1. Thermoelectric properties and crystal structures of　Au doped SiC/Si composites
H.Nakatsugawa1), K.Nagasawa1) and Y.Okamoto2)
1) Graduate School of Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya Word, Yokohama , Japan
2) Department of Material Science and Engineering, National Defense Academy, Hashirimizu, Yokosuka , Japan
Introduction
Silicon semiconductor devices have been progressed efficiency with gain of developed power. One of the most considerable problems interfering the development is that because of Joule heating, their devices are not capable of conduct an electric current of the spec. Yamaguchi.,et.al [1][2] suggested new cooling system using of the Peltier material plate named “Self-Cooling Semiconductor Device”. This technology carries out the Peltier cooling with the use of flowing current in silicon power device (e.g PowerMOSFET, IGBT) itself by using of thermoelectric material instead of cupper electrode in silicon power devices. Thermoelectric properties of this applicable material are demanded the higher electrical conductivity, thermal conductivity and seebeck coefficient different from conventional thermoelectric material. Silicon carbide has considerable promise as the Self-Cooling Device material since it takes advantage of these desirable features.
The purpose of this study is to investigate the thermoelectric properties and crystal structure for SiC/Si/Au composite in order to apply the Self-Cooling Semiconductor Device. In this work, SiC/Si/Au composites were investigated the thermoelectric property and characteristics of crystal structure depend on temperature.
Electrical and thermal transport properties at RT
Table 1 shows the electrical transport properties and thermal transport properties at room temperature. The rise of sample density result in an increase of mobility, and hence it causes an increase of electrical conductivity. Conclusively, it is seen that a high value of power factor S2σ and thermal conductivity κ　at room temperature is the peak in the composite of PSS 10wt.% additive SiC/Si/Au=55/40/5wt.%, and reaching a value of S2σ =2.54×10-4W/mK2 and κ=52.4W/mK. For “Self-Cooling Semiconductor Device”, high power factor and thermal conductivity are desirable properties differ from conventional thermoelectric materials.
Experimental procedure
Polycrystalline samples were prepared by a conventional solid-state reaction.β-SiC, Si and Au powder were used as starting material, and polysilastyrene ware mixed as sintering aids. The mixing powder ware formed pellet type by uniaxial pressing at 1×106kg/m2. Furthermore, the pellets were conducted the Cold Isostatic Pressing at 3.5×107kg/m2. All sample were sintered at 2373K for 120 min in Ar atmosphere.
The measurement of electrical conductivity and Seebeck coefficient was performed ranging from 80 to 385K, and Hall coefficient was carried out at room temperature. The thermal diffusivity and specific heat values were measured by using laser flash method, and thermal conductivity was calculated from their results and sample density. The microstructures of the samples were observed using SEM on the fractured surface. The crystal structure and phase of the samples ware studied by powder XRD and high temperature XRD. The identification of polycrystalline SiC/Si/Au composites ware analyzed using the Rietveld refinement program, RIETAN-2000 from XRD data.
X-ray measurement
The powder X-ray measurement was carried out for all samples at room temperature. X-ray diffraction pattern indicate that SiC, Si and Au phases, respectively exist independent phase, in other words, it can be explained the evaporation of PSS and no chemical reactions of each other compound in sintering process. The pure substance of SiC (no doping SiC) is known the phase transition from 3C-SiC to 6H-SiC at about 2273K. Though it is nonreversible change, diffraction pattern of fabricated SiC samples are shown the 3C-SiC phase despite of sintering at 2273K. We infer that this reason is due to the excessive additive of Si.
Table 1. electrical conductivity σ, Hall coefficient RH, carrier concentration n, Hall mobility μ, Seebeck coefficient S, power factor S2σ, thermal diffusivity α, sample density d, specific heat c and thermal conductivity κ　at room temperature.
Fig. 3. Rietveld refinement of powder XRD data at room temperature:
The bar line of upper (middle, lower) stand represent possible SiC (Si, Au) bragg reflections.
(a) SiC/Si=60/40+PSS10wt.% (b) SiC/Si/Au=55/40/5+PSS10wt.%.
