Strength and thermal shock resistance of fiber-bonded Si-Al-C-O and Si-Ti-C-O ceramics

Silicon carbide-based fiber-bonded ceramics, obtained from hot pressing of woven silicon carbide fibers, are a cost-effective alternative to ceramic-matrix composites due to their ease of fabrication, involving few processing steps, and competitive thermomechanical properties. In this work, we studied the high-temperature strength and thermal shock resistance of Si-Al-C-O and Si-Ti-C-O fiber-bonded SiC ceramics obtained from hot pressing of two types of ceramic fibers, by mechanical testing in four-point bending. The bending strength of Si-Al-C-O-based fiber-bonded ceramics at room temperature is ∼250–260 MPa and remains constant with temperature, while the bending strength of Si-Ti-C-O increases slightly from the initial 220 to ∼250 MPa for the highest temperature. Both materials retain up to 90% of their room temperature strength after thermal shocks of 1400°C and show no reduction in elastic moduli. After thermal shock, failure mode is the same as in the case of as-received materials.     

Oxidation-Induced Microstructural Change of Si-Ti-C-O Fibers

Two types of Si-Ti-C-O fibers STC(6HS) and STC(6H), with different C/Si molar ratios of 0.99 and 1.65, respectively, have been subjected to oxidation tests at high temperatures between 1000° and 1500°C. Both fibers showed different mass gain during the oxidation tests. In the unoxidized region, there was no change in the chemical composition. For STC(6HS), coarse ß-SiC grains were formed throughout the region, while the grain coarsening in STC(6H) was restricted to the vicinity of the film/fiber interface. In this paper, the influence of free carbon on the oxidation-induced microstructural change of Si-Ti-C-O fibers is discussed.     

Thermal Conductivity, Electrical Resistivity, and Seebeck Coefficient of Uranium Mononitride

Measurements are reported for the thermal conductivity, λ(80° to 400°K), electrical resistivity, ρ(4.2° to 400°K), and absolute Seebeck coefficient, Q(6° to 400°K), of pressed and sintered uranium mononitride. The measurements between 77° and 400°K were made using an absolute longitudinal heat flow apparatus. These data and literature values for the thermal conductivity and electrical resistivity at higher temperatures were used to separate the electronic and lattice portions of the thermal conductivity. The results indicate that the lattice conductivity peaks in the range 250° to 300°K and that the high-temperature limit of the Lorenz function may be greater than the Sommerfeld value of 2.443 × 10^-8 (V^2°K^-2). The electrical resistivity and the absolute Seebeck coefficient exhibit sharp slope changes near the Néel temperature, T _N(∼50° to 60°K). The Seebeck coefficient has a minimum at 34°K and then rises to a local maximum at 10°K. This low-temperature peak is probably due to magnon drag. The temperature dependence of the electrical resistivity is dominated by the magnetic contribution which increases as T ^2.38±0.08 between 10° and 52°K. The magnetic contribution is constant at high temperatures with an estimated value of 142 μΩ-cm.     

Tensile Creep Behavior and Thermal Stability of Orthogonal Three-Dimensional Woven TyrannoTM ZMI Fiber/Silicon-Titanium-Carbon-Oxygen Matrix Composites

This article presents experimental results for tensile creep behavior of orthogonal three-dimensional woven Tyranno™ ZMI fiber/Si-Ti-C-O ceramic matrix composites at 1300°–1450°C in air. The composite contained Tyranno ZMI (56% silicon, 1% zirconium, 34% carbon, and 9% oxygen) fibers with a BN coating layer to improve interface properties, and it exhibited excellent tensile properties at elevated temperature in air. For creep stresses between 60 and 140 MPa, the creep rate decreased continuously with time, with no apparent steady-state regime observed at 1300°–1450°C. Under the test conditions, the microstructure of the Tyranno ZMI fiber and Si-Ti-C-O matrix was unstable, resulting in weight loss and SiC grain growth. As a result, the viscosity of the fiber and matrix increased, because increased viscosity caused a creep rate that continuously decreased, which made steady-state creep impossible under these conditions.     

Thermal conductivity and electrical resistivity of poco grade AXF-Q1 graphite to 3300° K

Thermal conductivity and electrical resistivity of Poco Grade AXF-Q1 Graphite are presented from 110 to 3300°K. The temperature range was covered by two steady state techniques: (a) a comparative rod apparatus and (b) a radial inflow apparatus. Above 500°K the thermal conductivity data are represented quite well by a T^?1 type function, that is characteristic of Umklapp scattering processes which give rise to the thermal resistivity. We have further shown that the lattice conduction accounts for about 80 per cent of the heat transport even at 3300°K. For more highly graphitized graphite and other cokes, the results can be different. Our results are at variance with some literature that indicate a ‘strongly’ decreasing thermal conductivity above 3000°K or a ‘constant’ thermal conductivity with a ‘strong’ electronic conduction at up to 3300 °K.     

