Creep mechanisms and microstructure evolution of Nextel™ 610 fiber in air and steam

Creep rates of Nextel™ 610 alumina fibers were measured at 1100 °C and 100–500 MPa in air and steam. Steam increased creep rates and reduced fiber lifetimes. Fiber microstructures were characterized by TEM. The small amounts of grain growth, fiber-axis grain elongation, and pore growth that occur during creep were quantified. To separate the effects of stress and temperature on microstructural evolution, grain growth and elongation were also quantified for fibers heat-treated for 1–100 h in air at 1100–1500 °C. Grain growth laws were determined. The contributions of pore growth and grain elongation to creep strain were quantified. Grain elongation accounts for a large fraction of the strain during creep in air, but little in steam. Pore growth was more pronounced in steam, but does not create significant creep strain. Creep and failure mechanisms consistent with the observed microstructural changes are discussed.     

Study of the interactions between HfB2 and Hi-Nicalon™ fiber

This work aims at studying the interactions between Hi-Nicalon SiC fiber and HfB₂ in presence of various sintering additive, such as Si₃N₄ or ZrSi₂. Spark plasma sintering was used as suitable technique to interrupt the sintering at intermediate temperatures and thus analyze the microstructure evolution at the various densification stages. SEM investigations enabled to disclose the interaction mechanisms not only among fiber and matrix, but also revealed the curious mechanisms of formation of core–shell structures in HfB₂ grains when ZrSi₂ was employed.     

Mechanical behavior of zirconium diboride–silicon carbide ceramics at elevated temperature in air

The mechanical properties of zirconium diboride–silicon carbide (ZrB₂–SiC) ceramics were characterized from room temperature up to 1600 °C in air. ZrB₂ containing nominally 30 vol% SiC was hot pressed to full density at 1950 °C using B₄C as a sintering aid. After hot pressing, the composition was determined to be 68.5 vol% ZrB₂, 29.5 vol% SiC, and 2.0 vol% B₄C using image analysis. The average ZrB₂ grain size was 1.9 μm. The average SiC particles size was 1.2 μm, but the SiC particles formed larger clusters. The room temperature flexural strength was 680 MPa and strength increased to 750 MPa at 800 °C. Strength decreased to ～360 MPa at 1500 °C and 1600 °C. The elastic modulus at room temperature was 510 GPa. Modulus decreased nearly linearly with temperature to 210 GPa at 1500 °C, with a more rapid decrease to 110 GPa at 1600 °C. The fracture toughness was 3.6 MPa·m^½ at room temperature, increased to 4.8 MPa·m^½ at 800 °C, and then decreased linearly to 3.3 MPa·m^½ at 1600 °C. The strength was controlled by the SiC cluster size up to 1000 °C, and oxidation damage above 1200 °C.     

Creep properties of melt infiltrated SiC/SiC composites with Sylramic™-iBN and Hi-Nicalon™-S fibers

Isothermal tensile creep tests were conducted on 2D woven and laminated, 0/90 balanced melt infiltration (MI) SiC/SiC composites at stress levels from 48 to 138 MPa and temperatures to 1400°C in air. Effects of fiber architecture and fiber types on creep properties, influence of accumulated creep strain on in-plane tensile properties, and the dominant constituent controlling the creep behavior and creep rupture properties of these composites were investigated. In addition, the creep parameters of both composites were determined. Results indicate that in 2D woven MI SiC/SiC composites with Sylramic™-iBN or Hi-Nicalon™-S fibers, creep is controlled by chemical vapor infiltration (CVI) SiC matrix, whereas in 2D laminated MI SiC/SiC composites with Hi-Nicalon™-S fibers, creep is controlled by the fiber. Both types of composites exhibit significant variation in creep behavior and rupture life at a constant temperature and stress, predominantly due to local variation in microstructural inhomogeneity and stress raisers. In both types of composites at temperatures >1350°C, residual silicon present in SiC matrix to reacts with SiC fibers and fiber coating causing premature creep rupture. Using the creep parameters generated, the creep behaviors of the composites have been modeled and factors influencing creep durability are discussed.     

Thermal stability of CVI and MI SiC/SiC composites with Hi-Nicalon™-S fibers

The influence of high-temperature argon heat-treatment on the microstructure and room- temperature in-plane tensile properties of 2D woven CVI and 2D unidirectional MI SiC/SiC composites with Hi-Nicalon?-S SiC fibers was investigated. The 2D woven CVI SiC/SiC composites were heat-treated between 1200 and 1600 °C for 1- and 100-hr, and the 2D unidirectional MI SiC/SiC composites between 1315 and 1400 °C for up to 2000 hr. In addition, the influence of temperature on fast fracture tensile strengths of these composites was also measured in air. Both composites exhibited different degradation behaviors. In 2D woven CVI SiC/SiC composites, the CVI BN interface coating reacted with Hi-Nicalon?-S SiC fibers causing a loss in fast fracture ultimate tensile strengths between 1200 and 1600 °C as well as after 100-hr isothermal heat treatment at temperatures > 1100 °C. In contrast, 2D unidirectional MI SiC/SiC composites showed no significant loss in in-plane tensile properties after the fast fracture tensile tests at 1315 °C. However, after isothermal exposure conditions from 1315° to 1400°C, the in-plane proportional limit stress decreased, and the ultimate tensile fracture strain increased with an increase in exposure time. The mechanisms of strength degradation in both composites are discussed.     

