Wire Axis Research Articles

GaN nanocones field emitters with the selenium doping

The pure and selenium (Se)-doped GaN nanocones were grown via a vapor–liquid–solid (VLS) mechanism with Pt catalysts. The diameter of GaN nanocones gradually decreases from ~500 to ~100 nm along the wire axis,and the tips of GaN nanocones change to thin with sharp ends. The Se-doped GaN nanocones have exhibited impressive field emission properties and high stability with a lower turn-on field of 2.75 V/µm (at room temperature), which is lower than pure GaN nanocones (5.53 V/μm). Consequently, the incorporation of Se impurities can improve the field emission performance of GaN nanocones, which is sufficient for application in field electron emission devices and cold electron sources in display devices. Moreover, the synthesized Se-doped GaN nanocones will facilitate flexible design of device architectures for nanoelectronics.

Defining a relationship between pearlite morphology and ferrite crystallographic orientation

This study involved fully pearlitic wires of seven different diameters (5.5–1.6 mm). All samples were laboratory annealed to re-austenitize and were then air-cooled to reform the pearlite structure. Morphological alignment of the pearlite, along the wire axis, improved significantly, 32% to 93%, as the wire diameter decreased. This improvement coincided with increases in the <110> ferrite fiber texture, and falls in the axial residual stresses. In all the wires, the majority of the pearlite lamellae appeared to align, in a 2-D analysis, with minimum elastic stiffness (EMin under simple compression) for the ferrite (α). This correlation increased from 80% to 98% with decrease in wire diameter and fall in axial residual stresses. 3-D microstructures by serial sectioning, 3-D rotations seeking EMin and observations on coarse pearlite, indicated that {011}α and <001>α were, respectively, the pearlite interface (or habit plane) and growth direction.

The microstructures, growth mechanisms and properties of carbon nanowires and nanotubes fabricated at different CVD temperatures

Polycrystalline carbon nanowires (p-CNWs) has been synthesized via the CVD strategy using C2H4 as carbon source and Ni particles distributed in porous Si3N4 ceramic as growth seeds at 700°C. As-received p-CNWs showed the solid core and the basal planes of graphite nanosheets were perpendicular to the wire axis. Elevated growth temperatures encouraged the formation of hollow carbon nanotubes (CNTs) at 750°C and amorphous carbon above 800°C, which in turn altered the electrical conductivity and dielectric properties of carbon reinforced Si3N4 composite ceramics. The growth mechanisms and microstructures of solid wires and hollow tubes were discussed and compared detailedly. It is energetically favorable for p-CNWs, which shows high surface energy, to convert itself into crystallized CNTs during the annealing at 1000°C. The four-state transition process was accompanied by the diffusion of carbon atoms and the rearrangement and growth of nanosheets. The present research strategies and results are not only beneficial to investigate the growth and graphitization behavior of carbon nanoscale materials, but also probably extended to investigate other nanowire-nanotube systems.

Structural integrity of progressively cold-drawn pearlitic steels: From Raffaello Sanzio to Vincent van Gogh

Abstract: This papers deals with fracture behavior and structural integrity of progressively cold-drawn pearlitic steels on the basis of their microstructural evolution during manufacturing by multi-step cold drawing. It is seen that the manufacture technique by progressive cold drawing in several steps produces a microstructural evolution in the form of progressive slenderizing and orientation (in the wire axis or cold drawing direction) of the pearlitic colonies (first microstructural level), as well as increasing orientation and densification of the ferrite/cementite lamellae (second microstructural level) linked with a decrease of pearlite (ferrite/cementite) interlamellar spacing. Thus the microstructure of the cold-drawn pearlitic steel wires becomes progressively oriented as the cold-drawing degree increases and this microstructural fact influences their macroscopic behavior by inducing anisotropic fracture and crack path deflection. Therefore, this paper offers a micro- and macro-approach to the fracture and structural integrity of cold-drawn pearlitic steels, introducing the new concept of microstructural integrity .

