Oxidation of HIPed TiC Ceramics in Dry O2, Wet O2, and H2O Atmospheres

Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O₂ (dry oxidation), Ar/O₂/H₂O (wet oxidation), and Ar/H₂O (H₂O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H₂O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO₂ scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H₂O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO₂ and H₂ with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO₂ scale accompanied CO₂ evolution, and the H₂O-containing oxidation produced a small amount of H₂. A piece of single crystal TiC was oxidized in ¹⁶O₂/H₂¹⁸O to reveal the contribution of O from H₂O to the oxidation of TiC by secondary ion mass spectrometry.     

Discoloration of Fired Kaolinitic Clays (Study of Fe3+ Coordination by Mössbauer and UV-ViS-NIR Spectroscopy)

The final color of a ceramic body is generally determined by the contents of Fe³⁺ ions. The Central European market accepts kaolinitic clays for the production of tableware, electro-insulators, and wall tiles only if the Fe₂O₃ in chemical analyses of the kaolin does not exceed 1.0 wt%. The chemical analyses calculate the content of all iron components as if they were in the form of Fe₂O₃ and then their quantity excludes even good-quality clay from the ceramic industry of white bodies. We found that oxidizing firing of the samples fired at over 1180°C changes the color in cases where the initial Fe³⁺ component is in the form of hydro ferric oxide [FeO(OH)] or if the ferric ions were added to the white kaolin samples in the form of ferric nitrate [Fe(NO₃)₃·9H₂O]. The Mössbauer spectroscopy confirms the presence of only diluted Fe³⁺ ions. The UV-ViS-NIR spectroscopy confirms that even if the concentration of Fe₂O₃ is above 3.0 wt%, these ions of Fe³⁺ in the case of their initial hydro ferric oxide formed in clay are incorporated into the mullitic structure in tetrahedral coordination. The iron-coloring effect depends on the coordination of Fe³⁺ ions with the studied discoloring effect of fired bodies—the very small and well-distributed particles enter into the formatted mullitic structure.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Ion Exchange in Alkali Layers of Potassium β -Ferrite ((1 + x)K2O · 11Fe2O3) Single Crystals

The potassium ions in potassium β-ferrite ((1 + x)K₂O ·11Fe₂O₃) crystals were exchanged with Na⁺, Rb⁺, Cs⁺, Ag⁺, NH₄⁺, and H₃O⁺ in molten nitrates or in concentrated H₂SO₄. On the other hand, spinel and hexagonal ferrites were formed by soaking the crystals in the melt of divalent salts. The crystals of K⁺, Rb⁺, and Cs⁺β-ferrites decomposed to form α-Fe₂O₃ at high temperatures of 800° to 1100°C. In addition, H₃O⁺, NH₄⁺, and Ag⁺β-ferrites decomposed to form α-Fe₂O₃ at relatively low temperatures of 350° to 650°C, in accordance with the stabilities of the inserted ions. The electrical properties of some β-ferrites were measured.     

Formation of Sm2+ lons in Sol-Gel-Derived Glasses of the System Na2 O-Al2O3-SiO2

Samarium ions (Sm²⁺) incorporated into aluminosilicate glasses by a sol-gel process showed persistent spectral hole burning at room temperature. Gels of the system Na₂O-Al₂O₃SiO₂ synthesized by the hydrolysis of Si(OC₂H₅)₄, Al(OC₄H₉)₃, CH₃ COONa, and SmCl₃·6H₂O were heated in air at 500°C, then reacted with H₂ gas to form Sm²⁺ ions. Whereas Al³⁺ ions effectively dispersed the Sm³⁺ ions in the glass structure, Na⁺ ions were not effective. The Al₂O₃-SiO₂ glasses proved appropriate for reacting the Sm³⁺ ions with H₂ gas and exhibited the intense photoluminescence of Sm²⁺ ions. The reaction of Sm³⁺ ions with H₂ in the Al₂O₂-SiO₂ glasses was determined by first-order kinetics, and the activation energy equaled 95 kJ/mol. At 800°C, the maximum photoluminescence of the Sm²⁺ ions was achieved within 20 min.     

Reactive Ceria Nanopowders via Carbonate Precipitation

Nanocrystalline CeO₂ powders have been successfully synthesized via a carbonate precipitation method, using ammonium carbonate (AC) as the precipitant and cerium nitrate hexahydrate as the cerium source. The AC/Ce³⁺ molar ratio ( R ) affects significantly precursor properties, and spherical nanoparticles can be produced only in a narrow range of 2 < R ≤ 3. The precursor, having an approximate composition of Ce(OH)CO₃·2.5H₂O, decomposes to CeO₂ at temperatures ≥300°C. The CeO₂ powder calcined at 700°C exhibits high reactivity and can be densified to >99% of theoretical at 1000°C.     

