JP2004226177A - Humidity sensor and humidity measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel humidity sensor, usable even in an atmosphere in which corrosive gas exists, having a detection structure and measurement constitution prevented from being complicated, and being in no danger of causing irreversible change, and to provide a humidity measuring method. <P>SOLUTION: This humidity sensor is characterized in that it is made up of a quartz oscillator and electrodes installed on both sides thereof so as to interpose the oscillator between them, that the surfaces of the electrodes are each coated with a metal oxide film, or that the electrodes are made of metal oxide. This sensor is used in this humidity measuring method. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a humidity sensor and a method for measuring humidity.
[0002]
[Prior art]
Measuring the amount of water vapor contained in the air, that is, measuring the humidity, is an industrial or environmentally necessary matter. In recent years, not only in the environmental field, but also in the management and operation of manufacturing equipment and equipment in factories and business establishments, it is necessary to maintain a constant humidity, and the operation of precision machinery and equipment that may cause problems due to condensation. In terms of management, there is a high demand for accurate, small and weighing equipment.
Conventionally, various types of hygrometers have been used.
For example, (1) a method of measuring humidity by measuring elongation that occurs when hair absorbs water vapor (a so-called hair hygrometer), and (2) a method of using a phenomenon in which the electric resistance of an electrolyte is changed by water vapor (patent) Literature 1), (3) A method of determining a change in electric resistance by adsorbing moisture of an organic substance, a semiconductor, a ceramic, a metal oxide, or the like (Patent Document 2), or a method of determining a change in electric capacity (Patent Document 3) (4) A method of detecting a change in absorbance using infrared rays, (5) Utilization of humidity dependence of a visible absorption spectrum or a fluorescence intensity of a film containing an organic dye such as crystal violet or betaine dye in a polymer compound carrier. (6) a method using the visible absorption spectrum of cobalt chloride supported on a polymer carrier, (7) a quartz oscillator coated with polyamide or the like. Min and a method of detecting the weight change caused by the adsorption.
[0003]
However, all of these methods have problems. The problems are as follows.
The method (1) cannot be used because the operable atmosphere temperature is limited to around room temperature, and the hair changes in an atmosphere where corrosive gas such as acid or alkali is present.
In the method (2), as compared with other methods, the detection structure becomes complicated, and maintenance work of detection parts such as electrodes and electrolytes is required.
In the method (3), particularly when the resistance change is used, since the conductivity of H ⁺ ions generated by water adsorption is often used, the amount of adsorbed water is reduced. In a low humidity region, the resistance of the element tends to be excessive, and it is difficult to accurately measure the resistance value by a normal method. This phenomenon hinders humidity measurement. In addition, there is generally difficulty in measurement due to excessive element resistance. Further, when a hydrophilic polymer is used as the dry and wet material, there is a problem in use because a stable output and a problem of deterioration of the film structure are likely to occur in a high humidity region where there is a possibility of dew condensation.
The method (4) has a complicated measurement configuration such as a light-emitting unit, a light-receiving unit, and optical components to be used, becomes a large measuring device, and tends to be expensive.
In the methods (5) and (6), since the organic dye, the organic polymer, and the like lack heat resistance, the operating humidity range is relatively narrow, and irreversible changes may occur when highly reactive steam is present in the atmosphere. There is a problem such as there is.
The method (7) has a problem that even if molecules such as gas other than moisture are adsorbed, a signal is output as a change in the weight of the element, so that it is difficult to selectively detect only moisture.
For these reasons, a new type of sensor different from the conventional humidity sensor is demanded.
[0004]
[Patent Document 1]
Japanese Patent Application Laid-Open No. 2000-28563 [Patent Document 2]
JP 10-239264 A [Patent Document 3]
JP-A-9-43186
[Problems to be solved by the invention]
It is an object of the present invention to provide a new humidity sensor which differs from the conventional type.
[0006]
[Means for Solving the Problems]
The present inventors have studied the above-mentioned problems, and have found the following matters to complete the present invention.
