Dong-Su Kang

Bulk graphite: materials and manufacturing process

Graphite can be classified into natural graphite from mines and artificial graphite. Due to its outstanding properties such as light weight, thermal resistance, electrical conductivity, thermal conductivity, chemical stability, and high-temperature strength, artificial graphite is used across various industries in powder form and bulk form. Artificial graphite of powder form is usually used as anode materials for secondary cells, while artificial graphite of bulk form is used in steelmaking electrode bars, nuclear reactor moderators, silicon ingots for semiconductors, and manufacturing equipment. This study defines artificial graphite as bulk graphite, and provides an overview of bulk graphite manufacturing, including isotropic and anisotropic materials, molding methods, and heat treatment.

Fabrication of isotropic bulk graphite using artificial graphite scrap

Abstract Isotropic synthetic graphite scrap and phenolic resin were mixed, and the mixed powder was formed at 300 MPa to produce a green body. New bulk graphite was produced by carbon-izing the green body at 700°C, and the bulk graphite thus produced was impregnated with resin and re-carbonized at 700°C. The bulk density of the bulk graphite was 1.29 g/cm 3 , and the porosity of the open pores was 29.8%. After one impregnation, the density increased to 1.44 g/cm 3 while the porosity decreased to 25.2%. Differences in the pore distribution before and after impregnation were easily confirmed by observing the microstructure. In addition, by using an X-ray diffractometer, the degrees-of-alignment (Da) were obtained for one side perpendicular to the direction of compression molding of the bulk graphite (the “top-face”), and one side parallel to the direction of compression molding (the “side-face”). The anisot-ropy ratio calculated from the Da-values obtained was 1.13, which indicates comparatively good isotropy.

습식 식각에 의한 실리콘 웨이퍼의 표면 및 전기적 특성변화(1) - 불산 농도에 따른 표면형상 변화 -

The electrical properties and surface morphology changes of a silicon wafer as a function of the HF concentration as the wafer is etched were studied. The HF concentrations were 28, 30, 32, 34, and 36 wt%. The surface morphology changes of the silicon wafer were measured by an SEM (80˚ tilted at ×200) and the resistivity was measured by assessing the surface resistance using a four-point probe method. The etching rate increased as the HF concentration increased. The maximum etching rate 27.31 μm/min was achieved at an HF concentration of 36 wt%. A concave wave formed on the wafer after the wet etching process. The size of the wave was largest and the resistivity reached 7.54 ohm·cm at an 30 wt% of HF concentration. At an HF concentration of 30 wt%, therefore, a silicon wafer should have good joining strength with a metal backing as well as good electrical properties.

Thermal Emission Effect of Electronic Parts Using Carbon Materials

Abstract Recent high efficiency electronic devices have been found to have heat emission problems. As for LEDs, anexcessive increase in the device temperature causes a drop of the luminous efficiency and circuit lifetime. Therefore, heat rel eeasin the limited space of such electronic parts is very important. This is a study of the possibility of using a coating of carbo nmaterials as a solution for the thermal emission problem of electronic devices. Powdered carbon materials, cokes, carbon blacks ,amorphous graphite, and natural flakes were coated with an organic binder on an aluminum sheet and the subsequent thermalemissivity was measured with an FT-IR spectrometer and was found to be in the range of 5~20 µ m at 50 o C. The emissivityof the carbon materials coated on the aluminum sheet was shown to be over 0.8 and varied according to carbon type. Themaximum thermal emissivity on the carbon black coated-aluminum surface was shown to be 0.877. The emissivity of theanodized aluminum sheets that were used as heat releasing materials of the electronic parts was reported to be in the rangeof 0.7~0.8. Therefore, the use of a coating of carbon material can be a potential solution that facillitates heat dissipation f orelectronic parts.Key words thermal emissivity , electronic parts, carbon materials , coating.

