This review highlights the synthesis, processing and properties of non-oxide silicon-based ceramic materials derived from silazanes and polysilazanes. A comprehensive summary of the preparation of precursor compounds containing Si–N–Si units, including commercially available materials, is followed by the discussion of various processing techniques. The fabrication of dense bulk ceramics in the Si/E/C/N systems is reported which involves cross-linking of the polymeric ceramic precursor followed by a polymer-to-ceramic transformation step. The cross-linked precursor can be milled, compacted and pyrolysed to form dense, additive-free, amorphous silicon carbonitride monoliths or polycrystalline composites which withstand oxidation in air at 1600°C. Furthermore, an overview is given on the fabrication of silazane derived powders and coatings involving chemical vapour deposition (CVD) methods utilising volatile precursors. Fibre spinning and fibre properties, as well as other processing techniques like infiltration of preforms, the preparation of porous ceramics and joining are briefly discussed. A state of the art of the mechanical properties of polymer derived amorphous Si/C/N and Si/B/C/N ceramics with respect to hardness as well as high-temperature creep and oxidation resistance is summarised. Finally, some important aspects of industrial applications will be considered. The review is in part based on our own work related to the polysilazane derived ceramics, but will also cover a comprehensive state of the art including the published literature in this field.

Silazane derived ceramics and related materials

Silicon oxycarbide glasses have been synthesized by inert atmosphere pyrolysis at 1000°C of gel precursors obtained by cohydrolysis of triethoxysilane, HSi(OEt)3, and methyl-diethoxysilane, HMeSi(OEt)2. The oxycarbide structures have been carefully characterized by means of different techniques such as 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR) and Raman spectroscopies, X-ray diffraction (XRD), and chemical analysis. Experimental results clearly indicate that, depending on the composition of the starting gels, the resulting oxycarbide glass either is formed by a pure oxycarbide phase or contains an extra carbon or silicon phase. By increasing the temperature up to 1500°C, the oxycarbide glasses display compositional and weight stability; however, the amorphous network undergoes structural rearrangements that lead to the precipitation of nano-sized β-SiC crystallites into amorphous silica. Crystallization of metallic silicon is also clearly observed at 1500°C for the samples in which the presence of Si-Si bonds was postulated at 1000°C.

Structural Characterization and High‐Temperature Behavior of Silicon Oxycarbide Glasses Prepared from Sol‐Gel Precursors Containing Si‐H Bonds

Catalytic reactions that enable the formation of new bonds to carbon centres play a pervasive role in the state-of-the-art synthesis of organic molecules and macromolecules. In contrast, the development of analogous processes as routes to main group compounds and materials has been much slower. Nevertheless, recent advances have led to a broad expansion of this field and now allow access to a wide range of catenated structures based on elements across the p block. These breakthroughs have already impacted areas such as hydrogen storage and transfer, functional inorganic polymers and ceramic thin films. Dehydrogenation and dehydrocoupling processes are particularly well developed and may be mediated by either transition metal or main group catalysts. Such pathways represent an increasingly attractive and convenient alternative to traditional routes, such as salt metathesis and reductive coupling reactions. An overview of this emerging area is presented in this Review with a focus on recent developments and future challenges.

Catalysis in service of main group chemistry offers a versatile approach to p-block molecules and materials

A series of silsesquioxane copolymers synthesized by hydrolysis and condensation of phenyl- and methyltrimethoxysilanes have been studied as preceramic polymers. The pyrolytic conversion to ceramics was characterized by thermogravimetric analysis, 29Si and 13C nuclear magnetic resonance and Raman spectroscopy. The pyrolysed materials were further characterized by differential thermal analysis. X-ray diffractometry and transmission electron microscopy. The ratio of phenyl to methyl groups in the copolymer was found to control polymer structure and rheology, as well as ceramic composition and char yield. On pyrolysis to 1000 °C under inert conditions, silicon oxycarbides were formed, along with glassy carbon. On heating from 1200 °C to 1400 °C, the oxycarbide structure diminished, and the materials were comprised primarily of amorphous silica, amorphous Si-C, some small crystallite SiC and graphitic carbon. The carbon content increased, and char yield decreased, with increasing concentration of phenyl groups in the copolymer. The presence of free carbon appears to inhibit the crystallization of silica. Significant carbothermal reduction was observed only above 1500 °C. Oxidation studies of the pyrolysed materials indicated the presence of at least two forms of carbon.

