Jo May 18, 2022
Sim Jae Rim, a section head at the Faculty of Materials Science and Technology, has established a way of making WC-Co-B4C system electrode material for electric spark alloying and an alloying process by this material so that the service life of tools and machine parts could get more than 2.5 times as long.
B4C in the material has a good effect on the formation of alloying layers since it plays the role as flux which decreases the amount of oxide scale made during electric spark alloying. It also decreases the amount of oxides of low hardness and helps the formation of carbides of high hardness.
The reasonable electric conditions for electric spark alloying by WC-Co-B4C system electrode material are short circuit current I=9A, U=24V, C=420㎌ and t=2min/cm2. The thickness and hardness of an electric spark alloying layer by the new material are higher, that is, 90㎛ and 23.5GPa respectively, compared to 70㎛ and 19.5GPa by previous WC-Co system electrode material.
Electric spark alloying by WC-Co-B4C system electrode material forms alloying layers of high hardness on the surfaces of tools and machine parts, which leads to the lengthening of their working life.
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Jo May 16, 2022
The research team led by Ri Ju Hyok, a section head at the Faculty of Heat Engineering, has designed and built a cryogenic liquid tank for storage of oxygen liquid.
The tank is manufactured to store oxygen liquid produced by cryogenic distillation process for a certain period and to supply it in either liquid or gas form.
It is composed of an inner body for storage of oxygen liquid, an outer body, a pipeline for injection and exhaustion, safety factors including checker valves, safety valves, pressure control valves, a level meter and manometers and a self-evaporator for increasing the pressure inside the tank.
The space between the inner and outer bodies is filled with thermal insulation material expanded perlite and made vacuous to reduce evaporation loss and to keep cool.
The problem of preserving the cold to minimize cold loss by a heat bridge was solved, which decreased daily loss to only 0.3~0.8%.
The degree of vacuum in the insulated space is 1.3Pa, and the total heat conductivity of the expanded perlite is 0.017W/(m•K).
With all these advantages it can also be used for storage of other cryogenic liquids like nitrogen, argon, etc. as well as oxygen.
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Jo May 13, 2022
Kim Myong Il, a vice dean at the Faculty of Communications, has invented an indoor antenna for mobile communication.
Compared with previous antennas, this antenna has several advantages ― it ensures enough matching, gain and horizontal omni pattern in a wide band of frequencies; it is easy to feed, produce and assemble; it is light, good-looking and easy to install.
Consisting of a cone monopole, a double cone reflection panel, a curved plastic protection lid and coaxial feeding structure, the antenna assumes horizontal omni pattern in a wide range of frequency such as GSM, WCDMA, LTE, etc. With the gain of 3dBi, it can provide indoor mobile communication service in areas such as the underground of a building where mobile communication service cannot reach.
As a wide-range radiator, a cone monopole has top and bottom panels made of metal (Oblique angle is 25.5, radii are 12mm and 21mm respectively.), with an impedance of 50Ω and a wide radiation area. So it is of nearly omni directional pattern on the horizontal plane.
Vertical radiation can be controlled by a double cone reflection panel consisting of a primary cone reflection panel, a plain reflection panel, an upper cone reflection panel for coaxial feeding and it can bring a gain of 3dBi.
The inner conductor of a feeding coaxial cable is connected to the bottom panel and the outer conductor is linked to the reflection panel. The impedance of the antenna can be controlled by inserting an inclined line between the cone monopole and the reflection panel. Therefore, the impedance of the antenna remains constant in a wide frequency range. What is more, it is easy to install on the wall and it is covered tightly with a radome made of glass fiber plastic in order to prolong the life span and give a good-looking appearance.
In a wider frequency range(756~1 063/1 670~2 777MHz) than in the frequency range of former wire monopoles, the gain is 3.2dBi, which is 1.1dBi higher than former antennas (2.1dBi), so the service bound gets 1.4 times wider than before. The antenna is light, easy to install and meets aesthetic requirements.
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Jo May 11, 2022
Kim Ryul, a researcher at the Faculty of Electronics, has developed a new insulation bushing for a 10kV electric boiler that conforms to the operating conditions of the boiler and guarantees high reliability.
Unlike prior bushings of a cylindrical shape for a 3-phase 3kV electric boiler, a newly-developed one is of a conical shape, satisfies the operating conditions for a 10kV electric boiler and is more reliable and lighter. As its head has been changed into a ring type, they can increase yield, which means it can help reduce the level of environmental pollution and save raw and other materials
The new bushing is lighter by 0.7 times each, which means it has now become cheaper to make.
