JPS5860501A - Moisture sensor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a humidity sensor, and particularly to a silicon carbide humidity sensor that has a wide humidity measurement range and is excellent in sensitivity, responsiveness, reproducibility, and stability.

Conventionally, various kinds of electrical resistance type humidity sensors are known. For example, lithium chloride-based humidity sensors, which are currently the mainstream, are made by coating or impregnating a lithium chloride solution onto a substrate material such as resin or glass tape. This type of humidity sensor has a relatively narrow measuring humidity range, and when the humidity is high, the concentration of hygroscopic salts dilutes and drops as droplets, shortening its lifespan. There are many problems in terms of stability of properties, such as a decrease in moisture absorbing ability and a relatively low sensitivity (rate of change in resistance with respect to changes in relative humidity).

On the other hand, in recent years, various electrical resistance type humidity sensors using semiconductors, alumina films, or ceramic semiconductors have been developed, which are small, lightweight, and easy to handle. For example, a humidity sensor using silicon has a pair of comb-shaped electrodes attached to a steatite substrate, a semiconductor material mainly composed of silicon is printed thereon, and a resistor is formed by high-temperature sintering.

This humidity sensor has problems in terms of stability of characteristics, such as slightly poor accuracy, relatively short lifespan, and changes over time. MgCr20+ as other ceramic humidity sensor
TiO2 series, magnetite colloid (Fe304),
Nickel ferrite (N'FC204) + porous A
t, Os, and thin film S n O2, and some of them are in practical use, but some devices have high resistance at low temperatures, hysteresis occurs at high humidity, poor response, short lifespan, and manufacturing difficulties. Problems such as poor yield remain.

The present invention has been made in consideration of the above-mentioned current state of the prior art, and its purpose is to solve these problems, widen the measurement range of humidity, and improve sensitivity.

An object of the present invention is to provide a novel humidity sensor with excellent responsiveness, reproducibility, stability, etc.

To summarize the present invention, the humidity sensor of the present invention is an electrical resistance type humidity sensor characterized in that the moisture sensitive material is a porous silicon carbide sintered body, and the sintered body contains silicon carbide, boron, and A porous sintered body containing one or more boron-containing compounds is preferred.

As a result of extensive research into electric resistance type ceramic humidity sensors, the present inventors discovered that silicon carbide (SiC) itself or a porous sintered body containing silicon carbide (SiC) as a main component exhibits resistance changes that are sensitive to humidity. In particular, silicon carbide, which is the main component, contains a small amount of boron and/or a boron-containing compound as an additive to sinter the element and obtain the desired resistance value. The present invention was achieved by discovering that it is effective to include one or more of the following.

As the silicon carbide used in the present invention, it is appropriate to use a commercially available product having an average particle size of 2 μm or less.

The boron-containing compound in the present invention is not particularly limited, but
For example, boron oxide (B203), boron carbide (B, C)
and boron nitride (BN) are suitable.

In the present invention, the amount of boron and/or boron-containing compound added to silicon carbide, which is the main component, is based on the total amount of the porous silicon carbide sintered body, and the amount is 0 in terms of boron.
．． A suitable content is 1 to 5% by weight. This amount is 0.
If it is less than 1% by weight, sintering will not be sufficient and the mechanical strength of the device will be weak; if it exceeds 5% by weight, depending on the type of additive, the resistance value of the device at low temperatures may be too high or too low. This is not appropriate as it is too far outside the practical measurement range.

Further, it is appropriate that the relative density of the porous silicon carbide sintered body in the present invention is 60 to 90%, and this is 60%.
If it is less than 9, the sintered body will be in a biscuit state and its strength will be weak.
If it exceeds 0%, the sintered body will have fewer pores, the moisture absorption effect will be weak, and the sensitivity to humidity changes will decrease.

The porous silicon carbide sintered body in the present invention has an electrical resistivity of 102 to 1010 Ωcm depending on the type of additive.
is significantly different. Then, in response to changes in relative humidity,
Its electrical resistance value changes greatly.

However, depending on the selection of the additives and their amounts, the electrical resistance value may vary from 10' to 1O7, especially 102
It is possible to make a large change within the practical measurement range of an electrical resistance type humidity sensor of ~106Ω (see examples below).

Moreover, it has the advantage of extremely fast response speed and excellent durability.

In producing the humidity sensor made of the porous sintered body of the present invention, after adding and mixing a small amount of additives to silicon carbide powder, or without mixing, or adding an organic binder,
This mixture is press-molded and fired at a high temperature of 1800 to 2100°C to form a porous sintered body (porosity 10 to 35%), and the surface layer of both end faces of this sintered body is ground and then cut.
Gold on both ends.

A method may be used in which electrodes such as silver are baked and lead wires are attached.

Next, embodiments of the present invention and their effects will be explained using Examples, but the present invention is not limited thereto.

