Abstract
Two dominant crystalline phases of silicon carbide (SiC): α-SiC and β-SiC, differing in size and chemical composition, were investigated regarding their potential for dynamic nuclear polarization (DNP). (29)Si nuclei in α-SiC micro- and nanoparticles with sizes ranging from 650 nm to 2.2 μm and minimal oxidation were successfully hyperpolarized without the use of free radicals, while β-SiC samples did not display appreciable degrees of polarization under the same polarization conditions. Long T(1) relaxation times in α-SiC of up to 1600 s (∼27 min) were recorded for the (29)Si nuclei after 1 h of polarization at a temperature of 4 K. Interestingly, these promising α-SiC particles allowed for direct hyperpolarization of both (29)Si and (13)C nuclei, resulting in comparably strong signal amplifications. Moreover, the T(1) relaxation time of (13)C nuclei in 750 nm-sized α-SiC particles was over 33 min, which far exceeds T(1) times of conventional (13)C DNP probes with values in the order of 1-2 min. The present work demonstrates the feasibility of DNP on SiC micro- and nanoparticles and highlights their potential as hyperpolarized magnetic resonance imaging agents.