ASTM E2311-04(2009)
(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 E595 and E1559), 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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Designation: E2311 − 04(Reapproved 2009)
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 MIL-S-45743 Soldering, Manual Type, High Reliability
Electrical and Electronic Equipment
1.1 This practice provides guidance for making decisions
FED-STD-209EAirborne Particulate Cleanliness Classes in
concerningtheuseofaquartzcrystalmicrobalance(QCM)and
Cleanrooms and Clean Zones
a thermoelectrically cooled quartz crystal microbalance
(TQCM)inspacewherecontaminationproblemsonspacecraft
NOTE 1—Although FED-STD-209E has been cancelled, it still may be
used and designations in FED-STD-209E may be used in addition to the
are likely to exist. Careful adherence to this document should
ISO designations.
ensure adequate measurement of condensation of molecular
constituents that are commonly termed “contamination” on 2.3 ISO Standards:
spacecraft surfaces. ISO 14644-1 Cleanrooms and Associated Controlled
Environments—Part 1: Classification of Air Cleanliness
1.2 A corollary purpose is to provide choices among the
ISO 14644-2 Cleanrooms and Associated Controlled
flight-qualified QCMs now existing to meet specific needs.
Environments—Part 2: Specifications for Testing and
1.3 The values stated in SI units are to be regarded as the
Monitoring to Prove Continued Compliance with ISO
standard. The values given in parentheses are for information
14644-1
only.
3. Terminology
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the 3.1 Definitions:
responsibility of the user of this standard to establish appro- 3.1.1 absorptance, α,n—ratio of the absorbed radiant or
priate safety and health practices and determine the applica- luminous flux to the incident flux.
bility of regulatory limitations prior to use.
3.1.2 activity coeffıcient of crystal, Q, n—energy stored
during a cycle divided by energy lost during a cycle, or the
2. Referenced Documents
quality factor of a crystal.
2.1 ASTM Standards:
3.1.3 crystallographic cut,Φ,n—rotationanglebetweenthe
E595Test Method for Total Mass Loss and Collected Vola-
optical axis and the plane of the crystal at which the quartz is
tile Condensable Materials from Outgassing in a Vacuum
cut;typically35°18'ATcutforambienttemperatureuseor39°
Environment
40' cut for cryogenic temperature use.
E1559Test Method for Contamination Outgassing Charac-
3.1.4 collected volatile condensable materials, (CVCM),
teristics of Spacecraft Materials
n—tested per Test Method E595.
2.2 U.S. Federal Standards:
3.1.5 equivalent monomolecular layer, (EML), n—single
MIL-STD-883Standard Test Method, Microcircuits -8
layer of molecules, each3×10 cm in diameter, placed with
centers on a square pattern. This results in an EML of
15 2
approximately1×10 molecules/cm .
This practice is under the jurisdiction of ASTM Committee E21 on Space
3.1.6 field of view, (FOV), n—the line of sight from the
Simulation andApplications of SpaceTechnology and is the direct responsibility of
surface of the QCM that is directly exposed to mass flux.
Subcommittee E21.05 on Contamination.
Current edition approved Nov. 1, 2009. Published December 2009. Originally
3.1.7 irradiance at a point on a surface, n—E , E(E =
e e
approved in 2004. Last previous edition approved in 2004 as E2311–04. DOI:
-2
dI /dA),(wattpersquaremetre,Wm ),ratiooftheradiantflux
e
10.1520/E2311-04R09.
incident on an element of the surface containing the point, to
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
the area of that element.
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
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E2311 − 04 (2009)
3.1.8 mass sensitivity, S, n—relationship between the fre- depend upon such factors as knowing its contamination source
quency shift and the arriving or departing mass on the sensing andtheapproximatelevelofoutgassing,theabilityorinability
crystal of a QCM. As defined by theory: to place a sensor in the immediate area of concern, the
variation of the solar thermal radiation striking the sensor, the
∆m/A 5 ρ c/2f ∆f (1)
~ !
q
powerdissipationoftheQCMandhowitaffectscertaincritical
where:
spacecraft cooling requirements, cost to the program, and the
schedule.Therefore,itisnotdesirableorpossibletoincludeall
∆m = mass change, g,
QCMtestinginonetestmethod.Theengineersmustdetermine
A = area on which the deposit occurs, cm ,
f = fundamental frequency of the QCM, Hz, and provide the detailed monitoring procedure that will satisfy
ρ = density of quartz, g/cm , and
their particular requirements and be fully aware of the effects
q
c = shear wave velocity of quartz, cm/s.
of any necessary deviations from the ideal.
