Structural Evolution in Sol–Gel-Derived Yttrium Aluminum Garnet–Alumina Precursor Fibers

A sol leading to a eutectic Al₂O₃–Y₃Al₅O₁₂ composition was spun into fibers. These fibers were dried and pyrolyzed between 200° and 850°C in a nitrogen or water-vapor atmosphere. Pyrolysis in nitrogen resulted in dense, amorphous fibers with considerable residual carbon content. In water vapor, fibers also remained amorphous, but organics were almost completely removed. The loss of organics created micropores that grew as the pyrolysis temperature increased. The amorphous fiber structures were examined by nitrogen adsorption, helium pycnometry, and transmission electron microscopy. Young's moduli of the pyrolyzed fibers were measured, and porosities of the fibers were calculated from both nitrogen-adsorption data and elastic behavior. Sintering tests showed the best sinterability for fibers pyrolyzed in water vapor to 385°–500°C.     

Synthesis of Silicon Carbide through the Sol—Gel Process from Rayon Fibers

Silicon carbide whiskers (SiC( w )) are synthesized by the pyrolysis of rayon fibers impregnated with sol—gel-derived SiO₂. The influence of boric acid on whisker growth and the effect of varying the TEOS:rayon weight ratio are studied and reported. Evolution of SiC( w ) from impregnated rayon fibers in the carbonized and in the carbothermal stages is investigated.     

Alumina Prepared from Freeze-Dried Aluminum Isopropoxide

Preliminary experiments show that calcining freeze-dried aluminum isopropoxide at 300°C produces sinterable amorphous alumina powders. The resultant powders are soft, contain some carbonaceous residue, and have large surface areas (∼100 m²/g). On a micrometer size scale, they are similar in morphology to sulfate-derived materials although their surface areas are smaller (≤100 m²/g compared to ≥200 m²g for the sulfate-derived powders). Powders calcined at higher temperatures contain less carbon but are also less sinterable.     

Sol–Gel Processing of Tetramethylammonium Silicate

A new sol–gel silica material based on tetramethylammonium silicate (TMAS) is reported here. Both xerogels and aerogels were produced by gelling a solution of 18.7 wt% TMAS using a series of esters. The gelation kinetics was controlled by varying the type and concentration of the ester. The supercritically dried TMAS aerogel was seen to have a larger pore size distribution with high porosity (>90%). In contrast, the conventionally dried gel showed a narrow distribution of mesopores, indicating its potential as a host matrix for protein encapsulations and catalysts.     

Sol–Gel Preparation and Properties of Fluoride-Substituted Hydroxyapatite Powders

Hydroxyapatite (HA) and fluor-hydroxyapatite (FHA) powders were synthesized by a sol–gel method for usage as bone filler and drug carrier. Calcium nitrate and triethyl phosphite were used as precursors under an ethanol–water-based solution. Different amounts of ammonium fluoride (NH₄F) were incorporated for the preparation of FHA powders. With heat treatment above 400°C, a characteristic apatite phase was observed for all the sol–gel powders. However, the crystallization temperature decreased with increasing fluoride addition. The tricalcium phosphate (TCP) phase formed in the pure HA powder above 800°C was attenuated in the FHA powders, confirming an enhanced phase stability of the FHA powders. Increasing the F⁻ addition improved crystallinity and increased the crystallite size, as was determined from X-ray diffraction (XRD) analyses. The lattice parameters of the heat-treated powders varied corresponding to the fluoride addition, i.e., a gradual decrease in the a -axis, while little change in the c -axis was observed with increasing fluoride addition, indicating a nearly complete substitution of fluoride within the apatite lattice. However, little difference was observed with heat-treatment temperatures (400°–1000°C). The powders substituted with fluoride exhibited reduced dissolution rates in an in vitro solution as compared with the pure HA powder, suggesting the possibility of tailoring bioactivity with fluoride substitution.     

Mullite Whiskers Grown from Erbia-Doped Aluminum Hydroxide–Silica Gel

Mullite whiskers and anisotropic grains that were derived from erbia-doped aluminum hydroxide–silica gel were studied. Firing 3.0-mol%-erbia-doped isostatically pressed pellets at 1600°C for 1.0–8.0 h resulted in a high surface concentration of mullite whiskers. Their c -axes were aligned preferentially along the pellet surface; the maximum length was 50 μm, and the maximum aspect ratio was 23. The pellet surface was fully covered by mullite whiskers, and small anisotropic grains with a low aspect ratio were observed in the bulk. The voids that were observed in the fracture surfaces were covered fully by mullite whiskers. The large number of voids resulted in an apparent density of 1.60 g/cm³ in the sintered pellets. The molar ratio of alumina to silica in the whiskers was in the range of 1.30–1.45 (an average value of 1.31), regardless of whether the alumina/silica powder compositions were mixed in a 3:2 or 2:1 ratio.     

