Contribution to the Study of Moisture Expansion in Ceramic Materials

Measurements were made of the expansions produced by autoclaving coprecipitated and mixed gels of silica and alumina fired over the temperature range 850° to 1200°C. The effect of adding soda to gels of kaolinite composition was investigated and the compositions of the phases were determined. The results show that amorphous silica has a limited influence and must be modified by alumina or by soda and alumina to produce expansions comparable with those of ceramic bodies. The active material is an amorphous alkali aluminosilicate, to be distinguished from permutites and glass. Formation of glass and crystalline compounds reduces moisture expansion. At low firing temperatures (below 950° C.) the hydration of γyAl₂O₃ to boehmite produces high moisture expansions, but γ-Al₂O₃ modified by silica (silicon spinel) has only a limited influence. Some observations are made on the nature of cristobalite developed during the firing of pure amorphous silica and amorphous silica into which additives were introduced.     

Microstructure and Properties of Co2+: ZnAl2O4/SiO2 Nanocomposite Glasses Prepared by Sol–Gel Method

Transparent bulk Co²⁺: ZnAl₂O₄/SiO₂ nanocomposites containing nanocrystalline Co²⁺: ZnAl₂O₄ dispersed in silica glass matrix were obtained by the sol–gel method. The gels of composition 89SiO₂–6Al₂O₃–5ZnO− x CoO ( x =0.2, 0.4, 0.6, 0.8, 1.0) (mol%) were prepared at room temperature by using two different aluminum salts, aluminum nitrate and aluminum alkoxide (aluminum-iso-propoxide, Al(OPrⁱ)₃), as starting materials. The transparent gels were converted to the crystalline phase of gahnite by heating above 900°C. The microstructural evolution of gels was characterized. The effect of Co²⁺ concentration on spectroscopic properties was also discussed. Co²⁺: ZnAl₂O₄ nanocrystals dispersed in the SiO₂-based glass are formed at lower heat-treatment temperature and shorter heating time by using Al(OPrⁱ)₃ as raw material.     

Effect of Precursor Particle Size on the Densification and Crystallization Behavior of Mullite

Stoichiometric mullite, 3Al₂O₃·2SiO₂, has been prepared from a variety of colloidal and solution precursors. In order to change the level of mixing, the size of the silica particles has been varied while keeping the alumina (boehmite) particle size constant. The effect of varying particle size on the crystallization and densification behavior has been studied. Densification behavior was characterized by measuring the bulk density of pellets as a function of heat treatment temperature and by dilatometry. Phase development was examined by thermal analysis and X-ray diffraction. Results showed that the formation of crystalline phases including mullite inhibited densification. The formation of mullite was controlled by the initial segregation of alumina and silica in the gel. A heat treatment time-temperature profile designed to optimize the densification of the colloidal powders was investigated in order to compare the ability of the different gels to densify.     

Sol–Gel Synthesis and Characterization of Ag and Au Nanoparticles in SiO2, TiO2, and ZrO2 Thin Films

Silver and gold nanoparticles were synthesized by the sol–gel process in SiO₂, TiO₂, and ZrO₂ thin films. A versatile method, based on the use of coordination chemistry, is presented for stabilizing Ag⁺ and Au³⁺ ions in sol–gel systems. Various ligands of the metal ions were tested, and for each system it was possible to find a suitable ligand capable of stabilizing the metal ions and preventing gold precipitation onto the film surface. Thin films were prepared by spin-coating onto glass or fused silica substrates and then heat-treated at various temperatures in air or H₂ atmosphere for nucleating the metal nanoparticles. The Ag particle size was about 10 nm after heating the SiO₂ film at 600°C and the TiO₂ and ZrO₂ films at 500°C. After heat treatment at 500°C, the Au particle size was 13 and 17 nm in the TiO₂ and ZrO₂ films, respectively. The films were characterized by UV–vis optical absorption spectroscopy and X-ray diffraction, for studying the nucleation and the growth of the metal nanoparticles. The results are discussed with regard to the embedding matrix, the temperature, and the atmosphere of the heat treatment, and it is concluded that crystallization of TiO₂ and ZrO₂ films may hinder the growth of Ag and Au particles.     

