The Effect Grinding and Annealing Temperature on Microstructure and Magnetic Properties of The Pressed Powder Mixture of 20.0 wt% Fe and 80.0 wt% BaTiO3

The mixture of 20.0 wt% Fe and 80.0 wt% BaTiO3 powders was found to consist of a BaTiO3 matrix and composite particles comprising Fe cores and BaTiO3 shells. During the grinding of this mixture, several processes occur, such as pulverization of the particles of both powders; changes in the size, morphology and composition of aggregates, crushing of crystal grains, increase in the density of chaotically distributed dislocations; growth of internal microstrains; rise in the residual stress; decrease in the amount of crystalline and increase in the amount of amorphous phases, as well as oxidation of Fe into its oxides FeO, Fe3O4 and Fe2O3. These changes in chemical composition, morphology and microstructure substantially affect the magnetization of the pressed powder mixture. With increasing the grinding time from 0 to 90 min, the increase in magnetization is dominantly affected by crushing of Fe crystal grains. The decrease in magnetization during grinding from 90 to 120 min is caused by the decrease in the amount of metallic Fe by oxidation to FeO and increase in both the density of chaotically distributed dislocations and internal microstrains. Formation of Fe3O4 and Fe2O3 oxides has a prevailing effect on the increase in magnetization when grinding for longer than 130 min. Chemical and microstructural changes in the mixture of powders of 20.0 wt% Fe and 80.0 wt% BaTiO3 during heating affect the magnetization of cooled samples at 22°C. The powder is thermally stable up to 330°C. The magnetization of samples cooled to 22°C declines with increasing annealing temperature above 330°C. This decrease is caused by both the oxidation of Fe to FeO and formation of larger crystalline grains of Fe. The magnetization of samples heated up to 250°C remains unchanged. At higher temperatures, with increasing the temperature of the sample, its magnetization declines as a result of the transition of directed domains to a chaotic state caused by the effect of thermal energy, formation of larger crystal grains and oxidation of Fe into FeO.