Nitride metals forming

With hydrogen, the alkali metals form the mono-hydrides MeH, having salt-like properties and a partially ionic, Me H, NaCl-type structure. They are colourless crystalline solids having a fairly negative AH of formation. The mono-hydrides react with water. They may be prepared from hydrogen and the metal (heated at 700-800°C for Li, 350-400°C for the others) or through the reaction of hydrogen with the alkali mono-oxide, nitride, etc. 

Molecular nitrogen is nearly insoluble in liquid sodium, but the presence of lithium makes it more soluble thanks to stabilization of the nitride ion formed in the metal melt by coordination of Li ions [14) functioning towards the nitride ion as EPA units. 

Fire or explosion hazard may arise from the foUowing ammonia reactions Reaction with halogens produces nitrogen trihahdes which explode on heating its mixture with fluorine bursts into flame reacts with gold, silver, or mercury to form unstable fulminate-type shock-sensitive compounds similarly, shock-sensitive nitrides are formed when ammonia reacts with sulfur or certain metal chlorides, such as mercuric, or silver chloride liquid ammonia reacts violently with alkah metal chlorates and ferricyanides. 

Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

Alkaline earth-containing ternary nitrides make up the second largest group of ternary phases. Because the alkaline earth metals form stable binary nitrides, most alkaline earth containing ternary nitrides have been synthesized by the reaction of a binary nitride with a metal or by the reaction of two binary nitrides. This synthesis has resulted in a number of new ternary nitrides with a variety of structures. For example, the reaction of calcium nitride with Group 14 or 15 metals or metalloids forms a series of structurally related ternary nitrides with the anti-perovskite type structure. In Ca3MN (M = P, As, Sb, Bi, Ge, Sn, Pb) (Figure 8.5) the... 

Chemical bonds in metal nitrides are rich in variety. Alkaline and alkaline earth metals form ionic bonds to nitrogen in compounds such as... 

The free energies of the exchange reactions Eq. (7) AG7° must be positive, otherwise Si3N4 decomposes. Secondly, Si3N4 and Si02 should not react with the oxides or nitrides to form silicides or metals i.e., the reactions... 

The whiskers that are commonly added to silicon nitride to form composites are Si3N4 and SiC (common nomenclature is to add the suffix (w) to denote whiskers and this will be used where appropriate). Although other ceramic and metallic whiskers are available (e.g. BN, TaC, TiC, B4C and Fe), Si3N4 and SiC each have properties that make them prime candidates as reinforcements, not least of which is that they can sustain the high temperatures and reactivity of the sintering process without being degraded. 

Some pi-ocesses depond ou the fact that the oxide of a nitrido-forming metal is reduced to the metallic condition, when heated in the electric furnace with carbon in the requisite quantity, and if a stream of nitrogen be then blown through the fused mass, nitride is formed. 

Such a dependence is frequently observed experimentally in the oxida-tion of transition metals forming volatile oxides, and also of their alloys, carbides, nitrides, borides, silicides and other chemical compounds.350 365 An example taken from the work of E.A. Gulbransen and K.F. Andrew389 is presented in Fig. 5.22. 

The very active metals are capable of combining directly with nitrogen to form nitrides. In the air the oxide is formed so much more readily, that nitride formation is likely to escape notice but if the metal is presented in powdered form in a thick mass the oxygen is all combined in the surface layer and only nitrogen penetrates to the interior where pure nitride is formed. 

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