Jo Jul 28, 2024
Development and use of natural energy plays an important role in guaranteeing the sustainable development of economy and satisfying the energy demands. In this field, study on the solar energy is particularly important. Recently, the efficiency of perovskite solar cells (PSCs) which have become the focus of solar energy development almost reached the efficiency of silicon solar cells and therefore a lot of efforts are being made to ensure the stability of solar cells.
The stability of PSCs primarily depends on the stability of perovskite which performs the function of light absorber in devices. For this reason, many researchers are trying to improve the heat and humidity stability of perovskite.
Recently, it has been found that ion liquid, which has been studied in several fields including medicine due to its special properties such as high viscosity, low vapor pressure and incombustibility, can be used as an additive or a solvent for perovskite precursor to improve the performance and stability of PSCs.
Ion liquid, well known as an eco-friendly harmless solvent, has polarity that can solve perovskite precursor, so it can replace the toxic DMF or DMSO that have been conventionally used for fabrication of PSCs.
Kang Jin Hyok, a researcher at the Faculty of Information Science and Technology, investigated the influence of ion liquid methylammonium acetate (MAAc) on the characteristics of perovskite and the performance of hole transport material (HTM)-free PSCs with carbon electrode when it is used as a solvent of precursor for preparing perovskite.
The perovskite was prepared by solid–gas reaction between dimethylammonium lead iodide (DMAPbI3) and methylamine (MA). DMAPbI3 was solved by ion liquid MAAc instead of DMF and it was used as a precursor.
The perovskite film based on ion liquid was more uniform and denser than the one based on DMF, thus improving the crystallinity and crystal size of perovskite. The analysis result showed that the nonradiative recombination of cells decreased and the open circuit voltage could be improved when using MAAc instead of DMF.
For further details, you can refer to his paper “The characteristics of hole-transport material-free perovskite solar cells with carbon electrode made by using ion liquid methylammonium acetate solvent” in “Results in Optics” (SCI).
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Jo Jul 26, 2024
Increasing the measurable distance and the displacement measurement limit of a self-mixing displacement sensor poses one of the most important problems to improve the sensor’s performance.
In order to solve the problem, there has to be found an estimation method of the optical feedback factor for a strong feedback regime.
The hardship of the estimation in this regime is brought about by a strong speckle noise and the restriction of normalization methods presented hitherto.
Kim Chol Hyon, a section head at the Faculty of Physical Engineering, has found an approach to improving the accuracy of the self-mixing sensor, which can work even for long range and large displacement, by deciding the region of the optical feedback factor based on the ratio of fringe height and by using it and the fringe counting method.
The experiments showed that the SM displacement sensor based on the new approach will be a great help to the enlargement of the measurement range of the sensor.
If further information is needed, please refer to his paper “Amelioration of accuracy of displacement measurement in strong feedback regime based on region decision: enlargement of measurement range of self-mixing displacement sensor” in “Optics Communications” (SCI).
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Jo Jul 25, 2024
To raise the speed and accuracy of displacement retrieval by overcoming the weakness of time-consuming parameter estimation and complex displacement retrieval algorithm is very important in improving the real-time quality of measurement.
Kim Il Hyok, a student at the Faculty of Physical Engineering, has proposed a new method for parameter estimation and displacement retrieval which does not require complex normalization and is of high precision of displacement retrieval and incomparably less time-consuming than existing data fitting based methods.
It is a high-speed and high-precision method of displacement reconstruction based on data-fitting method in the near-half-fringe zone collaborating with the fringe-counting method. The method proposed is applicable to any optical feedback level and to any motion. The method provides the fastest estimation and the highest precision of displacement retrieval. Its displacement reconstruction error is less than 16nm and its estimation speed is over 30 times as fast as conventional methods.
You can find the details in his paper “Betterment of displacement retrieval method based on data fitting in near-half-fringe zone” in “Applied optics” (SCI).
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Jo Jul 23, 2024
Dielectric Barrier Discharge (DBD) has been applied in many fields such as ozone synthesis, degradation of VOCs, plasma etching, surface modification, actuators, sterilization of medical devices, etc., and it has been attracting a great deal of attention of researchers in recent years. In order to find and control the features of DBDs, a lot of efforts have been devoted to their theoretical, numerical and experimental investigation. The advance in numerical simulation encouraged the development of DBD technology.
DBD is generally produced between two electrodes, which have several shapes such as parallel plate, concentric cylinder, and so on. Parallel plate electrodes are preferred as it not only makes it easy to produce large area plasma, but is also flexible and convenient for treating film-like materials or gases.
