SrTiO3 glass ceramics

This is the first in a series of two papers describing the crystallization and dielectric properties of glass-ceramics derived from a particular strontium titanium aluminosilicate glass composition. This first paper concerns the development of crystalline phases and microstructure of glass-ceramics prepared under various crystallization conditions. In the following paper, the dielectric properties of these glass-ceramics are described and correlated with the characterization results.Perovskite strontium titanate (SrTiO₃) was the primary crystalline phase in glass-ceramics crystallized over the temperature range of 800 to 1100° C. At crystallization temperatures below 950° C, SrTiO₃ formed with a spherulitic or dendritic growth habit. X-ray diffraction suggested that the SrTiO₃ crystallized in a perovskite-like precursor phase which transformed to perovskite SrTiO₃ with further crystallization time. However, electron diffraction indicated that this precursor phase was cubic perovskite SrTiO₃. At higher crystallization temperatures, perovskite SrTiO₃ was present as individual crystallites without evidence of the spherulitic habit. The crystallization of SrTiO₃ was followed by that of other phases, the hexacelsian and anorthite forms of SrAl₃Si₂O₈, and the rutile and anatase forms of TiO₂. The crystallization sequence and microstructure of the glass-ceramics were determined by the competition for strontium and titanium between the crystallizing phases, SrTiO₃ and SrAl₂Si₂O₈, and TiO₂.     

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.     

Effect of 1 wt% CoO addition on dielectric and microstructural behaviour of (SrO·TiO2)-(2SiO2·B2O3) glass and glass ceramic

Glass of the nominal composition 64 wt%(SrO·TiO₂)·35 wt%(2SiO₂·B₂O₃)-1 wt%(CoO) was prepared. The glass samples were subjected to heat treatment at 900 and 950 C. The phase progression in these glass ceramics from X-ray diffraction studies shows the formation of Sr₂B₂O₅ as primary crystalline phase followed by rutile (TiO₂), Sr₃Ti₂O₇, SrB₂Si₂O₈ and Sr₃B₂SiO₈ as secondary phases. The first DTA exothermic peak of glass corresponds to the crystallization of Sr₂B₂O₅, rutile and Sr₃Ti₂O₇ phase while second crystallization peak may be assigned to the formation of SrB₂Si₂O₈ and Sr₃B₂SiO₈ phases. From microstructure studies we find that strontium borate grows with larger grain size whereas the other phases like Sr₃Ti₂O₇, TiO₂ appear smaller in size. Cobalt oxide content in the strontium titanate borosilicate glass ceramic gives the thermal stability to dielectric behaviour and decreases the dielectric loss. Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995     

Electrical and mechanical properties of alkali barium titanium alumino borosilicate glass-ceramics containing strontium or magnesium

The AC electrical data, measured in the frequency range 0.1 Hz–5 MHz, were used to study the electrical response of lithium barium titanium alumino borosilicate glass-ceramics containing strontium or magnesium. Complex plane plot from these electrical data for various glass ceramic samples reveal contributions from simultaneously operating polarization mechanisms to overall dielectric behaviour. The SrO/BaO or MgO/Al₂O₃ replacements resulted in the increase of the conductivity and dielectric constant of the corresponding glass-ceramic materials. The relationships between the crystalline phases formed and dielectric values of studied glass-ceramics are complicated. The complex impedance of the crystalline materials generally decreases with the addition of SrO instead of BaO and with increase of MgO at the expense of Al₂O₃. The obtained data were correlated to the internal microstructure, crystalline phases formed, glassy matrix and glass to crystal interface region. β-Spodumene together with varieties of Ba-containing phases [e. g. orthorhombic celsian—BaAl₂Si₂O₈, fresnoite—Ba₂(TiO)Si₂O₇], barium silicate Ba₂Si₃O₈, lithium borate—Li₆B₄O₉, lithium metasilicate—Li₂SiO₃ strontium titanate—SrTiO₃, and magnesium borate—MgB₄O₇ phases were mostly developed in the crystallized glasses. The microhardness characteristics of the crystalline glasses have been investigated by Knoop microhardness. The SrO/BaO or MgO/Al₂O₃ replacements resulted into decreasing the microhardness values. The obtained data of the glass-ceramic specimens were explained in relation to the crystalline phases, microstructure formed and the glassy matrix.     

