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The structure and properties of polysiloxane dendrimer melts are studied by extensive atomistic molecular dynamics simulations. Two homologous series differing in the spacer length are considered. In the first series the dendrimer spacers are the shortest ones, comprising only one oxygen atom, while in the second series the spacers consist of two oxygen atoms with the silicon atom in between. Melts of the dendrimers from the 3rd up to the 6th generation number are modelled in a wide temperature range from 273 to 600 K. A comparative study of the macroscopic melt characteristics such as the melt density and thermal expansion coefficients is performed for the two series. Analysis of the dendrimer structure in melts and in the isolated state shows that intermolecular interactions and interpenetration of dendrimer molecules in melts hardly affect the dendrimer interior organization. However, the presence of neighboring molecules significantly slows down their intramolecular dynamics in melts in comparison with that of isolated dendrimers. An increasing generation number causes an increase of the radius of the dendrimer interior region unavailable for neighboring molecules, which starts to exceed the length of the peripheral interpenetration layer for high-generation dendrimers; this fact could lead to different mechanisms of melt dynamics for lower and higher generation dendrimers.

Characteristics of instantaneous (I), vibration-average (V), and frozen (F) structures of liquid ethylene glycol (EG), monoethanolamine (MEA), and ethylenediamine (ED) are obtained by means of molecular dynamics in the temperature range of 273–453 K. Structures are described by plotting Voronoi polyhedra and Delaunay simplexes. The distributions of volumes of Voronoi polyhedra and the radii of the spheres of Delaunay simplexes were obtained in the temperature range of liquid phase EG, MEA, and ED. A comparative analysis of these characteristics of three studied liquids is performed with different averaging over time and space. It is shown that describing the structure of liquids according to Voronoi and Delaunay allows us to compare the characteristics of spatial networks of hydrogen bonds in them very clearly.

A simplified model of the in-plane molecular chain, allowing the description of folded and scrolled packings of molecular nanoribbons of different structures, is proposed. Using this model, possible steady states of single-layer nanoribbons scrolls of graphene, graphane, fluorographene, and fluorographane (graphene hydrogenated on the one side and fluorinated on the other side) are obtained. Their stability is demonstrated and their energy is calculated as a function of the nanoribbon length. It is shown that the scrolled packing is the most energetically favorable nanoribbon conformation at long lengths. The existences of scrolled packings for fluorographene nanoribbons and the existence of two different scroll types corresponding to left- and right-hand Archimedean spirals for fluorographane nanoribbons in the chain model are shown for the first time. The simplicity of the proposed model makes it possible to consider the dynamics of scrolls of rather long molecular nanoribbons at long enough time intervals.

This review summarizes the data published over the past two and a half decades on the mechanism of plastic deformation of bulk isotropic linear glassy polymers in uniaxial tension, compression, and shear at low deformation temperatures (Тdef < 0.6Тg) and moderate loading rates. The main attention is focused on studies concerning the numerical computer simulations of plasticity of organic polymer glasses. The plastic behavior of glassy polymers at nano-, micro-, and macrolevels in the range of macroscopic strains up to ≈100% is discussed. Plasticity mechanisms are compared for organic, inorganic, metallic, polymer, and nonpolymer glasses with different chemical structures and types of interparticle interactions. The general common mechanism of plasticity of glassy compounds, the nucleation of plasticity carriers in them, and the structure of such carriers and their dynamics are covered. Within the framework of the common plasticity mechanism, the specific features of deformation in glassy polymers are analyzed. Specifically, the participation of conformational transformations in macromolecules in the deformation response of polymer glasses, change in intensity of the yield peak with the thermomechanical prehistory of the sample, and the role of van der Waals interactions in the accumulation of excess potential energy by the sample under loading are considered. The role of free volume and structural and dynamic heterogeneities in the plasticity of glasses is also discussed.

Carbon nanofibers (CNFs) were grown in situ on porous glass at different temperatures and times using a Ni acetate catalyst and CH4/N2 as a carbon source. The porous glass was obtained by acid leaching of phase separated borosilicate glass, which generates a broad size distribution of mesopores (≈20 nm). Subsequent impregnation with Ni acetate reduces the pore size to ≈ 4 nm but also creates new micropores, thus increasing the surface area. During thermal treatment the surface area decreases as temperature rises, mainly due to shrinkage of the glassy matrix; however new pores are created at ≈ 70 nm (mainly at 600 oC) associated to the generation of CNFs on the glass surface, indicating this temperature offers the best conditions. The CNFs grow inside and fill in the micro-mesopores in the porous glass. They do not grow at 500 oC as the Ni acetate is not transformed into metallic Ni. Ni deactivation occurs at temperatures over 700 oC, thus reducing the formation of CNFs. At 1000 oC the degradation of CH4 leads to a thickening of the CNFs. The thermal degradation of the CNFs occurs in two steps, the first (360-416oC) corresponding to CNFs grown on the glass surface and the second (518-649oC) to CNFs grown inside the glass pores. Treatment times over 2 h lead to the deactivation of Ni, pore shrinkage and hence lower CNF yields.

