Focus on Thermoelectrics and Nanomaterials

Abstract

Thermoelectric (TE) materials and devices are a solid-state technology that has been in use for decades, but research has been rapidly expanding over the last ten years. The primary reason is that these materials are of great technological importance and could provide significant contributions as alternative energy materials. In addition to waste heat recovery, TE materials are part of solid-state devices that could convert solar energy directly into electricity. Current materials and devices exhibit low efficiency (≈7–8%) and have been used primarily in niche markets. Even though the conversion efficiency is low, this technology allows low quality waste heat to be turned into useful electrical energy. The conversion efficiency, ηTE is proportional to a quantity called the figure of merit, ZT, defined as: ZT = α2T/ϱκ. ZT is proportional to the Carnot efficiency and therefore requires a significant temperature difference to operate more efficiently. The material parameters are the thermopower α, the electrical resistivity ϱ and the thermal conductivity κ. Heat is carried by both electrons (κe) and phonons (κph), hence κ = κe + κph. An extensive review of TE materials along with applications is given in Ref. [1]. The rebirth in thermoelectric materials research started from two main infl uences: ↓ Glen Slack's suggestion of a phonon glass electron crystal (PGEC) approach to TE materials research such that in this PGEC material the electrons would behave as if in a crystal and the phonons would scatter as in a glassy or amorphous material [2]. This led to the discovery of filled skutterudites and their high TE performance. ↓ Mildred Dresselhaus' assertion that lower dimensional materials would lead to higher ZT values eventually led to the work of Rama Venkatasubramanian and Ted Harman in low dimensional structures [3–5]. TE devices have a long history of use in NASA's deep space probes using radioactive thermoelectric generators (RTGs) to serve as a long-term reliable power supply [1]. They have been working non-stop in many space missions for over 30 years. In other areas such as a heavy diesel truck where the waste heat is about 60%, thermoelectrics could play an important role in converting this into electricity. Even if the current TE materials are used in devices with limited efficiency on these heavy trucks that operate between 500 °C and ambient, then billions of dollars could potentially be saved each year in the US alone. Not only is money saved, as well precious energy is converted from a waste energy state into usable electrical energy. The potential of TE technology with respect to solar energy conversion and the challenges in materials advancement are also very promising. The goal is to utilize higher efficiency TE devices that would exploit the infrared spectrum of solar radiation and thus could be coupled directly onto a highly efficient solar collector and turn this solar thermal energy into electrical energy. As stated earlier, thermoelectrics have been used primarily in niche applications in the past, but with the advent of broader automotive applications and the need to utilize waste heat recovery technologies, TE technology is becoming ever more prominent. Current research into new materials, nanostructures and nanocomposites provides hope that significant increases in the efficiency may be possible within the next decade. The emerging field of TE nanocomposites, a mixture of nanomaterials in a bulk matrix, appears to be one of the most promising recent research directions and provides hope that significant increases in the efficiency may be possible within the next decade [6]. Many of the articles highlighted with this issue of pss – Rapid Research Letters [7] will emphasize some of the advances and remaining challenges in these scientifically interesting and technologically important materials. Over the past decade TE materials have improved their performance by about a factor of 2 and are in a position to be a significant contributor to addressing our energy needs, especially in waste heat or solar energy conversion. With new higher efficiency materials the field of harvesting waste energy via thermoelectric devices will become even more prevalent. Terry M. Tritt is Professor of Physics, Director of the Complex & Advanced Materials Laboratory at Clemson University, SC, USA and head of the Center of Excellence in Thermoelectric Materials Research at Clemson, one of the leading laboratories in the field. His research interests comprise the investigation of solid-state materials for thermoelectric applications; thermoelectric nanomaterials and nanostructures, thermal conductivity and electronic properties of low-dimensional conductors.