A heat treatment method for improving piezoelectric performance of a ferroelectric single crystal in a quadrature phase

Abstract

The present application relates to a kind of orthogonal phase ferroelectric single crystal piezoelectric performance promotion heat treatment method, belong to piezoelectric material technical field.To solve the problem that the performance of existing lead-free bulk piezoelectric material still does not meet the high-end use demand, the present application provides a kind of orthogonal phase ferroelectric single crystal piezoelectric performance promotion heat treatment method, potassium sodium niobate bulk crystal is placed in heating container, with 1 ℃ / min heating rate from room temperature to heat treatment temperature and keeps warm 1h, then with 1 ℃ / min cooling rate drops to room temperature, the heat treatment temperature is not less than the orthorhombic-tetragonal phase transition temperature of potassium sodium niobate bulk crystal.The present application improves the piezoelectric performance of potassium sodium niobate (KNN) crystal by heat treatment and controls polarization orientation distribution, after KNN crystal growth ends, it is modified again, more positive face and face in combination with polarization configuration with net polarization is induced by orthorhombic-tetragonal-orthorhombic phase transition, so as to improve piezoelectric performance.

Description

Technical Field

[0001] This invention belongs to the field of piezoelectric materials technology, and particularly relates to a heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals. Background Technology

[0002] Piezoelectric materials are crystalline materials that exhibit a voltage between their two ends when subjected to pressure. When these materials deform under external force, the relative displacement of positive and negative ions within the unit cell causes the centers of positive and negative charges to no longer coincide, leading to macroscopic polarization of the crystal and the generation of charges on its surface. Currently, lead-based piezoelectric materials, represented by lead zirconate titanate (PZT) ceramics and relaxor ferroelectric single crystals of magnesium niobate titanate (PMNT), occupy the vast majority of the piezoelectric market share. However, these materials contain the toxic and harmful heavy metal lead, which poses serious threats to human health and the ecological environment. Therefore, the development of environmentally friendly lead-free piezoelectric materials has become one of the important scientific frontiers and technological competition focuses in the international functional materials field.

[0003] Currently, the lead-free piezoelectric materials considered to have the greatest potential to replace lead-based materials mainly include sodium potassium niobate (K). 0.5 Na 0.5 Lead-free piezoelectric materials (NbO3, KNN) and barium titanate (BaTiO3, BT) are commonly used in piezoelectric applications. However, due to differences in crystal structure, doping and modification, preparation processes, and temperature stability, the piezoelectric properties of lead-free piezoelectric materials still lag significantly behind those of lead-containing materials. Spontaneous polarization, as a functional unit, greatly influences the piezoelectric properties of lead-free piezoelectric materials. Some studies have utilized the control of polarization orientation distribution to improve the piezoelectric properties of piezoelectric materials. For example, in materials such as BaTiO3, applying an electric field along a specific direction causes electric dipoles to align along the direction of the electric field, forming a single-domain structure, thereby enhancing piezoelectric performance. Furthermore, engineered domain structures formed by specific spontaneous polarization orientations can achieve better piezoelectric performance compared to single-domain structures.

[0004] Methods for achieving specific polarization configurations and engineered domains mainly focus on doping with other ions or constructing compositional gradients during crystal growth. For example, in sodium potassium niobate single crystals, compositional gradients are used to construct "fishbone-like" domain structures to induce piezoelectric properties. However, this method is only applicable during crystal growth and cannot be used after growth is complete. The grown crystals are typically reprocessed using heat treatment, with annealing releasing and eliminating internal stress to improve piezoelectric properties.

[0005] Since polarization orientation distribution is highly sensitive to temperature, it can also be controlled through heat treatment. For example, by controlling the cooling rate of PbTiO3 thin films, engineered domain structures with a specific ratio of in-plane and out-of-plane orientations can be achieved, thereby improving piezoelectric properties. However, this method is only applicable to thin-film piezoelectric materials. Compared with two-dimensional thin-film piezoelectric materials, the grown bulk single crystals have a three-dimensional structure. Due to factors such as size effects and internal defects, it is more difficult to precisely control the specific polarization configuration distribution of bulk structures, and the control of domain structures is characterized by inhomogeneity and unpredictability. Summary of the Invention

[0006] To address the issue that the performance of existing lead-free bulk piezoelectric materials still does not meet the requirements of high-end applications, this invention provides a heat treatment method for improving the piezoelectric performance of orthorhombic ferroelectric single crystals.

