The influence of protective atmosphere on oxidation of sintered SmCo5 magnetic materials

Abstract

Understanding of the thermal stability of intermetallic SmCo5 powder is essential for designing the working atmosphere in all phases of the technological procedure in the production of sintered SmCo5 magnets to obtain maximal magnetic properties. The thermal stability of the SmCo5 powder with defined chemical composition and particle size was investigated in the interval from 20 to 900*C. Commercial SmCo5 powder was used in this experiment. The powder was milled in anhydrous toluene in an agate mortar to fine powder of quality used in the production of sintered magnets. All the experiments were carried out with powder of an average particle size of 7.23*m, established by SEM. The thermal stability of the SmCo5 powder in static air atmosphere was investigated by thermogravimetric analysis (TGA) using a DuPont Thermal Analyzer. Investigation of the behavior of SmCo5 powder during heating was carried out using new samples of SmCo5 powder for each of the investigated temperature cycles. It was found by TGA that up to 200*C, the oxidation of SmCo5 was negligible. X-Ray diffraction of the thermogravimetric experimental residue of the SmCo5 powder, heated at 240*C, yielded only the presence of the SmCo5 phase. By X-ray diffraction different crystal forms were identified depending on the maximal heating temperature. The following phases were identified: Sm2O3, Co, CoO, Co3O4 and SmCoO3. According to TG and X-ray results, for each of the investigated temperatures, the corresponding chemical reactions were established. The experimental data from both the thermal and X-ray investigations confirm that the phases of pressing and aligning the SmCo5 powder, in the process of producing sintered SmCo5 magnets, may be performed without a protective atmosphere.

Истражена је могућност коришћења вакуума на граници између ниског и високог (10-3-10-4 bar) као заштитне атмосфере у фазама синтеровања и термичке обраде SmCo5 магнета. Испитан је утицај заштитне атмосфере на структурне промене SmCo5 магнета за различите температурне и временске интервале синтеровања, а за константну температуру и у време термичке обраде. Присуство оксидних и других инклузија у било ком делу синтерованих SmCo5 узорака, осим у површинском слоју, није доказано. Сканирајућом електронском микроскопијом (СЕМ), коришћењем Texture Analysis System-a ( TAS´+) мерена је дебљина оскидне зоне. Утврђен је регресиони модел према коме дебљина оксидне зоне расте са квадратом температуре синтеровања, а линеарно са временом синтеровања, за дефинисану температуру термичке обраде.