Microstructural changes of polyacrylonitrile-based carbon fibers (T300 and T700) due to isothermal oxidation (1): focusing on morphological changes using scanning electron microscopy

Polyacrylonitrile (PAN)-based carbon fibers have high specific strength, elastic modulus, thermal resistance, and thermal conductivity. Due to these properties, they have been increasingly widely used in various spheres including leisure, aviation, aerospace, military, and energy applications. However, if exposed to air at high temperatures, they are oxidized, thus weakening the properties of carbon fibers and carbon composite materials. As such, it is important to understand the oxidation reactions of carbon fibers, which are often used as a reinforcement for composite materials. PAN-based carbon fibers T300 and T700 were isothermally oxidized in air, and microstructural changes caused by oxidation reactions were examined. The results showed a decrease in the rate of oxidation with increasing burn-off for both T300 and T700 fibers. The rate of oxidation of T300 fibers was two times faster than that of T700 fibers. The diameter of T700 fibers decreased linearly with increasing burn-off. The diameter of T300 also decreased with increasing burn-off but at slower rates over time. Cross-sectional observations after oxidation reactions revealed hollow cores in the longitudinal direction for both T300 and T700 fibers. The formation of hollow cores after oxidation can be traced to differences in the fabrication process such as the starting material and final heat treatment temperature.

Changes in the porosity of bulk graphite according to the viscosity of resin for impregnation

When manufacturing bulk graphite, pores develop within the bulk during the carbonization process due to the volatile components of the fillers and the binders. As a result, the physical properties of bulk graphite are inferior to the theoretical values. Impregnants are impregnated into the pores generated in the carbonization process through pressurization and/or depressurization. The physical properties of bulk graphite that has undergone impregnation and re-carbonization processes are outstanding. In the present study, a green body was manufactured by molding with natural graphite flakes and phenolic resin at 45 MPa. Bulk graphite was manufactured by carbonizing the green body at 700 and it was subsequently impregnated with impregnants having viscosity of 25.0 cP, 10.3 cP, and 5.1 cP, and the samples were re-carbonized at 700°C. The above process was repeated three times. The open porosity of bulk graphite after the final process was 22.25%, 19.86%, and 18.58% in the cases of using the impregnant with viscosity of 25.0 cP, 10.3 cP, and 5.1 cP, respectively.

습식 식각에 의한 실리콘 웨이퍼의 표면 및 전기적 특성변화(2) - 표면거칠기와 전기적 특성의 상관관계 -

The relationship the between electrical properties and surface roughness (Ra) of a wet-etched silicon wafer were studied. Ra was measured by an alpha-step process and atomic force microscopy (AFM) while varying the measuring range 10×10, 40×40, and 1000×1000μm. The resistivity was measured by assessing the surface resistance using a four-point probe method. The relationship between the resistivity and Ra was explained in terms of the surface roughness. The minimum error value between the experimental and theoretical resistivities was 4.23% when the Ra was in a range of 10×10μm according to AFM measurement. The maximum error value was 14.09% when the Ra was in a range of 40×40μm according to AFM measurement. Thus, the resistivity could be estimated when the Ra was in a narrow range.

Effects of binder type and heat treatment temperature on physical properties of a carbon composite bipolar plate for PEMFCs

Abstract This study investigated a developed process for producing a composite bipolar plate having excellent conductivity by using coal tar pitch and phenol resin as binders. We used a pressing method to prepare a compact of graphite powder mixed with binders. Resistivity of the impregnated compact was observed as heat treatment temperature was increased. It was observed that pore sizes of the GCTP samples increased as the heat treatment temperature increased. There was not a great difference between the flexural strengths of GCTP-IM and CPR-IM as the heat treatment temperature was increased. The resistivity of GPR700-IM, heat treated at 700°C using phenolic resin as a binder, was 4829 µΩ·cm which was best value in this study. In addition, it is expected that with the appropriate selection of carbon powder and further optimization of process we can produce a composite bipolar plate which has excellent properties. Key words: polymer electrolyte membrane fuel cell, bipolar plate, graphite, resistivity, flexural strength