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CHAPTER IV
SYNTHESIS A N D CHARACTERISATION OF REBa,HfO 5.5 [RE=Y, Gd, Sm] - A NEW CLASS OF CERAMIC COMPOUNDS
IV.1 Introduction
In Chapter III it is established that the addition of HfO, in YBCO
superconductor has resulted in the formation of a secondary phase YBa,HfO,,,.
By replacing Y by Gd and Sm, we could develop a new group of complex
~erovskite REB~,H~O, , where RE-Y, Gd and Sm having the general formula
A,(BBt)O, [where A-Ba, B-Y, Gd and Sm and B'-Hfl which also could be
used as substrate materials for YBCO film.
In this chapter we present the synthesis and sintering of REBa,HfO,,*
as single phase material. The crystal structure, sintered density, resistivity
measurements, dielectric properties, surface morphology and their chemical
compatibility with YBCO are studied in detail and the results are presented.
IV.2 Preparation of REBa,HfO,,,
REBa,HfO,,, was prepared by mixing stoichiornetric amounts of high
* I'atented in the United Stares ofAmerica. 13rmpe mid in India.
purity [ 9 9 . 9 O / 0 ] rare earth oxides p,O,, Gd,O,, Sm,O,] BaCO, and HfO, in an
agate mortar with acetone as the mixing medium. The wet ~ o w d e r was dried
in an electric oven at 150°C for 2 h, followed by calcination a t 1200°C in air
for 20 h in an alumina crucible with two intermediate grindings. The calcined
material was powdered thoroughly and pessed in the form of pellets having
diameter l l m m and thickness 1.5 mm by uniaxial pressing at a pressure of
5 ton/cm2. These pellets were sintered at temperature 1450°C for 12 h. This
critical sintering condition was standardised from the results obtained in a
systematic study on the variation of physical properties like sintered density,
electrical resistivity, dielectric constant and dissipation factor of REBa,HfO,,,
samples processed at different temperatures.
IV.3 Crystal Structure of REBa,HfO,,
The crystal structure of REBa,HfO,," was studied by x-ray powder
diffraction method. The x-ray diffraction pattern taken on YBa,HfO,,,
GdBa,HfO,, and SmBa,HfO,, samples sintered at 1450°C for 28 values
between 5" and 90" are shown in Fig.IV.1. The computerised XRD data for
~hcsc materials arc given in Tables IV.1, IV.2 and IV.3 rcspcctivcly. l'hc XRD
patterns and the corresponding data clearly show that all these materials have
* The XI</) doto ond a brief sirmmorj. ofthe sjnthesis of Y H a A m , , . GdHo.HfU,,o~id SrnHo/ffU,, hove beeri accepted in JCPDSjjle prrblished by the Notioriol jisritirte qf Scierice ond l i . chno lo~ , United Srates Deporrrrienr o/Co~rrrrrcrce. Notiorla1 lristi/irte oJSrorlclorrl.s and lechnology, and will nppeor in one ofits jbrthcor~iing edrriorr.
