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Preparation and electric-field response of novel tetragonal barium titanate
Journal of Alloys and Compounds 574 (2013) 212–216
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation and electric-field response of novel tetragonal barium titanate Rui-jie Li a,b, Wen-xin Wei c, Jin-ling Hai a,b, Ling-xiang Gao a,b,⇑, Zi-wei Gao a,b, Yan-yu Fan a,b a
Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, Xi’an 710062, PR China School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China c College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, PR China b
a r t i c l e
i n f o
Article history: Received 2 February 2013 Received in revised form 27 April 2013 Accepted 30 April 2013 Available online 9 May 2013 Keywords: Barium titanate Tetragonal phase Hydrothermal method Electric-field response
a b s t r a c t The tetragonal barium titanate (BaTiO3) particles with novel shapes were synthesized by a surfactant-free hydrothermal method and fully characterized by Raman, XRD, SEM and contact angle analysis. The shape of particles are dependent on reaction temperature, pH value and mineralization time. The tetragonal BaTiO3 particles with different morphology possess different tetragonality, surface hydrophilicity and dielectric property, and so they exhibit distinct electric-field response. The high tetragonality and good dielectric property of the particles are proposed as a main power of enhancing the electric response of tetragonal BaTiO3 dispersed in hydrous elastomer. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Electrorheological (ER) materials as novel intelligent soft matters have precise and tunable response to an external electric field, which have wide applications in sensing [1,2], electronics [3], damper system [4,5], and vibration control [6]. ER elastomers (EREs), a family member of ER materials, have recently attracted considerable attention [7–10]. In EREs, ER particles (as dispersed phase) are dispersed into polymer network (as continuous phase). When an electric field applied, the polarized particles construct chain or column-like structures in the direction of the electric field. Therefore, the anisotropic microstructure and mechanical property (such as storage modulus) of EREs can be tuned by applying external electric field. So EREs exhibit ER effects which make EREs potentially important in numerous electromechanical devices, such as valves, dampers, clutches in automotive industries, and smart skins in bionic technology [11,12]. In the quest of excellent ER effect of ER materials, intense effort has been expended on preparing highly active ER particles [13–16]. Traditional ER particles, such as barium titanate (BaTiO3) and its analogs [17], have good chemical stability, high permittivity and low dielectric loss. According to literatures [18,19], the origin of
⇑ Corresponding author at: School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China. Tel.: +86 29 8153 0730; fax: +86 29 8153 0734. E-mail address: [email protected] (L.-x. Gao).
ER effect is widely attributed to the polarization of dispersed particles in suspension under an electric field. Comparatively, tetragonal BaTiO3 has a spontaneous polarization trend along the caxis at room temperature [20], so it is forecasted that tetragonal BaTiO3 particles might have stronger electric field response than cubic BaTiO3. Moreover, tetragonal BaTiO3 has excellent ferroelectric properties, regarded as the most important compound for the manufacture of multilayer ceramic capacitors (MLCC) [21,22]. However, in present, the electric-field response behavior of tetragonal BaTiO3 has rarely been investigated with respect to its’ morphologies. It is difficult to synthesize expediently tetragonal BaTiO3 with high tetragonality. Up to now, tetragonal BaTiO3 nano-powders, such as nano-sphere [23], nano-torus [24], nano-dendrite arrays [25] and bowl-like nanoparticles [26], have been prepared, but the procedures are too complicated. In this communication, we report a surfactant-free preparation of two tetragonal barium titanate particles with novel morphologies by hydrothermal methods, and firstly report, to the best of our knowledge, electric-field response performance of tetragonal BaTiO3. 2. Experimental 2.1. Materials Analytical grade tetrabutyl titanate (Ti(C4H9O)4) and barium nitrate (Ba(NO3)2) were used as starting materials. Analytical grade potassium hydroxide (KOH) was used as mineralizer. Deionized water was used in whole synthesis procedure.
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.203
R.-j. Li et al. / Journal of Alloys and Compounds 574 (2013) 212–216
213
Fig. 1. Flow chart for the preparation of tetragonal BaTiO3 samples (a: S1 and b: S2).
