A bismuth layer piezoelectric ceramic material and a method for manufacturing the same

Abstract

This invention provides a bismuth layered piezoelectric ceramic material and its preparation method. The raw materials of the bismuth layered piezoelectric ceramic material include a bismuth layered perovskite main phase and a composite sintering aid. The weight ratio of the composite sintering aid to the bismuth layered perovskite main phase is 1%-3%. The raw materials of the bismuth layered perovskite main phase include bismuth oxide, titanium dioxide, and niobium pentoxide, with a molar ratio of bismuth oxide, titanium dioxide, and niobium pentoxide of 4:3:2. The composite sintering aid is a mixture of sodium borate and alumina. By controlling the raw material ratio, main phase grain morphology and size, and composite sintering aid combination of the bismuth layered piezoelectric ceramic material, the technical problems of high porosity, low density, and piezoelectric performance degradation of traditional bismuth layered piezoelectric ceramics are solved. This effectively avoids the generation of impurity phases in the main phase, reduces grain stacking gaps, achieves omnidirectional dense filling of grain boundaries, and ultimately obtains a material with both excellent high-temperature stable piezoelectric performance and high density.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric ceramic materials technology, and more specifically, to a bismuth layered piezoelectric ceramic material and its preparation method. Background Technology

[0002] Piezoelectric ceramics, as core functional materials for electromechanical energy conversion, are indispensable in high-temperature piezoelectric devices in aerospace, oil exploration, and automotive electronics. Bismuth-layered piezoelectric ceramics, with their advantages of high Curie temperature (650~950℃), low dielectric loss, and high resistivity, have become the preferred material for piezoelectric vibration sensors in high-temperature environments. Their crystal structure, formed by alternating layers of perovskite-like minerals and bismuth-oxygen layers, possesses inherent high-temperature stability.

[0003] However, existing bismuth layered piezoelectric ceramics suffer from significant structural defects during fabrication: their grains naturally exhibit a lamellar morphology, and during sintering, these lamellar grains tend to randomly align and accumulate, leading to numerous voids and pores at the grain boundaries. This results in a material density generally below 95% and a porosity exceeding 5%. These structural defects have multiple negative impacts: firstly, they reduce the material's mechanical stability, making it prone to fracture under high-temperature and high-pressure conditions; secondly, they create leakage current channels, causing a significant decrease in high-temperature resistivity, making it impossible to apply high voltage during polarization, hindering the full activation of piezoelectric properties, and limiting the reliability of piezoelectric devices.

[0004] Current improvement methods in the industry have significant limitations: while ion doping can optimize some electrical properties, it easily sacrifices the Curie temperature; texturing processes such as hot pressing and hot forging can improve grain arrangement, but the equipment is complex, costly, and has low production efficiency; template grain growth methods require additional template preparation and tape casting, resulting in poor process repeatability and difficulty in large-scale application. Existing technologies cannot fundamentally solve the problems of high porosity and insufficient density while ensuring the high-temperature performance of materials, and thus cannot meet the stringent requirements of high-temperature piezoelectric devices for structural density and performance stability.

[0005] Therefore, a solution is urgently needed to address the problems in existing technologies. Summary of the Invention

[0006] The main objective of this invention is to provide a bismuth layered piezoelectric ceramic material and its preparation method, so as to at least solve the problems of high porosity and insufficient density in the prior art, which are difficult to guarantee the high-temperature performance of bismuth layered piezoelectric ceramics.

[0007] To achieve the above objectives, the present invention provides a bismuth layered piezoelectric ceramic material. The raw materials of the bismuth layered piezoelectric ceramic material include a bismuth layered perovskite main phase and a composite sintering aid. The weight ratio of the composite sintering aid to the bismuth layered perovskite main phase is 1%-3%. The raw materials of the bismuth layered perovskite main phase include bismuth oxide, titanium dioxide, and niobium pentoxide, with a molar ratio of bismuth oxide, titanium dioxide, and niobium pentoxide of 4:3:2. The bismuth layered perovskite main phase has a lamellar structure with a length of 2-5 μm, a thickness of 150-300 nm, and a diameter-to-height ratio of 10:1-20:1. The composite sintering aid is a mixture of sodium borate and alumina.

[0008] Optionally, the molar ratio of sodium borate to alumina in the composite sintering aid is 3:1.

[0009] Optionally, the alumina has a purity of ≥99.9% and a particle size of ≤1μm.

[0010] Optionally, bismuth oxide is monoclinic α-Bi₂O₃, titanium dioxide is rutile TiO₂, and niobium pentoxide is orthorhombic Nb₂O₅.

[0011] This invention also provides a method for preparing a bismuth layered piezoelectric ceramic material, which includes the following steps:

[0012] The bismuth oxide, titanium dioxide and niobium pentoxide weighed in proportion are mixed evenly with neutral salt, pre-calcined at 750-900℃ for 3-4 hours, washed with water until the conductivity of the filtrate is ≤3μS / cm, and dried to obtain plate-like crystalline powder, namely the main phase of bismuth layered perovskite structure.

[0013] A composite sintering aid and an organic binder are added to the flaky crystalline powder, mixed evenly, and then pressed to obtain a ceramic green body; the composite sintering aid is a mixture of sodium borate and alumina.

[0014] The ceramic green body is subjected to debinding at 650-700℃ for 2-3 hours to remove the organic binder;

[0015] The ceramic green body after debinding is sintered at 1050-1150℃ without pressure for 3-4 hours, and then cooled to room temperature in the furnace.

[0016] After electrode addition and polarization treatment, a low-porosity, high-density bismuth layered piezoelectric ceramic is obtained.

[0017] Optionally, the neutral salt is a mixture of NaCl and KCl in a molar ratio of 1:1, and the mass ratio of the piezoelectric ceramic raw material to the neutral salt is (1:2)-(2:1).

[0018] Optionally, the process of uniformly mixing and then pressing to obtain a ceramic green body specifically includes:

[0019] After mixing evenly, pre-press at 80-100MPa for 20-40s, then pressurize at 150-200MPa for 1.5-2.5min to obtain ceramic green body.

[0020] Optionally, the debinding heating rate is 1℃ / min, and the sintering heating rate is 5℃ / min.

