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This thesis investigates the synthesis and structural characterisation of AM(_2)O(_8) phases, many of which show negative thermal expansion (NTE); relevant literature is reviewed in Chapter One. Chapter Two describes the synthesis, structure solution, and mechanistic role of a new family of low-temperature (LT) orthorhombic AM(_2)O(_8) polymorphs (A(^TV) = Zr, Hf; M(^VI) = Mo, W). These materials are key intermediates in the preparation of cubic AM(_2)O(_8) phases from AM(_2)O(_7)(OH)(_2)(H(_2)O)(_2). The structure of LT-AM(_2)O(_8) has been elucidated by combined laboratory X-ray and neutron powder diffraction. Variable temperature X-ray diffraction (VTXRD) studies have shown LT- AM(_2)O(_8) phases exhibit anisotropic NTE. LT-ZrMo(_2)O(_8) has been shown to undergo spontaneous rehydration, allowing preparation of ZrMo(_2)O(_7)(OD)(_2)(D(_2)O)(_2) and assignment of D(_2)O/OD positions within the structure by neutron diffraction. Using this result, a reversible topotactic dehydration pathway from AM(_2)O(_7)(OH)(_2)(H(_2)O)(_2) to LT-AM(_2)O(_8)s is proposed. Chapter Three investigates the order-disorder phase transition with concurrent oxygen mobility in cubic AM(_2)O(_8) materials; studies include comprehensive VT neutron diffraction of cubic ZrMo(_2)O(_8) to reveal a static to dynamic transition at 215 K, and novel quench-anneal/quench-warm variable temperature/time diffraction experiments on ZrMo(_2)O(_8) which lead to an activation energy of 40 kJmol(^-1) for oxygen migration. In Chapter Four (^17)O-labelled cubic ZrW(_2)O(_8) has been prepared to understand the oxygen migration process by VT MAS NMR. In situ hydrothermal studies of cubicZrMo(_2)O(_8) using synchrotron radiation have shown direct hydration to ZrMo(_2)O(_7)(OH)(_2)(H(_2)O)(_2).. In Chapter Five VTXRD of trigonal a-AMo(_2)O(_8) phases reveals a previously unknown second-order phase transition at 487 K (A = Zr) or 463 K (A = Hf) from P31c to P3ml. Rigid-body Rietveld refinements have shown this is due to alignment of apical Mo-O groups with the c axis in the high-temperature, a' phase.

QC 351 A7 no. 48 / New materials of low thermal expansion are finding wide application. The expansion coefficient (a) is a function of temperature, and this function must be known for each material before its applicability can be assessed. A novel method for determining a, which is at once precise and easily implemented, has been devised. It is based on the dependence of mode frequencies in a Fabry-Perot interferometer on the mirror separation. The expansion sample is formed into an interferometer spacer with ends polished flat and parallel. Spherical mirrors are optically contacted to the ends, forming a confocal interferometer. The assembly is maintained at controlled temperatures in an environmental chamber. The two lowest -order transverse modes are probed by variable -frequency sidebands derived from a 633 -nm He- Ne laser by amplitude modulation. A change in sample temperature AT causes a change in interferometer length AL, which shifts the resonance frequencies by Av. Then a = (1 /AT) (AL /L) _ - (1 /AT)(iv /v). Thus, a can be measured with precision limited ultimately by the stability of the source laser, in practice 1:109 with presently available commercial lasers. For a sample of Owens -Illinois Cer -Vit, a has been measured at 10 temperatures in the range 3.0 to 32.4 °C, with a mean error of 2 x10-9 and a maximum error of 3 x10 -9. For a sample of Corning ULE silica, a has been measured at six temperatures in the same range, with a mean error of <1 x10 -9 and a maximum error of <1.3 x10 -9.

