RU2847022C1 - Method for polymer ferroelectric films polarization by means of their movement in plasma of glow discharge - Google Patents

Abstract

FIELD: performing operations.

SUBSTANCE: invention relates to the technology of producing polymer ferroelectric materials. Polarizing polymer ferroelectric films with thickness in range of 1 to 100 mcm is carried out at low pressure in a vacuum chamber while exposing the polarized film to negative ions and electrons of oxygen plasma of glow discharge, which is ignited at pressure of about 10 Pa, with cathode, to which negative potential is supplied, and anode with zero potential, in contact with which there is polarized film. At that, the polarisable film is continuously moved through the glow discharge combustion region, a two-electrode system is used without a mesh electrode, with at least 1/3 reduced distance between the cathode and the anode compared to the three-electrode system, and a bent anode with a bending radius R is used, determined by the formula R=(L²+4h²)/8h, where L is cathode width, and h varies in range from 5 to 10 mm.

EFFECT: invention provides high productivity, uniformity and efficiency of process of polarization of polymer ferroelectric films with thickness from 1 to 100 mcm, due to possibility of uniform treatment in one cycle of films of large area.

1 cl, 2 dwg, 2 tbl

Description

Field of technology

The invention relates to the technology of producing polymer ferroelectric materials, concerns a method for polarizing polymer ferroelectric films and is intended to increase the efficiency of this process as applied to film polymer ferroelectrics.

State of the art

Various methods of polarization of polymer ferroelectric materials are known, associated with the application of an electric field to the polarizable film.

A known method of contact (thermal) polarization involves applying high voltage to conducting electrodes deposited on both sides of a polymer ferroelectric film at elevated temperatures. The degree of polarization achieved, and consequently the piezoelectric effect occurring in the film, is determined by the temperature, time, and field strength during polarization. To prevent arcing, which could lead to breakdown and damage to the material, the film is placed in a vacuum or immersed in an insulating liquid. If the electrodes do not reach the edge of the film, polarization can be performed in air without arcing.

The invention METHOD FOR POLARIZING PIEZOELECTRIC FILM (patent TW I742850B, H01L 41/257, published 11.10.2021) is known, which describes a method for contact polarization of ferroelectric films, and which proposes installing insulating gaskets on the edge areas of the film sample to prevent electrical breakdown through air during the polarization process.

The disadvantage of the contact polarization method is the need to apply conductive layers to the samples, the termination of the polarization process when a breakdown occurs at one point, the need to heat the sample, the duration of the polarization process, the limited size of the polarized samples, and the impossibility of polarizing rolled materials.

A known method of polarization is a corona discharge (a self-sustaining gas discharge occurring in highly non-uniform fields near electrodes with a small surface curvature), implemented in corona triode-type circuits. This circuit includes an electrode in the form of one or more needles or blades, to which a potential of approximately 35 kV is applied; an accelerating grid (control electrode), which regulates the flow of electrons to a grounded electrode with a voltage of approximately 10 kV; and a grounded electrode on which the polarizable film is placed. The ferroelectric film must have good contact with the grounded electrode, which is sometimes accomplished by applying a thin-film electrode made of highly conductive materials such as Au, Ag, Cu, ITO, etc., to one of the film surfaces.

The invention METHOD FOR POLARIZING PIEZOELECTRIC FILM (patent US 20210320242A1, H01L 41/257, H01L 41/193, H01L 41/317, published 10/14/2021) is known, which describes a method for polarizing piezoelectric films in a corona discharge, and also proposes methods for improving the quality of the final product to ensure good adhesion of the polarizable film to the grounded electrode.

The invention METHOD FOR POLARIZING PIEZOELECTRIC FILM (patent TW I747556B, H01L 41/253, H01L 41/22, published 11/21/2021) is known, which also uses the corona polarization method and proposes solutions for removing air bubbles between the polarizable film and the grounded electrode in order to ensure good adhesion of the film and reduce the number of defects during processing.

Known is an application for an invention METHOD FOR POLARIZING A PVDF FILM IN A CORONA DISCHARGE FIELD (application for invention of the Russian Federation No. 2021127185, C08J 5/18, B29D 7/01, B29C 71/00, H01L 41/04, H01L 41/193, H01L 41/257, published on 03/14/2023), which is characterized in that the PVDF film is exposed to an electric field of a corona discharge only at room temperature until a polarizing potential is reached on the surface of the PVDF film, which ensures a stable electret state of the PVDF film, after which the electric field of the corona discharge is turned off and the PVDF film is heated to a temperature of at least 80 ° C, after which the heating of the PVDF film is immediately stopped and the PVDF film is cooled to room temperature.

