Stabilization of Tetragonal ZrO2 in ZrO2–SiO2 Binary Oxides

Gel-glasses of various compositions in the x ZrO₂^.(10 – x )SiO₂system were fabricated by the sol–gel process. Precipitation due to the different reactivities between tetraethyl orthosilicate (TEOS) and zirconium(IV) n -propoxide has been eliminated through the use of 2-methoxyethanol as a chelating agent. Thermal treatment of these gels produced crystalline ZrO₂particles. While monoclinic is the stable crystalline phase of zirconia at low temperatures, the metastable tetragonal phase is usually the first crystalline phase formed on heat treatment. However, stability of the tetragonal phase is low, and it transforms to the monoclinic phase on further heat treatment. In this study, it has been found that the transformation temperature increases as the SiO₂content in the ZrO₂–SiO₂ binary oxide increases. The most significant results were from samples containing only 2 mol% SiO₂, where the metastable tetragonal phase formed at low temperatures and remained stable over a broad temperature range. X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to elucidate the structure of these binary oxides as a function of temperature.     

Glass Formation in the System SiO2–PbO Containing Transition-Metal Oxides

Glass-forming regions, valence states, and viscosities in SiO₂–PbO systems containing various transition-metal oxides as a third component were investigated. The glasses were prepared by melting in an open atmosphere. The glass-forming regions ranged as follows: MnO≡ZnO > FeO_1.5>NiO. The ratios Fe²⁺/(Fe²⁺+ Fe³⁺) and Mn³⁺/ (Mn³⁺+ Mn²⁺) in the glasses were determined by chemical analysis. The Fe²⁺/ (Fe²⁺+ Fe³⁺) ratio in SiO₂–PbO–FeO_1.5 glasses ranged from 0.016 to 0.050. The Mn³⁺/ (Mn³⁺+ Mn²⁺) ratio in SiO₂–PbO–MnO glasses ranged from 0.056 to 0.30. The fraction of manganese (III) ions in the glasses varies considerably with the glass composition. The effects of transitionmetal oxides on the viscosity are discussed.     

Chemical Vapor Infiltration Rates of ZrO2 into MoSi2 Porous Bodies and Comparison with Model Calculation

Zirconia (ZrO₂) was chemical vapor infiltrated (CVI) into a partially sintered MoSi₂ body (preform) by using zirconium n -propoxide (Zr(OC₃H₇)₄) as a gas precursor. Infiltration distances at different conditions were compared with the calculated results. Chemical vapor deposition (CVD) film growth rates of ZrO₂ were measured, and the data were incorporated into the model calculations. Two models were used to analyze the observed infiltration distances. Initially a conventional model assuming a pore with constant radius (SP model) was used. With this model, it was possible to predict the approximate infiltration distance. However, the model cannot predict pore closure and the infiltration distances for a variety of CVI conditions. Secondly, a newly proposed model (PC model) from a previous paper was applied to calculate the infiltration distance. Using this model, it was possible to predict the occurrence of pore closure or the formation of the deposition layer on the preform surface.     

Formation of Convoluted Silica Precipitates during Amorphous Phase Separation in the Ca3(PO4)2–SiO2–MgO System

Dispersed aggregates of peculiar morphology have been obtained in phase-separated glasses of the system Ca₃(PO₄)₂–SiO₂–MgO, where the separated phase is amorphous silica. The formation of such convoluted aggregates is tentatively explained in terms of a fast coalescence process of initial isolated quasi-spherical droplets which behave as a dispersed phase in an emulsion-like system.     

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.     

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.     

Effect of Neodymium:Yttrium Aluminum Garnet Laser Irradiation on Crystallization in Li2O–Al2O3–SiO2 Glass

A 355-nm neodymium:yttrium aluminum garnet laser, produced by a harmonic generator, was used for the nucleation process in photosensitive glass containing Ag⁺ and Ce³⁺ ions. The pulse width and frequency of the laser were 8 ns and 10 Hz, respectively. Heat treatment was conducted at 570°C for 1 h, following laser irradiation, to produce crystalline growth, after which a LiAlSi₃O₈ crystal phase appeared in the laser-irradiated Li₂O–Al₂O₃–SiO₂ glass. The present study compares the effect of laser-induced nucleation on glass crystallization with that of spontaneous nucleation by heat treatment.     

