• Proceedings of the Korea Association of Crystal Growth Conference
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• 1996.06a
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• pp.5-18
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• 1996

The solid state cesium ion source os alumino-silicate based zeolite which contains cerium. The material is an ionic conductor. Cesiums are stably stored in the material and one can extract the cesiums by applying electric field across the electrolyte. Cesium ion bombardment has the unique property of producing high negative ion yield. This ion source is used as the primary source for the production of a negative ion without any gas discharge or the need for a carrier gas. The deposition of materials as an ionic species in the energy range of 1.0 to 300eV is recently recognized as a very promising new thin film technique. This energetic non-thermal equilibrium deposition process produces films by “Kinetic Bonding / Energetic Condensation" mechansim not governed by the common place thermo-mechanical reaction. Under these highly non-equilibrium conditions meta-stable materials are realized and the negative ion is considered to be an optimum paeticle or tool for the purpose. This process differs fundamentally from the conventional ion beam assisted deposition (IBAD) technique such that the ion beam energy transfer to the deposition process is directly coupled the process. Since cesium ion beam sputter deposition process is forming materials with high kinetic energy of metal ion beams, the process provider following unique advantages:(1) to synthesize non thermal-equilibrium materials, (2) to form materials at lower processing temperature than used for conventional chemical of physical vapor deposition, (3) to deposit very uniform, dense, and good adhesive films (4) to make higher doposition rate, (5) to control the ion flux and ion energy independently. Solid state cesium ion beam sputter deposition system has been developed. This source is capable of producing variety of metal ion beams such as C, Si, W, Ta, Mo, Al, Au, Ag, Cr etc. Using this deposition system, several researches have been performed. (1) To produce superior quality amorphous diamond films (2) to produce carbon nitirde hard coatings(Carbon nitride is a new material whose hardness is comparable to the diamond and also has a very high thermal stability.) (3) to produce cesiated amorphous diamond thin film coated Si surface exhibiting negative electron affinity characteristics. In this presentation, the principles of solid state cesium ion beam sputter deposition and several applications of negative metal ion source will be introduced.

• Progress in Superconductivity and Cryogenics
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• v.18 no.1
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• pp.59-63
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• 2016

The superconducting NMR magnets have used cryogen such as liquid helium for their cooling. The conduction cooling method using cryocoolers, however, makes the cryogenic cooling system for NMR magnets more compact and user-friendly than the cryogen cooling method. This paper describes the thermal and structural analysis of a cryogenic conduction cooling system for a 400 MHz HTS NMR magnet, focusing on the magnet assembly. The highly thermo-conductive cooling plates between HTS double pancake coils are used to transfer the heat generated in coils, namely Joule heating at lap splice joints, to thermal link blocks and finally the cryocooler. The conduction cooling structure of the HTS magnet assembly preliminarily designed is verified by thermal and structural analysis. The orthotropic thermal properties of the HTS coil, thermal contact resistance and radiation heat load are considered in the thermal analysis. The thermal analysis confirms the uniform temperature distribution for the present thermal design of the NMR magnet within 0.2 K. The mechanical stress and the displacement by the electromagnetic force and the thermal contraction are checked to verify structural stability. The structural analysis indicates that the mechanical stress on each component of the magnet is less than its material yield strength and the displacement is acceptable in comparison with the magnet dimension.

• Journal of Electrical Engineering and Technology
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• v.10 no.3
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• pp.1075-1080
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• 2015

A hybrid electric vehicle (HEV) powertrain has more than one energy source including a high-voltage electric battery. However, for a high voltage electric battery, the average current is relatively low for a given power level. Introduced to increase the voltage of a HEV battery, a compact, high-efficiency boost converter, sometimes called a step-up converter, is a dc-dc converter with an output voltage greater than its input voltage. The inductor occupies more than 30% of the total converter volume making it difficult to get high power density. The inductor should have the characteristics of good thermal stability, low weight, low losses and low EMI. In this paper, Mega Flux^® was selected as the core material among potential core candidates. Different structured inductors with Mega Flux^® were fabricated to compare the performance between the conventional air cooled and proposed potting structure. The proposed inductor has reduced the weight by 75% from 8.8kg to 2.18kg and the power density was increased from 15.6W/cc to 56.4W/cc compared with conventional inductor. To optimize the performance of proposed inductor, the potting materials with various thermal conductivities were investigated. Silicone with alumina was chosen as potting materials due to the high thermo-stable properties. The proposed inductors used potting material with thermal conductivities of 0.7W/m·K, 1.0W/m·K and 1.6W/m·K to analyze the thermal performance. Simulations of the proposed inductor were fulfilled in terms of magnetic flux saturation, leakage flux and temperature rise. The temperature rise and power efficiency were measured with the 40kW boost converter. Experimental results show that the proposed inductor reached the temperature saturation of 107℃ in 20 minutes. On the other hand, the temperature of conventional inductor rose by 138℃ without saturation. And the effect of thermal conductivity was verified as the highest thermal conductivity of potting materials leads to the lowest temperature saturations.

