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.     

Synthesis of Novel (Polymer Blend-Ceramics) Nanocomposites: Structural,Optical and Electrical Properties for Humidity Sensors

Fabrication of novel nanocomposites films of (PVA–CMC) blend and (PVA–CMC) blend doped by niobium carbide nanoparticles has been investigated. The structural, optical and electrical properties of (PVA–CMC–NbC) nanocomposites for humidity sensors have been studied. The (PVA–CMC–NbC) nanocomposites were prepared with different concentrations of (polyvinyl alcohol and carboxyl methyl cellulose) and Niobium carbide nanoparticles. The experimental results of optical properties for (PVA–CMC–NbC) nanocomposites showed that the absorbance, absorption coefficient, extinction coefficient, refractive index, real and imaginary dielectric constants and optical conductivity of (PVA–CMC) blend increase with increase in Niobium carbide nanoparticles concentrations. The transmittance and energy band gap decrease with increase in Niobium carbide nanoparticles concentrations. The DC electrical properties of (PVA–CMC–NbC) nanocomposites showed that the electrical conductivity of the blend increases with increase in NbC nanoparticles concentrations. The experimental results of novel (PVA–CMC–NbC) nanocomposites applications showed that the (PVA–CMC–NbC) nanocomposites have high sensitivity for relative humidity.     

Niobium carbide reinforced-Ti6Al4V composites via directed energy deposition

Directed energy deposition (DED) was used to produce niobium carbide (NbC)-reinforced Ti6Al4V (Ti64) metal–matrix-composite (MMC) structures. The objective was to improve upon Ti64's wear and oxidation resistance. The characterization techniques consisted of scanning electron microscopy (SEM), backscattered electron (BSE) imaging, energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), Vickers micro- and nanoindentation-derived hardness, as well as tribological testing at varying normal loads. DED produced compositions were of Ti64, Ti64 + 5 wt.% NbC (5NbC), and Ti64 + 10 wt.% NbC (10NbC). Electron micrographs revealed crack- and delamination-free structures. Tribological analysis revealed a 25.1% reduction in specific wear rate. XRD and EDS results indicated the presence of a Ti-Nb solid solution. It was deduced that the NbC particles coupled with the Ti-Nb solid solution aided in increasing Ti64's resistance to plastic shear as the superficial microstructure remained unchanged compared to pure Ti64. Additionally, TGA displayed a reduction in total oxidation mass gain and suppressed oxidation kinetics to parabolic behavior with increased NbC. Application-based composite structures with site-specific mechanical properties were fabricated in the form of a composite cylinder, gear and compositionally graded cylinder. The graded cylinder displayed a 0%–45%NbC presence—end-to-end—equating to a hardness increase from 161.6 ± 4.0HV_0.2 to 1055.9 ± 157.4HV_0.2.     

Wire-electrical discharge machinable alumina zirconia niobium carbide composites – Influence of NbC content

Alumina zirconia (ZTA) ceramics can be made electric discharge machinable by addition of a percolating network of an electrically conductive phase. In this study the influence of NbC content on the mechanical and electrical properties as well as the ED-machinability of ZTA-NbC ceramics containing 17 vol.% zirconia and 24–32 vol.% NbC were investigated. Samples were hot pressed from mixed and milled starting powders. Surface morphology and surface roughness of wire electrical discharge machined surfaces were studied by SEM and perthometry. Cutting speed was determined to benchmark the ED-machinability.Rising NbC contents progressively impede sinterability. Maximum strength, Young’s modulus and hardness were found at intermediate NbC contents. Conductivity evidently rises with NbC content, the cutting performance showed an adverse tendency. The surface quality of the materials was improved by increasing the content of conductive phase. Additional trimming operations can reduce the mean roughness of machined surfaces to 1 μm.     

The effects of NbC nanoparticles synthesized by combustion method on the mechanical properties of Cu–NbC nanocomposite

In the first stage of this work, the master nanocomposite of niobium carbide (NbC)–Cu (ceramic-based nanocomposite) was synthesized by a mechanically induced magnesiothermic combustion in the Nb₂O₅/CuO/Mg/C system. Ignition time in this system was recorded to be ∼28 min of milling. In the second stage, appropriate amounts of NbC–Cu nanocomposite powder were mixed with pure copper powder to prepare Cu-based nanocomposite with 0, 5, 10, 15, and 20 volume fraction of NbC. The final metal matrix nanocomposite powder was sintered by spark plasma sintering method. The density of nanocomposite specimens decreased with increasing the percentage of NbC nanoparticles, while the microhardness of specimens increased with increasing nano-NbC content. Regarding the tensile test, the sample Cu–10 vol.% NbC nanocomposite with a strength of 372 MPa (∼63% higher than that of nonreinforced copper) was the best composition, and the nanocomposite strength decreased at higher NbC concentrations, mostly due to the agglomeration and nonhomogeneous distribution of reinforcing nanoparticles.     