Solubility and Transport of NiO Cathodes in Molten Carbonate Fuel Cells

Equilibrium NiO solubility measurements were made in Li₂CO₃/K₂CO₃ mixtures as a function of temperature, ambient gas environment, and salt composition for gases containing 3.1% H₂O. The equilibrium solubility was found to increase with increasing temperature, CO₂ partial pressure, and cation fraction Li⁺ and to decrease slightly with increasing O₂ partial pressure. These results were coupled with theoretical predictions of (1) O₂, H₂, and CO₂ activities and (2) compositional gradients which develop across the electrolyte structure of an operating carbonate fuel to explain the two distinct regions of nickel precipitation across the fuel cell. Precipitation occurs near the cathode because NiO solubility decreases commensurate with electrolyte compositional gradients, whereas the second region deeper within the electrolyte matrix results from a decreased O₂ activity.     

Synthesis of Er3+ Doped Y2O3 Nanophosphors

The synthesis and characterization of yttrium hydroxyl carbonate (Y(OH)CO₃²⁻) and yttrium nitrate hydroxide hydrate (Y(OH)NO₃H₂O) precursor materials as well as Y₂O₃ nanoparticles are reported. The resultant precursor particle size is about 10–12 nm with a narrow size distribution by the enzymatic decomposition method, whereas the particle size was smaller than those acquired by the homogeneous and alkali precipitation methods. The formation of Y(OH)CO₃²⁻ and Y(OH)NO₃H₂O species was also evident from the fourier-transform infrared spectrometry (FT-IR) analysis. Precipitated Y(OH)CO₃²⁻ precursors have an amorphous nature whereas Y(OH)NO₃H₂O precursors have a crystalline nature, which was manifested from the XRD analysis. Moreover, precipitated Y(OH)NO₃H₂O precursors were found in the agglomerated form and Y(OH)CO₃²⁻ was established in the monodispersed form, as determined from the FE-SEM, TEM and DLS measurements. It was demonstrated that calcination of precursor materials at 900°C eventually removed the inorganic anions from the precursors and consequently produced crystalline Y₂O₃ nanoparticles, which was evident from the XRD and FT-IR analysis. The EDS analysis confirms Er³⁺ doping in the Y₂O₃ nanoparticles. The morphology and the size of the Y₂O₃ nanoparticles are almost unchanged before and after the calcination.     

Low-Temperature Synthesis of Praseodymium-Doped Ceria Nanopowders

Praseodymium-doped ceria (CeO₂) nanopowders have been synthesized via a simple but effective carbonate-coprecipitation method, using nitrates as the starting salts and ammonium carbonate as the precipitant. The precursors produced in this work are ammonium rare-earth double carbonates, with a general formula of (NH₄)_0.16Ce_{1− x} Pr _x (CO₃)_1.58·H₂O (0 < x ≤ 0.20), which directly yield oxide solid solutions on thermal decomposition at a very low temperature of ∼400°C. Praseodymium doping causes a gradual contraction of the CeO₂ lattice, because of the oxidation of Pr³⁺ to smaller Pr⁴⁺, and suppresses crystallite coarsening of the oxides during calcination. Dense ceramics have been fabricated from the thus-prepared nanopowders via pressureless sintering for 4 h at a low temperature of 1200°C.     

Stability of Molybdenum Disilicide in Combustion Gak Environments

The stability of MoSi₂ in combustion gas at 1370° and 1600°C was evaluated using SOLGASMIX-PV thermodynamic modeling, periodic weight measurements, and characterization via XRD, SEM, EDS, and image analysis. Passive oxidation occurred at both temperatures. During an initial stage of exposure, specimen surfaces oxidized to form MoO_3(g) and amorphous SiO₂ via reduction of CO₂ and H₂O. After a short time (<6.5 min at 1370°C, <1 min at 1600°C), the oxidation mechanism switched; Mo₅Si₃ and amorphous SiO₂ formed as oxidation products. The first mechanism esulted in the formation of 46.1 vol% at 1370°C and 42.6 vol% at 1600°C of the amorphous silica surface coating. The attainment of a near-terminal weight gain implied silica formation was limited by H₂O and CO₂ diffusion through the silica coating.     

