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.     

Solubility Limit of MgO in Al2O3 at 1600°C

The solubility of Mg in alumina was measured using wavelength-dispersive spectroscopy mounted on a scanning electron microscope. Careful calibration of the microscope's working conditions was performed in order to optimize the detection limit and accuracy. Measurements were conducted on water-quenched and furnace-cooled samples, without any thermal or chemical etching to avoid alteration of the bulk concentration. The results indicate the solubility limit of Mg in alumina to be 132±11 ppm at 1600°C.     

Activity–Composition Relations in FeCr2O4–FeAl2O4 and MnCr2O4–MnAl2O4 Solid Solutions at 1500° and 1600°C

Activity–composition relations of FeCr₂O₄–FeAl₂O₄ and MnCr₂O₄–MnAl₂O₄ solid solutions were derived from activity–composition relations of Cr₂O₃–Al₂O₃ solid solutions and directions of conjugation lines between coexisting spinel and sesquioxide phases in the systems FeO–Cr₂O₃–Al₂O₃ and MnO–Cr₂O₃–Al₂O₃. Moderate positive deviations from ideality were observed.     

Air Oxidation of UO2 Fuel Fragments at 175° to 400°C

The air oxidation at 175° to 400°C Of unirradiated and irradiated UO₂ fuel fragments was characterized by apparent activation energies of 140 ± 10 and 120 ± 10 kJ/mol, respectively. There was a small enhancement in the rate of weight gain for irradiated fragments compared with unirradiated fragments up to ∼300°C; from 300° to 400°C there was no significant difference in oxidation rate. Air availability had a strong effect on rate of weight change above 300°C.     

Ractions of SiC with H2/H2O/Ar Mixtures at 1300°C

The reactions of a sintered α-SiC with 5% H₂/H₂O/Ar at 1300°C were studied. Thermomchemical modeling indicates that three reaction regions are expected, depending on the initial water vapor or equivalently oxygen content of the gas stream. A high oxygen content ( P (O₂) > 10⁻²² atm) leads to a SiO₂ formation. This generally forms as a protective film and limits consumption of the SiC (passive oxidation). An intermediate oxygen content (10⁻²² atm > P (O₂) > 10⁻²⁶ atm) leads to SiO and CO formation. These gaseous products can lead to rapid consumption of the SiC (active oxidation). Thermogravimetric studies in this intermediate region gave reaction rates which appear to be controlled by H₂O gas-phase transport to the sample and reacted microstructures showed extensive grain-boundary attack in this region. Finally, a very low oxygen content ( P (O₂) < 10⁻²⁶ atm) is thermochemically predicted to lead to selective removal of carbon and formation of free silicon. Experimentally low weight losses and iron silicides are observed in this region. The iron silicides are attributed to reaction of free silicon and iron impurities in the system.     

Oxidation of CVD Si3N4 at 1550° to 1650°C

A thermo gravimetric study of the oxidation behavior of chemically vapor-deposited amorphous and crystalline Si₃N₄ (CVD Si₃N₄) was made in dry oxygen (0.1 MPa) at 1550° to 1650°C. The specimens were prepared under various deposition conditions using a mixture of SiCl₄, NH₃, and H₂ gases. The crystalline CVD Si₃N₄ indicated a parabolic oxidation kinetics over the whole temperature range, whereas the amorphous CVD Si₃N₄ changed from a parabolic to a linear law with increased temperature. The oxidation mechanism is discussed in terms of the activation energy for the oxidation and the microstructure of the formed oxide films.     

Strength Retention in Hot-Pressed Si3N4 Ceramics with Lu2O3 Additives after Oxidation Exposure in Air at 1500°C

The effect of oxidation exposure on room-temperature flexural strength was examined in 3.33- and 12.51-wt%-Lu₂O₃-containing hot-pressed Si₃N₄ ceramics exposed to air at 1500°C for up to 1000 h. After oxidation exposure, the room-temperature strength of the ceramics was degraded, and strength retention decreased with time at temperature, dependent on the amount of additive. The retention in room-temperature strength displayed by the two compositions after 1000 h of oxidation exposure was 75%–80%. The degradation in strength was attributed to the formation of new defects at and/or near the interface between the oxide layer and the Si₃N₄ bulk during oxidation exposure.     

Oxidation Behavior of Boron-Modified Mo5Si3 at 800°–1300°C

Mo₅Si₃ shows promise as a high-temperature creep-resistant material. The high-temperature oxidation resistance of Mo₅Si₃ has been found to be poor, however, limiting its use in oxidizing atmospheres. Undoped Mo₅Si₃ exhibits pest oxidation at 800°C. Mass loss occurs in the temperature range 900°–1200°C due to volatilization of molybdenum oxide, indicating that the silica scale that forms does not provide a passivating layer. The addition of boron results in protective scale formation and parabolic oxidation kinetics in the temperature range of 1050°–1300°C. The oxidation rate of Mo₅Si₃ was decreased by 5 orders of magnitude at 1200°C by doping with less than 2 wt% boron. Boron doping eliminates catastrophic pest oxidation at 800°C. The mechanism for improved oxidation resistance of borondoped Mo₅Si₃ is viscous sintering of the scale to close pores that form during the initial transient oxidation period, due to volatilization of molybdenum oxide.     

