• Composites Research
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• v.19 no.1
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• pp.9-14
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• 2006

Experimental investigation on heat transfer ratio was firstly performed with three types of sandwich panels such as the Carbon/Epoxy Skin-Aluminum Honeycomb and Balsa Core Sandwich Panel of 37mm thickness, the Carbon/Epoxy Aluminum Skin-Honeycomb Core Sandwich Panel of 57mm thickness (including insulator) and the Carbon/Epoxy Skin-Aluminum Honeycomb Core Sandwich Panel of 37mm thickness based on the KS F 2278:2003(Insulation test method of windows). In additional to this investigation, experimental tests were also done for evaluation of heat transportation ratio with the Aluminum Skin- Aluminium Honeycomb Sandwich Panels of 27mm and 35mm thickness, and Aluminum Skin-Foaming Aluminum Sandwich Panel of 27mm thickness by the KS F2277:2002 (Insulation measuring method of construction component-Calibration heat box method or protective heat box method). In this study, it was found that the larger net heat transfer cross sectional area between the skin and the sandwich core is given, the higher heat transportation ratio occurs. It was also found that the hybrid type insulation had better insulation characteristics compared to the non-hybrid type insulation.

• Composites Research
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• v.37 no.3
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• pp.209-214
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• 2024

Graphene oxide (GO), known for its high stiffness, thermal conductivity, and electrical conductivity, is being utilized as a reinforcement in nanocomposite materials. This study evaluates the mechanical properties of epoxy nanocomposites incorporating GO and edge modified GO (E-GO), which has hydroxyl groups substituted only on its edges. GO/E-GO was uniformly dispersed in epoxy resin using ultrasonic dispersion, and mechanical properties were assessed through tensile testing. The results showed that the addition of nanoparticles increased both tensile strength and toughness. The tensile strength of the epoxy without nanoparticles was 74.4 MPa, while the highest tensile strength of 90.7 MPa was observed with 0.3 wt% E-GO. Additionally, the modulus increased from 2.55 GPa to 3.53 GPa with the addition of nanoparticles. Field emission scanning electron microscopy of the fracture surface revealed that the growth of cracks was impeded by the nanoparticles, preventing complete fracture and causing the cracks to split in multiple directions. E-GO, with surface treatment only on the edges, exhibited higher mechanical properties than GO due to its superior dispersion and surface treatment effects. These results highlight the importance of nanoparticle surface treatment in developing high-performance nanocomposite materials.

• Korean Journal of Materials Research
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• v.10 no.3
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• pp.246-252
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• 2000

Thermoelectric properties of the $Bi_2(Te_{0.85}Se_{0.15})_3$ alloy, prepared by mechanical alloying and hot pressing, were investigated with the variation of the hot-pressing temperature ranging from $300^{\circ}C$ to $550^{\circ}C$. Contrary to the p-type behavior of single crystal, the hot-pressed $Bi_2(Te_{0.85}Se_{0.15})_3$ alloy exhibited n-type conduction without addition of donor dopant. When the $Bi_2(Te_{0.85}Se_{0.15})_3$ powders were annealed in $(50{\%}\;H_2+50{\%}\;Ar)$ atmosphere, the hot-pressed specimens exhibited a positive Seebeck coefficient due to the reduction of the electron concentration by removal of the oxide layer on the powder surface and annealing-out of the excess Te vacancies. Figure-of-merit of the hot-pressed $Bi_2(Te_{0.85}Se_{0.15})_3$ alloy was improved by hot pressing at temperatures above $450^{\circ}C$, and the maximum value of $1.92{\times}10^{-3}/K$ was obtained for the specimen hot-pressed at $550^{\circ}C$.

• Korean Journal of Materials Research
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• v.9 no.4
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• pp.349-354
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• 1999

The temperature dependences of the Hall coefficient, carrier mobility, electrical resistivity, Seebeck coefficient, thermal conductivity, and figure-of-merit of the undoped and $CdI_2$-doped 90% $Bi_2Te_3-10% Bi_2Se_3$, single crystals, grown by the Bridgman method, have been characterized at temperatures ranging from 77K to 600K. The saturated carrier concentration and degenerate temperature of the undoped 90% $Bi_2Te_3-10% Bi_2Se_3$ single crystal are $5.85\times10_{18}cm^{-3}$ and 127K, respectively. The scattering parameter of the 90% $Bi_2Te_3-10% Bi_2Se_3$ single crystal is determined to b -0.23, and the electron mobility to hole mobility ratio ($\mu_e/\mu_h)$ is 1.45. The bandgap energy at 0K of the 90% <$Bi_2Te_3-10% Bi_2Se_3$ single crystal is 0.200 eV. Adding $CdI_2$as a donor dopant, a maximum figure-of-merit of $3.2\times10^{-3}/K$$CdI_2$-doped specimen.

