Sung-Duk Kwon

Decomposition of magnetization processes in complex permeability spectra of 3% SiFe

Complex permeability spectra were measured as a function of the ac field amplitude h0 and the cut angle α with respect to the [001] direction in a rectangular 3% SiFe sample. The spectra for α=0° at a relatively low field amplitude showed Debye-type dispersion with a relaxation frequency of 26.3 kHz. However, at a relatively high field amplitude, the spectra changed to a superposition of two dispersions, one with a relaxation frequency at 3.3 kHz and the other with a relaxation frequency at 26.3 kHz. Irreversible and reversible 180° domain wall motions were considered as mechanisms responsible for the h0 dependent relaxation at 3.3 kHz and the h0 independent relaxation at 26.3 kHz, respectively. From the α dependence of the spectra, we obtained another relaxation of the magnetization rotation, with a relaxation frequency of 0.16 MHz, in the original 180° domains for α⩾70°.

Thermal properties of KCl in a modified Einstein model

Abstract The local dynamics of the lattice is taken into consideration to modify the original Einstein model of lattice dynamics, and the modified Einstein model is tested by evaluating thermodynamic properties of crystalline KCl. Specifically, the thermal expansion, thermal expansion coefficient and specific heat at constant volume have been evaluated as a function of temperature at atmospheric pressure for crystalline KCl using a realistic potential of the form A 1 R -9 + A 2 R -6 ± A 3 R -1 and using normal-mode frequencies obtained from the local dynamics of two particles, which are allowed to oscillate in a static potential field due to the rest of the particles assumed to be at rest in their mean positions. The temperature dependence of these thermodynamic functions is examined in comparison with the available experimental data, and a reasonable agreement is observed.

Determination of magnetization rotation and domain wall motion components in reversible complex susceptibility spectra of Co-based amorphous ribbons with various annealing times

The complex susceptibility spectra for the Co 66 Fe 4 Ni 1 B 14 Si 15 ribbons annealed at 380 C under external magnetic field of 3Oe with various annealing times were measured in reversible magnetization range. The susceptibility spectra originating from both relaxation processes of reversible domain wall motion and magnetization rotation are separated from the complex susceptibility spectra measured for the samples. The variation of static susceptibilities and relaxation frequencies of domain wall motion and magnetization rotation with annealing time indicate that the alignment of domains to external field direction onset at the annealing time is about 5 min and is accomplished at about 20 min. For short annealing time < 5 min, magnetic softening is the main effect on magnetization processes.

EVALUATION OF THIN COATING LAYERS USING RAYLEIGH‐LIKE WAVES

It is necessary to evaluate the characteristics of thin coating layers nondestructively for reliable use of coated components since performances of the components with thin coating layers are critically dependent on the bonding conditions between thin layers and substrates. To evaluate bonding quality or surface material properties, in this study, we have proposed a new approach analyzing the “minimum reflection profile” that can be obtained by measuring energy of reflected Rayleigh‐like waves with variation of incidence angle from normal to beyond Rayleigh angle in a pitch‐catch immersion setup. Using the proposed method, we evaluated the bonding quality of chemical vapor deposit (CVD) diamond coating layers having different bonding conditions. In this paper, the characteristics of Rayleigh‐like waves and performances of the minimum reflection profiles were discussed.

Quantitative Evaluation of CVD Diamond Coating Layer Using Rayleigh-like Waves

This work aims to explore the possibility of using generalized Lamb waves for nondestructive evaluation of the bonding quality of layered substrates. For this purpose, CVD diamond specimens with different cleaning conditions are prepared. A dispersion simulation performed for layered substrates with imperfect interfaces is carried out to find out the characteristics of dispersion curves that can be used for bonding quality evaluation. Then the characteristics of dispersion curves of the fabricated specimens are experimentally determined by using an ultrasonic backward radiation measurement technique. The results obtained in the present study show that the lowest velocity mode (Rayleigh-like) of the generalized Lamb waves is sensitively affected by the bonding quality. Therefore, the Rayleigh-like Lamb waves can be applied for nondestructive evaluation of imperfect bonding quality in various layered substrates.

Degradation Assessment of Thermoplastic Synthetic Resin Using Propagation Characteristics of Ultrasound

