Chang, W. Y.

Davies, R. E.

Dennison, John R.

Nickles, Neal

English

DigitalCommons@USU

Utah State University ground-based test facility for study ofelectronic properties of spacecraft materialsW. Y. Chang, N. Nickles, R. E. Davies, and J. R. DennisonPhysics Department, Utah State University, Logan, UtahAbstract.  Materials used for spacecraft and space structures in near-Earth orbit are subject to severe environmentaleffects including high vacuum conditions, hot and cold extremes temperature, strongly oxidizing atomic oxygenenvironments, and high fluxes of energetic electrons, ions, neutrals and photons.  Instrumentation developed at UtahState University is designed to simulate, at least to some level, all of these conditions and to study charged particleand photon interactions with spacecraft surfaces.  The facilities are particularly well suited to study electronemission as related to spacecraft charging, including secondary and backscattered yields, energy-spectra, and angle-resolved measurements as a function of incident energy, species, and angle.  There are capabilities to determine allparameters required for the NASCAP materials database.  Specifically, the chamber provides controlled neutralenvironments, controlled temperatures, electron fluxes, ion fluxes, photon fluxes, and a wide array of neutral andcharged particle and photon detectors.  In principle, these capabilities can be used simultaneously, allowing study ofsynergistic effects.  Extensive surface science characterization capabilities are also available to fully characterize thesamples in situ. Details of the instrumentation and representative measurements are presented.Project MotivationSpacecraft charging, due to interactions of spacecraftwith the surrounding natural space plasma, is known to bethe cause of many system anomalies and componentfailures [Bedingfield et al., 1996; Leach et al., 1995].  Toassist spacecraft designers in mitigating damage fromcharging effects, NASA, AFOSR, ESA, and RSA havedeveloped extensive sets of engineering tools to predict theextent of charging in various spacecraft environments (forexample, NASCAP/LEO, NASCAP/GEO, POLAR,SPENVIS [Heynderickx et al., 1999], and ECO-M [Danilovet al., 1999]).  However, these tools, along with theirmaterials database, are no longer sufficient in moredemanding environments (e.g., polar and geosynchronousorbits), for higher power and voltage solar arrays, and forvery sensitive modern (high-density, low-current, and low-voltage) electronics [Hastings and Garrett, 1996].Current NASCAP databases lack electronic properties ofmost spacecraft materials in use (only nine materials arepresently incorporated [Mandell et al., 1993]) and manynew spacecraft bulk materials or coatings need to becharacterized.  For the next generation of tools (enhancedNASCAP [Mandell et al., 1999]), more detailed materialproperties must also be incorporated into the databases foruse in enhanced models, such as electron induced energy-and angle-resolved electron yields [Nickles et al., 1999],environmental degradation and contamination of materialsleading to evolution of electronic materials properties whichaffect spacecraft charging [Davies and Dennison, 1997;Davies and Dennison, 1999a], incident-angle dependenceof electron yields, photon energy dependence of electronyields, and thermal coefficients of electrical conductivity.Project DescriptionTo ameliorate these concerns, the NASA SpaceEnvironments and Effects (NASA/SEE) Program hasfunded a three year study (July 1998 to July 2001) ofelectronic properties of spacecraft materials at Utah StateUniversity [Dennison, 1998].  The project time table issummarized as follows:Year 1: Measurement of NASCAP parameters forstandard conductive materials including elementalconductors, conducting alloys, and conductive coatings.Year 2: Measurement of NASCAP parameters forstandard insulators (bulk and coatings), energy- and angle-resolved secondary and back-scattered electron yields ofconductors via electron and photon excitation.Year 3: Measurement of NASCAP parameters forexisting and newly developed spacecraft materials, and timeevolution of secondary electron (SE) and backscattered(BS) yields from environment-induced surfacecontamination.For each sample, measurements will be made that willallow determination of all of the 19 parameters used tocharacterize a material in the current NASCAP database[Mandell et al., 1993].  