Daomin Min

Charge transport properties of dielectrics revealed by isothermal surface potential decay

Revealing the charge transport properties of space grade high insulation materials can benefit the mitigation of electrostatic discharges (ESD) on spacecraft. The charge transport properties of polyimide are investigated by an isothermal surface potential decay (ISPD) experiment under a simulated space environment chamber. After irradiated the sample by an electron gun, the 2-D surface potential distributions are measured by a non-contact potential probe. From the surface potential decay curves, we obtain current density at steady state ISPD. Analyzing the steady state current density against surface potential, we find two regimes. One is Ohmic regime and another is Space Charge Limited Current (SCLC) regime, which are separated at around -950 V with the sample thickness of 27 μm at 298 K. Ohmic resistivity and effective charge carrier mobility are calculated from these two regimes, respectively. In addition, the trap density of polyimide is derived from the SCLC theory, when the sample is charged to high initial surface potential.

The energy distribution of trapped charges in polymers based on isothermal surface potential decay model

Space charge formation in polymeric materials can cause some serious concern in real operation, because it has significant influence on the performance of polymers. For example, space charge in some insulating materials can severely distort the electric field, even lead to materials degradation. On the contrary, in the case of its applications, space charge stored in electrets can greatly improve their properties. It is therefore important to understand trapped charge distribution in materials as it is considered to be a novel indicator for effective evaluation of aging status and electric withstanding strength of insulating materials. In this paper, a model based on isothermal surface potential decay (ISPD) is proposed to study the distribution of trapped charges by considering the physical mechanism of the detrapping process. By measuring the ISPD characteristics of polymeric materials and fitting the data according to the assumption of shallow and deep traps, the distribution of trapped charges is obtained, which may be related to the change of aggregation structure of polymers. In order to verify the model, it is used to analyze different ISPD decay curves of polypropylene (PP) and low density polyethylene (LDPE), as well as the ISPD data of PP electrets with/without pressure expanding treatment. The results show that the proposed ISPD model is effective and convenient. Two peaks are observed on the curve of the trapped charge density versus the trap level. The obtained distribution of the trapped charges in polymers can reveal the different nature of electron/hole traps and the different transportation behavior of hole/electron carriers, i.e., the electron-type traps show an inter-chain character while the character of hole-type traps is intra-chain. In addition, the distribution of trapped charge is further related to aggregation structure of PP and LDPE, as well as PP electrets with/without pressure expanding treatment.

Understanding the conduction and breakdown properties of polyethylene nanodielectrics: effect of deep traps

Due to the variation of charge transport characteristics by introduction of nanostructured filler, the conduction and dielectric breakdown properties of nanodielectrics are poorly understood. This work studies on the effect of deep trap on the dc conduction and breakdown properties of nanodielectrics. X-ray diffraction technique is conducted to study the crystallization behavior of LDPE/Al2O3 nanocomposites. Thermally stimulated current (TSC) is applied to measure the trap parameters of specimens. Breakdown strength and volume resistivity are also measured. The results indicate that small amount of nanoalumina enhances the crystallinity, volume resistivity and breakdown strength, and decreases the crystallite size. The TSC results show that the deep trap level and density both increase at low nanoparticle loading samples (<;1 wt%). It is concluded that the independent interfacial region brought by small amount of nanoparticles generates a new potential barrier φ2. It interacts with the original trap sites in LDPE matrix, resulting in the increase of deep traps in nanocomposites. Nanoparticle may act as nucleating agents to modify the morphology and change the deep traps, leading to the reduction of electrical conduction and the improvement of breakdown properties. Both deep trap level and density are benefit to enhance the volume resistivity and breakdown strength. The reduction in mobility of charge carriers, the enhanced height of barrier, the decrease of low density region and the formation of homocharges caused by deep traps are of importance to the reduced conduction and high breakdown performance.

Numerical analysis of space charge accumulation and conduction properties in LDPE nanodielectrics

LDPE nanodielectrics show good space charge suppression performances, reducing the electric field distortions and improving the electric strengths. The decrease of space charge accumulation of LDPE nanodielectrics with increasing the nanoparticle loadings can be explained by the reduction of charge injection, the enhancement of conduction, and so on. However, the phenomena that the conductivities of LDPE nanodielectrics decrease firstly and then may increase with increasing the nanoparticle loadings has not been fully understood. A bipolar charge transport model consisting of charge injection, charge migration, and charge trapping, detrapping, recombination dynamics is used to investigate the space charge accumulation and conduction properties of LDPE nanodielectrics. Based on simulation results and existing experimental results, we discuss the influencing factors for space charge accumulation and conduction properties of LDPE nanodielectrics. It is found that the heightening of injection barrier plays a more important role in the suppression of space charges and the reduction of conductivities of LDPE nanodielectrics. Whereas, the variation of trap density and trap energy will regulate the nanoparticle loading dependent conduction properties.

Surface and volume charge transport properties of polyimide revealed by surface potential decay with genetic algorithm

It is very important to understand the surface and volume charge transportation properties of high insulating materials, such as polyimide, in order to find suitable method to mitigate the electrostatic discharge (ESD) of certain sensitive components on spacecraft. An isothermal surface potential decay (ISPD) experiment is performed inside a ground based vacuum chamber on polyimide under a simulated space environment. Immediately after low energy electron beam irradiation on polyimide, the 2D surface potential distributions are measured by a non-contact potential probe under five various temperatures from 298 to 338 K. The surface potential decay of the insulating material can be divided into two categories: transient process and steady state process. The steady state process is determined by the surface and volume charge transportation properties of dielectric. An ISPD model with genetic algorithm (GA) is developed to reveal the steady state surface potential decay experimental results. From the GA analysis, we obtain the surface resistivity, volume Ohmic resistivity, and charge carrier mobility of polyimide at various temperatures. After analyzing the surface and volume charge transportation properties of the material as a function of temperature, we find that the surface resistivity, volume Ohmic resistivity, and charge carrier mobility are well fitted with the Arrhenius law. Consequently, surface activation energy, volume activation energy, and trap energy of polyimide are found as 0.30 eV, 0.32 eV, and 0.54 eV, respectively.

