Nanostructured spark plasma sintered Ce-Tzp ceramics

Article abstract of DOI:10.1111/j.1551-2916.2011.04978.x

In this work, spark plasma sintering (SPS) of 10 mol% CeO₂-doped ZrO₂ nanocrystalline powders, obtained by a two-step synthesis procedure, allows the preparation of fully densified nanostructured ceramics. The CeO₂-ZrO₂ powders with particle size below 100 nm are obtained after CeO₂ deposition on hydrothermally synthesized ZrO ₂ particles by the impregnation method. Tetragonal CeO ₂-ZrO₂ ceramics are obtained when sintering at 1200°C without holding time. A graded material containing tetragonal, monoclinic, and pyrochlore phases are obtained when sintering at 1200°C and for 5 min holding time. This is explained in terms of the gradual reduction of Ce ⁴⁺ to Ce³⁺ species by carbon in the graphite environment during SPS. With the successful combination of the stabilizer coating technique and SPS, we achieve not only the stabilization of the tetragonal phase in the ceramics, but also good control of the grain size, by producing nanostructured ceramics with 40-70 nm grain sizes.

Full text of DOI:10.1111/j.1551-2916.2011.04978.x

J. Am. Ceram. Soc., 95 [3] 901–906 (2012)
DOI: 10.1111/j.1551-2916.2011.04978.x
©
2011 The American Ceramic Society
ournal
J
Nanostructured Spark Plasma Sintered Ce-TZP Ceramics
‡
‡,†
§
‡
Sylvia A. Cruz, Rosalıa Poyato, Francisco L. Cumbrera, and Jose A. Odriozola
´
‡
Instituto de Ciencia de Materiales de Sevilla, Universidad de Sevilla-CSIC, Avda, Ame
´
rico Vespucio 49, 41092 Sevilla, Spain
§
Departamento Fı
´
sica de la Materia Condensada, Universidad de Sevilla, 41080 Sevilla, Spain
1
4,15
In this work, spark plasma sintering (SPS) of 10 mol% CeO
doped ZrO nanocrystalline powders, obtained by a two-step
2
-
advantages as simple processing and equipment.
The pro-
2
cessing of very fine-grained, even nanostructured, ceramics
requires, besides the use of nanometric polycrystalline pow-
ders, the use of sintering techniques that allow densification
while retaining grain size. To overcome the problem of grain
growth, unconventional sintering and densification tech-
niques have been proposed. These include the use of grain
synthesis procedure, allows the preparation of fully densified
nanostructured ceramics. The CeO –ZrO powders with parti-
2
2
cle size below 100 nm are obtained after CeO
hydrothermally synthesized ZrO particles by the impregnation
method.Tetragonal CeO –ZrO ceramics are obtained when
2
deposition on
2
2
2
1
6
17,18
sintering at 1200°C without holding time. A graded material
containing tetragonal, monoclinic, and pyrochlore phases are
obtained when sintering at 1200°C and for 5 min holding time.
growth inhibitors or high-pressure densification.
How-
ever, although the nanometric grain size is achieved, second-
ary phases can be developed in the ceramic in the former
case, and extreme pressures as high as 8 GPa are needed in
the latter. The spark plasma sintering (SPS) method has
emerged as an effective technique for the processing of fully
densified nanostructured materials. In this technique, the
simultaneous application of pressure and electrical current
through the sample during heating allows fast densification
4
+
This is explained in terms of the gradual reduction of Ce
Ce
to
species by carbon in the graphite environment during
3+
SPS. With the successful combination of the stabilizer coating
technique and SPS, we achieve not only the stabilization of
the tetragonal phase in the ceramics, but also good control
of the grain size, by producing nanostructured ceramics with
1
9–21
40–70 nm grain sizes.
of ceramics at relatively low temperatures and pressures.
In the last few years, few attempts of processing CeO2–
ZrO solid solution or related systems by means of SPS have
2
I. Introduction
16,22,23
been carried out.
