Properties of nanocrystalline ZrO2-Y2O 3-CeO2-CoO-Al2O3 powders

Article abstract of DOI:10.1134/S0020168511100050

We have studied the properties of nanocrystalline ZrO₂-Y ₂O₃-CeO₂-CoO-Al₂O₃ powders prepared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210°C. A number of general trends are identified in the variation of the properties of the synthesized powders during heat treatment at temperatures from 500 to 1200°C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrystalline ZrO₂-based powders containing 1 to 5 mol % Al₂O₃ allows one to obtain composites with good sinterability at a reduced temperature (1200°C).

Full text of DOI:10.1134/S0020168511100050

ISSN 0020ꢀ1685, Inorganic Materials, 2011, Vol. 47, No. 10, pp. 1107–1110. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © E.V. Dudnik, V.V. Tsukrenko, A.V. Shevchenko, A.K. Ruban, L.M. Lopato, 2011, published in Neorganicheskie Materialy, 2011, Vol. 47, No. 10, pp. 1217–1221.
Properties of Nanocrystalline
ZrO2–Y2O3–CeO2–CoO–Al2O3 Powders
E. V. Dudnik, V. V. Tsukrenko, A. V. Shevchenko, A. K. Ruban, and L. M. Lopato
Frantsevich Institute of Materials Science Problems, National Academy of Sciences of Ukraine,
ul. Krzhizhanovskogo 3, Kyiv, 03680 Ukraine
eꢀmail: dep25@ipms.kiev.ua
Received March 9, 2011
Abstract—We have studied the properties of nanocrystalline ZrO2–Y2O3–CeO2–CoO–Al2O3 powders preꢀ
pared via hydrothermal treatment of a mixture of coprecipitated hydroxides at 210 C. A number of general
°
trends are identified in the variation of the properties of the synthesized powders during heat treatment at
temperatures from 500 to 1200 C. Our results demonstrate that the addition of 0.3 mol % CoO to nanocrysꢀ
talline ZrO2ꢀbased powders containing 1 to 5 mol % Al2O3 allows one to obtain composites with good sinterꢀ
°
ability at a reduced temperature (1200 C).
°
DOI: 10.1134/S0020168511100050
INTRODUCTION
a narrow particle size distribution, which offers
improved sinterability [7].
An important issue in the microstructural engiꢀ
neering of ZrO2ꢀbased composites is to create hip joint
endoprostheses. The high strength of such materials,
due to the transformation toughening effect, makes
them an attractive alternative to Al2O3ꢀbased materials
[1]. At the same time, the high probability of the soꢀ
called “aging” of ZrO2ꢀbased endoprostheses in the
aggressive medium of living organisms is a serious
obstacle to their potential applications. The aging is due
to the uncontrolled phase transition of tetragonal zircoꢀ
Nanocrystalline multicomponent ZrO2ꢀbased
powders are nonequilibrium thermodynamic systems
possessing a high free energy, which determines their
activity in the fabrication of composites. The study of
the properties of ceramic powders with the aim of
maintaining their sinterability is a key issue in the
development of advanced materials, including ZrO2
ꢀ
based bioimplants.
The purpose of this work was to study the properties of
nanocrystalline powders with the compositions (mol %)
95.2ZrO2–2.8Y2O3–0.7CeO2–0.3CoO–1Al2O3 (A1),
93.7ZrO2–2.8Y2O3–0.7CeO2–0.3CoO–2.5Al2O3 (A2),
and 91.2ZrO2–2.8Y2O3–0.7CeO2–0.3CoO–5Al2O3 (A5),
prepared from coprecipitated hydroxides under
hydrothermal conditions and heatꢀtreated at temperꢀ
nia (ТꢀZrO2 to monoclinic zirconia (МꢀZrO2) [2]. There
are several approaches to this problem, in particular,
grain size reduction in ZrO2ꢀbased composites to below
the critical level [3], ZrO2 stabilization through codoping
with yttria and ceria [4], and fabrication of ZrO2ꢀbased
composites containing 1 to 8 wt % Al2O3 [5].
The ZrO2–Y2O3–CeO2–CoO–Al2O3 system is of
interest for resolving the problem of the aging of hip
joint endoprostheses. The properties of ZrO2ꢀbased
composites in this system, governed by a combination
of the properties of materials in binary and ternary
constituent systems, meet the requirements for bioinꢀ
ert implants [6, 7]. Cobalt oxide is intended to color
the ceramic [8], which would enhance the contrast of
implants against living tissue.
atures from 500 to 1200 С.
°
EXPERIMENTAL
In our preparations, we used reagentꢀgrade chemiꢀ
cals: zirconyl nitrate (ZrО(NО3)2
nitrate (Y(NО3)3
6Н2O), aluminum nitrate (Al(NО3)3
cobalt nitrate (Co(NО3)3 2Н2O).
⋅
2Н2О), yttrium
6Н2O), cerium nitrate (Се(NО3)3
9Н2O), and
⋅
⋅
⋅
⋅
The desired combination of properties of ZrO2
ꢀ
based composites is determined by all fabrication
The above compositions were obtained through
steps: from the selection of the powder composition homogeneous coprecipitation from an appropriate
and preparation method to the forming process. One mixture of aqueous solutions of the starting salts, using
process for powder preparation is the hydrothermal aqueous NH4OH as the precipitant. We used a reverse
decomposition of a mixture of coprecipitated hydroxꢀ precipitation method. The process was run in a magꢀ
ides in an alkaline medium, which combines the netic stirrer, and the resultant suspension was boiled
advantages of the coprecipitation and sol–gel techꢀ for 3–4 h. In this way, we obtained dull, translucent
niques with hydrothermal treatment. This process gelꢀlike hydroxide mixtures, which were washed many
yield softly agglomerated nanocrystalline powder with times with distilled water by decantation.
1107

