Concave Interface Research Articles

Application of eddy current technique to vertical bridgman growth of CdZnTe

The eddy current technique was used to reveal the interface shape during vertical Bridgman growth of CdZnTe and to follow changes in the properties of the solidified ingot as it was cooled to room temperature after growth. Experiments were performed where partially solidified charges were decanted to show the interface shape. Eddy current analysis of the partially solidified charge indicated a concave interface shape in qualitative agreement with the shape of the decanted ingot. However, due to noise, interference, and possibly the inhomogeneous nature of the melt, only some of the eddy current signals could be analyzed empirically for interface shape; absolute values of conductivity could not be calculated from the eddy current data. Eddy current measurements made to follow changes in conductivity during post-growth cooling showed a minimum in the data during an 800°C annealing step indicative of a transition in the electrical properties of the ingot. On further cooling, a dramatic increase in the bulk conductivity of the ingot was noted. Such a transition can probably be described as a Mott transition.

Modeling the vertical Bridgman growth of cadmium zinc telluride I. Quasi-steady analysis of heat transfer and convection

A quasi-steady-state analysis of the vertical Bridgman growth of large-diameter, cadmium zinc telluride is conducted using a finite element model which accounts for the details of heat transfer, melt convection, and solid/liquid interface shape. Large radial gradients are shown to dominate the temperature field in the solid, while convection flattens the radial temperature distribution in the melt. Concave interface shapes are predicted to arise from the thermal conductivity mismatch between solid and liquid. The shape of the solid/liquid interface is sensitive to the growth rate due to the importance of latent heat release.

A general theoretical approach to the behaviour of foreign particles at advancing solid-liquid interfaces

In the present paper expressions for the viscous drag and disjoining forces upon a foreign spherical body at a solid-liquid interface are developed under general conditions, with clearly denned boundary conditions, and for various situations (planar, concave or convex interface, a smooth particle, a particle with coating or absorption).

Microstructural Evolution during the Sintering of TiC–Mo–Ni Cermets

TiC‐Ni‐Mo cermet specimens were prepared by using a mixture of fine (1.5 μm) and coarse (30 μm) TiC powders. When the fraction of fine TiC particles was 80%, a (Ti,Mo,Ni)C complex carbide phase was observed deposited on the coarse TiC particles and resulted in a typical cored structure. As the fraction of fine TiC particles decreased, the coarse TiC particles exhibited a unique microstructural evolution with the development of a concave interface. This microstructural change of the coarse TiC grains can be explained in terms of the coherency strain energy.

LEC growth of large GaAs single crystals

As the origin of polycrystallization in LEC grown GaAs crystals, accumulation of dislocations has been studied by microscopic observation of polycrystallized crystals. It has been found that dislocations have a tendency to propagate perpendicular to the solid⌜quid interface during growth. The shape of the interface near periphery of the growing crystal is usually concave towards the melt, this concave interface can lead to an accumulation of dislocations. This allocation causes polycrystallization. By control of the growing interface shape, polycrystalline growth has successfully been suppressed. Using this technology, GaAs single crystals of 75 mm diameter and 500 mm long, of 100 mm diameter and 320 mm long, and of 150 mm diameter and 170 mm long have been obtained with good stability.

The shape of the crystal/melt interface during the growth of lithium niobate crystals by the Czochralski technique

Abstract The influence of crystal orientation, melt height, and crystal rotation rate on the shape of the crystal/melt interface for LiNbO3 was investigated. While the crystal orientation and the melt height has little influence on the interface shape, the shape was found to be strongly affected by the crystal rotation rate. Above a critical rate, inversion from a convex to a concave interface occurs. The critical rate is inversely proportional to the crystal diameter. A comparison between critical rotation rates for LiNbO3 calculated by relations predicting such rates and those obtained by growth experiments is given.

Reflection and transmission of spherical waves incident on a concentric spherical interface

The traditional analysis of sound reflection and transmission at an interface between two lossless fluids is for plane waves and a plane interface. Considered here is reflection and transmission of a spherical wave at a concentric spherical interface, either concave or convex. The pressure reflection (R) and transmission (T) coefficients are found to be complex; the coefficients for a convex interface are complex conjugates of those for a concave interface. Among the (somewhat surprising) results are these. First, although at high frequency the expressions for R and T are the same as for plane waves, at low frequency R→−1 regardless of the value of the ratio ρ2c2/ρ1c1, (provided ρ2≠ρ1). Second, perfect transmission requires both ρ2=ρ1, and c2=c1, not just ρ2c2=ρ1c1. Third, if the source is a monopole and the interface radius a→0, the sound power transmitted into the second medium is that expected for a single fluid; the same is not true, however, if the source is a dipole. [Work supported by ONR, NASA, and Southampton University.]

