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Sonocatalyzed hydrothermal preparation of lead titanate nanopowders
Materials Letters 61 (2007) 4522 – 4524 www.elsevier.com/locate/matlet
Sonocatalyzed hydrothermal preparation of lead titanate nanopowders A. Rujiwatra a,⁎, C. Wongtaewan a , W. Pinyo a , S. Ananta b a
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b
Received 24 October 2006; accepted 19 February 2007 Available online 28 February 2007
Abstract Nanoparticles of perovskite lead titanate, PbTiO3, were successfully prepared in a stoichiometric proportion at conspicuously low temperature and short reaction time of only 130 °C and 3.5 h. The employment of ultrasonic treatment prior to hydrothermal reaction, or so-called “sonocatalyzed hydrothermal reaction”, was examined in detail. Roles of ultrasonic wave both as a catalyst to lower the hydrothermal reaction temperature affording the phase-pure PbTiO3 and as an external influence to provide better size homogeneity were discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Lead titanate; Hydrothermal; Sonochemical; Perovskites; Powder technology
1. Introduction As one of the most important ferroelectric materials with high spontaneous polarization and piezoelectric coefficients but low aging rate of the dielectric constant, lead titanate (PbTiO3 or PT)based compounds can serve a variety of high temperature and frequency applications. These include infrared sensors, electrooptic devices and ultrasonic transducers [1–3], as examples. The performance of these materials and devices thereof is frequently determined by characteristics of the precursor powders, which are intrinsically regulated by preparative techniques and parameters. Although various techniques are currently available for the preparation of PT powders, e.g. sol–gel [4], chemical precipitation [5], emulsion [6] and hydrothermal technique [7,8], only few of these offer reasonable controllability of the powder characteristics. Hydrothermal technique has been proved to possess such controllability on phase formation, stoichiometry, particle size and morphology. For instance, the powders prepared by hydrothermal technique can exhibit different crystal structures and morphologies, i.e. cubic, tetragonal primitive and tetragonal body-centered perovskites. This crucially depends on the preparative parameters [7,8]. The temperatures generally used in the preparation of PT powders under hydrothermal ⁎ Corresponding author. Tel.: +66 53 941906; fax: +66 53 892277. E-mail address: [email protected] (A. Rujiwatra). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.02.039
conditions are between 180 and 250 °C [9–11]. Recently, we reported the achievement of using an even milder temperature of 150 °C [8]. Lately, there have been few reports on the promotion of hydrothermal reactions by ultrasonic treatment for the preparation of fine oxide powders, e.g. Al-doped ZnO, ZrO2, TiO2 and NiFe2O4 [12–14]. The hybridization of these two techniques for the preparation of piezoelectric powders, however, has not yet been reported. In the present work, we report the success in the application of the sonochemical catalyzation concept with conventional hydrothermal process for the preparation of phase-pure PT powders. 2. Experimental Mixtures of lead (II) nitrate (Pb(NO3)2, Univar 99.0%) and titanium (IV) oxide (TiO2, Riedel-de Haen 99.5–100%) were prepared in aqueous media. The amount of PbII and TiIV was controlled to 1:1, and the final concentrations for each precursor were fixed at 1.32 mol dm− 3. The pellets of potassium hydroxide (KOH, Merck 85%) were gradually added to each reaction mixture to adjust the pH of the mixture to 14. This was reported to be the optimal pH to provide phase-pure PT powders [7,8]. The mixtures were transferred into Teflon liners, which were then sealed and ultrasonic irradiated at 70 (±5) °C for varied durations ranging from 1 to 12 h, using a laboratory ultrasonic bath (Bandelin Electronic RK255H, 160/320 W, 35 kHz). The
A. Rujiwatra et al. / Materials Letters 61 (2007) 4522–4524
