Development and characterisation of a Y2Ti2O7-based glass-ceramic as a potential oxidation protective coating for titanium suboxide (TiOx)

A silica-based glass-ceramic, with Y₂Ti₂O₇ as the major crystalline phase, is designed, characterised and tested as an oxidation-protective coating for a titanium suboxide (TiO_x) thermoelectric material at temperatures of up to 600 °C. The optimised sinter-crystallisation treatment temperatures are found to be 1300 °C and 855 °C for a duration of 30 min, and this treatment leads to a glass-ceramic with cubic Y₂Ti₂O₇ and CaAl₂Si₂O₈ as crystalline phases. An increase of ~270 °C in the dilatometric softening temperature is observed after devitrification of the parent glass, thus further extending its working temperature range.Excellent adhesion of the glass-ceramic coating to the thermoelectric material is maintained after exposure to a temperature of 600 °C for 120 h under oxidising conditions, thus confirming the effectiveness of the T1 glass-ceramic in protecting the TiO_x material.     

Effect of sintering temperature on solidification and migration of non-volatile and volatile heavy metals in glass-ceramic

Using tailings as material to prepare glass-ceramic is an excellent way to achieve the resource utilization of solid waste. However, at present, researches on the solidification and migration of heavy metals are limited. Therefore, in this study, ten groups of samples were prepared by controlling sintering temperatures. The solidification, migration, and leaching behavior of non-volatile and volatile heavy metals were studied. The research showed that, with the increase of temperature, the properties of the samples were improved. Fe participated in the phase transformation and evolved into insoluble iron pyroxenes solid solution, while Pb was homogeneously distributed in the glass matrix of glass-ceramics. The leaching concentrations of Fe and Pb in the glass-ceramics were 0.055 mg/L ～0.087 mg/L and 0.074 mg/L ～0.140 mg/L, which were far below the threshold value. The results showed that heavy metals can be effectively solidified in glass-ceramics and have good environmental benefits.     

Precipitating spinel into precursor glass and its assistance in crystallization

The crystallization phenomena of spinel in CaO-MgO-Al₂O₃-SiO₂-Fe₂O₃ glass have received much attention due to the particular role in preparation of glass-ceramic materials, which represent an effective option to manage hazardous waste. In this study, both preliminary spinel and secondary spinel were precipitated in the precursor glass. The formation of these spinel was meticulously assessed by a combination of X-ray diffractometry and scanning electron microscopy. The structure of the microenvironment in the precursor glass was characterized by Raman spectrums. These advanced techniques highlight the potential for one-step crystallization of the glass. The investigation, which focused on one-step crystallization, demonstrated the growth of pyroxene on spinel accompanying a migration of chromium. The results also show the microstructure of the obtained glass-ceramic was very dependent on the heat-treat temperature. This study not only unambiguously reveals the precipitation mechanisms of spinel but also provides more documentation for one-step crystallization in the glass-ceramics field.     

高掺量赤泥-粉煤灰微晶玻璃研究

赤泥是氧化铝生产中排放的碱性废渣,粉煤灰是燃煤电厂排放的高SiO2含量废渣.综合利用两种废渣制备高性能的微晶玻璃材料,控制SiO2含量在31%～44%,CaO含量在25%～31%,两种废渣总吃渣量能够达到90wt%以上,具有显著经济效益和环境效益.     

微晶玻璃板材工业化生产的探讨

本文探讨烧结成形的微晶玻璃板材的化学成分、原料、工艺流程、生产线设计、合理生产规模、产品成本、工业生产前景.     

Photoluminescence of transparent glass-ceramics based on ZnO nanocrystals and co-doped with Eu3+, Yb3+ ions

Glasses of the K₂OZnOAl₂O₃SiO₂ system co-doped with Eu₂O₃ and Yb₂O₃ were prepared by the melt-quenching technique. Transparent zincite (ZnO) glass–ceramics were obtained by secondary heat-treatments at 680–860 °C. At 860 °C, traces of Eu oxyapatite appeared in addition to ZnO nanocrystals. The average crystal size obtained from the X-ray diffraction data was found to range between 14 and 35 nm. Absorption spectra of the initial glasses are composed of an absorption edge and absorption bands due to electronic transitions of Eu³⁺ ions. With heat-treatment, the absorption edge pronouncedly shifts to the visible spectral range. The luminescence properties of the glass and glass-ceramics were studied by measuring their excitation and emission spectra at 300, 78, and 4.2 K. Strong red emission of Eu³⁺ ions dominated by the ⁵D₀–⁷F₂ (612 nm) electric dipole transition was detected. Changes in the luminescence properties of the Eu³⁺-related excitation and emission bands were observed after heat-treatments at 680 °C and 860 °C. The ZnO nanocrystals showed both broad luminescence (400–850 nm) and free-exciton emission near 3.3 eV at room temperature. The upconversion luminescence spectrum of the initial glass was obtained under excitation of the 976 nm laser source.     

