Complex topological configurations are fertile ground for exploring emergent phenomena and exotic phases in condensed-matter physics. For example, the recent discovery of polarization vortices and ...their associated complex-phase coexistence and response under applied electric fields in superlattices of (PbTiO
)
/(SrTiO
)
suggests the presence of a complex, multi-dimensional system capable of interesting physical responses, such as chirality, negative capacitance and large piezo-electric responses
. Here, by varying epitaxial constraints, we discover room-temperature polar-skyrmion bubbles in a lead titanate layer confined by strontium titanate layers, which are imaged by atomic-resolution scanning transmission electron microscopy. Phase-field modelling and second-principles calculations reveal that the polar-skyrmion bubbles have a skyrmion number of +1, and resonant soft-X-ray diffraction experiments show circular dichroism, confirming chirality. Such nanometre-scale polar-skyrmion bubbles are the electric analogues of magnetic skyrmions, and could contribute to the advancement of ferroelectrics towards functionalities incorporating emergent chirality and electrically controllable negative capacitance.
► A heat-integrated reforming process of glycerol using supercritical water is established. ► The syngas obtained is conditioned and entered in a methanol synthesis loop. ► Power is produced by a ...turbine and a fuel cell. ► The aim was to maximize the overall process efficiency in terms of total power and methanol production. ► Optimal conditions were systematically identified.
A process for producing methanol from the synthesis gas obtained by reforming of glycerol using supercritical water is studied. The process also produces power from the huge pressure energy of product gas just at the outlet of the reformer by a turbine. The expanded product gas is conditioned in a PSA system, which has three sections so as to produce a H2-rich gas stream, a CO-rich gas stream and CO2 for sequestration. Thus, it can be achieved the feed required for the methanol synthesis. The surplus hydrogen is sent to a fuel cell to generate power, and the PSA off-gas, purge from the methanol loop and gases separated from the crude methanol are burnt in a furnace to achieve an energy self-sufficient process. By changing the reforming temperature, the water-to-glycerol mass ratio and the purge from the methanol loop, the conditions for optimizing the overall process relative to methanol and power productions were achieved. Thus, by reforming at 1000°C and 240atm, and performing the methanol synthesis at 250°C and 85atm, the optimal conditions were a water-to-glycerol mass ratio of 1.68 with a purge ratio of 0.2. Under these conditions 0.270kg MeOH/kg glycerol and overall energy efficiency of 38.0% were obtained. The separated CO2 for sequestration is 0.38kg/kg of glycerol.
Topological solitons such as magnetic skyrmions have drawn attention as stable quasi-particle-like objects. The recent discovery of polar vortices and skyrmions in ferroelectric oxide superlattices ...has opened up new vistas to explore topology, emergent phenomena and approaches for manipulating such features with electric fields. Using macroscopic dielectric measurements, coupled with direct scanning convergent beam electron diffraction imaging on the atomic scale, theoretical phase-field simulations and second-principles calculations, we demonstrate that polar skyrmions in (PbTiO
)
/(SrTiO
)
superlattices are distinguished by a sheath of negative permittivity at the periphery of each skyrmion. This enhances the effective dielectric permittivity compared with the individual SrTiO
and PbTiO
layers. Moreover, the response of these topologically protected structures to electric field and temperature shows a reversible phase transition from the skyrmion state to a trivial uniform ferroelectric state, accompanied by large tunability of the dielectric permittivity. Pulsed switching measurements show a time-dependent evolution and recovery of the skyrmion state (and macroscopic dielectric response). The interrelationship between topological and dielectric properties presents an opportunity to simultaneously manipulate both by a single, and easily controlled, stimulus, the applied electric field.