Table 1 identifies each of theseparameters, along with the experimental methods andapparatus employed to measure the related physicalproperties. The measurements are categorized under threeheadings:(i) sample characterization, used to fully identify thespecific materials tested and to allow end users to moreaccurately assess which material is most closely relatedto their specific spacecraft materials;(ii) conduction related properties, used to model theresponse of the materials to accumulated charge; and6th Spacecraft Charging Technology Conference, AFRL-VS-TR-20001578, 1 September 200073(iii) electron emission induced by electrons, ions andphotons, which determine a material’s response tospace environment fluxes.A number of additional measurements, highlighted in italicsin column three of Table 1, are intended to provideinformation which may be incorporated in future versionsof engineering tools to enhance the modeling of spacecraftcharging.  The results of these measurements will beintegrated into the databases of current tools and newgeneration models.Table 1: Property measurements related to NASCAP modeling parameters.PropertyCategoryMeasured Property(Methods and Apparatus) Related NASCAP Parameters [(a),(b)]Density (Gravimetric). Density ρ [9,19].Bulk composition (AA, IPC). Mean atomic number <Z> [4].Mean atomic weight <A> [10].Surface contamination (AES, AES mapping plusUPS, SIMS, EDX, ESD as needed).Surface morphology (in situ LEED, SEM; ex situSTM, AFM, SEM, optical microscopy).SampleCharacterizationCoating thickness (in situ HEED, quartz micro-balance; ex situ STM, AFM, optical microscopy).Dielectric film thickness d [2].Dielectric constant (ex situ capacitivemeasurements).Relative dielectric constant εr [1].Bulk and surface conductivity (4- point resistanceprobe measurements).Bulk conductivity σ0 [3].Surface resistivity ρs [14].Temperature dependence of conductivity.Electrostatic discharge (I-V profiles of non-conducting films on conducting substrates).Maximum potential before discharge to space Vmax [15].Maximum surface potential difference before dielectricbreakdown discharge Vpunch [16].ConductionRelatedPropertiesHigh energy plasma-induced conductivity (4-point probe measurements of non-conductingsamples for flux of 5-30 keV electrons).Two parameter fit of radiation-induced conductivity σr: k and∆ [17,18].BS/SE total yield versus incident electron energy(Emission current from sample for 5 eV to 30keV monoenergetic electrons using hemi-spherical retarding field analyzer).Maximum SE yield δmax [5].Energy for δmax, E max [6].Parameterless fit for η(E0).Extended parameter fits for δ(E0) and η(E0).Incident angle dependence of δ(E0) and η(E0).Stopping power data. 4-parameter bi-exponential range law fit for primary electronenergy range derived from stopping power data: b1, n1, b2, n2[7-10].Electron-inducedEmissionEnergy- and angle-resolved BS/SE cross-sections(Energy and angle dependent emission cross-sections using rotatable Faraday cup retardingfield analyzer.  High resolution energy dependentemission current using HSA or TOF.Monoenergetic electrons from 5 eV to 30 keV.)Parameters for BS/SE angular distributions used by NASCAPat various incident energies.Deviation from NASCAP BS/SE angular emissiondistributions.Ion-inducedEmissionTotal electron yield versus incident ion energy(Emission current of biased sample from coldcathode ion guns, 500 eV to 5 keV, or PHI ionguns, 100 eV to 5 keV).SE yield due to 1 keV proton impact δH [11].Incident proton energy for Hmaxδ  and HmaxE [12].Energy spectra of emitted electrons.Photon-inducedEmissionTotal electron yield versus incident photon energy(Emission current of biased sample fromdischarge lamps, 0.5-11 eV, or He resonancelamp, 21.2 and 40.8 eV).Total electron yield from solar spectrum [13].Photon energy dependence of emitted electron yields.