• 대한전기학회:학술대회논문집
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• 대한전기학회 1999년도 하계학술대회 논문집 D
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• pp.1983-1985
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• 1999

The dielectric constant of a monolayer on a material surface was calculated with consideration of the local field acting on polar molecules with a permanent dipole moment, and the interaction working between the molecules and a material. It is revealed that the dielectric relaxation time r of monolayers in the isotropic polar orientational phase is determined using a linear relationship between the monolayers compression speed a and the molecular area. The dielectric relaxation time of phospholipid monolayers was examined on the basis of analysis developed here.

• Steel and Composite Structures
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• 제24권3호
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• pp.297-307
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• 2017

A unified mathematical model of phase-lag Green-Naghdi magneto-thermoelasticty theories based on fractional derivative heat transfer for perfectly conducting media in the presence of a constant magnetic field is given. The GN theories as well as the theories of coupled and of generalized magneto-thermoelasticity with thermal relaxation follow as limit cases. The resulting nondimensional coupled equations together with the Laplace transforms techniques are applied to a half space, which is assumed to be traction free and subjected to a thermal shock that is a function of time. The inverse transforms are obtained by using a numerical method based on Fourier expansion techniques. The predictions of the theory are discussed and compared with those for the generalized theory of magneto-thermoelasticity with one relaxation time. The effects of Alfven velocity and the fractional order parameter on copper-like material are discussed in different types of GN theories.

• 한국자기공명학회논문지
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• 제17권1호
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• pp.24-29
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• 2013

The structural characteristics of 4-coordinate $BO_4$ [B(1)] and 3-coordinate $BO_3$ [B(2)] groups in $BiB_3O_6$ were studied by $^{11}B$ magic angle spinning (MAS) and single-crystal nuclear magnetic resonance (NMR) spectroscopy. The spin-lattice relaxation time in the laboratory frame, $T_1$, for $^{11}B$ decreased slowly with increasing temperature, whereas the spin-lattice relaxation times in the rotating frame, $T_{1{\rho}}$, for B(1) and B(2), which differed from $T_1$, were nearly constant. Further, $T_{1{\rho}}$ for B(1) and B(2) showed very similar trends, although the $T_{1{\rho}}$ value of B(2) was shorter than that of B(1). The 3-coordinate $BO_3$ and 4-coordinate $BO_4$ were distinguished by $^{11}B$ MAS NMR spectrum and $T_{1{\rho}}$.

• 한국의류학회지
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• 제4권1_2호
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• pp.25-33
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• 1980

The studies on shrinkage and characteristics of the weft knitted fabrics were investigated under the various dry and wet treating conditions. Various relaxation values were found out according to treating conditions. The characteristics of knitted fabrics such as shrinkage rate, thickness, spirality, elongation and recovery were also measured. The used knitting yams were OE (open-end) cotton and POY (pre-oriented yarn)-DTY (draw textured yam) polyester. The conclusions obtained in this study are as follows. 1. In case of dry relaxation little change of Ks values was seen with increasing time after 48 hours. So it was found that relaxation shrinkage of dry relaxation reached its maximal state in about 48 hours. 2. In case of wet relaxation, higher Ks values were observed, in comparision with those of dry one and higher shrinkage rates were also observed. But when experimental temperature was constant, sudden marked increases in Ks values and shrinkage rates appeared through the initial 4 hours, and after that time little change was seen in them. 3. As Ks value increases, thickness also increased. But thickness showed to some degree stability around Ks value 23. 4. As Ks value increases, spirality values also increased gradually. But little change of spirality values was observed above a certain Ks values (cotton 22.5, polyester 21.5). 5. As Ks value increases, the elongation decreased under a certain load, and the recovery was random.

• 한국수소및신에너지학회논문집
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• 제34권2호
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• pp.178-186
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• 2023

Three activation methods (constant voltage, current cycling, and hydrogen pumping) were applied to investigate the effects on the performance of the membrane electrode assembly (MEA) loaded with PtCo/C catalyst. The current cycling protocol took the shortest time to activate the MEA, while the performance after activation was the worst among the all activation methods. The constant voltage method took a moderate activation time and exhibited the best performance after activation. The hydrogen pumping protocol took the longest time to activate the MEA with moderate performance after activation. According to the distribution of relaxation time analysis, the improved performance after the activation mainly comes from the decrease of charge transfer resistance rather than the ionic resistance in the cathode catalyst layer, which suggests that the existence of water on the electrode is the key factor for activation.

• 한국전기전자재료학회논문지
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• 제11권11호
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• pp.1007-1013
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• 1998

In this paper, the dielectric properties of fabricated Polyvinylidene fluoride(PVDF, $PVF_2$) thin film with substrate temperature from 30 to at vapor deposition. The dielectric properties of PVDF thin film had been studied in the frequency range from 10Hz to 4MHz at measuring temperature between 20 and $100^{/circ}C$. The anomalous increasing in dielectric constant and dielectric loss at low frequencies and high temperature was described for PVDF thin film containing ion impurities. In particularly, ion mobility of fabricated PVDF thin film at substrate temperature at $30^{/circ}C$ decrease from $2\times10^{-5}\;to\;3.07$\times10^{-7}cm^2/V.s$ On the other hand, ion density increase abruptly from 1.49\times$$10^{13}$ to $1.5\times$10^{16}$cm^{-3}$ In spite of decreasing of ion mobility, dielectric constants and dielectric loss for PVDF thin film increase rapidly with decreasing frequency and high temperature. It was concluded that the dielectric constants and dielectric loss was related to ion density than to ion mobility at low frequency and high temperatures.

