Non-hydrolytic sol-gel synthesis and characterization of materials of the type AA'M3O12

In recent years, there has been an increased interest in negative thermal expansion (NTE) materials, which contract upon heating. Materials exhibiting this property have the potential for achieving better control of thermal expansion through the synthesis of composite materials with more desirable expansion coefficients. By introducing NTE materials into these composites, it is possible to offset the positive thermal expansion of other components in the composite. As a result, these NTE materials can find use in a wide range of applications such as optics, electronics, tooth fillings and any other area where exact positioning of parts over a wide range of temperatures is crucial. A family of materials that has been known to show NTE are A 2M3O12 compounds, where A can be a variety of trivalent cations and M can be Mo or W. Previous work on this system has shown that the thermal expansion is highly dependent on the type of trivalent cation employed.;However, in spite of the interest in these A2M3O 12 compounds, little research has been dedicated to synthesizing materials containing two aliovalent cations instead of just one or two trivalent cations. In fact, the first example of a heterosystem with +2 and +4 cations was not reported until 2004. This dissertation presents results of investigation and characterization of these mixed cation systems, and the change in the thermal expansion properties.;The first goal of the research presented herein was to synthesize mixed cation systems using a lower temperature route, and then compare the materials synthesized using low temperature methods with those synthesized using the ball-milling method. This will ensure the validity of applying a lower temperature method to these mixed cation systems. A non-hydrolytic sol-gel (NHSG) method was used, which is based on the reaction of metal alkoxides with metal halides to form M-O-M linkages, with alkyl halides as byproducts.;With this method, MgHfW3O12 and MgZrW3O 12 were successfully synthesized. When compared to samples prepared by the ball-milling method, many distinct differences were observed. The first was that unlike with the ball-milling method where the desired materials required extended heat treatments at high temperatures (1050-1100°C, 17-24 h), the NHSG method allowed the synthesis of these compounds after reacting at 130°C for as little as 3 d and subsequent heat treatment to temperatures as low as 540°C for as little as 2 h. Furthermore, SEM showed that with the NHSG method, micronsized particles with defined morphology were formed, instead of the large, chunky particles observed when ball-milling was used. This significant change in particle size and morphology is very important for potential applications since it leads to better homogeneity of the components in a composite. Once these results were obtained, the same NHSG method was applied to other combinations of 2+ and 4+ cations.;The second goal of the project was to extend the use of the NHSG method to materials of the type A2M3O12 where A is a trivalent method. These types of materials have been previously made using traditional solid state methods. However, previous research in our group has shown that it is possible to access new metastable phases when using lower temperature routes. With the NHSG method, MgZrMo3O12 was made for the first time, as well as a new polymorph of Y2W 3O12.;Materials made were then characterized using a variety of analytical techniques. These included thermograviometric-differential thermal analysis, powder X-ray diffraction, variable temperature powder X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, and synchrotron experiments. High pressure studies were also carried out in an attempt to study how the synthesized materials behave when subjected to pressure.