liners were fitted in hydrothermal bombs for further hydrothermal reactions conducted under autogenous pressure at varied temperatures from 100 to 130 °C for 3.5 to 72 h. The powders were recovered by filtration and washed with deionized water until the pH of the filtrate was ca. 7, in order to remove any remaining hydroxide on the powder surface. Powder X-ray diffractometer (PXRD, Siemen D500/D501, CuKα, Ni filter, λ = 1.54 Å) was used to characterize the crystalline phases, and field emission scanning electron microscope (FESEM, JEOL JSM-6335F) equipped with an energy dispersive X-ray (EDX) analyzer was employed in the investigation of morphological and elemental composition. In order to study the particle agglomeration, particle size distributions of the aggregates were analyzed by laser diffraction technique (CLAS 1064 ranging from 0.04 to 500.00 μm/100 classes). 3. Results and discussion The PXRD patterns of the white-yellow powders obtained after ultrasonic treatment at 70 °C for various irradiation times indicated no formation of PT phase. Only well crystalline starting precursors of PbO and TiO2 in both rutile and anatase were clearly apparent. This suggested the absence of chemical reaction, which would lead to the formation of PT, which may be accounted for by the insufficient sound intensity and hence energy to promote the reaction [15]. Slightly smaller size of PbO and TiO2 particles with prolonged irradiation time was yet observed. Mean diameters of the particles after 1 and 6 h of ultrasonic treatment were 260 nm and 230 nm, respectively. The results agreed well with the postulation on the formation of local acoustic microjets piercing the particles to smaller sizes [15,16]. According to former reports on the preparation of PT powders under hydrothermal conditions, the minimum temperature affording phasepure PT powders was 150 °C [7,8]. In the present work, we therefore attempted the milder temperatures of 100 and 130 °C for hydrothermal reactions. The reactions were conducted on the reaction mixtures, which had been ultrasonic irradiated at 70 °C for an hour, under autogenous pressure at 100 °C for 6 to 24 h. Fig. 1 shows the PXRD patterns of the hydrothermally treated powders in comparison with those of the ultrasonic irradiated precursor. The presence of PbO and TiO2 precursors was clearly apparent even after 6 h of hydrothermal reaction. The prolongation of reaction time to 12 h though resulted in the
Fig. 1. The PXRD patterns of the powders obtained from hydrothermal reactions at 100 °C for (a) 0, (b) 6, (c) 12 and (d) 24 h with prior ultrasonic irradiation at 70 °C for an hour; = PbO, ● = TiO2 rutile, = TiO2 anatase, ○ = TixOy, and # = non-stoichiometric PT.
▪
4523
Fig. 2. The PXRD patterns of PT powders obtained from (a) hydrothermal reaction at 130 °C for 3.5 h with prior 6 h of ultrasonic irradiation, compared to those yielded from hydrothermal reactions at 130 °C for 6 h with prior ultrasonic irradiation at 70 °C for (b) 1, (c) 3, (d) 6 and (e) 12 h. The asterisks indicate unidentified peaks.
formation of various transformed oxide phases of precursors, possibly mixed with non-stoichiometric PT. The presence of these PT phases became more pronounced after 24 h of reaction, suggesting the feasibility in the preparation of the desired perovskite PT at the water boiling temperature. Influence of ultrasonic treatment time prior to hydrothermal reaction at 100 °C was not revealed by the PXRD technique. The employment of 1 and 6 h of ultrasonic treatment prior to hydrothermal reaction at 100 °C for 24 h gave similar PXRD results. Hydrothermal reactions of the irradiated reaction mixtures at 130 °C, on the other hand, always provided phase-pure PT powders. Fig. 2 shows the PXRD patterns of the powders obtained from the hydrothermal reactions at 130 °C for 3.5 and 6 h with prior ultrasonic treatment at 70 °C for an hour. Every peak present in the patterns could be indexed with tetragonal P4/mmm and refined cell parameters a = 3.88(2) Å, c = 4.16 (1)–4.21(2) Å. These parameters were well consistent with those generally obtained from hydrothermal reactions [7,8]. The EDX results confirmed the equimolar ratio of Pb:Ti for the powders characterized to be the phase-pure PT by PXRD technique. The phase-pure PT powder was therefore successfully prepared in a stoichiometric proportion at conspicuously low