NASA/TM-- 1998-208816

Microfabricated Chemical Sensors for

and Emission Control Applications

G.W. Hunter, P.G. Neudeck, and L.-Y. Chen

Lewis Research Center, Cleveland, Ohio

D. Knight

Cortez III Service Corporation, Cleveland, Ohio

C.C. Liu and Q.H. Wu

Case Western Reserve University, Cleveland, Ohio

Safet_Y

November 1998

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NASA/TM-- 1998-208816

Microfabricated Chemical Sensors for

and Emission Control Applications

Safet_Y

G.W. Hunter, P.G. Neudeck, and L.-Y. Chen

Lewis Research Center, Cleveland, Ohio

D. Knight

Cortez III Service Corporation, Cleveland, Ohio

C.C. Liu and Q.H. Wu

Case Western Reserve University, Cleveland, Ohio

Prepared for the

1?th Digital Avionics Systems Conference

cosponsored by the American Institute of Aeronautics and Astronautics

and the Institute of Electrical and Electronics Engineers

Bellevue, Washington, October 31--November 6, 1998

National Aeronautics and

Space Administration

Lewis Research Center

November 1998
Acknowledgments

The authors would like to acknowledge the contributions of Dr. J. Lei, Dr. H. Will, and Dr. D. Larkin of

NASA LeRC, Dr. D. Makel of Makel Engineering, and the technical assistance of D. Androjna and

C. Salupo of Cortez/NASA LeRC. L. Chen is a NRC Fellow.

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
Microfabricated Chemical Sensors for Safety and Emission Control Applications

Gary W. Hunter, Philip G. Neudeck, and Liang-Yu Chen*

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio USA 44135

Dak Knight

Cortez III Service Corporation

Cleveland, Ohio USA 44135

C. C. Liu and Q. H. Wu

Case Western Reserve University

Electronics Design Center

Cleveland, Ohio USA 44106

ABSTRACT

Chemical sensor technology is being

developed for leak detection, emission monitoring, and fire safety applications. The development of these sensors is based on progress in

two types of technology: 1) Micromachining

and microfabrication (MEMS-based) technology to fabricate miniaturized sensors. 2) The

development of high temperature semiconductors, especially silicon carbide. Using these

technologies, sensors to measure hydrogen,

hydrocarbons, nitrogen oxides, carbon monoxide, oxygen, and carbon dioxide are being

developed. A description is given of each

sensor type and its present stage of development. It is concluded that microfabricated

sensor technology has significant potential for

use in a range of aerospace applications.

INTRODUCTION

Aerospace applications require the development of chemical sensors with capabilities

beyond those of commercially available sensors.

Three areas of particular interest are leak detection, emission monitoring, and fire detection. In

launch vehicle safety applications, the detection

of hydrogen leaks is important for applications

involved with, for example, operation of the

Space Shuttle. In 1990, the leaks on the Space

Shuttle while on the launch pad temporarily

grounded the fleet until the leak source could be

identified. In response to the hydrogen leak

problems, NASA endeavored to improve

propellant leak detection capabilities during

assembly, pre-launch operations, and flight. In

particular, efforts were made to develop an

automated hydrogen leak detection system using

point-contact hydrogen sensors. However, no

commercial sensors existed at that time that operated satisfactorily in this and other space

related applications. For example, commercially available sensors often needed oxygen or

depended upon moisture to operate. 1 Such

sensors did not meet the needs of this application where hydrogen detection was necessary in

inert environments or upon exposure to

cryogenic environments. 2 Thus the development of new types of sensors was necessary.

* National Research Council Research Associate.

NASA/TM--1998-208816 1
The development of a new class of

sensors is also necessary for the monitoring of

emissions from aircraft engines. The control of

emissions from aircraft engines is an important

component of the development of the next

generation of these engines. The ability to

monitor the type and quantity of emissions being generated by an engine is important in not

only controlling those emissions but also in

determining the status of the engine. Ideally, an

array of sensors placed in the emission stream

close to the engine could provide information

on the gases being emitted by the engine. However, there are very few sensors available

commercially which are able to measure the

components of the emissions of an engine

in-situ. The harsh conditions and high temperatures inherent near the reaction chamber of

the engine render most sensors inoperable.

