JPS5848059B2 - gas sensing element - Google Patents

Description

[Detailed description of the invention] The present invention relates to a gas-sensitive element with excellent gas selectivity.

Gas-sensitive elements are known that utilize the fact that the specific resistance of the surface of an oxide semiconductor changes when gas comes into contact with the surface of the oxide semiconductor.

For example, ZnO, Sn02, F”e2 exhibiting N-type semiconductivity
When a reducing gas comes into contact with 03 or the like, the resistance value decreases, and when an oxidizing gas comes into contact with it, the resistance value increases.

In addition, in an oxide semiconductor exhibiting P-type semiconductivity, increases and decreases in resistance value exhibit an inverse relationship.

In the above-mentioned oxide semiconductors, the reactivity or selectivity with various gases is determined by the semiconductor surface temperature, surface electron level structure, porosity, pore size, etc.
Generally, an oxide semiconductor alone has low sensitivity as a gas-sensitive element and cannot be said to have sufficient selectivity.

Therefore, attempts have been made to increase the sensitivity by adding a catalyst such as Pt or Pd to the oxide semiconductor.

In this case, a certain degree of selectivity can be obtained by selecting the element temperature.

For example, a gas sensitive material in which pt is added has high sensitivity to isobutane gas, but does not react much to hydrogen gas or carbon monoxide gas.

On the other hand, when Pd is added, high sensitivity is exhibited to isobutane gas, hydrogen gas, and carbon monoxide gas.

However, both pt and Pd also exhibit high sensitivity to alcohol-based gases such as ethyl alcohol gas.

So above? For devices such as, especially in ordinary households,
In the same way as LP gas (liquefied propane gas), they are sensitive to alcohol-based gases generated by foods, hair styling products, cosmetics, etc., which has been a big problem in practical use.

In view of the above points, an object of the present invention is to provide a highly reliable gas-sensitive element for LP gas that is not sensitive to alcohol-based gas and has high sensitivity to isobutane gas.

The present invention comprises a pair of electrodes and a gas sensitive body provided between the electrodes, and the gas sensitive body includes an N-type metal oxide semiconductor and platinum (Pt) in an amount of 0.005 to 8% by weight. , palladium (Pd), and rhodium (Rh), and a molar ratio of 1.5 to the noble metal catalyst.
It is a gas-sensitive element containing ~30 times (however, 0.05 to 30 times when palladium alone is used as a noble metal catalyst) phosphorous, and in particular zinc oxide (ZnO)-based as an N-type metal oxide semiconductor. , a tin oxide (S no2) system, and an iron oxide (Fe203) system can be used to obtain a gas-sensitive element with excellent selectivity, and furthermore, by using silica, alumina, and a silica-aluna compound as a carrier for the noble metal catalyst, a stable gas-sensitive element can be obtained. A gas-sensitive element can be obtained.

In the present invention, the amount of noble metal catalyst added is 0. 0 0
The 5-8 weight section was set at 0. This is because if the weight ratio is less than 0.05, the sensitivity to isobutane gas becomes low, and if it exceeds 8 weight ratio, the sensitivity to alcohol gas becomes easy.

Also, the amount of P added is 1 molar ratio to the amount of precious metal catalyst added.
．． The reason why the ratio is 5 to 30 times is that if it is less than 1.5 times, there will be no noticeable difference between the sensitivity to inbutane gas and the sensitivity to alcohol-based gas, and the sensitivity to isobutane gas will also be small. L such as
This is because the sensitivity to P gas becomes smaller.

Note that when palladium alone is used as the noble metal catalyst, 0.05 to 30 times as much phosphorus may be added for the same reason as described above.

Note that the above-mentioned noble metal catalyst may be contained as a metal such as Pt, Pd, or Rh, or may be contained as a metal compound thereof.

In addition, zinc oxide (ZnO)-based, tin oxide (S
nO2)-based and iron oxide (Fe203)-based N-type metal oxide semiconductors are Z n O , Sno2, and Fe203, respectively? It is an N-type metal oxide semiconductor in which various sub-components are added, such as ZnO-Cr203, SnO2-SiO2, Fe203, etc.
- Shows A203 etc.

Similar effects can be obtained using other subcomponents.

The present invention will be explained in detail below using structural examples.

For example, as shown in cross section in FIG. 1, a pair of σ sleep electrodes 2 are provided on the outer peripheral surface of a cylindrical insulating substrate 1, and a gas sensitive body 3 is provided so as to cover the cylindrical insulating substrate 1 and the electrodes 2. A gas-sensitive element 4 according to the present invention is obtained.

The gas sensitive body 3 is an N-type metal oxide semiconductor to which a noble metal catalyst and phosphorus are added.

Further, the gas sensing element 4 constructed as described above may be used, for example, as a second
It is assembled on the pin foot as shown perspectively in the figure.

In FIG. 2, 5 represents a lead wire, 6 represents an insulating plate, and 7 represents a heater.

The heater 7 is provided to improve the sensitivity of the gas sensitive element, and can be provided as appropriate if necessary.

Furthermore, the stability can be further improved by incorporating the above-mentioned noble metal catalyst and phosphorus together with a carrier made of silica, alumina, or a silica-aluna type compound.

Next, the gas-sensitive element according to the present invention is manufactured, for example, as follows.

First, a predetermined amount of N-type metal oxide semiconductor material is weighed out, thoroughly mixed, and then water or a binder is added and mixed.

