JPH0332881B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field to Which the Invention Pertains] The present invention relates to a method for manufacturing a varistor for low voltage circuits containing zinc oxide (ZnO) as a main component. [Prior art and its problems] Ceramics made by mixing ZnO with a small amount of additives and sintering it as a main component are known to exhibit excellent voltage nonlinearity. It is widely used as a varistor to suppress abnormal voltage (surge) in electric circuits. The voltage nonlinearity of ZnO varistors is due to the Schottky barrier formed at the grain boundaries of ZnO particles. In practical varistors, the varistor voltage per grain boundary layer formed by bonding ZnO particles is almost constant regardless of the crystal grain size.
Its value is about 2V. Varistor voltage is the voltage between the terminals when a current of 1mA flows through the varistor, and is usually expressed as V1mA. Therefore, the varistor voltage of the voltage nonlinear resistance element is determined by the number of grain boundary layers existing between the electrodes provided on the ZnO sintered body. Therefore, for devices used in low-voltage circuits, it is necessary to reduce the thickness of the device or to sufficiently increase the ZnO particle size. For example, when applying a ZnO varistor to a DC12V 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. , the number of grain boundaries that can exist between the terminal electrodes of this device is at most 11 layers. On the other hand, the particle size of the ZnO varistor sintered body produced by the usual method is 10 to 20 μm, so the thickness of the element is 0.1 to 0.2 mm in order to obtain a varistor voltage of about 22 V.
must be done. However, sintered bodies such as ZnO varistors have low mechanical strength when the thickness is 0.1 to 0.2 mm, and there are problems such as cracking during manufacturing.
This method of making the device thin is not practical. To solve this problem, when making ZnO varistors, a small amount of ZnO single crystals with a particle size much larger than the ZnO powder is added to the raw material powder, and the ZnO single crystals (hereinafter referred to as core particles) are used as cores to form particles. Ingenious methods have been devised to encourage growth. According to this method, the crystal grain size grows to 50 μm or more,
Varistor voltage per 1 mm element thickness (below
V1mA/t) can be lowered to about 10V/mm. The following method is commonly used to produce core particles that promote grain growth. (1) ZnO powder to which a small amount of Ba compound or Sr compound is added is molded and fired, and the resulting sintered body is hydrolyzed. (2) A sintered body obtained by molding a powder obtained by adding and mixing a grain growth promoter such as Bi ₂ O ₃ or a rare earth oxide to ZnO powder and then firing it is pulverized. (3) Directly produce ZnO single crystal using vapor phase growth method. Among these methods for producing core particles, method (1) is capable of hydrolyzing and removing Ba compounds and Sr compounds used as grain growth promoters;
It is also the most commonly used because it is easy to control additives and the core particle size. Add the core particles obtained in this way
In order to reduce the value of V1mA/t and maintain other electrical properties well, the core particle diameter was set to 25
53 μm is disclosed in Japanese Patent Application Laid-Open No. 58-20773, which was invented by the present inventors. However, it is not possible to obtain core particles with a diameter of 25 to 53 μm with a good yield, and even with the most commonly used method (1) above, there is a large variation in the core particle diameter, and it is difficult to obtain core particles with the optimum diameter. The rate is only about 40%. In addition, in method (1), when firing ZnO powder mixed with Ba compound or Sr compound, ZnO
Particles tend to grow along the C axis in terms of crystal structure, so they tend to have an elongated shape. When sieving is performed to select only core particles with a diameter of 25 to 53 μm from core particles that include elongated ones, the longitudinal direction is
Core particles larger than 53 μm easily pass through the sieve mesh and get mixed in. ZnO added with such core particles
The varistor obtained by sintering the powder has non-uniform crystal grains, which inevitably leads to a deterioration in the characteristics of the varistor. [Object of the Invention] The object of the present invention is to increase the particle size of ZnO varistors used in low voltage circuits and reduce the value of V1mA/t. It is an object of the present invention to provide a manufacturing method that makes the particle diameter uniform. [Summary of the invention] The present invention is characterized in that a Ba compound or a Sr compound is added.
By adding and mixing rare earth compounds to ZnO powder and hydrolyzing the sintered body, which is formed and fired,
The rare earth element suppresses abnormal grain growth, making it possible to obtain rounded core particles with an optimal grain size at a high yield. [Examples of the Invention] The present invention will be described below based on Examples. BaCO ₃ , Al(NO ₃ ) ₃ , Co ₃ O ₄ , and La ₂ O ₃ powders were added to ZnO powder as additive elements.
Addition is made so that the content of Ba is 0.08 atomic%, Al is 20 atomic%, Co is 2 atomic%, and La is 0.1 atomic%.
After thorough mixing and pressure molding, the mixture was fired at 1150°C for 4 hours. The obtained sintered body has a structure in which a BaO layer precipitated at grain boundaries surrounds ZnO particles. Core particles are obtained by sufficiently boiling this sintered body in pure water to dissolve the BaO layer. Figure 1 shows the particle size distribution when this was classified using a sieve. For comparison, Figure 2 shows the particle size distribution of core particles produced by the conventional method, that is, without adding La ₂ O ₃ to ZnO powder.
As can be seen from the comparison between Figures 1 and 2, in the conventional method without the addition of La ₂ O ₃ , the core particles are distributed over a wide range, with a steep peak in the particle size range of 25 to 53 μm. In contrast, according to the present invention, by adding La 2 O 3, the particle size can be reduced to 25 to 100 μm by adding La ₂ O ₃ . The 53μm core particles are distributed in a narrow range with a prominent peak, and no abnormal grain growth occurs. The reason why the grain size distribution of the core particles differs depending on the presence or absence of La is that La precipitates at the ZnO grain boundaries and suppresses rapid grain growth. In this example, Al and Co are also added, but these are intended to increase the dissolution rate of the BaO layer during hydrolysis, that is, the separation rate of the core particles, and to improve the electrical characteristics of the varistor. It is essentially independent of the size and shape of the core particles. Table 1 shows the yield of core particles with a particle size of 25 to 53 μm and their shapes among _the core particles obtained using the above _method . Therefore, nuclear particles produced by conventional methods are also listed.
Abnormal grain growth may be suppressed with the addition of La ₂ O ₃ .