Formation of Nanocrystalline Transition-Metal Ferrites inside a Silica Matrix

Nanocrystalline transition-metal ferrites were synthesized inside an amorphous silica matrix by the sol–gel method. The formation of spinel ferrites began above 400°C, giving fine particles of about 10 nm at 800°C. This is associated with a specific role of the silica matrix, which facilitates the diffusion of the reacting cations, enhancing the ferrite formation. Above 1000°C the MnFe₂O₄ and CuFe₂O₄ nanoparticles lost their fine nature. The dried gels and crystalline materials were characterized by X-ray diffraction, thermal, FTIR, and BET analyses as well as by high-resolution scanning transmission electron microscopy.     

Comparison of Ion Irradiation Effects in Silicon-Based Preceramic Thin Films

Thin films of polymers (polysiloxanes, polycarbosilanes, and polysilazanes) and alkoxide-derived siloxane gels, precursors for SiC, SiCN, SiOC, and SiOBC ceramics, were irradiated with increasing fluences of C or Au ions to study the kinetics of their conversion into ceramics. Ion beam analyses showed that the main effect of irradiation on the composition of the films is the selective release of H₂ by radiolysis. During subsequent high-temperature annealing of films converted as much as possible by irradiation, CO _x , CH _x , or silane molecules do not evolve, contrary to what is observed during the pyrolysis of unirradiated precursor films. According to Raman analyses, a large proportion of the carbon atoms segregate into clusters after irradiation and in films converted by direct pyrolysis (or combined treatments). However, carbon particles formed during irradiation are more diamond-like, affording films with 2—3 times higher hardness, as shown by nanoindentation tests. In both types of ceramics (SiC or SiOC), the optimal properties (hardness, thermal stability, and photoluminescence) associated with C segregation are obtained for a C/Si ratio of the order of 1. Boron addition is detrimental to hardening of SiOC glasses, in contrast to nitrogen.     

Phosphorus-Implanted Glass for Radiotherapy: Effect of Implantation Energy

A chemically durable glass that contains a large amount of phosphorus is useful for in situ irradiation of cancers. It can be activated to be a β-emitter with a half-life of 14·3 d by using neutron bombardment. Microspheres of the activated glass that are injected to tumors can irradiate the tumors directly with β-rays without irradiating neighboring normal tissues. In the present study, P⁺ ions in various doses have been implanted into a pure silica glass in a plate form at 200 keV. Almost all the implanted phosphorus is present in the inner region of the glass rather than in the surface region, taking the form of phosphorus colloids for all the doses in the range of 5 × 10¹⁶-1 × 10¹⁸ cm^-2. A large number of amorphous phosphorus colloid particles with diameters of 10-150 nm are formed in the silica glass that has been implanted with a dose of 1 × 10¹⁸ cm^-2; these colloid particles are distributed widely in a layer that is centered at a depth of 200-250 nm. All the investigated glasses hardly release any phosphorus and silicon into water at a temperature of 95°C, even after 7 d. A silica glass that has been implanted with P⁺ ions at 200 keV with a dose of 1 × 10¹⁸ cm^-2 is believed to be useful as a radiotherapy glass with sufficient phosphorus content and high chemical durability.     

Optical Absorption in Sol-Gel-Derived Crystalline Barium Titanium Fine Particles

Pure, translucent, and highly crystallized barium titanate (BaTiO₃) monolithic gels were synthesized via a sol-gel technique at temperatures down to 90°C by using a high-concentration solution of barium alkoxide and titanium alkoxide. The gels consisted of fine particles with an average diameter of ~10 nm and showed the X-ray diffraction patterns of a pseudo-cubic BaTiO₃ system at room temperature; however, in bulk crystal, the tetragonal structure was more stable. The optical transmission spectra of the polycrystalline gels were similar to those for single-crystalline BaTiO₃. From the excitation energy dependence of the absorption-edge structures, the optical-gap energies for the gels were estimated to have values that were ~0.1 eV higher than those for the single crystal; these higher optical-gap energies may be attributed to size effects.     

