Method for enhancing pyroelectric property of ferroelectric single crystal

Abstract

The invention provides a method for enhancing the pyroelectric property of a ferroelectric single crystal, which comprises the following steps: applying uniaxial compressive stress less than or equal to 2 MPa to a ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material, then heating the ferroelectric single crystal material to 800-1200 DEG C, preserving heat, and continuously applying load to the single crystal in the heat preservation process until the strain of the ferroelectric single crystal material reaches 1.5-2.5%, thereby obtaining the ferroelectric single crystal. And then unloading the applied load until returning to the initial uniaxial compressive stress applied to the material, cooling to room temperature under the uniaxial compressive stress, and finally cutting the ferroelectric single crystal material along the direction forming 0-90 degrees with the preferred growth orientation of the ferroelectric single crystal material, thereby obtaining the ferroelectric single crystal material. According to the method, a controllable dislocation structure is introduced to the surface or the interior of a ferroelectric single crystal material, and an internal stress field and polarization behavior of the ferroelectric single crystal material are regulated and controlled, so that the pyroelectric response performance is greatly improved.

Description

Technical Field

[0001] The present invention belongs to the field of functional materials, and particularly relates to a method for enhancing the pyroelectric performance of ferroelectric single crystals. Background Art

[0002] Pyroelectric materials are a kind of functional materials that can spontaneously generate charges during temperature changes, and are widely used in fields such as infrared detectors, non-contact temperature sensors, and thermoelectric energy harvesters. Among them, ferroelectric single crystal materials are often used as the core materials of high-performance pyroelectric elements due to their high polarization intensity and excellent crystal orientation consistency. For example, potassium niobate (KNbO3), barium titanate (BaTiO3), lithium niobate (LiNbO3), etc. all exhibit good pyroelectric response performance.

[0003] However, in practical applications, the pyroelectric performance of ferroelectric single crystal materials is still significantly affected by factors such as the internal defect state, polarization uniformity, and internal stress field distribution of the materials. Traditional performance enhancement means mostly rely on methods such as chemical doping, composite modification, or polarization treatment. These methods may introduce impurities, change the intrinsic structure of the materials, and even affect their stability and reliability. In addition, with the increasing demand for device miniaturization and low power consumption, developing a new regulation technology that does not require changing the material composition and can significantly enhance the pyroelectric performance has become a research hotspot in the current field of functional materials.

[0004] The patent with the application number CN201110417766 discloses a lead zinc niobate-lead titanate single crystal material and its pyroelectric application, which uses the Bridgman method to grow single crystals containing multiple elements. However, this method is prone to cause uneven composition distribution, and there is still a lack of effective means to regulate the element content at present. The patent with the application number CN201611079479 discloses a pyroelectric relaxation single crystal ultra-thin sensitive chip and its preparation method. This method increases the infrared light absorption rate by spraying a blackening layer on the surface of the single crystal, and this method has strong applicability, but its pyroelectric performance still needs to be further improved. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a method for enhancing the pyroelectric performance of ferroelectric single crystals. The present invention introduces an ordered dislocation structure into the ferroelectric single crystal by means of high-temperature mechanical imprinting, regulates its internal stress field and polarization behavior, thereby effectively improving its pyroelectric performance. This method has a simple process, does not require a specific mold, the dislocation introduction process is controllable, is applicable to a variety of ferroelectric single crystal materials, and can significantly enhance its application performance in devices such as pyroelectric sensors.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a method for enhancing the pyroelectric performance of a ferroelectric single crystal. First, a uniaxial compressive stress of ≤2 MPa is applied to the ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material. Then, the ferroelectric single crystal material is heated to 800 - 1200 °C and held for a certain time. During the holding process, a load is continuously applied to the ferroelectric single crystal material until the strain of the ferroelectric single crystal material reaches 1.5 - 2.5%. Subsequently, the applied load is unloaded until the uniaxial compressive stress initially applied to the ferroelectric single crystal material is restored. Then, it is cooled to room temperature under this uniaxial compressive stress. Finally, the ferroelectric single crystal material is cut along a direction that forms an angle of 0 - 90° with the preferred growth orientation of the ferroelectric single crystal material, thus obtaining the desired product.