The role of TiO2 and composition in the devitrification of near-stoichiometric cordierite

Devitrification behavior and thermal expansion of glasses and glass-ceramics, doped with TiO₂, near the stoichiometric cordierite composition were investigated. The activation energy for growth of surface nucleated crystals was shown to be approximately 433 kJ/mol. and was independent of TiO₂ content in the glass. Volume nucleation was achieved by the addition of approximately 8 wt% TiO₂, but the mechanism of volume nucleation was different depending upon cordierite composition. Regions in the MgO-Al₂O₃-SiO₂ phase diagram near the stochiometric cordierite compound were investigated using thermal analysis, X-ray powder diffraction and dilatometry. Glass in glass phase separation was postulated for MgO-lean glasses, whereas precipitation of mullite and Al₂TiO₅ preceded devitrification in other compositions. In each case, the formation of a silica-rich glass is believed to initiate the devitrification. Coefficients of thermal expansion for the glass-ceramics increased with increasing TiO₂ content resulting from increasing levels of uncrystallized glass and the formation of mullite and rutile during crystallization.     

Crystallization study of (TiO2, ZrO2)-rich SiO2-Al2O3-CaO glasses Part I Preparation and characterization of zirconolite-based glass-ceramics

Nuclear power reactors generate long-lived radionuclides such as minor actinides (Np, Am, Cm) which are mainly responsible for the long term radiotoxicity of high level nuclear wastes obtained after reprocessing of nuclear spent fuel. Specific highly durable matrices such as glass-ceramics appear as good candidates for the immobilization of minor actinides. This work concerns the synthesis and the characterization of zirconolite (CaZrTi₂O₇) based glass-ceramics prepared by controlled devitrification of (TiO₂, ZrO₂)-rich SiO₂-Al₂O₃-CaO parent glasses for which neodymium was selected to simulate the radioactive trivalent minor actinides. The present study reports the effect of increasing TiO₂, ZrO₂ and CaO amounts in glass composition on the structure and the composition of the zirconolite crystals (formed as the only crystalline phase in the bulk of the glass), on their nucleation rate I(Z) and on the volume proportion of crystalline phase V of the glass-ceramics. It appears that I(Z) and V strongly increase when the parent glass composition changes. Neodymium electron spin resonance (ESR) shows that the total amount of Nd³⁺ ions incorporated in the zirconolite phase increases with TiO₂, ZrO₂ and CaO amounts in parent glass composition.     

Characterization of crystal phases,morphology and crystallization processes in lithium aluminosilicate glass-ceramic

The nucleation and crystallization processes of Li₂O-Al₂O₃-SiO₂ glass-ceramics were investigated by differential thermal analysis. The crystalline phases produced during thermal treatment at different temperatures and the residual glassy phase were characterized by X-ray diffraction, SEM and image analysis techniques. The activation energy of the crystallization process was calculated as E=380±20 kJ mol^–1. The influence of nucleating agents (TiO₂, ZrO₂) was evaluated to obtain glass-ceramics transparent to visible light. The stability of the glassy phase in cooling was determined by means of temperature-time-transformation curves.     

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.     

Crystallization and microstructures of Y-Si-Al-O-N glass-ceramics containing main crystal phase Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>

A glass with the nominal composition of 28Y48Si24Al83O17N (in equal percentage) was chosen as parent glass in this paper to prepare Y₃Al₅O₁₂-based glass-ceramics. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to assess the crystallization process of the parent glass. YAG as the only crystalline phase appears in all glass-ceramics produced under 1250°C. A small amount of O’-Sialon secondary phase starts to precipitate from parent glass samples as heat treatment temperature increases to 1250°C. Grain size of the dendrite crystal which corresponds to YAG phase increases and the dendrite branches get thickened as heat treatment temperature increases. Moreover, grain size of YAG phase resulting from two-stage heat treatment is much smaller than that of YAG phase obtained by one-stage heat treatment. The results are relevant to developing improved crystallization treatments for glasses with potential for crystallization to YAG-based glass-ceramics and for heat treatments of YAG/β-SiAlON materials.     

熔制保温时间对CAMS系矿渣微晶玻璃结构与性能的影响

以白云鄂博二次选后尾矿、高炉渣和粉煤灰为主要原料,采用熔融法制得CaO-Al_2O_3-MgO-SiO_2(CAMS)系微晶玻璃,利用DSC、XRD、SEM、EDS、Raman、红外光谱及理化性能测试手段,研究了熔制过程中不同保温时间对微晶玻璃结构与性能的影响。结果表明:随着熔制保温时间的延长,矿渣玻璃熔液中氟的挥发及对氧化铝坩埚的侵蚀导致基础玻璃中氟含量降低而Al2O3含量升高,基础玻璃析晶温度呈先上升后下降的趋势,微晶玻璃主晶相为透辉石,晶粒出现先细化后粗化的趋势,第二相萤石相随着保温时间的延长逐渐消失,最终导致微晶玻璃结构及理化性能发生改变。当玻璃熔制保温时间为5h时,制备的微晶玻璃综合性能最优,其密度、抗折强度、显微硬度及耐酸碱性分别为3.09g/cm3、201 MPa、7 021 MPa、97.78%和98.83%。     