A humidity sensor consisting of electrodes placed on both sides of a quartz oscillator, whose electrode surface is covered with a metal oxide film, or whose electrodes are made of a metal oxide, forms a humidity sensor. When contacted with a gas containing water, a physical adsorption / desorption reaction of water molecules (moisture) occurs on the surface of the metal oxide according to the content of water vapor. In the vibrating part formed by the electrodes of the crystal unit, a change in weight occurs according to the amount of water vapor adsorbed, and a phenomenon occurs in which the oscillation frequency of the crystal unit resonating at a constant frequency decreases, and this frequency change Can be caught. Therefore, the change in the content of water vapor, that is, the change in humidity can be measured by the change in the resonance frequency.
[0007]
In the metal oxide thin film covering the electrode surface, the metal oxide film or the electrode is composed of chromium oxide, or composed of copper oxide, or composed of a single metal component selected from nickel, cobalt, and manganese. Or an oxide semiconductor made of a mixture of metals selected from the above.
Alternatively, a layer or film in which an oxide semiconductor formed by adding nickel, cobalt, or manganese at a single ratio or an arbitrary ratio and an oxide semiconductor of tin is used can be used.
As the electrode surface material, it is desirable to use a semiconductor metal material such as chromium oxide and copper oxide, but depending on the environment to be measured, nickel, cobalt, manganese may be contained in a single or any ratio. A manufactured oxide semiconductor or a metal oxide film formed by stacking an oxide semiconductor formed by mixing nickel, cobalt, and manganese in a single or arbitrary ratio and an oxide semiconductor formed of tin can be used. .
The electrode material itself is a known material such as gold or silver which is generally used without any problem, and it is sufficient that the surface of the electrode to be detected is chromium oxide, copper oxide or the like. Therefore, it is sufficient that these metal oxides are coated on the surface of an electrode material such as gold by a known method. The coating thickness may be any thickness as long as the moisture adsorption phenomenon appears and the quartz resonator itself can electrically oscillate. Further, the surface covered with the electrode may be of any size, but is preferably equal to or less than the electrode area of the base. It is also possible to create and use an oxide film of its own on a surface of chromium, copper, nickel, or the like, which is generally used as an electrode material of a quartz oscillator, by a known method or the like.
[0008]
According to the present invention, the following inventions are provided.
(1) It is composed of a quartz oscillator and electrodes placed on both sides of the quartz oscillator, and the electrode surface is covered with a metal oxide film or the electrode is made of a metal oxide. Humidity sensor.
(2) The humidity sensor according to (1), wherein the metal oxide film or the electrode on the electrode surface is made of chromium oxide.
(3) The humidity sensor according to (1), wherein the metal oxide film on the electrode surface or the electrode is made of copper oxide.
(4) The metal oxide film or the electrode on the electrode surface is composed of a single metal component selected from nickel, cobalt, and manganese, or is an oxide semiconductor composed of a mixture of metals selected from these. The humidity sensor according to claim 1, wherein:
(5) The metal oxide film or the electrode on the electrode surface is composed of a single metal component selected from nickel, cobalt, and manganese, or is an oxide semiconductor composed of a mixture of metals selected from these. (1) The humidity sensor according to (1), wherein the humidity sensor is complexed with a tin oxide.
(6) A humidity measuring method, wherein the humidity sensor according to any one of (1) to (5) is brought into contact with a gas to be measured and a change in resonance frequency is measured.
[0009]
BEST MODE FOR CARRYING OUT THE INVENTION
The humidity sensor of the present invention is configured as follows.
It is composed of a crystal unit and electrodes provided on both sides of the crystal unit, and the electrode surface is covered with a metal oxide film, or the electrode is made of a metal oxide.
Quartz crystal oscillators are widely used in various electronic devices because they have excellent performance with respect to piezoelectric oscillators, particularly frequency accuracy and stability, as standards for frequency and time. The resonance frequency of the crystal resonator is determined by the outer shape. For example, in the case of an AT-cut thickness-sliding quartz crystal resonator generally used in a frequency band of several MHz to several tens of MHz, a bar-shaped artificial quartz is cut into a predetermined angle plate shape with respect to a crystal axis, and a circle is formed. It is a piece of crystal such as a plate, rectangle, or strip. In such an AT-cut crystal resonator, the resonance frequency is determined by the thickness of the crystal piece. The plate surface of the crystal blank is polished to a thickness corresponding to a desired resonance frequency. Then, excitation electrodes are formed on the opposing surfaces using a known means such as vacuum deposition to have a desired thickness, and the excitation electrodes are led to both side edges. Then, it is sandwiched by holding members fixed to the distal ends of a pair of terminals provided on portions based on both side edges of the crystal blank, and a conductive adhesive or the like is applied and fixed thereto, and mechanically held. In general, an electrical connection is made between the excitation electrode and the terminal. If the crystal oscillator is used as a standard element, not as a sensing element, but as a reference frequency for communication equipment and electrical equipment, etc. It is a common practice to cover with a vacuum and hermetically seal in a vacuum or inert gas atmosphere.
