Nd / W co-substituted Bi4Ti3O12 piezoelectric ceramic as well as preparation method and application thereof

Abstract

The invention provides piezoelectric ceramic which is obtained by replacing Bi < 3 + > in Bi4Ti3O12 with Nd < 3 + > and replacing Ti < 4 + > in Bi4Ti3O12 with W < 6 + >. The structural formula of the piezoelectric ceramic is Bi < 4-x > Nd < x > Ti < 2.925 > W < 0.075 > O < 12 >, wherein x is greater than or equal to 0.01 and less than or equal to 0.05. The invention also provides a preparation method and application of the piezoelectric ceramic. Nd < 3 + > with smaller ion radius is introduced to replace Bi < 3 + >, and W < 6 + > is combined to replace Ti < 4 + >, so that the crystal structure of the ceramic can be finely adjusted, and the electrical property of the ceramic can be improved. Nd doping has great influence on the microstructure and macroscopic performance of the ceramic. Preferably, when x is equal to 0.03, the piezoelectric ceramic with very high piezoelectric property and resistivity is obtained.

Description

Background Technology

[0001] Piezoelectric materials are widely used in the manufacture of sensors, filters, and transducers due to their unique piezoelectric effect (the ability to convert mechanical energy into electrical energy). These materials play a crucial role in various fields such as energy, nuclear power, electronics and information technology, aerospace, and healthcare. Currently, commercially available piezoelectric ceramic materials are primarily lead-based because of their excellent piezoelectric coefficient, high electromechanical coupling coefficient, and high mechanical strength. However, the manufacturing process of lead-based piezoelectric ceramics poses significant risks to human health and the natural environment. The operating temperature of these ceramics is typically limited to 50%-75% of the Curie temperature (TC). When the temperature exceeds this threshold, the piezoelectric properties of the ceramics degrade sharply. Therefore, lead-based piezoelectric ceramics are difficult to meet the requirements of high-temperature applications (>400℃).

[0002] In contrast, bismuth layered ferroelectrics (BLSFs) are attracting widespread attention as potential alternatives to lead-based piezoelectric ceramics, especially in high-temperature environments, due to their high Curie temperature (TC > 500 °C), excellent thermal stability, and high breakdown field strength. The general chemical formula of bismuth layered ferroelectrics is (Bi₂O₂). 2+ (A m 1B m O 3m+1 ) 2 Its crystal structure consists of a bismuth-oxygen layer (Bi₂O₂). 2+ and perovskite layer (A m 1B m O 3m+1 ) 2 The bismuth titanate is arranged in an alternating and regular manner along the c-axis. In this structure, the A-site cation is a dodecahedral coordinated metal ion, the B-site cation is an octahedral coordinated cation, and m represents the number of perovskite layers between adjacent bismuth-oxygen layers, usually taken as 1 to 5. Among them, bismuth titanate (Bi4Ti3O12) with a monoclinic structure and m=3 is a promising lead-free ferroelectric material. Its spontaneous polarization intensity (Ps) on the ab plane can reach 50 μC / cm², its Curie temperature is high (TC≈675℃), and it has excellent fatigue resistance [16,17], making it an ideal material for high-temperature piezoelectric devices that can work stably at temperatures up to 500℃. However, unmodified bismuth titanate (BIT) has some limitations in practical applications, including a large coercive field (Ec) and a relatively low piezoelectric coefficient (d33<7 pC / N)

[18] . In addition, bismuth volatilizes during ceramic sintering, resulting in a large number of oxygen vacancy defects inside the material. Defects are activated and undergo long-range migration under the influence of a thermal field, leading to a decrease in the resistivity (ρ) of BIT ceramics. Therefore, simultaneously improving both piezoelectric coefficient and resistivity remains a key challenge and research hotspot in the development of high-temperature piezoelectric ceramic materials.

[0003] Over the past two decades, researchers have discovered that chemical doping is a simple and effective method to improve the electrical properties of BIT ceramics. Typically, lanthanide rare earth elements, or higher-melting-point Ba and Sr, are doped at the A-site of the perovskite layer in a bismuth layered structure to replace the volatile Bi. This method helps reduce Bi volatilization during sintering, thereby lowering the oxygen vacancy concentration. Furthermore, due to differences in ionic radius, chemical valence state, and electronegativity, doping causes distortions in the crystal structure, which positively impacts the regulation of electrical properties. For B-site doping, an ionic radius similar to that of Ti is typically chosen. 4+ Similar high-valence ions, such as W 6+ 、Nb 5+ Ta 5+ and V 5+ To maintain electrical neutrality, the electrons of these ions react with oxygen vacancies, leading to a decrease in charge carriers (electrons and oxygen vacancies) within the ceramic, thereby increasing resistivity. In summary, the purpose of A-site or B-site substitution is to reduce leakage current and oxygen vacancy defect concentration, which can effectively improve the piezoelectric activity of the material and reduce its high-temperature conductivity.

