JPH0721477B2 - Humidity sensor - Google Patents

Description

TECHNICAL FIELD The present invention relates to a humidity sensor and a method for manufacturing a humidity sensor, and more specifically, it is sensitive to water vapor in an atmosphere and is sensitive to changes in humidity. The present invention relates to a humidity sensor that exhibits no hysteresis, is insensitive to other organic substances, and is selectively sensitive only to water vapor, and a method for manufacturing the same.

[Conventional technology]

The establishment of humidity control technology is demanded as one of the ways to make the living environment comfortable. However, at present, the so-called humidifier supplies moisture and the dehumidifier removes moisture unidirectionally, and humidity control has not been achieved. The cause of this is that the development of element materials that sensitively sense moisture and quickly convert it into electrical signals has been delayed. In recent years, an element containing magnetite as a main component has been developed and expected to be applied to a humidity sensor, but the following drawbacks have been pointed out.

The humidity sensor works by reversible adsorption and desorption of water vapor in the atmosphere, but magnetite is not completely desorbed and accumulates on the surface. A periodic heat treatment operation is required to remove this irreversible adsorbed water, but at the same time, the heat treatment operation promotes a structural change of the sensor surface, resulting in deterioration of the sensor characteristics. Further, since magnetite exhibits the same high sensitivity not only to water vapor but also to alcohol, it cannot be a highly accurate humidity sensor.

[Problems to be Solved by the Invention]

From the current situation as described above, specifically, there is a strong demand for development of an element material having a completely reversible adsorption property of water vapor, and excellent in a selective sensor property for water vapor,
The present invention provides a humidity sensor having excellent sensitivity characteristics as described above and a method for manufacturing the humidity sensor.

[Means for Solving the Problems]

The present invention relates to a humidity sensor which is sensitive to humidity and capable of converting humidity into an electric signal, wherein the niobium oxide thin film is formed on an electrically insulating and heat resistant substrate. And a method for producing the same, wherein the niobium alkoxide organic solvent solution is applied on an electrically insulating and heat-resistant substrate, and then heat-treated to form a niobium oxide thin film. It provides a method.

As a method for forming a niobium oxide thin film on a substrate, it is possible to use a conventionally well-known method for producing a functional ceramic thin film. Examples of those methods include physical methods, chemical methods, and those methods. Is roughly divided into physicochemical methods.

If the physical methods are further divided, the main ones are physical vapor deposition methods.
(PVD), which is a typical physical vapor deposition method, usually 1 x 1
A thermal evaporation method, in which niobium oxide is heated and evaporated in a high vacuum lower than 0 ^-6 Torr to deposit evaporated particles on a substrate to form a thin film on the substrate, also known as vacuum evaporation method, resistance heating, high frequency heating or electron By heating by impact, niobium oxide or niobium oxide is evaporated into a vacuum containing an inert gas such as argon or an active gas such as oxygen, ionized by DC discharge, RF discharge, electron impact, etc. and accelerated by an electric field, There are ion plating method that deposits on the substrate as it is or by reacting with atmospheric gas, sputtering method that deposits substances that are splashed from the target in atomic or molecular form on the substrate by accelerated ions, and other molecular beam epitaxy. , Electrostatic spraying, various coating methods, etc.

Further, as a typical chemical method, there is a chemical vapor deposition method in which a gaseous raw material is chemically reacted in a vapor phase or on the surface of a substrate to form a thin film.

Typical examples of the physicochemical method in which the both are combined include a plasma chemical vapor deposition method and a laser photochemical vapor deposition method.

Among various thin film forming methods, as a manufacturing method applicable to inexpensive substrate, low running cost, excellent reproducibility, and less restriction on the shape of the substrate, the method of the present invention described above, an organic solvent of niobium alkoxide Examples of the method include applying the solution to an electrically insulating and heat-resistant substrate and then performing heat treatment to form a niobium oxide thin film.

In the present invention, the niobium oxide is not particularly limited as long as it is a compound of niobium and oxygen, and includes various oxides having different structures due to the difference in the oxidation number, and can be used alone or in combination. Is.

The niobium alkoxide used in the present invention has the general formula Nb (OR) ₅
In the above, R is an alkyl group, and the carbon number thereof is not particularly limited, but from the viewpoint of easiness of synthesis, solubility in an organic solvent and the like, those having 1 to 5 carbon atoms are preferable, and R as methyl or ethyl. , Propyl, butyl and the like are exemplified.

