Microwave-hydrothermal synthesis of nanocrystalline zirconia powders

Article abstract of DOI:10.1111/j.1151-2916.2001.tb01084.x

Nanosized zirconium oxide (ZrO₂) powders were prepared by adding NaOH to a zirconyl chloride aqueous solution under microwave-hydrothermal conditions. The obtained results showed that the tetragonal polymorph increased with increasing NaOH concentration in the starting solution and reached the maximum value by using 1M ZrOCl₂. The microwave-assisted hydrothermal synthesis is expected to be able to process continuously, and may lead to energy saving because of rapid heating to temperature and increased kinetics of crystallization. This method is very simple and can lead to powders with desirable characteristics such as very fine size, narrow size distribution, and good chemical homogeneity.

Full text of DOI:10.1111/j.1151-2916.2001.tb01084.x

J. Am. Ceram. Soc., 84 [11] 2728–30 (2001)
journal
Microwave-Hydrothermal Synthesis of
Nanocrystalline Zirconia Powders
Federica Bondioli, Anna Maria Ferrari, Cristina Leonelli,* Cristina Siligardi, and
Gian Carlo Pellacani*
Department of Chemistry, Faculty of Engineering, University of Modena and Reggio Emilia, 41100 Modena, Italy
Nanosized zirconium oxide (ZrO ) powders were prepared by
digestion system (Model MDS-2000, CEM Corp., Matthews, NC).
This system uses 2.45 GHz microwaves and is controlled by
pressure. It can attain a maximum pressure of 200 psi, which is
equivalent to ϳ194°C, based on steam tables. The reaction vessel
was connected to a pressure transducer that monitors and controls
the pressure during synthesis.
2
adding NaOH to a zirconyl chloride aqueous solution under
microwave-hydrothermal conditions. The obtained results
showed that the tetragonal polymorph increased with increas-
ing NaOH concentration in the starting solution and reached
the maximum value by using 1M ZrOCl . The microwave-
2
assisted hydrothermal synthesis is expected to be able to
process continuously, and may lead to energy savings because
of rapid heating to temperature and increased kinetics of
crystallization. This method is very simple and can lead to
powders with desirable characteristics such as very fine size,
narrow size distribution, and good chemical homogeneity.
After preliminary tests, microwave-hydrothermal treatments
were conducted at 200 psi for 2 h. The time, pressure, and power
were computer controlled. After the synthesis reaction, the powder
was filtered, washed, and dried. After the last washing of the
synthesized powder, the supernatant was analyzed by ICP spec-
troscopy (Model Liberty 200, Variant, Sidney, Australia) to
evaluate the presence of sodium and the efficacy of the washing
step.
I. Introduction
(
2) Powder Characterization
All the synthesized products were analyzed with a computer
VER the years, zirconium oxide (ZrO ) ceramics have been
2
O
largely used because of their chemical and physical properties,
such as excellent chemical resistance, high refractoriness, and
ionic conductivity.
assisted X-ray (CuK␣) powder diffractometer (Model PW3710,
Philips Research Laboratories, Eindhoven, The Netherlands). The
XRD patterns were collected in a 2␪ range of 25°–90° at room
temperature, with a scanning rate of 0.005°/s and a step size of
0
To achieve such desirable properties, the synthesis conditions
must be well-controlled to obtain fine powders with a narrow
particle size distribution that enhance reactivity and densification.
7
.02°. Lattice parameters of monoclinic, [P2 ], and tetragonal,
1/a
8
1
–3
[P42/nmc], phases were determined by a least-squares refinement
Among the various methods, hydrothermal crystallization is an
interesting process used to directly prepare submicrometer- and
nanometer-sized crystalline powders with reduced contamination
and low synthesis temperature. A recent innovation to the hydro-
9
and the average grain size by using the Sherrer’s formula.
quantitative determination of the volume fraction in the mixture of
both monoclinic and tetragonal phases was made by using the
A
1
0
4
–6
following formula:
thermal method, developed by Komarneni et al.,
involves the
introduction of microwaves during the hydrothermal synthesis to
increase the kinetics of crystallization by 1–2 orders of magnitude.
The purpose of the present work is to report the synthesis of
ultrafine ZrO2 powders under microwave-assisted hydrothermal
conditions. The effects of various synthesis parameters, such as
solution pH, concentration of ions, temperature, and reaction time
on powder properties, such as the type of polymorph, crystallite
size, particle size distribution, and degree of agglomeration, were
investigated.
I͑111͒t
I(111) ϩ I(111) ϩ I(111)
m
X ϭ
t
(1)
m
m
where the subscripts m and t refer to the monoclinic and tetragonal
phases and I refers to the X-ray intensity of the corresponding
peaks. The sample morphology was examined by TEM (Model
EM400, Philips Research Laboratories). The surface area analysis
was conducted on the powders by BET (Model Gemini 2360,
Micromeritics Instrument Corp., Norcross, GA), using nitrogen as
an adsorbate. The particle size was also calculated, using the
specific surface area data, by the equation:
II. Experimental Procedure
(
1) Sample Preparation
The microwave-assisted hydrothermal synthesis of ZrO pow-
6
␾
ϭ
(2)
S␳
2
ders was conducted using various concentrations (from 0.5M to
0M) of ZrOCl ⅐8H O aqueous solutions. The solutions were
neutralized with NaOH (from 0.5M to 1M) to pH 9. The mixture
where ␾ is the average diameter of a spherical particle, S is the
surface area of a powder, and ␳ is the experimental density value
of powder measured by a helium picnometer (Model Accupic
1
2
2
was then treated in a Teflon-lined vessel using a microwave
1
330, Micromeritics Instrument Corp.). Finally, the thermal sta-
bility of the powders was evaluated by DTA (Model 404, Netzsch,
Selb, Germany).
G. S. Rohrer—contributing editor
III. Results
The as-prepared powders contained only ZrO as a crystalline
phase whose crystal symmetry varied with synthesis conditions.
Manuscript No. 188573. Received May 11, 2000; approved January 30, 2001.
2
*
Member, American Ceramic Society.
2
728

