The swelling and cation exchange properties of montmorillonite are fundamental in a wide range of applications ranging from nanocomposites to catalytic cracking of hydrocarbons. The swelling results from several factors and, though widely studied, information on the effects of a single factor at a time is lacking. In this study, density functional theory (DFT) calculations were used to obtain atomic-level information on the swelling of montmorillonite. Molecular dynamics (MD) was used to investigate the swelling properties of montmorillonites with different layer charges and interlayer cationic compositions. Molecular dynamics calculations, with CLAYFF force field, consider three layer charges (−1.0, −0.66 and −0.5 e per unit cell) arising from octahedral substitutions and interlayer counterions of Na, K and Ca. The swelling curves obtained showed that smaller layer charge results in greater swelling but the type of the interlayer cation also has an effect. The DFT calculations were also seen to predict larger d values than MD. The formation of 1, 2 and 3 water molecular layers in the interlayer spaces was observed. Finally, the data from MD calculations were used to predict the selfdiffusion coefficients of interlayer water and cations in different montmorillonites and in general the coefficient increased with increasing water content and with decreasing layer charge.
The reaction mechanisms in the Si|Ta|Cu and Si|TaC|Cu metallization systems are discussed based on the experimental results and the assessed ternary Si-Ta-Cu, Si-Ta-C and Ta-C-Cu phase diagrams. The ternary Si-Ta-N and Ta-N-Cu phase diagrams were also assessed in order to compare the thermodynamic properties of the TaC diffusion barriers to more widely investigated TaNx diffusion barriers. With the help of the sheet resistance measurements, RBS, XRD, SEM, and TEM the Ta barrier layer was observed to fail above 650 °C due to the formation of TaSi2. This was accompanied by the diffusion of Cu through the silicide layer and the resulting formation of Cu3Si precipitates. The stability of the TaC layers was better and the failure was observed above 750 °C due to the formation of Cu3Si and TaSi2. However, interdiffusion of Cu and Si was observed already at lower temperatures due to the presence of pinholes in the TaC layer. This emphasises the importance of the fabrication method and the quality of the TaC layers.
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