Thermal analysis in the field of high temperature superconductors

43

Thermochimica Acta, 133 (1988) 4348 Elsevier Science Publishers B.V., Amsterdam

THERMAL

ANALYSIS

IN THE FIELD OF HIGH TEMPERATURE

E. Kaisersberger,

J. Janoschek,

NETZSCH-Geratebau

GmbH, Wittelsbacherstrasse

SUPERCONDUCTORS

W. Hadrich 42, D-8672

Selb FRG

ABSTRACT

Thermoanalytical metry

methods

were used to study the preparation,

and phase transitions

A considerable

increase

ting oxide-carbonate

and loss between

0

balance.

in density

during

powder mixture

in the dilatometer.

The oxygen and 0

6

7

oxygen

stoichio-

of superconductors

of the type (Y, Eu)Ba Cu 0 . 2 37 the sintering process for the star-

was achieved

content

by rate controlled

and the reversibility

was investigated

with a symmetric

sintering

of oxygen

gain

thermo-micro-

INTRODUCTION

The ceramic prepared

technology

powders

used for manufacturing

allows

dilatometric,

sintesed

bodies

thermogravimetric

from specially

and DSC/DTA

measure-

ments. Mixtures

of oxides,

atmosphere l-2-3

carbonates

to follow

oxides.

The starting

from the fine grinded resulting

and oxalates

the reactions powders

oxides

superconductors

during

were heated

the preparation

were stoichiometrical

of Ytterbium

had critical

or Europium

temperatures

in air and in oxygen of superconducting mixtures

(by weight)

and CuO and BaCO

at 90 K to 95 K.

3

; the

EXPERIMENTAL

The sintering in a single

of powder

pushrod

dilatometer.

compressed

to small rods

and heated

in oxygen

shrinkage

by linear

The carbonate 6 mm, length

precursor

in the quasi isothermal

measurements

for Y6a2Cu3U7 150-200

was N/mm')

increase was programed

For rate controlled

at the desired

shrinkage

7-8mm, pressure

The temperature

up to 975'C.

raise was programmed

to hold the linear

was followed

(diameter

atmosphere.

at the rate of 10 K/min ature

compacts

sintering

or stepwise

isothermal

rate level.

Thermal Analysis Proc. 9th ETA congress, Jerusalem, Israel,21-25 Aug. 1988 0040-6031/88/$03.50 Q 1988 Elsevier Science Publishers B.V.

linear

the tempermode

44 Thermogravimetric microbalance.

tests were performed

The symmetrical

of very small mass changes or in a static or dynamic (stoichiometrical mosphere, hydrogen

in a newly developed

of this instrument

over a wide

were heated

product

Powders

thermo-

under a high vacuum

of Eu 0 , CUD and BaCD

in PtRh crucible:

was tempered

(97% He, 3% H2) atmosphere

symmetrical

allows determination

range of temperature

gas atmosphere.

mixture)

the resulting

design

in oxygen,

and afterwards

Sn dynamic reduced

again

oxygzn

at-

in a helium-

reoxidized.

RESULTSAND DlS&USSlON Dilatometry tering

is used for a long time in ceramic

temperatures,

changes.

Also

to a "ceramic

on the reactivity

the sintered

product

bonate

technology",

reactions

and structural

superconductors,

difatometry

of the raw materials

sin-

provides

which are preuseful

and on structural

fn-

changes

of

of sintering

curves

for powder

compacts

of the car-

precursor

heated

for YBa Cu 0 . The main sintering process starts at 78O'C 2 37 in oxygen at 10 K/min (curve 2), Minor shrinkage steps are recor-

ded at 34D'C and SlO'C.

The shrinkage

shown in fig. 1. Rate controlled age for the .same material and chemical

Fig.

mineral

to determine

fref 1).

Fig. 1 shows a comparison

when

in,firing,

in the field of high temperature

pared according formation

shrinkage

technology

1

stability

is not completed

sintering

(curve l), thus providing

of the sintered

Sintering

at the final temperature

leads to a 50% higher linear higher mechanical

shrink-

strength

body.

of YBa Cu 0 - carbonate precursors with linear heating 2 37 rate (curve 2) or stepwise isothermal mode (rate controlled sintering,

curve

1) In oxygen

atmosphere

45 The sintering w&ght

temperature

loss for the CO

780-C corresponds

evolution)

changes

2 in the expansion

tigated

up to now. The stepwise

with an intercept ent mechanisms isothermal

Preparation,

isothermal

further

are controlling

of the type EuBa2Cu307

mode

the sintering

reduction

resolves

tests are necessary

in thermogravimetry

tempering,

The smaller

curve at 340-C and 610'C have not been further

at 82O'C,

heating

to the main TG step (start of

for the same powder mixture.

