Yi Tan

Resistivity Distribution of Multicrystalline Silicon Ingot Grown by Directional Solidification

The effects of impurities on the resistivity distribution and polarity of multicrystalline silicon ingot prepared by directional solidification were investigated in this article. The shape of the equivalence line of the resistivity in the vertical and cross sections was determined by the solid-liquid interface. Along the solidification height of silicon ingot, the conductive type changed from p-type in the lower part of the silicon ingot to n-type in the upper part of the silicon ingot. The resistivity in the vertical section of the silicon ingot initially increased along the height of the solidified part, and reached its maximum at the polarity transition position, then decreased rapidly along the height of solidified part and approached zero on the top of the ingot because of the accumulation of impurities. The variation of resistivity in the vertical section of the ingot has been proven to be deeply relevant to the distribution of Al, B, and P in the growth direction of solidification.

Effect of Pulling Rate on Multicrystalline Silicon Ingot during Directional Solidification

In this paper, the structure and composition of multicrystalline silicon ingots prepared by directional solidification with different pulling rates were analyzed to investigate the effect of pulling rate on the multicrystalline silicon ingot. The results showed that the lower pulling rate will make the site taking place constitutional supercooling move to the upper part of ingots and make the high purity area become larger. Lowering the pulling rate will decrease the impurity effective segregation coefficient and the solid-liquid interface curvature.

Calcium Evaporation from Metallurgical Grade Silicon by an Electron Beam Melting

In order to investigate Ca evaporation behavior in the electron beam melting process, metallurgical-grade silicon was melted in an electron beam furnace with different experimental conditions. The results showed that the content of impurity Ca was significantly decreased in the early time, while these changed slowly with the extension of the melting. The removal rate of Ca was controlled by the transfer of Ca atoms from the bulk liquid silicon to liquid/gas phase interface within the range of experiment temperature.

Effect of the Si3N4 Coating on the Mc-Si Ingot in the Process of Multicrystalline Silicon Purification by Metallurgical Methods

The Si3N4 protective coating has an important impact on avoiding melting silicon from contacting with the crucible wall directly. A mixed Si/Si3N4 layer was formed on the interface of silicon and Si3N4 coating, and the declination of N content was observed in this mixed layer. With the ingots condition of 1500oC for 2 h, the large Si3N4 and SiC particles appeared in the mixed layer and the formation mechanism was discussed. The Si3N4 coating had significantly increased the lifetime of minority carriers by decreasing impurity content.

Effect of Microstructure on Mechanical Properties of Ti-Al Based Alloys

Three types of TiAl-Nb ternary alloys are obtained by arc-melting and heat treatment, which are γ-TiAl single phase, γ-TiAl + α2-Ti3Al duplex phase and γ-TiAl +α2-Ti3Al +Nb2Al multiple phase alloy. The phase stability is studied using X-ray diffractometer and electron probe microanalyzer. Mechanical properties are investigated through compress and tensile tests at RT to 1373K. It is found that, their mechanical properties are related to their microstructures. The deformability of alloy B is better than that of alloy A and C at RT due to small grain size reduced by α2 phase appearance, lamellar microstructure and interfaces. The deformability of alloy B increases significantly with increasing temperature, the elongation of alloy B achieves 40.4% at 1173K. The fracture mode of alloy B changed from brittle transgranular failure at room temperature to ductile intergranular failure at 1173K. While, both of alloy A and C have not increased its deformability at 1173K and showed brittle transgranular failure at both temperatures. Alloy C was a very brittle material at room and high temperature due to Nb2Al phase appearance, which reducing the continuity of (γ+α2) lamellar structure.

Investigation on the Removal of B Impurity in Metallurgical Grade Si by Electron Beam Melting

The growth in the solar energy technology caused inshortage solar grade Si. As a lowcost, environmental friendly technology, metallurgical method purity silicon is developed significantly. However, as a typical impurity in Si, B is difficult to be removed by directional refining or vacuum melting due to its large segregation coefficient and less evaporation coefficient. In this paper, the big difference of evaporation pressure between Si and B can be applied to separate B from Si, in which, B is remained in molten Si, while most of Si becomes evaporant. Electron beam is applied to scan molten Si and the Si existed in the form of the evaporant is gather on the watercooled crystallizer. The content B in the evaporant is undetectable by ICP-MS.

Effects of Impurities Distribution on the Crystal Structure and Electrical Properties of Multi-Crystalline Silicon Ingots

Multi-crystalline silicon ingots were prepared by directional solidification using vacuum induction melting furnace. The content of aluminum and iron deeply decreased in the columnar crystal region of the multi-crystalline silicon ingots. The columnar crystal growth broke off corresponded to the iron contents sharply increased. The height of columnar crystal in the silicon ingots related to the pulling rates had been clarified by the constitutional supercooling theory. The maximum of the resistivity and the minority carrier lifetime closed to the transition zone where the conductive type changed from p-type to n-type in silicon ingots. Further analysis suggested that the electrical properties were related to the contents of shallow level impurities aluminum, boron and phosphorus.

Fabrication and Properties of Al/Al-Cu Functionally Graded Material

Five-layered Al/Al-Cu functionally graded material (FGM) was fabricated by powder metallurgy technology. The microstructure and composition of the prepared specimen were studied. Vickers hardness, flexural strength and fracture surface morphology were also measured. The results showed that Al/Al-Cu graded material with dense structure and compositional continuous change was obtained by solution-precipitation method. The graded materials presented a compositional continuous change along the graded direction because of the diffusion effect, and the Vickers hardness was liner proportional to the distribution of Cu content. Compared with pure sintered Al, remarkable improvement on hardness and fracture strength was achieved due to the CuAl2 phase dispersively distributed in the matrix. With the increase of Cu content, the fracture mode changed from tough fracture to the tendency of brittle fracture.

Effect of Heating Treatment on the Resistivity of Polycrystalline Silicon

Effect of heat treatment in atmosphere on the resistivity of polycrystalline silicon has been investigated in this paper. After heat treatment at 1050oC for 10h, there is no obvious change of the resistivity in the N-type region of polycrystalline silicon, which could be contributed to the complicated influence factors, such as more impurities content and defects. On the other hand, an obvious increase of the resistivity was observed in the P-type region which could be contributed the redistribution of Al and B in the Si-SiO2 interface. The resistivity of the P-type region increased from less than 1Ω·cm to several hundreds Ω·cm.

Microstructure and Mechanical Properties of AlN Ceramics Sintered with Yttrium Nitrate

AlN ceramics were prepared by hot-pressing process with Y(NO3)3·6H2O as a sintering additive. Microstructure and mechanical properties of sintered AlN ceramics were measured and compared with that of monolithic AlN ceramic prepared by the same process. The results showed that the density and mechanical properties of monolithic AlN ceramics were drastically enhanced by adding Y(NO3)3·6H2O. Especially, the AlN ceramic with 3.24 wt% Y(NO3)3·6H2O (2 wt% Y2O3) displayed striking enhancement in mechanical propertie. However, such enhancement did not follow a consistent behavior with the increase of Y(NO3)3·6H2O content. A significant drop present at contents over inflection point (3.24 wt% Y(NO3)3·6H2O).