A sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure and a preparation method

Abstract

The application relates to the technical field of electronic functional materials and devices, and relates to a sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure and a preparation method. 0.47 Na 0.47 Ba 0.06 Ti 1‑x (Li 1 / 4 Nb 3 / 4 ) x O3, and the preparation method comprises the following steps: according to the chemical formula, mixing raw materials, and sequentially performing a first ball milling, discharging, drying, pre-sintering, a second ball milling, discharging, drying, roller milling, flow casting, laminating and pressing, degassing and sintering processes to obtain the ceramic material. Compared with the prior art, the material provided by the application is a multilayer composite lead-free material, the ceramic has a novel design structure, the high electrocaloric effect in a wide temperature range is excellent, has great application prospects in the field of novel solid-state refrigeration technology, is not easy to pollute and damage the environment, has low preparation cost, good stability and the like.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of electronic functional materials and devices, and in particular to a sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure and a preparation method. BACKGROUND

[0002] Unlike traditional gas compression refrigeration technology, new refrigeration technology generally uses solid materials as media to achieve refrigeration, solves the problems of low energy conversion efficiency and adverse effects on the ecological environment, and is easier to preserve. Compared with traditional gas compression refrigeration, solid-state refrigeration technology is more efficient, green, quiet and stable, and therefore provides a new way to solve the problems of ozone layer destruction and greenhouse effect caused by the refrigeration process. Although new refrigeration technologies are different, their basic principles are similar. They all change the structure and properties of the refrigeration material (generally a solid) by applying different external fields such as magnetic fields, force fields and electric fields to the material, mainly changing the disorder degree of the internal structure of the material, and the change is manifested in the change of temperature. Therefore, energy conversion can be achieved by continuously applying and removing the external field, thereby realizing refrigeration.

[0003] As a new type of solid-state refrigeration, electrocaloric refrigeration generally uses ferroelectric materials as media, changes the disorder degree of the internal dipoles of the refrigeration material by adjusting the electric field applied to the refrigeration material, and then changes the temperature of the material to achieve the refrigeration effect. The realization of electrocaloric refrigeration also follows the Carnot cycle, but does not require large field generating devices and is not easy to affect the surrounding environment, so it overcomes more shortcomings and has more important significance in application.

[0004] In combination with previous papers and reports on ferroelectric materials, lead-based materials have been widely concerned due to their good dielectric properties, and many studies have been carried out around them. However, as a heavy metal element, the toxicity of lead cannot be ignored. From production to use, and in the process of scientific research, it is more likely to have adverse effects on the human body. At the same time, the increasingly severe environmental problems have prompted people to strictly legislate harmful materials in various fields, so exploring an environmentally friendly lead-free ferroelectric material is a hot research direction at present. Sodium bismuth titanate-based relaxor ferroelectric ceramics (BNT-based ceramics) have relatively high polarization strength and dielectric constant, and excellent performance without easily polluting and damaging the environment, so they have good research value and application prospect. BNT-based materials have a rhombohedral structure, good ferroelectric properties, and exhibit special dielectric properties at a certain temperature, but achieving large adiabatic temperature change and wide working temperature range is still challenging. Modifying the composition and structure of BNT materials as a basic material is expected to further improve their performance, and using a gradient structure to composite multiple phase transition temperature components can achieve synergistic optimization of the electric refrigerator effect and the working temperature range. SUMMARY

[0005] The purpose of the present application is to overcome the defects of the prior art and provide a sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure and a preparation method. By replacing traditional gas compression refrigeration materials and lead-based ferroelectric materials, the problems of low energy conversion efficiency and adverse effects on the ecological environment in the refrigeration field are solved. A new gradient composite structure is used to achieve a wide working temperature range of high electric refrigerator effect, thereby promoting the further practical application of electric refrigerator materials.

[0006] The purpose of the present application can be achieved by the following technical solutions:

[0007] A sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure, the chemical formula of the sodium bismuth titanate-based relaxor ferroelectric ceramic material is Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3, wherein x = 0.005, 0.010 and 0.015.

