Investigation On Charge Particle Radiation-induced Conductivity And Dielectric Properties Of Polyimide

As important parameters to evaluate charging/discharging behaviors of space-applied dielectric-material polyimide （PI）, radiation-induced conductivity （RIC）behaviors of polyimide（PI） and nano-SiO₂surface modified polyimide（SiO₂/PI） werestudied under low energy （<170keV） electron irradiation. In order to investigate theirradiation damage of space radiation particles, RICs and dielectric properties werealso studied on PI and SiO₂/PI after proton irradiation with various proton energiesusing complementary techniques such as in-situ RIC measurements, broadbandfrequency dielectric spectroscopy and photo-stimulated discharge currentspectroscopy et. al. As results, it was determined the evolutional behaviors and thenshown the corresponding mechanisms of the RIC and dielectric properties ofpolyimide after proton irradiations. Furthermore, as an important application, theeffects of nano-SiO₂film were investigated on the RIC and dielectric properties ofthe modified polyimide.The results indicate that in polyimide under electron irradiations with differentenergies and fluxes, there appears the same dynamics RIC mode namely RICincreases firstly in a power law of electron radiation time and then reaches a steadystate. This dynamical RIC mode in polyimide is determined as a conducting processcontrolled by bipolar carrier transportation mechanism. It was found that the second-stage steady RIC values could be formulated as a power law of ionization dose rateduring electron radiation. While in the first RIC increasing stage with radiation time,the index of power law dependence α was measured as0.26without changing withthe electron energy and flux. It should be noted that the index α is a characteristicparameter to define the distribution of carrier traps with energy in polyimide, thusunchanged index α during the experiments indicates that the electron irradiationshows no influence on the distribution of carrier traps in polyimide. On the basis ofthe abovementioned analysis of dynamics RICs, a step analytical mathematic modelwas established on the RIC behavior in polyimide under electron irradiation. On theother hand, a cyclic electron irradiation procedure was designed and applied toinvestigate the pre-irradiation effects of RIC. The results show that compared withthe above continuous electron radiation, cyclic electron radiation could result inovershooting the RIC to much higher values than the corresponding steady RIC.Based on ionization processes during electron radiations the mechanism for theovershoot phenomena is due to that radiation-induced free radicals in polyimidecould modulate the carrier trapping, stimulation and recombination processes,changing the conducting behaviors. Further investigations indicate that after proton pre-irradiation, it was measuredlower RIC in polyimide due to the structural damage effects, but the correspondingdynamics RIC mode shows no change. By analysis of the dynamics parameters ofRIC model and photo stimulated discharge current spectra, there are two optionsarisen to explain the reasons on the decrease of RIC of polyimide after proton pre-irradiation. In one hand, proton pre-irradiation damage effect results in moregenerations of carrier traps with shallow energy-levels, thus decreasing transportationefficiency of carriers in polyimide during electron irradiation. On the other hand,proton pre-irradiation could degrade pyromellitimide groups which are the mainsource of carrier generation, thus decreasing the generation rate of carriers. Based onthe displacement damage mechanisms during proton irradiation, the change model ofsteady RIC values could be established as a function of displacement dose inpolyimide..For polyimide materials after high-energy （5MeV） and low-energy （<200keV）proton irradiations, the results on temperature/frequency dependence of dielectricproperties indicate that proton irradiation induced structural damage should be themain reason for the degradation of dielectric properties in polyimide: One is that theeffects of decarbonyl and removal of dipole groups as C-N result in decreasingthe dielectric polarization, hence, the dielectric constant decreases with increasingproton irradiation fluence; the other is that the inhomogeneous damagecharacteristics around the proton tracks in polyimide introduces the interfacialpolarization, which enhances the polarization in polyimide. The abovementionedtwo factors play reverse roles on the change of the dielectric constants of polyimideafter proton irradiations. The degradation of dipole groups induces the decrease ofdipole relaxation, and thus reduces the dielectric loss at high frequency band10⁴Hz).Meanwhile, displacement damage process during proton irradiations may produce alarge number of amorphous phase regions and then increase the energy loss fordipole polarization. This factor may act as a competitive mechanism to change thedielectric loss together with former factor related with dipole degradation. At lowerfrequency band （<10⁴Hz）, the track interfacial space charge relaxation processexerts influences the dielectric loss behaviors.For the case of polyimide modified with surface nano-SiO₂film, it is interestedto be noted that the conductive mechanism is changed from electron-hole carriersbipolar-controlled transportation mode under higher ionization dose rate （4.77×10⁴rad/s） to electron carriers unipolar one as the ionization dose rate is lower than3.1×10⁴rad/s. Meanwhile, the RIC of SiO₂/PI samples is measured lower than thoseof the pristine polyimide under electron irradiation with the same ionization dose rate.Theoretical analysis indicates that interfacial image potential well between nano-SiO₂film and PI substrate enhances the carrier recombination, inducing decrease of RIC in SiO₂/PI; while the quasi-interfacial states would trap more hole carriers at theinterface of SiO₂/polyimide, resulting in changing the conductive mode fromelectron-hole carriers bipolar transportation mechanism to electron carrier unipolartransportation one in SiO₂/PI as the ionization dose rate is low.