Dopant Segregation and Space Charge Effect in Nanostructured
Tetragonal Zirconia
F. Boulc’h, E. Djurado,
z
and L. Dessemond
Laboratoire d’Electrochimie et de Physico-chimie des Mate ´riaux et des Interfaces,
38402 Saint-Martin d’He `res Cedex, France
Dense, tetragonal single-phased and nanocrystalline zirconia ceramics containing 3 mol % R
2
O
3
without impurities and with
average grain sizes of 60 nm were prepared. Electrical properties were investigated by impedance spectroscopy as far as the ionic
radius is increased from 0.098 to 0.109 nm, respectively, from Yb
3+
,Y
3+
, Gd
3+
, to Sm
3+
. Conductivity variations were
attributed to an effect of R
2
O
3
dopant. Segregation phenomenon was demonstrated by comparison of these electrical data with
those of air quenched samples and was fully related to the dopant ionic radius size. The grain boundary resistance and the width
of the space charge layer were consequently increased while the bulk resistance was not changing and was in the same order of
magnitude compared to the microcrystalline zirconia.
© 2004 The Electrochemical Society. DOI: 10.1149/1.1764711 All rights reserved.
Manuscript submitted November 4, 2003; revised manuscript received January 12, 2004. Available electronically June 30, 2004.
High ionic conductivity, good mechanical properties, and long
term stability are basic requirements for application as solid electro-
lyte in solid oxide fuel cells SOFCs operating at intermediate tem-
perature IT-SOFCs from 400 to 700°C. Tetragonal ZrO
2
TZP,
which is stabilized in small grains and for low R
2
O
3
dopant contents
presents a large potential
1
for making progress in the development
of electrolytes for IT-SOFCs. Indeed, TZP is known to exhibit good
mechanical properties.
2,3
Recently, long-term stability has been
demonstrated at 700°C in humid atmosphere.
4
Moreover, enhanced
ionic conductivity has been suggested when grain size is decreased
to 300 nm and for a lower yttria content.
5
Nanostructured materials are distinguished from conventional
polycrystalline materials by the size of structural units and by their
different properties.
6,7
In many cases, this is the result of a greater
number of grain boundaries where defect formation energies are
decreased as shown by Gibbs.
8
For instance, the solubility of impu-
rities and dopants can be markedly enhanced in nanocrystalline sol-
ids as a result of grain boundary and surface segregations.
9
The
segregation enthalpy ( H
s
) can be defined as a sum of four
contributions
10
H
s
= H
+ H
+ H
+ H
with an interface energy contribution H
, a contribution due to
solute-solvent interaction H
, an elastic strain energy contribution
H
, and an electrostatic term H
.
Until now, the driving force for the grain boundary segregation
has been debated, especially in oxide ceramics.
10
Aoki et al.
11
have
shown that calcium segregation in CaO-stabilized ZrO
2
is grain size
dependent and strongly correlated with silicon segregation. Segrega-
tion can be driven by Coulombic interactions, as it was demon-
strated in gadolinium-doped cerium oxide by Tschope et al.
12
Elas-
tic driving forces can be significant for segregation phenomena
when a large size mismatch exists between dopant and host cations
as reported by Kilner.
13
The aim of this work is to study the segregation phenomenon in
nanostructured tetragonal zirconia ceramics, free of silicon and
doped by trivalent rare-earth cations (R
3+
). The influence of the
Zr
4+
substitution by a constant amount of 3 mol. % oversized R
3+
dopant such Yb
3+
,Y
3+
, Gd
3+
, and Sm
3+
on electrical properties
and especially on the grain boundary blocking effect in nanostruc-
tured ZrO
2
ceramics was investigated using impedance spectros-
copy. The doping with trivalent cations does not eliminate the Cou-
lombic segregation driving force which still plays a role in the
investigated segregation behavior. This force is identical for all
selected dopants. Therefore, any segregation difference mainly
originates from ion size mismatch. Quenching experiments were
performed to investigate the role of the dopant size on the segrega-
tion phenomenon.
The second aim of this study was to provide new experimental
data on the effect of space charge on the electrical properties of the
bulk and the grain boundary of nanostructured pure ionic conductor.
Moreover, a comparison with conventional polycrystalline ceramics
has been carried out. All ceramics will be referred to as xR-TZP
where x mol. % is the amount of R
2
O
3
.
Experimental
Physico-chemical characterizations of nanocrystalline 3R-TZP
ceramics.—3R-TZP, 2.5Y-TZP, and 3.5Sc-TZP nanocrystalline
powders were produced by a spray-pyrolysis process using a 1.7
MHz ultrasonic atomizer.
14
Single-phased tetragonal ceramics were
prepared by isostatic pressing at 300 MPa and sintering at 1500°C
for 2 h in air. Oversized R
3+
dopants from Yb
3+
,Y
3+
, Gd
3+
to
Sm
3+
were selected due to their lower valence ( +3) compared to
Zr
4+
and due to a systematic increase of their ionic radii, respec-
tively, ranging from 0.098 to 0.109 nm.
Two firing cycles were carried out for air quenched samples.
First, the pellets were heated to 1500°C for 2 h at a heating rate of
10°C min
-1
and then cooled to room temperature at 10°C min
-1
.
The pellets were subsequently heated to a temperature lower or
equal to 1500°C and then air-quenched at 6500°C min
-1
. Single-
phased tetragonal 2.5Y-TZP ceramics were successively air-
quenched from different temperatures: 1000, 1300, and 1500°C and
3Sm-TZP and 3.5Sc-TZP pellets from 1500°C.
X-ray diffraction XRD was carried out at room temperature
using a Siemens D500 diffractometer equipped with a linear detector
Cu K radiation, = 1.54 Å, 0.04°, 2 steps, 5 s counting time.
The average grain size was calculated using the Scherrer formula,
corrected with silicon for the 111 peak of tetragonal zirconia. Po-
sition and full-width at half-maximum fwhm of XRD peaks were
determined by a deconvolution of pseudo-Voigt-shaped peaks with
Profile fitting software Diffract-at, Socabim, Paris. Rietveld refine-
ments were carried out using FULLPROF software. The cell param-
eters a and c were refined in P4
2
/nmc space group for the tetrag-
onal form. Raman spectroscopy was performed using a DILOR XY
confocal Raman spectrometer with the 514.5 nm line of an argon ion
laser.
The microstructure of the disks was examined using scanning
electron microscopy SEM, Jeol JSM-35 before and after air
quenching. Nitrogen adsorption/desorption experiments were per-
formed at -196°C using a static volumetric method Micrometric
ASAP 2010 to determine pore size. Nitrogen adsorption/desorption
was carried out within the relative pressure range 10
-7
p/p
0
1. Before measurements, samples were cleaned in vacuum at
z
E-mail: Elisabeth.Djurado@lepmi.inpg.fr
Journal of The Electrochemical Society, 151 8 A1210-A1215 2004
0013-4651/2004/1518/A1210/6/$7.00 © The Electrochemical Society, Inc.
A1210