Preparation of bioactive glass ceramic nanoparticles by combination of sol-gel and coprecipitation method

SiO₂-CaO-P₂O₅ ternary bioactive glass ceramic nanoparticles were prepared via the combination of sol-gel and coprecipitation processes. Precursors of silicon and calcium were hydrolyzed in acidic solution and gelated in alkaline condition together with ammonium dibasic phosphate. Gel particles were separated by centrifugation, followed by freeze drying, and calcination procedure to obtain the bioactive glass ceramic nanoparticles. The investigation of the influence of synthesis temperature on the nanopartilce’s properties showed that the reaction temperature played an important role in the crystallinity of nanoparticle. The glass ceramic particles synthesized at 55 °C included about 15% crystalline phase, while at 25 °C and 40 °C the entire amorphous nanopowder could be obtained. In vitro testing showed that the bioactive glass ceramic nanoparticles can induce the formation of hydroxylaptite from simulated body fluid rapidly. As a result, this bioactive glass ceramic nanoparticle with excellent bioactivity would be a promising filler material for bone tissues engineering.     

Sol-gel synthesis of erbium-doped yttrium silicate glass-ceramics

Yttrium silicate glasses and glass-ceramics were prepared by the sol-gel process. Yttrium nitrate was added to tetraethyl orthosilicate in amounts representing between 0.2 and 20 mol%, as well as amounts corresponding to the disilicate composition. Some samples were doped with erbium acetate. The solutions underwent gellation in 2-7 days and were dried for 2 weeks. Differential thermal analysis was used to design a multi-step heat treatment to 1000 °C to densify samples to transparent or translucent monoliths. Above 1000 °C, samples crystallized to yttrium disilicate and cristobalite. Phase separation before crystallization influenced the formation of the crystal phases.     

Synthesis of ORMOSIL silica/rhodamine 6G: Powders and compacts

The sol-gel method was used to prepare a silica matrix with rhodamine 6G. This dye was chemically bonded to silica matrix by grafting reactions, obtaining ORMOSIL/R6G powder, at two different concentrations, 0.03 mmol of dye/g silica and 0.009 mmol of dye/g silica, called ORMOSIL1 and ORMOSIL2, respectively. Fluorescent compacts were also obtained, through the high-pressure processing of these powders, at 7.7 GPa and room temperature. The microstructure, chemical composition and thermal stability of the powders and compacts were comparatively studied, by N₂ adsorption/desorption isotherms, FTIR spectroscopy, thermogravimetric analyses (TGA) and elemental analyses for carbon, hydrogen, and nitrogen (CHN). The optical behavior was studied by fluorescence spectroscopy. The ORMOSIL1 compacts were completely opaque showing that the compaction was not effective, due to the high organic grade incorporated into the starting ORMOSIL1 powder. The ORMOSIL2 compacts were transparent showing an effective compaction, which are in agreement to the great surface area and porosity reduction. Additionally, it was observed in TGA results, the retention of the organics in these samples up to temperatures near 300 °C. The fluorescence emission showed that the dye was dispersed in molecular level in all samples.     

The study on various wet chemistry techniques on YBa2Cu3O7−δ superconducting oxides powder preparation

This paper reports on the results of four different wet chemical techniques used for the synthesis of superconducting oxides, YBa₂Cu₃O₇ (YBCO). YBCO samples were prepared by complexion citric gel route (CT), acetate-tartrate sol-gel (ASG), oxalate/ethanol coprecipitation (COP), and acetate-tartrate sol-gel solid-state reaction (ASG-SSR) techniques. Each of the YBCO powders was annealed for 4 h at 880 °C, 900 °C, 920 °C, and 950 °C to find the optimum temperature. The optimum sintering temperature for the formation of Y-123 phase is very much dependent on the type of wet chemical technique used to prepare the powders.     