Processing and Properties of Sol–Gel-Derived Alumina/Silicon Carbide Nanocomposites

Dense alumina/5 vol% SiC nanocomposites were prepared by sol–gel processing using nanosized (180 nm) precoated SiC powders and a commercial boehmite sol. The SiC powder was precoated with boehmite by a controlled heterogeneous precipitation from an aluminum nitrate solution. The coated SiC powder was then dispersed in a boehmite sol, gelled, calcined, and densified by gas pressure sintering under argon atmosphere at 7–8 MPa pressure. The dependence of the calcination conditions on densification, the effect of seeding on the microstructural development, as well as the mechanical behavior of the sintered specimens, are presented and discussed.     

Sol–Gel Alumina-Coated Carbon Film

A simple method for dipcoating porous reticular carbon foam with aluminum oxide thin-film coatings is presented. Weight gain versus temperature and number of dipcoatings is presented along with scanning electron micrographs of uniform, adherent alumina thin films. Composite foam structures of up to 57% alumina have been prepared.     

Aluminum Nitride Fibers Synthesized from Alumina Fibers Using Gas-Reduction–Nitridation Method

Aluminum nitride fibers were successfully synthesized from alumina fibers using an NH₃–C₃H₈gas mixture as a reduction–nitridation agent. Observation using SEM clearly demonstrated that the morphology of the nitrided fibers was exactly the same as that of the raw alumina fibers, retaining the original regular shape and smooth surface. Up to 95% of the starting alumina was converted to aluminum nitride at 1400°C within 0.5 h via a single-step synthesis process.     

Nonepitaxial Orientation in Sol–Gel Bismuth Titanate Films

Control over crystallographic orientation in ceramic thin films is important for highly anisotropic structures. Layered perovskites, like Bi₄Ti₃O₁₂, have interesting properties associated with their ferroelectric nature, which may be fully exploited only when films are highly textured. Textured films of this titanate were fabricated via a sol–gel technique without using epitaxial growth. Orientation in the film is confirmed by XRD and SEM, and supported by refractive index and dielectric measurements. In an attempt to explain the orienting effect, light scattering experiments were conducted to yield information about the molecular size, shape, and conformation of macromolecules as the sol–gel solution ages and condensation reactions proceed. These experiments clearly show an increase in the size of molecular clusters with time. We believe that it is the organization of these large clusters during spin coating, and the relationship of the backbone chemistry to the crystal structure of Bi₄Ti₃O₁₂, that are responsible for the observed orientation.     

Preparation of Aluminum Oxynitride by Nitridation of a Precursor Derived from Aluminum–Glycine Gel and the Effects of the Presence of Europium

Spinel-type AlON, Al_2.75□_0.25O_3.74N_0.26, was obtained by ammonia nitridation of an oxide precursor prepared by peptizing a glycine gel with aluminum nitrate. To achieve crystallization, the nitrided product had to be annealed at 1500°C for 10 min in flowing nitrogen. The use of glycine instead of citric acid was important for obtaining a white product without residual carbon. A similar preparation method was used for adding small amounts of europium below 10 mol%. A strong blue emission was observed for products ranging from 0.5 to 3.0 mol% Eu doping. The product with 0.5 mol% doping had a maximum emission intensity at 400 nm for an excitation of 254 nm. The products with 1 and 3 mol% doping showed double maxima at 475 and 520 nm. These three emissions were due to the presence of divalent europium in the EuAl₁₂O₁₉ magnetoplumbite-like aluminum oxynitride impurity mixed with the AlON spinel major phase. The 1 mol% Eu-doped product exhibited expanded hexagonal lattice parameters ( a =0.5591 and c =2.236 nm) compared with the values for EuAl₁₂O₁₉ magnetoplumbite itself, observed in the 7.7 mol%-doped product without any strong emission. The above spectrum change was discussed in relation to the coordination around Eu²⁺.     

Micropatterning on Methylsilsesquioxane– Phenylsilsesquioxane Thick Films by the Sol–Gel Method

Prismatic patterns with a pitch of 30 μm and a slant height of 30 μm were embossed successfully in 70:30 (in mol%) methylsilsesequioxane:phenylsilsesqeioxane (MeSiO_3/2–PhSiO_3/2) thick films on glass substrates by laminating an organic polymer sheet as a stamper with the patterns against the films. The embossed shape of the prismatic patterns precisely agreed with the negative shape of those of the stamper that was used. The resultant MeSiO_3/2–PhSiO_3/2 films were transparent, and the refractive index of the films was adjusted to 1.51, to match that of the glass substrate.     

Preparation of Mesoporous Silicon Nitride via a Nonaqueous Sol–Gel Route

A silicon diimide gel Si(NH) _x (NH₂) _y (NMe₂) _z was prepared by an acid-catalyzed ammonolysis of tris(dimethylamino)silylamine. Pyrolysis of the gel at 1000°C under NH₃ flow led to the formation of an amorphous silicon nitride material without carbon contamination. All of the gel and pyrolyzed products exhibited a mesoporous structure with a high surface area and narrow pore-size distribution. The effective surface area of the pyrolyzed silicon nitride residues decreases with increasing temperature, but the heating rate during pyrolysis has little influence on the surface area and pore-size distribution of the final mesoporous ceramic Si₃N₄ products because of the highly cross-linked structures of the precursor silicon diimide gel.     

