Nb Layers Research Articles

Vacuum Brazing of Alumina to Titanium for Implantable Feedthroughs Using Pure Gold as the Braze Metal

Pure gold brazing of 96% alumina ceramic with CP titanium using electrical resistance heating has been analyzed. Wetting studies of gold with niobium and titanium were performed. HTCC alumina ceramic was metalized with a 500nm Nb layer yielding hermetic joints with Ti ferrule having leak rate of the order of 1.6×10 -8 atm-cc/ sec. Wetting studies of Ti with Au and EDX analysis at the Au- Ti interface revealed that all the four Ti-Au intermetallic compounds forms at the interface and to avoid the growth of these intermetallic compounds brazing time should be kept minimum. Nb also shows good wetting behavior towards molten gold. Adhesion strength of Nb thin film over polished alumina substrate was measured using nanoscratch testing using a nanoindentor on 100nm Nb thin film on alumina. Scratch testing revealed that strong and adherent thin films of Nb over polished alumina can be deposited using sputtering. 

Magnetic Response of Ferromagnet-Superconductor-Ferromagnet (Gd/Nb/Gd) Trilayer

Magnetic measurements on Gd(30 nm)/Nb(70 nm)/Gd(30 nm) trilayer were performed with the field applied parallel to the plane of the film. Ferromagnetic transition was observed at temperature of about 285 K and the superconducting transition was suppressed to 6 K. A novel finding of the study is the observation of rearrangement of ferromagnetic domains in magnetic Gd layers induced by the traversal of the trilayer through the superconducting state in the adjoining Nb layer.

Vortex flipping in superconductor/ferromagnet spin-valve structures

We report in-plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni below the superconducting transition ${T}_{\mathrm{c}}$, our results show strong evidence of vortex flipping driven by ferromagnet magnetization. This is a direct consequence of the proximity effect that leads to vortex supercurrent leakage into the ferromagnets. Here, the polarized electron spins are subject to the vortices' magnetic field, occasioning vortex flipping. Such a novel mechanism has been made possible by fabrication of the ferromagnet/superconductor/ferromagnet/antiferromagnet multilayered spin valves with an S layer thin enough to barely confine vortices inside as well as F layers thin enough to align and control magnetization within the plane. When Co is used, the vortex flipping effect is not observed. This is attributed to the shorter coherence length of Co. Interestingly, a reduction in the pinning field of about 400 Oe is observed instead when the Nb layer is in the superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts, any explanation must be directly related to the proximity effect and thus a remarkable phenomenon that deserves further investigation.

Laser metal deposition of TiC/Inconel 718 composites with tailored interfacial microstructures

Abstract Laser metal deposition (LMD) was applied to deposit Inconel 718 metal matrix composites reinforced with TiC particles. The influence of laser energy input per unit length on constitution phases, microstructures, hardness, and wear performance of LMD-processed TiC/Inconel 718 composites was studied. It revealed that the LMD-processed composites consisted of γ Ni–Cr solid solution matrix, the intermetallic precipitation phase γ′, and the TiC reinforcing phase. For the laser energy input per unit length of 80–120 kJ/m, a coherent interfacial layer with the thickness of 0.8–1.4 μm was formed between TiC reinforcing particles and the matrix, which was identified as (Ti,M)C (M=Nb and Mo) layer. Its formation was due to the reaction of the strong carbide-forming elements Nb and Mo of the matrix with the dissolved Ti and C on the surface of TiC particles. The microstructures of the TiC reinforcing phase experienced a successive change as laser energy input per unit length increased: Relatively coarsened poly-angular particles (80 kJ/m) – surface melted, smoothened TiC particles (≥100 kJ/m) – fully melted/precipitated, significantly refined TiC dendrites/particles (160 kJ/m). Using the laser energy input per unit length ≥100 kJ/m produced the fully dense composites having the uniformly dispersed TiC reinforcing particles. Either the formation of reinforcement/matrix interfacial layer or the refinement in TiC dendrites/particles microstructures enhanced the microhardness and wear performance of TiC/Inconel 718 composites.