(a) Mandell et al., 1993.(b) The numbers of the materials database parameters used in the current version of NASCAP are indicated in square brackets.Proposed additions to the database are indicated in italics.6th Spacecraft Charging Technology Conference, AFRL-VS-TR-20001578, 1 September 200074Facilities and CapabilitiesThe primary instrument of the USU facility is a standardultra-high vacuum (UHV) chamber equipped with versatilesurface analysis and sample characterization abilities (seeFigure 1).  This chamber is suitable to simulate diversespace environments including controllable vacuum (<   10-10to 10-3 Torr), temperature (<100 to >1500 K), and ambientneutral gases conditions, electron fluxes (four guns with 5eV to 30 keV mono-energetic sources), ion fluxes (threeguns with <0.1 to 5 keV mono-energetic sources for inertand reactive gases), and solar irradiation (two lamps with0.5 to 11 eV IR/VIS/UV mono-energetic sources; 21.2 and40.8 eV helium resonance lamp).  A variety of detectors areavailable for property measurements of single orsimultaneous electron, ion, and photon, induced emission.These include a standard Faraday cup (FC) detector,hemispherical analyzer (VG VG100AX), cylindrical mirroranalyzer (Varian 981-2707), and time of flight (TOF)micro-channel plate detector (Jordan C-0701).  Specifically,they allow us to measure total emitted electron (ion) yield,backscattered/secondary yield, and energy spectra.Extensive sample characterization capabilities are alsoavailable, including Auger spectroscopy, photoelectronspectroscopy, and secondary ion mass spectroscopy forcontamination assessment and scanning electronmicroscopy, low energy electron diffraction, and ex situscanning tunneling (atomic force) microscopy formorphology examination. Capacitance, conductivity, andelectrostatic discharge equipment are available for ex situexamination of conduction-related properties.The facility is also equipped with a second, smaller UHVchamber (shown in Figure 2), dedicated primarily to angle-resolved SE emission measurements.  A retarding fieldanalyzer (RFA) Faraday cup type detector [Nickles et al.,1999], continuously rotatable about the sample is used toobtain angle-resolved SE yield and spectra for bothnormally and obliquely incident electrons in the range ofemission angles −16° < α < +76°.  Angular resolution of theinstrument is ∼1.5° and the energy resolution is 0.5 eV ±0.1% of the incident beam energy.  The chamber ispresently equipped with a 1-3 keV electron source (Varian981-2454) and a 100-500 eV ion source (Varian 981-2046).In addition to angle-resolved measurements, this chamber isused to study the dynamic evolution of SE yields as afunction of surface condition [Davies and Dennison, 1997]and sample potential [Davies and Dennison, 1999b].Materials Selected for StudyBased on discussions with spacecraft communityspecialists and careful consideration, a set of materials forstudy have been selected to meet two objectives: (1)extending the NASCAP database to include the mostcommon spacecraft materials and (2) investigatingrepresentative materials with wide ranges of physicalproperties.  A prioritized list is shown in Table 2.Figure 1.  AFOSR ultra-high vacuum surface analysischamber used for space environment simulation andcharged particle scattering studies at Utah State University[Riffe and Dennison, 1995].Figure 2.  Ultra-high vacuum chamber dedicated to angle-resolved SE measurements and contamination studies.  