Effect of MgO addition on the crystallization and in vitro bioactivity of glass ceramics in the CaO–MgO–SiO2–P2O5 system

CaO–MgO–SiO₂–P₂O₅ glass ceramics were successfully prepared by sintering the sol–gel-derived powders. The effects of MgO addition on the samples crystallization and structure were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). In addition, samples degradation and in vitro bioactivity assays were also evaluated. With more MgO addition, the glass ceramics crystallization kinetics under non-isothermal conditions changed from bulk crystallization to surface crystallization, and new crystal phases of Ca₂MgSi₂O₇ and SiO₂ were induced. In addition, it is observed that with increasing MgO concentration, the glass ceramics degradability gradually decreased and the formation of apatite was delayed.     

The influence of TiO2 content on the properties of glass ceramics: Crystallization,microstructure and hardness

The effects of compositional variation, crystallization behavior, crystalline phases and microstructure formed in the SiO₂3Al₂O₃3CaO (SAC) glass system using various amounts of TiO₂ as nucleating agent were investigated by Differential Thermal Analysis (DTA), X-ray powder diffraction (XRD), Scanning Electron Microscope (SEM), Energy-dispersive X-ray spectroscopy (EDAX) and Fourier transform infrared spectroscopy (FTIR) techniques. The crystallization kinetics and mechanical properties of SAC glass ceramics were studied using crystallization peak temperature (T_p) of three different glasses as obtained from DTA, the activation energy (E) and Avrami exponent (n) were also determined. The crystallization peak temperature (T_p) and activation energy (E) were found to increase with the increase in TiO₂ content. The major crystalline phases were anorthite and wollastonite along with gehlenite and titanite as the minor crystalline phases present in the glass ceramic system. The studies showed that the three dimensional crystalline structure and the microhardness increased with the increase of TiO₂ content in the glass ceramics system.     

Micro-crystallization of the infrared transmitting chalcogenide glass in GeSe2–As2Se3–PbSe system

Micro-crystallization of the chalcogenide glass 40GeSe₂–50As₂Se₃–10PbSe has been studied in order to obtain infrared transmitting glass ceramics. Differential scanning calorimetry, IR transmission spectroscopy, X-ray diffraction, scanning electronic microscopy and thermal dilatancy have been used for characterizing the crystallization process. Performing thermal treatment on the glass sample at 250 °C (40 °C higher than T_g) for 10 h, we obtained a glass ceramic containing well-dispersed micro-crystals (<50 nm) and the obviously decreased thermal expansion coefficient.     

Effect of A2O3 (A = La,Y, Cr,Al) on thermal and crystallization kinetics of borosilicate glass sealants for solid oxide fuel cells

Influence of various intermediate oxides on thermal, structural and crystallization kinetics of 30BaO–40SiO₂–20B₂O₃–10A₂O₃ (A = Y, La, Al, Cr) glasses has been studied. The highest glass transition temperature (T_g) with high thermal stability is observed in Y₂O₃ containing glasses as compared to other glasses. The thermal expansion coefficient (TEC) increases with increasing heat treatment duration in all the glasses. The maximum increase in TEC is observed in Cr₂O₃ containing glass ceramics. FTIR study showed that transmission bands due to silicate and borate chains become sharper with splitting after heat treatment. A selected glass sample (BaCr) has been tested for interaction and adhesion with Crofer 22 APU interconnect material for its application as a sealant in solid oxide fuel cell.     

Some features of the surface modification of MgO–Al2O3–TiO2–SiO2 glass and glass ceramics by Ag diffusion

Diffusion of Ag species into the surface layers of the MgO–Al₂O₃–TiO₂–SiO₂ glass stimulates the crystallization processes therein during subsequent thermal treatment. It was found that the silver doping significantly increases the microhardness of glass and glass ceramics. The effect of the Ag diffusion and the subsequent high-temperature treatment (T > T_g) on the structure and luminescence properties of the MgO–Al₂O₃–TiO₂–SiO₂ glass was studied. The thermal evolution, structure and properties of the glass and glass ceramics were investigated by the DSC method, XRD analysis and luminescence spectroscopy. It was found that the Ag diffusion into the surface layers leads to the formation of numerous different luminescent molecular Ag_n clusters. The MgO–Al₂O₃–TiO₂–SiO₂ glass and glass ceramics subjected to the Ag diffusion can operate as effective down-converters of the radiation demonstrating the effective transformation of UV and blue light into the radiation of red and NIR spectral range.     