Stress Corrosion Cracking of Progressively Cold-Drawn Pearlitic Steels: From Tintoretto to Picasso

Progressive cold drawing in eutectoid steels produces a preferential orientation of the pearlitic microstructure in the wire axis or drawing direction. This affects the posterior behaviour of the steels under conditions of stress corrosion cracking (SCC). The experimental results show that cold drawing induces strength anisotropy in the steel, and thus the resistance to SCC is a directional property that depends on the angle in relation to the drawing direction. Therefore, an initial transverse crack changes its propagation direction to approach that of the wire axis, thus producing mixed mode propagation, the deflection angle being an increasing function of the cold drawing degree. This experimental result may be explained by micro-mechanical considerations on the basis of the lamellar microstructure of the steels. A relationship is established between the microstructural angles and the deflection angles of the macroscopic SCC crack, thus providing a materials science type relationship between the microstructure and the macroscopic crack paths with regard to hydrogen assisted cracking (HAC) associated with the cathodic regime and in the matter of localised anodic dissolution (LAD) linked to the anodic regime. Whereas in hot rolled (not cold drawn at all) and slightly drawn steels the phenomenon of SCC develops in mode I, i.e., the SCC crack is a straight line linked with the classical linear perspective painting by Tintoretto, in the case of heavily drawn steels, the SCC deflected crack is a polygonal line associated with the multi-perspective cubist painting by Picasso, these conclusions being valid for both HAC and LAD.

Hydrogen Effects on Progressively Cold-Drawn Pearlitic Steels: Between Donatello and Michelangelo

This paper reviews previous research by the author in the field of hydrogen effects on progressively cold-drawn pearlitic steels in terms of hydrogen degradation (HD), hydrogen embrittlement (HE) or, at the micro-level, hydrogen-assisted micro-damage (HAMD), thus affecting their microstructural integrity and compromising the (macro-)structural integrity of civil engineering structures such as prestressed concrete bridges. It is seen that hydrogen effects in pearlitic microstructure (either oriented or not) are produced at the finest micro-level by plastic tearing in the form, in general, of hydrogen damage topography (HDT) with different appearances depending of the cold drawing degree, evolving from the so-called tearing topography surface (TTS) in hot-rolled (not cold-drawn at all) or slightly cold-drawn pearlitic steels to a sort of enlarged and oriented TTS (EOTTS) in heavily drawn steels (the pronounced enlargement and marked orientation being along the wire axis or cold drawing direction). Whereas the pure TTS mode (null or low degree of cold drawing) resembles the Michelangello stone sculpture texture (MSST), the EOTTS mode does the same in relation to the Donatello wooden sculpture texture (DWST).

Microstructure of NbTi superconducting alloy

A superconducting multifilament wire made of Nb-60 at.% Ti alloy was investigated by methods of atom probe/field ion microscopy (APFIM), three-dimensional atom probe (3DAP), high resolution electron microscopy (HREM) and computer simulation. The results of the APFIM and HREM analysis suggest, that the specimens contain two phases, a bcc structure (β-phase), and a hcp structure (α-phase). The mesascopic and nanoscale heterogeneities of element concentration in *-phase were observed. Atom probe analyses of annealed NbTi superconductors and a statistical processing of the chains of atoms, consistently registered by mass-analyses, indicated that no atomic clusters were formed in alloy prior to the onset of the second phase formation. The β-phase and niobium enriched phase have the configurations extended along the wire axis, the nanoscale heterogeneity inside the β-phases was nearly isotropic. An atomic structure of interphase boundaries has been determined using HREM and atomistic structure calculations. Interphase boundaries in Nb-Ti alloy are coherent or semi-coherent in spite of the existence of the irregular microprotrusions.The boundaries between α- and β-phases reveal the absence of the rigid-body translation and continuity of lattice planes across the interface. It was demonstrated that the method of simulation in reciprocal space is computationally efficient for investigations of the atomic structure and energy of coherent and non-coherent interphase boundaries. The extremely high critical current density in the optimized Nb-Ti can be due to the coherent structure of interphase boundaries.

Optimization of wire feed for GTAW based additive manufacturing

The problem of deposition accuracy is encountered during gas tungsten arc welding (GTAW) based additive manufacturing when wire is fed in side direction. A mathematical model was developed to calculate the wire flying distance in arc zone, according to which displacement compensation was designed to ensure the size accuracy. When arc length was 5mm and vertical distance of wire tip to tungsten electrode was larger than 3mm, the horizontal distance between melting wire tip and axis of tungsten electrode was 3.5mm. The displacement compensation was verified to be effective by forming cross-angle. The model can also be used to achieve bridging transfer, which is beneficial for smooth layer appearance.

