• Characterization of Thin Film Materials using SCAN meta-GGA, an Accurate Nonempirical Density Functional

      Buda, IG; Lane, C; Barbiellini, B; Ruzsinszky, A; Sun, J; Bansil, A (2017-03-23)
      We discuss self-consistently obtained ground-state electronic properties of monolayers of graphene and a number of 'beyond graphene' compounds, including films of transition-metal dichalcogenides (TMDs), using the recently proposed strongly constrained and appropriately normed (SCAN) meta-generalized gradient approximation (meta-GGA) to the density functional theory. The SCAN meta-GGA results are compared with those based on the local density approximation (LDA) as well as the generalized gradient approximation (GGA). As expected, the GGA yields expanded lattices and softened bonds in relation to the LDA, but the SCAN meta-GGA systematically improves the agreement with experiment. Our study suggests the efficacy of the SCAN functional for accurate modeling of electronic structures of layered materials in high-Throughput calculations more generally.
    • Nanoparticle-Infused UHMWPE Layer as Multifunctional Coating for High-Performance PPTA Single Fibers

      Zhang, Z; Zhao, Y; Li, H; Percec, S; Yin, J; Ren, F; Ren, Fei|0000-0002-8868-4536 (2019-12-01)
      © 2019, The Author(s). High-performance fibers made of poly-(p-phenylene terephthalamide) (PPTA) with high stiffness and high strength are widely used in body armor for protection due to their high degree of molecular chain alignment along the fiber direction. However, their poor mechanical properties in the transverse direction and low surface friction are undesirable for applications requiring resistance to ballistic impact. Here we provide a simple yet effective surface engineering strategy to improve both the transverse mechanical properties and the tribological property by coating PPTA fibers with ultra-high molecular weight polyethylene (UHMWPE) embedded with silica nanoparticles. The coated-PPTA fiber shows remarkable enhancement in transverse mechanical properties including ~127% increase of Young’s modulus, which is attributed to both the alignment of UHMWPE chains in the transverse direction and the embeded ceramic nanoparticles. Meanwhile, the surface friction of the coated fiber increases twofold as a result of the ceramic nanoparticles. In addition, the coated fibers exhibit an enhanced chemical resistance to external harsh environment. The improved transverse mechanical properties, surface frictional characteristics, and chemical resistance demonstrate that coating with UHMWPE and ceramic nanoparticles can be used as an effective approach to enhance the performance of PPTA and other high-performance polymer fibers for body armor applications.
    • Observation of superconducting vortex clusters in S/F hybrids

      Di Giorgio, C; Bobba, F; Cucolo, AM; Scarfato, A; Moore, SA; Karapetrov, G; D'Agostino, D; Novosad, V; Yefremenko, V; Iavarone, M (2016-12-09)
      © 2016 The Author(s). While Abrikosov vortices repel each other and form a uniform vortex lattice in bulk type-II superconductors, strong confinement potential profoundly affects their spatial distribution eventually leading to vortex cluster formation. The confinement could be induced by the geometric boundaries in mesoscopic-size superconductors or by the spatial modulation of the magnetic field in superconductor/ferromagnet (S/F) hybrids. Here we study the vortex confinement in S/F thin film heterostructures and we observe that vortex clusters appear near magnetization inhomogeneities in the ferromagnet, called bifurcations. We use magnetic force microscopy to image magnetic bifurcations and superconducting vortices, while high resolution scanning tunneling microscopy is used to obtain detailed information of the local electronic density of states outside and inside the vortex cluster. We find an intervortex spacing at the bifurcation shorter than the one predicted for the same superconductor in a uniform magnetic field equal to the thermodynamical upper critical field H c2. This result is due to a local enhanced stray field and a competition between vortex-vortex repulsion and Lorentz force. Our findings suggest that special magnetic topologies could result in S/F hybrids that support superconductivity even when locally the vortex density exceeds the thermodynamic critical threshold value beyond which the superconductivity is destroyed.
    • Structure-Mechanical Property Relations of Skin-Core Regions of Poly(p-phenylene terephthalamide) Single Fiber

