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.     

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.     

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).     

Metastable immiscibility and microstructural development during sintering of a SiO2-Al2O3 plasma prepared powder

The sintering characteristics of SiO₂-36.6 wt % Al₂O₃ powder, prepared by condensation from high frequency plasma, have been studied and microstructural changes occurring during sintering followed by transmission electron microscopy The as-prepared amorphous powder showed evidence of spinodal decomposition into an Al₂O₃-rich and SiO₂-rich glass consistent with the position of a previously reported metastable miscibility gap in the SiO₂-Al₂O₃ system. Mullite crystallized on an extremely fine scale at 1000° C and gradually coarsened at higher temperatures. Sintering occurred above 1100° C by a viscous flow mechanism with activation energy 87 kcal mol^–1 which corresponds to the activation energy for viscous flow of SiO₂-Al₂O₃ glass containing approximately 17 wt % Al₂O₃.     

The phase evolution of phosphoric acid-based geopolymers at elevated temperatures

The Al₂O₃-2SiO₂ powders for sol-gel synthesis have good phosphoric acid activation properties. The phase evolution of phosphoric acid-based geopolymers at elevated temperatures is similar to that of alkali-based metakaolin geopolymers. The chemosynthetic phosphoric acid-based geopolymers have been found to possess extremely good thermal stability, with no sign of melting up to 1550 °C. The onset temperature of crystallization of the specimens occurred at 900 °C. The amount of quartz and berlinite crystal reached a maximum at approximately 1000 °C. Further exposure to 1150 °C showed that quartz and berlinite phases were converted to cristobalite and aluminum phosphate.     

Synthesis of ultrafine zeolites by dry-gel conversion without any organic additive

ZSM-5, mordenite, cancrinite and Y zeolites were successfully crystallized by the dry-gel conversion (DGC) without any organic additive (or template), in which the crystal particles of ZSM-5, mordenite and Y zeolite were well controlled in the range of ultrafine size. With a molar ratio of SiO₂/Al₂O₃ ≤40, the pure ZSM-5 zeolite was obtained at 170 °C at the Na₂O/SiO₂ molar ratio <0.2, but the pure mordenite was formed at 150 °C at the Na₂O/SiO₂ molar ratio >0.25. When the dry-gel with a composition of 5.0Na₂O:Al₂O₃:5.0SiO₂ was crystallized at 140 °C for 24 h, pure cancrinite phase was obtained. TEM images showed that thus-crystallized zeolites consisted of ultrafine particles ranging 35-150 nm for ZSM-5, 50-200 nm for mordenite and 120-200 nm for zeolite Y, respectively. Different from ZSM-5, mordenite and cancrinite zeolites, the DGC synthesis of Y zeolite required more water in the dry-gel before crystallization.     

Effect of TiO2 addition on oxidation behavior of sintered bodies composed of Si3N4-Y2O3-Al2O3-AlN

The effect of TiO₂ content on the oxidation of sintered bodies from the conventional Si₃N₄-Y₂O₃-Al₂O₃-AlN system was investigated. Sintered specimens composed of Si₃N₄, Y₂O₃, Al₂O₃, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO₂ in the range of 0 to 5 wt% to Si₃N₄, were fabricated at 1775 °C for 4 h at 0.5 MPa of N₂. Oxidation at 1200 to 1400 °C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO₂ content. In the phases present in the specimens oxidized at 1300 °C for 100 h in dry air, Y₃Al₅O₁₂ and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y₂O₃·2TiO₂ (Y2T) appeared in their place and increased with increasing TiO₂ content. In those oxidized at 1400 °C, -cristobalite was dominant and very small amounts of Y₂O₃·2SiO₂ and Y2T were contained. There was a tendency for more -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO₂ formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO₂ decreases oxidation resistance.     

Dental castable glass ceramics: ceramming treatment and colour property

The magnitudes of colour change vector and lightness were examined for 20CaO/10P₂O₅/10MgO/10Al₂O₃/50SiO₂ glass ceramics (B₂O₃/CaF₂=0.01 wt % 0.01 wt %, mass fraction). The glass ceramics were thermally treated (cerammed) for crystallization at each selected temperature (800 to 980°C for each ceramming period) within a steel ring including the investment mould. Cristobalite-quartz investment mould exhibited no adhesive mould interfaces, showing that the magnitudes of colour change vector and lightness ranged from 17.60 (890°C) to 37.05 (980°C) and 55.77 (890°C) to 71.42 (980°C) for glass ceramics, respectively, after thermal treatment for 2 h (sample thickness 2 mm). The thermal treatment controlled the colour property measured by colour analyser as compared the cerammed samples with bovine enamel and human tooth enamel.     

