalyticmetalto

havemaximumeffectonthesemiconductor. Studiesof

this baseline system help determine limits of diode

sensitivity, potential material interactions between Pd and

SiC, and whether a barrier layer between the Pd and SiC is

necessary for long-term sensor stability. The details of

this work are reviewed elsewhere (22). The sensor

detects hydrogen and hydrocarbons in inert or oxygen

containing environments.

Figure 4 illustrates the advantage of SiC over Si in

hydrocarbon sensing applications. Shown is the zero bias

capacitive response of a Pd/SiC Schottky diode to one

hydrocarbon, propylene, at a range of temperatures. The

sensor temperature is increased from I0()"C to 400"C in

steps of 100°C and the response of the sensor is observed.

At a given temperature, the sensor is exposed to air for 20

minutes, N2 for 20 minutes, 360 ppm of propylene in N+ for

20 minutes, N2 for 10 minutes, and then 10 minutes of air.

The magnitude of sensor response to 360 ppm propylene

depends strongly on the operating temperature. A sensor

operating temperature of 100"C is too low lbr propylene to

dissociate on the Pd surface, so the device does not

respond at all. The three other curves for 200°C, 300°C,

and 400"C show that elevating the temperature increases

the sensor's response to propylene. The presence of

propylene can be detected at any of these higher

temperatures with 200°C being the minimum operating

temperature determined in this study. Since the standard

long-term operating temperature of Si is usually below

200°C. these results demonstrate the significant advantages

of using SiC rather than Si in gas sensing applications.

However, the sensor response is affected by extended high

temperature heating. Prolonged heating at 425 C has been

shown to change the sensor properties and to decrease

sensor sensitivity (22). Even after heating at 425C in air

for 140 hours, the Pd/SiC Schottky diode is still very sensitive to the presence of hydrogen: a factor of 1000 change

in forward current is observed upon exposure to 1000 ppm

hydrogen in He. The reason for this change in diode

properties is likely due to reactions between the Pd and

SiC at the interlace upon heating. Nonetheless, efforts

have been underway to stabilize the sensor structure for

long-term, high temperature operation. Two new structures have been demonstrated which improve the stability

of the Pd-based Schottky diode structure over that of

Pd/SiC (24).

16.5

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VOLTAGE = 0 V I PROPYLENE

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TEMPERATURE OQO-O._)O N2

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z • +ooc , •

,,¢

u'--:12"5 ,I...- .... " ...... i AIR

< N= _• -• • _-A ::i I

o_J_-_o.o_o-_-_-_e • o:-:.,,!,

9.5 .... -4

0 20 40 60 80

TIME(MIN)

Figure 4. The temperature dependence of the zero bias capacitance to various gas mixtures. The response to propylene is seen

to be strongly temperature dependent.

The first structure is PdCr directly deposited on SiC

(PdCr/SiC). The advantages of PdCr as a high temperature

alloy have been explored extensively in strain gage applications (25). It is a stable high temperature material which

is able to provide static strain measurements at temperature up to 1100°C. However, its use in a gas-sensing SiCbased structure depends on not only its inherent stability

but also such factors as the alloy's reactivity to SiC and the

catalytic interactions of PdCr alloy with the gases to

bc measured.

Figure 5 demonstrates that PdCr/SiC is a viable diode

structure for high temperature gas sensing applications

with improved stability compared to Pd/SiC (22)). Shown

is the response of a PdCr/SiC sensor as a function of

heating time. The sensor is heated at 425°C in air for extended periods and then characterized at 100°C in air and

upon exposure to a mixture of 120 ppm H2 in N2 (N3/H2

mix). While the air baseline current drifts lower with

heating time, the current in the N:/H2 mix is relatively stable after the initial heating period of 40 hours. This results

in the diode's sensitivity to H2 (change from baseline) being nearly two orders of magnitude larger at this voltage

after 250 hours of heating. Correspondingly, the magnitude of the signal in H2 is nearly constant (within a factor

of 3) after 40 hours.

