Conductive Ceramic Foams from Preceramic Polymers

Ceramic foams in the system Si-O-C, possessing different bulk densities and morphologies, were obtained from preceramic polymers using two different direct foaming approaches. The electric properties of the foams were varied by adding suitable fillers to the precursor mixtures in amounts up to 80 wt%. The electrical conductivity of the foams was varied by several orders of magnitude. The effects of the type of filler and preceramic polymer (methylsiloxane or methylphenylsiloxane resins), as well as the used filler precursor, on the properties of the ceramic foams were investigated.     

Pyrolysis Kinetics for the Conversion of a Polymer into an Amorphous Silicon Oxycarbide Ceramic

We present experimental and analytical results for the pyrolysis reactions underlying the conversion of a cross-linked polymer into an amorphous ceramic material. The activation energies, obtained from thermogravimetric data, and chemical analysis of the volatiles by mass spectroscopy are used to identify the reaction pathways. The reaction is determined to be first-order, which is consistent with its solid-state nature. The magnitude of the weight loss is analyzed to calculate the number of molecular sites in the polymer that participate in the reaction. The experiments were conducted on a polymer made from silsesquioxanes that convert into silicon oxycarbide ceramics on pyrolysis. The results show that <2.5% of the silicon atoms in the polymer are removed as volatile silanes, and less than one-half of the carbon atoms are lost as methane. These results are a first step in understanding the molecular basis for the ceramic yield, as well as the evolution of the nanostructure as the material changes from an organic into a ceramic state by reactions that can occur at <850°C.     

Comparison of Microwave Hybrid and Conventional Heating of Preceramic Polymers to Form Silicon Carbide and Silicon Oxycarbide Ceramics

Six different preceramic polymers were pyrolyzed via conventional and microwave hybrid heating; these polymers provide a range of carbon content and local atomic coordination. The products were compared with each other using X-ray diffractometry and transmission electron microscopy. Nanocrystalline β-SiC was the principal crystal phase detected, and the amount and size of the nanocrystals increased as the processing temperature increased. Differences were observed in the amount and size of the β-SiC nanocrystals and the graphitization of residual carbon between the microwave hybrid heating and the conventional oven heating of polycarbosilanes. Conventional heating of a high-carbon polysiloxane in an oven (in flowing argon) produced a greater amount of β-SiC from carbothermal reduction at high temperature. Microwave hybrid heating led to better β-SiC nanocrystal development for polyureasilazane.     

Stresses Occurring during Joining of Ceramics Using Preceramic Polymers

This paper reviews the current literature and describes issues related to using preceramic polymer precursors for joining ceramics for high-temperature applications. In particular, the effects of precursor yield, temperature, and heating rate on the maximum stress due to shrinkage associated with conversion of preceramic precursors to covalently bonded network structures are discussed. Conditions where the joint material viscosity and the shrinkage rates are high must be avoided. Fillers and prepyrolysis of the precursor can be used to decrease the stresses that occur during processing of joints.     

Role of Redistribution Reactions in the Polymer Route to Silicon–Carbon–Oxygen Ceramics

Redistribution reactions are used in the synthesis of several preceramic polymers. Moreover, redistributions play a major role in the pyrolysis of these precursors. Examples of thermal redistributions involving the exchange of Si—O/Si—X bonds (X = O, H, C, …) in polysiloxanes, precursors to silicon–oxygen–carbon ceramics, are given. Redistributions account for the escape of volatile organosilicon compounds, decreasing the yield and modifying the composition of the final ceramic. Moreover, they deeply modify the environment of Si atoms in the residue. At ∼900°C, Si—O/Si—C redistributions have reached equilibrium, leading to a random, entropically controlled distribution of sites. At higher temperatures, redistribution equilibria are displaced by the crystallization of SiC. Thus, the structure of the silicon oxycarbide phase (environment of Si atoms) is dependent on the O/Si ratio of the glass and the pyrolysis temperature, but is not directly dependent on the structure of the precursor.     

