Study On Slow Polarization Behavior Of Nano-SiO_x/ LDPE Composite

As a kind of new material, nano-filler/ polymer composite have increasingly been studied by many researchers. Lots of scholars have made great efforts to study microstructure, dielectric properties and charge transportation of nanocomposite. However, the theoretical model is still an absence for explaining many experimental phenomena between microstructure and dielectric characteristic in nanocomposites. The Quantum effect, small size effect, surface effect and tunnel effect of nano-particles are specific comparing with micro-particles. These features originate special interfaces between nano-particles and matrix polymer, which greatly affects charge transportation in the composite. The interface polarization (slow polarization or low frequency polarization) of nano-SiO_x/ LDPE (Low-density Polyethylene) composite are studied in this paper in detail, as well as that of micro-SiO₂/ LDPE composite. At the same time, the effects of temperature and electrical field on interface polarization in both composites are studied.Nano-SiO_x/ LDPE composite and micro-SiO₂/ LDPE composite were prepared via a method of double-solution mixture, and both of the composites were investigated with scanning electron microscope (SEM). The SEM images showed that nano-SiO_x and micro-SiO₂ could be dispersed in LDPE homogeneously.The discharging current of all samples after being prestressed at a DC electric field were recorded via Keithley 6517A. Theε′-ε_∞andε′′could be obtained by converting discharging current decaying with time via Fourier Transform Method (FEM) from the time domain to the frequency domain. The author analyzed the low frequency dielectric loss and found that 1) The polarization intensity of samples increased with electrical field, and the polarization mechanism might change as electrical field increased; 3) The polarization intensity of pure LDPE was higher than that of the nanocomposite and microcomposite, while the polarization peak location of pure LDPE was lower frequency than that of composites; 4) The slow polarization peak location shifted towards high frequency as temperature increased; 5) The interface polarization phenomenon was affected by electrical stressing history, for example, the interface depolarization of pure LDPE was strongly affected by electrical stressing history, while that of nanocomposite was slightly affected by electrical stressing history.The dielectric constant and other polarization mechanism of composite could be studied via analyzing wide frequency band dielectric spectrum measured with Novocontrol Concept 80. It was found that 1) the decrement of dielectric constant of pure LDPE was small at the frequency range of 1×10^-4～1×10⁷ Hz, while that of nanocomposite was large at the investigated frequency range, as well as that of the microcomposite. The decrement of dielectric constant of nanocomposite increased with the nano-filler load, while that of microcomposite increased rulelessly with the micro-filler load. 2) The dielectric loss peak of nanocomposite located at a higher frequency, and the peak value increased with the nano-filler load. The dielectric loss peak location of nanocomposite shifted towards high frequency as the nano-filler load increased. The dielectric loss peak of the microcomposite located at lower frequency, and the peak value increased with the micro-filler load. The dielectric loss peak location of the microcomposite did not shift with the load. No obvious dielectric loss peak appeared at the range of 1×10^-4～1×10⁷ Hz in pure LDPE, while the dielectric loss intensity increased at frequency lower than 1×10^-1Hz.Thermally Stimulated Currents (TSC) of all samples were measured from 173K to 353K with an instrument made by SEISAKU-SHD, LTD, Japan. It was found that the interface trap energy level of pure LDPE was the deepest, and the TSC current peak located at the highest temperature (about 320.323K); the interface trap energy level of microcomposite was shallower than that of pure LDPE, and the TSC current peak located at about 304K; the interface trap energy level of nanocomposite was the shallowest, and the TSC current peak located in the range of 250²88K and shifted towards a lower temperature as the nano-filler load increased.The space charge distributions of LDPE and composites were measured by Pulsed-electro-acoustic Method (PEA). It was found that the heteropolar space charges in composites were more than those in pure LDPE; Field-induced-thermally-ionization appeared in microcomposite obviously. The space charges originating from Field-induced-thermally-ionization appeared in 0.5% wt microcomposite when the electrical field was 2×10⁷ V/m, while it appeared in 1% wt microcomposite when the electrical field was 5×10⁶ V/m. The quantity of space charge originating from field-induced-thermally-ionization was directly proportional to the micro-SiO₂ load and electrical field, and these space charges were difficult to release.