Creep of a Nextel™720/alumina ceramic composite containing an array of small holes at 1200°C in air and in steam

Tensile stress-strain and tensile creep behaviors of an oxide-oxide composite containing an array of small circular holes were evaluated at 1200°C. The composite consists of Nextel™720 alumina-mullite fibers in a porous alumina matrix. Test specimens contained an array of 17 holes with 0.5-mm diameter drilled using a CO₂ laser. The presence of holes caused reduction in tensile strength and modulus. Tensile creep tests were conducted at 1200°C in air and in steam at creep stresses ranging from 38 to 140 MPa. Primary, secondary, and tertiary creep regimes were noted in air and in steam. The presence of the laser-drilled holes accelerates the steady-state creep rates. Creep run-out, defined as 100 hours at creep stress, was attained for stress levels <60 MPa in air and for stresses <40 MPa in steam. The presence of the laser-drilled holes significantly degrades creep resistance of the composite. The retained tensile properties of all specimens that attained run-out were determined. Composite microstructure was examined; the damage and failure mechanisms were considered. The degradation of tensile properties and creep resistance are attributed to damage caused to composite microstructure by laser drilling.     

Microstructure and mechanical performance of SiCf/BN/SiC mini-composites oxidized at elevated temperature from ambient temperature to 1500 °C in air

SiC_f/BN/SiC mini-composites comprising single tow SiC fibre-reinforced SiC with chemical vapour deposited (CVD) BN interface layers were fabricated. The mechanical performance and binding strength of the composites and the fibre/interface for the oxidized SiC_f/BN/SiC mini-composite samples (oxidation at 1000, 1200, 1300, 1400 and 1500 °C in air for two hours) were investigated by tensile tests and fibre push-out tests, respectively. The value of oxidation mass change was also measured. Some unusual phenomena for the SiC_f/BN/SiC mini-composites oxidized at 1000 °C were discovered in this work. The tensile strength reached a maximum value, and the mass gain rate showed as a singular negative value, while the shear strength between the fibre and the matrix was moderate. Scanning electron microscopy, energy dispersive spectrometry, infrared spectroscopy and X-ray diffraction characterization methods were used to reveal the microstructural evolution and investigate the unusual phenomenon during oxidation procedures. This work will provide guidance for predicting the service life of SiC_f/BN/SiC composite materials and may enable these materials to become a backbone for thermal structure systems in aerospace applications.     

Mechanical and microwave absorbing properties of Tyranno® ZMI fiber annealed at elevated temperatures

The tensile strength and microwave absorbing properties of the amorphous silicon carbide fiber (Tyranno-ZMI) annealed at different temperatures were studied. The tensile strength of the as-received ZMI fiber tows was 1.1 GPa; and the average real and imaginary parts of permittivities of the as-received ZMI/resin samples were 11.3 and 10.5 respectively. The major dielectric loss mechanism of the fibers was conduction loss, which was due to high electrical conductivity of the enriched carbon in ZMI fibers. The 2.0 mm thick ZMI/resin composites could absorb 80% microwave energy in X band, indicating good microwave absorbing property. After heat treatment, fibers degraded gradually and permittivities increased, which were mainly attributed to the decomposition of amorphous SiC_xO_y and the growth of the SiC nanocrystals and free carbon nanodomains.     

Fracture behavior of a melt-infiltration-processed SiC/Si composite at 900 °C in argon,air, and steam

The fracture behavior of a melt-infiltration-processed SiC/Si composite, used to mimic the matrices of industrial fiber-reinforced ceramic composites, was examined in different atmospheres and temperatures. Specimens tested in four-point bending at 900 °C in oxygen-gettered argon, dry air, or steam-rich atmospheres exhibited higher average fracture strengths than specimens tested at 25 °C. Higher mean fracture strength values were obtained for specimens tested in dry air or in a steam-rich atmosphere at 900 °C than for specimens tested in high-purity, oxygen-gettered argon at this temperature. The increased fracture strengths obtained in air and in steam-rich atmospheres coincided with increased specimen oxidation and apparent oxide filling and blunting of flaws in these composites. A transition in the location of catastrophic failure, from sites of preexisting damage created by Vickers indentations for tests in argon to other locations for tests in air or steam-rich atmospheres, was also consistent with such apparent oxide filling/blunting of indentation-induced flaws.     

