JP4423127B2 - Semiconductor gas detector - Google Patents

Description

  The present invention relates to a semiconductor-type gas detection element that includes a gas-sensitive layer that covers a detection electrode and is provided so as to be in contact with a gas to be detected.

  Conventionally, as this type of semiconductor gas detection element, for example, a substrate type semiconductor gas detection element has been known. In the gas detection element, for example, a pair of detection electrodes is vapor-deposited on an alumina substrate which is an insulating substrate, 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.

  Since the specific surface area of the constituent particles of the gas sensitive layer increases as the particle size is reduced, the oxidation sensitivity of the gas sensitive layer increases and the gas sensitivity increases. Further, when the particle diameter is reduced and the size of the interparticle neck is set to the same order as the thickness of the space charge layer, the change in electric resistance becomes sharp and the gas sensitivity is further increased. Therefore, in order to increase the sensitivity of the gas detection element, it is desired that the particle diameter of the constituent particles of the gas sensitive layer is small.

  On the other hand, the resistance change of the gas sensitive layer near the electrode is the most sensitive to the bulk resistance change of the entire gas sensitive layer, so in order to increase the sensitivity of the gas sensing element, the diffusibility of the gas to be detected inside the gas sensitive layer should be reduced. There is a need to improve. The diffusibility of the gas to be detected inside the gas sensitive layer is greatly influenced by the porous structure (porous structure) of the gas sensitive layer and the particle diameter of the constituent particles of the gas sensitive layer. As the particle diameter increases, the diffusibility of the gas to be detected is improved. On the other hand, the change in electric resistance of the gas sensitive layer is reduced, and the gas sensitivity is lowered. Therefore, it is required to optimize the relationship between the particle size and the structure of the gas sensitive layer that controls the diffusibility.

  The above-mentioned substrate type semiconductor gas detection element can be classified into a thick film type gas detection element and a thin film type gas detection element according to the film thickness of the gas sensitive layer.

In the thick film type gas detection element, a metal oxide is used as a constituent particle of a gas sensitive layer, and a gas sensitive layer is formed on an insulating substrate provided with a detection electrode so as to have a film thickness of about 10 to 50 μm.
On the other hand, in the thin film type gas detection element, the gas sensitive layer is formed so that the metal oxide is a constituent particle of the gas sensitive layer and the film thickness is about 1 μm or less. This gas-sensitive layer is formed by forming a metal oxide thin film on an insulating substrate provided with a detection electrode by vacuum deposition, sputtering, liquid phase deposition (LPD), or the like. Is very small, and high gas sensitivity can be obtained.

  In addition, since the semiconductor type gas detection element used as the prior art in this invention is a general technique, prior art documents, such as a patent document, are not shown.

  The above-mentioned thick film type gas sensing element has a larger particle diameter than the constituent particles of the gas sensitive layer of the thin film type gas sensing element. However, there is a problem that the gas sensitivity is lowered because the change in electric resistance of the gas sensitive layer is reduced.

  On the other hand, the thin-film gas sensing element has a smaller particle diameter than the constituent particles of the gas-sensitive layer of the thick-film gas sensing element, so the path of the gas to be detected between the particles is small, and the diffusibility of the gas to be detected is increased. Lack. Therefore, there has been a problem that the desired gas sensitivity and response speed cannot be obtained unless the film thickness is controlled to be very thin. In addition, since the gas detection element is always exposed and poisoned in an atmosphere where various types of gases exist, the thin film type gas detection element having a thin film thickness is more easily deteriorated than the thick film type gas detection element. There was a point.

  In other words, by optimizing the relationship between the particle size of the constituent particles of the gas-sensitive layer and the structure of the gas-sensitive layer that controls diffusivity, the gas sensitivity of the gas to be detected, the responsiveness at the time of gas detection, and the durability against the poisoned gas It was difficult to make a gas sensitive part excellent in the above.

  Accordingly, an object of the present invention is to provide a semiconductor type gas detection element having a gas sensitive part excellent in gas sensitivity of gas to be detected, responsiveness at the time of gas detection, and durability against poison gas.

