Sol–Gel Fabrication of Epitaxial and Oriented TiO2 Thin Films

Thin films of titania have been prepared by spin coating on fused silica, Si(100), and rutile(110), starting with a sol–gel process. The alkoxide solution was chelated with diisopropanolamine, and the resulting precursor solution was hydrolyzed prior to coating. Oriented rutile films were obtained on fused silica and Si(100), while epitaxially oriented film was formed on rutile (110). X-ray diffraction results indicated that the as-deposited films transformed to rutile via anatase with increasing temperature. The phase transformation temperature was found to be dependent on the substrate, and it was in general higher on the substrates than that observed for the gel powder. Microstructural studies revealed that these films consisted of finely dispersed grains of 0.05 to 0.15 μm in size.     

Nanostructured Dense ZrO2 Thin Films from Nanoparticles Obtained by Emulsion Precipitation

Nonagglomerated spherical ZrO₂ particles of 5–8 nm size were made by emulsion precipitation. Their crystallization and film-forming characteristics were investigated and compared with nanosized ZrO₂ powders obtained by sol–gel precipitation. High-temperature X-ray diffraction indicated that the emulsion-derived particles are amorphous and crystallize at 500°C into tetragonal zirconia, which is stable up to 1000°C. Crystallite growth from 5–20 nm occurred between 500°–900°C. Films of 6–75 nm thickness were made by spreading, spin coating, and controlled deposition techniques and annealed at 500°–600°C. The occurrence of t -ZrO₂ in the emulsion-precipitated powder is explained by the low degree of agglomeration and the corresponding low coarsening on heating to 500°–800°C, whereas the agglomerated state of the sol–gel precipitate powder favors the occurrence of the monoclinic form of zirconia under similar conditions.     

Sol–Gel Synthesis of High-k HfO2 Thin Films

HfO₂ films were prepared using alkoxy-derived precursor solutions. The effects of the chemical composition of precursor solutions on the microstructure development were investigated for HfO₂ films on Si substrates. The microstructure distinguished developed in the HfO₂ films prepared using the precursor solutions with and without diethanolamine. This result is considered to be due to the difference in the progress of organic decomposition and the behavior of nucleation and grain growth. The flatness and refractive index of the HfO₂ films were improved using diethanolamine-added solution. The refractive index and the dielectric constant of the HfO₂ film prepared at 400°C using a diethanolamine-added solution were about 1.85 and 17, respectively. A similar microstructure developed in the HfO₂ films on polyimide films. Much flat and uniform HfO₂ films are expected for application to integrated optical devices.     

Sol–Gel Route to Synthesis of Microporous Ceramic Membranes: Preparation and Characterization of Microporous TiO2 and ZrO2 Xerogels

This paper focuses on the preparation and characterization of pure TiO₂ and ZrO₂ xerogels. The preparation method is based on a sol–gel technique using metal tert -amyloxides as precursors to produce nano-sized metal oxide particles which are subsequently packed in a gelation process, eventually resulting in microporous xerogele. The unsupported TiO₂ and ZrO₂ xerogele produced in this manner have a mean pore diameter less than 2 nm and more than 50% microporosity. However, these gels, in their pure form, are thermally stable only to 350°C. Improved thermal stabilities of mixed metal oxide xerogels will be reported elsewhere.     

Sol–Gel Synthesis of λ–Na2O·xAl2O3 Films

Na₂O· x Al₂O₃ ( x = 9, 11)films have been obtained by sol–gel method. Crystallization processes during heat treatments have been investigated by X–ray diffraction analysis. A metastable phase with the mullite structure, λ–Na₂O· x Al₂O₃, has been observed starting from 800°C. Films remained stable after a heat treatment at 1000°C for 250 h. Impedance spectroscopy measurements showed that the films of λ-Na₂O· x Al₂O₃ possess a large three–dimensional ionic conductivity at 400°C.     

