Barium carbonate doped potassium sodium niobate based piezoelectric single crystal and preparation method thereof

Abstract

The invention discloses a barium carbonate doped potassium-sodium niobate-based piezoelectric single crystal and a preparation method thereof, the barium carbonate doped potassium-sodium niobate-based piezoelectric single crystal has a chemical general formula of (1-x) (K < 0.498 > Na < 0.498 > Li < 0.004 > Nb < 0.9955 > Bi < 0.004 > Fe < 0.0005 > O < 3 >)-xBaCO3, 0.001 < = x < = 0.009, and x represents a mole fraction. The preparation method comprises the following steps: taking Na2CO3, K2CO3, Nb2O5, Li2CO3, Bi2O3, BaCO3 and Fe2O3 as raw materials, adopting a seedless solid-phase crystal growth method, and carrying out primary refinement ball milling, pre-sintering, secondary refinement ball milling and sintering to prepare the barium carbonate doped potassium sodium niobate based single crystal. According to the method, the collaborative optimization of the piezoelectric property and the temperature stability is realized while the growth of the large-size single crystal is ensured.

Description

Technical Field

[0001] The present invention relates to a lead-free piezoelectric crystal material, a potassium sodium niobate-based piezoelectric single crystal, and specifically to a barium carbonate-doped potassium sodium niobate-based piezoelectric single crystal and a preparation method thereof. Background Art

[0002] Lead-based piezoelectric materials (such as lead zirconate titanate, PZT) have long occupied the core material position of piezoelectric devices due to their high piezoelectric constants and excellent electromechanical coupling coefficients. However, lead elements are prone to accumulate through the biological chain during preparation and disposal, causing irreversible ecological toxicity (such as nerve damage and soil pollution), resulting in strict restrictions on their applications in environmental protection regulations such as the EU RoHS directive. Therefore, the development of lead-free piezoelectric materials with both high performance and environmental friendliness has become the global research focus.

[0003] Among many lead-free piezoelectric systems, potassium sodium niobate (K 0.5 Na 0.5 NbO3, KNN)-based materials are regarded as the most potential candidate materials to replace PZT due to their high Curie temperature (Tc > 400 °C) and the characteristic of a coexisting phase boundary of a tunable orthorhombic-tetragonal phase. Compared with polycrystalline ceramics, KNN-based single crystals can significantly improve the piezoelectric response d C > 300 pC / N and reduce the dielectric loss tanδ < 2% due to the absence of the pinning effect of grain boundaries on the movement of domain walls. However, traditional single crystal growth techniques such as the melt method and the solution method are limited by the high volatility of the KNN system, the evaporation of Na / K elements, complex phase transition behaviors, and component segregation problems, and it is difficult to achieve the controllable preparation of large-size and highly homogeneous single crystals with a size greater than 10 mm. In recent years, the seed-free solid-state growth technique (SFSSCG) has become an effective way to break through the above bottlenecks due to its advantages such as low process cost and strong component controllability. For example, the patent with the publication number CN101913868A, titled "Preparation Method of Potassium Sodium Niobate Textured Ceramics and Potassium Sodium Niobate Single Crystals", prepared centimeter-scale KNN single crystals by introducing Bi-based composite oxides as grain boundary regulators and using the SFSSCG technique, and verified that its piezoelectric coefficient d 33 reached 250 pC / N; further, the patent with the publication number CN106087058A, titled "A K 33 Na 0.5 Na 0.5 NbO3-Based Ferroelectric and Piezoelectric Single Crystal and a Preparation Method Thereof", optimized the remanent polarization intensity (P r ) of the single crystal to 25 μC / cm 2 through Mn doping and annealing processes. However, the existing technologies still have the following key defects:

[0004] (1) Grain size limitation: Under the existing SFSSCG process, the maximum effective size of single crystals is usually less than 20 mm, making it difficult to meet the application requirements of high-power transducers and multi-layer actuators;

[0005] (2) Insufficient polarization performance: The coercive field (E c ) of single crystals has a wide distribution range, resulting in low polarization efficiency and easy occurrence of local breakdown;

[0006] (3) High-temperature stability defect: The dielectric loss (tanδ) fluctuates significantly in the wide temperature range of -50 to 200 °C, > 3%, limiting the reliability in extreme environments.

[0007] In view of the above problems, the publication number is CN118127633A, and the name is a sodium potassium niobate-based single crystal with stable piezoelectric properties in a wide temperature range and its preparation method, which proposes to increase the Curie temperature of KNN single crystals to 420 °C and broaden the stable temperature range of its ferroelectric phase through the co-doping strategy of BiAlO3 and Li2O. However, due to the significant difference between the ionic radius of Li + of 0.076 nm and that of K + / Na + of 0.138 nm / 0.102 nm, the lattice distortion is aggravated, and the piezoelectric coefficient d 33 is 377 pC / N. Therefore, how to achieve the coordinated optimization of piezoelectric properties and temperature stability while ensuring the growth of large-size single crystals is still a technical problem to be solved urgently in this field. Summary of the Invention

[0008] The object of the present invention is to provide a barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal and its preparation method in view of the above-mentioned deficiencies of the existing technology, so as to achieve the coordinated optimization of piezoelectric properties and temperature stability while ensuring the growth of large-size single crystals.

[0009] The technical solution to achieve the object of the present invention is:

[0010] A barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal, the chemical general formula of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal is:

[0011] (1-x)(K 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe 0.0005 O3)-xBaCO3, abbreviated as (1-x)(KNLNBFe)-xBaCO3,

[0012] where 0.001 ≤ x ≤ 0.009, and x represents the mole fraction.

[0013] The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal uses Na2CO3, K2CO3, Nb2O5, Li2CO3, Bi2O3, BaCO3 and Fe2O3 as raw materials. By using the seedless solid-phase crystal growth method, through primary refinement ball milling, pre-sintering, secondary refinement ball milling and sintering, the barium carbonate-doped sodium potassium niobate-based single crystal can be prepared.

[0014] Furthermore, the preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal includes the following steps:

[0015] S1, primary refinement ball milling of raw materials:

[0016] Dry the raw material powders of Na2CO3, K2CO3, Nb2O5, Li2CO3, Bi2O3, BaCO3 and Fe2O3. Weigh each raw material according to the stoichiometric ratio in (1-x)(KNLNBFe)-xBaCO3, where 0.001 ≤ x ≤ 0.009, and then mix them. Conduct primary refinement ball milling on the raw materials, and after the ball milling is completed, dry them to obtain the raw materials after primary refinement ball milling, simply referred to as primary ball milled materials. Here, x represents the molar fraction.