Formation of Na-A and -X zeolites from waste solutions in conversion of coal fly ash to zeolites

Na-A and -X zeolites were synthesized from waste solutions in conversion of coal fly ash (Fa) to zeolite. The amorphous SiO₂ and Al₂O₃ of Fa were completely dissolved to form Po, Pt, and Pc type zeolites in NaOH solutions at 85°C. Only 24% of Si⁴⁺ eluted from Fa were converted to the zeolites and the remaining waste solutions contained high Si⁴⁺ concentrations. When molar ratio SiO₂/Al₂O₃ of the waste solutions was modified at 1.0≤SiO₂/Al₂O₃≤2.0 by addition of NaOH-NaAlO₂ solutions and the solutions were agied at 85°C, a single phase of Na-A zeolite was formed. The Na-X zeolite was formed at SiO₂/Al₂O₃≥2.5 and its crystallinity was increased with increasing the SiO₂/Al₂O₃ ratio, whereas the crystallinity of Na-A zeolite was decreased. At SiO₂/Al₂O₃=7.3, a single phase of Na-X zeolite was produced.     

Nanocomposites in mullite-ZrO2 and mullite-TiO2 systems synthesised through alkoxide hydrolysis gel routes: microstructure and fractography

The sol-gel process allows preparation of very homogeneous and reactive monolithic, optically clear gels. Low-temperature thermal treatments (700–1000 °C) lead to amorphous optically clear samples (glass). Amorphous mullite compositions (0.4Al₂O₃-0.6SiO₂ to 0.8Al₂O₃-0.2SiO₂) retain large amounts of Ti and Zr elements. The crystallization has been studied by differential thermal analysis, dilatometry, X-ray and electron diffraction and Raman scattering. The nucleation begins above 1000 °C with the departure of the last protonic species, the amorphous matrix being completely crystallized only above 1400 °C. The addition of Zr and Ti elements leads to a homogeneous nucleation of phases with a composition close to ZrO₂ and Al₂Ti₃O₉ (EDX analysis) above the solubility limit. TEM and SEM analyses show that the precipitate size remains submicrometric over a wide temperature range (1000–1400 °C) and consequently glass-like mechanical properties, as well as toughening effects, caused by the presence of nanoprecipitates, are observed.     

Low temperature mullite crystallization in Al- and Si-alkoxide derived homogeneous gels

A homogeneous mullite gel (3:2 Al₂O₃:SiO₂ molar ratio) was made from tetraethoxysilane (TEOS) and aluminum isopropoxide (Al(OCH(CH₃)₂)₃) at room temperature within a relatively quick three days. Mullite was the only crystalline phase to form during calcination; metastable phases like aluminosilicate spinel did not appear. When heated at 20 °C/min, crystalline mullite (65 mol% Al₂O₃) started forming at 575 °C and reached 27 wt.% by 900 °C. Major crystallization occurred at ~ 1000 °C with a concurrent increase of Al₂O₃ concentration (68–69 mol%) in the mullite phase. The alumina content decreased towards stoichiometric (3Al₂O₃·2SiO₂) mullite at even higher temperatures. When the gel remained in the amorphous state, low temperature preheating significantly improved crystallization at higher temperatures. After preheating at 425 °C for 24 h, 78 wt.% of the final product was crystallized when it was subsequently annealed at 750 °C for 5 min. Only 20 wt.% crystallized without preheating.     

Solid-solution range of mullite up to 1800 °C and microstructural development of ceramics

Tetraethoxysilane (TEOS) and Al-sec-butylate (Al-O-Bu) were used for the sol-gel synthesis of mullite ceramics. The starting materials had bulk compositions corresponding to values between 72 and 78 wt% Al₂O₃, and 28 and 22 wt% SiO₂, respectively, and were calcined at 400 °C (A-series) and 1100 °C (B-series). B-series samples, despite their higher green densities, could only be sintered to about 65–70% TD (theoretical density) at 1650 °C, whereas A-series samples achieve values of about 93–98% TD. Ceramics with relatively high amounts of glass phase from large tabular mullite crystals, which are embedded in a finer-grained mullite matrix. As soon as the bulk Al₂O₃ content increases, equiaxed mullite grains appear and the mean grain size becomes smaller, showing a significant difference between the nucleation and crystal growth mechanisms of mullites formed in samples with the lower and higher Al₂O₃-bulk compositions. Depending on the bulk composition of the samples, the temperature-controlled solid-solution of mullite ranges between about 72.7 and 74.3 wt% Al₂O₃ at 1600 °C and 74.1 and 75.4 wt% Al₂O₃ at 1800 °C, indicating that the solid-solution region bends over towards the Al₂O₃-side of the Al₂O₃-SiO₂ phase diagram.     

