Low Temperature Bonding of Ceramics by Sol-Gel Processing

Sol-gel bonds were produced between smooth, clean silicon or polycrystalline alumina substrates by spin-coating solutions containing partially hydrolysed silicon alkoxides onto both substrates. The two coated substrates were assembled and the resulting sandwich was fired at temperatures ranging from 300 to 600°C. The influence of the sol-gel chemistry on the film microstructure and interfacial fracture energy was investigated using a wide range of techniques, including ellipsometry, FTIR, TG-DTA, rheology, TEM and micro-indentation. For silicon wafers, an optimum water-alkoxide molar ratio of 10 and hydrolysis water pH of 2 were found. Such conditions led to relatively dense films (>90%), resulting in bonds with significantly higher fracture energy (3.5 J/m²) than those obtained using classical water bonding (typically 1.5 J/m²). Aging of the coating solution was found to decrease the bond strength. Poly-crystalline alumina substrates were similarly bonded at 600°C; the optimised silica sol-gel chemistry yielded interfaces with fracture energy of 4 J/m².     

Synthesis and characterization of new semifluorinated linear and hyperbranched poly(arylene ether phosphine oxide)s through B2 + A2 and AB2 approaches

A new phosphorus containing trifluoromethyl-activated bisfluoro B₂ monomer has been synthesized successfully by coupling reaction of 4-methoxyphenylphosphonic dichloride and the Grignard salt of 5-bromo-2-fluorobenzotrifluoride. This monomer was converted to linear poly(arylene ether phosphine oxide)s by nucleophilic displacement of the fluorine atom on the benzene ring with several diphenols. The B₂ monomer was further demethylated to form an AB₂ monomer which on self condensation yielded hyperbranched poly(arylene ether phosphine oxide) with identical phosphorous containing moiety. The products obtained exhibit weight-average molecular weights as high as 600,000 g mol⁻¹ in SEC. These linear and hyperbranched poly(arylene ether phosphine oxide)s showed thermal stability as high as 516 °C for 10% weight loss in TGA in nitrogen and showed glass transition temperatures up to 253 °C in DSC. All the polymers were soluble in a wide range of organic solvents, e.g., CHCl₃, THF, NMP and DMF, however, the hb sample showed a significant lower solution viscosity compared to linear samples of similar molar mass. Transparent thin films of linear poly(arylene ether phosphine oxide)s casted from dichloromethane exhibited tensile strengths up to 50 MPa, a modulus of elasticity up to 0.95 GPa and elongation at break up to 36% depending on their exact repeating unit structures. No free standing films could be prepared from the hb analogue due to the missing entanglements, but stable thin polymer films on silicon wafers with high hydrophobicity were formed which showed water contact angles as high as 91°.     

Nanopore Size Growth and Ultrafiltration Performance of SnO2 Ceramic Membranes Prepared by Sol-Gel Route

Supported ceramic membranes have been produced by the sol-casting procedure from aqueous colloidal suspensions prepared by the sol-gel route. Coatings on a tubular alumina support have been successfully performed leading to crack free layers. Samples have been sintered at 400, 500 and 600°C, and the effect of heating treatment on the nanostructure and on the ultrafiltration properties are analyzed. The characterization has been done by high resolution scanning electron microscopy, nitrogen adsorption-desorption isotherms, water permeation and cut-off determination using polyethylene glycol standard solutions. The micrographs have revealed that grains and pore size increase with the temperature, whereas their shape remains invariant. This results is in agreements with N₂ adsorption-desorption analyses, which have revealed that the mean pore size diameter increases from 4 to 10 nm as the sintering temperature increases from 400 to 600°C, while the total porosity remains constant. Furthermore, the tortuosity, calculated from water permeability, is essentially invariant with the sintering temperatures. The membranes cut-off, determined with a retention rate equal to 95%, are 3500, 6500 and 9000 g·mol^–1 for 400, 500 and 600°C, respectively, showing that the permeation properties of SnO₂ ultrafiltration membranes can easily be controlled by sintering condition.     

