Synthesis of monoclinic and cubic ZrO2 nanoparticles from zircon

Article abstract of DOI:10.1111/j.1551-2916.2010.04277.x

Zirconia (ZrO₂) nanoparticles with nonstabilized monoclinic and sodium-stabilized cubic phase were produced from zircon sand using the ball mill-aided precipitation route. Characterization and a comprehensive study of nanocrystalline ZrO₂ particles were expressed by X-ray diffraction, particle size distribution (PSD), Fourier transform infrared spectroscopy, thermal analysis, Brunauer-Emmett-Teller surface area and pore size analysis, X-ray fluorescence spectrometry, scanning electron microscopy, and transmission electron microscopy. In this article, the influence of the processing parameters on the crystalline phase, particle size, PSD, aggregation, and morphology are reported. The experimental results prove that the precipitation leads to aggregated particles, which are disaggregated by the ball-milling process. The ball-milling process strongly influences the formation of uniform-sized spherical particles with a high surface area. Fully crystalline monoclinic ZrO₂ nanoparticles with an average particle size of 64 nm (d ₅₀) and the specific surface area of 126 m²/g were obtained. In addition, the sodium-stabilized cubic ZrO₂ nanoparticles with an average particle size of 39 nm (d₅₀) and the specific surface area of 227 m²/g were obtained with the help of the ball-milling process. In the present process, a simple reaction scheme is developed for the large-scale production of stabilized and nonstabilized ZrO₂ nanoparticles using inexpensive precursor obtained from zircon sand.

Full text of DOI:10.1111/j.1551-2916.2010.04277.x

J. Am. Ceram. Soc., 94 [5] 1410–1420 (2011)
DOI: 10.1111/j.1551-2916.2010.04277.x
r 2011 The American Ceramic Society
ournal
J
Synthesis of Monoclinic and Cubic ZrO2 Nanoparticles from Zircon
Palanisamy Manivasakan, and Venkatachalam Rajendranw
Centre for Nanoscience and Technology, K. S. Rangasamy College of Technology, Tiruchengode 637 215, Tamil Nadu,
India
Prema Ranjan Rauta, Bhakta Bandhu Sahu, and Bharati Krushna Panda
Dalmia Institute of Scientific and Industrial Research, Rajgangpur 770 017, Orissa, India
Zirconia (ZrO2) nanoparticles with nonstabilized monoclinic
and sodium-stabilized cubic phase were produced from zircon
sand using the ball mill-aided precipitation route. Characteriza-
tion and a comprehensive study of nanocrystalline ZrO2 parti-
cles were expressed by X-ray diffraction, particle size
distribution (PSD), Fourier transform infrared spectroscopy,
thermal analysis, Brunauer–Emmett–Teller surface area and
pore size analysis, X-ray fluorescence spectrometry, scanning
electron microscopy, and transmission electron microscopy. In
this article, the influence of the processing parameters on the
crystalline phase, particle size, PSD, aggregation, and morphol-
ogy are reported. The experimental results prove that the pre-
cipitation leads to aggregated particles, which are disaggregated
by the ball-milling process. The ball-milling process strongly in-
fluences the formation of uniform-sized spherical particles with a
high surface area. Fully crystalline monoclinic ZrO2 nanopar-
ticles with an average particle size of 64 nm (d50) and the specific
surface area of 126 m2/g were obtained. In addition, the sodium-
stabilized cubic ZrO2 nanoparticles with an average particle size
of 39 nm (d50) and the specific surface area of 227 m2/g were
obtained with the help of the ball-milling process. In the present
process, a simple reaction scheme is developed for the large-
scale production of stabilized and nonstabilized ZrO2 nanopar-
ticles using inexpensive precursor obtained from zircon sand.
cation of ceramics at lower sintering temperature and ultrafine-
grained ceramics.16 Dispersion of nanoscale particles into the
ceramic matrix is used to improve the mechanical properties of a
ceramic-based nanocomposite system. The incorporation of
nanoparticles in the ceramics matrix facilitates considerable
modification of the behavior of the ceramics, which exposes
new functions such as superplasticity or high machinability like
metal.17
In recent years, ZrO2 nanoparticles have attracted wide at-
tention due to their size-dependent properties and significant
technological applications.18–20 ZrO2 nanoparticles with a high
refractive index are used as fillers in transparent coatings such as
anticorrosive, antireflection, and scratch-proof coatings.1 In ad-
dition, there is a high level of interest in zirconia nanocoatings
have because of high-performance applications, such as wear-
resistant insulating coatings, sensors and thermal barrier
coatings.21,22 It is suitable for powder phosphorization and is
used to produce proton-conductive zirconium phosphate parti-
cles. Phosphorized zirconium oxide nanoparticles are used for
the preparation of composite polymer membranes.23 Nanocrys-
talline ZrO2 ceramics, with an average grain size of o100 nm,
has attracted more attention due to its exotic properties such as
sintering ability, mechanical toughness, and superplastic
behavior.24 Owing to their chemical inertness, thermal stability,
high hardness, and refractive index, they are used as an ideal
inorganic material for the fabrication of organic–inorganic
nanocomposites.25 Zirconia nanoparticles having 1–10 nm
pore size distributions are used in membrane applications and
this is often considered because of their stability in the aqueous
environment in a wide pH window.26 The addition of nanoscale
zirconia (ZrO2) particles into alumina matrix is used to enhance
the mechanical properties of Al2O3/ZrO2 composite system.27
Pure ZrO2 has three stable crystal forms, namely P21/c
monoclinic (m), P42/nmc tetragonal (t) and Fm3 m cubic fluo-
rite (c) at different temperatures.22 An orthorhombic polymorph
is also possible,28 but only occurs at room temperature in pure
zirconia under metastable conditions due to constraint.29 Zir-
