【作者】 岳龙；

【导师】 吴宜勇；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2013， 博士

【摘要】 本文以航天器介电材料充放电行为评价为研究背景，以空间应用聚酰亚胺及纳米SiO₂改性聚酰亚胺材料为研究对象，利用辐致电导原位测试、宽频介电谱测量、光激电流测试等现代测试技术及分析方法，研究了在电子辐照条件下（<170keV）聚酰亚胺辐致电导动力学行为以及质子辐照损伤效应对材料辐致电导和介电行为的影响，揭示了空间用聚酰亚胺辐致电导及介电行为演化规律和机制。研究结果表明，在不同能量和通量电子辐照条件下，聚酰亚胺辐致电导率动力学过程具有相似的特征，即由电子-空穴双极型传导控制电导动力学演化规律：辐致电导率随辐照时间以幂函数形式上升，然后到达稳态值，且辐致电导率稳态值是辐照电离剂量率的幂函数，辐致电导率上升过程的时间幂指数为材料中载流子陷阱浓度随能级分布的特征参数α，对于不同电子辐照条件下的聚酰亚胺辐致电导率上升过程，该幂指数均为0.26，表明电子辐照条件未改变聚酰亚胺中载流子陷阱分布。根据上述动力学特征分析，建立了描述聚酰亚胺辐致电导率随辐照电离剂量率的阶梯形式的动力学数学方程。研究发现，在短时循环电子辐照条件下，聚酰亚胺辐致电导率出现“过冲”现象，即循环辐照条件下，聚酰亚胺电导率可达到比连续辐照时更高的电导率值。基于电子辐照的电离效应特征揭示出在非连续辐照下，在材料中形成的自由基及其对载流子俘获、激发和复合过程的调制是产生这种“过冲”现象的主要原因。质子辐照后聚酰亚胺辐致电导行为研究结果表明，经质子辐照后，材料的辐致电导率会随辐照质子注量的增加而下降，但其辐致电导率动力学模式未发生变化。基于质子辐照后聚酰亚胺辐致电导率动力学规律以及光激电流谱分析表明，质子辐照损伤导致聚酰亚胺辐致电导率下降的原因包括两个方面，其一为质子辐照位移效应导致材料中均四苯二酐基团降解，造成聚酰亚胺载流子激发能力降低；另一方面，质子辐照损伤效应导致聚酰亚胺材料内部浅能级载流子陷阱增多，引起载流子传输效率下降。根据聚酰亚胺质子辐照损伤过程的特点，建立了质子辐照后聚酰亚胺辐致电导率随位移损伤剂量的衰减规律。对不同能量的质子辐照后聚酰亚胺介电性能的温谱/频谱演化行为研究表明，质子辐照导致聚酰亚胺结构损伤是引起聚酰亚胺介电行为变化的主要原因：质子辐照位移效应造成聚酰亚胺强偶极基团（如C=O和C-N等）的去除，降低了聚酰亚胺分子极化能力，使其介电常数下降；质子辐照在聚酰亚胺材料中形成非均匀损伤并产生异质界面，界面空间电荷极化可增强材料的极化能力，这与由于偶极基团衰减导致聚酰亚胺介电极化能力下降构成相互竞争机制。辐照损伤导致偶极基团的去除效应造成偶极弛豫减弱，从而引起高频（>10⁴Hz）介电损耗系数下降；但是，由于质子辐照损伤引入大量的无定形相区域，使偶极转向的难度增加，造成能量损失增大，这两个因素同时影响高频损耗行为。辐照产生的界面空间电荷极化弛豫过程主要影响聚酰亚胺低频（<10⁴Hz）损耗行为，引起聚酰亚胺低频介电损耗系数增加。对纳米SiO₂改性后聚酰亚胺辐致电导率研究表明，在低电离剂量率（<3.10×10⁴rad/s）电子辐照条件下，SiO₂改性聚酰亚胺辐致电导行为呈现单极型电子传导控制的动力学特征，而在高电离剂量率（4.77×10⁴rad/s）下为电子-空穴双极型传导控制动力学特征。同时在相同电离剂量率下，SiO₂改性聚酰亚胺稳态辐致电导率小于未改性材料的辐致电导率。分析结果表明，SiO₂与聚酰亚胺界面处的镜像势阱加剧载流子复合是造成SiO₂/PI辐致电导率下降的原因，同时SiO₂与聚酰亚胺界面处的类界面态对空穴载流子的俘获效应是造成SiO₂/PI辐致电导率动力学特征随辐照电离剂量率变化的主要原因。更多还原

【Abstract】 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.更多还原