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.     

Preparation of β-BaB2O4 Powders and Thin Films by Sol–Gel Method

β-BaB₂O₄ (β-BBO) powders and films were successfully synthesized by the sol–gel method using metal alkoxides. A homogeneous and stable solution was prepared by the reaction of barium metal with boron triethoxide in ethanol by addition of diethanolamine. The drip-coated precursor films began to crystallize to β-BBO on Pt substrates at 500°C and converted to β-BBO films with preferred orientation to the c -axis at 700°C.     

Sol–Gel Preparation of BaTi4O9 and Ba2Ti9O20

BaTi₄O₉ and Ba₂Ti₉O₂₀ precursors were prepared via a sol–gel method, using ethylenediaminetetraacetic acid as a chelating agent. The sol–gel precursors were heated at 700°–1200°C in air, and X-ray diffractometry (XRD) was used to determine the phase transformations as a function of temperature. Single-phase BaTi₄O₉ could not be obtained, even after heating the precursors at 1200°C for 2 h, whereas single-phase Ba₂Ti₉O₂₀ (as determined via XRD) was obtained at 1200°C for 2 h. Details of the synthesis and characterization of the resultant products have been given.     

Preparation of Li2B4O7 Films with Preferential Orientation by Sol–Gel Method

Li₂B₄O₇ films, a promising material for surface acoustic wave devices, were prepared by the sol—gel method using metal alkoxide precursors as starting materials. The Li₂B₄O₇ films on silicon (100) and (111) single crystals prepared from a coating solution to which acetic or hydrochloric acid was added were highly oriented along the (122) plane, whereas those without acid additive were randomly oriented. The results were interpreted based on the basic sol-gel chemistry and the lattice matching between the film and substrate.     

Preparation and Process Chemistry of SnO2 Films Derived from SnC2O4 by the Aqueous Sol–Gel Method

Nanocrystalline SnO₂ films were prepared from SnC₂O₄ by a sol–gel route. A clarified and stable Sn-containing sol was obtained by dissolving and chelating SnC₂O₄ with C₆H₈O₇ and H₂C₂O₄ in a C₆H₈O₇/triethanolamine (TEA) mixing aqueous solution with a pH of 6.5–7.0. The chelation and condensation reactions were deducted based upon infrared, Raman, and X-ray photoelectron spectra analysis. Results illuminated that a number of ionized-state carboxyl groups and active tin hydrate were produced in the mixing solution by amido association of TEA with H on –COOH of H₃L and H₂C₂O₄, supplying a precondition for tin sol formation. X-ray diffraction and field emission scanning electron microscope analysis indicated that SnO₂ film had a rutile structure and consisted of nanocrystals with a mean size of about 7 nm. Film thickness could be controlled by the number of dip coating—annealing cycles according to 30–45 nm/cycle for a Sn concentration of 0.25 mol/L.     

Optical Properties of Er-Doped Al2O3–SiO2 Films Prepared by a Modified Sol–Gel Process

Er-doped Al₂O₃–SiO₂ (1/9 in mol ratio of Al₂O₃/SiO₂) thin films were prepared by using a modified sol–gel process. The modified process entails the precipitation and digestion of Er(OH)₃, obtained from the reaction between Er ions and NH₄OH in solution. Thin films were deposited on Si wafers by using a spin coating technique (3000 rpm) and the coated films were heat treated at different temperatures for 1 h in an oxygen-purged furnace. All the films were structurally characterized by the X-ray diffraction technique using Cu K α radiation. Refractive indices and the morphologies of the films were studied using a spectroscopic phase modulated ellipsometer and atomic force microscopy, respectively. It was observed that the films were crack free and of about 0.4 μm thickness in a single spin coating and both the lifetime and the photoluminescence intensity of Er ions increased with increasing the annealing temperature. The luminescence properties of the Er-doped Al₂O₃–SiO₂ made by a conventional and our modified doping process were compared and discussed from the stand point of peak intensities and lifetimes as a function of annealing temperatures. It is to be noted here that our modified process was found to be more effective in reducing the clustering of Er ions in Al₂O₃–SiO₂ materials as compared to that of the conventional method.     

