Crystalline phase formation, microstructure and mechanical properties of a lithium disilicate glass–ceramic

In this work, crystalline phase formation, microstructure and mechanical properties of a lithium disilicate (LS2, Li₂Si₂O₅) glass–ceramic in the SiO₂–Li₂O–Al₂O₃–P₂O₅ system were investigated. A four-stage heat treatment process was used for crystallization of the glass. The effects of holding time and temperature at the final stage on the crystalline morphology of the glass–ceramic were studied. The experimental results revealed the two crystallization peaks at 672 and 839 °C. At a temperature lower than 770 °C and holding time of 20 min, the lithium metasilicate (Li₂SiO₃) phase dominates. On the other hand, when the glass was heated to a higher temperature or held for a longer time, the LS2 phase dominates and some other minor phases such as cristobalite and lithium phosphate emerge. Scanning electron microscopy and energy dispersive spectrometry revealed a large number of nanosized ZrO₂ particles when the crystallization temperature was above 790 °C. Vickers hardness of the LS2 glass–ceramic was about 8.1–8.4 GPa and flexural strength was in the range of 282–307 MPa. Crack deflection was observed along the LS2 cluster boundaries. The crystallization sequence was proposed to explain the observed microstructure and phases.     

Grain-size effects on the structural, electrical properties and ferroelectric behaviour of barium titanate-based glass–ceramic nano-composite

Grain-size effects on the structural and electrical properties as well as ferroelectric behaviour of 10BaTiO₃–70V₂O₅–20Bi₂O₃ glass–ceramic nano-composite have been studied by scanning electron micrographs (SEM), X-ray diffraction (XRD), differential scanning calorimeter (DSC), dc conductivity (σ) and dielectric (ε) measurements over a wide temperature range. The present glass has been transformed into glass–ceramic nano-composite by annealing at temperatures close to crystallization temperature (T_cr). The XRD and SEM observations have shown that by heat treating at T_cr, the sample under study undergoes structural changes: from amorphous to partly crystalline for 1 and 8 h and to colossal crystallization for 24 h. After heat treated at T_cr for 1 and 8 h, the samples under load consist of small nano-crystallites (average size ca. 20–50 nm) embedded in glassy matrix. However, when the glass heat treated at T_cr for 24 h, the microstructure of the sample changes considerably. It is found that the glass–ceramic nano-composite obtained by heat treated at T_cr for 1 and 8 h exhibit giant improvement of electrical conductivity that is up to four order of magnitude. The electrical conductivity increases with increasing grain-size. The major role in the conductivity enhancement of this glass–ceramic nano-composite is played by the developed interfacial regions “conduction tissue” between crystalline and amorphous phases, in which the concentration of V⁴⁺–V⁵⁺ pairs responsible for electron hopping, is higher than inside the glassy matrix. The heat treated at T_cr for 24 h leads to decrease of the electronic conductivity. This phenomena lead to disappearance of most “conduction tissue” for electrons and substantial reduction of electronic conductivity. The experimental results were discussed in terms of a model proposed in this contribution which is based on a “core–shell” concept. The glass heat-treated at different times (1, 8 and 24 h) exhibited broad dielectric anomalies in the vicinity of the ferroelectric-to-paraelectric transition temperature. The Curie temperature (T_c), corresponding to ferroelectric phase transition increases with increasing grain-size. The observation of the glass–ceramic nano-composite being studied here can be used to control BaTiO₃ grain-size and hence transition temperature by proper adjustment of annealing times.     

Rate dependence of the tensile and flexural strengths of glass–ceramic Macor

Understanding of the tensile and flexural strengths of the glass–ceramic Macor bears important applications in materials science, aerospace, defense, and other engineering disciplines. In this article, we systematically investigate the rate dependence of the tensile strength and the flexural strength of Macor utilizing two methods: the Brazilian disk (BD) test and semi-circular bend (SCB) test. Both static tests and dynamic tests are conducted to explore the rate dependence of tensile and flexural strengths of Macor. The static measurement is conducted with a servo-controlled material testing machine, and the dynamic experiment is carried out with a 6.35-mm diameter split Hopkinson pressure bar (SHPB) system. The pulse-shaping technique is used to achieve dynamic force balance, and thus eliminates the loading inertial effect and enables quasi-static stress analysis. The experimental results show that both the tensile strength and the flexural strength of Macor are loading rate dependent. The flexural strength is observed to be consistently higher than the tensile strength.     