Known inventions include CONTROLLED THIN-FILM FERROELECTRIC POLYMER CORONA POLARIZING SYSTEM AND PROCESS (patent US 10050419B2, H01T 19/04, published on 08/14/2018) and METHOD FOR MONITORING POLARIZATION QUALITY OF PIEZOELECTRIC FILM (patent TW I747360B, G01N 27/00, H01L41/22, published on 05/21/2021), also based on the polarization of polymer ferroelectric films in a corona discharge, where various methods for monitoring the polarization process directly during processing are proposed.

The disadvantages of the corona discharge method for polarizing ferroelectric films include the process's dependence on environmental conditions, as it occurs in air, which reduces the stability of polarizable films during mass production. The need to heat the polarized sample and apply high voltages (up to 35 kV) also leads to uneven polarization over a large film surface due to the point nature of the corona discharge. Furthermore, corona polarization can cause an arc discharge (electrical breakdown), damaging the polarizable film. To prevent arc formation, the electric field must be reduced, limiting the polarization effect.

The invention, "PVDF FILM ROLL-TO-ROLL COMPOSITE POLARIZATION DEVICE AND METHOD" (patent CN 111244264B, H10N 30/045, published April 18, 2023), proposes a roll-to-roll method for polarizing ferroelectric films using corona discharge, with the polarization process occurring during the unwinding and winding of the rolled material. This method enables the industrial production of piezoelectric polymer films, but is low-productivity, and the aforementioned drawbacks result in defective areas in the rolls.

The closest analogue (prototype) to the proposed invention in its technical essence and the achieved results is the invention of the authors "METHOD FOR POLARIZING POLYMER FERROELECTRICS USING GLOW DISCHARGE PLASMA" (patent for invention of the Russian Federation No. 2826131, C08F 214/22, H10N 30/045, published on September 4, 2024), the essence of which lies in carrying out the process of polarization of film polymer ferroelectrics in a vacuum chamber using glow discharge plasma ignited in a three-electrode system. This process allows for increased polarization efficiency due to the low probability of sample breakdown during polarization, reduced processing time, no need for heating, uniform polarization over the sample area, and high process efficiency. In addition, carrying out the process at reduced pressure (in a vacuum) makes it possible to combine it in a single vacuum cycle with the subsequent operation of forming electrodes using vacuum methods for applying thin-film coatings.

A disadvantage of the described method for polarizing polymer ferroelectric films using glow discharge plasma is the limited surface area of the film, which is determined by the area of the electrodes on which the samples are placed. The electrode area is limited by the volume of the vacuum chamber. Furthermore, some unevenness in the film edge treatment occurs due to edge effects.

Disclosure of invention

The objective of the invention is to increase the productivity, uniformity and efficiency of the process of polarization of polymer ferroelectric films with a thickness of 1 to 100 μm using glow discharge plasma ignited at a pressure of ≈10 Pa, due to the possibility of uniform processing of large-area films in one cycle.

The solution to this problem is achieved by carrying out the polarization process using glow discharge plasma (Fig. 1) at reduced pressure in a vacuum chamber 1 with a two-electrode system including a cathode 5 and anode 6. A negative potential is applied to the cathode, the anode with zero potential, and a polarizable film is in contact with the anode, which is continuously moved through the glow discharge combustion region. In this case, a two-electrode system without a mesh electrode is used, with a distance between the cathode and the anode reduced by at least 1/3 compared to the three-electrode system of the prototype, and the anode has a curved forum with a bending radius R determined by the formula R = (L ² + 4h ² ) / 8h, where L is the width of the cathode, and h varies in the range from 5 to 10 mm. The bending of the anode should ensure tight adhesion of the film to it during the polarization process with stable glow discharge combustion over the entire area of the cathode. Ferroelectric film 10, whose surface is in contact with the anode, continuously moves through the discharge combustion region, for example, by being rewound from slave shaft 8 to drive shaft 9, which is rotated by a stepper motor. Film polarization occurs through the accumulation of a negative electric charge on its surface, imparted by charged particles extracted from the glow discharge plasma by anode 6. Uniformity of the polarization process is ensured by the continuous movement of the polarized film through the glow discharge region, eliminating the need for a third mesh electrode, which ensures uniform polarization when the film is located over a limited area on the anode. The transition to a two-electrode system reduces the process cost and increases its efficiency.

The installation of a sputtering system in a vacuum chamber will also allow conductive electrode materials to be applied to a rolled ferroelectric film in a single vacuum cycle along with the polarization process, thereby obtaining a finished functional structure.

Thus, the technical result of the invention consists in increasing the productivity, uniformity and efficiency of the polarization process of large-area polymer ferroelectric films with a thickness of 1 to 100 μm using glow discharge plasma by reducing the polarization time, eliminating the process of forced heating and subsequent cooling, low probability of film breakdown during the polarization process, and reducing the process cost.

List of figures

Fig. 1 shows a diagram of a setup for polarizing a roll of ferroelectric film using glow discharge plasma.

Fig. 2 shows the technical implementation of a vacuum chamber with a diode electrode system for polarizing a roll of ferroelectric film using glow discharge plasma.

