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

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158 3 Experimental Investigations of HTSC

single crystals supports the idea that the incorporation of the 211 particles

into a 123 matrix is responsible for the subgrain formation.

The consequence of the proposed model of subgrain formation is the exis-

tence of an incubation period free of subgrains (e.g., subgrain-free region) at

the beginning of growth, when dislocation density is not high enough. Such

regions are observed at the seed and at the GSBs (Fig. 3.45). The regions free

from subgrains can also form along SGBs. In such regions, the grain growth

occurs perpendicularly to the main growth front, forming a step at the growth

front (Fig. 3.46 [197]). The region B in Fig. 3.46 is the barrier for GSBs to

continue from part A to part C of the [010] a-GS. Hence, the incubation pe-

riod free of subgrains appears at the beginning of the C part of the [010]

a-GS. At the same time, the presence of subgrain-free regions along the GSBs

and SGBs shows that cellular growth does not deﬁne subgrain formation in

melt-grown Y(RE)BCO bulks [197].

3.4.3 Behavior of 211(422) Disperse Phase

A uniformity of Y-211 phase distribution within a Y-123 matrix is necessary

to increase superconducting and mechanical properties of a Y-123 phase. For

this, two problems should be solved, namely (i) the spherical Y-211-free re-

gions [532] and (ii) 211 segregation along speciﬁc crystallographic orientations

of a Y-123 phase

[1106]. The formation of spherical Y-211-free regions is at-

tributed to the formation of spherical pores due to gas (oxygen) evolution

during incongruent melting of a Y-123 phase [532]. Figure 3.47 demonstrates

spherical pores in liquid, pores ﬁlled by liquid and Y-211-free regions. When a

Y-123 powder compact is heated above a peritectic temperature, oxygen gas

is released, forming spherical pores in the liquid. If oxygen gas diﬀuses out of

the pores, they will disappear by the liquid ﬁlling process. Compared to the

liquid motion to pores, the mobility of solid 211 particles is relatively slow.

It makes a non-uniform Y-211 distribution around the liquid pockets (these

regions are shown by circles in Fig. 3.47b). During peritectic reaction, liquid

pockets turn into spherical Y-211-free regions. At the same time, due to the

lower Y-211 density around the liquid pockets, the reaction to form a Y-123

phase is not easy. Therefore, unreacted liquid phase (BaCuO

and CuO) is

often observed in the center of Y-211-free regions [530]. The size and amount

of pores are dependent on the heating rate to a peritectic temperature [642]:

larger pores are developed at higher heating rate. Prolonged holding at the

partial melting state can eliminate spherical pores by providing enough time

for diﬀusion of oxygen gas out of the sample [1008]. However, the prolongation

of this process leads to signiﬁcant coarsening of Y-211 particles and decreasing

of J

[642].

Heterogeneity of 211 particle distribution can also be classiﬁed in the following

forms [197]: (i) an increasing of the 211 phase density along c-axis direction, (ii) a

development of particle heterogeneity at 123 grain and subgrain boundaries and

(iii) oscillations of 211 concentration perpendicular to the c-axis.

3.4 Melt-Processed Y(RE )BCO 159

Seed

GSB

–

growth

–

growth

300 μm

Fig. 3.45. Polarized optical micrograph of TSMG-processed Y-123/Y-211 [197]

160 3 Experimental Investigations of HTSC

Growth Front

[010]

[100]

a - a - SGB

a - Subgrains

Fig. 3.46. Schematic view of the a-a-SGB, formed by the growth in the [100]

direction at the step on the growth front of the [010] a-GS. The layer B, grown by

