Papers by Keyword: Crystal Growth

Abstract: Barium hexaferriteBaFe₁₂O₁₉single crystals hexagonal platelet shape and sizes of up to 8 mm were grown of lead oxide and sodium oxide based fluxes at 1260 °C. The unit cell parameters of single crystals grown using different fluxes is in good agreement with literature data. Substitution of Ba by Pb was detected, but with only negligible influence on unit cell parameters and Curie temperature.

Abstract: Tin monoselenide (Sn_0.3Se_0.7) crystals have been grown by direct vapour transport technique. These crystals were found exhibit orthorhombic structure with space group D2h16 (Pcmn). The electrical measurements such as thermoelectric power and resistivity were carried out at high temperature on these as-grown crystals. The parameters like See back co-efficient, Fermi energy and scattering co-efficient were calculated from experimental data of TEP. The primary studies show the semiconducting behaviour of these crystals. Activation energy was also calculated by employing Arrhenius equation for grown samples. These layered materials, due to different kinds of interactions exhibits anisotropic crystalline structure and this lead to an anisotropy in electrical transport. Anisotropy seems to be decrease with temperature for all grown crystals. A detailed study about electrical transport and anisotropy in all such crystals will be presented at the time of presentation.

Abstract: SiC powders having different purities were prepared by carbothermal reduction under different conditions from traditional process and SiC single crystals were grown by the PVT method from the powders. After crystal growth, boule was cut to wafers and they were polished for chemical and defect analyses. Total impurities including Al, B and Ti which were derived from powders decreased remarkably during crystal growth. The formation of defects including micropipe and dislocations such as TED, TSD and BPD was strongly influenced by impurity content. The effect of impurity seemed to be negligible at below 1ppm level for MPD. On the other hand, dislocations continuously decreased even more when higher purity SiC powder below 1ppm level was used.

Abstract: Lead and aluminum substituted barium hexaferrite (Ba,Pb)Fe_12–хAl_хO₁₉ single crystals were grown from lead oxide flux at 1260 °C as hexagonal plates with sizes of about 2 mm. A maximum substitution level of x = 4.82 was achieved for the first time for bulk single crystals. The variation of the unit cell parameters with Al content is in good agreement with literature data on exclusively Al substituted barium hexaferrite, while the substitution of Ba by Pb has hardly any influence. Similarly, Pb has only a negligible influence on the magnetic properties, while Al substitution significantly reduces the saturation magnetization in a very similar manner as known from Pb-free barium hexaferrite.

Abstract: In this work, Ga₂S₃ crystals were obtained by vertical Bridgman method. The presence of cracks in the grown crystals was interpreted as a result of phase transition into monoclinic structure during cooling. This suggests the use of another approach for the growth of high quality samples, e.g. chemical transport method or melt-solution method. Maximal transparency range of 0.48-22.5 μm and at least 10 times higher damage threshold to that for GaSe render anisotropic Ga₂S₃ crystal among the most prospective crystals for nonlinear applications.

Abstract: A large, crack-free ZnGeP₂ single crystal with size of Φ26 mm×70 mm was grown in a vertical three-zone tubular furnace by modified vertical Bridgman method, i.e. real-time temperature compensation technique with small temperature gradient in double-wall quartz ampoule. The as-grown single crystal was characterized by X-ray diffractometer (XRD), energy dispersive spectrometer (EDS), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). It was found that there is a face of (100) and its second-order XRD peaks were observed. The vertical elements distribution of the main part of the grown crystal has a stoichiometric ratio which is close to the ideal stoichiometry of 1:1:2. The IR transmittance of a sample of 2.5 mm thickness is above 58% in the range from 3500 to 800 cm^-1. All these results demonstrate that the quality of the ZnGeP₂ single crystal grown by the modified method is good, and could be used in the preparation of devices.

