Assistant Professor：Naomi Hirayama
At our laboratory, we have been developing materials for energy conversion to tackle the global warming that is being caused by the mass consumption of fossil fuels. In particular, considerable weight is currently being placed on the study of materials for power generation from exhaust heat. Using these materials, heat energy, which is the final phase of energy consumption, can be recycled into electrical energy. Concurrently, we have also been pursuing environmentally friendly semiconductor energy conversion materials, while studying environmentally “low-load” production processes. Environmental semiconductors are semiconductor materials that are abundant on the earth and which comprise of materials that are friendly to living creatures and to the environment.
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 which 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, of its constituent thermoelectric material, where ZT is consisting of the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and 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.
Magnesium silicide (Mg2Si) has emerged as the most promising thermoelectric (TE) material for automotive applications. This is mainly due to the light weight of Mg2Si, the relative abundance of its constituent elements (which have no associated national risk to material supply), and to its good TE properties with sufficient durability at elevated operational temperatures. We have been engaging in the establishment of fundamental fabrication techniques for the manufacture of Mg2Si TEGs, including the use of all-molten poly-crystalline source material, plasma activated sintering with tunable pulse wave current operation, sintering scalability with a metal binder, and monobloc sintering or a modified plating method for metal electrode termination to Mg2Si.
In the context of increasing energy prices and climate change, thermo-electric conversion is of the highest interest for producing electrical power from waste heat. The automotive industry is keenly awaiting the installation and development of thermoelectric generators (TEG) because of the strict fuel consumption regulations in the EU. Since ~70 % of the gasoline that is used in cars is emitted as waste heat, if some percentage of the discarded heat could be reused then fuel consumption would be improved. An on-board TEG system is one possible technique for conserving fuels and supplying electricity.