Jonathan Lipowitz

Fine-diameter polycrystalline SiC fibers

Abstract Various organosilicon polymers have been converted to small-diameter, polycrystalline silicon carbide fibers by melt-spinning, cross-linking and pyrolyzing to high-temperature in argon. Several wt% boron was doped into the fibers before pyrolysis. Use of polycarbosilane precursor gave 8–10 μm diameter fibers having up to 2.6 GPa tensile strength, 450 GPa elastic modulus, 3.1–3.2 g/cm3 density. The microstructure consists of >95 wt% β-SiC crystallites of 30–40 nm average crystallite size. Stoichiometric fibers or fibers having excess carbon content have been prepared. Fiber has been thermally aged under inert conditions at 1800°C for 12 h with minimal strength and microstructural change. Stoichiometric fiber maintains higher strength after oxidative aging at 1370°C. Current processing efforts are aimed at preparing the fiber in continuous tow form.

Characterization of nanoporosity in polymer-derived ceramic fibres by X-ray scattering techniques

Ceramic fibres with Si-C-O and Si-N-C-O compositions, prepared by pyrolysis of polymer precursors, generally have densities lower than those calculated from a volume additivity rule. However, techniques often used to detect porosity such as electron microscopy methods, surface area and porosimetry measurements show that little surface-connected porosity is present. X-ray scattering measurements, both wide-angle (WAXS) and small-angle (SAXS), show considerable scattering in the range 1° < 2θ < 10° (CuKα). Treatment of the scattering data by the classical Guinier (low angle limit) and Porod (high angle limit) methods indicate that closed, globular, nanometre-scale porosity (1 to 3 nm diameter) is present in all ceramic fibres examined. X-ray scattering power correlates quantitatively with the volume fraction porosity, as expected if porosity is the dominant facto affecting X-ray scattering. Nano-particles of excess carbon and ofβ-SiC nanocrystallites, though present, are minor contributors to the scattering of X-rays in these ceramic fibres. Fibres are three-dimensional, not of fractal dimension, and are not oriented. As density increases with increasing pyrolysis temperature, average pore size increases and pore volume fraction decreases. This results from a thermodynamically favourable reduction of surface free energy and apparently occurs by a viscous flow process.

Structure and properties of ceramic fibers prepared from organosilicon polymers

This paper is a review of structure and properties of ceramic fibers derived from organosilicon polymers, with emphasis on the authors research. Ceramic fibers are prepared from organosilicon polymers by melt-spinning, cross-linking, and pyrolysis. Desirable polymeric precursors display the following properties: high char yield of desired composition, thermal stability at melt-spinning temperature, stable rheology, high purity and freedom from particulate impurities, and ability to undergo rapid cure (cross-linking). Ceramic fibers in the Si-C-O or Si-C-N-O systems display a rich nanostructure consisting of some or all of the following metastable phases: (1) an amorphous, continuous siliconoxycarbide or siliconoxycarbonitride phase; (2) dispersed carbon nanocrystallites; (3) dispersed β-SiC or Si3N4 nanocrystallites; and (4) closed, globular nanopores. The crystalline phases increase in volume fraction and crystallite size as stoichiometry approaches the crystalline composition and as pyrolysis temperature increases. The Si-C-N-O fibers are amorphous. Pore size increases and total pore volume decreases with increasing pyrolysis temperature. Considerable variation in ceramic fiber composition can be achieved by varying cure conditions and pyrolysis atmosphere. Polycrystalline SiC fibers can be produced by pyrolysis above 1600°C. Fiber diameters range from 7 to 20 µm. Elastic moduli vary from 140 to >420 GPa (20 to >60 Msi) and are controlled by composition, nanostructure, and fiber density. Fiber densities range from ∼2.2 to >3.1 g/cm3. Tensile strengths range up to ∼5 GPa (700 ksi) and are Griffith flaw-controlled.