[100] a-growth, is a barrier for dislocation walls (a-SGBs) to travel from part A to

part C of the [010] a-GS

When stoichiometric Y-123 powder is used as a starting material for the

melt processing, Y-211 particles are formed due to incongruent melting. Most

of them are consumed completely to form a Y-123 phase during the peritec-

tic reaction. However, some of them often remain unreacted in the form of

trapped particles within Y-123 domains. In this case, the trapped Y-211 par-

ticles form X-like linear Y-211 tracks (Fig. 3.48) along the diagonal directions

of Y-123 domains [532, 534, 707, 1106]. The shape of the Y-211 track pattern

is dependent on the crystallographic orientation of the polished surfaces of

the Y-123 domains, but generally, the Y-211 tracks meet the corners of Y-123

domains. The boundary planes to produce the Y-211 tracks are {110} planes

of a Y-123 matrix [530]. Other secondary phases (BaCeO

, BaSnO

, etc.) that

are formed by impurity additions do not make such a pattern. When the par-

ticle density is low, most of them are segregated in liquid at the Y-123 domain

boundary [532]. When their density is high, they are trapped within the Y-123

domains normal to the growth fronts in the form of agglomerates [534].

In the case of using Y-211 excess powder, instead of linear Y-211 tracks, a

planar segregation mode is developed. Speciﬁc crystallographic parts of Y-123

domains are ﬁlled with Y-211 particles, but other parts are free from Y-211

particles. The planar segregation of Y-211 particles appears as a symmetrical

3.4 Melt-Processed Y(RE )BCO 161

(a)

Voids

50 μm

(c)

(b)

(a)

Y-211 + Liquid

Liquid

Phase

Y-211-Free Spherical Regions

50 μm

Fig. 3.47. (a) Spherical pores, developed in liquid, (b) liquid ﬁlling into pores and

162 3 Experimental Investigations of HTSC

50 μm

Fig. 3.48. X-like Y-211 pattern [530]

butterﬂy-like pattern presented by both (100) and (010) growth interfaces on a

polished surface [534]. The formation of planar Y-211-free regions is dependent

on the size of Y-211 particles and the growth rate of Y-123 domains. Large-

sized Y-211 particles are trapped randomly; at the same time, small-sized Y-

211 particles form a segregation pattern, containing Y-211-free regions. The

Y-211 segregation pattern is preferentially developed when the cooling rate is

low [530].

The Y-211 density in the liquid of the Y-211 excess system is relatively

higher than that of the stoichiometric Y-123 system. The presence of many

particles at the advancing solid interface decreases the critical velocity for

particle trapping by increasing the viscosity of the liquid (η) given in the

form [1015]:

∗

= η(1 + 2.5f

) , (3.3)

where η

∗

is the eﬀective viscosity and f

is a volume fraction of particles ahead

of the growing interface.

Even at a constant Y-211 content, the distribution of Y-211 particles can

be changed with growth rate of Y-123 fronts. As can be seen in Fig. 3.49,

Y-211 particles are trapped randomly at a cooling rate of 20

◦

C/h through a

peritectic temperature, while they make planar Y-211 segregation patterns at

arateof5

◦

C/h. This implies that the formation of Y-211 segregation pattern

is a function of the growth rate of Y-123 interfaces [530].

The spatial distributions of Y-211 particles have been studied in the sam-

ples obtained by using RE-seeds with application of the undercooling tech-

nique. A macrosegregation of Y-211 particles occurs during growth process

from seeded liquid. It depends on the interface direction and growth rate,

and also introduced undercooling of the sample [251, 252]. The Y-211 particle

density into Y-123 matrix is higher at lower degree of the undercooling sample

3.4 Melt-Processed Y(RE )BCO 163

Y-211-rich

Region

Y-211-free

Region

100 μm

5°C/h

20°C/h

(a)

(b)

