ASTM E2311-04
(Practice)Standard Practice for QCM Measurement of Spacecraft Molecular Contamination in Space
Standard Practice for QCM Measurement of Spacecraft Molecular Contamination in Space
SIGNIFICANCE AND USE
Spacecraft have consistently had the problem of contamination of thermal control surfaces from line-of-sight warm surfaces on the vehicle, outgassing of materials and subsequent condensation on critical surfaces, such as solar arrays, moving mechanical assemblies, cryogenic insulation schemes, and electrical contacts, control jet effects, and other forms of expelling molecules in a vapor stream. To this has been added the need to protect optical components, either at ambient or cryogenic temperatures, from the minutest deposition of contaminants because of their absorptance, reflectance or scattering characteristics. Much progress has been accomplished in this area, such as the careful testing of each material for outgassing characteristics before the material is used on the spacecraft (following Test Methods E 595 and E 1559), but measurement and control of critical surfaces during spaceflight still can aid in the determination of location and behavior of outgassing materials.
SCOPE
1.1 This practice provides guidance for making decisions concerning the use of a quartz crystal microbalance (QCM) and a thermoelectrically cooled quartz crystal microbalance (TQCM) in space where contamination problems on spacecraft are likely to exist. Careful adherence to this document should ensure adequate measurement of condensation of molecular constituents that are commonly termed "contamination" on spacecraft surfaces.
1.2 A corollary purpose is to provide choices among the flight-qualified QCMs now existing to meet specific needs.
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Standards Content (Sample)
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Designation: E2311 – 04
Standard Practice for
QCM Measurement of Spacecraft Molecular Contamination
in Space
This standard is issued under the fixed designation E2311; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope FED-STD-209E AirborneParticulateCleanlinessClassesin
Cleanrooms and Clean Zones
1.1 This practice provides guidance for making decisions
concerningtheuseofaquartzcrystalmicrobalance(QCM)and
NOTE 1—Although FED-STD-209E has been cancelled, it still may be
a thermoelectrically cooled quartz crystal microbalance
used and designations in FED-STD-209E may be used in addition to the
ISO designations.
(TQCM)inspacewherecontaminationproblemsonspacecraft
are likely to exist. Careful adherence to this document should
2.3 ISO Standards:
ensure adequate measurement of condensation of molecular
ISO 14644-1 Cleanrooms and Associated Controlled
constituents that are commonly termed “contamination” on
Environments—Part 1: Classification of Air Cleanliness
spacecraft surfaces.
ISO 14644-2 Cleanrooms and Associated Controlled
1.2 A corollary purpose is to provide choices among the
Environments—Part 2: Specifications for Testing and
flight-qualified QCMs now existing to meet specific needs.
Monitoring to Prove Continued Compliance with ISO
1.3 The values stated in SI units are to be regarded as the
14644-1
standard. The values given in parentheses are for information
3. Terminology
only.
1.4 This standard does not purport to address all of the
3.1 Definitions:
safety concerns, if any, associated with its use. It is the
3.1.1 absorptance, a, n—ratio of the absorbed radiant or
responsibility of the user of this standard to establish appro-
luminous flux to the incident flux.
priate safety and health practices and determine the applica-
3.1.2 activity coeffıcient of crystal, Q, n—energy stored
bility of regulatory limitations prior to use.
during a cycle divided by energy lost during a cycle, or the
quality factor of a crystal.
2. Referenced Documents
3.1.3 crystallographic cut, F, n—rotation angle between
2.1 ASTM Standards:
the optical axis and the plane of the crystal at which the quartz
E595 Test Method for Total Mass Loss and Collected
is cut; typically 35° 188ATcut for ambient temperature use or
Volatile Condensable Materials from Outgassing in a
39° 408 cut for cryogenic temperature use.
Vacuum Environment
3.1.4 collected volatile condensable materials, (CVCM),
E1559 Test Method for Contamination Outgassing Charac-
n—tested per Test Method E595.
teristics of Spacecraft Materials
3.1.5 equivalent monomolecular layer, (EML), n—single
-8
2.2 U.S. Federal Standards:
layer of molecules, each 3 3 10 cm in diameter, placed with
MIL-STD-883 Standard Test Method, Microcircuits
centers on a square pattern. This results in an EML of
15 2
MIL-S-45743 Soldering, Manual Type, High Reliability
approximately 1 3 10 molecules/cm .
Electrical and Electronic Equipment
3.1.6 field of view, (FOV), n—the line of sight from the
surface of the QCM that is directly exposed to mass flux.
3.1.7 irradiance at a point on a surface, n—E , E (E =
e e
-2
1 dI /dA),(wattpersquaremetre,Wm ),ratiooftheradiantflux
e
This practice is under the jurisdiction of ASTM Committee E21 on Space
incident on an element of the surface containing the point, to
SimulationandApplicationsofSpaceTechnologyandisthedirectresponsibilityof
Subcommittee E21. 05 on Contamination.
the area of that element.
