Castro, Porosity and magnetic losses in soft magnetic composites Porosity and Magnetic Losses in Soft Magnetic Composites Nicolau Apoena Castro 1 and Fernando JG Landgraf 1,2 1 IPT Institute for Technological Research 05508-901 Sao Paulo, Brazil 2 Centro Universitário da FEI - S. Bernardo do Campo, Brazil Keywords : Soft Magnetic Composites; Magnetic Losses; porosity. Abstract. The paper investigates the effect of porosity on the magnetic properties of Soft Magnetic Composites produced by constant compacting conditions, using niobium powders was a non-magnetic phase. Losses increased with the total non-magnetic volume fraction. Annealing under a nitrogen- hydrogen atmosphere decreased losses (due to stress relieving) despite a permeability decrease brought by a Nb phase transformation expansion. Introduction Soft Magnetic Composites (SMC) are gaining marketshare as higher frequencies are been used in electrical motors. When comparing SMC with electrical steel sheets, SMC show lower Total Magnetic Losses at high frequency due to the iron particle insulation, which lower the eddy current loss component [1]. It should be mentioned that there are some uncertainties about the estimation of classical and excess losses [2]. At lower frequencies, the magnetic behaviour of the Soft Magnetic Composites is severely impaired by their Hysteresis Losses, as they are 5 to 10 times larger than those of non-oriented electrical steel sheets. As a consequence, the frequency break even point is still too high for many applications. Decreasing hysteresis loss is an important objective in SMC development. The factors that are usually considered to influence the hysteresis behaviour of electrical steels are grain size, inclusion content, texture and residual stresses. In the case of SMCs, the effect of plastic deformation when the parts are pressed has to be taken in consideration. Preliminary results, showing that hysteresis losses of an iron powder pressed to 900 MPa were about 400 J/m 3 (at 0.5T) whereas the same powder, polymer bonded and pressed to 200 MPa (presumably with less plastic deformation), presented 500 J/m 3 , indicated that porosity (or the non-magnetic volume fraction) is also a factor to be investigated. In powder metallurgy processing of mechanically soft materials, such as iron powders, porosity is normally controlled by the compaction pressure. This process variable is inconvenient for the proposed investigation, as changing the compaction pressure would change the amount of plastic deformation in the particles, which would also affect the hysteresis loss. To avoid such interference, and supposing that pores are just non-magnetic regions inside the material, different volume fraction of “pores” could be produced by adding non-magnetic particles to the iron powder. Fine Niobium powder was used for that purpose. No effort was made to electrically isolate the particles, at the present stage. The effect of porosity decreasing magnetic permeability is well known [3]. This effect is usually attributed to an interparticle gap effect, as a source of demagnetizing field. The magnetization curve is “sheared” by the demagnetizing field of porosity as it is by gaps in the magnetic circuit. The “gap effect” also shears the hysteresis curve but it should not change the hysteresis area, unless effects other than a demagnetizing field are at work. As the effect of plastic deformation is superimposed in the intended investigation, it was considered interesting to try to separate those effects by stress relieving. At the same time, the possible effect of heat treatments on recrystallization and grain growth should also be investigated. Experimental procedures Iron powder was mixed with different quantities of Niobium powder, as shown in Table 1, to produce samples with different “non-magnetic volume fraction”. The iron powder was classified and the fraction between sieves mesh 70 and 120 were used, so the average iron particle is 150 µm . This rather large particle size was chosen aiming at favouring larger grain size. Niobium powder with average