1108
DUDNIK et al.
25
А2
А1
20
15
10 μm
10
10
10
μ
m
(b)
(d)
(f)
(а)
10
5
А5
10 μm
μ
m
(c)
0
500
700
900
1200
Temperature, C
°
Fig. 2. Effect of heat treatment on the crystallite size of
ZrO in the nanocrystalline ZrO –Y O –CeO –СоО–
2
2
2
3
2
Al O powders.
2
3
μ
m
10 μm
formed on a DRONꢀ1.5 powder diffractometer (Сu
radiation, continuous scan rate from 1 to 4 /min). The
K
α
(e)
°
crystallite size was determined using the Scherrer forꢀ
mula. Electronꢀmicroscopic examination was carried
out on a CAMEBAX SXꢀ50 Xꢀray microanalyzer. In
microstructural and phase analyses, we used an
MINꢀ8 optical microscope and a standard set of
immersion media (magnification from 60
×
to 620 ).
×
The specific surface area of the asꢀprepared and heatꢀ
treated nanocrystalline powders was determined by
BET analysis of nitrogen adsorption isotherms.
10 μm
(g)
Fig. 1. Morphology of the nanocrystalline powders after
RESULTS AND DISCUSSION
(a) hydrothermal treatment, (b–d) heat treatment at
700
°C, and (e–g) heat treatment at 1200°C: (b, e) A1,
After hydrothermal treatment, the three materials
consisted of a lowꢀtemperature, cubic ZrO2ꢀbased
solid solution (FꢀZrО2). The particles of powders A1–
(c, f) A2, (d, g) A5.
A5 formed “soft” rounded agglomerates with a complex
hierarchy. Their morphology is illustrated by Fig. 1a. The
crystallites are 7–8 nm in size (Fig. 2) and form firstꢀ
Hydrothermal treatment was performed in a laboꢀ
ratory autoclave at 210 for 3 h. In all cases, dehydraꢀ
°
С
tion under hydrothermal conditions led to the formaꢀ
tion of a wellꢀdefined interface between the mother
liquor and the suspended precipitate. The isolated
precipitate was washed many times with distilled
water, collected on a vacuum filter, and dried at 90–
level agglomerates, which range in size up to 1–2
in the three powders. These agglomerates, in turn,
form secondꢀlevel agglomerates, 3–10 m in size. The
dominant size fraction is 5–10 m in powder A1 and
3–5 m in powders A2 and A5. The agglomerates are
µ
m
µ
µ
µ
95
°
С
for 8 h.
To study the effect of heat treatment on the strucꢀ
ture and phase composition of the resultant nanocrysꢀ
talline powders, the dried samples were fired at 500,
700, 900, and 1200 for 1.5 h at each temperature.
isotropic and consist of two phases: fine ZrO2 particles
are embedded in a transparent matrix phase. The
refractive index of the matrix phase is smaller than that
of the fine particles, as evidenced by the sharp, conꢀ
trast (rough) boundaries of the particles. Therefore,
°
С
The processes induced in the nanocrystalline powꢀ after hydrothermal crystallization the powders contain
ders by heat treatment were followed using Xꢀray difꢀ an Xꢀray amorphous (isotropic) matrix phase. The
fraction (XRD), electron microscopy, and microꢀ fineꢀparticle phase is isotropic because of the small
structural analysis. XRD characterization was perꢀ particle size.
INORGANIC MATERIALS Vol. 47
No. 10
2011

