5 Oxygen Sensor Using Perovskite-Type Oxides Measurements of Electrical Characteristics Yasuhiro Shimizu, Yoshiki Fukuyama, Hiromichi Arai, and Tetsuro Seiyama Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University 39, Kasuga, Fukuoka 816, Japan
The perovskite-type oxides (ABO) were studied from the viewpoints of application for oxygen sensors. Among the perovskite-type oxides examined, SrTiO showed a high sensitivity to oxygen in the temperature range from 550 to 800 °C in the "lean-burn" region (10 Pa < PO 10 Paandshoweda p-type semiconductive nature. As for SrSnO, no signi ficant changes in the conductivity were observed in the "lean-burn" region, since the change of the con duction mechanism from p-type to n-type occurred in this region. The resistivity characteristics of the specimens were also investigated in the exhaust gas of propane-oxygen combustion. SrSnO was promising for a combustion monitoring sensor, judging from the magni tude of the decrease in the resistivity at λ= 1 with decreasing λand good reproducibility of resistivity characteristics ( λ ; oxygen excess ratio). On the other hand, the magnitude of the decrease in the resistivity of SrTiO at λ= 1 was smaller than that of SrSnO, because the conduction mechanism of SrTiO changed from p-type to n-type at PO = 10 Pa. The effects of B site partial substitution of SrTiO on the oxygen partial pressure dependence of the conductivity were also investigated. 3
3
2
4
2
1/4
2
3
2
2
3
3
3
3
3
-2
2
3
The combustion of hydrocarbon fuels i s widely used to obtain useful energy i n various industries. As for an engine or a furnace, i t i s necessary to control the a i r to fuel r a t i o from the viewpoints of fuel conservation and pollution control (1.). Oxygen sensors are widely used to detect an oxygen p a r t i a l pressure i n the exhaust gas (1-6). Among oxygen sensors, some are useful to monitor and control the stoichiometric a i r to fuel r a t i o as required f o r pollution control (JL-6) and others are capable of monitoring the "lean-burn" region as required for enhancement of an energy efficiency (^3). These oxygen sensors are c l a s s i f i e d into two types, i . e . , s o l i d 0097-6156/86/0309-0083S06.00/0 © 1986 American Chemical Society
FUNDAMENTALS
84
A N D APPLICATIONS O F C H E M I C A L SENSORS
electrolyte type such as c a l c i a - s t a b i l i z e d zirconia and y i t r i a stablized zirconia (7-10), and semiconductor type such as t i t a n i a U , 5, 6.) and mixed metal oxides of CoO and MgO (3.). In the past several years, there have been increasing interests i n the semiconductive type oxygen sensor, because of small size, simple structure and s l i g h t cost. This type of sensor u t i l i z e s e l e c t r i c a l conductivity changes due to oxygen adsorption or desorption. The conductivity i s proportional to oxygen p a r t i a l pressure as the following equation. °
-
OoPQ^/"
1
(1)
The m value depends on the semiconductive nature of metal oxides i n the surrounding oxygen p a r t i a l pressure. For example, m i s 4 or 6 for p-type semiconductors and - 4 or - 6 for n-type semiconductors (11). Modification of redox properties In order to use semiconductive metal oxides as an oxygen sensor, both thermal s t a b i l i t y at elevated temperatures and atmospheric s t a b i l i t y under reductive environments are required for reproducibility and accuracy of the sensor. It i s well known that redox properties of oxides can be modified by the formation of mixed oxides (12). Effect of mixed oxide formation on A G ° of M-0 dissociation i s i l l u s t r a t e d i n Figure 1. The free energy changes of the following reactions, A G ° and A G ° , are calculated from thermodynamic data (13, 14). A
