Casting and machining of devices of high temperature superconducting BSCCO
- PDF / 845,577 Bytes
- 6 Pages / 576 x 792 pts Page_size
- 9 Downloads / 178 Views
I. INTRODUCTION
The intense research in recent years on bulk high temperature superconductors has not yet led to many useful applications. Low / c 's have hindered the development of superconducting tapes and wires for magnets and motors. There are, however, a number of applications with only modest Jc requirements, where the new superconductors could replace low Tc materials mainly because of advantages associated with the higher Tc's. One device is a magnetic shield. In this paper we will show that high Tc (3=85 K) Bi-Sr-Ca-Cu-O (BSCCO)1"3 has another advantage over the other high Tc ceramic superconductors, and even some low temperature superconductors, in being readily machinable using conventional metal cutting techniques. Thus complex shapes with close dimensional control can be readily machined from billets. We began casting BSCCO alloys when difficulties were encountered consolidating powders into large sintered bodies, such as 7 cm diameter sputtering targets (disks). These tended to crack and deform during sintering. Sintering even larger bodies required larger dies and presses which were not conveniently accessible to us. Casting from the liquid state was viewed as a potentially easy and economical method to produce large and dense bodies in roughly final form.4'5 As-cast samples were found not to be superconducting and required a heat-treatment cycle to render them so. A significant part of the present study was to investigate the heat-treatment procedure which restored the
superconducting state. The heat treatment was complex in terms of the chemical and physical changes that occurred within the sample. Heat-treatment cycles, microstructures, and phase relationships for several nominal compositions were studied. The microstructure of heat-treated BSCCO was found to be a random network of platelets. This suggested to us that the material was machinable in the same sense that other materials with a platelet microstructure, such as graphite and MACOR™, are machinable.6 Properly heat-treated BSCCO has excellent machining properties and this paper also describes our experience with that aspect. In the early stages of this work, we found that dc and ac susceptibilities (both measured as a function of field) and resistivity were poor predictors of magnetic shielding capabilities. It was therefore necessary to measure shielding directly, using the resulting data in combination with microstructural and phase analysis, to vary composition and processing conditions to obtain improved shielding. II. PRELIMINARY CASTING AND HEAT-TREATMENT EXPERIMENTS
Many factors were found to complicate the casting and heat-treatment procedures. A number of experiments were undertaken to understand these procedures. From these experiments the following factors were found to affect the characteristics of the casting and heat-treatment processes. The melting temperature 467
J. Mater. Res., Vol. 6, No. 3, Mar 1991 http://journals.cambridge.org
Downloaded: 14 Mar 2015
IP address: 131.170.6.51
R. J. Rayne era/.: Casting and mac
Data Loading...
