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

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128 3 Experimental Investigations of HTSC

(a)

400 nm

A-layer

10 μm

(b)

2212

2223

Fig. 3.15. (a) Thin amorphous layer at the boundaries of colonies with two neigh-

boring basis planes (B) and amorphous structure (A) in triple-point junction of

colonies [14]; (b) inclusion of admixed non-superconducting phase of (Ca, Sr)

CuO

shown by using more dark color [828]

of CuO

plane, decreasing J

. At the same time, strain around these defects

can act as eﬀective magnetic ﬂux-pinning centers, improving superconducting

properties [793]. The dislocation density in monolithic tapes near Ag/Bi-2223

interface can exceed on one order of magnitude the corresponding value in the

core center [1117]. A more high plastic deformation near the interface in the

3.3 Superconducting Composites, Based on BSCCO 129

tape processing is explained. An addition of silver dispersion permits to form

relatively uniform density of core. In this case, an increased dislocation density

leads to higher and homogeneous residual stresses in the core, allowing the

preparation of superconductor with better properties. Moreover, an increas-

ing of the irreversible strain ε

irr

(the strain level, above which J

decreases

irreversibly) is caused by fact that the silver inclusions deﬂect crack and pin

their surfaces in the mechanism of crack bridging, preventing the growth of

crack into superconducting core [1117]. In this case, AgNO

additives render

better eﬀect on the formation of Bi-2223 phase and the value of J

compared

with Ag or Ag

O dispersion [1000]. Fabrication of layered tape compositions

of Bi-2212/Ag with intermediate buﬀer layer of silver provides the improved

superconducting properties at bending compared with single

tapes, having the same cross-sections of superconducting component [466].

The level of irreversible strain also changes in dependence on the number of

superconducting ﬁlaments. So, the bending strain above 0.1% at the surface

of monocore tape leads to decreasing of critical current (I

). At the same

time, the tape with 1296 ﬁlaments can sustain strains of 0.7% without actual

diminution of I

[925].

The test data (Fig. 3.16a) of tension of the 61 ﬁlamentary Bi-2223/Ag

tapes under T =20K and B = 0–8 T show small changes (5% decreasing of

at ε = 0) in the range of small strain (upon to ε

irr

=0.4%) [873]. Above

this level, there is intensive fracture of superconducting ﬁlaments, destroying

current paths and leading to sharp diminution of critical current. This behav-

ior is observed for all measured magnetic ﬁelds. In this case, corresponding

normalized values of I

(see Fig. 3.16b) state generalized curve for all

considered values of the magnetic ﬁeld, causing independence of mechanical

properties from ﬁeld. Moreover, the moment of irreversible decreasing of I

by straining does not depend on external magnetic ﬁeld, implying ε

irr

to be

an intrinsic parameter of superconductor, found by its material properties.

This result is conﬁrmed by comparison of corresponding curves I

= I

(ε),

obtained at various temperatures (see Fig. 3.16c). Small diﬀerences between

values of ε

irr

, observed in broad temperature range, are explained by material

diﬀerences of test samples, rather than by some other factors. It should also

be noted that a strain increasing much more that ε

irr

can demonstrate satu-

ration at decreasing of I

. In many cases, the critical current makes up 20%

from initial value even after a strain of 0.8% [246].

The most important eﬀects on superconducting properties are rendered by

voids and microcracks, forming in the BSCCO core during multi-stage ther-

mal treatments [831, 1196]. The distribution of microcracks in Bi-2223/Ag

tapes is introduced by mechanical deformation during superconductor process-

ing. Comparative studies of longitudinal-rolled and transversal-rolled “green”

(i.e., without thermal treatment) tapes, and also of tapes prepared by using

one-axis pressing, give the following results [359]. The longitudinal-rolled and

transversal-rolled tapes demonstrate smooth interfaces with a small number

of short cracks, respectively, in the direction of the tape width and along its

length. At the same time, the one-axis pressed tapes show wavy interfaces

130 3 Experimental Investigations of HTSC

0 0.2 0.4 0.6 0.8

/ I

(b)

B(T)

1.2

0.8

0.6

0.4

0.2

ε (%)

100

(A)

(a)

B(T)

ε (%)

0 0.2 0.4 0.6 0.8

ε (%)

/ I

0 0.2 0.4 0.6 0.8

1.2

0.8

0.6

0.4

0.2

4.2 К

20 К

77 К

(c)

Fig. 3.16. The test data of the Bi-2223/Ag tape tension: (a) critical current and (b)

its normalized value, obtained at T = 20 K and B = 0–8 T; (c) comparison results

for diﬀerent temperatures [873]

3.3 Superconducting Composites, Based on BSCCO 131

and long cracks directed to the sample length. These results are explained

by the concept that in the case of rolling, maximum shear stress will develop

along plane with normal vector, having component coinciding with the rolling

direction, promoting the formation of transversal cracks that block transport

currents. In the case of pressing, the stress state twists at 90

◦

about the nor-

mal to the tape plane, causing cracking along current direction (Fig. 3.17).

