High-impedance and high-stability piezoelectric crystal and preparation method and application thereof

Abstract

The invention belongs to the technical field of piezo-electric crystals, and relates to a growth method of piezo-electric crystals, in particular to a high-impedance and high-stability piezo-electric crystal and a preparation method and application thereof. The preparation method comprises the following steps: melting LTGA (La3Ta0. 5Ga5.5-xAlxTa0. 5O14, x is more than 0 and less than or equal to 0.5) polycrystal materials under the condition of inert atmosphere or oxygen-deficient atmosphere with the oxygen volume fraction of not more than 2%, then adding seed crystals, and carrying out single crystal growth by adopting a pulling method to obtain LTGA single crystals; and under the inert atmosphere condition, heating the LTGA single crystal to 900-1100 DEG C, and carrying out annealing treatment for at least 24 hours. The piezo-electric crystal obtained by adopting the preparation method disclosed by the invention can show high impedance characteristic, realizes excellent high-temperature piezoelectric property and temperature stability, and can meet the research and development and production requirements of a piezoelectric sensor capable of resisting the temperature of 850 DEG C.

Description

Technical Field

[0001] The present invention belongs to the technical field of piezoelectric crystals, and relates to a method for growing a piezoelectric crystal, and in particular to a high-impedance and high-stability piezoelectric crystal, and a preparation method and application thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention, and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] Piezoelectric sensors are widely used in aerospace, ship propulsion, nuclear power and other industrial fields. They need to meet special requirements for working in extreme environments (such as high temperature, vacuum or low oxygen partial pressure, etc.) to achieve the purpose of continuous monitoring of structural health. Piezoelectric crystal materials, as the core components of piezoelectric sensors, must meet the growing performance requirements.

[0004] Lanthanum gallium silicate (La3Ga5SiO 14 As a piezoelectric crystal material, LGS series crystals have high temperature stability and can maintain phase stability before reaching the melting point (about 1470°C). However, the impedance of this series of crystals varies greatly with temperature, making it difficult to meet the application requirements of high-temperature piezoelectric sensors. According to the distribution of cations, LGS series crystals can be divided into ordered and disordered categories. It is worth noting that the disordered crystal La3Ta 0.5 Ga 5.5 O 14 (LTG) exhibits excellent piezoelectric activity, with an effective piezoelectric coefficient d 11 6.5pC / N, far exceeding the structure-ordered crystals such as Ca3TaGa3Si2O 14 (CTGS) and Ca3TaAl3Si2O 14 (CTAS) etc., the latter 11 The resistivity of LTG crystals is relatively low at high temperatures (only 10 at 600°C). 6 Ω·cm), which will result in the minimum operating frequency (f LL ) is small, and the piezoelectric charge is easily lost, which seriously affects the detection performance of the sensor. 0.5 Ga 5.5-x Al x Ta 0.5 O 14 (LTGA, 0<x≤0.5) crystal, the resistivity at high temperature only increases slightly (7×10 4Ω·cm), low impedance is still a shortcoming that must be solved in the application of this crystal trend sensor. Summary of the invention

[0005] In order to address the deficiencies in the prior art, the purpose of the present invention is to provide a high-impedance and high-stability piezoelectric crystal and a preparation method and application thereof. The piezoelectric crystal obtained by the preparation method of the present invention can exhibit high impedance characteristics and achieve excellent high-temperature piezoelectric performance and temperature stability, which can meet the research and development and production needs of piezoelectric sensors resistant to 850°C.

[0006] In order to achieve the above object, the technical solution of the present invention is:

[0007] In a first aspect, a method for preparing a high-impedance and high-stability piezoelectric crystal comprises the following steps:

[0008] Under inert atmosphere, LTGA (La3Ta 0.5 Ga 5.5-x Al x Ta 0.5 O 14 , 0<x≤0.5) the polycrystalline material is melted, and then a seed crystal is added and a single crystal is grown by a Czochralski method to obtain a LTGA single crystal;

[0009] The LTGA single crystal is heated to 900-1100° C. and annealed for at least 24 hours.