Theretarding field analyzer Faraday cup detector, rotating aboutthe sample stage, is shown in inset.6th Spacecraft Charging Technology Conference, AFRL-VS-TR-20001578, 1 September 200075Table 2: Materials for spacecraft charging studies.Conducting MaterialsElemental Metals & Semiconductors PriorityAl*, Ag*, Au* 1C (HOPG graphite) 1a-C (arc evaporated amorphous carbon) 1Be (sintered) 2Cu 2d-C (sputtered diamond-like carbon) 2a-Si (amorphous), Mg* 2C (soot) 3Ge, Ti 3AlloysAl alloy 6061-T6 1Al alloy 2024-T3 2Stainless Steel 316, Ti-Al, Al alloy 7075-T6 3GaAs 3Al alloy 5667 3Conductive CoatingsAquadag (C)* 3ITO (vapor deposited In-Sn Oxide)* 3Insulating MaterialsBulk Insulators PriorityAl2O3, SiO2* 1Kapton*, Teflon*, Mylar 1Borosilicate glass 2Kevlar, Nylon 2BeO, TiO2 3Diamond 3Non-Conductive CoatingsIrridited Al 2MgF (Anti-reflection coating) 2Oxidized Al 2Anodized Al alloy 6061-T6 (Cr-acid) 3Anodized Al alloy 5656 (Sulfuric acid) 3Anodized Mg 3Specific Spacecraft Materials in UseConductors PriorityCarbon impregnated Teflon 1Graphite Al 6061-T61/P120 (4290) 1Black Kapton conducting thermal control blanket 1Magnetospheric plasma analyzer (MPA) materials 2Tether sphere base metal 2LDEF Al support 3InsulatorsCarbon fiber composite 1Graphite cyanate ester P75 tape 1Graphite epoxy plain weave T300/F263 1“Yellow” kapton thermal control blanket 1Fiberglass (antenna material) 2OSR solar cell coating 2Solar cell concentrator DC93500 2Tether sphere painted surface 2LDEF painted tray 3LDEF contaminated painted tray 3Specific Spacecraft Materials under DevelopmentConductors PriorityOther conductive paintsOther electrically conductive thermal control coatings 2InsulatorsAl on thermal control blanket 3AO resistant polymer films & threads 3ITO on thermal control blanket 3Other non-conductive paintsOther graphite compositesOther optical coatingsOther solar cell material 2Other thermal control coatings 2* Materials characterized in current NASCAP database.Extended StudiesOur project will also perform a series of investigations tomore thoroughly characterize the electronic properties ofmaterials and to enhance the next generation models ofspacecraft charging.The expanded set of materials in the database and theextended incident-energy range of our measurements willallow us to determine which of a number of empirically-based model functions for the reduced SE yield curveprovides the best fit to the full data set, particularly in thehigh energy tail.  Figure 3 illustrates this point for fourrepresentative functions and typical data.  The data can alsobe used to determine an appropriate model function for theback-scattered yield curve and to improve the modeling ofincidence-angle dependence of the SE and backscatteredyield curves.Nickles et al. [Nickles et al., 1999] describes initialresults of our studies regarding the effects of surfacepotential on energy-, and angle-resolved SE yields and theneed to include these effects in future charging models.  Atypical angular distribution of argon-sputteredpolycrystalline gold is shown in Figure 4 [Davies, 1999;Davies and Dennison, 1999b].  A parametric fit to theangular-resolved yield using a standard cosine fit (or,additionally, the deviations from such a fit) could proveuseful for modeling return current to charged spacecraftsurfaces under certain circumstances [Nickles et al., 1999].Finally, a recent study of the influence of electronstimulated desorption and electron stimulated adsorption ofcarbon on BS/SE yields from aluminum/aluminum-oxidesamples [Davies and Dennison, 1997] emphasizes thesignificance of surface modification (due to, e.g.,contamination, surface modification, or changes in surfacemorphology) on spacecraft charging.  Studies are underwayto look at similar evolution of BS/SE emission for othersample materials resulting from carbon deposition, organic6th Spacecraft Charging Technology Conference, AFRL-VS-TR-20001578, 1 September 200076Figure 3.  Plot of reduced SE yield curve, δ(Eo)/ δmax vs.Eo/Emax [adapted from Schwartz, 1990].  The hatchedregion indicates the extent of typical experimental data[Dionne, 1975].  Four theoretical models are shown, (i)Schwartz model with n=4/3 power law (dot), (ii) standardmodel with n=4/3 power law (circle), (iii) standard modelwith n=5/3 power law (square), and Vaughn’s empiricalequation (cross).Figure 4.  Angle-resolved secondary electron yield of argonsputtered polycrystalline gold at 1500 eV incident energy[Nickles et al., 1999].  