A mechanism of degradation of carbon coating under extreme conditions has been discovered by Kazan Federal University and OFS.
The study was part of a project titled “Synthesis and study of a new class of nanocomposite ceramics with a degenerate dielectric constant for optoplasmonic applications” (financially supported by Russian Science Foundation); Professor Sergey Kharintsev was the project lead.
As Kharintsev stated, quartz multimode optical fibers are extensively utilized in telecommunications for data transmission, as distributed sensors — for quantifying pressure, temperature, mechanical stresses, as well as distributed acoustic and spectroscopic probes in chemically aggressive media at high temperatures and pressures, for instance, in oil exploration.
At high ambient temperatures and pressures, molecular hydrogen H2 and hydroxyl groups OH– can easily penetrate to the core of an optical fiber and enter into chemical reactions. This inevitably leads to an increase in optical loss and, consequently, to a deterioration in the transparency of the fiber.
Svetlana Saparina, Study Co-Author and PhD Student, Kazan Federal University
“To solve this problem, amorphous carbon is usually used, which effectively protects the optical fiber from moisture and hydrogen. The thickness of the carbon layer is several tens of nanometers, and microstrains of the fiber do not cause a significant deterioration in optical transparency,” added Saparina.
An analysis of amorphous carbon coatings led to the discovery of an irreversible increase in electrical resistance (up to 20%) at the time of repeated heating and cooling cycles occurring in the air.
The increase in resistance is due to the interaction of amorphous carbon with water molecules from the environment. Dissociation of water molecules into H+ and OH- ions and their interaction with edge defects of amorphous carbon lead to the formation of C-H and COOH / C-OH functional groups.
Sergey Kharintsev, Professor, Kazan Federal University
“Using Raman scattering, we discovered the formation of a C = O carbonyl group when heated above 80 degrees, which annihilates when cooled to room temperature. Thus, the decomposition of water into H + and OH- and their irreversible binding to edge defects lead to the enrichment of the carbon layer with various functional groups,” added Kharintsev.
“This mechanism leads to an increase in the electrical resistance of amorphous carbon. Defect-enriched carbon provides enhanced diffusion of hydrogen and moisture to the fiber core, making it less transparent,” continued Kharintsev.
The specimens required for the study were offered by OFS. The amorphous carbon coatings can also be utilized for small-scale moisture adsorbents and detectors, say the researchers.
Saparina, S. V., et al. (2021) Water enrichment/depletion of amorphous carbon coatings probed by temperature-dependent dc electrical conductivity and Raman scattering. Applied Surface Science. doi.org/10.1016/j.apsusc.2021.151052.