The electron transfer in the donor-polymer-acceptor molecular complex

The electron transfer in the donor-polymer-acceptor molecular complex was investigated. The carbon chain with various numbers of atoms, which has two possible conformations (homogeneous and dimerizided), was taken as a polymer. A theoretical model that described such systems was proposed, and the probability of detection of the electron at the end of the chain that originally is localized on the first atom was theoretically investigated. Two types of electron transfer were revealed.     

Synthesis and optical properties of poly(2,11-triphenyleneethynylene-alt-m-phenyleneethynylene)s

Triphenylene units are introduced into the backbone of a poly(m-phenyleneethynylene) (mPE). The new polymer, mPPET, exhibits strong intra-chain folding in good solvents in which the polymer is soluble, such as chloroform, benzene, and THF. In poor solvents, where the polymer is insoluble, such as acetonitrile, hexane, or DMSO, interchain aggregation dominates. In a solvent mixture of a good solvent and a poor solvent with appropriate ratios, mPPET appears to adopt a random coil conformation. Among the three conformations (intra-chain folding, interchain aggregation, and random coil chain), the intra-chain polymeric foldamer exhibits the highest fluorescence quantum yield. These results indicate that introducing a planar polynuclear aromatic compound into mPE not only increases its intra-chain folding propensity, but also may enhance its fluorescent properties. This approach may be applied to synthesize a variety of conjugated foldamers containing planar polynuclear aromatic hydrocarbons with interesting photophysical properties.     

The modeling of interfaces: Atoms or continua?

Interfaces are a central microstructural unit responsible for properties ranging from the capacity for void nucleation to enhanced diffusion rates. Demand for materials with specific properties makes the tailoring of interfaces a central goal in materials modeling. In the consideration of detailed defect geometries or nanoscale effects, atomistic modeling provides reliable insights. On the other hand, the response of materials to macroscopic loads is often best treated within a continuum framework. This overview reviews both atomistic and continuum models of interfaces with an eye to the possible relationships between them. The problems of void nucleation and grain boundary segregation are used as examples to elucidate the progress that can be made on both the continuum and atomistic fronts as well as the limitations of each. These approaches are further contrasted through consideration of the example of elastic wave propagation at interfaces. The treatment of waves localized at interfaces yields distinct predictions depending upon whether an atomistic or continuum analysis is made.     

Simulation of Fracture Nucleation in Cross-Linked Polymer Networks

A novel atomistic simulation method is developed whereby polymer systems can undergo strain-rate-controlled deformation while bond scission is enabled. The aim is to provide insight into the nanoscale origins of fracture. Various highly cross-linked epoxy systems including various resin chain lengths and levels of nonreactive dilution were examined. Consistent with the results of physical experiments, cured resin strength increased and ductility decreased with increasing cross-link density. An analysis of dihedral angle activity shows the locations in the molecular network that are most absorptive of mechanical energy. Bond scission occurred principally at cross-link sites as well as between phenyl rings in the bisphenol moiety. Scissions typically occurred well after yield and were accompanied by steady increases in void size and dihedral angle motion between bisphenol moieties and at cross-link sites. The methods developed here could be more broadly applied to explore and compare the atomistic nature of deformation for various polymers such that mechanical and fracture properties could be tuned in a rational way. This method and its results could become part of a solution system that spans multiple length and time scales and that could more completely represent such mechanical events as fracture.     

光致弯曲聚合物的材料特性测试

一种具有双向形变记忆功能的光致弯曲聚合物材料,在光驱动型执行器中应用潜力较大。材料的力学性能及光致弯曲特性,是执行器结构设计和分析的必要依据。通过拉伸实验,了解到材料具有线弹性材料类似的力学性能,并得到了其弹性模量。通过对材料光致弯曲效应的分析,推导出光致弯曲曲率的表达式。利用该公式,对光致弯曲实验观测到的数据进行非线性拟合,得到材料的光致弯曲特性参数。计算曲率较好地符合了实验数据,证明了公式和参数的合理性。     

Determination of the elastic and plastic properties of materials through instrumented indentation with reduced sensitivity

By considering the complete loading and unloading response characteristics of multiple sharp indentations with differing indenter apex angles, a new approach has been developed to extract the elastic and plastic properties of materials. Considerable reduction in the sensitivity characteristics of all the indentation parameters invoked in the reverse analysis for the identification of the elastic and plastic properties of the indented material is realized. The reduction in sensitivity obtained using the present approach is attributed to an optimization process that identifies the material properties that best describe all the available information from multiple indentations. A comprehensive comparison of several multiple indentation methods for a large number of material combinations illustrates that the triple indentation method that does not utilize representative stresses and the quadruple indentation method that invokes representative stresses provide the least sensitivity in the determination of elastic and plastic properties.     