NA SA/TM-- 1999-209450 7
The secondstructureinvolvesthe incorporationof

chemicallyreactiveoxidesintotheSiC-basedSchottky

diodestructure.A widevarietyof materials, e.g.metal

oxidesuchasSnO2,aresensitivetoC_HrandNOxathigh

temperatures. Thesematerialscouldbeincorporatedasa

sensitivecomponent into MOSstructuresand,unlike

silicon,SiC-baseddevicescanbeoperatedathighenough

temperatures for thesematerialsto bereactiveto gases

suchasCxHyandNOx.Thisresultsinanewtypeof gas

sensitivestructure:a metal-reactiveoxide-semiconductor

structure(MROS).Theadvantages of thistypeofSiCbasedstructureinclude1) increasedsensorsensitivity

sincethedioderespondstogasreactionswithnotonlythe

catalyticmetalbutwiththereactiveoxideaswell, 2)

improved sensor stability since the gas reactive oxide can

act as a barrier layer between the metal and SiC potentially

stabilizing the sensor's structure, and 3) the ability to vary

sensor selectivity by varying the reactive oxide element.

1.00E-04

1.00E-05

1.00E-06

<[

_- 1.00E-07 _

z

_ 1.00E4)8 _

1.00E-I_ e

VOLTAGE = 0,7 V

T=I00C

---- tB I

N21H2 MIX

--&

AIR

A --•--

1.00E-10

I

I

1.00E-11 ! ! I

o 50 lOO 15o

TIME (HRS)

i

200 250

Figure 5. The forward current at 100°C vs heating time at 425°C

in air (&) and in 120 ppm H_ in N2 (I).

Figures 6-7 demonstrate the use of this type of MROS

sensor by comparing the operation of a Pd/Sn02/SiC sensor

with a Pd/SiC sensor on the same chip. A thin

(approximately 50 ,_) SnO2 layer covers the half the chip

while the other half does not have SnO2 present. Palladium

contacts are sputter deposited on both sides of the chip.

Two different carrier gases are used: pure N2 and an air/N,

mixture. The sensors are first exposed to air tbr 15 minutes,

then the carrier gas for 15 minutes, followed by 400 ppm of

a hydrogen-bearing gas in the carrier gas, 5 minutes of the

carrier gas, and finally 10 minutes in air. The air/N2 carrier

gas had a constant oxygen concentration of 10%.

The effect of the thin SnO2 layer is most easily seen in the IV curves of Figure 6 for the Pd/Sn02/SiC and Pd/SiC diodes

respectively. The I-V curve for the Pd/SnO2/SiC diode in

Figure 6 shows parallel shunt resistance for voltages below

1.0 V, and exponential Schottky behavior above 1.0 V until

series resistance effects begin to dominate. The barrier

height derived from the exponential portion of the curves

suggests that the SnO2 increases the barrier height of the

diode. The effect of the 400 ppm H2 in N2 on the I-V curve

was to increase the current lbr a given voltage, with the

increase in current in the shunt resistance region being

somewhat lower than the increase in the Schottky region.

This increase in current (resistance decrease) was also noted

when the resistance across,just the SnO2 was monitored with

probes under the same conditions. Thus, SnO2 affects the

response to hydrogen of the diode with higher sensitivity to

hydrogen noted in the exponential Schottky-like conduction

region. In contrast, the Pd/SiC diode shows two types of I-V

behavior: an exponential response in the low voltage regions

and a series resistance effects at higher voltages. These

results clearly show that the SnO2 changes the sensor's basic

electronic behavior and the sensor response to H,. Further

research will investigate the effects of the geometry of the

SnO2 deposition on the sensor response as well as other the

effect of other reactive materials beside SnO2.

1E-02

IE-03 ii

_L

1E-04 :

i

PdlSiC DIODE

: AIR

N21H2

1E_6 _&:'-' _,-- II_ 1E_7 L_Bm '

1E_8 _ ,':' Pd/Sn02/SiC DIODE

i • AIR I

1E-09 , -" • N2/H2 i

!

1E-10 ..... -_+ ! ! , I -_

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

VOLTAGE (V)

Figure 6. Current vs voltage at 350°C for a Pd/SiC diode in air

(O) and the 400 ppm N2/H2 mix (A), and a Pd/SnO2/SiC diode in

air (I) and the N2/H2mix (&).

Figure 7 shows the response of both the Pd/SiC and the

Pd/Sn02/SiC diodes to H2, methane, and propylene that

were aged over a several week period at 350°C. The

Pd/SiC sensor does not respond to 400 ppm of H2 (Figure

7) or propylene and methane (not shown) in the air/N_,

mixture. However, the Pd/Sn02/SiC sensor responds with

increasing signal strength to methane. H2, and propylene.