Centrifugal Casting of Thin-Walled Ceramic Tubes from Preceramic Polymers

Thin-walled (wall thickness, 100–2000 μm) mono- and bilayered ceramic tubes in the system Si–O–C–(N) were obtained by centrifugal casting of a polysiloxane/filler suspension. Si and SiC powders were dispersed in polyorganosiloxane/triethoxysilane solutions. After centrifugal casting in a Teflon tube with a rotational speed of 2000 rpm and subsequent cross-linking at 130°C and 60 rpm, the tubes were pyrolyzed in argon or in nitrogen at 1400–1600°C. Bilayered tubes with controlled variation of porosity were obtained by overcasting the monolayer green tubes with a modified slurry composition.     

SiOC Ceramic Foams through Melt Foaming of a Methylsilicone Preceramic Polymer

A process for the production of SiOC ceramic foams has been for the first time developed through melt foaming of a siloxane preceramic polymer with the help of a blowing agent, followed by pyrolysis under an inert atmosphere. The raw material consisted of a methylsilicone resin, a catalyst (which accelerated the cross-linking reaction of the silicone resin) and a blowing agent (which generated gas above 210°C). Methylsilicone resin foams were obtained through controlling the melt viscosity around 210°C, temperature where the blowing agent started to decompose, by varying the initial molecular weight of the preceramic polymer and the amount of the catalyst. The obtained SiOC ceramic foams exhibited excellent oxidation stability up to 1000°C, as shown by thermal gravimetric analysis (TGA). As expected, the mechanical properties of the SiOC ceramic foams varied as a function of their bulk density, possessing a flexural strength up to 5.5 MPa and a compression strength up to 4.5 MPa. The main steps in the process, namely foaming and pyrolysis, were analyzed in detail. The viscosity change was analyzed as a function of temperature by the dynamic shear measurement method. The pyrolysis process of foams was analyzed by TGA coupled with infrared spectroscopy (IR).     

Comparison of Ion Irradiation Effects in Silicon-Based Preceramic Thin Films

Thin films of polymers (polysiloxanes, polycarbosilanes, and polysilazanes) and alkoxide-derived siloxane gels, precursors for SiC, SiCN, SiOC, and SiOBC ceramics, were irradiated with increasing fluences of C or Au ions to study the kinetics of their conversion into ceramics. Ion beam analyses showed that the main effect of irradiation on the composition of the films is the selective release of H₂ by radiolysis. During subsequent high-temperature annealing of films converted as much as possible by irradiation, CO _x , CH _x , or silane molecules do not evolve, contrary to what is observed during the pyrolysis of unirradiated precursor films. According to Raman analyses, a large proportion of the carbon atoms segregate into clusters after irradiation and in films converted by direct pyrolysis (or combined treatments). However, carbon particles formed during irradiation are more diamond-like, affording films with 2—3 times higher hardness, as shown by nanoindentation tests. In both types of ceramics (SiC or SiOC), the optimal properties (hardness, thermal stability, and photoluminescence) associated with C segregation are obtained for a C/Si ratio of the order of 1. Boron addition is detrimental to hardening of SiOC glasses, in contrast to nitrogen.     

Pyrolysis of Preceramic Polymers in Ammonia: Preparation of Silicon Nitride Powders

The pyrolysis of cross-linked polycarbosilanes, polysilanes and polysilazanes in an ammonia atmosphere to 1473 K yields amorphous, low-carbon silicon nitride powders. The efficacy of carbon removal is independent of the polymer's structure or functionality but is partially dependent on the initial cross-linking temperature. The amorphous silicon nitride powders partially crystallize to α-Si₃N₄ by 1773 K.     

Mechanical Properties of Silicon Oxycarbide Ceramic Foams

The mechanical properties of ceramic foams obtained through a novel process that uses the direct foaming and pyrolysis of preceramic polymer/polyurethane solutions were investigated. The elastic modulus, flexural strength, and compressive strengths were obtained for foams in the as-pyrolyzed condition; values up to 7.1 GPa, 13 MPa, and 11 MPa, respectively, were obtained. The strength of the foam was virtually unchanged at temperatures up to 1200°C in air; however, long-term exposure at 1200°C led to a moderate degradation in strength, which was attributed to the evolution of intrastrut porosity during the oxidation of residual free carbon, as well as devitrification of the foams struts.