[0008] Further, when x = 0.005, 0.010 or 0.015, the ferroelectric-relaxor phase transition temperature of the material is 74℃, 58℃ and 45℃, respectively.

[0009] Further, the multi-layer composite refers to the composite of three structures of sodium bismuth titanate-based relaxor ferroelectric ceramic materials when x is 0.005, 0.010, and 0.015 respectively.

[0010] Further, when the multi-layer composite is performed, the quantity ratio of the three structures is 1:(1-2):(1-3).

[0011] Further, the multi-layer composite of the gradient structure includes a sandwich multi-layer structure and other possible multi-layer designs of gradient structures, thereby widening the phase transition temperature range and further optimizing the temperature stability of the electrocaloric effect.

[0012] Further, the ceramic material has a high electrocaloric effect, and the maximum electrocaloric negative temperature change is greater than 0.5K under a 6kV / mm electric field, close to lead-based materials.

[0013] Further, the ceramic material combines components with different ferroelectric-relaxor phase transition temperatures, so that continuous phase transition occurs in a wide temperature range, realizing a great widening of the electrocaloric working temperature, and the electrocaloric refrigeration working interval is up to 68℃.

[0014] A preparation method of a sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure, comprising the following steps:

[0015] According to the chemical formula, a bismuth source, a sodium source, a barium source, a titanium source, a lithium source, and a niobium source are mixed, and then sequentially subjected to a first ball milling, discharging, drying, pre-sintering, second ball milling, roller milling, laminating and pressing, degassing, and sintering processes to obtain the sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure.

[0016] Further, the bismuth source includes Bi2O3, the sodium source includes Na2CO3, the barium source includes BaCO3, the titanium source includes TiO2, the lithium source includes Li2CO3, and the niobium source includes Nb2O5.

[0017] Further, the ball milling time in the first and second ball milling processes is 10-12h.

[0018] Further, in the roller milling process, the roller milling time is 8-12h, and in the casting process, the adhesives used are ethanol, butanone, triolein, polyethylene glycol, dibutyl phthalate, and polyvinyl alcohol.

[0019] Further, in the pre-sintering process, the pre-sintering temperature is 800-900℃, and the pre-sintering time is 3-4h.

[0020] Further, in the laminating and pressing process, the forming pressure is 10MPa, the temperature is 50℃, and the pressing time is 20min.

[0021] Furthermore, the glue removal process specifically involves heating to 200-250℃ at a heating rate of 0.25-1℃ / min and calcining at a constant temperature for 1-3 hours, then heating to 500-600℃ at a heating rate of 0.5-2℃ / min and calcining at a constant temperature for 7-10 hours.

[0022] Furthermore, the ceramic green body used in the adhesive removal process has an area of ​​15mm × 15mm and a thickness of 0.48mm.

[0023] Furthermore, the sintering process specifically involves heating to 1150-1170℃ at a heating rate of 3-5℃ / min and calcining at a constant temperature for 2-3 hours.

[0024] Furthermore, the sintering process employs a method of lowering the sintering temperature, thereby suppressing defects caused by interlayer diffusion and volatilization of organic binders at high temperatures, while simultaneously inducing the ferroelectric-relaxation phase transition temperature to shift to near room temperature to achieve a high electrocaloric effect.

[0025] This method yielded a composition-gradient bismuth titanate-based relaxor ferroelectric ceramic material with a single perovskite phase via solid-state sintering. The x = 0.005, 0.010, and 0.015 compositions exhibit sharp dielectric anomaly peaks near the phase transition temperature, indicating a transition from an ergonomically relaxed phase to an ergonomically relaxed phase, and these peaks continuously shift towards lower temperatures with increasing doping concentration. Li + 、Nb 5+ The incorporation of [a specific substance] can induce a random distribution of ions of different sizes and valence states, leading to an increase in the local free electric field strength, a gradual decrease in the size of PNRs, and enhanced activity. Near the phase transition temperature, the remanent polarization Pr of the ceramic decreases sharply, but noteworthy is that a high saturation polarization Pm is still maintained. This is beneficial for achieving a large reversible entropy change, corresponding to a large electrocaloric response. Upon further heating, the local electric field suppresses the directional alignment of PNRs, resulting in a decrease in the electrocaloric response.