That the Pd/SnO2/SiC sensor response to propylene is

stronger than that to He is significant; the Pd/SiC response

in N_, (22) and in N,_/air to propylene and H2 was reversed.

Thus, the addition of the SnO2 layer makes possible the

detection of gases not detected without the layer. It should

bc noted that the response of the Pd/SiC sensor degraded

over the several week period of 350°C operation while the

Pd/SnO2/SiC sensor remained relatively stable.

NASA/TM-- 1999-209450 8
Otheresearchincludesthedevelopment ofcompletesensorpackagingfortheSiC-basedsensor. Theobjectiveisto

beabletopackagethesensor,withconnections toexternal

monitoringequipment, for operationin engineenvironments.Thusissuessuchasstablelectrical contactsoSiC

and the integrityof hightemperatureinsulationare

ofimportance.

1.0E-03

1.0E-04 PdfSnO2/SiC DIODE

• 400 PPM H2

__ 400 PPM METHANE

C 400 PPM PROPYLENE

i

Pd_SiCDIODE

• 400PPMH_ .0E-05

Z t

u.I

ne

el."

0_1'0E'06 _ AIPJN2

i i

1.0E-07

1.0E-08

VOLTAGE = 0.50 V

T = 350 C

AIR/N2 CARRIER

AIR/N2

GAS MIX

AIR

• ............ i

i

i

,A-A • A--A

b • :r

I

i AIR r = _:_- -_ : _,

<,i..,._., _-..... L ......

I-_-- _ _-t_-I-_F_---I H-_-I

0 10 20 30 40 50 60

TIME(MINi

Figure 7. Current vs time at 350°C for a Pd/SiC diode exposed

to 400 ppm H 2 (I) and a Pd/SnOe/SiC diode exposed to

400 ppm of H 2 (A), methane IA), and propylene (O).

Nitrogen Oxide (NOx) and Carbon Monoxide (CO)

Detection

Two approaches are being explored for sensitive detection

of NO_ (specifically NO and NO2) and CO. First, the development of an MS or MIS SiC-based Schottky diode

with a NO_ or CO sensitive structure as discussed in the

previous section. Such a structure could use a catalytic

gate (26) or a MROS structure. The general principles of

operation of these types of sensors were discussed in previous sections.

The second approach to NO_ and CO detection is to use a

microfabricated and micromachined Si-based structure. In

contrast to the SiC-based approach where the SiC is used

as a semiconductor, the Si in this approach is not an integrated part of the electrical sensing circuit. Rather, the Si

is used as a platform on which the structure necessary for

the sensor is fabricated. This sensor structure, shown in

Figure 8, includes a temperature detector, heater, and

sensing element. The microfabrication process allows the

sensor to be small in size with low heat loss and minimal

energy consumption. Energy consumption is further reduced by etching out the backside of the Si wafer so that

the sensor components (temperature detector, heater, and

sensing element) are over a diaphragm region. This minimizes the thermal mass of the sensing area thereby decreasing power consumption for heating and decreasing

the time for thermal equilibrium. The temperature detector and heater are doped into the Si substrate for operation

over a wide temperature range. The sensing element is

composed of interdigitated electrode elements across

which is deposited SnO,. Changes in conductivity of

doped SnO2 across the interdigitated electrodes is measured and correlated to NO:, concentration.

A major component of this development work is to stabilize

the SnO: for long-term, high temperature operation. Drift in

the properties of ShOe with long term heating due to grain

boundary annealing have been previously noted (27-28).

This drift results in changes in the sensor output with time

and reduces sensor sensitivity. In order to stabilize the SnOe

structure for long term operation, the fabrication of nanocrystalline SnO2, as shown in Figure 9, is being investigated.

The grain size of the SnO: grains shown in Figure 9 are approximately 10 nanometers. Nanocrystailine materials have

several inherent advantages over conventionally fabricated

materials including increased stability and sensitivity at high

temperature (29-30).

CATALYST DOPED

SnO2

ELECTRODE

ELECTRODE

S_2

LAYER

SILICON

Figure 8. The structure of a tin-oxide N()_ sensor including temperature detector, heater, and sensing element. The electrode

material is Pt and the sensor dimensions are approximately

300 microns on a side with a height of 250 microns.

Figure 9. Nanocrystalline SnO2 after annealing at 600C for

30 minutes.

