1、Designation: E 2311 04Standard Practice forQCM Measurement of Spacecraft Molecular Contaminationin Space1This standard is issued under the fixed designation E 2311; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last r
2、evision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice provides guidance for making decisionsconcerning the use of a quartz crystal microbalance (QCM) anda thermoel
3、ectrically cooled quartz crystal microbalance(TQCM) in space where contamination problems on spacecraftare likely to exist. Careful adherence to this document shouldensure adequate measurement of condensation of molecularconstituents that are commonly termed “contamination” onspacecraft surfaces.1.2
4、 A corollary purpose is to provide choices among theflight-qualified QCMs now existing to meet specific needs.1.3 The values stated in SI units are to be regarded as thestandard. The values given in parentheses are for informationonly.1.4 This standard does not purport to address all of thesafety co
5、ncerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 595 Test Method for Total Mass Loss
6、 and CollectedVolatile Condensable Materials from Outgassing in aVacuum EnvironmentE 1559 Test Method for Contamination Outgassing Charac-teristics of Spacecraft Materials2.2 U.S. Federal Standards:3MIL-STD-883 Standard Test Method, MicrocircuitsMIL-S-45743 Soldering, Manual Type, High ReliabilityEl
7、ectrical and Electronic EquipmentFED-STD-209E Airborne Particulate Cleanliness Classes inCleanrooms and Clean ZonesNOTE 1Although FED-STD-209E has been cancelled, it still may beused and designations in FED-STD-209E may be used in addition to theISO designations.2.3 ISO Standards:4ISO 14644-1 Cleanr
8、ooms and Associated ControlledEnvironmentsPart 1: Classification of Air CleanlinessISO 14644-2 Cleanrooms and Associated ControlledEnvironmentsPart 2: Specifications for Testing andMonitoring to Prove Continued Compliance with ISO14644-13. Terminology3.1 Definitions:3.1.1 absorptance, a, nratio of t
9、he absorbed radiant orluminous flux to the incident flux.3.1.2 activity coeffcient of crystal, Q, nenergy storedduring a cycle divided by energy lost during a cycle, or thequality factor of a crystal.3.1.3 crystallographic cut, F, nrotation angle betweenthe optical axis and the plane of the crystal
10、at which the quartzis cut; typically 35 188 AT cut for ambient temperature use or39 408 cut for cryogenic temperature use.3.1.4 collected volatile condensable materials, (CVCM),ntested per Test Method E 595.3.1.5 equivalent monomolecular layer, (EML), nsinglelayer of molecules, each 3 3 10-8cm in di
11、ameter, placed withcenters on a square pattern. This results in an EML ofapproximately 1 3 1015molecules/cm2.3.1.6 field of view, (FOV), nthe line of sight from thesurface of the QCM that is directly exposed to mass flux.3.1.7 irradiance at a point on a surface, nEe, E (Ee=dIe/dA), (watt per square
12、metre, Wm-2), ratio of the radiant fluxincident on an element of the surface containing the point, tothe area of that element.3.1.8 mass sensitivity, S, nrelationship between the fre-quency shift and the arriving or departing mass on the sensingcrystal of a QCM. As defined by theory:1This practice i
13、s under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21. 05 on Contamination.Current edition approved Sept. 1, 2004. Published September 2004.2For referenced ASTM standards, visit the ASTM website, www
14、.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3Available from U.S. Government Printing Office Superintendent of Documents,732 N. Capitol St., NW, Mail Stop: SDE, Wash
15、ington, DC 20401.4Available from American National Standards Institute (ANSI), 25 W. 43rd St.,4th Floor, New York, NY 10036.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.Dm / A 5 rqc /2f2! Df (1)where:Dm = mass change, g,A = area o
16、n which the deposit occurs, cm2,f = fundamental frequency of the QCM, Hz,rq= density of quartz, g/cm3, andc = shear wave velocity of quartz, cm/s.3.1.9 molecular contamination, nmolecules that remainon a surface with sufficiently long residence times to affect thesurface properties to a sensible deg
17、ree.3.1.10 optical polish, nthe topology of the quartz crystalsurface as it affects its light reflective properties, for example,specular (sometimes called “clear polish”) or diffuse polish.3.1.11 optical solar reflector, (OSR), na term used todesignate thermal control surfaces on a spacecraft incor
18、porat-ing second surface mirrors.3.1.12 quartz crystal microbalance (QCM), na piezoelec-tric quartz crystal that is driven by an external electronicoscillator whose frequency is determined by the total crystalthickness plus the mass deposited on the crystal surface.3.1.13 reflectance, r, nratio of t
19、he reflected radiant orluminous flux to the incident flux.3.1.14 surface of interest, nany immediate surface onwhich contamination can be formed.3.1.15 super-polish, npolish of a quartz crystal that pro-duces less than 10 root mean square (rms) roughness on thesurface.3.1.16 QCM thermogravimetric an
20、alysis, (QTGA),nraising the temperature of the QCM deposition surfacecauses contaminants to evaporate, changing the QCM fre-quency as a function of time and the mass loss. Relevant vaporpressures can be calculated for various species and can be usedto identify the molecular species.3.1.17 total mass
