Niobium activity

Specifications for plutonium product call for less than 100 ppm of uranium, a total gamma activity less than 40 iCi/g plutonium, and a zirconium-niobium activity less than 5 /uCi/g plutonium. Plutonium nitrate product 3PC, stream 55, meets the total activity specification but does not quite meet the zirconium-niobium specification with the hi burnup, 40,000 MWd/MT, feed used in this process example. 

The important (3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tantalum, and niobium of the P-isomorphous type and manganese, iron, chromium, cobalt, nickel, copper, and siUcon of the P-eutectoid type. The P eutectoid elements, arranged in order of increasing tendency to form compounds, are shown in Table 7. The elements copper, siUcon, nickel, and cobalt are termed active eutectoid formers because of a rapid decomposition of P to a and a compound. The other elements in Table 7 are sluggish in their eutectoid reactions and thus it is possible to avoid compound formation by careful control of heat treatment and composition. The relative P-stabilizing effects of these elements can be expressed in the form of a molybdenum equivalency. Mo (29) ... 

The stoichiometry of the prepared compounds depends not only on the composition of the initial mixture, but also on the initial oxide s fluorination activity. Unlike tantalum oxide, fluorination of niobium oxide by an ammonium hydrofluoride melt results in the formation of oxyfluoroniobates, but not of fluoroniobates. During the first step of Nb205 fluorination, (NH4)3NbOF6 is formed according to the following interaction [51, 52, 105, 111, 121, 122] ... 

Investigation of fluoride melts containing tantalum or niobium is a very difficult procedure that involves high temperatures, corrosion activity and the... 

Based on an analysis of the initial dissolution rate in different solutions at different temperatures, several very useful conclusions and recommendations were made. It was found that the apparent activation energies for the dissolution of niobium and tantalum in 10 mol/1 HF solution are 56.5 and 65.5 kJ/mol, respectively for columbite, and 42.7 and 61.1, respectively, in the case of tantalite. It was also concluded that the mechanism of dissolution is the same for both columbite and tantalite. In addition, the initial dissolution rate of niobiuth (RNb) from columbite is controlled primarily by the activities of the... 

Analysis of the volumetric effects indicates that as a result of such mechanical activation, iron and manganese are concentrated in the extended part of the crystal, while tantalum and niobium are predominantly collected in the compressed part of the distorted crystal structure. It is interesting to note that this effect is more pronounced in the case of tantalite than it is for columbite, due to the higher rigidity of the former. Akimov and Chernyak [452] concluded that the effect of redistribution of the ions might cause the selective predominant dissolution of iron and manganese during the interaction with sulfuric acid and other acids. 

These batteries have vanadium oxide as the active material of the positive electrode, niobium oxide for the active material of the negative electrode, and an organic solvent for the electrolyte. Lithium ions enter the vanadium oxide from the niobium oxide during discharge, and lithium ions enter the niobium oxide from the vanadium... 

Figure 16.11 Titania (TiO c)-suppoited (a) and niobium-doped titania (Nbo.o8Tio.920j /Au)-suppoited (b) Au particle-size-dependent specific activities for—CO oxidation at a potential step of 0.5 V vs. RHE. The current densities have been corrected for the TEM-deteimined...

Oxygen and carbon have substantial solid solubilities in niobium at the temperatures normally required for reduction. As the activity coefficients of both carbon and oxygen in niobium are low, their retention in the niobium metal produced by the carbothermic reduction of niobium oxide is expected. It is, however, possible (as explained later) to remove these residual impurities by extending the pyrovacuum treatment to still higher temperatures and lower pressures. 

Sample. This source places no restrictions on target material. Clusters of metals, produced. For example, polyethylene and alumina have been studied as well as refractory metals like tungsten and niobium. Molecular solids, liquids, and solutions could also be used. However the complexity of the vaporization process and plasma chemistry makes for even more complex mixtures in the gas phase. To date the transition metals(1-3) and early members of group 13 (IIIA) and 14 (IVA)( 11-16) have been the most actively studied. 

The similarity of the reactivity patterns for niobium and cobalt and the non-reacti vi ty of iron with nitrogen suggests that dissociative chemisorption is taking place. Dissociation of molecularly chemisorbed nitrogen is an activated process on all metals(35) and is most exothermic for the early metals in the periodic tab e(36). The limited observations on clusters seems to be consistent with these trends. 

For niobium and cobalt clusters structures have been proposed based upon the elements behavi or (71). Niobium s specific inertness has been associated with structures that are analogous to close-packed surface of W(110) which also has an activation barrier for hydrogen chemisorption. Since the IPs are also expected to be higher for closed packed structures these two sets of observations are in agreement. This model at its current stage of development requires different structures for each system and as yet has not been useful in making predictions. 

Figure 1.3 Left. Detailed view of the Nb K-edge XANES data of a pyridine salt of niobium-exchanged molybdo(vanado)phosphoric acid (NbPMo fVJpry) as a function of temperature [31]. A change in niobium oxidation state, from Nb5+ to Nb4+, is identified between 350 and 420°C by a relative increase in absorption about 19.002 keV, and can be connected with the activation of the catalyst for light alkane oxidation. Right. Radial Fourier-transform EXAFS function for the NbPMo (V)pyr sample heated to 420°C [31 ]. The two peaks correspond to the Nb-O (1.5 A) and Nb-Mo (3 A) distances in the heteropolymolybdate fragments presumed to be the active phase for alkane oxidation. (Reproduced with permission from Elsevier.)...

The right panel of Figure 1.3 displays the radial function obtained by Fourier transformation of the -weighed background-subtracted EXAFS data from the solid heated to 420°C [31], This spectrum shows two major peaks, one at about 1.5 A associated with backscattering from O neighbors, and a second at 3 A related to the Nb-Mo pairs. The measured distances are consistent with a combination of niobium oxo species and heteropolymolybdate fragments, presumably the catalytically active phase. 

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