Directional solidification of HgCdTe and HgZnTe in a transverse magnetic field

Hg0.80Cd0.20Te crystals were grown vertically in a transverse magnetic field by directional solidification. The effects of a magnetic field on the nature of fluid flow in the melts were investigated by measuring compositional variations along the axial and radial directions of the grown ingots. Magnetic field effects were shown to be significant over the entire field range employed (i.e. 2 to 5 kG). The axial compositional profiles (determined by precision densoty measurements) showed an abrupt decrease in the mole fraction of CdTe when the field was applied. Radial compositional mapping by infrared transmission and X-ray energy dispersion spectrometry indicated that the solid-liquid interface evolved through three stages when the field was applied, i.e. from a radially symmetric concave interface to an off-center concave shape when the field was initially applied, then to a tilted plane, and, finally, to an off-center concave interface. The axial compositional profile of an Hg0.84Zn0.16Te ingot showed similar field effects.

Focusing of an ultrasonic beam by a curved interface

We calculate the wavefields near the caustics, and their cusps, formed when a well collimated, ultrasonic beam scatters from a concave fluid-solid interface. The radius of curvature of the interface is assumed to be sufficiently large and the angle of incidence sufficiently small that only reflection and transmission of the beam need be considered. The incident beam is modeled as a two-dimensional wavefield whose initial profile is rectangular. The aperture is assumed to be sufficiently large (in wavelengths) that a well collimated beam is radiated, and the interface is assumed to lie close enough to the aperture that it is struck by a wavefield that has not yet evolved into a cylindrical wave. The scattered wavefields are represented as multiple integrals and are evaluated using a combination of asymptotic and numerical analysis. Special attention is given to the sometimes competing effects of the shadow boundary of each scattered beam with its corresponding caustic, and cusp if one is formed.

Characterization of indium-doped liquid-encapsulated Czochralski GaP crystals by Birefringent method

The (1 1 0) longitudinal slices of 1.7 at. % indium-doped liquid-encapsulated Czochralski GaP crystals grown with co-rotation conditions have been examined by both macroscopic and microscopic birefringent methods. The crystal defects such as dislocations, striations, facets and growth cells can be very well revealed by suitable arrangements of the polarizers and the specimens. The growth cells occurred owing to constitutional supercooling before the initiation of polycrystals in the crystal with a convex solid-melt interface. However, the terrace traces and twins, instead of growth cells, occurred before the initiation of polycrystals in the crystal with a slightly concave solid-melt interface. The dislocation density in most parts of the slices is low owing to the impurity hardening effect of indium. The slip lines, which consisted of densely piled-up dislocations, were usually found in the surface area of the shoulder, the area where the crystal radius fluctuated and the area around the remelted zones where the striations were discontinuous. The slip lines in the non-faceted region always terminated in the facet-non-facet interface, and the faceted area was almost dislocation free.

Computation of stress in Bridgman crystals

Abstract Finite element analysis has been used to calculate the stress distribution in crystals growing by the Bridgman method. It has been found that the temperature distribution, and particularly changes in temperature gradient, are the main factors governing the maximum thermally induced stress. The solid-liquid interface shape is a secondary factor but a concave interface is associated with the highest stress in a uniform temperature gradient. The application of an external stress during growth is also discussed.

Thermal convection during bridgman crystal growth

Numerical experiments are used to study thermally driven flows which occur during vertical Bridgman crystal growth of a single component fluid. The solid-liquid interface was specified as parabolic and flow patterns were calculated for various insulation thicknesses, Grashof, Prandtl and Biot numbers. When the melt is on top and the gravity vector is axially downward it was shown that flow persists as long as a radial temperature gradient is present. If the interface is convex, as viewed from the liquid, a single cell is observed. A concave interface exhibits multiple counterrotating cells. The insulation thickness and Grashof, Prandtl and Biot numbers influence the flow in a quantitative manner.

On facets existed on concave interfaces in Czochralski‐grown GaSb crystals

AbstractIt is tried to make facet regions grow on interfaces curved concavely toward the melt using the Czochralski method. By increasing rapidly the pulling rate of crystals, the whole growth interface is once remelted and a concave interface is formed in the following growth process. A facet region occurs from a (¯I¯I¯I) plane which appears by the remelt of interfaces. This facet continues to grow while it is being surrounded by concave interfaces of nonfacet regions. From the microscopic measurement of spreading resistance. the impurity concentration is higher in the facet region than in non‐facet ones. This suggests that the growth mechanism of the facet on concave interfaces is the same as that on convex ones.