Fig IV.1 X.ray diffraction patterns of single phase (A) YUa,NfO,, (U) GdUa,I-IlO,,, ( C ) SmUa,fIfO,,
* Table 1V.I X-ray diffraction data of sintered YBa,HfO,,,
S.No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1/10
4
6
100
4
3
24
4
5
27
3
10
9
4
9
28
18.310
21.310
30.260
35.620
37.300
43.380
47.440
48.660
53.820
57.710
63.100
71.540
79.480
87.420
hkl
111
200
220
311
222
400
331
420
422
333
440
620
444
642
d (calculated)
4.833
4.186
2.959
2.524
2.416
2.093
1.921
1.872
1.708
1.61 1
1.479
1.329
1.208
1.118
Width
0.285
0.435
0.750
0.270
0.240
0.510
0.212
0.405
0.570
0.345
0.540
0.524
0.285
0.360
d (observed)
4.841
4.166
2.951
2.518
2.408
2.084
1.915
1.870
1.702
1.596
1.472
1.318
1.205
1.115
Table IV.2 X-ray diffraction data of sintered GdBa,Hf06,6 *
-
hkl
111
200
220
311
222
400
331
420
422
440
620
444
642
111,
7
8
100
7
7
33
17
12
38
18
18
16
18
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
Width
0.138
0.135
0.450
0.128
0.139
0.390
0.240
0.143
0.285
0.105
0.195
0.195
0.120
20
18.380
22.220
30.190
35.600
37.200
43.280
47.390
48.630
53.570
62.720
71.340
79.490
87.300
d (observed)
4.823
4.183
2.958
2.520
2.41 5
2.089
1.917
1.871
1.704
1.480
1.321
1.205
1.116
d (calculated)
4.824
4.178
2.954
2.519
2.412
2.089
1.916
1.868
1.705
1.477
1.321
1.206
1.116
Table IV.3 X-ray diffraction data of sintered SmBa,HfO,,, *
-
hkl
111
200
220
311
222
400
331
420
422
333
440
620
444
642
1/10
13
14
100
10
12
31
12
8
28
11
18
15
10
16
-
S.No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
d (observed)
5.202
4.227
2.899
2.635
2.413
2.099
1.945
1.932
1.706
1.538
1.481
1.321
1.205
1.119
d (calculated)
4.847
4.198
2.968
2.531
2.423
2.099
1.926
1.877
1.713
1.615
1.484
1.327
1.211
1.121
-
28
17.030
21.000
30.820
34.000
36.020
43.060
46.670
47.000
53.690
60.110
62.690
71.370
79.500
87.000
Width
0.315
0.300
0.555
0.234
0.280
0.660
0.315
0.324
0.645
0.270
0.375
0.270
0.198
0.134
the same structure as judged by the similarity in their intensities and positions
of the lines on the x-ray powder diffraction patterns. The crystal structure
diagram of REBa,HfO,, is shown in Fig. IV.2. REBa,HfO,, were also found to be
@RE @Ba OHf 00
Fig IV.2. Crystal structure diagram of KEBa,I-If05,
isostructural with other rare-earth cubic perovskites will1 the general forrnula
A,(BB1)Ob such as EuBa,NbOb and YBa,SbOb(l-2) reported in the JCI'DS file,
in which doubling of the basic perovskite unit cell is observed. This doubling
78
is due to the ordering of B and B' on the octahedral sites. In a substitutional
solid solution BB', there is random arrangement of B and B' atoms in equivalent
positions in the crystal structure. If, upon suitable heat treatlnent, the random
solid solution rearranges into a structure in which the B and B' occupy the
same set of positions but in a regular way, the structure is described as a
superstructure (3). In the superstructure, the positions occupied by B and B'
are no longer equivalent and this is exhibited in the XRD pattern by the
presence of superstructure lines. The presence of superstructure lines in the
XRD patterns shown in Fig. IV.1 thus indicates, the ordering of thc basic
ABO, perovskite unit cell in the REBa,HfO,, materials. The XRD peaks
including the minor peaks for REBa,HfO,,, are now indexed for a complex
cubic perovskite s t ructure. T h e lattice constant values calculated for
YBa,HfO,,, GdBa,HfO,, and SmBa,HfO,, con~pounds were a-8.372, 8.364
and 8.396 A respectively.
IV.4 Sintered Density and Resistivity Measurements
The theoretical densities of REBa,HfO,, were calculated from atolnic
masses and crystal structure. The cubic perovskite cell of REha,IliO,,, contains
four unit formulae of atoms. The theoretical densities based on the number of
atolns pcr unit cell were calculated from the l a ~ t i c c constant values.
Experimentally the densities of sintered REBa,IlfO,,, pellets were measured
by Archimedes method and each was found to be about 98% of their theoretical
79
density values. The values of lattice constant, theoretical density and sintered
density are given in Table IV.4. The properties of polycrystalline materials
are highly dependent on their bulk density and hence the sintered density of
the specimen material should be approximately equal to the theoretical density.
