JPH0360163B2 - - 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 manufacturing 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, slurry with a binder such as PVA (polyvinyl alcohol) is added to a tornado reactor,
Dry granules are often produced using drying equipment such as spray dryers. 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 ,
Due to reasons such as silicon oxide density: 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 and fewer defects, as well as current-voltage characteristics and life characteristics, compared to those using ordinary powder as a starting material. It's also excellent. 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 lightning arresters, although it depends on the energizing conditions, the leakage current region roughly corresponds to (2), and is the current region defined as the so-called limited voltage (10KA−
20KA) corresponds to (3) and (4). That is, an element with excellent protection characteristics has good current stability (life) in the region (2), and it is desirable that the limiting voltage related to (3) or (4) be as small as possible. From the examination of the microstructure and reaction mechanism so far, 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 also form spinel particles with antimony oxide (or those that decompose at high temperatures to form antimony oxide, such as antimony tartrate). . From this, it can be seen that the components dissolved in zinc oxide particles are mainly in the high current region (the above-mentioned
Regarding (3) and (4)), those that do not easily react with zinc oxide and exist between particles, such as bismuth oxide, are mainly in the low current region ((1) and
(2)). 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 form of hydroxide; however, 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, if these slurries are allowed to stand and dry, the aqueous solution will evaporate to dryness except for the bismuth ions, but
Bismuth is dried from the solid phase as a white precipitate, and its extremely high density makes it extremely difficult to obtain a highly uniform dry powder for bismuth, which affects the current-voltage characteristics on the low current side. It was found that it worsened the condition and had a negative effect on the life characteristics. Although the use of bismuth nitrate has been mainly discussed here, the same is true when bismuth hydrochloride is used in an alcoholic solution because it easily undergoes hydrolysis and forms the same 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. As a result of various studies in consideration of mass productivity, and with the condition that drying could be performed with each element mixed as uniformly as possible, it was found that a method using a spray dryer was preferable. Furthermore, spherical granules can be obtained by adding a water-soluble binder as in the conventional method, and the granules are dried at a relatively high temperature. It was found that there are also advantages such as no need for a thermal decomposition process. This invention has been made based on the above findings, and includes a method for manufacturing a voltage nonlinear resistor containing zinc oxide as a main component.
A mixed slurry containing an aqueous metal salt solution of an additive, a precipitate of a part of an additive, and an aqueous binder is homogenized by stirring or agitation, and the mixed slurry is continuously spray-dried to remove precipitates other than the zinc oxide powder and The present invention resides in a method for producing a voltage nonlinear low antibody in which decomposition and oxidation of various salts and formation of granules are performed simultaneously. The mixed slurry is preferably prepared by adding a bismuth ion solution containing concentrated nitric acid to a slurry containing zinc oxide, an aqueous additive salt solution, and a water-soluble binder. Further, the metal salt is a compound that decomposes during spray drying, and either nitrate or tartrate can be suitably used. The temperature of the hot air during spray drying is preferably set to a temperature necessary to decompose the metal salt and a temperature that does not decompose the water-soluble binder. The present invention will be described below with reference to prototype experimental examples. Example FIG. 2 shows a schematic diagram of the mixing and granulation process in the case of a two-fluid nozzle type spray dryer as an example. In the figure, the slurry produced in the disper mill 4 is sprayed from a nozzle 11 in the spray dryer main body 6 by a slurry transport pump 6 together with compressed air from a compressor 10 (in the case of a two-fluid nozzle). The air that has passed through the heater 7 enters the main body 6 of the spray dryer (heating tower) and instantly dries the mist coming from the nozzle 11, and the dried powder is taken out from the granule outlet 12. The mixed slurry is prepared as follows. Pour about 1.5 liters of water into the disper mill and add 3% PVA.
Solution 2, surfactant 2% aqueous solution 1 and antifoaming agent
Add 20 c.c. and mix in advance. Next, predetermined amounts of nickel nitrate, cobalt nitrate, chromium nitrate, and aluminum nitrate are weighed out and dissolved in 1.0% water (first solution). Next, dissolve a predetermined amount of antimony oxide in an aqueous solution of tartaric acid to create a second liquid of 0.3 cm, and dissolve bismuth nitrate in a mixed solution of 200 c.c. of water and 100 c.c. of concentrated nitric acid to create a third liquid. Be prepared. Pour all of the first and second liquids into the disper mill and mix for about 15 minutes, then slowly mix in 10 kg of zinc oxide powder (over about 10 minutes) and mix for about 1 hour. . Finally, slowly mix in the third liquid over about 5 minutes, then mix in the third liquid for about 15 minutes.
Mix for ~1.5 hours. This mixed slurry is continuously fed to the spray dryer by a pump. The operating conditions of the spray dryer are a hot air temperature of about 250°C, an exhaust air temperature of 120°C, a spray pressure of 1.0 Kg/cm ² , and a discharge pressure of 0.8 Kg/cm ² . With this amount of slurry, drying is completed in about 15 minutes, and the powder is taken out from the bottom of the drying tower. The particle size distribution of the obtained powder is 150 μ or more: 40%, 100-150 μ: 40
%, 100-50μ: About 20%, almost no difference from powder mixture products. The flowability was almost the same as that of the powder mixture, and the bulk density was slightly smaller at 1.2 g/cm ³ .
When observed under a microscope, the shape was almost spherical, with some chain balls and other irregular shapes. When a part of this granulated powder is examined using a thermobalance, a weight loss is observed up to around 600°C, which is thought to be due to the decomposition of water, PVA, and surfactant, but a special No significant reaction was observed, and it is thought that the decomposition was almost complete at the time of spray drying. It was therefore found that no special pyrolysis step was required. Using the obtained powder, mold it in a regular uniaxial molder.
After pressing with a pressure of 300 kg/cm ² and baking at 1200° C. for 4 hours, a polishing electrode was attached and the current-voltage characteristics were examined (this is referred to as sample A). Various salt solutions,
Sample B is a case in which bismuth precipitate and zinc oxide powder slurry are left to dry in batches, and then an element is made in exactly the same process as sample B, and sample C is a powder mixture. When we investigated the shrinkage rates of these A, B, and C samples, we obtained the results as shown in the table, and it is clear that the A sample and B sample shrunk better than the powder mixture C sample.
It can be seen that uniform reaction sintering is occurring. Note that the zinc oxide particle diameter was constant at approximately 15 μm for samples A, B, and C.

