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

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78 2 Composition Features and HTSC Preparation Techniques

OPIT TIRT

Fig. 2.19. Schematic view of the cross-sections of the OPIT and TIRT tapes. The

mutual orientation of the ﬁlaments and that of the ﬁeld with respect to the tape

broad face are expressed by the angles β and α [850]

then three segments are stranded, drawn and formed to a ﬁnal round-shape

wire. Round wires fabricated by SAFM are more tolerant against bending

strain than the usual round wires and permit to reach high J

(Fig. 2.20)

[18].

– Wrapping method of BSCCO/Ag tapes around core to decrease the trans-

port current losses (Fig. 2.21) [1027].

– Rotation-symmetric-arranged tape-in-tube wire (ROSATwire) technique

with Bi-2212/Ag tape-shaped multiﬁlaments, possessing triple rotation

Fig. 2.20. Cross-section of a round wire fabricated by stranded-and-formed method

[18]

2.2 BSCCO Films, Tapes and Wires 79

Thin and Non-twisted

Pre-tape

Repetition

Same-direction

Wrapping

One or More

Pre-tapes for Each Layer

BSCCO

Opposite-direction

Wrapping

Round Wire Tape

After Pressing

or Rolling

Fig. 2.21. Wrapping method to fabricate BSCCO/Ag wires and tapes [1027]

symmetry and having good crystal alignment in each ﬁlament. Since the

present wire structure yields complete symmetrical arrangement of the

tape-shaped ﬁlaments, it is no longer necessary to use a rolling, but permits

to use a drawing (Fig. 2.22) [780].

– Manufacture of Bi-2223/Ag one- and multiﬁlament composite tapes with

oxide barrier layers between ﬁlaments to diminish electric and magnetic

losses (Fig. 2.23) [315,449,574].

– Development of multiﬁlament tapes with central part, that consists of

Bi-2223 ﬁlaments, ensuring transport current ﬂow, and surrounded by the

barrier ceramic layer and YBCO thin ﬁlm to screen external magnetic ﬁeld

and also to protect Bi-2223 ﬁlaments (Fig. 2.24) [45].

80 2 Composition Features and HTSC Preparation Techniques

Preparation of multi-

filamentary tapes with the

same dimensions

Stacking the tapes with a

diamond shape to form a

segment

Stacking the neighboring

3-segments with triple-

rotation symmetry

3 segments 12 segments 27 segments

Fig. 2.22. Fabrication process of rotation-symmetric-arranged tape-in-tube wire

with various Bi-2212/Ag segments [780]

Ag BaZrO

Bi-2223

Fig. 2.23. Typical cross-section of Bi − 2223/Ag/BaZrO

/Ag monocore tape [574]

2.3 Tapes and Wires, Based on Thallium and Mercurial Cuprates 81

Ag Coating

YBCO Film (500 nm)

SrTiO

Buffer Layer (100 nm)

Multifilament Bi-2223

c-axis

Fig. 2.24. Schematic diagram of Bi-2223 multi-ﬁlament tape with Y-123 screening

layer [45]

2.3 Tapes and Wires, Based on Thallium

and Mercurial Cuprates

Processing techniques of the thallium thin ﬁlms (Tl-1212, Tl-1223, Tl-2012,

Tl-2212 and Tl-2223) coincide in many details with corresponding fabrica-

tion techniques of YBCO thin ﬁlms. Best results have been obtained by using

the following substrates: LaAlO

and NdGaO

[432,604,753,754], and also

buﬀer layers: YSZ and CeO

[881] due to good agreement of their crystal-

lographic properties. Moreover, MgO, LaGaO

,SrTiO

have been used as

substrates [604]. Tl-1223 thick ﬁlms and wires with high superconducting

properties have been prepared on substrates from Ag and ZrO

[724,725,1025].

An expensive doping by ﬂuorine has permitted to improve considerably the

structure-sensitive properties of Tl-1223 samples [380].

Hg-1212 and Hg-1223 thick ﬁlms demonstrating maximum critical tem-

peratures, T

, have been prepared in two stages [723]: (1) a precursor layer,

that consists of Ba

and doping oxides, that is, PbO, Bi

and

ReO

has been deposted (2) superconducting phase has been formed by

heating the precursor ﬁlm at the partial pressure of mercury. The precur-

sor layers could be obtained by using the precipitation processes including:

(i) spray pyrolysis [990]; (ii) pulverization of powder with solvent [695]; (iii)

sol–gel method [1078]; and (iv) application of powder-polymeric suspension

[1178]. The majority of researches use mono-crystalline ceramic substrates

(e.g., YSZ) [990,1078,1178]. Moreover, Ni-based substrates with Cr/Ag buﬀer

layer have been used [695]. The increased superconducting properties and

82 2 Composition Features and HTSC Preparation Techniques

50 μm

(a)

(b)

Fig. 2.25. (a) SEM micrograph of (Hg, RE)-1212 sample synthesized at 820

◦

C, (b)

backscattered electron micrograph of the same sample [923]

2.4 BSCCO Bulks 83

improved formation of the mercurial conductor are reached by Re, Pb and

F doping [269,378,379]. Optimum synthesis conditions permit to obtain good

aligned (Fig. 2.25a) and phase purity (Fig. 2.25b) structures of thick ﬁlms.