Space Charge Modulated Electrical Breakdown

Electrical breakdown is one of the most important physical phenomena in electrical and electronic engineering. Since the early 20th century, many theories and models of electrical breakdown have been proposed, but the origin of one key issue, that the explanation for dc breakdown strength being twice or higher than ac breakdown strength in insulating materials, remains unclear. Here, by employing a bipolar charge transport model, we investigate the space charge dynamics in both dc and ac breakdown processes. We demonstrate the differences in charge accumulations under both dc and ac stresses and estimate the breakdown strength, which is modulated by the electric field distortion induced by space charge. It is concluded that dc breakdown initializes in the bulk whereas ac breakdown initializes in the vicinity of the sample-electrode interface. Compared with dc breakdown, the lower breakdown strength under ac stress and the decreasing breakdown strength with an increase in applied frequency, are both attributed to the electric field distortion induced by space charges located in the vicinity of the electrodes.

A comparison of numerical methods for charge transport simulation in insulating materials

Bipolar charge transport (BCT) model has been widely used to simulate time/space evolution of space charges in insulating materials. The BCT simulations are performed to investigate the relationships between space charge accumulation and conduction, electroluminescence (EL), charge packet formation, electrical breakdown, and surface potential decay (SPD) properties. Accordingly, the charge advection-reaction equation that contains shocks or high gradient regions should be solved by highly accurate and stable numerical methods to obtain high resolution. We use Runge-Kutta discontinuous Galerkin (RKDG) method and finite differential weighted essentially non-oscillatory (WENO) method to resolve the charge advection-reaction equation. Then, we calculate the SPD properties and space charge profiles of corona charged low-density polyethylene (LDPE) at various initial surface potentials. The simulated results of the two schemes are compared with analytical SPD results, and also compared with each other. It is found that the simulated SPD curves of RKDG and WENO in the case of single carrier injection are both consistent with the analytical results. Moreover, in the case of both single carrier injection and bipolar carrier injection, WENO scheme is more accurate than RKDG scheme at a given spatial discretization.

Linking traps to dielectric breakdown through charge dynamics for polymer nanocomposites

Polymer nanocomposites can change the density and/or energy of traps, suppress the accumulation of space charges, and enhance dielectric breakdown strength. It is of interest to reveal the influencing mechanism of trap properties on the dielectric breakdown of polymer nanocomposites. Results of thermally stimulated depolarization current and surface potential decay were reviewed, showing that incorporating a small amount of nanoparticles into a polymer can increase the density and/or energy of deep traps. Then, the relation between traps and dc breakdown field of several polymer nanocomposites were analyzed. It was found that the increase in the density and energy of deep traps contributes to the improved dielectric breakdown performance. The modifications of traps by nanoparticles and surface treatments affect the charge dynamics in the bulk of polymer nanocomposites. Then, the accumulation of space charges, the distortion of electric field, and the energy gain of free carriers are regulated to improve the performance of dielectric breakdown.

Numerical simulation on molecular displacement and DC breakdown of LDPE

It is generally known that the dc breakdown strength of low density polyethylene (LDPE) decreases with as the thickness and temperature of the sample increase. The breakdown strength is influenced by the charge transport and electric field distortion, and is also related to the molecular chain displacement and fracture. This paper investigates mutual relations among the charge transport, molecular chain displacement, and thickness dependent dc breakdown of LDPE. A model that combines the dynamics of charge transport and molecular displacement (CTMD) is used to calculate the space charge accumulation, molecular chain displacement, and dc breakdown properties of LDPE with various thicknesses at various constant voltage ramping rates. It is assumed that breakdown occurs when the molecular chain displacement reaches a critical value. The simulation results show that the breakdown field as a function of sample thickness satisfies an inverse power law with a power index of about 0.43 for various voltage ramping rates. This is consistent with experimental results. The CTMD model considers both the distortion of electric field and the displacement kinetics of molecular chains, resulting in a power index closer to the experiment than that calculated only from the electric field distortion. Adopted a Williams-Landel-Ferry type molecular chain mobility in the CTMD model, the simulation results are consistent with the results calculated by applying experimental results on polyisobutylene and polymethyl methacrylate to the free volume breakdown theory. It is also found that the CTMD model with temperature-dependent molecular chain mobility controlled by piecewise Arrhenius equations can explain well the temperature dependent breakdown experimental results of LDPE.

Simulation on the influence of bipolar charge injection and trapping on surface potential decay of polyethylene

The surface potential decay (SPD) properties of low-density polyethylene (LDPE) are determined by charge injection and volume charge transport properties. We adopt a bipolar charge transport (BCT) model to investigate the SPD properties of LDPE with the consideration of time dependent charging process. The non-linear charge continuity equation in the BCT model is solved by a finite differential weighted essentially nonoscillatory (WENO) method that is high stable and high order accurate. Poissons equation is resolved by Boundary Element method (BEM). We obtain the analytical and numerical SPDs in the case of single charge carrier injection and the agreement of analytical and numerical results is excellent. After analyzing the numerical SPD and space charge results at bipolar charge injection, we find that SPD crossover phenomena occur when the hole injection barrier between the insulator and the grounded electrode is relative low. The SPD crossover occurs when the grid voltage exceeds a threshold value. Moreover, it is also found that the SPD crossover is affected by charge trapping/detrapping properties.