Thermoelectric materials for middle and high temperature ranges
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Thermoelectric materials for middle and high temperature ranges Ryoji Funahashi,a)b) Tristan Barbier, and Emmanuel Combe Inorganic Functional Materials Research Institute, National Institute of Advanced Industrial Science & Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan (Received 19 March 2015; accepted 28 April 2015)
Thermoelectric generation is one of the strongest candidates for recovering the waste heat from industry and transportation. Some of oxides and silicides are considered to be promising thermoelectric materials because of their high oxidation resistance. Several types of modules using p-type Ca3Co4O9/n-type CaMnO3 and p-type MnSi1.75/n-type Mn3Si4Al2 have been prepared and shown around 4 kW/m2 of maximum power density. The present study described the challenging enhancement of the thermoelectric figure of merit ZT of both oxide and silicide compounds. Introduction of secondary phases and low bulk density using a partial melting method is found to be effective for reducing phonon thermal conductivity in the promising Bi2Sr2Co2Ox. The grain size and distribution of the secondary phases can be controlled by optimizing the parameters of the partial melting method. On the other hand, detailed crystallographic structure of a new n-type Mn3Si4Al2 is clarified and leads to the enhancement of the ZT values by elemental substitution.
I. ENERGY CRISIS AND WASTE HEAT
Contributing Editor: Terry M. Tritt a) Address all correspondence to this author. e-mail: [email protected] b) This author was an editor of this focus issue during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www. mrs.org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2015.145
not progress notably. This is due to not only technical issues but also initial cost, running cost, investment recovery, etc. Since the present heat engines are Carnot cycles, waste heat is always formed by energy conversion. The average of total efficiency utilized for the primary energy is as low as 30%, with 70% exhausted to the air as waste heat.3 To enhance the total energy efficiency and to reduce the carbon dioxide emission, such waste heat should be converted to the electrical energy. Technologies of heat storage and transport have been developing. However, since most sources of waste heat are widely and thinly dispersed, it is difficult to use such heat energy efficiently. Electricity is a convenient form of energy in points of view of transportation, storage, and conversion. Thus, the direct conversion of waste heat to electricity has placed high expectations. Thermoelectric conversion has paid attention as the strongest candidate to generate electricity from dilute waste heat. The waste heat is widely dispersed and can be found in the temperature region from 323 K to higher than 1273 K. Figure 1 shows the industry-classified waste heat energy with temperature higher than 473 K.4 The heat energy with temperature higher than 673 K occupies about 29%. However, the effectiv
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