T.B. Reed

Induction‐Coupled Plasma Torch

A new method of generating a stable plasma at atmospheric pressure using inductive coupling at a frequency of several Mc is described. Methods of starting and operating this plasma in argon, and mixtures of argon with helium, hydrogen, oxygen, and air are discussed. The Fowler and Milne method was used to measure the temperature profile of the plasma under various conditions of gas flow and composition, and at several power levels. Measured peak temperatures ranged from 14 000°–19 000°K. The power losses from the plasma in the form of convection, radiation, and conduction to the nozzle walls were measured under the same conditions. Total power transferred to the plasma ranged from 1.6–3.1 kw which was approximately 50% of the input power. The extent to which local thermal equilibrium prevails in the plasma is discussed; the available evidence indicates that under the operating conditions described herein, equilibrium is closely approached.

Growth of Refractory Crystals Using the Induction Plasma Torch

Apparatus is described for growing refractory crystals using the induction plasma torch in a Verneuil‐type geometry. The plasma torch makes possible crystal growth at higher temperatures than can be achieved with flames and in inert or reactive atmospheres. Conditions are given for growth of sapphire, stabilized zirconia, and niobium crystals.

TRI-ARC FURNACE FOR CZOCHRALSKI GROWTH WITH A COLD CRUCIBLE.

Abstract A crystal puller using the arc-melting technique has been developed for the Czochralski growth of single crystals. The circumference of the melt is heated by three arcs, and the crystal is pulled from the center of the melt with a water-cooled seed rod. The charge is contained in a hollow water-cooled copper hearth fitted with a piston so that the melt can be continuously fed. Use of this apparatus eliminates the difficulty of finding crucible materials suitable for high-temperature growth, since the melt is contained by a solid shell of the same substance in contact with the hearth. The furnace is economical, convenient to use, and requires a relatively small mass of material. Boules of Sn, Ge, Si, Cu2O, V, VO, Ti2O3, Ni, Nb, NbO, and TiC, with melting points between 232 and 3150°C, have been grown. A number of good quality crystals of NbO with various stoichiometries were grown for measurement of the electrical properties. Mass spectrographic analysis of grown crystals usually shows no copper or tungsten above 10 ppm, the limit of detection. Silicon crystals grown on a graphite hearth have a resistivity of about 10 ohm-cm. This suggests that the concentration of electrically active impurities introduced from the arc or hearth does not exceed 0.1 ppm. The tri-arc modification also has advantages for other uses such as ordinary arc melting, freeze purification, arc casting, “splat cooling”, arc-Verneuil crystal growth and zone refining.

Heat‐Transfer Intensity from Induction Plasma Flames and Oxy‐Hydrogen Flames

The heat transfer of flames generated in the induction plasma torch was measured with a water‐cooled copper split‐surface probe moved with respect to the flame. The resultant data were inverted mathematically to give the radial distribution of heat‐transfer intensity q.The distribution of q was measured for plasma flames as a function of distance from the nozzle and gas composition, using argon and mixtures of argon with oxygen or helium. For comparison, q was also measured for two oxy‐hydrogen flames. The peak q for the various flames ranged from 56 to 145 W/cm2, with total heat transfer ranging from 0.64 to 4.21 kW.

Diffusion and convection in vapor crystal growth

Abstract The dynamics of diffusion-limited vapor growth are analyzed for three cases: growth of microscopic crystallites immediately following nucleation, growth of crystals when vapor transport from the source is due entirely to diffusion, and growth of crystals when vapor transport is increased by natural or forced convection. In the convection case the rate-determining step is the diffusion of the vapor through a boundary layer in the gas phase adjacent to the growth interface. Forced convection decreases the thickness of this boundary layer and leads to rapid growth. It is shown that in this case the growth rate is proportional to the first power of the concentration difference between the source and the interface, and to the square root of carrier gas velocity. The growth rate expression contains one adjustable constant which depends on the geometry of the system. The stability of the growth interface is analyzed in terms of constitutional supercooling. It is shown that a heat source must be introduced between the source and the growth interface in order for a plane interface to be stable. The use of forced convection permits the introduction of a virtual heat source at the outer edges of the boundary layer. Crystals of iodine and camphor have been grown in a system in which forced convection makes it possible to control both carrier gas velocity and the temperature gradient at the interface. The observed growth rates exhibit the calculated dependence on concentration difference and gas velocity, provided that the interface gradient is suitably adjusted for smooth growth. A maximum smooth growth rate of 4 mm/hr was achieved for iodine and camphor, a value approaching rates of growth from the melt.

