Ribbon Length Research Articles

Viscosity measurement of mould fluxes using inclined plane test and development of mathematical model

During the current research and development activities at Tata Steel Teesside Technology Centre, UK, the inclined plane test (IPT) is adopted as a quick method to measure the viscosity/fluidity of mould powders being used currently for continuous casting of different steel qualities and section sizes. The usefulness of the IPT method was also validated by comparing the viscosities that were measured by a high temperature viscometer. It has been established that the IPT measured viscosity values are comparable with the mould powder supplier’s data. The IPT ribbon lengths of different powders have been correlated with the viscosities using an Arrhenius type relationship. The ribbon lengths of the solidified fluxes were found to have a good correlation with the molar ratios of the corresponding powders. Hence, the relationship was further tuned to develop a viscosity prediction model using the chemical compositions of the mould fluxes (i.e. the model can be used for a quick assessment of mould flux viscosity based on its chemical composition).

Thermal rectification and phonon scattering in asymmetric silicon nanoribbons

Thermal rectification is an interesting phenomenon and has important potential applications in improving the thermal management of electronics and saving energy. Exploring thermal rectification phenomena and understanding the mechanism are very necessary and important. This paper reports the investigation of the thermal conductivity and thermal rectification of asymmetric silicon nanoribbons by the non-equilibrium molecular dynamics simulation. The results indicate that the thermal conductivity of the nanoribbon is only on the order of 100 Wm−1K−1. Thermal rectification is observed in silicon nanoribbons at different temperatures, geometry aspects, and ribbon length. The thermal conductivity is apparently larger when heat flows from the thin end to the thick end. The larger the aspect ratio of the thick end to thin end is, the larger the thermal rectification. The rectification coefficient does not change much in the ribbon length ranges from 8.1 nm to 21.7 nm. The longitudinal phonon scattering in the silicon nanoribbon at different frequency is investigated by the phonon wave packet dynamic simulation. The results show that the phonon transmission coefficient is sensitive to frequency and transverse phonons are generated during the scattering. Decreasing the thin end width will reduce the transmission coefficient due to the scattering at the slope boundary.

The transport properties of the molecular-scale B2C and BC3 electronic devices

The transport properties of the molecular-scale B2C and BC3 electronic devices are investigated with an ab initio method combined with a nonequilibrium Green function technique. The effects of different BC graphenes and ribbon lengths on the transport properties of the devices are significant. The results show that the devices with different BC graphenes and sizes have unusual transmission coefficients, which leads to special current transport mechanisms for the devices. Notably, the current strength of the device with the shortest ribbon length is the largest in three B2C devices, but the current strength of the device with the shortest ribbon length is the smallest for BC3 device.

THE RELATIONSHIP BETWEEN HARD X-RAY PULSE TIMINGS AND THE LOCATIONS OF FOOTPOINT SOURCES DURING SOLAR FLARES

The cause of quasi-periodic pulsations (QPP) in solar flares remains the subject of debate. Recently, Nakariakov & Zimovets (2011) proposed a new model suggesting that, in two-ribbon flares, such pulsations could be explained by propagating slow waves. These waves may travel obliquely to the magnetic field, reflect in the chromosphere and constructively interfere at a spatially separate site in the corona, leading to quasi-periodic reconnection events progressing along the flaring arcade. Such a slow wave regime would have certain observational characteristics. We search for evidence of this phenomenon during a selection of two-ribbon flares observed by RHESSI, SOHO and TRACE; the flares of 2002 November 9, 2005 January 19 and 2005 August 22. We were not able to observe a clear correlation between hard X-ray footpoint separations and pulse timings during these events. Also, the motion of hard X-ray footpoints is shown to be continuous within the observational error, whereas a discontinuous motion might be anticipated in the slow wave model. Finally, we find that for a preferential slow wave propagation angle of 25-28 degrees that is expected for the fastest waves, the velocities of the hard X-ray footpoints lead to estimated pulse periods and ribbon lengths significantly larger than the measured values. Hence, for the three events studied, we conclude that the observational characteristics cannot be easily explained via the Nakariakov & Zimovets (2011) propagating slow wave model when only angles of 25-28 degrees are considered. We provide suggested flare parameters to optimise future studies of this kind.

