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arXiv2013

Estimation of mass thickness response of embedded aggregated silica nanospheres from high angle annular dark-field scanning transmission electron micrographs

In this study we investigate the functional behavior of the intensity in high-angle annular dark field (HAADF) scanning transmission electron micrograph (STEM) images. The model material is a silica particle (20 nm) gel at 5 wt%. By assuming that the intensity response is monotonically increasing with increasing mass thickness of silica, an estimate of the functional form is calculated using a maximum likelihood approach. We conclude that a linear functional form of the intensity provides a fair estimate but that a power function is significantly better for estimating the amount of silica in the z-direction. The work adds to the development of quantifying material properties from electron micrographs, especially in the field of tomography methods and three-dimensional quantitative structural characterization from a STEM micrograph. It also provides means for direct three-dimensional quantitative structural characterization from a STEM micrograph.

cond-mat.mtrl-sci↗

arXiv2013

A perturbation basis for calculating NMR Diffusometry

An approximative method for solving the Bloch-Torrey equation in general porous media is presented. The method expand the boundaries defining the porous media using electrostatic charges. As a result the eigenvalue problem of the Laplace operator in a confined geometry can approximately solved. Importantly the approximative solution is orthogonal in the low-frequent region of Fourier space. This gives a natural approach for studying spin magnetization in presence of magnetic fields. The error in the approximation scales with N^{-2} times the magnitude of each eigenvalue, where N is the size of the expansion matrix. From a computational point of view, the calculations scale quadratically with the number of basis functions using fast multipole methods.

cond-mat.soft↗

arXiv2013

Innate character and theory model about channel segregation in a sand mold steel ingot

The channel segregation is a severe casting defect in steel ingots. The formation of channel segregation is generally attributed to the solute partition and the interdendritic thermosolutal convection in the mushy zone. In this article, the channel segregation in a steel ingot was carefully characterized by detailed experimental observation. A different formation mechanism for the channel segregation was revealed. In the mushy zone of the ingot, large amount of separate MnS inclusions move laterally and upwards. Some MnS inclusions will remain in the moving trace of inclusions. Such residual MnS inclusions appear as large amount of separate MnS inclusion chains. In the subsequent solid phase transition process, promoted by the MnS chains, ferrite prefers to forms form the austenite near the MnS inclusions and shows as large amount of separate ferrite chains. Large amount of ferrite chains align in a strip-like zone, which results in so called channel segregation in macro-etching. The physical model about the driving force for the MnS movement is further theoretically analyzed. In the mushy zone, the interface tension resultant applied on the MnS inclusions can act as the drive force for the lateral movement of MnS inclusions. And the buoyance applied on the MnS inclusions act as the drive force for the upwards movement.

cond-mat.mtrl-sci↗

arXiv2013

Microsegregation and dendritic growth mode of Al-5wt%Cu alloy based on non-equilibrium mush zone model

The microsegregation and dendritic growth mode of Al-5wt%Cu alloy was investigated. In the early solidification stage, the crystal growth mode of interrupted growth and periodic boundary trapping will happen, which results in the segregationless dendritic grains. Microsegregation only exists at the final solidification stage with extremely tiny residual melt fraction. In the tiny residual melt zone, the diffusion of solute from the enriched boundary layer to the residual melt and the convergence of enriched boundaries produce the final microsegregation.

cond-mat.mtrl-sci↗

arXiv2013

Statistical Mechanics of Multistability in Microscopic Shells

Unlike macroscopic multistable mechanical systems such as snap bracelets or elastic shells that must be physically manipulated into various conformations, microscopic systems can undergo spontaneous conformation switching between multistable states due to thermal fluctuations. Here we investigate the statistical mechanics of shape transitions in small elastic elliptical plates and shells driven by noise. By assuming that the effects of edges are small, which we justify exactly for plates and shells with a lenticular section, we decompose the shapes into a few geometric modes whose dynamics are easy to follow. We use Monte Carlo simulations to characterize the shape transitions between conformation minima as a function of noise strength, and corroborate our results using a Fokker-Planck formalism to study the stationary distribution and the mean first passage time problem. Our results are applicable to objects such as such as graphene flakes or protein β-sheets, where fluctuations, geometry and finite size effects are important.

cond-mat.stat-mech↗

arXiv2013

Mechanism of formation and peculiarities of structure of bulk MgB2 compound specimens

The existence of two phases within one and the same hexagonal lattice of MgB2 compound, differing in Mg and B (in the homogeneity region) and especially in impurity oxygen content, as well as in microstructure, is demonstrated by various techniques. The regions corresponding to these two phases of MgB2 have the sizes of 100-500 μm, and they fill the whole bulk of specimens, alternating with each other. It is suggested that the two-phase state of MgB2 compound is caused by specific features of its formation mechanism (as a result of synthesis at 800-1000°C), including the stages of Mg melting, dissolution of solid boron in it up to the composition of MgB2 and further crystallization of the MgB2 compound from the melt with the formation of dendrite-like structure with corresponding redistribution of main components and impurities.

