Direct thermal-to-electric energy conversion material consisting of environmentally-benign Mg2Si synthesized using waste Si sludge and recycled Mg alloy
Authors: Honda Yasuhiko , Iida Tsutomu , Sakamoto Tatsuya , Sakuragi Shiro , Taguchi Yutaka , Mito Yohiko , Nemoto Takashi , Nakajima Tadao , Taguchi Hirohisa , Nishio Keishi , Takanashi Yoshifumi 
Published: September 2010

In order to perturb global warming and realize a sustainable global energy system, enhancements in the energy efficiency are required. One of the reliable technologies to reduce the greenhouse gas emissions and the consumption of fossil fuel that is attracting attention is thermoelectric technology, which can directly convert heat into electricity and consequently increase the energy conversion efficiency of power generation by combustion. Magnesium silicide (Mg2Si) has been identified as a promising advanced thermoelectric material operating in the temperature range from 500 to 800 K. Compared with other thermoelectric materials that operate in the same conversion temperature range, such as PbTe, TAGS (Ge-Te-Ag-Sb) and CoSb3, Mg2Si shows promising aspects, such as the abundance of its constituent elements in the earth’s crust and the non-toxicity of its processing by-products, resulting in freedom from care regarding prospective extended restriction on hazardous substances.

Here we have tried to introduce reusing of industrial waste of Si sludge as a source material for Mg2Si, because the current product inversion rate of Si for semiconductor LSI devices remains at 25 to 30 %, while most of the remainder is disposed of as a waste; this is mainly discharged as sludge from grinding and polishing processes. It is possible that the reuse of this waste Si could be effective in both reducing the cost of source Si and in the reduction of industrial waste. On the other hand, recycled materials of standard lightweight magnesium alloys based on the Mg-Al-Zn-Mn system, such as AZ91 or AM50, were also introduced as a Mg source for Mg2Si synthesis. The concept of this work is a production of wasted heat recovery device using environmentally-benign Mg2Si by means of industrial waste or less pure recycled sources.

The efficiency of a thermoelectric device is characterized by the dimensionless figure of merit, ZT=S2σT/κ, of its constituent thermoelectric material, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature. As a target for practical use, ZT value exceed unity, which gives about 10 % conversion efficiencies, is expected. So far, we succeeded to obtain the Mg2Si with ZT=1.08 using rather pure Si (99.999% : solar grade) and Mg (99.95%) sources. In this article, we report multifarious fabrication processes in order to realize ZT value as high as unity and the detailed thermoelectric properties concerning Mg2Si initiated from reused Si sludge and the recycled Mg-alloy sources. In conjunction, we will also discuss the practical output-power characteristics of the samples with the formation of Ni electrodes by monobloc sintering. A tentative generated power density from the wasted heat at 773 K was ˜2 KW/m2.