Effects of Heat Treatment on Photocatalytic Property of Sol-Gel Derived Polycrystalline TiO2

Polycrystalline TiO₂ in the bulk form was prepared by hydrothermal treatment of amorphous TiO₂ gel, and its photocatalytic activity for degradation of ethanol and physical properties were investigated after drying and calcination (300–650°C). The maximum photocatalytic activity reached when the polycrystalline TiO₂ was calcinated at temperatures from 300 to 500°C, which was almost the same as for a commercial TiO₂ powder. It was markedly reduced above 600°C. Photocatalytic activity of TiO₂ was affected by crystal growth of anatase, progress of sintering, anatase-rutile transformation and morphology of the polycrystalline TiO₂. It was a characteristic of the polycrystalline TiO₂ prepared in this study that the photocatalytic activity did not vary so much with the calcination temperature in the range of 300–500°C.     

Physico-Chemical and Catalytic Study of the Co/SiO2 Catalysts

This paper is focused on the physico-chemical and catalytic properties of Co/SiO₂ catalysts. Silica-supported cobalt catalysts were prepared by sol-gel and impregnation methods and characterized by BET measurements, temperature programmed reduction (TPR_H2), X-ray diffraction (XRD), and thermogravimetry-mass spectroscopy (TG-DTA-MS). The sol-gel method of preparation leads to metal/support catalyst precursor with a homogenous distribution of metal ions into bulk silica network or on its surface. After drying the catalysts were calcined at 500, 700, and 900°C. The reducibility of the supported metal oxide phases in hydrogen was determined by TPR measurements. The influence of high temperature—atmosphere treatment on the phase composition of Co/SiO₂ catalysts was investigated by XRD and TG-DTA-MS methods. At least five crystallographic cobalt phases may exist on silica: metallic Co, CoO, Co₃O₄, and two different forms of Co₂SiO₄ cobalt silicate. Those catalysts in which cobalt was chemically bonded with silica show worse reducibility as a result of strongly bonded Co-O-Si species formed during high-temperature oxidation. The TPR measurements show that a gradual increase in the oxidation temperature (500–900°C) leads to a decrease in low-temperature hydrogen reduction effects (<600°C). The decrease of cobalt oxide reduction degree is caused by cobalt silicate formation during the oxidation at high temperature (T 1000°C). The catalysts were tested by the reforming of methane by carbon dioxide and methanation of CO₂ reactions.     

Thermal behavior of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> mesoporous gels

Summary Titania-based photocatalytic materials were prepared by sol-gel method using Fe³⁺ and polyethyleneglycol (PEG₆₀₀) as additives. Thermogravimetry (TG), differential thermal analysis (DTA) and evolved gas analysis (EGA) with MS detection were used to elucidate processes that take place during heating of Fe³⁺ containing titania gels. The microstructure development of the Fe₂O₃/TiO₂ gel samples with and without PEG₆₀₀ admixtures was characterized by emanation thermal analysis (ETA) under in situ heating in air. A mathematical model was used for the evaluation of ETA results. Surface area and porosity measurements of the samples dried at 120&deg;C and the samples preheated for 1 h to 300 and 500&deg;C were compared. From the XRD measurements it was confirmed that the crystallization of anatase took place after thermal heating up to 600&deg;C.     

Glycol-based sol-gel process for the fabrication of ferroelectric PZT thin films

A new sol-gel system using ethylene glycol was developed for the fabrication of PZT thin films with compositions near the morphotropic phase boundary Pb(Zr_0.52Ti_0.48)O₃. Ethylene glycol was used as both a chelating agent and a solvent to replace the highly toxic methoxyethanol used in previous formulations. Thin films were deposited by spin coating the solutions onto platinized silicon substrates. Films were completely crystallized by about 600°C and contained the ferroelectric perovskite phase. A dielectric constant of about 750–800 at 1 KHz was obtained for thin films of 0.3 µm thickness. The hysteresis measurements revealed a remanent polarization of 15 mC/cm² with a coercive field of 60 kV/cm.     

