Engineering topics

Derby, J. J.

Publications and source records attributed to Derby, J. J..

8 records

NASA NTRS2009

Numerical Optimization of the Thermal Field in Bridgman Detached Growth

The global modeling of the thermal field in two vertical Bridgman-like crystal growth configurations, has been performed to get optimal thermal conditions for a successful detached growth of Ge and CdTe crystals. These computations are performed using the CrysMAS code and expand upon our previous analysis [1] that propose a new mechanism involving the thermal field and meniscus position to explain stable conditions for dewetted Bridgman growth. The analysis of the vertical Bridgman configuration with two heaters, used by Palosz et al. for the detached growth of Ge, shows, consistent with their results, that the large wetting angle of germanium on boron nitride surfaces was an important factor to promote a successful detached growth. Our computations predict that by initiating growth much higher into the hot zone of the furnace, the thermal conditions will be favorable for continued detachment even for systems that did not exhibit high contact angles. The computations performed for a vertical gradient freeze configuration with three heaters representative of that used for the detached growth of CdTe, show favorable thermal conditions for dewetting during the entirely growth run described. Improved thermal conditions are also predicted for coated silica crucibles when the solid-liquid interface advances higher into the hot zone during the solidification process. The second set of experiments on CdTe growth described elsewhere has shown the reattachment of the crystal to the crucible after few centimeters of dewetted growth. The thermal modeling of this configuration shows a second solidification front appearing at the top of the sample and approaching the middle line across the third heater. In these conditions, the crystal grows detached from the bottom, but will be attached to the crucible in the upper part because of the solidification without gap in this region. The solidification with two interfaces can be avoided when the top of the sample is positioned below the middle position of the third furnace.

Stelian, C.↗

NASA NTRS1993

Buoyancy-driven flows of a radiatively participating fluid in a vertical cylinder heated from below

The effect of radiative energy transport on the onset and evolution of natural convective flows is studied in a Rayleigh-Benard system. Steady, axisymmetric flows of a radiatively participating fluid contained in a rigid-walled, vertical cylinder which is heated on the base, cooled on top, and insulated on the side wall are calculated by using the Galerkin FEM. Bifurcation analysis techniques are used to investigate the changes in the flow structure due to internal radiation. The results of this two-parameter study - where the Rayleigh number, Ra and optical thickness, tau, are varied - apply to fluids ranging from opaque to nearly transparent with respect to IR radiation. For any nonopaque fluid, internal radiation eliminates the static state that without radiation, exists for all values of the Rayleigh number. This heat transfer mechanism also destroys a symmetry of the system that relates clockwise and counterclockwise flows. The connectivity between characteristic flow families and the range of Ra where families are stable are found to depend greatly on tau. Results demonstrate the inadequacy of characterizing the behavior of this system using simple notions of radiative transfer in optically thick or thin media; the nonlinear interaction of radiation and flow are far more complicated than these asymptotic limits would imply.

Salinger, A. G.↗

NASA NTRS1992

Heat transfer in vertical Bridgman growth of oxides - Effects of conduction, convection, and internal radiation

In the present investigation of crystalline phase internal radiation and heat conduction during the vertical Bridgman growth of a YAG-like oxide crystal, where transport through the melt is dominated by convection and conduction, heat is also noted to be conducted through ampoule walls via natural convection and enclosure radiation. The results of a quasi-steady-state axisymmetric Galerkin FEM indicate that heat transfer through the system is powerfully affected by the optical absorption coefficient of the crystal. The coupling of internal radiation through the crystal with conduction through the ampoule walls promotes melt/crystal interface shapes that are highly reflected near the ampoule wall.

