Correlation between microscopic and macroscopic

Solid State Ionics 136–137 (2000) 107–113
www.elsevier.com / locate / ssi
Correlation between microscopic and macroscopic properties of
yttria stabilized zirconia
1. Single crystals
a,
b
c
d
b
M. Hartmanova *, J. Schneider , V. Navratil , F. Kundracik , H. Schulz ,
E.E. Lomonova e
a
Institute of Physics, Slovak Academy of Sciences, 84228 Bratislava, Slovak Republic
b
Institute of Crystallography and Mineralogy of LMU, 80333 Munich, Germany
c
Department of Physics, Faculty of Education, Masaryk University, 60300 Brno, Czech Republic
d
Department of Physics, Faculty of Mathematics and Physics, Comenius University, 84215 Bratislava, Slovak Republic
e
General Physics Institute, Russian Academy of Sciences, 117942 Moscow, Russia
Abstract
This paper reports on the results of the research on the yttria-stabilized zirconia single crystals and on the defects obtained
in the wide range of yttria amounts, 1.13 , x , 33.7 mol%. The correlation among phase composition, electrical conductivity
and microhardness is described and discussed.  2000 Elsevier Science B.V. All rights reserved.
Keywords: Yttria-stabilized zirconia; Single crystals; Polymorphism; XRD; Electrical conductivity; Microhardness
1. Introduction
Zirconia (ZrO 2 ) is one of the best examples in
materials science where structure, microstructure,
defects and phase composition / transformations are
intimately connected with macroscopic properties
such as, for instance, electrical conductivity and
microhardness. It is well known that ZrO 2 can
crystallize in the different phases depending on the
temperature and doping amount (e.g. [1]). These
phases can be partially (PSZ) or fully (FSZ) stabilized at room temperature by the addition of suitable
dopants, e.g. yttria (Y 2 O 3 ). However, even in such
*Corresponding author. Fax: 1421-7-54776085.
E-mail address: [email protected] (M. Hartmanova).
well-studied systems, there has been considerable
confusion as to the exact location of phase
boundaries. This confusion probably arises as a
consequence of differences in materials, chemical
homogeneity and experimental techniques applied. In
general, the PSZ usually consists of two or more
phases (cubic, tetragonal, monoclinic). PSZ has
remarkable advantages over FSZ as to its mechanical
properties. In contrast to this fact, the electrical
properties of PSZ are rather poor in most cases as
compared to those of FSZ. At the present stage of
knowledge, a rather vague compromise between
doping, processing, structure and properties has to be
chosen for the practical device design. A better
understanding of the correlation between microscopic structure and macroscopic properties is obvi-
0167-2738 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved.
PII: S0167-2738( 00 )00358-1
108
M. Hartmanova et al. / Solid State Ionics 136 – 137 (2000) 107 – 113
ously needed for the improvement of device parameters.
The purpose of the present more detailed study of
zirconia polymorphism is based on the interest in a
better understanding of the correlation between
crystallographic structure and electrical conductivity
or microhardness of YSZ (and mainly PSZ) crystals,
respectively in the low-temperature region. This
correlation is to be compared with that expected for
the same compositions of YSZ (PSZ) deposited as
thin films by electron beam evaporation and magnetron sputtering methods in another work in preparation.
2. Experimental
The single crystals of ZrO 2 1 x ? Y 2 O 3 (x 5 1.13,
1.25, 3.24, 4.31, 9.21, 12.3, 16.3 and 33.7 mol%)
were grown by the directional solidification of melt
using the direct rf heating in a water-cooled crucible
(a skull melting technique) [2]. The crystals obtained
are milky (more or less), clear-colourless and clearlight brown coloured in dependence on the amount
of the dopant (stabilizer). The analytical determination of impurities in the initial materials, the
evaluation of their redistribution during the crystal
growth and their average concentration in the investigated samples were carried out by means of inductively coupled plasma atomic emission spectroscopy.
Zr, Y and O atoms were uniformly distributed
through the samples. In addition to these atoms, Ca,
Si, Mg, Nb and Sr atoms were found as impurity
background ( # 0.1).
