Study of microstructure and its effects on the complex permeability of Ni-Zn Ferrite

Abstract

Ni'_xZn,Fe204,with x=0.2 and 0.4 were prepared by a conventional solid smle reaction technique. The samples were sintercd at various temperatures (1200"C, 1250°C. 1300"C 1350"C and 1400"C) in air for 5 hours. StructllTal and surface morphology were studied by x-ray diffraction method and optical microscopy, The magnetic propcrties of the fcrrites were charaeteri7.ed with high frequency (lkHL-13MHz) complex pemleability and temperaturc dependent permeability measurcments. The cffects of microstructure, composition and sintering temperatures on the complex permeability of i\/;-Z/1 ferrite are discussed. A possible correlation among sintering temperature, grain "izc and density is also discussed. The X-ray diffraction patterns of both compo8itions clearly indlCate their single phase and formation of spinel structure. The lattice consmnt of Ni,,~'Lno,Fe204 is found to he 8.3638 A while 8.3705 A for 11/io67.no£e,O,. The increase in lattice parameter with increasing Zn content can he explained on the basis of the ionic radii. The radius of the Zn2+ (0.82 A) is greater than that of the Ni"~ (O.78 A). The microstructural study shows that grain size increases with sintering t~mperatllrc, The dcnsity of the Nio,s2nv IFel04 samples increases as the sinlering temperatnre increases from 1200'C to 1300°C, and above 1300'C the density begins to decrease. Thf ,Via6Zn",FelO, samples show similar behaviour to that of the NiosZnvFelO, samples cxcept that the density is fonnd to be maximum at 1350.C. During the sintering proces~, the thermal energy gen~rates a force that drives the grnin bonndarie~ to grow over pores, thercby decreasing the pore volume and increasing the density of the materia!. At higher sintering temperatures, the density dccreases because the intragranular porosity mcrcases resulting from discontinnous grain j,'TOwth. The initial permcability proportionally changes with sintering density. A sample with more uniformity and larger grain size as well as highest dcnsity is obtained at 1350'C sintering temperalurc (optimnm T,) for Nio.oZn04Fe204. The real part of permeability, p;'. is ob,erved 10 be maximum at this optimnm T, for Nin ,Zno.,Fe2()" The permeability value increases with increasing Zn content in Nil_x7.nxFelO" because the average grain size increases ",ilh increasing Zn content. Larger grnins tend to consist of a greater number of domain walls. Thus the domain ",all motion is affected by the grain ~ize and cnhanced with the VI '", increase of grain size. The P:, therefore, increases with grain size. For both the compositions, p,' is found to be maximum at optimum T, (depending on sample composition). If the sintering temperature is higher than that of the optimum T" 1/; decreases. Higher sinteJing temperatures (> optimum T,) may increa.~e the number of pores within the grains of the samples, which result in a decrease in permeability. The ferrite with high permeability tends to have its permeability decreases at a relatively low frequency. The p,' values for all 5ample5 are found independent of frequency below the resonance frequency. The relative quality factor (Q) for Nio6Zn04Fcl()' is found to be 2,1,,10' at 1250'C sintering temperature. The highest Q value at 1250'C for NioJ-nn ,FeZ04 while 1200"C for 11,'insZno,zFelO, is probably due to the gro\\1h of lesser imperfection and defects in them than those in the samples sintered at higher sintering temperatures. The Curie temperature of NiO,eZn01Fe204 is found to be S16:t(C, while 353:tl"C for Nio6ZnOJ'eP4.