Synthesis of mesocrystalline BT/BNT nanocomposite

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Fig. 2.3 (a, b, c) FE-SEM and (d, e, f) TEM images of (a) HTO precursor, (b) BT/HTO

nanocomposite obtained by solvothermal treatment of HTO-Ba(OH)2 mixtures with mole ratio of Ba/Ti = 0.5 at 150 ^°C for 12 h, (c, d, e, f) sample obtained by acid-treatment of BT/HTO nanocomposite with 2 M HCl solution for 12 h. (f) HRTEM image derived from white pane in TEM image (d).

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(BT/HTO)-Bi2O3-Na2CO3 reaction system, high purity BT/BNT can be obtained, and the XRD patterns of the products prepared at different temperatures are shown in Fig.

2.5. After heat-treatment at 500 ^°C, no obvious new phase is observed. The basal spacing of the HTO decreases from 0.871 nm to 0.726 nm because of the dehydration of its interlayer water. Except BT phase, all other starting material phases disappear when the temperature was elevated to 600 ºC. The main crystalline phase of the product is the BT phase, and a small amount of Bi12TiO20 phase that can be well identified by JCPDS File No. 34-0097 is also found.⁴⁰ At 700 ^°C, a mixture of BT and BNT phases are formed. These results indicate that the HTO firstly reacts with Bi2O3 to form Bi12TiO20, and then Bi12TiO20 reacts with Na2CO3 to form BNT in the reaction system.

Subsequently, the BT and BNT phases react together gradually to form Ba0.5Bi0.25Na0.25TiO3 (BBNT) solid solution with the increasing temperature above 800

C, and finally the formation reaction of BBNT almost completes at 1000 C.

Fig. 2.4 XRD patterns of the samples obtained by heating treatments of (BT/HTO)-Bi2O3-Na2CO3

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mixture with (a) stoichimetric mole ratio, (b) 10 % mole excess of Bi2O3 and Na2CO3, (c) 20 % mole excess of Bi2O3 and Na2CO3, and (d) 20 % mole excess of Bi2O3 and 40 % mole excess of Na2CO3

for formation of BT/BNT nanocomposite, respectively, at 700 ºC for 3 h.

Fig. 2.5 XRD patterns of (a) (BT/HTO)-Bi2O3-Na2CO3 mixture and samples obtained by heat-treatments of the mixture at (b) 500, (c) 600, (d) 700, (e) 800, (f) 900, and (g) 1000 ºC for 3 h, respectively.

In the (BT/HTO)-Bi2O3-Na2CO3reaction system, the platelike morphology of BT/HTO retains up to 800 ^°C, where the mixture of BT and BNT phases is formed, and almost destroyed at 1000 ^°C, where the formation reaction of BBNT solid solution is completed (Fig. 2.6). The formation reaction of BNT in the (BT/HTO)-Bi2O3-Na2CO3 system is investigated in detail using TEM and SAED, and the results are shown in Fig. 2.7. It can be clearly seen that the platelike particles constructed from the nanoparticles are formed at 600 and 700 ºC (Fig. 2.7(a, e)). At 600 ºC, the SAED spots patterns corresponding to

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the BT phase and the Bi12TiO20 phase respectively can be observed in one platelike particle (Fig. 2.7 (b)), indicating that BT phase and Bi12TiO20 phase coexist in one platelike particle. This result is consistent with the XRD result in Fig. 2.5. The nanocrystals with a size of about 5 nm, which are distributed uniformly on the surface of the platelike particle, can be confirmed to be Bi12TiO20 phase by HRTEM and FFT pattern (Fig. 2.7(c, d)). These results suggest that the Bi12TiO20 nanocrystals are formed on the BT nanocrystal surface by the heteroepitaxial growth mechanism.

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Fig. 2.6 SEM images of samples obtained by heat-treatments of the (BT-HTO)-Bi2O3-Na2CO3

mixture at (a) 500, (b) 600, (c) 700, (d) 800, (e) 900, (f) 1000, (g) 1100, and (h) 1200 ^°C for 3 h, respectively.

Fig. 2.7 (a, e) TEM images and (b, f) SAED patterns of samples obtained by heat-treatments of (BT/HTO)-Bi2O3-Na2CO3 mixture at (a, b) 600 ^oC and (e, f) 700 ^oC for 3 h, respectively. (c) HRTEM image is an enlarged image derived from white pane in TEM image (a) and (d) the FFT pattern is obtained from the whole region of the HRTEM (c).

