The spin dynamics of dissolution DNP samples consisting of 4.5 M [ 13 C]urea in a mixture of (1/1)Vol glycerol/water using 4-Oxo-TEMPO as a radical was investigated. We analyzed the DNP dynamics as function of radical concentration at 7 T and 3.4 T static magnetic field as well as function of deuteration of the solvent matrix at the high field. The spin dynamics could be reproduced in all cases, at least qualitatively, by a thermodynamic model based on spin temperatures of the nuclear Zeeman baths and an electron non-Zeeman (dipolar) bath. We find, however, that at high field (7 T) and low radical concentrations (25 mM) the nuclear spins do not reach the same spin temperature indicating a weak coupling of the two baths. At higher radical concentrations, as well as for all radical concentrations at low field (3.4 T), the two nuclear Zeeman baths reach the same spin temperature within experimental errors. Additionally, the spin system was prepared with different initial conditions. For these cases, the thermodynamic model was able to predict the time evolution of the system well.While the DNP profiles do not give clear indications to a specific polarization transfer mechanism, at high field (7 T) increased coupling is seen. The EPR line shapes cannot clarify this in absence of ELDOR type experiments, nevertheless DNP profiles and dynamics under frequency-modulated microwave irradiation illustrate the expected increase in coupling between electrons with increasing radical concentration.

Abstract. Electroplating the waveguide of a 7 T polarizer in a simple innovative way increased microwave power delivered to the sample by 3.1 dB. Silicon particles, while interesting for hyperpolarized MRI applications, are challenging to polarize due to inefficient microwave multipliers at the electron Larmor frequency at high magnetic fields and fast electronic relaxation times. Improving microwave transmission directly translates to more efficient EPR excitation at high-field, low-temperature conditions and promises faster and higher 29Si polarization buildup through dynamic nuclear polarization (DNP).

Abstract. Electroplating the waveguide of a 7 T polarizer in a simple innovative way increased microwave power delivered to the sample by 3.1 dB. Silicon particles, while interesting for hyperpolarized MRI applications, are challenging to polarize due to inefficient microwave multipliers at the electron Lamour frequency at high magnetic fields and fast electronic relaxation times. Improving microwave transmission directly translates to significantly higher EPR signal at high-field, low-temperature conditions and promises faster and higher 29Si polarization build-up through DNP.

Porous Si nanoparticles (NPs) with different doping degree were prepared using low-load metal assisted catalytic etching and subjected to dynamic nuclear polarization at 3.4 T and 6.7 T. Thermal oxidation of Si was applied to form paramagnetic centers of dangling bond type in Si/SiO2 interface, which were used to polarize 29Si nuclei. The doping significantly affected the gained polarization and buildup times: high doping degree generally led to lower and faster polarization compared to the low doping. On the other hand, slight p-type or n-type doping was necessary to achieve the highest polarization of about 11 %.