Lead-free electrocaloric ceramic material and preparation method thereof

Abstract

The invention discloses a lead-free electrocaloric ceramic material and a preparation method thereof. Comprising the following steps that BaCO3, Sm2O3, Li2CO3 and TiO2 are weighed according to the stoichiometric ratio of Ba (1-2x) SmxLixTiO3, ceramic raw materials are subjected to ball milling, ceramic powder is obtained, and x is larger than 0.0 and smaller than or equal to 0.1; drying the ceramic powder, sintering for the first time, and then sequentially carrying out secondary ball-milling mixing, drying, sieving, granulating, dry pressing and cold isostatic pressing to prepare a ceramic green body; and sintering the ceramic green body for the second time, and then cooling along with the furnace to obtain the lead-free electrocaloric ceramic material. A defect dipole is formed by equivalently introducing trivalent samarium ions and monovalent lithium ions into a lead-free barium titanate ceramic material, and the defect dipole not only serves as an exogenous polarization factor to improve the polarizability of barium titanate, but also improves a breakdown electric field, so that the internal balance between the high breakdown electric field and high polarization is effectively relieved; therefore, the electrocaloric temperature change of the barium titanate ceramic is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of electronic materials, and more specifically, relates to a lead-free electrocaloric ceramic material and a preparation method thereof. Background Art

[0002] Refrigeration technology is crucial in modern life, such as highly integrated circuits, high-power instruments, and cold chains for food and medicine. The currently dominant vapor compression refrigeration system in the market generates a large amount of greenhouse gases during use, damages the environment, has low refrigeration efficiency, and is difficult to further improve, resulting in an excessively high energy consumption ratio. The new electrocaloric refrigeration technology, as an alternative to the traditional vapor compression refrigeration technology, has received extensive attention and research due to its high refrigeration efficiency, environmental friendliness, and easy system integration. The electrocaloric refrigeration technology is based on the electrocaloric effect. By inducing a change in the polarization state of a ferroelectric material through an electric field, the change in the internal entropy value of the ferroelectric material is controlled, and heat transfer is achieved through the exothermic / endothermic behavior generated during the application / removal of the electric field, thereby realizing refrigeration. Compared with electrocaloric materials such as ferroelectric single crystals and organic polymers, ferroelectric ceramics, which have competitive advantages such as high electric breakdown strength, mature commercial preparation technology, and low cost, have become the most studied electrocaloric materials in recent years and have broad application prospects in solid-state refrigeration technology.

[0003] Currently, the research on the electrocaloric effect of ferroelectric ceramic materials is generally based on Maxwell relations. Most of the regulation methods are through constructing a solid solution system and adjusting the molar ratio it occupies to change the phase transition temperature, so that the phase transition occurs in the required working temperature range. At the same time, due to the formation of diffuse phase transition or continuous phase transition, the working temperature range of ferroelectric ceramic materials is broadened. Then, by controlling the sintering process, adjusting the grain size, increasing the material density, and breakdown field strength, a ferroelectric ceramic material with a giant electrocaloric effect is obtained. As the regulation of the intrinsic dipole polarization of ferroelectric ceramic materials by research reaches a certain level, it is more difficult to further improve. Therefore, there is an urgent need to provide a material with a giant electrocaloric effect and a higher breakdown electric field. Summary of the Invention

[0004] In view of the above defects or improvement requirements of the prior art, the present invention provides a lead-free electrocaloric ceramic material and a preparation method thereof, thereby solving the technical problem of the low electrocaloric effect of ferroelectric ceramic materials existing in the prior art.

[0005] To achieve the above object, according to one aspect of the present invention, a lead-free electrocaloric ceramic material is provided, and its chemical composition is Ba (1-2x) Sm x Li x TiO3, where 0.0 < x ≤ 0.1.