[0017] S2, pre-sintering of primary ball milled materials:

[0018] Conduct pre-sintering on the primary ball milled materials obtained in S1 to obtain the primary pre-sintered materials.

[0019] S3, secondary refinement ball milling of primary pre-sintered materials:

[0020] Conduct secondary refinement ball milling on the primary pre-sintered materials obtained in S2 to obtain the raw materials after secondary refinement ball milling, simply referred to as secondary ball milled materials.

[0021] S4, sintering of secondary ball milled materials:

[0022] Sieve the secondary ball milled materials obtained in S3; under a fixed pressure, press the secondary ball milled materials into round blanks; sinter the round blanks. The sintering conditions are: sintering temperature 1103 - 1120 °C, sintering time 18 - 24 h, to obtain the barium carbonate-doped sodium potassium niobate-based single crystal sample.

[0023] S5, annealing treatment of the sample:

[0024] Conduct ultrasonic treatment on the barium carbonate-doped sodium potassium niobate-based single crystal sample obtained in S4; then dry it, and conduct annealing treatment on the dried sample to obtain the barium carbonate-doped sodium potassium niobate-based single crystal. The size of the barium carbonate-doped sodium potassium niobate-based single crystal is 3.0 × 6.0 × 2.0 mm 3 ~20.0 × 18.0 × 2.0 mm 3 。

[0025] Further, in S1, the drying conditions of the raw materials are as follows: the drying temperature is 110 - 130 °C, and the drying time is 6 - 10 h.

[0026] Further, in S1, the conditions for the first fine ball milling are as follows: the grinding ball material is zirconia, the ball milling tank material is HDPE (high density polyethylene), the ball milling medium is absolute ethanol, the ball milling speed is 400 - 450 r / min, and the ball milling time is 20 - 24 h.

[0027] Further, in S1, the drying conditions after ball milling are as follows: the drying temperature is 70 - 80 °C, and the drying time is 10 - 12 h.

[0028] Further, in S2, the conditions for the first pre - sintering are as follows: the pre - sintering temperature is 700 - 800 °C, and the pre - sintering time is 6 - 8 h.

[0029] Further, in S3, the conditions for the second fine ball milling are the same as those for the first fine ball milling.

[0030] Further, in S4, the sieving condition is to pass through a 100 - 200 mesh sieve.

[0031] Further, in S4, the pressure for pressing the round blank is 1**00 - 120 Mpa.

[0032] Further, in S5, the holding temperature for the annealing treatment is 600 °C, and the holding time is 3 - 13 h.

[0033] The sodium potassium niobate - based single crystal doped with barium carbonate prepared by this method only has the K 0.5 Na 0.5 NbO3 phase and no impurity phase; the sodium potassium niobate single crystal is composed of a single crystal region and a ceramic region. Among them, the single crystal region has a single dense structure, and there are slight protrusions and undulations on the surface.

[0034] After testing, the dielectric loss of the prepared low - loss ferroelectric and piezoelectric single - crystal material is tanδ = 1.39%, the domain width is 300 - 500 nm, the piezoelectric constant is d 33 = 120 - 370 pC / N, the orthorhombic - tetragonal phase transition temperature T O-T = 120 - 175 °C, and the Curie temperature T C = 409 - 426 °C.

[0035] Preferably, in S1, x = 0.005; in S4, the sintering conditions are as follows: the sintering temperature is 1114 °C, and the sintering time is 24 h; the chemical formula of the sodium potassium niobate - based single crystal doped with BaCO3 is: 0.995(K 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe0.0005 O3)-0.005BaCO3,

[0036] Abbreviated as 0.995(KNLNBFe)-0.005BaCO3,

[0037] d 33 = 367 pC / N, P r = 36.1 μC / cm 2 , Q m = 25.478, k t = 0.4334, Curie temperature T C = 415 °C, orthorhombic-tetragonal phase transition temperature T O-T = 143 °C, and dielectric loss tanδ = 0.01913, crystal embryo ratio is 74.6%, crystal size is 20.0×18.0×2.0 mm 3 .

[0038] Based on the seed-free solid-state crystal growth (SFSSCG) technology and the Fe / Ba co-doping strategy, the present invention has successfully developed a high-performance, large-size KNN-based lead-free piezoelectric single crystal material. The core innovation points and technical advantages are as follows:

[0039] (1) Breakthrough in the controllable growth ability of large-size single crystals: By co-doping Fe / Ba to synergistically optimize the lattice dynamics and grain boundary migration behavior, the SFSSCG technology has achieved a significant increase in the single crystal size. When the BaCO3 doping amount x = 0.005, the single crystal size can reach 20.0×18.0×2.0 mm 3 , which is more than twice that of the existing KNN single crystals that are usually less than 9.3×10.0×2.0 mm 3 This process overcomes the size limitation caused by Na / K volatilization in the traditional melt method and provides a key material basis for the large-area integration of lead-free piezoelectric devices;

[0040] (2) Biphase structure regulation and performance co-optimization: Fe / Ba co-doping effectively regulates the orthorhombic-tetragonal phase transition behavior of KNN single crystals, stabilizes the two-phase coexistence structure in the range of 0 ≤ x ≤ 0.009, and realizes the following performance breakthroughs through the synergistic effect of the high coercive field orthorhombic phase and the high domain activity tetragonal phase:

[0041] 1) High piezoelectric response: Piezoelectric coefficient d 33 reaches 367 pC / N, approaching the commercial PZT ceramic level of d 33 = 400 - 600 pC / N;

[0042] 2) Excellent ferroelectric properties: Remnant polarization intensity P r = 36.1 μC / cm 2 , coercive field E cThe distribution is concentrated, and the polarization efficiency is increased by 40%; high-temperature stability: the Curie temperature T C = 415 °C, the orthorhombic-tetragonal phase transition temperature T O-T = 143 °C, which broadens the working temperature range and is suitable for high-temperature sensors and energy harvesting devices;

[0043] (3) Low dielectric loss and high mechanical quality factor: Through the acceptor doping of Fe 3+ and the oxygen vacancy inhibition effect of Ba 2+ , the crystal defect density is significantly reduced, achieving:

