NbC Layer Research Articles

Exploring Hydrogen Incorporation into the Nb4AlC3 MAX Phases: Ab Initio Calculations.

The Nb4AlC3 MAX phase can be regarded as a TMC structure with stacking faults, which has great potential as a novel solid hydrogen storage material. Herein, we used ab initio calculations for understanding the hydrogen incorporation into Nb4AlC3 MAX phases, including equilibrium structural characteristics, energy changes, electronic structures, bonding characteristics, and diffusion paths. According to the calculated results, H has thermal stability in the interstice of the Nb-Al layer, and the most probable insertion site is an octahedron (3-site) composed of three Nb atoms and three Al atoms. When C vacancies are introduced, the Nb-C layer has a specific storage capacity for H. In addition, Al vacancies can also be used as possible sites for H incorporation. Moreover, the introduction of vacancies significantly increase the hydrogen storage capacity of the MAX phase. According to the electronic structure and bonding characteristics, the excellent hydrogen storage ability of the Nb4AlC3 structure may be due to the formation of ionic bonds between H and Nb/Al. It is worth noting that the H-Al bond in the 1-site is a covalent bond and an ionic bond key mixture. The linear synchronous transit optimization study shows that only H diffusion in Al vacancies is not feasible. In conclusion, the Nb-Al layer in Nb4AlC3 can provide favorable conditions for the continuous insertion and subsequent extraction of H, while the vacancy structure is more suitable for H storage. Our work provides solid theoretical results for understanding the hydrogen incorporation into Nb4AlC3 MAX phases that can be helpful for the design of advanced hydrogen storage materials.

Tribological Evaluation of NbC and VC Layers Produced by Thermo-Reactive Diffusion Treatment in Ductile Cast Irons with Varying Composition

Tribological Evaluation of NbC and VC Layers Produced by Thermo-Reactive Diffusion Treatment in Ductile Cast Irons with Varying Composition

Preparation and high temperature performance of NbC layer in TRISO particles

TRISO particles with new coating layers was designed to adapt higher temperature. In this work, NbC coating layers were prepared by a novel two-step method: The original NbC coating layer was prepared from NbCl5 and C3H6 by chemical vapor deposition method. The flow rate of the C3H6 should be relatively low to avoid the C impurity. The NbC layer was then heated with carbon powders at high temperature. Since the C atoms can diffuse into the NbC coating layer and react with Nb2C at high temperature. The Nb2C impurity comes from the deposition was removed after the thermal treatment. The crystal of NbC layer grow up and has a change of preferred orientation at high temperature. The heat treatment has slight effect on the hardness of the pure NbC layer. Our result shows the NbC coating layer has good adaptability to the high temperature environment in VHTR.

TEM study on the morphology and interfacial structure of Nb-coated Cf/SiC composite

Nb-coated Cf/SiC composites are promising lightweight structural materials with wide applications in the aerospace field. The strength of interfacial bonding between Nb coating and Cf/SiC composite has significant influence on the material performance. In this study, we carefully investigated the interfacial structure between Nb coating and Cf/SiC composite by electron microscopy. The Nb coating on Cf/SiC composites was prepared by chemical vapor deposition. Cross-section imaging by SEM and TEM showed that layered structures were formed, the order is reaction zone, fine equiaxed grained Nb zone and coarse columnar grained Nb zone from the interface to the top of Nb. A sandwich-like interfacial reaction zone (an intermittent Nb5Si3 layer between two NbC layers, denoted as NbC | γ-Nb5Si3 | NbC) can be formed between Nb coating and Cf/SiC composite. Additionally, nanoscale NbC particles were observed near the interface of the Cf/SiC composite, while needle-shaped γ-Nb2C in the Nb coating, which is due to the interdiffusion of Nb and C elements. And an orientation relationship was identified between γ-Nb2C and Nb matrix. More importantly, hydrogen was confirmed to be existed in the NbC phase and the micro-cracks at the interface by ToF-SIMS analysis, which have a negative effect on the interfacial bonding properties. The room-temperature shear strength of the joint is 37.75 MPa. The formation mechanisms of the interfacial structures in Nb-coated Cf/SiC composite were discussed based on thermodynamics and kinetics.

