Pzt composite thin film for flexible piezoelectric ultrasonic sensor and preparation method thereof

Abstract

The application discloses a PZT composite film for a flexible piezoelectric ultrasonic sensor and a preparation method thereof. The piezoelectric effect of the PZT film is regulated through a TiO2 seed layer, so that the obtained flexible piezoelectric ultrasonic sensor exhibits excellent thermal stability and sensitivity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of sensors, and relates to a PZT composite film for a flexible piezoelectric ultrasonic sensor and a preparation method thereof. BACKGROUND

[0002] Piezoelectric materials can convert micro mechanical deformation into electrical signals, thereby realizing the conversion between force signals and electrical signals, and have obtained wide attention in the fields of material science and engineering.

[0003] The currently widely used organic piezoelectric material PVDF can realize the detection of ultrasonic signals at room temperature, but due to its organic characteristics, it is easy to decompose, and the prepared flexible piezoelectric film sensor is not suitable for use at high temperature, and has poor temperature stability. Inorganic piezoelectric materials have good temperature stability and are widely used in various sensing fields at high temperatures in recent years. Among these piezoelectric materials, lead zirconate titanate (PZT) has outstanding piezoelectric properties and is widely used in various fields including sensors, actuators, energy harvesting devices, etc., so it has been widely concerned in the research of piezoelectric materials in recent years. Among the various properties of PZT films, its piezoelectric property is particularly important in its practical application. In recent years, researchers have been constantly exploring methods to enhance the performance of PZT materials.

[0004] Compared with the traditional rigid PZT film sensor prepared with a Pt / Si hard substrate, the PZT flexible piezoelectric ultrasonic sensor prepared with a LaNiO3 / Mica substrate can be perfectly attached to the surface of complex power equipment and can adapt to more complex power equipment use environments. At the same time, the flexible piezoelectric ultrasonic sensor prepared from PZT / LNO / Mica has higher sensitivity than the PZT / Pt / Si rigid piezoelectric ultrasonic sensor, and thus is widely studied.

[0005] However, the flexible ultrasonic sensor prepared from the PZT / LNO / Mica film has a working temperature of 0-150 degrees Celsius, and high temperature can cause internal polarization failure and film depolarization of the sensor, so that the sensor cannot perceive ultrasonic signals. In addition, the limited piezoelectric coefficient d33 and large dielectric constant of the flexible ultrasonic sensor further limit its sensitivity to small ultrasonic signals, so the flexible ultrasonic sensor cannot meet the use conditions in the application scenarios of complex power equipment.

[0006] In view of the above reasons, it is urgent to conduct in-depth research on the flexible ultrasonic sensor prepared from the PZT film and explore a sensor with excellent temperature stability and high sensitivity. SUMMARY

[0007] In order to overcome the above problems, the present application provides a PZT composite film for a flexible piezoelectric ultrasonic sensor and a preparation method thereof, wherein the piezoelectric effect of the PZT film is regulated by a TiO2 seed layer, so that the prepared flexible piezoelectric ultrasonic sensor exhibits excellent thermal stability and sensitivity.

[0008] Specifically, the present application aims to provide the following aspects:

[0009] In a first aspect, a PZT composite film is provided, wherein the piezoelectric effect of the PZT film is regulated by a TiO2 seed layer.

[0010] The relative dielectric constant of the PZT composite film is 200-300.

[0011] The PZT composite film comprises, from top to bottom, a PZT film layer, a TiO2 seed layer and a substrate.

[0012] The thickness of the TiO2 seed layer is 50-500 nm.

[0013] The thickness of the PZT film layer is 500-2000 nm.

[0014] The substrate is a polymer substrate, a platinum metal substrate or an LNO / Mica substrate.

[0015] In a second aspect, a method for preparing the PZT composite film of the first aspect is provided, and the method comprises:

[0016] Step 1: depositing a TiO2 precursor colloid on a substrate to obtain a TiO2 precursor film;

[0017] Step 2: depositing a PZT precursor sol on the TiO2 precursor film to obtain the PZT composite film.

[0018] In step 1, the TiO2 precursor colloid is prepared by a sol-gel method.

[0019] In step 1, the film formed by depositing the TiO2 precursor colloid is a TiO2 seed layer, and the thickness of the TiO2 seed layer is 50-500 nm.

[0020] In a third aspect, a flexible piezoelectric ultrasonic sensor is provided, comprising the PZT composite film of the first aspect or the PZT composite film prepared by the method of the second aspect.

[0021] The present application has the following beneficial effects:

[0022] (1) The PZT composite film provided by the application can regulate the piezoelectric effect of the PZT film through the TiO2 seed layer, significantly improve the piezoelectric coefficient d33 of the PZT film, so that the prepared flexible piezoelectric ultrasonic sensor has stronger sensing ability and higher sensitivity to ultrasonic signals, and the relative dielectric constant of the PZT film is reduced, which is beneficial to improving the intensity of the output signal of the flexible piezoelectric ultrasonic sensor.