conia has a monoclinic phase at room temperature and it trans-
forms into tetragonal and cubic phase at 11701 and 23701C,
respectively.24 The tetragonal and cubic phases are unstable
in bulk forms at ambient temperature. Unfortunately, these two
high-temperature phases of zirconia are more valuable for
the technological applications mentioned above than at the
room temperature monoclinic phase. In particular, it has re-
ceived much attention due to their specific applications such as
thermal barrier coatings, high-fracture toughness and electro-
chemical devices.30,31 To stabilize the cubic and tetragonal
phases at room temperature, many divalent and trivalent cat-
ionic species, such as Mg21, Ca21, Y31, and Ce41, are embed-
ded in ZrO2 by doping with MgO, CaO, Y2O3, and CeO2 during
their synthesis.30,32 However, the cubic phase can also be stabi-
lized at room temperature by doping with Na.33
I. Introduction
IRCONIA-BASED advanced ceramics are immensely attractive
Z
because of their outstanding thermal stability, chemical re-
sistance, mechanical characteristics, and ionic conductivity.1–3
The ceramics system established with zirconia has been applied
to wide range of applications such as catalysis,4–6 chromato-
graphic materials,7,8 fuel cell technology,9,10 and gas sensors.6,11–13
Owing to their excellent properties such as high strength, tough-
ness, hardness, wear resistance, and thermal shock resistance,
zirconia-based materials are used in many engineering applica-
tions including automobile engine parts, wire-drawing dies, and
cutting tools. ZrO2 is an attractive oxide ceramic for thermal
barrier coatings on metals and metallic alloys due to its low
thermal conductivity and relatively high coefficient of thermal
expansion.14,15 The developments of functional ceramics require
the nanoscale oxide powders with uniform shapes and narrow
PSD. The production of nanoparticles has attracted consider-
able attention due to their potential applications in the densifi-
R. Cutler—contributing editor
Manuscript No. 28062. Received May 29, 2010; approved October 28, 2010.
This work was financially supported by the Department of Science and Technology,
New Delhi, India to carry out this research project (SR/S5/NM-40/2005 dated, June 26,
2007).
The development of inexpensive methods for producing mass
quantities of highly crystalline and monodispersed nano-ZrO2
wAuthor to whom correspondence should be addressed. e-mail: veerajendran@
gmail.com
1410

May 2011
Synthesis of Monoclinic and Cubic ZrO2 Nanoparticles
1411
particles, remains an area of extensive interest. A variety of
synthesis methods are being explored and developed for the
production of stabilized and nonstabilized nanoscale zirconia
particles, such as thermal decomposition,16,20 sol–gel,34,35 pre-
cipitation,35,36 emulsion precipitation,37 hydrothermal,38 reverse
microemulsion,39 solvothermal synthesis,31 gel combustion,30
plasma synthesis,40 freeze drying,32 sonication,41 and mechani-
cal milling.42 Reliable, high-quality nanometer-sized particles
are readily available using precipitation technologies. These
powders have high chemical purity but are relatively expensive.
Early work showed that zircon ore could be fused at 6001C us-
ing NaOH and mixed with water to form a hydrolyzed zircon-
ate, which was further treated with HCl to form a zirconium
oxychloride before precipitation.43 This is an inexpensive pro-
cess and has not been exploited to the authors’ knowledge. The
purpose of this paper is to use a slight variation on this process
and use nitric acid to form a zirconyl nitrate, which can be pre-
cipitated. The use of ball milling as a means for breaking down
loose agglomerates is also explored. It was hypothesized that
this route may be less expensive than hydrothermal44 or other
industrial processes currently in use45 and may be suitable for
applications requiring high-surface area where sodium impuri-
ties can be tolerated. The objective of the present paper is to
demonstrate a process for making soft, nonagglomerated, and
inexpensive nanometer-sized zirconia powder.
precipitated at pH 7. At this stage of the reaction, a character-
istic white precipitate of ZrO(OH)2 ꢀ xH2O was appeared. The
resultant precipitate was aged in the mother liquor at a temper-
ature of 251C for 24 h in a bath at constant temperature. After
the completion of the ageing period, the precipitate was filtered
and washed several times in double-distilled water until the pre-
cipitate was free from sodium nitrate and unreacted compo-
nents. Then, the precipitate was dried in a hot air oven at a
temperature of 1201C for 1 h. The synthesis of nanozirconia
particles was performed at three different pH values such as 7,
10, and 1370.1. The above process was repeated for pH 10 and
1370.1. All the synthesized samples were calcined at different
temperatures ranging from 5001 to 8001C for 6 h in a static air
atmosphere with a heating rate of 51C/min and were then cooled
to room temperature inside the oven.
(A) Ball Milling
Ball milling of soft aggregates of calcined (at 5001C) samples
were performed by mean of a planetary ball mill (PM100; Ret-
sch Corporation, Haan, Germany) in a dry medium at 500 rpm
for 3 h. Milling parameters such as ball to charge ratio (20:1)
and rotational speed (500 rpm) as well as grinding time (3 h)
were optimized. Milling was done in a 250 mL of zirconia grind-
ing jar with protective jacket of zirconium oxide. Zirconium ox-
ide balls of 10 mm were utilized for millings. The samples
calcined at 5001C were placed in the jar at room temperature
and atmospheric pressure then sealed and imposed to milling.
After grinding period, the jar was allowed to be cooled down to
room temperature. The particles were collected from grinding
jar and stored under nitrogen atmosphere to prevent particles
agglomeration. The overall yield of present process is 84 %, i.e.,
5.2 g out of 10 g of zircon (63 wt% ZrO2).