Microstructure and mechanical properties of zirconia-toughened lithium disilicate glass–ceramic composites

The lithium disilicate glass–ceramics composites reinforced and toughened by tetragonal zirconia (3Y-TZP) were prepared by hot-pressing at 800 °C with varying zirconia content from 0 to 30 wt.%. In the case of the composites of small zirconia content (below 10 wt.%), zirconia acted as nucleation agent primarily, and the microstructure was refined continuously. The morphology of Li₂Si₂O₅ crystals transformed from rod-shaped to spherical structure, and the mechanical properties decreased inevitably. For the composites of large zirconia content (from 15 wt.% to 30 wt.%), however, zirconia restrained the phase separation of glass. The morphology of Li₂Si₂O₅ crystals transformed to rod-shaped structure again. The mechanical properties of the composite at zirconia content of 15 wt.% increased up to 340 MPa and 3.5 MPa m^1/2 which were much higher than those of zirconia-free glass–ceramics. The improved properties were attributed mainly to compressive stress reinforcement, phase transformation and bridging toughening mechanisms.     

Preparation and properties of sintered glass–ceramics containing Au–Cu tailing waste

Au–Cu tailings (ACTs) are gold mining industry by-products that require further treatment before disposal to alleviate polluting the environment such as landfill, plant cover or using material production. This study focuses on the preparation and properties of a material called glass–ceramics that was prepared by a melting method with ACT, as raw materials, and without additional nucleating agents. The nucleation and crystallization temperature of the base glasses were determined by differential scanning calorimetry (DSC). The microstructure and properties of the glass–ceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), a bending strength test, a microhardness test, linear thermal expansion coefficient, and a wear-and-tear test, thereby obtaining the best optimum formula composition and the process parameters. The results show that the utilization of ACT can be up to 77.56 wt.%, which can make good use of ACT and develop decorated glass–ceramics with perfect properties. Two crystalline phases are diopside ((Mg_0.6Fe_0.2Al_0.2) Ca(Si_1.5Al_0.5)O₆) and hedenbergite (Ca(Fe_0.821Al_0.179) (SiAl_0.822Fe_0.178O₆)) which are formed in the glass–ceramic samples. The obtained glass–ceramics show the properties of maximum bending strength of 209.6 MPa, density of 3.23 g/cm³, hardness of 1008.7 kgf/mm² and wear resistance of 0.039 mg/g.     

Bioactivity potential of calcium alumino-silicate glasses and glass–ceramics containing nitrogen and fluorine

Calcium alumino-silicate glasses of general composition (in eq.%) 28Ca:57Si:15Al:[100 ? (x + y)]O:xN:yF (x = 0 or 20 and y = 0, 3 or 5) and their glass–ceramic counterparts were immersed in simulated body fluid (SBF) at 37 ± 0.5 °C for 28 days to assess their potential bioactivity. The glasses showed no Ca release or surface calcium phosphate deposition due to their high network connectivities (>2.55). The glass–ceramics all showed potential bioactivity, as the SBF became enriched in Ca and calcium phosphate deposits formed on their surfaces. This was a result of Ca release from crystalline phases (predominantly wollastonite in the case of CaSiAlOF glass–ceramics and gehlenite in the case of CaSiAlONF glass–ceramics). No aluminium was leached from the glass–ceramics into the SBF, due to its pH always exceeding 7.0.     

Low temperature sintering and properties of Ca–Al–B–Si–O glass/ceramic composites with various ceramic fillers

Low-temperature sintering and properties of low temperature co-fired ceramics materials based on a typical Ca–Al–B–Si–O glass and various ceramic fillers such as (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ were investigated. Densification, crystallization and dielectric properties are found to strongly depend on the type of filler. The densification process of glass/ceramic composites with different ceramic fillers is mainly from 600 to 925 °C, and the initial compacting temperature of samples is 600 °C. The initial rapid densification of samples starts after glass softening temperature of samples. The XRD patterns of (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ samples demonstrate crystalline phases, CaTiO(SiO₄) and CaTiSiO₅, respectively, as a result of firing at 875 °C for 15 min. The high dielectric constant fillers produce high ε_r values of the dielectric samples. The maximum dielectric constant of samples for (Zr_0.8Sn_0.2)TiO₄, (Ca_0.5Mg_0.5)TiO₃, BaSm₂Ti₄O₁₂ and CaTiO₃ filler is 14.02, 16.21, 18.64 and 23.78, respectively. Comparing with other samples, the specimens for (Ca_0.5Mg_0.5)TiO₃ and CaTiO₃ ceramic filler have lower dielectric loss. Especially, the sample for (Ca_0.5Mg_0.5)TiO₃ filler exhibits the lowest dielectric loss of 0.00011.     

