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Nanostructured Thermoelectric Materials Alexandre Cuenat and Laurie Winkless, NPL Nanomaterials What are thermoelectric materials? Thermoelectric (TE) materials can produce direct electrical power from an applied temperature difference. They are extremely reliable, ‘fuel-free’ solid-state devices with no moving parts. TE devices can play an important role in energy harvesting and energy efficiency. They are a long-proven technology – TE materials have been used for many years as generators for deep space probes. However, their everyday use is limited by low efficiency. Recent developments based on nanomaterials have shown dramatic improvements (by a factor 2 to 3) in the performance of TE devices. However, the accurate measurement of their efficiency is extremely difficult. Heat Source What impact can these materials have? Recent years have seen a huge increase in worldwide energy demand. Fossil fuels are a finite resource, so alternatives are actively sought after, along with more efficient ways of using fossil fuels. Most of our systems are also incredibly inefficient – consider for example than more than 70% of the energy generated from a typical car engine is wasted, mostly in the form of heat. The exhaust temperature can be up to 500°C, providing a large temperature difference – perfect for TE materials. NPL Nanomaterials group are working on two European projects to develop accurate and traceable characterisation techniques for Nanostructured TE Materials and devices The basics The Origins of Thermoelectricity The thermoelectric effect is the conversion of temperature difference to electric voltage (and vice versa). It refers to three separately identified effects – the Seebeck Effect, the Peltier effect and the Thomson Effect. Lord Kelvin was the first to realize the connection between these effects, and created the first non-equilibrium thermodynamic theory. e- Generating electrical current is based on the Seebeck effect, which is the creation of a voltage across a material with a temperature gradient. In materials the density of states (DOS) describes the number of states at each energy level that are available. The Fermi level (EF) is the median energy of of the electrons distributed along a Fermi-Dirac distribution (blue df line) df − D (E ) × D(E) f dE dE E h+ Ef This voltage can be used to drive a current through the material. The current will flow in different direction, depending of the type of doping (n or p-type, see inset) of the materials. • Conductivity requires large area under differential conductivity (Green line) • Thermopower (Seebeck coefficient) requires asymmetry in differential conductivity about the Fermi Level, Ef (Blue area) Thermoelectric Materials have been used for a number of years, but have, until recently, only operated with low thermodynamic efficiency. This efficiency is related to the Figure of merit Z of the TE material: • Thermal Conductivity, κ • Electrical Conductivity, σ ZT = S 2σT e2 df ⎞ ⎛ τ (E)v 2 (E) ⎜ −D(E) 0 ⎟ dE ⎝ 3∫ dE ⎠ σ= κ Thermoelectric generator (TEG) • Seebeck Coefficient, S Thermoelectric Materials are mostly semiconductor materials – both p and n-type (see text bubbles). The reason for this can be seen in the table below. The goal of the engineer is to MAXIMISE the electrical conductivity of a material, while MINIMISING the thermal conductivity. As these transport characteristics depend on interrelated material properties, there are great challenges surrounding the optimisation of the efficiency of the final device. Metals Semiconductors Insulators S (x10-6 VK-1) 5 200 1000 σ = ηeμ (Ω-1m-1) 108 105 10-10 λ tot = λ l + λ el λ tot (Wm-1K-1) λ tot ≈ λ el 10 - 1000 λ tot < λ el 1 - 100 λ tot ≈ λ l 0.1 - 1 ZT 10-3 0.1 - 1 10-14 e df ⎞ ⎛ τ (E)v 2 (E) ⎜ −D(E) 0 ⎟ (E − E f )dE ⎝ 3T σ ∫ dE ⎠ It has been shown that it is possible to enhancing Z using low-dimensional materials Electron Crystal-Phonon Glass Standard degenerate semiconductor Nanostructured semiconductor >50% of the heat is carried by phonons with mean free path over 40nm >90% of the heat is carried by phonons with mean free path over 5nm P-type Materials Dresselhaus, 2006 Impurity/Dopant Table and plot: Comparison of thermoelectric properties of metals, semiconductors and insulators (Fleurial, 1993, Kushch, 2004) The critical role of metrology N-type Materials The difficulty in TE materials metrology is that accurate efficiency measurements are as complex as building an entire device. Results can vary considerably, particularly above room temperature where thermal gradients in the measurement system add to systematic inaccuracies. An uncertainty of 50% is quite common in TE measurements. A doped semiconductor where charge is carried by electrons (which are negative) e- Phonon Electron • Increased Density of States near the Fermi Level Quantum confinement S↑ lph ↓ Bulk Six key elements of a thermoelectric waste heat recovery module for vehicle applications. NSF_DOE partnership for thermoelectric devices S=− D (E ) Single Nanocrystal D (E ) QD Superlattice D (E ) A doped semiconductor where P-type Materials charge is carried by A dopedare holes (which semiconductor where positive) charge is carried by holes (which are positive) h+ e- − d f d E • Heavy fermion compounds: S↑ s~ le Ideal DOS: Delta function several kT from EF will maximize power factor if mobility can be enhanced by eliminating ionized impurity scattering The NPL Context I NPL is involved in two European projects on the characterisation of thermoelectric materials and devices. This fits into two of NPL’s core priorities - Energy and Sustainability. Thermoelectric generators (TEGs) have wide-ranging applications in energy harvesting. Some of the obvious applications include: Power supply in remote locations, Wireless sensors, Wearable TEG, Body-heatpowered medical devices, Fuel reduction in cars, Industrial waste heat recovery, MEMS power, computer and IT heat management. TE devices have also been successfully deployed in space in the form of Radioisotope thermoelectric generators (RTGs) so are already considered stable and reliable. NPL’s aim is to develop accurate characterisation techniques for TE materials and the final devices. This work will go some way to removing the uncertainties surrounding TE efficiency measurement. Partners on the EMRP project are: PTB (Germany), LNE (France), INRIM (Italy), MIKES (Finland), CMI (Czech Republic), SIQ (Slovenia) Cold Source Cold Junction Partners on the FP7 project are: KTH (Sweden), Siemens (Germany), NCSR ”Demokritos” (Greece), Cidete (Spain), SEAT (Spain), ELECTROLUX (Sweden), CSIC (Spain), LEITAT (Spain), Cardiff University (UK), Fraunhofer Inst (Germany), Uppsala University (Sweden) I