Excess Niobium Research Articles

Effects of nitride precipitation on delta phase formation in additively manufactured nickel superalloys

Additively manufactured Inconel 625 is particularly susceptible to the formation of niobium-rich δ-phase precipitates in the interdendritic regions during high temperature exposure. With specific minor alloying element combinations and nitrogen mass fractions on the order of 0.1 %, the formation of δ-phase can be suppressed through the precipitation of nitrides. For example, mass fractions of 0.39 % silicon and 0.03 % titanium led to the precipitation of Z-phase and η-nitrides, which consumed the excess niobium in the interdendritic regions and limited δ-phase formation. Even when holding the material at a temperature of 870 °C for 1000 h, the δ-phase volume fraction was approximately 2 %, which is far below the 6 % level observed in the wrought condition. When the titanium mass fraction was increased to 0.21 % and the silicon mass fraction decreased to 0.05 %, titanium-rich MN nitrides formed within the interdendritic regions instead. Since much of the Nb in these regions was not consumed by the nitrides, δ-phase formation was promoted and reached volume fractions above 10 %. The addition of a hot isostatic pressing step prior to high temperature exposure in both alloys produced a more uniform niobium distribution and minimized δ-phase formation at shorter times. At extended times, trends in the δ-phase volume fractions were similar to those observed in the as-deposited condition, with the initial alloy compositions driving differences in the distribution of excess niobium available for δ-phase formation.

Bond ionicity, lattice energy, bond energy and the microwave dielectric properties of non-stoichiometric MgZrNb2+xO8+2.5x ceramics

The intrinsic relationship between crystal structure, surface topography, P-V-L theory and dielectric properties of non-stoichiometric MgZrNb2+xO8+2.5x (−0.08 ≤ x ≤ 0.08) ceramics prepared by a normative solid-state reaction route were determined. X-ray diffraction patterns demonstrated that the appropriate addition of excess or insufficient niobium ions could not destroy the crystal phase, but change the lattice parameters and unit cell volume. Meanwhile, the formation of pure phase with monoclinic wolframite structure and P2/c space group was confirmed. The Rietveld refinement was used to obtain structure parameters and investigate the crystal structure of the specimens. SEM showed that insufficient niobium ions didn't show any impact on the dense structure, and the excess niobium ions promoted the growth of grains, but caused the appearance of undesired rough surfaces. The microwave properties as a function of the niobium ions content were found to be closely related to bond ionicity, lattice energy and bond energy calculated according to the P-V-L theory. As a result, when x was taken as −0.04 for the MgZrNb2+xO8+2.5x ceramics sintered at 1320 °C, the best dielectric properties, εr = 25.62, Q × f = 80828.1 GHz, τf = −44.64 ppm/°C, were successfully obtained.

High Content of Niobium on the Properties of Barium Titanate Ceramics

This study reported the effect of adding excess niobium in higher amount (up to 1.5 mol% Nb2O5) than that reported in any other literature. Increasing in mol% Nb2O5led to a formation of secondary phase. The average grain size decreased with higher Nb2O5addition and it also inhibited grain growth and controlled grain uniformity in the microstructure. The presence of needle-like secondary phase when added >0.3mol% Nb2O5aided grain growth inhibition resulting in finer matrix grain. Addition of Nb2O5more than 0.9 mol% in regardless of sintering time showed two dielectric peaks which could be related to secondary phase in the microstructure.

Threshold concentrations in zinc-doped lithium niobate crystals and their structural conditionality

On the basis of precise X-ray diffraction study of lithium niobate single crystals of congruent composition and four zinc-doped (at 2.8, 5.2, 7.6, and 8.2 mol %) crystals, structural conditionality of the threshold concentrations of the dopant has been established. At these concentrations, the mechanism of zinc incorporation into crystal changes. As the zinc concentration increases, this element first substitutes excess niobium, localized in lithium positions, with a simultaneous decrease in the number of vacancies in these positions. Then zinc substitutes lithium with formation of new lithium vacancies. When a certain limit on the number of vacancies is reached, zinc begins to substitute niobium in its main positions. This process is naturally accompanied by a decrease in the number of vacancies to their complete disappearance and formation of a self-compensating crystal. The character of the dependence of the crystal physical properties on the dopant concentration changes specifically when the impurity concentration passes through the threshold values.

