JPS6133466B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention aims to prevent explosions, fire accidents _{, pollution} , etc. caused by flammable gas leaks in industrial activities or homes. This invention relates to a method for producing a sintered indium oxide semiconductor thick film doped with palladium (Pd) on an insulating substrate by a firing method from raw materials of palladium (PdCl ₂ ) and porcelain clay.

Conventional gas detection methods include optical interference method that uses changes in the refractive index of light, infrared spectroscopy, gas chromatography, electrochemical method that uses the electromotive force of batteries, and detection tubes that use chemical reactions. method, catalytic combustion method, and semiconductor sensor method. Among these, the catalytic combustion method and the semiconductor sensor method can extract the gas concentration as an electrical signal and are easy to use, so they have been used for leak detection of flexible gases.

The catalytic combustion method involves burning gas in contact with a heated platinum wire and detecting changes in the electrical resistance of the heated wire. It has been used for a long time in coal mines to detect methane gas, but it can cause deterioration of the element and poisoning. Since the sensitivity tends to decrease and the change in the obtained electrical signal is small, it is necessary to amplify this, which has the drawback of increasing the price. On the other hand, semiconductor sensors have a fast response speed and high sensitivity, so they can detect gases at low concentrations, and the necessary alarm measures can be created at low cost with a simple circuit. This kind includes _SnO2
The system includes gas sensors, ZnO, V ₂ O _s , NiO,
CoO-based materials and materials using rare-earth transition metal-based perovskite-type composite oxides have been proposed, but materials other than SnO ₂ have many problems. SnO ₂ -based gas sensors are widely used to detect household gas leaks, but because they are easily affected by water vapor, they deteriorate relatively quickly and have drawbacks such as the change in electrical resistance due to gas is not proportional to the gas concentration.
In addition, there are many unknowns about the detection principle, and because experimental facts preceded it and it was used without being technologically complete, one cannot help but feel that it is an unreliable device.

In view of these circumstances, the present invention uses In ₂ O ₃ as a host material, adds porcelain clay, and generates a sintered semiconductor thick film activated with PdCl ₂ on an insulating substrate. In addition to eliminating the drawbacks of the SnO ₂ semiconductor gas sensor to a nearly satisfactory state,
An object of the present invention is to provide a method for manufacturing an indium oxide gas sensor that has gas selective ability.

For this reason, the method of the present invention uses indium oxide as a base material, palladium chloride and porcelain clay as additive materials, and involves high-temperature pre-calcination at about 700°C and heating at about 600°C in air or a controlled oxygen atmosphere. The method is characterized in that a thick film containing indium oxide as a main component and oxides of palladium, silicon, aluminum, and zinc is sintered on an insulating substrate by performing low-temperature main firing.

The manufacturing method of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows a manufacturing process diagram of an In ₂ O ₃ based gas sensor according to the present invention. First, commercially available high-purity In ₂ O ₃ (99.9%) is calcined at about 700° C. for 1 hour to remove moisture and unnecessary impurities. Thoroughly crush this in an agate mortar and sieve it to a particle size of 200 mesh or less. PdC ₂ (5wt%) was added as an activation raw material to In ₂ O ₃ powder with uniform particle size.
(below), suppressing raw material porcelain clay (10wt% or less) is added and mixed, and stirred with distilled water to create a colloidal suspension. A certain amount of this suspension (approximately 10 mg)
Measure out and apply it on the steatite substrate,
After keeping it in the drying oven (about 100℃) for about 30 minutes,
Main firing is performed at approximately 600°C for 1 hour in an air atmosphere. Furthermore, in order to relieve the mechanical stress of the thick film and stabilize electrical changes over time, the thick film element after main firing is annealed in air at 200° C. for about 1 hour. When electrodes are constructed using silver paste so that the contact area between the thick film element and the gas to be measured is 1×10 (mm), an In ₂ O ₃ (Pd)-based semiconductor gas sensor is obtained. Note that in order to improve the sensitivity of the element, the insulating substrate is subjected to a heater treatment. Figure 2 shows a completed diagram of the indium oxide gas sensor. In Figure 2, numeral 1 is the sensor, 2 is the lead wire, 3 is the electrode, 4 is the insulating substrate, and 5 is the heating heater. There is.

