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

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178 3 Experimental Investigations of HTSC

Table 3.3. Crack spacing [198]

Sample λ

± σ(μm) λ

± σ (μm) d

211

(μm) V

211

(wt%) d

(μm) V

(wt%)

0wt% Ag

O1.8 ± 0.134.2 ± 17.0 1.95 21.1 – –

12 wt% Ag

O3.7 ± 0.4 133.5 ± 43.2 2.02 20.9 5.2 6.1

is the ab-microcrack spacing; λ

is the c-microcrack spacing; d

211

is the 211

particle size; d

is the Ag particle size; V

211

is the 211 volume percent; V

is the

Ag volume percent; σ is the standard deviation

particles (Fig. 3.64). In this case, signiﬁcant increase of spacing between both

ab-cracks and c-cracks (Table 3.3) [198] is observed.

The crack, initiated due to Vickers micro-indentation, deﬂects when one

reaches the gap between two silver inclusions (Fig. 3.65). After the crack

passes through the gap, it continues in the same direction of the initial growth.

Thus, a toughening is demonstrated, increasing intrinsic fracture toughness

of YBCO sample [652]. Silver increases both maximum size and incline of R-

curve (Fig. 3.66). This increase of fracture toughness is caused by reinforcing,

developing into so-called, wake zone, where a plastic strain of Ag inclusions-

bridges, pinning the crack surfaces, states most mechanism of toughening and

fracture resistance in the absence of material transformations [1171].

An addition of tetragonal ZrO

particles can signiﬁcantly reinforce mono-

lithic YBCO samples due to the martensitic (tetragonal-monoclinic) phase

transition [322]. In particular, when 10 mol.% ZrO

particles, coated with the

Y-211 phase by using a sol–gel process, were added to Y-123, only modest

improvement of fracture toughness (K

) was observed. However, 20 mol.%

ZrO

improved the K

by nearly 50%. In this case, for the samples with the

Crack

10 μm

Fig. 3.65. Crack deﬂection between silver particles, dispersed in YBCO supercon-

ducting matrix [652]

3.4 Melt-Processed Y(RE )BCO 179

[MPa

1/2

]

•

0.0 0.5 1.0

1.3

1.2

1.1

0.0

Crack Increment, Δc (mm)

(a)

[MPa

1/2

]

0.0 1.0 1.5 2.0

Crack Increment, Δc

(mm)

(b)

Chevron Notch

28%Ag

19%Ag

0%Ag

Slope

= 0.39

Slope

= 0.27

Slope = 0.14

0.5

Fig. 3.66. R-curves obtained for diﬀerent YBCO/Ag composites in samples with

single edge notch: (a) 123 superconductor; (b) three composites with various amount

of silver. Ag increasing improves both the level and incline of the fracture resistance

curves, showing cumulative character of the toughening with the crack growth [1171]

same density, the 10 mol.% ZrO

additions also increased K

by nearly 50%.

At the same time, the average strength of the Y-123 and Y-123/Y-211/ZrO

remained nearly constant despite the diﬀerences in K

. On the contrary,

a critical concentration of ZrO

additions (∼ 15 wt%) [791] was stated, at

which the maximum superconductor strength and plasticity were reached.

180 3 Experimental Investigations of HTSC

AA’

BB’

Section B−B’

Microcracks or

Voids

Resin-

Impregnate

Layer

Covering Layer

Section A−A’

(a)

(b)

500 μm

Fig. 3.67. (a) Structure of superconducting bulk reinforced by resin impregna-

tion [1070]; (b) optical micrograph for cross-section of Sm-123 bulk after the resin

impregnation [1073]

3.4 Melt-Processed Y(RE )BCO 181

•• • • •

1.0

(a)

(b)

0.8

−

0.6

−

0.4

−

0.2

−

0.0

12345

•• • • •

•

SmBCO with resin impregnation

GdBCO with resin impregnation

SmBCO without resin impregnation

GdBCO without resin impregnation

Number of Experiments

Normalized Peak Field

Number of Experiments

Normalized Peak Field

1.00

−

0.95

−

0.90

−

0.85

−

0.80

•

∅ 46 mm, with resin impregnation

∅ 80 mm, with resin impregnation

∅ 46 mm, without resin impregnation

∅ 80 mm, without resin impregnation

Fig. 3.68. Trapped ﬁeld values vs the number of thermal cycles (77–293 K): (a)

SmBCO and GdBCO with and without resin impregnation [1068]; (b)thesame

results for YBCO bulks with diﬀerent diameter of pellets [1070]

182 3 Experimental Investigations of HTSC

In this case, the plastic deformation occurs in range of high temperatures in

mechanism of intercrystallite sliding on nanophase with Zr-contents.

