Structure property relations of chemically synthesized nanocrystalline PZT powders

Materials Science and Engineering A304–306 (2001) 775–779

Structure property relations of chemically synthesized nanocrystalline PZT powders P. Pramanik∗ , R.N. Das Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, West Bengal, India

Abstract Nanocrystalline (1–100 nm) materials can be generated by a variety of different chemical synthesis routes. The present investigation deals with the improvement of electrical properties (dielectric) through nanocrystalline PZT materials. Chemical routes have prepared nanocrystalline (grain size 20–100 nm) PZT powders as well as large grained (190–480 nm) with Zr/Ti ratio of 60/40 at different temperatures having the average crystallite sizes 8–18 nm. It was observed that dielectric constant after sintering different nanocrystalline PZT systems attains very high value (∼10,000–12,000) in comparison to large grained materials (5000–7000). It was observed that dielectric property increases with decreasing particle size of the nanocrystalline powders. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline materials; Dielectric constant; Sintering

1. Introduction Nanostructured materials (1–100 nm) have now been investigated for more than a decade using a rather wide range of experimental methods [1]. Various investigation of their chemical, mechanical, electrical, magnetic and optical behavior have already demonstrated the possibilities to research the properties of nanostructured materials through the control of the sizes of their constituent clusters or powders and the manner in which the constituents are assembled. Nanocrystalline materials exhibit high diffusivity, increased solid solubility, high strength, and sufficient formability in the case of ceramics. There remain opportunities for relatively new tailored synthesis and processing methods. Some aspects of our present understanding is the inter-relationship between structure and properties of nanocrystalline materials that need to be characterized on both atomic and nanometer scale. The microstructural features of importance include particle/grain size, distribution, and morphology. The nanocrystalline ultrafine grained materials exhibit a variety of considerably improved property with respect to coarse-grained polycrystalline materials which includes increased hardness/strength, enhanced diffusivity, improved ductility/toughness, higher electrical resistivity, higher thermal expansion coefficient, lower thermal conductivity and

∗ Corresponding author. Tel./fax: +91-03222-55303. E-mail address: [email protected] (P. Pramanik).

soft magnetic properties. Only limited literature is available on the electrical properties of nanocrystalline materials. In our present discussion, we have introduced structure property (dielectric constant) relations using a new category of smart material, lead zirconate–titanate (PZT), which have wide industrial applications. PZT, a solid solution of ferroelectric PbTiO3 (Tc = 490◦ C) and antiferroelectric PbZrO3 (Tc = 230◦ C), belongs to the ferroelectric family of perovskite structure with a general formula of ABO3 (A: mono or divalent, and B: tri to hexavalent ions), discovered by Jaffe et al. [2] in 1954. The extensive work on synthesis [3–13] and characterization of pure and complexed PZT have emerged for their wide industrial applications such as transducers, computer memory and display, light valves, electro-optical modulators, oscillators and sensors, etc. In the present paper, we have introduced nanocrystalline PZT material and its influence over the dielectric property. Since nanocrystalline materials are containing very large number of atoms at the grain boundaries, the numerous interfaces provide a high density of short-circuit diffusion paths [14]. Consequently, they are expected to exhibit enhanced diffusivity in comparison to coarse-grained polycrystalline materials with the same chemical composition. This enhanced diffusivity can have a significant effect on density and hence dielectric property of the sample. Here, we have reported the preparation of PZT materials through chemical process with different grain morphology and distribution (from nano to submicron grain) and study the corresponding dielectric property of the system with changing dimensions.

