• Bulletin of the Korean Chemical Society
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• v.9 no.4
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• pp.213-217
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• 1988

A new paradigm for the viscosity of fluid is presented by considering the fact that the viscosity is equal to the shear stress divided by the shear rate. The shear stress is obtained from the sum of kinetic and internal pressures of fluid, and the shear rate is found from the phonon velocity divided by the mean free path of the phonon. The calculated viscosities for various simple substances are in excellent agreements with those of the observed data through the wide temperature range covered both of liquid and gas phase.

• Applied Chemistry for Engineering
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• v.31 no.1
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• pp.73-83
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• 2020

As electronic devices become more advanced and smaller, one of the biggest problems to solve is the heat affecting the efficiency and lifetime of instruments. High thermal conductivity materials, in particular, metal or ceramic ones, have been used to reduce the heat generated from devices. However, due to their low mechanical properties and high weight, thermally conductive composites composed with polymers having a light-weight and good mechanical properties as a matrix and carbon materials having high thermal conductivity as a thermally conductive filler have been attracting great attention. To improve the thermal conductivity of the composites, a phonon scattering must be suppressed to move phonon effectively. In this review, we classified researches related to phonon migration and scattering inhibition of carbon/polymer composites, and discussed various methods to improve thermal conductivity.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.191.2-191.2
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• 2015

Details of carrier dynamics in self-assembled quantum dots (QDs) with a particular attention to nonradiative processes are not only interesting for fundamental physics, but it is also relevant to performance of optoelectronic devices and the exploitation of nanocrystals in practical applications. In general, the possible processes in such systems can be considered as radiative relaxation, carrier transfer between dots of different dimensions, Auger nonradiactive scattering, thermal escape from the dot, and trapping in surface and/or defects states. Authors of recent studies have proposed a mechanism for the carrier dynamics of time-resolved photoluminescence CdTe (a type II-VI QDs) systems. This mechanism involves the activation of phonons mediated by electron-phonon interactions. Confinement of both electrons and holes is strongly dependent on the thermal escape process, which can include multi-longitudinal optical phonon absorption resulting from carriers trapped in QD surface defects. Furthermore, the discrete quantized energies in the QD density of states (1S, 2S, 1P, etc.) arise mainly from ${\delta}$-functions in the QDs, which are related to different orbitals. Multiple discrete transitions between well separated energy states may play a critical role in carrier dynamics at low temperature when the thermal escape processes is not available. The decay time in QD structures slightly increases with temperature due to the redistribution of the QDs into discrete levels. Among II-VI QDs, wide-gap CdZnTe QD structures characterized by large excitonic binding energies are of great interest because of their potential use in optoelectronic devices that operate in the green spectral range. Furthermore, CdZnTe layers have emerged as excellent candidates for possible fabrication of ferroelectric non-volatile flash memory. In this study, we investigated the optical properties of CdZnTe/ZnTe QDs on Si substrate grown using molecular beam epitaxy. Time-resolved and temperature-dependent PL measurements were carried out in order to investigate the temperature-dependent carrier dynamics and the activation energy of CdZnTe/ZnTe QDs on Si substrate.

• Proceedings of the Korean Institute of Information and Commucation Sciences Conference
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• 1999.11a
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• pp.552-554
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• 1999

Phonon scattering and impact ionization models have been presented to analyze hot carrier transport in high energy region, using full band model and Fermi's golden rule. We have investigated temperature dependent properties for impact ionization process of Si using realistic energy band structures at 77K and look. The realistic full band model, obtained from the empirical pseudopotential method with local from factors, is used to calculate scattering rate. The accurate calculation of impact ionization rate requires the use of a wavevector- and frequency-dependent dielectric function ξ ( q,$\omega$). The empirical phonon scattering rate P$\sub$ph/, is given by deriving from linear function for P$\sub$ph/ versus D(E) since the phonon scattering rate is linearly depended on density of states D(E). Impact ionization rate p,, is calculated from the first principle's theory. and fitted by modified Keldysh formula having power of above 2.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.20 no.7
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• pp.606-610
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• 2007

This paper describes the Raman scattering characteristics of polycrystalline (poly) 3C-SiC thin films, in which they were deposited on the oxidized Si substrate by APCVD method according to growth temperature. Since the phonon modes were not measured for $0.4{\mu}m$ thick 3C-SiC, $2.0{\mu}m$ thick 3C-SiC deposited on the oxidized Si at $1180^{\circ}C$, in which TO (transverse optical mode) and LO (longitudinal optical mode) phonon modes were appeared at 794.4 and $965.7cm^{-1}$, respectively. The broad FWHM (full width half maximum) can explain that the crystallinity of 3C-SiC deposited at $1180^{\circ}C$ becomes polycrystalline instead of disorder crystal. Additionally, the ratio of intensity $I_{LO}/I_{TO}{\approx}1.0$ of 3C-SiC indicates that the crystal disorder of $3C-SiC/SiO_2/Si$ is small. Compared poly $3C-SiC/SiO_2$ with $SiO_2/Si$ interfaces, $1122.6cm^{-1}$ phonon mode was measured which may belong to C-O bonding and two phonon modes, 1355.8 and $1596.8cm^{-1}$ related to D and G bands of C-C bonding in the Raman range of 200 to $2000cm^{-1}$.