Current edition approved Sept. 1, 2004. Published September 2004. DOI:
3.1.8 mass sensitivity, S, n—relationship between the fre-
10.1520/E2311-04.
quency shift and the arriving or departing mass on the sensing
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM crystal of a QCM. As defined by theory:
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
3 4
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E2311 – 04
QCMtestinginonetestmethod.Theengineersmustdetermine
Dm/ A 5 ~r c/2f ! Df (1)
q
and provide the detailed monitoring procedure that will satisfy
where:
their particular requirements and be fully aware of the effects
Dm = mass change, g,
of any necessary deviations from the ideal.
A = area on which the deposit occurs, cm ,
f = fundamental frequency of the QCM, Hz,
5. Significance and Use
r = density of quartz, g/cm , and
q
5.1 Spacecraft have consistently had the problem of con-
c = shear wave velocity of quartz, cm/s.
taminationofthermalcontrolsurfacesfromline-of-sightwarm
3.1.9 molecular contamination, n—molecules that remain
surfacesonthevehicle,outgassingofmaterialsandsubsequent
on a surface with sufficiently long residence times to affect the
condensation on critical surfaces, such as solar arrays, moving
surface properties to a sensible degree.
mechanical assemblies, cryogenic insulation schemes, and
3.1.10 optical polish, n—the topology of the quartz crystal
electrical contacts, control jet effects, and other forms of
surface as it affects its light reflective properties, for example,
expelling molecules in a vapor stream. To this has been added
specular (sometimes called “clear polish”) or diffuse polish.
the need to protect optical components, either at ambient or
3.1.11 optical solar reflector, (OSR), n—a term used to
cryogenic temperatures, from the minutest deposition of con-
designate thermal control surfaces on a spacecraft incorporat-
taminants because of their absorptance, reflectance or scatter-
ing second surface mirrors.
ing characteristics. Much progress has been accomplished in
3.1.12 quartz crystal microbalance (QCM), n—apiezoelec-
this area, such as the careful testing of each material for
tric quartz crystal that is driven by an external electronic
outgassing characteristics before the material is used on the
oscillator whose frequency is determined by the total crystal
spacecraft (following Test Methods E595 and E1559), but
thickness plus the mass deposited on the crystal surface.
measurementandcontrolofcriticalsurfacesduringspaceflight
3.1.13 reflectance, r, n—ratio of the reflected radiant or
still can aid in the determination of location and behavior of
luminous flux to the incident flux.
outgassing materials.
3.1.14 surface of interest, n—any immediate surface on
which contamination can be formed.
6. General Considerations
3.1.15 super-polish, n—polish of a quartz crystal that pro-
6.1 A QCM sensor is used to measure the molecular
duces less than 10Å root mean square (rms) roughness on the
contamination of critical surfaces on spacecraft at one or more
surface.
temperatures for an extended period of time. A piezoelectric
3.1.16 QCM thermogravimetric analysis, (QTGA),
crystalisexposednexttoa“surfaceofinterest”orintheplane
n—raising the temperature of the QCM deposition surface
where molecular flux is expected. It is then cooled to the
causes contaminants to evaporate, changing the QCM fre-
temperature at which the crystal should condense whatever
quencyasafunctionoftimeandthemassloss.Relevantvapor
molecular contaminant exists at that temperature (according to
pressurescanbecalculatedforvariousspeciesandcanbeused
the vapor-pressure characteristics of that constituent). By
to identify the molecular species.
measuring the frequency-shift of the crystal and knowing the
3.1.17 total mass loss, (TML), n—when tested per Test
mass sensitivity (frequency to mass-added factor for that
Method E595.
crystal), the mass accumulated can be determined. Sunlight
3.1.18 thermoelectric quartz crystal microbalance,
striking the solar panels may cause outgassing that intercepts
(TQCM), n—The temperature of the crystal is controlled with
the surface of interest. The probable source and extent of
a thermoelectric element so that the rate of deposition and the
contamination can be determined from known components of
species that condense onto the QCM can be related to the
the spacecraft and probable sources.
temperature.
6.2 PotentialcontaminationproblemareasareshowninFig.
3.2 Constants:
1.
3.2.1 density of quartz—at T = 25°C, r = 2.6485 g/cm
q
6.2.1 The performance of thermal control surfaces is de-
5 3
(1);at T=77K, r = 2.664 g/cm (2).
q
graded as a result of the accumulation of contaminants, which
3.2.2 mass sensitivity—AT or rotated cut crystal (3).
may increase the surfaces’ solar absorptance;
6.2.2 Optics may be degraded by increasing “light” scatter-
4. Summary of Practice
ing or reflectance loss;
4.1 Measurement of molecular contamination on spacecraft
6.2.3 Electronic modules with high rates of outgassing
can be performed in a variety of ways. The specific methods
components may have voltage arc-over;
depend upon such factors as knowing its contamination source
6.2.4 Internal to the spacecraft there may be outgassing
andtheapproximatelevelofoutgassing,theabilityorinability
sources which will degrade (for instance, mass spectrometer
to place a sensor in the immediate area of concern, the
causing signal overload conditions);
variation of the solar thermal radiation striking the sensor, the
6.2.5 Windows and optical elements may be degraded by
powerdissipationoftheQCMandhowitaffectscertaincritical
adsorption of a contaminant film leading to a loss of transmit-
spacecraft cooling requirements, cost to the program, and the
tance, reflectance, or an increase in scattered light; and
schedule.Therefore,itisnotdesirableorpossibletoincludeall
6.2.6 Solar arrays are adversely affected by the absorptance
of contaminants.
