High-impedance near-zero temperature drift high-temperature piezoelectric crystal material, growth method and application thereof

Abstract

The application discloses a high-impedance near-zero temperature drift high-temperature piezoelectric crystal material and a growth method and application thereof, and belongs to the technical field of piezoelectric crystal growth. x Gd 1‑x Ca4O(BO3)3, x=0.1-0.3, and the high-impedance near-zero temperature drift high-temperature piezoelectric crystal material can be obtained by using a conventional Czochralski method to grow a large-size high-quality single crystal, and the growth process is simple and easy to process. The piezoelectric special-shaped element made of the crystal material has an effective longitudinal piezoelectric constant d 33 The effective shear piezoelectric constant d 26 The high-temperature 1000 DEG C resistivity can reach 10 6 Ω.cm; the piezoelectric constant change rate in the range from room temperature to 1000 DEG C is below 5%, and the crystal material has the near-zero temperature drift characteristic and high-temperature resistance.

Description

Technical Field

[0001] This invention relates to the field of piezoelectric crystal growth technology, and in particular to a high-impedance near-zero temperature drift high-temperature piezoelectric crystal material, its growth method, and its applications. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] Piezoelectric sensors, due to their advantages such as simple structure, good stability, wide operating temperature range, and strong resistance to electromagnetic interference, have important applications in aerospace, automotive, nuclear power, and intelligent manufacturing. Under actual operating conditions, sensors often need to withstand temperatures of 482℃ or even above 650℃ for extended periods to monitor the health of their structure. This places high demands on the high-temperature resistance and operational stability of sensor materials. To address these application requirements, researchers have explored various piezoelectric materials, including piezoelectric ceramics and piezoelectric crystals. The operating temperature of piezoelectric ceramics is limited by their Curie temperature. Most piezoelectric ceramics typically have a Curie temperature below 350℃, while piezoelectric ceramics with high Curie temperatures (such as bismuth calcium niobate and bismuth calcium titanate) have lower resistivity at high temperatures, limiting their device operating temperature to no more than 482℃, thus restricting their application at even higher temperatures.

[0004] Compared to piezoelectric ceramics, piezoelectric crystal materials can avoid the influence of the Curie temperature, achieving low loss, high impedance, and good temperature stability of piezoelectric properties. Lithium niobate (LiNbO3, LN) crystals have a high Curie temperature of up to 1200℃, exhibiting excellent piezoelectric and mechanical properties, and an effective piezoelectric constant d0. eff The pC / N ratio is ~70, but due to oxygen loss and high-temperature degradation, the operating temperature is generally no more than 650℃. Lanthanum gallium silicate (La3Ga5SiO2) 14 Rare earth calcium oxyborate (RECa4O(BO3)3, RECOB, RE: rare earth element) series crystals exhibit no phase transition before reaching their melting point (~1470℃), possessing a large piezoelectric constant and good temperature stability, making them a preferred material for fabricating high-temperature piezoelectric sensors. However, their disordered structure leads to decreased high-temperature resistivity and increased high-temperature losses, and the high price of Ga2O3 raw material also limits their large-scale growth and application. Compared to the aforementioned high-temperature piezoelectric materials, rare earth calcium oxyborate (RECa4O(BO3)3, RECOB, RE: rare earth element) series crystals have a high melting point (~1500℃) and no phase transition before reaching their melting point, resulting in lower losses, higher resistivity, and better temperature stability of piezoelectric performance, providing an important material foundation for the design and development of high-temperature piezoelectric sensors. In addition, this series of crystals is easy to grow using the Czochralski method, readily yielding large-size, high-quality single crystals at a lower cost.

[0005] Currently, numerous studies have been conducted on RECOB series high-temperature piezoelectric crystals, revealing their excellent high-temperature piezoelectric and dielectric properties. However, the piezoelectric properties of existing RECOB series high-temperature piezoelectric crystals exhibit a large rate of change across the entire temperature range (20℃~1000℃), and their piezoelectric constant d... 26 The maximum rate of change even exceeds 30%, limiting the development and application of this type of piezoelectric crystal in high-temperature environments, while crystals with lower piezoelectric constants d... 26 Piezoelectric crystals with varying rates of change have relatively low high-temperature resistance. In 2014, Yu Fapeng and others from Shandong University developed several novel rare-earth calcium oxyborate crystals, including YGdCOB, YSmCOB, and GdLaCOB, among which Y... 0.3 Gd 0.7 The piezoelectric constant d of COB crystal 26 It exhibits a low rate of change of approximately -3% from room temperature to 900°C, but the inventors found that its impedance decreased significantly during thermal cycling experiments, making it difficult to meet the requirements of sensing applications that can withstand temperatures of 1000°C in real-world complex environments.

