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Singh, N. B.

Publications and source records attributed to Singh, N. B..

53 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Identification and control of a multizone crystal growth furnace

This paper presents an intelligent adaptive control system for the control of a solid-liquid interface of a crystal while it is growing via directional solidification inside a multizone transparent furnace. The task of the process controller is to establish a user-specified axial temperature profile and to maintain a desirable interface shape. Both single-input-single-output and multi-input-multi-output adaptive pole placement algorithms have been used to control the temperature. Also described is an intelligent measurement system to assess the shape of the crystal while it is growing. A color video imaging system observes the crystal in real time and determines the position and the shape of the interface. This information is used to evaluate the crystal growth rate, and to analyze the effects of translational velocity and temperature profiles on the shape of the interface. Creation of this knowledge base is the first step to incorporate image processing into furnace control.

Batur, C.↗
NASA NTRS1992

Thermodynamic reactivity, growth and characterization of mercurous halide crystals

Thermodynamic calculations were carried out for the Hg-X-O system (X = Cl, Br, I) to identify the potential sources of contamination and relative stability of oxides and oxy-halide phases. The effect of excess mercury vapor pressure on the optical quality of mercurous halide crystal was studied by growing several mercurous chloride crystals from mercury-rich composition. The optical quality of crystals was examined by birefringence interferometry and laser scattering studies. Crystals grown in slightly mercury-rich composition showed improved optical quality relative to stoichiometric crystals.

Singh, N. B.↗
NASA NTRS1991

Growth kinetics of physical vapor transport processes: Crystal growth of the optoelectronic material mercurous chloride

Physical vapor transport processes were studied for the purpose of identifying the magnitude of convective effects on the crystal growth process. The effects of convection on crystal quality were were studied by varying the aspect ratio and those thermal conditions which ultimately affect thermal convection during physical vapor transport. An important outcome of the present study was the observation that the convection growth rate increased up to a certain value and then dropped to a constant value for high aspect ratios. This indicated that a very complex transport had occurred which could not be explained by linear stability theory. Better quality crystals grown at a low Rayleigh number confirmed that improved properties are possible in convectionless environments.

Singh, N. B.↗
NASA NTRS1991

Flight experiment on acousto-optic crystal growth

The physical vapor transport method was used for growing mercurous chloride crystals in different convective conditions. Optical homogeneity is found to be extremely dependent on convection levels. Results of numerical studies indicate that for a gravity level of 0.001 g or less the Stefan wind drives the flow and no recirculating cells are observed.

Singh, N. B.↗
NASA NTRS1991

Development of a laser-Doppler system for measurement of velocity fields in PVT crystal growth systems

A laser-Doppler velocimetry (LDV) system capable of measuring velocities as low as 10 exp -5 m/s is presented, and a calibration system for determining the accuracy of the LDV system at these velocities is described. The results obtained in mercurous chloride crystal grown in cylindrical ampoules at 300 C, using physical vapor transport (PVT) methods, are presented. It is concluded that the overall flow pattern observed is a unicellular, asymmetric pattern between Rayleigh number of 125 and 250.

Jones, O. C.↗
NASA NTRS1990

Transport phenomena during vapor growth of optoelectronic material - A mercurous chloride system

Crystal growth velocity was measured in a mercurous chloride system in a two-zone transparent furnace as a function of the Rayleigh number by varying a/L, where a is the radius of the growth tube and L is the transport length. Growth velocity data showed different trends at low and high aspect ratio, a result that does not support the velocity-aspect ratio trend predicted by theories. The system cannot be scaled on the basis of measurements done at a low aspect ratio. Some change in fluid flow behavior occurs in the growth tube as the aspect ratio increases.

Singh, N. B.↗
NASA NTRS1989

Determination of the mean solid-liquid interface energy of pivalic acid

A high-confidence solid-liquid interfacial energy is determined for an anisotropic material. A coaxial composite having a cylindrical specimen chamber geometry provides a thermal gradient with an axial heating wire. The surface energy is derived from measurements of grain boundary groove shapes. Applying this method to pivalic acid, a surface energy of 2.84 erg/sq cm was determined with a total systematic and random error less than 10 percent. The value of interfacial energy corresponds to 24 percent of the latent heat of fusion per molecule.

Singh, N. B.↗
NASA NTRS1989

Effects of crystal-melt interfacial energy anisotropy on dendritic morphology and growth kinetics

Morphological and kinetic studies of succinonitrile, a BCC crystal with a low (0.5 percent) anisotropy and pivalic acid, and FCC crystal with relatively large (5 percent) anisotropy in solid-liquid interfacial energy, show clearly that anisotropy in the solid-liquid interfacial energy does not affect the tip radius-velocity relationship, but has a profound influence on the tip region and the rate of amplification of branching waves. Anisotropy of the solid-liquid interfacial energy may be one of the key factors by which the microstructural characteristics of cast structures reflect individual material behavior, especially crystal symmetry.

