JPH02178902A - Voltage dependent nonlinear resistance element - Google Patents

Abstract

PURPOSE:To extinguish shottky barrier between a sintered substance and a metallic electrode so as to make it excellent in mechanical strength and surge resistance by providing a contact layer, consisting of a specific zinc oxide, between the sintered substance and the metallic electrode. CONSTITUTION:Proper amounts of Pr, Co, B, etc., in the form of compounds such as oxides, etc., are added to ZnO powder, and then are mixed and granulated so as to make up granulated powder for ZnO varistor, and this granulated powder is formed into disc shape. Next, solution, wherein nitric acid indium In(No3).-9H2O is dissolved in acetylacetone, is applied on opposing both main faces of a sintered substance 3 which is made by firing, and is heat-treated after drying so as to disperse In into a ZnO sintered substance. Since the contact layers 2A and 2B, wherein In is introduced into zinc oxides, are low resistant layers, schottky barrier between the zinc oxides and the metallic electrodes 1A and 1B disappears, and mechanical strength and surge resistance become favorable.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention relates to a voltage nonlinear resistance element for low voltage circuits containing zinc oxide as a main component, and particularly relates to a contact layer between a zinc oxide sintered body and a metal electrode. .

[Conventional technology]

Ceramics, which are made by mixing ZnO with a small amount of additives and sintering it as a main component, are known to exhibit excellent voltage nonlinearity, and are useful for suppressing abnormal voltages (surges) in electrical circuits. It is widely used as a varistor for control purposes. Such a varistor is manufactured according to the steps shown in FIG.

The voltage nonlinearity of ZnO varistors is due to double Schottky barriers formed at grain boundaries of ZnO crystal grains. In a practical varistor, the varistor voltage per grain boundary layer formed by the combination of ZnO crystal grains is almost constant regardless of the crystal grain size, and its value is 2
That's about it. (Varistor voltage is 1mA to the varistor.
It is the voltage between the terminals when a current of ) Therefore, the varistor voltage of the voltage non-linear resistance element is determined by the number of grain boundary layers existing between the electrodes provided on the opposing surfaces of the ZnO sintered body.

[Problem to be solved by the invention]

By the way, silver and aluminum are used for the electrodes of ZnO varistors, and except for ZnO elements for lightning arresters, it is common to bake silver paste and use this as the electrode. In this case, a Schottky barrier is formed at the interface between the silver electrode and the ZnO sintered body, and when a current of 1 mA is passed, a voltage of about 3 V is generated due to the Schottky barrier at the electrode/ZnO sintered body interface. Therefore, this electrode/ZnO sintered body interface barrier becomes impossible to ignore as the varistor voltage becomes lower.

For example, when applying a ZnO varistor to a DC 12V circuit, a varistor voltage of 22V is generally used in consideration of circuit voltage fluctuations, but as mentioned above, the varistor voltage per grain boundary layer is approximately 2V. Therefore, 11 layers of grain boundaries exist between the opposing electrodes of this element. However, due to the barrier at the electrode/ZnO sintered body interface mentioned above, ZnO
The sum of the varistor voltage at the grain boundaries of the sintered body and the varistor voltage at the electrode/sintered body interface must be 22V. Therefore, in reality, as the varistor voltage decreases, the difference between the varistor voltage determined from the number of sintered bodies and the actually measured varistor voltage increases, so the number of grain boundaries in the sintered body must be reduced. For this reason, if the thickness of the sintered body is reduced, (1) the mechanical strength of the sintered body will decrease (2) the ability to absorb surge energy such as lightning surges and switching surges (
Since the surge resistance (surge resistance) is proportional to the volume of the sintered body, there were problems such as a decrease in surge resistance characteristics.

In addition, the Schottky barrier at the electrode/sintered body interface is
The Schottky barrier existing at the grain boundaries of the sintered body itself has a low relative Vesage resistance, so the lower the varistor voltage is, the greater the change in varistor voltage due to surges becomes. The problem with the Schottky barrier is that it tends to fluctuate during the assembly process, such as the soldering process and the resin molding process, making process control of the varistor voltage difficult.

This invention was made in view of the above points, and its purpose is to eliminate the Schottky barrier between the sintered body and the metal electrode, thereby providing a low-voltage non-conductor with excellent mechanical strength and surge resistance, and easy process control. An object of the present invention is to provide a linear resistance element.

[Means to solve the problem]

According to the present invention, the above-mentioned object is achieved in a voltage nonlinear resistance element comprising a metal electrode arranged on a sintered body made by mixing zinc oxide powder and a small amount of additive powder that causes voltage nonlinearity. This is achieved by providing a contact layer made of zinc oxide containing at least one element of , In, 8/, Ga and provided between the sintered body and the metal electrode.

