A method for measuring poisson's ratio of piezoelectric thin films

Abstract

The present application relates to the field of poisson ratio measurement, and relates to a method for measuring poisson ratio of piezoelectric film, which comprises the following steps: measuring or calculating the transverse compression modulus of piezoelectric film, the longitudinal compression of piezoelectric film, the longitudinal piezoelectric coefficient of piezoelectric film, and the transverse piezoelectric coefficient of piezoelectric film, and then substituting the above parameters into the formula to obtain the poisson ratio of piezoelectric film. Compared with the prior art, the present application has the advantages of obtaining the poisson ratio of piezoelectric film more accurately, avoiding the large error caused by the measurement of the film thickness change and the like.

Description

Technical Field

[0001] This invention relates to the field of Poisson's ratio measurement, and in particular to a method for measuring the Poisson's ratio of piezoelectric thin films. Background Technology

[0002] Some porous thin films based on nonpolar polymers exhibit a strong piezoelectric effect after appropriate polarization treatment. These materials are named piezoelectric electrets or ferroelectric electrets and are novel artificial intelligence materials. Piezoelectric electrets have outstanding characteristics such as low acoustic impedance very close to that of air, ultrathinness, ultralight weight, flexibility, large-area film formation, and low cost. Therefore, they have broad application prospects in wearable devices, air-coupled ultrasonic wave transmission and reception, security, and micro-energy fields.

[0003] The mechanical and electrical properties of piezoelectric thin films are closely related. One of the most important technical parameters characterizing the mechanical properties of piezoelectric thin films is the Poisson's ratio μ. 13 The overall tensile properties of the material (i.e., μ) 13 The Poisson's ratio can be calculated from the transverse and longitudinal strain during tension. The definition of Poisson's ratio is:

[0004]

[0005] in,

[0006] The strain in the tensile direction during transverse stretching.

[0007]

[0008] The strain in the tensile direction during longitudinal stretching.

[0009] Figure 1 and Figure 2 This is a schematic diagram of the physical quantities in the definitions of transverse and longitudinal strains, showing the application of a transverse tensile force F to the piezoelectric film. x The length of the piezoelectric film changes from its original transverse length x0 to x, and a longitudinal tensile force F is applied to it. y The length of the piezoelectric film changes from the original transverse length z0 to z. After transverse stretching, the transverse length of the piezoelectric film is currently measured by Poisson's ratio μ. 13 The method involves measuring the original length of the film in both the transverse and thickness directions, then stretching the material to achieve elongation in both directions, calculating the strain ε1 and ε3, and substituting these into equation (a) to obtain the Poisson's ratio μ. 13 Therefore, it can be seen that Poisson's ratio μ 13 The calculation results are closely related to the accuracy of the measurement of the transverse strain ε1 and the longitudinal strain ε3 of the material.

[0010] However, due to limitations in the accuracy of measuring instruments, this method can only obtain relatively accurate data when the sample material is relatively thick and has a large deformation. For piezoelectric electret films, their thickness is typically in the range of tens to hundreds of micrometers, and the deformation under operating conditions is generally <5%. Therefore, the strain ε3 in the thickness direction of the film is too erroneous when measured with common instruments, making it impossible to accurately obtain the Poisson's ratio μ. 13 The method does not provide accurate values ​​for Poisson's ratio, therefore it is not suitable for measuring the Poisson's ratio of piezoelectric films. Summary of the Invention

[0011] The purpose of this invention is to overcome the shortcomings of the prior art and provide a convenient and accurate method for measuring the Poisson's ratio of piezoelectric thin films. This method can accurately measure the lateral deformation coefficient of thin-thickness piezoelectric electret thin films.