[0007] The technical solution of the present invention:

[0008] A heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals involves placing a bulk sodium potassium niobate crystal in a heating container, heating it from room temperature to the heat treatment temperature at a heating rate of 1℃ / min and holding it at that temperature for 1 hour, and then cooling it down to room temperature at a cooling rate of 1℃ / min. The heat treatment temperature is not lower than the orthorhombic-tetragonal phase transition temperature of the bulk sodium potassium niobate crystal.

[0009] Furthermore, the orthorhombic-tetragonal phase transition temperature of the potassium sodium niobate bulk crystal is 200℃.

[0010] Furthermore, the heat treatment temperature is 250–400°C.

[0011] Furthermore, the method for preparing the bulk potassium sodium niobate crystals includes the following steps:

[0012] Step 1: Using K2CO3, Na2CO3 and Nb2O5 as raw materials, KNN polycrystalline raw materials are obtained through ball milling, drying and sintering.

[0013] Step 2: Place the KNN polycrystalline raw material obtained in Step 1 in a single crystal furnace and heat it. Observe and record the melting point temperature at which the polycrystalline raw material completely melts. Continue to heat it up and keep it at a temperature 50°C above the melting point temperature for 6 hours. Then, cool it down to a temperature 30°C above the melting point temperature and keep it at that temperature for 1 to 2 hours.

[0014] Step 3: Lower the seed crystal to the surface of the melt, hold it for a period of time, then lift the seed crystal off the liquid surface, ensuring that the end of the seed crystal melts to form a new atomic face;

[0015] Step 4: Gradually lower the temperature to near the melting point temperature, and immerse the seed crystal below the surface of the melt again. When a bright square edge appears around the seed crystal, start pulling it up.

[0016] Step 5: When the crystal shoulder reaches 8mm, raise the temperature by 1-5℃ to allow the crystal to naturally rotate to constant diameter growth. Due to the gradual decrease of the solid-liquid level and the influence of component segregation during crystal growth, a slight cooling is required for control.

[0017] Step 6: When the crystal pull-out length reaches 20-30 mm, pull the crystal out of the liquid surface by 15 mm, and then cool it to room temperature to obtain potassium sodium niobate block crystals.

[0018] Furthermore, the purity of K2CO3, Na2CO3 and Nb2O5 mentioned in step one is 99.99%, and the molar ratio of K2CO3, Na2CO3 and Nb2O5 is 0.55:0.55:1.

[0019] Furthermore, the ball milling in step one is a mixed alcohol ball milling, the ball milling time is 24 hours, and the sintering is sintering at 1000℃ for 9 hours.

[0020] Furthermore, in step three, the seed crystal rotates at a speed of 20 r / min.

[0021] Furthermore, in step four, the lifting speed during the shoulder placement process is 0.2 mm / h.

[0022] Furthermore, the temperature drop in step five is no more than 0.2℃, and the cooling rate is 0.2~0.4℃ / h.

[0023] Furthermore, the cooling rate described in step six is ​​30°C / h.

[0024] The beneficial effects of this invention are:

[0025] This invention proposes a method to improve the piezoelectric properties of potassium sodium niobate (KNN) crystals by controlling the polarization orientation distribution through heat treatment. Unlike modification methods such as doping or constructing compositional gradients, this invention modifies the KNN crystal after growth, using an orthogonal-tetragonal-orthogonal phase transition. This transforms a portion of the initial negative outward orientation within the KNN crystal into an in-plane orientation, inducing a greater combination of positive outward and in-plane polarization configurations with net polarization, thereby improving piezoelectric properties. This invention is simple and extremely low-cost, and can promote the development and application of transducers, sensors, energy harvesters, and other devices utilizing the piezoelectric effect of KNN crystals. Attached Figure Description

[0026] Figure 1 Micrographs of the internal domain structure of the crystals before heat treatment and after heat treatment in Comparative Example 2, Example 1 and Example 3;

[0027] a is the original KNN bulk crystal, b is the KNN bulk crystal after heat treatment in Comparative Example 2, and c is the KNN bulk crystal after heat treatment in Example 1.

