Dynamic Polarization Experiments Research Articles

Comparison of liquid and gel electrolytes for the investigation of pitting corrosion on stainless steels

Electrochemical methods have become established for the investigation and testing of stainless steels for their resistance to pitting corrosion. In contrast to liquid electrolytes, which are normally used for this purpose, gel-like electrolytes have clear advantages regarding their usability. However, it must be clarified whether both types of electrolyte lead to the same or different results. In the investigations presented here, various agar-based gel electrolytes were characterized regarding their corrosion-relevant properties, such as chemical composition, rheo-logical properties and the electrolyte film forming on the surface. To compare the two electrolyte types, electrochemical investigations were carried out with graduated agar and NaCl concentrations on the material 1.4301 (AISI 304). Pitting and repassivation potentials were determined by dynamic polarization experiments and the varied factors were statistically evaluated regarding their influence. The results show that liquid and gel electrolytes provide comparable results. However, the pitting corrosion phenomena under external polarization are shallower with gel electrolytes and therefore their repassivation is supported. Also, the use of gel electrolytes with indicators for iron ions, which at the same time act as oxidizing agents and allow polarization into critical areas, gives comparable results. Gel electrolytes are therefore a very good alternative to liquid electrolytes if the detection of corrosion-sensitive surfaces is in focus.

Dynamic deuteron polarization measurements performed in a new type of horizontal dilution cryostat

We have reached 31% deuteron polarization in D-ammonia (ND 3) and 27% in D-butanol (C 4D 10O). The dynamic polarization experiments were performed at a magnetic field of 2.5 T in a new type of horizontal dilution cryostat. This dilution cryostat, built for target asymmetry measurements with a photon beam, was developed with special regard to fast cool-down and easy loading of the target material. The cooling power is 5 mW at 0.2 K, 20 mW at 0.3 K and 34 mW at 0.4 K. Starting from room temperature the lowest temperature of 165 mK is reached in about 2 h including the loading of the target material.

Dynamic nuclear polarization in irradiated ammonia below 0.5 K

We have reached +90.5% and −93.6% dynamic proton polarization in solid NH 3 using paramagnetic radicals created by proton irradiation of 40 Mrad total dose. The dynamic polarization experiments were performed at 25 kG field in a dilution refrigerator. These results may indicate a breakthrough in the development of better polarized target materials for high-energy physics experiments.

Dynamic orientation of nuclei at low temperatures

A study has been made of the mechanisms of dynamic polarization of nuclei in organic materials at temperatures below 1 K and in magnetic fields up to 50 kG. After a summary of the mechanisms of dynamic polarization, the experimental results on 1,2-propanediol, doped with paramagnetic Cr V complexes, are compared quantitatively with a spin temperature model of Borghini, in which it is assumed that the nuclear polarization is obtained via thermal contact with a dynamically cooled electron spin-spin interaction reservoir. The strong influence of this latter reservoir was apparent from the transient behavior of different nuclear spin systems contained in the same sample. The abovementioned spin temperature model has the unique property of not using the simplifying “high-temperature approximation” and it was the first time that this model has been found to be in reasonable agreement with experimental data in the 100 mK region. Furthermore, dynamic polarization experiments with the free radical BDPA are reviewed. These measurements show clearly the contributions of different mechanisms of dynamic polarization. The results are found to be in agreement with formulas derived from the spin temperature theory after introduction of some simplifying assumptions in order to be able to deal with low temperatures.

The Nuclear-Electron Overhauser Effect of Carbon-13 as Studied by Multi-Field Dynamic Polarization Experiments in Solutions

Abstract Experimental dynamic nuclear polarization (DNP) studies at 176 gauss, 3420 gauss, and 13 200 gauss and at various temperatures have been concerned with the Overhauser effect of solvent carbon-13 nuclei in solutions of free radicals. Detailed measurements have been made in solutions of benzene and methyl iodide. The relative importance of scalar and dipolar coupling, the correlation times and spectral intensity functions have been of the greatest interest. The importance of the “three-spin-effect” has been investigated by applying triple-resonance irradiation. Dipolar and scalar hyperfine couplings between carbon-13 nuclei and odd electrons are of comparable magnitude, where the main difference of the DNP enhancement factors is caused by different shapes of the spectral intensity function for the fluctuations of the scalar interaction. The results are discussed in terms of the molecular structure and of the collision attitudes.

Dynamic Polarization in Solutions of Sodium in Liquid Ammonia

Dynamic polarization experiments have been carried out in dilute solutions of sodium in liquid ammonia (mole ratio R > 130) at room temperature in a magnetic field of 3.3 kG. A positive Overhauser enhancement corresponding to a leakage factor of 0.5 was measured for 14N, showing an important relaxation process due to the hyperfine interaction with electrons. The paramegnetic shift of 14N was measured in a direct way, and the results are in agreement with those of other authors. Overhauser shift measurements were performed for 14N and H; an experimental proof of a negative spin density at the proton was thus obtained.

Dynamic Polarization in Asphaltene Solutions

Dynamic polarization experiments in solutions of asphaltene in xylene have been performed at both low and high magnetic fields. The interpretation of the experiments leads to the conclusion that the EPR line of the asphaltene is inhomogeneously broadened. The characteristics of the spin packet are deduced. In particular, the spin-packet width is found to be constant for many different experimental conditions. Specific information is obtained on the residence time of a xylene molecule on the asphaltene particle.