[0004] Zhang et al. systematically studied the electrical properties of tungsten-doped BIT ceramics and determined the optimal composition. 33 The ratio is 19 pC / N, the total temperature (TC) is 643℃, and the density (ρ) is 1.3 × 10⁻⁶. 9 Ω·cm (at 400℃). Although the resistivity of the ceramic is significantly improved, its piezoelectric properties still need further enhancement. Summary of the Invention

[0005] The first objective of this invention is to provide a piezoelectric ceramic with high voltage properties.

[0006] The second objective of this invention is to provide a method for preparing piezoelectric ceramics with high voltage electrical properties.

[0007] The third objective of this invention is to provide an application of piezoelectric ceramics with high voltage electrical properties.

[0008] This invention is achieved through the following technical solution: A piezoelectric ceramic made of Nd 3+ Replacement of Bi4Ti3O 12 Bi in 3+ W 6+ Replacement of Bi4Ti3O 12 Ti in 4+ get; The piezoelectric ceramic has the structural formula Bi. 4-x Nd x Ti 2.925 W 0.075 O 12 , where 0.01≤x≤0.05.

[0009] The piezoelectric ceramic has the structural formula Bi. 3.99 Nd 0.01 Ti 2.925 W 0.075 O 12 Bi 3.97 Nd 0.03 Ti 2.925 W 0.075 O 12 Or Bi 3.95 Nd 0.05 Ti 2.925 W 0.075 O 12 .

[0010] The method for preparing the piezoelectric ceramic includes the following steps: Bi2O3, Nd2O3, WO3, and TiO2 are weighed according to the stoichiometric ratio, mixed, ball-milled, dried, granulated, pressed into shape, degummed, and sintered to obtain the final product.

[0011] The ball milling time is 4-6 hours.

[0012] The drying temperature is 50°C; The drying time is 3 hours.

[0013] The pressure for compression molding is 120-130 MPa.

[0014] The degumming temperature is 600℃; The degumming time is 2 hours.

[0015] The sintering temperature is 1020-1120℃.

[0016] The sintering time is 3 hours.

[0017] The piezoelectric ceramics described herein are used in the manufacture of sensors, filters, or transducers.

[0018] Compared to existing technologies, the present invention attracts Nd with a smaller ion radius. 3+ Replace Bi 3+ At the same time, combined with W 6+ Replace Ti 4+ Nd doping can fine-tune the crystal structure of ceramics and improve their electrical properties. Nd doping has a significant impact on the microstructure (including phase structure, microstructure, and domain structure) and macroscopic properties (electrical properties) of ceramics. Preferably, when x=0.03, piezoelectric properties (25.8 pC / N) and resistivity (7.85 × 10⁻⁶ at 500 °C) are obtained. 6 Piezoelectric ceramics with very high Ω·cm. Attached Figure Description

[0019] Figure 1 (a) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) Room temperature XRD patterns of ceramics; Figure 1 (b) shows Figure 1 (a) is a magnified view of the XRD pattern in which 2θ is in the range of 37°–48°; Figure 2 (a) shows the Rietveld refinement of the XRD curve of BIT; Figure 2 (b) shows Ti 2.925 W 0.075 O 12 Rietveld finish on ceramics; Figure 2 (c) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01) Rietveld finishing of ceramics; Figure 2 (d) shows Bi 4-xNd x Ti 2.925 W 0.075 O 12 (x=0.03) Rietveld finishing of ceramics; Figure 2 (e) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.05) Rietveld finishing of ceramics; Figure 2 (f) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) The results of the ceramic finishing process; Figure 3 (a) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 ceramics at room temperature 50cm -1 Up to 1000cm -1 Raman spectra within the range; Figure 3 (b) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) The Raman peak position of ceramics varies with x; Figure 4 (a) shows the surface microstructure of BIT ceramic; Figure 4 (b) shows Ti 2.925 W 0.075 O 12 The surface microstructure of ceramics; Figure 4 (c) shows Bi 4-x Nd x Ti 2.925 W 0.075 O12 (x=0.01) Surface microstructure of ceramics; Figure 4 (d) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.03) Surface microstructure of ceramics; Figure 4 (e) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.05) Surface microstructure of ceramics; Figure 4 (f) shows the grain size and aspect ratio of all samples; Figure 5 (a) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x = 0.01, 0.03, 0.05) Relative permittivity of the ceramics; the inset shows the TC of the samples; Figure 5 (b) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) Temperature dependence of dielectric loss of ceramics; the inset shows the tanδ of the sample at 500°C; Figure 6 (a) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) The piezoelectric coefficient d of the ceramic 33 ; Figure 6 (a) shows Ti 2.925 W 0.075 O 12 BIT and Bi4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) Thermal stability of ceramics; Figure 7 (a) shows the PE ring and IE curves of BIT ceramic; Figure 7 (b) shows Ti 2.925 W 0.075 O 12 PE ring and IE curve of ceramic; Figure 7 (c) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01) PE ring and IE curves of ceramics; Figure 7 (d) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.03) PE ring and IE curves of ceramics; Figure 7 (e) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.05) PE ring and IE curves of ceramics; Figure 7 (f) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) Component dependence of 2Pr in ceramics.