Control of interface shape by using heat reservoir in FZ growth with infrared radiation convergence type heater

A concave interface is unfavorable for the growth of single crystals from the melt, because it enhances the concentration of inclusions and dislocations along the core of growing crystals. The growth of single crystals is often interrupted by these inclusions and dislocations. In FZ growth using an infrared radiation convergence type heater, the growing interface of oxide materials which minutely absorb the near infrared radiation tends to become concave towards the melt. When a heat reservoir in the form of an alumina hollow cylinder is used, heated by radiation from a heating lamp, the temperature distribution in the vicinity of the growing interface is influenced. In the case of YAG, where the interface is concave without a heat reservoir, the interface can be changed to convex towards the melt by using a heat reservoir, thus single crystals of YAG with a low density of inclusions can be grown successfully.

Vertical bridgman growth of CdGeAs 2 with control of interface shape and orientation

Several factors influencing the quality of Bridgman-grown CdGeAs 2 have been studied. Decanting experiments have revealed that an unfavorable concave melt-solid interface shape occurs under normal growth conditions. Modification of the interface shape to a flat profile by the use of appropriate temperature gradients has been shown to greatly reduce the occurrence of polycrystallinity and consequent cracking due to thermal expansion anisotropy. Other mechanisms causing cracking in single crystals have been identified: sticking due to oxides and additional phases such as Cd 3As 2 on the boule surfaces, and the possibility of bending due to off-axis growth. Experiments to reduce surface contaminants and to control the growth direction by seeding are described. Conflicts in temperature profile requirements make it difficult to achieve optimum conditions in both areas simultaneously, however, and for this reason the yield of uncracked boules has remained low.

THE GROWTH OF FACETS AND VICINAL INTERFACES ON CZOCHRALSKI-GROWN YAG SINGLE CRYSTALS

In this paper, we studied the macro-morphology and micro-structure of crystal-molt interfaces, which was exposed by rapid separating the growing crystal from the melt. We have determined the indices of the facets revealed on the interface of Czochralski-pulled YAG single crystals with various orientations. The facets have been discovered on the concave interface as well as on convex interface of -oriented grown YAG single crystals. We have observed the micro-structure of vicinal interface, which is composed of sections of singular surface dispersed with steps. The experimental evidence for the step movement as well as the interaction of steps with inclusions on these interfaces is presented.The micro-structure of the interfaces and the correspondence between the facets and the core for concave interface growth are discussed, and it is implied that the macro-shapes of interface influence the micro-mechanism of growth. The growth mechanism for concave and convex interfaces based on these considerations is then developed.

Facets in GaSb crystals pulled under concave interface conditions

Central and peripheral facets are occasionally found in the end part of Te-doped GaSb crystals pulled under non-steady state growth conditions when the growth interfaces are concave toward the melt. The central faceted region is continuously grown and is a helical shape owing to the temperature fluctuation caused by crystal rotation. The microscopic growth rate in this facet changes periodically. The central facet is essentially different from that on the convex growth interfaces, and does not result from the shift of the location of a faceted region which originated on the concave interfaces at the crystal edge. The coexistence of both central and peripheral faceted regions is not recognized on the same concave curved planes. The growth mechanism of the central facet on the concave interfaces is not understood at present.

Localized interface breakdown in zone melting and the travelling heater method

The crystallizing interface of naphthalene was observed in the vertical zone melting configuration both with and without added solvent. For stepwise-increased travel rates, breakdown began near the center of the tube for convex interfaces. A concave interface first broke down on the periphery, but at steady state was only broken down at the center for moderate travel rates. These results were attributed to variations in temperature gradient along the interface and to convection currents in the zone which cause solute variations along the interface.

Mechanical behavior of rapidly solidified Al-Al2Cu and Al-Al3Ni composites

The eutectic alloys Al-Al2Cu and Al-Al3Ni have been unidirectionally solidified at rates from 1.05 to 6.80 in, per min by a semicontinuous casting technique, and then tested in tension at room temperature. In both alloys the flow stress and ultimate tensile strength increased with increasing solidification rate, except for the highest solidification rate. The increases in matrix work-hardening rate with solidification rate were too great to be accounted for by dislocation pileup mechanisms, but were found to correlate with elastic constraint effects of the matrix aluminum phase by the reinforcing phases. In the Al-Al2Cu eutectic the strength of the Al2Cu platelets increased as the platelet width decreased with increasing growth rate. Misalignment of the composite caused by either a cellular or a macroscopically concave solid-liquid interface resulted in a decrease in the ultimate strength, especially in the rod-like Al-Al3Ni alloy. This has been related to the fracture behavior of the composites. The very low fracture toughness of the lamellar Al-Al2Cu eutectic is consistent with models of composite materials, and seriously limits the alloy’s usefulness for engineering applications.