Papers by Keyword: Supersaturation

Abstract: An effective powder consumption is indispensable for enlarging the diameter and thickness of SiC crystals. We employed three types of filling designs for SiC source powder with different distances between the surface of the seed and the source powder. To maintain the shape of the designs, the SiC source powder was heat-treated in an Ar atmosphere at 680 torr within a temperature range of 1500 to 1600°C. The SiC source powder consumption and contribution to growth in well-structured layouts increased due to the increase in the surface area of SiC source powder, despite its lower initial powder filling. The numerical simulation showed that the well-structured layouts with a higher surface area of SiC source powder have a higher partial pressure of Si and SiC₂ gases (supersaturation of these gas phases) near the seed region compared to the without well-structured layouts. The computed tomography (CT) analysis of the cross-section of SiC source powder after the growth run clearly showed that the source powder was previously sublimated at the region of the crucible wall, and recrystallization at the surface region of the source powder physically retarded the pathway of SiC source gases to the region of the SiC seed crystal. The newly designed well-structured layouts of the source powder have an economical advantage in achieving effective powder consumption during crystal growth.

Abstract: When three welding conditions, including welding configuration, welding speed and laser power, are sequentially altered, supersaturation of liquid aluminum is crystallography-dependently determined to limit growth kinetics of dendrite tip with decease of thermo-metallurgical factors for solidification cracking to salvage the weld properties during emerging laser welding repair of multicomponent nickel-based single-crystal superalloy. After comparing growth crystallography between right side and left side of weld, the distribution of supersaturation of liquid aluminum is axis- symmetrically developed by favorite (001)/[100] dendrite growth kinetics, while the distribution is nonaxisymmetrically developed by detrimental (001)/[110] dendrite growth kinetics. High heat input is inappropriately procured by either high laser power or slow welding speed to insidiously boost supersaturation of liquid aluminum, worsen alloying partition and solute copiousness, heterogeneously exacerbate morphology and size of dendrite growth, whereby stray grain formation is strongly produced. In order to ameliorate dendrite growth, low heat input is usefully rendered by either low laser power or high welding speed to attenuate supersaturation of liquid aluminum, narrow solidification temperature range and obviate dendrite tip undercooling alongside columnar interface in order to augment crack-resistant dendrite of epitaxial growth. The overall supersaturation of liquid aluminum is crystallography-dependent. During across whole melt-pool solidification interface, supersaturation of nonsymmetric (001)/[110] welding configuration is not as small as symmetric (001)/[100] welding configuration under well-controlled heat input, thereby fosters the kinetic factors for stray grain development to obstruct columnar dendrite and consequently weaken unidirectional morphology. It is crystallographically favorable for relief of supersaturation with axis- symmetrical dendrite growth. The mechanism of crack-vulnerable dendrite growth elimination through circumspect improvement of supersaturation-controlled growth kinetics is constructively proposed for feasible crackless rejuvenation of laser welding or laser cladding. The theoretical predictions are scrupulously supported by corroborative metallograph observation.

Abstract: The contribution of crystallography-dependent metallurgical factors, such as supersaturation of liquid aluminum and minimum dendrite tip undercooling, to solidification behavior and microstructure development is numerically analyzed during Ni-Cr-Al ternary single-crystal superalloy molten pool solidification to better understand thermodynamic and kinetic driving forces behind solidification cracking resistance. The variation of supersaturation of liquid aluminum and minimum dendrite tip undercooling with location of solid/liquid interface is symmetrically consistent in (001)/[100] welding configuration. By comparison, the variation is asymmetrically consistent in (001)/[110] welding configuration. The different distribution is attributed to growth crystallography and dendrite selection. Significant increase of supersaturation of liquid aluminum and dendrite tip undercooling from [010] dendrite growth region to [100] dendrite growth region preferentially aggravates microstructure development as result of nucleation and growth of stray grain formation with the same heat input on each half of the weld pool in (001)/[110] welding configuration. High heat input (both increasing laser power and decreasing welding speed) exacerbates supersaturation of liquid aluminum and dendrite tip undercooling by faster diffusion to incur stray grain formation with severity of contributing thermometallurgical factors for susceptibility to solidification cracking, while low heat input (both decreasing laser power and increasing welding speed) ameliorates microstructure development and increases resistance to solidification cracking. Weld microstructure of optimum welding conditions, such as combination of low heat input and (001)/[100] welding configuration, is less susceptible to solidification cracking to suppress asymmetrical microstructure development and improve weld integrity potential rather than insidious welding conditions, such as combination of high heat input and (001)/[110] welding configuration. Severer supersaturation of liquid aluminum and wider dendrite tip undercooling occur in the [100] dendrite region as consequence of alloying enrichment, while smaller supersaturation of liquid aluminum and narrower dendrite tip undercooling occur in the [001] dendrite region as consequence of alloying depletion to spontaneously facilitate epitaxial growth of single-crystal essential. Symmetrical (001)/[100] welding configuration decreases growth kinetics of dendrite tip with smaller overall supersaturation of liquid aluminum and dendrite tip undercooling than that of asymmetrical (001)/[110] welding configuration regardless of combination of laser power and welding speed. Mitigation of supersaturation of liquid aluminum and dendrite tip undercooling simultaneously alleviate crack-susceptible microstructure development and solidification cracking. Additionally, the appropriate mechanism of solidification cracking resistance improvement through modification of crystallography-dependent supersaturation and undercooling of dendrite tip is proposed. Calculation analyses are sufficiently explained by experiment results in a reasonable way. The additional purpose of this theoretical analysis is to evaluate solidification cracking susceptibility of similar nickel-based or iron-based single-crystal superalloys.

