Abstract: Deep ultraviolet （DUV） light sources have promising application prospects in fields such as ultraviolet lithography， high-resolution spectroscopy， precise microfabrication， biomedicine， ultra-sensitive detection， and photochemistry. In practical applications， it is necessary to use deep ultraviolet birefringent devices such as polarizer/beam splitter， polarization beam splitter， phase retarder， optical isolation， and beam displacement to generate， modulate， separate and control polarized light. Currently， there is an extremely limited supply of commercial birefringent crystals suitable for the deep ultraviolet wavelength range. Calcium metaborate（Ca（BO₂）₂， abbreviated as CB₂） crystal is considered as a kind of deep ultraviolet birefringent crystal with promising application prospects， due to their large birefringence， short ultraviolet transmission cutoff edge and high ultraviolet band transmittance. Although the thermal properties of crystals are of paramount importance for both bulk crystal growth and subsequent device applications， the thermal behavior of CB₂ crystal has not yet been systematically investigated. Previously， only limited thermal data were available， with merely the thermal expansion coefficient reported within a narrow temperature range from 323 K to 873 K. CB₂ crystals were grown by the Czochralski method， and their thermal properties were systematically studied in this study. The thermal expansion coefficient was measured employing thermal dilatometer， specific heat capacity and thermal diffusivity were characterized by laser flash analyzer along different principal crystallographic directions. Thermal conductivity was calculated combining the measured specific heat capacity， thermal diffusivity， and the theoretical density of the crystal. The research results indicate that thermal expansion of the CB₂ crystal exhibits obvious anisotropy， and the average thermal expansion coefficients along the crystallographica，b， andc axes respectively areα₁₁=4.52×10^-6 K^-1，α₂₂=3.44×10^-6 K^-1 andα₃₃=2.91×10^-5 K^-1 within the temperature range from 323 K to 723 K. The microstructure analysis indicates that this is mainly attributed to the differences in the strength of chemical bonds along the different crystallographic axes. specific heat capacity shows a steadily increasing trend as a whole， ranging from 0.937 J/（g·K） to 1.242 J/（g·K）， indicating that it has a relatively high laser damage threshold. The thermal diffusivity and thermal conductivity gradually decrease as the temperature rises. Within the temperature range from 298 K to 723 K， the thermal diffusivities areA₁₁=0.734~1.755 mm²/s，A₂₂=2.199~4.361 mm²/s andA₃₃=0.529~1.196 mm²/s， respectively. Correspondingly， the calculated thermal conductivities areκ₁₁=2.366~4.353 W/（m·K），κ₂₂=7.089~10.818 W/（m·K） andκ₃₃=1.781~3.088 W/（m·K）， respectively. The thermal conductivity of CB₂ crystal also exhibits strong anisotropy. Due to the strong covalent bond （B—O bond） connecting along theb-axis direction， the phonon group velocity is extremely high in this specific direction， resulting in the maximum thermal conductivity. The research results of this paper provide detailed and reliable basic data for optimizing the temperature field design of single crystal growth， suppressing crystal cracking， and designing the thermal management for deep ultraviolet high-power optical devices.

Key words: thermal property; calcium metaborate; birefringent crystal; thermal expansion coefficient; thermal diffusivity; thermal conductivity