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Generalization of internal centrifugal zone growth of metal-ceramic composites. Progress report, December 1, 1974--August 31, 1975

Description: Objectives are (1) to develop a model of Internal Centrifugal Zone Growth (ICZG) and (2) to use this model to improve present ICZG systems. During the present year, models were developed for finite samples heated by finite induction coils. These models enable calculation of two-dimensional temperature profiles in solid samples. Molten zone shapes can be calculated provided that simplified boundary conditions for the rf field are employed. Experiments to test these models were conducted here and at ORNL. The heating instability phenomenon was possibly observed but not quantified. (DLC)
Date: January 1, 1975
Creator: Sekerka, R. F.
Partner: UNT Libraries Government Documents Department

Kinetics, morphology and thermodynamics of the solid-liquid transition of non-metals. Progress report, March 1, 1979-February 28, 1980

Description: Some previous work on Internal Centrifugal Zone Growth was documented. New calculations have been made to show that for large rf skin depths, the temperature of the sample depends in a systematic way on only three dimensionless parameters; these characterize the rf power level, the surface heat transfer coefficient, and the ambient temperature. Critical values are given for the ambient temperature below which curves of sample temperature versus RF power level are S-shaped. Based on this improvement in understanding, our previous numerical results, valid for arbitrary skin depths, are being prepared for publication. Work continues toward the measurement of the solid-liquid surface tensions of non-metallic materials via the grain-boundary groove technique. Degassed samples of GeO/sub 2/ have been obtained, but the necessary temperature caused damage to the quartz tube in our present apparatus, necessitating a new design. While the new apparatus is under construction, sodium sulfate will be used as a prototype material to enable work on the optical system. Use of an astronomical telescope in conjunction with the optical viewpoint leads to poor image quality so we are considering the substitution of a microscope with a large working distance. Previous difficulties with numerical calculation of the temperature profiles in the system have been alleviated by using finer grid sizes for the finite difference scheme. Further effort has been expended to form the basis of new work on the application of Onsager's theory of reciprocity to transport phenomena in solids.
Date: August 1, 1979
Creator: Sekerka, R.F.
Partner: UNT Libraries Government Documents Department

Kinetics, morphology and thermodynamics of the solid--liquid transition of non-metals. Progress report, March 1, 1978--February 28, 1979

Description: Apparatus is being assembled for measurement of the solid--liquid surface tension, ..gamma.., of two non-metallic materials, GeO/sub 2/ and NaCl, via the grain-boundary groove technique. According to this technique, a solid adjacent to its liquid is used in a temperature gradient and the shape of the solid--liquid interface near a grain boundary groove is measured. From this shape and a knowledge of the thermal field, ..gamma.., a parameter extremely important in nucleation and solidification kinetics, is calculated. GeO/sub 2/ is chosen for its low entropy of fusion and NaCl for its simple ionic bonding; both are transparent. The apparatus for GeO/sub 2/ involves a lineal geometry, a two-zone furnace and an optical viewport; that for NaCl involves an axial geometry. The thermal field for the lineal geometry was calculated by the solution of finite difference equations. If future results support the tendency for the crucible to impose its simple linear temperature gradient on thin samples, the theoretical analysis will be drastically simplified and the technique might be extendable to a broad class of materials. Previous work on the modeling of Internal Centrifugal Zone Growth has been communicated to personnel at Oak Ridge who have incorporated it in their ongoing research programs.
Date: January 1, 1979
Creator: Sekerka, R.F.
Partner: UNT Libraries Government Documents Department

Generalization of international centrifugal zone growth of metal-ceramic composites. Progress report, December 1, 1975--November 30, 1976

Description: Research conducted to develop a realistic model of Internal Centrifugal Zone Growth (ICZG) and to utilize the predictive capacities of this model to improve and extend present ICZG systems is reported. One-dimensional models for (hypothetical) infinitely large samples heated by long RF induction coils were previously developed and an S-curve instability phenomenon associated with spontaneous coupling of the RF field to materials whose electrical conductivity increases significantly with temperature was predicted. This model was extended to two dimensions to account for finite samples and coils. During the reporting period one-dimensional modeling was used in predicting additional instabilities (S shaped with a cusp) on melting which are ''piggy-back'' with the S-curve instabilities at lower temperatures. The low temperature S instability has been demonstrated for silicon; whereas, in cooperation with Oak Ridge, the predictive capacities of the modeling have been used to engineer around instabilities in the CrO/sub 3/-Mo systems and to extend ICZG to a number of other materials.
Date: August 1, 1976
Creator: Sekerka, R. F. & Hartzell, R. A.
Partner: UNT Libraries Government Documents Department

Generalization of international centrifugal zone growth of metal-ceramic composites. Technical report

Description: The electric and temperature fields within inductively heated materials have been calculated in order to better understand the essential features of the crystal growth process called Internal Centrifugal Zone Growth (ICZG). A complicated two-dimensional problem has been studied via a simple one-dimensional model which applies to infinitely long samples heated by infinitely long induction coils. Two such one-dimensional models are presented. In the first model, all material properties of the sample are assumed to be independent of temperature. The coupled differential equations determining the electric field and temperature distributions within the sample are solved analytically. Resulting profiles of temperature and electromagnetic fields within the sample provide a basis for more sophisticated models. The second model deals with materials (e.g., oxides) whose electrical conductivities are very low at room temperature but increase significantly with temperature. The differential equations for electromagnetic and temperature fields are consequently strongly coupled, resulting in multiple steady state solutions, some of which are unstable. An understanding of this instability is of utmost importance with regard to crystal growth via ICZG, because it has been observed that an increase in the electrical conductivity of the sample when melting occurs can cause a catastrophic increase in surface temperature. The modeling predicts, however, that the instability can be controlled through a judicious choice of RF frequency, sample size, and sample alloying. Progress has been made in solving the two-dimensional differential equations. Many problems are encountered that are not present in one-dimensional modeling. The method of solution is outlined, but no solutions have yet been obtained.
Date: August 1, 1976
Creator: Sekerka, R. F. & Hartzell, R. A.
Partner: UNT Libraries Government Documents Department