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.     

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.     

Sintering and Crystallization of a Glass Powder in the Li2O–ZrO2–SiO2 System

Sintering and crystallization of a 23.12 mol% Li₂O, 11.10 mol% ZrO₂, 65.78 mol% SiO₂ glass powder was investigated. By means of thermal shrinkage measurements, sintering was found to start at about 650°C and completed in a very short temperature interval (Δ T similar/congruent 100°C) in less than 30 min. Crystallization took place just after completion of sintering and was almost complete at about 900°C in 20 min. Secondary porosity prevailed over the primary porosity during the crystallization stage. The glass powder compacts first crystallized into lithium metasilicate (Li₂SiO₃), which transformed into lithium disilicate (Li₂Si₂O₅), zircon (ZrSiO₄), and tridymite (SiO₂) after the crystallization process was essentially complete. The microstructure was characterized by fine crystals uniformly distributed and arbitrarily oriented throughout the residual glass phase.     

Proton Conductivity in Zr4+-Ion-Doped P2O5–SiO2 Porous Glasses

The effect of zirconium ions on glass structure and proton conductivity was investigated for sol-gel-derived P₂O₅–SiO₂ glasses. Porous glasses were prepared through hydrolysis of PO(OCH₃)₃, Zr(OC₄H₉)₄, and Si(OC₂H₅)₄. Chemical bonding of the P⁵⁺ ions was characterized using ³¹P-NMR spectra. The phosphorous ions, occurring as PO(OH)₃ in the ZrO₂-free glass, were polymerized with one or two bridging oxygen ions per PO₄ unit with increased ZrO₂ content. The chemical stability of these glasses was increased significantly on the addition of ZrO₂, but the conductivity gradually decreased from 26 to 12 mS/cm at room temperature for 10P₂O₅·7ZrO₂·83SiO₂ glass. A fuel cell was constructed using 10P₂O₅·5ZrO₂·85SiO₂ glass as the electrolyte; a power of ∼4.5 mW/cm² was attained.     

Low-Fire Processing of ZrO2–SnO2–TiO2 Ceramics

Effects of a liquid-phase-sintering aid, BaCuO₂+ CuO (BCC), on densification and microwave dielectric properties of (Zr_0.8Sn_0.2)TiO₄ (ZST) ceramics have been investigated. The densification kinetics of ZST are greatly enhanced with the presence of 2.5–5 wt% BCC, but become retarded when the amount of BCC increases further. At a given BCC content, moreover, slower densification kinetics are observed with a larger particle size of ZST. The above results are attributed to a chemical reaction taking place at the interface of BCC/ZST during firing. The ZST dissolves into BCC, forming crystalline phases of ZrO₂, SnO₂, CuO, and BaTi₈O₁₆ which reduce the amount of BCC flux available for liquid-phase sintering. The crystallization kinetics become more significant, compared with densification kinetics, with increasing the amount of BCC and the particle sizes of ZST. For samples with 2.5–5 wt% BCC, a high relative sintered density is obtained at 1000°C and the resulting microwave ceramics have a dielectric constant and a value of Q at 7 GHz in the ranges of 35–38 and 2800–5000, respectively.     

Direct Measurement of Relative Partial Molar Enthalpy of SiO2 in SiO2–M2O (M=Li, Na, K, Cs) Binary and SiO2–CaO–Al2O3 Ternary Melts

The relative partial molar enthalpies, Δ _SiO2, of SiO₂ in SiO₂–M₂O (M = Li, Na, K and Cs) binary and SiO₂–CaO–Al₂O₃ternary melts were directly measured by drop-solution calorimetry at 1465 K and 1663 K. Δ _SiO2 changes from exothermic to endothermic as silica content increases, confirming the tendency toward immisciblity seen from activity measurements. It is concluded that Δ _SiO2 is negative due to acid-base reactions and charge-coupled substitutions when the melt is composed of fewer Q ⁴ and more Q ³ and Q ² species, but positive due to structural strain when the melt is composed of mostly Q ⁴ species. The Δ _SiO2 obtained by calorimetry is a useful measure of basicity, when comparing different alkali and alkaline earth oxides.     

