29 research outputs found
Thermoelectric efficiency has three Degrees of Freedom
Thermal energy can be directly converted to electrical energy as a result of
thermoelectric effects. Because this conversion realises clean energy
technology, such as waste heat recovery and energy harvesting, substantial
efforts have been made to search for thermoelectric materials. Under the belief
that the material figure of merit represents the energy conversion
efficiencies of thermoelectric devices, various high peak- materials have
been explored for half a century. However, thermoelectric properties vary
greatly with temperature , so the single value does not represent
device efficiency accurately. Here we show that the efficiency of
thermoelectric conversion is completely determined by \emph{three} parameters
, , and , which we call the \emph{thermoelectric
degrees of freedom}. The , which is an average of material
properties, is a generalisation of the traditional figure of merit. The
and , which reflect the gradients of the material properties, are
proportional to escaped heat caused by the Thomson effect and asymmetric Joule
heat, respectively. Our finding proposes new directions for achieving high
thermoelectric efficiency; increasing one of the thermoelectric degrees of
freedom results in higher efficiency. For example, thermoelectric efficiency
can be enhanced up to 176\% by tuning the thermoelectric degrees of freedom in
segmented legs, compared to the best efficiency of single-material legs.Comment: main articles with 9 pages, 4 figures, supplementary information with
35 pages, 9 figures, 6 table
Hybrid-functional and quasi-particle calculations of band structures of Mg2Si, Mg2Ge, and Mg2Sn
We perform hybrid functional and quasi-particle band structure calculations
with spin-orbit interaction to investigate the band structures of Mg2Si, Mg2Ge,
and Mg2Sn. For all Mg2X materials, where X = Si, Ge, and Sn, the
characteristics of band edge states, i.e., band and valley degeneracies, and
orbital characters, are found to be conserved, independent of the computational
schemes such as density functional generalized gradient approximation, hybrid
functionals, or quasi-particle calculations. However, the magnitude of the
calculated band gap varies significantly with the computational schemes. Within
density-functional calculations, the one-particle band gaps of Mg2Si, Mg2Ge,
and Mg2Sn are 0.191, 0.090, and -0.346 eV, respectively, and thus severely
underestimated compared to the experimental gaps, due to the band gap error in
the density functional theory and the significant relativistic effect on the
low-energy band structures. By employing hybrid-functional calculations with a
35% fraction of the exact Hartree-Fock exchange energy (HSE-35%), we overcame
the negative band gap issue in Mg2Sn. Finally, in quasi-particle calculations
on top of the HSE-35% Hamiltonians, we obtained band gaps of 0.835, 0.759, and
0.244 eV for Mg2Si, Mg2Ge, and Mg2Sn, respectively, consistent with the
experimental band gaps of 0.77, 0.74, and 0.36 eV, respectively.Comment: 23 pages, including 84 references, 5 tables, 3 figure
Counterintuitive example on relation between ZT and thermoelectric efficiency
The thermoelectric figure of merit ZT, which is defined using electrical
conductivity, Seebeck coefficient, thermal conductivity, and absolute
temperature T, has been widely used as a simple estimator of the conversion
efficiency of a thermoelectric heat engine. When material properties are
constant or slowly varying with T, a higher ZT ensures a higher maximum
conversion efficiency of thermoelectric materials. However, as material
properties can vary strongly with T, efficiency predictions based on ZT can be
inaccurate, especially for wide-temperature applications. Moreover, although ZT
values continue to increase, there has been no investigation of the
relationship between ZT and the efficiency in the higher ZT regime. In this
paper, we report a counterintuitive situation by comparing two materials:
although one material has a higher ZT value over the whole operational
temperature range, its maximum conversion efficiency is smaller than that of
the other. This indicates that, for material comparisons, the evaluation of
exact efficiencies as opposed to a simple comparison of the ZTs is necessary in
certain cases.Comment: 12 pages, 2 tables, 2 figure
Native point defects and low -doping efficiency in solid solutions: A hybrid-density functional study
We perform hybrid-density functional calculations to investigate the charged
defect formation energy of native point defects in , , and
their solid solutions. The band gap correction by hybrid-density functional is
found to be critical to determine the charged defect density in these
materials. For , interstitials are dominant and provide
unintentional -type conductivity. Additionally, as the vacancies can
dominate in -poor , -type conductivity is possible for . However, the existence of low formation energy defects such as
and in and their diffusion can cause
severe charge compensation of hole carriers resulting in low -type doping
efficiency and thermal degradation. Our results indicate that, in addition to
the extrinsic doping strategy, alloying of with under
-poor conditions would be necessary to enhance the -type conductivity
with less charge compensation.Comment: Main: 17 pages (including title, abstract, main, references, figure
captions. 4 figures). This manuscript is accepted for publication in JALCOM.
The article will be published as Gold Open Acces
Best Thermoelectric Efficiency of Ever-Explored Materials
A thermoelectric device is a heat engine that directly converts heat into
electricity. Many materials with a high figure of merit ZT have been discovered
in anticipation of a high thermoelectric efficiency. However, there has been a
lack of investigations on efficiency-based material evaluation, and little is
known about the achievable limit of thermoelectric efficiency. Here, we report
the highest thermoelectric efficiency using 13,353 published materials. The
thermoelectric device efficiencies of 808,610 configurations are calculated
under various heat-source temperatures (T_h) when the cold-side temperature is
300 K, solving one-dimensional thermoelectric integral equations with
temperature-dependent thermoelectric properties. For infinite-cascade devices,
a thermoelectric efficiency larger than 33% (~1/3) is achievable when T_h
exceeds 1400 K. For single-stage devices, the best efficiency of 17.1% (~1/6)
is possible when T_h is 860 K. Leg segmentation can overcome this limit,
delivering a very high efficiency of 24% (~1/4) when T_h is 1100 K.Comment: 32 pages (main+table+figure captions+figures), 7 additional pages for
6 high resolution figures, Supporting Data file is not public ye
Fusion cytokine IL-2-GMCSF enhances anticancer immune responses through promoting cell–cell interactions
Network Analysis of the Disaster Response Systems in the Waste of Electrical and Electronic Equipment Recycling Center in South Korea
Since dust and flammable gas are generated during the waste recycling process, there is always a risk of a fire accident. However, research on disaster management at recycling facilities deals only with the problem of processing systems from a technical standpoint and does not suggest concrete alternatives from a management aspect. Therefore, this study analyzed the influence of the disaster response network of a Waste of Electrical and Electronic Equipment (WEEE) recycling center at the organizational level based on the concept of the cognitive accuracy of a network considering administrative aspects. Accordingly, a survey was conducted using a structured questionnaire targeting 47 employees at the WEEE recycling center in South Korea and by applying the two-mode network analysis method using UCINET software, the centrality of the actor and the density of the network were quantitatively analyzed. Through this analysis, we confirmed that factors affecting the influence of the network exist, such that the entire network and the networks of different levels of position are different. We suggest that this can be improved by deploying safety and health management managers who perform formal tasks at the center of the network so that everyone can agree on the political approach and by empowering the safety and health management manager to conduct active education and training. Furthermore, we suggest that the network structure should be reorganized, centering on the person in charge of safety and health management to have a network system that matches each position