JPH0122230B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a moisture-sensitive element, and in particular to a ceramic moisture-sensitive element using a ZrO ₂ +Y ₂ O ₃ based moisture-sensitive material, which reduces its resistance and improves its moisture-sensing properties. It relates to a post-treatment method consisting of immersion in a KOH-containing liquid followed by calcination to prevent deterioration over time.
According to the present invention, a moisture sensing element that stably exhibits good moisture sensing characteristics over a long period of time can be obtained.

BACKGROUND OF THE INVENTION In recent years, moisture sensitive elements have come to be used in many fields. For practical garden products, it is used to control food cooking in microwave ovens, to detect dryness in clothes dryers,
It is widely used for humidity control in room air conditioners, for detecting condensation on VTR cylinders, and in industrial applications, for humidity control during the manufacture of various electronic components. In addition, attempts are being made to use it for air conditioning in agricultural houses and for preventing dew condensation on automobile rear windows. Humidity control is essential in addition to temperature control in various automated systems for food cooking, air conditioning, drying, etc., and there is a demand for the development of moisture-sensitive elements that operate with high reliability.

In order to meet these demands, electrical resistance type moisture sensing elements have been put into practical use. This detects changes in humidity as changes in electrical resistance.
Various types of moisture-sensitive materials have been proposed, including electrolyte materials such as lithium chloride, organic polymer materials, and ceramic materials. Electrical resistance type moisture sensing elements are basically required to have a low electrical resistance value, good linearity of resistance-humidity characteristics, have an appropriate operating range, and not deteriorate in the usage environment. Recently, ceramic moisture-sensitive materials have been attracting attention as a material that comprehensively satisfies these requirements.

There have been many proposals for ceramic moisture-sensitive materials, but the important properties are (a) low electrical resistance (the higher the current, the better the sensitivity), and (b) resistance-humidity characteristics. (c) good responsiveness is required. The present inventors believe that a ceramic moisture-sensitive material that basically satisfies these characteristics is
It has been concluded that the ZrO ₂ +Y ₂ O ₃ system exhibits particularly stable and favorable properties.

The moisture sensing element may be in the form of a bulk type in which a pair of electrodes are provided on opposing surfaces of the ceramic sintered body, but a preferred form is to form an electrode layer on at least one surface of the ceramic substrate, and to It is a thin film type in which a moisture sensitive layer is formed by coating a mixture of the ceramic and a binder on the layer, drying and firing. It is preferable to form the coating film by screen printing. More importantly, the electrode pattern in the electrode layer should be formed so that the spacing between the electrodes is as small as possible, 0.20 mm or less. For example, a pair of comb-shaped electrodes are meshed with the comb teeth alternately, and the spacing between the comb teeth is adjusted.
Good results can be obtained by setting it to 0.20 mm or less.

However, there is still a lot of room for improvement in the performance of such Y ₂ O ₃ +ZrO ₂ -based ceramic moisture sensing elements, and trials are currently being carried out. One of the points to be improved is the prevention of changes over time, and it is also desired to further reduce the resistance value.

PRIOR ART As methods for preventing aging of a moisture-sensitive element, there have been several proposals including the addition of additives, a baking method, etc., one of which is a surface treatment method. This is intended to prevent deterioration over time by impregnating the fired body with a surface treatment agent and modifying its surface. The surface treatment agent must be specifically selected depending on the ceramic moisture-sensitive material in question. For example, Japanese Patent Application Laid-open No. 59-47703 discloses that Na ₃ PO ₄・
It is disclosed that the resistance value can be reduced by applying a dip coating treatment using a 12MoO ₃ aqueous solution. Although it is stated that changes over time are reduced, no specific data is presented. Also, Tokukai Akira
No. 59-75601 discloses that after impregnating the open pore surface of a TiO ₂ -SnO ₂ ceramic porous body with a specific pore radius distribution and porosity with at least one of an alkali metal, phosphorus, or sulfur, A method for manufacturing a porous textured moisture resistor is disclosed, which comprises performing heat treatment at .degree. C. and forming at least one pair of electrodes on the surface of the resulting porous material.