November 2001
Communications of the American Ceramic Society
2729
Table I. Lattice Parameters of ZrO Powders Obtained by Different Synthesis Conditions
2
Monoclinic ZrO
2
tЈ-ZrO
2
Sample
a (Å)
b (Å)
c (Å)
␤ (°)
a (Å)
c (Å)
c/a
Zr1Na0.5
Zr1Na1
Zr1Na10
Zr0.5Na10
Zr0.05Na1
Zr0.5Na1
5.324(13)
5.287(15)
5.319(6)
5.325(7)
5.322(7)
5.317(5)
5.203(11)
5.208(15)
5.198(6)
5.202(6)
5.199(7)
5.194(5)
5.153(14)
5.119(15)
5.142(5)
5.143(4)
5.137(5)
5.138(3)
99.59(14)
99.42(19)
99.50(5)
99.62(5)
99.66(5)
99.59(4)
5.118(8)
5.118(8)
5.116(8)
5.130(8)
5.192(6)
5.183(8)
5.196(7)
5.209(8)
1.015
1.013
1.016
1.016
5.130(8)
5.209(8)
1.015
1
2,13
The absence of alkali ions in the last washing water indicated the
efficacy of the washing step. The lattice parameters of the powders
obtained are given in Table I. In this case, the axial ratio is in
agreement with that of the metastable tЈ phase. In particular, the
tetragonal polymorph increased with increasing NaOH concentra-
tions in the starting solution (Fig. 1) and reached the maximum
earlier work
on the tetragonal crystal evolution from amor-
phous zirconia gel materials in which topotatic crystallization on
nuclei in the amorphous zirconia has been proposed as the
mechanism of tetragonal zirconia formation under hydrothermal
conditions.
value by using 1M ZrOCl (Fig. 2). The volume fractions of the tЈ
2
phase and the average particle size calculated by a different
method are given in Table II. TEM analysis confirmed the effect
of concentration on particle size. In particular, the calculated
average particle size ranged from 16 (Ϯ 3) to 9 (Ϯ 3) nm, with the
ZrOCl concentration varying from 0.5M to 1M. TEM observation
Table II. Volume Fractions of the t؅-ZrO Phase and
2
Average Particle Size Obtained by Different Methods
2
of particles revealed spherical-shaped particles with no agglomer-
ation (Fig. 3), as also underlined by the small differences with the
average particle size determined by X-ray line broadening. Thus,
Component
concentration
(M)
Grain size (nm)
under microwave-hydrothermal conditions, the ZrO powder first
2
Volume fraction
crystallizes in the tЈ-ZrO2 phase, and then transforms into the
stable monoclinic phase on further heating. From this point of
view, the exothermic peak at ϳ400°C (Fig. 4) can be attributed to
a growth phenomenon, wherein small crystals coalesce into large
Sample code ZrOCl
2
NaOH
0.5
1
tЈ phase (%)
By XRD
By BET
Zr1Na0.5
Zr1Na1
1
1
1
75.16
83.36
48.26
14.89
8.36 (tЈ)
8.45
†
ND (m)
9.5 (tЈ)
8.50
1
1
particles containing one or more grain boundaries. As indicated
by thermal analysis at ϳ1200°C, the monoclinic phase trans-
formed into tetragonal. Our current study is in agreement with
†
ND (m)
Zr1Na10
10
9.66 (tЈ) 11.31
14.76 (m)
Zr0.5Na10 0.5
Zr0.05Na10 0.05
10
5.78 (tЈ) 21.67
1
4.88 (m)
10
1
0
16.65 (m) 18.69
Zr0.5Na1
0.5
24.51
11.57 (tЈ) 22.92
1
7.42 (m)
†
Cannot be accurately determined from XRD data.
Fig. 1. XRD patterns of powders obtained starting from a (a) 0.5M and
(b) 1M NaOH solution (0.5M ZrOCl2).
Fig. 2. XRD patterns of powders obtained starting from a (a) 0.5 M, (b)
M, and (c) 10M ZrOCl solution (1M NaOH).
1
Fig. 3. TEM micrographs (130 000ϫ) of the Zr1Na1 zirconia powders.
2

