Directional Solidification Method Research Articles

The influence of the freezing process on vapour transport during sublimation in vacuum-freeze-drying

The influence of the freezing process on mass transfer during subsequent sublimation in an aqueous polymer solution is investigated. Different textures of the frozen sample depending on the solidification parameters and the texture dependent diffusion coefficient for vapour transport out of the frozen, drying sample are studied. Experiments are performed, using a controlled directional solidification method (Bridgman technique) and isothermal sublimation, analysed through a light microscope. The dependence of the ‘ice finger’ primary spacing λ 1 with the solidification parameters v il −1 4 · G −1 2 (solidification interface velocity and temperature gradient, cf. Hunt, Solidification and Casting of Metals, Proc. Int. Conf. on Solidification, Sheffield, U.K., pp. 3–9. The Metals Society (1979); Kurz and Fisher, Acta Metall. 29, 11–20 (1981)) is shown. A proportionality between this combination of freezing parameters and the diffusion coefficient for vapour transport during sublimation is confirmed by the experiments.

Anisotropy of the reflectivity spectra of a BiSrCaCuO single crystal within the (001) plane

A BiSrCaCuO 2212 phase single crystal was grown by a directional solidification method. The ac susceptibility versus temperature curve is given. Some structural analyses were carried out. The measurements of reflectivity spectra by polarized light have been performed on the (001) plane in the wavelength range from 0.6 to 1.7 μm at room temperature. The spectra of both a and b-axis directions are characterized by a strongly damped plasma edge of similar shape. By fitting the Lorentz–Drude model we get some quantitative results, which may be related to the structural features of the BiSrCaCuO 2212 phase single crystal.

Modelling of directional solidification: from Scheil to detailed numerical simulation

Solute segregation in crystals grown by variations on the directional solidification method has long been correlated by the simple idea of a well mixed melt and a uniform stagnant film adjacent to the interface. All the details of convection in the melt are hidden in the single parameter, the diffusion layer thickness δ. Although extremely useful as a qualitative measure of convection, this description potentially oversimplifies the complex interactions of the furnace geometry, heat transfer and buoyancy driven convection in setting the flow pattern and solute segregation. Today, detailed numerical simulations of directional solidification are feasible that include all of the complexity introduced by the presence of the melt/crystal interface, convection in the melt and heat transfer throughout the system. This paper reports results of simulations of directional solidification of dilute alloys in a prototypical vertical Bridgman system. The predictions of these calculations are compared directly with the stagnant film model of segregation. It is demonstrated that although the diffusion layer thickness can be used to correlate the transition between intense convection in the melt and diffusion-controlled growth, it does not necessarily correspond to a physical picture of the solute transport in the bulk and does not predict the dependence of the radial uniformity of the composition on the flow. Direct comparison between the calculations and growth experiments for gallium-doped germanium demonstrate the accuracy of the numerical simulations for predicting the behavior of real systems.

DIRECTIONAL GROWTH OF Bi -Sr-Ca-Cu-O2212 PHASE SUPERCONDUCTING SINGLE CRYSTAL

Superconducting single crystals of Bi-Sr-Ca-Cu-O 2212 phase have been sucessfully grown by a directional solidification method. The maximum size of single crystal reached 19+3×mm3. The (001) plane of the crystals is parallel to the growth direction. The crystals tend to grow along the [010] direction. Back-reflection Laue pattern revealed that the large crystal is indeed one single crystal. Measurements of electrical resistivity and ac susceptibility showed that Tc(zero) of the single crystals annealed in O2 was 81.5 K.

Growth and properties of oxygen- and ion-doped Bi2Sr2CaCu2O8+ delta single crystals.

