Size Effects in Ferroic Solids

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9223847 Newnham This award is the initial funding for a new Materials Research Group at the Pennsylvania State University on the topic of size effects in ferroic solids. Scaling phenomena in representative oxide ferroics are being investigated theoretically and experimentally. Emphasis is being placed on determining the critical sizes for the following four regimes: (1) large particles that each contain many domain walls which reduce in number as the particle size decreases, (2) particles that contain a single domain, (3) particle sizes smaller than a single domain, (4) particles in the nanometer size scale. The material systems of choice are ferroelectrics, ferroelastics, and secondary ferroics. The goal is to determine the critical particle sizes for each system where the material and its behavior transitions from bulk behavior to single domain behavior to glass-like, and non-ferroelectric behavior. The experimental program involves preparation of fine-grain powders plus ceramic/polymeric, and ceramic/ceramic composite materials in the form of thin films; characterization of the particle/grain microstructure, domain configurations, phase transformations, and critical sizes using thermal analysis, x-ray diffraction, Raman spectroscopy, electron microscopy, and spectroscopic ellipsometry; measurement of permittivity, elasticity, as a function of applied field, temperature, and frequency. The theory component is based on the Landau-Ginsburg-Devonshire formalism. The effects of volume, external surfaces, and internal interfaces (both grain boundaries and domain walls) are considered. Ferroic crystals have domain walls which can be moved by electric fields (ferroelectrics), magnetic fields (ferromagnets), mechanical stress (ferroelastics), or some higher order phenomena involving a combination of the first three (secondary ferroics). These materials are used as transducers, capacators, transformers, sensors, and actuators, because of the lar ge contribution that domain walls make to piezoelectricity, permittivity, permeability, elastic compliance, and other property coefficients. A basic understanding of size effects is critical to the development of these devices, which are being made smaller in the form of thin films, fiber-filled composites, and multilayer ceramics with submicron grain sizes.