Magnetic frustration and dielectric relaxation in insulating Nd2NiMnO6 double perovskites

We have investigated structural, dielectric, and magnetic properties of polycrystalline double perovskite Nd2NiMnO6 compound. The compound crystallizes in monoclinic P2(1)/n symmetry and is partially B-site disordered depending on the synthesis conditions. It undergoes second-order ferromagnetic transition at 192K and shows glassy behaviour at low temperature. The glassy phase is due to anti-site disorder within the homogeneous sample. Temperature and frequency dependent dielectric measurements reveal colossal values of dielectric constant and is best interpreted using Maxwell-Wagner interfacial polarization model. Impedance spectroscopy has been used to analyse the intrinsic dielectric response. This enabled us to differentiate the conduction process at the grain and grain boundaries. Arrhenius behaviour is favoured at the grain boundary, while variable range hopping mechanism is considered most suitable within the grain region. dc conductivity measurements corroborate variable range hopping conduction. (C) 2015 AIP Publishing LLC.

Tailored interface and enhanced elastic modulus in epoxy-based composites in presence of branched poly(ethyleneimine) grafted multiwall carbon nanotubes

In this study, branched poly(ethyleneimine), BPEI, was synthesized from carboxylic acid terminated multi-walled carbon nanotubes (c-MWNTs) and characterized using FTIR, TEM and TGA. The BPEI was then chemically grafted onto MWNTs to enhance the interfacial adhesion with the epoxy matrix. The epoxy composites with c-MWNTs and the BPEI-g-MWNTs were prepared using a sonication and mechanical stirring method, followed by curing at 100 degrees C and post-curing at 120 degrees C. The dynamic mechanical thermal analysis showed an impressive 49% increment in the storage elastic modulus in the composites. In addition, the nanoindentation on the composites exhibited significant improvement in the hardness and decrease in the plasticity index in the presence of the BPEI-g-MWNTs. Thus, epoxy composites with BPEI-g-MWNTs can be further explored as self-healing materials.

Tailoring the dispersion of multiwall carbon nanotubes in co-continuous PVDF/ABS blends to design materials with enhanced electromagnetic interference shielding

Highly conducting composites were derived by selectively localizing multiwall carbon nanotubes (MWNTs) in co-continuous PVDF/ABS (50/50, wt/wt) blends. The electrical percolation threshold was obtained between 0.5 and 1 wt% MWNTs as manifested by a dramatic increase in the electrical conductivity by about six orders of magnitude with respect to the neat blends. In order to further enhance the electrical conductivity of the blends, the MWNTs were modified with amine terminated ionic liquid (IL), which, besides enhancing the interfacial interaction with PVDF, facilitated the formation of a network like structure of MWNTs. This high electrical conductivity of the blends, at a relatively low fraction (1 wt%), was further explored to design materials that can attenuate electromagnetic (EM) radiation. More specifically, to attenuate the EM radiation by absorption, a ferroelectric phase was introduced. To accomplish this, barium titanate (BT) nanoparticles chemically stitched onto graphene oxide (GO) sheets were synthesized and mixed along with MWNTs in the blends. Intriguingly, the total EM shielding effectiveness (SE) was enhanced by ca. 10 dB with respect to the blends with only MWNTs. In addition, the effect of introducing a ferromagnetic phase (Fe3O4) along with IL modified MWNTs was also investigated. This study opens new avenues in designing materials that can attenuate EM radiation by selecting either a ferroelectric (BT-GO) or a ferromagnetic phase (Fe3O4) along with intrinsically conducting nanoparticles (MWNTs).

Influence of yttria and zirconia additions on spark plasma sintering of alumina composites

The mechanisms of densification and creep were examined during spark plasma sintering (SPS) of alumina doped with a low and high level of zirconia or yttria, over a temperature range of 1173-1573 K and stresses between 25 and 100 MPa. Large additions of yttria led clearly to in situ reactions during SPS and the formation of a yttrium-aluminum garnet phase. Dopants generally lead to a reduction in the densification rate, with substantial reductions noted in samples with similar to 5.5 vol% second phase. In contrast to a stress exponent of n similar to 1 for pure alumina, the doped aluminas displayed n similar to 2 corresponding to an interface-controlled diffusion process. The higher activation energies in the composites are consistent with previous data on creep and changes in the interfacial energies. The results reveal a compensation effect, such that an increase in the activation energy is accompanied by a corresponding increase in the pre-exponential term for diffusion.

