Assessment of the Thermal Shock Resistance Figures of Merit of Al₂W₃O₁₂, a Low Thermal Expansion Ceramic

Abstract

Zero thermal expansion phases from the A 2 M 3 O 12 and related thermomiotic (negative thermal expansion) families are natural candidates for applications where high thermal shock resistance is the principal requirement. However, their mechanical properties are largely unknown, as are sintering routes for consolidation into bulk objects. Therefore, a preliminary case study on the effect of microstructure on mechanical strength and thermal shock resistance of Al 2 W 3 O 12 has been performed. All thermal and mechanical properties necessary for calculation of thermal shock resistance figures of merit have been measured experimentally. Tensile strengths were measured by four-point flexural test and analyzed by the Weibull method. The microstructure of bulk specimens, conventionally pressureless sintered at 1273 K, was coarse-grained, containing microcracks, and inhomogeneous with respect to density due to the agglomeration of nanoparticles, and led to low tensile strength. Despite this, thermal shock resistance features evaluated for Al 2 W 3 O 12 are encouraging. The Hasselman figure of merit for thermal shock resistance for severe heating conditions of Al 2 W 3 O 12 was 120 K, comparable to sapphire, the state-ofthe-art material for some advanced thermal shock resistance applications. This study shows that zero thermal expansion phases from the A 2 M 3 O 12 family have potential to be transformed into useful engineering ceramics for thermal shock resistance applications.

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

“…The impact of different microstructures with the same density (96% TD) on Young's modulus is outstanding (Figure ). Figure B shows the dependence of Young's modulus on density for the pellets obtained by TSS and SPS, and the literature values for samples prepared with conventional sintering . The Young's moduli of the TSS pellets followed a linear dependence on density found in conventionally sintered materials; however, the Young's moduli of the SPS pellets were considerably larger.…”

“…Based on this information, the stages of TSS were proposed to be executed at 900, 1075 and 950°C (for 6, 1 and 6 hours, respectively). Attempts to perform the first and the third stage of TSS at temperatures ≤820°C did not succeed in achieving densities higher than 92% TD, even when adopting much longer dwelling times (as high as 40 hours) …”

“…Prisco et al, reached 91% of theoretical density (TD) for Al 2 W 3 O 12 bodies with a coarse‐grained and inhomogeneous microstructure, starting from an initially agglomerated nanopowder synthesized by total evaporation, via conventional pressureless constant‐ramp‐and‐hold sintering. Despite the poor microstructure, this phase was demonstrated to be highly promising for applications that demand high thermal shock resistance . Jardim et al, studied the relationship between the density and mechanical properties (Young's modulus and hardness) of Al 2 W 3 O 12 in the range between 75% and 90% TD and reported a linear dependence.…”

“…Enhancement of the mechanical properties of near‐zero thermal expansion materials in consolidated form is fundamental for thermal shock resistance applications . The A 2 M 3 O 12 family of ceramics, where A and M are commonly occupied by trivalent and hexavalent cations, respectively, contains several phases known to have near‐zero thermal expansion.…”

Effects of low contents of A2M3O12 submicronic thermomiotic‐like fillers on thermal expansion and mechanical properties of HDPE‐based composites

“…In this context, Al 2 W 3 O 12 and Y 2 W 3 O 12 are the ideal candidates that fulfill these two requirements. Al 2 W 3 O 12 possesses low thermal expansion (CTE = 1.75 × 10 −6 °C −1 , for the temperature range between −6 and 800°C) , whilst Y 2 W 3 O 12 presents a negative thermal expansion (CTE = −7.0 × 10 −6 °C −1 , in the temperature range between −258 and 1,100°C) . The last phase is highly hygroscopic and forms a trihydrate monoclinic structure at room temperature (Y 2 W 3 O 12 .3H 2 O), nevertheless, it does not suffer amorphization and water molecules can be easily withdrawn from the structure .…”