Synthesis, vacuum sintering and dielectric characterization of zirconia (t-ZrO2) nanopowder

Article abstract of DOI:10.1016/j.jallcom.2011.03.089

Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition combustion method, the particles were nanometer sized (20 nm) and the size distribution was very narrow (3.4 nm). Systematic structural characterization revealed

Full text of DOI:10.1016/j.jallcom.2011.03.089

Journal of Alloys and Compounds 509 (2011) 6819–6823
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis, vacuum sintering and dielectric characterization of zirconia (t-ZrO2)
nanopowder
R. Pazhania, H. Padma Kumarb, Angeo Varghesec, A. Moses Ezhil Rajd, Sam Solomone, J.K. Thomasa,∗
a Electronic Materials Research Laboratory, Department of Physics, Mar Ivanios College, Nalanchira, Thiruvananthapuram 695015, Kerala, India
b Department of Physics, W.M.O. Arts and Science College, Muttil 673122, Wayanad, Kerala, India
c Mar Baselios College of Engineering & Technology, Thiruvananthapuram 695015, Kerala, India
d Department of Physics, Scott Christian College, Nagercoil 629003, Tamil Nadu, India
e Department of Physics, St. John’s College, Anchal 691306, Kollam, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Phase pure zirconium oxide powders have been synthesized using the single step auto-ignition com-
bustion method, the particles were nanometer sized (20 nm) and the size distribution was very narrow
(3.4 nm). Systematic structural characterization revealed the t-ZrO2 and indexed for its tetragonal struc-
Received 30 November 2010
Accepted 15 March 2011
Available online 22 March 2011
˚
˚
ture (a = 3.5975 A and c = 5.1649 A). Calculated microstrain in most of the plane indicated the presence of
compressive stress (65–288 MPa) along various planes of the particles. Observed space group (P42/nmc)
revealed the presence of cations in the 8e positions (0.75, 0.25, 0.75) and the anions in the 16 h posi-
tions (0.25, 0.25, 0.4534). The metal-oxide (Zr–O) band observed at the low wavenumber region further
confirmed the phase purity of the as-prepared ZrO2 nanopowders. Peaks at the binding energy posi-
tions 2.042 and 0.525 keV in the energy dispersive X-ray spectrum revealed oxygen deficient zirconia.
The particle size estimated by TEM was in good agreement with the results obtained through X-ray line
broadening (20.81 nm) measurements. The nanopowders were sintered to above 98% of the theoretical
density by using vacuum sintering technique at a relatively low temperature of 1300 ◦C. Stable tetragonal
ZrO2 experimentally yield the permittivity value of about 28 at 10 MHz.
Keywords:
t-ZrO2
Combustion synthesis
X-ray diffraction
TEM
Dielectrics
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
Such a technologically important zirconia nanostructure has
been synthesized by different methods which include various
Zirconium oxide (ZrO2) exhibiting high refractive index (2.17)
and high optical transparency (Eg = 5.2 eV) in the UV–Vis–IR has
instrumental in device fabrication in photonics [1]. In addition,
zirconia has unique combination of electronic and mechanical
properties that led to wide applications, as a structural material,
solid-state electrolyte, catalyst and thermal barrier coatings [2].
ZrO2 exists in three different phases namely room temperature
stable monoclinic phase and high temperature stable tetragonal
and cubic phases (metastable phases at room temperature). The
metastable phase of zirconia (t-ZrO2) is the focus of recent investi-
gations due to their high hardness, chemical inertness and optical
properties (Abbe number ≈ 35) make them important target mate-
rials for the fabrication of metal-halide lamps, solid state lasers,
scintillators and bar-code scanners [3–6]. Recently, t-ZrO2 has been
proved to be a highly luminescent material, which is photochemi-
cally stable with low phonon energy (470 cm−1) [7].
