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

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58 2 Composition Features and HTSC Preparation Techniques

obtained layer-by-layer (metal substrate–buﬀer layer–superconducting layer),

see Fig. 2.3. Every layer including metallic substrate plays a special role in

the long length conductors. The candidate materials and the thickness of each

layer are also presented in Fig. 2.3.

In general, thick YBCO-coated conductors consist of epitaxially grown

ﬁlms on appropriate substrate with or without few buﬀer layers. Therefore, the

substrate or the buﬀer layer must have a suitable textured surface structure to

provide the required in-plane alignment growth of the HTSC crystalline ﬁlms,

resulting in the avoidance of weak links due to the existence of only low-angle

grain boundaries. Typical problems, decided now at design and manufacture

of coated conductors, are presented in the Table 2.1.

The general problem of ﬁlms, prepared by the liquid phase epitaxy and

electrophoretic deposition, demonstrating high rates of precipitation and suf-

ﬁciently low cost, is the crack formation due to stress relaxation during sample

cooling and diﬀerent thermal properties of buﬀer layer and ﬁlm. If we assume

that ﬁxed cracks are extended through the whole ﬁlm thickness, we may fol-

low a calculation for cracks in brittle ﬁlms on elastic substrates [1057,1060].

According to the model, the total energy is the sum of the strain energy in the

Biaxial In-plane.

Alignment

Homogeneity.

High Production

Rate.

Thickening.

Alignment Control for HTSC

Layer. Prevention of Reaction.

Suppression of Crack Formation.

Homogeneity. Flatness. High

Production Rate.

High Strength. Non-magnetic

Materials. Thinning.

Alignment Control for Buffer

Layer.

Superconducting Layer

Y–123, Nd–123, Sm–123,

RE(mix)–123

Thickness: 1–10 µm

Buffer Layer YSZ, MgO, CeO

NiO, ZrO

, BaZrO

, etc

Thickness:< 3 µm

Substrate: Ni–Alloy (Hastelloy),

Ni, Ag, Ni–Cr Alloy, Zr, etc

Thickness: 25–125 µm

Fig. 2.3. The problems, solved in the design of high-qualitative-coated conductors

and including the development of basic structure, estimation of key factors and use

of candidate materials for each layer [985]

For simplicity and clearness, a thin passivation layer for stabilization, insulation

and encapsulation is excluded.

2.1 YBCO Films and Coated Conductors 59

Table 2.1. Nearest goals of R&D for three diﬀerent types of YBCO-coated conductors [985]

Type Structure or

technique

features

Substrate Buﬀer

layer

HTSC layer Key

processes

Target

Textured

substrate

Non-reactive,

high strength

Ni, Ag clad

materials:

Ni–Cr, Ni–V,

Ni-based alloys,

etc.

None or

NiO, ZrO

BaZrO

MgO,

YSZ,

CeO

etc.

Y-123,

RE-123,

etc.

Rolling/annealing;

surface

polishing;

surface oxidation

epitaxy; buﬀer

layer; HTSC

layer; evaluation

,etc.)

Length: 10–100 m;

substrate thickness:

≤ 100 μm;

≥ 10

− 10

A/cm

(77 K)

Aligned buﬀer

layer

ISPLD, IBAD Polycrys-talline

Hastelloy,

Ni-based alloys,

etc.

YSZ,

MgO,

CeO

,etc.

Y-123,

RE-123,

etc.

Substrate

polishing; IBAD

process; ISPLD

process; HTSC

layer; evaluation

,etc.).

Length: 100–1000 m;

substrate thickness:

≤ 100 μm;

≥ 10

–10

A/cm

(77 K); production

rate: > 1m/h

Rapidly grown

HTSC layer

MOD, LPE Ni, Ag clad

materials:

Ni-based alloys,

etc.

None or

MgO,

YSZ, NiO,

BaZrO

CeO

,etc.

Y-123,

RE-123,

etc.

Substrate

polishing;

buﬀer layer;

homogeneous

seed ﬁlm;

MOD process;

LPE process;

evaluation (J

etc.)