2.2. Synthesis of BaTiO3 particles by modified hydrothermal method
2.6. Measurement of BaTiO3 particles electric-field response in hydrous elastomers
BaTiO3 particles were synthesized via hydrothermal methods in the following description. Appropriate amounts of Ti(C4H9O)4 were dissolved in water to form a Ti precursor solution. Subsequently, fresh Ti precursor and one equivalent amounts of Ba(NO3)2 solution were transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed, heated up to certain temperatures and held for 24 h, and then cooled naturally down to room temperature. The products were filtered, washed with diluted formic acid and water for several times, and then dried in a vacuum oven at 60 °C for 12 h (Fig. 1). During preparation progresses, along the different adding time of mineralizer KOH, two different processes were adopted. One process was summarized in Fig. 1a through which sample 1 (S1) was synthesized, and another process was shown in Fig. 1b through which sample 2 (S2) was prepared.
A certain amount of BaTiO3 particles were ground with gelatin/glycerol/water mixture in the agate mortar which was bathed at 65 °C in an electric-heated thermostatic water bath. After BaTiO3 particles were dispersed evenly in a mixture of gelatin, glycerol and water, small amount of glutaraldehyde was added quickly as cross-linking agent. And then, the mixture was transferred to two plexiglas boxes (40 20 8 mm3) averagely and cured with or without an external dc electric field (1.2 kV/mm) [7,13] for 50 min from 65 °C to room temperature (remained 65 °C for 30 min, cooled down to room temperature for 20 min). The curing progress went on sequentially for 7 h at room temperature after removing the electric field. Finally, the BaTiO3/gelatin/glycerin composite hydrous elastomers were prepared, called A-elastomers (cured without any electric field) and B-elastomers (cured with 1.2 kV/mm electric field), respectively [9,29]. In the curing progress, dc voltage was applied by a high-voltage dc power supply (regulator range of 0–30 kV) which can deliver electric field strength up to 1.2 kV/mm. Storage modulus of A and Belastomers were recorded by Q800DMA dynamic viscoelastic spectrometer in static-press multi-frequency mode within the range of frequency 1–10 Hz at room temperature. Each measurement was carried out and repeated at least three times. The difference in modulus (DG) of a pair of A and B-elastomers were explored and the electric response of BaTiO3 particles was investigated indirectly.
2.3. Structure characterization of particles The phase composition of as-prepared BaTiO3 powders was identified with Raman spectroscopy (Raman, ALMEGA-TM, Therm Nicolet, America, using a 532 nm excitation of the Nd/YAG laser) and X-ray diffractometer (XRD, D/Max-3c, Rigaku, Kyoto, Japan, using Cu Ka radiation). The microstructures were investigated by scanning electron microscopy (SEM), with a Philips-FEI (Quanta 200) microscope.
3. Results and discussion 3.1. Microstructure and morphology analysis
2.4. Measurement of the dielectric property of particles The relative dielectric property of particles was indicated indirectly by the dielectric constants of the suspensions of the particles dispersed in silicone oil. In preparation process of the suspensions, the dried particles with volume fraction of 6% were dispersed in silicone oil (g = 50 mPa s at 25 °C) by mechanical stirring and ultra-sonification. The dielectric spectra of the suspensions was measured by an impedance analyzer (HP 4284A) in the frequency range from 20 Hz to 106 Hz using a measuring fixture (HP 16452A) for liquid [27,28]. 1 V of bias electrical potential was applied to the suspensions. It was so small that no chain formation within suspensions was induced, thus we could obtain the true behavior of interfacial polarization between particles and medium. Therefore, the dielectric properties of BaTiO3 particles can be compared.
2.5. Measurement of the surface hydrophile of particles Hydrophilicity of particles’ surface was measured by an OCA 20 video-based optical contact angle (CA) measuring instrument and the infiltration situation is measured by sessile drop method. The test procedure is described as follows: Firstly, 2.0 wt% BaTiO3 particles were dispersed into ethanol by ultrasonic wave forming a suspension. Secondly, a clean glass substrate (74 25 1.2 mm3) was immersed vertically into the ethanol suspension and then emersed out at a low constant speed, and naturally dried at room temperature. The dip coating was repeated three times to make particles well floor on the glass slides. Thirdly, the static contact angle was measured by following procedure. The syringe needle (there is water in it) was positioned 0.2 mm from the particles floor, and then a droplet of water 3 lL was dropped on the floor at a rate 1.00 lL/s. After dispensing, the drop shape was monitored by a CCD camera and an image analysis system for 30 s. CA, diameter, and volume of the water drop were recorded and quantified. The values of CA of each particle in text were represented an average value of three separate drops at least.
