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

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98 3 Experimental Investigations of HTSC

system, time-resolved observations are possible. This method ensures the di-

rected visualization of the superconducting currents and Abrikosov vortices,

penetrating the sample. In this case, the total picture of defects is identiﬁed

from voids and microcracks in polycrystal down to smallest heterogeneities in

single crystallites. MOI technique has been used to research the structure of

single crystals [209, 469, 559], bi-crystals [833, 860, 1088] and polycrystalline

materials [469, 832, 1141]. It should be noted that for the best understanding

of HTSC intrinsic properties, it is very important to carry out comparative in-

vestigations of the “perfect” (i.e., monocrystalline) material and intergranular

boundaries [523, 1128, 1129]. In particular, the discovery of the lamellar-like

defects, which are weak links in Bi-2212 crystallites obtained by the directed

solidiﬁcation technique [1081, 1129], can be used to interpret comparative data

on superconducting properties of Bi-2212 polycrystals and related materials.

The microscopic Hall probe array method is a promising novel technique to

measure the component of the local magnetic ﬁeld perpendicular to the sample

surface [1190, 1191]. With this aim, two-dimensional Hall sensors for research

of electronic gas are etched in a GaAs/Al GaAs hetero-structure. The active

area of the Hall elements was initially ≥ 10×10 μm

and was gradually reduced

to 3 × 3 μm

. In the last case, the arrays have been fabricated, including up

to 20 Hall elements with the magnetic ﬁeld sensitivity better than 10 μT.

In addition to a research of the vortex-lattice melting transition [1191], this

method has been utilized for measurements of local magnetic relaxation in

crystals of YCa

7−δ

[1190] and Nd

1.85

0.15

CuO

4−δ

[312].

The rapid development of the scanning probe microscopy, following the

pioneering work [68], has also contributed to vortex imagine research. The

achievements in the investigation of HTSC systems by using this method are

presented in the overview [183].

The scanning SQUID microscopy provides highest sensitivity of vortex

structures [548]. The acting microscope consists of a mechanical xy-scanning

system combined with an integrated miniature SQUID-magnetometer. The

SQUID is fabricated with 1 μmNb–AlO

–Nb junctions and octagonal pick-

up loop with a diameter of 10 μm integrated on a silicon substrate and has

a sharpened tip. The tip is brought in immediate contact with the sample

during the scanning and serves for distance regulation. The spatial resolution

(about 10 μm) is limited by the size of the pick-up loop.

By using scanning tunneling microscopy at low temperatures, the spatially

varying quasiparticle density of states may be measured, yielding important

information about electronic properties of vortices and vortex structures [412,

413]. Scanning tunneling spectroscopy in the mixed state of HTSC is diﬃ-

cult because of the problem of the sample surface quality and the strict re-

quirements for highly reproducible tunneling conditions. The ﬁrst successive

experiments have been fulﬁlled on YBCO single crystals with vector B ori-

ented along c-axis [656]. The experiments on Bi-2212 single crystals brought

an unexpected result; they showed that the spectra inside the vortices diﬀer

weakly from the zero ﬁeld spectra, in contrast to the situation in YBCO [882].

3.1 Experimental Methods of HTSC Investigations 99

Therefore, the detection of the vortices in BSCCO is much more diﬃcult than

in the YBCO system.

Magnetic force microscopy has been used to imagine vortex systems and

research their interaction with defects in the HTSC thin ﬁlms [1183]. The

imagines of Abrikosov- and Josephson-vortices trapped in thin-ﬁlm YBCO

washer dc-SQUIDs have been obtained, using low-temperature scanning elec-

tron microscopy at liquid nitrogen temperature [525]. During scanning of

the sample surface by the electron beam, the signal is generated due to the

beam-induced local displacement of the vortices, resulting in a change of the

ﬂux coupled from the trapped vortices into the SQUID loop. In this way,

1/f-noise sources in the device have been identiﬁed. The spatial resolution of

this method is about 1 μm.

Lorentz microscopy for vortex imaging uses a high-voltage ﬁeld emission

electron microscope, generating a coherent electron beam. The electron phase

shift due to vortices in a tilted specimen is detected in an appropriately

defocused image [389]. This method has been used to investigate the vor-

tex lattice in single crystals Bi-2212 [390] and include the imaging of vortex

motion [1074].

