Technical background
The piezoelectric property that the thin polymer film of loose structure is had outstanding performance after handling through suitable electric polarization, it has the characteristics of piezoelectric and electret simultaneously, be named as porous polymer piezo-electric electret thin film, piezo-electric electret or ferroelectric electret, it is the new class pickoff material that grows up around the nineteen ninety.The piezoelectric d of porous polymer electret piezoelectric membrane
33The piezoelectric activity of coefficient ratio ferroelectric polymers PVDF and copolymer p thereof (VDF/TrFE) exceeds a magnitude; Compare with piezoceramic material, porous polymer electret piezoelectric membrane removes has higher piezoelectric d
33Beyond the coefficient, but also have compliance large tracts of land film forming, low cost, the low permittivity of polymer and the outstanding characteristics such as low acoustic impedance of mating with air and water.At present unique commercial porous polymer electret piezoelectric membrane is to be stock with polypropylene (PP) resin, forms loose structure through chemistry or physical blowing technology.This porous PP electret piezoelectric film material is at pressure sensor, electroacoustic, acoustic-electric, ultrasonic sensor, and aspect such as alarm, clinical medicine obtained application, and is expected to have broad application prospects in fields such as communication, security personnel, control, life science and military affairs.
The piezoelectric activity of porous polymer piezo-electric electret thin film material comes from and is deposited on the up and down pore space structure of the opposite polarity space charge of two walls and material of inner hole.The volume ratio of the mechanical property of the electret properties of the piezoelectric susceptibility of this material and hole dielectric wall, hole film system, the pattern size of hole and medium and air gap is directly related, and the thermally-stabilised electret thermal stability that depends primarily on medium of piezoelectric activity.Commodity porous PP electret piezoelectric membrane is because the restriction of the electret thermal stability of PP material own, its working temperature generally can not be above 60 ℃, be higher than this temperature, its piezoelectric activity has limited the application of this class functional material because of the decay of electric charge aggravates to reduce even disappear.And the electret of some other high thermal stability (for example PTFE, FEP, COC, PET, PI etc.), owing to be subjected to the restriction of traditional filming technology can't form desirable loose structure, therefore still not only do not had high thermal stability at present but also had the report of the porous polymer piezo-electric electret thin film of high-tension electricity activity.
Summary of the invention
The objective of the invention is to propose a kind of preparation method who has high thermal stability and the active porous polymer electret piezoelectric membrane of high-tension electricity simultaneously.
The preparation method of the porous polymeric electret piezoelectricity thing film that the present invention proposes comprises the preparation of porous polymer film and to this film two parts of charging.Wherein, the preparation process of porous polymer film is: film (herein with a kind of polymer (for example PTFE, FEP, COC, PET, PI, PE, PEN or PP etc.) of or two kinds or two or more different melting points, if adopt two kinds and two or more thin polymer films, then minimum fusing point thin polymer film is called film 1 (the 1st film), and the peak melting point thin polymer film is called film 2 (the 2nd film); If only adopt a kind of thin polymer film, fusing point and the fusing point of then looking film 1 and film 2 equate.) the stacked compound film system that becomes, and place aperture plate at the one or both sides of film, be greater than or equal to film 1 fusing point then, be less than or equal under the certain value (T) in the warm area of film 2 fusing points to compound film system exert pressure (P), elapsed time (t) forms perforated membrane, the scope of P is 1kPa-10MPa, and the scope of t is 2min-120min.The preferable range of P is 1MPa-8MPa; The preferable range of t is 10-50 minute,
Among the present invention, piezo-electric electret thin film is made in charging to porous polymer film, and its charging modes can adopt conventional method, mainly contains:
(1) corona charging, the steps include: perforated membrane placed on the metal electrode and flatten (can the single face plated electrode, also can the two sides plated electrodes not), above perforated membrane, settle a corona electrode, corona electrode connects DC high-voltage power supply, under the condition of no grid voltage or grid voltage regulation and control, carry out corona charging, obtain porous polymer electret piezoelectric membrane; The corona charging condition is: voltage for+2kV~+ 100kV or-2kV~-100kV, maximum temperature is not higher than the fusing point of film 2, be 1 second-1 hour conduction time, the distance of electrode and perforated membrane is 2cm~50cm.
(2) contact method charging the steps include: the two sides of perforated membrane is plated electrode, and upper/lower electrode respectively is connected respectively to the two poles of the earth of a DC high-voltage power supply then, and charging obtains porous polymer electret piezoelectric membrane; The contact method charge condition is: voltage for+2kV~+ 100kV or-2kV~-100kV, maximum temperature is not higher than the fusing point of film 2, be 1 second-1 hour conduction time.
(3) electron beam charging, the electron beam charge condition is: electron beam can be 5keV~100keV, and maximum temperature is not higher than the fusing point of film 2, be 1 second-1 hour conduction time.
Among the present invention, the material of aperture plate can adopt metal or nonmetallic materials.
Among the present invention, the order number of aperture plate is the 2-2000 order, and the silk footpath is 0.001-2mm, and the aperture is 0.005-35mm.
Among the present invention, aperture plate can be independently, also can be compounded on pressure wheel or the pressure plare.
Among the present invention, can be lateral direction penetrating between the mesh of aperture plate, also can be closed.
Among the present invention, the number of plies of various types of films is 1 or multilayer.
Among the present invention, various types of films can be dense film, also can be perforated membranes.
Among the present invention, corona charging can adopt negative corona charging or positive corona charging.
Among the present invention, the corona charging electrode is a needle-like, or thread, or edge of a knife shape.
