A layered composite lead-free relaxor ferroelectric material and a preparation method thereof

Abstract

The application discloses layered composite lead-free relaxor ferroelectric material and a preparation method thereof, and belongs to the technical field of piezoelectric ceramics, and overcomes the defects that the strain excited by the lead-free relaxor ferroelectric material in the prior art is small under a low driving electric field. The layered composite lead-free relaxor ferroelectric material comprises a first relaxor ferroelectric layer, a ferroelectric layer and a second relaxor ferroelectric layer arranged in sequence; and in the layered composite lead-free relaxor ferroelectric material, 0 < ferroelectric layer mass percentage < 30%. The application provides the layered composite lead-free relaxor ferroelectric material which has low driving electric field and large strain.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric ceramic technology, specifically relating to a layered composite lead-free relaxor ferroelectric material and its preparation method. Background Technology

[0002] In recent years, lead-free relaxor ferroelectric materials have attracted widespread attention, mainly because they exhibit the physical phenomenon of electro-induced phase transition, which allows for the attainment of high electro-induced strain, making them highly suitable for applications such as piezoelectric actuators.

[0003] However, the commercialization of lead-free relaxor ferroelectric materials faces significant challenges because they require a high applied electric field (60–80 kV / cm) to excite a large electroinduced strain, while commercial applications generally require an applied electric field ≤30 kV / cm. Existing lead-free relaxor ferroelectric materials exhibit relatively small strain under low driving electric fields. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the defect of small strain excited by lead-free relaxor ferroelectric materials under low driving electric field in the prior art, thereby providing a layered composite lead-free relaxor ferroelectric material and its preparation method.

[0005] To this end, the present invention provides the following technical solution.

[0006] In a first aspect, the present invention provides a layered composite lead-free relaxor ferroelectric material, comprising a first relaxor ferroelectric layer, a ferroelectric layer and a second relaxor ferroelectric layer disposed sequentially; wherein in the layered composite lead-free relaxor ferroelectric material, 0 < ferroelectric layer mass percentage < 30%.

[0007] Furthermore, the first relaxor ferroelectric layer is mBi 0.5 Na 0.5 TiO3-(1-m)SrTiO3 ceramic, 0.7≤m≤0.8; optionally, 0.75Bi. 0.5 Na 0.5 TiO3-0.25SrTiO3 ceramics;

[0008] Furthermore, the ferroelectric layer is nBi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-(1-n)SrTiO3, 0.9≤n≤1; optionally, 0.96Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.04SrTiO3 ceramics;

[0009] Furthermore, the second relaxor ferroelectric layer is pBi 0.5Na 0.5 TiO3-(1-p)SrTiO3 ceramic, 0.7≤p≤0.8; optionally, 0.75Bi. 0.5 Na 0.5 TiO3-0.25SrTiO3 ceramics.

[0010] Furthermore, in the layered composite lead-free relaxor ferroelectric material, the first relaxor ferroelectric layer and the second relaxor ferroelectric layer have the same mass percentage.

[0011] Furthermore, a first electrode layer is disposed on the surface of the first relaxor ferroelectric layer, and a second electrode layer is disposed on the surface of the second relaxor ferroelectric layer. Both the first electrode layer and the second electrode layer are silver electrodes.

[0012] Secondly, the present invention provides a method for preparing a layered composite lead-free relaxor ferroelectric material, comprising the following steps:

[0013] The powder of the first relaxor ferroelectric layer is placed into the mold for the first pre-pressing.

[0014] The ferroelectric layer powder is added into the mold for a second pre-pressing process;

[0015] The powder of the second relaxor ferroelectric layer is added into the mold and finally pressed to obtain the ceramic blank.

[0016] A layered composite lead-free relaxor ferroelectric material is prepared by sintering a ceramic blank.

[0017] Furthermore, the pressures of the first and second pre-compressions are both lower than the pressure of the final compression.

[0018] Furthermore, at least one of the following conditions must be met:

[0019] (1) The pressure of the first pre-compression is 10-30 MPa;

[0020] (2) The pressure of the second pre-compression is 10-30 MPa;

[0021] (3) The final pressing pressure is 100-150 MPa.

