• The Bulletin of The Korean Astronomical Society
• /
• v.36 no.2
• /
• pp.94.2-94.2
• /
• 2011

Electron microbursts, energetic electron precipitation having duration less than 1 sec, have been thought to be generated by chorus wave and electron interactions. While the coincidence of chorus and microburst occurrence supports the wave-particle interaction theory, more crucial evidences have not been observed to explain the origin of microbursts. We propose the measurement of energy dispersion of microbursts could be an evidence supporting wave-particle theory. During chorus waves propagate along magnetic field, the resonance condition should be satisfied at different magnetic latitude for different energy electrons. If we observed electron microbursts at low altitude, the arrival time of different energy electrons should make unique dispersion structures. In order to observe such energy dispersion, we need a detector having fast time resolution and wide energy range. Our study is motivated from defining the time resolution and energy range of the detectors required to measure microburst energy dispersions. We performed test particles simulation to investigate how electrons interact with simple coherent waves like chorus waves. We compute a large number of electron's trajectories and successfully produce energy dispersion structures expected when microbursts are observed with 10 msec time resolution detectors at the altitude of 600 km. These results provide useful information in designing electron detectors for the future mission.

• Composites Research
• /
• v.32 no.3
• /
• pp.141-147
• /
• 2019

In this study, micro-sized and nano-sized pure aluminum (Al) powders were compressed by unidirectional pressure at room temperature. Although neither type of Al bulk was heated, they had a high relative density and improved mechanical properties. The microstructural analysis showed a difference in the process of densification according to particle size, and the mechanical properties were measured by the Vickers hardness test and the nano indentation test. The Vickers hardness of micro Al and nano Al fabricated in this study was five to eight times that of ordinary Al. The grain refinement effect was considered to be one of the strengthening factors, and the Hall-Petch equation was introduced to analyze the improved hardness caused by grain size reduction. In addition, the effect of particle size and dispersion of aluminum oxide in the bulk were additionally considered. Based on these results, the present study facilitates the examination of the effect of particle size on the mechanical properties of compacted bulk fabricated by the powder metallurgy method and suggests the possible way to improve the mechanical properties of nano-crystalline powders.

• Journal of Advanced Marine Engineering and Technology
• /
• v.23 no.3
• /
• pp.331-337
• /
• 1999

Cracks on the cylinder block of diesel engines will often happen due to cyclic load and thermal stress. According to the Classification Societies' rules welding reparis of cylinder block made of cast irons are generally not permitted. However such welding repairs became inevitable taking enormous cost and time for their renewal into consideration. In this study repair welding methods for the clinder blocks made of gray cast irons were reviewed and the tests of their welds were carried out in order to purpose the repair welding meth-ods of packing seat and o-ring seat of cylinder block and apply them to the practice. The following conclusions are botained :1 The tensile strength of weld of cast iron more than that of base metal can be obtained by means of preheating keeping temperature above $100^{\circ}C$ between welding pass-es preventing slag inclusion peening and cramping weld metal by studs. 2. The suspected crack by a magnetic particle test due to different magnetic permeability can be identified which are not associated with a mechanical discotinuity.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
• /
• 2003.10a
• /
• pp.313-317
• /
• 2003

Micro-Channel ultra-precision polishing is a new technology used in magnetic field-assisted relishing. In this paper, an electromagnet or the i18 of test system was designed and manufactured. A size of magnetic abrasive is used on 25~75${\mu}{\textrm}{m}$ and for the polish a micro-channel upper part. A surface of channel which is not even is manufactured using magnetic abrasive finishing at upper surface of micro-channel. As a result, the surface roughness rose by 80％ after upper surface of micro- channel was polished up 8 minutes by polishing.

• Investigative Magnetic Resonance Imaging
• /
• v.7 no.1
• /
• pp.39-46
• /
• 2003

