CaBi2Nb2O9 ceramic and preparation method and application thereof

Abstract

This invention provides a CaBi₂Nb₂O₉ ceramic, wherein the area of ​​ferroelectric domains in any cross-section of the CaBi₂Nb₂O₉ ceramic accounts for 14-18% of the cross-sectional area; and the length of the ferroelectric domains in the CaBi₂Nb₂O₉ ceramic is 234-596 nm, and the thickness is 41-196 nm; the inverse piezoelectric coefficient extrapolated from the zero electric field amplitude of the CaBi₂Nb₂O₉ ceramic is 6.5-6.8 pm / V; and the Rayleigh coefficient of the CaBi₂Nb₂O₉ ceramic is 0.15-0.18 × 10⁻⁶. ‑17 m 2 / V 2 Furthermore, the CaBi₂Nb₂O₉ ceramic at room temperature... 33 The C / N ratio is 13-13.5 pC. This invention also discloses the preparation method and application of the CaBi₂Nb₂O₉ ceramic.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ceramic preparation, in particular to a preparation method of bismuth layer-structured ferroelectric ceramic. BACKGROUND

[0002] Due to the ability of piezoelectric ceramics to convert electrical energy and mechanical energy, they are widely used in actuators, sensors, energy harvesters and other devices. Severe environmental applications such as nuclear reactors and aircraft turbines require a special type of piezoelectric ceramic that can work at high temperatures above 600℃. However, high-performance perovskite piezoelectric ceramics, including lead zirconate titanate (PZT), barium titanate (BT) and potassium sodium niobate (KNN), have a Curie temperature lower than 500℃. On the other hand, high Curie temperature piezoelectric ceramics, such as bismuth layer-structured ferroelectric (BLSF) ceramics, have poor piezoelectric coefficients (d 33 ) and low resistivity (ρ). Therefore, there is a strong driving force to improve the piezoelectric response of high Curie temperature BLSF ceramics.

[0003] BLSF ceramics are represented by the formula (Bi2O2) 2+ (A m-1 B m O 3m+1 ) 2- , where A represents Li + , Ca 2+ , Sr 2+ , Bi 3+ , La 3 + , Ce 3+ and other twelve-coordinate ions, and B represents octahedral coordination ions such as Cr 3+ , Co 3+ , Fe 3+ , Mn 4+ , Ti 4+ , Mo 5+ , W 6+ , etc. CaBi2Nb2O9 (CBN, m = 2) is a representative BLSF, bismuth calcium niobate, which exhibits almost the highest Curie temperature (about 940℃) in the BLSF series. CBN ceramics are a promising candidate material for use as a key material for vibration sensors, and CBN-made sensing elements can convert vibrations into electrical signals at temperatures above 600℃. However, the spontaneous polarization of CBN ceramics is limited along the a-b plane, resulting in poor d 33 and large coercive field. In addition, the volatilization of Bi2O3 during conventional high-temperature sintering often leads to a large number of defects, thereby reducing the resistivity of the ceramic. Current extensive research focuses on developing composition engineering methods to adjust its microstructure and electrical properties. For example, by A-site, B-site or AB-site doping, the d33 and p dc (600℃) are increased to 13-20 pC / N and 10 5 -10 6 Ω·cm, respectively. However, the conventional sintering requires a large amount of time and a high sintering temperature, and more importantly, can cause defects, such as vacancy defects, pores. In addition, researchers have also developed a textured densification sintering method, including spark plasma sintering (SPS), hot-press sintering (HP) and reaction template grain growth (TGG), to prepare a textured piezoelectric ceramic, which exhibits enhanced piezoelectric response in a specific direction. However, the larger piezoelectric response along the textured direction is usually accompanied by an unfavorable high electrical conductivity.

[0004] Therefore, how to improve the piezoelectric response of the high Curie temperature BLSF ceramic becomes a problem to be solved. SUMMARY

[0005] The purpose of the present application is to provide a preparation method of a high d 33 and p dc Bismuth layer-structured ferroelectric ceramic.