[0021] Optionally, the polarization treatment conditions are: polarization temperature 180-200℃, polarization voltage 10-13kV / mm, and polarization time 20-25min.

[0022] Optionally, the upper electrode is printed with silver paste, the silver firing temperature is 650℃, and the holding time is 10-15 minutes.

[0023] This invention discloses a bismuth layered piezoelectric ceramic material and its preparation method. The raw materials of the bismuth layered piezoelectric ceramic material include a bismuth layered perovskite main phase and a composite sintering aid. The weight ratio of the composite sintering aid to the bismuth layered perovskite main phase is 1%-3%. The raw materials of the bismuth layered perovskite main phase include bismuth oxide, titanium dioxide, and niobium pentoxide, with a molar ratio of bismuth oxide, titanium dioxide, and niobium pentoxide of 4:3:2. The bismuth layered perovskite main phase has a lamellar structure with a length of 2-5 μm, a thickness of 150-300 nm, and a diameter-to-height ratio of 10:1-20:1. The composite sintering aid is a mixture of sodium borate and alumina. By controlling the raw material ratio, main phase grain morphology and size, and composite sintering aid combination of bismuth layered piezoelectric ceramic materials, the technical problems of high porosity, low density and piezoelectric performance decay of traditional bismuth layered piezoelectric ceramics are solved. The generation of impurity phases in the main phase is effectively avoided, the grain stacking gap is reduced, and the grain boundaries are densely filled in all directions, ultimately obtaining a material with both excellent high-temperature stable piezoelectric performance and high density. Attached Figure Description

[0024] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0025] Figure 1 This is a flowchart of a method for preparing a bismuth layered piezoelectric ceramic material, which is optional according to an embodiment of the present invention. Detailed Implementation

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0027] This application provides a bismuth layered piezoelectric ceramic material, wherein the raw materials of the bismuth layered piezoelectric ceramic material include a bismuth layered perovskite structure main phase and a composite sintering aid;

[0028] The weight ratio of the composite sintering aid to the bismuth layered perovskite main phase is 1%-3%; the raw materials of the bismuth layered perovskite main phase include bismuth oxide, titanium dioxide and niobium pentoxide, and the molar ratio of bismuth oxide, titanium dioxide and niobium pentoxide is 4:3:2.

[0029] The bismuth layered perovskite main phase has a lamellar structure with a length of 2-5 μm, a thickness of 150-300 nm, and a diameter-to-height ratio of 10:1-20:1; the composite sintering aid is a mixture of sodium borate and alumina.

[0030] Specifically, the raw material system of bismuth layered piezoelectric ceramic materials uses a bismuth layered perovskite structure as the core functional phase and a composite sintering aid as an auxiliary dense phase. The bismuth layered perovskite structure as the main phase is composed of bismuth oxide, titanium dioxide, and niobium pentoxide in a molar ratio of 4:3:2. This ratio matches the lattice structure of bismuth layered perovskite, achieving stoichiometric balance of ions, ensuring the complete synthesis of the bismuth layered perovskite lattice, and avoiding the formation of impurity phases due to ion deficiency or excess, thus solidifying the piezoelectric function of the ceramic from the source. The weight ratio of the composite sintering aid to the bismuth layered perovskite structure as main phase is 1%-3%. If the amount of composite sintering aid added is too low, effective sintering assistance and grain boundary filling cannot be achieved; if it is too high, a non-piezoelectric second phase will form, encapsulating the main phase grains and reducing the piezoelectric response. A ratio of 1%-3% ensures that the aid is distributed only at the grain boundaries of the main phase, achieving functional retention and structural optimization.

[0031] The morphology of the main phase grains and the combination of composite sintering aids are both adapted to the anisotropic growth characteristics of bismuth layered perovskite and the densification mechanism of ceramic sintering. Bismuth layered perovskite crystals naturally exhibit anisotropy, with rapid growth along the direction parallel to the bismuth layer and slow growth perpendicular to it. Therefore, the main phase grains have a lamellar structure, with a size range of 2-5 μm in length, 150-300 nm in thickness, and a diameter-to-height ratio of 10:1-20:1. Lamellar grains within this size range can form a continuous and dense structure through close face-to-face stacking. Compared with equiaxed crystals, the stacking gap is smaller, which can reduce internal porosity from the perspective of grain morphology. At the same time, this size matches its conventional growth scale, avoiding the problems of excessive grain boundaries due to excessively small grains and uneven stacking due to excessively large grains, thus ensuring structural uniformity.

[0032] The composite sintering aid is a mixture of sodium borate and alumina. The two form a synergistic sintering aid and grain boundary filling effect. Sodium borate, as a low-melting-point sintering aid, melts during sintering to form a liquid phase that fills the intergranular gaps and promotes low-temperature sintering, avoiding abnormal grain growth caused by high temperature. Alumina, as a high-melting-point inorganic powder, can act as a grain boundary framework to inhibit excessive growth of the main phase grains, while further filling the tiny grain boundary gaps not covered by the liquid phase. The two work together to achieve all-round grain boundary filling, effectively reducing ceramic porosity and increasing density.

[0033] This application solves the technical problems of high porosity, low density, and piezoelectric performance degradation of traditional bismuth layered piezoelectric ceramics by controlling the raw material ratio, main phase grain morphology and size, and composite sintering aid combination of bismuth layered piezoelectric ceramic materials. It effectively avoids the generation of impurity phases in the main phase, reduces the grain stacking gap, and achieves all-round dense filling of grain boundaries, ultimately obtaining a material with both excellent stable piezoelectric performance and high density.

[0034] In one possible implementation, the molar ratio of sodium borate to alumina in the composite sintering aid is 3:1.