[0010] The problem of relatively low resistivity of LTG crystal at high temperature in the present invention is solved by first adding Al to replace part of Ga, that is, using LTGA to replace LTG, to reduce the conductivity to increase the resistivity. However, replacing part of Ga with Al will introduce Al Ta Anti-site defects are formed, and the anti-site defects are not conducive to the improvement of crystal resistivity; therefore, as mentioned above, the resistivity of LTGA at high temperature only increases slightly, and the impedance is still not high. To this end, the present invention adopts the Czochralski method to grow LTGA single crystals under inert atmosphere conditions, thereby obtaining oxygen vacancy defects and Al Ta The purpose of the LTGA single crystal with anti-site defects is to isolate oxygen through an inert atmosphere, thereby creating oxygen vacancy defects, and using oxygen vacancy defects to suppress Al Ta Antisite defects; finally, through high temperature and long time annealing, not only the thermal stress generated in the crystal growth process can be fully released, but also the concentration of microscopic defects in the crystal can be regulated, and the inhibitory effect of oxygen vacancy defects on antisite defects can be strengthened, thereby greatly improving the resistivity of LTGA crystals and high temperature (especially 850℃) resistivity.

[0011] During the annealing process, it can be carried out in an air atmosphere or in an inert atmosphere. Studies have shown that when the annealing process is carried out in an inert atmosphere, the inhibitory effect of oxygen vacancy defects on antisite defects can be better strengthened, so that the resistivity of the obtained piezoelectric crystal and the high temperature (especially 850°C) resistivity are more significantly improved. The inert atmosphere described in the present invention refers to a gas atmosphere formed by an inert gas such as nitrogen or helium, neon, argon, etc. that does not contain oxygen.

[0012] On the other hand, a high-impedance and high-stability piezoelectric crystal is obtained by the above preparation method.

[0013] In a third aspect, an application of the above-mentioned high-impedance and high-stability piezoelectric crystal in the preparation of a piezoelectric sensor.

[0014] Specifically, the piezoelectric sensor is used for detection under high temperature environmental conditions, and the temperature of the high temperature environmental conditions in the present invention refers to a temperature not lower than 600°C.

[0015] The beneficial effects of the present invention are:

[0016] The LTGA crystal obtained by the microscopic defect control preparation technology provided by the present invention has a relative dielectric constant of Dielectric loss is less than 10% (100kHz); piezoelectric constant d 11 =d 12 =7.0~7.5pC / N, the piezoelectric constant change rate in the whole temperature range is ≤8%; the resistivity can reach 10 at high temperature of 850℃ 6 Ω·cm, which is higher than the resistivity of LTG series crystals obtained by conventional preparation technology (1×10 4 The Ω·cm) is increased by nearly two orders of magnitude, and it has the characteristics of high impedance and high stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The accompanying drawings in the specification, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0018] Figure 1 It is La3Ta grown in Example 1 of the present invention 0.5 Ga 5.5-x Al x O 14 (x=0.2) piezoelectric crystal.

[0019] Figure 2 It is La3Ta grown in Example 3 of the present invention 0.5 Ga 5.5-x Al x O 14(x=0.1) piezoelectric crystal.

[0020] Figure 3 It is La3Ta grown in Example 4 of the present invention 0.5 Ga 5.5-x Al x O 14 (x=0.3) piezoelectric crystal.

[0021] Figure 4 It is La3Ta grown in Example 1 of the present invention 0.5 Ga 5.5-x Al x O 14 (x = 0.2) piezoelectric crystal and LTG (La3Ta 0.5 Ga 5.5 O 14 ) and the piezoelectric constant d of the ordered CTGS crystal 12 Changes in the range from room temperature to 850°C.

[0022] Figure 5 It is La3Ta grown in Example 1 of the present invention 0.5 Ga 5.5-x Al x O 14 (x = 0.2) and La3Ta grown in Comparative Example 4 0.5 Ga 5.3 Al 0.2 O 14 Quantitative analysis of oxygen vacancy concentration in piezoelectric crystals.