II. Experimental Procedure
(1) Extraction of Zirconyl Nitrate from Zircon Sand
Zircon sand (ZrSiO4) obtained from the Kanyakumari region, a
coastal part of Tamil Nadu, India, was used as a starting ma-
terial for precursor synthesis without further purification. Phys-
ical and chemical analysis of zircon sand is given in Table I.
Twenty-five grams of zircon sand that was obtained was
powdered and roasted at 1501C for 1 h to remove the moisture
content followed by boiling with 25 mL of concentrated hydro-
chloric acid (35%, Merck GR, Mumbai, India) at 501C for 1 h
on a hot plate. The water-soluble metal chlorides that were
present in the acid-treated sand were removed by washing with
double-distilled water. The residue was further fused with about
four times its weight (100 g) of sodium hydroxide (98%, Merck
GR) at 6001C for 3 h. The fused mass was cooled to room tem-
perature and then the hot cake that was formed was leached
with water and filtered. The insoluble residue consisted of zir-
conium hydroxide, which was dissolved in 250 mL of 6M nitric
acid (69%, Merck GR) and then filtered. The clear filtrate that
was obtained was repeatedly evaporated to dryness and the res-
idue was extracted using 250 mL of DD water. The extracted
zirconyl nitrate was used as the precursor for the synthesis of
nano-ZrO2 particles. The chemical reactions, which took place
during precursor extraction and synthesis of nano-ZrO2 parti-
cles, are shown in Scheme 1.
(4) Powder Characterization
(A) X-ray Powder Diffraction (XRD): Crystallinity and
the crystalline phase of synthesized zirconia samples were de-
termined by XRD (X0Pert Pro, PANalytical, Almelo, the Neth-
˚
erlands) using CuKa as a radiation source (l 5 0.15406 A). The
samples were scanned in the 2y range from 101 to 801 at a scan-
ning rate of 241/min. The average crystallite size of all the zir-
conia samples was calculated using the Scherrer’s formula.31
kl
b cosy
D ¼
(1)
where D is the crystal size, k the Scherrer’s constant (k 5 0.9), l
the wavelength of the X-ray, b the full-peak width at half of
the maximum intensity after correction for instrument-broad-
ening contributions and y the peak position. The percentage
composition of each phase was calculated from the integral peak
intensity ratio.46,47
(B) Particle Size Distribution (PSD): The PSD was de-
termined with a submicrometer particle size analyzer (Nan-
ophox, Sympatec, Clausthal-Zellerfeld, Germany) according to
the dynamic light scattering technique. The particle size of all the
samples was measured in the range of 1–1000 nm at the scatter-
ing angle of 901. The three-dimensional photon cross-correlation
technique was used for the simultaneous measurement of particle
size and stability. The He–Ne Laser with 10 mW maximum in-
tensity was used as a light source at a wavelength of 632.8 nm. As
synthesized ZrO2 nanoparticles were dispersed in an aqueous so-
lution, which contains 5 mM 4,5-dihydroxy-m-benzenedisulfonic
acid disodium salt (99%, Loba GR, Mumbai, India) under soni-
cation. To achieve stable dispersion, the pH of ZrO2 colloidal
solution was adjusted to pH 12 by adding a 0.1 N NaOH (98%,
Merck GR) solution. The sonochemically dispersed stable ZrO2
colloidal solution was used to obtain particles size distribution.
(C) Fourier Transform Infrared (FT-IR) Spectrome-
try: An FT-IR spectrum of as-synthesized zirconia samples
was obtained on a PerkinElmer infrared spectrometer (spectrum
100, PerkinElmer, Waltham, MA) using KBr pellets. The KBr
(2) Synthesis of Zirconia (ZrO2) Nanoparticles
Two-hundred and fifty milliliter of the extracted zirconyl nitrate
(ZrO(NO3)2) precursor was hydrolyzed by the drop-wise addi-
tion of 4M NaOH (98%, Merck GR) solution while being con-
stantly stirred and an amorphous hydrated zirconium oxide was
Table I. Physical and Chemical Analysis of Zircon Sand
Physical analysis
Property
XRF chemical analysis
Proportion
Component
Weight (%)
Apparent porosity (%)
Bulk density (g/cm3)
Apparent specific gravity
Particle size (mm)
0.20
4.65
4.20
12.50
5.00
0.10
ZrO2
SiO2
Na2O
CaO
Hf
62.970.1
35.270.1
0.370.1
0.0870.01
1.5070.1
—
Surface area (m2/g)
Loss on Ignition (%)
—

1412
Journal of the American Ceramic Society—Manivasakan et al.
Vol. 94, No. 5
Scheme 1. Schematic representation of the synthesis of nano zirconia particles from zircon sand using the conventional precipitation route.
discs were made by pressing the mixture, which contained 10 mg
of zirconia nanoparticles with 100 mg of KBr at a pressure of
125 kg/cm2.
(D) Thermal Analysis: Thermogravimetric (TG) and
differential thermal analysis (DTA) measurements were carried
out using thermal gravimetric/differential thermal analyzers (Di-
amond TG/DTA, PerkinElmer). The samples were heated con-
tinuously with a rate of 101C/min from 40 to 12001C in a static
air atmosphere.