As the organic solvent, any solvent capable of dissolving niobium alkoxide can be used, and hydrocarbons, alcohols,
Examples include ethers, esters, ketones, etc.
Alcohols are usually used.

It is also possible to add and use a compound having a chelating ability for niobium alkoxides such as glycols such as 1,3-butanediol.

The method for adjusting the organic solvent solution of niobium alkoxide is not particularly limited as long as a uniform solution can be formed, and stirring can be performed only at room temperature, but heating and ultrasonic irradiation accelerate the homogenization time. It is also possible to do so.

As a substrate to which the niobium alkoxide solution is applied,
Any material that is electrically insulating and can withstand the heating process described below can be used, and ceramics and glass are preferable. Also, the shape thereof is not particularly limited, and examples thereof include a sheet shape, a film shape, a fiber shape, and a block shape, but a film shape or a sheet shape is preferably used.

The method of applying the niobium alkoxide solution to the substrate is selected depending on the shape of the substrate. However, if the method is a dip coating method, a spray method or the like, there is little limitation due to the shape, and if the substrate is a film or sheet, the spinner method or the like is used. It is also possible, and among them, preferred methods include a dip coating method and a spinner method.

A coating film formed from a niobium alkoxide solution on various substrates becomes a niobium oxide thin film from a relatively low temperature of about 300 ° C. The heat treatment temperature is 300 ° C or higher, and the heat-resistant temperature of the substrate or the reaction between the substrate and the niobium oxide thin film. It is carried out under the condition of the starting temperature or less, and usually heat treatment is carried out in the temperature range of 300 ° C to 1000 ° C.

The humidity sensor of the present invention can be manufactured by disposing a conducting wire or the like formed of silver or ruthenium oxide on the niobium oxide thin film.

[Action]

The niobium oxide thin film formed on the electrically insulating and heat-resistant substrate of the present invention is extremely sensitive and reversible to water vapor in the atmosphere, and is resistant to repeated heat treatment. Since there is almost no change in the specific surface area, it is considered that it exhibits an excellent function as a humidity sensor.

〔Example〕

Hereinafter, specific examples of the present invention will be described, but the present invention is not limited thereto.

Example 1 1 gram of niobium pentaisopropoxide was dissolved in 10 ml of 2-propanol at 30 ° C. and stirred for 15 minutes, and then a (26 mm × 38 mm × 1 mm) quartz glass plate was immersed therein, and every minute. 1
It was pulled up at a speed of 60 mm. After drying this at 110 ℃, 4
It was baked at 00 ° C. for 4 hours to form a niobium oxide thin film on a glass plate. After measuring the electrical resistance in a dry helium atmosphere by the DC 4-terminal method, water vapor was introduced into the atmosphere so that the relative humidity was 60%, and the resistance change at that time was measured.
The resistance change when the atmosphere was quickly returned to dry helium was also measured. The situation is shown in FIG.

Comparative Example 1 In the same manner as in Example 1, the same measurement as in Example 1 was performed on the magnetite thin film prepared from the ethylene glycol solution of iron nitrate, and the results are also shown in FIG. Although the sensor characteristics due to the adsorption of water vapor are sensitive, tailing is observed in the desorption process, and there is a problem in sensitivity as a humidity sensor.

Comparative Example 2 The same measurement as in Example 1 was performed on a titanium oxide thin film prepared from a solution of titanium tetraisopropoxide in isopropanol by the same method as in Example 1, and the results are also shown in FIG. Although the sensor characteristics due to the adsorption of water vapor are sensitive, tailing is observed in the desorption process, and there is a problem in sensitivity as a humidity sensor.

Example 2 FIG. 2 shows the change in electric resistance when the atmosphere of the niobium oxide thin film used in Example 1 was changed from dry helium to an ethanol vapor atmosphere. From this, it was confirmed that the niobium oxide thin film has no sensor property for alcohol, that is, it has excellent selective sensor property for water vapor.

