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Description  |
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This invention relates to a combustible gas detecting element for detecting
a combustible gas by being changed in its resistivity upon being subjected
to a combustible gas, namely a reducing gas, and provides such an element
having a long stable life under actual operation even in a high humidity
atmosphere.
Recently, various metal oxide semiconductors were studied and suggested for
gas detecting elements. However, conventional gas detecting elements are
not completely satisfactory for actual social demands. For example, some
do not change their resistivity very much upon being brought into contact
with a gas, that is they are not very sensitive to a change of
concentration of gas. Some other conventional elements are not
satisfactory as to their stability to resistivity for long term use under
voltage application and/or high humidity. The stability, more specifically
insensitivity, of gas detecting elements to humidity is very much required
because they are often used in a very humid atmosphere such as in kitchens
and bath rooms.
It was recently found that gamma-type ferric oxide (.gamma.-Fe.sub.2
O.sub.3) having a spinel-type crystalline structure has excellent gas
detection characteristics. There are various crystalline structures of
ferric oxides which are very different from each other in their chemical
and physical properties. Among them, the best known one is alpha-type
ferric oxide (.alpha.-Fe.sub.2 O.sub.3) having a corundum-type crystalline
structure. Besides, .gamma.-Fe.sub.2 O.sub.3, .beta.-Fe.sub.2 O.sub.3,
.delta.-Fe.sub.2 O.sub.3, etc. are known. Among them, only the
.gamma.-Fe.sub.2 O.sub.3 has actually useful gas detection
characteristics.
The sensitivity of this .gamma.-Fe.sub.2 O.sub.3 to a reducing gas, namely
a combustible gas, is most remarkable when it has a temperature between
300.degree. and 400.degree. C., and is known to have highest sensitivity
at about 350.degree. C., which sensitivity is usually defined by the ratio
of Ra (its resistance in air) to Rg (its resistance in the gas to be
detected). A more important factor for a gas detecting element is the
change rate of resistance of the element unit concentration of gas in a
gas concentration range in which the gas detection should be performed.
This change rate of resistance will be called hereinafter "resistance
change ratio". The resistance change ratio is required to be as large as
possible. This resistance change ratio determined by how quantitatively
the gas concentration can be detected in the gas concentration for the gas
detection. In order to realize a gas detecting element of high performance
and high reliability, not only the sensitivity but also the resistance
change ratio of the element is required to be constant for a long time
even under severe operational conditions.
Further, usually an aging process is employed for producing gas detecting
elements. However, it is desired to avoid the necessity for such an aging
process in view of its productivity and its cost. In order to remove the
aging process, the produced gas detecting element is required to have
stable properties for a long time from just after the production. However,
conventional gas detecting elements were not satisfactory as to these
points.
Accordingly, it is an object of this invention to provide a combustible gas
detecting element which has a large resistance change ratio and can detect
gas concentration highly quantitatively, and which keeps its initial
properties stably for a long time even under severe conditions such as
high temperature and high humidity.
This object is achieved according to this invention by providing a
combustible gas detecting element comprising 99.0 to 25 mole percent of
gamma-type ferric oxide (.gamma.-Fe.sub.2 O.sub.3), 0.5 to 70 mole percent
of gamma-type aluminum oxide (.gamma.-Al.sub.2 O.sub.3), and 0.05 to 15
mole percent, in total, of at least one member selected from the group
consisting of lanthanum (La), cerium (Ce), praseodymium (Pr) and neodymium
(Nd) which are in terms of La.sub.2 O.sub.3, Ce.sub.2 O.sub.3, Pr.sub.2
O.sub.3 and Nd.sub.2 O.sub.3, respectively.
This invention will be more detailedly described hereinafter with the aid
of drawings, in which:
FIG. 1 is a perspective view of an example of a gas detecting element of
this invention in the form of a sintered film; and
FIG. 2 is a perspective view of an example of a gas detecting element of
this invention in the form of a sintered body.
