The crystallization of diabase glass

The crystallization behaviour of diabase glass at elevated temperatures was studied in samples prepared by melting the diabase rock. DTA and X-ray analyses revealed the crystallization of diopside (CaO · MgO · 2SiO₂) at 865 C and anorthite (CaO · Al₂O₃ · 2SiO₂) at 1060 C. Further, the kinetics of crystallization of diopside were studied. The phenomenological Johnson-Mehl-Avrami equation was used and the exponentn=3/2 determined from the dependency of the volume fraction of the crystal phase (diopside) on time. The activation energy of crystallization of diabase glass 248 kJ mol^–1 was estimated on the basis of DTA measurements carried out at different heating rates and found to be in good agreement with literature data for similar glass.     

Nucleation and crystallization kinetics of CaO-Al2O3-2SiO2 in powdered anorthite glass

The nucleation and crystallization kinetics of CaO-Al₂O₃-2SiO₂ crystals in powdered anorthite glass with particle size <44 m in which CaO-Al₂O₃-2SiO₂ crystals were found to crystallize in the heating process of the glass, were studied by nonisothermal measurements using differential thermal analysis (DTA). The temperature of maximum nucleation rate was determined from the DTA curves of samples heat treated at different temperatures. The activation energy and kinetic parameters were simultaneously calculated from the DTA data using previously reported kinetic models. The crystallization process of a sample heat treated at the temperature of the maximum nucleation rate was fitted to kinetic equations with an Avrami constant, n2 and a dimensionality of crystal growth, m2, indicating that the constant number of nuclei of CaO-Al₂O₃-2SiO₂ precipitated in a glass matrix grew two-dimensionally with an activation energy taken as an average of the values calculated by the Kissinger and also the Augis and Bennett method of 679±4 kJ mol^–1.     

Facile fabrication of SiO2/Al2O3 composite microspheres with a simple electrostatic attraction strategy

SiO₂/Al₂O₃ composite microspheres with SiO₂ core/Al₂O₃ shell structure and high surface area were prepared by depositing Al₂O₃ colloid particles on the surface of monodispersed microporous silica microspheres using a simple electrostatic attraction and heterogeneous nucleation strategy, and then calcined at 600 °C for 4 h. The prepared products were characterized with differential thermal analysis and thermogravimetric analysis (DTA/TG), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). It was found that uniform alumina coating could be deposited on the surface of silica microspheres by adjusting the pH values of the reaction solution to an optimal pH value of about 6.0. The specific surface area and pore volume of the SiO₂/Al₂O₃ composite microspheres calcined at 600 °C were 653 m² g⁻¹ and 0.34 ml g⁻¹, respectively.     

The effect of rare-earth oxides on the crystallization of CaO–Al2O3–SiO2 glasses

CaO–Al₂O₃–SiO₂ –La₂O₃–Nd₂O₃ glass was prepared and their physical properties, such as density, glass transition temperature and crystallization temperature, were measured. The heat treatment of these glasses precipitated Ca₂La₈(SiO₄)₆O₂ oxyapatite (CLS) crystal for 20CaO–10Al₂O₃–60SiO₂–10La₂O₃ (mol%) glass and Ca₂Nd₈(SiO₄)₆O₂ oxyapatite (CNS) crystal was precipitated with Nd₂O₃ substitution. Crystallization of these glasses was observed at the surface and internally within the samples. The spherical and stick-like crystals were observed throughout the bulk of the glasses and the surface crystal layer of oxyapatite crystals were strongly oriented along the c-axis. The apparent activation energies of crystal growth were estimated as 360 kJ mol^–1.     

Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part II Surface and internal crystallization processes investigated by differential thermal analysis (DTA)

