Vol.2, No.7, 694-706 (2010) Natural Science
Copyright © 2010 SciRes. OPEN ACCESS
Formation characterization and rheological properties of
zirconia and ceria-stabilized zirconia
Arvind K. Nikumbh*, Parag V. Adhyapak
Department of Chemistry, University of Pune, Ganeshkhind, India; *Corresponding Author: aknik@chem.unipune.ernet.in
Received 17 February 2010; revised 3 May 2010; accepted 8 May 2010.
Zirconia and ceria-stabilized zirconia (12Ce-TZP)
were synthesized by the dicarboxylate copre-
cipitation technique such as fumarate, succi-
nate, tartarate and adipate. The formation of
these dicarboxylate precursors was studied by
elemental analysis, thermal analysis and infra-
red spectroscopy. The precursors were further
decomposed at 650oC for 2 hours to form re-
spective zirconia and ceria-stabilized zirconia.
The composition of these oxides was checked
by Atomic absorption spectrometer and Energy
dispersive X-ray analysis. The structural and
morphological characterization of these oxides
was done by using X-ray diffraction analysis,
surface area, scanning electron micrographs
and particle size distribution analysis. These
properties were found depending to great extent
on the nature of the precursors. The zirconia
and ceria-stabilized zirconia obtained from adi-
pate precursor were found to be good for slip-
casting. The slips (i.e., suspensions) of these
oxides with different solid contents were pre-
pared at different pH with distilled water and
ethanol as dispersing agents, with and without
deflocculant. The suspension rheological flow
(i.e., variation of shear stress and viscosity with
shear rate) was determined. The minimum vis-
cosities were observed at pH = 10.16 for ZrO2-
water and pH = 10.26 for 12Ce–TZP-water sys-
tem. The slip, green and sedimentation bulk
density were measured.
Keywords: Oxides; Ceramics; Chemical Synthesis;
Surface Properties
Zirconia and zirconia-base ceramics are of both scien-
tific and technological interest as structural and func-
tional materials due to their superior properties [1].
These powders have attracted much interest recently due
to their specific optical and electrical properties [2-5]
and potential applications [6-11]. Zirconia has three
polymorphs: monoclinic, tetragonal and cubic [1]. The
former is the thermodynamically stable and the other
two are metastable polymorphs.
The addition of ‘stabilizing’ oxides, like CaO, MgO,
CeO2, Y2O3 to pure zirconia allows to generate multi-
phase materials known as partially stabilized zirconia,
whose microstructure at room temperature generally
consists of cubic zirconia as the major phase, with mo-
noclinic and tetragonal zirconia precipitated as the minor
phase [12]. The improvement in the mechanical strength
and toughness due to phase transformation in partially
stabilized zirconia was first reported by Garvie et al.
During the last decades an active research has been
undertaken in different laboratories, in order to obtain
zirconia and stabilized-zirconia powders with the re-
quired characteristics of size, purity, uniformity, crystal-
linity etc. A variety of methods have recently been de-
scribed for the preparation of ZrO2 and CeO2-ZrO2 for
potential applications. These methods includes conven-
tional methods like co-precipitation [14-20], citrate
process [17], polymerized complex process [18-21],
sol-gel [22], gel-combustion process [23,24], spray py-
rolysis [25], hydrothermal [26], surfactant templating
[27,28], ultrasound [29] and sonochemical [30] methods
A strong effort has been directed to increase the over-
all efficiency of CeO2-ZrO2 in various applications. One
of the keys to this success is the selection of appropriate
preparation methods and composition (i.e., Ce/Zr ratio),
which in turn determine homogeneity at a molecular
level and morphological properties. Considerable inter-
est is being shown in tetragonal zirconia polycrystalline
ceramics. Coyle et al. [31] were among the first re-
searchers to study tetragonal zirconia produced using the
ZrO2-CeO2 system. The presence of tetragonal phase is
an essential condition for zirconia toughening besides
A. K. Nikumbh et al. / Natural Science 2 (2010) 694-706
Copyright © 2010 SciRes. OPEN ACCESS
hindering or interrupting crack propagation [32].
In all the above technological applications, the control
of rheology is of paramount importance for zirconia and
partially stabilized zirconia [33-35], especially when
stabilization and milling processes are used [33,35,36].