Glass–ceramics and composites containing aluminum borate whiskers

Glass–ceramics and composites containing aluminum borate whisker crystals were developed using two different approaches: crystallization of an aluminum borosilicate glass or addition of an aluminum borate precursor powder to a glass frit. Two different glass frits were used, a commercially available borosilicate glass or the same aluminum borosilicate glass used in the crystallization experiments. X-ray diffraction analysis showed that Al₄B₂O₉ or Al₁₈B₄O₃₃ whiskers formed in all samples, indicating that the glass crystallized significantly with increasing heating temperature, and that the precursor can be effectively be used to generate in situ aluminum borate crystals within glass matrices. However, the samples produced by mixing the aluminum precursor with glass frits contained porosity after processing, indicating that pressureless viscous sintering was not efficient. The hardness of the glass–ceramic did not vary significantly with processing temperature, but the (indentation) fracture toughness measured showed a >100% increase (after heating at 1200 °C), demonstrating that whisker-shaped crystals are effective in increasing the mechanical toughness of the glass matrix. The hardness of the composites showed a dependence on the amount of aluminum borate crystals present.     

Influence of barium oxide on the crystallization, microstructure and mechanical properties of potassium fluorophlogopite glass-ceramics

The influence of barium oxide, heat treatment time and temperature on the crystallization, microstructure and mechanical behavior of the system Ba_x·K_1−2x·Mg₃·Al·Si₃O₁₀·F₂ (where x = 0.0, 0.3 and 0.5) was investigated in order to develop novel, high strength and machinable glass-ceramics. Three glasses were prepared and characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscope (SEM) techniques and some mechanical testing methods.The crystallization kinetics of glass-ceramics was also studied. Activation energy and Avrami exponent calculated for the crystallization peak temperature (T_p) of three different glass batches. The Vickers hardness decreased slightly on formation of the potassium fluorophlogopite and barium fluorophlogopite phases, but decreased significantly on formation of an interconnected ‘house of cards’ microstructure.     

Effect of soluble Cr2O3 on the silicate network,crystallization kinetics,mineral phase,microstructure of CaO-MgO-SiO2-(Na2O) glass ceramics with different CaO/MgO ratio

Cr₂O₃ is often used as a glass additive to prepare glass ceramics. Chromium element exists mainly in two parts in the glass ceramics: chromium-containing spinel and soluble chromium in glass matrix. Herein, effect of soluble chromium on the CaO-MgO-SiO₂-(Na₂O) glass system is researched. Glass and glass ceramics were characterized by Raman spectrum, X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry. It is found that the addition of Cr₂O₃ increased the Q²si structure unit in glass networks, especially in glass systems with high MgO content. The crystallization temperatures of the systems were increased with the addition of Cr₂O₃. Soluble chromium reduced the crystallization activation energy of the glass system slightly, but did not alter its crystallization behavior (surface crystallization). With the increase of MgO content, the mineral phases of the glass ceramics gradually changed from wollastonite to diopside. Cr₂O₃ reduced the lattice parameters of the mineral phases. The addition of Cr₂O₃ has a significant effect on grain refinement and structural compactness of the glass ceramics system with high MgO content.     

In vitro behavior of bioactive phosphate glass–ceramics from the system P2O5–Na2O–CaO containing titania

