Jo Jul 7, 2025
Aluminum and its alloys have light weight, good thermal and electrical properties, excellent mechanical properties and process ability, and low cost, so they are widely used in various fields of the national economy such as the mechanical industry, aerospace, daily necessities industry, electronic industry, building materials industry, etc. Aluminum and its alloys are easily oxidized in the atmosphere because of their strong chemical activity, resulting in a thin oxide film on the surface, which is protective because of its good corrosion resistance in dry air, but it is easily damaged in contaminated atmosphere, water, seawater, soil and various corrosive media, which is not protective. Therefore, surface treatment is carried out to enhance the surface performance of aluminum and its alloy, such as corrosion resistance, wear resistance and decorativeness.
At present, the sealing of anodized aluminum alloy is carried out at high temperature, which leads to high energy consumption and low productivity.
Kim Chang Sok, a researcher at the Science Engineering Institute, has investigated the sealing mechanism of Ni-F system to realize room temperature sealing, one of the ways to reduce energy consumption and shorten the production cycle in the sealing process for surface treatment of aluminum alloy, and investigated the influence of various factors on the room temperature sealing quality to confirm a reasonable room temperature sealing process index.
Through the room temperature sealing experiments of aluminum alloy 6061, he has found that the optimum conditions are Ni2+ concentration of 1.5g/L, F- concentration of 2g/L, sealing temperature of 20~30℃, sealing time of 15~20min and pH of 5.5~6.5.
The surface properties of the samples sealed at room temperature are superior to the surface properties of the existing sealed samples at high temperature. In addition, the sealing temperature can be lowered by over 70℃ and the time for sealing can be shortened by 15 minutes, which leads to the decrease in energy consumption for heating sealing solution and the increase in productivity.
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Jo Jul 4, 2025
Multistage centrifugal pumps are commonly-used pieces of turbomachinery for applications with high head and flow duty requirements. So, the improvement of their technological indexes is greatly significant for saving energy, ensuring production and raising economic benefit.
Since the axial force in high-pressure multistage pumps reaches tens of tons, it is essential to balance hydraulic axial thrust. Even then, an axial bearing is required; and its size depends largely on the prediction accuracy of axial thrust and on the type of the balancing device. Therefore, the size of bearings and the axial thrust balance systems of multistage pumps are frequently determined on the basis of axial force measurements carried out on each particular type and hydraulic pressure of pumps.
In general, balancing devices are used for balancing the axial thrust acting on the rotors of multistage centrifugal pumps. Balancing devices create forces that oppose the axial thrust generated by the impellers. When dimensioned correctly, balancing devices are able to completely balance the axial thrust, so the required pressure automatically builds up in the radial gap due to an axial displacement of the rotor. Therefore, the axial clearance at the device depends on the axial force acting on the rotor, i.e. on the operation point of the pump.
Balancing devices have two gaps: a radial gap and an axial clearance. The mutual coupling of these two gaps allows automatic balancing of the axial force. According to the variation of operation point of a centrifugal pump, the axial thrust changes. The rotor moves left and right in the axial direction to be in a new equilibrium state, when the axial clearance changes with the movement.
If the axial displacement of the rotor is too large, the balancing device may contact with the casing to cause wear. Thus, the rotor vibrates and the stability of operation is destroyed. Therefore, when balancing device is designed, even small axial displacement must yield large changes in the balancing force. In previous studies, this performance was estimated by sensitivity coefficient.
Sensitivity factor represents the variation of balancing force according to the pressure distribution in front and rear shrouds of impeller, but does not represent variation of balancing force when a rotor moves. Therefore, it can be more correct to estimate the automatic balancing ability of a balancing device by the parameter representing the variation of balancing force when a rotor moves a unit length.
Han Pok Nam, an institute head at the Faculty of Mechanical Science and Technology, has decided leakage through radial and axial clearance, pressure difference and balancing force based on the theoretical analysis of the gap flow of a balancing device. Then, he has defined a new concept of stiffness coefficient and analyzed the relationship between stiffness coefficient and geometrical parameters.
Thus, he has found that the stiffness coefficient decreases with too large or too small sensitivity coefficient, and a reasonable sensitivity coefficient should be chosen around 0.5.
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Jo Jul 3, 2025
It is well known that TEA-CO2 laser is a kind of high-power laser with enough pulse peak output, and study on TEA-CO2 laser has been consistently conducted for a long period of time. The design of electrode structure, which emits field electrons in TEA-CO2 laser, is a question of finding an effective method for increasing laser power by raising the emission efficiency of field electrons and their utilization.
In practice, the tips of cold-cathode are made in the shape of a needle or a blade. Such tips emit field electrons at low discharge voltage by raising the amplification degree of electric field. When the electric field strength at the tips is too high, the lifetime of tips will be shortened owing to the cathode sputtering. The field electron generator that contains cold-cathode provides high vacuum of 10-4Pa.
The Marx generator supplies electric power to the generator of field electrons. Here, such geometric parameters as the height and basal width of tips and the size of anode cells are all related to the amplification degree of electric field. The electric field strength at the tips is affected by not only the shape and size of each tip but also their arrangement. When high voltage is applied to the cathode, the existent electric fields in the vicinity of the tips interact with each other, which obviously influences the electric field strength at the tips.
