Modeling and numerical simulation of the resistance spot welding of zinc coated steel sheets using rounded tip electrode: Analysis of required conditions

This paper investigates the essential conditions to improve the accuracy of a resistance spot welding computational study of advanced zinc coated steel sheets using rounded tip electrode. An experimental analysis is performed to highlight the required considerations for a suitable simulation. A sequential Electrical-Thermal-Metallurgical and Mechanical (ETMM) finite element analysis with appropriate precautions of the contact conditions enables to accurately simulate the nugget development during the welding. A critical smooth evolution of the contact radius is required. A fine meshing with an interfacial mesh size of at least 0.05 × 10⁻³ m combined with a coupling time step of 0.0025 s between the electrical-thermal-metallurgical and the mechanical analysis allows a regular incrementation of the contact radius, without burdening the time computing. Accurate values of the contact resistance depending on the interfacial pressure and temperature are essential for a good simulation of the nugget size. The ETMM calculation is successfully extended to the simulation of the welding of a typical two sheets assembly.     

Diffusion maps for changing data

Graph Laplacians and related nonlinear mappings into low dimensional spaces have been shown to be powerful tools for organizing high dimensional data. Here we consider a data set X in which the graph associated with it changes depending on some set of parameters. We analyze this type of data in terms of the diffusion distance and the corresponding diffusion map. As the data changes over the parameter space, the low dimensional embedding changes as well. We give a way to go between these embeddings, and furthermore, map them all into a common space, allowing one to track the evolution of X in its intrinsic geometry. A global diffusion distance is also defined, which gives a measure of the global behavior of the data over the parameter space. Approximation theorems in terms of randomly sampled data are presented, as are potential applications.     

Pushing the speed limit in enantioselective supercritical fluid chromatography

Chromatographic enantioseparations on the order of a few seconds can be achieved by supercritical fluid chromatography using short columns packed with chiral stationary phases. The evolution of ‘world record’ speeds for the chromatographic separation of enantiomers has steadily dropped from an industry standard of 20–40 min just two decades ago, to a current ability to perform many enantioseparations in well under a minute. Improvements in instrument and column technologies enabled this revolution, but the ability to predict optimal separation time from an initial method development screening assay using the t_{min cc} predictor greatly simplifies the development and optimization of high‐speed chiral chromatographic separations. In this study, we illustrate how the use of this simple tool in combination with the workhorse technique of supercritical fluid chromatography on customized short chiral columns (1–2 cm length) allows us to achieve ultrafast enantioseparations of pharmaceutically relevant compounds on the 5–20 s scale, bringing the technique of high‐throughput enantiopurity analysis out of the specialist realm and into the laboratories of most researchers.     

Remarks on the Born approximation and the Factorization Method

In this paper, we compare the far-field operators for the full nonlinear inverse scattering problem with the Born approximation as its linearization. The Factorization Method shows that both operators share the same behavior with respect to illposedness of the inverse problem. The results are derived for acoustic and electromagnetic scattering problems and the corresponding problem in electrical impedance tomography.     

Time Discrete Approximation of Weak Solutions to Stochastic Equations of Geophysical Fluid Dynamics and Applications

As a first step towards the numerical analysis of the stochastic primitive equations of the atmosphere and the oceans,the time discretization of these equations by an implicit Euler scheme is studied.From the deterministic point of view,the 3D primitive equations are studied in their full form on a general domain and with physically realistic boundary conditions.From the probabilistic viewpoint,this paper deals with a wide class of nonlinear,state dependent,white noise forcings which may be interpreted in either the It6 or the Stratonovich sense.The proof of convergence of the Euler scheme,which is carried out within an abstract framework,covers the equations for the oceans,the atmosphere,the coupled oceanic-atmospheric system as well as other related geophysical equations.The authors obtain the existence of solutions which are weak in both the PDE and probabilistic sense,a result which is new by itself to the best of our knowledge.     

波士顿矩阵法在河南承接产业转移中的应用

承接产业转移是加速区域经济发展的"助推器",提升产业集聚能力是提高区域产业竞争力的关键,如何科学地选择承接产业,并提升产业集聚能力是河南亟待解决的问题.选取2010年和2014年两个时点,运用产业梯度系数和产业动态集聚指数对工业进行分行业分析,并借鉴波士顿矩阵分析法将其划分为明星、金牛、问题、瘦狗和衰退等产业.根据河南的自然资源、劳动力、区位条件、市场需求等比较优势,并考虑产业集聚发展的能力,来确定河南承接产业转移的重点产业.     

