Parinov I.A. Microstructure and Properties of High-Temperature Superconductors

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68 2 Composition Features and HTSC Preparation Techniques

In Region I, the conductor is heated above the melting point of Bi-2212,

held at this temperature for a short time and then cooled. In this stage,

Bi-2212 powder melts incongruently, forming liquid and non-superconducting

crystalline phase. At the end of this stage, Bi-2212 formation begins. At melt-

ing, the Bi-2212 powder releases oxygen, which is not a problem for ﬁlms

that are open to the environment, but can be a problem for tapes and wires,

where it can cause the silver sheath to bubble. On the other hand, at el-

evated temperatures, used during melting, Bi evaporation from the melt is

possible [924]. The silver sheath of tapes and wires prevents this process, but

this is actual danger for ﬁlms. Also, it should be noted that critical condi-

tion to prepare homogeneous highly aligned microstructure is the minimum

fraction of crystalline phases in the melt at the beginning of Bi-2212 phase

formation.

In Region II, Bi-2212 forms from the melt, where growth and alignment of

Bi-2212 grains occur. Since Bi-2212 melts incongruently, at cooling it should

be formed by a reaction between liquid and crystalline phases. In this case,

in order to form a homogeneous highly aligned microstructure, the melt must

contain small grain of non-superconducting phases when Bi-2212 begins to

form. The cooling rates, used for the melt process of Bi-2212 conductors

should be suﬃciently fast to intensify Bi-2212 phase formation and mini-

mize fraction of normal phases, which are present in the ﬁnal product always.

The problems with having non-superconducting phases in the fully processed

conductor are that the phases are too large to pin ﬂux, they block the su-

percurrent path and diminish useful properties of the conductor. In order to

obtain high superconducting properties, a highly aligned grain structure is

necessary. Plate-like Bi-2212 grains grow from the melt because growth of

Bi-2212 is faster in the ab-plane than in the c-direction. This two-dimensional

growth is critical for the alignment that develops during cooling. The experi-

ments have shown [409] that the cooling rate and thickness of the oxide melt

aﬀect the alignment: slower cooling and thinner oxide yield higher alignment.

The misorientation angle for a given grain size decreases with decreasing oxide

thickness. The alignment mechanism requires that the large, properly oriented

grains grow at the expense of the smaller misoriented grains. Films that are

< 20–25 μm thick align easily. In thicker ﬁlms, the alignment is usually not

uniform throughout, being higher close to the free surface than near the silver

interface. It has been suggested that in the ﬁlms the Bi-2212 growth and align-

ment begin at the free surface and proceed into the oxide layer. Since ﬁlms

are two-dimensional, one would expect a higher alignment in thinner ﬁlms be-

cause of the smaller misorientation angle a grain could have and still grow to

a given length. Moreover, as the aligned grains grow in ﬁlms, they may rotate

misaligned grains into alignment near the free surface more easily compared

with the nearest neighborhood of the superconductor/metal interface. The

free surface in ﬁlms is also important from the point of general alignment of

the ﬁlm structure. Thick ﬁlms (50–100 μm) can have a 20–25 μmthicklayerof

aligned Bi-2212 grains at the free surface with poorly aligned Bi-2212 below

2.2 BSCCO Films, Tapes and Wires 69

this layer [409]. In general, the grain alignment and J

are higher in ﬁlms

than tapes.

In Region III, the formation and alignment of Bi-2212 is maximized. At

the same time, in Region IV, where the sample cools to room temperature, the

critical alteration of microstructure occurs. High J

requires fast cooling rate

for ﬁlms (of order 1200

◦

C/h [984]) and analogous rates for tapes and wires

[977]. However, at very high cooling rates the conductors may crack, leading

to low J

[984]. At the same time, the Bi-2212 can decompose with slow cool-

ing rates (at < 300

◦

C/h) forming Bi-2201, possessing lower superconducting

properties.