NASA/TM-- 1999-209450 9
Theparameterswhichaffectsensorsensitivitytoagiven

gasincludedeviceannealingtemperatureanddopingof

thegrainboundaries. Forexample, annealingof thesensorat700°CforI houryieldsveryhighsensitivity(neara

factor10changeinresistanceto95ppmCOat an operating temperature of 350°C) without significantly affecting

the SnO2 grain size. However, annealing of the sensor at

8000C for 1 hour significantly decreases the sensitivity

(down to a factor of 3 for the same CO concentration under the same conditions) and generates a significant

amount of SnO2 grain growth.

Inclusion of materials in the grain boundaries can also

significantly affect sensor sensitivity and stability. Figure

10 shows the response of a sensor doped with 5% Pt to a

range of CO concentrations at 300°C in air. The sensor is

very sensitive with a change in conductance of a factor of

38 from baseline to 95 ppm CO followed by a quick recovery. This is an increase of a factor of near 4 from the

undoped sample. The detection of NO_, at 360C has also

been achieved down to the 5 ppm level for a sensor annealed at 600°C. The sensor tends to saturate at higher

NO2 concentrations but the highest level of sensitivity is in

the lower ppm range which is the region of interest. Work

is beginning to increase the sensitivity, range, and selectivity of this NO: sensor through selective doping.

Further, the inclusion of silicia has been shown to significantly decrease the grain growth of the SnO2 as shown in

Figure !1: the inclusion of SiO2 significantly inhibited

particle growth of the grains after heating at 700°C tbr 24

hours. Work will continue to optimize processing conditions and doping concentrations of metals and silicates to

improve sensor stability, sensitivity, and detection range

for a given gas.

.,.., lSO' .....................................................................

4

,,0:ZZZZZZIZIZZZZZZ III ZZ. 

::::::::::::::::::::::::::::::::::::::::::::::::::::::::

| ::::::::::::::::::::::::::::: :::::::i::

4_ ............................................. ........

_ ................... _ ..................... I

10 ........................... I

• i i , ! t

Time (min)

Figure 10. The response of a Pt-doped tin-oxide sensor to a

range of CO concentrations at 300C in air.

a) b)

Figure I I. Comparison of the effect of annealing at 700_C for

24 hours on an a) undoped SnO__film and b) 5 weight percent

SiO2 doped SnO2 film. The SiO., doped film shows significantly

improved stability (decreased grain growth) over the

undoped film.

Oxygen Detection

The development of a microfabricated 02 sensor has been

initiated for safety purposes in aerospace applications but.

as demonstrated in the automotive emissions control example, significant applications exist in the area of aeronautics emission control. Commercially available 02 sensors are typically electrochemical cells using zirconium

dioxide (ZrO2) as a solid electrolyte and Pt as the anode

and cathode. The anode is exposed to a reference gas

(usually air) while the cathode is exposed to the gas to be

detected. Zirconium dioxide becomes an ionic conductor

of O = at temperatures of 600C and above. This property

of ZrO2 to ionically conduct O2 means that the electrochemical potential of the cell can be used to measure the

ambient oxygen concentration at high temperatures. However, operation of these commercially available sensors in

this potentiometric mode limits the range of oxygen detection. Further, the current manufacturing procedure of this

sensor, using sintered ZrO2, is relatively labor intensive

and costly resulting in a complete sensor package with a

power consumption on the order of several watts.

The objective of this research is to develop a zirconium

dioxide solid electrolyte O, sensor using microfabrication

and micromachining techniques. As noted in the previous

two sections, the presence of 02 often affects the response

of hydrogen, hydrocarbon, and NO_ sensors. An accurate

measurement of the O_, concentration will help quantify the

response of other sensors in environments where the 02

concentration is varying. Thus, the combination of an 02

sensor with other microfabricated gas sensors is envisioned to optimize the ability to monitor emissions.

NASA/TM-- 1999-209450 10
A schematicof thesensordesignisshownin Figure12.

AsdiscussedintheNO_detectionsectionabove,microfabricatingthesensorcomponentsontoa micromachined

diaphragmregionallowsthesensortobesmallinsizeand

havedecreasednergyconsumptionadtimeforthermal

equilibrium.Whenoperatedintheamperometric mode,

thecurrentof thiscellisalinearfunctionof theambient

O_,concentration. Thislinearesponsetooxygenconcentrationsignificantlyincreasesthe02detectionrangeofthe

sensor.Achamber structurewithawell-definedorificeis

micromachined tocoverthesensingarea.This orifice provides a pathway to control oxygen diffusion which is important in amperometric measurements. This orifice also

protects the integrity of the sensing electrode from impinging particles. Preliminary testing of the complete 02 sensor has been accomplished and further improvements on

the design are planned.

