JPS5847663B2 - Gas detection element with selectivity of detection gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a gas detection element having selectivity of a detection gas.

It is widely known that the resistance value of metal oxide semiconductors changes depending on the atmospheric gas, and various applications have been proposed in Japanese Patent No. 568,957 and the like.

Generally, gas detection elements using metal oxide semiconductors of this kind are sensitive to various gases indiscriminately, so when they are actually used by incorporating them into alarms or other gas detection devices, Although the type of gas to be detected should naturally be limited to some extent depending on the application,
There was a risk that the device would become sensitive to other gases and cause the device to malfunction.

In other words, if the above-mentioned element is used in a device that detects gas leaks from propane gas for general household use, such a device would originally detect gases such as propane and butane, which are classified as household propane gas fuel. Although the device should only operate when gas is included in the atmosphere, the device is sensitive to gases that do not require detection, such as alcohol generated from alcoholic beverages in ordinary households and smoke generated during cooking. This often causes the device to malfunction in a way that is not intended for its intended purpose.

Conventionally, as a countermeasure to this problem, Japanese Patent Application Laid-Open No. 49-12959
Publication No. 6 proposes a method of discriminating gas sent to a gas sensing section using a filter made of an adsorption activator or a combustion filter made of palladium asbestos, and also discloses an element with a filter used in this method. ing.

The device shown in this publication has a pedestal 22 as shown in FIG.
A metal oxide semiconductor gas sensing section 21 supported by three pins 23 on top, palladium asbestos 24 surrounding the gas sensing section 21 at a constant interval so as not to come into contact with the gas sensing section 21, and a molecular sieve. adsorbent 25, wire mesh 26,
A metal cap 27 is provided.

However, according to this structure, since the filter (palladium asbestos 24) is separated from the gas sensing section 21, a means for holding the filter in a predetermined shape is required in addition to the pedestal 22 that supports the gas sensing section 21. Manufacturing is troublesome.

In addition, in terms of performance, there is a drawback that gases other than the gas that comes into contact with the gas sensing section 21 (gas that simply passes through the space between the gas sensing section 21 and the filter) are discriminated, which places an extra burden on the filter.

Furthermore, since heat conduction is hindered by the space between the gas sensing section 21 and the filter, the gas sensing section 21 simply
If the filter is simply heated by the radiant heat generated by the heating, it is actually difficult to heat the filter to a temperature suitable for activating the catalyst when a combustion type filter such as palladium asbestos 24 is used as the filter. However, there was a major problem in that the gas selection detection function was not fully demonstrated.

In addition, the earlier patent application No. 1983-24777
No. 120298) shows a device in which an oxidation catalyst filter is integrated with a gas detection semiconductor;
Since the filter used the same type of material as the gas detection semiconductor, there was little freedom in selecting the filter material, and it was difficult to choose a material that was inexpensive and had excellent oxidation activity.

The invention of this earlier application is based on the inference that when different types of metal oxides are integrated based on valence control theory, deterioration occurs due to a solid phase reaction between the two, and the filter uses the same type of material as the semiconductor for gas detection. That is.

In other words, conventionally, based on the above reasoning, when an oxidation catalyst filter is used integrally with a metal oxide semiconductor gas detection section, the filter and the gas sensing section are made of the same material, and the gas sensing section and the filter are made of different materials. It is common knowledge that when they are made of raw materials, they are separated from each other, and for this reason, the above-mentioned drawbacks of each of the prior art techniques have not been able to be overcome.

However, the present inventor has found that even if the metal oxide semiconductor gas sensing portion is integrally coated with different metal oxide catalysts, no significant solid phase reaction as previously assumed occurs, and at least the gas sensing portion is It was experimentally discovered that this effect does not affect the inside of the body.