[0020] Figure 8 (a1) shows the surface morphology of BIT ceramics; Figure 8 (a2) shows Ti 2.925 W 0.075 O 12 Surface morphology of ceramics; Figure 8 (a3) shows Bi 4-x Nd xTi 2.925 W 0.075 O 12 (x=0.03) Surface morphology of ceramics; Figure 8 (b1) shows the phase diagram of the BIT ceramic; Figure 8 (b2) shows Ti 2.925 W 0.075 O 12 Phase diagram of ceramics; Figure 8 (b3) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.03) Phase diagram of ceramics; Figure 8 (c1) shows the amplitude diagram of the BIT ceramic; Figure 8 (c2) shows Ti 2.925 W 0.075 O 12 Amplitude diagram of ceramics; Figure 8 (c3) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.03) Amplitude diagram of ceramics; Figure 8 (d) shows Ti 2.925 W 0.075 O 12 Phase and amplitude curves of the micro-region piezoelectric response of ceramics; Figure 8 (e) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.03) Phase and amplitude curves of the micro-region piezoelectric response of ceramics; Figure 8 All PFM test areas were 10μm × 10μm; Figure 9 (a) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12(x=0.01, 0.03, 0.05) Relationship between ρ and temperature for ceramics; Figure 9 (b) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) Plots of ceramics fitted to ρ using the Arrhenius function; Figure 9 (c) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) E in different temperature ranges of ceramics a Component dependence; Figure 10 (a) shows the impedance spectrum of the BIT ceramic; Figure 10 (b) shows Ti 2.925 W 0.075 O 12 Impedance spectrum of ceramics; Figure 10 (c) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01) Impedance spectrum of ceramics; Figure 10 (d) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.03) Impedance spectrum of ceramics; Figure 10 (e) shows Bi 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.05) Impedance spectrum of ceramics; Figure 10 (f) shows Ti 2.925 W 0.075 O 12 BIT and Bi 4-xNd x Ti 2.925 W 0.075 O 12 (x=0.01, 0.03, 0.05) Capacitance (C') spectrum of ceramics at 500°C. Detailed Implementation

[0021] Example 1 Bi was prepared using a traditional solid-state reaction method. 4-x Nd x Ti 2.925 W 0.075 O 12 (x=0.01) Piezoelectric ceramic sample.

[0022] A certain amount of raw materials [Bi₂O₃ (99.9%), Nd₂O₃ (99.99%), WO₃ (99.8%), TiO₂ (99.62%)] were weighed according to the stoichiometric ratio and ball-milled for 4-6 hours using a planetary ball mill. After ball milling, the powder was dried at 50°C for 3 hours. The powder was then granulated and pressed into small discs with a diameter of 10 mm and a thickness of 0.8 mm using a tablet press at a pressure of 127 MPa. The pressed discs were then treated at 600°C for 2 hours to remove the binder. Finally, the discs were sintered at 1020°C for 3 hours to obtain ceramic discs.

[0023] Example 2 Bi was prepared using a traditional solid-state reaction method. 4x Nd x Ti2. 925 W0. 075 O 12 (x=0.03) Piezoelectric ceramic sample.

