Time: Thursday,2:30p.m.,22th October
Place: A415 SINANO
Abstract: Grain size effects on the physical properties of polycrystalline ferroelectrics have been extensively studied for decades; however there are still major controversies regarding the dependence of the piezoelectric and ferroelectric properties on the grain size. Dense BaTiO3 ceramics with different grain sizes were fabricated by either conventional sintering or spark plasma sintering using micro- and nano-sized powders. The results show that the grain size effect on the dielectric permittivity is nearly independent of the sintering method and starting powder used. A peak in the permittivity is observed in all the ceramics with a grain size near 1 micrometer and can be attributed to a maximum domain wall density and mobility. The piezoelectric coefficient d33 and remnant polarization Pr show diverse grain size effects depending on the particle size of the starting powder and sintering temperature. This suggests that besides domain wall density, other factors such as back fields and point defects, which influence the domain wall mobility, could be responsible for the different grain size dependence observed in the dielectric and piezoelectric/ferroelectric properties. In cases where point defects are not the dominant contributor, the piezoelectric constant d33 and the remnant polarization Pr increase with increasing grain size.
Dr YAN Haixue is a Senior Lecturer in functional materials at Queen Mary University of London (QMUL). His research includes ferroelectrics [1-4], thermoelectrics [5,6] and Spark Plasma Sintering (SPS) [2,4,7,8]. He made original contributions in the research on lead-free high Tc ferroelectrics and size effect in ferroelectrics. He developed low cost nanotechnology to prepare new thermoelectric materials with high ZT. His current research on SPS is focused on materials with textured, nano- and metastable structures.