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Preparation and Characterization of Nanocrystalline ZrO2-Ga2O3 Solid Solutions
P. Barret, P. Berthet
To cite this version:
P. Barret, P. Berthet. Preparation and Characterization of Nanocrystalline ZrO2-Ga2O3 Solid So- lutions. Journal de Physique III, EDP Sciences, 1997, 7 (3), pp.483-490. �10.1051/jp3:1997136�.
�jpa-00249590�
Preparation and Characterization of Nanocrystalline Zr02-Ga203
Solid Solutions
P~ Barret and P. Berthet (*)
Laboratoire de Chimie des Solides (**), Bitiment 414, Universit# Paris-Sud, 91405 Orsay Cedex, France
(Receiied 6 May1996, reiised 15 July 1996, accepted 22 October 1996)
PACS 81.05 Je Ceramics and refractories PACS.81.05.Ys Nanophase materials
PACS.8120.Fw Sol-gel processing, precipitation
Abstract~ Nanocrystalline solid solutions of gallium oxide m zirconia were prepared by the
crystallization, between 500 and 800 °C, of amorphous coprecipitates containing up to 53 mol%
gallium The thermal evolution of the material from the amorphous state to its decomposition
was followed by DTA and TGA correlated with XRD The solid solutions are nanocrystalline (10-25 nm), their structure is tetragonal (Ga/Zr < 1/9) or cubic The cell parameter of the cubic materials decreases from 5.09 to 4 97 I with increasing gallium content These solid solutions decompose near 925 °C by diffusion of the gallium ions towards the crystallite surface.
1. Introduction
Zirconia have remarkable ionic and refractory< properties which led to their applications as oxygen sensors, solid electrolytes or chemical catalyst. Many researchs have already been re- ported on various aspects ofthese materials [I]. Many ofits properties only find an application
when zirconia is stabilized under one of its high temperature forms (tetragonal or cubic) by the substitution of the tetravalent zirconium ions by di- or trivalent ions such as yttrium, calcium
or magnesium The stabilization obtained by these substitutions is associated with the intro- duction of oxygen vacancies which, moreover, play an important role for the ionic conductivity
of these materials.
Some unusual solid solutions of Zr02 and other oxides as Fe203 or A1203 can be synthesized
from hydroxide coprecipitates or by nitrate codecomposition [2-5] These metastable solid
solutions are nanocrystalline. To our knowledge, the solubility of gallium oxide in zirconia has not be explored hitherto. In the present paper, the preparation of metastable solid solutions for a wide range of the Zr02 -Ga203 system is reported together with an investigation of their
thermal stability.
(* Author for correspondence (e-mail. Berthet@psisun u-psud fr) (**) URA 446 CNRS
@ Les #ditions de Physique 1997
484 JOURNAL DE PHYSIQUE III N°3
2. Experimental Procedure
2. I. SAMPLE PREPARATION. The required amounts of hafnium free zirconium oxychloride
(ZrOC12, 8H20) and gallium chloride (GaC13) were dissolved in de-ionized water to give a
total salt concentration of 0.5 mol l~~. This solution
was coprecipitated using an aqueous
ammonia solution ill mol l~~). The hydroxide precipitates was washed with de- ionized water until the chloride and the ammonia disappeared (pH m 6, the de-ionized water pH), filtered, dried at 45 °C for I day, crushed and ground in an agate mortar and then annealed at various
temperatures in air. Powders with compositions between 0 and 53 mol$lo in gallium were prepared.
2. 2. SAMPLE CHARACTERIzATION. Chemical analysis of the powders were carried out using ICP /AES (Inductively Coupled Plasma, Atomic Emission Spectroscopy). All the compositions
mentioned in this paper are those obtained by this analysis, they are in good agreement with the nominal compositions of the starting materials.
X-Ray Diffraction (XRD) patterns of the powders were recorded using a PHILIPS PW 1840 diffractometer and filtered CuKa radiation. Crystallite sizes were calculated by X-ray line
broadening analysis using the Scherrer formula on the (ill) peak for cubic and tetragonal phases and the (ill) peak for the monoclinic one.
