Synthesizing Nanocrystalline Pb(Zn1/3Nb2/3)O3 Powders fromMixed Oxides

John Wang,*,† Wan Dongmei, Xue Junmin, and Ng Wei Beng

Department of Materials Science, National University of Singapore, Singapore 119260

The attempt to synthesize a Pb(Zn1/3Nb2/3)O3 (PZN) pow-der of perovskite structure via both traditional ceramic andchemistry-based novel processing routes over the last threedecades has failed. Difficult-to-synthesize nanocrystallitePZN powders have, for the first time, been successfullyprepared via a mechanochemical reaction either amongPbO, ZnO, and Nb2O5 or between PbO and pre-reactedZnNb2O6 from ZnO and Nb2O5 for more than 15 h in ahigh-energy mechanochemical reaction chamber. The re-sulting PZN powders exhibit a well-established perovskitestructure and their crystallite sizes are in the range of 10 to15 nm, as has been indicated from the peak broadening ofX-ray diffraction and direct observation using a high-resolution transmission electron microscope.

I. Introduction

LEAD ZINC NIOBATE, Pb(Zn1/3Nb2/3)O3 (PZN), exhibits a par-tially disordered perovskite structure and is a relaxor type

of ferroelectric ceramic material.1 It undergoes a rhombohe-dral-to-cubic transformation at temperatures around 140°C andshows many desirable dielectric and piezoelectric propertiesfor actuator and transducer applications. To the best of ourknowledge, no one has succeeded in synthesizing such a finereactive-sintering perovskite PZN powder, although variousceramic and chemistry-based novel processing routes, such assol–gel, molten salt, and the combustion method,2,3 have beenused over the last three decades. The growth of PZN crystals ofvarious sizes from a molten salt containing excess PbO andconstituent oxides is the only technique which has been shownto result in the phase formation of perovskite structure.1,4All ofthe other processing routes lead to the occurrence of a pre-dominant pyrochlore phase with or without a minor amount ofperovskite phase in the resulting materials.5

It is also impossible to retain a high percentage of perovskitephase in a sintered PZN ceramic via pressureless sintering ofPZN crystals grown from the molten salt containing excessPbO.4 Hot pressing or hot isostatic pressing at a very highpressure (∼200 MPa) and at temperatures in the range of 1100°to 1200°C is the only viable consolidation scheme for retaining>90% perovskite phase.6 This is related to the thermodynamicinstability of PZN over the temperature range of 600° to1400°C,4 which has been suggested to originate from suchparameters as the tolerance factor7 and the site potential ratio.8

To stabilize the perovskite structure in PZN, an appropriateamount of additives of perovskite structure, such as PbTiO3,

BaTiO3, SrTiO3, and PbZrO3, is required for the partial sub-stitution of either A or B site cations or both.5,9

Mechanoalloying was developed for producing nanocrystal-line metal and alloy powders and it has been employed forsynthesizing nanosized ferrite-based magnetic materials.10 Fewceramic materials, however, have been successfully processedvia the novel fabrication technique.11 In this paper, we describea novel mechanochemical method for the synthesis ofnanocrystalline PZN powders of perovskite structure.

II. Experiments

The starting materials were widely available oxides: PbO(99% in purity, J. T. Baker Inc.), ZnO (99% in purity, Aldrich),and Nb2O5 (99% in purity, Aldrich). The perovskite phase wassynthesized via two slightly different routes: (i) mechano-chemical reaction among PbO, ZnO, and Nb2O5; and (ii)mechanochemical reaction between PbO and pre-reactedZnNb2O6 from ZnO and Nb2O5. In route (i), a powder mixtureof PbO, ZnO, and Nb2O5 of appropriate weight ratios wasprepared in a conventional ball mill using zirconia balls as themilling medium in ethanol for 48 h, followed by drying theslurry at 80°C. A 6 g sample of the powder mixture was loadedin a wear-resistant vial of 40 mm diameter and 40 mm lengthtogether with a high-density milling ball of 12.7 mm diameter.Mechanochemical reaction was subsequently carried out in ahigh-energy shaker mill operated at 875 rpm for various timeperiods ranging from 0 to 20 h.12 The resulting powders werethen characterized for phases present using an X-ray diffrac-tometer (Philips PW1729). A high-resolution transmissionelectron microscope (Philips CM300 FEG) and a BET specificsurface analyzer (Nova-2000, Quantachrome) were employedto study their particle characteristics.

In route (ii), the first step of the synthesis process involvedmixing appropriate amounts of ZnO and Nb2O5 powders byfollowing the same procedures as detailed for the mixture ofPbO, ZnO, and Nb2O5. The powder mixture was then calcinedat 1050°C for 4 h in order to develop the ZnNb2O6 phase. Itwas subsequently mixed with an appropriate amount of PbOpowder in a conventional ball mill. A 6 g sample of the result-ing powder mixture was loaded in the mechanochemical reac-tion vial and the mechanochemical treatment was carried outfor various time periods ranging from 0 to 23 h.

