Recent Advances in Gas Sensors in Japan - ACS Symposium Series

2 Recent Advances in Gas Sensors in Japan

Downloaded by UCSF LIB CKM RSCS MGMT on December 2, 2014 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch002

Tetsuro Seiyama and Noboru Yamazoe Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University 39, Kasuga, Fukuoka 816, Japan

The development of new gas sensors has shown great advances in Japan during the past decade. The first type of gas sensor has centered around the development of semiconductor gas sensors using SnO elements, the commercial production of which has been growing steadily as detectors or alarms for leakage of domestic fuel gases. Many efforts have also been directed to the development of new gas sensors from semiconductor materials. The second type has evolved from the development of solid electrolyte gas sensors. In addition to oxygen sensors based on stabilized zirconia which are produced on a massive scale for control of car engines and steel making processes, solid electrolyte sensors for these applications are under investigation. The third type involves humidity sensors which have been become indispensable parts of several electrical appliances. Recent advances in the development of several gas sensors, i.e., lean-burn sensors, CO sensors, FET humidity sensors and solid electrolyte H sensors are described. 2

2

Research

and development of gas sensors i n Japan started with semiconductor gas sensors. In 1962, Seiyama {1, 2) and shortly after Taguchi (3_) proposed independently gas sensors using semiconductive metal oxides, which aimed at the detection of inflammable or reducing gases such as LP ( l i q u i f i e d propane) gas and town gas. Taguchi and his colleagues i n Figaro Engineering Inc. expended utmost efforts to put semiconductor gas sensors into p r a c t i c a l use and succeeded i n commercial production of them i n 1968. Since then, many researchers in Japan have been stimulated to investigations of improved or new types of semiconductor gas sensors as well as basic understandings of their sensing mechanisms. Semiconductor gas sensor devices and their applications have been inceasing year after year as needs f o r them expand i n various f i e l d s as seen from the number of patents shown i n Figure 1. The success of semiconductor gas sensors has encouraged the development of other types of gas sensors such as the 0097-6156/86/0309-0039S06.00/0 © 1986 American Chemical Society

In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

F U N D A M E N T A L S A N D APPLICATIONS O F C H E M I C A L SENSORS

40

oxygen sensor, humidity sensor, c a t a l y t i c combustion type sensor, and so on. These sensors have marked great advances largely through the effective use i n the automobile, s t e e l , and electronics industries.


The present state of gas sensors i n Japan The production of gas sensors. The production records of various types of gas sensors for past five years i n Japan are l i s t e d i n Table I except for the oxygen and humidity sensors. The sensors produced i n the largest quantity are of the semiconductive type, followed by the c a t a l y t i c combustion and thermistor types. These sensors have been mostly applied to domestic uses such as gas leakage alarms or gas control systems for LP gas and town gas which are extensively used for cooking and heating i n Japanese houses. This i s why these sensors are manufactured on a large scale. Other electrochemical sensors have been developed mainly to monitor other gases. Table I.

1

Production records of various types of gas sensors ) (thousand units)

F i s c a l year 1979 Sensor Semiconductor gas 1,220 sensor Catalytic combus171 tion type sensor Galvanic c e l l type 29 sensor E l e c t r o l y t i c sensor 9 Solid electrolyte 26 sensor Thermistor sensor 39 Ion selective 14 electrode sensor Others 2

1980

1981

1982

1983 5,430

3,140

4,330

4,170

186

202

223

240

31

34

36

38

9 28

10 31

11 74

12 82

45 11

51 10

60 9

55 8

2

7

8

T) except oxygen s e n s o r s which are used i n automobile industries.

8 and

steel

Metal oxide semiconductor gas sensors. The production of semiconductor gas sensors has grown into a large industry, producing more than f i v e m i l l i o n pieces per year i n 1983. Besides Sn02,Y-Fe203 and a-Fe203 have been put into p r a c t i c a l use as sensor materials; y-Fe203 sensors for LP gas alarms and a-Fe203 sensors for town gas alarms were commercialized i n 1978 and 1981, respectively, by Matsushita Electronic Components Co., Ltd. However, the "FIGARO" sensor (a type of Sn0 sensor) s t i l l has an overwhelming market share over Fe203 sensors and the c a t a l y t i c combustion type sensors, as shown i n Figure 2. The application of semiconductor gas sensors (mainly the Sn02 sensor) i s ever expanding into various f i e l d s , domestic and indust r i a l , as shown i n Figure 3. Examples are the CO selective sensor and combustion monitor sensor, which are applicable to micro wave ovens or ventilation fans, kerosene or gas stoves, hot water supply systems, and so on. 2


2.

