552 research outputs found
Large enhancement of the thermoelectric power factor in disordered materials through resonant scattering
In the search for more efficient thermoelectric materials, scientists have
placed high hopes in the possibility of enhancing the power factor using
resonant states. In this study, we investigate theoretically the effects of
randomly distributed resonant impurities on the power factor. Using the
Chebyshev Polynomial Green's Function method, we compute the electron transport
properties for very large systems (10 million atoms) with an exact treatment of
disorder. The introduction of resonant defects can lead to a large enhancement
of the power factor together with a sign inversion in the Seebeck coefficient.
This boost depends crucially on the position of the resonant peak, and on the
interplay between elastic impurity scattering and inelastic processes. Strong
electron-phonon or electron-electron scattering are found detrimental. Finally,
the robustness of our results is examined in the case of anisotropic orbitals
and two-dimensional confinement. Our findings are promising for the prospect of
thermoelectric power generation.Comment: To appear in Phys. Rev.
Hydrogen production using Purple Non-Sulfur Bacteria (PNSB) cultivated under natural or artificial light conditions with synthetic or fermentation derived substrates
The aim of this thesis was to verify the feasibility of the hydrogen production process with purple non sulfur bacteria both under sunlight irradiation in an up-scaled system and with the use of low cost substrates. Among the products offermentations tested the best results were obtained with a medium derived from vegetable wastes. The use of a genetically modified strain of Rhodopseudomonas palustris insensitive to ammonium opened the way towards the use of wastes with attainment of high hydrogen yields also in inhibiting conditions. The experimentation carried out under natural irradiation demonstrated the full feasibility of the process using sunlight instead of artificial light in a semi-pilot reactor: the production rates were the highest so far reported for comparable outdoor systems
Absence of confinement in (SrTiO3)/(SrTi0:8Nb0:2O3) superlattices
The reduction of dimensionality is an efficient pathway to boost the
performances of thermoelectric materials, it leads to the quantum confinement
of the carriers and thus to large Seebeck coefficients (S) and it also
suppresses the thermal conductivity by increasing the phonon scattering
processes. However, quantum confinement in superlattices is not always easy to
achieve and needs to be carefully validated. In the past decade, large values
of S have been measured in (SrTiO3)/(SrTi0:8Nb0:2O3) superlattices (Nat. Mater.
6, 129 (2007) and Appl. Phys. Lett. 91, 192105 (2007)). In the -doped
compound, the measured S was almost 6 times larger than that of the bulk
material. This huge increase has been attributed to the two dimensional
confinement of the carriers in the doped regions. In this work, we demonstrate
that the experimental data can be well explained quantitatively within the
scenario in which electrons are delocalized in both in-plane and growth
directions, hence strongly suggesting that the confinement picture in these
superlattices may be unlikely.Comment: 5 figures, manuscript submitte
Unified modelling of the thermoelectric properties in SrTiO3
Thermoelectric materials are opening a promising pathway to address energy
conversion issues governed by a competition between thermal and electronic
transport. Improving the efficiency is a difficult task, a challenge that
requires new strategies to unearth optimized compounds. We present a theory of
thermoelectric transport in electron doped SrTiO3, based on a realistic tight
binding model that includes relevant scattering processes. We compare our
calculations against a wide panel of experimental data, both bulk and thin
films. We find a qualitative and quantitative agreement over both a wide range
of temperatures and carrier concentrations, from light to heavily doped.
Moreover, the results appear insensitive to the nature of the dopant La, B, Gd
and Nb. Thus, the quantitative success found in the case of SrTiO3, reveals an
efficient procedure to explore new routes to improve the thermoelectric
properties in oxides.Comment: 5 figures, manuscript submitte
The role of the dopant in the superconductivity of diamond
We present an {\it ab initio} study of the recently discovered
superconductivity of boron doped diamond within the framework of a
phonon-mediated pairing mechanism. The role of the dopant, in substitutional
position, is unconventional in that half of the coupling parameter
originates in strongly localized defect-related vibrational modes, yielding a
very peaked Eliashberg function. The electron-phonon
coupling potential is found to be extremely large and T is limited by the
low value of the density of states at the Fermi level
Investigating the high-temperature thermoelectric properties of n-type rutile TiO
Transition metal oxides are considered promising thermoelectric materials for
harvesting high-temperature waste heat due to their stability, abundance and
low toxicity. Despite their typically strong ionic character, they can exhibit
surprisingly high power factors , as in n-type SrTiO for
instance. Thus, it is worth examining other transition metal oxides that might
surpass the performances of SrTiO. This theoretical paper investigates the
thermoelectric properties of n-type rutile TiO, which is the most stable
phase of titanium oxide up to 2000 K. The electronic structure is obtained
through ab initio calculations, while the prominent features of strong
electron-phonon interaction and defects states are modelled using a small
number of parameters. The theoretical results are compared with a wealth of
experimental data from the literature, yielding very good agreements over a
wide range of carrier concentrations. This validates the hypothesis of band
conduction in rutile TiO and allows the prediction of the high-temperature
thermoelectric properties
Drought-tolerant cyanobacteria and mosses as biotechnological tools to attain land degradation neutrality
Abstract. The induction of biocrusts through inoculation-based techniques has gained increasing scientific attention in the last 2 decades due to its potential to address issues related to soil degradation and desertification. The technology has shown the most rapid advances in the use of biocrust organisms, particularly cyanobacteria and mosses, as inoculants and biocrust initiators. Cyanobacteria and mosses are
poikilohydric organisms – i.e., desiccation-tolerant organisms capable of
reactivating their metabolism upon rehydration – that can settle on bare soils in abiotically stressing habitats, provided that selected species are used and an appropriate and customized protocol is applied. The success of inoculation of cyanobacteria and mosses depends on the inoculant's physiology, but also on the ability of the practitioner to
identify and control, with appropriate technical approaches in each case
study, those environmental factors that most influence the inoculant
settlement and its ability to develop biocrusts. This review illustrates the current knowledge and results of biocrust
induction biotechnologies that use cyanobacteria or mosses as inoculants. At the same time, this review's purpose is to highlight the current
technological gaps that hinder an efficient application of the technology in the field
Intrinsic Low Temperature Paramagnetism in B-DNA
We present experimental study of magnetization in -DNA in
conjunction with structural measurements. The results show the surprising
interplay between the molecular structures and their magnetic property. In the
B-DNA state, -DNA exhibits paramagnetic behaviour below 20 K that is
non-linear in applied magnetic field whereas in the A-DNA state, remains
diamagnetic down to 2 K. We propose orbital paramagnetism as the origin of the
observed phenomena and discuss its relation to the existence of long range
coherent transport in B-DNA at low temperature.Comment: 5 pages, 4 figures, submitted to Physical Review Letters October 200
- …