Soda lime phosphate bioglass–ceramics with incorporation of small additions of TiO₂ were prepared in the metaphosphate and pyrophosphate region, using an appropriate two-step heat treatment of controlled crystallization defined by differential thermal analysis results. Identification and quantification of crystalline phases precipitated from the soda lime phosphate glasses were performed using X-ray diffraction analysis. Calcium pyrophosphate (β-Ca₂P₂O₇), sodium metaphosphate (NaPO₃), calcium metaphosphate (β-Ca(PO₃)₂), sodium pyrophosphate (Na₄P₂O₇), sodium calcium phosphate (Na₄Ca(PO₃)₆) and sodium titanium phosphate (Na₅Ti(PO₄)₃) phases were detected in the prepared glass–ceramics. The degradation of the prepared glass–ceramics was carried out for different periods of time in simulated body fluid at 37 °C using granules in the range 0.300–0.600 mm. The released ions were estimated by atomic absorption spectroscopy and the surface textures were measured by scanning electron microscopy. Investigation of in vitro bioactivity of the prepared glass–ceramics was done by the measurement of the infrared reflection spectra for the samples after immersion in the simulated body fluid for different periods at 37 °C. The result showed that no apatite layer was formed on the surface of the samples and the dominant phase remained on the surface was β-Ca₂P₂O₇, which is known for its bioactivity.     

Effects of alumina on the crystallization behavior, densification and dielectric properties of BaO–ZnO–SrO–CaO–Nd2O3–TiO2–B2O3–SiO2 glass–ceramics

The alumina addition effects on the crystallization, sintering behaviors and dielectric properties of BaO–ZnO–SrO–CaO–Nd₂O₃–TiO₂–B₂O₃–SiO₂ (Ba–Zn–Sr–Ca–Nd–Ti–B–Si) glass powder were investigated using the differential thermal analyzer (DTA), thermo-mechanical analyzer (TMA), X-ray diffractometer (XRD). The results showed that the addition of alumina powder into Ba–Zn–Sr–Ca–Nd–Ti–B–Si glass changed the crystallization sequence from Nd₂Ti₄O₁₁–Nd_0.66TiO₃ to Nd₂Ti₃O_8.7–Nd₂Ti₂O₇–Nd₂Ti₄O₁₁and increased the densification activation energy due to the dissolution of Al³⁺ ions into the glass structure. Fully densified 30 vol.% alumina-added Ba–Zn–Sr–Ca–Nd–Ti–B–Si glass can be obtained via glass viscous flow before the second and third crystalline phases, Nd₂Ti₂O₇ and Nd₂Ti₄O₁₁crystallization. The 30 vol.% alumina-added Ba–Zn–Sr–Ca–Nd–Ti–B–Si glass–ceramics sintered at 900 °C exhibited a high dielectric constant of 17 and a quality factor of about 820, which provided a promising candidate for LTCC applications.     

Synthesis and characterization of clinopyroxene based glasses and glass-ceramics along diopside (CaMgSi2O6)-jadeite (NaAlSi2O6) join

The crystallization behavior and mechanical characterization of glasses based upon the compositions along diopside (CaMgSi₂O₆)-jadeite (NaAlSi₂O₆) join has been investigated. Six glasses were obtained by the melt-quenching technique. Structural and thermal behaviors of these glasses were investigated by density and molar volume, infrared spectroscopy (FTIR) and dilatometry. The crystallization behavior of glasses was investigated by using differential scanning calorimetry (DSC). Sintering and crystallization behavior of the glass-ceramics were investigated under non-isothermal heating conditions up to temperatures of 850 °C. Mechanical characterization of glasses was investigated by using the measurement of Vickers indentation hardness and elastic constants such as Young's modulus (E), shear modulus (G), bulk modulus (K) and Poisson's ratio (ν). These data of the glasses were correlated with the structure of glasses, nature and role played by glass forming cations.     

Preparation and characterization of some multicomponent silicate glasses and their glass–ceramics derivatives for dental applications

The chemical corrosion and UV–vis absorption and infrared absorption spectra of binary and multicomponent lithium silicate glasses and corresponding glass–ceramics were investigated. The chemical durability of the glasses and derived glass–ceramics was found to be excellent to all leaching media. The IR absorption spectra of the glass and glass–ceramic samples reveal absorption bands of characteristic groups mainly due to major silicate network besides the possible sharing of network units due to some involving oxide constituents. X-ray analysis of glass–ceramics indicates the separation of lithium disilicate phase as the main constituent beside other phases according to the specimen chemical constituents. The obvious promising investigated chemical and physical properties are correlated with the presence of multioxides such as Al₂O₃, TiO₂, MgO and ZrO₂. Transmission and reflectivity properties reveal acceptable data. The prepared glass–ceramics are recommended for dental applications.     

