Annealing polarization method of lithium niobate crystal

Abstract

The present invention relates to an annealing polarization method for lithium niobate crystals. The annealing polarization method for lithium niobate crystals comprises the following steps: connecting the two ends of the lithium niobate crystal to the positive and negative electrodes of a polarization power supply respectively; grounding the positive and negative electrodes of the lithium niobate crystal, then annealing the lithium niobate crystal, and cooling to 1190°C to 1210°C after the annealing is completed; disconnecting the grounding wire, and then applying an electric field to the lithium niobate crystal based on a preset electric field application program; and discharging the positive and negative electrodes of the lithium niobate crystal, and after the discharge is completed, grounding the positive and negative electrodes of the lithium niobate crystal and cooling to room temperature. The present invention performs annealing and polarization as one process at the same time, and performs front-end processing after polarization is completed, which not only reduces the process and is conducive to improving efficiency, but also can improve the front-end processing yield, thereby reducing the problem of cracking in the crystal annealing polarization and furnace removal process due to the crystal pyroelectric effect.

Description

Technical Field

[0001] The invention relates to the technical field of lithium niobate crystal polarization, and in particular to an annealing polarization method for lithium niobate crystals. Background Art

[0002] Lithium niobate single crystal (LiNbO3, LN) is a multifunctional artificial crystal material with excellent performance. It is the ferroelectric with the highest known Curie temperature and the largest spontaneous polarization, and exhibits the characteristics of combining electrical, optical, and nonlinear properties. It is a rare and widely used artificial crystal. Due to the good piezoelectric properties, nonlinear optical properties, electro-optical and photorefractive properties of lithium niobate, it has a wide range of applications in surface acoustic wave devices, infrared detectors, optical switches, acousto-optics, piezoelectricity, etc.

[0003] The grown lithium niobate crystal is generally yellow-white or yellow-green. During the growth process, there is thermal stress inside the crystal, which affects the optical uniformity of the crystal. The residual thermal stress will increase the difficulty of crystal processing and bring inconvenience to the optical test of the crystal. Therefore, we need to solve the problem of residual stress inside the crystal through annealing process. Through annealing, the crystal has higher optical uniformity, and at the same time, it can also reduce the thermal stress inside the crystal, making the crystal have more stable physical and chemical properties.

[0004] The grown crystals generally present a multi-domain structure, which seriously affects the optical properties of lithium niobate crystals. Therefore, we need to pass current through the lithium niobate crystals to polarize them. Through the polarization process, the lithium niobate crystals are made single-domain, which improves the optical properties of the lithium niobate crystals.

[0005] At present, the processes used in the industry all require the crystal to be processed in the front process, that is, the head and tail of the crystal are cut off. In this process, the crystal is very easy to crack, causing the product to be scrapped. Based on this, the common practice in the industry is to first anneal the grown crystal, that is, heat it to about 1240℃ and keep it warm for 10h~30h, and then perform front-end processing to cut off the head and tail after annealing, and then polarize it. This solution has a long cycle, and there is still a risk of cracking during crystal annealing, especially in winter, when lithium niobate has good pyroelectric properties and is very easy to crack due to temperature changes. Summary of the invention

[0006] Based on this, it is necessary to provide an annealing polarization method for lithium niobate crystals in order to reduce the problem of crystal annealing polarization and cracking during the furnace removal process caused by the crystal pyroelectric effect.

[0007] A method for annealing and polarizing a lithium niobate crystal comprises the following steps:

[0008] Connect the two ends of the lithium niobate crystal to the positive and negative electrodes of the polarization power supply respectively;

[0009] The positive and negative electrodes of the lithium niobate crystal are grounded, and then the lithium niobate crystal is annealed, and after the annealing is completed, the temperature is lowered to 1190° C. to 1210° C.;

[0010] The grounding wire is disconnected, and then an electric field is applied to the lithium niobate crystal based on the following preset electric field application program: the current setting value I of the polarization power supply is changed once every 1s~60s, and increases linearly from 0 to I0 within 0.5h~4h, and the resistance R of the lithium niobate crystal is calculated according to the formula R=U1 / I1, wherein I1 and U1 are the current and voltage displayed by the polarization power supply, respectively; the voltage setting value U of the polarization power supply changes from the initial voltage setting value at a rate not exceeding 2V / min according to the formula U=I*R*1.1 with the change of current, wherein the initial voltage setting value of the polarization power supply does not exceed 1V, and when the current setting value increases to I0, the setting value of the polarization power supply temporarily stops changing, and the voltage setting value is U0 at this time; after the electric field is stabilized, the temperature is lowered to below 1000°C, and the voltage setting value is linearly reduced from U0 to 0 within 0.5h~4h; and

[0011] The positive and negative electrodes of the lithium niobate crystal are discharged. After the discharge is completed, the positive and negative electrodes of the lithium niobate crystal are grounded and cooled to room temperature.