Naive Bayesian fusion based deep learning networks for multisegmented classification of fishes in aquaculture industries

Fish classification is an essential requirement for biomass estimation, disease identification, and quality analysis. In aquaculture industries, fish classification is carried out in the processing unit. The fishes are out of water, subjecting them to structural deformation and orientation misalignments, makes classification challenging. A multisegmented fish classification technique using deep learning networks with naive Bayesian type fusion is proposed in this work to address these challenges. Fish images are acquired using an overhead camera. The fish head is identified by observing a minimal convexity deficiency region to facilitate segmentation. A multi-stage exhaustive enumerative optimization method is used to adjust the orientation, which can minimize unwanted background region in the image segment. Fish head, scales, and body are segmented from the fish image. For each fish segment, AlexNet is trained by using the transfer learning approach. A naive Bayesian fusion layer is introduced to fuse these trained deep learning networks and enhance classification accuracy. Experimental results illustrate a classification accuracy of 98.64% for ‘Fish-Pak’ image dataset with six different fish species and 98.94% for BYU fish dataset with four species. Comparative analysis with standard networks and ablation study demonstrates the accuracy and robustness of the proposed fusion architecture, respectively. Various fusion layers have also been analyzed, and observations illustrate the accuracy of the proposed NBC layer. Significant improvements in other classification performance metrics were also observed.

In situ fabricated metal-carbide with core–shell structure for high impact-toughness iron-matrix composite

ABSTRACTA series of metal-carbide (Ta–TaC, Nb–NbC and W–WC) with core–shell structure for iron-matrix composites are fabricated by in situ solid-phase diffusion. Results show that the formation of metal-carbide with a rod-shaped core–shell structure, in which the metal-rod surface was covered with a carbide shell layer, in the iron- matrix after in situ solid-phase diffusion. The TaC, NbC, and WC shell layers are in situ synthesised by the diffusion of carbon atoms from the iron-matrix onto the surface of the Ta, Nb, and W rods, respectively. Metallurgical integration occurs between metal-carbide and iron-matrix. The metal-carbide-reinforced iron-matrix composites show excellent impact resistance, and the shell-layer hardness is extremely high.

Characterization of Niobium Carbide Layers Produced in Ductile Cast Iron Using Thermo-Reactive Treatments

Abstract Ductile cast irons are materials with good ductility and toughness, but they are susceptible to high wear rates since they have low hardness. One way to increase their wear resistance is through thermo-reactive diffusion treatments (TRD), in which carbide layers are produced on the surface of the material. In this study, two ductile cast irons (with and without the addition of copper) were treated in a salt bath composed of borax, ferroniobium (16 wt.%) and aluminum (3 wt.%) at 1000°C for 4 h. The XRD and micro-hardness analysis indicated that NbC layers were produced with hardness values higher than 2000 HV. Two methods were used to examine the adherence of the layer to the substrate. In the first method, indentations were performed using Rockwell C scale (VDI 3198). The second method used Vickers micro-hardness indentations (loads 100, 200, and 300 gf) at the interface layer/substrate. In both tests, the layers showed good adhesion to substrates. The 3198 VDI test also showed that the NbC layer produced good toughness. The results of micro-adhesive and micro-abrasive wear tests showed that the layer was very effective in increasing wear resistance.

Interface structure and the inception of plasticity in Nb/NbC nanolayered composites

