JPH03216541A - Detector for gas sensor - Google Patents

Abstract

PURPOSE:To contrive a power saving and a smaller size by a method wherein a thin film of a metal heating body is formed on a ceramic fine line and a semiconductor layer is further formed thereon as gas sensitive film. CONSTITUTION:When a thin film 5 of a metal heating body formed on a ceramic fine line 3 is energized electrically to generate heat, a semiconductor film 9 is heated up to 200-500 deg.C through a insulating body layer 7 and as a reducing gas is adsorbed, oxygen in the gas is dissociated to lower an electric resistance of the semiconductor film 9. A change in the resistance is detected with a circuit connected to counter electrodes 11 and 11. Thus, the electric resistance can be increased by using the metal heating body in a thin film 5 to reduce a sectional area, which achieves a smaller size and a power saving. It also should be noted that material of the semiconductor film 9 can employ one or two or more of stannic oxide, iron oxide, titanic oxide, indium oxide and zinc oxide and moreover, it is desirable to dope to control the valence of a metal atom composing the semiconductor.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a detection element used in a gas sensor that detects gas in the air, and more specifically to a gas sensor that detects a reducing gas by a change in the electrical resistance of the element. The present invention relates to a sensing element used for.

[Problems to be solved by the prior art and the invention] Gas sensors used to detect various reducing gases, such as gas leak alarms for LP gas, can be broadly classified into:
There are catalytic combustion type and semiconductor type, both of which utilize the fact that when a gas to be detected comes into contact with a sensing element, the electrical resistance of the element changes.

As shown in FIG. 9, the catalytic combustion type heater has a ceramic layer 27 made of alumina or the like sintered around a platinum wire coil heater 25, and a catalyst is supported on the surface of the ceramic layer 27. The temperature rise caused by the flammable test gas interacting with the catalyst and burning on the heated platinum wire is detected as a change (increase) in the electrical resistance of the platinum wire.

There are two types of semiconductor type devices: one called a counter electrode type, and the other called a bulk type or hot wire type.

As shown in FIG. 10, the counter electrode type has a coil heater 25.
The n-type semiconductor SnD.
A thin layer 29 of a gas-sensitive sintered body made of or 2nO is provided, and counter electrodes 11 and 1l are provided at both ends of the thin layer 29 (19 and 19 are lead wires). On the other hand, the hot wire type is the 11th
As shown in the figure, a sintered body 31 made of the same semiconductor as above is fixed around a platinum wire coil heater 25. Both semiconductor types utilize the fact that the electrical resistance of the semiconductor decreases when the gas to be detected comes into contact with the semiconductor, but in the case of the hot wire type, the parallel combined resistance of the semiconductor and heater becomes the electrical resistance of the element.

For both catalytic combustion type and semiconductor type, the element temperature is 200 to 500℃.
It is equipped with a heater in order to operate at a high temperature. Therefore, the power consumption of the element is large, and particularly in portable sensors operated by batteries, the battery is consumed rapidly, which has been a problem.

For this purpose, we have to downsize the element and increase the resistance.
Efforts have been made to save power by lowering the operating temperature, etc., but because conventional devices use coil-shaped heaters as mentioned above, there is inevitably a limit to miniaturization. .

In contrast, the counter-electrode type described above does not coil the heater, but instead provides an insulating layer around a single straight heater wire, and coats a gas-sensitive film made of semiconductor on top of the insulating layer. A corresponding one has been proposed (Japanese Unexamined Patent Publication No. 1999-1-
No. 18052, Japanese Unexamined Patent Publication No. 118053). The heater wire that has been put into practical use is 20 μm thick and 1 mm long.
The power consumption has been reduced to about 70 mW.

However, in order to further reduce power consumption by making the heater wire thinner, there is a limit to the thinning of the heater wire, and there are difficulties in terms of manufacturing technology.

Therefore, an object of the present invention is to provide a detection element for a gas sensor that is further miniaturized and consumes less power.

