Method of detecting carbon dioxide gas

The presence of carbon dioxide contained in a gas can be detected through measurement of the change in the electrical resistance of a hydroxyapatite in contact with carbon dioxide gas. The sensitivity of the hydroxyapatite to carbon dioxide gas can be enhanced by the formation of a composite of the hydroxyapatite with an inorganic carbonate, e.g., sodium carbonate or calcium carbonate. The sensitivity to carbon dioxide gas can be further enhanced by the formation of a composite of the hydroxyapatite with an inorganic halide, e.g., calcium chloride.

FIELD OF THE INVENTION 
The present invention relates to a method for detecting carbon dioxide gas, 
an element for detecting the gas, and a process for producing the same. 
The present invention can be utilized in various fields, such as, for 
example, for the control of the concentration of carbon dioxide gas in a 
hothouse for agricultural cultivation (agricultural use), for the 
monitoring of exhaust gases (industrial use), for the control of living 
environments (use for environmental sanitation), for the early detection 
of fires (use for the prevention of disasters), and the like. 
BACKGROUND OF THE INVENTION 
Various sensors capable of detecting the presence of a specific component 
(e.g., hydrocarbons, oxygen or carbon monoxide) contained in a gas have 
been developed for such purposes as the prevention of disasters, effective 
operation of machines or plants, and the like. However, unlike such gases 
as hydrocarbons, oxygen and carbon monoxide, carbon dioxide is chemically 
stable and, therefore, it is difficult to detect with sufficient 
sensitivity with a gas sensor utilizing hitherto known principles, e.g., 
an adsorption reaction or a combustion reaction of such a gas (see, e.g., 
U.S. Pat. No. 4,343,768). 
It is known that carbon dioxide gas generates, when dissolved into water, 
hydrogen ions in proportion to the quantity dissolved therein and, hence, 
the concentration of carbon dioxide gas can be measured indirectly by 
measuring the concentration of hydrogen ion by use of a pH meter and a 
glass electrode (see U.S. Pat. No. 4,376,681). This method, however, 
requires a long period of time to dissolve carbon dioxide contained in a 
sample gas into water and remove it therefrom. In addition, the 
measurement tends to be strongly influenced by the presence of such 
foreign gases as SO.sub.x, NO.sub.x and NH.sub.3, which also could change 
the pH of the aqueous solution to be measured. 
There is also known a method for detecting carbon dioxide gas, in which a 
sample of gas or fluid containing carbon dioxide gas, bicarbonate ion 
and/or carbonate ion is allowed to come into contact with an acid 
extracting fluid; a carbon dioxide-free gas is passed through the fluid in 
order to carry the dissolved carbon dioxide gas onto a carbon dioxide 
absorbing tube provided with an alkaline solution with a resulting change 
in the electrical conductivity of the alkaline solution, thus making it 
possible to measure the concentration of carbon dioxide gas contained in 
the sample (see U.S. Pat. No. 4,321,545). However, this method, like the 
above method using a pH meter, not only requires a long period of time for 
dissolving and removing carbon dioxide gas, but also is unable to 
distinguish the kind of ions detected. In addition, an apparatus to be 
used for the measurement could hardly be small in size. 
There is also known a method utilizing the characteristic absorption of 
carbon dioxide gas in the infrared region of the spectrum. In general, a 
sensor utilizing this method consists of an IR ray generation section from 
which an IR beam with a wavelength of 4.25 .mu.m is emitted, a cell having 
a path length of several meters, an IR detector, and a fan which draws air 
through the cell. An apparatus utilizing the method, therefore, is 
expensive and could hardly be small in size. In addition, measurements 
utilizing the method are susceptible to the influence of dusts and other 
contaminants. 
It is, therefore, desired, to develop a small and light carbon dioxide gas 
sensor capable of detecting the gas with a high accuracy and a quick 
response, without being influenced by dusts or the like. 
In view of the above objective, the present inventors have conducted 
intensive investigations and found that carbon dioxide gas can be detected 
by utilizing a hydroxyapatite, which so far is known to be a porous 
ceramic usable as a moisture sensor since its electrical resistance 
changes in response to the change in moisture (see Japanese Patent 
Application (OPI) No. 166,249/83). (The term "OPI" as used herein refers 
to a "published unexamined Japanese patent application".) The moisture 
sensor of a hydroxyapatite utilizes the physical phenomenon that water 
absorbed on the surface of a hydroxyapatite penetrates into the pores of 
the porous ceramic and condenses therein. On the other hand, it has now 
been found that when a hydroxyapatite is brought into contact with carbon 
dioxide gas, carbonate apatite is formed therefrom in proportion to the 
concentration of carbon dioxide gas and, hence, the carbon dioxide gas can 
be detected through measurement of the change in its electrical resistance 
because the thus formed carbonate apatite has a greater electrical 
resistance than the hydroxyapatite. The present invention has been 
accomplished based on the above finding. 
SUMMARY OF THE INVENTION 
According to the present invention, the presence of carbon dioxide 
contained in a gas can be detected by bringing a hydroxyapatite 
represented by formula (I): 
EQU M.sub.10 (ZO.sub.4).sub.6 (OH).sub.2 (I) 
wherein M is an element selected from the group consisting of Ca, Ba, St, 
Pb and Cd; and Z is an element selected from the group consisting of P, As 
and V, into contact with a gas containing carbon dioxide, and measuring 
the change in the electrical resistance of the hydroxyapatite caused by 
the contact with the carbon dioxide gas. 
The detection of the presence of carbon dioxide gas can be effected with an 
increased sensitivity by bringing a carbon dioxide gas detection element 
comprising a composite of a hydroxyapatite represented by the above 
formula (I) and an inorganic carbonate into contact with a gas containing 
carbon dioxide in order to attain an increased change in its electrical 
resistance upon contact with carbon dioxide. 
The detection of the presence of carbon dioxide gas can be effected with a 
much increased sensitivity even at a low temperature by using a carbon 
dioxide gas detection element which comprises a composite of a 
hydroxyapatite represented by the above formula (I) and an inorganic 
halide and bringing it into contact with carbon dioxide in order to 
enhance the change in its electrical resistance. 
It is an object of the present invention to provide a method for detecting 
the presence of carbon dioxide contained in a gas. 
It is another object of the present invention to provide a carbon dioxide 
gas detection element capable of detecting the presence of carbon dioxide 
gas contained in a gas with a high sensitivity. 
It is a further object of the present invention to provide a carbon dioxide 
gas detection element which makes it possible to detect the presence of 
carbon dioxide gas contained in a gas by a simple means.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The hydroxyapatite represented by formula (I); 
EQU M.sub.10 (ZO.sub.4).sub.6 (OH).sub.2 (I) 
wherein M is an element selected from the group consisting of Ca, Ba, Sr, 
Pb and Cd; and Z is an element selected from the group consisting of P, As 
and V, may be prepared by known methods, including, e.g., wet, dry and 
hydrothermal processes. Any hydroxyapatite having formula (I) can be used 
in the present invention. However, it is preferable to use a 
hydroxyapatite represented by formula (I) wherein M is Ca and Z is P. 