[0028] KNN bulk crystal, d is the KNN bulk crystal after heat treatment in Example 3;

[0029] Figure 2 This is a schematic diagram of crystal polarization orientation;

[0030] Figure 3 The graph shows a comparison of the piezoelectric properties of KNN crystals before and after heat treatment in Comparative Example 1, Comparative Example 2, Example 1, Example 2, and Example 3. Detailed Implementation

[0031] The technical solution of the present invention will be further described below with reference to embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention. In the following embodiments, the process equipment or apparatus not specifically specified are all conventional equipment or apparatus in the art. Unless otherwise specified, the raw materials used in the embodiments of the present invention are all commercially available; unless otherwise specified, the technical means used in the embodiments of the present invention are all conventional means well known to those skilled in the art.

[0032] Example 1

[0033] This embodiment provides a heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals.

[0034] This embodiment first employs the top seed crystal fluxing method (TSSG) to prepare potassium sodium niobate KNN crystals, specifically including the following steps:

[0035] Step 1: Using K2CO3, Na2CO3 and Nb2O5 with a purity of 99.99% as raw materials, mix them in a molar ratio of 0.55:0.55:1 and put them into a ball mill with alcohol and ball mill for 24 hours. Dry the powder obtained from ball milling and put it into a crucible. Sinter it at 1000℃ for 9 hours to obtain KNN polycrystalline raw material.

[0036] Step 2: Place the KNN polycrystalline raw material obtained in Step 1 in a single crystal furnace and heat it. Observe and record the melting point temperature at which the polycrystalline raw material completely melts. Continue to heat it up and keep it at a temperature 50°C above the melting point temperature for 6 hours. Then, cool it down to a temperature 30°C above the melting point temperature and keep it at that temperature for 1 to 2 hours.

[0037] Step 3: Lower the seed crystal to the surface of the melt, hold it for a period of time, then lift the seed crystal off the liquid surface, ensuring that the end of the seed crystal melts to form a new atomic face. The rotation speed of the seed crystal is 20 r / min.

[0038] Step 4: Gradually lower the temperature to near the melting point temperature, and immerse the seed crystal below the surface of the melt again. When a bright square edge appears around the seed crystal, start pulling it up. The pulling speed during the shoulder formation process is 0.2 mm / h.

[0039] Step 5: When the crystal shoulder reaches 8mm, increase the temperature by 3℃ to allow the crystal to naturally rotate to constant diameter growth. Due to the gradual decrease of the solid-liquid level and the influence of component segregation during crystal growth, a small temperature drop is required for control. The small temperature drop is 0.2℃, and the cooling rate is 0.2℃ / h.

[0040] Step 6: When the crystal pull-out length reaches 20 mm, pull the crystal out of the liquid surface by 15 mm, and then cool it to room temperature at a rate of 30℃ / h to obtain potassium sodium niobate block crystals.

[0041] The orthorhombic-tetragonal phase transition temperature of the sodium potassium niobate bulk crystals obtained in this embodiment is 200℃.

[0042] The specific method for heat treatment of bulk potassium sodium niobate crystals in this embodiment is as follows:

[0043] Potassium sodium niobate block crystals were placed in a crucible and heated in a muffle furnace. The temperature was increased from room temperature to 250°C at a heating rate of 1°C / min and held for 1 hour. Then the temperature was reduced to room temperature at a cooling rate of 1°C / min.

[0044] During heat treatment, the heating and cooling rates should be avoided to prevent rapid temperature changes from causing crystal cracking.

[0045] Example 2

[0046] This embodiment provides a heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals.

[0047] This embodiment first employs the top seed crystal fluxing method (TSSG) to prepare potassium sodium niobate KNN crystals, specifically including the following steps:

[0048] Step 1: Using K2CO3, Na2CO3 and Nb2O5 with a purity of 99.99% as raw materials, mix them in a molar ratio of 0.55:0.55:1 and put them into a ball mill with alcohol and ball mill for 24 hours. Dry the powder obtained from ball milling and put it into a crucible. Sinter it at 1000℃ for 9 hours to obtain KNN polycrystalline raw material.