A practical current detection method can be used to measure the electrical characteristics of the In ₂ O ₃ (Pd)-based semiconductor gas sensor obtained by the above method. That is,
In a series circuit of a gas sensor, a standard resistor R, and a battery, when a direct current of 1 (mA) is passed through the gas sensor in advance and a constant concentration of gas is brought into contact with it, the element resistance r of the gas sensor decreases. , the current flowing through the element increases and the terminal voltage of the standard resistor increases. If this is observed with an XY recorder, a response curve can be obtained.

Figure 3 shows a typical response curve.
Point A in the figure shows the time when gas is injected, and point B shows when the gas injection is stopped. The time between A and B, that is, the time for the terminal voltage of the standard resistor to reach the saturation value, is approximately 9
(sec), and the terminal voltage is approximately 1 (sec) from point A.
10 (v), and the response speed of the In ₂ O ₃ based gas sensor according to the present invention is extremely fast, similar to that of the conventional SnO ₂ based gas sensor. Almost the same results were obtained with devices made by changing the raw material composition during the main firing, although there were differences in sensitivity.

Figure 6 shows the pre-firing temperatures of 0℃, 600℃, 700℃,
Graphs showing the relationship between main firing temperature and voltage sensitivity were obtained experimentally for each of the five cases of 800°C and 900°C. As is clear from this experimental result, when the preliminary firing temperature is about 700°C and the main firing temperature is about 600°C, as in the method of the present invention, the voltage sensitivity is most effective.

In addition, Figure 7 shows the amount of PdCl ₂ added at 0Wt% and 3Wt%.
%, 5W%, and 10W%, graphs showing the relationship between main firing temperature and voltage sensitivity were obtained experimentally _.
The voltage sensitivity is highest when the amount of addition is approximately 5 Wt% and the main firing temperature is 600°C.

Hereinafter, a method for manufacturing an In ₂ O ₃ gas sensor according to the present invention and an experimental example regarding its voltage sensitivity will be described.

Experimental example 1 A raw material containing In ₂ O ₃ powder pre-calcined at a temperature of 700°C in an air atmosphere as a suspension in distilled water with no additives of PdCl ₂ and porcelain clay was mixed at a temperature of 700°C.
In the devices obtained by main firing at 500°C, 600°C, 700°C, and 800°C, there is almost no voltage sensitivity to gas.

Experimental Example 2 PdCl ₂ 5Wt% and porcelain clay 1Wt% were added to powdered In ₂ O ₃ that had been calcined for 1 hour at a temperature of 700°C in the air, and the raw material was suspended colloidally in distilled water. After main firing at a temperature of 600°C for 1 hour, an In ₂ O ₃ -based gas sensor containing chlorine (Cl ₂ ), palladium oxide (PdO), and trace amounts of metal oxides is obtained.

FIG. 4 shows the gas concentration vs. voltage sensitivity characteristics of this device for propane (C ₃ H ₈ ) and carbon monoxide (CO) (device heating temperature: 150° C.).

Experimental example 3 Powder calcined under the same conditions as experimental example 2
_{In2O3 , PdCl3} 3Wt% and porcelain clay 10Wt%
FIG. 5 shows the gas concentration vs. voltage sensitivity characteristics for C ₃ H ₈ and CO (device heating temperature: 150° C.) of the device obtained by adding and main firing.

According to FIG. 4, it is clearly possible to detect C ₃ H ₈ and CO. Although not shown in the figure, we confirmed that this device has almost the same sensitivity to hydrocarbons such as butane (C ₄ H ₁₀ ), ethane (C ₂ H ₆ ), and methane (CH ₄ ). However, the amount of porcelain clay added was fixed at a constant amount in the range of 1 to 3 Wt%, and PdCl ₂ was added to 1 to 3 Wt%.
In the element obtained by adding a fixed amount in the range of ~10Ww%, regardless of the amount of porcelain clay added,
It was found that the sensitivity was low when the amount of PdCl ₂ added was 2 Wt% or 7 Wt% or more, and the highest sensitivity was achieved when the amount was 5 Wt%.

