A method for enhancing the pyroelectric properties of ferroelectric single crystals

Abstract

The present invention provides a method for enhancing the pyroelectric performance of a ferroelectric single crystal. The method comprises applying a uniaxial compressive stress of 2 MPa or less to the ferroelectric single crystal along its preferred growth orientation. The ferroelectric single crystal is then heated to 800-1200°C and held there. During the holding period, a load is continuously applied to the single crystal until the strain of the ferroelectric single crystal reaches 1.5-2.5%. The applied load is then unloaded until the initial uniaxial compressive stress applied to the material is restored. The material is then cooled to room temperature under the uniaxial compressive stress. Finally, the ferroelectric single crystal is cut along a direction at a 0-90° angle to the preferred growth orientation of the ferroelectric single crystal. The method introduces a controllable dislocation structure on the surface or inside the ferroelectric single crystal material to regulate its internal stress field and polarization behavior, thereby significantly improving the pyroelectric response performance.

Description

Technical Field

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

[0002] Pyroelectric materials are a class of functional materials that spontaneously generate charge in response to temperature changes. They are widely used in infrared detectors, non-contact temperature sensors, and thermoelectric energy harvesters. Ferroelectric single crystals, due to their high polarization strength and excellent crystal orientation consistency, are often used as core materials for high-performance pyroelectric components. For example, potassium niobate (KNbO3), barium titanate (BaTiO3), and lithium niobate (LiNbO3) all exhibit excellent pyroelectric response.

[0003] However, in practical applications, the pyroelectric properties of ferroelectric single crystal materials are still significantly affected by factors such as the material's internal defect state, polarization uniformity, and internal stress field distribution. Traditional performance enhancement methods often rely on methods such as chemical doping, composite modification, or polarization treatment, which may introduce impurities, change the material's intrinsic structure, and even affect its stability and reliability. In addition, with the increasing demand for device miniaturization and low power consumption, the development of a new control technology that can significantly enhance pyroelectric performance without changing the material composition has become a research hotspot in the field of functional materials.

[0004] Patent application number CN201110417766 discloses a lead zinc niobate-lead titanate single crystal material and its pyroelectric application. It utilizes the Bridgman method to grow single crystals containing multiple elements. However, this method can easily lead to uneven element distribution, and currently lacks effective means to control element content. Patent application number CN201611079479 discloses a pyroelectric relaxor single crystal ultra-thin sensitive chip and its preparation method. This method increases infrared light absorption by spraying a black layer on the single crystal surface. While this method has strong applicability, its pyroelectric performance still needs to be further improved. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for enhancing the pyroelectric properties of ferroelectric single crystals. This method uses high-temperature mechanical imprinting to introduce an ordered dislocation structure within the ferroelectric single crystal, regulating its internal stress field and polarization behavior, thereby effectively improving its pyroelectric performance. This method is simple, requires no specialized molds, and allows for a controllable dislocation introduction process. It is applicable to a variety of ferroelectric single crystal materials and can significantly enhance their performance in devices such as pyroelectric sensors.

[0006] In order 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. The method comprises the following steps: firstly applying a uniaxial compressive stress of ≤2 MPa to the 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°C and keeping the temperature, and continuously applying a load to the ferroelectric single crystal material during the holding process until the strain of the ferroelectric single crystal material reaches 1.5-2.5%, then unloading the applied load until the uniaxial compressive stress applied to the ferroelectric single crystal material returns to the initial state, then cooling the material to room temperature under the uniaxial compressive stress, and finally cutting the ferroelectric single crystal material along a direction 0-90° with the preferred growth orientation of the ferroelectric single crystal material to obtain the ferroelectric single crystal material.

[0008] The method provided by the present invention first applies a preload force of ≤2 MPa to the ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material, thereby pre-activating a specific slip system, reducing the slip initiation stress, and preventing the material from undergoing only elastic deformation. Uniaxial compressive stress is then continuously applied to the ferroelectric single crystal material during a heat preservation process until a specific strain is reached, thereby inducing dislocations to slip on the most slip-prone plane, causing dislocation movement or proliferation within the crystal, and obtaining a dislocation structure with a specific density distribution. Dislocation types include edge dislocations and screw dislocations, thereby reconstructing the internal stress field and polarization distribution. Finally, the ferroelectric single crystal material is cut along a direction 0°-90° with respect to the preferred growth orientation of the ferroelectric single crystal material to form a ferroelectric single crystal material with a specific density distribution of dislocations. The dislocation lines of the obtained ferroelectric single crystal material are aligned at 0°-90° with the direction of heat flow applied when applied to a pyroelectric material (the heat flow direction is perpendicular to the cutting surface). The treated ferroelectric single crystal material has a pyroelectric current peak value increased by 20% to 150%, and a pyroelectric coefficient increased by 10% to 450%.