[0023] Figure 6 It is La3Ta grown in Example 1 of the present invention 0.5 Ga 5.5-x Al x O 14 (x = 0.2) and La3Ta grown in Comparative Example 4 0.5 Ga 5.3 Al 0.2 O 14 The variation of the resistivity of a piezoelectric crystal from room temperature to 850°C. DETAILED DESCRIPTION

[0024] It should be noted that the following detailed descriptions are exemplary and are intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present invention belongs.

[0025] Explanation of terms

[0026] Piezoelectric crystal: refers to a non-centrosymmetric crystal that deforms under the action of mechanical force, causing the charged particles to undergo relative displacement, resulting in positive and negative bound charges on the crystal surface. Such a crystal is called a piezoelectric crystal.

[0027] Piezoelectric constant: Longitudinal piezoelectric constant d ii (i=1,2,3), indicating that when the wafer is subjected to longitudinal stress X i (i=1,2,3), polarization charges are generated on the upper and lower electrode surfaces of the chip, and the corresponding polarization intensity component is P i (i=1,2,3), and the polarization component P i (i=1,2,3), and stress X i (i=1,2,3) are proportional, and the proportionality coefficient is d ii (i=1,2,3), that is, P i =d ii X i .

[0028] Relative dielectric constant: Dielectric constant It indicates the change in electric displacement caused by a unit change in electric field intensity in the X direction under the condition of zero strain. The dielectric constant The ratio of the dielectric constant of vacuum to ε0 is called the relative dielectric constant. Relative dielectric constant and Same reason.

[0029] The crystal chemical formula of the piezoelectric crystal provided by the present invention is La3Ta 0.5 Ga 5.5-x Al x Ta 0.5 O 14 (LTGA, 0<x≤0.5). The crystal remains phase stable until its melting point (about 1470℃) and can be grown by the Czochralski method. The crystal belongs to the trigonal system and the space group is P 321 , chemical formula is A3BC3D2O 14 It includes four cationic sites: a decahedral site A, an octahedral site B, and two different tetrahedral sites C and D.

[0030] As introduced in the background technology, LTG has excellent piezoelectric activity but its resistivity under high temperature conditions is relatively low, which affects the detection performance of the piezoelectric sensor. Therefore, the present invention proposes a high-impedance and high-stability piezoelectric crystal and its preparation method and application.

[0031] A typical embodiment of the present invention provides a method for preparing a high-impedance and high-stability piezoelectric crystal, comprising the following steps:

[0032] Under inert atmosphere, LTGA (La3Ta 0.5 Ga 5.5-x Al x Ta 0.5 O 14 , 0<x≤0.5) the polycrystalline material is melted, and then a seed crystal is added and a single crystal is grown by a Czochralski method to obtain a LTGA single crystal;

[0033] The LTGA single crystal is heated to 900-1100° C. and annealed for at least 24 hours.

[0034] In some embodiments, the annealing process is performed under an inert atmosphere. Studies have shown that the high temperature impedance performance of the piezoelectric crystal obtained by annealing in an inert atmosphere is higher than that of the piezoelectric crystal obtained by annealing in an air atmosphere.

[0035] In some embodiments, the process of single crystal growth by the Czochralski method includes necking, shoulder release, equal diameter growth and lifting.

[0036] In some embodiments, the seed crystal is immersed in a polycrystalline melt until the top of the seed crystal is perpendicular to and just in contact with the melt, and single crystal growth begins along the Z direction.

[0037] In some embodiments, after the single crystal growth is completed, the crystal is subjected to a constant temperature treatment and then programmed to cool to room temperature.

[0038] In some embodiments, the annealing time is 24 to 30 hours. Under this condition, not only the thermal stress generated during the crystal growth process is more fully released, but also it is more conducive to the control of the concentration of microscopic defects in the crystal.

