Instrumentation and Measurements for Electron Emission from Charged Insulators

The electron was first discovered in 1898 by Sir John Joseph Thomson and has since been the subject of detailed study by nearly every scientific discipline. At nearly the same time Heinrich Rudolf Hertz conducted a series of experiments using cathode tubes, high potentials and ultraviolet light. When applying a large potential to a cathode he found that an arching event across the metal plates would occur. In addition, when shining an ultraviolet light on the metal he found that less potential was required to induce the spark. This result, taken together with other electrical phenomena brought about by the shining of light upon metal and was eventually termed the photoelectric effect. The work of Thomson and Hertz represent the beginning of electron emission studies and a body of ideas that pervade nearly all aspects of physics. In particular these ideas tell us a great deal about the nature of physical interactions within solids. In this thesis we will focus on the emission of electrons induced by an incident electron source over a range of energies, in which one can observe changes in emitted electron flux and energy distribution. In particular, when energetic particles impinge on a solid they can impart their energy, exciting electrons within the material. If this energy is sufficient to overcome surface energy barriers such as the work function, electron affinity or surface charge potential, electrons can escape from the material. The extent of electron emission from the material can be quantified as the ratio of incident particle flux to emitted particle flux, and is termed the electron yield

Nonlinear dynamics of piezoelectric high displacement actuators in cantilever mode

Experimental results of the nonlinear dynamic response of a piezoelectric high displacement actuator known as thin-layer composite unimorph ferroelectric driver and sensor were compared to a theoretical model, which utilizes the multiple scales method to connect the effective spring constant to higher-order stiffness constants c4 of the piezoelectric layer. This type of actuator has prestress gradients resulting from the manufacturing process that have been reported to play an important role in enhanced actuation. A value of c4=−4.7x1020 N/m2 was obtained for the higher-order lead zirconate titanate (PZT) stiffness coefficient, which is higher than other published results for PZT without prestress gradients. Peak resonance displacements over 1 mm were obtained for even small (100 Vpp) applied fields. The analysis showed a slight voltage dependence that was not specifically accounted for in the theory. This was confirmed by recasting data from other published results and further confirmed by dc offset studies reported here.

Comprehensive Theoretical Framework for Modeling Diverse Electron Transport Experiments in Parallel Plate Geometries

A unified set of parameters and dynamic equations have been developed to describe the time-dependent surface voltage and currents measured for a broad range of electron transport experiments conducted in parallel plate geometry with a dielectric slab above a grounded electrode and with either a floating or fixed voltage upper surface. The framework can model measurements of constant voltage, time-of-flight and AC conductivity; radiation induced conductivity; surface voltage accumulation and decay; electrostatic discharge; electron emission and electron-induced luminescence. The broad applications of the theoretical framework are outlined in terms a comprehensive classification of the ways in which charge is injected into or excited within a material; these classifications include surface deposition, bulk deposition and penetrating radiation for pulsed, stepped and periodic applied voltages/charge from either surface electrodes or electron beams. A set of equations are developed to model evolving electron transport and related phenomena in highly disordered insulating materials over large ranges of time, electric field, temperature, absorbed dose, and adsorbed dose rate. These analytic equations derived from physics-based theories predict the equilibrium and time-dependent accumulation, dissipation and transport of charge carriers; these basic equations are (i) Gauss’ law, (ii) a 1D electron continuity equation with Ohm’s law and source terms, (iii) a 1D continuity equation for holes with source terms, and (iv) the sum of currents due to various conduction mechanisms (including contributions from drift, diffusion, dispersion, polarization, and radiation-induced processes). The total conductivity is modeled as the sum of contributions from three independent conductivity mechanisms: thermally activated hopping, variable range hopping, and radiation-induced conductivity using a concise, unified set of independent fitting parameters. At a microscopic level, modeling and understanding these conduction mechanisms in disordered insulating materials is fundamentally based on a detailed knowledge of the distribution and occupation of the density of states (DOS) of nearly-free and trapped charged carriers. The conduction is controlled by transitions between extended valence and conduction band states, between localized trap states and the extended valence and conduction band states, and hopping between localized states; constant, linear, power law, exponential and Gaussian localized DOS are considered. By analyzing the observed temperature, field, dose rate and time dependent conductivities that result from both extended and localized trap state conduction, this theoretical framework provides new insight into the role of the localized trap state DOS in myriad ground-based materials testing methods

Defect-Driven Dynamic Model of Electrostatic Discharge and Endurance Time Measurements of Polymeric Spacecraft Materials

Charge buildup on insulating materials in the space environment can produce long exposure to electric fields, which can lead to Electrostatic Discharge (ESD). Charge buildup is the leading cause of spacecraft failure due to space environment interactions. ESD can be thought of as the point at which the buildup of charge in localized defects, found in polymeric insulating materials, leads to a catastrophic change in electrical conductivity, which can cause the materials to structurally breakdown. Defects produced by radiation, or prolonged exposure to electric fields, significantly alter the endurance time, the time it takes to produce enough defects to generate a current path to flow more readily. The literature discusses two competing theories for ESD in insulators, based on generation of either recoverable or irrecoverable defects. Such defects in the polymer chains can be produced by the electric field and result in localized trapped states for conduction electrons. Both mechanisms are characterized by the density of electron traps and the corresponding energy to create such defects. We propose a hybrid model for the aging process that predicts the endurance time as a function of electric field and temperature. The model incorporates both types of defects with an interdependence of the two mechanisms. Measurements of the endurance time dependence on electric fields in the insulating polymer Low Density Polyethylene (LDPE) are fit against this hybrid model. Understanding the electric field dependence of the time to ESD can assist designers in selecting appropriate materials for spacecraft construction and in mitigating destructive processes

