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

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138 3 Experimental Investigations of HTSC

that the sausaging increases with the increasing of superconductor thickness

in the process of intermediate mechanical deformation between heat cycles.

At this thermal mechanical regime, the rolling leads to greater sausaging in

comparison with the pressing (see Fig. 3.22c). Moreover, an increase of mi-

crohardness in the case of prolonged heating correlates almost linearly with

the growth of the critical current density (see Fig. 3.22a) [826]. The mea-

surement of the Knoop microhardness (H

) estimates diﬀerence of the J

behavior in ab-plane (that is parallel to the rolling plane) and along c-axis

direction (that is perpendicular to the rolling plane). So, in the ﬁrst case, the

microhardness H

remains approximately constant, at the same time; in the

second case, it increases with the annealing time (Fig. 3.23) [568]. Destroying

of the microhardness growth with the density increasing permitted to state

heterogeneities, linked with formation of more dense blocks of grains, sepa-

rated by cracks [519] that was caused by dominating mechanisms of the core

deformation, namely sliding and fracture of grains.

22.5

17.5

12.5

7.5

2.5

100 200 225 250 275 30075

Long Axis of Indenter is

Parallel to Wide Axis of

Tape

12 hours annealing, || ab

24 hours annealing, || ab

12 hours annealing, || c

Long Axis of Indenter is

Perpendicular to Wide

Axis of Tape

(kA/cm

)

(10g, 15s)

Fig. 3.23. J

(0 T, 77 K) as a function of Knoop microhardness of BSCCO transverse

cross-sections. The data to the left of the axis break is for H

parallel to the rolling

plane (i.e., approximately || ab-plane), and the data to the right is H

perpendicular

to the rolling plane (i.e., approximately || c-axis). The numbers point to the number

of heat treatments, which the tape is subjected to [568]

3.3 Superconducting Composites, Based on BSCCO 139

Key factors, stating critical strains and stresses for superconducting tapes,

are the following: (i) connectivity and alignment of grains [273, 600], (ii) uni-

formity and high texture of ceramic core [385, 519], (iii) interface regions

Ag/BSCCO and sheath material [626], (iv) temperature during loading [433].

For multi-ﬁlamentary tapes, number and distribution uniformity of ﬁlaments

play considerable role in the estimation of mechanical properties [854, 873].

Critical mechanical characteristics can be considerably improved, adding Ag

(Ag

O, AgNO

) dispersion in the superconducting core. For example, the

value of ε

irr

increases more than two times compared to the monolithic tape

in the case of addition into monocore tape of 7 wt% AgNO

.Inthecaseof

multi-ﬁlamentary tape, this value increases more than three times in compar-

ison with the monolithic monocore tape [1001]. This growth of mechanical

properties compensates well a small decreasing of J

, observed in the case of

AgNO

additives [32].

Based on MOI method, intensive researches of BSCCO/Ag samples under

bending and at diﬀerent values of strain permitted to observe the stage of

microcrack formation and also to understand how magnetic ﬂux penetrated

superconductor [558, 855]. It is conﬁrmed that the defects of the void type

or non-superconducting phase type are the sites of crack initiation [247, 836].

The bending, also as another strain, changes the microstructure and critical

current in two directions: (i) as a result, the intergranular contacts worsen,

decreasing critical current density, and (ii) preliminary existing defects are

developed that diminish local critical current due to decreasing of eﬀective

superconducting cross-section [451, 855]. Intergranular contact, destroyed at

bending, could be restored after straightening of tape and the tape cooling

down to cryogenic temperature [573]. The observed eﬀect is explained by the

core compression transferred by silver sheath on cooling. The compression

causes a sliding of BSCCO grains, restoring the broken contacts. Obviously,

this eﬀect will depend on the core density and texture. In this case, the lattice

of transversal microcracks, formed during the longitudinal rolling, possesses

elevated sensitivity to the tape bending [854]. In bending, most tensile and

compressing loading exist at external surfaces of the tape and render eﬀect

mainly on the metal (alloy)/ceramic interfaces. Fatigue tests of the tape bend-

ing/straightening type identify the mechanism of microcracking formation in

these interfaces and the microcrack growth in ceramic. It is found that tensile

loading favors formation of intercrystalline cracks. At the same time, the com-

pressing stresses form transcrystalline cracks, depending on the orientation of

the ab-plane [659]. When tape is subjected to cyclic strains, which are lower

than ε

irr

, the cracks are transcrystalline type irrespective of stress kind.