Example 3 and Comparative Example 3 150 thin films similar to the niobium oxide thin film used in Example 1 were prepared and their specific surface areas were calculated by the BET method by nitrogen adsorption. Soak these membranes in water, pull them up, and dry.
After heating and exhausting at ℃, the specific surface area was measured again by the BET method. By repeating such an operation, the water resistance of the niobium oxide thin film was examined. Although the results are shown in FIG. 3, a similar experiment was conducted for a magnetite thin film to compare and examine it.

From the figure, it was confirmed that the niobium oxide thin film prepared by the present invention has a large specific surface area of 400 m ² / g and that the surface structure is hardly changed by water adsorption / desorption.

Example 4 The niobium oxide thin film described in Example 1 was used to observe the change in electric resistance when the relative humidity in the atmosphere was changed. In FIG. 4, ○ indicates the change when the humidity increases, and ● indicates the change when the humidity decreases.

From the results shown in FIG. 4, it was found that the change of electric resistance was steep in the range of water vapor pressure of 30 to 100 mmHg.
Therefore, the niobium oxide thin film is the most suitable sensor for controlling this humidity range.

Example 5 When the thin film X-ray diffraction of the niobium oxide thin film prepared in Example 1 was measured, it was confirmed that its crystal structure was niobium dioxide, as shown in FIG.

Example 6 The surface smoothness of the niobium oxide thin film used in Example 1 was observed with a surface roughness meter using a diamond needle as a probe. Sixth
Although the results are shown in the figure, it was confirmed that the thin film formed by the present invention is a very smooth film having a surface roughness of about ± 50 Å or less. It was also confirmed that the thickness of this film was 3500 angstroms (Å).

Example 7 A quartz glass plate was dipped in the solution used in Example 1, pulled up at various speeds, dried at 110 ° C., and then baked at 400 ° C. for 4 hours. The thin films of these niobium oxide thin films were measured with a surface roughness meter. As shown in FIG. 7, the film thickness of the niobium oxide film could be controlled in the range of about 1000 to 4000 angstrom at the pulling rate in the range of 5 to 160 mm / min.

Example 8 A solution in which the amount of niobium pentaisopropoxide dissolved in 10 ml of 2-propanol was changed from 0.1 g to 1.5 g was prepared, and a quartz glass plate was immersed therein. The glass plate was pulled at a constant pulling rate of 160 mm / min, dried, and baked at 400 ° C. for 4 hours. The thickness of the niobium oxide film thus formed was measured and plotted against the concentration of niobium ions in the solution in FIG. From this, it is possible to control the film thickness of the niobium oxide film by changing the amount of niobium ions in the solution.

Example 9 The niobium oxide thin film prepared in Example 1 was again immersed in the solution used in Example 1, pulled up, dried, and baked. That is, the niobium oxide thin film was formed again on the niobium oxide thin film. In this way, the change in film thickness due to the repeated application of the niobium oxide thin film was observed, and the results shown in FIG. 9 were obtained. By repeatedly applying multiple layers,
A niobium oxide film with a maximum film thickness of 1 micron could be created on a glass substrate.

[Brief description of drawings]

1 is a graph showing sensor characteristics of Example 1, Comparative Examples 1 and 2, FIG. 2 is a graph showing selective sensor characteristics showing the results of Example 2, and FIG. 3 is Example 3 And a graph showing the results of experiments on water resistance of each sensor of Comparative Example 3, FIG. 4 is a graph showing the results of Example 4, and FIG.
The figure shows the thin film X-ray diffraction spectrum of the niobium oxide thin film of Example 1. 6 is a graph showing the surface smoothness of the niobium oxide thin film of Example 1, FIG. 7 is a graph showing the relationship between the pulling rate and the film thickness showing the results of Example 7, and FIG. 8 is a graph showing the relationship between the concentration of niobium pentaisopropoxide and the film thickness of the niobium oxide thin film of No. 8, and FIG.
11 is a graph showing the relationship between the number of times of overcoating and the film thickness, showing the results of Example 9.

Claims (2)

[Claims] 1. A humidity sensor which is sensitive to humidity and capable of converting the humidity into an electric signal, wherein a niobium oxide thin film is formed on an electrically insulating and heat resistant substrate. sensor. 2. A niobium oxide thin film is formed by applying an organic solvent solution of niobium alkoxide onto an electrically insulating and heat resistant substrate and then subjecting it to heat treatment.
A method for manufacturing the described humidity sensor.