[0024] A certain amount of raw materials [Bi₂O₃ (99.9%), Nd₂O₃ (99.99%), WO₃ (99.8%), TiO₂ (99.62%)] were weighed according to the stoichiometric ratio and ball-milled for 4-6 hours using a planetary ball mill. After ball milling, the powder was dried at 50°C for 3 hours. The powder was then granulated and pressed into small discs with a diameter of 10 mm and a thickness of 0.8 mm using a tablet press at a pressure of 127 MPa. The pressed discs were then treated at 600°C for 2 hours to remove the binder. Finally, the discs were sintered at 1020°C for 3 hours to obtain ceramic discs.

[0025] Example 3 Bi was prepared using a traditional solid-state reaction method. 4x Nd x Ti2. 925 W0. 075 O 12(x=0.05) Piezoelectric ceramic sample.

[0026] A certain amount of raw materials [Bi₂O₃ (99.9%), Nd₂O₃ (99.99%), WO₃ (99.8%), TiO₂ (99.62%)] were weighed according to the stoichiometric ratio and ball-milled for 4-6 hours using a planetary ball mill. After ball milling, the powder was dried at 50°C for 3 hours. The powder was then granulated and pressed into small discs with a diameter of 10 mm and a thickness of 0.8 mm using a tablet press at a pressure of 127 MPa. The pressed discs were then treated at 600°C for 2 hours to remove the binder. Finally, the discs were sintered at 1020°C for 3 hours to obtain ceramic discs.

[0027] Comparative Example 1 Bi4Ti3O prepared by traditional solid-state reaction method 12 (BIT) Piezoelectric ceramic sample.

[0028] A certain amount of raw materials [Bi₂O₃ (99.9%), TiO₂ (99.62%)] were weighed according to the stoichiometric ratio and ball-milled for 4-6 hours using a planetary ball mill. After ball milling, the powder was dried at 50°C for 3 hours. The powder was then granulated and pressed into small discs with a diameter of 10 mm and a thickness of 0.8 mm using a tablet press at a pressure of 127 MPa. The pressed discs were then treated at 600°C for 2 hours to remove the binder. Finally, the discs were sintered at 1020°C for 3 hours to obtain ceramic discs.

[0029] Comparative Example 2 Ti prepared by traditional solid-state reaction method 2.925 W 0.075 O 12 Piezoelectric ceramic sample.

[0030] A certain amount of raw materials [Bi₂O₃ (99.9%), WO₃ (99.8%), TiO₂ (99.62%)] were weighed according to the stoichiometric ratio and ball-milled for 4-6 hours using a planetary ball mill. After ball milling, the powder was dried at 50°C for 3 hours. The powder was then granulated and pressed into small discs with a diameter of 10 mm and a thickness of 0.8 mm using a tablet press at a pressure of 127 MPa. The pressed discs were then treated at 600°C for 2 hours to remove the binder. Finally, the discs were sintered at 1020°C for 3 hours to obtain ceramic discs.

[0031] To test the piezoelectric and electrical properties of the ceramic samples, ceramic discs were surface-treated, and then silver or platinum paste (0.3-0.5 mm thick) was screen-printed on both sides of the surface-treated discs, forming surface electrodes at 600°C-800°C. The phase structure of the ceramic samples was characterized using an X-ray diffractometer (MiniFlex600, Rigak, Japan). XRD data were collected using a Cu-Kα radiation source (λ=0.15406 nm), with a 2θ range of 10°C-60°C. The microstructure of the ceramic samples was observed using a scanning electron microscope (SEM, MIRA4 LMH, Czech Republic). The dielectric properties of the samples, such as dielectric constant and dielectric loss, were measured using an LCR bridge (Keysight E4990A, China). Raman spectroscopy was obtained using a Raman spectrometer (DXR3, Thermo Fisher Scientific, USA), with an excitation source of 532 nm and a range of 50-3500 cm⁻¹. -1 The DC resistivity versus temperature curves of the ceramic samples were acquired using a high-temperature resistivity testing system (HRMS-900, Partulab, China). Furthermore, the ferroelectric properties of the ceramics were measured using a ferroelectric measuring instrument (TF-2000, aixACCT, Germany). The ceramic samples were placed in a high-temperature polarization device with a silicone oil bath (MPD20KVWO, China) and polarized for 10-15 minutes at 150°C-180°C using a gradient voltage (14-20kV / mm). A static d33 analyzer (ZJ-3AN, Institute of Acoustics, Chinese Academy of Sciences, China) was used. The thermal stability of the ceramic samples was evaluated by measuring d33 at different temperatures ranging from 200-700°C and annealing for 2 hours. At room temperature, the ferroelectric domain structure of the ceramic samples was observed in piezoelectric response force microscopy (MFP-3D, Asylum Research, USA) in piezoelectric response force mode. The AC impedance diagram was measured in the temperature range of 300-600°C using a high-temperature impedance thermometer (DMS1000+WK6500).