Implementation of the invention

The following are indicated on the figures: 1 - vacuum chamber; 2 - pumping system; 3 - working gas filling system; 4 - high-voltage cathode power source; 5 - cathode; 6 - anode; 7 - film roll rewinding system; 8 - driven shaft; 9 - driving shaft, 10 - ferroelectric film roll; 11 - dielectric frame with the ability to change the relative position of the electrodes.

The method of polarization of ferroelectric film is carried out as follows. A roll of polymer ferroelectric film 10, wound on a driven shaft 8, is pulled over anode 6, and the end of the roll is fixed on a driving shaft 9. Vacuum chamber 1 is evacuated to a residual pressure of 1 Pa. Then, the working gas oxygen is supplied to a pressure of 10 Pa in the chamber. A high-voltage negative potential of up to 8 kV is applied to cathode 5 by power source 4, so that a glow discharge is ignited between the cathode and anode 6 bent with a bending radius R, the distance between which is from 20 to 30 mm. Film roll rewinding system 7 is started. The rotation speed of driving shaft 9 is such that each platform of the film roll is treated in glow discharge plasma for no more than 5 minutes. In this case, uniformity of film movement must be ensured with a speed variation of no more than 1%. Negative oxygen ions and electrons generated in the glow discharge plasma are extracted by anode 6 with zero potential and accumulate on the surface of slowly rewinding film 10, creating a surface potential that forms a virtual electrode. An electric field is generated in the system between the virtual electrode and anode 6, penetrating the film. The interaction of the electric dipoles of the ferroelectric film with the resulting electric field leads to the orientation of these dipoles along the field, which causes film polarization. The film roll polarization process is completed after the film is rewound from the driven shaft onto the driving shaft.

The higher the surface charge, the higher the electric field strength in the sample. Consequently, more dipoles will be oriented along this field in a shorter time, increasing the degree of polarization and the piezoelectric modulus. To maximize dipole orientation in the direction of the generated electric field, it is necessary to create a sufficient charge on the film surface, which is determined by the exposure time, sample thickness, and system geometry.

Example of implementation of the invention

Polarization of a 10 m long, 120 mm wide roll of mono-oriented, non-polarized (piezoelectric modulus d ₃₃ = 1.1 pC/N) PVDF-B0028 film with a thickness of 28 μm manufactured by Poly-K (State College, PA 16803 USA) was carried out on a laboratory bench assembled on the basis of an MRS plasma processing unit (developed by Bauman Moscow State Technical University), equipped with a diode system of internal electrodes and a film rewinding system (Fig. 2). The process parameters are given in Table 1.

Table 1

Parameter Meaning Cathode potential, kV 12 Distance between cathode and anode, mm 20 Electrode dimensions, mm 200x140 Maximum pressure in the chamber, Pa 1 Working pressure in the chamber, Pa 10 Working gas Oxygen Duration of polarization of each section of the film, min 3

The piezoelectric modulus _d33 after film processing was measured using the Berlincourt method on a YE2730A measuring instrument (Sinocera Piezotronics, China) at a calibrated load of 0.25 N and a frequency of 110 Hz at six points on five 20 x 20 mm samples cut from a roll of glow-discharge plasma-polarized PVDF film. The results are presented in Table 2.

Table 2.

Sample No. Point number Average d ₃₃ , pC/N Overall average
, pC/N Deviation, % 1 2 3 4 5 6 1 26.4 28.6 26.4 27.7 25.7 25.2 26.7 26.3 1.5 2 26.7 26.3 24.0 28.1 27.7 27.5 26.7 1.5 3 26.3 26.9 23.0 25.8 27.4 26.8 26.0 1.1 4 28.8 28.2 26.7 26.2 26.5 25.1 26.9 2.3 5 25.1 27.3 23.0 24.9 25.1 26.0 25.2 4

Analysis of the piezoelectric coefficient measurement results presented in Table 2 shows that the piezoelectric coefficient values _d33 for films polarized using the proposed method are fully consistent with the manufacturer's stated values ( _d33 >23 pC/H). Furthermore, the spread of _d33 values over the sample area does not exceed 4%, indicating high uniformity of the polarization process of polymer ferroelectric films.

Claims (1)

A method for polarizing polymer ferroelectric films with thicknesses in the range from 1 to 100 μm, carried out at reduced pressure in a vacuum chamber by exposing the polarizable film to negative ions and electrons of an oxygen plasma glow discharge ignited at a pressure of ≈10 Pa, with a cathode to which a negative potential is applied and an anode with zero potential, in contact with which the polarizable film is located, characterized in that the polarizable film is continuously moved through the combustion region of the glow discharge, while using a two-electrode system without a mesh electrode, with a distance between the cathode and the anode reduced by at least 1/3 compared to a three-electrode system, and using a curved anode with a bending radius R, determined by the formula R = (L ² + 4h ² ) / 8h, where L is the width of the cathode, and h varies in the range from 5 to 10 mm.