(E) Brunauer–Emmett–Teller (BET) Surface Area Anal-
ysis: The specific surface areas of all the samples were calcu-
lated according to the BET method48 using the BET surface area
analyzer (Autosorb AS-1MP, Quantachrome, Boynton Beach,
FL). The samples were degassed under vacuum at 2951C for 3 h
to remove the physisorbed moisture. The physisorption analysis
was done with N2 adsorption–desorption measurements at liquid
nitrogen temperature (ꢁ1961C). Very low temperature is used to
avoid any thermally induced changes on the surface of the par-
ticles. The average pore diameter and total pore volume were
calculated according to the Barret–Joyner–Halenda method.49
(F) X-ray Fluorescence Spectrometry (XRF): Qualita-
tive and quantitative elemental analysis of zircon and zirconia
samples was performed using XRF (EDX-720, Shimadzu,
Kyoto, Japan). The powder samples can be analyzed directly
without any preparation on it.
analysis using EDTA titration method.50 A weighted sample
(0.25 g) of zirconia was fused with a 1:1 borax and sodium car-
bonate fusion mixture, dissolved in 2 N HNO3 and filtered. The
filtrate was made up to 250 mL by using a 2 N HNO3 solution in
a standard measuring flask (SMF). An SMF solution (50 mL)
was added into a conical flask, which contained 60 mL of water,
and was boiled for 5 min; then it was titrated against a 0.02 N
EDTA solution using xylenol, an orange indicator in a hot con-
dition (601C). The end point was marked as the disappearance
of pink color.
(H) Electron Microscopic Analysis: The surface mor-
phology of the samples was inspected using an SEM (JSM-
6390LV, JEOL, Tokyo, Japan) with an accelerating voltage of
25 KV. Transmission electron microscope (TEM) pictures of the
samples were obtained using a TEM (CM200, Philips, Hills-
boro, OR) operating at 120 kV.
III. Results and Discussion
(1) TG/DTA Observation
Figures 1(a) and (b) displays the DTA and TGA curves of the
hydrous zirconia synthesized at pH 7 after being dried at 1201C.
TGA results indicate that the samples obtained by the wet
methods are hydrous samples. The controlled thermal decom-
position of hydrous zirconia (ZrO(OH)2 ꢀ xH2O) and its crystal-
lization temperature are established with the help of TG/DTA
(G) Chemical Analysis: The chemical purity of the as-
synthesized samples was determined through wet chemical

May 2011
Synthesis of Monoclinic and Cubic ZrO2 Nanoparticles
1413
analysis. When it is heated, a part of the structural H2O mol-
ecules is decomposed and leads to the ZrO2 ꢀ xH2O structure.
The self-controlled three-step thermal decomposition behavior
of hydrous zirconia can be written as:
results summarize the finding that the ZrO2 powder consists of
chemisorbed hydroxyl groups and adsorbed water molecules.
(3) XRD Studies
Three samples of zirconia were synthesized at three different pH
levels, respectively, pH 7, 10, and 1370.1 to investigate the
effect of pH on crystalline phase. The crystalline phase, phase
fraction and average crystallite size of all zirconia samples are
presented in Table II. Figures 3(a–d) shows the XRD patterns of
the zirconia sample precipitated at pH 770.1 after calcinations
at (a) 5001, (b) 6001, (c) 7001, and (d) 8001C for 6 h. Figure 4
displays XRD data of ZrO2 synthesized at pH 1070.1 after be-
ing calcined at 5001C for 6 h. Figures 5(a–d) shows the XRD
patterns of the zirconia sample precipitated at pH 1370.1 after
calcinations at (a) 5001, (b) 6001, (c) 7001, and (d) 8001C for 6 h.
The diffraction peaks of all the samples were identified and as-
signed with standard powder diffraction data using the JCPDS
Files 81–1314 (monoclinic), 87–2105 (orthorhombic), 49–1746
(orthorhombic), 50–1089 (tetragonal), and 65–0461(cubic).
From the XRD pattern (Fig. 3(a)), it is found that the ZrO2
particles synthesized at pH 770.1 yield a predominantly mono-
clinic (90%) phase along with some fraction of orthorhombic
(10%) zirconia. The XRD pattern (Fig. 4) of the sample syn-
thesized at pH 1070.1 results a monoclinic structure (86%) co-
existing with some fraction of cubic zirconia (14%), whereas the
XRD pattern (Fig. 5(a)) of the sample synthesized at pH 1370.1
exhibits a cubic phase (96%) along with some fraction of te-
tragonal (4%) zirconia. It can be seen from the XRD results that
the phase transformation of zirconia is influenced by the pH of
precipitation. A greater quantity (90%) of monoclinic zirconia
(m-ZrO2) is noticed at pH 7, which is reduced (86%) in the
sample synthesized at pH 10. In contrast, the minor orthorhom-
bic phase (10%) obtained at pH 7 disappeared at pH 10. The
minor orthorhombic zirconia obtained at pH 7 was transformed
into cubic zirconia (14%) at pH 10, while both the monoclinic
and orthorhombic structures disappeared in the sample synthe-
sized at pH 13. Instead of both monoclinic and orthorhombic
structures, cubic (96%) and tetragonal (4%) zirconia is detected
at pH 13.
In the DTA curve, the broad endothermic peak centered at
2201C is attributed to a loss of the physically adsorbed water
and the weight loss on heating is equal to 10.6% at 1481C and
12.8% at 1831C. The endothermic effect occurring at 4311C is
assigned to a loss of chemically coordinated (chemisorbed) wa-
ter molecules present in hydrous zirconia and the weight loss on
heating is equal to 23.7% at 4311C. The total 24.3% weight loss
on heating to 4801C results from the molecular decomposition
of ZrO(OH)2 ꢀ H2O into ZrO2 and 2H2O.
It is noted that hydrous zirconia is progressively decomposed
into amorphous zirconia due to dehydroxylation with increasing
temperature (4311C). A sharp exothermic peak that appeared at
4801C is associated with the enthalpy of the transformation of
the amorphous zirconia into crystalline zirconia. The kinetic
process of dehydration and crystallization of hydrous zirconia
appeared, respectively, at 4311 and 4801C. The above thermal
decomposition behavior of hydrous zirconia is in good
agreement with the data obtained for undoped-zirconia nano-
particles synthesized by other methods.24,51
(2) FT-IR Measurements
(4) Effect of Precipitation pH
Figure 2 displays the FT-IR spectra of zirconia samples precip-
itated at pH values of 7, 10, and 13 after calcination at 5001C.
