Fundamentals and Applications of Chemical Sensors - American

6 Principles and Development of a Thick-Film Zirconium Oxide Oxygen Sensor Shinji Kimura, Shigeo Ishitani, and Hiroshi Takao

Downloaded by UNIV OF QUEENSLAND on August 15, 2015 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch006

Materials Research Laboratory, Central Engineering Laboratories, Nissan Motor Company, Ltd., 1 Natsushima-cho, Yokosuka 237, Japan

The newly-developed oxygen sensor consists of laminated, porous, thick film zirconia, reference and measurement electrode layers on an alumina substrate in which a thick film heater is embedded. Measurement of the oxygen concentration is accomplished by positioning the sensor entirely in the exhaust gas, and sending a continuous flow of DC current through the porous zirconia layer between two electrodes. Reference oxygen gas instead of air or other standard materials is then generated electrolytically at the reference electrode/ziroconia interface. The sensor has voltage characteristics which are nearly identical to the usual crucible-type sensor. Detailed analysis of the steady-state voltage chatacteristics of both the thick film oxygen sensor and the crucible type oxygen sensor are shown. Monitoring oxygen content i n exhaust gas from an automotive i n t e r n a l combustion engine has been widely used as the basis f o r c o n t r o l l i n g the a i r - f u e l r a t i o of the combustible mixture fed to the engine. An oxygen sensor i s used t o produce an e l e c t r i c a l signal representing the oxygen content i n the exhaust gas. (j_>2.) The zirconia sensor operates primarily on the p r i n c i p l e of a concentration c e l l . It consists of a non-porous s o l i d e l e c t r o l y t e layer fabricated from z i r c o n i a s t a b i l i z e d with y t t r i a or c a l c i a and exhibits high oxygen ion mobility. This layer i s sandwiched between two porous and e l e c t r i c a l l y conductive electrodes. In one of the most common sensors, the non-porous s o l i d e l e c t r o l y t e layers takes the form of a crucible closed at one end so that a i r used as a reference gas can be introduced into the i n t e r i o r of the crucible while the outside of the crucible i s exposed to the exhaust gas. A schematic drawing and E versus the a i r - f u e l r a t i o curve for t h i s sensor are shown i n Figure 1. E i s the electromotive force (EMF) between the two electrodes i n accordance with the Nernst equation: _

RT

Po2(A)

0097-6156/ 86/ 0309-0101 $06.00/ 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.

m


102

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS

where R i s the gas constant, T i s the absolute temperature, F i s the Faraday constant, and Po2(A) and Po2(B) are oxygen p a r t i a l pressures i n a i r and i n exhaust gas, respectively. The sensor voltage varies greatly at the stoichimetric point which i s an a i r - f u e l r a t i o of approximately 1U.7 f o r an ordinary engine.In an atmosphere with an a i r - f u e l r a t i o smaller than 1^.7, the CO gas concentration i n the exhaust gas increases; Such an atmosphere i s c a l l e d a r i c h atmosphere. In an atmosphere with an a i r - f u e l r a t i o larger than 1U.7, the oxygen gas concentration i n the exhaust gas increases; This i s c a l l e d a lean atmosphere. The objectives of t h i s study were twofold; (1) to develop a new type of oxygen sensor which makes the crucible-type sensor more compact and which does not use a i r as a reference gas, (2) to analyze output c h a r a c t e r i s t i c s of both the newly-developed z i r c o n i a oxygen sensor and the crucible-type oxygen sensor. Thick Film Zirconia Oxygen Sensor and Measurements A cross-section schematic drawing of the newly-developed thick f i l m oxygen sensor i s shown i n Figure 2. The platinum f i l m heater i s embedded i n the alumina substrate. E l e c t r i c a l resistance of the heater i s about 6 ohms at room temperature. Arranged i n layered fashion on the alumina substrate are the z i r c o n i a underlayer, the platinum reference electrode, the z i r c o n i a s o l i d e l e c t r o l y t e s t a b i l i z e d with 5.1 mole % Y 2 O 3 , the platinum measurement electrode, and f i n a l l y , t h e protective spinel (A203 MgO) layer. The z i r c o n i a layer i s i+mm long, kmm wide and 30um t h i c k . The element i t s e l f measures 5mm by 9mm and i s 1.2mm thick. A plane schematic drawing of the thick f i l m oxygen sensor i s shown i n Figure 3. The protective layer i s eliminated. A part of each platinum lead wire i s embedded i n the alumina substrate. The earth l i n e of the heater and sensor i s common. Figure k shows a production flow chart f o r the thick f i l m oxygen sensor. The heater, the underlayer, the z i r c o n i a s o l i d e l e c t r o l y t e and the two electrodes are formed by screen p r i n t i n g and s i n t e r i n g . The s i n t e r i n g condition i s at 1,U80°C for 2HR i n a i r . The temperature of sensor surface r i s e s to 600°C with plasma spraying. As a r e s u l t , the z i r c o n i a s o l i d e l e c t r o l y t e , two electrodes and protective spinel layer become porous. Measurements of the oxygen concentration are made with the sensor positioned e n t i r e l y i n the exhaust gas from an ordinary engine. A direct current i s applied between the two electrodes from a DC power source. (See Figure 2.) Measurement conditions are as follows: o Atmosphere - In the exhaust gas from an ordinary engine. o Gas temperature - 1 ,000°K o Direct current 0 - 20yA o Air-Fuel Ratio (A) 12- 17 This oxygen sensor can function when the gas temperature i s higher than 200°C. Figures 5 and 6 present the experimental results obtained with the t h i c k f i l m oxygen sensor. Figure 5 shows the r e l a t i o n s h i p between DC sensor current and #