3.1.9 molecular contamination, n—molecules that remain
5. Significance and Use
on a surface with sufficiently long residence times to affect the
5.1 Spacecraft have consistently had the problem of con-
surface properties to a sensible degree.
taminationofthermalcontrolsurfacesfromline-of-sightwarm
3.1.10 optical polish, n—the topology of the quartz crystal
surfacesonthevehicle,outgassingofmaterialsandsubsequent
surface as it affects its light reflective properties, for example,
condensation on critical surfaces, such as solar arrays, moving
specular (sometimes called “clear polish”) or diffuse polish.
mechanical assemblies, cryogenic insulation schemes, and
3.1.11 optical solar reflector, (OSR), n—a term used to
electrical contacts, control jet effects, and other forms of
designate thermal control surfaces on a spacecraft incorporat-
expelling molecules in a vapor stream. To this has been added
ing second surface mirrors.
the need to protect optical components, either at ambient or
cryogenic temperatures, from the minutest deposition of con-
3.1.12 quartz crystal microbalance (QCM), n—a piezoelec-
taminants because of their absorptance, reflectance or scatter-
tric quartz crystal that is driven by an external electronic
ing characteristics. Much progress has been accomplished in
oscillator whose frequency is determined by the total crystal
this area, such as the careful testing of each material for
thickness plus the mass deposited on the crystal surface.
outgassing characteristics before the material is used on the
3.1.13 reflectance, ρ,n—ratio of the reflected radiant or
spacecraft (following Test Methods E595 and E1559), but
luminous flux to the incident flux.
measurementandcontrolofcriticalsurfacesduringspaceflight
3.1.14 surface of interest, n—any immediate surface on
still can aid in the determination of location and behavior of
which contamination can be formed.
outgassing materials.
3.1.15 super-polish, n—polish of a quartz crystal that pro-
6. General Considerations
duces less than 10Å root mean square (rms) roughness on the
6.1 A QCM sensor is used to measure the molecular
surface.
contamination of critical surfaces on spacecraft at one or more
3.1.16 QCM thermogravimetric analysis, (QTGA),
temperatures for an extended period of time. A piezoelectric
n—raising the temperature of the QCM deposition surface
crystalisexposednexttoa“surfaceofinterest”orintheplane
causes contaminants to evaporate, changing the QCM fre-
where molecular flux is expected. It is then cooled to the
quencyasafunctionoftimeandthemassloss.Relevantvapor
temperature at which the crystal should condense whatever
pressurescanbecalculatedforvariousspeciesandcanbeused
molecular contaminant exists at that temperature (according to
to identify the molecular species.
the vapor-pressure characteristics of that constituent). By
3.1.17 total mass loss, (TML), n—when tested per Test
measuring the frequency-shift of the crystal and knowing the
Method E595.
mass sensitivity (frequency to mass-added factor for that
3.1.18 thermoelectric quartz crystal microbalance,
crystal), the mass accumulated can be determined. Sunlight
(TQCM), n—The temperature of the crystal is controlled with
striking the solar panels may cause outgassing that intercepts
a thermoelectric element so that the rate of deposition and the the surface of interest. The probable source and extent of
species that condense onto the QCM can be related to the
contamination can be determined from known components of
temperature. the spacecraft and probable sources.
3.2 Constants: 6.2 PotentialcontaminationproblemareasareshowninFig.
3 5
1.
3.2.1 densityofquartz—atT=25°C,ρ =2.6485g/cm (1) ;
q
at T=77K, ρ = 2.664 g/cm (2). 6.2.1 The performance of thermal control surfaces is de-
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;
6.2.4 Internal to the spacecraft there may be outgassing
sources which will degrade (for instance, mass spectrometer
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof
this practice. causing signal overload conditions);
E2311 − 04 (2009)
FIG. 1 Examples of Spacecraft Component Degradation Due to Contamination
FIG. 2 Sources of Contamination and Transport Mechanisms
6.2.5 Windows and optical elements may be degraded by uncertain,thatis,backscatteringofoutgassedmoleculesdueto
adsorption of a contaminant film leading to a loss of atmospheric gas collisions, influence of free oxygen and
transmittance,reflectance,oranincreaseinscatteredlight;and
charged particles as they impact the spacecraft surface.