A molecular mechanical model has been proposed for nanoribbons of graphone (graphene with single-sided hydrogenation), which takes into account deformations of valence bonds, valence and torsion angles, as well as non-bonded van der Waals and Coulomb interactions of atoms in a graphone nanoribbon. The ground states of the nanoribbons have been found using the proposed model. It has been shown that a rectangular fragment of graphone on a substrate formed by an infinite planar graphene sheet forms a flat monolayer, whereas a fragment that does not interact with the substrate takes a convex shape, the outer side of which contains the attached hydrogen atoms. The simulation of the dynamics has demonstrated that the single-sided structure of a graphone sheet is resistant to thermal vibrations (at temperatures T < 900 K, hydrogen atoms do not migrate from one side of the sheet to the other). The difference between the sides leads to a rapid folding of long-length free-standing graphone nanoribbons into scrolled structures. Thermal vibrations do not prevent the formation of scrolls, and the scrolls themselves are resistant to these vibrations.

In order to obtain dense silicon oxycarbide (SiOC) materials that maintain the properties of glass, non-conventional spark plasma sintering was used to sinter SiOC powders from 1300 to 1700 °C and with 40 MPa of pressure. The concurrence of electrical current, high pressure and low vacuum while the material is being heating produces a dense SiOC-derived material composed of a SiO 2 glassy matrix reinforced with SiC nanowires grown in situ , graphene-like carbon and turbostratic graphite. SiOC materials with high electrical and thermal response are obtained as a result of this new processing technique. Electrical resistivity undergoes an extraordinary decrease of five orders of magnitude from 1300 (1.0 × 10 5 Ω m) to 1700 °C (0.78 Ω m), ranging from insulate to semiconductor material; and thermal conductivity increases by 30%, for these sintering temperatures.

The position of a graphene nanoribbon on a solid substrate allows the chemical modification of only one of the nanoribbon sides. A method was proposed that enables the chemical modification of the other side, too. It was numerically modeled how a nanoribbon separated from a substrate rolls up into a roll and how the roll unrolls on a flat substrate. The dependences of the number of coils and the radii of rolls forming by hydrogenation on the nanoribbon length and width were determined.

[En] Cobalt-silicon oxycarbide materials (Co–SiOC) have been produced through the polymer-derived ceramic route by incorporating Cobalt acetate in different proportions. HF etching of the obtained materials leads to obtaining C-rich Co–SiOC materials where the constituent phases present different characteristics. The SiO units that were the most vulnerable to the chemical attack were the most tensioned ones which are connected at the edges of the graphitic planes. The carbonaceous nanostructures in the HF-etched materials present a different ordering degree which is related with the microstructural characteristics of the pyrolyzed precursors. In addition, the electrochemical performance turns out to be dependent not solely on the Co content but also to the graphitization degree of the carbonaceous phase., A. Tamayo acknowledges the financial support from Fundacion General CSIC and the program COMFUTURO. This research has been developed under the frame of the project MAT2016-78700-R from the Spanish Ministry of Economics and Competitiveness. We want to acknowledge Lucia Salvador for her inestimable assistance in the electrochemical measurements.

Hierarchically micro/mesoporous silicon oxycarbide derived carbon (SiOC-DC) materials have been successfully prepared by HF selective removing of silica nano-domains present in SiOC. Dense carbon enriched SiOC materials are obtained by pyrolysis from 1100 to 1400 °C of novel sol-gel hybrids which contain both Si-H and Si-Ph bonds. The pyrolysis temperature deeply affects the etching process, major differences are observed between the sample pyrolyzed at 1100 °C and at higher temperatures. Those differences are directly associated to the evolution of the phase separation of SiOC into SiO2 and SiC, which facilitates the selective extraction of silica nano-domains and the formation of micro/mesoporous SiOC-DC materials with high specific surface areas (SSAs) and high pore volumes. The SiOC-DC sample pyrolyzed at 1200 °C displays very promising results of specific capacitance (90-100 Fg-1 at 40 Ag-1), relatively high energy density (28 W.h.Kg-1) at a low power density (0.10 kW.kg-1), and a significant performance at high power density (10 W.h.Kg-1 at 49 kW.kg-1), taking into account the low potential window of inorganic liquid electrolyte used (0.9 V). We have found that the presence of micropores but even more, the presence of slit-shape mesopores (Dmeso 3-10 nm) is crucial in order to explain the electrochemical response of these materials.

