Co sensor and its fabrication

Details Co sensor and its fabrication Co sensor and its fabrication

1999-06-15

2001-11-20

Warden, Jill (Department: 1743)

Chemical apparatus and process disinfecting, deodorizing, preser

Analyzer, structured indicator, or manipulative laboratory...

Means for analyzing gas sample

C422S098000, C422S083000, C422S080000, C422S089000

Reexamination Certificate

active

06319473

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a CO sensor using an SnO2 metal oxide semiconductor and a method of production thereof.
PRIOR ART
It is known that an SnO2-based CO sensor is subjected to a higher temperature and a lower temperature alternately and the level of CO is detected from an output of the CO sensor at the lower temperature. For example, in the case of the present applicant's SnO2-based gas sensor TGS203 (TGS203 is a trade name), a pair of electrodes also serving as heaters are buried in a sintered body of SnO2 and the gas sensor is operated at a cycle of 150 seconds. The gas sensor is heated to a higher temperature (the highest temperature is 300° C. approx.) in the first 60 seconds and to a lower temperature (the lowest temperature is 80° C. approx.) in the subsequent 90 seconds. CO level is detected from a sensor signal that is generated immediately before the end of the lower temperature period. In this gas sensor, 2 mg approx. of Pd in reduction to metal is added to 1 g of SnO2.
SnO2-based CO sensors, however, tend to increase in resistance about two times within the first several months after the start of service.
SUMMARY OF THE INVENTION
A primary task of the present invention is to prevent long-term increase in CO sensor resistance.
A secondary task of the present invention is to improve CO concentration dependency of the resistance of the CO sensor.
Another task of the present invention is to suppress temperature and humidity dependency of the CO sensor.
The CO sensor according to the present invention is a sensor that detects CO including SnO2 gas sensitive material subjected to cyclical temperature change, wherein 0.01~10 mg/gSnO2 of an electron-donative sulfur compound in reduction to simple S element is added in said SnO2.
Preferably, the addition of said sulfur compound is 0.1~5 mg/gSnO2 in reduction to simple S element.
Preferably, said electron-donative sulfur compound is at least one member of a group consisting of thiourea, thiosulfuric acid and its derivatives, thiocyanic acid and its derivatives, thiocyanogen and its derivatives, thiol, thiophenol, thioether, thiosugar and its derivatives, thiophene and its derivatives, thionaphthene and its derivatives, thiotolene and its derivatives, thiopyran and its derivatives, thiophthene and its derivatives, thioacetazone and its derivatives, thioxene and its derivatives, thioacetal and its derivatives, thioindigo and its derivatives, thiooxine and its derivatives, thiocarbazide and its derivatives, thiokinase, thioglucosidase and CS
2
. Most preferably, said electron-donative sulfur compound is at least a member of a group of compounds comprising thiourea, thiosulfuric acid and its derivatives, for example, ammonium thiosulfate, thiocyanic acid and its derivatives, for example, ammonium thiocyanate, thiocyanogen and its derivatives.
Preferably, in said SnO2, 5~500 &mgr;g/gSnO2 of Ir in reduction to metal is added.
Further, preferably, said Ir is added in the form of an Ir-Pt compound catalyst wherein the additions of Ir and Pt are respectively 5~500 &mgr;g/gSnO2 each in reduction to metal, and the weight ratio of Ir to Pt is from 1/5 to 5.
In the method of production of the CO sensor of the present invention, after SnO2 powder being a gas sensitive material is sintered, 0.01~10 mg/gSnO2 of an electron-donating sulfur compound in reduction to simple S element is added. Said electron-donative sulfur compound includes, for example, thiourea, thiosulfuric acid and its derivatives, thiocyanic acid and its derivatives, thiocyanogen and its derivatives, thiol, thiophenol, thioether, thiosugar and its derivatives, thiophene and its derivatives, thionaphthene and its derivatives, thiotolene and its derivatives, thiopyran and its derivatives, thiophthene and its derivatives, thioacetazone and its derivatives, thioxene and its derivatives, thioacetal and its derivatives, thioindigo and its derivatives, thiooxine and its derivatives, thiocarbazide and its derivatives, thiokinase, thioglucosidase and CS2. Preferably, said electron-donative sulfur compounds is at least a member of a group comprising thiourea, thiosulfuric acid and its derivatives, for example, ammonium thiosulfate, thiocyanic acid and its derivatives, for example, ammonium thiocyanate, thiocyanogen and its derivatives.
