JPH0743332A - TiO2-SnO2 gas sensor element - Google Patents

Abstract

PURPOSE:To measure gas response at relatively low temp. and to obtain high selectivity especially against hydrogen by forming an electrode on the surface of a TiO2-SnO2 solid soln. and using the change of the interface resistance between the solid soln. and the electrode. CONSTITUTION:A pair of electrodes 2, 3 are deposited and formed on the surface of a sensor medium 1 composed of a TiO2-SnO2 solid soln. The solid soln. is, for example, a perfect solid soln. wherein the ratio of TiO2 and SnO2 is 1:1 and an N-type semiconductor in itself. The work function of either one of the electrodes 2, 3 is larger than that of the medium and constituted of a metal of 4.5eV or more, for example, Au or Pt. In this case, the gas sensor element thus constituted responds to gas at relatively low temp. of 300 deg.C or lower. For example, the value of the current flowing through the element is changed by the introduction of hydrogen gas and the resistance of the element is changed. At this time, electrode interface resistance becomes 50 times as an absolute value of resistance as compared with the resistance change of the medium 1 and the change ratio due to the introduction of hydrogen is also large.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor element using a solid solution of TiO ₂ and SnO ₂ .

[0002]

_2. Description of the Related Art Conventionally, TiO ₂ has been studied as an oxygen sensor element, and SnO ₂ has been studied as a sensor element for various gases, and some of them have already been put to practical use. Also Ti
Research on a humidity sensor element using a mixed system of O ₂ and SnO ₂ has also been reported.

In a gas sensor element using a conventional metal oxide semiconductor represented by SnO ₂ as a gas sensor, the gas response mechanism is such that the metal oxide is brought into contact with the periphery of a detection line such as Pt line and the like. It is sandwiched between a pair of electrodes and a contact combustion type sensor element that detects the oxidation (combustion) of the detection gas in the physical layer by changing the resistance value of the detection line due to the heat released during the oxidation. In the metal oxide layer, the detection gas is oxidized by oxygen adsorbed on the surface of the metal oxide, and the electrons trapped by the adsorbed oxygen at this time are released into the metal oxide layer to change the resistance. It is divided into sensor elements.

[0004]

However, in the conventional gas sensor element, it is necessary to heat the sensor element up to 400 ° C. or higher, which makes the element structure complicated, for example, it is necessary to provide a heater for the element. There is. In addition, there is a problem that the metal oxide itself used as a gas sensitive body for the measurement at a high temperature is sintered and the element changes with time and the sensor characteristics change.

Therefore, it is an object of the present invention to provide a gas sensor element capable of measuring a gas response at a relatively low temperature.

[0006]

As a result of repeated studies on the above problems, the inventor of the present invention used a solid solution of TiO ₂ —SnO ₂ as a sensor medium and formed an electrode on this surface to form a solid solution. It is characterized by using the change in resistance value at the interface with the electrode, and is preferably characterized in that the electrode is made of a metal having a work function of 4.5 eV or more.

FIG. 1 is a schematic view showing the structure of a gas sensor element according to the present invention. According to FIG. 1, TiO ₂ --S
A pair of electrodes 2 and 3 is adhered and formed on the surface of a sensor medium 1 made of a solid solution made of nO ₂ . TiO ₂ -Sn
The O ₂ solid solution is, for example, 1: 1 of TiO ₂ and SnO _2.
Completely dissolved, and is itself an n-type semiconductor. Further, it is desirable that one of the electrodes 2 and 3 deposited on the surface of the sensor medium 1 has a work function larger than that of the sensor medium 1 and is made of a metal of 4.5 eV or more. Examples include Au and Pt.