A0
AB0
n
n + m
—>
A0n-1
+
1/2
—>
A0 -1
+
B0
n
0
2
AG °
(2)
AG °
(3)
A
+
m
R
1/2 0
2
R
where component oxide, A0 , i s more reducible than B0 . Figure 1 shows that G ° of CoO, ZnO, and T i 0 alone are usually smaller than A G ° of mixed oxides derived from the component oxides. The dotted lines, a), b), c ) , and d) are - AGR° for these reactions n
m
A
2
R
CO H 1/13 C H 1/10 C H 2
4
1 0
3
8
+ + + +
1/2 1/2 1/2 1/2
0 02 0 0 2
2
2
» C0 > H0 —> 4/13 C0 > 3/10 C 0
a) b) c) d)
2
2
2
2
+ +
5/13 H 0 2/5 H 0 2
2
which are some possible oxidation reactions that may occur i n an engine or a furnace. Among these oxides, Nb 03, T i 0 , and mixed oxides of CoO and MgO are now studied for a semiconductive oxygen sensor. CoO i s the most reducible among them and i t i s not suitable for an oxygen sensor to detect the a i r to fuel r a t i o due to a low s t a b i l i t y toward reduction with hydrogen or hydrocarbons. However, as shown i n Figure 1, the s t a b i l i t y w i l l be improved by means of mixing with other oxides. In fact, a mixed oxide of CoO and MgO was reported as an oxygen sensor to improve a s t a b i l i t y toward reduction i n the ^rich-burn" region where oxygen p a r t i a l pressure i s usually below 10 Pa (3.). Exactly speaking, the above calculation method might not be applicable to actual oxides, since we do not know what kind of composition w i l l be produced under reductive environments. However, i f we deal with this problem f i r s t , the results calculated 2
1 5
2
SH1M1ZU ET
5.
85
Perovskite-Type Oxides
AL.
from this method w i l l best meet our needs. These considerations prompted us to investigate the oxygen sensor that consisted of mixed oxides. The perovskite-type oxides were chosen because of the f o l lowing reasons : 1) The e l e c t r i c a l properties of these oxides can be modified easily by selecting an appropriate combination of the cation constituents. 2) They are stable under reductive environments at high temperatures (15). Experimental Perovskite-type oxides were prepared by calcining the mixtures of TiC>2, Sn02, A I 2 O 3 and alkaline earth carbonates i n a desired proportion at 800 - 1000 °C for 2 - 5 h (16.). The calcined powders were again ground with a b a l l m i l l and then pressed into discs of 10 mm i n diameter and 1 mm i n thickness. The disc was covered with powder of the same composition and sintered at 1200 °C for 6 h i n an alumina crucible. Some of the discs were crushed, and then the powder was i d e n t i f i e d by X-ray d i f f r a c t i o n with f i l t e r e d Cu ka radiation. The surface area of the specimen was measured by BET method using the nitrogen adsorption isotherms. The Pt paste was applied on both sides of these discs and was f i r e d at 900 °C for 10 min for conductivity measurements, while the Pt paste was applied on the same side for thermoelectromotive c o e f f i c i e n t measurements. Temperature differences, AT, between the two ends of the discs were established with a small heater near one end of the disc. The semiconductive nature of the specimens was determined by the thermoelectromotive c o e f f i c i e n t , i . e . , negative value means n-type semiconductor and positive one means p-type. Figure 2 shows the gas-flow diagram for measuring conductivity characteristics of the specimens. The specimen was mounted i n a quartz vessel located i n an e l e c t r i c a l furnace. The D.C. conductivity of the specimen was measured using a constant voltage supply and an electronic picoammeter i n the temperature range from 400 °C to 800 °C under oxygen p a r t i a l pressure between 10 