Modeling the texture evolution of Cu/Nb layered composites during rolling

Metallic based multi-layered nano-composites are recognized for their increased plastic flow strength and indentation hardness, increased ductility, improved radiation damage resistance, improved electrical and magnetic properties, and enhanced fatigue failure resistance compared to conventional metallic materials. One of the ways in which these classes of materials are manufactured is through accumulated roll bonding where the material is produced by several rolling and heat treatment steps during which the layer thickness is reduced through severe plastic deformation. In this article, a single rolling pass of the accumulated roll bonding process in which a Cu/Nb layered composite with an initial average layer thickness of 24μm subjected to a 50% height reduction is examined. Experimental morphological and crystallographic texture data is presented and used to initialize numerical models for these layered material systems. This study focuses on the Cu/Nb combination of fcc/bcc materials for incoherent material interfaces. A single crystal model based upon thermally activated dislocation motion is used and the evaluation of material parameters is presented. Since the initial state of the composite is not in a fully annealed condition, nano hardness tests for both the Cu and Nb layers are used to initialize the model for each of the two materials. EBSD data of the heat treated material is used to characterize the initial state of the composite and used to produce 40 combined morphological and crystallographic numerical model realizations of the material. Each of the models is then imposed to a plane strain compression deformation height reduction of 50%. Numerical data from the 40 simulations was then combined to arrive at the statistically comparable crystallographic texture for each of the Cu and Nb materials. This data was compared with the experimental data and the results suggest very good agreement between the predicted and experimental textures for both the materials. Selected stress profile predictions for two realizations are presented and demonstrate a strong difference in stress state between the Cu and Nb layers.

Study of nanometric superconducting multilayers for RF field screening applications

Vortex penetration severely limits high field performances in bulk niobium RF cavities for accelerators. To reach higher fields, Gurevich [Appl. Phys. Lett. 88, 012511 (2006)] proposed to deposit nanometric layers (d < λ) to partially screen of the field sensed by niobium. Model (NbN/MgO)n (n = 0 to 4) samples have been deposited on thick Nb layers. This paper presents the first complete characterization set (HC1 and RF surface resistance) of a new family of composite nanostructured superconducting layers deposited on Nb, which are liable to bring a breakthrough in the technology of superconducting accelerating RF cavities.

Improvement of Electrical Properties of Silicon Quantum Dot Superlattice Solar Cells with Diffusion Barrier Layers

We investigate the effects of a niobium-doped titanium dioxide (TiO2:Nb) diffusion barrier layer on the performance of silicon quantum dot superlattice (Si-QDSL) solar cells. The insertion of a 2-nm-thick TiO2:Nb layer significantly reduces phosphorus diffusion from a highly doped n-type layer into a Si-QDSL layer during thermal annealing at 900 °C. The phosphorous concentration in the Si-QDSL layer of the solar cell with the TiO2:Nb diffusion barrier layer was found to be less than 1018 cm-3, which is approximately two orders of magnitude lower than that of the solar cell without the diffusion barrier layer. The reduction in phosphorous concentration leads to the improvement of photo-generated carrier collection in the Si-QDSL layer. The short circuit current density of the solar cell with the diffusion barrier layer was dramatically improved to 1.6 mA/cm2 without the degradation of open circuit voltage and fill factor.

Synthesis and phase evolution in Nb/Si multilayers obtained by sequential laser ablation

The paper reports the synthesis of Nb/Si multilayers (48/27nm) deposited on Si single crystal substrate by sequential laser ablation of elemental Nb and Si. Significant amount of Nb is found in the amorphous Si layer (~25–35at.% Nb). The Nb layer is found to be polycrystalline. The phase evolution of the multilayer has been studied by annealing at 600°C for various times and carrying out cross sectional electron microscopic studies. We report the formation of amorphous silicide layer at the Nb/Si interface followed by the formation of the NbSi2 phase in the Si layer. Further annealing leads to the nucleation of hexagonal Nb5Si3 grains in amorphous silicide layers at Nb/NbSi2 interfaces. These results are different from those reported for sputter deposited multilayer.

Meso-Scale Modeling the Orientation and Interface Stability of Cu/Nb-Layered Composites by Rolling

Metallic-based multilayered nanocomposites are recognized for their increased plastic flow resistance and indentation hardness, increased ductility, improved radiation damage resistance, improved electrical and magnetic properties, and enhanced fatigue failure resistance compared to conventional metallic materials. One of the ways in which these classes of materials are manufactured is through accumulated roll bonding where the material is produced by several rolling and heat-treatment steps during which the layer thickness is reduced through severe plastic deformation. A single rolling pass of the accumulated roll bonding process in which a Cu/Nb-layered composite with an initial average layer thickness of 24 μm subjected to a 50% height reduction is modeled. A single-crystal model based upon thermally activated dislocation motion is used. Nanohardness tests for both the Cu and Nb layers are used to help initialize the model for each of the two materials. Electron backscatter diffraction (EBSD) data of the heat-treated material is used to characterize the initial state of the composite and to produce 40 combined morphological and crystallographic numerical model realizations of the material. The results suggest very good agreement between the predicted and experimental textures for both the materials. Highly oriented microstructure develops during severe plastic rolling deformation of Cu/Nb nanocomposites. The deformation textures significantly deviate from those expected when rolling Cu or Nb alone, and the Cu/Nb interfaces do not correspond to those with the lowest possible formation energies. We study the interfacial stability of specific Cu/Nb bicrystal configurations under rolling conditions using a finite-element crystal plasticity model. Specifically, we examine how slip activity and lattice reorientation are affected by the kinematic constraint imposed by the interface. Our results show that for certain configurations the slip activity and lattice rotation of the individual crystallites display some sensitivity to the kinematic constraint, yet the overall stability of a given bicrystal can be predicted by the stability of the individual single-crystal orientations. Future work will account for the influence of the bimetal interface on the interface stability and development of enhanced properties.