• Current Applied Physics
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• v.18 no.11
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• pp.1338-1344
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• 2018

Band parameters and superconductivity of yttrium hypocarbide ($Y_2C$) have been investigated. The computations are performed using first-principles pseudopotential method within a generalized gradient approximation. The equilibrium lattice parameters have been determined and compared with experiment. Moreover, the material of interest is found to be stiffer for strains along the a-axis than those along the c-axis. A band-structure analysis of $Y_2C$ implied that the latter has a metallic character. The examination of Eliashberg Spectral Function indicates that Y-related phonon modes as well as C-related phonon modes are considerably involved in the progress of scattering of electrons. By integrating this function, the value of the average electron-phonon coupling parameter (${\lambda}$) is found to be 0.362 suggesting thus that $Y_2C$ is a weak coupling Bardeen-Copper-Schrieffer superconductor. The use of a reasonable value for the effective Coulomb repulsion parameter (${\mu}^*=0.10$) yielded a superconducting critical temperature $T_c$ of 0.59 K which is comparable with a previous theoretical value of 0.33 K. Upon compression (at pressure of 10 GPa) ${\lambda}$ and $T_c$ are increased to be 0.366 and 0.89 K, respectively, showing thus the pressure effect on the superconductivity in $Y_2C$. The spin-polarization calculations showed that the difference in the total energy between the magnetic and non-magnetic $Y_2C$ is weak.

• Nuclear Engineering and Technology
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• v.55 no.7
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• pp.2556-2566
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• 2023

A first-principle approach within the framework of density functional theory was employed to study the effect of vacancy defects and fission products (FPs) doping on the mechanical, electronic, and thermodynamic properties of uranium monocarbide (UC). Firstly, the calculated vacancy formation energies confirm that the C vacancy is more stable than the U vacancy. The solution energies indicate that FPs prefer to occupying in U site rather than in C site. Zr, Mo, Th, and Pu atoms tend to directly replace U atom and dissolve into the UC lattice. Besides, the results of the mechanical properties show that U vacancy reduces the compressive and deformation resistance of UC while C vacancy has little effect. The doping of all FPs except He has a repairing effect on the mechanical properties of U_1-xC. In addition, significant modifications are observed in the phonon dispersion curves and partial phonon density of states (PhDOS) of UC_1-x, Zr_xU_1-xC, Mo_xU_1-xC, and Rh_xU_1-xC, including narrow frequency gaps and overlapping phonon modes, which increase the phonon scattering and lead to deterioration of thermal expansion coefficient (α_V) and heat capacity (C_p) of UC predicted by the quasi harmonic approximation (QHA) method.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2018.06a
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• pp.53-53
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• 2018

Thermoelectric materials can convert directly waste heat to electricity and vice versa. The improvement of the thermoelectric efficiency strongly depends on the dimensionless figure of merit, $ZT=S^2{\sigma}T/{\kappa}$, where S is the Seebeck coefficient, ${\sigma}$ is the electrical conductivity, T is the absolute temperature, and ${\kappa}$ is the thermal conductivity. The thermal conductivity consists of the electronic contribution (${\kappa}_e$) and phonon contribution (${\kappa}_{ph}$). It is very challenge to increase the power factor, $S^2{\sigma}$ and to reduce the thermal conductivity simultaneously because the power factor and electronic thermal conductivity are coupled. One strategy is to decrease the phonon thermal conductivity. The phonon thermal conductivity can be decreased by controlling the grain size and structural defects such as dislocations and twinning. In order to achieve enhancements in thermoelectric efficiency, microstructures that can form numerous interfaces have been investigated intensively for controlling the transport of charge carriers and heat carrying phonons. In this presentation, we report the heterogeneous microstructure of $Mg_2Si_{0.6}Sn_{0.4}$ thermoelectric materials and investigation of its influence on thermoelectric properties.

• Nuclear Engineering and Technology
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• v.6 no.1
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• pp.9-13
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• 1974

The exciton emission spectra of CdS single crystals excited by electrons were measured at 80$^{\circ}$K as a function o: the wave length. The measured dissociation energy of exciton bound to neutral donor was 2.0 meV, compared to the corresponding theoretical value of 2.4 to 3.2 meV. An exciton bound to neutral donor and a longitudinal optical (LO) phonon may not interact, but a free exciton dissociated from a neutral donor and a LO phonon is expected to interact each other. Therefore the origin of the spectra consisting of interaction term was located at the spectrum consisting of a free exciton dissociated from a neutral donor (I$_2$d). From the analysis of the spectra the LO phonon energy of CdS was found to be 40.5 meV.

• Progress in Superconductivity and Cryogenics
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• v.26 no.1
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• pp.10-13
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• 2024

We performed angle-resolved Raman spectroscopy experiments on lead-doped and undoped Bi₂Sr₂CaCu₂O_8+δ(Bi2212) samples using a 660 nm laser and analyzed the Raman tensor of the phonon modes. The phonon mode was clearly observed at the 60, 103, and 630 cm^-1 Raman shifts. The 60, 630 cm^-1 peaks were only clearly observed when the incident and scattered light polarizations were configured to be parallel. The polarization angle dependence of the amplitude of the 60, 630 cm^-1 peak on the parallel configuration shows a twofold symmetry; therefore, both peaks originate from Ag phonons and the crystal structure of Bi2212 should be considered orthorhombic. On the other hand, the 103 cm^-1 peak is clearly observed in both parallel and perpendicular configurations. Remarkably, the off-diagonal component of the Raman tensor of the 103 cm^-1 peak showed an anti-symmetry that could not be realized within the known crystal structure of Bi2212. The implications of our findings are discussed.