6.3 Some of the sources of contamination and mechanisms
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice. for transporting them are shown in Fig. 2. Pre-launch, vacuum
E2311 – 04
FIG. 1 Examples of Spacecraft Component Degradation Due to Contamination
FIG. 2 Sources of Contamination and Transport Mechanisms
-13 -2 -1
test-induced contamination remains a problem as well as vent-viewing OSR (4),2 3 10 gcm s for a mature large
-14
launch-induced contaminants. High-angle plume impingement satellite (4), and a projected Space Station budget of 1 3 10
-2 -1
from spacecraft orientation thrusters, as well as multi-layer gcm s (daily average) (5).
insulation surrounding cryogenic surfaces, are also sources of
7. Defining Molecular Contamination
contamination. Frequently, the largest long-term sources are
warm,relativelythick,non-metallicmaterialsofthespacecraft 7.1 The process termed outgassing is a combination of
construction. High vapor pressure (low molecular mass) mol- events (Fig. 4) including the solid state diffusion of molecules
eculesmayphotopolymerizeonsurfacestobecomelowvapor to the surface, followed by desorption into the high-vacuum
pressure (high molecular mass) stable contaminants. Vapor environment of space.When those molecules reach a sensitive
pressure-controlled self-contamination needs to be in the surface, either by line-of-sight or indirect (non-line-of-sight)
design engineer’s mind; however, some parameters are still transport and deposit, the deposit is termed “molecular con-
uncertain,thatis,backscatteringofoutgassedmoleculesdueto tamination.” At low altitudes atmospheric molecules some-
atmospheric gas collisions, influence of free oxygen and times play a role in these processes by scattering or deflecting
charged particles as they impact the spacecraft surface. molecular contamination.
6.4 Sometypicalspacecraftoutgassingratesandtheexperi- 7.2 The definition of equivalent monomolecular layer
mental determination of the resolution of QCMs are shown in (EML)ofwateronasurface(Fig.5)isbasedontheconceptof
-8
Fig. 3. Some actual deposition rate conditions on a spacecraft a uniform single layer of molecules, each 3 3 10 cm in
-12 -2 -1
have been observed to be 1.2 3 10 gcm s for a sunlit diameter, placed with centers on a square pattern. This results
E2311 – 04
FIG. 3 Typical Outgassing Rates
FIG. 4 Outgassing Combination of Events from Atmospheric Molecules on External Surfaces
inanEMLbeingdefinedasapproximately1 310 molecules/ 8.1.1 A piezoelectric quartz crystal (Fig. 6) is externally
cm . However, molecular deposits are not always formed as driven by an electronic oscillator attached to two metal plates
uniform films. (usually deposited by vacuum evaporation) placed on both
7.3 Given, for instance, water with a gram molecular mass sides of the quartz blank. This imposes a time dependent
of 18 g/mole andAvogadro’s number of 6 3 10 molecules/g electric field across the plate, which causes the crystal to
-8 -8 2
mole, this results in 3 3 10 g/EML or 3 3 10 g/cm . oscillateatafrequencydeterminedbythetotalthicknessofthe
crystal plus any mass on these electrodes. The oscillation
8. QCM Theory
appears as a Gaussian distribution of displacement, peaking at
8.1 Crystal Frequency: the center and vanishing at the electrode edge. The frequency
E2311 – 04
FIG. 5 Equivalent Monomolecular Layer (EML)
of the surface motion decreases as a layer of contaminant is crystal may be approximately 0.635 cm (0.25 in.) in diameter.
formed (mass addition), according to the degree to which each Thequartzplateelectrodemayhaveadifferentdiameteronthe
element is being displaced by the oscillation. The arriving or topmost surface than on the bottom because the a/´ value for
departing molecules (mass flux) are deposited or desorbed aluminum, which is commonly used as an electrode material,
randomly. Therefore, integrating the distribution of surface forirradiationfromthesunislowerthanforquartz.Electrodes
displacementsprovidesuswithavalidsensitivity(massfluxto of gold, platinum, and other metals are also often used.
change in frequency) for the quartz plate. Experimental con- Aluminum is commonly chosen because of it’s low absorp-
firmation that the mass sensitivity of the plano-plano (p-p) tance coefficient for solar radiation but gold resists the forma-
crystal is as predicted by theory (3, 6-11) has been provided tion of oxides between the lead wire and the gold electrode
many times. makingitmorereliableforlong-termspaceuse.Theelectrode-
8.1.2 The reso
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