Keun Yong Sohn

Microstructure and phase analyses of melt-spun Si-Ni base anode materials for Li-ion battery

Si-based anode composite materials have been studied to improve the performance and the durability of Li-ion secondary batteries in this study. Si-Ni-Al, Si-Ni-Cu and Si-Ni-Cu-Al base alloys were designed and rapidly solidified at the cooling rate of about 106 °C/sec by optimizing the melt spinning. The ribbons were characterized using FE-SEM equipped with EDS, X-ray diffractometer and HR-TEM. The thin ribbons of Si-Ni-Al alloy consisted of nano-sized Si particles and amorphous matrix, which was regarded as an ideal microstructure for the anode material. At the wheel side of the ribbon, 20–30 nm of Si particles were formed (Zone A); whereas at the air side relatively large Si particles were distributed (Zone B). The Si-Ni-Cu alloy showed coarser Si particles than the Si-Ni-Al alloy, and its matrix consisted of NiSi2, Cu3Si and amorphous structures. Finally, the microstructure of the Si-Ni-Cu-Al alloy strips was composed of coarse Si particles, CuNi, Al4Cu9, NiSi2, and unknown phases, and the size of those Si particles were too large to be used for the anode materials.

Electrochemical properties of rapidly solidified Si-Ti-Ni(-Cu) base anode for Li-ion rechargeable batteries

In this study, rapidly solidified Si-Ti-Ni-Cu alloys have been investigated as high capacity anodes for Li-ion secondary batteries. To obtain nano-sized Si particles dispersed in the inactive matrix, the alloy ribbons were fabricated using the melt spinning process. The thin ribbons were pulverized using ball-milling to make a fine powder of ∼ 4 µm average size. Coin-cell assembly was carried out under an argon gas in a glove box, in which pure lithium was used as a counter-electrode. The cells were cycled using the galvanostatic method in the potential range of 0.01 V and 1.5 V vs. Li/Li+. The microstructure and morphology were examined using an x-ray diffractometer, Field-Emission Scanning Electron Microscopy and High Resolution Transmission Electron Microscopy. Among the anode alloys, the Si70Ti15Ni15 electrodes had the highest discharge capacity (974.1 mAh/g) after the 50th cycle, and the Si60Ti16Ni16Cu8 electrode showed the best coulombic efficiency of ∼95.9% in cyclic behavior. It was revealed that the Si7Ni4Ti4 crystal phase coexisting with an amorphous phase, could more efficiently act as a buffer layer than the fully crystallized Si7Ni4Ti4 phase. Consequently, the electrochemical properties of the anode materials pronouncedly improved when the nano-sized primary Si particle was dispersed in the inactive Si7Ni4Ti4-based matrix mixed with an amorphous structure.

Effect of Ca addition on soft magnetic properties of nanocrystalline Fe-based alloy ribbons

The effect of Ca addition on the magnetic properties of a nanocrystalline Fe-based alloy was investigated. A small amount of Ca (0.06 wt%) was added to the Fe-based alloy, which was then melt spun to fabricate thin ribbons with a thickness of ∼30 μm. These ribbons were heat treated to obtain a nanocrystalline structure with a grain size of ∼10 nm, and the crystallization behavior was studied to optimize the grain structure. The characteristics of the ribbon alloys were analyzed using a B-H meter, a 4-point probe, a transmission electron microscope (TEM), and a scanning electron microscope (SEM). As a result, the optimum permeability and minimum core loss were obtained for the alloy containing Ca, when annealed at 520 °C for 1 h. The analyses revealed that a reduced core loss could be attributed to the high electrical resistivity and suppressed grain growth, which were caused by the Ca element distributed along the grain boundary. Based on the results, Ca addition to Fe-Si-B-Nb-Cu base nanocrystalline alloy was very effective in controlling the grain size, minimizing the eddy current loss, inducing an improved magnetization behavior, and reducing the core loss.

Study of Electromagnetic Wave Absorption Properties with Particle Size in Soft Magnetic Alloy Powder

The electromagnetic wave (EM) absorption properties of various particle size have been investigated in a sheet-type absorber using the alloy powder. With decreasing the average particle size, the complex permeability () and permittivity () increased and the matching frequency is shifted toward lower frequency. The fabricated EM wave absorbers showed permeability , permittivity for a mesh sample, and the calculated power absorption was as high as 80% in the frequency range over 2 GHz.

