Fluxing action of naf on oxidized silicon
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When Reaction 1 takes place above 650 ~ the reaction product consists of crystalline Si particles embedded in porous matrix of NaF. The Si particles range in size from submicron to a few millimeters, with an average value of 0.1 mm. Si can be extracted from the NaF matrix by heating the reaction products at temperatures above the melting point of Si (1412 ~ Molten Si particles sink through the NaF and coalesce into a pool beneath the NaF layer. In our investigation, the coalescence of the Si particles occurred rapidly, suggesting that the surface of each particle was very clean. Since the reaction products were stored in air, the Si particles were coated with a very thin layer of SiO2. The interaction between NaF and this SiO2 layer was responsible for the fluxing action that produced the oxide-free silicon. To examine the proposed interaction, we heated NaF to temperatures between 600 ~ and 1500 ~ in contact with preoxidized Si, as well as silica gel. A control heat treatment was applied to preoxidized Si in the absence of NaF. Products were analyzed after rapid quenching. MATERIALS Freshly crushed metallurgical grade (m.g.) Si (98.5 pct) was used with four average particle sizes prepared by crushing and sieving the resulting powder: 0.850 to 0.999 mm; 0.425 to 0.850 mm; 0.250 to 0.425 mm; and 0.150 to 0.250 mm. Part of the Si from these batches was purposely oxidized in air at 1000 ~ for 17 h. The thickness of the oxide layer was approximately 0.4/~m, as estimated by weight measurement assuming spherical particles. The NaF was reagent (Baker) grade and the SiO2 gel was gas chromatograph grade 30/60. EXPERIMENTAL Air-oxidized Si mixed with NaF in a 1:4 molar ratio to simulate SiF4-Na reaction products and mixtures of Si-NaF and SiOz-NaF were pelletized under 5,000 psi of L. NANIS, formerly with SRI International, Menlo Park, CA, is now Consultant, Electrochem. Systems, Palo Alto, CA. A. SANJURJO, is Materials Scientist, SRI International, Menlo Park, CA. S. WESTPHAL, formerly with SRI International, Menlo Park, CA is now Research Engineer at Fairchild, Palo Alto, CA. Manuscript submitted November 28, 1980. METALLURGICAL TRANSACTIONS B
pressure to form disks with 1.3 cm diam and 0.77 cm thick. The mixture was then loaded in a Grafoil-lined graphite boat, which was covered with graphite to minimize NaF loss by volatilization. The loaded boat was heated in an Ar atmosphere in an alumina tube furnace protected from NaF vapors with a Grafoil liner. The temperature was monitored by a Pt-PtRh (10 pct) thermocouple whose tip was placed in a well drilled into the base of the graphite boat. After the sample reached the furnace temperature, it was kept at temperature for a predetermined time interval, allowed to cool to approximately 600 ~ under Ar at the (cool) end of the furnace, and then quenched in air. The products were analyzed by X-ray, and examined with optical microscopy (OM), scanning electron microscopy (SEM), and X-ray emission spectroscopy (XES). Temperature, time at temperature, and particle size of the S
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