The surface treatment agent must be compatible with the specific ceramic moisture-sensitive material, and a surface treatment agent suitable for one type of moisture-sensitive material may be suitable for another type of moisture-sensitive material. There is no guarantee.

Summary of the Invention The present inventor has developed a ZrO ₂ + Y ₂ O ₃ based ceramic humidity sensing element, preferably a type of humidity sensing element in which a pair of comb-shaped electrode layers is formed on a substrate and a humidity sensitive coating is applied thereon. In response, we have repeatedly investigated surface treatment agents that prevent deterioration over time. As a result, it was concluded that a method of immersing in a KOH-containing liquid and post-calcining is suitable.
The firing temperature is preferably 750 to 870°C.

Immersion in a KOH-containing liquid is carried out in the above-mentioned Japanese Patent Application Laid-Open No.
Although this method falls within the concept of alkali metal impregnation disclosed in No. 75601, the type of moisture-sensitive ceramic targeted by the present invention itself is different, and the moisture-sensitive element and firing temperature that are preferred by the present invention are also different.

In summary, the present invention is a post-treatment method for preventing changes over time in the moisture-sensing characteristics of a moisture-sensing element using a ZrO ₂ + Y ₂ O ₃ -based moisture-sensitive material.
Provided is a method for post-processing a moisture-sensitive element, which comprises immersing it in a KOH-containing liquid and then performing a baking process. The preferred firing temperature is 750-870°C. The KOH concentration of the KOH-containing liquid is preferably 10 to 30%.
In addition, a preferred form of the moisture-sensitive element on which the present invention is based is that a comb-shaped electrode layer is formed on at least one surface of a ceramic substrate, and a ZrO ₂ +Y ₂ O _3- based moisture-sensitive material is mixed with a binder on the electrode layer. The moisture-sensitive layer is formed by screen printing and after drying is baked at a temperature of 750-870°C, followed by an aging treatment if necessary.
It is produced by heat treatment at a temperature of 750-870°C.

Specific Description The moisture-sensitive element used in the present invention can be realized in a so-called "pulk" type in which a pair of electrodes are formed on the facing surfaces of a ceramic sintered body, but it is preferable to form a moisture-sensitive film on a substrate. It is manufactured as a thin film type. FIG. 1 is a perspective view of a preferred example of a thin film type moisture sensitive element used in the method of the present invention. An electrode layer 3 is formed on one or both surfaces of the substrate 1, and a moisture sensitive layer 5 is formed thereon (the moisture sensitive layer is partially omitted). As a substrate, Al ₂ O ₃ , SiO ₂ ,
Ceramics such as ZrO ₂ are used. To form an electrode layer on a substrate, for example, nickel or the like is deposited as an under electrode, then gold, platinum, etc. is deposited on top of it as an upper electrode for rust prevention, and a desired electrode pattern is formed using photoetching technology. The ruthenium electrode can also be formed by a screen printing method using ruthenium paste. Recently, many microfabrication technologies have been put into practical use for manufacturing printed circuit boards for electronic circuits.
By applying them, fine electrode patterns can be formed. For example, the spatsuta method is also a hairy method. The electrode pattern is preferably one in which a pair of comb-shaped electrodes are interlocked with comb teeth as shown in FIG. The smaller the interval between the comb teeth, the lower the resistance value, so the sensitivity of the moisture sensing element can be increased. Good results were obtained using a comb tooth spacing of 0.05-0.20 mm.

After forming the electrode layer in this manner, a moisture sensitive layer made of a mixture of a Y ₂ O ₃ +ZrO ₂ (0.01 to 99.00%) type moisture sensitive material and a binder, which is the object of the present invention, is formed thereon.

Binders are used to provide good sintering at low temperatures. As the binder, substances known as sintering aids can be used, but lead-free borosilicate glass is particularly suitable. The binder is used in an amount of 5 to 15%, preferably 7 to 12% of the total moisture sensitive material.

