Papers by Author: Yi Tan

Abstract: Large temperature gradient was introduced to improve the removal rate of metal impurity in silicon ingot during direction solidification. The concentration of metal impurities in the silicon ingot with a large temperature gradient is 0.96 ppmw. The solidification time is reduced by 20% due to the fast speed of crystal growth improved; meanwhile the purity is increased by 64%.

Abstract: The microscopic morphologies of Bi₃₀Ga₇₀ immiscible alloy particles were investigated. Monosized microparticles with similar core-shell structures were fabricated for the first time by one step using the Pulsated Orifice Ejection Method. The EDS revels that the core and the shell consist of a Ga-rich phase (＞90 at. %) and a Bi-rich phase (＞80 at. %), respectively. The DSC testing at different temperatures is performed. Core-shell microstructures as well as endothermic peaks and exothermic peaks are observed after heating-cooling cycles when the working temperature is below the temperature of spinodal line, indicating good thermal stability after phase transformation. The thermal energy storage was preliminary tested, which is a good attempt for thermal energy storage. It is likely to use core-shell structures as microencapsulated phase change materials.

Abstract: Solar grade silicon (SoG-Si) is the basic material of photovoltaic industry. Metallurgical method as a way of purification isconsidered to reduce the cost of polysilicon production unanimously based on its low cost, short production cycle, relatively simple process, lowpollution and controllable scale size, and it has become a hot topic in the subject research around the world.In this paper Al-Si-Sn alloy is used to purifypolysiliconand influence on the purification efficiency of polysilicon with different compositions of Al-Si-Sn alloy is analyzed. In order to study the segregation of impurity B in purification process, we introduce final segregation coefficient k_B,final and analyze the relation between k_B,final and the content of Al,Snin Al-Si-Sn alloy thermodynamically.

Abstract: This article provides a method to determinate boron (B) impurity in silicon by inductively coupled plasma emission spectrometry (ICP-AES), in which the element spectral and analytical parameters were optimized. Three factors that influence testing results were discussed, including the amount of mannitol (C6H14O6) addition, concentration of nitric acid (HNO3) and evaporated temperature. As a result, the experimental parameters and conditions were optimized. The method of the detection limit, recovery and precision were all awarded with satisfactory results.

Abstract: A conventional hot-pressing method was used to produce Al/Al-4wt%Cu functionally graded material (FGM). Heat treatment, which included solid solution treatment (T4) and aging treatment (T6), was carried out on hot-pressed (F) specimens. The creep crack growth tests were performed under constant loading of the creep-testing machines. The distribution of copper composition was investigated by line analysis via electron probe microanalysis. Fracture morphology and creep crack paths were studied by scanning electron microscopy. During heat treatment, the thickness of the graded transition layer increased due to copper composition redistribution. Creep crack growth retardation was found when crack propagated from the graded transition region to the Al-4wt%Cu layer. Greater improvement in creep crack growth resistance was achieved by the T4 and T6 states of Al/Al-4wt%Cu FGM. For T4 and T6 state specimens, the micro-cracks and crack kinking in the transition region were observed, which prevented creep crack growth.

Abstract: Electron beam injection(EBI) is a process of gathering the electrons in materials using electron beam(EB). The EBI technology is proposed for purification of silicon particles by removing metal impurities through high-temperature oxidation, EBI, and HF acid washing processes. Analysis of silicon particle morphology after high-temperature oxidation using digital camera and after EBI using scanning electron microscope(SEM) were conducted. Then, the composition of silicon particles was analyzed using inductively coupled plasma(ICP). The silicon particle colours turned bright after EBI; therefore, EBI can change the thickness of SiO2 films in addition to increasing the temperature of the silicon particles. The results show that this technology is effective in removing metal impurities in silicon particles.

Abstract: A new method to evaluate the critical cooling rate, Rc of Fe-based metallic glass alloy was proposed and discussed. [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4 alloy particles were prepared with narrow size distribution and high sphericity by Pulsated Orifice Ejection Method in Ar, He and 50%Ar+50%He mixed atmosphere, respectively. Phase transition of a particle from amorphous to amorphous-crystalline and fully crystalline occurred with the increase of particle diameter. Rc of the formation of fully amorphous phase was estimated to be in the range of 700-1100 K/s, lower than that measured by time-temperature transformation diagram of bulk metallic alloy. No change of Rc occurred in Ar, He or 50%Ar+50%He mixed atmosphere, which proved it an effective method to evaluate the critical cooling rate of Fe-based metallic glass alloy.

Abstract: A simulation method was used to study the effects of physical parameters, including the contact angle between molten metal and material of orifice, surface tension and viscosity on particle formation of POEM. Droplets can be stably obtained only when the contact angel is at least 90° or larger, as well as the surface tension is adequate. Within a wide range, viscosity has little effect on droplet formation; as the viscosity increases, necking time is postponed and vibration time is shortened.

Abstract: Electron beam melting is an effective method to remove volatile impurities in silicon, during which impurities such as P, Al and Ca etc. can be removed to less than 0.3×10^-4wt.%. However, so far there is few research on the influence of electron beam parameters, such as beam density and beam size, on molten pool morphology, hence electron beam melting process has not been completely understood, which leads to low energy utilization. In this paper, on the basis of beam size calibration, the influence of beam density and beam size on molten pool morphology is investigated and the concept of melting angle is proposed to characterize molten pool morphology. At the same time, the optimal molten pool morphology for impurities removal and the corresponding electron beam parameters are also analyzed.

Abstract: The porous silicon layer was fabricated by electrochemical etching process using an aqueous HF-based electrolyte. The characterizations of porous silicon layer were investigated by Emission-type scanning electron microscope (SEM), Raman spectra and X-ray diffraction (XRD). With the current density increasing, the pore diameter and density become much bigger. This result also was confirmed by Raman spectra and XRD result of samples, which revealed the decreasing of grain size of silicon. The resistivity of crystalline silicon increased when the porous layer was removed after heat treatment at 850°C for 2.5h, which should be attributed to the gettering process of porous silicon.