• Structural Engineering and Mechanics
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• v.65 no.4
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• pp.491-504
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• 2018

Because of sandwich structures with low weight and high stiffness have much usage in various industries such as civil and aerospace engineering, in this article, buckling and free vibration analyses of coupled micro composite sandwich plates are investigated based on sinusoidal shear deformation (SSDT) and most general strain gradient theories (MGSGT). It is assumed that the sandwich structure rested on an orthotropic elastic foundation and make of four composite face sheets with temperature-dependent material properties that they reinforced by carbon and boron nitride nanotubes and two flexible transversely orthotropic cores. Mathematical formulation is presented using Hamilton's principle and governing equations of motions are derived based on energy approach and applying variation method for simply supported edges under electro-magneto-thermo-mechanical, axial buckling and pre-stresses loadings. In order to predict the effects of various parameters such as material length scale parameter, length to width ratio, length to thickness ratio, thickness of face sheets to core thickness ratio, nanotubes volume fraction, pre-stress load and orthotropic elastic medium on the natural frequencies and critical buckling load of double-bonded micro composite sandwich plates. It is found that orthotropic elastic medium has a special role on the system stability and increasing Winkler and Pasternak constants lead to enhance the natural frequency and critical buckling load of micro plates, while decrease natural frequency and critical buckling load with increasing temperature changes. Also, it is showed that pre-stresses due to help the axial buckling load causes that delay the buckling phenomenon. Moreover, it is concluded that the sandwich structures with orthotropic cores have high stiffness, but because they are not economical, thus it is necessary the sandwich plates reinforce by carbon or boron nitride nanotubes specially, because these nanotubes have important thermal and mechanical properties in comparison of the other reinforcement.

• Journal of Powder Materials
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• v.22 no.6
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• pp.413-419
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• 2015

Lanthanum zirconate, $La_2Zr_2O_7$, is one of the most promising candidates for next-generation thermal barrier coating (TBC) applications in high efficient gas turbines due to its low thermal conductivity and chemical stability at high temperature. In this study, bulk specimens and thermal barrier coatings are fabricated via a variety of sintering processes as well as suspension plasma spray in lanthanum zirconates with reduced rare-earth contents. The phase formation, microstructure, and thermo-physical properties of these oxide ceramics and coatings are examined. In particular, lanthanum zirconates with reduced rare-earth contents in a $La_2Zr_2O_7-4YSZ$ composite system exhibit a single phase of fluorite or pyrochlore after fabricated by suspension plasma spray or spark plasma sintering. The potential of lanthanum zirconate ceramics for TBC applications is also discussed.

• Journal of Powder Materials
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• v.19 no.4
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• pp.253-258
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• 2012

The characteristics of $Al_2TiO_5$ ceramics were influenced by the additives and the heat treatment that controls the microcrack behavior at grain boundaries. The effect of additives on $Al_2TiO_5$ ceramics were investigated in terms of mechanical properties and thermal expansion at high temperature. The $Al_2TiO_5$ were synthesized at $1500^{\circ}C$, $1550^{\circ}C$ and $1600^{\circ}C$ for 2h by reaction sintering. The formation of $Al_2TiO_5$ phase was increased by additives that enhanced the volume of the microcrack that can lead to low thermal expansion. The mechanical properties of the stabilized $Al_2TiO_5$ ceramics were increased remarkably at $1100^{\circ}C$, $1200^{\circ}C$ and $1300^{\circ}C$ due to the oneset of mechanical healing of grain-bondary microcracks at a high temperature. The amount of microcrack was decreased at lower sintering temperature that causes the increase of mechanical properties at high temperature.

