Time‐resolved investigation of Cu(In,Ga)Se2 growth and Ga gradient formation during fast selenisation of metallic precursors

Ga segregation at the backside of Cu(In,Ga)Se₂ solar cell absorbers is a commonly observed phenomenon for a large variety of sequential fabrication processes. Here, we investigate the correlation between Se incorporation, phase formation and Ga segregation during fast selenisation of Cu–In–Ga precursor films in elemental selenium vapour. Se incorporation and phase formation are analysed by real‐time synchrotron‐based X‐ray diffraction and fluorescence analysis. Correlations between phase formation and depth distributions are gained by interrupting the process at several points and by subsequent ex situ cross‐sectional electron microscopy and Raman spectroscopy. The presented results reveal that the main share of Se incorporation takes place within a few seconds during formation of In–Se at the top part of the film, accompanied by outdiffusion of In out of a ternary Cu–In–Ga phase. Surprisingly, CuInSe₂ starts to form at the surface on top of the In–Se layer, leading to an intermediate double graded Cu depth distribution. The remaining Ga‐rich metal phase at the back is finally selenised by indiffusion of Se. On the basis of a proposed growth model, we discuss possible strategies and limitations for the avoidance of Ga segregation during fast selenisation of metallic precursors. Solar cells made from samples selenised with a total annealing time of 6.5 min reached conversion efficiencies of up to 14.2 % (total area, without anti‐reflective coating). The evolution of the Cu(In,Ga)Se₂ diffraction signals reveals that the minimum process time for high‐quality Cu(In,Ga)Se₂ absorbers is limited by cation ordering rather than Se incorporation. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of cure kinetics in epoxy/multiwalled carbon nanotube nanocomposites

With the increased interest in thermoset resin nanocomposites, it is important to understand the effects of the material on nanoscale characteristics. In this study, a curing reaction of an epoxy resin, which contained 0.25, 0.50, or 1.00 wt % of multiwalled carbon nanotubes (MWCNTs), at different heating rates was monitored by differential scanning calorimetry; cure kinetics were also evaluated to establish a relationship between crosslinking (network formation) and mechanical properties. MWCNT concentrations above 0.25 wt % favored crosslinking formation and decreased the activation energy (E_a) in the curing reaction. Examination of the kinetic mechanism suggests that the MWCNT locally restricted the spatial volume and favored the formation of nodular morphology in the resin, especially for high MWCNT concentrations. The MWCNT exhibited some entanglement in the matrix, which hindered a more pronounced effect on the mechanical properties. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39857.     

Pulse profile and metal transfer in pulsed gas metal arc welding: droplet formation,detachment and velocity

The influence of current profile and pulse parameters on droplet formation and transfer was investigated. One profile has an exponential ramp up and down in the current pulse shape, while the second is nearly square shaped. High-speed photography, synchronised with a high-speed data acquisition system, was used to monitor the droplet formation and transfer. It was found that for long-tail current profile, most of droplet formation and detachment occurs before background current is reached. While, for the nearly square pulse, most of droplet formation and transfer occurs during background current, giving a stable and smooth metal transfer. The arc attachment position was found to vary for the different profiles. Droplet speed was measured, and it was found that it is proportional to the peak current and inversely proportional to background current. Dimensionless process parameters were defined and used to predict droplet speed using a neural networks algorithm.     

The bacterial actin nucleator protein ActA of Listeria monocytogenes contains multiple binding sites for host microfilament proteins

BACKGROUND: Several intracellular pathogens, including Listeria monocytogenes, use components of the host actin-based cytoskeleton for intracellular movement and for cell-to-cell spread. These bacterial systems provide relatively simple model systems with which to study actin-based motility. Genetic analysis of L. monocytogenes led to the identification of the 90 kD surface-bound ActA polypeptide as the sole bacterial factor required for the initiation of recruitment of host actin filaments. Numerous host actin-binding proteins have been localized within the actin-based cytoskeleton that surrounds Listeria once it is inside a mammalian cell, including alpha-actinin, fimbrin, filamin, villin, ezrin/radixin, profilin and the vasodilator-stimulated phosphoprotein, VASP. Only VASP is known to bind directly to ActA. We sought to determine which regions of the ActA molecule interact with VASP and other components of the host microfilament system. RESULTS: We used the previously developed mitochondrial targeting assay to determine regions of the ActA protein that are involved in the recruitment of the host actin-based cytoskeleton. By examining amino-terminally truncated ActA derivatives for their ability to recruit cytoskeletal proteins, an essential element for actin filament nucleation was identified between amino acids 128 and 151 of ActA. An ActA derivative from which the central proline-rich repeats were deleted retained its ability to recruit filamentous actin, albeit poorly, but was unable to bind VASP. CONCLUSIONS: Our studies reveal the initial interactions that take place between invading Listeria and host microfilament proteins. The listerial ActA polypeptide contains at least two essential sites that are required for efficient microfilament assembly: an amino-terminal 23 amino-acid region for actin filament nucleation, and VASP-binding proline-rich repeats. Hence, ActA represents a prototype actin filament nucleator. We suggest that host cell analogues of ActA exist and are important components of structures involved in cell motility.     

