JP5010541B2 - Substrate type gas sensing element and method for producing constituent particles of gas sensitive layer - Google Patents

Description

  The present invention provides a substrate-type gas detection element in which a gas sensitive layer mainly composed of a metal oxide semiconductor is provided on an insulating substrate provided with a pair of detection electrodes so as to cover the detection electrode, and the gas sensitive layer The present invention relates to a method for producing the constituent particles.

  2. Description of the Related Art Conventionally, a substrate type semiconductor gas sensing element has a pair of sensing electrodes deposited on an alumina substrate, which is an insulating substrate, for example, and a gas sensitive layer covering the electrodes is formed. The resistance of the gas sensitive layer is changed by releasing trapped electrons due to an oxidation reaction between adsorbed oxygen on the surface of the particles constituting the film material of the gas sensitive layer and the gas to be detected. The gas to be detected is detected by measuring the change in resistance.

For example, Patent Document 1 discloses a substrate-type gas detection element in which a gas sensitive layer is formed in a porous structure in which constituent particles composed of a core particle and a metal oxide fine particle layer covering the surface of the core particle are gathered. Is described.
The core particles use insulating particles such as alumina powder and silica powder, and the fine particle layer uses fine particles such as tin oxide, tungsten oxide, indium oxide, zinc oxide, and composite metal oxide.

The average particle diameter of the core particles is, for example, about several tens of nm to several μm.
The fine particle layer is obtained by precipitating metal oxide fine particles on the surface of the core particle by a liquid phase precipitation (LPD) method. When the metal oxide fine particles are tin oxide, the average particle diameter of the fine particles is about several to several tens of nm (for example, 5 to 20 nm). At this time, the film thickness of the fine particle layer covering the core particles is several tens to several It is about 100 nm (for example, 50 to 800 nm).

  Since the constituent particles of the gas-sensitive layer in Reference 1 are formed in a porous structure in a state where the surface of the core particles is coated with a metal oxide, the space between the constituent particles as the path of the gas to be detected becomes large. The gas to be detected can be easily diffused into the gas sensitive layer, and a fast response can be obtained.

JP 2006-38564 A

The substrate type gas detection element of Reference 1 has good responsiveness, but depending on conditions such as installation location,
In addition, good gas sensitivity and response speed may be required. Furthermore, it is desired that the gas detection element has a small humidity dependency.
Accordingly, an object of the present invention is to provide a substrate type gas detection element having a gas sensitive layer having excellent response characteristics, and a method for producing the constituent particles of the gas sensitive layer.

  In order to achieve the above object, the first feature of the present invention is that a gas sensitive layer mainly composed of a metal oxide semiconductor is provided on an insulating substrate provided with a pair of detection electrodes so as to cover the detection electrodes. In the substrate-type gas detection element, the gas sensitive layer is formed of core particles mainly composed of titanium oxide, and constituent particles including a fine particle layer mainly composed of tin oxide outside the core particles. In the point.

In this configuration, the main component of the core particle that is the constituent particle is titanium oxide, and the main component of the material of the fine particle layer is tin oxide. The resistance of titanium oxide is much higher than that of tin oxide.
When such constituent particles are used as the film material of the gas sensitive layer, the current flowing between the detection electrodes is transmitted through the fine particle layer. Therefore, the electrical resistance of the substrate type semiconductor gas detection element is determined by the fine particle layer.
As shown in the examples described later, the substrate type gas sensing element of this configuration has a very fast response speed and high gas sensitivity with respect to the gas to be detected. Further, for example, tin oxide is formed on the gas sensitive layer. Compared to conventional substrate type semiconductor gas sensing elements using powder, it has stable characteristics against humidity changes.

  The second characteristic configuration of the present invention is that the thickness of the fine particle layer is 1 to 5 nm in the substrate-type gas detection element.