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At 700 ºC, while two sets of single crystal-like SAED spots patterns corresponding to the BT phase and the BNTphase are observed simultaneously in one platelike particle (Fig. 2.7 (f)), revealing that the platelike particle is constructed from the BT and BNT nanocrystals which have the same crystal-axis orientation direction. Namely, the mesocrystalline BT/BNT nanocomposite is formed at 700 ºC. The SAED result also reveals that the BT and BNT nanocrystals in the mesocrystalline BT/BNT nanocomposite have the same crystal-axis orientation direction in [110]-zone axis, and lattice constant of BT phase is slightly larger than that of BNT phase, which also agrees well with the XRD result in Fig. 2.5. This result suggests the formation of a heteroepitaxial interface between BT and BNT nanoparticles in the mesocrystalline BT/BNT nanocomposite.

2.3.3 Formation reaction mechanism of mesocrystalline BT/BNT nanocomposite According to the results described above, a schematic representation for the formation reaction mechanism of the mesocrystalline BT/BNT nanocomposite from HTO by the two-step reaction process is given in Fig. 2.8. In the first step, firstly, Ba²⁺

ions are intercalated into the HTO bulk crystal through its interlayer pathway by a H⁺/Ba²⁺exchange reaction, subsequently, the Ba²⁺ions react with the TiO6octahedral layers of HTO to form the BT nanocrystals in the crystal bulk by the topochemical structural conversion reaction under the solvothermal conditions.³⁶In the topochemical solvothermal reaction, about 50% of the HTO phase is transformed to the BT nanocrystals due to 0.5 of Ba/Ti mole ratio in the reaction system, in which the BT nanocrystals are uniformly distributed in the HTO platelike particle (Fig. 2.2). The

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formation of BT phase by a dissolution-deposition reaction is also possible on the platelike particle surface.³⁹ In the present case, the topochemical conversion reaction is predominant, owing to the low concentration of Ba(OH)2 and low reaction temperature.¹⁵ There is a definite corresponding relationship between the crystal-axis directions of HTO precursor and formed BT product in the topochemical reaction system, in which [200] and [002] directions of HTO phase correspond to [002] and [1-10] directions of BT phase, respectively, as shown in Fig. 2.2(d). Therefore, all the formed BT nanocrystals in one platelike crystal of the BT/HTO nanocomposite present the same crystal-axis orientation in the [110]-zone direction which is consistent with the [010]-zone direction of the HTO matrix crystal, as shown in Fig. 2.2(d).

Fig. 2.8. Schematic representation for the formation mechanism of the mesocrystalline BT/BNT nanocomposite from the layered HTO single crystal by a two-step reaction process.

In the second step, firstly the Bi³⁺ ions immigrate into the HTO bulk crystal of the BT/HTO via the interlayer pathway of HTO, and react with the TiO6 octahedral layers

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of HTO framework to form the Bi12TiO20 nanocrystals on the BT nanoparticle surface, which causes the formation of the BT/Bi12TiO20 nanocomposite, as shown in Fig. 2.7(b).

Secondly, the Bi12TiO20 nanocrystals in the BT/Bi12TiO20 nanocomposite react with Na2CO3 to form the BNT nanocrystals by a heteroepitaxial growth mechanism, which results in formation of mesocrystalline BT/BNT nanocomposite, where all BT and BNT nanocrystals show the same crystal-axis orientation in the [110]-zone direction, as shown in Fig. 2.7(f). In the mesocrystalline BT/BNT nanocomposite, the heteroepitaxial interface between BT and BNT nanocrystals is formed, where the BT phase and BNT phase have little different lattice constants. The BT and BNT nanoparticles in the mesocrystalline BT/BNT nanocomposite can react together to form BBNT solid solution at the heteroepitaxial BT/BNT interface over 900 ^°C, and finally the BT and BNT nanocrystals are transformed completely to BBNT solid solution over 1000 ^°C.