[0022] Furthermore, at least one of the following conditions must be met:

[0023] (1) The powder of the first relaxor ferroelectric layer is mBi 0.5 Na 0.5 TiO3-(1-m)SrTiO3 powder, 0.7≤m≤0.8;

[0024] (2) The powder of the ferroelectric layer is nBi 0.5 (Na 0.84 K 0.16) 0.5 TiO3-(1-n)SrTiO3 powder, 0.9≤n≤1;

[0025] (3) The powder of the second relaxor ferroelectric layer is pBi 0.5 Na 0.5 TiO3-(1-p)SrTiO3 powder, 0.7≤p≤0.8;

[0026] (4) Granulate the powder of the first relaxor ferroelectric layer before placing it into the mold;

[0027] (5) Granulate the ferroelectric layer powder before adding it into the mold;

[0028] (6) Granulate the powder of the second relaxor ferroelectric layer before adding it into the mold.

[0029] Furthermore, the sintering process involves holding the ceramic at 1050–1150°C for 2–4 hours. This further improves the temperature stability of the sodium bismuth titanate-based lead-free piezoelectric ceramic.

[0030] Furthermore, the process also includes coating the surface of the layered composite lead-free relaxor ferroelectric material with silver paste and sintering it at 500–550°C for 1–3 hours.

[0031] The powder for the first relaxor ferroelectric layer (BNT-ST powder) is prepared using a traditional solid-state method. The specific preparation method includes: mixing raw materials Bi₂O₃, Na₂CO₃, SrTiO₃, and TiO₂, followed by ball milling, drying, pre-calcination, secondary ball milling, and drying of mBi. 0.5 Na 0.5 TiO3-(1-m)SrTiO3 ceramic powder.

[0032] The preparation method of the powder for the second relaxor ferroelectric layer (BNT-ST powder) is the same as that for the powder for the first relaxor ferroelectric layer.

[0033] The ferroelectric layer powder (BNKT-ST powder) is prepared using a traditional solid-state method. The specific preparation method includes: mixing, ball milling, drying, pre-calcining, secondary ball milling, and drying of raw materials Bi₂O₃, Na₂CO₃, K₂CO₃, TiO₂, and SrTiO₃ to prepare nBi. 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-(1-n)SrTiO3 powder.

[0034] The process for preparing BNT-ST powder and / or BNKT-ST powder meets at least one of the following conditions:

[0035] (1) The ball milling time is 16-24 hours;

[0036] (2) The drying process is to dry at 110-150℃ for 12-24 hours;

[0037] (3) The preheating temperature is 750-900℃, and the time is 2-3 hours;

[0038] (4) The secondary ball milling time is 16 to 24 hours.

[0039] Granulation of the powder of the first relaxor ferroelectric layer, the powder of the ferroelectric layer, and the powder of the second relaxor ferroelectric layer each independently includes: adding 3 wt% to 7 wt% of a polymer aqueous solution to the powder for granulation. The polymer includes at least one of polyvinyl alcohol (PVA), polyvinyl ketone (PVP), or polyacrylic acid (PAA), and the mass concentration of the polymer aqueous solution is 5% to 10%.

[0040] Before sintering the ceramic green body, a polymer removal step is included: the ceramic green body is held at 550–650℃ for 1–3 hours to remove the polymer. After polymer removal, the temperature is further increased to the sintering temperature for sintering. After cooling, silver paste is uniformly coated on the surface of the sintered product, and sintering is performed again.

[0041] The technical solution of this invention has the following advantages:

[0042] 1. The layered composite lead-free relaxor ferroelectric material of the present invention comprises a first relaxor ferroelectric layer, a ferroelectric layer and a second relaxor ferroelectric layer disposed sequentially; in the layered composite lead-free relaxor ferroelectric material, 0 < ferroelectric layer mass percentage < 30%.

[0043] This invention utilizes a relaxation / ferroelectric 2-2 composite method to design a layered composite lead-free relaxor ferroelectric material. It limits the content of the ferroelectric layer mass percentage to 30% within the layered composite lead-free relaxor ferroelectric material, thereby increasing the partial voltage of the relaxor ferroelectric layer ceramic. This allows the relaxor ferroelectric layer to undergo a phase transition from relaxor ferroelectric to ferroelectric under a lower electric field, resulting in greater strain performance at low electric fields. This invention provides a layered composite lead-free relaxor ferroelectric material that combines low driving electric field and large strain.