[0023] (2) The PZT composite film provided by the application not only improves the temperature stability, but also maintains the sensitivity of the flexible piezoelectric ultrasonic sensor to ultrasonic signals at high temperatures, ensures the reliability and effectiveness of the flexible piezoelectric ultrasonic sensor in a high-temperature environment, and is more suitable for monitoring requirements in a high-temperature environment such as complex power equipment.

[0024] (3) The PZT composite film provided by the application has a more compact and small crystal structure due to the addition of the TiO2 seed layer, which is beneficial to improving the mechanical strength and thermal stability of the PZT composite film, thereby improving the stability and reliability of the flexible piezoelectric ultrasonic sensor in a complex environment.

[0025] (4) The flexible piezoelectric ultrasonic sensor prepared from the PZT composite film provided by the application has a wider frequency range in the lead-breaking experiment and can detect more ultrasonic signal information.

[0026] (5) The preparation method of the PZT composite film provided by the application opens up a new way for the research and preparation of the flexible piezoelectric ultrasonic sensor, and helps to promote the development of sensor technology. BRIEF DESCRIPTION OF DRAWINGS

[0027] Various other advantages and benefits of the present application will become apparent to those of ordinary skill in the art upon reading the following detailed description of the preferred embodiments. The accompanying drawings are included solely for purposes of illustrating the preferred embodiments and are not to be construed as a limitation of the present application. It should be readily understood that the drawings depicted are only some embodiments of the present application and that any other drawings, which are similar to those included, can be derived from the included drawings without using inventive faculty.

[0028] In the drawings:

[0029] Figure 1 The SEM characterization diagram of the PZT composite film prepared in Example 1 is shown;

[0030] Figure 2 The SEM characterization diagram of the PZT composite film prepared in Comparative Example 1 is shown;

[0031] Figure 3FIG. 6 shows a comparison of X-ray diffraction patterns of the PZT composite thin film prepared in Example 1 and the PZT composite thin film prepared in Comparative Example 1;

[0032] Figure 4 FIG. 7 shows a voltage-amplitude curve of the PZT composite thin film prepared in Example 1;

[0033] Figure 5 FIG. 8 shows a voltage-amplitude curve of the PZT composite thin film prepared in Comparative Example 1;

[0034] Figure 6 FIG. 9 shows a comparison of relative dielectric constant of the PZT composite thin film prepared in Example 1 and the PZT composite thin film prepared in Comparative Example 1;

[0035] Figure 7 FIG. 10 shows a time-domain signal graph of a lead break experiment when the PZT composite thin film prepared in Example 1 and the PZT composite thin film prepared in Comparative Example 1 are respectively prepared into sensors;

[0036] Figure 8 FIG. 11 shows a frequency-domain signal graph of a lead break experiment when the PZT composite thin film prepared in Example 1 and the PZT composite thin film prepared in Comparative Example 1 are respectively prepared into sensors;

[0037] Figure 9 FIG. 12 shows a comparison of temperature stability of the flexible piezoelectric ultrasonic sensor prepared in Example 1, the flexible piezoelectric ultrasonic sensor prepared in Comparative Example 1, and the PVDF. DETAILED DESCRIPTION

[0038] The specific embodiments of the present application will now be described in Figures 1 to 9 Specific embodiments of the present application will be described herein below with reference to the accompanying drawings. The present application can, however, be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and fully convey the scope of the application to those skilled in the art.

[0039] It should be noted that certain terms have been used throughout the specification and claims which have been used for the purpose of clarity in describing the application. Those skilled in the art will appreciate that each term can relate to any one of the possible equivalents. The specification and claims are not to be limited to the exact terms used in the specification. The above description is presented to enable any person skilled in the art to make and use the application. Modifications in the components, the process steps, and the details of the described embodiments can be made with the scope of the application being defined by the appended claims.

[0040] In the description of the present application, it should be noted that the terms "upper", "lower", "inner", "outer", "front", "back" and the like indicate the orientation or positional relationship in the working state of the present application, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application. In addition, the terms "first", "second", "third", "fourth" are only for descriptive purposes and cannot be understood as indicating or implying relative importance.

[0041] In order to facilitate the understanding of the embodiments of the present application, the following will be further explained and described with specific examples in conjunction with the accompanying drawings, and each drawing does not constitute a limitation on the embodiments of the present application.

[0042] In a first aspect, a PZT composite film is provided according to the present application, which regulates the piezoelectric effect of the PZT film through a TiO2 seed layer.