[0035] Specifically, the molar ratio of sodium borate to alumina in the composite sintering aid is 3:1. This ratio is precisely determined based on the synergistic sintering aid and grain boundary filling mechanism of both, which can fully leverage their advantages and avoid their respective defects to achieve optimal densification. Sodium borate, as a low-melting-point sintering aid, can melt first during sintering to form a liquid phase, rapidly filling the intergranular gaps of the main phase, reducing the sintering activation energy, and preventing abnormal grain growth caused by high temperatures. Alumina, as a high-melting-point inorganic powder, can act as a rigid framework for grain boundaries, inhibiting excessive growth and agglomeration of the main phase grains, while simultaneously filling the fine gaps not covered by the liquid phase, thus improving grain boundary stability. The 3:1 molar ratio ensures that the amount of sodium borate liquid phase precisely covers and disperses the alumina powder, guaranteeing that the liquid phase fully plays its role in sintering and filling, while also using alumina to inhibit the excessive formation of a glassy phase by sodium borate, which would lead to loose grain boundaries. Simultaneously, it avoids excessive alumina agglomeration that would create new pores, further improving the density of the ceramic grain boundaries.

[0036] In one possible implementation, the alumina has a purity of ≥99.9% and a particle size of ≤1μm.

[0037] Specifically, the high purity and small particle size requirements allow alumina to fully exert its auxiliary densification effect while avoiding the introduction of impurities that interfere with the ceramic structure and properties. A purity of ≥99.9% minimizes impurities such as alkali metals and heavy metals in alumina, preventing these impurities from reacting with the main phase or sodium borate during sintering to form impurity phases. This avoids impurity phases disrupting the lattice integrity of the bismuth layered perovskite main phase and prevents impurities from accumulating at grain boundaries to form a loose phase, affecting the ceramic density and piezoelectric stability. A particle size ≤1μm allows alumina powder to be uniformly dispersed in the sodium borate liquid phase with a finer particle size, fully filling the tiny grain boundary gaps between the main phase lamellar grains. Compared to large-particle-size alumina, small-particle-size powder has a larger specific surface area, allowing for more thorough contact with the liquid phase and main phase grain boundaries. This enables it to more efficiently play its role as a grain boundary framework, uniformly suppressing excessive growth of main phase grains and avoiding structural inhomogeneity caused by abnormal local grain growth.

[0038] In one possible implementation, bismuth oxide is monoclinic α-Bi₂O₃, titanium dioxide is rutile TiO₂, and niobium pentoxide is orthorhombic Nb₂O₅.

[0039] Specifically, monoclinic α-Bi₂O₃ is selected as the bismuth oxide phase. This phase is a stable phase of bismuth oxide, and its crystal structure has a high degree of matching with the bismuth layer lattice of bismuth layered perovskite, which can provide a stable Bi₂O₃ phase for the synthesis of the main phase of bismuth layered perovskite. 3+ To avoid insufficient ion release due to phase instability, rutile TiO2 is selected as the source of titanium dioxide. This phase is a high-temperature stable phase of TiO2 with good chemical stability, and its lattice contains Ti... 4+ It can precisely embed into the perovskite lattice sites of bismuth layered perovskite, ensuring the integrity of the main phase lattice; niobium pentoxide is selected from the orthorhombic phase Nb₂O₅, which has Nb 5+ The coordination environment is highly compatible with the ion coordination requirements of bismuth layered perovskite layers and can cooperate with Ti. 4+ Synergistic filling of lattice sites enhances the stability of the main phase lattice. The selection of the aforementioned phase types is based on the lattice compatibility, phase stability, and ion release characteristics of each oxide with the bismuth layered perovskite. Monoclinic α-Bi₂O₃, rutile TiO₂, and orthorhombic Nb₂O₅ can all exist stably and efficiently release matching metal ions during sintering. The precise combination of each ion forms a complete lattice, avoiding lattice distortion and impurity phase generation caused by phase mismatch from the source. At the same time, it improves the orderliness of main phase grain growth, making the morphology and size of the lamellar grains more uniform. Furthermore, combined with the grain boundary filling effect of the composite sintering aid, it achieves a dual improvement in ceramic density and piezoelectric properties.

[0040] This application also provides a method for preparing a bismuth layered piezoelectric ceramic material, which includes the following steps:

[0041] Step 1: Weigh out bismuth oxide, titanium dioxide and niobium pentoxide according to the proportion and mix them evenly with neutral salt. Pre-calcine at 750-900℃ for 3-4 hours. Wash with water until the conductivity of the filtrate is ≤3μS / cm. Dry to obtain plate-like crystalline powder, which is the main phase of bismuth layered perovskite structure.

[0042] Step 2: Add a composite sintering aid and an organic binder to the flaky crystalline powder, mix evenly, and then press to form a ceramic green body; the composite sintering aid is a mixture of sodium borate and alumina;

[0043] Step 3: Remove the organic binder from the ceramic body at 650-700℃ for 2-3 hours.

[0044] Step 4: Sinter the debinded ceramic green body at 1050-1150℃ without pressure for 3-4 hours, and then cool it to room temperature in the furnace.

[0045] Step 5: After electrode application and polarization treatment, a low-porosity, high-density bismuth layered piezoelectric ceramic is obtained.

[0046] Specifically, in step 1, first weigh bismuth oxide, titanium dioxide, and niobium pentoxide according to a preset ratio. Place the weighed raw materials into a mixing device, add a preset amount of neutral salt, and start the device to stir until the three raw materials and neutral salt are fully mixed and there are no obvious lumps, resulting in a uniform mixture. Place the mixture into a crucible and place it in a muffle furnace. Set the furnace temperature to slowly rise from room temperature to 750-900℃ and hold for 3-4 hours to complete the pre-calcination. After the pre-calcination, wait for the material to cool to room temperature, take it out and place it in a container filled with deionized water. Stir and soak, then filter, collect the filtrate and test its conductivity. Repeat the water washing and filtration operations until the conductivity of the filtrate is ≤3μS / cm. Place the washed solid material into an oven and dry it at a suitable temperature to constant weight. Take it out, crush it, and sieve it to obtain uniform flaky grain powder, which is the main phase of the bismuth layered perovskite structure.

