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Machining superconducting BSCCO devices
Physica C 162-164 ( 1989 ) 879-880 North-Holland
MACHINING SUPERCONDUCTING BSCCO DEVICES R. Rayne, L. Toth and B. Bender Naval Research Laboratory, Washington D. C. 20375 We have machined cast BSCCO into SQUID and SQUID shield devices. Heat treatment of ascast samples results in a random three dimensional network of platelets. This microstructure is responsible for the machinability of BSCCO.
INTRODUCTION A commonly held misconception is that the high Tc superconductors are difficult to shape and fabricate into devices because they are ceramics. Our research shows that BSCCO is machinable with conventional metal working machines and steel tools. Here we describe how BSCCO is best processed to enhance its machinability. Only a very few other ceramics such as the glass ceramics marketed under the trade name Macor are machinable. BSCCO and these glass ceramics are machinable to precise tolerances and fine surface finishes because their microstructure contains a three dimensional network of platelets. PROCESSING Typically BSCCO is prepared using powder processing techniques. While this technique is suitable for small samples and many experimental studies in superconductivity, it is not an optimum process for producing machinable BSCCO. A better method is to cast pieces to be machined. We have cast BSCCO into many shapes. The as-cast piece has to be heat treated to develop a microstructure suitable for machining and its superconducting properties. The advantages of casting over powder processing techniques are the ability to process much larger parts, and to produce samples which are denser and whose microstructure consists of a more random, 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
three dimensional network of piatelets suitable for machining. In the present study the compositions 4334, 4336, 2212, and 2223 were cast, heat treated and subsequently machined. Stolchiometrtc amounts of Bi203, SrCO3, CaCO3, and CuO were thoroughly mixed and calcined. The mixture was then melted at 1050°C and poured into a metal mold which was preheated to 300°C. The casting was then removed from the mold and heat treated at 780°C in air for 12 to 36 hours. The casting procedure is unique in that when BSCCO melts it losses one oxygen atom per unit cell. The oxygen is regained during the heat treatment, at which time the platalet microstructure develops. A more complete description of the casting process is given elsewhere 1. RESULTS We have fabricated several high Tc superconducting devices from machined parts. Figure 1 shows the body of a BSCCO SQUID. Machining operations used to make the part included lathe cuffing, milling, drilling and tapping. The only parts of the SQUID not BSCCO were the 0-80 screws. Tolerances were maintained to :L-O.001 inches. The performance of this SQUID is currently under evaluation. Another part being evaluated is a SQUID shield. Here the shield was machined to exact sizes and tolerances of an existing niobium shield on a commercial SQUID.
880
R. Rayne et al. / Machining superconducting BSCCO devices
1) A machined BSCCO SQUID body, 1 cm in diameter, 3 cm high. The black circles are 4-40 tapped blind screw holes.
Figures 2 and 3 show the machined surface finish and the fracture surface microstructure of BSCCO. These photographs are very similar to ones obtained for commercial Macor. In regions where the surface of BSCCO was very smooth, the microstructure of platelets was random, three dimensional and resembled a "House of Cards'. In regions where the surface was rough, as if the tooling pulled out small chunks, the platelets had some preferred orientations. Fortunately, in cast samples such regions were infrequent. Machined sintered samples, on the other hand, showed much more surface roughness. This was caused by more frequent regions of non-random platelet micro.structures. SUMMARY We have determined that cast BSCCO is machinabie with conventional metal cutting tools to high tolerances and fine surface finishes. BSCCO is one of but a handful of machinable ceramics. SQUIDs and SQUID shields were fabricated from cast pieces. The machinability was correlated to the microstructure, which was found to be closely related to that for Macor.
2) The machined surface of a SQUID shield.
ACKNOWLEDGEMENTS We are indebted to our physicist colleagues at NRL, R. Soulen, D. Gubser, S. Wolf, M. Osofsky and others for providing designs of superconducting devices. D. Jones did the careful machining. T. Jones of NOSC,San Diego Ca. 92152, kindly machined 107K Tc Pb-BSCCO in his laboratory and provided us the sample. S. Lawrence gave many helpful suggestions. DARPA supported this project. REFERENCES 1. B. N. Das et al., Journal of Superconductivity, in print.
