ZrO2-SiO2 materials prepared by sol-gel

Gels with composition xZrO₂−(100−x)SiO₂, X = 10−55, were prepared in different conditions using zirconium acetylacetonate and TEOS as precursors.

Gels treated at different temperatures up to 1100°C were characterized by X-ray diffraction, IR spectroscopy and TEM. Preparation conditions determined the subsequent development of crystalline phases following thermal treatment.

Monoclinic zirconia segregation dispersed in a silica matrix occurred when the gels were prepared in a strong hydrocloric acid medium. Preparation with a lower acid content favours instead the formation of very small crystals of tetragonal zirconia.     

Coordination and optical attenuation of TiO2-SiO2 glass by electron energy loss spectroscopy

Electron energy loss spectroscopy (EELS) techniques have been applied to investigate both the coordination and optical attenuation of TiO₂-SiO₂ glass. The coordination was determined from the electron energy loss near edge fine structure (ELNES) of Ti L_2,3-edges, and the results show that Ti ions substitute for Si ions in the tetrahedral coordinated sites. The optical attenuation of the glass was obtained from low energy loss of EELS data through Kramers-Kronig analysis, and the results show that Ti-doped silica has an absorption edge near 4.0 eV.     

Investigation of the structure evolution process in sol-gel derived CaO-B2O3-SiO2 glass ceramics

A sol-gel method was used to prepare CaO-B₂O₃-SiO₂ (CBS) glass powder for making low-temperature cofired ceramics. This paper was focused on the mechanism of hydrolysis and polymerization and also on the structural evolution of xerogel at various temperatures. The xerogel was transformed into glass ceramics containing CaSiO₃ and CaB₂O₄ crystalline phases through nucleation and crystallization processes. The results indicated that the xerogel exhibits [BO₄] or [SiO₄] based three-dimensional network structure whose interstices Ca ions fill in, which becomes more orderly and stable after heat treatments. The CBS glass ceramics through controlled crystallization have a potential as electronic packaging materials.     

Non-isothermal crystallization kinetics of ZnO-BaO-B2O3-SiO2 glass

Glass of composition 40SiO₂-30BaO-20ZnO-10B₂O₃ (mol%) was made by conventional melting and casting process. Crystallization kinetics of above glass has been investigated under non-isothermal conditions, using the formal theory of transformations for heterogeneous nucleation. The procedure is applied to the experimental data obtained by differential thermal analysis (DTA), using continuous-heating techniques. The crystallization results are analyzed, and both the activation energy of crystallization process as well as the crystallization mechanism is characterized. Dilatometric measurement of this glass was also done and data obtained was used to calculate the viscosity of the formed glass. Development of crystalline phases on thermal treatments of the glass at various temperatures has been analyzed by X-ray diffraction. Microstructure of the crystalline phases was investigated by scanning electron microscopy (SEM) and the development of various structural features with variation in heat treatment cycle was observed. The nucleation and growth of these phases in the matrix of glass has been described and discussed.     

The role of PbO on crystallization in PbO-SrO-TiO2-SiO2 glass

An investigation of the crystallization in the PbO-SrO-TiO₂-SiO₂ glass system demonstrates that the crystallization temperature of the glass decreases with an increase of the mole fraction of PbO. The second crystalline phase, SrTiO₃, was present when the mole fraction of PbO is more than 0.35, and impaired the piezoelectric coefficient of the glass ceramics in the main crystalline phase, Sr₂TiSi₂O₈.     

Multilayer SiO2 and TiO2 coatings on glasses by the sol-gel process

Glass films of pure SiO₂ and TiO₂ have been prepared on sodalime silica flat slide glasses by the sol-gel process using the dip-coating technique from TEOS and Ti(OC₃H₇)₄ solutions. The various parameters such as chemicals concentrations, viscosity, type of catalyst, withdrawal speed and temperature of densification leading to the obtention of good and adherent coatings with definite film thicknesses are reported. The same technique has been used for the depositon of layers of colored films SiO₂---M_xO_y (M = Co, Mn, Nd and Cr). Brilliant yellow coatings have been obtained with TiO₂---CeO₂.     