[0047] Step 2: Place the prepared flaky crystalline powder into a mixing device, add the pre-set amounts of composite sintering aid (a mixture of sodium borate and alumina) and organic binder, and start the device to thoroughly stir, ensuring that the flaky crystalline powder, composite sintering aid, and organic binder are mixed evenly to form a lump-free mixture with suitable flowability. Pour the mixture into a mold of a pre-set size, place it in a pressure forming device, set appropriate pressure parameters, and start the device for pressure forming. After forming, slowly release the pressure to prevent cracks and deformation of the green body due to excessive internal and external pressure differences, ensuring the structural integrity of the green body. Remove the mold, and after demolding, obtain a ceramic green body with a regular structure, no cracks, and no looseness. The viscosity of the organic binder ensures that the green body is not easily loosened after forming, and the pre-mixing of the composite sintering aid ensures that it plays a uniform role in subsequent sintering. The organic binder can be selected from one or more of polyvinyl alcohol, carboxymethyl cellulose, and paraffin wax. The amount of organic binder added is 3%-6% of the mass of the flaky crystalline powder.

[0048] Step 3: Arrange the prepared ceramic green bodies neatly in the muffle furnace, ensuring a certain gap between them to prevent them from sticking together. This gap ensures uniform heat conduction within the furnace and prevents the green bodies from softening and sticking together after heating, thus avoiding damage to their shape. Set the muffle furnace heating program, slowly increasing the temperature from room temperature to 650-700℃, and maintain this temperature for 2-3 hours to perform binder removal. 650-700℃ allows the organic binder to fully decompose and volatilize, and is below the melting point and reaction temperature of the main phase and additives, avoiding damage to the green body structure. Slow heating also prevents the green body from cracking due to thermal stress. Since binder residue will decompose and generate gas during subsequent high-temperature sintering, forming pores and affecting the ceramic density and piezoelectric properties, the added organic binder needs to be removed. During the binder removal process, observe the furnace status in real time to ensure that the organic binder has fully decomposed and volatilized, leaving no residue. After binder removal, turn off the heating device and allow the muffle furnace to cool naturally to room temperature before removing the ceramic green bodies for later use.

[0049] Step 4: Place the debinded ceramic green body into the sintering furnace, arranging them neatly to ensure that the green body does not come into contact with the furnace wall or other high-temperature components. If the furnace wall and high-temperature components are not heated evenly, contact will cause local overheating or uneven heating of the green body, leading to deformation and cracking. Use pressureless sintering, set the heating rate of the sintering furnace, and slowly heat to 1050-1150℃, then hold at that temperature for 3-4 hours to complete the sintering. Pressureless sintering can avoid the deformation of the green body caused by pressure. 1050-1150℃ is suitable for the growth of the main phase grains and the sintering of the additives. Slow heating reduces thermal stress, and the holding time ensures that the main phase grains grow fully and stack tightly, and the additives fully play their role in filling grain boundaries to make the green body dense. During sintering, impurities can react with the main phase to produce impurity phases, which can damage the crystal lattice integrity and reduce the ceramic performance. Therefore, it is necessary to control the atmosphere inside the furnace to keep the impurities out. After sintering, the heating system of the sintering furnace is turned off and the furnace door is not opened. The ceramic blank is allowed to cool naturally to room temperature before being taken out.

[0050] Step 5: Take the cooled ceramic blank and uniformly deposit an electrode layer on both the upper and lower surfaces of the ceramic blank using vacuum deposition or sputtering deposition to complete the electrode application. A uniform electrode layer ensures uniform electric field conduction during subsequent polarization, resulting in a more uniform orientation of the electric domains within the ceramic and ensuring stable piezoelectric performance. Place the electrode-coated ceramic blank into a polarization device, set the polarization conditions, and polarize the ceramic blank. Polarization causes the disordered electric domains within the ceramic to align along the direction of the electric field, imparting a piezoelectric effect to the ceramic and enabling the conversion between mechanical and electrical energy. After polarization, turn off the polarization device, remove the ceramic, and after simple cleaning, obtain a low-porosity, high-density bismuth layered piezoelectric ceramic material.

[0051] The scheme of this application obtains uniform lamellar grain powder by neutral salt-assisted pre-firing and water washing to remove impurities. Combined with pre-mixing of composite sintering aids and pressure molding with controlled pressure release, a regular green body is prepared. The binder is completely removed by precise temperature control and the green body is free of defects. The green body is densified by pressureless precise temperature control sintering. Finally, the bismuth layered piezoelectric ceramic material prepared by uniform electrode plating and polarization treatment has low porosity and high density.

[0052] In one possible implementation, the neutral salt is a mixture of NaCl and KCl in a molar ratio of 1:1, and the mass ratio of the mixture of bismuth oxide, titanium dioxide and niobium pentoxide to the neutral salt is (1:2) to (2:1).

[0053] Specifically, a mixed neutral salt consisting of NaCl and KCl in a molar ratio of 1:1 is selected, and a mixture of bismuth oxide, titanium dioxide, and niobium pentoxide is mixed with this neutral salt in a mass ratio of 1:2 to 2:1. This mixed neutral salt provides a suitable melting reaction environment for the synthesis of the bismuth layered perovskite main phase. The 1:1 ratio of NaCl and KCl can form a eutectic system, which lowers the melting temperature and improves the coating and wettability of the melt on the raw materials, promoting full contact and uniform reaction of the three oxides, and directionally generating uniformly sized plate-like grains. The 1:2 to 2:1 mass ratio of raw materials to neutral salt ensures that the amount of neutral salt is sufficient to achieve full dispersion and reaction of the raw materials, avoiding uneven grain growth caused by raw material agglomeration, and does not increase the difficulty of subsequent water washing and impurity removal due to excessive neutral salt. At the same time, it can effectively inhibit abnormal grain growth. Combined with water washing, the neutral salt can be completely removed, and finally, pure and uniform plate-like grain powder is obtained.

[0054] In one possible implementation, the process of uniformly mixing followed by pressure molding to obtain a ceramic green body specifically includes:

[0055] After mixing evenly, pre-press at 80-100MPa for 20-40s, then pressurize at 150-200MPa for 1.5-2.5min to obtain ceramic green body.