Controlled crystallization of (TiO₂-ZrO₂)-rich calcium aluminosilicate glasses led to zirconolite in the bulk, and titanite and anorthite on the surface. Such glass-ceramics can be envisaged for minor actinides immobilization. In this study, the crystallization of three glass compositions with increasing TiO₂, ZrO₂ and CaO amounts was followed by differential thermal analysis (DTA). The effect of glass particle size and of heating rate on DTA curves was studied in order to investigate nucleation mechanisms and to extract the corresponding crystal growth activation energies E _c for the different crystalline phases. Exothermic effects associated with the crystallization of a phase having a defect-fluorite structure in the bulk and its consecutive transformation into zirconolite were only detected for the highly TiO₂, ZrO₂ and CaO enriched glasses due to their higher crystallization rate. Using an Avrami constant n = 3 and a dimensionality of crystal growth m = 3, the activation energy of defect-fluorite crystal growth was found to be E _c = 440 kJ · mol^–1 (modified Kissinger method). Titanite and anorthite grow only from glass surface with activation energies of respectively 493 and 405 kJ · mol^–1 (n = m = 1, Kissinger method). DTA study of melt crystallization during cooling showed that baddeleyite (ZrO₂) crystals firstly crystallize but become unstable versus zirconolite for higher undercooling.     

Effect of Al2O3 concentration on zirconolite (Ca(Zr,Hf)Ti2O7) crystallization in (TiO2,ZrO2,HfO2)-rich SiO2–Al2O3–CaO–Na2O glasses

Glass-ceramic matrices containing zirconolite (nominally Ca(Zr,Hf)Ti₂O₇) crystals in their bulk that would incorporate high proportions of minor actinides (Np, Am, Cm) or plutonium could be envisaged for their immobilization. Zirconolite-based glass-ceramics can be prepared by controlled crystallization of zirconolite in glasses belonging to SiO₂–Al₂O₃–CaO–Na₂O–TiO₂–ZrO₂–HfO₂ system. In this study, neodymium was used as trivalent actinides surrogate. Increasing Al₂O₃ concentration in glass composition had a strong effect on the nucleation rate I _z of zirconolite crystals in the bulk, on the amount of neodymium incorporated in zirconolite phase and on the crystal growth rate of silicate phases (titanite + anorthite) from glass surface. These results could be explained by the existence of competition—in favor of aluminum—between Al³⁺ and (Ti⁴⁺, Zr⁴⁺, Hf⁴⁺) ions for their association with charge compensators cations to facilitate their incorporation in the glassy network. Differential thermal analysis (DTA) was used to study exothermal effects associated with bulk and surface crystallization. ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra showed that aluminum enters glasses network predominantly in 4-fold coordination. Neodymium optical absorption and fluorescence spectroscopies showed that the Al₂O₃ concentration changes performed in this study had not significant effect on Nd³⁺ ions environment in glasses.     

A generalized method for determining the crystal nucleation and growth rates in glasses by differential thermal analysis

A generalized experimental method that uses differential thermal analysis (DTA) has been developed for determining the nucleation rate (I), the crystal growth rate (U), and the concentration of quenched-in nuclei (N _q) in glasses. The method is applicable even for glasses, whose I and U curves (as a function of temperature) overlap to a considerable degree. Measuring I by the conventional method may yield an overestimated value for I if the chosen crystal growth temperature is within the region of overlap between I and U. When applied to a Na₂O · 2CaO · 3SiO₂ glass, whose I and U curves are known to overlap considerably, the present DTA method yields values for I, U, and N _q that are in excellent agreement with the same values determined by the conventional methods.     

Structural and microstructural transitions during phase separation and crystallization of Al2O3-SiO2-P2O5-Y2O3 glass

A highly refractory glass in the system Al₂O₃-SiO₂-P₂O₅-Y₂O₃ has been designed and produced such that, upon heating, an essentially fully crystalline glass—ceramic evolves containing mullite (nominally 3Al₂O₃·2SiO₂) and xenotime (YPO₄) as the final principal phases. Phase separation in this glass occurred during cooling from the melt and continued during annealing. XPS of the Al 2p, P 2p and the Y 3d electrons revealed that the average chemical environment of each of these elements is measurably different in the annealed glass and in the completely crystallized material. This indicates that the compositions of the separated glass phases are very different from those of the crystal phases which form from them. Additional rearrangement of the glass structure was observed at 1173 K. Extensive formation of mullite was initially detected at 1223 K and was followed by the crystallization of xenotime and the transient compounds of Y₄Al₂O₉, Y₂Si₂O₇, AlPO₄ and YP₅O₁₄. The optimum crystal nucleation and crystallization temperatures of 1173 and 1473 K, respectively, were determined from DTA, XRD, SEM and TEM studies.     

