Synthesis and variable temperature electrical conductivity studies of highly ordered TiO<Subscript>2</Subscript> nanotubes

Rafts of aligned, high aspect ratio TiO₂ nanotubes were fabricated by an electrochemical anodization method and their axial electrical conductivities were determined over the temperature range 225–400 °C. Length, outer diameter, and wall thickness of the nanotubes were approximately 60–80 μm, 160 nm, and 30 nm, respectively. Transmission electron microscopy studies confirmed that the TiO₂ nanotubes were initially amorphous, and became polycrystalline anatase after heat treatment at temperatures as low as 250 °C in air. The activation energy for conductivity over the temperature range 250–350 °C was found to be 0.87 eV. The conductivity values are comparable to those of nanocrystalline and nanoporous anatase thin films reported in literature.     

Application of the homotopy method for the analytic approach of the nonlinear free vibration analysis of the simple end beams using four engineering theories

In this paper, nonlinear vibration analyses of Euler–Bernoulli, Rayleigh, Shear and Timoshenko beams with simple end conditions are presented using homotopy analysis method (HAM). Closed form solutions for natural frequencies, beam deflection, post-buckling load–deflection relation, and critical buckling load are presented. The calculated natural frequencies for all four cases were verified against some available results in the literature and very good agreement observed. Furthermore, obtained results for deflection, buckling, and post-buckling of each beam are presented and the effects of some parameters, such as slenderness ratio, the rotary inertia, and the shear deformation are examined.     

Transition from the Steady to the Oscillatory Thermocapillary Convection in a Rectangular Container Under Various Cold Wall Temperature Effects

The transition from the steady to the oscillatory thermocapillary flow of high Prandtl number fluid in the rectangular container configuration under various cold wall temperatures effects have been studied experimentally. The fluid is heated by a thin wire placed along the free surface. The effects of buoyancy on the transport phenomenon are carefully assessed. The thermocapillary flow field is described based on a flow visualization and temperature measurement. The critical temperature difference for the onset of oscillations varies when the cold wall temperature is varied. The heat loss at the liquid free surface is identified to be responsible for this effect.     

Applications of quantum dots in optical fiber luminescent oxygen sensors

The potential applications of luminescent semiconductor nanocrystals to optical oxygen sensing are explored. The suitability of quantum dots to provide a reference signal in luminescence-based chemical sensors is addressed. A CdSe-ZnS nanocrystal, with an emission peak at 520 nm, is used to provide a reference signal. Measurements of oxygen concentration, which are based on the dynamic quenching of the luminescence of a ruthenium complex, are performed. Both the dye and the nanocrystal are immobilized in a solgel matrix and are excited by a blue LED. Experimental results show that the ratio between the reference and the sensor signals is highly insensitive to fluctuations of the excitation optical power. The use of CdTe, near-infrared quantum dots with an emission wavelength of 680 nm, in combination with a ruthenium complex to provide a new mechanism for oxygen sensing, is investigated. The possibility of creating oxygen sensitivity in different spectral regions is demonstrated. The results obtained clearly show that this technique can be applied to develop a wavelength division multiplexed system of oxygen sensors.     

Failure analysis of woven kevlar fiber reinforced epoxy composites pinned joints

An investigation was performed to determine the failure mode and the failure load of mechanically fastened joints in woven kevlar epoxy composite plates. Two-dimensional finite element code is developed to predict damage initiation, progression and strength of joints. Hashin, Hoffman and Maximum Stress criteria were used in this failure analysis. Experiments were performed to find the failure load and to predict the failure mode. Parametric studies were also carried out to evaluate the effect of joint geometry on this analysis. The obtained results were compared each other and comparison showed good agreement between numerical and experimental methods.     

Influence of Al concentration on structural and optical properties of Al-doped ZnO thin films

Undoped ZnO and Al-doped zinc oxide (ZnO:Al) thin films with different Al concentrations were prepared onto Si (100) substrate by pulsed filtered cathodic vacuum arc deposition system at room temperature. The influence of doping on the structural and optical properties of thin films was investigated. The preferential (002) orientation was weakened by high aluminum doping in films. Raman measurement was performed for the doping effects in the ZnO. Atomic force microscopy images revealed that the surface of undoped ZnO film grown at RT was smoother than that of the Al-doped ZnO (ZnO:Al) films. The reflectance of all films was studied as a function of wavelength using UV–Vis–NIR spectrophotometer. Average total reflectance values of about 35 % in the wavelength range of 400–800 nm were obtained. Optical band gap of the films was determined using the reflectance spectra by means of Kubelka–Munk formula. From optical properties, the band gap energy was estimated for all films.     