[0056] Specifically, the process of uniformly mixing and then pressing is as follows: First, the flaky crystalline powder is mixed evenly with the composite sintering aid and organic binder, then placed into a mold of a preset size, and placed in a pressure forming device. It is held under a pre-pressure of 80-100MPa for 20-40 seconds, and then the pressure is adjusted to 150-200MPa and pressed continuously for 1.5-2.5 minutes to complete the pressure forming and demolding, thereby obtaining the ceramic green body. The process involves a step-by-step pressurization process. First, a lower pre-compression of 80-100 MPa for 20-40 seconds quickly removes air from the mixture, preventing air residue from causing defects such as bubbles and pores in the green body during subsequent high-pressure molding. This also allows the flaky crystalline powder to initially form a tight stack, preventing localized material accumulation or loosening. Next, a continuous high-pressure press of 150-200 MPa for 1.5-2.5 minutes further compacts the mixture, promoting full face-to-face contact and tight stacking of the flaky crystalline particles. This minimizes initial porosity within the green body. The pressure and time parameters are tailored to the viscosity and plasticity of the mixture, ensuring a high initial density while avoiding excessive pressure or time leading to cracking and grain breakage, or insufficient pressure or time resulting in a loose and unstable green body. This process yields a ceramic green body with a regular structure, high density, no cracks, no pores, and stable morphology.

[0057] In one possible implementation, the debinding heating rate is 1°C / min, and the sintering heating rate is 5°C / min.

[0058] Specifically, the low-rate heating of 1℃ / min during the debinding stage allows for uniform temperature conduction within the ceramic body, enabling the organic binder to gradually and slowly decompose and volatilize from the surface to the interior. This avoids the rapid decomposition of the binder due to excessive heating, which could generate a large amount of gas, causing blistering and cracking of the body. Simultaneously, the low-rate heating effectively reduces thermal stress caused by the temperature difference between the inside and outside of the body, ensuring the integrity and morphological stability of the body structure after debinding. The sintering stage employs a heating rate of 5℃ / min, balancing heating efficiency with allowing the body to gradually complete lattice rearrangement and grain growth as the temperature increases. Combined with the sintering aid effect of the composite sintering agent, this ensures uniform grain growth and tight stacking. This rate avoids localized overheating, abnormal grain growth, or thermal stress cracking caused by excessively rapid heating, and also prevents excessively long sintering cycles and insufficient grain growth caused by excessively slow heating. The precise setting of these two differentiated heating rates matches the process characteristics of debinding and sintering, mitigating body defects from a process perspective.

[0059] In one possible implementation, the polarization treatment conditions are: polarization temperature 180-200℃, polarization voltage 10-13kV / mm, and polarization time 20-25min.

[0060] Specifically, a polarization temperature of 180-200℃ can enhance the ion migration ability inside the ceramic, reduce the domain orientation resistance, and make it easier for disordered domains to orient themselves along the electric field direction. Moreover, this temperature range will not destroy the dense lattice structure already formed in the ceramic, and avoid the decrease in intergranular bonding force caused by high temperature. A polarization voltage of 10-13kV / mm provides sufficient electric field driving force for domain orientation, which can ensure that the deep domains inside the ceramic are fully oriented, and avoid insufficient domain arrangement and insufficient piezoelectric performance due to too low voltage. At the same time, it can prevent defects such as internal breakdown and microcracks caused by too high voltage. A polarization time of 20-25min can ensure that the domain orientation process is fully completed, so that the oriented domains form a stable arrangement. It can avoid domain bounce and disorder due to too short time, and it can also avoid increased energy consumption and reduced efficiency due to too long time. Appropriate polarization parameters enable the internal electric domains of ceramics to achieve full and stable directional alignment, maximizing the excellent piezoelectric conversion performance of ceramics, while effectively avoiding structural defects in the polarization process, ensuring that the structural advantages of low porosity and high density of ceramics are not compromised.

[0061] In one possible implementation, the upper electrode is printed with silver paste, the silver burning temperature is 650°C, and the holding time is 10-15 minutes.

[0062] Specifically, the sintering temperature of 650℃ allows the silver paste to sinter into a dense conductive silver layer, while being lower than the sintering temperature of the ceramic blank, thus avoiding damage to its dense structure. The holding time of 10-15 minutes ensures that the silver paste is fully sintered, allowing the silver electrode to bond firmly to the ceramic substrate, preventing electrode detachment, and avoiding thermal damage. Ultimately, this results in an electrode layer with excellent conductivity and tight bonding, providing stable conditions for polarization treatment.

[0063] The following experiments further illustrate this application.

[0064] I. Implementation Examples

[0065] Example 1

[0066] 1. Material formulation: The main phase raw materials of the bismuth layered perovskite structure are monoclinic α-Bi2O3, rutile TiO2, and orthorhombic Nb2O5, with a molar ratio of 4:3:2; the composite sintering aid is sodium borate and alumina (purity ≥99.9%, particle size ≤1μm), with a molar ratio of 3:1; the weight ratio of the composite sintering aid to the main phase is 2%; the neutral salt is a mixture of NaCl and KCl with a molar ratio of 1:1, and the mass ratio of the main phase raw materials to the neutral salt is 1:1; the organic binder is polyvinyl alcohol (PVA, degree of polymerization 1750±50).

[0067] 2. Preparation method:

[0068] Step 1: Weigh α-Bi₂O₃, rutile TiO₂, and orthorhombic Nb₂O₅ in a molar ratio of 4:3:2, mix with neutral salt, and place in a planetary ball mill (200 r / min). Add an appropriate amount of deionized water (conductivity ≤1 μS / cm), and ball mill for 2 hours until uniformly mixed and free of lumps. After drying, obtain a mixture. Place the mixture in an alumina crucible and place it in a muffle furnace. Heat to 830℃ at 5℃ / min and hold for 3.5 hours to complete the pre-calcination. After cooling to room temperature, place it in deionized water and stir for 30 minutes. Filter and use a conductivity meter to detect the conductivity of the filtrate. Repeat water washing and filtration until the conductivity of the filtrate is ≤3 μS / cm. Place the solid material in an oven and dry at 120℃ to constant weight. After pulverizing, pass through a 200-mesh sieve to obtain plate-like crystalline powder, which is the main phase of the bismuth layered perovskite structure.

[0069] Step 2: Add composite sintering aid (sodium borate: alumina = 3:1, weight ratio 2%) and 5wt% PVA binder to the flaky crystalline powder, ball mill for 1 hour to mix evenly, and obtain a mixture; pour the mixture into a standard mold (diameter 10mm, thickness 2mm), put it into a pressure forming machine, first pre-press at 80-100MPa for 30s, then press at 175MPa for 2min, and then slowly release the pressure and demold to obtain a ceramic green body.