[0032] Results and Discussion Figure 1 For BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 XRD patterns of the ceramics at room temperature within the 2θ range of 10°–60°. The diffraction peaks of all ceramic samples clearly match the orthorhombic structure (PDF#721019) of the bismuth layered solid fraction (BLSFs) BIT phase, with space group B2cb. This result indicates that Nd and W successfully integrate into the BIT lattice, forming a solid solution. Notably, the diffraction peak intensity is strongest on the (117) plane, confirming that Bi... 4-x Nd x Ti2.925 W 0.075 O 12 The ceramic exhibits a three-layer perovskite structure (m=3) between adjacent bismuth oxide layers. This observation is consistent with the strongest reflectance of BLSFs materials being (112m+1). Figure 1 (b) is an enlarged XRD pattern of 2θ in the range of 37° to 48°. The intensity of the (1115) diffraction peak gradually decreases. With increasing Nd doping concentration, the distance between the (220) and (1115) diffraction peaks decreases. This phenomenon can be explained by Nd... 3+ (1.27 Å, coordination number 12) and Bi 3+ This can be explained by lattice distortion caused by differences in ionic radius and electronic configuration between (1.36 Å, coordination number 12).

[0033] like Figure 2 (ae) is shown. The calculated results are consistent with the XRD data collected in the test, with refinement factors Rwp (6.36-7.92%), Rp (4.87-6.20%), and χ². 2 The values ​​(1.993-3.358) are all within a reasonable range. The refined lattice parameters (a, b, and c) and cell volume (V) are as follows: Figure 2 As shown in (f), it can be seen that after Nd doping, a and b show a decreasing trend, while c increases with increasing doping concentration. V decreases with increasing doping concentration, which is due to the smaller ionic radius of Nd. 3+ Replaced Bi 3+ This substitution leads to a decrease in interplanar spacing, which in turn causes the unit cell to shrink.

[0034] To better identify changes in vibration modes, the Gaussian-Lorentz function was used to fit the peaks of the Raman spectral data [2]. The Raman spectrum can be divided into three main regions: 200 cm⁻¹ -1 The following low wavenumber region is related to (Bi2O2). 2+ Bi in layer 3+ Vibration (62cm) -1 ) and the vibrations of A-site ions in the perovskite layer (89 cm) -1 118cm -1 and 149cm -1 Related; 200-600cm -1 In the mid-wavenumber region, it is associated with the bending, tensile, and torsional vibration modes of Ti-O; 600 cm⁻¹ -1 The high-frequency region mentioned above belongs to the TiO6 octahedral vibration mode, such as... Figure 3 As shown in (b).

[0035] After Nd doping, (Bi2O2) 2+ The vibration of the layer remained constant, but at site A (118 cm)-1 and 149 cm -1 A slight shift was observed, indicating that Nd did not replace (Bi2O2). 2+ Instead of layer, it replaced position A with Bi. 3+ BIT ceramics, 227cm -1 and 269cm -1 The vibrational modes at this location are attributed to the torsional vibrations of the Ti-O bonds. After Nd addition, these modes shifted to higher wavenumbers, indicating a change in the degree of structural distortion. 331cm -1 and 358cm -1 The peak at that location corresponds to the O-Ti-O bending mode. Compared to BIT ceramics, Bi... 4- x Nd x Ti 2.925 W 0.075 O 12 In ceramics, the intensity of these peaks decreases, and the peaks broaden. This change is due to W 6+ Replace Ti 4+ This altered the original O-Ti-O bending vibration. With increasing Nd doping concentration, the 614 cm⁻¹... -1 The vibrational modes at this location shift to higher wavenumbers, reflecting the structural distortion of the TiO6 octahedron. This lattice distortion enhances the Bi... 4-x Nd x Ti 2.925 W 0.075 O 12 The degree of spontaneous polarization of ceramics.

