Low temperature production of glass ceramics in the anorthite–diopside system via sintering and crystallization of glass powder compacts

The production of glass ceramics (GCs) with theoretical anorthite–diopside (An–Di) weight ratios of 60/40, 50/50 and 45/55 via sintering and crystallization of glass powder compacts was investigated at different temperatures between 800 and 950 °C. The investigated compositions are located in the cross-section of the ternary fluorapatite–An–Di system close to An–Di binary joint, with constant fluorapatite content of 4.8 wt.%. Two different groups of glass powders, with mean particle size of 2 and 10 μm, were used. The experimental results showed that sintering is almost complete at 800 °C, preceding crystallization, which takes place via surface crystallization mechanism. The properties values of the produced GCs, which are the best for the composition close to An–Di eutectic line, are discussed with respect to the evolution of crystalline phases and the microstructure over increasing firing temperature. Under the technology perspective, the investigated processing route is significantly superior in comparison to the attempts reported in earlier studies.     

Effect of MgO addition on the properties of PbO –TiO2–B2O3 glass and glass–ceramics

Glass samples with composition of (50?X) PbO–X MgO–25 TiO₂–25B₂O₃ (where X=0, 5, 10 and 15 mol%) were prepared using conventional quenching technique. The amorphous nature of glass samples were confirmed by XRD. The glass transition temperature, T_g and crystallization temperature T_c were determined from the DTA. It has been observed that the addition of MgO enhances the T_g. The rise in T_g with MgO content may be attributed to the greater field strength of Mg²⁺ cation (as compared to Pb²⁺) which leads to the formation of stronger bonds. These glass samples were converted to glass–ceramics by following a two-stage heat treatment schedule. It was observed that there was good correlation between the density and CTE results of the glass–ceramics. The XRD results revealed the formation of tetragonal lead titanate as a major crystalline phase in the glass–ceramics. The addition of MgO to the glass contributes to the formation of MgB₄O₇. The dielectric constant for all the glass–ceramic samples was observed to be higher than that of corresponding glass samples. Further, with addition of MgO the room temperature dielectric constant for glass–ceramic samples increases up to 10 mol% of MgO and then decreases for 15 mol%. It has been further observed that the variation of dielectric constant of glass–ceramic samples with MgO content is exactly opposite to the variation of crystallite size of PbTiO₃ embedded in the glass ceramic-samples.     

Effect of iron oxide content on the crystallisation of a diopside glass–ceramic glaze

The effect of iron oxide content on the crystallisation of a diopside glass–ceramic glaze was investigated using a glass–ceramic frit in the K₂O–ZnO–MgO–CaO–Al₂O₃–SiO₂ system and a granite waste glass. Measurements by X-ray diffraction (XRD) combined with scanning electron microscopy (SEM) and EDX microanalysis showed that the distribution of Fe³⁺ ions among different crystalline phases such as franklinite (ZnFe₂O₄) and hematite Fe₂O₃ depends on the iron content in the original diopside mixture. Thus, the original glaze crystallises to franklinite or hematatite when iron content is greater than 2 and 15%, respectively.     

Optimisation of sintered glass–ceramics from an industrial waste glass

Industrial plasma melting of municipal solid waste (MSW) incinerator fly ashes leads to a glass that may be easily crystallised to gehlenite glass–ceramics, by the sintering of fine glass powders. However, since the glass composition is not optimised for glass–ceramic manufacturing, the viscous flow is much hindered by a very significant surface crystallisation and dense glass–ceramics are feasible only by sintering above 1000 °C. This paper reports a new strategy for obtaining dense and strong glass–ceramics at 950 °C, with a holding time of only 30 min, consisting of the mixing of waste glass with a secondary recycled glass, such as soda-lime–silica glass or borosilicate glass. For an optimum balance between the two types of glass also the addition of kaolin clay, in order to favour the shaping, was found to be feasible. The approach had a positive effect, besides on the mechanical properties (e.g. bending strength exceeding 100 MPa), on the chemical stability.     

Modification of the glass surface by titanium dioxide films synthesized through the sol–gel method

The glass surface has been modified by titanium dioxide films synthesized using the sol-gel method. The sol-gel synthesis has been optimized by the experimental design with the use of the Latin square method. The film composition has been evaluated using X-ray powder diffraction analysis and optical measurements. The photoinduced hydrophilicity of the modified glass has been investigated.     

