4,472 research outputs found
Broadband and Wide Angle Nonreciprocal Thermal Emission from Weyl Semimetal Structures
Nonreciprocal thermal emission is a cutting-edge technology that enables
fundamental control over thermal radiation and has exciting applications in
thermal energy harvesting. However, so far one of the foremost challenges is
making nonreciprocal emission to operate over a broad wavelength range and for
multiple angles. In this work, we solve this outstanding problem by proposing
three different types of structures always utilizing only one Weyl semimetal
(WSM) thin film combined with one or two additional dielectric or metallic
layers and terminated by a metallic substrate. First a tradeoff relationship
between the magnitude and bandwidth of the thermal nonreciprocity contrast is
established based on the thickness of the WSM film. Then, the bandwidth
broadening effect is demonstrated via the insertion of a dielectric spacer
layer that can also be fine-tuned by varying its thickness. Finally, further
control on the resulting strong nonreciprocal thermal radiation is demonstrated
by the addition of a thin metallic layer in the proposed few layer designs. The
presented composite structures work for a broad frequency range and multiple
emission angles, consisting highly advantageous properties to various
nonreciprocal thermal radiation applications. Moreover, the proposed designs do
not require any patterning and can be experimentally realized by simple
deposition fabrication methods. They are expected to aid in the creation of
broadband nonreciprocal thermal emitters that can find applications in new
energy harvesting devices
Quantitative P-31 NMR Analysis of Lignins and Tannins
The development of sustainable biorefinery products is confronted, among others, with the challenge of lignin and tannin valorization. These abundant, renewable aromatic biopolymers have not been widely exploited due to their inherent structural complexity and high degrees of variability and species diversity. The lack of a defined primary structure for these polyphenols is further compounded with complex chemical alterations induced during processing, eventually imparting a large variety of structural features of extreme significance for any further utilization efforts.Consequently, a protocol for the rapid, simple, and unequivocal identification and quantification of the various functional groups present in natural polyphenols, is a fundamental prerequisite for understanding and accordingly tailor their reactivity and eventual utility.Quantitative P-31 NMR offers the opportunity to rapidly and reliably identify unsubstituted, o-mono substituted, and o-disubstituted phenols, aliphatic OHs, and carboxylic acid moieties in lignins and tannins with broad application potential.The methodology consists of an in situ quantitative lignin or tannin labeling procedure using a suitable P-31 containing probe, followed by the acquisition of a quantitative P-31 NMR spectrum in the presence of an internal standard. The high natural abundance of the P-31 nucleus allows for small amounts of the sample (similar to 30 mg) and short NMR acquisition times (similar to 30-120 min) with well-resolved P-31 signals that are highly dependent on the surrounding chemical environment of the labeled OH groups
Detection of velocity in high temperature liquid metals
Various efforts have been made to measure velocity in liquid metals. All of these efforts, however, share the same inherent limitation, namely, not being operative at the high temperatures required by liquid metals and liquid slags in an industrial application. In this paper, the current methods used were reviewed, and a new technique was presented for the measurement of velocity in high temperature liquid metals. In using this technique there are two stages. Starting with the calibration stage and then moving to the actual measurement stage by making use of the data obtained from calibration stage. Calibration proceeds in the following manner. Metallic spheres moving with a specific velocity are immersed in liquid metal held under isothermal conditions and at specific temperature. Their melting times are determined very accurately with a novel technique. These measurements are repeated for different metal bath temperatures and for different velocities of metallic spheres. In this manner it is possible to calculate the correlation between velocity and melting times for each metal bath temperature. During the actual measurement stage, when the metal bath temperature is known and its velocity is unknown, the magnitude of the unknown liquid metal velocity can be derived as follows: metallic spheres are immersed into the moving liquid metal and their melting times are determined. Using the above mentioned correlations, it will be shown that the magnitude of the unknown velocity in liquid metal can be deduced. This new technique was applied to high temperature liquid aluminum and liquid steel and these results were presented. The potential applicability of this technique in other liquid metals and liquid slags will also be discussed
An efficient and stereoselective dearylation of asarinin and sesamin tetrahydrofurofuran lignans to acuminatolide by methyltrioxorhenium/H2O2 and UHP systems
The synthesis of stereoisomers of acuminatolide is rare and requires complex and time-consuming multistep procedures. Asarinin (1) and sesamin (2), two diasteromeric tetrahydrofurofuran lignans, are efficiently mono-dearylated by methyltrioxorhenium (MTO, I) and hydrogen peroxide (H2O2) or urea hydrogen peroxide adduct (UHP) as primary oxidant to give (-)-(7R,8'R,8R)-acuminatolide (3A) and (+)-(7S,8R,8'R)-acuminatolide (3B), respectively, in high yield and diastereoselectivity (de > 98%). The oxidation of 1 was also performed with novel heterogeneous catalysts based on the heterogenation of MTO on poly(4-vinylpyridine) and polystyrene resins. In these latter cases 3A was obtained with a different yield and selectivity depending on the physical-chemical properties of the support. Cytotoxic effects of 3A and 3B in mammalian cell lines in vitro are also reported
A Theoretical Model of Underground Dipole Antennas for Communications in Internet of Underground Things
