Triboelectric nanogenerators (TENGs) represent an emerging technology in energy harvesting, medical treatment, and information technology. Flexible, portable, and self‐powered electronic devices ...based on TENGs are much desired, whereas the complex preparation processes and high cost of traditional flexible electrodes hinder their practical applications. Here, an MXene/polyvinyl alcohol (PVA) hydrogel TENG (MH‐TENG) is presented with simple fabrication, high output performance, and versatile applications. The doping of MXene nanosheets promotes the crosslinking of the PVA hydrogel and improves the stretchability of the composite hydrogel. The MXene nanosheets also form microchannels on surfaces, which not only enhances the conductivity of the hydrogel by improving the transport of ions but also generates an extra triboelectric output via a streaming vibration potential mechanism. The measured open‐circuit voltage of the MH‐TENG reaches up to 230 V even in a single‐electrode mode. The MH‐TENG can be stretched up to 200% of the original length and demonstrates a monotonical increasing relationship between the stretchable length and the short‐circuit voltage. By utilizing the MH‐TENG's outstanding stretchable property and ultrahigh sensitivity to mechanical stimuli, applications in wearable movement monitoring, high‐precision written stroke recognition, and low‐frequency mechanical energy harvesting are demonstrated.
A flexible and stretchable triboelectric nanogenerator (TENG) with polyvinyl alcohol (PVA) hydrogel encapsulated as electrodes is fabricated. Doping of MXene nanosheets into PVA can greatly promote the electrical properties of the TENG. Utilizing the TENG's outstanding stretchable property and ultra‐high sensitivity to mechanical stimuli, applications in wearable movement monitoring, high‐precision written recognition, and mechanical energy harvesting are demonstrated.
Contact electrification (CE) has been known for more than 2600 years but the nature of charge carriers and their transfer mechanisms still remain poorly understood, especially for the cases of ...liquid-solid CE. Here, we study the CE between liquids and solids and investigate the decay of CE charges on the solid surfaces after liquid-solid CE at different thermal conditions. The contribution of electron transfer is distinguished from that of ion transfer on the charged surfaces by using the theory of electron thermionic emission. Our study shows that there are both electron transfer and ion transfer in the liquid-solid CE. We reveal that solutes in the solution, pH value of the solution and the hydrophilicity of the solid affect the ratio of electron transfers to ion transfers. Further, we propose a two-step model of electron or/and ion transfer and demonstrate the formation of electric double-layer in liquid-solid CE.
For solving time-dependent convection-dominated partial differential equations (PDEs), which arise frequently in computational physics, high order numerical methods, including finite difference, ...finite volume, finite element and spectral methods, have been undergoing rapid developments over the past decades. In this article we give a brief survey of two selected classes of high order methods, namely the weighted essentially non-oscillatory (WENO) finite difference and finite volume schemes and discontinuous Galerkin (DG) finite element methods, emphasizing several of their recent developments: bound-preserving limiters for DG, finite volume and finite difference schemes, which address issues in robustness and accuracy; WENO limiters for DG methods, which address issues in non-oscillatory performance when there are strong shocks, and inverse Lax–Wendroff type boundary treatments for finite difference schemes, which address issues in solving complex geometry problems using Cartesian meshes.
It is well known that semi-discrete high order discontinuous Galerkin (DG) methods satisfy cell entropy inequalities for the square entropy for both scalar conservation laws (Jiang and Shu (1994) 39) ...and symmetric hyperbolic systems (Hou and Liu (2007) 36), in any space dimension and for any triangulations. However, this property holds only for the square entropy and the integrations in the DG methods must be exact. It is significantly more difficult to design DG methods to satisfy entropy inequalities for a non-square convex entropy, and/or when the integration is approximated by a numerical quadrature. In this paper, we develop a unified framework for designing high order DG methods which will satisfy entropy inequalities for any given single convex entropy, through suitable numerical quadrature which is specific to this given entropy. Our framework applies from one-dimensional scalar cases all the way to multi-dimensional systems of conservation laws. For the one-dimensional case, our numerical quadrature is based on the methodology established in Carpenter et al. (2014) 5 and Gassner (2013) 19. The main ingredients are summation-by-parts (SBP) operators derived from Legendre Gauss–Lobatto quadrature, the entropy conservative flux within elements, and the entropy stable flux at element interfaces. We then generalize the scheme to two-dimensional triangular meshes by constructing SBP operators on triangles based on a special quadrature rule. A local discontinuous Galerkin (LDG) type treatment is also incorporated to achieve the generalization to convection–diffusion equations. Extensive numerical experiments are performed to validate the accuracy and shock capturing efficacy of these entropy stable DG methods.
