Short time inversion recovery

This section discusses 3D-SPRITE which has been shown to be successful for studying short relaxation time systems and which is free from distortions due to susceptibility variations for coals with high resolution at a frequency of 400 MHz.55 The results obtained are discussed in relation to the three-dimensional distribution of the mobile component for two kinds of coal, Witbank and Goonyella. At the same time, inversion recovery preparation experiments (7 mapping), T2 mapping and T mapping based on SPRITE methods are presented in order to clarify the chemical heterogeneity of coals. 

If it is desired to emphasize Tt differences, one easy choice is to repeat the pulse sequence with a short repetition time TR, so that magnetizations are unable to relax completely back to equilibrium. Thus signals Corn water molecules with long Tj are reduced in intensity. Alternatively, an inversion recovery sequence (180°, T, 90°) can be applied instead of just a 90° pulse to excite the resonance. With careful selection of the value of r, as indicated in Fig. 14.4, T, differences can be emphasized. 

Fig. 3. Simulated partially relaxed powder spectra, calculated assuming a two-site jump and for an inversion recovery sequence with a solid echo for detection and recovery times of (a) 1.5 ms, (b) 5 ms, (c) 10 ms, (d) 20 ms and (e) 500 ms. Correlation times are 0.35 and 20 ns for the left and right columns, demonstrating the different Ti anisotropy for short and long correlation times. (Adapted with permission from Ref. 456.)...

It is possible to monitor relaxation times by modulating the microwave amplitude at a rate close to the inverse spin-lattice relaxation time, Tj (Misra, 2004). This is due to the fact that if the microwave amplitude is changed faster than that which the spins can follow as governed by the electron SLR time, the EPR signal is determined predominantly by T. This modulation technique permits measurement of relaxation times in the interval between the values that are slow enough to be measured by the recovery techniques and the values that are too fast to have any significant impact on continuous wave (cw) lineshape. These latter relaxation times can be as short as 10 s, which is substantially shorter than those that can be measured by the saturation/inversion recovery techniques. 

Due to the trace division procedure, the noise in the resulting RE time trace (Fig. 7(B)) is growing towards long times, therefore data with good SNR are essential for proper distance extraction. On the other hand, detection in inversion recovery experiment can be done with very short inter-pulse delay, only limited by the dead time of the spectrometer, which makes the detected echo much stronger than in a DEER experiment. At X band, where RE-based distance measurements were mainly performed so far, the concentration sensitivity of RE technique seems to be somewhat better that the one of DEER. ° ... 

Creep behavior is similar to viscous flow. The behavior in Equation 14.17 shows that compliance and strain are linearly related and inversely related to stress. This linear behavior is typical for most amorphous polymers for small strains over short periods of time. Further, the overall effect of a number of such imposed stresses is additive. Non-creep-related recovery... 

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