Monolithic copper oxide aerogel via dispersed inorganic sol-gel method

To avoid the use of rare copper alkoxides, copper oxide aerogels were prepared using copper chloride, polyacrylic acid and propylene oxide via the dispersed inorganic sol-gel method, a supercritical fluid drying process and a 500 °C thermal treatment. The morphology and composition of the aerogel without thermal treatment (copper-based aerogel) and copper oxide aerogel were both characterized and analyzed. Based on studies of the gelation mechanism, it was demonstrated that polyacrylic acid guides the sol formation including providing a steric effect. Analysis of the thermal treatment process shows that the copper-based aerogel has four primary thermal reactions and the major composition of samples treated above 420 °C is monoclinic copper oxide.     

Preparation and characterization of monolithic alumina aerogels

Alumina aerogels were prepared by a sol-gel method combined with the ethanol supercritical drying technique using aluminum tri-sec butoxide and nitric acid as the precursor and catalyzer respectively. This method affords high-surface-area alumina aerogel monoliths without the use of complexing agents. The structure and morphology of the aerogels were investigated by TEM, XRD, FTIR and BET techniques. The results confirmed that the as-prepared alumina aerogel possessed a network microstructure made up of pseudoboehmite fibers and a surface area of 690 m²/g. It was transformed to γ-Al₂O₃ after heat treatment at 800 °C without a significant loss in surface area. DMA analysis and hotdisk thermal analysis were performed to characterize the mechanical and thermal properties of the samples. The results indicated that the alumina aerogel was robust and exhibited excellent thermal insulating properties. The elastic modulus was up to 11.4 MPa after drying, which is the one of the highest modulus of alumina aerogels ever reported. The thermal conductivities at 30 °C and 400 °C were 0.028 W/mK and 0.065 W/mK respectively.     

Influence of heat treatment on spectroscopic properties of Er in multicomponent ZrF4-ZnF2-AlF3-YF3-MF2 (M = Ca, Sr, Ba) based glass

Er³⁺ doped multicomponent fluoride based glass was prepared. These precursor fluoride glass samples were then heated using different schedules. Crystalline phase particles were successfully precipitated in the multicomponent fluoride glass samples after heat treatment. The influence of heat treatment on the spectroscopic properties of Er³⁺ in multicomponent fluoride based glass samples were discussed. Small changes of the Judd-Ofelt parameters Ω_i (i = 2, 4, 6) were found in multicomponent fluoride glass samples before and after heat treatment compared to oxyfluoride telluride glass. Preparation conditions used to produce transparent multicomponent fluoride glass ceramics doped with rare-earth ions are discussed.     

Luminescence spectra of stilbene-3 doped lead-tin-fluorophosphate glass excited by VUV-UV synchrotron radiation

The stilbene-3 doped lead-tin-fluorophosphate glasses were successfully prepared and the luminescence spectra obtained from VUV-UV synchrotron radiation were investigated. The emission peaked about 440 nm from the undoped glass is observed at low temperature but quenches at room temperature. The doped glass exhibits strong characteristic emission of stilbenen-3 with 467 and 436 nm Gaussian bands under ultraviolet 250-330 nm excitation (characteristic excitation of stilbene-3) but weak emission under vacuum ultraviolet 190 nm excitation (host excitation). It is suggested that the energy transfer from host to the stilbene-3 dye can occur but the efficiency is still fairly low.     

A novel strategy for the preparation of yttria-stabilized zirconia powders: Deposition and scratching of thin films obtained by the sol-gel method

In the present work, an alternative to the traditional sol-gel method is reported. The gel is deposited on a wide flat glassy surface so that a very thin film of solid is obtained after evaporation of the solvent. The yttria-stabilized zirconia powders are recovered by removing the referred film from the glassy surface and subsequent milling. The samples have been characterized by Scanning Electron Microscopy (SEM), nitrogen adsorption at −196 °C and Fourier Transform Infrared (FT-IR) spectroscopy. The experimental results show that the 3 mol% yttria-stabilized zirconia (3YSZ) powders prepared by drying of thin films at 100 °C exhibit extremely low values of specific surface area and pore volumes. Furthermore, the proportion of solvent or the type of alcohol used in the preparation of the powders play a very important role in the textural properties of the 3YSZ powders. In this connection, for samples prepared by varying the propanol proportion an increase in the volume of this solvent leads to larger values of specific surface area and pore volumes. On the other hand, an increase in the molecular size of the alcohol used as the solvent leads to a noticeable development of the textural parameters.     