Inhibitory effect of pollen and propolis extracts

Bee pollen and propolis were collected from Apis mellifera colonies in five regions of Turkey. The antifungal properties of methanol extracts of pollen and propolis (2% and 5% concentrations) were determined on Alternaria alternata and Fusarium oxysporium f. sp. melonis. The least active concentration towards the tested fungi was 2% concentration of both extracts. The inhibitory effect of all propolis extracts on growth of F. oxysporium and A. alternata were generally higher when compared with pollen extracts. The growth of A. alternata and F. oxysporium were not affected at both concentrations of pollens. However, F. oxysporium against propolis extracts was more sensitive than A. alternata (P < 0.01). None of the pollen extracts tested completely inhibited mycelial growth of fungi used in our experiment. Percent inhibition of both pollen concentrations against A. alternata and F. oxysporium was lower than 50%. However, both concentrations of Alanya and Bey?ehir propolis extracts were 100% effective on mycelial growth of F. oxysporium until the 7th day of incubation (P < 0.01). 2% Alanya and Bey?ehir pollen extracts completely stimulated mycelial growth of F. oxysporium on the 7th day of incubation. Both concentrations of propolis extract showed more than 50% inhibition against E. oxysporium. It is suggested that high concentrations ofpropolis extract could be used as an antifungal agent against tested fungi.     

Research on the generation of electricity from the geothermal resources in Simav region, Turkey

One of the greatest problems in using renewable energy sources is the great variability of energy level, both in the short and long term. Geothermal energy, by nature, has high availability because the source is not dependent on weather conditions, so it is among the most stable renewable energy sources. Geothermal energy has the potential to play an important role in the future energy supply of Turkey. Although Turkey has the second-highest geothermal energy potential in Europe, electricity generation from geothermal energy is rather low.This study examines the use of geothermal energy in electricity generation and investigates the applicability of the existent geothermal energy resources to electricity generation in the Kütahya–Simav region, Turkey. The binary cycle is used in the designed power plant for electricity generation from geothermal fluid in which the percentage of liquid is high and which is at lower temperature. In this power plant, R134a is chosen as the secondary fluid, whose boiling point temperature is lower than that of water, and is used instead of geothermal fluid in a second cycle. The thermal efficiency of the designed power plant is measured to be 12.93%.     

A New Ultrasonic Thermostatic-Assisted Cloud Point Extraction/Spectrophotometric Method for the Preconcentration and Determination of Bisphenol A in Food,Milk, and Water Samples in Contact with Plastic Products

Bisphenol A (BPA) contamination in foods and beverages usually occurs as a result of migration from the packages that contain it. In this context, a simple, easy-to-use, and efficient method was developed for the spectrophotometric determination of BPA in food, milk, and water samples in contact with plastic products after preconcentration by ultrasonic-thermostatic-assisted cloud point extraction (UTA-CPE). The method is based on the charge transfer-sensitive complexation of BPA with 3-methylamino-7-dimethylaminophenothiazin-5-ium chloride (AzB) in the presence of cetyltrimethylammonium bromide (CTAB) at pH 8.5 and then extraction of the formed complex into the micellar phase of polyethylene glycol dodecyl ether (Brij 35). The effects of the analytical variables affecting complex formation and extraction efficiency were systematically studied and optimized. Under optimized conditions, a good linear relationship was obtained in the range of 1.2–160 μg L^?1 with a detection limit of 0.35 μg L^?1. After preconcentration of a sample of 20 mL, a sensitivity enhancement factor was found to be 180. The accuracy and reliability of the method were evaluated by recovery studies from the spiked quality control samples and intraday and interday precision studies. From the studies conducted, the extraction efficiency (E%) was in the range of 94–103% with a relative standard deviation lower than 5.2% (as RSD%, n = 5). The method was successfully applied to the preconcentration and determination of BPA from the selected sample matrices.

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Graphical Abstract Migration of bisphenol A into the foodstuffs

     

Enhancing thermal and ionic conductivities of electrospun PAN and PMMA nanofibers by graphene nanoflake additions for battery‐separator applications

The present study reports the fabrication and characterization of electrospun polyacrylonitrile (PAN) and polymethyl methacrylate (PMMA) nanofiber separators embedded with graphene nanoflakes. Different weight percentages (0, 2, 4, and 8 wt%) of graphene nanoflakes were dispersed in dimethylformamide (DMF) and ethanol using sonication and high‐speed agitations, and then PAN and PMMA powders were added to the dispersions prior to the mixing process. Ratios of 85:15 for DMF : PAN and 88:12 for ethanol : PMMA were chosen during the dispersion and dissolution processes. After the fabrication of the membranes via the electrospinning process, thermal, dielectric, ionic, and surface hydrophobic properties of the PAN and PMMA nanofiber separators were investigated in detail. Test results revealed that the physical properties, such as wettability, dielectric constant, ionic conductivity, and thermal conductivity values of the nanocomposite separators were significantly increased as a function of graphene concentrations. For example, the water contact angle, ionic conductivity, dielectric constant, and thermal conductivity values of the membranes were increased from 120° to 145°, 3.31 × 10^?4 to 5.52 × 10^?4 S/m, 3.5 to 8.5 W/m K, and 1.0 to 5.0 W/m K, respectively, when the graphene concentration was increased from 0% to 8% in PMMA. Similar trends were observed in the PAN fibers, as well. Lithium‐ion (Li‐ion) batteries have become the major source of power for portable electronic devices, and because separators are one of the major components of these batteries, the present alternatives can be an option for long‐lasting Li‐ion battery fabrications. Copyright © 2014 John Wiley & Sons, Ltd.