Collection of Transparent Ceramics and Scintillation Crystal（202312）
Microwave plasma chemical vapor deposition (MPCVD) technology is an ideal way to prepare large size and high-quality single crystal diamonds. However, the complexity of MPCVD single crystal diamond growth and the diversity of crystal growth requirements make it difficult to optimize the growth process. To address this issue, a systematic design method for MPCVD single crystal diamond growth based on plasma diagnostic technology was proposed, using plasma imaging and spectral analysis to quantitatively diagnose microwave plasma. The physical coupling characteristics and quantitative relationship between pressure, microwave(MW) power, plasma properties, and substrate temperature were studied by using home-made MPCVD system. And the size of major axis, precursor group concentration and distribution, energy density, and other data of the plasma under different parameters were obtained. Based on experimental data, the growth process map of single crystal diamond was obtained. According to this map, we selected process parameters by growth temperature and growth area. Through experimental verification, it is shown that this map is usful for guiding prediction with parameter error of less than 5%. Simultaneously, based on the predicted map, growth of single crystal diamond under different plasma energy densitiesis studied. At lower power (2600 W), a higher energy density (148.5 W/cm3) was obtained, and the concentration of carbon containing precursors was higher than that of the other parameters, resulting in a higher growth rate (8.9 μm/h). By this method system, effective plasma control and process optimization can be carried out meeting for different single crystal diamond growth.
Dy3+-doped SrGdGa3O7 crystal was successfully grown through the Czochralski method and investigated in detail for its structural and optical features. Its crystallographic lattice parameters were optimized by Rietveld refinement based on XRD data. Polarized absorption spectra, polarized emission spectra, and fluorescence decay curves of Dy: SrGdGa3O7 crystal were analyzed. Absorption cross-sections at 452 nm corresponding to π- and σ-polarization were computed as 0.594×10-21 and 0.555×10-21 cm2, respectively. Calculated effective J-O intensity parameters Ω2, Ω4, and Ω6 were 5.495×10-20, 1.476×10-20, and 1.110×10-20 cm2, respectively. J-O analysis and emission spectra show that transition 4F9/2→6H13/2 of Dy: SrGdGa3O7 crystal has the highest fluorescence branching ratio and fluorescence intensity under 452 nm excitation within the visible spectral region, the emission cross-sections of π- and σ-polarization were 1.84×10-21 and 2.49×10-21 cm2 at the wavelength of 574 nm, respectively. The measured radiative lifetime and fluorescence decay time of the Dy3+: 4F9/2 level were 0.768 and 0.531 ms with a quantum efficiency of 69.1%. All these results reveal that Dy3+: SrGdGa3O7 crystal is a promising material for yellow lasers pumped with blue laser diodes.
The uniform growth of large-size optical grade diamond films has been one of the hot spots and difficulties in the field of microwave plasma chemical vapor deposition (MPCVD) diamond research, in which the structure and position of the deposition platform are crucial to the uniformity of diamond films and the long-term stability of thick film growth. Here, the effect of deposition platform height on the electric field uniformity, plasma state and temperature uniformity on the substrate surface was investigated by COMSOL simulation combined with experiments to optimize the process parameters for the uniform growth of optical grade diamond films. The 2-inch diamond film with thickness of 337 μm and inhomogeneity <11% was obtained at the optimal deposition platform height of 2 mm. The full width of the Raman half-peak from the center to the edge of the film is in the range of 3-4 cm-1, and the maximum transmittance is 69%-70% in the visible light band and 70% in the infrared light at 10.6 μm. This indicates that thickness and quality of the diamond film are relatively uniform, thus the uniform deposition of 2-inch optical grade diamond film is achieved. Above results show that the deposition platform height has a great influence on the electric field distribution and plasma state on the substrate surface, and the electric field uniformity on the substrate surface is significantly improved with the increase of the deposition height, but the influence on the temperature uniformity is smaller.
Transparent ceramic materials have excellent strength, hardness and optical properties, which have important application prospect in light-weight transparent protective armor. However, the preparation of transparent ceramic components with large protection area and high transmittance properties is the main challenge to achieve application. In this work, large-size yttrium aluminum garnet (Y3Al5O12, abbreviated as YAG) transparent ceramics with low deformation and excellent optical quality were fabricated by reactive sintering in vacuum using domestic high-purity Al2O3 and Y2O3 powders as starting materials, and the key technologies including dry pressing, calcining, high-temperature vacuum sintering and optical performance were broken through. In addition, as the upgrading of molding and sintering equipment, the dimension of YAG transparent ceramic was enlarged to 1040 mm×810 mm×15 mm, laying a substantial foundation for future applications.
