Nitridation reactions

The direct nitridation of metal powders is commonly used to produce 

These and other nitrogen reactions are strongly endothermic. requiring 

This and the standard free energy make up the total free energy of reaction as shown in equation (5.6). In a gas mixture where Pm — 0.79 atm and the rest is an inert gas, all the metal nitrides are stable with respect to their metals, except Fe above 250 K and Cr above 1325 K. In air where Pn = 0.79 atm, this result is not true because tiie metals may also oxidize. Due to the presence of oi rgen in air, we must also consider the oxidation reactions at tiie same time as tiie nitridation reactions. This is done in the next section. 

In addition to metal nitridation, metal carbides may be reacted as follows  

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These nitridation reactions are also characterized by very high values of activation energy. As a result, the nitridation front represents a strongly nonlinear combustion wave which may be extremely thin and corrugated. 

Apparently thermodynamically unstable, because when the peroxodisulfuryl difluoride-boron nitride reaction mixture was heated to 40°C, detonations occurred. 

In the same way, if the gas pressure increases, the nitrogen concentration is not modified but the carbon concentration increases (mainly in the graphite or free form). This can suggest that the nitridation reaction is slower than the carbidation reaction. This assumption is experimentally confirmed by reaction of metal foils in a furnace. At similar reaction temperatures and times, the thickness of metal which has reacted is much greater with carbon than with nitrogen. 

Detailed experimental studies on these gas-solid combustion reactions reveal the dependence of combustion and propagation characteristics, like front propagation velocity, combustion temperature and degree of conversion, on operating parameters like nitrogen pressure, particle size and morphology of the reactant metal and dilution of the gas and solid phases. From these studies the optimum synthesis conditions for a variety of nitrides are determined and information about the mechanisms of several gas-solid combustion reactions is obtained. With the aid of combustion theory, the apparent values of activation energy for several nitridation reactions are calculated from measured combustion characteristics. 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

Polynuclear metal frameworks could also assist in N—N scission. Within this category fall the so-called nitriding reactions of metals (see Section XI). The cleavage of bridging diazene or hydrazine in well-defined reactions of their complexes has not been observed. 

Zirconium chloride is a white powder, and lithium nitride is a black powder. When this reaction proceeds, the lithium chloride forms as a liquid just as the iron forms as a liquid in the thermite reaction. The melting point of lithium chloride is 605°C (the melting point of iron is 1536°C ) The temperature of the zirconium chloride/lithium nitride reaction reaches 1370°C less than 1 second after the reaction is initiated. This liquid plays the same role as the flux agents already discussed It provides a medium for rapid mixing of reagents. 

This chapter discusses the fluid-solid and solid-solid reactions used to produce ceramic powders. The first aspect of this discussion is the spontaneity of a particular reaction for a given temperature and atmosphere. Thermodynamics is used to determine whether a reaction is spontaneous. The thermod3mamics of the thermal decomposition of minerals and metal salts, oxidation reactions, reduction reactions, and nitridation reactions is discussed because these are often used for ceramic powder synthesis. After a discussion of thermodynamics, the kinetics of reaction is given to determine the time necessary to complete the reaction. Reaction kinetics are discussed in terms of the various rate determining steps of mass and heat transfer, as well as surface reaction. After this discussion of reaction kinetics, a brief discussion of the types of equipment used for the synthesis of ceramic powders is presented. Finally, the kinetics of solid—solid interdiffusion is discussed. 

Using Figure 5.5, the standard Gibbs free energy, AG , for the nitridation reaction can be determined at 800°C, giving —112 kcal per mole. The [RgT In K] term for the nitridation is given by... 

Fig. 3.36. X-ray diffraction diagrams of Sm125Fe875 after mechanical alloying (top), after the formation of Sm2Fe17 by annealing in vacuum (middle), after the nitriding reaction (bottom)...

The above reaction is exothermic (AH = -77kJmor ) and occurs within 10 min at 230 °C. The authors propose that by analogy to the amide decomposition in the presence of lithium hydride, the amide-nitride reaction is a two step process which is ammonia mediated. In this case, Li3N (as opposed to LiH) acts as the trap for gaseous ammonia to form the imide in a fast reaction (Eq. 16.12) ... 

To lower the deposition temperature, CVD processes enhanced by plasma [48-58] and laser [55-58] have been investigated. Low-resistivity (< 40 pQ cm) TiN was deposited by Akahori et al. [59] using TiCL in an electron-cyclotron resonance (ECR) plasma process (Ts b = 540°C, microwave power = 2.8 kW). All films had stoichiometric composition with low chlorine concentrations of 0.16 at. % as determined by ICP-MS. This indicates that the nitridation reaction of TiCU is enhanced enormously by the ECR plasma. 

Identify the most promising combination of X additions, ternary and higher order alloying addition(s), and nitridation reaction conditions that result in the formation of an adherent, dense nitride surface layer. 

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