[0008] In the method provided by the present invention, first, a preloading force of ≤2 MPa is applied to the ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material to pre-activate specific slip systems, reduce the slip initiation stress, and prevent the material from only undergoing elastic deformation. Then, during the holding process, a uniaxial compressive stress is continuously applied to the ferroelectric single crystal material until a specific strain is reached, thereby inducing dislocations to slip on the most easily slip planes, resulting in dislocation movement or multiplication inside the crystal and obtaining a dislocation structure with a specific density distribution. The dislocation types include edge dislocations and screw dislocations, which reconstruct the internal stress field and polarization distribution. Finally, the ferroelectric single crystal material with a dislocation structure having a specific density distribution is cut along a direction that forms an angle of 0° - 90° with the preferred growth orientation of the ferroelectric single crystal material, so that the dislocation lines of the obtained ferroelectric single crystal material form an angle of 0° - 90° with the heat flow direction (the heat flow direction is perpendicular to the cutting plane) applied when the material is used as a pyroelectric material. After treatment, the peak value of the pyroelectric current of the ferroelectric single crystal material is increased by 20% to 150%, and the pyroelectric coefficient is increased by 10% to 450%.

[0009] Experiments have found that only when dislocations with a specific uniform relationship with the heat flow direction applied when the material is used as a pyroelectric material are finally obtained can the performance of the pyroelectric material be significantly improved. To obtain dislocations with a specific uniform relationship with the heat flow direction, the control of the preloading force before holding, the control of the temperature range, and the control of the final strain need to work together. If it is not within the scope of the present invention, it may not be possible to generate dislocations on specific crystal planes, but mixed dislocations may be formed on multiple crystal planes, making it impossible to obtain a controllable angle between the heat flow direction and the dislocation lines, and even causing the initiation of internal cracks in the material or even crystal fragmentation.

[0010] Preferably, the ferroelectric single crystal material is selected from one of KNbO3, BaTiO3, KTa 1-x Nb x O3, and preferably BaTiO3.

[0011] Further preferably, the ferroelectric single crystal material is BaTiO3 with a preferred growth orientation of

[110] or

[001] .

[0012] Preferably, the thickness of the ferroelectric single crystal material in the direction of preferred growth orientation is 0.1 - 8 mm. Experiments have found that the thickness of the ferroelectric single crystal material cannot be too thick. If it is too thick, heat transfer is too slow, and the performance may be dominated by heat transfer. Moreover, the influence of dislocations on the performance may be erased by the macroscopic size effect, resulting in the loss of the gain effect. On the other hand, if the thickness is too small, it is not conducive to cutting to obtain the final product.

[0013] Preferably, a uniaxial compressive stress of 1 - 2 MPa is applied to the ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material. In the present invention, a preloading force is applied before heating to allow the single crystal to adapt to the stress loading environment in advance, activate specific slip systems in advance, and reduce the slip initiation stress. However, the preloading force needs to be effectively controlled. If it is too small, the slip system may not be activated, and it will be difficult to introduce dislocations later.

[0014] In the actual operation process, the mechanical loading method is static pressure loading with a planar rigid indenter.

[0015] Preferably, the ferroelectric single crystal material is heated to 800 - 1200 °C, preferably 900 °C - 1150 °C, at a heating rate of 0.5 - 2 °C / min and held for 15 - 60 min. By adopting the above heating procedure and controlling the holding time within the scope of the present invention, an ideal dislocation density can be obtained. If the heating is too fast or the time is too short, dislocations may not have sufficient time to generate and proliferate, resulting in a low dislocation density.

[0016] Preferably, a load is continuously applied to the ferroelectric single crystal material at a loading rate of 0.01 - 0.02 MPa / s until the strain of the ferroelectric single crystal material reaches 1.5 - 2.5%, preferably 1.5 - 2%, and then the load applied during the deformation process is unloaded at a rate of 0.02 - 0.04 MPa / s until returning to the initial uniaxial compressive stress applied to the ferroelectric single crystal material.

[0017] By loading to the above strain at the above loading rate, dislocations with a specific density and direction can be successfully introduced, and then unloaded to the initially applied uniaxial compressive stress at a certain rate, rather than completely unloading, to avoid the rapid release of the stress of the material, resulting in the disappearance and annihilation of the dislocation structure. And by controlling the loading rate and unloading rate of the load within the scope of the present invention, the performance is optimal. If the loading rate is too fast, dislocations do not have time to proliferate, and stress concentration may occur in a short time, causing the crystal to break. If the unloading rate is too small and the time is too long, atomic rearrangement may occur, ultimately leading to the annihilation of dislocations; if the rate is too large and the time is too short, the stress may be released too quickly, causing the crystal to break.

