Molecular dynamics simulations and experimental measurements were used to investigate the thermal and mechanical properties of cross-linked phenolic resins as a function of the degree of ...cross-linking, the chain motif (ortho–ortho versus ortho–para), and the chain length. The chain motif influenced the type (interchain or intrachain) as well as the amount of hydrogen bonding. Ortho–ortho chains favored internal hydrogen bonding whereas ortho–para favored hydrogen bonding between chains. Un-cross-linked ortho–para systems formed percolating 3D networks of hydrogen bonds, behaving effectively as “hydrogen gels”. This resulted in differing thermal and mechanical properties for these systems. As cross-linking increased, the chain motif, chain length, and hydrogen bonding networks became less important. Elastic moduli, thermal conductivity, and glass transition temperatures were characterized as a function of cross-linking and temperature. Both our own experimental data and literature values were used to validate our simulation results.
The Li+–BF4 – and BF4 ––BF4 – interactions are studied using second order perturbation theory (MP2) and coupled cluster singles and doubles approach, including the effect of connected triples, ...CCSD(T). The MP2 and CCSD(T) results are in excellent agreement. Using only the MP2 approach, the interactions of Li+ with bis(trifluoromethane)sulfonimide anion (TFSI) and Li+ with bis(fluorosulfonyl)imide anion (FSI) are studied. The results of these high level calculations are compared with density functional theory (DFT) calculations for a variety of functionals and with the APPLE&P force field. The B3LYP approach well reproduces the accurate calculations using both a small and large basis set. The M06 and M06L functionals in the larger basis set are in good agreement with the high level calculations. While the APPLE&P force field does not outperform the best functionals, the APPLE&P results agree better with the accurate results than do some of the functionals tested.
► Simulations quantify the rolling/sliding behavior of carbon nano-onions during friction. ► Interfacial bond formations force particles to slide as contact pressures increase. ► The diamond core ...within nano-onions leads to sliding at all pressures. ► Transition from rolling to sliding increases friction coefficient by order of magnitude.
Carbon nano-onions have been shown to provide exceptional friction and wear reduction through a rolling and sliding mechanism of the nano-onions at the interface, but the influence of this behavior on the their tribological properties is not well documented. Understanding the lubrication mechanisms of such nanostructured materials aids in determining their frictional properties and promotes the use of these materials in tribological applications. Here, we characterize the mechanisms of rolling and sliding through which carbon nano-onions provide low coefficients of friction by performing atomistic molecular dynamics simulations of carbon nano-onions sliding between diamond-like carbon substrates. The results indicate that the ability of the nano-onions to roll is inhibited both by increased contact pressure and the presence of a diamond core within the nanoparticles that enhances the formation of interfacial bonds during friction. The transition from rolling to sliding behavior is accompanied by a significant increase in the coefficient of friction.
Interactions between pre-cured phenolic polymer chains and a solvent have a significant impact on the structure and properties of the final postcured phenolic resin. Developing an understanding of ...the nature of these interactions is important and will aid in the selection of the proper solvent that will lead to the desired final product. Here, we investigate the role of the phenolic chain structure and the solvent type on the overall solvation performance of the system through molecular dynamics simulations. Two types of solvents are considered: ethylene glycol (EGL) and H2O. In addition, three phenolic chain structures are considered, including two novolac-type chains with either an ortho–ortho (OON) or an ortho–para (OPN) backbone network and a resole-type (RES) chain with an ortho–ortho network. Each system is characterized through a structural analysis of the solvation shell and the hydrogen-bonding environment as well as through a quantification of the solvation free energy along with partitioned interaction energies between specific molecular species. The combination of simulations and the analyses indicate that EGL provides a higher solvation free energy than H2O due to more energetically favorable hydrophilic interactions as well as favorable hydrophobic interactions between CH element groups. In addition, the phenolic chain structure significantly affects the solvation performance, with OON having limited intermolecular hydrogen-bond formations, while OPN and RES interact more favorably with the solvent molecules. The results suggest that a resole-type phenolic chain with an ortho–para network should have the best solvation performance in EGL, H2O, and other similar solvents.
Ab initio techniques are used to study the interaction of ethylene glycol and water with a phenolic polymer. The water bonds more strongly with the phenolic OH than with the ring. The phenolic OH ...groups can form hydrogen bonds between themselves. For more than one water molecule, there is a competition between water–water and water–phenolic interactions. Ethylene glycol shows the same effects as those of water, but the potential energy surface is further complicated by CH2–phenolic interactions, different conformers of ethylene glycol, and two OH groups on each molecule. Thus, the ethylene glycol–phenolic potential is more complicated than the water–phenolic potential. The results of the ab initio calculations are compared to those obtained using a force field. These calibration studies show that the water system is easier to describe than the ethylene glycol system. The calibration studies confirm the reliability of force fields used in our companion molecular dynamics study of a phenolic polymer in water and ethylene solutions.