Formation of NiAl2O4 by Solid State Reaction

The growth of nickel-aluminum spinel, NiAl₂O₄, in diffusion couples of polycrystalline Al₂O₃ and NiO was investigated between 1200° and 1500°C. The growth kinetics for the spinel layer obeyed a parabolic rate law in this temperature range. Marker experiments showed that the spinel layer formed by counterdiffusion of nickel and aluminum ions. Comparison of experimental and theoretical values of the parabolic rate constants suggests that the diffusion of aluminum ions through the spinel layer is rate controlling.     

Preparation of La2Zr2O7 by Sol—Gel Route

The La₂Zr₂O₇ phase was prepared from metal acetylacetonates by a sol—gel route without any intermediate phase formation. X-ray peaks appeared at a temperature as low as 500°C at the positions expected for La₂Zr₂O₇, although they were broad. The crystal structure of La₂Zr₂O₇ was found to be of the fluorite type below 900°C and of thepyrochlore type above 1000°C. The substitution of a small amount of Eu for La was carried out to investigate the crystal structure from the viewpoint of fluorescence, and these results confirmed the formation of fluorite type La₂Zr₂O₇ below 900°C.     

Activity–Composition Relations in NiAl2O4─MnAl2O4 Solid Solutions and Stabilities of NiAl2O4 and MnAl2O4 at 1300° and 1400°C

Activities of NiO were measured in the oxide and spinel solutions of the system MnO–NiO–Al₂O₃ at 1300° and 1400° C with the aim of deriving information on the thermodynamic properties of the spinel phases. Synthetic samples in selected phase assemblages of the system were equilibrated with metallic nickel and a gas phase of known oxygen partial pressures at a total pressure of 1 atm. The data on NiO activities and directions of conjugation lines between coexisting oxide and spinel phases were used to establish the activity–composition relations in spinel solid solutions at 1300° and 1400°C. The MnAl₂O₄–NiAl₂O₄ solid solutions exhibit considerable negative deviations from ideality at these temperatures. The free energy of formation of MnAl₂O₄ from its oxide components (MnO + Al₂O₃) at 1300° and 1400°C is calculated to be −24.97 and −26.56 kJ. mol⁻¹, respectively. The activities determined in the stoichiometric spinel solid solutions are more negative as compared with those predicted from cation distribution models.     

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.     

Synthesis of MgAl2O4 Spinel Powder by Combination of Sol–Gel and Precipitation Processes

MgAl₂O₄ spinel precursor was prepared by a novel method combining a sol–gel process with the "traditional" precipitation process. The thermal decomposition and phase development of the precursor were analyzed, and the degree of agglomeration of the calcined powder was assessed by determining its particle size and crystal size. The optimum calcination temperature was determined based on the variation of specific surface areas, crystal size, and particle size. Completely crystallized ultrafine spinel powder ( d ₅₀=600 nm, specific surface area=105 m²/g) was obtained after calcination at 900°C.     

Equilibrium Cation Distribution in NiAl2O4, CuAl2O4, and ZnAl2O4 Spinels

Stoichiometric NiAl₂O₄, CuAl₂O₄, and ZnAl₂O₄ spinels were prepared and equilibrated at temperatures from 600° to 1400°C. The parameters u and x , denoting the oxygen position and fraction of divalent cations on tetrahedral sites, respectively, were determined from a detailed X-ray diffraction analysis. In NiAl₂O₄, x increased from 0.07 at 595° to 0.26 at 1391°C; in CuAl₂O₄, x decreased from 0.68 at 613° to 0.64 at 1195°C; and in ZnAl₂O₄, x decreased from 0.96 at 905° to 0.94 at 1197°C. The form of the temperature dependence of x could not be described using theoretically based equations advanced in the literature. A more general equation which allows for a non-distributional contribution to the configurational entropy was derived and observed to properly describe the temperature dependence; the results indicate that short-range order is of definite significance in these intermediate aluminate spinels.     

Low-Temperature Preparation of Ba5Nb4O15 Ceramics Through a Sol–Gel Process

Hexagonal Ba₅Nb₄O₁₅ nanorods and microdisks were synthesized by a sol–gel process at temperatures of 700°–900°C. The samples were characterized by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and UV–visible absorption spectroscopy. The visible light absorption edges of the Ba₅Nb₄O₁₅ nanorods and microdisks corresponded to the band gap energies of 3.63 and 3.70 eV, respectively. This shows that the nanorod and microdisk-type structures are promising candidates for application in the miniaturization of microwave components.     

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.     

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.     