Ce-TZP is the susceptibility to reduction of CeO
The main problem of SPS on
2 3
to Ce O
ETRAGONAL zirconia polycrystalline (TZP) ceramics have
excellent mechanical properties and attractive ionic con-
2
T
in vacuum or low oxygen partial pressure atmospheres,
which has an effect on the stability of the tetragonal zirconia
phase at room temperature. During SPS, a strongly reducing
atmosphere is created around the powder by the graphite die
and punches. Different studies have shown the catastrophic
degradation and cracking of samples when sintering in hot
ductivity, which allows their use in very different fields, such
as structural applications, fuel cell technologies, gas sensors,
1–7
or solid-state electrolytes.
Enhancements of electrochemi-
cal properties, and mechanical properties, such as plasticity,
4
5
6
7
wear resistance, and fracture toughness, have been reported
for doped-ZrO ceramics when decreasing the grain size.
In the last years, increasing attention has been paid to
2
2–25
2
pressing or graphite furnace.
Huang et al. have reported
the formation of macrocracks in 12 mol% CeO –ZrO
2
2
CeO -doped zirconia, due to its excellent hydrothermal sta-
2
bility, which makes it an interesting material to be used as
ceramics prepared by spark plasma sintering of commercial
co-precipitated powder at 1450°C for 2 min.
Xu et al. have reported the partial retention of tetragonal
2
3
8
,9
catalyst,
which makes it of considerable interest for advanced struc-
and remarkably high transformation toughness,
phase in 8 mol% CeO -doped ZrO , spark plasma sintered
2
2
1
0,11
tural applications.
mechanical properties when decreasing grain size has been
reported for this material. Wang et al. have reported that
both the fracture strength and fracture toughness increase
when decreasing grain size from 15 to 2 lm, for the composi-
In addition, an enhancement of
2
at 1300°C without holding time. Besides the t-ZrO phase, a
66 vol% monoclinic phase and a trace amount of a Zr–Ce–O
cubic solid solution were found. However, the nanostruc-
ture was not achieved in this study. A graded microstructure
1
1
22
has been reported in tetragonal 1 mol% Y
CeO -co-doped ZrO with 2 wt% Al as sintering additive,
spark plasma sintered at 1450°C for 2 min,
2 3
O + 6 mol%
tion containing 12% CeO
enhancement in mechanical properties for nanostructured
2
. However, despite the expected
2
2
2 3
O
1
7,23
with grain
1
2,13
CeO –ZrO ceramics, the published studies are scarce.
sizes of 0.44 lm in the core and 0.17 lm in the edge of the
ceramic. This microstructural gradient, together with the
2
2
The properties of sintered ceramics are highly dependent
on the initial powder properties as well-demonstrated in liter-
ature. Conventional synthesis routes, such as solid-state, ball
milling, sol–gel, combustion, or co-precipitation, have been
the most widely applied methods for the preparation of
increased t-ZrO
has been related to the partial reduction of CeO2 to Ce O
2
phase transformability found in the ceramic,
2
3
during sintering. When sintering at 1550°C, elongated grains
with CeAl11 18 composition have been observed in the sur-
O
CeO
2
–ZrO
2
solid solution nanocrystalline powders. Recently,
face region. The presence of this phase has been related to
the reduction of CeO2 to Ce O , as it is only found in the
nonconventional methods, such as microwave-induced com-
bustion or suspension drying, have been reported, with
2
3
more reduced edge layer.
In the present study, nearly fully densified nanostructured
0 mol% CeO -stabilized tetragonal ZrO was processed by
1
2
2
H.-E. Kim—contributing editor
means of a combination of a two-step powder synthesis route
and SPS. The effects of the SPS sintering conditions on the
phase composition and microstructure of the ceramics were
investigated. The formation of a graded microstructure and a
Manuscript No. 29718. Received May 11, 2011; approved October 29, 2011.
Author to whom correspondence should be addressed. e-mail: rosalia.poyato@
pyrochlore phase was discussed and was related to the CeO2
†
reduction.
icmse.csic.es.
901

902
Journal of the American Ceramic Society—Cruz et al.
Vol. 95, No. 3
II. Experimental Procedure
1) Powder Synthesis and Characterization
2
-stabilized ZrO powder was
without any texture correction. Subsequently, the texture
correction was included using the March–Dollase func-
tion.
