Various pulsed magnetization experiments employing peak fields of up to 2 T and pulse durations of 30 ms and 3 ms were carried out on YBCO samples at temperatures between 20 and 80 K. Trapped magnetic flux profiles were recorded. The highest remanent magnetizations were obtained for a multi-pulse technique with step-wise cooling. The shape and the absolute values of the trapped flux profile are discussed in terms of the dynamics of such pulsed magnetization processes.

A certain extent of orientation of the Col hd mesophase can be inferred from Figure 1G in the temperature interval corresponding to the very end of melting. Since the width of the mesophase peak significantly decreases at these temperatures, we suggest that larger and more perfectly ordered domains are being formed. Such domains could partially preserve their orientation upon a short passage in the isotropic melt.[39] Although the studied molecule does not contain any chiral centers, the helicity in the crystalline phase is believed to be caused by a slight non-planarity of the molecular core (out-of-plane tilt of the phenyl groups) inducing rotational shift of the neighboring molecules in the column Ceramics are polycrystalline materials that often exhibit isotropic physical properties even when the tiny crystal grains that constitute the dense compact are anisotropic. This is due to the fact that the grains are usually randomly oriented with respect to their crystallographic orientation. Efforts have been made in the past to align the anisotropic grains in ceramics in order to produce anisotropic ceramics that mimic the properties of anisotropic single crystals, i.e., to produce textured or grain-oriented microstructures. Two strategies to achieve such anisotropic microstructures have been examined so far, namely: i) preparation of powders, preferably with needle or platelet morphologies, aligned via a shear-flow process, followed by pressureless sintering and/or hot-forming for extended times at high temperatures in order to allow the growth of aligned grains according to the Ostwald ripening mechanism; [1,2] and ii) in order to improve the grain alignment even further, addition of a small part of well-developed, large (2±5 lm) needles or platelets as grain-growth templates. [3,4] With these established processing concepts, in order to ensure a high degree of grain alignment, an increase in the initial size of the particles is a prerequisite. However, this slows down both the speed of densification and grain alignment, implying an unfavorable prolongation of the processing time.The work reported here opens a new direction by starting with nanopowders instead of micrometer-sized powders. It was initially motivated by the two recent findings that i) rapid COMMUNICATIONS 676

With respect to the potential application as cryo-permanent magnets, melt-textured YBaCuO bulk parts were used to investigate various pulsed magnetization processes employing pulsed copper coils. Results for different multi-pulse processes including different peak fields, pulse durations, temperatures and geometries are compared. The strong current-carrying capabilities of HTS material, even for currents above the critical current J c , result in strong shielding effects, in particular for short pulse durations. It turns out that the inhomogeneities present in all real materials play an important role for the initial magnetic flux penetration. To fully magnetize the bulk samples, the non-stationary conditions and geometrical effects require peak fields, which substantially exceed twice the saturation magnetization measured at the sample surface. Transient magnetizations as well as shapes and absolute values of trapped flux profiles are reported.

No abstract

No abstract