Yong Zheng Fang

Growth of Undoped GaAs Single Crystal by Pulling-Down Method

Undoped GaAs single crystal has been grown in PBN crucibles by the pulling-down method. The temperature profile of the furnace was optimized with a narrow melting zone and a small temperature gradient at the solid-liquid interface. Quartz ampoules were used to protect the evaporation of As during the growth and the deformation of the ampoule was discussed as a function of temperature, time and pressure differential. A Ø56 mm×70 mm GaAs crystal with nearly 100 % single crystalline yield was obtained. X-ray rocking curve analysis revealed the excellent crystalline quality. The average EPD and electrical properties of the crystal were tested comparable to those of the crystal produced by the VGF method. Therefore, the pulling down method was a feasible approach for GaAs crystal production.

Investigation on Mechanical Properties of 6H-SiC Crystal

In this work, the mechanical properties, such as Vickers microhardness Hv, fracture toughness Kc, yield strength σv and brittleness index Bi, of <0001> oriented 6H-SiC crystal are systematically evaluated using a microindentation technique under 0.1-2 kg applied load. It is found the Hv is decreased as the applied load is increased which is mainly attributed to the effect of indenter penetration. The Hv value can be effectively presumed by Kicks law and the Meyers index n is determined to be 1.73. However, the Kc value is measured nearly a constant (~0.148 MPa.m1/2) which reveals the toughness of 6H-SiC(0001) crystal is much weaker than those of Si(100) and GaAs(100) crystals. The variation of σv to the load is similar to that of Hv. The brittleness index Bi also exhibits deceasing tendency as the applied load is added.

Crack Behavior of Si-Doped GaAs Crystals Grown by Pulling-Down Method

The crack behavior of Si-doped GaAs crystals produced by a novel pulling-down method was investigated. Some cracks were observed in the crystal tail part and no twins or polycrystals were observed. The cracking mechanism was discussed considering the growth parameters, such as the pulling-down rate, annealing time and cooling speed of the furnace. The crystal was easily broken if the cooling rate was too fast. To avoid cracking, the temperature profile and the growth parameters had been optimized. The cleavage property of the GaAs crystal was strongly related to its atomic arrangement and corresponding electron density map. Ultrasonic vibration or mechanical machine would make the crystal cleaved along (110) plane. GaAs crystal displayed a strong anisotropic crack property under the force of microindentation test.

Study on the Relationship between Transparency and Mineral Compositions of Shoushan Stone

The Shoushan Stone is one of the famous craft carving stones in China and transparency is one of the key indicators to evaluate the value of Shoushan Stone. The better the transparency is, the higher the value is. In this study, the mineral compositions and morphologies were investigated by XRD and SEM, respectively. Combining with the characterization of Shoushan Stone samples, the relationships between mineral composition, morphology and transparency were studied. The results show that the transparency of Shoushan Stone is determined by the mineral categories and microstructures. The sample with high transparency is fewer mineral composition numbers, similar mineral compositions and bigger crystal particles

Chemical Strengthening of Erbium Phosphate Laser Glass by Ion Exchange

Two kinds of Er-doped lithium phosphate glass have been chemically strengthened by ion exchange in a mixed binary melt of KNO3 and NaNO3 at temperature far below glass Tg. Ion depth profile has been determined with WDS technique. The effect of acid wash on surface morphology, hardness and fracture temperature of glass was investigated. It was found that acid wash following ion exchange could improve the surface morphology and strength of glass. Nevertheless, WDS analysis showed that the acid wash could cause the ion diffusion from glass surface to acid wash liquid, i.e. the back diffusion of K+ and Na+. The results revealed that the back diffusion of K+ was severer than that of Na+ during acid wash and the bond of K+ to glass network was weak.

Study on the Relationship between Transparency and Morphology of Shoushan Stone

The Shoushan Stone is one of the famous craft carving stones in China and the transparency is one of the decisive factors to appraise its value. The better the transparency is, the higher the value is. In this paper, the mineral compositions and morphology of some Shoushan Stone samples were investigated by XRD and SEM methods. Combining with the characterization of Shoushan Stone, the relationships between the morphology and the transparency of Shoushan Stone were studied. The results show that the transparency of Shoushan Stone samples was determined by their microstructures, such as the mineral particle sizes and its arrangement, the gaps and porosity. The Shoushan Stone samples with high transparency are those with good arrangement, bigger crystal particle and less porosity and gaps.

Sr1-xNdXFe12O19 Ferrite Synthesized through Sol-Gel Self-Combustion

Sr1-xNdxFe12O19 ferrites were Synthesized via sol–gel and self-combustion methods with Fe(NO3)3·9H2O，Sr(NO3)2 and citric acid. The as-synthesized powder was studied by differential scanning calorimeter (DSC). X-ray diffraction (XRD) and vibrating sample magnetometer (VSM) were applied to investigate the structure and magnetic properties. The powder was synthesized in different calcination temperature and doping content of Nd2O3. The experiment reveals that the magnetic properties of Nd3+ doped ferrites are less sensitive to calcination temperature comparing with no doping ferrites. The coercivity (Hcj) of Nd3+ doped ferrites reach 423.96 kA·m-1.

Surface and Bulk Crystallization Kinetics of Er3+ Doped Mixed Alkali Phosphate Glasses

DSC measurements have been carried out for the as-quenched xNa2O- (15-x)Li2O-4B2O3-11Al2O3-5BaO-65P2O5 (x=0,3.75,7.5,11.25 and 15 mol%) glasses with different particle size. Two crystallization peaks appear on the DSC curves for sample sized 90-110μm. The presence of two crystallization peaks is due to different crystallization mechanisms, surface and bulk (internal) crystallization. The X-ray diffraction measurements are also employed to investigate the crystallization of glasses. The results show that bulk crystallization is difficult to occur in the studied phosphate glasses. The effect of mixed alkali on glass thermal stability is also studied in this paper. The surface and bulk crystallization active energies are calculated according to Kissinger equation.

Core Courses for Undergraduate Students in Materials Science and Engineering

As a professional with strong theoretical and practical in a new college, the guidance major-materials science and engineering involves extremely broad field. Our reform ideas and practices are only to training science-based applied talents possessing engineering capabilities. Facing the new situation on the demand for materials engineering personnel, we reform curriculum and teaching to get rid of the drawbacks of the original content and build the professional training program with the characteristics and course system.