6,411 research outputs found
Analysing faults and SFCL response in electric aircraft
Aircraft technology moves towards electrification in order to achieve environmentally friendly goals. However, one of the main challenges facing the electrification of aircraft technology is the weight of the electric devices necessary for operating a fully electric aircraft. The National Aeronautics and Space Administration (NASA) proposed the use of the superconductive technology in electric aircraft (EA) to overcome this challenge in addition to its other benefits. The proposed EA is called N3-X and it has an on-board DC superconducting network including a superconducting fault current limiter (SFCL). The SFCL is a self-mechanism device that very effectively limits the current within a few milliseconds, thus improving the stability of the system. As the grounding of this network is different from traditional networks, analysing the behaviour of different fault types and how to manage them becomes critical. In this paper, one set of the EA's systems (one generator, AC/DC rectifier and four motors with their DC/AC variable frequency drives) has been modelled in MATLAB/Simulink environment to carry out the fault analysis and to demonstrate the effect of the SFCL on this network under different types of faults
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Promising thermoelectric performance in van der Waals layered SnSe2
SnSe as a lead-free IV–VI semiconductor, has attracted intensive attention for its potential thermoelectric applications, since it is less toxic and much cheaper than conventional PbTe and PbSe thermoelectrics. Here we focus on its sister layered compound SnSe2 in n-type showing a thermoelectric performance to be similarly promising as SnSe in the polycrystalline form. This is enabled by its favorable electronic structure according to first principle calculations, its capability to be effectively doped by bromine on selenium site to optimize the carrier concentration, as well as its intrinsic lattice thermal conductivity as low as 0.4 W/m-K due to the weak van der Waals force between layers. The broad carrier concentration ranging from 0.5 to 6 × 1019 cm−3 realized in this work, further leads to a fundamental understanding on the material parameters determining the thermoelectric transport properties, based on a single parabolic band (SPB) model with acoustic scattering. The layered crystal structure leads to a texture in hot-pressed polycrystalline materials and therefore anisotropic transport properties, which can be well understood by the SPB model. This work not only demonstrates SnSe2 as a promising thermoelectric material but also guides the further improvements particularly by band engineering and texturing approaches
Constitutive model of 7075 aluminum at high temperature
In order to obtain the accurate mechanical properties of 7075 aluminum alloy, the Gleeble-1500D thermal simulation test machine was used to perform compression test on 7075 aluminum alloy. The deformation temperature range is 490 °C~560 °C, and the strain rate is 0,001 s-1~1 s-1. At present, for the high temperature thermal compression process, the Arrhenius constitutive model with strain compensation is usually used. The results show that the correlation coefficient of the Arrhenius constitutive model of 7075 aluminum alloy with strain compensation is 0,9894, and the average relative error is 5,6 %, realizing the fitting of flow stress and prediction, verified the feasibility of the model
Constitutive model of 7075 aluminum at high temperature
In order to obtain the accurate mechanical properties of 7075 aluminum alloy, the Gleeble-1500D thermal simulation test machine was used to perform compression test on 7075 aluminum alloy. The deformation temperature range is 490 °C~560 °C, and the strain rate is 0,001 s-1~1 s-1. At present, for the high temperature thermal compression process, the Arrhenius constitutive model with strain compensation is usually used. The results show that the correlation coefficient of the Arrhenius constitutive model of 7075 aluminum alloy with strain compensation is 0,9894, and the average relative error is 5,6 %, realizing the fitting of flow stress and prediction, verified the feasibility of the model
Low-Symmetry Rhombohedral GeTe Thermoelectrics
High-symmetry thermoelectric materials usually have the advantage of very high band degeneracy, while low-symmetry thermoelectrics have the advantage of very low lattice thermal conductivity. If the symmetry breaking of band degeneracy is small, both effects may be realized simultaneously. Here we demonstrate this principle in rhombohedral GeTe alloys, having a slightly reduced symmetry from its cubic structure, to realize a record figure of merit (zT ∼ 2.4) at 600 K. This is enabled by the control of rhombohedral distortion in crystal structure for engineering the split low-symmetry bands to be converged and the resultant compositional complexity for simultaneously reducing the lattice thermal conductivity. Device ZT as high as 1.3 in the rhombohedral phase and 1.5 over the entire working temperature range of GeTe alloys make this material the most efficient thermoelectric to date. This work paves the way for exploring low-symmetry materials as efficient thermoelectrics. Thermoelectric materials enable a heat flow to be directly converted to a flow of charge carriers for generating electricity. The crystal structure symmetry is one of the most fundamental parameters determining the properties of a crystalline material including thermoelectrics. The common belief currently held is that high-symmetry materials are usually good for thermoelectrics, leading to great efforts having historically been focused on GeTe alloys in a high-symmetry cubic structure. Here we show a slight reduction of crystal structure symmetry of GeTe alloys from cubic to rhombohedral, enabling a rearrangement in electronic bands for more transporting channels of charge carriers and many imperfections for more blocking centers of heat-energy carriers (phonons). This leads to the discovery of rhombohedral GeTe alloys as the most efficient thermoelectric materials to date, opening new possibilities for low-symmetry thermoelectric materials. Cubic GeTe thermoelectrics have been historically focused on, while this work utilizes a slight symmetry-breaking strategy to converge the split valence bands, to reduce the lattice thermal conductivity and therefore realize a record thermoelectric performance, all enabled in GeTe in a rhombohedral structure. This not only promotes GeTe alloys as excellent materials for thermoelectric power generation below 800 K, but also expands low-symmetry materials as efficient thermoelectrics
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