JPH0434101B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention comprises an oxide semiconductor and a pair of electrodes, one of which is made of Pt or Pd, and has a junction with the oxide semiconductor that exhibits non-ohmic characteristics. This invention relates to a hydrogen detection element. (Prior Art and its Problems) It is well known that hydrogen is an energy source that is attracting attention as an alternative to petroleum. However, we have not yet reached the hydrogen era.
Many issues remain to be solved before hydrogen can be used in the world. One of them is
There is a hydrogen detection element. Several attempts have been made to develop hydrogen detection elements. For example, there are elements using _SnO2 . This element has a high operating temperature and is sensitive to gases other than hydrogen. Therefore, in order to use it as a sensor for a hydrogen detector, it requires several additional devices and has the disadvantage that the device becomes complicated. In addition, as another example, on the surface of a CdS single crystal,
There is one in which a CdS single crystal is doped with In to lower its resistance, and a Pd electrode is formed on it (Applied
Physics Letters Vol, 28. No. 11, 1976). This device has the disadvantage that it takes a long time, more than 15 minutes, to produce a desired response after exposure to hydrogen, and it lacks stability. Therefore, the present inventors have conducted extensive research with the aim of providing a hydrogen detection element that eliminates the problems of the above-mentioned conventional techniques. As a result, the inventors discovered that certain oxide semiconductors and platinum (Pt) or palladium (Pd) have non-ohmic characteristics that are highly sensitive to hydrogen, and based on this fact, the authors developed this book. Completed the invention. (Structure and operation of the present invention) The present invention provides (ZnO) _1-a (Al ₂ O ₃ ) _a , (ZnO) _1-b
(ZrO ₂ ) _b , (Fe ₂ O ₃ ) _1-c (TiO ₂ ) _c , Ba _1-d M _d TiO ₃
(However, M: At least one or more of La, Ce, Nd, Y, Sb, Gd, Bi) or BaTi _1-e M' _e O ₃ (However, M': At least one or more of Ta, Nb, W) Oxide semiconductors as constituent components (however, a,
b, c, d, e are 0≦a≦0.15, 0≦b, respectively
≦0.1, 0.001≦c≦0.08, 0.0001≦d≦0.007,
0.0005≦e≦0.005) and a pair of electrodes in contact with the oxide semiconductor, one of which is made of platinum (Pt) or palladium (Pd) and has non-ohmic characteristics between it and the oxide semiconductor. The hydrogen detection element is characterized in that the other electrode is made of a metal other than Pt or Pd and has ohmic characteristics. The present invention will be explained in detail below. The oxide semiconductor in the present invention includes zinc oxide (ZnO) alone, ZnO with aluminum oxide (Al ₂ O ₃ ) or zirconium oxide (ZrO ₂ ) added as a semiconductor agent, and ferric oxide (Fe ₂ O ₃ ) with titanium oxide (TiO ₂ ) as a semiconducting agent, or barium titanate (BaTiO ₃ ) with lanthanum (La), cerium (Ce), or neodymium (Nd) as a semiconducting agent. , ythonium (Y), antimony (Sb), gadolinium (Gd), bismuth (Bi), tantalum (Ta),
One or more of niobium (Nb) and tungsten (W) is added. The content of the semiconducting agent is within the following range.
When adding Al ₂ O ₃ to ZnO, the mole fraction of Al ₂ O ₃ a
is 0≦a≦0.15, when adding ZrO ₂ to ZnO, ZrO ₂
The mole fraction b of is 0≦b≦0.1, when adding TiO ₂ to Fe ₂ O ₃ , the mole fraction c of TiO ₂ is 0.001≦c≦0.08,
When adding M (where M is at least one of La, Ce, Nd, Y, Sb, Gd, Bi) to BaTiO ₃ ,
The molar fraction d of M is 0.0001≦d≦0.007, M′ (however,
M′ is at least one of Ta, Nb, and W)
When added to BaTiO ₃ , the molar fraction e of M′ is 0.0005
≦e≦0.005. If the values of a, b, c, d, and e are outside the above ranges, the resistivity of the oxide semiconductor becomes too large and hydrogen detection sensitivity decreases, which is not desirable. The oxide semiconductor forms a junction having a potential barrier with Pt or Pd. The potential barrier thus has properties that vary depending on the concentration of hydrogen surrounding the junction. That is, the potential barrier changes sharply due to the presence of hydrogen gas. Therefore, using this, hydrogen gas can be detected with high sensitivity. The oxide semiconductor in the present invention is preferably produced by a conventional firing method. That is, for example, (ZnO) _1-a・(Al ₂ O ₃ ) _a is added to (ZnO) powder,