[0006] The present application is realized by the following technical solutions:

[0007] A CaBi2Nb2O9 ceramic, the area of the ferroelectric domain in any cross section of the CaBi2Nb2O9 ceramic accounts for 14-18% of the cross section area; and the length of the ferroelectric domain in the CaBi2Nb2O9 ceramic is 234-596 nm, and the thickness is 41-196 nm; and the inverse piezoelectric coefficient of the zero electric field amplitude extrapolation of the CaBi2Nb2O9 ceramic is 6.5-6.8 pm / V; and the Rayleigh coefficient of the CaBi2Nb2O9 ceramic is 0.15-0.18×10 -17 m 2 / V 2 ; and the d 33 of the CaBi2Nb2O9 ceramic at room temperature is 13-13.5 pC / N. The preparation method of the CaBi2Nb2O9 ceramic comprises the following steps:

[0008] After adding 2-3 wt% of Bi2O3 to the raw material powder of CaCO3, Bi2O3 and Nb2O5 with a molar ratio of 1:1:1, a raw material is obtained;

[0009] After ball milling, drying and calcining the raw material, mixing with a binder, ball milling again, and forming a green body;

[0010] After degreasing the green body, microwave sintering at 935-945℃, the bismuth layer-structured ferroelectric ceramic is obtained.

[0011] The grinding balls used in the ball milling include zirconium dioxide balls;

[0012] The medium used in the ball milling includes ethanol;

[0013] The time of the ball milling is 20-30h.

[0014] The temperature of the drying is 40-60℃;

[0015] The time of the drying is 10-14h.

[0016] The forming includes the step of axial pressing.

[0017] The pressure of the axial pressing is 150MPa.

[0018] The temperature of the glue removing is 600-700℃.

[0019] The temperature of the calcining is 800-900℃;

[0020] The time of the calcining is 2-3h.

[0021] The holding time of the microwave sintering is 9-11min.

[0022] The frequency of the microwave used in the microwave sintering is 2.45GHz, and the power is 250-1250W.

[0023] The binder includes ethyl cellulose, hydroxymethyl cellulose or methyl cellulose.

[0024] The CaBi2Nb2O9 ceramic can be applied to prepare a vibration sensor.

[0025] The working temperature of the vibration sensor is 0-700℃.

[0026] Compared with the prior art, the present application has the following advantages:

[0027] The CaBi2Nb2O9 ceramic provided by the present application has thinner domain walls and higher domain wall density, so that the piezoelectric performance is significantly enhanced.

[0028] The preparation method of the CaBi2Nb2O9 ceramic provided by the present application can significantly improve the domain wall density of the CaBi2Nb2O9 ceramic and reduce the thickness of the domain wall by using microwave sintering (MS).

[0029] The CaBi2Nb2O9 ceramic provided by the present application can convert vibration into an electric signal at a temperature of 700℃, so that it can be used to prepare a vibration sensor that works at high temperature. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 A shows the XRD pattern of CS1060, MS920, MS940, MS960 and MS980.

[0031] Figure 1 B shows the Figure 1 A shows the enlarged peak pattern of the (008) peak.

[0032] Figure 1 C shows the Rietveld-refined XRD pattern of CS1060.

[0033] Figure 1 D shows the Rietveld-refined XRD pattern of MS940.

[0034] Figure 2 A shows the SEM image of the surface morphology of CS1060.

[0035] Figure 2 B shows the SEM image of the surface morphology of MS980.

[0036] Figure 2 C shows the SEM image of the surface morphology of MS960.

[0037] Figure 2 D shows the SEM image of the surface morphology of MS940.

[0038] Figure 2 E shows the SEM image of the surface morphology of MS920.

[0039] Figure 2 F shows the average grain size variation and length / thickness ratio of the surface of CS1060, MS980, MS960, MS940, MS920 measured by NanoMeasurer.

[0040] Figure 3 A shows the SEM image of the internal morphology of CS1060.

[0041] Figure 3 B shows the SEM image of the internal morphology of MS980.

[0042] Figure 3 C shows the SEM image of the internal morphology of MS960.

[0043] Figure 3 D shows the SEM image of the internal morphology of MS940.

[0044] Figure 3 E shows the SEM image of the internal morphology of MS920.

[0045] Figure 3 F shows the average grain size variation and length / thickness ratio plots for CS1060, MS980, MS960, MS940, MS920, measured by NanoMeasurer.

[0046] Figure 4 A shows the XPS spectra of the O 1s core level obtained from CS1060 and MS 940 samples and Gaussian-Lorentzian peak fitting plots.

[0047] Figure 4 B shows the spectra of the Bi 4f core level obtained from CS1060 and MS 940 samples and Gaussian-Lorentzian peak fitting plots.

[0048] Figure 5 A shows the d 33 variation as a function of the ceramic thermal depolarization temperature for CS1060, MS980, MS960, MS940, MS920.

[0049] Figure 5 B shows the relative d 33 variation as a function of the ceramic thermal depolarization temperature for CS1060, MS980, MS960, MS940, MS920.