21、 loss, (TML), nwhen tested per TestMethod E 595.3.1.18 thermoelectric quartz crystal microbalance,(TQCM), nThe temperature of the crystal is controlled witha thermoelectric element so that the rate of deposition and thespecies that condense onto the QCM can be related to thetemperature.3.2 Constants
22、:3.2.1 density of quartzat T = 25C, rq= 2.6485 g/cm3(1)5;atT =77K,rq= 2.664 g/cm3(2).3.2.2 mass sensitivityAT or rotated cut crystal (3).4. Summary of Practice4.1 Measurement of molecular contamination on spacecraftcan be performed in a variety of ways. The specific methodsdepend upon such factors a
23、s knowing its contamination sourceand the approximate level of outgassing, the ability or inabilityto place a sensor in the immediate area of concern, thevariation of the solar thermal radiation striking the sensor, thepower dissipation of the QCM and how it affects certain criticalspacecraft coolin
24、g requirements, cost to the program, and theschedule. Therefore, it is not desirable or possible to include allQCM testing in one test method. The engineers must determineand provide the detailed monitoring procedure that will satisfytheir particular requirements and be fully aware of the effectsof
25、any necessary deviations from the ideal.5. Significance and Use5.1 Spacecraft have consistently had the problem of con-tamination of thermal control surfaces from line-of-sight warmsurfaces on the vehicle, outgassing of materials and subsequentcondensation on critical surfaces, such as solar arrays,
26、 movingmechanical assemblies, cryogenic insulation schemes, andelectrical contacts, control jet effects, and other forms ofexpelling molecules in a vapor stream. To this has been addedthe need to protect optical components, either at ambient orcryogenic temperatures, from the minutest deposition of
27、con-taminants because of their absorptance, reflectance or scatter-ing characteristics. Much progress has been accomplished inthis area, such as the careful testing of each material foroutgassing characteristics before the material is used on thespacecraft (following Test Methods E 595 and E 1559),
28、butmeasurement and control of critical surfaces during spaceflightstill can aid in the determination of location and behavior ofoutgassing materials.6. General Considerations6.1 A QCM sensor is used to measure the molecularcontamination of critical surfaces on spacecraft at one or moretemperatures f
29、or an extended period of time. A piezoelectriccrystal is exposed next to a “surface of interest” or in the planewhere molecular flux is expected. It is then cooled to thetemperature at which the crystal should condense whatevermolecular contaminant exists at that temperature (according tothe vapor-p
30、ressure characteristics of that constituent). Bymeasuring the frequency-shift of the crystal and knowing themass sensitivity (frequency to mass-added factor for thatcrystal), the mass accumulated can be determined. Sunlightstriking the solar panels may cause outgassing that interceptsthe surface of
31、interest. The probable source and extent ofcontamination can be determined from known components ofthe spacecraft and probable sources.6.2 Potential contamination problem areas are shown in Fig.1.6.2.1 The performance of thermal control surfaces is de-graded as a result of the accumulation of contam
32、inants, whichmay increase the surfaces solar absorptance;6.2.2 Optics may be degraded by increasing “light” scatter-ing or reflectance loss;6.2.3 Electronic modules with high rates of outgassingcomponents may have voltage arc-over;6.2.4 Internal to the spacecraft there may be outgassingsources which
33、 will degrade (for instance, mass spectrometercausing signal overload conditions);6.2.5 Windows and optical elements may be degraded byadsorption of a contaminant film leading to a loss of transmit-tance, reflectance, or an increase in scattered light; and6.2.6 Solar arrays are adversely affected by
34、 the absorptanceof contaminants.5The boldface numbers in parentheses refer to the list of references at the end ofthis practice.E23110426.3 Some of the sources of contamination and mechanismsfor transporting them are shown in Fig. 2. Pre-launch, vacuumtest-induced contamination remains a problem as
35、well aslaunch-induced contaminants. High-angle plume impingementfrom spacecraft orientation thrusters, as well as multi-layerinsulation surrounding cryogenic surfaces, are also sources ofcontamination. Frequently, the largest long-term sources arewarm, relatively thick, non-metallic materials of the
36、 spacecraftconstruction. High vapor pressure (low molecular mass) mol-ecules may photo polymerize on surfaces to become low vaporpressure (high molecular mass) stable contaminants. Vaporpressure-controlled self-contamination needs to be in thedesign engineers mind; however, some parameters are still
37、uncertain, that is, back scattering of outgassed molecules due toatmospheric gas collisions, influence of free oxygen andcharged particles as they impact the spacecraft surface.6.4 Some typical spacecraft outgassing rates and the experi-mental determination of the resolution of QCMs are shown inFig.