[0006] According to another aspect of the present invention, there is provided a method for preparing the lead-free electrocaloric ceramic material as described above, comprising the following steps:

[0007] Step 1: Weigh ceramic raw materials of BaCO3, Sm2O3, Li2CO3 and TiO2 respectively according to the stoichiometric ratios of Ba element, Sm element, Li element and Ti element in Ba (1-2x) Sm x Li x TiO3, and ball-mill the ceramic raw materials to obtain ceramic powder, where 0.0 < x ≤ 0.1;

[0008] Step 2: Dry the ceramic powder and then conduct the first sintering, and then successively conduct secondary ball-milling and mixing, drying, sieving, granulation, dry pressing and cold isostatic pressing to make a ceramic green body;

[0009] Step 3: Conduct the second sintering on the ceramic green body, and then cool down with the furnace to obtain the lead-free electrocaloric ceramic material.

[0010] Preferably, the ball-milling conditions in both Step 1 and Step 2 are: ball-milling is carried out in the presence of a ball-milling medium and a ball-milling solvent; the mass ratio of the ball-milling medium, the ball-milling solvent and the ceramic raw materials is (8 - 10):(1 - 5):(1 - 5), and the ball-milling time is 0.1 - 24 h.

[0011] Preferably, the ball-milling medium is zirconia balls, and the ball-milling solvent is absolute ethanol.

[0012] Preferably, in Step 2, the drying conditions are: drying at 40 - 100 °C for 0.1 - 24 hours.

[0013] Preferably, the conditions for the first sintering are: heating to 700 - 1200 °C at a heating rate of 0.1 - 20 °C / min and holding for 0.1 - 24 hours.

[0014] Preferably, the sieving specifically is: sieving through an 8 - 300 mesh sieve, and taking the material below the 8 - 300 mesh sieve.

[0015] Preferably, the granulation specifically is: granulating after mixing a binder with the ceramic powder; the mass of the binder is 1 - 50% of the mass of the ceramic powder; the binder is preferably an aqueous solution of polyvinyl alcohol, and the mass fraction of polyvinyl alcohol in the aqueous solution of polyvinyl alcohol is 1 - 20%.

[0016] Preferably, in Step 2, the pressure for dry pressing is 1 - 100 MPa, and the time for dry pressing is 0.1 - 60 minutes.

[0017] Preferably, in step 2, the pressure of the cold isostatic pressing is 1 to 300 MPa, and the time of the cold isostatic pressing is 1 to 60 min.

[0018] Preferably, in step 3, the second sintering is specifically carried out as follows: heating at a heating rate of 1 to 20 °C / min to 1000 to 1400 °C, and then holding for 0.1 to 24 hours.

[0019] Generally speaking, compared with the prior art, the above technical solution conceived by the present invention can achieve the following beneficial effects:

[0020] 1. The present invention provides a lead-free electrocaloric ceramic material Ba (1-2x) Sm x Li x TiO3, which forms defect dipoles (Sm Ba · -Li Ba ′) by introducing trivalent samarium ions and monovalent lithium ions in an equal amount into the lead-free barium titanate ceramic material and substituting the divalent barium ions of barium titanate (BaTiO3) in a substitutional manner. On the one hand, the constructed defect dipoles, as exogenous polarization factors, increase the polarization rate of barium titanate from below 20 μC / cm 2 to above 20 μC / cm 2 . On the other hand, the constructed defect dipoles broaden the band gap of barium titanate, effectively increasing the carrier activation energy and raising the breakdown electric field of barium titanate to 80 to 200 kV / cm. The effective alleviation of the internal trade-off between the high breakdown electric field and high polarization of the lead-free electrocaloric ceramic material enables the electrocaloric temperature change of the material at room temperature to be increased to 1.54 K. Therefore, the present invention proposes a material that regulates the polarization behavior and breakdown electric field of ferroelectric ceramic materials by constructing defect dipoles, thereby enhancing its electrocaloric effect and obtaining a giant electrocaloric effect. The defect dipoles not only increase the polarization rate of barium titanate but also enhance the breakdown electric field of the material.