[0050] Figure 6 A shows the surface topography plot of CS1060 after polishing.

[0051] Figure 6 B shows the surface topography plot of MS940 after polishing.

[0052] Figure 6 C shows the amplitude plot of CS1060 after polishing.

[0053] Figure 6 D shows the amplitude plot of MS940 after polishing.

[0054] Figure 6 E shows the phase plot of CS1060 after polishing.

[0055] Figure 6 F shows the phase plot of MS940 after polishing.

[0056] Figure 7 A shows the monopolar strain-electric field plot of CS1060.

[0057] Figure 7 B shows the monopolar strain-electric field plot of MS940.

[0058] Figure 7 C shows the d 33 * = SP Calculated d 33 Data points in the graph, and linear fit of these data sets.

[0059] Figure 8 A shows the plot of resistivity versus temperature for CS1060 and MS940 ceramics.

[0060] Figure 8 B shows the plot of straight line fit by Arrhenius function for CS1060 and MS940 ceramics.

[0061] Figure 9 A shows the plot of impedance spectra for CS1060 ceramic in the temperature range 400-700℃.

[0062] Figure 9 B shows the plot of impedance spectra for MS940 ceramic in the temperature range 400-700℃.

[0063] Figure 10 A shows the plot of dielectric constant versus temperature at 1MHz for CS1060 and MS940 ceramics, Figure 10 The inset in A is an enlarged view at temperatures above 900℃.

[0064] Figure 10 B shows the plot of dielectric constant versus temperature at 10kHz, 100kHz, 1MHz for CS1060, Figure 10 The inset in B shows the plot of dielectric constant at 500-600℃.

[0065] Figure 10 C shows the plot of dielectric constant versus temperature at 10kHz, 100kHz, 1MHz for MS940, Figure 10 The inset in C shows the plot of dielectric constant at 500-600℃. DETAILED DESCRIPTION

[0066] The characterization method of the CaBi2Nb2O9ceramic provided by the application is as follows:

[0067] The bulk density of the ceramic is determined according to the Archimedes principle.

[0068] X-ray diffraction (XRD) patterns of the ceramic are collected using an X-ray diffractometer (MiniFlex600, Rigaku) with Cu-Ka radiation (λ = 0.15406 nm) from 20° to 60°, with a step of 0.02°, and the step for Rietveld refinement data is 0.01°.

[0069] XPS spectra were acquired using an Al target (h v = 1486.6 eV) x-ray photoelectron spectrometer (SHIMADZU, AXIS SUPRA+) and calibrated using the C1s peak at 284.8 eV.

[0070] The microstructure of sintered ceramics was characterized using a scanning electron microscope (ZEISS).

[0071] To investigate the electrical properties of the ceramics, the samples were polished to about 0.4 mm thickness on 600-grit sandpaper, and then coated with silver / palladium paste on both sides. The electrodes were obtained after heat treatment at 600°C for 10 minutes. 33 The piezoelectric d 33 33 was measured by quasi-static piezoelectric d

[0072] The thermal stability of the ceramic samples was evaluated by measuring the change in room temperature d 33 after annealing at different high temperatures from 200 to 930°C for 2 hours.

[0073] The ferroelectric domains under PFM mode were observed using an atomic force microscope (Asylum Research MFP 3D).

[0074] Unipolar S-E hysteresis loops were measured using a ferroelectric analyzer (TF-2000, aixACCT, Germany), and then the d 33 * as a function of the applied electric field.

[0075] The temperature dependence of the resistivity was collected from 200°C to 700°C at a heating rate of 5°C / min by a resistivity test system (RMS1000+6517B).

[0076] The change in dielectric constant with temperature at different frequencies was measured by a dielectric temperature measurement system (DMS1000+WK6500). The impedance spectroscopy was measured using a high-temperature impedance thermograph (DMS1000+WK6500) in the temperature range of 400 to 700°C. The method for preparing CaBi2Nb2O9 ceramics (abbreviated as CBN) according to the present invention is as follows:

[0077] The raw material powders [CaCO3(99.5%), Bi2O3(99.99%), Nb2O5(99.99%)] were weighed in stoichiometric ratio 1 : 1 : 1. In order to reduce the negative effect of Bi2O3evaporation during calcination and subsequent sintering, an excess of 2.7 wt.% Bi2O3was added to the raw material powders. Then, the powder mixture was milled in a ball mill for 24 hours with the aid of zirconia balls and ethanol (99.7%). The milled slurry was dried in an oven at 50 °C for 12 hours and then calcined at 850 °C for 2.5 hours. The resulting powder was again pulverized under the same conditions. Subsequently, green bodies with a diameter of 10 mm and a thickness of 0.8 mm were obtained by axial pressing at a pressure of 150 MPa after the addition of a binder. The binder can be ethyl cellulose. Finally, the green bodies were sintered using a silicon molybdenum furnace (i.e. conventional sintering method, abbreviated as CS) at 1060 °C for 2 hours (where the total heating and cooling time is about 10 hours), and after binder removal at 650 °C, using a microwave sintering furnace (i.e. microwave sintering, abbreviated as MS) at 920 °C, 940 °C, 960 °C, 980 °C for 10 minutes (where the total heating and cooling time is about 2 hours). In the MS experiments, the samples were heated to the designed sintering temperature at a heating rate of 10 °C / min, and after sintering for 10 min, the furnace was turned off and the samples were allowed to cool to room temperature. The sample sintered at 1060 °C is denoted as CS1060.

[0078] The sample sintered at 920 °C is denoted as MS920.

[0079] The sample sintered at 940 °C is denoted as MS940.

[0080] The sample sintered at 960 °C is denoted as MS960.

[0081] The sample sintered at 980 °C is denoted as MS980.

[0082] In particular, the furnace chamber of the microwave sintering furnace is provided with a silicon carbide plate. The microwave sintering furnace has a working frequency of 2.45 GHz, and the sample and the external silicon carbide plate are heated at a high power of 250-1250 W. The silicon carbide plate can also absorb microwave energy and rapidly increase the temperature as an auxiliary heater in the early stage of sintering.

[0083] For the CS method, heat is transferred to the green body from the surrounding heat source by convection, conduction or radiation. In other words, in the CS process of CBN ceramics, heat is transferred from the surface to the interior. In this way, a non-uniform temperature field is generally generated in the interior of the green body, leading to inconsistent grain crystallization kinetics and inconsistent shrinkage of the ceramic body. High sintering temperature (e.g. 1060 °C) and long sintering time (~10 hours) are not favorable for CBN ceramics, because the volatilization of bismuth oxide will produce oxygen vacancies in the sample. Therefore, the piezoelectric coefficient (d33 ) poor, resistivity (p dc ) low.

[0084] However, the sintering mechanism of MS is different from that of CS. The microwave source generates high-energy microwaves of a certain frequency (2.45 GHz) that resonate with the molecules inside the sample, providing a large amount of energy for uniform heating. In addition, the surrounding external silicon carbide plate acts as a supplemental heat source, helping to achieve a uniform temperature field. Compared with CS, MS has a lower atomic diffusion barrier, leading to accelerated atomic diffusion and rapid sintering. Therefore, MS is beneficial for shortening the sintering time (2 hours) and obtaining a dense, fine, and uniform grain morphology. Figure 1 A shows a comparison of XRD patterns obtained from CS 1060, MS 980, MS 960, MS 940, and MS 920 samples. The diffraction peaks of all ceramic samples can be indexed to the orthorhombic phase with the standard card PDF #49-0608. The highest diffraction peak is denoted as the (115) diffraction plane, which corresponds to the strongest diffraction plane (112m+1) in BLSF ceramics, as the m value of CBN is equal to 2. As shown in Figure 1 B, in addition to the (008) diffraction peak, another small peak can be observed at a higher angle on the shoulder of the (008) peak for all MS samples. Notably, other small peaks from impurities can be observed in other regions. Interestingly, the XRD pattern of CBN ceramics sintered at 940°C using the CS method also shows these small peaks. Therefore, due to the low microwave sintering temperature and Bi2O3 excess, this method can cause a small amount of unexpected bismuth-containing secondary phases to appear in CBN.

[0085] To further investigate the effect of MS on CBN ceramics, Rietveld refinement was performed on all samples in the 10-90° range according to the crystal model reported by Black et al. As shown in Figure 1 B and C, the calculated results are consistent with the collected XRD data, with R P (5.99-7.24%), R wp (9.27-10.3%), and Chi2 (4.24-5.11) are also within a reasonable range. Table 1 lists the calculated lattice parameters a, b, c, and orthorhombic parameter 2(a-b) / (a+b). The orthorhombic parameter of microwave sintered ceramics is lower than that of traditional sintered samples. In addition, the parameter of MS 920 is significantly reduced compared to other microwave sintered samples, which may be due to the sintering temperature being lower than the Curie temperature and the cooling rate being significantly faster than traditional sintering. In addition, the parameter of MS 920 is significantly reduced compared to other microwave sintered samples. These results indicate that MS is beneficial for improving the tetragonality of CBN.