[0012] The objective of this invention can be achieved through the following technical solutions:

[0013] Poisson's ratio μ of piezoelectric thin film 13 Not only can it be determined by its definition (a),

[0014]

[0015] It can also be calculated using other physical quantities of the thin film. For piezoelectric thin films, the Poisson's ratio can be related to four common material properties:

[0016] (1) Elastic modulus of transverse tension of the film Δp1 is the stress under lateral compression;

[0017] (2) Compressive modulus in the thickness direction of the film Δp3 represents the longitudinal compressive stress;

[0018] (3) Longitudinal piezoelectric coefficient of the thin film Δσ m This represents the change in induced charge during stretching;

[0019] (4) Transverse piezoelectricity of the thin film

[0020] From the definitions of elastic modulus Y1 and Y3, the strain can be determined. and Substituting into equation (a) and combining it with the piezoelectric coefficient d 33 and d 31 The Poisson's ratio μ of the piezoelectric thin film can be obtained from the definition. 13 The equation relating these four parameters is as follows:

[0021] -μ 13 Y3d 33 =Y1d31 (b)

[0022] thereby:

[0023]

[0024] Therefore, this invention proposes a method for accurately measuring the Poisson's ratio of piezoelectric thin films by determining the transverse compressive modulus Y1, longitudinal compressive modulus Y3, and transverse piezoelectric coefficient d. 31 Longitudinal piezoelectric coefficient d 33 The Poisson's ratio of the piezoelectric film is calculated according to equation (Ⅰ).

[0025] A method for measuring the Poisson's ratio of a piezoelectric thin film includes the following steps:

[0026] First, electrodes were coated onto the piezoelectric thin film and polarized. Tensile testing was performed to obtain Y1, complex capacitance spectroscopy was performed to obtain Y3, and piezoelectric coefficients were measured using quasi-static and dynamic methods to obtain d. 33 and d 31

[0027] According to equation (Ⅰ), the Poisson's ratio μ of the piezoelectric thin film can be calculated by substituting the data. 13 :

[0028]

[0029] Where Y1 is the compressive modulus in the transverse direction (parallel to the surface of the piezoelectric film), Y3 is the compressive modulus in the longitudinal direction (perpendicular to the surface of the piezoelectric film), and d 33 The piezoelectric coefficient, d, is the piezoelectric coefficient in the longitudinal direction, i.e., perpendicular to the surface of the piezoelectric film. 31 It is the piezoelectric coefficient in the transverse direction, that is, parallel to the surface of the piezoelectric film.

[0030] Furthermore, the method for testing the transverse compressive modulus of the piezoelectric film includes: performing stress-strain tests on the piezoelectric film and calculating the slope of the stress-strain curve, which is the transverse compressive modulus Y1.

[0031] Furthermore, when performing stress-strain testing on the piezoelectric thin film, the stretching rate is 20-50 mm / min.

[0032] Furthermore, the method for calculating the longitudinal compression of the piezoelectric film includes: measuring the anti-resonance frequency f of the piezoelectric film. a The longitudinal compressive modulus Y3 of the piezoelectric film was then calculated according to the following formula (II);

[0033]

[0034] Where ρ is the density of the piezoelectric film and t is the thickness of the piezoelectric film.

[0035] Furthermore, the method for testing the longitudinal piezoelectric coefficient of the piezoelectric film includes: applying a longitudinal pressure F to the piezoelectric film, measuring the amount of induced charge Q generated on the piezoelectric film electrode during the application of the longitudinal force F, and calculating the longitudinal piezoelectric coefficient d according to the following formula (Ⅲ). 33 :

[0036]

[0037] Furthermore, the longitudinal pressure F applied to the piezoelectric film is achieved by applying a counterweight with a mass of 100-120g.

[0038] Furthermore, the method for testing the transverse piezoelectric coefficient of the piezoelectric film includes: applying a transverse tensile force F1 to the piezoelectric film, measuring the amount of induced charge Q1 generated on the film electrode during the application of the transverse tensile force F1, and calculating the transverse piezoelectric coefficient d according to the following formula (V). 31 :

[0039]

[0040] Where t is the thickness of the piezoelectric film and L is the transverse length of the piezoelectric film.

[0041] Furthermore, the lateral tensile force F1 ranges from 6 to 7 N.

[0042] Furthermore, the electrode for coating the piezoelectric film includes one of a metal electrode, a carbon electrode, or a composite material electrode. Specifically, the metal electrode is selected from either an aluminum electrode or a zinc electrode, and the carbon electrode is a graphite electrode.

[0043] Furthermore, the polarization voltage for coating the piezoelectric film with electrodes and polarizing it is 10-30kV.

