Solar Cell Silicon: Synthesis, Production, and Refyning
Sponsored by: TMS Extraction and Processing Division, TMS: Recycling and Environmental Technologies Committee, TMS: Materials Characterization Committee
Program Organizers: Shadia Ikhmayies; Neale Neelameggham, IND LLC

Monday 8:00 AM
February 24, 2020
Room: Miramar
Location: Marriott Marquis Hotel

Session Chair: Shadia Ikhmayies, Isra University

8:00 AM Introductory Comments

8:05 AM
Molten Salt Electrolysis Production of Solar Silicon from Natural Quartzite: Aditya Moudgal¹; Sarat Buasai¹; Alexander McMahon¹; Yi Jie Wu¹; Adam Powell¹; Uday Pal²; Yu Zhong¹; ¹Worcester Polytechnic Institute; ²Boston University
    Polysilicon production for photovoltaics (PV) via the dominant Siemens process is costly due to numerous unit operations, energy-intensity, and inherent safety problems. Molten salt electrolysis reduction of silica could potentially achieve similar energy use and cost to the Hall-Héroult process which produces the world’s aluminum, i.e. 90% lower energy use and 80% lower cost vs. the Siemens process. Recently a new molten salt was engineered for this purpose comprised of CaF₂-MgF₂-YF₃-CaO-SiO₂ with low volatility <0.1 μg/cm²·s, low viscosity <5 mPa·s, high ionic conductivity >4 S/cm and SiO₂ solubility >5 wt%, and inert anode compatibility – an order of magnitude better on all metrics than prior work. This presentation will describe experimental and modeling work to engineer a silicon reduction process using this molten salt. The process produces solid silicon using periodic current reversal to suppress cathodic instabilities.

8:25 AM
Phase Diagrams of the Si-P Binary System: Shadia Ikhmayies¹; ¹Al Isra University
    Thermo-Calc software was used to produce the phase diagram the silicon-phosphorous (Si-P) binary system. The calculations were performed at a pressure of 1 bar and temperature range 300-2000 K. The temperature versus mass percent phase diagram showed three phases; Si (Diamond A4) solid solution, liquid, and SiP. There are two single phase regions and six double – phase regions. The melting points of Diamond Si and red P were determined and found to be Tm (Si) = 1687.0 K (1413.85°C), and Tm(red P) = 704.25 K (431.1 °C), which are close to those found in the literature. There is eutectic point at T = 1404.01 K (1130.86 °C) and Si mass percent of 65%. The metallic SiP line appears at 47.552 Si mass percent, and the melting point of SiP is Tm(SiP) = 1412.41 K (1139.26 °C).

8:45 AM
Thermo-calc Determination of the Phase Diagram of Si-B Binary System: Shadia Ikhmayies¹; ¹Al Isra University
    Thermo-Calc software was used to determine the temperature-composition phase diagram of Si-B binary system at a pressure of 1 bar. There are six phases; liquid, diamond-Si, β-B, SiB3, SiB6, and SiBn. In addition to the six single phase regions, the phase diagram shows nine mixed two-phase regions, which are all thoroughly discussed. There are two peritectic reactions; the first one is: Liquid + β-B ↔ SiBn at 2310.04 K and 8.165 Si mass percent, and the second is Liquid+ SiBn ↔ SiB6 at 2122.98 K, and 29.447 Si mass percent. The phase diagram also shows a eutectic reaction, which is: Liquid ↔ Diamond Si+SiB6 at 1657.56 K and 96.715 Si mass percent. In addition, it shows a peritectoid reaction, which is: Diamond Si + SiB6↔SiB3 at 1443.01 K and 99.715 Si mass percent. The melting point of β-B is 2347.85 K and that of diamond Si is 1687.00 K.

9:05 AM
Combustion Synthesis of Nanostructured Silicon: Sergio Cordova¹; Evgeny Shafirovich¹; ¹University of Texas at El Paso
    Nanoscale silicon (Si) is a promising material for many advanced technologies such as batteries, thermoelectrics, and solar cells. However, the current methods for fabricating this material are expensive, complex, and are difficult to scale up. The present work focuses on using mechanically activated self-propagating high-temperature synthesis (MASHS) of nanosilicon using silica (SiO₂) and magnesium silicide (Mg₂Si) as the precursors. During mechanical activation, SiO₂ and Mg₂Si are ball-milled together for a short time. Then the mixture is compacted and ignited, which results in a self-sustained propagation of the combustion wave over the sample leading to the formation of nanosilicon and magnesia. The magnesia is leached in diluted hydrochloric acid. Also, the addition of sodium chloride (NaCl) to the mixture, decreases combustion temperature and particle size of the formed Si.

9:25 AM
Zr Addition for Enhanced B Removal from Si by Si-Cu Solvent Refining: Yongsheng Ren¹; Kazuki Morita¹; ¹The University of Tokyo
    A novel attempt was made to remove B from Si by adding Zr as the trapping agent in solidification refining using a Si-Cu solvent. The premise of the study was based on the following: (i) low solid solubility of Cu in Si; (ii) significant density difference between Si and liquid Si-Cu; (iii) lower liquidus temperature of the Si-Cu system; (iv) high affinity of Zr to B for enhanced formation of boride. First, the solubility product of ZrB₂ in the Si-Cu melt was measured as 4.11 × 10⁻¹² (1345 K, Si-50 at.% Cu) and 2.55 × 10⁻¹² (1258 K, Si-57 at.% Cu). ZrB₂ precipitation was found at the bottom of the sample. Additionally, solidification refining experiments were carried out under different variables. The maximum removal fraction of B was 93.4%; further, the added Zr was almost completely eliminated and did not contaminate the refined primary Si.