[0012] The annealing and polarization method of lithium niobate crystal in the technical solution of the present invention is simple in process. Compared with the traditional annealing and polarization process, the present invention performs annealing and polarization as one process at the same time, wherein the current setting value and the voltage setting value are controlled simultaneously in the polarization process to prevent the lithium niobate crystal from cracking due to the change in resistance of the lithium niobate crystal or the sharp change in actual voltage caused by the power supply reading error. The present invention performs the front-end processing after completing the polarization, which not only reduces the process steps and is conducive to improving efficiency, but also can improve the front-end processing yield, thereby reducing the cracking problem of the crystal during the annealing polarization and furnace removal process caused by the crystal pyroelectric effect.

[0013] In a feasible implementation, the operation of connecting the two ends of a lithium niobate crystal to the positive and negative electrodes of a polarization power supply is as follows: the lithium niobate crystal is placed between a first polarization cap and a second polarization cap, the first polarization cap is covered with an electrode dish on the side away from the lithium niobate crystal, and the second polarization cap is covered with an electrode sheet on the side away from the lithium niobate crystal, and the electrode dish and the electrode sheet are respectively connected to the positive and negative electrodes of the polarization power supply.

[0014] In a feasible implementation, the first polarization cap comprises lithium niobate sand or lithium niobate mud, and the second polarization cap comprises lithium niobate mud;

[0015] The lithium niobate mud includes the following components according to average particle size and mass fraction:

[0016] First, 1 to 2 parts of lithium niobate polycrystalline sand [0.15 mm to 0.35 mm);

[0017] 1 to 2 parts of the second lithium niobate polycrystalline sand [0.35 mm to 0.65 mm); and

[0018] Third, 1 to 2 parts of lithium niobate polycrystalline sand [0.65mm~1mm].

[0019] In a feasible implementation, the operation of annealing the lithium niobate crystal is: heating the temperature to 1220°C~1260°C at a heating rate of 10°C / h~60°C / h and then keeping the temperature for 12h~36h for annealing.

[0020] In a feasible implementation, after the annealing is completed, the temperature is lowered to 1190° C. to 1210° C. and maintained at 1190° C. to 1210° C. for 4 h to 10 h.

[0021] In a feasible implementation, the lithium niobate crystal is placed vertically in the Z direction in the annealing furnace and polarized along the crystal growth direction. The calculation formula of the polarization current is as follows:

[0022] I0=crystal radius^2*3.14*current density.

[0023] In a feasible implementation, the lithium niobate crystal is placed horizontally in the annealing furnace in the X direction or the Y direction and polarized in a direction perpendicular to the crystal growth direction. The calculation formula of the polarization current is as follows:

[0024] I0=crystal length*crystal diameter*current density.

[0025] In a feasible implementation, the current density is 1 mA / cm^2~5 mA / cm^2.

[0026] In a feasible implementation, the operation of cooling the temperature to below 1000° C. after the electric field is stabilized is: after maintaining the electric field for 0.5 h to 4 h, cooling the temperature to below 1000° C. at a cooling rate of 10° C. / h to 60° C. / h.

[0027] In a feasible implementation, the initial voltage setting value of the polarization power supply is 0.1V, 0.2V, 0.3V, 0.4V, 0.5V, 0.6V, 0.7V, 0.8V or 0.9V. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 This is a flow chart of an annealing polarization method of a lithium niobate crystal according to an embodiment of the present invention;

[0029] Figure 2A schematic diagram of an annealing polarization device for a lithium niobate crystal and a lithium niobate crystal according to an embodiment of the present invention;

[0030] Figure 3 It is a front view of an annealing furnace, an electrode dish, a first polarization cap, a second polarization cap, an electrode sheet, and a lithium niobate crystal according to an embodiment of the present invention;

[0031] Figure 4 A top view of an annealing furnace, an electrode dish, a first polarization cap, a second polarization cap, an electrode sheet, and a lithium niobate crystal according to an embodiment of the present invention;