Abstract
Air-stability is one of the most important considerations for the practical application of electrode materials in energy-harvesting/storage devices, ranging from solar cells to rechargeable ...batteries. The promising P2-layered sodium transition metal oxides (P2-Na
x
TmO
2
) often suffer from structural/chemical transformations when contacted with moist air. However, these elaborate transitions and the evaluation rules towards air-stable P2-Na
x
TmO
2
have not yet been clearly elucidated. Herein, taking P2-Na
0.67
MnO
2
and P2-Na
0.67
Ni
0.33
Mn
0.67
O
2
as key examples, we unveil the comprehensive structural/chemical degradation mechanisms of P2-Na
x
TmO
2
in different ambient atmospheres by using various microscopic/spectroscopic characterizations and first-principle calculations. The extent of bulk structural/chemical transformation of P2-Na
x
TmO
2
is determined by the amount of extracted Na
+
, which is mainly compensated by Na
+
/H
+
exchange. By expanding our study to a series of Mn-based oxides, we reveal that the air-stability of P2-Na
x
TmO
2
is highly related to their oxidation features in the first charge process and further propose a practical evaluating rule associated with redox couples for air-stable Na
x
TmO
2
cathodes.
Hydrogen production from the supercritical water reforming of glycerol was studied in a tubular reactor without adding a catalyst. Experiments were carried out at a pressure of 240bar, temperatures ...of 750–850°C, and glycerol feed concentrations of 5–30wt.%. Likewise, the residence time was changed from 12 to 160s, by handling the feed flow-rate. The dry gas is mainly consisted of H2, CO2, CO, CH4. In addition, small concentrations of glycerol were measured in the liquid phase analysis, but barely traces of others like glycolaldehyde, glyceraldehyde, dihydroxyacetone and lactic acid were detected. Thus, two probable reaction pathways are discussed, which makes it possible to explain the experimental results by using a method applicable to other similar processes. The results showed that the glycerol conversion was almost complete, except at the highest glycerol feed concentration, in which the conversion was of 88%. Hydrogen yields from 2 to 4molH2/molglycerol were obtained at high and low glycerol feed concentrations, respectively, when operating at high temperature and residence time. Besides, it was verified the catalytic effect of the reactor material (Inconel 625) from the trend of the gas product yields with time and the structured carbon nanotubes encountered. The catalytic activity of the reactor material was decreasing to reach a steady state after a few tens of operating hours. This study illustrates that the reforming of glycerol using supercritical water without added catalyst is feasible to achieve a high-yield hydrogen production, and it encourages to continue the research line, to obtain a process economically interesting.
•Glycerol reforming using supercritical water without adding a heterogenous catalyst is feasible.•The use of a glycerol concentration as high as 30wt.% produces a gas with high hydrogen yield.•The effect of operating variables on the process performance and approach to chemical equilibrium was discussed.•Two probable reaction pathways were proposed that make it possible to explain the experimental results.•The reactor material (Inconel 625) showed a high catalytic activity, which was decreasing to reach a steady state.
•The quantification of mixed aleatoric and structural uncertainty in RANS with turbulence closure independence is studied.•A framework for the mixed UQ is suggested by combining probabilistic methods ...and the Eigenspace Perturbation Method.•The framework is applied to two complex flows (NASA ARN nozzle and impinging jet for heat transfer).•Uncertainty in the simulations is analysed against specific experimental data but also spatially in the whole domain.•It is demonstrated the importance of considering mixed uncertainty as reliable confidence estimates because of interactions.
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Reynolds Averaged Navier Stokes models are the most popular approach for Computational Fluid Dynamics simulations of turbulent flows. Despite their popularity, these numerical models still need an appropriate quantification of margins and uncertainty for reliable engineering practice. In turbulent flow simulations, there are two kinds of uncertainties: aleatoric (those arising due to errors in initial conditions, material parameters, etc.) and structural or epistemic (those arising due to the turbulence model used). While these uncertainties have been explored in isolation, there are no studies that properly consider both types of uncertainty together as is the case in real life engineering applications. Considering these two sources of uncertainty in isolation limits our knowledge regarding the sources of uncertainty, their relative magnitudes, their interaction and the accuracy of uncertainty estimates. For the reasons given, a methodology is necessary to amalgamate uncertainties of different nature. In this paper is outlined a framework to carry out uncertainty quantification for such mixed uncertainty cases. The results are compared to those from aleatoric only and epistemic only studies to analyze the manner in which aleatoric and epistemic uncertainties interact and affect final results in complex turbulent flows and heat transfer. We compare and contrast the relative contributions of these sources to the overall uncertainty. The results obtained for our test cases exhibit that only considering aleatoric or epistemic uncertainty sources in isolation tends to severely under-predict the uncertainty in simulations. Utilizing the framework, we show that considering both sources together leads to a satisfactory prediction of the uncertainty in simulation results because of the underlying relations between these sources uncertainty in their propagation.
