JPH05332980A - Production of moisture sensitive element - Google Patents

Abstract

PURPOSE:To ease measurement in a low-temperature area by necessitating no AC current or pulse current circuit because the element can be operated by DC current, and no linearizer circuit because electric resistance will not change in an index function against relative humidity changes. CONSTITUTION:A ZnTiO3 sintered body is applied with heat treatment in a reductive atmosphere of 600 to 1000 deg.C, and a crystal phase of composite oxide, representing Zn1-x TiO3 (where, X>0) is generated on the surface of the sintered body or the above ZnTiO3 crystal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a humidity sensitive element made of a novel compound.

[0002]

2. Description of the Related Art Hitherto, as a humidity sensor for detecting humidity in the atmosphere, soil, special atmosphere, etc., a method of converting humidity change into electric resistance has been used. As a humidity sensitive element used for such a humidity sensor, one in which an electrolyte aqueous solution such as LiCl is infiltrated in a porous base material, one in which carbon is dispersed in a hydrophilic organic polymer, or the like is known. There is.

Of these, a moisture sensitive element made of an electrolyte, particularly LiCl, is a moisture sensitive resistor having excellent performance.
It is known as a Dunmore type element, but its moisture-sensitive characteristics change due to condensation, droplets, and adsorption of hygroscopic gas.
Handling and maintenance becomes complicated. Further, the moisture-sensitive element made of an organic polymer has a problem in deterioration and heat resistance of the organic material itself, and its use condition is limited.

Therefore, in recent years, instead of these elements, a humidity sensitive element made of ceramics has been used, which does not have the above problems. As the moisture sensitive element made of such ceramics, for example,
The ones disclosed in JP-A-52-42297 and the like are known. This type of moisture sensitive element is MgCr ₂ O ₄ −.
It is composed of TiO ₂ -based complex oxide ceramics, and the ionic conduction of adsorbed water is utilized to capture a change in humidity in the atmosphere as a change in electric resistance.

FIG. 3 shows such MgCr ₂ O ₄ -Ti.
The figure shows the measurement principle of a humidity sensor using a humidity sensitive element made of O ₂ , and in the figure, electrodes 13 and 15 are provided at both ends of a moisture sensitive element 11 made of MgCr ₂ O ₄ —TiO ₂ . An AC power supply 17 is connected to the electrodes 13 and 15. In such a humidity sensor, the humidity is measured by utilizing ionic conduction using a proton (H ₃ O ⁺ ) generated by dissociation of H ₂ O adsorbed on the surface of the humidity sensitive element 11 as a carrier. Therefore, H ₂ O adsorbed on the surface of the humidity sensitive element 11 dissociates and adsorbs to form a hydroxyl group on the surface and forms a physically adsorbed water layer by hydrogen bonding on the hydroxyl group, but when the humidity of the atmosphere becomes high, Protons (H ₃ O ⁺ ) increase, and the resistance value between the electrodes 13 and 15 decreases.

Thus, the protons (H ₃ O
^{Since +} ) increases or decreases depending on the humidity, the humidity can be measured by detecting the resistance between the electrodes 13 and 15.

[0007]

[Problems to be Solved by the Invention] However, Mg
Since the moisture sensitive element made of Cr ₂ O ₄ —TiO ₂ ceramics utilizes the ionic conduction of adsorbed water,
When a direct current is applied, a polarization phenomenon occurs and the resistance of the humidity sensitive element increases, making it impossible to detect humidity. Therefore, there is a problem that it cannot be used with a direct current.

That is, in the moisture sensitive element made of the MgCr ₂ O ₄ —TiO ₂ system ceramics, the proton (H ₃ O ⁺ ) generated by the dissociation of the adsorbed water is used as a carrier. Side, the protons on the positive electrode side decrease, the electrical resistance in this reduced area increases, and the electrical resistance increases as a whole, causing a polarization phenomenon, which increases the resistance of the humidity sensing element and increases the humidity measurement. There was a problem that an error occurred. Since this problem is due to the moisture sensitivity mechanism, a fundamental solution is not possible.As a workaround, a DC power supply is avoided and an AC power supply or a pulse power supply is used, so a complicated and expensive accessory circuit is required. There was a problem that

Further, in the above humidity sensitive element, since the ionic conduction of the adsorbed water is utilized, the electric resistance is exponentially 1 with respect to the change of the relative humidity as shown in FIG.
There is a problem that the linearizer circuit is required to capture the electric resistance as data because of a large change from 0 ³ to 10 ⁹ Ω.