[0086] Table 1. Structural parameters of CS 1060, MS 920, MS 940, MS 960, and MS 980 ceramics.

[0087]

[0088] Figure 2 (A)-(E) show the surface morphology of CBN ceramics as revealed by SEM images. From the images, it can be seen that the CBN ceramics are tightly packed with plate-like or rod-like grains, and there are no obvious pores or intergranular impurities in the CBN ceramics. In traditional sintered ceramics, small rod-like grains are located at the junction of several large plate-like grains Figure 2 A), which leads to a wide grain size distribution and a large length / thickness ratio Figure 2 F). This phenomenon is mainly due to the inherent layered structure of BLSF and the inhomogeneous temperature field in the ceramic plate during traditional sintering, which leads to preferential growth along the a-b plane. In contrast, the grains of microwave sintered ceramics exhibit a uniform rod-like structure. Similar morphology, the grain size is about 1 μm Figure 2 (B-E)), which is much smaller than that of traditional sintered ceramics (3.5 μm). As the sintering temperature decreases, the average grain size and length / thickness ratio gradually decrease Figure 2 F). Direct volume heating in the green body will provide a relatively uniform temperature field, which is conducive to grain growth within the temporary microwave sintering duration (10 minutes). In addition, both low sintering temperature and residence time can produce small grain size and obvious porosity, as shown in the MS 920 sample. Since the sintering mechanism of the MS method is different from that of the CS method, the present application performs additional SEM characterization by polishing representative bulk samples, reducing their thickness from ~0.8 mm (surface) to ~0.5 mm (interior). After high-temperature etching at 920°C, the microstructure inside these samples is revealed using scanning electron microscopy techniques. As shown in Figure 3 , the microstructure inside the samples is consistent with the surface microstructure shown in Figure 2 . Compared with CS samples, the average grain size of MS samples is significantly reduced. In addition, the grain size of the MS 940 sample is the smallest, and the anisotropy is significantly reduced. Table 2 shows the density of CS 1060 and MS 940 ceramics. The relative density and theoretical density of MS 940 ceramics are both greater than those of CS 1060 ceramics, which further verifies that microwave sintering can reduce sintering defects and vacancies, and improve the degree of densification.

[0089] Table 2. Comparison of experimental density, theoretical density, and relative density of CS 1060 and MS 940 ceramics

[0090]

[0091] To detect the species of volatiles during sintering, the green bodies were put into a high-temperature furnace at 650 °C to burn off the binder, then their initial mass was measured, followed by CS and MS, respectively, and finally weighed again after sintering to obtain the mass loss during sintering. From Table 3, the mass volatilization ratios of CS1060 and MS 940 samples before and after sintering were 0.57% and 0.13%, respectively. We noticed that the melting points of raw materials Bi2O3, Nb2O5, and CaO were 825 °C, 1520 °C, and 2572 °C, respectively. Since Bi2O3 has the lowest melting point, which is much lower than the sintering temperature (940-1060 °C) of CS and MS, it can be preliminarily speculated that the reduced part is mainly Bi2O3 compounds. This result shows that the MS method can inhibit the volatilization of Bi2O3. To further confirm the volatilization of Bi2O3, the samples were characterized by XPS, and the spectral changes of O1s and Bi 4f in MS 940 and CS1060 samples were compared by Gaussian-Lorentz peak fitting. As shown in Figure 4 A, the area ratio [b / (a+b)] of the b peak at 531.6 eV decreased from 62.5% in the CS1060 sample to 52.6% in the MS 940 sample. This observation indicates that the oxygen vacancy concentration in the MS 940 sample is significantly lower than that in the CS1060 sample. In addition, as shown in Figure 4 A-B, the binding energies of O1s and Bi 4f peaks in the spectrum of the MS 940 sample are obviously shifted to a higher direction compared with the CS1060 sample, further indicating that the chemical bond between bismuth and oxygen atoms in the MS 940 sample is more compact than that in the CS1060 sample. This is another evidence for the lower oxygen vacancy concentration in the MS 940 sample.

[0092] Table 3. Mass volatilization ratios of CS1060 and MS 940 samples before and after sintering.