Wetting in an Electronic Packaging Ceramic System: II, Wetting of Alumina by a Silicate Glass Melt under Controlled pO2 Conditions

The wetting of polycrystalline alumina by a colored, calciamagnesia aluminosilicae glass was found to be dependent on temperatutre between 1300° and 1500°C, but independent of gas atmosphere effects. Neither the oxygen partial pressure, oveer the range of 10^-6 to 10^-10 Pa, the gas buffer system (Co/CO₂ or H₂/H₂O), nor pre-equilibration of the substrate surface with the atmosphere at tge exoperimental temperature before solid-liuid interface formation affected the stable contact angle. An initial drop in contact angle the stable contact angle. An initial drop in contact angle occurring within the first hour is attributed to repaid dissolution of alumina and the formation of a stable glass/alumina interface. The contact angle after an 8-h isothermal hold decreased from 1300° to 1500°C. The solid-liquid interfacial energy, μM_S1, controls the wetting behavior. Changes in μM_s1 are attributed to he breakup of the silica network as temperature increases.     

Wet-Chemical Routes Leading to Scandia Nanopowders

Two wet-chemical routes have been used to synthesize Sc₂O₃ nanopowders from nitrate solutions employing ammonia water (AW) and ammonium hydrogen carbonate (AHC) as the precipitants. The precursors and the resultant oxides are characterized by elemental analysis, X-ray diffractometry, differential thermal analysis/thermogravimetry, high-resolution scanning electron microscopy, and Brunauer-Emmett-Teller analysis. Crystalline γ-ScOOH· n H₂O ( n ≈ 0.5) is the only phase obtained by the AW method. This phase dehydrates to Sc₂O₃ at ∼400°C, yielding hard aggregated nanocrystalline Sc₂O₃ powders. Three types of precursors have been synthesized by the AHC method, depending on the AHC/Sc³⁺ molar ratio ( R ): amorphous basic carbonate [Sc(OH)CO₃·H₂O] at R ≤ 3, crystalline double carbonate [(NH₄)Sc(CO₃)₂·H₂O] at R ≥ 4, and a mixture of the two phases at 3 < R < 4. Among these precursors, only the basic carbonate shows spherical particle morphology, ultrafine particle size (∼50 nm), and weak agglomeration. Sc₂O₃ nanopowders (∼28 nm) with high surface area (∼49 m²/g) have been prepared by calcining the basic carbonate at 700°C for 2 h.     

Formation and Transformation of WO3 Prepared from Alkoxide

Amorphous WO₃ or WO₃ or H₂O is formed by hydrolysis of tungsten ethoxide. The temperature of hydrolysis influences the crystallization of WO₃·H₂O. Tungsten hydrate (WO₃·H₂O) has an orthorhombic unit cell with a=0.5235 nm, b = 1.0688 nm, and c=0.5123 nm. Orthorhombic WO₃ crystallizes at 350° to 500°Cfrom amorphous WO₃. Cubic WO₃ is formed at 200° to 310°C with dehydration of WO₃·H₂O. WO₃ transformations are examined by high-temperature X-ray diffraction. The kinetics of formation of the cubic modification have been studied by measuring the weight decrease with a thermobalcnce. Formation isotherms can be interpreted in terms of the first-order equation –In (1–f)=kt; activation energies are 110 and 80 kJ mol⁻¹ for initial and final stages, respectively.     

Synthesis of Titanate Derivatives Using Ion-Exchange Reaction

Two types of titanate derivatives, layered hydrous titanium dioxide (H₂Ti₄O₉· n H₂O) and potassium octatitanate (K₂Ti₈O₁₇) with a tunnellike structure, were synthesized using an ion-exchange reaction. Fibrous potassium tetratitanate (K₂Ti₄O₉· n H₂O) was prepared by calcination of a mixture of K₂CO₃ and TiO₂ with a molar ratio of 2.8 at 1050°C for 3 h, followed by boiling-water treatment of the calcined products for 10 h. The material then was transformed to layered H₂Ti₄O₉· n H₂O through an exchange of K⁺ ions with H⁺ ions using HCl. K₂Ti₈O₁₇ was formed by a thermal treatment of KHTi₄O₉· n H₂O. Pure KHTi₄O₉· n H₂O phase was effectively produced by a treatment of K₂Ti₄O₉ with 0.005 M HCl solution for 30 min. Thermal treatment at 250°–500°C for 3 h resulted in formation of only K₂Ti₈O₁₇.     

