The aim of this project is to explore the use of Atomic Oxygen (AO) — referred to hereafter as MOXY (MOnatomic OXygen) — in the restoration of cultural heritage artifacts. Atomic oxygen (denoted as O or O₁) is highly reactive due to its unpaired electron, making it prone to quickly bonding with nearby molecules. This reactivity can be utilized to remove built-up dirt from the surfaces of ancient artworks without damaging the original materials.
KPV’s role in the MOXY project is to develop a device capable of evaluating AO flow. Since direct spectrometric analysis of AO is complex and no suitable direct AO sensors currently exist, an indirect sensing approach is proposed. This involves observing how materials similar to surface pollutants found on artworks respond to AO exposure. The concept is to use a resonant quartz crystal microbalance (QCM) sensor, pre-coated with a specialized layer, to detect mass loss caused by AO interaction.
After a thorough analysis, a suitable sensor type was identified: a quartz crystal resonator used in deposition rate and thickness monitors. These sensors are capable of detecting thickness changes on the order of 1 Ångström when using sufficiently rigid materials. The sensor consists of a gold-coated quartz crystal (~14 mm in diameter) that operates in the thickness shear mode, with optimal resonance occurring in the central area.
To adapt this type of sensor for MOXY evaluation, several challenges must be addressed:
- Coating Material Selection:
- Identify a coating material for the quartz crystal that is sensitive to MOXY.
- The material should have good mechanical and chemical stability.
- The reaction rate with AO must be slow enough to ensure sensor longevity.
- Selective Sensitivity
- Ideally, a second layer with different sensitivity to MOXY/plasma/ozone should be added.
- A dual-layer sensor could improve selectivity and distinguish MOXY from other species.
- Sensor Housing and Product Removal:
- MOXY has a short lifetime and limited diffusion range.
- The sensor housing must ensure close proximity and effective exposure to MOXY.
- Efficient removal of reaction byproducts must also be ensured.
- Mechanical Influence from Gas Flow:
- The gas flow may exert force on the crystal, affecting its resonant frequency.
- The sensor holder design and data processing algorithms must compensate for this effect.
- Sensor Usability Criteria:
- Reliable criteria must be established for determining when a sensor is suitable for use or needs replacement.
- These criteria should be integrated into the device’s control system.
- Layer Configuration and Multi-Resonance Measurement:
- Due to MOXY’s short effective range, using two different sensors simultaneously is problematic.
- Manual sensor exchange is undesirable due to reliability and usability issues.
- Ideally, both layers would be applied to the same crystal, producing distinct local resonances.
- This would require the layers to have excellent mechanical properties (especially hardness) and a carefully engineered configuration.
- If successful, two separate resonant frequencies could be detected, each corresponding to a different layer.
To enable this, significant modifications to the device’s electronic circuitry and data processing algorithms are required to distinguish between multiple resonances and estimate the thickness of each coating layer individually. An alternative approach — using two separate back electrodes on the quartz crystal instead of one — may also be investigated.
