JPS6145368B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a compact, highly accurate, and highly responsive humidity sensing element. Among the various basic variables in nature, such as temperature, atmospheric pressure, and humidity, humidity is still difficult to measure with high precision. On the other hand, accurate and easy measurement of humidity and its adjustment are becoming necessary in the air conditioning, food industry, agriculture, and many other fields. Currently, there are methods to detect humidity as an electrical signal, such as a method that uses the amount of electricity required to dissociate adhered moisture by electrolysis, and a method that uses changes in resistance due to moisture adsorption. There is also a method that utilizes changes in capacitance due to moisture adsorption, such as the device developed by the present inventors. This humidity sensing element is superior to conventional methods in all aspects such as accuracy, sensitivity, responsiveness, hysteresis, change over time, chemical resistance, ease of handling, measurement range, and heat resistance. It is still not perfect, and has a particular drawback in that the change in capacitance value with respect to humidity does not have linearity over a range from low humidity to high humidity. The reason for this, as will be explained later, is that the silver paint layer itself on the surface of the humidity detection element has a moisture absorption and desorption effect, so that the capacitance value is accurate and accurate in response to changes in the net moisture content in the air. This is because they were unable to respond quickly. In order to improve this drawback, the present inventors have recently developed a method in which a silver layer made of pure metallic silver is formed using a silver mirror reaction to construct a humidity detection element. However, this method also has a drawback; when the humidity sensing element is immersed in Tollens reagent, the Tollens reagent may creep up the surface of the element due to capillary action, causing the silver layer to overlap with other layers. During this period, bridges due to metallic silver may be formed, increasing leakage current (hereinafter abbreviated as LC). Humidity sensing elements with large LC values can no longer maintain normal capacitance and must be disposed of as defective products. The present invention solves these conventional drawbacks, and is compact and easy to assemble, and has sufficient linearity in capacitance value with respect to relative humidity over a wide range from low humidity to high humidity. The purpose of this invention is to provide a low-cost humidity detecting element. below,
The humidity detection element according to the present invention will be explained using the drawings of FIGS. 1 to 8. FIG. 1 shows the basic configuration of the humidity detection element to which the present invention is applied. In FIG. A dielectric anodic oxide film 2 is formed on the surface of the metal base 1, and a semiconductor metal oxide film such as manganese dioxide is formed on the dielectric anodic oxide film 2. 3 is formed. However, the dielectric anodic oxide film 2 and the semiconductor metal oxide film 3 are adjacent to each other at a contact portion 4 and a non-contact portion 5. Further, on this semiconductor metal oxide film 3, an electrode 6 facing the metal base 1 is provided. Here, when moisture in the air passes through the counter electrode 6, if moisture absorption and desorption is performed by the counter electrode 6 itself, the linearity of the capacitance value with respect to humidity will be affected, and the accuracy as a humidity detection element will be affected. If water permeation is impeded, the responsiveness will be reduced, so either case is unfavorable, and the counter electrode 6 itself must have no effect on permeating water. However, in the past, the counter electrode 6 was formed from silver paint containing organic substances such as acrylic resin, and these organic substances adsorbed and desorbed moisture to some extent, causing a decrease in the accuracy of the humidity detection element. In order to solve such problems, in the present invention,
Silver mirror reaction is used as a means to form a counter electrode consisting only of pure metallic silver, but at this time, the creeping up of Tollens' reagent causes the counter electrode, metal substrate, dielectric anodic oxide film, and semiconductor material to form. If bridges due to metallic silver occur between each layer of the metal oxide film, it will cause LC failure. Below, in order to prevent LC failure as mentioned above, as a means to stop Tollens reagent from creeping up,
A specific example of the present invention in which a water-repellent paint is used will be described. First, to explain the reagents used for the silver mirror reaction, put 20 ml of 5% silver nitrate aqueous solution into a Teflon container, add 1 ml of 10% sodium hydroxide aqueous solution, and add 2% ammonia water while stirring. Dissolve silver precipitate. The resulting solution is called Tollen's reagent and contains silver in the form of an ammonia complex. Tollen's reagent was heated to 30 to 60°C, and the oxide film part of the humidity detection element, on which the semiconducting metal oxide film had been formed, was immersed in the reagent, and 1 ml of formalin solution was added at the same time for 3 to 20 minutes. put. At this time, if the humidity detection element is immersed as it is,
As shown in FIG. 2, the surface 2 of the semiconducting metal oxide film 3 and the dielectric anodic oxide film 2 is in contact with the outside air.
Since the portions a and 3a have fine irregularities, the Tollen's reagent creeps up along the concave portions to the surface 1a of the metal substrate 1 that is in contact with the outside air, and a metallic silver bridge 7 is generated along the path. Then, metal base 1