Microstructure and stresses in a keatite solid–solution glass–ceramic

The microstructure of a translucent keatite solid–solution glass–ceramic (keatite s.s.) of the LAS-system (Li₂O–Al₂O₃–SiO₂) has been analyzed with SEM, AFM, XRF, XRD, and TEM. The glass–ceramic consists mainly of keatite s.s. with minor secondary phases such as zirconium titanate, gahnite and probably rutile. Furthermore the resistance to temperature differences (RTD) of this glass–ceramic was investigated. It is shown that, in spite of the relatively high coefficient of thermal expansion (CTE) of about 1 × 10⁻⁶ K⁻¹, an improved RTD can be achieved by special ceramization treatment. With this, compressive stresses in the first 100 μm to 150 μm are induced. These stresses can presumably be contributed to a difference in CTE between the surface-near zone and the bulk. Said CTE difference is caused by chemical gradients of CTE-relevant elements, such as Zn, K, and supposedly additional alkali elements such as Li. These stresses are useful to increase the strength and application range of glass–ceramics based on keatite s.s.

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3-D high-strength glass–ceramic scaffolds containing fluoroapatite for load-bearing bone portions replacement

This study aimed to fabricate and investigate the structure, mechanical properties and bioactivity of three-dimensional (3-D) glass–ceramic scaffolds for bone tissue engineering. The scaffold material was a fluoroapatite-containing glass–ceramic synthesized by a melting–quenching route. Glass–ceramic powders were mixed with polyethylene particles acting as pore formers; the blend was pressed to obtain “green” compacts that were thermally treated to remove the organic phase and to sinter the inorganic one. The structure and morphology of the resulting scaffolds were characterized by X-ray diffraction, scanning electron microscopy, density measurements and capillarity tests. Crushing tests were carried out to investigate the mechanical properties of the scaffolds. The in vitro bioactivity was assessed by soaking the scaffolds in simulated body fluid for different time frames and by analyzing the modifications that occurred on the sample surface. The scaffolds had an interconnected macroporous structure with pores up to 50% vol. and they showed an orthotropic mechanical behaviour and strength well above 20 MPa. In addition, in vitro tests put into evidence the excellent bioactivity of the material. Therefore, the prepared scaffolds can be used in bone reconstructive surgery as effective load-bearing grafts thanks to their ease of tailoring, bioactive properties and high mechanical strength.     

Synthesis,structural and dielectric properties of 0.8PMN–0.2PT relaxor ferroelectric ceramic

A 0.8PMN–0.2PT solid-solution ceramic was synthesized by columbite processing technique. The effects of sintering temperature on the density, structure and microstructure and in turn on the dielectric properties were investigated. The ceramics sintered at and above 1050\(^{\circ }\hbox {C}\) resulted in single-phase perovskite formation. However, high density >90% is achieved only after 1170\(^{\circ }\hbox {C}\). Microstructural analysis revealed that grain size increases with increase in sintering temperature. A significant increase in the peak of dielectric permittivity only after 1150\(^{\circ }\hbox {C}\) owing to increase in density is noted in this study. The quadratic law applied to this ceramic demonstrates that the transition is diffused. The broadness in phase transition and lower dielectric relaxation obtained for the composition demonstrate that the ceramic exhibits characteristics of both relaxor and normal ferroelectrics. The ceramic of composition 0.8PMN–0.2PT exhibits excellent dielectric properties \(\varepsilon _{\mathrm{r}\text {-}\mathrm{max}} =\) 20294?27338 at 100 Hz with \(T_{\mathrm{c}} = 100\)–\(96^{\circ }\hbox {C}\) at low sintering temperature 1170–1180\(^{\circ }\hbox {C}\), respectively.     

Mechanical properties of silicate glass–ceramics containing tricalcium phosphate

The flexural strength, elastic moduli, Vickers hardness and fracture toughness for silicate glass-ceramics (anorthite and diopside) containing tricalcium phosphate (TCP) were measured. The microstructures of the silicate glass–ceramics containing TCP were shown to consist of a complex structure of rod-like silicate and TCP crystals. The flexural strengths of glass–ceramics containing 32 wt% TCP for anorthite and 38 wt% TCP for diopside corresponding to a eutectic composition in the phase diagram were 236 and 226 MPa. The Youngs modulus and fracture toughness of the eutectic compositions were 89.4 GPa and 2.5 MPa·m1/2 for anorthite and 126 GPa and 2.3 MPa·m1/2 for diopside, respectively. The anorthite glass–ceramic containing TCP has a lower Youngs modulus in spite of a high strength as compared to other silicate glass–ceramics containing apatite or TCP.     