Zirconium Dioxide _?

Silicon N_ide

Silicon Dioxdde

Platinum

ELECTROCHEMICAL

CELL REGION

i mn_l_ CHAMBER REGION

,n., ._ ....

/ \, 1

/ \_

/ \ i

SIUCON / \ SILICON t

/ '\\ I

i / 01AP.HAGMREGION ,

i L

Figure 12. The structure of a microfabricated amperometric

oxygen sensor. The dimensions ot this sensor are comparable to

that of the NO_ sensor shown in Figure 10.

CO2 Detection

The detection of CO2 is, like the detection of oxygen, based

on the use of a solid electrolyte. The significant difference

between the O_,sensor will be the use of NASICON (sodium

super ionic conductor) as the solid electrolyte. NASICON is

an ionic conductor composed of NasZr2Si2POl__ which has

previously been shown to be sensitive for CO_, detection.

The preparation of the NASICON will be performed using a

sol-gel technique. The sensor structure has similarities to

that of Figure 8: a microfabricated, miniaturized sensor

structure which can be incorporated with other sensors such

as the CO sensor. The response of a NASICON-based sensor to a ran,,e of low CO, concentrations is shown in Figure

13. The sensor signal is the peak current derived from

voltametric measurements. Effort is underway to further

miniaturize the sensor and improve the response time. A

combined CO2/CO sensor is of interest not only for []re

safety applications but for aeronautic combustion monitoring applications as well.

125 .....................................................................................................................................................

120

a.

=E

115

cA_

_ 110

105

100

0 200 400 600 800 1000

CARBON DIOXIDE (PPM)

Figure 13. The peak current of a NASICON based sensor to

range of CO2 concentrations at 350 C.

Hi_h-SelectiviW Gas Sensors Based on Ceramic

Membranes

Gas sensors often rely on either the variation of a material

property in response to a gaseous species, or the electrochemical transport of a species dependent upon its partial

pressure. However, these sensors may have a lack of selectivity and display similar response to several different

gaseous species. The selectivity of these sensors can be

improved through the use of a selective membrane over

the sensor to exclude other gaseous species which may

interfere with the measurement of the species of interest.

A selective membrane with a narrow distribution of Ix)re sizes

may separate gaseous molecules according to molecular size,

offering size selectivity to species of interest. Since the gas

molecules of interest are on the order of several angstroms, the

pore size would, likewise, need to be only several angstroms

in diameter. Zeolites seem particularly well suited to this application. Zeolites are a crystalline ceramic material which

contains open channels (or pores) as a natural part of its

structure: they arc also called microporous crystalline solids.

Zeolites are composed primarily of aluminum, silicon, and

oxygen. The elements combine to tbrm SiO4 and A104 tetrahedra which are arranged to tbrm three dimensional networks

with uniform channels through the crystal. While these channels are identical in size or diameter within each structure of a

zeolite, the diameters of theses channels can vary significantly

(from 3 to 12 angstroms), depending mainly on the ratio of

aluminum to silicon. Further, the effective diameters of the

crystalline channels can be modified by the size of ions siting

within the channels. Different ions may be incorporated into

the crystalline channels by ion exchange. Still further, a.s a

ceramic material, zeolites are capable of withstanding high

temperatures and harsh environments which may be encountered during gas measurement.

NASA/TM-- 1999-209450 1I
Compositemembranes containing zeolites are being prepared using various processing techniques to achieve desired selectivity, including sol-gel and electrochemical

vapor deposition methods. The developed gas-selective

membranes are also being integrated with various electrochemical sensors to further enhance the selectivity to gas

molecules of interest.

High Temperature Electronic Nose Concept

The successful development of the individual high

temperature sensors discussed above will allow the

formation a sensor array which will allow the detection of a

number of gases on a single chip. For example, the

formation of an array of the sensors discussed in this paper

will detect H2, CxHy, NOx, CO, CO2 and 02. These gases

will be detected using three different platforms: a Schottky

diode, a resistor, and an electrochemical cell. Further

sensitivities can be achieved using the SiC-based MROS

approach adding other materials (such as TiO2, WO3,

SrTiO2 etc.) into the SiC-based Schottky diode sensor

structure. Each diode having a different reactive oxide will

have a different sensitivity to the gases to which they are

exposed. Improved selectivity and differentiation can be

achieved using the ceramic membranes discussed above.

