New insights into the bioactivity of SiO2–CaO and SiO2–CaO–P2O5 sol–gel glasses by molecular dynamics simulations

The structures of binary xCaO · (100 ? x)SiO₂ glasses with x = 10, 20 and 30 mol-% and ternary (20 ? x)CaO · xP₂O₅ · 80SiO₂ glasses with x = 3, 10, 15, 17 and 20 mol-% have been studied by means of classical molecular dynamics simulations using both the melt-quenched and the sol–gel protocols. The structural picture derived correlates the bioactive behaviour to the combined effects of the connectivity of the extended silicate network and to the tendency to form (or not to form) non-homogeneous domains. In this context, a mathematical relationship that relates the Ca/P ratio in the Ca phosphate micro-segregation zones to the P₂O₅ content in ternary glasses has been developed and this has been used to fine-tuning the optimum amount of P in a glass for its highest in vitro bioactivity. The composition with optimal Ca/P ratio, 80Si–14.8Ca–5.2P, has been synthesized and the results of bioactivity tests have confirmed the prediction.     

Sintering properties of sol–gel derived lithium disilicate glass ceramics

Lithium disilicate (Li₂Si₂O₅) glass-ceramics were fabricated through two sol–gel methods: the nitrate route and the alkoxide route. Thermal analysis revealed different crystallization processing of two gel-derived powders. Li₂Si₂O₅ powders were obtained after heat treatment at 800?°C. These powders were pressed and pressureless sintered under 900–1030?°C. Microstructure of sintered samples revealed the grain size and morphology of Li₂Si₂O₅ ceramics. Although grain size in both samples increased with increasing sintering temperature, samples from the alkoxide route derived powders had more uniform grain size and pore distribution. In addition, open porosity decreased in both samples with increasing sintering temperature. Unlike familiar nucleation that resulted in grain growth mechanism, the Li₂Si₂O₅ particles developed into irregular large size grains at first, and then grew into rod-shaped grains.

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Surface characteristics and corrosion resistance of sol–gel derived CaO–P2O5–SrO–Na2O bioglass–ceramic coated Mg alloy by different heat-treatment temperatures

To improve the initial corrosion resistance and then make the degradation rate of magnesium alloys to meet the biomedical application, crack-free CaO–P₂O₅–SrO–Na₂O bioglass-ceramic coatings were synthesized on AZ31 magnesium alloy substrates using a sol–gel dip-coating technique followed by a heat-treatment in the temperature range of 400–500 °C. The effects of heat-treatment on the phase constituents, surface characteristics and corrosion resistances of the coatings were investigated. It was shown that the crystallization of Ca₂P₂O₇ occurred after the glass was treated at 400 °C. As the temperature increased from 400 °C to 450 °C, besides main phase Ca₂P₂O₇, β-Ca(PO₃)₂ and Ca₄P₆O₁₉ were identified as minor crystal phases in the glass–ceramic. No new phase was detected with the temperature increasing to 500 °C except for the further crystallization. Meanwhile, the water contact angles of the coatings decreased with the increase of heat-treatment temperature due to the great crystallization. The corrosion resistances of the coated magnesium alloys were studied by electrochemical corrosion techniques in the simulated body fluid. The results revealed that the coating heat-treated at 400 °C exhibited superior corrosion resistance because of less crystallization, suggesting that the calcium phosphate bioglass–ceramic coating can provide effective protection for magnesium alloy substrate to control its initial degradation in vivo and maintain the desired mechanical properties.     

Synthesis and characterization of Na2O–CaO–SiO2 glass–ceramic

The Na₂O–CaO–SiO₂ ternary glass–ceramic with the composition of 49 mass% Na₂O, 20 mass% CaO, and 31 mass% SiO₂ was prepared by the conventional method. The ternary glass–ceramic was characterized using X-ray diffraction (XRD), differential thermal analysis (DTA), thermogravimetric analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy techniques. The Na₂CaSiO₄ phase, having the cubic crystal system, with the crystallite size of 25.14 nm and lattice parameter of 0.7506 nm was determined from the XRD pattern. The activation energy of the glass–ceramic calculated from the DTA curves was found to be 162.02 kJ mol^?1. The Avrami exponent was found to be ~2 indicating a one-dimensional growth process. The mass loss percent from ambient temperature to 1,173 K is less than 1 %. The density was calculated to be 2,723 kg m^?3. The fine-grained microstructure with the particle sizes less than 1 μm was confirmed by the scanning electron microscope micrograph.     