wet-chemical processes [8], mechanochemical processing [9], salt-
assisted aerosol decompositions [10], emulsion precipitation [11]
and few more. Among the various wet-chemical processes, recently
the combustion synthesis has attracted much attention due to
its relatively low cost and better control of stoichiometry in
comparison to other methods. The process basically involves the
preparation of homogeneous fuel-oxidant precursor and subse-
quent combustion of an aqueous solution containing salts of the
desired metals (usually nitrates) and some organic fuel, such as cit-
ric acid, urea, glycine, etc. [12–14]. Phase purity and characteristics
of the combustion-synthesized powder are primarily depending
on certain parameters; redox reaction between nitrate ions and
the fuel, amount of the fuel used in the process, the enthalpy of
combustion, amount of gas evolution and the rate of combustion
[15,16]. In addition, fast decomposition rate, evolution of high vol-
ume of gaseous products and sufficient enthalpy of combustion
yield nanocrystalline particles after calcination.
Few reports have been involved in the combustion synthesis of
ZrO2 nanocrystals by using various fuels. However, present arti-
cle highlights the modified single step auto-igniting combustion
synthesis of zirconia using citric acid as the fuel, since it has good
∗
Corresponding author. Tel.: +91 9447205190; fax: +91 471 2530887.
E-mail address: jkthomasemrl@gmail.com (J.K. Thomas).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.03.089

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R. Pazhani et al. / Journal of Alloys and Compounds 509 (2011) 6819–6823
complexing ability, low ignition temperature (200–250 ◦C) and
controlled combustion reaction with starting precursors [17]. In the
present article we report the synthesis, structural, surface morpho-
logical, vacuum sintering and dielectric studies on the optimized
single phase pure zirconia nanopowders.
3500
3000
2500
2000
1500
1000
500
3500
3000
2500
2000
1500
1000
500
(101) peak
Gaussian fit
FWHM = 0.7206
2. Materials and methods
In modified single step auto-igniting combustion synthesis, an aqueous solu-
tion containing Zr2+ was prepared by dissolving typical amount of high purity
ZrOCl2·8H2O in distilled water (∼200 ml) in a glass beaker. Citric acid was then
added as fuel and the oxidant/fuel ratio of the system was adjusted by adding nitric
acid and ammonium hydroxide. Amount of citric acid was calculated based on total
valence of the oxidizing and reducing agents for maximum release of energy dur-
ing combustion. The resulting translucent solution was heated on a hot plate (at
about 200 250 ◦C) until it turned into a viscous solution. The solution boils upon
heating and undergoes dehydration accompanied by foam. On heating further, the
foam ignites by itself due to persistent heating giving a voluminous and fluffy prod-
uct from combustion. The combustion product was subsequently characterized as
single phase nanocrystals of ZrO2. Grey ashes obtained after combustion were then
collected for structural characterization and other morphological studies.
0
28
29
30
31
32
33
2θ (degrees)
0
20
30
40
50
60
70
80
90
X-ray diffraction (XRD) data collection on the combustion-synthesized powder
was carried out for phase identification and crystallite size determination using a
Bruker D-8 X-ray diffractometer (CuK␣ radiation, Ni-filter). The powder-XRD data
were collected in the 2ꢀ range of 20–90◦, in step scan mode with step width 0.02 and
step time 2.40 s. The metal-oxide phase formation was identified from the recorded
Fourier Transform Infrared(FTIR) spectrum in the range 400–4000 cm−1 (Thermo
Nicolet Avtar 370 DTGS). The purity and the stoichiometry of the prepared sample
were identified from the recorded EDAX spectrum using JEOL-6400 electron micro-
scope operating at 20 kV. The sample was dusted on an adhesive conductive carbon
disc attached to a mount and coated with a gold film prior to examination. The parti-
cle size and the morphology were recorded using Transmission Electron Microscopy
2θ (degrees)
Fig. 1. XRD patterns of as-prepared ZrO2 nanopowder sample. Inset shows the
(1 0 1) predominant peak.
to the overlap of indexed peaks (1 0 3) and (2 1 1) situated respec-
tively at 2ꢀ = 59.30◦ and 60.029◦.