Kinetics of the Reaction of UC2 and Nitrogen from 1500° to 1700°C

The kinetics of the reaction of UC₂ spheres with nitrogen was studied from 1500° to 1700°C. A metallographic method was used to determine the time-dependent conversion of UC₂ to U(C,N) and free C. The conversion appeared to be controlled by the diffusion of solid carbon in solution to sites where it could precipitate as free carbon. These sites were the surface of the sphere and particles of free carbon that existed within the original UC₂. An increased distribution of these internal sites decreased the distance for carbon diffusion and resulted in an increased rate of reaction. The UC₂ appeared to undergo a very rapid initial reaction that resulted in the uptake of 1 to 5 at.% nitrogen in the UC₂ at these temperatures.     

Oxidation State of Chromium in CaO–Al2O3–CrOx–SiO2 Melts under Strongly Reducing Conditions at 1500°C

Equilibrium ratios Cr²⁺/Cr³⁺ of chromium oxide dissolved in CaO–chromium oxide–Al₂O₃–SiO₂ melts have been determined by analysis of samples equilibrated at 1500°C under strongly reducing conditions ( p _o2= 10^−9.56 to 10^−12.50 atm). The majority of the chromium is divalent (Cr²⁺) under these conditions and Cr²⁺/Cr³⁺ ratios at given constant oxygen pressures decrease with increasing basicity of the melts, expressed as CaO/SiO₂ ratios. In addition, Cr²⁺/Cr³⁺ ratios, at a given CaO/SiO₂ ratio, are relatively unaffected by the amount of Al₂O₃ present.     

R-Curve Behavior and Stable Crack Growth at Elevated Temperature (1500°–1650°C) in a Si3N4/SiC Nanocomposite

The crack growth resistance behavior and the stable crack growth regime of a Si₃N₄/SiC composite have been examined at high temperature (1500°–1650°C). SENB specimens were used and the load/unloading technique, with high deflection rates to ensure an elastic behavior, has been employed to estimate the crack lengths. Rising R -curves have been obtained with a maximum crack growth resistance almost twice as high as the initial value. Above the T _g of the intergranular glassy phase, the behavior changes from brittle to visco-plastic and, consequently, the fracture characteristics become strongly rate dependent. It is observed experimentally that in the enhanced ductile region the crack extension velocity during the stable crack propagation from a preexisting flaw decreases rapidly with time. This phenomenon has been tentatively attributed to dynamic crack-tip stress relaxation resulting from the rapid flow of the glassy intergranular phase in the process zone. Thus, the rheological properties of the composite appear to be of major importance to gain insight into the mechanical behavior at such elevated temperatures.     

Fabrication and Properties of Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films near the morphotropic phase boundary were successfully fabricated on Pt(111)/Ti/SiO₂/Si substrates via an aqueous sol–gel method. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 50 nm. The dielectric, ferroelectric, and piezoelectric properties of the sol–gel-derived BSPT thin films were investigated. A remanent polarization of 74 μC/cm² and a coercive field of 177 kV/cm were obtained. The local effective piezoelectric coefficient d ^*₃₃ was 23 pC/N at 2 V, measured by a scanning probe microscopy system. The dielectric peak appeared at 435°C, which was 80°C higher than that of Pb(Ti, Zr)O₃ thin films.     

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.     

Orientation Control in Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films were fabricated via a sol–gel method on Pt(111)/Ti/SiO₂/Si(111) substrates. The effects of different factors on the orientation of the sol–gel-derived BSPT thin films were investigated. The results showed that a higher lead excess concentration, longer drying time, higher pyrolysis temperature, longer pyrolysis time, higher crystallization temperature, and longer crystallization time could enhance the (100) orientation of the BSPT thin films. Based on the experimental results, a mechanism for the orientation evolution in the sol–gel-derived BSPT thin films was proposed. The production of the (100) orientation was attributed to the (100)-oriented PbO nanocrystals forming during the pyrolysis process due to the lattice match.     

Determination of 1600° and 1700°C. Liquidus Lines in CaO-2Al2O3 and AI2O3 Stability Fields of the System CaO—Al2O3—SiO2

The 1600° and 1700°C. liquidus lines in the CaO·2Al₂O₃ and A1₂O₃ stability fields of the system CaO-Al₂O₃-SiO₂ are determined from the chemical analyses of saturated slags at these temperatures.     