Development of a such a microfabricated gas sensor array

operable at high temperatures and high flow rates would

be a dramatic step in allowing the monitoring/control of

emission produced by an aeronautic engine. This gas

sensor array would, in effect, be a high temperature

electronic nose and be able to detect a variety of gases of

interest. Several of these arrays could be placed around the

exit of the engine exhaust to monitor the emissions

produced by the engine. The signals produced by this

nose could be analyzed to determine the constituents of the

emission stream and this information then used to control

those emissions. The microfabrication of these sensors is

necessary: a conventional bulky system would add weight

to the aircraft and impede the flow gases leaving the

engine exhaust.

The concept of an electronic nose has been in existence for

a number of years. Commercial electronic noses presently

exist and there are a number of efforts to develop other

electronic noses. However, these electronic noses depend

significantly on the use of polymers and other lower

temperature materials to detect the gases of interest. These

polymers are generally unstable above 400°C and thus

would not be appropriate for use in harsh engine

environments. Thus, a separate development is necessary

tor a high temperature electronic nose.

Work has begun on this high temperature electronic nose

concept. As part of the Glennan Microsystem Initiative

(31), CWRU, Ohio State University (32), and NASA GRC

have begun to combine eflbrts in sensor technology,

microfabrication technology, packaging, and software

interpretation of sensor responses to develop a high

temperature electronic nose which meets the needs of

industrial applications.

COMMERCIAL APPLICATIONS

The gas sensors being developed by the Chemical Species

Gas Sensors team at NASA GRC and its collaborators at

CWRU, MEI, NASA KSC, and GIT are meant for aeronautics and aerospace applications but can be used in a variety

of commercial applications as well. For example, the PdAg

hydrogen sensors were developed for application on the

launch pad of the space shuttle. These sensors were not

completely developed for the space shuttle application due

to the change in behavior at higher hydrogen concentrations.

However, these sensors can be applied to an automotive

application. GenCorp Aerojet Corporation, in coniunction

with NASA Marshall Space Flight Center, has developed

hardware and software to monitor and control the NASA

GRC/CWRU sensors. The system can be customized to fit

the user's needs, e.g., to monitor and display the condition of

the tank of a natural gas vehicle. Several of these systems

have been purchased for use on the Ford Motor Company

assembly line lor natural gas vehicles (NGV). This complete system received a 1995 R&D 100 Award as one of the

1130most significant inventions of that year.

MEI is continuing this work with potential application of

hydrogen sensors in a variety of industrial environments.

For example, use of a hydrogen sensor to detect degradation of the health of transformers is being explored. As

electrical insulation in a transtbrmer breaks down prior to

its failure, hydrogen is released. Monitoring of hydrogen

in-situ in these transformers can provide early detection of

transformer degradation and avoid costly catastrophic failure of transformers in the field.

Likewise, the high temperature hydrogen, hydrocarbon,

NO_ and oxygen sensors are being developed for aeronautics applications but can be applied in commercial applications. For example, the conditions in an aeronautic engine

are similar to those of an automotive engine. Thus, sensors that work in aeronautic engine applications may be

operable in automotive engine applications. NASA GRC

has previously interacted with the Partnership for the Next

Generation of Vehicles (PNGV) program regarding

NASA/TM-- 1999-209450 12
possible use of SiC-based technology for the sensing of

hydrocarbons in automotive emissions. Other possible

applications include combustion process monitoring,

catalytic reactor monitoring, alarms for high-temperature

pressure vessels and piping, chemical plant processing,

polymer production, and volatile organics detection.

SUMMARY

The needs of space and aeronautic applications require the

development of sensors with capabilities beyond those of

commercial sensors. These requirements include operation

in harsh environments, high sensitivity, and minimal size

weight and power consumption. Sensor technology is being

developed to address these requirements using microfabrication and micromachining technology as well as SiC semiconductor technology. High temperature gas selective

membranes are also being developed to augment the capabilities of these sensors. The combination of these technologies allows for the fabrication of a wide variety of sensor designs with behavior and properties that can be tailored

to the given application. Several types of sensors have been

described all of which have aeronautic and space applications. Some of the sensor designs are relatively mature

while the development of others is ongoing. The combination of these technologies may allow the development of a

high temperature electronic nose to provide complex chemical analysis in harsh environments. Combination of these

sensor technologies with hardware and software has already

been accomplished in several applications.

