JPH043645B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a voltage nonlinear resistor containing zinc oxide as a main component and used in lightning arresters and surge absorbers.

Conventionally, when producing a zinc oxide voltage nonlinear resistor, the main component, zinc oxide, and additives such as bismuth oxide and cobalt oxide are generally used as oxide powder raw materials. These various powders are mixed by adding water and using a ball mill or other means, and after drying, a suitable binder is added to form granules, which are then press-molded and fired at a temperature of around 1200°C to obtain a sintered body. After forming a polished electrode on this, an element is obtained. Among these processes, in the granulation process to form granules, in the case of mass production, it is possible to make dry granules by using a drying device such as a tornado reactor or spray dryer to make slurry with a binder such as PVA (polyvinyl alcohol) added. Well done.

It is well known that the various electrical characteristics of voltage nonlinear resistors vary greatly depending on the manufacturing method and manufacturing parameters. For example, in the case of the mixing process, there are nearly 10 types of trace additives other than the main ingredients, and the density of each powder is quite different (zinc oxide density: 5.6 g/cm ³ , bismuth oxide density 8.9 g/cm ³ , silicon oxide density: Due to reasons such as the density of 2.2 g/cm ³ etc., it was quite difficult to achieve sufficiently uniform mixing. Poor mixing deteriorates the uniformity of the ceramic itself, which in turn appears as voltage nonlinearity, failure in life, and failure in various durability tests.

Although there is little mention of ensuring uniformity of mixing,
For example, a method has been proposed in which an aqueous additive ion solution is added to zinc oxide powder and mixed, as described in JP-A-48-97090. This mixture is well-mixed in a microscopic sense, and the resulting sintered body has a well-uniformed particle size, fewer defects, and excellent current-voltage characteristics compared to those using ordinary powder as a starting material. ing. Although the method of mixing these salts gives the device excellent properties, it has been found that there are problems in the drying method of the slurry in the actual manufacturing process.

I will say a few words about this. FIG. 1 shows a microstructure model of a voltage nonlinear resistor whose main component is zinc oxide. In the figure, the fine structure of the resistor includes particles 1 mainly composed of zinc oxide, a boundary layer 2 mainly composed of bismuth oxide, and particles 3 of spinel or the like produced when antimony oxide is added. The source of voltage nonlinearity is thought to be related to electrical barriers existing between zinc oxide particles or in a boundary layer between zinc oxide and bismuth oxide. Depending on the magnitude of the voltage applied to the barrier, it can be divided into (1) an ohmic region, (2) a shot region, (3) a tunnel region, and (4) a region where the resistance of zinc oxide particles controls the current-voltage characteristics. In the case of a lightning arrester, the leakage current region roughly corresponds to region (2), and the current region (10KA-20KA) defined as the so-called limited voltage corresponds to regions 3 and 4, although it depends on the energization conditions. That is, it is desirable that an element with excellent protection characteristics has good current stability (life span) in the region 2, and that the limiting voltage related to the region 3 or 4 is as small as possible.

Based on the examination of the microstructure and reaction mechanism, we have found that bismuth oxide (or compounds that become bismuth oxide at high temperatures, such as bismuth nitrate)
becomes a liquid phase during firing, promotes crystal growth of zinc oxide particles, dissolves ions such as chromium, and exists so as to cover the zinc oxide particles. Ions such as cobalt, nickel, manganese, chromium, and aluminum are dissolved in zinc oxide particles, but some of them form spinel particles together with antimony oxide (or substances that decompose at high temperatures to become antimony oxide, such as antimony tartrate). From this, it can be seen that the components dissolved in zinc oxide particles are mainly related to the large current region (3 and 4 above), do not easily react with zinc oxide, such as bismuth oxide, and are mainly present between particles. Low current region (1 and 2 above)
related to. The spinel particles of the second phase are considered to be involved in the distribution of various ions to the zinc oxide portion and the bismuth oxide portion.