Abstract: Based on the technology of combustion synthesis in high-gravity field, the TiC-TiB₂ ceramics with refined microstructures have been fabricated by adjusting the technological parameter and proportioning of raw starting powders. The effects of particle size of raw powders on crystal growth and mechanical properties were studied in this paper. It found that the propagation rate and the combustion temperature can be improved by decreasing the grain size of raw powders, as well as the heat exchange and mass diffusion were enhanced too, so that the microstructure homogenization, densification and mechanical properties of the TiC-TiB₂ ceramics were improved accordingly. The highest density, relative density, Vickers hardness and fracture toughness of TiC-TiB₂ composites measured 4.07 g/cm³, 87.5%, 22.5 ± 1.2 GPa and 9.5 ± 0.8 MPa·m^0.5, respectively, since that the fine B₄C powder with particle size < 3.5 µm and the fine Ti powder with particle size < 38 µm were chosed as raw starting powders. The fracture behaviour of TiC-TiB₂ composites was dominated by the strong coupled toughening mechanisms of crack deflection and crack-bridging, and the high fracture toughness of the TiC-TiB₂ ceramics benefits mainly from the achievement of micro-nanocrystalline microstructure in the full-density solidified ceramic.

Abstract: In recent years, silicon solar cells continue to remain the main stream in photovoltaic (PV) industry, particularly of made from multi-crystalline silicon (mc-Si). The progress of crystal growth technology for mc-Si ingot using directional solidification (DS) is particularly significant. With the breakthrough of the so-called high-performance (HP) mc-Si technology in 2011, the mc-Si solar cell efficiency had increased from 16.6% in 2011 to 18 % or beyond in 2013. Nowadays, HP mc-Si, solar cells from a normal screen-printing aluminum back surface field (Al-BSF) production line could easily reach 18.3%. With the passivated emitter and rear cell (PERC) structure using PECVDalumina passivation, an average efficiency of over 19.2% could also be obtained. The emerging of HP mc-Si almost blocked the development of mono-like technology in 2012, and pushed p-type mono-Si cells to higher efficiency by using advanced technology. Unlike the conventional way of having large grains and electrically-inactive twin boundaries, the growth of HP mc-Si is from small and uniform grains having more random GBs. The grains developed from such grain structures significantly relaxes the thermal stress and suppresses the massive generation and propagation of dislocation clusters. Currently, most of commercial mc-Si ingots are grown by this concept, which could be implemented by seeded with small silicon particles or using nucleation agent coatings. The seeded growth has been well adopted in industry. However, the melting control of the seed layer and the thick red zone induced remain key issues in mass production. Several methods have been considered to resolve these issues with some success. The use of nucleation agent layers is a simpler approach, but the control of initial grain structures remains challenging.

Abstract: The influences of solution flow and lateral temperature distribution on the surface morphology of the 4H-SiC single crystal grown from solution was investigated. A flat surface region was enlarged by the seed-rotation rate. The solution flow simulation indicated that the higher rotation rate made the outward solution flow ordered beneath the solution surface. Such a solution flow was thought to be effective to enlarge the flat region of growth front. Furthermore, a full-flat surface was obtained with a hollow-type graphite rod at a seed-rotation rate of 60 min^-1. The simulated results of temperature distribution showed the hollow-type graphite rod reduced the lateral temperature gradient at the SiC-solution interface. The ordered solution flow and the small temperature gradient at the growth front were found to be effective to make the growth front flat in the solution-growth method.

Abstract: PbWO₄ crystals with different morphologies were readily induced by duck egg membrane via biomimetic synthesis at room temperature. The size and morphologies of the PbWO₄ crystals could be controlled by outer or inner surface of duck egg membrane, the reactant concentration and the reaction time. The results show that spherical, flower-like and spindle-like crystals were obtained on the inner surface of membrane while rhombic, hexagon-like and chrysanthemum-like crystals were gained on the outer surface with the concentration of Pb²⁺ and WO₄^2- increases. Room-temperature fluorescence spectra indicate the products on the inner surface of the duck egg membrane have a slight blue shift compared to that on the outer surface at the same condition. The PbWO₄ crystals with small size obtained at a lower reactant concentration present a better fluorescence performance. The exploration of the reaction mechanism reveals that the interaction between Pb²⁺ ions and the proteins on the surface of duck egg membrane can make the conformation of the proteins more ordered. In general, the present synthesis route may be extended to prepare other inorganic functional micro-materials.