Differential Thermal Analysis (DTA) and ThermoGravimetric Analysis (TGA) were carried out in a SETARAM TGDTA 92-16 apparatus in the temperature range 20-1300 °C under an
oxygen flow with a heating rate of 10 °C min~~ and
a cooling rate of100 °C min~~.
Some samples were also observed by Transmission Electron Microscopy (TEM) with a JEOL 2000 EX apparatus equiped with a microanalysis device.
3. Experimental Results
The coprecipitated hydroxides are X-ray amorphous as shown in Figure I. Typical DTA
and TGA curves of Zr02-Ga203 solid solutions are presented in Figure 2. The DTA curve
shows a broad endothermal effect between 50 and 350 °C. It corresponds with a loss of water evidenced in the TGA curve. From these curves, it is not possible to separate the contribution of adsorbed water from that of structural water. A second peak, exothermic, is present for all
the samples, it corresponds to the crystallization of the solid solution. At this temperature, all the water of the starting coprecipitate has been removed. The gallium delays the zirconia
crystallization process; the crystallization onset is shifted from about 435 °C for gallium-free
zirconia to 755 °C for the sample with the highest gallium content (Tab. I). For the highest gallium concentrations (x > 0.39), another exothermic peak is observed upon heating at about 925 °C. It corresponds to the decomposition of the cubic solid solution with the formation of
fl-Ga203 and of a tetragonal zirconia based phase, as confirmed by XRD spectra. No other effect is detected up to 1300 °C. During cooling, a sharp exothermic peak appears, as shown in Figure 2, it corresponds to the tetragonal to monoclinic transformation, as indicated by
XRD spectra. The temperature of this transformation is strongly influenced by the gallium concentration, from 885 °C for gallium free zirconia to 400 °C for the solid solution containing 53 mol$lo gallium (Tab.1)
As DTA experiments indicate that the crystallization of the materials under~ study always
occurs at a temperature lower than 755 °C, a series of crystallized samples was prepared
by annealing dried coprecipitates at 775 °C for 15 min (heating rate 10 °C min~~). The
X-ray diffractograms of these samples exhibit broad reflection peaks which can be indexed on the basis of a fluorite type structure (Fig. I). No second phase such as Ga203 is detected.
ioooo IcountsJ
9000
8000
7000
e 6000
sore d
4000
c 3000
zero ~
1000
~ /
0
30 40
Fig. I XRD spectra of (a) 14 mol~o Ga dried coprecipitate and of samples annealed at 775 °C for 15 min with (b) 4, (c) 14, (d) 21, (e) 39 and (f) 48 mol% Ga.
Temperature (°c)
~~ 350 690 l024 896 552
lExo20 /~
10
0
~~
a
~ o
h~
-lo ~'
j
iO
~°f~~~~°~°'
b -2°
~
j
-20~~
-40 ~H20 ieav,ng
-50
2000 4000 6000 8000 10000
Time Is)
Fig. 2 (a) DTA and (b) TGA curves of the 48mol% Ga sample.
486 JOURNAL DE PHYSIQUE III N°3
Table I. Transformation temperatures deduced from the DTA curves.
mol§lo Ga crystallization transformation (upon cooling)
onset temperature (°C) tetragonal ~ monoclinic (°C)
0 435 885
4 465 735
10 510 685
14 550 640
21 630 615
39 720 515
48 745 425
53 755 400
Table II. Stritctural parameters of Zr02-Ga203 solid solittions annealed at 775 °C for
15 mm m air.
molit Ga Crystalline structure Cell parameters Crystallite size (nm)
0 monoclinic a
= 5 15, b
= 5 19, c = 5 31 fl
= 99.2° 25
4 tetragonal a = 5 08 et c = 5.16 23
10 cubic / tetragonal a = 5.09 21
14 cubic a = 5.08 20
21 cubic a = 5.05 18
39 cubic a
= 5 00 14
48 cubic a
= 4.98 11
53 cubic a = 4 97 10
The nature of the crystalline phases and their lattice parameters are reported in Table II The
exact symmetry (tetragonal or cubic) of the sample containing 10 mol% gallium is not clear
cut, therefore the limit between the tetragonal and cubic solid solutions must be very close to this composition. An important decrease of the lattice parameter is observed as the gallium
content increases (Fig. 3a). For cubic solid solutions this decrease is linear, which suggests that
gallium atoms substitute for zirconium atoms in the network. The line broadening in the XRD spectra shows that the materials consist of crystallites with a size always lower than 25 nm, as deduced using Scherrer formula. The crystallite size decreases as the Ga203 content increases
(Fig. 3b).