A rate theory study of helium bubble formation and retention in Cu–Nb nanocomposites

A spatially dependent rate theory model for helium migration, clustering, and trapping on interfaces between Cu and Nb layers is introduced to predict the evolution of the concentrations of He clusters of various sizes during implantation and early annealing. Migration and binding energies of point defects and small clusters in bulk Cu and Nb are found using conjugate gradient minimization and the nudged elastic band method. This model is implemented in a three-dimensional framework and used to predict the relationship between helium bubble formation and the nano-composite microstructure, including interfacial free volume, grain size, and layer thickness. Interstitial and vacancy-like migration of helium is considered. The effects of changing layer thickness and interfacial misfit dislocation density on the threshold for helium bubble nucleation are found to match experiments. Accelerated helium release due to interfaces and grain boundaries is shown to occur only when diffusion rates on interfaces and grain boundaries are greatly increased relative to the bulk material.

A DFT Calculation of Nb and Ta (001) Surface Properties

First principle calculations are performed using the super cell method with pseudopotentials and plane waves based on the Density Functional Theory (DFT) for the surface structural properties at T = 0 K. Thin slabs of 7 - 13 atomic layers of the clean Nb and Ta (001) surfaces are considered and relaxations, surface energies, and work functions of the fully relaxed slabs are presented. Consistent results are obtained with the Generalized Gradient Approximation (GGA) and the Local Density Approximation (LDA) for the exchange-correlation functional and they compare well with experimental and other theoretical works.

High Sensitive Magnetic Nanosensors Based on Superconducting Quantum Interference Device

Nanosized superconducting quantum interference devices (nano-SQUIDs) offer the possibility to investigate small spin populations and the magnetization of nanoparticles opening new horizons in the world of nanomagnetism. We will report recent results about the design, the fabrication and the characterization of high sensitive magnetic nanosensors based on nano-SQUID having a flux capture area of 0.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . In order to achieve a better magnetic flux resolution a proper device design has been developed. The fabrication process is based on the electron beam lithography and a lift-off procedure in a 25 nm thick Nb layer. Measurements of current-voltage and current vs. magnetic flux characteristics have been reported. The comparison between the nanodevice based on the improved design and a standard one has shown the effectiveness of the new design. Preliminary measurements at T=4.2 K on Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> nanoparticles having a size of 8 nm have been reported, showing the capability of such nanodevices to investigate the magnetism at a submicrometric scale.

Microwave Properties of Nb/PdNi/Nb Trilayers

We combine wideband (1–20 GHz) Corbino Disk and Dielectric Resonator (8.2 GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers, grown by UHV dc magnetron sputtering, composed by Nb layers of nominal thickness d S =15 nm, and a ferromagnetic PdNi layer of thicknesses d F =1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to H c2 were applied. The microwave resistivity at fixed H/H c2 increases with d F , eventually exceeding the Bardeen–Stephen flux-flow value. Vortex pinning is very weak at large d F .

Influence of stray fields and the proximity effect in ferromagnet/superconductor/ferromagnet spin valves

We have investigated two switching mechanisms, the proximity effect and the influence of stray fields, in Permalloy (Py)/Nb/Py spin valves. Samples were grown by using dc magnetron sputtering and were patterned to form a bar shape. A coexistence of the proximity effect and the stray-field effect in Py/Nb/Py was found and could be explained by three resistance states: a low-resistance antiparallel state, an intermediate-resistance parallel state, and a high-resistance domain state with multiple domains in both Py layers. In order to compare the stray-field effect with the proximity effect, we increased the bias current density to enhance the Lorentz force on flux lines in the Nb layer in which flux lines are induced by stray fields from the domain walls of the Py layers. An increase in the magnetoresistance due to stray-field-induced flux flow in the domain state with increasing bias current density was observed as expected; meanwhile, a resistance difference between the parallel and antiparallel states due to the proximity effect was also observed. Between the two switching mechanisms, we found that a switch operating on the stray field effect under a high bias current density on the order of 105 A/cm2 was more advantageous for using a superconducting spin-valve as a resistance switch. Finally, we show that the stray-field-induced flux flow can be stabilized by orienting the magnetization easy axis perpendicular to the bar axis.