Crystallisation behaviour of the iron oxide-devoid BOF slag with adding SiO2

The current research attempted to investigate the crystallisation mechanism of iron oxide-devoid basic oxygen furnace (BOF) slag with adding SiO₂. First, the glass sample was prepared by adding 29?wt-% of SiO₂ to BOF slag, followed by eliminating iron oxide by reduction process. Non-isothermal DSC analysis together with confocal laser microscopy, XRD and EPMA mappings were carried out to observe the crystallisation process. The glass sample showed that the crystallisation process started from the surface where the main phases were identified to be akermanite, merwinite and wollastonite. In addition, the crystallisation process was affected by the nucleation temperature which was decided by the heating rate because of the difference in the nucleation rate between wollastonite and Mg-rich phases. The current results could be used to propose the feasibility of utilising BOF slag as glass-ceramics by chemical modification with heat treatment, which controls the crystallisation behaviour.     

Effect of ZnO/K2O ratio on the crystallization sequence and microstructure of lithium disilicate glass-ceramics

The effect of ZnO/K₂O (Z/K) ratio on the crystallization sequence and microstructure of lithium disilicate (Li₂Si₂O₅: LS2) glass-ceramics was carefully investigated for the SiO₂-Li₂O-K₂O-ZnO-P₂O₅ system. The Z/K ratios of precursor glasses were varied from 0 to 3.5 while the nucleating agent of P₂O₅ and glass modifiers of ZnO plus K₂O were fixed to have 1.5 and 4.5 mol% relative to LS2, respectively. For the samples prepared by two-stage heat treatments of 500 °C for 1 h and 800 °C for 2 h in air, the LS2 nucleation rate was increased with increasing the Z/K ratio due to the variation in crystallization sequence from type II (Li₂SiO₃: LS) to type I (LS + LS2) in addition to an amorphous phase separation in base glass. Consequently, with increasing the Z/K ratio, the LS2 crystalline phase within the glass matrix continuously changed from larger acicular ones to smaller equiaxed ones.     

Combined sodium and fluorine promote diopside continuous growth to achieve one-step crystallization in CaO-Al2O3-SiO2-Fe2O3 glass-ceramics

In CaO-Al₂O₃-SiO₂-Fe₂O₃ (CASF) glass-ceramics, a dense silicon and aluminum tetrahedral network barrier layer is formed around the diopside nucleus, which hinders the continuous growth of the crystal. In this work, the continuous growth of the diopside crystals was achieved by introducing sodium and fluorine. A combination of FTIR, XRD, SEM and EDS were used to analyze the evolution mechanism of the precursor glass network structure and the growth process of the diopside. The modification mechanism of sodium and fluorine action on the glassy structure was revealed by ²⁹Si NMR. The results showed that sodium and fluorine can selectively destroy the [SiO₄]/[AlO₄] network structure. The Ca²⁺ diffusion channels formed around diopside, which facilitated the continuous growth of the crystals. The diopside crystals can grow to an expected size, approximately 1 μm, when the value of NBO/T reaches 1.44.     

Effect of CuO addition on crystallization and thermal expansion properties of Li2O–ZnO–SiO2 glass-ceramics

A Li₂O-ZnO-SiO₂ (LZS) glass system was modified with CuO, and its phase development, microstructure evolution, crystallization kinetics and thermal expansion properties were investigated as a function of heat treatment temperature. As a result of the X-ray diffraction study and microstructure observation, lithium zinc silicate formed as the first crystalline phase with increasing heat treatment temperature. Silica polymorph developed as minor crystalline phase at higher temperatures. From the X-ray diffraction patterns, CuO addition led to a decrease in both the crystallization temperature of lithium zinc silicate phase and the volume fraction of quartz phase. According to the crystallization kinetics, the crystallization activation energy for lithium zinc silicates is almost equal to the diffusion activation energy of Zn²⁺ in glass, it suggests that the diffusion of network modifier Zn²⁺ dominates the crystallization of lithium zinc silicates. Additionally, CuO addition caused the transition of Zn²⁺ from network modifiers to network formers. From the thermal expansion coefficient measurements, two abrupt changes in slope of the thermal expansion curves were observed and attributed to the phase transitions of cristobalite and lithium zinc silicate, respectively. Comparison of the thermal expansion coefficient of two types of glass-ceramics revealed that CuO addition in the LZS system can partly inhibit the formation of cristobalite at high temperatures.