As shown by DTA, the Zr02-Ga203 solid solutions decompose near 925 °C. In order to
study the decomposition process, samples containing 21 mol% gallium were heated for 15 min at several temperatures between 585 and 1300 °C. The results obtained from their XRD spectra are reported in Table III, they confirm the former analysis of the DTA curve. Above 865 °C, some growth of the cubic crystallites is observed and at 910 °C tetragonal crystallites
are detected: their size is lower than that of the cubic ones. The formation of fl-Ga203 is evidenced above 945 °C, but some amorphous or poorly crystallized Ga203 may be present
at a lower temperature since the gallium concentration in the tetragonal phase is certainly
lower than that in the cubic solid solution. The tetragonal phase obtained between 945 °C and
30 5.20
~~ 5,15
I b)
'~
~
C
( ~
j T
~
~°
~
e
~
~~~
§ "i
~ ~ ~
i
15
j
~
© 5,05
) ,~
~ a
10 5 00 §
5 4.95
10 20 30 40 50 60 0 10 20 30 40 50 60
Go (mo'%) Go (mo'%)
Fig. 3 a) Crystallite size and, b) Lattice parameters of the monoclinic [ml, tetragonal [t] and cubic [cl phases as a function of composition for samples annealed at 775 °C.
Table III. Structi~ral euoli~t~on with temperati~re of the 21 mot% Ga sample.
Temperature (°C) Phase Cell parameters Crystallite size (nm)
a (Ji) b (I)
c (h) fl(°)
585 partially crystallized
630 cubic 5.05 22
690 cubic 5.05 22
735 cubic 5.05 23
775 cubic 5 05 22
820 cubic 5 05 23
865 cubic 5 05 24
910 cubic + tetragonal 5 07 cubic 27
5 07 5.14 tetragonal:18
945 tetragonal +flGa203 5.08 5.16 22
1050 tetragonal +flGa203 5.08 5.16 29
l150 monoclinic + flGa203 5.16 5 20 5.33 99.1 25
1300 monoclinic + flGa203 5.15 5 18 5 30 99.1 27
1300 (22 h) monoclinic + flGa203 5.16 5.21 5.33 99.2 53
1050 °C can be quenched at room temperature, but for higher temperature heat treatments a monoclinic phase is formed upon cooling as evidenced by DTA The formation of a monoclinic
phase may be associated with a more complete decomposition of the solid solution or to some
growth of the tetragonal crystallites. Such a growth is suggested by the crystallite size of the material annealed for 22 h at 1300 °C.
488 JOURNAL DE PHYSIQUE III N°3
Fig. 4 TEM photograph of the 48 mol$l Ga sample annealed at 980 °C
A TEM photograph of a sample containing 48 molll gallium and heated at 980 °C is pre- sented in Figure 4. It shows agglomerated spheroidal nanometric particles (20-40 nm). The
particle size measured by TEM may be compared with that calculated from X-ray line broad-
ening (27 nm) for the tetragonal phase (a
= 5.09 and c
= 5.19 I) present in this sample. On
the other hand, it was not possible to evidence Ga203 rich domains on the TEM photograph
or by microsTEm analysis, although fl-Ga203 lines are clearly observed in the XRD spectra It is likely that most zirconia crystallites are embedded in a Ga203 layer coming from the
decomposition of the cubic solid solution. This hypothesis is supported by the observation of the largest particles in the MET photograph.
