Gas sensors and methods of manufacturing the same

In a gas sensor which uses CuO as a p-type semiconductor, by adding Na.sub.2 CO.sub.3 in excess of 1 wt % relative to CuO, sensitivity to gases such as H.sub.2, NO, NO.sub.2 and SO.sub.2 is suppressed, whereby selectivity for CO is increased. Sensitivity to CO.sub.2 can also be obtained. In addition, by adding a sodium salt of tungstic acid or molybdic acid, CO.sub.2 sensitivity can be made lower than the CO sensitivity, and CO gas in the exhaust gases discharged from gas-fired water heaters or other combustion equipment can be selectively detected. It is therefore possible to detect incomplete combustion.

TECHNICAL FIELD 
The present invention is utilized for the detection of gases contained in 
trace amounts in gaseous mixtures. 
BACKGROUND ART 
Sensors that make use of ceramic semiconductor materials to detect gases 
present in air have been known for some time. Whereas hitherto only 
sensors utilizing reactions involving n-type ceramic semiconductors were 
known, the inventors associated with the present application have 
discovered that gases can be detected using high-purity CuO, which is a 
p-type semiconductor. They have previously filed a patent application for 
this, which application will hereinafter be referred to as "the prior 
application." This prior application was laid open to public inspection as 
Japanese Kokai Patent 6-258270. In the gas sensor disclosed in this prior 
application, at least 99 wt % of the semiconductor component contributing 
to conductivity in the sintered product was CuO, and the amount of 
additive was no more than 1 wt %. It was also disclosed in the prior 
application that CO sensitivity is increased by adding an alkaline metal 
compound as this additive which is present at no more than 1 wt %. 
However, although CO sensitivity was increased by adding an alkali metal 
compound to the high-purity CuO, the CO sensitivity thus obtained was only 
about twice the sensitivity to the same concentration of H.sub.2 for 
example. A further consideration is that recent research on the connection 
between global warming and CO.sub.2 gas has highlighted the need for a gas 
sensor capable of measuring CO.sub.2 gas concentrations. 
Meanwhile, detection of gases in exhaust gases differs from detection in 
air in that the partial pressure of oxygen varies, and CO.sub.2 
concentration and water vapor partial pressure vary according to the state 
of combustion. Under such conditions there are no sensors with high enough 
sensitivity to CO alone that they can be used for CO detection without 
some modification. For example, a gas sensor comprising CuO with addition 
of Na.sub.2 CO.sub.3 is sensitive to CO.sub.2 as well, and at the CO.sub.2 
concentrations present in exhaust gas from gas-fired water heaters, its 
sensitivity to CO.sub.2 is close to its sensitivity to CO at the CO 
concentrations which have to be detected to give warning of a dangerous 
amount of CO. This means that selective detection of CO will sometimes be 
unsuccessful. 
For example, in experiments performed by the present inventors, it was 
found that at a CO.sub.2 concentration of 5.5% an experimental gas sensor 
comprising CuO with addition of Na.sub.2 CO.sub.3 gave about the same 
detection output for CO.sub.2 as the output in response to the 
approximately 2000-4000 ppm of CO which was generated during incomplete 
combustion. Given that both CO.sub.2 and CO are present in exhaust gas 
during incomplete combustion, and that even during normal combustion 
CO.sub.2 is generated at concentrations sufficiently large to be expressed 
in percent rather than ppm, it will be seen that such high sensitivity to 
CO.sub.2 is unsuitable for selective detection of CO. 
It is an object of the present invention to solve such problems by 
providing gas sensors capable of selective detection of CO and CO.sub.2 ; 
gas sensors in which the CO.sub.2 sensitivity has been made lower than the 
CO sensitivity; and manufacturing methods for these sensors. 
DISCLOSURE OF INVENTION 
According to a first aspect, the present invention is a gas sensor capable 
of selective detection of CO and CO.sub.2, and having a p-type member 
formed from a p-type semiconductor the main constituent of which is CuO, 
and two electrodes which are connected to this p-type member, said 
electrodes serving to extract changes in electrical characteristics 
resulting from the presence of a gas to be detected, wherein the p-type 
semiconductor contains, as an additive, Na.sub.2 CO.sub.3 in excess of 1 
wt % relative to CuO. 
The increase in CO gas selectivity due to adding a sodium compound to CuO, 
which is a p-type semiconductor, is the same as disclosed in the prior 
application. Nevertheless, when the prior application was filed it was 
thought that gas detection by means of a structure in which electrodes are 
attached to a p-type semiconductor consisting mainly of CuO was possible 
only if high-purity CuO was used, i.e., CuO in which additive-derived 
semiconductor constituents other than CuO amount to no more than 1 wt %. 