[0049] Step 2: Place the KNN polycrystalline raw material obtained in Step 1 in a single crystal furnace and heat it. Observe and record the melting point temperature at which the polycrystalline raw material completely melts. Continue to heat it up and keep it at a temperature 50°C above the melting point temperature for 6 hours. Then, cool it down to a temperature 30°C above the melting point temperature and keep it at that temperature for 1 to 2 hours.

[0050] Step 3: Lower the seed crystal to the surface of the melt, hold it for a period of time, then lift the seed crystal off the liquid surface, ensuring that the end of the seed crystal melts to form a new atomic face. The rotation speed of the seed crystal is 20 r / min.

[0051] Step 4: Gradually lower the temperature to near the melting point temperature, and immerse the seed crystal below the surface of the melt again. When a bright square edge appears around the seed crystal, start pulling it up. The pulling speed during the shoulder formation process is 0.2 mm / h.

[0052] Step 5: When the crystal shoulder reaches 8mm, increase the temperature by 3℃ to allow the crystal to naturally rotate to constant diameter growth. Due to the gradual decrease of the solid-liquid level and the influence of component segregation during crystal growth, a small temperature drop is required for control. The small temperature drop is 0.2℃, and the cooling rate is 0.2℃ / h.

[0053] Step 6: When the crystal pull-out length reaches 20 mm, pull the crystal out of the liquid surface by 15 mm, and then cool it to room temperature at a rate of 30℃ / h to obtain potassium sodium niobate block crystals.

[0054] The orthorhombic-tetragonal phase transition temperature of the sodium potassium niobate bulk crystals obtained in this embodiment is 200℃.

[0055] The specific method for heat treatment of bulk potassium sodium niobate crystals in this embodiment is as follows:

[0056] Potassium sodium niobate block crystals were placed in a crucible and heated in a muffle furnace. The temperature was increased from room temperature to 300°C at a heating rate of 1°C / min and held for 1 hour. Then the temperature was reduced to room temperature at a cooling rate of 1°C / min.

[0057] Example 3

[0058] This embodiment provides a heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals.

[0059] This embodiment first employs the top seed crystal fluxing method (TSSG) to prepare potassium sodium niobate KNN crystals, specifically including the following steps:

[0060] Step 1: Using K2CO3, Na2CO3 and Nb2O5 with a purity of 99.99% as raw materials, mix them in a molar ratio of 0.55:0.55:1 and put them into a ball mill with alcohol and ball mill for 24 hours. Dry the powder obtained from ball milling and put it into a crucible. Sinter it at 1000℃ for 9 hours to obtain KNN polycrystalline raw material.

[0061] Step 2: Place the KNN polycrystalline raw material obtained in Step 1 in a single crystal furnace and heat it. Observe and record the melting point temperature at which the polycrystalline raw material completely melts. Continue to heat it up and keep it at a temperature 50°C above the melting point temperature for 6 hours. Then, cool it down to a temperature 30°C above the melting point temperature and keep it at that temperature for 1 to 2 hours.

[0062] Step 3: Lower the seed crystal to the surface of the melt, hold it for a period of time, then lift the seed crystal off the liquid surface, ensuring that the end of the seed crystal melts to form a new atomic face. The rotation speed of the seed crystal is 20 r / min.

[0063] Step 4: Gradually lower the temperature to near the melting point temperature, and immerse the seed crystal below the surface of the melt again. When a bright square edge appears around the seed crystal, start pulling it up. The pulling speed during the shoulder formation process is 0.2 mm / h.

[0064] Step 5: When the crystal shoulder reaches 8mm, increase the temperature by 3℃ to allow the crystal to naturally rotate to constant diameter growth. Due to the gradual decrease of the solid-liquid level and the influence of component segregation during crystal growth, a small temperature drop is required for control. The small temperature drop is 0.2℃, and the cooling rate is 0.2℃ / h.

[0065] Step 6: When the crystal pull-out length reaches 20 mm, pull the crystal out of the liquid surface by 15 mm, and then cool it to room temperature at a rate of 30℃ / h to obtain potassium sodium niobate block crystals.

[0066] The orthorhombic-tetragonal phase transition temperature of the sodium potassium niobate bulk crystals obtained in this embodiment is 200℃.