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 (CxHy), nitrogen oxides (NOx), carbon

monoxide (CO), carbon dioxide (CO2) and oxygen (O2). 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 dual-use commercial applications makes this general area of sensor development a field of significant interest.

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. 3 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 chemical species are indicative of a fire, two species of interest are carbon monoxide and carbon dioxide. 4

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 (MEMS-based

SENSOR DEVELOPMENT

Hydrogen Sensor Technology

One component of the sensor development program at NASA Lewis Research Center

(LeRC) and Case Western Reserve University

(CWRU) involves the development of palladium (Pd) alloy Schottky diodes on silicon (Si)

substrates. A Schottky diode is composed of a

metal in contact with a semiconductor (MS) or a

metal in contact with a very thin insulator on a

semiconductor (MIS). 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: factors of over 1000 change in reverse

current for 0.5% H2. 5 Further, this type of sensor does not need oxygen for operation.

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

sensor is fabricated using a n-type silicon wafer

on which approximately 50 A of SiO2 is

NASA/TM--1998-208816 2
thermally grown in the sensor region. The

heater and temperature detector are platinum

resistors. Gold leads are applied by thermal

compression bonding and the sensor is mounted

on a TO5 header or on a ceramic flat package.

The surface area of the Schottky diode is

6.1x10 3 cm 2 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.

A hydrogen sensor using palladium

chrome (PdCr) as the H2 sensitive alloy is presently under development for use on the NASA

experimental vehicle, the X-33, in its hydrogen

leak detection system. 6 Palladium chrome was

chosen due its ability to detect H2 up to 100%.

A resistor, whose resistance is dependent on the

H2 concentration, has been added to the sensor

design to expand the detection range of the sensor (Figure 1); a Schottky diode provides sensitive detection of low concentrations of H2 while

the resistor (also made of I,dCr) provides sensitivity up to 100% H2. A complete system with

hardware and software included with the sensor

is included on the X-33 safety system. This

complete system is also scheduled for a demonstration flight on the Shuttle for the fall of

1998. 6

Pd-ALLOY SCHOTTKY

DIODE CONNECTORS

r

2.2 mi

HEATER 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 expands the sensitivity range of the sensor package to 100% hydrogen.

Hydrocarbon Detection

The development of hydrocarbon sensors

for use in harsh environments has centered on the

development of a stable SiC-based Schottky diode. The advantage of SiC over Si is its ability to

operate as a semiconductor at temperatures in excess of 600°C. This allows SiC-based gas

sensors to operate at temperature high enough to

allow the detection of hydrocarbons. 7 The first

Schottky diode structure developed at NASA

LeRC was I,d on SiC MS structures (i,d/SiC).

Direct contact between the catalytic metal and

the semiconductor allows changes in the catalytic

metal to have maximum effect on the semiconductor. Studies of this baseline system help

determine limits of diode sensitivity, potential

material interactions between I,d and SiC, and

whether a barrier layer between the i'd and SiC is

necessary for long-term sensor stability. The sensor detects CxHy in inert or oxygen containing

environments. The details of this work are

reviewed elsewhere. 7

Figure 2 illustrates the advantage of SiC

over Si in CxHy sensing applications. Shown is

the zero bias capacitive response of a Pd/SiC

Schottky diode to one hydrocarbon (propylene)

over a range of temperatures. The sensor temperature is increased from 100°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 N2 for

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

10 minutes.

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 for 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

NASA/TM--1998-208816 3
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 CxHy gas sensing applications.

However, the Pd/SiC 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. The reason for this change in diode properties is likely due to reactions between

the Pd and SiC at the interface upon heating.

16.5

15.5

._.14.5

M.