[0026] Simultaneously, by employing a gradient structure composite multilayer phase transition temperature composition method, the material undergoes multiple phase transitions during heating, corresponding to a sudden increase in the electrocaloric effect. This maintains a high negative electrocaloric temperature change over a relatively wide temperature range, achieving synergistic optimization of the material's electrocaloric effect and operating temperature range. This application uses a high-sensitivity resistance temperature detector (PT100) to directly obtain temperature changes, accurately collecting the electrocaloric temperature changes of the compositionally gradient structure bismuth titanate-based relaxor ferroelectric ceramic material at each temperature point.

[0027] This invention uses a solid-state sintering method to prepare lead-free Bi. 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 )x O3 ceramic material, the electric calorimetric temperature change is collected by direct test method. The ceramic has high electric calorimetric effect in a wide temperature range, has great application potential in the field of new solid-state refrigeration technology, has the advantages of low cost, high efficiency and easy miniaturization application, solves the problems of low refrigeration efficiency and environmental damage of traditional gas compression refrigeration method, and is expected to be applied in the field of information technology such as microelectronic devices and integrated circuits.

[0028] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0029] 1) The present application designs a new structure of ceramic, namely a composition gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material;

[0030] 2) The electric calorimetric refrigeration material in the present application has high electric calorimetric effect, and the highest electric calorimetric negative temperature change is up to 0.69K under an electric field of 6kV / mm, close to lead-based materials;

[0031] 3) The present application adopts the method of gradient structure composite multilayer phase transition temperature component, and the electric calorimetric refrigeration material maintains high electric calorimetric effect in a wide temperature range, and the temperature range of electric calorimetric negative temperature change greater than 0.5K under an electric field of 6kV / mm is up to 68℃, and the temperature stability is optimized;

[0032] 4) The electric calorimetric refrigeration material in the present application does not contain lead, is friendly to the environment, and is not easy to cause pollution and damage. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 The composition gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3 design A dielectric constant and dielectric loss temperature spectrum;

[0034] Figure 2 The composition gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3 design A temperature change P-E curve under an electric field of 6kV / mm;

[0035] Figure 3 The composition gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material Bi 0.47 Na0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x Temperature dependent I-E curve of Design A at electric field 6 kV / mm;

[0036] Figure 4 Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x Temperature dependent electrocaloric effect test curve of Design A at electric field 6 kV / mm;

[0037] Figure 5 Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x Temperature spectrum of dielectric constant and dielectric loss of Design B;

[0038] Figure 6 Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x Temperature dependent P-E curve of Design B at electric field 6 kV / mm;

[0039] Figure 7 Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x Temperature dependent I-E curve of Design B at electric field 6 kV / mm;

[0040] Figure 8 Bi 0.47 Na 0.47 Ba 0.06 Ti1-x (Li 1 / 4 Nb 3 / 4 ) x The temperature-variable electrocaloric effect test curve of design B under an electric field of 6 kV / mm;

[0041] Figure 9 The electric caloric effect change rate curve of the sodium bismuth titanate-based relaxor ferroelectric ceramic material obtained for example 2, design B and comparative example x = 0.005 under an electric field of 6 kV / mm. DETAILED DESCRIPTION

[0042] The application will be described in detail below with reference to the drawings and specific examples.

[0043] A sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure has a general chemical formula of Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3, wherein x = 0.005, 0.010 and 0.015, and the designed multilayer composite ceramic has a layer number ratio of 1:2:3 (denoted as A) and 1:1:1 (denoted as B) based on the layer number ratio of the sodium bismuth titanate-based relaxor ferroelectric ceramic material obtained when x is 0.005, 0.010 and 0.015.

