Jo May 28, 2024
Analyzing coercivity mechanism is one of the key problems to understand magnetic behavior and to improve the magnetic properties of ferromagnetic materials including permanent magnets.
Up to now, many studies have been done to analyze the coercivity mechanisms of different kinds of permanent magnets, especially rare earth nanocrystalline permanent magnets based on Nd-Fe-B or Sm-Co alloys.
The coercivity mechanism of nanocrystalline magnets also depends on microstructures, especially on grain boundary phase. A study on coercivity of Nd-Fe-B sintered magnets showed that existence of amorphous Nd-rich grain boundary between Nd2Fe14B grains strengthens domain wall pinning so that coercivity is enhanced. And a study on the magnetic properties of high performance Dy-doped Nd-Fe-B sintered magnet concluded that the Dy-rich shell increases the nucleation field from the grain boundary and the Nd-rich layers weaken the inter-granular exchange coupling to enhance coercivity.
Kim Kyong Gi, a researcher at the Faculty of Physical Engineering, evaluated and analysed the relationship between coercivity and α-Fe layer thickness in an exchange coupled Nd2Fe14B/α-Fe nanocomposite multilayer system by comparing the results of previous experimental and micromagnetic studies.
The results showed that the coercivity decreases with increasing α-Fe layer thickness, which is coincident with the results of prior micromagnetic and experimental studies.
For further information, please refer to his paper “An analytic study on coercivity mechanism of exchange coupled Nd2Fe14B/α-Fe nanocomposite magnets” in “Journal of Magnetism and Magnetic Materials” (SCI).
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Jo May 26, 2024
Blockchain is a distributed ledger that is shared and maintained by all participants on the network based on a consensus protocol. The most widely used consensus mechanism is Proof-of-Work (PoW), which has been deployed in public blockchain networks like Bitcoin and Ethereum. In PoW, block generation requires solving a cryptographic math puzzle whose solution is easy to verify but extremely hard to solve. The participants in the blockchain network exhaust their computing resources to solve the puzzle. Here, block generation is called mining and participants are called miners.
Once a miner successfully mines a block, he becomes eligible to receive a reward. For this reason, miners competitively participate in the mining process. To reduce the average block generation time, individual miners aggregate their computing powers into a mining pool, where all participating miners solve the puzzle together and share the reward. In a mining pool, the pool manager distributes less difficult partial PoW puzzles (pPoW) than the original full PoW puzzle (fPoW) to individual miners. If enough miners solve pPoW, some of these solutions are likely to become the solution to fPoW because each solution to pPoW has a probability of yielding a solution to the fPoW. However, mining pools undermine the decentralization and security of Blockchain. Especially, if a single mining pool owns more than half of the entire network mining capacity, this pool can generate the blocks faster than any other pool, reaping all rewards and choices of transactions to confirm (51% attack).
Meanwhile, in order to be used in bitcoin practically, any solution to the centralized mining problem must preserve the existing blockchain; preserve large investments many miners have made and are planning to make in their equipment; provide a seamless transition from the existing system to the new one, providing adjustable knobs that can be fine-tuned for a desired trade-off that fits the community’s needs.
However, none of the proposals satisfies all the requirements above. In other words, preceding techniques require some changes in the design of the cryptocurrency and so, they are not compatible with the current bitcoin system.
Kim Kyu Chol, a section head at the Faculty of Information Science and Technology, has proposed a hybrid PoW consensus protocol in order to discourage centralization and tackle the 51% attack. In the proposed scheme, miners can generate the block by solving either an original outsourceable bitcoin puzzle or a non-outsourceable puzzle.
The main feature of his system is that it is fully compatible with current bitcoin designs, i.e., it can be implemented right now, because it preserves both the existing blockchain and investments which have been made in mining hardware. In contrast to the current bitcoin system and any other preceding protocols, his scheme presents two puzzles, but is still single-tiered.
You can find further details in his paper “Single-Tiered Hybrid PoW Consensus Protocol to Encourage Decentralization in Bitcoin” in “Security and Communication Networks” (SCI).
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Jo May 24, 2024
Water inrush in mining fields is considered as one of the major dangerous factors for safe production and workers’ lives in coal mines. To forecast the water inrush in coal mines, various software related to flow simulation has been used in recent years. Especially, Visual MODFLOW (VMOD) is one of the powerful software that provides the tools for building three-dimensional groundwater conceptual and numerical models.
Visual MODFLOW (VMOD) can simulate the water flow distribution state with the hydrogeological conditions and detailed data in accordance with pumping well tests of the mine. In numerical simulations for underwater flow, repeated measurements of water level is essential for forecasting water inrush, so the control point drawdown must be clearly calculated in accordance with the mining construction plans, dewatering period and drainage time.
Song Myong Song, a researcher at the Faculty of Mining Engineering, has simulated the water flow distribution state considering the hydrogeological conditions and detailed data in accordance with pumping well tests of a mine.
First, he built a mathematical model to simulate the water inrush along the major faults of the mine by using three-dimensional simulation of groundwater flow analysis software (Visual MODFLOW). Then, on the basis of the full analysis of the geological and hydrogeological conditions, he set several major faults as boundary conditions and evaluated the water level distribution under steady flow conditions using the in-situ well pumping test data. After that, he determined optimal parameters in relation to the water inrush forecasting using the PEST-ASP module in Visual MODFLOW in order to get the accurate groundwater flow distribution at the control points including normal and maximum water inrush.