Radiation effect on the thermal conductivity and diffusivity of ramie fibers in a range of low temperatures by γ rays

To understand the influence on the thermal conductivity by the length of the molecular chain in the polymer fiber, the thermal conductivity and thermal diffusivity of ramie fibers and those irradiated by γ rays, which induced molecular chain scission of cellulose, were investigated in a range of low temperatures. The degrees of polymerization, crystallinities, and orientation angles of ramie fibers and those irradiated by γ rays (γ‐ray treatment) were measured by the solution viscosity method, solid‐state NMR, and X‐ray diffraction. Only the degree of polymerization decreased with the γ‐ray treatment, and the crystallinities and orientation angles were almost independent of the γ‐ray treatment. The thermal conductivities of the ramie fibers with and without γ‐ray treatments decreased with decreasing temperature. The thermal diffusivities of the ramie fibers and those irradiated by γ rays were almost constant from 250 to 100 K, increased slightly with the temperature decreasing from 100 to 50 K, and increased rapidly with the temperature decreasing below 50 K. The thermal conductivity and thermal diffusivity of the ramie fibers decreased with the γ‐ray treatment. The mean free path of the phonon in the ramie fibers was reduced by the γ‐ray treatment. This decrease of the thermal diffusivity and thermal conductivity was explained by the reduction of the mean free path of the phonon by molecular chain scission with γ rays. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 5007–5018, 2006     

Thermal Response and Oxidation of Tyranno™-Fiber-Reinforced Si-Ti-C-O Matrix Composites for a Thermal Protection System in High-Enthalpy Dissociated Air

The thermal response and oxidation of Tyranno™ Lox-M fiber-reinforced Si-Ti-C-O matrix composites in high-enthalpy dissociated air was investigated in an arc jet facility (an arc wind tunnel). The maximum surface temperature reached 1310–1670°C. Catalytic recombination of oxygen and nitrogen on the composite surface under dissociated air was not significant. Surface recession was insignificant below 1600°C surface temperatures and above 5 kPa of oxygen partial pressure at the stagnation point. Passive-to-active oxidation transition of the composite agreed with Balat's theory for monolithic silicon carbide. A glass sealant prevented active oxidation of the composite for short-time exposures.     

Electrical and Gas-Sensing Properties of a Thick Film Resistor of Nanosized SnO2 with Variable Percentage of Permanent Binder

Nanosized SnO₂ powder was synthesized by thermal decomposition of tin di acetate at 380°C. The effect of permanent binder (5, 10, 15, and 20 wt%) on the electrical and gas-sensing properties of thick film planar resistors shows sheet resistivity four to five orders of magnitude smaller, with comparatively low temperature coefficient of resistance. The sensors with 15 wt% of glass show the highest sensitivity toward H₂, CO, and liquefied petroleum gas (300 ppm) at optimal temperatures of 150°, 220°, and 190°C, respectively, giving selectivity. The sensors are tested for 10–300 ppm of gas with a response and recovery time of 10 and 20 s, respectively.     

Infiltration and Pyrolysis of Polytitanocarbosilane in an Si-Ti-C-O Fabric/Mullite Porous Composite

Incorporating Si-Ti-C-O fabric into a mullite matrix is expected to increase the fracture energy of mullite ceramics. The present paper describes the processing of an Si-Ti-C-O fabric/mullite/polytitanocarbosilane composite. A polytitanocarbosilane (a precursor of Si-Ti-C-O fiber)/xylene solution was infiltrated into a laminated porous mullite composite with 35–37 vol% fabric and thermally decomposed to an amorphous solid at 1000°C, in an argon atmosphere, to decrease the porosity and residual stress induced by the difference in thermal and mechanical properties between the Si-Ti-C-O fabric and the mullite. The decrease in porosity of the composite with pyrolysis of the precursor polymer was analyzed theoretically, and those results were used to control the effective experimental parameters. The infiltration/pyrolysis process was repeated eight times to produce a composite of 90.4% theoretical density. The composite exhibited significant pseudoductility, with a fracture energy of 11.4 kJ/m² and a flexural strength of 290 MPa.     