The solid line is a cosine fit to the SEyield data of the form δ(θ)=δ0⋅cos(θ).deposition and charging-induced contamination, oxidation,and AO/UV materials degradation.Acknowledgments.  Instrumentation described in this paperwas funded through grants from the AFOSR (DURIP) Programand the USU Research Office.  The research described here wassupported through the NASA/SEE Program, NASA’s GraduateStudent Researcher’s Program (Davies), and fellowships from theNASA Rocky Mountain Space Grant Consortium (Davies,Nickles).ReferencesBedingfield, K.L., R.D. Leach, and M.B. Alexander, Spacecraftsystem failures and anomalies attributed to the natural spaceenvironment, NASA Reference Publication 1390, NASAMarshall Space Flight Center, August 1996.Danilov, V., G. Drolshagen, V.M. Dvoryashin, A.M. Kramarenko,P. Pita Leria, V.S. Sokolov, and Y.V. Vasilyev, Numericalsimulation of high-voltage charging at high altitudes:comparison of NASCAP and ECO-M, to appear in theProceedings of 6th Spacecraft Charging Technology Conference,Boston, MA, 1999.Davies, R.E., An instrument for experimental secondary electronemission investigations, with application to the spacecraftcharging problem, MS Thesis, Utah State University, 1995.Davies, R.E. and J.R. Dennison, Evolution of secondary electronemission characteristics of spacecraft surfaces, J. Spacecraftand Rockets, 34, 4, 571-574, 1997.Davies, R.E. and J.R. Dennison, Evolution of secondary electronemission characteristics of spacecraft surfaces, to appear in theProceedings of 6th Spacecraft Charging Technology Conference,Boston, MA, 1999a.Davies, R.E. and J.R. Dennison, The effect of negative bias onsecondary electron emission from spacecraft surfaces, to besubmitted to J. Spacecraft and Rockets, 1999b.Davies, R.E., Measurement of angle-resolved secondary electronspectra, Ph.D. Thesis, Utah State University, 1999.Dennison, J.R., Electronic properties of materials with applicationto spacecraft charging, NASA Space Environments and EffectsProgram Grant, 7/1998-7/2001.Dionne, G.F., Origin of secondary-electron-emission yield-curveparameters, J. Appl. Phys., 46, 8, 3347-3351, 1975.Hastings, D. and H. Garrett, Spacecraft-environment interactions,Cambridge University Press, 1996.Heynderickx, D., B. Fontaine, B. Quaghebeur, W.C. Carey, andE.J. Daly, SPENVIS: ESA’s space environment informationsystem and models for spacecraft charging, to appear in theProceedings of 6th Spacecraft Charging Technology Conference,Boston, MA, 1999.Leach, R.D. and M.B. Alexander, Failures and anomaliesattributed to spacecraft charging, NASA Reference Publication1375, NASA Marshall Space Flight Center, August 1995.Mandell, M.J., P.R. Stannard, and I. Katz, NASCAP programmer’sreference manual, NASA Lewis Research Center, May 1993.Mandell, M.J., I. Katz, and D.L. Cooke, Toward a more robustspacecraft charging algorithm, to appear in the Proceedings of6th Spacecraft Charging Technology Conference, Boston, MA,1999.Nickles, N., R.E. Davies, and J.R. Dennison, Energy-angle-resolved secondary electron cross sections and applications tospacecraft charging, to appear in the Proceedings of 6thSpacecraft Charging Technology Conference, Boston, MA,1999.Riffe, D.M. and J.R. Dennison, Ultra-high vacuum electronscattering chamber for the characterization of materials insevere environments: surface and nanocrystalline solids, DOD-Defense University Research Instrumentation Grant, 8/1995-7/1996.Schwarz, A., Application of a semi-empirical sputtering model tosecondary electron emission, J. Appl. Phys., 68, 5, 2382, 1990.00.3-20020406080 SE yield data Cosine fit6th Spacecraft Charging Technology Conference, AFRL-VS-TR-20001578, 1 September 200077

Utah State University Ground-based Test Facility for Study ofElectronic Properties of Spacecraft Materials

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Utah State University Ground-based Test Facility for Study ofElectronic Properties of Spacecraft Materials

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