• Rapid Communication in Photoscience
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• 제4권1호
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• pp.25-28
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• 2015

The time-resolved photoexcitation dynamics of electrical conductivity of the magnetic organic superconductor ${\lambda}-(BETS)_2Fe_{0.45}Ga_{0.55}Cl_4$ has been studied with a nanosecond visible laser pulse at its three different phases, i. e., metallic phase, superconducting phase and insulating phase. A transient increase of the resistance is induced by photoirradiation at all the temperatures measured for all three phases, but the decay profile shows a significant temperature dependence. The relaxation rate in the metallic and insulating phase are different from each other, and the decay time is relatively faster and almost constant in the metallic phase. However, a prolongation of the relaxation time is observed at temperature just around the narrow superconducting phase. Nonbolometric (nonthermal) origin of the observed photoresponse of the electrical conductivity is confirmed in the superconducting phase.

• 대한치과교정학회지
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• 제48권6호
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• pp.384-394
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• 2018

Objective: This study was performed to investigate an appropriate degree of prestretch for orthodontic synthetic elastomeric chains focusing on time-dependent viscoelastic properties. Methods: Orthodontic synthetic elastomeric chains of two brands were prestretched to 50, 100, 150, and 200% of the original length in one and three cycles, and the hysteresis areas of the obtained stress-strain curves were determined. Acrylic plates were employed to maintain constant strain during the experiment. A total of 180 samples were classified into nine groups according to brand, and their stresses and permanent deformations were measured immediately after prestretch (0 hour), after 1 hour and 24 hours, and after 1, 2, 3, 4, 5, 6, 7, and 8 weeks. The relationship between stress relaxation and permanent deformation was investigated for various degrees of prestretch, and the estimated stress resulting from tooth movement was calculated. Results: The degree of prestretch and the stress relaxation ratio exhibited a strong negative correlation, whereas no correlation was found between the degree of prestretch and the average normalized permanent strain. The maximal estimated stress was observed when prestretch was performed in three cycles to 200% of the original length. Conclusions: Although prestretch benefited residual stress, it did not exhibit negative effects such as permanent deformation. The maximal estimated stress was observed at the maximal prestretch, but the difference between prestretch and control groups decreased with time. In general, higher residual stresses were observed for product B than for product A, but this difference was not clinically significant.

• Advances in Computational Design
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• 제2권3호
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• pp.123-141
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• 2017

Solving a system of linear or non-linear equations is required to analyze any kind of structures. There are many ways to solve a system of equations, and they can be classified as implicit and explicit techniques. The explicit methods eliminate round-off errors and use less memory. The dynamic relaxation method (DR) is one of the powerful and simple explicit processes. The important point is that the DR does not require to store the global stiffness matrix, for which it just uses the residual loads vector. In this paper, a new approach to the DR method is expressed. In this approach, the damping, mass and time steps are similar to those of the traditional method of dynamic relaxation. The difference of this proposed method is focused on the method of calculating the damping. The proposed method is expressed such that the time step is constant, damping is equal to zero except in steps with maximum energy and the concentrated damping can be applied to minimize the energy of system in this step. In this condition, the calculation of damping in all steps is not required. Then the volume of computation is reduced. The DR method for form-finding of membrane structures is employed in this paper. The form-finding of the three plans related to the membrane structures with different loading is considered to investigate the efficiency of the proposed method. The numerical results show that the convergence rate based on the proposed method increases in all cases than other methods.

• Composites Research
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• 제32권6호
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• pp.382-387
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• 2019

상대습도와 온도가 PET 필름의 점탄성 특성에 미치는 영향을 조사하기 위해 RH-DMA를 이용하여 single frequency strain mode 시험, stress relaxation mode 시험, creep 시험을 수행하였다. 상대습도는 10%, 30%, 50%, 70%, 90%를 적용하고 온도는 single frequency strain mode 시험의 경우 30~95℃, stress relaxation mode 시험의 경우 30℃ 와 70℃, creep 시험의 경우 5~95℃를 고려하였다. 연구결과에 따르면 상대습도가 높아지면 저장탄성계수와 손실탄성계수는 낮아지며 손실탄성계수의 최대값은 상대습도의 변화에 큰 영향을 받지 않고 거의 일정해진다. 이완탄성계수는 초기에 급격히 감소하다가 일정한 값을 가지며 높은 온도에서는 상대습도의 변화에 민감해진다. 변형률 회복는 초기에 급격히 증가하며 온도가 높아지면 이완 탄성계수와 마찬가지로 상대습도에 민감하게 변한다. 크리프 컴플라이언스의 증가 정도는 온도가 높아지면 커지며 유리전이온도보다 온도가 높아지면 증가 정도는 더욱 커진다. 시간-온도 중첩법을 통해 구해지는 마스터 선도를 이용하면 상대습도와 온도 등의 운용 조건에서의 장기 성능을 예측할 수 있는 정보를 얻을 수 있다.