THINKING OBJECTIVELY: AN OVERVIEW OF “THING THEORY” IN VICTORIAN STUDIES

For those of us whose life's work consists of the study of the Victorian Era, nothing is plainer than the fact that one cannot escape from “things.” Indeed, for many of us the wealth of detail and artefact available is one of the attractions of the era: who could resist the hats, coaches, buttons, newspapers, lengths of ribbon, packets of tea, bits of old lace, sugared plums, ink pots, keys, and pocket watches that clutter the pages of Dickens, overflow from Gaskell, and crowd in amongst the characters of Collins, Thackeray, Trollope, Eliot, and Brontë? The Victorians had a preoccupation with and predilection for the careful and considered acquisition and utilisation of objects, and this preoccupation has become a focus for critical trends in this area. In 2003, Lynn Pykett wrote that “The Victorians were fascinated with objects and things – but recent scholarship has proved equally fascinated with this Victorian obsession” (1). This obsession can be traced back to a turn in Victorianist criticism in the 1980s, beginning with Brigg's Victorian Things and based on theories of commodity culture in writers such as Jean Baudrillard, Roland Barthes, and Walter Benjamin, towards an interdisciplinary interest in material culture and particularly in consumer culture and theories of consumption and commodification. More recently critics have moved away from Marxist explorations of objects as commodities, to explore the possibilities of the object in contexts other than those generated by discourses of exchange value, production, and consumption, adopting cultural materialist and new historicist approaches to the objects of Victorian culture and literature. This movement has become known as “Thing Theory.” Starting with Bill Brown's seminal work, A Sense of Things, in 2003, critics have sought for ways of explaining the relationship between the subject and the object in terms other than those of the capitalist market system, in order to take account of the complexities of the object as a signifier. This review seeks to give an overview of these two critical perspectives on the objects of Victorian studies, from the roots of “thing theory” in consumer culture and commodity studies, to the key texts and indicative readings which have shaped “thing theory” as a discipline. Starting with a look at some key texts on consumer culture in the nineteenth century, I then move on to look at those “thing theory” works which have moved away from a focus on the object as commodity, towards cultural materialism and an understanding of the Victorian “object” beyond its role in consumer culture. Finally, I look at some readings indicative of the work currently being published in the field.

Coulomb gap in graphene nanoribbons

We investigate the density- and temperature-dependent conductance of graphene nanoribbons with varying aspect ratio. Transport is dominated by a chain of quantum dots forming spontaneously due to disorder. Depending on ribbon length, electron density, and temperature, single or multiple quantum dots dominate the conductance. Between conductance resonances, cotunneling transport at the lowest temperatures turns into activated transport at higher temperatures. The density-dependent activation energy resembles the Coulomb gap in a quantitative manner. Individual resonances show signatures of multilevel transport in some regimes, and stochastic Coulomb blockade in others.

Out-of-plane anisotropy and low field induced magnetic domain reorientation in Al/Metglas-2605S2/Al trilayer sensors

Local magnetism of Al/Metglas-2605S2/Al trilayer-like sensors prepared by dc sputtering has been studied at room temperature by both in-field and zero-field Mössbauer spectroscopy for a maximum field of 60 mT. Ferromagnetic domain reorientation from out-of-ribbon plane to in-plane, induced by low applied fields, was measured by monitoring the intensities of lines 2 and 5 in the Mössbauer sextet spectra obtained for different magnitudes of applied magnetic field. Coating the Metglas-2605S2 with Al layers of 20 μm thickness, a stress-field is induced, allowing to distinguish different in-plane magnetic anisotropy distributions along the ribbon length (∼4 mT) as well along its width (∼10 mT). Using a phenomenological model for the magnetization reversal, out-of-plane anisotropies ranging up to 3 kJ/m3 were estimated for the Metglas-2605S2 alloy ribbons. This anisotropy range is similar to that observed for the in-plane case previously reported. The methodology applied in this work can be used to determine native out-of-plane anisotropy distributions for different melt-spun ribbons. It also allows understanding magnetization reversal associated with the ferromagnetic domains reorientation caused either by induced stress or by applied magnetic field.