The IR active modes, between 490 and 1030 cmꢁ1, correspond
to the asymmetric and symmetric stretching frequencies of Zr–
O–Zr vibrational bands.52 The absorption peaks obtained at
1360 and 1630 cmꢁ1 show, respectively, the O–H stretching and
bending vibrations of ZrO2 ꢀ xH2O and H2O molecules. The OH
group is distinguished easily in the H2O molecule by its bend-
The above structural transitions are explained in terms of the
nonstabilized monoclinic and sodium-stabilized cubic zirconia.6
In an effort to understand the effect of pH and sodium ions
on the crystal structure of zirconia, the samples synthesized at
pH 7 and 13 were examined by qualitative elemental analysis.
ing vibrations that appear in a single band at B1630 cmꢁ1
.
Further, the O–H bending vibration of the hydroxyl group of
ZrO2 ꢀ xH2O molecule appears at B1360 cmꢁ1. The above
Fig. 2. FT-IR spectra of ZrO2 samples precipitated at (a) pH 7, (b) pH
10, and (c) pH 13 after calcination at 5001C and after ball milling for 3 h
at 500 rpm.
Fig. 1. (a) DTA and (b) TGA curves of the hydrous zirconia precip-
itated at pH 7 after dried at 1201C.

1414
Journal of the American Ceramic Society—Manivasakan et al.
Vol. 94, No. 5
Table II. Crystalline Phase, Phase Composition, and Average Grain Size of ZrO2 Samples Precipitated at pH 7, 10, and 13
Precipitation pH (70.1)
Calcinations temperature (1C)
Crystalline phase
Phase fraction % (71)
Average particle size (nm)
7
500
Monoclinic
Orthorhombic
Monoclinic
Orthorhombic
Monoclinic
Orthorhombic
Monoclinic
Orthorhombic
Monoclinic
Cubic
Cubic
Tetragonal
Cubic
Tetragonal
Cubic
90
10
92
08
93
07
95
05
86
14
96
04
96
04
95
05
92
08
22
600
700
800
500
500
600
700
800
27
32
38
18
11
16
19
22
10
13
Tetragonal
Cubic
Tetragonal
Table III shows that the chemical composition of ZrO2 samples
precipitated at pH 7 and 13 after calcinations at 5001C. It can be
observed from Table III that the major components present in
the sample precipitated at pH 770.1 are 97.8 wt% ZrO2,
0.2 wt% Na2O, 0.4 wt% SiO2, 0.012 wt% CaO, and 1.5 wt%
Hf, whereas the samples precipitated at pH 1370.1 are 95.3
wt% ZrO2, 2.6 wt% Na2O, 0.5 wt% SiO2, 0.016 wt% CaO, and
1.5 wt% Hf. It is obvious that the sample synthesized at pH
1370.1 has 2.6 wt% of sodium in its composition whereas the
sample synthesized at pH 770.1 has 0.2 wt% sodium in its
composition. When compared with pH 770.1, pH 1370.1 has
13 times higher sodium content in its composition.
It is used to frame the discussion that the presence of sodium
in the crystal structure is influenced by the phase formation of
zirconia. The above result agrees with the known fact that a
fraction of Na1 cations present in the mother liquor after the
precipitation at pH 1370.1 is electrostatically chemisorbed on
the surface of ZrO(OH)2 ꢀ xH2O. At basic pH 1370.1, it is
noteworthy that the strong negative surface potential53 of
ZrO(OH)2 ꢀ xH2O facilitates the chemisorptions of Na1 cation
on the precipitate surface. The chemisorbed sodium ions enter
the structure of zirconia during the crystallization to form so-
dium-stabilized cubic zirconia. Formation of sodium-stabilized
cubic zirconia under basic pH is studied experimentally in detail
by other methods developed from chemical precursors.6 At neu-
tral pH 770.1, the surface potential of ZrO(OH)2 ꢀ xH2O is
equal to zero, i.e. electro kinetically uncharged.53 In this case,
Na1 cations are not chemisorbed on the uncharged surface of
ZrO(OH)2 ꢀ xH2O and lead to monoclinic zirconia. The mono-
clinic-to-cubic phase transformation is clearly established by
varying the pH from 7 to 10 and 10 to 13. It is evident from
XRD results that the neutral pH 770.1 has no cubic phase
whereas the pH 10 and 1370.1 yields, respectively, 14% and
96% cubic phases. In conclusion, the pH of the medium is
higher than the pH of the isoelectric point, i.e. pH 7, the negative
charge developed on the surface of hydrous zirconia and
chemisorptions of sodium ions take place. An increase in pH
during precipitation results in an increase in the interaction be-
tween the sodium ions and hydrous zirconia, which leads to in-
creased sodium ion chemisorptions and formation of cubic
zirconia.
The XRD patterns of zirconia samples synthesized at pH
770.1 (Figs. 3(a–d)) and pH 1370.1 (Figs. 5(a–d)) indicate the
influence of calcination temperature on the crystalline phase and
size of zirconia in the temperature range of 5001–8001C. It can
be seen from Table II that the calcination temperature of the
sample that is synthesized at pH 770.1 increases from 5001 to
8001C, the percentage of monoclinic phase increases from 90%
to 95% while decreasing the percentage of the orthorhombic
phase from 10% to 5% (Figs. 3(a–d)). A complete (498%)
transformation of the orthorhombic to the monoclinic phase
was observed after calcination at 10001C. As shown in Fig. 3(a),
after calcination at 5001C for 6h, the average crystallite size of
monoclinic zirconia is about 22 nm, which increases to 27 nm at
Fig. 3. XRD patterns of ZrO2 samples precipitated at pH 7 after calcination at (a) 5001C, (b) 6001C, (c) 7001C, and (d) 8001C and after ball milling for
3 h at 500 rpm.

