ASTM E2311-04(2016)

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E2311 − 04 (Reapproved 2009) E2311 − 04 (Reapproved 2016)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. 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.
2. Referenced Documents
2.1 ASTM Standards:
E595 Test Method for Total Mass Loss and Collected Volatile Condensable Materials from Outgassing in a Vacuum
Environment
E1559 Test Method for Contamination Outgassing Characteristics of Spacecraft Materials
2.2 U.S. Federal Standards:
MIL-STD-883 Standard Test Method, Microcircuits
MIL-S-45743 Soldering, Manual Type, High Reliability Electrical and Electronic Equipment
FED-STD-209E Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones
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 ISO
designations.
2.3 ISO Standards:
ISO 14644-1 Cleanrooms and Associated Controlled Environments—Part 1: Classification of Air Cleanliness
ISO 14644-2 Cleanrooms and Associated Controlled Environments—Part 2: Specifications for Testing and Monitoring to Prove
Continued Compliance with ISO 14644-1
3. Terminology
3.1 Definitions:
3.1.1 absorptance, α, n—ratio of the absorbed radiant or luminous flux to the incident flux.
3.1.2 activity coeffıcient of crystal, Q, n—energy stored during a cycle divided by energy lost during a cycle, or the quality factor
of a crystal.
This practice is under the jurisdiction of ASTM Committee E21 on Space Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.05 on Contamination.
Current edition approved Nov. 1, 2009April 1, 2016. Published December 2009April 2016. Originally approved in 2004. Last previous edition approved in 20042009 as
E2311–04.– 04 (2009). DOI: 10.1520/E2311-04R09.10.1520/E2311-04R16.
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 Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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E2311 − 04 (2016)
3.1.3 crystallographic cut, Φ, n—rotation angle between the optical axis and the plane of the crystal at which the quartz is cut;
typically 35° 18' AT cut for ambient temperature use or 39° 40' cut for cryogenic temperature use.
3.1.4 collected volatile condensable materials, (CVCM), n—tested per Test Method E595.
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3.1.5 equivalent monomolecular layer, (EML), n—single layer of molecules, each 3 × 10 cm in diameter, placed with centers
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on a square pattern. This results in an EML of approximately 1 × 10 molecules/cm .
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.
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3.1.7 irradiance at a point on a surface, n—E ,E(E = dI /dA), (watt per square metre, Wm ), ratio of the radiant flux incident
e e e
on an element of the surface containing the point, to the area of that element.
3.1.8 mass sensitivity, S, n—relationship between the frequency shift and the arriving or departing mass on the sensing crystal
of a QCM. As defined by theory:
Δm/A 5 ~ρ c/2f ! Δf (1)
q
where:
Δm = mass change, g,
A = area on which the deposit occurs, cm ,
f = fundamental frequency of the QCM, Hz,
ρ = density of quartz, g/cm , and
q
c = shear wave velocity of quartz, cm/s.
3.1.9 molecular contamination, n—molecules that remain on a surface with sufficiently long residence times to affect the surface
properties to a sensible degree.
3.1.10 optical polish, n—the topology of the quartz crystal surface as it affects its light reflective properties, for example,
specular (sometimes called “clear polish”) or diffuse polish.
3.1.11 optical solar reflector, (OSR), n—a term used to designate thermal control surfaces on a spacecraft incorporating second
surface mirrors.
3.1.12 quartz crystal microbalance (QCM), n—a piezoelectric quartz crystal that is driven by an external electronic oscillator
whose frequency is determined by the total crystal thickness plus the mass deposited on the crystal surface.
3.1.13 reflectance, ρ, n—ratio of the reflected radiant or luminous flux to the incident flux.
3.1.14 surface of interest, n—any immediate surface on which contamination can be formed.
3.1.15 super-polish, n—polish of a quartz crystal that produces less than 10Å root mean square (rms) roughness on the surface.
3.1.16 QCM thermogravimetric analysis, (QTGA), n—raising the temperature of the QCM deposition surface causes
contaminants to evaporate, changing the QCM frequency as a function of time and the mass loss. Relevant vapor pressures can
be calculated for various species and can be used to identify the molecular species.
3.1.17 total mass loss, (TML), n—when tested per Test Method E595.
3.1.18 thermoelectric quartz crystal microbalance, (TQCM), n—The temperature of the crystal is controlled with a
thermoelectric element so that the rate of deposition and the species that condense onto the QCM can be related to the temperature.
3.2 Constants:
3 5 3
3.2.1 density of quartz—at T = 25°C, ρ = 2.6485 g/cm (1) ; at T = 77 K, ρ = 2.664 g/cm (2).
q q
3.2.2 mass sensitivity—AT or rotated cut crystal (3).
4. Summary of Practice
4.1 Measurement of molecular contamination on spacecraft can be performed in a variety of ways. The specific methods depend
upon such factors as knowing its contamination source and the approximate level of outgassing, the ability or inability to place
a sensor in the immediate area of concern, the variation of the solar thermal radiation striking the sensor, the power dissipation
of the QCM and how it affects certain critical spacecraft cooling requirements, cost to the program, and the schedule. Therefore,
it is not desirable or possible to include all QCM testing in one test method. The engineers must determine and provide the detailed
monitoring procedure that will satisfy their particular requirements and be fully aware of the effects of any necessary deviations
from the ideal.