The effect of Al2O3 on sintering and crystallization of MgSiO3-based glass-powder compacts

The influence of Al₂O₃ (8 wt.%) on sintering and crystallization features of glass powders based on magnesium silicate (MgSiO₃) was experimentally determined. The investigated compositions were Y_0.125Mg_0.875Si_0.875B_0.125O₃ and Y_0.125Mg_0.725Ba_0.15Si_0.875B_0.125O₃. For the experiments, glasses in bulk and frit forms were produced by melting in Pt-crucible at 1600 °C for 1.5 h. Glass-powder compacts were sintered at different temperatures between 900 °C and 1100 °C. The evolution of crystalline regime was determined by in situ recording of X-ray diffractograms of fine glass powders at elevated temperatures. The results and their discussion showed that addition of 8 wt.% Al₂O₃ in glass batches affected the thermal properties of the glasses and resulted in MgSiO₃-based glass ceramics well sintered between 900 °C and 1100 °C. In the BaO-free MgSiO₃ glass ceramics, clino- and orthoenstatite crystallize while the presence of BaO favours the formation of hexacelsian.     

Stress-dependent ferroelectric properties of Pb(Zr1/2Ti1/2)O3–Pb(Zn1/3Nb2/3)O3 ceramic systems

Effects of compressive stress on the ferroelectric properties of ceramics in PZT–PZN systems were investigated. (1 − x)Pb(Zr_1/2Ti_1/2)O₃(xPb(Zn_1/3Nb_2/3)O₃ or (1 − x)PZT–(x)PZN (x = 0.1–0.5) ceramics were prepared by a conventional mixed-oxide method. The ferroelectric properties under compressive stress of the PZT–PZN ceramics were observed at stress levels up to 170 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. It was found that with increasing compressive stress the area of the ferroelectric hysteresis (P–E) loops, the saturation polarization (P_sat), the remanent polarization (P_r), and the coercive field (E_c) decreased. These results were interpreted through the non-180° ferroelectric domain switching processes.     

Phase evolution and photoluminescence in Er3+ doped glass ceramics containing lutetium oxyfluoride nanocrystals

Crystalline phase evolution through merely adjusting composition was achieved in silicate glass ceramics containing Lu_nO_n-1F_n+2 (n = 5–10) nanocrystals. Orthorhombic or cubic phase nanocrystals were precipitated in the aluminosilicate glass matrix after thermal treatment together with varying the Na₂O/NaF ratio. Oxyfluoride nanocrystals with quasi-spherical shape show homogenous and dense distribution in glass matrix by transmission electron microscopy measurement. Intense upconversion and mid-infrared emissions were realized in these glass ceramics compared to the precursor glass, and the emission spectral shapes, relative emission intensity and fluorescence decay curves of Er³⁺ in cubic LuOF embedded samples exhibit remarkable differences due to the crystal phase dependent effect in glass ceramics. These results indicate that the crystallization and luminescence properties of oxyfluoride glass ceramics could be modified through the alteration of glass composition, which could be used for the development of novel glass ceramics and design of luminescent properties.     

Synthesis, characterization and in vitro bioactivity of magnesium-doped sol-gel glass and glass-ceramics

Bioactive glass and glass-ceramics in the system CaO-MgO-SiO₂-P₂O₅ have been prepared by the sol-gel and high temperature sintering techniques. The obtained samples were characterized by thermogravimetric and differential thermal analysis (TG/DTA), N₂-adsorption measurement, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). In vitro bioactivity tests were also conducted in simulated body fluid (SBF). The studies of crystallization kinetics under non-isothermal conditions showed the activation energy for crystallization to be 381 kJ/mol and the crystallization mechanism gradually changed from three-dimension growth to two-dimension crystallization with the increase of heating rate. Sintering temperature had great influence on the samples texture and structure. In addition, the apatite-formation on glass and glass-ceramics was confirmed by in vitro tests, and crystallization decreased the samples bioactivity.     