Ultra-thin carbon nanosheets coated with SnO2–NbC nanoparticles as high-performance anode materials for lithium-ion batteries

A SnO₂–NbC–C ternary composite was constructed using hydrothermal and ball milling methods. For this special design, the SnO₂–NbC nanoparticles were uniformly coated with ultra-thin carbon nanosheets. It should be noted that the NbC nanoparticles can accommodate the volume variation of SnO₂ particles and prevent Sn agglomeration, resulting in good electrical contact between particles and low charge transfer resistance. After 100 cycles of 0.2 Ag^-1, the SnO₂–NbC–C nanocomposite exhibited a high capacity of 956.0 mAhg^-1, and a rate capacity of 955.2 mAhg^-1, and after 500 cycles of 5.0 Ag^-1, it achieved a capacity of 704.1 mAhg^-1. In addition, the SnO₂–NbC–C composite remained stable after 200 and 700 cycles without agglomeration.     

TEM characterization and reaction mechanism of composite coating fabricated by plasma spraying Nb–SiC composite powder

Niobium carbide composite coatings with Nb₂C, NbC, Nb₃Si as the main phases were prepared in situ on the surface of TC4 titanium alloy by plasma spraying Nb–SiC composite powder. The microstructure of the coating was characterized in detail by TEM, and the reaction mechanism of Nb–SiC was revealed. Sub-micron and nano-scale NbC grains dispersed in Nb₃Si region, nano-Nb/Nb₃Si cellular eutectic region, and equiaxed Nb₂C nanograins region were formed in the coating. The research results show that Nb and SiC reacted firstly to form cubic NbC and Nb₃Si phases during the plasma spraying process. Then, NbC with a higher melting point took the lead in crystallization during the cooling process of the coating, forming sub-micron and nano-scale NbC granular fine grains. Nb₃Si with a lower melting point crystallized around the sub-micron and nano-scale NbC granular fine grains in the subsequent cooling process. In the plasma spraying process, the molten droplets formed Nb/Nb₃Si cellular eutectic structure under large temperature gradient and extremely fast cooling rate. The remaining Nb in the raw material powder formed a diffusion couple with NbC to generate fine and dispersed nano-equiaxed Nb₂C with cubic structure. The present investigation provides a reference for the reaction synthesis of advanced nanocomposites using Nb–SiC system.     

Comparing ablation properties of NbC and NbC-25 mol.% ZrC coating on SiC-coated C/C composites

In this work, ablation properties of NbC and NbC-25 mol.% ZrC coating, deposited on SiC-coated C/C composites by supersonic atmospheric plasma spraying, were tested by an oxyacetylene torch. Results showed that, for NbC coating, an unexpected smooth liquid film mostly composed of niobium suboxides (such as NbO₂ and NbO), rather than pure Nb₂O₅, generated during ablation for 45 s. Mechanical erosion resulted from the molten SiO₂, and the relatively low viscosity of the outer oxide layer owing to insufficiently high melting point of niobium suboxides were the key factors for the failure mechanism of NbC coating. While NbC–ZrC coating abated for 90 s has a 97.49 and 66.53% decrease of linear and mass ablation rate relative to NbC coating ablated for 45 s, since ZrO₂ hindering the evaporation of SiO₂ droplets, and more thermal-stable Nb–O–Zr liquid film endow (NbC–ZrC)/SiC/C/C composites with an outstanding anti-ablation property.     

Microstructure and wear properties of niobium carbide particulates gradient-distribution composite layer fabricated in situ

Niobium carbide (NbC) particulates gradient-distribution composite layer was successfully synthesized by in situ techniques. The formation mechanism, microstructure evolution and wear properties of the composite layer were investigated in detail. The results show that the thickness of the composite layer is about 1360 µm after the heat treatment at 1175 °C for 1 h. From the surface to the matrix, the volume fraction of NbC particulates continuously decreases while the average particulate diameter gradually increases from 213 nm to 1.46 µm. Correspondingly, the micro-hardness and relative wear resistance almost gradually decrease from the top surface to the matrix.In order to discuss the wear mechanism clearly, the composite layer was artificially divided into four regions according to the continuously variation of composition and microstructure. The wear mechanism of NbC particulates high-density zone (region A) is mainly characterized by the intergranular micro-cracks and micro-ploughing. The relative wear resistance of this region is 75 times higher than that of HT300. The wear mechanism of NbC particulates partly gathering zone (region B) is micro-ploughing and part of micro-cutting. NbC particulates dispersing zone (region C) is slight micro-cutting and a small amount broken NbC particulates. And near-matrix zone (region D) is severe micro-cutting with broken and re-embeding NbC particulates in the matrix. And the wear resistance of the four zones is higher than that of matrix.     