Crystallization and optical properties of a transparent mullite glass ceramic

Transparent mullite - based glass ceramics in the system SiO₂-Al₂O₃-B₂O₃-ZnO-K₂O were prepared. The activation energy of crystallization, the Avrami constants and the mechanism of crystallization of mullite were determined by the Matusita method through differential thermal analysis. X-ray diffraction and scanning electron microscopy methods were also used to determine the type and morphology of the precipitated crystalline phase. In addition, the degree of crystallinity was determined by the internal standard method. The results showed that bulk crystallization was the main mechanism for crystallization of the glasses. Optical absorption and photoluminescence properties of the obtained transparent glass-ceramics and their initial glasses were also studied. The glass-ceramics exhibited two emissions at 680 and 700 nm.     

Spectroscopic characterization of alkali borosilicate glasses containing iron ions

Structural properties of alkali borosilicate glasses containing iron ions were investigated using infrared, laser Raman and Mössbauer spectroscopy. Two types of glasses were prepared: SRL-type with the composition 18.5 wt% Na₂O, 10.0 wt% B₂O₃, 52.5 wt% SiO₂, 4.0 wt% Li₂O, 10 wt% TiO₂ and 5.0 wt% CaO, and sodium borosilicate glass with the composition 16.7 wt% Na₂O, 18.7 wt% B₂O₃ and 64.6 wt% SiO₂. Raman spectroscopy showed that orthosilicates are the dominant amorphous phase in the SRL-type of glass. Incorporation of iron in the SRL-type of glass induced polymerization of silicate units and -Si-O-Fe- copolymerization. It was concluded that different amorphous phases are simultaneously present in the SRL-type of glass containing iron ions. Interpretation of the Raman spectra is given. Incorporation of iron ions into the sodium borosilicate glass also affected the corresponding IR spectra. The valence state of iron and its coordination were determined by⁵⁷Fe Mössbauer spectroscopy.     

Incorporation of transition metal in porous glass-ceramics of TiO2-SiO2 system

Incorporation of transition metals in porous glass-ceramics of TiO₂-SiO₂ system was made by the phase separation and crystallization of the glasses of TiO₂-SiO₂-Al₂O₃-P₂O₅-CaO-MgO system containing various kinds of transition metals. The amount of transition metals incorporated in the skeleton of the porous glass-ceramics was dependent on both chemical composition of mother glass and conditions of heat treatment. In general the amount decreased with the increasing amount of rutile in the skeleton. In the glass of high TiO₂/SiO₂ ratio, the incorporation of relatively large amounts of transition metals was possible even if the precipitation of a fairly large amount of rutile occurred. The crystallization of rutile and in porous glass-ceramics was essential to fabricate rigid platelet porous glass-ceramics.     

Synthesis and crystallization of yttrium-aluminium garnet and related compounds

Amorphous oxide combustion products with compositions corresponding to Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ were synthesized by the glycine-nitrate process and heat-treated to induce crystallization. The crystalline structure of the resulting powders was determined by powder X-ray diffraction techniques. The phase stabilities of the crystalline phases were investigated as functions of the glycine-to-nitrate ratio, the yttrium-to-aluminium ratio, and the heat-treatment conditions. Heat treatment for short durations resulted in incompletely crystalline powders that consisted of a mixture of Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ phases, regardless of the chemical composition of the amorphous combustion product. However, heat treatment for longer durations or higher temperature generated both pure-phase, monoclinic Y₄Al₂O₉ and Y₃Al₅O₁₂ with the garnet structure. Prolonged heat treatment at high temperature failed to generate pure-phase orthorhombic YAlO₃. Subsequent analysis revealed a sluggish, complex crystallization process involving the formation and decomposition of several phases.     

Effect of fluorine content on crystallization of canasite glass-ceramics

The effect of fluorine (F) content in the parent glass on nucleation and crystallization of glass-ceramics with canasite as the principle crystalline phase have been studied using differential thermal analysis, X-ray diffraction, and transmission electron and scanning electron microscopies. CaF₂ precipitation occurred during the nucleation phase at all concentrations studied. However, canasite formation during subsequent crystallization was strongly dependent on F content. Canasite failed to form at both high and low concentrations of F, while with intermediate F concentrations canasite and CaF₂ were the only phases identified. Agrellite (NaCa₂Si₄O₁₀F) formed when the F content was high.     

