Interfacial properties and microfailure degradation mechanisms of bioabsorbable fibers/poly-l-lactide composites using micromechanical test and nondestructive acoustic emission

Interfacial properties and microfailure degradation mechanisms of the bioabsorbable composites for implant materials were investigated using micromechanical technique and nondestructive acoustic emission (AE). The tensile strength of absorbable fibers with hydrolysis was analyzed statistically using either uni- or bimodal Weibull distribution. As hydrolysis time increased, the tensile strength, the modulus and the elongation of poly(ester-amide) (PEA) and bioactive glass fibers decreased, whereas those of chitosan fiber almost did not change. Interfacial shear strength (IFSS) between bioactive glass fiber and poly-l-lactide (PLLA) was much higher than PEA or chitosan fiber/PLLA systems using dual matrix composite (DMC) specimen. The decreasing rate of IFSS was the fastest in bioactive glass fiber/PLLA composites whereas that of chitosan fiber/PLLA composites was the slowest. Work of adhesion, W_a between bioactive glass fiber and PLLA was the highest, and the wettability results were consistent with the IFSS. AE energies of PEA fiber decreased gradually, and their distributions became narrower than those in the initial state with hydrolysis time. In case of bioactive glass fiber, AE energies in tensile failure were much higher than those in compression. In addition, AE parameters at the initial state were much higher than those after degradation under both tensile and compressive tests. Interfacial properties and microfailure degradation mechanisms can be important factors to control bioabsorbable composite performance.     

Two-layer observer based control for a class of uncertain systems with multi-frequency disturbances

A novel disturbance estimation approach is presented for a class of uncertain systems subject to multiple-sinusoidal disturbances with unknown frequencies. Different from existing results on disturbance observer based control (DOBC), a new methodology with a two-layer observer structure is developed to effectively estimate and reject the disturbances. In the proposed control architecture, an auxiliary observer is derived to generate a disturbance representation in a parametric uncertainty form. Furthermore, the unknown parameters can be reduced to a constant vector with the dimension of the number of harmonic components in the disturbances. Then an augmented observer is designed to estimate the corresponding unknown parameters of the disturbances. As a result, the uncertain systems with disturbances constituting of multiple unknown-frequency sinusoidal components can be controlled within the DOBC framework, where asymptotic stability can be guaranteed. The proposed approach is successfully validated on a robotic manipulating example.     

A novel hybrid adaptive collaborative approach based on particle swarm optimization and local search for dynamic optimization problems

This paper proposes a novel hybrid approach based on particle swarm optimization and local search, named PSOLS, for dynamic optimization problems. In the proposed approach, a swarm of particles with fuzzy social-only model is frequently applied to estimate the location of the peaks in the problem landscape. Upon convergence of the swarm to previously undetected positions in the search space, a local search agent (LSA) is created to exploit the respective region. Moreover, a density control mechanism is introduced to prevent too many LSAs crowding in the search space. Three adaptations to the basic approach are then proposed to manage the function evaluations in the way that are mostly allocated to the most promising areas of the search space. The first adapted algorithm, called HPSOLS, is aimed at improving PSOLS by stopping the local search in LSAs that are not contributing much to the search process. The second adapted, algorithm called CPSOLS, is a competitive algorithm which allocates extra function evaluations to the best performing LSA. The third adapted algorithm, called CHPSOLS, combines the fundamental ideas of HPSOLS and CPSOLS in a single algorithm. An extensive set of experiments is conducted on a variety of dynamic environments, generated by the moving peaks benchmark, to evaluate the performance of the proposed approach. Results are also compared with those of other state-of-the-art algorithms from the literature. The experimental results indicate the superiority of the proposed approach.     

Organic small-molecule heterointerface for use in transistor-type non-volatile memory

A chargeable layer is an essential element for charge transfer and trapping in a transistor-based non-volatile memory device. Here we demonstrate that a heterointerface layer comprising of two different small molecules can show electrical memory characteristics. The organic heterointerface layer was fabricated with a pentacene and tris(8-hydroxyquinoline) aluminum (Alq3) layers by sequential vapor deposition without breaking the vacuum state. Pentacene was adopted as the active layer on the top, and Alq3 was used as the bottom layer for charge trapping. The bottom-gate top-contact transistor with an organic heterointerface layer showed distinct non-volatile memory behaviors and showed high air stability and reliability. We investigated the energy structure of the pentacene/Alq3 heterointerface layer to reveal the operation mechanism of the non-volatile memory and suggested that the writing/erasing gate bias-dependent energy barrier originating from the difference between the energy levels of the pentacene and Alq3 layers controls the charge transfer at the heterointerface layer. Our approach suggests a simple way to fabricate heterointerface layers for organic non-volatile memory applications with high air stability and reliability.     

Experimental investigation and thermodynamic modeling of the Mo–Co–B ternary system

Among the ternary borides, the Mo–Co–B system is of great interest because of its excellent hardness, toughness, and stability performance. Eight samples with 60 at.% Co were designed to investigate the isothermal section of Mo–Co–B system at 1073 K in the Co-rich portion. Scanning electron microscopy, energy-dispersion spectroscopy, electron probe microanalysis, differential scanning calorimetry, and X-ray diffraction were used to investigate the phase equilibria of the samples. The formation enthalpies of the ternary borides were obtained by first-principles calculations to serve as key information for thermodynamic assessment. By coupling the reviewed experimental data from the literature, the presently determined phase equilibria, and the calculated formation enthalpies of the compounds, the thermodynamic parameters for the Mo–Co–B ternary system were optimized and used to calculate the isothermal sections, vertical section, and liquidus projection of the system. Comprehensive comparisons showed that the calculated results are in reasonable agreement with the reported phase diagram and thermodynamic data.     

