Indian Journal of Engineering & Materials Sciences Vol. 15, April 2008, pp. 111-115

Preparation and characterization of PMN-PT nanocomposite

Shrabanee Sen*, S K Mishra, S Sagar & S K Das MST Division, National Metallurgical Laboratory, Jamshedpur 831 007, India

Received 10 November 2006; accepted 28 February 2008

Ferroelectric ceramic-polymer composites are considered as promising materials for applications in sensors, actuators and hydrophones. They are attractive for application as they exhibit high piezoelectric and pyroelectric response, low acoustic impedance matching with water and human skin and their properties can be tailored to various requirements. The advantage of composites over conventional ceramics is that they have better mechanical shock resistance and more durable. Lead magnesium niobate-lead titanate (PMN-PT with 35 mol% PT) ceramic powder is fabricated by citrate gel method. The calcinations temperature is optimized by thermal analysis. The formation of the ceramic powder is confirmed by XRD. The microstructural properties have been studied by, SEM and TEM. The particle size calculated from TEM was found to be between 50-55 nm and the homogeneous distribution of the powders was also observed. The composites have been prepared using solvent casting in which the powder is dispersed homogeneously in the polymer matrix. Different composites are made by varying the ceramic to polymer ratio. The structural, microstructural properties of the composite are studied.

Ferroelectric ceramic-polymer composites are considered as promising materials for applications in high-pressure sensors, hydrophones and actuators1-3. The advantage of composites over conventional ceramics is that they have better mechanical shock resistance and more durable. Reported work on PZT-PVDF composites of 0-3 connectivity prepared by both solvent cast technique and hot press technique showed that the results obtained from the composites prepared by the hot press technique exhibited better sensivity, reproducibility and durability4,5. Nowadays lead magnesium niobate-lead titanate (PMN-PT) with high piezoelectric properties has been widely studied6,7. PMN-PT is the solid solution of relaxor ferroelectric lead magnesium niobate (PMN) and normal ferroelectric lead titante (PT). It is found that with 35% PT, it is near to the morphotropic phase boundary (MPB) region and here the dielectric and piezoelectric properties are maximized because of the energy states of rhombohedral and tetragonal structures. At this region the outstanding piezoelectric properties have wide applications in transducers and actuators8,9. PVDF is a commonly used ferroelectric polymer, which possess good piezo and pyro properties9,10. This paper reports study on the 0-3 connectivity of PMN-PT-PVDF composites in different molar ratio (30,50, 70). The structural and microstructural properties of the prepared composites have been studied.

Experimental Procedure The ceramic powder (PMN-PT) was synthesized

by the citrate-gel method using moisture insensitive and inexpensive precursors like citrate salts (Ti and Nb) and nitrates (Pb and Mg). The citrate gel method was preferred because the homogenous mixing of the precursor solution takes place and the calcinations temperature is lowered which prevents lead loss. The details of the procedure for synthesis of ultra fine PMN-PT powders are described below: Preparation of niobium and titanium citrate solutions

Aqueous solution of niobium citrate complex was prepared staring from the oxide (Nb2O5), which was dissolved in HF (>7 M) by warming the mixture over water bath for 2 days to obtain a clear solution of niobium-fluoride complex (i.e NbF5

2- complex). The hydrous niobium oxide (Nb2O5.nH2O) was precipitated out from the clear solution of the niobium complex by addition of aqueous solution of ammonia. The precipitate was washed with 5% ammonia solution to make it fluoride free.

Finally, the precipitate was dissolved in aqueous solution of citric acid (2 M per unit mol of niobium ion) with continuous stirring to obtain a clear, aqueous solution of niobium citrate. The solution was assayed at 1000oC for 2 h to estimate the amount of niobium oxide. Similarly titanium citrate solution was made by dissolving titanium oxide (TiO2) in HF solution and further processing in the same way.

_____________ *For correspondence (E-mail: shrabaneesen@yahoo.co.in)

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Preparation of gel Required amount of magnesium carbonate was

taken and dissolved in minimum amount of nitric acid and to this solution required amount of lead nitrate was added. Further citric acid was added to in the molar ratio of 1:1. Proportionate amount of titanium and niobium citrate was taken and both the solution were mixed well. Ethylene glycol was added to the solution in the ratio of 0.25 M with respect to the total citrate ions. A white precipitate was formed and ammonium citrate solution was added to it in order to maintain a pH value of 6. The clear solution was heated at water bath for 6 h, which resulted in the formation of a semi solid gel. The formation of the gel takes place due to the esterfication of ethylene glycol and citric acid. This semi solid gel was heated overnight in an oven at 150P

oPC to get a hard gel, which

was crushed and calcined at the optimum temperature. Thermal analysis of the hard gel was done to optimize the calcinations temperature. The gel was calcined at 600P

oPC for 4 h to form ultrafine powders. XRD, SEM

and TEM of the fine powders were done to study the structural and microstructural properties. Composites of different molar ratios (30, 50 and 70) in the 0–3 connectivity were prepared by hot press technique. The required amount of polymer (PVDF) was taken and dissolved in methyl ethyl ketone to form a solution. After homogeneous mixing of the polymer in the solvent was completed, the ceramic powder was added to it slowly. The solution was continuously stirred well by a magnetic stirrer. After homogeneity was achieved, the solution was heated at 60 P

oPC along

with continuous stirring. This continued till a gel like formation takes place and then the gel was pressed between two metal discs. As the sample was pressed, the disk was heated up to 150P

oPC. Finally composites

on 1–2 mm were formed. The prepared composites were characterized by XRD, SEM and AFM. Results and Discussion

Thermal analysis (Model DT-40, Shimadzu Co, Japan) was carried out in static air at a heating rate of 10P

oPC/min upto 1000P

oPC in order to optimize the

calcination temperature. The TGA and DTA curves are shown in Fig 1. The weight loss occurred in two steps, first an initial weight loss which resulted from the evaporation of the absorbed water and then a rapid reduction up to 533P

oPC after which the mass loss

saturates. Above this temperature the TG curves show no weight loss. This was also confirmed from DTA curves, which shows two exothermic peaks at 327P

oPC

and 474P

oPC respectively.