The composition of the lead-free borosilicate glass preferred for use is 10-15% by weight of sodium oxide, 2-5% by weight of potassium oxide, 5-10% by weight of calcium oxide, and 5-10% by weight of magnesium oxide. The content is 0.5-3% by weight, the content of aluminum oxide is 5-10% by weight, the content of silicon dioxide is
30-45% by weight, and boron oxide content 20-45%
It is 30% by weight. This binder is preferable because it has a higher content of alkali metal and alkaline earth metal oxides and a lower melting point than general lead-free borosilicate glass.

After pulverizing and kneading the above mixture of ceramic and binder, the viscosity is adjusted with a resin paint, and then the thickness of the moisture sensitive layer is 5 to 200 μm, preferably 40 to 120 μm.
A moisture sensitive layer is applied and formed by screen printing so as to form the front and back. The thicker the film, the lower the electrical resistance of the final product.

Thereafter, after preliminary drying at a temperature of 130 to 190° C. for 0.2 to 2 hours, a firing treatment is performed at a temperature lower than conventional. The firing process sinters the ceramic powder particles,
It forms the skeleton structure of the moisture-sensitive membrane and provides structural strength. Previously, it was thought that a minimum firing temperature of 900°C was necessary to obtain the required structural strength, but it has been found that such high-temperature firing actually deteriorates the performance of the fabricated moisture-sensitive element, especially the electrical resistance. Ta. Therefore, the firing process is performed at a temperature of 750-870°C. It is believed that when high temperature firing is performed as in the prior art, the ceramic powder particles or binder melt (slag), reducing the porosity of the moisture-sensitive element, thereby increasing the electrical resistance.
Furthermore, it is thought that the ceramic that is the object of the present invention tends to undergo transformation into an unstable crystalline form, resulting in insufficient stability. This situation can be avoided by setting the upper limit of the firing temperature to 870°C. If the firing temperature is 750° C. or higher, the required structural strength of the moisture-sensitive element, which is the object of the present invention, is guaranteed. The firing holding time may be 5 to 1 hour, typically several tens of minutes.

A humidity sensing element with a certain level of performance can be produced by firing alone, but in order to make the humidity sensing element even more stable, it is necessary to perform an aging treatment on the humidity sensing element after firing, and then Heat treatment is preferred.

There are various approaches to aging processing, but as a result of many studies, the present inventors have found that, for example, 10%
A humidity increase/decrease cycle from low RH humidity to high humidity of, for example, 90% RH, and vice versa, is repeated alternately for 20 to 50 minutes, preferably around 30 minutes, at 30 to 90°C, preferably around 60°C. for a total of 1 to 3 weeks, preferably 2
I found that it is best to do it around a week or so. As a result of this aging treatment, a moisture-sensitive film is produced that can withstand long-term stable use without deteriorating even under harsh conditions during use.

Furthermore, it should be noted that this aging treatment creates very fine cracks in the moisture sensitive film. Although it is not clear whether this is due to repeated cycles of humidity increase/decrease or something unique to this material system, it is thought to produce a highly stable element.

After this, the device is subjected to a heat treatment at a temperature of 750-870°C. This heat treatment brings about complete dehydration, fixes the moisture sensitive membrane, and prevents an increase in electrical resistance and transformation. Above 870°C, melting of the glass or transformation of the crystal structure appears to occur and increases the electrical resistance of the device unacceptably. The reason why the temperature is set at 750°C or higher is that the moisture-sensitive element cannot be immobilized at a temperature lower than 750°C. In this way, a moisture sensitive element with low resistance and high sensitivity and stability is produced.

The immersion treatment in the KOH-containing liquid is carried out by immersing the produced moisture-sensitive element in the liquid for an appropriate period of time, such as 1 to 120 minutes. KOH concentration is 10~
30% by weight is suitable. If it is less than 10%, it will take a long time for impregnation, and if it exceeds 30%, the degree of surface modification will increase, causing harmful effects such as increased hysteresis.

After the dipping treatment, the device is preferably fired at a temperature of 750-870°C. Firing is necessary to prevent the KOH surface treatment agent from dripping during condensation and to produce a stable modified surface. Firing temperature range: 750~
The reason why the temperature is preferably 870°C is to prevent an increase in electrical resistance, as described above.