Based on repetitive experiments, we have found that the hydrothermal condition (P200 °C, 24 h) is guarantee for the formation of tetragonal BaTiO3 (verdict from XRD and Roman, not shown here). The concentration and adding time of mineralizer (KOH) played an important role in the morphology of BaTiO3 [30]. The schemes of two processes are shown in Fig. 1. According to path a in Fig. 1a (respectively 240 °C, 24 h), when mineralizer (KOH) of 0.1, 0.3, 0.7 and 1.0 mol/L was added, BaTiO3 particles with different shapes were obtained (Fig. 2a–d). It can be found that with increasing of KOH concentrations, BaTiO3 particles morphology change from amorphous to rods and sticks, and to homogeneous crystal druse, but then unexpectedly go back to unsightly shape (broke into irregular layer). As a result, the finer crystal druse (S1, Fig. 2c) was gained at KOH concentration of 0.7 mol/L. While KOH was added according to path b in Fig. 1b (respectively 220 °C, 24 h), new BaTiO3 particles were prepared and a corresponding obvious morphology evolution from blocky shape to dendrite was observed (Fig. 2e–h). Similarly, when the concentration of KOH was increased to 0.7 mol/L, the samples (S2) are well dendrite as depicted in Fig. 2g. The preparation conditions of two samples (S1 and S2) are presented in Table 1. 3.2. Structure characterization Fig. 3 shows the Raman spectra of as-synthesized BaTiO3 samples (S1–S2) obtained at different temperatures. Four typical
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Fig. 2. SEM images of the BaTiO3 samples synthesized with of KOH (a and e) 0.1, (b and f) 0.3, (c and g) 0.7 and (d and h) 1.0 (mol/L), respectively; at (a–d) 240 °C or (e–h) 220 °C.
Table 1 The synthesis of tetragonal BaTiO3 particles S1 and S2. Sample
Morphologies
Temperature (°C)
Time (h)
CKOH (mol/L)
S1 S2
Crystal druse Dendrite
240 220
24 24
0.7 0.7
Fig. 4. XRD patterns of BaTiO3 samples. a: S1, crystal druse and b: S2, dendrite.
Fig. 3. Raman patterns of BaTiO3 samples. a: S1, crystal druse and b: S2, dendrite.
Raman-active phonon modes of BaTiO3 can be identified from S1 to S2 samples: [A1(TO), E(LO)] at 260 cm1, [B1, E(TO + LO)] at 307 cm1, [A1(TO), E(TO)] at 515 cm1, and [A1(LO), E(LO)] at 715 cm1. A weak peak at 185 cm1 contributed from A1(LO) mode of BaTiO3 is observed in the Raman spectrum of S1. Generally, the tetragonal phase of BaTiO3 are identified by characteristic peaks at 307 cm1 [B1, E(TO + LO)] and 715 cm1 [A1(LO), E(LO)], and disappear when BaTiO3 crystals change from tetragonal to cubic phase [31,32]. All Raman spectra of S1–S2 have peaks at 307 cm1 and 715 cm1, indicating that the as-synthesized crystallites contain tetragonal BaTiO3. Further evidence for the tetragonal phase in the sample could be found in XRD analysis. For a wide range of XRD measurement from 20° to 60°, the scanning rate was 8°/min. A slow scan rate of 0.02°/min was used for a specific narrow range from 44° to 46°. The MDI Jade 5.0 version program was used to determine the lattice parameters [33,34]. XRD patterns of the as-synthesized products (S1–S2) are shown in Fig. 4. All the indexes are in good agreement with tetragonal BaTiO3 (JCPDS No. 5-0626).
Fig. 5. Dielectric spectra of suspensions containing tetragonal BaTiO3 particle. a: S1, crystal druse and b: S2, dendrite (6 vol.%, T = 25 °C).
Well-resolved diffraction peaks reveal that these products are tetragonal BaTiO3 crystallites. Moreover, the tetragonal phase of BaTiO3 crystallites can usually be identified from the splitting peaks at 2h 45° of (2 0 0)/(0 0 2) reflections in XRD pattern, while the cubic phase only has one single diffraction peak at 2h 45°
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Fig. 6. Contact angle of water drops (3 lL) on the floor of tetragonal BaTiO3 particle. a: S1, crystal druse and b: S2, dendrite.
