T he conversion of thermal energy to electrical energy is known as thermoelectric (TE) conversion. Th e TE eff ect can be used for both power generation and electronic refrigeration. When a temperature gradient (ΔT ) is applied to a TE couple consisting of n-type (electron-transporting) and p-type (hole-transporting) elements, the mobile charge carriers at the hot end tend to diff use to the cold end, producing an electrostatic potential (ΔV ). Th is characteristic, known as the Seebeck eff ect, where α = ΔV/ΔT is defi ned as the Seebeck coeffi cient, is the basis of TE power generation, as shown in Figure 1(a). Conversely, when a voltage is applied to a TE couple, the carriers attempt to return to the electron equilibrium that existed before the current was applied by absorbing energy at one connector and releasing it at the other, an eff ect known as the Peltier eff ect, as shown in Figure 1(b). Th ermoelectric technology and solid-state devices based on the TE eff ect have a number of advantages, including having no moving parts, and being reliable and scalable. Th e technology has therefore arouse worldwide interest in many fi elds, including waste heat recovery and solar heat utilization (power generation mode), and temperature-controlled seats, portable picnic coolers and thermal management in microprocessors (active refrigeration mode) [1].Th e effi ciency of TE devices is strongly associated with the dimensionless fi gure of merit (ZT) of TE materials, defi ned as ZT = (α 2 σ/κ)T, where σ, κ and T are the electrical resistivity, thermal conductivity and absolute temperature [2]. High electrical conductivity (corresponding to low Joule heating), a large Seebeck coeffi cient (corresponding to large potential diff erence) and low thermal conductivity (corresponding to a large temperature diff erence) are therefore necessary in order to realize high-performance TE materials. Th e ZT fi gure is also a very convenient indictor for evaluating the potential effi ciency of TE devices. In general, good TE materials have a ZT value of close to unity. However, ZT values of up to three are considered to be essential for TE energy converters that can compete on effi ciency with mechanical power generation and active refrigeration.High-performance TE materials have been pursued since Bi 2 Te 3 -based alloys were discovered in the 1960s. Until the end of last century, moderate progress had been made in the development of TE materials. Th e benchmark of ZT ≈ 1 was broken in the mid-1990s by two Tsinghua University, China Thermoelectric eff ects enable direct conversion between thermal and electrical energy and provide an alternative route for power generation and refrigeration. Over the past ten years, the exploration of high-performance thermoelectric materials has attracted great attention from both an academic research perspective and with a view to industrial applications. This review summarizes the progress that has been made in recent years in developing thermoelectric materials with a high dimensionless fi gure of...

bd p-Type PbTe is an outstanding high temperature thermoelectric material with zT of 2 at high temperatures due to its complex band structure which leads to high valley degeneracy. Lead-free SnTe has a similar electronic band structure, which suggests that it may also be a good thermoelectric material. However, stoichiometric SnTe is a strongly p-type semiconductor with a carrier concentration of about 1 Â 10 20 cm À3 , which corresponds to a minimum Seebeck coefficient and zT. While in the case of p-PbTe (and n-type La 3 Te 4 ) one would normally achieve higher zT by using high carrier density in order to populate the secondary band with higher valley degeneracy, SnTe behaves differently. It has a very light, upper valence band which is shown in this work to provide higher zT than doping towards the heavier second band. Therefore, decreasing the hole concentration to maximize the performance of the light band results in higher zT than doping into the high degeneracy heavy band. Here we tune the electrical transport properties of SnTe by decreasing the carrier concentration with iodine doping, and increasing the carrier concentration with Gd doping or by making the samples Te deficient. A peak zT value of 0.6 at 700 K was obtained for SnTe 0.985 I 0.015 which optimizes the light, upper valence band, which is about 50% higher than the other peak zT value of 0.4 for Gd z Sn 1ÀzT e and SnTe 1+y which utilize the high valley degeneracy secondary valence band.

Nanostructured Ag0.8Pbm+xSbTem+2 (m = 18, x = 4.5) system thermoelectric materials have been fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods followed by annealing for several days to investigate the effect on microstructure and thermoelectric performance. It was found that appropriate annealing treatment could reduce both the electrical resistivity and the thermal conductivity at the same time, consequently greatly enhancing the thermoelectric performance. A low electrical resistivity of 2 x 10-3 Ohm-cm and low thermal conductivity of 0.89 W m-1 K-1 were obtained for the sample annealed for 30 days at 700 K. The very low thermal conductivity is supposed to be due to the nanoscopic Ag/Sb-rich regions embedded in the matrix. A high ZT value of 1.5 at 700 K has been achieved for the sample annealed for 30 days.

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Solid solution is a potential way to optimize thermoelectric performance for its low thermal conductivity compared to those of the constituent compounds because of the phonon scattering from disordered atoms. Tin(II) sulfide (SnS) shows analogous band structure and electrical properties with tin selenide (SnSe), which was the motivation for investigating the thermoelectric performance of SnS and SnS-SnSe solid solution system. SnS compound and SnS 1Àx Se x (0 < x < 1) solid solution were fabricated using the melting method and they exhibited anisotropic thermoelectric performance along the parallel and perpendicular to the pressing directions. For the SnS compound, the maximum zT k value is 0.19 at 823 K along the parallel to pressing direction, which is higher than that along the perpendicular to the pressing direction (zT t ¼ 0.16). The zT values of SnS 0.5 Se 0.5 and SnS 0.2 Se 0.8 were higher than those of the SnS compound and a maximum zT value of 0.82 was obtained for SnS 0.2 Se 0.8 at 823 K, which is more than four times higher than that of SnS.

Graphene sheets are used for the first time to fabricate a new type of solid-contact ion-selective electrode (SC-ISE) as the intermediate layer between an ionophore-doped solvent polymeric membrane and a glassy carbon electrode. The new transducing layer was characterized by transmission electron microscopy, scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The performance of the new K(+-)selective electrodes was examined by a potentiometric water layer test, potentiometric measurements, and current reversal chronopotentiometry. The obtained potentiometric sensors were characterized with a calibration line of slope close to Nernstian (59.2 mV/decade) within the activity from 10(-4.5) to 0.1 M. The high capacitance of the graphene solid contacts results in a signal that is stable over one week. The short response time is less than 10 s for activities higher than 10(-5) M. The potential drift of the electrodes was calculated from the slope of the curves at longer times (ΔE/Δt = 1.2 × 10(-5) V s(-1) (I = 1 nA) and ΔE/Δt = 5.5 × 10(-5) V s(-1) (I = 5 nA)). All the results indicate that graphene is a promising material for use as a transducer layer for SC-ISEs.

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