Electrical conductivity and
Seebeck coefficient
Fig. 1(a) shows the temperature dependence of the electrical conductivity σ. As compared with Si 40wt.% additional SiC as primary composition sample, at room temperature, the value of σ of PSS 10wt.% additive primary composition (SiC/Si=60/40wt.%) was 3.1 times (2.62×104/Ωm) as large as that of the based sample. Moreover, that of PSS 10wt.% additive sample to add to SiC/Si/Au=55/40/5wt.% was 3.5 times (2.97×104/Ωm) as large as that of the primary sample. Fig. 1(b) shows the temperature dependence of the Seebeck coefficient. A maximum value of S in the range of all temperature occurs with SiC/Si=60/40wt.% sample, and at room temperature, it reached a value of approximately 111μV/K. As compared results of S value of PSS 10wt.% additive SiC/Si/Au=55/40/5wt.% to PSS 10wt.% additive SiC/Si=60/40wt.%, the effect of Au additive appears memorably in low temperature side (especially below 200K) as opposed to arising that effect with rising temperature in electrical conductivity shown in fig. 1(a) .
Scanning electron microscope
Microstructure variation with varying composition ratio is shown in fig. 2. the SEM results confirm that the respective samples are β-SiC (3C-SiC). When the PSS as the sintering aids additive samples (b), (c) is compared the primary sample (a), it appears that SiC grain growth is developed and porosity is diminished. This trend has a continual effect from 0.5 to 10wt.% by PSS additive. Adversely, PSS 20wt.% and Au 5wt.% additive sample (e) was observed an increase of porosity. It follows that PSS above 15wt.% and Au additive tend to depress the grain growth. It can be explained that a mass of PSS additive (above 15wt.%) have a little influence on sintering aids but strong volume affect, and optimum amount of PSS additive is 10wt.%.
High temperature X-ray measurement
Fig. 4. shows the temperature dependence of X-ray diffraction pattern. We find to appear the marginal diffraction peaks in the vicinity of 2θ=36.5 and 60.5° as the temperature rise to 1173K. We carried out the identification of this newly appearance peaks by determined their bragg reflection angles and there is a possibility that their peaks are 6H-SiC or 8H-SiC polytype. An additional remark which should be made here is that the diffraction peak in 2θ=26.5° is identified only bragg reflection of 8H-SiC. This peak has existed the region of all temperature and no significant changing of peak intensity; in other words,
8H-SiC has already existed at room temperature and has not been changing intensity as the temperature rise. Judging from the results so far obtained, the peak appearances in 2θ=36.5 and 60.5° as to temperature rise can be explained the phase transition from 3C-SIC to 6H-SiC.
(a)
(b)
(a)
(b)
(c)
Fig. 2. SEM micrographs of fracture
surface of
(a)SiC/Si=60/40wt.% (b)SiC/Si=60/40+PSS0.5wt.% (c)SiC/Si=60/40+PSS10wt.% (d)SiC/Si/Au=55/40/5+PSS10wt.% (e)SiC/Si/Au=55/40/5+PSS20wt.%
by sintering at 2373K for 120 min
in Ar atmosphere.
Fig. 1. Temperature dependence of electrical conductivity(a) and Seebeck coefficient(b) of SiC/Si=60/40+PSS(0, 0.5, 10)wt.% and SiC/Si/Au=55/40/5+PSS(10, 20)wt.%.
Fig. 4. Temperature dependence of XRD pattern of SiC/Si/Au=55/40/5+PSS10wt%:
(a) 2θ=31-41° (b) 2θ=55-65°(c) 2θ=21-34°.
Conclusions
Si, Au and PSS additive β-SiC sintered semiconductor appears a maximum value of electrical conductivity and power factor at the composite of PSS 10wt.% additive SiC/Si/Au=55/40/5wt.%. 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 in the vicinity of 2θ=36.5 and 60.5° as the temperature rise. We conclude that the development of the peak is the phase transition from 3C-SC as parent phase to 6H-SiC.
Acknowledgment
The Hall effect measurement system, scanning electron microscope and Archimedes’s method measuring device were used at the Instrumental Analysis Center in Yokohama National University. The high temperature XRD measurement was carried out at the School of Chemistry in University of Birmingham.
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).