III. Results and Discussion

Figure 1 shows the XRD patterns of the powder mixture ofPbO, ZnO, and Nb2O5 mechanochemically treated for 0, 5, 10,15, and 20 h, respectively. As expected, the powder that wasnot subjected to any mechanochemical treatment consists ofcrystalline PbO, ZnO, and Nb2O5, indicating that little or noreaction takes place among the three constituent oxides in theconventional ball mill. Two strong broadened diffraction peaksoccur over the 2u angle range of 28° to 32° for the powdermechanochemically treated for 5 h. The one at 29.08° corre-sponds to PbO (111) and the one at 31.13° is perovskite PZN(110). The very much broadened nature of PbO (111) at 29.08°implies that after the initial 5 h of mechanochemical treatment,

B. A. Tuttle—contributing editor

Manuscript No. 189903. Received September 2, 1998; approved November 20,1998.

Supported by the National University of Singapore under Grant No. RP3979900.*Member, American Ceramic Society.†Author to whom correspondence should be addressed. E-mail: maswangj@

nus.edu.sg.

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a significant refinement in particle sizes, together with a degreeof amorphization of constituent oxides, has taken place. At thesame time, nanocrystallites of perovskite PZN phase areformed as a result of the mechanochemical activation. Theintensity of perovskite PZN (110) increases with increasingmechanochemical treatment time up to 20 h, at the expense ofPbO (111). The increasing crystallinity of perovskite PZNphase with rising mechanochemical treatment time is clearlyshown by the sharpening of the PZN (110) peak. It has beenestimated using the Scherrer equation13 on the basis of thehalf-width of PZN (110) that the PZN crystallites in the powdermechanochemically treated for 20 h are in the range of 10 to 15nm in size. This suggests that the mechanochemical treatmentresults in the formation of PZN nuclei in a highly activatedmatrix of mixed oxides, followed by growth of these nucleiinto well-established PZN crystallites. Pyrochlore phases areapparently not involved in the formation process of perovskitephase.

Figure 2 shows the XRD traces of the powder mixture ofPbO and ZnNb2O6 (ZN) mechanochemically treated for 0, 10,15, and 23 h, respectively. Little or no reaction was observedbetween the two compounds after the conventional ball mill-ing, as demonstrated by the XRD pattern of the powder thatwas not subjected to any mechanochemical treatment. Thepowder mechanochemically treated for 10 h exhibits broadeneddiffraction peaks over the 2u angle range of 28° to 32°. How-ever, no substantial reaction between PbO and ZN is observed,suggesting that the 10 h of mechanochemical treatment onlyresults in a significant reduction in the particle sizes togetherwith a degree of amorphization in the two compounds. In a

remarkable contrast, perovskite PZN phase is the predominantphase in the powder mechanochemically treated for 15 h, asshown by a broadened and strong PZN (110) peak centered ata 2u angle of 31.13°. Its average crystallite size was estimatedto be∼10 nm using the Scherrer equation13 on the basis of thehalf-width of PZN (110). Similarly, the powder mechano-chemically treated for 23 h also shows fine perovskite PZNcrystallites as the only XRD detectable phase. In particular, it

Fig. 3. Average particle sizes of the powder mixture of PbO andZnNb2O6 mechanochemically treated for various times.

Fig. 1. XRD patterns of the powder mixture of PbO, ZnO, andNb2O5 mechanochemically treated for various times ranging from 0 to20 h: (s) PbO, (j) Nb2O5, (l) ZnO, (d) PZN.

Fig. 2. XRD patterns of the powder mixture of PbO and ZnNb2O6mechanochemically treated for various times ranging from 0 to 23 h:(s) PbO, (l) ZN, (d) PZN.

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exhibits a noticeably sharpened PZN (110) peak, due to theenchanced crystallinity with further increase in the mechano-chemical treatment time.

Figure 3 plots the average particle size for the powder mix-ture of PbO and ZnNb2O6 as a function of mechanochemicaltreatment time, as worked out on the basis of the specificsurface areas measured using BET. These results suggest thatthe initial 10 h of mechanochemical treatment significantlyreduces the particle sizes of mixed PbO and ZnNb2O6. Furtherincreasing the mechanochemical treatment time, up to 23 h,changes little in the particle characteristics. This agrees withwhat has been suggested by Boldyrev,14 that a mechanochemi-

cal reaction between two solid particles requires sufficient con-tacts between them, and therefore requires the fragmentation oflarge particles into small ones. Mechanical activation can occuronly when the particle size reaches a certain threshold value.

As shown in Figs. 4(a,b), the mechanochemically synthe-sized PZN powder (20 h) from mixed oxides of PbO, ZnO, andNb2O5 consists of perovskite crystallites of 10 to 15 nm in sizewhich occur as particle aggregates of 50 to 100 nm. An amor-phous layer of 2 to 5 nm in thickness was observed at theboundaries of these nanosized crystallites. A considerably highdegree of structural disorder was clearly visible in some ofthese fine PZN crystallites, as shown in Fig. 4(b).

IV. Conclusions

Using commercially available PbO, ZnO, and Nb2O5 pow-ders as the starting materials, ultrafine PZN powders of perov-skite structure have been synthesized via two slightly differentprocessing routes: (i) mechanochemical reaction of the threeconstituent oxides; and (ii) mechanochemical reaction betweenPbO and pre-reacted ZnNb2O6 from ZnO and Nb2O5. They aresingle-phase PZN and exhibit well-established perovskitestructure, and their crystallite sizes are in the range of 10 to 15nm. Pyrochlore phases were not involved in the mechano-chemical formation process of perovskite PZN phase.

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Fig. 4. High-resolution micrographs of mechanochemically synthe-sized PZN powder: (a) aggregates of PZN nanocrystallites in a largeparticle; and (b) the partially disordered perovskite structure.

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