1. Thermoelectric properties and crystal structures of Au doped SiC/Si composites
H.Nakatsugawa1), K.Nagasawa1) and Y.Okamoto2)
1) Yokohama National University, 79-5 Tokiwadai, Hodogaya Word, Yokohama , Japan
　　　　　　　　　　　　　　　2) National Defense Academy, Hashirimizu, Yokosuka , Japan
Introduction
Silicon semiconductor devices such as Power Metal oxide semiconductor field effect transistor (Power MOSFET), Insulated gate bipolar transistor (IGBT) and CPU are the most important devices to give us modern life in the present time, and this aspect will continue for future. In this system, the heat flux flow directions of the thermal conduction and the Joule heating are the same for the usual Peltier elements. On the contrary, the direction of the Peltier heat flow is the opposite for the usual Peltier element because of the cooling the high temperature objects. When we use these devices in the electric circuit, the heat removal or the cooling down is one of the most important issues because the silicon device must be the temperature of 423K or lower to keep its function. Thus we must simultaneously remove the heat from the silicon devices. Recently Yamagushi et al.[1][2] propose the self-cooling device, which does not need to use the additional power circuits because the Peltier cooling is done by its selfcurrent. This technology carries out the Peltier cooling with the use of flowing current in silicon power device itself by using of thermoelectric material instead of cupper electrode in silicon power devices. In particular, Silicon carbide has considerable promise as the self-cooling device material since it takes advantage of the higher electrical conductivity, thermal conductivity and Seebeck coefficient which are different from conventional thermoelectric material. The purpose of this study is to investigate the thermoelectric properties and crystal structure for Au doped SiC/Si composite in order to apply the self-cooling device .
Experimental
In this study, we synthesized SiC/Si and Au doped SiC/Si composites by a conventional solid-state reaction method. PSS (polysilastyrene) is one of the sintering additives in this system. Excess PSS thermally decomposes and evaporates during the sintering process and pores remain in the sample. β-SiC, Si, Au powder and PSS were used as starting materials. Slurries were made from mixed powder of wt.% SiC, 40 wt.% Si, wt.% Au and added PSS 10 wt.%. Then the slurries were mixed in polyethylene jars with nylon coated iron balls as the grinding media and with xylene solution as the mixing agent. After mixing for 24 hours and passing through 75 mesh sieve, these slurries were dried. The dried mixture were granulated using 500 mesh sieve and pressed into 20mmφ×6mm pellet under 100 kg/cm2 pressure. Then the pellets were sealed into an evacuated rubber tube and pressed isostatically at 3500 kg/cm2. Each pellet covered with the same composition powder was placed into a carbon crucible. Sintering was carried out in vacuum and atmosphere furnace heated by carbon. First, the furnace was heated up to 1000℃ at a rate of 20℃/min in vacuum, and Ar gas was introduced at normal pressure. The furnace temperature was further raised to 2100℃ at a rate of 10℃/min, and the sample was kept at this temperature for 2 hours and cooled naturally down to room temperature. Sintered bodies were cut to rectangular shaped specimens of 2×10×10 mm3 in dimensions for measurement of the thermoelectric properties.
The crystallographic properties of prepared sample are studied by using powder x-ray diffraction. The Au, C and Si constituents of prepared sampels are estimated by using Inductively Coupled Plasma (ICP) analysis, the direct combustion-gasometric method and the gravimetric technique, respectively. The microstructures of the samples were examined by a scanning electron microscope (SEM: VE-8800, KEYENCE) and simultaneously the dopant concentration of the samples after sintering process were clarified by using an electron probe x-ray microanalyzer (EPMA: JXA-8900R, JEOL). The crystal structure and phase of the samples were studied by using x-ray diffractometer (XRD: Bruker-AXS D5005, Siemens) ranging from room temperature to 900℃. The identification of polycrystalline SiC/(Si, Au) composites were analyzed using the Rietveld refinement program, RIETAN The measurements of electrical resistivity and Seebeck coefficient were carried out in the temperature range from 80 to 385 K. The electrical resistivity ρ was measured by a van der Pauw technique with a current of 10mA in He atmosphere. The Seebeck coefficient S measurement was carried out on a sample placed between two blocks of oxygen-free high-conductivity (OFHC) silver. The thermocouples were welded to the reverse sides of the OFHC silver to measure the temperature ΔT ～ 3K and thermoelectric power ΔV. The slope (dΔV/dΔT) obtained by a least-squares method yields the Seebeck coe.cient S at each measurement temperature. The Hall coefficient RH measurement was performed on flat square pieces of materials with a current of 100mA in a magnetic field of 8500 Oe by a van der Pauw technique at room temperature. The Hall carrier concentration n was determined from RH using n=1/eRH, where e is the electron charge, assuming a scattering factor equal to 1 and a single carrier model. Furthermore, the Hall mobility μ was determined from ρ and RH using μ=RH/ρ. The thermal diffusivity A, sample density D and specific heat C were measured at room temperature, where the thermal conductivity κ was calculated from κ=ACD. The thermal diffusivity A was measured by a laser flash method (TC-3000, ULVAC).
SEM & EPMA
Thermoelectric Properties
at room temperature
X-ray diffraction measurement at High temperature
β-SiC　phase
Si　phase
Au　phase
Electric Resistivity
Seebeck Coefficient
Summary
Si, Au and PSS additive β-SiC sintered semiconductor appears a maximum value of electrical resistivity and power factor at the composite of PSS 10wt.% additive [SiC]0.8[Si]0.175[Au] It is considered that the high sample density and Au additive result in an increase of mobility, and hence lead to enhancement of electrical conductivity. Furthermore, SEM observation to investigate the trend of the high sample density exhibits that there is a peak of the grain growth at PSS 10wt.% additive sample. In order to gain insight into the crystal characteristics of SiC/Si/Au composites, high temperature XRD carry out in the range of 300 to 1173K and confirm the appearance of the marginal diffraction peaks (α-SiC) in the vicinity of 2θ=36.5 and 60.5°as increasing temperature.
References
[1]S.Yamaguchi, Y.Okamoto, A.Yamamoto and M.Hamabe, Proc. 26th Int. Conf. Thermoelectrics, O-G-1, (2007).
[2]S.Yamaguchi, “Peltier cooling for Semiconductor Devices”, ULVAC 52 (2007) pp (Japanese).