[0032] Figure 5 A schematic diagram of an annealing polarization device for a lithium niobate crystal and a lithium niobate crystal according to another embodiment of the present invention;

[0033] Figure 6 A front view of an annealing furnace, an electrode dish, a first polarization cap, a second polarization cap, an electrode sheet, and a lithium niobate crystal according to another embodiment of the present invention;

[0034] Figure 7 A top view of an annealing furnace, an electrode dish, a first polarization cap, a second polarization cap, an electrode sheet, and a lithium niobate crystal according to another embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. In the following description, many specific details are set forth to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present invention, so the present invention is not limited by the specific embodiments disclosed below.

[0036] It should be noted that when an element is referred to as being "fixed to" another element, it may be directly on the other element or there may be a central element. When an element is considered to be "connected to" another element, it may be directly connected to the other element or there may be a central element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are for illustrative purposes only.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used herein in the specification of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0038] See also Figure 1The annealing polarization method of lithium niobate crystal according to one embodiment of the present invention comprises the following steps:

[0039] S10, connecting the two ends of the lithium niobate crystal to the positive and negative electrodes of the polarization power supply respectively.

[0040] Please combine Figure 2~Figure 4 , an annealing polarization device 100 for lithium niobate crystals according to an embodiment is provided, comprising an electrode dish 110, a first polarization cap 120, a second polarization cap 130, an electrode sheet 140, an annealing furnace 150, a polarization power supply 160 and a power supply control module 170. The electrode dish 110 and the electrode sheet 140 are respectively connected to the positive and negative electrodes of the polarization power supply 160. The annealing furnace 150 comprises a furnace body 151, a heating element 152 and a sintering plate 153. Specifically, the heating element 152 is a plurality of U-shaped silicon-molybdenum rods, and the plurality of U-shaped silicon-molybdenum rods are evenly distributed near the furnace body 151. The sintering plate 153 is located below the electrode dish 110, and the sintering plate 153 is used to support the electrode dish 110, the first polarization cap 120, the second polarization cap 130, the electrode sheet 140 and the lithium niobate crystal 200. In addition, the annealing furnace 150 may also include a temperature measuring element (not shown), such as a platinum-rhodium thermocouple.

[0041] In a feasible implementation, the operation of connecting the two ends of the lithium niobate crystal 200 to the positive and negative electrodes of the polarization power supply 160 is as follows: the lithium niobate crystal 200 is placed between the first polarization cap 120 and the second polarization cap 130, the side of the first polarization cap 120 away from the lithium niobate crystal 200 is covered with an electrode dish 110, and the side of the second polarization cap 130 away from the lithium niobate crystal 200 is covered with an electrode sheet 140, and the electrode dish 110 and the electrode sheet 140 are respectively connected to the positive and negative electrodes of the polarization power supply 160.

[0042] Among them, the preparation method of the first polarization cap 120 is as follows: select an electrode dish 110 of appropriate size, and connect a polarization wire to the electrode dish 110; fill the electrode dish 110 with lithium niobate sand of different particle sizes, with coarse sand at the bottom and fine sand at the top, for example, the lower layer is 0.65 mm lithium niobate polycrystalline sand, and the upper layer is 0.35 mm lithium niobate polycrystalline sand, to prevent the crystal from being cracked by the polycrystalline sand; for lithium niobate crystal 200 with severe tail swing or convex bottom, polarization glue and lithium niobate polycrystalline sand can be mixed in a certain proportion to make lithium niobate mud similar to plasticine, which is used to support and fix the lithium niobate crystal 200; ensure that the bottom of the lithium niobate crystal 200 is in full contact with the first polarization cap 120 located below, and no position of the lithium niobate crystal 200 is in direct contact with the electrode dish 110, and the first polarization cap 120 is completed.

[0043] Among them, the preparation method of the second polarization cap 130 is as follows: lithium niobate sand of different particle sizes and polarization glue are mixed in a certain proportion to make lithium niobate mud similar to plasticine, which is covered on the upper surface of the lithium niobate crystal 200, and the second polarization cap 130 is manually made to be close in size to the diameter of the lithium niobate crystal 200, in close contact with the lithium niobate crystal 200, able to maintain the shape unchanged and not easy to fall apart, and the thickness of the second polarization cap 130 is greater than the shoulder height of the lithium niobate crystal 200 to ensure that any position of the lithium niobate crystal 200 will not directly contact the electrode sheet 140 located above, and the electrode sheet 140 is covered on the second polarization cap 130 to ensure that the electrode sheet 140 is in close contact with the second polarization cap 130.