In order to achieve the above object, a semiconductor gas detection element according to the present invention is a semiconductor gas detection element that includes a gas sensitive layer that covers a detection electrode and is provided so as to be in contact with a gas to be detected. One characteristic configuration is that the gas sensitive layer includes constituent particles including core particles and a fine particle layer of a metal oxide that covers a surface of the core particles and has a porous structure, and the gas sensitive layer is configured as described above. It is in the point that it is formed in a porous structure in which particles are gathered.

  According to the first characteristic configuration described above, the constituent particles of the gas sensitive layer are particles in which the surface of the core particle is coated with the fine particle layer, and the average particle diameter of the fine particles covering the core particle is, for example, about several to several tens of nm. It is possible to obtain very fine particles. Therefore, the specific surface area where the fine particle layer comes into contact with the gas to be detected increases, and the oxidation activity of the gas sensitive layer increases, and the resistance value changes greatly due to the oxidation reaction with the adsorbed oxygen when the gas to be detected reaches the fine particle layer. It becomes easy to do. As a result, the bulk resistance of the gas sensitive layer is greatly changed to obtain high gas sensitivity.

Here, the electrical resistance of the semiconductor gas sensing element is determined by the metal oxide fine particle layer on the surface of the core particle. This is because the metal oxide that forms the surface of the constituent particle is sufficient if the core particle and the metal oxide fine particle layer constituting the constituent particle of the gas sensitive layer are both conductors, as long as there is a sufficient difference in resistance value. This is because current flows through the fine particle layer and current flowing through the core particle becomes small. Since very fine particles are applied to the fine particle layer, the change in electrical resistance of the fine particle layer becomes sharp, and a gas sensitive layer with higher gas sensitivity can be obtained.
Thus, in this configuration, in the constituent particles of the gas-sensitive layer, the vicinity of the surface layer of the constituent particles through which current flows is a metal oxide fine particle layer capable of controlling the specific surface area. As a result, a gas sensitive layer having high gas oxidation sensitivity and high gas sensitivity can be obtained.

  In addition, since the constituent particles of the gas sensitive layer are formed in a porous structure in which the surface of the core particle having a relatively large particle diameter is coated with a metal oxide, the constituent particles between the constituent particles that are paths of the gas to be detected are formed. The space becomes large, so that the gas to be detected can be easily diffused into the gas sensitive layer, and a quick response can be obtained.

  Further, since the gas to be detected can be easily diffused into the gas sensitive layer and a quick response can be obtained, the thickness of the gas sensitive layer can be increased to improve the durability against the poisoning gas. For example, the thickness of the gas sensitive layer can be several tens of μm, which has the same thickness as that of a conventional thick film type gas sensing element (about 10 to 50 μm). This is a semiconductor type gas detection element having a gas sensitive part that is not easily poisoned by the poison gas and has excellent durability.

Here, the conventional semiconductor gas detection element and the semiconductor gas detection element of the present invention will be compared by taking as an example the control of two characteristics of the gas sensitive layer in the gas sensitive layer, ie, diffusibility of the gas to be detected and gas sensitivity.
The diffusivity of the gas to be detected in the gas-sensitive layer of a conventional semiconductor gas detector can be improved by increasing the particle size of the constituent particles of the gas-sensitive layer. Inevitable to do. That is, since the two characteristics of the gas to be detected and the gas sensitivity are controlled by one parameter (the particle diameter of the constituent particles of the gas sensitive layer), there is a limit to improving both characteristics.

  However, in order to control the diffusivity of the gas to be detected by configuring as in the present invention, it is only necessary to adjust the particle diameter of the core particle, and to control the change in electrical resistance to increase the gas sensitivity. The particle diameter of the metal oxide fine particles may be adjusted. That is, unlike the conventional gas sensitive layer, the control of two characteristics of the diffusibility of the gas to be detected and the gas sensitivity is performed by different parameters (core particle diameter, metal oxide fine particle diameter). Therefore, both the diffusivity and gas sensitivity of the gas to be detected can be controlled independently, so that both characteristics can be improved.

  Therefore, if it is a semiconductor type gas detection element described in the first characteristic configuration of the present invention, the thick film gas detection element has excellent gas diffusibility and poisoning resistance, and the thin film gas detection element has high sensitivity characteristics. It is possible to provide a semiconductor type gas detecting element having a gas sensitive layer having both advantages.