      Chabi, S; Dikin, DA; Yin, J; Percec, S; Ren, F; Ren, Fei|0000-0002-8868-4536; Dikin, Dmitriy|0000-0001-8100-4502 (2019-12-01)
      © 2019, The Author(s). This study aims to elucidate the relationship between the mechanical properties and microstructures of poly(p-phenylene terephthalamide) (PPTA) single fibers at the micro/nano scale. The skin-core structure of Kevlar® 29 fiber was revealed through a focused electron beam experiment inside a scanning electron microscope (SEM) chamber. Cross sectional SEM images of the broken fiber showed that the thickness of the skin ranged from 300 to 800 nm and that the core region consisted of highly packed layers of fibrils. The skin and the core regions showed different mechanical behaviour and structural changes during nanoindentation and micro-tensile tests, indicating that the core region possessed higher stiffness, whereas the skin region could undergo more plastic deformation. Furthermore, micro-tensile testing results showed that the ultimate tensile strength, the elongation at failure, and the tensile toughness of single fibers could be significantly enhanced by cyclic loading. Such findings are important to understand the contribution of different microstructures of Kevlar® fibers to their mechanical performance, which in turn can be utilized to design high-performance fibers that are not limited by the trade-off between toughness and stiffness.
    • Tunable electroresistance and electro-optic effects of transparent molecular ferroelectrics

      Zhang, Z; Li, PF; Tang, YY; Wilson, AJ; Willets, K; Wuttig, M; Xiong, RG; Ren, S; Willets, Katherine A|0000-0002-1417-4656 (2017-08-01)
      Copyright © 2017 The Authors, some rights reserved. Recent progress in molecular ferroelectrics (MOFEs) has been overshadowed by the lack of high-quality thin films for device integration. We report a water-based air-processable technique to prepare large-area MOFE thin films, controlled by supersaturation growth at the liquid-air interface under a temperature gradient and external water partial pressure. We used this technique to fabricate ImClO4 thin films and found a large, tunable room temperature electroresistance: a 20-fold resistance variation upon polarization switching. The as-grown films are transparent and consist of a bamboo-like structure of (2, 1-, 0) and (1, 0, 2-) structural variants of R3m symmetry with a reversible polarization of 6.7 μC/cm2. The resulting ferroelectric domain structure leads to a reversible electromechanical response of d33 = 38.8 pm/V. Polarization switching results in a change of the refractive index, n, of single domains, Δn/n = 0.3. The remarkable combination of these characteristics renders MOFEs a prime candidate material for new nanoelectronic devices. The information that we present in this work will open a new area of MOFE thin-film technologies.
    • Visualizing domain wall and reverse domain superconductivity

      Iavarone, M; Moore, SA; Fedor, J; Ciocys, ST; Karapetrov, G; Pearson, J; Novosad, V; Bader, SD (2014-08-28)
      In magnetically coupled, planar ferromagnet-superconductor (F/S) hybrid structures, magnetic domain walls can be used to spatially confine the superconductivity. In contrast to a superconductor in a uniform applied magnetic field, the nucleation of the superconducting order parameter in F/S structures is governed by the inhomogeneous magnetic field distribution. The interplay between the superconductivity localized at the domain walls and far from the walls leads to effects such as re-entrant superconductivity and reverse domain superconductivity with the critical temperature depending upon the location. Here we use scanning tunnelling spectroscopy to directly image the nucleation of superconductivity at the domain wall in F/S structures realized with Co-Pd multilayers and Pb thin films. Our results demonstrate that such F/S structures are attractive model systems that offer the possibility to control the strength and the location of the superconducting nucleus by applying an external magnetic field, potentially useful to guide vortices for computing application. © 2014 Macmillan Publishers Limited.
    • Visualizing the structural evolution of LSM/xYSZ composite cathodes for SOFC by in-situ neutron diffraction

      Chen, Y; Yang, L; Ren, F; An, K; Ren, Fei|0000-0002-8868-4536 (2014-06-05)
      Thermal stability of composite cathodes for solid oxide fuel cells, the mixtures of (La0.8Sr0.2)0.95MnO 32-δ (LSM) and (Y2O3) x(ZrO2)1-x (xYSZ, x = 3, 6, 8 and 10), is determined using in-situ neutron diffraction. Thanks to the most advanced high flux neutron source, our work highlights the visualization of the phase evolutions in heterogeneous material systems at high temperatures, along with the analysis of the diffusion activities of transition metal ions that reveal the reaction mechanism and kinetics. It is found that the tetragonal-to-cubic phase transition in YSZ at T &gt'C leads to a heterogeneous redistribution of Mnions. The subsequent reaction of LSM and YSZ occurring at T > 1100°C is revealed as a three-stage kinetic process, yielding La2Zr 2O7, SrZrO3 and MnO. The diffusion activities of Y, Mn and La ions in the heterogeneous systems at elevated temperatures are derived by the structural analysis, and the three-stage reaction of YSZ and LSM is found strongly correlated to ions' behaviors as functions of temperature.