[0044] 1) Extremely low dielectric loss: tanδ = 0.01913, a 36% reduction compared to undoped KNN single crystals with tanδ > 0.03;

[0045] 2) Mechanical quality factor Q m = 25.478, indicating that the material has excellent energy conversion efficiency and anti-fatigue characteristics in resonant devices;

[0046] (4) Domain structure orientation and performance uniformity: Co-doping of iron / barium induces the formation of a regular layered domain structure mainly composed of 180° domains, and the average domain width reaches a minimum of 320 nm at x = 0.005. The advantages brought by this structural feature are as follows:

[0047] 1) High electromechanical coupling coefficient, the electromechanical coupling coefficient kp = 415.51, meeting the design requirements of broadband ultrasonic transducers;

[0048] 2) Performance uniformity: The uniformity of the single crystal composition, and the piezoelectric response fluctuation rate is less than 5%, ensuring the reliability of device mass production;

[0049] (5) Green preparation process and cost advantages:

[0050] 1) Using the SFSSCG technology combined with a low-temperature sintering process of ≤1200 °C, it has the following industrialization advantages: low energy consumption, a reduction in energy consumption compared to the melt method with a temperature greater than 1400 °C;

[0051] 2) Composition controllability: No seed crystal and melt sealing are required, avoiding the volatilization loss of Na / K;

[0052] 3) Environmental protection: Lead-free throughout the process, meeting the international environmental protection standards of RoHS and WEEE;

[0053] (6) Optimizing the microstructure: Annealing treatment is one of the key processes to optimize the performance of piezoelectric single crystal materials, which significantly affects the piezoelectric constant (d 33 ) by adjusting crystal defects, domain structures, and residual stresses. Description of the Drawings

[0054] Figure 1Test result graph of XRD (X-ray diffraction) test on KNLNBFe-Ba; Figure 1 Part (b) in it is Figure 1 An enlarged view of the dotted box in part (a);

[0055] Figure 2 Test result graph of macroscopic morphology test on KNLNBFe-Ba-5;

[0056] Figure 3 Test result graph of SEM (scanning electron microscope) test on KNLNBFe-Ba-5;

[0057] Figure 4 Test result graph of dielectric property test on KNLNBFe-Ba-5;

[0058] Figure 5 Test result graph of piezoelectric property test on KNLNBFe-Ba;

[0059] Figure 6 Test result graph of macroscopic morphology test on KNLNBFe-Ba, where the values in the graph represent the x values. Detailed implementation mode

[0060] The content of the present invention will be further described below in conjunction with the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0061] A barium carbonate-doped potassium sodium niobate-based piezoelectric single crystal, and the chemical general formula of the barium carbonate-doped potassium sodium niobate-based piezoelectric single crystal is:

[0062] (1 - x)(K 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe 0.0005 O3) - xBaCO3, abbreviated as (1 - x)(KNLNBFe) - xBaCO3, where 0.001 ≤ x ≤ 0.009 and x represents the mole fraction.

[0063] The preparation method of the barium carbonate-doped potassium sodium niobate-based piezoelectric single crystal includes the following steps:

[0064] Step 1: The raw materials used, namely Na2CO3, K2CO3, BaCO3, Nb2O5, Li2CO3, Fe2O3 and Bi2O3, are placed in an oven at 120 - 200 °C and dried for 4 - 6 h. Among them, the purity of Na2CO3 is 99.8%, the purity of K2CO3 is 99%, the purity of BaCO3 is 99.95%, the purity of Nb2O5 is 99.5%, the purity of Li2CO3 is >97%, the purity of Fe2O3 is 99.99%, and the purity of Bi2O3 is 99%. Weigh each raw material according to the stoichiometric ratio of (1 - x)(KNLNBFe)-xBaCO3, where 0.001 ≤ x ≤ 0.009, and then mix them. Load the mixture into a ball milling bottle made of HDPE and ball mill it for 24 - 48 h with absolute ethanol as the medium.

[0065] Step 2: Take out the ball milled product, dry it, and then pre-sinter it at 700 - 900 °C for 3 - 10 h.

[0066] Step 3: Ball mill it for 12 - 24 h with absolute ethanol as the medium and then dry it.

[0067] Step 4: After passing the dried powder through a 100-mesh sieve, press it into a circular blank with a diameter of 15 - 50 mm and a thickness of 2 - 5 mm under a pressure of 110 MPa, and sinter it at 1114 °C for 24 h to obtain a large-size ferroelectric and piezoelectric single crystal sample.

[0068] Step 5: After subjecting the obtained large-size ferroelectric and piezoelectric single crystal sample to ultrasonic treatment, dry it, and then put the dried sample into a muffle furnace and keep it at 600 °C for 3 - 13 h.

[0069] During the process of cutting the single crystal from the embryo, the edge position of the single crystal will be polished with sandpaper, and dust and impurities will be generated during the polishing process. This ultrasonic treatment process can make the surface of the single crystal sample cleaner.

[0070] Example 1:

[0071] The chemical formula of the barium carbonate-doped potassium sodium niobate-based piezoelectric single crystal is: 0.995(K 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe 0.0005 O3)-0.005BaCO3,

[0072] Abbreviated as 0.995(KNLNBFe)-0.005BaCO3, (x = 0.005).

[0073] Its preparation method includes the following steps:

[0074] Step 1: Primary refinement ball milling of raw materials:

[0075] Under the conditions of a drying temperature of 120 °C and a drying time of 6 h, dry the raw materials Na2CO3, K2CO3, Nb2O5, Li2CO3, Bi2O3, BaCO3 and Fe2O3. Then, weigh each raw material according to the stoichiometric ratio in 0.995(KNLNBFe)-0.005BaCO3 and mix them. The grinding ball material is zirconia, the ball mill tank material is HDPE, the ball milling medium is absolute ethanol, the ball milling speed is 420 r / min, and the ball milling time is 24 h. Under these conditions, carry out primary fine ball milling on the raw materials. After the ball milling is completed, dry them under the conditions of a drying temperature of 80 °C and a drying time of 12 h to obtain the raw materials after primary fine ball milling, simply referred to as primary ball milled materials;

[0076] Step 2, pre-sintering of the primary ball milled materials:

[0077] Under the conditions of a pre-sintering temperature of 750 °C and a pre-sintering time of 6 h, pre-sinter the primary ball milled materials obtained in Step 1 to obtain primary pre-sintered materials;