[0044] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0045] The advantage of this invention is that it allows for more accurate measurement of the Poisson's ratio μ of porous films. 13 By determining the common mechanical or electrical parameters of the sample film during the measurement process and performing simple calculations, the Poisson's ratio of the piezoelectric film can be obtained relatively accurately, avoiding large errors caused by measuring changes in film thickness. Attached Figure Description

[0046] Figure 1 This is a schematic diagram of deformation in the stretching direction during transverse stretching;

[0047] Figure 2 This is a schematic diagram of deformation in the stretching direction during longitudinal stretching;

[0048] Figure 3This is a schematic diagram illustrating the measurement of the longitudinal piezoelectric coefficient using a quasi-static method in this embodiment;

[0049] Figure 4 This is a schematic diagram of the transverse piezoelectric coefficient measurement in this embodiment;

[0050] Figure reference numerals:

[0051] F x : Lateral tensile force; x0: Original lateral length of the piezoelectric film; x: Lateral length of the piezoelectric film after lateral stretching; F y : Longitudinal tension; z0: Original longitudinal length of the piezoelectric film; z: Longitudinal length of the piezoelectric film after longitudinal stretching; 1: Weight; 2: Preload; 3: Aluminum electrode; 4: Piezoelectric film; F1: Force applied during transverse piezoelectric coefficient measurement; L: Length of the piezoelectric film in the stretching direction; t: Thickness of the piezoelectric film. Detailed Implementation

[0052] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0053] The following embodiments are implemented based on the above-described technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0054] The following are more detailed implementation examples, which further illustrate the technical solution of the present invention and the technical effects that can be obtained.

[0055] In the following embodiments, unless otherwise specified, the raw materials, reagents or processing techniques are all conventional commercial products or conventional processing techniques in the art.

[0056] Example

[0057] This embodiment provides a method for measuring the Poisson's ratio of a piezoelectric thin film, including the following steps:

[0058] (1) First, a 70μm thick PP film with an area of ​​4cm×0.4cm is deposited with a 60nm thick aluminum electrode, and the film is polarized by corona charging method with a polarization voltage of 10kV.

[0059] (2) The piezoelectric film was loaded onto a KJ-1065A tensile testing machine, and the stress-strain curve was obtained by scanning at a tensile rate of 30 mm / min. The slope of the curve in the linear region was calculated to obtain the transverse compressive modulus Y1, which is 9.1 × 10⁻⁶. 8 N / m 2 ;

[0060] (3) The complex capacitance spectrum of the sample was measured using an Agilent 4292A high-precision impedance analyzer, and the anti-resonance frequency of the piezoelectric thin film was found to be 351 kHz, with a film thickness of 70 μm and a density of 900 kg / m³. 3 The longitudinal compressive modulus Y3 of the piezoelectric film is calculated according to the following formula (II);

[0061]

[0062] Where ρ is the density of the piezoelectric thin film (900 kg / m³). 3 ), where t is the thickness of the piezoelectric film (70 μm), and the longitudinal compressive modulus Y3 of the film is calculated to be 2.2 × 10⁻⁶. 6 N / m 2 ;

[0063] (4) Place the piezoelectric film on the measuring platform of the Keithley 6514 electrometer and connect the circuit. Perform the measurement using a quasi-static method, such as... Figure 3 The diagram shows a common quasi-static method for measuring the longitudinal piezoelectric coefficient. The bottom three layers, from bottom to top, are aluminum electrode 3, piezoelectric film 4, and aluminum electrode 3. First, a preload 2 is placed on the film, then a weight 1 is placed on the preload 2. The change in charge Q during the application of weight 1 is measured. In this embodiment, a 5g preload is applied to the piezoelectric film, and then a 100g weight is placed on the film. The change in charge Q during the application of the weight is measured to be 140pC. The longitudinal piezoelectric coefficient d is calculated according to the following formula (Ⅲ). 33 :

[0064]

[0065] In this embodiment, F is the gravity of the applied 100g weight, and the longitudinal piezoelectric coefficient of the film is calculated to be 140pC / N.

[0066] (5) Load the piezoelectric film onto the KJ-1065A tensile testing machine and apply a lateral tensile force F1 to the piezoelectric film, such as... Figure 4 As shown, Figure 4 This diagram illustrates the measurement of the transverse piezoelectric coefficient. The bottom three layers, from bottom to top, are aluminum electrode 3, piezoelectric film 4, and aluminum electrode 3 again. F1, marked in the diagram, represents the transverse tensile force applied to the piezoelectric film during the measurement process. t is the film thickness, and L is the length of the film in the tensile direction.