The second characteristic configuration of the semiconductor gas detection element according to the present invention is that the core particles are insulative.
Therefore, in the case of the semiconductor type gas detection element according to the second characteristic configuration of the present invention, since no current flows through the core particles of the constituent particles of the gas sensitive layer, the metal oxide fine particle layer formed on the surface thereof Current flows reliably. Therefore, it becomes easy to control the electrical resistance of the constituent particles in the gas sensitive layer.

  A third characteristic configuration of the semiconductor gas detection element according to the present invention is that the electric resistance of the fine particle layer is set smaller than the electric resistance of the core particle.

If the electrical resistance of the core particle and the electrical resistance of the fine particle layer are about the same, the metal oxide of the fine particle layer has a higher ionization number than the metal in the fine particles of the N-type semiconductor, for example, to control the valence. The electric resistance of the fine particle layer is set small by doping with metal or the like. For example, when the metal in the fine particles is N-type semiconductor SnO _2, a group V element such as Sb is added. This makes it easier for the current to flow from the core particle to the fine particle layer, and the change in the electrical resistance of the fine particle layer can be sensitized.
Therefore, if it is a semiconductor type gas detection element as described in the 3rd characteristic structure of this invention, the electrical resistance of the structure particle of a gas sensitive layer can be controlled, and gas sensitivity can be improved.

Embodiments of the present invention will be described below with reference to the drawings.
The gas detection element of the present invention is a semiconductor type gas detection element that includes a gas sensitive layer that covers a detection electrode and is provided so as to be in contact with a gas to be detected. Examples of the semiconductor gas detection element include a substrate type semiconductor gas detection element in which a pair of detection electrodes are provided on an insulating substrate and a gas sensitive layer is provided across the pair of detection electrodes. It is not something that can be done. Examples of other semiconductor gas detection elements include a hot-wire semiconductor gas detection element, a direct heat semiconductor gas detection element, and an indirectly heated semiconductor gas detection element.

  As shown in FIGS. 1 and 2, the substrate-type semiconductor gas detection element 10 is provided with a pair of gold comb electrodes 2 and 3 deposited on the surface of the alumina substrate 1, and on these electrodes, A gas sensitive layer 5 containing a metal oxide semiconductor as a constituent component is provided.

As shown in FIG. 3, the constituent particles 51 of the gas sensitive layer 5 include core particles 51a and metal oxide fine particle layers 51b covering the surfaces thereof.
The core particles 51a are preferably exemplified by insulating particles such as alumina powder and silica powder, and the metal oxide fine particles are preferably exemplified by fine particles such as tin oxide, tungsten oxide, indium oxide, zinc oxide and composite metal oxide. However, it is not limited to this. In particular, the core particle 51a can be applied to any insulating or conductive material as long as it is a material having a somewhat lower electrical conductivity than metal oxide fine particles, such as various polymer compounds.
The average particle diameter of the core particles 51a is, for example, about several tens of nm to several μm.
The fine particle layer 51b is obtained by precipitating metal oxide fine particles on the surface of the core particle 51a by a liquid phase precipitation (LPD) method or the like. When the metal oxide fine particles are tin oxide, the average particle diameter of the fine particles is several to several tens of nm, for example, about 5 to 20 nm. At this time, the film thickness of the fine particle layer 51b covering the core particle 51a is several tens to several hundreds nm, for example, about 50 to 800 nm.
When the metal oxide fine particles are tungsten oxide (WO ₃ ), first, the surface of the alumina core is coated with titanium oxide (TiO ₂ ), and then tungsten oxide is coated thereon. In this case, since the resistance value of titanium oxide is much higher than that of tungsten oxide, the constituent particles 51 have the same properties as those obtained by directly coating the surface of the alumina core particles 51a that are insulators with tungsten oxide.
In this case, particles formed of both the alumina core and titanium oxide become the core particles 51a.

