Jo Nov 20, 2023
Recently, rare earth elements (REEs) are widely used in special material science, and therefore, preparation and application of REEs have been rapidly developed.
La, Sm, Pr and Nd among REEs are common additives. It is important to determine amounts of them in materials since they change the mechanical and electronic properties of materials.
Kim Pyong Hun, a section head at the General Assay Office, has studied atomization process of La, Sm, Pr and Nd by GFAAS. He also investigated the possibility of preconcentration of REEs by selective evaporation of matrix, and found proper atomization and pyrolization temperatures. Then, he made sure the use of chemical modifier and mini-flow of internal gas at atomization step resulted in relative enhancement in sensitivity.
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Jo Nov 18, 2023
Organic and inorganic compounds have been used as a modifier for enhancing bonding force and strength of sodium silicate sand at room and high temperatures. Sodium silicate is generally used as an inorganic binder for producing molds and cores which are hardened by CO2 or ester.
The additive amount of sodium silicate should be decreased by raising its binding capacity to improve the strength and collapsibility. Therefore, it is necessary to refine colloidal particles and change the structure of sodium silicate by modifying it.
Many studies on modification of sodium silicate have been concentrated on application of organic compounds with functional group.
O Kwang Hi, an institute head at the Faculty of Materials Science and Technology, has modified the size of colloidal particles of sodium silicate by sodium phosphate. Then, he has measured and compared the sizes of colloidal particles of modified and unmodified sodium silicate by using SEM. After that, he has observed the change of crystal of modified and unmodified sodium silicate by XRD.
For modification, he set the pressure at 0.4~0.5MPa, time at four to five hours and additive amount of modifier at 4%.
The main conclusions of his study are as follows:
Firstly, the heating temperature of sodium silicate core modified by sodium phosphate is 760~1 400℃ for cast iron and steel castings with wall thickness of 8 to 60mm, and after being heated at the temperature of 800~1 200℃ the retained compressive strength and the knockout energy of the modified sodium silicate sand are about 1/3 and about 1/2 lower than those of unmodified one.
Secondly, when the modified sodium silicate is heated and cooled, collapsibility of the mold is improved due to the difference in the shrinkage stresses between silicate glass and phosphate glass.
Thirdly, when sodium phosphate is added to sodium silicate as a modifier, Na2O-SiO2 binary system is changed into Na2O-SiO2-P2O5 ternary system, and net colloidal structure is formed to enhance the binding strength as a result of interaction between phosphate tetrahedroid and silicate tetrahedroid.
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Jo Nov 17, 2023
Practical application of manipulators needs determination of their dynamic errors to guarantee positioning accuracy.
Many researchers determined kinematic errors of Delta parallel manipulators, but not dynamic errors.
Pang Thae Jin, a researcher at the Robotics Institute, has established a new method for dynamic error determination of a Delta parallel manipulator with three actuators using a deformation model.
He wrote an algorithm based on the kinematic and dynamic models and developed a program for obtaining dynamic errors of a Delta parallel robot, using MATLAB software.
His method can be applied to construction of control systems for error compensation of complicated manipulators of different shapes.
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Jo Nov 14, 2023
Al-Si alloys are widely used in producing precise wear-resistant parts, such as air condition compressors, automobile engine pistons and cylinder bodies in automotive industry and machine-building industry due to the best cast ability among cast aluminum alloys. Especially, hypereutectic Al-Si alloys are rich in silicon. Therefore, they have some advantages such as small coefficient of heat expansion, good resistance to wear, high dimensional stability and lower casting cost.
At the same time, they have some disadvantages of lowering mechanical properties by primary crystal silicon as rough polygon or piece layers unless modified treatment is not enough in crystallization process because of high content of silicon.
Al alloys containing a trace of rare earth metal have high strength, hardness, wear resistance, corrosion resistance and electrical conductivity.
Preceding researches were mainly conducted on the effects of rare earth metal on the mechanical properties of Al-Si alloys.
Ri Hyon Mo, a researcher at the Faculty of Metal Engineering, has investigated the effects of Al-10RE master alloy as grain finer on the tensile strength, elongation percentage and resistance to wear of AiSi19Cu2주. Al-10RE master alloy was used as grain finer of AiSi19Cu2주.
The tensile strength, elongation percentage and resistance to wear of Al-Si hypereutectic cast alloy increase with increase in additive amount of Al-10RE master alloy up to 5%, but they decrease over 5%. After heat treatment, the tensile strength and elongation were 296MPa and 0.84%, respectively, when the additive amount of Al-10RE master alloy was 5%. They increase by 24.4% and 44.8% respectively more than AiSi19Cu2주 without addition of Al-10RE master alloy.
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Jo Nov 10, 2023
Core and Weber set are the most important set-valued solution concepts in TU-games. The core is a set of efficient payoff vectors that satisfy coalitional rationality i.e., a coalition receives at least its own worth, while the Weber set is a convex hull of all marginal vectors. In TU-games, core is always contained in Weber set. Furthermore, a game is convex if and only if its core coincides with its Weber set.
Inclusion of Weber set into core plays an important role in studies of stability of the Shapley value (inclusion of the Shapley value into the core) because the Shapley value is defined as a mean value of all marginal vectors.
O Un Suk, a lecturer at the Faculty of Applied Mathematics, has proved the stability of the extent/intent Shapley value using the inclusion of the extent/intent Weber set into the extent/intent core in games on concept lattices.
In games on extents, she introduced extent Weber set, newly defined strong convexity, and proved the inclusion of the extent Weber set into the extent core under this condition. Similarly, she proved that the intent Weber set is included into the intent core under the weak concavity in games on intents. Finally, she studied the relation between the game on extents and the game on intents, and derived a sufficient condition for stability of two Shapley values.
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Jo Nov 7, 2023
Game theory is a set of algebraic models to investigate behavior conflicting with each other. In practice, event players often have a game in uncertain conditions with lack of opponents’ strategies.
Especially, in such games as two-person zero-sum games, the payoff value of each player’s strategy is imperfect, so high accuracy of player modeling cannot be guaranteed. In general, statistical method processes uncertainty mathematically with lots of observed numerical value data, but observed data usually contain occasional fluctuation caused instantly by various reasons as well as essential information about opponents.
Being used as a technology of several variables which identifies common characteristic factors in an invariable set, statistical factor analysis uses probability methods for factor analysis. However, these methods include invariable assumptions about probability distribution so the results may not always be right. Also, there are some cases where modeling is not suitable in practice. Though modeling is not correct strictly, we can draw nearly right conclusions from those hypotheses if it is approximately suitable.
Game theoretical decision adoption is a decision-making problem which determines how the best result can be obtained when the result depends both on its own and on the opponent’s actions.
Some decision-making problems like a zero-sum game are inflexible in target weight according to the structure, so they are changed and affected by opponents. Information analysis methods used in the past are not sufficient to deal with these problems.
Kim Ok, a researcher at the Faculty of Information Science and Technology, has proposed an algorithm where a player’s model can be estimated by a small amount of game data in a two-person zero-sum game characterized by a monotonically decreasing function.
The algorithm is based on the analysis of game process profiles. First, she introduced ordered weight operators to the strategic sequence determination of players and defined a compensation function considering monotonically decreasing character of a two-person zero-sum game. Then, she evaluated the game payoff value of the strategy and conducted player modeling.
The proposed algorithm will contribute to enhancing the accuracy of player modeling in two-person zero-sum games.
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