The tensile tests of Bi-2223/Ag tapes demonstrate three typical stages,

namely (i) very narrow region of elastic behavior, (ii) suﬃciently broad stage

of microcrack initiation and growth and (iii) a macroscopic ﬂow, accompa-

nied by multiple cracking and macrocrack formation [786, 787]. Fatigue tests

of monocore and multi-ﬁlamentary tapes show that the microcracks do not

reach the macrofracture threshold in the second stage [552, 1023]. An eﬀect

140 3 Experimental Investigations of HTSC

of tension on the sharpness of transition, depicted in curves of the “transport

electric ﬁeld–current density” (E–J) dependence (that is very important for

devices, based on HTSC), may also be estimated experimentally [550]. Typ-

ical V –I curves dependent on strain for multi-ﬁlamentary Bi-2223/Ag tapes

are shown in Fig. 3.24. As shown in the ﬁgure, before sharp decreasing of I

its degradation takes place initially in the region of lower electric ﬁeld. The

test curves also estimate change of value n, deﬁning the dependence V –I

Joint eﬀect of strain and external magnetic ﬁeld deﬁnes considerable de-

creasing of I

even in the smallest growth of the ﬁeld [451]. Two current-

carrying paths co-exist in the tapes: one is through the Josephson junction

network, consisting of weakly linked grains, and the other is through the

strongly linked grains. Investigation of these path show that in addition to

material cracking, strain deteriorates the grain connectivity, leading to the

easier suppression of I

by magnetic ﬁeld in the low ﬁeld region. For the well-

connected current-carrying path, strain on one hand damages well-connected

grain boundaries through cracking or sliding. On the other hand, strain can

introduce new defects in grains, increasing intercrystalline components of the

magnetic ﬂux pinning. Typical dependencies of transport currents, ﬂowing

through weak-linked boundaries (i.e., the Josephson junction network) I

and

through strong-linked grains I

(the resulting critical current I

= I

+ I

on applied magnetic ﬁeld and strain are shown in Fig. 3.25 [451].

3.3.2 Irreversibility Lines for BSCCO

As has been noted in Sect. 1.6.4, the current-carrying capability of super-

conductor was found by the irreversibility line, presented by the “magnetic

ﬁeld–temperature” non-linear dependence. Above this line, there is a region

with the condition that direct current decreases rapidly, making superconduc-

tor to be useless for many applications. This property is connected with the

material anisotropy. Due to the greatest anisotropy of Bi family in comparison

–4

–5

–6

Current, I (A)

Voltage,V (V)

0.69

0.58

0.53

0.61 0.60

0.55

6 7 8 9 10 20 30 40

0.00–0.50%

Fig. 3.24. Characteristics of transport current V–I vs strain in multi-ﬁlamentary

Bi-2223/Ag [550]

3.3 Superconducting Composites, Based on BSCCO 141

0.8

0.6

0.4

0.2

0.001 0.01 0.1 1

netic Field (T)

Normalized I

and I

(B,

ε)/I

(0, ε)

(B,

ε)/I

(0, ε)