temperature and short reaction time of 130 °C and 3.5 h. The ultrasonic irradiation in the process therefore played an important role as a catalyst, leading to the term “sonocatalyzed hydrothermal reaction”, to reduce the hydrothermal reaction temperature affording phase-pure PT. The reduction of hydrothermal reaction temperature can be accounted for by cavitational bubble collapse phenomena. The collapse of such bubbles generated during ultrasonic irradiation reportedly leads to a launch of shock waves out into the liquid, which greatly enhances interparticle collisions velocity and particle fragmentation. This can substantially increase the available surface area of the powders and the ion diffusion process, leading to a reduction of the activation energy of the reaction and hence reaction temperature [17]. Scanning electron micrographs revealed the primary particles to be either cubic or tabular with wide size distribution of a few hundred nanometers to micrometers (Fig. 3). The primary particles were highly agglomerated with mean aggregate diameter of 10.87 μm determined from laser diffraction technique, and no improvement of particle dispersion was achieved, in comparison to the conventional cases. The influences of ultrasonic wave on phase purity, size and morphology of the obtained PT particles were also examined. The PXRD patterns of the powders obtained from the reactions conducted at 130 °C for 6 h with prior irradiation at 70 °C for 1 to 12 h are shown in
4524
A. Rujiwatra et al. / Materials Letters 61 (2007) 4522–4524
time up to 3 h led to a different size distribution pattern of the primary particles. The particle sizes were dispersed regularly in a relatively smaller region of 90–1170 nm (Fig. 3), which may be accounted for by concomitant occurrence of particle fragmentation and interparticle surface melting, depending on angle of interparticle collision point. These two competing phenomena may well lead to the average of particle sizes [12,17]. These results suggested an improvement in the characteristics of both primary particles and aggregates as provided by the sonocatalyzed hydrothermal reactions.
4. Conclusions In this work, the application of ultrasonic irradiation prior to hydrothermal reaction, or so-called sonocatalyzed hydrothermal technique, in the preparation of PT powders has proved to be very successful. In the process, ultrasonic wave played an important role as both catalyst, which helped in lowering the reaction temperature at which phase-pure PT could be prepared, and external influence in providing PT powders with better size homogeneity. Acknowledgements The authors gratefully acknowledge the National Nanotechnology Center (NANOTEC), Thailand, for financial support, and the Thailand Research Fund for providing instruments. Mr. C. Wongtaewan thanks the Royal Thai Government (Scientific Human Resources Development Program) for undergraduate scholarship. References
Fig. 3. Particle size distributions and scanning electron micrographs in the inlets of the particles present in the PT powders characterized by the PXRD patterns shown (a) in Fig. 2(a), and (b) in Fig. 2(d), respectively.
Fig. 2. Although irradiation times of up to 6 h showed negligible influence on phase purity and crystallinity of the PT powders, the longer irradiation time of 12 h resulted in the formation of another unidentified phase. This was more pronounced in the case of 24 h of ultrasonic irradiation, as indicated by asterisks in Fig. 2. Cell parameters of the PT powders could be well refined in P4/mmm with a = 3.88(1)–3.91(1) Å and c = 4.16(1)–4.22(2) Å. The widely distributed refine parameters may correspond to the rapid kinetics of the formation of PT framework under sonocatalyzed hydrothermal reactions. Although scanning electron micrographs showed no difference in particle morphology with different ultrasonic irradiation times, sizes of the aggregates were clearly reduced with prolonged irradiation time. Mean diameter of the aggregates obtained from an hour of ultrasonic irradiation followed by 6 h of hydrothermal reaction at 130 °C was 4.55 μm, compared to 3.83 μm of that obtained from 3 h of ultrasonic irradiation followed by hydrothermal reaction under the same conditions. The primary particle sizes and size distributions were also significantly influenced by ultrasonic irradiation durations. Sizes of the particles yielded from the reaction with only 1 h of irradiation before the hydrothermal reaction at 130 °C for 6 h were distributed between 120 and 1880 nm, with two most populated areas at ca. 190–260 nm and 900–1080 nm. The prolongation of the irradiation