Effects of MO (M = Mg, Ca, Ba) on crystallization and flexural strength of semi-transparent lithium disilicate glass-ceramics

In this study, semitransparent lithium disilicate glass-ceramics in the Li₂O-K₂O-Al₂O₃-Y₂O₃-La₂O₃-SiO₂ system was investigated by incorporation of P₂O₅ as nucleation agent and alkaline earth oxides as additive. The influence of alkaline earth oxides on the structure of glasses network, crystalline phases, microstructure and mechanical properties were investigated by means of Raman spectra, differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The mechanical strength was measured corresponding to norm ISO 6872. The Raman spectra predominantly showed that small additions of alkaline earth oxides not only form asymmetrical vibrations of the M-Si-O bonds, but also enhance the intensities of symmetrical vibrations of the P-O bonds, making the glass network more stable. And the small additions of CaO or BaO has more influence on the crystallization behaviour, crystalline phase, microstructure and mechanical properties of the glass-ceramics than the addition of MgO. The additions of alkaline earth oxides enhanced the first exothermic peak temperature but decreased the flexural strength of lithium disilicate glass-ceramics.     

Crystallization of sol-gel derived glass ceramic powders in the CaO-MgO-Al2O3-SiO2 system

Samples in the cordierite primary crystallization field were prepared in the CaO-MgO-Al₂O₃-SiO₂ quaternary system by sol-gel techniques, using both polymeric and colloidal gels. Dried gels were characterized by infrared spectroscopy, thermal analysis and electron microscopy. The crystallization of gel-like glass from both kinds of gels, as a function of temperature was followed by X-ray diffraction. Cordierite was detected as the only final crystalline phase, although some other crystalline phases, different for each kind of gel, were observed in the pathway reaction. The polymorphic modification of cordierite detected was pseudohexagonal, because no evidence of splitting was observed.     

Vitrification of copper flotation waste

The vitrification of an hazardous iron-rich waste (W), arising from slag flotation of copper production, was studied. Two glasses, containing 30wt% W were melted for 30min at 1400 degrees C. The first batch, labeled WSZ, was obtained by mixing W, blast furnace slag (S) and zeolite tuff (Z), whereas the second, labeled WG, was prepared by mixing W, glass cullet (G), sand and limestone. The glass frits showed high chemical durability, measured by the TCLP test. The crystallization of the glasses was evaluated by DTA. The crystal phases formed were identified by XRD resulting to be pyroxene and wollastonite solid solutions, magnetite and hematite. The morphology of the glass-ceramics was observed by optical and scanning electron microscopy. WSZ composition showed a high rate of bulk crystallization and resulted to be suitable for producing glass-ceramics by a short crystallization heat-treatment. WG composition showed a low crystallization rate and good sinterability; glass-ceramics were obtained by sinter-crystallization of the glass frit.     

Thermal analysis and phase evolution of ferroelectric PbTiO<Subscript>3</Subscript> obtained from silicate and borate based glasses

This article assessed the glass formability for crystallization of lead titanate (PbTiO₃) as the primary phase. It was found that silicate-based glasses formed glass as a block without the need for cold splatting, but B₂O₃-based glasses needed to be prepared by cold splatting to retard devitrification. Also, incorporation of BaO was favoured by an increase in the concentration of B₂O₃. DTA showed the crystallization of crystalline phases occurred above glass transition temperature. XRD traces supported the DTA data where the first exothermic peak in each case was presumed to be related to the crystallization of PbTiO₃ (PT) phase. XRD also showed the lower c/a ratios compared to pure PT ceramic, which suggested three possibilities for lower values: crystal clamping by the rigid glass matrix, intrinsic size effect and incorporation of impurities in the PT phase.     

The role of excess hydroxyl ions in the crystallization behaviour of an Al2O3-ZrO2-SiO2-Na2O glass-ceramic system

The crystallization behaviour of a gel-derived Al₂O₃-ZrO₂-SiO₂-Na₂O glass ceramic system under different heat-treatment atmospheres has been studied. The excess hydroxyl ions retained in the treated gel were found to play a crucial role during the crystallization of various phases. The evolution of phase transformations from gels to crystalline phases was monitored with infrared spectroscopy, X-ray diffraction and differential thermal analysis.