Thermodynamic stabilization of crystalline silicon carbide polymer‐derived ceramic fibers

Abstract

Three crystalline SiC fibers were studied: Tyranno, Hi-Nicalon, and Sylramic.Thermodynamic stability of the SiC fibers was determined by high temperature oxide melt solution calorimetry. Results shed light on the thermodynamic penalty or benefit associated with microstructural modification of the ceramic fibers, and how energetics correlate to mechanical properties. Enthalpies of formation from components (SiC, SiO 2 , Si 3 N 4 , and C, ∆H • f,comp ) for Tyranno, Hi-Nicalon, and Sylramic are −12.05 ± 8.71, −58.75 ± 6.93, and −71.10 ± 8.71 kJ/mol Si, respectively. The microstructure in Sylramic offers the greatest stabilizing effect, thus resulting in its much more exothermic enthalpy of formation relative to elements and crystalline components. In contrast, the microstructure in Tyranno offers the least stabilization. The thermodynamic stability of the fibers increases with increasing mixed bonding (Si bonded to both C and O). From mechanical testing, Young's moduli of Tyranno, Hi-Nicalon, and Sylramic are 112, 205, and 215 GPa, respectively. Greater thermodynamic stability is correlated with a higher Young's modulus.

Energetics and structure of SiC(N)(O) polymer‐derived ceramics

“…58,59 During dissolution of SiC(O) and SiCN(O) samples, N, and C evolve as N 2 , and CO 2 , respectively, and Si is oxidized and precipitates as high temperature cristobalite (SiO 2 ). 46 The dissolution of SiCN(O) is described as follows:…”

“…68,69 XRD permits identification of crystallization and can help determine structural differences in crystalline materials including some SiC PDC fibers. 46 Typically, PDCs pyrolyzed at lower temperatures are amorphous. 30,70 At high temperatures, crystallization in PDCs is promoted by carbothermal reduction.…”

“…D (∼1330 cm −1 ) and G (∼1600 cm −1 ) bands correspond to presence of sp 2 -hybridized disordered and graphitic-like carbons, respectively. 36,46 The integrated intensity of D TA B L E 3 Thermochemical cycle for calculation of enthalpy of formation of SiC(O) from elements ∆H • f, elem . XPS is efficient for identification of different Si 2p, C 1s, O 1s, and N 1s bonds; however, it is challenging to identify different SiN x C 4−x bonds by XPS alone.…”

“…Across all structures studied by calorimetry, each with a different composition, enthalpies from components range from −128 to +20 kJ/mol (see Table 7), note that results in Table 7 are in terms of per one mol of Si (kJ/mol Si). 46,58,[91][92][93] As PDCs are amorphous, their entropies of formation from crystalline components are expected to be positive, arising from disorder. Such positive entropy effects add to the stabilization relative to binary crystalline components.…”

“…[43][44][45] Nonetheless, spectroscopic techniques like Fourier transform infrared spectroscopy (FTIR), X-ray photoemission spectroscopy (XPS), and nuclear magnetic resonance (NMR) can be used to determine bonding speciation in the structure of PDCs. 46,47 Recently, PDCs fueled the development of commercial and industrial polymeric precursors for the synthesis of high temperature ceramics with tunable compositions. [48][49][50][51] Typically, SiC PDCs are less thermally stable than SiCN and SiOCs ceramics, which results in early crystallization of amorphous domains in binary SiC ceramics.…”