Other applications of the experimental methods for researching of HTSC

structures are as follows:

– methods for measuring electromagnetic properties (J

−H, E−H, magnetic

sensitivity, etc.), used to separate components of the magnetic ﬂux pinning

and grain connectivity, causing J

[243, 440, 830];

– optical and scanning electron microscopy methods, used to study inter-

crystalline boundary structure, surface morphology and phase composition

[306, 723, 724];

– atomic-force microscopy, used to research surface morphology [754];

– X-ray diﬀraction methods, used to research texture and phase composition

[724, 754];

– spectroscopy methods to study microstructure and phase composition [6,

626];

– electron probe microanalysis to investigate chemical composition of sur-

faces and interfaces [1186];

– using of the transparent organic analogs to study peritectic reaction in

YBCO [938];

– using of picture of the YBCO volume magnetization to identify internal

defects [707];

– research of individual ﬁlaments escaped from BSCCO tapes [116].

It should be noted that the broad spectrum of methods of the surface

and interface reconstructions and also experimental approaches developed for

semiconductor structures and nanomaterials [716] can be used to study HTSC

structures at nanosize scale.

100 3 Experimental Investigations of HTSC

The measuring methods of mechanical and strength properties of HTSC

repeat in many aspects the tests, which have been applied broadly for various

ceramics and composites. In particular, the so-called R-curves are used to

study fracture toughness of volume samples, changing with crack propagation

[1171]. The fracture toughness has also been estimated by using the method

of Vickers indentation [614,868]. In this case, there are speciﬁc features con-

nected, for example, with the cryogenic temperature eﬀects on superconduc-

tors or with very small characteristic sizes of superconducting components.

In order to estimate elastic and acoustic properties, ultra-sound methods are

used [323, 876]; anisotropic stresses at sample surface are computed by appli-

cation of X-ray diﬀraction [381]; mechanical behavior, strength and fracture

toughness of sample bulks are computed during tests fulﬁlled at room and

cryogenic (77 K) temperatures, using tension/compression [1187], schemes of

the three-point bending [712] and four-point bending [324], rapture [712] and

bending [322] of notched specimens, creep at compression [902] and also in-

vestigations of thermal cycling with cycles between room and cryogenic tem-

peratures [1070]. The concept of eﬀective volume and Weibull’s distribution

function are used for test estimations of strength characteristics [908]. In or-

der to study bulk sample density the methods of immersion [970], picnometry

[789] and Archimedes [770] are applied. The methods of system analysis are

developed, for example, for design of material properties and critical behav-

ior of HTSC materials and composites [804,487,1152]. In order to estimate

stress–strain state of superconducting bulks, special devices are designed, in

particular with integrated gauges, which are able to measure strains with

required precision under diﬀerent thermal and magnetic ﬁelds, taking into

account anisotropy of properties in ab-planeandinthec-axis direction [710,

711]. Several methods for the mechanical tests of BPSCCO bulks are shown in

Fig. 3.1. Figure 3.2 shows two schematic illustrations of how the strain gauges

and Hall probe for the measurements of the thermal and magnetic properties

are positioned on the YBCO surface.

BSCCO/Ag tape superconductors are tested at room and cryogenic tem-

peratures: on tension [915], longitudinal compression (stability loss) [1117],

transversal compression [246], compression and tension by using U-shaped

spring dais [1051], bend under three-point bending [776], by hand [573] or

by using special devices [575], around cylindrical surfaces (one side [575] and

two side [854]) and helical surfaces (an investigation of winding on tension

and bending) [768]. They are subjected to thermal cycling including change

of temperature from room to cryogenic value and back [1118], to fatigue tests,

including cycles of the tape bending and straightening [659], to cyclic loading

and unloading [552] or cyclic changing of external tension [1023], low-cyclic

[1050] and multi-cyclic (up to 10

cycles) [434] fatigue under longitudinal ten-

sion, to tension in the c-axis direction and multi-cyclic (up to 10

cycles)

fatigue [433]. Several test methods for HTSC tapes are shown in Fig. 3.3.

