Silica dynamic properties

Zhong, A., Rong, C., and Liu, S. 2007. Structural and dynamic properties of (Si02)6 silica nanostructures A quantum molecular dynamics study. J. Phys. Chem. A 111 3132-3136. 

A very large number of theoretical studies have been performed on MgO and AI2O3. Only some of the early studies and some of the most recent will be described here, in order to give some idea of the extent of progress over the past two decades. Important advances have recently been made in the application of ionic models to such materials as well as in band-theory studies and embedded-cluster studies. After reviewing the early work, contemporary studies of structure, stability, phase relations, and dynamic properties will be described, followed by recent studies of spectral properties and characteristics of the electron-density distribution for each of these materials. Attention is then turned to Si02, the silica polymorphs, and various compounds and clusters that may be used to model tetrahedrally coordinated Si in silica and the silicates. 

In many studies it is presumed that linear viscoelastic behaviour always occurs, but this is not the case for many reactive systems. Conventional experimental rheology utilizes a dynamic strain sweep, which examines the dynamic rheological response to varied strain amplitudes, at a fixed frequency. If the system shows an effect of strain amplimde on dynamic properties (such as G or G") the system is said to be exhibiting a non-linear (viscoelastic) response. If the properties are independent of strain amplitude, then the system is said to be exhibiting linear viscoelastic behaviour. Figure 4.2 shows the response of an industrial epoxy-resin moulding compound (approximately 70 wt.% silica) at 90 °C at strain amplitudes of 0.1% to 10% for frequencies of 1, 10 and lOOrad/s. 

Filler dispersion is a property that determines how well the filler partciles in a given rubber compound are dispersed as a result of the mixing process. This relates to carbon black dispersion as well as the dispersion of nonblack fillers such as silica, clay, calcium carbonate, titanium dioxide, etc. Also rubber curatives such as sulfur and accelerators can be poorly dispersed (commonly these ingredients are added late in the mixing cycle). Poor dispersion makes a mixed stock less uniform, and commonly the cured ultimate tensile strength will have more variability. Poor dispersion can affect other important cured physical properties such as abrasion, tear, and fatigue resistance, flexometer heat buildup, and other dynamic properties. 

To obtain an accurate estimate of thermodynamic properties for crystalline silica polymorphs, one needs an accurate description of the phonon density of states. Given the complexity of the problem, this is tractable only with several assumptions. For example, it is often assumed that the calculated phonon spectra are not strongly dependent on temperature. Certainly this will be a satisfactory assumption in the absence of any thermal expansion, and any changes in the interatomic potentials as a function of temperature. In this case, the force constants, and consequently the dynamical properties like phonon frequencies and density of states, will be independent of temperature. 

Applications of a molecular dynamics simulations to investigations of the microstructure and dynamic properties of nanoporous silica gels yielded densities down to 0.7 g ml and mass fractal dimensions of 2.6 or higher [84]. The specific surfaces that... 

The properties of silica-filled rubber and composites depends primarily on the association of individual primary silica particles in the final material. The association of primary particles is believed to be a reversible process responsible for the physical properties of the filled material at low strain levels. This process is furthermore believed to an essential mechanism explaining the improvement of dynamic properties of filled rubber, as opposed to carbon black, silica particles are characterized by a strongly polar surface able to generate a strong interaction. The interaction is reversible and leads to reduced hysteresis. 

The curing and dynamic properties of precipitated nano-silica on NR without and with the sulfur addition (NR with S), synthetic polyisoprene (IR), polybutadiene (BR) and SBR was investigated. Silica was treated with bis(3-triethoxysilylpropyl)tetrasulfane (TESPT) to form bonds at interfaces. Cure, Mooney viscosity, glass transition temperature, bound rubber, crosslink density and DMA were measured. The properties of silica-filled SBR and BR correlated with highest rolling resistance and SBR-silica correlated with best skid resistance. A Payne effect was observed in the loss modulus under some experimental conditions. In addition to possible filler de-agglomeration and network disruption, the nanoscale of the filler may have further contributed to the non-linear response typified by the Payne effect. ... 

Brus, J., Spirkova, M., Hlavata, D., and Strachota, A. 2004. Self-organization, structure, dynamic properties, and surface morphology of silica/epoxy films as seen by solid-state NMR, S AXS, and AFM. Macromolecules 37 1346-1357. 

Bemasconi, A., Sleator, T., Posselt, D., Kjems, J.K. and Ott, H.R., Dynamic properties of silica aerogels as deduced from speciflc-heat and thermal-conductivity measurements. Physical Review B, 1992. 45(18) p. 10363-10376. 

The presence of an LLCP and extrema lines in the supercritical region not only affects the thermodynamic properties of the liquid but also affects its dynamics. Recent computer simulations, based on atomistic [41], silica [39], and different water models [41], show that there is an intimate relationship between the C p line and the dynamic properties of the liquids, LDL and HDL. Specifically, it is found that in the more ordered liquid (i.e., the liquid with less entropy), the temperature dependence of the diffusion coefficient at constant pressure is given by D(T) exp(-EAT) (where Ep, is a constant), indicating that such a liquid is Arrhenius [51]. Instead, the less ordered liquid is found to be non-Arrhenius. Interestingly, in the supercritical region of the P-T plane, the dynamics of the fluid... 

With regard to dynamical properties, the behavior of silica and water models has also been shown to be in correspondence. A useful concept for studying the relationship between thermodynamics and dynamics, and testing the universality... 

For pure silica, the BKS potential model [5] was used to calculate the material properties from Equation 3.4.1. This model has been employed and it is easy to find some references in the literature. This allows to have a reference system for the calculation of the dynamic properties of the liquid silica [18, 19]. From the ab initio calculations from van Beest et al. [5], data for pure silica are given in the Table 3.4.1 with the calculated last term of Equation 3.4.2. 

Silanes as a group are essential for precipitated hydrated silica to impart better dynamic properties, such as lower tire rolling resistance, to a rubber compound. 

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