A new mineralogical approach is introduced to predict aggregate and concrete coefficient of thermal expansion (CoTE). Basically, a modeling approach is suggested based on the assumption that the CoTE of aggregate and concrete can be predicted from the CoTE of their constituent components. Volume percentage, CoTE and elastic modulus of each constituent mineral phase are considered as input for the aggregate CoTE model, whereas the same properties for coarse aggregate and mortar are considered for the concrete CoTE model. Methods have been formulated to calculate the mineral volume percentage from bulk chemical analysis for different type of rocks commonly used as aggregates in Texas. The dilatometer testing method has been established to measure the CoTE of aggregate, pure minerals, and concrete. Calculated aggregate CoTE, based on the determined CoTE of pure minerals and their respective calculated volume percentages, shows a good resemblance with the measured aggregate CoTE by dilatometer. Similarly, predicted concrete CoTE, based on the calculated CoTE of aggregate and mortar and their respective volume percentages compares well with the measured concrete CoTE by dilatometer. Such a favorable comparison between predicted and measured CoTE provided a basis to establish the composite model to predict aggregate and concrete CoTE. Composite modeling will be useful to serve as a check of aggregate source variability in terms of quality control measures and improved design and quality control measures of concrete.

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Despite the advantages that composites have over monolithic materials, their use has been restricted by some deficiencies in their properties. The goal of this study was to overcome deficiencies of unidirectional glass fibre epoxy resin composites by coating the fibres with a "tailored" interlayer. Polyimide–silica hybrids, also known as ceramers, based on hydrolysed tetraethoxysilane and a polyamic acid solution mixture were used to coat glass fibres for epoxy composites. The silica part of these hybrids appears to be present either as dispersed discrete particles or as continuous nano-sized domains trapped within the polyimide matrix. The structure of hybrids determines the final properties. In this study both types of morphologies for the interlayers were utilised to obtain different mechanical, thermal and thermomechanical properties.

This master's thesis deals with the challenges of undesirable thermal expansion in lightweight materials. Thermal expansion of parts or components can lead to malfunction or breakdowns of complete systems in demanding environment where a large temperature gradient often exists. This work investigates a class of lightweight materials of which the thermal expansion coefficient can be controlled. Moreover, an additive manufacturing approach to produce these thermal management materials with high fidelity and reliability are critical to reach this goal. To achieve these two major research objectives analytic predictions, simulations, and measurement of thermal expansion coefficient with respect to temperature changes are conducted. Design and optimization of a high precision multi-material manufacturing apparatus has been conducted, leading to significant increase in production quality including reliability, efficiency, and costs. / Master of Science / This master’s thesis deals with the challenges of undesirable thermal expansion in lightweight materials. Under thermal load parts or components usually expand and this can lead to malfunction or breakdowns. To encounter this issue of the undesired expansion this work investigates a class of lightweight materials of which the thermal expansion coefficient can be controlled. Moreover, an additive manufacturing approach to produce these thermal management materials with high fidelity and reliability are critical to reach this goal. To achieve these two major research objectives analytic predictions, simulations, and measurement of thermal expansion coefficient with respect to temperature changes are conducted. Design and optimization of a high precision multi-material manufacturing apparatus has been conducted, leading to significant increase in production quality including reliability, efficiency, and costs.

The A2Mo3O12 family, where A3+ is a large trivalent cation, can show interesting thermal properties such as negative thermal expansion, also known as thermomiotic behavior, where the overall volume of the material contracts with increasing temperature. A selection of compounds in this family, namely HfMgMo3O12, In2Mo3O12, Y2Mo3O12, Al2Mo3O12, In(HfMg)0.5Mo3O12, and In1.5(HfMg)0.25Mo3O12, have been synthesized using solid-state and mechanical activation techniques as well as a simplified sol-gel approach (Al2Mo3O12). Coefficients of thermal expansion were found to range from large-negative to low-positive in the orthorhombic phase, including near-zero in In(HfMg)0.5Mo3O12 and In1.5(HfMg)0.25Mo3O12. This set of materials provided insight into the role of low-frequency phonon modes in open-framework materials. Low-temperature heat capacity and thermal conductivity measurements confirmed that low-frequency modes were active in thermomiotic materials, and also present to some extent in all members of the open-framework A2Mo3O12 family examined. A clear correlation exists between the magnitude and sign of the coefficient of thermal expansion in the orthorhombic phase and the contribution of low-energy modes to the low-temperature heat capacity, with negative thermal expansion materials having a larger contribution. The low-frequency phonon modes result in low thermal conductivity and reduced phonon mean free paths when compared to conventional ceramics and indicate that these low values are characteristic of open-framework materials in NTE families even if the materials in the families are not thermomiotic themselves.

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