Reaction Mechanisms of Silicon Carbide Fiber Synthesis by Heat Treatment of Polycarbosilane Fibers Cured by Radiation: I, Evolved Gas Analysis.

SiC fibers were synthesized from polycarbosilane (PCS) fibers by heat treatment after electron beam irradiation curing. The pyrolysis reaction mechanisms from the organic PCS to ceramic SiC were investigated by the analysis of gases evolved during heat treatment. There were two steps in the major reaction: the first step was at 800–1200 K where H₂ and CH₄ evolved by scission of Si-CH₃ and Si-H and by rearrangement reactions, and the second step was at 1000–1700 K where H₂ evolved by reactions related to C atoms in the PCS main chain. H₂ evolution in the first step was reduced with increasing oxygen content in the cured PCS fibers.     

Synthesis and Characterization of β'-SiAlON Ceramics from Organosilicon Polymers

A polycarbosilane has been modified with aluminum alkoxide to obtain a new preceramic compound. The pyrolysis in NH₃ flow of this polyaluminocarbosilane leads to the formation of an amorphous Si-Al-O-N phase. The nitridation process has been followed by IR and ²⁹Si MAS-NMR spectroscopies. A fine-grained β-SiAION ceramic is obtained by firing the amorphous phase at 1500°C. The crystallization process has been studied by XRD, ²⁹Si MAS-NMR, and TEM techniques.     

Fabrication of Microcellular Ceramics Using Gaseous Carbon Dioxide

A microcellular ceramic with cell densities >10⁹ cells/cm³ and cells <10 μm was made with a preceramic mixture of polycarbosilane and polysiloxane. The preceramic compact was saturated with gaseous CO₂, a large number of cells were nucleated and grown by using a thermodynamic instability induced by a rapid pressure drop, and the microcellular preceramic was transformed into a microcellular ceramic by pyrolysis.     

Preparation of Silicon Carbonitrides from an Organosilicon Polymer: I, Thermal Decomposition of the Cross-linked Polysilazane

The conversion of a solid copolymer (ViSiHNH)_x─(MeSiHNH)_y into a ceramic was studied in the 20–1450°C temperature range under different atmospheres (inert or oxidative). The ceramic yield, the ceramic composition, and the gaseous evolution directly depend on the heating rate, the pyrolysis atmosphere, and the duration of pyrolysis. The implications of these observations to the pyrolysis mechanism are discussed. The silicon carbonitride obtained after pyrolysis is amorphous up to 1400°C with high carbon content. It will be shown in Parts II and III that either SiC or Si₃N₄ could be selectively crystallized depending upon processing conditions.     

Ceramization of Reflux-Treated Polymethylsilane Precursors to Silicon Carbide

The pyrolysis of polymethylsilane (PMS) in an argon gas environment with a flow rate of 1 L/min was investigated as a standard pyrolytic process, and the investigation showed SiSi network formation at 573 K. Subsequently, various condensed PMS resins were prepared by adjusting pre-heat-treatment or reflux conditions in the temperature range of 423–723 K. The effect of pre-heat treatment or refluxing on the ceramic yield at 1273 K was quantitatively evaluated. Structural evolution in the PMS resins prepared under various reflux conditions was investigated during pyrolysis up to 1873 K. The X-ray diffraction patterns of the pyrolysis products revealed crystallite growth of β-SiC and silicon at 1273–1473 K. ²⁹Si solid-state nuclear magnetic resonance with the single-pulse method was also conducted on the pyrolysis products at 1273 K.     

Dissolution of a Sphene Glass-Ceramic, and of Its Component Sphene and Glass Phases, in Ca-Na-Cl Brines

Leaching experiments for up to 569 d have been performed to examine the dissolution in Ca-Na-Cl groundwater of a sphene (CaTiSiO₅)-based glass-ceramic, a candidate material for immobilizing nuclear fuel recycle wastes. The experiments involved leaching of samples of a simulated-waste-loaded glass-ceramic, doped with ²²Na or ⁴⁵Ca, in a synthetic groundwater at 25° and 100°C. The results are compared with those from separate leaching experiments with ²²Na- or ⁴⁵Ca-doped samples of aluminosilicate glass and ceramic sphene, representing the component phases of the glass-ceramic. The comparison supports a model in which the glass-ceramic dissolution rate may be approximately derived from the weighted average of the separate dissolution rates of its component glass and ceramic phases. No synergistic effects in the leaching of the glass and ceramic phases in the glass-ceramic were found.     