The contact layer can be formed directly on the sintered body by thermal diffusion of In, Ki, Ga or ion implantation, or can be formed by a sputtering method or a method of coating zinc oxide containing In, AI, or Ga. .

[Effect]

Since the contact layer in which Jn, A/, and Ga are introduced into zinc oxide is a low resistance layer, the Schottky barrier between the zinc oxide and the metal electrode disappears.

〔Example〕

Next, embodiments of the present invention will be described based on the drawings.

FIG. 2 shows the manufacturing process of a resistance element according to an embodiment of the present invention. First, appropriate amounts of Pr, Co, E, etc. in the form of compounds such as oxides are added to ZnO powder, and then mixed and granulated to prepare granulated powder for ZnO varistors.

This granulated powder was molded using a mold with a diameter of 17 mm to a thickness of 25 mm.
Shape into n disc shape. Next, this molded body is fired at a temperature of 1350° C. for 4 hours in an oxidizing atmosphere. The size of the obtained sintered body was 14 mm in diameter and 1.0 mm in thickness.

A solution of indium nitrate (In(NO3) 9H,0) dissolved in acetylacetone was applied onto the opposing surfaces of the sintered body produced as described above, and after drying, it was heated at 700℃ for 4 hours.
h heat treatment was performed to diffuse In into the ZnO sintered body. In
The surface resistance of the surface on which it was diffused was less than IKΩ when a current of 1 mA was applied. When In is not diffused, the resistance is 20Ω or more.

A varistor was constructed by applying silver paste to the sintered body treated as described above and then baking it. A schematic cross-sectional view is shown in FIG. Subsequently, the varistor characteristics were measured. We also evaluated the varistor characteristics after assembly. The results are shown in Table 1.

The first table shows the surge resistance (A). However, the surge resistance was defined as the current at which the rate of change in VIffiA was ±10% after a standard current pulse of 8720 μS was passed through the device twice at 2-minute intervals.

Table 2 Table 1 shows the coefficient of variation of V I+nA + V ImA,
Voltage nonlinear coefficient α in the current 100 pA to 1 mA region
This is shown in comparison with the conventional method after one assembly after electrode attachment. Further, as reference data, characteristics when using a ZnO sintered body and an In-Ga electrode that does not form a Schottky barrier are also shown. From these results, it can be seen that the method of the present invention has no characteristic variation after electrode attachment and after assembly, and is superior to the conventional method.

As is clear from Table 2, the method of the present invention is superior after assembly when a surge is applied. The reason for this is thought to be as follows. That is, the surge withstand capacity of the sintered body itself is 1200OA, but in the case of an element with VlmA of 20V, since the barrier formed at the electrode/sintered body interface in conventional elements is easily changed by surges,
This is because a 2V change in the electrode/sintered body interface barrier results in a 10% change. In this way, the disadvantage that the lower the varistor voltage is, the lower the surge resistance of the ZnO varistor element manufactured by the conventional method appears to be is solved.

Although the above-mentioned embodiments described a method of coating and diffusing In, similar effects were also confirmed with Al and Ga elements. Further, although the embodiments have been described with respect to a disk-shaped sintered body, the effects of the present invention are not dependent on the shape, and can be confirmed not only when the electrodes are opposed to each other but also when two electrodes are provided on the same plane.

The concentrations of In, AI, and Ga are 10% compared to Zn in ZnO.
It is sufficient if it is 0 atomic ppm or more. Also, the thickness of the contact layer is 10
It is sufficient if the thickness is 00 Å or more.

〔Effect of the invention〕

According to this invention, in a voltage nonlinear resistance element in which metal electrodes are arranged on a sintered body made by mixing zinc oxide powder and a small amount of additive powder that causes voltage nonlinearity,
, kl, and a contact layer made of zinc oxide containing at least one element of Ga and provided between the sintered body and the metal electrode, so that a low resistance contact layer is interposed between the sintered body and the metal electrode. As a result, the Schottky barrier between the sintered body and the metal electrode disappears, and a voltage nonlinear resistance element with excellent mechanical strength and surge resistance and easy manufacturing process control is obtained.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a schematic sectional view showing a voltage nonlinear resistance element according to an embodiment of the present invention, and FIG. 2 is a flowchart showing the manufacturing process of a resistance element according to an embodiment of the invention. FIG. 3 is a flowchart showing a conventional device manufacturing process. IA, IB: metal electrode, record, 2B: contact layer, 3: sintered part ■

Claims (1)

[Claims] 1) In a voltage nonlinear resistance element in which metal electrodes are arranged on a sintered body made by mixing zinc oxide powder and a small amount of additive powder that causes voltage nonlinearity, In, Al, Ga
A voltage nonlinear resistance element comprising a contact layer made of zinc oxide containing at least one element and provided between the sintered body and a metal electrode.