JPH0943185A - Moisture sensor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately measure low to high moisture with a high sensitivity at a high response speed by using an active carbon fiber with a specific pore property as a moisture sensor element. SOLUTION: A moisture sensor element is constituted of an active carbon fiber with pore property where a volume occupied by a micro pore with a diameter of 20Å or less is equal to or more than 85% of a total pore area and a total power volume is equal to or more than 0.5cc/H. The pore of an active carbon fiber is made by activating carbon fiber within oxidizing gas atmosphere or carbonizing an organic precursor fiber in oxidizing gas atmosphere. A micro power whose diameter is 20Å or less rapidly absorbs or desorbs moisture and can smoothly and uniformly absorbs and desorbs water when an occupation volume is at least 85% of the total pore volume, thus reducing the scattering in electrical resistance change due to the absorption and desorption of water and accurately measuring moisture change. Further, since absorbed and desorbed water can be rapidly and uniformly absorbed in the pore of the micro pore even in a moisture atmosphere with less amount of absorbed water, thus accurately measuring moisture.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a humidity sensor element capable of measuring humidity in a wide humidity range from low humidity to high humidity and capable of detecting humidity with high accuracy and high sensitivity.

Humidity sensors detect humidity by combining the adsorption of water and the resulting physical changes.
Humidity sensors that use expansion and contraction due to moisture absorption and desorption of hair have long been known. However, this humidity sensor has drawbacks such as insufficient measurement sensitivity and large hysteresis during adsorption and dehumidification, and many detection elements for humidity sensors have been developed thereafter.

[0003]

2. Description of the Related Art Heretofore, electrolyte-based, metal-based, polymer-based, ceramic-based, etc. have been known as detection elements. Among them, the ceramic type is a sintered body mainly composed of a metal oxide and is thermally and physicochemically stable, so that a porous sintered body having appropriate pores and surface areas is often used. The ceramic humidity sensor has features such as a wide usable temperature range, a wide humidity measurement range, a fast response speed, and a low cost material, but it has a large drift at high humidity and is caused by chemisorption. In order to eliminate the effects of sensor deterioration and aging, there is a drawback in that it is necessary to heat the sensor element to release the chemically adsorbed water, or to apply a coating treatment to the sensor element surface.

The polymer type humidity sensor detects humidity from changes in electric resistance, dielectric constant and the like due to hydration swelling and moisture absorption of a moisture sensitive polymer film, and polystyrene sulfonic acid, alginic acid, polyacrylic acid, cellulose type. Detecting element materials such as polymer films have been developed. However, a material that can withstand long-term stable use in a high humidity atmosphere has not yet been developed. In addition, a detection element in which an electrolyte aqueous solution such as lithium chloride is impregnated in a porous matrix has a drawback that it cannot be used due to lithium chloride flowing out when it is used for a long time in a high humidity state or when it is condensed.

Further, a humidity sensor in which a hygroscopic substance is supported by using activated carbon fiber as a carrier has been developed. In Japanese Patent Publication No. Sho 61-22898, a fibrous shape having pores having an average diameter of 10 to 50 angstroms is developed. A humidity sensor in which activated carbon is loaded with a hygroscopic substance has been proposed. This humidity sensor is made by supporting fibrous activated carbon with a specific pore distribution with organic hygroscopic substances such as ethylene glycol and triethylene glycol, and inorganic hygroscopic substances such as lithium chloride and magnesium chloride. The humidity is detected by measuring the change in electric resistance.

[0006]

However, when the activated carbon fiber type humidity sensor is used for a long time in a high-humidity atmosphere or a high-temperature atmosphere, the hygroscopic substance may flow out or deteriorate due to deliquescent, resulting in high accuracy and high sensitivity. There is a problem that humidity cannot be measured.

In view of the problem that the function of the humidity sensor deteriorates due to the material properties of the hygroscopic substance, the present inventor has found that the porosity of the activated carbon fiber and the moisture absorption / dehumidification characteristics of the activated carbon fiber do not use the hygroscopic substance. As a result of diligent research on the relationship with, the use of activated carbon fiber having a specific porosity as a detection element, by measuring the change in electrical resistance due to adsorption and desorption of moisture absorption moisture, stable and highly accurate with high sensitivity It has been found that humidity can be detected.

The present invention has been completed based on the above findings, and its purpose is to be able to measure humidity in a wide range from low humidity to high humidity, and to have excellent response speed, high temperature stability, and temporal stability, and high accuracy. Another object of the present invention is to provide a humidity sensor element capable of detecting humidity with high sensitivity.