6. KIMURAETAL.

Zirconium Oxide Oxygen Sensor

Voltmeter 1.0


Exhaust pipe

103

Stoichimetric 1 i

point >

1,000°K > 0.8 w Q) CP 0.6 -P

rH o

Zirconia

> u

o

Electrodes-

0.4 Rich —

10

Exhaust gas

—

Lean

15

17

0.2

Protective layer

0.0 11

i 13

19

A i r - f u e l r a t i o (A/F) Figure

1.

Schematic drawing and v o l t a g e c r u c i b l e - t y p e oxygen sensor

curvef o r

DC power source Protective

in/

layer

(AÔ^ «MgO)

Measurement electrode (Pt) S o l i d e l e c t r o l y t e (Y203-Zr02)

k ,/ / / / n

Reference electrode (Pt) Under layer

Voltmeter

(Y203-Zr02)

\ J l e a t e r (Pt) Substrate

(AI2O3)

O2 + 4e

Exhaust gas

Figure

2.

Schematic sensor

drawing o f thick

film

oxygen




S

°

l

i

d

electrolyte

Through h o l e s

Heater lead wire

Figure

3.

Sensor lead wire

Earth lead wire

Schematic drawing of sensor

AI2O3 g r e e n

thick

film

oxygen

sheet

i Printing

(Pt h e a t e r )

Lamination Printing

(Y203~Zr02)

P r i n t i n g (Pt) Printing

(Y203-Zr02)

P r i n t i n g (Pt)

\

-

Sintering Vapor d e p o s i t i o n (Pt) Plasma s p r a y

#

(Al203 MgO)

Assembly

Figure

4.

Flow c h a r t f o r p r e p a r a t i o n o f oxygen s e n s o r

thick


film


KIMURA ET AL.

Figure

5.


Experimental voltage curves f o r thick f i l m oxygen sensor (Sensor c u r r e n t vs V curve)

stoichi 1.2

1

i

i

1.0 > >

0.8

CP

0.6

Rich — , —- Lean

u

8

0.4

0.2 1_ ..

0.0 11

13

i

.i

15

17

Air-fuel ratio Figure

6.

19

(A/F)

Experimental voltage curves f o r thick f i l m o x y g e n s e n s o r (A/F v s V c u r v e )


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

106

sensor voltage at 1 , 0 0 0 K . With a lean atmosphere (A/F= 1 6 ) , the sensor voltage r i s e s steadily as applied current increases. On the other hand, with a r i c h atmosphere (A/F= 1 3 ) , the sensor voltage r i s e s dramatically. The current which produces sudden change i n sensor voltage i s c a l l e d I Q R . In t h i s case, IcR= 2 y A . The output c h a r a c t e r i s t i c s for the sensor versus the a i r - f u e l r a t i o f o r a sensor current I^= 5yA larger than I Q R i s shown i n Figure 6 . This voltage curve correlates well with the output c h a r a c t e r i s t i c of the c r u c i b l e type oxygen sensor. (See Figure 1 . ) P