3.5. Electric-field response performance of tetragonal BaTiO3 particles
Fig. 7. The storage modulus/frequency curves of elastomers with different BaTiO3 particles (a–c) 0 kV/mm, (a0 –c0 )1.2 kV/mm; a, a0 : BaTiO3 of S1, b, b0 : BaTiO3 of S2, c, c0 : without BaTiO3.
[35]. The inserted magnified view in Fig. 4 shows that the two samples have obvious splitting peaks at 2h in the range of 44–46°, indicating that the synthesized powders are in a tetragonal phase. Based on XRD analyses, the lattice parameters of S1 are a = b = 3.9958 Å & c = 4.0324 Å and corresponding value of S2 are a = b = 3.9945 Å & c = 4.0275 Å. Calculated through the indexes of XRD, their tetragonalities (c/a ratio) are about 1.0092 and 1.0083, respectively. Therefore, it can be concluded that the as-obtained crystal druse and dendrite BaTiO3 possess high tetragonality (c/ a P 1.008) [36].
Fig. 7 shows the relationship between the storage modulus of A and B-elastomers via the frequency at a fixed strain of 1.0%. The curves a (a0 )–b (b0 ) are corresponding to the A- and B-elastomers containing 1.0 wt% S1–S2 cured in absence or presence of an applied electric field, c (c0 ) presents the storage modulus of the gelatin/glycerin hydrous elastomers without BaTiO3 particles cured in absence or presence of an applied electric field. It is shown that the curves c (c0 ) almost coincide with each other, indicating that pure gelatin/glycerin hydrous elastomer have no obvious response to an electric field. Curves a (a0 )–b (b0 ) are above c (c0 ), illustrating the filling role of BaTiO3 particles in the elastomers. The interval of the curves a (a0 )–b (b0 ) are much larger than that of c (c0 ) forcefully, suggesting that particles S1–S2 dispersed in gelatin/glycerin elastomers have obvious response to the electric field. Comparing the difference in modulus (DG) between B- and A-elastomers dispersed with S1 and S2, respectively, DG of the elastomers dispersed with S1 is much larger than that dispersed with S2, showed in inset of Fig. 7. The results indicate that BaTiO3 crystal druses have stronger electric response obviously than the dendrites. As far as the tetragonality, dielectric property and surface hydrophilicity of the two BaTiO3 particles concerned, it can be concluded that the particle’s high tetragonality and high dielectric property are the driving force of enhancing the electric response in hydrous elastomers. The main reason may be that the higher tetragonality of S1 in crystal druse shape makes itself easily polarized and conduces to its higher dielectric property. The higher dielectric property of S1 facilitates the stronger ‘‘pearl chains’’ effect [7,9,38,39] and strengthen the modulus of the corresponding B-elastomer, and DG of the elastomers dispersed with S1 particles turn larger. Consequently, the response property of the BaTiO3 particles to electric field is in the order of crystal druse > dendrite.
3.3. Dielectric property of tetragonal BaTiO3 particles
4. Conclusions
The dielectric spectra of suspensions were given in Fig. 5 which is not obviously affected by the frequency fluctuation at room temperature. The relative dielectric property of particles was presented indirectly. Higher dielectric constant of the suspension suggests a higher relative dielectric property of the corresponding dispersed particle. Comparing two BaTiO3 samples, the relative dielectric property of BaTiO3 crystal druse (S1) is stronger than BaTiO3 dendrite (S2), which suggests that the dielectric property increases with particle tetragonality (c/a ratio) increasing.
The BaTiO3 particles in tetragonal crystal structure with crystal druse and dendrite were prepared and characterized. They are different in morphology, tetragonality and dielectric property. It has been found that the morphology of BaTiO3 affects their tetragonality and dielectric property. In conclusion, the tetragonality of BaTiO3 determines the dielectric property, and the corresponding particles with higher dielectric property show stronger electric field response. References
3.4. Hydrophilicity of tetragonal BaTiO3 particles’ surface Fig. 6 shows the contact angles between water-drops and the BaTiO3 particles. The water contact angle of S1 and S2 are 17.8 ± 0.3° and 11.4 ± 0.3°, respectively. They are all less than 35°, which reveals that both S1 and S2 are super-hydrophilic and have a good dispersion property in aqueous medium and good compatibility with hydrous gel [37]. Obviously, BaTiO3 dendrite possesses predominant surface hydrophilic capacity and better dispersion property in aqueous medium.