2.3.4 Ferroelectric and piezoelectric responses of mesocrystalline BT/BNT

nanocomposite

To figure out the ferroelectric properties of the mesocrystalline BT/BNT nanocomposite, the pellet samples of the BT/BNT nanocomposite were prepared by heat-treatment of the (BT/HTO)-Bi2O3-Na2CO3 mixture pellets at different temperatures for ferroelectric studies. The pellet samples with lower leakage currents can be obtained by the cold isostatic pressing (CIP) treatment (Fig. 2.9). The samples prepared in a temperatures range from 600 to 900 ^oC show the closed ferroelectric-like P-E hysteresis loops (Fig. 2.10), revealing ferroelectricity of the mesocrystalline nanocomposites. The dependence of remanent polarization (Pr) on the fabrication heating temperature for the

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mesocrystalline nanocomposite is exhibited in Fig. 2.11(a). It is interesting that with elevating the calcination temperature, the Pr value increases, reaches a maximum value at 700 ^oC, and then decreases in the studied temperature range. BT/BNT-700 exhibites a larger ferroelectric response with a remanent polarization Pr value of 2.4 μC/cm² at an applied-electric field of 6 kV/cm than the mesocrystalline BT/ST nanocomposite with a Pr value of 0.6 μC/cm² at an applied-electric field of 17 kV/cm.¹⁵

Fig. 2.9 Plots of leakage current densities against time for the ferroelectric BT/BNT-700 pellet samples (a) with and (b) without CIP treatment at applied voltage of 6 kV/cm.

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Fig. 2.10 P-E hysteresis loops of the pellet samples obtained by heat-treatments of (BT/HTO)-Bi2O3-Na2CO3 mixture at different temperatures measured at 100 Hz.

Fig. 2.11 (a) Variations of remanent polarization and relative permittivity measured at 1 k Hz of frequency for pellet samples obtained by heat-treatments of (BT/HTO)-Bi2O3-Na2CO3 mixture at different temperature for 3 h. (b) Nanostructural models of the heteroepitaxial BT/BNT interface at diferent fabrication heating temperatures.

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The ferroelectric behavior of the mesocrystalline BT/BNT nanocomposites prepared at different heating temperatures can be explained by the variation of the nanostructure at BT/BNT interface in the nanocomposite, as shown in Fig. 2.11(b), based on the TEM results at the interface (Fig. 2.12). The enhancement of the Pr value from 600 to 700 ^oC is attributed to formation of mesocrystalline BT/BNT nanocomposite from BT/Bi12TiO20 nanocomposite, which generates the heteroepitaxial BT/BNT interface (Fig. 2.12). Around the heteroepitaxial BT/BNT interface, the BT lattice shrinks and BNT lattice expands because the lattice constant of BT is slightly larger than that of BNT with a lattice mismatching of 2.6 % (Table 2.1), which introduces a lattice distortion around the BT/BNT interface (Fig. 2.12(b)), therefore, the pseudo-cubic lattices of BT and BNT nanocrystals with paraelectric or weak ferroelectric responses are distorted to ferroelectric tetragonal and rhombohedral lattices by increasing or reducing the lattice constants in the direction paralleled to the interface, respectively.

The direction of ferroelectric spontaneous polarization around the interface become unstable and very sensitive to the applied-bias, which generates an enlarged ferroelectric response.^{15, 23, 28}

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Table 2.1. Piezoelectric constants of ferroelectric nanocomposites and single phase of nanostructured materials, and lattice mismatching in nanocomposites.

Nanocomposite Nanostructured single phase

Material *

d^*33

(pm/V)

Lattice mismatching

(%)

Compound

d^*33

(pm/V)

Polarization direction BT/ST-C ²⁰

BT/ST-M ¹⁵ BT/BNT-M **

BT/KN-S ¹² BT/CT-M ¹⁸

59 306 408 136 208

2.2 2.2 2.6 0.6 4.3

BaTiO3 (BT) ⁵¹ SrTiO3 (ST) CaTiO3 (CT) ¹⁸

Bi0.5Na0.5TiO3 (BNT) ⁵³ KNbO3 (KN) ⁵⁴

28.0 - 40.9 18.0 19.5

[001]

- [110]

[111]

[110]

* -C, -M, and -S represent nanocomposites prepared by nanocubes, mesocrystals, and surface coating, respectively.