3) A fractured surface showing the random, three dimensional network of BSCCO platelets in a heat treated casting.
MACHINING SUPERCONDUCTING BSCCO DEVICES R. Rayne, L. Toth and B. Bender Naval Research Laboratory, Washington D. C. 20375 We have machined cast BSCCO into SQUID and SQUID shield devices. Heat treatment of ascast samples results in a random three dimensional network of platelets. This microstructure is responsible for the machinability of BSCCO.
INTRODUCTION A commonly held misconception is that the high Tc superconductors are difficult to shape and fabricate into devices because they are ceramics. Our research shows that BSCCO is machinable with conventional metal working machines and steel tools. Here we describe how BSCCO is best processed to enhance its machinability. Only a very few other ceramics such as the glass ceramics marketed under the trade name Macor are machinable. BSCCO and these glass ceramics are machinable to precise tolerances and fine surface finishes because their microstructure contains a three dimensional network of platelets. PROCESSING Typically BSCCO is prepared using powder processing techniques. While this technique is suitable for small samples and many experimental studies in superconductivity, it is not an optimum process for producing machinable BSCCO. A better method is to cast pieces to be machined. We have cast BSCCO into many shapes. The as-cast piece has to be heat treated to develop a microstructure suitable for machining and its superconducting properties. The advantages of casting over powder processing techniques are the ability to process much larger parts, and to produce samples which are denser and whose microstructure consists of a more random, 0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
three dimensional network of piatelets suitable for machining. In the present study the compositions 4334, 4336, 2212, and 2223 were cast, heat treated and subsequently machined. Stolchiometrtc amounts of Bi203, SrCO3, CaCO3, and CuO were thoroughly mixed and calcined. The mixture was then melted at 1050°C and poured into a metal mold which was preheated to 300°C. The casting was then removed from the mold and heat treated at 780°C in air for 12 to 36 hours. The casting procedure is unique in that when BSCCO melts it losses one oxygen atom per unit cell. The oxygen is regained during the heat treatment, at which time the platalet microstructure develops. A more complete description of the casting process is given elsewhere 1. RESULTS We have fabricated several high Tc superconducting devices from machined parts. Figure 1 shows the body of a BSCCO SQUID. Machining operations used to make the part included lathe cuffing, milling, drilling and tapping. The only parts of the SQUID not BSCCO were the 0-80 screws. Tolerances were maintained to :L-O.001 inches. The performance of this SQUID is currently under evaluation. Another part being evaluated is a SQUID shield. Here the shield was machined to exact sizes and tolerances of an existing niobium shield on a commercial SQUID.
880
R. Rayne et al. / Machining superconducting BSCCO devices
1) A machined BSCCO SQUID body, 1 cm in diameter, 3 cm high. The black circles are 4-40 tapped blind screw holes.
Figures 2 and 3 show the machined surface finish and the fracture surface microstructure of BSCCO. These photographs are very similar to ones obtained for commercial Macor. In regions where the surface of BSCCO was very smooth, the microstructure of platelets was random, three dimensional and resembled a "House of Cards'. In regions where the surface was rough, as if the tooling pulled out small chunks, the platelets had some preferred orientations. Fortunately, in cast samples such regions were infrequent. Machined sintered samples, on the other hand, showed much more surface roughness. This was caused by more frequent regions of non-random platelet micro.structures. SUMMARY We have determined that cast BSCCO is machinabie with conventional metal cutting tools to high tolerances and fine surface finishes. BSCCO is one of but a handful of machinable ceramics. SQUIDs and SQUID shields were fabricated from cast pieces. The machinability was correlated to the microstructure, which was found to be closely related to that for Macor.
2) The machined surface of a SQUID shield.
ACKNOWLEDGEMENTS We are indebted to our physicist colleagues at NRL, R. Soulen, D. Gubser, S. Wolf, M. Osofsky and others for providing designs of superconducting devices. D. Jones did the careful machining. T. Jones of NOSC,San Diego Ca. 92152, kindly machined 107K Tc Pb-BSCCO in his laboratory and provided us the sample. S. Lawrence gave many helpful suggestions. DARPA supported this project. REFERENCES 1. B. N. Das et al., Journal of Superconductivity, in print.
3) A fractured surface showing the random, three dimensional network of BSCCO platelets in a heat treated casting.