Further, compounds of formula (I) may also contain small amounts of the 
elements Sc, Y, Tl, Bi, V, Ni, Mn, Fe, Sn, Rb, Na, K, and Cs in addition 
to those elements listed for element M and small amounts of the elements 
Si, Ge, Cr, Mn, Al, and B in addition those elements listed for element Z. 
A porous sintered material of a hydroxyapatite can be obtained by molding 
powders of a hydroxyapatite in a mold of, such as a pellet molding machine 
with the application of pressure and then sintering the molded product in 
an electric furnace at a temperature of from 800.degree. to 1,000.degree. 
C., preferably from 850.degree. to 950.degree. C., for a period of 1 hour 
or more, preferably from 1.5 to 2.5 hours. 
In the case where a thin layer sintered material is to be obtained, the 
hydroxyapatite may be mixed with water and an organic binder, e.g., a 
methyl cellulose, to form a slurry, which can be coated on a refractory 
substrate or base in the form of a thin layer. The coated product may then 
be sintered at a temperature of from 800.degree. to 1,000.degree. C., 
preferably from 800.degree. to 950.degree. C., for a period of 1 hour or 
more, preferably from 1.5 to 2.5 hours. 
The thus prepared porous sintered product can be immersed in an aqueous 
inorganic carbonate solution and then dried to allow the inorganic 
carbonate to attach on the surface of the hydroxyapatite, thereby forming 
a composite of a hydroxyapatite and an inorganic carbonate according to 
the invention. 
Any inorganic carbonate which increases the variation in electrical 
resistance due to the contact of the hydroxyapatite with carbon dioxide 
gas can be used in the present invention. It is preferred to use a sodium 
carbonate or calcium carbonate. 
Similarly, a hydroxyapatite-inorganic halide composite material of a carbon 
dioxide gas detection element according to the invention can be produced 
by immersing a porous sintered product of a hydroxyapatite in an aqueous 
inorganic halide solution and drying it to thereby deposit the inorganic 
halide on a surface of the hydroxyapatite. 
In the present invention, there can be used any inorganic halide which is 
capable of enhancing the change in the electrical resistance of the 
hydroxyapatite upon contact with gaseous carbon dioxide. It is, however, 
preferable to use calcium chloride, ammonium chloride or mixtures thereof. 
A carbon dioxide gas detection element can be produced by providing 
electrodes and wire leads at the both ends of the material comprising the 
hydroxyapatite, the hydroxyapatite-inorganic carbonate composite, or the 
hydroxyapatite-inorganic halide composite according to the invention. 
A carbon dioxide gas detection element according to the invention can also 
be produced by providing electrodes and wire leads at both ends of a 
porous sintered product of a hydraxyapatite and then immersing it in an 
aqueous inorganic carbonate solution, followed by drying it to form a 
composite of the hydroxyapatite and the inorganic carbonate. 
Similarly, a hydroxyapatite-inorganic halide composite material of a carbon 
dioxide gas detection element according to the invention can be produced 
by immersing a porous, sintered product of a hydroxyapatite, which is 
provided with electrodes and wire leads, in an aqueous inorganic halide 
solution and drying it to thereby deposit the inorganic halide on a 
surface of the hydroxyapatite. 
The immersion of the sintered hydroxyapatite into the aqueous solution of 
an inorganic carbonate or of an inorganic halide may be carried out in two 
stages: firstly under a reduced pressure, e.g., 1/10 of atmospheric 
pressure in the first stage, and then at atmospheric pressure in the 
second stage. When the immersion is carried out in two stages as above, 
the inorganic carbonate or the inorganic halide contained in the aqueous 
solution of the carbonate or the halide can be distributed throughout the 
pores of the sintered product to form a hydroxyapatite-inorganic carbonate 
composite or a hydroxyapatite-inorganic halide composite on the entire 
surface of the hydroxyapatite. 
The amount of the composites formed by the immersion can be increased by 
using in the immersing treatment an aqueous inorganic carbonate or halide 
solution containing the carbonate or halide in an increased amount. The 
electrical resistance of the detection element decreases with the increase 
in the amount of the composite contained therein. This makes the increase 
in the electrical resistance of the composites observed upon contact with 
gaseous carbon dioxide greater, i.e., makes the element more sensitive to 
gaseous carbon dioxide. Accordingly, the presence of carbon dioxide 
contained in a gas can be detected by the use of the detection element 
according to the invention even at a relatively low temperature, e.g., at 
around room temperature. 
In the hydroxyapatite, the electrical resistance is increased by the 
presence of carbon dioxide in a gas, but it is slightly decreased when 
moisture is present in the gas. Accordingly, in order to completely remove 
the influence by the moisture in the gas, it is required to remove the 
moisture in the gas in advance, or the detection may be carried out at an 
elevated temperature, e.g., as high as 500.degree. C. or above, so as to 
make the sensitivity of the element higher and, at the same time, to make 
the relative humidity of the gas low enough not to disturb the detection 
of gaseous carbon dioxide. 
However, since the detection element comprising the 
hydroxyapatite-carbonate or hydroxyapatite-halide composite according to 
the invention has a sufficiently high sensitivity to carbon dioxide gas, 
it can detect the carbon dioxide gas in the presence of moisture at a 
relatively low temperature, e.g., at a temperature around room 
temperature, or at a temperature around 300.degree. C. 
As described above, a carbon dioxide gas detection element can be prepared 
by providing electrodes and wire leads at the both ends of the 
hydroxyapatite or composites thereof, or by forming a thin layer of the 
hydroxyapatite or composites thereof on an insulating base, followed by 
bonding electrodes and wire leads to both ends of the thin layer. The thin 
layer formed on an insulating base can be mounted on or above a heater 
positioned on a refractory substrate or base to give a detection element 
that can be used at elevated temperatures. 
A pair of plates of electrodes can be provided on an alumina substrate or 
base, and then a platinum paste can be coated and dried thereon. On the 
electrodes of the alumina substrate or base can be placed the thin layer 
of the hydroxyapatite prepared as above, and then sintered at a 
temperature not lower than 500.degree. C., preferably from 700.degree. C. 
to 1,000.degree. C., to give a detection element for carbon dioxide gas 
with platinum as a noble metal positioned between the hydroxyapatite and 
the electrodes. 