Effect of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the crystallization behavior of SiO<Subscript>2</Subscript>–MgO–B<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

A series of glass comprising of SiO₂–MgO–B₂O₃–Y₂O₃–Al₂O₃ in different mole ratio has been synthesized. The crystallization kinetics of these glasses was investigated using various characterization techniques such as differential thermal analysis (DTA), thermo gravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Crystallization behavior of these glasses was markedly influenced by the addition of Y₂O₃ instead of Al₂O₃. Addition of Y₂O₃ increases the transition temperature, T _g, crystallization temperature, T _c and stability of the glasses. Also, it suppresses the formation of cordierite phase, which is very prominent and detrimental in MgO-based glasses. The results are discussed on the basis of the structural and chemical role of Y³⁺ and Al³⁺ ions in the present glasses.     

Sintering and crystallization of (SrO·SiO2)-(SrO·Al2O3·2SiO2) glass-ceramics

In the ternary SrO-Al₂O₃-SiO₂ system the pseudobinary join composition of 50 wt % SrO·SiO₂–50 wt % SrO·Al₂O₃·2SiO₂ (SS-SA2S) showed a glass melting temperature of 1500 °C and a crystallization peak temperature of 1100 °C. The (SS-SA2S) glass-ceramic pellets prepared by cold pressing and pressureless sintering, showed very low porosity. The (SS-SA2S) glass-ceramics containing B₂O₃ and those containing B₂O₃ and TiO₂ revealed crystallization peak temperatures of 1000 °C and unexpectedly high porosity. By applying Kissinger analyses to the DTA data the activation energy values for crystallization of the three glass-ceramics were determined to range from 196 to 255 kJ/mol. The Ozawa analyses on the DTA data gave the Avrami parameter values at 3.69 to 3.95. The X-ray diffraction (XRD) patterns from the three glass-ceramics revealed formation of the equilibrium crystalline phases of SrO·SiO₂ and SrO·Al₂O₃·2SiO₂ (monocelsian).     

Sintering and crystallization of off-stoichiometric BaO·Al2O3·2SiO2 glasses

Glass-ceramics with off-stoichiometric celsian composition of 50 wt% BaO·2SiO₂ – 50 wt% BaO·Al₂O₃·2SiO₂, (B2S-BA2S) were fabricated and investigated for their sintering and crystallization characteristics. (B2S-BA2S) glass powder showed a melting temperature much lowered compared to that of stoichiometric BaO·Al₂O₃·2SiO₂ (BA2S) glass powder and high sintering ability. (B2S-BA2S) glass powder containing B₂O₃, (B2S-BA2S)B and that containing B₂O₃ and TiO₂, (B2S-BA2S)BT revealed much lowered crystallization peak temperatures, but rather low sintered density. By applying Kissiger analysis to differential thermal analysis (DTA) data activation energy values for crystallization were determined as 265, 195 and 242 kJ/mol, respectively for (B2S-BA2S), (B2S-BA2S)B and (B2S-BA2S)BT glasses. X-ray diffraction (XRD) patterns from all the glass-ceramics crystallized at 1100°C for 4 h revealed formation of crystalline phases of -BaO·2SiO₂, monocelsian and hexacelsian. (B2S-BA2S) glass-ceramics crystallized at 1400°C for 4 h showed formation of -BaO·2SiO₂ and monocelsian phases with only trace of metastable hexacelsian phase.     

The role of fluorine in the devitrification of SiO2·Al2O3·P2O5·CaO·CaF2 glasses

A series of eight glasses based on a glass system with the generic composition 1.5(5–Z)SiO₂·(5–Z) Al₂O₃·1.5P₂O₅·(5–Z)CaO·ZCaF₂ were studied where Z = 2, 1.75, 1.5, 1.25, 1, 0.5, 0.25, 0. These glasses were characterised using differential scanning calorimetry (DSC) and x-ray powder diffraction (XRD). Glasses with high fluorine contents were found to crystallise readily to fluorapatite via a homogeneous nucleation route probably involving prior amorphous phase separation. These results are explained in terms of an approach which views glasses as being inorganic polymers where the presence of fluorine disrupts the glass network and thereby reduces the energy barrier to homogeneous nucleation and crystallisation of fluorapatite.     

