Realization of contact resolving approximate Riemann solvers for strong shock and expansion flows

The Harten–Lax–van Leer contact (HLLC) and Roe schemes are good approximate Riemann solvers that have the ability to resolve shock, contact, and rarefaction waves. However, they can produce spurious solutions, called shock instabilities, in the vicinity of strong shock. In strong expansion flows, the Roe scheme can admit nonphysical solutions such as expansion shock, and it sometimes fails. We carefully examined both schemes and propose simple methods to prevent such problems. High‐order accuracy is achieved using the weighted average flux (WAF) and MUSCL‐Hancock schemes. Using the WAF scheme, the HLLC and Roe schemes can be expressed in similar form. The HLLC and Roe schemes are tested against Quirk's test problems, and shock instability appears in both schemes. To remedy shock instability, we propose a control method of flux difference across the contact and shear waves. To catch shock waves, an appropriate pressure sensing function is defined. Using the proposed method, shock instabilities are successfully controlled. For the Roe scheme, a modified Harten–Hyman entropy fix method using Harten–Lax–van Leer‐type switching is suggested. A suitable criterion for switching is established, and the modified Roe scheme works successfully with the suggested method. Copyright © 2009 John Wiley & Sons, Ltd.     

Site-specific transition metal occupation in multicomponent pyrophosphate for improved electrochemical and thermal properties in lithium battery cathodes: a combined experimental and theoretical study

As an attempt to develop lithium ion batteries with excellent performance, which is desirable for a variety of applications including mobile electronics, electrical vehicles, and utility grids, the battery community has continuously pursued cathode materials that function at higher potentials with efficient kinetics for lithium insertion and extraction. By employing both experimental and theoretical tools, herein we report multicomponent pyrophosphate (Li(2)MP(2)O(7), M = Fe(1/3)Mn(1/3)Co(1/3)) cathode materials with novel and advantageous properties as compared to the single-component analogues and other multicomponent polyanions. Li(2)Fe(1/3)Mn(1/3)Co(1/3)P(2)O(7) is formed on the basis of a solid solution among the three individual transition-metal-based pyrophosphates. The unique crystal structure of pyrophosphate and the first principles calculations show that different transition metals have a tendency to preferentially occupy either octahedral or pyramidal sites, and this site-specific transition metal occupation leads to significant improvements in various battery properties: a single-phase mode for Li insertion/extraction, improved cell potentials for Fe(2+)/Fe(3+) (raised by 0.18 eV) and Co(2+)/Co(3+) (lowered by 0.26 eV), and increased activity for Mn(2+)/Mn(3+) with significantly reduced overpotential. We reveal that the favorable energy of transition metal mixing and the sequential redox reaction for each TM element with a sufficient redox gap is the underlying physical reason for the preferential single-phase mode of Li intercalation/deintercalation reaction in pyrophosphate, a general concept that can be applied to other multicomponent systems. Furthermore, an extremely small volume change of ~0.7% between the fully charged and discharged states and the significantly enhanced thermal stability are observed for the present material, the effects unseen in previous multicomponent battery materials.     

Superparamagnetic Fe3O4 nanoparticles-carbon nitride nanotube hybrids for highly efficient peroxidase mimetic catalysts

We report a facile route to synthesize size tunable Fe(3)O(4) nanoparticles (NPs)-carbon nitride nanotube (CNNT) hybrids. These hybrids showing the water-soluble property are proven to exhibit ultra high peroxidase mimetic activity compared to those of pure NPs, where a colorless peroxidase substrate 3,3,5,5-tetramethylbenzidine changes by H(2)O(2) to its blue colored oxidized state.     

Heterogeneity within order in crystals of a porous metal-organic framework

Generally, crystals of synthetic porous materials such as metal-organic frameworks (MOFs) are commonly made up from one kind of repeating pore structure which predominates the whole material. Surprisingly, little is known about how to introduce heterogeneously arranged pores within a crystal of homogeneous pores without losing the crystalline nature of the material. Here, we outline a strategy for producing crystals of MOF-5 in which a system of meso- and macropores either permeates the whole crystal to make sponge-like crystals or is entirely enclosed by a thick crystalline microporous MOF-5 sheath to make pomegranate-like crystals. These new forms of crystals represent a new class of materials in which micro-, meso-, and macroporosity are juxtaposed and are directly linked unique arrangements known to be useful in natural systems but heretofore unknown in synthetic crystals.     