  When such constituent particles 51 are used as the film material of the gas sensitive layer 5 of the substrate type semiconductor gas detection element 10, the current flowing between the detection electrodes 2 and 3 is transmitted through the fine particle layer 51b. The electric resistance of the substrate type semiconductor gas detection element 10 is determined by the metal oxide fine particle layer 51b on the surface of the core particle 51a which is an insulator. That is, in order to control the change in electrical resistance, the particle diameter of the metal oxide fine particles may be adjusted.

The electric resistance of the fine particle layer 51b is set to be smaller than the electric resistance of the core particle 51a.
If the electrical resistance of the core particle 51a and the electrical resistance of the fine particle layer 51b are approximately the same, in order to control the valence, a metal having a higher ionization number than the metal Sn in the fine particle in the fine particle layer 51b, such as Sb, is doped. By doing so, it is possible to reduce the electrical resistance of the fine particle layer 51b.
As a result, a current easily flows from the core particle 51a to the fine particle layer 51b, and the change in the electric resistance of the fine particle layer 51b can be sensitized. Therefore, gas sensitivity can be improved.

Thus, the constituent particles 51 having the core particles 51a and the fine particle layer 51b are formed in a porous structure to constitute the gas sensitive layer 5.
Here, the porous structure refers to forming a porous structure. Therefore, a space through which the gas to be detected flows is formed between the constituent particles 51.
This configuration can be obtained by adding ethylene glycol or the like to the constituent particles 51 to form a metal oxide semiconductor paste, and applying and baking this paste on the insulating substrate 1 provided with the detection electrodes 2 and 3.
The film thickness of the gas sensitive layer 5 thus obtained is set to several tens of μm, for example, about 40 to 50 μm.

  As described above, the constituent particles 51 of the gas sensitive layer 5 are particles obtained by coating the surface of the core particle 51a with the fine particle layer 51b. The average particle diameter of the fine particles covering the core particle 51a is very fine particles of about several to several tens of nm. Therefore, the specific surface area where the fine particle layer comes into contact with the gas to be detected is increased, the oxidation activity of the gas sensitive layer 5 is increased, and the resistance value is increased due to the oxidation reaction with the adsorbed oxygen when the gas to be detected reaches the fine particle layer 51b. It becomes easy to change greatly. As a result, the bulk resistance of the gas sensitive layer 5 changes greatly, and high gas sensitivity is obtained.

As described above, the electric resistance of the substrate type semiconductor gas detection element 10 is determined by the metal oxide fine particle layer 51b on the surface of the core particle 51a. That is, since the metal oxide fine particles, which are very fine particles, are applied, the change in the electric resistance of the fine particle layer 51b becomes sharp, and the gas sensitive layer 5 with higher gas sensitivity can be obtained.
Further, since the constituent particles 51 are formed in a porous structure by depositing a metal oxide on the surface of the core particle 51a having a relatively large particle diameter, the space between the constituent particles 51 is large, and the gas sensitivity of the gas to be detected is large. Diffusion into the layer 5 is facilitated and a fast response is obtained. Such a diffusion path of the gas to be detected can be freely controlled by selecting core particles 51a having different particle diameters. That is, in order to control the diffusibility of the gas to be detected, the particle diameter of the core particle 51a may be adjusted.
Furthermore, since the selectivity of the gas to be detected also depends on this diffusivity, it is possible to control the specific gas to be detected by adjusting the particle diameter of the core particle 51a.

In addition, the gas sensitive layer 5 has a film thickness of several tens of μm, and has the same thickness as that of the conventional thick film type gas detection element (about 10 to 50 μm). Thus, the substrate-type semiconductor gas detection element 10 having a gas-sensitive portion excellent in durability and having a high durability is obtained.
That is, the semiconductor gas detection element of the present invention combines the advantages of both the excellent gas diffusibility and poisoning resistance of the thick film gas detection element and the high sensitivity characteristics of the thin film gas detection element. .

  Further, as shown in FIG. 1, a platinum thin film heater 6 is provided on the back surface of the alumina substrate 1 in order to maintain the operating temperature of the substrate type semiconductor gas detection element 10.