Oxidation of the Carbon Interface in Nicalon-Fiber-Reinforced Silicon Carbide Composite

The recession rates for 10^-6-m-thick C interfaces in chemical vapor infiltrated SiC reinforced with Nicalon fibers were calculated from thermogravimetric data, assuming all of the mass losses were due to C oxidation, and found to be consistent with the measured recession distances of the C interface, which were surprisingly uniform across the composite. Agreement between the two approaches for a microstructurally complex material indicates thermogravimetric analysis could be an important tool for understanding environmental effects in ceramic composites with reactive interfaces. Mass losses were linear within the first 1.08× 10⁴ s to 2.16× 10⁴ s between 1073 and 1373 K and between 3.1× 10² and 2.5× 10³ Pa O₂. Calculated reaction orders with respect to O₂ were between 0.5 and 1.0 at 1373 K, and activation energies were about 50 kJmol^-1. Analysis of the kinetic data and estimates of gas boundary layer thickness suggest the mechanism for the C-interface oxidation involved reaction control, but the possibility of diffusion control for some conditions cannot be ruled out.     

Oxidation Behavior of Si-C-O Fibers (Nicalon) under Oxygen Partial Pressures from 102 to 105 Pa at 1773 k

Polycarbosilane-derived SiC fibers (Nicalon) were oxidized at 1773 K under oxygen partial pressures from 10² to 10⁵ Pa. The effect of oxygen partial pressure on the oxidation behavior of the Nicalon fibers was investigated by examining mass change, surface composition, crystal phase, morphology, and tensile strength. The Nicalon fibers were passively oxidized under oxygen partial pressures of >2.5 ×10² Pa and actively oxidized under an oxygen partial pressure of 10² Pa. Under oxygen partial pressures from 2.5 × 10² to 10³ Pa, active oxidation occurred at the earliest stage of oxidation, resulting in the formation of both a silica film and a carbon intermediate layer. Although the unoxidized core retained considerable levels of strength under the passive-oxidation condition, fiber strength was lost under the active-oxidation condition.     

Crystallization Behavior of Novel Silicon Boron Oxycarbide Glasses

Homogeneous silicon boron oxycarbide (Si-B-O-C) glasses based on SiO _x C_{4– x} and BO _y C_{3– y} mixed environments were obtained by pyrolysis under inert atmosphere of sol–gel-derived precursors. Their high-temperature structural evolution from 1000° to 1500°C was followed using XRD, ²⁹Si and ¹¹B MAS NMR, and chemical analysis and compared with the behavior of the parent boron-free Si-O-C glasses. The XRD study revealed that, for the Si-O-C and the Si-B-O-C systems, high-temperature annealing led to the crystallization of nanosized β-SiC into an amorphous SiO₂-based matrix. NMR analysis suggested that the β-SiC crystallization occurred with a consumption of the mixed silicon and boron oxycarbide units. Finally, by comparing the behavior of the Si-O-C and Si-B-O-C glasses, it was shown that the presence of boron increased the crystallization kinetics of β-SiC.     

A New Aluminum Coordination Site in Si-C-Al-N-(O) Ceramics

A variety of Si-C-Al-N-(O) ceramics with different AlN and SiC contents were prepared by pyrolysis of Si-Al-C-O precursors at 1700°C in nitrogen. In those ceramics containing small amounts of aluminum, a new ²⁷Al MAS NMR peak at 134.3 ppm was found. The existence of this peak is discussed in relation to recent findings in aluminum-containing complex ceramic systems.     