Now, when preparing aqueous solutions of various salts, nitrates such as nickel, cobalt, manganese, and chromium are easily soluble in water, and antimony tartrate is also easily soluble. On the other hand, bismuth nitrate is insoluble in water and produces a white precipitate in the hydroxide state, but by adding concentrated nitric acid to lower the pH, the white precipitate disappears and dissolves uniformly. Bismuth ions exist stably in sufficiently concentrated nitric acid, but when this solution is reacted with a large amount of zinc oxide powder, nitric acid quickly reacts with zinc oxide according to the following formula, so Z _o O + 2HNO ₃ →Z _o (NO ₃ ) ₂ +H ₂ O The pH of the solution increases and bismuth ions immediately form a white precipitate. In other words, an aqueous solution of bismuth nitrate containing nitric acid appears to be a uniform aqueous solution, but when this aqueous solution is mixed with zinc oxide powder, it immediately precipitates and forms a solid phase, so it is difficult to mix with other added ionic components. They cannot be handled in the same way, and problems arise with uniform dispersibility.

In particular, when these slurries are allowed to stand and dry, the aqueous solution becomes evaporated to dryness except for bismuth ions, but bismuth is dried from the solid phase as a white precipitate, and this density is extremely high. It has become extremely difficult to obtain a highly uniform dry powder for bismuth, and it has been found that this deteriorates the current-voltage characteristics on the low current side and has an adverse effect on the life characteristics.

Although the use of bismuth nitrate has been mainly discussed here, the same is true when bismuth chloride is used in an alcoholic solution because it easily undergoes hydrolysis and forms a solid phase if a large amount of water is present.

In any case, it is difficult to mix bismuth ions with zinc oxide in a solution state, so even if this white precipitate occurs, it is necessary to dry the bismuth ion in the best possible mixed state. It has been found that by doing this, since the dispersibility of other added ions is originally good, the overall uniformity is improved and the current-voltage characteristics on the low current side are also greatly improved.

This invention was made based on the above knowledge, and in the production of a voltage nonlinear resistor that also contains zinc oxide as a main component, zinc oxide powder, an aqueous metal salt solution of an additive, and a part of the additive are used. The method of manufacturing a voltage nonlinear resistor includes the steps of homogenizing a mixed slurry containing a reaction precipitate by stirring or agitation, and immediately drying the mixed slurry into a fine solid to obtain the resistor raw material powder.

As a method of drying in a better mixed state, several methods using a spray dryer, a fluidized bed heater, a freeze dryer, etc. are preferably used, but in any case, a method of rapid drying is desirable. Here, a case where a spray dryer is used and a case where a freeze-drying method is used will be explained based on Examples.

Example 1 A case where a laboratory type spray dryer is used will be explained.

A solution is prepared by dissolving 3.8 g of nickel nitrate, 7.5 g of cobalt nitrate, 5.2 g of chromium nitrate, and 1.85 g of manganese nitrate in 100 ml of water, and this is mixed with 100 g of zinc oxide powder to form a slurry. The obtained slurry is mixed for about 30 minutes using a stirrer or the like. Next, 13 g of tartaric acid and 3.8 g of antimony oxide are dissolved in 100 g of water and added to the slurry, and stirring is continued in the same manner.

8.8 g of bismuth nitrate is dissolved in a mixture of 10 ml of water and 10 ml of concentrated nitric acid, and this solution is slowly added to the stirring slurry containing zinc oxide, etc. Approximately 5
After stirring for a minute, the slurry is sent from the liquid transfer pump of the spray dryer to begin drying. In the present invention, as a matter of course, stirring of the slurry is continued during drying. FIG. 2 shows a schematic diagram of the two-fluid nozzle type commercially available spray dryer used. The spray dryer in the figure includes, for example, a slurry sample 4, a slurry transport pump 5, a nozzle 6, an air heater 7, a drying chamber 8, an exhaust fan 9, and a cyclone 1.
0, compressor 11, product containers 12 and 1
Including 3rd prize. The operating conditions in this experiment were: hot air inlet temperature 150℃, chamber exhaust air temperature 60℃, dry air amount 0.5
m ² /min, nozzle discharge pressure 3 Kg/cm ² , and liquid feed rate 10 ml/min.

The yield of the resistor raw material powder obtained was approximately 70%. This powder is granulated using normal ceramic methods.
After molding, it was fired. An aluminum electrode was formed by thermal spraying and the current-voltage characteristics were measured.