[0043] Among them, compared with the PZT film, the addition of the TiO2 seed layer introduces compressive deformation, resulting in a more compact structure of the PZT composite film, and better mechanical properties; at the same time, the stress compression causes the upper and lower structures (PZT film layer and TiO2 seed layer) to be asymmetric, introducing spontaneous polarization phenomenon, making it easier to reverse polarization, and thus improving d33; in addition, the TiO2 seed layer as a low dielectric constant layer reduces the overall dielectric constant, thereby improving the sensitivity of the prepared sensor.

[0044] Further, the relative dielectric constant of the PZT composite film is 200-300, the piezoelectric coefficient d33 is 6.0 pm / V, the piezoelectric response is increased by 6-7 times compared with the film without adding TiO2 seed layer, the mechanical strength is good, and the PZT composite film is not damaged after being bent for many times at 0-90°, and the PZT composite film is stable in the temperature range of 0-200℃, i.e. the relative dielectric constant and the piezoelectric coefficient remain unchanged.

[0045] In the present application, the PZT composite film comprises, from top to bottom, a PZT film layer, a TiO2 seed layer and a substrate.

[0046] Further, the substrate can be a polymer substrate such as polyimide (PI), polyethylene terephthalate (PET) and the like, or a metal platinum substrate such as copper foil, aluminum foil and the like, or an inorganic flexible substrate such as silicon nitride film, aluminum oxide film and the like, or an LNO / Mica substrate, preferably an LNO / Mica substrate.

[0047] In the LNO / Mica substrate, LNO (LaNiO3) has good electrical conductivity, thermal stability and chemical stability, and Mica (mica) as a natural crystal has good flexibility and heat resistance; the LNO / Mica substrate exhibits unique advantages in electrical conductivity, structural stability, interface characteristics, adhesion, lattice matching and thermal expansion coefficient matching, and using the same as the substrate is the best choice of the present application.

[0048] In the present application, the thickness of the PZT thin film layer and the TiO2 seed layer is determined according to actual requirements. Generally, the thickness of the PZT thin film layer is 500-2000 nm, preferably 1000-1500 nm, for example 1500 nm; and the thickness of the TiO2 seed layer is 50-500 nm, preferably 100-250 nm, for example 250 nm.

[0049] In the present application, the thickness of the PZT thin film layer and the TiO2 seed layer is determined according to actual requirements. Generally, the thickness of the PZT thin film layer is 500-2000 nm, preferably 1000-1500 nm, for example 1500 nm; and the thickness of the TiO2 seed layer is 50-500 nm, preferably 100-250 nm, for example 250 nm.

[0050] In a second aspect, the present application provides a method for preparing the PZT composite thin film according to the first aspect of the present application, and the method comprises:

[0051] Step 1: depositing a TiO2 precursor colloid on the substrate to obtain a TiO2 precursor thin film;

[0052] Step 2: depositing a PZT precursor sol on the TiO2 precursor thin film to obtain the PZT composite thin film.

[0053] Specifically:

[0054] Step 1: depositing a TiO2 precursor colloid on the substrate to obtain a TiO2 precursor thin film.

[0055] According to a preferred embodiment, the step 1 comprises the following steps:

[0056] Step 1-1: preparing a TiO2 precursor colloid by sol-gel method;

[0057] Step 1-2: depositing the TiO2 precursor colloid on the substrate to obtain a TiO2 precursor thin film.

[0058] In step 1-1, the sol-gel method comprises: dissolving a titanium precursor by an organic solvent to obtain a TiO2 precursor colloid.

[0059] In the present application, the titanium precursor is preferably tetrabutyl titanate and / or titanium n-propoxide, and more preferably tetrabutyl titanate.

[0060] The sol-gel method is generally simple to operate and can be completed at room temperature. The titanium precursor mainly provides a titanium source, and tetrabutyl titanate helps to reduce the agglomeration of TiO2 particles in the sol-gel process, thereby obtaining a TiO2 precursor colloid with better dispersibility.

[0061] In the present application, the organic solvent is preferably one or more of ethanol, ethylene glycol, and dimethyl ether, and more preferably ethanol.

[0062] The above-mentioned organic solvents can all dissolve the titanium precursor, but ethanol is more prominent in terms of lower toxicity, easy removal, and low cost.

[0063] In the present application, the mass ratio of the titanium precursor to the organic solvent is 1:(2-6), preferably 1:(3-6), for example 1:4.

[0064] The above-mentioned mass ratio can completely dissolve the titanium precursor in the organic solvent and ensure that the TiO2 precursor colloid can be used in the gel homogenization process.

[0065] In the present application, the dissolution time is 1-10 min, preferably 2-6 min, for example 5 min.

[0066] In the present application, taking tetrabutyl titanate as the titanium precursor and ethanol as the organic solvent as an example, tetrabutyl titanate reacts with ethanol to form colloidal particles with a three-dimensional network structure, which are TiO2 precursor colloids.

