Cordierite-based refractory ceramics from natural halloysite and peridotite: Insights on technological properties

Cordierite-based ceramics were fabricated from Moroccan natural halloysite clay by using a simple and low-cost manufacturing method. To this end, peridotite and halloysite samples, collected from Beni Bousera and Melilla sites, Morocco, were used as raw materials for ceramics manufacturing. A starting mixture was prepared (76.08 wt% of clay and 23.92 wt% of peridotite), molded and heated to the desired temperature (1250, 1300 and 1350 °C) to fabricate cordierite ceramic specimens. Both raw materials (peridotite and halloysite) and final ceramics were analyzed using routine characterization techniques including chemical analysis by XRF, mineralogical analysis by XRD, thermogravimetric analysis, and morphological characteristics using scanning electron microscopy (SEM). The prepared ceramics were investigated regarding their mineralogical composition, thermal and technological properties, chemical resistance, and microstructural characteristics. Our results indicated that peridotite sample is mainly composed of silica (40.25 wt%) and magnesia (38.05 wt%) while halloysite is consisted essentially of silica (38.00 wt%) and alumina (34.13 wt%). This was confirmed by XRD, TG-DTA and FTIR analyses. The prepared ceramic specimens at different sintering temperatures (i.e., 1250, 1300 and 1350 °C) have regular cylindrical forms, displaying good ceramic properties. This is consolidated with the main technological tests including porosity (4.56–3.11%), bulk density (2.45–2.78 g/cm³), shrinkage (6.51–10.31%), indirect tensile strength (20.35–27.60 MPa), and low linear thermal expansion coefficient (3.05–2.18 × 10^?6/°C). Cordierite specimen prepared at 1350 °C provided the best ceramic sample with the highest technological properties, good chemical resistance and thermal properties. Thus, naturally abundant halloysite and peridotite deposits are potential candidates for cordierite-based ceramic manufacture. Therefore, the achieved results have provided cost-effective ceramic bricks with physical, thermal and mechanical properties that are favorable to be used as refractory bricks.     

Structural relaxation in dense hard-sphere fluids

The long-time decay of the shear-stress autocorrelation function is shown to be quantitatively related to the decay of correlations between the orientation of bonds connecting colliding pairs of particles. Within computational uncertainties, we find that orientational correlations in high-density fluids decay as a stretched exponential in time, with an exponent that is independent of density. However, at low densities the decay is exponential. In two-dimensional systems the decay is exponential, even at high density.     

Friction and nanowear of polystyrene against hydrophobic and hydrophilic substrates

The ambition of this study is to analyze the role of interfacial interactions in friction and nanowear of polystyrene, by comparing friction against hydrophobic wafers (methyl‐terminated) and hydrophilic wafers (hydroxyl‐terminated) as a function of sliding velocity and normal force. Friction experiments are performed with a translation tribometer and nanowear investigation is achieved by using atomic force microscopy (AFM) analysis of the wafer surfaces after friction. Experimental results show that the friction coefficients measured on hydrophilic surfaces are always larger than those obtained with hydrophobic surfaces, indicating a relationship between friction and interfacial interactions. Elsewhere, AFM analysis shows that polystyrene transfer appears for a higher normal force in the case of hydrophobic substrates compared to hydrophilic one. However, the corresponding tangential (or friction) force necessary to detect transfer is quite similar for both types of substrates, indicating that the initial wear of polystyrene occurs for a similar threshold interfacial shear. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2449–2454, 2006     

Coordination properties of new bis(1,4,7-triazacyclononane) ligands: a highly active dizinc complex in phosphate diester hydrolysis

The synthesis and characterization of three new bis([9]aneN(3)) ligands, containing respectively 2,2'-bipyridine (L(1)), 1,10-phenanthroline (L(2)), and quinoxaline (L(3)) moieties linking the two macrocyclic units, are reported. Proton binding and Cu(II), Zn(II), Cd(II), and Pb(II) coordination with L(1)-L(3) have been studied by potentiometric titrations and, for L(1) and L(2), by spectrophotometric UV-vis measurements in aqueous solutions. All ligands can give stable mono- and dinuclear complexes. In the case of L(1), trinuclear Cu(II) complexes are also formed. The stability constants and structural features of the formed complexes are strongly affected by the different architecture and binding properties of the spacers bridging the two [9]aneN(3) units. In the case of the L(1) and L(2) mononuclear complexes, the metal is coordinated by the three donors of one [9]aneN(3) moiety; in the [ML(2)](2+) complexes, however, the phenanthroline nitrogens are also involved in metal binding. Finally, in the [ML(3)](2+) complexes both macrocyclic units, at a short distance from each other, can be involved in metal coordination, giving rise to sandwich complexes. In the binuclear complexes each metal ion is generally coordinated by one [9]aneN(3) unit. In L(1), however, the dipyridine nitrogens can also act as a potential binding site for metals. The dinuclear complexes show a marked tendency to form mono-, di-, and, in some cases, trihydroxo species in aqueous solutions. The resulting M-OH functions may behave as nucleophiles in hydrolytic reactions. The hydrolysis rate of bis(p-nitrophenyl)phosphate (BNPP) was measured in aqueous solution at 308.1 K in the presence of the L(2) and L(3) dinuclear Zn(II) complexes. Both the L(2) complexes [Zn(2)L(2)(OH)(2)](2+) and [Zn(2)L(2)(OH)(3)](+) and the L(3) complex [Zn(2)L(3)(OH)(3)](+) promote BNPP hydrolysis. The [Zn(2)L(3)(OH)(3)](+) complex is ca. 2 orders of magnitude more active than the L(2) complexes, due both to the short distance between the metal centers in [Zn(2)L(3)(OH)(3)](+), which could allow a bridging interaction of the phosphate ester, and to the simultaneous presence of single-metal bound nucleophilic Zn-OH functions. These structural features are substantially corroborated by semiempirical PM3 calculations carried out on the mono-, di-, and trihydroxo species of the L(3) dizinc complex.     

