Jo Jun 14, 2024
In the diamond tool industry, mechanical mixture of powders which consists of one kind of element was used as traditional binder material. The traditional material is prepared at high temperature and its composition is not homogeneous. What is more, it is difficult to attain perfect alloying. In order to overcome such disadvantages and increase the physical ability of binder material for diamond tools, many researchers started to study alloying of binder material powder. At present, considerable amount of prealloyed powder in addition to pure cobalt metal is used for manufacturing diamond crowns, natural stone material and tools for building material processing. Because all the particles of prealloyed powder contain various kinds of composition of alloy, it has good homogeneity. In addition, its melting point is lower than that of its major elements.
Now, prealloyed powder is mainly prepared by atomization. The powder prepared in this method has low sintering temperature and high alloying level. Also, it has good fluidity and wear resistance and high ability of binding with diamond. However, because alloy must be refined in advance, the demand for equipment is considerably high and it is difficult to control the composition of binder material. What is more, its particles are rough and it has high content of oxygen. It is also very expensive.
Recently, mechanochemical synthesis has become a method of preparing nano- and micrometer scale materials. This is simple and highly productive, so it is especially convenient for mass production of nano and superfine particles.
Ri Hyok Chol, a researcher at the Faculty of Chemistry, has proposed a method of preparing Fe-Cu-Co prealloyed powder for diamond binder material by reducing precursor powder prepared through the reaction of sulfate and sodium hydrate by mechanochemical synthesis, and compared it with the traditional binder material prepared by mechanical mixing.
The results showed that the binder material prepared from the prealloyed powder that underwent mechanochemical and reducing processes has the following characteristics. First, its particles are fine and the grain size distribution is uniform. Second, the alloy prepared with the use of binder material reaches the relative density of about 98%, the bending strength of about 1 120MPa and the hardness of about HRB110. Third, its mechanical property is superior to the materials prepared by mechanical mixing.
...
Jo Jun 13, 2024
Powder injection molding is advantageous for hard metal components with complex shapes or large h/d. In the production of cemented carbide by powder injection molding, the change in the carbon content of the alloy is greater due to the higher content of the moulding agent than in conventional die molding.
The carbon content in cemented carbides is a key factor in determining the properties of alloys. In general, carbon content is controlled by the composition of raw material, the atmosphere of thermal degreasing and the atmosphere of sintering.
Kim Song Il, a researcher at the Faculty of Materials Science and Technology, has succeeded in forming η phase on the microstructure by controlling the binder content and the solvent parameters of debinding process in WC-8Co hard alloy, which leads to the prolongation of life of alloy and cost reduction.
He studied the effect of binder content on the property of products by measuring the carbon content, shrinkage, hardness, transverse rupture strength (TRS) while changing the binder content between 40 and 50vol%. He performed solvent debinding in the mixed solvent of benzole and acetone in the ratio 3:2 at an extraction temperature of 45℃ with an immersion time of two to six hours. Thermal debinding was conducted together with sintering, and the final sintering was done at 1 450℃.
The hardness value and TRS were HV1 560 and 1 580MPa respectively, and the service life was extended by 1.3 times.
...
Jo Jun 11, 2024
A distributed air microbubble generator can produce microbubble solutions of uniform bubble diameter and high concentration. Two-phase fluid mixers of rotor-stator structure are usually used for generating microbubbles by distributed air. The structure design of its stator and rotor depends on the composition of two-phase medium to disperse. Both the stator and the rotor have independent effects on the characteristics of the two-phase fluid mixing device.
Rotor-stator mixers are of several types including the Silverson mixer, which is often used for homogenization and dissolution in liquid-liquid two-phase fluids. However, it reveals some defects in gas-liquid two-phase fluid mixtures. It is because the viscosity of gas-liquid two-phase fluid is relatively low compared to liquid-liquid two-phase fluid and the difference in gas and liquid density is very large. It is also because gas dispersion in the liquid is very difficult, and the conventional rotor-stator mixer configuration cannot provide sufficient pressure gradients in the shear gap, an important indicator of microbubble generation, due to the self-characteristics of gas-liquid two-phase fluid flow.
