8-9th August, 2008 School of Materials Science & Nanotechnology,Jadavpur University
Synthesis of nanocrystalline undoped tetragonal and cubic zirconia using poly-acrylamide as gel and matrix
J. Ghose* and Subir Roy#,
Chemistry Department. IIT Kharagpur, PIN-721302, India
*Guest faculty, School of Material Science & Tech. for M.Tech., Jadavpur University, India
# Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500058.
Abstract:Nanocrystalline zirconia with tetragonal and cubic structure has been prepared from ZrO2-polyacrylamide gel and by precipitation of zirconia in polyacrylamide matrix respectively. X-ray diffraction results of the samples annealed at high temperature show that the tetragonal and cubic form, obtained from the gel, are fairly stable in air upto 1173K and partially stable in inert atmosphere, upto 1273K. The stability at such high temperatures is due to the presence of oxygen vacancies in zirconia sample, incorporated in the decomposition of polymer.
Key Words: Tetragonal zirconia ; cubic zirconia; chemical synthesis; poly-acryl amide; gel
I. Introduction
Pure crystalline zirconia exists in three crystalline polymorphs: monoclinic (m), tetragonal (t) and cubic(c). The room temperature stable form is monoclinic [1]. The metastable tetragonal and cubic phases have important technological applications although the phases are unquenchable [2]. Tetragonal zirconia has become very important engineering ceramic material due to t→m phase transition and this phase transition is associated with transformation toughening [3]. Apart from its important mechanical properties the tetragonal phase of zirconia has attracted considerable attraction due to its application in some acid catalyzed reactions [4-6]. The cubic phase has drawn considerable attention for its application in automobile industry as an oxygen sensor [7] and in the fabrication of fuel cells [8].
Thus for wide application it is essential to stabilize the tetragonal and cubic phases, study their thermal stability and the factors that govern the phase transitions. Over the last two decades a large amount of work has been devoted to stabilize tetragonal and cubic phase of zirconia in pure form [9-18] and by doping with some oxides [19-22]. These reports have shown that the metastable phase in undoped nanocrystalline zirconia is tetragonal in most of the studies. The thermal stability of metastable tetragonal phases in doped zirconia is upto 1723K, although the tetragonal phase in pure nanocrystalline zirconia is thermally stable only upto 773K [9-12]. The applications of polymer as matrix for distribution of nanoclusters and capping agents in the synthesis of nanostructured ceramic oxides and sulphides have been studied by a number of researchers [23-26]. The
present work includes the synthesis of metastable nanocrystalline tetragonal and cubic zirconia using sulphonated poly-acrylamide in two different forms i.e. as polymer gel and polymer matrix. The same precursor chemicals were used with slight alteration of synthesis chemistry i.e the difference in the sequence of addition of the chemical reagents to obtain different metastable phases of zirconia. Attention has also been paid to study the interaction between the metal ions and the polymer both in matrix and gel. Moreover, it is interesting to note that the metastable phases of undoped ZrO2 are stable up to 1173K in air. * Corresponding Author, e mail: j_ghose@hotmail.com
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II. Experimentals
ZrOCl2.8H2O(98%) was used as the source of zirconium. Three solutions were used in the two different under mentioned techniques for the synthesis of tetragonal and cubic zirconia: aqueous ZrOCl2 solution (5 wt%), aqueous sulphonated polyacryl amide (SPAM) solution (2 wt%), and aqueous ammonia solution (5 wt%). The details of the synthesis procedure are given in the scheme-I and scheme-II of figure 1. X-ray diffraction analyses of the heat-treated samples were carried out with a Phillips X-ray diffraction unit (model-PW1710) using CuK∝(λ =1.5418A0) radiation with a Ni filter. Crystallite sizes of the zirconia samples were measured from XRD line broadening analyses. Thermal analyses of the dried gel were carried out with Shimadzu Thermal Analyzer DT-40 between 298K and 1173K in air. Infrared spectra of all the samples were recorded in KBr medium in the range 4000-400cm-1 for polymer gel and 1000-400cm-1 for annealed ZrO2 samples with a Perkin Elmer spectrophotometer. TEM microstructure analyses were carried out using a transmission electron microscope (Model JEM 200 CX) operating at a voltage of 120KV.