Length: 1–10 m;

substrate thickness:

≤ 100μm; HTSC

thickness: ≥ 5μm;

≥ 10

–10

A/cm

(77 K); production

rate: > 1m/h

60 2 Composition Features and HTSC Preparation Techniques

cracked region and the energy necessary to create the cracks, that is, the new

surface energy. The crack spacing, l, will be adjusted in order to minimize the

total energy of the system to [1060]

l ≈ 5.6



h/(Eε)

, (2.1)

where K

is the fracture toughness of the mode I, h the ﬁlm thickness, E

the Young’s modulus and ε the strain. The total energy of the cracked ﬁlm

cannot exceed the strain energy of the uncracked ﬁlm. This results in minimum

crack spacing and is consistent with the requirement that the energy release

by cracking should be higher than the energy necessary to create the cracks.

This means that below a critical thickness

=0.5K

/(Eε)

, (2.2)

no cracks will appear.

Based on this approach, the predicted dependence of the average crack

spacing l on the thickness h of a-axis-oriented YBCO ﬁlms on (100)SrPrGaO

substrates is shown in Fig. 2.4. At the same time, the decreasing micron-

sized particles to sub-micron-sized particles in colloids may diminish the ﬁlm

cracking at its electrophoretic deposition consideraldy (see Fig. 2.5).

In order to obtain YBCO samples with high properties, a preparation of

qualitative precursor powders is also very important. A number of techniques

for processing YBCO powders have been reported, including conventional

solid-state reaction, precipitation [106], plasma spray [415], freeze drying [495],

spray drying, combustion synthesis [571], sol–gel method [722], acetate method

[686] and ﬂame synthesis [889].

•

246810

Film Thickness, h (µm)

Average Crack Spacing,

l (µm)

Fig. 2.4. Theoretical crack spacing (solid curve) as a function of the ﬁlm thickness

compared with the experimental data and a ﬁt (dashed curve) to the data obtained

from a-axis-oriented YBCO LPE ﬁlms grown on (100) SrPrGaO

[05]

2.1 YBCO Films and Coated Conductors 61

Film A

Film B

500 µm

(a)

(b)

Fig. 2.5. Scanning electron microscopy images of sintered ﬁlms. The images of (a )

and (b) show ﬁlms fabricated from colloids, consisting of micron-sized particles and

sub-micron-sized particles, respectively. The heat treatment conditions for both are

the same [926]

The fabrication of a ceramic superconducting powder is presented in the

ﬂow chart of Fig. 2.6. First, the raw materials are weighed in a desired mo-

lar ratio and then mixed by conventional mixing/milling or liquid-solution

mixing. The homogeneity, obtained by conventional mixing, is limited by the

particle size of the powders, but the best mixing is generally obtained from

powders with a particle size less than 1 μm. For ultra-ﬁne powders (particle

size much smaller than 1 μm), the particles tend to segregate, thus resulting

in poor mixing. This problem may be minimized by liquid-solution mixing,

ensuring the precise compositional control and molecular-level chemical ho-

mogeneity. Moreover, this technique eliminates contamination from grinding

and milling media that occurs during a conventional mix/milling process. For

a multi-component system like HTSC, the mixing plays a key role in obtaining

high phase purity. Better mixing also translates into faster reaction kinetics.

62 2 Composition Features and HTSC Preparation Techniques

Raw Materials

Mixing Grinding

Drying or Solvent

Removal

Calcination

Grinding Comminution

Fig. 2.6. A ﬂow chart for the synthesis of ceramic superconducting powders

These powders can be calcined at lower temperatures and/or shorter time to

achieve the desired phase purity.

The next step is drying or solvent removal, that is necessary to preserve

the chemical homogeneity obtained by mixing. For multi-component systems,

solvent removal by slow evaporation can lead to a very inhomogeneous residue

due to the diﬀerence in solubilities of various components. In order to minimize

this eﬀect, various techniques are used, including, in particular, the processes

of ﬁltration, sublimation, etc. [960].