[Means to solve the problem]

In view of the above issues, as a result of intensive research, the present inventors succeeded in reducing the size and saving of the sensing element by using a thin film of a metal heating element on a thin ceramic wire instead of the conventional heater such as a platinum wire. They discovered that it was possible to realize electric power, and completed the present invention.

In other words, the first detection element for a gas sensor of the present invention has a thin film of a metal heating element formed on a thin ceramic wire, and a semiconductor layer whose electrical resistance changes depending on the gas to be measured is further formed thereon as a gas sensitive film. It is characterized by An insulating layer may be formed between the thin film of the metal heating element and the semiconductor layer, in which case counter electrodes are provided at both ends of the semiconductor layer.

Further, the second detection element for a gas sensor of the present invention is characterized in that a thin film that generates heat and whose electrical resistance changes depending on the gas to be measured is formed as a gas-sensitive film on the ceramic thin wire.

Furthermore, the third detection element for a gas sensor of the present invention is formed by forming a thin film of a metal heating element on a ceramic thin wire, and further forming a ceramic layer supporting a catalyst that burns the gas to be measured on top of the thin film, so that the gas is combusted. The gas is characterized in that the gas can be detected by a change in the electrical resistance of the thin film.

〔Example〕

Hereinafter, the present invention will be explained with reference to the drawings.

FIG. 1 shows a sensing element according to a first embodiment of the present invention. (
A) is a longitudinal sectional front view of the sensing element 1, and RA) shows a longitudinal lateral side along the vV line in FIG.

This embodiment corresponds to a counter electrode type semiconductor sensing element.

A thin film 5 of a metal heating element is formed on the entire outer periphery of a thin wire 3 made of ceramic, and a gas-sensitive semiconductor film 9 is further formed on the thin film 5 with an insulating layer 7 interposed therebetween. Further, counter electrodes 1l and 11 are provided at both ends of the semiconductor film 9.

During use, when the thin film 5 of the metal heating element is energized to generate heat, the semiconductor film 9 is heated to 200 to 500°C. When a reducing gas is adsorbed there, oxygen in the adsorbed gas is desorbed, and the electrical resistance of the semiconductor film 9 decreases. The resistance change is detected by a circuit connected to the opposing electrodes 11, 11.

The ceramic thin wire 3 must have heat resistance and electrical resistance higher than the thin film 5 of the metal heating element so as not to generate heat itself, and must have low thermal conductivity so as not to absorb the heat of the thin film 5 of the metal heating element. Must be.

Such materials include alumina, silicon carbide, silicon nitride, zirconium oxide, silicon oxide, aluminum nitride,
Ceramics such as mullite, magnesia, titania, titanium nitride, and boron nitride are used.

It is desirable that the diameter of the ceramic thin wire 3 is 1 + nm or less. If it is made thicker than the first cylinder, the amount of the thin film 5 of the metal heating element formed thereon will naturally increase, and the amount of heat generated will increase.

Then, the reference resistance of the semiconductor element becomes too small, resulting in increased power consumption.

As the thin film 5 of the metal heating element, a metal with high electrical conductivity and good corrosion resistance is used. Such metals include Pt
, Pt alloy, Ni-Cr alloy, Fe-Aj2 alloy, W
It is appropriate to use 1W alloy or the like.

The thickness of the thin film 5 of the metal heating element is desirably 20 μm or less, more preferably 500 A to 20 μs. If the thickness is less than 500, the performance of the thin film will become unstable. On the other hand, if it is thicker than 20 μm, there is no problem in terms of performance, but it is more advantageous to use a conventional metal heater wire with a thickness of about 20 tm.

As a method for coating the thin ceramic wire 3 with the thin film 5 of the metal heating element, a plating method, a vapor deposition method, a spatter method, etc. are used. In particular, when sputtering is used, the range of materials that can be selected for the thin film 5 of the metal heating element is widened.

As a material for the semiconductor film 9, one or more of tin oxide, iron oxide, titanium oxide, indium oxide, and zinc oxide, which are known as gas-sensitive substances, is used. Alternatively, it is preferable to further add impurities to these semiconductors to control the valence of metal atoms constituting the semiconductor to lower the electrical resistance and increase the sensitivity. For example, for tin oxide, replace Nb or sb with Nb203 or Sb2
It is added in the form of 0a to form a solid solution.