However, subsequent research has revealed that gas detection is still 
possible when the amount of additive is increased. Sensitivity to gases 
such as H.sub.2, NO, NO.sub.2 and SO.sub.2 can be kept low, and 
selectivity for CO increased, by adding Na.sub.2 CO.sub.3 in excess of 1 
wt %. It was also found that sensitivity to CO.sub.2 can be obtained as 
well. Such characteristics are maintained even when the amount of Na.sub.2 
CO.sub.3 added is increased to 40 wt %, but if the amount added is 
increased further, the sintered product tends to lose its molded shape and 
therefore cannot be used as a sensor. Given the strength required to 
withstand use as a sensor, it is preferable for no more than 20 wt % of 
Na.sub.2 CO.sub.3 to be added. 
According to a second aspect, the present invention is a method for 
manufacturing such a gas sensor by forming a member with electrical 
characteristics which change in accordance with the presence of a gas to 
be detected, said member being formed by adding to powdered CuO a sodium 
compound which will become Na.sub.2 CO.sub.3 as a result of firing, and 
then molding and firing, wherein the amount of sodium compound added 
results in the Na.sub.2 CO.sub.3 content exceeding 1 wt % of the CuO. 
The sintered mass can be used as a gas sensor either just as it is, or 
after machining to a suitable size. Alternatively, it can be formed as a 
thick film. Namely, a gas sensor can be obtained by first of all 
manufacturing a paste whereof the main solid constituents are powdered CuO 
and a sodium compound additive which becomes Na.sub.2 CO.sub.3 as a result 
of firing, then printing this paste onto a substrate and firing. A thick 
film can also be formed by grinding the sintered mass described above and 
using this as the raw material. 
The powdered CuO used as the raw material preferably has primary particles 
with a specific surface area of at least 2 m.sup.2 /g, and more preferably 
of at least 20 m.sup.2 /g. A specific surface area of at least 2 m.sup.2 
/g is equivalent to a particle size of 1 .mu.m or less, while a specific 
surface area of at least 20 m.sup.2 /g is equivalent to a particle size of 
0.25 .mu.m or less. In experiments performed by the inventors, when CuO 
with a specific surface area of less than 2 m.sup.2 /g was used, 
subsequent sintering did not go to completion and the sintered product 
obtained ended up being easily broken. It is therefore thought that a 
practical gas sensor cannot be obtained using CuO with such a small 
specific surface area. 
The maximum temperature during firing is preferably at least 400.degree. 
C., and preferably no more than 860.degree. C. A temperature of at least 
500.degree. C. and no more than 700.degree. C. is particularly desirable. 
In experiments performed by the inventors, when the firing temperature was 
lower than 400.degree. C., sintering did not go to completion and the 
sintered product obtained was easily broken when handled. It was also 
found that good properties were not obtained when firing was carried out 
at temperatures exceeding 860.degree. C. This may be because there is a 
relation between the temperature at which Na.sub.2 CO.sub.3 loses CO.sub.2 
and the temperature at which Na.sub.2 CO.sub.3 decomposes (see for example 
"Dictionary of Physics and Chemistry" published by Iwanami Shoten, 4th 
edition, p.761). 
A variety of methods can be used for adding Na.sub.2 CO.sub.3. For example, 
Na.sub.2 CO.sub.3 can be dissolved in water, CuO powder dispersed in the 
resulting solution, and then the water removed by drying. Alternatively, 
NaHCO.sub.3 can be added and then thermally decomposed in the firing 
process. 
According to a third aspect, the present invention is a gas sensor in which 
the CO.sub.2 sensitivity has been made lower than the CO sensitivity, said 
gas sensor having a p-type member formed from a p-type semiconductor the 
main constituent of which is CuO and to which a sodium compound has been 
added, and two electrodes which are connected to this p-type member, said 
electrodes serving to extract changes in electrical characteristics 
resulting from the presence of a gas to be detected, wherein the sodium 
salt of at least one acid selected from the group comprising tungstic acid 
and molybdic acid is included as the sodium compound. 
According to research by the present inventors, if the sodium salt of 
tungstic acid or molybdic acid is added, CO.sub.2 sensitivity decreases 
relative to CO sensitivity. By utilizing such materials in gas sensors, CO 
gas can be selectively detected in the exhaust gas discharged by gas-fired 
water heaters and other combustion equipment, and incomplete combustion 
can be detected. 
The content of the sodium compound is preferably from 0.5 to 23 wt % as 
tungsten relative to CuO when a sodium salt of tungstic acid is used, and 
preferably 0.4 to 16 wt % as molybdenum relative to CuO when a sodium salt 
of molybdic acid is used. 
According to a fourth aspect, the present invention is a method for 
manufacturing such gas sensors by forming a member with electrical 
characteristics which change in accordance with the presence of a gas to 
be detected, said member being formed by adding a sodium compound to CuO 
and then molding and firing, wherein a substance which will become a 
sodium salt of tungstic acid or a sodium salt of molybdic acid upon firing 
is added as the sodium compound. 
