Materials Processing Technology

Figure 1. A composite superconducting coil which could be used in generating magnetic fields. The black superconductor layer was made by the tape casting technique and consists of a Ag/Y-Ba-Cu-O mixture. The addition of silver to Y-Ba-Cu-O improves its strength and flexibility without degrading its superconducting properties. A strip of the superconductor is cut in the green state and cofired on the silver foil to give the composite coil. Photograph by Brian Ende.

INTRODUCTION The potential applications of the

new high-critical-temperature (high­To) ceramic superconductors range from microelectronic circuitry to large­scale power generation and storage. l

The lack of resistance to direct current offers the possibility that large, high­field magnets could be fabricated from the superconductors.2 Circuitry for computers and high storage factor (high-Q) cavities for particle beam accelerators are other areas of poten­tial application for the new supercon­ductors. These materials might even play a role in microwave and radio frequency (r.f.) applications in which their resistances are finite, but are nevertheless lower than the resis­tances of conventional conductors.3.4 Before many of these applications can be realized, however, the electrical and mechanical properties of the superconductors must be optimized. Improvements in flexibility and

1989 January. JOM

strength can be achieved by reducing the critical flaw size through proper ceramic processing. Likewise, proper processing can lead to improvements in the critical current density (Jo)'

The critical current density is the current density above which the su­perconductor becomes a normal con­ductor. While critical-current densities as high as 106 Alcm2 have been ob­tained in thin films, the critical cur­rent densities in bulk superconductors are generally less than _103 Alcm2 •

Clearly, improvements in Jc

must be made before many of the envisioned applications can be realized.

For many uses, it will be necessary to form an intimate, cohesive bond between the superconductor and a non-superconducting material (see this month's couer--4!d.). In high cur­rent density applications, it might be necessary to bond to a normal con­ductor so that an alternative electrical path exists in the event that the su-

~efore the potential of high-temperature superconductors can evolve from the realm of science fantasy to the marketplace of practical reality, materials scientists and engineers must first develop viable methods for fabricating these revolutionary materials into a wide assortment of functional shapes and sizes.

R.B. Poeppel, S.E. Dorris,

CA Youngdahl, J.P. Singh,

M.T. Lanagan, U. Balachandran,

J.T. Dusek and K.C. Goretta

Argonne National Laboratory

perconductor becomes normal. In the fabrication of magnetic coils, it will be necessary to bond to an insulating material, so that the individual turns of the coil are electrically isolated from one another. Production of these and other technologically useful devices requires the capability to economically and reproducibly fabricate both mono­lithic and composite conductors.

SHAPE-FORMING TECHNIQUES

Every technique used to produce bulk, high-To superconductors relies first on producing the superconductor powder. Powders of yttrium-barium­copper oxide (Y-Ba-Cu-O), bismuth­strontium-calcium-copper oxide (Bi­Sr-Ca-Cu-O), and thallium-barium­calcium-copper oxide (Tl-Ba-Ca-Cu-O) have all been produced by solid-state reaction. Formation by solid-state re­action entails intimately mixing the constituent oxides (or carbonates or

11

hydroxides in the case of barium), usu­ally by ban milling for about 12 hours. The mixture is then dried, pressed into pellets, and heated to a temperature which is below the melting point of the superconductor, but which is high enough to allow for formation of the superconductor in a reasonable length of time. For Y-Ba-Cu-O, this calcina­tion step is normally carried out in the temperature range of900-950°C in air. After calcining for about 16 hours, the pellets are cooled, ground up, again pressed into pellets, and re-calcined in the temperature range of 900-950°C. To produce phase-pure materials, the calcination steps must be repeated with intermittent grinding. Differen­tial thermal analysis has shown5 that phase-pure Y-Ba-Cu-O can be pro­duced after four calcinations at 900°C. While solid-state rjlaction is the most widely employed technique for produc­ing superconductor powders, sol-gel6

and liquid-mix? techniques have also been used successfully.

After the superconductor powder has been obtained, bulk superconduc­tors can be fabricated by solidification of the molten material or consolidation of the powder through sintering. Fab­rication by solidification is limited to thin wires and tapes, however, since the superconductors undergo incon­gruent melting. This leads to phase separation in large components, which have slow cooling rates. Consolidation of powders through sintering, on the other hand, is not limited to small com­ponents and can yield a variety ofintri­cate shapes. These shapes can be pro­duced by machining previously fired pieces, but it is far more convenient to shape the components in the green state. Forming processes which are being used for the superconductors in­clude tape casting, extrusion and slip casting.