A short circuit is formed between the electrode 6 and the counter electrode 6 via the metallic silver bridge 7, and LC increases. However, when the water-repellent paint 8 is applied to the interface between the surfaces 2a and 3a of the semiconducting metal oxide film 3 and the dielectric anodic oxide film 2 that are in contact with the outside air, the Tollens reagent becomes semiconducting as shown in FIG. Only the surface 3a of the metal oxide film 3 is wetted, and metallic silver as the counter electrode 6 is generated only on the surface 3a. The mechanism of the silver mirror reaction is that, as shown in the reaction below, the silver ammonia complex in the Tollens reagent is reduced to metallic silver, which is deposited on the sample surface. HCHO+2 [Ag(NH ₃ ) ₂ ] ⁺ OH ^- →2Ag+HCO ₂ NH ₄ +3NH ₃ +H ₂ O The silver layer thus formed as the counter electrode 6 still contains nitrate radicals, sodium hydroxide, ammonia, formalin, etc. Since this adversely affects the characteristics of the humidity detection element, the element after the silver layer is formed is thoroughly washed with water and then dried before use. The silver layer thus formed is very thin, with a thickness of 1 μm or less, and is brittle, so while the characteristics as a humidity detection element are improved, care must be taken when handling it. For this reason, a thin silver layer is applied on top of the silver layer through a carbon layer or directly using low-concentration silver paint, or a thermally sprayed metal layer,
A vapor deposited metal layer or the like may also be provided. When the humidity detection element of the present invention is used, the capacitance changes in response to changes in humidity from low humidity to high humidity regions with very good linearity, making it possible to accurately measure humidity over a wide humidity range. In addition, the low yield rate, which was mainly caused by LC defects in the humidity detection element, can be improved, and as a result, the unit manufacturing cost can be reduced. By the way, when the humidity detection element of the present invention is placed in humidity, moisture in the air passes through the counter electrode 6 and the semiconductor metal oxide film 3 and reaches the interface with the dielectric anodic oxide film 2. This interface includes a contact portion 4
and a non-contact part 5, and the moisture that reaches the interface,
In the non-contact part 5, due to manganese ions and other impurities in the semiconducting metal oxide film 3 such as carbon dioxide and nitrogen oxide dissolved in the air, it acts as if it were an electrolyte. As a result, the apparent contact area increases, resulting in an increase in capacitance. In this way, depending on the amount of moisture in the air, that is, the humidity, the dielectric anodic oxide film 2
Since the apparent contact area at the interface between the semiconductor metal oxide film 3 and the semiconductor metal oxide film 3 increases or decreases, a change in relative humidity in the air can be converted into a change in capacitance. Here, when a silver layer is produced as the counter electrode 6 by the method of the present invention, the silver layer produced by silver mirror reaction is pure silver and can be formed very thin and uniformly, so It has no effect on moisture permeation. When a silver layer is formed using silver paint as in the past, the resin contained as a binder itself has a certain degree of moisture absorption and desorption properties, so it absorbs moisture in an amount that is proportional to the amount of moisture in the air. However, the capacitance value did not reach the interface between the semiconducting metal oxide film 3 and the dielectric anodic oxide film 2, and therefore the capacitance value did not correspond accurately to humidity. The problem will be resolved. Moreover, as shown in FIG. 3, since the water-repellent paint 8 is applied at least to the interface between the surfaces 2a and 3a of the semiconducting metal oxide film 3 and the dielectric anodic oxide film 2 that are in contact with the outside air, , there is no need to strictly control the liquid level of Tollens' reagent. In other words, in the past, when nothing was applied to the boundary surface, it was necessary to control the immersion depth of the humidity detection element so that the liquid level of Tollens reagent was below the boundary surface, which required a lot of effort and effort. Even with such management, LC failures caused by capillary action or vibration of the liquid level still occurred at a rate of about 10%, but this number has been reduced to zero. Next, specific examples will be given and differences in characteristics from conventional ones will be explained. Figures 4 to 7 show specific examples of the humidity detection element of the present invention, with a diameter of 0.5 mm and a length of 10 mm.
A tantalum oxide film 10 is applied to the entire surface of the metal base 9, which is made of tantalum and has a thickness of 100 mm to 1 mm, except for a 1 mm tip.
A manganese dioxide film 11 is provided on the surface of the tantalum oxide film 10 at a distance of 1 mm from the interface.
is formed with a thickness of several tens to several hundred microns. Then the fourth
From the exposed tip of the metal base 9 as shown in the figure.
A 2.5 mm portion, that is, a 0.5 mm portion from the end of the manganese dioxide film 11, was immersed in silicone oil “KF96” (viscosity 30 cps) manufactured by Shin-Etsu Chemical Co., Ltd., as shown in Figure 5. A thin water-repellent film 12 of silicone oil is formed on the surface of the immersed part. Next, as shown in FIG. 6, the entire body is immersed in Tollen's reagent, and a silver layer 13 produced by a silver mirror reaction is applied to the part where the water-repellent film 12 is not formed, that is, the surface of the manganese dioxide film 11 that is in contact with the outside air. This silver layer 13 is formed as a counter electrode as shown in FIG.
An electrode lead 14 was connected to a part of the electrode through a solder layer 15 to constitute a humidity detection element of the present invention. The water-repellent paint used in this case is not limited to the silicone oil used in this example, but also includes silicone resins including silicone varnish, Teflon resins, and hydrophobic substances such as paraffin and oils. At least one kind selected from these may be used in combination.
Also, if the viscosity is less than 10 cps, there is a risk that the paint will spread to the area where the silver layer 13 is to be provided on the manganese dioxide film 11, and if it is more than 500 cps, uneven coating will occur, which is undesirable. You may choose from. Similarly, a silver layer 13 as a counter electrode is formed using a silver mirror reaction, but a humidity detection element without water-repellent paint is used as B, and a silver layer on the carbon layer as a counter electrode is formed using acrylic resin 2. Figure 8 shows the relationship between relative humidity and capacitance when C is a conventional humidity sensing element made of silver paint with a binder. Note that A represents the humidity detection element of the present invention.