Smooth titania nanotubes: Self-organization and stabilization of anatase phase

We present transmission electron microscopy and Raman scattering measurements showing that niobium inhibits the processes of nucleation and growth of anatase crystallites in the initial amorphous titania nanotubes and thus shifts the temperature of the complete amorphous-to-anatase phase transition to higher values up to 550°C. Niobium dopants stabilize the anatase phase in titania nanotubes up to 650°C. The size of anatase crystallites can reach 30–50nm. Excess niobium atoms which are pulled off from the volume of anatase crystallite form polymeric or monomeric Ti–O–NbO groups at the interface area. Slight shift and broadening of Eg (144cm−1), A1g (515cm−1) and Eg (630cm−1) modes in Raman spectra can be explained by niobium insertion into the anatase structure.

Thermally induced surface self-segregation of the chemical composition in sodium niobate: Auger spectroscopy and secondary electron emission study

The thermally induced self-segregation (TISS) of the chemical composition in sodium niobate single crystals has been investigated by Auger spectrometry and secondary-electron emissiometry. The high efficiency of these methods for studying fine features of TISS processes has been confirmed. Comparative analysis of TISS in sodium and potassium niobates, having perovskite structure, is performed within the liquid-phase TISS model. The difference for these compounds is that excess sodium is accumulated on the surface of sodium niobate, whereas excess niobium is accumulated on the surface of potassium niobate. This distinction is explained by the higher potassium vapor pressure in comparison with sodium.

Structural and magnetic ordering in the V xNb 1+yS 2 system

The influence of composition on the structural ordering and magnetism in the V x Nb 1+ y S 2 system has been investigated by X-ray diffraction and magnetic measurements. Stoichiometric V 1/3NbS 2 did not exhibit the structural ordering of vanadium between the NbS 2 layers. In the ordered structure, the vanadium composition deviated from the ideal value of x = 1 3 to both higher and lower values, while the niobium composition was in the range of 0.05⩽ y⩽0.18. Excess niobium, y>0, is thought to play an essential role in the structural ordering in this system. For samples with excess niobium and ordered structures, a magnetic transition was observed at 20–50 K, depending on the composition. The spontaneous magnetization of 3–5×10 −3 μ B/V atom is thought to be intrinsic to this system. The magnetization curves consisted of a constant and a proportional parts of the magnetic field, which correspond to the spontaneous magnetization and high-field susceptibility, respectively. The magnetization curves and the temperature dependencies of the high-field susceptibility were quite similar to those of the canted antiferromagnetic NiS 2. A correlation between the structural and magnetic ordering is suggested.

Study of the Surface Composition on Alumina-NbC Composites

The aim of this work is to analyse an alumina-NbC based composite ceramic made from a polymeric precursor (polysiloxane), alumina and metallic niobium. The materials have a fixed concentration ,of 60 wt% of polymer and 40% of a mixture of niobium and alumina. These materials are mixed and sintered at 1450°C for 6 h. Alumina based composites have been proposed as excellent materials for use as cutting tools, so knowledge of the superficial composition is extremely important because it is directly related to the hardness and abrasion resistance. Analysis of the surface composition was carried out by electron spectroscopy. It should be emphasized that there may be a meaningful difference between the surface and the interior composition due to a eventual processes such as element segregation to the surface and/or diffusion of elements from the surface to the sample bulk. The analysis was performed by XPS and Auger for three niobium concentrations 10, 20 and 40 wt%, and the results show the appearance of niobium on the surface only at the Nb composition of 10 wt%; this appears to he due to a process of niobium atom migration to the interior of the sample or one involving niobium bonds. For the 10 wt% sample after sintering, the formation of NbC on the surface and the presence of niobium Auger peaks were observed. However, for concentrations larger than 10 wt%, oxides and sub-oxides (NbO. NbO 2 , etc.) were formed which may result in the absence of niobium peaks in the spectra. The analysis of the ratio of niobium to carbon atoms at the surface shows a value of 0.1 which reveals that the quantity of niobium and carbon is not sufficient for ideal formation of niobium carbide (NbC). Under these conditions it is verified that there is only a slight formation of niobium carbide on the surface which is harmful to the hardness of the material. The excess niobium tends to diffuse towards the interior of the sample and react with the oxygen forming sub-oxides.

