Investigation of cured hydridopolysilazane-derived ceramic fibers via dynamic nuclear polarization
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During the pyrolysis of cured hydridopolysilazane (HPZ) polymer to form S i - C - N ceramic fibers, large quantities of unpaired electrons are produced. For such materials dynamic nuclear polarization (DNP) provides a means of enhancing the intensity of the NMR signal, and supplies information on the localization or delocalization of unpaired electrons. 29 Si, XH, and 13C DNP gives enhancements of 250 for 13C and at least 800 for 29 Si. 13 C and 29 Si DNP-DPMAS experiments yield the following results: (i) there are at least two distinct types of carbon, sp2 and sp3; (ii) there is a distribution of sp3 silicon environments; (iii) the unpaired electrons behave as fixed paramagnetic centers; and (iv) the unpaired electrons are distributed homogeneously throughout the sample. Proton DNP spectra can be obtained even though hydrogen is only a trace element in the finished ceramic. I. INTRODUCTION Attention has increasingly been focused on the synthesis of ceramics by pyrolysis of polymers.1"4 Hydridopolysilazane, or HPZ, has recently found industrial application as a pyrolysis precursor to ceramic silicon nitride1; however, silicon carbide, graphite, and uncharacterized hydrogen-containing impurities exist in the finished product. The pyrolysis process often leaves a solid amorphous material with numerous paramagnetic centers in the finished product. These types of materials can be difficult to characterize by traditional analytical methods. The three most prominent analytical techniques for analyzing these materials have been x-ray diffraction (XRD), vibrational spectroscopy, and solid-state NMR. Obtaining information about chemical structure (e.g., sp2/sp3 ratio) via XRD requires a large degree of crystallinity, or long-range order, in the sample under observation. The applicability of vibrational spectroscopy can be limited by complexity of the spectra and sample preparation requirements. For example, transmission infrared spectroscopy requires that the sample be ground to an appropriate particle size, or light scattering will give false bands.5 The other vibrational spectroscopy usually used in analysis of ceramics is attenuated total reflectance (ATR) infrared spectroscopy. Both ATR and XRD often do not adequately probe the bulk of a ceramic sample. ATR gives information on chemical species mainly within 0.5 nm of the surface6 and XRD is mainly useful for crystallites within the sample. Solid-state NMR techniques are not limited to systems near a surface or with a large degree of long-range a)
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Present address: Chemagnetics, Inc., 2555 Midpoint Drive, Fort Collins, Colorado 80525. 'Author to whom correspondence should be addressed. J. Mater. Res., Vol. 8, No. 3, Mar 1993 http://journals.cambridge.org
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order, and hence are able to probe the composition of the overall sample, i.e., surface, bulk, amorphous, and crystalline regions. NMR does not normally require extensive sample preparation prior to analysis. However, traditional NMR can be handicapped by the presence of paramagnetics in the
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