FIGS. 2(a) and 2(b) show a sensing element according to a second embodiment of the present invention. This embodiment corresponds to a hot wire type semiconductor sensing element. A thin film 5 of a metal heating element is formed on the entire outer periphery of a thin ceramic wire 3 similar to that of the first embodiment, and a semiconductor film 9 is further formed thereon. The same materials as mentioned above can be used for each part.

The operating principle is the same as in the first embodiment, and the change in electrical resistance of the semiconductor film 9 when gas is adsorbed is detected.

However, in this case, the parallel combined resistance of the semiconductor M9 and the metal heating element thin film 5 is the electrical resistance of the entire element.

Therefore, the electricity of the semiconductor film 9 and the metal heating element thin film 5? The closer the resistance is, the better the detection sensitivity will be. Generally, the electrical resistance of a semiconductor film is higher than that of a metal heating element, but there is a limit to lowering the resistance of the semiconductor film. However, in the sensing element of the present invention, the resistance of the thin film of the metal heating element can be increased by reducing the thickness of the thin film 5 of the metal heating element.

FIGS. 3(a) and 3(b) show a sensing element according to a third embodiment of the present invention. In this case, a layer 51 is formed over the entire outer periphery of the ceramic thin wire 3, which is a heating element and also has gas sensitivity because its electrical resistance changes in response to various gases. A material with such properties for layer 51 is Run■. This embodiment provides a sensing element with a simple configuration.

FIGS. 4(a) and 4(b) show a sensing element according to a fourth embodiment of the present invention. This embodiment corresponds to a catalytic combustion type sensing element. A thin film 5 of a metal heating element is placed on the entire outer periphery of the ceramic thin wire 3.
A porous ceramic layer 13 made of alumina or the like supporting a gas combustion catalyst such as platinum is further formed thereon. During use, the thin film 5 of the metal heating element is heated by electricity, and inside the heated ceramic layer 13,
The flammable gas to be detected interacts with the catalyst and burns. The resulting temperature rise changes the electrical resistance of the thin film 5 of the metal heating element (
increase)

FIGS. 5(a) and 5(b) show a sensing element according to a fifth embodiment of the present invention. This is a modification of the second embodiment, in which a metal heating element thin film 5 and a semiconductor film 9 are formed only on one half of the outer periphery of the ceramic thin wire 3. In this way, each layer provided on the ceramic thin wire 3 does not necessarily have to be provided on the entire outer periphery, but a sufficient function as a sensing element can be obtained. If such a structure is adopted when the difference in thermal expansion between the ceramic thin wire 3 and the films 5 and 9 thereon is large, separation between the layers can be prevented. This configuration in which each layer is provided only on half the circumference can be applied not only to this embodiment but also to the first, third, and fourth embodiments described above.

Example 1 As shown in FIG. 6, a platinum heating element film 5 with a thickness of 0.2 μm was formed on the surface of a fine alumina wire with a diameter of 10 μm by sputtering. Furthermore, on top of that, Sbz to Sn02
An n-type semiconductor film 9 containing 1% Oi as a solid solution was formed to have a thickness of about 1 μs and a length of about 2 mm by an RF sputtering method, thereby producing a hot wire type sensing element 1.

Further, the semiconductor element 1 is sintered and fixed to the ceramic support substrate 22 with platinum paste 17, and the lead wires 19
were connected to form the gas detection section 21.

Next, as shown in FIG. 7, the power supply 23 and reference resistor 24 are connected.
The gas detection unit 21 is incorporated into an electric circuit having a
supplied.

The results of measuring voltage changes at that time are shown graphically in FIG. As can be seen from this result, as the ring gas concentration increased, the voltage increased, and the gas sensitivity was good. Also,
When the gas concentration was 100 ppm, the power consumption of the sensing element was as low as 20 mW.