In order to evaluate supercondunting core density of tapes, the measuring

methods of microhardness, using Vickers diamond pyramid [1159] and Knoop

3.1 Experimental Methods of HTSC Investigations 101

Glass-Epoxy Tabs

(a)

(b)

Fiber

Matrix

Glass-Epoxy

Tabs

(c)

Fig. 3.1. Specimens for evaluation of the mechanical behavior of monolithic

BPSCCO and unidirectional Al

/BPSCCO composite in: (a) tensile test, (b)

three-point bending notched specimen and (c) single-edge notched fracture test [712]

non-symmetric indenter [827], are applied. Relative changing of density is esti-

mated for wire-drawing by using registration of its lengthening in dependence

on decreasing of the cross-section diameter [384, 568]. The microhardness pro-

ﬁles (i.e., monitoring of transversal cross-sections in diﬀerent points of their

diameters or main axes) are applied to estimate superconducting core density

of tapes and wires after deformation (drawing, extrusion, rolling and press-

ing; see Fig. 3.4) [458] or single ﬁlaments of multiﬁlament samples [457]. The

102 3 Experimental Investigations of HTSC

(a)

•

(b)

Strain Gauge

Hall Probe

(internal)

Hall Probe

(external)

Fig. 3.2. (a) Schematic illustration showing how the strain gauges are mounted on

the YBCO surface (gauges 1 and 2, positioned on the top surface, are used for the

measurements of the strain along the ab-plane, and gauges 3–6, ﬁxed on the side, are

used to measure the strain along the ab-axis on the ac-plane (gauges 3 and 4) and

along the c-axis (gauges 5 and 6) [711]; (b) schematic illustration of how the strain

gauges and Hall probe are attached to the YBCO sample for the measurement of

the mechanical properties and trapped magnetic ﬁeld (Nos. 1–3 are internal gauges

of strain, Nos. 4–7 are external gauges of strain) [710]

3.1 Experimental Methods of HTSC Investigations 103

HTSC Tape

Strain Gauges

Cross-section of dais

Fixed Caliper

Supporting Rig

Moving Caliper

Load

Sample

Rolling

Direction

Transversal

Direction

ab-t

(c)

(b)

(a)

ab-t-

ab-r

ab-r-

Fig. 3.3. Test methods for investigation of HTSC tapes: (a) on compression and

tension by using U-shaped spring dais with soldered sample [550], (b) on stability loss

at longitudinal compression [1117], (c) on tension or cyclic fatigue along c-axis [433]

104 3 Experimental Investigations of HTSC

•

Punch

View from the side

View from one side

HTSC

Tape

HTSC

Tape

HTSC Tape

Piezoelectric Transducer

(d)

(e) (f)

(g)

••

Fig. 3.3. (d) on transversal compression by using two punches with evaluation

volt–ampere characteristic [246], (e) on bending by using measuring devices of criti-

cal current depending on continuously decreasing (or increasing) diameter of bending

[1051], (f) on three-point bending by using a device in which loading is carried out

by the piezoelectric transducer of acoustic emission [820], (g) on bending around

helical surface [768]

3.1 Experimental Methods of HTSC Investigations 105

•••• •

••

•

•••

•

• •••

•

Adjacent Averaging

Measured Points

BSCCO

(a)

(b)

Fig. 3.4. Proper microhardness proﬁles of HTSC wires and tapes after: (a)drawing

and/or extrusion, (b) rolling and/or pressing [458]

ultrasound vibration method is used to evaluate the microcracking density

[21, 996]. The method for investigation of mechanical aging that model me-

chanical eﬀects, developing during a conductor stranding, was proposed in

[676]. This test models bending, twisting and tensile forces, experienced by

the HTSC wires during conductor stranding (Fig. 3.5). Moreover, to predict

thermal cracking, the wire aging is studied under conditions of elevated tem-

peratures [676].