In experiments performed by the present inventors, when Na.sub.2 
WO.sub.4.2H.sub.2 O or Na.sub.2 MoO.sub.4.2H.sub.2 O was used as the added 
substance, good results were obtained at additions of 1 to 40 wt % 
relative to CuO. These additions are equivalent to 0.5 to 23 wt % of 
tungsten relative to CuO, and 0.4 to 16 wt % of molybdenum relative to 
CuO. If the amount of addition exceeded 40 wt %, the sintered product did 
not maintain a solid shape and could not be used as a sensor. In 
particular, if the sintered product is used as a mass, then for both 
Na.sub.2 WO.sub.4.2H.sub.2 O and Na.sub.2 MoO.sub.4.2H.sub.2 O an addition 
of 20 wt % ensured that it could readily be handled without breaking. 
In these cases as well it is preferable for the maximum temperature during 
firing to be at least 400.degree. C. and no more than 860.degree. C. In 
experiments by the present inventors, the sintered product could be 
readily handled without breaking if the maximum temperature during firing 
was at least 500.degree. C. In addition, sensitivity to co-present gases 
could be kept low by ensuring that the maximum temperature during firing 
was at least 500.degree. C. and no more than 850.degree. C.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 to FIG. 3 show a first embodiment of a gas sensor according to the 
present invention. FIG. 1 is a plan view, FIG. 2 a side view and FIG. 3 a 
rear plan view. This gas sensor has a construction in which Au electrodes 
12 and 13 are formed on ceramic substrate 14, and p-type member 11 
comprising a sintered mass is in contact with these Au electrodes 12 and 
13. P-type member 11 is formed from a p-type semiconductor whereof the 
main constituent is CuO, and the two Au electrodes 12 and 13 are connected 
to this p-type member 11 so that they can extract changes in electrical 
characteristics resulting from the presence of a gas to be detected. 
Heater 15 for heating can be provided on the rear of ceramic substrate 14 
so that this gas sensor can be heated. When operating, a voltage is 
applied between Au electrodes 12 and 13, and changes in voltage or 
current, or in voltage and current, between Au electrodes 12 and 13 due to 
the presence of a gas to be detected are monitored. 
A p-type semiconductor containing, as an additive, Na.sub.2 CO.sub.3 in 
excess of 1 wt % relative to CuO is used as p-type member 11. 
Alternatively, a p-type semiconductor containing the sodium salt of at 
least one acid selected from the group comprising tungstic acid and 
molybdic acid is used as p-type member 11. 
The p-type semiconductor containing Na.sub.2 CO.sub.3 as an additive is 
obtained by adding to powdered CuO a sodium compound which will become 
Na.sub.2 CO.sub.3 as a result of firing, and then molding and firing this. 
The CuO used as the raw material has primary particles with a specific 
surface area of at least 2 m.sup.2 /g and preferably of at least 20 
m.sup.2 /g, and with a particle size of no more than 1 .mu.m and 
preferably of no more than 0.25 .mu.m. The amount of addition of the 
sodium compound is at least 1 wt % as Na.sub.2 CO.sub.3 relative to CuO, 
and preferably at least 1 wt % and no more than 40 wt %, and more 
preferably still at least 1 wt % and no more than 20 wt %. The maximum 
temperature during firing is at least 400.degree. C. and preferably no 
more than 860.degree. C., and more preferably still at least 500.degree. 
C. and no more than 700.degree. C. Bulk density can be controlled and CO 
selectivity increased by appropriate selection of the particle shape of 
the CuO raw material powder, and of the maximum firing temperature. 
The p-type semiconductor containing a sodium salt of tungstic acid or 
molybdic acid as an additive is obtained in similar manner by mixing with 
CuO a sodium compound of the sort that such an additive will be obtained 
by firing, and then molding and firing this. 
FIG. 4 and FIG. 5 show a second embodiment of a gas sensor according to the 
present invention. FIG. 4 is a plane view, and FIG. 5 is a sectional view. 
This gas sensor comprises interdigitated electrodes 22 and 23 which are 
formed on ceramic substrate 24, and p-type member 21 which has been formed 
as a thick film over these electrodes. P-type member 21 is formed by 
firstly printing a paste so as to be in contact with interdigitated 
electrodes 22 and 23, said paste having as its main solid constituents 
powdered CuO and an additive, the additive being either a sodium compound 
which becomes Na.sub.2 CO.sub.3 as a result of firing, or a sodium 
compound which becomes a sodium salt of tungstic acid or molybdic acid as 
a result of firing. This paste is then fired. Although not illustrated, as 
in the first embodiment a heater can be provided on the rear surface of 
ceramic substrate 24 in order to heat this gas sensor. It is also possible 
to form a thick film in the same manner as in the first embodiment, by 
using as the raw material the product obtained by grinding the sintered 
mass. Alternatively, a thick film can be formed by mixing the sodium 
compound with powdered CuO, dispersing the mixture in an organic solvent, 
using screen printing or other method to print the paste obtained, and 
then firing this printed paste. 