The crystallization of Gd2 3 -Al2O3 glasses

Glasses of Gd₂O₃ · x Al₂O₃ compositions where x represents 5/3, 4 and 6, were prepared using a rapid quenching apparatus and a laser beam. The crystallization process of the glasses was studied by means of differential thermal analysis (DTA), X-ray diffraction analysis and electron microscopy. The crystallizations of Gd₂O₃ · 5/3Al₂O₃, Gd₂O₃ · 4Al₂O₃ and Gd₂O₃ · 6Al₂O₃ are complex and exhibit one, two and three exothermic peaks in DTA measurement with increasing Al₂O₃ concentration, respectively. The crystallization process of Gd₂O₃ · 5/3Al₂O₃ glass involved the direct formation of the gadolinium aluminium garnet, 3Gd₂O₃ · 5l_{2 3} (GdAG), which is not obtained by the ordinary solid phase reaction. After crystallization of Gd₂O₃ · 4Al₂O₃ and Gd₂O₃ · l_{2 3} glass, both phases become a mixture of Gd₂O₃ · Al₂O₃ (perovskite type) and -Al₂O₃.     

Liquid immiscibility and crystallization in refractory phosphate glasses

The nature and evolution of liquid immiscibility and essentially complete crystallization in an optimized glass composition containing Al₂O₃, SiO₂ and P₂O₅ are examined from the results of electron microscopy, X-ray diffraction and density changes. Phase separation occurs during cooling from the melt; subsequent reheating to 850° C causes crystal nucleation followed by growth of the refractory phases of mullite (nominally 3Al₂O₃-2SiO₂) and AlPO₄ at approximately 1050 and 1160° C, respectively. Additional crystallization of these phases also occurs at 1400° C. Similar materials containing additions of ZrO₂ and Y₂O₃ are also discussed.When this work was conducted, the writers were Graduate Research Assistant and Associate Professor of Materials Engineering, respectively. Mr Lee is currently at Ohio State University.     

Synthesis and characterization of xMgO–1.5Al2O3–5SiO2 (x = 2.6–3.0) system using mainly talc and kaolin through the glass route

Three non-stoichiometric cordierite with compositions of xMgO–1.5Al₂O₃–5SiO₂ (x = 2.6–3.0 mol) were synthesized using mainly talc and kaolin through the glass route. The densification and crystallization behaviors of these glass powders were investigated using DTA, dilatometer, and XRD. Samples were then heat treated at 900 °C for 2 h and further analyzed using XRD, CTE, FESEM, density and porosity tests, and also dielectric test to further investigate their properties. The 2.8MgO·1.5Al₂O₃·5SiO₂ glass composition fully densified and crystallized into high purity α-cordierite at the heat treatment temperature. It exhibited lower CTE and dielectric constant compared to other composition, making it suitable for high frequency applications. Other formulations which contain multi crystalline phases require much higher temperatures for densification. The CTE value depends on the type of crystalline phase which exist in the sample, while the dielectric constant decreased with increasing MgO amounts in the sample compositions.     

Crystallization and properties of glasses prepared from Illinois coal fly ash

Glasses synthesized from Illinois coal fly ash by conventional melt quenching were recrystallized by suitable nucleation and crystal growth heat treatments. The microstructure and selected properties of the glasses and crystallized glasses were investigated using scanning and scanning transmission electron microscopy (SEM, STEM), Mössbauer spectroscopy, differential thermal analysis (DTA) and X-ray diffraction (XRD). Crystallization of the fly ash glasses without the aid of added nucleating agents was possible up to a maximum of 23 vol %. The crystalline phase was tentatively identified on the basis of STEM microanalysis as a combination of ferroaugite [(Ca, Fe²⁺)(Al, Fe³⁺)₂SiO₆] and potassium melilite [KCaAlSi₂O₇]. Comparative results of the thermal expansion, density and microhardness of the glass and crystallized glass are reported along with the Young's modulus of a glass fibre pulled from the fly ash melt.     

