Influência do tamanho médio de grão nas propriedades ferroicas de compósitos magnetoelétricos particulados
Abstract
In the case of composites materials, which presents the magnetoelectric coupling,
there are so many questions about the influence of the microstructure on the coupled
and uncoupled properties. Thereby, this work proposes the study of the grain size
influence, of ferroelectric and magnetic phases, on the magnetic, electric and coupled
properties in particulate magnetoelectric composites. In order to study the relationship
property/microstructure in volumetric magnetoelectric composites, it was necessary to
prepare samples with different average grain sizes, ie, the phases having average
grain size in the order of micrometers or nanometers. To perform this study it was used
as magnetic phase the CoFe2O4 and, as the ferroelectric phase, the [0,675]
Pb(Mg1/3Nb2/3)O3 – [0,325] PbTiO3, mixed in molar ratio 20/80 %, using techniques like
micro ball milling (to reduce particle size) and high energy ball milling (to mixture the
phases) during the powder syntheses, and Spark Plasma Sintering (SPS) to densify
the bulk. Due the presence of secondary phases in sintered composites generated
during the application of SPS, a processing protocol was created based on a post
sintering thermal treatment, in PbO rich atmosphere, which allowed the reduction of
secondary phases without, however, changing the average size of grain. These
procedures resulted in composite material suitable for the investigations of the
electrical, magnetic and magnetoelectric properties, considering the different
microstructures obtained. The magnetoelectric composites showed the self-biased
effect arising from the intrinsic strain generated in one phase over the other. It was
observed the properties dependence, coupled or uncoupled, with average grain size
of both phases. Furthermore, the magnetoelectric composites presented the selfbiased
effect that appears due to intrinsic strain generated from one phase over the
other. Moreover, the evolution of the magnetoelectric coefficient behavior as a function
of temperature and applied magnetic field has been more dependent on the variation
of the average grain size of the ferroelectric phase than the ferrimagnetic phase, which
can be attributed to the largest amount of ferroelectric phase in the structure
compound.