Control of Melt Flow and Oxygen Distribution Using Traveling Magnetic Field during Directional Solidification of Silicon
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ORIGINAL PAPER
Control of Melt Flow and Oxygen Distribution Using Traveling Magnetic Field during Directional Solidification of Silicon Ingots Yue Shao 1 & Zaoyang Li 1
&
Qinghua Yu 2,3 & Lijun Liu 1
Received: 3 June 2019 / Accepted: 25 November 2019 # Springer Nature B.V. 2019
Abstract Traveling magnetic field (TMF) is a potential method to control the melt flow and impurity transport during the directional solidification (DS) of silicon ingots. We numerically study the control mechanism of a downward and an upward TMF with different frequencies on the silicon melt flow and oxygen distribution in the DS process. Model experiment of generation and measurement of a TMF is first carried out to validate the magnetic field computing program. Then, the TMF frequency is varied to study its influence on the direction and magnitude of Lorentz force, pattern and intensity of silicon melt flow, and distribution of oxygen impurity. Results show that the downward TMF with large frequency can induce local melt flow above the melt-crystal interface and block the oxygen transport from the dominant flow to the interface. The upward TMF with small frequency and large amplitude can induce thoroughly upward melt flow along the crucible side wall and take the oxygen far away from the meltcrystal interface, which is better for reducing the oxygen content in the silicon melt and ingots. Keywords Crystalline silicon ingots . Directional solidification . Traveling magnetic field . Melt flow . Oxygen impurity
1 Introduction Silicon ingots manufactured by the directional solidification (DS) method are the main materials for commercial solar cells [1, 2]. Oxygen is an undesirable impurity in the silicon ingots, as it can cause the light-induced degradation and decrease the solar cell efficiency [3]. In the DS process, the oxygen impurity is dissolved from the silica crucible wall, and transported by the silicon melt flow to the crystallization interface and the melt free surface for segregation and evaporation, respectively. Therefore, control of the silicon melt flow is a possible way to decrease the oxygen content in the silicon ingots. * Zaoyang Li [email protected] * Lijun Liu [email protected] 1
School of Energy and Power Engineering, Xi’an Jiaotong University, Xi′an 710049, Shaanxi, China
2
Birmingham Centre for Energy Storage, School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK
3
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, Hubei, China
Recent research has proven that the Lorentz force induced by a traveling magnetic field (TMF) can be used to effectively control the conductive silicon melt flow [4–6]. The TMF can generate a Lorentz force in the gravity direction and induce a meridional flow, which will strengthen or weaken the thermal buoyancy flow, and therefore significantly change the flow patterns and the impurity distributions. Some researchers have conducted numerical simulations and experimental measurements for the DS of silicon ingots under
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