The rheological characteristics of aqueous suspensions
depends on several experimental parameters such as
particle size and particle size distributions, solids volume
fraction, pH of suspending media, type and amount of
added dispersing agents [37-40]. Despite the good
knowledge of stabilization mechanism [41,42] and the
methods to make a well dispersed and stable slip, there
is no general process applicable for ceria stabilized ZrO2
In this work we have prepared zirconia and ceria-sta-
bilized zirconia by dicarboxylate precursor method. This
method offers several advantages for processing ceramic
powders such as direct and precise control of stoi-
chiometry, uniform mixing of multicomponents on a
molecular scale, and homogeneity. This method is also
applied to synthesize ceria-stabilized zirconia showing to
be effective to prepare zirconia with a tetragonal struc-
ture. In addition, the rheology of zirconia and ceria-sta-
bilized zirconia is investigated in order to find out a
suitable stabilization method. Several critical factors
related to the slip rheology are studied, such as the in-
fluence of the amount of dispersant, and pH on viscosity
of concentrated aqueous and nonaqueous slips. Finally,
the density of green cast samples is studied and related
to the degree of slip dispersion.
2.1. Synthesis of Zirconyl Dicarboxylate
(a) Zirconyl fumarate dihydrate, ZrO(C4H2O4)·2H2O was
prepared by the precipitation method by taking high pu-
rity ZrO(NO3)2·nH2O (9.989 gm) in deionized water
(100 ml) in a beaker. The pH of the medium was ad-
justed to a low enough value (pH 6), so that hydroxide
precipitate does not form. The solution was stirred vig-
orously with a magnetic stirrer. To this fumaric acid
(0.15 M) solution was then added slowly with stirring till
a permanent precipitate occurred (precaution: don’t add
excess dicarboxylate). Acetone was added in equal
amounts to metal salts to get more homogenous, stoi-
chiometric, fine grained powders. The resultant precipi-
tate of ZrO(C4H2O4)·2H2O was white in colour. The so-
lution was filtered after stirring it for 30 minutes. The
filtrate was checked for Zr4+ whose absence ensured
complete precipitation. The precipitate was washed sev-
eral times with cold distilled water and then with acetone
to speed up the drying. It was air dried at the ambient
The similar experimental conditions were used for the
preparation of other dicarboxylates such as (b) succinate,
(ZrO(C4H4O4)·2H2O); (c) tartarate, (ZrO(C4H4O6)·2H2O);
and (d) adipate, (ZrO(C6H8O4)·2H2O).
2.2. Synthesis of Ceria Doped Zirconyl
Dicarboxylate Precursors
(a) Ceria doped zirconyl fumarate one and half hydrate,
Ce0.12Zr0.88O(C4H2O4)·1.5H2O was prepared according to
the similar procedure described in above Subsection 2.1.
by taking Ce(NO3)3·6H2O (3.076 gm), ZrO(NO3)2·nH2O
(12.012 gm) in deionized water (100 ml).
Other dicarboxylates such as (b) succinate, (Ce0.12
Zr0.88O(C4H4O4)·H2O); (c) tartarate, (Ce0.12Zr0.88O(C4H4
O6)·2H2O); and (d) adipate, (Ce0.12Zr0.88O(C6H8O4)·H2O)
were prepared by following the procedure given above.
2.3. Synthesis of Zirconia and
Ceria-Stabilized Zirconia
For the synthesis of zirconia and ceria-stabilized zirconia,
the above dicarboxylate precipitates were decomposed
and calcined slowly at 650oC for about 2 h in a platinum
crucible under static air atmosphere and then slowly
cooled (3oC/min) down to the room temperature. Thus
the heat treatment is sufficient for achieving a complete
decomposition of dicarboxylates. The powder obtained
was polycrystalline. This sample was then reground and
recalcined at the same temperature for another 2 h. The
furnace was turned off and sample was removed at room
temperature. The obtained samples of zirconia and ceria-
stabilized zirconia were stored in a desiccator.
2.4. Sample Characterization
The elemental analysis of carbon and hydrogen were
done by microanalytical technique. The metal analysis of
the samples was carried out by Atomic absorption spec-
trometer (AAS) and Energy dispersive X-ray analysis
(EDAX). The infrared spectra of precursor were re-
corded in the region 4000-450 cm-1 on the Perkin-Elmer
– 1600 series FTIR spectrophotometer using KBr pellets.
The TGA, DTG and DTA were recorded on Mettler
Toledo 850 instrument.
The X-ray powder diffraction patterns were deter-
mined on Rigaku Miniflex Diffractometer using CuKα
radiation (λ = 1.5405 Å; nickel filter). The BET surface
area was determined from N2 adsorption isotherms using
a Coulter (Omnisorp 100 CX) instrument. Morphologi-
cal studies were carried out using scanning electron mi-
croscope (SEM) Philips 30XL model. The particle size
distribution analysis was done using a dynamic light
scattering method (Ultrafine Particle Analyzer (UPA)
from Leeds and Northrup instrument).