Strength and deformation of concrete with variable content of moisture at elevated temperature up to 90°C

It is known, that the change of mechanical properties of concrete due to elevated temperature is also influenced by the moisture content. This change was primarily studied for prestressed concrete reactor vessels (PCRV). Because a PCRV is a mass concrete structure, the results of this research cannot be transfered to slender members. To simulate drying conditions of the latter, specimens of differing initial moisture content were subject to elevated temperatures and defined climates. The results of tests reveal the differing influence of moisture on strength and modulus of elasticity. The compressive strength is partly increased, partly decreased as the moisture content grows. Tensile strength and modulus of elasticity are weakened by decreasing moisture. Also the thermal strain as function of type of aggregate and moisture content was studied. The changes are caused by the alteration of structure of cement stone and by microcracks due to incompatibility.     

Interaction of ceria and erbia in air within temperature range 1500–600 °C

Phase equilibriums and structure transformations in the CeO₂–Er₂O₃ system have been studied within 1500–600 °C in the full concentration range using XRD and petrography methods. It was established that the system is characterized by the formation of solid solutions on the basis of cubic modification of Er₂O₃ (C-type) and fluorite CeO₂ (F-type). Solubility limits and concentration dependences of lattice parameters were determined for the phases forming in the system.     

Interaction of ceria and ytterbia in air within temperature range 1500 – 600 °C

Phase equilibria and structure transformations in the CeO₂–Yb₂O₃ system have been studied in air within the temperature range 1500 - 600 °C in the full concentration range using X-ray diffraction analysis (XRD) and petrography methods. It was established that the system is characterized by the formation of solid solutions on the basis of cubic modification of Yb₂O₃ (C- type) and fluorite CeO₂ (F- type) separated by two-phase (F + C) region. The systematic study that covered whole composition range excluded formation of new phases. Solubility limits and concentration dependences of lattice parameters were determined for the phases forming in the system.     

Hydrodynamics and simulation of air–water homogeneous bubble column under elevated pressure

A gas–liquid Eulerian computational fluid dynamics (CFD) model coupled with a population balance equation (PBE) was presented to investigate hydrodynamics of an air–water bubble column (1.8 m in height and 0.1 m in inner diameter) under elevated pressure in terms of pressure drop, gas holdup, mean bubble size, and bubble surface area. The CFD-PBE model was modified with three pressure correction factors to predict both the total gas holdup and the mean bubble size in the homogeneous bubbly flow regime. The three correction factors were optimized compared to experimental data. Increasing the pressure led to increasing the density, reducing the bubble size, and increasing the gas holdup. The bubble size distribution moved toward a smaller bubble size, as the pressure increased. The modified CFD-PBE model validated with experimental data and empirical models represented well hydrodynamics of the bubble column at P = 0.1, 1.5, and 3.5 MPa.     

Hydrodynamics of air–kerosene bubble column under elevated pressure in homogeneous flow regime

A multiphase computational fluid dynamics(CFD) model coupled with the population balance equation(PBE) was developed in a homogeneous air–kerosene bubble column under elevated pressure(P). The specific pressure drop(DP/L), gas holdup(a_G), and Sauter mean diameter(d_(32)) were experimentally measured in the bubble column with 1.8 m height and 0.1 m inner diameter, which was operated at a superficial gas velocity of 12.3 mm·s~(-1), and P = 1–35 bar(1 bar = 10~5 Pa). A modified drag coefficient model was proposed to consider the effect of bubble swarm and pressure on hydrodynamics of the bubble column.The Luo breakage model was modified to account for liquid density, viscosity, surface tension and gas density. The DP/L, a_G, and d_(32) obtained from the CFD model were compared with experimental data,and the gas density-dependent parameters of the CFD model were identified. With increasing P from 1 to 35 bar, the aGvaried from 5.4% to 7.2% and the d_(32) decreased from 2.3 to 1.5 mm. The CFD-PBE model is applicable to predict hydrodynamics of pressurized bubble columns for gas–organic liquid in the homogeneous regime.     

A survey of platinum hydrous oxide electrochemistry at elevated temperature: evidence for a new component in the β-deposit

Hydrous (or ) oxide. films produced on platinum in acid at room temperature usually give rise to two reduction peaks, in addition to the monolayer oxide response, on a negative sweep extending to about 0V. This has been attributed to the presence of two hydrous oxide components (designated as HO1 and HO2) in the film. It was confirmed in the present work that under more severe conditions, 3.0 m H₂SO₄ at 60°C, an additional, quite significant, peak — apparently due to the presence of a further hydrous oxide component (HO3) — may be observed. The approximate peak maximum potential values (RHE scale) were 0.4 V (HO1), 0.2 V (HO2) and 0.0 V (HO3); the E _p values are given here only as a guide: the processes involved occur under very irreversible conditions and the values vary with factors such as sweep rate, film thickness, temperature, etc. The differences in behaviour between these components is assumed to be due to factors such as the degree of aggregation and compactness in different regions of the gel-type, amorphous deposit. A brief account is given of the increasing evidence from different laboratories of unusual responses for platinum electrodes in aqueous acid solution.     