[Table] As described above, the humidity detection element of the present invention is extremely excellent in that it has excellent linearity over a wide range from low humidity to high humidity and has fewer LC defects.

[Brief explanation of the drawing]

Fig. 1 is a sectional view showing the basic structure of a humidity detection element targeted by the present invention, and Fig. 2 shows the main parts of a humidity detection element corresponding to a conventional example of the present invention in which a counter electrode is formed by a silver mirror reaction. FIG. 3 is a cross-sectional view showing essential parts of a humidity sensing element according to an embodiment of the present invention, and FIGS. FIG. 8, which is a sectional view showing the state, is a diagram showing a comparison of the relative humidity-capacitance characteristics of the humidity detecting element according to the present invention and a conventional humidity detecting element. 1... Metal substrate, 2... Dielectric anodic oxide film,
3... Semiconductor metal oxide film, 6... Counter electrode,
8...Water repellent paint.

Claims (1)

[Claims] 1. Tantalum, aluminum, titanium, niobium,
A dielectric anodic oxide film and a semiconducting metal oxide film are applied to the surface of a valve metal such as hafnium or zirconium or an alloy thereof, or a metal base such as silicon or germanium, so that the metal base and the end surface of each film are exposed to the outside air. A layer of water-repellent paint is formed on the interface between the dielectric anodic oxide film and the semiconducting metal oxide film, and a layer of water-repellent paint is formed on the semiconducting metal oxide film. 1. A humidity detection element comprising: a counter electrode having a silver layer formed by a silver mirror reaction; 2. The humidity sensing element according to claim 1, wherein at least one selected from silicone resin, Teflon resin, and hydrophobic substance is used as the water-repellent paint. 3. The viscosity of water-repellent paint is 10~10 at room temperature.
The humidity detection element according to claim 1, characterized in that the humidity detection element is 500 cps.