Gas sensor

Details Gas sensor Gas sensor

1998-12-14

2001-01-30

Tung, T. (Department: 1743)

Electrolysis: processes, compositions used therein, and methods

Electrolytic analysis or testing

For nitrogen or nitrogen containing compound

C204S424000, C204S426000, C205S784000, C205S786500

Reexamination Certificate

active

06179992

ABSTRACT:

The invention relates to a galvanic cell which is advantageously designed as a potentiometric sensor for determining gases such as CO
2
, SO
x
and NO
x
. The galvanic cell of the invention can be operated at elevated temperatures and is particularly well suited for applications in which the partial pressure of oxygen in the measuring medium varies with the content of the gases to be detected.
The use of oxygen-ion-conducting solid electrolytes for the potentiometric detection of gaseous anhydrides with the formula XO
x
(X=C, S, N) dates back to publications by Yamazoe and co-workers, who initially used the measuring principle to determine SO
x
, (Yan et al., Chem. Lett. (1992), 635; Yan et al., Sensors and actuators B. 12 (1993), Yan et al., Sensors and Actuators B. 20 (1994), 81), and later also to detect NO
x
(Kurosawa et al., Solid State Ionics 79 (1995), 338; Miura et al., J. Electrochem. Soc. 143 (1996), L33) and CO
2
(Miura et al., Sensors and Actuators B., 24-25 (1995), 260).
The measuring principle used in these galvanic cells is based on the arrangement:
Pt/Au, O
2
, XO
x
/Me
m
(XO
n
)
p
/stabilized ZrO
2
/O
2
, (XO
x
), Pt
where Me is a metal such as Li, Na or K, and the stabilized ZrO
2
serves as oxygen-ion-conducting solid electrolyte (O-SE). The O-SE is coated on one side with platinum and on the other side with a gas-sensitive layer of the salt Me
m
(XO
n
)
p
, or, in special embodiments, with a mixture of different salts of the gaseous anhydride to be detected. Both electrodes of the thus-prepared ZrO
2
pellet are in contact with the O
2
- and XO
x
-containing measuring gas.
According to Miura et al., vide supra, the functioning of the measuring electrode is based on the presence of a so-called ionic bridge. This ionic bridge, which provides an electrochemical connection between the metallic salt and the O-SE, is created by the formation of a solid phase containing both Me and oxygen ions.
The sensors proposed by the Yamazoe team have the disadvantage that the reproducible manufacture of functional examples thereof, ie, sensors that respond to the concentration of the gas to be detected, is difficult (cf. Yan et al., J. Electrochem. Soc. 143 (1996), 609). A sensor of this kind that does operate successfully is seemingly an exception due to a coincidental constellation. Our own research has shown that formation of the phase which is supposed to act as the ionic bridge is in fact unlikely for kinetic reasons, and that the electrodes used in the Yamazoe sensors do not respond at all to changes in the partial pressure of CO
2
.
The use of an O-SE in connection with the potentiometric detection of gases was known even before Yamazoe and his co-workers published their work. The advantage which the direct contact between an O-SE and a salt of the gaseous anhydride to be detected—in the case in question a sulfate and a carbonate for the determination of SO
x
and CO
2
respectively—provides with respect to eliminating the effect of the partial pressure of oxygen was first pointed out by Kleitz et al. (New Types of Solid-Electrolyte Gas Sensors, in: P. Vashishta, J. N. Mundy, G. K. Shenoy (publ.), Fast Ion Transport in Solids, Elsevier North Holland Inc., New York 1979, 69) and by Belanger et al. (J. Electrochem. Soc. 131 (1984), 579). However, in these sensors the O-SE only served as an aid. The sensor signal responding to the content of the gas to be determined in the measuring medium drops exclusively at the salt which acts as metal-ionic conductor (Me-SE). The O-SE merely measures the difference in oxygen potential between the measuring gas and a reference gas. As a result of the direct contact between O-SE and Me-SE, the cell voltages of the two measuring cells add together and the overall signal is rendered independent of the oxygen pressure. The disadvantage connected with the provision of a reference gas is avoided in a sensor