(
27
A quantity of 10 mol% CeO
2
Raman spectra were recorded on fracture surfaces using a
dispersive microscope (Horiba Jobin Yvon LabRam HR800,
Kyoto, Japan), with a 20-mW He-Ne green laser (532.14 nm),
without filter, and with a 600 g/mm grating. The microscope
used a 509 objective and a confocal pinhole of 100 lm. The
Raman spectrometer was calibrated using a silicon wafer.
Microstructural investigation was performed by scanning
electron microscopy (Hitachi S4800 SEM-FEG, Tokyo,
Japan).
obtained by a two-step synthesis procedure. In a first step, a
hydrothermal route was followed to synthesize nanocrystal-
line ZrO powder. Zirconyl nitrate hydrate (ZrO
2
NO )·xH O, 0.5 M) and sodium hydroxide (NaOH, 0.5 M)
2
(
were mixed together and sonicated for 30 min. Later, 10 mL
3
of the solution was disposed in the hydrothermal vessel with
2
2
mL of ethanol, closed and heated in a furnace for 5 h at
00°C. When the vessel achieved room temperature, the solid
was washed with distilled water and dried at 100°C for 1 h.
In a second step, cerium oxide was deposited onto the ZrO2
particles by the impregnation method. The adequate amount
III Results
of 1 M Ce(NO)
obtain a 10 mol% CeO –ZrO powder, the resulting slurry
was then dried for 12 h at 100°C. Finally, the ceramic pow-
der was calcined at 700°C for 4 h.
3 2
·6H O solution was added to the solid to
The XRD patterns for hydrothermally synthesized ZrO
calcined CeO -coated ZrO powder are presented in Fig. 1.
It is clear, the presence of crystalline monoclinic ZrO phase
2
and
2
2
2
2
2
(
JCPDS 01-078-0047) in the starting powder, and the tetrag-
onal solid solution (JCPDS 01-080-0) accompanied by the m-
ZrO phase in the CeO -coated ZrO powder. This is in
agreement with the phase diagram reported by Kaspar et al.
X-ray diffraction (PANalytical X’Pert Pro diffractometer,
Almelo, the Netherlands) was used for phase identification of
the powders. Diffraction patterns were recorded using CuKa
radiation over a 2h range of 10–90° and a position-sensitive
detector using a step size of 0.05 and a counting time of 1 s
per step.
The cerium and zirconium contents of the samples were
determined by X-ray fluorescence spectrometry (XRF-Pana-
lytical AXIOS PW4400, Almelo, the Netherlands) sequential
spectrophotometer with a rhodium tube as the source of
radiation.
2
2
2
28
for ZrO
2 2
–CeO solid solution.
29,30
According to Balducci et al.
the energetics of the
reduction of cerium cations is reduced on increasing the
amount of zirconium ions present in the solid solution. In
2 2
our case, 10 mol% CeO –ZrO , the reduction potential of
4+
3+
the Ce /Ce system reduces from 7.5 to 6.1 eV. Moreover,
in the synthesis process, the zirconia nanoparticles are
obtained by hydrothermal treatment in ethanol, which is
further partially eliminated by drying in air at 200°C. This
treatment results in the presence of adsorbed alcoxide species
The BET-specific surface areas were measured by nitrogen
adsorption at liquid nitrogen temperature (Micromeritics
ASAP 2000, Atlanta, GA). Before analysis, the samples were
degassed for 2 h at 150°C in vacuum.
that upon ceria addition and further calcination may also
species. These two processes
result in an easiest diffusion
4+
favor the reduction of Ce
favoring the presence of Ce
3
+
of the cerium cation into the ZrO
fairly low temperatures.
2
matrix, which occurs at
(2) Ceramics Processing and Characterization
The SPS (Model 515S; SPS Dr Sinter Inc., Kanagawa, Japan)
of the ceramic powder was performed in vacuum in a 15-mm
diameter cylindrical graphite die/punch setup, under a uniax-
ial pressure of 50 MPa at 1200°C for 5 min, or without hold-
ing time. The heating rate was 100°C/min. Temperature was
measured using a thermocouple, which was placed in a bore
hole in the middle part of the graphite die. The sintered
ceramics of ~15 mm in diameter and ~2 mm in thickness
were polished to eliminate the surface carbon. Density of the
ceramics was determined using the Archimedes’ method, using
water as the immersion liquid.
