Initiation and propagation behaviors of interior microflaws in the pyrolysis of an air-cured polycarbosilane precursor fiber

In the preparation of Nicalon-type Si—C—O fibers using a normal-pressure synthesized polycarbosilane precursor, a network of microflaws were detected on the cross-section of ceramic fibers obtained. Cross-section morphologies of the interior microflaws were investigated via SEM technique after certain temperature pyrolysis and their initiation and propagation behaviors were studied accordingly. The results showed that no microflaws came into being at the temperature up to 600°C due to the organic nature of the material. As a result of high weight loss and composition change, however, microflaws initiated over 600°C, formed at 700°C and propagated along the core-edge direction from 700–;900°C. Up to 1100°C, a microflaw network was formed because new microflaws generated across the oriented ones and interconnected them to form a network.     

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of -SiC nanocrystals (5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.     

Production mechanism of polytitanocarbosilane and its conversion of the polymer into inorganic materials

The reaction of polycarbosilane with tetra-alkyltitanate proceeded at 300° C in nitrogen atmosphere by the condensation of Si-H bonds in polycarbosilane and the substituent groups of the tetra-alkyltitanate accompanied by evolution of alkan gas, and then the formation of Si-O-Ti bonds occurred. In this condensation reaction using tetra-isopropyl titanate, tetra-n-butyl titanate and tetra-2-ethylhexyl titanate, activation energies of the initial rate of the increase in molecular weight were 17.04, 20.07 and 31.07 kcal mol^–1 respectively, and thus the more bulky the substituent group of tetra-alkyltitanate, the lower the reactivity became. Of these alkyltitanates, tetra-2-ethylhexyl titanate was found to be the most advantageous reactant for obtaining polytitanocarbosilane with a narrow molecular weight distribution, low gel fraction and high titanium concentration. Polytitanocarbosilane with high titanium concentration was converted into the densified amorphous inorganic material with high Si-C bonding energy in high yield. Titanium contained in the pyrolysed polytitanocarbosilane was furthermore found to have the effect of inhibiting crystalline grain growth of -type SiC up to high temperature.     

A study of tensile properties in Al–Si–Cu and Al–Si–Mg alloys: Effect of β-iron intermetallics and porosity

The effect of β-iron intermetallics and porosity on the tensile properties in cast Al–Si–Cu and Al–Si–Mg alloys were investigated for this research study, using experimental and industrial 319.2 alloys, and industrial A356.2 alloys. The results showed that the alloy ductility and ultimate tensile strength (UTS) were subject to deterioration as a result of an increase in the size of β-iron intermetallics, most noticeable up to β-iron intermetallic lengths of 100 μm in 319.2 alloys, or 70 μm in A356.2 alloys. An increase in the size of the porosity was also deleterious to alloy ductility and UTS. Although tensile properties are interpreted by means of UTS vs. log elongation plots in the present study, the properties for all sample conditions were best interpreted by means of log UTS vs. log elongation plots, where the properties increased linearly between conditions of low cooling rate–high Fe and high cooling rate–low Fe. The results are explained in terms of the β-Al₅FeSi platelet size and porosity values obtained.     

Electrical, optical and structural characterization of high-k dielectric ZrO2 thin films deposited by the pyrosol technique

High-k dielectric zirconium oxide (ZrO₂) thin films have been deposited on silicon substrates at temperatures from 400 to 600 °C using the spray pyrolysis technique. The films were deposited from two spraying solution concentrations (0.033 and 0.066 M) of zirconium acetylacetonate dissolved in N,N-dimethylformamide. These films were stoichiometric, transparent and with a very low surface roughness (5–40 ). The refractive index of these films was of the order of that obtained for a bulk material (2.12). Films deposited with high molar concentration presented the best electrical characteristic, have a dielectric constant in the range 12.5–17.5, depending on the deposition temperature, and can stand electric fields up to 3 MV cm^–1 without observing destructive dielectric breakdown. Transmission electron microscopy measurements, indicate that the films consist of nano-crystallites of the tetragonal ZrO₂ crystalline phase embedded into an amorphous matrix. Infrared spectroscopy measurements of the films show peaks associated with ZrO₂ and a peak related to silicon dioxide (SiO₂). The analysis of spectroscopic ellipsometry measurements on these films indicates the existence of a layer at the ZrO₂/Si interface composed of SiO₂ as well as ZrO₂ and crystalline silicon.     

