Electron backscatter diffraction study of polycrystallineYBa2Cu3O7�d ceramics

Anjela Koblischka-Veneva a,b, Michael R. Koblischka c,*, Petra Simon a,Frank M€uucklich a, Masato Murakami b

a Institute of Functional Materials, Universit€aat des Saarlandes, P.O. Box 151150, D-66041 Saarbr€uucken, Germanyb SRL/ISTEC, Division 3, 1-16-25 Shibaura, Minato-ku, Tokyo 105, Japan

c Experimental Physics, Universit€aat des Saarlandes, P.O. Box 151150, D-66041 Saarbr€uucken, Germany

Received 8 August 2001; received in revised form 4 January 2002; accepted 4 January 2002

Abstract

The electron backscatter diffraction (EBSD) technique was applied to polycrystalline YBa2Cu3O7�d ceramics. To

obtain good quality EBSD Kikuchi patterns on the ceramic superconductors, a new sample polishing route was es-

tablished. Samples with and without KClO3-doping were investigated; and the crystallographic orientations of the

grains were analyzed using pole figures and EBSD orientation mapping. On the polycrystalline KClO3-doped

YBa2Cu3O7�d samples we found two major orientations (0 0 1) and (1 0 0) normal to the sample surface, while the

undoped sample did not exhibit any orientation maxima. We ascribe this observation to the effect of a liquid phase due

to the KClO3-additive during the preparation process.

� 2002 Elsevier Science B.V. All rights reserved.

1. Introduction

The investigation of the microstructure of

ceramic, polycrystalline high-Tc superconductorsplays an important role in the understanding of the

magnetic properties, namely the critical current

density, jcðBÞ. The performance of bulk samples of

the RE-123 type (i.e. YBa2Cu3O7�d (YBCO) or

NdBa2Cu3O7�d (NdBCO)) relevant for applica-

tions (e.g. microwave resonators, trapped field

magnets, fault current limiters and wires) [1] are

especially severely limited by the presence of grain

boundaries [2] with different misorientation angles.

High angle grain boundaries were found early

to act as weak-links, but also low angle grain

boundaries and subgrain boundaries pose seriousproblems in large magnetic fields. Therefore, grain

boundaries form severe obstactles for the current

flow, thus in the past years a lot of effort was in-

vested to improve the texture and grain connec-

tions within these bulk samples [3,4].

An important step for the improvement of the

transport properties of the grain boundaries is

given by chemical doping, as described recently byMannhart and co-workers [5]. By means of doping

YBCO thin films with Ca, these authors showed

that the current density across the grain bound-

aries could rise by a factor of about 6. These in-

vestigations were, however, performed on YBCO

* Corresponding author. Tel./fax: +49-681-302-4867.

E-mail address: m.koblischka@mx.uni-saarland.de (M.R.

Koblischka).

0921-4534/02/$ - see front matter � 2002 Elsevier Science B.V. All rights reserved.

PII: S0921-4534 (02 )01244-3

Physica C 382 (2002) 311–322

www.elsevier.com/locate/physc

thin films with a single grain boundary. For the

development of bulk materials, it is therefore ex-

tremely important to study the effects of doping

on bulk, polycrystalline samples aimed for appli-

cations.

In earlier publications, we have reported on thepreparation and characterization of YBCO ce-

ramics doped by various alkali metals (K, Na, Rb)

[6,7]. By means of magnetic measurements (AC

and DC susceptibility, microwave surface resis-

tance) it was shown that in certain concentrations

of the dopant the grain growth and the inter-

granular properties of the samples improved con-

siderably. Therefore, a direct correlation of thelocal microstructure (grain size and orientation)

with the magnetic properties of the samples is a

very important task. An useful tool to achieve this

goal is given by the recently developed electron

backscatter diffraction (EBSD) technique [8–10],

which enables the orientation mapping of granular

samples by means of automatized recording of

Kikuchi patterns [11]. EBSD bridges the two tra-ditional methods of studying crystalline materials,

X-ray crystallography and transmission micro-

scopy, and adds the benefit of high spatial reso-

lution at any point on the surface of a bulk

specimen.

