Amorphous Silicoboron Carbonitride Ceramic with Very High Viscosity at Temperatures above 1500°C

Recently, the viscosity of a predominantly amorphous silicon carbonitride (Si_1.7C_1.0±0.1N_1.5) alloy with an apparent glass-transition temperature ( T _g) of 1400°–1500°C was studied. In this study, the creep behavior of silicoboron carbonitride (Si₂B_1.0C_3.4N_2.3), which seems to have a T _g value of >1700°C, was examined. Both materials exhibited a three-stage creep behavior. In stage I, the creep rate declined, because of densification. In stage II, the strain rate approaches a steady state. In stage III, it resumes a declining strain rate, which ultimately decreased below the measurement limit of the system. At 1550°C in stage II, the viscosity of silicoboron carbonitride was six orders of magnitude higher than that of fused silica. Among the Si-C-N ceramics, only chemical-vapor-deposited and reaction-bonded silicon carbides seem to have greater creep resistance than the silicoboron carbonitrides at temperatures >1550°C.     

Thermal Expansion of Various Ceramic Materials to 1500°C

Thermal-expansion data from room temperature to 1500°C. are given for a number of ceramic materials. Determinations were made with telemicroscopes on bars or rods 7 in. long     

Slow Crack Growth in Sapphire Fibers at 800° to 1500°C

Sapphire fibers with (near) c -axis orientation were tested in tension over a range of strain rates (10⁻⁵ to 0.5 min⁻¹) at elevated temperatures (800° to 1500°C). The strength of the fibers was dependent on the strain rate. Slow crack growth was confirmed as the degradation process by direct inspection of the fracture surfaces and estimation of fracture stresses from measured flaw sizes. The slow crack growth parameter, N , decreased with increasing temperature. At 1400°C a threshold in strength was observed; the threshold stress intensity factor was estimated to be ≅ 1 MP·m^1/2 at 1400°C. Thermally activated bond rupture (e.g., lattice trapping model) is postulated as the mechanism responsible for the slow crack growth.     

Synthetic Mica Investigations: III, Precision-Controlled Electric Furnaces for Temperatures to 1500°C

Three furnaces using silicon carbide heating elements are described, including (1) a utility furnace for melting and testing, (2) a precisioncontrolled (± 1/2°C.) crystallization furnace, and (3) a rectangular crystallization furnace. Schematic drawings of the furnaces and a simplified wiring diagram for the heating elements are included. One composite controller is described from a functional viewpoint, with a block-diagram presentation. A complete circuit diagram of this controller also is incorporated. The use of saturable core reactors in regulating power to electric furnaces is advocated; reactors permit the control of electricity in a manner similar to adjustment of the flow of water by means of a valve.     

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.     

Phase Equilibria in the System CaO-MgO-Iron Oxide at 1500°C

Phase relations in the system CaO-MgO-iron oxide were determined at 1500°C at two levels of oxygen pressure, 10^–9 atm and 0.2 atm (air). Particular attention was directed toward de termining the maximum amount of iron oxide which may be tolerated in CaO-MgO-iron oxide bodies before a liquid phase forms under equilib rium conditions at the two chosen oxygen pres sures. Inferences are made regarding the possible bearing of the results on the perform ance of tar-bonded dolomite brick used in steel making furnaces.     

Activity–Composition Relations in the Systems CaO–MnO and MgO–MnO at at 1500° and 1600°C

Activity–composition relations in CaO–MnO and MgO–MnO solid solutions were determined by equilibrating oxide solid solutions with platinum–manganese alloys under controlled oxygen partial pressures at 1500° and 1600°C. Both systems show considerable positive deviation from ideality.     