Thermal oxidation crosslinking in the blended precursors of organosilicon polymers containing polyvinylsilane with polycarbosilane

The polymer precursors containing polyvinylsilane (PVS) with polycarbosilane (PCS) in coating form were prepared. As the content of PVS increases, the onset temperature of the precursor oxidation decreases and the surfaces formed after the oxidation become smooth and hard. The role of Si–H bonds in the PCS–PVS systems during the oxidation is quantitatively analysed on the basis of the absorption of the Si–H stretching band in the precursor spectra. In the blended systems, the oxidation of the Si–H groups forms the dense crosslinked polymer layer which limits the oxygen diffusion into the precursor interior. © 1998 Chapman & Hall     

Fabrication and characterization of polymer-derived Si2N2O-ZrO2 nanocomposite ceramics

Amorphous Si-Zr-N-O powders, obtained by nitridation in an NH₃ flow of zirconium modified polycarbosilane, have been sintered to full density by hot pressing at 1500C. The resulting ceramic shows an extremely fine-grained microstructure composed of Si₂N₂O and ZrO₂ crystallites 20–30 nm in diameter. Thermal stability measured in air appears excellent up to 1300C for 48 h. Mechanical characterization pointed out good values of flexural strength (330 MPa), fracture toughness (4.1 MPam^0.5) and Weibull modulus.     

Nickel thin films prepared by chemical vapour deposition from nickel acetylacetonate

Nickel thin films were prepared by a low-temperature atmospheric-pressure chemical vapour deposition method. The raw material was nickel acetylacetonate. At a reaction temperature above 250 °C, polycrystalline nickel films can be obtained by hydrogen reduction of the raw material. The resistivity (8.1–13.3 cm) of the film was close to that of bulk nickel.     

Excellent heat resistance of Si-Zr-C-O fibre

In order to obtain the high heat-resistant fibre, Si-Zr-C-O fibre has been developed. Si-Zr-C-O fibre was produced by the use of polyzirconocarbosilane as the precursor polymer. In this paper, the difference in heat-resistance between Si-Ti-C-O and Si-Zr-C-O fibres was clarified. Si-Zr-C-O fibre showed excellent heat resistance (up to 1773 K) compared with Si-Ti-C-O fibre (up to 1573 K). Generally speaking, decomposition reaction of this type of fibre proceeds accompained by the release of CO gas which was formed by the reaction between excess carbon and oxygen included in the fibre. In the case of Si-Zr-C-O fibre, Zr can strongly capture the oxygen atoms, so that the aforementioned decomposition hardly proceeds up to 1873 K (ZrO2 + 3C = ZrC + 2CO; G<0 at over 1906 K).     

Synthesis and characterization of new precursors to nearly stoichiometric SiC ceramics: Part 1 The copolymer route

Organosilicon polymers with a (C/Si (atomic ratio)<1.28) exhibiting Si–Si and Si–CH2–Si linkages in their backbone have been prepared and characterized by multinuclear magnetic resonance and infrared spectroscopies, size exclusion chromatography and thermogravimetric analysis. These precursors give SiC ceramics with a C/Si (atomic ratio)1.14 and a relatively low oxygen content (between 1.5–3 at %).     

Fine SiC fiber synthesized from organosilicon polymers: relationship between spinning temperature and melt viscosity of precursor polymers

A very fine silicon carbide (SiC) fiber with diameter of 6 m, about a half of that of a commercially available SiC fiber, was synthesized from a polymer blend of polycarbosilane (PCS) and polyvinylsilane (PVS). The fine SiC fiber was obtained by optimizing the composition and the spinning temperature of PCS-PVS polymer blends. In order to determine these optimum conditions, the relationship between temperature and melt viscosities of the polymer blends was investigated. As a result, it was found that the optimum spinning temperature range was within a temperature range where the melt viscosity is 5–10 Pa · s. Moreover, by blending PVS with PCS, the spinning temperature of the polymer blends was lowered, the spinnability of polymer system was improved, and finer polymer fiber was obtained compared with PCS. The optimum content of PVS in the polymer blend was 15–20 wt%.     

