Hydrogen, in niobium

M. Mazzolai (2001). J. Alloys Compounds, 316, 189-192. A neutron spectroscopy study of the local vibrations, the interstitial sites and the solubility limit of hydrogen in niobium-molybdenum alloys. 

R. Cantelli, F. M. Mazzolai and M. Nuovo, Internal Friction due to Long-Range Diffusion of Hydrogen in Niobium (Gorsky Effect), Phys. Stat. Sol. 34 597 (1969). 

Niobium Oxides. The solubihty of oxygen in niobium obeys Henry s law to the solubiHty limit of the first oxide phase of 850—1300°C (123). The amount of oxygen in solution in niobium is 1.3 at. % at 850°C and nearly 2 at. % at 1000°C (124). Only three clearly defined anhydrous oxides of niobium have been obtained in bulk, ie, NbO, Nb02, and Nb20. Niobium monoxide, NbO, is obtained by hydrogen reduction of the pentoxide at... 

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. 

Fig. 7. Predicted diffusion coefficients for hydrogen (H) and deuterium (D) in niobium, as calculated by Schober and Stoneham (1988) from a model taking account of tunneling between various states of vibrational excitation and comparison with experimental measurements (solid lines). Theoretical curves are shown both for a model using harmonic vibrational wave functions (dashed lines) and for a model with anharmonic corrections (dashed-dotted lines).

ASh (nc), i.e., values at amax are offset by values at j3min such that their sum is approximately twice that at the critical composition. It can be shown that the approximation given in Equation 23 is good even though the partial entropy is not ideal, for example, as in niobium-hydrogen. 

The derivatives in the lower oxidation states Nb(II), Nb(HI), Ta(III) (known mainly for niobium) were obtained by the reduction of M(OR)5 by sodium amalgam [404, 1187], and bimetallic methoxoniobates (IV) by reduction with hydrogen in statu nascendi (Mg + MeOH) or on cathodic reduction in solution (method 7). The alkoxide chlorides ofNb(HI) and Nb(IV) crystallize on alcoholysis ofthe corresponding chlorides at low temperatures (method4) [416, 1731],... 

Figure 6.27. The potential curves for sudden (2) and adiabatic (3) hydrogen transfer between the tetrahedral interstitial positions in niobium crystal. Curve 1 is the potential in the unrelaxed lattice. (From Chakraborty et al. [1988].)...

AUcyl ligands in niobium and tantalum complexes are susceptible to attack by electrophiles (see Electrophilic Reaction). Hydrogenation (see Hydrogenation) of niobium or tantalum M R bonds to provide the metal hydrides is an important reaction of synthetic utility. Insertion reactions of unsaturated reagents into Nb- or Ta bonds are common. The unsaturated reagents include aUcenes, aUcynes, CO, NO, RN=C=NR, CNR, and others. 

Although dealing with high symmetry hydrogen sites is straightforward more sophisticated treatments become necessary as the symmetry is lowered. In niobium the tetrahedral sites in the metal are distorted to D2d symmetry and the single transition at high frequencies. 

The catalytic activities for ethene oligomerization, induced by the photoreduction of niobium oxide with ethene and other various compounds at room temperature, are listed in Table 1. The catalysts photoreduced with H2 at room temperature and reduced thermally with H2 at 823K show comparable activity, but with much lower values than the catalyst photoactivated with ethene. Not only ethene but also other olefins are effective for the photoactivation. Low activation ability of 2,3-dimethyl-2-butene, however, has been observed, suggesting that vinyl hydrogen in olefins may play a role in the photoactivation. 

It should be noted that low-temperature dynamical processes were also observed for hydrogen trapped by some substitutional impurities (Ti, Zr) in niobium using the internal friction [119, 120] and heat capacity [121] measurements. In particular, the heat capacity of a NbTio os ahoy doped with H(D) shows considerable hydrogen- or deuterium-induced contributions between 0.05 and 2 K [121]. However, the available experimental information is not sufficient to elucidate the microscopic picture of hydrogen motion in these systems. The same also refers to the low-temperature dynamical processes ofhydrogen in b.c.cVandTa [106, 107,122]. 

In the plasma experiments, the sticking coefficient of hydrogen on niobium must be equal to one if the membrane is to be classified as truly superpermeable. For highly energetic and ionized plasma species, including H2+, H3+, D2+ and D3+,... 

N. Boes, H. Zuechner, Study of the solubihty behavior of hydrogen in vanadium, niobium and tantalum using an electrochemical method, Ber. Bunsenges. Phys. Chetn. 80 (1976) 22—27. 

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 ... 

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. 

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