Topological Anderson insulator phenomena

We study the nature of the disorder-induced quantized conductance, i.e., the phenomena of topological Anderson insulator (TAI) induced in HgTe/CdTe semiconductor quantum well. The disorder effect in several different systems where anomalous Hall effect exist, is numerically studied using the tight-binding Hamiltonian. It is found that the TAI phenomena also occur in the modified Dirac model where the quadratic corrections $k^2\sigma_z$ is included and electron-hole symmetry is kept. It also occurs in the graphene system with the next nearest-neighbor coupling and staggered sublattice potential. Comparison between the localization lengths of the 2D ribbon and 2D cylinder clearly reveals the topological nature of this phenomena. Furthermore, analysis on the local current density in anomalous quantum Hall systems where the TAI phenomena can or can not arise reveals the nature of TAI phenomena: the bulk state is killed drastically and only the robust edge state survives in a moderate disorder. When the edge state is robust enough to resist the strong disorder that can completely kills the bulk state, TAI phenomena arise.

Structural Characterization of CVD-grown ZnO Nanocombs

We present the growth of ZnO nanocombs by using chemical vapor deposition (CVD) under atmospheric pressure. Two types of single- and double-side teethed nanocombs have been obtained. Structural analyses based on electron microscopy reveal the diameter and the period of teeth to be about 50 nm and 75 - 100 nm, respectively, while the length of the comb ribbons is 7 - 10 µm. For the single-teethed type, the entire nanocomb is a single crystal with the teeth grown along the [0001] direction, and the comb ribbon grown along the [10-10] direction. For double-side teethed nanocombs, they are twin crystals with twin boundaries of {10-13}. The comb ribbons grow along the directions of h10-11i while the teeth grow along the [0001] direction. With the results obtained, we believe that the growth of nanocombs follows the vapour-solid process. Studies of the optical properties basing on photoluminescence and Raman spectroscopy have indicated that the ZnO nanocombs have good crystal quality.

Nonlinear transport through ultra-narrow zigzag graphene nanoribbons: non-equilibrium charge and bond currents

The electronic nonlinear transport through ultra-narrow graphene nanoribbons (sub-10 nm) was studied. A stable region of negative differential resistance (NDR) appears in the I–V characteristic curve of odd zigzag graphene nanoribbons (ZGNRs) at both positive and negative polarity. This NDR originates from a transport gap induced by a selection rule that blocks the electron transition between disconnected energy bands of ZGNR. Based on this transition rule, the on/off ratio of the current increases exponentially with ribbon length up to 105. In addition, charging effects and the spatial distribution of bond currents were studied by using the non-equilibrium Green's function formalism in the presence of electron–electron interaction at a mean-field level. We also performed an ab initio density functional theory calculation of the transmission through a passivated graphene nanoribbon to demonstrate the robustness of the transport gap against hydrogen termination of the zigzag edges.

Electronic Transport in Z-Shaped Graphene-Nanoribbons: Shape and Size Effects

Based on the Green's function method and the Landauer-Buttiker formula, we studied the electronic transport properties of a graphene heterojunction. This was a Z-shaped graphene nanoribbon (GNR), which was connected by zigzag graphene nanoribbon leads. We show that the conductance of the Z-shaped GNRs is highly sensitive to the shape and size of the heterojunctions. The charge density is strongly localized on the zigzag edge sites of the leads and thereby a conductance dip or gap results at the Fermi energy. By varying the width of the graphene ribbons between the junctions, we found many more resonant peaks in the conduction because of the quasi-bound states. The number of resonant peaks has little to do with the length of the graphene ribbons between the junctions. Importantly, we show that in the low-energy region the electrons retain their ballistic transport characteristic in the width uniformity of Z-shaped GNRs with included angle θ of 60° or 150° turns. These findings show that the Z-shaped GNRs can be selected for future ballistic device applications.