Fig. 3.25. Normalized I

and I

as a function of magnetic ﬁeld, showing the

strain eﬀects on both the strong-link and weak-link current-carrying paths [451]

with other HTSC, its irreversibility line is the lowest among high-temperature

superconducting families (Fig. 3.26). Therefore, the irreversibility line is one

of the main problems of BSCCO techniques. Comparative test data, obtained

for single Bi-2212 crystals, Bi-2212/Ag and Bi-2223/Ag tapes [214], show that

the irreversibility line for Bi-2223/Ag tapes occupies a region of higher tem-

peratures than the irreversibility line for Bi-2212/Ag tapes, which is higher

than the one for Bi-2212 crystals (Fig. 3.27). Defects introduced, in particular

by using mechanical loading, act as pinning centers and are responsible for

trapping of vortex lines. The irreversibility lines are a measure of connectivity

of the CuO

superconducting planes and the vortices trapping (i.e., pinning

strength) [214, 472]. All three samples (Bi-2212 crystals, Bi-2212/Ag and Bi-

2223/Ag tapes) are superconductors of the same family and possess identical

spacing between CuO

planes. Moreover, the samples investigated in [214] had

comparative sizes. Hence, it may be assumed that shear of the irreversibility

line location characterizes directly the pinning strength and is thus maximum

for Bi-2223/Ag tapes, followed by Bi-2212/Ag tapes and smallest for Bi-2212

02040

60 80 100

T (K)

irr

(T)

H || c-axis

Bi-2212

Bi-2223

Tl-1223

Y-123

Fig. 3.26. Irreversibility lines for Bi-2212, Bi-2223, Y-123 [151] and Tl-1223 [1065]

142 3 Experimental Investigations of HTSC

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

peak

irr

(T)

Bi-2212 Crystal

Bi-2212 Tape

Bi-2223 Tape

Fig. 3.27. Irreversibility lines for single Bi-2212 crystals, Bi-2212/Ag and

Bi-2223/Ag tapes, possessing comparative sizes [214]

crystals. Elevated magnetic ﬂux pinning could be due to higher dislocation

density in tape in comparison with crystal. At the same time, the diﬀerence

between Bi-2212/Ag and Bi-2223/Ag tapes, in this case, is rather caused by

diﬀerent processing conditions. So, in order to obtain the Bi-2212/Ag tapes,

the melt-texturing technique has been used, diﬀerent from Bi-2223/Ag tapes

prepared by using the solid-phase reaction techniques and partial application

of liquid-phase sintering. In this case, the defects, introduced during mechan-

ical strain in Bi-2212/Ag tapes, are healed partially on melting. At the same

time, the solid-phase reaction in the Bi-2223/Ag tapes does not lead to consid-

erable decreasing of material damage [214]. The experiments show [472] that

the dislocation line density in Bi-2223/Ag tapes is approximately greater by

an order of magnitude, than in Bi-2212/Ag tapes and single Bi-2212 crystals.

The comparison of irreversibility lines for 27 ﬁlamentary tapes, processed

by using diﬀerent techniques with application of hot-pressing, rolling and cold-

pressing, has been carried out in [385]. All three tapes had the same sizes and

have been subjected to the same thermal and mechanical treatments before

ﬁnal sintering. Therefore, in this case also, shear of irreversibility line must be

attributed only to the characteristic of strength pinning. The cold strain must

create more defects than hot deformation because in the latter case, the grains

possess elevated mobility. Then, it may be expected that dislocation density in

the tapes, obtained by using cold-pressing technique, is more, than in the case

of hot-pressing technique. This states a higher location of the irreversibility

line in the tapes, obtained by cold-pressing method, in comparison with the

rolled tapes. The latter, in its turn, exceeds corresponding data for tapes,

fabricated by hot-pressing technique (Fig. 3.28).

Then, the dependence J

–H for the hot-pressed sample (Fig. 3.29) shows

improved behavior in the region of low magnetic ﬁeld in comparison with

other two samples. The fast diminution of J

at low external ﬁeld is proper

3.3 Superconducting Composites, Based on BSCCO 143

40 60 80 100

peak

(K)

irr

(T)

Rolling + Hot-Pressing

Rolling

Rolling + Cold-Pressing

Fig. 3.28. Irreversibility lines for the cold-pressed, rolled and hot-pressed

27-ﬁlamentary Bi-2223/Ag tapes, possessing the same sizes [385]

for Josephson weak links at intercrystalline boundaries. In this case, the rolled

tapes demonstrate maximum diminution of critical current under the low ﬁeld.

