Abstract
In this work, we investigated the processing-microstructure-property relationships for magnetoelectric (ME) particulate composites consisting of hard ferromagnetic CoFe2O4 (CFO) particles dispersed in a Nb-doped PbZrxTi1-xO3 (PZT) soft ferroelectric matrix. Several preparation steps, namely PZT powder calcination, PZT-CFO mixture milling and composite sintering were tailored and a range of microstructures was obtained. These included open and closed porosities up to full densification, PZT matrices with decreasing grain size across the submicron range down to the nanoscale and well dispersed CFO particles with bimodal size distributions consisting of submicron and micron sized components with varying weights. All samples could be poled under a fixed DC electric field of 4 kV/mm and the dielectric, piezoelectric and elastic coefficients were obtained and are discussed in relation to the microstructure. Remarkably, materials with nanostructured PZT matrices and open porosity showed piezoelectric charge coefficients comparable with fully dense composites with coarsened microstructure and larger voltage coefficients. Besides, the piezoelectric response of dense materials increased with the size of the CFO particles. This suggests a role of the conductive magnetic inclusions in promoting poling. Magnetoelectric coefficients were obtained and are discussed in relation to densification, piezoelectric matrix microstructure and particle size of the magnetic component. The largest magnetoelectric coefficient α33 of 1.37 mV cm−1 Oe−1 was obtained for submicron sized CFO particles, when closed porosity was reached, even if PZT grain size remained in the nanoscale.
Highlights
Up to the present time, the highest magnetoelectric coefficients have been achieved by combining a ferroelectric phase with high piezoelectric response and a ferromagnetic phase with large magnetostriction through strain mediation
Perovskite compounds close to the morphotropic phase boundary (MPB) are frequently employed due to Materials 2020, 13, 2592; doi:10.3390/ma13112592
We report the dielectric, piezoelectric, elastic and magnetoelectric properties of CFO/PbZrx Ti1-x O3 (PZT) particulate composites having a fixed volume content of CFO (x = 0.144) but a wide range of microstructures
Summary
The increase of the magnetoelectric (ME) response of multiferroic composite systems up to tens of V/cm·Oe has turned them into attractive candidates for the development of novel devices [1,2,3,4,5]. The problem is that densification becomes increasingly difficult even at moderate CFO contents This affects composite permittivity (εr ) and charge piezoelectric coefficients (d33 ) which can show a drastic decrease up to 60% and 80% respectively, even at only 0.14 CFO volume fraction. We report the dielectric, piezoelectric, elastic and magnetoelectric properties of CFO/PZT particulate composites having a fixed volume content of CFO (x = 0.144) but a wide range of microstructures. Densification values from 100 down to 62% were achieved, as well as grain size distributions of both phases covering the micron, submicron and nanometer ranges This was done by varying the calcination temperature for PZT, the mixing/milling process of CFO and PZT powders and the sintering cycle. This work has manifold implications because it shows how to tune the magnetoelectric response by the processing/microstructure and gives some guidelines to increase the magnetoelectric coefficient while keeping constant the CFO content
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