Measurements of Electronic Properties of Conducting Spacecraft Materials with Application to the Modeling of Spacecraft Charging

This paper describes the results of the first stage of this project, measurements of the electronic properties of conducting spacecraft materials. We begin with a description of the required measurements and specifics of the experimental methods used. A complete list of the conducting materials studied, justification of their selection for study, and a summary of the important results of the measurements is presented. This is followed by detailed measurements and analysis for one representative conductor, namely polycrystalline Au. We end with a description of incorporation of these measurements into the NASCAP database. INTRODUCTION Many spacecraft system anomalies and component failures are known to result from spacecraft charging which is due to the bombardments of spacecraft by energetic electrons, ions, and photons in natural space environment [Hastings and Garrett, 1996; Bedingfield et al., 1996; Leach et al., 1995]. To assist spacecraft designers in accommodating and mitigating the harmful charging effects on spacecraft, NASA has developed an extensive set of engineering tools to predict the extent of charging in various spacecraft environments (for example, NASCAP/LEO, NASCAP/GEO, and POLAR) [Mandell et al., 1993]. However, current NASCAP databases lack electronic properties of most spacecraft materials in use (only nine basic materials are presently incorporated) and many new spacecraft bulk materials and coatings need to be characterized. In an effort to improve the reliability and versatility of these models, the NASA Space Environments and Effects (NASA/SEE) Program has funded a study to measure the electronic properties of spacecraft materials related to NASCAP parameters [Dennison, 1998]. The objectives of the study are (i) to provide more accurate measurements together with sufficient materials characterization and (ii) to significantly extend the database to include a wider range of materials that are more representative of the myriad materials used in spacecraft design and to incorporate newly developed materials. This paper describes the results of the first stage of this project, measurements of the electronic properties of conducting spacecraft materials. We begin with a description of the required measurements and specifics of the experimental methods used. A complete list of the conducting materials studied, justification of their selection for study, and a summary of the important results of the measurements is presented. This is followed by detailed measurements and analysis for one representative conductor, namely polycrystalline Au. We end with a description of incorporation of these measurements into the NASCAP database. EXPERIMENT The NASCAP code designed to model spacecraft charging uses 19 parameters to characterize the electronic properties of a given material [Mandell et al., 1993]. For each sample studied, measurements are made to determine these 19 parameters. Table I identifies the experimental methods and apparatus employed to determine these physical properties. The measurements can be grouped under three headings: (i) sample characterization, used to fully identify the specific materials tested and to allow end users to more accurately assess which material is most closely related to their specific spacecraft materials; (ii) conduction related properties, used to model the response of materials to accumulated charge; and (iii) electron emission (induced by electrons, ions, photons) which determine a material’s response to space environment fluxes. The measurement methods and instrumentation specific for conducting samples are described below in more detail for each of these three groups. A number of additional property measurements, highlighted in italics in column three of Table I, are included in the study; the intent of these additional measurements is to extend the description of the electronic properties of the materials with the goal of improving the modeling of spacecraft charging in future codes. Further details of the instrumentation used for these measurements is found elsewhere [Chang et al., 1999]. Sample Preparation and Characterization Each conducting 1 cm diameter disk was polished using 0.25 :m diamond paste and cleaned using standard solvents prior to insertion into the vacuum chamber. Surface morphology was characterized ex situ using optical microscopy, scanning electron microscopy (SEM), and scanning tunneling and atomic force microscopy (STM/AFM). The disks were subsequently mounted on a sample carousel in a UHV chamber (base pressure <10 Torr). In situ characterization of surface morphology was accomplished with SEM and surface contamination was monitored with Auger electron spectroscopy (AES). Conduction Related Properties A standard four-point probe was used for ex situ measurements of bulk and surface conductivity.