Peter F. James

Kinetics of crystal nucleation in silicate glasses

Abstract The kinetics of volume nucleation are analysed in a number of “simple” systems in which the crystallising phase has the same composition as the parent glass. The compositions considered are Li2O·2SiO2, Na2O·2CaO·3SiO2, BaO·2SiO2, 2Na2O·CaO·3SiO2, Na2O·SiO2, 3BaO·5SiO2 and CaO·SiO2. In the first three cases classical homogeneous nucleation theory provides a satisfactory description of both the temperature dependence and magnitude of the nucleation rate if the crystal-liquid interfacial energy σ is temperature dependent. All seven compositions showed striking similarities. The values of TMAX/Tm, where TMAX is the temperature of maximum nucleation and Tm the melting point (or effective melting point) were all in the range 0.54 to 0.59. TMAX was always at, or somewhat above, Tg. Values of Td/Tm where Td is the “just detectable” nucleation temperature (I = 106 m−3 s−1) were 0.62, 0.64 and 0.66 in the first three systems. These represent somewhat higher undercoolings for the onset of homogeneous nucleation than observed in “droplet” nucleation studies of non-silicate systems. However, values of the Turnbull α parameter calculated from the σ results were in general accord with droplet studies for non-metals. Values of W ∗ /kT (W ∗ is thermodynamic barrier to nucleation) in the various systems also showed close similarities being ∼ 30 armatTMAX remarkably consistent pattern of the results suggests that the nucleation observed is predominantly homogeneous. Reasons for the failure to observe homogeneous nucleation in other “simple” compositions are discussed.

Experimental tests of the classical nucleation theory for glasses

Abstract New data for crystal nucleation rates and viscosities were obtained for Li2O·2SiO2 and BaO·2SiO2 glasses. Special efforts were made to minimise impurities in the glasses. Good general agreement with previously published nucleation results was found. The new and previous results were used to test the applicability of classical nucleation theory. For both lithium and barium disilicates the temperature dependence of nucleation rates was satisfactorily described by theory, ln (Iη/T) vs. 1/ΔG2T plots yielding straight lines over a wide range of temperatures (ΔG is the bulk free energy difference per mole between crystal and liquid at temperature T). However, for lithium disilicate, where experimental ΔG data were available, the experimentally determined preexponential factor A in the nucleation equation was much higher than the theoretical value, in agreement with previous studies. This was the case even allowing for reasonable variations in ΔG. A similar result was obtained for barium disilicate using calculated ΔG values. Possible reasons for the discrepancy in A between theory and experiment are discussed including transient nucleation, experimental errors in the nucleation rates and heterogeneous nucleation. None of these effects can account for the observed discrepancy. Other possible explanations are considered.

Glass ceramics: new compositions and uses☆

Abstract Glass ceramics, materials prepared by the controlled crystallisation of glasses, have a variety of established uses dependent on their uniform reproducible fine-grain microstructures, absence of porosity and wide-ranging properties which can be tailored by changes in composition and heat treatment. Moreover, during manufacture complex shapes may be produced using standard glass-forming techniques prior to crystallisation and dimensional changes are small. In recent years, new compositions, processing methods and applications have begun to emerge. Among the recent developments considered are new phosphate-based compositions, the use of sol—gel processing, glass ceramic matrix composites, glass ceramics in microelectronics packaging, glass ceramics bonded to metals and as joining media, high-strength and high-toughness materials and machineable compositions.

Liquid-phase separation in glass-forming systems

This review is concerned with the process of liquid-phase separation in glass-forming systems. In the first part a general account of phase equilibria is presented together with a discussion of the thermodynamic behaviour of systems exhibiting liquid-liquid immiscibility. The estimation of free energies from phase-boundary data and the location of the spinodal boundary are briefly considered. The origin of immiscibility in silicate solutions is discussed from a thermodynamic approach. The importance of association, particularly in silicate systems, is stressed. In the second part of the review, an outline of the theories of homogeneous nucleation and spinodal decomposition is given and a review of recent theoretical developments. The intersecting growth model is discussed and also the laterstage coarsening of both droplet and interconnected structures. The theories are compared with experimental results (including electron microscope and small-angle X-ray scattering data) for various systems. The effects of phase separation on crystallization processes in glasses and on the physical and chemical properties of glasses are outlined. Although the results considered are for oxide systems where sufficient data are available, much of the discussion is applicable to glass-forming systems in general.