[0039] The LTGA polycrystalline material described in the present invention can be obtained from the market or prepared by itself. In some embodiments, a lanthanum source compound, a tantalum source compound, a gallium source compound and an aluminum source compound are mixed in a stoichiometric ratio to obtain a mixed material; the mixed material is sintered once at 1000-1100°C, ground and mixed, and then pressed into blocks, and sintered twice at 1200-1320°C to obtain a LTGA polycrystalline material. The LTGA polycrystalline material obtained by this method has a higher purity and is more conducive to the subsequent preparation of LTGA single crystals.

[0040] In the present invention, the lanthanum source compound is selected from lanthanum oxide; the tantalum source compound is selected from tantalum oxide; the gallium source compound is selected from gallium oxide; and the aluminum source compound is selected from aluminum oxide. The present invention does not impose any special restrictions on the sources of the above raw materials, and the purity of all raw material compounds must reach more than 99.99%.

[0041] Specifically, the gallium source compound in the mixed material is in excess of 1-2 wt%. The present invention uses the method of excess gallium source compound to compensate for the component deviation caused by the volatilization of gallium during crystal growth, and "excess gallium source compound 1-2 wt%" is relative to the total mass of the mixed material.

[0042] Specifically, the primary sintering time is 10 to 20 hours.

[0043] Specifically, the time for mixing after grinding is 10 to 20 hours.

[0044] Specifically, the secondary sintering time is 30 to 50 hours.

[0045] Another embodiment of the present invention provides a high-impedance and high-stability piezoelectric crystal obtained by the above-mentioned preparation method.

[0046] A third embodiment of the present invention provides an application of the above-mentioned high-impedance and high-stability piezoelectric crystal in the preparation of a piezoelectric sensor.

[0047] Specifically, the piezoelectric sensor is used for detection under high temperature environmental conditions, and the temperature of the high temperature environmental conditions in the present invention refers to a temperature not lower than 600°C.

[0048] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in combination with specific embodiments and comparative examples.

[0049] Example 1

[0050] A method for preparing a high-impedance and high-stability piezoelectric crystal, the steps are as follows:

[0051] (1) La2O3, Al2O3, Ga2O3, Ta2O5 are mixed according to the chemical formula La3Ta 0.5 Ga 5.5-x Al x O 14 The mixture is prepared in a stoichiometric ratio of (x=0.2) to obtain a mixed material, wherein the excess gallium oxide accounts for 1.5% of the total mass of the mixed material, and then mixed in a mixer for 36 hours to make it fully mixed and uniform.

[0052] (2) The raw materials after being fully mixed in step (1) are placed in a ceramic crucible for the first sintering at a temperature of 1050°C for 15 hours to remove H2O; after cooling to room temperature, the raw materials of the first sintering are placed in a grinder for grinding for 1 hour and then mixed again for 16 hours, and pressed into a material block with a diameter of 50 mm under a pressure of 400 kN, and placed in a ceramic crucible for the second sintering to cause a solid phase reaction. The sintering temperature is 1300°C and kept at a constant temperature for 36 hours to obtain LTGA polycrystalline material.

[0053] (3) The LTGA polycrystalline material obtained in step (2) is placed in a platinum crucible in a single crystal furnace. The furnace is evacuated and filled with 100% volume fraction of argon. The polycrystalline raw material is heated to melt by medium frequency induction heating. After the polycrystalline material is completely melted, the temperature is lowered to condense, and then the temperature is raised to the melting point again to melt it completely. This is repeated twice to eliminate the bubbles generated in the melt. The melt is then overheated by 20°C and kept at a constant temperature for 10 hours to obtain a uniformly melted LTGA polycrystalline melt.