Brandon, S.↗

NASA NTRS1986

Thermal-capillary analysis of Czochralski and liquid encapsulated Czochralski crystal growth. II - Processing strategies

The pseudosteady-state heat transfer model developed in a previous paper is augmented with constraints for constant crystal radius and melt/solid interface deflection. Combinations of growth rate, and crucible and bottom-heater temperatures are tested as processing parameters for satisfying the constrained thermal-capillary problem over a range of melt volumes corresponding to the sequence occuring during the batchwise Czochralski growth of a small-diameter silicon crystal. The applicability of each processing strategy is judged by the range of existence of the solution, in terms of melt volume and the values of the axial and radial temperature gradients in the crystal.

Derby, J. J.↗

NASA NTRS1986

Thermal-capillary analysis of Czochralski and liquid encapsulated Czochralski crystal growth

Results are presented from finite element analysis of the Czochralski (CZ) and Liquid Encapsulated Czochralski (LEC) crystal growth processes based on a thermal-capillary model which governs the heat transfer in the system simultaneously with setting the shapes of the melt/solid interface, the melt and encapsulant menisci, and the radius of a steadily growing crystal. Calculations are performed for the small-scale growth of silicon (CZ) and gallium arsenide (LEC). The effects of melt volume and crucible position relative to the heater on the radius of the crystal and the shape of the melt/solid interface are predicted for the CZ system, and the importance of including an accurate representation of the melt meniscus for modeling the process is demonstrated. The additional effect of an encapsulant layer on heat transfer is treated for the LEC method for the cases of totally transparent and opaque encapsulant. The responses of these LEC prototype systems are examined for changes in pull rate and encapsulant volume.

Derby, J. J.↗

NASA NTRS1985

Thermal-capillary model for Czochralski growth of semiconductor materials

The success of efficiently calculating the temperature field, crystal radius, melt mensicus, and melt/solid interface in the Czochralski crystal growth system by full finite-element solution of the government thermal-capillary model is demonstrated. The model predicts realistic response to changes in pull rate, melt volume, and the thermal field. The experimentally observed phenomena of interface flipping, bumping, and the difficulty maintaining steady-state growth as the melt depth decreases are explained by model results. These calculations will form the basis for the first quantitative picture of Cz crystal growth. The accurate depiction of the melt meniscus is important in calculating the crystal radius and solidification interface. The sensitivity of the results to the equilibrium growth angle place doubt on less sophisticated attempts to model the process without inclusion of the meniscus. Quantitative comparison with experiments should be possible once more representation of the radiation and view factors in the thermal system and the crucible are included. Extensions of the model in these directions are underway.

Derby, J. J.↗

NASA NTRS1985

Dynamics of Bridgman-Stockbarger growth of non-dilute binary alloys

The coupling between heat and mass transfer in the Bridgman-Stockbarger growth of nondilute crystals induces growth rate transients that are longer than those caused by heat transfer alone. The dynamics of the diffusion-controlled growth for thermophysical properties representative of Pb(1-x)Sn(x)Te and Hg(1-x)Cd(x)Te are computed from a one-dimensional solidification model which includes melt-crystal phase behavior. Results show that the growth rate response to a step change in pull rate for both systems differs from the exponential decay observed for dilute alloys and that growth rates cannot be inferred from axial segregation data in nondilute systems due to the dependence of the segregation coefficient on composition.

Bourret, E. D.↗

NASA NTRS1982

Segregation effects during growth of pseudo-binary system with large liquidus-solidus separation

Results are reported for an experimental study of the seeded growth, of PbSnTe single crystals in a vertical Bridgman-Stockbarger (B-S) furnace. Profiles of axial and radial segregation of Sn are presented for crystal growth from melts with aspect ratios of 15 and 6.8; the profiles indicate intense convective mixing. A one-dimensional model of heat and mass transfer in an idealized B-S furnace with diffusion control is used to examine the importance of liquidus-solidus separation in determining growth-rate and composition transients in a pseudobinary system. It is shown that the coupling of heat and mass transfer in a pseudobinary system with a large liquidus-solidus separation causes the transient behavior in a nondilute system to differ markedly from such behavior in a dilute system and that PbSnTe crystals with a reasonable diameter can be grown under nearly convectionless conditions in a microgravity environment.

Bourret, E. D.↗