J. M. Schultz

Phases, morphology, and diffusion in CuInxGa1−xSe2 thin films

CuInxGa1−xSe2 thin films, with various Ga/(Ga+In) ratios, suitable for solar cells were processed by selenizing stacked Cu, Ga, and In precursor layers in a H2Se reactor in the temperature range of 400–500 °C. Cu/Ga/In and Cu/In/Ga precursors were obtained by sequential sputtering of the elemental layers. The Cu/Ga/In and Cu/In/Ga precursors, and the selenized films were characterized by scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, and Auger electron spectroscopy. The precursors contained only binary and elemental phases in the as-deposited condition and after annealing. The selenized films had a nonuniform distribution of Ga and In. The surface of the selenized films were In rich, while the Mo/film interface in these films was Ga rich. The selenized films with Ga/(Ga+In) ratios greater than 0.25 contain graded Ga and In compositions, and the selenized films with Ga/(Ga+In) ratios less than 0.6 contain a phase-separated mixture of CuInSe2 and CuGaSe2 with the CuInSe2 ne...

Preparation of homogeneous Cu(InGa)Se2 films by selenization of metal precursors in H2Se atmosphere

Homogeneous single phase Cu(InGa)Se2 films with Ga/(In+Ga)=0.25–0.75 were formed by reacting Cu–Ga–In precursor films in H2Se followed by an anneal in Ar. X‐ray diffraction and Auger analysis show that the metal precursors reacted only in H2Se were multiphase films having a layered CuInSe2/CuGaSe2 structure. Solar cells made with the multiphase films have properties similar to CuInSe2 devices. Cells made with the annealed single phase films behave like Cu(InGa)Se2 devices with the band gap expected for the precursor composition.

Microfibrillar reinforced composite from drawn poly(ethylene terephthalate)/nylon-6 blend

Abstract Homopolymer poly(ethylene terephthalate) (PET) and nylon-6 (PA-6) and a blend (1:1 by weight) of these polymers, are each extruded as strips and ultraquenched from the melt. After zone drawing and additional annealing at temperatures, Ta, of 220 or 240°C for 5 or 25 h in vacuum, the samples are studied by differential scanning calorimetry, small-angle and wide-angle X-ray scattering, and mechanical testing. It is found that all components in the blend are oriented and that the perfection of the structure develops with increasing Ta up to 220°C. At Ta = 240°C, isotropization of the PA-6 component is observed, while the orientation of the PET is preserved. A microfibrillar reinforced composite is obtained, with mechanical properties as high as those of glass-fibre-reinforced nylons and surprisingly large (about 100 ×) deformability. It is shown that during thermal treatment for longer time, in addition to physical processes (crystallization and relaxation), chemical interactions (additional condensation and exchange reactions) take place at the interfaces, resulting in the formation of PET-PA-6 block copolymers playing the role of compatibilizers. Further interaction between PET (partially) and PA-6 (completely) amorphous phases leads ultimately to the involvement of the entire amount of PA-6 in the block copolymers, thus converting the starting PA-6 matrix of the microfibrillar reinforced composite into a PET-PA-6 copolymer matrix.

Structural development during the early stages of polymer melt spinning by in-situ synchrotron X-ray techniques

Recent results from melt spinning of several polymers (nylon 6, polyethylene, poly(vinylidene fluoride) using in-situ simultaneous synchrotron small- and wide-angle X-ray scattering (SWAXS) techniques are summarized. Here, mechanisms of early stages of structural and morphological development during crystallization under melt flow are probed. We find that a model consisting of the formation of defective shish-like structures, followed by the formation of kebab crystals is most consistent with our results. We have further verified that the defective shish structure cannot be detected by the wide-angle X-ray scattering (WAXS) technique, but is visible by small-angle X-ray scattering (SAXS). This model has been proposed by Keller et al., for structural development during elongational flow.

Self-induced field model for crystal twisting in spherulites

The observed twisting of crystals about the growth direction in spherulites is modeled as a response to fields generated during the crystal growth process. These fields can be compositional or mechanical. In either case, the high local field value in the melt near the interface acts to reduce the growth velocity. Twisting of the crystal about the growth direction places all portions of the growth surface in contact with melt of lower field value than would be the case for untwisted growth, thereby increasing the growth velocity. The effect of a compositional field is analyzed here, using an extension of the moving point source to compute the composition in the melt ahead of a twisting crystal. The retarding effect of the elastic twisting of the crystal is included in the analysis. Simulations using parameters for a crystallizable/uncrystallizable polyethylene blend are carried out for a crystal with thin, rectangular cross-section. The results are quantitatively and qualitatively consistent with observation. Comments on simplifications within and implications of the model are given.