With the necessary equipment, EBSD Kikuchi

patterns can be used to obtain a crystallographic

orientation map of the grains on the surface ofthe specimen. Some preliminary studies using the

EBSD technique on high-temperature supercon-

ductor oxides have already been reported (YBCO

thin [12] and thick films [13] including coated

conductors [14], Tl-1223 [15,16] and Bi-2223 tapes

[17]), but the results were very limited. EBSD

patterns from only a few individual YBCO or

Bi-2223 crystalites were presented and the misori-entation of the Bi-2223 crystalites was semi-quali-

tatively estimated. Therefore, the results cannot

pictorially present the connectivity and crystallo-

graphic orientation distribution of the grains over

a bulk area, which would be essential for a fur-

ther understanding of the growth process of bulk

YBCO samples.

The aim of the present paper is to study thelocal microstructure of polycrystalline KClO3-

doped and undoped YBCO by means of EBSD.

We present EBSD Kikuchi patterns and discuss

the details of the necessary polishing procedure.

2. Experimental procedure

2.1. EBSD technique

Backscatter Kikuchi patterns are generated in a

commercial EBSD system (TSL TeXSEM Labo-

ratories [18]) using a phosphor screen mounted in

a commercial SEM microscope (CamScan Series

4) by back-diffraction of a stationary beam of

high-energy electrons from an almost perfect vol-ume of crystal. An EBSD pattern can be viewed as

a map of the angular relationships between atomic

planes in a crystal. Bragg diffraction occurs in

three dimensions from the lattice planes on either

side of the diverging electron beam, producing two

cones of radiation. When a phosphor screen is

placed to intercept the cones, two conic sections

are captured and appear as a pair of parallel linesknown as Kikuchi lines or Kikuchi bands that

represent the specific lattice planes. In order to

obtain intense Kikuchi patterns, the sample has to

be steeply tilted to about 70� from the horizontal.

To produce a crystallographic orientation map,

the electron beam is scanned over a selected sur-

face area and the resulting Kikuchi patterns are

automatically recognized by means of a modifiedHough transform [19], and indexed by the con-

trolling computer. The patterns are recorded by

means of a low-light CCD video camera, and an

additional background subtraction unit is required

as the Kikuchi pattern is superimposed on the

scattered electrons, which produce a background

about five times larger than the patterns. The data

are then analyzed using the OIM analysis software[18]. A schematical drawing of the EBSD setup is

given in Fig. 1.

2.2. Sample preparation and characterization

Polycrystalline KClO3-doped YBCO high-tem-

perature superconductors with nominal composi-

tion Yð1�0:2xÞBað2�0:2xÞKxCu3Oy ranging betweenx ¼ 0 and 0.75 were synthesized by a solid state

reaction in air [20]. Powders of Y2O3, BaCO3,

312 A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322

CuO, all with a purity 99.99% and KClO3 (more

than 99.9%), were mixed, pressed into pellets andheated at 880 �C for 24 h. The reacted pellets were

reground, pressed again and sintered at 915 �Cfor 20 h, followed by annealing at 600 �C for 14 h,

before being cooled to room temperature. The

samples chosen for this study (x ¼ 0:30) had a

cubic shape (1 � 1 � 1 mm3) and a Tc of 92.9 K,

as determined by AC susceptibility (f ¼ 250 Hz,

Ha ¼ 0:1 mT). Pure samples were prepared forcomparison, and cut to the same shape. Tc of the

pure samples was determined to be 91.3 K.

The phase content, microstructure and mor-

phology of the crystalline grains were character-

ized using X-ray powder diffraction and scanning

electron microscopy (SEM) coupled with an elec-

tron probe microanalyzer. Fig. 2 shows the X-ray

analysis of the doped sample. No secondary phaseup to the resolution inherent to the techniques

was present. Metal impurities stemming from the

starting powders and the crucible material can be

excluded in these samples. Based on the X-ray

patterns obtained, we studied the influence of the

additives on the lattice constants. All the K-doped

samples investigated remain orthorhombic. The

values are comparable to the published latticeconstants [21] (e.g. for the sample with x ¼ 0:30, we

obtain a ¼ 3:826ð�0:002Þ �AA, b ¼ 3:886ð�0:002Þ�AA, and c ¼ 11:698ð�0:005Þ �AA). The exact knowl-

edge of the lattice parameters of the sample to be

investigated is essential for the automatic orien-

tation mapping; even more so as in the case of

YBCO the c-axis length depends on the oxygen

content.