Fig. 3.49. Y-211 distribution within Y-123 domains of samples melt-processed at

cooling rates of (a)20

◦

C/hand(b)5

◦

C/h [530]

below peritectic temperature (ΔT = T

−T

) [251], showing that the particle

trapping is caused by the growth rate of Y-123 interface. However, the single

relationship between the growth rate and undercooling (ΔT )isnotsuﬃcient

to explain the macrosegregation. The interaction of the advancing solid–liquid

interfaces and the particles in the melt must be taken into account. By con-

sidering the particle radius, the growth rate and the number of particles per

unit volume in the vicinity of the growth front, two critical growth ranges

may be stated [188]. When 15

◦

C ≤ ΔT ≤ 25

◦

C both growth rate and number

of particles per unit volume exhibit low values. Therefore, only the largest

particles can be trapped by Y-123/Y-211 interface. When ΔT ≥ 25

◦

C, the

undercooling is increased, leading to an increase of the growth rate that causes

164 3 Experimental Investigations of HTSC

the smallest particle trapping also. The Y-211 particle density near seed can

be smaller than at the sample edge. It may be explained by the processes

of their pushing, trapping, ripening and coalescence, occurring in liquid, and

also by microcracks, observed between 123 lamellae and spherical pores near

seed, which cause lower density of this sample part.

When using the hot-seeding technique by Nd-123 crystals at lowered oxy-

gen gas pressure, the Nd-422 particle sizes increase considerably with distance

from seed (Fig. 3.50) that correlate with a time of crystallization [123]. How-

ever, in this case, an increase of density of the small trapped Nd-422 particles

is not observed, that is, processes of small particle pushing and large parti-

cle trapping are realized at the deﬁnite growth conditions of NdBCO grain.

Generally, the growth rate of Nd-123 grain is greater in the c-axis direction,

than in the ab-plane at any values of ΔT (Fig. 3.51). The growth rate in-

creases considerably with growth of ΔT and becomes non-linear in Nd-123

samples even at ΔT =5

◦

C, causing the processes of unstable solidiﬁcation

[123]. The observed gradual decreasing of growth rate together with duration

of the sample fabrication time at lowered content of oxygen gas can be ex-

plained by increased volume fraction of Nd-422 particles into liquid, which, in

this case restrains diﬀusive ﬂux directed from growing front to melt.

As ΔT increases, many undesirable subsidiary nuclei begin to form. The

growth mode of Y-123 crystal can be classiﬁed into four groups of mor-

phology (Fig. 3.52), namely: planar facet mode (ΔT ≤ 30

◦

C), cellular

growth mode (30

◦

C < ΔT ≤ 40

◦

C), cellular growth with undesirable nuclei

(40

◦

C < ΔT ≤ 45

◦

C) and formation of the polycrystalline sample by nuclei

with random orientation (ΔT>45

◦

C). Two-step undercooling technique,

Mean Diameter

500 µm

1000 µm

1500 µm

2000 µm

2.25 µm

2.49 µm

2.81 µm

3.08 µm

Diameter of Particle (μm)

Number of Particles

246810

Fig. 3.50. Size distribution for Nd-422 particles into Nd-123 matrix as function

of distance from NdBCO seed. The sample is grown at decreased pressure of O

1030

◦

C, during 50 h [123]

3.4 Melt-Processed Y(RE )BCO 165

Holding Time, t

(h)

Holding Time, t

(h)

Growth Length (mm)

(a)

Growth Length (mm)

(b)

1030°C (ΔT = 5 K)

1020°C(ΔT = 5 K)

Nd - 123 (1%O

)

c - axis

- axis

Nd - 123, 1%O

(ΔT = 15 K)

Y - 123 (ΔT = 15 K)

Nd - 123, in air, 1055°C (ΔT = 15 K)

10 20 30 40 50

6010 20 30

Fig. 3.51. Growth rate of NdBCO grain: (a) as function of undercooling ΔT and

(b) at diﬀerent processing conditions [123]

in which the Y-123 nucleus is stabilized at the ﬁrst undercooling step, where

the planar growth mode is maintained, and then crystal growth is accelerated

at a second undercooling step, permits to reduce considerably the required

processing time for single crystal fabrication compared to the conventional

technique without degradation of the sample magnetic properties and texture

166 3 Experimental Investigations of HTSC

Planar Facet Growth Cellular growth

Cellular Growth with

Undesirable Nuclei

Random

Nucleation

Fig. 3.52. Schematic illustrations of the growth morphology of Y-123 depending

on the degree of undercooling, ΔT [479]

[479]. The dependencies of the Y-123 phase growth rate at { 100} and {001}

facets on undercooling degree are subjected to parabolic law. Apparently, it

changes a direction of crystalline growth and combines orientation of the ab-

plane with direction of sample solidiﬁcation [251].