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2.5. Slip Preparation and Characterization
For studying suspension rheological flow characteristics
of oxide powders, several suspensions were prepared at
conditions where they could be cast. Rheological tests
permit to identify the conditions where the suspensions
are better dispersed. For these tests, zirconia and ceria-
stabilized zirconia suspensions were prepared in water
and ethanol with different solid loading, by using two
different deflocculants such as Darvan (i.e. polymeth-
crylic acid) and M2OC (i.e. ammonium polyacrylate), by
varying their concentrations and by varying the pH of
the suspension.
The suspensions were placed in a polyethylene bottle
with alumina balls and milled for 24 h to achieve good
homogenous dispersion. After dispersing, the suspen-
sions were degassed for several minutes under a rotary
vacuum pump. Elico pH-meter with a glass combination
electrode was used to measure the pH in all suspension
(i.e. slip). Hydrochloric acid (HCl) and tetramethyl am-
monium hydroxide (C4H13NO) solutions were used to
adjust the suspension pH. Suspension rheological flow
characteristics (i.e. variation of shear stress and viscosity
with shear rate) were determined with Brookfield vis-
cometer model RVT with small sample adaptor acces-
sory SC4 – 14/6R.
The degree of dispersion stability in the ceramic slur-
ries was determined by measuring slip and sedimenta-
tion bulk density. For measuring the slip density, the
suspension was poured in 5 ml specific gravity bottle
and suspension weight was measured. Sedimentation
bulk density was determined by pouring the suspension
i.e. slip (of known weight) into 5 ml measuring cylinder.
The measuring cylinder was covered with flexible film
(to prevent solvent evaporation) and the particles in sus-
pension were allowed to settle until the sediment height
no longer changed with time. The sedimentation bulk
volume was determined directly from sedimentation
height in the measuring cylinder. The weight of ZrO2 or
ceria-stabilized ZrO2 in the sedimentation volume was
determined by multiplying the known weight of suspen-
sion in the cylinder times the known weight percentage
of ZrO2 or ceria-stabilized ZrO2 in the suspension.
The suspension was then cast in a 20 mm × 20 mm × 10
mm acrylate mould placed on a plaster of Paris bricks. After
2 h, the cast solid (i.e. green body) was removed from the
mould. The obtained green bodies were dried at room tem-
perature for one day, and then at 110oC for one day.
For measuring the percent shrinkage, the dimension of
the green bodies dried at room temperature was meas-
ured using the Vernier caliper. The green bodies were
then dried at 110oC for one day in electric oven. After
drying again the dimension of the green bodies was
measured. The green density of the samples was meas-
ured using Archimedes method. Before determination of
the weight in water, the green bodies were immersed in
paraffin to close the pores.
3.1. Dicarboxylate Coprecipitation
During synthesis, several parameters which may influ-
ence the amount of zirconyl dicarboxylate precipitates at
25oC. In order to reach required stoichiometry in dicar-
boxylate precipitates, the control pH and concentration
of metal salt solution is very important.
1) Effect of zirconium concentration
A series of experiments has been made to determine
the most appropriate zirconium concentration values for
a maximum precipitation of dicarboxylates. Several di-
carboxylates such as fumarate, succinate, tartarate and
adipate have been prepared with the zirconium concen-
tration ranging from 0.05 to 0.4 M. The maximum
amount of zirconyl dicarboxylate precipitates (~90%)
are obtained for the zirconium content equals to 0.25 M.
2) Effect of dicarboxylate concentration
A second series of trials has been made to determine
the most appropriate concentrations of dicarboxylic ac-
ids such as fumaric, succinic, tartaric and adipic acid. It
is observed that in all cases the maximum (~92%) pre-
cipitation yield is obtained when dicarboxylic acid con-
centration is equals to 0.15 M. For more dicarboxylic
acid concentration, it shows a decrease of zirconyl pre-
cipitation. Therefore to balance this concentration and to
satisfy practical considerations, we use optimal concen-
tration, i.e., 0.15 M dicarboxylic acid is used for pre-
In case of zirconyl dicarboxylates, the effect of pH
could not studied, since for pH adjustment, precipitation
occur immediately by addition of 10% NH4OH. So the
pH and dicarboxylic acid concentration have been cho-
sen so that the solubility of zirconyl dicarboxylates is as
low as possible. It is difficult to give an exact figure for
new solubility values for dicarboxylate precipitate. The
best range of pH 4.5 to 6.0 is used for the synthesis of
zirconyl dicarboxylate (i.e. precursor).