2
730
Communications of the American Ceramic Society
Vol. 84, No. 11
from University of Modena and Reggio Emilia for performing the synthesis
experiments.
References
1
S. S o៮ miya and T. Akiba, “Hydrothermal Zirconia Powders: a Bibliography,” J.
Eur. Ceram. Soc., 19, 81–87 (1999).
2
S. S o៮ miya, M. Yoshimura, Z. Nakai, K. Nakai, K. Hishinuma, and T. Kumaki,
Microstructure; pp. 465–74. Edited by J. A. Pask and A. Evans. Plenum Press, New
York, 1986.
3
T. Tsukada, S. Venigalla, A. A. Morrone, and J. H. Adair, “Low-Temperature
Hydrothermal Synthesis of Yttrium-Doped Zirconia Pwders,” J. Am. Ceram. Soc., 82,
1
169–74 (1999).
4
S. Komarneni, R. Roy, and Q. H. Li, “Microwave-Hydrothermal Synthesis of
Ceramic Powders,” Mater. Res. Bull., 27 [12] 1393–405 (1992).
5
S. Komarneni, “Novel Microwave-Hydrothermal Processing for Synthesis of
Ceramic and Metal Powders”; pp. 103–17 in Novel Techniques in Synthesis and
Processing of Advanced Materials. Edited by J. Singh and S. M. Copley. The
Minerals, Metals, and Materials Society, Warrendale, PA, 1995.
Fig. 4. DTA/TGA curves of the Zr1Na1 zirconia powder.
6
S. Komarneni, M. C. D’Arrigo, C. Leonelli, G. C. Pellacani, and H. Katsuki,
V. Conclusion
“
Microwave-Hydrothermal Synthesis of Nanophase Ferrites,” J. Am. Ceram. Soc., 88,
3
041–44 (1998).
7
Nanosized and well-crystallized zirconium oxide (ZrO ) pow-
Powder Diffraction File, Card No. 37-1484. International Centre for Diffraction
2
ders were prepared by a microwave-hydrothermal method at a low
temperature (200°C) using zirconyl chloride salt and NaOH as a
precipitation agent. The size range of the powders was very narrow
from 10 to 20 nm) as observed by TEM and confirmed by BET
and XRD results.
Data, Newtown Square, PA, 1995.
8
Powder Diffraction File, Card No. 72-699. International Centre for Diffraction
Data, Newtown Square, PA, 1995.
9
B. Cullity, Elements of X-ray Diffraction; 2nd Ed. Addison–Wesley, Reading,
(
MA, 1977.
1
0
H. K. Schmid, “Quantitative Analysis of Polymorphic Mixes of Zirconia by
X-ray Diffraction,” J. Am. Ceram. Soc., 70, 367–76 (1987).
This study reveals that the phase stabilization process in the
1
1
R. Srinavisan and B. H. Davis, “Glow Phenomenon and Crystallization: Evidence
ZrO obtained by microwave-hydrothermal synthesis has a depen-
2
that They are Different Events for Hafnium–Zirconium Mixed Oxides,” J. Colloid
Interface Sci., 156, 400–405 (1993).
dence on the zirconium and NaOH concentration, and appears to
1
4
12
be because of a critical grain size effect. The powders obtained
E. Tani, M. Yoshimura, and S. Somia, “Formation of Ultrafine Tetragonal
ZrO
2
Powder under Hydrothermal Conditions,” J. Am. Ceram. Soc., 66, 11–14
from ZrOCl 1M and NaOH 1M had a finer particle size distribu-
2
(
1983).
tion and almost exclusively consisted of tЈ-ZrO2.
13
M. Z-C. Hu, R. D. Hunt, E. A. Payzant, and C. R. Hubbard, “Nanocrystallization
and Phase Transformation in Monodispersed Ultrafine Zirconia Zirconia Particles
from Various Homogeneous Precipitation Methods,” J. Am. Ceram. Soc., 82 [9]
2313–20 (1999).
Acknowledgments
1
4
R. Srinivasan, L. Rice, and B. H. Davis, “Critical Particle Size and Phase
Transformation in Zirconia: Transmission Electron Microscopy and X-ray Diffraction
Studies,” J. Am. Ceram. Soc., 73 [12] 3528–30 (1990).
We would like to express our thanks to Dr. Paola Miselli, DemoCenter,
Modena, Italy, for XRD study of the powders, and to Dr. Daniele Verucchi
Ⅺ