JPS5948637A - Humidity sensor - Google Patents

Abstract

PURPOSE:To enable accurate detection of relative humidity by reading changes in the oscillation frequency depending on a drop or a stop of the oscillation due to a dielectric relaxation based on the decrease in the resistance value due to humidity absorption of a piezo-electric body. CONSTITUTION:A piezo-electric body 2 is formed on the side of a tuning fork type vibrator body 1 and an electrode 3 thereon 2. The electrode 3' is preferably an island or net in the shape considering hygroscopicity and connected to a widely accepted oscillation circuit as surrounded by the broken line 4. As illustrated, the oscillation frequency decreases down to 65% in the relative humidity and the oscillation stops at this relative humidity of 65%. The percentage of the relative humidity can be confirmed by reading the oscillation frequency. The oscillation stops at the relative humidity of 65%, when the resistance value is close to 10<6>OMEGA. No piezo-electric property is indicated below this resistance value. Thus, the relative humidity can be detected exactly by reading changes in the oscillation frequency of the vibrator.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a humidity sensor having a function of stopping the oscillation of a vibrator when the humidity reaches a certain level.

There are two types of humidity sensors: one whose resistance changes as the humidity rises, and another whose capacitance changes as the humidity rises.

In addition, there is a humidity sensor that uses a piezoelectric vibrator and has a characteristic that oscillation stops as the humidity increases.

This utilizes the phenomenon that when absorbent mjrA is attached to the surface of a piezoelectric vibrator and the mass of the absorbent material increases due to rise in humidity, the oscillation is suppressed and covered.

The present invention relates to a type of humidity sensor using a piezoelectric vibrator, and provides a humidity sensor with a simple structure that can detect the humidity state based on changes in the oscillation state without using a moisture absorbing material as described above. It is.

That is, the gist of the present invention is to provide a vibrator composed of a vibrator body, a piezoelectric body formed on any surface of the vibrating part of the vibrator body, and an electrode formed on the piezoelectric body. The humidity sensor is characterized in that the piezoelectric material is a piezoelectric semiconductor made of a material with lattice defects or a material whose valence is controlled.

Among the components of such a humidity sensor, the vibrator body may be of a plate shape, a tuning fork shape, a vibrating piece shape, etc., and the shape may be particularly limited if it can be vibrated by forming a piezoelectric material and electrodes, which will be described later. is not limited.

Further, the piezoelectric body is formed in the vibrating portion of the vibrator body described above, and serves as a driving source for bending and vibrating the vibrator body.

Here, the types of piezoelectric materials include nb-vxb group compounds, such as ZnO, ZnS, CdS, CdSe, and Cd
mb-vb group compounds such as Te, e.g. GaP, Ga
In addition to As, *n1Sb, etc., there are also lithium niobate, lithium tantalate, lead zirconate titanate ceramics, and ceramics.

Methods for forming these piezoelectric bodies include vacuum evaporation, sputtering, ion blasting, and vapor deposition. In addition, a piezoelectric material may be bonded to the vibrator body.

A particularly important point in this invention is that the oscillation of the vibrator stops when the relative humidity exceeds a certain value.

In other words, even in piezoelectric materials, donor impurity units are generated in the forbidden band due to lattice defects due to stoichiometric misalignment or valence control through impurity addition, and while they take on semiconducting properties, they do not become activated. It absorbs moisture easily,
It has a property that the resistance value decreases as the relative humidity increases.

For example, in the case of zinc oxide, when the resistance value decreases to 10 6 Ω or less, its properties as a semiconductor become more pronounced than its properties as a piezoelectric material.

In general, whether a material exhibits piezoelectric or semiconducting properties can be determined by looking at the dielectric relaxation angular frequency (ωG). This dielectric relaxation angular frequency (ωC) is expressed by the following equation.

ωC= σ/ε0εP Here, σ: Electrical conductivity of the four piezoelectrics ε0: Dielectric constant of the vacuum εP: Dielectric constant of the piezoelectric heather Now, when the target angular frequency (ω) becomes ωC)ω,
The polarization generated within the crystal due to the free electrons is short-circuited and disappears, resulting in the crystal no longer exhibiting piezoelectricity.

Therefore, when the resistance value of the crystal decreases and the conductivity increases, the vibrator using the piezoelectric material as a driving source stops oscillating.

If we focus on the properties of piezoelectric materials, as humidity increases, resistance decreases, which leads to a decrease or stop of the oscillation frequency of the vibrator, making it possible to confirm changes in humidity.

In order to make the piezoelectric body a piezoelectric semiconductor in a lattice defect state, for example, when forming the piezoelectric body by sputtering method, 02 and A
There is a method of adjusting the gas pressure during sputtering by changing the mixing ratio of r. Furthermore, in order to control the valence and make a piezoelectric semiconductor, for example, for ZnO, there is a method of adding a trivalent or higher valence element such as AI.