XAFS Analysis for Niobium Carbide Particle Growth on Silica Support During Preparation Process

SiO₂-supported NbC catalysts were prepared by using temperature-programmed reactions (TPR). XAFS analysis confirmed that Nb₂O₅ was reduced to NbO₂ in the first TPR stage and converted into NbC in the second TPR stage. Nb particles grew only in the second TPR stage. Formation of highly dispersed NbC particles on SiO₂ surfaces was achieved.     

Effect of various morphology of in situ generated NbC particles on the wear resistance of Fe-based cladding

To improve the wear resistance of Fe-based cladding, NbC particles with different morphology are generated in situ by adding Nb, Cr₃C₂ and C with different content. The composition of samples and the morphology of NbC particles generated in situ are revealed by XRD, SEM and EDS. The wear resistance is studied by a reciprocating friction and wear testing machine. The wear mode and the wear mechanism of each sample are investigated. The results show that although NbC particles are generated in situ in samples with different Nb, Cr₃C₂ and C content, the morphology of NbC particles is varied. The wear resistance of samples containing cross-shaped NbC particles is more outstanding than that of samples containing only rectangular NbC particles. In addition, changing Nb, Cr₃C₂ and C content does not result in a change in wear mode, but leads to the formation of continuous lattice structure of Cr_0.19Fe_0.7Ni_0.11 and Cr₂₃C₆ compounds at grain boundaries and a change in the wear resistance. When the additions of Nb, Cr₃C₂ and C are 11.2 wt%, 8.6 wt% and 0.2 wt% respectively, the coefficient of friction of the sample is the lowest, and the wear resistance is the most outstanding.     

Effect of WC or NbC addition on lattice parameter of surrounding structure in Ti(C0.7N0.3)–Ni cermets investigated by TEM/CBED

The changes in the lattice parameters of the solid solutions in the Ti(C_0.7N_0.3)–WC–Ni and Ti(C_0.7N_0.3)–NbC–Ni systems were first shown quantitatively by the CBED (Convergent Beam Electron Diffraction) technique together with TEM (Transmission Electron Microscopy) microstructure characterization. The extent of the changes in the lattice parameters between core and rim differs in the case of WC and NbC additions. No change in the lattice parameters is observed in the Ti(C_0.7N_0.3)–WC–Ni cermets, in contrast to the Ti(C,N)–NbC–Ni cermets where significant changes in the lattice parameters are observed. The difference in the parameters is correlated with the core/rim structure, which disappears in the Ti(C,N)–NbC–Ni cermets when a large amount of NbC is added, and is discussed based on thermodynamic arguments. Large strain in the core and rim structure, especially near the core/rim interface, is also observed from the HOLZ (High Order Laue Zone) line splitting.     

Study of CrNx and NbC interlayers for HFCVD diamond deposition onto WC–Co substrates

Chromium nitride (CrN_x) and niobium carbide (NbC) films were deposited by magnetron sputtering on Co-cemented tungsten carbide (WC–Co) substrates and diamond deposition was performed by using hot-filament chemical vapor deposition (HFCVD) technique. The CrN_x and NbC interlayers have been deposited at different substrate temperatures (T_S = 400, 550 and 700 °C). The stability of these interlayers for diamond deposition has been studied by a heat treatment in H₂ atmosphere for 60 h at a temperature of 765 °C in the HFCVD reactor. X-ray diffraction (XRD), scanning electron microscopy (SEM) and glow discharge optical emission spectroscopy (GDOES) confirmed that due to this heat treatment the CrN_x films transformed into porous films composed of CrN_x, Cr₃C₂, Cr₇C₃ and Co phases, accompanied by a dramatic loss of nitrogen which is replaced by carbon. It was observed that higher nitrogen contents in the CrN_x films reduce the Co diffusion through the CrN_x layer. For NbC films, deposited by non-reactive magnetron sputtering from an NbC compound target, the heat treatment in the HFCVD reactor revealed that the films are absolutely stable during the heat treatment with some relaxation of residual stresses up to a factor of about 3. Furthermore it was found that Co diffuses through the NbC films with a T_S-dependant accumulation on the NbC film surface. By HFCVD it was possible to deposit adherent diamond coatings on the CrN_x and NbC interlayers. However, a reasonable adhesion of diamond on NbC was only obtained after different pre-treatments of the WC–Co substrates. The adhesion seems to be mainly governed by the topography of the WC–Co substrates.     