38、 3. Some actual deposition rate conditions on a spacecrafthave been observed to be 1.2 3 10-12gcm-2s-1for a sunlitvent-viewing OSR (4),23 10-13gcm-2s-1for a mature largesatellite (4), and a projected Space Station budget of 1 3 10-14gcm-2s-1(daily average) (5).7. Defining Molecular Contamination7.1
39、The process termed outgassing is a combination ofevents (Fig. 4) including the solid state diffusion of moleculesto the surface, followed by desorption into the high-vacuumenvironment of space. When those molecules reach a sensitivesurface, either by line-of-sight or indirect (non-line-of-sight)tran
40、sport and deposit, the deposit is termed “molecular con-tamination.” At low altitudes atmospheric molecules some-times play a role in these processes by scattering or deflectingmolecular contamination.7.2 The definition of equivalent monomolecular layer(EML) of water on a surface (Fig. 5) is based o
41、n the concept ofFIG. 1 Examples of Spacecraft Component Degradation Due to ContaminationFIG. 2 Sources of Contamination and Transport MechanismsE2311043a uniform single layer of molecules, each 3 3 10-8cm indiameter, placed with centers on a square pattern. This resultsin an EML being defined as app
42、roximately 1 3 1015molecules/cm2. However, molecular deposits are not always formed asuniform films.7.3 Given, for instance, water with a gram molecular massof 18 g/mole and Avogadros number of 6 3 1023molecules/gmole, this results in 3 3 10-8g/EML or 3 3 10-8g/cm2.8. QCM Theory8.1 Crystal Frequency
43、:8.1.1 A piezoelectric quartz crystal (Fig. 6) is externallydriven by an electronic oscillator attached to two metal plates(usually deposited by vacuum evaporation) placed on bothsides of the quartz blank. This imposes a time dependentelectric field across the plate, which causes the crystal tooscil
44、late at a frequency determined by the total thickness of theFIG. 3 Typical Outgassing RatesFIG. 4 Outgassing Combination of Events from Atmospheric Molecules on External SurfacesE2311044crystal plus any mass on these electrodes. The oscillationappears as a Gaussian distribution of displacement, peak
45、ing atthe center and vanishing at the electrode edge. The frequencyof the surface motion decreases as a layer of contaminant isformed (mass addition), according to the degree to which eachelement is being displaced by the oscillation. The arriving ordeparting molecules (mass flux) are deposited or d
46、esorbedrandomly. Therefore, integrating the distribution of surfacedisplacements provides us with a valid sensitivity (mass flux tochange in frequency) for the quartz plate. Experimental con-firmation that the mass sensitivity of the plano-plano (p-p)crystal is as predicted by theory (3, 6-11) has b
47、een providedmany times.8.1.2 The resonant frequency of the QCM used is normally10 MHz, 15 MHz, or up to 200 MHz, depending on theapplication. The p-p piezoelectric quartz crystal is approxi-mately 1.27 cm (0.5 in.) in diameter and 0.0112 cm (0.0044 in.)in thickness for the 15 MHz crystal, or 0.0168
48、cm (0.0066 in.)in thickness for the 10 MHz crystal, which is, as already statedabove, set in vibration by an oscillation circuit that measuresthe frequency change as mass flux occurs. In the case of thehigher frequency QCMs, such as the 25 MHz sensor, thecrystal may be approximately 0.635 cm (0.25 i
49、n.) in diameter.The quartz plate electrode may have a different diameter on thetopmost surface than on the bottom because the a/e value foraluminum, which is commonly used as an electrode material,for irradiation from the sun is lower than for quartz. Electrodesof gold, platinum, and other metals are also often used.Aluminum is commonly chosen because of its low absorp-tance coefficient for solar radiation but gold resists the forma-tion of oxides between the lead wire and the gold electrodemaking it more reliable for long-term space use. The electrode
