Thermoelectric materials are capable of converting heat and electricity to each other Thermoelectric devices can be miniaturized and highly integrated with existing semiconductor chip systems with microgenerators or microrefrigerators After years of research and accumulation, BiTe series, SnSe series, CuSe series, half-Heusler series, multicomponent oxides series, organic–inorganic composites series, and GeTe/PbTe series have been found to have excellent thermoelectric properties According to theoretical calculation, when the diameter of Bi2Te3 nanowires is 5 A, the ZT value reaches 14, and graphdiyne has a ZT value of 48 at 300 K Experimental measurements revealed that the ZT value of n-type SnSe reached 28 This review would focus on the updated experimental and theoretical achievements of seven kinds of materials, including BiTe series, SnSe series, CuSe series, multicomponent oxides, half-Heusler alloys, organic–inorganic composites, and GeTe/PbTe series The preparation method, microstructure characteristics, device structure, and thermoelectric properties of each material will be described in detail By analyzing the performance of these materials, three possible development directions are put forward for how to further improve the thermoelectric properties of materials

Review of current high-ZT thermoelectric materials

to Gerry Brown to explain his physics to the world; Brown and Chang-Hwan Lee edited this book. It makes no claim to be a comprehensive biography — it is more like a taster menu drawn from half-a-dozen different restaurants. Like Bethe’s work, the chapters range from extremely detailed physics to non-technical consideration of some of the biggest issues facing humanity. Much fascinating insight into some of the key figures can be gained without any scientific background, but some chapters require a knowledge of physics to degree level or beyond. A few of the later articles have a faint echo of axes being ground, but a book with no trace of controversy would be a poor reflection on Bethe’s life. I would especially recommend the book to anyone who has been involved in any way with the events described, as it brings alive many of the physicists that some of us knew only slightly, or by reputation alone. Reading about Bethe’s pure pleasure as he bulldozed problem after problem using the simplest tools he could get away with was an inspiration — I wanted to grab a piece of paper and have a go myself. He kept that pleasure to the end. A colleague who gave a seminar at Cornell in the week of Bethe’s death told me that Bethe closely followed the talk and obviously enjoyed it. His last words to a younger physicist, quoted in one of the best and most touching articles in the book, were simply “Carry on”. We will, Hans. ■

A bird's-eye view

Many of the renewable and sustainable energy technologies employ novel nanomaterials. For instance, thermal storage and thermoelectric conversion are in constant progress due to the emergence of new structures such as carbon-based materials, bulk nanostructures, 2D novel materials or nanowires. Thermal properties play a significant role to all these energy technologies as key parameters to evaluate the performance and efficiency of those materials in the final device. Understanding the effects of nanostructuring on thermal properties becomes critical, since a reduction in the thermal conductivity due to increased phonon scattering at interfaces is usually expected. Therefore, the determination of the thermal properties remains a critical aspect of material development effort, and measurement techniques are continuously developed or improved. Among those, non-contact heating methods are of importance since they bypass a frequent source of errors characteristic to contact-based thermal measurements, namely the thermal contact resistances, which can be dominant in nanoscale materials. Non-contact heating techniques are usually based on photothermal phenomenon, where heating is generated typically by incident radiation. This paper reviews non-contact heating measurement methods, providing an overview of basic principles for measurement along with associated theoretical model necessary for data reduction and their main applications. The techniques are categorized as time domain and frequency domain techniques, where the thermal response of the sample under study is analyzed as a function of time and frequency, respectively. Both types of methods study the transient response of the sample from a pulsed or modulated heating, and typical measurement output is thermal diffusivity. In addition, other non-contact techniques are also discussed, such as those based on steady-state response, from which the thermal conductivity is directly obtained, or those using AFM probe in the non-contact mode. Finally, main advantages and disadvantages of these techniques are summarized along with their associated uncertainties.

Non-contact methods for thermal properties measurement

Enhanced electrical properties of stoichiometric Bi0.5Sb1.5Te3 film with high-crystallinity via layer-by-layer in-situ Growth

Extracting the thermal conductivity of single nanowires is greatly important in investigating the phonon scattering phenomena that can occur due to the reduction of the dimensions of the material. In order to extract the thermal conductivity of a single nanowire from a measurement of the whole nanowire array, the effective medium theory (EMT) is employed. To appropriately use EMT, the template thermal conductivity must be known. However, the values reported in literature vary greatly. In this work, the photoacoustic technique was used to determine thermal conductivity of different anodic aluminum oxide (AAO) membranes as a function of the pore diameter and type of electrolyte used. To accurately obtain the thermal conductivity, values for the porosity, skeletal density and specific heat of each AAO membrane were measured. These values are critically needed in the most mathematical models to obtain the thermal conductivity of nanowires/AAO composites and theoretical works. Values of 1.07 W m–1 K–1 for the ...

Rules to determine thermal conductivity and density of Anodic Aluminum Oxide (AAO) membranes

Highly oriented [1 1 0] Bi2Te3 films were obtained by pulsed electrodeposition. The structure, composition, and morphology of these films were characterized. The thermoelectric figure of merit (zT), both parallel and perpendicular to the substrate surface, were determined by measuring the Seebeck coefficient, electrical conductivity, and thermal conductivity in each direction. At 300 K, the in-plane and out-of-plane figure of merits of these Bi2Te3 films were (5.6 ± 1.2)·10(-2) and (10.4 ± 2.6)·10(-2), respectively.

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Antonio M. Lopez Martinez

Papers

Anisotropic effects on the thermoelectric properties of highly oriented electrodeposited Bi2Te3 films