The coated magnets show blistering after use, which affects the use of the magnets, which often confuses people, depending on how they are produced and how to take precautions.
The moisture content of the blank, the uniform particle size of the ball mill, the pre-sintering and sintering temperature, and the atmosphere of the sintering furnace have a greater impact on the foaming rate of the magnet. The moisture in the base of the blank does not completely evaporate, which will also cause an increase in the foaming rate of the magnet. The specific discussion is as follows:
①If the difference between the inner and outer moisture content of the blank is relatively large after the blank is ejected from the mold, the magnet will simply bubble. This is because the initial surface moisture evaporation rate of the blank is faster than the internal moisture dissipation. With the passage of time, its surface moisture content decreases, and the difference between the inner and outer moisture content of the blank increases. Assuming that there is residual closed water at the base of the blank, It will foam at high temperatures. If the difference between the inner and outer moisture content of the blank is gradually reduced, the blistering is also gradually reduced.
②In the second fine grinding, the larger the particle size, the more simple the inner and outer capillary water will be dispersed, and the smaller the foaming rate of the magnet will be.
③During sintering, there is no blistering below a certain sintering temperature. This temperature is called the blistering critical temperature. Beyond this temperature, the blistering rate will gradually advance with the sintering temperature. As the burn-in temperature advances, the bubbling critical temperature also advances.
④ Another reason for bubbling is that after the pre-fired material is crushed, ground and mixed with iron to produce Fe2O3 in the crushing technology, it will be sintered according to the reaction formula of 6Fe203 = 4Fe304 + O22Fe2O3 = 4FeO + O2. When Fe2O3 is violently reduced to FeO, if the differentiated O2 cannot be discharged, the blank will also bubble.
⑤The sintering furnace with good ventilation, its magnets are also easy to produce bubbles.
The main solutions are: extend the drying time of the blank, increase the temperature of the pre-fired material, the uniform particle size of the ball mill, and reduce the secondary sintering temperature, and so on.
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