[0067] In this embodiment, the induced charge Q1 generated on the thin-film electrode during the application of force was measured to be 12.8 pC using a Keithley 6514 electrometer. The transverse piezoelectric coefficient d was calculated according to the following formula (V). 31 :

[0068]

[0069] The transverse piezoelectric coefficient d of the thin film was calculated. 31 -2pC / N;

[0070] (6) According to equation (I), the Poisson's ratio μ of the piezoelectric film is calculated by substituting the data. 13 :

[0071]

[0072] The Poisson's ratio of a 70 μm thick PP piezoelectric film was calculated to be 5.9.

[0073] Due to the inherent structural characteristics of piezoelectric films, which are typically very thin, the method of testing the Poisson's ratio of common materials in experiments is not accurate. Therefore, this method of calculating the Poisson's ratio of piezoelectric films using other quantities is proposed. This avoids the problem that the thickness change perpendicular to the film direction is difficult to measure when the piezoelectric film is very thin, and thus the Poisson's ratio of the piezoelectric film can be calculated more accurately.

[0074] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for measuring the Poisson's ratio of a piezoelectric thin film, characterized in that, Includes the following steps: First, electrodes were coated onto the piezoelectric thin film and polarized. Tensile testing was performed to obtain Y1, complex capacitance spectroscopy was performed to obtain Y3, and piezoelectric coefficients were measured using quasi-static and dynamic methods to obtain d. 33 and d 31 According to equation (I), the Poisson's ratio of the piezoelectric film can be calculated by substituting the data. : Equation (Ⅰ), Where Y1 is the compressive modulus in the transverse direction (parallel to the surface of the piezoelectric film), Y3 is the compressive modulus in the longitudinal direction (perpendicular to the surface of the piezoelectric film), and d 33 The piezoelectric coefficient, d, is the piezoelectric coefficient in the longitudinal direction, i.e., perpendicular to the surface of the piezoelectric film. 31 The piezoelectric coefficient is the piezoelectric coefficient in the transverse direction, i.e., parallel to the surface of the piezoelectric film. The method for testing the transverse compressive modulus of the piezoelectric film includes: performing a transverse tensile stress-strain test on the piezoelectric film, and calculating the slope of the stress-strain curve in the linear change region, which is the transverse compressive modulus Y1. The method for testing the transverse piezoelectric coefficient of the piezoelectric thin film includes: applying a transverse tensile force F1 to the piezoelectric thin film and measuring the amount of induced charge generated on the thin film electrode during the application of the transverse tensile force F1. Q 1 The transverse piezoelectric coefficient d is calculated according to the following formula (V). 31 : Formula (V), in t The thickness of the piezoelectric thin film. L denoted as the transverse length of the piezoelectric film.

2. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 1, characterized in that, When performing transverse tensile stress-strain tests on the piezoelectric thin film, the stretching rate is 20-50 mm / min.

3. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 1, characterized in that, The method for calculating the longitudinal compressive modulus of the piezoelectric film includes: measuring the anti-resonant frequency f of the piezoelectric film. a The longitudinal compressive modulus Y3 of the piezoelectric film is then calculated according to the following formula (II); Formula (II), in Density of the piezoelectric thin film t The thickness is the piezoelectric film thickness.

4. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 1, characterized in that, The method for testing the longitudinal piezoelectric coefficient of the piezoelectric film includes: applying a longitudinal pressure F to the piezoelectric film and measuring the amount of induced charge generated on the electrodes of the piezoelectric film during the application of the longitudinal force F. Q The longitudinal piezoelectric coefficient d is calculated according to the following formula (Ⅲ). 33 : Formula (III).

5. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 4, characterized in that, The longitudinal pressure F applied to the piezoelectric film is achieved by applying a counterweight, the mass of which is 100-120g.

6. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 1, characterized in that, The range of the lateral tensile force F1 is 6-7N.

7. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 1, characterized in that, The electrodes for coating the piezoelectric thin film include one of the following: metal electrodes, carbon electrodes, and composite material electrodes.

8. The method for measuring the Poisson's ratio of a piezoelectric thin film according to claim 1, characterized in that, The polarization voltage for coating the piezoelectric film with electrodes and polarizing it is 10-30kV.

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