Distribution Of Niobium Research Articles

The origin of the negative niobium tantalum anomaly in subduction zone magmas

In order to determine the distribution of niobium and tantalum between clinopyroxenes and aqueous fluids, we measured in two separate sets of experiments niobium and tantalum solubility in clinopyroxenes and in aqueous fluids, respectively. The solubility of niobium in clinopyroxenes was experimentally investigated in the system CaMgSi2O6–NaAlSi2O6–Nb2O5–H2O at 1.5 GPa and 700–1100 °C using piston-cylinder experiments. In the presence of excess Nb2O5, CaNb2O6 coexists with a clinopyroxene. The solubility of niobium in the pyroxene increases drastically with the aluminum content. While the solubility of niobium is in the order of 100–300 ppm by weight for Al-poor clinopyroxenes, it reaches 4 wt.% for clinopyroxenes containing 10 wt.% Al2O3. Microprobe analyses suggest that Nb is incorporated in the clinopyroxene as NaNbAl2O6 component. The solubility of CaNb2O6 in aqueous fluid was determined by the direct visual observation of the dissolution of CaNb2O6 crystals in an aqueous fluid using an externally-heated diamond anvil cell. At 1.5 to 1.7 GPa and 800–1000 °C, an aqueous fluid saturated with diopside dissolves only 20–100 ppm by weight of CaNb2O6, i.e. niobium solubility in the fluid is orders of magnitude below the solubility in aluminous clinopyroxenes. Experiments on the solubility of CaTa2O6 in clinopyroxenes and in aqueous fluid suggest that Ta behaves generally similar to Nb, but with the notable exception that the solubility of CaTa2O6 in aqueous fluids and aluminous pyroxenes is approximately a factor of five lower than the solubility of the corresponding niobium compound. Our results imply that (1) the fluid/clinopyroxene partition coefficient for Nb and Ta is between 0.1 and 0.001 for aqueous fluids containing little dissolved silicates and for clinopyroxenes of a composition realistic for the subducted slab or the subarc mantle. Only for very Al-poor clinopyroxenes may the partition coefficient approach unity. (2) The depletion of Nb and Ta in subduction zone magmas and fluids is related to the intrinsically low solubility of Nb and Ta in water-rich and solute-poor fluids in the shallow parts of a subduction zone. (3) The presence of rutile in the subducted slab is not a necessary requirement for the development of the negative Nb and Ta anomaly. (4) Fluid transport of niobium and tantalum will affect the Nb/Ta ratio in the subarc mantle only under exceptional circumstances. (5) Previous reports of high fluid/clinopyroxene partition coefficients for Nb and Ta are probably related to the Al-free or Al-poor composition of the systems studied.

Effect of niobium on the oxidation behavior of TiAl

Two TiO2layers formed in TiAl oxidation for 50 h at 900 °C were studied using scanning transmission electron microscopy. The main efforts were placed on the investigation of the distribution of niobium. It was found that Nb enriched in TiO2grains of mixture layer but did not exist in the outer TiO2layer. High-resolution electron microscopy (HREM) Z-contrast image revealed that Nb substitute for Ti site leading to Nb enrichment in TiO2grains of the mixture layer. The formation mechanism of the two TiO2layers and the potential effect of Nb doping in the mixture layer were also discussed.

Quantification of Large Scale Micro-X-Ray Fluorescence Elemental Images

Niobium is commonly alloyed with uranium to prevent surface oxidation, and determining how the niobium concentration is distributed throughout a sample is useful in explaining observed material properties. The niobium concentration distribution was determined across the surface of depleted uranium samples using micro-X-ray fluorescence (MXRF). To date, MXRF has been employed primarily as a qualitative tool for determining relative differences in elemental concentrations across a sample surface. Here, a process was developed to convert qualitative MXRF niobium distribution images from depleted uranium samples into images displaying concentration values. Thus, MXRF was utilized to determine elemental concentrations across a surface in a manner similar to that of the established method of electron microprobe X-ray analysis (EMPA). However, MXRF can provide such information from relatively large sample areas many cm2 in size that are too large to examine by the higher spatial resolution technique of EMPA. Although the sample surfaces were polished to the same degree as the standards, little or no sample preparation should be necessary for sample systems where a high energy analyte XRF line can be used for imaging.

Transport of niobium(V) through a TBP–Alamine 336 supported liquid membrane from chloride solutions

The transport of niobium(V) in chloride medium through a supported liquid membrane (SLM), using Alamine 336 and tributyl phosphate (TBP) as carriers, has been investigated. The work has been undertaken by first determining the liquid–liquid extraction distribution of niobium(V) between aqueous solution and different organic solutions. Optimum conditions for both extraction and stripping were established. The mixture of Alamine 336 and TBP provided the best extraction in high chloride medium, while low HCl concentration stripped the ion from the loaded organic phase. Transport experiments were carried out with a permeation cell having a continuous flow system and constant agitation. The study of the liquid membrane included the chemical conditions, i.e. carrier concentration and hydrochloric acid concentration in the feed solution. Maximum niobium recovery at 10% of Alamine 336 in TBP was obtained at HCl concentration between 6 and 7 mol/dm 3. The mechanism postulated is referred to a synergistic extraction of Nb(V) by means of a co-transport process. Nb(V) can be concentrated in the stripping solution and the SLM showed a good performance for Nb–Ta separation.