Stochastic Event-Driven Molecular Dynamics

A novel Stochastic Event-Driven Molecular Dynamics (SEDMD) algorithm is developed for the simulation of polymer chains suspended in a solvent. SEDMD combines event-driven molecular dynamics (EDMD) with the Direct Simulation Monte Carlo (DSMC) method. The polymers are represented as chains of hard-spheres tethered by square wells and interact with the solvent particles with hard-core potentials. The algorithm uses EDMD for the simulation of the polymer chain and the interactions between the chain beads and the surrounding solvent particles. The interactions between the solvent particles themselves are not treated deterministically as in EDMD, rather, the momentum and energy exchange in the solvent is determined stochastically using DSMC. The coupling between the solvent and the solute is consistently represented at the particle level retaining hydrodynamic interactions and thermodynamic fluctuations. However, unlike full MD simulations of both the solvent and the solute, in SEDMD the spatial structure of the solvent is ignored. The SEDMD algorithm is described in detail and applied to the study of the dynamics of a polymer chain tethered to a hard-wall subjected to uniform shear. SEDMD closely reproduces results obtained using traditional EDMD simulations with two orders of magnitude greater efficiency. Results question the existence of periodic (cycling) motion of the polymer chain.     

Decay of angular correlations in hard-sphere fluids

The decay of the collisional contribution to the shear-stress autocorrelation function is shown to be inconsistent with at ^–3/2 inverse-power law. The decay of the self part (a combination of pair and triplet correlations) indicates a stretched-exponential decay with a density-independent exponent. The pair contribution by itself also shows stretched-exponential behavior in both two and three dimensions, with different, but still density-independent, exponents. At very long times this stretched-exponential decay of the pair correlations switches over to an algebraic decay, consistent with the diffusional separation of pairs of particles.     

A hydrodynamically correct thermal lattice Boltzmann model

A three-dimensional lattice-Boltzmann model which yields correct hydrodynamics at the Navier-Stokes level of the Chapman-Enskog expansion requires a minimum of 26 velocities. We present results for a model with one additional velocity, determined by maximizing the equilibrium entropy. For compressible Rayleigh-Bénard convection the model is more accurate but considerably less stable, than a previous, approximate 21-speed model.     

T‐optimality and neural networks: a comparison of approaches for building experimental designs

This paper deals with optimal experimental design criteria and neural networks in the aim of building experimental designs from observational data. It addresses the following three main issues: (i) the introduction of two radically different approaches, namely T‐optimal designs extended to Generalized Linear Models and Evolutionary Neural Networks Design; (ii) the proposal of two algorithms, based on model selection procedures, to exploit the information of already collected data; and (iii) the comparison of the suggested methods and corresponding algorithms by means of a simulated case study in the technological field. Results are compared by considering elements of the proposed algorithms, in terms of models and experimental design strategies. In particular, we highlight the algorithmic features, the performances of the approaches, the optimal solutions and the optimal levels of variables involved in a simulated foaming process. The optimal solutions obtained by the two proposed algorithms are very similar, nevertheless, the differences between the paths followed by the two algorithms to reach optimal values are substantial, as detailed step‐by‐step in the discussion. Copyright © 2012 John Wiley & Sons, Ltd.     

Prospects for release-node quantum Monte Carlo

We perform release-node quantum Monte Carlo simulations on the first row diatomic molecules in order to assess how accurately their ground-state energies can be obtained. An analysis of the fermion-boson energy difference is shown to be strongly dependent on the nuclear charge, Z, which in turn determines the growth of variance of the release-node energy. It is possible to use maximum entropy analysis to extrapolate to ground-state energies only for the low Z elements. For the higher Z dimers beyond boron, the error growth is too large to allow accurate data for long enough imaginary times. Within the limit of our statistics we were able to estimate, in atomic units, the ground-state energy of Li(2) (-14.9947(1)), Be(2) (-29.3367(7)), and B(2)(-49.410(2)).