JP3171663B2 - Semiconductor type odor gas sensor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor type malodor gas sensor provided with a metal oxide semiconductor portion containing a valence-controlled tin oxide semiconductor as a main component, and relates to ammonia (N).
H _3), trimethylamine _{((CH 3) 3 N)} , hydrogen sulfide (H ₂ S), methanethiol (CH ₃ SH), butyric acid (C ₃
H ₇ COOH), etc., which are generated from chemical fertilizer manufacturing industry, slaughterhouse, livestock industry, compound fertilizer factory, waste treatment plant, sewage treatment plant, etc. The present invention relates to a semiconductor type malodor gas sensor for detecting malodor gas containing hydrogen gas and the like.

[0002]

2. Description of the Related Art Generally, in a semiconductor type malodor gas sensor, when a malodor gas is adsorbed on a semiconductor surface, the malodor gas is oxidized and the semiconductor surface undergoes a reducing action. At this time, since the resistance value of the semiconductor changes, the presence of the odorous gas can be detected by measuring the change in the resistance value. Further, the semiconductor surface which has undergone the reducing action is subjected to the air oxidizing action and reproduces the original detection function. At this time, the change in the resistance value can be detected using a bridge circuit. Heretofore, as a semiconductor type gas sensor containing tin oxide as a main component, there have been ones which carry only calcium oxide and those which carry palladium.

[0003]

However, these semiconductor-type gas sensors, for example, in which only calcium oxide is added, have a high sensitivity to molecules having a strong odor and a low sensitivity to molecules having a weak odor. Although it is a gas sensor with excellent odor detection characteristics, it has low sensitivity especially to ammonia, and has a problem in detecting odors containing ammonia as a main component. But high sensitivity to odorous gases such as hydrogen, all of which are generated from chemical fertilizer manufacturers, slaughterhouses, livestock industry, compound fertilizer factories, factory waste treatment plants, human waste treatment plants, sewage treatment plants, etc. Ammonia,
It is a problem for odor detection in places where odorous gas is generated, which is composed of components such as trimethylamine, hydrogen sulfide, methanethiol, and butyric acid, and is mainly composed of ammonia and also contains odorless gas such as hydrogen gas. was there.

[0004] In view of the above drawbacks, an object of the present invention is to provide a high sensitivity to an odor containing ammonia as a main component and a low sensitivity to an odorless gas such as hydrogen gas.
It is an object of the present invention to provide a semiconductor type odor gas sensor.

[0005]

In order to achieve this object, a semiconductor type malodor gas sensor according to the present invention is characterized in that a metal oxide semiconductor portion comprises a strongly basic metal oxide and a strongly acidic metal oxide. Both are carried. still,
The strongly basic metal oxide is lanthanum oxide, the strongly acidic metal oxide is chromium oxide, and the ratio of the added amount of the metal oxide is lanthanum oxide. The ratio of chromium to 8: 2 to 1: 9 and the total amount of lanthanum and chromium may be set to 0.03 to 10.0 atm% with respect to tin of tin oxide. The effects are as follows.

[0006]

In other words, it is considered that by supporting a metal oxide having a high basicity and a low oxidation activity, such as lanthanum oxide, on the semiconductor surface, the activity against an odorless gas having a reducing power can be reduced. By supporting a metal oxide having high acidity and high oxidizing activity, such as chromium, on the semiconductor portion, it is considered that the metal oxide can have a high activity with respect to adsorption to an electron donating gas and an oxidation reaction subsequent to the adsorption. Can be However, in order to improve sensitivity to a malodorous gas containing ammonia as an electron donating gas as a main component, each of them cannot be realized only by a single action, and by carrying these two types of metal oxides together, It is considered that the foul odor gas detection has been smoothly performed by the following reaction.

The surface of the metal oxide is in a state where the metal ion and the oxide ion are exposed, and the oxide ion is partially a hydroxide ion due to the adsorption equilibrium with water. By the way, on the surface of a metal oxide heated to a high temperature, oxide ions are generated by dehydration from an adjacent hydroxyl group. Here, on the surface of the acidic metal oxide, the exposed metal ion easily adsorbs an electron donating molecule (a lone electron pair or a molecule having a π bond), and when an electron moves from the electron donating molecule to the metal ion. Conceivable. On the other hand, in the case of a basic metal oxide, it is considered that an oxide ion generated on the surface easily causes a dehydrogenation reaction in which hydrogen is extracted from a reactive molecule. It is presumed that these reactions interact with each other and the reaction molecules such as ammonia are completely oxidized through decomposition and the like.

In other words, considering this reaction mechanism, it can be said that this semiconductor type malodor gas sensor has good detection conditions for malodorous gases such as amines and thiols including ammonia.

