Niobium Diboride Research Articles

Reaction spark plasma sintering of niobium diboride

Niobium diboride (NbB2) is synthesized and consolidated by the spark plasma sintering technique. Elemental reactants such as niobium (Nb) and boron (B) were subjected to two stage heat treatment, initially at 1200°C for synthesis and followed by densification at the temperatures in the range of 1700°C to 1900°C. High dense NbB2 (~97.7%ρth) is obtained at 1900°C after 15min holding period. Load application during heat treatment stage is found to improve the sinterability of the niobium diboride compacts. Hardness, elastic modulus and indentation fracture toughness of the high dense NbB2 are measured as 20.25GPa, 539GPa and 4MPam1/2 respectively.

Novel borothermal process for the synthesis of nanocrystalline oxides and borides of niobium

A new process has been developed for the synthesis of nanocrystalline niobium oxide and niobium diboride using an amorphous niobium precursor obtained via the solvothermal route. On varying the ratio of niobium precursor to boron and the reaction conditions, pure phases of nanostructured niobium oxides (Nb(2)O(5), NbO(2)), niobium diboride (NbB(2)) and core-shell nanostructures of NbB(2)@Nb(2)O(5) could be obtained at normal pressure and low temperature of 1300 °C compared to a temperature of 1650 °C normally used. The above borothermal process involves the in situ generation of B(2)O(2) to yield either oxide or diboride. The niobium oxides and borides have been characterized in detail by XRD, HRTEM and EDX studies. The core-shell structure has been investigated by XPS depth profiling, EFTEM and EELS (especially to characterize the presence of boron and the shell thickness). The niobium diboride nanorods (with high aspect ratio) show a superconducting transition with the T(c) of 6.4 K. In the core-shell of NbB(2)@Nb(2)O(5), the superconductivity of NbB(2) is masked by the niobium oxide shell and hence no superconductivity was observed. The above methodology has the benefits of realizing both oxides and borides of niobium in nanocrystalline form, in high purity and at much lower temperatures.

Superconductivity of Nb1−x Mg x B2: Impact of Stretched c-Parameter

We have performed a systematic study on the occurrence of superconductivity in Nb1−x Mg x B2 (0.0≤x≤0.40). X-ray diffraction and magnetization measurements are carried out to determine the changes in lattice parameters, superconducting transition temperature (T c ) and critical field (H c1). The substitution of Mg at Nb site results in considerable stretching of c-parameter with only a slight change in a parameter. Rietveld analysis on X-ray diffraction patterns gives a=3.11 A and c=3.26 A for pure NbB2 while a=3.10 A and c=3.32 A for Nb0.60Mg0.40B2. This increased c-parameter introduces superconductivity in niobium diboride. Magnetization measurements though indicate the absence of superconductivity in NbB2, the same shows a clear diamagnetic signal at about 10 K for Nb0.60Mg0.40B2 sample. The magnetization M(H) plots exhibit weak superconductivity like hysteresis loops. The stretching of c-parameter from around 3.26 to 3.32 i.e. by 0.06 cannot be explained solely by substitution of Nb by Mg in the lattice. It seems that some Nb deficiencies are introduced in the Nb1−x Mg x B2 as Mg is not substituted completely at the vacant Nb sites. This could be seen from XRD results, where one can clearly notice the presence of small amount of MgO in Nb1−x Mg x B2 samples.

A simple inorganic-solvent-thermal route to nanocrystalline niobium diboride

Nanocrystalline NbB2 has been prepared via a simple inorganic-solvent-thermal route by the reaction of metallic magnesium powders with niobium pentoxide and boric acid in an autoclave at 650 °C. X-ray powder diffraction pattern indicated that the product was hexagonal niobium diboride (NbB2), and the cell constant was a = 3.115 Å and c = 3.266 Å. Scanning electron microscopy image showed that it consisted of particles with an average size of about 30 nm. The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 500 °C in air. The molten salt, a mixture of sodium chloride and magnesium chloride, acting as inorganic solvent during the reaction, maybe plays an important role in the formation of nanocrystalline niobium diboride with a narrow particle size distribution.

Structural phase evolution and superconductivity in the non-stoichiometric intermetallic compound niobium diboride

The structural changes and superconductivity in the layered non-stoichiometric niobium diboride compound were systematically studied for both of niobium-deficient and boron-deficient phases by X-ray diffraction analysis and magnetic susceptibility measurements. The niobium and boron vacancies were found to have different effects on the crystal lattice parameters and superconducting properties; superconductivity only exists in niobium-deficient phases with a clear enhancement of inner-plane B–B bonding and the increase of inter-plane distance, while boron vacancies have the opposite effect of enhancing the Nb–B bonding and breaking the superconductivity. A main superconducting phase with Nb 0.83B 2 composition was obtained with T c ∼ 9.3–9.9 K, and a = 3.102(1) Å and c = 3.322(2) Å under the optimum sintering conditions. The low temperature specific heat was measured for three high-pressure synthesized superconducting samples to reveal the origin the increase of T c with the niobium deficiencies.

