An in-depth study of Al2W3O12 negative thermal expansion (NTE) ceramic was performed, focused on synthesis, phase mappings, and the underlying mechanisms shown to be responsible for NTE. Review of the literature has shown inconsistencies in reported values of the dilatometry measured coefficients of thermal expansion, and the temperature for the known monoclinic to orthorhombic phase transition. Two synthesis techniques are introduced: an ionic-liquid non-hydrolytic sol-gel synthesis route; and a low temperature solid state reaction synthesis for Al2W3O12. X-ray diffraction, Raman spectroscopy, and attenuated total reflectance Fourier-transform infrared spectroscopy (ATR-FTIR) were used to verify the techniques. Two differential scanning calorimetry (DSC) experiments (high and low temperature) were performed on the material showing the transition between -5 and -20 °C and no other phase changes until a reported degradation above 1100 °C. Extensive dilatometry on the material led to the discovery of elastic transitions occurring in the polycrystalline sample capable of explaining the inconsistencies in reported dilatometry results. This is further developed into a proposed model defining the regions between these transitions. Each region has a different thermal expansion as well as a direct effect on the reaction of the material upon cooling. This proposed model may allow more consistent reporting of dilatometry results for NTE materials. Raman spectroscopy was performed from 25-725 °C on the material showing both a joining in the tungsten-oxygen bending modes as well as a broadening in the tungsten-oxygen stretching modes. This is consistent with Al-O-W angle changes along the same temperature range reported in literature as well as the transverse vibrational modes responsible for NTE.

[pt] No desenvolvimento de materiais resistentes ao choque térmico, os materiais com expansão térmica negativa ou próxima a zero têm sido de vital importância nos últimos anos. A família de cerâmicas A2M3O12 (A = cátion trivalente, M = cátion hexavalente) apresenta características promissoras para evitar a ruptura por choque térmico, devido às suas propriedades diferenciadas de expansão térmica. Contudo, mudanças na rede cristalina causadas por defeitos pontuais podem gerar alterações neste material. A cerâmica Al2W3O12 pode conter defeitos como vacâncias de oxigênio, que afetam suas propriedades ópticas, térmicas e outras propriedades. O propósito deste trabalho é estudar a influência das vacâncias de oxigênio sobre as propriedades da cerâmica Al2W3O12, bem como, a importância da temperatura e da atmosfera sobre a geração de vacâncias de oxigênio. A fase Al2W3O12 foi exposto a duas atmosferas (H2 e Ar) sob temperaturas controladas de 300 graus C, 400 graus C e 500 graus C com o proposito de gerar vacâncias de oxigênio. A cerâmica obtida desta forma foi caracterizada por técnicas de difração de pó de raios-X (DPRX), espectroscopia por refletância de difusão (DRS), espectroscopia de ressonância paramagnética eletrônica (EPR), espectroscopia Raman, espectroscopia de fotoelétrons de raios-X (XPS) e teste de condutividade térmica. A influência das vacâncias de oxigênio no coeficiente de expansão térmica e na condutividade térmica nas amostras calcinadas em atmosferas não-oxidantes foi demonstrada. As amostras calcinadas a 500 graus C revelaram uma alta absorção do espectro ultravioleta e visível em relação às amostras calcinadas em 300 graus C e 400 graus C. Além disso, foram constatados diferentes estados reduzidos de valência do W (tais como W5+ e W4+) através da tecnica de XPS a fim de compensar vacâncias de oxigênio que foram confirmadas pela analise de EPR e Raman. / [en] In the development of materials resistant to thermal shock, materials with negative or near zero thermal expansion have been of vital importance in recent years. The A2M3O12 family of ceramics (A = trivalent cation, M = hexavalent cation) has promising characteristics to avoid thermal shock rupture, due to its atypical thermal expansion properties. Within this family is the Al2W3O12 ceramic, which has unique thermal properties and this allows its use in conditions of thermal shock. However, changes in the crystal lattice caused by point defects can generate alterations in this material. Al2W3O12 ceramic can contain defects such as oxygen vacancies, which affect optical, thermal and other properties. The goal of this study is to study the influence of oxygen vacancies on the properties of Al2W3O12 ceramics, as well as the importance of temperature and atmosphere in the generation of oxygen vacancies. The Al2W3O12 phase was exposed to two atmospheres (H2 and Ar) at controlled temperatures: 300 degrees C, 400 degrees C and 500 degrees C to generate oxygen vacancies. The ceramic obtained was characterized by X-ray power diffraction (XRPD), diffusion reflectance spectroscopy (DRS), electron paramagnetic resonance spectroscopy (EPR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermal conductivity test. The influence of oxygen vacancies on the coefficient of thermal expansion and thermal conductivity in calcined samples in non-oxidizing atmospheres was demonstrated. Samples calcined at 500 degrees C revealed high absorption of the ultraviolet and visible spectra compared to the samples calcined 300 degrees C and 400 degrees C. In addition, different states of reduced valence of W were found (such as W5+ and W4+), through the XPS technique, in order to compensate of oxygen vacancies, that were confirmed by EPR and Raman analysis.