雷 射 蒸 鍍

材 料 分 析

晶 體 成 長


  晶 體 成 長

Crystals grown by image furnace
(Traveling Solvent Floating Zone method)

The technique we are using is the so called Traveling Solvent Floating Zone (TSFZ) method. The major advantage of this technique is that crystals can be grown without getting into touch with other materials, such as crucibles, which eliminates a major source for contaminations. An infrared radiation furnace (model SC-15HD, NEC Nichiden Machinery) equipped with two ellipsoidal mirrors and two 1.5 kW halogen lamps positioned at the foci of the mirrors were applied in our lab. A picture of this furnace is displayed in Figure_1. With two halogen lamps (max 2×1500 W), this furnace can work at a temperature up to 2000 oC. Crystals with high melting point as well as a large length can be grown. The latter is particularly important in the case of incongruent melting systems, where large deviations between the nominal composition and that of the grown crystal are observed in the beginning of the growth experiment. Feed rod, melt and crystal are located within a sealed quartz tube, which allows performing crystal growth in different atmospheres.

The heating area forming the melt zone was at the foci of each mirror and was approximately 4 mm in width. The melt with solvent composition was suspended by surface tension between two rods. The surface tension is in reverse proportion to the temperature, which is dominated by the applied lamp power. The melt will drain away or collapse when the applied power is too high. On the other hand, the zone will shrink or the melt will even solidify when the lamp power is too low. The melt zone will sustain a curvature in balance between both rods after a fine tune of power is achieved. Unfortunately, the tuning tolerance of the lamp is only 0.1~0.3 V, and it probably corresponds to the temperature difference of only a few degrees. Therefore, this TSFZ growth process requires a tricky power control with painstaking care. Feed rod and grown crystal spin in opposite directions to guarantee a homogenous melt as well as a homogenous temperature distribution in the melt. As a result, a favorable slightly convex growth interface was maintained during the growth. As both rods were lowered simultaneously, crystallization occurred at the growth front between the lower rod and the melt. The optimum/maximum growth speed depends on the material and varies between a few tenth of a millimeter and several centimeters per hour.

Figure_1 Apparatus of the Floating Zone crystallization method.  Figure_2 A typical photograph of as-grown Bi2Sr2CaCu2Oy crystal boule. 

Figure_3 A photograph of Bi2Sr2RCu2Oy single crystals, where R represents Ca, Pr or Gd. Crystal (a) and crystal (b) are as-grown Bi2Sr2CaCu2Oy single crystals, and crystal (c) is Bi2Sr2GdCu2Oy single crystal. Crystals from (d) to (f) are Bi2Sr2PrCu2Oy single crystals. The actual size of a square in the background is 1 × 1 mm2. 


c2008 Company Institute of Physics, Superconductor Lab