Effect of cooling rate on microstructure of friction-stir welded AA1100 aluminum alloy

In this work, the microstructural changes occurring during cooling of friction-stir welded aluminum alloy AA1100 were evaluated. To this end, friction-stir welding (FSW) was performed in a wide range of cooling rates of 20–62 K/s and the evolved microstructures were studied by using electron backscatter diffraction. Below 0.6 T_m (T_m being the melting point), the stir zone material was found to experience no significant changes during cooling. At higher FSW temperatures, however, notable changes occurred in the welded material, including grain growth, sharpening of texture, reduction of the fraction of high-angle boundaries and material softening.     

Synthesis and characterization of iPP‐sPP stereoblock produced by a binary metallocene system

Stereoblock polypropylenes comprising of iPP and sPP segments are synthesized by polymerization of the following binary system of metallocenes: the C_s‐symmetric [2,7‐t‐Bu₂(Flu)₂Ph₂C(Cp)ZrCl₂] and the C₂‐symmetric rac‐Me₂Si(2‐Me‐4‐Ph‐Ind)₂ZrCl₂. Blends of samples made either by each catalyst individually (solution blend) with materials obtained with the mixed catalyst system (reactor blend) are compared. The simultaneous presence of MAO and DEZ, enhancing fast and reversible transfer of the growing chains between the two active centers, leads to the formation of a stereoblock microstructure. In this case, low molecular weight polymers are obtained. The junction between the blocks is qualitatively observed in ¹³C NMR. When made in toluene, the stereoblock material consists of a majority of syndiotactic sequences, whereas the ratio is more equilibrated when the polymerization was conducted in the more polar chlorobenzene. This is confirmed by the results obtained with ¹³C NMR, CRYSTAF, HT HPLC, DSC, SSA, WAXD, and optical microscopy. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1422–1434     

Solvent‐free inkjet printing process for the fabrication of conductive,transparent, and flexible ionic liquid‐polymer gel structures

A new and solvent‐free process for the fabrication of inkjet printed ionic liquid‐polymer gel microstructures with high‐resolution (line widths of ～40 μm), good electrical conductivity (5–30 mS cm^?1), optical transparency, and mechanical flexibility is presented. Carrying out the printing and polymerization process in nitrogen atmosphere eliminates the inhibiting influence of oxygen and guarantees homogeneously gelled structures. Careful selection and combination of ionic liquids (ILs) and unsaturated monomers makes it possible to achieve low viscosities which are printable with commercially available inkjet printers and printheads without adding extra solvents. By using different types and amounts of ILs and monomers the resulting properties of the printed IL‐polymer gels can be controlled in terms of ionic conductivity, optical transmission, and mechanical flexibility. Higher conductivities are possible by using a bifunctional instead of a monofunctional monomer, which allows one to lower the amount of monomer without loss in mechanical strength. Cast samples make it possible to obtain data of transmission (～90% for 170‐μm thick films) and mechanical flexibility (E = 0.02–0.23 MPa) of bulk material. Comparing electrical conductivity of printed and cast samples, the higher values of printed samples indicate the conductivity enhancing influence of moisture absorbed from the surrounding atmosphere after the fabrication process. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Aggregation of clay platelets in nematic liquid crystal, 5CB: microstructure,electrical conductivity and rheological investigations

The microstructure, electrical conductivity and rheological properties of a nematic liquid crystal (5CB) doped at concentrations up to 4.5 wt% of montmorillonite (MMT) or organomontmorillonite (OMMT) clay nanoplatelets, were investigated at temperatures between 293 and 310 K. Microscopy and electrical conductivity assessment revealed noticeable differences in aggregation in MMT and OMMT suspensions, MMT nanoplatelets showing a strong tendency to aggregation. The incubation of 5CB in the presence of MMT initially produced loose aggregation, followed by the formation of compact aggregates. The latter had practically no influence on the surrounding inter-aggregate regions. In the case of OMMT, a greater degree of integration of the nanoplatelets was observed within the liquid crystal structure of 5CB, resulting in a noticeable effect on electrical conductivity and activation energy of the composite material. Thixotropy was observed in suspensions of 5CB composites formed with either MMT or OMMT. A composite of 5CB with OMMT also exhibited anomalous viscous thinning at shear rates below 100 s^?1. A structural model is suggested to explain this behaviour.     

