Coatings produced from liquid Si-containing preceramic polymers, such as perhydropolysilazane (PHPS), have recently been actively studied and introduced as an alternative to traditional glasses and ceramics. This transition allows to retain most of the advantages of these materials, such as chemical and thermal stability, excellent optical and barrier properties. At the same time, new approaches in the formation of coatings open the door for compatibility with flexible and low-temperature technologies, expanding the functional capabilities of the protective material. Development of a nanocomposite films in a matrix/filler configuration is a proven strategy for modifying and tailoring the unique properties of the final material based on the requirements for its application.
Here we report on the technology for producing thin laminate-type nanocomposite coatings based on the SiOxNy matrix with horizontally oriented SWCNTs embedded in the matrix as the filler. The matrix film was formed by spin-coating from 20% PHPS solution in dibutyl ether. Thin SWCNT films collected at the outlet of the CVD reactor were embedded into the matrix by simple dry transfer method. Prepared composites were cured by two different low-temperature methods: (i) thermal curing in an oven at a T of 180 °C for 60 min and (ii) UV curing at the wavelengths of 185 and 254 nm for 40 min. Both processes exclude the high T pyrolysis, thereby increasing economic viability and production scalability of proposed methods.
We have already reported on the effect of the PHPS curing method on the morphological and functional properties of the resulting SiOxNy thin films [1]. As the next step, we studied the effect of the curing method on the optical properties of matrix films, as well as the effect of SWCNTs embedding on the optical properties of the composite coatings. The refractive indices (n) of the matrix and composite films were measured using spectral ellipsometry. These data complement obtained XPS, FTIR and SEM-EDX results illustrating the dependence of n on the nitrogen content in the film. Composite films show transmittance higher than 90%, and also exhibit an additional antireflection effect (~2%) vs. SWCNT films confirmed by the reflectance spectra. A dark spot in studies of PHPS-derived thin films is data on their long-term stability. Here we shed light on this issue by comparing measurements immediately after curing, 1 month after curing and 1 year after curing of prepared coatings. For all periods, thermally cured films exhibit a higher n from 1.47 to 1.56 compared to UV cured films (∆n = 0.1 - 0.4). Obtained data on the refractive indexes and appropriate thickness of the composite coatings allow effectively use them as antireflective protective coatings.
Acknowledgements: Estonian Ministry of Education and Research (TK210), Education and Youth Board of Estonia (ÕÜF5).
[1] Shmagina E. et al, Surface Engineering, 38:7-9, 769-777, 2022, DOI: 10.1080/02670844.2022.2155445