[0006] Therefore, how to provide a piezoelectric crystal with near-zero temperature drift characteristics and high impedance stability in high-temperature environments within a temperature range of 20~1000℃ is an urgent problem to be solved. Summary of the Invention

[0007] In view of this, the present invention provides a high-impedance near-zero temperature drift high-temperature piezoelectric crystal material, its growth method and application. The piezoelectric crystal material provided by the present invention can achieve near-zero temperature drift high-temperature piezoelectric performance, has a high piezoelectric constant, high resistivity and high piezoelectric performance stability, and can meet the development requirements of piezoelectric sensors with a temperature resistance of 1000℃.

[0008] In a first aspect, the present invention provides a high-impedance, near-zero temperature drift, high-temperature piezoelectric crystal material with the chemical formula Er. x Gd 1-x Ca4O(BO3)3, x=0.1~0.3.

[0009] Preferably, x=0.2.

[0010] In a second aspect, the present invention provides a method for growing the above-mentioned high-impedance near-zero temperature drift high-temperature piezoelectric crystal material, comprising the following steps:

[0011] A mixture of calcium source compound, boron source compound, erbium source compound and gadolinium source compound was prepared by mixing them in stoichiometric ratio, wherein the boron source compound was in excess by 1-2 wt%; then a polycrystalline material was prepared.

[0012] The polycrystalline material was melted, and after adding the seed crystal, the single crystal was grown along the (010) direction using the Czochralski method. After the growth was completed, the temperature was kept constant for a set time, and then the temperature was lowered to room temperature to obtain calcium erbium oxide gadolinium borate crystal.

[0013] Annealing calcium borate erbium-gadolinium crystals in an oxygen-containing atmosphere yields a near-zero temperature drift high-temperature piezoelectric crystal material.

[0014] Preferably, the calcium source compound is selected from calcium carbonate or calcium oxide; the boron source compound is selected from boric acid or boron oxide; the erbium source compound is selected from erbium oxide; and the gadolinium source compound is selected from gadolinium oxide.

[0015] Preferably, the steps for preparing the polycrystalline material are as follows: the mixture is sintered at 900~950℃ for 8~20h; after grinding, it is mixed again for 10~20h, then pressed into blocks, and sintered a second time at 1100~1150℃ for 20~30h to obtain the polycrystalline material.

[0016] Preferably, the step of melting the polycrystalline material specifically involves: heating the polycrystalline material to full melt under gas protection, then cooling it to solidify, repeating the steps of heating the polycrystalline material to full melt and cooling it to solidify 2 to 4 times, and then heating the polycrystalline material to 20 to 40°C above the melting point and holding it at that temperature for 2 to 5 hours.

[0017] Furthermore, the gas is a mixture of oxygen and an inert gas, wherein the volume fraction of oxygen is 1-5%, and the inert gas is selected from one or more of nitrogen or rare gases.

[0018] Preferably, the seed crystal is selected from GdCa4O(BO3)3, ErCa4O(BO3)3, or crystals isomorphic to GdCa4O(BO3)3 or ErCa4O(BO3)3.

[0019] Preferably, after the single crystal growth is completed, the temperature is kept constant for 40~120 min, and then cooled to room temperature at a rate of 15~30℃ / h to obtain gadolinium borate calcium oxide crystal.

[0020] Preferably, in the step of annealing calcium erbium oxide gadolinium crystals in an oxygen-containing atmosphere, the oxygen-containing atmosphere is a mixture of oxygen and nitrogen, wherein the volume fraction of oxygen is 20-30%; the annealing temperature is 900-1100℃, and the annealing time is 48-72h.

[0021] Thirdly, the present invention provides the application of the above-mentioned high-impedance near-zero temperature drift high-temperature piezoelectric crystal material or the high-impedance near-zero temperature drift high-temperature piezoelectric crystal material prepared by the above-mentioned preparation method, wherein the application is in piezoelectric ultrasonic transducers, piezoelectric vibration sensors, piezoelectric acceleration sensors or piezoelectric pressure sensor devices.

[0022] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0023] (1) The Er provided by this invention x Gd 1-x The relative permittivity of Ca4O(BO3)3 (x=0.1~0.3) is: =9~10, =11~14, =9~10, piezoelectric constant d 26 =8.5~10.0 pC / N, electromechanical coupling coefficient k 26 =20%~22%. Piezoelectric irregularly shaped elements made using this crystal material have an effective longitudinal piezoelectric constant d. 33 Up to 6.5~7.0 pC / N; effective shear piezoelectric constant d 26 Resistivity can reach 8.5~10.0 pC / N; at a high temperature of 1000℃, the resistivity can reach 10. 6 The piezoelectric constant is above Ω·cm; the rate of change of piezoelectric constant is below 5% in the range from room temperature to 1000℃, and can be as low as below 3%. It has near-zero temperature drift characteristics and good high temperature resistance. Even after 70 hours of thermal cycling at 1000℃, it still maintains the stability of piezoelectric constant and impedance value.