The thus prepared detection element material can then be immersed in an 
aqueous solution of an inorganic carbonate or halide, in order to 
impregnate the halide into the porous hydroxyapatite. Thereafter, it can 
be taken out of the solution and dried to give a detection element 
comprising composites of the carbonate apatite and the noble metal, or of 
the halide apatite and the noble metal. The resulting material can then be 
heated at a constant temperature in the range of from 100.degree. C. to 
600.degree. C., preferably at 400.degree. C., for at least 30 minutes, 
preferably from 1 to 3 hours, and cooled to give a highly sensitive 
detection element material according to the invention. 
In a carbon dioxide gas detection element according to the present 
invention, a noble metal such as platinum, palladium, rhodium, gold, or 
silver or a salt thereof and an inorganic carbonate or an inorganic halide 
can jointly form a composite with a hydroxyapatite. Any noble metal or 
salt thereof, or a mixture thereof which makes the variation in electrical 
resistance due to the contact of the hydroxyapatite with carbon dioxide 
gas great can be used in the present invention. It is, however, preferable 
to use platinum, palladium chloride or mixtures thereof. Since palladium 
chloride is soluble in water, a composite between the hydroxyapatite and 
palladium chloride as a noble metal can be formed by immersing the former 
in an aqueous solution of the latter, as in the case of the inorganic 
carbonate or the halide. 
In the preparation of the detection element for carbon dioxide gas 
according to the invention, the element comprising the composite with an 
inorganic halide can be brought, during the heating at a constant 
temperature in the range of from 100.degree. C. to 600.degree. C. (e.g., 
at a working temperature of 400.degree. C.), into contact with a 
conditioning gas which contains carbon dioxide gas, so as to further 
enhance its sensitivity to gaseous carbon dioxide. The concentration of 
carbon dioxide in the conditioning gas can be lower than in a simple gas 
to be actually examined. It can be preferable to use a conditioning gas 
which contains carbon dioxide gas in a concentration as close as possible 
to that in a sample gas to be actually examined. 
The contact between the composite apatites and a sample gas or a 
conditioning gas can be effected as a pretreatment just before the use of 
a carbon dioxide detection element prepared from the material according to 
the invention. 
In FIGS. 1, 2, 3 and 4, 1 is a thin layer of the hydroxyapatite or the 
composites thereof; 2 are electrodes provided at the both ends of the thin 
layer 1 for measuring its electrical resistance (impedance); 3 are wire 
leads to connect the electrodes 2 to an apparatus 16 (not shown in FIGS. 
1, 2, 3 and 4) for measuring electrical resistance (impedance); and 4 and 
41 are a refractory substrate or base to support the thin layer 1. In FIG. 
3, 5 is a heater provided on the refractory substrate or base 4 and having 
provided thereon the thin layer 1. 
In FIGS. 5 and 6, 11 and 12 are a porous hydroxyapatite or composites 
thereof; 21 and 22 are electrodes; and 31 and 32 are wire leads. In the 
element shown in FIG. 6, either or both of the electrodes 22 and 22 
function as a heater and, at the same time, as an electrode. 
When the hydroxyapatite is brought into contact with carbon dioxide at a 
temperature of from 500.degree. to 1,000.degree. C., it changes to a 
carbonate apatite with a significant change in its electrical resistance, 
which can be utilized for the detection of the presence of carbon dioxide. 
Further, in the case of the above-described composite material of the 
hydroxyapatite, the sensitivity for detecting the carbon dioxide gas can 
be increased by an increase in temperature at which it is brought into 
contact with the gas. Accordingly, since the thin layer 1 is used by 
heating or in a heating atmosphere, it is installed on the refractory 
substrate or base 4. It is preferred that the thin layer 1 has a thickness 
not greater than 300 .mu.m (most preferably not greater than 200 .mu.m) 
and is in a porous state. In cases where the element according to the 
invention is in the porous form, the wire leads may function as a support, 
as well. In this case, it is necessary that the wire leads have a 
sufficient strength and durability. 
The element according to the invention is used at an elevated temperature 
and, hence, the wire leads, as well as the substrate or base, to be used 
therein are preferably made of a material which can withstand a high 
temperature at which the detection is to be effected. 
FIG. 7 shows an example of a preferable flow chart illustrating the case 
where a gas to be examined is heated. In the flow sheet; 13 is a 
dehumidifier; 14 is a heater for a gas to be examined; 15 is a sensor for 
detecting the presence of carbon dioxide in which a detection element, 
which may be any of the types shown by FIGS. 1, 3, 4, 5 and 6, is housed; 
16 is an apparatus for measuring electrical resistance (impedance) which 
is connected to the wire leads 3, 31, and 32; and 17 is a line through 
which the gas flows. 
In the flow sheet shown in FIG. 7, the gas to be analyzed is passed through 
the line 17 and is allowed to enter into the dehumidifier 13, in which the 
moisture contained in the gas is removed. Thereafter, it is introduced 
into the heater 14 and then into the sensor 15 for the detection of the 
presence of carbon dioxide. If carbon dioxide is present in the gas, the 
measuring apparatus 16 will record a significant increase in the 
electrical resistance of the detection element, indicating the presence of 
carbon dioxide. In the case where the gas is heated to a sufficiently high 
temperature by the heater 14, the relative humidity of the gas could be 
reduced to an extremely low level even when a substantial amount of 
moisture is contained therein. In such a case, the influence of the 
moisture can be limited to a virtually negligible level and, therefore, 
the dehumidifier 13 may not be required. 
FIG. 3 shows an example of a carbon dioxide detection element provided with 
the thin layer 1 which is to be heated upon measurement. In the element, 
the heater 5 is provided in the refractory substrate or base 4, and the 
thin layer 1 provided with the electrodes 2 at the both ends thereof is 
formed on the heater 5. The thin layer 1 is heated by the heater 5 and is 
brought into contact with a gas to be examined. If the gas contains carbon 
dioxide, the electrical resistance of the thin layer 1 increases 
significantly to indicate the presence of carbon dioxide in the gas 
examined. 
FIG. 4 shows an example in which a heat-resistant electrical insulator 6 is 
provided between the thin layer 1 and the heater 5 which is provided on 
the heat-resistant substrate or base 41. In the case where an element of 
this type or an element of the type shown in FIG. 6, which is provided 
with a built-in heater, is used, the heater 14 shown in FIG. 7 need not be 
used. 
The electrical resistance increases with the increase in the concentration 
of carbon dioxide contained in the gas, irrespective of the type of the 
element used therefor. It is therefore possible to measure the 
concentration of the carbon dioxide. 
The increase in the electrical resistance of the element will be small (as 
shown in FIG. 10) in the case where the hydroxyapatite used therefor is 
represented by the following formula (I): 
EQU M.sub.10 (ZO.sub.4).sub.6 (OH).sub.2 (I) 
wherein M is an element selected from the group consisting of Ca, Ba, Sr, 
Pb and Cd; and Z is an element selected from the group consisting of P, As 
and V, and is incorporated with a compound (a minor component) represented 
by the formula (I), wherein M is an element selected from the group 
consisting of Sc, Y, Tl, Bi, V, Ni, Mn, Fe, Sn, Rb, Na, K and Cs; and Z is 
an element selected from the group consisting of Si, Ge, Cr, Mn, Al and B. 