[0021] 2. The present invention adopts two sinterings. The temperature of the first sintering is 700 to 1200 °C, and the purpose is to discharge CO2 from the ceramic raw material powder that has been ball-milled and mixed evenly for the first time and initially form a phase structure; the temperature of the second sintering is 1000 to 1400 °C, and the purpose is to make the formed ceramic green body finally form an electrocaloric ceramic material after high-temperature sintering.

[0022] 3. The present invention improves the electrocaloric effect of Ba (1-2x) Sm x Li x TiO3 by changing the ratio of samarium, lithium and barium. The preparation process is simple and increases the temperature change of the electrocaloric material. Description of the Drawings

[0023] Figure 1It is the SEM image of the lead-free electrocaloric ceramic material in Embodiments 1-5 of the present invention, where a is Embodiment 1; b is Embodiment 2; c is Embodiment 3; d is Embodiment 4; e is Embodiment 5.

[0024] Figure 2 It is the electrocaloric temperature change diagram of the lead-free electrocaloric ceramic material in Embodiments 1-5 of the present invention.

[0025] Figure 3 It is the polarizability diagram of the lead-free electrocaloric ceramic material in Embodiments 1-5 of the present invention.

[0026] Figure 4 It is the XRD diagram of the lead-free electrocaloric ceramic material in Embodiment 3 of the present invention. Detailed implementation manners

[0027] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] Embodiment 1:

[0029] Prepare Ba 0.98 Sm 0.01 Li 0.01 TiO3 electrocaloric ceramic material, including the following steps:

[0030] Step 1: Weigh the raw materials Ba 0.98 Sm 0.01 Li 0.01 TiO3 required for 0.15 mol, namely BaCO3 (29.07 g), Sm2O3 (0.26 g), Li2CO3 (0.06 g) and TiO2 (11.99 g), and configure the dosages. Use the wet ball milling process to mix all raw materials evenly. The mass ratio of the ball milling medium, anhydrous ethanol, and ball milling material is 8:4:3, the ball milling time is 6 hours, the ball milling medium is zirconia balls, and the ball milling solvent is anhydrous ethanol.

[0031] Step 2: After drying the powder evenly ball milled in Step 1, pre-sinter it at 1000 °C for 4 hours.

[0032] Step 3: Ball mill the material pre-sintered in Step 2 for another 6 hours to obtain ceramic powder.

[0033] Step 4: Dry the ceramic slurry in Step 3, pass it through an 80-mesh sieve, add a binder accounting for 10% of the mass of the ceramic powder. The binder is an aqueous solution of polyvinyl alcohol (PVA) with a mass fraction of 4%, granulate, and sieve.

[0034] Step 5: Dry-press the ceramic powder from Step 4 through a mold at a pressure of 4 MPa for a holding time of 10 seconds, and then obtain a green ceramic body through cold isostatic pressing.

[0035] Step 6: Sinter the green ceramic body obtained in Step 5 in air: Heat it to 1300 °C at a rate of 3 °C / min, hold for 3 hours, and then cool it to room temperature with the furnace to obtain Ba 0.98 Sm 0.01 Li 0.01 TiO3 electrocaloric ceramic material.

[0036] The SEM image of the obtained lead-free electrocaloric ceramic material is as shown in Figure 1 a in the figure, and the electrocaloric temperature change value is as shown in Figure 2 the figure.

[0037] Example 2:

[0038] Prepare Ba 0.96 Sm 0.02 Li 0.02 TiO3 electrocaloric ceramic material, including the following steps:

[0039] Step 1: Weigh 0.15 mol of Ba 0.96 Sm 0.02 Li 0.02 TiO3 required raw materials of BaCO3 (28.48 g), Sm2O3 (0.52 g), Li2CO3 (0.11 g) and TiO2 (11.99 g) for dosage configuration, and use the wet ball milling process to mix all raw materials evenly. The mass ratio of the ball milling medium, anhydrous ethanol, and ball milling material is 8:4:3, the ball milling time is 6 hours, the ball milling medium is zirconia balls, and the ball milling solvent is anhydrous ethanol.

