M. Aichele, Y. Gebremichael, F. W. Starr, J. Baschnagel, S. C. Glotzer; Publisher’s Note: “Polymer-specific effects of bulk relaxation and stringlike correlated
M. Aichele, Y. Gebremichael, F. W. Starr, J. Baschnagel, S. C. Glotzer; Publisher’s Note: “Polymer-specific effects of bulk relaxation and stringlike correlated
Three-dimensional laser scanning confocal microscopy has revealed that filamentous actin, when complexed with the cross-linking protein α-actinin, will spontaneously assemble on a micron scale into a structure compri...
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Three-dimensional laser scanning confocal microscopy has revealed that filamentous actin, when complexed with the cross-linking protein α-actinin, will spontaneously assemble on a micron scale into a structure comprised of a relatively rigid, frequently branching, 3D network of bundles with characteristic mesh size of the order of the persistence length of F-actin. In contrast, additional nanoscale ordering is observed, as synchrotron x-ray diffraction has revealed a disordered, distorted square lattice of actin fibers within the individual bundles.
A new loading apparatus was fabricated and used to conduct true-triaxial compression tests on KTB amphibolite, a crystalline basement rock. Tests in dry jacketed samples reveal that the intermediate principal stress h...
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The first step towards the full integration of fiber-reinforced polymer (FRP) composites in civil engineering applications is the complete understanding of the degradation mechanisms active during environmental exposu...
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The first step towards the full integration of fiber-reinforced polymer (FRP) composites in civil engineering applications is the complete understanding of the degradation mechanisms active during environmental exposure. The aim of this paper is to study the environmental durability of a unidirectional pultruded E-glass/vinyl ester composite under synergistic hygrothermal exposure and constant bending strain. Experimental results from the first 12 months of exposure, pertaining to the moisture absorption, glass transition temperature, longitudinal tensile properties and short beam shear properties of the composites, will be presented. It was observed that temperature has a significant influence on the diffusion coefficient as well as moisture saturation content, therefore, considerably affecting the retention strength of the composites. However, up to one year of exposure, the bending strains did not have any significant effects on the material properties. A correlation was established between the retention of tensile strength and the absorbed moisture content. Finally, a microscopic investigation of composite cross-sections showed the moisture-related damage to be primarily debonding with the level of debonding increasing with increasing exposure time or temperature.
The microstructure of epitaxial InAs thin films grown by MOCVD on mask-patterned “LEO” (lateral epitaxial overgrowth) GaAs and on unpatterned GaAs substrates was studied using double-crystal x-ray diffraction, scann...
The microstructure of epitaxial InAs thin films grown by MOCVD on mask-patterned “LEO” (lateral epitaxial overgrowth) GaAs and on unpatterned GaAs substrates was studied using double-crystal x-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. This paper describes the improvement in crystal quality (factor of 20 reduction in x-ray rocking curve width), the order of magnitude reduction in dislocation density, and the rearrangement of the remaining extended defects that were observed in the LEO material when compared to the film grown on the unpatterned wafer.
A massive magnetic-field-induced structural transformation in Gd5Ge4, which occurs below 30 K, was imaged at the atomic level by uniquely coupling high-resolution x-ray powder diffraction with magnetic fields up to 35...
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A massive magnetic-field-induced structural transformation in Gd5Ge4, which occurs below 30 K, was imaged at the atomic level by uniquely coupling high-resolution x-ray powder diffraction with magnetic fields up to 35 kOe. In addition to uncovering the nature of the magnetic field induced structural transition, our data demonstrate that the giant magnetocaloric effect, observed in low magnetic fields, arises from the amplification of a conventional magnetic entropy-driven mechanism by the difference in the entropies of two phases, borne by the concomitant structural transformation.
We present a detailed nanoindentation study of micron-scale thin films of polystyrene (PS), poly(phenylene oxide) (PPO), poly(methyl methacrylate) (PMMA), a metal-centered PMMA-Ruthenium block copolymer, and a PS-poly...