The desired gas sensitivity and response speed can be obtained by controlling parameters such as the particle diameter and thickness of the constituent particles forming the gas sensitive layer. In this configuration, the thickness of the fine particle layer is controlled.
If the thickness of the fine particle layer is 1 to 5 nm as in this configuration, the particle size of the coated fine particle layer becomes extremely small, so that the activity becomes high. Therefore, a gas sensing element having good gas diffusivity and high sensitivity can be obtained by the gas sensitive layer constituted by using the constituent particles having the ultra-thin particle layer as in the present configuration. Therefore, the substrate type gas detecting element of the present invention has a good response characteristic.
In the ultra-thin film state in which the thickness of the fine particle layer is 1 to 5 nm, the conduction path of conduction electrons at the neck part (grain boundary) between particles is almost closed, and the movement of conduction electrons passes through the space charge layer with high potential energy. As a result, the resistance of the gas detection element in the air increases. When surface adsorbed oxygen is consumed by the gas to be detected in such a state, the space charge layer is reduced, the transmission path of the neck portion is opened, and the resistance of the gas detection element changes drastically. Therefore, this configuration is highly sensitive.

  Furthermore, in the ultra-thin film state in which the thickness of the fine particle layer is 1 to 5 nm, the thermal diffusion of the metal element occurs at the time of firing in the vicinity of the interface between titanium oxide and tin oxide in the constituent particles. Unlike conventional tin oxide materials, the effect of humidity is less than that of conventional tin oxide sensors.

  A third characteristic configuration of the present invention is that, in the substrate-type gas detection element, palladium is added to the surface of the constituent particles.

  As in this configuration, by adding palladium to the surface of the constituent particles, the oxidation activity of the material can be further increased and the sensitivity of the substrate type gas detection element can be improved. Further, the gas selectivity of the substrate type gas detection element can be improved by reacting and removing interfering gases such as hydrogen and ethanol that are easily oxidized from the detection target gas on the surface of the gas sensitive layer.

  The fourth characteristic configuration of the present invention is that, in the substrate type gas detection element, antimony is added to the fine particle layer.

  The tin oxide in the fine particle layer is in an ultrathin state, and the electric resistance of the gas sensitive layer having such fine particles may be very high. Therefore, in this configuration, the electrical conductivity of the gas sensitive layer can be increased by adding antimony to the fine particle layer.

  The fifth characteristic configuration of the present invention is a method for producing the constituent particles of the gas sensitive layer, the first dispersion step of adding titanium oxide to the fluorostannic acid complex solution, which is the reaction mother liquor, and dispersing the boric acid aqueous solution A reaction initiation step for starting the precipitation reaction by adding a solution, a precipitation step for maintaining the temperature of the solution at room temperature and stirring the solution for 12 to 24 hours, and a structure formed by the precipitation step A first centrifugation step of centrifuging the particles, a washing step of washing the centrifuged constituent particles, a drying step of drying the washed constituent particles under reduced pressure, and the dried constituent particles from 500 to 750 And a firing step of firing at 1 ° C. for 1 to 3 hours.

According to this configuration, it is possible to produce a constituent particle in which the main component of the core particle is titanium oxide and the main component of the fine particle layer is tin oxide.
By controlling the time of the precipitation process and the temperature and time of the firing process as in the present configuration, the film thickness of the fine particle film of the constituent particles can be formed to a desired thickness. By using such constituent particles, for example, it is possible to form a substrate type gas detection element including a gas sensitive layer having desired response characteristics and humidity resistance characteristics.

  A sixth characteristic configuration of the present invention includes a second dispersion step of dispersing the cleaned constituent particles in an aqueous palladium chloride solution between the washing step and the drying step in the method for producing the constituent particles of the gas sensitive layer. Then, after the second dispersion step, a second centrifugation step of centrifuging the formed constituent particles is performed.

  According to this structure, the structure particle which added palladium to the surface can be formed. When a substrate type gas detection element having a gas sensitive layer using constituent particles in which palladium is added to the surface is formed, the oxidation activity of the material is further increased, and the gas sensitivity and gas selectivity can be improved.

  The seventh characteristic configuration of the present invention is that, in the method for producing the constituent particles of the gas sensitive layer, an antimonic acid fluoride solution is mixed with the reaction mother liquor.