Therefore, when the moisture sensitive element made of MgCr ₂ O ₄ -TiO ₂ series ceramics is used, an AC power source or a pulse power source and a linearizer circuit are required, and the humidity sensor becomes complicated and large in size. However, there was a problem of high cost.

Further, in the humidity sensitive element as described above, since the electric resistance is as high as 10 ⁸ to 10 ⁹ Ω in the low humidity region, the detection current becomes extremely small, and the measurement becomes difficult in this region. There was also a problem.

[0012]

As a result of various studies on the above problems, the present inventors have found that the composition formula is Zn
By preparing a sintered body of TiO ₃ and heat-treating the sintered body in a reducing atmosphere to partially vaporize Zn from the sintered body or ZnTiO crystal, Zn _{1 -X} TiO ₃ (where X>
It is characterized in that a crystal phase represented by 0) is generated.

The present invention will be described in detail below.

According to the manufacturing method of the present invention, first, ZnTiO
Create a sintered body consisting of ₃ . Such a sintered body is obtained by mixing and molding, for example, ZnO and TiO ₃ powder at a ratio of 1: 1 and sintering the powder at a temperature of 1150 to 1420 ° C. in an oxidizing atmosphere.
It is obtained by firing for about 10 hours. The density of the sintered body is not particularly limited, but there is no problem if the porosity is 1% or less.

This sintered body has an electric resistivity of 1 at room temperature in the air.
It shows 0 ⁸ Ωcm or more, and shows no humidity-sensitive property with respect to relative humidity in the atmosphere.

Therefore, according to the present invention, this sintered body is heat-treated in a reducing atmosphere.

Specifically, the oxygen partial pressure is 10 ^-18 torr.
3 at a temperature of 600 ° C to 1000 ° C in the following reducing atmosphere
Heat treatment is performed for 10 hours.

[0018] subjected to a heat treatment in the reducing atmosphere, it is Zn atoms are released from the sintered body or the surface of individual crystals of ZnTiO ₃ of ZnTiO _3, ZnTiO ₃
A Zn-deficient crystal phase is generated with respect to the stoichiometric composition. This crystal phase is represented by the chemical formula: Zn _1-X TiO
₃ (however, X> 0).

According to this heat treatment, Zn volatilization is more remarkable on the surface of the sintered body. Therefore, depending on the degree of heat treatment, a TiO ₂ phase may be formed on the surface of the sintered body. At this time, the sintered body had TiO _{2 on the} surface.
In the center, the ZnTiO ₃ phase remains as it is, and in the meantime, it is a crystal of Zn _{1 -X} TiO ₃ and has a composition gradient in which the X value gradually decreases from the surface to the center.

[0020]

According to the present invention, the ZnTiO ₃ sintered body is
By heat-treating in a reducing atmosphere, the ZnTiO ₃ sintered body becomes moisture-sensitive to relative humidity in the atmosphere. This humidity-sensitive characteristic shows a behavior that the electric resistance becomes lower as the relative humidity becomes lower, unlike the humidity-sensitive behavior that the electric resistance becomes 100 times or more as the relative humidity becomes lower as in the conventional humidity-sensitive element. This humidity-sensitive behavior is such that the electric resistance value of the element with respect to the relative temperature has a linear function relationship, and in order to A / D-convert the electric resistance value in the arbitrary humidity-sensing which was necessary in the conventional humidity-sensitive element. It has the advantage that the necessary logarithmic conversion is unnecessary.

Moreover, such a humidity-sensitive element operates with a DC power supply, does not require a linearizer circuit, an AC power supply, or the like, and facilitates measurement in a low humidity region.

