JPH0569376B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a humidity sensor that uses changes in the resistance value of a proton conductor.

[Prior art] LaConti et al. reported that the carrier of a proton conductor is a hydrated ion of protons, mainly
It has been reported that H ₃ O ⁺
No. 115293, U.S. Patent Application No. 773136). This means that
This suggests that the electrical conductivity of proton conductors changes with humidity.

The inventors investigated various proton conductors and found that phosphoric acid silica (H ₂ SiP ₂ O ₈ ) has high sensitivity to water vapor. Silica phosphate is sensitive to water vapor over a wide temperature range;
It can withstand temperatures up to about 500℃.

A humidity sensor with such characteristics can be used, for example, to detect water vapor generated during cooking and to control a microwave oven. It can also be used to detect water vapor in various plants, piping, etc. Furthermore, it can be used to control humidification and dehumidification in air conditioners and the like. Here, the fact that the sensor is highly sensitive, heat resistant, and heat-cleanable improves detection accuracy and enables use in a contaminated atmosphere.

Phosphated silica is an inorganic
General Chemistry Journal Zeitschrift fur Anorganishe und
Allgemein Chemie Vol. 346, p. 92, 1966), H. Makart Helvetica Chimica Acta
Helvetica Chimica Acta Vol 50 Fasciculus2,
No. 47, 399, 1967) et al. The inventors have discovered that this compound is a proton conductor and can be used as a humidity sensor.

Humidity detection using a proton conductor uses the increase or decrease of carriers, mainly H ₃ O ⁺ , inside the conductor as the partial pressure of water vapor changes in the atmosphere. It does not use the increase or decrease of electrons or holes due to

[Problem of the Invention] An object of the present invention is to provide a proton conductor humidity sensor that has high density and excellent heat resistance.

[Structure of the Invention] In the present invention, humidity is detected from the internal resistance of a proton conductor made of phosphoric acid silica (H ₂ SiP ₂ O ₈ ).

The phosphoric acid silica is molded into a disk shape, for example, and the internal resistance is measured by covering the front and back surfaces of the disk with breathable noble metal electrodes. Any film-like silica phosphate can be used, and any electrode can be used.

The resistance value of phosphoric acid silica can be measured using either direct current or alternating current, but excluding detection errors due to hydrogen,
In order to prevent deterioration due to overvoltage, it is preferable to measure with an alternating current of about 30 to 1 MHz.

[Example] (Synthesis of phosphoric acid silica) Tetraethoxysilicate (Si(OC ₂ H ₅ ) ₄ ) was
Phosphoric acid and water were mixed and stirred at a molar ratio of 1:15:15, and hydrochloric acid was added dropwise to obtain a gel-like compound. The reaction solution was kept at 150°C for 15 hours to precipitate the phosphoric acid silica, which was then washed with acetone and used as a sample. Any other method for producing phosphoric acid silica can be used.

The obtained sample was subjected to X-ray diffraction, and compared with literature values using an ASTM card, it was confirmed that the sample was phosphoric acid silica (H ₂ SiP ₂ O ₈ ). The results are shown in FIG. All the peaks from the ASTM card are included in the sample, but there are other weak peaks.
In addition, the inventors prepared several types of samples by changing the reaction time and washing conditions of the reaction solution, but the characteristics were the same.

Figure 2 shows data on weight changes due to heating and dehydration. The dehydration peak is gradual and is due to the desorption of interlayer water and structural water inside the crystal, and the sample is reversible and will not be destroyed by temperature changes within this range. Therefore, if the sensor is up to about 500℃,
It can be heated and used. Furthermore, it was confirmed from electromotive force etc. that phosphoric acid silica is a proton conductor.

(Structure of sensor) The structure of the proton conductor humidity sensor 2 is shown in FIG. In the figure, reference numeral 4 denotes a disk-shaped phosphoric acid silica molded body with a diameter of 10 mm and a thickness of 2 mm, and film-shaped gas permeable electrodes 6 and 8 having platinum powder pressed onto the front and back surfaces thereof were attached. Platinum lead wire 1 is attached to electrodes 6 and 8.
Connect 0 and 12 so that the resistance value of phosphoric acid silica can be taken out.

14 is a thermocouple made of platinum-platinum/rhodium, etc., and is attached to the side of the disk 4. 16 is a coil-shaped heater for heating the sensor 2 to an appropriate temperature. These lead wires 10, 12
and thermocouple 14 are welded to the stem provided on the base 18 to complete the sensor 2.

This sensor 2 was manufactured by laminating lead wire 10, platinum powder, phosphoric acid silica powder, platinum powder, and lead wire 12 in this order, and pressing at 2700 kg/cm ² . Further, the thermocouple 14 was attached using platinum paste.

Phosphoric acid silica may be mixed with other substances, such as other proton conductors such as antimonic acid, or various organic binders, etc., as long as the resistance value of phosphoric acid silica is the main one. good. The phosphoric acid silica may be in any shape, such as one coated in a film form on a substrate, and the shaping may be performed by sintering or the like.