Al ₂ O ₃ powder is added, mixed, crushed, and compacted into a predetermined shape, and then fired to form a sintered body. Note that instead of oxides, the raw materials may be carbonates such as Zn, nitrates, acetates, chlorides, and other organometallic compounds that become oxides when heated in air. The firing temperature is preferably 1000 to 1200°C for about 2 hours. This sintered body may be used as it is, but in order to obtain a more homogeneous sintered body, it is preferable to crush the sintered body once again to form a powder, then press it into powder and perform main firing. Alternatively, the above powder may be mixed with an organic vehicle and glass frit to form a paste-like material, which may be applied onto an insulating substrate by a method such as screen printing and fired to form a thick film of an oxide semiconductor. When performing main firing, add 500 to 3000 of the mixed powder.
It is preferable to compact the powder into the desired shape using a pressure of Kg/cm ² . After powder compaction, (ZnO) _1-a (Al ₂ O ₃ ) _a (ZnO) _1-b (ZrO ₂ ) _b ,
For (Fe ₂ O ₃ ) _1-c (TiO ₂ ) _c , approximately 1200°C, 2 hours; for Ba _1-d MaTiO ₃ or BaTi _1-e M′eO ₃ , 1200-1400°C, 30 minutes-1 hour. Heating is sufficient. A pair of electrodes in the present invention are used to apply a voltage to the oxide semiconductor and cause a current to flow therethrough, and one electrode is made of Pt or Pd. The junction between the Pt or Pd electrode and the oxide semiconductor has non-ohmic characteristics, that is, a potential barrier, and the height of this barrier changes depending on the amount of hydrogen at room temperature. As a method for forming the Pt or Pd electrode, Pt or Pd may be formed on an oxide semiconductor by a method such as vapor deposition. Furthermore, a method may be used in which a paste made by adding an organic vehicle and a frit to powdered Pt or Pd is applied onto the oxide semiconductor and baked by heating to 400 to 600°C. When the heating temperature exceeds 600°C, the detection sensitivity for hydrogen decreases. Another electrode is Ag-ln, Ag-Sn or
It is made of metal such as Ag-Ti and has the role of supplying current to the oxide semiconductor. The electrode includes
It is a so-called ohmic electrode in which a current proportional to the voltage flows. Specifically, an Ag-Sn paste made by kneading a mixed powder of Ag and Sn with an organic vehicle may be applied to the surface of the oxide semiconductor, dried, and baked. When a constant voltage is applied to the hydrogen detection element configured as described above and the element is exposed to hydrogen-containing gas, the resistance of the non-ohmic interface between the Pt or Pd electrode and the oxide semiconductor increases depending on the concentration of hydrogen. As the voltage changes, the current flowing through the element changes. This current change is proportional to the hydrogen concentration in the gas. Therefore, hydrogen around the element can be detected from the current value, and its concentration can be measured. In Figure 3, air containing 5% by volume (V%) of hydrogen and 100% air are shown.
The relationship between the voltage applied between the electrodes of the element (applied so that the Pt or Pd electrode becomes a positive voltage) and the current flowing through the element will be illustrated when the element is exposed at room temperature. As can be seen from this figure, when the applied voltage is kept constant, the current value increases due to the presence of hydrogen. Next, embodiments of the hydrogen detection element according to the present invention will be described. One embodiment of the hydrogen detection element is a film made of an oxide semiconductor. The device of this embodiment is shown in FIGS. 1 and 2. The device shown in FIG. 1 has a film-like oxide semiconductor 2 formed on the surface of a substrate 1 made of an electrical insulator such as Al ₂ O ₃ , and a non-ohmic film made of Pt or Pd on the oxide semiconductor. Electrode 3