Hierarchically structured Bi2MoxW1-xO6 solid solutions with enhanced piezocatalytic activities

Hierarchical structure Bi₂Mo_xW_1-xO₆ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) solid solutions with high surface area are prepared by hydrothermal method without employing any surfactant. All the as-prepared products are hierarchical microspheres self-assembled by nanosheets. The piezocatalytic performance of Bi₂Mo_xW_1-xO₆ solid solutions are investigated by the degradation of RhB under ultrasonic vibration. The experimental results show that the x value of Bi₂Mo_xW_1-xO₆ has great influence on the piezocatalytic efficiency. Among which, the Bi₂Mo_0.4W_0.6O₆ presents a high value of the rate constant k of 0.119 min^-1 by degrading 97.5% of RhB dye solution in 30 min. It is about 43.4% higher than that of pure Bi₂WO₆ sample, for which k is 0.083 min^-1. Radical trapping test indicates that holes (h⁺) are the dominating active species in the degradation process. The improved piezocatalytic performance for Bi₂Mo_0.4W_0.6O₆ sample ascribes to the larger piezoelectric potential generated under a strained state, which facilitates the transfer of charge carriers and accelerate the piezocatalytic process. Our work presents a new design strategy of piezocatalysts for remediation of water pollution.     

Defect energetics and stacking fault formation in high-entropy carbide ceramics

Inspired by the concept of entropy stabilization, multicomponent transition metal carbide (MTMC) ceramics have received increasing attention due to their extraordinary performances. However, the role of partial disorder in the transition metal sublattice on the defect properties in MTMCs is still elusive. In this work, we study defect formation and generalized stacking fault energies (GSFEs) in MTMCs. In both cases, we compare the results in MTMCs to binary TMCs. Our results suggest that C-related defects generally exhibit lower formation energies in MTMCs, suggesting that MTMCs are prone to off-stoichiometry with C vacancies. We further show that lower formation energies of C interstitials and higher migration energies of C vacancies account for the experimentally-observed delayed defect evolution. Finally, our calculated GSFEs in different MTMCs are close to the averages from all the constituent binary TMCs, indicating that the rule of mixture (ROM) can be applied to estimate the stacking fault energies for stoichiometric MTMCs.     

Powder-binder separation in injection moulded green parts

For powder injection moulding (PIM) the ceramic powder is mixed with a thermoplastic binder system to achieve an injectable feedstock. In contrast to injection moulding of polymeric components, the binder must be removed after the shaping step before sintering the ceramic part to full density. During the mould filling process shear forces act on the blend that might cause separations of powder particles and binder. In this case polymer films form at the mould surface and at internal interfaces which induce microstructural defects in the debinded part. In particular for multi-component parts this effect is critical since binder films in the joining zone weaken the bonding strength between the two components that might even lead to delamination.For detecting binder separations within the injection moulded bulk material and at joining zones of two-component parts the microstructure of green samples has been studied. Since conventional machining techniques like grinding and polishing modify the original structure, e.g. when particles are pulled out of the matrix and binder smears onto the surface, a special ceramographic method for the preparation of cross-sections was applied. This approach bases on broad ion beam techniques and enables the simultaneous polishing of hard ceramic particles and soft polymer molecules without destroying the structure or producing a relief at the surface. In the analysed samples binder accumulations were found along flow lines, at weld lines, at boundaries of so-called dead water regions and at the interface of two-component parts.     

45钢送丝激光熔覆温度场及应力场模拟

针对激光熔覆过程的特点,基于有限元分析软件ANSYS,对45钢送丝激光熔覆过程的温度场和应力场进行三维数值动态模拟.建模时采用实体单元和表面单元相结合的方式,采用参数化设计划分网格,施加热源载荷,设定初始条件和基本边界条件,对流边界条件进行求解.分析了激光熔覆过程中温度场在工件上的分布规律,揭示了整个时间历程上应力强度的变化规律,为提高激光熔覆质量提供了理论依据.     

Microstructure and high temperature tensile properties of Al–Si–Cu–Mn–Fe alloys prepared by semi-solid thixoforming

The differences in the microstructure and elevated temperature tensile properties of gravity die cast, squeeze cast, and semi-solid thixoformed Al–Si–Cu–Mn–Fe alloys after thermal exposure at 300 °C were discussed. The results demonstrate that the elevated temperature tensile properties of semi-solid thixoformed alloys were significantly higher than those of gravity die cast and squeeze cast alloys, especially after thermal exposure for 100 h. The ultimate tensile strength (UTS) of semi-solid thixoformed alloys after thermal exposure at 300 °C for 0.5, 10 and 100 h were 181, 122 and 110 MPa, respectively. The UTS values of semi-solid thixoformed alloys were higher than those of heat resistant aluminum alloys used in commercial applications. The enhanced elevated temperature tensile properties of semi-solid thixoformed experimental alloys after thermal exposure can be attributed to the combined reinforcement of precipitation strengthening and grain boundary strengthening due to thermally stable intermetallic phases as well as suitable grain size.