cond-mat.supr-con↗

arXiv2013

Pressure-induced phase transitions of multiferroic BiFeO3

Pressure-induced phase transitions of multiferroic BiFeO3 have been investigated using synchrotron radiation X-ray diffraction with diamond anvil cell technique at room temperature. Present experimental data clearly show that rhombohedral (R3c) phase of BiFeO3 first transforms to monoclinic (C2/m) phase at 7 GPa, then to orthorhombic (Pnma) phase at 11 GPa, which is consistent with recent theoretical ab initio calculation. However, we observe another peak at 2θ=7° in the pressure range of 5-7 GPa that has not been reported previously. Further analysis reveals that this reflection peak is attributed to the orthorhombic (Pbam) phase, indicating the coexisting of monoclinic phase with orthorhombic phase in low pressure range.

cond-mat.mtrl-sci↗

arXiv2013

Imaging electrical activity of neurons with metamaterial nanosensors

A technology for recording electrical activity of large neuron populations at arbitrary depth in brain tissues with less than cell spatial and millisecond temporal resolutions was the most craving dream of neuroscientists and a long pursued goal of engineers for decades. Even though many imaging techniques have been devised up to date, none of them is capable to deliver either quantitatively valid data nor able to meet contradictory requirements posed for sensors to be safe, non-invasive and reliably working either within cultured cell populations or during chronic implantations in vivo. In my research project, I design and justify a novel nanobiosensors, capable to detect and optically report the electric fields across cellular membrane and investigate properties of that specially engineered plasmonic nanoantennas. In the following literature survey, I observe the current state of electrophysiology methods and after recalling the basics of fluorescence, discuss benefits and drawbacks of today's voltage sensitive labelling and electric fields imaging methods. This review is wrapped up by a brief outlook of prospective applications of this technology.

physics.bio-ph↗

arXiv2013

Electron microscopy and diffraction study of high-temperature diamond-like Si-based ferromagnetic with self-organized super-lattice distribution of Mn impurity

New structure data for the high-temperature diluted magnetic semiconductor Si:Mn, synthesized by laser method are presented. The higher resolution transmission electron microscopy and diffraction was applied in two crystal directions of epitaxial layers of DMS Si:Mn with elimination of the contribution from GaAs substrate and interface. It is established, that the DMS Si:Mn can be represent as compound with variable composition Si2+xMn1-x (0 <x <1), single-phase diamond like structure, high crystal perfection and the self-organized formation of the super-lattice structures with the period equal to triple the distance between the nearest (110) atomic layers and interval between (110) layers which are doped by Mn atoms and orientated along the direction of Si:Mn film growth. The Si with 15 percent of Mn (or Si2.5Mn0.5) films consist of blocks with the 15-50 nm sizes and with mutually perpendicular orientations of the super-lattice modulations. Atoms of manganese in the (110) layers, doped by these impurity, settle down in the form of strips of an one-atom thickness. These manganese strips in the given (110) layer alternate occupy about half of area of the layer (110) in the full consent with the spectral X-ray analysis and ferromagnetic resonance data.

cond-mat.mtrl-sci↗

arXiv2013

Scaling in Modeling of Core Losses in Soft Magnetic Materials Exposed to Nonsinusoidal Flux Waveforms and DC Bias

Assuming that core loss data of Soft Magnetic Materials obey scaling relations, models describing the power losses in materials exposed to non-sinusoidal flux waveforms and DC Bias conditions have been derived. In order to test these models, the measurement data for two materials have been collected and the core losses calculated. Agreement between the experimental data and the model predictions is satisfactory.

cond-mat.mtrl-sci↗

arXiv2013

Simulation study on cavity growth in ductile metal materials under dynamic loading

Cavity growth in ductile metal materials under dynamic loading is investigated via the material point method. Two typical cavity effects in the region subjected to rarefaction wave are identified: (i) part of material particles flow away from the cavity in comparison to the initial loading velocity, (ii) local regions show weaker negative or even positive pressures. Neighboring cavities interact via coalescence of isobaric contours. The growth of cavity under tension shows staged behaviors. After the initial slow stage, the volume and the dimensions in both the tensile and transverse directions show linear growth rate with time until the global tensile wave arrives at the upper free surface. It is interesting that the growth rate in the transverse direction is faster than that in the tensile direction. The volume growth rate linearly increases with the initial tensile velocity. After the global tensile wave passed the cavity, both the maximum particle velocity in the tensile direction and the maximum particle velocity in the opposite direction increase logarithmically with the initial tensile speed. The shock wave reflected back from the cavity and compression wave from the free surface induce the initial behavior of interfacial instabilities such as the Richtmyer-Meshkov instability, which is mainly responsible for the irregularity in the morphology of deformed cavity. The local temperatures and distribution of hot spots are determined by the plastic work. Compared with the dynamical process, the heat conduction is much slower.