All the observed peaks in Fig. 1 are indexed assuming t-ZrO2
polymorph (Table 1). This table gives a brief account of all the peaks
about its position, Miller indices, interplanar distances (observed
and standard), their deviations and the microstrain on the grains.
The microstrain on the grains can be visualized from the line shift-
ing in the XRD spectrum. It can be seen that the observed d-spacing
of the diffraction planes are very close to the standard values and
therefore the ꢁdh k l values are very small in the order of 10−3 nm.
The deviation in the d-spacing is the measure of line shifting which
gives the value of microstrain in the nano powder sample. If d0 is
the observed d-spacing of the prepared sample and ds, the spacing
in the standard sample, the microstrain in the particles in the direc-
tion normal to the diffraction plane is ꢁdh k l/ds. If d0 > ds, then the
microstrain is positive which indicates that the residual stress is
tensile and if d0 < ds, microstrain is negative indicating generation
of residual compressive stress in the surface. In the present study,
calculated microstrain in most of the plane is negative, indicating
the presence of compressive stress on the surface of the particles.
Observed positive values of microstrain for the (0 2 2) and (1 0 3)
planes may be due to induced error in the measurement of the 2ꢀ
value, since the respective peaks are nonsymmetrical.
˚
TEM (model: PHILIPS-CM 200, resolution 2.4 A) operating at 200 kV. The sample for
TEM analysis was prepared by ultrasonically dispersing the powder in methanol and
allowing a drop to dry on a carbon-coated copper grid.
Vacuum sintering has been done on the pelletized nanopowders prepared under
the optimized preparation conditions. The final sintered product was produced
by the two-step sintering techniques – pre-sintering in hydrogen atmosphere (at
550 ◦C) to avoid agglomeration and final sintering by vacuum sintering. Proper care
has been taken not to exceed the melting point of the sintered base material. More-
over, control over heating rate, time, temperature and atmosphere has been taken
into consideration to have reproducible results. Initially the pellets were heated
in hydrogen ambient and then it was transferred to the vacuum furnace. In vac-
uum furnace the heat-treating process takes place inside an airtight vessel, where a
vacuum is created. This helps to alleviate surface reactions. Furnace has high tem-
perature refractory lining (Alumina–silica) to hold the process material and hold
in the heat without breaking down during the several hours that they usually run.
The heater arrangement optimizes temperature uniformity within the furnace hot
zone. Indirect heating electrical resistance coils are used to heat the pellets placed
inside a pre-evacuated sealed silica crucible at vacuum p < 13 Pa (10−1 mm/Hg). In
the present study, heating rate was maintained at 10 ◦C/min and after attaining
the predefined sintering temperature (1300 ◦C), the sample was heated continu-
ously for a sintering time of 3 h. The sintered density was then calculated following
Archimedes method.
The variation in the dielectric constant (εr) and loss factor (tan ı) of the sintered
ZrO2 have been studied using an Impedance Analyser (Agilent HP 4192A) in the
frequency range 10KHz–10 MHz at room temperature.
The microstress present in the nanopowder sample can be
defined as [19],
ε
3. Results and discussions
ꢂstress
=
E
(1)
2
Fig. 1 shows the diffraction pattern of the as-prepared ZrO2 pow-
der sample. The crystallinity of the sample is evidenced by sharper
diffraction peaks at respective diffraction angles which can be read-
ily indexed for its tetragonal structure (t-ZrO2). Obtained tetragonal
phase is comparable with the standard JCPDS data (Card No. 81-
1544) [18]. The sample exhibits only the tetragonal phase and the
major peaks appears at 2ꢀ = 30.27◦, 35.23◦, 50.27◦, 60.18◦, 62.95◦,
and 74.54◦. It is further observed that there is no indication of low
temperature monoclinic or the high temperature cubic phases. Evi-
dence of the tetragonal symmetry might be again confirmed from
the non-symmetric line shape around 2ꢀ = 35◦ and 60◦ regions of
the XRD pattern. In the 2ꢀ = 35◦ region, the nonsymmetrical line
shape originated from the splitting between the (0 0 2) and (1 1 0)
peaks situated near by 2ꢀ values 34.634◦ and 35.160◦ respectively.