[0040] Step 2: After drying the powder evenly ball milled in Step 1, pre-sinter it at 1000 °C for 4 hours.

[0041] Step 3: Ball mill the material pre-sintered in Step 2 for another 6 hours to obtain ceramic powder.

[0042] Step 4: Dry the ceramic slurry in Step 3, pass it through an 80-mesh sieve, add a binder accounting for 10% of the mass of the ceramic powder. The binder is an aqueous solution of polyvinyl alcohol (PVA) with a mass fraction of 4%, granulate it, and then pass it through a sieve.

[0043] Step 5: Dry-press the ceramic powder from Step 4 through a mold at a pressure of 4 MPa for a holding time of 10 seconds, and then obtain a green ceramic body through cold isostatic pressing.

[0044] Step 6: Sinter the green ceramic body obtained in Step 5 in air: Heat it up to 1290 °C at a rate of 3 °C / min, hold for 3 hours, and then cool it down to room temperature with the furnace to obtain the Ba 0.96 Sm 0.02 Li 0.02 TiO3 electrocaloric ceramic material.

[0045] The SEM image of the obtained lead-free electrocaloric barium titanate-based ceramic material is as shown in Figure 1 b, and the electrocaloric temperature change value is as shown in Figure 2 .

[0046] Example 3:

[0047] Prepare the Ba 0.94 Sm 0.03 Li 0.03 TiO3 electrocaloric ceramic material, including the following steps:

[0048] Step 1: Weigh the raw materials Ba 0.94 Sm 0.03 Li 0.03 TiO3 required, namely BaCO3 (27.89 g), Sm2O3 (0.78 g), Li2CO3 (0.17 g) and TiO2 (11.99 g), according to the stoichiometric ratio. Use the wet ball milling process to mix all the raw materials evenly. The mass ratio of the ball milling medium, absolute ethanol, and ball milling material is 8:4:3. The ball milling time is 6 hours. The ball milling medium is zirconia balls, and the ball milling solvent is absolute ethanol.

[0049] Step 2: After drying the powder evenly ball milled in Step 1, pre-sinter it at 1000 °C for 4 hours.

[0050] Step 3: Ball mill the material pre-sintered in Step 2 for another 6 hours to obtain ceramic powder.

[0051] Step 4: Dry the ceramic slurry in Step 3, pass it through an 80-mesh sieve, add a binder accounting for 10% of the mass of the ceramic powder. The binder is an aqueous solution of polyvinyl alcohol (PVA) with a mass fraction of 4%. Granulate and sieve.

[0052] Step 5: Dry press the ceramic powder in Step 4 through a mold at a pressure of 4 MPa for a holding time of 10 seconds, and then obtain a green ceramic body through cold isostatic pressing.

[0053] Step 6: Sinter the green ceramic body obtained in Step 2 in air: Heat it up to 1280 °C at a rate of 3 °C / min, hold for 3 hours, and then cool it down to room temperature with the furnace to obtain the Ba 0.94 Sm 0.03 Li 0.03 TiO3 electrocaloric ceramic material.

[0054] The SEM image of the obtained lead-free electrocaloric ceramic material is as shown in Figure 1 Figure c, and the electrocaloric temperature change value is as shown in Figure 2 Figure

[0055] Example 4:

[0056] Prepare Ba 0.92 Sm 0.04 Li 0.04 TiO3 electrocaloric ceramic material, including the following steps:

[0057] Step 1: According to the stoichiometric ratio, weigh 0.15 mol of Ba 0.92 Sm 0.04 Li 0.04 The raw materials required for TiO3, namely BaCO3 (27.29 g), Sm2O3 (1.05 g), Li2CO3 (0.23 g) and TiO2 (11.99 g), are configured in terms of dosage, and all raw materials are mixed evenly by wet ball milling process. The mass ratio of the ball milling medium, absolute ethanol and ball milling material is 8:4:3, the ball milling time is 6 hours, the ball milling medium is zirconium balls, and the ball milling solvent is absolute ethanol.

