Rheology and properties of melt-processed poly(ether ether ketone)/multi-wall carbon nanotube composites

Poly(ether ether ketone) (PEEK)/multi-wall carbon nanotube (MWNT) composites containing up to 17 wt% filler were prepared using a twin screw extruder. Transmission electron microscopy (TEM) images reveal that the MWNTs were homogeneously dispersed in the PEEK matrix. Linear viscoelastic measurements show that both complex viscosity and moduli increase with increasing MWNT concentration. The storage modulus, G^′ exhibits a dramatic seven order increase in magnitude around 1 wt%, leading to a solid-like low-frequency behaviour at higher loadings; the effect can be attributed to network formation at a rheological percolation threshold. Rheotens measurements show that the melt strength also increases significantly on addition of nanotubes, however, the drawability decreases. An analytical Wagner model was used to calculate the apparent elongational viscosity over a wide range of elongational rates, and to reveal significant increases on addition of MWNTs, with a similar threshold behaviour. The electrical response is also dominated by percolation effects, increasing by nearly 10 orders of magnitude from 10⁻¹¹ to 10⁻¹ S/cm, on the addition of only 2 wt% MWNTs. In contrast, the thermal conductivity and tensile elastic modulus of the composites increased linearly with nanotube content, rising by 130% and 50%, at 17 wt% MWNTs, respectively.     

Calibration of a TLD-100 powder dosimetric system to verify the absorbed dose to water imparted by 137Cs sources in low dose rate brachytherapy at the oncology unit in the Hospital General de Mexico

A thermoluminescence dosimetry (TLD) system was characterised at SSDL-ININ to verify the air-kerma strength (S(K)) and dose-to-water (D(W)) values for (137)Cs sources used in low dose rate (LDR) brachytherapy treatments at the Hospital General de Mexico (HGM). It consists of a Harshaw 3500 reader and a set of TLD-100 powder capsules. The samples of TLD-100 powder were calibrated in terms of D(W) vs. nC or nC mg(-1), and their dose response curves were corrected for supralinearity. The D(W) was calculated using the AAPM TG-43 formalism using S(K) for a CDCSM4 (137)Cs reference source. The S(K) value was obtained by using a NE 2611 chamber, and with two well chambers. The angular anisotropy factor was measured with the NE 2611 chamber for this source. The HGM irradiated TLD-100 powder capsules to a reference dose D(W) of 2 Gy with their (137)Cs sources. The percent deviations between the imparted and reference doses were 1.2% < or = Delta < or = 6.5%, which are consistent with the combined uncertainties: 5.6% < or = u(c) < or = 9.8% for D(W).     

Exploring the Selective Demethylation of Aryl Methyl Ethers with a Pseudomonas Rieske Monooxygenase

Biocatalytic dealkylation of aryl methyl ethers is an attractive reaction for valorization of lignin components, as well as for deprotection of hydroxy functionalities in synthetic chemistry. We explored the demethylation of various aryl methyl ethers by using an oxidative demethylase from Pseudomonas sp. HR199. The Rieske monooxygenase VanA and its partner electron transfer protein VanB were recombinantly coexpressed in Escherichia coli and they constituted at least 25 % of the total protein content. Enzymatic transformations showed that VanB accepts NADH and NADPH as electron donors. The VanA–VanB system demethylates a number of aromatic substrates, the presence of a carboxylic acid moiety is essential, and the catalysis occurs selectively at the meta position to this carboxylic acid in the aromatic ring. The reaction is inhibited by the by-product formaldehyde. Therefore, we tested three different cascade/tandem reactions for cofactor regeneration and formaldehyde elimination; in particular, conversion was improved by addition of formaldehyde dehydrogenase and formate dehydrogenase. Finally, the biocatalyst was applied for the preparation of protocatechuic acid from vanillic acid, giving a 77 % yield of the desired product. The described reaction may find application in the conversion of lignin components into diverse hydroxyaromatic building blocks and generally offers potential for new, mild methods for efficient unmasking of phenols.     