4. Discussion
Zr02-Ga203 metastable solid solutions were synthesized with up to 53 mol% gallium This
solubility has to be taken with some cautions, since amorphous or poorly crystallized Ga203 may be not detected on XRD spectra. Moreover, for a cubic solid solution, the more intense
peak of fl-Ga203 would be found in the foot of the (ill) line of the cubic phase. Even if it may be lower than 53%, the solubility of gallium in the metastable solid solutions must be close to this value, since a regular decrease of the lattice parameter is observed with increasing gallium
content The cell parameter (4.97 1) measured for the highest gallium content is unusually
low A similar solubility for aluminum was reported by Stocker [4] in a study of Zr02-A1203
solid solutions also prepared from coprecipitated hydroxides, but in this case the cell parameter
was found higher (5.09 1) than that found in the present study. The .higher cell parameter
found for the solid solution of alumina in zirconia is quite surprising since the ionic radius of Al~+ is lower than that of Ga~+. Their respective values are 0.675 and 0.76 1 for their highest
coordination number (CN
= 6) on the Shannon scale. However, in both cases these radii are smaller than the ionic radius of zirconium in fluorite derived structures~ 0 92 1 (CN
= 7)
or 0.98 1 (CN
= 8). The high solubility observed in the metastable solid solutions is not a sufficient condition to allow the formation of stable solid solutions as those formed with usual
substitutes for zirconium like yttrium or calcium. These dopants have higher ionic radii than zirconium, 1.04 1 and 1.14 1 (CN = 6) respectively. They have also a lower electronegativity
on the Pauling scale (Zr: 1.33, Y: 1.22, Ca: 1 00), whereas those of aluminum (1.61) and gallium (1.81) are higher.
The high solubility of gallium in the metastable solid solution suggests that the initial co-
precipitate is very homogeneous. The crystallization mechanism may consist of reorientations
or distortions of the oxygen polyhedra surrounding the zirconium and the gallium ions. As the solid solution crystallizes in the fluorite structure in which the gallium is never found in a stable state, the two cations are not equivalent in relation to the crystallization process. As the gallium content increases, the formation of the crystallized solid solution becomes more and more difficult, which explains that the crystallization temperature becomes higher.
The decrease of the crystallite size with increasing gallium contents could be due to an evolution of the coprecipitate texture depending on its composition It could also be related to a higher nucleation rate since the nucleation may occur at a higher temperature.
The decomposition of the solid solutions occurs around 925 °C At this temperature, the gallium ions diffuse towards the crystallite surface. A thin Ga203 layer is formed and is
embedding a zirconia richer tetragonal crystallite which has a lower size than the initial cubic
particle. The formation of the Ga203 layer around the zirconia grain makes difficult a further grain growth. Therefore, the decomposition of this kind of solid solution appears as a route for the preparation of nanocomposite materials.
5. Conclusion
The coprecipitation technic makes it possible to prepare metastable solid solutions of gallium oxide in zirconia. A high solubility, close to 50 mol% gallium, is obtained with a very low cell parameter (a
=
4.971). Despite this high solubility observed in a metastable state, the preparation of a stable solid solution seems impossible to achieve because of the small ionic radius or of the high electronegativity of the gallium ion. The metastable solid solutions are nanocrystalline (10 to 25 nm). Their decomposition by a convenient thermal treatment above 925 °C leads to the formation of nanocrystalline zirconia particles embedded in a Ga203 layer.
Acknowledgments
The authors thank C. Haut for TEM observations.
References
[1] Science and Technology of Zirconia I, A-H- Heuer and L-W- Hobbs, Eds., Advances in Ceramics Vol. 3 (American Ceramic Society, Westerville, OH, 1981).
[2] Stocker H-J, Contribution h l'4tude des propr14t4s des solutions solides r4fractaires h base de zircone et de la stabilisation de la zircone cubique, ThAse d'(tat, Ann. Chim. 5 (1960)
1459.
490 JOURNAL DE PHYSIQUE III N°3
[3] Smith K-E-, Kershaw R., Dwight K. and Wold A., Preparation and Properties of Cubic Zr02 Stabilized with Ni(II), Mat Res. Bi~ll. 22 (1987) l125.
[4] Davison S., Kershaw R, D~~ight K and ivold A., Preparation and Characterization of Cubic Zr02 Stabilized with Fe(III) and Fe(II), J. Solid State Chem. 73 (1988) 47.
[5] Berthet P., Berthon J. and Revcolevschi A., EXAFS Study of the Metastable Cubic Solid Solution of Fe203 in Zr02> Physica B158 (1989) 506.