[0058] Step 2: After drying the powder uniformly ball milled in Step 1, pre-sinter it at 1000 °C for 4 hours.

[0059] Step 3: Ball mill the material pre-sintered in Step 2 for another 6 hours to obtain ceramic powder.

[0060] Step 4: Dry the ceramic slurry in Step 3, pass it through an 80-mesh sieve, add a binder accounting for 10% of the mass of the ceramic powder. The binder is an aqueous solution of polyvinyl alcohol (PVA) with a mass fraction of 4%, granulate it, and then sieve it.

[0061] Step 5: Dry press the ceramic powder in Step 4 through a mold at a pressure of 4 MPa for a holding time of 10 seconds, and then obtain a ceramic green body through cold isostatic pressing.

[0062] Step 6: Sinter the ceramic green body obtained in Step 2 in air: heat it up to 1270 °C at a rate of 3 °C / min and hold for 3 hours, then cool it down to room temperature with the furnace, and thus obtain Ba 0.92 Sm 0.04 Li 0.04 TiO3 electrocaloric ceramic material.

[0063] The SEM image of the obtained lead-free electrocaloric ceramic material is as shown in Figure 1 Figure d, and the electrocaloric temperature change value is as shown in Figure 2 Figure

[0064] Example 5:

[0065] Prepare Ba0.90 Sm 0.05 Li 0.05 The preparation method of Ba

[0066] Step 1: Weigh 0.15 mol of BaCO3 (26.70 g), Sm2O3 (1.31 g), Li2CO3 (0.28 g) and TiO2 (11.99 g) required for Ba 0.90 Sm 0.05 Li 0.05 TiO3 according to the stoichiometric ratio, and use the wet ball milling process to mix all raw materials evenly. The mass ratio of the ball milling medium, absolute ethanol, and ball milling material is 8:4:3, the ball milling time is 6 hours, the ball milling medium is zirconia balls, and the ball milling solvent is absolute ethanol.

[0067] Step 2: After drying the powder evenly ball milled in Step 1, pre-sinter it at 1000 °C for 4 hours.

[0068] Step 3: Ball mill the material pre-sintered in Step 2 for another 6 hours to obtain ceramic powder.

[0069] Step 4: Dry the ceramic slurry in Step 3, sieve it through a 80-mesh sieve, add a binder accounting for 10% of the mass of the ceramic powder. The binder is an aqueous solution of polyvinyl alcohol (PVA) with a mass fraction of 4%, granulate it, and sieve it.

[0070] Step 5: Dry press the ceramic powder in Step 4 through a mold at a pressure of 4 MPa for a holding time of 10 seconds, and then obtain a green ceramic body through cold isostatic pressing.

[0071] Step 6: Sinter the green ceramic body obtained in Step 2 in air: heat it to 1260 °C at a rate of 3 °C / min and hold for 3 hours, then cool it to room temperature with the furnace, and Ba 0.90 Sm 0.05 Li 0.05 TiO3 electrocaloric ceramic material can be obtained.

[0072] The SEM image of the obtained lead-free electrocaloric ceramic material is as shown in Figure 1 e, and the electrocaloric temperature change value is as shown in Figure 2 shown.

[0073] Result analysis: According to the test results of the samples obtained in Examples 1-5 Figures 1 to 3 It can be seen that the lead-free electrocaloric ceramic material of the present invention has the following characteristics:

[0074] Figure 1 Fig. is the SEM image of the lead-free electrocaloric ceramic material in Examples 1-5, and the results show that all samples have a dense and uniform grain structure.