2.4 BSCCO Bulks

Bi-2212 superconducting coverings for tubes and rods are synthesized by the

diﬀusion reaction between Sr-Ca-Cu oxide substrate, possessing high melting

temperature, and Bi-Cu oxide coating layer [1032] (or mixing Bi–Cu and AgO

[1156,1157,1160]) with low melting temperature (Fig. 2.26). This technique

permits to fabricate diﬀusive superconducting layer with thickness 150 μm

(for roads of diameter 3 mm), demonstrating dense homogeneous structure of

plate-like grains that are aligned almost perpendicular to the substrate surface

(Fig. 2.27).

The weakening of pinning properties at above 20 K is intrinsic for Bi-2212

system. The practical requirement is to enhance this threshold that causes

Pb doping of bulks in the partial-melting technique [08]. In order to obtain

high-dense-textured Bi-2212 bulks, the hot-forging technique is used [324] and

also the solid-state reaction method [876]. The addition of PbO increases frac-

tion, formation rate and stability of Bi-2223 phase [1037]. However, fabrication

of Bi-2223 necessary bulks with for applications properties is the very diﬃ-

cult problem. Hot-pressing technique, used during processing of Bi-2223 bulks,

promotes void elimination under pressure and high temperature. Moreover, it

improves the crystallite alignment in the sample, causing an increasing of the

critical current density [448]. At the same time, J

can decrease at prolonged

sintering of superconducting bulks due to corresponding decrease of the ce-

ramic density. Hot pressing causes high density near the theoretical level for

Bi-2223 (∼ 6.31 g/cm

). The suﬃciently long sintering before hot pressing

rises Bi-2223 phase content in the sintered samples. At the same time, in or-

der to reach necessary properties of ﬁnal superconducting ceramics, special

control of processing parameters is required.

Room temperature pressing is

not eﬀective because of high resistance to strains. By increasing the pressing

temperature, this problem becomes less sharp. In this case, closed porosity

is formed and the sample density increases. This increases the contact area

between Bi-2212 and secondary phases. The formation of Bi-2223 phase is

based on the epitaxial growth of the Bi-2223 crystallites into Bi-2212 matrix

in accordance with the chemical reaction:

Bi−2212 + secondary phases → Bi−2223. (2.3)

Therefore, the elevated contact area causes an acceleration of chemical re-

action and Bi-2223 formation phase. The sample densiﬁcation also increases

This, in the ﬁrst place, is explained by that Bi-2223 phase is stable only in very

small temperature intervals (in diﬀerence from Bi-2212 phase) and kinetics of its

formation are very slow to obtain mono-phase material.

84 2 Composition Features and HTSC Preparation Techniques

Substrate

Bi-free Oxide

Bi : Sr : Ca : Cu

0 : 2 : 1 : 2

Calcining

Cold

Isostatic

Pressing

Sintering

Coating Layer

Bi-Cu Eutectic Oxide

Bi : Sr : Ca : Cu

2 : 0 : 0 : 1

O Addition

Calcining

Dip

Coating

Diffusion

Reaction

Bi-2212 Oxide

Superconductor

Annealing in Ar, N

Substrate

Tube Rod

Molten Bi-Cu-Ag

Oxide

Rod Tube

Coating Layer

Substrate

Bi-2212 Diffusion

Layer

Fig. 2.26. Specimen preparation procedure [1156,1160]

a connectivity of superconducting grains (Bi-2223 or Bi-2212), which is found

by a quality of intercrystalline boundaries. The increase of the grain con-

nectivity causes a rise of the critical current in the sample. However, during

the hot pressing, an excessive elimination of liquid phase, that is rich by Cu

and Pb, is possible. This leads to considerable phase segregation in the sam-

ple, but a deﬁciency of the secondary phases prevents total transformation of

Bi-2212 phase into Bi-2223 phase during hot pressing, decreasing T

and J

Preliminary sintering causes a frame formation from Bi-2212 crystallites, pre-

venting the secondary phase losses. After that, the hot pressing redistributes

secondary phases around Bi-2212 grains, accelerating the phase transforma-

tion considerably. Displacement of amorphous Bi-2212 phase and secondary

phases at intergranular boundaries heals intercrystalline defects. At the same

time, preliminary prolonged sintering can lead to the expense of larger part

of secondary phases on the growth of arbitrary oriented Bi-2223 crystallites.

2.4 BSCCO Bulks 85

Bi–2212

Substrate

100 μm

Fig. 2.27. SEM micrograph taken on the fractured cross-section of the Bi-2212 rod

(Bi-2212 diﬀusion layer and substrate). The sample is obtained at 850

◦

C for 20 h on

the (0213) substrate [1157]

In this case, amorphous Bi-2212 phase has suﬃcient time to re-crystallize.