Superconductivity in 4-Angstrom carbon nanotubes--a short review

We give an up-to-date review of the superconducting phenomena in 4-Angstrom carbon nanotubes embedded in aligned linear pores of the AlPO(4)-5 (AFI) zeolite, first discovered in 2001 as a fluctuation Meissner effect. With the introduction of a new approach to sample synthesis around 2007, new data confirming the superconductivity have been obtained. These comprise electrical, specific heat, and magnetic measurements which together yield a consistent yet complex physical picture of the superconducting state, largely owing to the one-dimensional (1D) nature of the 4-Angstrom carbon nanotubes. For the electrical transport characteristics, two types of superconducting resistive behaviors were reproducibly observed in different samples. The first type is the quasi 1D fluctuation superconductivity that exhibits a smooth resistance drop with decreasing temperature, initiating at 15 K. At low temperatures the differential resistance also shows a smooth increase with increasing bias current (voltage). Both are unaffected by an applied magnetic field up to 11 Tesla. These manifestations are shown to be consistent with those of a quasi 1D superconductor with thermally activated phase slips as predicted by the Langer-Ambegaokar-McCumber-Halperin (LAMH) theory. The second type is the quasi 1D to 3D superconducting crossover transition, which was observed to initiate at 15 K with a slow resistance decrease switching to a sharp order of magnitude drop at ～7.5 K. The latter exhibits anisotropic magnetic field dependence and is attributed to a Berezinskii-Kosterlitz-Thouless (BKT)-like transition that establishes quasi-long-range order in the plane transverse to the c-axis of the aligned nanotubes, thereby mediating a 1D to 3D crossover. The electrical data are complemented by magnetic and thermal specific heat bulk measurements. By using both the SQUID VSM and the magnetic torque technique, the onset of diamagnetism was observed to occur at ～15 K, with a rapid increase of the diamagnetic moment below ～7 K. The zero-field-cooled and field-cooled branches deviated from each other below 7 K, indicating the establishment of a 3D Meissner state with macroscopic phase coherence. The superconductivity is further supported by the specific heat measurements, which show an anomaly with onset at 15 K and a peak at 11-12 K. In the 3D superconducting state, the nanotube arrays constitute a type-II anisotropic superconductor with H(c1)≈ 60 to 150 Oe, coherence length ξ≈ 5 to 15 nm, London penetration length λ≈ 1.5 μm, and Ginzburg-Landau κ≈ 100. We give a physical interpretation to the observed phenomena and note the challenges and prospects ahead.     

Ti(C,N)-based cermets with strengthened interfaces: Roles of secondary cubic carbides

Core-rim structure, concerning its interface structure and internal stress, plays an important role in mechanical performance of Ti(C,N)-based cermets. Four types of cermets containing equimolar TaC, VC, ZrC, and NbC were fabricated, in order to reveal the synergetic relationship between core-rim structure and mechanical properties. VC and ZrC effectively inhibited the grain growth, while TaC and NbC favored for coarser core-rim grains. A thin distortion layer at the rim-binder interface was confirmed, formed during the solidifying stage. Compared to the TaC, ZrC and NbC, VC shrunk the lattice parameters of rims and effectively decreased the lattice misfit of the distortion zone from around 4.0%-0.8%. Cermets containing VC showed satisfactory bending strength of 2099 MPa and toughness of 10. 3 MPa m^1/2, owing to the strengthened rim-binder interface. TaC and NbC exhibited the similar roles in cermets with comparable mechanical properties, while addition of ZrC increased the lattice misfit of core-rim structure, resulting in poor mechanical performance. Decreased lattice misfit of rim-binder interface changed the fracture mode of cermets from intergranular to transgranular fracture, with in-situ formed dimples.     

MA-SHS of NbC and NbB2 in air from the Nb/B/C powder mixtures

When the Nb/B/C powder mixture with different molar ratios was mechanically activated by grinding for 60–150 min in a planetary ball mill and exposed to air, it self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) of NbC and NbB₂ was induced. This mechanical activation assisted SHS (MA-SHS in air process) depended strongly on the particle size of Nb metal, the mixing ratio of Nb/B/C and the weight ratio of sample to balls. The product of NbC and NbB₂ with fine, homogeneous microstructure is expected to be a promising candidate as precursor of NbC–NbB₂ composites.     