Fin-and-tube heat exchangers are the mostly used heat exchangers for thermal energy conversion with wide range of applications such as air conditioning, refrigeration, automotive industry, electronic ...devices, and the like. Demand of more efficient cooling by more compact heat exchangers leads to tremendous researches on this subject. In this paper, a detailed review of experimental and numerical researches upon different mechanisms of heat transfer enhancement in fin-and-tube heat exchangers are performed and the relevant influences and operating conditions are thoroughly reviewed. Effects of different geometrical parameters on heat transfer and pressure drop in each mechanism are also discussed in details. Furthermore, comparisons between different mechanisms of heat transfer improvement and some novel compound designs of fin-and-tube heat exchangers are discussed. In addition, some special researches on surface treatment, particle deposition, thermal contact, and fabrication material in fin-and-tube heat exchangers are described. Finally, some developed correlations for calculation of heat transfer and pressure drop characteristics of fin-and-tube heat exchangers with their ranges of validation are classified and compared.
•Heat transfer enhancement mechanisms in fin-and-tube heat exchangers are reviewed.•Different researches are tabulated including major results and operating conditions.•Combining different mechanisms leads to novel compound geometries.•Special researches such as fouling and surface treatment are discussed.•Various correlations for calculation of j and f factors are listed and compared.
Display omitted
•Processing pyrolysis and catalytic decomposition of polypropylene for CNTs and H2.•Novel sol–gel Fe/Ni catalysts were synthesized.•Catalyst activity was highly dependent on synthesis ...method and composition.•The formation mechanism for as-grown CNTs from polypropylene was discussed.•FeNi(SG) generated 25.14 mmol/gplastic H2 and 360 mg/gplastic carbon nanomaterials.
Thermo-chemical conversion of waste plastics into clean energy and valuable products can be a promising technology from both economic and environmental perspectives. In this work, pyrolysis and in-line catalytic decomposition of polypropylene was performed for production of hydrogen and carbon nanomaterials. A series of novel Fe/Ni catalysts were prepared, and the effects of catalyst active metal component (Fe, Ni, FeNi) and synthesis method (sol–gel and impregnation) were explored. Results show that the production of hydrogen and solid products was in a descending order as Fe-Ni, Fe and Ni loading, while sol–gel prepared catalysts were more catalytic effective than their impregnated counterparts. FeNi(SG) exhibited an optimal activity with productions of 25.14 mmol/gplastic of hydrogen and 360 mg/gplastic of high quality carbon nanomaterials, which was attributed to (i) uniform mesoporous structure with 212.30 m2/g specific surface area (ii) high dispersion degree (42.02%) of active metals and (iii) enhanced reducibility originated from the synergistic effect between Fe and Ni. Carbon nanomaterials which contained the majority of carbon nanotubes were comprehensively characterized in terms of structure complexity, morphology and graphitization degree, and the formation mechanism of bamboo-like multi-walled carbon nanotubes from pyrolysis and catalytic decomposition of polypropylene was also discussed.
High order accurate weighted essentially nonoscillatory (WENO) schemes are relatively new but have gained rapid popularity in numerical solutions of hyperbolic partial differential equations (PDEs) ...and other convection dominated problems. The main advantage of such schemes is their capability to achieve arbitrarily high order formal accuracy in smooth regions while maintaining stable, nonoscillatory, and sharp discontinuity transitions. The schemes are thus especially suitable for problems containing both strong discontinuities and complex smooth solution features. WENO schemes are robust and do not require the user to tune parameters. At the heart of the WENO schemes is actually an approximation procedure not directly related to PDEs, hence the WENO procedure can also be used in many non-PDE applications. In this paper we review the history and basic formulation of WENO schemes, outline the main ideas in using WENO schemes to solve various hyperbolic PDEs and other convection dominated problems, and present a collection of applications in areas including computational fluid dynamics, computational astronomy and astrophysics, semiconductor device simulation, traffic flow models, computational biology, and some non-PDE applications. Finally, we mention a few topics concerning WENO schemes that are currently under investigation.
