Preparation and Physical Properties of Radiofrequency-Sputtered Amorphous Films in the System AlPO4–TiO2

Amorphous films in the system AlPO₄–TiO₂ were prepared by an rf-sputtering method, and their physical properties, such as density, refractive index, and thermal expansion coefficient, and the infrared absorption spectra were measured. The thermal expansion coefficient increased linearly with increasing TiO₂ content. The results of the molar refractivity and the infrared absorption spectra indicated that the coordination number of titanium ions in these films is higher than that in SiO₂–TiO₂ glasses with a negative thermal expansion, in which Ti⁴⁺ ions are tetrahedrally coordinated. In order to confirm the coordination state of the titanium ions in these amorphous films, titanium K -band emission spectra were obtained by X-ray emission spectroscopy, revealing sixfold coordination. The higher coordination state of Ti⁴⁺ was considered to account for these amorphous films not exhibiting negative thermal expansion, as in the SiO₂–TiO₂ system.     

Characterization and Electrophoretic Deposition of Poly(Phenylsilsesquioxane)–Titania Hybrid Particles Prepared by the Sol–Gel Method

Poly(phenylsilsesquioxane)–titania (PhSiO_3/2–TiO₂) hybrid particles were prepared from phenyltriethoxysilane and titanium tetra- n -butoxide by the sol–gel method. Fourier transform infrared spectra showed that PhSiO_3/2 and the TiO₂ components were hybridized through Si–O–Ti bonds. The refractive index of the particles was monotonically increased from 1.57 to 1.62 with an increase in the TiO₂ content. The PhSiO_3/2–TiO₂ particles were electrophoretically deposited on indium tin oxide (ITO)-coated glass substrates to form opaque, thick films about 3 μm in thickness. When the mole ratio x in (1− x )PhSiO_3/2· x TiO₂ was equal to or less than 0.05, the deposited PhSiO_3/2–TiO₂ films became transparent with a heat treatment at 400°C because of the thermal sintering of the particles.     

Formation of Ring-Like Si–O–Zr Bonds at Intergranular Interfaces in Silica-Doped Zirconia

Possible cluster models of the intergranular interfaces phase for SiO₂/ZrO₂ binary oxides were optimized by the density-functional theory (DFT/B3LYP). New results on the formation of the interfacial ring-like Si–O–Zr bonds are validated by the analyses of reaction Gibbs free energies and computational infrared vibration spectra. Moreover, the Mulliken charge population of interfacial Si–O–Zr structures shows that the increased temperature and critical size of the tetragonal-to-monoclinic phase transformation for SiO₂-doped ZrO₂ mainly come from the reduced charge of oxygen atoms located at the interface and coordinated with the neighboring zirconium atoms, which accords well with the previous theoretical results.     

Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides

Metastable tetragonal ZrO₂ phase has been observed in ZrO₂–SiO₂ binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO₂ tetragonal stabilization in binary oxides such as Y₃O₂–ZrO₂, MgO–ZrO₂, or CaO–ZrO₂. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO₂ lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO₂–SiO₂ binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO₂ crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O⁻ groups). In this paper, FT-IR and ²⁹Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O⁻ is responsible for tetragonal phase stabilization. Moreover, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.     

Effect of Sb2O3 on Thermal Properties of Glasses in Bi2O3–B2O3–SiO2 System

Glasses with compositions 50Bi₂O₃– x Sb₂O₃–10B₂O₃–(40– x ) SiO₂ ( x =0, 1, 3, 5, 8, 10) have been prepared by conventional melt quench technique. Substitution of Sb₂O₃ for SiO₂ exerted an obvious effect on properties of glasses, especially, increased glass transition temperature ( T _g) and crystalline temperature ( T _c) greatly. Results of infrared transmission spectra attributed the effect to the formation of new bridging bonds of Sb–O–B and Sb–O–Si in glass network.     

Structural Changes of Sol–Gel-Derived TiO2–SiO2 Coatings in an Environment of High Temperature and High Humidity

Structural changes in sol–gel-derived TiO₂–SiO₂ coatings were found to proceed in an environment of high temperature and high humidity as follows: (1) dissociation of Si–O–Ti bonds in the coating by the attack of water vapor, (2) formation of Ti–O–Ti bonds, and (3) nucleation and growth of anatase TiO₂. The coating obtained with the addition of poly(ethylene glycol), PEG, reacts with water vapor more easily than the coating obtained without PEG, since the former is more porous than the latter due to the decomposition of PEG during heat treatment.     

