Effect of V2O5 addition on the crystallisation of glasses belonging to the CaO–ZrO2–SiO2 system

The crystallisation of CaO–ZrO₂–SiO₂ glasses doped with V₂O₅ (0.1–5 mol%) has been investigated in terms of microstructure and thermal parameters. Results indicate that crystallisation is predominantly controlled by a surface nucleation mechanism, even though a partial bulk nucleation has been encountered in compositions containing more than 2 mol% of doping oxide. As detected from differential thermal analysis curves, glass transition temperature and crystallisation temperature, are strongly dependent upon V₂O₅ content varying from 0.0 to 2.0 mol%, while the crystallisation activation energy values decrease with a parabolic trend from B-glass (0.0 mol% V₂O₅ content, 495±7) to V-0.7 (0.7 mol% V₂O₅ content, 420±6) composition, increasing again to 442±5 kJ/mol K with higher amount of V₂O₅. The microstructure of the glass-ceramic materials clearly showed a marked dependence upon the amount of V₂O₅, also due to the presence of phase separation for content higher than 0.7 mol%. Wollastonite, CaO·SiO₂, and a calcia–zirconia–silicate, 2CaO·4SiO₂·ZrO₂, are the main crystalline phases whose ratio slightly varies with vanadium oxide content. The glass ceramics obtained from the studied materials are greenish and bluish coloured, so it is possible to use the studied glasses as coloured frits for tile glazes.     

The annealing of alkaline-earth metal polymetaphosphate powders (I). Crystallisation and sintering processes

Amorphous barium, strontium, calcium and magnesium polymetaphosphate powders (MP₂O₆)_n, n = 20 were prepared by dehydration of the corresponding polymetaphosphate hydrate precipitates. These powders were annealed by different continuous and isothermal heat treatments over the temperature range 450° to 700 °C, the glass transition temperatures T_g to above (T_g + 120) °C. The morphologies at different degrees of crystallisation were studied by scanning electron microscopy. For the main crystallisation process (ten to sixty-seventy percent crystallisation), the powder particles retained their original pea-pod form; then after seventy percent crystallisation, these crystallised particles sintered laterally to lozenge-shaped twin-hexagonal crystals of lengths 0.5 to 3 μm. Differential thermal analysis confirmed that a markedly exothermic crystallisation process (overall enthalpy changes from about 30 to 45 kJ mol⁻¹) was occurring within the powder particles. Crystallisation rates varied from < 0.005 min⁻¹ at temperatures near T_g to > 0.5 min⁻¹ at higher temperatures; the activation energies for this process varied from 360 to 560 kJ mol⁻¹. The completely annealed crystals were studied by scanning electron microscopy, X-ray diffraction and further differential thermal analysis to 1000 °C. The X-ray diffraction d value patterns, the fusion temperatures and the enthalpies of fusion were all in close agreement with the literature values for the corresponding beta alkaline-earth metal polymetaphosphates prepared by melt crystallisation.     

An in situ synchrotron X-ray diffraction investigation of lepidocrocite and ferrihydrite-seeded Al(OH)3 crystallisation from supersaturated sodium aluminate liquor

Lepidocrocite and ferrihydrite-seeded Al(OH)₃ crystallisation from supersaturated sodium aluminate liquor at 70 °C was investigated using in situ synchrotron X-ray diffraction. The presence of iron oxides and oxyhydroxides in the Bayer process has implications for the nucleation and growth of scale on process equipment, and a greater understanding of the effect they have on Al(OH)₃ crystallisation may allow for development of methods for Al(OH)₃ scale prevention. The early stages of both crystallisation reactions were characterised by nucleation of gibbsite on the seed material. This was followed by a rapid increase in gibbsite concentration, which coincided with the appearance of the bayerite and nordstrandite polymorphs of Al(OH)₃. The lepidocrocite-seeded reaction then proceeded via a mechanism similar to that which has been observed previously for goethite, hematite and magnetite-seeded Al(OH)₃ crystallisation. Different behaviour was observed in the ferrihydrite-seeded experiment, with nucleation as well as growth occurring during the period of rapid increase in gibbsite concentration, followed by a period of diffusion controlled growth.     