Using SEM photographs, we carried out a

statistical analysis of the grain size in the samples.The YBCO grains have the specific shape of par-

allelepipeds. Here, we compare the grains with

different longitudinal and similar transversal di-

mensions, and calculated the average longitudinal

dimension. Fig. 3 shows the determined average

grain size as a function of the KClO3-addition, x,

together with the SEM images of samples with

x ¼ 0, 0.30 and 0.60. The figure illustrates that thegrain size reaches a maximum for x ¼ 0:30. Here, a

highly homogeneous large-grain ceramic structure

is achieved. Higher concentrations of the dopant

Fig. 2. X-ray diffraction pattern of the KClO3-doped YBCO

sample with x ¼ 0:30.

Fig. 1. Schematical drawing of the EBSD setup for automated orientation mapping.

A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322 313

(xP 0:60) lead to a small grain structure andchanges in the grain boundaries, and thus, de-

coupling of the grains. These results will be com-

pared to the grain sizes determined by EBSD in

Section 3.2.

The sample with x ¼ 0:30 and a pure YBCO

sample were chosen for the following EBSD in-

vestigations.

3. Results and discussion

3.1. Polishing technique of ceramic HTSC for

EBSD

Firstly, we discuss the requirements of the

sample polishing procedure. For use with theEBSD setup, the samples need to have a shiny, flat

surface and preferably, an almost perfect crystal-

lographic structure. The necessary polishing pro-

cedure is evidently somewhat delicate as several

authors have attempted to perform EBSD on bulk

superconducting samples (see for example, Refs.

[22,23]), but failed even though investigations of

the microstructure itself were possible by means oflight microscopy or SEM. This illustrates the high

sensitivity of the EBSD technique, as the back-

scattered electrons are generated from a small

depth under the sample surface, typically between

20 and 200 nm [9,24]. Backscatter Kikuchi pat-

terns have been found to disappear when a thin

foreign surface layer of thickness about two times

the Rutherford elastic mean free path is present at

a given beam energy [24]. Therefore, the polishingprocedure of the bulk ceramic samples plays an

important role to obtain high-contrast Kikuchi

patterns on polycrystalline samples.

For YBCO superconductors, several polishing

recipes are given in metallography journals [25,26],

but it is important to note that the main goal of

such recipes was to observe the microstructure it-

self. In this case, thin surface layers of differentmaterial or distorted YBCO will not be a problem.

However, for EBSD it is crucially important to

avoid such layers. In a former publication [6], we

found in a combined SEM and X-ray analysis of

the sample surfaces of doped YBCO that there is a

low-density surface layer of about 200 lm thick-

ness. This layer must be completely removed in

order to reveal the bulk properties of the sample.Furthermore, embedding the samples into resins is

not wanted for the EBSD measurements. Due to

these reasons, a sample polishing route different to

the standard metallography preparation technique

has to be established.

Magneto-optic (MO) investigations of flux dis-

tributions [27] were successfully performed since

many years on a variety of high-Tc samples, in-cluding polycrystalline samples [28–30]. All these

samples need to be polished, but superconductivity

is not allowed to be affected by the employed

procedure. Two slightly different polishing pro-

cedures were established for use with the MO

imaging: (1) The samples were dry polished on

polishing/lapping papers (3M Inc.) with SiO2

particles with sizes from 9 to 0.3 lm. Only forcleaning of the samples between the polishing

steps, ethanol is employed. (2) The samples were

ground on corundum papers (800–4000 mesh), and

for the last finishing step, a colloidal silica solution

(Struers OP-S) was used on the corresponding

polishing cloths (Struers OP-CHEM). For clean-

ing, again only ethanol is employed. Both methods

produce a very clean, shiny surface and, mostimportantly, the superconducting properties of

the samples are retained, as illustrated in Fig. 4

Fig. 3. The average grain size as a function of the KClO3-

doping, x. The insets present the corresponding SEM images of

polished surfaces as indicated by the arrows.