Aluminum Phosphate–Mullite Composites for High-Temperature Radome Applications

An AlPO₄–SiO₂ powder with a composition of Al:P:Si=1.5:1:0.1 was synthesized by the sol–gel method using aluminum nitrate, phosphate acid, and tetraethoxysilane. The structural evolution of this material was characterized by thermal gravimetric analysis-differential scanning calorimetry, Fourier transform infrared, and X-ray diffraction. By adding silica in AlPO₄, the sinterability of the AlPO₄ was enhanced because of the reactions between excess alumina and silica to form mullite. The sintered composites have a high strength and good dielectric properties at 10 GHz. Because of the formation of mullite at high temperatures, the composites showed a hydrophobic property. These unique properties indicate that the sintered AlPO₄–mullite composites are suitable for the radome application.     

Low-Temperature Crystallization of Forsterite and Orthoenstatite

MgO–SiO₂ precursor gels were prepared by mixing tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS), H₂O, and magnesium metal in methanol. Forsterite (Mg₂SiO₄) and orthoenstatite (MgSiO₃) were crystallized from their precursors at temperatures as low as 500° and 700°C, respectively. The chemical compositions of the crystallized phases were richer in MgO content than those of the starting materials. Inductively coupled plasma analysis showed that an amorphous SiO₂-rich phase was present, together with crystalline phases. We speculate that the amorphous SiO₂-rich phase has an important role in the low-temperature crystallizations of these magnesium silicates. Characterization of the preparation process via liquid-state ²⁹Si nuclear magnetic resonance (NMR) indicated that the breakage of the ≡Si–O–Si≡ bond was caused by the addition of magnesium metal. Solid-state ²⁹Si NMR showed that the Mg–O–Si bond might form in as-prepared specimens.     

Physical Properties and Structure of rf-Sputtered Amorphous Films in the System Al2O3–Y2O3

Amorphous films in the system Al₂O₃–Y₂O₃ were prepared by the rf sputtering method in the range of 0–76 mol% Y₂O₃, and their density, refractive index, and elastic constants were measured. All of the physical properties of the amorphous Al₂O₃–Y₂O₃ films had a similar compositional dependence; that is, they increased continuously, but not linearly with increasing Y₂O₃ content. To confirm the coordination states of aluminum and yttrium ions in the amorphous Al₂O₃–Y₂O₃ films, the Al K α X-ray emission spectra and the X-ray absorption near edge structures (XANES) were measured. The average coordination number of aluminum ions in the amorphous films containing up to about 40 mol% Y₂O₃ content was 5, that is a mixture of 4-fold- and 6-fold-coordinated states. In the region of more than about 50 mol% Y₂O₃, the fraction of the 6-fold-coordinated aluminum ions increased with increasing Y₂O₃ content, while the results led to the conclusion that the coordination number of yttrium ions was always 6, regardless of composition. These results indicate that, in amorphous films in the system Al₂O₃–Y₂O₃, the change of the coordination state of aluminum ions has an important effect on physical properties.     

Reaction and Formation of Crystalline Silicon Oxynitride in Si–O–N Systems under Solid High Pressure

Oxidized amorphous Si₃N₄ and SiO₂ powders were pressed alone or as a mixture under high pressure (1.0–5.0 GPa) at high temperatures (800–1700°C). Formation of crystalline silicon oxynitride (Si₂ON₂) was observed from amorphous silicon nitride (Si₃N₄) powders containing 5.8 wt% oxygen at 1.0 GPa and 1400°C. The Si₂ON₂ coexisted with β-Si₃N₄ with a weight fraction of 40 wt%, suggesting that all oxygen in the powders participated in the reaction to form Si₂ON₂. Pressing a mixture of amorphous Si₃N₄ of lower oxygen (1.5 wt%) and SiO₂ under 1.0–5.0 GPa between 1000° and 1350°C did not give Si₂ON₂ phase, but yielded a mixture of α,β-Si₃N₄, quartz, and coesite (a high-pressure form of SiO₂). The formation of Si₂ON₂ from oxidized amorphous Si₃N₄ seemed to be assisted by formation of a Si–O–N melt in the system that was enhanced under the high pressure.     