Growth and Properties of Potassium Holmium Double Tungstate KHo(WO4)2

Potassium holmium tungstate KHo(WO₄)₂ crystal, as other tungstates, shows the high temperature irreversible structural phase transition. Owing to this, KHo(WO₄)₂ single crystals were grown due to spontaneous crystallisation with use of the High Temperature Solution Growth technique, which allows to lower the temperature of crystallisation below the temperature of the phase transition. K₂W₂O₇ was used as a solvent. It provides a very wide temperature range of crystallisation and does not introduce additional impurities into the melt. The starting flux contained 20 mol% of KHo(WO₄)₂ dissolved in K₂W₂O₇. It was found that potassium holmium double tungstate is pleochroic. The two different optical spectra: one spectrum for electrical vector of linearly polarised light parallel to optical Y axis (main spectrum) and second one for electrical vector perpendicular to Y axis were measured.     

Application of D‐optimal experimental design in nano‐sized ZSM‐5 synthesis for obtaining higher crystallinity

D‐optimal experimental design with three levels of SiO₂/Al₂O₃, template/SiO₂, H₂O/SiO₂, SiO₂/Na₂O and TPABr/TPAOH ratio parameters was used to optimize the experimental parameters by the analysis of variance (ANOVA). The effects of above mentioned ratios in the initial synthetic mixture on the crystallinity of the ZSM‐5 zeolite were studied. The synthesized samples were characterized by XRD, FE‐SEM, and TEM analysis. Fischer test results showed that SiO₂/Al₂O₃ and H₂O/SiO₂ molar ratios are the most and least effective parameters, respectively, in the range studied. The most important two‐way interaction variable was that of template/SiO₂ and Na₂O/SiO₂ molar ratios. The optimum composition of the gel compound to achieve relative maximum crystallinity is SiO₂/Al₂O₃ = 99.96, template/SiO₂ = 0.16, H₂O/SiO₂ = 34.68, Na₂O/SiO₂ = 0.02 and TPABr/TPAOH = 1.44.     

Kinetics of crystal growth of strontium tungstate from solutions in sodium tungstate melts by continuous cooling

The crystallisation kinetics of strontium tungstate from unstirred saturated solutions in sodium tungstate melts was studied by continuous cooling from initial crystallisation temperatures T₀ = 1000° to 800°C to room temperature at cooling rates R_T = 0.67° to 3.3°C min⁻¹. The main crystal growth was diffusion rate-controlled; the final crystal growth was rate-controlled by the development rate of excess solute concentration. The estimated diffusion rate constant (k_D) values increased with cooling rates and initial crystallisation temperatures. They are higher than the rate constants for diffusion-controlled growth of calcium tungstate from sodium tungstate melts, but very much smaller than those for strontium tungstate from lithium chloride melts.     

The crystallisation of alkaline-earth metal metasilicates from metal chloride melts: Preliminary experimental studies

The crystallisation of alcaline-earth metal metalsilicates was studied by slow cooling of saturated solutions in the metal chloride melts at T₀ = 830° to 1300°C, down to ambient temperature; cooling rates were varied from 20° to 200°C hr⁻¹. Calcium metasilicate crystallised as β-CaSiO₃ prisms, strontium metasilicate as α-SrSiO₃ platelets and barium metasilicate as α-BaSiO₃ platelets. Final crystal lengths increased with reduction in cooling rate and with increase in initial crystallisation temperature.     

Effects of LiPO3 glass crystallisation

The LiPO₃ glass crystallisation has been investigated with calorimetry, scanning electron microscopy, optical microscopy and X-ray powder diffraction. It was discovered that the crystallisation at a given temperature depends on the thermal prehistory of the sample. It is shown that the cooled samples can change over the course of several hours. From the microphotographs, it is clear that the surface crystallisation mechanism is predominant for this glass. The velocities of the crystallisation front expansion for 425, 440, 460 °C are 4.9, 18.6 and 34.5 μm/min, respectively.     

