Study of Grain Size Effect on Microstructural Conduction Mechanism and Multiferroism of Na0.5Bi0.5TiO3-BaFe12O19 Particulate Composites

Abstract

Four novel polycrystalline magnetoelectric composite systems: S1, S2, S3 and S4 having composition [90 wt% Na0.5Bi0.5TiO3 (NBT) - 10 wt% BaFe12O19 (BaM)] considering the variation of grain size of both the phases [NBT(Lg) - BaM(Lg)-[S1], NBT(Lg) - BaM(Sg)-[S2], NBT(Sg) -BaM(Lg)-[S3] and NBT(Sg) - BaM(Sg)-[S4]] are successfully synthesized by using solid state reaction method. Here Lg is larger size grain and Sg is smaller size grain. The Rietveld refinement of X-ray diffraction pattern confirms the pure phase formation of our desired systems. The SEM,FESEM, STEM and TEM images provide the information about the size, geometry and connectivity of grains of both the phases. The microstructural conduction mechanism and multiferroic behaviour are examined by complex impedance spectroscopy, PE loop, MH loop and magnetoelectric coupling (MEC) coefficient (αME).
For analysis of electric and magnetic properties of our defined composite systems, both bulk and nano grain size BaFe12O19 (BaM) &Na0.5Bi0.5TiO3 (NBT) as well as polycrystalline (1-x)NBT- x BaM; x in wt% = 10, 20, 30, 40 and 50 composites are synthesized by using solid state reaction & auto combustion methods. These systems are further subjected to electric and magnetic characterizations.
From the results it is observed that, like doping and substitution, the effect of grain size, geometry and connectivity can tailor the electric as well as magnetic properties of a system (single phase/ composite). Larger grain size reduces the surface as well as pinning of grain boundaries effect due to which saturation magnetization (Ms) increases and coercive field (Hc) decreases for bulk BaM system. Due to larger grain size bulk BaM system has shown co-contribution of grain and grain boundary effect at room temperature whereas nano BaM system has shown only grain boundary effect. Nano BaM system has shown metallic nature with temperature due to dominated cation-cation interaction over cation-anion-cation interaction.
Larger grain size of NBT system has shown two types dipolar relaxations in 30ºC ≤ T ≤ 350 ºC range which is absent for smaller grain (nano) size NBT. Reduced grain size (nm range) NBT system has shown a ferromagnetic loop at room temperature (RT) which is absent for bulk.The variation of grain size, tailored the electric and magnetic properties of [50 wt% BaFe12O19–50 wt% Na0.5Bi0.5TiO3] composite system. Apart from grain size effect, it is also perceived that the variation of BaM wt%, changes both connectivity and interfaces density. This in turn changes the microstructural conduction mechanism as well as multiferroic behaviour of composites. From the microstructural conduction mechanism, it is observed that different interfaces (BaM-BaM, BaMNBT and NBT-NBT) are activated at different temperature ranges of composite systems. It is also found that due to lack of connectivity between BaM-BaM and NBT-NBT interfaces, [90 wt% Na0.5Bi0.5TiO3-10 wt% BaFe12O19] system has shown Positive temperature coefficient resistance(PTCR) behaviour. From the M-H loop study it is further found that, Ms increases with increase ofBaM wt% but Hc decreases. From PE loop it has been found that, remnant polarization (Pr)increases with increase of BaM wt% but up to a certain limit.
As specified earlier the electric, magnetic & multiferroic behaviours of earmarked [90 wt% Na0.5Bi0.5TiO3-10 wt% BaFe12O19] system along with variation of grain size in both the phases are carried out. Many interesting results exhibited by them are recorded. To name the few [NBT (Sg)-BaM (Lg)] composite system shows highest Pr as compared with others. A similar type of result is also observed from αME data. These observations are explained on the basis of connectivity and momentum transfer mechanisms. It is surprisingly found that, Ms drastically increases for the composite systems in which NBT phase has smaller grain size. This is explained on the basis of inter-grain-exchange interaction.
In summary, the entire work incorporated in this thesis justifies the title “Study of Grain Size Effect on Microstructural Conduction Mechanism and Multiferroism of Na0.5Bi0.5TiO3- BaFe12O19 Particulate Composites’’.