The effects of substrate materials and powder type on the properties of plasma sprayed ferrite

Abstract

The effect of the method of powder production and substrate materials on the physical and magnetic properties of plasma sprayed nickel ferrite films is discussed, including the production of films by spraying an oxalate powder and a solution of nitrates in methanol. Films deposited onto substrates having a larger thermal expansion coefficient than ferrite were found to exhibit better magnetic properties, including squarer hysteresis loops. An optimum choice is proposed for good surface finish, porosity and magnetic properties, consisting of spraying fully fired ferrite powder, of particle size 10 to 30 Μm, onto fosterite substrates. The magnetic properties were then comparable with bulk nickel ferrite. If surface finish is not a prime consideration satisfactory results could be obtained from the oxalate powder and the method could be used to produce non-stochiometric ferrites, possibly with improved properties. Films sprayed with the nitrate solution exhibited very favourable grain structures after annealing and it is probable that the method could be useful once problems associated with atomizing the liquid are overcome. Attempts to spray mixed oxides led to films with extreme roughness, due, in part, to a tendency of the powder to agglomerate. The films, as sprayed, were strongly magnetic demonstrating that solid state reaction had occurred, probably on the surface of the hot substrate.