Zirconia Oxygen Sensor Model Many types of oxygen sensor models have been proposed. (3.-8) A schematic drawing of the z i r c o n i a oxygen sensor model used i n t h i s study i s shown i n Figure 7 . The steady-state voltage c h a r a c t e r i s t i c of the thick f i l m oxygen sensor can be explained a n a l y t i c a l l y using t h i s schematic drawing. In t h i s analysis the following assumptions are made: (Note: symbols used i n t h i s paper are l i s t e d i n the legend of symbols.) (T) , Gases are CO, O 2 , CO2 and N 2 . Total pressure on the measurement electrode and i n the exhaust gas i s 1 atm, and p a r t i a l pressure of nitrogen gas i s a constant O.87 atm. (?) , At the reference electrode and the measurement electrode, chemical equilibrium of the following reaction i s maintained: C0+

1/2 0

Pco *{^2

—

2

I

p

C0 cc2

(2)

2

(3)

= K

( 3 ) , The d i s t r i b u t i o n s of P o, P 0 2 and P c o 2 i n the porous s o l i d e l e c t r o l y t e and porous protective layer are l i n e a r i n the steady state. (See Figure 8 . ) ® 5 ° 2 gas i s generated e l e c t r o l y t i c a l l y at the interface between the reference electrode and the s o l i d e l e c t r o l y t e layer. The mass of the O2 gas i s equal to the mass of O2 gas which diffuses through the porous thick f i l m z i r c o n i a , plus the mass of O2 gas which reacts with CO gas at the reference electrode. (5) , At the reference electrode/solid e l e c t r o l y t e layer interface, the mass of the CO gas which diffuses from the porous thick f i l m z i r c o n i a i s equal to the mass of CO2 gas which diffuses into the porous thick f i l m z i r c o n i a . © , At the measurement electrode/solid e l e c t r o l y t e layer interface, the mass of O2 gas which diffuses from the porous protective layer i s equal to the mass of O2 which changes to oxygen ion plus the mass of CO gas which diffuses from the porous protective layer. © , At the measurement electrode/solid e l e c t r o l y t e layer interface, the mass of CO gas which diffuses from the porous protective layer i s equal to the mass of CO2 gas which diffuses into the porous protective layer. (§) , Sensor voltage V can be expressed by the following equation C

V= E+ IRo RT PQ2 E= In kF Po2


(5)



6. KIMURAETAL.

Figure 8,

P a r t i a l pressure d i s t r i b u t i o n model


107

108


From assumption (T) P o 2 ( 0 ) Pco(0)+ P c o 2 ( 0 ) + P N 2 ( 0 ) = 1

(6)

+

From assumption (g) P c o ((0) Q) * >'f o 2 ( ") Pco2 K

( )

Pco(l) ' > f P o 2 ( l ) _

T r

( )

P

Q

T

8

From assumption (3) Pco(l) = - b i l + P ( 0 ) Po2(l)= b 2 l + P 2 ( 0 ) Pco2(D = ^31+Pco2(0)

(9)

C O


O

From assumption 5^=

©

b D ( S E ) +~ b i D ( S E ) 2

o2

(12)

co

Derivation of Equation 12 i s shown i n the appendix. From assumption (5) b

)

< 2 5 ,

We can obtain the calculated values of the sensor voltage and the i n t e r n a l pressure Prp from Equations 2 0 , 2 1 , 2k and 2 5 . 9

Calculated Results Crucible-type oxygen sensor with c a t a l y t i c electrode. In t h i s case, the s o l i d e l e c t r o l y t e i s non-porous and the sensor current 1 = 0 . A i r i s used as a reference gas, P 2 ( i ) i s constant 0 . 2 1 a t m ( X ) . Thus, only Equation 21 i s considered. a

2

0

Substituting 0 into I i n Equation 21 y i e l d s

Considering that the value of K i s extremely small (K= 6 . 3 3 1 6 at 1 , 0 0 0 ° K ) , solutions for Y i n Equation 2 6 are divided into the following three cases: Regarding A as the c o e f f i c i e n t of Y, i , e. x

A

I)

E

2

^

^

(27)

Pco(g)-Po ( ) 2

1 1

g

A < 0 (lean atmosphere) The terms of both Y and the constant may be desregarded. 2

Y> = P ( 0 ) = P o 2

o 2

(g)-|g^

Pco(g)

(28) 1

= P (g)

2

9

0 2

'

IE) A = 0 (stoichiometric point) The terms of Y

2

may be disregarded.