To obtain the lattice parameters (the cell dimensions) and the phase composition the Rietvald analysis of XRD data was performed. This analysis can
refine the structural parameters through a leastsquare method using the entire powder pattern which
includes the information on the XRD integrated
intensities and peak positions. The single crystals
were milled into the powders, which then were well
mixed with an internal standard silicon powder
(99.999% pure, a 5 0.543090 nm) for angular calibration. The XRD profiles of the mixed samples
were collected at room temperature by using a
powder XRD diffractometer (Model STOE Transmission Diffractometer with the sample holder for
flat transmission samples). The results obtained from
Mo Ka1 -transmission data are shown in Table 1.
AC conductivity measurements were performed by
an impedance technique in air. A Solartron SI 1260
impedance / gain phase analyzer interfaced to a computer and run through a Lab-view program was used.
The impedance measurements were made in the
frequency range 10 Hz–1 MHz and at temperatures
180–5208C. The temperature was stabilized by a
Chinoterm 10 A digital temperature controller with
the accuracy of 60.58C. The platinum paste electrodes were applied to the entire faces of a sample.
The measurements were analysed according to the
method of electrical equivalent circuit [3].
Vickers microhardness testing was used to explore
the hardness indentation deformation. Vickers microhardness value H was obtained as the ratio of the
load applied to the area of the resulting indentation.
The microhardness H, related to the given pyramid
geometry, is expressed by the relation H 5 2 ? cos
228(L /d 2 ), where L is the load applied and d is the
indentation diagonal. As the geometry of the indentation is independent on its size, in principle, the
microhardness H should be independent of the load
applied. In practice, there is a load dependence of
microhardness, particularly for small loads (indentation-size effect-ISE) [4,5]. That is why we can
compare the microhardness of various materials only
beyond the ISE region where H 5 H (L) is a
constant. The microhardness H measurements were
performed in air at room temperature by means of a
Hanneman (Vickers) microhardness tester along with
a Zeiss–Neophot microscope. Specimen indentations
were made with ten loads ranging 0.1–1.1 N and five
impressions were made at each load. Both diagonals
were measured to estimate the influence of substrate
as well as the asymmetry of the diamond pyramid
(every diagonal was measured at least five times).
3. Results and discussion
3.1. Phase composition and structural parameters
Using compositionally homogeneous samples, the
structural changes of zirconia (ZrO 2 ) via doping with
yttria (Y 2 O 3 ) have been investigated by Rietveld
analysis (program WYRIET [6,7]) of powder X-ray
M. Hartmanova et al. / Solid State Ionics 136 – 137 (2000) 107 – 113
109
Table 1
Lattice parameters and phase compositions of ZrO 2 single crystals doped with various concentrations of Y 2 O 3
Composition (mol%Y 2 O 3 )
1.13
Phase (s)
Lattice parameters (nm)
Phase composition (wt.%)
Monoclinic
a50.51632 (6)
b50.52082 (7)
c50.53245 (6)
b 599.103 (8)
a50.3598 (2)
c50.5197 (5)
a50.51614 (3)
b50.52093 (3)
c50.53241 (3)
b 599.121 (3)
a50.3612 (1)
c50.5183 (3)
a50.3619 (3)
c50.5159 (5)
0.51347 (5)
96.3 (2.4)
a50.36081 (3)
c50.51706 (6)
0.51453 (2)
0.51562 (1)
0.51666 (1)
0.52216 (1)
40.2 (3.4)
1
Tetragonal
1.25
Monoclinic
1
Tetragonal
3.24
Tetragonal
4.31
Cubic
1
Tetragonal
9.21
12.3
16.3
33.7
Cubic
Cubic
Cubic
Cubic
diffractometry (XRD) data. This technique is very
effective for the structural analyses of complicated
powder diffraction data. The weight percent of the
corresponding crystallographic phases of YSZ (phase
composition) together with the corresponding lattice
parameters can be found in Table 1. According to the
literature, the undoped ZrO 2 is monoclinic and single
phase (e.g. [8]). As it is shown in Table 1, the
samples investigated in the present work, ZrO 2 1x?
