Thesis (Ph.D.)--University of Delaware, 2005. / Principal faculty advisor: Darrin J. Pochan, Dept. of Materials Science. Includes bibliographical references.

Chemical strengthening via ion exchange is commonly used in the industry to produce ultra-thin and resistant glasses used for high-tech applications. This process results in the formation of an ion-exchanged region near the glass surface that is under significant compressive stress, which tremendously improves the mechanical strength and the scratch resistance of the glass. The aim of this thesis is to deepen our fundamental understanding of the ion-exchange mechanisms. Since the mechanical strength against brittle fracture of ion-exchanged glasses depends on the compressive stress at the surface and the depth of interdiffusion, special attention was paid to the two main parameters affecting those properties: the parent glass composition and stress relaxation processes.First, the effect of the parent glass composition on interdiffusion properties was analyzed. We focused our study on the effect of the alkali oxide:alumina ratio and the ionic potential (charge:radius) of divalent cations on the interdiffusional properties. An increase of the Al2O3 content in Na+/K+ ion-exchanged silicate glasses leads to a reduction of the depth of interdiffusion layer and an enhancement of the surface compressive stress. These trends are shown to be consistent with the compositional variation of the glass connectivity and hence, the stress relaxation occurring during the ion-exchange. It is also found that the depth of interdiffusion is higher for glasses containing divalent cations with a higher ionic potential such as Mg and Zn. This is attributed to the larger free volume available and the weaker Na-O bonds in those glasses. Second, the structural origin of stress relaxation is investigated. In the Na+/K+ ion exchanged glasses, the K+-foreign cations are introduced in the cages of the Na+-host cations. Due to stress relaxation, the size of the K-O coordination shell in ion-exchanged glasses increases. This increase is achieved by two structural adaptation mechanisms: a contraction of the Na-O, Ca-O and Mg-O coordination shell and both a shortening of the Si-NBO distances and an opening of the Q4 Si-O-Si angles. The greater knowledge developed in this thesis on the effect of parent glass composition on interdiffusion mechanisms and stress relaxation processes provides further insight into practical ways to optimize the mechanical strength of ion-exchanged glasses. / Doctorat en Sciences de l'ingénieur et technologie / info:eu-repo/semantics/nonPublished

