Sublimation dissociative

A considerable amount of research has been conducted on the decomposition and deflagration of ammonium perchlorate with and without additives. The normal thermal decomposition of pure ammonium perchlorate involves, simultaneously, an endothermic dissociative sublimation of the mosaic crystals to gaseous perchloric acid and ammonia and an exothermic solid-phase decomposition of the intermosaic material. Although not much is presently known about the nature of the solid-phase reactions, investigations at subatmospheric and atmospheric pressures have provided some information on possible mechanisms. When ammonium perchlorate is heated, there are three competing reactions which can be defined (1) the low-temperature reaction, (2) the high-temperature reaction, and (3) sublimation (B9). 

Recently, Jacobs (48) proposed a unified mechanism incorporating this dissociative sublimation step. He imagines the step towards the adsorbed state to be accomplished by proton transfer. At low temperatures, both NH3 and HCIO4 desorb but recombine in the gas phase to form gaseous NH4CIO4—i.e., pure sublimation occurs. At low temperatures and increased pressures, NH3 does not desorb but remains physically... 

Ammonium perchlorate is a monopropellant which is exothermic in its decomposition to the extent of 270 cal./gram (1, 30, 54), based on measured decomposition products, and has a measured adiabatic flame temperature of 950°C. (1, 54) (see Table I). The dissociative sublimation step is zero order and is endothermic to the extent of 500 cal./gram... 

The surface decomposition process of AP propellants is net exothermic, with heat absorbed at the AP and fuel surfaces by endothermic pyrolysis but with more heat liberated in the gas phase close to the AP crystals. In detail, decomposition at the AP surface consists of endothermic pressure-independent solid-to-gas phase dissociative sublimation, followed by exothermic pressure-dependent gas-phase oxidation. The over-all reaction for the AP decomposition is pressure dependent. 

If surface equilibrium prevails, then it is relatively straightforward to generalize the interface condition to chemical processes that are more complex than equations (1) and (8). This of interest, since propellant materials often experience processes of this type for example, NH4CIO4 undergoes dissociative sublimation into NH3 and HCIO4 [33]. For a general process in which the condensed material is transformed to 1 the surface equilibrium condition (for an ideal gas mixture and a solid whose thermodynamic properties are independent of pressure) is... 

Endothermic zeroth order pyrolysis of solid fuel and dissociative sublimation of AP to ammonia and perchloric acid... 

A factor of sublimation should also be considered. Guirao and Williams 71] described the deflagration of AP at pressure between 20 and 100 atm. and suggested that a dissociate sublimation process is likely to exist at the interface between the gaseous and condensed phases. 

L vov and Novichikhin [62] have explored the possibilities of treating decomposition as dissociative sublimation (see Section 2.4.5, for detail). Some of the reactants considered were binary compounds. Magnitudes of calculated for the following reactions ... 

As most of the rate constants for the reactions invoked are unknown, to fully substantiate the 3-phase combustion model requires a systematic study of the chemistries occurring in each of the three phases by high-level quantum-chemical calculations for the key processes involved. They include in particular the determination of the dissociative sublimation rate, AP(s) NH3 + HCIO4, the chain initiation reaction rate, HCIO4 -> OH + CIO4, as a function of temperature and pressure, and the rates of ensuing chain reactions involving ClOx (x = 0 - 4), NHy and NOy (y = 1 - 3) and HOz (z =... 

In many instances the evaporation and deposition of solids is kinetically hindered. This applies particularly to associative and dissociative sublimation in which case evaporation and condensation coefficients down to 10 were found Examples are ... 

The properties of the deposit, in particular its stoichiometry, can be influenced also in the case of dissociative sublimation if one of the gaseous species forms a relatively stable molecule. Nitrides, eq. (34), represent a classical example. In most of these systems, especially in the case of dielectrics on which dissociative chemisorption... 

The most diverse data exist about the best-known compound silicon carbide (SiC), perhaps because oxidation (in air), dissociation, sublimation, and phase change obscure the picture. No aluminum silicide has been described. 

NH4N03(s) Ammonium nitrate dissociation/ sublimation N2O formation NO2-SCR Fast SCR... 

Here is the energy of dissociation of one mole of Oj or Nj (N = 8) or of atomisation of one mole oi C (N = 4). This correction aUows for the fact that formation of the MX0 75 compound takes place with the starting metalloid dissociated (sublimated) 25%. As distinct from Eyf, is of thermodynamic origin, as it is the energetic part of the changes in the free energy occurring on vacancy formation. Results of E and E calculations for refractory compounds are discussed in Chapter 4. 

EuSe crystals were also grown from the vapor phase. The Mo ampules, filled with EuSe powder, were sealed under vacuum ( 10 Pa, 7.5x10" Torr) by an electron beam. Sublimation was carried out with a temperature gradient, 2120 1960 K or 2320- 2070 K. Monocrystalline blocks with up to 10 mm in one direction could be cut from the coarsegrained crystals. Crystals contained up to 10" wt% Mo. Experimentally observed mass transport rates were compatible with a diffusion mechanism associated with dissociative sublimation. The limiting step in the mass transport at 1960 to 2320 K was the diffusion of Se2 molecules, Vel mizov et al. [20]. 

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