After drying, the powders are calcined in a controlled atmosphere for re-

action until reaching a ﬁnal composition and phase assemblage. The reaction

Melt

Melt +

BaCuO

Melt +

YBa

7–x

Melt +

CuO + BaCuO

BaCuO

YBa

7–x

BaCuO

CuO + BaCuO

BaCuO

YBa

7–x

CuO + BaCuO

Fig. 2.7. A phase diagram of Y

–CuO–BaO system [318]

2.2 BSCCO Films, Tapes and Wires 63

path for HTSC systems depends on various processing parameters, such as

calcination temperature and time, heating rate, atmosphere (oxygen partial

pressure) and starting phases. The powders can also be synthesized directly

from solution using pyrolysis techniques [889] or an electro-deposition tech-

nique [65].

The phase diagram, presented for YBa

in Fig. 2.7, shows that even

small ﬂuctuations of compositions can lead to the formation of normal (non-

superconducting) phases, namely Y

BaCuO

, CuO and BaCuO

. An applica-

tion of precursors with carbon content also complicates the formation of the

YBa

phase and may lead to diminishing superconducting properties.

In detail, the carbon problem will be considered in Chap. 4.

2.2 BSCCO Films, Tapes and Wires

Now for the preparation of Bi-2212 thick ﬁlms on Ag and MgO substrates,

the set of techniques are applied, namely melt-processing [405,407], elec-

trophoretic deposition [49], doctor-bladed [520], dip-coated [1067] and organic

precursor [397] ﬁlms. Last three procedures are presented in Fig. 2.8. It should

be noted that the fabrication of Bi-2223, possessing higher superconducting

properties, by using melt processing is impossible because of PbO loss in high-

temperature process [253]. The powder for doctor-bladed and dip-coated ﬁlms

can be made using any of the processes listed in Table 2.2.

In the doctor-bladed process, one makes a “green” ﬁlm (i.e., ﬁlm before heat

treatment) from the organic/powder mixture by pouring a pool of the slurry

on a ﬂat surface (e.g., a piece of glass), then leveling the slurry with a straight-

edged blade, located above the ﬂat surface at the desired ﬁlm thickness (this

allows to carry out corresponding control during its preparation). The ﬁlm

is dried, cut into strips that are placed on the silver foil and ﬁnally melt

processed.

For the dip-coated ﬁlms, the silver foil is passed through the organic/powder

mixture, which adheres to the foil. The thickness of the ﬁlm is controlled,

changing the organic compounds, modifying their proportions in the mixture

and adjusting the solids loading in the mixture. After passing through the or-

ganic/powder mixture, the coating is dried, the organics are burned out and

the ﬁlm is melt processed.

For the organic precursor ﬁlms, a solution of organometallic compounds

of Bi, Sr, Ca and Cu is deposited on the Ag foil, the solvent is burned out and

the process is repeated until the desired layer thickness is built up. Finally,

the ﬁlm is melt processed. A Bi-2212 ﬁlm can also be made, painting Bi-2212

powder onto a silver foil. Here, the powder is mixed with an organic liquid

having a high vapor pressure (e.g., butanol). This slurry is brushed onto the

foil and ﬁnally melt processed.

Since the pioneering work of Heine et al. [405] on Bi-2212 conductors, al-

most all Bi-2212 conductors have been melt processed. This method is also

64 2 Composition Features and HTSC Preparation Techniques

(a)

(b)

(c)

Oxide

Powder

Organic

Formulation

Mixer

‘‘Green’’ Tape

“Green” Tape

Doctor-Blade

Slurry

Ag Foil Substrate

Carrier Sheet

Furnace

Organic Binder

Calcined Powder

Organic Solvent

Mixing

Continuous Dipping

Cooling

Heat Treatment

Bi(C

COO)

Ca(C

COO)

Sr(C

COO)

Cu(C

COO)

Organo-metallic

Compounds

Dissolve into

Organic Solution

Solution

HTSC Tape

Heat

Treatment

Pyrolyze

500ºC, in Air

Fig. 2.8. Schematic diagram to make (a)doctor-bladed,(b) dip-coated and (c)

organic precursor ﬁlms [397]

2.2 BSCCO Films, Tapes and Wires 65

Table 2.2. Synthesis techniques to make Bi-2212 and Bi-2223 powders

Method Description Advantages Disadvantages

12 3 4

Solid-state

reaction

[409]

Mix oxides, peroxides, carbonates or nitrates of

Bi, (Pb), Sr, Ca and Cu. React at elevated

temperature where no melting occurs. Grind

sample and reﬁre. Repeat until reaction is

complete

Simple, inexpensive

technique

Large grain size of reactants

can cause slow reactions.