Preparation,structural and thermo-mechanical properties of lithium aluminum silicate glass–ceramics

Lithium aluminum silicate glasses of composition (wt%) 12.6Li₂O–71.7SiO₂–5.1Al₂O₃–4.9K₂O–3.2B₂O₃–2.5P₂O₅ were prepared by the melt quench technique. These glasses were converted to glass–ceramics based on DTA data. X-ray diffraction (XRD) and Fourier transform infra-red spectroscopy (FTIR) were used to discern the phases evolved in the glass–ceramics. Phase morphology was studied using scanning electron microscopy (SEM). Thermal expansion coefficient (TEC) and glass transition temperature (T_g) of all samples were measured using thermo-mechanical analyzer (TMA). It was found that 3 h dwell time at crystallization temperature yielded samples with good crystallinity with a TEC of 9.461 × 10⁻⁶ °C⁻¹. Glass–ceramic-to-metal compressive seal with SS-304 was fabricated using LAS glass–ceramic. The presence of metal housing and compressive stresses at the glass–ceramic-to-metal interface reduced average grain size and changed the overall microstructure.     

SiO2/Al2O3 ratio dependence of microstructures and dielectric properties in barium strontium titanate glass ceramics

(Ba, Sr)TiO₃-Al₂O₃-SiO₂ glass ceramic system with various SiO₂/Al₂O₃ ratios was investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), dielectric spectroscopy and impedance spectroscopy. The XRD results demonstrated that the proper SiO₂/Al₂O₃ ratio could promote the crystallization of the major crystalline phase from the glass matrix. The dielectric property investigations showed that the dielectric constant passes through a maximum value while the dielectric breakdown strength has a minimum value with increasing SiO₂/Al₂O₃ ratio. Meanwhile, the complex impedance analyses suggest the resistance of the glass-crystal interface rapidly decreases and the capacitance of the crystal slightly decreases with the increase of SiO₂/Al₂O₃ ratio. The relaxation mechanisms of the (Ba, Sr)TiO₃ glass ceramics changed from localized relaxation to long range conductivity as the SiO₂/Al₂O₃ ratio was increased from 1.43 to 1.83. The variations in the dielectric response and the activation energy of the glass-crystal interface in the (Ba, Sr)TiO₃ glass ceramics with the ratio of 2.40 could be attributed to the crystallization of fresnoite phase.     

Isothermal crystallization kinetics and effect of crystallinity on the optical properties of nanosized CeO2 powder

The isothermal crystallization kinetics and effect of crystallinity on the optical properties of cerium dioxide (CeO₂) nanopowders synthesized using a coprecipitation route at 293 K and pH 9 were investigated using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, and ultraviolet–visible absorption spectrophotometry. The activation energy of CeO₂ crystallization from dried cerium dioxide precursor powders by isothermal method of 64.1±3.24 kJ/mol was obtained. The average value of the growth morphology parameter (n) is 1.94, meaning that two-dimensional growth with plate-like morphology was the primary mechanism of CeO₂ crystallization from cerium dioxide precursor powders. The indirect band gap energy (E_i) of CeO₂ decreased from 3.03 eV to 2.83 eV when the crystallinity increased from 18% to 82%, and the direct band gap energy (E_d) of CeO₂ also decreased from 3.76 eV to 3.64 eV.     

Crystallization behavior and properties of K2O–CaO–Al2O3–SiO2 glass-ceramics

In order to obtain high-strength anorthite glass-ceramics, K₂O–CaO–Al₂O₃–SiO₂ quaternary glass and relevant glass-ceramics were prepared and investigated. The results show that anorthite along with kalsilite or leucite was precipitated from the parent glass. Kalsilite crystals were formed firstly and then converted into leucite through reacting with SiO₂ in the glass phase. The morphology of the crystals was dependent on the heat-treatment temperature. Column crystals were transformed into fine granular grains when the sintering temperature changed from 900 °C to 1100 °C. The activation energy (E_α) and avrami constant (n) were also calculated as 463.81 KJ/mol and 3.74 respectively, indicating that bulk nucleation and three-dimensional crystal growth were the dominating mechanisms in the temperature range 1000–1100 °C. The maximum value of the flexural strength for the glass-ceramics containing leucite was 248 MPa and the coefficient of thermal expansion (CTE) was in the range 5.69~11.94×10⁻⁶ K⁻¹. The leucite is the main reason for the high CTEs and high flexural strength of glass-ceramics.