Enhanced magnetocrystalline anisotropy in an ultra-dense array of air-exposed crystalline cobalt nanowires

The magnetic anisotropy of an ultradense array of crystalline cobalt nanowires is investigated by means of broadband ferromagnetic resonance and magnetic torque measurements. The array is grown epitaxially in solution on a Pt(111) film and consists of single crystalline metallic wires with a diameter of 6.2 nm and a center-to-center interwire distance of 9.6 nm. The shape anisotropy and the Co hexagonal compact structure with the c-axis along the wire axis combine with each other to impose a perpendicular magnetic anisotropy despite the high density of 8 × 1012 wires/in.2. The intrinsic uniaxial magnetocrystalline anisotropy constants K1 and K2 are extracted from the ferromagnetic resonance and torque measurements using a mean field approach accounting for the interwire dipolar interactions. At room temperature, and despite air exposure, an unexpected increase of K1 and K2 of more than 40% with respect to the bulk is evidenced.

On the exact solutions of nonlinear Schrödinger equation with spin current

An integrable nonlinear Schrödinger (NLS) equation driven by spin polarized current governing the magnetization dynamics of a ferromagnetic nanowire is considered. The exact soliton solution of the NLS equation propagating along the direction of wire axis which is also the current direction along which nonuniform magnetization occurs is obtained through the application of exponential function method. The solution of the system admits a class of solitons such as kink and periodic solitons in the nanowire along the direction of the electric current.

Application of the Picosecond Magnetic Pulses for Inducing the Electron Motion in the Bilayer Nanowire

We theoretically study the effects of interaction of the single electron confined in a piece of bilayer nanowire with the picosecond magnetic pulse. It is shown that the transverse time-varying magnetic field hybridizes temporarily the vertical modes in the electron wave function which eventually stimulates its motion along the wire axis. The dynamics of such magnetically driven motion depends on the geometry of the confining potential, the effective mass of the electron and the time duration of the magnetic pulse. We show that this effect can potentially be utilized for fabrication of the magnetic valve.

The arc characteristics of cold metal transfer welding with AZ31 magnesium alloy wire

The arc characteristics of cold metal transfer (CMT) welding with AZ31 magnesium alloy wire were investigated. A high-speed CCD camera and an electrical signal acquisition system were used to obtain welding arc images and electrical signals respectively. For DC-CMT, stable welding process could be achieved when wire feed speed (WFS) was no more than 5m/min and for pulsed-CMT, WFS should be no more than 4m/min. The welding arc was formed between weld pool and molten droplet and exhibited as an asymmetrical bell shape. Evaporation of molten droplet gave itself a recoil force, pushing it away from wire axis. At mid to upper power range, repelled droplet transfer process was observed. Molten droplets deviated from wire axis and detached from wire tip under the influence of the recoil force of droplet evaporation, falling outside the weld pool and compromising arc stability. The WFS of 4m/min under pulsed-CMT mode could gain good clad appearance and a favorable contact angle, which could be capable of realizing multi-pass cladding of magnesium alloys.

Ultra-strong and damage tolerant metallic bulk materials: A lesson from nanostructured pearlitic steel wires.

Structural materials used for safety critical applications require high strength and simultaneously high resistance against crack growth, referred to as damage tolerance. However, the two properties typically exclude each other and research efforts towards ever stronger materials are hampered by drastic loss of fracture resistance. Therefore, future development of novel ultra-strong bulk materials requires a fundamental understanding of the toughness determining mechanisms. As model material we use today’s strongest metallic bulk material, namely, a nanostructured pearlitic steel wire, and measured the fracture toughness on micron-sized specimens in different crack growth directions and found an unexpected strong anisotropy in the fracture resistance. Along the wire axis the material reveals ultra-high strength combined with so far unprecedented damage tolerance. We attribute this excellent property combination to the anisotropy in the fracture toughness inducing a high propensity for micro-crack formation parallel to the wire axis. This effect causes a local crack tip stress relaxation and enables the high fracture toughness without being detrimental to the material’s strength.