Photoinduced degradation of fluorescence and formation of copper nanoparticles in sol-gel silica doped with flavins

Copper nanoparticles were formed by photoirradiation of doped sol-gel silica, which was prepared by mixing Cu²⁺ ions, ethylenediaminetetraacetic acid (EDTA), and riboflavin into sol-gel solutions of tetramethoxysilane. The doped silica exhibited broad absorption bands at 442 nm due to riboflavin and 740 nm due to Cu²⁺-EDTA complexes. After photoirradiation, the sol-gel silica showed a reddish brown color and the absorption peak around 580 nm due to the plasmon band of copper nanoparticles. Copper nanoparticles were also formed from other sol-gel silica doped with lumichrome or lumiflavin. The photostability of the flavins dyes obtained from the fluorescence intensities was in the order of lumichrome > lumiflavin > riboflavin in the sol-gel silica without Cu²⁺ ions. On the other hand, the fluorescence intensities were considerably reduced by photoirradiation of the sol-gel silica doped with Cu²⁺ ions, irrespective of the flavin dyes doped. Considering the absorption and fluorescence spectral changes during the photoirradiation, we concluded that copper nanoparticles are produced by the photoinduced electron transfer from the flavin dyes in the sol-gel silica.     

Sol-gel deposition of silica films on silicate glasses: Influence of the presence of lead in the glass or in precursor solutions

Many lead silicate historical glasses suffer degradation phenomena often observed as color changes and iridescence caused by lead ions leaching from the outer layers of the glass. In order to repair and to prevent these phenomena, glasses with large amounts of lead (6.7 and 14.3 at.% of lead) have been coated with silica films at neutral pH by dipping them in a precursor solution of TEOS (tetraethyl orthosilicate), ethyl alcohol and deionized water without any other acid or basic catalyst. Experiments with long dipping times (24 h) and temperatures around 20 °C have been performed to evaluate the role of lead ions of the glass as a catalyst. Silica films of very good quality and optical transparency have been also obtained on lead-free, soda-lime glasses by adding catalytic amounts of Pb(NO₃)₂ instead of HCl to the precursor solution. The films have been characterized by optical microscopy, AFM (Atomic Force Microscopy), XPS (X-ray Photoelectron Spectroscopy) and SIMS (Secondary Ion Mass Spectrometry).     

A study about the stabilization of anatase phase at high temperatures on sol-gel cerium and copper doped titania and titania-silica powders

Copper and cerium oxides doped titania samples, as well as silica-titania samples were prepared through sol-gel process by using titanium butoxyde as precursor. The obtained samples were characterized by X-ray diffractometry and scanning electron microscopy. The main objective of the proposed study is to promote the stabilization of anatase phase at high temperatures. The obtained experimental results show that cerium doped titania samples stabilizes the anatase phase until 800 °C, whereas copper doped samples calcinated at the same temperature are a mixture of anatase and rutile phases. So, CeO₂ is a better stabilizer for anatase phase than CuO. On the other hand, based on previously reported data it can be concluded that both, CeO₂ and CuO are better stabilizers for anatase phase than SnO₂. It is also verified that the total amount of silica in the titania-silica double oxide increases the temperature required for crystallization, and as a consequence, the anatase phase is stabilized at higher temperatures i.e. up to 1200 °C.     