_13.5

Z

<

_12.5

<

O_

O<11.5

10.5

9.5

VOLTAGE = 0 V PROPYLENE

TEMPERATURE

100 C

• 200 C

© 300 C

• 400 C

AIR

N2

l l l

N2

0 20 40 60 80

TIME(MIN)

Figure 2. The temperature dependence of the zero bias

capacitance to various gas mixtures. The response to

propylene is seen to be strongly temperature dependent.

Nitrogen Oxide and Carbon Monoxide

Detection

A microfabricated and micromachined

Si-based structure with a chemically sensitive resistor is being used to measure NOx and CO detection. The Si is used as a platform on which the

structure necessary for the sensor is fabricated.

This sensor structure, shown in Figure 3, 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

electrode material is Pt and the sensor dimensions

are approximately 300 microns on a side with a

height of 250 microns.

CATALYST DOPED

sno_ .....

k, CATALYST DOPED /

\ s,,o2 ]

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. 8 These structures involve

1) The use an alloy as the catalytic metal.

2) The use of a MIS structure where the "insulator" stabilizes the structure and is also reactive

to the gases of interest. Preliminary testing of

both of these structures suggests significantly

improved stability and response over Pd/SiC.

Figure 3. The structure of a tin-oxide NO_ and CO

sensor including temperature detector, heater, and

sensing element.

The sensing element is composed of

interdigitated electrode elements across which is

deposited SnO2. Changes in conductivity of

doped SnO2 across the interdigitated electrodes

is measured and correlated to NOx

NASA/TM--1998-208816 4
concentration. A major component of this development work is to stabilize the SnO2 for

long-term, high temperature operation. Drift in

the properties of SnO2 with long term heating

due to grain boundary annealing have been previously noted. 91o This drift results in changes

in the sensor output with time and reduces sensor sensitivity. In order to stabilize the SnO2

structure for long term operation, the fabrication

of nanocrystalline SnO2 is being investigated.

Nanocrystalline materials have several inherent

advantages over conventionally fabricated materials including increased stability and sensitivity

at high temperature. 1112

Several prototypes of these devices

using nanocrystalline SnO2 have been fabricated

and evaluated. The sensitivity of the SnO2 to

NO2 at 360°C is shown in Figure 4. The sensor

is shown to be able to detect NO2 down to the

5 ppm level. 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. Similar results are seen

with the detection of CO.

3°F....................... I.....................................

24 •

"R° 111215

3

°o .......... 1'0......... 20 ......... 30 ......... 40 ......... 50 ....

NO2Concentration (ppm)

Figure 4. The response of a tin-oxide sensor to a

range of NOz concentrations at 360°C.

Oxygen Detection

The development of a microfabricated

O2 sensor has been initiated for safety purposes

in aerospace applications but significant

applications also exist in the area of aeronautics

emission control. Commercially available O2

sensors are typically electrochemical cells using

zirconium dioxide (ZrO2) as a solid electrolyte

and Pt as the anode and cathode. 13 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 600°C 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 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 02

sensor using microfabrication and micromachining techniques. The combination of a

fuel with O2 produces a hazardous environment.

Thus, the measurement of both the fuel and O2

simultaneously is useful in leak detection applications. Also, the presence of O2 often affects

the response of H2, CxHy, and NOx sensors. An

accurate measurement of the O2 concentration

will help quantify the response of other sensors

in environments where the O2 concentration is

varying. Therefore, the combination of an O2

sensor with other microfabricated gas sensors is

envisioned to optimize the ability to detect leaks

and monitor emissions.

A schematic of the sensor design is

shown in Figure 5. As discussed in the NOx

detection section above, microfabricating the

sensor components onto a micromachined

diaphragm region allows the sensor to be small

in size and have decreased energy consumption

and time for thermal equilibrium. When

NASA/TM--1998-208816 5
operated in the amperometric mode, the current

of this cell is a linear function of the ambient 02

concentration. This linear response to oxygen

concentration significantly increases the O2

detection range of the sensor. A chamber

structure with a well-defined orifice is micromachined to cover the sensing area. 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 O2 sensor

has been accomplished and further improvements on the design are planned.