Processing, microstructure and mechanical properties of hot-pressed SiC continuous fibre/SiC composites

SiC (SCS-6TM) continuous fibre/SiC composites were fabricated by hot-pressing at 1700°C in vacuum using an Al sintering additive. Analytical transmission electron microscopy was used to investigate the microstructure of the composites. The room-temperature mechanical and high-temperature creep properties of the composites were investigated by four-point bending. The SiC powders used were sintered at a relatively low sintering temperature to high density (97% of theoretical density) with the addition of the Al sintering additive. It is believed that the Al additive is very efficient for the densification of SiC. The SiC fibres maintained their original form and microstructure during fabrication. The SiC matrix reacted with the outermost carbon sublayer in the fibre, forming a thin (1.8–4.8m) interfacial layer, which was composed of Al4C3, Si–Al–C, and Si–Al–O phases. The incorporation of SiC fibre into a dense SiC matrix significantly increased the room-temperature failure strain and improved the high-temperature creep properties. In addition, the incorporation of SiC fibre into a porous SiC matrix increased the room-temperature failure strain, but did not contribute to the high-temperature creep properties.     

Synthesis and formation mechanism of submicrometre spherical cordierite powders by ultrasonic spray pyrolysis

Cordierite powders containing very pure submicrometre spherical particles have been synthesized by the ultrasonic spray pyrolysis. Aqueous solutions of silicic acid, Al(NO3)3·9H2O and MgCl2·6H2O were used as precursors. Scanning electron micrographs have shown that particle surfaces were smooth and the mean particle diameter was 0.834 m. For the estimation of chemical and phase composition and phase transformation temperatures, differential thermal analysis, thermogravimetric analysis, X-ray diffraction, energy dispersive spectroscopy and infrared analysis have been applied. It was found that during spray pyrolysis, the condensation of silicic acid mostly occurred while aluminium and magnesium ion remained incorported between Si–O–Si chains. By subsequent heating to over 800°C, Si–O–M bonds (M=Al, Mg) were formed. The synthesis of cordierite occurred by the crystallization of -cordierite from the amorphous phase at 900°C followed by the phase transformation of - into -cordierite in the temperature range 1100–1200°C.     

A batch process to deposit amorphous metallic Mo–Si–N films

A process for depositing amorphous electrically conducting Mo–Si–N films in a batch-type reactive sputtering system has been developed. Each elemental constituent in the film is individually adjustable: molybdenum and silicon through the electrical power applied to the separate targets, and nitrogen through the gas flow rate. Argon is used for the tuning of the intrinsic stress. The amorphous structure of a Mo₃₁Si₁₈N₄₅ film is confirmed by cross-sectional transmission electron microscopy and electron diffraction. The structure remains unchanged up to at least 700 °C for 1 min of annealing in an argon ambient. In the process, the room-temperature resistivity decreases from an initial value of about 1.1 to about 1.0 m cm with no change in the film thickness. After 1100 °C for one minute, grains nucleate and the film resistivity falls by two-thirds. The intrinsic stress in Mo–Si–N films is significantly more uniform throughout the film area than in polycrystalline molybdenum films. These results hold promise for applications of Mo–Si–N films in micromechanical devices. Self-supported beams and membranes have been successfully delaminated from their silicon substrates; molybdenum-rich films are more ductile than silicon-rich films.     

Processing of microcellular SiC foams

Curing kinetics of commercial polycarbosilane have been measured using differential scanning calorimetry in air between 140 and 220C. It was found that the total heat of curing increases linearly with temperature. At constant temperature, the rate of heat release can be described by the sum of a first-order kinetic term, and a transient term of much higher order, both depending on the fraction of remaining reaction enthalpy. Constants for both terms increase according to the Arrhenius law, with roughly the same activation energy of about 1.2 kJ mol^–1. The heat released in an additional non-isothermal experiment is well described by integration of the isothermal heat-release equations, suggesting that the rate of reaction can be described solely in terms of the fraction transformed and temperature.     

Zirconium Temperature Measurements from the Melting Point to 4100 K Involving the Use of Blackbody Models in the Liquid State

The temperature (2128–4100 K) dependence of specific enthalpy Eat atmospheric pressure is obtained under conditions of fast (microseconds) pulsed electric heating of zirconium foil samples. The enthalpy dependence of the spectral density of radiation at the wavelength of 0.855 m is measured for flat surfaces and for two blackbody models made of zirconium foil. The temperature is calculated by Planck's formula. The temperature plateau on the curve of the dependence of Ton Ein the melting of zirconium is used for calibration, and the value of the plateau temperature is taken to be equal to the equilibrium melting point of 2128 K. The blackbody models are assembled of separate flat strips of zirconium. The objective of this study is to develop and experimentally verify a blackbody model suitable for both metals and graphite and to derive the temperature dependence of the thermal properties of liquid metals and liquid carbon at high temperatures under conditions of fast pulsed heating. A Tektronix TDS 754C four-channel digital oscilloscope was used in the measurements.     