In such a case, a simpler electrical circuit can be applied to the 
detection element, irrespective of the type of the element used. 
The present invention will further be explained by means of the following, 
non-limiting Examples. 
EXAMPLE 1 
Preparation of hydroxyapatite 
To 79 g of (NH.sub.4).sub.2 HPO.sub.4 was added 1,000 ml of distilled 
water, and the phosphate was completely dissolved. To this solution was 
added 5% ammonia water to adjust the pH to 12. As a result, 1,600 ml of an 
aqueous ammonium phosphate solution was obtained. 
Separately, to 236 g of Ca(NO.sub.3).sub.2.4H.sub.2 O was added 1,000 ml of 
distilled water, and the nitrate was completely dissolved. To this 
solution was added 5% ammonia water to adjust the pH to 12. As a result, 
1,200 ml of an aqueous ammonium calcium nitrate solution was obtained. To 
the resulting solution was added with stirring the aqueous ammonium 
phosphate solution prepared above, whereby a white precipitate was formed. 
The precipitate was filtered off, washed and dried at 250.degree. C. White 
powders of 100 g of hydroxyapatite were obtained. 
EXAMPLE 2 
Preparation of porous sintered material of hydroxyapatite 
To 50 g of the hydroxyapatite powders prepared in Example 1 was added 20 ml 
of an aqueous 5% methyl cellulose solution. The resulting mixture was well 
kneaded to form a slurry of the hydroxyapatite powder. This slurry was 
coated on a glass plate (200.times.200.times.5 mm) at a coverage of 0.05 
g/cm.sup.2, air dried for 24 hours and then peeled off. Thereafter, it was 
cut into a size of 15 mm.times.10 mm, placed on an alumina plate 
(25.times.25.times.0.5 mm), and sintered in an electric furnace at a 
temperature of 1,000.degree. C. for one hour. The thus prepared 
hydroxyapatite material had a thickness of 300 .mu.m. 
EXAMPLE 3 
Change in electrical resistance of hydroxyapatite due to the change in its 
temperature 
A carbon dioxide detection element was prepared by providing electrodes at 
the both ends of the thin layer of the porous sintered hydroxyapatite 
prepared in Example 2. 
The electrical resistance (Ro) of the element was measured in the air. 
Thereafter, the element was placed in an electric furnace, the air in the 
furnace was replaced with carbon dioxide gas, and the temperature in the 
furnace was raised to 500.degree. C., whereby the change in its electrical 
resistance (R) with a lapse of time was measured, and the ratio R/Ro was 
recorded. 
The above measurement was repeated in the same manner as above, except that 
the temperature in the furnace was changed to 600.degree., 700.degree., 
800.degree., 900.degree., or 1,000.degree. C. 
Results obtained are shown in FIG. 8. 
EXAMPLE 4 
Change in electrical resistance of hydroxyapatite due to the change in its 
thickness 
Carbon dioxide detection elements were prepared in the same manner as in 
Example 3, except that the thickness of the thin layer of the sintered 
hydroxyapatite formed on the alumina plate was adjusted to 100, 300, or 
500 .mu.m. 
The electrical resistances Ro and R of each element were measured in the 
same manner as in Example 3, except that the temperature in the electric 
furnace was maintained at 800.degree. C. 
Results obtained are shown in FIG. 9. 
EXAMPLE 5 
Preparation of sodium hydroxyapatite 
Powders of sodium hydroxyapatite were prepared In the same manner as in 
Example 1, except that Na.sub.2 HPO.sub.4 was used in combination with 
(NH.sub.4).sub.2 HPO.sub.4 in such an amount that Na was contained therein 
in such proportions, based on Ca, as shown in FIG. 10. 
Preparation of porous sintered sodium hydroxyapatite compacts 
Porous sintered sodium hydroxyapatite materials having different Na 
contents were prepared in the same manner as in Example 2, except that 
sodium hydroxyapatite powders obtained above were used instead of the 
hydroxyapatite powder. 
Measurement of electrical resistance of carbon dioxide detection element 
Carbon dioxide detection elements were prepared by providing electrodes at 
the both ends of the porous sintered material of the sodium hydroxyapatite 
obtained above. 
For comparison, the electrical resistance (Ro) of the element prepared in 
Example 3, which did not contain Na, was measured. 
Thereafter, the electrical resistance (R) of each element obtained as 
described above was measured, and the ratio R/Ro was recorded. 
Results obtained are shown in FIG. 10. It can be understood that the sodium 
hydroxyapatite has electrical resistance lower than that of the 
hydroxyapatite and that the electrical resistance decreases with an 
increase in Na content. 
EXAMPLE 6 
Change in electrical resistance of hydroxyapatite due to the change in the 
concentration of carbon dioxide 
The porous sintered hydroxyapatite detection element prepared in Example 3 
was placed in a tube having an inner diameter of 40 mm, and its electrical 
resistance (Ro) was measured while blowing air thereinto. Thereafter, a 
gas containing 1% by volume of carbon dioxide and heated to 1,000.degree. 
C. was introduced into the tube, the change in its electrical resistance 
was measured with a lapse of time, and the ratio R/Ro was recorded. Forty 
minutes after the introduction of the gas, the kind of gas blown into the 
tube was changed to one containing 10% by volume of carbon dioxide and 
heated at 1,000.degree. C., the change in the electrical resistance (R) of 
the element was measured, and the ratio R/Ro was recorded. Forty minutes 
after the changeover, the kind of gas blown into tube was again changed to 
one containing 50% by volume of carbon dioxide and heated at 1,000.degree. 
C. The change in its electrical resistance (R) was measured with a lapse 
of time, and the ratio R/Ro was recorded. Forty minutes after the 
changeover, the kind of gas blown into the tube was further changed to a 
100% carbon dioxide gas heated at 1,000.degree. C. The change in its 
electrical resistance (R) was measured with a lapse of time, and the ratio 
R/Ro was recorded. 
Results obtained are shown in FIG. 11. It can be understood from the 
results that the presence of carbon dioxide can be detected through an 
increase in electrical resistance of the element when the content of 
carbon dioxide contained in the gas exceeds 10% by volume although no 
significant change in electrical resistance was observed even after 40 
minutes in the case where the gas contained carbon dioxide of only 1% by 
volume. 