Zirconium Dioxide

Silicon Nitride

SiliconDioxide

Platinum

ELECTROCHEMICAL

CELL REGION

i _ CHAMBER REGION

I ,LIC0' / ,UCON

I

DIAPHRAGM REGION

Figure 5. The structure of a microfabricated amperometric oxygen sensor. The dimensions of this sensor are comparable to that of the NO,, sensor shown in

Figure 4.

C02 Detection

The detection of CO2 is, like the detection of oxygen, based on the use of a solid

electrolyte. The significant difference from the

O2 sensor will be the use of NASICON (sodium

super ionic conductor) as the solid electrolyte.

NASICON is an ionic conductor composed of

Na3ZrzSizPOl2 which has previously been

shown to be sensitive for CO2 detection. The

preparation of the NASICON will be performed

using a sol-gel technique. The sensor structure

will be similar to that of Figure 5: a microfabricated, miniaturized sensor structure which can

be incorporated with other sensors such as the

CO sensor. A combined C02/C0 sensor is of

interest not only for fire safety applications but

for combustion monitoring applications as well.

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, 02

and CO2. These gases will be detected using

three different platforms: a Schottky diode, a

resistor, and an electrochemical cell.

Development of such a micro-fabricated

gas sensor array operable at high temperatures

and high flow rates would be a dramatic step in

allowing the monitoring and control of

emissions 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 used 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.

The concept of an electronic nose has

14

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

NASA/TM--1998-208816 6
environments. Thus, a separate development

effort is necessary for a high temperature

electronic nose.

COMMERCIAL APPLICATIONS

The gas sensors being developed at

NASA LeRC and CWRU are meant for aeronautics and aerospace applications but can be

used in a variety of commercial applications as

well. For example, hydrogen sensors using

palladium silver that were initially developed

for application on the launch pad of the space

shuttle. These sensors can be applied to an

automotive application. GenCorp Aerojet

Corporation, in conjunction with NASA

Marshall Space Flight Center, has developed

hardware and software to monitor and control

the NASA LeRC/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 for natural gas vehicles

(NGV). This complete system received a

1995 R&D 100 Award as one of the 100 most

5

significant inventions of that year.

Likewise, the high temperature H2,

CxHy, NOx, and 02 sensors are being developed

for aeronautical 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.

Other possible applications include catalytic reactor monitoring, alarms for high-temperature

pressure vessels, and chemical plant processing.

SUMMARY

The needs of aerospace 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. 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.

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.

NASA/TM--1998-208816 7
REFERENCES

1. Hunter, Gary W.; "A Survey and Analysis of

Commercially Available Hydrogen Sensors",

NASA Technical Memorandum 105878,

November 1992.

2. Hunter, Gary W.; "A Survey and Analysis of

Experimental Hydrogen Sensors", NASA Technical

Memorandum 106300, October 1992.

3. "Proceedings of the 1995 Workshop on Fire

Detection Research", W. L. Grosshandler, Editor,

June, 1995, Gaithersburg, MD, NISTIR 5700.

4. Grosshandler, W. L.; "A Review of Measurements and Candidate Signatures for Early Fire

Detection," NISTIR 555, Nat. Inst. Of Stand. And

Tech., Gaithersburg, MD, January 1995.

5. Hunter, G. W.; Neudeck, P. G.; Chen, L.-Y.;

Liu, C. C.; and Wu, Q.H.; Makel, D. B.; and Jansa,

E.; "A Hydrogen Leak Detection System for Aerospace and Commercial Applications," 31st

IAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, San Diego, CA, July 10-12, 1995

Tech. Rep AIAA-95-2645, 1995.