2 2
The textural properties of ZrO and calcined CeO -coated
ZrO2 powders are summarized in Table I. The chemical
composition of the latter, obtained by XRF, was 10 mol%
Ce and 90 mol% Zr.
The SEM micrographs of the hydrothermally synthesized
ZrO powder and the calcined CeO -coated ZrO powder are
2
2
2
shown in Fig. 2. The ZrO2 powder shows an ellipsoidal
shape [Fig 2(a)], whereas a spherical shape is observed in the
calcined CeO –ZrO powder [Fig 2(d)]. To elucidate whether
2
2
this change in shape is related to the impregnation process or
to the calcination at 700°C, micrographs of calcined ZrO2
powder and of CeO –ZrO powder before calcination at
Powder X-ray diffractometry is perhaps the most useful
method for the quantitative analysis of the phases present
in multicomponent ceramics. However, the determination
of accurate compositions is sometimes difficult because of
significant overlap of Bragg reflections, as well as possible
texture effects. For this reason, results obtained using
traditional quantitative XRD methods generally are unsat-
isfactory. To approach phase identification and Rietveld
quantitative analysis on the sintered ceramics, diffraction
patterns were recorded using a step size of 0.02 and a
counting time of 10 s per step on the polished cross-sec-
tioned surfaces of the ceramics. The profile refinements
2
2
700°C are also shown in Fig. 2. It is observed that an ellip-
soidal shape is also observed in the powder after CeO2
impregnation [Fig. 2(c)], whereas both ZrO starting powder
2
and CeO -coated ZrO powder present spherical shape after
2
2
2
6
were performed using the Rietveld program FULLPROF.
This code is derived from a classical Rietveld program
DBW3.2S). In this work, the peak shape was assumed to
(
be a pseudo-Voigt function and the refinement included
the following aspects: (i) the background, which was mod-
eled as a third-order polynomial function; (ii) the scale fac-
tors; (iii) the global instrumental parameters (zero-point 2h
shift and systematic shifts, depending on transparency and
off-centering of the sample); (iv) the lattice parameters for
all phases; and (v) the profile parameters [Caglioti half-
width parameters and the mixing parameter (h) of the
pseudo-Voigt function]. This refinement was performed first
Fig. 1. XRD patterns of the hydrothermally synthesized ZrO
powder (a), and the calcined 10 mol% CeO -coated ZrO powder
(b).
2
2
2

March 2012
Nanostructured Spark Plasma Sintered Ce-TZP Ceramics
903
Table I. Textural Properties of the Powders
The Raman spectra of the ceramics sintered at 1200°C
without holding time, and for 5 min are shown in Figs. 4
and 5, respectively. In both ceramics, a band at around
Pore
BET surface
2
area (m /g)
volume
3
(cm /g)
Average pore
˚
diameter (A)
À1
2
optical transition band, related to
100 cm is observed. This band has been attributed to the
Powder
2
2
F
ion. The Raman bands below 700 cm
7/2
↔
F
5/2 electronic
3
+
31
À1
transition of Ce
are attributed to phonon modes. For the ceramic sintered
without holding time, peaks at 165, 265, 320, 465, 612, and
ZrO
Calcined 10 mol%
CeO -coated ZrO
2
49.0
18.0
0.25
0.18
192.7
404.0
2
2
À1
6
43 cm
appeared. These peaks correspond to the six
Raman bands predicted for theoretical tetragonal zirconia,
3
2,33
and have been found for previous authors.
No signifi-
Table II. Density and Relative Density of the Ceramics
cant differences were observed in the spectra measured at
the surface and core area of the cross-sectioned ceramic.
However, a slight difference in the intensity ratio of peaks
Density
3
(g/cm )
Sintering time (min)
Relative density (%)
À1
at 265 and 320 cm is observed, I265/I320 = 1.72 and 1.34
for the surface and core area, respectively. This may be
related with the symmetry change associated with the
observed metastable tetragonal phases (see Rietveld analysis
below).
0
5
5.78
5.85
97.9
99.0
Raman spectra for the ceramic sintered for 5 min show
significant differences when measured in the core area or the
surface of the ceramic, according to the observed differences
in the microstructure. For the core area, where the ceramic
presents the smaller grain size, six bands corresponding to
tetragonal phase are found. However, the spectrum measured
calcination at 700°C. Thus, it can be concluded that the
change in shape is related to the calcination process. The
grain size of both ZrO
ZrO powder remains below 100 nm.
