Causes and preventive measures for common defects in zinc oxide varistor ceramic resistors

1 Introduction

It is well known that zinc oxide varistor ceramic resistors are scrapped due to defects caused by various processes such as process and environment, such as pores, air interlayers, spallation and damage. In particular, the observable pores have a great influence on the ability of the resistor to withstand the energy of the current surge; the outstanding performance is that when the square wave current is impacted, the breakdown will be caused from the visible pores, thereby causing the resistor to be scrapped. Therefore, it is highly valued by the manufacturers of resistors.

Based on more than 20 years of practical experience, the author discusses the causes and preventive measures of common defects in zinc oxide varistor ceramic resistors. I hope to discuss this with the industry.

2 Reasons for the generation of air holes in the resistor sheet and preventive measures

2.1 Causes of accidental large-scale atmospheric holes

The pores in the resistor sheet can be divided into two cases: eye visible (>100 μm) and invisible (50 μm or less). The former is simply referred to as atmospheric pores, and the latter is referred to as small pores. Atmospheric pores have the greatest influence on current surge, which is one of the main causes of breakdown of resistors.

Many examples have shown that the main cause of atmospheric pores is caused by foreign organic impurities. These impurities mainly occur in the following cases:

(1) Zinc oxide is mixed with coal powder during its production.

Because most of the zinc oxide raw material manufacturers have used coal as fuel, and zinc oxide is melted and sublimated by the refined zinc ingot in the coal heating furnace. At the same time, it is oxidized by oxygen in the air and then sucked into the zinc oxide collecting bag; The heating part of the furnace body did not take environmental protection measures isolated from the zinc oxide furnace, causing a large amount of coal powder to be diffused in the surrounding air environment, so the coal powder was sucked into the zinc oxide.

In the case of using zinc oxide containing pulverized coal, the pulverized coal cannot be completely screened out even if the sieving measures are prepared by preparing the mixed slurry of zinc oxide and the additive, and the fine pulverized coal will be present in the zinc oxide powder. Therefore, when the resistor piece is fired at a high temperature, the pores remain because the pulverized coal is burned out, and sometimes there is a bubble on the surface of the porcelain after sintering, and some of the pores are seen when the sheet is polished. The above situation has appeared in several manufacturers due to the batch scrapping of the stomata of the resistor.

Based on these lessons, in order to prevent similar problems from occurring, the most effective measures are: for zinc oxide suppliers to inspect, to examine the production environment, analytical methods and other quality assurance system certification, and to sign supply technical conditions with suppliers. Agreement with the Quality Assurance Agreement.

The production process should be observed for the residue after sieving of each batch of zinc oxide mixed slurry. If there is residual pulverized coal, do not use it again, take the return and investigate the supplier's liability. In this case, there have been precedents for the loss of a large number of resistors due to the presence of pulverized coal in zinc oxide, returning and compensating for losses.

(2) Because the insulating glass fiber of the furnace body is mixed with the powder

When a company fails due to a tunnel furnace (the dome is in the high temperature zone of the furnace), it has to take measures to remove the insulating glass fiber from the top of the furnace. Because of the hot air, these fibers diffuse into the powder placed in the spray drying room. The material container is mixed into the powder when it is contained in water, and then enters the body. These fibers leave voids or bubbles after vaporization at the time of firing of the resistor. In this case, after a company handled the furnace accident for 10 days, a large number of stomatal defects were lost due to the resistance piece. Later, it was found that the ceramic fiber fell into the surface of the bag containing the powder, and the water was not removed into the powder. of.

(3) Foreign dust falls into the slurry

The zinc oxide resistor production workshop of a factory is located directly below the chimney of the furnace gas and boiler workshop. Due to the window opening of the resistance sheet material factory in summer, the dust of the chimney falls into every corner of the powder preparation room, especially into the pulp. In the surface of the mixing tank and various tools and in the container, and then into the slurry or powder; resulting in the occurrence of pinhole-shaped pores on the end face of the resistor sheet after the grinding, the square wave test will cause the pinhole Wear, a large number of resistors are scrapped every year; and after changing to natural gas in a few years, there has never been any such situation.

More typical is the phenomenon of a large amount of foaming after the resistance sheet of the plant was fired in 1995. In order to verify the reason for the simulation test, 20 g of dust was mixed into 500 g of powder, mixed and pressed into a D3-type resistor sheet, and the same phenomenon was found after firing. This fully proves that the dust is the cause of the foaming of the resistor sheet, which was probably caused by artificially dusting the powder into the powder.

In addition, due to the long-distance transportation and loading and unloading process of zinc oxide, a large amount of dust is deposited in the surface of the woven bag and the woven bag, and if the zinc oxide is added to the preparation slurry, the woven bag is not removed and the plastic bag is wiped off. The dust on the surface will fall into the slurry, that is, into the powder, which will also cause similar pore defects of the above similar size.

In view of the above situation, in the preparation of the mixed slurry plus zinc oxide must first smash the woven bag, while wiping the dust on the plastic bag with a wet rag, and then pouring the zinc oxide measures can be prevented.

