Correlation between crystallinity and thermal expansion in LiAlSiO4 ceramics prepared by spark plasma sintering

Abstract

LiAlSiO4 (LAS) ceramics are prepared by using the sol-gel method followed by spark plasma sintering. XRD patterns and SEM images verify that the ceramics contain amorphous and LAS phases and that microcracks appear in the sample prepared at 900 °C due to its larger grain size. Compared with applied pressure and soaking time, sintering temperature has a greater impact on the crystallinity and density of the ceramics during sintering. High-temperature XRD results reveal that the LAS phase exhibits its intrinsic negative thermal expansion independently in all samples regardless of crystallinity. The coefficients of thermal expansion (CTE) measured by the dilatometric method change from positive values in samples prepared at 600 and 650 °C to near zero in samples prepared at 700 and 800 °C and then to a negative value in the sample prepared at 900 °C. The combined effects of an amorphous phase with a positive CTE and the LAS phase with a negative CTE are responsible for the observed transformation of thermal expansion in the samples. The calculated total CTEs of the glass-ceramic bulks are in agreement with the results measured through the dilatometric method in samples prepared at 650–800 °C. Microcracks in the sample prepared at 900 °C cause a more negative bulk CTE than the calculated CTE.