FIG. 6 to FIG. 9 show several ways for utilizing a gas sensor in which a 
sodium salt of tungstic acid or molybdic acid has been added to CuO. A gas 
sensor in which Na.sub.2 CO.sub.3 has been added to CuO is capable of 
selective detection of CO and CO.sub.2, and it is assumed that such 
sensors will mainly be used in the air. As opposed to this, because a gas 
sensor in which a sodium salt of tungstic acid or molybdic acid has been 
added to CuO has a CO.sub.2 sensitivity which is relatively low compared 
with its CO sensitivity, and is capable of selective detection of CO gas 
in the exhaust gases discharged from combustion equipment, its utilization 
will involve being fitted inside, or to the exhaust system of, various 
types of combustion equipment. FIG. 6 shows forced draft type combustion 
equipment which uses a blower to feed room air to the burner and to 
discharge exhaust gas to the outside. FIG. 7 shows forced exhaust type 
combustion equipment which uses air taken in from the room and from 
outside for combustion, and discharges exhaust gases to the outside by 
means of a blower. FIG. 8 shows natural feed and exhaust type combustion 
equipment which takes in outside air for combustion and discharges the 
exhaust gases to the outside. FIG. 9 shows forced feed and exhaust type 
combustion equipment which takes in outside air and forcibly discharges 
exhaust gases. 
The results of measurements of experimental gas sensors will now be 
explained. 
FIG. 10 shows the configuration of apparatus for measuring sensitivity to 
gases. This measuring apparatus is operated as follows. Gas sensor 40 to 
be tested is placed inside quartz tube 43 and a flow of air plus a gas to 
be detected, such as CO or H.sub.2, is passed into this quartz tube 43 via 
solenoid valves 41 and mass flowmeter 42. The temperature of the gas flow 
is controlled by temperature controller 44. The voltage applied to gas 
sensor 40 and the current flowing through it are measured by multimeter 
45, and the measured values are processed by personal computer 46 and 
stored in external storage device 47. Solenoid valves 41 are configured to 
be capable of selecting the gas to be detected, adding it to air, and 
supplying the mixture to quartz tube 43. They are operated by control 
signals supplied from controller 48 via relays 49. When personal computer 
46 has acquired the value of the current detected by multimeter 45, after 
a suitable time has elapsed it outputs to controller 48 a control signal 
for switching over the gas being supplied. The operating temperature of 
gas sensor 40 being measured is set to 230-260.degree. C. If a heater has 
been provided on gas sensor 40, this operating temperature is controlled 
by this heater. If a heater is not provided, the operating temperature is 
controlled from outside quartz tube 43. 
FIG. 11 shows an example of the results of measurements obtained by 
multimeter 45. These results are the same as those shown in FIG. 6 of the 
prior application. In this example, a sintered mass comprising 0.5 wt % 
addition of Na.sub.2 CO.sub.3 to CuO was maintained at 260.degree. C., a 
fixed voltage applied, and CO, H.sub.2 and C.sub.3 H.sub.8 supplied as 
test gases to be detected. In each case, the supplied gas flow consisted 
of 4000 ppm of the test gas in air. When each test gas was introduced, a 
change in electric current was observed. The supply of the test gas was 
then discontinued, and after the current had stabilized again, the next 
test gas for detection was introduced. The larger the current fluctuation, 
the higher the sensitivity to the gas being detected. 
However, although the difference in sensitivity of a given gas sensor to 
each test gas can be indicated by comparing the sizes of the current 
fluctuations, comparisons cannot be made with other gas sensors. 
Accordingly, in the present specification the value defined as: 
[(R.sub.gas /R.sub.0)-1].times.100 is used to indicate sensitivity, where 
R.sub.0 is resistance in air and R.sub.gas is resistance under a flow of 
the gas to be detected. Alternatively, the value defined as: 
[(R.sub.gas /R.sub.base)-1].times.100 is used to indicate sensitivity when 
use in a location other than in the air is being considered. Here, 
R.sub.base is resistance in the base gas and R.sub.gas is resistance under 
a flow of the gas to be detected. 
"Sensitivity ratio" will be used to express gas selectivity. For each test 
gas, this sensitivity ratio is the sensitivity to that gas normalized by 
the sensitivity to a particular test gas. For a given gas sensor, this 
sensitivity ratio coincides with the ratio of current change at a 
particular voltage. 
MEASUREMENT EXAMPLE 1 
A gas sensor was fabricated by attaching electrodes to the sintered mass 
obtained by adding 10 wt % of Na.sub.2 CO.sub.3 to CuO with primary 
particles having a specific surface area of 2.36 m.sup.2 /g, and then 
firing in air at 700.degree. C. for 30 minutes. FIG. 12 to FIG. 14 show 
the structure of this gas sensor. FIG. 12 is a perspective view, FIG. 13 a 
side view, and FIG. 14 a view facing one of the electrodes. This gas 
sensor has a structure wherein sintered mass 31 is sandwiched between 
electrodes 32 and 33. Lead wires 34 and 35 are attached to electrodes 32 
and 33 respectively. Mass 31 was 2.3 mm thick and its lateral surface area 
was approximately 50 mm.sup.2. 
