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- 저자 : Frantisek (검색결과 16 건)
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A Numerical Model of the Temperature Field of the Cast and Solidified Ceramic Material
Frantisek Kavicka,Jana Dobrovska,Bohumil Sekanina,Karel Stransky,Josef Stetina 한국소성가공학회 2010 기타자료 Vol.2010 No.6
Corundo-baddeleyit material (CBM) ? EUCOR ? is a heat- and wear-resistant material even at extreme temperatures. This article introduces a numerical model of solidification and cooling of this material in a non-metallic mould. The model is capable of determining the total solidification time of the casting and also the place of the casting which solidifies last. Furthermore, it is possible to calculate the temperature gradient in any point and time, and also determine the local solidification time and the solidification interval of any point. The local solidification time is one of the input parameters for the cooperating model of chemical heterogeneity. This second model and its application on samples of EUCOR prove that the applied method of measurement of chemical heterogeneity provides detailed quantitative information on the material structure and makes it possible to analyse the solidification process. The analysis of this process entails statistical processing of the results of the measurements of the heterogeneity of the components of EUCOR and performs correlation of individual components during solidification. The crystallisation process seems to be very complicated, where the macro- and microscopic segregations differ significantly. The verification of both numerical models was conducted on a real cast 350 x 200 x 400 mm block.
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Creep processes in silicon nitride ceramics
Frantisek Lofaj,Sheldon M. Wiederhorn 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.3
Recent advances in our understanding of creep deformation in silicon nitride ceramics are reviewed and compared to two different models of creep. The classical models adopted from the metals literature are based on the assumption that creep occurs primarily by diffusion of atoms either through the grains, or along grain boundaries. The cavitation model of creep was developed specifically to explain creep in materials that consist of rigid grains with a mobile secondary phase at the grain boundaries, materials having structures similar to that of silicon nitride. Well-known effects such as creep asymmetry and a very wide range of stress exponents in the early commercial grades of silicon nitride can be fully understood within the framework of the cavitation models. The work discussed includes an identification of critical types of creep cavities in silicon nitride, the evolution of cavities with tensile strain, and an analysis of possible mechanisms involved in cavity formation. The analysis amplifies the cavitation creep model of Luecke and Wiederhorn and assumes that creep occurs via a combination of grain boundary sliding, viscous flow and solution-precipitation of the crystalline secondary phase, resulting in a redistribution of this phase among the multigrain junctions of the solid. The increase in creep resistance in the latest generation of silicon nitride materials was found to be related to the suppression of cavitation and a shift toward non-cavitation creep mechanisms. Differences between volume conservative mechanisms in tension and compression depend on the existence of different driving forces for creep: local tensile/compressive stresses and/or dilatational stresses. Increasing the viscosity of residual glassy films at the grain boundaries is believed to be an effective way to suppress cavitation and increase creep resistance. The addition of Lu+3 and N−3 to the bulk oxynitride glasses, similar to those at the grain boundary films, increases their viscosity. Thus, the suppression of cavitation and the higher creep resistance of the Lu-containing silicon nitride can be explained by the combined effect of Lu+3 and N−3 in the residual glass. Recent advances in our understanding of creep deformation in silicon nitride ceramics are reviewed and compared to two different models of creep. The classical models adopted from the metals literature are based on the assumption that creep occurs primarily by diffusion of atoms either through the grains, or along grain boundaries. The cavitation model of creep was developed specifically to explain creep in materials that consist of rigid grains with a mobile secondary phase at the grain boundaries, materials having structures similar to that of silicon nitride. Well-known effects such as creep asymmetry and a very wide range of stress exponents in the early commercial grades of silicon nitride can be fully understood within the framework of the cavitation models. The work discussed includes an identification of critical types of creep cavities in silicon nitride, the evolution of cavities with tensile strain, and an analysis of possible mechanisms involved in cavity formation. The analysis amplifies the cavitation creep model of Luecke and Wiederhorn and assumes that creep occurs via a combination of grain boundary sliding, viscous flow and solution-precipitation of the crystalline secondary phase, resulting in a redistribution of this phase among the multigrain junctions of the solid. The increase in creep resistance in the latest generation of silicon nitride materials was found to be related to the suppression of cavitation and a shift toward non-cavitation creep mechanisms. Differences between volume conservative mechanisms in tension and compression depend on the existence of different driving forces for creep: local tensile/compressive stresses and/or dilatational stresses. Increasing the viscosity of residual glassy films at the grain boundaries is believed to be an effective way to suppress cavitation and increase creep resistance. The addition of Lu+3 and N−3 to the bulk oxynitride glasses, similar to those at the grain boundary films, increases their viscosity. Thus, the suppression of cavitation and the higher creep resistance of the Lu-containing silicon nitride can be explained by the combined effect of Lu+3 and N−3 in the residual glass.