[0009]

Thus, according to the semiconductor type of offensive odor gas sensor of the present invention, the reactivity with respect to odorous gas such as ammonia is increased, and the detection capability with respect to odorless gas such as hydrogen is reduced. And its responsiveness is also improved. In addition, since long-term stability and high sensitivity characteristics at high temperatures can be obtained, a more reliable semiconductor odor gas sensor can be provided. Further, by maximizing the sensitivity characteristic to ammonia, it becomes possible to detect malodor with particularly high sensitivity to malodor containing ammonia as a main component.

[0010]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a semiconductor type of offensive odor gas sensor according to an embodiment of the present invention. In the description, the structure of the sensor, the method for preparing the tin oxide semiconductor, the method for supporting the metal oxide, the sensor circuit, and the characteristics of the sensor will be described in this order.

[Structure of Sensor] FIG. 1 shows the structure of a hot-wire type semiconductor odor gas sensor according to the present invention. In the figure, a part of the sensor is shown in cross section to show the internal configuration of the sensor. As shown in the figure, the sensor includes a metal oxide semiconductor portion on a platinum coil as the noble metal wire coil 1, and the metal oxide semiconductor portion supports both lanthanum and chromium oxide. I have. More specifically, the metal oxide semiconductor portion is formed by sintering tin oxide, and carries both lanthanum oxide and chromium oxide.

[Preparation of Tin Oxide Semiconductor Layer] A commercially available aqueous solution containing tin tetrachloride at a constant concentration is prepared, and antimony pentachloride is added to obtain an appropriate electric conductivity. 0.6 for tin tin
Atm% is added. When aqueous ammonia is added dropwise to this solution, tin hydroxide precipitates due to hydrolysis. This is washed with distilled water and dried to obtain a gelled tin hydroxide. This tin hydroxide is placed in an electric furnace at 600
When pyrolysis is performed at 4 ° C. for 4 hours, a lump of tin oxide is obtained. The tin hydroxide obtained in this way is
It is made into fine powder by a crusher. Distilled water was added to this to form a paste, and as shown in FIG. 1, a diameter of 0 mm was applied so as to cover the periphery of a noble metal wire coil (20 μmφ) of platinum or the like.
After being adhered in a 45 mmφ spherical shape and dried, an electric current is applied to the coil, and the coil is baked at 600 ° C. for 1 hour by the heat generated. Thus, a layer serving as a base of the metal oxide semiconductor portion of the semiconductor type offensive odor gas sensor of the present invention is formed.

[Method of Supporting Metal Oxide] First, commercially available lanthanum nitrate and chromium nitrate are made into an aqueous solution, and a mixed aqueous solution thereof is prepared. This mixed aqueous solution is impregnated into the tin oxide semiconductor, dried, heated again by applying a current to the coil, and thermally decomposed and fired at 600 ° C. for 1 hour, whereby each metal salt is decomposed. The oxide can be supported on the surface of tin oxide, and the tin oxide semiconductor supporting the lanthanum oxide and chromium oxide becomes the metal oxide semiconductor portion 2.

[Sensor Circuit] The sensor 3 is used by being incorporated in a Wheatstone bridge circuit shown in FIG. In the figure, reference numeral 4a denotes a resistance connected in series as a load resistance for the sensor 3;
Are reference resistors connected in series with each other to determine the reference potential of this circuit. The sensor 3 and the series resistor 4a are paralleled with respect to the power supply 4d with respect to the other reference resistors 4b and 4c, and the output of this sensor is obtained in the form of a voltage (mV) by the potential difference between the intermediate points A and B of the resistors. Can be done.

[Characteristics of the Sensor] The performance characteristics of the semiconductor type of offensive odor gas sensor of the present invention will be described below. Hereinafter, the dependence of the amount of lanthanum / chromium added, the composition, and the sensitivity characteristics will be described in order. Here, an embodiment will be described in which the characteristics for ammonia are set to the maximum.

(1) Dependence of added amount of lanthanum / chromium

[0017]

[Table 1]

Table 1 shows that the composition ratio of La and Cr is less than or equal to the composition of lanthanum and chromium (La: Cr).
｝ expressed as a ratio of tm% (La: Cr) = (50: 5)
0), lanthanum and chromium, respectively,
0.01 atm% to 5 atm with respect to tin of tin oxide
%, The sensitivity of ammonia (50 ppm) and hydrogen (1000 ppm), their sensitivity ratio (ammonia / hydrogen), and the dependence of the sensor resistance in clean air on the amount of lanthanum chromium added. (The sensor temperature is 400 ° C.). Ammonia sensitivity is 0.01atm% to 3% for lanthanum and chromium.
It was maintained high up to about atm%, and tended to decrease above that. In addition, the hydrogen sensitivity is such that the addition amount of each of lanthanum and chromium is 0.01 atm% to 0.1 atm%.
In the range up to the extent, it declined remarkably, and after that, it remained at a low value.