Sensors designed for aeronautic and space applications

also have significant commercial applications. One example is the use of the hydrogen sensor in automotive applications. This application involved a sensor not completely

developed for a space application but with excellent properties for use in the automotive application. Further, given

the similarities of aeronautic engine environments to those

of automotive engine environments and chemical process

monitoring, sensors developed for aeronautic applications

also have a wide range of applicability. Although each

application is different and the sensor needs to be tailored

for that environment, the base technology being developed

for aeronautic and space applications can have significant

impact on a range of fields.

REFERENCES

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Available Hydrogen Sensors", NASA Technical Memorandum

105878. November 1992.

2. Hunter. Gary W.: "A Survey and Analysis of Experimental

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Octobcr 1992.

3. Logothetis, E.M.: "Automotive Oxygen Sensors,". Chemical

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Salupo, C.S.: and Matus, L.G.: "Electrical Properties of Epitaxial

3C- and 6H-SiC p-n Junction Diodes Produced Side-by-Side on

6H-SiC Wafers," IEEE Transactions on Electron Devices,

vol. 41, pp. 826-835, 1994.

NASA/TM-- 1999-209450 13
15.SecondInternational HighTemperature Electronics Conference,SessionI,Charlotte, NC,June5-10,1994.

16.Lundstrom, I. Armgarth. M.:andPetersson, L.:"Physics

withCatalyticMetalGateChemical Sensors, CRCCriticalReviewsin SolidStateandMaterialsSciences", vol. 15.

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17. Hughes,R.C.; Schubert,W.K.; Zipperian,T.E.:

Rodriguez, J.L.;andPlut,T.A.:"ThinFilmPalladiumandSilver

AlloysandLayersforMetal-Insulator-Semiconductor Sensors",

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Huntsville, AL.May17-19,1994,NASAConference Publication3282.Vol.1,pp.224-233.

19.Hunter.G.W.:Neudeck, P.G.:Chen,L.-Y.'Liu,C.C.:and

Wu.Q.H.;Makel,D.B.:andJansa, E.; "AHydrogenLeakDetectionSystemforAerospace andCommercial Applications,"

presented at31stIAA/ASME/SAE/ASEE JointPropulsionConferenceandExhibit.SanDiego.CA.July10-12,1995 (American

Instituteof Aeronautics and Astronautics, Washington, DC)

Tech. Rep AIAA-95-2645,1995.

20. Barnes, H.L.: and Makel, D.B.: "Quantitative Leak Detection

Using Microelectronic Hydrogen Sensors" presented at the 31 _

AIAA Joint Propulsion Conference, San Diego. CA. AIAA

Paper 95-2648, 1995.

21. Baranzahi, A.: Spetz, A.L.; Glavmo. M.: Nytomt. J.; and

Lundstrom I.: "'East Responding High Temperature Sensors For

Combustion Control". Proceedings, 8ChInternational Conference

on Solid-state Sensors and Actuators, and Eurosensors IX.

Stockholm Sweden, pp. 741-744, 1995.

22. Chen, L.-Y.; Hunter, G.W.: Neudeck. P.G.: Knight, D.:

Liu, C.C.: and Wu. Q.H.: "'SiC-Based Gas Sensors". in Proceeding of the Third International Symposium on Ceramic Sensors, Anderson, H.U.: Liu. M.: and Yamazoe, N., ed., Electrochemical Society Inc., pp. 92-105, 1996.

23. Hunter, G.W.; Neudeck, P.G.: Chert, L.-Y.: Knight, D.:

Liu, C.C.: and Wu. Q.H.: "'Silicon Carbide-Based Hydrogen and

Hydrocarbon Gas Detection", presented at the 31 _t AIAA

Joint Propulsion Conference, San Diego, CA. AIAA Paper

95-2647, 1995.

24. Hunter, G.W.: Neudeck, P.G.: Chert, L.-Y.: Knight, D.:

Liu, C.C.: and Wu, Q.H.: "SiC-Based Schottky Diode Gas Sensors", Proceedings of International Conference on SiC and Related

Materials. Stockholm. Sweden, Sep. 1-7, 1997.