Characterization of the pyrolytic conversion of polysilsesquioxanes to silicon oxycarbides

Organoactinides in catalysis

: The synthetic utility of homogeneous titanium catalyzed redistribution of the cyclomers-(MeHSiO) (x)-(where x=4 or 5) and the linear oligomer -MeHSiO(x) - MeHSiO(x)-(M(n) approx. 2,200 D) was explored as a route to MeSiH3 and a methylsilsesquioxane-methylhydridopolysiloxane copolymer of approximate composition- MeHSiO 0.35 MeSi(O)1.5)0.65-. The high temperature behavior of the titantium derived methylsilsesquioxane copolymer follows closely that of similar polymers prepared by sol-gel processing. Heating under nitrogen to 800-1000 C at a heating rate of 5 C/min gives a black glass with an apparent composition of: SiO2 (70%); SiC (19%) and C (10%). The apparent composition bellies the true nature of the material which is probably a metastable glass, best described by the various Si-X bonding arrangements (X= O, C, H). Thus, MAS NMR and DRIFT spectroscopy show the existence of species containing 4,3 and 2 Si-O bonds with the remaining bonds either Si-C or SiH. At 600 C and below (under N2), the Si-C bonds are almost exclusively Si-H3, which is an indication of the extraordinary thermal stability of silsesquioxane polymers. Above 600 C, the CH3 groups react with Si-O bonds to generate Si-OH bonds, new Si-C bonds and Si-H bonds. This type of reactivity is illustrative of the basic chemistry involved in the carbothermal reduction of SiO2 to Si and/or SiC as well as the degradation mechanisms of the polymer. Keywords: Ceramic materials; Silica glass. (aw)

Synthesis and High-Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligo- and -polysiloxanes

https://apps.dtic.mil/sti/pdfs/ADA183824.pdf

Catalytic Synthesis of Oligosilazanes. 2

Silicon and aluminum complexes having formulae (I) and (II), wherein x is 0 or 1, T is H or (III) each R is independently selected from the group consisting of H, OH, C1-6 alkyl, C1-6 alkoxy, C2-6 alkene, C6-12 aryl, C1-6 hydroxyalkyl, C1-6 thioalkyl, C2-12 alkoxyalkyl, C3-20 heteroaromatic, and combinations thereof, wherein R may further contain one or more atoms of a non-carbon element such as Si, Ge, Sn, P, and the like; and Y is a cation, are prepared by reacting silica or alumina with a diol, in the presence of a base, while removing water formed during the reaction Methods for producing such complexes starting with silica or alumina, and methods for converting such complexes into other silicon or aluminum-containing compounds, are disclosed

Silicon and aluminum complexes

Working principles are developed as guidelines for the selection and/or design of organometallic polymers for processing fiber precursors to metal oxide fibers. These principles form the basis for the selection of metal carboxylate preceramics as an optimal approach to processing yttrium barium cuprate (123) ceramic superconducting fibers. A variety of candidate yttrium, barium, calcium, strontium, bismuth, and copper metal carboxylates were synthesized. Solubility and empirical rheology tests were conducted to screen these compounds to choose spinnable precursor systems. Simple extrusion studies confirmed that THF solutions of mixtures of yttrium, barium, and copper isobutyrates with some quantity of barium 2-ethyl-hexanoates can be used to successfully form 60–70 μm diameter 123 precursor fibers.

Superconducting fibers from organometallic precursors. Part II: Chemistry and low temperature processing1

Pyrolyses of a set of silicon and nitrogen substituted polysilazancs arc conducted under a set of standard conditions (5°C/min to 900°C in a nitrogen atmosphere) The apparent ceramic compositions and ceramic yields for pyrolyzed samples of the polysilazancs -[Ph(H)SiNH]x-, -[C6H13(H)SiNH]x-, -[Me(H)SiNH]x-, and -[H2SiNMc]x- arc determined Preliminary conclusions concerning structure/reactivity relationships are discussed based on the pyrolysis results

/pdf/new-catalytic-routes-to-preceramic-polymers-ceramic-31wmc9rfj9.pdf

Kay A. Youngdahl

Papers

Synthesis and High-Temperature Chemistry of Methylsilsesquioxane Polymers Produced by Titanium-Catalyzed Redistribution of Methylhydridooligo- and -polysiloxanes

Catalytic Synthesis of Oligosilazanes. 2

Silicon and aluminum complexes

Superconducting fibers from organometallic precursors. Part II: Chemistry and low temperature processing1

New Catalytic Routes to Preceramic Polymers: Ceramic Precursors to Silicon Nitride and Silicon-Carbide Nitride