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The anomalous density-temperature relationship of vitreous silica with low hydroxyl content is explained by the formation of medium-range ordering structure in the glass transition process. The ordered medium-range structure has the shape of a "nanoflake" and consists of two layers of SiO4 tetrahedra, bonded by O atoms located in the middle of the structure. The nanoflakes interact with their surrounding structures through both covalent chemical bonds and van der Waals bonds. In the formation of the van der Waals bonds, the orientation of SiO4 tetrahedra can change, which results in an increase in distance between the nanoflakes and their surrounding structures. Thus, there is a slight volume enlargement associated with the formation of nanoflakes. Since the nanoflakes' formation starts at a temperature near 1480 °C, and the population of the nanoflakes grows continuously as temperature decreases until about 950 °C, the bulk volume of silica glass increases in the temperature range from about 1480 °C to 950 °C. Therefore, the density anomaly of silica glass can be explained as a byproduct of forming of medium-range ordering structure in the silica glass transition.

Although the densities of crystal quartz and vitreous silica differ only by about 17% (2.65 and 2.20 g/cm3, respectively), the helium permeability of silica glass is six orders more elevated than that of crystal quartz. This vast difference has puzzled researchers for decades considering that silica glass and quartz crystal have the same chemical composition. This work discusses the mechanism of high helium permeation through silica glass. It briefly reviews the experimental data and its contradictions with the continuous random network theory. A recently proposed nanoflake model for silica glass structure is utilized to explain the origin of glass permeation to helium. According to the nanoflake model, the formation of nanoflakes not only brings a one-dimensional medium-range ordering structure into silica glass but simultaneously creates regions where van der Waals bonds replace the oxygen-silicon covalent bonds. It is the weakness of van der Waals bonds that causes the helium mobility in these areas to increase. [ABSTRACT FROM AUTHOR]

We examined the influence of interfacial Van der Waals vis-à-vis covalent bonds on the crystallographic and physical properties of reduced-graphene-oxide/BiFeO 3 nanocomposites. The crystallographic, magnetic, and ferroelectric properties have been measured and it was found that the presence of Van der Waals bonds have rather moderate influence on the structural properties such as distortion, bond lengths and angles, and structural noncentrosymmetry. However, in sharp contrast to the observation made in the case of the nanocomposites containing covalent bonds, presence of Van der Waals bonds results in dramatic decrease in the remanent ferroelectric polarization determined from direct electrical measurements. Van der Waals bonds appear to have changed the characteristics of the BiFeO 3 -graphene layer interfaces due to either large carrier mobility of the graphene layer and/or generation of strong dead layer. Enhanced functionalities, therefore, could be possible only for covalent bonded oxide-graphene interfaces. • Reduced-graphene-oxide/BiFeO 3 nanocmposites have been synthesized by different solution chemistry routes to yield samples containing either covalent or van der Waals bonds at the interface. • The presence or absence of the covalent bonds at the interface was examined by high resolution x-ray photoelectron spectroscopy. • The structural properties such as bond lengths, angles, distortion,noncentrosymmetry do not appear to depend dramatically on the types of bonds at the interface – covalent or van der Waals. • However, the magnetic and ferroelectric properties exhibit remarkable changes – spontaneous exchange bias switches from negative to positive and remanent polarization drops dramatically in the sample containing van der Waals bonds at the interface. • This work, highlights the utility of the types of bonds-covalent or van der Waals; in enhancing the functionalities of rGO/BiFeO 3 nano-composites. [ABSTRACT FROM AUTHOR]

By means of DFT calculations, we have individuated a minimum-energy path connecting two energy minima of clean graphene on clean and relaxed oxygen-terminated (0001)'SiO [ABSTRACT FROM AUTHOR]

Cementing mechanism of bio-phosphate cement was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), thermogravimetric-differential scanning calorimetry (TG-DSC) analysis and scanning electron microscopy (SEM). The results of FTIR and XPS show that Si-O bond and Si(2p) electron binding energy in sandstone cemented via chemical and microbiological methods are changed by the binding effects of barium hydrogen phosphate with quartz sand. Compared with barium hydrogen phosphate precipitated in solution, there were higher decomposition temperatures or melting points in sandstone. The FTIR, XPS, and TG-DSC results indicate that the microbial-induced and chemical precipitation of barium hydrogen phosphate can interact with quartz sand to generate van der Waals bond, which plays a role in the binding function between loose sand particles and barium hydrogen phosphate. SEM results show that barium hydrogen phosphate after chemical precipitation in sandstone has better dispersion than microbiological deposition. Therefore, barium hydrogen phosphate via chemical precipitation did not bind loose sand particles into sandstone. [ABSTRACT FROM AUTHOR]

The vibrational (Raman) spectra of H2O molecules isolated in cavities of beryl, cordierite, bikitaite, natrolite, scolecite, lawsonite, and hemimorphite have been measured in the temperature range of 4–295 K. The influence of van der Waals and hydrogen bonds on the values of frequency, intensity, and half-width of stretching and bending modes of H2O is considered. The spectra of translational vibrations of H2O molecules in crystal cavities are discussed. For the firsts time, the ratio between the frequencies of translation and stretching vibrations of H2O and the dependence of frequencies of bending vibrations on the angle H-O-H in H2O molecule are presented. [ABSTRACT FROM AUTHOR]