[0093]

[0094] To examine the difference in piezoelectric response between microwave-sintered ceramics and conventionally sintered ceramics, d 33 and thermal depoling were characterized. As shown in Figure 5 A and Figure 5 B, the d 33 of the CS1060 ceramic was 7.2 pC / N. In the MS series ceramics, when the sintering temperature was 940 °C, the d 33 was as high as 13.4 pC / N. By using microwave sintering, the significant enhancement of d 33 is attributed to the smaller grain size and the improved densification degree of the MS ceramics. First, the domain size depends on the grain size, with the relationship domain size ∝ (grain size) mThe increase in domain wall density in small-sized ceramics is beneficial to the improvement of piezoelectric response. Secondly, smaller grains and higher densification usually allow CBN ceramics to sustain higher breakdown electric field. Therefore, these fine-grained samples are more likely to be fully poled. In the present invention, the MS-prepared CBN samples have smaller grains and denser morphology, which can sustain higher poling electric field (18 kV / mm) compared to the CS-prepared CBN samples (14 kV / mm).

[0095] Since thermal stability is crucial for high-temperature applications, the present invention measured d 33 The variation of d Figure 5 As the annealing temperature increased to 400 °C, d 33 Only dropped by about 10%, which can be attributed to unstable non-180° domains. As the temperature rose above 800 °C, both the conventionally sintered and the microwave sintered ceramics established severe thermal depoling, and as the depoling temperature further increased to 930 °C close to the Curie temperature and microwave sintering, d 33 Dropped significantly. As for MS 920, d 33 Continued to drop as the annealing temperature increased, which is most likely caused by incomplete sintering. These results strongly suggest that microwave sintering is a viable method to improve the piezoelectric coefficient and thermal stability of CBN ceramics.

[0096] To gain insight into the improvement of piezoelectric response, the present invention observed the ferroelectric domains of MS 940 and CS 1060 samples using piezoresponse force microscopy (PFM) as shown in Figure 6 The amplitude image shows that the amplitude intensity of the MS 940 sample is nearly twice as large as that of the CS 1060 sample, which indicates that the piezoelectric response of the MS 940 sample is larger. This observation is in very good agreement with the results of strain-field experiments (see the Rayleigh analysis below). In addition, the MS 940 sample shows thinner and more uniformly distributed stripe domains. The PFM study clearly indicates that the domain wall density of the MS 940 sample is increased compared to the CS 1060, and the thinner domain walls and higher domain wall density are beneficial to the enhancement of piezoelectric performance.

[0097] To demonstrate the contribution of domain wall motion to the piezoelectricity, the present invention tested the unipolar S-E hysteresis loops of the samples using a ferroelectric analyzer (TF-2000, aixACCT, Germany) and obtained d 33 * as a function of the applied electric field. The inverse piezoelectric strain coefficient under different electric fields is obtained from the following equation. As shown in Figure 7As shown in AB, within the measured electric field range, both MS 940 and CS1060 samples exhibit unsaturated unipolar SE curves, indicating that MS 940 and CS1060 can trigger domain wall motion under the action of an applied electric field. This invention also uses formula (1) to calculate the inverse piezoelectric strain coefficient under various electric field amplitudes.

[0098] Formula (1) is d 33 * =S P / E0. Where S P Here, E0 represents the peak strain at each maximum electric field amplitude, and d represents the corresponding maximum electric field amplitude. 33 * It is the inverse piezoelectric coefficient.

[0099] Formula (2) can be used to study the Rayleigh behavior of piezoelectric strain coefficient and electric field amplitude.

[0100] Formula (2) is d 33 * =d init +αE where d init α is the inverse piezoelectric coefficient extrapolated from zero electric field amplitude, α is the Rayleigh coefficient, E is the electric field amplitude, and αE is the contribution of irreversible domain wall motion.

[0101] like Figure 7 As shown in Figure C, compared to the CS1060 sample, the MS 940 sample has a larger d... init (~6.76 pm / V) and α (~0.17 × 10⁻⁶) -17 m 2 / V 2 The value is given by αE, where αE is the contribution of irreversible domain wall motion, including intrinsic lattice strain and intrinsic reversible domain wall motion. Therefore, the d... init The increase in α value indicates that domain wall motion is more readily observed. Furthermore, domain wall motion, including both reversible and irreversible domain wall motion, leads to a nonlinear piezoelectric response and strain hysteresis. Larger strain hysteresis and residual strain also confirm the enhanced domain wall motion in the MS 940 specimen. PFM measurements indicate that the enhanced domain wall motion is primarily due to the reduction in domain size. 33 *Greater than d 33 This may be due to the large coercive field in CBN ceramics, making complete polarization difficult during the polarization process. (Except for d...) 33 In addition, resistivity (ρ) at high temperatures dc This is also crucial for high-temperature vibration sensor applications. The ρ of the MS 940 and CS1060... dc Relationship with temperature as follows Figure 8 As shown in Figure A, the MS 940 sample exhibits a larger ρ... dc ρ of MS 940 and CS1060 at 600℃dc 7×10 5 and 4×10 5 Ω·cm, which is better than the existing CBN value (~6×10). 4 The value of Ω·cm is nearly an order of magnitude larger. ρ dc The enhancement is attributed to improved densification, as demonstrated by the increased density achieved using MS compared to the CS method. In principle, the conduction mechanism in different temperature regions can be explained using the Arrhenius equation, as follows:

[0102]

[0103] Where ρ0 refers to the pre-factor, E a It is the activation energy for conduction, k B is Boltzmann's constant, and T is the absolute temperature. Figure 8 B shows the ρ of MS 940 and CS1060. dc As temperature changes, these curves can be divided into low-temperature (≤300℃) and high-temperature (≥300℃) regions. In the low-temperature region, the E values ​​of MS 940 and CS1060... a The values ​​are 1.114 eV and 1.096 eV, respectively. a Values ​​in the range of 0.6–1.4 eV indicate that conductivity is primarily due to oxygen vacancies and holes. In the high-temperature region, the Ea values ​​of MS 940 and CS1060 increase to 1.463 eV and 1.366 eV, respectively. a The values ​​are in the range of 1.4–1.8 eV, indicating that the conduction mechanism of MS is intrinsically thermally activated conduction, while CS is still mainly composed of oxygen vacancies and holes. MS 940 exhibits higher E values ​​at high temperatures. a This is likely because the MS method can mitigate the volatilization problem of bismuth oxides, thereby suppressing the generation of oxygen vacancies, as revealed by XPS analysis and mass loss measurement.

[0104] Figure 9 Figures A and B show the impedance spectra of CS1060 and MS 940 ceramics at 400-700℃, respectively. The impedance decreases with increasing temperature, which is consistent with... Figure 8 The temperature dependence of ρdc is consistent. The impedance spectrum of CS1060 ceramics exhibits three semicircles, while MS 940 shows almost only one semicircle. (See attached image.) Figure 9 As shown in Figure A, the grain effect (R) in the CS sample... G ), grain boundary effect (R GB The grain boundary effect (R) and ion diffusion effect (W) formed three semi-circular patterns in the CS sample at high temperatures. GB Both the ion diffusion effect (W) and the ion diffusion effect appear to be suppressed. Figure 9 B).

[0105] To investigate the effect of microwave sintering on the dielectric properties of CBN ceramics, the temperature dependence of the dielectric constant of CS1060 and MS 940 ceramics at 1 MHz was collected. Figure 10 As shown in A, the dielectric constant of CS1060 and MS 940 ceramics increases with increasing temperature, and rapidly decreases after reaching the Curie point, which is related to the transition of CBN ceramics from ferroelectric phase to paraelectric phase. The Curie point also increases slightly from 931℃ of CS to 935℃ of MS. As shown in B-C, the dielectric peak of CS1060 sample appears between 500℃ and 600℃, and weakens with increasing frequency. In MS 940 sample, the dielectric peak disappears, which corresponds to the suppression of oxygen vacancies in the ceramic. Figure 10

[0106] In summary, the present application first provides a CaBi2Nb2O9 ceramic, the area of ferroelectric domain in any cross section of the CaBi2Nb2O9 ceramic accounts for 14-18% of the cross section area; and the length of the ferroelectric domain in the CaBi2Nb2O9 ceramic is 234-596nm, and the thickness is 41-196nm; due to the above-mentioned thinner and more elongated domain structure, and more quantity, the domain wall motion is easier. The increase of domain wall motion can improve the piezoelectric coefficient of CaBi2Nb2O9 ceramic. Therefore, the d 33 of the CaBi2Nb2O9 ceramic at room temperature is 13-13.5pC / N. The inverse piezoelectric coefficient of the CaBi2Nb2O9 ceramic extrapolated from the zero-field amplitude is 6.5-6.8pm / V; and the Rayleigh coefficient of the CaBi2Nb2O9 ceramic is 0.15-0.18×10 -17 m 2 / V 2 . The increase of the inverse piezoelectric coefficient d init and the Rayleigh coefficient α value indicates that the domain wall motion is easier.