Microstructure and mechanical properties improvement of the Nextel™ 610 fiber reinforced alumina composite

The alumina slurry with high solid content was prepared, and a rapid lamination route for fabricate the Nextel? 610 fiber reinforced alumina composite was proposed in this work. The microstructure and mechanical properties of the as-received all-oxide composite were investigated by a series of techniques. The shrinkage cracks in matrix were reduced, while porous structure in composite was maintained. The N610/alumina composite has weak matrix and weak interface, as the Young’s modulus of the alumina matrix and the interfacial shear strength of the composite are 140.8±2.5GPa and 129.1±14.6MPa. The mechanical properties of the composite are much higher than lots of oxide/oxide composites, given its flexural strength, interlaminar shear strength and the fracture toughness are 398.4±5.7MPa、27.0±0.5MPa and 14.1±0.9MPa·m^1/2, respectively. The flexural strength of the virgin composite keep stable at 25–1050 °C, while dramatically decrease at 1100–1200 °C.     

Compressive creep testing of refractories at elevated loads—Device,material law and evaluation techniques

In order to cost-effectively characterize the high temperature compressive creep behaviour of refractories a testing device was designed for application at elevated loads. Special measures have been taken necessary to enable an even stress distribution within the specimen. To identify Norton-Bailey strain hardening creep law parameters a general inverse procedure using a Levenberg–Marquardt algorithm was developed. Satisfying experimental results could be received from the creep measurement in a wide range of temperatures and loads for both shaped and unshaped materials. By fitting the strain/time curves the creep law parameters of refractories under various temperatures can be precisely identified. The measurements also reveal that at elevated loads all three creep stages can be observed.     

Char gasification in steam at 1123 K catalyzed by K,Na, Ca and Fe—effect of H2, H2S and COS

Gasification of unloaded and loaded Saran char was followed at 1123 K and 0.1 MPa total pressure in a N₂ or H₂ atmosphere to which ~ 3 kPa H₂O and, in some cases, ~ 500 ppm H₂S or COS were added. Potassium, Na and Ca are excellent catalysts for the gasification of Saran char by steam. Hydrogen inhibits catalysis by K, Na and Ca but promotes catalysis by Fe. The extent of the effect of H₂S (or COS) on inhibiting char gasification by steam depends upon whether it is added to wet N₂ or wet H₂. The inhibitory effect is much more marked in wet H₂ in all cases. Likewise, the time required to recover catalyst activity, following exposure to H₂S or COS, is much greater in the presence of wet H₂ than in the presence of wet N₂.     

Effect of oxidation on the bending fatigue behavior of an advanced SiC/SiC CMC component at 1000 °C in air

The present study elucidates the effect of oxidation during static and fatigue loading in SiC/SiC CMC structured component, which shows damage in the stress-concentrated region. It is made of Tyranno SA3 fiber, BN (Boron nitride) interphase, and CVI (chemical vapor infiltration) + PIP (polymer impregnation and pyrolysis) hybrid matrix. The comparison based on strength and fracture morphology was made. After annealing, the as-received sample showed minute oxidation and slightly enhanced strength. The fatigued sample without annealing under low stress showed higher retained strength than the as-received sample due to smooth debonding. The fatigued sample with annealing under high stress showed a loss in strength than the as-received sample owing to the formation of a significant amount of borosilicates glasses, which further promoted SiO₂ formation between fiber and matrix and caused the brittle failure. However, simultaneous filling borosilicate glasses into the pores oppositely aided in maintaining the retained strength.     

Change in reactivity of kerogen-containing rock samples in air at temperatures of 450–650 K

Experiments on studying chemical interaction of kerogen-containing rock samples with an atmospheric air flow are described. The crushed rock was placed into a flow reactor with a controlled heating temperature. The fact of rock interaction with oxygen contained in air was detected by a gas analyzer, based on changes in the relative concentrations of O₂ and CO₂ molecules in the gas flow at the reactor exit. The measurements are performed in the temperature range from 450 to 650 K. The lower limit of temperatures is found (480 K), at which the change in the gas composition is still observed. The change in the chemical activity of the rock bed is estimated under the assumption of a first-order reaction in terms of the oxidizer concentration; the activation energy of the process is found to be 12 kcal/mole.