Nitrogen initial density dependence

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

The basic Brophy and Ingraham technique was studied by several other authors. Bayer and Trivedi " found that the effectiveness of the technique depended more on the nature of the coating than on its thickness, and that retained moisture in the electroplate was essential for effective conversion. They recommended a current density of 21.5 A/m for 5 minutes to produce a coating thickness of 1.25 to 2.5 tjm. Nishimura and co-workers found that the presence of air or water or both in the conversion gas improved the wear life. Table 9.5 compares the properties of the in situ films with those of burnished and sputtered films, and shows superior wear life for the in situ films. Their friction results were curious, in that they found that the initial films which were formed gave low friction in air or nitrogen but not in vacuum. Low friction in vacuum was obtained when the initial product was heated in vacuum to 400°C. 

Two different types of reactors are used depending on the product synthesized. The first type can maintain pressures up to 150 atm, and is widely used for production of powders in gasless and gas-solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 2000 atm), is used for the production of nitride-based articles and materials, since higher initial sample densities require elevated reactant gas pressures for full conversion. For example, well-sintered pure BN ceramic with a porosity of about 20-35% was synthesized at 100 to 5000-atm nitrogen pressure (Merzhanov, 1992). Additional examples are discussed in Section III. 

The dependence of the combustion temperature and burning rate on nitrogen pressure in the reactor was investigated on the sample of mixture (AlFg — SNaNj), having a diameter of 30 mm over pore density of initial mixture 5 = 0.34) and stoichiometric relationship of the components in the system. The results of the dependence investigation are presented in Table 8.3. No less than five experiments were carried for each dependence point. 

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