  As shown in FIG. 4, the substrate type semiconductor gas detection element 10 configured in this way is incorporated in a gas detection circuit 20 and constitutes a gas detection device X so as to obtain a gas detection output. Further, the gas detection output obtained from the substrate type semiconductor gas detection element 10 is input to the control unit 30, and it is determined whether or not the detected gas concentration has reached a level requiring an alarm. Here, when it is determined that an alarm is required, the control unit 30 outputs an alarm signal to the alarm unit 40 and causes the alarm unit 40 to sound an alarm buzzer, an alarm sound, and the like.

Embodiments of the present invention will be described below with reference to the drawings.
The substrate-type semiconductor gas detection element 10 having a powder obtained by depositing tin oxide (metal oxide fine particles) on alumina powder (core particles) by the LPD method is used as the constituent particles 51 of the gas sensitive layer 5. A manufacturing method is shown below.

Stannous fluoride was dissolved in ion-exchanged water, and a tin oxide precipitate obtained by oxidation was dissolved in a hydrofluoric acid solution to prepare a reaction mother liquor (tin fluoride complex).
Alumina powder was added to the reaction mother liquor, stirred, and uniformly dispersed with an ultrasonic vibrator, and then a boric acid aqueous solution as a reaction initiator was added. Tin oxide was precipitated for 10 hours with stirring. At this time, the thickness of the tin oxide particle layer (fine particle layer 51b) deposited on the surface of the alumina powder (core particle 51a) was about 100 nm, and the average particle diameter of the deposited tin oxide was 5 nm.

  Thereafter, particles having a tin oxide particle layer deposited and coated on the surface of the alumina powder from the reaction mother liquor were separated with a centrifuge (2500 rpm, 5 minutes) and washed with ion-exchanged water. The centrifugal separation and the washing treatment are repeated several times to perform sufficient washing, and then the separated particles are dried in a thermostatic bath (80 ° C., 180 minutes) and further baked at 200 ° C. to constitute the constituent particles of the gas sensitive layer 5 A powder of 51 was obtained.

Ethylene glycol is added to the powder to form a metal oxide semiconductor paste, and this paste is applied onto the insulating substrate 1 provided with the comb-shaped detection electrodes 2 and 3 and then baked at 800 ° C. for 2 hours to form a substrate type semiconductor. A gas detector 10 was prepared. At this time, the constituent particles 51 of the gas sensitive layer 5 have a porous structure and constitute the gas sensitive layer 5.
Moreover, the thickness of the tin oxide particle layer (fine particle layer 51b) deposited on the surface of the alumina powder (core particle 51a) was about 100 nm, and the average particle diameter of tin oxide was 15 nm. Moreover, the film thickness of the gas sensitive layer 5 was 40 micrometers, and the operating temperature of the gas detection element 10 was 450 degreeC.

  Using the substrate-type semiconductor gas detection element 10 produced in Example 1, the response characteristics when toluene as the gas to be detected was detected were examined. The results are shown in FIG.

The substrate-type semiconductor gas detection element 10 was exposed to toluene having different concentrations for 2 seconds, 47 seconds, 91 seconds, 138 seconds, 181 seconds, and 225 seconds after power-on. The toluene concentration at each time was 0.01 ppm, 0.03 ppm, 0.05 ppm, 0.1 ppm, 0.5 ppm, and 1 ppm, respectively. As a result, a change in the electrical resistance was detected immediately at each time, and immediately after that, the electrical resistance was stabilized. As a result, the semiconductor 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 5 is a thick film (40 μm). It is recognized.
This is because the metal oxide fine particle layer 51b of tin oxide on the core particle 51a has high oxidation activity, and the porous structure composed of the fine particle layer of 100 nm facilitates diffusion of the gas to be detected into the gas sensitive layer 5. Therefore, it is considered that the gas sensitivity and response speed are remarkably improved.

Using the substrate-type semiconductor gas detection element 10 produced in Example 1, sensitivity characteristics when toluene, ethylbenzene, and paraxylene were detected as gases to be detected were examined. The results are shown in FIG. In the figure, “Rair” means “sensor resistance in the air” and “Rs” means “sensor resistance at different concentrations of the gas to be detected”.

  As a result, it has been found that the sensor output increases as the ratio of the detected gas concentration in the air increases. Therefore, it is recognized that the semiconductor gas detection element of the present invention has excellent sensitivity characteristics with respect to various detected gases.