Air-annealing
Air-annealing
Sample B
Metastable cubic zirconia (Scheme-I)
Cl – free precipitate
Filtering and washing with de-ionized water
Sample A
Precipitation of hydrated zirconium oxide in presence of SPAM at constant pH of 9.0
Zirconium oxy-chloride aqueous solution (5 wt%) from a burette
Ammonia solution (5 vol %) from a burette
SPAM aqueous ammoniacal solution (2 wt%)
Oven-drying
and calcination at 873K
Oven-drying and decomposition at 873K
Metastable tetragonal zirconia (Scheme-II)
Zirconia -SPAM gel
Ammonia solution (5%)
Cl- free precipitated dispersed inde-ionizedwater
Filtering and washing withde-ionizedwater
Precipitate of zirconium-SPAM coordinated complex
Zirconium oxy-chloride aqueous solution (5 wt%)
SPAM aqueous solution (2 wt%)
Figure1. Flow diagram for synthesis of metastable cubic and tetragonal zirconia
III. Results and Discussion
III.A. Structural characterization by XRD
Fig.2 shows the X-ray diffraction patterns of the decomposed gel at 873K (as- burnt powder-Sample-B) (Fig.2a), as-burnt powder annealed for 6 hours at 873K(Fig.2b), 973K(Fig.2c), 1073K(Fig.2d), 1173K(Fig.2e), and 1273K(Fig2f) respectively. The diffraction patterns show all the samples are tetragonal, except the 1273K-annealed sample. The samples were characterized as tetragonal by the tetragonal splitting of the
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peaks at different values of 2θ [27]. This splitting of the peaks is not observed for the sample annealed at 873K but the crystal symmetry in this sample and even the local order in the as-burnt amorphous phase may be assumed to be tetragonal as per the assumption made by Bokhimi et al. [17]. The sample annealed at 1273K shows lines of a monoclinic phase. XRD pattern of zirconia sample A annealed up to 1073K showed no splitting of the peaks, which indicated the presence of cubic phase [24]. The 1173 K annealed sample-A shows a small amount of monoclinic phase along with the cubic as the major phase, which transforms to monoclinic phase completely at 1273K[24].
TEM bright field micrograph of ZrO2 (Sample B) annealed at 873K is given in figure 3. The micrograph shows the fine spherical features of the particles with round grain junctions. The average particle size calculated manually from the micrograph is 20nm and it was observed from TEM analysis that the average particle size does not change significantly for the sample annealed at 1173K although the crystallite size showed marked changes between 1073 and 1173K(Table1).
Variation of the crystallite size and lattice parameters with annealing temperature of the zirconia samples is given in table I. From the table it is evident that the crystallite size increases slowly between 873K and 1073K and then size increases rapidly between 1073K and 1273K. The lattice parameters of the annealed tetragonal samples increases slightly with annealing temperature (Table I)
Figure 2. XRD patterns of (a) gel decomposed at 873K and decomposed powder annealed for six hours in air at (b) 873K, (c) 973K, (d) 1073K, (e) 1173K and (f) 1273K respectively.
Figure 3. Transmission electron micrographs of tetragonal ZrO2 sample annealed for six hours at 873K in air.
III.B. Chemical reactions between the polymer and the metal ions
Sulphonic acid group is incorporated in polyacrylamide chain by chemical reaction deliberately to increases the hydrophilic character and thus increasing solubility of the polyacrylamide in water which is otherwise having poor solubility in water. When
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Zr4+ ions are added to the sulphonated polyacrylamide (SPAM) in presence of ammonium hydroxide, the metal ions react with OH- ions forming hydrated oxides of zirconium in preference to the SPAM. There is no chemical interaction between the polymer and the metal ions in presence of the OH- ions. The polymer plays the function of the matrix and minimizes the agglomeration of hydrated zirconium oxide(Fig.1, scheme-I). In contrast, when the metal ions are added to water solution of SPAM in absence of ammonium hydroxide, the former form complex with the polymer. In aqueous medium sulfonate groups of the polymer form complexes with metal lions. The essential feature of the complexes is that the metal ion coordinates only with one polymer ligand group [28]. The zirconium ions attract the hydrophilic polar groups of SPAM towards itself so strongly that the remaining hydrophobic chain of SPAM separates out of the water and discrete precipitate forms which can be separated out by filtering (Fig.1, scheme-II). The complex may be represented as [Zr(SPAM)x]4+. The coordination number of the metal ion determines the value of ‘x’. The addition of ammonium hydroxide to the complex precipitate will start the competition between the OH- ions and the polar linkages of SPAM for room in the inner coordination sphere. OH- ion being smaller in size and a stronger ligand penetrates through the inner coordination sphere of the metal-SPAM complex pushing the polar groups of SPAM to the outer coordination sphere of the complex. The complex may be represented as [Zr(OH-)x(SPAM)y]4+ where OH- ion satisfies both the primary valences and the coordination number. SPAM satisfies only the coordination number. Moreover, addition of OH- ions results crosslinking among the polymer chains. The crosslinking of poly acrylamide by Zr4+ at high pH has also been reported earlier [29]. This chemical reaction brings remarkable physical changes of the [Zr(SPAM)x]4+ precipitate in the ammonia solution. The precipitate dissolves in ammonia solution and forms progressively continuous gel.