(4) Emery enters the slurry or powder

During the simultaneous production of silicon carbide and zinc oxide surge arresters in a factory, due to the narrowness of the entire plant, SiC particles with flash crystals are scattered everywhere in the room where the zinc oxide resistors are manufactured and in the factory; the resistors produced in this environment exist for a long time. Foaming and porosity defects. Later, it was found that SiC powder is the main cause of pores, which is undoubtedly the result of decomposition of these SiC particles.

The above practical examples fully demonstrate that the impact of environmental sanitation on the manufacturing yield of zinc oxide resistors is very important. The lessons learned from these practical examples are worth learning, indicating that various external organic and inorganic impurities must be prevented by strict management control measures.

2.2 There are often a few reasons for small and small pore defects

These small porosity defects are due to the poor industrial hygiene of the process and the strict control of process operation, mainly in the following cases:

(1) Production of resistance sheet workshops, especially in the environmental hygiene of the process from the production of powder to the molding process

Most of this happens in factories with more wind and sand in the north. Because the ground can not be kept clean and the various containers used fall into the dust, it will also pollute the slurry or zinc oxide granulated powder. Even in manufacturers with less sand, if you enter the workshop without changing work clothes and shoes, the dust will be brought into the workshop, especially in the slurry or powder of the previous process. It is not difficult to understand from the above reasons that the stomata of the resistor sheet is also caused.

(2) The polyvinyl alcohol (PVA) solution used did not reach sufficient solubility and was not fully sieved.

Since the full dissolution of PVA must be in the range of 90 ° C - 100 ° C, and it needs to be kept for a certain period of time to fully dissolve; if it is not fully dissolved and not fully sieved (200 ~ 250 mesh sieve) to remove the undissolved agglomerates, then These agglomerates are difficult to disperse and disperse in a low-temperature mixed slurry process; this will undoubtedly enter the spray granulated powder, and thus will cause relatively large pores.

(3) The mixed slurry is not fully passed through the 200 mesh screen when it is injected into the storage tank.

In the various processes from the fine grinding of the additive to the process of preparing the slurry, foreign impurities may enter the slurry (such as plastic, paper dust, fiber strands of the packaged zinc oxide bag, etc.), especially in the presence of insufficient oxidation in the zinc oxide. Large particle powder, usually this zinc oxide particle has the same shape and hardness as sand. These will cause pores in the resistor sheet, the reason for which need not be described.

(4) The powder particles obtained by spray drying are too thick and too hard

It has been proved that the average particle size of the powder obtained by spray drying is preferably 80-100 μm. Tests have shown that if the maximum particle size is above 130 μm, and since the solid concentration of the slurry is less than 67%, the voids forming the hollow spherical particles are relatively large.

Furthermore, according to the author's observation of the end faces and cross-sections of the resistor blanks pressed by several manufacturers, it is found that even in the case where the density of the green body reaches 3.20 g/cm3, there are many un-crushed particles, as shown in the figure. 1 Obviously visible particle state.

As can be seen from Figure 1, all of the particles were not crushed and remained intact in shape, only in a state of being crushed and deformed, and voids of varying sizes were retained at the grain boundaries. Figure 2 shows the cross section of the blank similar to the unbroken particle state of Figure 1.

In order to ascertain whether the state of the particles shown in Fig. 2 is still present after firing, uncompressed particles and pores are still observed after the sample is fired, but its size is reduced because of its volume shrinkage. As shown in Figure 3.

The reason why these particles are not crushed is mainly because no lubricant is added to the slurry, and the amount of PVA added is relatively large, and the hardness of the particle shell is relatively large, so even in the case where the density reaches 3.20 g/cm3 or more, It is also difficult to break. These voids and voids are relatively large pores when the resistive sheet is sintered because it cannot be filled with a limited liquid phase.

In view of the current common problem, the proposal of using both dispersible and lubricious dispersing agents has been proposed [1], that is, using a tetradecyl acetate (cationic) dispersant, or adding, for example, glycerol (glycerol) Lubricants are used to reduce the hardness of the powder particles; at the same time, the moisture content of the powder is appropriately increased to reduce the molding pressure of the green body. Before adding the above lubricious organic matter, appropriate reduction of the amount of PVA should be taken. For example, it is sufficient to add 700 grams per 100 kg of zinc oxide mixed slurry in the clinker process; and 600 g of raw material is sufficient; The moisture content of the material also has a certain good effect.

In addition, the inlet temperature of the heat source of the drying tower is reduced as much as possible to below 320 ° C, and the negative pressure in the column is reduced to reduce the outlet temperature to about 100 ° C to extend the drying time of the powder in the tower as much as possible. The reason is that the droplets of the slurry formed by the spray will quickly cure and close the surface if it is exposed to high temperature, but when the temperature is further increased, the moisture in the closed liquid will inevitably break through the weak part; Large voids; otherwise the remaining voids are reduced. In the case that the particles are not crushed during molding, it will undoubtedly cause an increase in the atmospheric pores in the sintered porcelain body, thus affecting the square wave flow capacity.