[0078] Step 3, secondary fine ball milling of the primary pre-sintered materials:

[0079] Under the same ball milling conditions as the primary fine ball milling in Step 1, carry out secondary fine ball milling on the primary pre-sintered materials obtained in Step 2 to obtain the raw materials after secondary fine ball milling, simply referred to as secondary ball milled materials;

[0080] Step 4, sintering of the secondary ball milled materials:

[0081] Under the condition of a 100-mesh sieve, sieve the secondary ball milled materials obtained in Step 3. Then, under the conditions of a pressure of 110 MPa, a die diameter of 25 mm, and a die thickness of 2.5 mm, press the secondary ball milled materials into a round blank. Finally, under the conditions of a sintering temperature of 1114 °C and a sintering time of 24 h, sinter the round blank of the secondary ball milled materials to obtain a large-size ferroelectric and piezoelectric single crystal sample of sodium potassium niobate doped with barium carbonate, simply referred to as KNLNBFe-Ba. The single crystal obtained in Example 1 is KNLNBFe-0.005Ba, simply referred to as KNLNBFe-Ba-5;

[0082] Step 5, annealing treatment of the sample:

[0083] After subjecting the sodium potassium niobate single crystal sample doped with BaCO3 obtained in Step 4 to ultrasonic treatment, dry it. Then, put the dried sample into a muffle furnace and keep it at 600 °C for 10 h to obtain a sodium potassium niobate single crystal doped with barium carbonate with a microstructural organization tending to be uniform and stable piezoelectric properties.

[0084] To prove that no impurity phase is generated in KNLNBFe-Ba-5, XRD testing is carried out. The results are asFigure 1 As shown, the diffraction peaks of KNLNBFe-Ba-5 correspond to the standard characteristic peaks of KNN, and there are no impurity peaks. The test results show that KNLNBFe-Ba-5 has a perovskite structure, that is, the addition of Bi, Fe, Ba, and Li does not change the crystal structure of KNN, and has been completely diffused into the KNN lattice to form a new solid solution, and no impurity phase is generated.

[0085] To prove the successful preparation of KNLNBFe-Ba-5, macroscopic morphology tests were carried out. The test results are as Figure 2 and Figure 6 shown. KNLNBFe-Ba-5 consists of a ceramic region and a single crystal region. Among them, the maximum single crystal size of the single crystal region is 20×18×2 mm 3 .

[0086] To further prove the difference between KNLNBFe-Ba-5 and the ceramic region, SEM tests were carried out. The test results are as Figure 3 shown. This region is at the boundary between the ceramic region and the single crystal region. Among them, the single crystal region has a dense structure, with slightly protruded and undulating surfaces. The ceramic region is composed of many regularly shaped rectangular small grains. The test results show that the structure of the single crystal region is more dense, with obvious differences from the ceramic region.

[0087] To prove the temperature range of the tetragonal phase in KNLNBFe-Ba-5, dielectric property tests were carried out. The test results are as Figure 4 shown. The relative dielectric constant of KNLNBFe-Ba-5 has two abnormal peaks with the change of temperature. The first peak from low to high temperature is the dielectric anomaly peak that appears when the orthorhombic phase transforms into the tetragonal phase. The phase transition temperature T O-T is 143 °C; the second peak is the dielectric anomaly peak when the tetragonal phase transforms into the cubic phase. The phase transition temperature T C is 423 °C. Therefore, the temperature range of the tetragonal phase of KNLNBFe-Ba-5 is 280 °C.

[0088] To further prove the piezoelectric properties of KNLNBFe-Ba-5, piezoelectric property tests were carried out. The test results are shown in Table 1. The d 33 of KNLNBFe-Ba-5 is 398 pC / N. The test results show that KNLNBFe-Ba-5 has good piezoelectric properties.

[0089] Table 1 Electrical Property Tests of KNLNBFe-Ba

[0090]

[0091] To prove the addition of BaCO3 to K 0.498 Na 0.498 Li 0.004Nb 0.9955 Bi 0.004 Fe 0.0005 Regarding the influence of BaTiO₃, i.e., the role of Ba in expanding the temperature range of the ferroelectric phase of the crystal and enhancing the piezoelectric properties, Comparative Example 1 is provided, which is a KN 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe 0.0005 O₃ piezoelectric single crystal.

[0092] Comparative Example 1:

[0093] A method for preparing a sodium potassium niobate-based piezoelectric single crystal without adding Ba. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0094] The raw materials in Step 1 do not contain BaCO₃, that is, it satisfies the chemical formula (1 - x)(KNLNBFe) - xBaCO₃, where x = 0. The obtained material is named a sodium potassium niobate-based single crystal without adding Ba, that is, KNLNBFe. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe - Ba - 0.

[0095] To prove that no impurity phase is generated in KNLNB - F - Ba - 0, XRD testing is carried out. The results are as Figure 1 shown. The peak positions of the diffraction peaks of KNLNBFe - Ba - 0 are the same as the standard characteristic peaks of KNN, and there are no impurity peaks. The test results show that KNLNBFe - Ba - 0 has a perovskite structure. Further comparison with the test results of Example 1 shows that the peak positions of the diffraction peaks have not changed, but the intensities of the diffraction peaks have changed. This phenomenon indicates that adding Ba does not change the crystal structure of KNN, and at the same time, no impurity phase is generated.

[0096] To prove the successful preparation of KNLNBFe - Ba - 0, macroscopic morphology testing is carried out. The test results are as Figure 6 shown. KNLNBFe - Ba - 0 is also composed of a ceramic region and a single crystal region. However, the grains are very fine and evenly distributed in the ceramic powder. Further comparison with the test results of Example 1 shows that although a single crystal can still be successfully prepared without adding Ba, the size of the single crystal is much smaller than that in Example 1, and the crystal blank ratio is 7.9%.

[0097] To prove the temperature range of the tetragonal phase in KNLNBFe - Ba - 0, dielectric property testing is carried out. The test results are as Figure 5 shown. The two abnormal peaks of the relative dielectric constant of KNLNBFe - Ba - 0 changing with temperature respectively correspond to T for the orthorhombic to tetragonal phase transition O-T of 175 °C and T for the tetragonal to cubic phase transitionC is 426 °C. Therefore, the temperature range of the tetragonal phase of KNLNBFe-Ba-0 is 251 °C. Further comparing with the test results of Example 1, it can be known that the temperature range of the tetragonal phase of KNLNBFe-Ba-0 is smaller than that of KNLNBFe-Ba-5.

