Li₆Gd(BO₃)₃ (LGBO) crystal is an effective scintillator for neutron detection as well as a laser host and phosphor material. However, it is difficulty to grow large-size and high-quality crystals because of high viscosity of the melt and crystal cracking. Although LGBO crystals are grown by the Czochralski and other methods, the crystal size is small and the quality is poor for its practical application. The vertical Bridgman method is suitable for growing borate crystals due to its slow growth rate and small temperature gradient. This paper was to investigate the crystallization behavior of deep supercooling melt and grow LGBO crystals prepared by a modified vertical Bridgman method. In addition, the thermal properties of crystals were also analyzed to provide a reference for practical application.

LGBO polycrystalline materials were synthesized with high-purity chemicals (i.e., Li₂CO₃, 99.99%; H₃BO₃, 99.99% and Gd₂O₃, 99.99%) by a high-temperature solid-state method. LGBO crystal was grown by a modified vertical Bridgman method. The crystals were characterized by X-ray diffraction, ultraviolet excitation and emission spectroscopy, and differential scanning calorimetry. The thermal expansion coefficient of the crystals was measured by a model TMA 402 F1 thermomechanical analyzer. The thermal diffusivity coefficient of the crystals was measured by a model LFA 457 laser thermal conductivity meter, and the thermal conductivity of the crystals was calculated accordingly.

LGBO crystal was grown in a Platinum crucible with a seed well. Seed crystal was obtained via spontaneous nucleation in advance. The DSC curves of LGBO show that the melting point is 845 ℃, and the crystallization temperature is 731 ℃. Thus, the seeding and slow growth rates are the key issues for the growth of LGBO crystal because of the deep supercooling melt. The growth parameters are optimized with a <001>-oriented seed, a slope of 30 degrees at shoulder, a small temperature gradient of 30 ℃/cm at solid-liquid interface and a very slow descent rate of 2.4 mm/d. The transparent LGBO crystal with the sizes of 20 mm in diameter and 30 mm in length is obtained and the transmittance is close to 85% in a range of 320-800 nm. The excitation-emission spectra of the crystals are tested and the mechanism of luminescence is discussed.

The thermophysical properties of the crystal are investigated via measuring the specific heat capacity, thermal expansion, thermal conductivity and diffusion coefficient. Since the specific heat capacity of the crystal is large and changes significantly with temperature, the temperature change is relatively small when the crystal absorbs heat, and the ability to withstand thermal shock is high. The thermal expansion coefficient is an important parameter in the thermal properties of crystals, which has an important reference value for the selection of seed orientation and crystal processing in crystal growth. The gap between the thermal expansion coefficients of the two directions becomes larger with the increase of temperature. The thermal diffusivity and thermal conductivity are both physical quantities that characterize the heat transfer of materials. The thermal diffusivity of crystal phases (020) and (100) decreases with increasing temperature at 25-300 ℃. The thermal conductivity curve of LGBO crystal at 25-300 ℃ obtained by the thermal conductivity calculation formula indicates that the thermal conductivity of LGBO crystal is larger in the similar crystals.

Large-size LGBO crystals were grown by a modified vertical Bridgman method. The high viscosity of the melt and the extra-large supercooling determined the extremely slow growth rate of LGBO crystals. The specific heat capacity of the crystal at room temperature was 0.898 J/(g·K). The thermal expansion coefficients for the crystal phases (100) and (020) were 12.768×10^-6 K^-1 and 21.146×10^-6 K^-1, respectively. The thermal conductivities for crystal phases (100) and (020) at room temperature were 2.72 W/(m·K) and 2.60 W/(m·K), respectively. The thermal expansion had a great anisotropy as the temperature increased, and the thermal diffusivity decreased with the increase of temperature. The results indicated LGBO crystal under a high power could have a potential application due to its large specific heat.