[0036] For BIT and Bi 4-x Nd x Ti 2.925 W 0.075 O 12 The microstructure of the natural surface of the ceramic was characterized. Statistical analysis of the ceramic particle size was performed using NanoMeasurement software to determine the aspect ratio of the particles, such as... Figure 4 As shown in (af). All ceramic samples exhibited tightly packed grains with no visible pores. BIT ceramics showed a lamellar grain morphology, while Bi... 4- x Nd x Ti 2.925 W 0.075 O 12 Ceramics exhibit a more uniform anisotropic lamellar grain morphology. Furthermore, with the development of Ti... 4+ W 6+ The substitution resulted in a sharp decrease in average grain size from 3.12 μm (length) to 1.96 μm (length). This microstructure evolution is attributed to W 6+ Replace Ti 4+The reduction in oxygen vacancy concentration in the ceramic later inhibits ion diffusion during densification and slows down grain boundary diffusion rates. Furthermore, W 6+ This promotes grain growth along the ab direction, resulting in more uniform anisotropic grains. Therefore, Bi 4-x Nd x Ti 2.925 W 0.075 O 12 The reduction in grain size of ceramics increases the resistivity of ceramics, promotes a more complete polarization process, and helps to obtain excellent piezoelectric properties.

[0037] BIT and Bi were measured at 100 kHz. 4-x Nd x Ti 2.925 W 0.075 O 12 The relative permittivity (εr) and dielectric loss (tanδ) of ceramics change with temperature, as shown in the following figures. Figure 5 As shown. It can be observed that the εr curves of all ceramics initially change steadily, then increase with increasing temperature, until a distinct dielectric anomalous peak appears. This indicates that the bismuth layered ceramic material undergoes a phase transition from the ferroelectric phase to the paraelectric phase, with the corresponding temperature being the Curie temperature (TC)

[43] . Using W 6+ Replace Ti 4+ Subsequently, the TC of the ceramic dropped sharply from 681℃ to 654℃. This change is attributed to the Ti in the 6-coordinate environment. 4+ and W 6+ The ionic radii are 0.605 Å and 0.60 Å, respectively, which leads to an increase in the tolerance factor (t) after doping: Use W 6+ Replace Ti 4+ Subsequently, the Curie temperature of the ceramic dropped sharply from 681°C to 654°C. This change is attributed to the Ti in the 6-coordinate environment. 4+ and W 6+ The ionic radii are 0.605 Å and 0.60 Å, respectively, which leads to an increase in the tolerance factor (t) after doping: where R A R B and R O These represent the ionic radii of the A-site cation, B-site cation, and O ion, respectively. Generally, the larger the tolerance factor, the lower the Curie temperature. After Nd doping at the A-site, the Curie temperature of the ceramic decreases slightly, due to compositional fluctuations caused by doping and changes in A-site polarity.

[0038] like Figure 5 As shown in (b), Bi 4-x Nd x Ti 2.925 W0.075 O 12 The tanδ of the ceramic is significantly lower than that of the BIT ceramic. A small but broad dielectric anomaly peak was observed in the dielectric spectrum of the BIT ceramic, which is attributed to the point defect dielectric response caused by bismuth oxide volatilization. However, due to the reduced oxygen vacancy concentration after doping, the tanδ of the Bi ceramic is significantly lower than that of the BIT ceramic. 4-x Nd x Ti 2.925 W 0.075 O 12 This phenomenon was not observed in ceramics. The lower tanδ improves the reliability and stability of signal transmission in the sensor at high temperatures. Furthermore, when the temperature exceeds 550°C, tanδ increases with increasing temperature, due to enhanced carrier motion and increased leakage current at high temperatures. The significant increase in dielectric loss is due to a phase transition occurring after the ceramic absorbs energy, leading to changes in the crystal structure and reduced polarization. Additionally, the increased carrier motion at high temperatures also contributes to the increase in tanδ.

[0039] To investigate the piezoelectric properties of ceramics, BIT and Bi were subjected to piezoelectricity at room temperature. 4-x Nd x Ti 2.925 W 0.075 O 12 piezoelectric coefficient of ceramics (d 33 Measurements were performed, and the results are as follows: Figure 6 As shown in (a), BIT has relatively poor piezoelectric properties, and its d 33 The value is 5.7 pC / N, which is consistent with previously reported values ​​(<7 pC / N). After W doping, d 33 The value increased to 20.8 pC / N. As the Nd doping concentration increased to 0.03, d... 33 It further increased to 25.8 pC / N, reaching its maximum value. To evaluate Bi... 4-x Nd x Ti 2.925 W 0.075 O 12 To assess the thermal stability of the ceramics, all samples underwent a 2-hour staged annealing treatment, and their residual d was measured at room temperature. 33 Value, result as Figure 6 As shown in (b), all ceramic samples exhibit good temperature stability. The piezoelectric properties of the samples gradually decrease with increasing temperature. When the temperature exceeds TC, d 33 The value approaches zero. This phenomenon is attributed to the transformation of the ceramic crystal structure into a highly symmetrical tetragonal phase, which does not exhibit a piezoelectric response. After heat treatment at 500°C, the piezoelectric coefficient of the x=0.03 sample retained 86% of its initial properties (approximately 22.2 pC / N).