2 2
starting powder and calcined CeO –
2
The Table II shows density and relative density of the
ceramics as a function of holding time. The relative density
of the sintered ceramics is higher than 97% in both cases. A
gradual color change along the axial direction from brown in
the core to orange at the edge was observed on the cross-
sectioned ceramic sintered at 1200°C for 5 min, whereas a
homogeneous brown color was observed for the ceramic
sintered without holding time. No micro- or macrocracks
were observed in the ceramics, conversely to the reported
in the surface shows major bands at 185, 338, 480, 625, and
639 cm with minor peaks at 228, 267, 309, 387, 509, 542,
and 565 cm . The major bands at 185, 338, and 480 cm
À1
À1
À1
3
2–34
are assigned to the monoclinic phase. The bands at 267,
25, and 640 cm are assigned to the tetragonal phase. Two
À1
6
main differences have been reported for Raman spectra cor-
3
responding to the monoclinic and tetragonal phases : the
3
À1
À1
band at 480 cm is stronger than the band at 640 cm for
the monoclinic phase and there are some small bands
between 480 and 640 cm for this phase, but these weak
2
3
result by Huang et al. for similar materials.
The SEM micrographs of the 10 mol% CeO
À1
-doped ZrO
2 2
bands are absent for the tetragonal one. Regarding the minor
bands at 309 and 387 cm , they can be related to the
ceramics sintered at 1200°C for 5 min, and without holding
time are shown in Fig. 3. We analyzed the fracture surface
to avoid possible sample oxidation during polishing and
etching. In the ceramic sintered for 5 min, a graded micro-
structure is observed. The average grain size in the edge area
is in the range 160–260 nm, whereas in the core area, signifi-
cantly finer grains with sizes between 60 and 100 nm are
observed. On the other hand, a homogeneous microstructure
with grain sizes about 40–70 nm in the whole ceramic is
observed in the ceramic sintered without holding time.
À1
3
5,36
pyrochlore phase Ce
2
Zr
2
O
7
. It has been reported
that
this phase gives a strong band at 295 cm and four weak
À1
À1
bands at 391, 448, 497, and 590 cm . It can be concluded
that the Raman spectrum measured at the ceramic surface is
due to mixed monoclinic, tetragonal, and pyrochlore phases.
When the spectrum is measured in an intermediate region
between the core area and the surface of the ceramic, the
six bands corresponding to tetragonal phase are clearly
(
a)
(b)
(
c)
(d)
Fig. 2. SEM micrographs of the hydrothermally synthesized ZrO
powder (c), and calcined 10 mol% CeO -coated ZrO powder (d).
2 2 2 2
starting powder (a), calcined ZrO powder (b), 10 mol% CeO -coated ZrO
2
2

904
Journal of the American Ceramic Society—Cruz et al.
Vol. 95, No. 3
(
a)
(b)
(
c)
(d)
Fig. 3. SEM micrographs of the surface (a) and core (b) area of the ceramic sintered at 1200°C for 5 min, and the surface (c) and core (d) area
of the ceramic sintered at 1200°C without holding time.
À1
observed, however, small bands between 480 and 640 cm
appear, pointing to a trace of monoclinic phase. In addition,
the band observed at 387 cm is related to a trace of py-
À1
rochlore phase. According to these results, it can be con-
cluded that this ceramic shows a graded phase distribution
from core area to surface area.
Rietveld refinements corresponding to the ceramics sin-
tered at 1200°C without holding time, and for 5 min are
given in Fig. 6.
The Rietveld analysis corresponding to the ceramic sin-
tered without holding time was initially modeled as com-
2
pletely tetragonal (space group P4 /nmc), assuming Ce/Zr are
2À
randomly distributed at the special positions 2a and O ions
occupy the 4d positions. Under this assumption, a large value
of the weighted residual, Rwp, was obtained in spite of the
excellent quality of the diagram (expected residual
e
R = 3.46). Moreover, the high value of the observed
FWHM (0.2565° 2h) and asymmetry of the more relevant
Bragg reflection (at about 30° 2h) let us suspect the existence
of a second phase highly overlapped with the tetragonal
phase. As a first choice, we introduced in the model, the
cubic phase (fluorite structure, space group Fm3m). However,
this hypothesis was rejected as the Rwp residual increased.