EXAMPLE 7 
Preparation of porous sintered hydroxyapatite material and its 
characteristics as a sensor 
To 30 g of the hydroxyapatite powders prepared in Example 1 was added 10 ml 
of an aqueous 5% methyl cellulose solution. The mixture was well kneaded 
to form a slurry of the hydroxyapatite. The thus prepared slurry was 
charged into a mold (inner size: 50.times.20.times.20 mm). After 6 hours, 
the bottom of the mold was removed, and the molded article was pushed out 
of the mold and air dried for 48 hours. The dried molded article was 
placed in an electric furnace and sintered at a temperature of 
1,000.degree. C. for a period of 1 hour. At the both ends of the thus 
prepared porous sintered material were then formed electrodes by coating 
with a Pt paste, followed by baking at a temperature of 850.degree. C. for 
a period of 15 minutes. 
Measurement of response characteristics of porous sintered material to the 
change in concentration of carbon dioxide 
The electrical resistance (Ro) in air of the porous sintered element 
obtained above was measured. The element was placed in the center of a 
tube having an inner diameter of 40 mm, and air heated at a temperature of 
900.degree. C. was introduced into the tube and flowed through it. After 
10 minutes, a pure carbon dioxide gas heated at 900.degree. C. was 
introduced into the tube and flowed through it. The change in the 
electrical resistance (R) of the porous sintered element was measured, and 
the ratio R/Ro was recorded. Forty minutes after the introduction of the 
carbon dioxide gas, the kind of gas flowed through the tube was changed to 
air heated at 900.degree. C. The change in the electrical resistance (R) 
of the element was measured, and the ratio R/ROD was recorded. Forty 
minutes after the introduction of air, i.e., 80 minutes after the 
introduction of the carbon dioxide gas, the kind of gas flowed through the 
tube was again changed to a carbon dioxide gas heated at 900.degree. C. 
The change in electrical resistance (R) of the element was measured, and 
the ratio R/Ro was recorded. 
Results obtained are shown in FIG. 12. In this figure, the abscissa 
indicates the time elapsed after the first introduction of the carbon 
dioxide gas. 
FIG. 12 shows that the electrical resistance of the element increases 
quickly after the introduction of carbon dioxide gas and decreases quickly 
after the introduction of air. It can therefore be understood that the 
element made of a porous sintered material exhibits sharp response 
characteristics to carbon dioxide gas. 
EXAMPLE 8 
Preparation of thin layer type detection element of hydroxyapatite and its 
characteristics as a sensor 
The thin layer porous sintered hydroxyapatite element (which was formed on 
an alumina plate) prepared in Example 3 was placed in a tube having an 
inner diameter of 40 mm, and its electrical resistance (Ro) was measured 
while air was flowed through the tube. After air heated at 900.degree. C. 
was flowed through the tube for 10 minutes, the gas being flowed through 
it was changed to a pure carbon dioxide gas heated at 900.degree. C. The 
change in electrical resistance (R) of the element was measured with a 
lapse of time, and the ratio R/Ro was recorded. Forty minutes after the 
introduction of the carbon dioxide gas, the gas being flowed through the 
tube was changed to air heated at 900.degree. C. The change in electrical 
resistance (R) of the element was measured, and the ratio R/Ro was 
recorded. Forty minutes after the introduction of the air, i.e., 80 
minutes after the first introduction of the carbon dioxide gas, the gas 
being flowed through the tube was again changed to a carbon dioxide gas 
heated at 900.degree. C. The change in electrical resistance (R) of the 
element was measured with a lapse of time, and the ratio R/Ro was 
recorded. 
Results obtained are shown in FIG. 13. It can be understood that a thin 
layer hydroxyapatite element formed on an alumina plate also exhibits 
sharp response characteristics to carbon dioxide gas, as in the case of 
Example 7. 
EXAMPLE 9 
(1) Preparation of Sample 
(1-1) Reference sample 
One gram of powders of a high purity hydroxyapatite (AN 830425, 
manufactured by Central Glass Co., Ltd.) was charged into a pellet molding 
machine and pressed at a pressure of 200 kg/cm.sup.2 to form a pellet 
having a diameter of 20 mm. The pellet was heated in air at a temperature 
of 900.degree. C. for a period of 2 hours by means of an electric furnace 
to give a sintered material. 
The thus obtained sintered material was cut into sections with a size of 15 
mm (in length) by 10 mm (in width) and then polished. A ruthenium oxide 
paste was coated on the cut piece to form an electrode, and a platinum 
wire lead was bonded thereto. The paste was allowed to dry in a drier at a 
temperature of from 90.degree. to 100.degree. C. Thereafter, the wired 
piece was baked in air at a temperature of 850.degree. C. for a period of 
15 minutes to give a carbon dioxide gas detection element provided with 
electrodes as shown in FIG. 5. 
(1-2) Sample incorporated with sodium carbonate 
Into 100 ml of distilled water maintained at 20.degree. C. was dissolved 
3.50 g of sodium carbonate (Na.sub.2 CO.sub.3). In the resulting aqueous 
sodium carbonate solution was immersed the detection element prepared in 
(1-1) above, the immersing treatment being effected in two stages: at a 
reduced pressure of 1/10 atm. for a period of 30 minutes and then at 
atmospheric pressure for 16 hours. The resulting element was placed in a 
drier, and dried at a temperature of from 90.degree. to 100.degree. C. for 
a period of 2 hours to give a carbon dioxide gas detection element 
incorporated with sodium carbonate. 
Aqueous sodium carbonate solutions having different sodium carbonate 
concentrations were prepared. Carbon dioxide detection elements prepared 
as in (1-1) above were immersed in one of the sodium carbonate solutions 
and treated in the same manner as above to a give carbon dioxide gas 
detection elements incorporated with different amounts of sodium 
carbonate. 
(1-3) Sample incorporated with sodium carbonate 
Into 20 ml of 3N HCl maintained at 20.degree. C. was dissolved 3.31 g of 
calcium carbonate (CaCO.sub.3). The pH of the resulting solution was 
adjusted to from 9 to 10 by the addition of 2N ammonia water. Thereafter, 
distilled water was added thereto to make its total volume 100 ml. 
In the resulting aqueous calcium carbonate solution was immersed a 
detection element prepared as in (1-1) above and then treated in the same 
manner as in (1-2) above to give a carbon dioxide gas detection element 
incorporated with calcium carbonate. 
Aqueous calcium carbonate solutions containing different amounts of calcium 
carbonate were prepared in the same manner as above. The carbon dioxide 
detection element prepared in (1-1) above was immersed in each of the 
calcium carbonate solutions and treated in the same manner as above to 
give a carbon dioxide gas detection element incorporated with a different 
amount of calcium carbonate. 
(2) Test Method 
(2-1) Measurement of electrical resistance 
(i) Reference sample 
The reference sample prepared in (1-1) above was placed in an electric 
furnace maintained at 600.degree. C., and its electrical resistance (Ro) 
in air was measured. 
(ii) Sample incorporated with sodium carbonate 
The sample incorporated with sodium carbonate prepared in (1-2) above was 
placed in the electric furnace, in place of the reference sample, and its 
electrical resistance (R) was measured under the same conditions as in (i) 
above. The ratio of the electrical resistance (R/Ro) was calculated. 