0921-5093/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 5 6 4 - 1

776

P. Pramanik, R.N. Das / Materials Science and Engineering A304–306 (2001) 775–779

Table 1 The details of the chemicals required for the PZT preparative routes Metal salts/preparation condition (i) Initially homogeneous solution with desired metal ion ratio is formed (ii) The homogeneous solution on complete evaporation produce the precursor powder

Minimum furnace temperature require to produce single PZT phase

DEA process. Stoichiometric amount of hydrated metal oxides (Pb, Zr, Ti) were reacted with 8 mol DEA per mole of PZT at 200◦ C and produced a homogeneous red solution of metal ion–DEA complexes

200◦ C

25

TEA process. Stoichiometric amount of hydrated metal oxides of Ti and Zr were reacted with 8 mol of TEA to produce a red solution of metal ion–TEA complexes. The hot metal ion–TEA solution was then mixed with aqueous 30% (w/w) Pb(NO3 )2 mixture to produce a homogeneous mixture

450◦ C for 2 h

20

Tartarate process. 1 mol aqueous Pb(EDTA) + stoichiometric amount of Ti-tartarate and Zr-tartarate solution mixed to produce homogeneous solution

400◦ C for 2 h

30

PVA process. The stoichiometric amount of hydrated metal oxides (Ti, Zr) were slowly dissolved in concentrated HNO3 . The metal nitrate thus produced was mixed with 1 mol aqueous Pb(NO3 )2 solution. The homogeneous metal nitrate solution was then mixed with 4 mol 10% (w/v) aqueous PVA solution

500◦ C for 2 h

100

Sucrose process. The stoichiometric amount of hydrated metal oxides (Ti, Zr) were slowly dissolved in concentrated HNO3 . The metal nitrate thus produced were mixed with 1 mol aqueous Pb(NO3 )2 solution. The homogeneous metal nitrate solution was then mixed with 4 mol sucrose containing 10 mol% aqueous PVA solution

480◦ C for 2 h

80

Coprecipitation. Homogeneous metal nitrate solutions in accordance with the PZT composition were precipitated using ammonia. The precipitate was dried at 60◦ C over water bath and produced the precursor powder

500◦ C for 5 h

190

Hydrothermal. Coprecipitate mass was hydrothermally treated using aqueous KOH (40%) solution

220◦ C for 5 h

480

2. Experimental In all the chemical routes, the metal oxides of the corresponding PZT composition are introduced as a complex form in solution and the complex mixture on decomposition to the ambient temperature gives the single phasic PZT. The details of the chemical routes are given in Table 1. A series of PZT powders were prepared by the various heat treatments of metal-complex precursor material, coprecipitate material and hydrothermally treated material, and the corresponding particle sizes and crystallite sizes were measured by TEM and X-ray diffraction (XRD), respectively. For dielectric constant measurements for the PZT system, the heat-treated powders were pelletized using a pressure of 3.2×107 Pa. The pellets were sintered at 1100◦ C for 6 h in lead oxide-rich environments provided by PbZrO3 in the closed alumina crucible. The pellet density was measured using Archimedes principle, and for lattice parameter which is used to calculate theoretical density, all the reflection peaks were indexed, and lattice parameter of the PZT samples were calculated using a computer package based on a least-squares refinement method. The microstructure and the particle size distribution were studied by transmission electron microscopy (TEM) (TM-300, Phillips).

Corresponding average TEM particle size (nm)

orated dry carbonaceous mass (referred to as the precursor powder) were carried in air atmosphere with a heating rate of 10◦ C/min. Fig. 1 illustrates the thermogram of PZT precursor powder prepared through sucrose method as a typical example. The precursor powders may be considered to be constituted of some metal carboxylate and the respective metal oxide trapped in pyrolyzed mass produced by the oxidation of complexing agents (such as TEA, DEA, tartaric acid, EDTA), PVA and sucrose. The general thermograms reveal a strong exothermic peak between 300 and 500◦ C accompanied by the release of high amount of heat due to the combustion of the carbonaceous residue from the decomposition of complexes. The whole thermal process is accompanied by the evolution of large amounts of gas that is

3. Results and discussions Simultaneously recorded thermal gravimetry (TG) and differential thermal gravimetry (DTG) studies of the evap-

Fig. 1. TG/DTG curve of PZT precursor powder obtained by the sucrose process.