Effects of a wall on the self-ignition patterns and flame propagation of high-pressure hydrogen release through a tube

High-pressure hydrogen gas is gaining more attention as the next-generation energy carrier, but its safe handling and storage remains an important issue. The possibility of self-ignition near an obstacle is a realistic concern in practical applications such as a hydrogen car, but only a few studies related to this issue have been conducted. In this paper, experimental investigations were carried out to understand the effects of a wall on ignition patterns by high-speed imaging. This study was conducted using extension tubes of different lengths at burst pressures up to 30 MPa. The wall height, distance of the wall from the tube exit and the burst pressure were considered as the main wall parameters affecting self-ignition. The results showed that the existence of a wall could not change the type of ignition patterns (i.e., the wall does not initiate or extinguish a flame) regardless of the wall height and burst pressure, but only when the tube was too short to generate a strong flame inside the tube. When the tube was long enough to induce a strong flame in the tube, however, the wall promoted flame stabilization, e.g., the flame stabilization time outside the tube was shortened by locating the wall near the tube. But its effect disappeared when the distance of the wall from the tube exceeded 10D.     

Templated syntheses of Pt thin films with feature sizes of 3, 6 and 9 nm and their effects on SERS

We report on the syntheses of nanostructured Pt films with three different feature sizes of 3, 6 and 9 nm and their effects to the surface enhance Raman spectroscopy of rhodamine 6G (Rh6G) analytes. The syntheses of the Pt films have been achieved through templating the pore structures of mesoporous silica thin films (MSTFs). For the purpose of the present study, we synthesized three MSTFs with pore sizes 3, 6 and 9 nm. Other than the pore sizes, the silica thin films have the same pore structures. The Pt films have been synthesized by depositing Pt into the pores of the template MSTFs and removing the templates after the deposition. The Pt films produce strong Raman spectra of Rh6G with the intensity increasing as the feature size of the Pt film increases. Simulations of the induced electromagnetic fields on the Pt films show the same trend as the observed Raman intensity with the feature size. Copyright © 2012 John Wiley & Sons, Ltd.     

Theoretical study of the V(4F)?+?NO(2��r)?��?VO(4��?)?+?N(4S��) reaction compared with the Sc(2D) and Ti(3F) cases

To understand the title reaction, the MRCI, CCSD, CCSD(T), and DFT calculations have been done. A large domain of the ground-state potential energy surface has been explored including the activation energy barrier to form the triatomic complex, two stable intermediate complexes, V[NO] and NVO, the transition state connecting these two conformers, and the detachment of the nitrogen atom. We compared this reaction with the similar ones involving the Sc and Ti atoms. The activation barrier to form the VNO complex made from the ionic-covalent coupling decreases to approach the experimental data when the electron correlation effect is better included as in the Sc and Ti systems. The transition state connecting the two conformers was calculated to be higher than in the Sc and Ti cases probably due to larger number of nonbonding valence electrons and is probably too high with respect to the reactant energy level to allow the interconversion between the two conformers in the VNO. The direct concerted substitution (abstraction) reaction is improbable because this process will have to overcome a too high potential barrier. We have also found the transition state connecting two conformers of ScNO.     

Influence of residual impurities on ring‐opening metathesis polymerization after copper(I)‐catalyzed alkyne‐azide cycloaddition click reaction

Bottlebrush polymers (BBPs) are three‐dimensional polymers with great academic and industrial potential owing to their highly tunable and intricate architecture. The most popular method to synthesize BBPs is ring‐opening metathesis polymerization (ROMP) with Grubbs' catalyst, allowing living grafting‐through polymerization of macromonomers of up to ultrahigh molecular weights with narrow molecular weight distribution. In this case, it has been well recognized that the purity of macromonomers (MMs) is critical for a successful ROMP reaction. For MMs synthesized from reversible‐deactivation radical polymerization, Grubbs and Xia demonstrated that the better control of ROMP reaction can be achieved when they are prepared via “growth‐then‐coupling” method that is coupling a norbornenyl group to end‐functionalized prepolymers. However, these MMs can also contain various residual impurities from previous synthetic steps, which can potentially poison the catalyst and hamper the ROMP reaction. Herein, we intentionally doped possible impurities into purified MMs to identify the most poisoning species. As a result, it was found that alkyne‐functionalized norbornene most significantly retarded the ROMP reaction due to a formation of Ru‐vinyl‐carbene intermediates having low catalytic reactivity, whereas the other reagents such as solvent, Cu‐catalyst, ligands, and azido‐terminated prepolymers were relatively inert. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 726–737     

Multiple Ionic-Covalent Couplings in Molecules and Clusters

由电正和电负组元构成的分子的电子态是共价构型和离子构型相互干涉作用的结果. 研究揭示了在Li2H、Li2F和Li4F等小分子中离子-共价耦合的复杂性. 将这一类分析扩展至金属团簇或者金属表面对亲电分子的吸附将会对于观测到的物理和化学性质提供全新的解释.     

Minimal-basis-adapted pseudopotentials for transition metal atoms

A pseudopotential technique is presented to be used with a single function constructed from the three primitive GTOs for the representation of d orbitals of transition-metal atoms. Applied to Cu, CU₂, and CuF, it has permitted a reduction in the number of 3d-type GTO primitives without reducing the quality of the results.