Nitrogen in niobium

Literature contains a variety of data on the solubility of nitrogen in niobium. They do not always agree Table 1-6 summarizes some of them. 

The solubility of nitrogen in niobium varies with temperature according to Sievert s square root law. Nitrogen can therefore be removed by vacuum fusion. 

The transition temperature depends on the - especially interstitial -impurities in the metal. Transition temperatures measured under comparable test-conditions (creep rate, grain size), depending on kind and concentration of the impurity elements, are compiled in Table 1-7. 

The values mentioned show that the influence of nitrogen exceeds that of oxygen. For a recristallized metal of average purity (150 g/g 0  

The hardness also depends to a large extent on the Interstitial impurities, but up to now the influence of nitrogen has not been differentiated from that of other impurities. 

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Similar relationships can be written for the dissolution of hydrogen and oxygen. These relationships are expressions of Sievert s law which can be stated thus the solubility of a diatomic gas in a liquid metal is proportional to the square root of its partial pressure in the gas in equilibrium with the metal. The Sievert s law behaviour of nitrogen in niobium is illustrated in Figure 3.8. The law predicts that the amount of a gas dissolved in a metal can be reduced merely by reducing the partial pressure of that gas, as for example, by evacuation. In practice, however, degassing is not as simple as this. Usually, Sievert s law is obeyed in pure liquid metals only when the solute gas is present in very low concentrations. At higher concentrations deviations from the law occur. 

Figure 3.8 Sievert s law behaviour of nitrogen in niobium at high temperatures.

The behavior of nitrogen in niobium is illustrated in Figure 4.30 as an example. The extent to which nitrogen can be removed from the metal at a given temperature can be readily calculated from such diagrams. Alternatively, the relationship linking the equilibrium pressure, the nitrogen content in niobium and the temperature can expressed as... 

Friedrich, K., Lassner, E., Paesold, E. Determination of oxygen and nitrogen in niobium and tantalum. J. Less-Common Metals 22, 429 (1970). 

The equilibrium of nitrides with ammonia + hydrogen mixtures is not very efficient because of the relatively low stability of ammonia gas. Direct dissociation of the nitride will often produce satisfactory results provided that a suflSciently high temperature of measurement can be attained experimentally. Thus, Pemsler was able to measure the dissociation pressures of solid solutions of, for example, nitrogen in niobium ... 

In 1968 Friedrich et al. (23), who also tested flux methods (Pt flux or bath, or Ni-Ce-Mg) and used an empty crucible for each sample, were still unable to report any improvement in this situation. Their conclusion was that "since none of the techniques investigated up to the present time has given satisfactory nitrogen results, we must advise against the determination of nitrogen in niobium and tantalum by vacuum fusion at present, unless an empirical factor can be tolerated". 

In 1970 Friedrich et al. (24) reached the following conclusion "The determination of nitrogen in tantalum using the nickel-magnesium-cerium technique is possible if the nickel. -tantalum ratio is above 8 1. It has still not been possible to find a method suitable for the determination of nitrogen in niobium". 

Harris (29) determines nitrogen in niobium by melting a powdered or finely milled sample, contained in a nickel boat resting in a Vycor tube, at 600°C by means of solid sodium hydroxide in a slow stream of hydrogen. The time required is 1/2 to 1 hour. This converts any nitrogen present into ammonia, which is absorbed in water or in a dilute solution of boric acid and is determined titrimetrically or photometrically. 

Because of favourable nuclear properties of these matrices, several nuclear reactions can be used for the instrumental determination of nitrogen in niobium and tantalum. 

The conditions for the determination of nitrogen in niobium and tantalum (42)(45)(46) using the N(p,n) 0 reaction are similar to those described under 2.3 except for the irradiation (12 MeV protons, 0.5 lA for Nb and 3 to 4 /.A for Ta) and measuring conditions (No lead absorber needed, detector with 5 % relative detection efficiency). 

Niobium metal absorbs nitrogen, similar to hydrogen, forming interstitial solid solution. The absorption occurs at 300°C and the solubility of nitrogen in the metal is directly proportional to the square root of the partial pressure of nitrogen. The reaction is exothermic and the composition of such interstitial solid solution varies with the temperature. When the metal is heated with nitrogen at temperatures between 700 to 1,100°C, the product is niobium nitride, Nb2N or (NbNo.s) [12033-43-1]. When heated with ammonia at these temperatures, niobium forms this nitride. Another niobium nitride exists, NbN [24621-21-4], with a face-centered cubic crystalline structure. 

In contrast, the combustion temperatures recorded for the tantalum-nitrogen and niobium-nitrogen systems were much lower than the melting points of the respective metals. Photographs of the cross-sections of undiluted samples do not indicate any macroscopic effect of melting, and neither do SEM photographs of the products. 

Powdered niobium metal, 20.0 g. (—200 mesh), and tin(II) fluoride, 52.0 g. (40 mesh),t are mixed in a molybdenum crucible in an Inconel- or nickel-pipe reactor approximately 3 in. in diameter and 10 in. long and heated to 400-500°C. in a stream of dry nitrogen. The niobium(V) fluoride volatilizes from the reaction mixture and condenses on the water-cooled lid of the reactor, which leaves metallic tin in the crucible. The yield of niobium(V) fluoride is 21.1 g., or 95% of theoretical. A very small amount of blue niobium oxyfluoride (composition of variable oxygen and fluorine content) often forms as an impurity because of the presence of minute amounts of oxygen. Anal. Calcd. for NbFs Nb, 49.44 F, 50.56. Found Nb, 49.43 F, 50.2. 

In fact, apart from controlled laboratory atmospheres, the gas is always complex in the multi-oxidant sense since even nitrogen in air can form nitrides with some alloy systems in addition to the oxides formed by the oxygen. This is seen particularly in alloys containing metals such as chromium, titanium, and niobium, where the formation of nitrides in air atmospheres interferes with the simple oxidation situation that is observed when using pure oxygen, or oxygen-argon mixtures. ... 

In this design, the UN fuel will be located in the drum of the core, clad in a layer of rhenium and niobium 1% zirconium. This rhenium layer is required because of incompatibilities between nitrogen and niobium 1% zirconium. The nitrogen out-gassing... 

Compared with titanium, the solubility of nitrogen in both metals is low. According to Fromm and Gebhardt (1), the figures at I000°C are 0.3 at.% for niobium and 5 at.% for tantalum. The nitrogen content of industrial metals is usually very low (< 20 Mg/g) and only seldomly reaches values above 100 t g/g. 

Nitrogen and carbon are the most potent solutes to obtain high strength in refractory metals (55). Particulady effective ate carbides and carbonitrides of hafnium in tungsten, niobium, and tantalum alloys, and carbides of titanium and zirconium in molybdenum alloys. 

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