Phase evolution and electrical properties of BaO–SrO–TiO2–SiO2–Al2O3-based glass ceramics prepared by sol–gel process

Crystallization of BaO–SrO–TiO₂–SiO₂–Al₂O₃-based glass ceramics, prepared by sol–gel process, was evaluated in terms of the effect of sintering temperature on phase evolution and electrical properties. The characterization of the samples was performed by X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM) studies and impedance spectroscopy analysis. The XRD results demonstrate that fresnoite phase starts to crystallize at 700 °C and perovskite phase appears at 900 °C. The glass ceramic samples sintered at high temperatures contained three crystalline phases, including perovskite, feldspar and fresnoite. In addition, SEM observation showed that the average grain size increased and the porosity decreased with increasing sintering temperature. Furthermore, the measurement of impedance spectroscopy suggests that there is a minimum value of the activation energy associated with the sintering temperature of the glass ceramics. The possible explanation of the sintering temperature dependence was discussed.     

Crystallization kinetics of Li2O–Al2O3–GeO2–P2O5 glass–ceramics system

Journal of Thermal Analysis and Calorimetry - Lithium aluminum germanium phosphate glass–ceramics with NASICON structure find potential application in the field of energy storage...     

Preparation of ZnO–B2O3–SiO2:Mn2+ optical-storage glass–ceramics with different ZnF2 dopant by sol–gel method

Journal of Sol-Gel Science and Technology - ZnO–B2O3–SiO2:Mn2+ (ZBSM) optical-storage glass–ceramics doped with ZnF2 were successfully prepared via sol–gel method. The...     

Dielectric and energy storage properties of barium strontium titanate based glass–ceramics prepared by sol–gel method

Ba_0.6Sr_0.4TiO₃ based glass–ceramics were prepared by sol–gel process. Influences of B–Si–O glass content on the microstructure, dielectric, and energy storage properties of the BST based glass–ceramics have been investigated. Perovskite barium strontium titanate phase was found at annealing temperature 800 °C. A secondary phase Ba₂TiSi₂O₈ was detected and lowered by declining the mole ratio of element Si (from 50 to 25 mol%) in glass additive. Microstructural observation indicated that the microstructure homogeneity can be improved by glass addition till 2 mol%, while worsened by excessive glass concentrations. Due to relatively homogeneous microstructure, the maximum discharged energy density and breakdown strength were also obtained in samples with 2 mol% glass additive, which were found to be 0.553 J/cm³ and 43.2 kv/mm, respectively. Microscopic observation of the breakdown area was performed and the mechanical failure, including the formation and accumulation of micro-cracks during the dielectric breakdown process, was considered to be the main cause of dielectric breakdown. Results of the charging and discharging energy densities show that the BST based glass–ceramics prepared by sol–gel method has a potential for pulse power applications.     

Raman spectroscopic study of CuO–V2O5–P2O5–CaO glass system

In order to evidence the structural changes induced by CuO and V₂O₅ in the phosphate glass network and their modifier or former role, x(CuO·V₂O₅)(100 − x)[P₂O₅·CaO] glass system was prepared and investigated using Raman spectroscopy (0 ≤ x ≤ 40 mol%).Raman spectra of the studied glasses present the specific bands of the phosphate glasses at low concentration of transition metal (TM) ions, but at higher concentration (x > 7 mol%) a strong depolymerization of the phosphate network appears; non-bridging oxygen atoms are involved in VOP and CuOP bonds and new short units are formed. For a high concentration of V₂O₅ (x > 10 mol%) the Raman bands of V₂O₅ prevail in the spectra; this fact suggests that vanadium oxide imposes its structural units in the network acting thus as a network glass former.2D correlation analysis was also applied for the concentration-dependent Raman spectra in order to verify the assignments of the vibration modes and to find correlations in the changes induced by TM ions content. 2D correlation maps indicate a good correlation between the bands at ∼705 cm⁻¹ assigned to POP stretching vibration and at ∼1175 cm⁻¹ assigned to PO₂ groups which suggest the depolymerization of the phosphate network. The correlation between the 1270 cm⁻¹ and 930 cm⁻¹ bands also suggests that V₂O₅ oxide is responsible for PO bonds breaking and POV formation.     