The d-c resistivities of REBa,HfO,,, were measured using a d-c
Table IV.4 Lattice constant, theoretical density, sintered density and d-c resistivity of REBa2Hf06,,
electrometer (Keithley 602) and were found to be highly dependant on the
Material
YBa,HfO,,
GdBa2Hf0,,
SmBa,HfO,,
sintering temperatures. The resistivity values at room temperature were found
to bc of the order of 101OR cm for each of REBa,I-IfO,,, pellets and are also
Lattice constant [a:
(4
8.372
8.364
8.396
givcn in l'able IV.4 along with the sintcring temperatures.
IV.5 Measurement of Dielectric Properties
Theoretical density
( gem.')
7.129
7.944
7.759
Dielectric properties give the response of the material medium to various
electric fields passing through them. They are highly dependant on crystal
Sintered density
(gem.')
6.986
7.745
7.588
dc resisistivity
( ~ 1 ~ r n )
5x1 OtO
8x1 OjO
9x1 010
structures and polarisability of the material. In the case of polycrystalline
materials the dielectric properties, in addition, depend on the grain size, grain
boundaries, their distribution and nature of the impurity phases present. I-Ience
it is clear that the dielectric properties measured on polycrystalline materials
with inhomogeneties and other impurities are not the actual dielectric
properties of the material but, a modulated picture of all the effects. O n the
other hand, materials in its single crystal form give more accurate values of
dielectric properties.
Dielectric properties of polycrystalline REBa,MfO,,, were studied in
the frequency range of 30 H z to 13 MHz at room tenlperature and at liquid
nitrogen temperature by using an im~edence analyser. A brief description of
the instrument and the method of measurement of dielectric constant and
dielectric loss factor have been given in chapter 11. The variation of dielectric
constant (E') for sintered REBa,HfO,., pellets with frequency measured at room
temperature is given in Fig. IV.3 and the variation of dissipation factor (tan6)
is given in Fig. IV.4. The values of the dielectric constant were found to be
constant in the KHz to MHz region and the dissipation factor of the materials
were considerably reduced in these frequency ranges for each REBa,HfO,,,
samples. The values of E' in the H z to K H z region were slightly higher and
Fig JY.3 Variation of dielectric constant (E') with frequency for (A) YBa,lIfO,, (D) GdBa,HfO,,(C) SmnBa,HfO,, measured at 300K
z 2 4 - w C
t (IJ w 2 20- 0 0 0 .-
16- 0 a, - a, .- a
12
8
-
I "I
- l a c :
:A
-
I I
2 3 4 5 6 7
Log f
Log f
Fig IV.4 Variation of loss factor (tan 8 ) with frequency for (A) YBa,I-IfO,, (I)) C;cl1%a,II1O5,, (C) StnBa,I IfO,, tncasurcd nt 300K
83
decreased as the frequency increased to MHz region. This slightly high value
of E' at lower frequencies can be attributed to the space charge polarisation
taking place when a time varying electric field passes through the samples.
This space charge polarisation becomes inoperative as the frequency increases.
Since the value of E' is steady from KHz to MHz region the magnitude of the
orientation polarisation and ionisation polarisation is very low. At MHz and
higher frequency region only electronic polarisation mechanism is operative
and its magnitude is relatively large. The dielectric loss factor is a measure of
loss of electrical energy in the dielectric medium. Since the tan6 values for
REBa,IlfO,, samples are very low, these compounds can be considered as
good dielectric materials.
In view of their applications as substrate for YBCO superconducting
films, we have also conducted low temperature dielectric measurements in
REBa,HfO,,, samples. Figure IV.5 gives the variation of dielectric constant
values with frequencies measured at 77K. Figure IV.6 gives the variation of
dielectric loss factor with frequencies at the same temperature. The measured
values of E' at this temperature were almost same as those obtained at room
temperature. In MHz region the tan6 values of REBa,I-IfO,, measured at
77K were found to be relatively lower than that taken at room temperature.