[0098] To prove the piezoelectric properties of KNLNBFe-Ba-0, piezoelectric property tests were carried out. The test results are shown in Table 1. The d of KNLNBFe-Ba-0 33 is 126 pC / N. Further comparing with the test results of Example 1, it can be known that the d of KNLNBFe-Ba-0 33 is lower than the d of KNLNBFe-Ba-5 33 is lower.

[0099] Comparative Example 2:

[0100] The preparation method of sodium potassium niobate-based piezoelectric single crystal added with 0.001 BaCO3. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0101] It satisfies the chemical formula (1-x)(KNLNBFe)-xBaCO3, where x = 0.001. The obtained material is named KNLNBFe-Ba-1. For the convenience of comparison with Example 1, it is simply called KNLNBFe-Ba-1.

[0102] To prove that no impurity phase is generated in KNLNBFe-Ba-1, XRD tests were carried out. The results are as Figure 1 shown. The peak positions of the diffraction peaks of KNLNBFe-Ba-1 are the same as the standard characteristic peaks of KNN, and there are no impurity peaks. The test results show that KNLNBFe-Ba-1 has a perovskite structure. Further comparing with the test results of Example 1, it can be known that the peak positions of the diffraction peaks have not changed. However, the intensity of the diffraction peaks has changed. This phenomenon indicates that adding Ba does not change the crystal structure of KNN, and at the same time, no impurity phase will be generated.

[0103] To prove the successful preparation of KNLNBFe-Ba-1, macroscopic morphology tests were carried out. The test results are as Figure 6 shown. KNLNBFe-Ba-1 is also composed of a ceramic region and a single crystal region. However, the grains are very fine and evenly distributed in the ceramic powder. Further comparing with the test results of Example 1, it can be known that although adding Ba can successfully prepare single crystals, the size of the single crystals is much smaller than that in Example 1, and the crystal blank ratio is 4.1%.

[0104] To prove the temperature range of the tetragonal phase in KNLNBFe-Ba-1, dielectric property tests were carried out. The test results are as Figure 5As shown, the two anomalous peaks of the relative dielectric constant of KNLNBFe-Ba-1 varying with temperature respectively correspond to T for the orthorhombic to tetragonal phase transition O-T which is 120 °C and T for the tetragonal to cubic phase transition C which is 413 °C. Therefore, the temperature range of the tetragonal phase of KNLNBFe-Ba-1 is 293 °C. Further comparing with the test results of Example 1, it can be seen that the temperature range of the tetragonal phase of KNLNBFe-Ba-1 is larger than that of the tetragonal phase of KNLNBFe-Ba-5.

[0105] In order to prove the piezoelectric properties of KNLNBFe-Ba-1, piezoelectric property tests were carried out. The test results are shown in Table 1. The d of KNLNBFe-Ba-1 33 is 189 pC / N. Further comparing with the test results of Example 1, it can be seen that the d of KNLNBFe-Ba-1 33 is lower than the d of KNLNBFe-Ba-5 33 .

[0106] Comparative Example 3:

[0107] The preparation method of a sodium potassium niobate-based piezoelectric single crystal added with 0.003 BaCO3, the steps not specifically described are the same as those in the preparation method of Example 1, and the differences are as follows:

[0108] It satisfies the chemical formula (1 - x)(KNLNBFe) - xBaCO3, where x = 0.003. The obtained material is named KNLNBFe-Ba-3. For the convenience of comparison with Example 1, it is simply called KNLNBFe-Ba-3.

[0109] In order to prove that no impurity phase is generated in KNLNBFe-Ba-3, XRD tests were carried out. The results are as Figure 1 shown. The peak positions of the diffraction peaks of KNLNBFe-Ba-3 are the same as the standard characteristic peaks of KNN, and there are no impurity peaks. The test results show that KNLNBFe-Ba-3 has a perovskite structure. Further comparing with the test results of Example 1, it can be seen that the peak positions of the diffraction peaks have not changed. However, the intensity of the diffraction peaks has changed. This phenomenon indicates that adding Ba does not change the crystal structure of KNN, and at the same time, no impurity phase will be generated.

[0110] In order to prove the successful preparation of KNLNBFe-Ba-3, macroscopic morphology tests were carried out. The test results are as Figure 6 shown. KNLNBFe-Ba-3 is also composed of a ceramic region and a single crystal region. However, the grain size is smaller and evenly distributed in the ceramic powder. Further comparing with the test results of Example 1, it can be seen that although adding Ba can successfully prepare a single crystal, the size of the single crystal is smaller than that in Example 1, and the crystal blank ratio is 38.4%.

[0111] In order to prove the temperature range of the tetragonal phase in KNLNBFe-Ba-3, dielectric property tests were carried out. The test results are as Figure 5 shown. The two abnormal peaks of the relative dielectric constant of KNLNBFe-Ba-3 changing with temperature respectively correspond to T O-T of 147 °C for the orthorhombic to tetragonal phase transition and T C of 409 °C for the tetragonal to cubic phase transition. Therefore, the temperature range of the tetragonal phase of KNLNBFe-Ba-3 is 262 °C. Further comparison with the test results of Example 1 shows that the temperature range of the tetragonal phase of KNLNBFe-Ba-3 is greater than that of KNLNBFe-Ba-5.

[0112] In order to prove the piezoelectric properties of KNLNBFe-Ba-3, piezoelectric property tests were carried out. The test results are shown in Table 1. The d 33 of KNLNBFe-Ba-3 is 253 pC / N. Further comparison with the test results of Example 1 shows that the d 33 of KNLNBFe-Ba-3 is lower than the d 33 of KNLNBFe-Ba-5.

[0113] Comparative Example 4:

[0114] The preparation method of a sodium potassium niobate-based piezoelectric single crystal added with 0.007 BaCO3, the steps not specifically described are the same as those in the preparation method of Example 1, and the differences are as follows:

[0115] It satisfies the chemical formula (1-x)(KNLNBFe)-xBaCO3, where x = 0.007, and the obtained material is named KNLNBFe-Ba-7. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-7.

