Structure and Connectivity in an Amorphous Silicon Oxycarbide Polymer-Derived Ceramic: Results from 2D ²⁹Si NMR Spectroscopy

Abstract

The atomic structure of the Si–O–C tetrahedral network of an amorphous silicon oxycarbide polymer-derived ceramic (PDC) of the composition SiO0.94±0.11C1.13±0.08 was studied at both the short range and the intermediate range using 1D and 2D 29Si nuclear magnetic resonance (NMR) spectroscopic techniques, respectively . The 1D 29Si magic angle-spinning NMR spectrum of the PDC indicates that the Si–O–C network consists of SiO4, SiO3C, SiO2C2, and SiC4 units with relative abundances of approximately 26, 25, 20, and 29%, respectively. The 2D 29Si extended CSA amplification spectrum of this PDC shows that the chemical shift anisotropy (Δ) of the mixed-bond SiO x C4–x units is significantly higher than that of the SiO4 units. On the other hand, the unusually high Δ-value for the SiC4 units was interpreted to be indicative of its role as the connecting element between the Si–O–C network and the free-carbon nanodomains. The 2D 29Si double-quantum correlation NMR spectrum of this PDC indicates that there is extensive direct linking between SiO4 and SiO3C units in the Si–O–C network besides the connectivity between like SiO x C4–x units, while the SiO4 and SiO2C2 units are only linked via a SiO3C unit. In contrast, the SiO3C units show no restriction in linking with the other SiO x C4–x units in the network. Finally, the SiC4 units show significant clustering, which is consistent with their spatial localization at the interface between the Si–O–C network and the sp2 C nanodomains. Such a spatial distribution of the SiO x C4–x units is argued to be consistent with their mass-fractal dimensions measured in previous studies.

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

“…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 71–73 .…”

“…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. NMR is powerful for characterization of short range order in ceramics, and analysis of the spectra permits differentiation of SiN x C 4− x mixed bonding environments in PDCs 58,70,79 . Typically, 29 Si MAS NMR peak shifts are associated with different coordination environments (oxygen in SiO x C 4− x or nitrogen in SiN x C 4− x mixed bonds) 81 .…”

“…46 Typically, PDCs pyrolyzed at lower temperatures are amorphous. 30,70 At high temperatures, crystallization in PDCs is promoted by carbothermal reduction. [71][72][73] The XRD patterns of the PDCs are summarized in Figure 4 and the results point to growth of poorly crystalline β-SiC grains in the SiC(O) derived from SMP-10.…”

“…NMR is powerful for characterization of short range order in ceramics, and analysis of the spectra permits differentiation of SiN x C 4−x mixed bonding environments in PDCs. 58,70,79 Typically, 29 Si MAS NMR peak shifts are associated with different coordination environments (oxygen in SiO x C 4−x or nitrogen in SiN x C 4−x mixed bonds). 81 29 Si MAS NMR experiments are performed on all samples and results are shown in Figure 6.…”

Analysis of the Lithium Storage Mechanism in the SiOx/C Composite Based on the Performance Variation Applied to a Lithium-Ion Battery

“…Consistent with the XPS results before cycling, two resonances assigned to SiO 4 (−108 ppm) and SiO 3 C (−76 ppm) are observed, with SiO 4 accounting for 81.7% of the total fitted peak area and SiO 3 C making up 18.7% . Furthermore, no signals of SiO 2 C 2 (−37 ppm), SiOC 3 (−5 ppm), or SiC 4 (−10 ppm) are detected during the entire cycling process, indicating the absence of conversion among SiO x C 4– x units . After the first lithiation, the curve shifts to a high field as a result of the insertion of lithium ions and moved in the opposite direction when lithium ions are extracted.…”