Preparation of Ultra-Fine Nickel Manganite Powders and Ceramics by a Solid-State Coordination Reaction

A solid-state coordination reaction was adopted to prepare negative temperature coefficient ceramics. A mixed oxalate NiMn₂(C₂O₄)₃·6H₂O, a coordination compound, was synthesized by milling a mixture of nickel acetate, manganese acetate, and oxalic acid for 5 h at room temperature. An ultrafine NiMn₂O₄ powder was obtained by calcining the mixed oxalate in air at 850°C for 2 h. Ceramics with a relative density of more than 97% were achieved by sintering powder compacts at a temperature as low as 1050°C for 5 h. The specific electrical resistivity ρ_25°C and the thermal constant B _25°/85°C were 2174 Ω·cm and 3884 K, respectively. The drift of the resistivity after aging at 150°C for 1000 h was 3.0%.     

Negative Temperature Coefficient Material with Low Thermal Constant and High Resistivity for Low-Temperature Thermistor Applications

The negative temperature coefficient material with low thermal constant B and high resistivity were explored for low-temperature thermistor applications. The spinel oxide Fe_0.5Cu_0.2Ni_0.66Mn_1.64O₄ had a required low B value, but its resistivity was much lower than the required value. To increase the resistivity, yttrium-stabilized zirconia (YSZ) was introduced into the spinel oxide. The spinel oxide in composite with YSZ (80 mol%) exhibited a much increased resistivity (10 479 Ω·cm at 25°C) while without causing much change in the B value (2842 K). It was found that the electrical conduction of the composite can be described quantitatively using the percolation theory.     

Low-Temperature Oxidation, Hydrothermal Corrosion, and Their Effects on Properties of SiC (Tyranno) Fibers

The effects of oxidation in air and corrosion in high-temperature, high-pressure water on the mechanical properties of three commercially available amorphous Si-Ti-C-O (Tyranno) fibers with different oxygen contents (12%–18%) and diameters (8–11 μm) were investigated. The fibers were exposed to isothermal treatments at elevated temperatures and subsequently tested at room temperature. Structural changes in the fibers after oxidation and corrosion were also studied in order to understand better the degradation mechanisms of the fibers. Oxidation resulted in the formation of vitreous silica films and decreases of strength and Young's modulus of the fibers. Hydrothermal corrosion under 100 MPa water pressure started above 300°C and resulted in the formation of a carbon layer on the surface of the fibers. Dissolution of silica in water during the treatment was observed. Corrosion at temperatures above 400°C led to the formation of relatively thick carbon films which delaminated easily. It caused a decrease of strength and Young's modulus of the fibers. The hydrothermal method can be used for producing carbon coatings with thickness up to 2 μm on the surface of silicon carbide fibers. The degrading of the mechanical properties after oxidation and corrosion was controlled by the thickness of the oxide or carbon layer. Based on this fact, it is possible to predict changes in the mechanical properties from the oxidation data.     

Plastic Deformation of Ceramic-Oxide Single Crystals

It was found that plastic deformation takes place in periclase above 1100°C., in rutile above 600°C., and in sapphire above 900°C. The mechanism is slip; in sapphire (0001) is the slip plane and [1120] is the slip directiog. All creep curves for sapphire in tension show the same qualitative features. Each consists of three stages: a stage of increasing creep rate (sometimes called an incubation period), a stage of large but decreasing creep rate (sometimes called first-stage creep), and a stage of small and nearly constant creep rate (sometimes called second-stage creep). The so-called third-stage creep, characteristic of metal behavior, has not been noted. Plastic deformation increases the electrical resistivity of sapphire at constant temperature.     

High-Temperature Thermophysical Properties of Zirconium Carbide

An apparatus based on the method of Jain and Krishnan was developed for property measurements in the temperature range from measurable radiant emission to the thermal stability limit of metallic materials. Apparatus and experimental techniques for cylindrical specimens with length-to-diameter ratios >4 are described. The theory of the method is reviewed, practical formulations are developed, and error sources are analyzed. The following properties were determined for zirconium carbide in the range 1000° to 2500°K: thermal conductivity, spectral thermal emittance at 0.65μ spectral thermal emittance at 0.65μ as a function of time in vacuum, electrical resistivity, and total thermal emittance. A compilation of experimental values for other physical properties of ZrC is given.     

Plastic Deformation of Ceramic-Oxide Single Crystals, II

Sapphire single crystals exhibit the same qualitative creep properties over the temperature range 900° to 1400°C. as do comparable metal single crystals at room temperature. The creep of sapphire under constant load has four portions: ( a ) a period of increasing creep rate, ( b ) a period of decreasing creep rate, ( c ) a period of constant creep rate, and ( d ) a final period of increasing creep rate. The stress required to initiate creep falls smoothly from about 780 kg. per sq. cm. at 900°C. to about 130 kg. per sq. cm. at 1400°C. After creep is initiated, it will continue at a lower stress (the addition of chromia increases the stress required to initiate creep). The electrical resistivity of sapphire is apparently increased by plastic deformation and decreased by subsequent heating near 1800°C. Slip lines in periclase and rutile were studied and slip systems were identified.     

