Hybrid Adaptive Large Neighborhood Search for the Optimal Statistic Median Problem

In this paper, the problem of maximizing the median of a convex combination of vectors having important applications in finance is considered. The objective function is a highly nonlinear, nondifferentiable function with many local minima and the problem was shown to be APX hard. We present two hybrid Large Neighborhood Search algorithms that are based on mixed-integer programs and include a time limit for their running times. We have tested the algorithms on three testbeds and showed their superiority compared to other state-of-the-art heuristics for the considered problem. Furthermore, we achieved a significant reduction in running time for large instances compared to solving it exactly while retaining high quality of the solutions returned.     

An iterative approach to determine the complexity of local models for robust identification of nonlinear systems

In this paper, a new multi-model approach is proposed for identification of nonlinear systems. In similar identification methods, the operating space is partitioned and a local model is suggested for each partition. In such approaches, since the same linear structure is often used for all local models; huge number of local linear models is usually required to reasonably model an operating region with severely nonlinear dynamics. Therefore the size of the global model may exponentially increase; and as a result model robustness may decrease. In the proposed approach the best model structure is selected for the particular nonlinear study system in an iterative approach. At each iteration, a choice is made to increase number of local models and/or increase the local model complexity. Furthermore, it determines the complexity of local models based on increasing the model accuracy and ensuring the model robustness. In order to optimize the model approximation capability and model robustness, a model term selection approach based on a forward orthogonal least squares algorithm and a criterion that minimizes the sum of the variance of the parameter estimates is applied. Simulation results show that the proposed method results in an excellent validation performance with fewer parameters.     

Vapor-mediated melt infiltration for synthesizing SiC composite matrices

A two-step synthesis of SiC involving initial exposure of carbon surfaces to Si vapor, followed by Si melt infiltration, is investigated in this article with emphases on the mechanisms, kinetics, and microstructure evolution. Interrupted differential thermal analysis of amorphous C and Si powder mixtures and microstructure characterization are used to generate insight into the stages of the reaction. Exposure to Si vapor yields a SiC layer with nano-scale porosity driven by the volume change associated with the reaction. This forms a continuous pore network that promotes subsequent melt access to the reaction front with the C. While the pores remain open, the vapor phase reaction proceeds at a nearly constant rate and exhibits a strong temperature sensitivity, the latter due in part to the temperature sensitivity of the Si vapor pressure. The implications for enhancing the reactive melt infiltration process and fabrication of SiC matrices for ceramic composites are discussed.     

Synthesis and physicochemical investigation of imide-functionalized silica nanocomposites

The incorporation of inorganic nanoparticles into polymers have gained significant attention to improving functional properties. The ultimate nanocomposite behaviors are influenced by many parameters, such as microstructural distribution that are produced during the treatment process. Herein, a hybrid material integrating a modified network into a polyimide PI matrix was produced via the sol–gel method by the reaction of pyromellitic dianhydride, 4, 4-oxydianaline, and 1, 5-diaminonaphthalene to synthesize copolyimides nanocomposite. The modified polyimide and unmodified polyimide silica (SiO₂) nanoparticles were incorporated in the polyimide matrix to have polyimide silica nanocomposite. In modified silica nanoparticles, 3-aminopropyltriethosilane was introduced to have better compatibility among inorganic–organic hybrid with similar chemical contact due to their flexible alkyl group. The surface morphology or structure of silica and polyimide was affirmed by scanning electron microscopy, Fourier transforms infrared spectroscopy confirmed the synthesis of pure polyimide, unmodified polyimide, and modified polyimide silica via presence and absence of certain peaks. Thermogravimetric analysis (TGA) results showed high thermal stability of nanocomposites as silica content increases. In contrast to unmodified silica, the modified silica provides more thermal stability to the nanocomposites. Dynamic mechanical analysis was used to investigate the tensile stress of pure polyimide, unmodified, and modified silica nanocomposites. Thermal stability, storage modulus, and moisture absorption of these hybrid materials were improved with silica nanoparticles. The TG mass spectrum confirms the successful synthesis of modified silica networks. The substituted silica nanoparticles show higher mechanical toughness and storage in modified compared to unmodified silica nanocomposite, which exhibits stronger binding attraction between silica nanoparticles and polyimide matrix.     