Abstract A nondestructive ultrasonic technique was applied to evaluate the thermal characteristics and degradation of synthetic polymer resin (plastics) with better cost-effectiveness and functionality than glass and metal. Thermo- plastic and transparent acrylic resin (PMMA) specimens were annealed at below the glass transition temperature (T g ), and the propagation characteristics (attenuation and velocity) were measured. The attenuation increased and the velocity decreased with thermal degradation. The results showed that the thermal aging of the specimens could be evaluated quantitatively and that the T g could be evaluated qualitatively. Keywords: Plastics, PMMA, Thermal Degradation, Ultrasonic Attenuation, Ultrasonic Velocity[접수일: 2013. 10. 23, 수정일: 2013. 12. 13, 게재확정일: 2013. 12. 25] *안동대학교 물리학과, ✝Corresponding Author: Department of physics, Andong National University, Andong-Si 760-749, Korea (E-mail: sdkwon@andong.ac.kr)ⓒ 2014, Korean Society for Nondestructive Testing 1. 서 론가소성(plasticity)을 가진 고분자 합성수지(polymer synthetic resin)인 플라스틱(plastics)은 다른 고분자 응용물질인 섬유나 고무와는 다르게 실온에서 분자 사슬이 고정되어 있고, 내부식성, 투명성, 경량성, 경제성, 비중대비 고강도, 가공성 등으로 인해 유리 및 금속의 대체 물질로 각광을 받고 있다. 플라스틱은 크게 열경화성(thermosetting)과 열가소성(thermoplastic) 재료로 분류되며, 열가소성은 그 용도에 따라 범용(common) 플라스틱, 기능성(engineering) 플라스틱(EP), 고기능성(super engin- eering) 플라스틱(SEP)으로 구분되거나 고분자 중합체의 구조에 따라 비정질(amorphous) 및 결정성(crystalline)으로 분류된다. 범용 비정질 플라스틱으로는 우리에게 잘 알려진 PMMA, PVC, ABS 등이 있고 결정성으로는 PE, PP 등이 있다. 범용 플라스틱의 단점을 보완하고 금속의 대체효과를 위해 개발된 기능성 플라스틱으로는 PC, PBT, PET 등이 있다[1]. 플라스틱이 높은 온도와 자외선 같은 환경에 장기간 노출되게 되면 물리화학적 영향을 받아 고유의 특성이 변하여 기대되는 성능을 만족하지 못하는 열화(thermal degradation, aging)가 진행된다. 고분자로 이루어진 플라스틱은 열에 의해서 변형, 유동 등의 물리적 변화와 함께 고분자 주

New Approaches to Backscattered Rayleigh Wave Signal Analysis for the Evaluation of CVD Diamond Coating Layers

Nondestructive evaluation of chemical vapor deposit (CVD) diamond coating layers has to be performed since delamination between the CVD diamond coating layer and the silicon substrate occurs frequently due to a large difference in material properties. The backscattered Rayleigh waves have been applied and showed a possibility for evaluation of the bonding quality of CVD diamond coating layer. However, it is necessary to develop precise and quantitative method that can evaluate very thin coating layers since the thickness of CVD diamond coating layer is only a few micrometers. To address such a need, in this study, we proposed new approaches to backscattered Rayleigh wave signal analysis for the evaluation of CVD diamond coating layers precisely. Using the proposed methods, we evaluated the bonding quality of CVD coating layers with few micrometers of thickness.

Phase Behaviors of Colloids in Polymer Solutions

A density functional theory (DFT) based on the weighted density has been developed for studying the polymer-induced depletion interaction and phase behaviors of colloids in polymer solutions. The depletion potentials between two colloids induced by the polymer fluids and the colloid-colloid second virial coecients have been calculated. The calculated depletion potentials are compared with the simulation results. The phase diagram shows that short chains can give a more stable dispersion for colloids than long chains. The large colloid/polymer size ratio develops strong attractive interactions. This leads to a broadening of the coexistence region between the solid and the fluid phases of the colloids in a polymer solution.

Determination of Phase Velocity Dispersion Curves and Group Velocity in a Plate Using Backward Radiated Leaky Lamb Waves

The guided wave has been widely employed to characterize thin plates and layered media. The dispersion curves of phase and group velocities are essential for the quantitative application of guided waves. The technique using leaky Lamb wave (LLW) is one of the excellent methods to obtain dispersion curves. In the present work, a fully automated system for the measurement of backward radiation of LLW has been developed. The specimen moves in two dimensional planes as well as in angular rotation. The signals of backward radiation of LLW were measured from an elastic plate in which specific modes of Lamb wave were strongly generated. Phase velocity of the corresponding modes was determined from the incident angle. The generated Lamb waves propagated forward and backward with the leakage of energy into water. Backward radiated LLW was detected by the same transducer and its frequency components were analyzed to extract the related information to the dispersion curves. The dispersion curves of phase velocity we...

Evaluation of Corrosion Degradation Characteristics of Turbine Blade Material Using Backward Radiated Ultrasound

The corrosion degradation characteristics of the 12Cr alloy steel, which is widely used in fossil power plants as a turbine blade material, are evaluated nondestructively by use of the backward radiated Rayleigh surface wave. In order to evaluate corrosion degradation characteristics, we constructed automated system for the backward radiation, and the frequency dependency of the Rayleigh surface wave is investigated indirectly by measuring the angular dependency of the backward radiation of the incident ultrasonic wave in the specimens. The velocity of the surface wave decrease as the increase of the aging time in the backward radiation profile, which seems to result from the increase of the effective degrading layer thickness. And, amplitude of the surface wave increase as the aging time, which seems to result from the increase of the intergranular corrosion. The result observed in this study demonstrates high potential of the backward radiated ultrasound as a tool for the nondestructive evaluation of the corrosion degradation characteristics of the aged materials.