Katsuhiko Sasaki

Improvements of Thermal Conductivity of Aluminum Based Composites Containing VGCF-CNT Network by Heat Treatments of CNT

Improvements of thermal conductivity of aluminum based composites containing CNTVGCF networks, which may be used for the cooling device of Insulated Gate Bipolar Transistors (IGBT) for aerospace, are discussed in this paper. The composite is fabricated by Spark Plasma Sintering (SPS) from aluminum powder and vapor grown carbon fiber (VGCF). It has been found that the small amounts of CNT addition to the composites increase in the thermal conductivity of the VGCF-aluminum composites because the VGCFCNT network is constructed. To improve the thermal conductivity of the VGCF/CNTaluminum composites, CNT is subjected to additional heat treatments using SPS. The measurements of Raman spectrophotometry shows that the defect band dramatically decreases and that the unit surface and G/D ratio are improved by heat treatment. The observation of scanning electron microscope (SEM) also shows that CNT is bundled by the heat treatment. Finally, it is found that the composites containing the heat-treated CNT have the larger thermal conductivity than the composite containing the non-heat-treated CNT.

IGBT Cooling System Using High Thermal Conductive Aluminum Based Composite Containing VGCF-CNT Network

Cooling of Insulated Gate Bipolar Transistors (IGBT) is a huge problem to improve the energy efficiency. To improve the cooling efficiency, a material that has the high thermal conductivity is expected to be employed for the cooling system of IGBT. The authors have fabricated an aluminum based composite material containing a network of vapor grown carbon fiber (VGCF) and carbon nanotube (CNT). The composite has the high thermal conductivity more than three times of pure aluminum. The composite also has the thermal anisotropy caused by VGCF. In this paper, the composite fabricated by authors is employed to a cooling system of IGBT. First, the effect of the thermal conductivity on the thermal efficiency of the cooling system of IGBT is investigated by finite element analyses (FEM). The effect of the thermal anisotropy is, especially, focused on to show the important role of the thermal anisotropy for the cooling system. A method to improve the thermal conductivity of the composite is also shown to decrease the fabrication costs by decreasing in the usage of VGCF. The control of alignments of VGCF is chosen for the method. The alignment is controlled by the contraction flow made by a contraction channel. The experiments show that the contraction flow is effective to control the alignment of VGCF.

FEM analysis on balloon expandable stent considering viscoplasticity

Finite element method (FEM) in analysis of stenting process capable to reveal many information, which cannot be accessed in experiment. To obtain qualified result, the procedure should be verified and the analyses results should be validated. In the case of stent simulation, the verification of FEM procedure is more important than the validation of FEM results due to the difficulties in validating FEM results using experimental method. In this study, ANSYS Student R18.0 is used to compare two inelastic behavior analyses in studying the effect of dislocation creep, i.e. rate-independent and rate-dependent plasticity during the same time history. Rate-independent plasticity uses only multilinear behavior (von Mises potential) as isotropic hardening plasticity while rate-dependent plasticity uses creep (Bailey-Norton law) combined with multilinear von Mises potential. Cyclical symmetry of stent and balloon geometry is generated to save computational time. A Palmaz and a Sinusoidal stent type is chosen for the stent models because these stents represent rigid and flexible structure of a slotted-tube stent. To learn the influence of the time hardening formulation, three different periods of creeping time with the equal pressure rate and three different periods of expanding time with the equal creeping time are set in the loading. The study presented that the creep behavior tends to affect the nominal radii of flexible stent (Sinusoidal stent), but no influence to the stent with rigid structural geometry (Palmaz stent). Besides, valuable finding is identified as various expanding periods applied in the stent deployment.Finite element method (FEM) in analysis of stenting process capable to reveal many information, which cannot be accessed in experiment. To obtain qualified result, the procedure should be verified and the analyses results should be validated. In the case of stent simulation, the verification of FEM procedure is more important than the validation of FEM results due to the difficulties in validating FEM results using experimental method. In this study, ANSYS Student R18.0 is used to compare two inelastic behavior analyses in studying the effect of dislocation creep, i.e. rate-independent and rate-dependent plasticity during the same time history. Rate-independent plasticity uses only multilinear behavior (von Mises potential) as isotropic hardening plasticity while rate-dependent plasticity uses creep (Bailey-Norton law) combined with multilinear von Mises potential. Cyclical symmetry of stent and balloon geometry is generated to save computational time. A Palmaz and a Sinusoidal stent type is chosen for th...