Bioactive ceramics in the CaO-Al2O3-P2O5-ZnO system prepared by the sol-gel technology

The sol-gel method was applied to the synthesis of porous bioactive glass-ceramic materials in the Ca-Al₂O₃-P₂O₅-ZnO system when compositions were chosen in the glass formation range. Aluminium sol and soluble inorganic salts of the other components have been used as starting materials. The sol-gel transition was achieved by controlling the pH value of the medium. After drying the gel products were thermally treated up to 800°C, Phase formation was studied by X-ray diffraction, IR spectroscopy and electron microscopy.It was found that the main crystalline phases in the glass ceramics were C(PO₃)₂ and Ca₂P₂O₇. The amorphous powder was sintered to form ceramic materials with average pore size distribution of 150–200 µm by isostatic pressing and heat treatment at 500–600°C. This glass ceramic is more bioactive than hydroxylapatite in the initial stage after implantation.     

Sol-gel processed barium titanate waveguides

Monolayer monomode and multilayer multimode BaTiO₃ waveguides have been prepared on amorphous silica substrates by using the dip-coating technique and the sol-gel process. After heat treatment, these waveguides were hard and of good optical quality (losses as low as 2.6 dB/cm were measured) but, their structure was found to depend strongly on the number of layers. Monolayer waveguides were totally amorphous even when heated at 1000°C, while multilayer ones exhibited the tetragonal BaTiO₃ structure at an annealing temperature of 600°C.     

Improvement of the Pd/Al2O3 Catalyst by the Control of the Sol-Gel Preparative Parameters

In this work, the metal dispersion of the Pd/Al₂O₃ catalyst prepared by sol-gel method is improved by an adequate optimisation of the preparative variables. First, the gelation temperature and the ageing time are selected, in order to avoid the reduction of the metal precursor (palladium acetylacetonate, Pd(acac)₂) by the solvent (sec-butanol, sB). The metal sintering effect on the catalysts treated in oxygen at 500°C is then minimized when the alumina pore size is controlled by the variation of the alumium alkoxide (AsB) concentration and the acetic acid amount ([AcA]/[AsB]). The appearance of new palladium particles on the alumina surface and the matching between the particle diameters and the pore sizes were also effective for the metallic surface area improvement on the samples treated in oxygen at 800°C. Compared to the reference catalysts, the higher metal dispersion obtained on the sol-gel ones was the determinant factor for their higher catalytic activity in methane combustion.     

Barium Sodium Niobate Powders and Thin Films: Preparation and Ferroelectric Behavior

Powders and thin films of barium sodium niobate, Ba₄Na₂Nb₁₀O₃₀, of filled tungsten bronze type ferroelectric were processed by a sol-gel route using barium metal, niobium ethoxide and sodium methoxide as precursors. Polycrystalline powder resulted after heat treating the gel powder at or above 650°C. Thin films of Ba₄Na₂Nb₁₀O₃₀ showed no preferred orientation on Si(100), Pt/Si(100) and sapphire substrates. Hysteresis measurements at 1 kHz for the thin films annealed at 750°C, obtained using a prehydrolyzed precursor solution, and gave remanent polarization of 17.34 µC/cm² and coercive field of 62.5 kV/cm. Microstructural investigation of surface morphology of these films revealed grains about 0.3 µm in size. Prehydrolysis of the precursor solution was found to be necessary to achieve dense films with ferroelectric properties.     

Formation of α-Alumina Around 500°C in Alkoxy-Derived Alumina Gels Under Ambient Pressure—Effects of Starting Solution Composition and Seeding

The hydrolysis of aluminum-sec-butoxide (ASB) in the ASB-H₂O-C₂H₅OH-HCl solutions at 0°C for 18 h and subsequent aging of the sol for 30 days at 5°C provided an alumina gel which was partially (20–30 wt%) transformed to -Al₂O₃ when heat-treated at 500°–600°C under ambient pressure. In addition to the hydrolysis and aging conditions, the amounts of water and HCl in the starting solution were found to be very important factors for the low temperature formation of -Al₂O₃ in the resultant gel. Some difference in the chemical environment of involved OH groups was found between the alumina gel forming -Al₂O₃ at 500°–600°C and the conventional gel. Corundum seeding was found to be effective; thus, 1 wt% seeding brought about an increase in the fraction of -Al₂O₃ formed at 600°C to 40–50 wt%.     