Small-Angle Neutron Scattering and Rheological Characterization of Aluminosilicate Hydrogels

Structure and formation of aluminosilicate hydrogels were investigated as a function of chemical composition. Oscillatory rheometry was used to measure the dynamic moduli, G ' and G ", of the gels during setting. The formation of aluminosilicate hydrogels was monitored over time as a function of the storage modulus for gels of low solids volumes, φ < 0.030. Overall gel strength correlated well to the fractal dimension, D , observed by neutron scattering. Scattering data were interpreted using the power law, and the Guinier relationship was used to determine the size of the constructing colloidal/gel particles. Aluminosilicate hydrogels were observed to consist of large fractal aggregates of indeterminable size constructed from smaller subunits of ∼45 Å in dimension. Structure and formation of the gels were highly dependent on [Al₂O₃] and [Na₂O] content.     

Preparation of CdS-doped Glasses from Gels Containing Diethyldithiocarbamatocadmium

CdS-doped SiO₂ glasses were prepared via silica gels containing diethyldithiocarbamatocadmium Cd(S₂CN(C₂H₅)₂)₂. Heat treatment of the gels gave transparent yellow SiO₂glasses doped with hexagonal CdS crystals. In optical absorption and fluorescence spectra, the optical absorption edge and the emission peak clearly exhibited a blue shift, which was attributed to the quantum size effect of the carrier confinement, as the CdS content was decreased. In the fluorescence spectra of the CdS-doped silica glasses, the emission peak was observed only near 500 nm and not observed at longer wavelengths which were known to be present if there were sulfur vacancies.     

Micropatterning of Transparent Poly(Benzylsilsesquioxane) Thick Films Prepared by the Electrophoretic Sol–Gel Deposition Process Using a Hydrophobic–Hydrophilic-Patterned Surface

Poly(benzylsilsesquioxane) (BnSiO_3/2) particles show glass transition behavior, and the particles become a supercooled liquid when they are heated at temperatures above the glass transition temperature. Contact angles of BnSiO_3/2 molten liquid on the hydrophobic surface with fluoroalkylsilane and hydrophilic surface with silica were found to be around 77° and 12°, respectively. Using the difference in wettability for BnSiO_3/2 molten liquid between the hydrophobic and hydrophilic surfaces, micropatterns of transparent BnSiO_3/2-thick films were formed by the electrophoretic deposition of BnSiO_3/2 particles on indium tin oxide substrates with hydrophobic–hydrophilic patterns and subsequent heating above the glass transition temperature.     

Structural Evolution of Alkoxide Silica Gels to Glass: Effect of Catalyst pH

Structural and energetic changes of alkoxide silica during densification were followed in situ using photoacoustic Fourier-transform infrared spectroscopy (PAS-FTIR), transposed temperature drop calorimetry, and drop solution calorimetry. Acid-catalyzed silica gels contained higher concentrations of adsorbed water, silanol groups, and unreacted alkoxy groups than did the base-catalyzed gels/precipitates. The surface species in acid-catalyzed gels were removed easily, allowing this material to be densified at 800°C. The densified material was both structurally and energetically equivalent to fused silica glass. Base-catalyzed gels had fewer silanol groups in the green state, but free silanol groups emerged during drying at 500°C. The base-catalyzed gels did not densify below 1000°C, maintaining a high surface area of 100 m²/g. Base-catalyzed alkoxide gels comprised strained ring structures; their IR spectra were similar to those of fumed silica, but they were energetically much less stable.     