Y 2 O 3 are mainly monoclinic with a small amount of
tetragonal form, a mixture (m1t) phase, in the
compositional region x51.13–1.25 mol%. Samples
with x53.4 mol% are tetragonal single phase. The
further increasing of amount of yttria in ZrO 2 results
in the transformation of some of the tetragonal single
phase to the cubic phase, a mixture (c1t) phase,
what is a case of sample with x54.31 mol%. The
samples with x59.21–33.7 mol% exhibit cubic
single-phase structure. The compositional dependence of the lattice parameter can be attributed to the
ordering effect of oxygen vacancies, in agreement
with the behaviour of electrical conductivity (Section
3.2) and microhardness (Section 3.3).
3.7 (0.3)
94.6 (1.1)
5.4 (0.2)
59.8 (2.6)
3.2. Electrical conductivity: correlation with
structural changes
The structure–conductivity relation has been investigated extensively for the fluorite-related structures, because the oxygen ion conductivity is strongly dependent on the existing phases and their crystal
structures (e.g. [9]). The stabilized zirconia solid
solutions conduct ionically over a wide range of
temperatures and oxygen partial pressure. The defects such as oxygen vacancies generated by the
presence of substitutional Y 31 ions and the association of defect complexes play a significant role in the
ionic conduction.
The effect of a wide range of yttria additions on
the electrical conductivity s and activation energy Ea
was studied on the single and mixed phase compositions. The purpose was to compare this knowledge to that found for the systems (compositions)
investigated in the form of thin films (in preparation). The results obtained are shown in the form of
Arrhenius plots (Fig. 1), isothermal conductivity s
vs. yttria content x dependencies (Fig. 2) as well as
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M. Hartmanova et al. / Solid State Ionics 136 – 137 (2000) 107 – 113
Fig. 1. Arrhenius plots of ac conductivity s of zirconia single crystals doped with various amounts of Y 2 O 3 : (m) 1.13 mol%, (쏻) 1.25
mol%, (j) 3.24 mol%, (s) 4.31 mol%, (d) 9.21 mol%, (h) 12.3 mol%, (.) 16.3 mol% and (♦) 33.7 mol%, measured in air.
Fig. 2. Isothermal dependencies of conductivity s of YSZ on the amount of Y 2 O 3 in the low-temperature region: (.) 3208C, (m) 3608C,
(d) 4228C and (j) 5038C.
in the form of a change of the activation energy Ea in
dependence on the mol% Y 2 O 3 (Fig. 3). In each
case, the effect of yttria content in the zirconia by
means of the phase composition changes is seen to
have a detrimental influence on the electrical parameters and not only on them (see Section 3.3). As it is
possible to see in Figs. 1 and 2, the small additions
of yttria enhance the conductivity starting from the
M. Hartmanova et al. / Solid State Ionics 136 – 137 (2000) 107 – 113
111
Fig. 3. Influence of the amount of Y 2 O 3 on the activation energy Ea of YSZ in the low-temperature region.
mixed (m1t) phase composition, 1.13–1.25 mol%
Y 2 O 3 , through (t) single phase, 3.41 mol% Y 2 O 3 up
to the highest conductivity observed in our case for
the (t1c) mixed phase composition, 4.31 mol%
Y 2 O 3 . Further increase of yttria content in ZrO 2 ,
x.4.31 mol% Y 2 O 3 , results in the presence of (c)
single phase composition (Table 1) and a decrease of
conductivity. The increase of conductivity in the
compositional region 1.13,x,4.31 mol% Y 2 O 3 is
probably due to an increase of the fraction of (t)phase with higher conductivity in comparison with
that of the (m)-phase (Table 1). In the (c) single
phase region, the electrical conductivity decreases
starting at |9.21 mol% Y 2 O 3 and proceeding up to
the highest yttria content used, 33.7 mol% (Figs. 1
and 2). This observed decrease in conductivity is
probably attributable to the decrease of free oxygen
vacancy concentration in the anion sublattice
Y x Zr 12x O 22x / 2 solid solution. Several hypotheses
related to the increased interactions between dopant
cations and vacancies at low temperatures (e.g. [10])
as well as different models to explain the conductivity decrease behaviour observed in zirconiabased electrolyte systems have been proposed. For
instance, Strickler and Carlson [11] suggested that
the decrease in conductivity can be attributed to 1)
the interaction or clustering of the vacancies, 2)
ordering of the vacancies and 3) the formation of a
secondary phase. Catlow [12] suggested that the
clusters provided deeper traps for the vacancies,
resulting in a reduction in vacancy mobility. Thus,
despite the increasing number of vacancies when the
dopant concentration is raised, the vacancy mobility
will decrease and the latter factor starts to dominate
at sufficiently high concentrations leading to the
decrease of conductivity.