Finally, we introduced the metastable t′ phase, and the
decrease in the Rwp residual showed that the system is
formed by two phases, of which the major crystalline phase
is metastable tetragonal, t′, 90.94%, and the minor phase is
tetragonal 9.06% (Table III). In the last cycle, the number of
fitted parameters was 57.
Fig. 4. Raman spectra measured in the ceramic sintered at 1200°C
without holding time. Full Raman spectra (a) Range of interest (b).
t, tetragonal phase.
For the ceramic sintered for 5 min, the Rietveld refine-
ment indicates the formation of three phases (Table IV), of
which the major crystalline phase is tetragonal (82.5%),
and the minor phases are monoclinic (15.1%) and Ce Zr O
7
2
2
(
ometric pyrochlore, A (space group Fd3m), the Ce and
pyrochlore structure, 2.4%). In the structure of the stoichi-
2 2 7
B O
Zr atoms occupy the 16c and 16d positions, and oxygen
atoms occupy the 48f, 8a, and 8b positions. However, refine-
ment of the respective occupation factors revealed that Ce
ions are placed preferentially in the 16c position, whereas Zr
ions occupy the 16d Wycoff position. The structure of
pyrochlore oxides may be described as an ordered cubic
closed-packed array of cations with the oxygen ions occupy-
ing 7/8 of the tetrahedral sites. The stoichiometric phase is
a semiconductor. However, it has been reported that by
Fig. 5. Raman spectra measured in the ceramic sintered at 1200°C
for 5 min. Full Raman spectra (a) Range of interest (b). t, tetragonal
phase; m, monoclinic phase; and p, pyrochlore phase.

March 2012
Nanostructured Spark Plasma Sintered Ce-TZP Ceramics
IV. Discussion
905
The differences in microstructure and phases distribution
observed in the ceramics prepared with the two different
sintering conditions reveal the remarkable effect of the sinter-
4
+
3+
ing time. This is related to the reduction of Ce
to Ce
,
which is promoted by the strongly reducing environment and
the very low oxygen partial pressure created by the die and
punches during sintering. Different studies have shown that
the content of Ce O depends on the atmosphere (N , Ar,
2
3
2
vacuum), the partial oxygen pressure, and the temperature,
and amounts of Ce as high as 90% have been reported for
3
+
2
5,39
Ce-TZP ceramics sintered in graphite furnace.
The very fast SPS cycle used when sintering the ceramic at
1
of Ce . Although some degree of reduction is pointed out
200°C without holding time inhibits the complete reduction
4
+
À1
by the Raman band observed at 2100 cm (Fig. 4), and also
the tetragonal
1
7,23
by the brown/orange color in the samples,
phase stabilization is achieved in the CeO
2
–ZrO solid solu-
2
tion. The t and t′ phases have been found to be the only
phases in the different areas of the ceramic, which is in agree-
ment with the phase diagram reported by Yashima et al.
4
0
for CeO –ZrO ceramics. For 10% CeO –ZrO composition,
2
2
2
2
at the sintering temperature (1200°C) and room temperature,
the phases in the diagram are t and t′, respectively. In our
work, the fast cooling ramp achieved in SPS allows the reten-
tion, 10% of the t-phase, at room temperature. On the other
hand, this very fast sintering cycle inhibits the grain growth
giving place to a nanostructured ceramic with grain size simi-
lar to the initial nanometric particle size.
4
+
When sintering at 1200°C for 5 min, the reduction of Ce
3
+
to Ce propagates from the edge area to the inside of the
ceramic, giving place to the observed microstructural differ-
4
+
ences in the ceramic. In the edge area, where all Ce
ions
Fig. 6. Rietveld refined powder XRD data for the ceramic sintered
at 1200°C without holding time (a), and for 5 min (b).