[0044] Since the dielectric constant of the relaxor ferroelectric layer is much larger than that of the ferroelectric layer, the voltage across the relaxor ferroelectric layer in the composite material is higher than the applied electric field, while the electric field across the ferroelectric layer is lower than the applied electric field. This allows the relaxor ferroelectric layer to undergo a phase transition from relaxor ferroelectric to ferroelectric under a lower electric field, thereby obtaining greater strain performance under a low electric field.

[0045] The relaxor / ferroelectric layered composite structure of the present invention diffuses less during sintering, and will not deteriorate the matrix properties due to diffusion causing sintering into a single phase and failure to achieve composite.

[0046] 2. The layered composite lead-free relaxor ferroelectric material of the present invention has a simple elemental composition, which can improve the consistency of product performance.

[0047] 3. In the preparation process of the layered composite lead-free relaxor ferroelectric material of the present invention, the pressures of the first and second pre-pressing are both lower than the pressure of the final pressing. By using two pre-pressings with lower pressures and a final pressing with higher pressures, the powder added sequentially is prevented from being subjected to different levels of high pressure, which could cause uneven powder distribution and improve the uniformity of ceramic green body powder. Attached Figure Description

[0048] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0049] Figure 1 This is a schematic diagram of the layered composite lead-free relaxor ferroelectric material structure of the present invention;

[0050] Figure 2 The electric field strain curve of the layered composite lead-free relaxor ferroelectric material in Example 1 is shown.

[0051] Figure 3 The electric field strain curve of the layered composite lead-free relaxor ferroelectric material in Example 2 is shown.

[0052] Figure 4 The electric field strain curve of the layered composite lead-free relaxor ferroelectric material is shown in Comparative Example 1.

[0053] Figure 5 The electric field strain curves of the layered composite lead-free relaxor ferroelectric material are shown in Comparative Example 2.

[0054] Figure 6 The electric field strain curve of the ceramic material in Comparative Example 3 is shown.

[0055] Figure 7 The electric field strain curve of the ceramic material in Comparative Example 4 is shown. Detailed Implementation

[0056] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0057] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0058] Example 1

[0059] This embodiment provides a layered composite lead-free relaxor ferroelectric material, such as Figure 1 As shown, from bottom to top, it includes a first electrode layer, a first relaxor ferroelectric layer, a ferroelectric layer, a second relaxor ferroelectric layer, and a second electrode layer.

[0060] This embodiment also provides a method for preparing a layered composite lead-free relaxor ferroelectric material, including the following steps:

[0061] 1. Preparation of BNT-ST powder: Bi2O3, Na2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.75Bi 0.5 Na 0.5 TiO3-0.25SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0062] Preparation of BNKT-ST powder: Bi₂O₃, Na₂CO₃, K₂CO₃, SrTiO₃, and TiO₂ were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.96Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.04SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0063] 2. Add 4 wt% of polyvinyl alcohol (PVA) aqueous solution to BNT-ST powder and BNKT-ST powder respectively for granulation to obtain BNT-ST granules and BNKT-ST granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0064] Take 0.45g of BNT-ST granules and press them in a metal mold (10mm inner diameter) at 10MPa for 10s. Then take 0.1g of BNKT-ST granules and add them to the mold, and press them at 10MPa for 10s. Finally, take 0.45g of BNT-ST powder and add it to the mold, and press it at 150MPa for 60s to make a ceramic green body with a sandwich structure (0.45BNT-ST / 0.1BNKT-ST / 0.45BNT-ST).

[0065] 3. The ceramic preform is heated at 600℃ for 2 hours to remove PVA, and then the temperature is raised to 1100℃ and held for 2 hours to obtain a layered composite lead-free relaxor ferroelectric material.

[0066] 4. After cooling, silver paste is uniformly applied to the surface of the layered composite lead-free relaxor ferroelectric material and sintered at 500℃ for 2 hours.