No matter which of the three form­ing processes is used, the ceramic powder must be mixed with several additives to make a formulation that is fluid enough to be easily formed into various shapes, yet still has satisfac-

10 I!m Figure 2. The microstructure of 13 vol.% sil­ver in V-Sa-Cu-O.

12

30,---~--------------------,

-

200 400 600 BOO 1000 1200

TIKI

Figure 3. Typical shrinkage profile of V-Sa­Cu-O tape.

tory strength in the green state. This formulation, known as a slip, consists in general of a solvent, a dispersant, a binder, a plasticizer and the ceramic powder. The solvent provides a fluid medium for the powder and the other additives. Water is known to attack the Y-Ba-Cu-O superconductor by preferentially dissolving barium, so organic solvents such as methyl ethyl ketone and xylene are used.? The dis­persant reduces the extent of agglom­eration in the slip, thereby promoting uniform densification during sinter­ing. The binder consists of long-chain organic molecules that impart strength to the green body by estab­lishing a polymeric cross linkage of the ceramic particles. The plasticizer is made up of somewhat shorter organic molecules that partially disrupt the cross linkage of the binder, providing the green body with a degree offlexibil­ity as a result. Ratios of the various constituents of a slip vary, depending on the forming process and on charac­teristics ofthe ceramic powder such as particle size and specific surface area.

Tape Casting In tape casting, the fluid slip is

poured onto a smooth, flat surface and is then spread evenly with a doctor blade. After allowing the solvent to evaporate, the tape can be stripped and stored for subsequent shaping. With this method, tapes can be pro­duced with thicknesses in the range of 50-500 ~m. Tapes have been cast with Y -Ba-Cu-O and the 4-3-3-6 and 2-2-1-2 compositions of the Bi-Sr-Ca-Cu-O system. (The numbers refer to the sub­scripts of the first four elements.) Tape casting can also be used to produce composite materials by successively casting slips with different composi­tions, one layer on top of the other. Superconductor/insulator composites are currently being produced by this technique for use in the fabrication of magnetic coils.

Tape casting is also used to produce tapes from a mixture of silver and Y-Ba-Cu-O. The addition of silver to Y-Ba-Cu-O has been shown to improve the mechanical properties of the superconductor without having deleterious effects on the electrical

properties.8 In an experiment done at Argonne, a tape of 13 vol. % silver in Y-Ba-Cu-O was fired on a silver sub­strate. The resulting sample was ca­pable of withstanding a strain of -2%, as compared to Y-Ba-Cu-O without silver, which typically fractures at 0.1-0.2% strain. Figure 1 shows the Ag/y-Ba-Cu-O composite in the strained state, and Figure 2 shows its microstructure.

Extrusion The solids content for extrusion is

much higher than that for tape cast­ing, so that the resulting plastic mass must be forced at high pressure through an assortment of dies. Extru­sion can be used to produce complex shapes such as a honeycomb structure. Wires of the Y-Ba-Cu-O material have been extruded with diameters be­tween 0.1 and 3.0mm and with lengths of well over 200 cm. As with the green tapes, extruded wires exhibit great flexibility in the green state, and have been used to fabricate superconduct­ing coils 3 cm in diameter. Composite structures can be made quite easily from extruded wires by dip-coating or spray-coating the wire with a slip made from another material. Super­conductor/insulator composite wires are currently being studied for the possible fabrication of magnetic coils.

Both tape casting and extrusion subject the fluid slip to shear stresses during forming. The superconductor particles have a large aspect ratio and tend to align themselves with the shear stresses. As a result, the super­conducting grains tend to lie in the plane of tapes and along the axes of wires. Because the directions of pref­erential growth are the same as the directions of high current density, su­perconducting properties can be en­hanced by the tendency of ceramic grains to align themselves. In fact, im­provements in J

c of about 50% over un­

aligned bodies have been measured.9

Slip Casting Another method for producing bulk

superconductors is slip casting, a ver­satile means of making simple or com­plex shapes. It involves pouring a slip containing ceramic powder, binder and dispersant into a porous mold. Fluid is drawn into the mold, leaving within the mold cavity a compact of powder held together by the binder. A slight degree of texturing can also result through slip casting because of forces within the fluid on each particle. Capillary action at the mold wall causes particles to lie flat against the wall and, as particles settle, those with appreciable aspect ratios are in a sta­ble orientation only when the largest surface is perpendicular to the settling direction.

JOM • January 1989

SINTERING After fonning a green body by any of

the aforementioned techniques, sin­tering is required to produce a dense, strong superconductor. Total shrink­age of the component during firing typically ranges from 15 to 25%. In the temperature range of 220-350°C, the organic constituents volatilize, result­ing in a shrinkage of 2-5%. Tempera­ture must be increased very slowly in this range to avoid the cracking, blis­tering and warping that result from excessively rapid evolution of the or­ganic components. After burnout of the organics, the sample dimensions remain nearly constant until the sin­tering temperature is reached, at which point the sample shrinks an additional 10-20%. The shrinkage pro­file of an Y-Ba-Cu-O tape is shown in Figure 3. In the fabrication of com­posite conductors, it is important that each of the materials has a similar shrinkage profile to avoid cracking, curling or warping during firing.

Sintering of superconducting ma­terials can occur either by solid-state diffusion or in the presence of a liquid phase. Solid-state sintering is rela­tively slow, and in the absence of a liquid phase, it is difficult to obtain densities much greater than 75% of theoretical. Sintering with liquid for­mation tends to produce dense com­ponents, but because melting is in­congruent, phase separation occurs, and the superconducting grains be­come coated with non-superconduct­ing phases. It is expected that high­density materials can be produced without the fonnation of a liquid phase. Manipulation of the particle size distribution of the starting pow­ders should lead to higher green densi­ties, which should, in turn, produce improvements in fired densities.