FIG. 15 gives an example of the results of measurements of sensitivity 
ratio for various gases. For these measurements, 0.1 V was applied between 
electrodes 32 and 33. The sensitivity ratio for each test gas being 
detected was then obtained by taking the current change measured for 4000 
ppm of CO as "1" and normalizing the current change measured for each test 
gas. In addition to CO, the following gases were tested: H.sub.2, CH.sub.4 
(as a representative combustible gas which is present in air), NO and 
NO.sub.2 as nitrogen oxides, SO.sub.2 as a sulphur oxide, and CO.sub.2. 
The following concentrations were used for the test gases: 4000 ppm for 
H.sub.2 and CH.sub.4 ; 50 ppm for NO; 10 ppm for NO.sub.2 ; 5 ppm for 
SO.sub.2 ; and 5.5% for CO.sub.2. The base gas was air. It was found that 
the selectivity for CO was high, with the sensitivity ratio of H.sub.2 
relative to CO being approximately 1/10. It was also found that there was 
considerable sensitivity to CO.sub.2 as well. 
MEASUREMENT EXAMPLE 2 
Gas sensors similar to that of Measurement Example 1 were fabricated with 
different amounts of Na.sub.2 CO.sub.3 addition, and measurements made of 
variations in sensitivity to CO, H.sub.2 and CH.sub.4. The results of 
these measurements are given in FIG. 16. When the amount of Na.sub.2 
CO.sub.3 addition exceeded 1 wt %, sensitivity to CO became high, but 
sensitivity to the other gases became very low. 
MEASUREMENT EXAMPLE 3 
Gas sensors similar to that of Measurement Example 1 were fabricated using 
as the raw material CuO of different specific surface areas, and 
measurements made of variations in sensitivity ratio. An example of the 
results of these measurements is given in FIG. 17. This figure shows some 
of the measurement results already given in FIG. 15, side by side with the 
results of similar measurements made on a sintered mass fired under the 
same conditions but using as the raw material CuO having primary particles 
with a specific surface area of 52 m.sup.2 /g. It will be seen that 
although the additive and firing conditions were identical, the 
sensitivity ratios of both H.sub.2 and NO were lower as a result of using 
as the raw material a CuO with a larger specific surface area. 
FIG. 18 shows the relation between sensitivity and the specific surface 
area of the CuO raw material. CO, H.sub.2, CH.sub.4, NO, NO.sub.2, 
SO.sub.2 and CO.sub.2 were used as the test gases, at concentrations of 
4000 ppm, 4000 ppm, 4000 ppm, 50 ppm, 10 ppm, 5 ppm and 5.5%, 
respectively. The base gas was air. The vertical axis on the left-hand 
side of FIG. 18 shows the sensitivity to CO and CO.sub.2, while the 
vertical axis on the right-hand side shows the sensitivity to the other 
co-present gases. Note that the scale of these two axes differs by a 
factor of approximately 10. It was found that the larger the specific 
surface area of the primary particles of powdered CuO used as the raw 
material, the lower the sensitivity to gases other than CO and CO.sub.2. 
In particular, when the specific surface area exceeded 10-20 m.sup.2 /g, 
whereas the sensitivity to CO and CO.sub.2 increased, the sensitivity to 
other gases such as H.sub.2 underwent a marked decrease. 
FIG. 19 shows the relation between particle size and specific surface area 
of CuO. A specific surface area of 2 m.sup.2 /g or greater is equivalent 
to a particle size of less than about 1 .mu.m. 
MEASUREMENT EXAMPLE 4 
Gas sensors similar to that of Measurement Example 1 were fabricated using 
a variety of firing temperatures, and measurements made of variations in 
sensitivity ratio. An example of the results of these measurements is 
given in FIG. 20. The sensors were fired after 10 wt % of Na.sub.2 
CO.sub.3 had been added to CuO having primary particles with a specific 
surface area of 52 m.sup.2 /g. CO, H.sub.2 and CH.sub.4 were used as the 
test gases, in each case at a concentration of 4000 ppm. The base gas was 
air. The vertical axis on the left-hand side of FIG. 20 shows sensitivity 
to CO, while the vertical axis on the right-hand side shows sensitivity to 
the other co-present gas. When the firing temperature was 500.degree. C. 
to 700.degree. C., sensitivity to CO was high, and more than ten times the 
sensitivity to H.sub.2. However, the sensitivity to CO decreased when the 
firing temperature was outside this temperature range. 
FIG. 21 shows the relation between firing temperature when 10 wt % of 
Na.sub.2 CO.sub.3 was added to CuO, and the density of the sintered 
product obtained under these conditions. It will be seen that density 
increases abruptly when firing temperature exceeds 700.degree. C. 
MEASUREMENT EXAMPLE 5 
A gas sensor with the construction shown in FIG. 1 to FIG. 3 was fabricated 
by cutting to shape, and attaching electrodes to, the sintered mass 
obtained by adding 8 wt % of Na.sub.2 WO.sub.4.2H.sub.2 O to CuO and 
firing at a maximum temperature of 550.degree. C. An alumina substrate was 
used as ceramic substrate 14. 
