Optically Transparent Protective Coating for ITO-Coated PET-Based Microwave Metamaterial Absorbers

In spite of indium tin oxide (ITO)-coated polyethylene terephthalate (PET) sheets being a thriving candidate for flexible and transparent microwave metamaterial absorbers (MMAs), these materials are not robust under severe environmental and handling conditions. The ITO coatings are brittle and have poor scratch resistance when coated over flexible PET sheets; hence, their electrical performance degrades in time. Here, thin films of polyvinyl butyral (PVB) and also its composite with 3-aminopropyltriehoxysilane (APTES) processed at room temperature have been investigated as protective coatings over ITO-coated PET sheets, particularly for MMA application. Prior to coating, the effectiveness of various surface treatments to increase the ITO's wettability by exposure to UV light, chemicals, and air plasma was demonstrated by contact angle measurements. Furthermore, the consequences of surface treatments on the adhesion of the protective coatings to the ITO surface were studied using microindentation, pencil scratch test, ultrasonication test, water soak test, and shower test. Measurements of microwave absorption of the coated and uncoated substrates (PET and ITO-PET) over the X-band (8.2-12.4 GHz) showed a similar electromagnetic (EM) response approximately. Finally, free-space measurements of the MMA before and after PVB coating at its top and bottom were compared from 4 to 16 GHz, and the maximum EM absorption was found to have increased from 95.94% to 98.56%. Optical transparency of coated ITO-PET was quantified by measuring the optical transmittance at 632.8-nm wavelength using red laser and found to be equal to 68.9 ± 6.9. The mechanical strength of the developed coatings is analyzed by nanoindentation technique by obtaining values of hardness and elastic modulus.