and 10 Pa. The oxygen p a r t i a l pressure between 10 and 10 Pa was established by a continuous flow of a mixed gas of nitrogen and oxygen of a t o t a l pressure of 10 Pa. An oxygen pump using a c a l c i a - s t a b i l i z e d zirconia was adopted to obtain an oxygen p a r t i a l pressure between 10~ to 10 Pa. The r e s i s t i v i t y characteristics of the specimen were also investigated at 700 °C i n the exhaust gas of propane-oxygen combustion. Propane and oxygen were mixed at an appropriate r a t i o , and then a flow rate of 200 ml/min with a t o t a l pressure of 10$ p was established by balancing with nitrogen. The mixture gas was burned at 400 °C over a Pt/Al203 catalyst and the exhaust gas was introduced into the quartz vessel i n which the specimen had been installed. Water vapor originating from propane-oxygen combustion was trapped with dry-ice and ethanol mixture before reaching the test chamber. A c a l c i a - s t a b i l i z e d zirconia oxygen sensor was i n s t a l l e d i n the test chamber to obtain an actual equilibrium oxygen p a r t i a l pressure. A stoichiometric point i s defined as X = 1. At this point there i s just enough oxygen to convert a l l of propane to CO2 and H2O. 2
2
5
5
5
10
2
a
Results and discussion In
order to find a material well suited to application for a
"lean-
F U N D A M E N T A L S A N D APPLICATIONS
0
O F C H E M I C A L SENSORS
20 40 60 80 100 AG^ (component o x i d e ) k cal/mol 0
Figure 1. Effect of mixed oxide formation on AG° of M-0 dissociation (298 K). Lines l)-3) show the mixed oxide systems based on CoO, ZnO, and T i 0 , respectively, a) - d) are - AG for the following reactions. a) CO + 1/2 0 -» C0 b) H + 1/2 0 -» H 0 c) 1/13 C H + 1/2 0 - * 4/13 C0 + 5/13 H 0 d) 1/10 C H + 1/2 0 3/10 C0 + 2/5 H2O 2
R
2
2
l>
2
4
1 0
3
8
2
2
2
2
2
2
1. 3-way valve 2. Stop valve 3. F i l t e r 4. Double pattern needle valve 5. Needle valve 6. Mixer 7. Soap f i l m meter 8. Active alumina trap Figure 2.
2
2
9. 10. 11. 12. 13. 14.
Catalyst ( P t / A l 0 ) Dry ice-ethanol trap 4-way valve Sample holder Zirconia oxygen sensor Zirconia oxygen pump 2
3
Gas-flow diagram for measuring sensor characteristics.
5.
SHIMIZU E T A L .
87
Perovskite-Type Oxides
burn" sensor, the D.C. conductivity of the specimen was measured i n the temperature range from 400 °C to 800 °C under oxygen p a r t i a l pressure between 10 and 10 Pa. Characteristics of specimens used for e l e c t r i c a l measurements are l i s t e d i n Table I (17). Among the perovskite-type oxides examined, SrTi03 showed a high s e n s i t i v i t y to oxygen as shown i n Figure 3. Since the conductivity of SrTi03 was found to be proportional to Po2 ^ above 10 Pa of oxygen p a r t i a l pressure i n the temperature range from 550 °C to 800 °C, the e l e c t r i cal conductivity of SrTi03 showed a p-type semiconductive nature under these conditions (18), as also expected from the result of thermoelectromotive c o e f f i c i e n t . On the other hand, as for SrSn03, no s i g n i f i c a n t changes i n the conductivity were observed i n the oxygen p a r t i a l pressure between 10 and 10 Pa as shown i n Figure 4, while thermoelectromotive coefficient of this specimen indicated a ptype semiconductor at 2.1X10 Pa. These results suggest that the change of the conduction mechanism of the specimen from p-type to n-type occurs i n the oxygen p a r t i a l pressure from 10 to 10 Pa. The change of the conduction mechanism was also observed for an undoped specimen of BaTi03 (19, 20), shown i n Figure 5. The effect of A s i t e p a r t i a l substitution on the oxygen pressure dependence of the conductivity was examined for this specimen. Plotting the logarithm of the conductivity against the logarithm of P Q shown i n Figure 6, a slope of approximately 1/5 was found i n B a o . 9 7 N a o . o 3 3 specimen, a 3 % sodium substituted compound derived from BaTi03. This result suggests that the increase of positive hole concentration by p a r t i a l l y substituting a univalent N a for a divalent B a results i n the enhancement of a p-type conduction mechanism. 2