[0024] (2) Er of the present invention x Gd 1-x Ca4O(BO3)3 (x=0.1~0.3) crystals can be grown into large-size, high-quality single crystals using the conventional Czochralski method. The growth process is simple and easy to process. Attached Figure Description

[0025] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0026] Figure 1 Er is grown in Example 1 of this invention. 0.2 Gd 0.8 COB high-temperature piezoelectric crystal and the cutting shape used for testing;

[0027] Figure 2 Er is grown in Example 1 of this invention. 0.2 Gd 0.8 COB high-temperature piezoelectric crystal and Y in Comparative Example 4 0.3 Gd 0.7 COB crystal longitudinal piezoelectric constant d 33 The maximum value varies from room temperature to 650°C, with the inset showing Er.0.2 Gd 0.8 COB high-temperature piezoelectric crystal longitudinal piezoelectric constant d 33 The curve showing the change in rotation angle when rotating around the Z-axis;

[0028] Figure 3 Er is grown in Example 1 0.2 Gd 0.8 COB high-temperature piezoelectric crystal and Y in Comparative Example 4 0.3 Gd 0.7 High-temperature resistivity comparison chart of COB crystals;

[0029] Figure 4 Er is grown in Example 1 0.2 Gd 0.8 COB and Y of Comparative Example 4 0.3 Gd 0.7 The piezoelectric constant d of the COB high-temperature piezoelectric crystal was measured after 70 hours of thermal cycling at 1000°C. 26 The change graph;

[0030] Figure 5 Er is grown in Example 1 0.2 Gd 0.8 COB and Y of Comparative Example 4 0.3 Gd 0.7 Impedance curves of ZX-cut COB high-temperature piezoelectric crystals after thermal cycling at 1000℃ for 0h and 70h.

[0031] Figure 6 It uses Er 0.2 Gd 0.8 Image of irregularly shaped piezoelectric elements made from COB high-temperature piezoelectric crystals. Detailed Implementation

[0032] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0033] Terminology Explanation

[0034] Piezoelectric crystals are non-centrosymmetric crystals that deform under mechanical force, causing relative displacement of charged particles and resulting in positive and negative bound charges on the crystal surface.

[0035] Piezoelectric constant: Longitudinal piezoelectric constant d ii (i=1,2,3) indicates that when the wafer is subjected to longitudinal stress X iAfter (i=1,2,3), polarization charges are generated on the upper and lower electrode surfaces of the wafer, and the corresponding polarization intensity components are P. i (i=1,2,3), and polarization component P i (i=1,2,3), and stress X i (i=1,2,3) are proportional, and the proportionality constant is d. ii (i=1,2,3), i.e., P i =d ii X i .

[0036] Relative permittivity: Permittivity The dielectric constant represents the change in electric displacement caused by a one-unit change in the electric field intensity in the X-direction under zero strain conditions. With vacuum permittivity The ratio of the two is called the relative permittivity. Relative permittivity and Similarly.

[0037] Temperature drift: refers to the amount of drift of the piezoelectric coefficient of a crystal with temperature (with room temperature data as a reference). This parameter describes the stability of the performance of piezoelectric components or crystal elements within a temperature variation range. The near-zero temperature drift proposed in this invention means that the rate of change of the measured parameter relative to the value at room temperature is close to zero throughout the entire test temperature range.

[0038] Electromechanical coupling coefficient: During the vibration process, a piezoelectric crystal converts mechanical energy into electrical energy, or vice versa. This degree of energy conversion is represented by the electromechanical coupling coefficient.

[0039] Thickness shear electromechanical coupling coefficient k 26 : Refers to the parameters of the electro-coupling effect when a rectangular sheet of piezoelectric crystal is polarized along its length and the direction of the excitation electric field is perpendicular to the polarization direction, and the sheet undergoes thickness shear vibration.

[0040] Er x Gd 1-x Ca4O(BO3)3 (Er x Gd 1-x COB) Piezoelectric physical axes: For crystals with the m-point group, the physical coordinate axis Z-axis is parallel to the crystallographic coordinate axis c-axis, the physical coordinate axis Y-axis is parallel to the crystallographic coordinate axis b-axis, and the X-axis is determined by the right-hand screw rule. It should be further noted that for Er... x Gd 1-x For Ca4O(BO3)3 crystal, the +Z direction is related to the piezoelectric constant d. 33 The positive direction is consistent with that of the square of +X and the piezoelectric constant d. 11The positive direction is consistent; refer to the relevant regulations of the Institute of Electrical and Electronics Engineers (IEEE) regarding piezoelectric materials, which are derived from the quasi-static d 33 The tester has been confirmed.