Effects of Interface Treatment on the Fatigue Behavior of Shape Memory Alloy Reinforced Polymer Composites

Interfacial properties of Shape Memory Alloy (SMA) reinforced polymer matrix composites can be enhanced by improving the interfacial bonding. This paper focuses on studying the interfacial stresses developed in the SMA-epoxy interface due to various laser shot penning conditions. Fiber-pull test-setup is designed to understand the role of mechanical bias stress cycling and thermal actuation cycling. Phase transformation is tracked over mechanical and thermal fatigue cycles. A micromechanics based model developed earlier based on shear lag in SMA and energy based consistent homogenization is extended here to incorporate the stress-temperature phase diagram parameters for modeling fatigue.

Mapping the intriguing transient morphologies and the demixing behavior in PS/PVME blends in the presence of rod-like nanoparticles

The demixing behavior, transient morphologies and mechanism of phase separation in PS/PVME blends were greatly altered in the presence of a very low concentration of rod-like particles (multiwall carbon nanotubes, MWNTs). This phenomenon is due to the specific interaction of one of the phases (PVME) with the anisotropic MWNTs, which creates a heterogeneous environment in the blend. This specific interaction alters the chain dynamics in the interfacial region as against the bulk. A comprehensive analysis using isochronal temperature sweep was performed to understand the demixing temperature in the blends. The evolution of phase morphology as a function of time and temperature was assessed by polarizing optical microscopy (POM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). The addition of MWNTs increased the rheological demixing temperature and the spinodal temperature in almost all the compositions. The intriguing transient morphologies were mapped, which varied from nucleation and growth to coalescence-induced viscoelastic phase separation (C-VPS) in PVME-rich blends, to spinodal decomposition in the near-critical compositions, to transient gel-induced VPS (T-VPS) in the PS-rich compositions. Mapping of the morphology development displayed two types of fracture mechanisms: ductile fracture for near-critical compositions and brittle fracture for off-critical composition. The change in the phase separation mechanism in the presence of MWNTs was due to the variation in dynamic asymmetry brought about by these anisotropic particles. All these observations were correlated by POM, SEM and AFM studies. The length of the cooperatively rearranging region (CRR), as evaluated using modulated differential scanning calorimetry (MDSC) measurements, was found to be composition-independent. The observed variation of effective glass transition of PVME (low T-g component) on blending with PS (high Tg component) and by the addition of MWNTs accounts for the dynamic heterogeneity introduced by MWNTs in the system.

Damage mechanisms in stainless steel and chromium carbide coatings under controlled environment fretting conditions

Fretting is of a serious concern in many industrial components, specifically, in nuclear industry for the safe and reliable operation of various component and/or system. Under fretting condition small amplitude oscillations induce surface degradation in the form of surface cracks and/or surface wear. Comprehensive experimental studies have been carried out simulating different fretting regimes under ambient and vacuum (10(-9) MPa) conditions and, temperature up to 400 degrees C. Studies have been carried out with stainless steel spheres on stainless steel flats, and stainless steel spheres against chromium carbide, with 25% nickel chrome binder coatings. Mechanical responses are correlated with the damage observed. It has been observed that adhesion plays a vital role in material degradation process, and its effectiveness depends on mechanical variables such as normal load, interfacial tangential displacement, characteristics of the contacting bodies and most importantly on the environment conditions. Material degradation mechanism for ductile materials involved severe plastic deformation, which results in the initiation or nucleation of cracks. Ratcheting has been observed as the governing damage mode for crack nucleation under cyclic tangential loading condition. Further, propagation of the cracks has been observed under fatigue and their orientation has been observed to be governed by the contact conditions prevailing at the contact interface. Coated surfaces show damage in the form of brittle fracture and spalling of the coatings. Existence of stick slip has been observed under high normal load and low displacement amplitude. It has also been observed that adhesion at the contact interface and instantaneous cohesive strength of the contacting bodies dictates the occurrence of material transfer. The paper discusses the mechanics and mechanisms involved in fretting damage under controlled environment conditions. (C) 2015 Elsevier B.V. All rights reserved.

