JPS6127602A - Method of producing zno single crystal particles - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] 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 and sintering ZnO with trace amounts of additives as its main component are known to exhibit excellent voltage nonlinearity, and are known to exhibit excellent voltage nonlinearity, which can prevent abnormal voltages in electrical circuits. It is widely used as a varistor to suppress surges.

The voltage nonlinearity of the ZnO varistor is caused by spots and toky barriers formed at the grain boundaries of ZnO particles. In a practical varistor, the varistor voltage per layer of grain boundaries formed by bonding of ZnO particles is almost constant regardless of the size of the crystal grains, and its value is about 2. The varistor voltage is the voltage between the terminals when a current of 1 mA is passed through the varistor, and is usually expressed as V 1 mA.

Therefore, the varistor voltage of the voltage nonlinear resistance blade is ZnO
It is determined by the number of grain boundary layers existing between the electrodes provided on the sintered body. Therefore, for devices used in low-voltage circuits, the thickness of the device must be made thinner, or Zn
It is necessary to make the O particle size sufficiently large.

For example, when applying a ZnO varistor to a DC12V circuit, the varistor voltage is generally 2.
A varistor voltage of 2V is used, but as mentioned above, the varistor voltage per grain boundary layer is about 2V, so the number of grain boundaries that can exist between the terminal electrodes of this element is at most 11 layers.

On the other hand, since the particle size of a ZnO varistor sintered body produced by a conventional method is 10 to 20 μm, the thickness of the device must be 0.1 to 0.2 mm in order to obtain a varistor voltage of about 22V. However, sintered bodies such as ZnO varistors have low mechanical strength at a thickness of 0.1 to 0.2 JIm.
This method of making the device thinner is not practical due to problems such as cracks occurring during manufacturing.

To solve this problem, when making ZnO varistors, the raw material powder contains Zn with a much larger particle size than the ZnO powder.
An ingenious method has been devised in which a small amount of O single crystal is added and grain growth is promoted using the ZnO single crystal (hereinafter referred to as a core particle) as a nucleus. According to this method, the crystal grain size grows to (capital) μm or more, and the varistor voltage (hereinafter referred to as V 1 mA/t) per device thickness law is IOV/m.
It can be lowered to a certain extent.

The following method is commonly used to produce core particles that promote grain growth.

A powder obtained by adding and mixing a small amount of Ba compound or Sr compound to ill Z n O powder is molded and then fired, and the obtained sintered body is hydrolyzed.

(A sintered body obtained by molding a powder obtained by adding and mixing a grain growth promoter such as Bi2O3 or a rare earth oxide to 21Z n O powder and then firing it is pulverized.

(3) A ZnO single crystal is directly formed using a 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, and is also effective in controlling additives and controlling the core particle diameter. It is the most commonly used because it is easy to control.

The core particles thus obtained were added to V imA/l.
In order to reduce the value of and maintain good electrical properties, the core particle diameter is in the range of δ to Showa μm, as disclosed in Japanese Patent Application Laid-Open No. 20773, which was invented by the present inventors. Disclosed.

However, core particles with a diameter of 25 to 53 μm cannot be obtained with a good yield, and the most common method is the above-mentioned method (1).
Even with the method (2), the core particle size varies widely and the yield of core particles having the optimum particle size is only about 40 layers. Further, in the method (1), when ZnO powder mixed with a Ba compound or a Sr compound is fired, the ZnO particles tend to grow along the C axis in terms of crystal structure, so they tend to have an elongated shape. 25 to 53 μm from the core particle, including elongated ones
When sieving is performed to select only core particles having a diameter, core particles having a longitudinal direction of 53 μm or more easily pass through the sieve mesh and are 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.

[Purpose of the invention]

The purpose of the present invention is to increase the particle size of a ZnO varistor used in a low voltage circuit and to reduce the value of V l mA/l. The objective is to provide a uniform manufacturing method.

[Key points of the invention]

The present invention is based on ZnO containing Ba compound or Sr compound.
By hydrolyzing the sintered body that is formed by adding and mixing a rare earth compound to the powder and then firing it, core particles with a rounded shape and an optimum particle size can be obtained with a high yield because the rare earth element suppresses abnormal grain growth. It was designed so that

[Embodiments of the invention] The present invention will be explained below based on examples.