An ohmic electrode made of Ag-Sn or the like is formed. The device shown in FIG. 2 has an ohmic electrode formed on an insulating substrate 1, and an oxide semiconductor, Pt or Pd electrode successively formed thereon. When an oxide semiconductor is formed into a film, the response time of the device becomes less than one minute, making the device easy to use. An example of the method for manufacturing the above-mentioned film-like oxide semiconductor is as follows. First, the oxide semiconductor is thoroughly ground into particles. Next, an organic vehicle consisting of terpineol, ethyl cellulose, etc. and a frit of lead borosilicate glass are added to the obtained particles to form a paste, and the paste is applied onto a substrate by a method such as screen printing. after that
It is fired at a temperature of 600-1200℃ and baked onto the substrate.
The thickness of the obtained film-like oxide semiconductor is 3 to 30 μm.
Good condition. The particle size of the oxide semiconductor powder is preferably about 1 to 20 μm; if it is less than 1 μm, the electric resistance becomes too high when baked into a film, making it difficult to use as an element. Furthermore, if the thickness is 20 μm or more, it becomes difficult to apply the film in the form of a film, and the electrical properties of the fired film-like body will vary greatly. The amounts of the organic vehicle and glass frit added to the oxide semiconductor are preferably about 10 to 80 g and 1 to 10 g, respectively, per 100 g of the oxide semiconductor. The organic vehicle is added to make it easier to apply onto the substrate, and the glass frit is used to strengthen the film-like oxide semiconductor and to make it adhere to the substrate. Finally, an electrode is formed on the film-like oxide semiconductor. The formation method may be similar to the method described above. (Effects of the Present Invention) The hydrogen detection element according to the present invention has excellent detection sensitivity for hydrogen gas at room temperature, that is, at a relatively low temperature. Further, a film formed from an oxide semiconductor has good responsiveness, can be mass-produced, and is inexpensive. (Example) Hereinafter, an example of the present invention will be described. Example 1 Zinc oxide (ZnO), aluminum oxide (Al ₂ O ₃ ), and zirconium oxide (ZrO ₂ ) as starting materials
The powders were mixed into the composition shown in sample numbers 1 to 12 in Table 1, and after thoroughly mixing them, a diameter of 12 mm,
The powder was compacted and preformed onto a disk with a thickness of 2 mm. The molding pressure is 500-1000Kg/ ^cm2 . Next, the preform was calcined at 1100° C. for 2 hours. next,
The obtained calcined body was thoroughly ground with an alumina mortar and pestle, and formed into a disk shape with a diameter of 10 mm and a thickness of 1 mm under a pressure of 1000 to 2000 Kg/cm ² . Thereafter, main firing was performed at 1200° C. for 2 hours to obtain the oxide semiconductor of the present invention. #1500 on both sides of the sintered body of the above oxide semiconductor
After polishing with emery paper and cleaning with acetone, Ag-Sn paste was applied. And 500
The Ag-Sn ohmic electrode was baked by heating at ℃ for 5 minutes. The specific resistance of the sintered body was measured using the electrode. The results are shown in Table 1. After that, one electrode was scraped off and washed with acetone.
Pd paste was applied and heated at 450° C. for 5 minutes to form a non-ohmic electrode, thereby obtaining a hydrogen detection element according to the present invention. Next, connect the sample numbers 1 to 12 in series with the hydrogen detection elements.
A constant resistor of 50Ω was attached, and a voltage from OV to 1V was applied so that the Pd electrode became a positive voltage, and the voltage-current characteristics of the device were measured. FIG. 3 shows examples of measurement results when the device is placed in air and when it is placed in air containing 5V% hydrogen.
Curve A is a characteristic curve in air, and curve B is a characteristic curve in hydrogen-containing air. In air, the Pd electrode exhibits rectifying characteristics, but in hydrogen-containing air, the potential barrier at the Pd electrode is removed, resulting in current-voltage characteristics that follow Ohm's law. The magnitude of ΔV shown in this figure represents the sensitivity to hydrogen gas, and the larger ΔV is, the more sensitive the element is. The ΔV measured for each element is shown in the rightmost column of Table 1. Note that even when exposed to CH ₄ and CO gas, ΔV remained approximately zero.