Electrochemical polishing of Ti–13Nb–13Zr alloy

This work presents the results of the investigations on electropolishing of the Ti–13Nb–13Zr titanium alloy. The electropolishing was conducted in the solution containing ammonium fluoride and sulfuric acid. The influence of electropolishing process parameters on the texture (AFM, SEM) and chemical composition (EDX, XPS) of the surface layer was established. The Ti–13Nb–13Zr alloy after the electropolishing is covered with a thin layer composed mainly of titanium, niobium and zirconium oxides. Electrochemical impedance spectroscopy in 5% NaCl solution was used for the determination of the corrosion resistance of the alloy. The application of the electropolishing increases the corrosion resistance of the Ti–13Nb–13Zr alloy significantly. The increase of the corrosion resistance of the alloy is connected with the smoothing of the surface and with the formation of Ti, Nb and Zr oxides layer on the surface.

Direct Observations of Confined Layer Slip in Cu/Nb Multilayers

In situ nanoindentation of a 30 nm Cu/20 nm Nb multilayer film in a transmission electron microscope revealed confined layer slip as the dominant deformation mechanism. Dislocations were observed to nucleate from the Cu-Nb interfaces in both layers. Dislocation glide was confined by interfaces to occur within each layer, without transmission across interfaces. Cu and Nb layers co-deformed to large plastic strains without cracking. These microscopy observations provide insights in the unit mechanisms of deformation, work hardening, and recovery in nanoscale metallic multilayers.

Nano Superconducting QUantum Interference Device Sensors for Magnetic Nanoparticle Detection

In this paper, Superconducting QUantum Interference Devices (SQUIDs) based on single layer Nb nanobridge Josephson junctions are described. Devices, with loop area ranging from 4 to 0.5 microm2, have been patterned by Electron Beam Lithography (EBL) in a 20 nm thick Nb layer, achieving a responsivity of about 30 microA/phi0. Magnetization measurements have been performed via switching current measurements at a temperature T = 4.2 K. Preliminary detection of Silica-magnetite (Fe3O4-SiO2) core/shell nanoparticle cluster has been proven.

Superconducting Microdisk Cavities for THz Quantum Cascade Lasers

We present superconducting waveguides for terahertz (THz) quantum cascade lasers (QCLs). Double-metal waveguides provide high confinement of the optical mode and low waveguide losses which are dominated by absorption of the radiation in the metal layers. Implementing novel waveguide materials like superconductors is one way to reduce these losses. In order to prove the compatibility with the THz QCL active region and waveguide we have replaced the commonly used gold or copper layers by superconducting niobium (Nb). We have simulated the temperature distribution inside the THz QCL in order to evaluate the operation conditions at which the critical temperature of the Nb layers is not exceeded. Experimental results of THz QCLs with Nb waveguides are presented which show lasing emission despite the fact that the energy of the THz radiation of the investigated active region f = 2.5 THz = 10.3 meV is higher than the superconducting energy gap of Nb 2Δ = 2.8 meV. Calculations show that improvements in terms of lower waveguide losses can be achieved using a superconductor with higher critical temperature and thus wider superconducting gap e.g., NbTiN or MgB2.

Perpendicular magnetic anisotropy induced by a cap layer in ultrathin MgO/CoFeB/Nb

Perpendicular magnetic anisotropy has been observed in MgO/CoFeB/Ta but not in MgO/CoFeB/Ru, indicating that the metal cap layer is crucial. In this study, we grew MgO/CoFeB (1.2 nm)/Nb (1–2.5 nm) by sputtering and found that the magnetic anisotropy depends upon the Na thickness. The easy axis is in-plane when x = 1 nm but changes to perpendicular when x ≥ 1.2, and a transition from in-plane to perpendicular is observed at x = 1.1 nm. Perpendicular magnetic anisotropy gradually decays as the x increases, showing that it only exists in a small window of the cap Nb layer within ∼2 nm.

Diffusion bonding of ZrB2–SiC/Nb with in situ synthesized TiB whiskers array

Abstract The ZrB 2 –20 vol.% SiC (ZS) composites were diffusion bonded to Nb using pure Ti interlayer. Effects of joining temperature on the microstructure and mechanical properties of the joints were investigated. The results show that Ti reacted with Nb and ZS to form a typical three layers in the joint. An in situ synthesized TiB whiskers array which consisted of two types of TiB was produced in the reaction layer. The formation mechanism of TiB was analyzed. Mutual diffusion between Ti and Nb led to a ductile β-(Ti, Nb) layer on Nb side. Joining temperature influenced the thickness of reaction layers and distribution of TiB seriously. The maximum shear strength reached 158 MPa with bonding temperature at 1200 °C for 60 min.