[Table] Most of the grains are circular, and those produced by conventional methods suffer from abnormal grain growth, and many of the grains are elliptical, so the yield of particles with a diameter of 25 to 53 μm is significantly different.
Also, when SrCO ₃ is added instead of BaCO ₃
The effect of La is exactly the same, and although not shown in Table 1, even if BaCO ₃ and SrCO ₃ are added simultaneously, the same results as when each is added individually can be obtained. Table 2 shows the results when various rare earth oxides different from La ₂ O ₃ were used in the same amount added, in the same format as Table 1. ●No. 3 to No. 9 added BaCO ₃ No. 10 to No. 16 are cases where SrCO ₃ was added.

【table】

[Table] As shown in Table 2, the effects of the present invention are observed even when various rare earth elements other than La are added. Next, Table 3 uses La ₂ O ₃ and the amount added is 0.01 ~
The results for core particles obtained by changing the concentration to 2.0 at% are shown in the same format as Tables 1 and 2, but Table 3 adds the separation time of core particles by hydrolysis. Filled out. No. 17 to No. 21 in Table 3 are those with BaCO ₃ added, but these include No. 1 in Table 1 according to the amount of La added, and similarly No. 22 to No. .26 is for the case of SrCO ₃ addition, and the one shown in No. 2 of Table 1 is reprinted here.

〔Effect of the invention〕

Conventionally, the most common method for producing core particles used in manufacturing ZnO varistors for low-voltage circuits is to hydrolyze a sintered body made by adding Ba or Sr compounds to ZnO powder. As explained in the example, by adding rare earth elements in addition to Ba and Sr to ZnO powder, this rare earth element plays a role in suppressing rapid grain growth, resulting in a rounder and more uniform grain size. The yield of the core particles is significantly improved since the core particles are obtained and the particle size distribution has a steep peak in the optimum particle size range of 25 to 53 μm. As a result, variations in the characteristic values of varistors obtained by adding these core particles can be reduced.

[Brief explanation of the drawing]

FIG. 1 shows the particle size distribution of core particles obtained by the present invention, and FIG. 2 shows the particle size distribution of core particles obtained by the conventional method.

Claims (1)

[Claims] 1. Hydrolyzing a sintered body obtained by press-molding and firing a mixed powder of zinc oxide (ZnO) powder and at least one of a barium (Ba) compound and a strontium (Sr) compound. ZnO varistor raw material
In the method of manufacturing ZnO single crystal particles,
A method for producing ZnO single crystal particles, which comprises adding a rare earth element compound to ZnO powder. 2. In the method described in claim 1,
Rare earth elements in compounds of rare earth elements are 0.05 to 1.0
A method for producing ZnO single crystal particles characterized in that atomic percent is added.