Nitrides, dissociation energy

Before we present a survey on the adsorption of several simple gases some general aspects of the dissociative adsorption will be treated. Table 4.1 tabulates the dissociation energies of reactants in the gas phase. Table 4.2 shows some examples of the heats of formation of hydrides, nitrides, carbides and oxides. The following relevant conclusions emerge ... 

Table XXXIIL Dissociation Energies, Intemuclear Distances and Vibration Frequencies of Diatomic Nitrides...

The dissociation energy of chemical compounds is in the order of 10 to 100 eV. Of course, such a compound can be expected to decompose if bombarded with keV ions. Part of the volatile products thus arising are pumped away with the sputtering gas so that the growing film shows a deficit of these components. This is why stoichiometric compound films as oxides, nitrides, sulphides, etc. usually can only be prepared by sputtering by the addition of small amounts of O2, N2, H2S, etc., to the sputtering gas, hence in a form of reactive sputtering which replaces the losses. 

Thiazyl monomer is a radical with one unpaired electron. In contrast to nitric oxide, it polymerizes so readily that it cannot be isolated as a monomeric solid or liquid and has only a transient existence in the gas phase. Thiazyl monomer may be generated in a number of ways, including the volatilization of (SN) c (2) or pyrolysis of S4N4 (1) over quartz wool above 300 °C. The emission spectrum and ESR spectrum have been reported. The bond length is calculated to be 149.7 pm from its spectroscopic moment of inertia. This indicates a bond order of between two and three. The dissociation energy of this strong sulfur-nitrogen bond is estimated to be 463 kj mol from spectroscopic data. Nevertheless, like other sulfur nitrides, NS is endothermic and unstable with respect to its elements. Thiazyl monomer exhibits an IR band at 1209 cm T The experimental dipole moment is 1.83 0.03 D and the ionization potential is 9.85 eV. 

Gingerich, K. A., Gaseous metal nitrides. III. On the dissociation energy of thorium mononitride and predicted dissociation energies of diatomic group III-VI transition-metal nitrides, J. Chem. Phys., 49, (1968), 19-24. Cited on pages 311, 322, 506. 

It is clear from the above that h-BN materials possess a variety of unique properties and demands on such ceramics consistently increase. However, the production methods are energy and time consuming and thus expensive. Indeed, being extremely refractory material, similar to graphite, with nitride dissociation, it is difficult to sinter this type of compound. Can one suggest a novel approach to produce BN ceramics The author truly believes that direct synthesis of boron nitride net shape articles by combustion-based approach described below is an effective alternative to the conventional technologies. 

The cubic crystalline form of boron nitride has a diamond-like structure, each boron atom is bonded to four nitrogen atoms at a bond distance of 156 pm. The mean NB bond energy calculated from the energy of atomization is 321 kJ mol i.e. more than 2.5 times larger than the NB bond dissociation energy of amine borane. We conclude that the NB bond in amine borane is not only unusually long, but also unexpectedly weak. 

The formation of surface hydrides, oxides, and nitrides is usually a result of the dissociative chemisorption of the molecules of these gases. As can be seen from Fig. 9, which gives the potential curves for such an adsorption, the heat of adsorption is given by the difference in energy level between A and E, this difference being given by... 

Why are nitrides not as numerous as oxides The answer is that although generally nitrides are comparable in stability to oxides with respect to dissociation to atoms, the greater stability of N2 as compared to 02 means that nitrides are less stable with respect to dissociation to the combined elements. Thus compare the free energy changes [1] in kJmol"1 for the following processes at 1500 K ... 

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