5
ly
4
2
2
5
4
2
5
a s
2
T i 0
2 +
+
The r e s i s t i v i t y characteristics of the specimens were also i n vestigated i n the exhaust gas of propane-oxygen combustion. As shown i n Figure 7, the r e s i s t i v i t y of a l l specimens decreased dramatically at the stoichiometric point of combustion with decreasing x . From these results, we can c l a s s i f y these specimens into the following three groups. 1) SrTi03, CaTi03, and B a o , 9 7 N a o . 0 3 3 » the r e s i s t i v i t y increased with decreasing x at f i r s t , decreased abruptly at X = 1, and then decreased with decreasing X. The change i n the r e s i s t i v i t y at X = 1 was smaller than that of other specimens. 2) SrSn03, BaTi03, and CaSn03 ; the r e s i s t i v i t y increased s l i g h t l y with decreasing X down to X = 1, however, the magnitude of the change i n the r e s i s t i v i t y above X = 1 was small compared with that of the f i r s t group, and then the r e s i s t i v i t y decreased dramatically at X = 1. 3) BaSn03 and S r o . g L a o . l S n 0 3 ; the r e s i s t i v i t y decreased s l i g h t l y with decreasing X and then decreased dramatically at X = 1. These r e s i s t i v i t y characteristics depend on the semiconductive nature of the specimens. As stated previously, S r T i 0 showed the p-type conductivity i n the oxygen p a r t i a l pressure of 10 to 10 Pa and the change of the conduction mechanism into n-type occurred below 10 Pa. This phenomenon results i n a s l i g h t decrease i n the r e s i s t i v i t y at X =1. On the other hand, this change for SrSn0 and BaTi0 was found in the oxygen p a r t i a l pressure range from 10 to 10 Pa. The enhancement of p-type conduction mechanism of B a 9 7 ^ 0 . 0 3 ^ 0 3 by the p a r t i a l substitution of a univalent Na for a divalent B a i n the oxygen p a r t i a l pressure region from 10 to 10 Pa resulted i n a s l i g h t decrease i n the r e s i s t i v i t y at X = 1 compared with an undoped BaTi0 . In the case of BaSn03 and S r L a . i S n 0 , the n-type conduction mechanism was found to prevail even i n a high oxygen p a r t i a l T i o
3
2
5
2
3
3
2
5
Q#
+
2 +
2
3
Q # 9
0
5
3
Ba
3
3
Tio
3
743 973 873
0.32
0.65
0.21
745
- 0o08 953
773
- 0.20
0.50
833
983
0.47
0.41
1
P
P
P
P
1.5
1.8
54 55
0.9
0.4
57 63
5.9
0.6
1.0
2.7
65
63
n n
70
69
m^.g~l
%
6
6
3
6.9 xio-6
8.0 x l o "
4.6 x 1 0 "
2.4 x i o "
0
1.8 x l - 4
9.1 x i o - 8
1
S-cm"
b )
c )
a
a
4.4
4,8
- 5.1
4.2
C)
m value ^
measurements
Conductivity ^
electrical
Surface area
P
P
a
for
Relative dencity
specimens used
a) A t 973 K i n a i r . b) A t 995 K . c) A t 873 K . Source: Reproduced w i t h p e r m i s s i o n from Ref. 17. C o p y r i g h t 1985 The Chemical S o c i e t y o f Japan.
CaTi0
Na
0.97 0.03 CaSn0
3
BaTi0
^
sintered
Thermoelectromotive Semiconductive coefficient i n a i r type ) d9/dT (mV-K" ) T/K
C h a r a c t e r i s t i c s of
1 5 1 1
3
3
3
^O.^O.l BaSn0
SrTi0
SrSn0
Specimen
Table I.
SHIMIZU ET AL.
Perovskite-Type Oxides
- 6
eu o
w \
7 -
o 774 K
- 8 1
2
3 log
4 (P
0 2
JL 5
6
/Pa )
Figure 4. E l e c t r i c a l conductivity of SrSn03 as a function of oxygen p a r t i a l pressure. "Reproduced with permission from Ref. 17. Copyright 1985, 'The Chemical Society of Japan'."
