Preparation and Transition Temperature of Ca-Substituted Yba 2 Cu 4 O 8
- PDF / 232,479 Bytes
- 4 Pages / 420.48 x 639 pts Page_size
- 51 Downloads / 256 Views
PREPARATION AND TRANSITION TEMPERATURE OF Ca-SUBSTITUTED YBa2Cu408 T.Miyatake, K.Yamaguchi, T.Takata, S.Gotoh, N.Koshizuka and S .Tanaka Superconductivity Research Laboratory, International Superconductivity Technology Center, Shinonome, Koto-ku, Tokyo 135, Japan ABS TRACT Samples of Yl-xCaxBa2Cu408 are prepared by means of a new high pressure technique employing the oxygen-HIP. We find that the transition temperature increases with Ca content, and that Tc=90K is realized at x=0.1. These data are explained by the hole concentration, i.e. average charge per [Cu-O] site. KEY-WORDS: YBa2Cu408, Hole concentration i.
Ca-substitution,
Transition
temperature,
Introduction
It is well-known that a 90K superconductor YBa2Cu307-y (I2-3) has a dramatic dependence of the superconducting properties on the oxygen deficiency, y. Tokura et al. have given a broader view of the YBa2Cu307-y -like compounds with a wide range of both oxygen content and average [Cu-O]+P charge [I] . They have concluded that the principal parameter concerned with the transition temperature Tc in this system is the [Cu-O] charge in the sheet, and that the charge in the Cu-O chain provide an insulating reservoir [i] . Whereas a similar study of YBa2Cu408 (1-2-4), originally prouced in a thin film form by Marshall et al.[2], has not been so far conducted because of the difficulty of the bulk synthesis. However, the bulk synthesis of the 1-2-4 compound has recently been performed by high oxygen pressure technique [3,4] and also by ambient oxygen annealing method employing alkali carbonates[5]. In this work, we describe preparation and the enhancement of Tc in YBa2Cu408 by Ca-substitution for Y, in particular the dependence of the Tc on the hole concentration, i.e. average charge per [Cu-O] site derived from the total charge associated with the non Cu cations. 2. Sample preparation and characterization Samples with nominal compositions of Yl-xCaxBa2Cu408 (x=0.0-0.1) were prepared by a solid state reaction method using an oxygen-HIP (Hot Isostatic Pressing) technique. Starting materials of Y203, Ba(NO3)2, CuO and CaCO3 with a purity of 99.9% were mixed, and then fired at 900°C in flowing oxygen for 12h. After grinding, the resulting powders were compacted into a rectangular shape at 100MPa, and lightly sintered at 800°C in an oxygen atmosphere. The samples were HIP-treated in a gas environment of argon with 20% oxygen at 100MPa twice. First, the HIP treatment at 950°C for 6h was conducted. At this stage, we checked produced phases using a powder x-ray diffractometer.
Mat. Res. Soc. Syrup. Proc. Vol. 169. ©1990 Materials Research Society
242
Subsequently, the second HIP treatment at 1050*C for 3h was carried out. Powder x-ray diffraction data were taken for samples before and after oxygen-HIP treatments. Figs.l(a),(b) and (c) show the x-ray diffraction patterns for a Ca-free sample before HIP, after 950°C-HIP and 1050°C-HIP, respectively. Before HIP, the sample mainly consists of the 1-2-3 phase and CuO (Fig.l(a)). The diffraction pattern for the sample
Data Loading...