It should be noted that the contact length (of roller with material deformed)

during rolling in the rolling direction is far lower than in perpendicular direc-

tion (along the axis of rollers) and agrees with acting friction force. Then, the

strains ε

>> ε

develop in the central part of oxide core under conditions of

Rolling Pressing

Planes of Maximal

Shear Stress

Width

Thickness

Length

Microcracking

Longitudinal

Rolling

Transversal Rolling

One-axis Pressing

Sausaging

(a)

(b)

Fig. 3.17. (a) Stress state at rolling and pressing of “green” tapes; (b) corresponding

behavior of microcracking and sausaging

132 3 Experimental Investigations of HTSC

restraining of the surrounding material. However, the situation is diﬀerent at

the tape edges because of absence of the restraining in the direction outside

of the free side surface. It leads to side extension of the tape, strains ε

>ε

and possible longitudinal cracking of the sample (Fig. 3.17b).

In the case of one-axis pressing, the situation is complicated because of

high friction between tape and punches, causing considerable heterogeneity of

stress–strain state, which grows with decreasing of the ratio h

,where

and d

are the thickness and width of silver layer in the tape, respec-

tively. Therefore, the pressing eﬀect on the core density will be directly found

by the pressing parameters (i.e., pressure and friction) and also by the tape

composition and sizes.

Decreasing of microcracks and porosity may be reached by using modiﬁca-

tion of standard rolling [450, 565, 1133], alternative methods of pressing [32,

592, 672], intermediate deforming [341, 831], regulating the rate of the tape

thickness decreasing under mechanical loading [637], changing radii of rollers

in rolling [995], applying excessive pressure at temperature of Bi-2223 phase

formation [888], controlling the process of the Bi-2223 phase decomposition–

restoration at high temperatures [828], sintering samples after their deforming

[59, 857] and optimizing the cooling rate [440, 586, 829]. In these cases, a hot

deformation in BSCCO systems improves weak links, but a cold deformation

increases a magnetic ﬂux pinning [636].

Pressing of Bi-2223/Ag tapes under cryogenic temperatures permits to

transfer more mechanical deformation energy to the oxide core, due to a rel-

ative increasing in the hardness of silver to oxide in this thermal treatment.

Then, the fracturing of grains during deformation can produce multiple new

surfaces, promoting better ﬁnal alignment of superconducting grains [164].

In this sense, it may be assumed that the mechanism of the grain alignment

under pressing would be expected to involve at least two steps:

(1) Initial fracturing of the grains to produce multiple surfaces. The degree

of fracture depends on the deformation introduced into the grains.

(2) Rotation and/or movement of the fractured grains into (00l) alignment.

It is suggested that powder ﬂow is the main mechanism for the rotation

and/or movement of the granular bulk. Powder ﬂow also exposes the mul-

tiple surfaces created by the fracturing step.

The room temperature process (RTP) of deformation includes the follow-

ing steps. A small amount of force is applied and a small degree of fracture

occurs. Further force is applied and the silver sheath begins to deform plas-

tically. As a result, there are both powder ﬂow and fracturing of the grains.

At this step only few new surfaces are created and a ridge formation starts to

occur (the ridge formation mechanism in the RTP tapes is shown in Fig. 3.18).

The tape density increasing in the ﬁnal step reduces the amount of powder

ﬂow, and the fractured grains are further broken into small pieces.

The cryogenic process (CP) of deformation (77 K) includes the following

steps. First, a force is applied that fractures the platelets. The silver sheath is

3.3 Superconducting Composites, Based on BSCCO 133

(a) (b)

Fig. 3.18. A schematic representation of the steps, involved in the ridge formation

in the RTP tapes [164]

harder and powder ﬂow does not occur at the same amount of applied force as

for the RTP tapes. This causes the platelets to fracture, but with very small

powder ﬂow. This step creates many new surfaces in the superconductor, but

the surface density of the tape increases, as there is no lateral powder ﬂow.

As the force increases, the silver sheath becomes signiﬁcantly deformed and

both powder ﬂow and fracturing occur. As the force continues to increase, the

silver continues to plastically deform, and the tape density increases, reducing

the amount of powder ﬂow. The fractured grains are further broken into small

pieces. The RTP and CP are demonstrated in Fig. 3.19. In conclusion, note

that the increased silver sheath hardness at cryogenic deformation increases

rate of deformation in the Bi-2223/Ag processing compared to usual condi-

tions. As a result, an optimum process of tape fabrication improves connec-

tivity and texture of grains, and also J

[558]. Nevertheless, a densiﬁcation of

superconducting core may be accompanied by aggravation of Bi-2223 texture

and diminishing of J

[1061]. Then, the texture of Bi-2223 superconducting

grains could be improved as a result of directed growth of the Bi-2212 grains

due to residual stresses during tape [1198] or during reaction of Bi-2212 with

secondary phases [696].