[0044] The preparation method of the above-mentioned sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure comprises the following steps:

[0045] S1: pure Bi2O3, Na2CO3, BaCO3, TiO2, Li2CO3 and Nb2O5 with a purity of more than 99% are selected as raw materials for preparing the sodium bismuth titanate-based relaxor ferroelectric ceramic with a composition gradient structure, and are mixed after being weighed according to the above-mentioned general chemical formula to obtain primary mixed powder;

[0046] S2: anhydrous ethanol and zirconium dioxide grinding balls are added into a ball mill tank, and the primary mixed powder is added, and is ball milled for 10-12 h in a planetary ball mill, and is dried in a blast drying oven at 80-120 ℃ after discharging to obtain dried powder;

[0047] S3: the dried powder is placed in a muffle furnace and is pre-fired at 800-900 ℃ for 3-4 h to obtain pre-fired powder;

[0048] S4: The pre-sintered powder is subjected to secondary ball milling for 10-12 h, discharging, drying, adding ethanol, butanone, glycerol trioleate, polyethylene glycol, dibutyl phthalate, and polyvinyl alcohol, and roll milling for 8-12 h, and then casting, drying, and laminating according to the gradient structure design, and then pressing at a pressure of 10 MPa and a temperature of 50℃ for 20 min to form a green ceramic body, which is then sliced;

[0049] S5: The green ceramic body is transferred to a muffle furnace, heated to 200-250℃ at a heating rate of 0.25-1℃ / min, and calcined at a constant temperature for 1-3 h, and then heated to 500-600℃ at a heating rate of 0.5-2℃ / min, and calcined at a constant temperature for 7-10 h to obtain a degummed ceramic body;

[0050] S6: The degummed ceramic body is heated to 1150-1170℃ at a heating rate of 3-5℃ / min, and sintered at a constant temperature for 2-3 h, and then cooled to room temperature to obtain the sodium bismuth titanate-based relaxor ferroelectric ceramic material with a gradient structure.

[0051] The following examples are implemented on the basis of the above technical solutions of the present application, and detailed implementation modes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0052] The following are more detailed implementation cases, which further illustrate the technical solutions of the present application and the technical effects that can be obtained.

[0053] In the following examples, if there is no special description of raw materials or processing technology, it means that they are all conventional commercially available products or conventional processing technologies in the art.

[0054] Example 1

[0055] The present example provides a sodium bismuth titanate-based relaxor ferroelectric ceramic material with a gradient structure, which has three layer structures, and the chemical formula of each layer material is Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3, wherein the first x=0.005, the second x=0.010, and the third x=0.015, and the designed multilayer composite ceramic is based on the number ratio of the three components as 1:2:3 (the present example has 24 layers in total, which is collectively referred to as A), and the preparation method is as follows:

[0056] S1: Bi2O3, Na2CO3, BaCO3, TiO2, Li2CO3 and Nb2O5 with purity greater than 99% were selected as raw materials of the component gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic, and were accurately weighed and mixed according to the above chemical formula and the proportion of the three components by using an analytical balance to obtain a primary mixed powder;

[0057] S2: 80 g of anhydrous ethanol and 48 g of zirconium dioxide grinding balls were added as grinding media in a ball mill jar, 40 g of the primary mixed powder was added, and one-time ball milling was performed in a planetary ball mill for 12 h. After discharging, drying was performed in a 100 ℃ air drying oven to obtain a dried powder;

[0058] S3: The dried powder was placed in a crucible and compacted, and then was placed in a muffle furnace and heated to 850 ℃ at a heating rate of 3 ℃ / min for 4 h of pre-sintering to obtain a pre-sintered powder. Steps S1-S3 were repeated to obtain three pre-sintered powders with x = 0.005, 0.010 and 0.015;

[0059] S4: The three pre-sintered powders were respectively subjected to secondary ball milling for 12 h, discharging and drying. 9 g of the raw material powder was selected, 20 g of zirconium dioxide balls, 4.5 g of ethanol, 9 g of butanone, 0.3 g of glyceryl trioleate, 0.3 g of polyethylene glycol, 0.3 g of dibutyl phthalate, and 0.9 g of polyvinyl alcohol were added, and 8 h of rolling was performed. The thickness was set to 20 μm, and after air drying, the first, second and third components were stacked according to the layer ratio of 1:2:3, and then the ceramic green body with an area of 15 mm x 15 mm and a thickness of 0.48 mm was formed by pressing at a pressure of 10 MPa and a temperature of 50 ℃ for 20 min, and then was cut into small pieces;

