Improved trapped fields are reported for bulk melt-textured YBa2Cu3O 7−δ (YBCO) material in the temperature range between 20 K and 50 K. Trapped fields up to 12.2 T were obtained at 22 K on the surface of single YBCO disks (with Ag and Zn additions). In YBCO mini-magnets, maximum trapped fields of 16 T (at 24 K) and of 11.2 T (at 47 K) were achieved using (Zn + Ag) and Zn additions, respectively. In all cases, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequence of flux jumps due to thermomagnetic instabilities in the temperature range betweeen 20 K and 30 K. 7460Ge, 7462Bf, 7480BjBulk type II superconductors can trap high magnetic fields that are generated by superconducting persistent currents circulating macroscopically within the superconductor. The main features of the resulting field distribution are a maximum trapped field B 0 in the center of the superconducting domain and a field gradient towards the sample edge which is determined by the critical current density j c of the supercurrents. Therefore, high trapped fields B 0 in bulk superconductors require a high critical current density and a large size of the current loops. Large single-domain bulk YBa 2 Cu 3 O 7−δ based (YBCO) samples can be produced by melt texture processing, especially by using SmBa 2 Cu 3 O 7−δ (Sm-123) as a seed crystal.[1] The pinning effect in melt textured samples can be improved by irradiation methods [2,3] and alternatively, by zinc-doping. [4,5] Cracking of the samples was found to limit the trapped field of bulk YBCO at temperatures below 77 K which can be explained by tensile stresses that occur during the magnetization process due to the stored flux density gradient and may exceed the tensile strength of the material.[6] The mechanical properties of bulk YBCO and its tensile strength can be improved considerably by the addition of Ag. [7,8] Another possibility to avoid cracking during magnetizing is to encapsulate the bulk YBCO disks in steel tubes. [9] A steel tube leads to stress compensation by generating a compressive stress on YBCO after cooling from 300 K to the measuring temperature which is due to the higher thermal expansion coefficient of steel compared to that of YBCO in the a, b-plane. Maximum trapped fields B 0 of 11.5 T at 17 K and of 14.4 T at 22 K were reported for single YBCO disks and mini-magnets consisting of two single YBCO disks, respectively. [9] In both cases, Ag was added to the YBCO disks which were placed into austenite Cr-Ni steel tubes. Previous attempts to combine the two beneficial effects of Ag and Zn additions failed due to the solubility of Zn in Ag in oxygen atmosphere. High trapped fields have also been reported for bulk Sm-123 with additions of Ag. At the surface of Sm-123 disks, trapped fields of 2.1 T at 77 K and of 8 T at 40 K have been observed. [10] In the present paper, t...

The electrical conductivity and the Hall effect are investigated in the temperature region fiom 77 to loo0 K on undoped ZnO single crystals grown by vapour phase transport. The growth conditions were systematically altered to get crystals with definite non-stoichiometry. The electrical properties are related to these growth conditions. The analysis of the Hall data yields the phase boundary ZnO/Zn and a formation enthalpy of 1.5eV for the dominant native donor. This donor is suggested to be the oxygen vacancy. The Hall mobility is analysed with regard of new aspects giving a smaller anisotropy than in earlier analyses. An undotierten durch Gasphasentransport geziichteten ZnO-Einkristallen werden elektrische Leitfiihigkeit und Halleffekt im Bereich von 77 bis 1000 K untersucht. Durch Anderung der Ziichtungsbedingungen konnen definierte Grade der Nichtstoichiometrie erzeugt werden, was in den elektrischen Eigenschaften deutlich zum Ausdruck kommt. Die Analyse der Hallmessungen ergab die Phasengrenze ZnO/Zn sowie eine Bildungsenthalpie von 1,5 eV fur den dominierenden Eigendonator, der als Sauerstoffvakanz interpretiert wird. Die Analyse der Hallbeweglichkeit ergibt eine geringere Anisotropie als in friiheren Arbeiten.

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YBCO bulk materials were crystallized by a multi-seeded melt growth process. Up to four biaxial oriented Sm-123 seeds were placed on the top of rectangular shaped YBCO bars (up to ). The oriented grown single grains are joined by partially coherent grain boundaries with an improved current transport capability and high levitation forces with overall values of 55-65 N. Segregation of nonsuperconducting rest melt and microcracks limit the current transport across the grain boundary.

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