PROPERTIES OF NANOCRYSTALLINE
1109
Figure 3 illustrates the effect of heat treatment at
temperatures from 500 to 1200 on the specific surꢀ
100
80
°
С
face area of the powders. As seen, the specific surface
area of powder A5 after hydrothermal treatment is
twice that of powders A1 and A2. The likely reason for
this is that the agglomerates differ in porosity: accordꢀ
ing to microstructural analysis results, the porosity in
powder A5 exceeded that in powders A1 and A2.
It is worth pointing out that the variation in the
morphology of the hydrothermal powders A1, A2, and
A5 during heat treatment is topologically continuous:
the nearly spherical shape of the firstꢀ and secondꢀ
level agglomerates persists during heat treatment.
А5
60
40
20
А1
А2
According to XRD results, heat treatment at 500
°С
had no effect on the phase composition ( ꢀZrО2) and
F
0
500
700
900
C
1200
crystallite size (Fig. 2) of the powders. At the same
time, the specific surface area of powders A1 and A2
increased almost twofold (Fig. 3), which was probably
also related to changes in the porosity of the agglomerꢀ
ates. In the three powders, the secondꢀlevel agglomerates
Temperature
,
°
Fig. 3. Effect of heat treatment on the specific surface area
of the nanocrystalline ZrO –Y O –CeO –СоО–Al O
powders.
2
2
3
2
2 3
became looser and increased in size to 10–20 m. These
µ
changes can be accounted for by the removal of the
adsorbed water and the water of hydration from the mulꢀ
ticomponent hydrothermal powders during heat treatꢀ
agglomerates 0.5–1
µ
m in size form secondꢀlevel
agglomerates ranging in size up to 10–15 (A1 and A2)
or 30 m (A5). These processes in the temperature
range 500–700 are accompanied by phase transforꢀ
ment below 500
occur in the temperature ranges 60–120 and 320–
500°С, depending on composition, which leads to loosꢀ
°
С. As shown earlier [9], these processes
µ
°
С
mations of the lowꢀtemperature forms of Al2O3 [10],
Со3О4 formation [11], and a reduction in the specific
surface area of the powders (Fig. 3).
ening of the agglomerates and increases their porosity.
The percentage of isotropic, twoꢀphase regions in
the powders decreases. At the same time, we observe
the formation of fineꢀparticle, isotropic agglomerates
with milky polarization. It is of interest to note that the
powders have zones of low birefringence (A1) or individꢀ
ual anisotropic particles with rough surfaces (A2, A5). It
Raising the heatꢀtreatment temperature to 900
allows the ꢀZrO2 ꢀZrO2 lowꢀtemperature
phase transition in the powders to reach completion:
the XRD pattern shows no reflections from ꢀZrO2
°С
F
T
F
.
is reasonable to assume that heat treatment at 500
°С
The crystallite growth accelerates (Fig. 2). The perꢀ
centage of anisotropic ZrO2 particles in the powders
increases, and the milky polarization of fineꢀparticle
isotropic agglomerates persists, indicating that the
agglomerates contain an anisotropic phase with a parꢀ
ticle size under the resolving power of the optical
microscope used. The percentage of transparent isoꢀ
tropic lenticular zones increases in going from powder
A1 to A5 and the specific surface area of the powder
initiates a lowꢀtemperature ꢀZrO2 ꢀZrO2 phase
F
T
transition in powders A1–A5, which can only be idenꢀ
tified from crystalꢀoptical characteristics.
Two more points are worth mentioning. In isotroꢀ
pic, transparent zones of powder A2, up to 3 m in
µ
size, we observed a twoꢀ or threeꢀlayer rim of particles
with a rough surface, and powder A5 consisted of aligned
particles. This is a distinctive feature of the crystallization
processes at 500°С in the nanocrystalline powders conꢀ drops sharply (Fig. 3), which is due to the complex
taining 2.5 and 5 mol % Al2O3, which are accompanied
by only slight particle growth (Fig. 2).
nature of the processes that take place in the nanocrysꢀ
talline powders А1 А5 at temperatures above
850 . The phase transitions of ZrO2 and sintering
processes are accompanied by the formation of
°С
According to XRD results, after heat treatment at
700
ture metastable solid solutions:
Microstructural examination indicated an increase in
the percentage of ꢀZrO2 anisotropic particles with a
°
С
powders A1–A5 consisted of two lowꢀtemperaꢀ
α
ꢀAl2O3 [10] and CoO [11].
F
ꢀZrO2 and ꢀZrO2
T
.
The morphology of powders A1–A5 after heat
T
treatment at 900
ment at 700
agglomerates continues, the area of contacts between
them further increases, and the average size of the secꢀ
ondꢀlevel agglomerates reaches 40 m.
°
С
is similar to that after heat treatꢀ
rough surface. As seen in Figs. 1b–1d, in all of the
powders sintering processes were initiated and were
accompanied by a reduction in average agglomerate
size. This led to the formation of lenticular transparent
sintered zones. Their volume fraction increases in the
°
С. The densification of the firstꢀlevel
µ
order А1
А2
А5. The densification of the firstꢀ
After hydrothermal treatment and firings at 500,
700, and 900 , powders A1, A2, and A5 were white in
tallite growth (Fig. 2). The spherical firstꢀlevel color.
level agglomerates is essentially unrelated to the crysꢀ
°С
INORGANIC MATERIALS Vol. 47
No. 10 2011