Similarly in the 2ꢀ = 60◦ region, non symmetrical peak shape is due
where E is the elastic constant or generally known as Young’s
modulus of the material. Young’s modulus of zirconium oxide is
186.21 GPa [20] and therefore the compressive stress on the sur-
face of the nanopowder is of the order 65–288 MPa along various
planes of the particles.
Accurately measured d values for the (4 0 0) and (0 0 4) peaks
are tabulated in Table 2. These values are again verified by use
of the cell refinement software (Celcal) for the same tetragonal
structure by considering all the indexed peaks. Obtained values
˚
˚
a = 3.5975 A and c = 5.1649 A are consistent with the standard JCPDS
˚
˚
values (a = 3.6060 A, c = 5.1758 A). Similarly, refined axial ratio c/a
exactly matches with the standard value (Table 2) and is in agree-
ment with the reported values. The calculated and refined values
of unit cell volume of the crystal system also matches well with the
standard values. Calculated and refined values of the density are

R. Pazhani et al. / Journal of Alloys and Compounds 509 (2011) 6819–6823
6821
Table 1
Interplanar dh k l spacings and microstrain in the t-ZrO2 nanopowder.
Peak position 2ꢀ (◦)
(h k l)
Inter planar distance dh k l (nm)
Deviation in dh k l (ꢁdh k l = d0 − ds) (nm)
Microstrain (ε = ꢁdh k l/ds)
Observed (d0)
Standard (ds)
30.2738
34.5578
35.2298
50.2658
59.1698
60.1778
62.9498
74.5418
81.7658
85.0418
(1 0 1)
(0 2 2)
(1 1 0)
(1 1 2)
(1 0 3)
(2 1 1)
(2 0 2)
(2 2 0)
(2 1 3)
(3 1 0)
2.9499
2.5934
2.5454
1.8136
1.5602
1.5364
1.4753
1.2720
1.1769
1.1397
2.9591
2.5879
2.5503
1.8164
1.5563
1.5399
1.4795
1.2751
1.1782
1.1405
−0.0092
0.0055
−0.0049
−0.0028
0.0039
−0.0035
−0.0042
−0.0031
−0.0013
−0.0008
−0.0031
0.0021
−0.0019
−0.0015
0.0025
−0.0023
−0.0028
−0.0024
−0.0011
−0.0007
listed in Table 2. The density of the powder sample is more than
the standard value, implied that the prepared powder is compact
and densely packed with few unit cells of the material. Moreover,
from the observed space group (P42/nmc) and the reported data
[21,22], it is assumed that the cations are located in the 8e position
(0.75, 0.25, 0.75) and the anions are located in the 16h positions
(0.25, 0.25, 0.4534).
hedron [27,28]. Significantly all the above said bands have been
reported in m-ZrO2 [28]. The broad characteristic band at 513 cm−1
appears in t-ZrO2 while at 628 cm−1 in c-ZrO2 [27,28]. In general,
all the four ZrO2 polymorphs have very similar vibrational struc-
tures. A minor variation in their frequencies or relative intensities
occurs in different Zr4+ distribution in the interstitial sites. They are
sensitive to the oxygen vacancies and other defects [29].
The calculated crystallite size was about 20.81 nm, which
reveals the nanocrystalline size of the prepared powder. From this
value, the specific surface area was calculated on assuming the
presence of spherical particles, by using the relation [13]:
These results reveal that the phase formation is complete for
the as-prepared ZrO2 nanopowder and there is no evidence for the
presence of any organic intermediates in the sample. However, the
observed weak bands at 3445 and 1587 cm−1 can be assigned to
the stretching and bending vibrations of the O–H bands that reveal
the presence of surface hydration.