Electrical conductivity of NbN-SiO2 films obtained by ammonolysis of Nb2O5-SiO2 sol-gel derived coatings

The studies of electrical conductivity of NbN-SiO₂ films are reported. To obtain these films, sol-gel derived xNb₂O₅-(100 − x)SiO₂ (where x = 100, 90, 80, 70, 60, 50 mol%) coatings were nitrided at 1200 °C. The nitridation process leads to the formation of some disordered structures, with NbN metallic grains dispersed in insulating SiO₂ matrix. The structure of the samples was studied using X-ray diffraction (XRD) and atomic force microscopy (AFM). The electrical conductivity was measured with the conventional four-terminal method in the temperature range from 5 to 280 K. The superconducting transition was not observed even for the sample that does not contain silica. All the samples exhibit negative temperature coefficient of resistivity. The results of conductivity versus temperature may be described on the grounds of a model proposed for a weakly disordered system.     

Apatite formation on TiO2 anatase microspheres

Titanium dioxide (TiO₂) coatings have been long considered as biocompatible interfaces to promote the physico-chemical bonding between the bone tissues and implant material (e.g., titanium and stainless steel). Monodispersed TiO₂ (anatase, the low temperature polymorph of TiO₂) microspheres, produced in the form of colloidal precipitates, were deposited on different substrates and apatite formation was induced on the resulted surface by immersing the coated substrates in simulated body fluid solution. Analytical and microstructural investigations, conducted by X-ray diffraction, energy depressive X-ray spectroscopy and scanning electron microscopy techniques, showed considerable higher rates of apatite formation, in vitro, on the anatase microspheres compared to the sol-gel-derived thin films of the same oxide. We concluded that the particular surface morphology of the packed TiO₂ microspheres, promotes a faster apatite formation in vitro.     

Inclusion of carbon nanotubes in a TiO2 sol-gel matrix

We report here the successful inclusion of carbon nanotubes (CNs) into a TiO₂ matrix prepared by a sol-gel method. The presence of CNs in the sol-gel matrix and the structure of the film were analyzed principally by transmission electron microscopy. Complementary information about the behavior of embedded carbon nanotubes versus heat treatment and ion irradiation were obtained by X-ray photoelectron spectroscopy. The elaboration of an inorganic matrix containing embedded carbon nanotubes leads to a new nanocomposite. The possible applications of this nanocomposite are discussed.     

Study of the structural insertion of Al in the Al2O3-SiO2 and Nd2O3-Al2O3-SiO2 glass systems

Glasses in the Al₂O₃-SiO₂ and Nd₂O₃-Al₂O₃-SiO₂ systems were prepared by the sol-gel method. The gel-glass evolution was studied using near-infrared (NIR) and ultraviolet-visible (UV-VIS) spectrophotometry, X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier infrared spectroscopy (FTIR) and nuclear magnetic resonance spectroscopy (NMR). The results obtained indicate a relationship between the sample compositions, the treatment temperature and the Al coordination. In the samples of the Nd₂O₃-Al₂O₃-SiO₂ system the densification of the structure, when the treatment temperature increases, leads to the segregation of neodymium oxide particles.     

Low temperature synthesis of monolithic silica glass from the system Si(OC2H5)4---H2O---HCl---HOCH2CH2OH by the sol-gel method

To avoid breaking of the gel during drying any organic solvent such as ethanol has not been used. When the amount of HCl is increased and the mixture is stirred vigorously, a transparent silica sol can be prepared immediately with only Si(OC₂H₅)₄ and H₂O. By adjusting the amount of HCl and the temperature, the sol may gel at any time, even in 5 min at room temperature. Such a gel may be dried without cracking under completely open conditions. The addition of glycol as a complex agent can apparently increase the pore size of the gel and decrease its capillary force and surface tension. So, large dried gels can be made in a rather short time. For example, a dried gel of diameter 68 mm and thickness 13 mm can be made in only 4 days. Because of the large pores, the dried gels can be sintered to monolithic silica glasses of practical size with a rather fast heating rate. The mechanisms in the whole process have been explored.     