In situ study of structure development in poly(trimethylene terephthalate) fibers during stretching by simultaneous synchrotron small- and wide-angle X-ray scattering

In this study, simultaneous synchrotron small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) on as-spun and on spun and subsequently drawn poly(trimethylene terephthalate) (PTT) fibers under cold and hot stretch-hold operations were performed. Crystallinity and orientation analyses were performed on WAXD patterns, while correlation function analysis was used to analyze SAXS patterns to determine the long period. For spun and drawn fibers, WAXD patterns suggested that unit cell dimensions, crystallinity and orientation were dependent on stretching temperature and strain. From orientation analysis, it was reasonable to infer that a rigid amorphous phase existed in the drawn fibers. From SAXS patterns, a defectively stacked lamellar structure was induced at 120°C. Further application of strain up to 100% at 120°C triggered a lamellar to fibrillar transition, as suggested by the formation of a pair of very weak narrow streaks perpendicular to the meridian in the WAXD pattern. These streaks imply a microfibrillar entity. For as-spun fibers, strain could only induce micro-crazes, whose planes are perpendicular to the fiber axis, but could not induce crystallization. At 120°C, WAXD patterns showed that a new transitional phase was formed, which could be transformed to the equilibrium structure with application of strain over 100%.

Correct determination of crystal lamellar thickness in semicrystalline poly(ethylene terephthalate) by small-angle X-ray scattering

Abstract For the purpose of resolving an uncertainty over the correct determination of the crystalline lamellar thickness in semicrystalline poly(ethylene terephthalate), PET, via small-angle X-ray scattering (SAXS) analysis, a gel-crystallization method from oligomeric poly- (ethylene glycol) solution was used to prepare samples with high crystallinity (57%). By using simultaneous synchrotron SAXS and wide-angle X-ray diffraction (WAXD) measurements, the heating and cooling processes of the gel-crystallized PET sample were monitored. Results support the assignment of the larger thickness value from the SAXS correlation function analysis as the lamellar crystal thickness. Analysis of WAXD 0 1 1 reflection line broadening gives the minimum lamellar thickness (in the chain axis) and verifies the thickness assignment for gel and melt crystallized samples. This assignment is critical as it affects the correct interpretation of the crystallization behavior in semicrystalline polymers of relatively low crystallinity.

Study of the structure development during the melt spinning of nylon 6 fiber by on-line wide-angle synchrotron X-ray scattering techniques

The melt spinning of nylon 6 has been studied with on-line wide-angle synchrotron X-ray scattering techniques. The apparatus consisted of a single screw extruder and a metering pump mounted on a horizontal platform that could be translated in the vertical direction allowing a range of distances to be sampled with the X-ray beam. The structure development, equatorial crystallinity index, and crystalline orientation were studied as a function of take-up speed and position along the spinline. For low-speed (50 mpm) situations, the nylon chains crystallize into independent hydrogen bonded sheets that start to interact with each other as their concentration starts to increase. For higher speed situations, the nylon chains crystallize directly into the interacting hydrogen-bonded sheet structure. Upon conditioning at room temperature for 24 h, this interacting hydrogen-bonded sheet structure transforms into the well-known three-dimensional alpha and gamma phases of nylon 6, probably existing in a shish-kabob structure. The equatorial crystallinity index increases as distance from the spinneret increases and as take-up speed decreases. The crystalline orientation function is constant along the spinline for a constant take-up speed, and increases as take-up speed is increased. Conditioning further increases both the crystallinity and crystalline orientation of the fibers.

Transverse screw dislocations: A source of twist in crystalline polymer ribbons

Abstract A new model to explain the twisting of ribbonlike crystals within spherulites is proposed. The model is based on the inherent twisting perpendicular to the axis of a screw dislocation. In the proposed model a sequence of transverse screw dislocations of the same sign are spaced along the ribbon axis. The magnitude of the twist given by such a string of dislocations is calculated, and the mean dislocation spacings needed to give the measured degrees of twist are noted and found to be not unreasonable. The model also predicts the correct form of the temperature dependence of the degree of twist. An interpretation of the extant model of polymer ribbons is presented, whereby the nature of the orientation of the polymer chains within the crystals and the folding of chains at the ribbon surfaces cause the sign of the dislocations to be maintained and control the sign of the twist throughout the spherulite.

Microstructure and fracture behaviour of short and long fibre-reinforced polypropylene composites

Microstructure and fracture mechanical behaviour of injection-moulded, longer glass fibrereinforced polypropylene (Verton* aspect ratio ≈ 320) were studied as a function of fibre volume fraction and compared to that of shorter fibre-filled polypropylene (aspect ratio ≈ 70). Toughness was measured using instrumented notched lzod and falling weight impact tests, as well as compact tension specimens. It was found that the addition of longer fibres generally increased the toughness of the material, although more significant increases were seen in the impact tests than were seen in the compact tension test. For the latter results, a correlation between toughness improvement and microstructural details was performed on the basis of the microstructural efficiency concept, a semi-empirical approach of the formKc,C = (a* +nR)Kc,M, where,Kc,C andKc,M are the fracture toughnesses of the composite and the matrix, respectively,a* is a matrix stress correction factor,n is a scaling parameter andR is a fibre reinforcement effectiveness factor. The latter corrects for differences in the composite microstructures, and incorporates effective fibre orientation factors, layering of injection moulded parts, and fibre volumes in the different layers.