The effect of additives on the crystallization and sintering of 2MgO-2Al2O3-5SiO2 glass-ceramics

Crystallization and sintering behaviour of three cordierite (2MgO-2Al₂O₃-5SiO₂) glasses containing different amount of additives were investigated and compared by using differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and the Archimedes method. The stoichiometric 2MgO-2Al₂O₃-5SiO₂ (MAS) glass and the 2MgO-2Al₂O₃-5SiO₂ glass containing 3 wt% of B₂O₃ and 3 wt% of P₂O₅ (MASBP) showed two exotherms (one for -cordierite formation from a glass and the other for -cordierite formation from the -cordierite phase), whereas the 2MgO-2Al₂O₃-5SiO₂ glass containing 2 wt % of B₂O₃, 2 wt% of P₂O₅, and 2 wt % of TiO₂ (MASBPT) showed only a single exotherm representing -cordierite formation. By using Kissinger, Augis-Bennett, Ozawa, and modified Kissinger methods, the activation energy values for -cordierite formation in the MASBP and MASBPT glasses were determined as 310±6 and 326±13 kJ mol^–1, respectively, whereas that in the MAS glass was determined as 868±5 kJ mol^–1. The MASBPT glass showed the lowest peak temperature value for -cordierite formation (980 °C) amongst the three glasses. Both the MASBP and MASBPT glasses showed excellent sintering behaviour (> 99.7% of theoretical density).     

Solid state reactions in the ZrO2 · SiO2 - αAl2O3 system

Solid state reactions between ZrO₂· SiO₂ and Al₂O₃ in mixed powders were studied by quantitative X-ray diffraction, density measurements and qualitative EDAX. Data were obtained at temperatures ranging from 1400 to 1600° C for 5 h; the initial molar ratios of the reactants (Al₂O₃/ZrO₂ · SiO₂) varying from 0 to 5. The results indicate that: (1) ZrO₂· SiO₂ and Al₂O₃ react and form ZrO₂, crystalline 3Al₂O₃ · 2SiO₂ and a noncrystalline mullite phase; (2) the non-crystalline mullite phase is an important transitional phase towards equilibrium under subsolidus conditions. In the experimental conditions used the amount of the non-crystalline phase varies by as much as about 15%. This phase is of great importance in the mechanisms of reaction sintering between ZrO₂ · SiO₂ and Al₂O₃.     

Nucleation and growth of aluminium oxide on silicon in the CVD process

Films of aluminium oxide have been formed on single crystal silicon substrates using AlCl₃-CO₂-H₂ gas mixtures in a cold-walled chemical vapour deposition (CVD) reactor. The nucleation and subsequent growth of the deposit have been observed under the varying process parameters. It is found that the nucleation and growth of the Al₂O₃ are dependent on the H₂O flux and H₂O supersaturation. An activation energy of 34.8 Kcal mol^–1 is obtained for the growth rate indicating that the CVD of Al₂O₃ on silicon is a thermally activated process and limited by surface reaction. Scanning electron micrographs (SEM) show that the deposited films are amorphous at low temperature, 850° C, but change to fine grained polycrystalline structure at high temperature, 1000° C.     

Sintering and crystallization of off-stoichiometric SrO·Al2O3·2SiO2 glasses

Glass-ceramics with the celsian-corundum binary join composition of 88.8 wt% SrO·Al₂O₃·2SiO₂ – 11.2 wt% Al₂O₃, (SA2S-A), were fabricated by pressureless sintering and investigated for their sintering and crystallization behaviors. The (SA2S-A) glass powder showed crystallization peak and melting temperatures of 1059 and 1550 °C, respectively and high sintering ability. The (SA2S-A) glass powders containing B₂O₃, (SA2S-A)B and those containing B₂O₃ and TiO₂, (SA2S-A)BT showed lowered crystallization peak temperatures of 1033 and 997 °C, respectively. By applying Kissiger analyses to the DTA data of the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders, the activation energy values for crystallization were determined as 488, 370 and 333 kJ/mol, respectively. The Ozawa analyses on the DTA data gave the Avrami parameter values at 1.2, 1.1 and 1.9, respectively for the (SA2S-A), (SA2S-A)B and (SA2S-A)BT glass powders. The x-ray diffraction (XRD) patterns of the (SA2S-A) glass-ceramics, crystallized at 1100 °C for 4 h, showed formation of both the monocelsian and hexacelsian phases. The (SA2S-A)B and (SA2S-A)BT glass-ceramics crystallized at 1100 °C for 1 h, showed formation of the phase-pure monocelsian and did not show any evidence of the hexacelsian formation prior to the monocelsian formation.