3.2. Characterization of Dicarboxylate
The composition of precursor is characterized at first
stage by elemental analysis and Atomic absorption spec-
trometry (AAS). The elemental analysis of dicarboxylate
precursors were made in wt.%: for fumarate, ZrO (C4H2
O4)·2H2O (C, 18.83 (18.67); H, 2.22 (2.33); Zr, 35.86
(35.45)) and Ce0.12Zr0.88O(C4H2O4)·1.5H2O (C, 18.32
(18.90); H, 2.06 (1.96); Zr, 31.60 (31.57); Ce, 6.51
(6.61)). For succinate, ZrO(C4H4O4)·2H2O (C, 18.67
A. K. Nikumbh et al. / Natural Science 2 (2010) 694-706
Copyright © 2010 SciRes. OPEN ACCESS
(18.52); H, 2.54 (3.08); Zr, 35.19 (35.18)) and
Ce0.12Zr0.88O(C4H4 O4)·H2O (C, 19.53 (19.43); H, 2.65
(2.42); Zr, 32.55 (32.47); Ce, 6.93 (6.80)). For tartarate,
ZrO(C4H4 O6)·2H2O (C, 16.65 (16.49); H, 2.84 (2.74);
Zr, 31.18 (31. 31)) and Ce0.12Zr0.88O(C4H4O6)·2H2O (C,
16.45 (16.17); H, 3.01 (2.69); Zr, 27.16 (27.00); Ce, 5.73
(5.65)). For adipate, ZrO(C6H8O4)·2H2O (C, 24.81
(25.07); H, 3.71 (4.17); Zr, 31.95 (31.74)) and
Ce0.12Zr0.88O(C6H8O4)·H2O (C, 26.24 (26.16); H, 3.12
(3.63); Zr, 29.18 (29.15); Ce, 6.07 (6.11)). The values in
the parenthesis are calculated ones. The observed values
are found to be ± 0.5% of the nominated values.
The energy dispersive X-ray analysis (EDAX) further
confirms all the cations were present in a perfect cationic
ratio in the precursor. The peaks pertaining to all the
cations were present in the EDAX spectrum. The ele-
mental composition analysis at several spots was uni-
form, which is indicative of a highly homogeneous ma-
terial. This is due to the fact that all the cations are uni-
formly mixed.
The presence of water of crystallization for these pre-
cursors was confirmed on the basis of the thermal analysis
curves under static air atmosphere. These results are fur-
ther supplemented by infrared spectroscopic measurements.
The bidentate linkage of dicarboxylate group with metal
was confirmed on the basis of the difference between the
antisymmetric and symmetric stretching frequencies. A
chain like polymeric octahedral structure has been as-
signed by infrared spectra for these precursors.
Thermal analysis (TGA, DTG and DTA) of the pre-
cursors shows that all the complexes dehydrate and de-
compose in the temperature range 30-600oC. It is ob-
served that the weight loss in TGA corresponds to the
formation of respective zirconia or ceria-stabilized zir-
3.3. Characterization of Zirconia and
Ceria-stabilized Zirconia
After characterizing the above precursor was decom-
posed slowly at 650oC to form respective oxides. The
X-ray diffraction (XRD) patterns of zirconia obtained
from all precursors shows the tetragonal phase. The ex-
perimentally observed d-spacing values and relative in-
tensities are compared with those reported in the litera-
ture [43]. The lattice parameters for each compound are
calculated and are listed in Table 1.
Similarly the addition of 3, 6, 9, 12 mol.% of ceria to
the system, allows stabilization of zirconia matrix in
tetragonal phase. It is observed that for 3 to 9 mol.%
ceria in ZrO2, the diffraction peak to XRD spectra reveal
that the material crystallizes in face centered cubic fluo-
rite type structure. However, for 12 mol.% ceria in ZrO2
powder, the diffraction lines are matched to the tetrago-
nal phase, on the other hand above 12 mol.% ceria the
tetragonal phase is collapse. Thus 12 mol.% ceria stabi-
lized ZrO2 retained tetragonal phase at room temperature.
The experimentally observed d-spacing values and rela-
tive intensities are compared with those reported in the
literature [43]. The lattice parameter for 12 mol.% ceria
stabilized zirconia prepared from different dicarboxylate
are then calculated and shown in Table 1.
The morphology of ceria-stabilized ZrO2 powders was
also analyzed by scanning electron micrographs (SEM),
which revealed agglomerates with irregular shape and
variable packing density of their primary particles (Fig-
ure 1). According to literature [44], these agglomerates
can be classified as ‘soft’ or ‘hard’, the hard agglomer-
ates consisting of close-packed particles with high densi-
ties. As can be seen from Figures 1(a) and (b), the 3 and
6 mol% ceria stabilized zirconia shows agglomerated
structures, while 9 mol% ceria content zirconia show
fused agglomerates (Figure 1(c)). However, for 12 mol%
ceria content zirconia, the SEM show less dense ag-
glomerates, suggesting soft agglomerates with particles
connected by Van der Waals forces. Thus increasing
ceria content, the concentration of intra-aggregate pore
are decreased meaning a reduction of the aggregates
content, up to the disappearance for powder contain-
ing12 mol.% CeO2.
