Jo Aug 18, 2024
Determination and description problems are two basic problems of Formal Concept Analysis (FCA). Since most methods of determining the concept lattice are based on generation of non-neighbor concepts, the process of analyzing the neighboring relation between generated concepts is required to solve the description problem.
Pak Chol Hong, a researcher at the Faculty of Applied Mathematics, has proposed some new efficient algorithms for simultaneously describing the concept lattice and its hierarchy-matrix without this process. The hierarchy-matrix is a successful description of the concept lattice, by which any software can autonomously understand the information of hierarchy of the concepts. The focus of his algorithms is on generation of neighbor concepts tested for canonicity and registering the neighbor information in the hierarchy-matrix sequentially.
He drew the following conclusions.
First, the concepts of subcontexts induced by a given concept are lower (or upper) ones of the concept and all lower (or upper) neighbor concepts of the concept are denoted by the antitone Galois connections on the subcontexts.
Second, all lower (or upper) neighbor concepts of a given concept are generated by the antitone Galois connections on the subcontexts and restricted by the rank of the subcontex-matrices.
Third, the key aspects of the efficiency of the concept lattice and its hierarchy-matrix based on the generation of lower neighbor concepts (BCLHMLN) and the concept lattice and its hierarchy-matrix based on the generation of upper neighbor concepts (BCLHMUN) are generation of neighbor concepts on the subcontext, the equivalence class of the object or the attribute with respect to adjoin mappings of Galois connections, the design of canonicity test and the utilization of the histories of generated lower (or upper) concepts.
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Jo Aug 16, 2024
In recent years, a number of modal identification methods based on continuous wavelet transform (CWT) have been developed. The time variation of the instantaneous amplitude and phase of each mode component within measured signals can be considered by time-frequency representation. In the time-frequency plane, wavelet analysis decomposes the measured signals into a series of curves called ridges which directly express the amplitude and phase of each mode component within the measured signals. Thus, by the extraction of ridges and the value of CWT along the ridges, modal parameters are identified from the measured signals. The measured signals are free-decay responses of mechanical structures which are processed by time-frequency transform. In this process, the challenge is to determine ridges exactly. When the frequency window of CWT includes closely spaced natural frequencies, the ridge is distorted and a pseudo mode is generated.
Existing methods for modal identification by CWT give correct results when the frequency window includes one natural frequency. Thus, it is necessary to build a theoretical basis for correctly determining closely spaced natural frequencies by considering the distortion of ridges and determining the reasonable bandwidth when the frequency window includes two closely spaced natural frequencies.
Ri Yong Ho, a researcher at the Science Engineering Institute, has proposed a formula for mathematically considering the distortion of characteristics of a ridge when the frequency window includes two close mode components, and conducted a numerical simulation on MATLAB to verify it.
The simulation verified the accuracy of the proposed formula.
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Jo Aug 15, 2024
Most rare earth minerals contain uranium, thorium and their decay products, so the technology for removing radioactivity plays a key role in the production of rare earths, especially in reduction of production cost, environmental protection and application of rare earth products. Furthermore, complete removal of trace radioactive substances existing in rare earth products is highly significant for wide application of rare earths and in medicine for health protection and cure.
Up to now, many methods for separating and removing radioactive substances have been reported: ion exchange method, solvent extraction method, co-precipitation method, etc. among which co-precipitation method is very effective for infinitesimal amount of radioactive materials such as radium and actinium. In an extremely dilute system, trace radioactive substances are usually separated by co-precipitation with carriers. Porous materials such as diatomaceous earth, activated carbon, fire brick, silica gel and alumina, barium sulfate, strontium sulfate , iron hydroxide(Ⅲ) and aluminum hydroxide are used as carriers.
In an attempt to remove trace radioactive substances existing in industrially produced rare earth carbonate, Kim Chol Ju, a researcher at the Faculty of Chemical Engineering, has conducted an experimental study of the radioactivity removal characteristics of [Ba-Fe] co-precipitation method, the combination of BaSO4 and Fe(OH)3 co-precipitation methods.
In the [Ba-Fe] co-precipitation process, the total removing rate is over 92% and loss rate of rare earths is less than 0.1% in the case of industrially produced rare earth carbonate.
As this method enables prompt and effective removal of radioactive elements such as uranium, thorium, radium, radiolead, etc., remaining in extremely small quantities in rare earth compounds, it could be applied in the field of hydrometallurgy in which rare earth compounds are produced from several kinds of rare earth minerals and in other fields of production and application of rare earth materials from rare earth compounds.