Sol-Gel Derived Bismuth Titanate Thin Films with c-Axis Orientation

Bismuth titanate (Bi₄Ti₃O₁₂), a member of the layered perovskite family, has a unique set of ferroelectric properties, which include a high remanent polarization, low coercive field, and high Curie temperature, that make it a possible candidate for data storage applications. For this investigation, bismuth titanate, or BiT, films were fabricated via sol-gel method to examine the effect of processing on phase development and orientation. Solutions were deposited onto platinized silicon, and then heat treated for one hour at temperatures ranging from 550°C to 700°C in 100% O₂. It was found that c-axis orientated BiT films could be formed at temperatures as low as 550°C by using bismuth oxide template layers, while films without bismuth oxide templating possessed a random orientation over the same temperature range.     

Er3+/Yb3+ Co-Activated Silica-Alumina Monolithic Xerogels

Monolithic silica xerogels doped with different concentrations of Er³⁺, Yb³⁺ and Al³⁺ were prepared by sol-gel route. Densification was achieved by thermal treatment in air at 950°C for 120 h with a heating rate of 0.1°C/min. We studied the luminescence properties of the ⁴I_13/2 ⁴I_15/2 emission band of Er³⁺ as a function of the Al/Er/Yb concentration and we paid particular attention to the alumina effects. Raman spectroscopy and Vis-NIR absorption were used to monitor the degree of densification of the glasses and the residual OH content.     

Enthalpies of Dilution of Aqueous Solutions of HCl, MgCl2}, CaCl2, and BaCl2 at 300, 325, and 350°C

Dilution enthalpies, measured using isothermal flow calorimetry, are reported for aqueous solutions of BaCl₂ at 300°C and 11.0 MPa, MgCl₂, CaCl₂, and BaCl₂ at 325°C and 14.8 MPa, and at 350°C and 17.6 MPa. Previously collected dilution enthalpies for aqueous solutions of MgCl₂ and CaCl₂ at 300°C and 10.3 MPa and for aqueous solutions of HCl at 250, 275, and 300°C at 10.3 MPa and 320°C at 12.8 MPa were included with the new data at 300°C and 11.0 MPa and at 350°C and 17.6 MPa when fitting the Pitzer parameters. The concentration range of the chloride solutions was 0.5 to 0.02 molal. Parameters for the Pitzer excess Gibbs ion–interaction equation were determined from the fits of the experimental heat data. Equilibrium constants, enthalpy changes, entropy changes, and heat-capacity changes for the association of alkaline earth metal ions and H⁺ with chloride ion were estimated from the heat data. For all systems, the enthalpy and entropy changes are positive and show accelerating increases with temperature. The resulting equilibrium constants show significant, but smaller, increases with temperature.     

Structure and dielectric properties of HfO<Subscript>2</Subscript> films prepared by a sol–gel route

Mono- and multilayer HfO₂ sol–gel thin films have been deposited on silicon wafers by dip-coating technique using a solution based on hafnium ethoxide as precursor. The densification/crystallization process was achieved by classical annealing between 400 and 600 °C for 0.5 h (after drying at 100 °C). Systematic TEM studies were performed to observe the evolution of the thin film structure depending on the annealing temperature. The overall density of the films was determined from RBS spectrometry correlated with cross section (XTEM) thickness measurements. After annealing at 450 °C the films are amorphous with a nanoporous structure showing also some incipient crystallization. After annealing at 550 °C the films are totally crystallized. The HfO₂ grains grow in colonies having the same crystalline orientation with respect to the film plane, including faceted nanopores. During annealing a nanometric SiO₂ layer is formed at the interface with the silicon substrate; the thickness of this layer increases with the annealing temperature. Capacitive measurements allowed determining the value of the dielectric constant as 25 for four layer films, i.e. very close to the value for the bulk material.     