The moisture-sensitive element product thus post-treated exhibits low resistance and excellent responsiveness, and exhibits stable performance over a long period of time due to little change over time.

Effects of the Invention A very simple surface treatment operation can prevent the moisture-sensing characteristics of the moisture-sensitive element from changing over time, and at the same time lower the resistance value, so that a more excellent moisture-sensitive element can be manufactured.

Reference Example (Production of Moisture Sensing Element) A comb-shaped ruthenium electrode as shown in FIG. 1 was formed by screen printing on an Al ₂ O ₃ substrate with a length of 18 mm and a width of 9 mm. The spacing between each electrode comb tooth was 0.20 mm, and printing was performed over a length of 12 mm. on the electrode layer
Mixed ceramic 93 with Y ₂ O ₃ : ZrO ₂ in a 50:50 weight ratio
% + 7% lead-free borosilicate glass was formed. The moisture-sensitive layer was formed as follows: The above mixture was crushed and kneaded in an automatic mortar made of alumina, the viscosity was adjusted with butyl carbitol and epoxy resin paint, and a screen was applied so that the thickness of the moisture-sensitive layer was about 20 μm. It was applied by printing. Thereafter, it was pre-dried at a temperature of 170°C for 1 hour and fired at a temperature of 800°C for 15 minutes.

Next, aging treatment was performed at 60°C for 2 weeks while repeating a humidity increase/decrease cycle of 10% RH and 90% RH every 30 minutes, followed by heat treatment at 870°C.

Example Post-treatment was carried out by immersing the moisture-sensitive element obtained in Reference Example in a 20% KOH aqueous solution for 5 minutes and then baking it at 800° C. for 20 minutes.

Figure 2 shows the initial value (no post-processing), KOH (20%)
It is a graph showing the resistance-relative humidity characteristics of the humidity sensing element immediately after immersion and immediately after 800° C. baking after KOH immersion. The change in resistance value is measured at a measurement frequency of 1000Hz and a measurement voltage.
Measured as 1V. It can be seen that the resistance value decreases by the KOH immersion treatment compared to the initial value.
Although the resistance value increases by firing after the KOH immersion treatment, it still maintains a value lower than the initial value.

FIG. 3 is a graph showing the change in electrical resistance value over time when left in the atmosphere under various humidity conditions shown in the graph. The initial value, the value immediately after immersion, and the value immediately after firing are also shown on the left end of the graph. It can be seen that the electrical resistance value immediately after firing was substantially maintained even after being left for 35 days.

Figure 4 shows the humidity sensing element after post-treatment when the relative humidity is reduced from 85% to 33% and from 33%.
It is a graph showing the responsiveness when increasing to 85%. A highly acceptable high response is observed.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a preferred specific example of the humidity sensing element, FIG. 2 is a graph showing resistance-humidity characteristics of the humidity sensing element, and FIG. 3 is a graph showing changes in resistance value over time. FIG. 4 is a graph showing the response characteristics. 1: Substrate, 3: Electrode layer, 5: Moisture sensitive layer.

Claims (1)

[Claims] 1. A post-treatment method for preventing changes over time in the moisture-sensing properties of a moisture-sensing element using a ZrO ₂ +Y ₂ O _3- based moisture-sensitive material, which method comprises soaking the moisture-sensing element in a KOH-containing solution. A method for post-processing a moisture-sensitive element, which comprises immersing it and then firing it. 2. The method according to claim 1, wherein the firing temperature is 750 to 870°C. 3. The method according to claim 1, wherein the KOH concentration of the KOH-containing liquid is 10 to 30% by weight. 4. The moisture sensing element has a comb-shaped electrode layer formed on at least one surface of a ceramic substrate, and ZrO ₂ +
A moisture-sensitive layer made by mixing Y ₂ O ₃ -based moisture-sensitive material with a binder is formed by screen printing, and after drying.
2. A method according to claim 1, which is produced by calcination at a temperature of 750-870<0>C followed by, if necessary, an aging treatment and then a heat treatment at a temperature of 750-870<0>C.