Yigal D. Blum

Preceramic Polymer Pyrolysis. I. Pyrolytic Properties of Polysilazanes.

The physicochemical behaviour of characterized polysilazanes has been examined during their pyrolytic transformation into amorphous silicon-based ceramics. Selected polysilazanes bearing different substituents at silicon and nitrogen were synthesized by ruthenium catalysed dehydrocoupling of Si-H bonds with N-H bonds. The relationships between the structure and chemical content of polymers and their pyrolysed ceramic compositions and yields are discussed. Possible reactions occurring during pyrolysis are described in terms of a set of mechanisms based on known behaviour of silazane monomers. The decomposition product patterns at different temperature levels and the compositions of the final ceramics suggest specific kinetically or thermodynamically controlled thermolysis pathways. Additional chemical reactivity has been observed when the amorphous ceramic products at 800° C are heated and crystallized at 1600° C.

Novel polysiloxane and polycarbosilane aerogels via hydrosilylation of preceramic polymers

We report new polysiloxane and polycarbosilane aerogels, which have been obtained by crosslinking Si–H-containing polymers with a CC-containing crosslinker via hydrosilylation reactions. The crosslinking reaction has been carried out in a highly diluted solution using up to 97 vol% of solvent. The obtained aerogels have a colloidal structure with meso- and macropores. Density as low as 0.17 g cm−3 has been reached, which implies a porosity of ca. 84 vol%.

Chemical reactivities of hafnium and its derived boride, carbide and nitride compounds at relatively mild temperature

MB2/SiC composites are materials of choice for ultra-high-temperature structural applications, primarily in the aerospace arena. These composites are processed in a hot-press operation at a temperature range of 1900 to 2200°C. This article assesses potential “mild-temperature” (below 1500°C) chemical reactions that may lead to structures and coatings made of HfB2/SiC under pressureless or mild-pressure conditions. The reactions are anticipated to be involved in reactive and shape-forming processes, where ceramic precursors and/or reactive powders are incorporated. This article pays special attention to exothermic reactions as well as to formers of a liquid phase; both can aid the desired phase formation, microstructure development, and sintering of the composite under milder conditions than currently practiced. Reactions between loosely mixed powders with melting points significantly above 1500°C were detected by X-ray diffraction (XRD) analyses. Significant solid-phase reactions of the loose powder mixtures were observed at this mild temperature in powder form. Preliminary microstructural studies using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray Spectroscopy (EDX) techniques have confirmed the presence of unique reaction mechanisms between the loosely connected particles.Good examples are the reactions between Hf powder and powders of BN or B4C, all having melting points above 2200°C, which form at 1500°C, or below HfB2/HfN and HfB2/HfC crystalline domains, respectively. These reactions are less intuitive than the reaction with B2O3, which forms HfB2/HfO2, potentially via molten or gaseous phases of boron oxide.

Evolution of C-rich SiOC ceramics Part II. Characterization by high lateral resolution techniques: electron energy-loss spectroscopy, high-resolution TEM and energy-filtered TEM

Abstract In this Part II of the paper on the evolution of carbon-rich Si–O–C polymer-derived ceramics (PDCs), emphasis is placed on the strengths and limitations of transmission electron microscopy (TEM) techniques with high lateral resolution, i.e., electron energy-loss spectroscopy (EELS), high-resolution TEM (HRTEM) and energy-filtered TEM (EFTEM). Here, the identical SiOC materials, as described in Part I. EELS studies confirmed the progression of structural rearrangements within the SiOC matrix temperatures exceeding 1200°C. High-resolution TEM imaging showed that the SiOC matrices are indeed predominantly amorphous even upon high thermal treatment. Energy-filtered TEM analysis revealed, in contrast to the results obtained by Raman spectroscopy (Part I), that the excess free carbon phase undergoes a pronounced rearrangement within the amorphous microstructure. HRTEM characterization revealed the distribution of phases within the amorphous SiOC matrix; information that is not accessible by integral spectroscopic techniques. Discrepancies between the interpretation of experimental results obtained by local versus integral characterization tools are discussed in detail.