FUNDAMENTALS A N D APPLICATIONS O F C H E M I C A L
SENSORS
- 4 1027 K
-
H I
- 5 927 K
g o
w \ 6 - 827 K O
727 K - 7 -
-A
A—Ar-
672 K
- 8
1
2 log
3 4 5 (P /Pa ) 0 2
F i g u r e 5. E l e c t r i c a l conductivity of BaTi0 oxygen p a r t i a l p r e s s u r e .
3
as a f u n c t i o n o f
5. SHIMIZU ET AL.
91
Perovskite-Type Oxides
1
i 973
r
K
—0-
o •
SrSn03
O
SrTi03
•
Sr
•
BaSn03
La
0 # 9
•
BaTi0
O
Ba
SnO
CaSnC>3
A
CaTi0
3
0 # 0 3
TiO3
3
I
1.2
1.4 C
i
3
•
1.0
0 # 1
Na
0 # 9 7
•
- 4 0.9
o-
1
L.
1.6
H
Q / 3 8
X =
2
(0 /C H ) toich 2
3
8
s
Figure 7. Dependence of the r e s i s t i v i t y of perovskite-type oxides on the X value. "Reproduced with permission from Ref. 17. Copyright 1985, 'The Chemical Society of Japan'."
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
92
pressure. Therefore, specimens whose conduction mechanism change from p-type to n-type below 10 Pa can not be used for monitoring of the stoichiometric r a t i o of a i r to f u e l . Among the perovskite-type oxides examined, SrSn0 was promising for a combustion monitoring sensor, judging from the magnitude of the decrease i n the r e s i s t i v i t y at X = 1 and from excellent reproducibility of the r e s i s t i v i t y characteristics. In order to c l a r i f y the r e s i s t i v i t y characteristics of the specimens, we obtained the relationship between an equilibrium oxygen p a r t i a l pressure and the oxygen excess r a t i o from both theoretical calculations and measurements using the oxygen sensor. The complete propane oxidation can be described by the following reaction. 2
3
C H 3
+
8
5 0 ->
3 C0
2
+
2
4 H0
(4)
2
I f there i s excess propane, CO w i l l be one of residue products combustion, while i f there i s excess oxygen, free oxygen w i l l present i n the exhaust gas. The oxygen p a r t i a l pressure of the haust gas, P Q , at 700 °C was calculated using the following equations (13, 14), ignoring the presence of CO. 2
AG
^73
C
= ~ 513.03 kcal/mol = - RT In
2
^° C H
^3^2*0
F
3
P
P
0
5
= 0 i "
2
P
2
C H i 3
8
of be extwo
(5)
2
x
(6)
8
a n d
where AGg i s the Gibbs free energy change of eq. (4), Po i C H i ^ i n i t i a l p a r t i a l pressure of oxygen and propane, respectively, and x i s the fraction of propane consumed. The free energy change (AGg ) was obtained by interpolation of values both at 900 K and at 1000 K. A desired r a t i o of propane to oxygen was established by the following procedure. The flow rate of propane was maintained at 7.5 ml/min, that of oxygen was changed to obtain a desired value of \ and then the t o t a l flow rate was maintained at 200 ml/min balancing with nitrogen. Therefore, the oxygen excess ratio, X, i s described by the following equation as a function of an i n i t i a l p a r t i a l pressure of oxygen. 73
p
a r e
3
2
t
le
8
73
p
0 i
/ p
2
X = p
p
C H i 3
£
8
=
p
3
8
f 5
( 0 / C H ) toich 2
0 i 2
£° s
P
P
5.33
=
0
2
i
(7)
C H i 3
8
When X i s above unity and propane i s oxidized completely, the equilibrium oxygen p a r t i a l pressure can be given by the combination of eqs. (6) and (7) P
0
2
=
5
P
C
3
H
8
i
(x
-
1)
(8)
On the other hand, when X i s below unity, the i n i t i a l oxygen p a r t i a l pressure can be written from eqs. (5) and (6) as a function of x. p
0 i = 1.02x10-24 2
+
5P
C
3
H
8
i
x
(9)
And then the equilibrium oxygen p a r t i a l pressure can be derived the combination of eqs. (6) and (9).
by
5.
93
Perovskite- Type Oxides
SHIMIZUETAL.