In this paper, a new type of high-order finite difference and finite volume multi-resolution weighted essentially non-oscillatory (WENO) schemes is presented for solving hyperbolic conservation laws. ...We only use the information defined on a hierarchy of nested central spatial stencils and do not introduce any equivalent multi-resolution representation. These new WENO schemes use the same large stencils as the classical WENO schemes in 25,45, could obtain the optimal order of accuracy in smooth regions, and could simultaneously suppress spurious oscillations near discontinuities. The linear weights of such WENO schemes can be any positive numbers on the condition that their sum equals one. This is the first time that a series of unequal-sized hierarchical central spatial stencils are used in designing high-order finite difference and finite volume WENO schemes. These new WENO schemes are simple to construct and can be easily implemented to arbitrary high order of accuracy and in higher dimensions. Benchmark examples are given to demonstrate the robustness and good performance of these new WENO schemes.
•A new class of high order finite difference and finite volume WENO schemes are constructed.•These schemes are based on the multi-resolution idea, and a series of unequal-sized hierarchical central spatial stencils.•These schemes can use arbitrary positive linear weights, and are easy to implement for one and multi-dimensions.•These schemes have a gradual degrading of accuracy near discontinuities.
We construct uniformly high order accurate discontinuous Galerkin (DG) schemes which preserve positivity of density and pressure for Euler equations of compressible gas dynamics. The same framework ...also applies to high order accurate finite volume (e.g. essentially non-oscillatory (ENO) or weighted ENO (WENO)) schemes. Motivated by Perthame and Shu (1996)
20 and Zhang and Shu (2010)
26, a general framework, for arbitrary order of accuracy, is established to construct a positivity preserving limiter for the finite volume and DG methods with first order Euler forward time discretization solving one-dimensional compressible Euler equations. The limiter can be proven to maintain high order accuracy and is easy to implement. Strong stability preserving (SSP) high order time discretizations will keep the positivity property. Following the idea in Zhang and Shu (2010)
26, we extend this framework to higher dimensions on rectangular meshes in a straightforward way. Numerical tests for the third order DG method are reported to demonstrate the effectiveness of the methods.
A CO2‐mediated hydrogen storage energy cycle is a promising way to implement a hydrogen economy, but the exploration of efficient catalysts to achieve this process remains challenging. Herein, ...sub‐nanometer Pd–Mn clusters were encaged within silicalite‐1 (S‐1) zeolites by a ligand‐protected method under direct hydrothermal conditions. The obtained zeolite‐encaged metallic nanocatalysts exhibited extraordinary catalytic activity and durability in both CO2 hydrogenation into formate and formic acid (FA) dehydrogenation back to CO2 and hydrogen. Thanks to the formation of ultrasmall metal clusters and the synergic effect of bimetallic components, the PdMn0.6@S‐1 catalyst afforded a formate generation rate of 2151 molformate molPd−1 h−1 at 353 K, and an initial turnover frequency of 6860 molH2
molPd−1 h−1 for CO‐free FA decomposition at 333 K without any additive. Both values represent the top levels among state‐of‐the‐art heterogeneous catalysts under similar conditions. This work demonstrates that zeolite‐encaged metallic catalysts hold great promise to realize CO2‐mediated hydrogen energy cycles in the future that feature fast charge and release kinetics.
Sub‐nanometer Pd–Mn clusters were encaged within silicalite‐1 zeolites by a ligand‐protected method under direct hydrothermal conditions. The obtained zeolite‐encaged metallic nanocatalysts exhibited a record formate generation rate of 2151 molformate molPd−1 h−1 at 353 K, and an excellent initial turnover frequency of 6860 molH2
molPd−1 h−1 for CO‐free formic acid decomposition at 333 K without any additive.
PDF