[0070] Step 3: Place the ceramic blank into a muffle furnace, heat it to 680℃ at a rate of 1℃ / min, hold it at that temperature for 2.5 hours to remove the binder, and then remove it after it has cooled naturally to room temperature.

[0071] Step 4: Place the debinding blank into a sintering furnace, heat it to 1100℃ at a rate of 5℃ / min, sinter it without pressure for 3.5 hours, and then cool it to room temperature with the furnace to obtain a ceramic green body.

[0072] Step 5: Using silver paste printing (silver content ≥99%), electrodes are printed on the upper and lower surfaces of the ceramic blank. The blank is placed in a muffle furnace and heated at 650℃ for 12 minutes to burn silver. The ceramic blank with the electrodes is then placed in a polarization device and polarized at 190℃, 11.5kV / mm for 22 minutes. After cooling, the surface is cleaned to obtain a bismuth layered piezoelectric ceramic sample.

[0073] Example 2

[0074] 1. Material formulation: The main phase raw materials are monoclinic α-Bi2O3, rutile TiO2, and orthorhombic Nb2O5 in a molar ratio of 4:3:2; the composite sintering aid (sodium borate: alumina = 3:1) has a weight ratio of 1% to the main phase; the neutral salt is NaCl and KCl in a molar ratio of 1:1, and the mass ratio of the main phase raw materials to the neutral salt is 1:2; the alumina purity is ≥99.9% and the particle size is ≤1μm; the organic binder is PVA (degree of polymerization 1750±50).

[0075] 2. Preparation method: Except for the following parameters, the rest are the same as in Example 1:

[0076] Step 1: Preheat to 750℃ and hold for 3 hours;

[0077] Step 2: Pre-compression at 80MPa for 20 seconds, followed by high pressure at 150MPa for 1.5 minutes;

[0078] Step 3: Degas at 650℃ and maintain the temperature for 2 hours;

[0079] Step 4: Sintering temperature 1050℃, hold for 3 hours;

[0080] Step 5: Polarization temperature 180℃, voltage 10kV / mm, time 20min; silver firing and holding for 10min.

[0081] Example 3

[0082] 1. Material formulation: The main phase raw materials are monoclinic α-Bi2O3, rutile TiO2, and orthorhombic Nb2O5, with a molar ratio of 4:3:2; the composite sintering aid (sodium borate: alumina = 3:1) has a weight ratio of 3% to the main phase; the neutral salt is NaCl and KCl with a molar ratio of 1:1, and the mass ratio of the main phase raw materials to the neutral salt is 2:1; the alumina purity is ≥99.9%, and the particle size is ≤1μm; the organic binder is PVA (degree of polymerization 1750±50).

[0083] 2. Preparation method: Except for the following parameters, the rest are the same as in Example 1:

[0084] Step 1: Preheat to 900℃ and hold for 4 hours;

[0085] Step 2: Pre-compression at 100MPa for 40 seconds, followed by high pressure at 200MPa for 2.5 minutes;

[0086] Step 3: Degas at 700℃ and maintain the temperature for 3 hours;

[0087] Step 4: Sintering temperature 1150℃, hold for 4 hours;

[0088] Step 5: Polarization temperature 200℃, voltage 13kV / mm, time 25min; silver firing and holding for 15min.

[0089] II. Comparative Example

[0090] The preparation and detection methods of all comparative examples are consistent with those of Example 1, with only a single variable deviating from the limitations of this invention.

[0091] Comparative Example 1

[0092] The difference from Example 1 is that the main phase raw materials are tetragonal β-Bi2O3, anatase TiO2, and monoclinic Nb2O5, while the formulations and preparation methods of the other materials are completely consistent with those of Example 1.

[0093] Comparative Example 2

[0094] The difference from Example 1 is that the weight ratio of the composite sintering aid (sodium borate:alumina = 3:1) to the main phase is 0.5%, while the formulation and preparation method of the other materials are completely consistent with those of Example 1.

[0095] Comparative Example 3

[0096] The difference from Example 1 is that the weight ratio of the composite sintering aid (sodium borate:alumina = 3:1) to the main phase is 4%, while the other material formulations and preparation methods are completely consistent with Example 1.

[0097] Comparative Example 4

[0098] The difference from Example 1 is that the molar ratio of sodium borate to alumina in the composite sintering aid is 1:1, while the formulation and preparation method of the other materials are completely consistent with those in Example 1.

[0099] Comparative Example 5

[0100] The difference from Example 1 is that the neutral salt is a single NaCl (deviating from the limitation of the 1:1 molar ratio of NaCl to KCl in this invention), the mass ratio of the main phase raw material to the neutral salt is still 1:1, and the formulation and preparation method of the other materials are completely consistent with Example 1.

[0101] Comparative Example 6

[0102] The difference from Example 1: the pressure molding uses a single pressure of 175MPa for 2.3min, while the other material formulations and preparation methods are completely consistent with Example 1.

[0103] Comparative Example 7

[0104] Differences from Example 1: No composite sintering aid; the neutral salt is a single KCl, and the mass ratio of the main phase raw material mixture to the neutral salt is 3:1; the pressure molding is a single pressure of 150MPa for 2 minutes; the debinding heating rate is 3℃ / min, and the sintering heating rate is 10℃ / min; the upper electrode is coated by sputtering; the polarization temperature is 170℃, the voltage is 9kV / mm, and the time is 18 minutes; the remaining preparation steps are the same as in Example 1.

[0105] III. Experimental Verification

[0106] 1. Verification Indicators

[0107] 1.1 Density: The density was determined by microscopic analysis, a method commonly used in the ceramics field, and was calculated by observing the pore distribution in the cross-section of the sample. Five parallel samples were tested for each group of samples, and the average value was taken. The result was retained to two decimal places.

[0108] 1.2 Porosity: It was detected simultaneously with the density by microscopic analysis, and the proportion of pore area in the cross section was directly calculated; each group of samples corresponded one-to-one with the density parallel sample, and the results were retained to two decimal places.