and oxidation

evolved

differ-

(e.g. stepwise

gas analysis).

of superconducting

in subsequent

ranges

to show whether

in these ranges

and coupled

was investigated

two shrinkage

inves-

material

thermogravimetric

ex-

periments. Fig. 2 shows the temperature pering,

starting

program

with the carbonate

and TG-OTG

curves

for preparation

and tem-

precursor. 7

Fig. 2

Total

graphic

symmetrical

for preparation

thermobalance

and tempering

in flowing

of EuBa2Cu307

It can be seen.from

the one graph that the final weight

tempering

is higher

(+ 0.42%)

er weight

gain is achieved

ermal

segments

The preparation

of the selected

was evaluated

The Eu203-CuO-&CO3 low temperature, Therefore tions.

mixture most

this weight

compared

to level

in the linear tempering

separately

of the sample after

after preparation,

cooling

segments

and the high-

compared

to the isoth-a

program.

according

shows an unexpected

probable

in a

oxygen.

due to impurities

to fi'g. 3.

weight

loss of 1.52 % at

in the starting

step is not used for the further

powders.

stoichiometric

calcula-

46 The main weight

loss starting

at 760-C us evluated

for the formation

of

from the carbonate. The theoretical mass change of 10.88 X Cu 0 2 3 6.5 is exceeded in the experiment by impurities and inaccuracy in the stoichi-

EL&

ometry

of the starting mixture.

PREPARATION

Fig. 3

Preparation

(6 = x = 6.5) from the carbonate-oxide

of EuBa2Cu30x

mixture,

Above

109O'C a sharp mass change

is recorded

to 0 * This TG step is reversible tempgrature exact

range

for heating

(see Fig. 2). With

stoichiometric

mixture

starting

shown in fig. 2.

tent after

to 0

stoichiometry

6.92' seems to be the reduction

taining

atmosphere.

strongly

depending

ison between change

during

results

The reaction

reduction,

evaluated

loss from 0

in oxygen to 6th?s

of high purity and an

the oxygen

stoichiometry

content

only

In above example the oxygen conproof for oxygen

of the superconductor in the reduction

of the atmosphere,

seem to be critical.

gen in the amount of 0

powders

But the better

temperatures

on the hydrogen

laboratories

and cooling

one could evaluate

from this type of experiment tempering

for the oxygen

in hydrogen process are therefore

compar-

Fig. 4 shows the total mass

to 7.51 %. This corresponds

With the general

assumption

to the re'ful: %%n

above,

to loss of oxy-

for the composition

3.42' BaO and Cu, the stoichiometry of the of the reduced material to be Eu Cu 2 3' before the reduction exsuperconductor is calculated as EuBa Cu 0 periment.

This corresponds

con-

after tempering.

Fig:4

Reduction

The reduction

covers

of the relatively gas, sharp

of EuBa Cu U 2 3x

The reoxidation

a broad temperature

low hydrogen

reduction

in hydrogen

content

mass changes

containing

range between

ZOO'C and 8UO'C because

in the atmosphere.

are achieved

of the same material

atmosphere,

around

Using pure hydrogen

400-C.

is shown in fSg. 5. The oxygen content

is calculated as FuBa Cu 0 after this run, but there 2 3 6.89 are intermediate stages with higher oxygen content (6UO'C to 8OO'C). Further

of the sample

tests are necessary

Fig, 5

for identificatSon

of these apparently

in flowing Reoxidatiofl Of Eu6a Cu 0 2 3 3.6

cxygen.

unstable

products.

48 The highest

reaction

imum at 570-C. tetragonal

rate for reoxidation

The latter

transition

A newly developed for reduction

corresponds

described

software

is found at 41O'C and a second max-

to the temperature

by Gallagher

of the orthorhombic

(ref. 2).

program will be used to study the reaction kinetics

and reoxidation.

CONCLUSION

Densification

during

for Y6a2Cu307

can be improved

Thermogravimetry ometry

sintering

intermediate

from the carbonate

sintering

tempering,

superconductors.

products

reduction

Impurities

may influence

and reoxidation

in starting

the exact

of oxygen

stoichi-

experiments

powders

stoichiometric

precursor

in dilatometers.

is shown to be useful for the determination

from preparation,

EuBa*Cu~07-type

of a powder compact by rate controlled

on

and unstable

calculations.

REFERENCES 1

P.K. Gallagher

2

Cr. Sageev (received

a.o. Mat. Re. Bull. 22 (7) 1987

Grader,

P.K. Gallagher

as manuscript

to be published

from the author)

ACKNOWLEDGEMENT We thank Prof. GUntherroth, and the fruitful

discussions

University

of Kdln, for the supply of raw materials

of the test program.