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1.
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. 相似文献
2.
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. 相似文献
3.
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). 相似文献
4.
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. 相似文献
5.
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. 相似文献
6.
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. 相似文献
7.
M. Kobayashi T. Meguro K. Komeya T. Yokoyama J. Funahashi T. Kameda 《Journal of Materials Science》2000,35(16):4129-4136
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. 相似文献
8.
K. Wakasa A. Ikeda Y. Yoshida M. Yamaki 《Journal of materials science. Materials in medicine》1995,6(1):29-31
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. 相似文献
9.
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. 相似文献
10.
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. 相似文献
11.
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. 相似文献
12.
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. 相似文献
13.
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. 相似文献
14.
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. 相似文献
15.
Oswaldo Burciaga-DiazJose Ivan Escalante-Garcia Alexander Gorokhovsky 《Cement and Concrete Composites》2012,34(1):18-24
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. 相似文献
16.
Jae Won LeeJung-Yeul Jung Soon-Geul LeeYong Tae Kang 《International Journal of Refrigeration》2011,34(8):1727-1733
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. 相似文献
17.
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. 相似文献
18.
Jean Rocherullé 《Journal of Materials Science》2003,38(7):1425-1429
The crystallisation of a Li0.6Al0.1Si0.6O1.575N0.05 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 Li0.6Al0.6Si2.4O6 (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. 相似文献
19.
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. 相似文献
20.
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. 相似文献