Author name used in this publication: Y. ChenAuthor name used in this publication: H. L. W. ChanAuthor name used in this publication: C. L. ChoyVersion of RecordPublishe

Chan, HLW

Chen, Y

Choy, CL

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Properties of Nanocrystalline PT Powder and PT/P(VDF-TrFE) 0-3 Nanocomposites Y. Chen, H.L.W. Chan and C.L. Choy Department of Applied Physics and Materials Research Centre The Hong Kong Polytechnic University, Kowloon, Hong Kong. Abstract - Nanometer-size powder of lead titanate (F'T) was prepared using the sol-gel process. The PT gel was annealed at various temperatures to produce powder with different particle sizes. The structural variation of the PT powder with annealing temperatures was studied by X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis @TA) and differential scanning calorimetry (DSC). The crystallite size and particle size of the PT powder were measured by XRD and a particle size analyzer, respectively. PTIPOF-  TrFE) 0-3 composites with a PT volume fraction of 0.4 were prepared by using PT powder of size 30 nm to 270 urn. The permittivities of the composites were measured using an impedance analyzer. After poling, the piezoelectric and pyroelectric coefficients of the composites were also determined. I. INTRODUCTION Lead titanate (PT) is an important ferroelectric material possessing good piezoelectric and pyroelectric properties. In the conventional approach, PT is prepared by ball milling followed by calcination above 850°C. Because of the high synthesis temperature, the PT particle produced are too large to be used in integrated sensor applications. In recent years, there has been considerable interest in using the sol-gel process to prepare ceramics from metal alkoxides [l-31 because high-purity ultrafine particles and large-area thin films can be produced at relatively low temperature. The present authors are interested in preparing nanocrystalline powder and incorporating them into a piezoelectric vinylidene/ trifluoroethylene copolymer (P(VDF-TrFE)) matrix to form 0-3 nanocomposites for integrated pyroelectric sensor applications. 11. EXPERIMENTAL PROCEDURES The procedure for the preparation of PT nanocrystalline powder is outlined in Fig. 1. First, lead acetate trihydrate was dissolved in 2- methoxyethanol (CjHs02) at 80°C in a three-neck reaction flask. After the lead acetate trihydrate had dissolved, the reaction flask was fitted with a reflux condenser and the solution was heated to approximately 11 8°C to remove the adsorbed water. The rise of the temperature of the vapour reaching the top of the reflux condenser from 100°C to 124°C (the boiling point of methoxyethanol) indicated the complete removal of residual water. Then the solution was cooled to 100°C and titanium isopropoxide was added to give a 1:l molar ratio of lead to titanium. Volatile isopropyl acetate was produced and when it was completely removed, the temperature of the vapour reaching the top of the reflux condenser would increase from 85°C (b.p. of isopropyl acetate) to about 124°C. PT gel was obtained by controlling the hydrolysis condition of the PT solution in which the PH was adjusted to about 11 by adding water, ammonia and alcohol. The gel was dried at 100°C for 24h and then heated to various temperatures (500°C to 900°C) to give particle of various sizes. The crystallization process of the PT gel was studied by differential thermal analysis (DTA, Perkin-Elmer 1 700), thermogravimetric analysis (TGA, Perkin-Elmer TGS- 2) and x-ray diffraction ( Philips X'pert XRD system) with Ni filtered CuK a radiation. The particle size distributions were measured using a particle size analyser (Horiba CAPA- 700). The Curie temperature Tc of the annealed powder was determined by differential scanning calorimetry (DSC, Perlun-Elmer DSC-2 ). To prepare 0-3 composites, P(VDF-TrFE) (65/35) copolymer powder supplied by Piezotech in France was fist  dissolved in methyl-ethyl-ketone (h4EK) and then a suitable amount of PT powder was blended into the solution. The mixture was then poured onto a petri-dish