Finite Element Analysis of Press Forging Process of AZ31 Sheet

In this paper, the distribution of temperatures on specimen and the variations in punch forces on friction coefficients during press forging of AZ31 sheets have been analyzed by Finite Element Analysis technique. Results show that the top of rib and boss areas showed lower temperatures than other areas because of the heat dissipation to air by radiation. The foot regions of the rib and boss showed greater temperature rise due to greater plastic flow. The applied punch force significantly increased at 473K or lower temperatures indicating the forming temperature of AZ31 sheet should not lower than 473K. The friction condition of the punch influenced more significantly on the materials flow pattern compared to those of the die. Using a lubricant on the contact area between the punch and specimen would effectively reduce the punch force and enhance the forming capability.

Variable cubic-polynomial memristor based canonical Chua's chaotic circuit

In this letter, the Chua’s circuit with an active variable memristor is presented for the first time. The central concept behind the variable memristor is that its memductance could be controlled by changing the cubic function which describes a relationship between the flux and charge of the memristor. It was found that chaotic dynamics of the variable-memristorbased Chua’s circuit could be easily controlled according to the memductance profile of the memristor. The controllable chaotic dynamics in the circuit were studied thoroughly by the numerical analysis. The rich dynamics of the Chua’s circuit using the proposed variable memristor were verified in terms of the time series, frequency spectra, phase states, bifurcation and Lyapunov exponents. These results were also confirmed by laboratory experiments.

Microstructural Change and Magnetic Properties of Nanocrystalline Fe-Si-B-Nb-Cu Based Alloys Containing Minor Elements

The effect of minor element additions (Ca, Al) on microstructural change and magnetic properties of Fe-Nb-Cu-Si-B alloy has been investigated, in this paper. The Fe-Si-B-Nb-Cu(-Ca-Al) alloys were prepared by arc melting in argon gas atmosphere. The alloy ribbons were fabricated by melt-spinning, and heat-treated under a nitrogen atmosphere at 520-570oC for 1 h. The soft magnetic properties of the ribbon core were analyzed using the AC B-H meter. A differential scanning calorimetry (DSC) was used to examine the crystallization behavior of the amorphous alloy ribbon. The microstructure was observed by X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM). The addition of Ca increased the electrical resistivity to reduce the eddy current loss. And the addition of Al decreased the intrinsic magnetocrystalline anisotropy K1 resulting in the increased permeability. The reduction in the size of the α-Fe precipitates was observed in the alloys containing of Ca and Al. Based on the results, it can be concluded that the additions of Ca and Al notably improved the soft magnetic properties such as permeability, coercivity and core loss in the Fe-Nb-Cu-Si-B base nanocrystalline alloys.

Effect of crystallization condition on the Microwave properties of Fe-based amorphous alloy flakes and polymer composites

The electromagnetic (EM) wave absorption properties with a variation of crystallization temperature have been investigated in a sheet-type absorber made of the amorphous Fe73Si16B7Nb3Cu1Finemet powder. With the variation of the annealing temperature, the magnetic and dielectric properties of the crystallized Fe-based absorber with a nano-structure were changed. The complex permittivity increased with increasing the annealing temperature, whereas the complex permeability was maximized after annealing at 530°C for 1 hour. The absolute value of the reflection parameter, |S11|, increased with increasing annealing temperature of the nanocrystalline alloy powder. On the contrary, the transmission one, |S21|, showed the highest value after annealing at 530°C for 1 hour, which is regarded as the optimum temperature for the improvement of EM wave absorption properties.

The Characteristic Changes of Electromagnetic Wave Absorption in Fe-based Nanocrystalline P/M Sheets Mixed with Ball-Milled Carbon Nanotubes

【Electromagnetic wave energies are consumed in the form of thermal energy, which is mainly caused by magnetic loss, dielectric loss and conductive loss. In this study, CNT was added to the nanocrystalline soft magnetic materials inducing a high magnetic loss, in order to improve the dielectric loss of the EM wave absorption sheet. Generally, the aspect ratio and the dispersion state of CNT can be changed by the pre-ball milling process, which affects the absorbing properties. After the various ball-milling processes, 1wt% of CNTs were mixed with the nanocrystalline

Mechanical Properties and Deformation Features of AZ31-Sb Alloy

Fe_{73}Si_{16}B_7Nb_{3}Cu_1