Effect of glass–ceramic microstructure on its in vitro bioactivity

Two routes were used to obtain a glass–ceramic composed of 43.5 wt % SiO₂ – 43.5 wt % CaO – 13 wt % ZrO₂. Heat treatment of a glass monolith produced a glass–ceramic (WZ1) containing wollastonite-2M and tetragonal zirconia as crystalline phases. The WZ1 did not display bioactivity in vitro. Ceramizing the glass via powder technology routes formed a bioactive glass–ceramic (WZ2). The two glass–ceramics, WZ1 and WZ2, were composed of the same crystalline phases, but differed in microstructure. The in vitro studies carried out on WZ2 showed the formation of an apatite-like layer on its surface during exposure to a simulated body fluid. This paper examined the influence of both chemical and morphological factors on the in vitro bioactivitity. The interfacial reaction product was examined by scanning and transmission electron microscopy. Both instruments were fitted with energy-dispersive X-ray analyzers. Measurements of the pH made directly at the interface of the two glass–ceramics were important in understanding their different behavior during exposure to the same physiological environment.     

Biocompatible glass–ceramic materials for bone substitution

A new bioactive glass composition (CEL2) in the SiO₂–P₂O₅–CaO–MgO–K₂O–Na₂O system was tailored to control pH variations due to ion leaching phenomena when the glass is in contact with physiological fluids. CEL2 was prepared by a traditional melting-quenching process obtaining slices that were heat-treated to obtain a glass-ceramic material (CEL2GC) that was characterized thorough SEM analysis. Pre-treatment of CEL2GC with SBF was found to enhance its biocompatibility, as assessed by in vitro tests. CEL2 powder was then used to synthesize macroporous glass–ceramic scaffolds. To this end, CEL2 powders were mixed with polyethylene particles within the 300–600 μm size-range and then pressed to obtain crack-free compacted powders (green). This was heat-treated to remove the organic phase and to sinter the inorganic phase, leaving a porous structure. The biomaterial thus obtained was characterized by X-ray diffraction, SEM equipped with EDS, density measurement, image analysis, mechanical testing and in vitro evaluation, and found to be a glass–ceramic macroporous scaffold with uniformly distributed and highly interconnected porosity. The extent and size-range of the porosity can be tailored by varying the amount and size of the polyethylene particles.     

Micromechanical testing of glass–ceramic sealants for solid oxide fuel cells

The understanding of the mechanical behavior of sealants is a prerequisite for the improvement of the integrity and reliability of solid oxide fuel cell stacks. The glass–ceramic sealant material in a SOFC stack is usually a thin layer; hence, micromechanical testing methods need to be applied for characterization. Indentation testing is used in the current study to determine elastic modulus and fracture toughness. The properties of sealant materials in stack typical thin layer geometry are compared with sintered bars. In addition to tests of as-joined material, the effect of stack operation on the properties is assessed.     

Tb3+/Yb3+ codoped silica–hafnia glass and glass–ceramic waveguides to improve the efficiency of photovoltaic solar cells

In this paper we present the investigation of the energy transfer efficiency between Tb³⁺ and Yb³⁺ ions in silica–hafnia waveguides. Cooperative energy transfer between these two ions allows to cut one 488 nm photon in two 980 nm photons and could have important applications in improving the performance of photovoltaic solar cells. Previous works revealed that for a given concentration of donors (Tb³⁺), increasing the number of acceptors (Yb³⁺) located near to the Tb³⁺ ion can increase the Tb–Yb transfer probability. However, when increasing the density of active ions, some detrimental effects due to cross-relaxation mechanisms become relevant. On the basis of this observation the sample doping was chosen keeping constant the molar ratio [Yb]/[Tb] = 4 and the total rare earths contents were [Tb + Yb]/[Si + Hf] = 5%, 7%, 9%. The choice of the matrix is another crucial point to obtain an efficient down conversion processes with rare earth ions. To this respect a 70SiO₂–30HfO₂ waveguide composition was chosen. The comparison between the glass and the glass–ceramic structures demonstrated that the latter is more efficient since it combines the good optical properties of glasses with the optimal spectroscopic properties of crystals activated by luminescent species. A maximum transfer efficiency of 55% was found for the highest rare earth doping concentration.     

