• PDF / 644,016 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 78 Downloads / 230 Views

DOWNLOAD

REPORT

---

Microstructural evolution of La-doped SiB6 high-temperature thermoelectric material during a Spark Plasma Sintering D.W.Lee, J.H.Won, K.H.Kim, J.Matsushita*, K.B.Shim CPRC, Hanyang Univ., Seoul 133-791, Korea *Dept. of Mat. Sci., Tokai Univ., Hirasuka 259-1292 , Japan TEL:+82-2-2290-0501

FAX:+82-2-2291-7395

e-mail: [email protected]

ABSTRACT

SiB6 has proved to a potentially useful material because of its excellent thermoelectrical properties above 700

℃, low specific gravity, high degree of hardness, and moderate melting

point. SiB6, which has poor sinterability with a conventional sintering technique due to the covalent characteristic, has been successfully densified fully using a spark plasma sintering(SPS) method. The SPS-processed specimens consisted of SiB6, SiB4 and SixBy phases. Pure SiB6

℃

powder were densified fully at the sintering temperature of 1600 . In particular, it was found that the rare earth element was very effective in evolving the microstructure of SiB6 phase, resulting in reducing the sintering temperature and controlling grain growth. These effects were discussed in details in terms of microstructure evolution during the SPS process.

INTRODUCTION

Silicon-borides have been attractive in various industrial application because of their high temperature properties, high hardness and excellent electrical conductivity. Of several silicon boride(SiXBY) phases which were previously reported, SiB6 is a promising material for use as a thermoelectric semiconductor at high temperature[1] because it has large Seebeck coefficient, a low thermal conductivity, excellent electrical conductivity, high degree of hardness.[2,3] However, the application has been limited by the poor sinterability of SiB6 using conventional sintering technique. A spark plasma sintering (SPS) is one of the best candidates to densify fully poorly sinterable materials. The SPS technique is a newly developed sintering process that can heat the material G8.13.1

several to tens of thousands of degrees Celcius by spark discharges between the particles generated by a pulsed direct current which is applied through electrodes. Due to these discharges, acceleration of mass-transfer by the evaporation or melting on the particle surface occurs and the densification can be achieved rapidly at lower overall temperatures. Therefore, the SPS process can control the grain growth by using a shorter sintering time than that of the conventional process, such as, hot-pressing and hot-isostatic pressing .[4,5,6] This work presents the sintering mechanism of SiB6 produced by a spark plasma sintering technique in order to improve the microstructure for thermoelectric applications at high temperature. The effect of rare earth element(La) on microstructure also were investigated.

EXPERIMENTAL Commecially available SiB6 powder has been used as starting materials. Densification was performed by the spark plasma sintering system (Sumitomo Coal Mining: SPS-515S). The processing was carried out in vacuum using a graphite die with a diameter of 10