Opaque Glass-ceramics Research Articles

Glass and glass-ceramics from red mud tailings: Understanding the evolution mechanism

Large quantities of red mud tailings cause serious negative impacts to both the environmental sustainability and public health. Eco-friendly utilization of tailings can address this issue and achieve its commercial value. We reported an approach to preparing transparent basic glass and opaque glass-ceramics utilizing red mud tailings as the starting material. Crystallization kinetics, phase transition, microstructure evolution, and performances of glass-ceramics were studied. The crystallization activation energy was calculated to be only 125.5 kJ/mol, which made the transformation from red mud tailings to glass-ceramics much easier. The Avrami coefficient of crystallization was calculated to be around 2, revealing that one-dimensional bulk crystallization occurred in glass-ceramics. Fluorapatite crystals having a one-dimensional morphology were found to generate in glass-ceramics, which endowed the glass-ceramics with superior performances, such as the density of 2.7 g/cm3, the crack resistance of 2.43 MPa m1/2, and the Vickers hardness of 643.67 HV. These results and findings demonstrate that hazardous red mud tailings could become eco-friendly glass and glass-ceramics for construction applications.

Crystallization and microstructure in K2O substituted SiO2–MgO–Al2O3–Li2O–AlPO4 glass-ceramics

The substitution of K+ ion by Ca2+ or Sr2+ in K2O–SiO2–MgO–Al2O3–B2O3–MgF2–Li2O–AlPO4 glass is executed and the relevant effects on crystallization and microstructure are explored. The glass monoliths obtained by single-step melt-quenching at 1500 ± 10 °C were transparent in nature. Density of base glass was evaluated as 2.66 ± 0.03 g cm−3 and found to be decreased by in presence of CaO or SrO. Opaque glass-ceramics were derived from the transparent glasses by controlled heat-treatment at 1000 ± 10 °C and the predominant crystalline phase was identified as fluorophlogopite mica, KMg3AlSi3O10F2 by X-ray diffraction. Field emission scanning electron microscopy revel that the plate like fluorophlogopite mica crystals predominated in base glass-ceramic (without CaO or SrO doped) but changed to nanocrystalline morphology containing droplet like spherical shaped mica crystallites (average size 200–400 nm) on addition of CaO and SrO. Highest density (=2.74 ± 0.03 g.cm-3) observed for SrO containing glass-ceramic is attributed to the nanocrystalline morphology.

Remarkable photoluminescence enhancement and tuning in Eu/CdS co-doped chloroborosilicate glass-ceramics

The effect of heat treatment was studied on Eu/CdS co-doped chloroborosilicate glass having a new composition 37.8SiO2–27B2O3–18BaO–3.6K2O–3.6Al2O3–10BaCl2 (mol%) recently developed in our group. The base glass has been proven to be a suitable host for various rare-earth ions and metallic nanoparticles, also upon controlled crystallization, because it has the potential to yield low-phonon energy (350 cm−1) BaCl2 crystals. The glasses were heat treated at 650 °C for several time durations. Transmission electron microscopy (TEM) revealed the presence of CdS quantum dots (QDs) and BaCl2 crystals in the heat-treated glasses. X-ray diffraction studies confirmed the formation of glass-ceramics, with the presence of BaCl2-based phases. When heat-treated, all the Eu/CdS co-doped glasses showed an intense enhancement in Eu3+/Eu2+ emissions up to 30 or 40 h of heat treatment, followed by a gradual decrease. The initial increase is due to the formation of low-phonon energy BaCl2-based crystalline phases as well as the generation and growth of CdS QDs at elevated temperature. The subsequent decrease in PL intensity is attributed to the concentration quenching effect of the QDs. The rate of enhancement was significantly higher when glasses formed opaque glass-ceramics due to the change in environment of the Eu ions and scattering effect. The maximum intensity enhancement due to the CdS co-doping and glass crystallization was observed to be 400 times as compared to the non heat-treated glass containing the same amount of Eu. This enhancement is quite remarkable in rare-earth/semiconductor co-doped glasses and can be useful in various photonic applications.

Wide thermal expansion in Ag0/Au0 nanoparticle doped SiO2-MgO-Al2O3-B2O3-K2O-MgF2 glass-ceramics

