Cold-crucible technique

Arc furnaces (cold crucible technique). Small, commercial, laboratory arc-melting equipment generally includes a high-vacuum/aigon atmosphere pumping... 

In several cases authors have reported growing crystals by the recrystallization technique, but did not state a size for the crystals grown. Whittaker (1%8) prepared rods by a cold-crucible technique in which the samples were levitated and melted by high frequency induction. The rods were then heated by induction to 100°C below their melting point for 8 hours. Grains up to 20 mm in length and 7 mm diameter were prepared. In his studies Whittaker (1%8) noted an inverse relationship between the size of crystals grown by this technique and... 

Melt growth techniques can be classified into three sub-groups according to the method of melt containment hot crucible, cold crucible and crucible-less, and in the following discussion of individual methods an attempt will be made to identify the type of melt containment used in specific crystal growth experiments in the literature. 

Fig. 8. Schematic representation of the Czochralski technique (a) seeded growth from a hot crucible (b) random orientation growth by necking (c) induction heated cold crucible, and (d) tri-arc cold crucible.

In practice, crystal growing experience based on experimentation counts for much in choosing suitable growth parameters for the wide variety of physical properties to be encountered in rare earth metals and compounds. There are three main versions of the Czochralski technique currently used throughout the rare earth crystal growing community. One employs a hot crucible, the other two use cold crucible containment. 

The normal spectral emissivity Sn was measured by Lange and Schenck at 650 nm for the first time [39]. Since then, over ten measurements have been reported [40-50], as shown in Fig. 4.12. Shvarev et al. [40] reported that thermal emission from molten silicon can be explained by Drude s free-electron model. Pulse lasers have been used to melt silicon. This method has advantages, in that measurement can be carried out in a very short time and a furnace is not required [42-44]. However, it is difficult to measure the sample temperature accurately as long as laser heating is employed. Recently, measurements using a cold crucible or levitator have been attempted these techniques assure measurement conditions without optical contamination, because there is no crucible wall at high temperature, which causes disturbing emission and reflection [47-50]. 

Elements dissolved in boron influence its crystal structure. Dissolved impurities also influenee the physical and chemical properties of boron, especially the electrical properties, because boron is a semiconductor. Preparation of solid solutions in jS-rh boron requires a careful choice of crucible material. To avoid contamination, boron nitride or a cold, coinage-metal crucible should be used or the levitation or floating-zone melting techniques applied. 

Small solid seuaples can be analyzed directly by dynamic headspace sampling using a platinum coil and quartz crucible pyrolyzer and cold trap coupled to an open tubular column (341,369,379). This method has been used primarily for the analysis of mineral samples and of additives, catalysts and byproducts in finished polymers which yield unreliable results using conventional headspace techniques owing to the slow release of the volatiles to the headspace. At the higher temperatures (450-1000 C) available with the pyrolyzer the volatiles are more readily and completely removed from the sample providing for quantitative analysis. 

The technique and past history of skull melting has been described in several articles from this and other laboratories.12-19 Basically, radio frequency power from a commercial power supply (50 KW at 3-5 MHz) is transferred to a work coil tightly wrapped about a skull crucible consisting of a set of cold fingers that... 

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