From Negative to Zero Thermal Expansion IR Transparent Materials Using the Defect Chemistry of ReO3-Type Fluorides

热膨胀，即尺寸随温度的变化，在确定材料是否适用于特定应用方面起着非常重要的作用。当尺寸稳定性很重要时，低或零热膨胀是必要的，例如在精密光学器件的制造中，并且在暴露于温度快速变化的陶瓷中也非常有益，例如在炊具和一些发动机和航空航天应用中使用的陶瓷。在加热时收缩的材料（显示负热膨胀）可用于补偿其他组件中的正热膨胀。这项研究的重点是提高我们对原子结构和材料性质之间关系的基本理解，寻找具有有趣性质的新组合物，开发控制热膨胀的化学策略，并利用它们来创建红外透明零热膨胀材料。这种材料在热成像、通信和激光应用中具有潜在的价值。研究生和本科生将接受各种合成和表征技术的培训，这些技术是制备和理解这些材料的特性所必需的。学生将被介绍到材料化学/科学的重要概念和方法，并从事发展一般专业技能的活动。技术摘要在固态和材料化学计划的支持下，该项目将集中在具有ReO 3型骨架的金属氟化物中的热膨胀控制，因为这种结构类型相对简单，这有助于促进实验和计算方法来阐明基本原理，可以制备各种不同的化学组成，并且金属氟化物通常在红外线中具有良好的透明度。2015年初，该团队报告说，氟化钙锆在很宽的温度范围内显示出极强的负热膨胀。氟化锆钙所属的材料家族具有相当大的组成和结构灵活性。这通过1）阳离子取代、2）阳离子无序和3）远离理想化学计量的移动以形成氟化物过量固体来提供热膨胀的控制，所述氟化物过量固体具有间隙氟化物或阳离子空位。通过故意产生这种结构缺陷来控制热膨胀是新颖的，并且可能非常强大。可变温度/压力中子和X射线散射方法将用于建立原子级结构和属性之间的联系。

Non-technical AbstractThermal expansion, which is change in dimensions with temperature, plays a very important role in determining if a material will be suitable for a particular application. Low or zero thermal expansion is necessary when dimensional stability is important, such as in the fabrication of precision optical devices, and it is also highly beneficial in ceramics that are exposed to rapid changes in temperature, such as those used in cookware and in some engine and aerospace applications. Materials that shrink on heating (display negative thermal expansion) can be used to compensate for positive thermal expansion in other components. This research focuses on enhancing our fundamental understanding of the relationship between atomic structure and a material's properties, finding new compositions with interesting properties, developing chemical strategies for controlling thermal expansion, and using them to create infrared transparent zero thermal expansion materials. Such materials are potentially of value in thermal imaging, communications and laser applications. Graduate and undergraduate students will be trained in the wide variety of synthetic and characterization techniques needed to prepare and understand the properties of these materials. They will be introduced to important concepts and methods in materials chemistry/science, and engaged in activities that develop general professional skills.Technical AbstractWith support of the Solid State and Materials Chemistry program, the project will focus on the control of thermal expansion in metal fluorides with an ReO3-type framework, as this structure type is relatively simple, which helps facilitate both experimental and computational approaches to the elucidation of underlying principles, a wide range of different chemical compositions can be prepared, and metal fluorides typically have good transparency in the infrared. In early 2015, the team reported that calcium zirconium fluoride displays extremely strong negative thermal expansion over a wide temperature range. The family of materials which calcium zirconium fluoride belongs to has considerable compositional and structural flexibility. This provides for the control of thermal expansion by 1) cation substitutions, 2) cation disorder and 3) movement away from the ideal stoichiometry to form fluoride excess solids, which either have interstitial fluoride or cation vacancies. The control of thermal expansion by the deliberate generation of such structural defects is novel and potentially very powerful. Variable temperature / pressure neutron and x-ray scattering methods will be used to establish the links between atomic level structure and properties.