Pak Sin Hyok, a researcher at the Institute of Nano Science and Technology, has designed an electrode structure of cold cathode and grid anode with a high amplification degree of electric field strength at the tip, in consideration of the tip’s parameters, the arrangement of electrode structure, etc.
He designed the tips as a triangle, and simulated the electric field in the discharge region by the COMSOL Multiphysics software while changing the size and array interstice of tips and the dimension of grid anode corresponding to the tips. On this basis, he found out the optimum design parameters of electrode for raising the amplification degree of electric field strength at a tip to the maximum.
The simulation result shows that the optimum interstice of tips that provides the maximum amplification degree of electric field was 8mm, and the amplification degree of electric field strength was 1.2.
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Jo Jun 27, 2025
Graphene has extraordinary mechanical strength and excellent thermal and electrical conductivity. With these merits, graphene has been an ideal reinforcement for high-performance metal matrix composites, attracting a great deal of attention.
In particular, graphene-reinforced Al matrix composites (AMCs) are superior to Cu-based composites and other various Al alloys due to their advantages such as high strength, high conductivity, light weight and low cost. In accordance with the desired terms, the optimal properties of graphene-reinforced AMCs are obtained by manipulating the content and dispersion of graphene layers. However, it still remains difficult to determine the optimal content and realize the uniform dispersion of graphene in the Al metal matrix.
Jon Sin Hyok, a researcher at the Faculty of Materials Science and Technology, in cooperation with some other researchers, has conducted a first-principles study of the interface binding nature, mechanical strength, and electronic properties of aluminum/graphene (Al/G) composites, using superlattice models with varying graphene content.
Their calculations have revealed the weak binding between Al and graphene layers with no new chemical bonding at the interface and the gradual decrease in binding strength with increasing graphene content.
You can find the details in his paper “Influence of interposed graphene sheets on mechanical and electronic properties of Al/graphene superlattice” in “Applied Physics Letters” (SCI).
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Jo Jun 26, 2025
Accurate determination of blood glucose concentration has a great significance in the prevention and treatment of diabetes as well as in the food processing and fermentation.
Glucose oxidase (GOx) modified electrodes play an important role in blood glucose detection by electrochemical methods. Enzyme-based sensors have disadvantages such as low enzyme stability and complicated immobilization.
To eliminate these disadvantages, non-enzymatic sensors with metal and metal oxide nanoparticles have been used. However, these sensors have some disadvantages such as poor sensitivity and selectivity and low catalytic activity. Some researchers eliminated these disadvantages by decorating the metal / metal oxide nanostructures on active carbon supports including carbon nanotubes (CNTs), graphene, etc.
Fe3O4 MNPs with good magnetic property are widely used for electrochemical biosensors and, in particular, they are also used for glucose detection as a direct non-enzymatic biosensor with no glucose oxidase.
Pak Wi Song, a section head at the Faculty of Chemical Engineering, has prepared an electrochemical sensing platform for glucose sensing by magnetic loading of Fe3O4/GO nanocomposites on graphite-epoxy composite electrode (GECE).
He characterized the Fe3O4/GO/GECE modified electrode by scanning electron microscopy (SEM), X-ray diffraction analysis (XRD) and cyclic voltammetry (CV).
The fabricated biosensor exhibited excellent electrocatalytic activity. The linear range for glucose is from 0.5 to 6.5mM with the detection limit of 268.5μM (S/N =3) and the sensitivity of 63.5mA/mM.
For more information, please refer to his paper “Magnetic Fe3O4/GO nanocomposite-modified graphite-epoxy composite electrode for high-performance glucose sensing” in “International Journal of Electrochemical Science” (SCI).
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Jo Jun 24, 2025
Thermoelectric phenomenon is a physical process in which electric current flows by heat diffusion when there exists a temperature gradient in thermoelectric materials such as conductors or semiconductors. Therefore, it is very important to enhance the thermoelectric performance of corresponding material in thermoelectric applications. The performance of thermoelectric material describes how much thermal energy can be directly converted into electrical energy.
Thus, semiconductors whose electrical conductivity and Seebeck coefficient are between conductors and insulators were used in thermoelectric applications. In general, as electrical conductivity and thermal conductivity are proportional to the concentration of carriers such as electrons and phonons, the better the electrical conductivity of material is, the better its thermal conductivity is. However, it is difficult to obtain materials with both high electrical conductivity and low thermal conductivity. Fortunately, the idea that thermal conductivity can be reduced by high entropy design has attracted a great deal of interest of researchers who were making efforts to develop high entropy materials (HEMs) with good thermoelectric property.
Pang Chol Ho, a researcher at the Faculty of Materials Science and Technology, has newly developed an improved residual error non-homogeneous grey model and estimated the thermoelectric performance parameters of high entropy materials (HEMs) using this model.
Firstly, by combining the non-homogeneous grey model, residual error processing method and Markov model, he improved the forecasting accuracy of the model.
Secondly, he performed a comparative analysis of several HEMs using the proposed IRENHGM (1, 1) model and other grey models. The results showed that the Mean Absolute Percentage Error (MAPE) value of the proposed model is less than 0.02, which is the highest in the forecasting accuracy.
For more details, you can refer to his paper “Estimating the Thermoelectric Performance Parameters of High Entropy Materials by the Improved Residual Error Non-homogeneous Grey Model(1, 1)” in “The Journal of Grey System” (SCI).
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