Under high magnetic ﬁeld, there is a plateau due to magnetic ﬂux pinning

in single grains. The dependence J

–H for the hot-pressed and cold-pressed

tapes is almost parallel to one another, when applied ﬁeld is parallel to the

tape surface. In the case of perpendicular ﬁeld to the tape surface, prepared

by using cold pressing, the critical current density decreases slowly in exter-

nal ﬁeld, exceeding 100 mT, in comparison with the hot-pressed and rolled

tapes. This result proves that hot-pressing increases grain connectivity and

H (mT)

Rolling+ Hot-Pressing

Rolling

Rolling+ Cold-Pressing

0.1

0.01

0 200 400 600 800 1000

H || Tape Plane

H ⊥ Tape Plane

Fig. 3.29. Dependencies J

− H for the cold-pressed, rolled and hot-pressed 27

ﬁlamentary Bi-2223/Ag tapes at 77 K in magnetic ﬁelds that are parallel and per-

pendicular to the surface tape [385]

144 3 Experimental Investigations of HTSC

therefore improves weak link behavior in low ﬁelds. At the same time, cold

isostatic pressing forms a great number of defects, forcing the magnetic ﬂux

pinning [385].

3.3.3 BSCCO Bulks

An addition in BPSCCO (Bi-2223) bulks of thermo-mechanical and chemical

compatible reinforcement improves the HTSC microstructure, superconduct-

ing and mechanical properties. In particular, an addition of MgO whiskers,

oriented parallel to ab-plane of superconducting grains [1186], together with

carrying out cyclic procedures of hot-pressing and annealing, increase the av-

erage grain size both in the ab-plane and on the sample thickness. At the

same time, when a whisker concentration (V

) is 20% and more, there is con-

siderable constraint of the BPSCCO phase grain growth, especially in the

ab-plane, which is most favorable for improvement of superconducting prop-

erties. By introducing whiskers, the current-carrying capability of supercon-

ductor increases in comparison with analogous monolithic sample at the same

grain size (Fig. 3.30).

Additional pinning centers, for example, microdam-

ages, defects of crystalline lattice or low chemical substitutions, can form at

the MgO/BPSCCO interfaces. These interfaces remain clear without chemical

reaction zone even after prolonged annealing (Fig. 3.31), which is an additional

factor of J

increasing. Long thermal treatment can also contribute to obtain

denser BPSCCO microstructure with lower content of admixtures, causing

better superconducting properties [1184].

•

(MgO)

/BPSCCO (V

= 10%)

Monolithic BPSCCO

Average Grain Size, l (µm)

(kA/cm

)

Second

Hot-Pressing

10 12 14 16 188

Fig. 3.30. Eﬀects of average grain size (l) on critical current density (J

) in mono-

lithic BPSCCO and (MgO)

/BPSCCO composite with V

= 10% [1186]

An addition of Al

ﬁlaments is less advantageous due to physical and chem-

ical incompatibility of BPSCCO with Al

leading to chemical reactions on

interfaces at elevated temperatures of processing, decreasing J

[1147].

3.3 Superconducting Composites, Based on BSCCO 145

Bi-2223

(MgO)

Bi-2223

2 µm

Fig. 3.31. High-magniﬁcation SEM micrograph of the interface region in a polished

and etched cross-section of a (MgO)

/BPSCCO composite with V

= 10% [1186]

In order to reach desired phases, microstructures and structure-sensitive

properties of superconductor, it is important to control technological param-

eters. In the case of hot-pressing technique, these parameters include tem-

perature, pressure and process duration. Based on the tests, the optimum

temperature of hot-pressing for BPSCCO bulk is equal to 800–855

◦

C [1019].