Unified Model of Charge Transport in Insulating Polymeric Materials

We present a preliminary report on the theoretical and experimental study of transport models in highly insulating materials. The report is developed in four sections; first we give background on the nature of the problems in space craft charging, the contributions and connections made by the Utah State material physics group. Second we discuss the density of states to explore the connections between material composition and the microscopic and macroscopic transport equations. Third from Maxwell’s equations we present an overview of the transport equations. Finally we present preliminary results using experimental data on KaptonTM, the transport equations and relevant expressions for the density of states

Charge Dynamics in Highly Insulating Space Craft Materials

We present a preliminary report on the theoretical and experimental study of transport models in highly insulating materials. The report is developed in four sections; first we give background on the nature of the problems in space craft charging, the contributions and connections made by the Utah State material physics group. Second we discuss the density of states to explore the connections between material composition and the microscopic and macroscopic transport equations. Third from Maxwell’s equations we present an overview of the transport equations. Finally we present preliminary results using experimental data on KaptonTM, the transport equations and relevant expressions for the density of states

Electric Field Dependence of the Time to Electrostatic Breakdown in Insulating Polymers

Electrostatic breakdown can be thought of as the point at which a buildup of local defects in insulators leads to a catastrophic change in electrical conductivity. Defects can be produced by temperature, radiation, or a prolonged exposure to constant electric fields. The endurance time is the time it takes to generate enough defects to create a conduction path for electric current to flow more readily. The literature for electrostatic breakdown in polymeric highly disordered insulating materials discusses two competing theories for electrostatic breakdown, based on generation of either recoverable defects or irrecoverable defects. Such defects in the polymer chains can be produced by the electric field and result in localized trapped states for the conduction electrons. Both mechanisms are characterized by the density of electron traps and the corresponding energy to create such defects. We propose a hybrid thermodynamic model for the electric field aging process that predicts the mean time to failure (the endurance time) as a function of applied electric field and temperature. The hybrid model incorporates both types of defects, and proposes an interdependence of the two production mechanisms. Measurements of the dependence of endurance time on electric field in the insulating polymer Low Density Polyethylene (LDPE) were fit against this hybrid model. Higher electric fields produced breakdown times of 4 s to 1 hr and were associated with creation of irrecoverable defects. Lower electric fields resulted in breakdown times on the order of 2 hours to several months; these were associated with recoverable defect generation. Intermediate range electric fields produced interesting results that illustrate the interdependence of the two types of defects. We end with consideration of an important application of the research. Charge buildup on insulating materials in the space environment can produce long exposure to electric fields, which can lead to breakdown at lower fields. This charge buildup is the leading cause of spacecraft failure due to space environment interactions [1]. Understanding the electric field dependence of the time to electrostatic breakdown can assist designers in selecting appropriate materials for spacecraft construction and in mitigating these destructive processes

Evolution of the Electron Yield Curves of Insulators as a Function of Impinging Electron Fluence and Energy

Electron emission and concomitant charge accumulation near the surface of insulators is central to understanding spacecraft charging. A study of changes in electron emission yields as a result of internal charge buildup due to electron dose is presented. Evolution of total, backscattered, and secondary yield results over a broad range of incident energies are presented for two representative insulators, Kapton and Al2O3. Reliable yield curves for uncharged insulators are measured, and quantifiable changes in yields are observed due to \u3c100-fC/mm2 fluences. Excellent agreement with a phenomenological argument based on insulator charging predicted by the yield curve is found; this includes a decrease in the rate of change of the yield as incident energies approach the crossover energies and as accumulated internal charge reduces the landing energy to asymptotically approach a steady state surface charge and unity yield. It is also found that the exponential decay of yield curves with fluence exhibit an energy-dependent decay constant alpha(E). Finally, physics-based models for this energy dependence are discussed. Understanding fluence and energy dependence of these charging processes requires knowledge of how charge is deposited within the insulator, the mechanisms for charge trapping and transport within the insulator, and how the profile of trapped charge affects the transport and emission of charges from insulator

Defect-Driven Dynamic Model of Electrostatic Discharge and Endurance Time Measurements of Polymeric Spacecraft Materials

Charge buildup on insulating materials in the space environment can produce long exposure to electric fields, which can lead to Electrostatic Discharge (ESD). Charge buildup is the leading cause of spacecraft failure due to space environment interactions. ESD can be thought of as the point at which the buildup of charge in localized defects, found in polymeric insulating materials, leads to a catastrophic change in electrical conductivity, which can cause the materials to structurally breakdown. Defects produced by radiation, or prolonged exposure to electric fields, significantly alter the endurance time, the time it takes to produce enough defects to generate a current path to flow more readily. The literature discusses two competing theories for ESD in insulators, based on generation of either recoverable or irrecoverable defects. Such defects in the polymer chains can be produced by the electric field and result in localized trapped states for conduction electrons. Both mechanisms are characterized by the density of electron traps and the corresponding energy to create such defects. We propose a hybrid model for the aging process that predicts the endurance time as a function of electric field and temperature. The model incorporates both types of defects with an interdependence of the two mechanisms. Measurements of the endurance time dependence on electric fields in the insulating polymer Low Density Polyethylene (LDPE) are fit against this hybrid model. Understanding the electric field dependence of the time to ESD can assist designers in selecting appropriate materials for spacecraft construction and in mitigating destructive processes