Gas Sensors in Japan

SEIYAMA A N D YAMAZOE

41

400-

300-


200-

100-

O ID

H (N I D ID ID

CT L A LD N OOCJl O i D ID ID I D I D ID N

r—I C M h O C T i n I D N OO C D O rv. r \ l \ N N N N N K C X 3 O

H 0

CM ^ 0000

cn en cn cn o~i cn en en cn cn en en en cn cn cn cr> cn cn cn cn a i en o i

F i g u r e 1. Change i n annual number of p a t e n t s c o n c e r n i n g gas s e n s o r s .

"FIGARO" (Sn02

semiconductor

sensor

sensor) 90%

Catalytic type

combustion

sensor

Fe203

sensor

3%

7%

F i g u r e 2. Market share of s e n s o r s equipped i n h o u s e h o l d leakage alarms f o r both LP and town g a s e s . *

the number of gas

leakage

alarms i n s t a l l e d

up to

now.


gas



F i g u r e 3.

A p p l i c a t i o n of semiconductor gas s e n s o r s .



2.

43



Oxygen sensors. The production of oxygen sensors using s t a b i l i z e d zirconia has been increasing rapidly since 1977, and as a whole about one million pieces were produced i n Japan i n 1980, as shown i n Figure 4. About 96 % of oxygen sensors are used for automobile exhaust control. They are primarily supplied by two companies, Japan Electronic Control Systems and Nippon Denso Co., Ltd., as shown i n Figure 5, though the share of the former company includes oxygen sensors imported from U.S.A. More recent s t a t i s t i c s have not been disclosed, but i t i s said that Nippon denso Co., Ltd. produced 2.4 m i l l i o n oxygen sensors i n 1983. In other applications, about two hundred thousand oxygen sensors are consumed annually f o r steel making process control i n the Japanese steel industries. The production of oxygen sensors i s increasing mainly for application i n car engine systems. Humidity sensor. There exists a strong demand for humidity sensors especially i n Japan because of humid weather i n summer. The use of humidity sensors has been widely spreading i n i n d u s t r i a l f i e l d s as well as domestic. The production record i s l i s t e d i n Table I . Humidity sensors consist of two major types, i . e . , one which measures r e l a t i v e humidity and the other which detects dew condensation. The sensors for r e l a t i v e humidity made of ceramic or organic polymers are produced i n amounts exceeding one m i l l i o n . They are applied to controling systems i n a i r conditioning, micro wave ovens, and so on (4). Dew condensation sensors are usually made of hydrophilic or swelling organic polymers i n which carbon p a r t i c l e s are dispersed (j>). This type of sensor u t i l i z e s a drastic increase i n resistance at the point of dew condensation, since water absorption swells the polymers to counteract ohmic contact between carbon p a r t i c l e s . This type sensor i s now widely applied to humidity controling systems of video tape recorders or car windows. Table I.

Fiscal year 1983 1984 1) estimated

Production Record of Humidity Sensors i n Japan (thousand unit) Relative humidity Ceramic Organic polymer 750 560 900 ) 1

Dew condensation 8,500 12,000 ) 1

Examples of recently developed sensors Lean-burn sensor. Conventional oxygen sensors u t i l i z i n g c a l c i a - or y t t r i a - s t a b i l i z e d zirconia have been based on a concentration c e l l (potentiometric) method. When the sensor i s located i n car engine exhaust, the output emf undergoes a steep change at the s t o i c h i o metric a i r / f u e l r a t i o as shown i n Figure 6, and enables us to o p t i mize the operating conditions of the three way catalysts. On the other hand, recent investigations (6, 7) for fuel economy have focused attention on the combustion under excess a i r (or lean fuel) conditions, which i s also advantageous for depressing N0 formation as shown i n Figure 7. For control of lean-burn conditions, however, the conventional oxygen sensor i s not sensitive enough so that a lean-burn sensor based on an amperometric method i s of current interest. X



FUNDAMENTALS A N D APPLICATIONS O F C H E M I C A L

F i g u r e 5.