Based on these facts, the present invention makes it possible to selectively detect a specific type of gas by using a metal oxide semiconductor as a gas sensing part and a filter, and in particular, provides the filter with inexpensive and excellent oxidation activity. A gas sensing element that uses a transition metal oxide catalyst with a transition metal oxide catalyst and integrates this filter and a gas sensing part, which facilitates production and improves performance, and which does not cause adverse effects due to solid phase reactions, etc. It provides:

That is, the present invention includes a gas detection element main body in which a pair of electrodes are connected to a metal oxide semiconductor whose resistance value changes depending on gas, and a gas selective permeability filter using a transition metal oxide catalyst different from the semiconductor. and the filter,
A heater is provided which is located outward from the current path between the electrodes and is substantially separated from the current path, and which directly covers the gas sensing element body and heats the gas sensing element and the filter. It is characterized by

Materials for the gas selective permeability filter used in the present invention include Nip, Co203, Cod, ZnO,
Various transition metal oxidation catalysts such as Fe203, V205, Ag20, CuO, etc., and Pd, Pt in these transition metal oxides
, Ag, Rh, Ir, Ru, or the like may be added.

Depending on its ability as an oxidation catalyst, such a filter oxidizes a specific type of flammable gas with 02 in the atmosphere and converts it into an inert gas, thereby preventing its passage as a combustible gas. It selectively allows only gases that are difficult to oxidize to pass through. In this case, the type of gas that selectively passes through the filter depends on the oxidizing ability of the filter, which is determined by conditions such as the type of filter material and the heating temperature of the filter. do.

In order to clarify the selective gas permeability of such a filter, FIG. 1 is a graph showing the relationship between the oxidation ability of a filter using an oxidation catalyst and the permeability to various gases.

In the same graph, the vertical axis represents the gas permeability, and the horizontal axis represents the oxidation ability of the filter, that is, the activity as an oxidation catalyst. Curves A to E in the same graph represent CO gas and B fJ5H2 gas, respectively. , C shows the relationship between the filter's oxidizing ability and transmittance for ethanol, D for isobutane, and E for methane.

As shown in the same graph, among these gases, CO gas is most easily oxidized, followed by H2 gas, ethanol, isobutane, and methane, which have lower oxidation reactivity in that order. This determines the type of gas that is blocked and the type of gas that passes through it.

In other words, if the oxidation ability of the filter is low, for example as indicated by the symbol a in the same graph, CO gas will be oxidized and converted to CO2, so the permeation as CO gas will be almost completely blocked, and other The gas passes through almost unchanged.

In addition, if it is a level indicated by a slightly higher sign b than this, H2 gas, which is the next most easily oxidized together with CO gas, is H2 gas.
20, the permeation is blocked, and gases other than these are selectively permeated.

Furthermore, at the level indicated by symbol C, alcohol is also oxidized and isobutane and methane are permeated, and at the degree indicated by symbol d, only methane is selectively transmitted.

In addition, various metal oxide semiconductors can be used as the material for the gas sensing portion, but it is particularly preferable to use SnO2 or In203 because high sensitivity can be obtained.

An element of the present invention is an integrated form of such a filter and a gas sensing section, and embodiments of the present invention will be described in detail below with reference to the drawings.

In Fig. 2, 1 is a cylindrical container made of an insulating material such as ceramic or glass, which has an appropriate density and thickness to prevent gas from permeating through its peripheral wall, and is open at both ends. There is a formal precept.

The center of the interior of the container 1 is filled with a gas sensing section 2 whose main material is a metal oxide semiconductor whose resistance value changes by adsorbing a reducing gas.

ALI I P t 2P is provided at both ends of the sensing section 2.
It is equipped with porous electrodes 3, 3 made of a noble metal such as d, etc., which are sufficiently porous to have air permeability.

The electrodes 3, 3 are made by impregnating a rough-textured alumina or glass material with chloride such as Au, Pt, or Pd dissolved in water and baking it, or by baking alumina powder or glass powder into gold paste. It is shaped into a certain shape by mixing it with an appropriate mixture and baking it.