Example 99 parts by weight of silicon carbide (SiC) powder (manufactured by Fujimi Abrasives Industry Co., Ltd., average particle size 2 μm) and 1 part by weight of boron carbide (B4C) powder (manufactured by Takasago Sangyo Co., Ltd., average particle size 1 μm) (calculated as boron) 0.788% by weight of each was weighed and mixed, and a xylene diluted silicone resin solution (concentration 5%) was added as an organic binder.
0%) was added at a rate of 20 mt per 100 g of raw material mixed powder, and mixing was performed using an agitator. Then,
After sizing this mixture into granules, use a mold to
Press molded at a pressure of kg/cm2 and has a diameter of 5Qmm.

It was made into a disk shape with a thickness of about 2 mm. This disk-shaped molded body was prepared in a vacuum using a vacuum hot press device (10-3 to 10-
Firing treatment was performed at a temperature of 1,800 to 2,100° C. for 1 hour while applying a load of 200 kg/cm 2 at a temperature of 1,800 to 2,100° C. The obtained sintered body had a porosity of 10 to 35 cm, and was a porous sintered body having fine pores suitable for adsorbing moisture. After lightly grinding the surface layer on both end faces, this sintered body was cut into 3 mm square chips with a diamond cutter, silver electrodes were baked on both end faces, and lead wires were attached to achieve a relative density of 65%. A humidity sensor made of a porous sintered body was fabricated.

Regarding the obtained humidity sensor, the relationship between relative humidity and electrical resistance value was investigated. The results are shown in the attached drawings.

That is, the attached drawing is a graph showing the relationship between the relative humidity (4) (horizontal axis) and the electrical resistance value (2) (vertical axis) at 25° C. of the humidity sensor obtained in this example. As is clear from the drawing, the relationship between relative humidity and electrical resistance value is linear over a wide range from low humidity of 5%R,H to high humidity of 95%R,H, and almost total relative humidity Humidity detection is possible over the area. In addition, the amount of change in electrical resistance value with respect to changes in relative humidity is large, (103~
106Ω), has high sensitivity, and furthermore, the characteristics can be shifted in parallel depending on the element dimensions.

On the other hand, in terms of responsiveness, when the relative humidity of the atmosphere was changed from 90% RH to 20% RH at a measurement temperature of 25°C, it took about 50 seconds for the resistance value to reach equilibrium; It can be seen that when changing from 90 cm Ft, H△, the time to reach equilibrium is also extremely short, approximately 45 seconds, and the response speed of the sensor to humidity is extremely fast.

Furthermore, various durability tests were conducted under the following conditions, and the humidity characteristics before and after the tests were investigated. As a result, a high temperature load life test was conducted in which AC5V was continuously applied to the device for 8,700 hours in a constant temperature bath at a constant temperature of 85C at 60C.

Continuously apply 5V AC to the device in an atmosphere of 90 to 95% RH.
High temperature and high humidity load life test for 700 hours and -10~
AC5V applied to the device in an atmosphere of 35℃ and 10 to 95℃RH.
The change in humidity after performing a normal temperature and normal humidity load life test, etc., in which 1.
At 34RH or lower, hardly any deterioration was observed.

The above characteristic description uses boron carbide (B4C) as an additive.
However, even when boron (B), boron oxide (B203), and boron nitride (BN) other than B4C are used as additives, the humidity characteristics are very similar to those of the humidity sensor with all B4C added.

It was experimentally confirmed that it has responsiveness and durability.

As explained in detail above, the humidity sensitive material of the humidity sensor of the present invention is a porous silicon carbide sintered body, and it is possible to measure humidity over a wide humidity range from low humidity to high humidity. Do you get it. Furthermore, the electrical resistance value changes greatly with respect to humidity changes, and the sensitivity to humidity changes is high. In addition, the response to humidity changes is fast, and
The characteristics are stable even under high humidity conditions and do not deteriorate, allowing stable measurements with high reproducibility.

Therefore, by using the humidity sensor of the present invention, it is possible to measure relative humidity with high reproducibility, excellent sensitivity, and quick response.Furthermore, the characteristics are stable and it can be used for a long period of time. Many effects can be obtained. or,
The manufacturing method of the humidity sensor of the present invention has the advantage that it is possible to mass-produce products with stable characteristics at a high yield, and the manufacturing cost is low.

[Brief explanation of the drawing]

Claims (1)

[Claims] 1. An electrical resistance humidity sensor characterized in that the moisture sensitive material is a porous silicon carbide sintered body. 2. The porous silicon carbide sintered body has a relative density of 6
0-90%, electrical resistivity 102-10102c
The electrical resistance type humidity sensor according to claim 1, which is m. 3. Claim 1, wherein the porous silicon carbide sintered body contains boron and/or a boron-containing compound.
The electrical resistance type humidity sensor according to item 1 or 2. 4. The electrical resistance humidity sensor according to claim 3, wherein the content of the boron and/or the boron-containing compound is 0.1 to 5% by weight in terms of boron substitution.