[0041] This invention provides a high-impedance, near-zero temperature drift, high-temperature piezoelectric crystal material with the chemical formula Er. x Gd 1-x Ca4O(BO3)3, x=0.1~0.3.

[0042] The Er provided by this invention x Gd 1-x Ca4O(BO3)3 piezoelectric crystal material (abbreviated as Er) x Gd 1-x The melting point of COB is approximately 1450℃. There is no phase transition from room temperature to the melting point. It has a non-centrosymmetric structure and belongs to the monoclinic crystal system, point group m. In this crystal, rare earth gadolinium and rare earth erbium occupy each other's lattice sites. The angle between the crystallographic axis a and the piezoelectric physical axis X is approximately (a,X)=11.2°, and the angle between the crystallographic axis c and the piezoelectric physical axis Z is (c,Z)=0°. The crystallographic axis b is parallel to the piezoelectric physical axis Y. The piezoelectric physical axes X, Y, and Z follow the right-hand screw rule.

[0043] The present invention has found that the value of x, i.e. the molar ratio of Er and Gd elements, has a significant impact on the temperature stability of the obtained piezoelectric crystal material. If x is too large or too small, it will lead to an excessively high rate of change of the piezoelectric constant, making it impossible to use stably in a high-temperature environment of 1000℃.

[0044] In this invention, x = 0.2, meaning the chemical formula of the high-impedance near-zero temperature drift high-temperature piezoelectric crystal material is Er. 0.2 Gd 0.8 Ca4O(BO3)3. This invention reveals that the piezoelectric crystal material exhibits optimal piezoelectric constant stability when x is close to 0.2.

[0045] The present invention also provides a method for growing the above-mentioned high-impedance near-zero temperature drift high-temperature piezoelectric crystal material, comprising the following steps:

[0046] A mixture of calcium source compound, boron source compound, erbium source compound and gadolinium source compound was prepared by mixing them in stoichiometric ratio, wherein the boron source compound was in excess by 1-2 wt%; then a polycrystalline material was prepared.

[0047] The polycrystalline material was melted, and after adding the seed crystal, the single crystal was grown along the (010) direction using the Czochralski method. After the growth was completed, the temperature was kept constant for a set time, and then the temperature was lowered to room temperature to obtain calcium erbium oxide gadolinium borate crystal.

[0048] Annealing calcium borate erbium-gadolinium crystals in an oxygen-containing atmosphere yields a near-zero temperature drift high-temperature piezoelectric crystal material.

[0049] The high-impedance near-zero temperature drift high-temperature piezoelectric crystal material Er of the present invention x Gd 1-x Ca4O(BO3)3 has a non-centrosymmetric m-point group crystal structure, good mechanical properties, is not easily deliquescent, melts uniformly, and can be grown into large-sized high-quality single crystals in a short time using the Czochralski method.

[0050] In this invention, the calcium source compound is selected from calcium carbonate; the boron source compound is selected from boric acid or boron oxide; the erbium source compound is selected from erbium oxide; and the gadolinium source compound is selected from gadolinium oxide. This invention does not impose any special restrictions on the source of the above raw materials, but the purity of all raw material compounds must reach 99.99% or higher.

[0051] After the ingredients are prepared, the present invention also includes a step of mixing the various materials. The present invention does not impose any special restrictions on the mixing step, as long as a uniformly mixed mixture can be obtained.

[0052] This invention uses an excess of boron source compound to compensate for the composition deviation caused by boron volatilization during crystal growth. "1-2 wt% excess of boron source compound" refers to the total mass of the mixture.

[0053] In this invention, the steps for preparing the polycrystalline material are as follows: the mixture is sintered once at 900-950℃ for 8-20 hours; after grinding, it is mixed again for 10-20 hours, then pressed into blocks, and sintered a second time at 1100-1150℃ for 20-30 hours to obtain the polycrystalline material. The first sintering in this invention is to pre-decompose the carbonate materials in the raw materials to reduce the generation of bubbles in the subsequent polycrystalline material melt. The particle size of the grinding product after the first sintering is preferably 10-20 μm to ensure the quality of crystal growth.

[0054] In this invention, the step of melting the polycrystalline material specifically involves: heating the polycrystalline material to a complete melt under gas protection, then cooling it to solidify, repeating this process 2-4 times; then heating the polycrystalline material to a temperature 20-40°C above its melting point and holding it at that temperature for 2-5 hours. This repeated heating-melting-cooling-solidification process thoroughly removes bubbles generated in the melt, reducing crystal growth defects (bubbles and inclusions, etc.) and improving crystal growth quality. This invention does not impose special limitations on the equipment used in the polycrystalline material melting process; commonly used equipment in the field is acceptable. The gas used in the polycrystalline material melting process is preferably a mixture of oxygen and an inert gas, with an oxygen volume fraction of 1-5%, and the inert gas selected from one or more of nitrogen or rare gases. The oxygen content during YGdCOB crystal growth affects the change in its microstructural disorder, thereby affecting the electroelastic properties of the crystal material.