cond-mat.mtrl-sci↗

arXiv2013

High-pressure cupric oxide: a room-temperature multiferroic

Multiferroic materials, in which ferroelectric and magnetic ordering coexist, are of fundamental interest for the development of multi-state memory devices that allow for electrical writing and non-destructive magnetic read-out operation. The great challenge is to create multiferroic materials that operate at room-temperature and have a large ferroelectric polarization P. Cupric oxide, CuO, is promising because of its large P ~ 10^{2} μC.m^{-2}, but is unfortunately only multiferroic in a temperature range of 20 K, from 210 to 230 K. Here, using a combination of density functional theory and Monte Carlo calculations, we establish that pressure-driven phase competition induces a giant stabilization of the multiferroic phase of CuO, which at 20-40 GPa becomes stable in a domain larger than 300 K, from 0 to T > 300 K. Thus, under high-pressure, CuO is predicted to be a room-temperature multiferroic with large polarization.

cond-mat.mtrl-sci↗

arXiv2013

Creation of ensembles of nitrogen-vacancy centers in diamond by neutron and electron irradiation

We created dense ensembles of negatively charged nitrogen-vacancy (NV-) centers in diamond by neutron and electron irradiation for applications in hybrid quantum systems and magnetometry. We characterize fluorescence intensity, optical and coherence properties of the resulting defects by confocal microscopy, UV/Vis and FTIR spectroscopy, optically detected magnetic resonance and small angle X-ray scattering. We find the highest densities of NV- at neutron fluences on the order of 10^17 cm^-2 and electron doses of 10^18 cm^-2, with spin resonance linewidths of 6 MHz. Lower electron energies increase the ratio of centers in the desired negative charge state to those in the neutral one. Annealing at 900 °C during the irradiation reduces the spin resonance linewidth. Electron irradiation furthermore results in substantially higher optical transparency compared to neutron irradiation.

quant-ph↗

arXiv2013

Break-up phenomena of liquid metal thin film induced by high electric current

The room temperature liquid metal related electronics has been found important in a wide variety of emerging areas over the past few years. However, its failure features under high electrical current densities are not clear until now. Here we show that a liquid metal thin film would break-up as the applied current increases to a critical magnitude. The break-up phenomenon is attributed to be caused by the so-called electromigration effect. This problem could be one of the major hurdles that must be tackled with caution in the research and application of future liquid metal film electronics.

cond-mat.mtrl-sci↗

arXiv2013

Electrophysical characteristics of near-surface layers in p-Si crystals with sputtered Al films and subjected to elastic deformation

The deposition of Al film onto the (111) surface of a p-Si crystal was shown to induce a deformation in the near-surface layer of the latter. Provided that the crystal strain is elastic and uniaxial, the gettering of defects in the near-surface layer is observed, which is confirmed by a change in the dependence of the specimen resistance on the elastic strain magnitude. The maximum depth of the defect capture has been calculated on the basis of the energy of interaction between the deformed layer and dislocations.

physics.chem-ph↗

arXiv2013

Novel Giant Magnetoresistance Model using Multiple Barrier Potential

This paper proposes a new model for Giant Magnetoresistance (GMR) and calculates its typical graph qualitatively. The model foundation is the microscopic mechanism in GMR, where the GMR effect can be explained by intrinsic and extrinsic potential. The potentials are spin-dependent. The potentials determine the transmission probability then it will give conductance value. Here, the multiple barrier potential model is proposed as the new GMR model. The transmission probability is determined using transfer matrix method. It was found that this model is fit qualitatively with the typical GMR graph.

cond-mat.mtrl-sci↗

arXiv2013

Unexpected colloid-like supernatant from liquid-phase ball-milling graphite using miscible solutions as solvents: a failure analysis

Ball-milling graphite was conducted in miscible solutions with the purpose to exfoliate graphene. Colloid-like stable supernatants were unexpectedly obtained. Followed were the characterizations with Scanning electronic microscopy, Thermogravimetric analysis, X-ray diffraction and Elemental dispersive X-ray mapping to recognize them. As a result, strongly against the initial judgments, the components of colloid were mainly impurities of mixture of zirconia, silicate and yttrium oxide, other than nano-graphite.

cond-mat.mtrl-sci↗

arXiv2013

Splashing phenomena of room temperature liquid metal droplet striking on the pool of the same liquid under ambient air environment

In this article, the fluid dynamics of room temperature liquid metal (RTLM) droplet impacting onto a pool of the same liquid in ambient air was investigated. A series of experiments were conducted in order to disclose the influence of the oxidation effect on the impact dynamics. The droplet shape and impact phenomenology were recorded with the aid of a high-speed digital camera. The impact energy stored in the splash structures was estimated via a theoretical model and several morphological parameters obtained from instantaneous images of the splash. It was observed that the droplet shape and the splashing morphology of RTLM were drastically different from those of water, so was the impact dynamics between room temperature LM pool and high temperature LM pool. The energy analysis disclosed that the height of the jet is highly sensitive to the viscosity of the fluid, which is subjected to the oxidation effect and temperature effect simultaneously, and thus perfectly explained the phenomena. These basic findings are important for the application of RTLM in a series of newly emerging technologies such as liquid metal based spray cooling, ink-jet printed electronics, interface material painting and coating, metallurgy, and 3D packages, etc.

physics.flu-dyn↗