[0009] Experiments have found that only by ultimately obtaining dislocation lines that have a specific uniform relationship with the direction of heat flow applied when used as a pyroelectric material can the performance of the pyroelectric material be significantly improved. To obtain dislocation lines that have a specific uniform relationship with the direction of heat flow, it is necessary to coordinate the control of the preload force before insulation, the control of the temperature range, and the control of the final strain. If this is not within the scope of the present invention, dislocations may not be generated on a specific crystal plane, and mixed dislocations may be formed on multiple crystal planes, making it impossible to obtain a controllable heat flow direction and dislocation line angle, and even causing the initiation of cracks in the material or even crystal breakage.

[0010] In a preferred embodiment, the ferroelectric single crystal material is selected from KNbO3, BaTiO3, KTa 1-x Nb x One of O3, preferably BaTiO3.

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

[110] or

[001] .

[0012] In a preferred embodiment, the thickness of the ferroelectric single crystal material in the preferred growth orientation direction is 0.1-8 mm. Experimental findings indicate that the thickness of the ferroelectric single crystal material should not be too thick. If it is too thick, heat transfer is too slow, and performance may be dominated by heat transfer. The effect of dislocations on performance may be eliminated by macroscopic size effects, thereby losing the gain effect. If the thickness is too small, it is not conducive to cutting to obtain the final product.

[0013] A preferred approach involves applying a uniaxial compressive stress of 1-2 MPa to the ferroelectric single crystal along its preferred growth orientation. In the present invention, a preload is applied before heating to allow the single crystal to acclimate to the stress-loading environment, pre-activate specific slip systems, and reduce the slip initiation stress. However, the preload force must be effectively controlled; if the slip system is too small, it may not be activated, making it difficult to introduce dislocations later.

[0014] In actual operation, the mechanical loading method is static pressure loading with a flat rigid pressure head.

[0015] In a preferred embodiment, the ferroelectric single crystal material is heated to 800-1200°C, preferably 900-1150°C, at a heating rate of 0.5-2°C / min and held at that temperature for 15-60 minutes. Using this heating schedule and controlling the holding time within the scope of the present invention can achieve an ideal dislocation density. If the temperature is increased too quickly or the holding time is too short, dislocations may not have sufficient time to generate and multiply, resulting in a low dislocation density.

[0016] In a preferred embodiment, the load is continued to be applied to the ferroelectric single crystal material at a 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 the initial uniaxial compressive stress applied to the ferroelectric single crystal material is returned.

[0017] By loading at the aforementioned loading rate to the aforementioned strain, a specific density and orientation of dislocation lines can be successfully introduced. Unloading is then performed at a specific rate to the initially applied uniaxial compressive stress, rather than completely unloading, to avoid excessively rapid stress release in the material, which can lead to the disappearance and annihilation of dislocation structures. Optimal performance is achieved by controlling the loading and unloading rates within the scope of the present invention. If the loading rate is too fast, dislocations will not have enough time to multiply, potentially causing stress concentration and crystal fragmentation within a short period of time. Unloading at too low a rate and for too long a time can lead to atomic rearrangement and ultimately dislocation annihilation. Unloading at too high a rate and for too short a time can lead to excessive stress release, causing crystal fragmentation.

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

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

[0020] When the ferroelectric single crystal material is cut along a direction at 90° to the preferred growth orientation of the ferroelectric single crystal material, a sample in which the dislocation lines are parallel to the heat flow direction is obtained.

[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. 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] , the uniaxial compressive stress is applied, and when it is cut along a direction 90° to the preferred growth orientation of the ferroelectric single crystal material, that is, a plane cut parallel to the (001) crystal plane, a sample with dislocation lines parallel to the heat flow direction will be obtained, and the crystal polarization direction

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

[0023] Principles and advantages

[0024] The method provided by the present invention first applies a preload force of ≤2 MP to the ferroelectric single crystal material in a preferred growth orientation, thereby pre-activating a specific slip system, reducing the slip initiation stress, and preventing the material from undergoing only elastic deformation. Uniaxial compressive stress is then continuously applied to the ferroelectric single crystal material during a heat preservation process until a specific strain is reached, thereby inducing dislocations to slip on the most slip-prone plane, causing dislocation movement or proliferation within the crystal, and obtaining a dislocation structure with a specific density distribution. Dislocation types include edge dislocations and screw dislocations, thereby reconstructing the internal stress field and polarization distribution. Finally, the ferroelectric single crystal material is cut along a direction 0°-90° with respect to the preferred growth orientation of the ferroelectric single crystal material to form a ferroelectric single crystal material with a specific density distribution of dislocations. The dislocation line of the obtained ferroelectric single crystal material is 0°-90° with respect to the heat flow direction applied when applied to the pyroelectric material (the heat flow direction is perpendicular to the cutting surface). The treated ferroelectric single crystal material has a pyroelectric current peak value increased by 20% to 150%, and a pyroelectric coefficient increased by 10% to 450%.