(Invited) Large Mobility Modulation Due to Discrete Impurities in Nanowires

Semiconductor nanowires have been receiving great attention over the past decades, owing to their possible applications to chemical/biological sensors as well as future electronic/photonic devices [1,2]. Among other characteristics, the locality of ionized impurities plays a crucial role in optimizing the device performance; carrier transport is critically dependent on where the localized impurities are positioned inside nanoscale channels. In fact, this is the physical origin of the random dopant fluctuation found in threshold voltage, which is now of most serious concerns in integrating nanoscale Si-based devices [3]. Recently, we have theoretically carried out systematic investigations on the effects of the locality of multiple impurities on transport characteristics under quasi-one dimensional (quasi-1D) nanowire structures [4]. We have shown that both phase interference and phase randomization simultaneously play a dominant role in determining the impurity-limited resistance even under the fully coherent circumstances, in which no energy dissipating scattering is involved. In this presentation, we extend our approach further to study the polarity dependence of impurities on transport characteristics. We show that the impurity-limited mobility could be modulated over three orders of magnitude by changing the axial separation among impurities provided that the charge polarity of impurities is opposite to that of carriers. We employ a cylindrical nanowire with radius of 2 nm and ionized acceptors or donors are distributed at random in the Si channel. Typical potential profiles in the channel region consist of the long-range potential modulation spread over the channel and the localized short-range potential modulation. Here, we would like to point out that some confusion exists in many theoretical analyses, regarding the channel resistance caused by such potential profiles: The long-range potential modulation and the short-range scattering potential are mixed up. The former is caused mainly by the applied gate voltage and the work function difference between the substrate and gate, whereas the latter is attributed to the screened Coulomb potential of impurities and carriers. The channel resistance is dominated by the short-range scattering potential and the long-range potential simply modulates the carrier density in the channel region, rather than the channel resistance. Therefore, in order to calculate the impurity-limited resistance, it is essential to eliminate the long-range potential modulation from the channel potential. In the present calculations, the average impurity density in the channel is fixed at 2 x 1019 cm-3, so that the channel length varies according to the number of impurities doped in the channel, namely, L = 4 nm for a single impurity, 8 nm for two impurities, etc. The conductance is obtained from the Landauer formula in terms of the transmission probability calculated from the Lippmann-Schwinger equation with the short-range scattering potential. The impurity-limited resistance and mobility are then extracted by subtracting the quantum resistance from the inverse of the conductance. We show that the impurity-limited resistance (mobility) due to two ionized impurities located on the wire axis is greatly modulated as a function of the distance Δ between the impurities: The constructive phase interference is very strong at small Δ even at room temperature, whereas this phase interference diminishes at large Δ. Since no phase randomizing scattering is included in the present calculations, this randomization is attributed to the broadness of the energy spectrum of the incoming electrons from the reservoirs [4]. The most striking feature is that the impurity-limited resistance (mobility) is greatly suppressed (enhanced) at some particular value of Δ (around 1 nm) in the case of donors. We have also carried out more elaborate NEGF simulations with the screened Coulomb potential by placing the donor impurities at random inside the channel region. The above feature is still clearly visible, though the modulation is reduced owing to random impurity distributions. Nevertheless, this large mobility modulation is expected to affect significantly the device performance, especially, in the case of Junctionless-FETs. This study was supported in part by the MEXT under Grant- in-Aid for Scientific Research (B) (No. 15H03983). REFERENCES [1] V. Schmidt, et al., Adv. Mater. 21, 2681 (2009). [2] R. Rurali, Rev. Mod. Phys. 82, 427 (2010). [3] A. Asenov, IEEE ED-45, p.2505 (1998); N. Sano, et al., IEDM Tech. Dig. p.275 (2000). [4] N. Sano, J. Appl. Phys. 118, 244302 (2015); EUROSOI-ULIS 2016, p. 216 (2016).

Hybrid-impurity resonance in a quantum wire placed in a magnetic field

An analytical expression for the absorption coefficient of electromagnetic radiation in a quantum wire is obtained. We consider the case of a field lying in the plane perpendicular to the wire axis. The calculation is performed using second-order perturbation theory with consideration of electron scattering by ionized impurities. We demonstrate the resonant character of the absorption and find the position of resonance frequencies in the dependence on the magnitude and direction of the magnetic field.