[0060] S5: The ceramic body was transferred to a muffle furnace and heated to 200 ℃ at a heating rate of 0.25 ℃ / min, and then was annealed for 2 h. Then, the temperature was increased to 600 ℃ at a heating rate of 0.5 ℃ / min, and then was annealed for 8 h to obtain a degummed ceramic body;

[0061] S6: The degummed ceramic body was heated to 1150 ℃ at a heating rate of 3 ℃ / min, and then was sintered for 2 h, and then was naturally cooled to room temperature to obtain the above-mentioned component gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material.

[0062] The present embodiment also includes testing the electrical properties of the obtained component gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material, which specifically includes:

[0063] 1) After the obtained ceramic material was sequentially ground, polished and coated with silver electrodes on both sides, the dielectric constant and dielectric loss temperature spectrum test was performed, and the test results were as follows:

[0064] For example,Figure 1 The dielectric constant and dielectric loss temperature spectrum of the obtained ceramic material in the temperature range of 25-400℃ is shown in the figure. It can be seen from the figure that the ferroelectric-relaxation phase transition peak of the ceramic is merged into a flat wide peak at test frequencies of 1 kHz, 10 kHz and 100 kHz, and the peak width span is greater than 17℃ at room temperature.

[0065] 2) After the obtained ceramic material is sequentially ground, polished and coated with silver electrodes on both sides, temperature-dependent P-E curve, temperature-dependent I-E curve and temperature-dependent electric calorimetric effect test are carried out in the temperature range of 25-130℃, and the test results are as follows:

[0066] As shown in Figure 2 The temperature-dependent P-E curve of the polarization intensity of the obtained ceramic material under a saturated electric field of 6kV / mm during the heating process is shown in the figure. It can be seen from the figure that the saturation polarization and the residual polarization of the material decrease slowly with the increase of temperature;

[0067] As shown in Figure 3 The temperature-dependent I-E curve of the current value of the obtained ceramic material under a saturated electric field of 6kV / mm during the heating process is shown in the figure. It can be seen from the figure that the double peaks of the coexistence of ferroelectric and relaxation phases weaken and slowly evolve into a platform current peak with the increase of temperature;

[0068] As shown in Figure 4 The temperature-dependent electric calorimetric negative temperature change curve of the obtained ceramic material under a saturated electric field of 6kV / mm during the heating process is shown in the figure. It can be seen from the figure that the electric calorimetric negative temperature change of the material first increases and then decreases with the increase of temperature, and the maximum value is 0.61K. The electric calorimetric negative temperature change is greater than 0.5K in the temperature range of 39-94℃, and the temperature range width is 55℃.

[0069] Example 2

[0070] This embodiment provides a sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure, which has three layer structures. The chemical general formula of each layer is Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3, wherein the first x=0.005, the second x=0.010, and the third x=0.015. The designed multilayer composite ceramic is based on the number ratio of the three components is 1:1:1 (this embodiment has 24 layers, denoted as B), and the preparation method is:

[0071] S1: Select Bi2O3, Na2CO3, BaCO3, TiO2, Li2CO3 and Nb2O5 with purity greater than 99% as raw materials for preparing the component gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic, and mix them according to the above chemical formula and the proportion of the three components to obtain the initial mixed powder;

[0072] S2: Add 80 g of anhydrous ethanol and 48 g of zirconium dioxide grinding balls as grinding media into the ball mill jar in sequence, and add 40 g of the initial mixed powder, transfer it to the planetary ball mill for 12 h of primary ball milling, and after discharging, dry it in a 100℃ air drying oven to obtain the dried powder;

[0073] S3: Put the dried powder into the crucible and gently compact it, then transfer it to the muffle furnace and heat it to 850℃ at a heating rate of 3℃ / min for 4h of pre-burning to obtain the pre-burned powder;