Besides its application as nonlinear optical devices, La2CaB10O19 (LCB) crystal has been extensively studied as a host crystal due to excellent properties. Nevertheless, rare-earth (RE) ions doped LCB crystals for ultraviolet (UV) lasers have not been studied yet. In this work, Ce3+ doped La2CaB10O19 (Ce3+:LCB) crystal with the size of 40 mm×21 mm×6 mm was grown by top-seeded solution growth (TSSG) method. Its lattice parameters are slightly different from that of the LCB crystal, and its X-ray rocking curve indicates that the Ce3+:LCB is of high crystalline quality. Transmittance spectrum and UV absorption spectrum measured at room temperature show intense absorption in the ranges of 200-288 nm and 305-330 nm,and Sellmeier equations for the refractive indices were determined by least-squares method. The excitation and fluorescence spectra show that there are two broad excitation peaks at 280 nm and 316 nm, corresponding to transitions of Ce3+ ions from 4f to 5d. Four emission peaks were obtained at 290, 304, 331, and 355 nm, which correspond to transitions from 5d state to 2F5/2 state and 2F7/2 state. Ce3+:LCB crystal exhibits high thermal conductivity (6.45 W/(m·K)) at 300 K, and keeps good thermal stability with the increase of temperatures. Its thermal expansion coefficients and lattice parameters of c direction linearly enlarge from 2.94×10-6 /K and 0.91240 nm to 5.3×10-5 /K and 0.91246 nm in the temperature range from 358 K to 773 K, respectively. These results demonstrate that Ce3+:LCB crystal has excellent optical properties and good thermal stability, which is conducive to its application for UV lasers.
The Gd2O2S:Tb scintillation ceramics is extensively used for neutron radiography and industrial non-destructive testing due to its bright green emission, high intrinsic conversion efficiency and high thermal neutron capture cross-section. However, the existence of Gd2O3 secondary phase in Gd2O2S ceramics impedes the scintillation property. In this work, The Gd2O2S:Tb precursors were synthesized in water-bath with H2SO4 and Gd2O3 as starting materials. Molar ratio of H2SO4 to Gd2O3 defined as n was adjusted to synthesize the precursors., which influence on the properties of the precursors and powders was studied. Chemical composition of the precursors changes with the increase of n, from 2Gd2O3·Gd2(SO4)3·xH2O (n<2.00) to Gd2O3·2Gd2(SO4)3·xH2O (2.25≤n≤2.75), and to Gd2(SO4)3·8H2O (n=3.00). After being calcined and reduced, all the powders form pure Gd2O2S phase. Morphology of the Gd2O2S:Tb powders is closely related to the phase composition of the precursor. Increasement of the XEL intensity shows two stages with n increase, corresponding to the phase transition of the precursor, respectively. The Gd2O2S:Tb scintillation ceramics were therefore fabricated by vacuum pre-sintering and HIP post-treatment. The ceramics were fabricated from the powders prepared with different n, achieving high relative density and XEL intensity, except the ceramics fabricated from the powders prepared with the n=2.00, 2.25, 2.50. The increase of n is beneficial to the removal of the Gd2O3 secondary phase from the Gd2O2S:Tb ceramics. This work provides a way for eliminating the secondary phase in Gd2O2S:Tb scintillation ceramics.
Transparent AlON possesses good mechanical and optical properties, which shows great potential for application. However, high fabrication cost seriously restricts its wide usage. To solve this problem, gel-casting and pressureless sintering of transparent AlON was tentatively studied here, with emphasis on low temperature synthesis and anti-hydrolysis treatment of AlON powder. It was found that fine AlON powder could be readily synthesized at a low temperature of 1700 ℃ by a novel carbothermal nitridation technique, using polymer coated AlN/Al2O3 mixture as the starting materials. The powder obtained was submicron in size and its hydrolysis resistance could be significantly improved after surface coating with a polyurethane layer. On the basis of these findings, transparent AlON ceramics was successfully prepared through gel-casting and pressureless sintering. The material sintered at 1850 ℃ showed good optical and mechanical properties, with a high in-line transmittance of 83.1%-86.2% from ultraviolet to mid-infrared and three-point bending strength of 310 MPa.