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Floating zone refining of thermal sensitive intermetallic compounds
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CURRENT TOPICS
Floating Zone Refining of Thermal Sensitive Intermetallic Compounds.An improved technique for growing single crystals of binary semiconductors which decompose on melting has been described by J. M. Whelan of Bell Telephone Laboratories. The basic experimental work was performed on gallium arsenide, but the method should be applicable to a variety of compounds which are thermally unstable at their melting points. According to the paper presented by Dr. Whelan to the 133rd National Meeting of the American Chemical Society in San Francisco, the floating zone method appears to be superior to other methods for growing GaAs single crystals. Composition of the liquid phase at the melting point is strongly dependent on the partial pressure of arsenic. This is most easily controlled by using a sealed system containing excess arsenic and regulating its minimum temperature. The rate of growth is easily controlled and spurious nucleation is greatly reduced by the thermal symmetry of the freezing interface. Other advantages of the method are the reproducibility, regularity of impurity distributions, and the minimization of apparatus contamination. The relatively large surface-to-volume ratio favors the approach to equilibrium between the liquid and vapor phases. In addition, the small liquid volume minimizes temperature variations in the melt, and the consequent concentration gradients of the principal components. The technique should be most useful with binary compounds in which only one of the component elements has a considerable vapor pressure at its melting point. The compound must have a high enough electrical conductivity to allow heating by radio frequency induction. The surface tension and density of the molten ma-
[J. F. I.
terial must also be such as to support a molten zone during the process. In the basic floating zone refining technique, a rod is supported vertically. A heat source, for example an induction coil operated at radio frequencies, is moved relative to the rod, melting a liquid zone as it moves. Surface tension supports the liquid zone. Usually by this method, a single crystal can be grown and purification achieved in the zone refining.
Supercharged Hydraulic Oil System Eliminates Surging.--The Yale & Towne Manufacturing Company has announced the perfection of a supercharged hydraulic oil system for lift trucks to eliminate surging in the hydraulic tank and the danger of getting air bubbles into the lines and pump. This new development is being made a standard integrated design feature in all Yale rider-type gas, LP-Gas and electric powered lift trucks. The presence of air in excessive amounts in a lift truck hydraulic system can lower hoist speeds, cause hoist cavitation and, in extreme cases, lead to pump failure. In the new Yale system, the free flow hydraulic return line brings the hydraulic oil into the tank at a midpoint on the inboard side. A metal pipe conveys the oil straight down into one end of a channel-shaped passage welded to the bottom of the tank. The suction line leading up out of the tank into the hydraulic system is located at the other end of this passage. Action of the hydraulic pump pulls the oil through the passage and into the suction line in a smooth, steady flow. A series of apertures along the sides of the channel passage bleed off excessive oil and pressure smoothly, preventing a bubbling action in the oil remaining in the tank.
CURRENT TOPICS
Floating Zone Refining of Thermal Sensitive Intermetallic Compounds.An improved technique for growing single crystals of binary semiconductors which decompose on melting has been described by J. M. Whelan of Bell Telephone Laboratories. The basic experimental work was performed on gallium arsenide, but the method should be applicable to a variety of compounds which are thermally unstable at their melting points. According to the paper presented by Dr. Whelan to the 133rd National Meeting of the American Chemical Society in San Francisco, the floating zone method appears to be superior to other methods for growing GaAs single crystals. Composition of the liquid phase at the melting point is strongly dependent on the partial pressure of arsenic. This is most easily controlled by using a sealed system containing excess arsenic and regulating its minimum temperature. The rate of growth is easily controlled and spurious nucleation is greatly reduced by the thermal symmetry of the freezing interface. Other advantages of the method are the reproducibility, regularity of impurity distributions, and the minimization of apparatus contamination. The relatively large surface-to-volume ratio favors the approach to equilibrium between the liquid and vapor phases. In addition, the small liquid volume minimizes temperature variations in the melt, and the consequent concentration gradients of the principal components. The technique should be most useful with binary compounds in which only one of the component elements has a considerable vapor pressure at its melting point. The compound must have a high enough electrical conductivity to allow heating by radio frequency induction. The surface tension and density of the molten ma-
[J. F. I.
terial must also be such as to support a molten zone during the process. In the basic floating zone refining technique, a rod is supported vertically. A heat source, for example an induction coil operated at radio frequencies, is moved relative to the rod, melting a liquid zone as it moves. Surface tension supports the liquid zone. Usually by this method, a single crystal can be grown and purification achieved in the zone refining.
Supercharged Hydraulic Oil System Eliminates Surging.--The Yale & Towne Manufacturing Company has announced the perfection of a supercharged hydraulic oil system for lift trucks to eliminate surging in the hydraulic tank and the danger of getting air bubbles into the lines and pump. This new development is being made a standard integrated design feature in all Yale rider-type gas, LP-Gas and electric powered lift trucks. The presence of air in excessive amounts in a lift truck hydraulic system can lower hoist speeds, cause hoist cavitation and, in extreme cases, lead to pump failure. In the new Yale system, the free flow hydraulic return line brings the hydraulic oil into the tank at a midpoint on the inboard side. A metal pipe conveys the oil straight down into one end of a channel-shaped passage welded to the bottom of the tank. The suction line leading up out of the tank into the hydraulic system is located at the other end of this passage. Action of the hydraulic pump pulls the oil through the passage and into the suction line in a smooth, steady flow. A series of apertures along the sides of the channel passage bleed off excessive oil and pressure smoothly, preventing a bubbling action in the oil remaining in the tank.