[0109] 1.3 Piezoelectric constant d 33 A piezoelectric constant tester (d) was used. 33 The piezoelectric properties at room temperature were tested using a testing instrument with an accuracy of ±1 pC / N; five parallel samples were tested for each group of samples, and the average value was taken and rounded to the nearest integer.

[0110] 1.4 Main phase grain size: The length (μm level), thickness (nm level), and aspect ratio (length / thickness) of the lamellar grains were measured; three parallel samples were set up for each group of samples, and each parallel sample was tested three times independently. The average value of all data was taken, and the error of the parallel samples was controlled within ±5nm (thickness) and ±0.1μm (length).

[0111] 1.5 High-Temperature Piezoelectric Stability: The piezoelectric constant d at 550℃ is used as the reference value. 33 Retention rate is the evaluation index; retention rate = (d at 550℃) 33 Value / room temperature d 33 (value) × 100%; 5 parallel samples were selected for each group of samples (d at room temperature) 33 (If the parallel samples are consistent, take the average value and keep two decimal places.)

[0112] 2. Testing Steps

[0113] 2.1 Sample Pretreatment

[0114] The ceramic samples prepared in Examples 1-3 and Comparative Examples 1-7 were cleaned with deionized water to remove stains and silver paste residue, dried in an oven at 105°C for 30 minutes, and then screened after cooling to room temperature.

[0115] 2.1.1 Samples used for density and porosity testing: Select samples without cracks or damage, cut them into 2mm thick cross-sectional samples along the central axis, and grind and polish them (to ensure the cross-section is flat and free of scratches). Each group has 5 parallel samples.

[0116] 2.1.2 Used for piezoelectric constant d 33 Samples for (room temperature + high temperature) testing: Select samples with regular size (uniform diameter 10mm, thickness 2mm), no cracks, and no damage, with 5 parallel samples in each group;

[0117] 2.1.3 Samples for main phase grain size detection: Select ceramic samples without cracks or damage, crush them and pass them through a 200-mesh sieve to obtain powder samples. Each group has 3 parallel samples for later use.

[0118] 2.2 Density and Porosity Detection (Microscopic Analysis)

[0119] The pretreated cross-sectional sample was placed on the stage of a 4000x metallurgical microscope, and the instrument was calibrated and the focus was adjusted until the cross-sectional structure was clear.

[0120] Five different fields of view were selected for each sample (evenly distributed in the center and surrounding areas of the cross section), and cross-sectional images were captured using the microscope's built-in image analysis software.

[0121] Image analysis software was used to calculate the percentage of pore area in each field of view (pore area / total field of view × 100%).

[0122] The average pore area ratio of each sample (the average pore area ratio of the 5 fields of view) is the porosity of that sample.

[0123] Calculate density using the formula: Density (%) = 100% - Porosity (%);

[0124] After completing the testing of 5 parallel samples, the average values ​​of density and porosity were calculated and used as the test results for this group of samples.

[0125] 2.3 Room temperature piezoelectric constant d 33 Detection

[0126] The pretreated sample was placed in d 33 On the testing stage of the testing instrument, adjust the position to ensure good contact between the sample electrode and the testing instrument probe;

[0127] After setting the instrument's detection parameters and calibrating the instrument, perform three tests on each sample and take the average of the three tests as the room temperature d for that sample. 33 value;

[0128] After completing the testing of 5 parallel samples, calculate the room temperature d for each group of samples. 33 The average value was used as an indicator of the room temperature piezoelectric performance of this group of samples.

[0129] 2.4 High-Temperature Piezoelectric Constant d 33 Detection

[0130] Select the completed room temperature d 33 Five parallel samples were placed in a high-temperature constant temperature chamber (temperature control accuracy ±2℃), set at 550℃, and kept at that temperature for 30 minutes to ensure uniform temperature across the entire sample.

[0131] Quickly remove the sample and place it into the calibration d that has undergone high-temperature adaptation treatment within 1 minute. 33 The testing instrument (with detection parameters consistent with room temperature testing) avoids sample cooling and ensures instrument detection accuracy.

[0132] Each sample was tested three times, and the average value was taken as the 550℃d value for that sample. 33 value;

[0133] The high-temperature piezoelectric constant retention rate of each sample was calculated using the formula: Retention rate = (d at 550℃) 33 Value / room temperature d 33 (value) × 100%;

[0134] After completing the testing of 5 parallel samples, the average retention rate of the high-temperature piezoelectric constant of each group of samples was calculated as the result of the high-temperature stability evaluation.

[0135] 2.5 Main phase grain size detection

[0136] Take the pretreated powder sample, add an appropriate amount of deionized water, add 1-2 drops of polyethylene glycol dispersant, and ultrasonically disperse for 15 minutes to obtain a uniformly dispersed suspension.

[0137] The suspension was dropped into the detection cell of a laser particle size analyzer (accuracy ±0.01μm). After calibrating the instrument, each parallel sample was tested three times to confirm that there was no obvious agglomeration of crystals (pass standard: equivalent particle size distribution is 0.2-4μm, with no agglomerates larger than 5μm).

[0138] Use a pipette to draw 1 μL of properly dispersed suspension and slowly drop it onto the center of a clean glass slide (avoiding crystal overlap). Place the slide in a vacuum drying oven and dry it at 45°C for 15 minutes to prepare a sample slice with uniformly distributed crystals.

[0139] Place the sample section on the stage of a metallurgical microscope (magnification 4000x, accuracy ±1nm), calibrate the instrument, and adjust the focus until the grain outline is clear.

[0140] Ten representative lamellar grains with complete outlines, no damage, and no overlap were selected. Using the microscope's built-in precision measurement software, the longest side (defined as length) and the shortest distance perpendicular to the longest side (defined as thickness) of each grain were measured. The diameter-to-height ratio of each grain was calculated (diameter-to-height ratio = length / thickness, rounded to the nearest integer).

[0141] Replace the glass slide area or re-prepare the sample slice, and perform three independent tests on the same parallel sample. Take the average value of the grain length, thickness and diameter-to-height ratio of the three tests as the test data of the parallel sample.

[0142] The same method was used to test two more parallel samples. The average value of the grain length, thickness and diameter-to-height ratio of each sample was calculated to verify whether it conformed to the range of plate-like grain parameters defined in this invention.