Sol–Gel Route to Porous Lanthanum Cobaltite (LaCoO3) Thin Films

Sol–gel-derived LaCoO₃ thin films were deposited on yttria-stabilized zirconia (YSZ) substrates from a lanthanum isopropoxide–cobalt acetate (with 2-methoxyethanol) precursor solution. A chelating agent (2-ethylacetoacetate) and polyethylene glycol (PEG) were used to modify the above-mentioned precursor solution. The La-Co precursor solution was sufficiently viscous, and transparent LaCoO₃ gel films were prepared successfully using a spin-coating technique. Crystallization behavior and microstructure evolution were investigated using X-ray diffractometry (XRD) and scanning electron microscopy (SEM). A single-phase perovskite thin film with the grain size of ∼50 nm was obtained by heat-treating the spin-coated gel film at a temperature of 600°C. SEM observations revealed that the microstructure of LaCoO₃ thin films that were prepared from the precursor solution with PEG was porous, and the LaCoO₃ thin film maintained its porous microstructure to a temperature of 800°C.     

Sol–Gel Processing and Characterization of Pure and Metal-Doped SnO2 Thin Films

A chloride-based inorganic sol–gel route was used for preparing pure and metal (osmium, nickel, palladium, platinum)-doped SnO₂ sol. SnCl₄ was first reacted with propanol, then the resulting compound was hydrolyzed and subsequently mixed with solutions of the metal dopants. The obtained sols were used for depositing thin films by spin coating or for preparing powders by solvent evaporation at 110°C. FTIR spectroscopy and thermal analysis of the powders revealed that chlorine still bound to tin stabilized the sol against gelation by hindering the condensation reactions. Film characterizations showed that platinum and palladium, unlike nickel and osmium, were likely to form nanoparticles in the SnO₂ lattice. This result was discussed with regard to the different ways that platinum and palladium, on one hand, and nickel and osmium, on the other, modified the growth of SnO₂ grains and the film roughness and morphology. Dopants that formed nanoparticles (platinum, palladium) resulted in the roughest film, while dopants that did not form particles (nickel, osmium) resulted in SnO₂ grain size very close to that of pure SnO₂.     

Sol–Gel Synthesis and Photocatalytic Activity of CeO2/TiO2 Nanocomposites

Uniform CeO₂ / TiO₂ composite nanoparticles with different Ce/Ti molar ratios have been successfully synthesized via the sol–gel method. The samples were characterized using differential thermal analysis (DTA), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The surface state analysis by means of X-ray photoelectron spectroscopy (XPS) shows that the Ti element mainly exists as a chemical state of Ti⁴⁺, while the Ce element exists as a mixture of Ce³⁺ and Ce⁴⁺ oxidation states. The photocatalytic degradation of methyl orange (MeO) in CeO₂ / TiO₂ suspension was investigated. The results indicate that the CeO₂/TiO₂ nanocomposites show higher photocatalytic activity than pure TiO₂. Photodegradation of MeO can be improved by increasing the Ce/Ti molar ratio in the initial 15 min.     

Direct Formation and Photocatalytic Performance of Anatase (TiO2)/Silica (SiO2) Composite Nanoparticles

Anatase (TiO₂)/silica (SiO₂: 23.9–27.7 mol%) composite nanoparticles were directly synthesized from (i) the reaction of titanyl sulfate (TiOSO₄) and sodium metasilicate (Na₂SiO₃) under mild hydrothermal conditions, (ii) the acidic precursor solutions of TiOSO₄ and tetraethylorthosilicate (TEOS) by thermal hydrolysis, and (iii) the metal alkoxides, i.e., tetraisopropoxide (TTIP) and TEOS, by the sol–gel method. Their photocatalytic activities were evaluated by measurements of the relative concentration of methylene blue after UV irradiation. The as-prepared TiO₂/SiO₂ composite nanoparticles showed far more improved photocatalytic activity than the pure anatase-type TiO₂. The composite nanoparticles formed from (i) TiOSO₄ and Na₂SiO₃ as well as those from (ii) TiOSO₄ and TEOS showed fairly good photocatalytic activity, and it was better than that of those synthesized from (iii) the metal alkoxides, which was suggested to be due to the difference in crystallinity of the anatase.     

A Study of the Phase Relations of ZrO2–TiO2 and ZrO2–TiO2–SiO2

The phase relations of the systems ZrO₂–TiO₂ and ZrO₂–TiO₂–SiO₂ were investigated. X-ray diffraction techniques served as the principal means of analysis. The binary system ZrO₂–TiO₂ was found to be one of partial solid solutions with no intermediate compounds. A eutectic point was found to exist at 50 to 55 weight % ZrO₂ and 1600°C. A preliminary investigation of the ternary system ZrO₂–TiO₂–SiO₂, although not extensive, resulted in a better understanding of this system, with a fairly accurate location of some of its boundary lines. A eutectic point was located at 2% ZrO₂, 10% TiO₂, and 88% SiO₂ at approximately 1500°C.     