The system kaolinite - carbon black consisting of cheap precursors has been investigated, in terms of its potential to form A1₂ O₃ - SiC composites. The carbothermal reduction process of mullite and silica was studied, in detail, in the range 1275° to 1810° C and over different periods, both under sintering as well as hot-pressing conditions. It was established that the reduction of mullite and silica starts around 1450° C, where the rate of reaction is very slow. Until about 1800° C during the reduction of mullite, SiO₂ gets preferentially reduced, thus forming a composite ceramic consisting of SiC and A1₂ O₃ phases. The kinetics of the formation of SiC + A1₂ O₃ were followed in the range 1590° - 1660° C and it was noted that under hot-pressing conditions they follow a contracting cylinder model. The rate of reaction increased with the increase in temperature and followed a parabolic path with time because of the geometry of the hot-pressed specimens at each temperature. This indicated that the gas diffusion in and out of the system along the edges of the cylindrical specimens is the rate controlling step. The activation energy of the reduction process was calculated to be 922 KJ/mole. The application of pressure prior to the carbothermal reduction process seemed to be not favourable for the formation of SiC and A1₂ O₃, however, when applied after the beginning of soaking period, this greatly improved the densities and formation of SiC and A1₂ O₃. The microstructure of the samples was analysed using SEM and TEM. It was found that the grain size of the composite ceramic was of the order of 0.2μm. SiC was present mainly in the form of fine platelets. Finally, the isothermal compaction behaviour of the system was studied under a constant pressure in the temperature range 1200° C - 1800° C, during which the formation and carbothermal reduction of mullite and silica took place. A mathematical model based on the least squares fitting was used to fit the compaction curves. Due to the complex nature of the compaction data an empirical approach was used to interpret the data and a viscoelastic model was developed. It was found that the interactive-double-Kelvin unit having two elastic and two viscous components explained the type of compaction behaviour observed in the kaolinite + C system. One of the viscous components (η₁) and one of the elastic components (M₁) were found to be temperature sensitive. It is concluded that starting from the cheap precursors (kaolinite and carbon black) a particulate composite of A1₂ O₃,-SiC can be produced by hot-pressing technique. SiC-whisker formation is not encountered in this system. The very fine grain size of the paniculate composite, resulting in a small flaw size, should provide the composite ceramic with good mechanical properties. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Thesis (M.Sc. (Chemistry)) -- University of Limpopo, 2015 / This dissertation investigated the properties of ferrierite synthesised using different SiO2 sources under identical conditions. The SiO2 sources used were TEOS, water-glass, Aerosil 200 and Ludox LS-30. The synthesis procedure comprised preparation of a gel with molar composition: 20 Na2O : Al2O3 : 37 pyrrolidine : 66:5 SiO2 : 6:3 H2SO4 : 1460 H2O. This was followed by hydrothermal treatment at 160 oC in a stainless steel autoclave for 72 h. The solid products were characterised by XRD, SEM, NH3-TPD and BET techniques. The ferrierite prepared using sodium silicate was more crystalline than ferrierite zeolites synthesised using Ludox LS-30, Aerosil 200 and tetraethyl orthosilicate as SiO2 sources. An amorphous phase was produced when ferrierite was synthesised using unhydrolysed TEOS as a sole SiO2 source. The physicochemical properties of the materials were not only affected by the nature of the SiO2 source, but also some synthesis manipulations such as acid-hydrolysis and water-glass addition when TEOS was used as a primary silica source. There were improvements in the materials produced when the TEOS was pre-hydrolysed with HCl and also mixed with water-glass in equal proportions. The SEM images of ferrierite materials synthesised using water-glass and Ludox LS-30 were uniform. The water-glass-based materials were thin sheets, flake-like images and Ludox LS-30-based produced thin-plate-like morphologies. The micrograph of ferrierite synthesised using TEOS as the SiO2 source showed hexagonal-type morphology and aggregates of smaller particles. There were two types of shapes in the ferrierite synthesised using Aerosil 200 as the silica source, namely, octagonal prismatic and hexagonal type morphologies. An equimolar mixture of TEOS and water-glass showed octagonal prismatic shape with triangular faces along certain edges of the material. The NH3-TPD acid site distribution profiles showed two peaks of weak acid strength at low temperatures (≤ 350 oC) for the representative H-ferrierite investigated. The ferrierite materials synthesised using unhydrolysed TEOS and Ludox LS-30 as SiO2 sources, showed NH3 desorption peaks at higher temperatures (≥ 350 oC). These peaks correspond to ammonia eluted from strong acid sites. The BET surface area of ferrierite synthesised using water-glass was high, while the material synthesised using unhydrolysed TEOS had the lowest surface area. Novel crystal shapes, comprising octagonal prisms with additional triangular phases, were observed in ferrierite samples prepared by the use of TEOS/water-glass mixture as silica source. The zeolitic materials prepared in this study were tested for the efficiency in the removal of Cu2+ from simulated wastewater, using a batch method. The effects of initial pH, initial concentration, contact time and adsorbent dose on Cu2+ adsorption were studied. All the materials showed maximum metal uptake efficiency at pH 5, and this pH was fixed in further studies involving other variables. It was observed that the metal uptake from aqueous solution increased with contact time and adsorbent dose. The Na-form of ferrierite synthesised using water-glass was the poorest Cu2+ adsorbent with respect to the four variables investigated (pH, contact time, adsorbent dose and initial metal ion concentration). KEY CONCEPTS Acidity, Adsorption, Ferrierite, Morphology, Silica source.

The degradability of wollastonite, montmorillonite and talc in an aqueous solution of acetic acid at different temperatures is main object of this study. Mineral carbonation, i.e. the reaction of calcium and magnesium presented in these three silicates, is a novel and promising approach to carbon dioxide capture and long-term storage. The kinetic of wollastonite, montmorillonite and talc dissolution is studied due to assessment of their efficiency for CCS technologies. The dissolution kinetic is discovered with using measured time dependence of Ca2+ and Mg2+ leached ions concentration.

<p> A number of silicate compounds BaTSiO4 (T = Co, Mg, Zn), Na0.5K0.5GaSiO4 and Sr1-xCaxBeSiO4 (x = 0.0 ~ 0.4) have been characterized by a combination of diffraction techniques. These compounds crystallize with the same (√3XA, C) superstructure of the hexagonal kalsilite (KAlSiO4) structure and belong to the large structural family of stuffed tridymite-derivatives. Their crystal structures have been refined by using powder neutron data (BaTSiO4, T = Mg, Zn), powder X-ray data (Na0.5K0.5 GaSiO4) and single crystal X-ray data (BaCoSiO4 and Sr1-xCaxBeSiO4, x = 0.0 and 0.27). This study shows that these kalsilite-like structures can accommodate cavity and tetrahedral atoms of variable sizes by relatively minor framework distortions and atomic displacements. The formation of the (√3XA, C) superstructure can be correlated with the relative sizes of the tetrahedral and cavity atoms.</p> / Thesis / Master of Science (MSc)