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Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation and electric-field response of novel tetragonal barium titanate Rui-jie Li a,b, Wen-xin Wei c, Jin-ling Hai a,b, Ling-xiang Gao a,b,⇑, Zi-wei Gao a,b, Yan-yu Fan a,b a
Key Laboratory of Applied Surface and Colloid Chemistry, Shaanxi Normal University, Ministry of Education, Xi’an 710062, PR China School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China c College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, PR China b
a r t i c l e
i n f o
Article history: Received 2 February 2013 Received in revised form 27 April 2013 Accepted 30 April 2013 Available online 9 May 2013 Keywords: Barium titanate Tetragonal phase Hydrothermal method Electric-field response
a b s t r a c t The tetragonal barium titanate (BaTiO3) particles with novel shapes were synthesized by a surfactant-free hydrothermal method and fully characterized by Raman, XRD, SEM and contact angle analysis. The shape of particles are dependent on reaction temperature, pH value and mineralization time. The tetragonal BaTiO3 particles with different morphology possess different tetragonality, surface hydrophilicity and dielectric property, and so they exhibit distinct electric-field response. The high tetragonality and good dielectric property of the particles are proposed as a main power of enhancing the electric response of tetragonal BaTiO3 dispersed in hydrous elastomer. Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved.
1. Introduction Electrorheological (ER) materials as novel intelligent soft matters have precise and tunable response to an external electric field, which have wide applications in sensing [1,2], electronics [3], damper system [4,5], and vibration control [6]. ER elastomers (EREs), a family member of ER materials, have recently attracted considerable attention [7–10]. In EREs, ER particles (as dispersed phase) are dispersed into polymer network (as continuous phase). When an electric field applied, the polarized particles construct chain or column-like structures in the direction of the electric field. Therefore, the anisotropic microstructure and mechanical property (such as storage modulus) of EREs can be tuned by applying external electric field. So EREs exhibit ER effects which make EREs potentially important in numerous electromechanical devices, such as valves, dampers, clutches in automotive industries, and smart skins in bionic technology [11,12]. In the quest of excellent ER effect of ER materials, intense effort has been expended on preparing highly active ER particles [13–16]. Traditional ER particles, such as barium titanate (BaTiO3) and its analogs [17], have good chemical stability, high permittivity and low dielectric loss. According to literatures [18,19], the origin of
⇑ Corresponding author at: School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710062, PR China. Tel.: +86 29 8153 0730; fax: +86 29 8153 0734. E-mail address: [email protected] (L.-x. Gao).
ER effect is widely attributed to the polarization of dispersed particles in suspension under an electric field. Comparatively, tetragonal BaTiO3 has a spontaneous polarization trend along the caxis at room temperature [20], so it is forecasted that tetragonal BaTiO3 particles might have stronger electric field response than cubic BaTiO3. Moreover, tetragonal BaTiO3 has excellent ferroelectric properties, regarded as the most important compound for the manufacture of multilayer ceramic capacitors (MLCC) [21,22]. However, in present, the electric-field response behavior of tetragonal BaTiO3 has rarely been investigated with respect to its’ morphologies. It is difficult to synthesize expediently tetragonal BaTiO3 with high tetragonality. Up to now, tetragonal BaTiO3 nano-powders, such as nano-sphere [23], nano-torus [24], nano-dendrite arrays [25] and bowl-like nanoparticles [26], have been prepared, but the procedures are too complicated. In this communication, we report a surfactant-free preparation of two tetragonal barium titanate particles with novel morphologies by hydrothermal methods, and firstly report, to the best of our knowledge, electric-field response performance of tetragonal BaTiO3. 2. Experimental 2.1. Materials Analytical grade tetrabutyl titanate (Ti(C4H9O)4) and barium nitrate (Ba(NO3)2) were used as starting materials. Analytical grade potassium hydroxide (KOH) was used as mineralizer. Deionized water was used in whole synthesis procedure.
0925-8388/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.203
R.-j. Li et al. / Journal of Alloys and Compounds 574 (2013) 212–216
213
Fig. 1. Flow chart for the preparation of tetragonal BaTiO3 samples (a: S1 and b: S2).