Among the present invention, during corona charging, can adopt aperture plate between corona electrode and the porous membrane, control the surface potential of perforated membrane by the aperture plate bias voltage, also can be without aperture plate.
Corona charging operation principle of the present invention is as follows: under certain air pressure, in constant voltage, or under the constant current charge condition, utilize the inhomogeneous field that exists between electrode to cause gas partial discharge between electrode, it is corona discharge, the ion that corona discharge produces is by being deposited on porous film surface or carrying out charge-exchange with porous film surface, make perforated membrane produce certain electrical potential difference in two surfaces up and down, meet or exceed the discharge voltage of perforated membrane internal air gap when this electrical potential difference, to cause the internal air gap discharge, the opposite polarity ion of discharge generation by being deposited on the inner hole of perforated membrane dielectric surface or carry out charge-exchange with dielectric surface, make two cornices up and down of inner hole have opposite polarity electric charge to reach the purpose of charging.
Contact method charging operation principle among the present invention is as follows: under certain air pressure, rationally under the direct voltage of (controlled), after the electrical potential difference that is coated with the perforated membrane upper and lower surface of electrode when the two sides reaches certain value, cause the bubble-discharge of air gap in the film internal porosity, the opposite polarity ion of discharge generation by being deposited on the inner hole of perforated membrane dielectric surface or carry out charge-exchange with dielectric surface, make two cornices up and down of inner hole have opposite polarity electric charge to reach the purpose of charging.
Among the present invention, can improve the thermal stability of porous polymer electret piezoelectric membrane by the regulation and control charging temperature.
Technical process of the present invention is simple, and is novel, can access the porous polymer piezo-electric electret thin film of high thermal stability and high-tension electricity activity.
Embodiment
The invention is further illustrated by the following examples.
Embodiment 1, adopts metal grid mesh, and mesh is 1mm * 1mm square aperture.Film 1 adopts FEP film (two-layer), thickness 12.7 μ m; Film 2 adopts PTFE film (three layers), thickness 3 μ m.The temperature of exerting pressure is 280 ℃; Time 15min exerts pressure; Pressure 4kPa exerts pressure.Corona electrode is a needle-like; Corona voltage 32kV; No gate control voltage; Charging temperature is 20 ℃; Charging interval is 60s; The distance of corona electrode and sample is 7cm.The aluminium electrode of each vacuum evaporation 100nm of perforated membrane two sides.
Embodiment 2, adopt metal grid mesh, and mesh is 0.125mm * 0.125mm square aperture.Film 1 adopts FEP film (one deck), thickness 12.7 μ m; Film 2 adopts PTFE film (two layers), thickness 3 μ m.The temperature of exerting pressure is 280 ℃; Time 5min exerts pressure; Pressure 4kPa exerts pressure.Corona electrode is a needle-like; Corona voltage 64kV; No gate control voltage; Charging temperature is 25 ℃; Charging interval is 40s; The distance of corona electrode and sample is 4cm.The perforated membrane two sides is the aluminium electrode of vacuum evaporation 100nm respectively.
Embodiment 3, adopt metal grid mesh, and mesh is 0.125mm * 0.125mm square aperture.Film 1 adopts FEP film (one deck), thickness 12.7 μ m; Film 2 adopts PTFE film (two layers), thickness 3 μ m.The temperature of exerting pressure is 280 ℃; Time 10min exerts pressure; Pressure 4kPa exerts pressure.Corona electrode is a needle-like; Corona voltage 5kV; No gate control voltage; Charging temperature is 20 ℃; Charging interval is 10 minutes; The distance of corona electrode and sample is 10cm.The aluminium electrode of each vacuum evaporation 100nm of perforated membrane two sides.
Embodiment 4, adopt metal grid mesh, and mesh is 0.125mm * 0.125mm square aperture.Film adopts PP film (three layers), thickness 6 μ m.The temperature of exerting pressure is 190 ℃; Time 2min exerts pressure; Pressure 10kPa exerts pressure.Corona electrode is a needle-like; Corona voltage 20kV; No gate control voltage; Charging temperature is 20 ℃; Charging interval is 30s; The distance of corona electrode and sample is 4cm.The aluminium electrode of each vacuum evaporation 100nm of perforated membrane two sides.
Embodiment 5, adopt metal grid mesh, and mesh is 0.125mm * 0.125mm square aperture.Adopt FEP film (one deck, thickness 6 μ m); PP film (one deck, thickness 6 μ m); And PTFE film (one deck, thickness 3 μ m).The temperature of exerting pressure is 230 ℃; Time 35min exerts pressure; Pressure 1kPa exerts pressure.Corona electrode is a needle-like; Corona voltage 32kV; No gate control voltage; Charging temperature is 20 ℃; Charging interval is 60s; The distance of corona electrode and sample is 7cm.The perforated membrane two sides is the aluminium electrode of vacuum evaporation 100nm respectively.
Test result to above-mentioned porous FEP/PTFE composite electret piezoelectric membrane is:
1. the sem photograph of porous polymer film cross section (as shown in Figure 2).
2. porous polymer electret film piezoelectric d
33The pressure characteristic result of the test (as shown in Figure 3) of coefficient
3. porous polymer electret film piezoelectric d
33The frequency run result (as shown in Figure 4) of coefficient
4. porous polymer electret film piezoelectric d
33The isothermal die-away test result (as shown in Figure 5) of coefficient
5. the porous polymer electret film piezoelectric d handled of preaging
33The isothermal die-away test result (as shown in Figure 6) of coefficient