What is more, it is 2.4 times higher in dielectric strength, 3.3 times in working voltage, 1.25 times in output power, 2.7 times in 5-minute upkeep water pressure and 0.65 times lower in maximum current, which offers a scientific guarantee of the operating conditions and reliability of a 10kV electric boiler.
The yield has increased up to 93%.
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Jo Apr 29, 2022
In general, knowledge on the mechanical properties of a rock mass is a prerequisite for the numerical simulation and the design of the underground structure, opening-up of mineral deposits and mining processes.
Since the early 1990s, many scholars have proposed a variety of methods to determine the strength and deformation parameters of a rock mass using geological strength index (GSI). The standard GSI chart considers qualitatively the surface condition and blockness of a rock mass, and it is used to estimate values between 0 and 100 representing the overall geotechnical quality of the rock mass. The best outcomes can be achieved only by the collaboration between experienced engineering geologists and geotechnical engineers.
To quantitatively determine the GSI, you have to detect the joints in two-dimensional (2D) photographs of a rock mass surface using image processing technology, then determine the fractal dimension, and finally predict the GSI using artificial neural network (ANN).
The detailed steps for joint detection on the rock mass surface are as follows.
Step 1: Converting the color image of a rock mass into a black and white one
Step 2: Smoothing and sharpening
Step 3: Binary encoding
Step 4: Noise removal
Step 5: Detection of the joints
The fractal dimension of a 2D rock mass surface can be calculated with previous research findings.
A 3-layer BP ANN is used for predicting the GSI of the surface of a jointed rock mass.
On the basis of the GSI chart, an ANN model is established, in which the input neurons are the fractal dimension and surface condition index (i.e. roughness and weathering condition), and the output neuron is a GSI value. And the number of neurons of a hidden layer is first set to three and it is finally determined via learning procedure to establish the most effective ANN structure.
In this way, the GSI of a jointed rock mass can be determined quantitatively and objectively by the interface, coded by using built-in tools of MATLAB 7.0, such as image processing, fractal analysis and ANN.
The details of this can be found in the essay, “Determination of geological strength index of a jointed rock mass based on image processing” by Hong Kun Ui, dean of the Faculty of Mining Engineering, presented to the SCI Journal “Journal of Rock Mechanics and Geotechnical Engineering”.
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Jo Apr 26, 2022
Materials selection involves a multiattribute decision making whereby materials engineers choose optimal materials among more than two alternatives on the basis of more than two attributes.
Technique for order preference by similarity to ideal solution (TOPSIS) is a well-known multi attribute decision making (MADM) method and it has been widely used in materials selection.
It is based on the principle that the best alternative must have the shortest distance from the positive ideal solution (PIS) and the farthest distance from the negative ideal solution (NIS).
However, the TOPSIS has a non-negligible drawback: rank reversal. It refers to the change in the ranks of the alternatives when one alternative is removed from or added to the list of alternatives.
Although many works have been conducted, the MADM methods such as TOPSIS still suffer from rank reversal, and it is necessary to give further study to perfectly overcome it.
According to the traditional TOPSIS, when the composition of the alternatives is changed, the normalized decision-matrix is changed. The PIS and the NIS are determined from the maximum or minimum value among the attribute values of the present alternatives. When the alternative with maximum or minimum value is removed or added, the PIS and the NIS are changed. Hence, the distances from the alternatives to the PIS and the NIS are changed, and the relative closeness values are also changed, and therefore, rank reversal may be generated.
To overcome the rank reversal, the normalization method and the determination method of the PIS and the NIS should be irrelevant to the composition of alternatives. Therefore, there is a need to improve the normalization method and the determination method of the PIS and NIS.
The normalization of decision-matrix by the linear max-min normalization method can solve this problem.
The application of the improved TOPSIS without rank reversal to select the best absorbent layer material for thin film solar cells (TFSCs) showed that it could fully overcome rank reversal and that it could be applied to many real materials selection problems in practice.
You can find more information about this in “Materials selection method using improved TOPSIS without rank reversal based on linear max-min normalization with absolute maximum and minimum values” presented by Yang Won Chol, a researcher at the Faculty of Materials Science and Technology, to the SCI Journal “Materials Research Express”.
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