Polymer-derived Ceramic Fibers Research Articles

Polymer-Derived Ceramic Fibers

SiC-based ceramic fibers are derived from polycarbosilane or polymetallocarbosilane precursors and are classified into three groups according to their chemical composition, oxygen content, and C/Si atomic ratio. The first-generation fibers are Si-C-O (Nicalon) fibers and Si-Ti-C-O (Tyranno Lox M) fibers. Both fibers contain more than 10-wt% oxygen owing to oxidation during curing and lead to degradation in strength at temperatures exceeding 1,300°C. The maximum use temperature is 1,100°C. The second-generation fibers are SiC (Hi-Nicalon) fibers and Si-Zr-C-O (Tyranno ZMI) fibers. The oxygen content of these fibers is reduced to less than 1 wt% by electron beam irradiation curing in He. The thermal stability of these fibers is improved (they are stable up to 1,500°C), but their creep resistance is limited to a maximum of 1,150°C because their C/Si atomic ratio results in excess carbon. The third-generation fibers are stoichiometric SiC fibers, i.e., Hi-Nicalon Type S (hereafter Type S), Tyranno SA, and Sylramic™ fibers. They exhibit improved thermal stability and creep resistance up to 1,400°C. Stoichiometric SiC fibers meet many of the requirements for the use of ceramic matrix composites for high-temperature structural application. SiBN3C fibers derived from polyborosilazane also show promise for structural applications, remain in the amorphous state up to 1,800°C, and have good high-temperature creep resistance.

Hierarchically structured polymer-derived ceramic fibers by electrospinning and catalyst-assisted pyrolysis

Abstract Hierarchically structured polymer-derived ceramic fibers were successfully produced by electrospinning a commercially available preceramic polymer to which a cobalt-based catalyst precursor was added, followed by pyrolysis in nitrogen at temperatures ranging from 1250 to 1400 °C. The nanowires formed via the vapor–liquid–solid (VLS) mechanism, involving the reaction of SiO and CO gases, generated from the decomposition of the polymer-derived-ceramic at high temperature, with the heating atmosphere assisted by the presence of nano-sized CoSi droplets. The main crystalline phase for the nanowires was Si 3 N 4 below 1350 °C, and Si 2 N 2 O at 1400 °C, and the amount of nanowires increased with increasing heating temperature. Hierarchically structured fiber mats possessed a higher specific surface area (14.45 m 2 /g) than that of a sample produced without the cobalt catalyst (4.37 m 2 /g).

Polymer-Derived Ceramic Composite Fibers with Aligned Pristine Multiwalled Carbon Nanotubes

Polymer-derived ceramic fibers with aligned multiwalled carbon nanotubes (MWCNTs) are fabricated through the electrospinning of polyaluminasilazane solutions with well-dispersed MWCNTs followed by pyrolysis. Poly(3-hexylthiophene)-b-poly (poly (ethylene glycol) methyl ether acrylate) (P3HT-b-PPEGA), a conjugated block copolymer compatible with polyaluminasilazane, is used to functionalize MWCNT surfaces with PPEGA, providing a noninvasive approach to disperse carbon nanotubes in polyaluminasilazane chloroform solutions. The electrospinning of the MWCNT/polyaluminasilazane solutions generates polymer fibers with aligned MWCNTs where MWCNTs are oriented along the electrospun jet by a sink flow. The subsequent pyrolysis of the obtained composite fibers produces ceramic fibers with aligned MWCNTs. The study of the effect of polymer and CNT concentration on the fiber structures shows that the fiber size increases with the increment of polymer concentration, whereas higher CNT content in the polymer solutions leads to thinner fibers attributable to the increased conductivity. Both the SEM and TEM characterization of the polymer and ceramic fibers demonstrates the uniform orientation of CNTs along the fibers, suggesting excellent dispersion of CNTs and efficient CNT alignment via the electrospinning. The electrical conductivity of a ceramic fibers with 1.2% aligned MWCNTs is measured to be 1.58 x 10(-6) S/cm, which is more than 500 times higher than that of bulk ceramic (3.43 x 10(-9) S/cm). Such an approach provides a versatile method to disperse CNTs in preceramic polymer solutions and offers a new approach to integrate aligned CNTs in ceramics.