The coarse-grained model of polyethylene and alkanes (the united-atom model, in which each CH2 group is represented by a single bead) was proposed several decades ago. It is widely applied in molecular dynamics simulations. For different tasks, the models with different geometrical and force parameters are used. Until now, it was thought that the coarse-grained model of polyethylene cannot reproduce the orthorhombic crystalline phase, which is typical of this polymer. In the present study, we analyze the simplest coarse-grained model of polyethylene. In this model, the Lennard-Jones potential (6–12) is adopted for van der Waals interactions between the beads of different chains. Of the bonded interactions, only the “valence” bonds between beads and the “bond” and “torsion” angles are taken into account, whereas the cross terms between them are disregarded. We consider the model variation in which the bead (the force center with the mass of a CH2 group) is displaced from the center of the carbon atom and all the interactions, both bonded and nonbonded, are defined by the positions of these beads. For this model, we find the area of geometrical parameters (the displacement value and the van der Waals radius of the bead) in which all the three known crystalline phases of polyethylene are at equilibrium at low temperatures. We choose the force field constants for the model so that its oscillation spectrum reproduces the low-frequency part of the inelastic neutron scattering spectrum of the orthorhombic polyethylene. It proved to be that this choice can be made unambiguously. We compare the dispersion curves in the terahertz range with experimental data on the Raman scattering and infrared spectroscopy, and discuss the advantages and disadvantages of the analyzed simplest coarse model.

Extensive molecular dynamics simulations of polybutylcarbosilane dendrimer melts were performed in a wide temperature range from 300 to 600 K. The melt macroscopic and structural characteristics were analyzed for the third up to the eighth generation dendrimers for the systems consisting of 8 and 27 dendrimer molecules in the simulation box. For every system, averaging was performed over 8 independent simulation runs and along equilibrium time trajectories of up to 5 ns. Calculated values of the thermal expansion coefficients, heat capacity, and self-diffusion coefficients are in a good agreement with experimental observations. Analysis of the molecular mass dependence of the gyration radius and shape factor, detailed radial density distributions of dendrimer structural units, mobility of the branching points, and intermolecular interaction energy allowed to shed light on the basics of distinction in behavior of low and high generation dendrimer melts and formulate the directions of further research.

Highly densified bulk silicon oxycarbide—carbon composites (SiOC-C) reinforced with carbon black (CB), active carbon (AC) or graphite (GR) have been obtained through spark plasma sintering. Structural, mechanical, tribological and thermal properties of SiOC-C composites were researched. The SiOC-C composites reinforced with GR and CB tended to give β-SiC. Wear behavior at room temperature under dry conditions was determined by using a ball on plate configuration and stainless steel balls as counter bodies. The coefficient of friction of the composites decreases down to 40% and the wear rate is also reduced by 1 in the order of magnitude when compared to SiOC. A graphite-like carbon tribolayer is probably formed in the composites; this layer could be responsible for the observed enhancement of such tribological properties. These SiOC-C composites are promising candidates for use in harsh enviroments where an adequate tribological response is required at high temperatures.

The COMPASS force field has been successfully applied in a large number of materials simulations, including the analysis of structural, electrical, thermal, and mechanical properties of carbon nanoparticles. This force field has been parameterized using quantum mechanical data and is based on hundreds of molecules as a training set, but such analysis for graphene sheets was not carried out. The objective of the present study is the verification of how good the COMPASS force field parameters can accurately describe the frequency spectrum of atomic vibrations of graphene, graphane and fluorographene sheets. We showed that the COMPASS force field allows us to describe with good accuracy the frequency spectrum of atomic vibrations of graphane and fluorographene sheets, whose honeycomb hexagonal lattice is formed by sp 3 hybridization. On the other hand, the force field does not describe very well the frequency spectrum of graphene sheet, whose planar hexagonal lattice is formed by sp 2 banding. In that case the frequency spectrum of out-of-plane vibrations differs greatly from the experimental data — bending stiffness of a graphene sheet is strongly over estimated. We present the correction of parameters of out-of-plane and torsional potentials of the force field, which allows achieving the coincidence of vibration frequency with experimental data. After such corrections, the COMPASS force field can be used to describe the dynamics of flat graphene sheets and carbon nanotubes.

