JPS5846699B2 - gas sensing element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas sensing element obtained by mixing carbon (charcoal coke, activated carbon, etc.) into an oxyacid compound semiconductor and sintering it, and in particular can sense gas even at room temperature. Furthermore, gas sensing sensitivity and sensing accuracy, gas selectivity, moisture resistance, etc. are improved.

Most conventional gas sensing elements are made of amorphous composite metal oxide semiconductors, but these elements are susceptible to humidity and are unstable, and sensitivity cannot be increased unless heated to high temperatures. There were problems such as poor gas selectivity.

This invention solves the above problem and is characterized in that carbon (charcoal, coke, activated carbon, etc.) is mixed into an oxyacid compound semiconductor and sintered.

Oxylate compound semiconductors of the present invention include compounds such as silicates, phosphates, molybdates, vanadates, and tungstates.

These semiconductors are composed of an oxyacid ion group and a metal ion, and the oxyacid ion group in particular has a large Lewis acid effect, that is, an electron accepting effect, and exhibits a unique action.

In addition, to explain the gas selectivity due to the Lewis acid effect, the Lewis acid effect is the action of a base that acts to give an electron pair and an acid that acts to accept or accept an electron pair.
For example.

There is a limited number of fixed oxygen (atoms or molecules) on the surface of an oxylate compound semiconductor depending on the constituent substances of the semiconductor, and the equilibrium is usually maintained depending on conditions such as temperature and gas fairness. In a gas sensing element, the gas adsorption (sensing) form differs depending on the type of gas molecules to be adsorbed and the n-type or p-type of the semiconductor.

It also depends on the dependence of the bonding properties such as ionic bonds and covalent bonds of the substances constituting the semiconductor, and also depends on the activation energy level near the surface as well as the geometric shape of the pore structure on the surface of the semiconductor element. Accordingly, the adsorption form of the fixed oxygen with gas molecules can be classified into physical adsorption, physicochemical adsorption, and chemisorption, or these adsorption forms may coexist.

There are various forms of gas sensitivity (adsorption), such as the history of the semiconductor element affecting its adsorption form.

In this way, the fixed oxygen is in a state that matches the surrounding conditions on the surface of the semiconductor element, and since most of the gas sensing elements of this invention are n-type semiconductors, the detected gas molecules adsorbed to the semiconductor are gives an electron to

The semiconductor that accepts electrons increases its conduction electrons and lowers its electrical resistance.

When the adsorbed gas molecules disappear, the state returns to the fixed oxygen state determined by the surrounding conditions, and the conduction electrons also become constant, resulting in the original resistance value.

When these reaction behaviors are applied to the Arrhenius reaction rate equation, k...reaction rate or reactivity, A... frequency factor, E
a... Activation energy, R... Gas constant, T...
・Absolute temperature, A and Ea can be determined as a function of temperature, and the activation energy Ea of the oxylate compound semiconductor of the present invention is lower than that of the amorphous metal oxide semiconductor of the conventional gas sensing element. The influence of

That is, gas selection becomes easy.

Furthermore, due to the bonding properties of the constituent materials of the semiconductor of this invention, that is, the ratio of ionic bonds to covalent bonds, conduction electron/ion conduction increases at a certain temperature, the reactivity k becomes dog, and the activation energy Ea becomes small. be.

In particular, in a certain temperature range in which both the electron conduction mechanism and the ion conduction mechanism are taken into consideration when adsorbed gas is reacted, the reactivity k becomes significant.

This corresponds to an increase in the electron transfer/Lewis acid effect on the surface of the gas sensing element, and this results in a specific temperature depending on the degree of reducibility of the gas molecules adsorbed on the semiconductor to be sensed and the difficulty in transferring surface electrons. Once set, the reactivity of the adsorbed gas molecules can be approximately determined, making it possible to selectively sense gases.

Among them, those with ion groups such as 5i3084-2SIO44- have acid effects greater than acetic acid, especially 5IO2・
2M20a and the like have an acid effect equivalent to sulfuric acid.

These oxygen acid ion groups have highly reactive points on their surfaces similar to active centers, and react with functional groups (aldehydes, alcohols, etc.) in gas molecules and perform dehydrogenation reactions.

Applying such Lewis acid effect, if a specific temperature of the sensing element is selected and set depending on the reducing power of the gas molecule to be sensed and the difficulty of transferring surface electrons, the reactivity of the adsorbed gas molecule can be determined. can be approximately determined, thus enabling selective sensing of gases.

Oxylate compound semiconductors, which have the above-mentioned excellent properties as gas sensing elements, have extremely high resistance (1
08 to 10'', c), the electrical circuit is complicated and the cost is high.

To solve this problem, it was possible to obtain a stable gas sensing element with high electrical conductivity by mixing carbon into an oxyacid compound semiconductor and sintering it.

Carbon has generally been known to have good adsorption properties for organic gases, but this means that the majority of carbon particles (charcoal, coke, activated carbon, etc.) have a hexagonal ring structure unique to carbon. This is because the specific surface area of carbon is porous and extremely large.

