Thermal,mechanical, and acoustic properties of silica‐aerogel/UPVC composites

The properties of silica‐aerogel/UPVC composites have been investigated with emphasis on sound and heat insulation. UPVC is a material of construction for window profiles and drainage pipes. Hydrophobic silica aerogels were synthesized using silicate sodium as a precursor through a two‐step sol–gel process. The physical and textural properties of the synthesized silica aerogels such as density, surface area, and particle size were analyzed using SEM and BET analysis. Then, the synthesized aerogels were mixed with Unplastisized Polyvinyl Chloride (UPVC) compound at five different weight ratios in an internal mixer to find out the effects of silica aerogels on the thermal, mechanical, and acoustical characteristics. The prepared UPVC/aerogel composites were characterized for tensile properties, impact strength, hardness, Vicat softening temperature, thermal conductivity, sound absorption, and sound transmission loss. The results revealed that adding silica aerogel in to the matrix of UPVC increases its hardness and softening temperature while decreases impact strength. The thermal conductivity of UPVC was decreased by up to 50% using silica aerogel. The sound absorption property of UPVC was increased up to three times by using silica aerogels due to its high porosity. Silica aerogel increased the maximum sound transmission loss of UPVC in the low frequency range. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44685.     

Comparison of pristine and polyaniline‐grafted MWCNTs as conductive sensor elements for phase change materials: Thermal conductivity trend analysis

Phase change materials (PCMs) function based on latent heat stored on or released from a substance over a slim temperature range. Multiwalled carbon nanotubes (MWCNTs) and polyaniline are important elements in sensor devices. In this work, pristine and polyaniline‐grafted MWCNTs (PANI‐g‐MWCNTs) were applied as conductive carbon‐based fillers to make PCMs based on paraffin. The attachment of PANI to the surface of MWCNTs was proved by Fourier transform Infrared analysis. Dispersion of MWCNTs in paraffin was studied by wide‐angle X‐ray scattering. Heating and solidification of PCM nanocomposites were investigated by differential scanning calorimetry, while variation in nanostructure of PCMs during heating/solidification process was evaluated by rheological measurements. It was found that after 30 min of sonication, the samples filled with 1 wt % MWCNTs have melting and solidification temperatures of 29 and 42 °C, respectively. It was also found that PANI attachment to MWCNTs significantly changes thermal conductivity behavior of PCM nanocomposites. The developed MWCNTs‐based sensor elements responded sharply at low MWCNTs content, and experienced an almost steady trend in conductivity at higher contents, while PANI‐g‐MWCNTs sensor followed an inverse trend. This contradictory behavior brought insight for understanding the response of PCMs against thermal fluctuations. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45389.     

Influence of preheating of fly ash precursors to produce geopolymers

This study has been focused on the effect of pretreatment of FA and POFA after geopolymerization on the mechanical properties. The aim of this work was to simplify the pretreatment that it can easily industrial be applied, using existing technology. Previous work has shown that a reduction in particle size increases the mechanical properties. However, this method involves a milling process which is not applicable for a wide industrial application. Hence, FA and POFA particles have been heated to 300°C, 500°C, and 800°C but applying different pretreatments: (i) predried at 110°C (reference sample); (ii) as received; (iii) prewetted; (iv) prewetted and later quenched in cold water. It was found that during the treatment the particle size increased due to thermal stress cracking. During fast heating, trapped pore water cannot be removed as fast as it evaporates and hence the particles crack. This increase in particle size caused an increase in compressive strength. In addition, heating to 300°C and 500°C caused a dehydroxylation of FA and POFA. This dehydroxylation resulted in a higher initial reactivity, reducing the setting time and improved mechanical strength.     

