Yan Qingzhi

Effect of helium implantation on SiC and graphite

Effects of helium implantation on silicon carbide (SiC) and graphite were studied to reveal the possibility of SiC replacing graphite as plasma facing materials. Pressureless sintered SiC and graphite SMF-800 were implanted with He+ ions of 20 keV and 100 keV at different temperatures and different fluences. The He+ irradiation induced microstructure changes were studied by field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM).

Comparison of Characteristics of Gd2BaCuO5 Powders Prepared by Different Processes

Abstract Ultra-fine Gd 2 BaCuO 5 (Gd211) powders were prepared by three different routes including low temperature combustion synthesis (LCS), coprecipitation and solid state reaction. The powders were characterized in terms of morphology, chemical purity, specific surface area and particle size. The results show that LCS process yields the most weakly agglomerated powder with smallest average particle size. The microstructure and superconductor property of GdBa 2 Cu 3 O 5 (Gd123) bulk material fabricated from the precursor of Gd211 powders were also studied. It is found that the employment of fine Gd211 starting powder is effective for the size reduction of Gd211 particles in the bulk samples. Gd123 bulk produced from LCS powder achieved the largest critical current density ( J c ) value compared with those from other two powders.

The influence of ablation products on the ablation resistance of C/C–SiC composites and the growth mechanism of SiO2 nanowires*

Ablation under oxyacetylene torch with heat flux of 4186.8 (10% kW/m2 for 20 s was performed to evaluate the ablation resistance of C/C?SiC composites fabricated by chemical vapor infiltration (CVI) combined with liquid silicon infiltration (LSI) process. The results indicated that C/C?SiC composites present a better ablation resistance than C/C composites without doped SiC. The doped SiC and the ablation products SiO2 derived from it play key roles in ablation process. Bulk quantities of SiO2 nanowires with diameter of 80 nm?150 nm and length of tens microns were observed on the surface of specimens after ablation. The growth mechanism of the SiO2 nanowires was interpreted with a developed vapor?liquid?solid (VLS) driven by the temperature gradient.

Atomic origins of solid helium bubbles in tungsten

Solid helium bubbles were directly observed in the helium ion implanted tungsten (W), by different transmission electron microscopy (TEM) techniques at room temperature. The diameters of these solid helium bubbles range from 1 nm to 8 nm in diameter with the mean bubble size about 3 nm. The selected area electron diffraction (SAED) and fast Fourier transform (FFT) images revealed that solid helium bubbles possess body-centered cubic (bcc) structure with a lattice constant of 0.447 nm. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images further confirmed the existence of helium bubble in tungsten. The present findings provide an atomic level view of the microstructure evolution of helium in the materials, and revealed the existence of solid helium bubbles in materials.