Structural Stability and Kinetics of Hydrogen in TiZrNi Quasicrystals

When hydrogen atoms slowly diffuse into a metastable structure, a coherence length of thin TiZrNi quasicrystal metallic ribbons is increased. Improved atomic order with addition of hydrogen is an uncommon phenomenon in metals, and may reflect the unordinary structural property of quasicrystals. Metastable TiZrNi quasicrystals prepared by rapid quenching of molten ingots were hydrogenated in a low pressure of hydrogen (lower than 1000 Torr) at high temperature. After completion of the pressure-composition-temperature measurements between 200 to 300 degrees C, X-ray diffraction data revealed that the full width at half maximum values of the diffraction peaks were decreased suggesting that the quasicrystal phase may stabilized by hydrogen inclusion. Annealing of the same sample at 200 degrees C without hydrogen yields no change of the relative intensity of diffraction peaks.

Atomistic quantum transport modeling of metal-graphene nanoribbon heterojunctions

We calculate quantum transport for metal-graphene nanoribbon heterojunctions within the atomistic self-consistent Schr\"odinger/Poisson scheme. Attention is paid on both the chemical aspects of the interface bonding as well the one-dimensional electrostatics along the ribbon length. Band-bending and doping effects strongly influence the transport properties, giving rise to conductance asymmetries and a selective suppression of the subband formation. Junction electrostatics and $p$-type characteristics drive the conduction mechanism in the case of high-work-function Au, Pd, and Pt electrodes while contact resistance becomes dominant in the case of Al.

GMI Effect of FeCoSiB Amorphous Ribbon

The effects of AC frequency, magnetic field strength, ribbon length and test temperature on the Fe72Co8Si15B5 amorphous ribbon’s magnetic impedance were studied. The results showed that the resistance, reactance and impedance of amorphous ribbons increased with the increase of AC frequency, but decreased as magnetic field strength decreased; the rangeability of resistance, reactance and impedance increased as AC frequency and magnetic field strength increased; the resistance and reactance increased as ribbon length increased; the ribbon had a good temperature stability in resistance.

Texturation of Ni–Co–Mn–In Ribbons by Melt Spinning

AbstractNi45Co5Mn36.6In13.4 ribbons were prepared by the melt spinning technique. They are in the austenitic state at the room temperature with a Cubic L21 structure and are composed of columnar grains growing perpendicular to the ribbon plane. A nearly single {001}〈100〉 cube component is formed at low wheel‐rotating speed due to the preferential growth of nuclei in the ribbon plane normal and length directions. Appropriate control of the melt spinning condition offers a promising solution to the texturation of polycrystalline ferromagnetic shape memory alloys.

SV2 Acts via Presynaptic Calcium to Regulate Neurotransmitter Release

Synaptic vesicle 2 (SV2) proteins, critical for proper nervous system function, are implicated in human epilepsy, yet little is known about their function. We demonstrate, using direct approaches, that loss of the major SV2 isoform in a central nervous system nerve terminal is associated with an elevation in both resting and evoked presynaptic Ca(2+) signals. This increase is essential for the expression of the SV2B(-/-) secretory phenotype, characterized by changes in synaptic vesicle dynamics, synaptic plasticity, and synaptic strength. Short-term reproduction of the Ca(2+) phenotype in wild-type nerve terminals reproduces almost all aspects of the SV2B(-/-) secretory phenotype, while rescue of the Ca(2+) phenotype in SV2B(-/-) neurons relieves every facet of the SV2B(-/-) secretory phenotype. Thus, SV2 controls key aspects of synaptic functionality via its ability to regulate presynaptic Ca(2+), suggesting a potential new target for therapeutic intervention in the treatment of epilepsy.

