Inhomogenous Amorphous Content and Enhanced Piezoelectric Properties in Bi12TiO20-based Composite Ceramics

Bi 12 TiO 20 -based composite ceramics were prepared by sintering a mixture of presynthesized Bi 12 TiO 20 and a second phase around the melting point of Bi 12 TiO 20 . These ceramics can exhibit direct as well as inverse piezoelectricity without electrical polarization, and become competitive piezoelectric material with high depoling temperature and stable thermal behavior. However, their piezoelectric constants were, in general, relatively low and do not remain consistent for both the symbol and magnitude over the entire surface. To prevent the permeation of melting Bi 12 TiO 20 into substrate and thus create the inhomogenous amorphous content, samples were sintered on Al 2 O 3 (0001) single crystal substrate. Compared to samples sintered on Al 2 O 3 ceramics, piezoelectric coecient (d 33 ) of these samples was increased by 80%. Moreover, the symbol of d 33 is identical on the whole surface. Here, the enhanced piezoelectricity might be ascribed to increased alignment of distorted BiO 5 polyhedra in amorphous Bi 12 TiO 20 phase. These results provided a method to improve the piezoelectric properties of Bi 12 TiO 20 -based ceramics and provided some inspiration for further development of Bi 12 TiO 20 -based ceramics suitable for practical applications.


Introduction
Traditionally, inorganic piezoelectric materials can be divided into the following two categories: a) ferroelectric single and poly crystals (such as BaTiO 3 , K 0.5 Na 0.5 NbO 3 , Na 0.5 Bi 0.5 TiO 3 and Pb(Ti, Zr)O 3 piezoelectric ceramics, single crystals, and thin lms); b) non ferroelectric but piezoelectric single crystals and oriented thin lms (such as SiO 2 , La 3 Ga 5 SiO 14 single crystals, ZnO, AlN oriented lms) [1][2][3][4][5]. Since piezoelectricity cannot exist in central materials, all the above materials are noncentrosymmetric. However, the discovery of exoelectricity (coupling between strain gradient and poling) extended piezoelectric materials to all insulators [6][7][8]. According to Lubomirsky et al., some amorphous lms (including BaTiO 3 , BaZrO 3 and SrTiO 3 ) can also show piezoelectricity after passing a temperature gradient [9][10][11][12]. These polar amorphous phases are called quasi amorphous phases, and the appearance of polarity is considered as the partial arrangement of local bonding units induced by plastic strain gradient [13]. Yudin and Tagantsev named this effect as plastic exoelectricity in their review paper, and the design of inorganic materials was then expanded to centrosymmetric amorphous thin lms [14].
Recently, we reported a new bulk ceramic that exhibits both positive and inverse piezoelectricity without electric eld polarization. These ceramics were prepared by high-temperature interfacial reaction of Bi 12 TiO 20 with a second phase such as Na 0.5 Bi 0.5 TiO 3 , BaSnO 3 and BaTiO 3 [15][16][17][18]. Since Bi 12 TiO 20 phase is necessary at present, we call them Bi 12 TiO 20 -based ceramics. According to previous studies, there is no obvious oriented grains and the ferroelectric phase is not essential for these polar Bi 12 TiO 20based ceramics [15,16]. Inspired by polarization theory of quasi amorphous thin lms, this abnormal piezoelectricity may originate from the orientation of distorted BiO 5 polyhedron of the amorphous Bi 12 TiO 20 phase at the grain boundary [17,18]. Therefore, the polarization effect could also be termed as plastic exoelectricity [18]. Bi 12 TiO 20 -based ceramics have the advantages of low dielectric constant, low dielectric loss, high depoling temperature, and stable thermal behavior. This makes them competitive piezoelectric material for high-temperature applications. However, the polarization mechanism has not been fully elucidated and their piezoelectric constants were, in general, relatively low and do not remain consistent for both the symbol and magnitude over the entire surface. It is an urgent and important task to further verify the origin of this abnormal piezoelectricity and then improve the piezoelectric properties.
Previously, the piezoelectric properties were usually improved by changing the main composition or content of the second phase, but with little success [15][16][17][18]. This paper presented an effective method to improve the piezoelectricity of Bi 12 TiO 20 -based ceramics and discussed the reasons for the improved piezoelectric properties.