Characterization of mono- and diphasic mullite precursor powders prepared by aqueous routes. 27Al and 29Si MAS-NMR spectroscopy investigations

The structural evolution from amorphous to crystalline mullite, for different 3Al₂O₃ · 2SiO₂ mono- and diphasic precursors, has been investigated by ²⁹Si and ²⁷Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. The crystallization has also been studied by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The chemical composition in the aluminosilicate network of the diphasic precursors and in the crystallized phases has been determined from the ²⁹Si NMR spectra. A close agreement is found with the composition deduced from the lattice parameters measured by XRD. For monophasic precursors the amount of hexa-coordinated aluminium atoms decreases when the temperature increases while Al(IV) and Al(V) increase. Al(VI) practically completely disappears just before the crystallization at 980 °C. An alumina-rich mullite 2Al₂O₃ · SiO₂ (21 mullite) is then formed through a strong exotherm. An enthalpy of 75 kJ per mol is determined for the crystallization of the 21 mullite. At higher temperatures the segregated silica is progressively reincorporated into the mullite lattice. For diphasic precursors the ²⁹Si NMR spectroscopy shows the segregation of silica. The aluminosilicate network is then richer in alumina and the amount of remaining AlO₆ octahedra before the crystallization at 980 °C is higher. Spinel crystallizes and continues to become richer in alumina until it reacts with silica to form the stoichiometric 32 mullite at 1260–1275 °C. The nature of the crystallization is related to the local composition of the amorphous alumino-silicate network and to the amount of AlO₆ octahedra on approaching 980 °C.     

Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y₂O₃, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO₃ and Al₂O₃ will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y₂O₃-77 mol% Al₂O₃ and 1702±7° C. YAlO₃ (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO₃ formed during metastable solidification transforms to YAG in the presence of Al₂O₃ at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.     

Geopolymers based on a coarse low-purity kaolin mineral: Mechanical strength as a function of the chemical composition and temperature

A coarse mineral with 70% kaolinite and 30% quartz was calcined and chemically activated by alkaline solutions of Na₂SiO₃ and NaOH. The compressive strength evolution was investigated as a function of the curing temperature at 20 and 80 °C, and the molar ratios of SiO₂/Al₂O₃ (2.64-4.04) and Na₂O/Al₂O₃ (0.62-1.54). For curing at 20 °C, the best composition was SiO₂/Al₂O₃ = 2.96 and NaO/Al₂O₃ = 0.62, reaching 85 MPa at 28 days. Curing at 80 °C had a positive effect on the strength development only in the first 3 days. X-ray diffraction of the geopolymeric formulations showed the formation of amorphous silicoaluminates of similar nature. The microstructure consisted of unreacted quartz and metakaolinite particles in a matrix of silicoaluminate polymer and condensed silica gel from the unreacted sodium silicate.     

CO2 bubble absorption enhancement in methanol-based nanofluids

In this study, the nanoparticles (i.e. SiO₂ and Al₂O₃ nanoparticles) and methanol are combined into SiO₂/methanol and Al₂O₃/methanol nanofluids to enhance the CO₂ absorption rate of the base fluid (methanol). The absorption experiments are performed in the bubble type absorber system equipped with mass flow controller (MFC), mass flow meter (MFM) and silica gel (which can remove the methanol vapor existing in the outlet gases). The parametric analysis on the effects of the particle species and concentrations on CO₂ bubble absorption rate is carried out. The particle concentration ranges from 0.005 to 0.5 vol%. It is found that the CO₂ absorption rate is enhanced up to 4.5% at 0.01 vol% of Al₂O₃/methanol nanofluids at 20 °C, and 5.6% at 0.01 vol% of SiO₂/methanol nanofluids at −20 °C, respectively.     

Glass formation and phase transformations in plasma prepared Al2O3-SiO2 powders

The structure of Al₂O₃-SiO₂ sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al₂O₃ and partially amorphous in the range 52 to 88 wt % Al₂O₃. The crystalline phase was mullite up to 75 wt % Al₂O₃ but at higher Al₂O₃ contents a metastable solid solution of SiO₂ in -Al₂O₃ was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al₂O₃ end of the Al₂O₃-SiO₂ system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al₂O₃ contents up to 60 wt %. The viscosity change is relatively small as the Al₂O₃ content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al₂O₃ concentrations greater than approximately 80 wt %, -Al₂O₃ is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al₂O₃. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO₂ in -Al₂O₃, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO₂ in solution in metastable Al₂O₃ increases the temperature of transformation to -Al₂O₃ to greater than 1500° C compared with 1230° C for pure Al₂O₃.     

Isothermal devitrification of an oxynitride LAS glass composition monitored by X-ray powder diffraction

The crystallisation of a Li_0.6Al_0.1Si_0.6O_1.575N_0.05 glass matrix has been studied by means of X-ray powder diffraction at 730°C. Data diffraction and EDS analysis have shown the simultaneous formation of two crystalline phases Li₂SiO₃ and Li_0.6Al_0.6Si_2.4O₆ (so-called virgilite) with no evidence for the presence of nitrogen in these phases. The Avrami exponent has been determined for each phase from several time-dependent X-ray diffraction studies. The values of the Avrami exponent, close to 2.25, suggest that crystallisation is tri-dimensional and controlled by a diffusion process with a decreasing nucleation rate at 730°C. With respect to the aluminium-containing phase, a long time heat treatment at this temperature has not revealed a phase transition from the metastable hexagonal symmetry to the stable tetragonal one. The XRD correlation length values, from the Scherrer equation, are close to 450 Å. Furthermore, SEM micrographs have confirmed the volume crystallization of this glass composition and the approximate linear dimension of the regions of diffracting materials.     

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.     

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.