By using some seeds (MSMG technique), accelerating processing of melted

Y(RE)BCO samples, a decreasing of levitation force due to residual products

of melt at intergranular boundaries [932] is possible. In order to study this

problem samples were prepared by using two seeds with diﬀerent crystallog-

raphy [531]. The obtained superconductors with junctions (100)/(100) and

(110)/(110) demonstrated better properties (i.e., levitation force and trapped

magnetic ﬁeld) than the samples with junctions (100)/(001) and (110)/(001).

In melted YBCO samples with and without PtO

additives, 211 parti-

cles are disposed so that 211 longer axes are parallel or perpendicular to the

ab-direction or the c-axis of crystalline lattice of 123 phase [124]. The orienta-

tion relations between 123 matrix and trapped 211 particles ([001]

123

|| [001]

211

3.4 Melt-Processed Y(RE )BCO 167

and [110]

123

|| [111]

211

) are stated in [37]. In contrast to these results, investi-

gations with application scanning electron microscopy [1053] show absence of

deﬁnite relationships between crystallographic orientations of 123 matrix and

211 inclusions. Due to own magnetic properties, the 211 particles align along

an applied magnetic ﬁeld, when sample is prepared by the melt-processing

technique in the presence of the magnetic ﬁeld [237]. However, it may be ob-

served that Nd-422 particles [1168] and Sm-211 particles [1026] are arranged

parallel to growth direction in one-direction solidiﬁed YBCO system even

without magnetic ﬁeld. The 211 particles have polygonal form with faceted

surfaces, possessing interface anisotropy. The particle anisotropy increases

with addition of PtO

[1015] and CeO

[535] that are used to control the

211 particle sizes. Additives form 211 needle-shaped particles in liquid. In

order to diminish a free energy of interface, 211 particles should be reoriented

in liquid to dispose with the least surface energy before the moment of their

trapping by the growing 123-front. An alignment of 211 particles also depends

on the growth rate of 123 interfaces, size and shape of 211 particles [530].

3.4.4 Eﬀects of Doping Additives

To improve the structure-sensitive properties of HTSC, metal oxide additions

are used. Among the impurity elements, PtO

[777] and CeO

[534] are known

to be eﬀective materials to increase J

through the reﬁnement of Y-211 parti-

cles and/or possible substitution in Y-123 lattices. The impurity elements not

only reduce the Y-211 size eﬀectively [777], but also change its morphology

[1106]. During heating of Y-123 powder with doping additives to a peritectic

temperature, the additives react with a Y-123 phase to form BaCeO

[537]

and Ba

CuPt

[1180]:

YBa

7−x

+CeO

→ BaCeO

; (3.4)

YBa

7−x

+PtO

→ Ba

CuPt

. (3.5)

This causes Ba deﬁciency in the composition of the remainder and forma-

tion of the free CuO phase. The reaction of the latter with Y-123 phase leads

to the pseudo-peritectic reaction, forming a Y-211 phase prior to peritectic

melting [530]:

YBa

7−x

+CuO→ Y

BaCuO

+ liquid . (3.6)

These Y-211 particles, formed before melting, serve as seeds for the Y-211

phase during peritectic reaction, increasing the number of peritectic Y-211

particles [349].

One of the ways to reduce the size of Y-211 particles is to provide het-

erogeneous nucleation sites for peritectic Y-211 particles. The Ba

CuPt

compound, which is formed as result of PtO

addition in accordance with re-

action (3.5), can act in this role due to the similar lattice parameters between