The reason why the addition of an appropriate amount of PdCl ₂ has _a remarkable sensitizing effect is that the chlorine generated by the thermal decomposition of _{PdCl 2} is oxidized to the OH _group or H
Activating the action of the group, an electronic energy exchange takes place with the contacting gas, while at the same time
A new impurity level of Pd or PdO is formed in In ₂ O ₃ and electronic transition from this level increases the carrier density in the conduction band, increasing the electrical resistance of the In ₂ O ₃ based thick film. This is due to the fact that the

According to Figure 5, the sensitivity to CO decreases only slightly, but the decrease in sensitivity to C ₃ H ₈ is remarkable, and as a result, the sensitivity to CO is high, so it is clear that both gases It has selective ability in detection.

Compared to the element of Experimental Example 2, the amount of addition in the element of Experimental Example 3 is that PdCl ₂ is decreased and porcelain clay is increased, but the porcelain clay contains SiO,
This is because metal oxides such as Al ₂ O ₃ and SrO are contained (for example, clay from Mashiko), so these act in some way to suppress the activation effects of chlorine and PdO mentioned above. As a result, electron transfer between the element and the contact gas can be made inactive and the gas sensitivity of C ₃ H ₈ can be reduced. This thing is
It is also common to other carbohydrates, including C ₃ H ₈ .

However, if the amount of porcelain clay added is increased too much, the suppression effect increases and sensitivity to all gases is lost.

As detailed above, according to the method for manufacturing an indium oxide gas sensor of the present invention, three raw materials are used: powdered In ₂ O ₃ as a base material, PdCl ₂ as an activating raw material, and porcelain clay as a suppressing material. A sintered In ₂ O ₃ -based gas sensor can be produced by the sintering method.

As is clear from Experimental Examples 2 and 3, by adding appropriate amounts of activating material and suppressing material porcelain clay to the main component In ₂ O ₃ ,
An element with high sensitivity to C ₃ H ₈ and CO, and an element with a sensitive detection ability for CO but low sensitivity to C ₃ H ₈ and a selective ability for gas types. is obtained.

Furthermore, according to the method of the present invention, an indium oxide gas sensor with high voltage sensitivity can be manufactured at low cost through a simple process of setting the high temperature pre-firing temperature to approximately 700°C and the low-temperature main firing temperature to approximately 600°C. It has the advantage of

[Brief explanation of the drawing]

The drawings show an outline of the implementation of the present invention and an example of the characteristics of the device obtained by the method of the present invention.
The figure is a manufacturing process diagram of an In ₂ O ₃ (Pd)-based semiconductor gas sensor, and Figure 2 is a plan and cross-sectional view of the completed device.
Figure 3 is a graph showing an example of a response curve to combustible gas, and Figure 4 is a graph showing an example of a _{response curve} for combustible gas _.
5Wt%, porcelain clay 1Wt% added and temperature 600℃
Figure 5 shows the gas concentration vs. voltage sensitivity characteristics of an element sintered at 600°C with In ₂ O ₃ as the main component and 3 Wt% of PdCl ₂ and 10 Wt% of porcelain clay added. FIG. 6 is a graph showing the relationship between high temperature preliminary firing temperature and low temperature main firing temperature and sensitivity, and FIG. 7 is a graph showing the relationship between PdCl ₂ addition concentration and sensitivity. DESCRIPTION OF SYMBOLS 1...Sensor, 2...Lead wire, 3...Electrode, 4...Insulating substrate, 5...Heating heater.

Claims (1)

[Claims] 1. Using indium oxide as a base material and palladium chloride and porcelain clay as additive materials, perform high-temperature pre-firing at about 700°C and low-temperature final firing at about 600°C in air or in a controlled oxygen atmosphere. Accordingly, a thick film containing indium oxide as a main component and oxides of palladium, silicon, aluminum, and zinc is sintered on an insulating substrate. A method for manufacturing an indium oxide gas sensor.