Materials Science Forum Vol. 847

Abstract: It has long been recognized that a need for practical methods for maintaining optimum cesium pressure within a thermionic energy converter has been pursued for space applications. An attractive solution to the problem is the use of a sorption type reservoir where cesium absorbed into a lattice. Four graphite materials were initially investigated for the potential usage as cesium storage media. Primary research focusing on the characteristics of loading ability, structural integrity, and dimensional change of these materials was performed by synthesizing proper cesium-graphite lamellar compounds with a two zone vapor transport method. Graphite foams show promising prospect for thermionic application as integral cesium reservoir material.

Abstract: The tungsten products including tungsten electrode, tungsten filament, tungsten crucible have been widely used in national production. To study their properties at high temperature can provide a basis for improving the production process and the quality of processing as well as reducing production defects, which has important significance for the optimization of the performance and life extension of tungsten products. In this paper the development of tungsten and tungsten products is briefly introduced, the effects of many kinds of doped elements and doped compounds on high temperature mechanical properties is summarized, the research status of high temperature properties of tungsten products described in detail. And the development direction and application prospects for optimizing the high temperature performance of tungsten products has been out looked,in order to provide a reference for the research on the mechanical properties under high temperature of tungsten products.

Abstract: Tungsten is the dominant resources in China, and its reserves and consumption ranks the first place in the world. The tungsten electrode material, as an important manufacture of tungsten product, widely used in metal inert-gas welding, plasma welding, cutting, plasma spraying, is a kind of important industrial material [1]. The radioactive thorium tungsten electrode material is now gradually substituted by rare earth tungsten electrode material since it is excellent in thermal electron emission property. Through many years' study, we have mastered the key technology for the tungsten electrode material. After the extension and application of the key preparation technology, the authors group has developed the rare earth tungsten electrode materials and rare earth tungsten products, and the adjustment of industrial structure and upgrading of the product been realized.

Abstract: Pyrite (FeS₂) is an important semiconductor material which shows various excellent optical and electrical properties and extensive applied prospect as a new-type, photoelectrical functional materials. In this study, a low cost and efficient simple hydrothermal two-step synthetic method was given to obtain FeS₂ microspheres with 2-3 μm in diameter. The obtained products were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and ultraviolet and visible spectrophotometer (UV-Vis). XRD showed that the synthetic sample consisted of two crystal structures of FeS₂, pyrite and marcasite. SEM observation indicated that FeS₂ microspheres were well crystallized and had good uniformity. UV-Vis spectrum had a strong optical absorption in the region of 200-400 nm wave length. The reaction temperature had an impact on the size of FeS₂ microspheres. A possible mechanism for the size of the FeS₂ microspheres generated at high temperature is smaller than that at low temperature is discussed.

Abstract: Porous silicon (PS) was prepared with catalyst of Cu nanoparticles in HF/H₂O₂ solutions. The effects of the key fabrication parameters (amount of copper, etching temperature and concentration of H₂O₂) on the nanostructure of PS were systematically investigated and discussed. The experimental results indicated that the porosity of PS increased with the amount of copper, the reaction temperature and the concentration of H₂O₂, respectively. Copper can be used as catalyst to assist in etching silicon and the evolution process of copper nanoparticles was explored showing that the mass of copper experienced a drastic reduction in the first 60 seconds. A mechanism is proposed to explain the formation of PS by Cu-assisted chemical etching.

Abstract: As a promising absorber layer, semiconducting Cu₂ZnSnS₄ (CZTS) can be applied to replace the organic dye in dye-sensitized solar cell for photovoltaic application because of its good stability, suitable band gap energy (~1.5eV) and large absorption coefficient (~10⁴ cm^-1). In this paper, successive ionic layer adsorption and reaction (SILAR) method was utilized to deposit CZTS nanoparticles on mesoporous titanium dioxide (TiO₂). This solar cell with carbon film coated FTO glass slide as the back electrode showed an open circuit voltage of 380 mV. Preliminary results obtained for solar cells fabricated with this material are promising.

Abstract: CoMoB film was prepared on Si substrate via electroless deposition as the diffusion barrier for ULSI-Cu metallization. Annealing experiments of CoMoB(30nm) film and CoMoB(10nm)/Cu (40nm)/CoMoB(30nm)/SiO₂/Si multi-films were carried out in the temperature range from 400^◦C to 700^◦C. Failure temperature and mechanism of Cu diffusion in CoMoB film were discussed. The composition, sheet resistance and morphology of the film were investigated by X-Ray Diffractometer (XRD), Four Point Probe (FPP) and Atomic Force Microscopy (AFM), respectively. It can be concluded that the failure temperature of CoMoB film is 600^◦C. The main reason of failure is that a large number of Cu particles passed through CoMoB grain boundary and reacted with Si substrate to generate Cu₄Si with high resistance.

Abstract: Stain etching, one step metal assisted chemical etching (1-MACE) and two-step metal assisted chemical etching (2-MACE) were used for preparing porous silicon powders (PSPs) based on metallurgical silicon powder. The influences of different oxidants species and concentrations on the structure of PSP were discussed. The results indicated that the different oxidant species has an important effect on the morphology and structure of PSP. In stain etching, there is still a challenge for fabricating PSP with uniform and controlled pore size structure. In contrast, metal-assisted chemical etching method is easier to prepare PSPs sample with uniform depth and pore size than stain etching, In 1-MACE, the growth rate of the PSPs pore was between 0.05 and 0.10 μm/min, which is far less than that of 2-MACE (about 0.2~0.5 μm/min). Furthermore, 2-MACE showed more advantages than stain etching and 1-MACE in controlling of pore size range and structure.

Abstract: In this work, directional solidification was performed for multicrystalline silicon (mc-Si) ingot casting. The initial nucleation at the bottom of the silicon melt could be controlled by changing the cooling rate from 9 to 20μm/s. Metallographic microscope, X-Ray Diffraction (XRD), Microwave photoconductivity decay meter (μ-PCD) and four-point probe resistivity tester were used to investigate the microstructure, crystal orientation and electrical properties of the mc-Si ingots. The obtained results showed that cooling rate at 17μm/s is the optimum condition for the mc-Si ingots casting, under which the prepared ingot has lower dislocation density of 6×10^-3 cm^-2, better electrical properties, more uniformer resistivity distribution with an average value of 0.68 Ω×cm and higher minority carrier lifetime with a maximum value of 1.8 μs than that of in the other cooling rate conditions.

Abstract: A simple and efficient method has been developed to eliminate the internal stress in molybdenum oxides by an ultraviolet ozone treatment. The results of X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and Raman spectroscopy indicate that oxygen vacancy was a determining factor of the compressive stress in MoO₃, which can be released by ultraviolet ozone treatment. Based on this hole-transporting layer, the photovoltaic power conversion efficiency up to 3.91% was achieved, which is 22% higher than that without ultraviolet ozone treatment. And ultraviolet ozone treatment on MoO₃ is a useful method to embellish the interface to enhance the ability of collecting hole of hole-transporting layer to improve the performance of OSC with MoO₃ film as hole-transporting layer.