[0054] (4) slowly immersing the seed crystal into the polycrystalline melt of step (3) until the top of the seed crystal is perpendicular to and just in contact with the melt, and starting single crystal growth along the Z direction;

[0055] (5) The crystal growth process includes four stages: necking, shoulder release, equal diameter growth and stripping. In the necking process, the pulling speed is 1.5mm / h and the rotation speed is 3rpm; when the seed crystal diameter is reduced to 1mm, the temperature is slowly lowered at a rate of 0.3℃ / h to release the shoulder; in the shoulder release process, the pulling speed is reduced to 0.5mm / h; when the diameter of the crystal shoulder reaches the predetermined size, the temperature is raised or lowered at a rate of 1℃ / h to grow at an equal diameter; the pulling speed during equal diameter growth is 0.5mm / h; the specific operation of the stripping process is as follows: the temperature is slowly raised at a rate of 15℃ / h, and the pulling speed is increased to 6mm / h and the rotation speed is 12rpm. When the bottom of the crystal is observed to have a tendency to shrink inward, it is manually pulled to separate the crystal from the melt. After the single crystal growth is completed, the crystal is kept at a constant temperature in the temperature field for 1 hour and cooled to room temperature at a rate of 20℃ / h to obtain an LTGA crystal.

[0056] (6) After taking out the crystal, it was placed in a tube furnace for annealing. The annealing atmosphere was 100% by volume argon. The annealing temperature was 1100°C and the annealing time was 30 hours. The obtained piezoelectric crystal was as follows: Figure 1 shown.

[0057] Example 2

[0058] This embodiment is the same as Embodiment 1, except that the annealing atmosphere in step (7) is an air atmosphere.

[0059] Example 3

[0060] This embodiment is the same as embodiment 1, except that: the piezoelectric crystal prepared is

[0061] La3Ta 0.5 Ga 5.5-x Al x O 14 (x=0.1), such as Figure 2 shown.

[0062] Example 4

[0063] This embodiment is the same as embodiment 1, except that: the piezoelectric crystal prepared is

[0064] La3Ta 0.5 Ga 5.5-x Al x O 14 (x=0.3), such as Figure 3 shown.

[0065] Example 5

[0066] This embodiment is the same as embodiment 1, except that: the piezoelectric crystal prepared is

[0067] La3Ta 0.5 Ga 5.5-x Al x O 14 (x=0.4).

[0068] Example 6

[0069] This embodiment is the same as embodiment 1, except that: the piezoelectric crystal prepared is

[0070] La3Ta 0.5 Ga 5.5-x Al x O 14 (x=0.5).

[0071] Comparative Example 1

[0072] The difference between this comparative example and Example 1 is that the piezoelectric crystal material prepared is lanthanum gallium tantalate (LTG), whose chemical formula is La3Ta 0.5 Ga 5.5 O 14 That is, no Al2O3 raw material is added during the synthesis process; in step (3), the furnace is evacuated and filled with 98% by volume argon and 2% by volume oxygen; in step (6), the annealing atmosphere is an air atmosphere.

[0073] Comparative Example 2

[0074] The difference between this comparative example and Example 2 is that in step (3), the furnace is evacuated and filled with 98% by volume argon and 2% by volume oxygen; and in step (6), the annealing atmosphere is air atmosphere.

[0075] Comparative Example 3

[0076] This comparative example is the same as Example 1, except that the prepared piezoelectric material is La3Ta 0.5 Ga 5.5 O 14 (LTG), i.e., no Al2O3 raw material is added in step (1).

[0077] Comparative Example 4

[0078] This comparative example is the same as Example 1, except that in step (3), the furnace is evacuated and filled with 98% by volume argon and 2% by volume oxygen.

[0079] The piezoelectric crystals grown in the embodiments and comparative examples were made into piezoelectric vibrators, and the resistivity of the upper limit of the test temperature of the corresponding piezoelectric crystals was measured using a digital multimeter. The results are shown in Table 1.

