Gian Domenico Sorarù

Microstructural and mechanical characterization of sol gel-derived Si-O-C glasses

The mechanical properties of three silicon oxycarbide glasses pyrolysed under inert (Ar) atmosphere were studied as a function of the pyrolysis temperature. The silicon oxycarbide glasses were prepared from various alkyl substituted alkoxysilanes such as HSi(OEt)3 and HMeSi(OEt)2 in different ratios by using the sol-gel method. The Si–O–C-glasses obtained were respectively: (i) silicon oxycarbide network with excess carbon, (ii) stoichometric SiCxO2(1� x) where x=0.30 and (iii) silicon oxycarbide matrix with an excess of Si. Si–C bonds introduced in the starting silica gel network can be partially retained in the final glass after pyrolysis under inert atmosphere. After pyrolysis at temperatures between 600–1500 � C, the presence of tetracoordinated C atoms in the silica network results in an improvement of mechanical properties and thermal stability compared with silica glass. By using elemental analysis, density, SEM, BET and XRD (combined with Rietveld-analysis), the glass characterization was performed. Flexural strength (MOR), elastic modulus (E) and Vickers hardness (HV) were measured and will be discussed in terms of glass composition and microstructure. # 2002 Published by Elsevier Science Ltd.

High temperature stability of sol-gel-derived SiOC glasses

The high temperature stability of sol-gel-derived SiOC glasses has been investigated in order to understand the role played by the presence of a free-carbon phase in the amorphous oxycarbide network. Therefore, pure and excess free-carbon containing SiOC glasses have been prepared from hybrid gels and used for the high temperature stability experiments. Moreover, the possible effects of the sample size with respect to local kinetics and thermodynamics and of the testing environment have also been investigated. Results show that all these parameters play a crucial role in defining the high temperature behavior of SiOC glasses.

XPS characterization of gel-derived silicon oxycarbide glasses

Abstract The results of an XPS study of sol-gel-derived silicon oxycarbide glasses are presented. Si(2p) XPS peak showed the presence of all the possible mixed silicon oxycarbide units, i.e. SiCxO4 − x, 0 ≤ × ≤ 4. C(1s) XPS peak suggested the presence of three components at 283, 284.5–285.0 and 285.5 eV. The first two are due respectively to carbon atoms into CSi4 sites (283 eV) and aromatic carbon environment and/or aliphatic CHx, x = 1, 2 (284.5–285.0 eV), whereas the last one has been assigned to carbon atoms sharing bonds simultaneously with silicon and oxygen atoms forming SiOC units. It was also found that the fracture surface is richer in carbon compared with the bulk.

Silicon oxycarbide glasses from gels

Silicon-oxycarbide glasses can be synthesized from modified silica gels through a pyrolysis process in inert atmosphere. A critical analysis of the literature suggests a close relationship between oxygen content in the gel precursors and the amount of carbon atoms covalently bonded to silicon atoms in the corresponding silicon-oxycarbide phase. The problem of the formation of a free carbon phase will be discussed and related to the amount of carbon in the starting gel. An example of successful synthesis of an almost pure carbon-rich oxycarbide phase will also be presented.

Hybrid RSiO1.5/B2O3 Gels from Modified Silicon Alkoxides and Boric Acid

A new sol-gel route, without any external water addition, for the preparation of hybrid RSiO1.5/B2O3 gels (R = Me, Et, Vi) is reported. The gels are easily synthesized by reacting B(OH)3 with the liquid silicon alkoxide. The gels have been characterized mainly by FT-IR and 11B MAS NMR spectroscopy. The results show that this new sol-gel process allows the formation of a homogeneous borosilicate gel in which trigonal BO3 units are incorporated into the siloxane network via B—O—Si bonds.