Reaction Between Titanium and Zirconia Powders During Sintering at 1500°C

Zirconia–titanium (ZrO₂–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y₂O₃ partially stabilized ZrO₂ in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as-sintered composites were characterized by X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO₂, resulting in the formation of α-Ti(O, Zr), Ti₂ZrO, and/or TiO. These oxygen-containing phases extracted oxygen ions from ZrO₂, whereby oxygen-deficient ZrO₂ was generated. For relatively small Ti/ZrO₂ ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO₂. For the specimens with ≥50 mol% Ti, lamellar Ti₂ZrO was precipitated in α-Ti(Zr, O), with no TiO being found. Both m -ZrO_{2− x} and t -ZrO_{2− x} were found in specimens with ≤50 mol% Ti; however, only c -ZrO_{2− x} was formed in the specimen with 70 mol% Ti. As ZrO₂ was gradually dissolved into Ti, yttria was retained in ZrO₂ because of the very limited solubility of yttria in α-Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO₂ increased with the Ti content. The complete dissolution of ZrO₂ into Ti was followed by the precipitation of Y₂Ti₂O₇ in the specimen with 90 mol% Ti.     

Superplastic Alumina at Temperatures below 1300°C Using Charge-Compensating Dopants

To achieve low-temperature superplasticity in alumina, we have introduced charge-compensating dopants, Ti⁴⁺ and Mn²⁺, which jointly have a high solubility and significantly enhance the diffusion and deformation processes during sintering and forming. Zirconia as a second-phase pinning agent has also been incorporated to impart microstructural stability against static and dynamic grain growth. The superplastic alumina obtained can be shape-formed under biaxial tension to 100% engineering strain at temperatures below 1300°C. Deformation characteristics of this alumina at temperatures from 1200° to 1400°C and at strain rates from 4 × 10^-6 to 3 × 10^-3/s are described. The origin of enhanced kinetics is attributed to the formation and dissociation of dopant-defect complexes.     

Glasses Resistant to Sodium Vapor at Temperatures to 700°C

Glasses resistant to attack by sodium vapor at temperatures up to 700°C were developed from the CaO-Al₂O₃-MgO-BaO-B₂O₃ system. The resistance of these glasses decreases sharply above the transformation temperature. The glasses have a high water content, have good wetting properties, and adhere well to materials such as aluminum oxide and niobium; thus they can be used as solder glasses and metalizing components. The surface layer formed on the glass during attack by sodium vapor has a specific electrical resistance which is lower by 2 to 3 orders of magnitude than that of glass not exposed to sodium vapor.     

Sintering of Alumina at Temperatures of 1400°C. and Below

Previous investigators have indicated that both small particles and the addition of certain oxides promote the sintering of alumina at temperatures below 1700°C. By utilizing combinations of oxides and small particle size, the sintering temperature of 96% alumina bodies was reduced in this investigation to the 1300° to 1400°C. range. It is proposed that this low-temperature sintering is aided by the formation of a liquid phase. Thin sections of the alumina sintered at low temperatures revealed bodies with small grain size whose bulk densities were above 3.80 gm. per cc.     

Activity–Composition Relations in Platinum–Chromium and Platinum–Vanadium Alloys at 1500°C

Activity–composition relations in Pt–Cr and Pt–V alloys at 1500°C were determined from equilibration of these alloys with Cr₂O₃ and V₂O₃, respectively, in Co₂–H₂ atmospheres of known oxygen partial pressures, followed by quenching to room temperature and microprobe analyses of the quenched alloys. The activities of Cr and V were calculated from known standard free energies of formation of Cr₂O₃ and V₂O₃ from the respective elements, and the oxygen pressures were calculated from the C–H–O equilibrium in the gas phase. Both alloy systems show strong negative deviations from Raoultian behavior.     

Oxidation of Zirconium Diboride–Silicon Carbide at 1500°C at a Low Partial Pressure of Oxygen

The oxidation behavior of zirconium diboride containing 30 vol% silicon carbide particulates was investigated under reducing conditions. A gas mixture of CO and ∼350 ppm CO₂ was used to produce an oxygen partial pressure of ∼10⁻¹⁰ Pa at 1500°C. The kinetics of the growth of the reaction layer were examined for reaction times of up to 8 h. Microstructures and chemistries of reaction layers were characterized using scanning electron microscopy and X-ray diffraction analysis. The kinetic measurements, the microstructure analysis, and a thermodynamic model indicate that oxidation in CO–CO₂ produced a non-protective oxide surface scale.     