[0080] Table 1 Resistivity test data of high temperature piezoelectric crystals

[0081] sample Test temperature upper limit 850℃ resistivity (Ω·cm) <![CDATA[Example 1 (La3Ta 0.5 Ga 5.3 Al 0.2 O 14 )]]> 850℃ <![CDATA[2.4×10 6 ]]> <![CDATA[Example 2 (La3Ta 0.5 Ga 5.3 Al 0.2 O 14 )]]> 850℃ <![CDATA[5.1×10 5 ]]> <![CDATA[Comparative Example 1 (La3Ta 0.5 Ga 5.5 O 14 )]]> 850℃ <![CDATA[1.0×10 4 ]]> <![CDATA[Comparative Example 2 (La3Ta 0.5 Ga 5.3 Al 0.2 O 14 )]]> 850℃ <![CDATA[5.2×10 4 ]]> <![CDATA[Comparative Example 3 (La3Ta 0.5 Ga 5.5 O 14 )]]> 850℃ <![CDATA[5.0×10 4 ]]> <![CDATA[Comparative Example 4 (La3Ta 0.5 Ga 5.3 Al 0.2 O 14 )]]> 850℃ <![CDATA[7.4×10 4 ]]>

[0082] Figure 4 It shows that La3Ta with disordered structure 0.5 Ga 5.3 Al 0.2 O 14 (LTGA) Crystal and La3Ta 0.5 Ga 5.5 O 14 (LTG) Ca3TaGa3Si2O with a relatively ordered crystal structure 14 (CTGS) crystal has a higher piezoelectric constant d 12 , while LTGA crystal d 12 The temperature stability is also better than that of LTG crystals. Figure 5 It shows the quantitative analysis of oxygen vacancy concentration of Example 1 and Example 2. The peak intensity of EPR spectrum of LTGA crystal grown and annealed in argon environment is significantly higher than that of LTGA crystal sample grown in oxygen, indicating that the former has a higher oxygen vacancy concentration. Figure 6 It shows that the resistivity of LTGA crystals grown and annealed in an argon environment is greatly improved compared with the LTGA crystal samples grown in oxygen, and the resistivity is increased by about two orders of magnitude at 850°C.

[0083] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a high-impedance and high-stability piezoelectric crystal, characterized in that: The steps include: In an inert atmosphere or an oxygen-deficient atmosphere with an oxygen volume fraction of no more than 2%, the LTGA polycrystalline material is melted, and then a seed crystal is added and a single crystal is grown by a Czochralski method to obtain a LTGA single crystal; wherein LTGA is La3Ta 0.5 Ga 5.5-x Al x Ta 0.5 O 14 , 0<x≤0.5; Under inert atmosphere conditions, the LTGA single crystal is heated to 900-1100° C. and annealed for at least 24 hours.

2. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 1, characterized in that: The process of obtaining LTGA single crystals is carried out under inert atmosphere conditions.

3. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 1, characterized in that: The process of single crystal growth by the Czochralski method includes necking, shoulder release, equal diameter growth and lifting.

4. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 1, characterized in that: The seed crystal is immersed in the polycrystalline melt until the top of the seed crystal is perpendicular to and just in contact with the melt, and single crystal growth begins along the Z direction.

5. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 1, characterized in that: After the single crystal growth is completed, the crystal is subjected to constant temperature treatment and then programmed to cool to room temperature.

6. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 1, characterized in that: The annealing time is 24 to 30 hours.

7. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 1, characterized in that: A lanthanum source compound, a tantalum source compound, a gallium source compound and an aluminum source compound are mixed in a stoichiometric ratio to obtain a mixed material; the mixed material is sintered once at 1000-1100° C., ground and mixed again, and then pressed into blocks, and sintered twice at 1200-1320° C. to obtain LTGA polycrystalline material.

8. The method for preparing a high-impedance and high-stability piezoelectric crystal according to claim 7, characterized in that: The primary sintering time is 10 to 20 hours; Or, the time of mixing after grinding is 10 to 20 hours; Alternatively, the secondary sintering time is 30 to 50 hours.

9. A high impedance and high stability piezoelectric crystal, characterized in that: Obtained by the preparation method according to any one of claims 1 to 8.

10. Use of the high-impedance and high-stability piezoelectric crystal according to claim 9 in the preparation of a piezoelectric sensor.

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