3.1.2 Acoustic Emission Method

Acoustic emission (AE) is the physical phenomenon connected with irradi-

ation of elastic waves by a solid under loading due to dynamic local refor-

mation of the material’s internal structure. This method has been applied to

research diﬀerent physical and mechanical properties of HTSC, accompanying

106 3 Experimental Investigations of HTSC

Twisting

on 360°

Back

(Double)

Bend

Spiral

Winding

Back

Tension

Device

Fig. 3.5. Schematic of mechanical aging test, designed to simulate the conductor

stranding process [676]

microstructure and phase transformations during fabrication and loading of

the superconductors at various internal and external loading. Intensive AE

has been displayed at YBa

7−x

sintering [230] and thermal treatment

[1011]. It is observed due to microcracking, caused by anisotropic compression

of crystalline lattice in narrow temperature range at cooling after sintering.

Acoustic emission has been studied in YBCO ceramic at the sample heating

from liquid nitrogen temperature [1017]. The observed AE has been correlated

with the microstress relaxation at the grain boundaries due to the thermal ex-

pansion anisotropy of grains. As possible mechanisms of stress relaxation, the

dislocation displacement and microcracking initiation at most unfavorably ori-

ented grain boundaries have been assumed. The oxidation kinetics of YBCO

ceramic has also been studied by using AE method [231]. The oxidation has

submitted to exponential law. The results showed that a narrow surface layer

oxidized initially, but the oxygen solubility into material was controlled by

the rate of the volume diﬀusion. YBCO ceramic has been investigated in the

25–700

◦

C range [229]; AE irradiation has been registered in the 260–300

◦

and it has been shown that this was caused by the structure phase transition

of 90

◦

phase into 30

◦

one during redistribution of oxygen into material. AE

method has been used to register the structure transformations in HTSC dur-

ing thermal cycling [963]. The eﬀect of the cyclic change of current in YBCO

ceramic has been studied by the AE method at 77 K [233]. In this case, a pen-

etration of magnetic ﬂux lines into sample under the action of own magnetic

ﬁeld has been stated. A pick of acoustic emission in Tl

CuO

6+x

sample

at critical temperature T

[788] has been observed. The relaxation anomalies

stated have been correlated with a change of charge state of linear defects and

parametric changes of the cuprate layer structure.

3.1 Experimental Methods of HTSC Investigations 107

The processes of microcracking and microplasticity of GdBa

and

monocrystals and also of YBCO ceramic have been studied

by the local indentation method accompanied by registration of AE signals

[80]. Comparison of AE data and fracture pictures permitted to estimate a

character of inelastic strain of HTSC. Then, it may be concluded that AE

method can operatively control mechanical properties of HTSC. In the case

of GdBa

monocrystals, the AE signals have been caused by formation

of characteristic microcracks at the angles of the indenter imprint. In the pro-

cess of local loading of Bi

crystals, formation of wedge-shaped

defects, accompanied by initiation of low-frequency component of AE signals,

was observed. It may be assumed that these defects are result of joint pro-

cesses of microcracking and de-lamination in BSCCO. The AE method was

also applied to research inelastic strains of HTSC ceramics during macroscopic

mechanical tests [79] and microindentation [340]. AE was used to investigate

secondary sintering of Bi-2223/Ag tape during heating after primary sinter-

ing and rolling [232]. In this case, it has been assumed that the process of

liquid-phase healing of cracks, formed during rolling, has appeared as a pos-

sible source of AE. This result leads to the conclusion on the possibility of

application of the AE method as the non-destructive control method in the

processing of Bi-2223/Ag tapes.

In order to answer the issue of the possibility to apply the AE method for

estimation of microdamage initiation and propagation during bending of Bi-

2223/Ag tapes, a test device has been designed, which is presented in Fig. 3.6

[820]. In the tests carried out at room temperature, the following parameters

were estimated: (i) sample displacement under indenter (Δl), (ii) the applied

loading (P

), (iii) the number of AE impulses (ΣN), (iv) a sum of AE sig-

nal amplitudes (ΣA), (v) AE signal activity (

N), (vi) the loading time (t).

ΣN

ΣA

Δl

Fig. 3.6. Schematic illustration of test device. Dinamometer (1), cylindrical in-

denter of 20 mm diameter with AE transducer (2), measurer of displacements (3),

Bi-2223/Ag tape (4), supports (5) with 10 mm diameters and 35 mm spacing, ten-

sometric devices of agreement (6,8), preliminary ampliﬁer (7), system block of AE

signal treatment (9) and registering device (10)