[0067] Example 2

[0068] This embodiment provides a method for preparing a layered composite lead-free relaxor ferroelectric material, including the following steps:

[0069] 1. Preparation of BNT-ST powder: Bi2O3, Na2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.75Bi 0.5 Na 0.5 TiO3-0.25SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0070] Preparation of BNKT-ST powder: Bi₂O₃, Na₂CO₃, SrTiO₃, K₂CO₃, and TiO₂ were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.96Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.04SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0071] 2. Add 4 wt% of polyvinyl alcohol (PVA) aqueous solution to BNT-ST powder and BNKT-ST powder respectively for granulation to obtain BNT-ST granules and BNKT-ST granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0072] Take 0.4g of BNT-ST granules and press them in a metal mold (10mm inner diameter) at 10MPa for 5s. Then take 0.2g of BNKT granules and add them to the mold, and press them at 10MPa for 5s. Finally, take 0.4g of BNT-ST powder and add it to the mold, and press it at 150MPa for 60s to make a ceramic green body with a sandwich structure (0.4BNT-ST / 0.2BNKT-ST / 0.4BNT-ST).

[0073] 3. The ceramic preform is heated at 600℃ for 2 hours to remove PVA, and then the temperature is raised to 1100℃ and held for 2 hours to obtain a layered composite lead-free relaxor ferroelectric material.

[0074] 4. After cooling, silver paste is uniformly applied to the surface of the layered composite lead-free relaxor ferroelectric material and sintered at 500℃ for 2 hours.

[0075] Example 3

[0076] This embodiment provides a method for preparing a layered composite lead-free relaxor ferroelectric material, including the following steps:

[0077] 1. Preparation of BNT-ST powder: Bi2O3, Na2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.72Bi 0.5 Na 0.5 TiO3-0.28SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0078] Preparation of BNKT-ST powder: Bi₂O₃, Na₂CO₃, SrTiO₃, K₂CO₃, and TiO₂ were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.96Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.04SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0079] 2. Add 4 wt% of polyvinyl alcohol (PVA) aqueous solution to BNT-ST powder and BNKT-ST powder respectively for granulation to obtain BNT-ST granules and BNKT-ST granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0080] Take 0.4g of BNT-ST granules and press them in a metal mold (10mm inner diameter) at 10MPa for 5s. Then take 0.2g of BNKT granules and add them to the mold, and press them at 10MPa for 5s. Finally, take 0.4g of BNT-ST powder and add it to the mold, and press it at 150MPa for 60s to make a ceramic green body with a sandwich structure (0.4BNT-ST / 0.2BNKT-ST / 0.4BNT-ST).

[0081] 3. The ceramic preform is heated at 600℃ for 2 hours to remove PVA, and then the temperature is raised to 1100℃ and held for 2 hours to obtain a layered composite lead-free relaxor ferroelectric material.

[0082] 4. After cooling, silver paste is uniformly applied to the surface of the layered composite lead-free relaxor ferroelectric material and sintered at 500℃ for 2 hours.

[0083] Example 4

[0084] This embodiment provides a method for preparing a layered composite lead-free relaxor ferroelectric material, including the following steps:

[0085] 1. Preparation of BNT-ST powder: Bi2O3, Na2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.75Bi 0.5 Na 0.5 TiO3-0.25SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0086] Preparation of BNKT-ST powder: Bi₂O₃, Na₂CO₃, SrTiO₃, K₂CO₃, and TiO₂ were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.93Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.07SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0087] 2. Add 4 wt% of polyvinyl alcohol (PVA) aqueous solution to BNT-ST powder and BNKT-ST powder respectively for granulation to obtain BNT-ST granules and BNKT-ST granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0088] Take 0.4g of BNT-ST granules and press them in a metal mold (10mm inner diameter) at 10MPa for 5s. Then take 0.2g of BNKT granules and add them to the mold, and press them at 10MPa for 5s. Finally, take 0.4g of BNT-ST powder and add it to the mold, and press it at 150MPa for 60s to make a ceramic green body with a sandwich structure (0.4BNT-ST / 0.2BNKT-ST / 0.4BNT-ST).

[0089] 3. The ceramic preform is heated at 600℃ for 2 hours to remove PVA, and then the temperature is raised to 1100℃ and held for 2 hours to obtain a layered composite lead-free relaxor ferroelectric material.

[0090] 4. After cooling, silver paste is uniformly applied to the surface of the layered composite lead-free relaxor ferroelectric material and sintered at 500℃ for 2 hours.