[0143] 3. Verification Results and Analysis

[0144] (1) The verification results are shown in Table 1 (average value of each group of parallel samples, grain size: length / thickness / diameter-to-height ratio)

[0145] Table 1. Detection results of various properties and main phase grain size of bismuth layered piezoelectric ceramic samples from Examples 1-3 and Comparative Examples 1-7.

[0146]

[0147] (2) Conclusion

[0148] According to the data in Table 1, the density of Examples 1-3 of the present invention is ≥97%, the porosity is ≤2.85%, and the piezoelectric constant d is... 33 All grains exhibit a piezoelectric constant retention rate of ≥172 pC / N at 550℃ of ≥86%. Furthermore, analysis using a laser particle size analyzer and a 4000x metallographic microscope shows that the main phase grain size conforms to the specifications of "length 3-4 μm, thickness 200-250 nm, and aspect ratio 12:1-18:1." This indicates that the present invention can stably prepare high-performance bismuth layered piezoelectric ceramics with target-sized lamellar main phase grains and excellent high-temperature performance. The combination of lamellar grain length of 3-4 μm and thickness of 200-250 nm reduces grain agglomeration, improves grain boundary bonding during sintering, contributes to increased density and reduced porosity, and enhances the high-temperature stability of grain boundaries, laying the structural foundation for excellent piezoelectric and high-temperature performance. The aspect ratio controlled at 12:1-18:1 ensures the lattice integrity of the bismuth layered perovskite structure, reduces impurity defects, optimizes grain arrangement regularity, and improves the piezoelectric constant d. 33 This method ensures high-temperature piezoelectric stability and avoids performance degradation caused by excessively large aspect ratios (too thin grains are prone to breakage and deformation at high temperatures) or too small aspect ratios (too thick grains cannot form an effective piezoelectric response and have poor high-temperature stability). This size range is highly compatible with the composite sintering aids and step-by-step pressurization schemes of this invention, enabling uniform grain growth and ensuring stable performance of ceramic materials at both room temperature and high temperature. This contrasts with the performance degradation at both room temperature and high temperature caused by abnormal grain size in the comparative example (such as thickness exceeding 200-250nm and aspect ratio deviating from 12:1-18:1).

[0149] The performance of Comparative Examples 1-7 was significantly worse than that of the Example. Among them, Comparative Examples 1-6 had deviated parameters such as phase type, amount and ratio of additives, type of neutral salt, and pressurization method, resulting in decreased density, increased porosity, deteriorated piezoelectric properties, and abnormal grain size. Moreover, the high-temperature piezoelectric constant retention rate was less than 78%. Comparative Example 7 had the worst performance in all aspects, with a high-temperature piezoelectric constant retention rate of only 65.99%. This also shows that the solution of the present invention can significantly improve the high-temperature stability of bismuth layered piezoelectric ceramics while ensuring room temperature performance, making up for the shortcomings of the existing technology in high-temperature performance.

[0150] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A bismuth layered piezoelectric ceramic material, characterized in that, The raw materials for the bismuth layered piezoelectric ceramic material include a bismuth layered perovskite main phase and composite sintering aids. The weight ratio of the composite sintering aid to the bismuth layered perovskite main phase is 1%-3%; the raw materials of the bismuth layered perovskite main phase include bismuth oxide, titanium dioxide and niobium pentoxide, and the molar ratio of bismuth oxide, titanium dioxide and niobium pentoxide is 4:3:

2. The bismuth layered perovskite main phase has a lamellar structure with a length of 2-5 μm, a thickness of 150-300 nm, and a diameter-to-height ratio of 10:1-20:1; the composite sintering aid is a mixture of sodium borate and alumina. The molar ratio of sodium borate to alumina in the composite sintering aid is 3:1; Bismuth oxide is a monoclinic α-Bi₂O₃, titanium dioxide is rutile TiO₂, and niobium pentoxide is an orthorhombic Nb₂O₅.

2. The bismuth layered piezoelectric ceramic material according to claim 1, characterized in that, The alumina has a purity of ≥99.9% and a particle size of ≤1μm.

3. A method for preparing a bismuth layered piezoelectric ceramic material, characterized in that, The method for preparing the bismuth layered piezoelectric ceramic material according to any one of claims 1 or 2 comprises the following steps: The bismuth oxide, titanium dioxide and niobium pentoxide weighed in proportion are mixed evenly with neutral salt, pre-calcined at 750-900℃ for 3-4 hours, washed with water until the conductivity of the filtrate is ≤3μS / cm, and dried to obtain plate-like crystalline powder, namely the main phase of bismuth layered perovskite structure. A composite sintering aid and an organic binder are added to the flaky crystalline powder, mixed evenly, and then pressed to obtain a ceramic green body; the composite sintering aid is a mixture of sodium borate and alumina. The ceramic green body is subjected to debinding at 650-700℃ for 2-3 hours to remove the organic binder; The ceramic green body after debinding is sintered at 1050-1150℃ without pressure for 3-4 hours, and then cooled to room temperature in the furnace. After electrode addition and polarization treatment, a low-porosity, high-density bismuth layered piezoelectric ceramic is obtained.

4. The preparation method according to claim 3, characterized in that, The neutral salt is a mixture of NaCl and KCl in a molar ratio of 1:1, and the mass ratio of the piezoelectric ceramic raw material to the neutral salt is (1:2)-(2:1).

5. The preparation method according to claim 3, characterized in that, The process of mixing thoroughly and then pressing to form a ceramic blank specifically includes: After mixing evenly, pre-press at 80-100MPa for 20-40s, then pressurize at 150-200MPa for 1.5-2.5min to obtain ceramic green body.

6. The preparation method according to claim 3, characterized in that, The debinding heating rate is 1℃ / min, and the sintering heating rate is 5℃ / min.

7. The preparation method according to claim 3, characterized in that, The polarization treatment conditions are: polarization temperature 180-200℃, polarization voltage 10-13kV / mm, and polarization time 20-25min.

8. The preparation method according to claim 3, characterized in that, The upper electrode is printed with silver paste, the silver firing temperature is 650℃, and the holding time is 10-15 minutes.

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