Therefore, one could not be used to increase the grain connectivity during hot

pressing. Thus, preliminary super-long sintering before the hot pressing, do

not permit to fabricate bulks with high J

. Moreover, expensive liquid phase,

not dissolved into Bi-2223, forms non-superconducting phases and diminishes

[783].

One-axis cold pressing can lead to partial orientation of BSCCO plate-

late grains in the pressing direction, taking into account great microstructure

anisotropy [959]. Therefore, the sample re-crystallization, presenting an ori-

entated grain growth, provides maximum texture during next sintering, cor-

responding to external loading. There is a good correlation between the ﬁnal

angle distribution of BSCCO plates and sample squashing,namelyalower

misorientation corresponds to higher loading [770,900]. Then, the hot forging

followed by squashing of sample leads to improvement of alignment of Bi-

2223 plates [901]. These samples show J

=8kA/cm

(zero magnetic ﬁeld).

This presents interest for current conductors and current limiters. Another

multistage processing of Bi-2223 bulks includes one-axis cold pressing of su-

perconducting powders, then annealing without pressure and ﬁnally the hot

forging [1043].

Taking into account the plate-late morphology of BSCCO grains, magnetic-

melt-processing-texturing (MMPT) technique has been developed, combining

at high temperature two diﬀerent parameters, namely an application of in

situ magnetic ﬁeld (8 T) and one-axis loading (60 MPa) [771]. This technique

has permitted to obtain materials with a good texture. At the same time, hot

plastic deformation technique permits the formation of sharp crystallographic

texture and high density of defects in the crystalline lattice, which are eﬀective

86 2 Composition Features and HTSC Preparation Techniques

pinning centers, in the BSCCO bulks. As the result, the critical current density

> 10

A/cm

has been reached in Bi-2212 ceramic [173,467,468].

In order to form microstructure features – pinning centers – in BSCCO

systems, following methods are used: (i) irradiation of samples after processing

by protons [584], heavy ions [1055] or neutrons at doping HTSC by uranium

[392], that forms super-thin amorphous cylindrical inclusions, the so-called

“columnar defects” (Fig. 2.28); (ii) introducing defects, connected with dop-

ing additions, disperse particles and dislocations [04,292,669]; and (iii) tech-

nique change, supposing an introduction of the mixed particles (e.g., Pb) into

Bi-2212 phase [658]. The enhanced pinning due to irradiation of supercon-

ductor by high-energetic ions, leads to signiﬁcant increasing of the critical

current density, J

, sometimes in the order of magnitude and more. However,

the critical temperature, T

, in this case, as a rule decreases, according to

the common law of the superconductivity suppression by structure. At small

irradiation doses, small (∼ 1K) increasing of T

(so-called “eﬀect of small

doses”) is sometimes observed. Usually, the irradiation dose is selected so as

to reach the maximal value of J

at small decreasing of T

.First,in[04],very

signiﬁcant (on 15 K) growth of T

in Bi-Pb-Sr-Ca-Cu-O thick ﬁlms after their

irradiation by argon ions has been found. The origin ﬁlms consisted of mixing

of Bi-2212 and Bi-2223 phases with the domination of the Bi-2212 phase (they

has T

= 85 K). The ionic irradiation stimulated the processes of local melting

of the ﬁlm and diﬀusion of atoms, resulting in the formation of high-T

Bi-

2223. Thus, the ionic irradiation can be an alternative technique to fabricate

Bi-2223 phase, which is known for its great sensitivity to conditions of sample

synthesis and annealing.

Fig. 2.28. Traces of heavy ions forming amorphous cylindrical regions in HTSC

2.4 BSCCO Bulks 87

An intensive eﬀect in improvement of superconducting properties is

decreasing of cooling rate of the sample that increases J

and T

and

forms more perfect microstructure of superconductor [829]. In order to im-

prove mechanical and superconducting properties of BSCCO during process-

ing, the next additions are introduced into the samples: inclusions of Ag

(Ag

O, AgNO

[999], Al

strengthening ﬁlaments of Al

[679,1147] and

ZrO

• Y

[679], whiskers of MgO [1185,1186,1187], particle dispersion of

MgO and additions of high-dense polyethylene [97]. A schematic description

of the solid-state processing method of the (MgO)

/BPSCCO composite is

shown in Fig. 2.29, and SEM micrographs taken from etched surfaces of pol-

ished monolithic and composite specimens are presented in Fig. 2.30. More-

over, in order to strengthen BSCCO bulks, the glass-epoxy tapes are used

[1044]. At the same time, an application of doping additions of TiO

and

ZrO

, which leads to toughening of superconductor, can simultaneously cause

considerable decomposition of Bi-2223 phase and diminish T

[351,702].

MgO Whiskers

Bi–2223 Powder

Wet-mixing Bi-2223 Powder with MgO

Whiskers and Drying

Hot-pressing

Annealing

HTSC composite

Repeated Hot-pressing

and Annealing

Pre-sintering

“Green” Specimen Forming

Fig. 2.29. Solid-state processing of (MgO)

/BPSCCO composite [1185]