Stressed Skin Design Versus Braced Frame Design Through Efficient Numerical Modelling

International Journal of Steel Structures - Steel frame structures are traditionally designed with bracings that stabilize the main bearing structure. Another approach is to apply the...     

The properties of free-standing epoxy polymer multi-mode optical waveguides

The paper reports on the fabrication and characterisation of free-standing multimode optical epoxy polymer waveguides consisting of a core made of EpoCore and EpoClad polymer cladding and cover protection layers. The 50 × 50 μm² rectangular waveguides are intended for short-reach optical interconnection and optimised for an operating wavelength of 850 nm. The waveguides of the proposed shapes were fabricated by a standard photolithography process on a silicon substrate provided with a Poly(vinyl alcohol) thin layer. The free-standing structure was then achieved by peeling the deposited EpoClad/EpoCore/EpoClad structures of that substrate. The optical scattering losses of the created planar waveguides, measured by the fibre probe technique at 632.8 and 964 nm, were 0.30 dB cm⁻¹ at 632.8 nm and 0.17 dB cm⁻¹ at 964 nm. Propagation optical loss measurements for rectangular waveguides were performed by the cut-back method and the best samples had optical losses below 0.55 dB cm⁻¹ at 850 and 1310 nm.

     

Cavitation Bubbles Generated by Vibrating Quartz Tuning Fork in Liquid $$^$$He Close to the $$\lambda $$λ-Transition

We report direct optical observation of cavitation bubbles in liquid helium, both in classical viscous He I and in superfluid He II, close to the \(\lambda \)-transition. Heterogenous cavitation due to the fast-flowing liquid over the rough surface of prongs of a quartz tuning fork oscillating at its fundamental resonant frequency of \(4\,\mathrm {kHz}\) occurs in the form of a cluster of small bubbles rapidly changing its size and position. In accord with previous investigators, we find the cavitation threshold lower in He I than in He II. In He I, the detached bubbles last longer than one camera frame (10 ms), while in He II the cavitation bubbles do not tear off from the surface of the fork up to the highest attainable drive.     

Solvothermal synthesis of Ag/ZnO micro/nanostructures with different precursors for advanced photocatalytic applications

Micro/nanostructured systems based on metallic oxide (ZnO) with noble metal (Ag) on the surface (Ag/ZnO) are synthesized by solvothermal method from zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), zinc acetate dehydrate (Zn(CH₃COO)₂·2H₂O), zinc acetylacetonate hydrate (Zn(C₅H₇O₂)₂·xH₂O) and silver nitrate (Ag(NO₃)) as precursors. In these systems, polyvinylpyrrolidone (PVP) is used as surfactant for controlling particle morphology, size and dispersion. The obtained materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), UV–vis diffuse reflectance spectroscopy (DRS), N₂ gas adsorption–desorption (BET) and Raman spectroscopy (RS). By XRD results, all major peaks are indexed to the hexagonal wurtzite-type structure of the ZnO and samples with noble metal, extra diffraction peaks are detected which correspond to the face-centered-cubic (fcc) structure of the metallic Ag. Depending on used precursor, different morphologies have been obtained. Mainly, ZnO prims-like rods – NRs (with 0.8 ? aspect ratio ? 3.4) – have been observed. Quasi-spherical particles of metallic Ag (with diameters between 558 ± 111 μm and 22 ± 1 nm) have been detected on the ZnO surface. Photocatalytic results (all samples studied >30% MB degradation) verify the important effect of surfactant and the viability of synthesized Ag/ZnO micro/nanocomposites for environmental applications.     

Target-based coherent beam combining of an optical phased array fed by a broadband laser source

The target-based phasing of an optical phased array (OPA) fed by a broadband master oscillator laser source is investigated. The specific scenario examined here considers an OPA phasing through atmospheric turbulence on a rough curved object. An analytical expression for the detected or received intensity is derived. Gleaned from this expression are the conditions under which target-based phasing is possible. A detailed OPA wave optics simulation is performed to validate the theoretical findings. Key aspects of the simulation set-up as well as the results are thoroughly discussed.