A gate-induced switch in zigzag graphene nanoribbons and charging effects

Using the non-equilibrium Green’s function formalism, we investigate nonlinear transport andcharging effects of gated graphene nanoribbons (GNRs) with an even number of zigzag chains.We find a negative differential resistance (NDR) over a wide range of gate voltages with anon/off ratio ∼ 106 for narrow enough ribbons. This NDR originates from the parity selection rule and alsoprohibition of transport between discontinuous energy bands. Since the external field is wellscreened close to the contacts, the NDR is robust against the electrostatic potential.However, for voltages higher than the NDR threshold, due to charge transfer through theedges of the zigzag GNR (ZGNR), screening is reduced such that the external potential canpenetrate inside the ribbon giving rise to smaller values of off-current. Furthermore, theon/off ratioof the current depends on the aspect ratio of the length/width and also edge impurity. Moreover, theon/off ratio displays a power law behavior as a function of ribbon length.

Disorder-induced gap behavior in graphene nanoribbons

We study the transport properties of graphene nanoribbons of standardized 30 nm width and varying lengths. We find that the extent of the gap observed in transport as a function of Fermi energy in these ribbons (the "transport gap") does not have a strong dependence on ribbon length, while the extent of the gap as a function of source-drain voltage (the "source-drain gap") increases with increasing ribbon length. We anneal the ribbons to reduce the amplitude of the disorder potential, and find that the transport gap both shrinks and moves closer to zero gate voltage. In contrast, annealing does not systematically affect the source-drain gap. We conclude that the transport gap reflects the overall strength of the background disorder potential, while the source-drain gap is sensitively dependent on its details. Our results support the model that transport in graphene nanoribbons occurs through quantum dots forming along the ribbon due to a disorder potential induced by charged impurities.

Anomalous Enhancement of the Boltzmann Conductivity in Disordered Zigzag Graphene Nanoribbons

We study the conductivity of disordered zigzag graphene nanoribbons in the incoherent regime by using the Boltzmann equation approach. The band structure of zigzag nanoribbons contains two energy valleys, and each valley has an excess one-way channel. The crucial point is that the numbers of conducting channels for two propagating directions are imbalanced in each valley due to the presence of an excess one-way channel. It was pointed out that as a consequence of this imbalance, a perfectly conducting channel is stabilized in the coherent regime if intervalley scattering is absent. We show that even in the incoherent regime, the conductivity is anomalously enhanced if intervalley scattering is very weak. Particularly, in the limit of no intervalley scattering, the dimensionless conductance approaches to unity with increasing ribbon length as if there exists a perfectly conducting channel. We also show that anomalous valley polarization of electron density appears in the presence of an electric field.

Effects of magnetism and electric field on the energy gap of bilayer graphene nanoflakes

We study the effect of magnetism and perpendicular external electric field strengths on the energy gap of length confined bilayer graphene nanoribbons (or nanoflakes) as a function of ribbon width and length using a first-principles density-functional electronic structure method and a semilocal exchange-correlation approximation. We assume $AB$ (Bernal) bilayer stacking and consider both armchair and zigzag edges, and for each edge type, we consider the two edge alignments, namely, $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ edge alignment. For the armchair nanoflakes we identify three distinct classes of bilayer energy gaps, determined by the number of carbon chains in the width direction ($N=3p$, $3p+1$ and $3p+2$, $p$ is an integer), and the gaps decrease with increasing width except for class $3p+2$ armchair nanoribbons. Metallic-like behavior seen in armchair bilayer nanoribbons are found to be absent in armchair nanoflakes. Class $3p+2$ armchair nanoflakes show significant length dependence. We find that the gaps decrease with the applied electric fields due to large intrinsic gap of the nanoflake. The existence of a critical gap with respect to the applied field, therefore, is not predicted by our calculations. Magnetism between the layers plays a major role in enhancing the gap values resulting from the geometrical confinement, hinting at an interplay of magnetism and geometrical confinement in finite size bilayer graphene.