Thermodynamics nitridation reactions

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

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. 

This reaction is catalyzed by iron, and extensive research, including surface science experiments, has led to an understanding of many of the details (72). The adsorption of H2 on iron is fast, and the adsorption of N2 is slow and characterized by a substantial activation energy. N2 and H2 are both dis so datively adsorbed. Adsorption of N2 leads to reconstmction of the iron surface and formation of stmctures called iron nitrides that have depths of several atomic layers with compositions of approximately Fe N. There is a bulk compound Fe N, but it is thermodynamically unstable when the surface stmcture is stable. Adsorbed species such as the intermediates NH and NH2 have been identified spectroscopically. 

The third aspect, the stability range of solid electrolytes, is of special concern for alkaline-ion conductors since only a few compounds show thermodynamic stability with, e.g., elemental lithium. Designing solid electrolytes by considering thermodynamic stability did lead to very interesting compounds and the discovery of promising new solid electrolytes such as the lithium nitride halides [27]. However, since solid-state reactions may proceed very slowly at low temperature, metasta-... 

In many CVD reactions for the deposition of nitrides, ammonia (NH3) is used as a source of nitrogen, rather than nitrogen. This is preferable from a thermodynamic standpoint but may present a problem in that any NH3 — either unreacted or present in the regions of furnace where the temperature is below its dissociation temperature— tends to combine with the HCl effluent gas and forms NH4CI, which may clog the exhaust line. 

Nitridoborates of lanthanum and the lanthanides were obtained from reactions of lanthanide metal or lanthanide metal nitride with layer-like (a-)BN at elevated temperatures (3>1200°C). These reactions require elaborated techniques in the inert gas sample-handling and the use of efficient heating sources, such as induction heating. Only some compounds remain stable in this high-temperature segment, and the yields of such reactions are often limited due to the competing stability of binary phases, allowing only the most (thermodynamically) stable compounds to exist. 

Nitrides, Phosphides, and Arsenides. The phase diagram for the Cr-N system between 900 and 1350°C has been constructed and thermodynamic relationships in this system have been calculated. These latter studies indicated the extreme sensitivity of the kinetics of Cr-N reactions to oxygen impurity. ... 

In general, carbides, nitrides, and borides are manufactured in the vapor phase in order to form high-purity powders. This procedure is fundamentally different than a strict CVD process, since in powder synthesis reactors, deposition on seed particles may be desirable, but deposition on the reactor walls represents a loss of product material. As we will see, in CVD, heterogeneous deposition on a surface will be sought. Aside from this issue of deposition, many of the thermodynamic and kinetic considerations regarding gas phase reactions are similar. 

In general, the stable thermodynamic products of ordinary flames have little worth, but many of the uncommon flames have products of value. The chlorination of hydrocarbons may be carried out in a flame process which was recently announced (A4). A most fascinating example is the formation of boron nitride from the flame reaction between diborane and hydrazine, two compounds which are ordinarily thought of as fuels (B2, VI). The stabilization of this flame depends upon the proper preparation of the premixed gases, since a solid adduct between the reactants prevents flame stabilization if the preflame residence time is too great. 

A theoretical analysis of combustion synthesis of refractory nitrides was presented by Munir and Holt in 1987.37 They predicted the existence of an activation energy due to chemical reaction or mass-transport. Glassman et al. in 1987,38 in their thermodynamic analysis of TiN formation, examined the possibility of creating TiN by a self-sustained reaction of the metal particles and nitrogen gas in a rocket motor. They reported that for the stoichiometric ratio of 0.5 mole N2/mole titanium, the reaction has... 

Both gases accelerate the mass and heat transport through the pores and the local removal of the Si02 surface layer (it follows from thermodynamic calculations that only in H2 or NH3 containing atmospheres can Si02 be nitrided [440]). Factors influencing reaction and micro structure are summarised in Table 15. It is obvious that the nitridation is very complex and can modelled only partially at present [547, 548],... 

TiC is thermodynamically not compatible with Si3N4 under sintering conditions. TiC reacts with Si3N4 forming TiCxN x where the x values are in the range of 0.3 [568, 569] to 0.24 [567], The x value in TiCxN x depends on nitrogen pressure and temperature. As an additional reaction product, SiC may be formed or the oxide nitride liquid phase can be reduced by carbon... 

Thermodynamic considerations are useful for determining conditions for carburization and nitridation. For example, in the nitridation of niobium the particular phase of the product (e.g. NbN, / -Nb2N, or y-Nb4N3) will depend on temperature and pressure. This holds true also for carbides. However, for carbides, competing reactions such as... 

The thermodynamic considerations above are useful guides to the preparation of carbides and nitrides, but are not limiting as the syntheses can be carried out at nonequilibrium conditions. For example, in the carburization or nitridation of oxides it is possible to lower the temperature of reaction by sweeping away the product gases [23] ... 

Fluid-solid reactions include thermal decomposition of minerals, roasting (oxidation) of sulfide ores, reduction of metal oxides with hydrogen, nitridation of metals, and carburization of metals. Each t3 e of reaction will be discussed finm the thermodynamic point of view. Then reaction kinetics for all of the various rate determining steps in fluid-sohd reactions will be discussed for two general models shrinking core and shrinking particle. 

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