Experimental Section
Here, the starting materials include high purity Bi 2 O 3 , TiO 2 and BaCO 3 powders. Firstly, barium titanate and Bi 12 TiO 20 were synthesized by conventional solid-state reaction route at 1050°C and 750°C, respectively. Secondly, barium titanate and Bi 12 TiO 20 powders were mixed in the molar ratio of 12:1 and then milled into a disc. Finally, the Bi 12 TiO 20 -BaTiO 3 composite was placed on the Al 2 O 3 (0001) single crystal substrate, and then sintered in 860°C air atmosphere for about 2 hours. Figure 1 shows the sketch map of the sintering process. According to the thermocouple 1 and thermocouple 2, the temperature difference along thickness direction during sintering process was about 1 ~ 3°C.
The surface morphology and distribution of elements were examined by high-resolution scanning electron microscopy (SEM) (HitachiS-4800). The piezoelectric coe cient (d 33 ) was tested by YE2730A d 33 Meter. The dielectric constants were determined by the LCR analyzer (Agilent 4294A). The element content was analyzed by XPS (Thermo Escalab 250). The local structure and chemical bonding environment were studied by Raman spectroscopy (LaBRAM HR800). the top and bottom surfaces of the sintered ceramics decreased, indicating that the volume fraction of the Bi 12 TiO 20 crystal phase decreased. Since no diffusion halo was found under the diffraction peak, the content of the amorphous phase was limited [19,20]. There was no signi cant difference in the relative intensity of Bi 12 TiO 20 and BaTiO 3 phases between the top and bottom surfaces of the sintered sample, so the volume fraction of Bi 12 TiO 20 and BaTiO 3 crystal phases of the top and bottom surfaces had little difference. For a better comparison, detailed XRD locations of the unsintered composites and sintered sample are given in Fig. 2 (d). After sintering, the XRD peaks of Bi 12 TiO 20 and BaTiO 3 on the top surface of sintered samples obviously moved to a small direction of 2θ, indicating the slight lattice expansion for crystalline Bi 12 TiO 20 and BaTiO 3 phases [21,22]. However, the peak shift and lattice expansion were not obvious for the bottom surface. In this case, the lattice expansion may be caused by the residual stress formed during sintering [22].