and, after evaporation of the solvent, the mixture was moulded into a 0.2 mm thick sheet by hot pressing at 170°C and at a pressure of 0.83 kg/m'. Composites with a PT volume fraction of 0.4 were fabricated using PT powder annealed at various temperatures. Dics samples of 12.5 mm diameter were prepared. In order to elicit piezoelectric activity, the composites must be poled by subjecting it to an electric field for a period of time at a suitable temperature to align the dipoles in both phases. The 0-3 composite was poled at 105°C with a field of 26 kV/mm for 2h. The electric field was switched off at 105°C (Curie temperature of the copolymer =102"C) before cooling the sample to room temperature. By this procedure, only the ceramic phase was poled. In order to pole thc copolymer, the sample was heated fiom room temperature to 90°C and a field of 27 kV/mm was applied for lh, then the sample was cooled to room temperature with the field kept on. The above procedure was repeated in order to better polarize the copolymer. The sample was short-circuited and annealed for 17 hours at 60°C before measurements. 0-7803-3355-1/96/$5.00 0 1996 IEEE. 619 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 20:36 from IEEE Xplore.  Restrictions apply.Pb(OAc)~*3H*O w 50- I Heat to 118OC to remove residual water Dehydrated solution cool down to 100°C Ti(0 C 4H9)4 Reflex at 124°C for 3h (Pb.Ti) Complex metal alkoxide Cool down to room temperature Water, ammonia, alcohol Stirring Precipitation Keep at 100°C for 24h PT Gel Dry and anneal sC4Nfw-E 1Dckgi-m Fig. 1 A flowchart showing the procedure for preparing PT nanocrystalline powder by the sol-gel process. The ceramic poling ratio a was caculated from the peak of the (200) and (002) x-ray reflections heights I(200) and associated with PT using the relation: The capacitance of the samples was measured at 1 kHz using an HP4194A impedance analyzer and the permittivity was calculated. The pyroelectric coefficient p was measured using the digital integration method, [4] and the dj3 coefficient of the samples was measured using a Pennebaker model 8000 piezo djj tester ftom American Piezo-Ceramics Inc.. The piezo- and pyro-electric coefficients of the polarized samples were measured and results given in Table 1. 111. RESULTS AND DISCUSSION DTA and TGA were performed on the PT powder at a heating rate of 10"C/min. The DTA curve shows an endothermic peak at 80°C (Fig. 2a) which is accompanied by a small weight loss (Fig. 2b), primarily due to the removal of 37k ' 1 5 0 '  & '  & ' A' & '  & '  7b - Tenperatwe ("C) Fig 2a DTA curve of PT gel. 103.8 (95.56%) 2121 (%.E%) 288.8 (90.94%) -9 (87.9h) 360 jOi 620 Fig.2b. TGA curve of PT gel. Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 20:36 from IEEE Xplore.  Restrictions apply.TABLE 1 THE RELATIONSHIP BETWEEN THE A " E A L R i G  TEMPERATURE. THE AVERAGE CRYSTALLITE SEE. PARTICLE sm AhD Tc OF PT NANOCRYSTALLINE POWDER a. E, d33 AND p ARE PROPERTIES OF 0-3 PTmVDF-TRFE) COMPoSlTES WITH A CERAMIC VOLUME FwcnoN OF 0 4 USING PT ANNEALED AT VARIOUS TEMPERATURES Annealing crystallite particle T, (T) of PT E €1  a1 (dj3)l PI PlISl Temp ("C) dia (nm) dia (nm) @cm) (pClm2K) (pC/m2K) 500 26 S 30 434 5 39 2 32 6 0 73 I 1  2 44 I 135  600 35 0 40 472 5 43  0 32 5 0 73 14 7 49 4 I 5 2  650 36 5 50 _ _ _ _ _ _  44 0 3; 6 0 48 6 9  54 1 I61 700 40 3 70 479 5 40 0 30 9 0 68 14 0 47 2 1 5 3  800 44 s I20 475 4 40 3 31 4 0 73 I4 0 46 0 146 900 54 I 270 481 4 44 0 31 9 0 72 13 6 42 6 1 34 (25°C) (25OC) Subscript 1 indicates both phases are polarized water and alcohol trapped in micropores. The exothermic peak at 312"C, together with the corresponding weight loss. can probably be attributed to the oxidation of acetate groups or the decomposition of unhydrolyzed alkoxy groups. The exothermic peak at 51OoC, not accompanied by a significant decrease in weight, probably reflects the crystallization of the PT powder. Fig. 3 shows the XRD patterns of PT powder after annealing at various temperatures for lh. Samples annealed at temperatures lower than 500°C exhibit the typical amorphous pattern. When