Synthesis, characterization, and applications of shaped single crystals

Abstract

In this paper we will review techniques for the growth of single crystal mostly of insulating material which are shaped in the growth process. In particular, we will focus on the growth of single crystal fibers of optically activated materials; the emphasis will be placed on the so-called Laser Heated Pedestal Growth (LHPG) method of pulling crystalline fibers. LHPG offers a number of logistical advantages which can be exploited as a tool for materials research. Progress in the synthesis of materials using LHPG is described, as are spectroscopic techniques which are employed in characterizing the optical and physical properties of the crystal fibers obtained through this method.There has been interest on crystalline materials prepared in fiber form for a sustained period of time; this is partially because single crystal fibers occurring naturally in the form of whiskers often possess near ideal physical properties in crystallinity and in tensile strength. Earlier work addressed itself mostly to metallic materials, for example, in 1922 von Gompers [1] was successful in pulling single crystal metallic filaments directly from the melt. Later, in the 1950's, much of the work centered on the magnetic and mechanical properties of metallic whiskers [2]; however the size and composition of these could not be controlled accurately.One method which allows the growth of crystalline fibers is the so-called Stepanov method in which the melts are drawn through shapers and crystallization is made to occur after passage through a die [3]. Another method that enabled us to grow single crystal fibers of the desired length and diameter, in the proper crystallographic orientation and of proper composition and doping is the Laser Heated Pedestal Growth (LHPG) method of fiber synthesis [4].LHPG and the related float zone growth technique are micro-variants of the Czochralski growth method; the feed stoch used is generally in the shape of a rod and the melt is in the form of a self supporting bubble at the tip of the rod. A number of heating sources have been used to produce the melt; the most common method by far has been laser heating with focused single or multiple beams. A seed is dipped into the melt and is wetted by in; as the seed is pulled out, surface tension of the molten materials forms a pedestal around the seed, hence the name of pedestal growth. The melt is kept in place solely by surface tension, hence, this fiber growth method does not require crucibles and eliminates one source of sample contamination. This type of container-less growth also permits the synthesis of materials with extremely high melting points. The laser heated version of pedestal growth (LHPG) is illustrated in Fig. 1 [5].In a typical LHPG fiber pulling system, a stabilized CO 2 cw laser typically with an output of between 15-75 W is used as a heating source. The usual focusing and turning optics for the beam are shown in Fig. 2 along with the pulling and feeding mechanisms. The fiber pulling assembly may be enclosed in a vacuum-tight chamber allowin...

Experiment and simulation on interface shapes of an yttrium aluminium garnet miniature molten zone formed using the laser-heated pedestal growth method for single-crystal fibers

“…The main advantages of this crucible-free technique include high pulling rates, low production cost, and the feasibility of growing materials with very high melting points, high purity and low stress. The syntheses of a range of materials and properties of crystal growth have been explored using the LHPG method (Yen, 1999).…”

Microstructural Evolution in α-Al2O3 Compacts During Laser Irradiation

“…2009;Ferkel et al, 1997;Shishkovskii, 2005;Shishkovsky et al, 2007;Subramanian & Marcus, 1995;Subramanian et al, 1993). Among these methods are SLS of both α-Al 2 O 3 (Da Shen et al, 2007;Zhao et al, 2003) and α-Al 2 O 3 -α-Cr 2 O 3 (Nubling & Harrington, 1997;Quispe Cancapa et al, 2002;Yen, 1999).…”