[0067] In step 1-2, the substrate can be a polymer substrate such as polyimide (PI), polyethylene terephthalate (PET), etc., a metal platinum substrate such as a copper foil, an aluminum foil, etc., or an inorganic flexible substrate such as a silicon nitride film, an aluminum oxide film, etc., or an LNO / Mica substrate, and is preferably an LNO / Mica substrate.

[0068] In step 1-2, the deposition can be any one of atomic deposition, magnetron sputtering, and spin coating, and is preferably spin coating.

[0069] Spin coating uses relatively simple and easy-to-operate equipment, and precise control of the thickness of the deposited film can be achieved by controlling the rotation parameters. For example, at 3000 revolutions per second, the film thickness formed by the gel homogenizer in one operation is about 50 nm.

[0070] In step 1-2, after deposition, the TiO2 precursor film is preferably further heat-treated. The heat treatment is used to dry the excess organic solvent and moisture in the TiO2 precursor colloid, and most importantly, to promote the formation and crystallization of the titanium dioxide crystals.

[0071] Further, the heat treatment is performed at a temperature of 500-700°C, preferably 600-700°C, for example 650°C, and for a time of 1-3h, preferably 1.5-2.5h, for example 2h. The heat treatment temperature of 500-700°C is advantageous for promoting the growth and crystallization of the crystals, and improving the quality of the TiO2 seed layer. If the temperature is too high, the crystals may grow too large or be non-uniform, and if the temperature is too low, the rutile phase may be formed. The time of 1-3h provides sufficient time for the growth and crystallization of the crystals, while avoiding the possibility of overgrowth of the crystals or unnecessary structural changes due to too long a time.

[0072] In the present application, the film formed by depositing the TiO2 precursor colloid on the TiO2 precursor film is the TiO2 seed layer.

[0073] Step 2: depositing a PZT precursor sol on the TiO2 precursor film to form the PZT composite film.

[0074] In step 2, the PZT precursor sol is prepared by a sol-gel method, which specifically includes:

[0075] Step 2-1: mixing a co-solvent and a catalyst to obtain a mixed sol;

[0076] Step 2-2: adding lead compounds, zirconium compounds and titanium compounds to the mixed sol to obtain the PZT precursor sol.

[0077] In step 2-1, the co-solvent is used to dissolve the lead compounds, zirconium compounds and titanium compounds. The co-solvent is preferably 2-methoxyethanol, which not only dissolves the above compounds, but also helps to stabilize the sol and prevent the formation of precipitates.

[0078] In step 2-1, the catalyst is preferably acetic acid, which not only acts as a catalyst, but also better controls the reaction rate.

[0079] In step 2-1, the mass ratio of the co-solvent to the catalyst is (2-8):1, preferably (4-5):1, for example 4:1.

[0080] If the proportion of the co-solvent is too low, some compounds may not be fully dissolved, resulting in precipitation; if the proportion is too high, although the solubility increases, it may adversely affect the subsequent reaction steps or the properties of the product. The catalyst can accelerate the reaction rate, but its amount needs to be appropriate. If the proportion of the catalyst is too low, the catalytic effect is not enough, and the reaction rate is too slow; if the proportion is too high, the reaction rate may be too fast, making the reaction difficult to control, and may produce unwanted by-products or affect the structure of the final product. The mass ratio of the co-solvent and the catalyst is appropriate.

[0081] In step 2-1, the longer the mixing time of the co-solvent and the catalyst, the more uniform the mixed sol formed. Generally, the mixing time of the two is 1-3 h, preferably 1-2 h, for example, 1 h.

[0082] In step 2-2, the lead compound is selected from any one of lead acetate trihydrate, lead oxide, lead acetate trihydrate, preferably lead acetate trihydrate [Pb(CH3COO)2·3H2O] with excellent solubility and stability.

[0083] In step 2-2, the zirconium compound is selected from any one of zirconium n-butylate, zirconium oxide, zirconium n-propylate, preferably zirconium n-propylate [Zr(OC3H9)4] with strong solubility and controllability.

[0084] In step 2-2, the titanium compound is selected from any one of tetrabutyl titanate, titanium oxide, titanium isopropylate, preferably tetrabutyl titanate [Ti(OC3H9)4] with excellent solubility and reactivity.

[0085] In step 2-2, the mass ratio of the lead compound, the zirconium compound and the titanium compound is 1:(0.3-0.6):(0.2-0.4), preferably 1:(0.5-0.6):(0.2-0.3), for example, 1:0.56:0.3.

[0086] The amount of the lead compound is excessive to compensate for the loss of lead caused by the formation of pyrochlore phase during crystallization.

[0087] According to the preferred embodiment, the lead compound, the zirconium compound and the titanium compound are sequentially added to the mixed sol.

[0088] The lead compound hydrolyzes and condenses in the mixed sol to form a basic framework, providing a stable reaction environment for the addition of the zirconium compound and the titanium compound; the addition of the zirconium compound is more conducive to adjusting the crystal phase of PZT; and the addition of the zirconium compound helps better control the gelation process, because titanium is more prone to hydrolysis, causing the sol to gel prematurely, affecting the quality of the final product.

