Synthesis and characterization of nanosized powders of yttria-doped zirconia

Article abstract of DOI:10.1016/S0925-8388(02)00337-7

Zirconia-3 mol% yttria nanosized powders have been prepared following a chemical route. The aim of this work is to optimize the techniques of solid solution synthesis and processing to obtain nanophase sintered ceramics without using milling or other special procedures. To ascertain this, several characterization techniques have been used. The electrical resistivity of polycrystalline specimens was compared to that of a single crystal of similar chemical composition. The main results show that a monomodal distribution of pore sizes has been obtained. Total stabilization of the tetragonal phase and nanophase sintered ceramics with relative densities of 98% were prepared by sintering at temperatures lower than 1200°C. The main conclusion is that the optimization of the synthesis and processing techniques play a key role for improvement of chemical and physical properties of solid electrolytes prepared by chemical techniques. Elsevier Science B.V. All rights reserved.

Full text of DOI:10.1016/S0925-8388(02)00337-7

Journal of Alloys and Compounds 344 (2002) 186–189
L
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Synthesis and characterization of nanosized powders of
yttria-doped zirconia
*
S.K. Tadokoro , E.N.S. Muccillo
Energy and Nuclear Research Institute-CCTM, Multidisciplinary Center for the Development of Ceramic Materials, C.P. 11049, Pinheiros,
˜
Sao Paulo, SP 05422-970, Brazil
Abstract
Zirconia–3 mol% yttria nanosized powders have been prepared following a chemical route. The aim of this work is to optimize the
techniques of solid solution synthesis and processing to obtain nanophase sintered ceramics without using milling or other special
procedures. To ascertain this, several characterization techniques have been used. The electrical resistivity of polycrystalline specimens
was compared to that of a single crystal of similar chemical composition. The main results show that a monomodal distribution of pore
sizes has been obtained. Total stabilization of the tetragonal phase and nanophase sintered ceramics with relative densities of 98% were
prepared by sintering at temperatures lower than 1200 8C. The main conclusion is that the optimization of the synthesis and processing
techniques play a key role for improvement of chemical and physical properties of solid electrolytes prepared by chemical techniques.
2002 Elsevier Science B.V. All rights reserved.
Keywords: Nanostructures; Precipitation; Scanning electron microscopy; Ionic conduction
1. Introduction
tritor milling [6–8] and hydrothermal recrystallization
[7,9].
Zirconia–yttria ceramics have been proposed for a
number of applications due to the combination of me-
chanical, thermal, electrical and optical properties. Several
efforts have been made over the last years for the
optimization of synthesis and processing procedures to
improve chemical and microstructural homogeneities. As a
result, the preparation of ultrafine powders has lead to
unusual properties like superplasticity and others [1].
Several techniques have been proposed for the synthesis of
ultrafine powders allowing for obtaining nanophase
ceramics. Nanophase zirconia doped with 3 mol% yttria
(Y-TZP) sintered ceramics have been prepared by the
pyrolysis of alcoholic solutions [2], sol–gel [3], combus-
tion [4], and inert gas condensation [5] techniques. How-
ever, the coprecipitation of salts is the most used technique
for preparing zirconia-based solid solutions for industrial
scale. Some works in the literature suggest the use of this
technique for the synthesis of nanophase Y-TZP ceramics.
In these works, however, the formation of hard agglomer-
ates during the synthesis process induces one of the
additional step procedures: ultrasonic dispersion [6], at-
In this work, zirconia–3 mol% yttria solid solutions
have been prepared by the coprecipitation technique. The
main purpose is to obtain highly sinterable nanosized
powders with controlled agglomeration in order to produce
nanophase ceramics without using special procedures
during processing.
2. Experimental
ZrOCl2?8H2O (.99%) and Y2O3 (.99.9%) were used
as starting materials. Yttrium nitrate solution was prepared
by dissolution of the starting material in nitric acid. All
other reagents were of analytical grade. For the synthesis
of the solid solutions containing 3 mol% yttria, the
coprecipitation technique in water medium followed by
azeotropic distillation were used. After precipitation com-
pletion, the gel remained under vigorous stirring for 2 h.
The precipitate was washed with a 10 vol%. ammonium
hydroxide solution until complete elimination of Cl2 ions.
Dehydration was carried out with alcoholic solutions for
the interchange of hydroxyl and alkoxy groups. Then, the
gel was dispersed in n-butyl alcohol and distilled. The
precipitate was dried at 45 8C and gently deagglomerated
*
Corresponding author.
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2002 Elsevier Science B.V. All rights reserved.