Considerations on the structure and physical properties of B2O3-SiO2 and GeO2-SiO2 glasses

Data of density, refractive index and thermal expansion coefficient for B₂O₃-SiO₂ and GeO_2-SiO₂ glasses have been analyzed. The volumes of the structural units are the same found for the vitreous B₂O₃, GeO₂ and SiO₂. The volume of any structural unit is constant over the entire composition region of the glass system. The same has been found for the differential refraction and unit refraction of the structural units in these glasses. Different features are observed for the differential expansion of the structural units. There is a considerable change with composition in the differential expansion of BO₃, GeO₄ and SiO₄ units. The effect is attributed to a change in the asymmetry of vibrations with the number of Si-O-B or Si-O-Ge linkages in the matrix. The thermal expansion coefficient is mainly determined by the contribution of B₂O₃ or GeO₂ in the concerned glasses.     

TiO2/蒙脱石复合物中纳米TiO2的相变与光催化性能

以TiOSO4.2H2O为TiO2的前驱体、蒙脱石为基体,采用水解-插层复合法,在不同热处理温度下制备了系列TiO2/蒙脱石纳米复合样品。采用XRD研究了样品中TiO2物相和晶粒大小等属性的变化,研究了样品对亚甲基蓝溶液的吸附脱色性和光催化脱色性。结果表明,热处理温度对TiO2/蒙脱石中TiO2物相与相对含量、晶粒大小和光催化脱色性等有很大影响。经700℃热处理的TiO2/蒙脱石复合样品锐钛矿晶粒仅为13.7 nm,光催化脱色率高达42.7%,总脱色率达79.8%;经1100℃热处理后,锐钛矿相对含量高达55.7%,锐钛矿晶粒为55.8 nm,光催化脱色率仍为32.5%,总脱色率高达46.6%;TiO2/蒙脱石复合样品中的TiO2与纯纳米TiO2样品相比,具有更高的锐钛矿相向金红石相转变的温度、更小的TiO2晶粒尺寸、更强的光催化脱色性。蒙脱石结构层对TiO2的相转变和晶粒长大具有明显的阻滞作用。     

Phonons in TiO2---SiO2 glasses

We report the infrared and Raman studies of titanium doped vitreous silica glasses for a number of titanium concentrations. The vibrational modes associated with the randomly oriented chains of SiO₄ tetrahedra show broadenings and shifts. The LO-TO splitting of some Raman active modes decreases with increasing titanium concentration. This is attributed to the decrease in long-range coulomb fields associated with the chains of SiO₄ tetrahedra which are broken by the titanium atoms. The results are discussed in the context of random network models. An increase in the average intertetrahedral angle of the SiO₄ network is calculated from the data. This explains the anomalous decrease in the density of TiO₂---SiO₂ glasses with increasing titanium content. We identify two new modes associated with the distorted titanium tetrahedra. A polarized Raman mode at 1115 cm⁻¹ which is infrared inactive and an unpolarized Raman mode at 945 cm⁻¹ which is infrared active are observed.     

Effects of Li replacement on the nucleation, crystallization and microstructure of Li2O-Al2O3-SiO2 glass

The Li replacement including the Li₂O replaced by other oxides and the expensive Li₂CO₃ replaced by low-cost spodumene mineral was studied to lower the product cost of (Li₂O-Al₂O₃-SiO₂, LAS) glass ceramic, and the effects of Li replacement on the nucleation, crystallization and microstructure of LAS glass were investigated by the differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that Li₂O replacement increases the crystallization activation energy, lowers the crystal growth, and increases the nucleation and crystallization temperature by restraining the formation of crystalline phases. The Li₂CO₃ replacement decreases the crystallization activation energy, promotes the crystal growth, without affecting the nucleation, and lowers the crystallization temperature by adding some beneficial compositions with mixed alkali effect.     

Gradient-index glass rods of PbO---K2O---B2O3---SiO2 system prepared by the sol-gel process

A glass rod of the PbO---K₂O---B₂O₃---SiO₂ system having a radial gradient of refractive index has been prepared by the sol-gel process using aqueous solutions of lead acetate and potassium nitrate as the sources for index-modifying cations. A gel prepared by gelatinizing a sol from the mixture of tetramethoxysilane, tetraethoxysilane, and the aqueous solutions of boric acid and lead acetate was placed in an aqueous solution of potassium nitrate to form the concentration gradient of modifier cations by diffusion through the micropores of the gel. The glass rod of about 7 mm in diameter obtained by drying and sintering the gel had a refractive index of parabolic profile changing from the center toward the perimeter with a maximum difference of about 0.04.     