Synthesis of Ba(Mg1/3Ta2/3)O3 Microwave Ceramics through a Sol–Gel Route Using Acetate Salts

A sol–gel process to unsophisticatedly synthesize Ba(Mg_1/3Ta_2/3)O₃ (BMT) ceramics at low cost has been developed in the present work. This process involves the reaction of TaCl₅ with acetates of Ba and Mg in the presence of citric acid. Pure BMT polycrystalline powders can be obtained by calcining the synthesized products at 1000°C. The BMT powders were found to have a primary particle size as small as 100 nm. BMT ceramics with favorable structural characteristics can be obtained from sintering of the sol–gel BMT at temperatures much lower than that for the conventional solid-state BMT. Sintering the sol–gel BMT in pellet form at 1300°C resulted in an ordering parameter of 0.72 for the pellet, and a relative density of >95% was achieved with sintering at 1500°C. The grain size of the sintered sol–gel BMT was large and uniform in comparison with the products from the solid-state method. Using the sol–gel route, sintering at temperatures as low as 1400°C gave ceramics with acceptable microwave dielectric properties (a dielectric constant of 16 and Qf factor of 14 400 GHz), while higher temperatures (>1600°C) are needed for the solid-state route to give similar properties.     

Sol–Gel-Processed SiO2/TiO2/Methylcellulose Composite Materials for Optical Waveguides

SiO₂–TiO₂–methylcellulose (MC) composite materials processed by the sol-gel technique were studied for optical waveguide applications. Dense, crack-free and homogeneous films as thick as 2 μm were obtained via the organic binder MC-assisted sol–gel process and single coating with low-temperature treatment. Light waveguiding in such hybrid film was demonstrated at a wavelength of 650 nm. About 1.1 dB/cm or lower propagation loss for the SiO₂ (80 mol%)–TiO₂ (20 mol%)–MC (22 wt%) film can be achieved. The effects of thermal treatment on the structure and properties of the gel films were also investigated.     

Immiscibility Area in the System TiO2–ZrO2–SiO2

A furnace for use in conjunction with the X-ray spectrometer was developed which was capable of heating small powdered specimens in air to temperatures as high as 1850°C. This furnace was also used for the heating and quenching of specimens in air from temperatures as high as 1850°C. An area of two liquids coexisting between 20 and 93 weight % TiO₂ above 1765°± 10°C. was found to exist in the system TiO₂–SiO₂, which is in substantial agreement with the previous work of other investigators. The area of immiscibility in the system TiO₂–SiO₂ was found to extend well into the system TiO₂–ZrO₂–SiO₂. The two liquids were found to coexist over a major portion of the TiO₂ (rutile) primary-phase area with TiO₂ (rutile) being the primary crystal beneath both liquids. The temperature of two-liquid formation in the ternary was found to fall about 80°C. with the first additions of ZrO₂ up to 3%. With larger amounts of ZrO₂ the change in the temperature of the boundary of the two-liquid area was so slight as to be within the limits of error of the temperature measurement. Primary-phase fields for TiO₂ (rutile), tetragonal ZrO₂, and ZrTiO₄ were found to exist in the system TiO₂–ZrO₂–SiO₂. SiO₂ as high cristobalite is known to exist in the system TiO₂–ZrO₂–SiO₂.     

Preparation and Optical Nonlinear Property of Sol–Gel-Derived Cu─SiO2 Thin Films

Cu-metal-doped glass films having a Cu:Si atomic ratio of 0.05 ± 0.002 were successfully prepared by a sol-gel method using a dipping technique. The appearance of surface plasmon of Cu metal at about 570 nm was observed after heat treatment at or above 700°C. The third-order nonlinear susceptibility (x³) was as high as 5.0 × 10^–8 esu at 570 nm.     

PbTiO3–PbO–SiO2 Glass-Ceramic Thin Film by a Sol–Gel Process

PbTiO₃(PT)-PbO-SiO₂ glass-ceramic thin films were pro-duced by a sol-gel process. The crystallization of PT oc-curred at ∼700°C and was higher than that in PT-PbO-B₂ O₃ sol-gel glass-ceramics. A pinhole-free thin film was obtained by a rapid thermal annealing process when the designed glass-forming phase content in the thin film was >24 vol%. The measured dielectric constants of the films fairly agreed with the predicted values, based on a parallel mixing model. The dielectric constant was 219 and the di-electric loss was 0.04 in the 0.6PT-0.4(PbO-SiO₂) film that was fired at 700°C.     

