Thermal expansion plays a very important role in determining if a material will be suitable for a particular application. The proposed work will lead to an enhanced understanding of both structure property relationships in low and negative thermal expansion (NTE) materials, and strategies for controlling thermal expansion. The pressure dependence of thermal expansion (TE) in a variety of low and negative TE materials will be examined to establish the factors that lead to highly pressure dependent coefficients of thermal expansion (CTEs) in such materials. It is hypothesized that low pressure phase transitions in NTE materials will lead to the quite widespread occurrence of extreme pressure sensitivity. The pressure dependence of CTEs is a design consideration for composites where a NTE filler may experience stresses. The control of thermal expansion, by modifying the O:F ratio, in oxyfluorides with a ReO3 framework structure, will be examined and the underlying structure property relationships established by separately interrogating the response of M-F-M and M-O-M links to temperature and pressure using total scattering methods. Substitution of fluoride for oxide, as a means of controlling thermal expansion, is an unexplored arena with great potential for interesting findings. The local structures of AX2O7 (A - Zr, Hf; X - P, V) will be examined using total scattering to better understand their high temperature phase transitions and how the nanostructure (local structure) of their disordered high temperature phases can lead to low or negative thermal expansion, as only the disordered forms of these materials display interesting expansion characteristics
NON-TECHNICAL SUMMARY: The thermal expansion characteristics of a material play a very important role in determining if it is suitable for use in a wide variety of applications. The proposed work will lead to an enhanced understanding of strategies for controlling thermal expansion, and the preparation of new materials. This will be of value in the search for new useful engineering materials. As an integral part of this work, graduate and undergraduate students will be trained in a wide variety of synthetic and materials characterization techniques, introduced to important concepts in materials chemistry/science, and engaged in activities that develop professional skills. These skills are of considerable value to the US economy. A significant component of the proposed experimental work will be conducted using major x-ray and neutron scattering facilities located at Department of Energy (DOE) national laboratories. The work at DOE laboratories enhances the educational experience of students, and leads to collaborations that professionally benefit both university and government laboratory employees.
Most materials expand, or show "positive thermal expansion", when they are heated. Materials that contract on heating, or display "negative thermal expansion", are intrinsically interesting as the behavior is atypical. Both positive and negative positive thermal expansion can lead to problems with the construction of devices. For example, the performance of precision optics in telescopes, other scientific instruments, communications systems etc. can be degraded by the dimensional changes associated with temperature variations. These difficulties can be avoided by either very carefully controlling temperature, fabricating key components out of zero thermal expansion materials, or compensating for positive expansion using negative thermal expansion materials. The creation of composites from intimate mixtures of positive and negative thermal materials is one potential path to tunable thermal expansion parts. The manufacture and use of such composites potentially subjects the component materials to stresses, which can change their structures and properties. With NSF support we have worked with partners to develop new methods and equipment for studying the effect of pressure (stress) on materials. Our development of a "Background Reducing Internal Collimator" (BRIM) has allowed us to examine the effects of very carefully controlled pressure on the thermal expansion properties of materials. We have found that the application of quite low pressures strongly modifies the thermal expansion characteristics of the very well known negative thermal expansion materials ZrW2O8 and HfW2O8. These results have implications for any attempt to use them in a composite at elevated temperatures. We have also worked with scientists at the Advanced Photon Source to help develop tools for studying the changes in short range, or local, structure that occur in materials as they are subjected to high pressures. Our approach has provided insight into the mechanism by which the crystalline negative thermal expansion material ZrMo2O8 becomes a glass, with quite different properties, on exposure to high pressures. The equipment and methods that we have developed are suitable for use by other workers to study a wide range of materials. Under NSF support we discovered that scandium fluoride (ScF3), which has a very simple atomic structure, displays very strong negative thermal expansion below room temperature, and positive thermal expansion above ~1100 K. This material, although expensive, is optically isotropic and transparent over a wide range of wavelengths. We have examined several different pathways to chemically tune or modify its properties. We have also closely studied the oxyfluorides TiOF2 and TaO2F to better understand the potential role of defects and atomic disorder in determining the thermal expansion properties of materials. This body of work was motivated by two considerations: 1) an understanding of the fundamentals underpinning the properties of these materials is of intrinsic scientific interest and will help facilitate the rational design of controlled thermal expansion materials, and 2) zero thermal expansion materials, which are structurally related to ScF3, could be of use in optical devices. Five graduate students and one undergraduate have received training with the aid of this support. They have gained experience with modern methods for preparing and studying materials. They have also spent time working side by side with established researchers at major national facilities such as the Advanced Photon Source and the High Flux Isotope Reactor.