Comparison of microstructure, second phases and texture formation during melt processing of Bi-2212 mono- and multifilament wires

Based on simultaneous in situ high energy synchrotron micro-tomography and x-ray diffraction (XRD) measurements we compare the microstructural changes and the formation of second phases and texture during the processing of Bi-2212 round wires with 15 μm filament diameter (multifilament) and 650 μm filament diameter (monofilament) fabricated using identical Bi-2212 precursor. The monofilament tomograms show in unprecedented detail how the distributed porosity agglomerates well before Bi-2212 melting decomposition to form lenticular voids that completely interrupt the filament connectivity along the wire axis. When the Bi-2212 phase completely melts connectivity in the axial wire direction is established via the changes in the void morphology from the lenticular voids to bubbles that remain when Bi-2212 crystallises out of the melt. By measuring the attenuation of the monochromatic x-ray beam, the associated Bi-2212 mass density changes have been monitored during the entire heat cycle. The XRD results reveal that the wire architecture can have a strong influence on the phase evolution during the melt processing heat treatment affecting the reversibility of Bi-2212 melting decomposition reaction. A strong Bi-2212 texturing is only achieved in the multifilament wire, while in the monofilament wire Bi-2212 crystallites grow with nearly random orientation.

Tunable High-Field Magnetization in Strongly Exchange-Coupled Freestanding Co/CoO Core/Shell Coaxial Nanowires.

The exchange bias properties of Co/CoO coaxial core/shell nanowires were investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. First, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting predefined magnetization states to the antiferromagnetic (AFM) shell, as corroborated by micromagnetic simulations. Second, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops attributed to the AFM easy axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, was thus experimentally and theoretically demonstrated.

Magnetostatic interaction in electrodeposited Ni/Au multilayer nanowire arrays**Project supported by the National Natural Science Foundation of China (Grant No. 11204246) and the Natural Science Foundation of CQCSTC (Grant No. cstc2014jcyjA50027).

Ordered Ni/Au multilayer nanowire arrays are successfully fabricated inside the nanochannels of anodic aluminum oxide template by pulse electrodeposition method. The thickness of the alternating layers is controlled to examine the magnetostatic interaction in Ni/Au multilayer nanowires. The magnetic easy axis parallel to the nanowires indicates that here the magnetostatic coupling along the wire axis dominates over the interactions perpendicular to the nanowires. However, the magnetostatic interaction between adjacent nanowires with larger magnetic layers is enhanced, leading to the existence of an optimum coercivity value.

Orbital effect of magnetic field on the Majorana phase diagram

Studies of Majorana bound states in semiconducting nanowires frequently neglect the orbital effect of magnetic field. Systematically studying its role leads us to several conclusions for designing Majoranas in this system. Specifically, we show that for experimentally relevant parameter values orbital effect of magnetic field has a stronger impact on the dispersion relation than the Zeeman effect. While Majoranas do not require a presence of only one dispersion subband, we observe that the size of the Majoranas becomes unpractically large, and the band gap unpractically small when more than one subband is filled. Since the orbital effect of magnetic field breaks several symmetries of the Hamiltonian, it leads to the appearance of large regions in parameter space with no band gap whenever the magnetic field is not aligned with the wire axis. The reflection symmetry of the Hamiltonian with respect to the plane perpendicular to the wire axis guarantees that the wire stays gapped in the topologically nontrivial region as long as the field is aligned with the wire.

Effect of the tilted magnetic field on the magnetosubbands and conductance in the bilayer quantum wire

We theoretically study the single electron magnetotransport in GaAs and InGaAs vertically stacked bilayer nanowires. In considered geometry, the tilted magnetic field is always perpendicular to the main (transport) axis of the quantum wire and, therefore its transverse and vertical components allow separately for changing the magnitude of intralayer and interlayer subbands mixing. We study the changes introduced to energy dispersion relation E(k) by tilted magnetic field of strength up to several tesla and analyze their origins for symmetric as well as asymmetric confining potentials in the growth direction. Calculated energy dispersion relations are thereafter used to show that the value of a conductance of the bilayer nanowire may abruptly rise as well as fall by few conductance quanta when the Fermi energy in nanosystem is changed. It is also shown that such conductance oscillations, in conjunction with spin Zeeman effect, may give a moderately spin polarized current in the bilayer nanowire.