FIG. 22 shows the results of measurements of sensitivity to various test 
gases. For these measurements, the apparatus illustrated in FIG. 10 was 
used, and instead of being heated externally, gas sensor 40 being measured 
was heated by means of a heater provided on its rear surface. A voltage of 
2.5 V was then applied between the electrodes and measurements made of the 
ratio of R.sub.gas to R.sub.base for a sequence of test gases, where 
R.sub.gas is the sensor resistance in the presence of a given test gas, 
and R.sub.base is the sensor resistance in the base condition. The base 
condition was an atmosphere comprising 5% CO.sub.2 and the added water 
vapor equivalent of the partial pressure of saturated water vapor at 
50.degree. C. and atmospheric pressure. CO.sub.2, CO, H.sub.2, CH.sub.4, 
NO and NO.sub.2 were used as the test gases. The measurements were made 
under two CO.sub.2 conditions: a first in which only CO.sub.2 was tested 
and the CO.sub.2 concentration was varied within the range 2.5-10%, and a 
second in which another test gas was added to 5% CO.sub.2. The test gases 
were used in the following concentrations: 500, 1000, 1500, 2000 and 4000 
ppm for CO; 500, 1000, 2000 and 4000 ppm for H.sub.2 ; 2000 ppm for 
CH.sub.4 ; 25, 50 and 100 ppm for NO; and 10, 20 and 30 ppm for NO.sub.2, 
and these gas concentrations were changed stepwise. It will be seen from 
the measurement results that there was hardly any sensitivity to CO.sub.2, 
and that CO selectivity was very high. 
MEASUREMENT EXAMPLE 6 
In order to measure sensor sensitivity during incomplete combustion in a 
water heater, gas sensors of the type used in Measurement Example 5 were 
used for measurements of sensitivity characteristics in a gas composition 
designed to simulate the exhaust gases resulting from incomplete 
combustion. The composition of the simulation gas is given in Table 1, and 
the results of the measurements are shown in FIG. 23 and FIG. 24. FIG. 23 
gives the results obtained when a voltage of 5 V was applied across a 
series-connected 1 k.OMEGA. reference resistance Rr and the gas sensor 
described above, and measurements made of the changes in sensor terminal 
voltage as a function of CO concentration. FIG. 24 shows these measurement 
results as changes in sensitivity (i.e., the ratio of the resistance 
change) due to changes in CO concentration. The measurements of voltage 
change revealed that voltage changed considerably up to the onset of 
incomplete combustion (a state corresponding to a CO concentration of 
approximately 2000-4000 ppm), thereby indicating that measurement of 
voltage change is effective for detecting incomplete combustion. In 
practice, the output of the gas sensor can also be utilized after 
correction by a microprocessor or the like. 
TABLE 1 
__________________________________________________________________________ 
simulation 
water vapor 
gas NO ppm 
NO.sub.2 ppm 
SO.sub.2 ppm 
CH.sub.4 ppm 
CO.sub.2 % 
CO ppm% 
H.sub.2 ppm 
mm Hg 
__________________________________________________________________________ 
normal 
1 30 10 5 20 7 9.5 
125 0 90 
40 15 
5 300 
65 
90 
50 15 
5 500 
200 
90 
60 15 
5 1000 
500 
90 
incomplete 
5 
60 15 
5 1500 
800 
90 
combustion 
6 
65 15 
5 2000 
1100 
90 
70 15 
5 3000 
2000 
90 
__________________________________________________________________________ 
MEASUREMENT EXAMPLE 7 
Sensor output characteristics during incomplete combustion in a forced 
draft type gas-fired water heater such as shown in FIG. 6 were measured 
using gas sensors of the type employed in Measurement Examples 5 and 6. 
The results of these measurements are shown in FIG. 25. The vertical axis 
shows the ratio of R.sub.gas, sensor resistance during combustion, to 
R.sub.base, sensor resistance when air is blown through the heater. 
Incomplete combustion is equivalent to a CO concentration of the order of 
2500 ppm. FIG. 25 shows that incomplete combustion can be detected by this 
gas sensor. Moreover, taking into consideration the measurement results of 
Measurement Examples 5 and 6 (FIG. 23 and FIG. 24), it will be seen that 
this gas sensor detects mainly CO gas. 
MEASUREMENT EXAMPLE 8 
A 2 mm.times.3 mm.times.1 mm sintered mass was obtained by adding 8 wt % 
Na.sub.2 WO.sub.4.2H.sub.2 O to CuO and firing at a maximum temperature of 
600.degree. C. Four gas sensors each having the structure shown in FIG. 1 
to FIG. 3 were fabricated using this mass just as it was and attaching 
electrodes to the 2 mm.times.3 mm faces. These four gas sensors were then 
used for measurements of sensor resistance as a function of the 
concentration of CO, H.sub.2, O.sub.2, water vapour and NO. The results 
are shown in FIG. 26 to FIG. 30. At any given concentration, the measured 
values obtained by the four gas sensors are nearly the same, and FIG. 26 
to FIG. 30 show the range of these measured values. 