Sintering Behavior of Hot-Pressed Ca–αSialon Ceramics

On the basis of phase relationships in the Ca–Si–Al–O–N system, a Ca––sialon ceramic was synthesized using the hot-pressing technique. The reaction sequences and densifications of the Ca––sialon vs. firing temperatures have been characterized in detail. The present experiments reveal a reaction sequence as follows: at 1250°C the reactant mixture started to soften, at 1300°C a gehlenite phase was produced, at 1500°C the gehlenite phase was resolved into a liquid phase and a Ca––sialon started to form, and at 1600°C the formation of Ca––sialon was complete. The product was stable and almost entirely single phase Ca––sialon. Accompanying to the above sequences, densification also proceeded via a liquid-phase sintering, particle rearrangement, solution–reprecipitation, and grain growth process. In the final microstructure elongated grains of Ca––sialon were obtained, improving the fracture toughness of this Ca––sialon ceramic.     

Cyclic fatigue crack growth behaviour in β-(Si–Al–O–N) at ambient and elevated temperatures

Four-point bending fatigue tests on a hot-pressed sintered Sm–-(Si–Al–O–N) ceramic were conducted at room temperature, 900 °C and 1000 °C in air under different load ratios and cyclic frequencies. The growth of indentation cracks was measured during the fatigue tests. The results indicate that the cyclic fatigue crack growth threshold is lower and crack growth rates are higher, for given values of K_max, at 1000 °C than those at room temperature. The cyclic fatigue crack growth behaviour at 900 °C is similar to that at room temperature. It was found that the crack growth retardation due to cyclic fatigue loading is much more pronounced at higher frequencies. An increase in cyclic frequency from 1 to 10 Hz cause a reduction of up to two orders of magnitude in crack propagation rates. High-temperature cyclic fatigue crack growth rates increased and threshold stress intensity factor ranges decreased with increasing load ratio. Possible mechanisms for cyclic crack growth are discussed.     

Preparation of continuous zirconia fibres from polyzirconoxane synthesized by the facile one-pot reaction

A one-pot reaction of zirconium oxychloride octahydrate (ZOC) with ethyl acetoacetate (Hetac) in the presence of triethylamine was investigated. The reaction was carried out in molar ratios of Hetac/ZOC=1.0, 1.5, 2.0 to give highly viscous solutions with good spinnability and stability to self-condensation. Polyzirconoxanes (PZOs) were isolated as white powders by reprecipitation of the solutions with tetrahydrofuron (THF)-hexane that were soluble in acetone, methanol and THF. The molecular weight of PZO was Mn=1000–2000, and was dependent on the molar ratio and the reaction time. Dry spinning of the solution (Hetac/ZOC=1.5) containing 3mol% tris(acetylacetonato)yttrium [Y(acac)3] gave continuous precursor fibres (3.0Y–PZO). Yttria stabilized zirconia fibres of 12–18 m diameter with a tensile strength of 1.4GPa were obtained by heat treatment of the 3.0Y–PZO at 1100 or 1200 °C. The fibre consisted of submicrometre-sized particles of tetragonal crystallites. © 1998 Chapman & Hall     

Curing and pyrolysis of polysiloxane/divinylbenzene and its derived carbon fiber reinforced Si—O—C composites

The curing and pyrolysis of hydrogen-containing polysiloxane (PSO) and divinylbenzene (DVB) were investigated in this paper. It was found that H₂PtCl₆ was an effective catalyst for the curing of DVB/PSO. The mass ratio of DVB/PSO had great effect on ceramic yield. The cured DVB/PSO with a mass ratio of 0.5:1 had the highest ceramic yield (76%) at temperature up to 1000°C, and its pyrolysates consisted of 38.3 wt% silicon, 27.4 wt% oxygen, and 34.3 wt% carbon of which 26.3 wt% was free carbon. The composition and structure of pyrolysates of DVB/PSO were changed with increasing pyrolysis temperature. The pyrolysis behavior of DVB/PSO was characterized by thermal analysis. DVB/PSO-derived Si–O–C composites reinforced with carbon fiber cloth (C_f/Si–O–C) were fabricated. The results showed that the flexural strength of C_f/Si–O–C composites could be increased from 118.00 ± 5.00 MPa to 139.78 ± 7.68 MPa if the pyrolysis temperature was elevated from 1000 to 1400°C, which was ascribed to the weakened interfacial bonding.