JPH0224464B2 - - Google Patents

Description

TECHNICAL FIELD OF THE INVENTION The present invention relates to a moisture-sensitive element, and more particularly to a moisture-sensitive element that is made by impregnating porous ceramic with a conductive compound and has little deterioration over time and is excellent in mass production. .

BACKGROUND ART Moisture sensing elements are used in a wide range of fields, including air conditioning in living environments, household appliances such as microwave ovens, agriculture, forestry, stockpiling, and medical care. Such moisture-sensitive elements include electrolyte-based elements using moisture-sensitive materials such as lithium chloride and polyvinyl alcohol, organic material-based elements using moisture-sensitive materials such as butyl cellulose acetate, and moisture-sensitive elements such as _{MgCr2O4 -} based ceramics. Metal oxide based materials have mainly been used.

Among these, Dunmore uses a polyvinyl acetate film containing lithium chloride as a moisture-sensitive material.
Type humidity-sensing elements are excellent as humidity-sensing sensors for intermediate humidity regions, with good accuracy and little change over time, but as many as five elements are required to measure humidity between 30% and 90% relative humidity. However, it also has the disadvantage of low mass productivity.

On the other hand, on porous substrates such as porous ceramics,
Moisture-sensitive elements impregnated with lithium chloride were also relatively widely used, but when used for long periods of time under high temperature and high humidity conditions, the humidity characteristics changed significantly, salt elution due to deliquescence was observed, and The drawback was that the humidity response speed was slow. Moreover, this moisture-sensitive element has a problem in that it has a low yield due to large manufacturing variations and is not suitable for mass production.

Also, Cr ₂ O ₃ system, Mn ₃ O ₄ -TiO ₂ system, Si-Na ₂ O
- Metal oxide-based humidity sensing elements such as V ₂ O _5- based ones are also relatively widely used, but each of these elements has advantages and disadvantages. At present, many problems remain.

The present inventors have conducted research on ceramic moisture-sensitive elements that have excellent humidity characteristics and thermal stability, and in the process, impregnated porous ceramics with hygroscopic aqueous solutions such as lithium chloride and potassium acetate. We prepared a moisture-sensitive element made of the same material and examined its characteristics as a humidity sensor. the result,
If these humidity sensing elements are left in an atmosphere with a humidity of 33% for a long period of time, the electrical resistance value will increase, or if they are exposed to both low and high humidity environments alternately, the relative humidity value at 30% will change significantly. This was recognized.
As a result of investigating the cause of such changes in electrical resistance, we found that the distribution of salts such as lithium chloride present in the humidity sensing element changes even though the humidity is maintained within a constant level. It was found that this was caused by Therefore, the present inventors
As a result of research aimed at solving these problems, it was discovered that these problems could be solved by using a certain type of surfactant to improve the "wettability" with the porous ceramic carrier. Further research revealed that, to our complete surprise, a porous ceramic carrier was simply impregnated with a surfactant without impregnating it with lithium chloride or potassium acetate. The present invention was completed by discovering that

Purpose of the Invention Therefore, the present invention aims to solve the drawbacks of the prior art at once, and provides a device that has uniform humidity characteristics, excellent reproducibility and stability, and that can be used under high temperature and high humidity conditions. It is an object of the present invention to provide a humidity sensing element for use in an intermediate humidity region, which shows almost no change over time and is mass-producible.

Summary of the Invention The moisture sensing element according to the present invention has a specific resistance of 150 at 20°C in a 30% aqueous solution made of porous ceramic with electrodes attached.
It is characterized by being impregnated with an ionic surfactant of Ωcm or less. This moisture sensing element is
If necessary, it may contain electrolytes such as lithium chloride, potassium acetate, and sodium sulfate. Further, a nonionic surfactant may be mixed with the ionic surfactant.

DETAILED DESCRIPTION OF THE INVENTION The ceramic used in the present invention is preferably porous and has a certain degree of porosity as well as a certain degree of physical strength. Examples of such porous ceramics include:
A material obtained by sintering silicon carbide powder and glass that does not contain a large amount of water-soluble alkali metal as disclosed in Japanese Patent Publication No. 51-42745 can be used. Of course, other porous ceramics or porous metal oxide films can also be used, for example:
Fe ₃ O ₄ colloid, Cr ₂ O ₃ system, Fe ₂ O ₃ , Al ₂ O ₃ ,
ZnO, CoO, _Mn3O4 − _TiO2 , _{Ni2O3 , ZnO−}
Li ₂ O−V ₂ O ₅ series, CuO−Cu ₂ O−Fe ₂ O ₃ , MgCr ₂ O ₄
Examples include ZnCr ₂ O ₄ series, corundum-glass, and the like.