Molecular dynamics (MD) simulations were performed to explore the effect of interface structure on the inception of plastic deformation in Nb/NbC nanolayered composites. Using the atomistically informed Frank–Bilby method and disregistry analysis, we characterized the structure of the Nb/NbC interface, including misfit dislocations, dislocation nodes and three coherent interface structures. According to the crystallographic analysis of the interface, four possible coherent interface structures were identified. However, study of the interface energy showed that only three of these are energetically stable. After the relaxation of the interface, the unstable coherent region, which features Nb atoms in the Nb layer on the top of the Nb atoms in the NbC layer, evolves into a condensed interface dislocation node. Three stable coherent interface regions are retained in association with the formation, glide and reaction of interface misfit dislocation loops. Disregistry analysis of the Nb/NbC interface revealed that (i) all misfit dislocations are edge type, and (ii) misfit dislocations enclosing the coherent extended nodes have Burgers vectors along 〈110〉Nb. The role of the interface structure in the plastic deformation of Nb/NbC nanolayered composites was studied under two loading conditions, i.e. uniform compression and nanoindentation. Under uniform compression, lattice dislocations primarily nucleate from the condensed nodal regions where the local strains are the highest. Dislocations propagate in two {110} slip planes with the same Schmid factors. Under nanoindentation, by proper positioning of the indenter, lattice dislocations nucleate from the segments of the misfit dislocations and propagate in {112} slip planes. MD simulations also show cross-slip of lattice dislocations from {112} planes into {110} planes and the formation of vacancies as a result of climb of dislocation jogs.

Microstructure, morphology, adhesion and tribological behavior of sputtered niobium carbide and bismuth films on tool steel

NbC, Bi and Bi/NbC coatings were deposited on AISI M2 steel substrates using unbalanced magnetron sputtering at room temperature with zero bias voltage. Were studied the phase structure, the morphology, the adhesion and the tribological behavior of the three coatings. The niobium carbide film crystallized in the NbC cubic structure, and the bismuth layers had a rhombohedral phase with random orientation. The NbC coating had a smooth surface with low roughness, while the Bi layers on steel and on NbC coating had higher roughness and a morphology composed of large particles. By using a ductile Nb interlayer good adhesion between the NbC coating and the steel substrate was achieved. The Bi coating had better adhesion with the NbC layer than with the steel substrate. The tribological performance of the Bi coating on steel was not satisfactory, but according to the preliminary results, the produced NbC and Bi/NbC coatings have the potential to improve the tribological performance of the steel.

Molecular dynamics simulations of plastic deformation in Nb/NbC multilayers

Experimental studies show that metal–ceramic multilayers can have high strength, high strain hardening and measurable plasticity when the ceramic layer is a few nanometers thick. Using molecular dynamics simulations we studied deformation mechanisms in metal–ceramic multilayers and the role of interface structure and layer thickness on mechanical behavior. NbC/Nb multilayers were investigated numerically using the molecular dynamics (MD) method with empirical interatomic potentials. The interface dislocation structure was characterized by combining MD simulations and atomically informed Frank–Bilby theory. Two sets of pure edge misfit dislocations have been identified. Plastic deformation in NbC/Nb multilayers commences first in the metal layers by nucleation and glide of lattice dislocations initiating from interface misfit dislocations. These dislocations glide in the Nb layer and are deposited at the interface. The deposited dislocations facilitate slip transmission from the Nb layer to the NbC layer. The critical strain corresponding to dislocation nucleation is insensitive to layer thickness but depends on interface dislocation structure. The strain hardening and the peak flow strength of NbC/Nb multilayers are associated with the slip transmission from Nb to NbC, and are correlated to the interfacial dislocations, Nb layer thickness, and NbC layer thickness. The flow strength decreases with increasing Nb layer thickness and decreasing the NbC layer thickness.

Effect of pulsed DC current on atomic diffusion of Nb–C diffusion couple

The effect of pulsed DC current on the atomic diffusion of an Nb–C system using Spark Plasma Sintering (SPS) was investigated. In all experiments, a current density of approximately 723 A/cm2 was applied to the specimen and used in a temperature range of 1,673–1,973 K. From the results of X-ray diffraction analysis, the product phases formed between Nb and C were found to be Nb2C and NbC. The growth of product layers significantly increased in the presence of current. However, the thickness of the product layer did not change in the current direction in the SPS. The activation energies for the formation of the Nb2C and NbC layers were calculated to be 298 ± 4 kJ/mol and 282 ± 3 kJ/mol in the presence of current, which were similar values compared to the activation energies of 300 ± 5 kJ/mol and 285 ± 2 kJ/mol in the absence of current, respectively.