3
+
are reduced to Ce , the relevant phase diagram is the ZrO –
2
4
Ce O system, reported by Leonov et al. According to this
1
2
3
phase diagram, a cubic Ce
lore structure is in equilibrium with m-ZrO
2
Zr
2
O
7
compound with a pyroch-
–Ce solid solu-
2
2 3
O
2 2
Table III. Rietveld Refinement of the 10 mol% CeO –ZrO
tion below 1000°C. The Raman spectra measured in the
surface area confirm the presence of monoclinic and pyroch-
lore phases. In an intermediate area, the Raman spectra con-
firm the presence of monoclinic and pyrochlore phases,
together with traces of tetragonal phase, pointing to a partial
Ceramic Sintered at 1200°C without Holding Time
Phase
Tetragonal
t′
Fraction (%)
˚
˚
c (A)
9.06
3.65
5.20
90.94
3.62
5.21
4
+
3+
reduction of Ce to Ce . The stabilization of the t-phase in
the CeO –ZrO solid solution in the core area of this ceramic
a = b (A)
2
2
indicates a low degree of reduction, similar to the one
described in the ceramic sintered without holding time.
The coarser grain size observed in the surface area of this
ceramic is also a consequence of the complete reduction of
R
R
R
v
O
p
12.7
20.9
4.26
4.91
wp
exp
2
4
+
3+
Ce
generated in the structure by the trivalent cation Ce
motes a higher species diffusion, which results in enhanced
sintering and grain coarsening. Thus, the graded CeO reduc-
to Ce
in this area. The high amount of vacancies
1
z
0.061
0.022
3
+
pro-
2
tion that takes place when the sintering temperature is
allowed to stabilize for a short time results in a graded mate-
rial with a gradient in microstructure, grain size, and phases
distribution.
2 2
Table IV. Rietveld Refinement of the 10 mol% CeO –ZrO
Ceramic Sintered at 1200°C for 5 min
Phase
Tetragonal
Monoclinic
Pyrochlore I
Fraction (%)
˚
˚
b (A)
˚
c (A)
82.50
3.62
3.62
5.21
15.10
5.18
5.20
5.32
7.13
9.01
4.53
3.95
0.36
2.40
10.63
10.63
10.63
a (A)
V. Conclusions
The combination of a two-step powder synthesis technique
and spark plasma sintering allows the processing of nearly
fully densified 10 mol% CeO -doped ZrO . The very fast
R
Rwp
p
2
2
SPS cycle used when sintering the ceramic at 1200°C without
holding time allows the stabilization of the tetragonal phase,
despite the highly reducing atmosphere generated during
sintering, and the retention of the nanostructure, with grain
sizes of 40–70 nm and high relative density, 97.9%. When
stabilizing the sintering temperature for 5 min, a graded
material with presence of monoclinic and pyrochlore phases
in the surface area and tetragonal phase in the core area is
obtained. This graded phase distribution, together with the
R
v
exp
2
O
1
z
0.048
0.12
modifying the stoichiometry, the electrical properties can be
A detailed
3
7,38
changed from insulating to metallic behavior.
study of the electric properties will be given elsewhere.

906
Journal of the American Ceramic Society—Cruz et al.
Vol. 95, No. 3
2
0
observed gradient in grain size, with 60–100 nm in the core
area and 160–260 nm in the surface area, is consequence of
the gradual reduction of Ce
ronment during SPS.
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Electric Field and Pressure on the Synthesis and Consolidation of Materials:
4
+
3+
to Ce in the graphite envi-
A Review of the Spark Plasma Sintering Method,” J. Mater. Sci., 41, 763–77
(
2006).
21
U. Anselmi-Tamburini, J. E. Garay, and Z. A. Munir, “Fast Low-Temper-
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8
2
2
Acknowledgments
T. Xu, P. Wang, P. Fang, Y. Kan, L. Chen, J. Vleugels, O. Van der Biest,
and J. Van Landuyt, “Phase Assembly and Microstructure of CeO -Doped
ZrO2 Ceramics Prepared by Spark Plasma Sintering,” J. Eur. Ceram. Soc., 25,
2
Dr. Cruz thanks Junta de Andalucıa (TEP-1048) for her fellowship. The finan-
´
cial support for this work has been obtained from the Spanish Ministerio de
3437–42 (2005).
2
3
Ciencia e Innovacio
co-financed by FEDER funds from the European Union and from Junta de
Andalucıa (P09-TEP-5454).
´
n (ENE2009-14522-C05-01) and (ENE2009-14522-C05-03)
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