Preparation of niobium borides NbB and NbB 2 by self-propagating combustion synthesis

Preparation of niobium borides NbB and NbB 2 was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts in this study. Effects of the sample green density, preheating temperature and starting stoichiometry on combustion characteristics, as well as on the composition of final products were studied. Experimental evidence indicates the self-sustained reaction zone propagating along a spiral trajectory for the reactant compacts without prior heating or preheated at 100 °C. The increase of initial sample temperature to 200 and 300 °C by prior heating brings about a planar flame-front propagating in a steady mode. As the preheating temperature or sample green density increased, the combustion temperature was found to increase and the propagation rate of combustion wave was correspondingly enhanced. According to the temperature dependence of combustion wave velocity, the activation energies associated with the Nb + B and Nb + 2B reactions were determined to be 151.8 and 132.4 kJ/mol, respectively. As indicated by the XRD analysis, the final composition of burned products was essentially governed by the starting stoichiometry of reactant compacts. Synthesized products composed of a single boride phase NbB and a small amount of unreacted Nb were obtained from the reactant compacts of Nb:B = 1:1. In addition, the SHS reaction of powder compacts with an initial composition Nb:B = 1:2 yielded niobium diboride NbB 2 as the dominant phase, along with another boride phase Nb 3B 4 in a minor quantity.

Low temperature synthesis of NbB2 nanorods by a solid-state reaction route

Niobium diboride (NbB2) nanorods have been successfully synthesized by the reaction of NbCl5 with NaBH4 in the temperature range of 550–650 °C in an autoclave. X-ray diffraction (XRD) and electron diffraction show that the as-prepared products are NbB2. Transmission electron micrographs shows that the average diameter of samples are in the range of 50–60 nm and the length is up to 600 nm. The mechanism for the formation of crystalline NbB2 is proposed.

Superconductivity and crystal growth of NbB 2

The niobium diboride NbB 2 has the same crystal structure as MgB 2 (space group: P6/mmm). Since the transition temperature T c of NbB 2+ x depends on the chemical composition x or the lattice parameter, we examined a NbB 2+ x ( x = 1) nominal sample, which was prepared by melting the Nb and B chunks by a tri-arc furnace. The X-ray diffraction (XRD) studies showed that our sample was single phase and had no impurities. However, magnetization measurements with a SQUID magnetometer revealed that the superconducting transition occurred twice at different temperatures. We consider that two-step superconducting transitions correspond to a phase separation in NbB 2+ x ( x = 1). Attempts to obtain a single-phase sample by slow cooling from melt yielded the reduction of the B-rich domain in the sample.

Synthesis of ultrafine NbB 2 powder by rapid carbothermal reduction in a vertical tubular reactor

Niobium diboride (NbB 2) powder was synthesized by carbothermal reduction of Nb 2O 5B 2O 3C precursor prepared with Nb 2O 5, H 3BO 3 and cornstarch. The reaction was consecutive and Nb 2O 5 was formed while NbC was consumed. The apparent activation energy of NbB 2 formation for the Nb 2O 5B 2O 3C system was about 360 kJ mol −1, which is in good agreement with that for the NbCB 2O 3C system. These results indicate that the rate-determining step of the carbothermal reduction was the formation of NbB 2 from NbCB 2O 3C mixture. For continuous production of NbB 2 particles a vertical tubular reactor was used in which the pulverized Nb 2O 5B 2O 3C precursor particles were entrained downwards with an argon gas flow. The size and purity of the NbB 2 formed were strongly dependent on the calcination temperature of the precursor, precursor size and temperature of carbothermal reduction. The carbonaceous material in the precursor particles was quite reactive when cornstarch was calcined at 400°C rather than 700°C. The smaller the precursor size, the longer the precursor particles could stay in the reaction zone, giving a better yield of NbB 2. Sintering of NbB 2 particles occurred at 1800°C. Under optimum conditions the NbB 2 crystallites were as small as 40–50 nm and the residual carbon content was 2.65% by mass.

Chemical vapour deposition of niobium diboride (NbB2)

Niobium diboride was prepared by chemical vapour deposition from a gas mixture of NbCl5, BCl3, H2 and Ar on a quartz substrate heated to a temperature of 950–1200°C. Homogeneous films were formed from 950 to 1050°C, and pillar crystal from 1050 to 1200°C. Niobium diboride was found to react with the quartz substrate above 1200°C to form a silicide, Nb5Si3. The optimum molar compositions of the gas mixture used to grow crystals of NbB2 were 2.5–5.0 vol% of BCl3, and 2.5–5.5 vol% of NbCl5. In the presence of impurities of nickel regular hexagonal shaped pillar crystals of maximum 10 μm in a edge and 50 μm in length were obtained.