Robust technology chain design: considering undesired interactions within the technology chain

Existing methodologies in technology chain design are used to plan the deployment of manufacturing technologies under consideration of interactions between these technologies and workpiece properties. Present methodologies focus on workpiece characteristics which the technologies are designated to change. However, workpiece properties can also be negatively affected because of interactions between the manufacturing technologies and features which the technologies are not supposed to change. These undesired interactions can cause a lower quality of the produced parts and an increased amount of defective parts. In this paper, a new methodology is presented which enables the user to identify undesired interactions during the technology chain design process. Firstly, the product to be manufactured is analyzed and described as a set of individual features. Secondly, feature-specific technology chains are designed under consideration of possible undesired interactions. Thirdly, the individual feature-specific technology chains are merged to generate a robust technology chain for the manufacturing of the analyzed product. Since undesired interactions usually occur during production ramp-ups for the first time, the methodology is applied to a case study concerning a ramp-up in the automotive industry. In this context, improving the process stability by preventing the occurrence of undesired interactions is of high economic importance.     

Modeling the mechanical properties of optimally processed cordierite–mullite–alumina ceramic foams by X-ray computed tomography and finite element analysis

Bulk and cellular cordierite ceramics were prepared from a non-stoichiometric powder consisting of corundum, talc (triclinic), α-quartz, K-feldspar, kaolinite, mullite and a small amount of a glass phase. The optimal sintering processing route was evaluated to obtain good mechanical properties. A high flexural strength of 120 MPa and a Young’s modulus of 99 GPa were achieved. The ceramic foams were fabricated by impregnation of polymer preforms with the optimized stock suspension. The mechanical properties of ceramic foams were studied by impulse excitation and compression tests. The Gibson–Ashby model predicted the ceramic foam’s effective modulus and its elastic limit strength well, as measured experimentally. In addition, the actual three-dimensional (3-D) structure obtained from X-ray computed tomography (CT) coupled with the finite element method (FEM) was used to calculate the Young’s modulus and the elasticity limit of the ceramic foam; however, this did not produce aby better agreement between the calculated values and the experimental results. The discrepancy between the Gibson–Ashby model and FEM could probably be attributed to the accuracy and small volume of representative reconstructed 3-D cellular structure. Taking account of the effect of the internal hollow structure on the stress localization in the ceramic struts, the CT–FE modeling provides a good measure of the adaptability and predictability of actual ceramic foam structures for realistic damage modeling.     

Flat indentation of a viscoelastic polymer film on a rigid substrate

A systematic method of flat indentation was developed to measure the elastic and viscoelastic properties of polymer films. A flat indentation problem on an elastic film perfectly bonded to a rigid substrate was revisited, from which the relationship between the applied force and the penetration depth was obtained in a simple form. Application of the elastic–viscoelastic correspondence principle converts the force–depth relationship for elastic films to the Laplace transform of that for viscoelastic films. Indentation experiments with a flat diamond tip were performed on polymer films (SU-8 and NR4-8000P). Analysis of the measured data with the viscoelastic force–depth relationship provides the shear moduli, Poisson’s ratios, and relaxation moduli of these films. Viscoplastic deformations produced in the films that underwent the flat indentation process were quantified by measuring the residual deformation after unloading with an atomic force microscope.     

Electronic and structural consequences of n-doping: bithiazole oligomers and partially reduced bithiazolium cations

N-methylated-poly(diakyl-bithiazoles), NPABTs, can be reduced to give reasonably stable n-doped polymers. In order to understand the changes in the electronic structure and polymer conformations upon reductive doping of NPABTs, a series of oligomers of increasing length was studied by self-consistent field (AM1) and configuration interaction (ZINDO/S) calculations. Large red shifts in the optical transitions are predicted upon n-doping, along with significant planarization of chain segments. Full planarization was not realized for n-doped materials, however, as a variety of localized units were observed.     

