Surface nitrides

Hafnium begins to react with nitrogen at about 900°C to form a surface nitride film, and reacts rapidly with hydrogen at about 700°C to form hydrides (qv). The hydrogen diffuses rapidly and converts the bulk metal into the brittle hydride. 

The preliminary plastic deformation considerably effects on the phase formation, structure, microhardness and thickness of nitrided layers in -Fe and Fe-Ni alloys. The high microhardness of the diffusion layers results from the formation of the s- and y- nitrides. Iron doping with Ni leads to changing of the s-, f-phases composition. The existence of narrow intervals of deformations of 3-8 % and 20-30 %, in which the considerable (about 2 times) rise of microhardness of surface nitrided layers due to accelerated formation of s- and f-phases, was found. 

Puurunen et al. have investigated the effect of surface nitridation of y-AI2O3 supports by the atomic layer deposition process on the activity of chromium catalysts for isobutane dehydrogenation. Nitridation was observed to suppress activity and it was argued that oxide ions were more active for the dissociation of isobutane. 

On the other hand, surface nitriding or case hardening of a steel specimen can improve its durability. Both N and C atoms are introduced inter-stitially to the Fe lattice on the surface of the steel by immersing it in, for example, a solution of sodium cyanide (NaCN) in a molten Na2C03/NaCl mixture at 870 °C. 

The kinetics of the formation of surface nitride layers are discussed in terms of a reaction involving nitrogen-hydrogen molecular ions with the substrate. Diffusion of nitrogen into the bulk determines the rate of growth of the nitride layer. 

There are reports [24,25] that it is possible to decrease the MOVPE growth temperature to 550°C by using the nitridated surface of sapphire without the subsequent buffer layer. Tokuda et al [24] reported the GaN 00.2 FWHM dependence on the nitridation time of sapphire at 940°C in a plasma-excited N2 atmosphere. Using this technique, the best MOVPE GaN layers of FWHM less than 20 arc min were grown on a surface nitridated for 10 - 20 min. In a similar experiment, Kim et al [25] obtained GaN of the best 00.2 FWHM of 0.4 degrees for 10 min nitridation time. 

Whereas it seems justified to assume that the most unsaturated parts of the surface of iron catalysts are the most active in the synthesis, it may well be that on Mo and W, which are supposed to bind nitrogen stronger, an intermediate part of the energy spectrum functions most actively in the catalytic reaction, and it may therefore be more nearly correct to say that in this case surface nitrides are the catalysts. 

The discussed examples clearly point out that the different interaction of for example particle surface oxides compared to particle surface nitrides and carbides with the electrolyte and the metal surface significantly affects particle incorporation. Note that, although it is often observed, it should not be concluded from the presented examples that oxide or oxide-covered particles will always codeposit more easily than non-oxides. Changing the surface of the BN particles to a silicon oxide does not prevent aggregation or enhance codeposition.107... 

K. (2009) Adhesion study of pulsed laser deposited hydroxyapatite coating on laser surface nitrided titanium. Thin Solid Films, 517, 5496-5501. 

A further role of hydrogen in the reaction mixture is to prevent catalyst deactivation by formation of surface nitride, carbide, or unsaturated oligomeric deposit... 

Figure 3b. Optical micrograph of HPO Ge02 film on (111) germanium surface nitrided under low ammonia flow condition.

Further ways for improving the coating adhesion are via deposition of thin adhesive interlayers between substrate and coating (Bouzakis et al, 2010b) and through surface nitriding, prior to the PVD film deposition (Erkens et al. 2011). 

This structure is very similar to that of the (002) plane of (bulk) Fe4N and hence no reconstruction and the assignment of surface nitride. (It should be noted that the known bulk iron nitrides will never be thermodynamically stable under the conditions of ammonia synthesis but may be readily formed in NH3/H2 mixtures providing high virtual N2 pressures.)... 

Alkali promotion is vital for Fe catalysts. The basicity of the surface determines its activity and, in particular, the selectivity toward longer chain hydrocarbons (see the section Control of Selectivity ). Thus, the effect of K2O is much more pronounced than that of the weaker base Li20. The presence of silica, which reacts with the alkali to form less basic silicates, lowers the basicity of alkali-promoted catalysts (20). Thus, the presence of silica in alkali-promoted Fe catalysts lowers the heavy hydrocarbon selectivity. CO2 chemisorption can be used as a measure of the surface basicity. It has been found that chemisorbed nitrogen lowers the amount of CO2 that can subsequently be chemisorbed, which indicates that surface nitrides lower the basicity. This is in agreement with the observation that nitriding iron catalysts results in a lower heavy hydrocarbon selectivity (14). CO2 chemisorption data nevertheless need to be interpreted with care. For instance, promotion with CaO increases the CO2 chemisorption but it has little effect on the FT selectivity. 

Kida, K., Tsujimura, H., Nishikiori, T., and Ito, Y. (2010) Surface nitriding of pure iron and stainless steel by using molten salt electrochemical process. Extended Abstract of The 42nd Conference on Molten Salt Chemistry, pp. 55-56. 

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