The reinforcement by surrounding the Y(RE)BCO bulk with a metal ring

is also eﬀective in increasing mechanical properties [727]. In this case, the bulks

can be pre-strained in compression, since the thermal expansion coeﬃcient of a

metal ring is much smaller than that of the Y(RE)BCO bulks. The epoxy resin

impregnation into REBCO bulk eﬀectively improves mechanical properties

[647, 1069, 1070]. In this case, almost all defects (cracks, pores and scratches)

can be ﬁlled by the epoxy resin within a distance of about 2 mm from the sam-

ple surface (for superconducting pellets of 3 cm in diameter and 2 cm in thick-

ness) (Fig. 3.67) [1069]. These bulks do not show a decreasing of the trapped

magnetic ﬁeld even after intensive thermal cycling (cycles 77–293 K) [1070]

and demonstrate an increasing of levitation force [1072]. Compared to usual

samples, the characteristics of reinforced specimens are presented in Fig. 3.68.

Moreover, a large current of 1000 A could be passed along the REBCO bulk

without the transition of the superconductor in normal state [1073]. The epoxy

resin could be successfully reinforced by dispersing quartz ﬁllers which also

promote a decreasing of thermal expansion diﬀerence between polymer and su-

perconductor [1070]. Finally, an additional high pressure at high temperature

permits to obtain during short time non-porous practically REBCO bulks with

improved mechanical properties at simultaneous preservation or improvement

of superconducting properties [868]. The mechanical properties of disc-shaped

YBa

B magnets have been reinforced signiﬁcantly due to an epoxy

resin impregnation and introducing of carbon ﬁlaments [1071]. Moreover, an

aperture has been drilled in the sample center, which has been ﬁlled by

Bi–Pb–Sn–Cd alloy with aluminum wire. As a result, the static trapped

ﬁeld has been increased up to 17.24 T at magnet cooling with the absence

of mechanical damage.

Carbon Problem

4.1 YBCO System

During all stages of HTSC preparation from calcining of precursor powder

up to fabrication of ﬁnal product, there is a problem of carbon segregation

into sample deteriorating structure-sensitive properties. So, during calcining

in preparation of YBCO, the BaCO

and CuO powders form 123 phase

reacting according to the following [958]:

BaCO

(solid) + CuO (solid) → CO

(gas) + BaCuO

(solid) (4.1)

4BaCuO

(solid) + Y

(solid) + 2CuO (solid) → 2YBa

(solid)

(4.2)

When BaCO

decomposes, according to reaction (4.1), CO

is released,

and its localized concentration quickly reaches equilibrium value. The localized

pressure, depending on the temperature and other thermodynamic con-

ditions, can cause other decomposition reactions, forming undesired phases,

which in turn contribute to reduce the superconducting properties of the ﬁnal

product. It has been reported [825, 1098] that in the superconductors car-

bon remains in an amount of 0.1–1.0% or more, even if the powder is ﬁred

at high temperatures. In particular, an existence of residual carbon in oxide

superconductors, synthesized by a sol–gel method [585, 687, 1095], is possi-

ble. However, the residual carbon content of the powder, synthesized by the

conventional solid-state reaction method, can be about three times as much

as that of the powder, prepared by the sol–gel method for a similar condition

of heat (Fig. 4.1).

The powders, synthesized by the sol–gel method, demonstrate supercon-

ductivity, when they are heated at temperatures higher than 900

◦

Candthe

total carbon content is less than 0.4%. Dependencies of the T

c,onset

property

and the magnetic susceptibility of YBCO powders on total carbon contents

are shown in Fig. 4.2. Obviously, diamagnetism volumes are increased and

c,onset

is improved as the carbon contents are reduced.

184 4 Carbon Problem

2.0

1.0

0.2

0.1

≈ ≈

600°C

800°C

900°C

950°C

01020

Heating Time (h)

Carbon Content (wt.%)

Fig. 4.1. Change of carbon content with heating time for various ﬁring temper-

atures. (•)inO

ﬂow, (o) in air ﬂow, (♦1) and (♦2): samples synthesized by the

conventional method [675]

The next hot isostatic pressing (HIP) can lead to the following improve-

ment of the powder’s superconducting properties. So, the powder, heated

at 500

◦

C with HIP, has a higher T

c,onset

and a larger volume of diamag-

netism than the powder without HIP treatment (Fig. 4.3). In this case, the

orthorhombic distortion increases when the HIP treatment is performed at

500

◦

C, but the distortion decreases when the temperatures of the HIP treat-

ment are raised to 800 and 900

◦

C (Fig. 4.4).

It should be noted that the carbon content of the powder, prepared by

the sol–gel method, decreases more eﬀectively than that of the powder by the

conventional solid-state reaction, in spite of the presence of many more car-

bonaceous materials in the starting materials used in the sol–gel method. The

reason for this behavior may be explained by assuming that BaCO

,intro-

duced in the powder synthesized by the sol–gel method, would be decomposed

rapidly on heating at higher temperatures than 900

◦

C due to the very small

size (some dozens of nanometers) of BaCO

particles. It is possible that such

ﬁne particles may be highly reactive.