[0044] The polarized glue is obtained by mixing a polymer compound (molecular weight 2000-50000) with pure water, and the polymer compound may be, for example, starch, cellulose powder, carboxymethyl cellulose, etc. Furthermore, the lithium niobate mud includes the following components according to the average particle size and mass fraction:

[0045] First, 1 to 2 parts of lithium niobate polycrystalline sand [0.15 mm to 0.35 mm);

[0046] 1 to 2 parts of the second lithium niobate polycrystalline sand [0.35 mm to 0.65 mm); and

[0047] Third, 1 to 2 parts of lithium niobate polycrystalline sand [0.65mm~1mm].

[0048] In this embodiment, the average particle sizes of the first lithium niobate polycrystalline sand, the second lithium niobate polycrystalline sand and the third lithium niobate polycrystalline sand increase successively, and the three can match each other, so that the prepared lithium niobate mud can be in close contact with the crystal, maintain its shape and is not easy to disperse.

[0049] The lithium niobate crystal 200 with the first polarization cap 120 and the second polarization cap 130 is placed in the annealing furnace 150. According to the actual conditions of the lithium niobate crystal 200, such as whether there are microcracks on the top and bottom, the positive and negative potential of the pyroelectric potential, etc., the lithium niobate crystal 200 is connected to the positive and negative electrodes of the polarization power supply 160 through polarization wires.

[0050] Specifically in this embodiment, the lithium niobate crystal 200 is placed vertically in the annealing furnace 150 in the Z direction, and polarization is performed along the crystal growth direction (i.e., vertical polarization). It should be noted that in the annealing polarization method of the lithium niobate crystal of the present invention, the lithium niobate crystal 200 can also be placed horizontally in the annealing furnace 150 in the X direction or the Y direction, and polarization is performed perpendicular to the crystal growth direction (i.e., horizontal polarization), such as Figure 5~Figure 7 shown.

[0051] S20, grounding the positive and negative electrodes of the lithium niobate crystal, and then annealing the lithium niobate crystal. After the annealing is completed, the temperature is lowered to 1190°C~1210°C.

[0052] Since lithium niobate crystals have discharge behavior, the positive and negative poles of the lithium niobate crystals must be grounded before applying the electric field.

[0053] In a feasible implementation, the operation of annealing the lithium niobate crystal is: heating the temperature to 1220°C~1260°C at a heating rate of 10°C / h~60°C / h, and then keeping the temperature for 12h~36h for annealing. Further, the heating rate can be but not limited to 10°C / h, 20°C / h, 30°C / h, 40°C / h, 50°C / h or 60°C / h, the temperature after heating can be but not limited to 1220°C, 1230°C, 1240°C, 1250°C or 1260°C, and the holding time can be but not limited to 12h, 15h, 18h, 20h, 24h, 30h or 36h.

[0054] In a feasible implementation, after the annealing is completed, the temperature is lowered to 1190°C to 1210°C, and the temperature is kept at 1190°C to 1210°C for 4 to 10 hours. Further, the holding temperature may be, but is not limited to, 1190°C, 1195°C, 1200°C, 1205°C or 1210°C, and the holding time may be, but is not limited to, 4 hours, 5 hours, 6 hours, 7 hours, 8 hours, 9 hours or 10 hours.

[0055] S30, disconnect the ground wire, and then apply an electric field to the lithium niobate crystal based on the following preset electric field application program: the current setting value I of the polarization power supply changes once every 1s~60s, and increases linearly from 0 to I0 within 0.5h~4h, and the resistance R of the lithium niobate crystal is calculated according to the formula R=U1 / I1, wherein I1 and U1 are the current and voltage displayed by the polarization power supply, respectively; the voltage setting value U of the polarization power supply changes from the initial voltage setting value at a rate not exceeding 2V / min, according to the formula U=I*R*1.1 with the change of current, wherein the initial voltage setting value of the polarization power supply does not exceed 1V, and when the current setting value increases to I0, the setting value of the polarization power supply temporarily stops changing, and at this time the voltage setting value is U0; after the electric field stabilizes, the temperature is lowered to below 1000°C, and the voltage setting value is linearly reduced from U0 to 0 within 0.5h~4h.