FIG. 1 is a schematic diagram according to an embodiment of the present invention, showing a state in which a tuning fork type vibrator is incorporated into an oscillation circuit. In the figure, reference numeral 1 denotes a tuning fork-shaped vibrator main body, a piezoelectric body 2 is formed on the side surface of the vibrating piece, and an electrode 3 is further formed on the piezoelectric body 2. As electrode 3, ALI, AI
For example, it is formed by a method such as a vacuum evaporation method or a sputtering method. Considering the hygroscopicity of the electrode 3, it is preferable to form it into an island shape or a net shape.

The tuning fork type camera having such a configuration is connected to a known oscillation circuit as surrounded by a broken line 4.

The piezoelectric body 2 adsorbs and desorbs moisture from around the electrode 3, or via the electrode 3 if the electrode 3 has hygroscopic properties.

FIG. 2 shows an example in which a vibrating piece type vibrator is used as the vibrator, and a piezoelectric material is attached to the surface of the vibrating piece type vibrator 13 supported by the frame 11 via the support part 12. 14 and further an electrode 15 is formed. 16 is electrode 1
5 to the frame 12.

Although the vibrating piece type vibrator having this configuration is not shown, the first
As shown in the figure, it is incorporated into an oscillator circuit during use.

A description will be given below based on specific examples.

First, a tuning fork-shaped vibrator body was prepared. Next, a piezoelectric material made of zinc oxide was formed on the part of the vibrator body that generates vibration, that is, on the side surface of the tuning fork leg. To form this zinc oxide piezoelectric body, a reactive sputtering method was used, the atmosphere in the vacuum chamber was set to Ar:02=50:50, the sputtering pressure was set to 5X102 Torr, and oxidation was performed using a metallic zinc target. A film-like piezoelectric material made of zinc was formed. Its resistance was 1×10 8 Ω at a relative humidity of 30%. Further, the thickness at this time was 30 μm.

This humidity sensor has a reference oscillation frequency of approximately 76 KH2 at a relative humidity of 30%. And we got this humidity. We also measured the resistance value of the piezoelectric material at each relative humidity,
The results are also shown in Figure 3.

As is clear from FIGS. 3 and 3, the oscillation frequency decreases until the relative humidity reaches 65%, and the oscillation stops at the relative humidity of 65%. Therefore, by reading the oscillation frequency, it is possible to check what percentage the relative humidity is.

Furthermore, although the emission Fim 414 stops at a relative humidity of 65%, the resistance value at that time is around 106Ω. This corresponds to the fact that the resistance value decreases, dielectric relaxation occurs, and piezoelectricity is no longer exhibited.

Moreover, FIG. 4 shows the change in oscillation frequency at each relative humidity when the thickness of the zinc oxide piezoelectric material is changed. By changing the thickness as is clear from FIG. 4, it is possible to set the value of relative humidity at which oscillation stops. In addition, sputtering conditions such as 02 vs. A
The properties can be changed by adjusting the r ratio - the gas pressure during sputtering. Further, the characteristics can be changed by adjusting the impurity addition needle.

Therefore, this humidity sensor can also be used as a dew condensation sensor.

In the above embodiments, a piezoelectric material made of zinc oxide was explained, but other piezoelectric materials such as ZnS, Cd
55 Even if a ■b-■b group compound such as 55CdSe, CdTe, etc. or an Ib-Vb group compound such as GaP, GaAs, In5b, etc. are used, wet it.
can be configured.

As described above, according to the present invention, it is a humidity sensor that utilizes the decrease or stop of oscillation due to dielectric relaxation phenomena, based on the decrease in resistance value due to moisture absorption of the piezoelectric material, and the change in output voltage or the oscillation frequency of the vibrator. It is possible to accurately detect relative humidity by reading changes in the relative humidity.

A humidity sensor having such a function is useful for detecting not only the indoor and outdoor humidity conditions but also the humidity conditions of electrophotographic copying machines and temperature control equipment.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an embodiment of the present invention, Fig. 2 is a plan view showing a vibrating element type vibrator, and Fig. 3 is a relative oscillation frequency of a humidity sensor obtained by an embodiment of the invention. FIG. 4 is a diagram showing humidity characteristics and resistance value-relative humidity characteristics of a piezoelectric material, and FIG. 4 is a diagram showing oscillation frequency-relative humidity characteristics when the thickness of the piezoelectric material is changed. 1-tuning fork type vibrator, 2-piezoelectric body, 3-electrode. Patent applicant Murata Manufacturing Co., Ltd. Figure 1 L-, --J Figure 2 215

Claims (1)

[Claims] A humidity sensor comprising a vibrator body, a piezoelectric body formed on an arbitrary surface of a vibration-generating part of the vibrator body, and an electrode formed on the piezoelectric body, the humidity sensor comprising: A humidity sensor characterized in that the body is a piezoelectric semiconductor made of a lattice defect material or a valence-controlled material.