314 A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322

presenting a flux patterns obtained by MO imag-

ing on the KClO3-doped sample with x ¼ 0:30 at

T ¼ 50 K. This sample was polished using route

(2). Here, it should be noted that at low temper-atures, the sample does not exhibit the typical

flux pattern of a polycrystalline sample, where the

flux enters individually into the superconducting

grains. More details of these MO investigations

are given in Refs. [31,32].

On the same sample, very intense EBSD Ki-

kuchi patterns were obtained, thus demonstrating

that the same polishing procedure can be em-ployed also for EBSD. Furthermore, this allows to

avoid a carbon coating of the sample surfaces,

which is otherwise common for EBSD investiga-

tions on ceramic samples [33,34].

3.2. EBSD patterns and mapping

In Fig. 5, we present EBSD Kikuchi patternsand indexation of the YBCO samples. Fig. 5(a)

presents the EBSD Kikuchi patterns and index-

ation of the polycrystalline YBCO sample. The

dominating star in the upper middle part of the

Kikuchi pattern is the (1 0 0) direction; the star at

the bottom right is (1 � 10). Fig. 5(b) shows the

EBSD Kikuchi pattern of KClO3-doped YBCO

sample. Here, the (0 0 1) axis is oriented towards

the top right of the pattern with its characteristicfourfold rotation symmetry. Note that the Kikuchi

pattern shows more contrast as compared to the

pure YBCO sample, but is otherwise not different.

By means of EBSD on YBCO thin films, we ver-

ified that there is no visible difference between the

Kikuchi patterns, so the chosen polishing proce-

dure does not introduce defects to the polycrys-

talline sample.Now, we discuss the results of the automated

mapping of EBSD Kikuchi patterns. Firstly, we

present contour pole figures and inverse pole fig-

ures (IPFs) of the samples under study. Pole fig-

ures show the orientation of particular types of

crystallographic poles, with respect to a Cartesian

reference frame in stereographic projection. For

historical reasons, the axes of the reference framehave been labeled ‘‘rolling direction [1 0 0]’’ (RD),

‘‘transverse direction [0 1 0]’’ (TD) and ‘‘normal

direction [0 0 1]’’ (ND). Conventionally, the latter

lies normal to the polished sample surface. In Fig.

6(a), a contour pole figure of the KClO3-doped

YBCO sample is shown in the directions [1 0 0],

[0 0 1] and [0 1 0] . From this figure, it is clearly

visible that the KClO3-doped sample exhibits tex-ture, with the two directions (1 0 0) and (0 0 1)

being dominating.

Fig. 6(b) and (c) shows the corresponding IPFs

of the KClO3-doped and the pure YBCO sample

in all three reference directions [1 0 0], [0 0 1] and

[0 1 0]. On the triangles, the real crystallographic

directions are given. From these figures, we can

deduce that the two orientations (0 0 1) and (1 0 0)are dominant in the KClO3-doped YBCO sample,

which is also visible in the mapping shown in Fig.

7(c). This result is in contrast with the results on

the pure YBCO sample (c), where the grains are

more or less randomly distributed. This observa-

tion confirms our earlier results [35] and provides

an explanation for the improved magnetic prop-

erties as discussed in Refs. [31,32].Fig. 7 presents results of the automated mapping.

A typical map consists of about 10 000–20 000

Fig. 4. MO flux patterns of the KClO3-doped YBCO sample

with x ¼ 0:30 at 50 K and an applied field of 100 mT. The

magnetic field is imaged as bright areas; the Meissner phase

remains dark. Only in the lower part of the image, flux entering

along grain boundaries can be observed. The marker is 500 mm

long.

