Magnetic field frequency

Figure -4. The RYDMR spectrum for TBPDA-fCgoh- Microwave magnetic field frequency v=8.96 GHz.

The action of the microwave UHF magnetic field results in a change in microhardness and, therefore, in occurrence of the magnetomechanical effect (MME). The magnitude of MME depends on the duration of UHF irradiation (Fig. 1). At durations less than a certain critical value (U => 60 min) the MME enhances, while at t > tcr the MME saturates. It has been also found that the MME magnitude is a linear function of the magnetic field frequency. 

Figure 2.7. Influence of different magnetic field frequencies on the heating behavior of iron containing high density polyethylene...

More recently, another intermediate timescale was explored by NMR relaxom-etry [102]. The spin-lattice relaxation times of water molecules were determined in the range of 20 ns to 20 ps by varying the magnetic field frequency from 10 kHz to 20 MHz and depending on the water content. This technique is well suited for the study of ionomer membranes because of its extreme sensitivity to water-polymer interactions, but it requires a structural and dynamic model to extract characteristic features. The effect of confinement is predominant in polyimides even at high water content (algebraic law with a slope of —0.5 characteristic of porous materials), whereas the diffusion quickly reaches a bulk behavior in Nafion (a plateau is observed at low magnetic fields). 

Table 3.9 gives the resonance frequencies in a 9.4 tesla magnetic, field. . . 

The sample is placed in a cqnst a nt magnetic field, Bq, and the variation in frequency throughout the t/omain Tieing expfored excites one by one the different resonances. The scan lasts a few minutes. Inversely, one can maintain a constant frequency and cause the magnetic field to vary. 

The sample is again subjected to a constant magnetic field but all the nuclei are excited by a very short radio frequency pulse. The frequency e (e.g., 400 MHz for a proton at 9.4 tesla) is applied over a period of several... 

A SQUID [2] provides two basic advantages for measuring small variations in the magnetic field caused by cracks [3-7]. First, its unsurpassed field sensitivity is independent of frequency and thus dc and ac fields can be measured with an resolution of better than IpT/VHz. Secondly, the operation of the SQUID in a flux locked loop can provide a more than sufficient dynamic range of up to 160 dB/VHz in a shielded environment, and about 140 dB/>/Hz in unshielded environment [8]. 

Figure Bl.7.18. (a) Schematic diagram of the trapping cell in an ion cyclotron resonance mass spectrometer excitation plates (E) detector plates (D) trapping plates (T). (b) The magnetron motion due to tire crossing of the magnetic and electric trapping fields is superimposed on the circular cyclotron motion aj taken up by the ions in the magnetic field. Excitation of the cyclotron frequency results in an image current being detected by the detector electrodes which can be Fourier transfonned into a secular frequency related to the m/z ratio of the trapped ion(s).

The original method employed was to scan eitiier the frequency of the exciting oscillator or to scan the applied magnetic field until resonant absorption occiined. Flowever, compared to simultaneous excitation of a wide range of frequencies by a short RF pulse, the scanned approach is a very time-inefficient way of recording the spectrum. Flence, with the advent of computers that could be dedicated to spectrometers and efficient Fourier transfomi (FT) algoritluns, pulsed FT NMR became the nomial mode of operation. 

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