Controllability of non-linear delay systems

In this paper we examine the approximate controllability in the space W² × L² of non-linear systems with delays in states and controls. If the uncontrolled system is asymptotically stable and if the linear part of the control system is controllable, then the non-linear delay system is approximately controllable. An upper bound on the magnitude of retardation for the system to be controllable is estimated. An example is given to demonstrate the practical verification of the new conditions     

Engineering Design Processes: A Comparison of Students and Expert Practitioners

In this paper we report on an in‐depth study of engineering design processes. Specifically, we extend our previous research on engineering student design processes to compare the design behavior of students and expert engineers. Nineteen experts from a variety of engineering disciplines and industries each designed a playground in a lab setting, and gave verbal reports of their thoughts during the design task. Measures of their design processes and solution quality were compared to pre‐existing data from 26 freshmen and 24 seniors. The experts spent significantly more time on the task overall and in each stage of engineering design, including significantly more time problem scoping. The experts also gathered significantly more information covering more categories. Results support the argument that problem scoping and information gathering are major differences between advanced engineers and students, and important competencies for engineering students to develop. Timeline representations of the expert designers' processes illustrate characteristic distinctions we found and may help students gain insights into their own design processes.     

A novel antibacterial resin composite for improved dental restoratives

A novel furanone-containing antibacterial resin composite has been prepared and evaluated. compressive strength (CS) and Streptococcus mutans viability were used to evaluate the mechanical strength and antibacterial activity of the composites. The modified resin composites showed a significant antibacterial activity without substantially decreasing the mechanical strengths. With 5-30 % addition of the furanone derivative, the composite kept its original CS unchanged but showed a significant antibacterial activity with a 16-68 % reduction in the S. mutans viability. Further, the antibacterial function of the new composite was not affected by human saliva. The aging study indicates that the composite may have a long-lasting antibacterial function. Within the limitations of this study, it appears that the experimental antibacterial resin composite may potentially be developed into a clinically attractive dental restorative due to its high mechanical strength and antibacterial function.     

Sterol mutants ofSaccharomyces cerevisiae: Chromatographic analyses

The sterols accumulated by ergosterol deficient mutants of the geneserg6, erg2, erg3, anderg5 (formerlypo11, po12, po13, andpo15) have been analyzed by gas liquid chromatography. Together with pure sterols obtained from the mutants, they were characterized on SE-30, OV-17, and OV-225. The effects of molecular structure on the retention characteristics of a range of C₂₈ ergostane sterols have been studied. The double mutants obtained by crossing the single mutants were also analyzed and their sterols identified where possible. The effects of theerg mutations on the control of sterol biosynthesis in yeast are discussed.     

Additive Manufacturing of Metallic Materials: A Review

In this review article, the latest developments of the four most common additive manufacturing methods for metallic materials are reviewed, including powder bed fusion, direct energy deposition, binder jetting, and sheet lamination. In addition to the process principles, the microstructures and mechanical properties of AM-fabricated parts are comprehensively compared and evaluated. Finally, several future research directions are suggested.     

Stiffness characteristics of twisted cords for cord-rubber composites

A finite element model is presented to predict the stiffness characteristics of twisted cords by treating them as structures and considering the stiffness couplings due to extension-bending-torsional deformations. The axial, bending, and torsional stiffnesses are calculated for both the aramid-cord and steel-cord, and compared to an approximate expression with good agreement where applicable. To illustrate the stiffness behavior, all the three stiffnesses are presented with variations in the number of twists per unit length, the surrounding rubber modulus and the thickness. The stiffness couplings among the extension, bending, and twisting deformations are presented for aramid-cords with varying number of twists per unit length and rubber thickness. The results illustrate that stiffness characteristics are strongly dependent upon the number of twists per unit length, type of cord, surrounding rubber layer thickness and modulus. The stiffness coupling presented illustrate the mechanisms of load transfer, which are important for understanding failure mechanisms of cords, and cord-rubber composites.     