[0010]
In the present invention, a metal oxide is used for the electrode surface or the electrode material. As a characteristic of the metal oxide, any metal oxide can be used as long as a physical adsorption / desorption reaction of water molecules (moisture) occurs on the surface of the metal oxide in accordance with the content of water vapor. .
Metal oxide is composed of chromium oxide, or composed of copper oxide, nickel, cobalt, a single oxide of manganese, or an oxide semiconductor prepared by containing at an arbitrary ratio, nickel, cobalt , Manganese may be used alone or in an arbitrary ratio, and an oxide semiconductor and a tin oxide semiconductor may be stacked and used.
[0011]
When only the electrode surface is made of the above-mentioned material, the electrode material itself is a known material such as gold or silver which is generally used without any problem. And so on. Therefore, it is sufficient that these metal oxides are coated on the surface of an electrode material such as gold by a known method. The coating thickness may be any thickness as long as the moisture adsorption phenomenon appears and the quartz resonator itself can electrically oscillate.
Further, the surface covered with the electrode may be of any size, but is preferably equal to or less than the electrode area of the base. It is also possible to create and use an oxide film of its own on a surface of chromium, copper, nickel, or the like, which is generally used as an electrode material of a quartz oscillator, by a known method or the like.
When the electrode material is composed of the above-mentioned materials, it is not necessary to dare further cover the electrode surface with the same material.
[0012]
When nickel and cobalt are contained in an arbitrary ratio, they may be in any ratio, but are preferably 1:99 to 1: 1.
When nickel and manganese are contained at an arbitrary ratio, they may be at an arbitrary ratio, but are preferably 1:99 to 1: 1. In the case of manufacturing by containing cobalt and manganese at an arbitrary ratio, each may be at an arbitrary ratio, but it is preferable that the atomic ratio of cobalt and manganese is about 1: 1.
In the case where nickel, cobalt, and manganese are contained in an arbitrary ratio, the nickel, cobalt, and manganese may be in an arbitrary ratio, but may be 1:99:99 to 1: 1: 1 in atomic ratio. preferable.
Nickel, cobalt, composed of a single metal component selected from manganese, or, for an oxide semiconductor consisting of a mixture of metals selected from these, when further tin oxide is multi-layered Any thickness may be used as long as the crystal resonator itself can electrically oscillate, but is preferably about 100 nm.
[0013]
In the present invention, other components (organic components, metal salts, and the like) may be contained in addition to the above-described composite oxide as long as the effects of the present invention are not impaired.
[0014]
The method for forming the thin film is not particularly limited, and a known thin film forming method can be employed as it is. For example, a vapor phase method such as a sputtering method, a vacuum evaporation method, and a CVD (Chemical Vapor Deposition) method, an inorganic acid metal salt (nickel nitrate, copper nitrate, etc.), an organic metal salt (copper naphthenate, nickel octylate, naphthenate) Any method such as a method of applying a solution such as nickel or the like or a metal alkoxide (dimethoxy copper, acetylacetone copper or the like) on the device electrode and thermally decomposing the solution can be employed.
[0015]
In the method of thermally decomposing a solution of a metal or the like, for example, each solution is mixed at a metal component ratio such that a desired composite oxide is obtained, and the obtained mixed solution is applied on an electrode surface and dried. Usually, an atmosphere containing oxygen such as air (atmosphere) may be used. In the thermal decomposition method, an organic compound can be added to the mixed solution. In this case, a solid organic compound such as camphor (C ₁₀ H ₁₆ O) or urea (CO (NH ₂ ) ₂ ) may be added at room temperature, and sublimation or decomposition by heating may cause vaporization. Since they are scattered, a porous material having a larger surface area can be obtained by adding them. As a result, a large amount of water can be adsorbed, and the frequency change becomes more sensitive due to the humidity change. The amount of addition may be such that the ratio in the raw material is usually about 1: 0.01 to 1: 5, preferably 1: 0.05 to 1: 1. Note that two or more of these organic compounds can be used.