In order to minimize the number of intergranular weak links, that are

proper for oxide superconductors, a high degree of crystallographic texture

must be obtained. One possible route by which a strong crystallographic

texture can be produced is to melt process the material under the eﬀect

of an elevated magnetic ﬁeld [275]. In this case, the driving force for grain

alignment is provided by the anisotropic paramagnetic susceptibility, exhib-

ited by the superconductor grains. When a superconductor grain is placed in

a magnetic ﬁeld, the axis of maximum susceptibility aligns with the magnetic

ﬁeld direction. As a result, in the case of superconductors, such as BSCCO and

YBCO the grains should align with the c-axis parallel to the external magnetic

ﬁeld [53,1100]. Scanning electron microscopy (SEM) backscattered images of

polished cross-sections of Bi-2212 thick ﬁlms with two diﬀerent thicknesses,

processed in zero ﬁeld and at 10 T magnetic ﬁeld, are shown in Figs. 2.10

100 µm

Bi-2212

(a)

(b)

Fig. 2.10. SEM images of cross-sections of Bi-2212 ﬁlms melt processed in the

absence of magnetic ﬁeld [275]

70 2 Composition Features and HTSC Preparation Techniques

10T

100

µm

Bi-2212

(a)

(b)

Fig. 2.11. SEM images of cross-sections of Bi-2212 ﬁlms melt processed under 10 T

magnetic ﬁeld [275]

and 2.11, respectively. In the ﬁrst case, the degree of texture decreases with

the increasing ﬁlm thickness. In Fig. 2.12, the transport critical current den-

sities of the ﬁlms measured at 4.2 K are plotted as function of their thickness

for ﬁlms processed under zero and 10 T magnetic ﬁeld. Analogous results of

a higher degree of alignment at the higher magnetic ﬁeld are also observed in

Bi-2212 tapes (see Fig. 2.13).

The superconductors Bi-2212/Ag are also prepared by using melt-solidiﬁca-

tion method [396]. In its modiﬁcation, pre-annealing and intermediate rolling

are applied to improved the alignment of superconducting grains [699]. At

the same time, monocore and multiﬁlament superconducting tapes and wires

in silver sheath (Bi-2212/Ag and Bi-2223/Ag) are prepared most successively

by using the oxide-powder-in-tube (OPIT) method [216,1091,1159]. Short Bi-

2223/Ag multiﬁlament tapes have demonstrated J

> 80 kA/cm

at 77 K us-

ing this technique [287]. However, this multistage method is characterized

by numerous technical parameters and procedures which deﬁne (together

with initial composition) useful properties of the ﬁnal sample. In this case,

it is very important to prepare superconducting Bi-2212 and Bi-2223 pow-

ders, possessing better phase composition, grain size and shape, and also

chemical purity. Corresponding technical operations have been presented in

Table 2.2. Today, the standard approaches to obtain Bi-2223 precursor pow-

ders include the so-called methods of one-powder synthesis and two-powder

2.2 BSCCO Films, Tapes and Wires 71

•

20 40 60 80 100 120

•

0 T

10 T

140

Thickness (µm)

(10

A/cm

)

Fig. 2.12. Transport current densities at 4.2 K, zero ﬁeld, as a function of thickness

for ﬁlms processed under 0 and 10 T magnetic ﬁelds [275]

synthesis [213]. In the ﬁrst case, the precursor is prepared as the result

of calcination of the oxides and carbonates mixture. In the second case,

sintering of the mixture of two cuprate compositions is carried out. The

OPIT method for making conductors is shown schematically in Fig. 2.14

and includes the following procedures. After the preparation of precursor

powder, it is packed in a tube from Ag or its alloy. Here, silver is used,

in particular, because of its property to diﬀuse oxygen at high tempera-

tures. This permits to control the oxygen pressure during superconductor

fabrication [409]. The tube is sealed and mechanically worked into desired

conductor form. Usually the Ag tube is drawn to a small diameter (∼ 2 mm),

using set of cone holes. Then, this wire is rolled into a ﬂat tape with

thickness ∼ 0.1 mm. Multiﬁlament superconductors are prepared, placing

separate wires in a tube with a large diameter and carrying out analo-

gous technical operations. At ﬁnal stage, the samples are subjected to set

of thermal treatments. During chemical reaction, necessary superconduct-

ing phases form at this stage. Mechanical deforming helps to align and con-

struct the texture of crystalline structure in this technological link. Thus, the

brittle superconducting oxide is encircled by the sheath from silver or its

alloy which protects the superconducting core from chemical and thermody-

namic inﬂuences. Figure 2.15 shows diﬀerent OPIT conductor forms that have

been made.