Azimuthal orientational correlations due to excluded volume epitaxy in growing anisotropic grains

The microstructure of anisotropically shaped grains can strongly influence a range of material properties, including transport, mechanical and electro-optical. A grain-structure-related phenomenon, known as excluded volume epitaxy (EVE), is reported in this study. EVE is a local, inter-grain orientational correlations effect, which results from a combination of continuous nucleation of anisotropic grains and impingement of growing grains. Due to EVE, anisotropically shaped grains have a tendency to be similarly aligned in a local neighbourhood, despite the absence of any forced global orientation in the sample. The effect has been repeatedly observed by the authors in block copolymers, as illustrated by a representative TEM image. Optical microscopy of anisotropically shaped non-polymeric crystals revealed the generality of this effect. The simulation study revealed a tendency for azimuthal, inter-grain orientational correlation and re-confirmed the experimental observation of EVE.     

Micro-/Mesoporous conjugated polymer based on star-shaped triazine-functional triphenylamine framework as the performance-improved cathode of Li-organic battery

A novel organic conjugated polymer based on star-shaped triazine-functional triphenylamine framework poly[1,3,5-tris(4-diphenylamino-phenyl)triazine] (PTDAPTz) is designed and synthesized successfully by FeCl₃-catalysted chemical oxidative polymerization. The polymer PTDAPTz powder exhibits a compactly packed pleated skirt shape-like morphology with a high surface area (~930 m² g⁻¹) and a bimodal pore size distribution ranging from micropores (~0.55 nm) to small diameter mesopores (~2–6 nm). As explored as the cathode material, the obtained PTDAPTz presents the double charge–discharge process characteristics of both the free radical redox of triphenylamine unit and the bipolar redox of triazine unit in the polymer and a well-defined multistage charge/discharge voltage plateau (~3.8 V for p-doped and ~2.0 V for n-doped) during the charge–discharge process. Also, the PTDAPTz demonstrates an improved capacity (stabilized at 123 mA h g⁻¹ until 50th cycle) and the enhanced rate performance compared to polytriphenylamine (PTPAn). Specially, the discharge curve for the part of triphenylamine unit presents an obviously improved discharge plateau (~3.8 V for PTDAPTz compared to ~3.6 V for PTPAn) due to the electron-withdrawing effect of the triazine unit to triphenylamine. The elaborate structural design and created micro-/mesoporous morphology with the double charge–discharge process make PTDAPTz a potential candidate as the performance-improved cathode of Li-organic battery. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2574–2583     

Crystallographic and microstructural aspects of the superconductor YBa2Cu3O7-y —an overview

Crystal structures and structural inhomogeneities observed in YBa₂Cu₃O_7-y are reviewed. It is brought out that a proper understanding of the nature of the structural inhomogeneities is essential for exploiting the technological potential of this material. The need for an adequate characterization of specimens used for experiments is emphasized. Practical implications of the ferroelastic nature of the material are discussed.     

Arraying Cell Cultures Using PEG‐DMA Micromolding in Standard Culture Dishes

A robust and effortless procedure is presented, which allows for the microstructuring of standard cell culture dishes. Cell adhesion and proliferation are controlled by three‐dimensional poly(ethylene glycol)‐dimethacrylate (PEG‐DMA) microstructures. The spacing between microwells can be extended to millimeter size in order to enable the combination with robotic workstations. Cell arrays of microcolonies can be studied under boundary‐free growth conditions by lift‐off of the PEG‐DMA layer in which the growth rate is accessible via the evolution of patch areas. Alternatively, PEG‐DMA stencils can be used as templates for plasma‐induced patterning.

     

Microstructural evolution during oxidative ablation in air for polyacrylonitrile based carbon fibers with different graphite degrees

Polyacrylonitrile‐based carbon fibers with different graphite degrees were oxidative ablated at 500 and 600 °C in air. By Thermal gravimetric (TG), Raman spectroscopy, X‐ray diffraction, and SEM, the mass loss, microstructure, and surface morphology of carbon fibers were investigated. The mass loss of carbon fiber increases linearly with increasing oxidative ablated time under 500 and 600 °C. The carbon fiber with higher graphite degree shows higher oxidative resistance, and the surface roughness increases gradually because of chemical ablation during the whole oxidation. A gloss morphology appears on the surface primarily because of physical denudation for carbon fibers with lower graphite degree and then burn off according to carbon and oxygen reaction. The crystallite size (La) decreases significantly, while interlayer spacing(d₀₀₂) remains nearly unchanged. SEM observation suggests the two kinds of ablation mechanisms for carbon fibers with different graphite degrees indicating that CC band in sp³ hybridization prefers to be attacked by oxygen molecule more than that in sp² hybridization during oxidation ablation in air. Copyright © 2012 John Wiley & Sons, Ltd.