Method Of Making A Moisture-sensitive Element

Abstract

Disclosed is a method of making a moisture-sensitive element having an increased sensitivity to humidity with minimum secular variation of electric characteristics. The method comprises the steps of subjecting an aluminum matrix to electrolytic treatment to produce a film of aluminum oxide on the matrix, removing the film from the matrix, heat treating the film, and attaching electrodes to opposite surfaces of the film.

Description

This invention relates to a method of making a moisture-sensitive element essentially consisting of a porous film of aluminium oxide having electrodes thereon and particularly to a method of making such electroded humidity sensor whose sensitivity to moisture will now lower with time.

In producing the humidity sensor of the character mentioned above, a porous aluminum oxide film is formed on an aluminum matrix or body by subjecting the aluminum matrix to anodic oxidation in an electrolytic bath of oxalic acid, sulfuric acid, cromic acid or any other acid which is proper for the purpose. A moisture-permeative electrode is formed on the aluminium oxide film by vacuum evaporating or sputtering. Thus, a moisture detecting device consists of a composite of "moisture pervious electrode-porous film of aluminium oxide-aluminium." Such moisture detector is shown in U.S. Pat. Nos. 3,440,372 and 3,574,681. It is very sensitive, and quick in response. However, disadvantageously it takes a relatively long period (ranging from a few months to 1 year) before a new device after produced, has reached a stable condition after the termination of inherent secular variation. More specifically, the impedance of the device tends to rise with time. The rise rate depends on the surrounding humidity. As the surrounding moisture increases, the impedance of the element rises at an increased rate.

The X-ray diffraction image of the porous film of aluminium oxide appears as undefined spread pattern rather than a definite and well-defined line. From this it appears that the aluminium oxide film be composed of very minute crystals, and specifically of those of .gamma.-alumina and/or .gamma.'-alumina.

It appears to the inventor that the cause for secular variation of the electric characteristics is that: .gamma.-alumina or .gamma.'-alumina is hydrated by water in the surrounding air so that the moisture-sorption capability is changed. It is well known that hydrated alumina when heated at an ever increasing temperature, will be changed first, to .gamma.-alumina at 500.degree. C and higher temperatures, and then it will be again changed to .delta.-alumina, .theta.-alumina and finally to .alpha.-alumina in the order given at 900.degree. C and higher temperatures.

This invention is based on those facts above mentioned, and the object of this invention is to provide a method of producing an electroded moisture detector essentially composed of an aluminium oxide film with minimum secular variation.

To attain this object there is provided a method according to this invention essentially comprising the steps of: subjecting an aluminum matrix to electrolytic treatment to produce an aluminium oxide film on said matrix; removing said aluminium oxide film from said matrix; subjecting said aluminium oxide film to a heat treatment; and attaching metal electrodes to the opposite surfaces of said aluminium oxide film.

This invention will be better understood from the following description which is made with reference to the accompanying drawing which shows the secular variation of admittance of different moisture detector elements produced according to the teaching of the invention. An aluminium matrix is subjected to the anodic oxidation in an electrolytic bath of oxalic acid, sulfuric acid or other acids which are proper for the purpose. As a result an aluminium oxide film is formed on the aluminium matrix. The heating process is performed at a temperature higher than the melting point of aluminium. Therefore preferably the aluminium oxide film is removed from the matrix, and the aluminium oxide foil thus removed is subjected to heat treatment.

Speaking of the aluminium oxide film a barrier layer is formed on the matrix, and a porous film of aluminium oxide is formed on the barrier layer. Either a single layer of aluminium oxide of a composite layer of aluminium oxide plus barrier layer can be equally used. Therefore, the words, "aluminium oxide film" herein used should be understood as an aluminium oxide film with or without associated barrier layer. For example, an aqueous solution of mercuric chloride or methanol solution of bromide or iodine may be used as solvent for aluminium matrix. An aluminium oxide film is subjected to heat treatment, and finally a pair of electrodes of metal, such as gold or aluminium are attached to the opposite sides of the aluminium oxide film. Thus, a moisture detector element results.

If the aluminium oxide film is heated at a temperature lower than 900.degree. C, film is of .gamma.-alumina, and the moisture-sensitive element using such aluminium oxide film shows the same tendency of secular variation as found in the element using an aluminium oxide film which is not subjected to heat treatment.