[0089] In the present application, after adding the lead compound, under the condition of water bath heating and once stirring for 10-20 minutes, such as 15 minutes, then removing the heat source and continuing to stir until room temperature, under the condition of room temperature, adding the zirconium compound and stirring for 5-10 minutes, such as 10 minutes, then adding the titanium compound and stirring for 20-40 minutes, such as 30 minutes.

[0090] Further, the temperature of the water bath heating is 50-90℃, preferably 60-80℃, for example 70℃.

[0091] In which, under the condition of heating, the decomposition of the lead compound into lead ions and acetate ions in the hydrolysis process can be accelerated, and then the metal oxide network structure is formed; heating can also reduce the agglomeration of the lead compound. In the above temperature range, the reaction rate can be better controlled and the occurrence of side reactions can be reduced.

[0092] In the present application, after the above stirring is completed, aging for 18-48 hours, such as 24 hours, is performed to promote the formation of the gel network.

[0093] In step 2, the deposition can be any one of atomic deposition, magnetron sputtering, and spin coating, and is preferably spin coating.

[0094] Further, the spin coating speed is 2000-4000 rps, preferably 2500-3500 rps, for example 3000 rps.

[0095] In step 2, 1-1.5 mL is dropped each time, and then the PZT precursor sol is uniformly spin coated at the above speed, and then heat treatment is performed according to the following procedure, after the heat treatment is completed, the PZT precursor sol is spin coated and heat treated again, that is, the operation is repeated until the required thickness of the PZT thin film layer is obtained. In fact, the thin film formed by depositing the PZT precursor sol is the PZT thin film layer.

[0096] Further, the heat treatment includes:

[0097] Phase I: heating from room temperature to 300-400℃ at a heating rate of 80-120℃ / min, and holding for 1-3 min;

[0098] Phase II: continuing to heat from the holding temperature of the first phase to 450-550℃ at a heating rate of 30-70℃ / min, and holding for 1-3 min.

[0099] In which, first pre-crystallizing in the PZT thin film at a low temperature of 300-400℃ to reduce internal stress, and then heating to avoid the generation of the process pyrochlore phase.

[0100] In a further preferred embodiment, the heat treatment includes:

[0101] Stage I: heating from room temperature to 330-380°C at a heating rate of 90-110°C / min, holding for 1.5-2 min;

[0102] Stage II: continuing heating from the holding temperature of Stage I to 480-520°C at a heating rate of 40-60°C / min, holding for 2-3 min.

[0103] In a further preferred embodiment, the heat treatment before annealing preferably comprises:

[0104] Stage I: heating from room temperature to 350°C at a heating rate of 100°C / min, holding for 2 min;

[0105] Stage II: continuing heating from the holding temperature of Stage I to 500°C at a heating rate of 50°C / min, holding for 2 min.

[0106] In Step 2, the heat treatment before annealing preferably comprises:

[0107] Stage I: heating from room temperature to 300-400°C at a heating rate of 80-120°C / min, holding for 1-3 min;

[0108] Stage II: continuing heating from the holding temperature of Stage I to 450-550°C at a heating rate of 30-70°C / min, holding for 3-8 min;

[0109] Stage III: continuing heating from the holding temperature of Stage II to 600-700°C at a heating rate of 30-70°C / min, holding for 8-15 min, and then naturally cooling down.

[0110] Wherein, the pyrochlore phase is avoided by the multi-stage heating, and the internal stress of the TiO2 seed layer and the PZT thin film layer is optimized by the heat treatment before annealing.

[0111] In a further preferred embodiment, the heat treatment before annealing preferably comprises:

[0112] Stage I: heating from room temperature to 330-380°C at a heating rate of 90-110°C / min, holding for 1.5-2 min;

[0113] Stage II: continuing heating from the holding temperature of Stage I to 480-520°C at a heating rate of 40-60°C / min, holding for 4-6 min;

[0114] Stage III: continuing heating from the holding temperature of Stage II to 620-660°C at a heating rate of 40-60°C / min, holding for 9-12 min, and then naturally cooling down.

[0115] In a still further preferred embodiment, the heat treatment prior to annealing preferably comprises:

[0116] First stage: heating from room temperature to 350℃ at a heating rate of 100℃ / min, and holding for 2min;

[0117] Second stage: continuing heating from the holding temperature of the first stage to 500℃ at a heating rate of 50℃ / min, and holding for 5min;

[0118] Third stage: continuing heating from the holding temperature of the second stage to 650℃ at a heating rate of 50℃ / min, and holding for 10min, and then naturally cooling down.

[0119] In the present application, the room temperature is the normal temperature.

[0120] In a third aspect, a flexible piezoelectric ultrasonic sensor is provided, which employs the PZT composite film of the first aspect or the PZT composite film prepared by the method of the second aspect.