[0075] Figure 2 For the electrocaloric temperature change values of the lead-free electrocaloric ceramic materials in Examples 1-5, the results show that: when x = 0.03, Ba 0.94 Sm 0.03 Li 0.03 TiO3 ceramic materials exhibit a high breakdown electric field (80-200 kV / cm) and a giant electrocaloric effect (1-3 K), which are suitable for fabricating electrocaloric refrigeration devices.

[0076] Figure 3 For the polarizability of the lead-free electrocaloric ceramic materials in Examples 1-5, the results show that: when x = 0.03, Ba 0.94 Sm 0.03 Li 0.03 TiO3 ceramic materials have the highest polarization intensity of 25-40 μC / cm at 80-150 kV / cm 2 .

[0077] Figure 4 For the XRD pattern of the Ba 0.94 Sm 0.03 Li 0.03 TiO3 electrocaloric ceramic material in Example 3, all characteristic peaks in the figure, such as: (100), (110) and (111) characteristic peaks, correspond to the structural characteristic peaks of Ba 0.94 Sm 0.03 Li 0.03 TiO3 material, and there are no peaks of impurity phases, indicating the successful preparation of the Ba 0.94 Sm 0.03 Li 0.03 TiO3 electrocaloric ceramic material.

[0078] It is easy for those skilled in the art to understand that the above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A lead-free electrocaloric ceramic material, characterized in that, Its chemical composition is Ba (1-2x) Sm x Li x TiO3, where 0.0 < x ≤ 0.

1.

2. The preparation method of a lead-free electrocaloric ceramic material as described in claim 1, characterized in that, It includes the following steps: Step 1: Weigh ceramic raw materials of BaCO3, Sm2O3, Li2CO3 and TiO2 respectively according to the stoichiometric ratios of Ba element, Sm element, Li element and Ti element in (1-2x) Sm x Li x BaTiO3, and ball-mill the ceramic raw materials to obtain ceramic powder, where 0.0 < x ≤ 0.1; Step 2: After drying the ceramic powder, conduct the first sintering, and then successively carry out secondary ball milling and mixing, drying, sieving, granulation, dry pressing and cold isostatic pressing to obtain a green ceramic body; Step 3: Conduct the second sintering on the green ceramic body, and then cool down with the furnace to obtain the lead-free electrocaloric ceramic material.

3. The preparation method of a lead-free electrocaloric ceramic material as described in claim 2, characterized in that, The ball milling conditions in both Step 1 and Step 2 are: ball milling is carried out in the presence of a ball milling medium and a ball milling solvent; the mass ratio of the ball milling medium, the ball milling solvent and the ceramic raw material is (8-10):(1-5):(1-5), and the ball milling time is 0.1-24 h.

4. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, characterized in that, In Step 2, the drying conditions are: drying at 40-100 °C for 0.1-24 hours.

5. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, characterized in that, In Step 2, the conditions for the first sintering are: heating up to 700-1200 °C at a heating rate of 0.1-20 °C / min and holding for 0.1-24 hours.

6. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, characterized in that, In Step 2, the sieving specifically is: sieving through an 8-300 mesh sieve and taking the material under the 8-300 mesh sieve.

7. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, characterized in that, In Step 2, the granulation specifically is: mixing a binder with the ceramic powder and then carrying out granulation; the mass of the binder is 1-50% of the mass of the ceramic powder; the binder is preferably an aqueous solution of polyvinyl alcohol, and the mass fraction of polyvinyl alcohol in the aqueous solution of polyvinyl alcohol is 1-20%.

8. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, characterized in that, In Step 2, the pressure for dry pressing is 1-100 MPa, and the time for dry pressing is 0.1-60 min.

9. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, characterized in that In Step 2, the pressure for cold isostatic pressing is 1-300 MPa, and the time for cold isostatic pressing is 1-60 min.

10. The preparation method of a lead-free electrocaloric ceramic material according to claim 2, wherein, In Step 3, the second sintering specifically is: heating up to 1000-1400 °C at a heating rate of 1-20 °C / min, and then holding for 0.1-24 h.

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