[0074] S4: Perform 12h of secondary ball milling on the pre-burned powder, discharge and dry it; select 9g of raw material powder, add 20g of zirconium dioxide balls, 4.5g of ethanol, 9g of butanone, 0.3g of glyceryl trioleate, 0.3g of polyethylene glycol, 0.3g of dibutyl phthalate, 0.9g of polyvinyl alcohol, roll for 8h and then perform casting, naturally air dry, and then according to the layer ratio of the first, second and third components being 1:1:1, perform lamination in sequence, then set the pressure to 10MPa and the temperature to 50℃ and press for 20min to form a ceramic green body with an area of 15mm×15mm and a thickness of 0.48mm, and cut it into small pieces;

[0075] S5: Transfer the ceramic green body to the muffle furnace, first heat it to 200℃ at a heating rate of 0.25℃ / min and then heat it to 600℃ at a heating rate of 0.5℃ / min, and then heat it at a constant temperature for 8h to obtain a degassed ceramic green body;

[0076] S6: Heat the degassed ceramic green body to 1150℃ at a heating rate of 3℃ / min and then sinter it at a constant temperature for 2h, and after cooling to room temperature, the above component gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material is obtained.

[0077] The present embodiment also includes testing the electrical properties of the obtained component gradient structure sodium bismuth titanate-based relaxor ferroelectric ceramic material, which specifically includes:

[0078] 1) After grinding, polishing and double-sided silver electrode coating of the obtained ceramic material, test the dielectric constant and dielectric loss temperature spectrum, and the test results are as follows:

[0079] For example, Figure 5The dielectric constant and dielectric loss temperature spectrum of the obtained ceramic material in the temperature range of 25-400℃ are shown in the figure, and it can be seen from the figure that the ferroelectric-relaxation phase transition peak of the ceramic is merged into a flat wide peak at the test frequencies of 1kHz, 10kHz and 100kHz, and the peak width span is greater than 22℃ at room temperature.

[0080] 2) After the obtained ceramic material is sequentially ground, polished and coated with silver electrodes on both sides, the temperature-dependent P-E curve, temperature-dependent I-E curve and temperature-dependent electric calorimetric effect test in the temperature range of 25-130℃ are carried out, and the test results are as follows:

[0081] As shown in Figure 6 The temperature-dependent P-E curve of the polarization intensity of the obtained ceramic material under the saturated electric field of 6kV / mm during the heating process is shown in the figure, and it can be seen from the figure that the P-E curve presents a square curve with stronger ferroelectricity at room temperature due to the increase of the content of x=0.005 component, and the saturation polarization and the remanent polarization of the material slowly decrease with the increase of temperature;

[0082] As shown in Figure 7 The temperature-dependent I-E curve of the current value of the obtained ceramic material under the saturated electric field of 6kV / mm during the heating process is shown in the figure, and it can be seen from the figure that the double peaks of the coexistence of ferroelectric and relaxation phases are more obvious at room temperature due to the increase of the content of x=0.005 component, and the double peaks of the coexistence of ferroelectric and relaxation phases are continuously weakened and slowly evolve into a platform current peak with the increase of temperature;

[0083] As shown in Figure 8 The temperature-dependent electric calorimetric negative temperature change curve of the obtained ceramic material under the saturated electric field of 6kV / mm during the heating process is shown in the figure, and it can be seen from the figure that the electric calorimetric negative temperature change of the material first increases and then decreases with the increase of temperature, and the maximum value is 0.69K, and the electric calorimetric negative temperature change is greater than 0.5K in the temperature range of 39-94℃, and the temperature range width is 68℃.

[0084] According to the test results of Example 1 and Example 2, it can be known that the Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4Nb 3 / 4 ) x O3 ceramic in the application can maintain high electric calorimetric effect in a wide temperature range, realize the synergistic optimization of material electric calorimetric effect and working temperature range, and the ceramic material with high electric calorimetric effect and wide working temperature range can effectively promote the electric calorimetric refrigeration material to further go to practical application.

[0085] Comparative Example

[0086] The comparative example provides a single-component sodium bismuth titanate-based relaxor ferroelectric ceramic material. The comparative example has 24 layers, and has a general chemical formula of Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3(x=0.005). The difference between the preparation and the example 2 is that in S1, only Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3(x=0.005) is contained in the initial mixed powder, and other steps are the same as those in the example 2.