Sol–Gel Route to Synthesis of Microporous Ceramic Membranes: Thermal Stability of TiO2–ZrO2Mixed Oxides

In this paper concerning the synthesis of microporous ceramic membranes, we focus on the preparation and ther- mal stability of unsupported microporous TiO₂-ZrO₂, mixed-oxide membranes. It has been observed that, by add- ing up to 20% ZrO₂, into TiO₂, or up to 10% TiO₂, into ZrO₂, these microporous membranes display improved thermal stability. They can be fired up to 500°C for 0.5 h without closing micropores. However, membranes containing almost equal percentages of each component have lost microporous features and have low surface areas and low porosities. A phase diagram of a two-component TiO₂- ZrO₂, mixed-oxide membrane has been prepared based on DTA and X-ray diffraction data in order to better under- stand the microstructure changes upon firing.     

Low-Temperature Sol–Gel Synthesis of NaZr2P3O12

The NZP family of new low-expansion materials has attracted wide interest for its potential in advanced technological applications. NaZr₂P₃O₁₂, which is the parent composition of this family, has been synthesized by the solution sol-gel method using special precursor solutions, which led to its formation (although poorly crystalline) at temperatures as low as 120°C. The lowest temperature of formation of a single phase of NaZr₂P₃O₁₂ with a high degree of crystallinity was found to be 600°C.     

Synthesis and Characterization of (Zn,Cd)TiO3 by a Modified Sol–Gel Method

Ilmenite-type (Zn_{1− x} Cd _x )TiO₃ (0≤ x ≤0.15 and 0.8≤ x ≤1.0) was synthesized by a modified sol–gel route including the Pechini process via two-step heat treatments. The thermal stability of (Zn_{1− x} Cd _x )TiO₃ depended on the amount of cadmium content. The as-synthesized (Zn_{1− x} Cd _x )TiO₃ (0≤ x ≤0.15 and 0.8≤ x ≤1.0) showed higher thermal stability than that of ZnTiO₃. The variation of the dielectric constant of all synthesized (Zn_{1− x} Cd _x )TiO₃ samples for all measurement frequencies showed a similar tendency. The dielectric constant of each (Zn_{1− x} Cd _x )TiO₃ sample decreased first with increasing frequencies and then increased slightly when the frequency was up to 10⁷ Hz. Moreover, the dielectric loss tangent of all synthesized (Zn_{1− x} Cd _x )TiO₃ samples for all measurement frequencies also changed in similar patterns. The dielectric loss tangent decreased with increasing measurement frequencies. The microwave dielectric properties of (Zn_{1− x} Cd _x )TiO₃ were changed with the cadmium doping content in the range of microwave frequency.     

Sol–Gel Synthesis of Silica-Based Oxyfluoride Glass-Ceramic Thin Films: Incorporation of Eu3+ Activators into Crystallites

Transparent oxyfluoride glass-ceramic thin films were prepared using a sol–gel method starting from rare-earth trifluoroacetate/silicon alkoxide solutions. SiO₂–LaF₃ and SiO₂–LaOF glass-ceramics were formed by heating at temperatures of 300°–500°C and 600°–900°C, respectively. Eu³⁺ activators were successfully incorporated into oxyfluoride crystals, as evidenced by their luminescent properties, such as capability of a charge-transfer (O^2––Eu³⁺) excitation, suppression of a multiphonon relaxation, and occurrence of a cross-relaxation at low Eu³⁺ concentrations. As a result, the films exhibited strong red emission by ultraviolet excitation. The incorporation supposedly originated from decomposition of the (La,Eu)-trifluoroacetates in the silica-gel matrix.     

Solubility of TiO2 in ZrO2

The solubility of TiO₂ in tetragonal ZrO₂ is 13.8±0.3 mol% ui 1300°C, 14.9±0.2 mol% at 1400°C, and 16.1±0.2 mol% at 1500°C. These solid solutions transform to metastable monoclinic solid solutions without compositional change on cooling to room temperature.