Novel ‘inorganic gel casting’ process for the manufacturing of glass foams

A new technique for the production of glass foams was developed, based on alkali activation and gel casting. The alkali activation of soda-lime waste glass powders allowed for the obtainment of well-dispersed concentrated suspensions, undergoing gelification by treatment at low temperature (75 °C). An extensive direct foaming was achieved by mechanical stirring of partially gelified suspensions, comprising also a surfactant. The suspensions were carefully studied in terms of rheological behavior, so that the final microstructure (total amount of porosity, cell size) can be directly correlated with the degree of gelification.A sintering treatment, at 700–800 °C, was finally applied to stabilize the foams, in terms of leaching of alkaline ions. Considering the high overall porosity (88–93%), the newly obtained foams exhibited a remarkable compressive strength, in the range of 1.7–4.8 MPa.     

The influence of nano alumina additions on the coefficient of thermal expansion of a LZS glass–ceramic composition

In this work a 19.58Li₂O·11.10ZrO₂·69.32SiO₂ (mol%) glass–ceramic matrix was prepared and milled in order to determine its coefficient of thermal expansion (CTE) and to study how it is influenced by the addition of nanosized Al₂O₃ particles (1–5 vol%) and submicrometric Al₂O₃ particles (5 vol%). Comminution studies from the LZS parent glass frit showed that a powder with an adequate particle size (3.5 µm) is achieved after 120 min of dry milling followed by a second step of 60 h wet milling. The obtained LZS glass–ceramic samples (fired at 900 °C/30 min) showed an average relative density of ∼98% with zirconium silicate and lithium disilicate as main crystalline phases. Prepared composites with 1, 2.5 and 5 vol% of nanosized Al₂O₃ and 5 vol% submicrometric Al₂O₃ showed average relative densities varying from 97% to 94% as the alumina content increased. The formation of β-spodumene in the obtained composites leads to reduce the CTEs, whose values ranged from 9.5 to 4.4×10⁻⁶ °C⁻¹. Composites with 5% nanosized alumina showed a CTE lower than that of the equivalent formulation with submicrometric alumina.     

Influences of powder morphology on the densification and microstructure of a ZrO2-based nanocrystalline glass–ceramic

Morphology is an important characteristic of raw powder utilized for ceramic sintering but the role of powder morphology is mostly overlooked. In this study, two types of ZrO₂–SiO₂ powder with different morphologies (fiber and particle) were synthesized by blow spinning and sol–gel method, respectively, followed by direct current electric field-assisted hot pressing (FAHP) to obtain nanocrystalline glass–ceramics (NCGCs). Results showed that the two as-synthesized powders had different pyrolysis behaviors. The two types of as-synthesized powders were amorphous and tetragonal-ZrO₂ nanocrystallites first formed after calcination at 800°C. During FAHP, the particle powder can be densified at a lower temperature than that of the fiber powder, due to the facts that the particle powder showed higher specific surface area and higher densification driving force. The fiber powder was predominately densified by fiber fusion and plastic deformation, whereas the particle powder was densified via particle fusion. Both the two types of powder can be fully densified to obtain ZrO₂–SiO₂ NCGCs at 1230°C for 4 min. Tetragonal-ZrO₂ nanocrystallites in the NCGCs with particles as raw powder showed higher stability than those in the NCGCs with fibers as raw powder.     

Influence of the interfacial reaction on interfacial adhesion in polyarylacetylene resin–silica glass composites

The influence of interfacial reaction on interfacial adhesion in silica glass/polyarylacetylene resin composites was studied. In order to achieve chemical reaction at the interface, vinyltrimethoxysilane was grafted onto silica glass surface to react with polyarylacetylene resin. The reaction between polyarylacetylene resin and vinyltrimethoxysilane was proved based on the model reaction between phenylacetylene and vinyltrimethoxysilane. At the same time, the modified silica glass surface characteristics were evaluated by contact-angle measurements and surface energy determination. The interfacial adhesion in silica glass/polyarylacetylene resin composites was evaluated by shear strength testing and fracture morphology analysis. It was concluded that polyarylacetylene resin reacted with vinyltrimethoxysilane. Furthermore, due to the reaction between polyarylacetylene resin and vinyltrimethoxysilane at the interface, the interfacial adhesion in composites was significantly increased. The improvement in interfacial adhesion was solely attributed to the interfacial reaction.