Thermoelectric power generation is foreseen to play a much larger role in the near future, considering the need for alternative energies because of declining natural resources as well as the ...increasing efficiency of thermoelectric materials. The latter is a consequence of the discoveries of new materials as well as of improvements of established materials by, for example, nanostructuring or band structure engineering. Within this review, two major classes of high-temperature thermoelectrics are presented: clathrates formed by silicides and germanides, and complex antimonides including but not limited to the filled skutterudites. The clathrates and the skutterudites are cage compounds that exhibit low thermal conductivity, reportedly related to the rattling effect of the guest atoms, whereas the other antimonides achieve low thermal conductivity via defects or simply via the high complexity of their crystal structures.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
This work focuses on the synthesis and properties of quaternary ZnSnP2-yAsy chalcopyrite solid solutions. Full miscibility of the solid solution is achieved using ball milling followed by hot press ...sintering. The measured electrical conductivity increases substantially with As substitution from 0.03 S cm−1 for ZnSnP2 to 10.3 S cm−1 for ZnSnAs2 at 715 K. Band gaps calculated from the activation energies show a steady decrease with increasing As concentration from 1.4 eV for ZnSnP2 to 0.7 eV for ZnSnAs2. The Seebeck coefficient decreases significantly with As substitution from nearly 1000 μV K−1 for ZnSnP2 to −100 μV K−1 for ZnSnAs2 at 650 K. Thermal conductivity is decreased for the solid solutions due to alloy phonon scattering, compared to the end members with y = 0 and y = 2, with the y = 0.5 and y = 1.0 samples exhibiting the lowest values of 1.4 W m−1 K−1 at 825 K. Figure of merit values are increased for the undoped solid solutions at lower temperatures when compared to the end members due to the decreased thermal conductivity, with the y = 0.5 sample reaching zT = 1.6 × 10−3 and y = 1 reaching 2.1 × 10−3 at 700 K. The largest values of the figure of merit zT for the undoped series was found for y = 2 with zT = 2.8 × 10−3 at 700 K due to the increasing n-type Seebeck coefficient. Boltztrap calculations reveal that p-doping could yield zT values above unity at 800 K in case of ZnSnAs2, comparable with ZnSnP2.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Considering the effect of CO2 emission together with the depletion of fossil fuel resources on future generations, industries in particular the transportation sector are in deep need of a viable ...solution to follow the environmental regulation to limit the CO2 emission. Thermoelectrics may be a practical choice for recovering the waste heat, provided their conversion energy can be improved. Here, the high temperature thermoelectric properties of high purity Bi doped Mg2(Si,Sn) are presented. The samples Mg2Si1–x–ySn x Bi y with x(Sn) ≥ 0.6 and y(Bi) ≥ 0.03 exhibited electrical conductivities and Seebeck coefficients of approximately 1000 Ω–1 cm–1 and −200 μV K–1 at 773 K, respectively, attributable to a combination of band convergence and microstructure engineering through ball mill processing. In addition to the high electrical conductivity and Seebeck coefficient, the thermal conductivity of the solid solutions reached values below 2.5 W m–1 K–1 due to highly efficient phonon scattering from mass fluctuation and grain boundary effects. These properties combined for zT values of 1.4 at 773 K with an average zT of 0.9 between 400 and 773 K. The transport properties were both highly reproducible across several measurement systems and were stable with thermal cycling.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Various compounds in the (SnTe)x(Bi2Te3)y series are explored by a number of techniques including diffractometry and physical property measurements. Through variation in the x and y integers, one is ...able to obtain a series of ternary compounds: SnBi2Te4, SnBi4Te7, SnBi6Te10, and Sn2Bi2Te5 each show complex layering motifs similar to that of Bi2Te3. These compounds' increased complexity makes them of interest with respect to thermoelectric studies. The ZT values for cold‐pressed annealed pellets are at room temperature 0.33 for SnBi2Te4, 0.27 for SnBi4Te7, and 0.15 for SnBi6Te10.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
To prepare Ni0.05Mo3Sb5.4Te1.6, the starting materials were loaded in the stoichiometric ratio into silica tubes and then heated at 1000K. The reaction products were mixed and divided into four equal ...parts. The first sample was used as a reference sample called bulk. For the remaining samples, 1%, 2% and 3% by mass of MWCNT (multi-wall carbon nanotubes) were added by ball-milling. These materials were then subjected to consolidation by hot-pressing at 850K and 56MPa. Their transport properties were determined and compared to study the influence of MWCNT on the transport properties of Ni0.05Mo3Sb5.4Te1.6. Scanning and transmission electron microscopy were used to study the microstructural and nanostructural features of the samples, and Raman characterization was performed to look for changes induced by ball-milling and hot-pressing of the nanotubes. Mainly due to a largely reduced thermal conductivity by 40% and a slightly reduced power factor, the figure-of-merit was improved by 25% after addition of 3mass% of MWCNT.