Preferably, a solution of said electron-donative sulfur compound is impregnated into a sintered body of SnO2, and after that, the sintered body is dried and heat-treated. Most preferably, both Ir and Pt, 5~500 &mgr;g/gSnO2 in reduction to metal each, are added, the weight ratio of Ir/Pt being from 1/5 to 5, by impregnating a mixed solution of an Ir compound and a Pt compound into a sintered body of SnO2, then decomposing the Ir compound and the Pt compound impregnated.
With regard to the kind of CO sensor, for example, both the higher temperature and the lower temperature of a CO sensor may be higher than room temperature, and electric power may be given to the heater of the CO sensor in both the lower temperature period and the higher temperature period. The higher temperature of a CO sensor may be, for example, 300° C. approx., and a heating time to achieve the higher temperature may be, for example, from 10 ms to 10 s, and a standing time to the lower temperature may be, for example, from 1 s to 100 s, and the heater power may be set at 0 in the lower temperature period, allowing naturally cooling of the CO sensor down to around room temperature. In other words, with regard to the kind itself of CO sensor, it is sufficient that the sensor is an SnO2-based CO sensor that uses cyclic temperature change.
According to the present invention, 0.01~10 mg/gSnO2 of an electron-donative sulfur compound, in reduction to simple S element, is added being a gas sensitive material. This suppresses the long-term resistance increase of the CO sensor. Addition of 0.1 mg/gSnO2 or more of an electron-donative sulfur compound can eliminate the long-term resistance increase almost entirely. As a result, a CO sensor of extremely high reliability can be obtained. The addition of an electron-donative sulfur compound at said rate increases the CO concentration dependency a of the resistance of the CO sensor, and in turn, improves its quantitativeness. When the sensor resistance is denoted by Rs, the CO concentration dependency &agr; is defined by
Rs=k·∝[CO]
−&agr;
(where
k
is a constant of proportion)  (1)
The sulfur compound to be added is at least one member of a group comprising, for example, thiourea, thiosulfuric acid and its derivatives, for example, ammonium thiosulfate, thiocyanic acid and its derivatives, for example, ammonium thiocyanate and isothiocyanic acid, methyl thiocyanate and ethyl thiocyanate, allyl isothiocyanate, thiocyanogen and its derivatives, for example, various rhodan compounds and dichloroiminodithiazolidine, thiol, thiophenol, thioether, thiosugar such as D glucothiose, methyl thioadenosine and its derivatives, thiophene and its derivatives, thionaphthene and its derivatives, thiotolene and its derivatives, thiopyran and its derivatives, thiophthene and its derivatives, thioacetazone and its derivatives, thioxene and its derivatives, thioacetal and its derivatives, thioindigo and its derivatives, thiooxine and its derivatives, thiocarbazide and its derivatives, thiokinase, thioglucosidase and CS2. Preferably, the sulfur compound to be added is at least one member of a group consisting of thiourea, thiosulfuric acid and its derivatives, for example, ammonium thiosulfate, thiocyanic acid and its derivatives, for example, ammonium thiocyanate, and thiocyanogen and its derivatives. These sulfur compounds are electron-donative. For example, in the case of thiosulfuric acid, an S atom at a vertex of a tetrahedron around the central sulfur atom is electron-donative. These sulfur compounds have a sulfur atom of which oxidation number is 2 or under, in particular, −2. Of these compounds, thiourea, thiosulfuric acid and its derivatives, thiocyanic acid and its derivative

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