If the aluminium oxide film is heated at an elevated temperature higher than 900.degree. C, it is of .delta.-alumina or .theta.-alumina, and the moisture-sensitive element having an aluminium oxide film thus baked showes the substantial improvement in secular variation of electric characteristics, compared with the element having an aluminium oxide film which was not subjected to heat treatment. If the aluminium oxide film is heated at 1,200.degree. C or higher temperature, it is of .alpha.-alumina, and a moisture detector having an aluminium oxide film thus modified is almost free from secular variation.

After an aluminium oxide film was subjected to the heat treatment at 900.degree. C or higher temperature according to this invention, it is boiled or exposed to water vapour or steam. The inventor found that the moisture-sensitive element having an aluminium oxide film thus treated showed the same improvement as the element having an aluminium oxide film baked at 1,200.degree. C or higher temperature. As seen from this, the hot water treatment is effective to allow the aluminium oxide film to reach the permanent and stable condition ion a possible minimum time. This is contrary to that found in boiling an aluminium oxide film without heat treatment. In this case the pores of the aluminium oxide film are sealed, thus losing the sensitivity to moisture.

EXAMPLE 1

An aluminium oxide film as thick as 20 microns was produced on an aluminium matrix sheet as thick as 0.3 milimeters by subjecting the sheet to anodic oxidation in an oxalic acid bath of 0.3 percent concentration at a current density equal to 1 ampere/square-decimeter. The aluminium oxide film thus produced was removed from the aluminium sheet by dissolving the sheet in a methanol solution of iodine. The size of the aluminium oxide film thus obtained was about 1 .times. 1 cm.sup.2. The aluminium oxide film was heated at 1,000.degree. C for 1 hour, and then a pair of gold electrodes were applied to the opposite surfaces of the aluminium oxide film by vapourizing the metal onto the aluminium oxide film. The drawing shows the secular variation of the admittance of a moisture-sensitive element at a relative humidity of 80 percent. The admittance is given in terms of the ratio of the instantaneous value to the initial one. The curve 1 shows the secular variation of the element using an aluminium oxide film which was heated at 800.degree. C. The curve II shows the secular variation of the moisture-sensitive element according to this invention. This graph shows substantial reduction of secular variation attained by this invention.

EXAMPLE 2

An aluminium oxide film was produced on an aluminium substrate and removed therefrom in a similar way to Example 1. The aluminium oxide film thus obtained was heated at 1,200.degree. C for 1 hour. Then, gold was vapour-deposited onto the opposite surfaces of the aluminium oxide film. Curve III shows the secular variation of a moisture-sensitive element having such aluminium oxide film.

EXAMPLE 3

An aluminium oxide film was prepared in a similar way to Example 1. The aluminium oxide film was heated at 1,000.degree. C for 1 hour, and then soaked in the boiling water. Curve III shows the secular variation of a moisture detector having such aluminium oxide film.

Also, curve III shows the secular variation of a detector using an aluminium oxide film which was subjected to a heat treatment at an elevated temperature (1,200.degree. C). Therefore, this curve shows that the aluminium oxide film which was heated at a relatively low temperature (1,000.degree. C) will be improved, by subjecting it to a boiling water or steam treatment, to the same degree as found in the aluminium oxide film which was heated at a relatively high temperature (1,200.degree. C). The aluminium oxide film which was subjected to a heat treatment at a relatively low temperature is more sensitive to humidity than the aluminium oxide film which was subjected to a heat treatment at a relatively high temperature.

Thus, according to Example 3 there is provided a moisture-sensitive element having an increased sensitivity with minimum secular variation.

Claims

What is claimed is:

1. A method of making a moisture-sensitive element comprising the steps of: subjecting an aluminium matrix to electrolytic treatment to produce a porous film of aluminium oxide on said matrix; removing said aluminium oxide film from said matrix; subjecting said aluminum oxide film to a heat treatment; and attaching metal electrodes to the opposite surfaces of said aluminium oxide film.

2. A method of making a moisture-sensitive element according to claim 1 wherein it further comprises the steps of subjecting said aluminium oxide film to a hot water treatment as a subsequent to said heat treatment.

3. A method of making a moisture-sensitive element according to claim 1 wherein said heat treatment is conducted at 900.degree. C or higher temperature.

4. A method of making a moisture-sensitive element according to claim 2 wherein said hot water treatment includes exposing said aluminium oxide film to steam and soaking said aluminium oxide film in boiling water.

5. A method of making a moisture-sensitive element according to claim 1 wherein said electrolytic treatment is anodic oxidation.

6. A method of making a moisture-sensitive element according to claim 1 wherein an aqueous solution of mercuric chloride or methanol solution of bromide or iodine is used to remove said aluminium oxide film from said aluminium matrix.