Results And Discussion
XRD results showed that sintered sample was mainly composed of crystalline BaTiO 3 , crystalline Bi 12 TiO 20 , and amorphous Bi 12 TiO 20 . Figure 3(a) shows a typical SEM image of the sintered sample. It mainly included two distinct forms: small grains which were easy to be identi ed with grain size of about several hundred nanometers; large size matters that have an irregular shape with the size of about several to a dozen micrometers. According to the EDS spectrum shown in Fig. 3 (b), the small grains corresponded to crystalline BaTiO 3 , and the large-sized materials corresponded to Bi 12 TiO 20 crystal phase with amorphous Bi 12 TiO 20 phase distributing on the surface. Because the samples were sintered near the melting temperature of Bi 12 TiO 20 , a part of Bi 12 TiO 20 was liquid during sintering process. During the cooling process, the ne BaTiO 3 grains may be wrapped in the irregular Bi 12 TiO 20 phase.
The DSC curves of the sintered sample and pure Bi 12 TiO 20 ceramic were shown in Fig. 4. For the pure Bi 12 TiO 20 ceramic, a strong endothermic peak was present around 860°C, and the calculated enthalpy (∆H 1 ) was 282.5 kJ mol − 1 . This endothermic peak is the melting peak of Bi 12 TiO 20 crystalline phase. In comparison, a smaller endothermic peak at approximately 836°C was present for the sintered sample.
The calculated enthalpy (∆H 2 ) was approximately 137.5 kJ mol − 1 . This only emerging endothermic peak is also the melting peak of crystalline Bi 12 TiO 20 . The melting point of crystalline Bi 12 TiO 20 decreased because of the expansion of lattice [23]. Assuming that Bi 12 TiO 20 phase in sintered sample was 100% crystalline, the content of crystalline Bi 12 TiO 20 phase can be calculated by the ratio of ∆H 2 /∆H 1 [24].
According to calculation, about 48% of Bi 12 TiO 20 was crystalline. Therefore, we can further estimate an approximately 20% volume fraction of amorphous Bi 12 TiO 20 in the sintered sample. Although the sintered sample contained no oriented grains, it can present relatively large piezoelectricity without undergoing an electric poling course. Most of the piezoelectric strain constants (d 33 ) were in the range of 13−15pC/N, and the maximum value was increased to 20pC/N. In addition, the symbol of d 33 remained almost unchanged over the entire surface. In comparison to Bi 12 TiO 20 -based ceramics reported in the past, the piezoelectric constants were greatly improved [18]. There was no macroscopic ferroelectricity in BaTiO 3 phase, above this temperature [25]. At room temperature, the dielectric constant of the sample was only 280, which was far lower than samples sintered on Al 2 O 3 ceramics (about 600). It was observed that the k value had little change between − 50°C and 350°C, and there was no decreasing trend even though the temperature was much higher than the Curie temperature of BaTiO 3 . Therefore, this unusual piezoelectricity appears to have no relationship with crystalline BaTiO 3 . Figure 5(b) shows the piezoelectric resonance frequencies of BaTiO 3 ceramics and the sintered sample as functions of temperature, and the illustration shows the details of piezoelectric resonance frequencies of the sintered sample at 400°C. The experimental results showed that the piezoelectric resonance signal of BaTiO 3 ceramic can be observed at about 180°C. Below this temperature, the tendency of the piezoelectric resonance frequency of the sintered sample was similar to that of BaTiO 3 ceramics. While, above this temperature, the resonance frequency changed slowly with the increase of temperature. The resonance frequency curve can be tted into a straight line from 250 to 600°C, and the calculated temperature coe cient was about 100×10 − 6 /°C. This is an fascinating feature for Bi 12 TiO 20 -basded ceramics.
The Bi and Ba elements of the top and bottom surfaces of the sintered sample were quantitatively estimated by XPS. By using C1s signal (284.6eV) as a reference, the high-resolution spectra of the elements determined in the measurement scanning were corrected [27]. The percentages of Bi and Ba elements were calculated by corresponding software. The calculation results showed that the Bi/Ba ratio for the top surface was 1.27, while for the bottom surface was 1.94. Because some small BaTiO 3 grains may be wrapped in Bi 12 TiO 20 , the calculated Bi/Ba ratio may have errors. However, the difference of the Bi/Ba ratio between the top and bottom surfaces indicated that the Bi content of the bottom surface was higher than that of the top surface. The results of XRD showed that the contents of crystalline Bi 12 TiO 20 and BaTiO 3 phase had very little difference. The difference of Bi/Ba ratio according to XPS spectra indicates the different contents of amorphous Bi 12 TiO 20 phase. As shown in Fig. 6, the Ba 3d spectra of the top and bottom surfaces were the same, but for Bi 4f, the peak positions in the bottom surface shifted to the higher binding energies than that obtained in the top surface. The peak shift showed that the content of the amorphous Bi 12 TiO 20 phase in the bottom surface was higher than that in the top surface.
This result further proved the inhomogeneity of the content of Bi 12 TiO 20 amorphous phase [12,27,28].
Since the amorphous content is limit, the inhomogeneous distribution of amorphous Bi 12 TiO 20 along thickness direction is the main reason for the very low dielectric constant of the sintered sample.
In general, repeated annealing is an effective way of releasing elastic stress [29][30][31]. The piezoelectric constant of the sintered sample remained unchanged for repeated as well as long time annealing. Figure 7(a) shows detailed XRD comparisons of the unsintered composites and the annealed sample.
The XRD peak position of annealed sample was similar to that of unsintered composites, which con rmed the release of the elastic residual stress. Thus, the elastic deformation of crystalline phase should not be the main contributor of this abnormal piezoelectricity. Figure 7(b) shows the Raman spectra of Bi 12 TiO 20 standard sample and the annealed sample. The Raman peaks of the annealed sample can be divided into two groups: p1-p10 which is related to Bi 12 TiO 20 [31,32] and p11-p13 which is related to BaTiO 3 [33] (Bi 12 TiO 20 and BaTiO 3 peaks completely overlapped at approximately 720cm − 1 ).
The vibration modes of Bi 12 TiO 20 include the TiO 4 tetrahedron mode which appeared above 700cm − 1 and the BiO 5 polyhedron mode at low frequency. In order to study the detailed contrast of Raman spectra, an extended view near p1, p2, p10/p13 peaks is shown in the illustration. Compared with the Raman peak of Bi 12 TiO 20 standard sample, p10/p13 peak had no obvious change. This suggests that the TiO 4 tetrahedra remained unchanged. However, the p1 and p2 peaks in the top and bottom surfaces of the annealed sample moved almost the same degree towards lower wavenumber direction. As a re ection of Bi-O bond vibrations of BiO 5 polyhedra, the movement of these peaks indicates the asymmetric elongation of the Bi-O bond and further con rms the expanded distortion of the BiO 5 polyhedra.
Combining with the XRD spectra, the expansion was related with amorphous structure. By comparing the Raman spectra near the top and bottom surfaces, the BiO 5 polyhedra distorted at approximately the same degree. The uniform deformation degree of BiO 5 polyhedra along thickness direction may result in the enhancement of piezoelectric constant.
The difference of the preparation process between the sintered sample and the previously reported  [34]. As the substrate was Al 2 O 3 ceramics that had defects, the liquid Bi 12 TiO 20 phase will gradually ll the apparent pores of the substrate, thus the composition gradient was greatly reduced.

Conclusions
In conclusion, the Al 2 O 3 single crystal substrate sintering method is an effective way to enhance the piezoelectric properties of the Bi 12 TiO 20 -based ceramics. In this study, the piezoelectric strain constant was increased to 20pC/N. In addition, the prepared sample has high mechanical coupling coe cient, high depoling temperature and temperature stability of resonance frequency in a certain temperature range, which is an ideal material for high temperature piezoelectric materials. These results provided a method to improve the piezoelectric properties of Bi 12 TiO 20 -based ceramics, and provided inspiration for further research on developing Bi 12 TiO 20 -based ceramics suitable for practical applications. Figure 1 The schematic illustration of the sintering process.