FIG. 26 shows sensor resistance as a function of CO concentration. For 
these measurements, the base gas was an atmosphere comprising N.sub.2 to 
which 7.5% CO.sub.2, 7.5% O.sub.2, and the water vapour equivalent of the 
partial pressure of saturated water vapour at 50 .degree. C. and 
atmospheric pressure (12% water vapour) had been added. For a CO 
concentration at 500 ppm, the test gas used comprised 500 ppm of CO and 
250 ppm of H.sub.2. For a CO concentration of 1000 ppm, the test gas 
comprised 1000 ppm CO and 500 ppm H.sub.2. For a CO concentration of 2000 
ppm the test gas comprised 2000 ppm CO and 1000 ppm H.sub.2, and for a CO 
concentration of 3000 ppm it comprised 3000 ppm CO and 1500 ppm H.sub.2. 
FIG. 27 shows sensor resistance as a function of H.sub.2 concentration. For 
these measurements, 1000 ppm of CO was added to a base gas comprising 
N.sub.2 to which 7.5% CO.sub.2, 7.5% O.sub.2 and 12% water vapour had been 
added, and the H.sub.2 concentration was varied from 200 to 800 ppm. 
FIG. 28 shows sensor resistance as a function of O.sub.2 concentration. For 
these measurements, 1000 ppm of CO and 500 ppm of H.sub.2 were added to a 
base gas comprising N.sub.2 to which 7.5% CO.sub.2 and 12% water vapour 
had been added, and the O.sub.2 concentration was varied from 5 to 10%. 
FIG. 29 shows sensor resistance as a function of water vapour 
concentration. For these measurements, 1000 ppm of CO and 500 ppm of 
H.sub.2 were added to a base gas comprising N.sub.2 to which 7.5% CO.sub.2 
and 7.5% O.sub.2 had been added, and water vapour was varied from 10 to 
14%. 
FIG. 30 shows sensor resistance as a function of NO concentration. For 
these measurements, 1000 ppm of CO and 500 ppm of H.sub.2 were added to a 
base gas comprising N.sub.2 to which 7.5% CO.sub.2, 7,5% O.sub.2 and 12% 
water vapour had been added, and the NO concentration was varied from 0 to 
150 ppm. 
These measurement results show that the change in sensor resistance as a 
function of CO concentration is larger than its changes as functions of 
the concentration of H.sub.2, O.sub.2, water vapour and NO, thereby 
indicating suitability for detection of CO gas. 
MEASUREMENT EXAMPLE 9 
10 wt % of Na.sub.2 WO.sub.4.2H.sub.2 O was added to CuO and the resulting 
mixture fired at a maximum temperature of 650.degree. C. A gas sensor with 
the same structure as in Measurement Example 5 was fabricated by cutting 
the sintered mass obtained and attaching electrodes. The sensitivity of 
this gas sensor to CO, H.sub.2, CH.sub.4 and C.sub.3 H.sub.8 was then 
measured. The results are shown in FIG. 31. The base gas used in these 
measurements was dry air to which water vapour equivalent to saturated 
water vapour at 25.degree. C. and atmospheric pressure had been added, and 
the measurements were made with the concentration of each gas set at 2000 
ppm. The measurement results given in FIG. 31 show that this gas sensor 
has a high degree of selectivity for CO gas. 
MEASUREMENT EXAMPLE 10 
A thick film gas sensor with the structure shown in FIG. 4 and FIG. 5 was 
fabricated by forming a paste comprising 5 wt % of Na.sub.2 WO.sub.4 
(anhydride) added to CuO, screen printing this onto interdigitated 
electrodes, and firing at a maximum temperature of 650.degree. C. An 
alumina substrate was used as ceramic substrate 24, and a heater for 
heating the sensor was provided on the rear surface. 
FIG. 32 shows the results of measurements of sensitivity to various test 
gases. These measurements were performed in the same manner as in 
Measurement Example 5, using 7.5% CO.sub.2, 7.5% O.sub.2 and 12% water 
vapour added to N.sub.2 as the base gas, and 500, 1000, 2000, 3000 and 
4000 ppm of CO; 500, 1000, 2000, 3000 and 4000 ppm of H.sub.2 ; 50 ppm of 
NO, and 10 ppm of NO.sub.2 as the test gases. These measurement results 
show that the CO selectivity is very high. 
MEASUREMENT EXAMPLE 11 
Gas sensors with the same structure as in Measurement Example 1 (see FIG. 
12 to FIG. 14) were fabricated by adding 8 wt % of Na.sub.2 
WO.sub.4.2H.sub.2 O to CuO and firing at a variety of firing temperatures. 
