CONICET | Buscador de Institutos y Recursos Humanos

Aluminum borate (Al18B4O33) is one of the mixed oxides with extensive applications as due to its good thermomechanical properties. Its application requires of accessible processing techniques to provide controlled microstructures and shapes. This work presents an aqueous colloidal route to manufacture porous aluminum borate ceramics from micrometric Al18B4O33 powder (∼2.0 μm) obtained by high temperature reaction of alumina and boric acid. Zeta potential measurements determine that isoelectric point of the Al18B4O33 powders suspended in water happens at pH = 8.5 achieving a maximum positive value of 60 mV for pH < 4. Rheological characterization of concentrated slurries showed a pseudoplastic flowing behavior with a thixotropic cycle, being the yield point and the thixotropy value strongly dependents on the solid content. Shear thinning curves were fitted to a Windhab model that confirmed a face-edge interactions of the particles in the suspension. Porous ceramic with a green density of 50 % and a narrow micronic (12 μm) porosity are obtained by slip-casting of the optimal concentrated suspension (33 vol%). Sintering above the formation temperature showed a decrease in the density of the compacts as a consequence of the acicular grain growth. Four sintering conditions were tested to follow the structural and compositional evolution. It has been observed that thermal treatments up to 1300 °C do not affect the composition but the microstructure. At 1400 °C presence of alumina is detected indicating the initial decomposition of Al18B4O33 and evaporation of boron. At 1500 °C decomposition and evaporation phenomena are massive. The porous compacts sintered below the decomposition temperature are characterized by scanning electron microscopy, mercury intrusion porosimetry and mechanical strength, indicating that at 1300 °C Al18B4O33 achieves the more uniform microstructure and the better mechanical response. It should be noted that for the first time the Zeta potential curve for aluminum borate powder is reported. The processing methodology employed in this study, has potential application to the manufacture of porous materials with insulating applications to replace traditional ceramic materials working in harsh conditions, especially in nuclear energy systems with complex geometry by casting techniques and even by 3D printing.

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