So far, finite element method (FEM) has been dominant in simulating DBD plasma. Despite the fact that FEM is well-defined and powerful for unstructured grid, it is essentially non-conservative and complicated. In order to avoid checkboard instability due to unstructured grids, special care is needed with its integrals.
In recent years, the LB method has been successfully applied to some hydrodynamic problems such as Navier–Stokes and advection–diffusion equations. It is based on the well-known Boltzmann equation rather than hydrodynamic equations. Unlike other methods, the LB method chases distribution of particles within each discretized node, which ensures that non-linearity is local and non-locality is linear. Although the LB method is more memory-sensitive and requires weak compressibility, it is preferred for simulating multicomponent flow in complex geometries such as porous media. Parallel computation is available in this method, which means it is faster than others. LB method is not only simple, but also relatively stable to implement. Its stability depends mainly on lattice velocity and advection velocity, which allows us to adjust only two parameters for stability.
Choe Yong Son, a researcher at the Faculty of Physical Engineering, applied a lattice Boltzmann (LB) D1Q3 scheme to the numerical simulation of multi-component plasma by dielectric barrier discharge (DBD).
DBD in atmospheric argon is generated in a small gap of 0.5mm between two parallel plate electrodes, and is driven by high voltage AC of about 20kHz.
In order to find the characteristics of the DBD plasma, he coupled the LB numerical model with continuity and Poisson equations. The simulation results showed that LB method can be used for simulation of DBD plasma.
For more information, please refer to his paper “One-dimensional lattice Boltzmann simulation of parallel plate dielectric barrier discharge plasma in atmospheric argon” in “Mathematics and Computers in Simulation” (SCI).
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Jo Jul 22, 2024
Fasteners are widely used in all industrial sectors and the auto industry alone consumes 2 800 to 3 100 fasteners for assembly of a vehicle. Externally threaded fasteners comprise the bulk of fasteners used in these applications with over 90% of being produced by thread rolling.
Thread rolling is a cold forming or chipless machining operation where a matched set of dies with reverse form of thread displaces material to produce external threads on cylindrical or conical blanks with no material loss. It ensures specialization of production and increased production.
Hence, it is very important to determine reasonable process parameters by simulating a thread rolling process and analyzing the influence of process parameters, for the improvement of product accuracy and lengthening of a roll’s life. Actually, it is very difficult to survey the change of force acting on the roll and blank fitting apparatus. Therefore, the influence of parameters on thread rolling process cannot be correctly determined by real experiments. Thus, prior researchers simulated the thread rolling process by computer modeling. However, their simulations were restricted to 2D simulation due to the difficulties arising in 3D simulation of metric thread rolling process.
Pae Ung Chol, a researcher at the Faculty of Materials Science and Technology, has conducted a 3D simulation of thread rolling process to find influence factors and predict their tendency.
He analyzed the influence of process parameters such as blank diameter, roll diameter and fitting height of blank using a finite element code DEFORM.
The results of the study showed that the blank diameter has a little influence on product accuracy and the life of thread rolling die (roll) while roll diameter has a significant impact on its life, and that fitting height of blank has a remarkable influence on wear of roll screw thread and product accuracy.
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Jo Jul 19, 2024
In a three-phase power system, the voltages at the generation side are sinusoidal waves equal in magnitude with 120° phase difference. At the load side, however, voltages may become unbalanced due to phase angle deviations or unequal distribution of single phase loads caused by unequal voltage magnitudes of each phase. Voltage unbalance is a major power quality issue because a small unbalance in the phase voltages can even cause a larger unbalance in the phase currents.
A completely balanced three-phase system contains only positive sequence components of voltage, current and impedance, whereas an unbalanced system contains both positive and negative sequence components of voltages and currents. The negative sequence component of current in the unbalanced system increases temperature loss in the equipment. Hence, it is necessary to mitigate this problem by removing the negative sequence current at the load side and keep the source side balanced.
Reactive power compensation can be ensured by using Static Var Compensators (SVCs), DSTATCOMs, Static Synchronous Series Compensators (SSSCs) and Unified Power Quality Conditioners (UPQCs), thus improving power quality.
Jong Il Bok, a researcher at the Faculty of Electrical Engineering, proposed a controller to determine the difference value between load current and positive sequence current as the standard value of compensating current and a proportional complex integral controller to control the current of each phase for compensating asymmetry and harmonics of load using a distribution static compensator (DSTATCOM). Then, he studied the performance of the controller by simulating the entire system in the MATLAB/Simulink environment.
This technique makes it possible to compensate asymmetry or harmonics of reactive power of load and load current. In addition, it makes switching frequency constant and improves the current tracking ability.
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