[0091] Comparative Example 1

[0092] This comparative example is basically the same as Example 1, except that in step 2, 0.35g of BNT-ST granules are pressed in a metal mold at 10 MPa for 10s, then 0.3g of BNKT granules are added to the mold and pressed at 10 MPa for 10s, and finally 0.35g of BNT-ST powder is added to the mold and pressed at 150 MPa for 60s to produce a ceramic green body with a sandwich structure (0.35BNT-ST / 0.3BNKT-ST / 0.35BNT-ST).

[0093] Comparative Example 2

[0094] This comparative example is basically the same as Example 1, except that in step 2, 0.3g of BNT-ST granules are pressed in a metal mold at 10 MPa for 10 seconds, then 0.4g of BNKT granules are added to the mold and pressed at 10 MPa for 10 seconds, and finally 0.3g of BNT-ST powder is added to the mold and pressed at 150 MPa for 60 seconds to produce a ceramic green body with a sandwich structure (0.3BNT-ST / 0.4BNKT-ST / 0.3BNT-ST).

[0095] Comparative Example 3

[0096] This comparative example provides a method for preparing a ceramic material, including the following steps:

[0097] 1. Preparation of BNT-ST powder: Bi2O3, Na2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.75Bi 0.5 Na 0.5 TiO3-0.25SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0098] 2. Add 4 wt% of polyvinyl alcohol (PVA) aqueous solution to BNT-ST powder for granulation to obtain BNT-ST granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0099] Take 1g of BNT-ST granules and press them in a metal mold (10mm inner diameter) at 150MPa for 60s to form a single-layer ceramic green body (BNT-ST).

[0100] 3. Heat the ceramic body at 600℃ for 2 hours to remove PVA, and then continue to heat to 1100℃ and hold for 2 hours.

[0101] 4. After cooling, apply silver paste evenly to the ceramic surface and sinter at 500℃ for 2 hours.

[0102] Comparative Example 4

[0103] This comparative example provides a method for preparing a ceramic material, including the following steps:

[0104] 1. Preparation of BNKT-ST powder: Bi2O3, Na2CO3, K2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.96Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.04SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0105] 2. Add 4 wt% polyvinyl alcohol (PVA) aqueous solution to BNKT-ST powder for granulation to obtain BNKT granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0106] Take 1g of BNKT-ST granules and press them in a metal mold (10mm inner diameter) for 60s at 150MPa to form a single-layer ceramic green body (BNKT-ST).

[0107] 3. Heat the ceramic body at 600℃ for 2 hours to remove PVA, and then continue to heat to 1100℃ and hold for 2 hours.

[0108] 4. After cooling, apply silver paste evenly to the ceramic surface and sinter at 500℃ for 2 hours.

[0109] Comparative Example 5

[0110] This comparative example provides a method for preparing a composite lead-free relaxor ferroelectric material, including the following steps:

[0111] 1. Preparation of BNT-ST powder: Bi2O3, Na2CO3, SrTiO3, and TiO2 were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.75Bi 0.5 Na 0.5TiO3-0.25SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0112] Preparation of BNKT-ST powder: Bi₂O₃, Na₂CO₃, K₂CO₃, SrTiO₃, and TiO₂ were mixed, ball-milled, dried, pre-calcined, ball-milled again, and dried to prepare 0.96Bi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-0.04SrTiO3 powder. The ball milling time is 24h, the drying process is 110℃ for 24h, the pre-calcination process is 800℃ for 2h, and the secondary ball milling time is 24h.

[0113] 2. Add 4 wt% of polyvinyl alcohol (PVA) aqueous solution to BNT-ST powder and BNKT-ST powder respectively for granulation to obtain BNT-ST granules and BNKT-ST granules; the concentration of polyvinyl alcohol (PVA) aqueous solution is 7%.

[0114] Mix 0.9g of BNT-ST granules and 0.1g of BNKT-ST granules and add them to a metal mold (10mm inner diameter). Press at 150MPa for 60s to form a ceramic body.

[0115] 3. The ceramic preform is heated at 600℃ for 2 hours to remove PVA, and then the temperature is raised to 1100℃ and held for 2 hours to obtain a composite lead-free relaxor ferroelectric material.

[0116] 4. After cooling, silver paste is uniformly coated on the surface of the composite lead-free relaxor ferroelectric material and sintered at 500℃ for 2 hours.

[0117] Test case

[0118] 1. The strain properties of the materials obtained in Examples 1-4 and Comparative Examples 1-5 were tested. The testing equipment was from aixACCT Systems GmbH, Germany, with a test electric field of 30 kV / cm and a test frequency of 1 Hz.