Since the sensitivity ratio can be practically used as long as the sensitivity ratio is at least 0.5 or more, as shown in Table 1, the addition amount of lanthanum oxide and chromium oxide is 0.015 atm% or more with respect to tin. That would be good. Further, although the addition amount of each is 5 atm%, it is maintained high from the viewpoint of the sensitivity ratio, but it is desirable to keep the addition amount at this level from the viewpoint of lowering the ammonia sensitivity and the production process and cost. As a result, the total amount of lanthanum and chromium is from 0.03 atm% to 10 a
It would be desirable to set it to tm%.

(2) Composition dependence of lanthanum / chromium

[0021]

[Table 2]

Table 2 shows the total amount of lanthanum and chromium added.
With respect to tin oxide, the sensitivity of ammonia (50 ppm) and hydrogen (1000 ppm) at 1.20 atm%, their sensitivity ratio (ammonia / hydrogen), and
It shows the dependence of the sensor resistance in clean air on the composition (La: Cr) of lanthanum and chromium (sensor temperature is 400 ° C.). That is, the sensitivity of ammonia is (L
a: Cr) has one peak with respect to the change, and the maximum value of the peak is that (La: Cr) = (60:
40). On the other hand, the hydrogen sensitivity could be kept low irrespective of the composition. The sensitivity ratio is such that the composition ratio of lanthanum / chromium is (La: Cr) =
From (80:20) to (10:90), it is kept high, and the ammonia sensitivity is 30 m which is practically usable.
v or so. FIG. 3 illustrates the result. From FIG. 3, the composition ratio of lanthanum / chromium is (La:
(Cr) = (80:20) to (10:90), the semiconductor type of offensive odor gas sensor was able to obtain high performance.

(3) Sensitivity Characteristics Sensitivity characteristics will be described in the order of sensor temperature dependence, odor gas selectivity, response, and stability over time.

(A) Sensor Temperature Dependence FIG. 4 shows that, when (La: Cr) = (60:40) and the total amount of supported is 1.2 atm% with respect to tin of tin oxide, FIG. It shows the sensor voltage dependence and sensor temperature dependence of the sensitivity of ammonia (50 ppm) and hydrogen (1000 ppm) when the value of the resistor 4a is 10Ω. As a result, the ammonia sensitivity decreases with an increase in the temperature of the sensor. However, since the hydrogen sensitivity is kept low, a high ammonia selectivity is maintained in the sensor temperature range of 360 ° C. to 470 ° C.

(B) Ammonia selectivity FIG. 5 shows that when (La: Cr) = (60:40) and the total carried amount is 1.2 atm% with respect to tin oxide, a typical odorous gas is obtained. Ammonia, trimethylamine,
FIG. 6 is a graph plotting the sensor sensitivities of hydrogen sulfide, methanethiol, butyric acid, and hydrogen, methane, and carbon monoxide as typical odorless gases with respect to the logarithm of the gas concentration. (The sensor temperature is 400 ° C.) It can be seen that both exhibit good linearity. Further, since the hydrogen sensitivity is kept low, it can be said that there is a higher ammonia selectivity than the gas sensitivity at an allowable concentration of ammonia of 5 ppm. In addition, the gas sensitivity of the other odorous gas components at the allowable concentration of 1 ppm is maintained at a high value, so that it can be said that the gas has high sensitivity characteristics.

(C) Responsiveness FIG. 6 is a response waveform to ammonia when (La: Cr) = (60:40) and the total amount of supported is 1.2 atm% with respect to tin oxide. This indicates that the sensor has a practically desirable response speed.

(D) Stability over time FIG. 7 shows that ammonia (50 ppm) and hydrogen when (La: Cr) = (60:40) and the total supported amount is 1.2 atm% with respect to tin oxide. (1000 ppm) shows changes in sensor output with time in respective sensitivities and clean air (sensor temperature is 400 ° C.). This shows that practically stable performance is maintained for a long period of time.

Considering the above-described lanthanum / chromium composition, addition amount dependency, and sensitivity characteristics together with the above-mentioned catalytic effect, the best ammonia activity can be obtained near (La: Cr) = (50:50). This was demonstrated both theoretically and experimentally, and the practical range was also demonstrated experimentally. In addition, an appropriate amount for increasing the ammonia sensitivity has been experimentally shown to the extent that the responsiveness and the stability over time are not impaired.