[0056] In step S30, the current setting value and the voltage setting value are controlled simultaneously in the polarization process. To prevent the voltage setting value from being insufficient, the present invention has two setting values, namely the current setting value and the voltage setting value. The actual current and voltage of the electric field will not exceed the setting value. The voltage setting value U=I*R*1.1 of the present invention is set in this way because the current is preferred to meet the polarization requirements, but the crystal resistance is changing during the polarization process. The voltage setting value is usually to protect the crystal, and the maximum voltage is limited according to the real-time current size and crystal resistance. After the inventor's creative research, it is found that the coefficient 1.1 is the most suitable, and there is also a maximum change rate limit of 2V / min. If the coefficient is greater than 1.1, the voltage setting value will always change at 2V / min. Therefore, based on the rate that the coefficient 1.1 can be lower than 2V / min, it can not only ensure that the maximum voltage setting value is sufficient, but also prevent the crystal resistance from changing, or the actual voltage from changing sharply due to power supply reading errors.

[0057] In a feasible implementation, the initial voltage setting value of the polarization power supply is 0.1 V, 0.2 V, 0.3 V, 0.4 V, 0.5 V, 0.6 V, 0.7 V, 0.8 V or 0.9 V. Of course, the initial voltage setting value of the polarization film is not limited thereto, and can also be any value between greater than 0 and less than 1 V.

[0058] In a feasible implementation, the lithium niobate crystal 200 is placed vertically in the annealing furnace in the Z direction and polarized along the crystal growth direction (i.e., vertical polarization), such as Figure 2~Figure 4 As shown, the calculation formula of polarization current is as follows:

[0059] I0=crystal radius^2*3.14*current density.

[0060] In a feasible implementation, the lithium niobate crystal 200 is placed horizontally in the annealing furnace in the X direction or the Y direction and polarized in a direction perpendicular to the crystal growth direction (i.e., horizontal polarization), such as Figure 5~Figure 7 As shown, the calculation formula of polarization current is as follows:

[0061] I0=crystal length*crystal diameter*current density.

[0062] In a feasible implementation, the current density is 1 mA / cm^2 to 5 mA / cm^2. Further, the current density may be, but is not limited to, 1 mA / cm^2, 2 mA / cm^2, 3 mA / cm^2, 4 mA / cm^2 or 5 mA / cm^2.

[0063] In a feasible implementation, the operation of cooling to below 1000°C after the electric field is stabilized is: after maintaining the electric field for 0.5h to 4h, cooling to below 1000°C at a cooling rate of 10°C / h to 60°C / h. Further, below 1000°C may be, but is not limited to, 990°C, 980°C, 970°C, 960°C, 950°C, 940°C, 930°C, 920°C, 910°C or 900°C.

[0064] In this implementation, automatic control of current and voltage is achieved, and the control result is more accurate, which is beneficial to the polarization effect.

[0065] S40, discharging the positive and negative electrodes of the lithium niobate crystal. After the discharge is completed, the positive and negative electrodes of the lithium niobate crystal are grounded and cooled to room temperature.

[0066] The operation of discharging the positive and negative electrodes of the lithium niobate crystal is as follows: using a variable resistor, the two ends of the variable resistor are respectively connected to the positive and negative electrodes of the lithium niobate crystal, the initial resistance is about 100Ω~500Ω, and in about one minute, the resistance is evenly reduced to 0.

[0067] After the positive and negative electrodes of the lithium niobate crystal 200 are grounded, the annealing furnace 150 is powered off and the temperature is rapidly lowered. After the temperature drops to room temperature, the annealed and polarized lithium niobate crystal 200 is taken out. The room temperature may be, for example, 15°C to 35°C.

[0068] The annealing and polarization method of lithium niobate crystal in the technical solution of the present invention is simple in process. Compared with the traditional annealing and polarization process, the present invention performs annealing and polarization as one process at the same time, wherein the current setting value and the voltage setting value are controlled simultaneously in the polarization process to prevent the lithium niobate crystal from cracking due to the change in resistance of the lithium niobate crystal or the sharp change in actual voltage caused by the power supply reading error. The present invention performs the front-end processing after completing the polarization, which not only reduces the process steps and is conducive to improving efficiency, but also can improve the front-end processing yield, thereby reducing the cracking problem of the crystal during the annealing polarization and furnace removal process caused by the crystal pyroelectric effect.