* ' - ^ < ° > - | T ^ K ( * * < « > •

EE) A > 0 (Rich atmosphere) The terms of both Y and Y 3

2

w.jj}

2 / 3

>

may be disregarded.

r , 0 ^|Pco (g) • K / P o ( g ) j 2

2

(30

C


g (

3

1

)

(32)

110


The calculated P 2 ( ° ) / F curve and E vs A/F curve are shown i n Figure 9 . Total pressure at both the measurement and the reference electrode i s 1 atm. v

s

A

0

Crucible-type oxygen sensor with non-catalytic electrode. A nonc a t a l y t i c electrode (e. g. Au) i s thought to delay the reaction rate i n the following reaction 1/20

C0+

2

C0

=

2

Ideally, the reaction to produce C0 cannot proceed. Therefore, the value of Pco (g) decreases and the value of K ( = P o * ^ o / c o ) increases. Dividing Equation 2 6 by K 2

p


2

c

The terms of both Y i s large. Then

3

2

and Y may be desregarded since the value of K

2

Y =P (0) = P (g) O 2

p

2

O2

P

+

c o 2

(3k)

(g)

(35)

= Po (g) 2

This result produces a continuous sensor voltage change at the stoichimetric point. Thick f i l m oxygen sensor with c a t a l y t i c electrode (when I=Q)« Substituting 1 = 0 into Equation 2 0 yields X

3

+

••

( x

^

-

J

Y )

{

-

2

( x + Y )

+

(

( ^

x +

Pco(0)-T.)x

îfii ) K

+

P c o ( 0 )

)

= 0

( 3 T )

Then (38)

X=Y

W

l

n

PollOT " 2F

Y

-°

(

3

9

}

when I equals 0 , the sensor voltage i s always 0 . The dependence of X and Y on the oxygen concentration i n the exhaust gas i s the same as that for the measurement electrode of the crucible-type oxygen


111


6. KIMURAETAL. Table I.

Calculation results f o r P (Rich atmosphere)

0 2

(l)

a

n

d

Po2(D

electrode)

(Reference electrode)

DCO(SE)K(P 2(0) O

D


D

c

o

(PL)K(po ( 2

g

)

+

o (°) 2

Po2(°) (Measurement

p

|^|p

c

o

2

+

2DO2(SEJ

-2(SE)(î^Pco(0)-Po2(0) RT1J UFSDQ2(SE) RTII UFSD (SE)

( ) g

(U1)

O2

D

_

c

° 2 ( P L ) ^ ^

Pco( )-Po ( ) g

2

RThI V UFSD (PL)y RThI 1+FSD 2(PL) 02

Pco2(0)

g

(

(kO)

0

2/3 +

UFSDo2(SE)

(U2)

* o 2 < 0 > - i g ^ P c o < 0 > RTII UFSDQ2'(SE)

(U3)

Non-existent

Non-existent

Discussion Sensor voltage characteristics of the crucible type oxygen sensor. According to the oxygen sensor model used i n t h i s analysis, the oxygen p a r t i a l pressure P o ( 0 ) at the measurement electrode can be expressed by Equations 2 8 - 3 2 , 3h and 3 5 . Calculated results f o r the sensor voltage are shown i n Figure 9 D. S. Eddy calculated the sensor voltage characteristics using a chemical reaction equilibrium model. (_1_) His results correlate well with the results shown i n Figure 9 . In the analysis of t h i s work, Po2(0) ^ expressed using both p a r t i c a l pressure and the d i f f u s i o n c o e f f i c i e n t of each gas. The oxygen sensor with a c a t a l y t i c electrode shows abrupt change i n sensor voltage at the stoichiometric point as shown i n Figure 1. On the other hand, the oxygen sensor with a non-catalytic electrode shows continuous change of sensor voltage at that point, t h i s c o n t i nuity of sensor voltage can be explained with the K gap from an ideal value i n equilibrium condition. 2

9

c a n

e

Sensor voltage characteristics of the thick f i l m oxygen sensor. Experimental data show that sensor voltage characteristics of the thick f i l m oxygen sensor vary greatly with the value of the sensor current.



112

Calculation results f o r P 0 2 U ) (Lean atmosphere)

Table IE.