May 2011
Synthesis of Monoclinic and Cubic ZrO2 Nanoparticles
1415
accompanied by an increase in crystallite size, which induced the
phase transformation of zirconia. The present work shows close
agreement with early studies54,33 that cubic zirconia has been
stabilized by incorporation of sodium in zirconia gel during the
precipitation of hydrous zirconia under alkaline conditions.
Conversely, Chang et al.55 showed that the monoclinic phase
with increased crystallite size was formed at 10001C while in-
creasing the sodium content. However, our results indicated that
increasing the sodium content is detrimental to the stabilization
of cubic zirconia with decreased crystalline size at 5001C. This is
due to effect of high-energy ball milling which leads to incor-
poration of sodium in zirconia crystal structure and form cubic
zirconia with reduced crystalline size. Further, the present work
shows that the sodium stabilized cubic zirconia has been desta-
bilized at higher calcination temperature (410001C) to form
monoclinic zirconia with increased crystallite size. It is in line
with early work56 that the sodium destabilizes the metastable,
i.e., tetragonal zirconia and lead to the formation of monoclinic
phase at higher sintering temperature.
Fig. 4. XRD patterns of ZrO2 sample precipitated at pH 10 after
calcination at 5001C and after ball milling for 3 h at 500 rpm.
(4) Chemical Purity
It is evident from the chemical analysis that the sample synthe-
sized at pH 770.1 contains 97.6% of ZrO2, whereas the sample
synthesized at pH 1370.1 contains 92.5% of ZrO2. It can be
seen from XRF (Table III) results that the samples synthesized
at pH 770.1 contains 97.8% of ZrO2, whereas the sample syn-
thesized at pH 1370.1 contains 95.3% of ZrO2. The results ob-
tained from quantitative elemental analysis (XRF) are in close
agreement with the results obtained from the chemical analysis.
From the above studies, it was concluded that the nonstabilized
monoclinic zirconia synthesized at pH 770.1 shows 97.6%
chemical purity whereas the sodium-stabilized cubic zirconia
synthesized at pH 1370.1 shows 92.5% chemical purity. Com-
mercial high-purity zirconia powders have Na contents, which
are o0.05 wt%. Not only do high sodium contents impede
densification56 but they are detrimental to strength and ionic
conductivity, which are important properties of zirconia-based
materials. While zirconia-containing sodium would still be use-
ful for thermal barrier coatings, it is clear that the sodium con-
tents in the present powders are too high to be acceptable.
Keeping the pH low and lowering the sodium content is there-
fore critical issues to address if this method is to be pursued.
6001C, 32 nm at 7001C and 38 nm at 8001C (Table II). Table II
indicates that crystallite size and orthorhombic-to-monoclinic
phase transformation are inter-related. When increasing the ca-
lcinations temperature, the monoclinic phase fraction starts to
increase significantly with an increase in the crystallite size. The
major monoclinic zirconia appears only in the samples synthe-
sized at pH 7, but even at pH 13, cubic zirconia is predominant
in the sample. The enhanced durability of the cubic phase in this
case can be explained by partial incorporation of sodium into
the zirconia structure.6 When the calcination temperature of the
sample synthesized at pH 1370.1 from 5001 to 8001C (Figs.
5(a–d)) is increased, the percentage of the cubic phase decreases
from 96% to 92%, whereas the percentage of the tetragonal
phase increases from 4% to 8%. A complete (496%) transfor-
mation of the cubic to the monoclinic phase was observed after
calcinations at 12001C.
Table II shows that the average crystallite size of cubic zir-
conia is 11 nm at 5001C, 16 nm at 6001, 19 nm at 7001, and
22 nm at 8001C. The above results indicate that the grain size of
cubic zirconia is increased considerably with an increase in the
calcination temperature, which notably induces the cubic-to-
tetragonal phase transformation. In general, the results that are
observed show that the degree of crystallinity and the crystallite
size of both monoclinic and cubic zirconia increase with increas-
ing calcination temperature. Further, the temperature seems to
be the dominant factor for the grain size and phase contents.
The results that are obtained in accordance with earlier
reports6,35 are that the increase in calcination temperature is
(5) PSD
Figure 6 presents the PSD of ZrO2 samples after calcination at
5001C, (a) pH 7 and (b) pH 13 samples before milling, (c) pH 7
and (d) pH 13 after milling for 3 h at 500 rpm. It can be seen
from Figs. 6(a) and (c) that the monoclinic zirconia before ball
milling yields a particle size in the range of 269–444 nm
(Fig. 6(a)), whereas the ball-milled monoclinic zirconia yields a
Fig. 5. XRD patterns of ZrO2 samples precipitated at pH 13 after calcination at (a) 5001C, (b) 6001C, (c) 7001C, and (d) 8001C and after ball milling for
3 h at 500 rpm.

1416
Journal of the American Ceramic Society—Manivasakan et al.
