HRTEM of High T c Superconductor Materials
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HRTEM OF HIGH Tc SUPERCONDUCTOR MATERIALS J.C. BARRY*, B.L. RAMAKRISHNA** AND Z. IQBAL*** *Center for Solid State Science, Arizona State University, Tempe, AZ 85287 **Department of Chemistry and Center for Solid State Science, Arizona State University, Tempe AZ 85287. ***Corporate Technology, Allied-Signal Inc., Morristown, New Jersey 07960 ABSTRACT In this talk we will discuss the use of high resolution transmission electron microscopy (HRTEM) in the study of high Tc superconducting oxides. HRTEM has played an important part in the characterization of microstructure in YBa 2Cu3 O7_x, and in structure refinement of mixed phase Bi-Sr-Ca-Cu-O and Tl-Ba-Ca-Cu-O compounds. It is unlikely that HRTEM will contribute to any great extent to the understanding of why these materials are superconductors. But HRTEM will continue to make vital contributions to the studying and understanding of defect structures (such as grain boundaries and planar defects) which interfere with the flow of supercurrents. INTRODUCTION With papers coming out at the rate of (at least) ten per day it is not possible to properly review the field. This paper is not so much a review, but is rather an attempt to draw together some of the threads of this most labyrinthine subject. In 1971 Matthias stated that "room temperature superconductivity will always remain a pipedream"'. In those days a high Tc superconductor was anything with a T. > 10 K. Nowadays, Matthias' statement seems less certain than it once did. Nevertheless his statement is well taken, since room temperature superconductivity is, and may continue to be, the domain of science fiction - present research is focussed on the problems of increasing Tc, critical current density (Jc), and critical field (H.2) rather than achieving room temperature superconductivity. In the current terminology, a high Tc superconductor is one with a Tc > 30K. The era of the new high Tc superconductors began with the discovery by Bednorz and Muller 2 of a T. of 30 K in the La-Ba-Cu-O system. It is now known that there are a great many materials that are superconducting at above 30 K. These new ceramics can be grouped into 2 seven categories: T, = 30 K - 50 K3 La 2 _,BaxCu0 4 _6, i) Tc = 60 K - 90 K ,4 ii) YBa 2 Cu 3 O7 -x, Tc = 60 K - 115 K5 ,6, 7 , 8 ,9,10,11 iii) Bi-Sr-Ca-Cu-0,
iv)
TI-Ba-Ca-Cu-0,
v) Pb 2 Sr 2 (Ln,Ca)Cu30 8 +8 , vi) Ba0 . 6K0 . 4BiO3, vii) Tl.BaCu02, x = 0.1-0.2
Tc = 40 K - 125 K12,13,14,15,16,17 Tc = 70 K18 Tc = 30 K19 0 T, = 30 K2
(Tl doped BaCu0 2)
All the known high Tc oxides, with the exception of material type (vi), contain copper. All material types except material type (vi) and (vii) are layered structures which contain Cu-0 sheets (the T1 doped BaCuO2 will be discussed later in this paper). Materials type (vi) and (vii) are cubic. Theoretical Models Ideally, we would like to use electron microscopy to study the relationship between the structure and properties of the high Tc materials. In order to do this we need to discover what structural and chemical features are Mat. Res. Soc.
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