In the gas sensor of the present invention, for example, a TiO ₂ —SnO ₂ solid solution film is formed on the surface of a predetermined substrate. The solid solution film is formed by T-coating these by, for example, a coprecipitation method using chloride or an alkoxide method using alkoxide.
A mixed powder having a ratio of iO ₂ : SnO ₂ of 1: 1 is obtained, which is molded into a desired shape by a desired molding means such as a mold press, a cold isostatic press, an extrusion molding, and a doctor blade method. It can be obtained by firing at 1300 to 1600 ° C. and then quenching.

Further, the electrode formed on the surface of the solid solution is coated with a slurry containing metal powder, and 900 to 1
Baking at 500 ° C, sputtering, resistance vapor deposition, beam vapor deposition, laser ablation, etc.
Alternatively, it can be formed by a combination thereof.

[0010]

According to the present invention, when a solid solution of TiO ₂ —SnO ₂ having n-type semiconductor characteristics is used as a sensor medium and an electrode is formed on this, a response to a gas is obtained at a relatively low temperature of 300 ° C. or lower. Indicates.

Therefore, using hydrogen gas as an example, the response principle thereof was examined. FIG. 2 shows the response of the gas sensor element according to the present invention to hydrogen gas.
(A) is a diagram showing a change in the current flowing through the sensor element due to the introduction of hydrogen gas. From this figure, the value of the current flowing through the element changes due to the introduction of hydrogen gas and the element resistance changes due to hydrogen. I understand. Also, FIG.
FIG. 2B shows the resistance change between the sensor media at that time, and FIG. 2C shows the resistance change at the interface between the sensor media and the electrodes. According to FIGS. 2B and 2C, the electrode interface resistance is 50 times in absolute value of resistance as compared with the resistance change value of the sensor medium, and the rate of change due to introduction of hydrogen is large. It can be seen that the response mechanism of the sensor element is due to the change in the electrode interface resistance.

Since the gas sensor element utilizes the resistance change at the electrode interface, the material forming the electrode is important. That is, the gas sensitivity changes depending on the material of the electrode, and the work function is 4.5 eV as will be apparent from the examples described later.
The combination of the above metals showed the highest sensitivity, and among them, the combination of Al and Pt had the highest sensitivity.

As shown in Table 2, the gas responsiveness to hydrogen at a low temperature of 200 ° C. is not to methane, propane or carbon monoxide, but at a measured temperature of 500 ° C. Then, it becomes possible to respond to propane greatly, and it can be seen that the selective gas response is exhibited depending on the measurement temperature.

[0014]

【Example】

Example 1 Titanium chloride (TiCl ₄ ) powder and tin chloride (SnC)
l ₄ ) A powder was used as a starting material, and aqueous ammonia was added dropwise to the starting material for neutralization to precipitate a metal hydroxide. Then, this hydroxide was freeze-dried and then calcined in the atmosphere at 900 ° C. for 1 hour to obtain a mixed powder of titanium oxide and tin oxide. The mixed powder is molded into pellets at a molding pressure of 1 t / cm ² , and further molded by cold isostatic pressing at a pressure of 3 t / cm ² , followed by firing at 1500 ° C. for 12 hours, then taking out from the furnace into the air and quenching. Then, a TiO ₂ —SnO ₂ solid solution pellet sample was prepared.

An element for measuring resistance by the four-terminal method is cut out from a pellet into a 2 mm square prism, and four Pt wires are wound around this. Furthermore, in order to enhance the connection between the sample and the electrode, the Pt wire of the electrode and the sample In between, a Pt paste was applied and baked at 900 ° C.