An initial hot-pressing leads to J

increasing in HTSC directly after this proce-

dure, as well as following annealing (Fig. 3.32). However, as has been discussed

in Sect. 2.4 single hot-pressing alone cannot create a dense and alignment mi-

crostructure with large grains. Moreover, it is impossible to reach high purity

of the Bi-2223 phase. Sometimes, a diminution of its content is observed be-

cause of partial decomposition, caused by combination of high temperature

and high pressure [1185]. Due to the above causes, it is necessary to carry out

additive annealing or cyclic procedures of “annealing–pressing” to increase

(Fig. 3.33). Next, annealing improves superconductor properties, but it

should be noted that exceeding its deﬁnite duration (this mark is equal to 160

in Fig. 3.32) causes a diminution of J

. There are other causes for decreasing

of the critical current density: (i) stopping of densiﬁcation in the absence of

additional pressure [801, 1185] and (ii) deﬁnite alterations of the supercon-

ductor chemical composition because of a possible evaporation of Bi and Pb

during prolonged annealing [670, 671].

High texture along c-axis due to one-axis cold pressing could be formed in

the samples with MgO dispersion and in the superconductors with polymeric

matrix (BPSCCO/polyethylene) [97]. An addition of silver in diﬀerent forms

into the bulk leads to improved superconducting and mechanical properties. In

this case, AgNO

additives show greater eﬀect on formation of superconduct-

ing phase and J

in comparison with Ag and Ag

O particles [999]. Introducing

ﬁlaments into BPSCCO matrix does not lead to considerable decrease

in superconducting properties [1147], but provokes substantial toughening

146 3 Experimental Investigations of HTSC

250

Annealing Time (h)

(kA/cm

)

Monolithic BPSCCO, HP(8.28 MPa)

Monolithic BPSCCO, HP(13.8

MPa)

Composite,V

= 10%, HP(8.28 MPa)

Composite,V

= 10%, HP(13.8 MPa)

Composite,V

= 20%, HP(13.8 MPa)

0 50 100 150 200

Fig. 3.32. Eﬀect of annealing time on J

(77 K and zero ﬁeld) for monolithic

BPSCCO and (MgO)

/BPSCCO composite (ﬁrst, hot-pressed at 820

◦

Cwith

diﬀerent pressures and then continuously annealed at 832

◦

Cin8%O

) [1185]

of superconductor simultaneously, caused by non-regular and rough surfaces

of fracture [712]. In the case of room temperature (293 K), macroscopically

plane surfaces of crack are observed corresponding to brittle fracture with

small de-laminations (Fig. 3.34a) and microscopically short pulled-out ﬁbers

(see Fig. 3.35a). At the same time, at cryogenic temperature (77 K), there

Annealing Time (h)

Monolithic BPSCCO

Composite,V

= 10%

Composite,V

= 20%

First HP

Second HP

Third HP

(kA/cm

)

50 100 150 200 250 300

Fig. 3.33. J

(77 K and zero ﬁeld) of monolithic BPSCCO and (MgO)

/BPSCCO

composite with V

= 10% and V

= 20% in 2-cycle and 3-cycle hot-pressing and

annealing processes [1185]

3.3 Superconducting Composites, Based on BSCCO 147

1 mm

(a)

(b)

400 µm

Fig. 3.34. Macroscopic failure modes in Al

/BPSCCO composite at: (a) 293 K

and (b) 77 K [712]

are intensive de-laminations at the fracture surface (Fig. 3.34b) and pulled-

out long ﬁbers (Fig. 3.35b). The pulled-out ﬁbers are covered in both cases

by BPSCCO grains oriented preferably along ﬁbers (Fig. 3.36), demonstrating

strong interfaces between ﬁbers and matrix. Due to brittleness of the BPSCCO

monolithic sample at 77 K and as its fracture is controlled by microstructure

defects, a considerable weakening of Al

/BPSCCO composite takes place

at 77 K because of intensive cracking of matrix, caused by thermal stresses.

Corresponding weakening of interface region between matrix and ﬁbers causes

greater increasing of fracture toughness at 77 K in comparison with room

temperature. The toughening mechanisms, acting at cryogenic temperature,

include the macrocrack deﬂection, branching and blunting and also crack

propagation along the ﬁber direction. Thus, the diﬀerences of the fracture