Abstract: Free standing 3C-SiC wafers with a dimeter of 50 mm and a thickness of ca. 0.8 mm have been grown on a regular base using 3C-SiC CVD seed transfer from Si wafers to a poly-SiC-carrier and a sublimation epitaxy configuration. Up to the thickness of almost 1 mm, stable growth conditions of the cubic polytype have been achieved. The high supersaturation was kept stable by the proper design of the hot zone that enables a high axial temperature gradient at the growth interface. The Sirich gas phase was realized by the application of a Tantalum getter that was integrated into the graphitebased growth cell. Furthermore, an adaption of the growth setup allowed the growth of 3C material with a diameter of 95 mm and bulk material up to 3 mm on 25 mm diameter. Computer simulations were used to determine the supersaturation of the growth setup for different source-to-seed distances. The minimum supersaturation necessary for stable growth of cubic SiC was found to be higher 0.1 for seed already containing the required 3C polytype.

Abstract: Two 3inch SiC boules were grown in a PVT setup using source material of different packing density. During the growth, in-situ computed tomography of the growing boules showed differences in the development of the growth interface. A slightly bent growth interface was found for the smaller packing density. For the higher packing density the resulting crystal exhibits the onset of 6 pyramidal facets on its flanks. Besides that, strong anisotropic lateral growth was found on its (000-1) facet. Numerical simulations show an impact of the powder on the thermal gradient in the growth cell and therefore on the supersaturation. It is discussed that a higher supersaturation can account for the anisotropy in the growth rate of the [1-100] and the [11-20] direction.

Abstract: The study of diffusion processes in the aluminum - copper system was carried out at the temperature 350 and 520 °C. Special attention was paid on the chemical composition of the system near Al/Cu interface. It was determined that the intermediate phases in the system, corresponding to the equilibrium phase diagram, were not formed at low temperature. At high temperature the intermediate phases forms starting with Cu - rich phases. In both cases supersaturated solid solution of copper in aluminum could be observed near the interface.

Abstract: The article deals with the developed of the model of supersaturated moist air flow with condensation. The proposed model is suitable especially for the cases where the rate of change of state variables in the flow field is slow like the flow in natural draft wet-cooling towers and in general the low Mach number flow. Void fraction of gas phase is included in governing equations. Homogeneous equilibrium model, where the two phases are well mixed and have the same velocity, is used. The results of the numerical solutions of the convergent nozzle are presented. The numerical fluxes are based on AUSM + -up scheme and temporal discretization is based on four stage order three explicit strong stability preserving Runge-Kutta method with non-negative coefficients.

Abstract: We have investigated the growth of 3C-SiC using sublimation growth in the temperature range from 1800°C to 1950°C. The supersaturation was determined using numerical modeling of the temperature field and gas phase composition by applying quasi-equilibrium thermodynamic conditions. Analysis of the 3C-SiC yield was carried out by optical microscopy, optical absorption, Raman spectroscopy and x-ray analysis. Quantitative data on supersaturation are compared with most stable 3C-SiC nucleation and growth condition. Finally the application to large area growth in a physical vapor transport growth reactor is briefly addressed.

Abstract: It is generally agreed that during diffusion process between two phases e.g. matrix/diffusant layer the thermodynamic equilibrium state is fulfilled and the supersaturated solid solutions can not formed by diffusion. Nevertheless, in many cases the formation of equilibrium phases does not occur due to kinetic and other reasons, and metastable phases can be formed. The analysis of the concentration profiles for bulk diffusion obtained at low temperatures consistent with B-regime for grain boundary diffusion in systems Cu/Al, Fe and Co/Cu is performed. It is shown that in these systems the supersaturated solutions are formed and the extent of supersaturation is the more as the diffusion temperature is lower. The concentration of diffusant may be 5-10-times greater than solubility according to phase diagram.

Abstract: Nano-structured hydrotalcite based mixed oxides have been synthesized using coprecipitation method under variable pH and low supersaturation condition. XRD technique has been used to confirm the hydrotalcite structure and its derived different phase of mixed oxides. The metal dispersion of mixed oxides was analyzed using ICP-MS. The nanostructures of the mixed oxides have been investigated using FESEM and HRTEM. The textural properties of mixed oxides were analyzed using N₂ adsorption-desorption (BET) technique. The Characterizations have revealed that the developed mixed oxides were consisted with hexagonal/rhombohedral well dispersed nano-particles. Polycrystalline mixed oxides formed mesopore surface and narrower pore size distribution.