Fig IV.5 Varia~ion of dielectric constant (z') xvirll Ircquc~lcy for (A) Ylla,IITO,, (U) GdUa,I-IIO,, (C) S I~U~, I - I~O, , , measured at 771L
C a 2 4 - w w r= 'sl w cn r 2 0 - 0 0 0 . - L
5 16- a, - a, . - n
12
8
-
- ------ -I,
-
-
c--*-L--.-.----.-L.-. c - @--~.-*. A -
- I I I
2 3 4 5 6 7 Log f
Log f
Fig IV.6 Variation of loss factor (tan 6 ) with frequency for (A) YDa,IIfO 5.5
(I%) Gd~a,I I fO, , (C) SmUa,I IfO,,, mea~ured at 77I<
For a comparitive study, the dielectric constant and loss factor values for
REBa,I-IfO,, measured at 10 MHz at room temperature and at liquid nitrogen
temperatures are given in Table IV.5.
Table IV.5 Dielectric constant (E') and loss factor (tan6)of REBa,HfO,,,
IV.6 Surface Morphology of REBa,HfO,.,
Microstructure of polycrystalline materials are very important to assess
their measured properties. Optical micrographs of sintered and highly polished
samples of REBa,HfO,,, are shown in Fig.IV.7 [A, B, C]. YBa,IlfO,,,
GdBa,I-IfO,,, and SmBa,HfO,,, samples form unifrom patterns with no
Material
YBa,HfO,,
GdBa,HfO,,
SmBa,HfO,,
additional phases. A few white spots in each of the micrographs represent
pores, which agree with the sintered density measurements. Since there are
very few pores combined with uniform distribution of grains as observed in
the micrographs, it gives approximately the true values of material properties
measured on these samples.
tan6 (at 10MHz-77K)
-lxlo4
-3x1 O4
-2x104
d (at lOMHz-300K)
-1 0
-1 9
-1 4
tan6 (at lOMHz300K)
-8x1 O4
-5x1 OJ
-4x1 OJ
E' (at 10MHz-77K)
-9
-1 9
-1 3
Fig. N.7 Optical micrographs of (A) YBa,HfO,, (B) GdBa,HfO,, (C) SmBa,HfO,, (Magnification 800)
IV.7 Chemical Compatibility of REBa,HfO,, with YBa,Cu,O,,
One of the most important criteria for the selection of any material as
substrate for YBCO superconductors is the chemical non-reactivity between
the substrate and the film at the processing temperature(4-9). In order to see
whether REBa,HfO,, is chemically compatible with YBCO, the chemical
reactivity of REBa,HfO,, withYBCO was studied at temperatures upto 950°C.
Superconducting YBCO powder was mixed with REBa,HfO,,, in 1:l volume
ratio and pressed in the form of pellets. The pellets were then annealed a t
950°C for 15 h and cooled slowly. If YBCO reacted with REBa,HfO,,, a t
such annealing conditions, new additional phases besides YBCO and
REBa,HfO,,, could be observed in the x-ray diffraction patterns. O n the other
hand, if YBCO does not react with REBa,HfO,,, the crystalline phases after
annealing will be just two phases of YBCO and REBa,HfO,,,. Powder
diffraction patterns of annealed samples of 1:l volume mixtures of YBCO
and REBa,HfO,, are shown in Fig.IV.8 [A-Dl. X-ray diffraction patterns
of the two phases in the annealed samples [Figs. IV.8(B-D)] were
compared with those of pure YBCO [Fig.IV.g(A)] and pure REBa-I-IfO,,
(Fig.IV.l). It is clear f rom these figures [IV.8(B-D)] that no new
additional phase was formed (within the precision of the XRD
technique) besides YBCO and REBa,HfO,,, in the YBCO-REBa,IlfO,,,
composites. This indicates that there is no detectable chemical reaction
8(r
Fig rV.8 X-ray diffraction patterns of (A) pure YBCO (B) 1:l volume mixture of YBCO and YBa,HfO,, (C) 1:l volume mixture of YBCO and GdBa,IIfO,,, (D) 1:l volume mixture of YBCO and SmBa,IIfO,,,
taking place between YBCO and REBa,HfO,,, when under severe heat
treatment.
T h e effect of REBa,HfO,,, addi t ion on t h e superconduct ing
properties of YBCO was studied by temperature-resistivity measurements.