Effect of CeO2-doping on surface and catalytic properties of CuO-ZnO system

The effect of doping CuO-ZnO system with CeO₂ on its surface and catalytic properties was investigated using nitrogen adsorption at −196 °C, EDX technique and catalysis of CO oxidation by O₂ at 100-200 °C. Pure mixed solids were prepared by thermal decomposition of copper/zinc mixed hydroxides at 400 °C. The doped solids were obtained by impregnating a known mass of mixed hydroxides with calculated amount of cerium ammonium nitrate followed by drying then calcination at 400 °C. The dopant concentration was 1.5, 3.0 and 4.5 mol% CeO₂. The results revealed that CeO₂-doping modified the surface atomic Cu/Zn ratio of the system investigated and changed the crystallite size of both CuO and ZnO phases. The increase of the amount of dopant added changed the major phase present. This treatment decreased the specific surface area of doped solids. The doping process modified also the catalytic activity in a manner dependent on both mode of preparation and dopant concentration. However, CeO₂-doping did not modify the mechanism of the catalytic reaction but changed the concentration of catalytically active sites involved in the catalyzed reactions.     

Electrical conduction mechanism and gas sensing properties of Pd-doped TiO2 films

Undoped and Pd-doped titanium oxide thin films (0.5 wt.%) were prepared by the sol-gel technique (dip-coating) on glass and silicon substrates. The as-deposited thin films were compacted by subjecting them to different annealing temperatures (300 °C, 500 °C). The dependences of the electrical conductivity vs. inverse temperature were investigated in air and in vacuum. A study of the effects of Pd-doping, annealing temperature and ambient conditions on their electrical properties was performed. The sensing behavior of titanium oxide thin films exposed to some reducing gases (methane, acetone, ethanol, formaldehyde and liquefied petroleum gas) was carried out, by means of electrical conductivity measurements. All the studied films are most sensitive to formaldehyde, with a special remark for the Pd-doped ones deposited on silicon substrates.     

Fluorescent compacts prepared by the entrapment of benzoxazole type dyes into a silica matrix at high pressure

We have produced silica-gel compacts doped with 2,5-Bis(benzoxazol-2^′-yl)-4-methoxyphenol dye using high-pressure processing of powders synthesized by the sol-gel technique. The high-pressure compaction of powders with three different dye concentrations was done at 4.5 GPa and room temperature. We have measured optical and mechanical properties of the obtained compacts. They were very stable, transparent, crack free, hard (3.56 ± 0.07 GPa) and dense (1.95 ± 0.03 g/cm³), being resistant to polishing and leaching, which enables its use in optical applications. The Stokes shift observed was higher than 100 nm indicating that the intramolecular proton-transfer in the electronically excited state (ESIPT) of this dye is maintained, even in an OH rich environment like silica. A shift to higher wavelength in the fluorescence spectra of the compacts, attributed to the increasing in the conjugation of the π system, was observed.     

Photo-quenched of the luminiscence signal in Co(II)-doped alumina prepared by the sol-gel method

Co(II)-doped alumina monoliths prepared by the sol-gel method were laser irradiated producing fluorescence. The intensity of this defect-induced fluorescence was exponentially reduced with the irradiation time to practically disappear. The rate the fluorescence intensity decays was modeled as a double exponential function of the irradiation time; the characteristic times associated with the decays are in the range of seconds. The suppression of the luminescence was associated with the local heating produced by the highly focused laser beam in a small area (≈ 2 μm in diameter) on the sample. This heating process reduces physical (grain boundaries and surface states) and chemical (oxygen vacancies) defects present in the sol-gel samples. Some residual fluorescence still remains after long periods of illumination. The characteristic times for alumina samples are compared with those obtained for other metal oxides prepared also by the sol-gel method.     