Note that when manufacturing a moisture-sensitive element by combining refractory powder and glass, it is desirable to use glass that does not contain alkali or glass that contains alkali but has an extremely low water-soluble alkali content. This is because when using a humidity sensitive element in a high humidity area, alkali may be leached from the glass.
This is because this alkali significantly impairs the characteristics of the moisture sensitive element. Furthermore, when determining the combination of refractory powder and glass, it is necessary to select the combination so that there is no large difference in the coefficient of thermal expansion of the two.

The ionic surfactant used in the present invention is (i)
A cationic surfactant, (ii) an anionic surfactant, (iii) an amphoteric surfactant, or (iv) a mixture thereof, or (v) any of the above (i), (ii), (iii), and (iv). It is a mixture of surfactants and nonionic surfactants. It is desirable that the specific resistance of this surfactant in a 30% aqueous solution is 150 Ωcm or less, preferably 100 Ωcm or less, when measured at 20°C. If the specific resistance of the surfactant exceeds 150 Ωcm, the resistance value during humidity measurement will become too large, causing various inconveniences, which is not preferable.

Further, it is desirable that the ionic surfactant be in a liquid state when used by being impregnated into porous ceramics. However, even if a surfactant becomes semi-solid or solid by itself under the above conditions, it can be used by mixing this ionic surfactant with other surfactants such as nonionic surfactants. This prevents the surfactant from becoming semi-solid or solid. If the surfactant impregnated into the porous ceramic becomes semi-solid or solid under usage conditions, this is undesirable, since this will lead to an increase in the resistance value and deterioration of the humidity response as a moisture-sensitive element.

Specific examples of such ionic surfactants include the following.

(1) Cationic surfactants (a) Primary to tertiary fatty amine salts RNH ₂ HX, R ₁ R ₂ NHHX, R ₁ R ₂ R ₃ NHX (b) Quaternary ammonium salts For example, alkyltrimethylammonium chloride [RN (CH ₃ ) ₃ ]Cl ha Trialkylbenzylammonium salt For example, alkyldimethylammonium chloride 2 Alkylpyridinium salt 2-Alkyl-1-alkyl-1-hydroxyethylimidazolinium salt f N,N-dialkylmorpholinium salt (2) Anionic surfactant alkylbenzene sulfonate (b) Alkylnaphthalene sulfonate C Alkyl sulfonate (R-SO ₃ ) nMm D α-sulfonated fatty acid salt Alkyl sulfate (ROSO ₃ ) nMm Polyoxyethylene alkyl phenyl ether sulfate Polyoxyethylene alkyl ether sulfate [RO(CH ₂ CH ₂ O)nSO ₃ ]nMm Thi alkyl phosphate (3) Zwitterionic surfactant A N,N-dimethyl-N-alkyl-N-carboxymethylammonium betaine B N,N-dialkylaminoalkylenecarboxylic acid C N,N,N-trialkyl-N-sulfonealkylene ammonium betaine 2-alkyl-1-hydroxyethyl-1-
Carboxymethylimidazonium betaine In addition, when the surfactant exhibits a semi-solid or solid state under the usage conditions, or even if it does not exhibit a semi-solid or solid state, in order to improve the humidity characteristics of the moisture sensing element, together with the ionic surfactant, Nonionic surfactants shown below can also be used.

A Polyoxyethylene alkyl ether RO (CH ₂ CH ₂ O) nH B Polyoxyethylene alkyl phenyl ether C Polyoxyethylene polystyrylphenyl ether D Polyoxyethylene polyoxypropylene glycol (e) Polyhydric alcohol fatty acid partial ester For example, Shugar ester Polyoxyethylene polyhydric alcohol fatty acid partial ester For example, Tweet Polyoxyethylene fatty acid ester RCOO (CH ₂ CH ₂ O) nH H Polyoxyethylene alkylamine In the present invention, the porous ceramic is impregnated with the above surfactant, but if necessary, the porous ceramic may be further impregnated with an electrolyte such as lithium chloride, potassium acetate, or sodium sulfate. good.

As the electrode attached to the porous ceramic, it is preferable to use one that does not have a large difference in thermal expansion coefficient from that of the ceramic and has good adhesion to the ceramic. Examples of such electrodes include ruthenium oxide, indium oxide, gold, palladium, platinum, alloys thereof, or glass pastes containing fine powders of these alloys.

Porous ceramics can be impregnated with the surfactant as described above, for example, by dropping a certain amount of a surfactant aqueous solution onto the porous ceramic, or by immersing the porous ceramic in a surfactant aqueous solution. etc.

As shown in FIG. 4, the moisture-sensitive element according to the present invention includes an electrode 2 such as a comb-shaped electrode formed on an insulating substrate 1, and then a porous ceramic film 3 such as a metal oxide film formed thereon. The porous ceramic 3 is then impregnated with a surfactant. Alternatively, as shown in FIG. 5, porous metal electrodes 5 are provided on both sides of the porous ceramic plate 4, and then the porous ceramic plate 4
It can also be produced by impregnating a surfactant with a surfactant.