According to this invention, by using .gamma.-Fe.sub.2 O.sub.3 and
.gamma.-Al.sub.2 O.sub.3 and adding various additives thereto, a gas
detecting element having a large resistance change ratio and capable of
highly quantitatively detecting gas concentration and of keeping its
properties for a long time even under severe conditions such as high
temperature and high humidity can be realized.
Hereinafter, this invention will be described by using Examples with
reference to FIGS. 1 and 2.
EXAMPLE 1
Various compositional ratios of mixtures of Fe.sub.3 O.sub.4 (having an
average particle size of 0.3 micron), .gamma.-Al.sub.2 O.sub.3 and
Nd.sub.2 O.sub.3 were employed in amounts listed in Table 1. Each of the
thus prepared mixtures was wet-milled by using stainless steel pot with
stainless balls, and was then pulverized. The thus made mixture was dried
in vacuum at a temperature of 80.degree. C., and was then sintered in
vacuum at a temperature of 800.degree. C. for 1 hour. Polyethyleneglycol
was added to the thus made sintered mixture to make a paste.
Meanwhile, an alumina substrate 1, having a width of 5 mm and a breadth of
5 mm and a thickness of 0.5 mm, for supporting a gas detecting element was
prepared. On the alumina substrate 1, the above prepared paste was printed
in a thickness of about 70 microns, and was dried in air at room
temperature. Then, the paste-applied substrate was sintered at a
temperature 400.degree. C. for 1 hour in air, whereby a gas detecting
element 2 was made. On the element 2, comb-type electrodes 3 were formed
by vacuum-evaporating gold, with the distance between the electrodes being
0.5 mm. On the opposite surface of the substrate was applied a
commercially available glaze resistor paste of ruthenium oxide, and the
paste was sintered to form a glaze resistor 4 as a heater. On the heater 4
were printed gold electrodes 5, and these electrodes were sintered as
electrodes for the heater. Lead wires 6 were bonded to the electrodes 3
and 5 by means of conductive paste.
At this stage, the starting Fe.sub.3 O.sub.4 had been oxidized to
.gamma.-Fe.sub.2 O.sub.3, and solvents in the pastes had been evaporated
off, whereby a sintered film 2 having practically sufficient mechanical
strength was obtained. The thickness of the thus made gas detecting
element 2 was about 50 microns.
By using the thus made various gas detecting elements, gas detection
measurements were carried out. The temperature of each of the elements for
measurements was controlled by controlling the current to flow in the
heater 4. The resistance (Ra) of the element in air was measured in a
container of 50 liters in which dry air was slowly stirred in a manner not
to produce turbulent air flow. The resistance (Rg) of the element in a gas
was measured in the above container by introducing an isobutane gas of
higher than 99% purity into the container at a rate of 10 ppm/sec in terms
of volume percent. The resistance (Rg) was measured at two points, when
the introduced gas got to 0.05 volume percent and 0.5 volume percent,
respectively, as Rg(0.05) and Rg(0.5), respectively. The reason why the
volume percents of 0.05 and 0.5 were selected is because the lower
explosion limit (LEL) of isobutane is about 2 volume percent, and thus a
combustible gas detecting element is required to detect a gas in the range
of from about one fiftieth to about one fifth of the LEL value, from a
practical point of view. The gas detection characteristics of each element
were measured with the element being kept at a temperature of 350.degree.
C. by applying a current to the heater thereof.
In this type of gas detecting elements, the resistance (Rg) of the element
is substantially proportional to C.sup.-n (where C is the gas
concentration of the gas to be detected) in such a concentration range as
of 0.05-0.5 volume percent. Therefore, the above-mentioned resistance
change ratio can be evaluated by this constant n (which will hereinafter
be called concentration factor). For example, when the resistance change
ratio is 6.02, n=0.780.