In general, the isolated oxygen vacancies (V O?? )
control the bulk ionic conductivity of the lightly
doped samples (in our case the single crystals doped
with 1.13 up to |9.21 mol% Y 2 O 3 ) while the
conductivity of the heavily doped samples (in our
case 12.3 and 16.3 mol% Y 2 O 3 ) is consistent with
transport properties controlled by the associated
9 V ??O )?. The complexes (Y 9ZrV O?? ). are
point defects (Y Zr
dominant defects at low temperatures (,5608C). In
the case of the highest Y 2 O 3 content used, 33.7
mol%, the formation of clusters, aggregates up to the
secondary phase of yttria is possible; according to
our XRD investigation, no remarkable amount of
Y 2 O 3 secondary phase was found in this sample. The
conditions under which the defect interactions become significant in the fluorite oxides are found in
the detailed study of Nowick et al. (e.g. [13]).
A change in the slope of Arrhenius plots was
observed after changing the phase composition related to the yttria contents in ZrO 2 (Figs. 1 and 3).
112
M. Hartmanova et al. / Solid State Ionics 136 – 137 (2000) 107 – 113
For T ,5608C, the activation energy Ea has a
substantial contribution from the energy required to
dissociate the dopant–vacancy cluster. The formation
of defect complexes is to be expected when the
concentration of point defects increases. This is
confirmed by the highest amounts of yttria used
(Figs. 1 and 2). Indeed, in this case the amount of
oxygen vacancies (V ??O ) and Y 31 ions at zirconia
sites is high enough to generate the formation of
9 V ??O )? over the
dopant–vacancy associations (Y Zr
whole temperature range considered. For the lower
amounts of yttria, e.g. 3.24 mol%, the amount of
oxygen vacancies is not large enough for the concentration of complex defects to be significant.
Qualitatively, the number of associated defects increases with the increasing amount of the dopant and
attempts made to explain this effect using models
based on the defects clusters were already mentioned
above. In the measured low-temperature region
(extrinsic conductivity), there was a clear trend such
that the calculated activation energy Ea decreased
with increasing the small amounts of Y 2 O 3 , 1.13,
x,4.31 mol%. The minimum of Ea was observed at
the (t) single phase composition, x(3.24 mol%
Y 2 O 3 . In the region of (t1c) mixed phase composition and (c) single phase, i.e. at 4.31,x,33.7
mol%, the increase of Ea is gradually reaching the
saturated value at x(16.3 mol% Y 2 O 3 . The values
obtained are in a good agreement with the data from
literature (e.g. [14]).
3.3. Microhardness as a function of phase
composition
The microhardness H of YSZ samples obtained at
the load L51.1 N applied on the indentor is shown,
as a function of various yttria contents in ZrO 2 and
thereby of the different phase compositions, in Fig.
4. In general, the microhardness H of partially
stabilized zirconia depends on the content and the
type of the stabilizing agent used (e.g. [15]). In our
measurements, we can see (Fig. 4) that in the (m1t)
mixed phase composition region the microhardness
H increases with increasing amount of yttria present
in ZrO 2 . In our case, the maximum value of microhardness H was reached at x(4.31 mol% Y 2 O 3 ,
i.e. in the (t1c) mixed phase composition. In the (c)
single phase region, i.e. at the x$12.3 mol% Y 2 O 3 ,
the value of microhardness H is not dependent on
the amount of yttria dopant. It was very difficult to
measure the microhardness H of the sample with
x516.3 mol% Y 2 O 3 due to the poor-quality surface
Fig. 4. Microhardness H of zirconia single crystals as a function of the Y 2 O 3 amount, measured at room temperature, in air and at L51.1 N.
M. Hartmanova et al. / Solid State Ionics 136 – 137 (2000) 107 – 113
113
area of the sample; this is probably the reason for the
H value not consistent with the other H values of
samples with the higher amounts of Y 2 O 3 used.
targets with well defined phase compositions during
the preparation process of YSZ thin films from these
single crystals.