The strong exothermic peak indicates the release of a high amount of heat due to combustion of the charred mass obtained from the decomposition of the metal citrate-ethylene glycol complex. It is also observed that above 550P

oPC no significant thermal effect was

observed in the DTA curve, which implies that complete ultrafine powders.

The phase formation of the powder was confirmed by XRD (Fig. 2). XRD spectra revealed that the formation of the desired phase has taken place with a small amount of pyrochlore phase (PbB2.31BNbB2BOB7B and PbNbB2BOB6B) marked in the figure.

The particle morphology, size and distribution was of the particles was studied by high-resolution transmission microscope (JEOL-JSM 2010 Electron Microscope). Figure 3a shows the distribution of the particles and also depicts that the particle shape is non-spherical [Fig. 3b]. After statistic analysis the average particle size was found to be 55±2 nm.

Fig. 1—TGA and DTA pattern of PMN-PT nanocomposite.

Fig. 2—Wide angle X-ray diffraction pattern of PMN-PT nanocomposite.

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This is in agreement with the particle size calculated from diffraction peak. The corresponding selected area electron diffraction pattern of the same sample (Fig. 3c shows symmetrically dotted pattern, implying that the nanoparticles are well crystallized.

The HRTEM pattern of the selected area indicates that there are very similar orientations in the particle (Fig. 3d). The lattice spacing in the particle was found to be 2.3 Å, which was comparable with the value

Fig. 3a—TEM of PMN-PT nanocomposite.

Fig.3b—TEM of PMN-PT nanocomposite from a different region.

Fig. 3c—Selected area diffraction pattern of PMN-PT.

Fig. 3d—HRTEM image of PMN-PT nanocomposite.

Fig. 4—Isotherm reveals that is of Type I in IUPAC system, which indicates the formation of non-porous material.

Fig. 5—XRD pattern for different concentration of PMN-PT.

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obtained from XRD analysis. The surface area of the fine powder was measured

by the Brunauer-Emmet-Teller (BET) method in a Beckman Counter Analyzer (Miami, FL). Before N B2 B adsorption- desorption measurement, each sample was degassed with a N B2B purge at 77 K for 3 h. The specific surface area was found to be 68 mP

2P/g P

-1P. The

isotherm shown in Fig. 4 reveals that is of Type I in IUPAC system, which indicates the formation of non-

porous material. The characterization of the prepared composites

was done by XRD, SEM and AFM. Figure 5 shows the room temperature XRD pattern of the composites using CoKBαB target. The first one shows the diffraction pattern of PVDF polymer. The amorphorous nature of the polymer is more prominent there. The next three graphs show the diffraction pattern of the composites in different ratios, i.e., 10, 30 and 50%. For the lowest concentration one peak of PVDF is visible. The diffraction peaks are more or less same with change in

Fig. 6—SEM micrographs of the composite PMN-PT-PVDF 10% at (a) 500 and (b)1000 magnification.

Fig. 7a—Topographic images of PVDF using atomic force microscopy (AFM).

Fig. 7 (b-d)—AFM of the composite PMN-PT and PVDF.

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intensity or Bragg angle value. This is due to the change in the ratio of the polymer and the ceramic. A rough estimation of the particle size was done using Scherer’s equation and it was found to be 22, 24 and 28 nm respectively for 10, 30 and 50% ratio of ceramic to polymer. The pattern suggests good crystallinity and the interaction of the ceramic with the polymer is also visible.

Figures 6a and 6b show the SEM micrographs of the composite PMN-PT-PVDF 10%. In Fig. 6a the polymer matrix is clearly visible with some ceramic powders embedded inside it. Further with higher magnification (Fig. 6b) the ceramic powders within the polymer matrix is more prominently seen.

Topographic images of the composites were taken by atomic force microscopy (AFM). The first figure (Fig. 7a) shows the homogeneous distribution of the polymer (PVDF) in the solvent. It also indicates the single-phase component of the polymer only. The other three figures (Figs 7b-7d] show the presence of two phases denoted by light and dark gray region. The increase in the ceramic to polymer ratio is also clearly evident from the micrographs. In Fig. 7d the distribution of the ceramic powders is uniform throughout the polymer matrix as the ratio between them is 50%.

Conclusions PMN-PT ultrafine powders were prepared by

citrate gel method. The formation of the compound was confirmed by XRD analysis. The crystallite size was found to be 50 nm which was comparable with that obtained from TEM micrographs, i.e., 55 nm. The composites were prepared by taking different ratios of polymer (PVDF) and the ceramic powders by hot press technique. AFM micrographs show the well distribution and ratio of the ceramic to polymer matrix in the composite. References 1 Skinner D P, PhD Thesis, 1978. 2 Lubitz K, Wolff A & Peru G, Ferroelectrics, 133 (1992) 21. 3 Shrout T R & Schulze W A, Mater Res Bull, 14 (1979) 1553. 4 Satish B, Sridevi K & Vijaya MS, J Phys D: Appl Phys, 35

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