6.2.6 Solar arrays are adversely affected by the absorptance
6.4 Sometypicalspacecraftoutgassingratesandtheexperi-
of contaminants.
mental determination of the resolution of QCMs are shown in
6.3 Some of the sources of contamination and mechanisms
Fig. 3. Some actual deposition rate conditions on a spacecraft
-12 -2 -1
for transporting them are shown in Fig. 2. Pre-launch, vacuum
have been observed to be 1.2 × 10 gcm s for a sunlit
-13 -2 -1
test-induced contamination remains a problem as well as
vent-viewing OSR (4),2×10 gcm s for a mature large
-14
launch-induced contaminants. High-angle plume impingement
satellite (4), and a projected Space Station budget of1×10
-2 -1
from spacecraft orientation thrusters, as well as multi-layer
gcm s (daily average) (5).
insulation surrounding cryogenic surfaces, are also sources of
contamination. Frequently, the largest long-term sources are
7. Defining Molecular Contamination
warm,relativelythick,non-metallicmaterialsofthespacecraft
construction. High vapor pressure (low molecular mass) mol- 7.1 The process termed outgassing is a combination of
events (Fig. 4) including the solid state diffusion of molecules
eculesmayphotopolymerizeonsurfacestobecomelowvapor
pressure (high molecular mass) stable contaminants. Vapor to the surface, followed by desorption into the high-vacuum
pressure-controlled self-contamination needs to be in the environment of space.When those molecules reach a sensitive
design engineer’s mind; however, some parameters are still surface, either by line-of-sight or indirect (non-line-of-sight)
E2311 − 04 (2009)
FIG. 3 Typical Outgassing Rates
FIG. 4 Outgassing Combination of Events from Atmospheric Molecules on External Surfaces
transport and deposit, the deposit is termed “molecular con- 7.3 Given, for instance, water with a gram molecular mass
tamination.” At low altitudes atmospheric molecules some- of 18 g/mole andAvogadro’s number of6×10 molecules/g
-8 -8 2
times play a role in these processes by scattering or deflecting mole, this results in3×10 g/EML or3×10 g/cm .
molecular contamination.
8. QCM Theory
7.2 The definition of equivalent monomolecular layer
(EML)ofwateronasurface(Fig.5)isbasedontheconceptof 8.1 Crystal Frequency:
-8
a uniform single layer of molecules, each3×10 cm in 8.1.1 A piezoelectric quartz crystal (Fig. 6) is externally
diameter, placed with centers on a square pattern. This results driven by an electronic oscillator attached to two metal plates
inanEMLbeingdefinedasapproximately1×10 molecules/ (usually deposited by vacuum evaporation) placed on both
cm . However, molecular deposits are not always formed as sides of the quartz blank. This imposes a time dependent
uniform films. electric field across the plate, which causes the crystal to
E2311 − 04 (2009)
FIG. 5 Equivalent Monomolecular Layer (EML)
oscillateatafrequencydeterminedbythetotalthicknessofthe above, set in vibration by an oscillation circuit that measures
crystal plus any mass on these electrodes. The oscillation the frequency change as mass flux occurs. In the case of the
appears as a Gaussian distribution of displacement, peaking at higher frequency QCMs, such as the 25 MHz sensor, the
the center and vanishing at the electrode edge. The frequency crystal may be approximately 0.635 cm (0.25 in.) in diameter.
of the surface motion decreases as a layer of contaminant is Thequartzplateelectrodemayhaveadifferentdiameteronthe
formed (mass addition), according to the degree to which each topmost surface than on the bottom because the α/ε value for
element is being displaced by the oscillation. The arriving or aluminum, which is commonly used as an electrode material,
departing molecules (mass flux) are deposited or desorbed forirradiationfromthesunislowerthanforquartz.Electrodes
randomly. Therefore, integrating the distribution of surface of gold, platinum, and other metals are also often used.
displacementsprovidesuswithavalidsensitivity(massfluxto Aluminum is commonly chosen because of it’s low absorp-
change in frequency) for the quartz plate. Experimental con- tance coefficient for solar radiation but gold resists the forma-
firmat
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