Market share of oxygen sensors i n Japan.


SENSORS




F i g u r e 7. Gas c o n c e n t r a t i o n s i n engine exhaust at v a r y i n g a i r - t o f u e l r a t i o ( A / F ) . Reproduced w i t h p e r m i s s i o n from Ref. 6. C o p y r i g h t 1984 Japan A s s o c i a t i o n of Chemical Sensors.




46

This very sort of lean-burn sensor has been developed successf u l l y by a Japanese automobile company, Toyota Motor Co. (6>, 7). This sensor u t i l i z e s electrochemical oxygen pumping, that i s , electrochemical transfer of oxygen from cathode to anode under an applied voltage, as depicted schematically i n Figure 8 . I f an oxygen-containing gas i s fed on the cathode through a small pin hole and the applied voltage i s large enough, the rate of oxygen pumping i s determined by the diffusion of gasous oxygen through the pin hole, and the external current (output) becomes proportional to the oxygen concentration outside the pin hole, as shown i n Figure 9. The I-V characteristics as well as the basic mechanism of this amperometric oxygen sensor are quite similar to those of polarography. Hence, the sensor may be c l a s s i f i e d as a l i m i t i n g current type. The diffusion limited current phenomenon also appears with a simple modification of the usual zirconia sensor. . That i s , a coating of the cathode with a porous ceramic layer, as carried out i n the sensor for pract i c a l use (see Figure 10), i s enough to make an effective diffusion layer of oxygen. The output current of the p r a c t i c a l sensor i s a l most proportional to excess a i r r a t i o , X, i n the lean-burn region as shown i n Figure 11, giving r i s e to responses much superior to the potentiometric sensor. The actual use of this type sensor has been begun i n Toyota Motor Company. One remaining problem i s i t s r e l a t i v e l y high operating temperature of about 700 °C. Carbon monoxide sensor. Carbon monoxide i s a toxic a i r pollutant originating from incomplete combustion of fuels i n burners or engines. Despite a strong demand for a very r e l i a b l e carbon monoxide sensor, the only available sensor was based on an electrochemical type u n t i l recently. In this type sensor, s e l e c t i v i t y and s e n s i t i v i ty for CO can often be enhanced by selecting elecrode materials and electrode potential appropriately, but several disadvantages are encountered, such as, short l i f e , d i f f i c u l t maintenance and a rather expensive price. In this circumstance, several investigations have been made to use Sn02 elements for carbon monoxide detection. For instance, a Sn02 element doped with a small amount of Th02 has been reported to show high s e n s i t i v i t y to carbon monoxide at room temperature. However, usually i t i s d i f f i c u l t to attain stable s e n s i t i v i t y and s e l e c t i v i t y under these operation conditions. Recently carbon monoxide sensors have been commercialized by Figaro Engineering Inc. and Yazaki Meter Co., Ltd. independently. The sensor u t i l i z e s sintered Sn0 elements which are essentially the same as those for LP gas detection but are operated i n a different mode which combines low temperature CO sensing and periodic heat cleaning. As i s well known, the gas s e n s i t i v i t y of semiconductor elements are influenced by operation temperature as well as by noble metal doping. The sensitivity-temperature characteristics for the TGS 203 element i s shown i n Figure 12 ( 8 J . Generally speaking, the s e n s i t i v i t y to CO, which i s low at high temperature, increases with lower temperature, and the response becomes higher than other gases such as ethanol and hydrogen below 100 °C. At lower temperatures, on the other hand, the semiconductor surface tends to suffer more seriously from contamination due to adsorption of gases such as water vapor, resulting in poor s t a b i l i t y and reproducibility i n the sensor response. In 2


2.



47

Cathode 02

+ 4e-2

\

0

2

- p

2

J ^

H

°

,

e


•O-i Zr02-Y 0 2

3

Anode 2 02~—

Oo +

4e

F i g u r e 8. Amperometric oxygen sensor u t i l i z i n g e l e c t r o c h e m i c a l pumping. Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1985 I n s t i t u t e o f E l e c t r i c a l E n g i n e e r i n g o f Japan.

