The effect of microstructure on thermal expansion coefficients in powder-processed Al2Mo3O12

Orthorhombic Al2Mo3O12 was investigated as a model anisotropic phase to understand the influence of powder preparation routes and bulk microstructure (mean grain size) on the bulk coefficient of thermal expansion (CTE) and to compare it to the intrinsic CTE of powder samples. A co-precipitation route was used for the synthesis of pure single-phase nanopowders, while a polyvinyl alcohol-assisted sol–gel method was utilized for the synthesis of micron-sized powders. Sintered samples prepared from both powders exhibited different microstructures in terms of mean crystal sizes and porosity. Bulk samples obtained from nanopowders were highly porous and contained crystals of approximately 100-nm diameter, while the bulk pieces produced from the micron-sized powders were denser, contained crystals larger than 5 μm, and showed occasional intergranular and transgranular microcracks. Such different microstructures hugely impact the bulk CTE: the nanometric sample possesses a bulk CTE (0.9 × 10−6 °C−1, from 200 to 700 °C) closer to the instrinsic CTE (2.4 × 10−6 °C−1) than for the micrometric sample, which showed a negative CTE (−2.2 × 10−6 °C−1) from 200 to 620 °C, and an even more negative CTE above 620 °C (−35 × 10−6 °C−1). A finite element analysis showed that the local maximum thermal tensile stresses could be as high as 220 MPa when simulating a temperature drop of 700 °C as an example of thermal treatment following sintering. This tensile stress is expected to exceed the tensile strength of Al2Mo3O12, explaining the origin of microcracks in bulk samples prepared from the micron-sized powders. The thermal behavior of the microcracks leads to differences between the intrinsic and bulk thermal expansion; we show experimentally that such differences can be reduced by nanostructuring.