The common notation used in tetragonal zirconia
Table 1. X-ray Powder diffraction and particulate properties data of zirconia and ceria-stabilized zirconia powders.
Compound Lattice Parameters (nm)
a0 c0
Mean Grain size
<D> Xray nm ± 10 %
Surface Area
m2/g Average particle size (by SEM) nm
(a) zirconia
Fumarate 0.511 0.518 21.03 4.87 900
Succinate 0.510 0.519 22.37 1.00 427
Tartarate 0.510 0.519 22.11 1.83 569
Adipate 0.511 0.518 16.77 7.26 222
(b) ceria-stabilized zirconia
Fumarate 0.509 0.513 18.03 10.33 178
Succinate 0.507 0.511 19.24 12.88 357
Tartarate 0.510 0.513 14.35 11.26 476
Adipate 0.507 0.512 13.46 10.55 264
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(a) (b)
(c) (d)
Figure 1. Scanning electron micrographs (SEM) of zirconia powders containing: (a) 3 mol.% ceria; (b) 6 mol.%
ceria; (c) 9 mol.% ceria and (d) 12 mol.% ceria.
polycrystal (TZP), literature [45] involves placing the
cation symbol of the stabilizing oxide before the TZP
abbreviation. In some cases the molarity of the stabiliz-
ing oxide will be indicated by a number before the cation
symbol e.g.
ZrO2 – 12 mol.% CeO2 = 12 Ce – TZP.
The mean grain sizes of ZrO2 and 12Ce–TZP are also
calculated from Debye-Scherrer equation [46]. The ob-
served mean grain size <D>X-ray for ZrO2 and 12Ce–TZP
compounds are respectively in the range of 16.77 to
22.36 nm, and 13.46 to 19.24 nm (Table 1). The BET
specific surface areas are also listed in Table 1. Under
the experimental conditions, the CeO2 content affected
both grain size and surface area of the powders. As ob-
served in Table 1, the grain size decreases with the in-
crease of CeO2 content, i.e., grain growth of zirconia is
inhibited by the ceria doping in zirconia. It is interesting
to observe here that, surface areas increased with a in-
crease of CeO2 content because of the crystallite coars-
ening. This suggests that increasing ceria content re-
duces the surface free energy of zirconia particles and so
increases the surface area, which is accompanied by
more effective tetragonal phase stabilization.
Figure 2 shows scanning electron micrographs (SEM)
of ZrO2 and 12Ce–TZP powders obtained from fumarate,
succinate, tartarate and adipate precursors at 650oC. The
ZrO2 powder obtained from fumarate precursor shows
higher degree of agglomerations, whereas ZrO2 obtained
from tartarate shows apparent agglomeration of very fine
particles (Figure 2(a)). Highly irregular shaped large
particles are observed in case of ZrO2 obtained from
succinate. Moderate sized particles with less agglomera-
tion are observed in ZrO2 obtained from adipate. The
average particle size for all ZrO2 powders is in the range
of 222 to 900 nm.
All 12Ce–TZP powders show less agglomerated stru-
ctures as compared to ZrO2 powders (Figure 2(b)).
Among these powders the 12Ce–TZP obtained from
adipate precursor shows moderate size less agglomerated
particles than 12Ce–TZP obtained from fumarate, suc-
cinate and tartarate precursor. The average particle sizes
of 12Ce–TZP powders obtained from different dicar-
boxylates are in the range of 178 to 476 nm.
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Fumarate Succinate
Tartarate Adipate
Fumarate Succinate
Tartarate Adipate
Figure 2. Scanning electron micrographs (SEM) of (a) zirconia (ZrO2) (b) ceria-stabiized
zirconia (12 Ce–TZP) powders obtained from different dicarboxylate precursors.
3.4. Rheological Studies
3.4.1. Powder Characteristics
Particle size distribution was measured using laser based
particle size analyzer. The preliminary experiments in-
dicated that distilled water and ethanol could be used to
disperse the powder. Figure 3 displays the particle size
distribution of ZrO2 and 12Ce–TZP powders obtained
from different dicarboxylates. From the particle size
distribution measurements occurred that the powder ex-
hibits a wide size distribution and 90% powder being
below 2.5 μm for ZrO2 and 0.9 μm for 12Ce–TZP (Fig-
ures 3(a) and (b)). The particle size distribution of ZrO2
obtained from adipate precursor show 30 to 40% parti-
cles below 1.6 μm and remaining particle have above 1.6
μm. While, 12Ce–TZP powders shows 30 to 40% parti-
cles of 0.4 μm range and remaining have above 0.4 μm.
