• Journal of Sensor Science and Technology
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• v.29 no.4
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• pp.220-226
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• 2020

Graphene has been widely considered a promising candidate for high-quality chemical sensors, owing to its outstanding characteristics, such as sensitive gas adsorption at room temperature, high conductivity, high flexibility, and high transparency. However, the main drawback of a graphene-based gas sensor is the necessity for external heaters due to its slow response, incomplete recovery, and low selectivity at room temperature. Conventional heating devices have limitations such as large volume, thermal safety issues, and high power consumption. Moreover, metal-based heating systems cannot be applied to transparent and flexible devices. Thus, to solve this problem, a method of supplying the thermal energy necessary for gas sensing via the self-heating of graphene by utilizing its high carrier mobility has been studied. Herein, we provide a brief review of recent studies on self-activated graphene-based gas sensors. This review also describes various strategies for the self-activation of graphene sensors and the enhancement of their sensing properties.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.04b
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• pp.23-23
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• 2009

It has been challenging to increase the thermoelectric figure of merit ($ZT=S^2{\sigma}T/\kappa$) of materials, which determine the efficiency of thermoelectric devices, because the three parameters Seebeck coefficient (S), electrical conductivity ($\sigma$), and thermal conductivity ($\kappa$) of bulk materials are inter-dependent. With the development of nanotechnology, ZT values of nanostructured materials are predicted to be enhanced by classical size effects and quantum confinement effects. In particular, Bi nanowires were suggested as one of ideal thermoelectric materials due to the expected quantum confinement effects for the simultaneous increase in Sand. In this work, we have investigated the thermal conductivity of individual single crystalline Bi nanowires with d = 98 nm and d = 327 nm in the temperature range 40 - 300 K using MEMS devices. The for the Bi nanowire with d = 98 nm was observed to be ~ 1.6 W/m-K at 300 K, which is much lower than that of Bi bulk (8 W/m-K at 300 K). This indicates that the thermal conductivity of the Bi suppressed due to enhanced surface boundary scattering in one-dimensional structures. Our results suggest that Bi nanowires grown by stress-induced method can be used for high-efficiency thermoelectric devices.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.03a
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• pp.221-228
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• 2010

A sliding accident on the road have a high percentage by road freezing, especially, it is often appeared at bridges and Tunnel of freezing areas. Thus, the stability of road operations is enhanced by preventing a partial freezing phenomenon. According to the geothermal snow melting system analysis, a pattern of thermal conductivity is found out about pavement materials of concrete and asphalt when it is buried. The thermal conductivity study is essential that be applied the geothermal snow melting system according to heating exchanger pipe laying of lower pavements. The model tests are conducted on low temperature in freezer using the manufactured test model which is equal to pavement materials. And Many variables are discovered from numerical analyzes of the same conditions with model test.

• Korean Membrane Journal
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• v.9 no.1
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• pp.34-42
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• 2007

A perovskite-type ($La_{0.6}Sr_{0.4}Ti_{0.2}Fe_{0.8}O_{3-{\delta}}$) dense ceramic membrane was prepared by polymerized complex method, using citric acid as a chelating agent and ethylene glycol as an organic stabilizer. Effect of Ti addition on lanthanum-strontium ferrite mixed conductor was investigated by evaluating the thermal expansion coefficient, the oxygen flux, the electrical conductivity, and the phase stability. The thermal expansion coefficient in air was $21.19\;{\times}\;10^{-6}/K$ at 473 to 1,223 K. At the oxygen partial pressure of 0.21 atm ($20%\;O_2$), the electrical conductivity increased with temperature and then decreased after 973 K. The decrement in electrical conductivity at high temperatures was explained by a loss of the lattice oxygen. The oxygen flux increased with temperature and was $0.17\;mL/cm^2{\cdot}min$ at 1,223 K. From the temperature-dependent oxygen flux data, the activation energy of oxygen ion conduction was calculated and was 80.5 kJ/mol at 1,073 to 1,223 K. Also, the Ti-added lanthanum-strontium ferrite mixed conductor was structurally and chemically stable after 450 hours long-term test at 1,173 K.

• Journal of the Korean institute of surface engineering
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• v.45 no.2
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• pp.75-80
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• 2012

A sputter-sublimation source was tested for high rate deposition of protective coating of PEMFC(polymer electrolyte membrane fuel cell) with high electrical conductivity and anti-corrosion capability by DC biasing of a metal rod immersed in inductively coupled plasma. A SUS(stainless steel) tube, rod were tested for low thermal conductivity materials and copper for high thermal conductivity ones. At 10 mTorr of Ar ICP(inductively coupled plasma) with 2.4 MHz, 300 W, the surface temperature of a SUS rod reached to $1,289^{\circ}C$ with a dc bias of 150 W (-706 V, 0.21 A) in 2 mins. For 10 min of sputter-sublimation, 0.1 gr of SUS rod was sputter-sublimated which is a good evidence of a high rate deposition source. ICP is used for sputter-sublimation of a target material, for substrate pre-treatment, film quality improvement by high energy particle bombardment and reactive deposition.

