15.06.2021What is a Temperature controlled Quartz Crystal Microbalance (TQCM)?
A Temperature controlled Quartz Crystal Microbalance (TQCM) is a type of sensor which uses changes in the resonant frequency of a quartz crystal to determine the amount and rate at which mass is deposited onto the crystals surface. A common variation on this type of sensor is used for monitoring thin film deposition processes. With the addition of a reference crystal and with thermal control, the sensor can be used for monitoring the outgassing of hardware when subject to vacuum.
A vacuum bakeout is performed to accelerate the outgassing of volatile molecules from hardware in a controlled environment, this is done to prevent contamination of highly sensitive flight equipment during operation.
Why do we perform vacuum bakeout?
The purpose of the vacuum bakeout process is to accelerate the outgassing of volatile molecules from the equipment being baked-out. This is achieved by placing equipment into a thermal vacuum chamber under high vacuum and at elevated temperatures. The absence of atmospheric pressure and increases in the kinetic energy of particles due to heating, provide the correct conditions for volatile molecules to be emitted from the surface of the equipment.
With AEON’s expertise we have been able to design, build and commission our own thermal vacuum chamber dedicated to performing the bakeout process for flight hardware to ECSS standards (ECSS-Q-ST-70-01) Read more about this here . This knowledge and experience mean we are able to have greater control over the bakeout process by fine-tuning pressure and temperature profiles within the bakeout process, even to the extent of being able to preferentially increase or decrease the heating of equipment in different areas to accommodate especially sensitive equipment.
It is important to note that these volatile molecules are commonly found in both raw materials as well as any applied finished. The exact chemical composition will be dependant of the materials used, and will have different affects if not removed, but generally will have a negative impact on electrical sensors, optics, and cryogenic systems. Ultimately, the vacuum bakeout process assures that the end-product will function as required.
How does the TQCM work?
The quartz crystals used in a TQCM sensors operate on the same principals as quartz crystal oscillators used in all modern electronics to provide a highly stable timing signal. Modern crystals are typically small in size and encased in a protective package to preserve their long-term stability. In contrast the crystals used in a TQCM sensor are significantly larger in size to increase collecting area and are directly exposed to the environment to monitor the bakeout.
The fundamental resonant frequency of the quartz crystal is dependent on how it is manufactured, with frequencies between 30 kHz and 100 MHz. When exposed to environmental conditions the actual resonant frequency will be affected by changes in pressure, temperature, and mass.
Since a TQCM sensor is designed to detect changes in mass deposited onto the crystal, the effects of pressure and temperature need to be removed so an accurate measurement of the changes due to deposited mass can be taken. This poses a difficult problem if only a single crystal is used inside the thermal vacuum chamber as the change in resonant frequency due to the vacuum and elevated temperature would make the change due to deposited mass impossible to isolate.
The solution to this is to introduce a second matched reference quartz crystal that operates identically to the first, however is placed in an enclosure within the vacuum chamber that allows the crystal to operate at the same vacuum and temperature conditions as the first but is shielded from any deposited mass. Through analysis of the frequency response of the two crystals it is possible to isolate the effect of deposited mass from any environmental factors.
Fundamental to the monitoring of the bakeout process is the knowledge of when the rate of outgassing of volatile molecules has reduced to an acceptably low level. This can be determined directly by observing the rate of change of frequency over a period of time.
To aid in the volatile molecules depositing onto the exposed crystals surface, the TQCM sensor is cooled to -20°C. This quickly removes kinetic energy from the molecules when they contact the crystals surface, improving the likelihood that they ‘stick’ to the surface and add to the crystals mass.
AEON’s TQCM sensors have been designed from fundamentals to ensure measurement accuracy and reliability. Having a modular design also means we have the flexibility to position and orient the TQCM sensor within the vacuum chamber. Therefore, if required a TQCM sensor could be placed near a critical location on the equipment being baked out. Alternatively, a novel solution using multiple TQCM sensors could be implemented to locally monitor outgassing at multiple locations along a piece of equipment. This information can then be fed back into the thermal vacuum chamber control system to increase or decrease heating in a particular region to ensure uniform and complete bakeout of the equipment to a level previously not possible.