Size Effects in Ferroic Solids

9223847纽汉姆这个奖项是对宾夕法尼亚州立大学新材料研究小组的初步资助，该小组的主题是铁性固体中的尺寸效应。对典型氧化物铁氧体中的标度现象进行了理论和实验研究。重点是确定以下四个区域的临界尺寸：(1)每个包含多个磁区壁的大颗粒，其数量随着颗粒尺寸的减小而减少，(2)包含单个磁区的颗粒，(3)小于单个磁区的颗粒尺寸，(4)纳米尺度的颗粒。可选择的材料体系有铁电材料、铁弹性材料和二次铁质材料。我们的目标是确定每个系统的临界颗粒尺寸，在每个系统中，材料及其行为从块状行为过渡到单域行为，再到玻璃样行为和非铁电行为。该实验计划包括制备薄膜形式的细颗粒粉末和陶瓷/聚合物以及陶瓷/陶瓷复合材料；使用热分析、X射线衍射、拉曼光谱、电子显微镜和椭圆偏振光谱来表征颗粒/颗粒的微观结构、磁区结构、相变和临界尺寸；测量介电常数和弹性，作为外加电场、温度和频率的函数。理论部分以兰道-金斯伯格-德文郡的形式主义为基础。考虑了体积、外表面和内部界面(晶界和磁场壁)的影响。铁性晶体具有可以被电场(铁电性)、磁场(铁磁体)、机械应力(铁弹性)或涉及前三者的组合的一些更高阶现象(次铁性)移动的磁畴壁。这些材料被用作换能器、电容器、变压器、传感器和致动器，因为磁区壁对压电性、介电常数、磁导率、弹性柔量和其他性能系数有很大的贡献。对尺寸效应的基本了解对这些器件的开发至关重要，这些器件正在以薄膜、纤维填充复合材料和亚微米级颗粒尺寸的多层陶瓷的形式变得更小。

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.