〔Effect of the invention〕

As explained above, in the sensing element of the present invention, instead of the conventional heater made of metal such as platinum wire, a thin film of a metal heating element is provided on a thin ceramic wire. The thin film of the metal heating element is formed by a plating method, vapor deposition method, sputtering method, etc., and its thickness can be freely controlled from submicron order to micron order.

Therefore, even materials that are difficult to make into thin wires due to manufacturing technology, or materials whose specific resistance is so low that they cannot be made sufficiently high in electrical resistance even when made into thin wires, can be applied as thin-film metal heating elements with high electrical resistance. Expands the selection range.

Furthermore, by making the metal heating element a thin film and reducing its cross-sectional area, electrical resistance can be increased, so a gas sensor with low power consumption can be manufactured.

Furthermore, as a method of miniaturizing sensors, a method using fine pattern formation technology used in the manufacture of integrated circuits has been attempted, but compared to such methods, the method of the present invention requires a much simpler process. It has the advantage that it can be manufactured using

[Brief explanation of drawings]

FIGS. 1(a) and 1(b) show a sensing element according to a first embodiment of the present invention, where (a) is a longitudinal sectional front view, and (b) is a vertical sectional front view.
FIGS. 2(a) and 2(b) show a sensing element according to a second embodiment of the present invention, FIG. 2(a) is a longitudinal sectional front view, and FIG. is (a)
3(a) and 3(b) show a sensing element according to a third embodiment of the present invention, FIG. 3(a) is a longitudinal sectional front view, and FIG. FIG. 4(a) is a longitudinal sectional side view taken along the line xx in FIG. 4(a), and FIG. ] is a longitudinal sectional side view along Y-Y-1m in (a), and FIG.
) and (b) show a sensing element according to a fifth embodiment of the present invention, (a) is a longitudinal sectional front view, and (b) is a Z in (a).
6 is an embodiment of the hot wire type sensing element according to the present invention; FIG. 7 is an electric circuit diagram incorporating the sensing element of FIG. 6; FIG. FIG. 8 is a graph showing the results of measuring gas sensitivity using the sensing element of the present invention, and FIGS. 9, 10, and 11 are perspective views showing conventional sensing elements. 1... Detection element 3... Ceramic thin wire 5... Metal heating element thin film 51... Heating element layer 7... Insulator layer 9... Semiconductor film 11... Counter electrode 13...・Ceramic layer 17...Platinum paste 19...Lead wire 21...Gas detection section 22...Ceramic support substrate 23...Power source 24...Reference resistor 25...Coil heater 27...・Ceramic layer 29...Sintered body layer 31...Sintered body Applicant Riken New Technology Corporation Agent Br Tachibana Takaishi, Physician Figure 1 (a) (b) Figure 2 (a) ( b) 1-w (a) (b) Figure 6 Figure 9 Figure 19 Figure 11

Claims (6)

[Claims] (1) A sensing element for a gas sensor, characterized in that a thin film of a metal heating element is formed on a ceramic thin wire, and a semiconductor layer whose electrical resistance changes depending on the gas to be measured is further formed thereon as a gas-sensitive film. (2) The sensing element according to claim 1, wherein an insulating layer is formed between the thin film of the metal heating element and the semiconductor layer, and counter electrodes are provided at both ends of the semiconductor layer. . (3) The sensing element according to claim 1 or 2, wherein the semiconductor layer is made of one or more of tin oxide, iron oxide, indium oxide, and zinc oxide. (4) The sensing element according to claim 3, wherein the semiconductor layer is further doped with an impurity that controls the valence of metal atoms constituting the semiconductor. (5) A sensing element for a gas sensor, characterized in that a thin film that generates heat and whose electrical resistance changes depending on the gas to be measured is formed as a gas-sensitive film on a ceramic thin wire. (6) Forming a thin film of metal heating element on ceramic thin wire,
A detection element for a gas sensor, characterized in that a ceramic layer supporting a catalyst for burning the gas to be measured is further formed thereon, and the gas can be detected by a change in the electrical resistance of the thin film due to combustion of the gas.