The realization of Internet of Underground Things (IOUT) relies on the establishment of reliable communication links, where the antenna becomes a major design component due to the significant impacts of soil. In this paper, a theoretical model is developed to capture the impacts of change of soil moisture on the return loss, resonant frequency, and bandwidth of a buried dipole antenna. Experiments are conducted in silty clay loam, sandy, and silt loam soil, to characterize the effects of soil, in an indoor testbed and field testbeds. It is shown that at subsurface burial depths (0.1-0.4m), change in soil moisture impacts communication by resulting in a shift in the resonant frequency of the antenna. Simulations are done to validate the theoretical and measured results. This model allows system engineers to predict the underground antenna resonance, and also helps to design an efficient communication system in IOUT. Accordingly, a wideband planar antenna is designed for an agricultural IOUT application. Empirical evaluations show that an antenna designed considering both the dispersion of soil and the reflection from the soil-air interface can improve communication distances by up to five times compared to antennas that are designed based on only the wavelength change in soil
Unraveling the formation dynamics of metallic femtosecond laser induced periodic surface structures
Femtosecond laser surface processing (FLSP) is an emerging fabrication
technique to efficiently control the surface morphology of many types of
materials including metals. However, the theoretical understanding of the FLSP
formation dynamics is not a trivial task, since it involves the interaction of
various physical processes (electromagnetic, thermal, fluid dynamics) and
remains relatively unexplored. In this work, we tackle this problem and present
rigorous theoretical results relevant to low-fluence FLSP that accurately match
the outcomes of an experimental campaign focused on the formation dynamics of
laser induced periodic surface structures (LIPSS) on stainless steel. More
specifically, the topology and maximum depth of LIPSS trenches are
theoretically and experimentally investigated as a function of the number of
laser pulses. Moreover, precise LIPSS morphology measurements are performed
using atomic force microscopy (AFM). The proposed comprehensive simulation
study is based on two-temperature model (TTM) non-equilibrium thermal
simulations coupled with fluid dynamic computations to capture the melting
metal phase occurring during FLSP. Our rigorous simulation results are found to
be in excellent agreement with the AFM measurements. The presented theoretical
framework to model FLSP under low-fluence femtosecond laser pulses will be
beneficial to various emerging applications of LIPSS on metallic surfaces, such
as cooling high-powered laser diodes and controlling the thermal emission or
absorption of metals
Electrochemical characterization of organic coatings for protection of historic steel artefacts
Figuras en el archivo zipElectrochemical techniques are mainly known in the field of cultural heritage conservation as a
tool for the elimination of corrosion layers or the removal of chlorides. However, these techniques are also
a valuable tool for assessing the anti-corrosive efficiency of protective coatings. The aim of this study was
to evaluate the performance of different coatings for their use in metallic heritage conservation using
polarization resistance (Rp) and electrochemical impedance spectroscopy (EIS). Carbon steel samples were
prepared to simulate the surface composition and morphology of historic steel artefacts, and coated by a
conservator-restorer following the common practices in conservation treatments. Three commercial organic
coatings have been studied: a microcrystalline wax (RenaissanceTM) and a methyl acrylate/ethyl
methacrylate copolymer resin (ParaloidTM B-72) dissolved in acetone –both them commonly used in
conservation and restoration treatments– and a ethylene copolymer wax emulsion in water (PoligenTM ES-
91009), that has not been used so far for this purposes. Four commercial corrosion inhibitor additives were
added to the ParaloidTM B-72 resin and PoligenTM ES-91009 wax. The additives were commercial
preparations with the following known active components: a blend of triazoles (M435), an ammonium salt
of tricarboxylic acid (M370), a calcium sulphonate (M109), and a bis-oxazoline (Alkaterge-TTM). Rp and
EIS results showed that the best protection of the steel specimens was afforded by PoligenTM ES-91009
when applied in thick layers. None of the additives have shown a clear improvement of the protection
properties of the coatings, and one of them impaired the barrier effect of the coating.Acknowledgements The authors express their gratitude to the Sixth
Framework Programme of the European Commission for financial
support of PROMET Project (Contract 509126). D.M. Bastidas
expresses his gratitude to the CSIC of Spain for his contract under
the I3P Programme, co-financed by the European Social Fund.Peer reviewe
Dependence of e-cloud on the longitudinal bunch profile: studies in the PS & extension to the HL-LHC
Recent studies have shown that the prospects for significantly increasing
bunch intensities in the LHC for the luminosity upgrade (HL-LHC) may be
severely limited by the available cryogenic cooling capacity and the
electron-cloud (EC) driven beam instability. However, it is planned that during
the HL-LHC era the bunch intensities in the LHC will go up by nearly a factor
of two compared to the LHC-design values. This motivates the exploration of
additional EC mitigation techniques that can be adopted in addition to those
already in place. Preliminary simulations indicated that long flat bunches can
be beneficial over Gaussian bunches to reduce the EC build up. Rigorous studies
using realistic bunch profiles have never been done. Therefore, we have
undertaken an in-depth investigation in the CERN 26 GeV PS to see if we can
validate the previous findings and, in particular, if flattening the bunch can
mitigate the EC. Here we present the results from dedicated EC measurements in
the PS using a variety of bunch shapes and a comparison with simulations.
Finally, we investigate if reshaping the bunch profiles using a 2nd harmonic rf
cavity can mitigate EC in the HL-LHC
- …