The above measurement was repeated, using the samples incorporated with 
different amounts of sodium carbonate prepared in (1-2) above, and the 
ratio R/Ro was calculated for each sample. 
(iii) Sample incorporated with calcium carbonate 
The ratio R/Ro was measured for each of the samples incorporated with 
calcium carbonate prepared in (1-3) above, in the same manner as in (ii) 
above. 
(2-2) Determination of sensitivity to carbon dioxide gas 
(i) Reference Sample 
A reference sample was placed in an electric furnance maintained at 
500.degree. C., and its electrical resistance (Ro) in air was measured. 
Thereafter, the air in the electric furnance was replaced with a pure 
carbon dioxide gas heated to the same temperature. The change in the 
electrical resistance of the sample was measured for the period of time 
shown in FIGS. 14 and 15, and the ratio R/Ro was calculated. 
The above measurement was repeated, except that the temperature of the 
electric furnance was maintained at 600.degree. C., and the ratio R/Ro at 
600.degree. C. was also calculated. 
(ii) Sample incorporated with sodium carbonate 
The above measurement conducted at 500.degree. C. and 600.degree. C. was 
repeated in the same manner as above, using the sample subjected to the 
immersing treatment in a solution of 3.50 g of sodium carbonate in 100 g 
of water, instead of the reference sample, and the ratio R/Ro was 
calculated. 
(iii) Sample incorporated with calcium carbonate 
The above measurement conducted at 500.degree. C. and 600.degree. C. was 
repeated in the same manner as above, using the sample subjected to the 
immersing treatment in the solution of 3.31 g of calcium carbonate in 100 
ml of water, instead of the reference sample, and the ratio R/Ro was 
calculated. 
(3) Test Results 
(3-1) Electrical resistance 
Sample incorporated with sodium carbonate: 
Results obtained are shown in FIG. 14. 
In FIG. 14, the ordinate indicates the ratio (R/Ro) of the electrical 
resistance (R) of the sample incorporated with sodium carbonate to that 
(Ro) of the reference sample; and the abscissa indicates the amount of 
sodium carbonate used in the immersing treatment in the preparation 
(indicated as grams of sodium carbonate contained in 100 ml of water used 
for the treatment). 
Sample incorporated with calcium carbonate: 
Results obtained are shown in FIG. 15. 
In FIG. 15, the ordinate indicates the ratio (R/Ro) of the electrical 
resistance (R) of the sample incorporated with calcium carbonate to that 
(Ro) of the reference sample; and the abscissa indicates the amount of 
calcium carbonate used in the immersing treatment in the preparation 
(indicated as grams of calcium carbonate used therefor). 
It can be understood from FIGS. 14 and 15 that the electrical resistance of 
the samples measured at 600.degree. C. is markedly reduced due to the 
incorporation of sodium carbonate or calcium carbonate into the 
hydroxyapatite. It can be seen through the comparison of the results shown 
in FIGS. 14 and 15 that the reduction in electrical resistance of the 
samples incorporated with sodium carbonate is slightly superior to that of 
the samples incorporated with calcium carbonate. 
(3-2) Sensitivity to carbon dioxide gas 
Results obtained in the measurement at 500.degree. C. are shown in FIG. 16, 
and the results obtained in the measurement at 600.degree. C. are shown in 
FIG. 17. 
In FIGS. 16 and 17, the ordinate indicates the ratio (R/Ro) of the 
electrical resistance (R) in carbon dioxide gas to that (Ro) in air; and 
the abscissa indicates the time (in minutes) elapsed between the 
measurement and the time when the air in the furnace was replaced with a 
pure carbon dioxide gas. In FIGS. 16 and 17, the continuous curves 101 and 
111 show the results with the reference sample; the dotted curves 102 and 
112 show the results with the sample incorporated with sodium carbonate; 
and the chained curves 103 and 113 show the results with the sample 
incorporated with calcium carbonate. 
It can be understood from FIGS. 16 and 17 that the increase in electrical 
resistance due to the presence of carbon dioxide gas in contact with the 
detection element becomes more significant as a result of the 
incorporation of sodium carbonate or calcium carbonate and, therefore, 
carbon dioxide contained in a gas can be detected with an increased 
sensitivity. The improvement in sensitivity to carbon dioxide in a gas in 
the sample incorporating with calcium carbonate is more significant than 
that in the sample incorporated with sodium carbonate. It can be seen by 
comparing the results in FIGS. 16 and 17 that the sensitivity for 
detecting carbon dioxide gas at 600.degree. C. is higher than that at 
500.degree. C. However, in the comparison with the reference sample, the 
improvement in the sensitivity at 500.degree. C. is greater than that at 
600.degree. C. The presence of carbon dioxide gas can, therefore, be 
readily detected at a temperature not higher than 500.degree. C., e.g., at 
a temperature of from 300.degree. to 500.degree. C. 
EXAMPLE 10 
The effect of the incorporation of calcium chloride into the hydroxyapatite 
on the sensitivity for detecting carbon dioxide gas. 
(1) Preparation of Sample 
(1-1) Reference sample 
Into 45 ml of ethanol was dissolved 20 g of butylcarbinol. Twenty grams of 
powdered, high purity hydroxyapatite (AN 830425, manufactured by Central 
Glass Co., Ltd.) was added to the solution and well admixed to form a 
uniform dispersion. The thus prepared dispersion was shaped into a sheet 
having a thickness of 200 .mu.m by a doctor blade method. The resulting 
sheet was then cut into a piece of 1 cm.times.1 cm. The piece was placed 
on electrodes on an alumina plate prepared by forming combshaped 
electrodes 23 and 23, as shown in FIG. 18, of ruthenium oxide on an 
alumina substrate or base 42 with a size of 2.5 cm 2.5 cm by means of a 
screen printing. After drying at room temperature, it was placed in an 
electric furnace filled with air. The temperature in the furnace was 
raised to 500.degree. C. and maintained at the same temperature for a 
period of 2 hours, and then it was raised to 800.degree. C. and maintained 
at the same temperature for a period of 2 hours. To the electrodes of the 
thus prepared sintered thin plate sample were bonded wire leads to give a 
carbon dioxide gas detection element. 
(1-2) Sample incorporated with calcium chloride 
Into 50 ml of distilled water was dissolved 1.84 g (0.02 mole) of calcium 
chloride, and the pH of the resulting solution was adjusted to between 9 
and 10 by adding aqueous ammonia. Thereafter, distilled water was added to 
the solution to make its total volume 100 ml. The carbon detection element 
prepared as in (1-1) above was immersed in the resulting calcium chloride 
solution (concentration of calcium chloride: 0.2 mol/l) under a reduced 
pressure of 1/10 atm. for a period of 30 minutes and then at atmospheric 
pressure for a period of 16 hours. It was dried in a drier at a 
temperature of from 90.degree. C. to 100.degree. C., and then provided 
with wire leads at the electrodes thereof to give a carbon dioxide gas 
detection element (Ca-1). 