The use of silver as sheath material of BSCCO tapes and wires is caused by

the compatibility of Ag with superconducting powders at high temperatures

of sintering. However, for tapes with silver sheath, there is the problem of

72 2 Composition Features and HTSC Preparation Techniques

0 T

10 T

Fig. 2.13. SEM images of cross-sections of Bi-2212 tapes melt processed under 0

and 10 T magnetic ﬁelds [275]

formation of an undulating oxide/Ag interface, which is known as sausaging.

This occurs along the tape after its drawing, rolling and pressing (Fig. 2.16)

[385,665], and also silver creeps at high temperatures [320]. Moreover, HTSC

tape sheath, used in concrete devices or samples, must satisfy deﬁnite de-

mands, namely (i) they must possess suﬃcient mechanical strength in order

to bear strains, stated by high electric and magnetic ﬁelds, that may crack

brittle superconducting core; (ii) they must possess low heat conduction in

current conductor and high electric resistance to decrease current losses in

cables [1177]. In order to solve these problems, the silver alloys with some

metals, namely Au, Cu, Mg, Mn, Pd, Zr, (Ni, Y), (Zr, Al) and (Mg, Ni) are

used [06,16,954,991,1063]. Thus, in total, there is a problem to obtain optimum

microstructures of superconducting composite, taking into account diﬀerent

behaviours of the silver (alloy) sheath and oxide core at thermal treatment

[385,1130].

Now, OPIT method is generally applied to prepare BSCCO superconduc-

tors. The advantage of this HTSC system is that the texturing and alignment

of crystallites, inﬂuencing signiﬁcantly superconducting properties of tapes,

may be simply achieved at the ﬁnal stage of the sample fabrication. In this

2.2 BSCCO Films, Tapes and Wires 73

Powder

Sintering

Packaging

Tube

Drawing

Multi-filamentary Samples

Restack

Second Drawing

Annealing

Rollin

Re-rolling

Re-annealing

Fig. 2.14. Schematic diagram of the oxide-powder-in-tube method to make HTSC

wires and tapes [409]

case, the tape deformation leads to shear in BSCCO structure along the double

layer Bi–O, acting as glass phase, due to the weak joints of the adjacent layers

to each other. As a result, the grains form an aligned structure at the next

heating. The YBCO family has a worse structure of grains and demonstrates

higher isotropy compared with BSCCO and also possesses small intergranular

transport currents that are caused by weak links. Therefore, all attempts to

use OPIT method for this family were not successful [649,1021].

74 2 Composition Features and HTSC Preparation Techniques

(a)

(b)

(c)

(f)

(d)

(e)

Fig. 2.15. Representative conﬁgurations for Ag-sheathed wires and tapes. Black

regions are Ag and white regions are BSCCO: (a) monocore tape, (b) coaxial mul-

tiﬁlament wire, (c) wire with two BSCCO cores, (d) tape rolled from the wire in

(c), (e)ﬁlamentwireand(f)ﬁlamenttape

At long sintering of Bi-2223/Ag monocore tapes, a diminishing of magnetic

ﬂux pinning with corresponding decrease in critical current, I

(Fig. 2.17

which could be caused by decreasing Pb content in the superconducting

core at increasing calcination time [670,671], is observed. However, we pro-

pose the more probable causes decreasing I

to be inevitable processes of

pore transformation at intergranular boundaries with pore displacement into

Obviously, the initial increase in critical current is connected with improvement

in quality of intercrystalline boundaries. However, this factor has secondary value

at longer sintering.

2.2 BSCCO Films, Tapes and Wires 75

BSCCO

Rolling Direction

Fig. 2.16. Longitudinal cross-section of a BSCCO tape, showing undulations in the

Ag/oxide interface. This is known as sausaging

growing grains, forming close porosity at prolonged sintering of the sample

[801,1185].

In total, an increasing of low pinning, which is intrinsic to BSCCO family

(compared to YBCO family), is one of the main goals of thermal treatment,

applied to prepare BSCCO/Ag samples by OPIT technique [87,282,493].