Table I. Details of thermal treatment, phase and lattice parameters of the annealed
tetragonal ZrO2 samples
Lattice parameters (±.003Ǻ)
Annealing
temperature(K)
in air/argon
Annealing
time
(hour)
Crystallite size(nm)
Phase(s)
a
b
c
β
(degree)
873
6
10.3
t
- -
973
6
11.2
t
5.0961 5.1750
1073
6
12.5
t
5.1006 5.1869
1173
6
19.0
t
5.1080 5.1990
1273
6
37.5
m
5.1670 5.2200 5.3370 99.290
1273
1
35.5
m
5.1705 5.2521 5.3405 98.740
1273(argon)
1
-
m(major) +t
5.1892 5.2290 5.3243 99.030
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Chemical reactions:
Scheme –I: Zr4+ + NH4OH + SPAM+ H2O ZrO(OH)2 + SPAM(matrix)+ H2O
Scheme-II: Zr4++ SPAM [Zr(SPAM)x]4+
[Zr(SPAM)x]4++ NH4OH +H2O [Zr(OH-)x(SPAM)y] 4+ + H2O
Thus Zr4+ encounters two different chemical environments in the two different schemes of reactions and crystallizes as two different metastable forms. These two different chemical environments have been created only by the difference in time of addition of ammonia. This slight alteration in synthesis chemistry has been proved enough to change the crystal structure of the finally crystallized ZrO2.
III.C. Thermal stability of tetragonal ZrO2 in inert atmosphere
The effect of annealing in inert atmosphere on the gel derived zirconia samples was studied by annealing the decomposed gel sample at, 1273K, in argon, for one hour. The results show that the tetragonal phase is partially retained when the samples are annealed in argon for one hour (Fig.4b). In contrast, the air-annealed sample transforms completely to the monoclinic phase in one hour (Fig.4a). These results clearly show that the annealing in inert atmosphere affects the stability of the tetragonal phase of zirconia. It appears that during the decomposition of zirconia-polymer gel, a reducing atmosphere is generated due to burning of carbonaceous parts of the gel in air, which is able to generate
Figure 4. XRD patterns of the calcined ZrO2 powder annealed for one hour at 1273K (a) in air, (b) in argon.
some oxygen vacancies in the sample. These oxygen vacancies are responsible for the stability of the metastable tetragonal phase in the zirconia samples up to 1173K. In argon, at 1273K some of the oxygen vacancies are stabilized and hence in argon annealed samples, tetragonal zirconia is found to be partially stable upto 1273K. Air annealing at 1273K, however, removes the oxygen vacancies from the samples and hence the sample undergoes complete t→m phase transition. Several theories have been put forward to explain the stability of the metastable phases in zirconia. Garvie [30] realized the tetragonal monoclinic phase transition as the particle size effect. Garvie’s hypothesis of particle size effect on the phase transition has been questioned by several researchers [12,31-33]. Murase and Kato [31] suggested that there is no clear relationship between
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the particle size and the t→m phase transition. Mitsuhasi et al.[12] assumed that anion impurities and domain structure have an effect on phase stability. Literature shows that the monoclinic phase can be prepared in the form of very fine particles of about 6 nm size [32]. The role of oxygen vacancies to stabilize the metastable phase in zirconia was also proposed by some researchers [32,33].
The present experimental results strongly supports the role of oxygen vacancy to stabilize the metastable tetragonal phase up to 1173K.
IV. Conclusions
Tetragonal ZrO2 of average particle size ~20 nm has been synthesized ZrO2-polymer gel. IR spectra of ZrO2-polymer gel indicate strong electrostatic interaction between the metal ion and the polar linkages of polymer. Tetragonal phase of zirconia can be stabilized up to 1173K without any foreign stabilizing agent. The stability of pure tetragonal zirconia, synthesized from the zirconia-polymer gel, is due to the presence of oxygen vacancies. The oxygen vacancies are formed in the samples as a result of the reducing atmosphere created during decomposition of the gel. The partial retention of the tetragonal phase at 1273K, in argon, is due to partial stabilization of the oxygen vacancies in argon atmosphere.
Acknowledgement
The authors wish to thank DST, Government of India, for financial support.
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