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.     

Synthesis of Nanocrystalline Enstatite Fiber Via Alkoxide Sol–Gel Process

An alkoxide sol–gel route to precursors to transparent enstatite fibers is described. The fibrous gels were drawn from acetic acid-modified viscous solutions. The amount of acetic acid was found to be the crucial factor in the spinnability. Infrared studies indicated that the acetate anion behaved as a ligand and modified the molecular structure of the enstatite gel. The fibrous gel was amorphous after drying by X-ray, and began to convert to enstatite at temperature as low as ∼600°C. The enstatite fibers were nanocrystalline up to 1100°C. At 1300°C, transparent and dense enstatite fibers were produced, with the room temperature dielectric constants of around 5.2–5.8 (at 1 MHz).     

Synthesis of Bismuth Sodium Titanate Nanosized Powders by Solution/Sol–Gel Process

(Bi_1/2Na_1/2)TiO₃ (BNT) is a prominent candidate for a lead-free piezoelectric material. In this study, BNT was synthesized using the solution/sol–gel method, in which a solution of Bi₂O₃ and Na₂CO₃ was dissolved in HNO₃ as starting materials. The solution then was mixed with ethylene glycol and titanium tetraisopropoxide. The obtained BNT powder was analyzed using FT-IR, DTA-TG, Raman spectroscopy, and high-temperature XRD. Results showed that BNT crystallization occurred above 600°C. TEM investigation showed that 100–200 nm BNT particles were formed by heat-treating the sol–gel-derived BNT sol at 700°C for 6 h.     

Synthesis and Characterization of Lead Zirconate Titanate Fibers Prepared by the Sol–Gel Method: The Role of the Acid

Lead zirconate titanate (PZT) fibers are prepared by the sol–gel method using acetic acid and methacrylic acid to control the pH of the PZT precursor sol. The chemical evolution of the precursors, the thermal and crystallization behavior of the PZT gel fibers, and the microstructure of the samples with and without acid addition are analyzed and compared. It is observed that the properties of the fibers are improved when acids are used; particularly longer PZT ceramic fibers (4–5 cm) are obtained after heat treatment and a single perovskite phase is obtained at 550°C when using methacrylic acid. The mechanisms of the acid effect on the strengthening and crystallization of PZT fibers are discussed.     

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.     

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.     

Preparation and Characterization of Nanocrystalline Misch-Metal-Substituted Yttrium Iron Garnet Powder by the Sol–Gel Combustion Process

Nanocrystalline Y_{3− x} MM _x Fe₅O₁₂ powders (MM denotes Misch-metal, x =0.0, 0.25, 0.5, 0.75, and 1.0) were synthesized by a sol–gel combustion method. Magnetic properties and crystalline structures were investigated using X-ray diffraction (XRD), a vibrating sample magnetometer (VSM), and a scanning electron microscope. The XRD patterns showed that the single-phase garnet of Y_{3− x} MM _x Fe₅O₁₂ was formed at x values ≤1.0. The saturation magnetization of powders increased with decreasing MM content and reached the maximum value at Y₃Fe₅O₁₂. The crystallite size of powders calcined at 800°C for 3 h was in the range of 38–53 nm.     

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.     

Characterization and Investigation of the Tribological Properties of Sol–Gel Zirconia Thin Films

Sol–gel zirconia thin films were prepared by dip coating in an ethanol solution of zirconium oxychloride. The zirconia films consisted of a completely tetragonal phase and exhibited nanoscale uniformity. They displayed excellent antiwear and friction-reduction performance in sliding against steel. The friction coefficient (0.13–0.15) and the wear life over 5000 sliding cycles were recorded for the films at a sliding speed of 90 mm/min and a load of 0.5 N. The film was characterized by slight scuffing and abrasion at low loads and sliding speeds.     

Phase Separation in Sol–Gel Process of Alkoxide-Derived Silica-Zirconia in the Presence of Polyethylene Oxide

Phase separation in a sol–gel process of SiO₂–ZrO₂ in the presence of polyethylene oxide is investigated. An amorphous gel with interconnected macroporous morphology is obtained when phase separation and sol–gel transition concur to fix a transitional structure of spinodal decomposition. Macropore size, together with connectivity of the pores and gel skeleton, can be controlled precisely by selecting an appropriate starting composition for preparation at a zirconium content ≤11.7 mol%. The macroporous gel retains additional mesopores <4 nm and exhibits typical bimodal pore size distribution. The addition of ZrO₂ in SiO₂ improves the thermal stability of both macroporous and mesoporous structures.     

