Synthesis and Characterization of Silicon Oxycarbide Derived Nanocomposites Obtained through Ceramic Processing of TEOS/PDMS Preceramic Materials

Abstract

Bulk silicon oxycarbide derived ceramic nanocomposites have been prepared by the application of the conventional ceramic processing to preceramic materials. Tetraethylortosilicate/ polydimethylsiloxane preceramic materials obtained by sol-gel process were thermally treated and attrition milled to 4 micrometers. Subsequently, the preceramic powders were pyrolized at 1100 °C to obtain silicon oxycarbide powders that were pressed and sintered at 1550 °C up to 16 hours. Silicon oxycarbide glasses obtained at 1100 °C from pyrolysis of preceramic materials consist of a Si-O-C network and a carbon like graphite phase well dispersed. At annealing temperatures higher than 1100°C silicon oxycarbide glasses undergo a rearrangement which involves a phase separation to silica and silicon carbide and a segregation of carbon like graphite phase. At these temperatures the material can be considered as a glassy matrix nanocomposite. At temperatures higher than 1500 °C the carbothermal reduction occurs with the consumption of both silica and free carbon phase. However, the nanocomposite structure is maintained but with different constituents. The silicon oxycarbide glasses obtained at 1100 °C are amorphous. However, as a result of all involving processes taken place during the ceramic process, the nanocomposites formed at 1550 °C comprise a silica matrix and nanodomains of carbon like graphite and silicon carbide both of them displaying an incipient crystallization. Structure and crystalline size evolution, from preceramic materials to silicon oxycarbide derived nanocomposites, have been determined by FT-IR and Raman spectroscopies, XRD and 29Si-MASNMR.

Surface and Structural Modification of Nanostructured Mesoporous Silicon Oxycarbide Glasses Obtained from Preceramic Hybrids Aged in NH4OH

“…The results deduced from the analysis of the infrared spectra are similar to that found by means of 29 Si NMR spectroscopy. Here, the r-SiOC glass presents a large amount of Si-C bonds and free silanol groups in the form of Q 2 and Q 3 species, whereas more crosslinked networks with a predominance of Q 4 and Q 3 species are obtained in the materials aged in NH 4 OH solutions.…”

“…The polycondensation reactions that take place in solution to form the preceramic material have been described in the system TEOS/PDMS by Velasco et al . whereas the full description of the reactions in the system TREOS/PDMS can be also found in the literature . Mazo compared the reactions involved in the two systems and concluded that the introduction of the Si–H functionalities only produces an increase in the crosslinking degree at temperatures above 300°C because of the condensation reaction of Si–H with Si–OH.…”

“…The polycondensation reactions that take place in solution to form the preceramic material have been described in the system TEOS/PDMS by Velasco et al 48 whereas the full description of the reactions in the system TREOS/PDMS can be also found in the literature. 21,29 Mazo 21 compared the reactions involved in the two systems and concluded that the introduction of the Si-H functionalities only produces an increase in the crosslinking degree at temperatures above 300°C because of the condensation reaction of Si-H with Si-OH. In this work, the addition of NH 4 OH in the preceramic precursors only produces a larger concentration of free silanol groups as a product of the disproportionation of the Si-H-containing species, which is translated in an increase in the crosslinking degree in the hybrid state.…”

“…The structural characterization of the obtained ceramics was carried out by means of 29 Si nuclear magnetic resonance (NMR), Raman and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The 29 Si NMR experiments were carried out in a Bruker AV 400 WB (Dresden, Germany) equipment operating at 79.41 MHz. Raman spectra were collected in a Renishaw In Via spectrometer (Glowcestershire, UK) under the excitation of a 514 cm À1 wavelength laser.…”

“…[26][27][28] In this study, we report on the nanostructured silicon oxycarbide obtained with a narrow bimodal pore size distribution from TREOS and polydimethlylsiloxane (PDMS). Based on previous work, 21,29 the higher reactivity of the silanes with the hydrogen functionality compared with nonfunctionalized silanes such as tetraethoxysilane (TEOS) allowed the obtaining of silicon oxycarbide glasses with higher surface area than in the materials obtained from the well-known system TEOS-PDMS. [18][19][20] Here, we study the effects produced in both the structural characteristics and the surface of the obtained ceramics because of the different carbon content and its nature with regard to the potential application of this sort of material in catalytic applications and heterogeneous chemistry.…”