Jo Jun 20, 2024
Addition of Zn element into CdS improves the short circuit current density and open circuit voltage to increase the efficiency of hetero-junction based solar cells such as CdTe and CIGS, decreasing the absorption losses of the window materials in p-n junction devices and the lattice mismatch.
Over the past three decades, there have been a number of reports on the deposition and properties of Cd1-xZnxS ternary compound with a various range of Zn concentration (low Zn content, midst content and overall Zn content), using different methods like chemical bath deposition, spray pyrolysis, SILAR, vacuum evaporation, MOCVD, RF sputtering, vapor zinc chloride treatment, etc.
Ki Tong Hun, a student at the Faculty of Electronics, has deposited Cd1-xZnxS thin films (0<x<1) by the CBD method and investigated the influence of Zn content on the structural, morphological, optical and electrical properties of films by using XRD, SEM, UV-VIS spectroscopy and other analysis instruments.
The analysis results can be summarized as follows;
XRD studies revealed that the crystal phase of Cd1-xZnxS thin film changed from the hexagonal to hexagonal/cubic mixed phase at lower Zn contents, and then transformed to cubic ZnS phase as Zn content further increased.
Through the SEM measurements, he could observe certain-sized granules formed by the agglutination of Cd and Zn compounds around the nucleus due to the absorption of Cd2+ and Zn2+ ions with initial increase of Zn concentration. On the other hand, for the Zn content greater than 0.6, Cd1-xZnxS particle size decreased and the mixture phase in thin film became stable.
For further details, please refer to his paper “Investigation on the influence of Zn content on the structural, optical, morphological and electrical properties of ternary compound Cd1-xZnxS window layer for CdTe solar cell” in “Functional Materials Letters” (SCI).
...
Jo Jun 18, 2024
There have been some studies related to the chemical and electrochemical behaviour of cerium in molten salts. The eutectic LiCl-KCl, the eutectic NaCl-KCl, the eutectic LiF-KF-NaF and room temperature AlCl3-1-methyl-3-ethylimidazolium chloride are used most for this kind of study, and some differences have been found in the stability and the electrochemical properties of cerium ions in these media.
Kim Pyong Hun, a section head at the General Assay Office, has studied the electrochemical reduction of CeO2 in eutectic CaCl2-NaCl melt by means of the cyclic voltammetry and the constant voltage electrolysis.
One cathodic current peak in the cyclic voltammogram was observed and a one-step electrochemical reduction mechanism of CeO2 appeared. For different hours of electrolysis at the temperature 800℃ and at the constant voltage 3.0V, the products of CeOCl and CeO2 were observed by XRD. This result confirms the electrochemical reduction sequence: CeO2→CeOCl.
...
Jo Jun 17, 2024
Redox flow battery (RFB) has been developed in popularity since 1970s and become a promising large-scale energy storage system recently. RFB is an electrochemical device that stores and releases energy through redox reactions of active ion species in electrolyte. The main advantage of RFB over other electrochemical devices such as lead acid battery and lithium ion battery is that the power system could be made very flexible because of the independence of its power from its energy and that its cost per energy in high-capacity systems is particularly low. The drawback is that the ion selective membrane for these RFBs is very expensive and it should satisfy very strict requirements for several aspects including ion selectivity, electric conductivity and chemical stability.
This has led to the recent trend of not using membranes. The advantage of the soluble lead flow battery is the absence of a membrane because only one electrolyte is used.
Kim Ju Song, a researcher at the Faculty of Chemistry, has suggested lead fluoroborate dissolved in aqueous solution of fluoroboric acid as electrolyte of SLFB and investigated the electrochemical characteristics. He chose paste of waste lead acid batteries as a source of lead and investigated its effects on the performance of electrolyte.
The results are as follows.
The conductivity of 1.5 mol/L electrolyte prepared with recycled lead has a peak when the concentration of HBF4 is less than 1.0 mol/L. The kinetic characteristics of Pb2+/Pb and Pb2+/PbO2 are almost the same in both reagent grade lead and recycled lead electrolytes. When electrolytes prepared from recycled lead are used, the average charging efficiency and the average voltage efficiency are all shown to be superior to those of methanesulfonic acid cells.
...
Jo Jun 16, 2024
Electrospinning is a method of producing micro/nano fibers by injecting polymer solutions in electrostatic fields. In an electric field, the polymer droplets at the nozzle tip form a Taylor cone. When the electric field force to overcome the surface tension of the polymer solution is applied, a fine spinning flow is emitted from the tip of the Taylor cone, and during its flying towards the collector, solvent evaporation and cooling of the melt result in solid nanofibers deposited onto the collector.
Recently, highly productive electrospinning methods such as multi-nozzle electrospinning and needleless elecrospinning have been proposed. Multi-nozzle electrospinning is a method with many overlapped nozzles. It is difficult to realize stable and continuous spinning because corona discharge is created by strong electrical interaction between the nozzles, due to the nonuniform distribution of the electric field at the nozzle tip. Therefore, creating a uniform electric field between nozzles is very important for multi-nozzle electrospinning. Prediction of the diameter of electrospun fibers is also important in their applications.
Not only some methods of controlling the structure and diameter of electrospun fibers by changing the structure and arrangement of nozzles, but also a number of attempts to improve the structure and orientation degree of electrospun fiber by changing the setup of collectors have been proposed. However, few attempts have been made to obtain uniform electrospun fibers by changing the shape of collector geometry.
Pak Yong Ho, a researcher at the Nano Physics Engineering Institute, has determined a new type of curved profile for a collector to obtain uniform fibers in an arched multi-nozzle electrospinning device.
Applying the calculation model for polymer jet in an electrostatic field using Calculating Fluid Dynamics (CFD), he calculated the velocity field distribution in electrospun polymer jet fluid, and determined the uniform velocity profile. Taking a curved surface determined by the uniform velocity profile as a collector, he constructed an arched multi-nozzle electrospinning system. To obtain uniform electrospun nanofiber membranes suitable for preparation of nanofiber ion exchange membrane for electrodialysis applications, he proposed an equation of the curve for collector geometry. Curved collectors need no additional means for obtaining electrospun fibres in uniform diameter.
...
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.
...
