Synthesis of silicon microwire
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K. Han National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32306 (Received 21 June 2001; accepted 1 August 2001)
Silicon microwires have been synthesized by the Taylor microwire process. In this process, silicon is melted inside a glass tube by a local heating source and fine microwire is then drawn out by mechanical pulling. The silicon microwire is encapsulated in a silica glass coating. Flexible 10–25-m diameter polycrystalline silicon microwires were synthesized by this method in continuous lengths up to 460 mm.
With the miniaturization of many engineering applications, there is an increasing need for fine-scale products such as tapes and wires. There are a number of techniques based on vapor deposition of multilayer structures, rapid solidification, and wire drawing which have been used previously to produce suitable structures. However, an elegantly simple technique is the Taylor microwire technique of melting materials in glass tubes and drawing the tubes to obtain very small diameter wires.1–5 A schematic of the Taylor microwire approach is shown in Fig. 1. This approach is an experimentally convenient technique that combines both rapid solidification and deformation. In this technique, material is melted inside a glass tube and the softened glass with the molten material core is then mechanically drawn out into a fine microwire. Key aspects are that the working temperature of the glass be higher than the melting point of the material and that the material not be reactive with the glass. In many respects, the Taylor microwire process is similar to the drawing of optical glass fibers.6 To our knowledge, the Taylor microwire process has never before been applied to silicon. In this work we have demonstrated the feasibility of this process for the synthesis of silicon microwire.7 Since the melting point of pure silicon is 1410 °C, a higher temperature glass is required. The glass employed was Corning 7913 high silica (Vycor), which has a softening temperature of 1530 °C and a working temperature of approximately 1900 °C. An optical floating-zone single-crystal growth system was employed to heat the Vycor glass to its working temperature, so that the Taylor microwire could be pulled. Pure silicon chunks [Alfa Aesar, 99.9999% (metals basis)] were loaded into evacuated and sealed Vycor glass tubes. These Vycor tubes J. Mater. Res., Vol. 16, No. 10, Oct 2001
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were then heated to approximately 1900 °C using focused halogen lamp heating, and the silicon Taylor microwire was synthesized by mechanical pulling. Figure 2 shows the pulled morphology of the silicon microwire. Due to a fluid instability during the pulling of the molten glass, two parallel silicon microwires were synthesized. A maximum 460-mm continuous length of silicon microwire was produced in the present study. Because of the very fine diameter of the microwire, it possessed considerable flexibility under elastic strain conditions. The cross-sectional areas of
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