Preparation of NiAl2O4/SiO2 and Co2+-Doped NiAl2O4/SiO2 Nanocomposites by the Sol–Gel Route

NiAl₂O₄/SiO₂ and Co²⁺-doped NiAl₂O₄/SiO₂ nanocomposite materials of compositions 5% NiO – 6% Al₂O₃– 89% SiO₂ and 0.2% CoO – 4.8% NiO – 6% Al₂O₃– 89% SiO₂, respectively, were prepared by a sol–gel process. NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals were grown in a SiO₂ amorphous matrix at around 1073 K by heating the dried gels from 333 to 1173 K at the rate of 1 K/min. The formations of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in SiO₂ amorphous matrix were confirmed through X-ray powder diffraction, Fourier transform infrared spectroscopy, differential scanning calorimeter, transmission electron microscopy (TEM), and optical absorption spectroscopy techniques. The TEM images revealed the uniform distribution of NiAl₂O₄ and cobalt-doped NiAl₂O₄ nanocrystals in the amorphous SiO₂ matrix and the size was found to be ∼5–8 nm.     

Structure of Na2O–CaO–P2O5–SiO2 Glass–Ceramics with Multimodal Porosity

We have investigated the evolution of the structure of nano–macro porous CaO–Na₂O–P₂O₅–SiO₂ bioactive glass–ceramics by Fourier transform infrared (FTIR) and Raman spectroscopies, and X-ray diffraction (XRD). A controlled devitrification, followed by a chemical leaching treatment is used to produce a multimodal distribution of nano/macro pores that are expected to improve cell attachment. Data show that the leaching process removes the sodium- and calcium-containing crystalline phases that are formed during the ceramming heat treatment. The primary Si–O peaks in the infrared spectra blue shift with leaching, indicating that the sample becomes SiO₂ rich. In parallel, the fraction of nonbridging oxygen decreases. These results suggest a restructuring of the glass network far below the glass transition temperature. The stresses from leaching, capillary forces, and subsequent restructuring develop and grow, eventually producing cracks in the sample.     

Structure, Thermal Behavior, Chemical Durability, and Optical Properties of the Na2O–Al2O3–TiO2–Nb2O5–P2O5 Glass System

The FTIR, Raman, UV-Vis, ³¹P MAS-NMR, DTA, and refractive index measurements have been combined to investigate a series of glasses with the general formula 20Na₂O–5Al₂O₃− x TiO₂–(45− x )Nb₂O₅–30P₂O₅, 15≤ x ≤45. The glass structure, as well as thermal, optical, and chemical durability properties, were then described as functions of the f _Nb/ f _Ti ratio. An increase of the f _Nb/ f _Ti ratio correlates with a decrease in length of the average phosphate chains linked through Nb–O–P and Ti–O–P bonds, with an increase in the glass stability and with increase in the linear refractive indices at 632.8 nm from 1.79 to 1.89. Furthermore, niobium is more effective than titanium in improving chemical durability.     

Mullite–Aluminum Phosphate Laminated Composite Fabricated by Tape Casting

Mullite–aluminum phosphate (3Al₂O₃·2SiO₂/AlPO₄) laminated composites were fabricated by tape casting. AlPO₄ had a density of 1.56 g/cm³, which corresponds to 61% of theoretical density, and a bending strength of 1.5 MPa after sintering at 1600°C for 10 h. The aluminum phosphate functioned as a porous, weak, and chemically stable interphase which was able to deflect cracks in a laminated composite. To increase the strength of the weak interphase material, 10 and 30 vol% of mullite were added.     

Novel Processing for Mesoporous Silica Films with One-Dimensional Through Channels Normal to the Substrate Surface

Mesoporous silica films with one-dimensional through channels perpendicular to the substrate surface have been fabricated successfully by a novel process, which was a combination of eutectic decomposition of amorphous films and subsequent chemical etching. In the case of the Fe–Si–O system, amorphous precursor films annealed under oxidizing conditions were decomposed to a regular array of needlelike hematite (Fe₂O₃) crystals with a diameter of ∼4 nm surrounded by an amorphous silica matrix. Then, hematite crystals were preferentially removed by chemical etching. The pore surface areas of the remaining mesoporous SiO₂ films were found to be more than 1000 m²/g by an isothermal N₂ gas adsorption and desorption measurement.     