P OS-KI

P C L

a

n

d

Po2(°)

Po2(0)

Po2(D

(Measurement electrode)

(Reference electrode)

D

(rr) C 0 ( P L )

, , P

°2(g)-2Do2(PL)

c

o

(

g

)

RThI (kk)


UFSDo2(PL) Dco(PL) }Dco2(PL)

, ^(g)

K ( r

1

. DcopCPL),. rco2(g) \

2Do2(PD

RThI 1 2/3 UFSDQ2(PL) |

f Dco(PL)K^Po (g)^ 2

P

(U5)

2i5ol(PL)

Pc02(g)

°2(°)+^SPC0(°) RTII 1*FSD 2(SE)

+

(1*7)

0

[Dco2(PLK^Tij - )- o2(g) p

I C L

0 ) . The calculated results are shown i n Tables I and H. In Tables I and IE, ICR, ILR, ICL and I I I can be expressed by the following equations: ( £ 1 ^


ICRS b

Pco(0)-P (0)) O2

2FSD (SE)Pco(0) RTl

/ jiQ \

co

*

UFSDo (PL)^ D C O ( P L )

_

2

ILR-

t„\\

/„x .p

/c \ n

V2Do2(PL) c o ( g ) Po2(g); P

RTl

(50)

+

._ 2FSDco(PL) Pco(g) "• RTl

.

=

/c-x

l+FSDo2(PL)Po2(g) RTh

/„x

_ i+FSD 2(PL) , v ILL= R^T- (, 02(g) T

0

;

P

+

D o2(PL) « 2 f c f w C

P

c

0

2

(

/ \\ V g

2FSD Q2(PL) Pcoig) HBS C

"

(.8)

fah\ (

5

M

/c-c\

Using the r e s u l t s i n Tables I and IE y i e l d s the calculated curves i n Figures 11 and 1 2 . The sensor current vs sensor voltage (V) curve and A/F vs sensor voltage (V) curve are shown i n Figure 1 1 . The thick l i n e s show the calculated results and the t h i n l i n e s show the experimental r e s u l t s . Tendencies shown by the calculated results correlate well with the experimental data. The sensor current vs the sensor voltage (E) curve f o r the l a r ger value of the sensor current i s shown i n Figure 1 2 . In the case of a lean atmosphere with the larger value of the sensor current, i t i s seen that sensor voltage (E) changes greatly. This voltage c h a r a c t e r i s t i c gives the A/F vs sensor voltage (E) c h a r a c t e r i s t i c shown i n Figure 1 3 . Internal t o t a l pressure P T at the reference electrode can also be calculated using the results i n Tables I and IE snf Equation 2 5 . Results of t h i s c a l c u l a t i o n are shown i n Figure Ik. When the sensor current I i s small, P T i n the lean atmosphere i s larger than P T i n the r i c h atmosphere. As the sensor current I becomes larger, P T becomes independent of the oxygen p a r t i a l pressure in the exhaust gas.


KIMURA ETAL.


1.2]

1


1

1

Sensor current

Figure

11.

1

1

115

1.2

(UA)

A i r - f u e l r a t i o (A/F)

C a l c u l a t e d and Experimental t h i c k f i l m oxygen sensor

results f o r

Rich (A/F=13) Lean (A/F=16)

KA)

Figure

12.

Calculated E vs I

curve




Stoichi.

1

O.CI

11

U

13

15

1

.

1

17

19

21

A i r - f u e l r a t i o (A/F)

Figure 13.

7

10

C a l c u l a t e d E vs A i r - f u e l r a t i o

6

10

5

10"

10"

4

10"

3

curve

10"

1(A)

Figure 14.

C a l c u l a t e d P T V S I curve


6.

KIMURAETAL.

117


CR« o2( ) i s expressed by Equation 1*3. As I becomes l a r g e r , i t can generally be expressed by the following equation. I : > I