A directional solidification method for growing large single crystals in the ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ system is reported. Ion doping, with replacement of La for Sr and Y for Ca, as well as oxygen doping in these crystals has been explored. Doped and undoped crystals have been characterized using microprobe analysis, x-ray diffraction, thermogravimetric analysis, and magnetic and Hall measurements. Ion doping results in little change of the superconducting transition for substitution levels below 20--25%, while beyond this level the Meissner signal broadens and the low-temperature Meissner signal decreases. Microprobe analysis and x-ray diffraction performed on these more highly substituted single crystals provide evidence for inhomogeneity and phase segregation into regions of distinct composition. Annealing unsubstituted crystals in increasing partial pressures of oxygen reversibly depresses the superconducting transition temperature from 90 (as made) to 77 K (oxygen pressure annealed), while the carrier concentrations, as determined from Hall effect measurements, increase from n=3.1(3)\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (0.34 holes per Cu site) to 4.6(3)\ifmmode\times\else\texttimes\fi{}${10}^{21}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ (0.50 holes per Cu site).No degradation of the Meissner transition or other indications of inhomogeneity or phase segregation with doping are noted, suggesting that oxygen-doped ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8+\mathrm{\ensuremath{\delta}}}$ is a suitable system for pursuing doping studies. The decrease in ${\mathit{T}}_{\mathit{c}}$ with concentration for 0.34\ensuremath{\le}n\ensuremath{\le}0.50 indicates that a high-carrier-concentration regime exists in which ${\mathit{T}}_{\mathit{c}}$ decreases with n and suggests that this decrease does not arise from material inhomogeneity or other materials problems. An examination of the variation of ${\mathit{T}}_{\mathit{c}}$ with the density of states and lattice constants for all of the doped and undoped superconducting samples considered here indicates that changes in ${\mathit{T}}_{\mathit{c}}$ with doping are primarily affected by changes in the density of states (or carrier concentration) rather than by structural variation induced by the doping.

High temperature superconductor composites by a modified bridgman method

We have investigated single-crystal growth of high temperature superconductor (111z) Bi-Sr-Ca-Cu materials by using a directional solidification method. We show that for a certain range of z, quaternary crystals having an essentially homogenous atomic composition can be obtained. For larger z values, dense composites are formed by alternate layers along the c-axis of the superconducting quaternary oxide and a semiconducting ternary Sr-Ca-Cu oxide. Preliminary transport measurements of these sandwiches reveal a superconducting behaviour within the ab plane but semiconductor-like along the c-axis. Plate-like single crystals of the HTSC Bi2.2Sr2Ca0.7Cu2Oy with dimensions up to 5 × 7 mm2 × (50–100) μm have been prepared by this method.

ELECTRICAL ACTIVITY OF GRAIN BOUNDARIES IN SILICON GROWN WITH AN ALUMINIUM CONTENT GRADIENT

In order to discriminate between the role of the structure and chemistry on the electrical activity of boundaries, p-type silicon bicrystals, characterized by a chemical gradient in aluminium from bottom to top of the ingot, were grown by a directional solidification method. The crystallographical and chemical analysis of symmetrical tilt boundaries namely Σ9, 13 and 25 was carried out by TEM and STEM X-ray spectroscopy. The electrical activity was studied by the scanning transmission electron beam induced current technique (STEBIC) which allowed the direct correlation with the microstructure to be achieved. It is shown that these coincidence tilt boundaries do not show intrinsic activity and that their behaviour is related to chemical decoration. The specific electrical activity of each type of decoration is considered.

Growth and characterisation of polycrystalline silicon ingots doped with Cu, C, B or Al by directional solidification for photovoltaic application

Polycrystalline silicon ingots using electronic grade (EG) silicon as charge and Cu, C, B or Al as dopant impurity have been grown by the static directional solidification method. A silicon-nitride-coated silica crucible has been used as container and the directional solidification has been achieved by helium extracting the heat from the bottom using a heat exchanger. The growth conditions, impurities distribution in the ingots and their electrical properties are discussed. Solar cells from these ingots have been made after epilayer depositions and it has been shown that silicon containing Cu (11 μg/g), C (376 μg/g), B (12.5 μg/g) or Al (2g2 μ/g) can be used to obtain silicon solar cells with an efficiency of ≈ 10%.