Purpose : To develop a theoretical model for magnetic relaxation behavior of the superparamagnetic nano-particle agent, which demonstrates multi-functionality such as liver- and lymp node-specificity. Based on the developed model, the computer simulation was performed to clarify the relationship between relaxation time and the applied magnetic field strength. Materials and Methods : The ultrasmall superparamagnetic iron oxide (USPIO) was encapsulated with biocompatiable polymer, to develop a relaxation model based on outsphere mechanism, which was resulting from diffusion and/or electron spin fluctuation. In addition, Brillouin function was introduced to describe the full magnetization by considering the fact that the low-field approximation, which was adapted in paramagnetic case, is no longer valid. The developed model describes therefore the T1 and T2 relaxation behavior of superparamagnetic iron oxide both in low-field and in high-field. Based on our model, the computer simulation was performed to test the relaxation behavior of superparamagnetic contrast agent over various magnetic fields using MathCad (MathCad, U.S.A.), a symbolic computation software. Results : For T1 and T2 magnetic relaxation characteristics of ultrasmall superparamagnetic iron oxide, the theoretical model showed that at low field (<1.0 Mhz), $\tau_{S1}(\tau_{S2}$, in case of T2), which is a correlation time in spectral density function, plays a major role. This suggests that realignment of nano-magnetic particles is most important at low magnetic field. On the other hand, at high field, $\tau$, which is another correlation time in spectral density function, plays a major role. Since $\tau$ is closely related to particle size, this suggests that the difference in R1 and R2 over particle sizes, at high field, is resulting not from the realignment of particles but from the particle size itself. Within normal body temperature region, the temperature dependence of T1 and T2 relaxation time showed that there is no change in T1 and T2 relaxation times at high field. Especially, T1 showed less temperature dependence compared to T2. Conclusion : We developed a theoretical model of r magnetic relaxation behavior of ultrasmall superparamagnetic iron oxide (USPIO), which was reported to show clinical multi-functionality by utilizing physical properties of nano-magnetic particle. In addition, based on the developed model, the computer simulation was performed to investigate the relationship between relaxation time of USPIO and the applied magnetic field strength.

• Transactions of Materials Processing
• /
• v.9 no.5
• /
• pp.494-503
• /
• 2000

The electro-magnetic stirring and mechanical process were applied to fabricate particulate metal matrix composites(PMMCs) with various particle size. The mechanical test on PMMCs was carried out in order to clarify the effect of 76 heat treatment on tensile behaviors. In order to study the thixoforming of PMMCs, fabricated billet are reheated by using the coil designed as a function of length between PMMC billet and coil surface, coil diameter and billet length. The effect of reinforcement distribution on billet temperature variation has been investigated with the calculated solid fraction theory based on a function of matrix alloy and volume fraction of reinforcement.

• Journal of Power System Engineering
• /
• v.14 no.4
• /
• pp.63-67
• /
• 2010

This paper introduces the eddy current signal characteristic of magnetic and non-magnetic gas turbine rotor. In the past, Magnetic particle inspection method was used in magnetic material for qualitative defect evaluation and the ultrasonic test method was used for quantitative evaluation. Nowadays, eddy current method is used in magnetic gas turbine rotor inspection due to advanced sensor design technology. We are studying on the magnetic gas turbine rotor by using eddy current method. We prepared diverse depth specimens made by magnetic and non-magnetic materials. We select optimum frequency according to material standard penetration data and experiment results. We got the signal on magnetic and non-magnetic material about 0.2 mm, 05 mm, 1.0 mm, 1.5 mm 2.0 mm and 2.5 mm depth defects and compare the signal amplitude and signal trend according to defect depth and frequency. The results show that signal amplitudes of magnetic are bigger than non-magnetic material and the trends are similar on every defect depth and frequency. The detection and resolution capabilities of eddy current are more effective in magnetic material than in non-magnetic materials. So, the eddy current method is effective inspection method on magnetic gas turbine rotor. And it has the merits of time saving and simple procedure by elimination of the ultrasonic inspection in traditional inspection method.

• Journal of the Korean Society for Railway
• /
• v.8 no.5
• /
• pp.419-424
• /
• 2005

This paper has performed fatigue and nondestructive test of bogie frame for Korean tilting train. Before the fatigue test, static tests were carried out. From the test, the structural safety was investigated using Goodman diagram. After the static test, the fatigue test were conducted under tilting load conditions. The fatigue test was conducted for $10{\times}10^6$ cycles. During the fatigue test, the nondestructive tests using magnetic particle and liquid penetrant were performed at $6{\times}10^6$ cycle and $10{\times}10^6$cycle. From the crack detection tests, it was known that there was no fatigue crack in the bogie frame.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.30 no.2 s.245
• /
• pp.210-218
• /
• 2006

This paper has performed static, fatigue and nondestructive test of tilting' bolster frame for Korean tilting train. From the static test, the structural safety was investigated using Goodman diagram. After the static test, the fatigue test were conducted under tilting load conditions. The fatigue test was conducted for $10\times10^6$ cycles. During the fatigue test, the nondestructive tests using magnetic particle and liquid penetrant were performed at $6\times10^6$cycle and $10\times10^6$cycle. From the crack detection tests, it was known that there was no fatigue crack in the tilting bolster.

• Proceedings of the KIEE Conference
• /
• 1989.07a
• /
• pp.447-450
• /
• 1989

Single particle orbit in plasma is obtained by drift Hamiltonian formulation in magnetic coordinate. The collisional effect is implied by Monte Carlo Method and the velocity space diffusion, energy transfer to the back ground plasma and the variation of energy distribution of test particles are investigated from many particles analysis.