Thermo-optic properties of B2O3 doped Li2O-Al2O3-SiO2 glass-ceramics

Reduction in the temperature coefficient of the optical path length, dS/dT of Li₂O-Al₂O₃-SiO₂ glass-ceramics with near-zero thermal expansion coefficient was attempted using control of the temperature coefficient of electronic polarizability, ?, and the thermal expansion coefficient, α. The dS/dT value of 2.6 mol% B₂O₃-doped glass-ceramic was 12.5  × 10⁻⁶/°C, which was 0.9 ×  10⁻⁶/°C smaller than that of B₂O₃-free glass-ceramic. On the other hand, reduction in dS/dT through B₂O₃ doping was not confirmed in precursor glasses. Results showed that reduction in dS/dT of the glass-ceramic through B₂O₃ doping is caused by the reduction in ?. The reduction in ? from B₂O₃ doping was probably attributable to numerical reduction in non-bridging oxide ions with larger ? value by the concentration of boron ions in the residual glass phase. In addition, application of hydrostatic pressure during crystallization was effective to inhibit precipitation of β-spodumene solid solution, which thereby decreases dS/dT. The dS/dT value of B₂O₃-doped glass-ceramic crystallized under 196 MPa was 11.7 ×  10⁻⁶/°C. That value was slightly larger than that of silica glass. The α value of this glass-ceramic was smaller than that of silica glass.     

Density-structure correlations in Na2O-CaO-P2O5-SiO2 bioactive glasses

Relations for Na₂O-CaO-SiO₂ glasses have been developed to calculate the density of Na₂O-CaO-P₂O₅-SiO₂ bioactive glasses. The calculation makes use of NMR results of O’Donnell et al. indicating that P₂O₅ forms a separate phase, containing Na₃PO₄ and Ca₃(PO₄)₂, in the investigated glasses. The volume of the silicate units is the same as that found in Na₂O-CaO-SiO₂, Na₂O-SiO₂, and CaO-SiO₂ glasses. Similarly, the volume of PO₄ units is equivalent to that in Na₃PO₄ and Ca₃(PO₄)₂. Calculated densities are consistent with the experimental data.     

Refractive index-structure correlations in Na2O-Al2O3-SiO2 glasses

The current structural models have been used to analyse the refractive index data of Na₂O-Al₂O₃-SiO₂ glasses (Al₂O₃/Na₂O?1). The SiO₂ content is the sole factor that controls the refractive index. Values could be obtained for the factors with which each structural unit contributes in the refractive index. The content of Al₂O₃ or Na₂O has no effect on the refractive index. The factors (differential refraction) are constant and do not change with composition. They have the same values for Na₂O-SiO₂ glasses. The differential refraction of a structural unit increases linearly with increasing the number of non-bridging oxygen ions. The difference of the contribution to the refractive index from a silicate unit to the next equals to a half of that for AlO₄ tetrahedron. The effect could be attributed to the change in both the concentration and differential refraction of structural units. The obtained factors for the structural units are useful in calculating the refractive index with a high degree of accuracy.     

Effects of CaO on crystallization properties in BaO-SrO-TiO2-SiO2 glass-ceramics

The role of calcium oxide (CaO) was investigated in the crystallization of BaO-SrO-TiO₂-SiO₂ glass. The CaO dopant altered the oriented growth facets of crystal unit cells in the glass-ceramics. The crystallites are acicular in micron scale in the samples having as small as 1% CaO dopant, but are long granular in the nanometer scale in the sample without CaO. The glass-ceramics which have nano crystallites are observed with a less crystallization density qualitatively, a higher transmissivity in the wavelength range of 200-2600 nm, and 30% lower piezoelectric coefficient, d₃₃, at 9.5 ± 0.3 pC/N than those glass-ceramics having CaO in the study.