the annealing temperature reaches 5OO0C, crystalline peaks appear, in agreement with the DTA result. The crystallite sizes of PT powder annealed at various temperatures were calculated from the full width at half maximum (FWHM) of the (lo]), (111) and (200) x-ray di&action peaks using the Scherrer's equation [5] D = (KA)/(Bcose) (2) where D is the crystallite diameter, A is the wavelength, 6 is the diEaction angle, B is the FWHM of a diffraction peak, and K is Scherrer's constant (= 0.89). By comparison with the diffiaction peaks of a standard material (Si), the width due to the instrument can be evaluated, and the true width arising f?om finite crystallite size can be obtained. The crystallite diameter, obtained by averaging the results for the (loo), (1 1 1) and (200) peaks, is given as a function of the annealing temperature in Table 1. The average crystallite diameter increases from 27 nm to 54 nm as the annealing temperature increases !?om 500°C to 900°C. The particle size distributions of the PT nanocrystalline powder annealed at various temperatures were measured using a particle size analyzer. The size of the PT powder (dispersed by ultrasonic agitation in ethanol) f a h  in the range of 30 to 270 nm indicating that each PT particle consists of a number of crystallites. The measured Curie transition temperature T, of the annealed powder (Table 1) was 435°C for powder annealed at 5OO0C and increased to 475°C- 481°C for annealing temperatures higher than 700°C. &C. 1 h h il, O C ,  1 h 2J 3 40 60 m 30) R Fig. 3 XRD patterns of PT powder annealed at various temperatures for 1 h. 62 1 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 20:36 from IEEE Xplore.  Restrictions apply.The permittivity E of the unpoled composites varied slightly with annealing temperature. After poling, the permittivity E,  of the samples decreased by about 23%. The polarized composite with PT powder previously annealed at 65OoC has the lowest poling ratio a, and hence the lowest piezoelectric coefficient (d;j),. We had tried to improve a1 by poling this sample several times but a1 showed little improvement. However, maximum values of the pyroelectric coefficient pI and p1k  were achieved in composites with the PT phase annealed at about 650OC. IV. CONCLUSION PT nanocrystalline powder has been prepared by the sol- gel process using metal alkoxides. The sizes of both the PT particles and the crystallites within each particle increase with annealing temperature. The PT powder annealed at 650°C has average particle and crystallite diameters of 50nm and 3 7 ~ 1 ,  respectively. The ultrafine PT powder has been incorporated into a P(VDF-TrFE) matrix to form 0-3 composites, and the nanocomposite with PT powder annealed at 650°C has the highest pyroelectric coefficient pand the highest gure of merit pis. Work is in progress to produce 1p.m thick 0-3 composite films for integrated pyroelectric sensor and array applications. ACKNOW‘LEDGMEhT Ths  research was supported by the Hong Kong Research Grant Council. Thanks are due to Dr. Qifa Zhou for his technical assistance. REFERENCES [ 11 Y Xu and J D Mackenzie, “Ferroelectric thin films prepared by sol- gel processing”, Integrared Ferroelectrics, vol 1, pp 17-42, 1992 [2] K Ishikawa, K Yorhikawa and N Okada, “Size effect in the ferroelectnc phase transition in PbTi03 Ultrafine particles”, Physical Review B, vol 37, pp 5832-5835, April, 1988 M H Lee, A Halliyal and R E Newnham, “Poling of coprecipitated lead titanate-expoxy 0-3 piezoelectric composites”’ J Am Ceram SOC , vol 72, pp 986-990, 1989 141 Y Xu, Ferroelectric Materials and Their Applications, Elsevier Science Publishers, Amsterdam, 199 1, pp 7 1-72 [ 5 ]  L S Birks and H Friedman, “Particle Size Determination from X-ray line Broadening” J Appl Phys, vol 17, pp 687-692, I946 [ 3 ]  622 Authorized licensed use limited to: Hong Kong Polytechnic University. Downloaded on December 1, 2008 at 20:36 from IEEE Xplore.  Restrictions apply.

Properties of nanocrystalline PT powder and PT/P(VDF-TrFE) 0-3 nanocomposites

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