Superconducting Glass-Ceramics in the Bi-Sr-Ca-Cu-O System

Bi_1.5SrCaCu₂O _z was prepared in the glassy state by rapid quenching of the melt. The recrystallization of the glass during various heat treatments was studied by differential thermal analysis, X-ray diffraction, scanning electron microscopy, and resistivity measurements. Activation energies and frequency factors for the crystallization events below 600°C were determined by nonisothermal differential scanning calorimetry. Heating at 450°C formed mainly the Bi_{2+ x} Sr_{2− x} -CuO _z solid solution ("R"). Between 765° and 845°C, R reacted slowly with the glass to form the 80 K superconductor Bi₂(Sr,Ca)₃Cu₂O _z and CuO. Heating for 7 d at 845°C, followed by slow cooling, eliminated a low-temperature resistive "tail" and raised the temperature of zero resistance to 77 K. Heating at 867°C caused partial melting, with segregation of a Bi- and Cu-rich liquid, and loss of the superconducting phases.     

三角沟槽高温热管变热流传热特性

针对高温热管在交变功率下的传热性能进行了试验研究,考虑功率的变化周期、振幅和热管的倾角等因素,分析了热管的温度、热阻及其热响应时间的变化。结果表明：在本实验条件下,恒定加热功率启动的时间与温度,和平均功率与之相等的周期交变功率启动时基本相等。交变功率的周期越长、功率振幅越大,温度波动越大,对热管的影响越大。三角沟槽高温热管在交变热通量下运行良好,性能稳定,能承受6 min内功率变化3000 W（760～3800 W）的恶劣工况。三角沟槽热管在交变功率下水平运行较45°倾角时均温性更好。高温热管具有较好的热响应和热缓冲作用,可显著提高所在系统的可靠性。     

Multiple Cracking and Tensile Behavior for an Orthogonal 3-D Woven Si-Ti-C-O Fiber/Si-Ti-C-O Matrix Composite

This paper presents experimental results for the multiple microcracking and tensile behavior of an orthogonal 3-D woven Si-Ti-C-O fiber (Tyranno™ Lox-M)/Si-Ti-C-O matrix composite with a nanoscale carbon fiber/matrix interphase and processed using a polymer impregnation and pyrolysis route. Based on microscopic observations and unidirectional tensile tests, it is revealed that the inelastic tensile stress/strain behavior is governed by matrix cracking in transverse (90°) fiber bundles between 65 and 180 MPa, matrix cracking in longitudinal (0°) fiber bundles between 180 and 300 MPa, and fiber fragmentation above 300 MPa. A methodology for estimation of unidirectional tensile behavior in orthogonal 3-D composites has been established by the use and modification of existing theory. A good correlation was obtained between the predicted and measured composite strain using this procedure.     

Specimen Size Effect of the Thermal Diffusivity/Conductivity of Aluminum Nitride

It has been generally found that the thermal diffusivity/conductivity of AIN measured by the laser-flash technique decreases with decreasing specimen size. The results of this study indicate that much of this effect can be attributed to the relatively large temperature rises for thin specimens, especially for high-energy laser pulses, and results from the combination of the noninearity of the commonly used type of IR detector and the strongly negative temperature dependence of the thermal diffusivity of AIN. The experimental results indicate that for reliable data, the specimen temperature rise at thermal equilirium should be kept to less than 1°C by using specimens with a thickness near 3 mm in combination with keeping the pulse energy to a reasonable minimum by attenuation of the laser beam by passing it through an aqueous solution of CuSO₄.     

Thermal Treatment of Titanate Derivatives Synthesized by Ion-Exchange Reaction

TiO₂ fibers were formed by thermal treatment of layered H₂Ti₄O₉ (hydrous titanium dioxide) and KHTi₄O₉ synthesized by ion-exchange reactions. The calcination of the former at 900° and 1050°C for 3 h yielded TiO₂ fibers with anatase and rutile phases, whose length and diameter were 15–20 and 2–5 μm and 10–15 and 3–5 μm, respectively. The thermal treatment of the latter at temperatures of 250° to 500°C yielded pure K₂Ti₈O₁₇, which tended to decompose to K₂Ti₆O₁₃ and TiO₂ at temperatures >600°C. At 1050°C, K₂Ti₆O₁₃ phase was formed with rutile TiO₂ fibers, whose length and diameter were 10–20 and 1–3 μm, respectively.