Product can have large

grain size. Impurities can

introduce during grinding

Co-

precipitation

[409]

Dissolve Bi, (Pb), Sr, Ca and Cu compounds in

acid. Add base to precipitate cations. Fire

precipitate to yield the desired phase

Intimate mixing of

cations

Not all cations may

precipitate out at the same

rate, causing segregation.

Initial composition and

precipitate composition may

be diﬀerent

Aerosol spray

pyrolysis [409]

Make solution, containing cations. Produce ﬁne

mist of the solution and pass it through a hot

furnace to form a powder of mixed oxides. Fire

mixed powder to yield the desired phase

Intimate mixing of

cations. Product has

very ﬁne grain size

(1–2 μm). Product can

have low carbon

content (using nitrates)

Species can lose, partially

Pb, during pyrolysis.

Powder, formed in pyrolysis

is not fully reacted to the

desired phase

Burn

technique [07]

Form nitrate solution of cations. Add organic

species, such as sugar, to solution. Heat solution

to remove water then heat powder at elevated

temperature. The sugar (fuel) and nitrate

ion oxidant react (i.e., burn) at elevated

temperature, yielding a high temperature that

forms mixed oxides. Fire this powder to yield

the desired phase

Intimate mixing of

cations. Powder can

have ﬁne grain size

Species can lose, partially

Pb, during burn process.

Powder formed in burn

process is not fully reacted

to the desired phase.

Product may contain carbon

(continued)

66 2 Composition Features and HTSC Preparation Techniques

Table 2.2. (continued)

Method Description Advantages Disadvantages

12 3 4

Freeze drying

[409]

Spray aqueous nitrate solution of Bi, (Pb),

Sr, Ca and Cu into liquid nitrogen. Collect

frozen droplets and freeze dry them to

remove water. Fire dried powder to yield

the desired phase

Intimate mixing of

cations. Product can

have low carbon

content

Cations may demix during

freeze drying, if the

temperature is not carefully

controlled. Nitrates,

presented after freeze drying,

may melt during ﬁring,

leading to large grains of

non-superconducting phases

Liquid mix

method [838]

Form nitrate solution of cations then add

glycol or citric acid. Heat to remove water

and form polymerized gel, then heat to

elevated temperature to yield the desired

phase

Intimate mixing of

cations. Powder can

have ﬁne grain size

Product may contain carbon

Micro-

emulsion

[587]

Form suspension of micro-droplets of

aqueous nitrate solution of Bi, (Pb), Sr,

Ca and Cu in oil. Add base to form

precipitates. Separate precipitate from oil

by washing in solvent. Fire precipitate to

yield the desired phase

Intimate mixing of

cations. Powder can

have ﬁne grain size

Product may contain carbon

Sol–gel [91] Form alkoxide solution of cations. Add

water or alcohol to cross-link molecules,

forming gel through polymerization and

condensation reactions. Heat to elevated

temperature to burn the organics and yield

the desired phase

Intimate mixing of

cations. Powder can

have ﬁne grain size

Method is better suited to

making ﬁlms, than bulk

powders

2.2 BSCCO Films, Tapes and Wires 67

named by partial melt processing, that reﬂects the fact that Bi-2212 melts

incongruently forming liquid and crystalline phases (i.e., partial melt).

Nu-

merous heating schedules that are currently used for the melt process of Bi-

2212 can be generalized for tapes and wires (Fig. 2.9a), and also for ﬁlms

(Fig. 2.9a). Both schedules could be divided into four stages [409].

max

30 min

5 min

10°/h

~840°C

72 hours

5°/min

III

Room Temperature

Temperature

Time

max

10°/h

100°/h

840°C

2 hours

III

Room Temperature

Temperature

Time

600°C

1 h

(a)

(b)

Fig. 2.9. Generic Bi-2212 melt-processing schedules used for (a) tapes and wires

and (b) ﬁlms. The schedules have been divided into four regions that are described

in the text [409]

It should be noted that the term melt processing clearly distinguishes from a

lower-temperature processing, where only a portion of the Bi-2212 is melted,

which is called liquid-assisted processing [410].