Thermoelectric (TE) generators that efficiently recycle a large portion of waste heat will be an important complementary energy technology in the future. While many efficient TE materials exist in the lower temperature region, few are efficient at high temperatures. Here, we present the high temperature properties of high-entropy alloys (HEAs), as a potential new class of high temperature TE materials. We show that their TE properties can be controlled significantly by changing the valence electron concentration (VEC) of the system with appropriate substitutional elements. Both the electrical and thermal transport properties in this system were found to decrease with a lower VEC number. Overall, the large microstructural complexity and lower average VEC in these types of alloys can potentially be used to lower both the total and the lattice thermal conductivity. These findings highlight the possibility to exploit HEAs as a new class of future high temperature TE materials. V
Refractory high-entropy alloys (RHEAs), comprising group IV (Ti, Zr, Hf), V (V, Nb, Ta), and VI (Cr, Mo, W) refractory elements, can be potentially new generation high-temperature materials. However, most existing RHEAs lack room-temperature ductility, similar to conventional refractory metals and alloys. Here, we propose an alloy design strategy to intrinsically ductilize RHEAs based on the electron theory and more specifically to decrease the number of valence electrons through controlled alloying. A new ductile RHEA, Hf 0.5 Nb 0.5 Ta 0.5 Ti 1.5 Zr, was developed as a proof of concept, with a fracture stress of close to 1 GPa and an elongation of near 20%. The findings here will shed light on the development of ductile RHEAs for ultrahigh-temperature applications in aerospace and power-generation industries. Published by AIP Publishing.
Perovskite-type cobaltates in the system La 2 Co 1+z (Mg x Ti 1-x ) 1-z O 6 were studied for z = 0 ≤ x ≤ 0.6 and 0 ≤ x < 0.9, using X-ray and neutron powder diffraction, electron diffraction (ED), magnetic susceptibility measurements and X-ray absorption nearedge structure (XANES) spectroscopy. The samples were synthesised using the citrate route in air at 1350°C. The space group symmetry of the structure changes from P2 1 /n via Pbnm to c R3 with both increasing Mg content and increasing Co content. The
global positioning system (GPS) devices. [2] Moreover, to further improve sensitivity and for use in controlled heating applications (e.g., in furnaces) by resistance heating, a high electrical resistivity (ρ) is desired. [1,3,4] Designing novel materials with little or no change in ρ over a very wide range of temperatures, however, remains a big challenge, which is indicated by the fact that more than a century after their discovery, Constantan (Cu-Ni) and Manganin (Cu-Mn-Ni) based alloys are still the most widely used materials in these contexts. [1,3,4] A crucial material parameter for the abovementioned applications is the temperature coefficient of resistance (TCR), Δρ/ρ 0 ΔT, which measures the variation of the electrical resistivity within a certain temperature range where ρ 0 , Δρ (= ρ − ρ 0 ) and ΔT (= T − T 0 ) are the resistivity at the lower temperature (typically the resistivity near 0 K, unless stated otherwise), difference in resistivity between the higher and the lower temperatures, and the temperature difference between the high and low temperatures, respectively. [5,6] Metals commonly display positive TCR values, which originate from an increasing probability of electrons to experience thermally induced scattering events. For simple metals, phonons represent the dominating scattering channel already at ambient temperatures exhibiting a linear temperature dependence for T > T D /3, where T D is the Debye temperature. [7,8] At very low temperatures, on the other hand, the resistivity is dominated by scattering from impurities and defects giving rise to the socalled residual resistivity, ρ 0 . In this work we will refer to the resistivity obtained at the lowest measurement temperature ≈2-5 K as the residual resistivity, ρ 0 . For the Manganin reference, we use the ρ 0 and ρ 300K that are obtained by fitting (cf. the Supporting Information). [9] Moreover, in metals with magnetic impurities, scattering from magnons (spin-waves) can reach significant levels at low temperatures. [10] In specific cases, where magnetic atoms are embedded in nonmagnetic host metals, the Kondo effect (temperature dependent scattering of conduction electrons by magnetic impurities at low temperatures) can play a dominant role at temperatures near zero Kelvin. [9] The origin of sign and magnitude of the TCR is still a subject of debate. [5][6][7][8]11] In the 1960s, Ioffe and Regel defined the minimum mean-free path (l min ) of the charge carrier to be ≈a (i.e., the interatomic spacing). [7,8,12] This limit, commonly known as Designing alloys with an accurate temperature-independent electrical response over a wide temperature range, specifically a low temperature coefficient of resistance (TCR), remains a big challenge from a material design point of view. More than a century after their discovery, Constantan (Cu-Ni) and Manganin (Cu-Mn-Ni) alloys remain the top choice for strain gauge applications and high-quality resistors up to 473-573 K. Here, an average TCR is demonstrated that is up to ≈800 times smaller in the tempera...
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