2.2. Synthesis of BaTiO3 particles by modified hydrothermal method
2.6. Measurement of BaTiO3 particles electric-field response in hydrous elastomers
BaTiO3 particles were synthesized via hydrothermal methods in the following description. Appropriate amounts of Ti(C4H9O)4 were dissolved in water to form a Ti precursor solution. Subsequently, fresh Ti precursor and one equivalent amounts of Ba(NO3)2 solution were transferred into a Teflon-lined stainless steel autoclave. The autoclave was sealed, heated up to certain temperatures and held for 24 h, and then cooled naturally down to room temperature. The products were filtered, washed with diluted formic acid and water for several times, and then dried in a vacuum oven at 60 °C for 12 h (Fig. 1). During preparation progresses, along the different adding time of mineralizer KOH, two different processes were adopted. One process was summarized in Fig. 1a through which sample 1 (S1) was synthesized, and another process was shown in Fig. 1b through which sample 2 (S2) was prepared.
A certain amount of BaTiO3 particles were ground with gelatin/glycerol/water mixture in the agate mortar which was bathed at 65 °C in an electric-heated thermostatic water bath. After BaTiO3 particles were dispersed evenly in a mixture of gelatin, glycerol and water, small amount of glutaraldehyde was added quickly as cross-linking agent. And then, the mixture was transferred to two plexiglas boxes (40 20 8 mm3) averagely and cured with or without an external dc electric field (1.2 kV/mm) [7,13] for 50 min from 65 °C to room temperature (remained 65 °C for 30 min, cooled down to room temperature for 20 min). The curing progress went on sequentially for 7 h at room temperature after removing the electric field. Finally, the BaTiO3/gelatin/glycerin composite hydrous elastomers were prepared, called A-elastomers (cured without any electric field) and B-elastomers (cured with 1.2 kV/mm electric field), respectively [9,29]. In the curing progress, dc voltage was applied by a high-voltage dc power supply (regulator range of 0–30 kV) which can deliver electric field strength up to 1.2 kV/mm. Storage modulus of A and Belastomers were recorded by Q800DMA dynamic viscoelastic spectrometer in static-press multi-frequency mode within the range of frequency 1–10 Hz at room temperature. Each measurement was carried out and repeated at least three times. The difference in modulus (DG) of a pair of A and B-elastomers were explored and the electric response of BaTiO3 particles was investigated indirectly.
2.3. Structure characterization of particles The phase composition of as-prepared BaTiO3 powders was identified with Raman spectroscopy (Raman, ALMEGA-TM, Therm Nicolet, America, using a 532 nm excitation of the Nd/YAG laser) and X-ray diffractometer (XRD, D/Max-3c, Rigaku, Kyoto, Japan, using Cu Ka radiation). The microstructures were investigated by scanning electron microscopy (SEM), with a Philips-FEI (Quanta 200) microscope.
3. Results and discussion 3.1. Microstructure and morphology analysis
2.4. Measurement of the dielectric property of particles The relative dielectric property of particles was indicated indirectly by the dielectric constants of the suspensions of the particles dispersed in silicone oil. In preparation process of the suspensions, the dried particles with volume fraction of 6% were dispersed in silicone oil (g = 50 mPa s at 25 °C) by mechanical stirring and ultra-sonification. The dielectric spectra of the suspensions was measured by an impedance analyzer (HP 4284A) in the frequency range from 20 Hz to 106 Hz using a measuring fixture (HP 16452A) for liquid [27,28]. 1 V of bias electrical potential was applied to the suspensions. It was so small that no chain formation within suspensions was induced, thus we could obtain the true behavior of interfacial polarization between particles and medium. Therefore, the dielectric properties of BaTiO3 particles can be compared.
2.5. Measurement of the surface hydrophile of particles Hydrophilicity of particles’ surface was measured by an OCA 20 video-based optical contact angle (CA) measuring instrument and the infiltration situation is measured by sessile drop method. The test procedure is described as follows: Firstly, 2.0 wt% BaTiO3 particles were dispersed into ethanol by ultrasonic wave forming a suspension. Secondly, a clean glass substrate (74 25 1.2 mm3) was immersed vertically into the ethanol suspension and then emersed out at a low constant speed, and naturally dried at room temperature. The dip coating was repeated three times to make particles well floor on the glass slides. Thirdly, the static contact angle was measured by following procedure. The syringe needle (there is water in it) was positioned 0.2 mm from the particles floor, and then a droplet of water 3 lL was dropped on the floor at a rate 1.00 lL/s. After dispensing, the drop shape was monitored by a CCD camera and an image analysis system for 30 s. CA, diameter, and volume of the water drop were recorded and quantified. The values of CA of each particle in text were represented an average value of three separate drops at least.