[0116] In order to prove that no impurity phase is generated in KNLNBFe-Ba-7, XRD tests were carried out. The results are as Figure 1 shown. The peak positions of the diffraction peaks of KNLNBFe-Ba-7 are the same as the standard characteristic peaks of KNN, and there are no impurity peaks. The test results show that KNLNBFe-Ba-7 has a perovskite structure. Further comparison with the test results of Example 1 shows that the peak positions of the diffraction peaks have not changed, but the intensities of the diffraction peaks have changed. This phenomenon indicates that adding Ba does not change the crystal structure of KNN, and at the same time, no impurity phase will be generated.

[0117] In order to prove the successful preparation of KNLNBFe-Ba-7, macroscopic morphology tests were carried out. The test results are as Figure 6As shown, KNLNBFe-Ba-7 is also composed of a ceramic region and a single-crystal region. However, the crystal grains are relatively small and are scattered in the ceramic powder, and the crystal embryo ratio is 30.2%. Further comparison with the test results of Example 1 shows that the size of the single crystal is smaller than that of Example 1.

[0118] To prove the temperature range of the tetragonal phase in KNLNBFe-Ba-7, dielectric property tests were carried out. The test results are as Figure 5 shown. The two abnormal peaks of the relative dielectric constant of KNLNBFe-Ba-7 changing with temperature respectively correspond to T O-T for the orthorhombic to tetragonal phase transition of 130 °C and T C for the tetragonal to cubic phase transition of 413 °C. Therefore, the temperature range of the tetragonal phase of KNLNBFe-Ba-7 is 283 °C. Further comparison with the test results of Example 1 shows that the temperature range of the tetragonal phase of KNLNBFe-Ba-7 is larger than that of KNLNBFe-Ba-5.

[0119] To prove the piezoelectric properties of KNLNBFe-Ba-7, piezoelectric property tests were carried out. The test results are shown in Table 1. The d 33 of KNLNBFe-Ba-7 is 302 pC / N. Further comparison with the test results of Example 1 shows that the d 33 of KNLNBFe-Ba-7 is lower than the d 33 of KNLNBFe-Ba-5.

[0120] Comparative Example 5:

[0121] The preparation method of a sodium potassium niobate-based piezoelectric single crystal added with 0.009 BaCO3. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0122] It satisfies the chemical formula (1 - x)(KNLNBFe) - xBaCO3, where x = 0.009. The obtained material is named KNLNBFe-Ba-9. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-9.

[0123] To prove that no impurity phase is generated in KNLNBFe-Ba-9, XRD tests were carried out. The results are as Figure 1 shown. The peak positions of the diffraction peaks of KNLNBFe-Ba-9 are the same as the standard characteristic peaks of KNN, and there are no impurity peaks. The test results show that KNLNBFe-Ba-9 has a perovskite structure. Further comparison with the test results of Example 1 shows that the peak positions of the diffraction peaks have not changed. However, the intensity of the diffraction peaks has changed. This phenomenon indicates that adding Ba does not change the crystal structure of KNN, and at the same time, no impurity phase will be generated.

[0124] To prove the successful preparation of KNLNBFe-Ba-9, macroscopic morphology tests were carried out. The test results are as Figure 6 shown. KNLNBFe-Ba-9 is also composed of a ceramic region and a single crystal region. However, the single crystals are embedded in the ceramic powder and have not been completely converted into single crystals. A small part of the crystals has been ceramized, and the crystal blank ratio cannot be measured, that is, excessive Ba doping will inhibit the conversion of ceramics into single crystals.

[0125] To prove the temperature range of the tetragonal phase in KNLNBFe-Ba-9, dielectric property tests were carried out. The test results are as Figure 5 shown. The two abnormal peaks of the relative dielectric constant of KNLNBFe-Ba-9 varying with temperature respectively correspond to T O-T for the orthorhombic to tetragonal phase transition at 166 °C and T C for the tetragonal to cubic phase transition at 414 °C. Therefore, the temperature range of the tetragonal phase of KNLNBFe-Ba-9 is 248 °C. Further comparison with the test results of Example 1 shows that the temperature range of the tetragonal phase of KNLNBFe-Ba-9 is smaller than that of KNLNBFe-Ba-5.

[0126] To prove the piezoelectric properties of KNLNBFe-Ba-9, piezoelectric property tests were carried out. The test results are shown in Table 1. The d 33 of KNLNBFe-Ba-9 is 193 pC / N. Further comparison with the test results of Example 1 shows that the d 33 of KNLNBFe-Ba-9 is lower than the d 33 of KNLNBFe-Ba-5.

[0127] To prove the influence of the annealing time on the sodium potassium niobate-based single crystals, Comparative Example 6, Comparative Example 7, Comparative Example 8, Comparative Example 9, Comparative Example 10, and Comparative Example 11 were provided. The KN-LNBFe-Ba-5 single crystal samples prepared were annealed under the conditions of annealing times of 3 h, 5 h, 7 h, 9 h, 11 h, and 13 h respectively.

[0128] Comparative Example 6:

[0129] A preparation method of barium carbonate-doped sodium potassium niobate-based piezoelectric single crystals with an annealing time of 3 h. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0130] After Step 4, the obtained single crystal sample was annealed. The annealing temperature was 600 °C, and the holding time was 3 h. The obtained material was named KNLNBFe-Ba-5-3, and for the convenience of comparison with Example 1, it was simply referred to as KNLNBFe-Ba-5-3.

[0131] To prove the successful preparation of KNLNB-F-Ba-5-3, piezoelectric performance tests were carried out. The test results are shown in Table 2. The d 33 of KNLNBFe-Ba-5-3 is 361 pC / N. Comparing with the test results of Example 1, compared with the d 33 = 367 pC / N of Example 1, the increase amplitude of d 33 is < 2%, indicating that short-time annealing for ≤ 3 h is not sufficient to fully release residual stress or optimize domain structure.

[0132] Comparative Example 7:

[0133] A preparation method of a sodium potassium niobate-based piezoelectric single crystal doped with barium carbonate with an annealing time of 5 h. The steps not specifically described are the same as those of the preparation method of Example 1. The differences are as follows:

[0134] After step 4, the obtained single crystal sample was annealed at an annealing temperature of 600 °C for a holding time of 5 h. The obtained material was named KNLNBFe-Ba-5-5. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-5-5.

[0135] To prove the successful preparation of KNLNB-F-Ba-5-5, piezoelectric performance tests were carried out. The test results are shown in Table 2. The d 33 of KNLNBFe-Ba-5-5 is 372 pC / N. Comparing with the test results of Example 1, there is still no significant difference, indicating that 5 h annealing is still at the critical stage of performance improvement.

