On the Mass Fractal Character of Si-Based Structural Networks in Amorphous Polymer Derived Ceramics
The intermediate-range packing of SiNxC4−x (0 ≤ x ≤ 4) tetrahedra in polysilycarbodiimide and polysilazane-derived amorphous SiCN ceramics is investigated using 29Si spin-lattice relaxation nuclear magnetic resonance (SLR NMR) spectroscopy. The SiCN network in the polysilylcarbodiimide-derived ceramic consists predominantly of SiN4 tetrahedra that are characterized by a 3-dimensional spatial distribution signifying compact packing of such units to form amorphous Si3N4 clusters. On the other hand, the SiCN network of the polysilazane-derived ceramic is characterized by mixed bonded SiNxC4−x tetrahedra that are inefficiently packed with a mass fractal dimension of Df ~2.5 that is significantly lower than the embedding Euclidean dimension (D = 3). This result unequivocally confirms the hypothesis that the presence of dissimilar atoms, namely, 4-coordinated C and 3-coordinated N, in the nearest neighbor environment of Si along with some exclusion in connectivity between SiCxN4−x tetrahedra with widely different N:C ratios and the absence of bonding between C and N result in steric hindrance to an efficient packing of these structural units. It is noted that similar inefficiencies in packing are observed in polymer-derived amorphous SiOC ceramics as well as in proteins and binary hard sphere systems.
Review: Silicon oxycarbide based materials for biomedical applications
Abstract Silicon oxycarbide (SiOC) based materials are of interest due to their unique properties, including high-temperature resistance, mechanical strength, chemical durability and corrosion resistance. The versatility of the silicon oxycarbide chemistry allows the modification of properties for different fields. In the last decade, attention has been paid to the potential use of SiOC materials in biomedical applications. Surface functionalization of SiOC is a promising approach to create efficient and controllable drug delivery systems with adequate molecule adsorption and release kinetics. The analysis of the literature reveals that silicon oxycarbides have also potential applications as protective biocompatible coatings, for example in blood-contact devices. Additionally, by incorporating metallic ions (i.e. Ca2+, Mg2+, Ag+) in the material network structure, promising antibacterial properties and bioactive behavior have been observed despite the high network connectivity values compared to silicate bioactive glasses. This review covers studies which have reported the use of silicon oxycarbide based materials for biomedical applications. Advances in the evaluation of biocompatibility, bioactivity, surface functionalization and antibacterial effects are discussed in detail, highlighting advantages and limitations of SiOC systems in the biomedical field.
Microstructural instability and the resultant strength of SiCO (Nicalon) and SiNCO (HPZ) fibres
Abstract Microstructural changes and the related strength degradation were investigated for SiCO Nicalon and SiNCO HPZ fibres during heat treatments in air and argon at 1000–1400 °C for 0.5 to 90 h. While the as-received Nicalon fibre contains β-SiC and carbon nanocrystals in an amorphous SiO 1.12 C 0.44 phase, the HPZ fibre is completely amorphous. It is also inhomogeneous in surface composition compared with that of the bulk. The time-dependent strength degradation of the Nicalon fibre in argon is related to the gradual decomposition of the SiO 1.12 C 0.44 phase from the surface which produces surface defects, β-SiC grain growth and intergranular porosity. The strength degradation of the HPZ fibre results from surface crystallization into α-SiO 2 , Si 2 N 2 O and β-SiC. On the other hand, the HPZ fibre core — which has composition close to SiN 1.02 C 0.23 — shows structural stability for all heat treatment conditions.