[Table] Next, the results of measuring the current/voltage characteristics of each sample are shown in Figure 2. As is clear from the figure, sample A exhibits excellent current/voltage characteristics over the entire current range, sample B particularly in the low current range, and sample C inferior in both. In sample B, non-uniform distribution of bismuth ions during static drying led to deterioration of characteristics in the low current region, and sample C had the worst results with powder mixing. The spray drying method has already been generally confirmed to be suitable for mass production in the drying process, and the prototype shown here is also extremely easy to scale up. Furthermore, although a two-fluid type spray dryer has been described here, the same is essentially true for disk-type and other types of spray dryers. As described above, by using aqueous solutions of various added ions and examining the drying method of slurry, we have provided a manufacturing method with excellent mass productivity. Improvements in properties were achieved.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing the microstructure of a voltage nonlinear resistor, Fig. 2 is a schematic diagram illustrating the mixing granulation process based on this invention, and Fig. 3 is an element manufactured based on this invention and a conventional method. FIG. 3 is a diagram showing current-voltage characteristics of an element manufactured by the method. In the figure, 1...Particles whose main component is zinc oxide, 2
...Boundary layer mainly composed of bismuth oxide, 3...
Spinel particles, 4...Dispersil, 5...Sludge transport pump, 6...Spray dryer main body, 7...
Air heater, 8...cyclone, 9...exhaust fan,
10... Compressor, 11... Nozzle, 12...
...Granule outlet.

Claims (1)

[Claims] 1. A method for manufacturing a voltage nonlinear resistor containing zinc oxide as a main component, comprising zinc oxide powder, an aqueous metal salt solution of an additive, a precipitate of a part of the additive, and a water-soluble binder. The mixed slurry is homogenized by stirring or agitation, and the mixed slurry is continuously spray-dried to simultaneously decompose and oxidize precipitates and various salts other than the zinc oxide powder, and form granules. , a method for manufacturing a voltage nonlinear resistor. 2. The voltage nonlinear resistor according to claim 1, wherein the mixed slurry is produced by finally adding a bismuth ion solution containing concentrated nitric acid to the slurry containing zinc oxide, an additive salt aqueous solution, and a water-soluble binder. Production method. 3. The method for manufacturing a voltage nonlinear resistor according to claim 1, wherein the metal salt is a compound that decomposes during spray drying and is either a nitrate or a tartrate. 4. The method of manufacturing a voltage nonlinear resistor according to claim 1, wherein the hot air temperature during spray drying is set to a temperature necessary for decomposing the metal salt and a temperature that does not decompose the water-soluble binder.