  According to this configuration, antimony can be added to the fine particle layer. When the substrate type gas sensing element having the gas sensitive layer using the constituent particles having the fine particle layer to which antimony is added is formed, the electric conductivity of the gas sensitive layer can be increased.

Embodiments of the present invention will be described below with reference to the drawings.
As shown in FIGS. 1 to 3, the substrate type gas detection element 10 of the present invention has a metal oxide semiconductor as a main component on an insulating substrate 13 such as an alumina substrate provided with a pair of detection electrodes 11 and 12. A gas sensitive layer 14 for covering the detection electrodes 11 and 12 is provided. The gas sensitive layer 14 is formed of constituent particles P including a core particle p1 mainly composed of titanium oxide and a fine particle layer p2 mainly composed of tin oxide outside the core particle p1.

The resistance of titanium oxide is much higher than that of tin oxide.
When such constituent particles are used as the film material of the gas sensitive layer, the current flowing between the detection electrodes is transmitted through the fine particle layer. Therefore, the electrical resistance of the substrate type semiconductor gas detection element is determined by the fine particle layer.

The fine particle layer p2 has a thickness of 1 to 5 nm.
If the thickness of the fine particle layer p2 is 1-5 nm as in this configuration, the particle diameter of the coated fine particle layer p2 is extremely small (<thickness of the fine particle layer p2) and becomes highly active. Therefore, the gas sensitive layer 14 using the constituent particle P having the ultrathin particle layer p2 as in the present configuration can provide a gas sensing element having good gas diffusivity and high sensitivity characteristics. . Therefore, the substrate type gas detection element 10 of the present invention has a good response characteristic.
Further, in the constituent particles P, thermal diffusion of the metal element occurs near the interface between titanium oxide and tin oxide during firing, and as a result, the gas sensitivity characteristics of the material differ from those of conventional tin oxide materials. The effect of humidity is reduced.
The range in which the thickness of the fine particle layer p <b> 2 is 1 to 5 nm may be in the case where it covers the entire surface of the constituent particle P or only a part of the entire surface of the constituent particle P. .

Palladium is added to the surface of the constituent particles P.
By adding palladium to the surface of the constituent particles P, the oxidation activity of the material can be further increased and the sensitivity of the substrate-type gas detection element 10 can be improved. Further, the gas selectivity of the substrate type gas detection element 10 can be improved by reacting and removing interfering gases such as hydrogen and ethanol that are easily oxidized from the detection target gas on the surface of the gas sensitive layer 14.
Palladium is immobilized on the surface of the constituent particle P in an ultrafine particle state.

Antimony is added to the fine particle layer p2.
Tin oxide in the fine particle layer p2 is in an ultra-thin state, and the electric resistance of the gas sensitive layer 14 having such fine particles may be very high. Therefore, in this configuration, the electrical conductivity of the gas sensitive layer 14 can be increased by adding antimony to the fine particle layer p2.

  A platinum thin film heater 15 is provided on the back surface of the insulating substrate 13 in order to maintain the operating temperature of the substrate type semiconductor gas sensing element 10.

<Method for producing gas-sensitive layer constituent particles>
The constituent particles P in the gas sensitive layer 14 are formed by a liquid phase precipitation (LPD) method on the surface of the core particles p1 of titanium oxide (TiO ₂ ) serving as the core, and the fine particle layer p2 containing tin oxide (SnO ₂ ) as a main component. Is formed.

In FIG. 4, the manufacturing method of the constituent particle P of the gas sensitive layer 14 of this invention is shown.
That is, a first dispersion step A in which titanium oxide is added to and dispersed in a fluorostannic acid complex solution that is a reaction mother liquor, a reaction initiation step B in which a boric acid aqueous solution is added to start a precipitation reaction, and the temperature of the solution is set to room temperature. A precipitation step C for maintaining the precipitation reaction for 12 to 24 hours while stirring the solution, a first centrifugation step D for centrifuging the constituent particles formed by the precipitation step C, and the centrifugation Washing step E for washing the structured particles, drying step H for drying the washed component particles under reduced pressure, and firing step I for firing the dried component particles at 500 to 750 ° C. for 1 to 3 hours, I do.