There are other problems demanding their solution during tape processing by

using OPIT technique, namely (i) an initiation of the local heterogeneities into

powders during mechanical deformation (e.g., pressing) [673]; (ii) the bubble

formation into silver sheath due to isolation of gases [384,568]; (iii) a possi-

bility of the microstructure perturbations because of violation in crystallite

alignment, initiated near secondary phases due to the formation of crack-like

500 100 150 200 250

120

Time (h)

Ic(A)

T = 4.2 K

T = 77 K

B = 0 T

Fig. 2.17. The critical current vs sintering time (B =0T,T =4.2and77K)

76 2 Composition Features and HTSC Preparation Techniques

defects during inelastic strain of pressed powder [831], and also due to various

deviations from optimum-phase composition [783]; (iv) diﬃculties in the tex-

ture formation of HTSC connected with the existence of secondary phases, het-

erogeneity and redistribution of material during thermal treatment [385,624];

(v) impossibility to totally heal the microcracks formed during mechanical de-

formation using high-temperature treatment [854]. Finally, an increased length

of tape initiates an action of scaling factor, which inﬂuences considerable de-

crease in the critical current density. Due to this, the doping additions of Ag

(Ag

O, AgNO

) are added to HTSC for improvement of structure-sensitive

properties [915,1001]. The complexity and multistage character of the OPIT

technique, and also smaller more than order of magnitude the critical cur-

rent density of tapes of compare with ﬁlms, deﬁne modiﬁcations of this stan-

dard technique of the BSCCO/Ag tape processing. As examples, we point the

following:

– using the hot extrusion (Fig. 2.18a) to obtain long-scale (l>150 m) Bi-

2223/Ag tapes [673];

– intermediate mechanical deforming (one-axis pressing [341] (Fig. 2.18b) or

cold rolling [831]) of tapes after ﬁrst sintering, that form Bi-2223 phase in

order to increase the density of superconducting core and corresponding J

– application of deformation methods which are alternative to rolling, namely

semi-continuous pressing [592], consecutive pressing [32] and periodic

pressing [672];

– modiﬁcations of the standard rolling: (i) out-center rolling with eccentric-

ity, using two concentric rollers [565], (ii) sandwich rolling, in which a

tape locates between two thick steel plates fastened with springs [1133],

(iii) transversal (diﬀering from standard or longitudinal) rolling [359], (iv)

two-axis rolling, that replaces the wire-drawing [450], (v) groove rolling

(Fig. 2.18c) [457];

– pressing Bi-2223/Ag tapes at cryogenic temperatures, based on relative

increase in silver hardness compared with superconducting oxide at liquid

nitrogen temperatures [164];

– application of hot isostatic pressing (Fig. 2.18d) to solve the problem of

porosity and microcracks in Bi-2223/Ag tapes [77];

– using an expensive pressure to pack a superconducting core at the tem-

peratures of Bi-2223/Ag phase formation [888];

–usingthewind-and-react (W&R), react-and-wind (R&W) and react-wind-

sinter (RWS) techniques of continuous winding at the ﬁnal stage of Bi-

2212/Ag tape processing, which decrease the damage of long-scale samples

during their winding [78];

–usingthewind-react-and-tighten (WRAT) technique, that combines the

advantages of the W&R and R&W techniques and consists of free wind-

ing and reaction in the winding with ﬁnal introduction of isolation after

completion of thermal processes and tightness of the winding to its ﬁnal

size [765].

2.2 BSCCO Films, Tapes and Wires 77

(a)

(c)

(d)

(e)

(b)

Fig. 2.18. Some additional technical operations used to improve quality of HTSC

prepared by OPIT technique: (a)extrusion,(b) one-axis pressing (c) groove rolling

(a cross-section of seven-ﬁlament wire is shown), (d) isostatic pressing and (e)con-

tinuous winding

Finally, new techniques to prepare the composite tapes and wires are

designed, that use optimum geometry and placement of components or

advantages of superconducting and ceramic materials, namely:

– Tape-in-rectangular-tube (TIRT) technique to prepare superconducting ﬁl-

aments with c-axis oriented in the direction that is not perpendicular to

the tape surface (Fig. 2.19) [850].

– Processing round Bi-2212/Ag wires by using stranded-and-formed method

(SAFM), in which ﬁrst, ﬁlaments are bundled to consist of a segment, and