Active Oxidation of SiC-Based Ceramics in Ar–2% Cl2-O2 Gas Mixtures at 1000°C

The corrosion in Ar–2% CI₂ gas mixtures of four low-cost SiC-based materials suitable for use in high-temperature heat exchangers has been invesigated. The oxygen potential was controlled by addition of O₂ or H₂ at 1000°C. Little attack was observed in the reducing environment composed of Ar–2% Cl₂–1% H₂ or the oxidizing environment composed of Ar–2% Cl₂–20% O₂ but all of the materials were subject to active corrosion at intermediate oxygen potentials. Selective attack of the free silicon phase was obsrved for the siliconized silicon carbide materials. The severity of the active oxidation and the oxygen potentital at which the corrosion changed from active to passive were affected by the nature of the sintering aids used in the materials     

The System HfO2-TiO2

The system HfO₂-TiO₂ was studied in the 0 to 50 mol% TiO₂ region using X-ray diffraction and thermal analysis. The monoclinic ( M ) ⇌ tetragonal ( T ) phase transition of HfO₂ was found at 1750°± 20°C. The definite compound HfTiO₄ melts incongruently at 1980°± 10°C, 53 mol% TiO₂. A metatectic at 2300°± 20°C, 35 mol% TiO₂ was observed. The eutectoid decomposition of HfO_2,ss) ( T ) → HfO_2,ss ( M ) + HfTiO_34,ssss occurred at 1570°± 20°C and 22.5 mol% TiO₂. The maximum solubility of TiO₂ in HfO_2,ss,( M ) is 10 mol% at 1570°± 20°C and in HfO_2,ss ( T ) is 30 mol% at 1980°± 10°C. On the HfO₂-rich side and in the 10 to 30 mol% TiO₂ range a second monoclinic phase M of HfO₂( M ) type was observed for samples cooled after a melting or an annealing above 1600°C. The phase relations of the complete phase diagram are given, using the data of Schevchenko et al. for the 50% to 100% TiO₂ region, which are based on thermal analysis techniques.     

Kinetics of the UO2-C-N2 Reaction at 1700°C

The mechanism of the reaction of UO₂ with carbon in the presence of N₂ at 1700°C and the rate of formation of the carbonitride product were determined. Uranium carbonitride forms at specific O₂ and N₂ chemical potentials by reactions such as (1) UO₂( s ) + 0.67HCN( g )→UO_1.33N_0.45( s ) + 0.67CO( g ) + 0.11N₂( g ) + 0.335H₂( g ) and (2) UO_1.33N_0.45( s ) + 1.58HCN( g )→UO_0.25N_0.75( s ) + 1.33CO( g ) + 0.79H₂( g ) + 0.64N₂( g ). At P _H2=2×10^-5 atm, HCN formed, permitting a gas-phase transport of reactions not observed in the UO₂-C reaction. Reaction (1) is completed in 0.01 to 0.1 of the time for complete conversion to carbonitride; reaction (2), which proceeds as soon as oxynitride is available, is controlled by solid-state diffusion across the carbonitride layers after they become continuous on the entire specimen. The reaction rates and product compositions depend on the P _N2 and P_CO in the system.     

Structural and Electric Properties of Sol–Gel-Derived Ba0.9Sr0.1TiO3 Multilayers

Crack-free Ba_0.9Sr_0.1TiO₃ (BST) and Mn-doped Ba_0.9Sr_0.1TiO₃ (BSTM) multilayers with thickness over 2 μm have been prepared by chemical solution deposition based on one single precursor. Both multilayers exhibit good performance as Bragg reflectors. Mn doping tends to suppress the leakage current in BST multilayers effectively by smoothing the layers and the reduction of the charge carries. The Mn-doped BST multilayer displays an excellent ferroelectric property, with an average remnant polarization ( P _r⁺– P _r⁻)/2 of 12.69 μC/cm² and an average coercive field ( E ⁺ – E ⁻ )/2 of about 72.95 kV/cm under an applied field of 440 kV/cm.     

Homogeneity Region of Strontium- and Magnesium-Containing LaGaO3 at Temperatures between 1100° and 1500°C in Air

Phase stability studies were performed within the quasi-ternary system LaGaO₃-SrGaO_2.5-"LaMgO_2.5". Emphasis was cast on the temperature dependence of the homogeneity region of La_{1− x} Sr _x Ga_{1− y} Mg _y O_3−δ perovskite solid solutions. Isothermal sections were determined at 1100°, 1250°, 1400°, and 1500°C in a static air atmosphere. The single-phase homogeneity region was found to considerably diminish with decreasing temperature, indicating a reduction of the solid solubility of Sr and Mg, and below 1100°C the doped perovskite becomes unstable. Consequently, the cubic perovskite phase was found to exist only at elevated temperatures and for high Sr and Mg amounts. Sample preparation was performed by the mixed-oxide process as well as by a modified combustion synthesis.