[0040] BIT and Bi were obtained at 180°C and 10 Hz. 4-x Nd x Ti 2.925 W 0.075 O 12 PE loops and IE curves of ceramics, such as Figure 7 As shown in (ae), the PE loop of BIT ceramics exhibits an arc shape, which can be attributed to the large leakage current caused by the low resistivity of the ceramic. This indicates high activity of oxygen vacancy defects, resulting in poor ferroelectric properties. In contrast, Bi... 4- x Nd x Ti 2.925 W 0.075 O 12 The PE loops in the ceramic are square but asymmetrical. This asymmetry is attributed to the presence of a bias electric field within the ceramic, caused by the pinning effect of defect dipoles and the reduction in grain size. The IE curves exhibit distinct current peaks, indicating ferroelectric domain flipping. Bi0.05 was extracted. 4-x Nd x Ti 2.925 W 0.075 O 12 The remanent polarization of ceramics 2Pr (Pr +- Pr - ),like Figure 7 As shown in (f), it is clear that 2Pr first increases and then decreases with increasing Nd doping concentration, with the composition at x=0.02 exhibiting the highest 2Pr. Therefore, introducing an appropriate amount of Nd is beneficial to improving the ferroelectric properties of BIT-based ceramics.

[0041] To reveal the contribution of external factors (domain structure) to piezoelectric properties, piezoelectric power microscopy (PFM) was used to evaluate the piezoelectric effect of BIT, x=0 and x=0.03 ceramics related to d33. Figure 8 The surface morphology of the samples shown in (a1-a3) indicates that all samples have good flatness. Compared with BIT ceramics, the x=0 and x=0.03 ceramics have smaller grain sizes. Figure 8The phase diagrams (b1-b3) reveal a large number of strip-shaped domains in the BIT ceramics, a characteristic of the microstructure of bismuth layered materials. Notably, no clear strip-shaped domains were observed in the ceramics at x=0 and x=0.03. This absence is attributed to the disruption of the original domain structure following W and Nb doping, leading to a significant reduction in domain size. For the ceramic at x=0.03, the domain walls are more uniform, and the domain density is significantly increased. The reduction in domain size inevitably leads to a decrease in domain wall energy (D∝(EDW)1 / 2, where D and EDW represent domain size and domain wall energy, respectively). The formation of high-density domain walls promotes the movement and flipping of domain walls under the influence of an external electric field, thereby improving the piezoelectric and ferroelectric properties of the ceramics.

[0042] Figure 8 (c1-c3) shows the amplitude images of BIT, x=0, and x=0.03 ceramics. The amplitude of the ceramics increases significantly with W and Nd doping. Generally, higher PFM amplitudes correspond to stronger piezoelectric responses and higher d... 33 The value, which is related to the macroscopically measured d 33 The changes are consistent. Figure 8 (de) shows the phase and amplitude curves of the microscopic piezoelectric response of x=0 and x=0.03 ceramics. Interestingly, the amplitude reached by the x=0.03 ceramic at 20V is higher than that of the x=0 ceramic at 40V, indicating that the piezoelectric response of the x=0.03 ceramic is much stronger than that of the x=0 ceramic. The amplitude-voltage curves exhibit asymmetric behavior, which can be attributed to the generation of a large number of oxygen vacancies during the sintering process of the bismuth-containing ceramics. These vacancies lead to domain wall pinning under the drive of the applied voltage.