A new model is proposed for the structure and properties of porous metal oxide scales (aka Chalk River Unidentified Deposits (CRUD)) observed on the nuclear fuel rod cladding in Pressurized Water ...Reactors (PWR). The model is based on the thermodynamically-driven expansion of agglomerated octahedral nickel ferrite particles in response to pH and temperature changes in the CRUD. The model predicts that porous nickel ferrite with internal {111} surfaces is a thermodynamically stable structure under PWR conditions even when the free energy of formation of bulk nickel ferrite is positive. This explains the pervasive presence of nickel ferrite in CRUD, observed CRUD microstructures, why CRUD maintains its porosity, and variations in porosity within the CRUD observed experimentally. This model is a stark departure from decades of conventional wisdom and detailed theoretical analysis of CRUD chemistry, and defines new research directions for model validation, and for understanding and ultimately controlling CRUD formation.
As technologies progress, the development of new mechanical systems demands the rapid determination of friction coefficients of materials. Data mining and materials informatics methods are used here ...to generate a predictive model that enables efficient high-throughput screening of ceramic materials, some of which are candidate high-temperature, solid-state lubricants. Through the combination of principal component analysis and recursive partitioning using a small dataset comprised of intrinsic material properties, we develop a decision tree-based model comprised of if-then rules which estimates the friction coefficients of a wide range of materials. This data-driven model has a high degree of accuracy with an
R
2
value of 0.8904 and provides a range of possible friction coefficients that accounts for the possible variability of a material’s actual friction coefficient.
The chemical modification of amorphous polystyrene (PS) by the deposition of atomic S, SC, and SH with 25, 50, and 100 eV of incident kinetic energy is examined using classical molecular dynamics ...simulations. The forces are determined using the second-generation reactive empirical bond-order (REBO) potential that has been extended to include sulfur. In all cases, the S atoms or S-containing dimers are deposited randomly on the PS surface with a flux of about 0.4 × 1024 ions/(cm2 s), which is comparable to experimental values. The simulations predict the way in which the depth profiles vary as a function of the identity and kinetic energy of the incident atom or dimer. We also quantify the ways in which the surface is chemically modified and provide a profile of the chemical products formed on the surface, within the substrate, or in the material sputtered from the surface. The simulations predict that the maximum density of deposited atoms throughout the surface substrate, 3.32 × 1018 /cm3, occurs for S deposition with 50 eV of incident energy. We further predict that the highest molecular weight products are formed as a result of S deposition with 100 eV of energy. Additionally, the chemical reactions that occur during the deposition are found to depend on the beam energy for all the incident atoms or dimers considered. Negligible change in the surface roughness is predicted to occur as a result of these deposition processes.
Pyrophyllite, Al
2
Si
4
O
10
(OH)
2
, a layered aluminosilicate material, has several properties which suggest that it might exhibit desirable frictional properties for solid-state lubrication. These ...include a lamellar structure and a low-energy basal plane. Here, we utilize atomic force microscopy to investigate the tribological performance of mineralogical pyrophyllite samples and compare the results to those of other minerals and solid lubricants. The results indicate that pyrophyllite exhibits low friction forces and shear stresses, on the same order of other commonly used solid lubricant materials, and a high resistance to deformation within a single layer. These fundamental findings highlight the potential use of pyrophyllite as a possible tribological material and alternative solid lubricant.
Interactions between pre-cured phenolic polymer chains and a solvent have a significant impact on the structure and properties of the final post-cured phenolic resin. Developing an understanding of ...the nature of these interactions is important and will aid in the selection of the proper solvent that will lead to the desired final product. Here, we investigate the role of phenolic chain structure and solvent type on the overall solvation performance of the system through molecular dynamics simulations. Two types of solvents are considered, ethylene glycol (EGL) and H
2
O. In addition, three phenolic chain structures were considered including two novolac-type chains with either an ortho-ortho (OON) or ortho-para (OPN) backbone network and a resole-type (RES) chain with an ortho-ortho network. Each system is characterized through structural analysis of the solvation shell and hydrogen bonding environment as well as through quantification of the solvation free energy along with partitioned interaction energies between specific molecular species. The combination of the simulations and analyses indicate that EGL provides a larger solvation free energy than H
2
O due to more energetically favorable hydrophilic interactions as well as favorable hydrophobic interactions between CH element groups. In addition, phenolic chain structure significantly impacts solvation performance with OON having limited intermolecular hydrogen bond formations while OPN and RES interact more favorably with the solvent molecules. The results suggest that a resole-type phenolic chain with an ortho-para network should have the best solvation performance in EGL, H
2
O, and other similar solvents.
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