P. Pramanik, R.N. Das / Materials Science and Engineering A304–306 (2001) 775–779

Fig. 2. Room temperature powder X-ray diffractograms (using Cu K␣ radiation, λ = 0.15418 nm) of the PZT powders after heat treatment (for 2 h) at 400◦ C (pattern “a”), 450◦ C (pattern “b”), 500◦ C (pattern “c”), 600◦ C (pattern “d”), and 700◦ C (pattern “e”) (Cu K␣) obtained from TEA precursors.

manifested in the weight loss in the TG curve. The evolution of various gas (such as water vapor, CO, CO2 , NO2 , etc.) not only helps the precursor material to disintegrate but also to dissipate the heat of decomposition, thus inhibiting sintering of the fine particle. The temperatures of heat treatment significantly influence the reactivity and the particle size. XRD data were obtained at room temperature for the different chemical processes after heating to various temperatures. The typical example of XRD of the metal–TEA precursor showed the single phase PZT pattern initiated at 400◦ C for 2 h and produced single phase PZT at 450◦ C for 2 h (in Fig. 2). The crystallite size for the series of heat-treated powders were calculated from X-ray (d1 1 0 ) line broadening studies using the Scherrer equation [15] and the corresponding crystallite size was found to range between 8 and 18 nm. The details of formation temperature from XRD are summarized in Table 1. The finer details of the particles and their morphology were investigated by TEM. The bright field electron micro-

777

Fig. 3. TEM micrograph and corresponding electron diffraction (as inset) of PZT powder obtained from tartarate precursor after heat treatment at 400◦ C for 2 h.

graph of the precursor powders heat-treated at different temperatures reflected the basic powder morphology where the smallest visible particle can be identified with the crystallite and/or their aggregates. Each micrograph exhibited electron diffraction pattern, which corresponds to the diffraction pattern from an assembly of nanocrystallites. Typical bright field micrographs of PZT powders obtained from tartarate route after it was heat treated at 400◦ C for 2 h is shown as a typical representative example in Fig. 3. The corresponding electron diffraction indicates the assembly of nanocrystallites. Table 2 summarizes the average particle and crystallite sizes of the materials obtained by TEM and XRD analyses, respectively. XRD and TEM studies show that (from Table 2) X-ray crystallite sizes are comparable for different PZT samples prepared by different methods but the grain (particle) size is different, which indicates that the particles are crystallite agglomerates with different degree of agglomeration. As in normal ferroelectric materials, the dielectric constant () of PZT, with Zr/Ti ratio 60/40, increases gradually with temperature and attains a maxima ( max ) at the Curie temperature (Tc ). The  max measurement of the prepared yel-

Table 2 Comparison of general characteristics of PZT ceramics Pb(Zr 0.6 Ti0.4 )O3 ≡ PZT systems

Calcination temperature

Average crystallite Average particle size/XRD (nm) size/TEM (±10 nm)

Relative density, Dr (±1%)a

Dielectric constant,  (±200)

Tc (±1◦ C)

Metal–DEA precursor Metal–TEA precursor Metal–tartarate precursor Metal–PVA precursor Metal–sucrose precursor Coprecipitate material hydrothermally heated at 200◦ C for 4 h Coprecipitate material hydrothermally heated at 220◦ C for 5 h Coprecipitate material

500◦ C 450◦ C 400◦ C 500◦ C 480◦ C 350◦ C

15 17 14 10 12 17

30 20 30 100 80 140

97.5 97.9 97.3 94.8 95.3 91.74

11518 12400 11900 10121 10700 8696

364 362 371 362 365 360

–

17

480

88.5

4549

360

500◦ C for 5 h

18

190

90.2

7528

360

a

for for for for for for

Relative density (%): % theoretical density.