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We present a detailed nanoindentation study of micron-scale thin films of polystyrene (PS), poly(phenylene oxide) (PPO), poly(methyl methacrylate) (PMMA), a metal-centered PMMA-Ruthenium block copolymer, and a PS-poly(ethylene-propylene) (PS-PEP) block copolymer with lamellar morphology. The results show that size-dependence is most readily noticeable for the lamellar PS-PEP film, indicating that the nanoidentation approach has sufficient sensitivity to capture scale dependence on scales in the range of tens of nanometers. The less pronounced scale-dependence (or lack thereof) in the other films is discussed in the context of identifying the physical length-scale of elementary processes of plastic deformation. The results indicate that the upper limit on the size of plastic shear zones in amorphous polymers is approximately 1200-9600 nm3 (i.e. a sphere with a diameter in the range of 20-40 nm).
The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a ...
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The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO2 pressure-induced devitrification transition, Pg, which is film thickness dependent, Pg(h)=ΔPg+Pgbulk. Pgbulk is the bulk glass transition and ΔPg can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent.
Single-phase Ba(Cd1/3Ta2/3)O3 ceramics have been produced using conventional powder processing methods. In our initial investigations, 2wt% ZnO powder was added to act as a sintering aid since a high-density ceramic w...
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Single-phase Ba(Cd1/3Ta2/3)O3 ceramics have been produced using conventional powder processing methods. In our initial investigations, 2wt% ZnO powder was added to act as a sintering aid since a high-density ceramic was not formed from solid-state diffusion alone. The resulting Ba(Cd0.327Zn0.006Ta2/3)O3 material sintered at 1550°C exhibits a dielectric constant of ∼33 and loss tangent of -5 at 2 GHz. In our more recent work, we have used boron as a sintering aid to facilitate sintering at temperatures as low as 1300°C, enhance the structural quality and improve the microwave properties of Ba(Cd1/3Ta2/3)O3 dielectrics. TEM results indicate that the liquid sintering mechanism is an important factor for boron concentrations exceeding 0.5wt%, while a point defect mechanism plays the dominant role at lower boron concentrations. The presence of superstructure peaks and splitting of the (220) and (214) peaks in X-ray diffraction spectra are direct evidence for the distortion from cubic symmetry as a result of Cd and Ta ordering on the B-site. Ab-initio electronic structure calculations within the local density functional approximation have been used to give insight into the unusual properties of this class of materials. In both Ba(Zn 1/3Ta2/3)O3 and Ba(Cd/3Ta 2/3)O3, the conduction band maximum and valence band minimum are composed of mostly weakly itinerant Ta 5d-and Zn-3d/Cd-4d levels, respectively. The covalent nature of the directional d-electron bonding in these high-Z oxides plays an important role in producing a more rigid lattice with higher melting points and enhanced phonon energies, and possibly inherently lower intrinsic microwave loss than comparable ionic materials.
Ordered structures of Ba(Cd1/3Ta1/2)O3 ceramics with and without boron additive were investigated systemically by electron diffraction and high resolution transmission electron microscopy. The results showed a well-or...
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Ordered structures of Ba(Cd1/3Ta1/2)O3 ceramics with and without boron additive were investigated systemically by electron diffraction and high resolution transmission electron microscopy. The results showed a well-ordered structure of 1:2 with hexagonal symmetry for Ba(Cd1/3Ta1/2)O3 with boron additive. No significant changes in ordered structures were observed after long-period annealing. The 1:2 ordered domain structures (average domain size ∼ 18 nm) and high-density domain boundaries induced by ordering were observed for Ba(Cd1/3Ta1/2)O3 without boron additive sintered at relatively high temperature. The sintering process has a profound influence on the microstructure of Ba(Cd1/3Ta2/3)O 3 ceramics.
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