[0018] Preferably, the temperature is decreased to room temperature at a rate of 0.5 - 1.5 °C / min.

[0019] Preferably, the ferroelectric single crystal material is cut along a direction that is 0° or 90°, preferably 90°, with respect to the preferred growth orientation of the ferroelectric single crystal material.

[0020] When the ferroelectric single crystal material is cut along a direction that is 90° with respect to the preferred growth orientation of the ferroelectric single crystal material, a sample with dislocation lines parallel to the heat flow direction is obtained at this time.

[0021] Further preferably, when the preferred growth orientation of the ferroelectric single crystal material is

[110] , it is finally cut along the (001) crystal plane, and when the preferred growth orientation of the ferroelectric single crystal material is

[001] , it is finally cut along the (110) crystal plane.

[0022] Experiments have found that when the preferred growth orientation of the ferroelectric single crystal material is

[110] , after the uniaxial compressive stress is applied and completed, when cutting along a direction that is 90° with respect to the preferred growth orientation of the ferroelectric single crystal material, that is, cutting along a plane parallel to the (001) crystal plane, a sample with dislocation lines parallel to the heat flow direction will be obtained at this time. The crystal polarization direction

[001] crystal direction is consistent with the temperature gradient. At this time, the pyroelectric performance will be greatly improved. The peak value of the pyroelectric current can be increased by up to 150%, and the pyroelectric coefficient is as high as 450%.

[0023] Principle and advantages

[0024] For the method provided by the present invention, first apply a preloading force of ≤2 MP to the ferroelectric single crystal material with respect to the preferred growth orientation of the ferroelectric single crystal material, activate specific slip systems in advance, reduce the slip initiation stress, and avoid the material only undergoing elastic deformation. Then, during the heat preservation process, continue to apply a uniaxial compressive stress to the ferroelectric single crystal material until a specific strain is reached to induce the dislocation to slip on the most easily slip plane, resulting in dislocation movement or multiplication inside the crystal, obtaining a dislocation structure with a specific density distribution. The dislocation types include edge dislocations and screw dislocations, reconstructing the internal stress field and polarization distribution. Finally, cut the ferroelectric single crystal material with a specific density distribution of dislocation structure along a direction that is 0° - 90° with respect to the preferred growth orientation of the ferroelectric single crystal material, so that the dislocation lines of the obtained ferroelectric single crystal material are at 0° - 90° with respect to the heat flow direction (the heat flow direction is the vertical direction of the cutting surface) applied when used as a pyroelectric material. After the treatment, the peak value of the pyroelectric current of the ferroelectric single crystal material is increased by 20% to 150%, and the pyroelectric coefficient is increased by 10% to 450%.

[0025] The method provided by the present invention enhances the pyroelectric performance only by regulating the dislocation structure without introducing external doping, chemical modification, or change in the intrinsic composition. The method of the present invention has the advantages of simple operation, wide application range, and significant performance regulation effect, and can be widely applied to fields such as infrared detection, temperature sensing, and energy harvesting. Description of the drawings

[0026] Figure 1 TEM images of the ferroelectric single-crystal materials obtained by the methods in Examples 1-2 and Comparative Example 1, where Figure 1 (a) is the TEM image of the ferroelectric single-crystal material provided in Comparative Example 1, Figure 1 (b) is the TEM image of the dislocated ferroelectric single-crystal material obtained after cutting in Example 1, Figure 1 (c) is the TEM image of the dislocated ferroelectric single-crystal material obtained after cutting in Example 3.

[0027] Figure 2 Pyroelectric current density-time curves of the ferroelectric single-crystal materials obtained by the methods of the examples and comparative examples, where Figure 2 (a) is the pyroelectric current density-time curve of the ferroelectric single-crystal material in Example 1, Figure 2 (b) is the pyroelectric current density-time curve of the ferroelectric single-crystal material in Example 2, Figure 2 (c) is the pyroelectric current density-time curve of the ferroelectric single-crystal material in Example 3, Figure 2 (d) is the pyroelectric current density-time curve of the ferroelectric single-crystal material in Comparative Example 1.