Ion exchange and charge transport properties of polymeric tris(4-vinyl-4′-methyl-2,2′-bipyridine) ruthenium(II) films

Multiply charged electroactive anions [IrCl ₆ ^2? , Fe(CN) ₆ ^3? , and W(CN) ₈ ^4? ] are electrostatically incorporated in polymeric films of tris(4-vinyl-4′-methyl-2,2′-bipyridine) ruthenium(II) [poly-Ru(vbpy) ₃ ²⁺ ] from aqueous trifluoroacetate solution. Values of apparent diffusion coefficients (D _ct) and heterogenous electron transfer rates (k _et) are measured for these anions as a function of their relative concentration (Γ _M/Γ _Ru) in the film.D _ct andk _et decrease systematically asΓ _M/Γ _Ru increases in a manner that is independent of charge and chemical identity of the ion. This result suggests that a nonchemical process, presumably electrostatic cross-linking, limits diffusional motion and is responsible for the decrease inD _ct andk _et with increasing anion content. Protonated polyvinyl-pyridine films exhibit similar ranges and variations inD _ct andk _et, which suggest similar structures and mechanisms of charge transport for these films and poly-Ru(vbpy) ₃ ²⁺ .     

Synthesis and evaluation of novel light‐curable amino acid‐constructed glass–ionomer cements

Poly(alkenoic acid)s constructed with β‐alanine and glutamic acid were synthesized and grafted with glycidyl methacrylate (GM). The purified polymers were further formulated with water, acrylic acid (AA), and Fuji II LC glass filler to form photocurable dental cements. Compressive strength (CS) and flexural strength (FS) of the cement, and viscosity of the resin liquid were used as screening tools to find the optimal formulation. The specimens were conditioned in distilled water at 37°C for 24 h, prior to testing. The results show that the effects of grafting ratio, polymer content, powder liquid (P/L) ratio, initiator concentration, and light exposure time were significant, but the effect of molecular weight was not. Aging study shows that the experimental cement showed a constant increase in mechanical strength for up to 1 week and then kept constant over 1 month. The formulation with 50/25/25 (liquid composition of polymer/AA/water), 70% (grafting ratio of GM), 3.2/1 (P/L ratio) as well as 0.9% (camphorquinone) and 1‐min light exposure time were found to be the optimal. The experimental cement was 19% higher in CS, 47% higher in diametral tensile strength and 176% higher in FS, compared to Fuji II LC. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 1587–1595, 2007     

Synthesis of high molecular weight polyoxyethylene with a quaternary catalyst and study of its conductive blends with poly(2‐vinyl pyridine)

High molecular weight polyoxyethylene (PEO) was synthesized by using a quaternary catalyst composed of triisobutyl aluminum, phosphoric acid, water, and N,N‐dimethylaniline (DMA). Optimum synthesis conditions and some properties of the product were studied. This catalyst showed high activity and the molecular weight of the polyoxyethylene obtained can approach one million. The activity of polymerization mainly depends upon the composition of catalyst. The optimum composition is as follows: i‐Bu₃Al:H₃PO₄:H₂O:DMA = 1 : 0.17 : 0.17 : 0.10–0.15 (molar ratio).The active centers of the catalyst was thus proposed. The high molecular weight PEO synthesized by this catalyst was blended with poly(2‐vinyl pyridine) (PVP) and then doped with LiClO₄ and TCNQ to obtain a conductive elastomeric material. Ionic, electronic, and mixed (ionic–electronic) conductivities of blends were investigated. At a Li/EO molar ratio of 0.1 and a TCNQ/VP molar ratio of 0.5, the mixed conductivity of the blend of PEO/PVP/LiCIO₄/TCNQ is higher than the sum of ionic conductivity of PEO/PVP/LiCIO₄ and electronic conductivity of PEO/PVP/TCNQ, when the weight ratio of PEO to PVP is 6/4 or 5/5. It can reach 4 × 10^?6 S/cm at room temperature. Differential scanning calorimetry, thermal gravimetric analysis, and the appearance of the blend showed that both TCNQ and LiClO₄ can complex with PEO and PVP, thus enhancing the compatibility between PEO and PVP. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007