The Evolutionary Process during Pyrolytic Transformation of Poly(N-methylsilazane) from a Preceramic Polymer into an Amorphous Silicon Nitride/Carbon Composite

The pyrolytic evolution of poly(N-methylsilazane), –[H₂SiN-Me] _x –, from preceramic polymer to ceramic product is followed by heating samples of the partially cross-linked polymer, in 200°C increments, from ambient temperature to 1400°C. The intermediate products are characterized by chemical analysis, diffuse reflectance Fourier transform IR spectroscopy (DRIFTS), Raman spectroscopy, and ²⁹Si and ¹³C magic-angle spinning (MAS) solid-state NMR. Spectro-scopic characterization indicates that the 1400°C pyrolysis products are amorphous silicon nitride mixed with amorphous and graphitic carbon (as determined by Raman spectroscopy), rather than silicon carbide nitride, as expected based on the presence of up to 20 mol% retained carbon. Efforts to crystallize the silicon nitride through heat treatments up to 1400°C do not lead to any crystalline phases, as established by transmission electron microscopy (TEM) and small-area electron diffraction (SAD). It appears that the presence of free carbon, along with the absence of oxygen, strongly inhibits crystallization of amorphous silicon nitride. These results contrast with the isostructural poly-(Si-methylsilazane), –[MeHSiNH] _x –, which is reported to form silicon carbide nitride on pyrolysis.     

Thermal Stability, Phase Evolution, and Crystallization in Si-B-C-N Ceramics Derived from a Polyborosilazane Precursor

Amorphous Si-B-C-N ceramic powder samples obtained by thermolysis of boron-modified polysilazane, {B[C₂H₄Si(H)NH]₃} _n , were isothermally annealed at different temperatures (1400–1800°C) and hold times (3, 10, 30, and 100 h). A qualitative and semiquantitative analysis of the crystallization behavior of the materials was performed using X-ray diffraction (XRD). The phase evolution was additionally followed by ¹¹B and ²⁹Si MAS NMR as well as by FT-IR spectroscopy in transmission and diffuse reflection (DRIFTS) modes. Bulk chemical analyses of selected samples were performed to determine changes in the chemistry/phase composition of the materials. It was observed that silicon carbide is the first phase to nucleate around 1400–1500°C, whereas silicon nitride nucleates at and above 1700°C. Crystallization accelerates with increasing annealing temperature and proceeds with increasing annealing time. Furthermore, the surface area of the powders strongly influences the thermal stability of silicon nitride and thus controls overall chemical and phase composition of the materials on thermal treatment.     

Poly(methylsilane)—A High Ceramic Yield Precursor to Silicon Carbide

Preceramic polymers offer exceptional potential for low-temperature processing of both oxide and non-oxide ceramics. In addition, shapes such as fibers, films, and membranes that are not commonly available using standard processing techniques are readity available using preceramic polymers. In non-oxide ceramics, the ceramic products generally available from preceramics do not exhibit all of the typical properties associated with the same materials produced by standard, high-temperature processing approaches. In part, this appears to be because there are very few preceramic polymers that lead to high-purity, single-phase materials. Poly(methylsilane), (–[MeHSi] _x –), produced from MeSiH₃, can be used to produce relatively pure, bulk SiC at temperatures below 1000°C. The transformation process from polymer to ceramic is followed by ²⁹Si NMR and diffuse reflectance IR. The polymer first undergoes a major rearrangement from poly(silane) to poly(carbosilane) at 400°C. Above 400°C, the resulting poly(carbosilane) decomposes to a hydrogenated form of SiC as shown by spectroscopic analysis of the 600°C material. Further heating, to 1000°C for 1 h, provides very narrow ²⁹Si peaks indicative of β-SiC mixed with small amounts of α-SiC polytypes. Chemical analysis, when coupled with the ²⁹Si and XRD results, suggests that poly(methylsilane) produces resonably pure, nanocrystalline SiC at temperatures much lower than previously observed for other SiC preceramic polymers.     

