Jia Yue Xu

Morphology-Controllable Synthesis and Characterization of ZnMoO4 Nanoparticles

In this work, nano-sized zinc molybdate (ZnMoO4) with several morphologies were prepared through introducing different template agents into hydrothermal system, in which ZnCl2 and Na2MoO4 were used as raw materials. Their structures and morphologies were characterized by XRD, FTIR and SEM techniques. Results showed that after constant temperature of 150°C for 4h, ZnMoO4 nanoparticles (~ 200 nm) were obtained in water medium. While in citric acid, CTAB and EDTA reaction templates, corresponding products were needle-like (400 nm *4000 nm), plate-like(350 nm*1500 nm*2000 nm) and microflower-like assembled by nanorods (300 nm*600 nm), respectively. All of them were monoclinic wolframite-type structure, and their photoluminescent (PL) spectra were also observed.

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.

Study on the SiC-Al Co-Continuous Phase Composites

SiC/Al co-continuous phase composites were fabricated by pressureless infiltration technology with reticulated silicon carbide porous ceramics as matrix and aluminum or magnesium as reinforcement phase. The effect of magnesium addition and holding time on the relative density, mechanical properties and macro morphology of the composites was investigated. The results showed that the porosity of SiC/Al co-continuous phase composites decreased with the increase of the holding time. The wettability of SiC and Al was improved with the addition of Mg. The hardness of SiC/Al co-continuous phase composites increased with the increase of the holding time. SiC and Al combined closely at longer holding time.

Fabrication and Properties of Zinc Silicate Phosphors through Solid State Reaction

Zinc silicate-based (Zn2SiO4:Eu3+) long afterglow phosphors were produced by solid state reaction method. The effects of borax and Eu2O3 additive on the properties of fabricated products have been studied. The results show that, there is not much difference in phase compositions within the borax additive amount; however, their SEM morphologies are different. Borax additive can increase the grain size of the product. Some sintering phenomena could be observed in the sample with Eu2O3 addition. The fluorescence spectroscopy results indicate that, the emission peak of the sample with Eu3+ additive located at 612nm, which may be a good candidate for red phosphor applications. The luminescent mechanism of Zn2SiO4:Eu3+ is also discussed.

Defects of GaAs Crystals Grown by the Pulling-Down Method

Compound semiconductor GaAs crystal was grown by the pulling-down method and the Φ2 GaAs boules were obtained. The growth defects, such as growth stripes, small facets, pits and dislocations, were observed. The pits on the surface of the boule was attributed to the water content of B2O3 encapsulant and the growth defects on the tail were associated with the evaporation of As due to GaAs decomposition. The average EPD (etch pit density) was measured less than 5000 cm-2, which shows the pulling-down method was a potential technique to grow high quality GaAs crystals.

Fabrication of VO2 Film in the Spin-Coating Way

VO2 thin films were obtained from V2O5 thin film in the thermal decompose process. V2O5 thin films were prepared in the spin-coating way and with V2O5 sol-gels as raw materials fabricated by the reformative inorganic sol-gel method. The effects of the viscosity of colloid on the fabrication of V2O5 film and the thermal decomposing temperature on the fabrication of VO2 film were studied. The results show that the appropriate viscosity of colloid to obtain well-distributed V2O5 film was in the range of 30g/L-40g/L and suitable thermal decomposing temperature was about 500°C.

Fabrication of Silicon Carbide Reticulated Porous Ceramics through Organic Foam Infiltration Process

Reticulated silicon carbide porous ceramics was fabricated through organic foam infiltration process by using silicon carbide slurry prepared from silicon carbide, alumina and clay and other raw materials. The effect of solid content on the viscosity and Zeta potential of the slurry were investigated. The results showed that the viscosity of slurry increased with the increase of the solid content. The absolute value of Zeta potential increased with the increase of pH value of the slurry from acid to alkaline aptitudes, and it became smaller with the further increase of the pH value. The maximum was reached at the pH around 10. The viscosity of slurry in the range of 4000mPas ~ 5000mPas was appropriate to prepare reticulated silicon carbide porous ceramics. The sintered body collapsed or cracked apparently, when the viscosity was too low. On the other hand, the viscosity could not be too high because its hard for slurry to impregnate foam. The performance of the samples coated once and recoated was compared. The porosity of the sample with one layer was higher than that of the samples with two layers. However, the recoating process improved the compressive strength and flexural strength of the reticulated silicon carbide porous ceramic.

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.

Preparation of CsI(Tl) Scintillation Film by RF Magnetron Sputter Method and its Structural and Optical Characterization

Thallium-doped cesium iodide CsI(Tl) scintillation film has been manufactured by radio frequency (RF) magnetron sputter method onto the quartz glass substrates. The X-ray diffraction (XRD) pattern of the film shows preferable growth of the crystalline film in the (200) orientation. The optical and scintillation properties of CsI(Tl) film were investigated, including photoluminescence excitation (PLE), photoluminescence (PL), X-ray excited luminescence (XEL) spectra and decay curve. The main emission peak at about 2.28 eV is related to the radiative relaxation from the strong-off configuration of localized excitons around Tl+ ions. Under UV excitation, the 2.28 eV emission of CsI(Tl) film presents a single exponential decay with 545 ns.

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.