Oxygen

Concentration

F i g u r e 9. I-V c h a r a c t e r i s t i c s of amperometric oxygen sensor (700 °C). Reproduced w i t h p e r m i s s i o n from Ref. 6. C o p y r i g h t 1984 Japan A s s o c i a t i o n o f Chemical Sensors.

American Chemical Society Library 1155 15th St., N.W. Washington, O.C. 20036 In Fundamentals and Applications of Chemical Sensors; Schuetzle, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

F U N D A M E N T A L S A N D APPLICATIONS

O F C H E M I C A L SENSORS


resistor

Pt

electrode(alr

side)

heater Zr02-Y203 exhaust

Pt

electrode(exhaust

porous

coating

protective

side)

layer

cover

F i g u r e 10. P r a c t i c a l amperometric oxygen sensor w i t h porous ceramic c o a t i n g . Reproduced w i t h p e r m i s s i o n from Ref. 6. C o p y r i g h t 1984 Japan A s s o c i a t i o n o f Chemical Sensors.

F i g u r e 11. I -X c h a r a c t e r i s t i c s o f amperometric oxygen sensor. excess a i r r a t i o . Reproduced w i t h p e r m i s s i o n from Ref. 7. C o p y r i g h t 1985 I n s t i t u t e of E l e c t r i c a l E n g i n e e r i n g of Japan.



2.

49


order to overcome this disadvantage, the commercial sensor adapts intermittent heating (60 seconds) of the element up to ca 300 °C to keep the surface clean, before gas sensing at low temperatures f o r a period of 90 seconds. The sensor performance thus observed i s shown i n Figure 13. The e l e c t r i c resistance of the element to 100 ppm CO i s similar to the response from 2000 ppm H or 1000 ppm ethanol. This means that the sensor i s free of false alarms due to H and ethanol i n ordinary use. An additional c i r c u i t device for temperature compensation can diminish the temperature effect i n the temperature range 0 - 5 0 °C. An activated charcoal f i l t e r i s also added to avoid the i n t e r f e r i n g effect of N0x gas. 2


2

FET type humidity sensor. Although sensors based on a f i e l d - e f f e c t transistor (FET) appear to hold promise as a small and low-cost i n t e l l i g e n t sensor, r e l a t i v e l y few people have been engaged i n the research on FET type sensors i n Japan. In t h i s respect, i t i s r e markable that a FET type humidity sensor was developed recently by Hijikigawa of Sharp Corp (_9). The sensor i s also worth notice as a new type of humidity sensor, which u t i l i z e s changes i n e l e c t r i c capacitance of humidity sensitive membrane interposed between double gate electrodes. A schematic cross-section of the FET humidity sensor integrated with a temperature sensing diode, which i s about 1.5 mm square i n size, i s shown i n Figure 14. The host FET device i s an n-channel MIS-FET with a meandering-gate structure. The gate insulator i s composed of double layers of s i l i c o n dioxide and s i l i c o n n i t r i d e films, the l a t t e r covering the whole surface to protect the FET characteristics i n the highly humid environment. The humidity sensitive polymer membrane of one micron i n thickness i s stacked between the lower-gate electrode and upper-gate electrode (porous gold electrode). The equivalent c i r c u i t of this humidity sensor i s shown in Figure 15. A D.C. voltage V , and a small A.C. voltage A V i s applied to the upper gate electrode. The upper- and lower-gates are e l e c t r i c a l l y connected with a s u f f i c i e n t l y large fixed resistance R . It can be shown that under adequate conditions, the output voltage, out» depends on the capacitance of the membrane, C , as follows. 0

0

B

v

s

v

o u t = ^o-RL'gn/d + Ci/C )

(1)

s

Where R i s the load r e s i s t o r connected with the drain electrode, g the transconductance of the FET , and C the capacitance of the gate insulator. It has been shown experimentally that V ut i s l i n e a r l y dependent upon r e l a t i v e humidity i n the almost whole humidity range. L

m

±

Q

Solid-Electrolyte Hydrogen Sensor. Most of s o l i d gas sensors so f a r developed need high temperature operation because of limited ionic conductivities when the electrolyte i s near room temperature. If s o l i d electrolytes with s u f f i c i e n t l y large ionic conductivities are available, unique gas sensors operative near room temperature can be fabricated. An example i s the following proton conductor hydrogen sensor proposed by our group (10, 11). The sensor construction i s shown i n Figure 16. As a proton conductor, Nafion (DU PONT, Nafion 117 H-type) membrane or disks of several inorganic ion exchangers were used. With sample gas and reference a i r passing over the respective electrodes, the e l e c t r i c



50

F U N D A M E N T A L S A N D APPLICATIONS

0

100

200

300

S e n s o r Temp,

F i g u r e 12. Temperature dependence element t o sample gases.