[0022]

EXAMPLE A raw material powder of ZnO and TiO ₂ was used in a molar ratio of 1 :.
After weighing so as to be 1, methanol was added as a solvent to the raw material powder, the mixture was put into a rotary pot with a zirconia ball having a diameter of 3 mm, and wet mixed for 20 hours. Then, the obtained mixed slurry is dried at 120 ° C.
It was calcined in the atmosphere of 0 ° C. for 6 hours. The obtained calcined product is crushed in an alumina mortar, and the crushed product that has passed through 40 mesh is ball-milled for 40 hours. After drying the crushed slurry at 120 ° C, P as a binder
1 wt% of VA (polyvinyl alcohol) is added and mixed for granulation, and the particle size is made uniform by sieving. 0.5 g of a raw material having a uniform particle size is weighed and molded into a pellet at a pressure of 1 ton / cm ² using a mold having a diameter of 12 mm. These compacts were placed on an alumina substrate and sintered in the air at 1200 ° C. for 3 hours to prepare a ZnTiO ₃ sintered compact.

Next, the sintered body obtained as described above was treated with 2000 cc / min of N ₂ gas and 30 cc / m of H ₂ gas.
In a mixed gas of in, reduction heat treatment was performed at 800 ° C. for 5 hours to form a moisture sensitive element.

As shown in FIG. 1, the obtained moisture-sensitive element is formed into a cylindrical shape having a diameter of 10 mm and a thickness of 1.5 mm, the moisture-sensitive element 1 is placed on an insulating substrate 2, and both ends of the moisture-sensitive element 1 are placed. Electrode 3
Install. Further, a copper plate 4 is attached to each of the electrodes 3, a copper wire 5 is connected to the copper plate 4, and the copper wire 5 is connected to a DC power source (not shown).
Connected to. On the other hand, by putting P ₂ O ₅ powder or a saturated aqueous solution of LiCl, CaCl ₂ , and Na ₂ CO ₃ into the desiccator, the relative humidity is about 0% RH and 15% R
A relative humidity atmosphere of H, 34% RH, 88% RH was obtained. The humidity sensitive element was placed in these atmospheres, and a current value when a constant DC voltage of 5 V was applied to both electrodes was measured to obtain the electric resistance.

FIG. 2 is a graph showing the relationship between relative humidity and resistance when the above humidity sensitive element is used. This FIG. 2 shows the relative humidity (% RH) on the horizontal axis and the resistance value (Ω) on the vertical axis, and shows the results measured under the condition of 25 ° C. According to this figure, the relationship between the humidity and the resistance value is linear, and it can be seen that the resistance value increases as the relative humidity increases.

[0026]

As described in detail above, according to the present invention, it is possible to easily operate with a DC power supply without using an AC power supply or a pulse power supply, and the electric resistance change is exponentially large with respect to the relative humidity change. Since it does not change, the linearizer circuit can be dispensed with, which can facilitate simplification of the humidity sensor. Moreover, the measurement in the low humidity region can be facilitated.

[Brief description of drawings]

FIG. 1 is a diagram showing wiring for evaluating characteristics of a moisture sensitive element of the present invention.

FIG. 2 is a graph showing the relationship between relative humidity and resistance of the humidity sensitive element of the present invention.

FIG. 3 is an explanatory diagram showing the measurement principle of a humidity sensor using a conventional moisture sensitive element made of MgCr ₂ O ₄ —TiO ₂ system ceramics.

FIG. 4 is a graph showing the relationship between relative humidity and resistance when humidity is measured by a humidity sensor using a conventional MgCr ₂ O ₄ —TiO ₂ type humidity sensitive element.

[Explanation of symbols]

 1 Moisture Sensitive Element 2 Insulating Substrate 3 Electrode 4 Copper Plate

Claims (1)

[Claims] 1. A composite oxide represented by Zn _{1 -X} TiO ₃ (where X> 0) is applied to a surface of the sintered body or the ZnTiO ₃ crystal by heat treating the ZnTiO ₃ sintered body in a reducing atmosphere. A method for manufacturing a moisture-sensitive element, which comprises producing a crystalline phase of an object.