Papers by Keyword: Temperature Gradient

Abstract: At the high growth temperatures of the PVT method, thermal radiation from the graphite crucible surface to the seed region is the dominant mode of heat transfer. In this study, we propose a newly designed crucible structure with a thinner graphite wall compared to the conventional design for SiC crystal growth. The SiC ingot grown using the conventional crucible exhibited the smallest thickness variation (flat top surface) between the center and the edge of the ingot, accompanied by polytype inclusions, which led to an increase in defect density. In contrast, SiC ingots grown using the newly designed crucibles (Design A and Design B) showed a convex top surface and a significantly lower defect density due to the improved heat transfer efficiency. Thinning the graphite crucible wall helps maintain a relatively higher temperature at the seed edge region, thereby effectively enhancing thermal radiation in the radial direction inside the crucible. These results indicate that thermal radiation in the radial direction can be achieved through appropriate optimization of the graphite wall thickness.

Abstract: The hot-zone design using an air-pocket was adopted to produce uniform temperature gradient in horizontal direction. In order to investigate the change of temperature gradient toward horizontal direction with growth time, the front shape of SiC growing crystal was measured with different growth stages such as initial, growing and finished stage. While SiC ingot grown in conventional hot-zone design exhibited inhomogeneous growth front in the initial stage of growth and multi facet formation in final stage, which could result in increased defect density, a homogeneous temperature gradient and improved crystal quality was obtained in the modified hot-zone design. Based on the mapping measurement of FWHM (Full width at half maximum) value in X-ray rocking curve, the crystal quality of SiC crystals grown with the modified hot-zone design was observed to be definitely better than conventional design.

Abstract: The paper studies the strength properties of sand-cement rod with the water-cement ratios of 0.4, 0.44, 0.49 and 0.54 under thermal gradient conditions. Experimental research concerns the influence of the temperature gradient within 60 to –20 ^оС on the mechanical properties of the sand-cement mortar with the different water-cement ratio. It is shown that the strength gain rate of sand-cement specimens varies in different periods of curing. The strength index of the sand-cement rod also varies in the conditions of heat and mass transfer that is supported by the theoretical background. The indicated temperature gradient significantly affects the curing process of the sand-cement specimens. Lower intensity of the strength gain is observed in specimens after 4-hour curing. The increase in the curing time from 8 to 12 hours leads to more intensive strength gain starting from the third specimen, when positive temperature begins. After 8-hour curing, the strength gain rate grows starting from the fifth specimen. At last, 12-hour curing results in the higher rate of the strength gain. Further increase in the curing time can lead to the highest strength gain rate. The rupture point drops with increasing water-cement ratio for all the specimens, independently of their position in the rod. Investigation of these processes will provide a better understanding of the negative effect of thermal gradient on the concrete structures and allow finding ways to increase their service life.

Abstract: The paper considers the issue of mathematical simulating the stress-strain state of a roll in the course of its heat treatment. It is shown that a sound heat treatment schedule affects significantly the economic efficiency of a metallurgical enterprise. The mathematical apparatus is provided to estimate the thermal interaction in the casting-furnace system, based on which a program for calculating the thermal state of a roll during heat treatment has been developed. Using this program allows evaluating the thermal stresses occurring in a roll during the heat treatment cycle and reducing the risk of discontinuity in the roll cast billet. Also, using the program developed, allows significant reducing the engineer-technologist man-hours for the design of the process cycle of the casting heat treatment. An example of calculating the thermal state of a roll with a diameter of 930 mm in casting is given. The change in the dynamics of the maximum temperature gradient along the product radius is shown, while correlating it with the furnace setting.

Abstract: Due to the temperature and concentration determine the kinetic undercooling of interface growth and nucleation undercooling inside the melt, they play an important role in the solidification microstructure of the alloy. In this paper, the effect of temperature gradient and cooling rate on the dynamic undercooling was studied and the mechanism of the concentration at the solid-liquid interface on the kinetic undercooling during the continuous cooling process was analyzed. A calculation method for the coupling of temperature and concentration during Inconel 718 alloy solidification was developed, which can solve the problem that the concentration and temperature are difficult to be calculated at the same time in the numerical calculation.

Abstract: This study investigated the crystallization position and formation mechanism of globular crystals at the center area in the thickness direction of aluminum alloy strips cast by a high-speed twin roll caster. Twin roll casters for single strips and clad strips were used, as well as twin roll casters equipped with a cooling slope. The globular crystals were formed from dividing arms of dendrites of the solidified layer facing the center area at the roll gap. The arms of isolated dendrite also divided. No globular crystals were formed at the interfaces of clad strips with different solidification temperatures because of the temperature gradient at the interface which inhibited division of the dendrite arms. It was demonstrated that globular crystals at the center area of the thickness direction were formed by dendrite-arm-dividing at the roll gap by the strip casting clad strip. Experiments by semisolid-strip casting with the cooling slope showed that globular crystals in the molten metal existed in the solidification layers.

Abstract: The aim of this study is to develop a finite element analysis technique to characterize the distributions of temperature and stress in the process of multilayer deposition of metallic powders by laser additive manufacturing (LAM). Simulation results indicate the residual normal stress in the laser moving direction is greater than that in other directions due to a larger temperature gradient, and it increases with number of deposited layers. Highly residual stresses are present in the LAM build and at the base nearby the interface between the build and base.

Abstract: The article contains a description of various growing conditions for isoparametric heterostructures based on InP. It is shown that the structural perfection of solid solutions grown on InP substrates is influenced by parameters, such as the temperature of the epitaxy process, the temperature gradient, and the composition and thickness of the liquid zone. By analyzing the quality of the surface and the structural perfection of isoparametric AlGaInAsP solid solutions, based on InP, optimal parameters of the zone recrystallization process in the temperature gradient field (TGZR), at which the epitaxial films had minimal roughness and high crystalline perfection, were found.

Abstract: The technology for manufacturing corundum-carbide-silicon refractories with increased slag resistance has been developed. It is shown that one of the directions for improving the quality of carbon-containing products is the creation of protective antioxidant coating on the surface of particles. A dense protective film is formed during the processing on the surface of carbon-containing particles. The coating does not break down when heated to the melting temperature of steel. Therefore, the number of heat-conducting particles in the bond can be increased without the risk of loss of strength. An increase of the number of high-heat-conducting particles reduces the value of the temperature gradient inside the refractory and reduces the zone impregnated with the melt. A technique has been developed that makes it possible to determine the degree of interaction of a refractory with metal (slag) in ground samples under the binocular microscope MBS-9. The proof of high slag resistance is the depth of penetration of metal and slag into the structure of the sample by not more than 1 to 2 mm without changing the lateral surface of the conical hole.

Abstract: The work is devoted to the study of defect formation processes in the near-surface layers of silicon under thermal shock conditions and to the effect of preliminary exposure in a constant magnetic field on this process. As a result of investigations it was established that dislocation half-loops near the local heat source are formed in the near-surface layers of Si (with a depth of up to 30 μm) after a current pulse of density j> 5.1010 A / m2 passing through the metallized film on the silicon surface. In addition, it was found that preliminary exposure of samples in a constant magnetic field leads to an increase in the dislocation density compared to samples not exposed in a magnetic field.