A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322 315

individual data points, i.e. EBSD Kikuchi pat-

terns. For the present mapping, we have chosen

a lateral spacing of 3 lm, covering an area of

654 � 106 lm2. The elapsed time for such a map-

ping is about 7–8 h in total. In (a), the selected area

of the sample is shown in a regular SEM image;

note that the sample is inclined by 70� from the

horizontal; (b) shows an image quality (IQ) map.The image quality is measured for each individual

Kikuchi pattern during the indexation. The lighter

the grey tone in the map, the better the quality of

the measured Kikuchi patterns; black denotes a

bad IQ. Bad IQ implies that the pattern recogni-

tion software did not obtain a solution, that is, no

Kikuchi pattern could be recognized. Along grain

boundaries and crystallographic defects, there will

be a larger scatter of the electrons, so the pattern

quality is reduced. Thus, cracks and pores are

clearly visible in this representation, so a standard

SEM image is closely reproduced by an IQ map.

Therefore, such IQ maps allow a direct compari-

son with the SEM images, polarization microscopy

images or, with MO images of field distributions.Fig. 7(c) shows the so-called IPF map in (0 0 1)

direction, i.e. the ‘‘grains’’ are shaded by their re-

spective crystallographic orientation normal to the

sample surface. The corresponding grain orienta-

tions are given in the associated stereographic tri-

angle. To obtain this map, a filter procedure [18]

is applied to ‘‘clean-up’’ questionable data points

Fig. 5. The EBSD Kikuchi patterns and indexing of YBCO. (a) Presents the EBSD Kikuchi patterns and indexation of the poly-

crystalline YBCO sample. The dominating star in the upper middle part of the Kikuchi pattern is the (1 0 0) direction; the star at the

bottom right is (1 � 10). (b) EBSD Kikuchi pattern of KClO3-doped YBCO sample. Here, the (0 0 1) axis is oriented towards the top

right of the pattern with its characteristic fourfold rotation symmetry.

316 A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322

Fig. 6. (a) Pole figure and (b) IPF of the KClO3-doped YBCO sample. (c) IPF of the pure YBCO sample. It is clearly visible that the

two orientations (0 0 1) and (1 0 0) are dominant in the KClO3-doped YBCO sample. This result is in contrast with the results on the

pure YBCO sample (c), where the grains are more or less randomly distributed.

A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322 317

originating from grain boundaries or surface de-

fects. Clean-up level two allowed the removal ofundefined points without altering the grain shapes

significantly.

From such an IPF map, it is clearly visible that

the two orientations, (0 0 1) and (1 0 0) are domi-

nating. This type of map summarizes the crystal-

lographic informations which can be obtained

from an EBSD mapping. Furthermore, we find

that 35% of the grains are of (0 0 1) orientation,54% of (1 0 0), and 0% of (1 1 0) orientation.

Fig. 8 presents an IQ map as in Fig. 7(b), but in

each map (a)–(e) the grain boundaries are indi-

cated in white with distinct levels of misorienta-

tion, i.e. 1–5� (a), 5–15� (b), 15–40� (c), 40–50� (d),

and finally, 60–70� (e). It is visible from this map-

Fig. 7. Results of the automated mapping on the KClO3-doped YBCO sample. (a) SEM image of the sample, inclined by 70� from the

horizontal. (b) Presents an IQ map. The IQ is calculated from the confidence index by the processing software. The lighter the grey tone

in the map, the better the quality of the measured Kikuchi patterns; black denotes a bad IQ. Cracks and pores are clearly visible in this

representation. (c) Shows the so-called IPF map in (0 0 1), direction, i.e. the grains are shaded by their respective crystallographic

orientation normal to the sample surface. The crystallographic orientations are given in the stereographic triangle.

318 A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322

ping that small angle grain boundaries are present

throughout the selected area. Grain boundaries

with larger angles are located along the crack in

the centre right of the map, and, as an important

result, around the pores in the YBCO matrix. This

grain boundary recognition is an important resultof the automated EBSD mapping. Such EBSD

mapping on superconductors, combined with MO

studies of the same area will enable an exact de-

termination of the nature of the weak links in a

superconducting sample. A further analysis of the

grain boundaries as described in Ref. [36] will be

performed in future experiments.

Fig. 9 presents a grain size histogram of the

KClO3-doped YBCO sample. In this figure, the

grain size as determined by EBSD is given as grainarea in lm2, where no assumption about the grain

shape is made. Here, it is important to note that

the definition of a grain in EBSD measurements

is different from that in e.g. optical microscopy.