• Journal of the Korean Chemical Society
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• v.40 no.1
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• pp.81-86
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• 1996

The physical properties of epoxy molding compound (EMC) according to the change of softening point of epoxy resin have been investigated in order to study the relationship between the properties of o-cresol novolac epoxy resin, which is main component of EMC for semiconductor encapsulation, and EMC. The softening points of used epoxy resin are 65.1 $^{\circ}C$, 72.2 $^{\circ}C$, and 83.0 $^{\circ}C$, respectively. The flexural strength and flexural modulus as mechanical properties were measured, and thermal expansion coefficient, thermal conductivity and glass transition temperature (Tg) as thermal properties, and spiral flow as moldability have been investigated to see the change of physical properties of EMC. The flexural modulus, thermal expansion coefficients in the glass state (${\alpha}_1$), and thermal conductivity of EMC were found to be keep constant value irrespective of the change of softening point, but Tg increased with softening point of epoxy resin, and the spiral flow decreased with that. It can be considered that these phenomena are due to the increase of crosslinking density of EMC according to the increase of softening point. The transition points were found out in the thermal expansion coefficient data in the rubbery state (${\alpha}_2$) and the flexural strength data. These can show the decrease of filler dispersion according to increase of epoxy resin viscosity.

• Journal of the Korea Concrete Institute
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• v.19 no.3
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• pp.283-292
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• 2007

A research projects is currently being conducted to develop a nonlinear finite element analysis methods for predicting the structural behavior of reinforced concrete frame structures, exposed to fire. As part of this, reinforced concrete frames subjected to fire loads were analyzed using the nonlinear finite-element program DIANA. Two numerical steps are incorporated in this program. The first step carries out the nonlinear transient heat flow analysis associated with fire and the second step predicts the structural behavior of reinforced concrete frames subjected to the thermal histories predicted by first step. The complex features of structural behavior in fire conditions, such as thermal expansion, plasticity, cracking or crushing, and material properties changing with temperature are considered. A concrete material model based on nonlinear fracture mechanics to take cracking into account and plasticity models for concrete in compression and reinforcement steel were used. The material and analytical models developed in this paper are verified against the experimental data on simple reinforced concrete beams. The changes in thermal parameters are discussed from the point of view of changes of structure and chemical composition due to the high temperature exposure. Although, this study considers codes standard fire for reinforced concrete frame, any other time-temperature relationship can be easily incorporated.

• Proceedings of the Materials Research Society of Korea Conference
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• 2010.05a
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• pp.26.2-26.2
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• 2010

최근에는 차세대 조명용 후보광원인 고출력 백색 LED를 개발하기 위한 경쟁이 치열하며, 이를 위해 업체가 고심하고 있는 가장 큰 문제 중의 하나가 칩에서 발생하는 열을 어떻게 관리하는가 하는 방열의 문제이다. 따라서, LED의 가장 큰 특징인 장수명을 손해보지 않기 위해서는 칩에서 발생되고 있는 열을 외부에 확산시키기 위한 기술 개발이 필수적이다. 다양한 방열소재 중 W-Cu 복합재는 W의 낮은 열팽창계수와 Cu의 높은 열전도도로 인해 방열소재로써 유망한 소재로 주목받고 있으나, 우수한 열적 특성을 발현하기 위해서는 고치밀화를 갖는 W-Cu 복합재 제조가 우선적으로 필요하다. W-Cu 복합체는 일반적으로 액상소결법을 통해 균일한 미세조직을 얻을 수 있으나, 열팽창계수를 낮추기 위해 Cu 함량이 적어지게 되면 치밀화가 어려우며 이를 해결하기 위해 나노입자를 갖는 분말을 이용하고자 하는 연구가 많이 진행되고 있다. 본 연구에서는 W과 Cu 산화물을 이용하는 것이 구성성분끼리의 편석이 발생하지 않으며, 소결성도 우수하여 양산화에 가장 접근한 방법으로 알려져 있다. 그러나, 지금까지의 얻어진 W-Cu 복합체의 경우, 분말상태에서의 얻어진 나노입자가 승온시에 마이크로 크기로 과도한 입자성장이 일어나기 때문에 소결 후에도 나노크기를 유지하기 어려울 뿐만 아니라, 구성상끼리의 응집체가 형성된다. 본 연구에서는 액상소결후에 W 입자가 Cu 기지내에 균일하게 분산되는 동시에 나노크기의 입자를 가지는 고분산 W-Cu 소결체를 얻고자 하였다. 이를 위해 금속산화물 분말의 분쇄를 위해 효과적인 방법으로 알려진 습식상태에서의 고에너지 볼밀링을 통하여 혼합된 텅스텐과 구리 산화물 분말의 수소환원공정을 통해 얻어진 100nm 이하의 입자를 가지는 W-20wt%Cu 나노복합분말을 출발분말로 사용하였다. W-20wt%Cu 나노복합분말의 성형체를 $1050^{\circ}C-1250^{\circ}C$의 온도범위에서 소결거동을 조사하였다. 그 결과, $1100^{\circ}C$ 온도에서 이론밀도에 가까운 소결밀도를 나타내었으며, 이는 기존에 비해 $100^{\circ}C$ 정도 치밀화 온도를 낮추는 결과이다. 소결체의 미세구조 관찰결과, 소결 후 약 200nm의 텅스텐 입자가 Cu내에 균일하게 분산되어 있었다. 제조된 W-Cu 시편에 대해서는 LED 응용성을 조사하기 위해 열전도도와 열팽창계수 등을 평가하였다.