100

10

1

1

I 1 1

1

O F C H E M I C A L SENSORS

400

(°C)

of the response of TGS 203

I

I

IT "

1

'

1

-

1.

-

s • oo

/

to

\

RS(STD); 0.1

-

Rs

>v

i n CO 100 ppm

C 2 H 5 0H ^

CO

I 10

•I

i•

i• i• i'

100

i•

.i . i .

iI

,I

1000

Concentration

F i g u r e 13. Gas c o n c e n t r a t i o n dependence i s t i c s of TGS 203 element.

\

.I

. i III

10000

(ppm)

o f the response c h a r a c t e r -


SEIYAMA A N D

YAMAZOE

humidity Si 3 N 4

Gas Sensors in Japan Idouble gates s t r u c t u r e l

sensitive membranee

/ ,

m

e

gqte(moisture

r

permeabie) electrode


(temperature

FET

Humidity

sensor

1.5

Temperature mm

sensor)

sensor

J

F i g u r e 14. S t r u c t u r e o f FET h u m i d i t y sensor. Reproduced w i t h p e r m i s s i o n from Ref. 9. C o p y r i g h t 1985 I n s t i t u t e o f E l e c t r i c a l E n g i n e e r i n g o f Japan.

F i g u r e 15. E q u i v a l e n t c i r c u i t o f FET h u m i d i t y sensor. Reproduced w i t h p e r m i s s i o n from Ref. 9. C o p y r i g h t 1985 I n s t i t u t e o f E l e c t r i c a l E n g i n e e r i n g of Japan.

Glass

tube

Au s p r i n g

Cu

tube

F i g u r e 16. S t r u c t u r e of p r o t o n conductor sensor. Reproduced w i t h p e r m i s s i o n from Ref. 10. C o p y r i g h t 1983 Kodansha L t d . (Tokyo).



52

potential of the sensing electrode was measured against the counter electrode (reference) by means of an electrometer. Figure 17 shows the response curves to H i n a i r at room temperature of the sensor using a Nafion membrane and Pt black electrodes. The response time to reach 2000 ppm H was as short as about 10 seconds and the sensing electrode potential reached about - 200 mV. This sensor was also sensitive to CO, but insensitive to methane and propane. As shown i n Figure 18, the potential i s almost proportional to the logarithm of H 2 concentration diluted i n a i r . When H is diluted i n N , the observed potential corresponds to the electromotive force of a H -0 fuel c e l l , and i n fact the EMF was as large as about 1.0 V with a theoretical slope of 30 mV/decade, as shown i n the same figure. I t has been shown that i n the case of H diluted i n a i r , the following electrode reaction, i . e . , electrochemical oxidation of hydrogen (2) and electrochemical reduction of oxygen (3), are important. 2

2

2

2

2

2


2

+

H —» 2 H + 2 e 1/2 0 + 2 H+ + 2 e 2

2

—» H 0 2

(anodic) (cathodic)

(2) (3)

When the c e l l responds to the sample gas, both reactions should take place on the sensing electrode to form a l o c a l c e l l , which results i n a mixed potential of the electrode. This has been well confirmed by the measurements of respective polarization curves. Low temperature operation has been shown to give an advantageous effect on the sensor structure. The above sensor requires the separation of the sample gas from the reference a i r . We have found that at room temperature, the s i l v e r electrode i s active to oxygen, but inert to low content of H i n a i r . This indicates that, i f one uses the sensing Pt electrode together with the refernce Ag electrode, a reference a i r flow i s not necessary, and therefore s i m p l i f i c a t i o n and miniaturization of sensor structure are possible as shown i n Figure 19. The sensor element was fabricated by mixing proton conductors such as zirconium hydrogen phosphate with Teflon dispersion, followed by applying the resulting paste on the alumina tube. This modified sensor exhibited good performance with a longterm s t a b i l i t y as shown i n Figure 20. I t i s also possible to f a b r i cate an amperometric type of proton conductor hydrogen sensors. 2