Growth of crystals of V2O3 and (V1−xCrx)2O3 by the tri-arc czochralski method

Abstract (V 1−x Cr x ) 2 O 3 for x = 0.0 to 0.018 crystals were grown by the tri-arc Czochralski technique. They were of good chemical purity and were found to have negligible solubility. For 0.010 ≲ x ≲ 0.015 there is evidence for the coexistence of both the paramagnetic insulating and the paramagnetic metallic phases.

Arc techniques for materials research

Abstract The paper discusses the use of thermal plasmas—in particular, the inert gas arc and cold hearth melting—in materials research. A simple laboratory size arc furnace is described, together with melting experiments and associated techniques—casting, quenching, temperature measurements, annealing and welding. Arcs are compared with other heat sources for materials preparation. The apparent intimate contact between molten specimen and cold hearth is explained in terms of thermal contact resistance and microscopic puckering of the interface.

Growth of EuO, EuS, EuSe and EuTe single crystals☆

Abstract Single crystals of the europium monochalcogenides weighing up to 60 g have been grown from Eu-rich solutions and from stoichiometric melts by using a gradient cooling technique, Since all four compounds have high Eu partial pressures near their melting points, the crystals were grown in weld-sealed tungsten crucibles 1.25 or 2.5 cm in diameter. The oxide was synthesized and grown in the same run. Dried Eu2O3 was heated with an excess of Eu metal to about 2200 °C, and the solution obtained was directionally frozen from the bottom by cooling to room temperature at 5–50 °C/hr. The resulting boules were single crystals with total impurities ranging from 250 to 5000 ppm. These crystals were then remelted, either with or without additional Eu metal, and directionally frozen to obtain crystals containing total impurities of about 200 ppm primarily C and Ca. The crystals were also used subsequently for growing crystals doped with Gd or La. The other compounds were synthesized in sealed quartz ampoules by the reaction of Eu metal at about 600 °C with vapor of the other element supplied by a reservoir at a lower temperature. Single crystals similar in size and purity to those of the oxide were then grown by the gradient cooling technique. The effect of deviations from stoichiometry on optical and electrical properties are discussed. The melting point of EuO, measured with an optical pyrometer and a W-(W, 26% Re) thermocouple, was 1980 ± 20 °C. EuS, EuSe and EuTe all melt between 2250 and 2500 °C.

Anomalous Transport Phenomena of Photoelectrons in the Ferromagnetic Semiconductor EuO

Anomalous transport phenomena of photoelectrons in pure EuO crystals have been studied over the temperature range from T =4.2 K to 80 K and in the magnetic fields up to H =50 kOe by using a fast-pulse technique with blocking electrodes. The Redfield geometry was used to determine directly the value of Hall mobility of photocarriers, µ H . Anomalous behaviours of photoconductivity such as an abrupt increase below the Curie temperature T c =69.5 K and a large negative-magnetoresistance effect around T c were observed. The value of µ H has been measured as a function of both the temperature and the magnetic field. It was directly found that the value of µ H varies with H around T c ; the µ H -value at T =63.5 K increases from 90 cm 2 /V. sec at H =0 to 160 cm 2 /V. sec at H =15 kOe. Thus, the anomalous transport phenomema in pure EuO crystals around T c have been experimentally confirmed, for the first time, to be mainly due to the mobility change. Experimental results are discussed in terms of the transport...

Growth of single Ti2O3 crystals from the melt

Procedures are described for growth of large single crystals of TiOx, (1.501 ⩽ x ⩽ 1.512) that are reasonably free of cationic impurities and of C and N. Data concerning the stoichiometry and the segregation of impurities during crystal growth are presented. The electrical properties of doped and pure single crystal materials are briefly described.