This report demonstrates high temperature sealing tendency of alumino-silicate glasses (ASG) on the system SiO2-MgO-Al2O3-B2O3-K2O-MgF2 doped with silver (Ag) and gold (Au) nanoparticles. Density of the melt-quenched (1550 °C) glasses were in 2.51–2.55 g.cm−3. The glasses were heat-treated at 900 ± 10 °C over which the opaque glass-ceramics were obtained with major crystalline phase (XRD) fluorophlogopite mica [KMg3(AlSi3O10)F2] with secondary phase norbergite (Mg2SiO4.MgF2). FESEM study indicated the development of Rod-like and rock-like crystallite particles (size ∼1–4 µm) randomly dispersed in mother glass–ceramic which was restructured into nanocrystalline morphology (crystallite size ∼100–400 nm) with the aid of Ag- or Au- nanoparticles. The significant change in microstructure influenced the corresponding density (2.51–2.55 g.cm−3) and thermal expansion characteristics. Coefficient of thermal expansion (CTE) for ASG was estimated to be 9.96 ± 0.10×10−6/K (50–800 °C), which increased to 10.46–11.01×10−6/K when doped with Ag- or Au- content. Such wide CTE (11.01 ± 0.11×10−6/K) value matches well with the CTE of ceramic/metallic materials used for high temperature application thus Ag- doped SiO2-MgO-Al2O3-B2O3-K2O-MgF2 glass can be a suitable for high temperature sealing application (like SOFC).

Microstructural characterization and wear properties of silver and gold nanoparticle doped K-Mg-Al-Si-O-F glass-ceramics

In ascertaining the effects of silver (Ag) and gold (Au) nanoparticles on crystallization of boro-alumino-silicate system; the K2O-MgO-Al2O3-SiO2-B2O3-F glasses doped with/without 0.2 wt% Ag- and Au- content were melt-quenched at 1550 °C. Doping of nanoparticles considerably increased the glass-transition temperature and softening point but decreased the thermal expansion. A sharp crystallization exotherm in differential scanning calorimetry (DSC) is observed at 750 °C ( ± 1 °C) for glass without nanoparticle and that broadened to 800–855 °C when contains nanoparticle. Opaque glass-ceramics were derived from the glasses by controlled heat-treatment at 1050 °C with predominant crystalline phase fluorophlogopite (KMg3AlSi3O10F2) mica. Traces of Ag- and Au- particles were also identified from X-ray diffraction (XRD) technique. The activation energy (Ec) of crystallization (344 ± 17 kJ/mol) is decreased to 233 ( ± 12) and 307 ( ± 15) kJ/mol (Kissinger method) on doping with Ag- and Au- nanoparticles, respectively. Compact microstructure (FESEM) composed of rock like and plate-like mica crystals are developed in base glass-ceramic and that gets restructured to interlocked type morphology in presence of Ag- nanoparticle. Significant microstructural change induced by nanoparticle addition caused the decrease in microhardness (4.31–4.66 GPa) and increase in thermal expansion. Friction and wear testing under reciprocative sliding (using WC-Co ball) exposed that the average coefficient of friction (COF) is 0.60 ± 0.2 for all glass-ceramics at 20 N load and 10 Hz frequency. At a lower load of 5 N, the average COF value is increased from 0.69 to 0.92 on use of Au-nanoparticle. A Similar trend was also observed at 10 N load as COF increased from 0.62 to 0.78.

Zn2+- Controlled Crystallization and Microstructure in K-Li-Mg-B-Si-Al-F Glass

Abstract Text:The crystallization of (9-X) K2O-1Li2O-12MgO-10B2O3-40SiO2-16Al2O3-12MgF2-X PbO/BaO/ZnO (X =0/5) composition (wt.%) were studied by means of dilatometry, DSC, XRD, SEM and microhardness analysis. Density of base K-Li-Mg-B-Si-Al-F glass (2.59 g.cm–3) is found to be increased on addition of the network modifier oxides PbO, BaO and ZnO content. Addition of Pb2+, Ba2+ and Zn2+ furthermore increased the glass transition temperature (Tg.). A characteristic exothermic hump is found to be appeared in DSC thermograph at the temperature range 800-950°C; and that is ascribed to the formation of crystalline phase fluorophlogopite mica, KMg3(AlSi3O10)F2. Opaque glass-ceramics were prepared from K-Li-Mg-B-Si-Al-F glasses (with and without containing PbO, BaO and ZnO content) by controlled heat-treatment at 1000°C. Interlocked type microstructure combined of flake like fluorophlogopite mica crystals is obtained in ZnO-containing K-Li-Mg-B-Si-Al-F glass-ceramic; and such microstructural pattern is ascribed to cause large thermal-expansion (>11.5×10-6/K, 50-800°C).Vickers Microhardness of base glass-ceramic (5.12 GPa) is increased when contains ZnO (5.26 GPa). ZnO-containing boroaluminosilicate glass-ceramic is, hence, considered with potential interest as they can exhibit the microcrack resistivity in high temperature recycling operation (like SOFC).