[1] A.J. Moulson, J.M. Herbert, Electroceramic, 2nd ed. Wiley, New York, 2003. [2] Y. Xu, Ferroelectric Materials and Their Applications, North-Holland, New York, 1991. [3] F. Jona, G. Shirane, Ferroelectric Crystals, Dover Publications, New York, 1993. [4] A. Udomporn, K. Pengpat, S. Ananta, J. Eur. Ceram. Soc. 24 (2004) 206. [5] J. Tartaj, J.F. Fernandez, F. Villafuerte-Castrejon, Mater. Res. Bull. 36 (2001) 479. [6] J. Fang, J. Wang, L.M. Gan, S.C. Ng, Mater. Lett. 52 (2002) 304. [7] A. Rujiwatra, J. Jongphiphan, S. Ananta, Mater. Lett. 59 (2005) 1871. [8] A. Rujiwatra, N. Thammajak, T. Sarakonsri, R. Wongmaneerung, S. Ananta, J. Cryst. Growth 289 (2006) 224. [9] M.C. Gelabert, B.L. Gersten, R.E. Riman, J. Cryst. Growth 211 (2000) 497. [10] X. Zeng, Y. Liu, X. Wang, W. Yin, L. Wang, H. Guo, Mater. Chem. Phys. 77 (2002) 209. [11] S.-K. Lee, G.-J. Choi, U.-Y. Hwang, K.-K. Koo, K.-K. Park, Mater. Lett. 57 (2003) 2201. [12] P.E. Meskin, V.K. Ivanov, A.E. Barantchikov, B.R. Churagulov, Y.D. Tretyakov, Ultrason. Sonochem. 13 (2006) 47. [13] R.R. Piticescu, R.M. Piticescu, C.J. Monty, J. Eur. Ceram. Soc. 26 (2006) 2979. [14] P.E. Meskin, A.E. Baranchikov, V.K. Ivanov, D.R. Afanas'ev, A.I. Gavrilov, B.R. Churagulov, N.N. Oleinikov, Inorg. Mater. 40 (2004) 1058. [15] L.H. Thompson, L.K. Doraiswamy, Ind. Eng. Chem. Res. 38 (1999) 1215. [16] K.S. Suslick, Science 247 (1990) 1439. [17] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295.
Sonocatalyzed hydrothermal preparation of lead titanate nanopowders A. Rujiwatra a,⁎, C. Wongtaewan a , W. Pinyo a , S. Ananta b a
Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b
Received 24 October 2006; accepted 19 February 2007 Available online 28 February 2007
Abstract Nanoparticles of perovskite lead titanate, PbTiO3, were successfully prepared in a stoichiometric proportion at conspicuously low temperature and short reaction time of only 130 °C and 3.5 h. The employment of ultrasonic treatment prior to hydrothermal reaction, or so-called “sonocatalyzed hydrothermal reaction”, was examined in detail. Roles of ultrasonic wave both as a catalyst to lower the hydrothermal reaction temperature affording the phase-pure PbTiO3 and as an external influence to provide better size homogeneity were discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Lead titanate; Hydrothermal; Sonochemical; Perovskites; Powder technology
1. Introduction As one of the most important ferroelectric materials with high spontaneous polarization and piezoelectric coefficients but low aging rate of the dielectric constant, lead titanate (PbTiO3 or PT)based compounds can serve a variety of high temperature and frequency applications. These include infrared sensors, electrooptic devices and ultrasonic transducers [1–3], as examples. The performance of these materials and devices thereof is frequently determined by characteristics of the precursor powders, which are intrinsically regulated by preparative techniques and parameters. Although various techniques are currently available for the preparation of PT powders, e.g. sol–gel [4], chemical precipitation [5], emulsion [6] and hydrothermal technique [7,8], only few of these offer reasonable controllability of the powder characteristics. Hydrothermal technique has been proved to possess such controllability on phase formation, stoichiometry, particle size and morphology. For instance, the powders prepared by hydrothermal technique can exhibit different crystal structures and morphologies, i.e. cubic, tetragonal primitive and tetragonal body-centered perovskites. This crucially depends on the preparative parameters [7,8]. The temperatures generally used in the preparation of PT powders under hydrothermal ⁎ Corresponding author. Tel.: +66 53 941906; fax: +66 53 892277. E-mail address: [email protected] (A. Rujiwatra). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.02.039
