Nonisothermal suspension polymerization of vinyl chloride for enhanced productivity

In the present study, a mathematical model is developed to numerically predict nonisothermal batch suspension polymerization of vinyl chloride. Free volume theory was used to consider diffusion‐controlled reactions. Model predictions were validated against field data obtained in a pilot scale stirred tank reactor. Variable temperature trajectory was considered during the course of the reaction to improve productivity by reducing the polymerization time for a certain conversion. Variable temperature during the course of the polymerization was successfully implemented by considering the predefined K value. By using variable temperatures during the course of the reaction, the density of the short branches per 1,000 monomer units as a criterion for structure defect remained relatively unchanged. Maximum reduction in reaction time relative to the isothermal case with the same K value and final conversion was 44% for the best temperature trajectory. J. VINYL ADDIT. TECHNOL., 22:470–478, 2016. © 2015 Society of Plastics Engineers     

Magnetic behavior of nickel–bismuth ferrite synthesized by a combined sol–gel/thermal method

The bulk NiFe_2?xBi_xO₄ ferrites with various Bi³⁺ concentration (x=0, 0.1, 0.15) were synthesized via sol–gel procedure, starting from nickel, bismuth and iron nitrate powders, followed by the conventional thermal treatment. The structural and magnetic properties of the as-prepared ferrites were studied by means of X-ray diffraction, alternating gradient force magnetometry and Faraday balance. The anisotropy constant was determined by the law of approach to saturation (LAS) model. An increasing Bi³⁺ concentration in NiFe_2?xBi_xO₄ leads to a decrease in the saturation magnetization, Néel temperature and the anisotropy constant of the material.     

Electrical properties of pre-extended FEF carbon black loaded rubbers

The electrical conduction was investigated in different rubber composites, subjected to various values of longitudinal extension. The study involves styrene-butadiene-rubber (SBR), butadiene-rubber (BR), acrylonitrile-butadiene-rubber (NBR), butyl-rubber (IIR), and natural rubber (NR). Each type was separately blended with 100 phr of fast extruding furnace black. The parameters of the model of Polley and Boonstra are recalculated for a specific example and good fitting is found.     

Volume Changes of Iron Oxide Compacts under Isothermal Reduction Conditions

Compacts made from chemically grade Fe₂O₃ were fired at 1473K for 6 hrs. The fired compacts were isothermally reduced either by hydrogen or carbon monoxide at 1073–1373K. The O₂ weight‐loss resulting from the reduction process was continuously recorded as a function of time using TGA technique, whereas the volume change at different reduction conditions was measured by displacement method. Porosity measurements, microscopic examination and X‐ray diffraction analysis were used to characterize the fired and reduced products. The rate of reduction at both the initial and final stages was increased with temperature. The reduction mechanism deduced from the correlations between apparent activation energy values, structure of partially reduced compacts and application of gas‐solid reaction models revealed the reduction rate (dr/dt) at both the initial and final stages. At early stages, the reduction was controlled by a combined effect of gaseous diffusion and interfacial chemical reaction mechanism, while at the final stages the interfacial chemical reaction was the rate determining step. In H₂ reduction, maximum swelling (80%) was obtained at 1373K, which was attributed to the formation of metallic iron plates. In CO reduction, catastrophic swelling (255%) was obtained at 1198K due to the formation of metallic iron plates and whiskers.     

Cost Performance Comparison of Two Public Sector Project Procurement Techniques

Increased industry discussion of the potential benefits of alternative forms of construction project delivery often presents little cost analysis of options. This work presents the results of a cost comparison study of completed public sector municipal facilities design and construction projects—a portion of which delivered using a traditional design-bid-build procurement approach and a portion of which delivered using the procurement technique of a negotiated lump sum construction contracts with in-house construction forces. The study compares costs for all phases of all the projects (design, bid and award, construction, and total project cost) across both methods of project procurement. Conclusions examine the cost trade-offs of using negotiated contracts with in-house forces, outline key success factors identified through this study in order to optimize the use of this form of project procurement, and identify areas for future research.     

Hybrid channel modelling for ultra-wideband portable multimedia applications

An efficient approach in modelling the ultra-wideband channel (UWB) by combining deterministic and statistic techniques is presented. The hybrid approach aims at lowering the complexity of modelling the UWB indoor channel by combining simple 2.5D ray tracing and measurement statistics. A UWB measurement campaign in an office environment is reported. The mean signal level and the rms delay spread values obtained from the ray-tracing tool are compared with measurements at several locations in the studied environment. Average statistical difference terms are added to the values of mean signal level and rms delay spread obtained from ray tracing to improve predictions. This approach results in the hybrid model and achieves computational efficiency by eliminating the need of modelling complex details of scatterers in the environment.     

Static Capacity Prediction by Dynamic Methods for Three Bored Piles

Three bored piles were built and tested at the National Geotechnical Experimentation Sites, at Texas A&M University, to gather data on the reliability of large-strain dynamic methods to predict the static capacity of bored piles. The three piles had a nominal diameter of 0.915 m, a nominal length of 10 m, and some planned and unplanned defects. The piles were first subjected to a static load test and then four companies were asked to perform dynamic tests—namely, Statnamic and drop weight tests—and predict the static load test results. The paper shows the comparison between predicted and measured results.     

Cathodoluminescence study in AlxGa1 − xN structures

In this work, we consider a 2D model for calculation of cathodoluminescence in GaN-based structures. This model is developed using an extended generation profile and taking into account the influence of the carrier diffusion process, internal absorption and some radiative recombination processes. First, we have investigated the effect of hole diffusion length and the surface recombination velocity on the CL spectra of GaN sample grown at 800 °C by MOVPE method. Then, we have calculated the dependence of CL intensity from AlGaN alloys as a function of Al content and the electron beam energy.

Results show a red shift of the CL peaks when the beam energy is varied from 2 to 10 keV at room temperature. The band-edge emission of Al_xGa_1 − xN shifts about 0.49 eV when the Al composition is increased from x = 0.18 to 0.38. Comparison of the experimental spectra with simulations shows a good agreement.     

Influence of hydrostatic pressure on the electrical properties of unvulcanized FEF-loaded SBR

Styrene butadience rubber (SBR-1502) loaded with different concentrations of FEF carbon black was tested to find out the effect of pressure on its electrical properties during vulcanization. Using the thermodynamically calculated values of the pressure coefficient (K), the thermoelastic coefficient (L = ?T/?P) was also estimated and compared with that obtained by other workers. The thermoelastic coefficient was found to be strongly dependent on the carbon black concentration.