Figure-of-merit of various Ni0.05Mo3Sb5.4Te1.6/CNT composites. Display omitted
•Various composites of Ni0.05Mo3Sb5.4Te1.6 with carbon nanotubes were synthesized and characterized.•In each case, the thermoelectric figure-of-merit increases rapidly with increasing temperature.•With increasing amount of carbon nanotubes, the thermal conductivity decreases more than the electrical conductivity.•The composite with 3% carbon nanotubes performs better than the bulk material by 25%.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPUK
Invoking Effective Media Percolation theory (EMPT), Hasselaman-Johnson effective media theory (EMT), and Nan and Birringer EMT, the effect of addition of SiC and Al2O3 nanoparticles on Kapitza ...resistance (RBd) of Ni0.05Mo3Sb5.4Te1.6 was investigated. Pore size and their volume distribution, and surface area were characterized using BET technique to correlate pore effect and surface area on RBd. Bounds for effective thermal conductivity were determined using Lipton–Vernescu model. Variation of thermal conductance with respect to temperature was studied and compared with the results of other materials. According to EMPT, RBd in Ni0.05Mo3Sb5.4Te1.6/SiC composites ranged from 3.84 × 10-7 to 5.42 × 10-7 m2KW–1 and 3.36 × 10-7 to 3.86 × 10-7 m2KW–1 for Ni0.05Mo3Sb5.4Te1.6/Al2O3 composites. Kapitza radius (aK) for SiC samples was ranged between 2.01 – 2.84 μm; for Al2O3 samples it was 1.86 μm. Hasselman-Johnson model gave RBd values 55%, 51%, and 8% more than what EMPT is predicting, but of the same order and aK values 3.5 μm, 4 μm, 3 μm for SiC samples and 1.2 μm, 0.6 μm, 0.55 μm for Al2O3 samples. Nan-Birringer model yielded large aK of 7.25 μm and RBd ∼ 1.4 × 10–6 m2KW–1 for Ni0.05Mo3Sb5.4Te1.6/SiC. So obtained parameters are reasonable estimates. Variation of effective thermal conductivity in Al2O3 samples is more sensitive to particle size compared to SiC samples. Mechanical properties were studied using micro–indentation technique and their effect on effective thermal properties was ascertained. Addition of Al2O3 nanoparticles have aided in enhancing mechanical properties of bulk material.
For bulk thermoelectrics, improvement of the figure of merit
ZT
to above 2 from the current values of 1.0 to 1.5 would enhance their competitiveness with alternative technologies. In recent years, ...the most significant improvements in
ZT
have mainly been due to successful reduction of thermal conductivity. However, thermal conductivity is difficult to measure directly at high temperatures. Combined measurements of thermal diffusivity, specific heat, and mass density are a widely used alternative to direct measurement of thermal conductivity. In this work, thermal conductivity is shown to be the factor in the calculation of
ZT
with the greatest measurement uncertainty. The International Energy Agency (IEA) group, under the implementing agreement for Advanced Materials for Transportation (AMT), has conducted two international round-robins since 2009. This paper, part II of our report on the international round-robin testing of transport properties of bulk bismuth telluride, focuses on thermal diffusivity, specific heat, and thermal conductivity measurements.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Recent research and development of high-temperature thermoelectric materials has demonstrated great potential for converting automobile exhaust heat directly into electricity. Thermoelectrics based ...on classic bismuth telluride have also started to impact the automotive industry by enhancing air-conditioning efficiency and integrated cabin climate control. In addition to engineering challenges of making reliable and efficient devices to withstand thermal and mechanical cycling, the remaining issues in thermoelectric power generation and refrigeration are mostly materials related. The dimensionless figure of merit,
ZT
, still needs to be improved from the current value of 1.0 to 1.5 to above 2.0 to be competitive with other alternative technologies. In the meantime, the thermoelectric community could greatly benefit from the development of international test standards, improved test methods, and better characterization tools. Internationally, thermoelectrics have been recognized by many countries as a key component for improving energy efficiency. The International Energy Agency (IEA) group under the Implementing Agreement for Advanced Materials for Transportation (AMT) identified thermoelectric materials as an important area in 2009. This paper is part I of the international round-robin testing of transport properties of bulk thermoelectrics. The main foci in part I are the measurement of two electronic transport properties: Seebeck coefficient and electrical resistivity.
Full text
Available for:
EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
A large variety of different interactions between the chalcogen atoms, Q, occur in the solid state structures of polyselenides and polytellurides, including both molecular and infinite units. The ...simplest motifs are classical Q(2)(2-) dumbbells and nonlinear Q(n)(2-) chains (n = 3, 4, 5, ..), e.g. found in alkali metal polychalcogenides. In addition, nonclassical so-called hypervalent motifs exist in the form of linear Q(3)(4-) units or within larger units such as Q(4)(4-) and Q(5)(4-). Infinitely extended Q units include zigzag, cis/trans and linear chains, as well as planar and slightly puckered layers. Several of those are susceptible to Peierls distortions, leading to the formation of both commensurate and incommensurate superstructures and anomalies in transport properties, including metal-nonmetal transitions.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
PDF