[0009]

That is, the humidity sensor element according to the present invention for achieving the above object has a diameter of 2 mm.
The volume occupied by micropores of 0 Å or less is 85% or more of the total pore volume, and the total pore volume is 0.5 cc / g.
The structural feature is that the activated carbon fiber has the above-mentioned porosity.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Activated carbon fibers are those in which pores are generated by a method of activating a carbon fiber in an oxidizing gas atmosphere or a method of carbonizing an organic precursor fiber in an oxidizing gas atmosphere. Since these pores can give a large adsorption performance, they are used as various adsorbents.

In order to use this activated carbon fiber as a humidity sensor element, it is necessary that water is adsorbed and the adsorbed water is easily removed, that is, moisture absorption and dehumidification are performed smoothly. Further, it is also an important factor that the change in electric resistance due to water adsorption / desorption is large, a large amount of water can be adsorbed, and that the water adsorption / desorption is not irreversible adsorption (chemical adsorption).

Among the pore characteristics specified in the present invention, the diameter is 2
Minute micropores of 0 angstrom or less function effectively to absorb and dehumidify water quickly. The micropores having a diameter of 20 angstroms or less need to have a volume of 85% or more of the total pore volume. If there are many large-diameter pores, it becomes difficult for water adsorption and desorption to proceed smoothly and evenly, and variations in electrical resistance change due to water adsorption tend to occur. 8 in volume
By setting the porosity to be 5% or more, moisture can be smoothly adsorbed and desorbed. As a result, variations in electric resistance change due to water adsorption / desorption can be reduced, and humidity change can be accurately measured. Further, not only in a high humidity atmosphere, but also in a low humidity atmosphere with a small amount of adsorbed moisture, the absorbed moisture is quickly and evenly adsorbed in the pores of the micropores, so that the humidity can be accurately measured.

Further, as a porosity, the total pore volume is 0.5.
It must be cc / g or higher. Total pore volume is 0.5
If it is less than cc / g, the amount of water that can be adsorbed is small, and therefore, if the measurement is performed for a long time in a high humidity atmosphere, the pores will be saturated with the adsorbed water and the humidity cannot be measured.

The activated carbon fiber having the porosity of the present invention is obtained by subjecting an organic precursor fiber obtained by melt-spinning a phenol resin or a furan resin which is converted into a glassy carbonaceous substance by firing to a carbonizing treatment in a non-oxidizing atmosphere. It can be produced by activating the glassy carbon fiber.

In this case, the firing carbonization treatment of the organic precursor fiber is performed by heat-treating at a temperature of 1000 ° C. or lower, preferably 900 ° C. or lower in a non-oxidizing gas atmosphere such as nitrogen gas or argon gas. To make. If the firing carbonization temperature exceeds 1000 ° C., the semiconducting properties of the glassy carbon fiber become weak, making it difficult to measure the change in electrical resistance due to water adsorption. Diameter 20
It becomes difficult to form fine micropores of angstrom or smaller.

The activation treatment is carried out by using glassy carbon fiber in air,
It is possible to apply a normal method of heat treatment in an oxidizing gas atmosphere such as carbon dioxide, water vapor or ozone,
The porosity is controlled by adjusting the temperature of the heat treatment, the heating rate, the heating time, and the like. The preferred activation conditions are a heating temperature of 800 to 1000 ° C. and a heating rate of 100 to 30.
The heating time is 0 ° C./hr and the heating time is 1 to 3 hr. Thus, the activated carbon fiber having the porosity specified in the present invention is manufactured.

Among the pore properties specified in the present invention, the diameter is 2
The volume occupied by micropores of 0 angstroms or less is 85% or more of the total pore volume, which is characterized by a large distribution of fine pores, and these fine pores quickly and evenly adsorb moisture during moisture absorption. be able to. Therefore, the electric resistance due to moisture absorption changes accurately and promptly, and as a result, the response speed is fast and the humidity can be detected with high accuracy and high sensitivity.

Also, since activation treatment is carried out after firing and carbonization, the pore structure formed does not become complicated, and since it has a simple shape, the adsorption and desorption of water can be carried out smoothly. Since physical adsorption is dominant and chemical adsorption hardly occurs, humidity can be detected with good reproducibility without causing hysteresis accompanying adsorption / desorption.

Further, the pores having a total pore volume of 0.5 cc / g or more have a large water adsorption capacity and can be measured for a long time in a high humidity atmosphere. Due to the nature of the property, moisture can be adsorbed quickly and evenly in the low humidity region, so that the humidity can be detected with high accuracy and high sensitivity even in the low humidity atmosphere. Therefore, it is possible to measure a wide range of humidity from low humidity to high humidity.