• Korean Journal of Materials Research
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• v.8 no.11
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• pp.1055-1060
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• 1998

Thermoelectric properties of ($Pb_{1-x}Sn_x$)Te ($0\leq{x}\leq{0.4}$) alloys, fabricated by mechanical alloying and hot pressing, were investigated with variation of the SnTe content. For the hot-pressed PbTe and ($Pb_{0.9}Sn_{0.1}$)Te. transition from p-type to n-type occurred at $200^{\circ}C$ and $300^{\circ}C$, respectively. However, the specimens containing SnTe more than 0.2mole exhibited p-type conduction up to 450'C. In extrinsic conduction region, the Seebeck coefficient and electrical resistivity of the hot-pressed ($Pb_{1-x}Sn_x$)Te decreased with increasing the SnTe content. The temperature at which the hot-pressed (Pbl-,Sn,)Te exhibited a maximum figure-of-merit was shifted to higher temperature with increasing the SnTe content The hot-pressed (Pbo ,Sno dTe exhibited a maximum figure-of-merit of $0.68\times10_{-3}/K$ at $200^{\circ}C$.

• Korean Journal of Materials Research
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• v.10 no.4
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• pp.257-263
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• 2000

Thermoelectric properties of the 0.05wt% $SbI_3$-doped n-type $Bi_2(Te_{0.95}Se_{0.05})_3$ alloy, prepared by melting/grinding and hot pressing, were investigated with variation of the annealing time up to 36 hours. The electron concentration of the 0.05wt% SbI$_3$-doped n-type $Bi_2(Te_{0.95}Se_{0.05})_3$ alloy decreased with increasing the annealing time. The figure-of-merit of the 0.05wt% $SbI_3$-doped n-type $Bi_2(Te_{0.95}Se_{0.05})_3$ alloy was improved from $2.1{\times}10^{-3}/K$ to $2.35{\times}10^{-3}/K$ by annealing at $500^{\circ}C$ for 3 hours. When annealed longer than 12 hours, however, the figure-of-merit decreased substantially due to the increase of the electrical resistivity.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.20 no.3
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• pp.638-644
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• 2019

In this study, the fluid flow and heat transfer characteristics within sandwich panels are investigated using computational fluid dynamics. Within the sandwich panels having periodic cellular cores, air can freely move inside the core section so that the structure is able to perform multi-functional roles such as simultaneous load bearing and heat dissipation. Thus, there needs to examine the thermal and flow analysis with respect to design variables and various conditions. In this regard, ANSYS Fluent was utilized to explore the flow and heat transfer within the pyramidal truss sandwich structures by varying the truss angle and inlet velocity. Without the entry effect in the first unitcell, the constant rate of pressure and the constant rate of Nusselt number was observed. As a result, it was demonstrated that Nusselt number increases and friction factor decreases as the inlet velocity increases. Moreover, the rate of Nusselt number and friction factor was appreciable in the range of V=1-5m/s due to the transition from laminar to turbulent flow. Regarding the effect of design variable, the variation of truss angle did not significantly influence the characteristics.