Filters 4, 4 using a transition metal oxide catalyst are integrated with the gas sensing element main body, which is composed of the gas sensing section 2 and electrodes 3, 3, outward from the current path between the electrodes 3, 3. In this embodiment, both electrodes 3,
Filters 4, 4 are filled on the outside of 3.

Lead wires 3a, 3a connected to the electrodes 3, 3 are led out to the outside of the filters 4, 4.

Further, a coil-shaped heater 5 is provided on the outer peripheral surface of the container 1, which allows the gas sensing section 2 and the filter 4,
4 is heated to an appropriate temperature.

In the illustrated example, the heater 5 is wound around the entire outer circumferential surface of the container 1 at regular intervals to uniformly heat the gas sensing part 2 and the filters 4, 4, but the heater 5 is wound around the entire outer circumferential surface of the container 1 at regular intervals to uniformly heat the gas sensing part 2 and the filters 4, 4. It is also possible to make the temperature conditions different between the gas sensing section 2 and the filters 4, 4 by changing the spacing between the heater coils, or by partially equipping the center or both ends of the outer peripheral surface of the container with a heater.

FIG. 3 shows another embodiment of the device of the invention.

In this embodiment, a rod-shaped core body 16 formed of an insulating material such as ceramic or glass is attached to the center axis of the cylindrical container 11.
is inserted, and electrodes 13, 13 made of noble metals such as Au, Pt, Pd, etc. are adhered to the circumferential surface of the core body 16, leaving only the central part, and the core body 16 and the container 11 are connected to each other. A gas sensing section 12 is filled in the center of the space between the gas sensing section 12 and the surrounding wall, spanning both the electrodes 13, 13, and filters 14, 14 are filled on both sides thereof.

Further, a film-like heater 15 is coated on the outer peripheral surface of the container 1.

This may be a coil-shaped heater as described above, but if a membrane heater is used, a high resistance one can be obtained, so there is an advantage that a commercial power source can be used as is without using a transformer.

In such an element, for example, an electrode 1 is first attached to the core body 16.
3 and 13 by printing or the like, then the material of the gas sensing part 12 is applied to a predetermined thickness by printing or other means and then baked, and then the filters 14 are attached on both sides. .14 and then compressed into the container 1 for final baking.

In this structure, both the electrodes 13, 13 are connected to the core 16.
Since it is led out of the element in a state where it is carried by a holder, it is convenient to incorporate it into a circuit if this part is clamped by pins 17 and 17 for leads.

FIG. 4 shows yet another embodiment, in which the core body 16' is shaped like a pipe in the structure shown in FIG.
A coil heater 15' is inserted into it.
Other structures and cutting means are substantially the same as those in the embodiment shown in FIG. 3 above.

In each of the embodiments shown in FIG. 3 and FIG. It has a structure located outward from the current path and substantially separated from the current path.

According to the element configured in this way, a specific type of gas can be selectively detected depending on the application.

That is, by the filters 4, 4 or 14, 14,
Depending on its oxidizing ability, only one or several specific gases that cannot be oxidized are transmitted and sensed by the gas sensing section 2 or 12.

In this case, the oxidizing ability of the filter is increased by the addition of noble metals even at the same set temperature, and increases with the amount of the added amount, and even with the same material, the higher the heating temperature is, the higher the oxidation ability is.

Therefore, by appropriately selecting the material and heating temperature of the filters 4, 4 or 14, 14, for example, the symbol C in the graph of FIG.
If it has the oxidizing ability shown in , isobutane,
Since methane is selectively detected, it is suitable for a propane gas leak detection device.

Further, if the device has an oxidizing ability as indicated by the symbol d in the graph of FIG. 1, a selective gas detection function suitable for various uses, such as being suitable for a gas detection device exclusively for methane gas, can be obtained.

In particular, the present invention significantly improves the manufacturability and performance of an element having such a gas selection function.