Crystallization modifies osteoconductivity in an apatite–mullite glass–ceramic

The response to implantation of novel apatite glass–ceramics was evaluated using a weight bearing in vivo bone implant model. Five novel glasses with varying calcium to phosphate ratios were cast as short rods and heat-treated to crystallize principally apatite. One glass ceramic had an apatite stoichiometry (Ca : P=1.67); three were phosphate-rich and one calcium-rich. One of the phosphate-rich glasses was also tested in its glassy state to determine the effect of crystallization on the biological response. Rods were implanted into the midshaft of rat femurs and left for 28 days. The femurs were then harvested and processed for scanning electron microscopy, energy dispersive X-ray microanalysis and conventional histology as ground and polished sections. Four of the materials exhibited evidence of osseointegration and osteoconduction. However, there was a marked inflammatory response to one of the phosphate-rich glass–ceramics, and to the non-crystallized glass. Crystallization of the latter significantly improved the bone tissue response. The glass–ceramic with an apatite stoichiometry elicited the most favorable response and merited further study as an osteoconductive bone substitute in maxillofacial and orthopedic surgery.     

Fabrication of nano-macroporous glass–ceramic bioscaffold with a water soluble pore former

Recently, several methods have been reported for fabricating tailored amorphous multi porosity bioscaffolds for bone regeneration and tissue engineering. In particular, the melt-quench-heat-etch method appears attractive for making large and/or complex shape structures or fibers for flexible products. However, often the macropore size has been limited to <100 μm. In this paper we report an improved method for fabricating nano-macroporous soda lime phosphosilicate glass using sucrose as a macropore former. The composite compact consisting of soda lime phosphosilicate glass and sucrose powders is pressed in a die at room temperature. 3D interconnected macroporous structure is formed first by dissolving the sucrose part in water at room temperature, and then sintering the compact at temperatures above the glass transition temperature. Thus, interconnected macropores with controlled size (≥100 microns) are formed readily. The sintering heat-treatment also induces nanoscale phase separation, which is then exploited for introducing nanoscale porosity. For the latter goal, the sample is leached in HCl under optimized conditions to yield desired nano-macroporous glass for bone scaffold or other applications.     

Glass–ceramic scaffolds containing silica mesophases for bone grafting and drug delivery

Glass–ceramic macroporous scaffolds were prepared using glass powders and polyethylene (PE) particles of two different sizes. The starting glass, named as Fa-GC, belongs to the system SiO₂–P₂O₅–CaO–MgO–Na₂O–K₂O–CaF₂ and was synthesized by a traditional melting-quenching route. The glass was ground and sieved to obtain powders of specific size which were mixed with PE particles and then uniaxially pressed in order to obtain crack-free green samples. The compact of powders underwent a thermal treatment to remove the organic phase and to sinter the Fa-GC powders. Fa-GC scaffolds were characterized by means of X-Ray Diffraction, morphological observations, density measurements, image analysis, mechanical tests and in vitro tests. Composite systems were then prepared combining the drug uptake-delivery properties of MCM-41 silica micro/nanospheres with the Fa-GC scaffold. The system was prepared by soaking the scaffold into the MCM-41 synthesis batch. The composite scaffolds were characterized by means of X-Ray Diffraction, morphological observations, mechanical tests and in vitro tests. Ibuprofen was used as model drug for the uptake and delivery analysis of the composite system. In comparison with the MCM-41-free scaffold, both the adsorption capacity and the drug delivery behaviour were deeply affected by the presence of MCM-41 spheres inside the scaffold.     

Effects of glass components on crystallization and dielectric properties of BST glass–ceramics

Crystallization behavior was studied for glass powders in which some portions of AlF₃ in the net composition of 60(Ba_0.7Sr_0.3)TiO₃–25SiO₂–15AlF₃ were replaced with Ga₂O₃ or Bi₂O₃. The replacement with Ga₂O₃ resulted in a progressive increase in crystallization temperature, which effectively assisted the viscous sintering of glass powders to produce densified BST glass–ceramics at relatively lower temperatures. For the Bi₂O₃-replaced glass powders, an increasing amount of Bi₂O₃ replacement lowered the crystallization temperature and yielded less densified glass–ceramics containing a considerable amount of glassy phase. The temperature dependence of permittivity was estimated for the Ga₂O₃- and Bi₂O₃-replaced glass–ceramics as a function of sintering conditions and the amount of replacement, respectively.