Sample A is the case where an element was created using spray-dried powder, and sample B is a comparative example where the element was created starting from powder that was dried in an oven or the like after leaving the slurry still without spray drying. sample, also
A case where all oxide powders other than zinc oxide were used is shown as sample C. Note that the manufacturing steps other than dry powder preparation are all the same for samples A, B, and C.

The shrinkage percentage of the element obtained by firing at 120°C for 4 hours is shown in the table below.

Table: Differences in element shrinkage rates depending on manufacturing method Element A Element B Element C 23.5(%) 22(%) 18(%) Obviously, samples A and B shrunk more than sample C, and this was due to the addition of various salts in aqueous solutions. It can be seen that the reactivity of various reactions during the ceramic production process inside the device has improved.

FIG. 3 shows the measurement results of the current-voltage characteristics of the three types of samples mentioned above. It can be seen that sample A exhibits the best characteristics in the entire current range, sample B has poorer characteristics in the low current region, and sample C has poorer characteristics in both the large current and small current regions.

Note that the fluidized bed dryer is similar to the spray dryer, and the current and voltage characteristics of the resulting devices were also similar.

Example 2 A case will be described in which a method already known as a freeze-drying method is used. For example, Am.Ceram.Soc.
This is the method described in Vol. 51, p. 159 of Bull. (1972). A conceptual diagram of this method is shown in FIG.

In the dryer in the figure, for example, a cryogen 14 using dry ice, acetone, etc., a hexane solution 15,
It includes a stirring device 16 such as a stirrer, a mixed slurry 17 containing all the additives such as various salts, zinc oxide, and bismuth compounds described in Example 1. Nitrogen gas is sent from the nozzle 18, and the slurry in the form of a fine mist is put into a hexane solution and instantly frozen. The frozen slurry will accumulate at the bottom of the hexane solution, so collect it on a mesh and quickly dry it under reduced pressure. The degree of vacuum is approximately 10 ^-1 to ^{10 -2} mmgH.

When we examined the current-voltage characteristics of a device made from the thus dried resistor raw material powder using normal ceramic methods as sample D, we confirmed that it had almost the same characteristics as sample A shown in Figure 3. It was done.

As mentioned above, improving the homogeneity of ceramics,
In addition, in order to improve current-voltage characteristics, a slurry consisting of various ion aqueous solutions, bismuth white precipitate, and zinc oxide is homogenized by stirring without being allowed to stand still, finely dried and solidified, and this is used as a raw material to generate voltage. By manufacturing non-linear resistors, improvements in their properties have been realized.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing the fine structure of a voltage nonlinear resistor, Fig. 2 is a schematic diagram of a spray dryer used in the present invention, and Fig. 3 is a current-voltage diagram of devices manufactured by the method of the present invention and the conventional method. Figure 4, which shows the characteristics, is a schematic diagram of the freeze-drying method used in the present invention. In the figure, 1...particles whose main component is zinc oxide, 2...
Boundary layer containing bismuth oxide as a main component, 3... Spinel particles, 4... Sample slurry, 5... Slipper transport pump, 6
... nozzle, 7 ... air heater, 8 ... drying chamber,
9... Ventilator, 10... Cyclone, 11... Compressor, 12 and 13... Product storage container, 14... Dry ice-acetone (cold agent), 15... Hexane solution, 16... Stirring device, 17... Mixed slurry, 18...
Nitrogen gas, 19...nozzle, 20...insulating material.

Claims (1)

[Claims] 1. In the production of a voltage non-linear resistor containing zinc oxide as a main component, a mixed slurry containing zinc oxide powder, an aqueous metal salt solution of an additive, and a reaction precipitate of a part of the additive is stirred or A method for manufacturing a voltage nonlinear resistor, comprising the steps of homogenizing by agitation and immediately drying the mixed slurry into a fine solid to obtain the resistor raw material powder. 2. The method for manufacturing a voltage nonlinear resistor according to claim 1, wherein drying is performed using evaporation in a spray dryer or a fluidized bed heater. 3. The method for manufacturing a voltage nonlinear resistor according to claim 1, wherein drying is performed using sublimation in a freeze dryer.