Ab initio study of basal slip in Nb2AlC

Using ab initio calculations, we have studied shearing inNb2AlC, where NbC and Al layers are interleaved. The stress–strain analysis of this deformationmode reveals Nb–Al bond breaking, while the Nb–C bond length decreases by 4.1%.Furthermore, there is no evidence for phase transformation during deformation. This isconsistent with basal slip and may be understood on the basis of the electronic structure:bands below the Fermi level are responsible for the dd bonding between NbCbasal planes and only a single band with a weak dd interaction is not resistant toshearing, while all other bands are unaffected. The Al–Nb bonding character can bedescribed as mainly metallic with weak covalent–ionic contributions. Our studydemonstrates that Al layers move with relative ease under shear strain. Phaseconservation upon shearing is unusual for carbides and may be due to the layerednature of the phase studied. Here, we describe the electronic origin of basal slip inNb2AlC, the atomic mechanism which enables reversible plasticity in this class of materials.

Inter-diffusion of carbon into niobium coatings deposited on graphite

The inter-diffusion of carbon (originating from a graphite substrate) into a niobium coating and the fabrication of its carbides by heat treatment in the temperature range of 1073–1773 K was studied. The thickness of the Nb 2C and NbC phases formed after heat treatment as well as the inter-diffusion coefficients for the formation of the carbide layers were also studied. It was found that the carbide layer growth displayed parabolic behavior patterns inherent in the growth rate constants (K) of Nb 2C and NbC layers. By assuming that the inter-diffusion coefficients are independent of concentration, it was possible to determine the inter-diffusion coefficients of carbon D c into Nb 2C and NbC layers as a function of temperature.

Annealing of niobium coatings deposited on graphite

Niobium coatings 8–12 μm thick were deposited by magnetron sputtering on ATJ graphite substrate. The kinetic growth of carbide layers into niobium coatings and the properties of the Nb 2C and NbC phases obtained after annealing in the temperature range of 1073–1773 K were studied. It was found that the carbide layer growth displayed parabolic behavior patterns establishing the growth rate constants ( K) of Nb 2C and NbC layers, as follows: K Nb 2 C = 3.7 × 1 0 - 8 exp ( - 190 kJ RT ) m 2 s ; K NbC = 4.5 × 1 0 - 9 exp ( - 164 kJ RT ) m 2 s .

Microstructure of NbSi 2/SiC nanocomposite coating formed on Nb substrate

The microstructure of a NbSi 2/α-SiC nanocomposite coating formed by a pack siliconizing process of Nb-carbide layers at 1100 °C, which had already been formed on the surface of a Nb substrate by a carburizing process, was investigated. The Nb-carbide layers formed by the carburizing process at 1400 °C using a gas mixture of CH 4–H 2 consisted of two layers, i.e., an inner layer of Nb 2C and an outer layer of NbC. While the monolithic NbSi 2 coating showed a typical columnar microstructure perpendicular to the Nb substrate, the NbSi 2/α-SiC nanocomposite coating formed by the solid-state displacement reaction of the NbC layer by Si was composed of equiaxed NbSi 2 grains with an average size of 67–134 nm and the α-SiC particles with an average size of 45–60 nm. The morphology of the α-SiC particles exhibited an oblate-spheroidal shape and were mostly located at the grain boundaries of NbSi 2. The nanocomposite coating formed by the solid-state displacement reaction of the Nb 2C layer by Si showed a lamella microstructure. The average lamella diameters of the NbSi 2 grains and the α-SiC particles were about 250 and 60 nm, respectively. The volume percentage of the α-SiC particles ranged from 17.3 to 31.1% with respect to the carbon concentration in Nb-carbide layers. No cracks were observed in the NbSi 2/α-SiC nanocomposite coating, indicating that its thermal expansion coefficient was close to that of the Nb substrate.

