Low-Temperature Sintering of High-Strength β-Eucryptite Ceramics with Low Thermal Expansion Using Li2O-GeO2 as a Sintering Additive

Abstract

Low‐temperature sintering of high‐strength β‐eucryptite ceramics with low positive coefficient of thermal expansion (CTE) was attained using Li2O–GeO2 sintering additive. High‐purity β‐eucryptite could be synthesized using high‐purity and fine amorphous silica, α‐alumina, and lithium carbonate powder mixture at 950°C via the solid‐state reaction route. Accordingly, the mixture was calcined at 900°C, pulverized, compacted, and sintered at 1020°C for 2 h with the sintering additive. The sintering temperature could be lowered by about 300°C, compared with the sintering temperature previously reported. The relative density of the sintered sample reached 99% of true density. Its flexural strength and indentation fracture toughness were 214 MPa and 2.5 MPa·m1/2, respectively. Its CTE (from room temperature to 800°C) was 1.2 × 10–6/K. Its dielectric constant was 5.5 with a loss tangent of 7.5 × 10–3. However, prolonged sintering time resulted in large negative CTE and mechanical weakness in the sintered samples. Microcracks were introduced with prolonged sintering time, which caused their large negative CTE with mechanical weakness.

Fabrication of near-zero thermal expansion of fully dense β-eucryptite ceramics by microwave sintering

“…Another way to obtain fully dense eucryptite ceramics was using Li 2 O-GeO 2 as a sintering additive [22]. Using microwave sintering method, high densities and comparable mechanical properties are obtained in a more simple method.…”

Thermal expansion of β-eucryptite in oxide-based ceramic composites

“…Consequently, hexagonal quartzderivative ␤-eucryptite structure exhibits only a slightly negative intrinsic CTE of −0.39 × 10 −6 / • C. However, CTEs ranging from +1.77 × 10 −6 / • C to −11 × 10 −6 / • C are reported on pure sintered ␤-eucryptite ceramics. 12,[27][28][29] Such highly negative values indicate the occurrence of spontaneous micro-cracking due to the ␤-eucryptite thermal expansion anisotropy. 30 The strategy of combining ␤-eucryptite and an oxide like alumina 31,32 or zirconia 33,34 to prepare a near zero CTE material has been already reported.…”

Microwave, Spark Plasma and Conventional Sintering to Obtain Controlled Thermal Expansion β‐Eucryptite Materials

“…β-eucryptite is the most negative thermal expansion phase in the lithium aluminosilicate ceramic system and, consequently, β-eucryptite has been thoroughly studied [5,6]. However, the study has focused on glass-ceramic materials [7,8], due to the difficulty in obtaining this system as a fully dense ceramic material in the solid state [3,9]. This is important because as far as possible obtaining glass-free dense materials would improve the mechanical properties such as modulus of elasticity compared with glass-ceramic.…”