[0025] The method provided by the present invention achieves enhanced pyroelectric performance by regulating the dislocation structure without introducing exogenous doping, chemical modification or changes in intrinsic components. The method of the present invention has the advantages of simple operation, wide application range and significant performance regulation effect, and can be widely used in infrared detection, temperature sensing and energy harvesting and other fields. BRIEF 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, wherein Figure 1 (a) is a TEM image of the ferroelectric single crystal material provided in Comparative Example 1, Figure 1 (b) is a TEM image of the dislocation-containing ferroelectric single crystal material obtained after cutting in Example 1, Figure 1 (c) is a TEM image of the dislocation-containing ferroelectric single crystal material obtained after cutting in Example 3.

[0027] Figure 2 The pyroelectric current density-time curve of the ferroelectric single crystal material obtained by the methods of the embodiment and the comparative example, wherein 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 The figure is a comparison of the pyroelectric properties of the ferroelectric single crystal materials obtained by the methods of the embodiment and the comparative example, wherein Figure 3 (a) is a comparison chart of the pyroelectric current density peak values ​​of the embodiment and the comparative example, Figure 3 (b) is a comparison chart of the pyroelectric coefficient peaks of the embodiment and the comparative example. DETAILED DESCRIPTION

[0029] Example 1

[0030] High-quality BaTiO3 single crystals with a preferred growth orientation of

[110] were selected, with a size of 4 × 4 × 8 mm. 3 First, a uniaxial compressive stress of 1.25 MPa is applied as a preload along its preferred growth orientation. Then, while maintaining the uniaxial compressive stress, the BaTiO3 single crystal is heated to 1150℃ at a heating rate of 1℃ / min and kept at this temperature for 30 minutes. During the holding process, the uniaxial compressive stress load is added to the ferroelectric single crystal material at a loading rate of 0.0125 MPa / s until the dislocation is successfully introduced after reaching 2% strain. The added load is then unloaded until it returns to 1.25 MPa. The uniaxial compressive stress unloading rate is 0.03 MPa / s. Then, the material is continued to be cooled to room temperature at a rate of 1℃ / min under a residual compressive stress of 1.25 MPa. Finally, it is cut along the (001) crystal plane to obtain a ferroelectric single crystal material containing dislocations.

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

[0032] The pyroelectric performance of the treated ferroelectric single crystal material is tested using a pyroelectric testing system (such as the ferroelectric analyzer TF2000). The measured pyroelectric current data is obtained, and the pyroelectric coefficient of the material is calculated by combining the temperature change data recorded by the synchronous temperature sensor.

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

[0034] Example 2

[0035] High-quality BaTiO3 single crystals with a preferred growth orientation of

[110] were selected, with a size of 4 × 4 × 8 mm. 3 First, a uniaxial compressive stress of 1.25 MPa is applied as a preload along its preferred growth orientation, and then it is heated to 1150℃ at a heating rate of 1℃ / min and kept at this temperature for 30 minutes. During the holding process, uniaxial compressive stress is continued to be applied to the crystal at a loading rate of 0.0125 MPa / s until the dislocation is successfully introduced after reaching 1.5% strain. The uniaxial compressive stress is then unloaded at an unloading rate of 0.03 MPa / s, and then the temperature is continued to be cooled to room temperature at a rate of 1℃ / min under 1.25 MPa residual compressive stress. Finally, the ferroelectric single crystal containing dislocations is obtained by cutting along the (001) crystal plane.

[0036] The pyroelectric performance of the treated ferroelectric single crystal material is tested using a pyroelectric testing system (such as the ferroelectric analyzer TF2000). The measured pyroelectric current data is obtained, and the pyroelectric coefficient of the material is calculated by combining the temperature change data recorded by the synchronous temperature sensor.

