6.9 Influence of HTS bulk geometry on the properties Ë 179of 30 mm and thickness of 13.7 mm was ground, and the levitation forces of HTS bulk
were measured with each 0.5-mm-thickness removal of material. During the grinding,
many cracks, holes, and impurities were present. In one case, a mass of copper which
was not oxidized appeared (d). The appearance of the YBCO upper surface (a) and
inside surface that was ground (b–d) is shown in Fig. 6.32. From (a) and (b), we
can see clearly that when the bulks was ground thin enough, the appearance of the
upper surface and the grinding surface was very similar except for a small quantity of
holes that were scattered on the ground surface. All of these defects must affect the
properties of the YBCO material. If the internal structure can be improved, the property
of the HTS bulks can be greatly improved. Figure 6.32 shows the influence of thickness
of HTS YBCO bulk using the grinding method on the levitation forces.
Table 6.10 lists the relationship between levitation forces and thickness of HTS
bulk [40], and the levitation force of cylinder 2 over the PMG was much larger
than that of cylinder 1. We expected that the levitation forces would increase with
increasing thickness of the sample in an applied magnetic field, especially in a high
applied magnetic field. When the thickness exceeded a certain value, the thickness
dependence of the levitation forces was not clear (see Fig. 6.33).
The levitation forces on part of a ground YBCO were not measured in the grin-
ding method of reducing thickness [40]. Cutting the sheet to reduce the thickness
can measure the levitation forces on section cut [43]. The HTS bulk sample in this
cutting experiment was an MTG YBCO bulk with a diameter of 30 mm and thickness of
18 mm. The sample was cut sequentially removing 3.2, 3.4, and 3.4 mm from face not
seeded. The residual thickness of the sample was 14.3, 10.1, and 6 mm, respectively.
Figures 6.34 and 6.35 show the measured results of the levitation forces on the slice
Fig. 6.32:Appearance Of The YBCO upper and grinding inside surface [39]. The appearance of YBCO
bulk (a) upper surface, ground (b) 2 mm, (c) 7.5 mm and (d) 9 mm.
Tab. 6.10:Relationship between thickness and levitation forces of YBCO bulk samples [40].
Parameters Cylinder 1 Cylinder 2
Thickness (mm) 12 14
Diameter (mm) 30 30
F 15 (N) 28. 8 41. 2
F 20 (N) 22. 9 29. 8