conditions are between 180 and 250 °C [9–11]. Recently, we reported the achievement of using an even milder temperature of 150 °C [8]. Lately, there have been few reports on the promotion of hydrothermal reactions by ultrasonic treatment for the preparation of fine oxide powders, e.g. Al-doped ZnO, ZrO2, TiO2 and NiFe2O4 [12–14]. The hybridization of these two techniques for the preparation of piezoelectric powders, however, has not yet been reported. In the present work, we report the success in the application of the sonochemical catalyzation concept with conventional hydrothermal process for the preparation of phase-pure PT powders. 2. Experimental Mixtures of lead (II) nitrate (Pb(NO3)2, Univar 99.0%) and titanium (IV) oxide (TiO2, Riedel-de Haen 99.5–100%) were prepared in aqueous media. The amount of PbII and TiIV was controlled to 1:1, and the final concentrations for each precursor were fixed at 1.32 mol dm− 3. The pellets of potassium hydroxide (KOH, Merck 85%) were gradually added to each reaction mixture to adjust the pH of the mixture to 14. This was reported to be the optimal pH to provide phase-pure PT powders [7,8]. The mixtures were transferred into Teflon liners, which were then sealed and ultrasonic irradiated at 70 (±5) °C for varied durations ranging from 1 to 12 h, using a laboratory ultrasonic bath (Bandelin Electronic RK255H, 160/320 W, 35 kHz). The
A. Rujiwatra et al. / Materials Letters 61 (2007) 4522–4524
liners were fitted in hydrothermal bombs for further hydrothermal reactions conducted under autogenous pressure at varied temperatures from 100 to 130 °C for 3.5 to 72 h. The powders were recovered by filtration and washed with deionized water until the pH of the filtrate was ca. 7, in order to remove any remaining hydroxide on the powder surface. Powder X-ray diffractometer (PXRD, Siemen D500/D501, CuKα, Ni filter, λ = 1.54 Å) was used to characterize the crystalline phases, and field emission scanning electron microscope (FESEM, JEOL JSM-6335F) equipped with an energy dispersive X-ray (EDX) analyzer was employed in the investigation of morphological and elemental composition. In order to study the particle agglomeration, particle size distributions of the aggregates were analyzed by laser diffraction technique (CLAS 1064 ranging from 0.04 to 500.00 μm/100 classes). 3. Results and discussion The PXRD patterns of the white-yellow powders obtained after ultrasonic treatment at 70 °C for various irradiation times indicated no formation of PT phase. Only well crystalline starting precursors of PbO and TiO2 in both rutile and anatase were clearly apparent. This suggested the absence of chemical reaction, which would lead to the formation of PT, which may be accounted for by the insufficient sound intensity and hence energy to promote the reaction [15]. Slightly smaller size of PbO and TiO2 particles with prolonged irradiation time was yet observed. Mean diameters of the particles after 1 and 6 h of ultrasonic treatment were 260 nm and 230 nm, respectively. The results agreed well with the postulation on the formation of local acoustic microjets piercing the particles to smaller sizes [15,16]. According to former reports on the preparation of PT powders under hydrothermal conditions, the minimum temperature affording phasepure PT powders was 150 °C [7,8]. In the present work, we therefore attempted the milder temperatures of 100 and 130 °C for hydrothermal reactions. The reactions were conducted on the reaction mixtures, which had been ultrasonic irradiated at 70 °C for an hour, under autogenous pressure at 100 °C for 6 to 24 h. Fig. 1 shows the PXRD patterns of the hydrothermally treated powders in comparison with those of the ultrasonic irradiated precursor. The presence of PbO and TiO2 precursors was clearly apparent even after 6 h of hydrothermal reaction. The prolongation of reaction time to 12 h though resulted in the
Fig. 1. The PXRD patterns of the powders obtained from hydrothermal reactions at 100 °C for (a) 0, (b) 6, (c) 12 and (d) 24 h with prior ultrasonic irradiation at 70 °C for an hour; = PbO, ● = TiO2 rutile, = TiO2 anatase, ○ = TixOy, and # = non-stoichiometric PT.