Modeling of dynamic micro-milling cutting forces

This paper investigates the mechanistic modeling of micro-milling forces, with consideration of the effects of ploughing, elastic recovery, run-out, and dynamics. A ploughing force model that takes the effect of elastic recovery into account is developed based on the interference volume between the tool and the workpiece. The elastic recovery is identified with experimental scratch tests using a conical indenter. The dynamics at the tool tip is indirectly identified by performing receptance coupling analysis through the mathematical coupling of the experimental dynamics with the analytical dynamics. The model is validated through micro end milling experiments for a wide range of cutting conditions.     

Thermal Modeling of Direct Digital Melt-Deposition Processes

Additive manufacturing involves creating three-dimensional (3D) objects by depositing materials layer-by-layer. The freeform nature of the method permits the production of components with complex geometry. Deposition processes provide one more capability, which is the addition of multiple materials in a discrete manner to create “heterogeneous” objects with locally controlled composition and microstructure. The result is direct digital manufacturing (DDM) by which dissimilar materials are added voxel-by-voxel (a voxel is volumetric pixel) following a predetermined tool-path. A typical example is functionally gradient material such as a gear with a tough core and a wear-resistant surface. The inherent complexity of DDM processes is such that process modeling based on direct physics-based theory is difficult, especially due to a lack of temperature-dependent thermophysical properties and particularly when dealing with melt-deposition processes. In order to overcome this difficulty, an inverse problem approach is proposed for the development of thermal models that can represent multi-material, direct digital melt deposition. This approach is based on the construction of a numerical-algorithmic framework for modeling anisotropic diffusivity such as that which would occur during energy deposition within a heterogeneous workpiece. This framework consists of path-weighted integral formulations of heat diffusion according to spatial variations in material composition and requires consideration of parameter sensitivity issues.     

Three-dimensional phase-field modeling of martensitic microstructure evolution in steels

In the present work a 3-D elastoplastic phase-field (PF) model is developed, based on the PF microelasticity theory proposed by A.G. Khachaturyan and by including plastic deformation as well as anisotropic elastic properties, for modeling the martensitic transformation (MT) by using the finite-element method. PF simulations in 3D are performed by considering different cases of MT occurring in an elastic material, with and without dilatation, and in an elastic perfectly plastic material with dilatation having isotropic as well as anisotropic elastic properties. As input data for the simulations the thermodynamic parameters corresponding to an Fe-0.3%C alloy as well as the physical parameters corresponding to steels acquired from experimental results are considered. The simulation results clearly show autocatalysis and morphological mirror image formation, which are some of the typical characteristics of a martensitic microstructure. The results indicate that elastic strain energy, anisotropic elastic properties, plasticity and the external clamping conditions affect MT as well as the microstructure.     

Mechanical properties and bonding feature of the YAg,CeAg, HoCu,LaAg, LaZn,and LaMg rare-earth intermetallic compounds: An ab initio study

Full-potential linearized augmented plane wave (FLAPW) method has been employed within the generalized gradient approximation (GGA) to investigate structural and elastic properties of YAg, CeAg, HoCu, LaAg, LaZn, LaMg compounds. The calculated ground state properties such as lattice constants, bulk Modulus and elastic constants agree well with the experiment. The ductility or brittleness of these intermetallic compounds is predicted. The calculated results indicate that LaAg is the most ductile amongst the present compounds. For HoCu and LaZn compounds, the mechanical properties and Debye temperature are predicted from calculated elastic constants. In addition, chemical bonding of these compounds has been investigated in the light of topological analysis approach based on the theory of atoms in molecules.     

Investigation of residual stress by the indentation method with the flat cylindrical indenter

Experiments and numerical simulations have been carried out to study residual stress of copper specimens by the identation method with a flat cylindrical indenter. Copper specimens were annealed at different temperatures for 35 min to obtain different residual stress levels. The experiments carried out on these specimens demonstrated the influence of residual stress on indentation behavior. The influence of annealing temperature on the elastic-plastic transition region is quite obvious. A method has been presented to determine material properties, such as elastic modulus and Poisson ratio. This method can also be applied to determine residual stress with the assumption of knowing the yield stress in advance. The advantage of this method is that it can avoid calculating the contact area. In the finite element modeling (FEM), residual stresses on copper specimens are simulated by preapplying stresses. The influence of residual stress on the indentation load-depth curves has been studied by FEM. There is good agreement between experimental and FEM numerical results. A numerical method has also been presented to determine residual stress. In addition, Mises stress and plastic distribution ahead of the indenter have also been studied to help us further understand the influence of residual stress.     