Comparing the simulation results with in-situ data of water inrush, the relative error was lower than 0.74%, showing that this method is the most effective for forecasting water inrush in coal mines.
For further details, please refer to his paper “Forecast of Mine Inflow using In-situ Well Test and Visual Modflow in an Underground Mine” in “CHALLENGES IN ROCK MECHANICS & ROCK ENGINEERING”, proceedings of the 15th ISRM Congress 2023 & 72nd Geomechanics Colloquium.
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Jo May 22, 2024
Today, demand for energy has been increasing and the environment pollution has become serious with the rapid growth of economy day by day. The reality urgently requires reduction of fossil fuel consumption and wide use of renewable energy resources.
Geothermal energy, a kind of the renewable energies such as solar energy and waste heat from the industry, has been widely used for power generation as an energy source unaffected by seasons and environment. Organic Rankine cycle (ORC) is recognized as a practical solution for small power production using low grade geothermal resources thanks to its simplicity, reliability, and flexibility.
Selection of the working fluid, cycle configuration and operation parameters most suitable for given temperature of heat source is a key process to improve the performance of ORC. For this reason, many researchers have reviewed the optimal combination of ORC configurations, working fluid and cycle parameters under the conditions of heat source above 100℃ and below 100℃. However, it is difficult to identify the optimal ORC configuration, working fluid and cycle parameters for particular areas from the reported data since the best working fluid and cycle parameters vary with the temperatures of heat and cooling source and the system configuration.
Ri Hung Nam, a section head at the Faculty of Heat Engineering, has performed thermodynamic optimization of ORC based on the net work output and exergy efficiency in areas with different geothermal and cooling water temperatures.
He determined optimal ORC system configuration, best working fluid, optimal evaporation temperature and the optimum area through comparative analysis of 8 different ORC systems using 8 working fluids in the two areas. He proposed a multi-objective function combined with the net work output and exergy efficiency to evaluate the overall thermodynamic performances of ORC.
As a result, he found that even though the exergy efficiency is relatively low in the area with lower temperatures of geothermal and cooling water, the net work output is greater and the overall thermodynamic performance is more powerful. It was shown that the area where geothermal and cooling water temperatures are relatively low is the optimal one in terms of overall thermodynamic performance of ORC.
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Jo May 20, 2024
Recently, aluminum has been widely used in various industries including aerospace, mechanical and wire cable industries, for its good mechanical properties and engineering properties.
Aluminum used in the wire cable industry has relatively low density, low melting point and good corrosion resistance compared to other metallic materials such as copper, steel, etc. However, it has worse mechanical properties.
It is also possible to increase the mechanical properties of aluminum by adding various alloying elements. However, in pure aluminum (more than 99.5%), mixed elements such as Fe, Si, Cu, etc. bring several negative effects. Therefore, it is important to investigate and introduce new ways to improve the mechanical properties of pure aluminum.
Carbon nanotubes have been widely used as reinforcement material for composites because of their mechanical, physicochemical and electrical properties. From the mechanical point of view, the combination of sp2 hybridization mode of carbon nanotubes is one of the very strong chemical bonds. Therefore, carbon nanotubes with ideal structure have very good mechanical properties.
Hence, addition of carbon nanotubes, an ideal reinforced material for composites, can be the most suitable method for achieving grain refinement of the matrix and enhancing the mechanical properties.
Jon Song Won, a researcher at the Faculty of Materials Science and Technology, has investigated the effect of carbon nanotubes on the properties of aluminum-carbon nanotube composites fabricated by the melting method.
He has found that the tensile strength, elongation, bending number and hardness are all high when the carbon nanotube content is 0.3%.
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Jo May 19, 2024
Rotor system, which plays an important role in modern industries, is used in many machines such as internal combustion and jet engines, various types of motors, generator turbines and so on. High speed and high power are the development direction of modern rotating machinery, and fatigue crack is apt to happen as one of the most serious damages in the heavy loading environment. Therefore, detecting the crack in a timely way is very important for safe operation.
Kim Yu Jong, a researcher at the Faculty of Mechanical Science and Technology, has proposed a finite element model of a hollow shaft with open cracks. She derived the time-varying stiffness matrix of finite element based on the transformation matrix by which node displacements are converted from the rotating coordinates into the fixed ones. She employed the harmonic balance method (HBM) to find the responses of a cracked rotor system.
She analyzed the nonlinear vibration of rotor system and compared the results with some published results, considering the eccentricity of disk, the dimensionless crack depth, the ratio between the inner radius and outer radius of the rotor and the angle between the crack and imbalance directions.
As a result, she drew the following conclusions.
The subcritical rotational speeds corresponding to the first critical forward whirling speeds are not excited in the open cracked hollow rotor system. For the deep crack, the first forward whirling rotational speeds decrease more rapidly than the first backward ones as the dimensionless crack depth increases.
The whirl orbits near the subcritical backward whirl speeds have the same shape and behavior for both breathing and open crack models. However, the variations of the whirl orbits shape with the change in the rotational speeds are slower at the deep crack than at the shallow crack.
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