6
S =
(2)
ꢃD
The chemical composition of the ZrO2 nanopowders prepared
at the optimized condition was extracted from the energy disper-
sive X-ray spectrum (EDAX) which is shown in Fig. 3. The different
elements of the spectrum are distinguished by different pulse
amplitude produced in the detector proportional to the character-
istic X-ray quantum energy. Peaks due to the presence of zirconium
and oxygen appeared at the positions with the binding energy
2.042 and 0.525 keV respectively. The atomic percentages of the
elements present in the prepared sample are respectively 74.45
and 25.55. EDAX result reveals oxygen deficient zirconia powders
in the present conditions of preparation.
Numerous studies on nanocrystalline ZrO2 have shown that
pure metastable tetragonal and cubic phases can be stabilized at
room temperature when the crystallite size is below a critical
size primarily due to very high surface energy associated with it.
When the crystallite size exceeds this size, the transformation of
where ꢃ, the calculated density of the material and D, the particle
size of the sample. The specific surface area is a dominant param-
eter in case of permeability and in the transport of a species that
can adsorb on the mineral surfaces. The specific surface area of the
spherical particles of the synthesized powder sample is 47 m2/g. In
general, decreased crystallinity with improvement in specific sur-
face area leads to higher reaction rate. From the observed value,
prepared material can be engaged in catalytic reactions.
In other combustion processes reported for the preparation of
nanocrystals of some ceramic oxides, polyvinyl alcohol and urea
were used as chelating agents and fuel, respectively and in all these
cases phase pure powder was obtained only after high temperature
calcination [23,24]. But in the present modified single step auto-
ignition combustion method, citric acid was used as the chelating
agent instead of polyvinyl alcohol and urea was replaced with
ammonia. By changing the complexing agent and oxidant fuel sys-
tem it was possible to get a single phase ZrO2 as nanocrystals in a
single step combustion.
100
FTIR spectrum of the ZrO2 nano powder synthesized at the opti-
mized preparation condition is shown in Fig. 2. For ZrO2 nano
crystals, three intense peaks centered at 420, 513, 758 cm−1 were
observed, which is the characteristic band of the tetragonal ZrO2
phase [25]. According to the available reports, observed bands at
420 and 513 cm−1 are attributed to the Zr–O stretching in tetrago-
nal ZrO2 [26,28]. Also the band at 758 cm−1 is the characteristic of
Zr–O bending vibrations of ZrO2 in ZrOı, usually ı = 4 and 6, poly-
1587.56
80
3445.22
60
40
20
0
758.76
Table 2
Unit cell parameters of the as-prepared ZrO2 powder.
Unit cell parameters
Unit cell edge
Calculated
JCPDS standard
Cell refinement
3.5975
513.31
420.85
500
˚
a (A)
3.5849
3.6067
4000 3500 3000 2500 2000 1500 1000
Wavenumber (cm-1)
Unit cell edge
˚
c (A)
5.1919
1.448
66.7237
6.1311
5.1758
1.435
67.3283
6.0760
5.1649
1.436
66.8442
6.1200
Axial ratio c/a
2
3
˚
Volume V = a c (A )
Density (gm cm−3
)
Fig. 2. FT-IR spectrum of as-prepared ZrO2 nanocrystals.

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R. Pazhani et al. / Journal of Alloys and Compounds 509 (2011) 6819–6823
1.2
1.0
0.8
0.6
0.4
0.2
0.0
120
100
80
εr
tan
δ
60
40
20
4
5
6
7
log(f)
Fig. 3. Energy dispersive absorption X-ray spectrum of the as-prepared ZrO2
nanocrystals.