▪
4523
Fig. 2. The PXRD patterns of PT powders obtained from (a) hydrothermal reaction at 130 °C for 3.5 h with prior 6 h of ultrasonic irradiation, compared to those yielded from hydrothermal reactions at 130 °C for 6 h with prior ultrasonic irradiation at 70 °C for (b) 1, (c) 3, (d) 6 and (e) 12 h. The asterisks indicate unidentified peaks.
formation of various transformed oxide phases of precursors, possibly mixed with non-stoichiometric PT. The presence of these PT phases became more pronounced after 24 h of reaction, suggesting the feasibility in the preparation of the desired perovskite PT at the water boiling temperature. Influence of ultrasonic treatment time prior to hydrothermal reaction at 100 °C was not revealed by the PXRD technique. The employment of 1 and 6 h of ultrasonic treatment prior to hydrothermal reaction at 100 °C for 24 h gave similar PXRD results. Hydrothermal reactions of the irradiated reaction mixtures at 130 °C, on the other hand, always provided phase-pure PT powders. Fig. 2 shows the PXRD patterns of the powders obtained from the hydrothermal reactions at 130 °C for 3.5 and 6 h with prior ultrasonic treatment at 70 °C for an hour. Every peak present in the patterns could be indexed with tetragonal P4/mmm and refined cell parameters a = 3.88(2) Å, c = 4.16 (1)–4.21(2) Å. These parameters were well consistent with those generally obtained from hydrothermal reactions [7,8]. The EDX results confirmed the equimolar ratio of Pb:Ti for the powders characterized to be the phase-pure PT by PXRD technique. The phase-pure PT powder was therefore successfully prepared in a stoichiometric proportion at conspicuously low temperature and short reaction time of 130 °C and 3.5 h. The ultrasonic irradiation in the process therefore played an important role as a catalyst, leading to the term “sonocatalyzed hydrothermal reaction”, to reduce the hydrothermal reaction temperature affording phase-pure PT. The reduction of hydrothermal reaction temperature can be accounted for by cavitational bubble collapse phenomena. The collapse of such bubbles generated during ultrasonic irradiation reportedly leads to a launch of shock waves out into the liquid, which greatly enhances interparticle collisions velocity and particle fragmentation. This can substantially increase the available surface area of the powders and the ion diffusion process, leading to a reduction of the activation energy of the reaction and hence reaction temperature [17]. Scanning electron micrographs revealed the primary particles to be either cubic or tabular with wide size distribution of a few hundred nanometers to micrometers (Fig. 3). The primary particles were highly agglomerated with mean aggregate diameter of 10.87 μm determined from laser diffraction technique, and no improvement of particle dispersion was achieved, in comparison to the conventional cases. The influences of ultrasonic wave on phase purity, size and morphology of the obtained PT particles were also examined. The PXRD patterns of the powders obtained from the reactions conducted at 130 °C for 6 h with prior irradiation at 70 °C for 1 to 12 h are shown in
4524
A. Rujiwatra et al. / Materials Letters 61 (2007) 4522–4524
time up to 3 h led to a different size distribution pattern of the primary particles. The particle sizes were dispersed regularly in a relatively smaller region of 90–1170 nm (Fig. 3), which may be accounted for by concomitant occurrence of particle fragmentation and interparticle surface melting, depending on angle of interparticle collision point. These two competing phenomena may well lead to the average of particle sizes [12,17]. These results suggested an improvement in the characteristics of both primary particles and aggregates as provided by the sonocatalyzed hydrothermal reactions.
4. Conclusions In this work, the application of ultrasonic irradiation prior to hydrothermal reaction, or so-called sonocatalyzed hydrothermal technique, in the preparation of PT powders has proved to be very successful. In the process, ultrasonic wave played an important role as both catalyst, which helped in lowering the reaction temperature at which phase-pure PT could be prepared, and external influence in providing PT powders with better size homogeneity. Acknowledgements The authors gratefully acknowledge the National Nanotechnology Center (NANOTEC), Thailand, for financial support, and the Thailand Research Fund for providing instruments. Mr. C. Wongtaewan thanks the Royal Thai Government (Scientific Human Resources Development Program) for undergraduate scholarship. References
Fig. 3. Particle size distributions and scanning electron micrographs in the inlets of the particles present in the PT powders characterized by the PXRD patterns shown (a) in Fig. 2(a), and (b) in Fig. 2(d), respectively.