A carbon dioxide gas detection element (Ca-2) was prepared in the same 
manner as above, except that a solution containing 0.02 mol/l of calcium 
chloride was used instead of the solution containing 0.2 mol/l of calcium 
chloride. 
A carbon dioxide gas detection element (Ca-3) was prepared in the same 
manner as above, except that a solution containing 0.002 mol/l of calcium 
chloride was used instead of the solution containing 0.2 mol/l of calcium 
chloride. 
(2) Test Method 
(2-1) Measurement of sensitivity of reference sample 
The reference carbon dioxide detection element prepared in (1-1) above was 
placed in an electric furnace maintained at 400.degree. C. by a 
temperature controller and filled with air, and the electrical resistance 
(Ro) of the reference element in air was measured. Thereafter, the air in 
the electric furnace was replaced with air containing 0.1%, 1.0% or 10% of 
carbon dioxide, and the electrical resistance (R) of the reference element 
was measured in each of the gases. The ratio R/Ro, i.e., [electrical 
resistance of the reference element measured in air containing carbon 
dioxide gas]/[electrical resistance of the reference element measured in 
air], was calculated for each case. 
(2-2) Sensitivity of element incorporated with calcium chloride 
The carbon dioxide detection element (Ca-1) was placed in an electric 
furnace maintained at 400.degree. C. by a temperature controller and 
filled with air, and the electrical resistance (Ro) of the element in air 
was measured. Thereafter, the air in the electric furnace was replaced 
with air containing 1% of carbon dioxide, the electrical resistance (R) of 
the element was measured, and the ratio R/Ro was calculated therefrom. 
The carbon dioxide gas detection elements (Ca-2) and (Ca-3) were subjected 
to the same measurement as above to measure their electrical resistances 
Ro and R, and the ratio R/Ro was calculated therefrom. 
(3) Test Results 
(3-1) Reference sample 
Results as shown in FIG. 19 were obtained. 
In FIG. 19, the ordinate indicates the ratio R/Ro, i.e., [electrical 
resistance of the reference sample measured in air containing carbon 
dioxide gas]/[electrical resistance of the reference sample measured in 
air]; and the abscissa indicates the period of time elapsed after the 
replacement of the air in the electric furnace with air containing carbon 
dioxide. 
In FIG. 19, the continuous curve 201 shows the results obtained in the air 
containing 0.1% of carbon dioxide; the dotted curve 202 the results in the 
air containing 1% of carbon dioxide; and the chained curve 203 the results 
in the air containing 10% of carbon dioxide. 
(3-2) Sample incorporated with calcium chloride 
Results as shown in FIG. 20 were obtained. 
In FIG. 20, the ordinate indicates the ratio R/Ro, i.e., [electrical 
resistance of the sample incorporated with calcium chloride measured in 
air containing carbon dioxide]/[electrical resistance of the sample 
incorporated with calcium chloride measured in air]; and the abscissa 
indicates the period of time elapsed after the replacement of the air in 
the electric furnace with air containing carbon dioxide. 
In FIG. 20, the continuous curve 301 shows the results obtained by the 
carbon dioxide detection element (Ca-3); the dotted curve 302 the results 
by the carbon dioxide detection element (Ca-2); and the chained curve 303 
the results by the carbon dioxide detection element (Ca-1). 
EXAMPLE 11 
The effect of the incorporation of calcium chloride and platinum into the 
hydroxyapatite on the sensitivity for detecting carbon dioxide gas. 
(1) Preparation of samples 
Ruthenium oxide electrodes were formed by means of a screen printing on the 
same alumina substrate as in Example 10 having a size of 2.5 cm.times.2.5 
cm, and a platinum paste was coated on the electrodes. A piece of the 
hydroxyapatite sheet prepared in (1-1) in Example 10 was placed on the 
electrodes and treated in the same manner as in (1-1) in Example 10 to 
give a thin plate sample. Wire leads were attached to the electrodes of 
the thin plate sample to give a carbon dioxide gas detection element 
incorporated with platinum. 
The thus prepared element was immersed in a solution containing 0.2 mol/l 
of calcium chloride in the same manner as in (1-2) in Example 10 to give a 
carbon dioxide gas detection element (Ca-Pt-1). 
Carbon dioxide gas detection elements (Ca-Pt-2) and (Ca-Pt-3) were prepared 
in the same manner as above, except that a solutions containing 0.02 mol/l 
and 0.002 mol/l of calcium chloride was used, respectively, instead of the 
solution containing 0.2 mol/l of calcium chloride. 
(2) Test Method 
The electrical resistances Ro and R of the carbon dioxide gas detection 
elements (Ca-Pt-1), (Ca-Pt-2) and (Ca-Pt-3) were measured in the same 
manner as in (2-2) in Example 10, and the ratio R/Ro of the electrical 
resistance was calculated therefrom. 
(3) Test Results 
Results as shown in FIG. 21 were obtained. 
In FIG. 21, the ordinate indicates the ratio R/Ro, i.e., [electrical 
resistance of the sample incorporated with calcium chloride and platinum 
measured in the air containing carbon dioxide gas]/[electrical resistance 
of the sample incorporated with calcium chloride and platinum measured in 
air]; and the abscissa indicates the period of time elapsed after the 
replacement of the air in the electric furnace with the air containing 
1.0% of carbon dioxide. 
In FIG. 21, the continuous curve 401 shows the results obtained by the 
carbon dioxide gas detection element (Ca-Pt-3); the dotted curve 402 the 
results by the element (Ca-Pt-2); and the chained curve 403 the results by 
the element (Ca-Pt-1). 
EXAMPLE 12 
The effect of the incorporation of calcium chloride, palladium chloride and 
platinum into the hydroxyapatite on the sensitivity for detecting carbon 
dioxide gas. 
(1) Preparation of Sample 
Into 50 ml of distilled water were dissolved 1.84 g (0.02 mol) of calcium 
chloride and 0.1 g (5.times.10.sup.-4 mol) of palladium chloride, and the 
pH of the resulting solution was adjusted to 9 to 10 by adding aqueous 
ammonia. Thereafter, distilled water was added to the solution to make its 
total volume 100 ml. The carbon dioxide gas detection element prepared in 
(1) in Example 11 was immersed in the resulting solution (concentration of 
calcium chloride: 0.2 mol/l; and concentration of palladium chloride: 
5.times.10.sup.-3 mol/l) under a reduced pressure of 1/10 atm. for a 
period of 30 minutes and then at atmospheric pressure for a period of 16 
hours. It was dried in a drier at a temperature of from 90.degree. C. to 
100.degree. C., and then provided with wire leads at the electrodes to 
give a carbon dioxide gas detection element (Ca-Pd-Pt-1). 
