An extremely low thermal conduction ceramic: RE9.33(SiO4)6O2 silicate oxyapatite

Complex rare-earth silicate oxyapatite RE9.33(SiO4)6O2 (RE = La, Nd, Sm, Gd, Dy) ceramics have been synthesized and their thermal conduction characteristics investigated. When evaluated using a steady-state laser heat-flux technique under conditions ranging from room temperature to 1000 °C the materials demonstrated very low thermal conductivities (0.96-1.49 W m-1 K-1), especially Gd9.33(SiO4)6O2, which shows a value of 1.10-1.14 W m-1 K-1 in the measured temperature range. Phonon mean free path and Raman spectra were used to investigate the thermal transfer mechanism. The source of low thermal conductivity was determined to be the strong intrinsic scattering in the crystal cell, which is due to the phonon mean free path being on the inter-atomic level. Furthermore, a connection between the full width at half maximum Raman spectra and the thermal conductivity of RE9.33(SiO4)6O2 ceramics at room temperature was established. The insensitivity of the thermal conduction properties to temperature for RE9.33(SiO4)6O2 ceramics have allowed it to show great potential in high temperature thermal insulation applications.