Tantalum-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. 

Figure 12.9 Tantalum-nitrogen system Effect of nitrogen pressure and solid phase dilution on combustion temperature. Particle size 85 pm.

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. 

The natural niobites and tantalites axe usually black, and form iso-morphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8 2 with increase in tantalum content.6 When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen.7 Small quantities of helium have also been found occluded in them. 

Figure 1.13 Mechanism of tantalum nitride formation (a) nitrogen concentration along TaN sample and (b) phase formation stages in T-N system.

It has been shown that solutions of transition metal complexes of metals, such as chromium and iron, in organolithium or Grignard reagents will also react with molecular nitrogen [125, 126]. The only other non-bio-logical systems which are known to react with nitrogen under mild conditions are metals in the pure state such as lithium and many of the transition metals, e.g. tantalum. 

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