[0067] The specific method for heat treatment of bulk potassium sodium niobate crystals in this embodiment is as follows:

[0068] Potassium sodium niobate block crystals were placed in a crucible and heated in a muffle furnace. The temperature was increased from room temperature to 400°C at a heating rate of 1°C / min and held for 1 hour. Then the temperature was reduced to room temperature at a cooling rate of 1°C / min.

[0069] Comparative Example 1

[0070] The heat treatment method provided in this comparative example differs from that in Example 1 only in that the potassium sodium niobate block crystals in this comparative example are placed in a crucible, heated in a muffle furnace, heated from room temperature to 180°C at a heating rate of 1°C / min and held at that temperature for 1 hour, and then cooled to room temperature at a cooling rate of 1°C / min.

[0071] Comparative Example 2

[0072] The heat treatment method provided in this comparative example differs from that in Example 1 only in that the potassium sodium niobate block crystals in this comparative example are placed in a crucible, heated in a muffle furnace, heated from room temperature to 200°C at a heating rate of 1°C / min and held at that temperature for 1 hour, and then cooled to room temperature at a cooling rate of 1°C / min.

[0073] 1. The changes in the internal domain structure of the crystal before and after heat treatment were observed using a polarizing microscope. The results are as follows: Figure 1 As shown.

[0074] Figure 1 Micrographs of the internal domain structure of the crystals before heat treatment and after heat treatment in Comparative Example 2, Example 1 and Example 3;

[0075] a is the original KNN bulk crystal, b is the KNN bulk crystal after heat treatment in Comparative Example 2, and c is the KNN bulk crystal after heat treatment in Example 1.

[0076] KNN bulk crystal, d represents the KNN bulk crystal after heat treatment in Example 3; the domain wall size and orientation remain almost unchanged after heat treatment in Comparative Example 2, corresponding to an unaffected piezoelectric coefficient. However, after heat treatment in Examples 1 and 3, the domain wall orientation changes to the

[010] direction, and the domain size is significantly reduced. The results indicate that beyond T... o-t Heat treatment can significantly alter the domain structure of a crystal.

[0077] Figure 2 This is a schematic diagram of crystal polarization orientation; as shown below. Figure 2 As shown, the spontaneous polarization direction in the unit cell is closely related to the lattice state. A longer lattice structure facilitates the eccentricity of B-site ions. During the orthorhombic-to-tetragonal phase transition in the annealing process, the polarization vector tends to rotate towards the longer side (c), while the likelihood of rotation towards the shorter side (a and b) is lower. This makes the two out-of-plane polarization orientations in the untreated sample more inclined to... Figure 2 The evolution trend shown by the dashed lines in (a) and (c) leads to the formation of partial in-plane polarization in the tetragonal phase at high temperatures.

[0078] In a tetragonal lattice, all sides except the longest side c are equal. When the temperature decreases and the crystal transforms from a tetragonal to an orthorhombic phase, the polarization vectors become four equally probable orthorhombic polarization vectors. The evolution trend of the polarization vectors during the transformation from an orthorhombic to a tetragonal phase is as follows: Figure 2 As shown in (b) and (d), some in-plane orientations are preserved.

[0079] Untreated KNN crystals inherently possess a certain piezoelectric coefficient, indicating that in the initial state, the number of positive out-of-plane orientations is greater than that of negative out-of-plane orientations. This is related to factors such as the compositional gradient or internal stress within the crystal. These factors continue to play a significant role in the polarization vector evolution caused by the phase transition, making negative out-of-plane orientations more likely to transform into in-plane orientations compared to positive out-of-plane orientations. Furthermore, there is a certain correlation between polarization orientations; negative out-of-plane polarizations are fewer, thus the correlation is weaker, making them more likely to transform into in-plane orientations. Considering these factors, through orthogonal-tetragonal-orthogonal phase transitions, some of the negative out-of-plane orientations in the initial KNN crystal transform into in-plane orientations. Heat treatment induces more net-polarized polarization configurations combining positive out-of-plane and in-plane orientations, thereby improving piezoelectric properties.