[0121] The present application is further described below by specific examples, which are merely exemplary and do not constitute any limitation on the scope of protection of the present application.

[0122] Example 1

[0123] (1) 5g of tetrabutyl titanate was dissolved in 20g of anhydrous ethanol, and the dissolution time was 5min, to obtain a TiO2 precursor colloid; then 1g of the TiO2 precursor colloid was spin-coated on an LNO / Mica substrate at a speed of 3000r / min, and then heat-treated at 650℃ for 2h to obtain a TiO2 precursor film, and a TiO2 seed layer with a thickness of 250nm was deposited on the TiO2 precursor film;

[0124] (2) Acetic acid and 2-methoxyethanol were mixed in a mass ratio of 1:4, and stirred for 1h to obtain a mixed sol; 4.332g of lead acetate was added to the mixed sol, heated in a water bath at 70℃ and stirred for 15min at a time, then the heat source was removed and stirring was continued until room temperature, 2.4334g of zirconium n-propylate was added at room temperature, stirred for 10min for the second time, then 1.3200g of tetrabutyl titanate was added and stirred for 30min for the third time, after the above stirring was completed, aging was performed for 24h to obtain a PZT precursor sol;

[0125] 20 drops (1.5mL) of the PZT precursor sol were dropped on the TiO2 precursor film prepared in step (1), and the PZT precursor sol was uniformly spin-coated at 3000rps, and then heat-treated according to the following procedure:

[0126] Stage I: heating from room temperature to 350℃ at a rate of 100℃ / min, holding for 2 min;

[0127] Stage II: heating from the holding temperature of stage I to 500℃ at a rate of 50℃ / min, holding for 2 min;

[0128] After the thermal processing, the PZT precursor sol is dropped again for the above-mentioned spin-coating and thermal processing until the desired thickness of the PZT thin film layer is reached, and the last thermal processing before annealing is performed according to the following procedure:

[0129] Stage I: heating from room temperature to 350℃ at a rate of 100℃ / min, holding for 2 min;

[0130] Stage II: heating from the holding temperature of stage I to 500℃ at a rate of 50℃ / min, holding for 5 min;

[0131] Stage III: heating from the holding temperature of stage II to 650℃ at a rate of 50℃ / min, holding for 10 min, and then naturally cooling down;

[0132] After the above-mentioned operation, the PZT composite thin film is prepared, wherein the thickness of the PZT thin film layer is 1500 nm.

[0133] Example 2

[0134] (1) 5 g of tetrabutyl titanate is dissolved in 30 g of anhydrous ethanol for 5 min to obtain a TiO2 precursor colloid; then 1 g of the TiO2 precursor colloid is spin-coated on an LNO / Mica substrate at a speed of 2000 r / min, and then a TiO2 seed layer with a thickness of 250 nm is formed on the TiO2 precursor thin film by heat treatment at 650℃ for 2 h;

[0135] (2) Acetic acid and 2-methoxyethanol are mixed at a mass ratio of 1:4, and stirred for 1 h to obtain a mixed sol; 4.332 g of lead acetate is added to the mixed sol, heated in a water bath at 70℃ and stirred for 15 min, then the heat source is removed and the stirring is continued until room temperature, 2.4334 g of zirconium n-propylate is added at room temperature, stirred for 10 min, then 1.3200 g of tetrabutyl titanate is added and stirred for 30 min, and then the above-mentioned stirring is completed, and the PZT precursor sol is obtained after aging for 24 h;

[0136] 20 drops (1.5 mL) of the PZT precursor sol are dropped on the TiO2 precursor thin film prepared in step (1), and the PZT precursor sol is uniformly spin-coated at 3000 rps, and then heat processed according to the following procedure:

[0137] Stage I: heating from room temperature to 350°C at a rate of 100°C / min, holding for 2 min;

[0138] Stage II: continuing heating from the holding temperature of Stage I to 500°C at a rate of 50°C / min, holding for 2 min;

[0139] After the thermal processing is finished, the above-mentioned spin-coating and thermal processing operations are performed again by dropping the PZT precursor sol until the desired thickness of the PZT thin film layer is reached, and the last thermal processing before annealing is performed according to the following procedure:

[0140] Stage I: heating from room temperature to 350°C at a rate of 100°C / min, holding for 2 min;

[0141] Stage II: continuing heating from the holding temperature of Stage I to 500°C at a rate of 50°C / min, holding for 5 min;

[0142] Stage III: continuing heating from the holding temperature of Stage II to 650°C at a rate of 50°C / min, holding for 10 min, and then naturally cooling down;

[0143] After the above-mentioned operations are completed, the PZT composite thin film is prepared, wherein the thickness of the PZT thin film layer is 1500 nm.