Mesoporous Nb2O5 Films: Influence of Degree of Crystallinity on Properties

Nanocrystalline Nb₂O₅ films were prepared by an extended sol-gel method. The synthesis is based on the hydrolysis of a modified Nb-alkoxide precursor. Reaction of the modified precursor (Nb(OEt)₅ + 2 2,4-pentanedione) with water in ethanol leads to a homogeneous hydrolyzed solution, which is stable against precipitation of niobium oxide after evaporation of the ethanol and in the whole pH-range investigated (1–10). Autoclaving leads to amorphous gels, from which homogeneous nanocrystalline niobium oxide films of up to 15 m can be made. During annealing crystalline phases are first observed above 500°C with fully crystalline films of orthorhombic T-phase Nb₂O₅ attained at 600°C. The microstructural, crystallographic, optical and photoelectrical properties of the films were characterized by means of SEM, XRD, UV-VIS spectroscopy and surface photovoltage spectroscopy, respectively.     

Surface Modification of Colloidal Silica by Sulfonation Route for Super-Hydrophilicity

Colloidal silica was chemically modified by a two-step method including an olefin sulfonation route as a preliminary study for super-hydrophilic applications. The hydrophobic vinyl groups were initially bonded chemically to the hydroxylated silica surface using the trichlorosilane coupling agents. The vinyl-terminated silica was then sulfonated by addition reaction with chlorosulfonic acid. The modified silica was investigated using DRIFT, TGA, element analysis, solid-state ²⁹Si- and ¹³C CP-MAS NMR. The vinyl-terminated specimen showed a characteristic IR absorption band at 1600 cm^–1 and a weight loss of approximately 3% starting at 350°C while the ²⁹Si NMR peaks at 70.9 and 79.8 ppm and ¹³C NMR at 136 ppm and 129.8 ppm were assigned to a vinyl group bonded to silica. Elemental analysis of the sulfonated silica indicated the presence of sulfur, carbon and hydrogen. Thermal decomposition in range 150–600°C was due to the presence of sulfonated organics and unreacted vinyl groups while the new signals on ¹³C NMR, which were in the range 70–15 ppm, were assigned to sulfonated carbons.     

Fabrication and Characterization of Sol-Gel Derived Potassium Sodium Strontium Barium Niobate

Potassium sodium strontium barium niobate (K_0.2Na_0.2Sr_0.48Ba_0.32Nb₂O₆, KNSBN) powder and thin films dip coated onto Si(100) substrates have been prepared by sol-gel route. The thermal evolution of these sol-gel derived KNSBN was studied by differential thermal analysis (DTA) and thermogravimetry (TG). Their structural changes at different annealing temperatures were examined by X-ray diffraction (XRD) and Raman spectroscopy. Our results suggest that the KNSBN tetragonal tungsten bronze (TTB) phase is formed via an intermediate orthorhombic phase. Pure TTB phase KNSBN was obtained at annealing temperatures of 1200°C and 600°C for powder and films respectively.     

Crystallization Processes in Composite Biomaterials

The crystallization processes in composite biomaterials containing fluorapatite (FA) and gel glasses (GG) have been investigated. Using the sol-gel method three gel-glasses have been synthesized: ZnO·SiO₂, ZnO·Al₂O₃·2SiO₂ and ZnO·CaO·2SiO₂. The composites have been prepared from GG and FA in different mole ratios and each of them treated at 950°C, 1050°C, 1150°C and 1250°C. From the XRD analysis of the composites it has been established that fluorapatite is present as a major crystalline phase. The XRD patterns and the IR spectra of the pure GG systems have been studied at 200°C, 600°C and 800°C. The surface of the samples has been analyzed before and after the in vitro experiments using SEM. An amorphous layer has been observed on the surface of each of the treated samples. The element concentration and the solubility of the samples have been analyzed after the in vitro experiments by Atomic Absorption Spectroscopy (AAS).