Polymer on Top: Current Limits and Future Perspectives of Quantitatively Evaluating Surface Grafting

Well‐defined polymer strands covalently tethered onto solid substrates determine the properties of the resulting functional interface. Herein, the current approaches to determine quantitative grafting densities are assessed. Based on a brief introduction into the key theories describing polymer brush regimes, a user's guide is provided to estimating maximum chain coverage and—importantly—examine the most frequently employed approaches for determining grafting densities, i.e., dry thickness measurements, gravimetric assessment, and swelling experiments. An estimation of the reliability of these determination methods is provided via carefully evaluating their assumptions and assessing the stability of the underpinning equations. A practical access guide for comparatively and quantitatively evaluating the reliability of a given approach is thus provided, enabling the field to critically judge experimentally determined grafting densities and to avoid the reporting of grafting densities that fall outside the physically realistic parameter space. The assessment is concluded with a perspective on the development of advanced approaches for determination of grafting density, in particular, on single‐chain methodologies.     

导热绝缘PE-LD/PE-g-MAH/h-BN泡沫塑料的制备及性能研究

采用熔融共混法制备了一系列导热绝缘的低密度聚乙烯/马来酸酐接枝聚乙烯/六方氮化硼(PE-LD/PE-g-MAH/h-BN新型泡沫塑料，研究了相容剂PE-g-MAH的加入、h-BN含量对PE-LD/PE-g-MAH/h-BN泡沫体系导热性能、绝缘性能、力学性能及热稳定性的影响。结果表明，PE-g-MAH有利于增加PE?LD与h?BN的界面黏结，增强泡沫体系拉伸强度和断裂伸长率，显著提高其热导性能；当h-BN含量为30 %时， PE-LD/PE-g-MAH/h-BN泡沫体系的导热率为0.256 W/(m·K)，相对于PE-LD/h-BN泡沫体系的0.217 W/(m·K) 和纯PE-LD泡沫体系的0.039 W/(m·K)，热导率分别提高1.18和6.57倍，同时保持较好的绝缘性和热稳性。     

Bayesian threshold selection for extremal models using measures of surprise

Statistical extreme value theory is concerned with the use of asymptotically motivated models to describe the extreme values of a process. A number of commonly used models are valid for observed data that exceed some high threshold. However, in practice a suitable threshold is unknown and must be determined for each analysis. While there are many threshold selection methods for univariate extremes, there are relatively few that can be applied in the multivariate setting. In addition, there are only a few Bayesian-based methods, which are naturally attractive in the modelling of extremes due to data scarcity. The use of Bayesian measures of surprise to determine suitable thresholds for extreme value models is proposed. Such measures quantify the level of support for the proposed extremal model and threshold, without the need to specify any model alternatives. This approach is easily implemented for both univariate and multivariate extremes.     

Assessment of plastic damage in high strength structural steel using electromagnetic technique

ABSTRACT

High strength structural (HSS) steels are widely used in many construction equipment and infrastructures. Currently, there is a substantial demand of Non-Destructive Testing of these steel structures. In this research, the electromagnetic technique was utilised to assess the plastic damage in a typical HSS steel (i.e. Q690 steel). The results showed that the magnetic hysteresis loop of this material changed apparently after the tensile deformation, and the relevant magnetic parameters such as coercivity, remanence, and maximum permeability were sensitive to the cumulative plastic strain. The results demonstrated that there is a good coherence between the plastic damage evolution and the magnetic parameters variation, which indicated that the electromagnetic technique is an effective method to assess the plastic damage of HSS steel.     

Generalized F-discrepancy-based point selection strategy for dependent random variables in uncertainty quantification of nonlinear structures

In the performance evaluation of structures under disastrous actions, for example, earthquakes, it is important to take into account the randomness of structural parameters. Generally, these random parameters are treated either as independent or perfectly dependent, but practically they are partly dependent. This article aims at developing a point selection strategy for uncertainty quantification of nonlinear structures involving probabilistically dependent random parameters characterized by copula function. For this purpose, the point selection strategy for structures involving independent basic variables is first revisited. As an improvement, a generalized F-discrepancy diminishing oriented iterative screening algorithm is proposed. Then, combining with the conditional sampling method, a conditional point set rearrangement method and a conditional iterative screening-rearrangement method are proposed for probabilistically dependent variables. These new point selection strategies are readily incorporated into the probability density evolution method for uncertainty quantification of nonlinear structures involving dependent random parameters, which is characterized by copula function. The proposed methods are illustrated by two examples including a shear frame with hysteretic restoring forces and a reinforced concrete frame structure with the damage constitutive model of concrete, where the material parameters are probabilistically dependent. The results demonstrate the effectiveness of the proposed method. Problems to be studied are discussed.     

Estimation of plasma parameters in the process of micro-scale powder plastic and characteristics of its products

The temperature and density of plasma jets were estimated with a Boltzmann plot and Stark broadening of Ar I (696.54 nm) lines by optical emission spectroscopy (OES) in the process of plasma plastic, and the morphology and microstructure of tungsten (W) powders were investigated by scanning electron microscope (SEM) and x-ray Diffraction (XRD), respectively. The results show that the assumption of local thermodynamic equilibrium (LTE) was invalid at the end of the plasma jets, and earlier than this after the injection of tungsten powder. The temperature and electron density of the plasma jets were up to about T=6797 K with Q_c=50 slpm and n_e=1.05×10¹⁶ cm⁻³ with Q_s=115 slpm at Z=60 mm, respectively, and both dropped rapidly with the injected tungsten powders of 20 μm. After the plasma plastic process, the spherical tungsten powders were prepared and there were some satellite particles on the surface of the spherical products. The tungsten powders were both composed of a single equilibrium α-W phase with a body centered cubic (bbc) crystal structure before and after plasma treatment.     