Measurements were then made of gas sensitivity as a function of maximum 
firing temperature. The results are shown in FIG. 33. Air with a CO.sub.2 
concentration of 5% was used as the base gas, and 4000 ppm of CO, 4000 ppm 
of H.sub.2 and 50 ppm of NO were used as the test gases. The vertical axis 
on the left-hand side of FIG. 33 shows CO sensitivity, while the vertical 
axis on the right-hand side shows H.sub.2 sensitivity and NO sensitivity. 
A satisfactorily high CO sensitivity is obtained if the maximum firing 
temperature is 400.degree. C. or over, but there is a conspicuous decrease 
in CO sensitivity if the maximum firing temperature exceeds 500.degree. C. 
(at which temperature the strength of the sintered mass is high). Provided 
that the maximum firing temperature is no more than 800.degree. C., the CO 
sensitivity is higher than the H.sub.2 sensitivity. 
MEASUREMENT EXAMPLE 12 
Thick film gas sensors of the same construction as in Measurement Example 
10 were fabricated using a paste comprising 5 wt % Na.sub.2 WO.sub.4 
(anhydride) added to CuO, and employing a variety of firing temperatures. 
Measurements were then made of gas sensitivity as a function of maximum 
firing temperature. The results are shown in FIG. 34. N.sub.2 to which 
7.5% CO.sub.2, 7.5% O.sub.2 and 12% water vapour had been added was used 
as the base gas, and 3000 ppm of CO, 3000 ppm of H.sub.2, 50 ppm of NO and 
10 ppm of NO.sub.2 were used as the test gases. It will be seen from these 
measurement results that satisfactorily high CO sensitivity is obtained if 
the firing temperature is 450.degree. C. or over, but that there is a 
pronounced decrease in CO sensitivity if the firing temperature exceeds 
550.degree. C. Provided that the firing temperature is no more than 
700.degree. C., the CO sensitivity is higher than the H.sub.2 sensitivity. 
MEASUREMENT EXAMPLE 13 
Gas sensors were fabricated with different amounts of Na.sub.2 
WO.sub.4.2H.sub.2 O addition, and their sensitivity measured. The maximum 
firing temperature was set at 600.degree. C. The structure of the gas 
sensors was the same as in Measurement Example 12 (see FIG. 12 to FIG. 
14). The results of these measurements are shown in FIG. 35. Air with a 
CO.sub.2 concentration of 5% was used as the base gas, and 4000 ppm of CO, 
4000 ppm of H.sub.2, and 50 ppm of NO were used as the test gases. The 
vertical axis on the left-hand side of FIG. 35 shows the CO sensitivity 
and H.sub.2 sensitivity, while the vertical axis on the right-hand side 
shows NO sensitivity. It will be seen from FIG. 35 that CO sensitivity 
increases remarkably when the amount of addition of the Na.sub.2 
WO.sub.4.2H.sub.2 O is 2 wt % or more. 
MEASUREMENT EXAMPLE 14 
A gas sensor was fabricated by adding 10 wt % of Na.sub.2 
MoO.sub.4.2H.sub.2 O to CuO, firing at a maximum temperature of 
700.degree. C., and attaching electrodes to the sintered mass thereby 
obtained. Measurements were then made of the resistance of this sensor. 
The structure of the sensor was the same as in Measurement Example 12 (see 
FIG. 12 to FIG. 14). The measurement results are shown in FIG. 36. Air 
with a CO.sub.2 concentration of 5% was used as the base gas, and 1000, 
2000 and 4000 ppm of CO, 2000 and 4000 ppm of H.sub.2, 4000 ppm of 
CH.sub.4, 50 ppm of NO, and 10 ppm of NO.sub.2 were used as the test 
gases. An initial measurement was also made for air only, and it was found 
that there was a small change in sensor resistance according to whether 
CO.sub.2 was present or not. Although the H.sub.2 sensitivity of this gas 
sensor is close to the CO sensitivity, it is fully capable of detecting 
actual incomplete combustion. 
MEASUREMENT EXAMPLE 15 
A gas sensor with the same structure as in Measurement Example 5 (see FIG. 
1 to FIG. 3) was fabricated from a sintered mass which had been fired 
under the same conditions as Measurement Example 14, and measurements were 
made of its resistance. The results of these measurements are given in 
FIG. 37. Air was used as the base gas, and 4000 ppm of CO, 4000 ppm of 
H.sub.2, 4000 ppm of CH.sub.4 and 5% CO.sub.2 were used as the test gases. 
MEASUREMENT EXAMPLE 16 
Gas sensors were fabricated with different amounts of Na.sub.2 
MoO.sub.4.2H.sub.2 O addition, and measurements made of the resulting 
changes in sensitivity. The maximum firing temperature was set at 
600.degree. C. and the structure of the sensors was the same as in 
Measurement Example 12 (see FIG. 12 to FIG. 14). The results of these 
measurements are shown in FIG. 38. Air was used as the base gas, and 4000 
ppm of CO, 4000 ppm of H.sub.2, and 5% CO.sub.2 were used as the test 
gases. These measurement results show that CO sensitivity increases 
remarkably when the amount of addition is greater than 1 wt %.