Niobium-nitrogen system

The titanium-nitrogen system was chosen as a case study for systems that exhibit melting, and for further clarification, experiments in the zirconium-nitrogen system were also conducted. Similarly, the tantalum-nitrogen system was extensively studied as an example of a non-melting metal, and further evidence was collected from experiments on the niobium-nitrogen system. 

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. 

Figure 12.8 Niobium-nitrogen system Effect of nitrogen pressure and solid phase dilution on propagation velocity. Particle size 36 pm.

The plethora of structures known in the niobium-nitrogen system appears to result from varying amounts of oxygen contamination [300, 301]. The phase diagram has been established [302, 303. 304, 305]. NbN, [S-Nb2N and y-Nb4N, are known from neutron diffraction experiments [306. 307], but the fully nitrided Nb,N5 has not been observed [ 308],... 

The magnets do require expensive helium refrigeration systems to cool the niobium-titanium coils to superconducting temperatures. Thus, a liquid nitrogen system should be less expensive, simpler to operate, and more reliable. But some scientists are dubious. Said John Hulm ... 

Metals which are subject to oxidation or attack by nitrogen can be sprayed in a closed system so that air is exluded. The heat necessary to melt the wire is produced by current generated in the wire itself by high-frequency currents flowing in small water-cooled coils. By this means, titanium, niobium and even uranium, can be sprayed without gaseous contamination. 

Loop Tests Loop test installations vary widely in size and complexity, but they may be divided into two major categories (c) thermal-convection loops and (b) forced-convection loops. In both types, the liquid medium flows through a continuous loop or harp mounted vertically, one leg being heated whilst the other is cooled to maintain a constant temperature across the system. In the former type, flow is induced by thermal convection, and the flow rate is dependent on the relative heights of the heated and cooled sections, on the temperature gradient and on the physical properties of the liquid. The principle of the thermal convective loop is illustrated in Fig. 19.26. This method was used by De Van and Sessions to study mass transfer of niobium-based alloys in flowing lithium, and by De Van and Jansen to determine the transport rates of nitrogen and carbon between vanadium alloys and stainless steels in liquid sodium. 

In a typical experiment, 4.63 g. (0.025 mole) of niobium(V) fluoride is mixed with 0.348 g. (0.012 mole) of -200-mesh silicon and loaded into the reactor inside a glove box under dry nitrogen. The assembled reactor is connected to a helium tank and repeatedly pressurized and depressurized to rid the system of air. The entire system is pressurized to 50 p.s.i., the Monel reactor valve is closed, and the reaction chamber is placed in a vertical furnace at 350°C. The pressure in the external lines is left at 50 p.s.i. to prevent entry of air. 

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. ... 

It has been established that certain elements, specifically chromium, mol)d)denum, and silicon, are ferrite formers. Aluminmn and niobium may also act as ferrite formers depending upon the alloy system. Other elements, such as nickel, manganese, carbon, and nitrogen, tend to promote the formation of austenite. 

Zak] Zakharov, A.M., Pshokin, V.P., Ivanova, E.I., Niobium Comer of the System Nb-B-C , Russ. Metall, 5, 192-195 (1985) (Experimental, Crys. Stracture, Phase Relations, 10) [1986Smi] Smid, L, Rogl, R, Rhase Equilibria and Structural Chemistry in Ternary Systems Transition Metal - Boron - Nitrogen , Inst. Phys. Conf. Ser. No. 75, Ch. 4, Adam Hilger Ltd., 249-257 (1986) (Experimental, Crys. Stracture, Rhase Relations, 17)... 

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