Aluminum titanate (Al 2 TiO 5) materials and aluminum titanate-mullite-zirconium titanate (Al 2 TiO 5-3Al 2 O 3 .2SiO 2-ZrTiO 4) composite materials were successfully processed from fine commercial powders and characterized. This was achieved by zircon (ZrSiO 4) addition to stoichiometric alumina-titania mixtures. Zircon addition was the principal processing variable explored. This additive stabilizes the unstable aluminum titanate phase, enhances the sintering process, restricts microcrack development and improves the mechanical properties of the bulk material, but has a slight detrimental effect on its thermal expansion behavior (α app from-1.5 to 2.5 x 10-6 °C-1 in the RT-800 °C range). With a clear microstructure configuration change, all the technological properties are directly (linearly) correlated with zircon proportion in the initial formulation in the range between 5 and 30 wt%. Developed phases were established, relatively dense ceramics were produced, and complex microstructures with multiphasic interlocked grains were identified. Also, an interconnected microcrack matrix was observed with no material integrity loss which explained the low or even negative thermal expansion behaviors observed in the developed materials. This, together with the mechanical behavior detected, encourages structural applications with high thermomechanical solicitations. The triplex composite material presented an excellent thermomechanical behavior and low porosity, 48 MPa flexural strength, low stiffness and high sintering grade with low thermal expansion.
Ceramic materials were satisfactorily processed through a dry scalable process from binary clay-boric acid (H 3 BO 3 ) mixtures. Relevant thermal parameters were established by a multitechnique approach that included thermogravimetric, differential thermal analysis, dilatometric analysis and structural and microstructural characterization of fired samples. Both clay and boric acid thermal processes were described and correlated. The experimental textural properties evidenced a porosity decrease with sintering temperature and acid addition in the 1100-1300 °C range. The amount of glass was strongly increased by the boron oxide incorporation, confirming its fluxing capacity. X-ray diffraction, supplemented by Rietveld-Le Bail refinement, verified the presence and thermal evolution of crystalline and glassy phases. The observed microstructure was similar to other clay-based ceramics, with quartz, cristobalite and mullite grains imbibed in the silicabased glassy phase. The observed mullite phase was actually a boron mullite solid solution. Boric acid was confirmed as an adequate boron oxide source. The present study gives information for further clay-based materials design with boron oxide as fluxing agent. The dry route hypothesis was confirmed. Both formulation and firing programs can be optimized. High boron addition (5 mass%) is not recommended due to the observed partial rehydration.
Aluminum titanate Al 2 TiO 5 materials were successfully processed from different fine commercial powders and characterized. Particularly, two calcined aluminas were compared through a multitechnique approach including differential thermal analysis and dilatometry together with structural, microestructural, and mechanical characterization. This allowed the description of all the thermochemical processes during thermal treatment. Developed phases were established. Relatively dense ceramics were obtained, and complex microstructures were described with interlocked grains and an interconnected microcrack matrix that do not jeopardize the material integrity. Multistep sintering and reaction sintering processes were observed in both samples. The first stage consists of the sintering of the starting powders (alumina and titania). A second sintering stage of the starting powders was observed for both samples as well. Once advanced, the second one is overlapped with Al 2 TiO 5 formation that starts at 1380 °C and finishes at 1440 °C. They affect crack development and, in consequence, the thermal behavior. The lower alumina particle size enhances the sintering and reaction advance processes. In the technological temperature range (room temperature-1000 °C), low or even negative thermal expansion behaviors were observed in the developed materials. This, together with the mechanical behavior, encourages structural applications with high thermomechanical solicitations of Al 2 TiO 5 based materials.
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