Structure Characterization of HSQ Films for Low Dielectrics Uses D4 as Sacrificial Porous Materials

Hydrogensilsesquioxane (HSQ) (low- k ) films were prepared by spin-on deposition using D4 (octamethyl cyclotetrasiloxane) as a sacrificial porous material. The dielectric constant of silica films significantly changed from 3.0 to 2.2. Fourier transform infrared spectroscopy was used to identify the network structure and cage structure of the Si–O–Si bond and other bonds that may appear. We studied the structural and electrical properties of the spin-coated films prepared by mixing HSQ and D4 films after oxygen plasma exposure for 5 min, and studied the structural recovery of the damage by annealing at 350°C for 1.5 h in a nitrogen (N₂) ambient. This structure results in significant lowering of the dielectric constant ( k ) on annealing at 350°C for 1.5 h in an N₂ ambient and improvement in the leakage current density.     

Attenuated Toatal Reflectance Fourier-Transform Infrared Spectra of a Hydrated Sodium Soilicate Glass

Attenuated total reflectance Fouriertransform infrared (ATR-FTIR) spectra were measured in the region from 4300 to 400 cm⁻¹ for a hydrated Na₂O–SiO₂ glass containing 35 wt% water. The Si–OH bending vibration mode was observed. It was found that the incorporated water, molecular water as well as hydroxyls, affected the Si–O vibrations. The effect of incorporated water upon the glass structure is discussed.     

Effects of Heating Rate on Stress Evolution in Alkoxide-Derived Silica Gel-Coating Films

Silica gel films were deposited by spin coating on single-crystal Si wafers using an acid-catalyzed Si(OC₂H₅)₄ solution as a coating solution. The gel films were heated at various rates, where in situ stress measurement was conducted. In-plane tensile stress developed during the course of heating, and was found to be larger at lower heating rates at temperatures up to 350°C. The larger stress was thought to cause cracking at lower temperatures, which was previously observed at lower heating rates in in situ observation. The larger stress at lower heating rates was basically ascribed to the larger degrees of densification, which was revealed in the larger extent of reduction in thickness as well as in Si–OH/Si–O–Si and O–H/Si–O–Si infrared absorption band area ratios at lower heating rates. The difference in stress at different heating rates appeared to originate mainly in the difference observed at low temperatures below 130°C, suggesting that the heating rate particularly affects the densification that occurs via solvent evaporation. The increment in stress was reduced over 400°C when the heating rate was low, which was thought to result from the higher degree of densification already achieved below 400°C as well as the structural relaxation occurring at such high temperatures.     

Photoluminescence Characteristics of Sol–Gel Materials Prepared from Bimetallic Erbium Alkoxides Coordinated by Al or Ti

Er³⁺-doped Al₂O₃–SiO₂ and TiO₂–SiO₂ powders were prepared by the sol–gel process using bimetallic erbium isopropoxides coordinated with Al or Ti. The local environment surrounding Er³⁺ ions was controlled orderly at the precursor stage. The phase development of two different systems as a function of temperature was characterized by XRD, and the amount of OH groups remaining within the samples was investigated by a Fourier-transformed infrared spectrometer with increasing annealing temperatures. The photoluminescence spectra and lifetimes of two systems annealed at different temperatures were measured and discussed. The strong emission and long lifetime were observed in Er³⁺-doped Al₂O₃–SiO₂ system, ascribed to the homogeneous distribution of Er³⁺ ion.     

Coordination and Bond Character of Silicon and Aluminum Ions in Amorphous Thin Films in the System SiO2-Al2O3

The coordination and bond character of the cations in amorphous SiO₂-Al₂O₃ films prepared by rf sputtering were examined by studying chemical shifts in the SiK and AlK X-ray emission spectra. The coordination number of Si ions in these films was always 4, regardless of composition, whereas the average coordination number of Al ions changed from 4 to 5, depending onAl₂O₃ content. TheSi-O-Al bond type seemed to appear as Al₂O₃ was introduced into amorphous SiO₂ films or vice versa.