The Crystallisation of Calcium Molybdate and Tungstate from Lithium Chloride Melts by Continuous Cooling. The Kinetics of Crystal Growth in Alumina Crucibles

The kinetics of crystallisation of calcium molybdate and tungstate from unstirred supersaturated solutions in lithium chloride melts — in alumina crucibles — was studied by continuous cooling from initial temperature T₀ = 800°C down to room temperature at cooling rates R_T = 20° to 200°C hr⁻¹. The solutions were analysed chemically and the crystals were examined by optical microscopy. Crystal growth started practically immediately after the onset of cooling: at first, the amount of material deposited onto crystals was far less than the amount of excess solute developed within the supersaturated solutions but crystallisation rates then increased as the crystal sizes increased. Then, after some time t* (at about seventy percent crystallisation), all excess solute was deposited onto growing crystals.     

Bulk crystallisation of (00l) oriented fresnoite Sr2TiSi2O8 in glass-ceramics of the Sr-Ti-Si-K-B-O system

This paper shows that glass-ceramics containing highly surface and bulk preferentially oriented fresnoite Sr₂TiSi₂O₈ crystals can be synthesised by a simple isothermal heat treatment of suitable glass compositions in the Sr-Ti-Si-K-B-O system. For all tested compositions, crystallisation starts from the free surfaces of the specimens and propagates to bulk with time. If most of these compositions lead to (00l) preferential orientation at the specimens' surfaces, bulk crystal texture is very composition dependent. The effects of variation in K₂O and B₂O₃ contents on the crystallisation have been studied. It is shown that low K₂O and high B₂O₃ contents are required to keep the (00l) orientation from the surface into the bulk. This result seems to be explained by the viscosities of the initial and residual glasses at the temperature of crystallisation: a low viscosity leads to a fine and homogeneous microstructure with small and strongly (00l) bulk oriented crystals.     

Evaluation of degree of crystallization by optical microscopy and chemical analysis in crystallization of alkaline-earth metal tungstates from solutions in lithium chloride melts

A comparative study of the accuracy of optical microscopy and chemical analysis of residual solutions for evaluation of degree of crystallisation was carried out to ascertain their suitability for kinetics study of CaWO₄ and BaWO₄ crystallisation from LiCl melts (i) by continuous cooling at a constant cooling rate, and (ii) at constant temperature from a supersaturated solution. Chemical analysis of residual solution does not give desirable accuracy for the case (i) for α_t values <0.30, and for the case (ii) for α_t values <0.60. Optical microscopy was more accurate and suitable for both cases.     

Mechanisms controlling primary crystallisation of Ga20Te80 glasses

The kinetic study of the crystallisation process of Ga₂₀Te₈₀ glass from isothermal and continuous heating calorimetric data have been performed applying a recently developed procedure. The kinetic information was complemented with X-ray diffraction measurements. With this scope, three crystallisation patterns, with three-dimensional isotropic growth have been analysed: (i) site saturation and interface controlled growth. (ii) homogeneous nucleation with interface controlled growth and (iii) homogeneous nucleation with two simultaneous modes of crystal growth (interface- and diffusion-controlled). A complex model with two simultaneous modes of three-dimensional isotropic crystal growth with decreasing homogeneous nucleation and soft impingement has been applied for modelling primary crystallisation of the Ga₂₀Te₈₀ glass. The model goes beyond the isokinetic hypothesis when coupling isothermal and continuous heating kinetic data. The apparent activation energy E_a = (2.06 ± 0.03) eV/at obtained for the primary crystallisation of the phase Te is shown to correspond to an activation energy for nucleation E_I = (2.85 ± 0.03) eV/at and an interface controlled activation energy for growth E_u = (1.90 ± 0.03) eV/at at the crystallisation onset.     