[0136] Comparative Example 8:

[0137] A preparation method of a sodium potassium niobate-based piezoelectric single crystal doped with barium carbonate with an annealing time of 7 h. The steps not specifically described are the same as those of the preparation method of Example 1. The differences are as follows:

[0138] After step 4, the obtained single crystal sample was annealed at an annealing temperature of 600 °C for a holding time of 7 h. The obtained material was named KNLNBFe-Ba-5-7. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-5-7.

[0139] To prove the successful preparation of KNLNBFe-Ba-5-7, piezoelectric performance tests were carried out. The test results are shown in Table 2. The d 33 of KNLNBFe-Ba-5-7 is 373 pC / N. The increase amplitude slows down compared with Comparative Example 7, Δd 33 ≈ 0.3%, indicating that 7 h annealing is close to the performance saturation point at this temperature.

[0140] Comparative Example 9:

[0141] Preparation method of barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal with annealing time of 9 h. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0142] After step 4, the obtained single crystal sample is annealed at an annealing temperature of 600 °C for a holding time of 9 h. The obtained material is named KNLNBFe-Ba-5-9. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-5-9.

[0143] To prove the successful preparation of KNLNBFe-Ba-5-9, piezoelectric performance tests are carried out. The test results are shown in Table 2. The d of KNLNBFe-Ba-5-9 33 is 384 pC / N. It is improved by 2.9% compared with Comparative Example 8, proving that extending the annealing time to 9 h can further promote the increase of the piezoelectric constant.

[0144] Comparative Example 10:

[0145] Preparation method of barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal with annealing time of 11 h. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0146] After step 4, the obtained single crystal sample is annealed at an annealing temperature of 600 °C for a holding time of 11 h. The obtained material is named KNLNBFe-Ba-5-11. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-5-11.

[0147] To prove the successful preparation of KNLNBFe-Ba-5-11, piezoelectric performance tests are carried out. The test results are shown in Table 2. The d of KNLNBFe-Ba-5-11 33 is 397 pC / N. It is improved by 8.4% compared with Example 1, attributed to the synergistic effect of the completion of lattice relaxation and the 180° domain orientation arrangement.

[0148] Comparative Example 11:

[0149] Preparation method of barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal with annealing time of 13 h. The steps not specifically described are the same as those in the preparation method of Example 1. The differences are as follows:

[0150] After step 4, the obtained single crystal sample is annealed at an annealing temperature of 600 °C for a holding time of 7 h. The obtained material is named KNLNBFe-Ba-5-13. For the convenience of comparison with Example 1, it is simply referred to as KNLNBFe-Ba-5-13.

[0151] To prove the successful preparation of KNLNBFe - Ba - 5 - 13, piezoelectric performance tests were carried out. The test results are shown in Table 2. The d 33 of KNLNBFe - Ba - 5 - 13 is 372 pC / N. It decreased by 5.1% compared with Comparative Example 11, confirming that annealing for >11 h leads to the regeneration of lattice defects and performance deterioration.

[0152] Table 2 Variation of d 33 (pC / N) with the prolongation of heat preservation time

[0153]

[0154] From the test results of the above Comparative Examples 1 - 11 and Example 1, the following conclusions can be drawn:

[0155] (1) Ba doping synergy mechanism: Through doping with 0.005 BaCO3, the synergistic effects of alleviating lattice distortion, suppressing oxygen vacancies, and regulating phase boundaries are achieved. The single - crystal size reaches the 20 - mm level and d 33 = 398 pC / N, reaching the international leading level of lead - free piezoelectric materials;

[0156] (2) Optimization of annealing process: The annealing time is limited to 9 - 11 h, preferably 10 h, breaking through the performance bottleneck of the traditional process ≤6 h. d 33 increases by 8 - 10%, and element loss caused by excessive annealing is avoided;

[0157] (3) Industrialization advantages: The SFSSCG technology combined with low - temperature sintering at 1114 °C reduces energy consumption by 25% compared with the melt method at >1400 °C, and no seed crystal is required, which is suitable for large - scale production;

[0158] (4) Application potential: The single - crystal material has stable performance in the wide temperature range of 20 - 400 °C, and tanδ < 0.02, which is suitable for high - precision sensors, ultrasonic transducers, and high - temperature energy - harvesting devices.

[0159] The applicant found through research that the addition of Ba 2+ has the following advantages:

[0160] (1) Lattice matching advantage: Ba 2+ has a high ion - radius matching degree with K + / Na + , which can reduce lattice distortion caused by doping, lower the internal stress during single - crystal growth, and provide a structural basis for large - size crystals.

[0161] (2) Volatilization inhibition and stoichiometry control: The strong oxygen affinity of Ba 2+ can effectively anchor K + / Na +Ions are inhibited from volatilizing during high-temperature sintering to ensure the compositional uniformity of the single crystal.

[0162] (3) Phase structure stabilization: Ba 2+ Doping can regulate the phase transition path of KNN, stabilize the orthorhombic-tetragonal phase boundary, and avoid cracks or grain fractures caused by phase transition stress during the growth process.

[0163] (4) Grain boundary kinetics optimization: Ba 2+ Segregation at grain boundaries reduces the activation energy of grain boundary migration, promotes the preferred growth of grains, and realizes the directional expansion of single crystal size.

[0164] (5) Defect engineering synergy: Ba 2+ Co-doping with Fe 3+ can form charge compensation, reduce the oxygen vacancy concentration, and improve the insulation and electromechanical coupling efficiency of the single crystal. That is, Ba 2+ doping solves the problems of compositional segregation, phase transition stress, and grain size limitation in the growth of KNN-based single crystals through lattice matching, volatilization inhibition, phase boundary regulation, grain boundary kinetics optimization, and defect engineering synergy, providing key technical support for the preparation of high-performance large-size lead-free piezoelectric single crystals.

[0165] The present invention relates to a large-size ferroelectric piezoelectric single crystal material of KNN-based doped with barium carbonate (BaCO3) and a solid-phase seedless growth preparation method. In its technical solution,

[0166] (1) Ba 2 + gradient doping design: By controlling the doping concentration of BaCO3 in the KNN precursor powder from 0 to 0.009 mol, using the radius matching of Ba 2 + with a radius of 0.135 nm and K + / Na + to relieve lattice distortion and inhibit Na / K volatilization;

[0167] (2) Polarization-annealing synergistic process: Combining electric field polarization and low-temperature annealing treatment at 500-600 °C to achieve the directional arrangement of domain structures and the release of residual stress.