[0107] The CaBi2Nb2O9 ceramic provided by the present application is prepared by a microwave sintering method. The microwave sintering can shorten the holding time of sintering and thus reduce the volatilization of bismuth oxide, which makes the domain structure of CaBi2Nb2O9 ceramic thinner and more elongated, and easier to produce domain wall motion. The domain in CaBi2Nb2O9 ceramic is a small area with the same spontaneous polarization direction in ferroelectric body. Obviously, the oxygen vacancies caused by the volatilization of bismuth oxide will prevent the formation of this structure. At the same time, the control of sintering temperature is also important. When the CaBi2Nb2O9 ceramic is sintered at about 940℃, the sample can quickly complete the sintering, saving a lot of time and energy, and obtaining a higher d 33 and resistivity, and the piezoelectric ceramic with more uniform and smaller grains and higher density.

[0108] ​Specifically, the microwave sintering comprises the following steps:

[0109] The raw material powder is obtained by adding 2-3wt% Bi2O3 to CaCO3, Bi2O3, Nb2O5 raw material powder with a molar ratio of 1:1:1; the raw material is ball milled, dried and calcined, and then ball milled with a binder to form a green body; the green body is degreased and then microwave sintered at 935-945℃ to obtain the bismuth layer-structured ferroelectric ceramic.

[0110] The CaBi2Nb2O9 ceramic can be applied to prepare a vibration sensor. Since the piezoelectric coefficient of the CaBi2Nb2O9 ceramic starts to decrease at 800℃, the working temperature of the vibration sensor prepared from the CaBi2Nb2O9 ceramic is 0-700℃.

[0111] Meanwhile, the relative density of the microwave sintered CBN ceramic is 99.6%, higher than 98.5% of the conventional sintered ceramic. After microwave sintering, the average grain size is reduced from 3.5μm to 1μm, and the grain size distribution is more uniform. The d 33 (13.4pC / N) of the microwave sintered CBN ceramic is much higher than the d 33 (7.2pC / N) of the conventional sintered CBN ceramic. The significant enhancement of d 33 is due to the increased domain wall density and more complete polarization of the CBN ceramic with reduced grain size and densification. At 600℃, the ρ dc (7×10 5 Ω·cm) of the MS 940 is higher than the ρ dc (4×10 5 Ω·cm) of the CS 1060, which is due to the inhibition of Bi2O3 volatilization and the reduction of the number of vacancy defects. In summary, compared with the CBN ceramic prepared by CS, the CBN ceramic prepared by MS has higher piezoelectric response and resistivity at high temperature, and can be applied to prepare high-temperature vibration sensors.

Claims

1. A CaBi2Nb2O9 ceramic, characterized in that: the area of ferroelectric domains in any cross section of the CaBi2Nb2O9 ceramic accounts for 14-18% of the cross section area; the length of the ferroelectric domains in the CaBi2Nb2O9 ceramic is 234-596 nm, and the thickness is 41-196 nm; the inverse piezoelectric coefficient extrapolated from the zero-field amplitude of the CaBi2Nb2O9 ceramic is 6.5-6.8 pm / V; and the d33 of the CaBi2Nb2O9 ceramic at room temperature is 13-13.5 pC / N.

2. The CaBi2Nb2O9 ceramic of claim 1, characterized in that: the grinding balls used in the ball milling include zirconia balls; the medium used in the ball milling includes ethanol; the ball milling time is 20-30 h; and the binder includes ethyl cellulose, hydroxymethyl cellulose or methyl cellulose.

3. The CaBi2Nb2O9 ceramic of claim 1, characterized in that: the drying temperature is 40-60 °C; and the drying time is 10-14 h.

4. The CaBi2Nb2O9 ceramic of claim 1, characterized in that: the forming includes the step of axial pressing. And the CaBi2Nb2O9 ceramic has a Rayleigh coefficient of 0.15-0.18x10 -17 m 2 / V 2 ; 5. The CaBi2Nb2O9 ceramic of claim 4, characterized in that: the axial pressing pressure is 150 MPa.

6. The CaBi2Nb2O9 ceramic of claim 1, characterized in that: the degumming temperature is 600-700 °C.

7. The CaBi2Nb2O9 ceramic of claim 1, characterized in that: the calcination temperature is 800-900 °C; the calcination holding time is 2-3 h; the microwave sintering holding time is 9-11 min; and the microwave used in the microwave sintering has a frequency of 2.45 GHz and a power of 250-1250 W.

8. The CaBi2Nb2O9 ceramic of claim 1, characterized in that: it is applied to prepare a vibration sensor.

9. The CaBi2Nb2O9 ceramic of claim 8, characterized in that: the working temperature of the vibration sensor is 0-700 °C. ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​

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