[0028] Figure 3 Comparison diagrams of the pyroelectric properties of the ferroelectric single-crystal materials obtained by the methods of the examples and comparative examples, where Figure 3 (a) is the comparison diagram of the peak values of the pyroelectric current density of the examples and comparative examples, Figure 3 (b) is the comparison diagram of the peak values of the pyroelectric coefficients of the examples and comparative examples. Detailed implementation manners

[0029] Example 1

[0030] A high-quality BaTiO3 single crystal with a single-crystal preferred growth orientation of

[110] and a size of 4 × 4 × 8 mm was selected. 3 , First, a uniaxial compressive stress of 1.25 MPa was applied along its preferred growth orientation as a preloading force. Then, under the condition of maintaining this uniaxial compressive stress, the BaTiO3 single crystal was heated to 1150 °C at a heating rate of 1 °C / min. It was held at this temperature for 30 minutes. During the holding process, a uniaxial compressive stress load was increased on the ferroelectric single-crystal material at a loading rate of 0.0125 MPa / s until dislocations were successfully introduced after reaching 2% strain. Subsequently, the increased load was unloaded until it returned to 1.25 MPa, and the uniaxial compressive stress unloading rate was 0.03 MPa / s. Then, it was continued to cool to room temperature at a rate of 1 °C / min under a residual compressive stress of 1.25 MPa. Finally, it was cut along the (001) crystal plane to obtain a dislocated ferroelectric single-crystal material.

[0031] The microstructure of the sample was characterized by transmission electron microscopy (TEM) for the type, density, and spatial distribution of dislocations; as Figure 1 (b) shows the TEM image of the prepared ferroelectric single crystal containing dislocations. The dislocation lines are distributed near the electric domains and perpendicular to the observed crystal plane, i.e., the cut crystal plane, indicating that it is parallel to the heat flow direction. This shows that under the heating state of the present invention, by applying mechanical force, a dislocation line structure consistent with the crystal polarization direction and the heat flow direction is induced to form.

[0032] The pyroelectric properties of the treated ferroelectric single crystal material were tested using a pyroelectric test system (such as the ferroelectric analyzer TF2000) to obtain the measured pyroelectric current data. Combining with the temperature change data recorded by the synchronous temperature sensor, the pyroelectric coefficient of the material was calculated.

[0033] As Figure 2 (a) and Figure 3 shown, the pyroelectric property test shows that the peak value of the pyroelectric current density of the treated sample is 16.9 μA / m 2 , and the peak value of the pyroelectric coefficient is 62.8 nC·cm -2 ·K -1 .

[0034] Example 2

[0035] A high-quality BaTiO3 single crystal with a single crystal preferred growth orientation of

[110] was selected, with dimensions of 4 × 4 × 8 mm 3 , first a uniaxial compressive stress of 1.25 MPa was applied along its preferred growth orientation as a preloading force, then it was heated to 1150°C at a heating rate of 1°C / min, held at this temperature for 30 minutes. During the holding process, a uniaxial compressive stress was continuously applied to the crystal at a loading rate of 0.0125 MPa / s until dislocations were successfully introduced after reaching a 1.5% strain. Subsequently, the uniaxial compressive stress was unloaded at a rate of 0.03 MPa / s. Then it was continuously cooled to room temperature at a rate of 1°C / min under a residual compressive stress of 1.25 MPa. Finally, it was cut along the (001) crystal plane to obtain a ferroelectric single crystal containing dislocations.

[0036] The pyroelectric properties of the treated ferroelectric single crystal material were tested using a pyroelectric test system (such as the ferroelectric analyzer TF2000) to obtain the measured pyroelectric current data. Combining with the temperature change data recorded by the synchronous temperature sensor, the pyroelectric coefficient of the material was calculated.

[0037] As Figure 2 (b) and Figure 3 shown, the pyroelectric property test shows that the peak value of the pyroelectric current density of the treated sample is 14.9 μA / m 2 , and the peak value of the pyroelectric coefficient is 16.5 nC·cm-2 ·K -1 。

[0038] Example 3

[0039] A high-quality BaTiO3 single crystal with a single-crystal preferred growth orientation of

[110] is selected, with dimensions of 4 × 4 × 8 mm 3 , first apply a uniaxial compressive stress of 1.25 MPa along its preferred growth orientation, then heat it to 1150 °C at a heating rate of 1 °C / min, hold it at this temperature for 30 minutes, and during the holding process, continue to apply a uniaxial compressive stress to the crystal at a loading rate of 0.0125 MPa / s until dislocations are successfully introduced after reaching a strain of 1.5%. Subsequently, unload the uniaxial compressive stress at an unloading rate of 0.03 MPa / s, and then continue to cool it to room temperature at a rate of 1 °C / min under a residual compressive stress of 1.25 MPa. Finally, cut it along the (110) crystal plane to obtain a ferroelectric single crystal containing dislocations.

