Fringe-field gradient

The use of pulsed gradients and fringe field gradients in NMR diffusion measurements has been widely documented in the research literature and is reviewed in other articles of this encyclopedia. Provided certain conditions are fulfilled, such as factorization of the effects of relaxation and diffusion and translational invariance, the pulsed-gradient techniques permit measurement of the average displacement... 

Since the fringe field gradient has a fixed strength at the position of the probe head, the only experimental parameters of the pulse schemes shown in Fig. 3 that can be varied are the time intervals between the RF pulses. That is, the echo attenuation by relaxation must be considered and compensated for. This can conveniently be done with the aid of self-compensating pulse schemes [14]. 

Fig. 8.1.1 [Mal3] Principle of STRAFl imaging. The STRAFl plane is defined by a region of constant field gradient in the fringe field outside the magnet. The plane can be shifted through the sample by energizing sweep coils, or the sample is moved through the STRAFl plane for pointwise acquisition of projections.

PNMR = Pulse gradient-field spin echo (PGSE) NMR, DNMR = diffusion ordered spectroscopy (DOSY) NMR, Elec = Electrochemistry, ENMR = fringe field NMR. 

Centre field (T) Centre frequency (MHz) Bore (mm) STRAFI (V Fringe frequency Gradient (MHz) (Tm- ) ... 

Some care must be taken with fringe field methods if motional averaging of dipolar interactions is incomplete. Echoes then tend to be modulated by the so-called dipolar correlation effect [15, 16]. One can account for this phenomenon by dividing the (normalized) echo amplitudes recorded with and without gradient at the same Larmor frequency and at the same pulse intervals [11,12]. Any influence by relaxation and the dipolar correlation effect can be eliminated in this way. 

In the short gradient pulse limit of the fringe-field NMR diffusometry technique, which is of particular interest in context with slowly diffusing polymers (see Fig. 3), the effective diffusion time is given by twA=r2- Under this prerequisite the stimulated-echo attenuation factor can be analyzed according to... 

The coherent gradient sensor (CGS) method is a full-field interferometric technique that produces fringe patterns by laterally shearing an incident wavefront. This method, developed by Rosakis et al. (1998) for curvature measurement in film-substrate systems, is amenable for use in a variety of experimental configurations in either a reflection or a transmission arrangement. 

In the present context of thin films on substrates, the quantity of interest is the local curvature. The curvature tensor field, Hap, can be determined directly from the CGS patterns recorded in reflection by differentiating the fringes of the in-plane gradient ... 

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