=1.02
(10)
2
XlO- ^
Substituting some values for X i n eq. (7), P i cam be obtained as a function of X value. Using this P o i Q« ( )» o the relationship between X value and x value. In our experimental condition, x i n eq. (9) can be substituted for X approximately from the above calculations. Thus, when X i s below unity, eq. (10) can be rewritten by the following equation. 0 2
a
n
d
e
9
w
e
c
a
n
D t a i n
2
=1.02 X 1 0 "
(11)
24
As shown i n Figure 8, the theoretical equilibruim oxygen p a r t i a l pressure of the exhaust gas as a function of X i s given by eqs. (8) and (11) with the negative value of the logarithm of the oxygen p a r t i a l pressure being shown on the l e f t v e r t i c a l axis. The observed oxygen p a r t i a l pressure i s also shown on the right v e r t i c a l axis i n the same figure. The difference between the theoretical and the observed oxygen p a r t i a l pressures resulted from ignoring the presence of carbon monoxide i n the "rich-burn region i n this calculation. Although we did not have an actual r a t i o of carbon dioxide to carbon monoxide i n the rich-burn region, P Q = 10~ Pa i s estimated at 700 °C assuming that the r a t i o of carbon dioxide to carbon monoxide i s about 10. In both cases, however, the oxygen p a r t i a l pressures i n the exhaust gas decreased dramatically from 10 to 1 0 ~ Pa at X = 1. In order to confirm the relationship between the r e s i s t i v i t y characteristics and the observed oxygen p a r t i a l pressure, the elect r i c a l conductivities of three typical specimens, SrTi03, SrSn03 and BaSn03, were investigated at 700 °C i n the oxygen p a r t i a l pressure range from 1 0 ~ to 10 Pa using the oxygen pump. The r e s i s t i v i t y characteristics of these specimens can be explained c l e a r l y by the conductivity dependence of the observed oxygen p a r t i a l pressure as shown i n Figure 9. The change of the conduction mechanism from ptype to n-type f o r SrTi03 i s found at 10~ Pa. This phenomenon results i n a s l i g h t decrease i n the r e s i s t i v i t y at X = 1, as stated previously. However, this change f o r SrSn03 i s found i n the oxygen p a r t i a l pressure range from 10 to 10 Pa. As f o r BaSn03, only the n-type conduction mechanism i s observed i n the conductivity dependence of the oxygen p a r t i a l pressure. In general, the e l e c t r i c a l conductivity of semiconductive metal oxides can be i l l u s t r a t e d as a function of oxygen p a r t i a l pressure, as shown i n Figure 10. The figure shows that equilibruim conductivi t y results of specimens are characterized by an oxygen-deficient, ntype region where the conductivity increases with decreasing P o » and an oxygen excess, p-type region where the conductivity increases with increasing P Q , separated by a conductivity minimum at a Po , which w i l l be designated as P o ° . The oxygen p a r t i a l pressure, P o ° at which the change of the conduction mechanism i s observed, depends on the variation of defect concentration of f u l l y ionized atomic defects, electrons (n) , and electron holes (p) i n the specimen. The oxygen p a r t i a l pressure P o ° , the type of conduction mechanism both i n the "lean-burn" and i n the "rich-burn" and the magnitude of the decrease i n the r e s i s t i v i t y at X = 1 were summarized i n Table I . The materials whose conduction mechanism i s unchangeable i n the oxygen p a r t i a l pressure below 10 Pa are useful f o r an oxygen 15
2
3
10
15
5
2
2
5
2
2
2
2
2
2
2
FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS
—1
I — T
T~
™ r -
T
1
1
4
r
2 0
-5
CO A O
ft
\
CM
ft O
- 10 -10 —
- 15 h
0> o
973 K -15
- 20 " 1
i
i
i
1. 0
i
i
1.2
^ _
i
1.4
1.6
0 /C H 2
3
(0 /C H ) 2
3
i
8
8
s t o i c h
Figure 8. Relationship between equilibrium oxygen p a r t i a l pressure and X value.
1
T
lo