• Journal of the Korean Society of Manufacturing Process Engineers
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• v.6 no.3
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• pp.98-104
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• 2007

The carbon fiber reinforced carbon composite (CFRC) materials are necessary for the advanced industries that require the thermal resistance. And the development and research for CFRC has been in progress in the field of aerospace and defense industry. CFRC have several advantages and special properties such as excellent anti ablation, outstanding strength retention at very high temperature, high heat capacity and thermal transport, high specific stiffness and strength, and high thermal shock resistance. They have been used as aircraft brake, rocket nozzle, nose cones, jet engine turbine wheels, and high speed craft. Since the technology related to CFRC was prohibited from importing and exporting, we developed our own technology to produce F-16 B32 brake disk made out of CFRC, and then we performed various tests to observe the characteristics of CFRC-based brake disk developed in this study in view of density, strength, friction, specific heat, and heat conductivity.

• Journal of the Korean Ceramic Society
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• v.50 no.6
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• pp.551-556
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• 2013

AlN films with c-axis orientation and thermal conductivity characteristics were deposited by using Pulsed Laser Deposition and the films were characterized by changing the deposition conditions. In particular, we investigated the optimal conditions for the application of a heat sinking plane AlN thin film. Epitaxial AlN films were deposited on sapphire ($c-Al_2O_3$) single crystals by pulsed laser deposition (PLD) with an AlN target. AlN films were deposited at a fixed pressure of $2{\times}10^{-5}$ Torr, while the substrate temperature was varied from 500 to $700^{\circ}C$. According to the experimental results of the growth temperature of the thin film, AlN thin films were confirmed with a highly c-axis orientation, maximum grain size, and high thermal conductivity at $650^{\circ}C$. The thermal conductivity of the AlN thin film was found to increase compared to bulk AlN near the band gap value of 6.2 eV.

• Journal of Ocean Engineering and Technology
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• v.10 no.2
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• pp.106-119
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• 1996

The ceramic has various high mechanical properties such as heat, abrasion, corrosion resistance and high temperature strength compared with metal. It also has low speciffic weight, low thermal expansibillity, low thermal conductivity. However, it could not be used as structural material since it is brittle and difficult for the machining. Therefore, there have been many researches to attempt to join ceramic with metal which is full of ductillity in order to compensate the weakness of ceramic.The problem is that residual stress develops around the joint area while the ceramic/metal joint material is cooled from high joining temperature to room temperature due to remarkable difference of thermal expansion coefficients between ceramic and metal. Especially, the residual stress at both edges of the specimen reduces the strngth of joint to a large amount by forming a singular stress field. In this study, two dimensional finite element method is attempted for the thermal elastic analysis. The joint residual stress of ceramic/metal developed in the cooling process is investigated and the change of joint residual stress resulted from the repetitive heat cycle is also examined. In addition, it is attempted to clarify the joint stress distribution of the case of tensile load and of the case of superposition of residual stress and actual loading stress.

• Journal of the Korean Institute of Illuminating and Electrical Installation Engineers
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• v.28 no.11
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• pp.26-32
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• 2014

In electrolysis smelting plants that using high DC current, the bus bar is most important facility for delivering the high current. The copper made bus bar is widely used for various advantages as good electrical and thermal conductivity, resonable malleability, ductility, and not rust easily. However, when high current in copper bus bar, temperature rises and maximum allowable current capacity is restricted by temperature of bus bar. In this paper, we investigated temperature variation of copper bus bar by putting cooling water channel imposed to bus bar construction. For the validity, various simulations were carried out.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.31 no.10
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• pp.1046-1052
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• 2007

Elastic-plastic structural analysis for regenerative cooling chamber of gas generator was performed. Uniaxial tension test was conducted for STS316L at room and high temperature conditions to get the material data necessary for the structural analysis of the chamber which was operated under thermal load and high internal pressure. Physical properties including thermal conductivity, specific heat and thermal expansion were also measured. The structural analysis for four different types of regenerative cooling chamber of gas generator revealed that increased cooling performance decreased the thermal load and strain of the cooling channel structure. The results propose that in order for the regenerative cooling gas generator chamber to have high structural stability with endurance to high mechanical and thermal loads, it is important for the chamber to be designed to have high cooling performance.