Vitrification and derived glass-ceramics from mining wastes containing vermiculite and lithium aluminium phosphate

The waste vitrification of abandoned open sky vermiculite deposits has been considered by combining with a natural phosphate mineral residue. Several batches haven been designed from the composition system: Li2O-MgO-Al2O3-P2O5-SiO2 including some Fe2O3 and Fluoride. The resulting glasses are transparent and smooth green coloured, giving rise after TTT treatments to several opal, opaque glass-ceramics with iridescent surface. Full characterization has been carried out by XRD and electron microscopy with EDS, as well as by XPS spectroscopies, concluding that the main crystalline phases formed were α-cordierite and β-spodumene. The surface of these glass-ceramics from vermiculite-amblygonite is enriched in Fe2O3. Compared to the parent glasses, the final glass-ceramics exhibited and improvement in fracture toughness.

Rare earth ion controlled crystallization of mica glass-ceramics

In understanding the effects of rare earth ions to control the crystallization and microstructure of alkaline boroaluminosilicate system, the CeO2, Nd2O3, Sm2O3 and Gd2O3 doped K2OMgOB2O3Al2O3SiO2F glasses were synthesized by melt-quenching at 1550 °C. Higher density (2.82–3.06 g cm−3) and thermal stability (glass phase) is experiential on addition of rare earth content, which also affects in increasing the glass transition temperature (Tg) and crystallization temperature (Tc). Decrease of thermal expansion in glasses with rare earth ion content is maintained by the stabilization of glass matrix owing to their large cationic field strength. A significant change in the non-isothermal DSC thermogram observed at 750–1050 °C is attributed to fluorophlogopite crystallization. Opaque glass-ceramics were prepared from such glasses by single step heat-treatment at 1050 °C; and the predominant crystalline phases are identified as fluorophlogopite mica, KMg3(AlSi3O10)F2 by XRD and EDX analysis. The compact glass-ceramic microstructure by the agglomeration of fluorophlogopite mica crystallites (crystal size ∼ 100–500 nm, FESEM) is achieved in attendance of rare earth ion; and such microstructure controlled the variation of density, thermal expansion and microhardness value. Higher thermal expansion (11.11–14.08×10−6/K at 50–800 °C and 50–900 °C) of such glass-ceramics approve that these rare earth containing glasses can be useful for high temperature vacuum sealing application with metal or solid electrolyte. The increase of Vickers microhardness (5.27–5.61 GPa) in attendance of rare earth ions is attributed to the compact crystallinity of fluorophlogopite mica glass-ceramic microstructure.

Spectroscopic properties of Eu3+/Nd3+ co-doped phosphate glasses and opaque glass–ceramics

This paper reports the fabrication and characterization of Eu3+/Nd3+ co-doped phosphate (PNE) glasses and glass–ceramics as a function of Eu3+ concentration. The precursor glasses were prepared by the conventional melt quenching technique and the opaque glass–ceramics were obtained by heating the precursor glasses at 450°C for 30h. The structural and optical properties of the glass and glass–ceramics were analyzed by means of X-ray diffraction, Raman spectroscopy, UV–VIS–IR absorption spectroscopy, photoluminescence spectra and lifetimes. The amorphous and crystalline structures of the precursor glass and opaque glass–ceramic were confirmed by X-ray diffraction respectively. The Raman spectra showed that the maximum phonon energy decreased from 1317cm−1 to 1277cm−1 with the thermal treatment. The luminescence spectra of the glass and glass–ceramic samples were studied under 396nm and 806nm excitation. The emission intensity of the bands observed in opaque glass–ceramic is stronger than that of the precursor glass. The luminescence spectra show strong dependence on the Eu3+ ion concentration in the Nd3+ ion photoluminescence (PL) intensity, which suggest the presence of energy transfer (ET) and cross-relaxation (CR) processes. The lifetimes of the 4F3/2 state of Nd3+ ion in Eu3+/Nd3+ co-doped phosphate glasses and glass–ceramics under 806nm excitation were measured. It was observed that the lifetimes of the 4F3/2 level of Nd3+ of both glasses and glass–ceramics decrease with the increasing Eu3+ concentration. However in the case of opaque glass–ceramics the lifetimes decrease only 16%.

Multi-color long-lasting phosphorescence in Mn 2+-doped ZnO–B 2O 3–SiO 2 glass–ceramics

Multi-color LLP phenomenon was observed in Mn 2+-doped ZnO–B 2O 3–SiO 2 glass–ceramics after the irradiation of a UV lamp at room temperature. Transparent ZnO–B 2O 3–SiO 2 glass emitted reddish LLP while opaque glass–ceramics prepared by the glass sample after heat treatment emitted yellowish or greenish LLP. The change of the phosphorescence is due to the alteration of co-ordination state of Mn 2+. The phosphorescence of the samples was seen in the dark with naked eyes even 12 h after the irradiation with a UV lamp ( λ max=254 nm) for 30 min. Based on the approximative t −1 decay law of the phosphorescence, we suggest that the LLP is attributed to the thermally assisted electron–hole recombination.