Recent Developments in Polymer‐Derived Ceramic Fibers (PDCFs): Preparation, Properties and Applications – A Review

This review states recent developments in the preparation of ceramic fibers using a chemical procedure denoted as Pyrolysis of Preceramic Polymers. Synthesis strategies which are used to prepare Polymer‐Derived Ceramics (PDCs) fibers from molecular and polymeric precursors are presented by refering to the crystalline boron nitride (BN) fibers and amorphous silicoboron carbonitride (SiBCN) fibers as models. The influence of the structure of the molecular and polymeric precursors, in which specific functional groups and structural motifs are incorporated on the melt‐spinnability, polymer‐to‐ceramic conversion and fiber properties is discussed. A minimum cross‐linking degree combined with the presence of flexible structural patterns and plasticizing functional groups yield controlled melt‐viscoelastic properties and appropriate thermal stability at low temperature to produce high quality fine‐diameter endless green fibers in a stable melt‐spinning process. These structural motifs and functional groups are also capable to undergo cross‐linking reactions and provide fiber curability allowing the polymer backbone to be interlocked, the fiber integrity to be thereby maintained. As‐cured fibers are then pyrolyzed in a controlled atmosphere with retention of the shape to produce the ceramic fibers in the desired composition and structure. †Member of IUF (Institut Universitaire de France)

Development of Polymer-Derived Ceramic Fibers

Development of Polymer-Derived Ceramic Fibers

Thermal stability of the low-oxygen-content silicon carbide fiber, Hi-Nicalon TM

The low-oxygen SiC fiber, Hi-Nicalon TM, was prepared by the pyrolysis of polycarbosilane fibers cured with electron-beam irradiation. This SiC fiber is continuous, in multi-filament form, and consists of Si-1.39C-0.010 by atomic ratio. Hi-Nicalon TM fiber has a high tensile strength and an elastic modulus of 2.8 and 270 GPa, respectively. This SiC fiber retains high strength and modulus even after exposure for 10 h at 1873 K in argon. It exhibits outstanding thermal stability as compared to other commercially available polymer-derived ceramic fibers.

Crystallization process of polymer-derived ceramic fibers with nano-structure

The crystallization processes of 2 kinds of Si-Ti-C-O fibers with different C/Si molar ratios of 0.99 and 1.65 were characterized. Both the as-received fibers have a microstructure where β-SiC nanocrystals (3–5 nm) are dispersed over an amorphous matrix, but the fiber with C/Si = 1.65 additionally contains some carbon units ( < 3 nm) around β-SiC nanocrystals. For the fiber with C/Si = 0.99, the crystallization proceeded drastically with time, and was involved with a rapid gas evolution. On the other hand, for the fiber with C/Si = 1.65, the crystallization was controlled by the presence of free carbon, and it resulted in a slower gas evolution. However, the free carbon subsequently began to disappear with time; instead, a new carbon layer was formed at the fiber surface. The electrical resistivity reflected the above microstructural changes.

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 N 2 pressure (5 MPa) to be removed. Treatment in argon caused Si 3N 4 decomposition, as well as C and SiC removal through reaction with Si or SiO 2. 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 Si 3N 4, SiC and C components, leaving a surface Si + O layer.

Microanalyses Of Structural Defects In Polymer-Derived Ceramic Fibers

Microanalyses were performed with a Cameca MBX scanning electron microprobe and a JEM 2000FX analytical electron microscope to determine the composition of foreign defect structures found on primary fracture surfaces of ceramic fibers derived from melt-spun Si-N-C-O polymeric precursors. The defects were the critical flaws responsible for deterioration of tensile strength in these fibers after thermal exposure in the range of 1400°C and above. High fiber strength is necessary to provide reinforcement and toughening in structural ceramic composites during high temperature use.Among the defects found were roughly spherical internal nodular flaws which contained Pb (Fig. 1) as well as others (Fig. 2) containing both Pb and Sn. Granular aggregates were also seen in which Fe and Cr were detected. Other nodular surface defects contained Ca, minor Al, and levels of 0 higher than that in the surrounding fiber (Fig. 3) . Extremely fine whisker growths at discrete sites along the length of some fibers were identified as silicon oxynitride by electron diffraction.