Segregation of niobium solute in nickel toward grain boundaries and free surfaces

The spatial redistribution of niobium atoms near the (100) and (111) free surfaces and selected grain boundaries (GBs) of pure nickel has been considered in the low niobium concentration limit. There is one key difference between the situation for placing a niobium atom at a free surface and at a GB. At the free surface, an energetic compromise is required between having space for the large niobium atom and being able to place that atom at a position of high electron density. For a GB, no such compromise is required. An extremely interesting feature is the presence of a region around the third layer of (111) and the fourth layer of (100) free surfaces where the substitutional internal energy reverses its sign. The authors’ simulations show a significant depletion in concentration of niobium immediately at free surfaces. However, under the first two or three layers of pure nickel, there exists a niobium-enriched region with a strongly temperature-dependent concentration. This predicted nonmonotonic distribution of niobium in the surface region may be important for many applications and calls for experimental confirmation. In contrast, at the grain boundaries, the concentration of niobium, which is pertinent to GB oxidation embrittlement, is predicted to be much higher than in the bulk. It monotonically decreases with the distance from the GB until reaching the bulk value. The calculation of the free energy uses atomistic potentials based on ab initio quantum mechanical calculations, includes lattice relaxation around niobium atoms by using molecular dynamics (with 1440 or 2880 atoms in the modeling cell), and includes vibrational entropy phenomenologically within the local harmonic approximation. The entire approach is ab initio based and does not require any empirical information.

Distribution of niobium in an Fe–Pt–Nb magnet

The role of Nb addition to improve the hard magnetic properties in FePt magnet was investigated by characterizing microstructures of both Fe–39.5 at.%Pt binary and Fe–39.5 at.%Pt–0.75 at.%Nb ternary alloys using transmission electron microscopy and atom probe field ion microscopy. In the as-quenched ternary alloy, the FePt ordered domains with a tweed contrast were observed as well as in the binary alloy. However, the ordered domain size of the ternary alloy was slightly smaller than that of the binary alloy and comparable to the thickness of magnetic domain wall. Although the coercivity of the as-quenched ternary alloy was small, subsequent annealing produced high coercivity. Nb was rarely solved in the FePt ordered phase and was concentrated in spherical particles of sub-microns in size. These particles may work as a pinning site against magnetic domain wall motion. Another possibility is that the change in composition ratio between Fe and Pt may be responsible to the magnetic property change.

Strain localization, slip-band formation and twinning associated with deformation of a Ti─24 at.% Al─11 at.% Nb alloy

Abstract In order to understand the behaviour of Ti–24 at.% A1–11 at.% Nb alloys during high-temperature deformation it appears relevant to have some information on the microstructures produced by high-temperature anneals of heavily deformed materials. In this work these microstructures have been studied after heat treatments between 700 and 1000°C. The influence on niobium distribution within the different phases present and the variation in the degree of order caused by this distribution have been studied. The deformation mechanisms occurring, leading to strain hardening at the early stages of deformation, have been compared according to the different phases obtained. Slip-band formation and twinning have been observed to occur within the D019 phase (α2) according to the different degrees of order measured. The B2 grains can effectively be softened by slip bands traversing them, and the stress concentrations produced at boundaries with the α2-phase can be responsible for initiating new deformation modes...

Distribution of niobium or tantalum between fluorine-containing slag and iron in blast furnace smelting

The distribution of Nb or Ta between iron and slag in blast furnace smelting is closely related to Si-Nb or Si-Ta equilibrium. Most of the Nb or Ta enters the iron when the equilibrium distribution of Nb or Ta is reached. The equilibrium distribution ratios of Nb and Ta are substantially independent of both the basicity and the CaF2 content of the slag but are markedly affected by the reaction temperature. The Ta contents in slag and iron were measured by the radioactive isotope182Ta.

Deformation Mechanisms Associated with Annealed Structures in a Ti-24Al-Nb Alloy

ABSTRACTCrystallographic and chemical analysis of the microstructures obtained by annealing a heavily deformed Ti-24Al-11 Nb alloy have been performed. The influence of the niobium distribution within the different phases present and the variation of degree of long range order caused by this distribution have been related to the deformation mechanisms leading to strain hardening at the early stages of deformation. In particular, slip band formation and twinning have been observed to occur within the (α2 phase according to the different degree of order measured.

Distribution of niobium and tantalum on Dowex-1 and polyurethane foam in HF−H2SO4 and HF−HCl medium

The sorption of niobium and tantalum on Dowex-1 and open-cell polyurethane foam polyether-type in HF−H2SO4 and HF−HCl medium has been investigated. The mechanism of the sorption as well as the composition of adsorbable complexes is discussed. A selective adsorption of tantalum was achieved from solutions, in which the concentration range of HF was 0.05–2M, of H2SO4≤5M and of HCl≤4M. The most interesting separation possibilities have been tested under dynamic conditions.