For Y-Ba-Cu-O superconductors, sintering must be followed by an an­nealing step. Annealing is necessary for Y-Ba-Cu-O because oxygen is evolved during sintering and must be reincorporated to achieve good super­conducting properties. lo Annealing is nonnally carried out by holding the component in the vicinity of 450°C for 5-10 hours. Annealing of the Bi -Sr-Ca­Cu-O materials might be unneces­sary-it has been reportedll that these materials lose very little oxygen at high temperatures.

Tapes ofY-Ba-Cu-O have been sin­tered to densities greater than 95% of theoretical density and with J in the range of 300-500 Afcm2 • E~truded wires have been produced with J ranging up to 1,300 Afcm2 • Sinteringof the newer Bi-Sr-Ca-Cu-O materials is not strongly influenced by oxygen ac­tivity and is nonnally done in air in the temperature range of 880-900°C.

1989 January • JOM

TECHNIQUES FOR BETTER PROPERTIES The methods discussed in the main text are capa­

ble of producing a wide variety of shapes of the su­perconductors in the green state; however, the me­chanical and electrical properties of the fired bodies are, to date, unsatisfactory. While improvements in these properties are to be expected as processing techniques are optimized, many other techniques are also being investigated as possible means for pro· ducing superconductors with the desired properties.

Spinning Spinning is a process, similar to plastic processing,

for making thin filaments. In this technique, a viscous fluid that contains the required ceramic constituents is formulated. Filaments are then drawn from the s0-lution by a lechnique such as extrusion, I I or dipping and withdrawing of a rod, IS both of which have pro­duced Y ·Ba-Cu.Q filaments. The spinning methods of the textile industry are also viable techniques for producing the filaments." The wet filaments are dried and then heated to high temperatures to form the ceo ramie. Heating burns off the solutions and causes the necessary reactions or sintering to form dense final products. Processing can be tailored to yield a wide range of diameters (from about 5 to 1,000 11m). \3 IS

Spinning solutions are of two general types. They may consist of ceramic powders mixed with organic constituents, or the metals needed to form the ce­ramic may be incorporated into the organics. Either type of solution is, in principle, capabie of producing high-quality filaments. Spinning techniques have been very successful in producing structural ceramic filaments. The filaments are thin, continuous over lengths of several meters, and have excellent me­chanical integrity, They tend, however, to be poly­phase, and many of their properties are determined by the minor phases.,s.11 To date, spun filaments of high-Te superconductors have been relatively imper­fect in both phase purity and mechanical integrity.12." This technique for making filaments has potential, but will require time and effort to perfect.

Formation In Tubes Several research groups have fabricated wires or

tapes of high-Te superconductors by a powder-in· tube process.II' 11 In this process, superconductor powder is placed within a metallic tube. The tube is then swaged, rolled or drawn. The result is a long tube filled ~h powder compacted to a high density. Heat

treatment sinters the powder, and a sheathed super­conductor wire results. The sheath material is gener­ally silver or gold, since the high·Te superconductors react with most other metals.H .11 Long continuous lengths of wire have been fabricated by this process and the resultant wires have yielded some of the best critical current densities obtained in bulk high-T, suo perconductors. These wires have the added advan· tage of a metallic sheath which protects the super· conductor from mechanical damage and chemical degradation.

Sheathed wires have also been produced by a new process. Fine Y-8a-Cu.Q particles entrained in a gas have been deposited directly by thermophoresis onto the inner surface of a copper tube. Subsequent sin­tering produces a sheathed wire.IO The principal ad­vantage of this technique is that it allows the substi­tution of Iower-cost copper for silver or gold.

Melting Techniques The highest critical current densities for bulk high·

T, superconductors have been obtained in wires or filaments fabricated by a melting technique. In one method, filaments of bismuth-based superconduc­tors" have been fabricated by laser-heated pedestal growth.12 The end of a thin rod of source material is meRed by a tightly focused laser beam. A single­crystal seed is then lowered into the molten pool. As the seed is withdrawn, a filament is formed, which has a high degree of favorable texturing. Critical current densities of ~50,000 AJcm2 have been produced. A second technique, which has yielded similar micro­structures and critical current densities in Y-Ba-Cu.Q wires, is called meR-textured growth.2l A sintered wire of Y-8a·Cu·O is heated to a liquid + solid phase field in a furnace with a large temperature gradient. Liquid flows to the hotter zone, and long highly oriented crystals are left in its wake. This high degree of favorable texture is thought to be responsible for the excellent critical current densitiesP

MeRing techniques have produced materials with novel microstructures and exceptional electrical pr0p­erties. It has yet to be established, however, whether these techniques are capable of fabricating useful lengths of conductor. In addition, monolithic wires are made by this process, and ~ may prove necessary for most applications to make use of shielded wires, or wires with a tough substrate.

ACKNOWLEDGEMENT This work was supported by the United States Department of Energy, Office of

Energy Storage and Distribution, under contract W31-109-ENG-38.

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