Vol. 94, No. 5
Table III. Chemical Compositions of ZrO2 Samples Precipitated at pH 7 and 13
XRF chemical composition (wt%)
Precipitation pH (70.1)
ZrO2
Na2O
SiO2
CaO
Hf
7
13
97.870.1
95.370.1
0.270.1
2.670.1
0.470.1
0.570.1
0.01270.01
0.01670.01
1.570.1
1.570.1
particle size in the range of 5–193 nm (Fig. 6(c)). Before ball
milling, the mean diameter of monoclinic zirconia is about 327
nm, which decreases to 64 nm after ball milling (Figs. 6(a) and
(c)). Hence, it is evident that precipitation followed by calcina-
tion yields aggregated zirconia particles, which are further dis-
aggregated by ball milling. Four to five times reduction of the
mean diameter of zirconia particles results with the aid of the
ball-milling process. It can be seen from Fig. 6(b), before ball
milling the calcined cubic zirconia yields a particle size in the
range of 122–300 nm with an average diameter of 193 nm,
whereas the ball-milled cubic zirconia (Fig. 6(d)) yields a particle
size in the range of 26–54 nm with an average diameter of 39 nm.
From the above observations it is clear that the calcined cubic
zirconia yields highly aggregated particles (193 nm), which are
disaggregated (39 nm) by the ball-milling process. From the
above results, it can be understood that the ball-milling process
helps the diminution of the particle size of the aggregated zir-
conia by four to five times. The effect of ball milling on the PSD
and average diameter was studied with help of PSD analysis. In
addition, the width of the PSD can be directly derived from the
scattering parameters obtained from the PSD results. Figure 6
show that calcined zirconia has a wide PSD width when com-
pared with the ball-milled samples. From the PSD results, we
conclude that the precipitation, drying and calcination steps lead
to aggregated particles that are further disaggregated by a sub-
sequent ball-milling process. While planetary mixing at a high
ball to charge ratio was effective in increasing the surface area,
two problems are obvious: (1) as the process is scaled up from a
laboratory process to a commercially viable process it is not
possible to keep the ball to charge ratio as high as in this process,
and (2) high surface areas mean that particle packing is difficult.
Commercial powders are typically having surface areas on the
order of 5–20 m2/g. The important point from the present study
is that it shows that this process results in soft agglomerates,
which are easy to break down.
(6) Effect of Calcinations Temperature
Figure 7 presents the effect of calcinations temperature on the
particle size of ZrO2 samples synthesized at (a) pH 7, (b) pH 10,
Fig. 6. Particle size distribution of ZrO2 after calcination at 5001C, (a) pH 7 and (b) pH 13 samples before ball milling, (c) pH 7 and (d) pH 13 samples
after ball milling for 3 h at 500 rpm.

May 2011
Synthesis of Monoclinic and Cubic ZrO2 Nanoparticles
1417
and (c) pH 13. Figure 7 shows a steady progression toward
larger particle size as the temperature increases. It is noted
(Fig. 7) that the variation in the calcination temperature from
5001 to 8001C leads to variations in average particle size, which
is in the range of 64–135, 56–110, and 39–86 nm, respectively,
for the samples synthesized at pH values of 7, 10, and 13. The
above result indicates that the powders that are obtained by
high-temperature calcination are aggregated because the aggre-
gates will be hard, which will be difficult to break efficiently by a
subsequent ball-milling process. To obtain a well-dispersed
nanosized particle product, the processing temperature must
be kept as low as possible and high-temperature calcination
should be avoided.
(7) SEM and TEM Observation
It is interesting to note that the PSD results are in good agree-
ment with SEM studies. The influence of ball milling on particles
size and morphology is also well established with SEM obser-
vations. Figure 8 displays the SEM images of ZrO2 samples after
calcination at 5001C, (a) pH 7 and (b) pH 13 samples before
milling, (c) pH 7 and (d) pH 13 after milling for 3 h at 500 rpm.
Similarity with PSD (Fig. 6(a)), the SEM micrograph of mono-
clinic zirconia before ball milling (Fig. 8(a)) is shows a highly
aggregated surface. Particle aggregation is apparent, suggesting
the formation of a continuous particulate network in the
suspension structure, whereas the SEM micrograph of the
Fig. 7. Effect of calcination temperature on the particle size of ZrO2
samples, (a) pH 7, (b) pH 10, and (c) pH 13.
Fig. 8. SEM images of ZrO2 after calcination at 5001C, (a) pH 7 and (b) pH 13 samples before ball milling, (c) pH 7 and (d) pH 13 samples after ball
milling for 3 h at 500 rpm.

1418
Journal of the American Ceramic Society—Manivasakan et al.
Vol. 94, No. 5
Fig. 9. Transmission electron microscopic images of ZrO2 samples precipitated at (a) pH 7 and (b) pH 13 calcined at 5001C after ball milling for 3 h at
500 rpm.
ball-milled monoclinic zirconia (Fig. 8(c)) shows the disaggre-
gated particles with almost spherical morphology. It can be seen
from Fig. 8(b) that synthesized cubic zirconia has an aggregated
surface with higher particles size, whereas the ball-milled
(Fig. 8(d)) cubic zirconia gets disaggregated particles with quite
reduced particles size and highly shaped spherical morphology.
Aggregated and nonaggregated zirconia could be well distin-
guished from the SEM results. The SEM findings not only jus-
tify the aggregation of the particles but also show the particle
morphology. Figure 9 displays the TEM images of ZrO2 sam-
ples precipitated at pH 7 and pH 13 calcined at 5001C after ball
milling for 3 h at 500 rpm. It can be seen from the TEM images
that cubic zirconia (pH 13) has predominantly spherical parti-
cles with a slightly agglomerated surface, whereas monoclinic
zirconia (pH 7) shows almost spherical morphology with some
rectangular-shaped particles. The above results are in good
agreement with PSD and SEM observations, which are pre-
sented in Figs. 6 and 7.
are 126, 190, and 227 m2/g, respectively, with increasing the pH
value of precipitation from 7 to 10 and 10 to 13 (Table IV).
