The influence of non-idealities on the thermoelectric power factor of nanostructured superlattices

Cross-plane superlattices composed of nanoscale layers of alternating potential wells and barriers have attracted great attention for their potential to provide thermoelectric power factor improvements and higher ZT figure of merit. Previous theoretical works have shown that the presence of optimized potential barriers could provide improvements to the Seebeck coefficient through carrier energy filtering, which improves the power factor by up to 40%. However, experimental corroboration of this prediction has been extremely scant. In this work, we employ quantum mechanical electronic transport simulations to outline the detrimental effects of random variation, imperfections, and non-optimal barrier shapes in a superlattice geometry on these predicted power factor improvements. Thus, we aim to assess either the robustness or the fragility of these theoretical gains in the face of the types of variation one would find in real material systems. We show that these power factor improvements are relatively robust against: overly thick barriers, diffusion of barriers into the body of the wells, and random fluctuations in barrier spacing and width. However, notably, we discover that extremely thin barriers and random fluctuation in barrier heights by as little as 10% is sufficient to entirely destroy any power factor benefits of the optimized geometry. Our results could provide performance optimization routes for nanostructured thermoelectrics and elucidate the reasons why significant power factor improvements are not commonly realized in superlattices, despite theoretical predictions

The fragility of thermoelectric power factor in cross-plane superlattices in the presence of nonidealities : a quantum transport simulation approach

Energy filtering has been put forth as a promising method for achieving large thermoelectric power factors in thermoelectric materials through Seebeck coefficient improvement. Materials with embedded potential barriers, such as cross-plane superlattices, provide energy filtering, in addition to low thermal conductivity, and could potentially achieve high figure of merit. Although there exist many theoretical works demonstrating Seebeck coefficient and power factor gains in idealized structures, experimental support has been scant. In most cases, the electrical conductivity is drastically reduced due to the presence of barriers. In this work, using quantum-mechanical simulations based on the nonequilibrium Green’s function method, we show that, although power factor improvements can theoretically be observed in optimized superlattices (as pointed out in previous studies), different types of deviations from the ideal potential profiles of the barriers degrade the performance, some nonidealities being so significant as to negate all power factor gains. Specifically, the effect of tunneling due to thin barriers could be especially detrimental to the Seebeck coefficient and power factor. Our results could partially explain why significant power factor improvements in superlattices and other energy-filtering nanostructures mainly fail to be realized, despite theoretical predictions

On the effectiveness of the thermoelectric energy filtering mechanism in low-dimensional superlattices and nano-composites

Electron energy filtering has been suggested as a promising way to improve the power factor and enhance the ZT figure of merit of thermoelectric materials. In this work, we explore the effect that reduced dimensionality has on the success of the energy-filtering mechanism for power factor enhancement. We use the quantum mechanical non-equilibrium Green's function method for electron transport including electron-phonon scattering to explore 1D and 2D superlattice/nanocomposite systems. We find that, given identical material parameters, 1D channels utilize energy filtering more effectively than 2D as they: (i) allow one to achieve the maximal power factor for smaller well sizes/smaller grains which are needed to maximize the phonon scattering, (ii) take better advantage of a lower thermal conductivity in the barrier/boundary materials compared to the well/grain materials in both: enhancing the Seebeck coefficient; and in producing a system which is robust against detrimental random deviations from the optimal barrier design. In certain cases, we find that the relative advantage can be as high as a factor of 3. We determine that energy-filtering is most effective when the average energy of carrier flow varies the most between the wells and the barriers along the channel, an event which occurs when the energy of the carrier flow in the host material is low, and when the energy relaxation mean-free-path of carriers is short. Although the ultimate reason for these aspects, which cause a 1D system to see greater relative improvement than a 2D, is the 1D system's van Hove singularity in the density-of-states, the insights obtained are general and inform energy-filtering design beyond dimensional considerations

Use of licorice plant extract for controlling corrosion of steel rebar in chloride-polluted concrete pore solution

The possibility of using licorice extract as a green inhibitor for steel reinforcements in chloride-contaminated simulated concrete pore solution was explored in this study. Different licorice amounts were added to alkaline solutions and then 1% NaCl was also added. Electrochemical studies, up to 24 h, and surface analysis (X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy) were performed. Results demonstrated the inhibition effectiveness of the plant extract on steel rebar corrosion, regardless of the concentration, being detected an inhibition efficiency higher than 80 % with electrochemical techniques for 0.1% licorice extract, which showed the most effective performance. Surface analysis methods confirmed the presence of licorice on the surface, through the deposition of organic molecules present in the plant extract on the surface oxide/hydroxide. DFT calculations confirmed that compounds present in licorice can be chemically adsorbed on steel oxide surface.This research was funded by the European Union Horizon 2020 research and innovation MSCA-IF-2019 programme under grant agreement No 892074 (NATCON project). Support from the Ministerio de Ciencia, Innovación y Universidades of Spain (RTI2018-096428-B-I00) is also acknowledged

Reduction of the Dark-Current in Carbon Nanotube Photo-Detectors

Abstract-Carbon nanotubes have been considered in recent years for future opto-electronic applications because of their direct band-gap and the tunability of the band-gap with the CNT diameter. The performance of infra-red photo-detectors based on carbon nanotube field-effect transistors is analyzed, using the non-equilibrium Green's function formalism. The relatively low ratio of the photo-current to the dark current limits the performance of such devices. We show that by employing a double gate structure this ratio can be significantly increased. Carbon nanotubes (CNTs) have been extensively studied in recent years due to their exceptional electronic, optoelectronic, and mechanical properties. CNTs can be considered as a graphene sheet which has been wrapped into a tube. The way the graphene sheet is wrapped is represented by a pair of indices (n, m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m = 0, the CNT is called zigzag. If n = m, the CNT is called armchair. Otherwise, it is called chiral. CNTs with n−m = 3 are metals, otherwise they are semiconductors. Semiconducting CNTs can be used as channels for transistors. Depending on the work function difference between the metal contact and the CNT, carriers at the metal-CNT interface encounter different barrier heights. Fabrication of devices with positive [1] and zero Some of the interesting electronic properties of CNTs are quasi-ballistic carrier transport [2], suppression of shortchannel effects due to one-dimensional electron transport IR photo detectors based on carbon nanotube field effect transistors (CNT-FETs) have been reported i

An Investigation of the Geometrical Effects on the Thermal Conductivity of Graphene Antidot Lattices

In this work we investigate the thermal conductivity of graphene-based antidot lattices. A third nearest-neighbor tight-binding model and a forth nearest-neighbor force constant model are employed to study the electronic and phononic band structures of graphene-based antidot lattices. Ballistic transport models are used to evaluate the electronic and the thermal conductivities. Methods to reduce the thermal conductivity and to increase the thermoelectric figure of merit of such structures are studied. Our results indicate that triangular antidot lattices have the smallest thermal conductivity due to longer boundaries and the smallest distance between the neighboring dots