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Prospects of the practical use of silk sericins
Frantisek Sehnal 한국곤충학회 2008 Entomological Research Vol.38 No.-
In Lepidoptera and Trichoptera, a blend of sericin proteins seals the pair of fibroin filaments into the silk fiber that is coated by additional layers of adhesive sericins. The structure and function, and even the number of sericin proteins, are poorly known. Most sericin applications to date employ mixtures of proteins extracted by hot alkaline water during the cocoon processing in the reeling mills. The extracts were shown to have antioxidant, moistening, UV-shielding and also moderate antibiotic properties. They are used as additives to various cosmetic products and in some countries also as remedies in human medicine. Sericins were reported to suppress tumor promotion, provide protection against ulcers and exert other health supporting effects. They also support proliferation and can provide a basic matrix for the differentiation of the cells cultured in vitro. Commercial production of sericin hydrolyzates as components of the tissue culture media will probably expand with the use of stem cells for the tissue repair. Sericins can polymerize to fibers, films and 3-D structures that provide scaffolds for complex tissue reconstructions. The use of recombinant sericin-type proteins for these applications is in the phase of basic investigations.
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Study of the Adhesive Coefficient Effect on the Hydraulic Losses and Cavitation
Pochly, Frantisek,Fialova, Simona,Kozubkova, Milada,Zavadil, Lukas Korean Society for Fluid machinery 2010 International journal of fluid machinery and syste Vol.3 No.4
The article is focused in three areas. In the first part there are analyzed the adhesion forces at the liquid and solid surface interface. There are shown the measured values of surface energy for different types of surfaces. The value of surface energy is decisive for determining the extent of the surface wettability by the liquid. The second part points to the possible negative effects of partly wettable surfaces, showing susceptibility to cavitation. The third section describes the practical aspects of surface wettability by the liquid. Under the new boundary conditions bases, expressing the effect of adhesion forces, there are determined the centrifugal pump characteristics.
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Recent Curriculum Development in the Early Childhood Geometry in Czech Republic
Kuoina, Frantisek Korean Society of Mathematical Education 1998 수학교육연구 Vol.2 No.1
The paper deals with some aspects of early childhood geometry in the Czech Republic. Children's first geometrical experiences come from real life. In our opinion, there exist four types of geometrical experience which can be called the partition of space, the filling of space motion in space and the dimension of space. We distinguish three levels of the mathematical learning process: a spontaneous level, an operational level and a theoretical level.
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Numerical Optimization of the Method of Cooling of a Massive Casting of Ductile Cast-Iron
Jana Dobrovska,Frantisek Kavicka,Karel Stransky,Bohumil Sekanina,Josef Stetina 한국소성가공학회 2010 기타자료 Vol.2010 No.6
The numerical models of the temperature field of solidifying castings, according to various authors, have been observing two main goals ? directed solidification as the basic assumption for the healthiness of a casting and the optimization of the technology while maintaining the optimal product properties. The achievement of these goals is conditioned by the ability to analyze and, successively, to control the effect of the deciding factors, which either characterize the process or accompany it. An original application of ANSYS simulated the forming of the temperature field of a massive casting from ductile cast-iron during the application various methods of its cooling using steel chills. The numerical model managed to optimize more than one method of cooling but, in addition to that, provided serious results for the successive model of structural and chemical heterogeneity, and so it also contributes to influencing the pouring structure. The file containing the acquired results from both models, as well as from their organic unification, brings new and, simultaneously, remarkable findings of causal relationships between the structural and chemical heterogeneity (i.e. between the sizes of the spheroids of graphite, the cells, density of the spheroids of graphite, etc.) and the local solidification time in any point of the casting. The determined relations therefore enable the prediction of the face density of the spheroids of graphite in dependence on the local solidification time. The calculated temperature field of a two-ton 500x500x1000 mm casting of ductile cast-iron with various methods of cooling has successfully been compared with temperatures obtained experimentally. The casting was cast in sand mould. The calculated model of the kinetics of the temperature field of the casting was verified during casting with temperature measurements in selected points.This has created a tool for the optimization of the structure with an even distribution of the spheroids of graphite in such a way so as to minimize the occurrence of degenerated shapes of graphite, which happens to be one of the conditions for achieving good mechanical properties of castings of ductile cast-iron.
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