[0040] Characterize the microstructure of the sample for dislocation type, density, and spatial distribution by transmission electron microscopy (TEM); as Figure 1 (c) shows the TEM image of the ferroelectric single crystal containing dislocations prepared in Example 3. The dislocation lines are distributed near the electric domains and are parallel to the observed crystal plane, that is, the cutting plane, indicating that it is perpendicular to the heat flow direction.

[0041] Use a pyroelectric test system (such as the ferroelectric analyzer TF2000) to test the pyroelectric properties of the processed ferroelectric single crystal material, obtain the measured pyroelectric current data, and calculate the pyroelectric coefficient of the material by combining the temperature change data recorded by the synchronous temperature sensor.

[0042] As Figure 2 (c) and Figure 3 shown, the pyroelectric property test shows that the peak value of the pyroelectric current density of the processed sample is 12.8 μA / m 2 , and the peak value of the pyroelectric coefficient is 5.6 nC·cm -2 ·K -1 。

[0043] Comparative Example 1

[0044] All other conditions are the same as in Example 1, except that no uniaxial compressive stress is applied to the single crystal under constant temperature conditions, that is, the strain is 0. As Figure 1 (a) shows, this ultimately results in the failure to successfully introduce dislocations; as Figure 2 (d) and Figure 3 shown, the peak value of the pyroelectric current density is 6.37 μA / m 2 , and the peak value of the pyroelectric coefficient is 3.19 nC·cm -2 ·K -1, with low performance.

[0045] Comparative Example 2

[0046] All other conditions were the same as in Example 1, except that the strain reached 5%, which ultimately caused the crystal to break and made it impossible to conduct pyroelectric performance tests and applications.

[0047] Comparative Example 3

[0048] All other conditions were the same as in Example 1, except that the temperature was cooled to room temperature at a rate of 10 °C / min. Ultimately, due to large thermal stress, cracks appeared in the crystal, making it impossible to conduct pyroelectric performance tests and applications.

Claims

1. A method for enhancing the pyroelectric performance of a ferroelectric single crystal, characterized in that: First, apply a uniaxial compressive stress of ≤2 MPa to the ferroelectric single-crystal material along its preferred growth orientation. Then, heat the ferroelectric single-crystal material to 800 - 1200 °C and hold for a certain time. During the holding process, continue to apply a load to the ferroelectric single-crystal material until the strain of the ferroelectric single-crystal material reaches 1.5 - 2.5%. Subsequently, unload the applied load until returning to the initial uniaxial compressive stress applied to the ferroelectric single-crystal material, and then cool it to room temperature at this uniaxial compressive stress. Finally, cut the ferroelectric single-crystal material along a direction that forms an angle of 0 - 90° with the preferred growth orientation of the ferroelectric single-crystal material, and it is obtained.

2. The method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1, characterized in that: The ferroelectric single crystal material is selected from one of KNbO3, BaTiO3, KTa 1-x Nb x O3.

3. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: The ferroelectric single-crystal material is BaTiO3 with a preferred growth orientation of [110] or [001].

4. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: The thickness of the ferroelectric single-crystal material in the direction of its preferred growth orientation is 0.1 - 8 mm.

5. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: First, apply a uniaxial compressive stress of 1 - 2 MPa to the ferroelectric single-crystal material along its preferred growth orientation.

6. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: Heat the ferroelectric single-crystal material to 800 - 1200 °C at a heating rate of 0.5 - 2 °C / min and hold for 15 - 60 min.

7. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: Continue to apply a load to the ferroelectric single-crystal material. The loading rate of the load is 0.01 - 0.02 MPa / s until the strain of the ferroelectric single-crystal material reaches 1.5 - 2.5%. Subsequently, unload the load applied during the deformation process at a rate of 0.02 - 0.04 MPa / s until returning to the initial uniaxial compressive stress applied to the ferroelectric single-crystal material.

8. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: Cool it to room temperature at a rate of 0.5 - 1.5 °C / min.

9. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 1 or 2, characterized in that: Cut the ferroelectric single-crystal material along a direction that forms an angle of 0° or 90° with the preferred growth orientation of the ferroelectric single-crystal material.

10. A method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 3, characterized in that: When the preferred growth orientation of the ferroelectric single-crystal material is [110], finally cut along the (001) crystal plane. When the preferred growth orientation of the ferroelectric single-crystal material is [001], finally cut along the (110) crystal plane.

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