In addition to platinum, the electrodes 6 and 8 are made of gold, silver, or
Any metal oxide semiconductor such as SnO ₂ or LaNiO ₃ can be used, and any shape such as a comb-shaped electrode can be used.

The thermocouple 14 can be replaced with a thermistor, a platinum temperature measuring resistor, or the like, and the thermocouple 14 and heater 16 may not be provided.

FIG. 4 shows an example of the detection circuit. Silica phosphate 4
The detection resistor R1 is connected to an AC power source 20 of about 30 Hz to 1 MHz, and the voltage to the detection resistor R1 is taken out with a voltmeter 22. The electromotive force of the thermocouple 14 is compared with a reference value by a comparator circuit 24, and an analog switch 26 is turned on and off. A power supply 28 is connected to the heater 16 via an analog switch 26, and the sensor 2
is heated to a constant temperature.

In reality, the power source 20 may be a direct current, but an alternating current is preferable in order to avoid electromotive force due to the presence of hydrogen and to avoid deterioration of the electrodes and conductor 4 due to the application of overvoltage.

(Characteristics) Fig. 5 shows the electrical conductivity of the sensor 2 in water vapor of 1 torr, 20 torr, and 100 torr at 50 to 250°C. Further, regarding typical temperatures, this data is converted into relative temperature and is shown in FIG. Note that the AC power source 20 used has a frequency of 200 Hz, the measured values mean steady values, and the other atmosphere is air.

From FIG. 5, it can be seen that the characteristics of the sensor 2 change at about 150°C. At lower temperatures than this,
The temperature dependence is small, and the output between 1 torr and 20 torr is large, but the output between 20 torr and 100 torr is relatively small. Next, above 150°C, the output decreases linearly with temperature, but the difference in output between each water vapor pressure is large. The small temperature dependence below 150°C cannot be considered from surface conduction due to adsorbed water, and the sharp decrease in electrical conductivity above 150°C is
The properties are significantly different from those of metal oxide semiconductors.

It can be seen from FIG. 6 that the logarithm of electrical conductivity varies linearly with the logarithm of relative humidity. 80 again
The straight lines at ~150°C are very similar, and the temperature dependence is small. For example, the relative humidity at an electrical conductivity of 1/10 ⁴ S/cm only changes by a factor of 10 from 80 to 150°C. Therefore, by using the thermocouple 14, heater 16, etc., it is easy to compensate for the temperature dependence of the sensor 2.
Next, at about 200℃, the gradient to humidity decreases,
The sensitivity itself is great.

The reason why the resistance value of phosphoric acid silica changes due to water vapor is that the actual carrier is not protons.
This is thought to be due to the presence of H ₃ O ⁺ etc. The carrier mobility depends on the water content inside the crystal, and the water content depends on the humidity of the atmosphere, so the resistance value of phosphoric acid silica probably changes depending on the humidity.

Synthesizing proton conductors other than phosphoric acid silica,
Characteristics were evaluated. First, antimony oxide (Sb ₂ O ₃ ) was reacted with 15 equivalents of hydrogen peroxide,
Antimonic acid (Sb ₂ O ₅ .2H ₂ O) was obtained.

The influence of water vapor partial pressure on the resistance value of antimonic acid at 105°C is shown in Figure 7. The water vapor sensitivity of the plain material (a) is lower than that of phosphoric acid silica. We also synthesized antimonic acid in which some of the hydrogen atoms were replaced with sodium ions, but the sensitivity did not increase (samples (b), (c), and (d)).

FIG. 8 shows the water vapor sensitivity of zirconium phosphate. Phosphoric acid (H ₃ PO ₄ ), hydrogen chloride, and zirconium chloride were reacted to obtain a gel of zirconium phosphate (Zr(HPO ₄ ) ₂ ). gel in phosphoric acid
The mixture was refluxed at 120°C for one week to obtain crystalline zirconium phosphate. Sensitivity was measured at 105°C and 220°C, but only low values were obtained.

From Na ₂ CO ₃ , ZrO ₂ , SiO ₂ , NH ₄ H ₂ PO ₄ ,
NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) was synthesized.
0.1N sulfuric acid was dropped into NASICON to replace some of the sodium ions with hydrogen ions, making it a proton conductor. Figure 9 shows the relationship between water vapor pressure and resistance value at 100°C. Changes in resistance value are small and there is hysteresis.

[Effects of the Invention] According to the present invention, a proton conductor humidity sensor with high sensitivity and high heat resistance can be obtained.

[Brief explanation of the drawing]

Figures 1 and 2 are characteristic diagrams of the proton conductor humidity sensor of the example, Figure 3 is a front view with a partial cutout of the humidity sensor of the example, and Figure 4 is the detection circuit used in the example. It is a circuit diagram. 5 and 6 are characteristic diagrams of the embodiment, and FIGS. 7 to 9 are characteristic diagrams of the conventional example.

Claims (1)

[Claims] 1. A humidity sensor using a change in the resistance value of a proton conductor, characterized in that the proton conductor is phosphoric acid silica (H ₂ SiP ₂ O ₈ ). Humidity sensor.