** Result of present study.

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Fig. 2.12 (a, d, g) TEM images and (b, e, h) HRTEM images of BT/BNT-700, BT/BNT-800, and BT/BNT-1000, respectively. (b, e, h) HRTEM images are an enlarged image derived from white pane in (a, d, g) TEM images, respectively, and (c, f, i) FFT pattern obtained from the whole region of HRTEM (b, e, h), respectively.

With further elevating the heating temperature to 800 ^oC, BBNT solid solution phase is formed at the BT/BNT interface by diffusing Ba²⁺ ions of BT phase to BNT phase and Na⁺, Bi³⁺ ions of BNT phase to BT phase, then a BT/BBNT/BNT interface is formed (Fig. 2.12(e, f)). The lattice distorton effect of the BT/BBNT/BNT interface on the ferroelectric response is less than that of the BT/BNT interface because the differences of the lattice constants of BT and BBNT, and BBNT and BNT are less than that of BT and BNT. The diminution of the lattice distortion causes the dropping of ferroelectric response. The BT/Bi12TiO20 nanocomposite sample prepared at 600 ^°C shows a weak ferroelectric response (Fig. 2.10), which may be ascribed to the formation of heteroepitaxial interface between Bi12TiO20 and BT nanocrystals, which causes the lattice distortion in the pseudo-cubic lattice of the BT nanocrystals.

In comparison with the mesocrystalline nanocomposite sample of BT/BNT-700, the BT single phase mesocrystal sample of BT-700 and the BNT single phase mesocrystal sample of BNT-700 exhibit a much weaker ferroelectric response (Fig. 2.13(a)) because BT-700 and BNT-700 mesocrystals are constructed from small BT or BNT nanocrystals with a size of 60 or 50 nm (Fig. 2.14), where without lattice distortion at nanocrystals interface. It is well know that the BT and BNT prepared by the low temperature process have the pseudo-cubic structure due to their small crystal sizes or low crystallinities.^15,

50-52 These pseudo-cubic lattices of BT and BNT nanocrystals exhibit the paraelectric or

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weak ferroelectric responses. The BT and BNT nanocrystals in BT-700 and BNT-700 mesocrystals also have the pseudo-cubic structure and exhibit the weak ferroelectric response. To further confirm the effect of the heteroepitaxial interface of BT/BNT nanocomposite on ferroelectric response, the ferroelectric BT-1250 and BNT-1050 samples were also prepared. The BT-1250 and BNT-1050 exhibit ferroelectirc responses (Fig. 2.13(b)). However, the BT/BNT-700 sample shows an amazing ferroelectirc response compared to the ferroelectirc BT-1250 and BNT-1050. These results suggest that the the lattice distortion at BT/BNT interface is highly useful for strengthening ferroelectric response in the BT/BNT nanocomposite.

Fig. 2.13 P-E hysteresis loops of the pellet samples of (a) BT-700, BNT-700 and BT/BNT-700 as well as (b) BT/BNT-700, BNT-1050 and BT-1250 measured at 100 Hz.

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Fig. 2.14. (a, c) TEM images and (b, d) SAED spots patterns of (a, b) BT single phase mesocrystal sample BT-700 and (c, d) BNT single phase mesocrystal sample BNT-700.

The piezoelectric response of the mesocrystalline nanocomposite BT/BNT-700 that shows the largest ferroelectric response was investigated by using piezoresponse force microscopy (PFM), and compared with the single phase mesocrystals of BT-700 and BNT-700 prepared at same temperature. The Displacement-applied voltage (D-V) loops and d^*33-applied voltage (d^*33-V) loops are presented in Fig. 2.15, where converse piezoelectric constant (d^*33) is calculated from D/V value of the D-V loop. It is noted that the d^*33 value of 408 pm/V at 10 V of applied voltage for the BT/BNT-700 nanocomposite is much larger than those of 60 and 50 pm/V for BT-700 and BNT-700 mesocrystals, respectively. The d^*33 value of BT/BNT-700 nanocomposite is one order of magnitude larger than d^*33 values of a nanostructured BaTiO3 (28 pm/V)⁵¹ and a

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highly oriented BNT thin film (25 pm/V) fabricated by PLD method, ⁵² and even higher than that of a high performance oriented BBNT ceramic (332 pC/N).¹⁴ This remarkably enhanced piezoelectric response can be attributed to the introduced lattice distortion at the heteroepitaxial BT/BNT interface, which makes the polarization rotation sensitive.^22,

23

Fig. 2.15. Displacement-applied voltage loops and d^*33-applied voltage loops for (a) BT-700, (b) BNT-700, and (c) BT/BNT-700 mesocrystalline samples.