Elimination of porosity in heavily rare-earth doped sol–gel derived silicate glass films

The combination of spin-coating and rapid thermal annealing is a very important sol–gel technique to prepare high quality silicate glass films, widely used in the fabrication of waveguides, photonic bandgap structures and other film-based optical devices. This work found that high rare-earth concentrations will seriously affect the optical quality of the films prepared by the spin-coating/rapid thermal annealing process, with pores with hundreds of nanometres in size being found in heavily rare-earth doped aluminosilicate glass films, causing significant light scattering. However, it was also found that a new recipe using acetylacetone was able to dramatically eliminate these pores and to improve the film optical quality, even for rare-earth concentrations as high as 15 mol%. This result will be useful for the fabrication of sol–gel derived devices based on rare-earth doped silicate glass films like active waveguides, functional films and photonic bandgap structures.     

Thermal properties of Na2O–P2O5–Fe2O3 polyphosphate glasses

Journal of Thermal Analysis and Calorimetry - Iron phosphate glasses are materials that can have many applications like durable matrixes in waste immobilization techniques, biomaterials,...     

Structural investigation of sol–gel derived SiO2–GeO2 noncrystalline and nanocomposite materials

Noncrystalline and nanocomposite materials of (1?x)SiO₂·xGeO₂ system, with Si:Ge ratio from 8:1 to 2:1 (x?=?0.111; 0.142; 0.2; 0.333), initially obtained by sol?Cgel method, were characterized by thermal analyses, X-ray diffraction, nuclear magnetic resonance and Raman spectroscopy. According to DTA and XRD results, the noncrystalline state of the as-prepared samples is stable up to 1,000?°C and only after 30?min heat treatment at 1,200?°C the samples become partial crystalline, due to development of cristobalite and quartz nanocrystals. Solid-state ²⁹Si MAS-NMR was employed in order to characterize the local structure around silicon as a function of composition and thermal history of the samples. The NMR data indicate the presence of Q², Q³ and Q⁴ units in all samples. The fraction of the highly interconnected SiO₄ tetrahedra increases both with germanium content and with annealing temperature. The Raman spectroscopy results evidence structural changes related to silicon- and germanium-oxygen units but also to their interconnection, that depend on Si:Ge ratio and annealing temperature.     

Effect of strontium substitution on the composition and microstructure of sol–gel derived calcium phosphates

Sr-doped calcium phosphates have been prepared by sol–gel chemistry. All samples exhibit two phases: hydroxyapatite (HAp) and tricalcium phosphate (β-TCP). With respect to undoped sample, the Sr-doped samples exhibit higher proportion of β-TCP phase but the quantity appears to be quite independent of the doping level. To explain the mismatch with the nominal stoichiometry, the presence of amorphous CaO and SrO compounds have been postulated and their proportions evaluated. The insertion of Sr²⁺ ions in the two crystalline phases HAp and β-TCP is almost total for low doping levels but quite incomplete for the highest doping level. The majority of the inserted Sr²⁺ ions are in the β-TCP phase. Considering the acknowledged beneficial effect of Sr²⁺ on the bone regeneration process, the effective partial substitution of Sr in biphasic calcium phosphate makes these materials very interesting for clinical applications. The Sr-substituted HAp and β-TCP cell parameters agree fairly well with the Vegard’s law and Sr²⁺ ions substitute preferentially for Ca²⁺ in the Ca2 site for hydroxyapatite and in the Ca4 site for β-TCP. The microstructural parameters confirm the previous observation and give a new evidence of clear stabilizing effect of Sr²⁺ ions towards the β-TCP structure.