Influence of heat treatment on dielectromagnetics loss
Tomasz Janta, Bogumił Węgliński Politechnika Wrocławska, Instytut Maszyn, Napędów i Pomiarów Elektrycznych, ul. Smoluchowskiego 19, 50-372 Wrocław
Annals 2 No. 3, 2002 pages 91-96
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abstract Dielectromagnetics find more and more attempts of applications on magnetic cores of electrical devices. Profitable for their uses enlargements is, among other things, diminution of their losses and improvement of other magnetic parameters essential for definite application. Comparison of losses of classical dielectromagnetics and of electrical sheets, at frequencies 50 Hz, is unprofitable for dielectromagnetics. Proportions these of losses change however with frequency. Losses of dielectromagnetics and of electrical sheet become even at frequencies of work of magnetic cores carrying out about 400 Hz. Research showed, that in case when epoxy resin is used as a dielectric, principle part of a dielectromagnetic losses (about 90%) are hysteresis loss, while eddy current loss are resoluteness lower. Well-founded is so endeavor to diminution, as of essential, loss from hysteresis. On magnitude of these losses, in considerable degree, have influence mechanical stresses introduced into soft magnetic particles during compacting of a magnetic core. Transferring technology of electrical sheets to dielectromagnetics one can accept, that thermal treatment, removing unprofitable results these stresses should drive to diminution of hysteresis loss. Temperature of proposed thermal treatment is connected with temperature of first recrystallization of iron and contains oneself in borders 550÷850oC. Lower temperatures of recrystallization will demand usage of longer times. Unfortunately, dielectrics used at present in dielectromagnetics, are mostly organic (epoxy, phenol resins, etc.) do not hold out temperatures required to recrystallization. Aim of presented research is to proof that recrystallization treatment drives to profitable diminution of hysteresis loss. Its application requires however use of dielectric resistant to applied temperature of recrystallization. Aluminum oxide (Al2O3) in dispersion form was used as dielectric. Dependence of hysteresis loss on temperature of thermal treatment for different dielectric content is presented in Figure 8. One can observe distinct dependence of hysteresis loss on temperatures of treatment for all examined dielectric contents. Courses of hysteresis loss of dielectromagnetics after thermal treatment have distinct minimum at temperatures about 700oC. This justifies advisability of thermal treatment application for diminution of value of dielectromagnetics losses. Effect of lowering of hysteresis loss is diminished in relation to what could be reached. This testifies lowering of resistivity executed dielectromagnetics with increase of thermal treatment temperature (Fig. 3). This is probably caused by effect of aluminum diffusion into magnetic particles of iron. Such diffusion drives to changes of structure of iron and in effect to its magnetic hardening, what is univocal with enlargement of hysteresis loss. Two basic effects overlap so on oneself during thermal treatment: diminution of hysteresis loss in consequence of removals of mechanical stress, and enlargement of this loss in consequence of mentioned diffusion leading to magnetic hardening of particles. At first influence of first effect checks the weight, because in temperatures about 700oC steps out minimum of hysteresis loss (Fig. 8). In higher temperatures second effect begins to check the weight and hysteresis loss grows. This is logical, because use of higher temperature than temperature of recrystallization should not to have influence on further magnetic improvement of iron particles, instead intensity of diffusion grows together with increase of temperature. Obtained courses of curves, shown in Figure 8 confirm this reasoning. On mentioned earlier these two effects overlaps greater magnetic charge of iron particles at greater content of dielectric. It causes increase of hysteresis loss due to higher magnetic induction in iron particles. In effect hysteresis loops have greater surface, what is univocal with greater hysteresis loss. Obtained results justify investigative works leading to elaboration of dielectric which could be able to withstand, without destruction and influences on magnetic particles, temperatures of recrystallization of used magnetic powder, for iron this is about 700oC.