Thereafter, each element was subjected to load life test. That is, current
was applied to the heater of each element to keep the element at a
temperature of 350.degree. C. and 10 V of d.c. voltage was applied between
the electrodes 3. These conditions were kept, where the element was placed
in ambient atmosphere of a temperature of 60.degree. C. and a relative
humidity of 95 RH%. At two time points (20 hours and 2000 hours) after the
start of the application of the above current and the d.c. voltage, each
element was subjected to the measurements of gas detection
characteristics. Table 1 shows the thus measured initial values Rg, n, and
change rates .DELTA.Rg, .DELTA.n.
As apparent from the above experimental results, the gas detecting element
according to this invention comprising 0.5 to 70 mole percent of
.gamma.-Al.sub.2 O.sub.3, 0.5 to 15 mole percent of Nd.sub.2 O.sub.3 and
the remainder of .gamma.-Fe.sub.2 O.sub.3 has a very large resistance
change ratio in the practical gas concentration range for detection, and
can highly quantitatively detect the gas. Moreover, its gas detection
characteristics are stable for a long time from just after the production
of the element, even under severe conditions such as a temperature of
60.degree. C. and a relative humidity of 95 RH%. Therefore, the gas
detecting element of this invention is not necessary to be subjected to
usual aging process.
Hereinabove, .gamma.-Al.sub.2 O.sub.3 and Nd.sub.2 O.sub.3 have been
exemplified as additives to .gamma.-Fe.sub.2 O.sub.3. However, similar
results can be obtained when Nd.sub.2 O.sub.3 is replaced by La.sub.2
O.sub.3, Ce.sub.2 O.sub.3 or Pr.sub.2 O.sub.3. In such case also, the
effective compositional ratio between one of them and .gamma.-Al.sub.2
O.sub.3 and .gamma.-Fe.sub.2 O.sub.3 coincides with that of the case of
using Nd.sub.2 O.sub.3. That is, when .gamma.-Al.sub.2 O.sub.3 is less
than 0.5 mole percent or when Nd.sub.2 O.sub.3, La.sub.2 O.sub.3, Ce.sub.2
O.sub.3 or Pr.sub.2 O.sub.3 is less than 0.05 mole percent, the effect of
these additive additions is not significant for obtaining a gas detecting
element having a highly stable gas detection characteristics over a long
term load life test under high temperature and high humidity. Likewise,
when the amount of .gamma.-Al.sub.2 O.sub.3 is more than 70 mole percent,
the resistance of the gas detecting element becomes abnormaly high both
where the element is in the form of a sintered body and in a form of a
sintered film, and furthermore, the element does not have sufficient
mechanical strength. When the amount of Nd.sub.2 O.sub.3, La.sub.2
O.sub.3, Ce.sub.2 O.sub.3 or Pr.sub.2 O.sub.3 is more than 15 mole
percent, the resultant gas detecting element does not have a sufficiently
long life under a high temperature and a high humidity.
In this Example 1, the single use of one of Nd.sub.2 O.sub.3, La.sub.2
O.sub.3, Ce.sub.2 O.sub.3 and Pr.sub.2 O.sub.3 has been described.
However, similar results can be obtained when two, three or all of these
additive elements are combined for use as additives to .gamma.-Al.sub.2
O.sub.3 and .gamma.-Fe.sub.2 O.sub.3, if these combined additives are, in
total, in an amount of from 0.05 to 15 mole percent. Details of such
combined additives will be exemplified in the following Example 2.
EXAMPLE 2
70 mole percent of a Fe.sub.3 O.sub.4 powder (having an average particle
size of 0.5 micron), 25 mole percent of .gamma.-Al.sub.2 O.sub.3 powder
and 5 mole percent, in total, of at least one of La.sub.2 O.sub.3,
Ce.sub.2 O.sub.3, Pr.sub.2 O.sub.3 and Nd.sub.2 O.sub.3 (in various
combinations) were mixed, wet-milled and pulverized in the same manner as
in Example 1 to obtain various mixtures as listed in Table 2. Each mixture
was dried in vacuum at a temperature of 80.degree. C. Each of the thus
obtained mixture powders was pressure-molded to a rectangular
parallelopiped form, and was sintered in nitrogen atmosphere at
800.degree. C. for 1 hour. It was then furnace-cooled to room temperature,
and was then again gradually heated to 400.degree. C. in air, and kept at
400.degree. C. for 1 hour, whereby the mixture material was oxidized. At
this stage, Fe.sub.3 O.sub.4 becomes .gamma.-Fe.sub.2 O.sub. 3.