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To impove piezoelectric responses of lead-free piezoelectric materials, some challenges on the enhanced piezoelectric response using the lattice distortion at heteroepitaxial interfaces of nanostructured nanocomposites have been reported, and some d^*33 values are summarized in Table 2.1 for the comparison. Most studies have focused on the BaTiO3/SrTiO3 (BT/ST) heteroepitaxial interface because Harigai et al.

have reported a giant dielectric response of BT/ST heteroepitaxially stacked thin film.²⁷ We think except the nanostructure of the heteroepitaxial interface, the combinations of the nanocomposite for build-up of heteroepitaxial interface, such as the combinations of ferroelectric/paraelectric phases and ferroelectric/ferroelectric phases, the combinations between tetragonal, cubic, rhombohedral, orthorhombic crystal systems, and their lattice mismatchings at the heteroepitaxial interface, will also affect piezoelectric response.

In the BT/ST combination nanocomposite system, Mimura et al. have reported a BT/ST nanocomposite (BT/ST-C in Table 2.1) constructed by self-assembling nanocubes of BT and ST, and it only gives a d^*33 value of 59 pm/V.²⁰ The largest d33

value of 306 pm/V in the BT/ST system have been achieved using the mesocrystalline BT/ST nanocomposite (BT/ST-M in Table 1) by our group.¹⁵ The larger piezoelectric response of mesocrystalline BT/ST nanocomposite can be attributed to the formation of a perfect and higher density BT/ST heteroepitaxial interface than those in other cases.

Therefore, it can be concluded that the nanostructure of the mesocrystalline nanocomposite is advantageous for the enhancement of piezoelectric response by the lattice strain engineering.

In the mesocrystalline nanocomposites, the piezoelectric response increases in a order of BT/CT-M (208 pm/V) < BT/ST-M (306 pm/V) < BT/BNT-M (408 pm/V) (Table 1).

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In this case, the difference of the piezoelectric responses can be attributed to the combinations of the heteroepitaxial interfaces. The BT, CT, ST, and BNT are tetragonal ferroelectric, orthorhombic ferroelectric, cubic paraelectric, and rhombohedral ferroelectric phases, respectively, at room temperature. Therefore, the BT/CT, BT/ST, and BT/BNT heteroepitaxial interfaces can be assigned to Tetra-Ferro/Orth-Ferro combination with a lattice missmatching of 4.3 %, Tetra-Ferro/Cub-Para combination with a lattice missmatching of 2.2 %, Tetra-Ferro/Rho-Ferro combination with a lattice missmatching of 2.6 %, respectively. The BT/ST combination exhibits a larger piezoelectric response than that of BT/CT, which may be ascribed to that the lattice mismatching of the BT/CT is too large for the formation of a stable heteroepitaxial interface in the nanocomposite. We think the much larger piezoelectric response of BT/BNT combination than that of BT/ST combination can be attributed to the optimized lattice mismatching of about 2.6 % and the Tetra-Ferro/Rho-Ferro interface combination. It is well known that PZT exhibits excellent piezoelectric performance at the morphotropic phase boundary (MPB), in which a Tetra-Ferro/Rho-Ferro interface is formed also.⁴ The combination of the polarization directions at the Tetra-Ferro/Rho-Ferro interface, namely [001]/[111], is important to obtain a large piezoelectric response. Therefore, we think BT/BNT interface is one of optimized combination for the enhanced piezoelectric performance.

Wada et al. have reported a BT/KNbO3 (BT/KN-S in Table 1) nanocomposite with a d33 value of 136 pC/N, although the nanocomposite is fabricated by a simple method, namely, coating BT grain surface in a porous ceramic with KN layer.¹² The BT/KN-S nanocomposite exhibits a larger d33 value than that BT/ST nanocomposites, except the mesocrystalline BT/ST nanocomposite, even with its small lattice mismatching of 0.6 %.

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It hints that the Tetra-Ferro/Orth-Ferro ([001]/[110]) combination may be also a promising system for the enhanced piezoelectric performance if the lattice mismatching is appropriate.