[0144] Comparative Example 1

[0145] The PZT composite thin film is prepared in a similar manner as in Example 1, except that there is no TiO2 seed layer, and the specific steps are as follows:

[0146] Acetic acid and 2-methoxyethanol are mixed in a mass ratio of 1:4, and stirred for 1 h to obtain a mixed sol; 4.332 g of lead acetate is added to the mixed sol, heated in a water bath at 70°C and stirred for 15 min at a time, then the heat source is removed and stirring is continued until room temperature is reached, 2.4334 g of zirconium n-propylate is added at room temperature, stirred for 10 min twice, then 1.3200 g of tetrabutyl titanate is added and stirred for 30 min three times, after the above-mentioned stirring is completed, the PZT precursor sol is obtained after aging for 24 h;

[0147] 20 drops (1.5 mL) of the PZT precursor sol are dropped on the LNO / Mica substrate, and the PZT precursor sol is uniformly spin-coated at 3000 rps, and then thermal processing is performed according to the following procedure:

[0148] Stage I: heating from room temperature to 350°C at a rate of 100°C / min, holding for 2 min;

[0149] Stage II: continue to increase the temperature from the temperature of the first stage to 500℃ at a rate of 50℃ / min, and keep the temperature for 2min;

[0150] After the thermal processing, the PZT precursor sol is dropped again for the above-mentioned spin-coating and thermal processing until the desired thickness of the PZT thin film layer is reached. The last thermal processing before annealing is performed according to the following procedure:

[0151] Stage I: increase the temperature from room temperature to 350℃ at a rate of 100℃ / min, and keep the temperature for 2min;

[0152] Stage II: continue to increase the temperature from the temperature of the first stage to 500℃ at a rate of 50℃ / min, and keep the temperature for 5min;

[0153] Stage III: continue to increase the temperature from the temperature of the second stage to 650℃ at a rate of 50℃ / min, and keep the temperature for 10min, and then naturally cool down;

[0154] After the above-mentioned operation, the PZT composite thin film is prepared, wherein the thickness of the PZT thin film layer is 1500nm.

[0155] Figure 1 The SEM characterization diagram of the PZT composite thin film prepared in Example 1 is shown, Figure 2 The SEM characterization diagram of the PZT composite thin film prepared in Comparative Example 1 is shown, and it can be found that the PZT composite thin film prepared in Example 1 has finer and more compact crystal grains, which is the reason for its stronger mechanical performance; at the same time, the growth of columnar crystals leads to a more obvious piezoelectric performance than that of Comparative Example 1.

[0156] Figure 3 The X-ray diffraction (XRD) comparison diagram of the PZT composite thin film prepared in Example 1 and the PZT composite thin film prepared in Comparative Example 1 is shown, and it can be found that the PZT composite thin films prepared in Example 1 and Comparative Example 1 both have good crystallization quality and no impurity phase is generated; but the diffraction peaks of the PZT composite thin film prepared in Example 1 are all shifted to low angles, and the piezoelectric coefficient d33 reaches 6.0pm / V, which indicates that the addition of the TiO2 seed layer introduces compressive stress, so that the piezoelectric performance and mechanical strength of Example 1 are much higher than those of Comparative Example 1.

[0157] The PZT composite thin films prepared in Example 1 and Comparative Example 1 are tested for piezoelectric response under the condition of an atomic force microscope, Figure 4 The voltage-amplitude curve diagram of the PZT composite thin film prepared in Example 1 is shown, Figure 5The voltage-amplitude curve of the PZT composite film prepared in Comparative Example 1 is shown, and it can be seen that the response of the PZT composite film prepared in Example 1 reaches about 6 nm, which is significantly higher than 1.2 nm of the PZT composite film prepared in Comparative Example 1, and is about 5 times of the PZT composite film prepared in Example 1, which indicates that the PZT composite film prepared in Example 1 can better convert the force signal into an electrical signal, and is more suitable for preparing a high-performance flexible piezoelectric ultrasonic sensor.

[0158] Figure 6 The relative permittivity comparison diagram of the PZT composite films prepared in Example 1 and Comparative Example 1 is shown, and it can be seen that the relative permittivity of the PZT composite film prepared in Example 1 is lower, which indicates that Example 1 has better sensitivity response, and is more suitable for making a high-performance flexible piezoelectric ultrasonic sensor.

[0159] The PZT composite films prepared in Example 1 and Comparative Example 1 are respectively prepared into sensors according to the following steps: the PZT composite films prepared in Example 1 and Comparative Example 1 are respectively put into positive and negative electrodes through lead wires, and then connected to a BNC copper shaft line, and the PZT composite films are clamped and tightly adhered to the lead wires through a PEI flexible plate, and then wrapped with a copper foil sheet to form electromagnetic shielding. Then, the two sensors are respectively subjected to lead breaking experiments according to the following method: the two sensors are respectively placed on a polyethylene plate with a radius of 30 centimeters and a thickness of 10 centimeters, so that they are kept at a distance of 15 centimeters from the center of the circle, and silicone grease is used to ensure that the sensors are tightly attached to the plate. Subsequently, a 1-centimeter-long automatic pencil lead is pressed to break at a 45-degree angle at the center of the plate. At this time, the two sensors will synchronously collect the signals generated by the breaking of the pencil lead.