Fig. 8. IQ map as in Fig. 7(b), but in each map (a)–(e) the grain boundaries are indicated in white with distinct levels, i.e. 1–5� (a), 5–

15� (b), 15–40� (c), 40–50� (d), and finally, 60–70� (e). Low angle grain boundaries can be found throughout the selected area; high

angle grain boundaries are located mainly around the pores and a crack in the middle of the map.

A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322 319

Grains in EBSD measurements are formed by analgorithm that groups sets of connected and sim-

ilarly oriented points into grains; thus these grains

do not necessarily correspond to the grains seen in

e.g. optical or standard SEM measurements. From

the grain analysis, the software excludes ‘‘edge

grains’’, which are only partially included in the

map. The average grain area A is calculated to be

397 lm2. The software calculates from this agrain diameter by assuming a circular grain shape,

yielding an average grain size diameter of 22.5 lm.

Comparing this result to our optical analysis pre-

sented in Fig. 3, where an average grain size of 42

lm is obtained, shows that the EBSD procedure

underestimates the grain size as the grains of

YBCO are parallelepipeds, and not circles. How-

ever, calculating the average grain area from theoptical measurements with an average grain width

of 10 lm yields A ¼ 420 lm2, which is indeed

comparable to the EBSD result.

These results clearly demonstrate that EBSD on

bulk high-Tc superconductors is possible to per-

form after a careful polishing of the sample sur-

faces. During our mapping, we have not observed

problems concerning charging of the samples, sothe application of an additional carbon layer is not

required at all; even though the presented maps

take about 7–8 h measurement time. Based on the

present results on doped, polycrystalline samples,

we may say that a main use of the EBSD technique

on bulk high-Tc superconductors will be the rein-

vestigation of the growth process of melt-textured

YBCO, where the results of some investigations

using optical microscopy [37,38] could be extended

considerably. This concerns mainly the formation

of grains and subgrains during the melt-growth,but also the influence of green phase particles (Y-

211) on the growth of the YBCO phase.

The influence of KClO3 on the properties and

microstructure of YBCO was discussed in detail in

our earlier studies [7,39]. We have associated the

effect of KClO3 with the presence of a liquid me-

dium such as KCl and a Cu-rich phase during

the technological process. The present EBSD studynow confirms this; and reveals the increased tex-

ture of the KClO3-doped samples as compared to

the pure YBCO. The increased texture of the

KClO3-doped samples is also manifested in the

flux distributions as shown in Fig. 4. The doped

samples are, therefore, more similar to melt-tex-

tured samples. This also provides an explaination

of the improved magnetic properties of the opti-mally doped samples with x ¼ 0:30 as observed

in Refs. [35,40]. The KClO3-doping is, therefore, a

very important step for the preparation of tech-

nological interesting YBCO superconductors such

as melt-textured bulks and coated conductors.

Together with the results of Mannhart et al. [5],

this shows a pathway to further improve the

properties of YBCO superconductors.

4. Conclusions

We have established a polishing route to suc-

cessful EBSD experiments on polycrystalline su-

perconductors, avoiding any additional coating

with a carbon layer. On KClO3-doped YBCO wefound an increased texture with the (0 0 1) and

(1 0 0) directions dominating, which is a result

of the presence of a liquid phase during the pro-

cessing stage. This observation confirms our ear-

lier results and provides an explanation for the

improved magnetic properties of the optimally

KClO3-doped samples with x ¼ 0:30. EBSD is a

powerful technique to detect the grain orintationsalso in bulk high-Tc superconductors. Therefore,

EBSD will play an important role for the under-

Fig. 9. EBSD grain size (area) histogram of the KClO3-doped

YBCO sample with x ¼ 0:30 for comparison with Fig. 3.

320 A. Koblischka-Veneva et al. / Physica C 382 (2002) 311–322

standing of the growth mechanism of bulk YBCO

samples and the development of larger, bulk melt-

textured samples for applications.

Acknowledgements

AVK and MRK would like to thank the Japa-

nese Science and Technology Agency (STA) for

providing the fellowship for the stay at SRL/IS-

TEC. This work was performed as part of DFG

project no. MU959/6.

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