That is, using a transition metal oxide catalyst that is inexpensive and has excellent oxidation activity in the filters 4, 4 or 14, 14,
In addition, the filters 4, 4 or 14, 14 are directly coated and integrated with the main body of the gas sensing element consisting of the metal oxide semiconductor gas sensing section 2 or 12 and the electrodes 3, 3 or 13, 13.

Therefore, the structure is simple and compact, and the filter can be easily attached and manufactured.

Furthermore, since the filters 4, 4 or 14, 14 select only the gas that comes into contact with the gas sensing section 2 or 12, no extra burden is placed on the filters.

Further, since the filter is efficiently heated by the heating action of the heater and the heat conduction from the gas detection element body, the catalyst performance is also improved.

In addition to the fact that the present inventor has noted that in this element, even if dissimilar metals come into contact with each other due to the coating, the filter 4
, 4 or 14, 14 are located outside the current path between the electrodes and are covered in a manner that does not substantially contact the current path, so structurally speaking, it is certain that there will be no adverse effects such as deterioration of the gas sensing part. is prevented.

That is, according to the structure shown in FIG. 2, the gas sensing section 2 and the filters 4, 4 are partitioned by the porous electrodes 3, 3, and solid phase reactions are prevented.

In addition, as shown in FIGS. 3 and 4, filters 4 and 4
Even if the metal oxide is coated in direct contact with the gas sensing part 2, the reaction will not be as pronounced as when dissimilar metal oxides are mixed uniformly, and some solid phase reaction will occur at the boundary between the two. However, since this part is located outside the current path,
There is no side effect on the gas detection function.

In addition, the device of the present invention can be modified in various designs in addition to the above-mentioned embodiments. For example, the device may be designed without using the container 1 or 11, and with the surface of a metal oxide semiconductor gas sensing portion having an electrode and a heater embedded therein. A structure may also be adopted in which the entire filter is covered with a transition metal oxide semiconductor.

Furthermore, when forming a gas detection device by combining the element of the present invention with various electric actuators, etc., by arbitrarily adopting various gas detection circuit configurations as known in the past, When this element detects gas, it is possible to operate the electric actuator accordingly, but various other circuit configurations are also possible.

For example, by connecting a resistor to each of two elements, the element of the present invention and the conventional element, and arranging an electric actuator between them to form a bridge circuit, the output for the gas detected by both elements can be adjusted. If it is canceled out, the output for operating the electrically actuated object will be:
Conversely, only the output for gases that are not sensed by the element of the present invention is extracted, and in this case, it is possible to selectively detect CO gas and the like that are easily oxidized and absorbed.

Alternatively, by assembling a bridge circuit as described above using two elements of the present invention, and making the oxidation abilities of the filters in both elements different to make the degree of gas selectivity of both elements different, one element can be If an output corresponding to only a specific gas that is detected by one element and not detected by the other element is extracted, it becomes possible to detect any specific single gas.

As described above, the element of the present invention is capable of selectively detecting a specific type of gas by the gas screening effect of the filter using a transition metal oxide catalyst, and moreover, in particular, Since the above-mentioned filter is integrally coated on the outside of the current path, manufacturing efficiency can be improved and costs can be reduced, the performance of the filter has also been improved, and adverse effects such as deterioration of the gas sensing part can be prevented. It is something that can be done.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the relationship between the oxidizing ability of the filter used in the device of the present invention and the permeability to various gases, and FIGS. 2 to 4 are cross-sectional views showing examples of the device of the present invention, and FIG. FIG. 5 is a sectional view showing an example of a conventional element. 2, 12... Gas sensing section, 3, 13...
・Electrode, 4 14... Filter.

Claims (1)

[Claims] 1. A gas detection element body comprising a pair of electrodes connected to a metal oxide semiconductor whose resistance value changes depending on gas, and a gas selective permeability filter using a transition metal oxide catalyst different from the semiconductor, the filter is located outward from the current path between the electrodes and is substantially separated from the current path, and is directly coated on the gas sensing element body, and a heater that heats the gas sensing element and the filter. 1. A gas detection element having selectivity of a detection gas.