1110
DUDNIK et al.
According to XRD results, heat treatment at posites with good sinterability at a reduced temperaꢀ
1200 improved the crystallinity of ꢀZrO2 ture (1200 ).
°
С
Т
.
°
С
Figures 1e–1g demonstrate the formation of a sintered
skeletal structure made up of firstꢀ and secondꢀlevel
agglomerates. The milky polarization of the fineꢀpartiꢀ
cle agglomerates persisted. The size of individual anisoꢀ
ACKNOWLEDGMENTS
We are grateful to V.P. Red’ko for performing the
Xꢀray work, V.M. Vereshchaka for performing the
electronꢀmicroscopic work, and L.D. Bilash for deterꢀ
mining the specific surface area of the powders.
tropic
Тꢀ
ZrO2 particles increased to 3–4 m, and the
µ
three powders turned bright blue, indicating the formaꢀ
tion of cobalt aluminate spinel, CoAl2O4 [12]. The
CoAl2O4 formation is accompanied by a twofold increase
in ZrO2 crystallite size (Fig. 2) and a sharp decrease in the
specific surface area of the powders (Fig. 3).
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Al2O3 system are nonequilibrium thermodynamic sysꢀ
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reactivity during heat treatments at 500, 700, 900, and
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°
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°
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the growth of each other (Fig. 2), an effect typical of
hydrothermal nanocrystalline powders [13]. After heat
treatment at 1200°С, the three powders were bright
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blue in color, which was due to the formation of cobalt
aluminate spinel, CoAl2O4. The spinel reduced the
mutual inhibition of structural changes and caused a
twofold increase in ZrO2 crystallite size. In the powꢀ
ders under consideration, a “skeletal” microstructure
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Synthesis and Properties of Nanocrystalline 90 wt %
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forms at a temperature 100
talline ZrO2–Y2O3–CeO2–Al2O3 powders [10]. This
indicates that the powders studied here retain sinterꢀ
ability at a reduced temperature (1200 ) and that one
can produce composites sufficiently stable to aging
because no ꢀZrO2 was detected in the powders.
°
C below that in nanocrysꢀ
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Effect of Heat Treatment on the Properties of Nanocꢀ
rystalline 80 wt % Al2O3–20 wt % ZrO2(CeO2, Y2O3)
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°
С
М
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fabricate hip joint endoprostheses and multilayered
bioimplants possessing enhanced strength.
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,
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CONCLUSIONS
13. Dudnik, E.V., Shevchenko, A.K., Ruban, A.K., et al.,
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The addition of 0.3 mol % CoO to nanocrystalline
ZrO2–Y2O3–CeO2–Al2O3 powders containing 1 to
5 mol % Al2O3 allows one to obtain bright blue comꢀ
INORGANIC MATERIALS Vol. 47
No. 10
2011