6. Hunter, G. W.; Neudeck, P. G.; Chen, L.-Y.;

Makel, D. B, Liu, C. C.; and Wu, Q.H. and "A

Hazardous Gas Detection System for Aerospace and

Commercial Applications," 34 th IAA/ASME/

SAE/ASEE Joint Propulsion Conference and Exhibit, Cleveland, OH, July 1998, Tech. Rep

AIAA-98-3614, 1998.

7. Chen, L.-Y.; Hunter, G. W.; Neudeck, P. G.;

Knight, D.; Liu, C. C.; and Wu, Q. H.; "SIC Based

Gas Sensors", in Proceedings of the Third International Symposium on Ceramic Sensors, Anderson,

H.U.; Liu, M.; and Yamazoe, N., ed., Electrochemical Society Inc., pp. 92-105, 1996.

8. Hunter, G. W.; Neudeck, P. G.; Chen, 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.

9. 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.

10. Xu, C.; Tamaki, J.; Miura, N.; and Yamazoe,

N.; "Grain Size Effects on Gas Sensitivity of Porous

SnOz-Based Elements, Sensors and Actuators, B,

vol. 3, pp. 147-155, 1991.

11. 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.

12. Vogel, R.; Hoyer, P.; and Weller, H.;

"Quantum-Sized PbS, CdS, Ag2S, Sb2S3, and Bi2S3

Particles as Sensitizers for Various Nanoporous

Wide-Bandgap Semiconductors", Journal of

Physical Chemistry, vol. 98, pp. 3183-3188, 1994.

13. Logothetis, E. M.; "Automotive Oxygen

Sensors", Chemical Sensor Technology, Vol. 3,

N. Yamazoe, ed., Kodansha Ltd., pp. 89-104, 1991.

14. Gardner, Julian W.; Bartlett, Philip N.;

"A Brief History Of Electronic Noses." Sensors and

Actuators, B, 18(1-3), 211-20,1994.

NASA/TM--1998-208816 8
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4. TITLE AND SUBTITLE

Microfabricated Chemical Sensors for Safety and Emission Control Applications

6. AUTHOR(S)

G.W. Hunter, P.G. Neudeck, L.-Y. Chen, D. Knight, C.C. Liu and Q.H. Wu

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

National Aeronautics and Space Administration

Lewis Research Center

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-523-26-13-00

8. PERFORMING ORGANIZATION

REPORTNUMBER

E-11418

10. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--1998-208816

11. SUPPLEMENTARY NOTES

Prepared for the 17th Digital Avionics Systems Conference cosponsored by the American Institute of Aeronautics and

Astronautics and the Institute of Electrical and Electronics Engineers, Bellevue, Washington, October 31--November 6,

1998. G.W. Hunter, RG. Neudeck, and L.-Y. Chen, NASA Lewis Research Center D. Knight Cortez III Service

Corporation, Cleveland, Ohio 44135; C.C. Liu and Q.H. Wu, Case Western Reserve University, Electronics Design Center,

Cleveland, Ohio 44106. Responsible person, G.W. Hunter, organization code 5510, (216) 433-6459.

12a. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category: 35 Distribution: Nonstandard

This publication is available from the NASA Center for AeroSpace Information, (301) 6214)390.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Chemical sensor technology is being developed for leak detection, emission monitoring, and fire safety applications. The

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

(MEMS-based) technology to fabricate miniaturized sensors. 2) The development of high temperature semiconductors,

especially silicon carbide. Using these technologies, sensors to measure hydrogen, hydrocarbons, nitrogen oxides, carbon

monoxide, oxygen, and carbon dioxide are being developed. A description is given of each sensor type and its present

stage of development. It is concluded that microfabricated sensor technology has significant potential for use in a range of

aerospace applications.

14. SUBJECT TERMS

Gas; Hydrogen; Leak; Fire; Emission; Detection

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

15. NUMBER OF PAGES

14

16. PRICE CODE

A03

18. SECURITY CLASSIFICATION 19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF THIS PAGE OF ABSTRACT

Unclassified Unclassified

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std. Z39-1B

298-102