Glicksman, M. E.↗
NASA NTRS1989

Free dendritic growth in viscous melts - Cyclohexanol

Experiments were carried out to measure the growth speed, V, and dendritic tip radius, R, of highly purified cyclohexanol. The data show that VR-squared = constant over the entire experimentally observed supercooling range, Delta T is between 0.1 and 1 K. The stability parameter estimated from this result indicates that sigma(asterisk) = 0.027, a value in good agreement with the values of sigma(asterisk) found for the cubic plastic crystals succinonitrile pivalic acid. Cyclohexanol differs from other carefully measured plastic crystals in that the viscosity of its melt at the melting point is about 20 times higher, so gravity-induced convection remains weak even at small supercoolings.

Singh, N. B.↗
NASA NTRS1989

Effect of temperature gradient on the optical quality of mercurous chloride crystals

Single crystals of mercurous chloride were grown at temperature gradients of 8, 11 and 17 K/cm by the physical vapor transport method. The optical quality of these crystals was evaluated by measuring bulk scattering and inhomogeneity of refractive index by birefringence interferometry. It was observed that a high temperature gradient at the solid-vapor interface induced thermal stresses and crystals showed higher scattering and irregular fringes.

Singh, N. B.↗
NASA NTRS1989

Evaluation of transport conditions during physical vapor transport growth of opto-electronic crystals

Transport conditions were evaluated during the vapor phase growth of mercurous chloride crystals in a closed tube. Experimentally observed growth rates were much smaller than those calculated by the Hertz-Knudsen (H-K) equation. The Arrhenius behavior of growth rate with the temperature was used to derive the sticking coefficient. A one-dimensional diffusion model was used to calculate the total mass flux and was compared with the condensing flux. It was predicted that growth occurred in the convecto-diffusive range.

Singh, N. B.↗
NASA NTRS1988

Dendritic solidification in binary alloys

Alloys generally solidify dendritically, and associated with that is the microsegregation of impurities. Pure metals also solidify in dendritic form as 'thermal' dendrites, which actually segregate the system's enthalpy. In this investigation, small additions of solute to succinonitrile have been studied and dendritic growth observed in a supercooled melt. This free dendritic growth-mode is similar to that experienced by equiaxed dendrites found in alloy castings. Observations of these free dendrites include measurement of velocity and tip radius of the dendrites at different supercoolings and solute concentrations.

Chopra, M. A.↗
NASA NTRS1988

Measurement of the diffusion coefficient of acetone in succinonitrile at its melting point

The diffusion coefficient of acetone in liquid succinonitrile at 331.1 K was determined using the method of McBain and Dawson (1935). Only dilute mixtures of SCN-acetone were studied. The interdiffusion constant was determined to be 0.0000127 sq cm/s and was essentially independent of the acetone concentration over the range investigated (0.5 to 18 mol pct acetone).

Chopra, M. A.↗
NASA NTRS1988

Preparation of multistage zone-refined materials for thermochemical standards

This paper describes the steps of a two-step zone purification procedure for completed minicells filled with ultrahigh-purity succinonitrile (SCN) in which the CNS is under its own vapor pressure. The solid-liquid equilibrium, as determined by melting and freezing point measurements, is therefore considered to be a realization of the triple-point. The freezing-point plateaus measured at a variety of bath temperatures demonstrated both the fundamental reproducibility of the measurement for a given sample and the lack of dependence of that measurement on the bath temperature which controls the freezing rate. The measurement reproducibility and the sample consistency indicate that the method described is indeed suitable for the preparation of pure material which can be used for the purposes of thermistor and thermometer calibration at 58.0796 + or - 0.0015 C.

Rubinstein, E.↗
NASA NTRS1983

Gravitational effects in dendritic growth

The theories of diffusion-controlled dendritic crystallization will be reviewed briefly, along with recently published critical experiments on the kinetics and morphology of dendritic growth in pure substances. The influence of the gravitational body force on dendrite growth kinetics will be shown to be highly dependent on the growth orientation with respect to the gravity vector and on the level of the thermal supercooling. In fact, an abrupt transition occurs at a critical supercooling, above which diffusional transport dominates the growth process and below which convective transport dominates. Our most recent work on binary mixtures shows that dilute solute additions influence the crystallization process indirectly, by altering the interfacial stability, rather than by directly affecting the transport mode. Directions for future studies in this field will also be discussed.

Glicksman, M. E.↗
NASA NTRS1982

Influence of diffusion and convective transport on dendritic growth in dilute alloys

Experimentation has been carried out in which the kinetics and morphology of dendritic growth were measured as a function of thermal supercooling, solute concentration, and spatial orientation of the dendritic growth axis. The crystal growth system studied is succinonitrile, NC(CH2)2CN, with additions of argon (up to 0.1 mole percent). This system is especially useful as a model for alloy studies because kinetic data are available for high purity (7-9's) succinonitrile. The influence of the solute, at fixed thermal supercooling, is to increase the growth velocity and correspondingly decrease intrinsic crystal dimensions. Morphological measurements are described in detail relating tip size, perturbation wavelength, supercooling, and solute concentration. The analysis of these effects based on morphological stability theory is also discussed, and experiments permitting the separation of convective and diffusive heat transport during crystal growth of succinonitrile are described. The studies underscore the importance of gravitationally-induced buoyancy effects on crystal growth.

Glicksman, M. E.↗