[0080] 2. Au electrodes were deposited on both surfaces of the KNN crystal using magnetron sputtering, and the piezoelectric properties of the samples were characterized using a quasi-static d33 measuring instrument. The results are as follows: Figure 3 As shown.

[0081] Comparative Examples 1 and 2 were heated at temperatures lower than the orthorhombic-tetragonal phase transition temperature (TO-T, 200℃) of bulk sodium potassium niobate crystals, and the piezoelectric coefficients of the KNN single crystals showed no significant change. After heat treatment at 180℃, the piezoelectric coefficient of Comparative Example 1 changed from 104 pC / N to 100 pC / N; after heat treatment at 200℃, the piezoelectric coefficient of Comparative Example 2 changed from 114 pC / N to 118 pC / N.

[0082] The heat treatment temperatures of Examples 1, 2, and 3 are higher than the orthorhombic-tetragonal phase transition temperature (TO-T) (200℃) of bulk sodium potassium niobate crystals, which can increase the KNN single crystal coefficient by 73%-98%. After heat treatment at 250℃, the piezoelectric coefficient of Example 1 changed from 120 pC / N to 208 pC / N; after heat treatment at 300℃, the piezoelectric coefficient of Example 2 changed from 108 pC / N to 212 pC / N; and after heat treatment at 400℃, the piezoelectric coefficient of Example 3 changed from 101 pC / N to 200 pC / N.

Claims

1. A heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals, characterized in that, Potassium sodium niobate bulk crystals are placed in a heating container and heated from room temperature to the heat treatment temperature at a heating rate of 1℃ / min and held at that temperature for 1 hour. Then, the temperature is lowered to room temperature at a cooling rate of 1℃ / min. The heat treatment temperature is not lower than the orthorhombic-tetragonal phase transition temperature of the potassium sodium niobate bulk crystals, specifically 250~400℃. The method for preparing the bulk potassium sodium niobate crystals includes the following steps: Step 1: Using K2CO3, Na2CO3 and Nb2O5 as raw materials, KNN polycrystalline raw materials are obtained through ball milling, drying and sintering. Step 2: Place the KNN polycrystalline raw material obtained in Step 1 in a single crystal furnace and heat it. Observe and record the melting point temperature at which the polycrystalline raw material completely melts. Continue to heat it up and keep it at a temperature 50°C above the melting point temperature for 6 hours. Then, cool it down to a temperature 30°C above the melting point temperature and keep it at that temperature for 1-2 hours. Step 3: Lower the seed crystal to the surface of the melt, hold it for a period of time, then lift the seed crystal off the liquid surface, ensuring that the end of the seed crystal melts to form a new atomic face; Step 4: Gradually lower the temperature to near the melting point temperature, and immerse the seed crystal below the surface of the melt again. When a bright square edge appears around the seed crystal, start pulling it up. Step 5: When the crystal shoulder reaches 8mm, raise the temperature by 1~5℃ to allow the crystal to naturally rotate to constant diameter growth. Due to the gradual decrease of the solid-liquid level and the influence of component segregation during crystal growth, a small temperature drop is required for control. Step 6: When the crystal pull-out length reaches 20-30mm, pull the crystal out of the liquid surface by 15mm, and then cool it to room temperature to obtain potassium sodium niobate block crystals; The ball milling in step one is a mixed alcohol ball milling, with a milling time of 24 hours, and the sintering is sintering at 1000℃ for 9 hours.

2. The heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals according to claim 1, characterized in that, The purity of K₂CO₃, Na₂CO₃, and Nb₂O₅ mentioned in step one is 99.99%. 3、 The molar ratio of Na2CO3 to Nb2O5 is 0.55:0.55:

1.

3. The heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals according to claim 1, characterized in that, In step three, the seed crystal rotates at a speed of 20 r / min.

4. The heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals according to claim 3, characterized in that, In step four, the lifting speed during the shoulder placement process is 0.2 mm / h.

5. The heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals according to claim 4, characterized in that, The temperature drop in step five shall not exceed 0.2℃, and the cooling rate shall be 0.2~0.4℃ / h.

6. The heat treatment method for improving the piezoelectric properties of orthorhombic ferroelectric single crystals according to claim 5, characterized in that, The cooling rate described in step six is ​​30℃ / h.

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