Sandor L. Lehoczky

Carbon nanotube growth on carbon fibers

Abstract Carbon nanotubes (CNT) were synthesized on graphite fibers by thermal CVD. On the fiber surface, iron nanoparticles are coated and act as catalysts for CNT growth. The growth temperature ranges from 550 to 1000 °C at an ambient pressure. Methane and hydrogen gases with methane contents of 10–100% are used for the CNT synthesis. At high growth temperatures (>800 °C), the rapid inter-diffusion of the transition metal iron on the graphite surface results in a rough fiber surface with no CNT grown on the surface. When the growth temperature is relatively low (650–800 °C), CNT are fabricated on the graphite surface with catalytic particles on the nanotube top ends. Using micro Raman spectroscopy in the breath mode region, single-walled or multi-walled CNT can be determined, depending on methane concentrations.

Effect of variable thermal conductivity on isotherms in Bridgman growth

Abstract A change in thermal conductivity associated with melting or solidification can have a profound influence on the isotherms near the solidification interface if the material is being directionally solidified in an ampoule whose walls carry a substantial portion of the heat. This analysis was prompted by a recent discovery that the thermal conductivity of Hg1−xCdxTe increased dramatically as the material is heated above the solidus curve. An illustrative example is shown in which the sample is approximated as an infinite cylinder with constant but different thermal properties in the solid and melt. The boundary conditions are fixed on the surface by a conductive ampoule in a two-zone Bridgman furnace with an adiabatic region separating the two zones. The effect of the adiabatic zone in this case is to intensify the curvature of the interface rather than to lessen it.

A method for interface shape control during Bridgman type crystal growth of HgCdTe alloys

A method is described for interface shape control of Hg1 - xCdxTe alloys during Bridgman-type crystal growth. The authors have used this method for several years to obtain the flat or convex interface shapes required for simultaneously reducing radial compositional variations and crystal defects. The method turns to advantage the thick-walled, fused-silica ampules required for growing these alloys. A proper combination of furnace geometry (two isothermal zones seperated by a thin barrier), upper- and lower-zone temperatures, and growth rate is required. Examples of results are reported.

Magneto-hydrodynamic damping of convection during vertical Bridgman-Stockbarger growth of HgCdTe

Abstract In order to quantify the effects of convection on segregation, Hg 0.8 Cd 0.2 Te crystals were grown by the vertical Bridgman-Stockbarger method in the presence of an applied axial magnetic field of 50 kG. The influence of convection, by magneto-hydrodynamic damping, on mass transfer in the melt and segregation at the solid-liquid interface was investigated by measuring the axial and radial compositional variations in the grown samples. The reduction of convective mixing in the melt through the application of the magnetic field is found to decrease radial segregation to the diffusion-limited regime. It was also found that the suppression of the convective cell near the solid-liquid interface results in an increase in the slope of the diffusion-controlled solute boundary layer, which can lead to constitutional supercooling.

Growth of HgZnTe alloy crystals by directional solidification

Abstract A series of Hg1-xZnxTe crystals were grown by directional solidification with x ranging from 0.15 to 0.22. The axial and radial compositional variations were determined by precision density measurements, infrared (IR) transmission spectra, and energy dispersive X-ray spectroscopy (EDX). Comparison between the axial compositional profiles and the calculated results of a one dimensional diffusion model gives a value for the effective diffusion coefficient in the HgZnTe melt of 2 to 4x10-6 cm2/s.

Growth of ZnTe by physical vapor transport and traveling heater method

ZnTe crystals were grown by horizontal physical vapor transport (PVT) and a Te-solution vertical traveling heater method (THM). The grown crystals were examined by X-ray Laue diffraction technique and Hall measurements to determine the growth orientation and the electrical properties of the crystals. They were also characterized by low temperature infrared (IR) absorption measurements. Several sets of distinct peaks were observed in the IR absorption spectra for the THM samples and were identified as resulting from Cu 2+ impurities. Similar measurements on vapor grown ZnTe showed featureless absorption spectra. Chemical analyses were carried out to measure the impurity content in various ZnTe samples and synchrotron radiation topography was used to study crystalline microstructure of the (111) ZnTe single crystals grown by PVT.

Effect of growth parameters on compositional variations in directionally solidified HgCdTe alloys

Abstract A series of Hg 1 - x Cd x Te alloy crystals was grown by directional solidification with compositions ranging from x = 0.2 to x = 0.4. The measured axial compositional profiles were interpreted in terms of an exact numerical solution of the appropriate diffusion equation that takes into account both the variations of the segregation coefficient and solidification rate with composition. The solutions for all growth rates agree generally well with the experimental data and confirm earlier observations that employed approximate analytical solutions. The effective mass diffusion coefficients showed no significant composition or melt temperature dependence. Slightly higher diffusion coefficients were obtained, however, for the highest growth rates.

Effect of residual accelerations during microgravity directional solidification of mercury cadmium telluride on the USMP-2 mission

Abstract Directional solidification of mercury cadmium telluride (MCT) requires that the temperature gradient to growth rate ratio be high to avoid constitutional supercooling. With the optimum gradient condition for solidifying MCT in NASAs advanced automated directional solidification furnace (AADSF), it is necessary to use translation rates as low as 0.2 μm/s. The result is that any fluid flow with a velocity comparable to or higher than this will dominate the solidification characteristics, particularly the compositional distribution in an alloy such as this which has a large solidus-liquidus separation. In an effort to reduce fluid flow velocities a space experiment was performed. On the second United States Microgravity Payload Mission (USMP-2), the AADSF made its maiden flight and successfully completed growth of a MCT boule 16 cm long. The furnace was located approximately 3 m away from the center of gravity of the space shuttle, and this combined with the drag component of residual acceleration present during flight, resulted in quasisteady residual accelerations of the order of 1 μg0 where g0 is the earths natural gravity. Of more importance is that different orbiter attitudes during the mission produced significant differences in the resultant residual acceleration vector, in both magnitude and direction and that these differences caused large compositional variations both across the radii of the boule and along the surfaces of the boule. Comparison will be made with examples grown on the ground and in magnetic fields.

Numerical simulation of heat transfer during the crystal growth of HgCdTe alloys

The effects of growth parameters on the thermal distribution during a Bridgman-Stockbarger-type crystal growth of HgCdTe alloys using short gradient zones are analyzed numerically. The analysis takes into account the change in the thermophysical properties upon freezing and considers both the temperature and composition dependences of the properties as well as translation rate effects. The calculated results compare favorably with previously published empirical results.

Growth orientation of carbon nanotubes by thermal chemical vapor deposition

Carbon nanotubes (CNT) were synthesized using thermal chemical vapor deposition. The diameters of these CNT were quite uniform with the length of the CNT up to several tens of micrometers. With the substrate surface normal either along or against the gravity vector, different growth orientations of CNT were observed by scanning electron microscopy. The Raman spectra were similar for samples synthesized at different locations. These results suggest that gravity affects the growth of long CNT with small diameters.