Mechanical Activation of Heterogeneous Sol–Gel Precursors for Synthesis of MgAl2O4 Spinel

MgAl₂O₄ spinel precursor was prepared using a heterogeneous sol–gel process. The effect of high-energy milling on the precursor decomposition and spinel formation was investigated. The milling decreased the Al(OH)₃ dehydroxylation temperature from 190° to about 130°C. The activation energy for spinel formation decreased from 688 kJ/mol for the as-prepared precursors to 468 kJ/mol for the precursors milled for 5 h. Milling of the precursor lowered the incipient temperature of spinel formation from 900° to 800°C, and the temperature of complete MgAl₂O₄ spinel formation from >1280° to ∼900°C.     

Spectroscopic Properties of Er3+-Doped Na2O–La2O3–Al2O3–SiO2 Glasses

Er³⁺-doped sodium lanthanum aluminosilicate glasses with compositions of (90− x )(0.7SiO₂·0.3Al₂O₃)· x Na₂O·8.2La₂O₃· 0.6Er₂O₃·0.2Yb₂O₃·1Sb₂O₃ (in mol%) ( x = 12, 20, 24, 40, 60 mol%) were prepared and their spectroscopic properties were investigated. Judd–Ofelt analysis was used to calculate spectroscopic properties of all glasses. The Judd–Ofelt intensity parameter Ω _t ( t = 2, 4, 6) decreases with increasing Na₂O. Ω₂ decreases rapidly with increasing Na₂O while Ω₄ and Ω₆ decrease slowly. Both the fluorescent lifetime and the radiative transition rate increase with increasing Na₂O. Fluorescence spectra of the ⁴ I _13/2 to ⁴ I _15/2 transition have been measured and the change with Na₂O content is discussed. It is found that the full width at half-maximum decreases with increasing Na₂O.     

Sol–Gel Processed Y-PSZ Ceramics with 5 wt% Al2O3

Y-PSZ ceramics with 5 wt% Al₂O₃ were synthesized by a sol–gel route. Experimental results show that powders of metastable tetragonal zirconia with 2.7 mol% Y₂O₃ and 5 wt% Al₂O₃ can be fabricated by decomposing the dry gel powder at 500°C. Materials sintered in an air atmosphere at 1500°C for 3 have high density (5.685 g/cm³), high content of metastable tetragonal zirconia (>96%), and high fracture toughness (8.67 MPa.m^1/2). Compared with the Y-PSZ ceramics, significant toughening was achieved by adding 5 wt% Al₂O₃.     

Mesoporous AlPO4 Glass from a Simple Aqueous Sol–Gel Route

Transparent and colorless AlPO₄ gel and glass are prepared via a simple aqueous sol–gel route using aluminum lactate and phosphoric acid as precursors. The stoichiometric AlPO₄ glass derived from this sol–gel route has a mesoporous structure with a surface area of 504 m²/g after calcination to 600°C. With increasing gel-to-glass processing temperature, the average degree of P/Al connectivity increases. After sample calcination at 600°C for 4 h, ²⁷Al MAS NMR spectra indicate that aluminum is almost completely converted into AlO₄ units. ²⁷Al{³¹P} rotational echo double resonance and ³¹P{²⁷Al} rotational echo adiabatic passage double resonance NMR experiments as well as ²⁷Al and ³¹P MAS NMR results further confirm that the prepared AlPO₄ glass has a three-dimensional network based on alternating Al(OP)₄ and P(OAl)₄ tetrahedral units in analogy to the local structure of crystalline AlPO₄.     

Spark Plasma Sintering of Sol–Gel Derived Amorphous ZrW2O8 Nanopowder

Dense ZrW₂O₈ was prepared by spark plasma sintering (SPS), using amorphous ZrW₂O₈ nanopowder as a raw material, at 873 K for 10 min. We investigated the effects of SPS conditions, such as sintering temperature, heating rate, and the discharge power that is expressed as the product of pulsed direct current and voltage, on the densification process of ZrW₂O₈. The relative density and microstructure of ZrW₂O₈ prepared by SPS were compared with those of ZrW₂O₈ prepared by hot pressing (HP). The relative density of ZrW₂O₈ prepared by HP at 873 K for 1 h was 63.1%. On the contrary, the relative density of ZrW₂O₈ prepared by SPS at 873 K for 10 min at a heating rate of 50 K/min was 98.6%. These results show that the discharge pressure that is proportional to discharge power enhances the densification and grain growth of ZrW₂O₈ in the SPS process.