Influence of different concentrations of glycidylisobutyl‐POSS on the glass transition of cured epoxy resin

In this study, polyhedral oligomeric silsesquioxane glycidylisobutyl‐POSS was dispersed in epoxy resin by ultrasound, and the parameters of a phantom model, the Williams–Landel–Ferry (WLF) and Vogel–Fulcher–Tammann (VFT) equations were modeled using dynamic mechanical analysis (DMA) to evaluate their influence on the glass transition state. The relaxation and retardation time distributions were estimated using a nonlinear regularization method, and the estimated physical parameters were discussed based on the results obtained from transmission electron microscopy (TEM). The TEM analysis indicated higher POSS dispersion with a spherical shape. The POSS dispersion was associated with the formation of micelles due to their hybrid character. The micelles favored the interconnections of the nodular microstructure of the epoxy thermosetting, which led to an increase in their T_g values. These interconnections increased the structure's percolation, promoted a reduction in the thermal expansion coefficient and resulted in a more homogeneous glass transition, in terms of a cooperative distribution in the relaxation times at the time scale measured. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41453.     

Scratch and indentation tests on polyoxymethylene (POM)

Scratch and indentation tests were performed on a polyoxymethylene homopolymer that was molded using three different mold temperatures. The different microstructures that developed during processing were studied by examining two samples for each mold temperature at different depths. During the scratch tests, the normal and the tangential force were recorded. The tests on the “as-molded” surface, in comparison to those that were polished until 1 mm of the material was removed, showed extrema of the normal force at a scratch depth that correspond to the interface of different regimes in the microstructure. The first extremum was considered to be caused by the breaking of the skin layer of the polymer (as a result of the rapid cooling process in the mold). Further extrema correspond to the permeation of the transition zone of the indenter into the spherulitic core of the sample. Indentation tests that were made on the same samples showed that the skin had a slightly lower hardness than the bulk of the polymer and that the difference in hardness decreases with increasing mold temperature. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1985–1996, 1997     

Thermoplastic polyurethane synthesis with modified montmorillonite prepared by torque rheometry: Investigation of morphological,thermal, chemical,and physical properties

In this study, samples of thermoplastic polyurethane (TPU) were synthesized by reactive processing in an instrumented batch mixer with 0, 1, 2, 5, and 10 wt % of organophilic montmorillonite (OMMT) clay. Fourier transform infrared spectroscopy confirmed the obtainment of TPU. Transmission electron microscopy showed several types of clay dispersion. Analysis by X‐ray diffraction revealed modifications in the TPU microstructure, a reduction in the degree of crystallinity, and an increase in the crystal size. Dynamic mechanical properties showed that the incorporation of OMMT has a strong influence on the storage and loss moduli obtained for the TPU matrix. The incorporation of OMMT also altered the crystallization and thermal stability of the TPU. With the use of the Flynn–Wall–Ozawa method it was observed that the nanoclay had a higher apparent activation energy. The results obtained applying the Criado method indicated that the solid state reaction is essentially controlled by geometric contraction, random nucleation with one nucleus for each individual particle and diffusion. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42640.     

Advanced X-ray methods for chalcogenide thin film analysis

This contribution reports advances made by very useful X-ray methods for the analysis of chalcopyrite based thin films. First, a suitable grazing incidence X-ray diffraction (GIXRD) setup which effectively enables depth sensitive analysis of chalcogenide thin films is described. A novel peak profile analysis method facilitates compositional depth profiling of Cu(In,Ga)(S,Se)₂ thin films. In situ growth studies of chalcogenide thin films using energy dispersive X-ray diffraction (EDXRD) require collimated, high flux X-ray excitation available at synchrotron light sources. The benefits of time resolved EDXRD analysis for the understanding of growth mechanisms are demonstrated. Finally the progress in the quantification of thin bilayer systems by soft X-ray emission spectroscopy (S-XES) is reported. Using the information of the relative emission intensities of particular elements in chalcogenide materials it is possible to accurately determine cover layer thickness in the nanometer scale of thin and even rough layers.