The factors affecting the thickness of sol-gel derived silica coatings prepared by dipping

Abstract A partially hydrolysed solution of tetraethoxysilane was used to prepare thin glassy films on glass microscope slides by dipping. The thickness of these films after heat treatment to 500 °C was determined as a function of the withdrawal speed and the properties of the coating solution, such as viscosity, density and surface tension. The results were fitted to available theories of dip coating and acceptable agreement was obtained for coating solution viscosities below around 8 mPas and withdrawal speeds in the range 5 to 15 cm/min. In addition, the thickness of selected coatings were obtained at a range of heat treatment temperatures. Differences in the thickness of coatings prepared after different maturing times of the solution are also discussed.

The gel to glass transition: Chemical and microstructural evolution

Abstract The chemical and microstructural changes occurring in the conversion of a died gel to fully dense glass are reviewed. The main emphasis is on gels prepared from alkoxide precursors, including silica and more complex silicate compositions. The gel to glass conversion in these systems is contrasted with that in colloidal systems. The processes occurring in the conversion are crucially dependent on the composition of the starting solution and the chemistry of the sol to gel transformation. Shrinkage is governed by four processes operating at different stages during the gel to glass transition: capillary contraction, condensation-polymerization, structural relaxation and viscous sintering. A variety of techniques have recently been applied to study the changes in the porous gel as a result of heat treatment, including dilatometry, gas adsorption, DTA, TGA, TEM, infra-red spectroscopy (to monitor OH content, in particular), Raman spectroscopy, resonance techniques and SAXS. The conversion of dried gels into monolithic glass samples using the slow drying and firing method is discussed, including removal of hydroxyl content and prevention of bloating. Other processing routes are also briefly reviewed including hypercritical drying and sintering, the use of drying control additives prior to sintering, and colloidal techniques.

Influence of water content on the rates of crystal nucleation and growth in lithia-silica and soda-lime-silica glasses

Two sets of glasses were studied with compositions close to Li2O·2SiO2 and Na2O·2CaO·3SiO2, and with water contents ranging from 0.019 to 0.136 wt% and 0.007 to 0.040 wt%, respectively. The crystal nucleation and growth rates increased markedly with increase in water content, whereas the viscosities of the glasses decreased. For the lithia glasses, increases in nucleation rates at various temperatures closely corresponded to reductions in viscosity, indicating that the main effect of water was to lower the kinetic barrier to nucleation (ΔGD), rather than to alter the thermodynamic barrier to nucleation (W*). For the soda-lime glasses, ΔGD was also lowered by water content but additional effects due to differences in base compositions were observed. The kinetic barriers to growth were lowered by water content for both sets of glasses, increases in growth rates corresponding closely to reductions in the viscosities. It is suggested that the large effects of water on nucleation and growth may be due to an increase in the oxygen ion diffusion coefficient. In the soda-lime glasses addition of sodium fluoride produced similar effects to the addition of water. Liquidus temperature measurements, and the results of DTA, optical microscopy and electron microscopy are also reported.

Characterization of sol-gel surfaces for biomedical applications

The aim of the present study was to characterize sol-gel derived titania coatings prepared by dipping. The surface characterization was carried out using X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF SIMS), combined with X-ray diffraction (XRD) and thermal analysis. Sol-gel derived titania surfaces mimicked the surface chemistry of the natural oxide layer found on commercial titanium implants. These surfaces could be used to develop an in vitro model of the osseointegration process. Similar analytical techniques were applied to apatite-like coatings and preliminary results suggest that hydroxyapatite coatings can be produced from a sol-gel route.

Silica and silica-titania glasses prepared by the sol-gel process

Clear monolithic samples of silica and silica-titania glasses were prepared by the sol-gel process from alkoxides as starting materials. The effects of the composition of the initial alcoholic solutions on the gelation of the silica materials and the effects of using different titanium compounds on the formation of silica-titania gels and glasses were investigated. DTA and TGA revealed losses of water and organic volatiles during heat treatment of the gels at lower temperatures (up to 400°C) and the glass transformation and crystallization behaviour at higher temperatures (up to 1500°C). The effects of using atmospheres with varying oxygen contents on the DTA peaks caused by oxidation reactions were also studied. Structural changes occurring during heat treatment were monitored by infra-red spectroscopy which indicated that the water contents of the glasses after heat treatment to 900°C were about 1000 ppm. Transmission electron microscopy of ion beam thinned foils of a 80 SiO220TiO2 composition showed a microstructure of extremely fine pores for heat treatments up to 1000°C. However, after extended heat treatments above 950–1000°C, the porosity appeared to decrease and a high concentration of fine crystallites of anatase (approximately 100 A in diameter) embedded in a silica-rich glass matrix were obtained.

Thin silica films prepared by dip coating

Abstract Thin silica films were deposited by dip coating and an expression developed to relate the densified thickness to the coating parameters. Friction experiments indicated failure by plastic deformation for films heated below 400°C and brittle failure for 400°C and above. The hazy appearance of some coatings prepared from solutions after intermediate ageing times was explained in terms of phase separation of the drying films.