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

[0038] Example 3

[0039] High-quality BaTiO3 single crystals with a preferred growth orientation of

[110] were selected, with a size of 4 × 4 × 8 mm. 3 First, a uniaxial compressive stress of 1.25 MPa is applied along its preferred growth orientation, and then it is heated to 1150℃ at a heating rate of 1℃ / min and kept at this temperature for 30 minutes. During the holding process, uniaxial compressive stress is continued to be applied to the crystal at a loading rate of 0.0125 MPa / s until the dislocation is successfully introduced after reaching 1.5% strain. The uniaxial compressive stress is then unloaded at an unloading rate of 0.03 MPa / s, and then the temperature is continued to be cooled to room temperature at a rate of 1℃ / min under 1.25 MPa residual compressive stress. Finally, the ferroelectric single crystal containing dislocations is obtained by cutting along the (110) crystal plane.

[0040] The microstructure of the samples was characterized by transmission electron microscopy (TEM) on the dislocation type, density and spatial distribution; Figure 1 (c) shows the TEM image of the dislocation-containing ferroelectric single crystal prepared in Example 3. The dislocation lines are distributed near the electric domain and are parallel to the observed crystal plane, that is, the cutting crystal plane, indicating that they are perpendicular to the heat flow direction.

[0041] The pyroelectric performance of the treated ferroelectric single crystal material is tested using a pyroelectric testing system (such as the ferroelectric analyzer TF2000). The measured pyroelectric current data is obtained, and the pyroelectric coefficient of the material is calculated by combining the temperature change data recorded by the synchronous temperature sensor.

[0042] like Figure 2 (c) and Figure 3 As shown in the figure, the pyroelectric performance test shows that the peak pyroelectric current density of the treated sample is 12.8 μA / m 2 The peak pyroelectric coefficient is 5.6 nC·cm -2 ·K -1 .

[0043] Comparative Example 1

[0044] The other conditions are the same as those 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. Figure 1 As shown in (a), the dislocation is not successfully introduced; Figure 2 (d) and Figure 3 As shown, the peak value of the pyroelectric current density is 6.37 μA / m 2 The peak pyroelectric coefficient is 3.19 nC·cm -2 ·K -1, the performance is lower.

[0045] Comparative Example 2

[0046] All other conditions were the same as those in Example 1, except that the strain reached 5%, which eventually caused the crystal to break, making it impossible to test and apply the pyroelectric performance.

[0047] Comparative Example 3

[0048] Other conditions were the same as those in Example 1, except that the temperature was lowered to room temperature at a rate of 10°C / min. Ultimately, due to the large thermal stress, cracks appeared in the crystal, making it impossible to test and apply the pyroelectric performance.

Claims

1. A method for enhancing the pyroelectric properties of a ferroelectric single crystal, characterized by: A uniaxial compressive stress of ≤2 MPa is first applied to the ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material, and then the ferroelectric single crystal material is heated to 800-1200°C and kept warm. During the holding process, a load is continuously applied to the ferroelectric single crystal material at a rate of 0.01-0.02 MPa / s until the strain of the ferroelectric single crystal material reaches 1.5-2.5%. Subsequently, the load applied during the deformation process is unloaded at a rate of 0.02-0.04 MPa / s until the initial uniaxial compressive stress applied to the ferroelectric single crystal material is returned. The material is then cooled to room temperature at a rate of 0.5-1.5°C / min under the uniaxial compressive stress, and finally the ferroelectric single crystal material is cut along a direction of 0-90° to the preferred growth orientation of the ferroelectric single crystal material to obtain the obtained material.

2. The method for enhancing the pyroelectric properties of a ferroelectric single crystal according to claim 1, wherein: The ferroelectric single crystal material is selected from KNbO3, BaTiO3, KTa 1-x Nb x One of O3.

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

5. The method for enhancing the pyroelectric properties of a ferroelectric single crystal according to claim 1 or 2, characterized in that: A uniaxial compressive stress of 1-2 MPa is first applied to the ferroelectric single crystal material along the preferred growth orientation of the ferroelectric single crystal material.

6. The method for enhancing the pyroelectric properties of a ferroelectric single crystal according to claim 1 or 2, characterized in that: The ferroelectric single crystal material is heated to 800-1200°C at a heating rate of 0.5-2°C / min and kept at this temperature for 15-60 minutes.

7. The method for enhancing the pyroelectric properties of a ferroelectric single crystal according to claim 1 or 2, characterized in that: The ferroelectric single crystal material is cut along a direction that is 0° or 90° to the preferred growth orientation of the ferroelectric single crystal material.

8. The method for enhancing the pyroelectric performance of a ferroelectric single crystal according to claim 3, wherein: When the preferred growth orientation of the ferroelectric single crystal material is [110], it is finally cut along the (001) crystal plane. When the preferred growth orientation of the ferroelectric single crystal material is [001], it is finally cut along the (110) crystal plane.

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