Thus bimodal type of particle distribution is observed in
both the oxides obtained from adipate precursor. The
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Figure 3. Particle size distribution analysis of (a) zirconia
ZrO2 and (b) ceria stabilized zirconia 12 Ce-TZP powders
obtained from different dicarboxylate precursors.
particle packing is based on the concept of filling the
voids in a bed of large spheres with smaller sized sph-
eres. The remaining pores between the smaller spheres
are then filled with still smaller spheres, etc. to give
good particle packing [47]. The particle size distribution
of ZrO2 and 12Ce–TZP powders obtained from fumarate,
succinate and tartarate shows either smaller or larger
particles (Figure 3). Thus ZrO2 and 12Ce–TZP obtained
from adipate are considered to be good for slip-casting
and hence we prepared in large quantity (~ 200 gm).
3.4.2. Optimization of Rheological Properties of
The state of particulate dispersion is affected mainly by
particle size and particle size distribution, specific sur-
face area of powder and on chemistry of solid/liquid
interface [48], dispersion mechanism [49], and so on.
The viscosity of the suspensions increases drastically
and abruptly, when the weight percent of solid in the
suspension is increased beyond a critical value.
1) Effects of solid loading
The present study shows the viscosity versus shear
rate (Figure 4) for aqueous (distilled water) and non-
aqueous (ethanol), ZrO2 and 12Ce–TZP (obtained from
adipate precursor) suspensions prepared with various
solid loadings (solid/liquid ratios of 75/25, 80/20 and
70/30 by weight and 1.0 wt.% M20C deflocculant). It
can be seen that the aqueous and non-aqueous suspen-
sions with 80 wt.% maximum loading can be achieved a
low viscosity for ZrO2 powder, while 75 wt.% maximum
solid loading can be seen the suspension exhibit low
viscosity for 12Ce–TZP powder. Above these weight
percent, the suspension has a high viscosity and pseudo-
plastic behavior.
2) Effect of dispersant concentration
In the present study, Darvan and M20C are used as a
dispersants (sometimes called deflocculants). The viscosity
versus shear rate curves of the suspensions with constant
solid loading (80 wt.% ZrO2 and 75 wt.% 12Ce–TZP) in
the presence of different added amounts of Darvan and
M20C dispersants are presented in Figures 5 and 6 re-
spectively. From the plots, an optimum Darvan concen-
tration with 1 wt% for ZrO2-water and ethanol system
and 0.8 wt% for 12Ce–TZP-water and ethanol system
could be determined to be minimum viscosity (Fig-
Figure 4. Plots of viscosity against shear rate with different
percentage solid loading. (a) zirconia and (b) 12 Ce–TZP pow-
ders in water and ethanol system.
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Copyright © 2010 SciRes. OPEN ACCESS
ure 5). Similarly, in the presence of M20C dispersant,
the optimum concentration with 1.2 wt% for ZrO2-water
and ethanol system and 1.0 wt% for 12Ce–TZP-water
and ethanol, the suspension exhibits nearly constant vis-
cosity and could be fitted to Newtonian flow behavior
(Figure 6). With further increasing the amount of dis-
persant, the suspensions show pseudoplastic behavior
and the viscosity level also increases. It is interesting to
observe that the M20C is the best dispersant, which
show minimum value of viscosity than Darvan dispers-
ant. Similarly, slurries prepared in distilled water along
with M20C dispersant show low viscosity than slurries
prepared in ethanol medium. Thus, distilled water and
M20C dispersant are used for rheological studies for
ZrO2 and 12Ce–TZP powders.
In above two cases, with low solid loading, the
mag-nitude of the average Van der Waals forces could be
small due to the relatively large distance between sus-
pended particles, so a small addition of dispersant could
lead to repulsion forces with enough magnitude to coun-
terbalance Van der Waals forces [50], and the frequency
of the collisions between separated particles is lower.
Figure 5. Plots of viscosity against shear rate for different concentration of Darvan deflocculant (dispersant) in
water and ethanol system and plots of viscosity against Darvan concentration (at constant shear rate of 40
sec-1). (a) ZrO2; (b) 12 Ce–TZP powders.