For example, P dC4 , P d (NO s )
2 Add a noble metal catalyst in the form of a solution, etc., and add phosphorus to H2P.
A paste-like gas sensitive material having an appropriate pH and viscosity is obtained by adding inorganic phosphorus compounds such as O4 and NH4H2PO4.

Thereafter, the paste-like gas sensitive material 3 produced as described above is applied to an insulating substrate 1 provided with a pair of electrodes 2 as shown in FIG. A gas-sensitive element according to the present invention is obtained.

Below, various characteristics of the gas-sensitive element according to the present invention will be measured and shown.

First, isobutane gas (0.2 Vol%) and alcohol gas (
0.2vol%)
As shown in the figure.

In addition, Figures 3 to 11 show the sensitivity to isobutane gas.
Figures 12 to 20 show the sensitivity to alcohol, and Figures 3 to 5 and 12 to 14 show the sensitivity to alcohol when ZnO-Cr203 is used as the N-type metal oxide semiconductor. Figure 8 and Figures 15 to 17 are Sn02-S.
When i02 is used, FIGS. 9 to 11 and FIGS. 18 to 20 respectively show the case when F"e203-At203 is used.

As a result, the precious metal catalysts such as Pd, Rh, and Pt were 0.00
It was confirmed that excellent sensitivity to isobutane gas was exhibited when the amount of phosphorus added was 5 times or more by weight, and when the amount of phosphorus added was less than 30 times that of the noble metal catalyst in terms of molar ratio.

However, when the noble metal catalyst exceeds 8% by weight and the molar ratio of phosphorus is less than 1.5 times (or 0.05 times when palladium alone is used as the noble metal catalyst), it shows high sensitivity to isobutane gas. , the effect of the present invention cannot be exerted because the same sensitivity is shown to alcohol.

Furthermore, as shown in Figures 3 to 20, if the amount of the noble metal catalyst is 0.005 to 8 Wt%, the sensitivity to gas is determined approximately by the molar ratio of the noble metal catalyst to phosphorus. It is hardly affected by quantity.

Next, we measured the sensitivity to the type of detected gas when ZnO Cr203 was used as the N-type metal oxide semiconductor and the noble metal catalyst was appropriately selected.
Shown in the table.

The gas-sensitive element was constructed as shown in Fig. 1 by the above-mentioned manufacturing method. , that is, Ro/R
It was shown as g.

As is clear from Table 1, the overall sensitivity is low in the case of no catalyst, but rather the sensitivity to alcohol gas is higher than to isobutane gas.

In addition, in the case of only P or only noble metal catalyst, it is similarly sensitive to isobutane gas and alcohol.

On the other hand, the gas-sensitive element according to the present invention is hardly sensitive to alcohol and is significantly sensitive to isobutane gas, so it is clear that it exhibits excellent selectivity for isobutane gas such as LP gas. .

Furthermore, similar to Table 1, the sensitivity to detection gas when tin oxide (Sno2 Si02) is used as the N-type metal oxide semiconductor (Table 2) and iron oxide (Fe
203-Al803) is used (Table 3).

As a result, as is clear from Tables 2 and 3, by using a noble metal catalyst and phosphorus in combination as in the present invention, an excellent gas-sensitive element was obtained that was not sensitive to alcohol and was extremely sensitive to isobutane. .

In particular, when Rh is used as a noble metal catalyst in the gas-sensitive element according to the present invention, it has the following effects. RL? is hardly affected, eliminating the need for a heater voltage stabilization circuit.

2) Eliminates malfunctions (initial alarms) due to adsorption of miscellaneous gases in the air when left unused for long periods of time.

In other words, in the past, if the sensor was left unused for a long period of time, miscellaneous gases would be adsorbed to the surface of the gas sensor, and an alarm could be issued even if there was no gas to be detected when the power was turned on again.

On the other hand, in the present invention, although the element resistance decreases when left unused for a long period of time, the width of the decrease is extremely small, so no alarm is issued, and the return to the steady level is quick.

As described above, the gas-sensitive element according to the present invention is hardly sensitive to alcohol, but is significantly sensitive to isobutane gas, so when used to detect LP gas, it can perform accurate gas detection without malfunction, and can be put to practical use. It can be said that it has great utility value.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an example of the structure of a gas-sensitive element according to the present invention, FIG. 2 is a perspective view showing an example of a device using the gas-sensitive element according to the present invention, and FIGS. FIG. 3 is a curve diagram showing an example of the characteristics of the gas-sensitive element according to the invention. 3... Gas sensitive element, 4... Gas sensitive element.

Claims (1)

[Scope of Claims] 1 Comprising a pair of electrodes and a gas sensitive body provided between the electrodes, the gas sensitive body is made of an N-type metal oxide semiconductor with a weight ratio of 0.005 to 8. A noble metal catalyst consisting of at least one of platinum (Pt), palladium (Pd), and rhodium (Rh) is added and the molar ratio is 1.5 to 30 times that of this noble metal catalyst (however, palladium alone is used as the noble metal catalyst). 0.05 to 30 times) of phosphorus (9
) is added thereto. 2 In claim 1, zinc oxide (ZnO)-based, tin oxide (Sn02) is used as the N-type metal oxide semiconductor.
A gas-sensitive element characterized by using an iron oxide (Fe203) system or an iron oxide (Fe203) system. 3. A gas-sensitive element according to claim 1 or 2, characterized in that silica, alumina, or a silica-aluna compound is used as a carrier for a noble metal catalyst.