Abstract
Layered transition metal oxides are the most important cathode materials for Li/Na/K ion batteries. Suppressing undesirable phase transformations during charge-discharge processes is a ...critical and fundamental challenge towards the rational design of high-performance layered oxide cathodes. Here we report a shale-like Na
x
MnO
2
(S-NMO) electrode that is derived from a simple but effective water-mediated strategy. This strategy expands the Na
+
layer spacings of P2-type Na
0.67
MnO
2
and transforms the particles into accordion-like morphology. Therefore, the S-NMO electrode exhibits improved Na
+
mobility and near-zero-strain property during charge-discharge processes, which leads to outstanding rate capability (100 mAh g
−1
at the operation time of 6 min) and cycling stability (>3000 cycles). In addition, the water-mediated strategy is feasible to other layered sodium oxides and the obtained S-NMO electrode has an excellent tolerance to humidity. This work demonstrates that engineering the spacings of alkali-metal layer is an effective strategy to stabilize the structure of layered transition metal oxides.
•The municipal solid waste reject fraction may be valorized by producing biofuels and electricity.•The novel process consists of three sub-processes to produce bio-oil and hydrogen, and an ...upgrading.•The effect of main operating variables on the process performance has been studied.•A techno-economic analysis has been carried out to assess the viability of the proposal.•The process seems to be technically and economically feasible.
Nowadays, the waste generation increases more and more, especially of the municipal solid waste. The municipal solid waste reject fraction may be valorized from an energy point of view. With this aim, a conceptual design of a process was developed by considering material and energy integration, which mainly consists of three sub-processes: fast pyrolysis of municipal solid waste to produce bio-oil, supercritical water reforming of the bio-oil aqueous phase to produce hydrogen to be used in the third section, which is the upgrading of the organic phase of the bio-oil by hydrodeoxygenation. The overall system was simulated using Aspen Plus software to achieve the highest process performance and the lowest utilities requirement. The former was assessed by the biofuel production (liquefied fuel gas, gasoline and diesel) and the net electrical power. In addition, the potential economic profitability of the plant was performed by specifying the main process units. Thus, for a feeding of 50 t/h of municipal solid waste reject fraction, a generation of a net electric power equal to 10.65 MWe and a production of 5.2 t/h biofuels (21.1% of the carbon present in the municipal solid waste) may be achieved, thus obtaining a very low gate fee (16.7 €/t) using the same industrial selling prices that those of fossil fuels and electricity in a full plant. Therefore, the process seems to be technically and economically feasible.
•The most relevant novelty of this work is the development and investigation of simple and effcient mechanical mixing devices using the non-uniform finite difference approximation.•The non-uniform ...finite difference approximation is adapted and applied for the first time to a numerical investigation in uid mixing mechanics.•This study is the first investigation of its kind on the use of parallel wall jets for scalable heat/mass transfer enhancement.•Several configurations of the parallel wall jets as well as Reynolds regimes are studied.•Oscillatory motion of the ow downstream can be predicted via simulations and effciently boosted with certain configurations.•The best mixing performance is observed for the smallest wall jet width tested.
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Many different approaches have been investigated in the literature to achieve efficient heat and mass exchangers in applications in several fields such as mechanical, aeronautical or biomedical engineering. Some of these engineering devices need to efficiently mix two fluids at different temperature or concentrations, achieving a decent mixing at a low power cost, and using passive mechanical elements if possible. This paper introduces and analyses numerically the novel application of two parallel wall jets to enhance mixing by vortex shedding induced mechanically. The numerical approach is the non-uniform finite difference approximation, used for the first time in the study of mixing mechanics. The parallel wall jets are constrained within a channel, which can be a simple and passive easily manufactured machine with high potential as scalable laminar mixing device. Different wall jet geometries and Reynolds numbers have been tested with a verified&validated CFD code based on non-uniform finite difference approximation. This analysis allowed to predict configurations that lead to unsteady oscillatory motion. In general, it has been observed that for a range of laminar wall jets, a small channel ratio seems a very efficient option for heat/mass transfer, thanks to the intense oscillation generated in the flow downstream, which enhances mixing with low power requirement.
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