Another Embodiment In the present invention, the main component of the metal oxide semiconductor portion of the thermal linear semiconductor sensor is tin oxide, but another metal oxide, for example, indium oxide may be used.

In this embodiment, the noble metal wire coil is also made of platinum wire, but may be replaced with gold, platinum-rhodium alloy wire, platinum-iridium alloy wire or the like.

In this embodiment, the strongly acidic metal oxide and the strongly basic metal oxide are lanthanum and chromium, respectively. A highly basic metal oxide, an oxide of an alkaline earth metal (Be, Mg, Ca, Sr, Ba) or an alkali metal (Li, Na, K, Rb, Cs) or a mixture of these and lanthanum oxide The same gas sensor can be used by selecting a combination of metal oxides having the same catalytic effect as in the above-mentioned combinations.

For example, a mixture of lanthanum and calcium metal oxides having an atomic composition ratio of 1: 2 is set to 1.2 atm% with respect to tin, and the addition amount of chromium oxide is fixed. 0.004at for tin
m% to 1.2 atm% (sensor temperature is 400 ° C.), ammonia (50 ppm), hydrogen (10 ppm)
Table 3 shows the respective sensitivities and their sensitivity ratio (ammonia / hydrogen).

[0033]

[Table 3]

According to the above-described embodiment, a higher sensitivity ratio can be obtained, and the initial stability and the response waveform tend to be better than those of the present embodiment.

Further, the characteristics of a malodorous gas containing a gas other than ammonia as a main component can be determined in the same process as in this embodiment.

Further, in the present embodiment, a hot wire sensor as shown in FIG. 1 is used, but other types may be used.
It may be a substrate type sensor. FIG. 8 shows an example. FIG.
As for a, instead of the noble metal wire coil 1 in the present embodiment, a rectangular wave-shaped electrode 1b deposited on a substrate 5 of alumina or the like is used, and the metal oxide semiconductor portion 2 is sintered in a spherical shape as in the present embodiment. Instead, the metal oxide semiconductor portion 2 is sintered on the substrate 5 so as to cover the electrode 1b in the same manner as in the present embodiment.
Is used to heat the sensor.
FIG. 8B shows a configuration in which the heater of FIG.
6 is provided so that the sensor can be heated.

In the claims, reference numerals are provided for convenience of comparison with the drawings, but the present invention is not limited to the configuration shown in the attached drawings.

[Brief description of the drawings]

FIG. 1 is a partially cutaway perspective view of a semiconductor type offensive odor gas sensor of the present invention.

FIG. 2 is a basic electric circuit diagram of the sensor of the present application.

FIG. 3 shows the (La: C) of the semiconductor type of offensive odor gas sensor of the present invention.
r) Diagram showing dependency

FIG. 4 is a graph showing the concentration-dependent characteristics of the semiconductor type of offensive odor gas sensor of the present application.

FIG. 5 is a diagram showing the sensor voltage dependence of the semiconductor type offensive odor gas sensor of the present application.

FIG. 6 is a diagram showing a response waveform to ammonia of a semiconductor type of offensive odor gas sensor of the present application.

FIG. 7 is a diagram showing the stability over time of the semiconductor type of offensive odor gas sensor of the present invention.

FIG. 8 is a diagram showing another embodiment of the semiconductor type of offensive odor gas sensor of the present invention.

[Explanation of symbols]

 2 Metal oxide semiconductor section

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-26045 (JP, A) JP-A-59-26044 (JP, A) JP-A-59-26041 (JP, A) JP-A 54-26041 39197 (JP, A) JP-A-51-94599 (JP, A) JP-A-49-66191 (JP, A) Japanese Utility Model Application 52-3591 (JP, U) Japanese Utility Model Application No. 51-44993 (JP, U) (Japanese) 51-44994 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 27/12

Claims (3)

(57) [Claims] 1. A semiconductor type malodor gas sensor comprising a metal oxide semiconductor part (2) containing a tin oxide semiconductor whose valence is controlled as a main component, wherein a base is added to the metal oxide semiconductor part (2). A semiconductor-type odor gas sensor that carries both a highly acidic metal oxide and a strongly acidic metal oxide. 2. The semiconductor according to claim 1, wherein said strongly basic metal oxide is lanthanum oxide (La ₂ O ₃ ), and said strongly acidic metal oxide is chromium oxide (Cr ₂ O ₃ ). Type odor gas sensor. 3. The lanthanum / chromium atomic composition ratio of the lanthanum oxide addition amount and the chromium oxide addition amount is set to 8: 2 to 1: 9, and their total addition amount is reduced to tin oxide tin. On the other hand, the total amount of lanthanum and chromium is 0.03a
3. The semiconductor-type offensive odor gas sensor according to claim 2, which is set to tm% to 10.0 atm%.