Materials used as solar receivers in concentrated solar power technology must withstand severe operational conditions caused by concentrated solar radiation. For this solar technology several ceramic material candidates (silicon carbide, porous and dense silicon oxycarbide) have been subjected to thermal shock resistance test by using Fresnel lens, the equipment available, that concentrates the solar radiation more than 2600 times. Fast heating (37 °C s¿1) and cooling rates (28 °C s¿1) from 100 to 1200 °C and dwelling time of 10 min are employed. The evolution of materials surface has been evaluated during test by spectroscopic methods and both confocal and electronic microscopies. It has been obtained the surface map of each analyzed sample in order to evaluate the effect of the concentrated solar radiation on the surface and the relationship with their durability. The absorptance values were also determined before and after the ageing test using normal direction between 400 and 1100 nm. Concentrated solar radiation facilitates the decomposition of tested materials producing the formation of gaseous species (mainly CO and CO2) and a dense SiO2 layer which is formed over the material surface. SiC and porous silicon oxycarbide materials fail the thermal shock resistance test. SiC experiences a catastrophic break and in the case of porous silicon oxycarbide, gases generated can evolve easily through the pores producing a severe degradation of the material surface. However, dense silicon oxycarbide resists 100 cycles at 1200 °C due to the formation of a protective SiO2 layer over the material surface and its dense microstructure that slow down the diffusion of the gases preventing bulk material from being degraded. The surface studies confirm the formation of a crystalline SiO2 phase all over the surface. Furthermore, the very similar coefficients of thermal expansion of silicon oxycarbide and silica (¿0.4×10¿6 °C), protects material against catastrophic failure. Finally, the absorptance values remain fairly constant before and after thermal shock test (94.78 to 95.96¿96.09%). The high resistance of dense silicon oxycarbide materials to thermal shock under concentrated solar radiation makes these materials suitable candidates of being used as high temperature solar receivers., Authors thank Dr. J. García-Hierro for the technical assistance with temperature measurements. This work was supported by projects ENE2012-39385-CO3-01 of the Ministerio de Economía Industria y Competitividad of Spain, and by the project MAT2016-78700-R financed by Spanish Research Agency and European Regional Development Fund (AEI/FEDER, UE). A. Tamayo also acknowledges to the Fundación General CSIC (Programa ComFuturo) for financial support.

We propose a simplified 2D model of the molecular chain that allows to describe molecular nanoribbon’s scrolled packings of various structures as spiral packaging chain. The model allows to obtain the possible stationary states of single-layer nanoribbons scrolls of graphene, graphane, fluorographene, fluorographane, graphone C\(_4\)H and fluorographone C\(_4\)F. We show the stability of scrolled packings and calculate the dependence of energy, the number of coils, inner and outer radius of the scrolled packing on the nanoribbon length. It is shown that a scrolled packing is the most energetically favorable conformation for nanoribbons of graphene, graphane, fluorographene, and fluorographane at large lengths. A double-scrolled packing when the nanoribbon is symmetrically rolled into a scroll from opposite ends is more advantageous for longer lengths nanoribbons of graphone and fluorographone. We show the possibility of existence of scrolled packings for nanoribbons of fluorographene and existence of two different types of scrolls for nanoribbons of fluorographane. The simplicity of the proposed model allows to consider the dynamics of molecular nanoribbon scrolls of sufficiently large lengths and at sufficiently large time intervals.

Crystalline materials are deformed plastically through crystallographic mechanisms based on the lattice’s periodicity. However, plasticity of disordered solids can’t be described in these terms due to an absence of regular lattices. To find the best way of the description of plastic response for disordered solids (DSs) became a serious challenge for material science and solid-state physics. This paper discusses current views on mechanism of plastic deformation in DSs and touches some problems in the field. It is broadly accepted now that one, common mechanism of plasticity operates in all DSs, independent on their chemical nature and interaction potentials. Such mechanism is dictated by the structural disorder of glasses. Many details of the mechanism are not well understood yet. Important features of the mechanism are discussed in this paper, and several problems, which do not permit the field to develop further successfully are considered.

