Analysis of exothermal process of large sized nanocrystalline cores during mass annealing

In mass annealing process of nanocrystalline cores, it is more difficult to control temperature for large sized cores than the small ones owing to the large amount of heat given out during crystallization transition period. This paper aims at seeking the cause for property inhomogeneity of mass annealed Fe73.5Cu1Mo3Si13.5B9 and Fe73.5Cu1Nb3Si13.5B9 nanocrystalline cores of large sizes. perimental results shown in the paper proves that the divergent properties of large sized nanocrystalline cores upon annealing are the results of uncertainty of the onset temperature and the amount of heat generated in exothermal process arising from the variation of the ribbon casting conditions. The effects of heat transfer conditions (such as the diameter of the tube furnace, furnace heating speed, core volume, surface area/volume ratio, ambient temperature as well as the type of protective gases) on the temperature rise of the cores were also investigated. This paper provides a basis for making necessary improvement on annealing process so as to raise the rate of qualified cores with excellent properties in mass production.