Effects of Melt Chemistry on the Spectral Absorption Coefficient of Molten Aluminum Oxide

Values of the spectral absorption coefficient (α) of liquid aluminum oxide were determined by transmission of a pulsed dye laser beam incident on continuous-wave (CW) CO₂-laser-melted pendant drops attached to sapphire filaments. Measurements were made on molten drops of Verneuil sapphire at wavelengths of 0.450 and 0.633 μm, at ambient oxygen partial pressures from 10^-10 to 1 bar in eight pure gases (Ar, CO, CO₂, H₂, H₂O, HCI, N₂ and O₂), in CO/CO₂ mixtures, and in H₂/H₂O mixtures, and at a temperature of ca. 2400 K. Specimens contaminated with iron, magnesium, silicon, and tungsten were also investigated in an oxygen atmosphere. At a wavelength of 0.633 μm, the value of α was greater than 50 cm^-1 under reducing or inert gas conditions. It decreased to a minimum at intermediate oxygen partial pressures of 5 × 10^-5 bar in CO/CO₂ mixtures and 5 × 10^-3 bar in H₂/H₂O mixtures, and increased at larger oxygen partial pressures. The specimens were opaque (α > 55 cm^-1) in hydrogen, in HCI at pressures above 0.04 bar. Specimens contaminated with 5000-10000 ppm of Fe, Mg, Si, or W were also opaque. At a wavelength of 0.45 μm the liquid aluminum oxide specimens were opaque in Ar and oxygen, and gave α= 46 cm^-1 in CO₂. The dynamic response when the ambient gas was changed from CO₂ to argon showed that the transmission maximum for = 0.45 μm was at p (O₂) < 0.1 bar.     

Reduction of Ions in Glass by Hydrogen

The reduction by hydrogen of Fe³⁺, Ce⁴⁺, and Sn⁴⁺ in soda-lime-silica glass was found to be diffusion-limited in the glass transition temperature range (500°C). The reduction was studied in fibers by chemical analysis and in bulk samples by optical absorption. Reduction to Fe²⁺, Ce³⁺, and Sn²⁺ proceeds by a reaction of the type H₂+2(ɛSi-O)^-+2Fe³⁺→2Fe²⁺+2(ɛSi–OH). Since the rate of reduction by hydrogen is quite similar for these systems and since reduction cannot be accomplished with CO, it is concluded that hydrogen is the diffusing species. A mechanism is developed in which hydrogen diffuses through the glass until it is trapped by a reducible ion.     

Phase Transformation in Barium Orthoferrate, BaFeO3-x

The compound BaFeO_{3- x} exists in many forms, the hexagonal phase having a wide range in oxygen content (BaFeO_2.63–2.92). The other phases have the limited compositions and the distinct structures of perovskite: triclinic I, BaFeO_2.50; triclinic II, BaFeO_2.64–2.67; rhombohedral I and II, BaFeO_2.62–2.64; and tetragonal, BaFeO_2.75–2.81. The phases contain Fe⁴⁺ ions correlating with the oxygen content. The hexagonal phase shows a continuous change in oxygen content with temperature down to BaFeO_2.63. The perovskitelike phases, however, show characteristic transformations. The triclinic I phase oxidizes to the triclinic II form at 320° to 500°C and to the hexagonal form at 720° to 915°C in oxygen. These transformations are related to oxidation-reduction of iron ions (Fe³⁺⇌ Fe⁴⁺).     

Waveguides in Lithium Niobate

Lithium niobate (LiNbO₃ is considered to be the leading electrooptical material for fabrication of active waveguides, modulators, and switches for application in integrated optical circuits. LiNbO₃ is a ferroelectric material with a high Curie temperature of 1210°C. This is essential to permit rapid diffusion at high temperature, e.g., >1000°C, without domain reversal. Its high electrooptic coefficient allows modulation at relatively low voltages. Waveguides are fabricated in LiNbO₃ by Ti indiffusion around 1000°C, by Li₂O outdiffusion, and, more recently, by Li⁺/H⁺ ion exchange. We report results of a study on diluted Li⁺/H⁺ ion exchange. This process is attractive because it allows control of the index change between 0.003 and 0.1 Furthermore, the fabricated waveguides are polarization selective, have high optical damage resistance, and do not exhibit index instability as is the case with complete Li⁺/H⁺ exchange. The variation in both the index of refraction and the diffusion coefficient with composition is very nonlinear. Mechanisms contributing to these nonlinear properties are discussed.     

Synthesis and Characterization of (Ce0.67Tb0.33)MnxMg1−xAl11O19 Phosphors Derived by Sol–Gel Processing

A (Ce_0.67Tb_0.33)Mn _x Mg_{1− x} Al₁₁O₁₉ phosphor powder was synthesized, using a simple sol–gel process, by mixing citric acid with CeO₂, Tb₄O₇, Al(NO₃)₃·9H₂O, Mg(OH)₂·4MgCO₃·6H₂O, and Mn(CH₃COO)₂. The phosphor crystallized completely at 1200°C, and the phosphor particle size was between 1 and 5 μm. The excitation spectrum was characteristic of Ce³⁺, while the emission spectrum was composed of lines from Tb³⁺ and Mn²⁺. The Mn²⁺ gave a green fluorescence band, and concentration quenching occurred when x > 0.10. The luminescent properties of the phosphor were explained by a configurational coordinate model.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.