When the HIP treatment is performed at 900

◦

C, a degradation of super-

conducting properties is observed. It is assumed that the degradation is caused

4.1 YBCO System 185

0.01 0.1 1.0

0.3

0.2

0.1

Carbon Content (wt.%)

c, onset

(K)

Susceptibility (emu/g)

Fig. 4.2. Dependencies of T

c,onset

and magnetic susceptibility on carbon contents.

The susceptibility is measured at 10 K [675]

by an increase of the tetragonal 123 phase over the orthorhombic 123 phase.

This agrees with test observation that orthorhombic distortion of the lattice

parameters decreases as HIP temperatures increase from 500 to 900

◦

YBCO superconductor reacts during sintering at diﬀerent temperatures

and conditions of gas mixtures. Its reaction with CO

may be carried out in

two stages. At 815

◦

C, it is given as [307]

2YBa

7−x

(solid) + 4CO

(gas) →4BaCO

(solid) + Y

(solid)+

4CuO(solid) + (0.5 − x)O

(gas) .

(4.3)

The complete reaction of YBCO with CO

at 950

◦

C is [307]

2YBa

7−x

(solid) + 3CO

(gas) →3BaCO

(solid) + Y

BaCuO

(solid)+

5CuO(solid) + (0.5 − x)O

(gas) .

(4.4)

For both cases, the decomposition of YBCO is only partially completed

and starts at grain boundaries. In the sample annealed at 815

◦

C, the reaction

products consist of two phases: BaCO

and Y

. The primary reaction

products are all insulators, which essentially coat all grain boundaries. It can

make the material lose its superconductivity even if the major phase is still

186 4 Carbon Problem

500

Temperature of HIP (°C)

c,onset

(K)

Ratio of Susceptibility

1.5

1.0

0.5

Heated Powder

without HIP

900800

Fig. 4.3. Inﬂuence of HIP treatment in O

on T

c,onset

and magnetic susceptibility

at 77 K. The HIP was performed with 100 MPa and the duration time 10 h [675]

superconducting. In the case of sintering in a 5% CO

mixture, the reac-

tion products may be nucleated at grain boundaries as separate precipitates

rather than as a thin layers, which coat the grain boundaries. In the case of

these individual precipitates at grain boundaries, the J

value may not de-

crease, as long as most boundaries remain superconducting. On the contrary,

since defects, such as twin boundaries and tiny secondary phases, can act as

ﬂux pinning centers, the J

value may in fact increase due to the presence of

small precipitates at grain boundaries.

The interaction between YBCO and CO

at a total pressure of 1 atm

(0.999 atm of O

and 0.001 atm of CO

) leads to two reaction mechanisms

[285]:

YBa

6+x

(solid) + 2CO

(gas) →2BaCO

(solid) + 0.5Y

(solid)+

3CuO(solid) + (2x − 1)/4O

(gas) ,

(4.5)

YBa

6+x

(solid) + 2CO

(gas) →2BaCO

(solid) + 0.5Y

(solid)+

2CuO(solid) + (2x − 1)/4O

(gas) .

(4.6)

Both of these reactions lead to the formation of non-superconducting

phases. The partial pressure of CO

can aﬀect the partial pressure of O

,which

4.1 YBCO System 187

500

3.9

3.8

Heated Powder

without HIP

Lattice Parameter (Å)

c/3

Temperature of HIP (°C)

900800

Fig. 4.4. Change of the lattice parameters of the YBCO powder by HIP treatment

in O

[675]

in turn aﬀects the oxygen content of the YBCO compound formed. It is well

known [498, 499, 589] that for x ≈ 1, YBa

6+x

is in the orthorhombic

phase and exhibits superconductivity at 90 K. As the oxygen content, which

is dependent on both the temperature and oxygen partial pressure, decreases,

the transition temperature of the orthorhombic phase decreases to 60 K, at

0.6 <x<0.7. At x ≈ 0.4, the material becomes tetragonal, and superconduc-

tivity is destroyed. The critical current density is aﬀected by both sintering

temperature and CO

content in the sintering atmosphere (Fig. 4.5). J

values

decrease with decreasing sintering temperature and increasing CO

content.

The crystals of the sintered samples in CO

atmospheres with 50 or

500 ppm CO

can demonstrate twins, whereas those sintered in 5% CO

not show any twinning [958]. The twin structure in YBCO determines its

orthorhombic superconducting properties [1031]. Hence, the samples sintered

in 5% CO

are not orthorhombic, but the extent of the orthorhombic struc-

ture decreases with increasing CO

in the sintering atmosphere. Moreover,

increasing CO

contents in the sintering atmosphere increases the electrical

resistivity of the sample [958].