Fluoride-containing bioactive glass-ceramics

Fluorapatite glass-ceramics are osteoconductive, and glass-ceramics containing fluorapatite crystals in a bioactive silicate glass matrix can combine the benefits of fluorapatite with the bone-regenerative properties of bioactive glasses. High phosphate content (around 6 mol% P₂O₅) bioactive glasses (SiO₂–P₂O₅–CaO–Na₂O–CaF₂) were prepared by a melt-quench route. Structural investigation using density measurements and calculations confirmed the presence of phosphorus as orthophosphate. Upon heat treatment, the glasses crystallised to mixed sodium calcium fluoride orthophosphates (sodium-containing compositions) and fluorapatite (sodium-free composition). Fluoride suppressed spontaneous crystallisation, allowing formation of glass-ceramics by controlled crystallisation. A notable feature is that silicate network polymerisation and network connectivity did not change during crystallisation, resulting in orthophosphate and fluorapatite crystals embedded within a bioactive glass matrix. By keeping the phosphate content high and the sodium content low, fluorapatite glass-ceramics can be obtained, while not affecting the structure of the bioactive silicate glass phase.     

Multicomponent phosphate invert glasses with improved processing

Owing to their structure of small phosphate units, phosphate invert glasses have high crystallisation tendencies, which make processing of the melt challenging. The aim was to improve their processing by (1) increasing the number of glass components and (2) incorporating intermediate oxides (TiO₂, MgO and ZnO). Glasses (P₂O₅–CaO–MgO–Na₂O) were produced by a melt-quench route. In series 1, TiO₂ was partially substituted for Na₂O, and the number of components was increased by partially substituting strontium for calcium, zinc for magnesium and potassium for sodium on a molar base. In series 2, the MgO + ZnO content in the multicomponent glass was varied between 0 and 20 mol% in exchange for CaO + SrO. Differential scanning calorimetry showed a significant increase of the processing window in the multicomponent glasses, explained by an increased energy barrier for crystallisation owing to increased entropy of mixing. The MgO + ZnO content also significantly improved the processing window from 117 K (0 mol% MgO + ZnO) to 185 K (20 mol%), owing to their large field strength. These results show that the processing of phosphate invert glasses for biomedical applications can be improved significantly by incorporating ions such as strontium or zinc which are also known to have therapeutic effects.     

Nonisothermal crystallisation kinetics of amorphous selenium prepared by high-energy ball milling: A comparison with the melt-quenching and thin-film techniques

Differential scanning calorimetry (DSC) studies were performed under nonisothermal conditions at various heating rates for glassy Se made by high-energy ball milling. Comparisons were made between the ball-milling technique and the melt-quenching and thin-film techniques. Well-defined endothermic and exothermic peaks were observed at the glass-transition temperature, T_g, and the onset temperature of crystallisation, T_c. The isoconversional method of Vyazovkin was used to determine the variation in the activation energy for crystallisation with temperature, E_α(T). The value of E_α(T) was dependent on the sample preparation method. The thermal stability of the Se glasses was evaluated by calculation of the temperature difference (T_c ? T_g) and the S-parameter. In addition, the glass-forming ability was estimated by the criteria of reduced glass-transition temperature, T_rg, and the Hruby number, H_R. The structures of the Se samples that resulted from the DSC analysis were identified using an X-ray diffractometer. The glasses formed using the thin film technique were the most stable.     

