Effect of porosity on the thermal-electric properties of Al-doped SiC ceramics

Abstract

Silicon carbide has being used in many practical applications because of its many excellent properties, such as extreme hardness, excellent thermal and mechanical properties and high thermal shock resistance. There are many reports in the literature on silicon carbide. Recently, as high-temerature thermoelectric materials, silicon carbide ceramics have attracted much attention in the literature [1±6]. Koumoto et al. [1] reported that porous SiC ceramics showed high ®gures of merit for thermoelectric energy conversion. Since then, the thermoelectric properties of porous SiC ceramics fabricated by using different starting SiC powders (e.g. synthesized by carbothermal reduction of silica contained in rice hull ash [2] and the gas-phase reaction method [3]) and different processing routes (e.g. sintering in nitrogen or argon atmosphere [4] and reaction sintering [5]) have been investigated. Despite efforts to investigate the porous SiC thermoelectric ceramics, the effect of porosity on the thermoelectric properties is still unclear. In this letter, the effect of porosity on the thermal-electric properties of porous Al-doped SiC ceramics by hot pressing and pressureless sintering methods is reported. Commercial a-SiC powders (grain size ,10 im, purity .98.5%) were ball milled with different contents of Al powder (400 mesh, purity .99%) for 24 h. The samples were prepared in two different ways: (i), pressureless sintering (PS) under argon for 1 h at 1770 8C after the mixtures were cold isostatic pressed at 200 MPa and (ii) hot-pressing (HP) under argon at 20 MPa and 1900 8C for 0 h or 1 h in highstrength graphite dies. During the sintering, graphite end-caps were bonded to the silicon carbide cylinders, which facilitated attachment of thermocouples and electrical leads for Seebeck coef®cient and electrical conductivity measurements. The relative density of the ceramic samples, measured by the liquid displacement method, is shown in Fig. 1. The porosity of the samples decreased with Al dopant concentration, which is supported by scanning electron microscope (SEM) observation. X-ray diffraction (XRD) analyses showed that there existed only a single SiC phase and no other new phase. The average grain size was around 8 im from SEM. Electrical conductivity measurements were carried out under argon by a two-probe a.c. method. Two platinum lines were sealed on two platinum slices, which were pressed to ®rmly attach to the polished graphite electrodes on the two ends of the samples. The electrical conductivity was obtained from two resistance values measured under two opposite current directions (biased at about 0.2 V a.c. voltage). Fig. 2 shows the electrical conductivity of the 4 wt % Al-doped SiC ceramics with different densities. It is clear that the electrical conductivity increases with the decrease of porosity, as expected. The thermal conductivity was measured using a