Effects of Amorphous and Crystalline SiO2 Additives on γ-Al2O3-to-alpha-Al2O3 Phase Transitions

This paper focused on the effects of various phases of SiO₂ additives on the γ-Al₂O₃-to-α-Al₂O₃ phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO₂, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO₂, such as quartz and cristobalite. Amorphous SiO₂ was considered to retard the γ-to-α phase transition by preventing γ-Al₂O₃ particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al₂O₃ surface. On the other hand, crystalline SiO₂ accelerated the α-Al₂O₃ transition; thus, this SiO₂ may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO₂ additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al₂O₃-to-α-Al₂O₃ phase transition.     

Structural and Optical Properties of Sol–Gel-Derived Au/BaTiO3 Nanocomposite Thin Films

Au/BaTiO₃ nanocomposite thin films with different Au concentrations were prepared by a sol–gel process. The films were characterized using X-ray diffraction, thermal analyses, X-ray absorption spectroscopy, UV-vis spectroscopy, and transmission electron microscopy. The effects of Au concentration and annealing temperature on the structural and optical properties of composite films were investigated. Gold doping lowered the crystallization temperature of as-synthesized amorphous BaTiO₃ and enhanced its crystallinity in post-deposition annealing. The Au–BaTiO₃ interface was also investigated and no alloying occurred between Au and BaTiO₃. The evolution of Au surface plasmon resonance spectra with increasing annealing temperature was observed in the 10 mol% Au/BaTiO₃ thin films. The variations of band-gap energy for Au/BaTiO₃ films were also discussed.     

Structure and Properties of Silica Derived from Silicon Alkoxide Reacted with Tartaric Acid

The effects of tartaric acid that was used in sol–gel processing on specific surface area, pore-size distribution, and gel structure of sol–gel-derived silica gels were investigated. The specific surface area of silicas that were calcined at 450°C increased from ∼600 m²/g to ∼1200 m²/g as the amount of tartaric acid that was used increased. The pore-size distribution changed as the surface area increased, and only the gels that had a surface area of 930-990 m²/g showed a very sharp pore-size distribution in the mesopore range. The difference in gel structure and properties was explained in terms of the acidity of tartaric acid and the inhibition of condensation among primary particles through the coordination or adsorption of tartaric acid on the particle surface. From the thermal behavior of the gels with different features, it was concluded that gel properties are determined not only by the structure of the precursor gel but also by the surface activity. The gel with uniform mesopores gave a high surface area over a wide range of calcination temperatures.     

Synthesis and Characterization of Silicon Oxycarbide Glasses

It has been found that X-ray amorphous silica glasses containing up to 18% carbon can be synthesized using a sol/gel process. In this study, the sols were prepared using four different organometallic precursors—methyl-, ethyl-, propyl-, and phenyltrimethoxysilanes. ¹H NMR,¹³C NMR, and TGA revealed that the methoxy groups are hydrolyzed in the solutions and, therefore, are absent in the gels. But the alkyl groups are retained in the dry gels. The ²⁹Si NMR data verified that the Si—C bonds associated with these alkyl groups are intact in the dry gels. Most importantly, however, Si—C bonds are found in the glasses obtained after heat-treating the gels at high temperature in an inert atmosphere; i.e., the synthesis does, in fact, create an oxycarbide network structure. TGA showed that the dense oxycarbide glasses are stable to 1000°C in argon and in air.     

Preparation of High-Purity ZrSiO4 Powder Using Sol–Gel Processing and Mechanical Properties of the Sintered Body

Effects of the concentration of ZrOCl₂, calcination temperature, heating rate, and the size of secondary particles after hydrolysis on the preparation of high-purity ZrSiO₄ fine powders from ZrOCl₂.8H ₂ O (0.2 M to 1.7 M ) and equimolar colloidal SiO₂ using sol–gel processing have been studied. Mechanical properties of the sintered ZrSiO₄ from the high-purity ZrSiO₄ powders have been also investigated. Single-phase ZrSiO₄ fine powders were synthesized at 1300°C by forming ZrSiO₄ precursors having a Zr–O–Si bond, which was found in all the hydrolysis solutions, and by controlling a secondary particle size after hydrolysis. The conversion rate of ZrSiO₄ precursor gels to ZrSiO₄ powders from concentrations other than 0.4 M ZrOCl₂.8H₂O increased when the heating rate was high, whereupon the crystallization of unreacted ZrO₂ and SiO₂ was depressed and the propagation and increase of ZrSiO₄ nuclei in the gels were accelerated. The density of the ZrSiO₄ sintered bodies, manufactured by firing the ZrSiO₄ compacts at 1600° to 1700°C, was more than 95% of the theoretical density, and the grain size ranged around 2 to 4 μm. The mechanical strength was 320 MPa (room temperature to 1400°C), and the thermal shock resistance was superior to that of mullite and alumina, with fairly high stability at higher temperatures.     