Adsorption of cations by clays has been a subject of investigation for many years. Many proposals have been advanced to explain this process of adsorption. Two proposals have been prominent in recent years. The Marshall "charged lattice" theory attributes adsorption of cations by clays to an electrostatic attration of the clay particles for cations. This charge on the clay particles is attributed to various isomorphic substitutions in the lattice of the clay which result in an unbalanced charge on the particle. This isomorphic substitution can take place either in the silicon-oxygen tetrahedra of the silicate structure or with the aluminum layer of the particle. The Cook "ion-pair" theory make the assumption that the charge on the clay particle is caused by the neutral clay chemisorbing hydroxyl molecules in the compact double layer thus giving the particle its negative charge. Cations are then attracted electrostatically to the diffuse outer layer. Experimental adsorption data is used to evaluate the validity of these two theories. Equations are set up representing mathematically the two mechanisms of adsorption and are tested graphically with the experimental data. These various graphs, in every case, show that the Marshall "charged lattice" mechanism is inconsistent with the experimental data. The Cook theory adequately represents a consistent agreement with this rather complete set of experimental data.

No description available.

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Bibliography: leaves 147-157. / Ph.D.

Assuming a simple force field involving only short range Non-Coulombic molecular energy terms along with P1 symmetry, a variation of the SQLOO model (Boisen and Gibbs, 1993) successfully generates the structure types of both α and β1 quartz along with at least five alternative structure types of silica not yet observed to our knowledge. These structure types are identified by the existence of symmetry elements represented in the optimized atomic coordinates and cell parameters that define a minimizer in the model. A family of minimizers is discovered through the combined use of Monte Carlo simulated annealing followed by quasi-Newton minimization techniques. The results are in contrast with the assertion made by Tse et al. (1992) that new structure types of SiO₂ can only be arrived at by Molecular Dynamic methods. By varying the parameters used in the minimization process, different families of structure types are discovered. Several structure types were found to have high symmetries. These results are in contrast with the findings by Kramer et aL (1991) that the stability of high symmetry structures of silica are stabilized in part by ionicity. The results reported here are for calculations involving Z = 3 and 6 formula units. This strategy may be useful in the prediction of possible high silica zeolite structure types. An examination of the atomic displacement parameters (ADPs) obtained for TO₄ tetrahedra (T = Si, Al) suggest rigid TO bonds are more common in non-framework than in framework silicates. Correlated motion is found among the ADPs that is consistent with TLS rigid body motion. For these data, the translational motion is represented by the ADPs of the central T atom while both the librational and translational motion is contained in those of the surrounding O atoms. The libration angle for rigid tetrahedra is linearly dependent on the difference between the isotropic equivalent displacement parameter of the T and O atoms, B(T) and B(O), respectively. The value of B(O) is on average twice that of B(T) with a maximum value of ~ 2.0Ų. Variations in the SiO bond lengths of rigid tetrahedra in the silica polymorphs is related only to f_s(O). Rigid TO and OO bonds are a necessary but not sufficient condition for rigid body motion. Nonrigid tetrahedra may represent crystals containing disorder or problems with the refinement. The computer program EXCALIBR (Bloss and Riess, 1973; Bloss, 1981, p. 202) has been rewritten and markedly improved. Like EXCALIBR, EXCALIBR II solves optical extinction data, as determined with a spindle stage, and determines the optic axial angle 2V and the orientation of the crystal’s optical indicatrix. EXCALIBR II uses a modification to Joel’s equation as a means of obtaining the optic axes of a crystal. Furthermore, EXCALIBR II successfully solves extinction data where one optic axis of a biaxial crystal is 90° to the spindle axis, an orientation that had thwarted its predecessor. EXCALIBR II also accurately determines the optical indicatrix orientation for uniaxial crystals. After solving extinction data for several different wavelengths and/or temperatures, EXCALIBR II calculates the angular change of each optic direction with wavelength and/or temperature along with the error on the angle. Using a simple t-test, it then computes a p-value to aid in the decision as to whether the optical direction truly exhibits dispersion. This is a more valid and sensitive procedure than the χ² test used by EXCALIBR, particularly because the covariance in each optic vector’s coefficients are taken into consideration and the results are invariant to the vector’s orientation. / Ph. D.