[0016]
The thickness of the thin film is not particularly limited as long as a change in frequency can be detected. Generally, in the case of a homogeneous and dense thin film such as a thin film formed by a sputtering method, the surface area is relatively small, so that it is usually about 1 to 200 nm, preferably 1 to 100 nm. On the other hand, in the above method of applying a solution of a metal salt or the like, since the surface area of a thin film generally formed is large, a change in frequency can be detected even if the thin film is thicker than the above range (for example, about 5 to 500 nm). In addition, these conditions are the same also about the case where two or more layers are formed.
[0017]
Here, the following mechanism is considered as a reason that the frequency change due to the metal oxide of the present invention changes according to the humidity change in the air. Water having a higher dielectric constant than the oxide is physically adsorbed on the surface of the metal oxide (when metal oxide is used as the thin film), so that the weight of the entire film increases and the frequency change increases accordingly. Can be considered.
[0018]
In the measurement method using a quartz oscillator, since the measured value is read from an electric signal, there is no difference in the read value between the measurers. In addition, the quartz oscillator is a general-purpose product used for electric equipment and the like, and a portable small-sized and low-cost measuring instrument can be manufactured.
[0019]
As a conventional humidity measuring means using a quartz oscillator, there has been a method using an organic or polymer film such as polyamide, lipid, or amine as a detection film. However, in a measurement method using a quartz oscillator using these substances as a detection film, it is difficult to measure only humidity because an adsorption reaction occurs not only in humidity but also in a gas contained in an object to be measured. Therefore, in order to perform high-sensitivity detection, it is necessary to take sufficient measures, and the device becomes expensive.
On the other hand, in the measurement method using the electrode surface of the quartz oscillator such as chromium oxide or the electrode material itself, the process for manufacturing the detection film can be largely omitted. Further, since almost no adsorption reaction by the gas contained in the measurement object occurs, it is possible to detect and measure efficiently and effectively.
Adsorption due to humidity hardly occurs on the surface of commonly used electrodes such as gold, silver, and silver oxide due to humidity, or a slight change.
[0020]
【Example】
Example 1
FIG. 1 schematically shows a humidity measurement method according to an embodiment of the present invention. In this case, an example is shown in which measurement is performed using a simple humidifying unit by bubbling water with air (nitrogen) in a laboratory. It is to be noted that such a technique is not necessarily used, and measurement can be performed even when the apparatus is left in a measurement atmosphere.
This time, a commercially available nitrogen (bomb) was used as the supply gas (air). First, nitrogen is output from the cylinder with a decompressor at an appropriate pressure, and a filter 1 is installed in the measuring section 2 so that dirt such as dust is not taken into the element. It is not always necessary to provide such a filter, and may be omitted depending on the measurement environment. Nitrogen that has passed through the filter is adjusted to an arbitrary flow rate by the flow meter 3 or cut off (cut off) as necessary. One pipe is connected to the measuring section 2 as it is, and the other pipe is connected to a humidifying section 4 for generating humidity by putting pure water or the like in a closed container and bubbling nitrogen flowing from the flow meter 3 there. It is composed of Here, low-humidity nitrogen and nitrogen having a humidity close to saturation by the humidifying unit 4 are supplied to the measuring unit 2. Therefore, an arbitrary humidity is supplied to the measurement unit 2 by mixing one or both at an arbitrary flow rate according to the required humidity.
Next, nitrogen containing arbitrary humidity is sent to the measurement unit 2 and the humidity in the measurement environment is measured on the electrode surface of the crystal unit 5, for example, coated with chromium oxide, installed on the measurement probe. The principle is that a weight change is caused by the adsorption of water molecules as a sample, and the amount of change is converted into a frequency change for measurement. The frequency change can be measured by a counter or the like as a known measuring device. The chromium oxide element used here was an AT-cut, 9 MHz gold-based crystal resonator electrode coated with chromium oxide on one side as a thin film of about 100 nm by a vacuum evaporation method. In addition, in order to confirm the change in humidity, a known and commercially available hygrometer and a thermometer for measuring a change in electric resistance and electric capacity were installed in the same flow system and checked. In the case of performing more stable measurement, for example, a constant temperature device (part) 6 capable of heating and / or cooling is used to obtain the oscillation frequency of the crystal unit 5 as a stable and constant signal. It is desirable to have a structure that can make the temperature of the substrate constant or controllable. Note that such a device is not necessarily required in the present measurement technique.