Effects of the Invention Since the moisture-sensitive element according to the present invention is constructed by impregnating a porous ceramic with an ionic surfactant, the ionic surfactant itself is used as a moisture-sensing agent, and its ionic conductivity can be used as is, and the wettability between it and the porous ceramic carrier can be improved, resulting in less variation in the humidity characteristics of the device and excellent stability and reproducibility.
Furthermore, it has excellent properties such that almost no change over time is observed even when used under high temperature conditions.
Furthermore, the ionic surfactant used here has a specific resistance of 150 Ωcm or less at 20°C in a 30% aqueous solution.
The resistance value when measuring humidity is a suitable value, and no inconvenience occurs.

The present invention will be further explained below with reference to Examples.
The present invention is not limited to these examples.

Example 1 70g of silicon carbide fine powder (average particle size 2.5μ) and zinc borosilicate glass (SiO ₂ 10-11%, B ₂ O ₃ 20-25%,
ZnO50-60%, PbO7%, _K2O , _Na2O ,
30g of Li ₂ O (50ppm or less) were mixed well and press-molded into a disk shape. This was fired at 750°C, and a paste obtained by mixing ruthenium oxide and the above-mentioned glass powder was printed on both sides, a platinum wire was attached to this, and the paste was sintered at 700°C. In this way, a porous ceramic with electrodes was prepared.

Next, the porous ceramic was immersed in an aqueous solution containing 9% each of coconut alkyldimethylbenzylammonium chloride and N,N-dipolyoxyethylenedodecylamine. After about 5 hours, the porous ceramic was taken out, the droplets on the surface were removed, and it was left at room temperature. Dry.

The humidity-electrical resistance characteristics of 100 moisture-sensitive elements thus manufactured were measured, and the results are shown in FIG. In Figure 1, curve a is the element
The curve b shows the highest resistance value among 100 elements, and the curve b shows the lowest resistance value among 100 elements. As is clear from FIG. 1, the humidity-sensitive element according to the present invention has extremely small variations in humidity characteristics.

Furthermore, even when the humidity-sensitive elements manufactured in this way were left in an atmosphere with a relative humidity of 33% and 93% for a long period of time, no change was observed in the humidity characteristics of the humidity-sensitive elements, and the stability and It was proven that the reproducibility was excellent.

Example 2 A porous ceramic produced in the same manner as in Example 1 was immersed in an aqueous solution containing 4.8% each of sodium polyoxyethylene dodecyl ether sulfate and polyoxyethylene sorbitan monolaurate, and after about 5 hours, it was taken out and the surface liquid The drops were removed and dried at room temperature.

The humidity-electrical resistance characteristics of 100 moisture-sensitive elements thus manufactured were measured, and the results are shown in FIG. In Figure 2, curve a is the element
The curve b shows the highest resistance value among 100 elements, and the curve b shows the lowest resistance value among 100 elements. As is clear from FIG. 2, the humidity-sensitive element according to the present invention has extremely small variations in humidity characteristics.

Furthermore, even if the humidity-sensitive element manufactured in this way is left in a low-humidity or high-humidity atmosphere for a long period of time, no change is observed in the humidity characteristics of the humidity-sensitive element, and it has excellent stability and reproducibility. has been proven.

Comparative Example 1 A porous ceramic produced in the same manner as in Example 1 was immersed in a 0.02% aqueous solution of lithium chloride, taken out after about 5 hours, and dried at room temperature after removing droplets on the surface.

The humidity electrical resistance characteristics of the humidity sensitive element thus manufactured were measured, and the results are shown in FIG. In Figure 3, curve a is the value immediately after manufacture, curve b is the value when left in an atmosphere with relative humidity of 33% for one week, and curve c is the value when the relative humidity is left for one week.
This is the value when placed in a 33% atmosphere.

As can be seen from FIG. 3, the humidity characteristics of this humidity sensing element are unstable and non-reproducible.

[Brief explanation of drawings]

1 and 2 are graphs showing the humidity characteristics of the humidity sensing element according to the present invention, and FIG. 3 is a graph showing the humidity characteristics of the humidity sensing element of the comparative example. Moreover, FIG. 4 is a perspective view of a humidity sensing element according to the present invention, and FIG. 5 is a sectional view of another humidity sensing element according to the invention.

Claims (1)

[Scope of Claims] 1. A moisture-sensitive element formed by impregnating a porous ceramic with electrodes with an ionic surfactant having a specific resistance of 150 Ωcm or less at 20°C in a 30% aqueous solution. 2. The ionic surfactant is (i) a cationic surfactant, (ii) an anionic surfactant, (iii) an amphoteric surfactant, or (iv) a mixture thereof, or (v) the above.
The moisture-sensitive element according to claim 1, which is a mixture of the ionic surfactant (i), (ii), (iii), or (iv) and a nonionic surfactant. . 3. The moisture sensing element according to claim 1, wherein the specific resistance is 100 Ωcm.