Based on repetitive experiments, we have found that the hydrothermal condition (P200 °C, 24 h) is guarantee for the formation of tetragonal BaTiO3 (verdict from XRD and Roman, not shown here). The concentration and adding time of mineralizer (KOH) played an important role in the morphology of BaTiO3 [30]. The schemes of two processes are shown in Fig. 1. According to path a in Fig. 1a (respectively 240 °C, 24 h), when mineralizer (KOH) of 0.1, 0.3, 0.7 and 1.0 mol/L was added, BaTiO3 particles with different shapes were obtained (Fig. 2a–d). It can be found that with increasing of KOH concentrations, BaTiO3 particles morphology change from amorphous to rods and sticks, and to homogeneous crystal druse, but then unexpectedly go back to unsightly shape (broke into irregular layer). As a result, the finer crystal druse (S1, Fig. 2c) was gained at KOH concentration of 0.7 mol/L. While KOH was added according to path b in Fig. 1b (respectively 220 °C, 24 h), new BaTiO3 particles were prepared and a corresponding obvious morphology evolution from blocky shape to dendrite was observed (Fig. 2e–h). Similarly, when the concentration of KOH was increased to 0.7 mol/L, the samples (S2) are well dendrite as depicted in Fig. 2g. The preparation conditions of two samples (S1 and S2) are presented in Table 1. 3.2. Structure characterization Fig. 3 shows the Raman spectra of as-synthesized BaTiO3 samples (S1–S2) obtained at different temperatures. Four typical
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Fig. 2. SEM images of the BaTiO3 samples synthesized with of KOH (a and e) 0.1, (b and f) 0.3, (c and g) 0.7 and (d and h) 1.0 (mol/L), respectively; at (a–d) 240 °C or (e–h) 220 °C.
Table 1 The synthesis of tetragonal BaTiO3 particles S1 and S2. Sample
Morphologies
Temperature (°C)
Time (h)
CKOH (mol/L)
S1 S2
Crystal druse Dendrite
240 220
24 24
0.7 0.7
Fig. 4. XRD patterns of BaTiO3 samples. a: S1, crystal druse and b: S2, dendrite.
Fig. 3. Raman patterns of BaTiO3 samples. a: S1, crystal druse and b: S2, dendrite.
Raman-active phonon modes of BaTiO3 can be identified from S1 to S2 samples: [A1(TO), E(LO)] at 260 cm1, [B1, E(TO + LO)] at 307 cm1, [A1(TO), E(TO)] at 515 cm1, and [A1(LO), E(LO)] at 715 cm1. A weak peak at 185 cm1 contributed from A1(LO) mode of BaTiO3 is observed in the Raman spectrum of S1. Generally, the tetragonal phase of BaTiO3 are identified by characteristic peaks at 307 cm1 [B1, E(TO + LO)] and 715 cm1 [A1(LO), E(LO)], and disappear when BaTiO3 crystals change from tetragonal to cubic phase [31,32]. All Raman spectra of S1–S2 have peaks at 307 cm1 and 715 cm1, indicating that the as-synthesized crystallites contain tetragonal BaTiO3. Further evidence for the tetragonal phase in the sample could be found in XRD analysis. For a wide range of XRD measurement from 20° to 60°, the scanning rate was 8°/min. A slow scan rate of 0.02°/min was used for a specific narrow range from 44° to 46°. The MDI Jade 5.0 version program was used to determine the lattice parameters [33,34]. XRD patterns of the as-synthesized products (S1–S2) are shown in Fig. 4. All the indexes are in good agreement with tetragonal BaTiO3 (JCPDS No. 5-0626).
Fig. 5. Dielectric spectra of suspensions containing tetragonal BaTiO3 particle. a: S1, crystal druse and b: S2, dendrite (6 vol.%, T = 25 °C).
Well-resolved diffraction peaks reveal that these products are tetragonal BaTiO3 crystallites. Moreover, the tetragonal phase of BaTiO3 crystallites can usually be identified from the splitting peaks at 2h 45° of (2 0 0)/(0 0 2) reflections in XRD pattern, while the cubic phase only has one single diffraction peak at 2h 45°
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Fig. 6. Contact angle of water drops (3 lL) on the floor of tetragonal BaTiO3 particle. a: S1, crystal druse and b: S2, dendrite.