[0055] In this invention, the seed crystal is selected from GdCa4O(BO3)3, ErCa4O(BO3)3, or crystals isomorphic to GdCa4O(BO3)3 or ErCa4O(BO3)3, preferably GdCa4O(BO3)3. The present invention uses Er... x Gd 1-x Ca4O(BO3)3 crystals grow faster and produce higher quality along the crystallization axis (010), i.e., the b-direction. Therefore, growing crystals along the (010) direction is preferred. The b-direction seed crystal can be obtained by the following method: [The text abruptly ends here, likely due to an incomplete sentence or missing information.] 3、 GdCa4O(BO3)3 or other isomorphic crystals can be processed along their crystallization axis b to obtain b-direction seed crystals.

[0056] The single crystal growth process of this invention includes four stages: necking, shoulder formation, constant diameter growth, and crystal removal. During necking, the pulling speed is 1-3 mm / h, and the rotation speed is 4-8 rpm. When the seed crystal diameter is reduced to 1-1.5 mm, the temperature is slowly lowered at a rate of 0.5-5 °C / h to form the shoulder. During shoulder formation, the pulling speed is reduced to 0.5-1 mm / h, and the rotation speed is 6-8 rpm. When the crystal shoulder diameter reaches a predetermined size of 20-40 mm, the temperature is increased or decreased at a rate of 0-2 °C / h to form constant diameter growth. During constant diameter growth, the pulling speed is 0.3-0.6 mm / h, and the rotation speed is 6-8 rpm. When the crystal is pulled to a height of 40-60 mm, preparations are made to remove the crystal. The preferred extraction process of this invention is as follows: the temperature is slowly increased at a rate of 20~50℃ / h, while the pulling speed is increased to 5~10mm / h and the rotation speed is 8~12rpm. When it is observed that the bottom of the crystal has a tendency to shrink inward, the crystal is manually pulled to separate it from the melt.

[0057] In this invention, after the single crystal growth is completed, it is kept at a constant temperature for 40-120 minutes, and then cooled to room temperature at a rate of 15-30℃ / h to obtain gadolinium borate calcium oxide crystal. A suitable cooling rate is beneficial to obtaining crystals with higher piezoelectric stability.

[0058] In this invention, the step of annealing calcium erbium oxide gadolinium crystals in an oxygen-containing atmosphere is a mixture of oxygen and nitrogen, wherein the volume fraction of oxygen is 20-30%; the annealing temperature is 900-1100℃, preferably 1000℃; and the annealing time is 48-72h, preferably 55-65h, so that the thermal stress generated during crystal growth is fully released and the disorder of its microstructure reaches a large level.

[0059] This invention provides the application of the above-mentioned high-impedance near-zero temperature drift high-temperature piezoelectric crystal material or the high-impedance near-zero temperature drift high-temperature piezoelectric crystal material prepared by the above-mentioned preparation method. The application is in piezoelectric ultrasonic transducers, piezoelectric vibration sensors, piezoelectric acceleration sensors or piezoelectric pressure sensor devices. It can work stably in the range of 20~1000℃, and is especially suitable for high temperature environments of 850~1000℃.

[0060] The technical solution of the present invention will be further described below with reference to specific embodiments. The purity of the raw materials used in the following embodiments is greater than 99.99%.

[0061] Example 1

[0062] This embodiment provides a high-temperature piezoelectric crystal material Ernst-calcium oxide gadolinium borate. 0.2 Gd 0.8 COB, its chemical formula is Er 0.2 Gd 0.8 Ca4O(BO3)3. The growth methods include the preparation of polycrystalline materials and Czochralski crystal growth, with the following steps:

[0063] (1) Using CaCO3, H3BO3, Er2O3, and Gd2O3 as raw materials, according to the chemical formula Er 0.2 Gd 0.8 The mixture was prepared by stoichiometric proportioning of Ca4O(BO3)3, with boric acid in excess at 1.5% of the total mass of the mixture. The mixture was then mixed in a mixer for 32 hours to ensure thorough and uniform mixing.

[0064] (2) The thoroughly mixed raw materials are placed into a ceramic crucible for the first sintering at a temperature of 930℃ and kept at a constant temperature for 10 hours to decompose and remove CO2 and H2O. After cooling to room temperature, the raw materials from the first sintering are ground in a grinder for 1 hour and then mixed again for 16 hours. The mixture is then pressed into blocks with a diameter of 50 mm and placed into a ceramic crucible for the second sintering. A solid-phase reaction occurs at a sintering temperature of 1150℃ and kept at a constant temperature for 24 hours to obtain calcium erbium borate gadolinium oxide polycrystalline material.