[0160] Figure 7 The time-domain signal diagram of the lead breaking experiment of the sensors prepared by the PZT composite films prepared in Example 1 and Comparative Example 1 is shown, and it can be seen that the response ability of the sensor prepared in Example 1 to the same lead breaking signal is 6-7 times of that of Comparative Example 1, which indicates that Example 1 has better sensitivity response, and is more suitable for making a high-performance flexible piezoelectric ultrasonic sensor.

[0161] Figure 8 The frequency-domain signal diagram of the lead breaking experiment of the sensors prepared by the PZT composite films prepared in Example 1 and Comparative Example 1 is shown, and it can be seen that the sensitivity of the film sensor prepared in Example 1 to the ultrasonic signals of different frequency bands of the same lead breaking signal is 6-7 times of that of Comparative Example 1, which indicates that Example 1 has better sensitivity response, and is more suitable for making a high-performance flexible piezoelectric ultrasonic sensor. Figure 7 and Figure 8 It can be seen that the flexible piezoelectric ultrasonic sensor prepared by the PZT composite film in Example 1 is more sensitive to ultrasonic signals, and can monitor a wider range of ultrasonic signal frequency domain.

[0162] The flexible piezoelectric ultrasonic sensor made of Example 1, Comparative Example 1 and PVDF (the flexible piezoelectric ultrasonic sensor made of PVDF is simply marked as PVDF) respectively were put into an oven, and the temperature was raised in steps, starting from 25℃ and setting a temperature gradient every 20℃ to 260℃, and each temperature was kept for 20 min, and then the lead breaking experiment was carried out. Figure 9 Figure 9 The temperature stability comparison chart is shown, and it can be seen that the flexible piezoelectric ultrasonic sensor made of PZT composite film in Example 1 exhibits much better temperature stability than the other two flexible piezoelectric ultrasonic sensors in the heating process, indicating that the addition of the TiO2 seed layer can significantly improve the temperature stability of the PZT flexible piezoelectric ultrasonic sensor.

[0163] In summary, it is obvious that by regulating the performance of the PZT film through the TiO2 seed layer, the PZT composite film obtained has higher piezoelectric coefficient, lower dielectric constant, and can withstand higher temperature, and the flexible piezoelectric ultrasonic sensor prepared has higher sensitivity and thermal stability.

[0164] The above describes the present application in combination with preferred embodiments and exemplary examples. However, it needs to be declared that these specific embodiments are only illustrative explanations of the present application, and do not constitute any limitation on the protection scope of the present application. Various improvements, equivalent replacements or modifications can be made to the technical content and embodiments of the present application without exceeding the spirit and protection scope of the present application, and these all fall within the protection scope of the present application. The protection scope of the present application is subject to the appended claims.​

Claims

1. A PZT composite film, characterized in that, The piezoelectric effect of PZT thin films was modulated using a TiO2 seed layer; The PZT composite film comprises, from top to bottom: a PZT film layer, a TiO2 seed layer, and a substrate; The addition of the TiO2 seed layer introduces compressive deformation into the PZT film layer and causes structural asymmetry between the PZT film layer and the TiO2 seed layer, thereby introducing spontaneous polarization and improving d33. The TiO2 seed layer serves as a low dielectric constant layer, thereby reducing the dielectric constant of the PZT composite film. The relative permittivity of the PZT composite film is 200~300; The substrate is an LNO / Mica substrate; The PZT composite film is used in a flexible piezoelectric ultrasonic sensor.

2. The PZT composite film according to claim 1, characterized in that, The thickness of the TiO2 seed layer is 50~500nm.

3. The PZT composite film according to claim 1, characterized in that, The thickness of the PZT thin film layer is 500~2000 nm.

4. A method for preparing the PZT composite film according to any one of claims 1 to 3, characterized in that, The method includes: Step 1: Deposit TiO2 precursor colloid on the substrate to obtain TiO2 precursor film; Step 2: Deposit PZT precursor sol on TiO2 precursor film to obtain the PZT composite film.

5. The method according to claim 4, characterized in that, In step 1, the TiO2 precursor colloid is prepared by the sol-gel method.

6. The method according to claim 5, characterized in that, In step 1, the thin film formed by the deposition of TiO2 precursor colloid is a TiO2 seed layer with a thickness of 50~500nm.

7. A flexible piezoelectric ultrasonic sensor, characterized in that, This includes the PZT composite film according to any one of claims 1 to 3 or the PZT composite film prepared by the method according to any one of claims 4 to 6.

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