A carbon dioxide gas detection element (Ca-Pd-Pt-2) was prepared in the 
same manner as above, except that a solution containing 0.2 mol/l of 
calcium chloride and 5.times.10.sup.-4 mol/l of palladium chloride was 
used instead of the solution containing 0.2 mol/l of calcium chloride and 
5.times.10.sup.-3 mol/l of palladium chloride. 
A carbon dioxide gas detection element (Ca-Pd-Pt-3) was prepared in the 
same manner as above, except that a solution containing 0.2 mol/l of 
calcium chloride and 5.times.10.sup.-5 mol/l of palladium chloride was 
used instead of the solution containing 0.2 mol/l of calcium chloride and 
5.times.10.sup.-3 mol/l of palladium chloride. 
(2) Test Method 
The electrical resistances Ro and R of the carbon dioxide gas detection 
elements (Ca-Pd-Pt-1), (Ca-Pd-Pt-2) and (Ca-Pd-Pt-2) were measured in the 
same manner as in 2-2) in Example 10, and the ratio R/Ro of the electrical 
resistance was calculated therefrom. 
(3) Test Results 
Results as shown in FIG. 22 were obtained. 
In FIG. 22, the ordinate indicates the ratio R/Ro, i.e., [electrical 
resistance of the sample incorporated with calcium chloride, palladium 
chloride and platinum measured in air containing carbon dioxide 
gas]/[electrical resistance of the sample incorporated with calcium 
chloride, palladium chloride and platinum in air]; and the abscissa 
indicates the period of time elapsed after the replacement of the air in 
the electric furnace with the air containing carbon dioxide. 
In FIG. 22, the continuous curve 501 shows the results obtained by the 
carbon dioxide gas detection element (Ca-Pd-Pt-3); the dotted curve 502 
the results by the element (Ca-Pd-Pt-2); and the chained curve 503 by the 
element (Ca-Pd-Pt-1). 
EXAMPLE 13 
The effect of a heating-and-cooling treatment on the sensitivity of an 
element incorporated with calcium chloride, palladium chloride and 
platinum. 
(1) Preparation of Sample 
A carbon dioxide gas detection element (Ca-Pd-Pt-1) was prepared in the 
same manner as in (1) in Example 12, using a solution containing 0.2 mol/l 
of calcium chloride and 5.times.10.sup.-3 mol/l of palladium chloride. 
(2) Test Method 
The carbon dioxide gas detection element (Ca-Pd-Pt-1) was placed in an 
electric furnace maintained at 400.degree. C. by means of a temperature 
controller, and the electrical resistance (Ro) of the element in air was 
measured. Thereafter, the air in the electric furnace was replaced with 
air containing 0.1% of carbon dioxide, and the change in the electrical 
resistance (R.sub.1) of the element was measured with a lapse of time. 
The carbon dioxide gas detection element (Ca-Pd-Pt-1) was placed in an 
electric furnace maintained at 400.degree. C. by means of a temperature 
controller, and the electrical resistance (Ro) of the element in air was 
measured. The element was taken out of the furnace and allowed to cool to 
room temperature. The element was again placed in the electric furnace, 
and the air in the furnace was replaced with air containing 0.1% of carbon 
dioxide. The change in the electrical resistance (R.sub.2) of the element 
was measured with a lapse of time. 
(3) Test Results 
Results as shown in FIG. 23 were obtained. 
In FIG. 23, the ordinate indicates the ratio R1/Ro or R2/Ro, i.e., 
[electrical resistance of the element measured in air containing carbon 
dioxide]/[electrical resistance of the element measured in air]; and the 
abscissa indicates the period of time elapsed after the replacement of the 
air in the electric furnace with the air containing carbon dioxide gas. 
In FIG. 23, the continuous curve 601 shows the results obtained with 
respect to the detection element in which the heating-and-cooling 
treatment was not carried out; and the dotted curve 602 shows the results 
obtained with respect to the detection element in which the 
heating-and-cooling treatment was carried out. 
EXAMPLE 14 
The effect of carbon dioxide gas conditioning on a carbon dioxide gas 
detection element incorporated with calcium chloride, palladium chloride 
and platinum. 
(1) Preparation of Sample 
A carbon dioxide gas detection element (Cd-Pd-Pt-1) was prepared in the 
same manner as in (1) in Example 12, using a solution containing 0.2 mol/l 
of calcium chloride and 5.times.10.sup.-3 mol/l of palladium chloride. 
(2) Test Method 
The carbon dioxide gas detection element (Ca-Pd-Pt-1) was placed in an 
electric furnace maintained at 400.degree. C. by means of a temperature 
controller, and the electrical resistance (Ro) of the element in air was 
measured. Thereafter, the air in the electric furnace was replaced with an 
air containing 0.1% of carbon dioxide, and the change in the electrical 
resistance (R.sub.3) of the element was measured with a lapse of time. The 
element was taken out of the furnace and allowed to cool to room 
temperature. The element was again placed in the electric furnace, and the 
air in the furnace was replaced with air containing 0.1% of carbon 
dioxide. The change in the electrical resistance (R.sub.4) of the element 
was measured with a lapse of time. 
(3) Test Results 
Results as shown in FIG. 24 were obtained. 
In FIG. 24, the ordinate indicates the ratio R.sub.3 /Ro or R.sub.4 /Ro, 
i.e., [electrical resistance of the element measured in air containing 
carbon dioxide gas]/[electrical resistance of the element measured in 
air]; and the abscissa indicates the period of time elapsed after the 
replacement of the air in the electric furnace with the air containing 
carbon dioxide gas. 
In FIG. 24, the continuous curve 701 shows the results obtained with 
respect to the detection element in which the treatment with carbon 
dioxide gas was not carried out; and the dotted curve 702 shows the 
results obtained with respect to the detection element in which the 
treatment with carbon dioxide gas was carried out. 
It can be understood by comparing the results shown in FIGS. 20 to 22 that 
the sensitivity of carbon dioxide gas detection elements utilizing a 
hydroxyapatite can be improved by the incorporation of calcium chloride, 
and the improvement in sensitivity becomes remarkably when calcium 
chloride and platinum are incorporated thereinto. It can also be 
understood that the improvement becomes more significant when calcium 
chloride, palladium chloride and platinum are incorporated thereinto. 
It can further be understood by comparing the results shown in FIGS. 23 and 
24 that the sensitivity of a carbon dioxide gas detection element can be 
improved further when it is given a heat history, or subjected to a 
heating-and-cooling treatment, and that the improvement becomes more 
significant when it is given a heat history, wherein it is heated in a gas 
containing carbon dioxide and then cooled. 
While a preferred embodiment of the invention has been shown and described 
in detail, it will be apparent that various modifications and alternations 
may be made thereto without departing the scope and spirit of the 
invention.