1h 1h 2h 2h 2h 4h

778

P. Pramanik, R.N. Das / Materials Science and Engineering A304–306 (2001) 775–779

Fig. 4. Variation of dielectric constant () with temperature at 10 kHz frequency for the PZT (60/40) obtained from PVA precursor method.

low PZT was carried out using pellets. The yellow colored sintered pellets were ground and lapped to make the surface flat and parallel and subsequently electroded by applying a silver paste to both flat surfaces. The plot of dielectric constant against temperature for PZT (60/40) powder obtained from PVA precursor powder is shown as a typical example in Fig. 4. The summaries of dielectric constants are depicted in Table 2. It was observed that the dielectric constant under similar sintering condition is increasing with decreasing particle size and giving a maximum where the particle (grain) size equals the crystallite size. Similarly, the relative density of pellets prepared from PZT powders increases (from Table 2) with decreasing particle size and attains maximum

relative density where particle size equals to the crystallite size. The plot of dielectric constant (Fig. 5) versus the difference between the particle size (denoted as ‘d’) and X-ray crystallite size (denoted as ‘dc ’) shows that the dielectric constant increases with decreasing (d − dc ) value, whereas the plot of dielectric constant versus relative density (Fig. 5) shows that the dielectric constant increases with increasing relative density. The relative density graph shows that with increasing relative density the dielectric constant increases by 1000 units.

4. Conclusions

Fig. 5. Variation of dielectric constant with the average particle size (d) and crystallite size (dc ) and the relative density (Dr ).

1. Nanocrystalline materials introduced more efficient sintering. 2. Chemical synthesis routes are technically simple and economical for large-scale production of nanocrystalline PZT materials. 3. In the nanocrystalline PZT materials, X-ray crystallite sizes are comparable for different samples prepared by different methods but the grain (particle) size which is an assemble of nanocrystallites is varied. The dielectric property of sintered nanocrystalline PZT material is improving with decreasing grain size. It can be concluded that when the particle size is comparable with crystallite size, then the densification reaches maximum, which leads to maximum dielectric constant.

P. Pramanik, R.N. Das / Materials Science and Engineering A304–306 (2001) 775–779

Acknowledgements We are grateful to CSIR, New Delhi, India for financial assistance. References [1] [2] [3] [4]

G.M. Whitesides, J.P. Mathias, C.T. Setu, Science 254 (1991) 1312. B. Jaffe, R.S. Roth, S. Marzullo, J. Appl. Phys. 25 (1954) 809. Y. Matsuo, H. Sasaki, J. Am. Ceram. Soc. 48 (1965) 289. B.V. Hiramath, B.V. Kingon, J.V. Biggers, J. Am. Ceram. Soc. 66 (II) (1983) 790. [5] S.S. Chandatreya, R.M. Fulrath, J.A. Pask, J. Am. Ceram. Soc. 64 (1981) 422.

779

[6] T.R. Shrout, P. Papet, S. Kim, G.S. Lee, J. Am. Ceram. Soc. 73 (7) (1990) 1862. [7] S.Y. Chem, S.Y. Cheng, Ch.-M. Wang, J. Am. Ceram. Soc. 73 (1990) 232. [8] T.R.N. Kutty, R. Balachandan, Mater. Res. Bull. 19 (1984) 1479. [9] M.M. Lenka, A. Anderko, R.E. Riman, J. Am. Ceram. Soc. 78 (1995) 2609. [10] H. Hirashima, E. Onishi, R. Nakagowa, J. Non-Cryst. Solids 121 (1990) 404. [11] V.R. Palkar, M.S. Muttani, Mater. Res. Bull. 14 (1979) 1353. [12] K. Rama Mohana Rao, A.V. Prasada Rao, S. Komarneni, Mater. Lett. 28 (1996) 463. [13] S.K. Saha, P. Pramanik, Br. Ceram. Trans. 96 (1997) 21. [14] H. Gleiter, Nanostruct. Mater. 1 (1992) 1. [15] M.P. Klug, L.E. Alexander, X-ray Diffraction Procedure, Wiley, New York, 1974, p. 634.