Piezoelectric properties of niobium-doped 18PMN-41PZ-41PT ceramics

The effects of various amounts of excess niobium ions added to the 18PMN-41PZ-41PT three-component ceramics system on the crystal structure, microstructure, polarization versus electric field, strain versus electric field and piezoelectric properties, have been studied. It was found that addition of niobium ions to 18PMN-41PZ-41PT induced a change in the crystal structure from rhombohedral to tetragonal phase. The excessive niobium ions also formed a micro-second phase which existed in the grain boundary, which could refine and uniform the grain. The refined and uniformed microstructure was found to increase the remanent polarization, saturation strain, planar coupling coefficient, Kp, and mechanical quality factor, the best value of Kp being ⩽ 69%.

Single Source Evaporation of a Niobium Based Alloy Containing Volatile Constituents

By adding excess niobium to the molten metal pool of a rod-fed evaporation source in order to approximate a steady state condition, reproducible transfer of source material containing niobium, titanium, chromium, and aluminum has been achieved. Single source evaporative coating of this material has not been considered practical prior to this work because of the factor of 106 difference in vapor pressure between the most volatile element and the least volatile element. As produced, the coatings had a banded structure. A heat treatment homogenized the coatings. A model is suggested to explain the banded structure.

93Nb NMR Linewidths in Nonstoichiometric Lithium Niobate

The angle variation of the 93Nb NMR linewidths is obtained for an off-stoichiometry lithium niobate crystal. It is shown that the widths can be calculated on the basis of a model in which both the magnitude and direction of the electric field gradient are treated as random quantities. A second type of niobium, with a near zero quadrupole coupling constant, is shown to exist in off-stoichiometric crystals. This amounts to roughly 6% of the total niobiums. This new resonance is undoubtedly associated with the mechanism of charge compensation and suggests that the excess niobium substitutes for lithium.

High temperature conproportionation of niobium pentahalide and niobium metal; A convenient route to hydrated cluster halides Nb 6Cl 14·8H 2O and Nb 6Br 14·8H 2O

We report the preparation of Nb 6Cl 14·8H 2O via Na 4Nb 6Cl 18, which is synthesized in a one-step reduction from NaCl, NbCl 5, and excess niobium metal. Nb 6Br 14·8H 2O is prepared via K 4Nb 6Br 18 from KBr, NbBr 5, and Nb. The reactions are smooth and straightforward and supply the hydrated metal cluster halides in almost quantitative yields.

Some effects of melt stoichiometry on the optical properties of barium sodium niobate

Single crystals of the compound barium sodium niobate (Ba 2NaNb 5O 15), currently of interest for its non-linear optical properties, undergo an apparent symmetry alteration when grown from melts rich in niobium pentoxide. The structure although still of the “tungsten-bronze” type changes from the usual orthorhombic biaxial form to a tetragonal uniaxial one. The symmetry change occurs rather abruptly when crystals are grown from melts containing 52.8 mole% niobium pentoxide and persists up to 58 mole%, the limits of the present study. Unlike the orthorhombic form which appears to have a consistent Curie temperature of about 560 °C, the ferroelectric Curie temperature of the uniaxial form shows a nearly linear relationship to the niobium pentoxide content in the melt. Curie temperatures ranging from 560 °C to 420 °C were observed and correspond to a Curie temperature depression of about 20 °C for each one mole% excess niobium pentoxide in the melt. Associated with the depression of the Curie temperature is a depression in the second harmonic phase matching temperature. Crystals grown at a nominal composition of 53 mole% niobium pentoxide appear to phase match for second harmonic generation at approximately -80 °C as compared to ∽ + 90 °C for the usual stoichiometric barium sodium niobate.