25. Lei, J.F.: Plat. Met. Rev., 35, p. 65-72, 1991.

26. Baranzahi. A.: Spetz, A.L: Jonansson, R.; Hamrefors, G.:

and Lundstrom I.. "" Influence of the Interaction between Molecules on the Response of a Metal-Oxide-Silicon Carbide, MOSiC, Sensor". Proceedings, 8'h International Conference on

Solid-state Sensors and Actuators, and Eurosensors IX. Stockholm Sweden, pp. 722-725, 1995.

27. Ogawa, H.: Nishikawa, M.; and Abe, A.; "'Hall Measurement

Studies and an Electrical Conduction Model of Tin Oxide Ultrafine Particle Films", Journal of Applied Physics. vol. 53,

pp. 4448-4455, 1988.

28. Xu, C.: Tamaki, J.: Miura, N.: and Yamazoe, N.: "Grain Size

Effects on Gas Sensitivity of Porous SnO_-Based Elements, Sensors and Actuators, B, vol. 3, pp. 147-155, 1991.

29. Yoo. D,J.:, Tamaki, J.; Park, S.J.:. Miura. N.: and

Yamazoe. N.: "Effects of Thickness and Calcination Temperature on Tin Dioxide Sol-Derived Thin-Film Sensor", Journal of

Electrochemical Society, vol. 142. pp. L105-L107, 1995.

30. Vogel, R.: Hoyer, P.: and Weller, H.: "Quantum-Sized PbS.

CdS, AgeS. Sb2S 3. and Bi2S 3 Panicles as Sensitizers lbr Various

Nanoporous Wide-Bandgap Semiconductors". Journal of Physical Chemistry, vol. 98, pp. 3183-3188, 1994.

31. For more information, see the web address:

http\\www.glennan.org

32. Center Ior Industrial Sensors and Measurements. Ohio State

University, http://cism.ohio-state.edu/mse/cism/home_page.html

NASA/TM-- 1999-209450 14

REPORT DOCUMENTATION PAGE J Form OMB Approved No. 0704-0188

Pubtic reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

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1. AGENCY USE ONLY (Leave blank) 2. REPORT DATE !3. REPORT TYPE AND DATES COVERED

October 1999 Technical Memorandum

4. TITLE AND SUBTITLE

Chemical Gas Sensors for Aeronautics and Space Applications III

6. AUTHOR(S)

G.W. Hunter, P.G. Neudeck, L.Y. Chen, C.C. Liu, Q.H. Wu, M.S. Sawayda

Z. Jim J. Hammond, D. Makel, M. Liu, W.A. Rauch, and G. Hall

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135-3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

5. FUNDING NUMBERS

WU-242-82-62-00

8. PERFORMING ORGANIZATION

REPORT NUMBER

E-I1943

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1999-209450

11. SUPPLEMENTARY NOTES

Prepared for the Sensors Exposition sponsored by Sensors Magazine, Cleveland. Ohio, September 14-16. 1999. G.W. Hunter and

P.G. Neudeck, NASA Glenn Research Center: L.Y. Chen, AYT Corporation, Cleveland, Ohio 44135: C.C. Liu, Q.H. Wu, M.S. Sawayda,

Z. Jin, and J. Hammond, Case Western Reserve University, Electronics Design Center, Cleveland Ohio 44106: D. Makel, Makel Engineering,

Chico, California 95973: M. Liu and W.A. Rauch, Georgia Institute of Technology. Altanta, Georgia 30332: G. Hall, NASA John E Kennedy

Space Center, Kennedy Space Center, Florida 32899. Responsible person. Gary W. Hunter, organization code 55 I0, (216) 433-6459.

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Subject Category: 35 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information. (301) 621-0390.

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13. ABSTRACT (Maximum 200 words)

Aeronautic and space applications require the development of chemical sensors with capabilities beyond those of.commercially available sensors. Areas of interest include launch vehicle safety monitoring, emission monitoring, and fire

detection. This paper discusses the needs of aeronautic and space applications and the point-contact sensor technology

being developed to address these needs. The development of these sensors is based on progress in two types of technology: 1) Micromachining and microfabrication technology to fabricate miniaturized sensors. 2) The development of high

temperature semiconductors, especially silicon carbide. Sensor development for each application involves its own

challenges in the fields of materials science and fabrication technology. The number of dual-use commercial applications

of this microfabricated gas sensor technology make this area of sensor development a field of significant interest.

14. SUBJECT TERMS

Gas; Hydrogen; Leak; Fire; Emission; Detection

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2O

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A03

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