[0021]
In the measuring apparatus configured as described above, nitrogen of about 20% humidity (the one that does not pass through the humidifying unit 2) is used for 5 minutes, and nitrogen of 95% humidity (the one that passes through the humidifying unit 2) is used for 5 minutes. The frequency change was measured at a flow rate of 200 ml / min. Note that air or other gas may be used instead of nitrogen. For comparison, a quartz oscillator having only a gold electrode was set in the measuring section 2 and measurement was performed at the same humidity concentration as described above.
[0022]
FIG. 2 shows an example of the relationship between the response characteristic and the frequency change at the time of the alternate switching measurement of 20% to 95% relative humidity. In this example, the abscissa represents time (unit: hours: minutes: seconds), and the ordinate represents chromium oxide as a thin film having a thickness of about 100 nm on a 9-MHz gold-based crystal oscillator electrode by a vacuum evaporation method on one side. The graph shows the frequency change (unit: Hz) with respect to humidity when a coated element is used.
First, it is shown that when nitrogen of about 20% relative humidity is passed through the measuring unit 2 for about 5 minutes, the frequency change is almost zero. Next, when nitrogen having a relative humidity of about 95% that has passed through the humidifying section 4 is similarly flown to the measuring section 2 for about 5 minutes, a rapid frequency change (frequency drop) occurs within a few seconds, and after about 5 minutes, It is shown that the frequency has changed by about 200 Hz. Next, the process of flowing nitrogen having a relative humidity of about 20% to the measuring section 2 for about 5 minutes and then flowing nitrogen having a relative humidity of about 95% to the measuring section 2 for about 5 minutes was repeated three times. Represents. As a result of humidification, in this example which was repeated three times, almost the same frequency change occurred, and the response was also the same. It was also found that the frequency returned to almost the same when nitrogen of a relative humidity of about 20% as a reference was passed.
For reference, an experiment was conducted on the frequency change by the gold electrode (AT cut, 9 MHz gold-based crystal resonator element) under the same humidity and process as in this example, but the frequency change hardly changed to about several Hz. I found out. That is, in the case of using the chromium oxide electrode element, it was found that in this example, the detection characteristic was about 200 Hz at maximum (basic frequency 9 MHz, 20 ° C.).
[0023]
【The invention's effect】
As described above, in such a humidity measurement method, since moisture is directly adsorbed on the electrode surface, measurement can be performed with high sensitivity using a known technique such as increasing the basic resonance frequency of the crystal unit. . In addition, since it is possible to carry the device in a field environment and perform quick measurement there, the effects of labor saving and cost reduction can be obtained.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram illustrating a configuration of a humidity measurement method according to an embodiment of the present invention.
FIG. 2 is a graph illustrating an example of a relationship between a response characteristic and a frequency change at the time of alternate switching measurement of 20% to 95% relative humidity.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Filter 2 Measuring part 3 Flow meter 4 Crystal oscillator 5 Humidifying part 6 Constant temperature device (part)

Claims (6)

A humidity sensor comprising a quartz oscillator and electrodes provided on both sides of the quartz oscillator, and the electrode surface is coated with a metal oxide film or the electrode is formed of a metal oxide. 2. The humidity sensor according to claim 1, wherein the metal oxide film on the electrode surface or the electrode is made of chromium oxide. The humidity sensor according to claim 1, wherein the metal oxide film or the electrode on the electrode surface is made of copper oxide. The metal oxide film or the electrode on the electrode surface is composed of a single metal component selected from nickel, cobalt, and manganese, or is an oxide semiconductor made of a mixture of metals selected from these. The humidity sensor according to claim 1. The metal oxide film or the electrode on the electrode surface is composed of a single metal component selected from nickel, cobalt, and manganese, or is an oxide semiconductor composed of a mixture of metals selected from these, and further oxidizes tin. 2. The humidity sensor according to claim 1, wherein the humidity sensor is combined with a substance. A humidity measuring method, comprising: bringing the humidity sensor according to any one of claims 1 to 5 into contact with a gas to be measured to measure a change in resonance frequency.

Images