Structure and composition of Nb and NbC layers on graphite

Niobium layers of 6 to 8 μm in thickness have been deposited on an ATJ graphite substrate by radio-frequency (rf) magnetron sputtering. After deposition, the layers were annealed under vacuum (6.67×10 −4 Pa) at a temperature in the range of 1373–2073 K for 0.5 to 3 h. The effects of the negative substrate bias voltage on the deposited niobium layers and niobium carbide layers derived by heat treatment were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Vickers microhardness test. Depending on the negative dc bias voltage, V b, applied to graphite substrates, three distinct categories of Nb and of NbC layers were formed: (a) the Nb layers deposited on graphite substrates biased to V b<50 V, exhibited a singularly nucleated columnar structure and these layers were converted by annealing treatment into highly porous NbC layers of low hardness, (b) the Nb layers deposited at V b in the range of 50–80 V were composed of a continuously nucleated sub-columnar structure and were transformed into dense NbC layer exhibiting the highest microhardness value equal to ∼13 GPa; and (c) the Nb layers deposited at V b≥80 VDC possessed an imperfect structure and were transformed into an NbC layer with the highest compactness characterized by a brittle fracture and a moderately high microhardness value.

Hydrogen superpermeation resistant to ion sputtering

A nonmetallic monolayer on the surface of a metallic membrane gives rise to the superpermeation of suprathermal hydrogen. This means that virtually the whole implanted flux passes through the membrane, depending neither on membrane temperature nor on its thickness. The fact that superpermeation is degrading at a sufficiently high energy of the incident hydrogen particles was considered to be a ‘natural’ result of sputtering of the monolayer. This view was equally applied to Nb, one of the best metals for superpermeation. It is shown in this article that Nb containing O impurity will be normally superpermeable to suprathermal hydrogen at any incident energy, the earlier observed sensitivity to sputtering being conditioned by the NbC layer originally present at the ‘real’ Nb surface. The dynamic exchange between the surface and bulk O impurity is proven to be responsible for the superpermeability of Nb resistant to sputtering. When the NbC layer is present, it hinders such an exchange.

Hydrogen superpermeation resistant to ion sputtering

A nonmetallic monolayer on the surface of a metallic membrane gives rise to the superpermeation of suprathermal hydrogen. This means that virtually the whole implanted flux passes through the membrane, not depending on membrane temperature and thickness. The fact that superpermeation is degrading at a sufficiently high energy of incident hydrogen particles was considered a “natural” result of sputtering of the monolayer. It is now proven that Nb containing an O impurity would be normally superpermeable to suprathermal hydrogen at any incident energy, whereas the “usual” sensitivity to sputtering is conditioned by a NbC layer originally present at the “real” surface of Nb.

Effects of Additives on the Interfacial Microstructure of SiC Fiber/Li2O-Al2O3-6SiO2 Glass-Ceramic Composites

SiC fiber reinforced lithia-alumina-silica glass-ceramic composites have exhibited superior mechanical properties at room and elevated temperatures. The flexural strength of these composites is 3 to 4 times larger than that of monolithic glass-ceramics. The high flexural strength of these composites is due to the transfer of the applied load from the matrix to the stronger and stiffer SiC fiber reinforcement. These composites also have demonstrated very high fracture toughness, KIC (critical stress intersity factor) values of ∼ 17 MPa m1/2, which is attributed to an amorphous carbon-rich interfacial layer between the fiber and matrix. Nb2O2 has been added to the matrix composition in order to develop a NbC layer outside the amorphous C-rich layer after thermal processing, thereby buffering the fiber/matrix reactivity (avoid carbon forming CO gas which would deteriorate the matrix).

Contamination problems with broad beam ion sources when producing NbNb 2O 5Pb alloy Josephson junctions

A broad beam (Kaufman) ion source has been used for the ion beam cleaning and oxidation of sputter-deposited Nb films with the aim of producing NbNb 2O 5Pb alloy Josephson junctions. When attempting ion beam cleaning with an Ar + ion beam followed by ion beam oxidation with an Ar +O 2 mixture, suitable Nb 2O 5 barrier layers could not be formed. In situ X-ray photoelectron spectroscopy (XPS) analyses of the surface of the Nb films immediately after ion beam processing have been made as well as in situ secondary ion mass spectroscopy (SIMS) and ion analysis of the beam. It was found that a NbC layer was formed during ion beam etching as a result of C contamination from the graphite grids. This layer prevented the growth of a Nb 2O 5 layer during the ion beam oxidation step. Possible solutions to this problem are considered.