Temperature-dependent anisotropic material modeling of the sheet metal component within the polymer injection forming process

In the course of this work, an extended material model for a carbon steel sheet metal has been developed based on Hill’48 yield criterion, considering temperature-dependent plastic anisotropy coefficients. This material model is applied on a polymer injection forming process in which the sheet metal heats up to a critical forming temperature through the contact with the plastic melt. At this temperature range blue brittleness occurs. The elastic properties, the yield stress as well as the plastic anisotropy coefficients of the sheet material become significantly different compared to those at room temperature. It should be emphasized that especially temperature-dependent anisotropy coefficients are not yet considered in most common material models. With the help of the presented modelling approach a more precise modelling of the temperature-dependent carbon steel material behavior can be realised.     

A mechanistic approach to extract the mechanical properties of individual constituents in plasma-sprayed metal matrix composite coatings

A mechanistic approach to determine the in-situ properties of individual constituents in a plasma sprayed metal matrix composite (MMC) coating was proposed. The approach was based on micro-indentation and inverse analysis techniques. Utilising the indentation data obtained from the micro-indentation experiments, elastic moduli of each constituent were calculated using a well-established method whereas yield strength and hardening exponent were extracted using the inverse procedure based on finite element analysis. Finite element results gave a satisfactory agreement between the numerically simulated and the measured indentation load-depth curves. Further studies using three dimensional finite element analyses of Vickers indentation on the MMC coating based on its actual microstructure also showed that the indentation behaviour of the MMC coatings is strongly dependent on its morphology, volume fraction, size and distribution of the reinforcing phase.     

OLEDs based on new oxadiazole derivatives

The absorption, photoluminescence and electroluminescence properties of systematically modified poly(1,3,4-oxadiazole)s are reported and compared with low molar mass compounds. Poly(1,3,4-oxadiazole)s are functionalised by introducing pendent alkyl side chains and tetraphenylsilane or hexafluoroisopropylidene group (6F) into the main chain, respectively. The photo-and electroluminescence of single layer devices was found to be in the blue and green spectral range. A further strategy was to start with the optimisation of the substituents of low molar mass compounds followed by bonding the optimised structure to a flexible PMMA main chain. It was demonstrated that it is possible to preserve the optical properties of the oxadiazole unit and at the same time to improve film forming properties of the final polymer.     

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 硬度是一个衡量材料性能的重要物理量,是体现材料弹性、塑性、强度和韧性等力学性能的综合指标。主要从硬件及软件上讨论了针对软质泡沫聚合材料硬度的数字测试系统。该系统应用了STC12系列带A/D转换的单片机,高精度压力传感器以及可控电机,来保证测试的稳定性和高精度。实践证明,该系统满足GB/T 10807－2006/ISO 2439:1997要求,可广泛用于在国家标准要求的A法、B法、C法下对标准尺寸的海绵、泡沫等试样的硬度进行准确的测试。     

Stress-assisted martensitic transformations in steels: A 3-D phase-field study

A 3-D elastoplastic phase-field model is developed for modeling, using the finite-element method, the stress-assisted martensitic transformation by considering plastic deformation as well as the anisotropic elastic properties of steels. Phase-field simulations in 3-D are performed by considering different loading conditions on a single crystal of austenite in order to observe the microstructure evolution. The thermodynamic parameters corresponding to an Fe–0.3% C steel as well as the physical parameters corresponding to commercial steels, acquired from experimental results, are used as input data for the simulations. The simulation results clearly show the well-known Magee effect and the Greenwood–Johnson effect. The results also show that even though the applied stresses are below the yield limit of the material, plastic deformation initiates due to the martensitic transformation, i.e. the well-known transformation-induced plasticity (TRIP) phenomenon. It is concluded that the loading conditions, TRIP as well as autocatalysis play a major role in the stress-assisted martensitic microstructure evolution.