Figure IV.9 [A-C]. shows temperature versus resistivity curves for the YBCO-
REBa,HfO,, composite system containing 50 vol% of REBa,HfO,, annealed at
950°C for 15 hours.
Fig. IV.9. Temperature-resistivity curves for YBCO-REBa,HfO,,,compositcs
containing 50 vol. O/O of (A) YBa,HfO,,, (B) GdBa,HfO,,, (C) SmBa,HfO,,,
a1
A superconducting transition temperature of 92K in all these composite
samples indicates that a substantial addition of REBa,HfO,, in YBCO did not
have any detrimental effect on the superconducting transition temperature of
YBCO even after severe heat treatment.
IV.8 Discussion
Three new compounds YBa,HfO,,,", GdBa,I-IfO,,," a l ~ d SmBa,HfO,,
have been synthesised, characterised and sintered as single phase materials by
solid state reaction method. X-ray diffraction analysis shows that these
materials are isostructural and have an ordered cubic perovskite structure of
type A,(BB1)06. The presence of superstructure lines in the XRD patterns of
REBa,HfO,,, indicates the ordering of the basic ABO, perovskite unit cell.
Though REBa,HfO,,, materials have the A,(BB1)06 structure, taking into
account the valency of Hf at 4+, the chemical formulae for the compounds are
written as YBa,HfO,,j, GdBa,HfO,, and SmBa,HfO,,. The electrical resistivity
measurements show that these compounds are good insulators. The dielectric
constant and loss factor values of REBa,HfO,, samples are found to be in a
range suitable for their use in microelectronic applications when compared to
those of some conventional ceramic dielectric materials like MgO, SrTiO,
'"~lrblished in Mater. Letters. 11, 278 (1992), 21, 301 (1771) Norh ifollaiid
" Published in Supercond. Sci. Technol. 8,121 (1991) Uti
92
and A1,0, which arc commonly used as substrate materials for the
iabrication of YBCO thick and thin films(l0-17). The IZEBa,HfO,,, was
found t o be chemically compatible with YBCO. A substantial addition of
these tnaterials in YBCO did not show a detrimental effect on the superconducting
properties of YBCO even after a prolonged and severe heat treatment upto 950°C.
O n e of the most unique characteristics which make these materials of
tremendous technological importance is their chemical non-reactivity with
YBCO superconductors even under severe processing conditions, making these
materials suitable as substrates for YBCO superconductors. REBa,HfO,,,
materials are highly stable under atmospheric conditions and no degradation
was observed in the stability of the samples, even if they were kept in boiling
water for 1 h. The samples were mechanically strong and could be sliced into
pieces of 0.5 mm thickness with a diamond cutter. Good reflecting surfaces
were obtained by mechanical polishing. Organic solvents such as alcohol,
acetone and carbon tetrachloride could be used as effective cleaning agents.
I n addition to chemical conlpatibility and iavourable dielectric
properties and phase stability, the lattice matching of REBa,HfO,,, with YBCO
is also an important factor for the epitaxial growth of YBCO films on single
crystal substrates. REBa,HfO,,, has a complex cubic perovskite structure with
lattice constants between 8.364 and 8.396A [Table IV.41. Even though the
lattice matching of these materials with YBCO is not perfect, the latticc
constant values of REBa,HfO,, based on the doubling of the simple perovskite
unit cell are in a range comparable to that of MgO [a-4.208K] which is an
extensively used substrate for epitaxial of YBCO films. As single
crystals are needed for substrate applications in the fabrication of YBCO thin
films, melting experiments were carried out to examine whether REBa,HfO,,,
materials melt congruently for possible single crystal growth from melt. Single
phase REBa,HfO,,, was placed in a platinum crucible and the material was
melted completely in air at a temperature of 1750 3C using a muffle furnace.
The melted samples were withdrawn from the furnace and quenched in air to
the room temperature. The quenched samples were then examined by x-ray
diffraction. The XRD patterns were found to be identical tothose of sinterd
samples indicating that REBa,HfO,,, materials melt congruently and could be
grown from the melt as single crystals.
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