Coated boron layers by boronization and a real-time boron coating using an impurity powder dropper in the LHD

In the Large Helical Device(LHD),diborane(B2H6)is used as a standard boron source for boronization,which is assisted by helium glow discharges.In 2019,a new Impurity Powder Dropper(IPD)system was installed and is under evaluation as a real-time wall conditioning technique.In the LHD,which is a large-sized heliotron device,an additional helium(He)glow discharge cleaning(GDC)after boronization was operated for a reduction in hydrogen recycling from the coated boron layers.This operational time of 3 h was determined by spectroscopic data during glow discharges.A flat hydrogen profile is obtained on the top surface of the coated boron on the specimen exposed to boronization.The results suggest a reduction in hydrogen at the top surface by He-GDC.Trapped oxygen in coated boron was obtained by boronization,and the coated boron,which has boron-oxide,on the first wall by B-IPD was also shown.Considering the difference in coating areas between B2H6 boronization and B-IPD operation,it would be most effective to use the IPD and B2H6 boronization coating together for optimized wall conditioning.

Theoretical study of the effects of vacancy and oxygen impurity on Ti_2GaC

This paper presents the mono-vacancy formation and migration energies of each element Ti, Ga, and C in the MAX phase Ti2GaC, which are obtained by first principles calculations. We also calculate the formation energies of oxygen substituting for Ti, Ga, and C and two formation energies of oxygen interstitial in different sites. The results show that the formation energy of oxygen substituting for Ti is the highest, and the formation energies of the O substitution for Ga atoms decrease as the oxygen concentration increases. The two different formation energies of one oxygen interstitial show that the stable site for the oxygen interstitial is at the center of the triangle composed by three Ga atoms. The effects of vacancy,oxygen substitution, and the interstitial on the electronic properties of Ti2GaC are also discussed in light of the density of states and the electron charge density.

Evolution of infrared spectra and optical emission spectra in hydrogenated silicon thin films prepared by VHF-PECVD

A series of hydrogenated silicon thin films with varying silane concentrations have been deposited by using very high frequency plasma enhanced chemical vapor deposition （VHF-PECVD） method. The deposition process and the silicon thin films are studied by using optical emission spectroscopy （OES） and Fourier transfer infrared （FTIR） spectroscopy, respectively. The results show that when the silane concentration changes from 10% to 1%, the peak frequency of the Si-H stretching mode shifts from 2000 cm-1 to 2100 cm-1, while the peak frequency of the Si-H wagging-rocking mode shifts from 650 cm-1 to 620 cm-1. At the same time the SiH^＊/Ha intensity ratio in the plasma decreases gradually. The evolution of the infrared spectra and the optical emission spectra demonstrates a morphological phase transition from amorphous silicon （a-Si：H） to microcrystalline silicon （μc-Si：H）. The structural evolution and the p-c-SiH formation have been analyzed based on the variation of Ha and SiH^＊ intensities in the plasma. The role of oxygen impurity during the plasma process and in the silicon films is also discussed in this study.

Crystal Phases and Chemical Stabilities of YSi2 Powders Fabricated from Low and High Purity Si and Y Powders

Y-Si compounds with the composition of Y:Si = 1:2 were fabricated using Yttrium and Silicon raw powders with low and high purity in various atmospheres and temperatures. Although the latest Y-Si phase diagram shows that the α- and β-YSi2 phases are the stable phases for the stoichiometric composition of Y:Si = 1:2, the current experimental results suggest that the high temperature phase with the hexagonal structure, β-Y3Si5, would be the stable phase for this composition, and that the high temperature phase with the orthorhombic structure, β-YSi2, would be the meta-stable phase with high oxygen impurity content. It was demonstrated that YSi2 powders possess much superior chemical stability than Yttrium metal. It was found that the best dispersing solvent was 2-propanol for YSi2 powder.

Equiatomic 9-cation high-entropy carbide ceramics of the IVB,VB,and VIB groups and thermodynamic analysis of the sintering process

The preparation of high-entropy(HE)ceramics with designed composition is essential for verifying the formability models and evaluating the properties of the ceramics.However,inevitable oxygen contamination in non-oxide ceramics will result in the formation of metal oxide impurity phases remaining in the specimen or even escaping from the specimen during the sintering process,making the elemental compositions of the HE phase deviated from the designed ones.In this work,the preparation and thermodynamic analysis during the processing of equiatomic 9-cation HE carbide(HEC9)ceramics of the IVB,VB,and VIB groups were studied focusing on the removing of the inevitable oxygen impurity existed in the starting carbide powders and the oxygen contamination during the powder mixing processing.The results demonstrate that densification by spark plasma sintering(SPS)by directly using the mixed powders of the corresponding single-component carbides will inhibit the oxygen-removing carbothermal reduction reactions,and most of the oxide impurities will remain in the sample as(Zr,Hf)O_(2)phase.Pretreatment of the mixed powders at high temperatures in vacuum will remove most part of the oxygen impurity but result in a remarkable escape of gaseous Cr owing to the oxygen-removing reaction between Cr_(3)C_(2)and various oxide impurities.It is found that graphite addition enhances the oxygen-removing effect and simultaneously prevents the escape of gaseous Cr.On the other hand,although WC,VC,and Mo2C can also act as oxygen-removing agents,there is no metal-containing gaseous substance formation in the temperature range of this study.By using the heat-treated powders with added graphite,equiatomic HEC9 ceramics were successfully prepared by SPS.

Generation Mechanism of Inhomogeneous Minority Carrier Lifetime Distribution in High Quality mc-Si Wafers and the Impacts on Electrical Performance of Wafers and Solar Cells

To find out the causation of inhomogeneous minority carrier lifetime distribution in high quality multicrystalline silicon （mc-Si） wafers, impurities and lattice defects were systematically studied by means of Fourier transform infrared （FTIR） spectroscopy and metallography, Inhomogeneously distributed oxygen impurity and dislocations were demonstrated to be key leading factors, and the restriction mechanism was discussed. Scattering process caused by ionized impurities and dislocations decreased carrier mobility, while carrier concentration was not significantly affected. Measurements showed that resistivity was higher and more dispersive in low lifetime area. Solar cells were fabricated with these wafers. Cells＇ efficiency of inhomogeneous ones exhibited averagely 0.27% lower than the regular ones in absolute terms. Recombination centers and leakage loss induced by dislocations and impurities led to the reduction in shunt resistors and open-circuit voltage, and then affected the performance of cells.