3.5. Electric-field response performance of tetragonal BaTiO3 particles
Fig. 7. The storage modulus/frequency curves of elastomers with different BaTiO3 particles (a–c) 0 kV/mm, (a0 –c0 )1.2 kV/mm; a, a0 : BaTiO3 of S1, b, b0 : BaTiO3 of S2, c, c0 : without BaTiO3.
[35]. The inserted magnified view in Fig. 4 shows that the two samples have obvious splitting peaks at 2h in the range of 44–46°, indicating that the synthesized powders are in a tetragonal phase. Based on XRD analyses, the lattice parameters of S1 are a = b = 3.9958 Å & c = 4.0324 Å and corresponding value of S2 are a = b = 3.9945 Å & c = 4.0275 Å. Calculated through the indexes of XRD, their tetragonalities (c/a ratio) are about 1.0092 and 1.0083, respectively. Therefore, it can be concluded that the as-obtained crystal druse and dendrite BaTiO3 possess high tetragonality (c/ a P 1.008) [36].
Fig. 7 shows the relationship between the storage modulus of A and B-elastomers via the frequency at a fixed strain of 1.0%. The curves a (a0 )–b (b0 ) are corresponding to the A- and B-elastomers containing 1.0 wt% S1–S2 cured in absence or presence of an applied electric field, c (c0 ) presents the storage modulus of the gelatin/glycerin hydrous elastomers without BaTiO3 particles cured in absence or presence of an applied electric field. It is shown that the curves c (c0 ) almost coincide with each other, indicating that pure gelatin/glycerin hydrous elastomer have no obvious response to an electric field. Curves a (a0 )–b (b0 ) are above c (c0 ), illustrating the filling role of BaTiO3 particles in the elastomers. The interval of the curves a (a0 )–b (b0 ) are much larger than that of c (c0 ) forcefully, suggesting that particles S1–S2 dispersed in gelatin/glycerin elastomers have obvious response to the electric field. Comparing the difference in modulus (DG) between B- and A-elastomers dispersed with S1 and S2, respectively, DG of the elastomers dispersed with S1 is much larger than that dispersed with S2, showed in inset of Fig. 7. The results indicate that BaTiO3 crystal druses have stronger electric response obviously than the dendrites. As far as the tetragonality, dielectric property and surface hydrophilicity of the two BaTiO3 particles concerned, it can be concluded that the particle’s high tetragonality and high dielectric property are the driving force of enhancing the electric response in hydrous elastomers. The main reason may be that the higher tetragonality of S1 in crystal druse shape makes itself easily polarized and conduces to its higher dielectric property. The higher dielectric property of S1 facilitates the stronger ‘‘pearl chains’’ effect [7,9,38,39] and strengthen the modulus of the corresponding B-elastomer, and DG of the elastomers dispersed with S1 particles turn larger. Consequently, the response property of the BaTiO3 particles to electric field is in the order of crystal druse > dendrite.
3.3. Dielectric property of tetragonal BaTiO3 particles
4. Conclusions
The dielectric spectra of suspensions were given in Fig. 5 which is not obviously affected by the frequency fluctuation at room temperature. The relative dielectric property of particles was presented indirectly. Higher dielectric constant of the suspension suggests a higher relative dielectric property of the corresponding dispersed particle. Comparing two BaTiO3 samples, the relative dielectric property of BaTiO3 crystal druse (S1) is stronger than BaTiO3 dendrite (S2), which suggests that the dielectric property increases with particle tetragonality (c/a ratio) increasing.
The BaTiO3 particles in tetragonal crystal structure with crystal druse and dendrite were prepared and characterized. They are different in morphology, tetragonality and dielectric property. It has been found that the morphology of BaTiO3 affects their tetragonality and dielectric property. In conclusion, the tetragonality of BaTiO3 determines the dielectric property, and the corresponding particles with higher dielectric property show stronger electric field response. References
3.4. Hydrophilicity of tetragonal BaTiO3 particles’ surface Fig. 6 shows the contact angles between water-drops and the BaTiO3 particles. The water contact angle of S1 and S2 are 17.8 ± 0.3° and 11.4 ± 0.3°, respectively. They are all less than 35°, which reveals that both S1 and S2 are super-hydrophilic and have a good dispersion property in aqueous medium and good compatibility with hydrous gel [37]. Obviously, BaTiO3 dendrite possesses predominant surface hydrophilic capacity and better dispersion property in aqueous medium.
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