Characterization and Tribological Investigation of Sol–Gel Titania and Doped Titania Thin Films

Titania (TiO₂) and doped TiO₂ ceramic thin films were prepared on a glass substrate by a sol–gel and dip-coating process from specially formulated sols, followed by annealing at 460°C. The morphologies of the original and worn surfaces of the films were analyzed with atomic force microscopy (AFM) and scanning electron microscopy. The chemical compositions of the obtained films were characterized by means of X-ray photoelectron spectroscopy (XPS). The tribological properties of TiO₂ and doped TiO₂ thin films sliding against Si₃N₄ ball were evaluated on a one-way reciprocating friction and wear tester. The AFM analysis shows that the morphologies of the resulting films are very different in nanoscale, which partly accounts for their tribological properties. XPS analysis reveals that the doped elements exist in different states, such as oxide and silicate, and diffusion took place between the film and the glass substrate. TiO₂ films show an excellent ability to reduce friction and resist wear. A friction coefficient as low as 0.18 and a wear life of 2280 sliding passes at 3 N were recorded. Unfortunately, all the doped TiO₂ films are inferior to the TiO₂ films in friction reduction and wear resistance, primarily because of their differences in structures and chemical compositions caused by the doped elements. The wear of the glass is characteristic of brittle fracture and severe abrasion. The wear of the TiO₂ thin film is characteristic of plastic deformation with slight abrasive and fatigue wear. The doped TiO₂ thin films show lower plasticity than the TiO₂ thin film, which leads to large cracks. The propagation of the cracks caused serious fracture and failure of the films.     

Sol–Gel Route to Nanocrystalline Lithium Metasilicate Particles

Crystalline lithium metasilicate (Li₂SiO₃) nanoparticles have been synthesized using a sol–gel process with tetraethylorthosilicate and lithium ethoxide as precursors. The particle size examined by using transmission electron microscopy and BET-specific surface area techniques is in the range 5–50 nm, depending on the temperature at which the material is calcined. The crystalline Li₂SiO₃ forms at ambient temperature (∼40°C), and it remains in this phase after calcination at temperatures up to 850°C. The BET-specific surface area is ∼110 m²/g for material calcined at temperatures below 500°C, decreasing to ∼29 and ∼0.7 m²/g following calcination at 700° and 850°C, respectively. Solid-state ²⁹Si NMR spectroscopy shows the presence of only Q ² structural units in the material. The lithium metasilicate is further characterized using differential scanning calorimetry and thermogravimetric analysis, and Fourier transform infrared spectroscopy.     

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.     

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.     

Structural Aspects of SBA-1 Cubic Mesoporous Silica Synthesized via a Sol–Gel Process Using a Silatrane Precursor

Silatrane prepared from fumed silica and triethanolamine was used as a precursor for SBA-1 synthesis at room temperature using cationic surfactants, derived from alkyltrimethylammonium bromides, C _n TMAB ( n =14, 16, and 18), as templates in dilute solutions. The influence of acidity, alkyl chain length of the surfactant, and synthesis temperature was studied. The shape of the SBA-1 crystals was dependent on the alkyl chain length of the surfactant. At a high surfactant concentration and an elevated reaction temperature (50°C), three-dimensionally ordered mesopores were invariably produced. Both X-ray diffraction and transmission electron microscopic results showed characteristics of the three-dimensional cubic structure. Scanning electron microscopic images of SBA-1 indicated the crystalline-like appearance of an octadecahedron (18-hedron) consistent with six square {100} and 12 hexagonal {110} planes with a cubic symmetry. The surface area of the product was as high as 1000–1500 m²/g with an adsorption volume of 0.6–1.0 cm³/g.     

Sol–Gel Synthesis of Cadmium Telluride-Microcrystal-Doped Silica Glasses

The sol–gel process has been applied to the preparation of small-sized CdTe-doped silica glasses. Gels synthesized by the hydrolysis of a complex solution of Si(OC₂H₅)₄, Cd(CH₃COO)₂· 2H₂O, and Te were heated from 350° to 600°C in a H₂─N₂ atmosphere to form fine cubic CdTe crystals. The size of CdTe crystals, determined from the line broadening of X-ray diffraction pattern, increases from 4 to 9 nm in diameter with increasing heat-treatment temperature. The optical absorption edge shifts to the higher-energy side as the size of the CdTe crystals decreases. This phenomenon is interpreted in terms of a quantum confinement effect of electron and hole in the CdTe microcrystals.     

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.     

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.     

Effect of Tween-80 on the Control of Particle Size and Shrinkage Properties of Nanoscale α-Alumina Synthesized by Sol–Gel Processing

The concept of tailored interfaces has been applied to the synthesis of nanoscale α-Al₂O₃. Tween-80 (poly-oxyethylene(20) sorbitan monooleate, T-80) was used as a surface modifier in the sol–gel process for this purpose. High-resolution transmission electron microscopy study of the powder obtained with T-80 confirmed the particle size of α-Al₂O₃ (∼55 nm) and morphology (spherical). The exothermic peak temperature in the differential thermal analysis was shifted to a lower temperature (∼917°C) when the powder was derived from a T-80 modifier content of 10 wt%. X-ray diffraction showed that the α-Al₂O₃ phase was the major phase that existed in modifier-derived powder that was sintered at 1000°C. The experiments, based on linear shrinkage, indicated that the powder with T-80 (10 wt%) could be densified at a low temperature.     

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.