Estimation of niobium

Estimation of Niobium and Tantalum when Present Together. 

Cupferron can be employed for the estimation of niobium and tantalum together, but does not differentiate between them any titanium present is also simultaneously predpitated.8... 

Niobium Pentoxide, Niobic Anhydride, Nb208, is probably the commonest compound of niobium. It is obtained in the treatment of niobium-bearing minerals (see p. 124), and is the starting material in the preparation of other niobium compounds. It is precipitated in the hydrated state by the hydrolysis of nearly all pentavalent niobium salts, and is formed in the gravimetric estimation of niobium (see p. 130). 

In the aluminothermic reduction of niobium oxides, the products must reach a temperature of at least about 2470 °C, and hence the heat required to raise niobium metal and alumina from room temperature to this temperature must be estimated. Using the values of the heat capacities and the heats of fusion for niobium and alumina, the following figures can be obtained ... 

The abundance of niobium in the earth s crust is estimated to be in the range 20 mg/kg and its average concentration in sea water is 0.01 mg/L. The metal also is found in the solar system including the lunar surface. Radionucleides niobium-94 and -95 occur in the fission products of uranium-235. 

The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K4Ta405F14 (see p. 132), is thrown down when a solution of potassium tantalum fluoride, KaTaF7, is boiled.7 Powell and Schoeller 8 find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly add solution) for the detection and estimation of traces of tantalum in niobium compounds. 

Niobium. Wade and Wood (2001) have suggested that niobium, potentially the most side-rophile of the RLEs, is depleted in the PM by some extraction into the core. The depletion of niobium estimated from Nb/Ta would be 15 15% (Kamber and Collerson, 2000). As an RLE, niobium would have a mantle abundance of 690 ppb. Considering that 15% is in the core reduces this number to 588 ppm, which is hsted in Table 2. 

Rehnements of the Taylor and McLennan (1985) model are provided by McLennan and Taylor (1996) and McLennan (2001b). The latter is a modihcation of several trace-element abundances in the upper crust and as such, should not affect their compositional model for the bulk crust, which does not rely on their upper crustal composition. Nevertheless, McLennan (2001b) does provide modihed bulk-crust estimates for niobium, rubidium, caesium, and tantalum (and these are dealt with in the footnotes of Table 9). McLennan and Taylor (1996) revisited the heat-flow constraints on the proportions of mahc and felsic rocks in the Archean crust and revised the proportion of Archean-aged crust to propose a more evolved bulk crust composition. This revised composition is derived from a mixture of 60% Archean cmst (which is a 50 50 mixture of mahc and felsic end-member lithologies), and 40% average-andesite cmst of Taylor (1977). McLennan and Taylor (1996) focused on potassium, thorium, and uranium, and did not provide amended values for other elements, although other incompatible elements will be higher (e.g., rubidium, barium, LREEs) and compatible elements lower in a cmst composition so revised. 

Niobium metal is typically gray or dull silver in color. It is one of the refractory metals along with tantalum, tungsten, molybdenum, and rhenium, due to its very high melting point. It is estimated that niobium has a natural occurrence in Earth s crust of approximately 20 parts per milhon (ppm). The largest niobium-containing mineral reserves are located in Brazil and Canada. 

Niobium is found in niobite, niobite-tanta-lite, pyrochlore, carbonatites, and euxenite. Pyrochlore represents the major source of known world reserves of niobium (Bartlett 1973). As of 1998, Brazil was the major source of US-imported niobium ores listed at 79%, with 8%, 3%, 4%, and 6% originating from Canada, Germany, France and others, respectively (Cunningham 1998). The 1998 estimated world production of niobium (in the form of NbjOs concentrates at a range of US 6.1 to US 7.0 per kg) was about 18 500 tonnes, with 3600 tonnes consumed in the United States (Cunningham 1998). 

Chemical effects of X-ray intensity ratios have been studied in a series of niobium compounds by electron and proton bombardments Molecular orbital calculation using the GAMESS method has been applied to estimate MO occupation of the bonding electrons. The X-ray intensity ratio linearly increased with increase... 

The radius of a niobium atom is 0.147 nm. Niobium adopts the A2 stmcture. Estimate the unit cell parameter of niobium crystals. 

Let us further specialize to the case of a thin niobium film deposited on 1/2 mil Mylar. Although g of niobium has been reported as high as 8 kilo-oersteds, a more conservative estimate is to take B kilo-oersteds for this hard superconductor. This magnet would then produce 90 kilo- oersteds with a current of 3.0 amp per centimeter of length at zero power dissipation. K it is assumed that one could obtain a winding density of 1600 turns per inch of radius, the windings would have a radial thickness of 15 in. this means that the physical size of the magnet would be very reasonable. 

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