Densification and Microstructure of Sodium-Doped Colloidal Mullite

A series of mullite precursor gels to prepared by hetero-coagulation of boehmite and colloidal silica. Samples were doped with up to 1.5 wt% Na₂O. Na₂O additions as low as 0.16 wt% led to the formation of α-alumina and a stable glassy phase in addition to mullite. No evidence for the incorporation of sodium into the mullita structure was found by either XRD or ²⁷AlMAS NMR. SEM showed mullite doped with 0.16 wt% and greater Na₂O had an inter-granular glassy phase and large crystalline grains. It is thought that the majority of the sodium was present in the glassy phase. Sodium addition did not decrease the mullite crystallization temperature and did not lead to enhanced sintering.     

Probing the Nature and the Structure of Pores in Silica Xerogels by Water Sorption: The Tetramethyl Orthosilicate-Hydrogen Chloride/Fluoride System

A series of silica xerogels were synthesized in a closed system by hydrolyzing tetramethyl orthosilicate (TMOS) with various concentrations of hydrochloric acid solution but without the addition of alcohol. The molar ratio of H₂O to TMOS was fixed at 4.96. The gels were characterized by water and nitrogen sorption measurements. The gelation time peaked at 50 to 80 h with 5 × 10⁻² to 10⁻¹ M HCl, apparently a consequence of the isoelectric point of silica. Surface area, pore size, and sorption capacity were, however, at a minimum for samples prepared with 5 × 10⁻² to 10⁻¹ M HCl. The BET c constant and hence the net heats of sorption were at a maximum for these samples. Although water sorption of all the samples prepared with HCl exhibited moderate Type I isotherms, the concavity of the isotherms to the P/P ₀ axis increased with increasing concentrations of HCl up to 10⁻¹ M in accordance with the decreased pore size and increased heat of sorption. Addition of F⁻ ions (HF or NaF) in the presence of 10⁻¹ M HCl steadily decreased the gelation time. Addition of F⁻ in the absence of HCl, however, caused rapid gelation and an increase in pore size, nitrogen BET surface area, and hydrophobicity. Consequently, the shape of water isotherms gradually converted to Type IV and/or Type V. Adsorption and desorption measurements on differently activated gels exhibited various types of hystereses as a result of development of hydrophobie sites at as low as 200°C, inelastic distortion induced by adsorption, and chemical reaction and aging of gels when exposed to water vapor.     

THIS ARTICLE HAS BEEN RETRACTEDEffect of Silica Sol on the Properties of Alumina-Based Duplex Ceramic Cores

A series of alumina-based ceramic cores sintered at 1300°C, 1400°C, and 1500°C for 5 h were prepared, and the phases and microstructures were characterized by X-ray diffraction and scanning electron microscopy. The effect of colloidal silica sols on the properties of ceramic core was discussed. The properties of these materials were determined. The results indicated that the microstructure of the core is characterized by the presence of substantially unreacted Al₂O₃ particles having a polycrystalline composition consisting essentially of in situ synthesized 3Al₂O₃·2SiO₂ on the surface of the Al₂O₃ particles. The colloidal silica sol contents do not have an appreciable effect on the densification and shrinkage of the alumina ceramic core. The ceramic cores of 5 wt% colloidal silica sol contents sintered at 1500°C for 5 h showed the smallest creep deformation in the present research.