Fig. 2. Although irradiation times of up to 6 h showed negligible influence on phase purity and crystallinity of the PT powders, the longer irradiation time of 12 h resulted in the formation of another unidentified phase. This was more pronounced in the case of 24 h of ultrasonic irradiation, as indicated by asterisks in Fig. 2. Cell parameters of the PT powders could be well refined in P4/mmm with a = 3.88(1)–3.91(1) Å and c = 4.16(1)–4.22(2) Å. The widely distributed refine parameters may correspond to the rapid kinetics of the formation of PT framework under sonocatalyzed hydrothermal reactions. Although scanning electron micrographs showed no difference in particle morphology with different ultrasonic irradiation times, sizes of the aggregates were clearly reduced with prolonged irradiation time. Mean diameter of the aggregates obtained from an hour of ultrasonic irradiation followed by 6 h of hydrothermal reaction at 130 °C was 4.55 μm, compared to 3.83 μm of that obtained from 3 h of ultrasonic irradiation followed by hydrothermal reaction under the same conditions. The primary particle sizes and size distributions were also significantly influenced by ultrasonic irradiation durations. Sizes of the particles yielded from the reaction with only 1 h of irradiation before the hydrothermal reaction at 130 °C for 6 h were distributed between 120 and 1880 nm, with two most populated areas at ca. 190–260 nm and 900–1080 nm. The prolongation of the irradiation
[1] A.J. Moulson, J.M. Herbert, Electroceramic, 2nd ed. Wiley, New York, 2003. [2] Y. Xu, Ferroelectric Materials and Their Applications, North-Holland, New York, 1991. [3] F. Jona, G. Shirane, Ferroelectric Crystals, Dover Publications, New York, 1993. [4] A. Udomporn, K. Pengpat, S. Ananta, J. Eur. Ceram. Soc. 24 (2004) 206. [5] J. Tartaj, J.F. Fernandez, F. Villafuerte-Castrejon, Mater. Res. Bull. 36 (2001) 479. [6] J. Fang, J. Wang, L.M. Gan, S.C. Ng, Mater. Lett. 52 (2002) 304. [7] A. Rujiwatra, J. Jongphiphan, S. Ananta, Mater. Lett. 59 (2005) 1871. [8] A. Rujiwatra, N. Thammajak, T. Sarakonsri, R. Wongmaneerung, S. Ananta, J. Cryst. Growth 289 (2006) 224. [9] M.C. Gelabert, B.L. Gersten, R.E. Riman, J. Cryst. Growth 211 (2000) 497. [10] X. Zeng, Y. Liu, X. Wang, W. Yin, L. Wang, H. Guo, Mater. Chem. Phys. 77 (2002) 209. [11] S.-K. Lee, G.-J. Choi, U.-Y. Hwang, K.-K. Koo, K.-K. Park, Mater. Lett. 57 (2003) 2201. [12] P.E. Meskin, V.K. Ivanov, A.E. Barantchikov, B.R. Churagulov, Y.D. Tretyakov, Ultrason. Sonochem. 13 (2006) 47. [13] R.R. Piticescu, R.M. Piticescu, C.J. Monty, J. Eur. Ceram. Soc. 26 (2006) 2979. [14] P.E. Meskin, A.E. Baranchikov, V.K. Ivanov, D.R. Afanas'ev, A.I. Gavrilov, B.R. Churagulov, N.N. Oleinikov, Inorg. Mater. 40 (2004) 1058. [15] L.H. Thompson, L.K. Doraiswamy, Ind. Eng. Chem. Res. 38 (1999) 1215. [16] K.S. Suslick, Science 247 (1990) 1439. [17] K.S. Suslick, G.J. Price, Annu. Rev. Mater. Sci. 29 (1999) 295.