In the case of substitution of silver sheath by a silver alloy, all alloying

additions form in the sheath particles of oxides as a result of the internal

134 3 Experimental Investigations of HTSC

CP RTP

(b)

(a)

(c)

(d)

Fig. 3.19. A schematic representation of the steps, involved in the grain alignment

mechanisms for both cryogenically pressed (CP) and room temperature pressed

(RTP) tapes [164]

3.3 Superconducting Composites, Based on BSCCO 135

0.75

0.50

0.25

0.00

0 50 100 150 200 250

(a)

1.00

Toughened

Ag-matrix

Ag Mg

0.5

Ag Mn

0.15

Ag Mg

0.1

Normalized Critical Current

Tensile Stress (MPa)

(b)

Ag Mg

0.5

Ag Mg

0.3

Ag Mg

0.2

Ag Mg

0.1

1.05

1.00

0.95

0.90

0.85

0.80

0.75

0 20 40 60 80 100

Normalized Critical Current

Bending Radius (mm)

Fig. 3.20. Improved mechanical properties of the alloyed Ag-sheath Bi-2223 tapes

with 55 ﬁlaments in comparison with pure Ag-sheath Bi-2223 tapes: (a) tension and

(b) bending [282]

oxidation and promote formation of Ag

O particles, serving as seeds. More-

over, they reduce grain sizes of the sheath and also decrease pre-stress of

oxide core, arising from diﬀerences in thermal expansion coeﬃcients of sheath

and ceramics that is revealed at tape cooling from sintering temperature to

cryogenic one of HTSC application [16]. All these factors result in micro-

hardness enhancement of the alloyed silver sheaths, as well as an increase of

the ultimate strength and the yield strength of a composite as a whole. It is

136 3 Experimental Investigations of HTSC

accompanied by the growth of J

stability relative to thermal cycling. Proper

test data (tension and bending) demonstrating improved mechanical proper-

ties of tapes with sheath from silver alloy in comparison to tapes possessing

silver sheath are shown in Fig. 3.20. At the same time, the doped Ag-sheath

leads to some decreasing of critical current density, amount of Bi-2223 phase

and T

, and also to growth of secondary phase and the core thickness vari-

ability (sausaging) [826]. Besides, there is little silver in the superconducting

core in the case of the alloyed Ag-sheath. At the same time, Ag intensively

penetrates from the sheath into ceramics of an unalloyed sheath composite

during annealing and forms particles of pure silver there. These particles pro-

vide an enhancement of J

. The above-mentioned decreasing of Bi-2223 phase

amount and T

is the result of the internal oxidation of the alloyed Ag-sheath

and accompanying decrease of oxygen ﬂow to the ceramics [626]. It should

be noted that the critical current density in multi-ﬁlamentary tapes is very

sensitive to technical parameters and microstructure properties. In the case

of silver sheath, the value of J

in the central part of the tape is found to

be considerably higher relative to the side sites [290] owing to heterogeneous

strain caused by rolling. The silver alloys with improved mechanical properties

decrease heterogeneous strains during rolling and increase J

in the side sites

[1193]. Further decreasing of sausaging and diﬀerences in the values of critical

current density in the central and side sites causes fabrication of the alloyed

Ag-sheath multi-ﬁlamentary tapes, possessing higher J

in comparison with

corresponding counterparts, having pure silver sheath (Fig. 3.21).

Investigations of the after-heat-treatment BSCCO-core Vickers microhard-

ness, assuming directed conformity between Vickers microhardness (H

)and

volume density of the material [1159], state a correlation between high density

of superconducting phase and high values of J

[458, 826]. These measure-

ments, carried out at diﬀerent stages of the oxide-powder-in-tube process, esti-

mate an eﬀectiveness of diﬀerent thermal treatments. In particular, a pressing

0.0 0.2 0.4 0.6 0.8 1.0

Bi-2223/Ag + AgMnNi

Bi-2223/Ag

netic Field, B (T)

Critical Current Density, J

(A/cm

)

77 K

B || Tape Plane

Fig. 3.21. Critical current density vs magnetic ﬁeld at 77 K [1193]

3.3 Superconducting Composites, Based on BSCCO 137

leads to a greater increasing of J

in comparison with rolling due to a greater

oxide core density after ﬁrst processing, than after the second one. More-

over, the degree of transformation from Bi-2212 to Bi-2223 phase is always

found to be greater in the pressed tapes [458, 826]. Corresponding results

for diﬀerent thermal treatment (i.e., cycles of “heat–strain”) are shown for

rolled and pressed Bi-2223/Ag monocore tapes in Fig. 3.22. It is also noted

Rolled

Pressed

Total Annealing Time (h)

(kA/cm

)

(kA/cm

)

(a)

•

130

110

02468101214

110 100 90 80 70 60 50

Rolled

Pressed

(10g, 15s)

•

(b)

(c)

•

Rolled

Pressed

•

BSCCO Core Thickness

Variability

(%)

BSCCO Core Average Thickness (μm)

50 100 150

200

250 300 350

Fig. 3.22. Comparative results of intermediate rolling and pressing between heat

cycles for Bi-2223/Ag tapes: (a) J

(0 T, 77 K) vs total heat treatment time; (b)

Vickers hardness vs critical current density; (c) BSCCO core thickness variability

as a function of average core thickness. The numbers point to the number of heat

treatments, which the tape is subjected to [826]