[0168] This method not only provides new ideas for the research of lead-free piezoelectric materials but also lays a solid foundation for their application in high-end devices such as sensors and actuators.

[0169] Although the embodiments of the present invention have been shown and described, for those of ordinary skill in the art, it can be understood that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The protection scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal, characterized in that, The chemical general formula of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal is as follows: (1 - x)(K 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe 0.0005 O3)-xBaCO3, abbreviated as (1 - x)(KNLNBFe)-xBaCO3, where 0.001 ≤ x ≤ 0.009 and x represents the mole fraction.

2. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 1, characterized in that, Using Na2CO3, K2CO3, Nb2O5, Li2CO3, Bi2O3, BaCO3 and Fe2O3 as raw materials, by the seedless solid-phase crystal growth method, through primary fine ball milling, pre-sintering, secondary fine ball milling and sintering, the barium carbonate-doped sodium potassium niobate-based single crystal can be prepared.

3. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 2, wherein It includes the following steps: S1, primary fine ball milling of raw materials: Dry the raw material powders of Na2CO3, K2CO3, Li2CO3, Nb2O5, Bi2O3, BaCO3 and Fe2O3, and weigh each raw material according to the (1-x)(KNLNBFe)-xBaCO3, where 0.001 ≤ x ≤ 0.009 in the stoichiometric ratio, then mix them. Conduct primary fine ball milling on the raw materials, and after ball milling, dry them to obtain the raw materials of primary fine ball milling, simply referred to as primary ball milling materials, and x represents the molar fraction; S2, pre-sintering of primary ball milling materials: Pre-sinter the primary ball milling materials obtained in S1 to obtain primary pre-sintered materials. The primary pre-sintering conditions are: the pre-sintering temperature is 700 - 800 °C, and the pre-sintering time is 6 - 8 h; S3, secondary fine ball milling of primary pre-sintered materials: Conduct secondary fine ball milling on the primary pre-sintered materials obtained in S2 to obtain the raw materials after secondary fine ball milling, simply referred to as secondary ball milling materials; S4, sintering of secondary ball milling materials: Pass the secondary ball milling materials obtained in S3 through a 100 - 200 mesh sieve; under a pressure of 100 - 120 Mpa, press the secondary ball milling materials into round blanks; sinter the round blanks. The sintering conditions are: the sintering temperature is 1103 - 1120 °C, and the sintering time is 18 - 24 h to obtain the barium carbonate-doped sodium potassium niobate-based single crystal sample; S5, annealing treatment of the sample: Conduct ultrasonic treatment on the barium carbonate-doped sodium potassium niobate-based single crystal sample obtained in S4; then dry it, and conduct annealing treatment on the dried sample to obtain the barium carbonate-doped sodium potassium niobate-based single crystal.

4. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 3, characterized in that, In S1, the primary fine ball milling conditions are: the grinding ball material is zirconia, the ball milling tank material is HDPE, the ball milling medium is anhydrous ethanol, the ball milling speed is 400 - 450 r / min, and the ball milling time is 20 - 24 h.

5. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 3, wherein, In S2, the primary pre-sintering conditions are: the pre-sintering temperature is 700 - 800 °C, and the pre-sintering time is 6 - 8 h.

6. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 3, characterized in that In S4, the sintering conditions are: the sintering temperature is 1114 °C, and the sintering time is 24 h.

7. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 3, wherein, In S5, the holding temperature for annealing treatment is 600 °C, and the holding time is 3 - 13 h.

8. A barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal, characterized in that, The chemical formula of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal is as follows: 0.995(K 0.498 Na 0.498 Li 0.004 Nb 0.9955 Bi 0.004 Fe 0.0005 O3)-0.005BaCO3, Abbreviated as 0.995(KNLNBFe)-0.005BaCO3.

9. The preparation method of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to claim 8, characterized in that, It includes the following steps: Step 1, primary fine ball milling of raw materials: Under the conditions of a drying temperature of 120 °C and a drying time of 6 h, dry the raw materials Na2CO3, K2CO3, Nb2O5, Li2CO3, Bi2O3, BaCO3 and Fe2O3. Then, weigh each raw material according to the stoichiometric ratio in 0.995(KNLNBFe)-0.005BaCO3 and mix them. The grinding ball material is zirconia, the ball mill tank material is HDPE, the ball milling medium is absolute ethanol, the ball milling speed is 420 r / min, and the ball milling time is 24 h. Under these conditions, carry out primary fine ball milling on the raw materials. After the ball milling is completed, dry them under the conditions of a drying temperature of 80 °C and a drying time of 12 h to obtain the raw materials after primary fine ball milling, which is simply referred to as primary ball milled material; Step 2, pre-sintering of the primary ball milled material: Under the conditions of a pre-sintering temperature of 750 °C and a pre-sintering time of 6 h, pre-sinter the primary ball milled material obtained in Step 1 to obtain the primary pre-sintered material; Step 3, secondary fine ball milling of the primary pre-sintered material: Under the same ball milling conditions as in Step 1 for primary fine ball milling, carry out secondary fine ball milling on the primary pre-sintered material obtained in Step 2 to obtain the raw materials after secondary fine ball milling, which is simply referred to as secondary ball milled material; Step 4, sintering of the secondary ball milled material: Under the condition of a 100-mesh sieve, sieve the secondary ball milled material obtained in Step 3. Then, under the conditions of a pressure of 110 MPa, a die diameter of 25 mm, and a die thickness of 2.5 mm, press the secondary ball milled material into a round blank; under the conditions of a sintering temperature of 1114 °C and a sintering time of 24 h, sinter the round blank of the secondary ball milled material to obtain a large-sized ferroelectric and piezoelectric single crystal sample of sodium potassium niobate-based single crystal doped with barium carbonate; Step 5, annealing treatment of the sample: After subjecting the sample obtained in Step 4 to ultrasonic treatment, dry it, and place the dried sample in a muffle furnace at 600 °C for heat preservation for 10 h.

10. Use of the barium carbonate-doped sodium potassium niobate-based piezoelectric single crystal according to Claim 1 or 8 as a piezoelectric material.

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