Future scope Many problems and tasks, both s c i e n t i f i c and technological, still remain i n the f i e l d of gas sensors. For example, microsensors, i n t e l l i g e n t sensors, cord-less sensors, etc., seem to be very important as well as attractive. However, these are not covered here, and only trends i n sensor application anticipated i n the near future in Japan are pointed out below. 1. For domestic use, more useful humidity sensors, CO sensors and combustion monitoring sensors w i l l be developed with their increased integration into safety systems for homes and o f f i c e s . 2. Gas sensors for automotive use , such as the X sensor, leanburn sensor and sensors for car-room air-conditioning w i l l be u t i l i z e d more extensively. 3. In the i n d u s t r i a l f i e l d , the appropriate and effective application of sensors for 0 , S0 , CO, NH3 and so on w i l l be advanced for 2

2


SEIYAMA AND YAMAZOE



2.

F i g u r e 18. Sensing e l e c t r o d e p o t e n t i a l v s . H2 c o n c e n t r a t i o n i n a i r or i n N2. Reproduced w i t h p e r m i s s i o n from Ref. 10. Copyright 1983 Kodansha L t d . (Tokyo).


53

54


L— Sample

8mm

*

gas


Alumina :

5?": ! /

f Pr01on

tube conductor

Teflon

Ag

Pt

wire

powder

wire

F i g u r e 19. S t r u c t u r e of t h e m o d i f i e d sensor w i t h r e f e r e n c e Ag e l e c t r o d e . Reproduced w i t h p e r m i s s i o n from Ref. 10. C o p y r i g h t 1983 Kodansha L t d . (Tokyo).

600 alr 500

~°

O

o°0

Q

o

o n

rj

Q

°o

0

MOO

1 u. 5

300 2000ppm H 2

200

In

_o

air o

Q

100 0

1.3%

-6

H2

e-o-

In

air n

o

-100 10 Operation

20

30

tlme/d

F i g u r e 20. Long-term s t a b i l i t y of the m o d i f i e d s e n s o r s . Reproduced w i t h p e r m i s s i o n from Ref. 10. C o p y r i g h t 1983 Kodansha L t d . (Tokyo).


2. SEIYAMA AND YAMAZOE


55

process control and security monitoring. For instance, sensors detecting arsine gas and phosphine gas are needed in the electronics industries. Literature Cited 1.


2. 3. 4. 5. 6. 7. 8. 9. 10. 11.

Seiyama, T. ; Kato, A. ; Fujiishi, K. ; Nagatani, M. Anal. Chem.,1962. 34, 1502. Seiyama, T. ; Kagawa, S. Anal. Chem., 1966, 38, 1069. Taguchi, N. Japanese Patent 45-38200 (applied in 1962). Nitta, T. ; Terada, Z. ; Hayakawa, S. J. Am. Cerm. Soc., 1980, 63, 295. Ishida, T. ; Kobayashi, T ; Kuwahara, K. ; Hatanaka, H. National Technical Reports, 1978, 24, 436. Igarashi, I. ; Takeuchi, T. Preprint of 24th meeting of Japan Association of Chemical Sensors, 1984 ; p. 5. Saji, K. ; Takahashi, H. ; Kondo, H. ; Takeuchi, T. ; Igarashi, I. Proc. of the 4th Sensor Symposium, 1984, p.147. Chiba, A., personal communication. Hijikigawa, M. ; Furubayashi, H. ; Miyoshi, S. ; Inami, Y. Proc. of the 4th Sensor Symposium, 1984, p.135. Miura, N. ; Kato, H. ; Yamazoe, N. ; Seiyama, T. Proc. of the Int. Mtg. on Chemical Sensors, Kodansha/Elsevier, 1983, p. 233. Miura, N. ; Kato. H. ; Yamazoe, N. ; Seiyama, T. Chem. Lett., 1983, 1573.

RECEIVED October 31, 1985


Recent Advances in Gas Sensors in Japan - ACS Symposium Series

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