Tailoring the interface in graphene/thermoset polymer composites: A critical review

With the emergence of scientific interest in graphene oxide (GO) in recent times, researchers have endeavored to incorporate GO in thermoset polymeric matrix to develop composites with extraordinary set of properties. The current state of research in graphene/thermoset polymer composites is highlighted here with a focus on the role of interface in dictating the overall properties of the composites. Different strategies like covalent and non-covalent functionalization of GO have been discussed with respect to improvement in mechanical, electrical, thermal and rheological properties. In addition, future prospects have been outlined. By assessing the current state of research in graphene/thermoset composites, it is obvious that graphene derivatives are promising materials in enhancing the structural properties of the nanocomposites at extremely low levels of filler loading. This opens new avenues in designing lightweight composites for myriad applications and by tailoring the interfacial adhesion with the polymer, ordered structure can be achieved at macroscopic processing scales. (C) 2015 Elsevier Ltd. All rights reserved.

Evolutionary and structural analyses of heterodimeric proteins composed of subunits with same fold

Heterodimeric proteins with homologous subunits of same fold are involved in various biological processes. The objective of this study is to understand the evolution of structural and functional features of such heterodimers. Using a non-redundant dataset of 70 such heterodimers of known 3D structure and an independent dataset of 173 heterodimers from yeast, we note that the mean sequence identity between interacting homologous subunits is only 23-24% suggesting that, generally, highly diverged paralogues assemble to form such a heterodimer. We also note that the functional roles of interacting subunits/domains are generally quite different. This suggests that, though the interacting subunits/domains are homologous, the high evolutionary divergence characterize their high functional divergence which contributes to a gross function for the heterodimer considered as a whole. The inverse relationship between sequence identity and RMSD of interacting homologues in heterodimers is not followed. We also addressed the question of formation of homodimers of the subunits of heterodimers by generating models of fictitious homodimers on the basis of the 3D structures of the heterodimers. Interaction energies associated with these homodimers suggests that, in overwhelming majority of the cases, such homodimers are unlikely to be stable. Majority of the homologues of heterodimers of known structures form heterodimers (51.8%) and a small proportion (14.6%) form homodimers. Comparison of 3D structures of heterodimers with homologous homodimers suggests that interfacial nature of residues is not well conserved. In over 90% of the cases we note that the interacting subunits of heterodimers are co-localized in the cell. Proteins 2015; 83:1766-1786. (c) 2015 Wiley Periodicals, Inc.

Three-Dimensional Nanoarchitecture of BiFeO3 Anchored TiO2 Nanotube Arrays for Electrochemical Energy Storage and Solar Energy Conversion

Here, we report the synthesis of TiO2/BiFeO3 nano-heterostnicture (NH) arrays by anchoring BiFeO3 (BFO) particles on on TiO2 nanotube surface and investigate their pseudocapacitive and photoelectrochemical properties considering their applications in green energy fields. The unique TiO2/BFO NHs have been demonstrated both as energy conversion and storage materials. The capacitive behavior of the NHs has been found to be significantly higher than that of the pristine TiO2 NTs, which is mainly due to the anchoring of redox active BFO nanoparticles. A specific capacitance of about 440 F g(-1) has been achieved for this NHs at a current density of 1.1 A g(-1) with similar to 80% capacity retention at a current density of 2.5 A g(-1). The NHs also exhibit high energy and power performance (energy density of 46.5 Wh kg(-1) and power density of 1.2 kW kg(-1) at a current density of 2.5 A g(-1)) with moderate cycling stability (92% capacity retention after 1200 cycles). Photoelectrochemical investigation reveals that the photocurrent density of the NHs is almost 480% higher than the corresponding dark current and it shows significantly improved photoswitching performance as compared to pure TiO2 nanotubes, which has been demonstrated based the interfacial type-II band alignment between TiO2 and BFO.

Investigation of Annealing Induced Yttria Segregation in Sputtered Yttria-Stabilized Zirconia Thin Films

8mol% yttria-stabilized zirconia (8YSZ) is an extensively studied solid electrolyte. But there is no consistency in the reported ionic conductivity values of 8YSZ thin films. Interfacial segregation in YSZ thin films can affect its ionic conductivity by locally altering the surface chemistry. This article presents the effects of annealing temperature and film thickness on free surface yttria segregation behavior in 8YSZ thin film by Angle Resolved XPS and its influence on the ionic conductivity of sputtered 8YSZ thin films. Surface yttria concentration of about 32, 20, and 9mol% have been found in 40nm 8YSZ films annealed at 1273, 1173, and 1073K, respectively. Yttria segregation is found to increase with increase in annealing temperature and film thickness. Ionic conductivities of 0.23, 0.16, and 0.08Scm(-1) are observed at 923K for 40nm 8YSZ films annealed at 1073, 1173, and 1273K, respectively. The decrease in conductivity with increase in annealing temperature is attributed to the increased yttria segregation with annealing. Neither segregation nor film thickness is found to affect the activation energy of oxygen ion conduction. Target purity is found to play a key role in determining free surface yttria segregation in 8YSZ thin films.