On the thus made sintered body 7 gold was vacuum evaporated to form a pair
of comb electrodes 8. On the opposite surface of the sintered body 7 a
platinum heater 9 was bonded by using an inorganic adhesive. Lead wires 10
were bonded to the the comb electrodes 8 and the heater 9 by using a
conductive paste. The operational temperature of each gas detecting
element was controlled by controlling the electric current applied to the
heater 9. Thereby, the element temperature was kept at 350.degree. C., and
measurements of gas detection characteristics of the gas detecting
elements above prepared were carried out under the same conditions as used
in Example 1. Thereafter, the gas detecting elements were placed in an
atmosphere of 60.degree. C. and 95 RH% for 2000 hours, with the element
temperature being kept at 350.degree. C. After this load life test, each
gas detecting element was taken out of the atmosphere, and was subjected
to the measurement of gas detection characteristics. These measured
characteristics were compared with the above initially measured
characteristics. The results of these measured characteristics are shown
in Table 2. As apparent from Table 2, the gas detecting elements obtained
in this Example 2 have very good load life properties under a high
temperature and a high humidity, just as in the case of Example 1.
In Example 1, the form of the gas detecting element mainly shown therein is
a sintered film. On the other hand, it is a sintered body in Example 2. As
apparent from the foregoing, in both cases, very good results can be
obtained as to initial properties and load life properties under severe
conditions of a high temperature and a high humidity, although the
absolute values of resistances vary from the sintered film form to the
sintered body form.
In the above Examples, a glaze heater of ruthenium oxide or a platinum
heater was exemplified as a heater. However, other heaters can also be
used, such as a coil type heater. Further, in the above Examples, La, Ce,
Pr and Nd were added in the form of oxides. However, any other form of the
compounds can be used. For example, they can be added in the form of
carbides, if the carbides can be converted to the corresponding oxides
upon being heated to a high temperature. Further, in the above Examples,
the starting material for the gamma-ferric oxide was Fe.sub.3 O.sub.4.
However, any other starting material therefor can be used, if they can
become .gamma.-Fe.sub.2 O.sub.3 in the resultant gas detecting element.
As apparent from the foregoing, the gas detecting element according to this
invention has a large resistance change ratio, namely a large
concentration factor. Further, it has an excellent load life properties
under high temperature and a high humidity, to which the gas detecting
element would be confronted in actual use. Therefore, the initial
properties of the gas detecting element can be kept for a long time e.g.
at a place directly exposed to sunshine, such as a kitchen and a bathroom
where the humidity is very high, and other similar places.
Further, if necessary, other additives which are not exemplified in the
above Examples can be used if the effects obtained by the above
exemplified gas detecting elements are not damaged thereby. Further, the
gas detecting element of this invention is effective not only to isobutane
but also to other combustible gases such as ethane, propane and hydrogen,
although only isobutane has been used for the showing.
TABLE 1
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Change rate of properties after
Compositional ratio load life test
(mole %) Initial value
after 20 hours
after 2000 hours
Sample
.gamma.
.gamma.
Ra Rg(0.5)
.DELTA.Rg(0.5)
.DELTA. n
.DELTA.Rg(0.5)
.DELTA. n
No. Fe.sub.2 O.sub.3
Al.sub.2 O.sub.3
Nd.sub.2 O.sub.3
(K .OMEGA.)