[0065] (3) Place the calcium borate erbium-gadolinium polycrystalline material obtained in step (2) into a platinum crucible in a single crystal furnace. Vacuum the furnace and fill it with nitrogen and oxygen as protective gases. The volume fraction of oxygen is controlled at 2%. Heat the polycrystalline material to melt using medium frequency induction heating. After the polycrystalline material is completely melted, cool it down to solidify it. Then heat it up again to melt it completely. Repeat this process twice to remove the bubbles generated in the melt. Then overheat the melt by 20°C and keep it at a constant temperature for 10 hours to obtain a uniformly melted calcium borate erbium-gadolinium polycrystalline melt.

[0066] (4) The seed crystal taken from the GdCa4O(BO3)3 crystal in the (010) direction is slowly immersed into the polycrystalline melt in step (3) until the top of the seed crystal is perpendicular to the melt and just in contact, and single crystal growth begins along the (010) direction.

[0067] (5) The crystal growth process includes four stages: necking, shoulder formation, constant diameter growth, and removal. During the necking process, the pulling speed is 1.5 mm / h and the rotation speed is 5 rpm. When the seed crystal diameter is reduced to 1 mm, the temperature is slowly reduced at a rate of 0.5 ℃ / h to form the shoulder. During the shoulder formation process, the pulling speed is reduced to 0.5 mm / h. When the diameter of the crystal shoulder reaches the predetermined size of 30 mm, the temperature is increased or decreased at a rate of 1 ℃ / h to form constant diameter growth. During constant diameter growth, the pulling speed is 0.5 mm / h. When the crystal is pulled to a height of 50 mm, the crystal is ready to be removed. The specific operation of the removal process is as follows: the temperature is slowly increased at a rate of 20 ℃ / h, while the pulling speed is increased to 5 mm / h and the rotation speed is 10 rpm. When it is observed that the bottom of the crystal tends to shrink inward, the crystal is manually pulled to separate it from the melt.

[0068] (6) After the crystal is extracted, the crystal is kept at a constant temperature in a temperature field for 1 hour and then cooled to room temperature at a rate of 20℃ / h to obtain calcium erbium oxide gadolinium crystal.

[0069] (7) After taking out the crystal, put it into a tube furnace for annealing. The annealing atmosphere is 80% N2 + 20% O2, the annealing temperature is 1000℃, and the annealing time is 60h, so that the thermal stress generated during the crystal growth process can be fully released.

[0070] Example 2

[0071] This embodiment provides a high-temperature piezoelectric crystal material Ernst-calcium oxide gadolinium borate. 0.2 Gd 0.8 COB, its chemical formula is Er 0.2 Gd 0.8 Ca4O(BO3)3. The growth methods include the preparation of polycrystalline materials and Czochralski crystal growth, with the following steps:

[0072] (1) Using CaCO3, H3BO3, Er2O3, and Gd2O3 as raw materials, according to the chemical formula Er 0.2 Gd 0.8 The mixture was prepared by stoichiometric proportioning of Ca4O(BO3)3, with boric acid in excess at 1% of the total mass of the mixture. The mixture was then mixed in a mixer for 32 hours to ensure thorough and uniform mixing.

[0073] (2) The thoroughly mixed raw materials are placed into a ceramic crucible for the first sintering at a temperature of 930℃ and kept at a constant temperature for 10 hours to decompose and remove CO2 and H2O. After cooling to room temperature, the raw materials from the first sintering are ground in a grinder for 1 hour, mixed again for 16 hours, pressed into blocks with a diameter of 50mm, and placed into a ceramic crucible for the second sintering to produce a solid-phase reaction. The sintering temperature is 1150℃ and kept at a constant temperature for 24 hours to obtain calcium erbium borate gadolinium oxide polycrystalline raw materials.

[0074] (3) Place the calcium borate erbium-gadolinium polycrystalline material obtained in step (2) into a platinum crucible in a single crystal furnace. Vacuum the furnace and fill it with nitrogen and oxygen as protective gases. The volume fraction of oxygen is controlled at 2%. Heat the polycrystalline material to melt using medium frequency induction heating. After the polycrystalline material is completely melted, cool it down to solidify it. Then heat it up again to melt it completely. Repeat this process 3 times to remove the bubbles generated in the melt. Then overheat the melt by 30°C and keep it at a constant temperature for 3 hours to obtain a uniformly melted calcium borate erbium-gadolinium melt.

[0075] (4) Take the seed crystal taken from GdCa4O(BO3)3 crystal in the (010) direction and slowly immerse it into the polycrystalline melt in step (3) until the top of the seed crystal is perpendicular to the melt and just in contact with it, and start single crystal growth along the (010) direction.