  Preferably, between the washing step E and the drying step H, a second dispersion step F in which the washed constituent particles are dispersed in an aqueous palladium chloride solution, and the constituent particles formed after the second dispersion step are And a second centrifugation step G of centrifuging.

(First dispersion process)
The reaction mother liquor (fluorinated stannic acid complex solution) is obtained by dissolving stannous fluoride in ion-exchanged water and dissolving a tin oxide precipitate obtained by oxidation in a hydrofluoric acid solution. The concentration of tin in the fluorostannic acid complex solution is, for example, 0.01 to 0.3 mol / L.
The reaction mother liquor may be mixed with a fluorinated antimonic acid solution. The concentration of the antimony fluoride (HSbF ₆ ) acid solution is adjusted, for example, so that the antimony content ratio (Sb / Sn) of the precipitate is 0.01 to 0.3.
Titanium oxide uses fine particles having an average particle diameter of about 30 to 40 nm.
In the first dispersion step A, the titanium oxide ultrafine particle powder is added to the reaction mother liquor and stirred, and uniformly dispersed with an ultrasonic vibrator.

(Reaction start process)
An aqueous boric acid solution is a reaction initiator. The concentration of the boric acid aqueous solution is, for example, 0.4 to 0.6 mol / L. In the reaction start step B, the boric acid aqueous solution is added to the (dispersion) solution prepared in the first dispersion step A to start the precipitation reaction.

(Precipitation process)
The precipitation step C is performed, for example, in a thermostatic bath or the like in order to maintain the solution whose reaction has been started in the reaction start step B at room temperature. At this time, in order to prevent the core particles p1 and the constituent particles P from precipitating, precipitation is performed for 12 to 24 hours while stirring with a stirring means such as a stirrer or by inversion mixing while the reaction vessel is completely sealed. Sustain reaction. The deposition time is set so that a desired film thickness of the fine particle layer p2 can be obtained. The concentration of the reaction mother liquor and the deposition time are important parameters for determining the film thickness. In the method for producing the constituent particles P of the gas sensitive layer 14 of the present invention, the film thickness of the fine particle layer p2 can be controlled to 1 to 5 nm.

(First centrifugation step)
In the first centrifugation step D, the constituent particles P are separated from the reaction liquid after the precipitation step C by a centrifuge. The conditions for centrifugation are, for example, 2000-2500 rpm, 5-10 minutes.

(Washing process)
In the washing step E, the constituent particles P separated in the first centrifugal separation step D are washed with ion exchange water. The constituent particles P are added to an appropriate amount of ion-exchanged water, and the water and the constituent particles P are separated by a centrifuge. The centrifugation conditions are the same as in the first centrifugation step D. This washing operation is performed a plurality of times.

(Second dispersion step)
In the second dispersion step F, the fine particles washed in the washing step E are dispersed in a palladium chloride (PdCl ₂ ) aqueous solution. The density | concentration of palladium chloride aqueous solution shall be 0.01 to 0.1 weight%, for example.

(Second centrifugation step)
In the second centrifugation step G, the fine particles are separated from the dispersion solution obtained in the second dispersion step F by a centrifuge. The centrifugation conditions are the same as in the first centrifugation step D.

(Drying process)
In the drying step H, the fine particles obtained by the above treatment are dried under reduced pressure. The decompression conditions are, for example, 25 to 50 ° C. and 300 hPa or less.

(Baking process)
In the firing step I, the dried constituent particles P are fired at 500 to 750 ° C. for 1 to 3 hours. The constituent particles P obtained by pulverization after firing are used as the constituent material of the gas sensitive layer.

  The film thickness of the fine particle film of the constituent particles can be formed to a desired thickness by controlling the time of the precipitation process and the temperature and time of the firing process as in this configuration. Therefore, by using such constituent particles, it is possible to form a substrate type gas detection element having a gas sensitive layer having desired response characteristics and humidity resistance characteristics.

