The Effects of Heat Treatment on the Corrosion Behavior of HVOF-sprayed WC-17 wt % Co Coatings

Protection of Metals and Physical Chemistry of Surfaces - A WC-17 wt % Co coating was deposited onto a ST37 mild steel substrate by HVOF-spray technique and then vacuum heat treated at 1100°C....     

Regioselective synthesis of new 5-methyl-5H-pyrimido[4′,5′:4,5][1,3]thiazino [3,2-a]perimidines

A convenient and efficient regioselective synthesis of new pyrimido[4′,5′:4,5] [1,3]thiazino[3,2-a]perimidines is described through intermolecular heterocyclization of 2,4-dichloro-5-(chloromethyl)-6-methylpyrimidine and 1H-perimidine-2(3H)-thione in short reaction times under mild conditions.     

An optical channel modeling of a single mode fiber

The evaluation of the optical channel model that accurately describes the single mode fibre as a coherent transmission medium is reviewed through analytical, numerical and experimental analysis. We used the numerical modelling of the optical transmission medium and experimental measurements to determine the polarization drift as a function of time for a fixed length of fibre. The probability distribution of the birefringence vector was derived, which is associated to the ‘Poole’ equation. The theory and experimental evidence that has been disclosed in the literature in the context of polarization mode dispersion – Stokes & Jones formulations and solutions for key statistics by integration of stochastic differential equations has been investigated. Besides in-depth definition of the single-mode fibre-optic channel, the modelling which concerns an ensemble of fibres each with a different instance of environmental perturbation has been analysed.     

Structural Behavior and In Vitro Bioactivity Evaluation of Sol-Gel Derived Glass-Ceramics Based on SiO<Subscript>2</Subscript>- CaO- P<Subscript>2</Subscript>O<Subscript>5</Subscript>- ZnO System

Glass and glass-ceramics, which are important bioceramics, have established an essential group of silicon-based materials having wide applications in medicine. Bioactive glass-ceramics have proved to be able to chemically bond to living bones due to forming an apatite-like layer on their surfaces. In this study, bioactive glass-ceramics of (64-X) SiO₂- 31 CaO- 5 P₂O₅- X ZnO based on mol% were synthesized by the sol–gel process. The bioactive glass-ceramics were analyzed using X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) before and after being exposed to simulated body fluid (SBF) solution. The results show that an increase in ZnO will cause the ability of calcium phosphate apatite formation on the glass-ceramic surfaces to first rise by 8 % but subsequently to decline by 12 %. The bioactivity of synthesized glass-ceramics can be confirmed by SEM and XRD due to the presence of a rich bone-like apatite layer.     

Design of MMIC oscillators using GaAs 0.2 μm PHEMT technology

We propose a feedback type oscillator and two negative resistance oscillators.These microwave oscillators have been designed in the S band frequency.A relatively symmetric resonator is used in the feed...     

Evaluation of two new white silicone thermal control paints under atomic oxygen

The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Thermal control paints are resistant to the conditions encountered at low earth orbit (LEO): vacuum, atomic oxygen, thermal cycling, and ultraviolet radiation. In this paper two white paints were prepared by ultrasonic dispersing method for application to space structures. The white paints include zinc-oxide-pigmented silicone elastomer SilGel 612 (U1) and a zinc-oxide-pigmented silicone elastomer RT604 (U2). These coatings are electrically non-conductive. We present the behavior of thermal control paints under atomic oxygen (AO). The effects of AO exposure were studied by the mass loss of paint specimens and complementary techniques including scanning electron microscopy (SEM) and atomic force microscopy (AFM). Also the optical degradation in the reflectance spectra, solar absorptance and thermal emittance for U1 and U2 samples before and after AO exposure were investigated. The paints have promising AO resistance properties that could be suitable for space applications.     

Shrinking of Chemically Cross-Linked Polymer Networks in the Postgel Region

Summary In this paper, the syneresis behaviors in the post-gel region of hydrogels of hydroxyethylcellulose (HEC) and its hydrophobically modified analogue (HM-HEC) were investigated by means of a new high precision swell-ratio-tester. The gels were prepared by cross-linking cellulose ether derivatives with divinyl sulfone (DVS) in alkaline solution at various cross-linker concentrations and temperatures. Increasing the cross-linker density promotes a faster shrinkage of the gel, and a more compressed gel. The compression of the 1 wt % HEC gel starts at an earlier time at 40 °C than at 25 °C, because the increased mobility of the chains is more favorable to a faster formation of interpolymer cross-links. The results from the deswelling measurements show that the hydrophobic modification of the polymer yields less contracted gels. This novel finding is ascribed to the fact that some of the hydroxyl groups for the formation of intermolecular cross-links are deactivated through the incorporated hydrophobic groups at these sites.     

Prepolymerization of ethylene with a Ziegler–Natta catalyst

The slurry prepolymerization of ethylene using TiCl₄/MgCl₂ as a catalyst was investigated. A 2³‐factorial experimental design method was employed to study the effects of the temperature, hydrogen, and active cocatalyst‐to‐catalyst molar ratio (Al/Ti) on the catalyst activity, prepolymer melt flow index, and powder bulk density. All dependent variables increased when the active Al/Ti ratio increased from 1 to 2. The hydrogen–Al/Ti interaction had a significant effect on the prepolymer melt flow index and catalyst activity. The hydrogen (partial pressure ranging from 0.5 to 1.5 bar) and temperature (ranging from 60 to 80°C) variables as well as the hydrogen–temperature and hydrogen–temperature–Al/Ti interactions increased the prepolymer powder bulk density, which ranged from 0.11 to 0.373 g/cc. To find the reason for the prepolymer powder bulk density variation with the operating conditions, the particle size distribution and crystallinity of the prepolymers were determined. The increasing catalyst activity, ranging from 132 to 660 g of polyethylene/mmol of Ti, enhanced the weight percentage of fines, which ranged from 3 to 60, and decreased the average particle size, which ranged from 562 to 120 μm. This was the reason for the increasing prepolymer powder bulk density and could be due to the breakup of the prepolymer particles during prepolymerization. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Numerical modeling of natural convection in an open cavity with two vertical thin heat sources subjected to a nanofluid

This paper presents a numerical study of natural convection cooling of two heat sources vertically attached to horizontal walls of a cavity. The right opening boundary is subjected to the copper–water nanofluid at constant low temperature and pressure, while the other boundaries are assumed to be adiabatic. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for the Rayleigh number in the range 10⁴ ≤ Ra ≤ 10⁷, and for solid volume fraction 0 ≤ φ ≤ 0.05. In order to investigate the effect of heat source location, three different placement configurations of heat sources are considered. The effects of both Rayleigh numbers and heat source locations on the streamlines, isotherms, Nusselt number are investigated. The results indicate that the flow field and temperature distributions inside the cavity are strongly dependent on the Rayleigh numbers and the position of the heat sources. The results also indicate that the Nusselt number is an increasing function of the Rayleigh number, the distance between two heat sources, and distance from the wall. In addition it is observed that the average Nusselt number increases linearly with the increase in the solid volume fraction of nanoparticles.