Thermally Stimulated Discharge Technology And Its Application In The Research Of Polymer Electret Saw Material

In recent years, with the rapid development of science and technology, SurfaceAcoustic Waves (SAW) devices have replaced many traditional analog electronic inthe field of communications, radar, and electronic warfare, and showing abviousadvantage, meanwhile, all kinds of new SAW piezoelectric materials have become ahot research point. Polymer electret is a new type of electret material which was riseup from 1940s. For its excellent piezoelectric properties, the material is widely used incommercial production and played a significant role in the industrial and medicalfields. Polymer electret can leaves out the expensive non-piezoelectric substrate aswell as reforms the crisp characteristic of traditional piezoelectric substrate when usedin SAW device. Nevertheless, SAW devices have high requirement of substrates’piezoelectric, and polymer electret’s piezoelectric performance is mainly derived fromits charge storage capacity, in order to increase piezoelectric factor of polymer electret,we must improve the charge storage density and stability first.Thermally Stimulated Discharge(TSD) is a basic method in the research ofelectret’s macro regularity and microscopic characteristics, it can gives the depolarizerule of the electret in a short time, and help people understand the storage andattenuation process of charge in electret, so it is a important tool to develop newelectret. TSD technology is a valid way to study the charge storage characteristics ofpolymer electret, which can easily access the information of polarization charge’s typeand storage mechanism, optimize the way of polarization based on these information,the polymer electret will get a better charge storage capacity by optimize the chargingcondition.Firstly, this paper designed and developed a TSC-DH thermal stimulateddischarge apparatus, and researched the application of Artificial Intelligencealgorithms in the apparatus temperature control system, then developed an automateddata acquisition and mapping software for the instrument to simplify thepost-experiment’s data processing work. Further more, we analyzed the feasibility ofpolymer electret as the piezoelectric material for SAW devices, selected three types ofprimary polymer films, and made the polymer electret samples through coronacharging and hot polarization method. The results of the electret samples’ surfacepotential track and TSD experiment draw the following conclusion： (1) The current peak position of TSD experiment will be affected by the heatingrate, in order to improve the results’credibility of TSD experiment, the paper studiedthe application of Artificial Intelligence control algorism in DH-TSC apparatus, theresult show that this algorism realized the linearly temperature rising of the heatingsystem, and improve the accuracy of the TSD experiment.(2) In order to simplify the post-TSD experimental data processing work, a dataacquisition and mapping software has been developed. This software accomplishedthe simultaneously collection of temperature and current in TSD experiment, as wellas automatic map drawing.(3) In order to prevent the complex charging phenomena happened in TSDexperiment due to the air gap between electrode and polymer film, and ensure theelectret is well grounded through the metal electrode, a uniform and complete metalelectrode layer on the electret surface is needed, the enlarged images of aluminumsurface derived from Scanning Electron Microscope show that, the aluminum layerplated by vacuum coating method met the requirements for the electrode.(4) In order to obtain the accurate and effective TSD results, after the preparationof polymer electret, the surface potential of the electret should be tracked until it isstability, according to the state of electrets’surface potential decay, we obtained that:corona charging method will deposit many unstable surface charge on the films, whilehot polarization method will not; high temperature can increases the trap charge’scapture rate and improves the electret’s surface charge density; the corona chargesFEP and PTFE electret have a better charge storage stability than PP electret.(5) We made the polymer electret’s TSD experiment by use of DH-TSCapparatus, analysis their TSD curve we got that: the corona charged PTFE and FEPhave higher rate of deep-trap charge capture than PP; the FEP electrets charged bynegative corona method have the best charge storage stability than that of hotpolarization and positive corona charge.In short, the thermally stimulated discharge technology is a powerful tool tostudy the electret material, and polymer electret has excellent charge storageproperties and piezoelectricity, further more, its smooth surface and low dielectric lossis also very suitable for the SAW propagation, so it is a promising SAW piezoelectricmaterial.