Relationship between sintering methods and physical properties of the low positive thermal expansion material Al2W3O12

Ceramic materials from the A(2)M(3)O(12) family with near-zero thermal expansion are good candidates for applications requiring high thermal shock resistance. Considering their inherently low thermal conductivity, the bulk forms of A(2)M(3)O(12) have to present Young's moduli and mechanical strength close to 100 GPa and 100 MPa, respectively, in order to compete with the state-of-the-art materials used to avoid thermal shock. The relationship between sintering, microstructure, and physical properties within the A(2)M(3)O(12) family is generally unknown while the preparation of bulks with high mechanical resistance remains a great challenge. Bulk samples of dense Al2W3O12 (96%TD) have been obtained by pressureless three-stage sintering (TSS) and spark plasma sintering (SPS). The Young's moduli and hardness of samples prepared by SPS were 50% higher than that measured for TSS samples and more than 100% in comparison to the Al2W3O12 bulk (91%TD). UV-Vis spectroscopy confirmed that A(2)M(3)O(12) phases are wide band-gap semiconductors (3.11 eV). When prepared by SPS, black Al2W3O12 absorbed light within the visible spectrum due to the introduction of donor sites within the band-gap. No enhancement of the mechanism causing negative thermal expansion was observed for black Al2W3O12. The mechanical properties achieved were significantly improved over those previously reported in literature for Al2W3O12.