The measurement was carried out at a constant flow rate (3
(00 ml / min) gas (synthetic air, 1% hydrogen) was introduced, and the current value flowing through the current terminal in air and hydrogen and the potential difference applied to the voltage terminal were measured. The results are shown in Fig. 2. As is apparent from FIG. 2A, by changing the gas atmosphere around the element from air to hydrogen, the current value which was stable at 0.07 μA in the air increased to 0.55 μA. When the gas atmosphere was returned to the air again, the current value was restored to the original level. Also, the resistance value of the entire element is calculated from the voltage of 30 V applied to the current terminal and the current value at the current terminal, and the resistance value of the sample itself is calculated from the potential difference applied to the voltage terminal and the current value at the current terminal. The resistance value was calculated by subtracting the resistance component of the sample itself from the resistance value of the entire device in consideration of the shape of the device. The results are shown in FIGS. 2 (b) and 2 (c). As is clear from FIGS. 2B and 2C, the change in sample resistance is several MΩ, while
The change in resistance value at the electrode interface is several hundred MΩ, and it can be seen that the change in current value obtained as a gas response is due to the change in electrode interface resistance of the sample.

Example 2 TiO ₂ —SnO ₂ produced as shown in Example 1
One electrode is Pt or A on the flat surface of the solid solution pellet.
An electrode was formed by combining u and the other electrode was Al or Ti to form an element. 1. Put the manufactured element in a furnace at 200 ° C., and set the Pt or Au electrode side to be positive.
A voltage of 5 V was applied and the change in the value of the current flowing through the device was measured. The gas atmosphere around the element is a constant flow rate (200 ml / min) from the gas cylinder, and synthetic air (nitrogen 80%,
Oxygen 20%) and 1% hydrogen were switched and introduced. The sensitivity to hydrogen (current value in gas / current value in air) was compared, and the results are shown in Table 1.

[0018]

[Table 1]

As is clear from Table 1, the sensitivity to hydrogen was exhibited at a low temperature of 200 ° C. In particular, as is clear from Table 1, the combination of Pt-Al electrodes showed the highest sensitivity.

Example 3 Samples prepared by forming electrodes on Pt and Al on TiO ₂ —SnO ₂ solid solution pellets prepared according to Example 1 were 2
The gas response to 1% hydrogen, 1% methane, 0.6% propane and 0.1% carbon monoxide was measured at 00 ° C.
The measurement voltage is 1.5 V, and the Pt electrode is positive.
Gas was introduced at a constant flow rate of 0 ml / min, and changes in the current value flowing through the device were measured. The results are shown in Table 2.

[0021]

[Table 2]

As is clear from Table 2, the sensitivities to the respective gases were not markedly sensitive to methane, propane and carbon monoxide, but were responsive to gas only to hydrogen.

Example 4 A sample was cut out from a TiO ₂ —SnO ₂ solid solution pellet into a 2 mm square prism, and a pair of electrodes was prepared using Pt wire and Pt paste. 300 ml / m at 300 ° C
0.6% propane was introduced at a flow rate of in, and the change in the current value flowing through the device was measured with 30 V applied. At this time, the element showed a sensitivity of 1.2, but when the temperature was raised to 500 ° C., the sensitivity improved to 6.7 as shown in FIG.

[0024]

As described in detail above, TiO ₂ --SnO ₂
By forming an electrode in a solid solution, a gas can be detected by a change in electrode interface resistance, and particularly, it exhibits high sensitivity to hydrogen even at a low temperature and has very high selectivity to hydrogen.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of a gas sensor element of the present invention.

FIG. 2 is a diagram for explaining the response mechanism of the gas sensor element of the present invention, in which (a) is a change in the current value of the sensor element,
(B) shows a change in resistance of the sensor medium, and (c) shows a change in resistance at the electrode interface.

FIG. 3 is a diagram showing a change in current value of a gas sensor element with respect to propane at 500 ° C.

[Explanation of symbols]

 1 sensor medium 2 and 3 electrodes

Claims (2)

[Claims] 1. A gas sensor element having a pair of electrodes formed on the surface of a TiO ₂ —SnO ₂ solid solution, wherein the detection gas is detected by a change in interfacial resistance between the solid solution and the electrodes. And a TiO ₂ —SnO ₂ gas sensor element. _2. The TiO ₂ —SnO ₂ gas sensor element according to claim 1, wherein at least one of the pair of electrodes is made of a metal having a work function of 4.5 eV or more.