[0087] As Figure 9 shown in the figure are the change rate curves of the electrocaloric effect of the ceramics of the example 2: design B and the comparative example: single component x=0.005, under a saturation electric field of 6 kV / mm in the process of temperature rise. It can be seen from the figure that the composition gradient structure design effectively widens the working temperature interval of the electrocaloric effect of the ceramics, and greatly optimizes the temperature stability.

[0088] The electrocaloric temperature change in the present application is directly collected by a high-sensitivity thermistor (PT100), and has higher accuracy.

[0089] The above description of the comparative examples is for the purpose of facilitating the understanding and use of the invention by ordinary skilled persons in the art. It is obvious that those skilled in the art can easily make various modifications to the examples, and apply the general principles described herein to other examples without creative labor. Therefore, the present application is not limited to the above examples, and the improvements and modifications made by those skilled in the art according to the disclosure of the present application without departing from the scope of the present application should be within the protection scope of the present application.

Claims

1. A method for preparing a sodium bismuth titanate-based relaxor ferroelectric ceramic material having a compositionally graded structure, characterized in that, The relaxor ferroelectric ceramic material is stacked and compounded by three layered sodium bismuth titanate-based relaxor ferroelectric ceramic materials stacked in sequence, the chemical general formula of each sodium bismuth titanate-based relaxor ferroelectric ceramic material is Bi 0.47 Na 0.47 Ba 0.06 Ti 1-x (Li 1 / 4 Nb 3 / 4 ) x O3, wherein the first x =0.005, the second x =0.010, and the third x =0.015; when the multilayer is compounded, the number ratio of the first layer structure, the second layer structure and the third layer structure is 1:(1-2):(1-3). The preparation method comprises the following steps: according to the chemical general formula of three kinds of sodium bismuth titanate-based relaxor ferroelectric ceramic materials, mixing a bismuth source, a sodium source, a barium source, a titanium source, a lithium source and a niobium source, and sequentially performing primary ball milling, discharging, drying, pre-sintering, secondary ball milling, roller milling and casting to obtain three kinds of casting sheets; after drying, the casting sheets are stacked according to the layer number ratio of the first, second and third components, and then pressed, degassed and sintered, so that the sodium bismuth titanate-based relaxor ferroelectric ceramic material with a composition gradient structure is obtained.

2. The method of claim 1, wherein the method is characterized by: The bismuth source comprises Bi2O3, the sodium source comprises Na2CO3, the barium source comprises BaCO3, the titanium source comprises TiO2, the lithium source comprises Li2CO3 and the niobium source comprises Nb2O5.

3. The method of claim 1, wherein the method is characterized by: In the primary ball milling and secondary ball milling processes, the ball milling time is 10-12 h.

4. The method of claim 1, wherein the method is characterized by: In the pre-sintering process, the pre-sintering temperature is 800-900 DEG C, and the pre-sintering time is 3-4 h.

5. The method of claim 1, wherein the method is characterized by: In the roller milling and casting process, the roller milling time is 8-12 h, and the adhesive used in the casting process is ethanol, butanone, glycerol trioleate, polyethylene glycol, dibutyl phthalate and polyvinyl alcohol.

6. The method of claim 1, wherein the method is characterized by: In the stacking and pressing process, the forming pressure is 10 MPa, the temperature is 50 DEG C, and the forming time is 20 min.

7. The method of claim 1, wherein the method is characterized by: In the degassing process, the temperature is heated to 200-250 DEG C at a heating rate of 0.25-1 DEG C / min, and then isothermal calcined for 1-3 h; then the temperature is heated to 500-600 DEG C at a heating rate of 0.5-2 DEG C / min, and then isothermal calcined for 7-10 h.

8. The method of claim 1, wherein the method is characterized by: In the sintering process, the temperature is heated to 1150-1170 DEG C at a heating rate of 3-5 DEG C / min, and then isothermal calcined for 2-3 h.

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