PbTe-SnTe mutual diffusion coefficient at just above the Pb 0.8Sn 0.2Te solidus temperature

The PbTe-SnTe mutual diffusion coefficient at just above the Pb 0.8Sn 0.2Te solidus temperature is measured using a directional solidification method, with a capillary used to suppress convective flows. The diffusion coefficient is determined to be (5.3±0.3) X10 -5 cm 2/s.

Alloy solidification in systems containing a liquid miscibility gap

Directional solidification methods have been used to examine the growth of fibrous, or tubular, composite samples in alloys close to the monotectic reaction: Liquid 1 Solid + Liquid 2. The binary systems Al-In, Cu-Pb and Cd-Ga have been examined and some use made of a transparent analogue, succinnonitrile-water. The solidification behavior appears to relate to the height of the miscibility gap in such systems and it is demonstrated how ternary additions which change this height (Sn to Al-In, Al to Cu-Pb) modify the microstructures dramatically. These structural changes are discussed with reference to the relative surface energies between two immiscible liquids and a third (solid) phase. The phase spacings and transitions in microstructure are discussed in terms of diffusion processes at the monotectic front.

Alloy solidification in systems containing a liquid miscibility gap

Directional solidification methods have been used to examine the growth of fibrous, or tubular, composite samples in alloys close to the monotectic reaction: Liquid 1 Solid + Liquid 2. The binary systems Al-In, Cu-Pb and Cd-Ga have been examined and some use made of a transparent analogue, succinnonitrile-water. The solidification behavior appears to relate to the height of the miscibility gap in such systems and it is demonstrated how ternary additions which change this height (Sn to Al-In, Al to Cu-Pb) modify the microstructures dramatically. These structural changes are discussed with reference to the relative surface energies between two immiscible liquids and a third (solid) phase. The phase spacings and transitions in microstructure are discussed in terms of diffusion processes at the monotectic front.

Crystal growth of KSbSe 2

Centimeter-size single crystals of the compound KSbSe 2 were prepared for the first time by the Bridgman method of directional solidification. Experimental growth conditions and procedure are described along with the characterization of the crystals produced. Preliminary results from growth experiments on KSbS 2 and RbSbS 2 are also presented.

STUDIES ON SOLIDIFICATION AND CONTRACTION AND THEIR RELATION TO THE FORMATION OF HOT TEARS IN STEEL CASTINGS.*

ABSTRACT.The following studies, resulting from a compilation of data and expressions, consider the relation of solidification and contraction to the formation of hot tears. Casting solidification involving the principle of equal cooling throughout is undesirable. Such conditions should be replaced by the more natural method of directional solidification. Controlled directional solidification is successful only when certain principles of casting design are adopted. These principles stress the importance of feeding smaller sections through heavier ones and by studying mass effect by the use of inscribed circles. Metal contraction and hindered contraction resulting from mold resistance is responsible in the major part for the existence of hot‐tear cracks in steel castings. Other important factors are cavities and the physical properties of the steel. There are two types of hot‐tear cracks, (a) the internal hot tear that develops during the solidification of the casting, and (b) the external hot tear that develops after solidification is complete, but while the temperature is about 1300 degrees C. Intelligent application of certain principles in casting design is one of the most important of seven methods that may be adopted to obviate the danger of hot tears, or at least limit the hot tear formation.1. There are many conflicting opinions on all phases of casting procedure and various remedies for all the numerous problems arising in the industry. These remedies often have been applied by rule of thumb or as experience has dictated their use. It is surprising to note how little experimental data are available on the actual physical processes which occur during the solidification and contraction of steel castings.2. It is not possible to review as large a field as the entire casting process in one article and do justice to all of its phases. The authors have selected, therefore, only one of the important phases, that of solidification, contraction and the formation of hot tears, and have attempted to bring together the best of available data and to supplement them with the results of some experiments.3. In the industry today there are so many definitions for the several terms that in order to clarify the discussion it has been found necessary to define certain of these terms.