Mirror Constant for Tyranno® Silicon-Titanium-Carbon-Oxygen Fibers Measured in Situ in a Three-Dimensional Woven Silicon Carbide/Silicon Carbide Composite

The strength, S , of ceramic and glass fibers often can be estimated from fractographic investigation using the fracture mirror radius, r _m, and the relationship S = A _m/( r _m)^1/2, where A _mis the "mirror constant." The present work estimates the value of A _mfor Tyranno^® Si-Ti-C-O fibers in situ in a three-dimensional woven SiC/SiC-based composite to be 2.50 ± 0.09 MPa·m^1/2. This value is within the range of 2–2.51 MPa·m^1/2 previously obtained for nominally similar Nicalon^® Si-C-O fibers.     

Effect of Hydrogen Atmosphere on Pyrolysis of Cured Polycarbosilane Fibers

SiC-based fibers with various chemical compositions were synthesized using an irradiation-curing process. Polycarbosilane (PCS) fibers were cured by irradiation with an electron beam in a helium atmosphere. The cured PCS fibers were pyrolyzed at 1300°C under controlled hydrogen or argon atmospheres, and SiC fibers with C/Si of 0.84 to 1.56 were obtained. The fibers consisted of <1.0 wt% O, <0.2 wt% N, <0.1 wt% H, with the balance being Si and C. The mechanism of pyrolytic transformation of cured PCS to SiC-based ceramics was investigated using TG/DTA analysis. Greater mass losses were observed during pyrolysis in a hydrogen atmosphere than in argon. This result suggests that the hydrogen atmosphere suppresses H₂ evolution and helps to remove excess carbon as CH₄ during pyrolysis. The microstructure and mechanical properties of the resulting SiC-based fibers were found to be very dependent on their C/Si chemical compositions.     

Preparation of Lead Lanthanum Zirconate Titanate (PLZT, (Pb,La)(Zr,Ti)O3) Fibers by Sol-Gel Method

Lead lanthanum zirconate titanate (PLZT, (Pb,La)(Zr,-Ti)O₃) ceramic fibers were prepared by the sol-gel method from a solution of lead acetate trihydrate, lanthanum isopropoxide, zirconium n -propoxide, and titanium isopropoxide that contained 2-methoxyethanol as the solvent. The sols obtained from the solution were concentrated at 156°-174°C for 2 h. Concentration at higher temperatures resulted in more-viscous sols of higher specific gravities. The concentration resulted in the formation of spinnable sols, which had viscosities >10⁵ mPas and exhibited Newtonian flow properties. These spinnable sols were formed to be so stable that no change in viscosity and spinnability was observed for more than three months when stored in a sealed container at room temperature. Gel-to-ceramic fiber conversion was investigated by means of X-ray diffractometry, infrared spectroscopy, and thermal analysis. Single-phase perovskite PLZT ceramic fibers 5-200 μm in diameter and >20 cm in length were obtained. Scanning electron microscopy (SEM) observation and Brunauer-Emmett-Teller (BET) measurement showed that heat treatment of the fibers at a lower rate resulted in the formation of fibers of denser microstructure. Although the SEM image of the cross section of the fibers revealed a relatively dense microstructure and a laser beam could be transmitted through a fiber 6 mm in length, BET measurement of the fibers indicated that the fibers had more than a few percent of open porosities, and scattering of light was observed in the laser-beam guiding test.     

GC-MS and 13C NMR Investigation of Lead Zirconate Titanate Precursor Sols for Fiber Preparation

Different macroscopic properties of PZT fibers have been obtained when using acetic acid and methacrylic acid to modify the PZT precursor. In order to clarify the role of the acids the molecular structure of the acidified PZT precursors was investigated and compared by gas chromatography-mass spectrometry, Fourier transform infrared, ¹³C nuclear magnetic resonance (NMR) spectroscopy (solution and solid state ¹³C NMR) and the reason for obtaining long PZT fibers is discussed. The results indicate that when methacrylic acid was used, long gel and ceramic fibers have been obtained because strongly co-ordinating carboxylate groups of methacrylic acid were formed. Linear chains, like those of methacrylic acid propyl ester and methacrylic acetate, have been formed in the PZT precursor sols. In addition, after heat treatment the polymer decomposed quickly so that pure perovskite could be obtained at low temperature in the PZT fibers. When acetic acid was used short fibers were obtained. Acetic acid may act as chelate agent to form oxo acetate in the precursors; this oxo acetate nature also resulted in PZT fibers drawing. However, the longest gel and ceramic fibers have been prepared from precursors with methacrylic acid.     