  The gas sensitive layer 14 of the sensing element is produced from the constituent particles P obtained by depositing the fine particle layer p2 mainly composed of tin oxide on the core particle p1 mainly composed of titanium oxide by the LPD method, and methane or the like. A substrate-type gas detection element 10 for detecting the flammability was prepared. Examples thereof will be described below.

Low crystalline tin oxide was synthesized by reacting stannous fluoride (SnF2) with hydrogen peroxide. 0.56 g of this tin oxide was dissolved in 40 mL of a 1.6% hydrofluoric acid aqueous solution and adjusted to a total volume of 120 mL with ion-exchanged water to prepare a reaction mother liquor (fluorinated stannic acid complex solution: 0.125 mol / L). ) Was produced.
0.1 g of titanium oxide fine particles having an average particle diameter φ of 36 nm was added to the reaction mother liquor and stirred, and uniformly dispersed in the reaction mother liquor with an ultrasonic vibrator (first dispersion step A).

  To the dispersion solution prepared in the first dispersion step A, 80 mL of a 0.5 mol / L boric acid aqueous solution as a reaction initiator was added (reaction start step B).

  The precipitation reaction was carried out while maintaining the temperature of the solution at 30 ° C. inside the thermostat. The precipitation reaction was continued for 24 hours while stirring with a stirrer (precipitation step C).

  After the precipitation reaction was completed, the constituent particles P in which tin oxide was precipitated on the titanium oxide core were separated from the reaction mother liquor by a centrifuge (first centrifugal separation step D). Centrifugation was performed at 2500 rpm for 5 minutes.

  The centrifuged constituent particles P were washed with ion exchange water (washing step E). The operation of adding fine particles to an appropriate amount of ion-exchanged water and separating the water and the constituent particles P with a centrifuge was repeated 6 times.

  The washed constituent particles P were dispersed in a 0.055 mol / L palladium chloride aqueous solution (second dispersion step F). The constituent particles P were centrifuged from the dispersion solution prepared in the second dispersion step F (second centrifugation step G). The separated constituent particles P were dried under reduced pressure (drying step H), baked at 500 ° C., and then pulverized to obtain the constituent material P of the gas sensitive layer.

Ethylene glycol is added to the obtained constituent particles P to form a paste, which is applied to an alumina insulating substrate 13 provided with comb-shaped electrodes 11 and 12, and baked at 750 ° C. for 2 hours (baking step I). A gas detection element 10 was produced.
The thickness of the gas sensitive layer 14 produced at this time is 55 μm. Thus, by making the thickness of the gas sensitive layer 14 equal to that of the thick film sensor, it is possible to have good long-term stability and resistance to poisoning.

  If the operating temperature of the gas detection element is set to a high temperature such as 550 ° C., for example, a combustible gas detection element with improved oxidation activity can be obtained. On the other hand, if the operating temperature of the gas detection element is set to 300 ° C. or lower, for example, a CO detection element with a fast response speed can be obtained.

Using the substrate-type semiconductor gas detection element 10 manufactured in Example 1, the sensitivity characteristics when methane as the gas to be detected and hydrogen and ethanol as the interference gases were detected were examined. The results are shown in FIG.
As a result, it was found that the sensitivity to methane is superior to any sensitivity to hydrogen and ethanol which are interfering gases.
For methane, the sensitivity characteristics at higher concentrations were investigated. The methane concentration was set to 0 to 10,000 ppm. The results are shown in FIG.

  As a result, methane at a high concentration of 10000 ppm can be detected without any problem, and it was recognized that the substrate type gas detection element of the present invention can be suitably used even in practical use.

Using the substrate-type semiconductor gas detection element 10 produced in Example 1, the response characteristics to methane, which is the gas to be detected, were examined. The results are shown in FIG. After the substrate type semiconductor gas detection element 10 was activated, gas was injected and measured every about 100 seconds. As a result, a change in the electrical resistance was detected immediately at each time, and immediately after that, the electrical resistance was stabilized. The full response time of 90% showed a very fast response in 3 seconds or less.
As a result, the substrate type gas detection element of the present invention has a very fast response speed and high gas sensitivity to the gas to be detected even though the gas sensitive film 14 is a thick film (55 μm). It is recognized.