Solid State NMR and TG/MS Study on the Transformation of Methyl Groups During Pyrolysis of Preceramic Precursors to SiOC Glasses

The sol-gel method was used to prepare two different starting gels containing SiCH3-groups for the preparation of SiOC ceramics. To understand the role of Si—H bonds in the incorporation of carbon into the SiOC network, gels prepared from a 1:2 mixture of triethoxysilane and methyldiethoxysilane (THDH2) and solely methyltriethoxysilane (TMe) were investigated. Thermogravimetric analysis coupled with mass spectroscopy (TG-MS) in inert atmosphere was performed to attain an insight into the decomposition reactions involved during gel-glass transformation. Samples calcined at different temperatures up to 1000°C were characterized by 29Si and 13C magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The presence of SiH groups in the starting gel allows an efficient conversion of Si—CH3 groups into CSi4 sites at lower temperatures. As a result, despite a much lower amount of carbon in the starting THDH2 gel (C/Si = 0.33) compared to the TMe gel (C/Si = 1), the amount of carbon inserted into the SiOC network of both glasses is equivalent, but the TMe sample contains the 10 fold amount of free carbon.

Pyrolysis study of methyl-substituted Si-H containing gels as precursors for oxycarbide glasses, by combined thermogravimetry, gas chromatographic and mass spectrometric analysis

Monolithic and transparent gels were prepared by mixing various ethoxide silicon precursors containing Si—CH3 and Si—H groups, the composition ensuring the same number of C—H and Si—H bonds. Pyrolysis of these samples was followed under helium flow by connecting thermogravimetry, gas chromatographic and mass spectrometric analysis, to study the conversion of the gels into oxycarbide materials. In addition to the usual direct thermal and mass spectra analysis (TG–MS), a TG–GC–MS arrangement, allowing gas chromatographic separation of the species simultaneously evolving during thermodecomposition followed by mass spectral analysis, was successfully achieved. Experimental results indicate that mass loss occurs in three steps, each characterized by specific reactions. At low temperatures, densification of the siloxane network derives from further condensation reactions. At intermediate temperatures, a remarkable rearrangement of the siloxane chains occurs, with the release of volatile silanes and several siloxane fragments due to Si—H and Si—O bond exchanges. At higher temperatures, the development of methane was detected and attributed to Si—C bond cleavage. Pyrolysis of gels containing only Si—CH3 or Si—H groups was also studied for comparison.

Synthesis of a polycyclic silazane network and its evolution to silicon carbonitride glass

Abstract Polysilazanes have been described as excellent polymeric precursors to amorphous silicon carbonitride (SiCN) glasses. In this work, the synthesis, thermal cross-linking and pyrolysis of a polycyclic silazane network precursor for a SiCN glass, were studied. This polymeric precursor was prepared by free radical polymerization of 1,3,5-trimethyl-1′,3′,5′-trivinylcyclotrisilazane, using dicumyl peroxide as radical initiator. The polymer-to-ceramic conversion was investigated by 29 Si and 13 C magic angle spinning nuclear magnetic resonance (MAS NMR) and infrared spectroscopies, X-ray diffraction and by simultaneous thermogravimetric and differential thermal analyses. Based on the results the polymeric precursor can be characterized as a polycyclic silazane network, containing aliphatic carbon segments produced by the vinyl polymerization. 29 Si and 13 C NMR data of this polymeric precursor heated at different temperatures indicated the occurrence of side-reactions in addition to the vinyl polymerization. The SiCN glass was obtained as an amorphous solid up to 1400 ° C . Above this temperature, the crystallization process of the ceramic product was promoted by rearrangement of the different Si sites in the amorphous network. Heat treatment at 1600 ° C , under N2 atmosphere, produced a ceramic containing 85% silicon carbide (SiC) and 15% silicon nitride, determined by 29 Si MAS NMR in relation to the total silicon sites, and under argon atmosphere, SiC.

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%.

Si nanocrystals obtained through polymer pyrolysis

In this letter, we report the formation of bulk samples of silica-based glass containing Si nanocrystals (Si-ncs) by pyrolysis of a preceramic precursor. The starting precursor is a sol–gel-derived polysiloxane containing only Si–H groups which leads, after annealing in a controlled atmosphere in the range 1000–1200 °C, to the precipitation of Si-ncs. Characterization of the nanostructure was performed by x-ray diffraction and Raman scattering analyses. Room-temperature luminescence experiments show the interesting optical properties of the Si-ncs/SiO2 material.