Since the humidity sensor element of the present invention is composed of activated carbon fibers having such a porosity, it is stable even in a high temperature and corrosive gas atmosphere, and can measure humidity in a wide range from low humidity to high humidity. It is possible, excellent in response speed, high temperature stability, stability over time, and capable of detecting humidity with high accuracy and high sensitivity.

[0021]

Hereinafter, examples of the present invention will be described in comparison with comparative examples.

Examples 1-5, Comparative Examples 1-5 Phenolic resin fiber [Kynol fiber manufactured by Gunei Chemical Co., Ltd.]
KR0240] was calcined and carbonized in a nitrogen gas atmosphere at different heat treatment temperatures to produce glassy carbon fibers. Then
The glassy carbon fiber was heated in carbon dioxide gas to be activated. The activation treatment was performed by adjusting the heating temperature, heating rate and heating time to produce activated carbon fibers having different porosity. The porosity was measured by mercury porosimetry.
The porosity of the activated carbon fiber thus obtained is shown in Table 1 in comparison with the treatment conditions.

[0023]

[Table 1] (Table Note) * 1 [(volume of micropores with a diameter of 20 angstroms or less) / (total pore volume)] x 100

Humidity sensors were assembled using these activated carbon fibers as detection elements, set in constant humidity chambers having different humidities, and the electric resistance after 1 minute was measured. The results are shown in Table 2.

[0025]

[Table 2]

Regarding Example 1 and Comparative Example 3 in Table 2,
FIG. 1 is a graph showing the relationship between relative humidity (%) and electric resistance (Ω). 1 to Example 1, the electric resistance change due to humidity is large over a wide humidity range from low humidity to high humidity and shows a good correlation. However, in Comparative Example 3, the change amount of electric resistance due to humidity is small and adsorption It can be seen that it sometimes takes time to reach an equilibrium state and the sensitivity is poor as a humidity sensor element.

Next, in order to evaluate the response performance and reproducibility as a humidity sensor, the change in electric resistance due to adsorption and desorption was measured by the following method. Humidity sensor 10% humidity
After reaching the equilibrium state by placing it in a desiccator adjusted to No. 2, it was set in a closed container filled with water whose humidity was adjusted to 98% and kept for 10 minutes. Then, the humidity sensor was returned to the desiccator and held for 10 minutes. In this operation cycle, the change with time of the electric resistance was measured when it was held in a closed container (adsorption process) and in a desiccator (desorption process).

Using the humidity sensors in which the activated carbon fibers of Example 1 and Comparative Example 3 in Table 1 were assembled as detection elements, the electrical resistance associated with adsorption / desorption was measured by the above method, and the adsorption time, desorption time and electric The change in resistance is shown in FIG. From FIG. 2, when the activated carbon fiber of Example 1 is used as the detection element, water adsorption occurs rapidly in the adsorption process, so that the electric resistance rapidly decreases and the equilibrium state is reached within 1 minute of the adsorption time. In the process, desorption of adsorbed water rapidly progresses to increase electric resistance, similarly, an equilibrium state is reached within 1 minute, and no change in electric resistance after equilibration is observed. That is, it can be seen that there is little variation in measured values and the response performance is excellent. Furthermore, since the amount of change in electric resistance due to adsorption and dehumidification is large, humidity can be detected with high accuracy and high sensitivity.
On the other hand, when the activated carbon fiber of Comparative Example 3 is used as the detection element, both the adsorption process and the desorption process take a long time to reach the equilibrium state, the electric resistance change amount at the equilibrium time is small, and the response performance and the detection accuracy are poor. It turns out.

[0029]

As described above, according to the present invention, by using the activated carbon fiber having a specific pore property as the humidity sensor element, the adsorption and desorption of water occur quickly and smoothly, and the response speed is fast and high. Humidity can be detected with high accuracy and high sensitivity. Furthermore, it has a wide measurable humidity range from low humidity to high humidity, and can be stably used for a long period of time even in a high temperature and corrosive gas atmosphere, and is therefore extremely useful as a humidity sensor element.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between relative humidity and electric resistance.

FIG. 2 is a graph showing a relationship between adsorption time and desorption time and electric resistance.

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[Procedure amendment]

[Submission date] September 5, 1995

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Claims (1)

[Claims] 1. The volume occupied by micropores having a diameter of 20 Å or less is 85% or more of the total pore volume, and the total pore volume is composed of activated carbon fibers having a porosity of 0.5 cc / g or more. Humidity sensor element.