NMOSFET ESD self-protection strategy and underlying failuremechanism in advanced 0.13-μm CMOS technology

In this paper, the high current characteristics encountered during electrostatic discharge (ESD) stress using nMOS/L_npn protection devices in a 0.13-μm CMOS technology are investigated for different device parameters: channel length, channel width, gate-oxide thickness, and drain/source contact to gate (DCG/SCG) spacing. From leakage current measurements following ESD stress, it is concluded that the shorter (0.13 μm) devices fail because of source/drain filamentation, whereas longer (0.3 μm) devices with thin (22 Å) oxide gate fail because of oxide breakdown. This conclusion is consistent with and supported by numerical simulations of the electric field. It is also supported by the observed effect of hot carrier stress on I_t2. Hot carrier stress experiments additionally revealed that ESD stress can and does affect subsequent hot carrier degradation of the device     

Electrochemical sensor investigation of carbon-supported PdCoAg multimetal catalysts using sugar-containing beverages

Novel PdCoAg/C nanostructures were successfully synthesized by the polyol method in order to develop electrocatalysts, related to the glucose sensor performance of the high glycemic index in beverages. The characterization of this novel PdCoAg/C electrocatalyst was performed by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy equipped with energy dispersive X-ray. The characterization results revealed that electronic state of the PdCoAg/C electrocatalyst was modified by the addition of the third metal. The electrochemical performances of the sensor were investigated by cyclic voltammetry and differential pulse voltammetry. The prepared enzyme-free sensor exhibited excellent catalytic activity against glucose with a wide detection range (0.005 to 0.35 mmol∙L⁻¹), low limit of detection (0.003 mmol∙L⁻¹), high sensitivity (4156.34 µA∙mmol⁻¹∙L∙cm⁻²), and long-term stability (10 days) because of the synergistic effect between the ternary metals. The glucose contents of several energy drinks, fruit juices, and carbonated beverages were analyzed using the novel PdCoAg/NGCE/C sensor system. These results indicate the feasibility for applications in the foods industry.     

Strong carbon nanofibers from electrospun polyacrylonitrile

Strong carbon nanofibers with diameters between 150 nm and 500 nm and lengths of the order of centimeters were realized from electrospun polyacrylonitrile (PAN). Their tensile strength reached a maximum at 1400 °C carbonization temperature, while the elastic modulus increased monotonically until 1700 °C. For most carbonization temperatures, both properties increased with reduced nanofiber diameter. The tensile strength and the elastic modulus, measured from individual nanofibers carbonized at 1400 °C, averaged 3.5 ± 0.6 GPa and 172 ± 40 GPa, respectively, while some nanofibers reached 2% ultimate strain and strengths over 4.5 GPa. The average tensile strength and elastic modulus of carbon nanofibers produced at 1400 °C were six and three times higher than in previous reports, respectively. These high mechanical property values were achieved for optimum electrospinning parameters yielding strong PAN nanofibers, and optimum stabilization and carbonization temperatures, which resulted in smooth carbon nanofiber surfaces and homogeneous nanofiber cross-sections, as opposed to a previously reported core–shell structure. Turbostratic carbon crystallites with average thickness increasing from 3 to 8 layers between 800 °C and 1700 °C improved the elastic modulus and the tensile strength but their large size, discontinuous form, and random orientation reduced the tensile strength at carbonization temperatures higher than 1400 °C.     

Hygrothermal effects on tensile and fracture properties of epoxy filled with inorganic fillers having different reactivity to water

The study investigated the effects of ageing on the tensile properties and fracture behaviour of an epoxy filled with inorganic fillers having different reactivity to water, such as fly ash, calcium carbonate (CaCO₃), and Portland cement. CaCO₃ is insoluble in water, whereas fly ash and cement are reactive to water; however, fly ash is less reactive than cement. The water absorption, tensile properties, and mode-I fracture toughness of the epoxy containing 7?wt% of filler were compared after ageing the composites in distilled water at 50?°C. Gravimetric analyses showed that compared to neat epoxy, CaCO₃ slightly decreased the equilibrium water uptake of the filled epoxy, whereas fly ash and cement increased the equilibrium water uptake. Compared to the other fillers, the tensile strength of CaCO₃-filled epoxy was inferior in both dry and wet conditions. However, in dry condition, the elastic modulus of CaCO₃-filled epoxy was slightly higher. Overall, the fly ash-filled epoxy showed better tensile and fracture properties in both dry and wet conditions although it absorbed more water than the other fillers did. Scanning electron microscopy revealed that crack deflection played a dominant role in the toughening of filled epoxies in both dry and wet conditions.     

Exploitation of Nanobifiller in Polymer/Graphene Oxide–Carbon Nanotube,Polymer/Graphene Oxide–Nanodiamond,and Polymer/Graphene Oxide–Montmorillonite Composite: A Review

In this article, a comprehensive review is presented regarding structure, synthesis, and properties of nanofillers such as graphene oxide, nanobifiller of graphene oxide, and their polymeric nanocomposite. The information about hybrid properties and synthesis of graphene oxide–carbon nanotube, graphene oxide–montmorillonite, and graphene oxide–nanodiamond is presented. Use of nanobifiller in polymer/graphene oxide–carbon nanotube, polymer/graphene oxide–montmorillonite, and polymer/graphene oxide–nanodiamond composites was summarized. Area of polymer and graphene oxide-based nanobifiller composites is less studied in literature. Therefore, nanobifiller technology limitations and research challenges must be focused. Polymer/graphene oxide nanobifiller composites have a wide range of unexplored potential in technological areas such as automobile, aerospace, energy, and medical industries.