Salt Metathesis: Tetrafluoroborate Anion Rapidly Fluoridates Organoboronic Acids to give Organotrifluoroborates

Tetrafluoroborate (BF₄⁻) has long been used as a spectator counter anion. Herein, we report an unprecedented salt metathesis between a variety of BF₄⁻ salts and a series of organoboronic acids yielding the corresponding organotrifluoroborates. We identified conditions for fast and efficient fluoridation (<1 h) with minimal workup. Fundamentally, this work discloses the proclivity of BF₄⁻ to exchange fluoride atoms with organoboronates, highlighting the lability of BF₄⁻.     

Dopant-free NiOx Nanocrystals: A Low-cost and Stable Hole Transport Material for Commercializing Perovskite Optoelectronics

Advancing inverted (p-i-n) perovskite solar cells (PSCs) is critical for commercial applications given their compatibility with different bottom cells for tandem photovoltaics, low-temperature processability (≤100 °C), and promising operational stability. Although inverted PSCs have achieved an efficiency of over 25 % using doped or expensive organic hole transport materials (HTMs), their synthesis cost and stability still cannot meet the requirements for their commercialization. Recently, dopant-free and low-cost non-stoichiometric nickel oxide nanocrystals (NiO_x NCs) have been extensively studied as a low-cost and effective HTM in perovskite optoelectronics. In this minireview, we summarize the synthesis and surface-functionalization methods of NiO_x NCs. Then, the applications of NiO_x NCs in other perovskite optoelectronics beyond photovoltaics are discussed. Finally, we provide a perspective for the future development of NiO_x NCs for the commercialization of perovskite optoelectronics.     

Self-Accelerating Diels–Alder Reaction for Preparing Polymers of Intrinsic Microporosity

It is a formidable challenge in polycondensation to simultaneously construct multiple covalent bonds to prepare double-stranded polymers of intrinsic microporosity (PIMs) with fused multicyclic linkages. To the best of our knowledge, this is the first study to develop a self-accelerating Diels–Alder reaction for successfully preparing double-stranded PIMs with fused multicyclic backbone structures. A self-accelerating Diels–Alder reaction was developed based on the [4+2] cycloaddition of sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) and ortho-quinone compounds. In this reaction, the cycloaddition of ortho-quinone with the first alkyne of DIBOD activates the second alkyne, which reacts with ortho-quinone at a rate constant 192 times larger than that of the original alkyne. Using this self-accelerating reaction to polymerize DIBOD and spirocyclic/cyclic difunctional ortho-quinone monomers, a novel stoichiometric imbalance-promoted step-growth polymerization method was developed to prepare PIMs. The resultant PIMs possess intrinsic ultramicropores with pore sizes between 0.45 to 0.7 nm, high specific surface areas above 646 m² g⁻¹, and good H₂ separation performance.     

Multidisciplinary green approaches (ultrasonic,co-precipitation,hydrothermal, and microwave) for fabrication and characterization of Erbium-promoted Ni-Al2O3 catalyst for CO2 methanation

The dispersion of nickel catalysts is crucial for the catalytic ability of CO₂ methanation, which can be influenced by the fabrication method and the operation process of the catalysts. Therefore, a series of fabrication methods, including ultrasonic, hydrothermal, microwave, and co-precipitation, have been applied to prepare 25Ni-5Er-Al₂O₃ catalysts. The fabrication method can partially influence the structural and catalytic activity of the nickel aluminate catalysts. Among the catalysts modified by Erbium prepared with various methods, the catalyst fabricated by ultrasonic pathway exhibited better catalytic performance and CH₄ selectivity especially, at a temperature (400 ℃). The impact of the temperature of the reaction (200–500 °C) was examined under a stoichiometric precursor ratio of (H₂:CO₂) = 4: 1, atmospheric pressure, and space velocity (GHSV) of 25000 mL/gcath. The results demonstrate that the ultrasonic method is strongly efficient for fabricating Ni-based catalysts with a high BET surface area of about 190.33 m²g^?1. The catalyst composed via the ultrasonic technique has 69.38 % carbon dioxide conversion and 100 % methane selectivity at 400 °C for excellent catalytic performance in CO₂ methanation reactions. The fabrication effect can be associated with its high surface area, which is achieved via the hot spot mechanism. Besides, the addition of Erbium promotes the Ni dispersion on the supports and stimulates the positive reaction because of the erbium oxygen vacancies.