Education Exchange
- PDF / 479,864 Bytes
- 2 Pages / 576 x 777.6 pts Page_size
- 42 Downloads / 210 Views
Low-Cost, Hands-On, Bridgman Crystal Growth Demonstrated for University Students at All Levels
fication rate. Due to the change in thermal conductivity, the heat flux propagates into the cold zone, encountering thermal resistance. The corresponding heat flow Editor's Note: With this article, MRS Bulletin zs expanding Education Exchange, which formerly diverges outward through the walls of the crucible. When the heat flux diverges, the focused on scientific educational experiences with local schools (K-12). The department now includes articles on community and university programs. This issue's article describes a university-surfaces of constant temperature, isolevel experiment on Bridgman crystal growth which was demonstrated at the Materials Research therms, curve up at the edges. The growth face occurs at the freezing temperature Society's 1995 Fall Meeting in Boston, November 27-December 1. isotherm. It should be noted that crystal growth propagates perpendicular to the isotherm representing the solidification Theory Introduction temperature.4 The crucible walls form a To understand the interface shape Our simple experiment in vertical thermal "short circuit" in the system. Bridgman-Stockberger (denoted as between the solid, liquid, and crucible Bridgman) growth can be easily and safe- wall, we refer to the thermal conditions in The thermal flux and thermal gradient Bridgman growth solidification configura- boundary conditions at boundaries 1, 2, ly performed by university students at all tions. The theory proposed by T. Jasinski levels or can be done as a demonstration. and 3 (from Figure 1) defines the ruling and A.F. Witt and later clarified by L. Inspired by M.A. Azouni's experiment,1 relationship. At the crucible wall (bounddefines the solid-liquid interface we use water for melt and ice for crystal. Holland aries 1 and 2), sufficiently close to the melt2 3 shape. " We use the solidification of water solid interface, the thermal gradients obey This system mimics semiconductors which have high conductivity melts (e.g., to model the Bridgman growth. For good (dT/dz)m=(dT/dz)s Si, GaAs, Te, HgCdTe). Our materials are crystallinity, a convex crystal face is (1) required as shown by the solidification of nontoxic and the temperatures are easily where subscripts m and s refer to the melt lead. We apply Holland's theory to underaccessible. We use a clear plastic tube for and solid phase of the load, respectively. stand the problems of II-VI, HI-V, and IV the crucible, mounted on an aluminum The z-component of the thermal flux, the semiconductor crystal growth whose cold finger secured in a Styrofoam plug in thermal conductivity (k) times the therthe top of a 20 1 liquid nitrogen dewar. interface propagates with a concave shape. mal gradient, at the solid-melt interface Since crucible, solid, and melt are transFigure 1 illustrates a typical Bridgman (boundary 3) obeys parent, students can view the solidificasolidification system. Bridgman solidification. The interface tends to be flat. The tion is directional cryst
Data Loading...