• Korean Journal of Materials Research
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• v.18 no.5
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• pp.235-240
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• 2008

This paper presents results and observations obtained from a study of the optical and thermal properties of alkali tellurite depending on the composition. Fourier transform infrared (FT-IR) spectra showed evidence of chemical modification from $TeO_4$ trigonal bipyramids (tbp) to $TeO_3$ trigonal pyramids (tp) in tellurite glasses. The optical band gaps of the different glass samples calculated using Tauc's method were found to range from 3.5-3.8 eV. The glass transition temperature (Tg) and glass stability (${\Delta}T$) of alkali tellurite glasses were investigated, as $M_2O$ [M: Li, Na, K] amounted to 25 mol%, through the use of differential thermal analysis (DTA). The coefficient of thermal expansion (CTE) was measured in a thermo mechanical analysis (TMA) with a slow heating rate after the glass samples were annealed. The results confirm that the optical band gap of alkali tellurite glasses depends on the Te-O-Te structural relaxation related to the ratio of bridging/non bridging oxygen (BO/NBO). In contrast, the thermal properties are related to the ionic field strength of the Te-O-M and M-O-M bonds, and the Te-O-Te breakage depends on the ratio of BO/NBO.

• Journal of the Korean Geotechnical Society
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• v.27 no.11
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• pp.55-69
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• 2011

The importance of unsaturated state in various geo-engineering problems has led to the advance of mechanical constitutive model emulating behavior of unsaturated soils in response to thermo-hydro-mechanical loading. Elasto-plastic mechanical constitutive model for unsaturated soil is formulated based on Bishop's effective stress. Effective stress and temperature are main variables in constitutive equation, and incremental formulation of constitutive relationship is derived to compute stress update and stiffness tensor. Numerical simulations involving coupled THM processes are conducted to discuss numerical stability and applicability of developed constitutive model: one-dimensional test, tri-axial compression test, and clay-buffering at high level radioactive waste disposal. Numerical results demonstrated that developed model can predict very complex behavior of coupled THM phenomena and is applicable to geo-engineering problems under various environmental conditions, as well as interpret typical behavior of unsaturated soils.

• Journal of Microbiology and Biotechnology
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• v.17 no.2
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• pp.364-368
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• 2007

Enterobacter sakazakii may be related to outbreaks of meningitis, septicemia, and necrotizing enterocolitis, mainly in neonates. To reduce the risk of E. sakazakii in baby foods, thermal characteristics for Korean E. sakazakii isolates were determined at 52, 56, and $60^{\circ}C$ in saline solution, rehydrated powdered infant formula, and dried baby food. In saline solution, their D-values were 12-16, 3-5, and 0.9-1 min for each temperature. D-values increased to 16-20, 4-5, and 2-4 min in rehydrated infant formula and 14-17, 5-6, and 2-3 min in dried baby food. The overall calculated z-value was 6-8 for saline, 8-10 for powdered infant formula, and 9-11 for dried baby food. Thermal inactivation of E. sakazakii during rehydration of powdered infant formula was investigated by viable counts. Inactivation of cultured E. sakazakii in infant formula milk did not occur for 20 min at room temperature after rehydration with the water at $50^{\circ}C$ and their counts were reduced by about 1-2 log CFU/g at $60^{\circ}C$ and 4-6 log CFU/ml with the water at 65 and $70^{\circ}C$. However, the thermo stability of adapted E. sakazakii to the powdered infant formula increased more than two times. Considering that the levels of E. sakzakii observed in powdered infant formula have generally been 1 CFU/100 g of dry formula or less, contamination with E. sakazakii can be reduced or eliminated by rehydrating water with at least $10^{\circ}C$ higher temperature than the manufacturer-recommended $50^{\circ}C$.

• Polymer(Korea)
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• v.32 no.6
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• pp.580-586
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• 2008

Copolyimides were synthesized from 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 1,3-bis(3-aminophenoxy)benzene (BAPB) with different mole ratios of 2,2-bis[4-(4-aminophenoxy)pheny1]hexafluoropropane (BAPP). The solution cast film of poly(amic acid) (PAA) was heat treated at different temperatures to create copolyimide films. The PI copolymer films were found to exhibit good optical transparencies. The thermomechanical properties, morphology, and optical transparency of PI films were examined using fourier transform infrared spectroscopy (FT-IR), wide-angle X-ray diffraction (XRD), scanning electron microscopes (SEM), differential scanning calorimeter(DSC), thermo-gravimetric analyzer (TGA), universal tensile machine (UTM), and IN - Vis. spectrometer. The glass transition temperature ($T_g$), ultimate strength, and initial modulus linearly increased with increasing BAPP mole fraction. However, thermal stability($T_D{^i}$) of the copolyimide remains constant regardless of BAPP loadings. It was found, however, that the optical transparency decreases slightly upon increasing the BAPP content because of the formation of the charge transfer complexes.