(K .OMEGA.)
on (%) (%) (%) (%)
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1* 100.0
0 0 640 31 0.59
-3.4 -4.2
+38.2
-17.3
2* 90.0
10.0
0 692 34 0.74
+10.6
-1.0
+24.6
-12.4
3* 95.0
0 5.0 648 32 0.60
-4.1 -6.3
+27.4
-10.7
4* 99.9
0.1 0.01
643 30 0.61
-3.2 -5.7
+38.1
-16.8
5 99.45
0.5 0.05
652 32 0.68
-1.1 -1.1
+4.3 -3.2
6* 98.99
1.0 0.01
680 34 0.63
+3.4 -2.2
+11.4
-11.6
7* 94.9
0.1 5.0 648 33 0.59
-2.7 -2.1
+20.3
-9.7
8 90.0
5.0 5.0 684 36 0.69
+2.6 -1.4
+3.5 -4.8
9 85.0
10.0
5.0 693 37 0.76
+2.7 -0.9
+3.0 -4.1
10 75.0
20.0
5.0 815 39 0.77
+0.8 -0.5
+2.1 -0.7
11 45.0
50.0
5.0 887 41 0.70
+0.8 -1.9
+3.1 -2.0
12 15.0
70.0
15.0
1247
58 0.68
+4.3 -4.0
+4.8 -4.9
13* 20.0
75.0
5.0 3400
194 0.55
+4.8 -5.1
+6.2 -9.8
14 79.5
20.0
0.5 810 41 0.77
+2.9 .+-.0
+3.4 -0.8
15 79.0
20.0
1.0 813 38 0.76
.+-.0
-0.7
+2.8 -1.1
16 70.0
20.0
10.0
876 40 0.74
+3.4 .+-.0
+3.4 -1.0
17* 60.0
20.0
20.0
858 48 0.64
+4.2 -0.9
+16.9
-7.8
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(*samples for comparison)
TABLE 2
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Resistance
Sample
Compositional ratio (mole %)
Ra Rg(0.5)
Rg(0.5)
change
Concentration
No. La.sub.2 O.sub.3
Ce.sub.2 O.sub.3
Pr.sub.2 O.sub.3
Nd.sub.2 O.sub.3
(K.OMEGA.)
(K.OMEGA.)
(K.OMEGA.)
ratio factor
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21*
0 0 0 0 Initial values
604
193 37 5.17 0.71
Values after
test 560
206 45 4.57 0.66
22 5.0 0 0 0 Initial values
612
197 33 5.94 0.77
Values after
test 634
209 35 5.91 0.77
23 0 5.0 0 0 Initial values
889
274 53 5.16 0.71
Values after
test 861
288 57 5.01 0.70
24 0 0 5.0 0 Initial values
684
201 36 5.63 0.75
Values after
test 644
213 38 5.52 0.74
25 0 0 0 5.0 Initial values
778
226 39 5.81 0.76
Values after
test 749
230 40 5.78 0.76
26 2.5 2.5 0 0 Initial values
777
231 38 6.08 0.78
Values after
test 782
233 39 5.97 0.78
27 0 2.5 2.5 0 Initial values
807
235 39 6.02 0.78
Values after
test 809
238 40 5.95 0.77
Initial values
833
252 40 6.30 0.80
28 0 0 2.5 2.5 Values after
845
256 41 6.24 0.79
test
Initial values
667
208 36 5.78 0.76
29 2.5 0 0 2.5 Values after
656
201 36 5.58 0.74
test
Initial values
786
233 37 6.29 0.80
30 2.0 1.0 1.0 0 Values after
790
235 38 6.18 0.79
test
Initial values
816
239 40 5.98 0.77
31 0 2.0 2.0 1.0 Values after
828
239 41 5.82 0.76
test
Initial values
844
253 41 6.17 0.79
32 1.0 0 2.0 2.0 Values after
845
255 42 6.07 0.78
test
Initial values
690
215 37 5.81 0.76
33 2.0 1.0 0 2.0 Values after
699
210 37 5.67 0.75
test
Initial values
843
263 42 6.26 0.79
34 2.0 1.0 1.0 1.0 Values after
856
260 43 6.04 0.78
test
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(*samples for comparison)
(Al.sub.2 O.sub.3 : 25mole% (constant), remainder: .gamma.-Fe.sub.2
O.sub.3)
* * * * *
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