Kazan Federal University and OFS found a mechanism of degradation of carbon coating under extreme conditions. The publications was part of a project titled 'Synthesis and study of a new class of nanocomposite ceramics with a degenerate dielectric constant for optoplasmonic applications' (supported by Russian Science Foundation); the project lead is Professor Sergey Kharintsev.
As he says, quartz multimode optical fibers are widely used in telecommunications for data transmission, as distributed sensors – for measuring temperature, pressure, mechanical stresses, as well as distributed spectroscopic and acoustic probes in chemically aggressive media at high pressures and temperatures, for example, 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. 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," comments co-author, PhD student Svetlana Saparina.
While investigating amorphous carbon coatings, the scientists found an irreversible increase in electrical resistance (up to 20%) during repeated heating and cooling cycles in 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. 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. 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," continues Professor Kharintsev.
The specimens for research were provided by OFS.
As the authors say, amorphous carbon coatings can also be used for small-scale moisture detectors and adsorbents.