Application Of Superconducting High Gradient Magnetic Separation Technology On Silica Extraction From Iron Ore Beneficiation Tailings

Comprehensive utilization of tailings resources not only solves environmental problems but also creates huge economic benefits. In this paper, fine weakly magnetic particles were extracted from iron ore beneficiation tailings and silica (SiO₂) was separated and purified using a superconducting high gradient magnetic separator. Based on plenty of contrast experiments, SiO₂ content of the magnetic concentrate could be increased from 68.67% to 92.57% with the selected optimal parameter combination, i.e., a magnetic flux density of 3.2 T, a pulp concentration of 40 g/L and a slurry flow velocity of 500 mL/min. The concentrate can be utilized as low-grade normal SiO₂ sands for industrial use, or act as the raw materials for preparing high-purity SiO₂ product.     

Fatigue strength of steel rollers with failure occurring at the weld root based on the local strain energy values: modelling and fatigue assessment

Weldments geometry with failures occurring at the weld toe or at the weld root cannot, by its nature, be precisely defined. Parameters such as bead shape and toe or root radius vary from joint to joint even in well-controlled manufacturing operations. The worst case configuration can be achieved by modelling as a sharp, zero radius, notch both the toe and the weld root. The intensity of asymptotic stress distributions obeying Williams’ solution is quantified by means of the Notch Stress Intensity Factors (NSIFs). For steel welded joints with failures originated from the weld roots, where the lack of penetration zone is treated as a crack-like notch, units for NSIFs are the same as conventional SIFs used in LEFM. The different dimensionality of NSIFs for different notch opening angles does not allow a direct comparison of failures occurring at the weld toe or at the weld root. In order to overcome the problem related to the variability of the V-notch opening angle, a simple scalar quantity, i.e. the value of the strain energy density (SED) averaged in the structural volume surrounding the notch tip, has been introduced. This energy is given in closed form on the basis of the relevant NSIFs for modes I, II and III. The radius R_c of the averaging zone is carefully identified with reference to conventional arc welding processes being equal to 0.28 mm for welded joints made of steel.The local-energy based criterion is applied here to steel welded rollers produced by Rulmeca and subjected to prevailing mode I (with failures at the weld root). The aim of the paper is firstly to describe the employed methodology for the fatigue assessment and secondly to show the first synthesis of fatigue data by means of local SED for a specific geometry.     

Predictions of phase stabilities,electronic structures and optical properties of potential superhard WB3

For better synthesis and use of WB₃ triborides surface coatings, structural, electronic and optical properties of hexagonal and trigonal hP4, hP8, hP16 and hR24-WB₃ triborides were predicted through the first-principles calculation. The formation enthalpies revealed that all of these WB₃ are energetically stable, and hR24-WB₃ is the best stable phase. Electronic properties showed that these WB₃ have strong B-B covalent bonds mixed with W-B covalent-ionic bonds. The calculated reflectivity that these WB₃ triborides can be used as optical shielding devices for FUV radiation. Moreover, Nd-YAG laser with wavelength of 1064?nm is available for synthetizing hP4, hP8 and hP16-WB₃ coatings, while the laser with wavelength of 532?nm is suitable for hR24-WB₃ coatings. The calculated static dielectric constants and refractive index indicated that the optical anisotropy for these WB₃ is in a sequence of hP8-WB₃ >?hP4-WB₃ >?hP16-WB₃ >?hR24-WB₃.     

Influence of the grain structure of yttria thin films on the hydrogen isotope permeation

Steel components are required in the infrastructure and the facilities of the hydrogen economy. The high hydrogen pressures in the hydrogen economy lead to embrittlement and surface corrosion of the steels. For the functionality of the facilities it is necessary to suppress the embrittlement and the surface corrosion of the steels by protective layers, e.g. ceramic thin films. With regard to fusion power plants ceramic thin films on the structural steel materials are also required. These thin films work as a tritium permeation barrier that is necessary to prevent the loss of the radioactive fuel inventory. Oxide thin films, e.g. Al₂O₃, Er₂O₃, and Y₂O₃, are promising candidates as tritium permeation barrier layers. In terms of the application in the first wall, this is especially true for yttrium due to its favorably short decay time after neutron activation compared to the other candidates. The Y₂O₃ layers with thicknesses of 0.5 μm–1 μm are deposited on both substrate sides by RF magnetron sputter deposition. Since the microstructure of the barrier layer plays an important role for the permeation reduction, layers with three different magnetron process modes and thus three different microstructures are prepared. After annealing the cubic crystal structure of all thin films is verified by X-ray diffraction and the different microstructures are investigated by scanning electron microscopy and transmission electron microscopy. The Y₂O₃ stoichiometry of all thin films and a chromium oxide material segregation at the interface are verified by analysis methods such as X-ray photoelectron spectroscopy. The permeation reduction factors of all thin films are determined in gas-driven deuterium permeation experiments. Corresponding to the three different microstructures, reduction factors of 25, 45, and 1100 are identified. Thus, the permeation reduction is strongly dependent on the Y₂O₃ microstructure. The measurement results suggest that a high density of grain boundaries leads to a high hydrogen permeation.