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Jo Aug 12, 2024
It is very important to correctly model and identify damping in all vibration systems of physical devices that involve damping.
A variety of methods and techniques of damping identification have been developed, most of which can be classified into frequency domain (circle-fitting method, half-power bandwidth method, wavelet transform method, etc.) and time domain (logarithmic decrement method, least squares complex exponential (LSCE) method, Smith least squares method, Hilbert transform method, etc.). Some of the damping identification methods in frequency domain have high accuracy, but the algorithm is very complicated, which restricts its wide application in practice.
Matrix method and modal method are also used for damping identification. In the modal method, modal identification is a must as the damping characteristics are decided by means of modal damping ratio. Modal identification is complicated and it is difficult to ensure the correctness of modal parameters. In the matrix method, which directly identifies the damping matrix by experimentally measured FRF, the imaginary part of experimentally measured FRF is decided for damping identification. This method is highly accurate and it is possible to decide the damping ratio of mechanical structures of lower DOF. However, it is very difficult for all numbers of DOF as mechanical structures are of high DOF in practice.
Kim Won Ju, a researcher at the Faculty of Mechanical Science and Technology, has proposed a method for correctly and simply deciding the damping characteristics of the whole system using the experimentally measured FRF and the FE model. Using the experimentally measured frequency response function matrix, he built an experimental model and combined it with the FE model to enlarge the experimental identity data to the whole system.
Comparing his method with preceding ones through simulations and error analysis, he verified its precision and effectiveness.
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Jo Aug 8, 2024
Unijunction transistor (UJT) is a semiconductor with two ohmic base contacts and an emitter p-n junction, so it is called double-base diode. Its static(S-shaped) I-V characteristic has a region with negative differential resistances. Therefore, UJTs are used in pulse generators, sawtooth generators and converters. UJT-based circuits are well known to be simpler and more reliable than those based on diodes and bipolar transistors.
Recently, transducers where the output signal frequency depends on physical quantities such as temperature, light, magnetic field, stress, concentration of gas and radiation level, have been designed with UJTs. Thus, improvement of the switching characteristic of UJT leads to that of the characteristics of these transducers.
To improve the switching characteristic of UJT, a number of researchers have investigated the radiation effect on it. To the best of our knowledge, there are no reports in the literature regarding how the neutron irradiation of UJT affects its peak point voltage.
Pae Kyong Il, a researcher at the Semiconductor Institute, has proposed a new analytical expression for the peak point voltage of UJT considering the effect of neutron irradiation, and proved its validity by comparing the calculated values and those measured from the neutron irradiation on an Si planar UJT.
His study has shown that the calculated values of the peak point voltage of UJT are in good agreement with the measured ones, that the peak point voltage of UJT decreases after neutron irradiation, and that the peak point voltage of UJT decreases gradually with the growing intensity of neutron fluence.
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Jo Aug 4, 2024
The silicon steel plate is widely used as core materials of various devices such as a transformer and an electromotor. However, in order to solve the energy problem, it is very important to develop soft magnetic materials with less iron loss.
Fe-based amorphous and nano-crystalline alloys are widely used because of its low cost and iron loss about one fifth to one third of silicon steel plates. But, they are restricted to some degree in application because the value of maximum flux density of these materials is about 80% of silicon steel plates. Therefore, researches to develop soft magnetic materials with higher maximum flux density and lower core loss have been performed. As a result, new Fe-based nano-crystal soft magnetic materials have been developed. Among them, Fe-Cu-B system nano-crystalline soft magnetic materials are attracting a great deal of attention.
In most studies, the experimental results of the influence of copper, silicon and boron on the magnetic properties were analyzed, but the thermodynamic changing process was not described in detail.
Jo Chol Min, a researcher at the Faculty of Materials Science and Technology, has conducted a study to thermodynamically solve the influence of Cu addition on the crystallization of the Fe-based nano-crystalline soft magnetic alloy fabricated by the melt spinning method. In addition, he has analyzed the influence of silicon and boron on the magnetic properties of Fe-based nano-crystalline soft magnetic alloys to determine the reasonable composite.
He found that the crystallization activation energy of Fe-based alloys containing copper was lower than copper-free alloys, which indicates that copper is effective to accelerate the crystallization of alloys and increase the magnetic property. He also demonstrated that in crystallization heat treatment, the maximum flux density and coercive force of the Fe-based nano-crystalline soft magnetic alloy containing copper were the best with 2% of Si.
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