SPRITE technique

Fig. 3.4.4 Schematic description of the one-dimensional double half k (DHK) SPRITE technique. The phase encode magnetic field gradient, Gz, ramped through half of /(-space beginning at the center and a single data point is acquired at a fixed time (tp) after the rf excitation pulse. The second half of /(-space is acquired after a 5T time delay. The time between rf pulses is TR.

Fig. 3.4.5 Schematic description of the two-dimensional SPIRAL-SPRITE technique. Cx and Gy are the phase encode magnetic field gradients that are amplitude cycled to traverse /(-space along an Archimedean Spiral. A single data point is acquired from the FID at a fixed encoding time tp after an rf excitation pulse. TR is the time between rf pulses.

The SPI and the SPRITE techniques have been applied to numerous concrete material problems. Some of these applications are presented in the following discussion. It is very important, however, to point out that these measurement techniques are not limited to cement-based material problems, but are fairly general in application. They can be, and have been, applied to many different material science problems. 

It is well known that the melting point of water confined to small pores is depressed [30, 31]. Therefore in concrete as the temperature decreases, the amount of frozen water will increase. Under normal temperature variation not all water in the pore structure will be frozen. The change from water in the liquid form to solid ice drastically reduces the Tfk of hydrogen (T2 ice <9 ps [32]). Ice will not be observed in an image, even with the SPRITE techniques, and our experimental images will be maps of unfrozen water distribution. 

Fig. 4 shows the application of a 1-D SPRITE technique to map the distribution of coke within a model fixed bed. In this particular example two layers of coked HZSM-5 were separated by a layer of fresh HZSM-5 (Fig. 4a).21 Each layer was 3.3 cm in length. Fig. 4b shows the profile of coke within the sample recorded by 1H SPRITE imaging these data were recorded in 15 min. The coke had a EEC ratio of 0.5, as determined by elemental analysis and the T2 of the sample was 65 ps—far shorter than could be studied using conventional spin-echo methods. It is worth emphasizing that SPRITE is mapping the coke directly in this experiment, although it should be noted that since it is imaging the H signal, as the coke becomes more graphitic in nature the amount of available H species that can be imaged will...

Fig. 4 Mapping coke distribution with the SPRITE technique (21). (a) Photograph of the sample used for the study. Two layers of coked HZSM-5 (areas a) were separated by a layer of fresh HZSM-5 (area b). Each layer was about 3.3 cm in length, (b) One dimensional SPRITE profile for the sample shown in (a).

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