In order to increase the pressure gradient in the shear gap, Cha Su Song, a researcher at the Nano Physics Engineering Institute, has proposed a stator structure with a shear blade and rotor-stator engagement relationship. Then, he has evaluated the bubble diameter distribution formed in gas-liquid two-phase fluid flowing through the generator by the CFD-PBM method and compared it with experimental results.
Both the rotor and stator are of a shear blade structure. He analyzed the bubble diameter distribution in the fluid analysis program ANSYS Fluent 18.1. Then, he built a microbubble generator with the same structure as in the analysis and did some experiments. After that, he measured bubble diameter distribution by zeta potential analyzer (Malvern Zetasizer Nano ZS) and compared it with the analytical results.
The average bubble diameters obtained by CFD analysis and experiments were about 107nm and 113nm, respectively, which were in good agreement. In conclusion, the improvement of stator structure in the distributed air microbubble generator has a significant effect on generator characteristics.
...
Jo Jun 10, 2024
In recent years, motion analysis technology based on inertial measurement units (IMU) has been widely used in the field of sports science due to its advantages such as its low cost and good adaptability in both indoor and outdoor environments. Xsens MVN BIOMECH, one of the motion analysis technologies that are being widely used all over the world, generally uses several IMUs and processes the rotary motion or position changes of body segments quantitatively.
This technology has been applied to motion analysis in several sporting events including horse riding, ski, basketball, tennis, etc.
Choe Nam Chol, a researcher at the Faculty of Electronics, has applied it to the motion analysis of a table tennis player. As the upper limb plays an important role in the table tennis player’s motion using a racket, he set it as a target.
Every player has their own modality in 3D-changes of wrist, elbow and shoulder joints during the stroke of each type. Those unique modalities would be very important for managers to assess and improve each player’s performance. Therefore, he analyzed the modalities of individual players by using the motion analysis system.
First, he mounted four IMUs on the thoracic spine, the upper limb, the forearm and the back of the hand of a player to measure in real time the 3D-angles of rotation of three joints during the forehand, backhand and topspin forehand strokes. Next, he analyzed the motion pattern and the range of motion (ROM) of wrist, elbow and shoulder joints on the basis of the measured data.
The results showed that the proposed analysis method is effective for improvement of technical skills of players.
...
Jo Jun 7, 2024
With the development of large CFB boilers, a cyclone separator with a downward gas exit that provides convenience for the construction and compact layout of boilers appeared. Later, a square cyclone separator was suggested for Π-arrangement in CFB boilers and for convenient installation of heat transfer surfaces on the inner separator walls.
Previous studies of the flow field in cyclone separators with a downward gas exit that focused only on the space between the vertex and the gas exit failed to get flow characteristics in each section of the separator.
Ryom Jun Ho, a researcher at the Faculty of Heat Engineering, has conducted a detailed simulation of the flow characteristics in a square cyclone separator with a downward gas exit.
He used the computational fluid dynamic (CFD) method to predict and evaluate the flow field and collection performance inside the square cyclone separator. In addition, he employed the Reynolds stress model (RSM) for flow field analysis in two cyclone separators with different dust exits.
The results demonstrate that the flow characteristics and collection performance of the square cyclone separator with a vertical dust exit are better than that with an inclined exit.
...
Jo Jun 5, 2024
A research team led by Kim Mun Hui, a researcher at the Faculty of Electrical Engineering, has developed a small wind power generation system (SWPGS) for producing maximum power using a permanent magnet generator with two coils.
The generator with two coils ensures not only higher efficiency but also lower expense of the SWPGS than that with one coil, because it can automatically make the load curve more closely approach to the maximum power characteristic curve of a wind turbine without any control circuits.
The system consists of a three-blade horizontal axis wind turbine, a permanent magnet generator (PMG) with two coils, a rectifier, a battery, a load, etc.
PMG with two coils ensures higher accuracy of power control than that with one coil.
In the PMG, only the first coil works at low wind speed, and if the wind speed becomes higher, the second coil works, too.
As a result, the power generation system is driven at its maximum power because it is able to steadily approximate the load curve to the maximum power curve of the wind turbine.
Thus, this system can produce more electric power from the same wind resources in low wind speed areas.
...