Furthermore, the solids concentration of the slurry must be strictly controlled to be as high as possible to reduce the voids in the particles. It is necessary to control the particle size range of the granulation material, and the maximum average particle diameter should not exceed 100 μm to reduce the voids of the hollow particles. The larger the particle size, the lower the square wave capacity. It has been tested to separate the blanks with different thicknesses, that is, the resistors formed by the 60 mesh sieve powder, the square wave flow capacity is much lower than the flow capacity of the powder under the 60 mesh sieve.

(5) The moisture content of the shaped green body powder is seriously uneven

The water content of the shaped green body powder is seriously uneven, mainly refers to: 1 The water-containing process is not well atomized or heavily dripped, forming agglomerates or agglomerates with high local moisture content, but not dispersed during the mixing process; 2 The mixing time is not enough, resulting in uneven moisture content of the powder. In the above two cases, the unevenness of the moisture of the powder is caused, and cracks or pores may be caused by the inconsistency in the shrinkage in the compact.

(6) The shape of the spray-dried powder is not in the shape of an apple (see Figure 4), especially the leakage of the pusher plate and the powder of the blank, which may affect the uniformity of the powder-filled mold due to its poor fluidity. The pores will also be generated due to uneven density. The breakdown rate of the resistors formed by this powder is significantly higher than that of the normal materials.

Fig. 5 shows that the pore state of the end face of the green body is pressed by the irregular powder of the powder particles of Fig. 4. Some blanks also exhibit uneven edges and different colors, as shown in Figure 6. The unevenness of the end face may be related to the unevenness of the die; the color inconsistency is related to the coarseness of the additive and the uneven mixing of the zinc oxide. These are all factors that affect the porosity and final performance of the resistor.

(7) When the clinker process uses a refractory 匣钵 calcination additive, the slag falls into the additive.

Since some of the crucibles are not sufficiently sintered, the crucibles are liable to fall into the additive due to breakage or damage of the crucible during handling of the additive. The slag is a refractory material. It reacts with the components of the resistor during the high-temperature sintering process of the resistor, which may cause voids due to its melting, which is also the cause of the pores.

In order to prevent this from happening, it is recommended to use sintered pure quartz glass crucible. Even if a trace amount of antimony slag enters the additive, it will exist in the form of silicon oxide by fine grinding, which will not have a big influence on the performance of the resistor. .

In order to prevent the unevenness of the powder moisture, it is necessary to check the nozzle for leaking water frequently; the second is to check whether the nozzle has poor atomization; the third is that the first two conditions are good and the water content is constant. Next, make sure that the mixing time is sufficient to ensure uniform moisture content at each time. Fourth, the powder containing good water must be strictly passed through a 40-60 mesh sieve to screen out the agglomerates with high water content.

Swedish Beate Balzer et al. [2] demonstrated that the fracture surface reveals the most common type of critical defects by studying the mechanical strength of zinc oxide ceramics. The larger pores in the vicinity are flat, and more or less dangerous void diameters are introduced. Between 50-150μm. Their outer dimensions correspond to the voids of the large spray granulated powder particles, and are therefore attributed to either voids or particles like "cake shape" or low particle compactness. These two defects are representative of ceramics that are press-formed with spray granules. Once formed, they are not "healed" once they are formed; in contrast, they may even expand. This fully demonstrates that the void size of the spray granulated powder particles is an important factor affecting the pore size of the resistor sheet.

3 The main cause of air interlayer caused by the blank and preventive measures

3.1 The cause of the air interlayer caused by the blank

(1) When multiple compression exhaust procedures are used, the compressed air is not discharged during the last compression. The reason is that the density of the circumference of the blank is too large due to the excessive amount of compression at the last or the previous compression, and the dense powder has blocked the passage of the air, resulting in the air being discharged during the last compression. Aggregation is caused in the middle of the body or in the middle and lower parts. If the air interlayer is severe, it may break into two halves when the blank is pushed out; when it is slightly light, it will be in the rubber discharge (or cracking during the calcination process), it will be exposed until it is very mild, even after the square wave test. come out.

(2) When the die contacts the powder after molding, the compression speed is too fast, or the compression amount is too large, which causes the periphery of the blank to be tightly closed, so that the air is not urgently eliminated. It also causes the same defect condition as the cause (1).

Controlling the amount of compression (and the increase in density) of the powder blank in the mold sleeve, or slowing down the compression speed, is an important measure to prevent the air interlayer of the blank.

(3) The blank causes the interlayer to be mostly compressed when the die contacts the powder, and the air is too late to be eliminated; or even if the compression is slow, the interlayer is caused by the unreasonable setting of multiple compressions. Therefore, a reasonable compressed exhaust procedure must be set. This requires adjustment by pressure control.

Since the amount of compression of the blank is determined by the function of the adjustment press program, it is necessary to set the number of exhausts and the exhaust delay time according to the size of the blank. If the procedure of three times of exhaust gas is taken, the reasonable pressure distribution should be the lowest of the first pressure and gradually increase after the second time; even then, since the first compression is between the powder gaps air