Intrinsically Formed Trivalent Titanium Ions in Sol–Gel Titania

Electron paramagnetic resonance (EPR) room-temperature observation of trivalent titanium in sol–gel titania has been reported. This unprecedented detection of stable paramagnetic signals in this titania, which was treated at 200°–800°C, occurred regardless of whether the atmosphere was reducing, inert, or oxidating. The possibility that the paramagnetic signals may originate from an metal impurity other than titanium was completely excluded by analyzing the samples using three different ion-beam-analysis techniques: Rutherford backscattering spectroscopy, particle-induced X-ray emission resonance, and energy-recoil detection analysis. The paramagnetic signals appeared at precisely the temperature range of the sample dehydroxylation, which suggests a mechanism for explaining these Ti³⁺ EPR spectra.     

Low-Temperature Fine-Grained Alumina–Aluminum Titanate Composites Produced from a Titania-Coated Alumina Precursor

A method for the preparation of alumina–aluminum titanate (AT) composites, which can be sintered to high density with a fine-grained microstructure at <1450°C, is reported. The composite precursor is alumina particles coated by sol–gel-derived titania, which reacts during sintering to form AT in situ at temperature as low as 1300°C. The composite can be sintered at 1350°C to 98% density with 1.5–2.0 μm grain size. Other composites containing 5–50 wt% AT are also investigated.     

Sol—Gel Synthesis of Strontium-Doped Lanthanum Manganite

Strontium-doped lanthanum manganite powders were prepared using a peroxide acetate salt based solution. The stable sol was peptized by reacting ammonium hydroxide with the precursor solution. The amorphous dried gel powders exhibit a high energy level, due to their high cations coordination and small particles, to develop the perovskite phase. This crystalline phase development from powders containing monocarboxylate ligands was characterized by thermal analysis (TG, DTG, DTA), X-ray diffraction, and IR spectroscopy. The transformation from amorphous powders into a crystallized homogeneous oxycarbonate phase in a first stage corresponds to an exothermal DTA peak at 270°C. X-ray diffraction patterns and IR spectra showed similar behavior of the powders after complete organic removal, during the conversion into perovskite phase starting at approximately 630°C and achieved about 700°C and achieved about 700°C, as well as during the sintering process.     

Synthesis of Hydroxyapatite Nanopowders via Sucrose-Templated Sol–Gel Method

The present research describes synthesis of hydroxyapatite (HAp) nanopowders using a sol–gel route with calcium nitrate and ammonium hydrogen phosphate as calcium and phosphorous precursors, respectively. Sucrose is used as template material, and alumina is added as a dopant to study its effects on particle size and surface area. Synthesized powders are characterized using X-ray diffractometry, BET surface-area analysis, and transmission electron microscopy. Results show that alumina stabilizes the HAp crystalline phase. Average particle size of mesoporous HAp samples is between 30 and 50 nm with surface area of 51–60 m²/g.     

Characterization of Nanocrystalline Forsterite Fiber Synthesized via the Sol–Gel Process

An alkoxide sol–gel route was developed to prepare forsterite fibers. Transparent precursor gel fibers were converted to the crystalline phase of forsterite by heating above 550°C. Fibers heated at 1100°C for 2 h showed a porous microstructure with nanocrystalline sizes of about 50–100 nm. The structural evolution of fibrous gel was characterized. Effects of water and ethanol content on microstructures and strengths of fired fibers are also discussed. Some properties of the fired fibers were investigated.     

Low‐Temperature Synthesis of Aluminum Nitride from Transition Alumina by Microwave Processing

Aluminum nitride (AlN) was synthesized by carbothermal reduction and nitridation method from a mixture of various transition alumina powders and carbon black using 2.45 GHz microwave irradiation in N₂ atmosphere. We achieved the synthesis of AlN at 1300–1400°C using 2.45 GHz microwave irradiation for 60 min. Our results suggest that θ‐Al₂O₃ is more easily nitrided than γ‐, δ‐, and α‐Al₂O₃. On the other hand, nitridation ratio of samples synthesized in a conventional furnace under nitrogen atmosphere were zero or very low. These results show that 2.45 GHz microwave irradiation enhanced the reduction and nitridation reaction of alumina.     

Reactive Synthesis and Phase Stability Investigations in the Aluminum Nitride–Silicon Carbide System

The effect of AlN on the structure formation of SiC was investigated. SiC was synthesized in the presence of AlN under vacuum at 1500°C, and the result was cubic SiC. The synthesis of AlN–SiC composites through the reaction Si₃N₄+ 4Al + 3C = 3SiC + 4AlN was also investigated and compared with synthesis via field-activated self-propagating combustion (FASHS). Reactants were heated in a vacuum furnace at temperatures ranging from 1130° to 1650°C. Below 1650°C, the reaction is not complete and at this temperature the product phases are AlN and cubic SiC. At 1650°C, the product contained an outer layer which contained β-SiC only and an inner region which contained AlN and cubic SiC. 2H-SiC and AlN composites synthesized via field-activated self-propagating combustion were annealed at 1700°C under vacuum. The AlN dissociated and evaporated and the 2H-SiC transformed to the cubic β phase. Reasons for the differences in products of furnace heating and FASHS are discussed.