Preparation of organic-inorganic nano-composites for antireflection coatings

In this research, bi-layer thin film stacks that served as an antireflective coating were developed. The top layer was synthesized using tetraethoxysilane and 3-(trimethoxysilyl) propyl methacrylate by the well-known sol-gel technique. Its refractive index was lower than that of the bottom layer, which was prepared by reaction between tetrabutoxyltitanium and γ-glycidoxy propyl trimethoxysilane. Antireflective coatings were obtained by spin-coating of the synthesized sols on a glass substrate, followed by pre-drying, UV-curing, and post-baking. Fourier transform infrared spectrometer was employed to investigate the evolution of chemical bonds during the UV-curing and the sol-gel processes. The size of the inorganic/organic hybrid particles in the sol was found to be less than 10 nm, as measured by transmission electron microscope and dynamic light scattering. Thermo gravimetric analyzer was used to find out the thermal degradation temperature of the two layers and the effect of post-baking. The results indicated that the thermal stability increased after post-baking at 200 °C for 15 min. The reflectance of the antireflection coating was controlled by the relative refractive indices and the thickness of the top and bottom layers. Under optimal synthesis condition, we obtained an antireflection coating, exhibiting a low reflection, 1% at 550 nm, in the visible range.     

Synthesis optimization of organic xerogels produced from convective air-drying of resorcinol-formaldehyde gels

Resorcinol-formaldehyde gels were produced at 50, 70 and 90 °C and with three different R/C ratios (500, 1000 and 2000). The effect of these variables combined with that of aging time was studied in order to optimize the synthesis conditions. The convective air-drying process was used, and the drying duration was studied with regard to the synthesis conditions. The aging time has no effect on the pore texture after 24 h at 90 °C or 48 h at 70 °C, whatever the R/C value. The synthesis-aging step can be shortened by increasing the temperature. Nevertheless, the pore size tends then to decrease, especially when R/C is high, but this can be counterbalanced by increasing R/C. Moreover, bubbles often appear in the gel at high synthesis temperature, which limits the temperature to about 70 °C in the case of monolithic parts. At 70 °C and with an air velocity of 2 m/s, the elimination of 90% of the solvent requires 1 h drying when the pore size reaches 400-600 nm, 2.5 h for 50 nm wide pores and 3 h when the pore size decreases to 15-20 nm. The drying duration does not exceed 8 h in all cases and could be shortened by increasing the temperature at the end of the process.     

Preparation and characterization of ZnO-TiO2 films obtained by sol-gel method

The sol-gel route has been applied to obtain ZnO-TiO₂ thin films. For comparison, pure TiO₂ and ZnO films are also prepared from the corresponding solutions. The films are deposited by a spin-coated method on silicon and glass substrates. Their structural and vibrational properties have been studied as a function of the annealing temperatures (400-750 °C). Pure ZnO films crystallize in a wurtzite modification at a relatively low temperature of 400 °C, whereas the mixed oxide films show predominantly amorphous structure at this temperature. XRD analysis shows that by increasing the annealing temperatures, the sol-gel Zn/Ti oxide films reveal a certain degree of crystallization and their structures are found to be mixtures of wurtzite ZnO, Zn₂TiO₄, anatase TiO₂ and amorphous fraction. The XRD analysis presumes that Zn₂TiO₄ becomes a favored phase at the highest annealing temperature of 750 °C. The obtained thin films are uniform with no visual defects. The optical properties of ZnO-TiO₂ films have been compared with those of single component films (ZnO and TiO₂). The mixed oxide films present a high transparency with a slight decrease by increasing the annealing temperature.     

Synthesis and characterization of SiO2-PEG hybrid materials

Sol-gel is a promising technique for the synthesis of organic-inorganic hybrid materials both of class I and II. In materials of class I organic molecules are physically entrapped in an inorganic matrix, while in those of class II organic and inorganic parts are connected by covalent bonds. In this paper a sol-gel procedure to obtain SiO₂-PEG hybrids of class I, in which PEG is simply mixed at the sol stage, is compared to a sol-gel procedure to obtain SiO₂-PEG hybrid materials of class II, where a particular sol-gel Si-C precursor is synthesized. XPS analyses showed the different distribution of the organic phase in the SiO₂ matrix and the bond between PEG and SiO₂ for hybrids of class II. The PEG molecule in hybrid of class II showed an enhanced thermal stability up to 350 °C. Doping with a lithium salt was performed on hybrids of class II, and the ionic conductivity was measured.