variant proposed later (Maruyama et al., Solid State Ionics 23 (1987), 107 and Saito et al., Solid State Ionics 28-30 (1988), 1644 and DE-OS 41 12 301). In this sensor variant, only the measuring gas is required to flow round the O-SE. Despite this, the sensor is still a combination of an oxygen and a metal-ion concentration cell, the only difference being that the two electrolytes in the sensor are sintered onto one another and thus form a so-called two-layer electrolyte. According to this measuring principle, the electronic conductivity of the metallic-ion conductor must be negligibly small compared with the ionic conductivity. In sensors of this kind, unlike those of Kleitz et al., vide supra, and Belanger et al., vide supra, where the salt itself serves as Me-SE, an independent material which is a comparatively better ionic conductor and which is in equilibrium with the salt Me
m
(XO
n
)
p
forms the measuring electrode.
The type of sensor described by Maruyama et al., vide supra, and Saito et al., vide supra, have the disadvantage that they are always a combination of a metallic-ion and an oxygen concentration chain. This is of special significance under the aspect of the non-negligible electronic conduction in the Me-SE (N{umlaut over (a)}fe, Sensors and Actuators vol. 21 (1994), 79 and N{umlaut over (a)}fe, Solid State Ionics 68 (1994), 249), which is usually associated with an impairment of the sensor measuring properties.
The object of the invention is thus the reproducible manufacture of a galvanic cell, especially a potentiometric sensor for detecting gases, which is based exclusively on an oxygen concentration chain incorporating an O-SE and which thus prevents the measuring properties of the sensor from being impaired by electron conduction in the electrolyte.
This object is established in that, between the oxygen-ion-conducting solid electrolyte and the metallic salt Me
m
(XO
n
)
p
acting as gas-sensitive layer, at least one intermediate layer of a material is introduced which seals off the potential-determining area of the solid electrolyte surface from the surroundings, making it impervious to gases therein, and which exhibits both high ionic conductivity (metallic ions Me) and high electronic conductivity; under the conditions in which the cell is used, the electronic conduction is preferably of a similar magnitude as or even greater than the ionic conduction. In addition, the material of the intermediate layer should preferably be inert under the conditions in which the sensor is operated and should allow the metal oxide Me
y
O to dissolve in it to a finite extent. The mobility of oxygen or the oxide Me
y
O in the material of the intermediate layer should be negligibly small.
One subject of this invention is thus a galvanic cell comprising:
a) an oxygen-ion-conducting solid electrolyte,
b) a gas-sensitive material which contains at least one salt having the structural formula Me
m
(XO
n
)
p
, where Me is a metal, X stands for C, S or N, and the symbols m, n and p characterize the respective stoichiometric relations,
c) a material, interposed between the oxygen-ion-conducting solid electrolyte and the gas-sensitive material, which allows conduction both by cations of the metal Me and by electrons and which seals off a potential-determining area of the solid electrolyte surface from the surroundings, making it impervious to gases therein, and
d) two electronically conductive potential taps at surface areas of the oxygen-ion-conducting solid electrolyte.
As oxygen-ion-conducting solid electrolyte, use may be made, for example, of a material based on ZrO
2
, ThO
2
, CeO
2
, HfO
2
or Bi
2
O
3
. It is especially beneficial if, as oxygen-ion-conducting solid electrolyte, use is made of a material based on cubic, tetragonal or partially stabilized ZrO
2
, eg, a YO
1.5
-stabilized ZrO
2
.
The salt with the structural formula Me
m
(XO
n
)
p
is preferably a salt of an alkali metal or an alkaline earth metal, eg, a Li, Na, K, Rb, Cs, Ca, Sr or Ba salt. The metallic salt is preferably a carbonate, sulfate or nitrate, depending on the gas to be detected.
The cationically conductive material may be an alkali-metal-ion or

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