Furthermore, inorganic salts, water vapor, carbon dioxide gas,
Due to the treatment with air, carboxyl groups, aldehyde groups, hydroxyl groups, etc., as well as surface oxides and some inorganic salts adhere to the carbon terminals, activating the carbon surface, resulting in poor gas adsorption. It's good.

In other words, the fact that the surface oxide is fixed to carbon means that oxygen is held at the end of carbon's hexagonal ring structure and becomes fixed oxygen, and this fixed oxygen affects electrical conduction changes when gas is adsorbed to carbon. It gives.

Furthermore, one of the characteristics of carbon is that it is less affected by humidity, which makes it extremely effective in terms of moisture resistance when used as a gas sensing element.

Next, a method for manufacturing a gas sensing element in which carbon is mixed into an oxyacid compound semiconductor of the present invention will be described.

As an example, 400 to 500 mesh powdered activated carbon is washed with alcohol, washed with water and thoroughly dried at 120'C, and then metal salts (Pd, Rh,
Ru, Au, Ni, Co, Cr, Mn.

A, 9 salts) or a mixture of two or more thereof.

If necessary, a gel is generated by adjusting the pH to efficiently attach the metal salt.

After washing with water and drying, add a dilute phosphate solution (
0.05 to 0.1 mol %) and dried again.

The purpose of immersing this carbon in a dilute phosphate solution is to improve the heat resistance of the carbon and to improve its bonding with the oxysate compound semiconductor due to the impregnated phosphate groups.

Note that it is desirable to use hard carbon, as this makes it possible to stabilize the electrical conductivity and to obtain a uniform element.

The carbon obtained as described above is mixed into the oxysulfate compound semiconductor. There are many types of oxysulfate compounds, and here we will use zinc orthosilicate (Z n 2 ) as an example.
S104) will be shown and explained.

Since zinc orthosilicate itself is an insulator with a resistance of 10168 or more, La2O3j T102 and Sn02 are each 0.05 to 0.1 mol% and 0.1 to 0, respectively.
．． 2 mol% is added and sintered at 1100 to 1300°C for 2 hours to convert it into a semiconductor.

This results in a specific resistance of 10 to 10 7 Ω.

This zinc orthosilicate semiconductor is pulverized to a size of 300 mesh or less, and thoroughly mixed with the above carbon using a ball mill.

The mixing weight ratio is carbon/zinc silicate-1/~.
10 2 This powdery mixture was heated at 600 to 1000°C for 2 hours after element molding.
It is sintered for ~3 hours.

The carbon-mixed oxylate compound semiconductor described above can sense organic gas even at room temperature, so by providing the detection electrodes 1 and 2 in the element S as shown in FIG. It has a function as a sensing element.

That is, at room temperature, the resistance change of the gas sensing element with respect to the n-butane gas concentration is expressed as follows: ambient humidity is (a) 30%, (
b) The characteristic curve in the case of 60% is shown in Fig. 2.

As is clear from FIG. 2, the gas sensing element of the present invention has high gas sensing sensitivity even at room temperature, but as shown in FIG. If the heating element P, such as a positive temperature coefficient thermistor whose resistance changes rapidly at a temperature of Furthermore, gas detection sensitivity, gas selectivity, and moisture resistance are improved.

Figure 4 shows the change in resistance of the gas sensing element with respect to the concentration of hydrogen gas appearing at the detection electrodes 1 and 2 in Figure 3, with the heating temperature of the element being 80°C, the ambient humidity (a) being 30%, and (b) 60
% is a characteristic curve diagram.

Although a positive temperature coefficient thermistor is shown as an example of the heating body P, heating bodies such as a coil heater, a membrane heater, and a print heater can also be used.

As explained above, the carbon-containing oxylate compound semiconductor gas sensing element of the present invention utilizes the surface-fixed oxygen of carbon and its extremely large specific surface area, and also utilizes the Since it is sintered as a polycrystal with sensitivity and gas selectivity taken into consideration, it also has higher gas sensitivity than conventional metal oxide semiconductor devices.
It has excellent reproducibility, moisture resistance, and gas selectivity.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing one embodiment of the configuration of the gas sensing element of the present invention, FIG. 2 is a characteristic curve diagram showing the relationship between gas concentration and resistance of one embodiment of this gas sensing element, and FIG. This figure is a perspective view showing another embodiment of the structure of the gas sensing element of the present invention, and FIG. 4 is a characteristic curve diagram showing the relationship between gas concentration and resistance of this other embodiment of the gas sensing element. P... Heating body, S... Gas sensing element,
1, 2......Detection electrode, 3, 4...
・A pair of electrodes.

Claims (1)

[Claims] 1 Oxylate compound consisting of any one of silicates, phosphates, molybdates, vanadates, and tungstates, combined with one or more metal salts having a gas-sensitive catalytic effect. A gas sensing element comprising an oxyacid compound semiconductor mixed with carbon treated with a mixed liquid and sintered.