A comparative analysis of intramolecular dynamics of four types of isolated dendrimers from the fourth to the seventh generations belonging to the siloxane and carbosilane families, differing in spacer length, core functionality, and the type of chemical bonds, has been performed via atomic molecular dynamics simulations. The average radial and angular positions of all Si branching atoms of various topological layers within the dendrimer interior, as well as their variations, have been calculated, and the distributions of the relaxation times of their radial and angular motions have been found. It has been shown that the dendrons of all the dendrimers elongate from the center and decrease in a solid angle with an increasing generation number. The characteristic relaxation times of both angular and radial motions of Si atoms are of the order of a few nanoseconds, and they increase with an increasing generation number and decrease with temperature, with the angular relaxation times being larger than the radial ones. The relaxation times in the carbosilanes are larger than those in the siloxanes. The rotational angle dynamics of the carbosilane dendrimers show that the chain bending is mainly realized via trans-gauche transitions in the Si branching bonds.

Molecular nanoribbons with different chemical structures can form scrolled packings possessing outstanding properties and application perspectives due to their morphology. Here, we propose a simplified two-dimensional model of the molecular chain that allows us to describe the molecular nanoribbon's scrolled packings of various structures as a spiral packaging chain. The model allows us to obtain the possible stationary states of single-layer nanoribbon scrolls of graphene, graphane, fluorographene, fluorographane (graphene hydrogenated on one side and fluorinated on the other side), graphone ${\mathrm{C}}_{4}\mathrm{H}$ (graphene partially hydrogenated on one side), and fluorographone ${\mathrm{C}}_{4}\mathrm{F}$. The obtained states and the states of the scrolls found through all-atomic models coincide with good accuracy. We show the stability of scrolled packings and calculate the dependence of energy, the number of coils, and the inner and outer radius of the scrolled packing on the nanoribbon length. It is shown that a scrolled packing is the most energetically favorable conformation for nanoribbons of graphene, graphane, fluorographene, and fluorographane at large lengths. A double-scrolled packing when the nanoribbon is symmetrically rolled into a scroll from opposite ends is more advantageous for longer length nanoribbons of graphone and fluorographone. We show the possibility of the existence of scrolled packings for nanoribbons of fluorographene and the existence of two different types of scrolls for nanoribbons of fluorographane, which correspond to the left and right Archimedean spirals of the chain model. The simplicity of the proposed model allows us to consider the dynamics of molecular nanoribbon scrolls of sufficiently large lengths and at sufficiently large time intervals.

Molecular dynamics simulations of two types of isolated siloxane dendrimers of various generations (from the 2nd to the 8th) have been performed for temperatures ranging from 150 K to 600 K. The first type of dendrimer molecules has short spacers consisting of a single oxygen atom. In the dendrimers of the second type, spacers are longer and comprised of two oxygen atoms separated by a single silicon atom. A comparative analysis of molecular macroscopic parameters such as the gyration radius and the shape factor as well as atom distributions within dendrimer interior has been performed for varying generation number, temperature, and spacer length. It has been found that the short-spacer dendrimers of the 7th and 8th generations have a stressed central part with elongated bonds and deformed valence angles. Investigation of the time evolution of radial displacements of the terminal Si atoms has shown that a fraction of the Si groups have a reduced mobility. Therefore, rather long time trajectories (of the order of tens of nanoseconds) are required to study dendrimer intramolecular dynamics.

[EN] Highly micro-mesoporous silicon oxycarbide-derived carbon materials (SiOC-DC) with elevated specific surface area up to 895 mg and high pore volumes of mesopores (0.3 cmg) and micropores (0.2 cmg) have been obtained by direct etching of SiOC with HF. Initial organic-inorganic hybrids were synthesized by the sol-gel method employing triethoxysilane and polydimethylsiloxane (PDMS) of different molecular weights (550, 1750 and 4200 gmol). These hybrids were pyrolyzed from 1100 to 1400 °C. Short PDMS chains (550 gmol) within the hybrid structure rendered dense SiOC materials after pyrolysis, however long PDMS chains (1750 and 4200 gmol) produced highly micro-mesoporous SiOC materials up to 1300 °C. The etching of SiOC employing HF produced the selective removing of silica nano-domains and as a result highly micro-mesoporous SiOC-DC materials were created. In dense SiOC the HF attack is favoured at the highest temperatures (1400 °C) when the SiOC is more phase separated. On the other hand, in porous SiOC there is an agreement between the pyrolysis temperature and the porosity and, therefore the SiOC materials are etched easier at 1200 °C. SiOC-DC materials are composed by a porous glassy network enriched with carbon (graphene-like carbon and turbostratic carbon) and SiC which present an incipient crystallization., This work was supported by project MAT2016-78700-R financed bySp anish Research Agency and European Regional Development Fund (AEI/FEDER, UE). The authors are also grateful to C. Díaz Dorado forher help with FE-SEM and HR-TEM images.