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  • 1
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    Interface demarcation in Bridgman-Stockbarger crystal growth of II-VI compounds (1991)
    In:  Other Sources
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    Publication Date: 2011-08-24
    Description: In preparation for anticipated microgravity space missions carrying the Advanced Automated Directional Solidification Furnace, experiments were conducted to determine growth of interface in growing crystals of cadmium-doped germanium (test case), cadmium telluride, and mercury zinc telluride, using three different techniques. It was found that compositional variations detectable by microprobe and EDS techniques were most successful for detecting interfaces in materials with extensive transient regions, large segregation coefficients, and high alloy content.
    Keywords: MATERIALS PROCESSING
    Type: In: Growth and characterization of materials for infrared detectors and nonlinear optical switches; Proceedings of the Meeting, Orlando, FL, Apr. 2, 3, 1991 (A93-26892 09-76); p. 2-10.
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  • 2
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    Microhardness variations in II-VI semiconducting compounds as a function of composition (1990)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A comparison is made of microhardness variations as a function of composition in directionally solidified alloys of Hg(1-x)Cd(x)Te, Hg(1-x)Zn(x)Te, Hg(1-x)Cd(x)Se, and Hg(1-x)Zn(x)Se, using the same growth method for all four systems in order to eliminate preparation-induced mechanical property variations. In each case, compositional analysis was performed next to a microhardness indentation. Due to the 14-percent shorter bond length of ZnTe relative to HgTe, Zn-base alloy mechanical strength should exceed that of Cd-base systems. Experimental verification is furnished for this hardening effect in both Hg(1-x)Zn(x)Te and Hg(1-x)Zn(x)Se.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 99; 1-4,
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  • 3
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    A method to eliminate wetting during the homogenization of HgCdTe (1986)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Adhesion of HgCdTe samples to fused silica ampoule walls, or 'wetting', during the homogenization process was eliminated by adopting a slower heating rate. The idea is to decrease Cd activity in the sample so as to reduce the rate of reaction between Cd and the silica wall.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Applied Physics (ISSN 0021-8979); 60; 3777-377
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  • 4
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    Numerical simulation of heat transfer during the crystal growth of HgCdTe alloys (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: 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.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 86; 1-4,
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  • 5
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    Internal temperature gradient of alloy semiconductor melts from interrupted growth experiments (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: Interrupted growth experiments on Hg(0.8)Cd(0.2)Se alloys were performed in an attempt to estimate the actual internal temperature gradient in the melt during directional solidification. The results have been analyzed in terms of a theoretical model which assumes an axial composition profile characteristic of diffusion controlled solute redistribution during growth. A comparison of the calculated and measured values suggests a reduction in the applied (empty furnace) temperature gradient by about a factor of three. Values of the interface segregation coefficient (k) determined from the phase diagram, predict values of solute concentration build up in the solid during growth interruptions which are inconsistent with measured results. It appears that the k values used tend to underestimate the actual k values for higher alloy compositions and overestimate k for the lower alloy compositions.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 86; 1-4,; 100-105
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  • 6
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    Compositional redistribution during casting of Hg(0.8)Cd(0.2)Te alloys (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A series of Hg(0.8)Cd(0.2)Te ingots was cast both vertically and horizontally under well-defined thermal conditions by using a two-zone furnace with isothermal heat-pipe liners. The main objective of the experiments was to establish correlations between casting parameters and compositional redistribution and to develop ground-based data for a proposed flight experiment of casting of Hg(1-x)Cd(x)Te alloys under reduced gravity conditions. The compositional variations along the axial and radial directions were determined by precision density measurements, infrared transmission spectra, and X-ray energy dispersion spectrometry. Comparison between the experimental results and a numerical simulation of the solidification process of Hg(0.8)Cd(0.2)Te is described.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 91; 1-2,; 20-26
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  • 7
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    Growth of HgZnTe alloy crystals by directional solidification (1988)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: A series of Hg(1-x)Zn(x)Te 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 4 x 10 to the -6th sq cm/s.
    Keywords: MATERIALS PROCESSING
    Type: Journal of Crystal Growth (ISSN 0022-0248); 86; 1-4,; 87-92
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  • 8
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    Growth of solid solution single crystals (1988)
    In:  CASI
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    Publication Date: 2013-08-31
    Description: Based on the thermophysical properties of Hg sub 1-x Cd sub x Te alloys, the reasons are discussed for the failure of conventional Bridgman-Stockbarger growth methods to produce high quality homogeneous crystals in the prescence of Earth's gravity. The deleterious effects are considered which arise from the dependence of the thermophysical properties on temperature and composition and from the large amount of heat carried by the fused silica ampules. An improved growth method, developed to optimize heat flow conditions, is described and experimental results are presented. The problems associated with growth in a gravitational environment are discussed. The anticipated advantages of growth in microgravity are given and the implications of the requirements for spaceflight experiments are summarized.
    Keywords: MATERIALS PROCESSING
    Type: NASA, Washington, Microgravity Science and Applications Flight Programs, January - March 1987, Selected Papers, Volume 1; p 377-390
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  • 9
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    Growth of solid solution single crystals (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Based on the thermophysical properties of Hg sub 1-x Cd sub x Te alloys, the reasons are discussed for the failure of conventional Bridgman-Stockbarger growth methods to produce high quality homogeneous crystals in the presence of Earth's gravity. The deleterious effects are considered which arise from the dependence of the thermophysical properties on temperature and composition and from the large amount of heat carried by the fused silica ampules. An improved growth method, developed to optimize heat flow conditions, is described and experimental results are presented. The problems associated with growth in a gravitational environment are discussed. The anticipated advantages of growth in microgravity are given and the implications of the requirements for spaceflight experiments are summarized.
    Keywords: MATERIALS PROCESSING
    Type: NASA-TP-2787 , NAS 1.60:2787
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  • 10
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    Viscosity Relaxation in Molten HgZnTe (2001)
    In:  Other Sources
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    Publication Date: 2019-07-18
    Description: Rotating cup measurements of the viscosity of the pseudo-binary melt, HgZnTe have shown that the isothermal liquid with zinc mole fraction 0.16 requires tens of hours of equilibration time before a steady viscous state can be achieved. Over this relaxation period, the viscosity at 790 C increases by a factor of two, while the viscosity at 810 C increases by 40%. Noting that the Group VI elements tend to polymerize when molten, we suggest that the viscosity of the melt is enhanced by the slow formation of Te atom chains. To explain the build-up of linear Te n-mers, we propose a scheme, which contains formation reactions with second order kinetics that increase the molecular weight, and decomposition reactions with first order kinetics that inactivate the chains. The resulting rate equations can be solved for the time dependence of each molecular weight fraction. Using these molecular weight fractions, we calculate the time dependence of the average molecular weight. Using the standard semi-empirical relation between polymer average molecular weight and viscosity, we then calculate the viscosity relaxation curve. By curve fitting, we find that the data imply that the rate constant for n-mer formation is much smaller than the rate constant for n-mer deactivation, suggesting that Te atoms only weakly polymerize in molten HgZnTe. The steady state toward which the melt relaxes occurs as the rate of formation of an n-mer becomes exactly balanced by the sum of the rate for its deactivation and the rate for its polymerization to form an (n+1)-mer.
    Keywords: Inorganic, Organic and Physical Chemistry
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