Derivatization of Preformed Polymers to Produce Ceramic Precursors

The feasibility of producing derivatized preformed polymers to be used as ceramic precursors was explored. A borane derivative of polyethyleneimine (PEI) was prepared through a transamination reaction involving Me₃NBH₃. Higher boron loadings were possible with Me₃NB₃H₇. The borane derivative could be cast into films and produced boron nitride upon pyrolysis. Similarly, polyethyleneimine hydrochloride was converted into the cyanohydroborate derivative through a reaction with NaBH₃CN. This derivative was also accessible through a transamination reaction. Both materials produced boron nitride upon pyrolysis. Polyallylamine hydrochloride could not be derivatized. Silyl derivatives of PEI were also prepared, but the products obtained afforded low ceramic yields upon pyrolysis.     

α-Decay Damage Effects in Curium-Doped Titanate Ceramic Containing Sodium-Free High-Level Nuclear Waste

A polyphase titanate ceramic incorporating sodium-free simulated high-level nuclear waste was doped with 0.91 wt% of ²⁴⁴Cm to accelerate the effects of long-term self-irradiation arising from α decays. The ceramic included three main constituent minerals: hollandite, perovskite, and zirconolite, with some minor phases. Although hollandite showed the broadening of its X-ray diffraction lines and small lattice parameter changes during damage ingrowth, the unit cell was substantially unaltered. Perovskite and zirconolite, which are the primary hosts of curium, showed 2.7% and 2.6% expansions, respectively, of their unit cell volumes after a dose of 12 × 10¹⁷α decays.g^-1 Volume swelling due to damage ingrowth caused an exponential (almost linear) decrease in density, which reached 1.7% after a dose of 12.4 × 10¹⁷α decays.g^-1. Leach tests on samples that had incurred doses of 2.0 × 10¹⁷ and 4.5 × 10¹⁷ a decays g^-1 showed that the rates of dissolution of cesium and barium were similar to analogous leach rates from the equivalent cold ceramic, while strontium and calcium leach rates were 2–15 times higher. Although the curium, molybdenum, strontium, and calcium leach fates in the present material were similar to those in the curium-doped sodium-bearing titanate ceramic reported previously, the cesium leach rate was 3–8 times lower.     

M-Doped Al2TiO5 (M=Cr, Mn, Co) Solid Solutions and their Use as Ceramic Pigments

New ceramic pigments based on the tialite (Al₂TiO₅) structure, doped with Co (pink), Cr (green), or Mn (brown), were prepared through the pyrolysis of aerosols followed by calcination of the obtained powders at 1400°C. The expected decomposition of Al₂TiO₅ into a mixture of Al₂O₃ and TiO₂ on refiring was inhibited by Cr-doping and also by co-doping with Mg the Mn- or Co-doped samples. Microstructure and phase evolution during pigment preparation were monitored by scanning electron microscopy and XRPD. Unit cell parameters of tialite were determined by Rietveld refinement of the X-ray diffraction patterns, revealing in all cases the formation of solid solutions where the solubility of dopants in the Al₂TiO₅ lattice followed the trend Co3+ ions in a large interstitial site of the tialite lattice with a distorted octahedral geometry, and of Mn³⁺ and Co²⁺ ions in the Al³⁺ octahedral sites of the tialite lattice in the former case, and in both Al³⁺ and Ti⁴⁺ octahedral sites in the latter. Testing the ceramic glazes assessed the technological behavior of pigments, which found that the color stability was reasonably good for the Mn-doped tialite and the Cr-doped pigment, although the latter suffered a small loss of green hue. The Co-doped pigment was found to be not stable in glazes, undergoing a cobalt-leaching effect.