Distribution of Nb, P, Mn between liquid iron and MgOsat.-FetO-SiO2-NbOx-MnO slags.

Experiments have been conducted to study the niobium, phosphorus and manganese distribution between MgOsat.-FetO-MnO-SiO2 NbOx slaps and liquid iron at 1600°C. It is shown that the niobium distribution ratio increases with the iron oxide content in slag, whereas the phosphorus and manganese distribution ratios remain almost constant. Also, the thermodynamic criteria governing the selective oxidation of niobium, phosphorus and manganese in the extractive process of niobium from hot metal have been discussed.

Liquid-liquid distribution of niobium in malonate solutions with liquid anien exchangers

Abstract Niobium was quantitatively extracted at pri 3-4.5 with 0.08 M Amoerlite LA-2 in xylene from 0.02 M malonic acid. It was stripped from the organic phase with 0.25 M potassium carbonate solution and was determined spectrophotometrically with TAR at 540 nm. Apart from Amberlite LA-2, Primene JM-T quantitatively extracted niobium and xylene were the only satisfactory dilutents. Wiobium was separated from various elements which formed weak complexes with malonic acid by stripping these elements before extraction of niobium. By the process of selective stripping it was separated from elements forming strong complexes. The method was used for the determination of niobium from stainless steel.

Niobium(V) extraction by butyl lauryl phosphoric acid-dodecane from nitric acid

Extraction of niobium by tributyl phosphate (TBP) in the Purex process is inefficient; and in spite of this, niobium is found in solvent streams. Alternate mechanisms for niobium transfer into the solvent were sought. Butyl lauryl phosphoric acid (HBLP), a model compound of secondary degradation products of the solvent used in the process, was found to enhance niobium extraction above a threshold concentration of 5 × 10 −4 M. A study of the niobium distribution coefficient dependences in the system HNO 3-HBLP-dodecane on the HBLP, hydrogen ion and nitrate ion concentration suggests an extraction mechanism involving neutralization of an hydroxyl group bound to the niobium and no incorporation of nitrate into the extracted species. These results are supported by chemical analysis of the extracted species which show an empirical formula corresponding to Nb(OH) 4BLP. Silica and zirconium monobutyl phosphate, under certain conditions, disperse adsorbed niobium in the solvent phase in the form of emulsions or suspensions. Centrifugation of these give no net enhancement of niobium transfer.

On the solidification microstructure of copper-rich niobium alloys

The microstructure of copper-rich niobium alloys has been studied as a function of cooling rate. The main alloy composition investigated was Cu-7 wt% Nb. The cooling rate was varied from 40° C sec−1 to approximately 7×104° C sec−1. The microstructure as observed in the scanning electron microscope changes from well-dispersed niobium spheres for fast-cooling to an heterogeneous distribution of niobium “flowers” for slow-cooling.

Distribution of niobium and tantalum in ultrabasic inclusions in kimberlites and basalts

Distribution of niobium and tantalum in ultrabasic inclusions in kimberlites and basalts

Temperature-stable Barium Titanate Ceramics Containing Niobium Pentoxide

Niobium is known to lower the Curie point and broaden the permittivity peak of barium titanate. However, the distribution of niobium in sintered barium titanate is often not homogeneous, being influenced by such preparation variables as large cation/small cation stoichiometry and firing conditions. In addition to these factors, it was found that, at least up to about 5 mol% Nb, the distribution of niobium in the barium titanate grains could be regulated by small amounts (<1 mol%) of oxides of Co, Mg, Ni or Mn. These oxides appear to form niobates that produce high permittivity solid solutions in the “skins” of the barium titanate grains and permit dielectrics to be obtained with permittivity (K≃¯2000), stable to within ±10% over the temperature range −55℃ to 125℃.

Distribution of niobium in stream sediments related to heavy-mineral sand dunes, near pemberton, Western Australia

Abstract After the completion of a programme of regional stream sediment sampling to observe base metal distributions in drainage systems in the southwest of Western Australia, it was noted that above background Pb and Sn contents in minus 80-mesh fractions of samples from the Warren River area, were related to unusually high Nb contents in the same samples. Further investigations revealed that the anomalous samples were from streams draining eroded sand dunes containing heavy mineral sands, related to a former shore line. Assays of heavy mineral differentiates from these dune sands gave values up to 0.12% Nb in samples of altered ilmenite.

Distribution of niobium in intrusive rocks in the northern part of the Dzhungaro-Balkhash folded system

Distribution of niobium in intrusive rocks in the northern part of the Dzhungaro-Balkhash folded system

Use of radioactive isotopes of arsenic and niobium for investigations on metallurgical reactions

Methods for investigation of arsenic and niobium distribution between iron and slag by means of radioactive isotopes of these elements (As 76 and Nb 95) are studied. Special emphasis is placed on the technique used for the removal of radioactive admixtures and for the identification of radioactive isotopes. Thermodynamic data characteristic of the oxidation of arsenic and niobium dissolved in liquid iron are given.