[0043] To investigate the effect of Nd doping on the electronic conductivity of ceramics, DC resistivity measurements were performed in the temperature range of 200°C to 600°C. Figure 9 As shown in (a), the ρ of all ceramics decreases with increasing temperature, indicating that charge carriers are activated at high temperatures. Using W... 6+ Replace Ti 4+ Subsequently, the ρ of the ceramic increased by two orders of magnitude. This improvement is attributed to the decrease in oxygen vacancy concentration, as the excess electrons generated by substitution compensate for the oxygen vacancies created by the volatilization of bismuth oxide. The defect reaction equation is as follows: When Nd replaces Bi, the resistivity increases slightly, reaching its maximum at composition x=0.03 (7.85×10⁻⁶ at 500°C). 6 Ω·cm). To elucidate the electrical conductivity mechanism of ceramics, the activation energy for thermal conduction (E) was calculated based on the Arrhenius equation. a ),like Figure 9As shown in (bc): Where ρ0 refers to the exponential factor, and k B Here, is Boltzmann's constant, and T is the absolute temperature. Clearly, ceramics exhibit different conductivity mechanisms in the low-temperature range (LT, 200-400°C) and the high-temperature range (HT, 400-600°C). In the low-temperature region, the activation energy of ceramics ranges from 1.11 to 1.19 eV, indicating that its conductivity mechanism is mainly dominated by intrinsic carriers (p-type, such as oxygen vacancies). In the high-temperature region, the activation energy ranges from 1.52 to 1.79 eV, approximately half the theoretical band gap (3.3 eV) of BIT-based ceramics. This indicates that intrinsic conductivity dominates in the high-temperature region.

[0044] The impedance spectra of all ceramics were measured at selected temperatures and within a frequency range of 20 Hz to 1 MHz. Figure 10 As shown in (af). Typically, ceramic impedance spectra can be divided into three regions: low frequency, mid frequency, and high frequency, corresponding to the conductivity effects at grain boundaries, grain boundaries, and electrode interfaces, respectively. The impedance spectra of all ceramic samples exhibit a semi-circular shape, with the center of the arc located below the real part of the impedance spectrum. This indicates that the ceramic possesses excellent electrical homogeneity and exhibits non-Debye relaxation behavior. Figure 10 (f) shows the capacitance (C') of the ceramic at 500°C as a function of frequency. Notably, at this temperature, the conductivity of all ceramic samples is primarily dominated by the grain response. Furthermore, the impedance arc radius gradually decreases with increasing temperature, indicating that the ceramic exhibits negative temperature coefficient behavior. The impedance values ​​of all doped ceramic samples are significantly higher than BIT, consistent with the change in resistivity.

[0045] In summary, Bi was successfully synthesized via a solid-state reaction method. 4- xNd x Ti 2.925 W 0.075 O 12 Piezoelectric ceramics. The effects of A-site Nd doping and B-site W substitution on the structure and electrical properties of BIT ceramics were systematically investigated. Results show that this doping method effectively reduces oxygen vacancy concentration and inhibits grain growth, significantly improving high-temperature resistivity. Due to the mismatch between the ionic radius and chemical valence state of the dopant elements, the crystal structure undergoes significant distortion. Simultaneously, doping disrupts the original domain structure, leading to a reduction in domain size and a significant enhancement in piezoelectric response. Notably, when x = 0.03, the ceramic exhibits a high dc / N ratio of 25.8 at 500°C. 33 and 7.85×10 6 The material exhibits excellent ρ values ​​of Ω·cm. Furthermore, its high TC (temperature coefficient) of 646°C and good thermal stability make it a potential candidate for high-temperature energy harvesting and conversion applications.

Claims

1. A piezoelectric ceramic, characterized in that: By Nd 3+ Replacement of Bi4Ti3O 12 Bi in 3+ W 6+ Replacement of Bi4Ti3O 12 Ti in 4+ get; The piezoelectric ceramic has the structural formula Bi. 4-x Nd x Ti 2.925 W 0.075 O 12 , where 0.01≤x≤0.

05.

2. The piezoelectric ceramic as described in claim 1, characterized in that: The piezoelectric ceramic has the structural formula Bi. 3.99 Nd 0.01 Ti 2.925 W 0.075 O 12 Bi 3.97 Nd 0.03 Ti 2.925 W 0.075 O 12 Or Bi 3.95 Nd 0.05 Ti 2.925 W 0.075 O 12 .

3. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: Includes the following steps: Bi2O3, Nd2O3, WO3, and TiO2 are weighed according to the stoichiometric ratio, mixed, ball-milled, dried, granulated, pressed into shape, degummed, and sintered to obtain the final product.

4. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: The ball milling time is 4-6 hours.

5. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: The drying temperature is 50°C; The drying time is 3 hours.

6. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: The pressure for compression molding is 120-130 MPa.

7. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: The degumming temperature is 600℃; The degumming time is 2 hours.

8. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: The sintering temperature is 1020-1120℃.

9. The method for preparing piezoelectric ceramics as described in claim 1, characterized in that: The sintering time is 3 hours.

10. The application of the piezoelectric ceramic as described in claim 1, characterized in that: It is used in the manufacture of sensors, filters or transducers.

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