Al2O3–ZrO2–SiO2 Ternary Glasses for Molybdenum Oxidation Barriers

The wettability of binary and ternary glasses belonging to SiO₂–Al₂O₃–ZrO₂ diagram has been studied using the sessile drop technique at 1750° and 1800°C. The ternary SiO₂–Al₂O₃–ZrO₂ (90–5–5 wt%) glass has proved to be well appropriated as a molybdenum oxidation barrier coating. The addition of 5 wt% of MoO₂ slightly improves its wettablity at higher temperatures without affecting its oxidation barrier properties. The Mo comes into the glass network as a mixture of Mo⁵⁺, Mo⁴⁺, and Mo⁶⁺. After oxidation at 1000°C in oxygen atmosphere, the molybdenum remains in the glass network as Mo⁶⁺.     

Porous Silica Matrix Obtained from Pyrex Glass by Hydrothermal Treatment: Characterization and Nature of the Porosity

A novel porous silica matrix has been prepared from Pyrex glass, using hydrothermal treatment under saturated-steam condition. This process makes it possible to obtain, in one step, a silica support formed of a homogeneously distributed and interconnected macropore microstructure. The new matrix contains silanol groups that can be used in reactions of surface modification to provide a hybrid material and a selective macrofiltration membrane, and also it can improve chemical inertness. The porous matrix is noncrystalline as obtained and, after thermal treatment at temperatures higher than 950°C, exhibits an X-ray pattern characteristic of α-cristobalite and low volume contraction. The present samples were characterized by scanning electron microscopy, mercury intrusion porosimetry, nitrogen adsorption-desorption isotherms, infrared spectroscopy, X-ray powder diffractometry, atomic absorption, and high-resolution solid-state nuclear magnetic resonance. The results present a new way of producing a macroporous silica matrix.     

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.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Solubility of Si3N4 in Liquid SiO2

The nitrogen solubility in the SiO₂-rich liquid in the metastable binary SiO₂-Si₃N₄ system has been determined by analytical TEM to be 1%–4% of N/(O + N) at 1973–2223 K. Analysis of the near edge structure of the electron energy loss peak indicates that nitrogen is incorporated into the silicate network rather than being present as molecular N₂. A regular solution model with a positive enthalpy of mixing for the liquid was used to match the data for the metastable solubility of N in the presence of crystalline Si₃N₄ and to adjust the computed phase diagram. The solubility of Si₃N₄ in fused SiO₂ is far less than reported in liquid silicates also containing Al, Mg, and/or Y. Apparently, these cations act as modifiers that break anion bridges in the silicate network and, thereby, allow further incorporation of Si₃N₄ without prohibitive amounts of network cross-linking. Finally, indications emerged regarding the diffuse nature of the Si₃N₄-SiO₂ interface that leads to amorphous regions of higher N content.     

Sintering and Crystallization of CaO–Al2O3–ZrO2–SiO2 Glasses Containing Different Amount of Al2O3

In this work several complementary techniques have been employed to carefully characterize the sintering and crystallization behavior of CaO–Al₂O₃–ZrO₂–SiO₂ glass powder compacts after different heat treatments. The research started from a new base glass 33.69 CaO–1.00 Al₂O₃–7.68 ZrO₂–55.43SiO₂ (mol%) to which 5 and 10 mol% Al₂O₃ were added. The glasses with higher amounts of alumina sintered at higher temperatures (953°C [lower amount] vs. 987°C [higher amount]). A combination of the linear shrinkage and viscosity data allowed to easily find the viscosity values corresponding to the beginning and the end of the sintering process. Anorthite and wollastonite crystals formed in the sintered samples, especially at lower temperatures. At higher temperatures, a new crystalline phase containing ZrO₂ (2CaO·4SiO₂·ZrO₂) appeared in all studied specimens.