[0069] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0070] The above-mentioned embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for annealing and polarizing a lithium niobate crystal, characterized in that: The steps include: Connect the two ends of the lithium niobate crystal to the positive and negative electrodes of the polarization power supply respectively; The positive and negative electrodes of the lithium niobate crystal are grounded, and then the lithium niobate crystal is annealed, and the temperature is lowered to 1190° C. to 1210° C. after the annealing is completed; The grounding wire is disconnected, and then an electric field is applied to the lithium niobate crystal based on the following preset electric field application program: the current setting value I of the polarization power supply is changed once every 1s~60s, and increases linearly from 0 to I0 within 0.5h~4h, and the resistance R of the lithium niobate crystal is calculated according to the formula R=U1 / I1, wherein I1 and U1 are the current and voltage displayed by the polarization power supply, respectively; the voltage setting value U of the polarization power supply changes from the initial voltage setting value at a rate not exceeding 2V / min according to the formula U=I*R*1.1 with the change of current, wherein the initial voltage setting value of the polarization power supply does not exceed 1V, and when the current setting value increases to I0, the setting value of the polarization power supply temporarily stops changing, and the voltage setting value is U0 at this time; after the electric field is stabilized, the temperature is lowered to below 1000°C, and the voltage setting value is linearly reduced from U0 to 0 within 0.5h~4h; and Discharging the positive and negative electrodes of the lithium niobate crystal, and after the discharge is completed, grounding the positive and negative electrodes of the lithium niobate crystal and cooling them to room temperature; The preset electric field application program controls the current setting value and the voltage setting value simultaneously to prevent the lithium niobate crystal from cracking due to a sudden change in actual voltage caused by a change in resistance of the lithium niobate crystal or an error in power supply reading.

2. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: The operation of connecting the two ends of the lithium niobate crystal to the positive and negative poles of the polarization power supply respectively is as follows: the lithium niobate crystal is placed between a first polarization cap and a second polarization cap, the first polarization cap is covered with an electrode dish on the side away from the lithium niobate crystal, and the second polarization cap is covered with an electrode sheet on the side away from the lithium niobate crystal, and the electrode dish and the electrode sheet are respectively connected to the positive and negative poles of the polarization power supply.

3. The annealing polarization method of lithium niobate crystal according to claim 2, characterized in that: The first polarization cap comprises lithium niobate sand or lithium niobate mud, and the second polarization cap comprises lithium niobate mud; The lithium niobate mud includes the following components in parts by mass: First, 1 to 2 parts of lithium niobate polycrystalline sand; 1 to 2 parts of the second lithium niobate polycrystalline sand; and Third, 1 to 2 parts of lithium niobate polycrystalline sand; Wherein, the average particle size of the first lithium niobate polycrystalline sand is [0.15mm, 0.35mm); The average particle size of the second lithium niobate polycrystalline sand is [0.35mm, 0.65mm); The average particle size of the third lithium niobate polycrystalline sand is [0.65 mm, 1 mm].

4. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: The operation of annealing the lithium niobate crystal is: heating the temperature to 1220°C~1260°C at a heating rate of 10°C / h~60°C / h and then keeping the temperature for 12h~36h for annealing.

5. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: After the annealing is completed, the temperature is lowered to 1190°C ~ 1210°C and maintained at 1190°C ~ 1210°C for 4h ~ 10h.

6. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: The lithium niobate crystal is placed vertically in the annealing furnace in the Z direction and polarized along the crystal growth direction. The calculation formula of the polarization current is as follows: I0=crystal radius^2*3.14*current density.

7. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: The lithium niobate crystal is placed horizontally in the annealing furnace in the X direction or the Y direction and polarized in a direction perpendicular to the crystal growth direction. The calculation formula of the polarization current is as follows: I0=crystal length*crystal diameter*current density.

8. The annealing polarization method of lithium niobate crystal according to claim 6 or 7, characterized in that: The current density is 1mA / cm^2~5mA / cm^2.

9. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: The operation of cooling the temperature to below 1000° C. after the electric field is stabilized is: after maintaining the electric field for 0.5 h to 4 h, cooling the temperature to below 1000° C. at a cooling rate of 10° C. / h to 60° C. / h.

10. The annealing polarization method of lithium niobate crystal according to claim 1, characterized in that: The initial voltage setting value of the polarization power supply is 0.1V, 0.2V, 0.3V, 0.4V, 0.5V, 0.6V, 0.7V, 0.8V or 0.9V.

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