ZnO powder contains BaC0B, At(NOa)s,
Cog (J4 and further La2O3 Raku powder, B breath is 0.08 at% and At is 20% as additive elements, respectively.
Atomic ppm, Co 2 at%, La 0.1 at%
After mixing thoroughly and press molding, it was fired at 1150'IC 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, FIG. 2 shows the particle size distribution of core particles produced by the conventional method, that is, without adding La2O3 to ZnO powder.

As can be seen from the comparison between Figures 1 and 2, in the conventional method without the addition of La2O3, the core particles are distributed over a wide range, and there is a mountain with a steep peak in the particle size range of 25 to 53 μm. 10 due to abnormal grain growth.
While there are many core particles with a particle size exceeding 0 μm, according to the present invention, by adding La2O3, the core particles with a particle size of 25 to 53 μm are distributed in a narrow range with a prominent peak, and abnormalities are observed. No 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, AZI C
o is also added, but these are used to speed up 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. is essentially unrelated.

Table 1 shows the yield of core particles of 6 to 53 μm in particle size and their shape among the core particles obtained using the above method, and shows the shapes of core particles with SrCO3 added instead of BaCO3 and for comparison. Nuclear particles produced by conventional methods are also listed. In the case of adding La20B, abnormal grain growth is suppressed and most of them have a circular shape as shown in Table 1, whereas in the case of the conventional method, abnormal grain growth occurs and many of them have an elliptical shape.
The yields for ~53 μm particle sizes are significantly different.

Furthermore, the effect of La is exactly the same when SrCO3 is added instead of BaCO3, and although it is not shown in Table 1, adding BaCO5 and SrCO3 at the same time gives the same results as when each is added individually. can get.

Table 2 shows the results when using various rare earth oxides different from La10g at the same addition amount in the same format as the g1 table.
ia is the case when 5rCOB is added.

As shown in Table 2, the effects of the present invention can be observed even when various rare earth elements other than Lm are added.

Next, Table 3 uses La20m and the amount added is 0.01~
The results of the nuclear particles obtained by changing the concentration to 2.0 atom% are shown in the same format as Rattan Tables 1 and 2, but Table 3 shows the results for the separation of the core particles by hydrolysis. I added the time.

MLs 17 to *21 in Table 3 are those with BaCOs added, but these include Nal in Table 1 according to the amount of La added, and similarly Na22 to Na26 are 5r.
In the case of CO1 addition, the amount shown as 1 m2 in Table 1 is reprinted here.

From Table 3, if the amount of La added is 0.05 atomic % or more, the yield of core particles with an appropriate particle size is high and the shape becomes circular, but if the amount of La added exceeds 1 atomic %, It can be seen that the separation time of the core particles during hydrolysis increases rapidly, which makes it impractical.

Therefore, the most preferable range of addition amount of La is 0.05
~1.0 at%. This value did not change even when oxides of various rare earth elements other than La were used as shown in Table 2.

In this example, the case where the rare earth element is added as an oxide has been described, but it goes without saying that other compounds such as carbonate compounds may be used as long as they become oxides during firing.

C Effects of the Invention] Conventionally, the most commonly used method for producing core particles when manufacturing ZnO varistors for low-voltage circuits is to hydrolyze a sintered body of ZnO powder to which a Bm compound or Sr compound is added. , in the present invention, as explained in the embodiment, Z
By adding rare earth elements in addition to Eia and 8r to the nO powder, core particles with roundness and a more uniform particle size can be obtained because the rare earth elements play a role in suppressing rapid grain growth. and optimum particle size box s25~5
Since the particle size distribution has a steep peak at 3 μm, the yield of core particles is significantly improved. As a result, variations in the characteristic values of varistors obtained by adding these core particles can be reduced.

[Brief explanation of drawings]

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 (Br) compound. A method for producing ZnO single crystal particles as a raw material for a ZnO varistor, the method comprising: adding a compound of a rare earth element to the ZnO powder. 2) In the method according to claim 1, the rare earth element in the rare earth element compound is 0.05 to 1.0 at%.
A method for producing ZnO single crystal particles, characterized in that ZnO is added.