Clusters identification and characterization in a gas–solid fluidized bed by the wavelet analysis

The local solid flow structure of the bubbling fluidized bed of sand particles was investigated in order to identify and characterize the clusters. Extensive experiments were carried out using an optical fibre probe, measuring the velocity and the diameter of clusters. Under all operating conditions, ascending and descending clusters co‐existed at all measurement locations. The locus of the inversion point at which the directions of cluster motion changed was determined. The velocity of the ascending clusters was a function of both superficial gas velocity and the radial and axial position. With increasing superficial gas velocity, both the velocity and the diameter of ascending clusters decreased near the wall. However, the velocity of descending clusters depended mainly on superficial gas velocity and the largest clusters existed closer to the wall. The results of this study help to explain cluster hydrodynamics in fluidized beds.     

Synthesis and properties of polybenzoxazine modified polyurethanes as a new type of electrical insulators with improved thermal stability

A new class of electrical insulating materials with improved thermal stability was prepared from combination of polybenzoxazine with epoxy terminated polyurethane prepolymer (EPU). EPU was prepared by the reaction of glycidol with NCO‐terminated urethane prepolymer. The prepolymer was prepared from polycaprolactone polyol (molecular weight 1000), and hexamethylene diisocyanate. Bisphenol‐A and aniline were used for preparation of benzoxazine monomer. Mode of reactions and optimum curing condition were determined by DSC and FTIR analysis. Upon these analysis as well as DMTA findings, formation of interpenetrating polymer networks was suggested for blends consisted different weight ratios of two components. Viscoelastic behavior, mechanical, thermal, and electrical properties of the prepared samples was studied and their relation to the chemical structure and composition of blends were elucidated. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Sub-wavelength resolution in air-coupled ultrasound images of spot welds

Resistance spot welds joining steel sheets were examined using air-coupled ultrasound at 1 MHz. Transmission C-scan images were obtained using two axially aligned transducers, with each focused on its adjacent surface of the specimen. Because of the enormous impedance mismatch between air and steel, the transmitted signal was exceedingly weak, requiring several hours to collect an image. Although the air-coupled images resembled the weld shapes as determined by an independent technique, surface topography introduced considerable image distortion. The smearing of the images was substantially less than the 5.9-mm wavelength in steel, with the steepness of the transition between welded and unwelded areas indicating a transverse resolution about 1 mm. Similarities to near-field imaging are mentioned as a possible explanation for the observed sub-wavelength resolution.     

Evolution of number concentration of nano- particles undergoing Brownian coagulation in the transition regime

Evolution of number concentration of nanoparticles undergoing Brownian coagulation in the transition regime is studied theoretically and numerically.The results show that the curves of particle size distribution move toward the area with large particle diameters, the curve peak becomes lower and the range that particle diameters cover becomes wider as time elapses.In the process of coagulation the particles with small diameter disappear gradually and the particle size distribution remains a log-normal distribution.The change rate of the particle size distribution is more appreciable at the initial stage than that at the final stage.The initial Knudsen number has a significant effect on the coagulation rate which increases with decreasing the initial Knudsen number.The larger the initial geometric standard deviation is, the smaller the curve peak is, and the wider the area that curves cover is.The initial geometric standard deviation has a significant effect on the particle size distribution which can remain a self-preserving state when the initial geometric standard deviation is smaller than 2.With the increase of the diversity of initial particle size, the particle size distribution does not obey the log-normal distribution any more as time elapses.     

Multi-mode of vortex-induced vibration of a flexible circular cylinder

The vortex-induced vibration of a flexible circular cylinder is investigated at a constant Reynolds number of 1 000.The finite-volume method on moving meshes is applied for the fluid flow, and the Euler-Bernoulli beam theory is used to model the dynamic response of a flexible cylinder.The relationship between the reduced velocity and the amplitude response agrees well with the experimental results.Moreover, five different vibrating modes appear in the simulation.From the comparisons of their vortex structures, the strength of the wake flow is related to the exciting vibrating mode and different vortex patterns arise for different vibrating modes.Only 2P pattern appears in the first vibrating mode while 2S-2P patterns occur in the other vibrating modes if monitoring at different sections along the length of the cylinder.The vibration of the flexible cylinder can also greatly alter the three-dimensionality in the wake, which needs further studies in our future work, especially in the transition region for the Reynolds number from 170 to 300.