5. Significance and Use
5.1 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
The boldface numbers in parentheses refer to the list of references at the end of this practice.
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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.
6. General Considerations
6.1 A QCM sensor is used to measure the molecular contamination of critical surfaces on spacecraft at one or more temperatures
for an extended period of time. A piezoelectric crystal is exposed next to a “surface of interest” or in the plane where molecular
flux is expected. It is then cooled to the temperature at which the crystal should condense whatever molecular contaminant exists
at that temperature (according to the vapor-pressure characteristics of that constituent). By measuring the frequency-shift of the
crystal and knowing the mass sensitivity (frequency to mass-added factor for that crystal), the mass accumulated can be
determined. Sunlight striking the solar panels may cause outgassing that intercepts the surface of interest. The probable source and
extent of contamination can be determined from known components of the 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 degraded as a result of the accumulation of contaminants, which may
increase the surfaces’ solar absorptance;
6.2.2 Optics may be degraded by increasing “light” scattering or reflectance loss;
6.2.3 Electronic modules with high rates of outgassing 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 causing
signal overload conditions);
6.2.5 Windows and optical elements may be degraded by adsorption of a contaminant film leading to a loss of transmittance,
reflectance, or an increase in scattered light; and
6.2.6 Solar arrays are adversely affected by the absorptance of contaminants.
6.3 Some of the sources of contamination and mechanisms for transporting them are shown in Fig. 2. Pre-launch, vacuum
test-induced contamination remains a problem as well as launch-induced contaminants. High-angle plume impingement from
spacecraft orientation thrusters, as well as multi-layer insulation surrounding cryogenic surfaces, are also sources of contamination.
Frequently, the largest long-term sources are warm, relatively thick, non-metallic materials of the spacecraft construction. High
vapor pressure (low molecular mass) molecules may photo polymerize on surfaces to become low vapor pressure (high molecular
mass) stable contaminants. Vapor pressure-controlled self-contamination needs to be in the design engineer’s mind; however, some
parameters are still uncertain, that is, back scattering of outgassed molecules due to atmospheric gas collisions, influence of free
oxygen and charged particles as they impact the spacecraft surface.
6.4 Some typical spacecraft outgassing rates and the experimental determination of the resolution of QCMs are shown in Fig.
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3. Some actual deposition rate conditions on a spacecraft have been observed to be 1.2 × 10 g cm s for a sunlit vent-viewing
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OSR (4), 2 × 10 g cm s for a mature large satellite (4), and a projected Space Station budget of 1 × 10 g cm s (daily
average) (5).
FIG. 1 Examples of Spacecraft Component Degradation Due to Contamination
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FIG. 2 Sources of Contamination and Transport Mechanisms
FIG. 3 Typical Outgassing Rates
7. Defining Molecular Contamination
7.1 The process termed outgassing is a combination of events (Fig. 4) including the solid state diffusion of molecules to the
surface, followed by desorption into the high-vacuum environment of space. When those molecules reach a sensitive surface, either
by line-of-sight or indirect (non-line-of-sight) transport and deposit, the deposit is termed “molecular contamination.” At low
altitudes atmospheric molecules sometimes play a role in these processes by scattering or deflecting molecular contamination.
7.2 The definition of equivalent monomolecular layer (EML) of water on a surface (Fig. 5) is based on the concept of a uniform
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single layer of molecules, each 3 × 10 cm in diameter, placed with centers on a square pattern. This results in an EML being
15 2
defined as approximately 1 × 10 molecules/cm . However, molecular deposits are not always formed as uniform films.
7.3 Given, for instance, water with a gram molecular mass of 18 g/mole and Avogadro’s number of 6 × 10 molecules/g mole,
-8 -8 2
this results in 3 × 10 g/EML or 3 × 10 g/cm .
8. QCM Theory
8.1 Crystal Frequency:
8.1.1 A piezoelectric quartz crystal (Fig. 6) is externally driven by an electronic oscillator attached to two metal plates (usually
deposited by vacuum evaporation) placed on both sides of the quartz blank. This imposes a time dependent electric field across
the plate, which causes the crystal to oscillate at a frequency determined by the total thickness of the crystal plus any mass on these
electrodes. The oscillation appears as a Gaussian distribution of displacement, peaking at the center and vanishing at the electrode
edge. The frequency of the surface motion decreases as a layer of contaminant is formed (mass addition), according to the degree
to which each element is being displaced by the oscillation. The arriving or departing molecules (mass flux) are deposited or
desorbed randomly. Therefore, integrating the distribution of surface displacements provides us with a valid sensitivity (mass flux
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FIG. 4 Outgassing Combination of Events from Atmospheric Molecules on External Surfaces
to change in frequency) for the quartz plate. Experimental confirmation that the mass sensitivity of the plano-plano (p-p) crystal
is as
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