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Study of the Removal Technology and Mechanism of Impurities in Tungsten by Electron Beam Melting

Author: LiuYe
Tutor: MaYunZhu
School: Central South University
Course: Materials Science and Engineering
Keywords: electron beam melting high purity tungsten finite elementsimulation interstitial impurity non-interstitial impurity removemechanism
CLC: TF841.1
Type: Master's thesis
Year: 2013
Downloads: 14
Quote: 0
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Abstract


Abstract:High purity tungsten has been widely used in semiconductor electronic components, because of the outstanding performances of electron mobility capabilities and high temperature stability. For the high purity materials, even a very small quantity of impurities will significantly deteriorate the final properties. To date, electron beam melting has been considered as one of the key processes for preparing high purity tungsten, and the determination of process parameters is the choke point of this technology. In this paper, the melting power (temperature) was calculated and analysed based on the heat balance, which was combined with the finite element method to simulate the temperature field in the electron beam melting process. Finally the removal effect and mechanism of impurities during the melting process were investigated.The morphology, organizational characteristics, purity and composition of the tungsten were observed and measured by scanning electron microscopy (SEM), Optical microscope (OM), inductively coupled plasma atomic emission spectrometry (ICP-MS) and energy dispersive X-ray spectroscope (EDS), respectively. The results showed that:(1) In this paper, the process of melting tungsten by electron beam was simulated, thereby determining the melting process parameters. And the experimental results matched the computer simulation very well.(2) The purity of tungsten bars was increased from99.93%to99.98%after electron beam melting, the average removal rate of interstitial impurities i.e. C and O reached45.8%and55.6%. The removal rates of non-interstitial impurities, such as Si, Al, Ni, Fe, As, Cd, K,Mg were52%,50%,50%,54.3%,90%,95%,75%and71.4%respectively, and there were two cases of the axial impurities distribution, As, Al, Cd, K, Mg, Ni, Mo uniform distribution, Fe, Si, Ca content is axially gradually increased from the bottom to up; The radial Cr, Fe and Ca content of tungsten ingot from the edge to the center of the bath was gradually increased.(3) The interstitial impurities C and O were mainly removed in the form of gaseous, whereas non interstitial impurities such as Cd, As and K were mainly eliminated by surface evaporation. The removal of Fe and Si were under the control of directional solidification, while the removal of the other elements was in the charge of evaporation and directional solidification.

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CLC: > Industrial Technology > Metallurgical Industry > Nonferrous metal smelting > Rare metal smelting > Refractory metals smelting ( high - temperature molten metal ) > Tungsten
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