Humidity characteristics were examined using the substrate type semiconductor gas detection element 10 produced in Example 1. The results are shown in FIG.
FIG. 8 shows the change in the base resistance of the sensor when the humidity in the measurement chamber (20L) is replaced from RH 40% to a completely dry atmosphere with dry air (replacement speed 3.3 L / min). The solid line shows the base resistance of the substrate type semiconductor gas sensing element 10 of the present invention using the constituent particles P in the gas sensitive layer 14, and the broken line shows the conventional substrate type semiconductor gas using tin oxide powder in the gas sensitive layer. The base resistance of each sensing element is shown. Here, the sensor resistance value Rs under an arbitrary humidity and the ratio Rs / Rs (RH 40%) of the sensor resistance value Rs (RH 40%) at the humidity RH 40% indicate the fluctuation of the sensor resistance value with respect to the humidity change.
From the results of FIG. 8, it was recognized that the substrate-type semiconductor gas detection element 10 of the present invention has stable characteristics against changes in humidity.

[Another embodiment]
In the above-described embodiment, the case where the semiconductor gas detection element is a substrate type gas detection element has been described. However, the present invention is not limited to this, and it may be a hot-wire semiconductor gas detection element, a direct heat semiconductor gas detection element, an indirectly heated semiconductor gas detection element, or the like.

  The present invention can be used particularly for improving the response characteristics of a substrate type gas detection element.

Schematic of the substrate type gas detection element of the present invention Schematic diagram of constituent particles Schematic diagram of the main part of the substrate type gas detection element The figure which showed the outline of the manufacturing method of the constituent particle of the gas sensitive layer of this invention The figure which showed the result of having investigated the sensitivity characteristic of the board | substrate type | mold gas detection element of this invention The figure which investigated the sensitivity characteristic of the board | substrate type gas detection element of this invention The figure which investigated the response characteristic of the board | substrate type | mold gas detection element of this invention The figure which investigated the humidity characteristic of the board | substrate type | mold gas detection element of this invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Substrate-type gas detection element 11, 12 Detection electrode 13 Insulating substrate 14 Gas sensitive layer P Constituent particle p1 Core particle p2 Fine particle layer

Claims (7)

A substrate-type gas detection element in which a gas sensitive layer mainly composed of a metal oxide semiconductor is provided on an insulating substrate provided with a pair of detection electrodes so as to cover the detection electrode,
The gas sensitive layer is a substrate type gas detection element formed of core particles mainly composed of titanium oxide and constituent particles including a fine particle layer mainly composed of tin oxide outside the core particles.   The substrate type gas detection element according to claim 1, wherein the fine particle layer has a thickness of 1 to 5 nm.   The substrate-type gas detection element according to claim 1, wherein palladium is added to the surface of the constituent particles.   The substrate type gas detection element according to any one of claims 1 to 3, wherein antimony is added to the fine particle layer. A first dispersion step in which titanium oxide is added to and dispersed in the fluorostannic acid complex solution that is a reaction mother liquor,
A reaction initiation step of initiating a precipitation reaction by adding an aqueous boric acid solution;
A precipitation step for maintaining the temperature of the solution at room temperature and maintaining the solution for 12 to 24 hours while stirring the solution;
A first centrifugation step of centrifuging the constituent particles formed by the precipitation step;
A washing step for washing the centrifuged constituent particles;
A drying step of drying the washed constituent particles under reduced pressure;
A method for producing the constituent particles of the gas-sensitive layer, comprising a firing step of firing the constituent particles dried at 500 to 750 ° C. for 1 to 3 hours. During the washing step and the drying step,
A second dispersion step of dispersing the washed constituent particles in an aqueous palladium chloride solution;
The method for producing the constituent particles of the gas sensitive layer according to claim 5, wherein after the second dispersion step, a second centrifugation step of centrifuging the formed constituent particles is performed.   The method for producing the constituent particles of the gas sensitive layer according to claim 5 or 6, wherein the reaction mother liquor is mixed with a fluorinated antimonic acid solution.

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