Fig. 5. Dielectric response of sintered ZrO2.
carried out at various frequencies ranging from 10 kHz to 10 MHz
at room temperature. As evidenced from the observed variations,
the dielectric constant (εr) decreases as the frequency increases. At
higher frequencies, the variations in εr almost ceases and attains
a constant value of 28. This value of dielectric constant is compa-
rable to the reported values [35,36]. However, dielectric constant
of ZrO2 and few of its solid solutions have been reported in the
literature, and among these there is a considerable spread in val-
ues for nominally similar materials. Thus different sources give the
permittivity for the same material varying by up to a factor of two;
for example, for calcium-stabilized zirconia has been reported as
being 16 or 26, and for monoclinic zirconia values are quoted from
13.5 to 24 [36]. The inconsistency in the observed values of per-
mittivity may be due the measurement techniques used and also
due the available crystalline phases of ZrO2. Zirconia has a mono-
clinic unit cell at room temperature, but the added cations stabilize
a higher symmetry tetragonal or cubic unit cell with corresponding
changes in lattice parameters. This in turn alter the material proper-
ties specifically the dielectric constant. Present study deals mainly
the stable tetragonal ZrO2, which experimentally yield the permit-
tivity value of about 28 at the frequency of 10 MHz. Fig. 5 shows
the loss tangent variation with respect to the applied frequency
in kHz to MHz range. The dielectric loss is about 1.2 at 10 kHz,
which is comparatively higher than that observed at the MHz range.
Observed higher loss in the low frequency region may be due to a
high defect charge density existing in the material, which would
lead to the loss of leakage conductance. At 10 MHz, the observed
dielectric loss is only 0.17 and therefore zirconia can be used as
dielectric resonators applicable to both terrestrial and satellite
communications.
metastable phases to the monoclinic one occurs due to decrease in
surface energy [30,31]. Hence particle size analysis finds a place in
deciding and transformation of crystalline phase.
Fig. 4 shows the TEM micrograph of as-prepared ZrO2 powder,
which reflects the highly agglomerated sub-micrometer powder
having nearly spherical nanocrystals with facets of size around
20 nm. The crystallite size observed by TEM is in good agreement
with the result estimated by X-ray line broadening (20.81 nm).
The particle distribution is almost uniform and the deviation from
the mean particle size is not exceeding 3.4 nm. The crystallite size
obtained in the combustion-synthesized ZrO2 powder seems to
be above the critical size needed to stabilize the metastable cubic
phase [32–34]. This study again favors for the evolution of tetrag-
onal phase of the product for the present preparative conditions
probably due to high degree of agglomeration which can reduce
the surface energy. In addition, high enthalpy generated due to the
fuel citric acid used for combustion also facilitates agglomeration.
In the present vacuum sintering technique, fully dense powder
compacts were obtained with ultrafine and nano sized particles.
Obtained density value 5.98 g/cc is in agreement with the JCPDS
standard value of 6.076 g/cc, which is about 98.42% of the bulk value
of the same material.
Fig. 5 shows the frequency dependence of dielectric constant
and dielectric loss for the sintered ZrO2 pellets. Measurements were
4. Conclusion
A simple, cost-effective and time-saving low-temperature sin-
gle step auto igniting combustion synthesis has been developed
to obtain nanocrystalline t-ZrO2 powder. XRD results revealed
the tetragonal phase with nano sized particles. The phase con-
firmation and the elemental analysis have been performed to
judge the purity and stoichiometry of the prepared nanopowders.
TEM image shows that the particle size is uniform and is around
20 nm. The vacuum sintered sample shows high sintered density
of about 98% of the theoretical density at a relatively low tem-
perature of 1300 ◦C. A high permittivity of about 28 is obtained
at higher frequencies (10 MHz). The nanocrystalline ZrO2 has the
advantage of lower sintering temperature and better microwave
Fig. 4. TEM micro image of the as-prepared ZrO2 nanopowder.

R. Pazhani et al. / Journal of Alloys and Compounds 509 (2011) 6819–6823
6823
dielectric properties than that prepared by conventional tech-
niques.
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