[Comparative Example 1]
Except that the constituent particles 51 of the gas-sensitive layer 5 are only tin oxide, a conventional thin-film gas detection element similar to the substrate-type semiconductor gas detection element 10 manufactured in Example 1 is used to detect the gas. The sensitivity characteristic when a certain toluene was detected was investigated. The gas sensitive layer was controlled by the LPD method so that the film thickness was 100 nm and 300 nm.
The result when the film thickness is 100 nm is shown in FIG. 7, and the result when the film thickness is 300 nm is shown in FIG. The operating temperature of the thin film gas sensing element was 450 ° C.
The toluene concentration at the time of gas detection by the thin film type gas detection element is 0.03 ppm, 0.07 ppm, respectively at 10 seconds, 120 seconds, 240 seconds, 360 seconds, 480 seconds, and 605 seconds after power-on. They were 0.1 ppm, 0.3 ppm, 0.7 ppm, and 1 ppm.

As a result, even when the thin-film gas sensing element was exposed to toluene, a rapid change in electrical resistance could not be detected, and even after toluene detection, the electrical resistance could not be stabilized quickly. That is, the thin film type gas detection element has a low sensitivity characteristic with respect to the gas to be detected and a slow response speed.
In particular, as shown in FIG. 8, as the film thickness increases, the diffusion of the gas to be detected in the dense thin film becomes very slow, and the gas to be detected cannot be diffused into the gas sensitive layer and is oxidized at the surface of the thin film. Therefore, the sensitivity is significantly reduced and the response speed is very slow.

[Comparative Example 2]
A gas to be detected using a conventional thick film type gas detection element similar to the substrate type semiconductor gas detection element 10 produced in Example 1 except that the constituent particles 51 of the gas sensitive layer 5 are only tin oxide. The sensitivity characteristics when toluene was detected were investigated. The gas sensitive layer was controlled by the LPD method so that the film thickness became 10 μm. The results are shown in FIG.
The toluene concentration at the time of gas detection by the thick film type gas detection element was the same as in Comparative Example 1.

  As a result, even when the thick film type gas detection element was exposed to toluene, a rapid change in electric resistance could not be detected. That is, the thick film type gas detection element has a large average particle diameter of the film and low oxidation activity, so that the sensitivity characteristic with respect to the detection gas is low and the response speed is low.

Schematic diagram of semiconductor gas detection element of the present invention Schematic cross-sectional view of the semiconductor gas detection element of the present invention Schematic cross-sectional view of the constituent particles of the gas sensitive layer in the semiconductor gas detection element of the present invention Schematic of a gas detection device using the semiconductor type gas detection element of the present invention The figure which showed the result of having investigated the response characteristic when detecting the toluene which is a to-be-detected gas using the semiconductor type gas detection element of this invention. The figure which showed the result of having investigated the sensitivity characteristic when detecting the detected gas toluene, ethylbenzene, and paraxylene using the semiconductor type gas detection element of the present invention. The figure which showed the result of having investigated the sensitivity characteristic when detecting the toluene which is a detection gas using the conventional thin film type gas detection element (film thickness 100nm) The figure which showed the result of having investigated the sensitivity characteristic when detecting the toluene which is a gas to be detected using the conventional thin film type gas detection element (film thickness 300nm) The figure which showed the result of having investigated the sensitivity characteristic when detecting the toluene which is a detection gas using the conventional thick film type gas detection element.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Insulating substrate 2, 3 Detection electrode 5 Gas sensitive layer 10 Semiconductor type gas detection element 51a Core particle 51b Fine particle layer

Claims (3)

A semiconductor type gas detection element that includes a gas sensitive layer that covers a detection electrode and is provided so as to be in contact with a gas to be detected.
The gas sensitive layer includes constituent particles composed of a core particle and a metal oxide fine particle layer which covers the surface of the core particle and has a porous structure ,
A semiconductor type gas sensing element in which the gas sensitive layer is formed in a porous structure in which the constituent particles are aggregated.   The semiconductor gas detection element according to claim 1, wherein the core particles are insulative.   2. The semiconductor type gas detection element according to claim 1, wherein an electric resistance of the fine particle layer is set smaller than an electric resistance of the core particle.

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