A. K. Nikumbh et al. / Natural Science 2 (2010) 694-706
Copyright © 2010 SciRes. OPEN ACCESS
Figure 6. Plots of viscosity against shear rate for different concentration of M20C deflocculant (dispersant) in
water and ethanol system and plots of viscosity against M20C concentration (at constant shear rate of 40 sec-1).
(a) ZrO2 (b) 12 Ce–TZP powders.
This explains why the rheological properties of the sus-
pensions are almost unaffected by the amount of dis-
persant at low solids loading. However, it should also be
pointed out that due to insufficient surface coverage at
lower dispersant additions, the balance between Van der
Waals forces and steric forces is not completely stable,
and agglomerates would easily form through the uncov-
ered surface sites due to Brownian motion. With in-
creasing solid loading, the importance of Van der Waals
forces also increases due to the shorter distances be-
tween suspended particles. Therefore, agglomerates will
readily form if insufficient amount of dispersant is added,
under which conditions Van der Waals forces will domi-
nate the interaction between suspended particles, and
significant shear thinning will be observed due to the
breakdown of agglomerates because of the applied shear
3.4.3. Rheology of Slips
One of the widely practiced methods to stabilize the
slips is electrostatic stabilization, achieved by the repul-
sion of equally charged particles. The repulsive interac-
A. K. Nikumbh et al. / Natural Science 2 (2010) 694-706
Copyright © 2010 SciRes. OPEN ACCESS
tion results from the development of an electric double
layer around the particles up on dispersing a powder into
a aqueous (polar) media and is a function of pH, the con-
centration of specifically adsorbed ions and the ionic
strength of the suspension [52]. The repulsive force de-
creases with increasing separation between the particles.
The possibility of electrostatic stabilization can be eva-
luated by measuring viscosity-shear rate as a function of
To study the rheological properties, the slips were
prepared in the following way:
1) 80 wt% ZrO2 and 20 wt% distilled water along with
1.2 wt% M20C dispersant.
2) 75 wt% 12Ce–TZP and 25 wt% distilled water
along with 1.0 wt% M20C dispersant.
After homogeneous mixing of slips, the pH was ad-
justed to various levels by adding HCl or tetramethyl
ammonium hydroxide. In slip-casting processing, the
amount and rate of dissolution of ions is important [53].
The dissolution of Zr is lower than that of Si and is
slightly reduced with increasing mixing time, i.e., above
110 h [42]. In other report [54], cerium dissolution rate
of Ce-ZrO2 powder under slightly acidic conditions (pH
= 3 to 5.5), only small amounts (1-2 mg L-1 after 10 h
stirring) of cerium were dissolved. No dissolution of
cerium or zirconium was found under basic conditions.
According to these literatures, in the present case the
slurries were prepared by mechanical stirring at room
temperature for 4 h before the viscosity measurements.
Thus, the dissolution of Zr and Ce are totally negligible
in our samples (ZrO2 and 12Ce–TZP powders). The
slurry stability was also evaluated by measuring the set-
tling of powder (sedimentation height as a percentage of
the total suspension height) for different pH of the slips.
The viscosity behavior and sedimentation density are
given in Table 2.
Figures 7(a) and (b) show the rheological behavior
(i.e. viscosity-shear rate) at several pH values for ZrO2-
water and 12Ce-TZP-water systems, with M20C dis-
persant (i.e. deflocculant). The viscosity dependence of
various pH values at constant shear rate (D = 40 sec-1)
are also shown in Figure 7. It is seen that, at neutral pH
the viscosity shows a maximum value, while for acidic
or basic range of pH, the viscosity shows a minimum
value. At these minimum viscosity values, the rheologi-
cal behavior is Newtonian, i.e., viscosity is independent
of shear rate. At this pH range, the particles are well
dispersed and show high slip and sedimentation density.
These results are also presented in Table 2. At remaining
pH values, viscosity is high and slip has pseudoplastic
behavior. Thus, the high sedimentation density and low
viscosity values tend to form good green bodies. Usually,
basic slips are preferred to prevent the rapid formation of
strong bond between the contracted particulate suspen-
sion and mould during casting process [53]. On the other
hand a high concentration of acid slips is corrosive and
attacks the mould used for casting, which can cause
contamination by dissolution of gypsum.