[0076] (5) The crystal growth process includes four stages: necking, shoulder formation, constant diameter growth, and removal. During the necking process, the pulling speed is 1.5 mm / h and the rotation speed is 5 rpm. When the seed crystal diameter is reduced to 1 mm, the temperature is slowly reduced at a rate of 0.5 ℃ / h to form the shoulder. During the shoulder formation process, the pulling speed is reduced to 0.5 mm / h. When the diameter of the crystal shoulder reaches the predetermined size of 30 mm, the temperature is increased or decreased at a rate of 1 ℃ / h to form constant diameter growth. During constant diameter growth, the pulling speed is 0.5 mm / h. When the crystal is pulled to a height of 50 mm, the crystal is ready to be removed. The specific operation of the removal process is as follows: the temperature is slowly increased at a rate of 20 ℃ / h, while the pulling speed is increased to 5 mm / h and the rotation speed is 10 rpm. When it is observed that the bottom of the crystal tends to shrink inward, the crystal is manually pulled to separate it from the melt.

[0077] (6) After the crystal is extracted, the crystal is kept at a constant temperature in a temperature field for 1 hour and then cooled to room temperature at a rate of 20℃ / h to obtain calcium erbium oxide gadolinium crystal.

[0078] (7) After taking out the crystal, put it into a tube furnace for annealing. The annealing atmosphere is 80% N2 + 20% O2, the annealing temperature is 1000℃, and the annealing time is 48 hours, so that the thermal stress generated during the crystal growth process can be fully released.

[0079] Comparative Example 1

[0080] The difference between this comparative example and Example 2 is that the high-temperature piezoelectric crystal material in this comparative example is erbium calcium oxide borate (ErCOB), whose chemical formula is ErCa4O(BO3)3, that is, Gd2O3 raw material is not added in the synthesis process.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 2 is that the high-temperature piezoelectric crystal material in this comparative example is gadolinium borate (GdCOB), whose chemical formula is GdCa4O(BO3)3, that is, Er2O3 raw material is not added during the synthesis process.

[0083] Comparative Example 3

[0084] The difference between this comparative example and Example 2 is that the chemical formula of the calcium erbium oxide gadolinium high-temperature piezoelectric crystal material in this comparative example is Er 0.5 Gd 0.5 Ca4O(BO3)3.

[0085] Comparative Example 4

[0086] The chemical formula of the calcium borate yttrium oxide gadolinium high-temperature piezoelectric crystal material in this comparative example is Ya. 0.3 Gd 0.7 Ca4O(BO3)3.

[0087] Test case

[0088] The piezoelectric crystals grown in the examples and comparative examples were fabricated into piezoelectric oscillators of different sizes. The resistivity of the corresponding piezoelectric crystals at the upper limit of the test temperature was measured using a digital multimeter. The relative permittivity, electromechanical coupling coefficient and piezoelectric constant of each sample were measured using the balanced bridge method, impedance method and quasi-static method. The results are shown in Table 1.

[0089] Table 1 Test performance data of high temperature piezoelectric crystal materials

[0090]

[0091] Note: The resistivity in Table 1 refers to the resistivity of the piezoelectric crystal material at 1000℃, and the rate of change of piezoelectric constant refers to the piezoelectric constant d of the piezoelectric crystal material from 25℃ to 1000℃. 26 Rate of change.

[0092] As can be seen from Table 1, Er in the embodiments of the present invention 0.2 Gd 0.8 COB exhibits good temperature stability of its piezoelectric constant, with a piezoelectric constant d0 in the range of 25~1000℃. 26 The rate of change is below 5%, and it can withstand a high temperature environment of 1000℃, while the resistivity at 1000℃ can still reach 10.6 With a resistance of Ω·cm or higher, it has higher impedance and better piezoelectric constant temperature stability and high temperature resistance compared to the comparative piezoelectric crystal material. It can work stably in the range of 20~1000℃, and is especially suitable for high temperature environments of 850~1000℃.

[0093] Figure 1 Er is grown in Example 1 of this invention. 0.2 Gd 0.8 COB high-temperature piezoelectric crystal and the cutting shape used for testing. Figure 2 Er is grown in Example 1 of this invention. 0.2 Gd 0.8 COB and Y of Comparative Example 4 0.3 Gd 0.7 COB high-temperature piezoelectric crystal longitudinal piezoelectric constant d 33 The rate of change of the maximum value from room temperature to 650°C (when the longitudinal piezoelectric constant d) 33 (It reaches its maximum value when rotated 52° around the Z-axis), indicating that Er 0.2 Gd 0.8 COB is more than Y 0.3 Gd 0.7 COB has better stability. Figure 3 Er is grown in Example 1 0.2 Gd 0.8 COB and Y of Comparative Example 4 0.3 Gd 0.7 A comparison of the high-temperature resistivity of COB high-temperature piezoelectric crystals shows that at high temperatures of 500~1000℃, Er 0.2 Gd 0.8 The resistivity of COB is always higher than that of Y. 0.3 Gd 0.7 The resistivity of COB. Figure 4 Er is grown in Example 1 0.2 Gd 0.8 COB and Y of Comparative Example 4 0.3 Gd 0.7 The piezoelectric constant d of the COB high-temperature piezoelectric crystal after 70 hours of thermal cycling at 1000℃. 26 Change diagram Figure 5 Er is grown in Example 1 0.2 Gd 0.8 COB and Y of Comparative Example 4 0.3 Gd 0.7 The impedance curves of the COB high-temperature piezoelectric crystal after 0h and 70h thermal cycling show that the Er grown in Example 1... 0.2 Gd 0.8 COB high-temperature piezoelectric crystals exhibit excellent cycling performance, with very small changes in piezoelectric constant and impedance, while Y... 0.3 Gd0.7 The piezoelectric properties and impedance of the COB high-temperature piezoelectric crystal decreased significantly after 70 hours of cycling.

