Nanodomain Structure and Energetics of Carbon Rich SiCN and SiBCN Polymer-Derived Ceramics

This Ph.D. thesis focuses on the synthesis, processing, solid state structure, nanodomain structure, structural evolution, thermodynamic stability, and functional properties of carbon rich SiCN and SiBCN ceramics derived from preceramic polymers with tailored compositions and structures. The main objective of the studies is to better understand the effects of the composition and structure of the starting precursors, on the behavior of the resultant ceramics. First, a set of preceramic polymers with systematically varied compositions and structures were synthesized. They are linear polysilylcarbodiimides and polysilazanes, and their boron modified counterparts; branched polysilsesquicarbodiimide and its 13C/15N isotope-enriched counterpart; and branched polysilsesquiazane. The synthesis of these polymers was investigated using NMR, FT-IR, Raman and TG/DTG. The results demonstrate that the obtained precursors exhibit the expected compositions and structures. Then, the effects of processing route on the thermal stability of PDCs were studied by comparing bulk ceramics with their powder counterparts. The thermal transformation was investigated using FT-IR, Raman spectroscopy, XRD, TG/DTG and TEM. The results reveal that bulk ceramics are more thermally stable than their powder counterparts in terms of resistance to crystallization and decomposition. The Si(B)CN ceramic powders derived from poly(boro)phenylvinylsilylcarbodiimide contain α/β-SiC crystallites when heat treated at 1400ºC, while their bulk counterparts prepared at the same temperature remain completely amorphous. It is also found that bulk ceramics exhibit less weight loss than their powder analogues at temperatures up to 2100°C, especially in the case of SiCN bulk ceramics. The higher thermal stability of bulk ceramics as compared with the powder counterparts is likely due to the powders have greater surface area which enhances the carbothermal reaction and silicon carbide crystallization. In addition, the boron modification impedes the degradation of silicon nitride in both bulk and powder samples, similar to previously reported results. Next, the energetics of PDCs was investigated using high temperature oxidative drop solution calorimetry on (i) ceramics derived from branched and linear polysilylcarbodiimide; (ii) ceramics derived from poly(boro)silazanes pyrolyzed at different temperatures. The results reveal that the ceramics derived from the branched polymer is energetically more stable than those from the linear polymer. Structural analysis using MAS NMR suggests that the increased energetic stability of the ceramics derived from the branched polymer is likely due to the presence of hydrogen at the mixed bonding environments consisting of N, C and Si atoms. These environments make up the interfacial region between the Si3N4 and “free” carbon nanodomains. For both the linear and branched polymers, the ceramics derived at 800 ºC are energetically more stable than those derived at 1100 ºC. The study of group (ii) reveals the effect of the pyrolysis temperature on the structural evolution and energetics of poly(boro)silazane-derived Si(B)CN ceramics. These ceramics contain mixed SiCxN4-x(x=0-4) tetrahedra. MAS NMR spectroscopy of the 1100ºC and 1400ºC ceramics reveals that the structural evolution involves the following processes: (i) the demixing of SiCxN4-x mixed bonding environments, (ii) the cleavage of mixed bonds at the interdomain regions, and (iii) the coarsening of domains. Calorimetry results demonstrate that this structural evolution is favorable in both enthalpy and free energy. We also investigated the solid state structures and nanodomain structures of the ceramics derived from the tailored polymers. The MAS NMR results indicate that the SiCN ceramics derived from polyphenylvinylsilylcarbodiimide and polymethylvinylsilylcarbo- diimide both contain nanodomains of silicon nitride and “free” carbon. In the ceramics prepared from the phenyl-containing polymer, the “free” carbon and silicon nitride domains are basically isolated, while in the ceramics derived from the methyl-containing polymer, “free” carbon and silicon nitride domains are connected via C-N bonds. The SiBCN ceramics derived from boron-modified polysilylcarbodiimides have an additional B-containing phase which is located at the interface between the silicon nitride and the “free” carbon phase. On the other hand, the SiCN ceramics derived from polysilazanes contain “free” carbon and mixed bonded SiCxN4-x(x=0-4) nanodomains. The SiCxN4-x (x=0-4) domain consists of a core of SiN4 tetrahedra, which connect to “free” carbon domain via SiN3C, SiN2C2, SiNC3, and SiC4 tetrahedra. SiC4 tetrahedra make up the most outer shell of mixed bonded SiCxN4-x(x=0-4) nanodomains. The SiBCN ceramics derived from boron-modified polysilazanes have an additional B-containing phase which connects the SiC4 with the “free” carbon domains. SAXS results indicate that (i) the size of Si-containing nanodomains in SiCN ceramic is larger than in their SiBCN counterparts; (ii) the ceramics derived from phenyl-containing polymers exhibit smaller Si-containing nanodomains than those derived from methyl-containing polymers; and (iii) the size of Si-containing domains increases with pyrolysis temperature in all ceramics. Finally, some functional behaviors of the resultant ceramics were investigated. The electrochemical properties of the ceramics were investigated to explore their applicability as anode materials for lithium-ion batteries. The results reveal that the SiCN ceramics derived from both linear and branched polymers are suitable anode materials for lithium-ion batteries. In particular, the SiCN ceramics derived from linear polyphenylvinylsilazane demonstrate outstanding performance in terms of cycling stability and capacity at high currents. In addition, the AC conductivity was characterized using impedance spectroscopy. It is found that the impedance spectra vary in accordance to the different microstructures of the ceramics, which, in turn, are related to the chemistry of the precursors. Conduction in SiCN ceramics is dominated via “free” carbon and SiC phases in series, as analyzed by two semicircles in the Nyquist plot. Conduction in SiBCN ceramics is dominated by one phase, and is thus represented by one semicircle in the Nyquist plot. Nonconducting BN forms the interfacial phase between the “free” carbon and SiC phases, and isolates the discontinuous phase from the continuous phase. Therefore, conduction in SiBCN ceramics is dominated by the continuous phase, whether it is “free carbon” or SiC