Microstructure of Ti(CN)–WC–NbC–Ni Cermets

An investigation of the microstructural evolution and dissolution phenomena in a Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system is reported. In Ti(C_0.7N_0.3)– y NbC–20Ni systems, a phase separation occurs between the Ti(CN) core and the (Ti,Nb)(CN) rim phases when the system contains >15 wt% NbC. This phase separation results from the increased misfit between the cores and the solid-solution rim phases with the addition of NbC. Based on data obtained from a previous study and compositional analyses of the rim structure of the Ti(C_0.7N_0.3)– y NbC–20Ni system, the average dissolution rates of WC and NbC appear to be approximately the same with respect to that of Ti(CN), under given sintering conditions (1510°C for 1 h). In addition, compositional changes in the rim structure of the Ti(C_0.7N_0.3)– x WC– y NbC–20Ni system are compared with those for a Ti(C_0.7N_0.3)– x WC–20Ni system to explain the effect of NbC on WC dissolution in the Ti(C_0.7N_0.3)–WC–NbC–Ni system. The presence of NbC in the Ti(C_0.7N_0.3) –x WC–20Ni system is found to suppress the dissolution of WC.     

Microstructure and properties of niobium carbide composite coatings prepared by plasma spraying

Niobium carbide composite coatings were prepared on titanium alloy surface by plasma spraying NbC–Al₂O₃, Nb–SiC and Nb–SiC–Al composite powders, respectively. The phase composition, microstructure and formation mechanism of the three composite coatings were analyzed and their microhardness, toughness and scratch resistance were compared. The phases of the NbC–Al₂O₃ system did not change during the plasma spraying process, and new phases (Nb₂C, NbC and Nb₃Si) were formed in the Nb–SiC and Nb–SiC–Al systems. TEM results of the Nb–SiC composite coating indicate that the new phases nanocrystalline Nb₂C, submicron NbC and nanocrystalline Nb₃Si were formed during the plasma spraying process. Compared with the NbC–Al₂O₃ composite coating, the microstructure of the Nb–SiC and the Nb–SiC–Al composite coatings were uniform, and the porosity were relatively low, and the hardness was higher. The Nb–SiC–Al composite coating was denser than the Nb–SiC composite coating, the lamellar structure was obvious and the number of pores in the coating was the least, which is attributed to the better molten state of the composite powder by the addition of the Al to the Nb–SiC system. The Nb–SiC–Al composite coating had better toughness and scratch resistance.     

Nano-structured carbon obtained by chlorination of NbC

New carbon materials have been synthesized by chlorinating niobium carbide at different temperatures. During the reaction, the volatile niobium chloride, mainly NbCl₅, is formed and eliminated leaving the carbon in the shape of spherical particles. TEM examination of the remaining particles revealed that at 400 °C and 700 °C they are composed of a NbC core with an amorphous carbon shell outside. The NbC cores are very small (∼20 nm) at 700 °C and they are not observed at 900 °C. Carbon particles without the NbC core present a more ordered structure composed of concentric wavy graphene layers. This structure seems intermediate between carbon onions and carbon blacks. The 900 °C sample presents a very high BET surface area (1292 m² g⁻¹) and the lower temperature samples exhibit a hysteresis phenomenon that can be attributed to the existence of large pores in the interspace between the NbC core and the carbon shell.     

Effect of synthesis parameters on structural and thermal properties of NbC/C nano composite synthesized via in-situ carburization reduction route at low temperature

Core-shell NbC/C nano composite has been synthesized by reacting niobium pentaoxide (Nb₂O₅) with metallic magnesium and acetone (C₃H₆O). The ensued powder samples are characterized by X-ray diffraction (XRD), differential scanning calorimetry/thermal gravimetric analysis (DSC/TGA), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and Brunauer-Emmett-Teller (BET) analysis. The synthesis temperature, holding time and heating rate has been optimized to facilitate the in-situ reduction-carburization of Nb₂O₅. The broadening of XRD diffraction peaks corresponding to NbC is analyzed using Williamson-Hall (W-H) analysis by calculating various parameters such as strain, stress, strain energy density for all the samples. The mean particle size of nano NbC is estimated by Scherrer's criterion which is highly correlated with W-H analysis in less stressed system. The electron microscopy analysis showed that particles have a partial faceted morphology along with carbon layer and having wide particle size distribution. BET analysis depicted large surface area having a combination of micropores and mesopores. All the results of various characterizations have been used to predict the mechanism of formation of core-shell NbC/C nano composite.