p

I

P m-

RTII

0

Clearly, i t becomes an almost constant oxygen p a r t i a l pressure and i s independent of the oxygen p a r t i a l pressure i n the exhaust gas. Therefore, i t can be used as the reference value f o r the on-off type stoichimetric point oxygen sensor. I depends on D o(SE), T, S and 1 . As D ( S E ) becomes larger, i . e., the z i r c o n i a layer becomes more porous, and I Q R becomes larger. In t h i s c a l c u l a t i o n , the value of D ( S E ) i s assumed to be 10" cm / sec. The value of D ( S E ) obtained i n experimental results and calculated results closely matches. As the sensor current I becomes larger, P o 2 ( 0 ) i r i c h atmosphere becomes smaller. (See Equation 1*0). But when I i s larger than lOOyA, the effect of a current on P o 2 ( 0 ) must be considered. When I i s larger than ICR> the cubic Equation 21 produces no answers. I C R means the mass of CO gas that diffuses through the porous protective layer. As the protective spinel layer i s more porous than z i r c o n i a layer, I L R > I C R . The depencence of P 0 2 U ) and P 2 ( 0 ) sensor current I i n a lean atmosphere d i f f e r s from that i n a r i c h atmosphere. In a r i c h atmosphere, P c ^ ) changes abruptly. On the other hand, i n a lean atmosphere, P o 2 ( 0 ) changes greatly. This fact suggests the p o s s i b i l i t y f o r a "lean oxygen sensor." In the relationship between the sensor current and sensor voltage (See Figure 1 2 . ) , sensor voltage changes at ICL« ^CL means the mass of O2 gas that diffuses through the porous protective layer toward the measurement electrode. The change of sensor voltage at ICL shows a l i m i t i n g current c h a r a c t e r i s t i c s by oxygen gas d i f f u s i o n . The stoichimetric point i n the A/F vs sensor voltage curve (See Figure 1 3 . ) i s s h i f t e d toward a lean atmosphere. C R

C

co

7

2


co

co

na

o

n

O

1

In the case of a different combustible gas atmosphere. Tests by Takeuchi, et a l produced the following esperimental phenomena: (£) The z i r c o n i a oxygen sensor showed an on-off voltage characteri s t i c at A = 1 i n N2 - O2 - 1% CO gas. p

p

X= . o p / c o (Po2/Pco)o

(57)

(Po2/Pco)o= The stoichiometric point of P o 2 / c o p

(58)

On the other hand, the c h a r a c t e r i s t i c was seen at X= k i n N gas. In our model, t h i s phenomena can be explained as follows: when N 2 ~ O2 - CO gas i s used, A must be zero at the on-off point. (See Equation 27) 2

A

=^^J

Pco( )-Po (g) g

2

( 2 7 )

When N 2 - O 2 - H 2 gas i s used, M must be zero at the on-off point. M i s defined by


118


^ - ^ m

T

h

U

S

p

H ( )-Po 2

g

)'

2 ( g

(

9

)

(60)

^ = ^ L l

D

i g m r

H

2

(

P

L

)

(6D

DH2( ) i considered larger than D ( P L ) . Assuming D H ( P L ) / D C O (PL) = 4, P ( g ) VPo2(g) = k when P o(g) = F (g) = > differs from each gas atmosphere, i . e. the difference of d i f f u s i o n c o e f f i cient i n the protective layer of combustible gas makes X d i f f e r e n t . pL

s

co

2

T n u s

02


5

c

x

E2

Conclusions (T) A more compact thick f i l m z i r c o n i a oxygen sensor with a b u i l t i n heater has been developed. In t h i s sensor, the reference oxygen gas i s not a i r ; the oxygen gas i s generated e l e c t r o l y t i c a l l y at the interface between the reference electrode and the porous zirconia electrolyte. (g) The voltage c h a r a c t e r i s t i c s of the sensor are almost i d e n t i c a l to those of the conventional crucible-type oxygen sensor. (5) Analysis of the steady-state voltage c h a r a c t e r i s t i c s of the thick f i l m oxygen sensor and conventional crucible-type oxygen sensor indicates agreement between the t h e o r e t i c a l curves and the experiment a l curves. © In the case of a larger sensor current, the model f o r the thick f i l m oxygen sensor used i n t h i s analysis showed favorable p o s s i b i l i t i e s f o r a "lean oxygen sensor." Arroendix Dervation of Equation 12 Equation 12 can be derived as follows: The mass of oxygen which i s converted to oxygen ions= 1/kFS. The mass of oxygen gas which diffuses through the porous z i r c o n i a layer = J T - -n 3 ° - n 9 / P \ _ D oP _ D , / 3P 15x~~ SlT RT' ~ RT " RT "?X" ^ =

b

From the assumption

2

(

of chemical equilibrium i n the following reaction

CO + 1 /2 02 =

C02

the c o e f f i c i e n t of the mass of CO gas i s 1/2. We obtain

I

_ D o(SE) ,

Therefore,

I||

0

=

D

o

2

(

S

E

)

B

2

+

D (SE) ,

A

co

1

/

2

D ^ S E U M

This equation i s the same as equation 1 2 . introduced i n t h i s way.