Crystallisation kinetics,glass forming ability and thermal stability in glassy Se100 − xInx chalcogenide alloys

Differential scanning calorimetry (DSC) studies have been done under non-isothermal conditions at different heating rates for glassy Se_100 ? xIn_x (5 ≤ x ≤ 20) alloys. DSC traces with well-defined endothermic and exothermic troughs and peaks at glass transition (T_g), crystallisation (T_c) and melting (T_m) temperatures were observed. The crystallisation kinetics parameters, Avrami index (n), activation energy for crystallisation (E_c) and frequency factor (K_o), have been calculated on the basis of the classical Johnson–Mehl–Avrami (JMA) model and related methods derived by Kissinger, Augis–Bennett and Mahedevan. Activation energy for glass transformation (E_t) has been evaluated on the usual two different non-isothermal methods developed by Moynihan and Kissinger. An extension of the Augis–Bennett method well known for evaluating E_c to calculate E_t has been explored with satisfactory results. Results obtained from these methods are in close agreement with each other. Close correlation between E_t, E_c and heating rate (β) was observed. The glass forming ability (GFA) and thermal stability parameters have been calculated for each glass system. It was found that the proportion of indium additive changed significantly the values of glass/crystal transformation, GFA and thermal stability of the studied system.     

Formation and stability of nickel-zirconium glasses

Glasses may be formed in two composition ranges: 33–42 and 60–76 at.% Zr. The crystallisation temperatures (T_x), activation energies (Q_x), crystallisation morphologies, densities, Young's moduli and peak in the X-ray diffraction pattern (Q_p) are reported as a function of composition. T_x, Q_x and Q_p do not interpolate smoothly between the two composition ranges. Maximum thermal stability occurs at 36 at.% Zr, well away from the value predicted by the free electron approach of Nagel and Tauc. The glasses are denser than would be expected from a simple dense random packing of hard spheres. Some implications for the structure of all-metal glasses are discussed.     

The crystallisation of alkaline-earth metal oxides from metal chloride melts: Solubility-temperature phase diagram analyses and preliminary experimental studies

The solubility v temperature phase diagrams for magnesium, calcium, strontium and barium oxide solutions, in lithium, sodium and potassium chloride melts and in the alkaline earth metal chloride melts, have been analysed. The solutions in the alkali metal chlorides are pseudo-binary reciprocal ternary mixtures; the more soluble barium and strontium oxides showed small deviations, calcium oxide showed larger deviations while the extremely sparingly-soluble magnesium oxide showed extensive deviations from ideality. These deviations were related to electrostatic interactions in solution, that depended in turn on some function of reciprocal solute and solvent caiton radii. The solutions in the alkalineearth metal chlorides are binary mixtures with MO · 4 MCl₂ solvate formation. — Some preliminary crystallisation experiments were carried out, by slow cooling of saturated solutions in the metal chloride melts: calcium and strontium oxide crystallisation from lithium chloride melts and barium oxide crystallisation from all three alkali metal chloride melts would be worth further investigation.     

The crystallisation of chromite-magnesiochromite spinels from a calcium magnesium aluminosilicate glass: Nucleation,crystal growth and final crystal sizes

The crystallisation of chromite-magnesiochromite spinels was studied from a calcium magnesium aluminosilicate glass (a simulated slag) containing 3 to 12 percent total iron oxides and 0.3 to 1.5 percent chromium(III) oxide, at temperatures from 1400° to 700 °C. – Spinel crystallisation occurred in glasses with 3–7 percent FeO and 0.7–1.1 percent Cr₂O₃. At temperatures 1100 °C and above, the nucleation was rapid and crystal numbers very high, at FeO contents above 3 percent and Cr₂O₃ contents above 0.7 percent; at 1056° and 1000 °C however, the crystal numbers reached some optimum values but then decreased as clinopyroxene crystals grew onto and enveloped the spinel microcrystals. In these glasses, the crystal lengths varied with growth time according to the relation, l_t = 2 k_g t^α = R_g1 t^α, where α = 0.7–1.0: this time dependence was a compromise between a relation for dendritic growth and one for facetted growth. The growth rates generally increased about five to seven times for 160 °C temperature rise: the energy of activation for the spinel crystal growth was then estimated as 180 ± 60 kJ mole⁻¹. – No spinel crystals were observed in glasses with more than 7 percent FeO content, only clinopyroxene crystals. Probably, these latter had nucleated rapidly and grown onto spinel microcrystals, while the spinel microcrystals were still of < 0.1 μm size.