Study On Preparation Of C54-TiSi₂ By Electromagnetic Directional Solidification Of Hypoeutectic Ti-Si Allo

C54-TiSi2 is a high melting point and low resistivity silicide,which can be used as a basic material for aerospace,ultra-large scale integrated circuits and microelectronic device preparation.The traditional methods of preparing C54-TiSi2 include chemical deposition,high-temperature self-propagation and solid-phase reaction methods,which have the drawbacks of complex reaction mechanism and the reaction process is not easy to control,which easily lead to other Ti-Si intermetallic compounds and C49-TiSi2,making the resistivity of C54-TiSi2 higher and cannot be applied to high-end materials.It is necessary to develop a C54-TiSi2 preparation method that avoids the generation of other Ti-Si intermetallic compounds and C49TiSi2.The electromagnetic directional solidification technique,which enhances phase separation and impurity removal,is commonly used for the preparation and purification of metals or metal compounds.Therefore,experiments for the preparation and purification of C54-TiSi2 were carried out based on the phase separation of electromagnetic directional solidification,solidification and precipitation characteristics of C54-TiSi2,and adjustment of melt composition.The C54-TiSi2 products were successfully prepared while avoiding the formation of other intermetallic compounds and C49-TiSi2.The main research of this paper is as follows:(1)Theoretical study of TiSi2 generation.Based on the alloy phase diagram,the principle of directional solidification,the principle of electromagnetic reinforced mass transfer and the equilibrium bias coefficient model of impurities,the laws of TiSi2 phase and impurity phase generation from different alloy melts are discussed.Theoretical analysis shows that electromagnetic directional solidification technology can be used for the preparation and separation of TiSi2 from high-silicon Ti-Si alloy melt,and the sub-eutectic Ti-Si alloy composition is more conducive to the precipitation of TiSi2.(2)Experimental study of electromagnetic directional solidification of subeutectic Ti-Si alloy melt to separate C54-TiSi2.The lattice parameters of C54-TiSi2,the phase separation law and the quantification of impurity bias were confirmed by analyzing the XRD physical phase diffraction,microstructure and impurity bias thermodynamics of the enriched layers.The results showed that C54-TiSi2 was successfully prepared during the electromagnetic directional solidification of Ti-53.98 wt.%Si melt,and no other Ti-Si intermetallic compounds and C49-TiSi2 were found to exist.The purity of C54-TiSi2 in the L1 layer of the ingot could reach more than 99.66 wt.%at a pull-down speed of 3 μm/s.The best removal rates of impurities Fe and Al were 90.36%and 84.78%,respectively.The equilibrium segregation coefficients of Fe,Al and Si were calculated as kFe=2.25×10-4,kAl=2.69× 10-3 and kSi=2.2×10-1,respectively.Since the segregation coefficients of Si are much larger than those of Fe and Al,resulting in a much larger content of Si than Fe and Al at the same height,Si is the key impurity that hinders further purification of C54-TiSi2.(3)Experimental study on the purification of C54-TiSi2 by de-Si in NaOH solution.Based on the microscopic morphological characteristics and distribution pattern of Si in C54-TiSi2,thermodynamic analysis and experiments with different alkali leaching parameters were carried out to determine the optimal alkali leaching parameters and the purity and performance parameters of C54-TiSi2 after leaching.The results show that the content of impurity Si increases with the increase of ingot thickness,and the morphology of Si is determined by the intercrystalline pores of C54-TiSi2.Leaching experiments show that alkali leaching can desiliconize without affecting the lattice structure of C54-TiSi2 crystals.With the optimal leaching parameters of 15%NaOH concentration,20 min leaching time,70℃ reaction temperature and 4:1 liquid-to-solid ratio,the purity of C54-TiSi2 crystals reached more than 99.5%,and the removal of Si and Al impurities reached 99%and 90%.In addition,the electrical resistivity of C54TiSi2 was reduced from 8.35×10-5 Ω·cm to 4.75×10-5 Ω·cm after alkali leaching,which improved the electrical properties of C54-TiSi2 crystals.This study provides a new method for the preparation of C54-TiSi2 without producing other Ti-Si intermetallic compounds and C49-TiSi2,whose resistivity after simple alkali leaching purification is much lower than the commercially available C54TiSi2 parameters required.