3.4.4. Slip Casting
The packing ability of the dispersed particles on the slip
casting is a good index of the dispersion degree achieved
in the suspensions, and was evaluated by pouring the
required amounts of the slips into a plaster mould. The
ZrO2 and 12Ce–TZP slips obtained at different pH are
then de-aired under a mild vaccum for 15 min. These
slips are then poured into a square plaster mould (20 mm
× 20 mm × 10 mm). The liquid is draw into pores by
capillary action. After 2 h the rectangular green bodies
were removed from the mould and dried at room tem-
perature for 24 h and then put in an oven at 110oC for
another 24 h period. The percent shrinkage was calcu-
Table 2. Rheological properties of ZrO2 and 12Ce–TZP slips at several pH values
pH Viscosity Behaviour Slip density (gm cm-3) Bulk sedimentation density
(gm cm-3) % Shrinkage Green density
(% Theorotical)
(a) 80 wt% ZrO2 + 20 wt% water + 1.2 wt% M20C
1.26 Pseudoplastic 2.86 2.47 0.62 59.99
3.11 Newtonian 2.86 2.58 0.08 71.98
5.76 Pseudoplastic 2.86 2.47 1.03 45.06
8.58 Near Newtonian 2.87 2.54 1.40 72.99
10.16 Newtonian 2.86 2.66 0.16 77.94
13.34 Pseudoplastic 2.86 2.44 0.13 50.27
(b) 75 wt% 12Ce–TZP + 25 wt% water + 1.0 wt% M20C
2.11 Pseudoplastic 2.93 2.58 0.21 47.51
3.41 Near Newtonian 2.90 2.61 0.18 60.14
6.06 Pseudoplastic 2.92 2.52 1.28 42.12
8.16 Near Newtonian 2.89 2.69 1.01 72.19
10.26 Newtonian 2.90 2.71 0.98 75.86
13.04 Pseudoplastic 2.92 2.57 1.52 45.70
A. K. Nikumbh et al. / Natural Science 2 (2010) 694-706
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Figure 7. Plots of viscosity against shear rate for suspensions with pH values indicated and plots of vis-
cosity against several pH values (at constant shear rate of 40 sec-1). (a) 80 wt.% zirconia–20 wt.% wa-
ter–M20C deflocculant; (b) 75 wt.% 12 Ce–TZP –25 wt.% water–M20C deflocculant.
lated. The density of green bodies was also measured
according to the Archimedes method. These results are
also summarized in Table 2. For both the systems, the
highest green density and low percent shrinkage corre-
sponds to minimum viscosity of Newtonian behavior are
observed. In these systems, the highest theoretical green
density was 77% for ZrO2 and 75% for 12Ce–TZP. The
acidic slips of ZrO2 and 12Ce–TZP (both in water-M20C
system) are found to be minute pits on the mould surface
after casting. Basic slips however have a general rough-
ening of the surface rather than pitting. The solid casting
obtained from ZrO2 is white, good in nature and higher
strength than the solid-cast obtained from 12Ce–TZP
and it has pale yellow in colour.
The results presented in this work enable us to draw the
following conclusions:
1) Zirconia (ZrO2) and ceria-stabilized zirconia (12
mol% ceria in ZrO2) powders are synthesized by differ-
ent dicarboxylates such as fumarate, succinate, tartarate
and adipate precursor method.
2) Based on the results obtained from X-ray diffrac-
A. K. Nikumbh et al. / Natural Science 2 (2010) 694-706
Copyright © 2010 SciRes. OPEN ACCESS
tometry, particle size distribution, scanning electron mi-
croscopy and BET surface area measurements, ZrO2 and
ceria-stabilized ZrO2 (12Ce–TZP) powders obtained
from adipate precursor are found to be good for slip
casting. Both powders have tetragonal structure. The
ceria addition is affected the powder characteristics. The
grain size decreases with ceria content, which is accom-
panied by more effective tetragonal phase stabilization in
this powder.
3) The intensity of shear during slip preparation, as
controlled by the solid volume fraction, has a great in-
fluence on the dispersion efficiency, which, in turn, is
reflected in the rheological characteristics of the suspen-
sions and their packing ability on slip-casting. Prelimi-
nary test on the rheological behavior shows 80 wt%
ZrO2 and 75 wt% 12Ce–TZP powders in distilled water
and M20C dispersant gives minimum viscosity and
Newtonian flow behavior.
4) Rheological properties show minimum viscosity for
the system ZrO2-water-M20C at pH = 10.16 and 12Ce–
TZP-water-M20C at pH = 10.26. At these pH range, the
particles are well dispersed and show high slip, green
and sedimentation density. The green density seems even
more reliable in the evaluation of the dispersing degree.
5) The solid casting obtained from ZrO2 powder is
white, good in nature and higher strength than 12Ce–
TZP powder and it has pale yellow colour.
The authors thank Dr Grathwhol from Universitat Bremen, Germany,
for suggestions and helpful discussions. They also thank the Executive
Director, Centre for Materials for Electronics Technology (C-MET)
and Dr. C. V. Rode, National Chemical Laboratory (NCL), Pashan
Road, Pune-411008, India, for the facilities given for part of the work.
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