RESUMEN
Molybdenum disulfide (MoS2) is a solid lubricant used in various forms, such as a dry lubricant by itself or as a component of a more complex coating. In both these forms, the effect of oxygen contamination on the sliding properties of the MoS2 coatings is traditionally considered detrimental, resulting in expensive technological processes to produce pure MoS2. Here, it is shown that the high oxygen content does not necessarily hinder the solid lubricant properties and may even result in a lower friction and wear when compared to pure MoS2. Mo-S-O coatings were fabricated by unbalanced magnetron sputtering and tribologically tested under vacuum conditions. Oxygen caused amorphization of the as-deposited coatings but did not prevent the triboactivated formation of an ultra-thin crystalline MoS2 tribolayer with the incorporated oxygen. Such an imperfect tribolayer was found to reduce the coefficient of friction to 0.02, a value lower than that of pure MoS2. Moreover, owing to the higher density and hardness of oxygen-containing films, the wear rate was also found to be lower. Molecular dynamics simulations performed using a newly developed Mo-S-O force field confirmed that such an imperfect tribolayer can mitigate friction in a manner comparable to that of MoS2.
RESUMEN
The present study introduces a facile single-source precursor preparative access to bamboo-like multiwalled carbon nanotubes (MWCNTs) highly dispersed within a mesoporous silica-rich matrix. The metal-free single-source precursor was synthesized via a one-pot sol-gel process using tetramethyl orthosilicate (TMOS) and 4,4'-dihydroxybiphenyl (DHBP) and converted subsequently via pyrolysis under an argon atmosphere into MWCNT/silica nanocomposites. The in situ segregation of the highly defective bamboo-like MWCNTs was carefully investigated and has been shown to occur within the mesopores of the silica-rich matrix at relatively low temperatures and without the use of a metal catalyst. The experimental results have been supported by extensive computational simulations, which correlate the molecular architecture of the single-source precursor with the structural features of the carbon phase segregating from the silica matrix. Furthermore, the role of hydrogen in the stability of the prepared nanocomposites as well as in the high-temperature evolution and morphology of the segregated MWCNTs has been discussed based on vibrational spectroscopy, calorimetric studies and empirical potential calculations. The results obtained within the present study may allow for designing highly-defined nanocarbon-containing composites with tailored structural features and property profiles.
RESUMEN
We investigate 29Si nuclear magnetic resonance (NMR) chemical shifts, δiso, of silicon nitride. Our goal is to relate the local structure to the NMR signal and, thus, provide the means to extract more information from the experimental 29Si NMR spectra in this family of compounds. We apply structural modeling and the gauge-included projector augmented wave (GIPAW) method within density functional theory (DFT) calculations. Our models comprise known and hypothetical crystalline Si3N4, as well as amorphous Si3N4 structures. We find good agreement with available experimental 29Si NMR data for tetrahedral Si[4] and octahedral Si[6] in crystalline Si3N4, predict the chemical shift of a trigonal-bipyramidal Si[5] to be about -120 ppm, and quantify the impact of Si-N bond lengths on 29Si δiso. We show through computations that experimental 29Si NMR data indicates that silicon dicarbodiimide, Si(NCN)2 exhibits bent Si-N-C units with angles of about 143° in its structure. A detailed investigation of amorphous silicon nitride shows that an observed peak asymmetry relates to the proximity of a fifth N neighbor in non-bonding distance between 2.5 and 2.8 Å to Si. We reveal the impact of both Si-N(H)-Si bond angle and Si-N bond length on 29Si δiso in hydrogenated silicon nitride structure, silicon diimide Si(NH)2.