Catalytic synthesis of oligosilazanes part 2

Abstract Preliminary studies on transition metal-catalyzed dehydrocoupling of Si-H bonds with H-N bonds to form Si-N bonds, silazanes and H 2 are described. A number of transition metal complexes have been tested as catalyst precursors for the dehydrocoupling reaction. Of these, Ru 2 (CO) 8 -(Et 3 Si) 2 proved to be the most active homogeneous catalyst, and Pd particles produced by the in situ reduction of Pd(OAc) 2 were the most active heterogeneous catalysts. Kinetic studies of the ruthenium-catalyzed reaction of Et 3 SiH with primary amines, RNH 2 (R = n-Pr, n-Bu, s-Bu and t-Bu) were run at 70 °C in THF. The effects of changes in Et 3 SiH, amine and catalyst concentrations on rates of reaction were examined. The data indicate that the dehydrocoupling catalytic cycle involves an extremely complex set of equilibria wherein the rate-determining step is dependant on the steric requirements of the amine and fragmentation of the starting Ru 3 (CO) 12 cluster.

Strong surface diffusion mediated glancing-angle deposition: Growth, recrystallization and reorientation of tin nanorods

Different from usual glancing-angle deposition where low surface diffusion is necessary to form nanorods, strong surface diffusion mediated glancing-angle deposition is exemplified by growing tin nanorod films on both silicon and glass substrates simultaneously via thermal evaporation. During growth, the nanorods were simultaneously baked by the high-temperature evaporator, and therefore re-crystallized into single crystals in consequence of strong surface diffusion. The monocrystalline tin nanorods have a preferred orientation perpendicular to the substrate surface, which is quite different from the usual uniformly oblique nanorods without recrystallization.

Investigation on the oxidation process of SiCO glasses by the means of non-Rutherford backscattering spectrometry

Ion beam analysis was applied for the first time to characterize the oxidation process of silicon oxycarbides (SiCO) glasses (belonging to the class of polymer-derived ceramics). The technique unveiled the structure of the interface between the silica scale and the SiCO substrate. At the early oxidation stage, a smooth concentration profile was observed between the surface silica layer and the substrate. The role of the free carbon on the kinetics of the oxidation process was studied by preparing samples with different carbon content. A high amount of free C in the SiCO glasses promotes the transition, after long oxidation periods, to a sharp interface between the silica scale and the SiCO substrate.

Low-cost matrix development for an Oxide-Oxide composite

Continuous oxide fiber/oxide matrix composites are attractive for use as high temperature structural materials because they can combine composite properties with long-term oxidative stability. The development of a matrix for such a composite and prevention of matrix-fiber coating interaction is described here. The goal use temperature of this composite is 1100°C to 1200°C. The composite is being developed by the M C Consortium comprising 3M, Rockwell International, and SRI International. The composite consists of an alumina-based woven-tow fiber preform, coated with lanthanum phosphate (monazite) to promote fiber debonding and pullout, in an oxide matrix derived from a preceramic-polymer slurry filled with active and inert powders. This approach to the matrix enables conventional polymer matrix composite technology to be used in composite part fabrication. Only one infiltration of the matrix is required, a critical factor in keeping the cost low.

A New Catalytic Method for Producing Preceramic Polysilazanes

A transition metal (e.g., Ru 3 (CO) 12 , Pt/C) catalyzed process for Si-N bond formation is discussed that provides a new route to mono-, oligo-, and polysilazanes. The catalysts function by activating Si-H bonds in the pres-ence of ammonia. Polymeric silazanes can also be produced from oligomers in the presence of ammonia at low temperatures. This method allows us to control or modify the composition of the polysilazane during or after the polymeriza-tion. A variety of polysilazanes were prepared and converted to Si 3 N 4 with ceramic yields ranging from 55%-85%. By varying the monomers and reaction conditions, we can control the nitrogen and carbon content in the preceramic polymers, which enables us to obtain ceramic products that are primarily Si 3 N 4 and simultaneously minimizes the coproduction of SiC and C.

Evolution of C-rich SiOC ceramics

Abstract Carbon-rich Si–O–C polymer-derived ceramics (PDCs) were investigated by various spectroscopic techniques, in order to characterize the evolution of their predominantly amorphous microstructure upon thermal treatment up to 1450°C. Particular attention was addressed to modifications of the excess free carbon phase present in these materials. Surprisingly, the carbon clusters exhibited high stability above the pyrolysis temperature. Despite the high volume fraction of carbon, only a very limited carbothermal reduction process was detected. This study is divided into two parts: PartI deals with characterization tools that reveal a rather low lateral resolution and are hence termed here as integral spectroscopic techniques, i.e., solid-state NMR and Raman spectroscopy. In contrast, PartII illustrates the experimental results obtained from the very same materials characterized by spectroscopic and imaging techniques with high lateral resolution, i.e., electron energy-loss spectroscopy (EELS), high-resolution transmission electron microscopy (HRTEM), and energy-filtered TEM. In addition to materials characterization, emphasize of both papers is also to compare the information gained by either integral or local spectroscopy techniques and to highlight the strengths and weaknesses of either approach.