[0119] Test results are available Figure 2-7 See Table 1.

[0120] Table 1 Strain properties of ceramic materials

[0121]

[0122] As shown in Table 1, the strain excited by the layered composite lead-free relaxor ferroelectric ceramic material of the present invention under a low driving electric field is significantly improved.

[0123] 2. Consistency Testing

[0124] Example 1 was repeated 5 times, and the obtained layered composite lead-free relaxor ferroelectric ceramic materials were tested respectively. The strain properties were 0.25%, 0.24%, 0.24%, 0.25%, and 0.25% respectively. It can be seen that the layered composite lead-free relaxor ferroelectric ceramic materials prepared by the present invention have high consistency in performance and good repeatability.

[0125] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A layered composite lead-free relaxor ferroelectric material, characterized in that, It includes a first relaxor ferroelectric layer, a ferroelectric layer, and a second relaxor ferroelectric layer arranged sequentially; in the layered composite lead-free relaxor ferroelectric material, 0 < ferroelectric layer mass percentage < 30%; The first relaxor ferroelectric layer is mBi 0.5 Na 0.5 TiO3-(1-m)SrTiO3 ceramic, 0.7≤m≤0.8; The ferroelectric layer is nBi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-(1-n)SrTiO3, 0.9≤n≤1; The second relaxor ferroelectric layer is pBi 0.5 Na 0.5 TiO3-(1-p)SrTiO3 ceramic, 0.7≤p≤0.

8.

2. The layered composite lead-free relaxor ferroelectric material according to claim 1, characterized in that, In the layered composite lead-free relaxor ferroelectric material, the first relaxor ferroelectric layer and the second relaxor ferroelectric layer have the same mass percentage.

3. A method for preparing the layered composite lead-free relaxor ferroelectric material according to claim 1 or 2, characterized in that, Includes the following steps: The powder of the first relaxor ferroelectric layer is placed into the mold for the first pre-pressing. The ferroelectric layer powder is added into the mold for a second pre-pressing process; The powder of the second relaxor ferroelectric layer is added into the mold and finally pressed to obtain the ceramic blank. A layered composite lead-free relaxor ferroelectric material is prepared by sintering a ceramic blank.

4. The method for preparing the layered composite lead-free relaxor ferroelectric material according to claim 3, characterized in that, The pressures of the first and second pre-compressions are both less than the pressure of the final compression.

5. The method for preparing the layered composite lead-free relaxor ferroelectric material according to claim 4, characterized in that, At least one of the following conditions must be met: (1) The pressure of the first pre-compression is 10~30MPa; (2) The pressure of the second pre-compression is 10~30 MPa; (3) The final pressing pressure is 100~150 MPa.

6. The method for preparing the layered composite lead-free relaxor ferroelectric material according to any one of claims 3-5, characterized in that, At least one of the following conditions must be met: (1) The powder of the first relaxor ferroelectric layer is mBi 0.5 Na 0.5 TiO3-(1-m)SrTiO3 powder, 0.7≤m≤0.8; (2) The powder of the ferroelectric layer is nBi 0.5 (Na 0.84 K 0.16 ) 0.5 TiO3-(1-n)SrTiO3 powder, 0.9≤n≤1; (3) The powder of the second relaxor ferroelectric layer is pBi 0.5 Na 0.5 TiO3-(1-p)SrTiO3 powder, 0.7≤p≤0.

8.

7. The method for preparing the layered composite lead-free relaxor ferroelectric material according to any one of claims 3-5, characterized in that, At least one of the following conditions must be met: (1) Granulate the powder of the first relaxor ferroelectric layer before placing it into the mold; (2) Granulate the ferroelectric layer powder before adding it into the mold; (3) Granulate the powder of the second relaxor ferroelectric layer before adding it into the mold.

8. The method for preparing the layered composite lead-free relaxor ferroelectric material according to any one of claims 3-5, characterized in that, The sintering process involves holding the temperature at 1050~1150℃ for 2~4 hours.

9. The method for preparing the layered composite lead-free relaxor ferroelectric material according to any one of claims 3-5, characterized in that, It also includes coating the surface of the layered composite lead-free relaxor ferroelectric material with silver paste and sintering it at 500~550℃ for 1-3 hours.

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