4. Conclusions
Acknowledgements
Experiments have been performed with zirconia
single crystals doped with yttria from a wide range
of amounts, 1.13,x,33.7 mol%. This has allowed
us to specify the phase compositions and the defects
obtained under the used experimental conditions. In
this way we have obtained the basis for the investigation of equivalent YSZ systems (compositions) prepared in the form of thin films. The
experiments have provided valuable data:
The work was partially supported by the research
grants No. 2 / 5081 / 98, 1 / 4253 / 97 of the Slovak
Grant Agency and No. 106 / 96 / 0322 of the Grant
Agency of Czech Academy of Sciences. One of the
authors (M.H.) thanks the Deutscher Akademischer
Austauschdienst for providing a fellowship, during
which period the XRD measurements were performed.
• XRD results of all YSZ samples indicate the
polymorphous structure in dependence on the
amount of yttria dopant present in ZrO 2 : (m1t)
mixed phase composition for x51.13, 1.25 mol%
Y 2 O 3 , (t) single phase for x53.24 mol% Y 2 O 3 ,
(t1c) mixed phase composition for x54.31
mol% Y 2 O 3 and (c) single phase for x59.21–
33.7 mol% Y 2 O 3 ,
• Electrical conductivity s of investigated systems
exhibits its maximum value close to the tetragonal–cubic phase boundary, in our case at x(4.31
mol% Y 2 O 3 ,
• The most expressive change in the slope of
Arrhenius plots, minimum value of the activation
energy Ea , was observed at x(3.24 mol% Y 2 O 3 ,
(t) single phase region
• Behaviour of microhardness H as a function of
yttria content in ZrO 2 was found to be in agreement with the estimated phase compositions and
defects obtained; maximum value of H was
observed in the (t1c) mixed phase composition
region, at x(4.31 mol% Y 2 O 3 .
References
In general, a careful control of the phase composition in YSZ can result in the excellent properties
reported in the literature. However, the question is to
what extent is it possible to maintain the phase
composition of the systems (single crystals) as the
[1] M. Kuwabara, T. Murakami, M. Ashizuka, Y. Kubota, T.
Tsukidate, J. Mater. Sci. Lett. 4 (1985) 467.
[2] V.I. Aleksandrov, V.V. Osiko, A.M. Prochorov, V.M. Tatarintsev, Uspekhi Khimii 47 (1978) 385.
[3] B.A. Boukamp, Equivalent Circuit (EQUIVCRT.PAS) users
manual, University of Twente, Department of Chemical
Technology, The Netherlands, 1989.
[4] F.G. Yost, Metall. Trans. 14 (1981) 947.
[5] V. Navratil, V. Stejskalova, Phys. Stat. Sol. A 157 (1996)
339.
[6] J. Schneider, Proc. Int. Workshop on Rietveld method,
Petten, Netherlands, 1989, p. 71.
[7] J. Schneider, R.E. Dinnebier, Mater. Sci. Forum 79–82
(1991) 177.
[8] M. Yashima, K. Ohtake, M. Kakihana, H. Arashi, M.
Yoshimura, J. Phys. Chem. Solids 57 (1996) 17.
[9] E.C. Subbarao, H.S. Matti, Solid State Ionics 11 (1984) 317.
[10] J.A. Kilner, R.J. Brook, Solid State Ionics 6 (1982) 237.
[11] D.W. Strickler, W.G. Carlson, J. Am. Ceram. Soc. 48 (1965)
286.
[12] C.R.A. Catlow, in: T.O. Sorensen (Ed.), Nonstoichiometric
Oxides, Materials Science Series, Academic Press, New
York, 1981, p. 61.
[13] R. Gerhardt-Anderson, W.K. Lee, A.S. Nowick, J. Phys.
Chem. Solids 48 (1987) 563.
[14] M. Filal, C. Petot, M. Mokchah, C. Chaeau, J.L. Carpentier,
Solid State Ionics 80 (1995) 27.
[15] M. Hirano, S. Watanabe, E. Kato, Y. Mizutani, M. Kawai, Y.
Nakamura, Solid State Ionics 111 (1998) 161.