[Table] Example 2 The hydrolysis products of ferric oxide (Fe ₂ O ₃ ) and titanium tetraisopropoxide (Ti[O(CH ₃ ) ₂ ] ₄ ) as starting materials were prepared using the sample numbers in Table 2. A hydrogen detection element according to the present invention was manufactured in the same manner as in Example 1 by preparing the composition shown in . For comparison, a sample having the composition of sample number C-1 was also produced. The specific resistance and ΔV of each element were measured in the same manner as in Example 1, and the results are shown in Table 2.

【table】

[Table] It can be seen that by adding a small amount of (TiO ₂ ) to (Fe ₂ O ₃ ), △V increases, making the element highly sensitive to hydrogen. Note that ΔV was almost zero even when exposed to CH ₄ and CO. Example 3 Semiconducting elements shown in sample numbers 1 to 10 in Table 3 were added as oxides of each element to barium carbonate (BaCO ₃ ) and titanium oxide (TiO ₂ ) as starting materials, and the same process as in Example 1 was carried out. Preliminary sintering was performed using the method. After that, the obtained calcined body is crushed, mixed, and compacted, and then
C. for 0.5 to 1 hour to obtain the oxide semiconductor of the present invention. After baking an ohmic electrode similar to that in Example 1 onto the oxide semiconductor, Pt paste was applied to the oxide semiconductor to create a non-ohmic electrode, and heating was performed at 550°C for 10 minutes to produce a hydrogen detection element according to the present invention. I got it. Thereafter, specific resistance and ΔV were measured in the same manner as in Example 1. Table 3 shows the specific resistance and ΔV range for each semiconductor element.

[Table] FIG. 4 also shows the relationship between the amount of Y added and the specific resistance when the semiconductor element is Y. This figure shows that as the amount of Y added increases, the resistivity tends to decrease once and then increase again. A similar tendency is shown in the case of other semiconductor-forming elements. The sintered bodies of sample numbers 1 to 7 have the general formula Ba _1-d MdTiO ₃ (M is a semiconductor element), and the sintered bodies of sample numbers 8 to 10 have the general formula BaTi _1-e M′eO ₃ (M’ is It is expressed by the general formula of the semiconducting element). The sample containing the semiconductor element in the amount shown in each test number had excellent hydrogen detection characteristics. Also,
It showed almost no sensitivity to CH ₄ and CO gas. Example 4 Each of the sintered bodies produced in Examples 1 to 3 was thoroughly ground to obtain powder having a particle size of about 2 to 10 μm. 55% organic vehicle to 100g of these various powders
g, and 3 g of lead borosilicate glass frit were added and thoroughly kneaded to obtain an oxide semiconductor paste. These pastes were printed in the form of a film on an Al ₂ O ₃ substrate using a 250-mesh stainless steel mesh. the board
After drying at 180°C for 10 minutes, it was further heated at 950°C for 20 minutes to bond the film-like oxide semiconductor to the baked substrate. Next, a Pd, Ag-Ti-Sn electrode or a Pt, Ag-Ti-Sn electrode was applied onto the above film in the manner shown in FIG. 1 to produce a hydrogen detection element according to the present invention. Note that the interval between the Pd or Pt electrode and the Ag-Ti-Sn electrode was 1 mm. The produced hydrogen detection elements were used in Examples 1 to 2, respectively.
Similar to that shown in No. 3, it showed excellent hydrogen detection characteristics. Furthermore, since the oxide semiconductor was formed into a film, the response time to changes in the amount of hydrogen in the air was around 1 minute, which was 1/3 to 1/10 times shorter than in the case of a sintered body.

[Brief explanation of drawings]

1 and 2 are perspective views showing embodiments and examples of the hydrogen detecting element according to the present invention, and FIG. 3 is a diagram showing an example of voltage-current characteristics of the hydrogen detecting element according to the present invention. Curve A is a characteristic curve in air, and curve B is a characteristic curve in air containing 5V% hydrogen. FIG. 4 shows the results of the example,
FIG. 3 is a diagram showing the relationship between the amount of Y in BaTiO ₃ and specific resistance. 1... Substrate, 2... Film oxide semiconductor, 3...
Pt or Pd electrode, 4...ohmic electrode.

Claims (1)

[Claims] 1 (ZnO) _1-a (Al ₂ O ₃ ) _a , (ZnO) _1-b (ZrO ₂ ) _b ,
(Fe ₂ O ₃ ) _1-c (TiO ₂ ) _c , Ba _1-d M _d TiO ₃ (However, M:
(At least one of La, Ce, Nd, Y, Sb, Gd, Bi) or BaTi _1-e M′ _e O ₃ (M′: Ta,
An oxide semiconductor containing at least one of Nb, W) as a main component (a, b, c, d, e
are 0≦a≦0.15, 0≦b≦0.1, 0.001≦, respectively.
c≦0.08, 0.0001≦d≦0.007, 0.0005≦e≦0.005
) and a pair of electrodes in contact with the oxide semiconductor, one electrode is made of platinum (Pt) or palladium (Pd) and has non-ohmic characteristics with the oxide semiconductor, and the other electrode is made of platinum (Pt) or palladium (Pd) and has non-ohmic characteristics with the oxide semiconductor. A hydrogen detection element characterized in that the electrode is made of a metal other than Pt or Pd and has ohmic characteristics. 2. The hydrogen detection element according to claim 1, wherein the oxide semiconductor is a film-like body.