Effect of heat treatment on the elemental distribution of Si,C,N,O fibers

Nicalon® and hydridopolysilazane (HPZ) polymer-derived ceramic fibers have been heat treated in various environments at 1200–1700°C, in attempts to understand their deterioration processes and to modify their surface layer composition, which is critical for their use in ceramic and metal matrix composites. The elemental distribution at and below the surface layer has been determined, before and after treatments, by a scanning Auger microprobe (SAM). Annealing in nitrogen resulted in its penetration deep into Nicalon fibers to replace carbon and oxygen. Carbon associated with Si could be easily replaced by N, while ‘free’ carbon was more stable and required higher temperature (1700°C) and N2 pressure (5 MPa) to be removed. Treatment in argon caused Si3N4 decomposition, as well as C and SiC removal through reaction with Si or SiO2. Only Si + O remained in the outer layer of HPZ fibers, and only a porous Si + C material remained in the outer layer of carbon-enriched HPZ fibers. Treatment of C-HPZ fibers in wet air at 1200°C resulted in the oxidation of the Si3N4, SiC and C components, leaving a surface Si + O layer.

Fiber Synthesis Processes

This chapter reviews the preparation, composition, structure and properties of ceramic fibers derived from polymeric precursors. Those fibers which have reached a commercial or developmental status will be emphasized. The intent is to show the close relationship between process, structure and properties for ceramic fibers which have been derived from polymers (Figure 17.1).

29Si AND 13C MAGIC ANGLE SAMPLE SPINNING NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY OF CERAMIC FIBERS PREPARED BY PYROLYSIS OF ORGANOSILICON POLYMERS

Silicon oxycarbide (Si-C-O) and oxycarbonitride (Si-N-C-O) fibers, prepared by pyrolysis of organosilicon polymers, have been characterized by 29Si and 13C magic angle spinning(MAS) NMR. The Si-C-O fibers are partially amorphous and the Si-N-C-O fibers are totally amorphous. A single-pulse FT-NMR technique using ≥ 60s pulse delay with phase cycling and with multiple signal acquisitions (>200) was used. A spinning rate > 4 KHz is required to remove spinning sideband interference. Boron nitride rotors were used for sample spinning in 13C NMR experiments. 29Si chemical shifts of the five possible tetrahedral species about silicon (SiC4, SiC3O, SiC2O2, SiCO3, SiO4) in silicon oxycarbides (NILCALON™ fibers) can be partially resolved at 6.3 Tesla (29Si resonance frequency of 53.8 MHz). These Si-C-O compositions were previously shown to contain β-SiC nanocrystallites, 1 to 3 nm in size, and a carbon nanophase distributed throughout the continuous amorphous phase. Chemical shift assignments, relative to external TMS, are based on analogous organosilicon compounds and on the various silicon carbide polytypes.

Structure and Properties of Ceramic Fibers Prepared From Polymeric Precursors [1]

Ceramics can be prepared by pyrolysis of organosilicon polymers. Advantages of this method of ceramics preparation are; the ability to prepare shapes difficult to achieve by other methods such as fibers and films; the ability to achieve high purity because reagents used to prepare the polymer can be purified by well established chemical methods; processing at lower temperature than conventional methods [2].

Fire safety properties of some transformer dielectric liquids

This paper reviews the fire hazard properties of some transformer fluids of reduced flammability. Fluids examined include a silicone (polydimethylsiloxane), a high molecular weight aliphatic hydrocarbon, and a polyalphaolefin. Fire hazard properties considered include ease of ignition, flame spread, fire growth and rate of heat release (large scale pool burns), rate of smoke evolution, fire gas and smoke toxicity, and extinguishment behavior.

Characterization of the Reaction Products of Symmetrical Dimethyltetrachlorodisilane with Hexamethyldisilazane by29 Si NMR Spectroscopy

Abstract Ceramic fiber-based composites are becoming an increasingly important group of structural materials (3). Useful ceramic fibers can be prepared by melt-spinning, cure and pyrolysis of a polymethyldisilylazane polymer precursor (4, 5), which is, in turn, the reaction product of mixtures of the following methylchlorodisilanes (I) with the indicated approximate compositions: This mixture is reacted with excess hexamethyldisilazane (HMDZ) to generate polymeric species with the approximate composition II, along with trimethylchlorosilane [Me3SiC1] and ammonium chloride [NH4C1] as by-products.