S.K. Tadokoro, E.N.S. Muccillo / Journal of Alloys and Compounds 344 (2002) 186–189
187
in an agate mortar. During the experiments, 26 variables
were controlled to ensure reproducibility. These variables
have been selected from previous works [10–13]. How-
ever, most of the combinations of these variables found in
the literature do result in powders containing a significant
amount of hard agglomerates. These hard agglomerates are
responsible for the poor densification of ceramic compacts.
This is the reason for the additional step (ultrasonic
dispersion, milling or hydrothermal treatment) used in
most works where solid solutions are prepared by the
coprecipitation technique. After many combinations of the
variables involved in the precipitation process, we have
found best results for: low-concentrations of the cation (0.1
g l21) and precipitant (1 g l21) solutions; precipitation pH
greater than 9; a suitable washing cycle with ammonium
solution–absolute ethanol–isopropyl alcohol of 4:2:2. The
azeotropic distillation was quite important to remove
residual water from the gel. The size of the prepared batch
is also important as many of these variables are best
controlled employing low-size batches.
Fig. 1. Pore size distribution curve obtained by the BJH method after
calcining the powder.
method assuming a cylindrical pore model is shown in Fig.
1. A monomodal distribution of pore sizes can be seen in
this curve. In general, zirconia-based solid solutions pres-
ent a bimodal distribution of pore sizes assigned to intra-
and interagglomerate pores. The latter are hardly removed
and usually cause an increase in time and/or temperature
required for complete densification of these ceramics. The
present result is attributed to the use of optimized parame-
ters during synthesis and to the complete elimination of
residual water from the precipitate by azeotropic distilla-
tion.
The linear shrinkage of a powder compact and its
derivative are shown in Fig. 2. Total shrinkage is 27% and
occurs in a temperature interval of 550–1255 8C. The
maximum shrinkage rate occurs at 1075 8C.
Fig. 3 is a representative micrograph of the fracture
surface of a sintered specimen (sintering temperature,
1170 8C). The main characteristic features are low-porosity
and grains with uniform shape and similar average size.
Sintering of the compacted powders at temperatures
between 1170 and 1200 8C resulted in high densification.
The specimen sintered at 1200 8C for 5 h, for example,
Cylindrical specimens were prepared by uniaxial and
isostatic pressing. The sintering was carried out in air for 5
h at selected temperatures.
The specific surface area of calcined powders was
obtained by the BET (Brunauer, Emmett, Teller) method
(ASAP 2010, Micromeritics). The BJH (Barret, Joyner,
Halenda) method was used for obtaining the pore size
distribution. The linear shrinkage (DIL 402 E/7, Netzsch)
of powder compacts was followed up to 1500 8C with
heating and cooling rates of 8 8C and 15 8C min21
,
respectively.
Fractured or polished and thermally etched surfaces of
sintered specimens were observed in a scanning electron
microscope (LEO 440I, Oxford). The grain size distribu-
tion was calculated over |1000 grains by a home made
program based on the Saltikov statistical analysis. Raman
spectroscopy (Renishaw Raman Microscope System 3000
coupled to an Olympus BH-2 microscope and to a CCD
detector cooled by Peltier) was used to characterize the
phase composition in sintered specimens. An Ar1 laser
(Omnichrome, model 170) with an exciting radiation of
514.5 nm was used.
Electrical resistivity measurements were carried out in a
HP4192A impedance analyzer between 5 Hz and 13 MHz
in the 280–540 8C temperature range. The results were
analyzed in the impedance mode, using a special software
[14]. Silver paste was used as electrode material.
3. Results and discussion
The value of specific surface area determined by the
BET method is 132.5 m2 g21 after calcining at 500 8C for
1 h. This calcination profile was selected from thermal
analysis results.
The pore size distribution determined by the BJH
Fig. 2. Curves of linear shrinkage and its derivative for powder compacts.

188
S.K. Tadokoro, E.N.S. Muccillo / Journal of Alloys and Compounds 344 (2002) 186–189
Fig. 3. Scanning electron micrograph of a fracture surface of Y-TZP sintered specimen. Sintering temperature, 1170 8C.
reached 99.5% relative density (assuming a theoretical
density value of 6.12 g cm23), and an average grain size of
130650 nm.
Fig. 4 shows the Raman spectrum of a sintered speci-
men. Six bands of the zirconia–yttria solid solution can be
seen, including that centered at 460 nm, which is charac-
teristic of the tetragonal phase. This result shows that total
stabilization of the tetragonal phase has been achieved for
this composition and sintering condition.
Typical impedance diagrams for polycrystalline and
single crystal specimens are shown in Fig. 5. Numbers
positioned over experimental points are the decimal
logarithm of the frequency, in Hz. The high-frequency
semicircle is due to lattice or intragranular resistivity either
for the single crystal or sintered specimens. The low-
frequency semicircle is assigned to the intergranular
resistivity due to the blocking of charge carriers at grain
boundaries. The comparatively large diameter of this low-
frequency semicircle is a consequence of the small average
grain size of this specimen. As shown in the insert (Fig. 5),
there is an overlap of both semicircles, and a careful
resolution of these diagrams is quite important for obtain-
ing a precise value for the activation energy of the
intragranular resistivity.
The activation energy values for intra- and intergranular
resistivity were calculated from the Arrhenius plots of the
electrical resistivity resulting in 0.86 and 1.07 eV, respec-
tively. These figures are in general agreement with values
obtained for a sintered specimen prepared by inert gas
condensation technique [15]. The activation energy for the
lattice resistivity of the single crystal is 0.91 eV. Although
single- and polycrystalline specimens have the same
chemical composition, the exact phase composition of the
single crystal cannot be ascertained because the cooling
rate of the process for growing this crystal is not known.
This is a possible reason for the difference observed in the
activation energy values for conduction in both materials.
Fig. 5. Impedance spectroscopy diagrams for single crystal and poly-
Fig. 4. Raman spectrum of a specimen sintered at 1170 8C.
crystalline specimen sintered at 1170 8C.

S.K. Tadokoro, E.N.S. Muccillo / Journal of Alloys and Compounds 344 (2002) 186–189
189
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A careful selection of the synthesis and processing
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distribution of pore sizes have been obtained without using
any milling or other special procedures. Reactive ceramic
powders with these characteristics allow for manufacturing
ceramics with high sinterability, reduced average grain
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To FAPESP (92/02962-6, 94/05929-5, 95/05635-4,
99/10798-0) that sponsored most of the facilities used in
this work. S.K.T. acknowledges FAPESP (98/09976-9) for
the scholarship. To Ceres Co. for the single crystal. To
CNEN and CNPq (300934/94-7).
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