Clive A. Randall
Clive A. Randall is a Professor of Materials Science and Engineering and Director of the Materials Research Institute at The Pennsylvania State University. He received a B.Sc. with Honors in Physics in 1983 from the University of East Anglia (UK), and a Ph.D. in Experimental Physics from the University of Essex (UK) in 1987. He was Director for the Center for Dielectric Studies 1997-2013, and Co-Director of the Center for Dielectrics and Piezoelectrics 2013-2015, still serving as Technical Advisor. He has authored/co-authored over 450 technical papers (20,000 citations H-index 76) and holds 15 patents (with 1 pending) in the field of electroceramics. His research interests are in the area of discovery, processing, material physics, and compositional design of functional materials; with different processing and characterization methods. Prof. Randall has received a number of awards from various societies, including the American Ceramic Society Fulrath Award, Fellow of the American Ceramic Society, Academician of the World Academy of Ceramics; Spriggs Phase Equilibria Award; Friedberg Lecture at the American Ceramic Society; Edward C. Henry Best Paper of the Year from the American Ceramics Society Electronics Division (2012 and 2017), IEEE UFFC-S Ferroelectrics Recognition Award (2014), Robertson Breakthrough of the Year Award (College of Earth and Mineral Sciences, Penn State University, 2017).
A Discussion of Defects, Crystal Chemistry, Thermochemistry, Non-equilibrium Processing and the Impact on Properties of Ferroelectric Materials
Ferroelectric and related materials are very sensitive to compositional design. Perovskite structured ferroelectrics can be compositionally guided through understanding the inter-relationship between crystal chemistry and phase transition behavior. Several demonstrations of this design approach through the Goldschmidt tolerance factor will be given including high temperature morphotropic phase boundaries, high temperature relaxor ferroelectrics, and developing antiferroelectrics solid solutions.
A subtler perturbation to ferroelectric phase transitions and properties is understanding of non-stoichiometric partial Schottky reactions, the associated defect and defect clusters that can control Curie points, the electronic conduction, and associated degradation mechanisms. Co-doping strategies utilizing rare earth ion dopants that are amphoteric in nature (occupying A and B-sites of the perovskite structure) can help the stabilization of the mobile oxygen vacancy defects. Comprehension of these defects and quantification of their dynamics can be used to mediate internal bias. The details of these charge distributions need to be considered at a defect complex within the lattice and/or macroscopically across a grain or series of grain boundaries. Experimental insights into this behavior can be determined through careful electron paramagnetic resonance (EPR) and thermally stimulated depolarization current (TSDC) measurements. Data from these observations can be modeled with a phenomenological theory, that points to local electrostatic potentials modifying the probability of ionic hopping. Understanding the science and engineering of these defect dynamics is critical, particularly as applications push to higher temperatures and higher electric field operation.
Many of the above concepts are applicable to solid state processing methods that involve high temperature sintering. However, there is a possibility that we may have to reexamine the defect and dopant strategies if we continue to make advances with low temperature synthesis of bulk and multilayer devices. With the introduction of cold sintering, a process that limits the temperatures to below 300°C, there are many new concepts that will impact the future designs of ferroelectrics and related materials. Recently, we have densified ferroelectrics such as (Na,K)NbO3, BaTiO3, and Pb(Zr,Ti)O3 under these cold sintering conditions. The properties and future trends of these materials for capacitors, piezoelectrics and other applications will be discussed, together with co-sintering with polymers and hybrid organic/inorganic perovskites.