Equation 18 can also be


6. KIMURAETAL.


Legend of Symbols A a-| a 3 A/F (A/F) b-j b b3 C D D (PL) D (PL) D o2( ) DJJ (PL) D (SE) D (SE) co2( ) E F h I IQL IQR III ^LR K 1 M Po P P o2 Pco(g) 02(g) Pco2(g) PN2(g) H2(g) Po2(g)' Pco(0) ^02(0) Pco2(0) PN2(0) Pco(l) P 2(l) co2(- -) N2(l) PT R Ro S T V 2

a

0


2

C0

02

pL

c

2

co

o2

D

S E

C

0 2

C

p

p

0

p

p

1

Equation 2 7 Pressure c o e f f i c i e n t of CO gas i n protective layer Pressure c o e f f i c i e n t of 0 gas i n protective layer Pressure c o e f f i c i e n t of C0 gas i n protective layer A i r Fuel r a t i o Ideal a i r f u e l r a t i o Pressure c o e f f i c i e n t of CO gas i n s o l i d electrolyte Pressure c o e f f i c i e n t of 0 gas i n s o l i d electrolyte Pressure c o e f f i c i e n t of C0 gas i n s o l i d electrolyte Gas concentration P/RT Diffusion c o e f f i c i e n t Diffusion c o e f f i c i e n t of CO gas i n protective layer Diffusion c o e f f i c i e n t of 0 gas i n protective layer Diffusion c o e f f i c i e n t of C0 gas i n protective layer Diffusion c o e f f i c i e n t of H2 gas i n protective layer Diffusion c o e f f i c i e n t of CO gas i n s o l i d electrolyte Diffusion c o e f f i c i e n t of O2 gas i n s o l i d electrolyte Diffusion c o e f f i c i e n t of CO2 gas i n s o l i d electrolyte Equation 1 Faraday constant Thickness of protective layer Current Equation 52 Equation kQ Equation 5^ Equation 50 Equation 3 Thickness of s o l i d electrolyte Equation 5-9 P a r t i a l pressure of CO gas P a r t i a l pressure of 02 gas P a r t i a l pressure of C02 gas P a r t i a l pressure of CO gas i n exhaust gas P a r t i a l pressure of 02 gas i n exhaust gas P a r t i a l pressure of CO2 gas i n exhaust gas P a r t i a l pressure of N2 gas i n exhaust gas P a r t i a l pressure of H2 gas i n N - H2 - 02 gas P a r t i a l pressure of 02 gas i n N2 - H2 - O2 gas P a r t i a l pressure of CO gas at measurement electrode P a r t i a l pressure of O2 gas at measurement electrode P a r t i a l pressure of CO2 gas at measurement electrode P a r t i a l pressure of N2 gas at measurement-electrode P a r t i a l pressure of CO gas at reference electrode P a r t i a l pressure of O2 gas at reference electrode P a r t i a l pressure of CO2 gas at reference electrode P a r t i a l pressure of N2 gas at reference electrode Total pressure at reference electrode Gas constant Resistance of sensor Area of electrode Absolute temperature Equation k 2

2

2

2

2

2

2


120 x X Y X

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS Diffusion length Equation 22 Equation 23 Equation 57


Literature Cited 1. Eddy, D. S., IEEE Transactions on Vehicular Tech., VT-23,1974, 125 2. Hamann, E., Manger, H. and Steinke, L., SAE paper770401,1977 3. Fleming, W. J., SAE paper770400,1977 4. Fleming, W. J., J. Electrochem. Soc., 1977, 124, 21 5. Fleming, W. J., SAE paper 800020, 1980 6. Wang, D. Y. and Nowick, A. S., J. Electrochem, Soc., 1979, 126, 1155 7. Verkerk, M. J. and Burggraaf, A. J., J. Electrochem. Soc., 1983, 130, 78 8. Mizusaki, J., Amano, K., Yamauchi, S. and Fueki, K., Proceedings of the International National Meeting on Chemical Sensor, Fukuoka, 1983, 279 9. Takeuchi, T., Saji, K. and Igarashi, I., abstract 74, p196, The Electrochemical Society Extended abstracts, Pittsburgh, Oct., 1978 RECEIVED December 12, 1985


Fundamentals and Applications of Chemical Sensors - American

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