Micromechanical analysis of self compacting concrete

The micro-level properties of different self compacting concrete (SCC) mixes with and without mineral admixures are studied. The study considers SCC as a two phase material consisting of matrix and aggregate. Micro indentation technique is employed to obtain the hardness of individual phases and to compute the micro-property (modulus of elasticity). Using a self consistent homogenization procedure, the micro-property is scaled-up to obtain the macro-property which is shown to agree with the experimentally obtained macro values. It is seen that there exists a smaller interfacial transition zone at different ages of curing across all the mixes due to the presence of more fines in SCC. Also, there is no significant change in the property of the SCC having no fly ash or silica fume beyond 28 days whereas a substantial change in the micro and macro properties are seen in the SCC having fly ash and silica fume.

Weak conservation of structural features in the interfaces of homologous transient protein-protein complexes

Residue types at the interface of protein-protein complexes (PPCs) are known to be reasonably well conserved. However, we show, using a dataset of known 3-D structures of homologous transient PPCs, that the 3-D location of interfacial residues and their interaction patterns are only moderately and poorly conserved, respectively. Another surprising observation is that a residue at the interface that is conserved is not necessarily in the interface in the homolog. Such differences in homologous complexes are manifested by substitution of the residues that are spatially proximal to the conserved residue and structural differences at the interfaces as well as differences in spatial orientations of the interacting proteins. Conservation of interface location and the interaction pattern at the core of the interfaces is higher than at the periphery of the interface patch. Extents of variability of various structural features reported here for homologous transient PPCs are higher than the variation in homologous permanent homomers. Our findings suggest that straightforward extrapolation of interfacial nature and inter-residue interaction patterns from template to target could lead to serious errors in the modeled complex structure. Understanding the evolution of interfaces provides insights to improve comparative modeling of PPC structures.

Effect of Surface Energy Anisotropy on the Stability of Growth Fronts in Multiphase Alloys

Eutectic growth offers a variety of examples for pattern formation which are interesting both for theoreticians as well as experimentalists. One such example of patterns is ternary eutectic colonies which arise as a result of instabilities during growth of two solid phases. Here, in addition to the two major components being exchanged between the solid phases during eutectic growth, there is an impurity component which is rejected by both solid phases. During progress of solidification, there develops a boundary layer of the third impurity component ahead of the solidification front of the two solid phases. Similar to Mullins-Sekerka type instabilities, such a boundary layer tends to make the global solidification envelope unstable to morphological perturbations giving rise to two-phase cells. This phenomenon has been studied numerically in two dimensions for the conditions of directional solidification, by Plapp and Karma (Phys Rev E 66:061608, 2002) using phase-field simulations. While, in the work by Plapp and Karma (Phys Rev E 66:061608, 2002) all interfaces are isotropic, in our presentation, we extend the phase-field model by considering interfacial anisotropy in the solid-solid and solid-liquid interfaces and characterize the role of interfacial anisotropy on the stability of the growth front through phase-field simulations in two dimensions.

Pattern-Formation During Self-organization in Three-Phase Eutectic Solidification

Eutectic growth is an interesting example for exploring the topic of pattern-formation in multi-phase systems, where the growth of the phases is coupled with the diffusive transport of one or more components in the melt. While in the case of binary alloys, the number of possibilities are limited (lamellae, rods, labyrinth etc.), their number rapidly increases with the number of components and phases. In this paper, we will investigate pattern formation during three-phase eutectic solidification using a state-of-the art phase-field method based on the grand-canonical density formulation. The major aim of the study is to highlight the role of two properties, which are the volume fraction of the solid phases and the solid-liquid interfacial energies, in the self-organization of the solid phases during directional growth. Thereafter, we will show representative phase-field simulations of a micro-structure in a real alloy (Ag-Al-Cu) using an asymmetric phase diagram as well as interfacial properties.