Disaggregated particles have six to seven times higher specific
surface area than the aggregated particles. Aggregated and non-
aggregated particles are well distinguished from the observed
surface area of the zirconia samples. When increasing the pH (7–
13) values of precipitation, a decrease in the mean aggregate and
particle size and an increase in the surface area of the zirconia
samples are noticed. The above results conclude that the mono-
clinic-to-cubic phase transformation leads to higher surface area
and lower particle size.
The particle size of zirconia depends on the nucleation of hy-
drous zirconia sols during the precipitation process. Further, the
nucleation of hydrous zirconia depends on the experimental
conditions, including pH, temperature, concentration of reac-
tants, nature of anions, and stirring and ageing time.57 It is used
to frame the discussion that the pH value of precipitation affects
the size distribution of the zirconia particles. In a neutral value
of pH, the surface potential of hydrous zirconia is almost zero,
i.e., near iso-electric,53 which induce surface aggregation of hy-
drous zirconia sols because of hydrogen bonding caused by van
der Waals attraction between the hydrolyzed hydrous zirconia
molecules. At the basic pH value of 10 and 13, the surface of
hydrous zirconia has a negative z potential, which, in turn, re-
duces the surface aggregation of hydrous zirconia sols because
of the strong electrostatic repulsion between the negatively
charged hydrous zirconia molecules. Thus, the aggregation be-
havior of hydrous zirconia is strongly dependent on the pH
value according to the Derjaguin and Landau, Verwey and
Overbeek theory.58 In addition, above a pH value of 7, i.e., ba-
sic pH value of 10 and 13, the present process has free sodium
ions in the mother liquor, which protects the coalescence of hy-
drous zirconia molecules because of the chemisorption of so-
dium ions on the negative surface of hydrous zirconia. The
(8) Textural Properties and Particle Size
Table IV summarises BET surface area, mean aggregate, and
particle size of ZrO2 samples before and after ball milling for 3 h
at 500 rpm. Before ball milling, the mean aggregate size of zir-
conia is 327, 220, and 193 nm, respectively, for the samples pre-
cipitated pH 7, 10, and 13. After ball milling, the mean particle
size of zirconia is 64, 56, and 39 nm, respectively, for the samples
precipitated with pH 7, 10, and 13 (Table IV). Whereas the av-
erage crystalline size decreases from 22 to 18 and 18 to 11 nm
with an increase in the pH value of precipitation from 7 to 10
and 10 to 13 (Table II). It can be noticed from Table IV that the
surface area of aggregated particles are 18, 26, and 35 m2/g, re-
spectively, for the samples precipitated pH 7, 10, and 13 (Table
IV). After milling, the surface area of the disaggregated particles
Table IV. BET Surface Area, Mean Aggregate, and Particle Size of ZrO2 Samples Before and After Ball Milling for 3 h at 500 rpm
Before milling
After milling
Precipitation pH
Calcination temperature
Mean aggregate size
BET surface area
(m2/g)
Mean particle size
BET surface area
(m2/g)
(70.1)
(1C)
(nm)
(nm)
7
10
13
500
500
500
327
220
193
18
26
35
64
56
39
126
190
227

May 2011
Synthesis of Monoclinic and Cubic ZrO2 Nanoparticles
1419
16S. Jiang, G. C. Stangle, V. R. W. Amarakoon, and W. A. Schulze, ‘‘Synthesis
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Tetragonal Zirconia/Ni Nanocomposite Prepared by the Modified Internal
Reduction Method,’’ J. Am. Ceram. Soc., 88 [6] 1468–73 (2005).
justification of the above argument shows that the basic pH
value of 13 has a positive effect on particle size reduction.
Summary
18J. Joo, T. Yu, Y. W. Kim, H. M. Park, F. Wu, J. Z. Zhang, and T. Hyeon,
‘‘Multi-Gram Scale Synthesis and Characterization of Monodisperse Tetragonal
Zirconia Nanocrystals,’’ J. Am. Chem. Soc., 125, 6553–57 (2003).
Nonstabilized monoclinic and sodium stabilized cubic zirconia
are synthesized from raw zircon sand using a chemical extrac-
tion process followed by a ball mill-aided precipitation route.
Detailed characterization using XRD and EDS revealed that the
sample synthesized at pH 7 affords nonstabilized monoclinic
zirconia, whereas pH 13 affords sodium-stabilized cubic zir-
conia. This method yields monoclinic zirconia with chemical
purity of 97.6% and cubic zirconia with a chemical purity of
92.5%. The pH value of precipitation is the key controlling fac-
tor for the crystalline phase and particle size of the product.
Although increasing calcination temperature can induce growth
of crystals, which leads to higher particle size. The stable
m-ZrO2 and c-ZrO2 can be synthesized in a large quantity un-
der neutral and strong basic conditions with an average size of
6473 and 3973 nm, respectively. The specific surface area of m-
ZrO2 was 126 m2/g for the powder synthesized at a pH value of
7, whereas the powder synthesized at pH 13 yields c-ZrO2 of a
specific surface area of 227 m2/g. It is observed that precipitation
yields aggregated particles that are disaggregated by a ball-
milling process. It is worth noticing that the ball mill process
develops reduced particle size with spherical morphology. The
results are useful for understanding the surface chemistry and
properties of hydrous zirconia precipitated at different pH val-
ues and its design for the synthesis of nonstabilized m-ZrO2 and
sodium stabilized c-ZrO2 nanoparticles. The effect of planetary
ball milling on particle size, surface area, and morphology has
been addressed. It may be noted that this method has some ad-
vantages such as quite simple to conduct and easy scaling-up the
process to commercial production rates than many of the other
commercially available methods. This process has the potential
to be inexpensive when this reaction scheme is developed for
large-scale tonnage production of nanosized zirconia particles.
The major limitation of this process, at present, is the residual
sodium in the material.
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