Oxidation behavior of oxidation protective coatings for PIP-C/SiC composites at 1500 °C

Three-dimensional carbon fiber reinforced silicon carbide (C/SiC) composites were fabricated by precursor infiltration and pyrolysis (PIP) with polycarbosilane as the matrix precursor, SiC coating prepared by chemical vapor deposition (CVD) and ZrB₂-SiC/SiC coating prepared by CVD with slurry painting were applied on C/SiC composites, respectively. The oxidation of three samples at 1500 °C was compared and their microstructures and mechanical properties were investigated. The results show that the C/SiC without coating is distorted quickly. The mass loss of SiC coating coated sample is 4.6% after 2 h oxidation and the sample with ZrB₂-SiC/SiC multilayer coating only has 0.4% mass loss even after oxidation. ZrB₂-SiC/SiC multilayer coating can provide longtime protection for C/SiC composites. The mode of the fracture behavior of C/SiC composites was also changed. When with coating, the fracture mode of C/SiC composites became brittle. When after oxidation, the fracture mode of C/SiC composites without and with coating also became brittle.     

Microhardness of Mullite at Temperatures to 1000°

The microhardness of mullite single crystals was measured at temperatures to 1000°C. The hardness decreased from about 15 GPa at room temperature to about 10 GPa at 1000°C. Thus, the mullite at 1000°C had a higher hardness than sapphire, approaching the value of SiC, but at room temperature was softer than both materials.     

Mullitization from a Multicomponent Oxide System in the Temperature Range 1200°–1500°C

Mullitization from a multicomponent oxide system (alumina–kaolin–quartz–feldspar–talc) was analyzed as a function of firing temperature from 1200° to 1500°C based on quantitative XRD and SEM. In the present study, whisker forms of mullite grew in three characteristic stages. In the first stage (1255°–1295°C), mullitization (nucleation) took place from glass via alumina dissolution into glass under the condition of no apparent change in glass content. The reaction in this stage was rate-limited by alumina dissolution into glass. Extensive mullitization occurred in the 1295°–1335°C range (second stage) directly from glass. Unlike in the sol–gel-based binary system, alumina dissolution into glass was not shown to be the rate-controlling mechanism during this extensive mullitization stage. Finally (>1335°C, third stage), the reaction was saturated, accompanied by an apparent decrease in glass consumption rate. The impingement of mullite whiskers by other whiskers and crystals was speculated to cause mullite to grow in the transverse direction, yielding a diminished reaction rate in the final stage. Mullitization stages in this work were compared with those of the alumina–silica binary system shown in the literature.     

Activity–Composition Relations of Chromium Oxide in Silicate Melts at 1500°C under Strongly Reducing Conditions

The solubility and activity–composition relations of chromium oxide in melts of the systems CaO–CrO _x –SiO₂ and CaO–Al₂O₃–CrO _x –SiO₂ have been determined at 1500°C by equilibrating melts with Pt–Cr alloys at known oxygen pressures. It is shown that the increase in the concentration of divalent chromium ions, as the oxygen pressure and the basicity of melt decrease, results in a dramatic increase in the solubility of chromium oxide in the liquid phase. An increase in the Al₂O₃ content of the melt leads to a decrease in the solubility of chromium oxide over the whole composition range studied. The activity coefficient of CrO has been found to increase with increasing melt basicity and decreasing oxygen pressure whereas the activity coefficient of CrO_1.5 decreases sharply with increasing melt basicity for siliceous melts but levels off at a basicity ratio (wt% CaO/wt% SiO₂) of about 0.7. An increase in the Al₂O₃ content of the melts results in an increase in the activity coefficient of CrO.