[0094] Application examples

[0095] Er from Example 1 0.2 Gd 0.8 COB is used to fabricate irregularly shaped piezoelectric elements, and its macroscopic image is shown below. Figure 6 As shown.

[0096] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A high-impedance, near-zero temperature drift, high-temperature piezoelectric crystal material, characterized in that, The chemical formula of the near-zero temperature drift high-temperature piezoelectric crystal material is Er. x Gd 1-x Ca4O(BO3)3, x=0.1~0.

3.

2. The near-zero temperature drift high-temperature piezoelectric crystal material as described in claim 1, characterized in that, x=0.2。 3. The method for growing high-impedance near-zero temperature drift high-temperature piezoelectric crystal materials as described in any one of claims 1 to 2, characterized in that, Includes the following steps: A mixture of calcium source compound, boron source compound, erbium source compound and gadolinium source compound was prepared by mixing them in stoichiometric ratio, wherein the boron source compound was in excess by 1-2 wt%; then a polycrystalline material was prepared. The polycrystalline material was melted, and after adding the seed crystal, the single crystal was grown along the (010) direction using the Czochralski method. After the growth was completed, the temperature was kept constant for a set time, and then the temperature was lowered to room temperature to obtain calcium erbium oxide gadolinium borate crystal. Annealing calcium borate erbium-gadolinium crystals in an oxygen-containing atmosphere yields a near-zero temperature drift high-temperature piezoelectric crystal material.

4. The growth method as described in claim 3, characterized in that, The calcium source compound is selected from calcium carbonate or calcium oxide; the boron source compound is selected from boric acid or boron oxide; the erbium source compound is selected from erbium oxide; and the gadolinium source compound is selected from gadolinium oxide.

5. The growth method as described in claim 3, characterized in that, The steps for preparing the polycrystalline material are as follows: the mixture is sintered at 900~950℃ for 8~20h; after grinding, it is mixed again for 10~20h, then pressed into blocks, and sintered a second time at 1100~1150℃ for 20~30h to obtain the polycrystalline material.

6. The growth method as described in claim 3, characterized in that, The specific steps for melting the polycrystalline material are as follows: under gas protection, the polycrystalline material is heated to full melting, and then cooled to solidify. The steps of heating the polycrystalline material to full melting and cooling to solidify are repeated 2 to 4 times. Then, the polycrystalline material is heated to 20 to 40°C above the melting point and kept at a constant temperature for 2 to 5 hours.

7. The growth method as described in claim 6, characterized in that, The gas is a mixture of oxygen and an inert gas, wherein the volume fraction of oxygen is 1-5%, and the inert gas is selected from one or more of nitrogen or rare gases.

8. The growth method as described in claim 3, characterized in that, The seed crystal is selected from GdCa4O(BO3)3, ErCa4O(BO3)3, or crystals with the same structure as GdCa4O(BO3)3 or ErCa4O(BO3)3.

9. The growth method as described in claim 3, characterized in that, After the single crystal growth is completed, it is kept at a constant temperature for 40~120 min, and then cooled to room temperature at a rate of 15~30℃ / h to obtain calcium erbium oxide gadolinium borate crystals; in the step of annealing the calcium erbium oxide gadolinium borate crystals in an oxygen-containing atmosphere, the oxygen-containing atmosphere is a mixture of oxygen and nitrogen, wherein the volume fraction of oxygen is 20~30%; the annealing temperature is 900~1100℃, and the annealing time is 48~72 h.

10. The application of the high-impedance near-zero temperature drift high-temperature piezoelectric crystal material according to any one of claims 1 to 2 or the high-impedance near-zero temperature drift high-temperature piezoelectric crystal material prepared by the growth method according to any one of claims 3 to 9, characterized in that, Applications in piezoelectric ultrasonic transducers, piezoelectric vibration sensors, piezoelectric accelerometers, or piezoelectric pressure sensors.

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