DEVELOPMENT OF RADAR ABSORBING NANOCOMPOSITE COATINGS USING ELECTROLESS TECHNOLOGY

Abstract

The field of nanocomposite coating involves the co-deposition of multiphase material where at least one of the constituent phases has one dimension less than 100 nm in size. The promise of nanocomposites lies in their multifunctionality, the possibility of realizing unique combinations of properties unachievable with traditional materials. The challenges in obtaining this sophisticated goal are tremendous. They include control over the distribution in size and dispersion of the nanosize constituents, tailoring and understanding the role of interfaces between structurally and/or chemically dissimilar phases and their effect on bulk properties. Electromagnetic (EM) absorbers are an essential part of defense system for their contribution to survivability of air vehicles and for use as commercial products for the electromagnetic interference (EMI) shielding. The emergence of Nanoscience and Technology opened the door for new opportunities to further improve the functionality of electromagnetic absorbers. However, the challenge of incorporating nanoparticles into a coating matrix is to overcome the difficulty of dispersing larger volume fractions of nanoparticles into the suitable matrix without sacrificing the mechanical properties of the resulting composite. The use of nanoparticles in matrix system has become a subject of interest in engineering applications due to potential changes in physical properties of nanocomposites. These changes in properties come from two aspects of nanoparticles: increased surface area and quantum effects associated with nano-dimensional particles structure. These factors can change or enhanced properties such as reactivity, strength, magnetic and dielectric properties etc. of nanocomposites. Out of various composite/nanocomposite coating technologies, electroless, EL coating technology is one of the best technology that withstands with recent development in the Nanosciene and Technology to provide coatings for various applications. Last five years, several researchers have controlled these parameters for EL Ni-P based composite coatings successfully and are able to co-deposit second phase nano particles in the EL Ni-P matrix that led this classical EL coating technology to a further step that is EL nanocomposite coatings (ELNCC). all diffraction peaks and also seen under TEM after irradiations. This systematic process of nano crystal's growth (10-70 nm) and evolution of morphologies (spherical to pyramidal faces) seen under FESEM and TEM micrographs and further explained in terms of Ostwald ripening and quantum size effect. The RL enhancement mechanism is explained by quantum size effect in the case of as synthesized and annealed (at 200 °C) stable spherical nanocrystals of 10 nm size. NRAMs annealed at 600 °C have shown the worst RL having zero bandwidth (the range of stable or unfluctuated RL) and all are metastable RL peaks. It is ascribed that the pyramidal faced morphology in nano scale range of 10-70 nm has shown excellent enhancement in RL in comparison to other shapes observed so far. The fluctuation in e" and p" at 600 and 800 °C is the metastable stage due to non uniform shape and size distribution of nano crystals. Such fluctuations are also observed for MWA NRAMs powders. Microwave absorbing paints have been fabricated by mixing NRAM powders into epoxy resin. 75 % of NRAMs powder, 25 % of an epoxy resin by weight (Mat Sol 698) and methyl ethyl ketone (MEK) as thinner were taken to make the paint. This composite paint of thickness —2mm was then applied on the standard aluminum sheet of 86 mm x 54.5 mm for microwave properties measurements in Ku-band. Reflection Loss, RL values less than —20 dB were obtained at 14.25 GHz with increasing microwave irradiation of 160-360 watts respectively due to metastable shape of crystals. On the other hand, the strongest RL of —53.69 dB is observed at 14.75 GHz for completely grown nano crystals of pyramidal shape. Furthermore, the position of minimum RL peak is found to move towards higher frequency region with increased microwave irradiation power from 160 to 760 watts. Chapter 4 deals with The Development of a Universal EL Ni-P Bath.' This chapter describes a universal EL Ni-P bath which has been successfully utilized for various substrates with constant surface area like powders of macro to nano size and sheets of aluminum, glass, polymer and Si-wafer. The duration of experimental processes and EL bath parameters like pH and temperature were optimized to obtain an amorphous deposition. As synthesized NRAM powder of — 10 nm was coated with amorphous Ni-P nano layer (-5-10 nm) by EL technique to develop EL Ni-P/NRAM nanocomposite powders. All of these properties attribute a wide range of EL Ni-P coating thicknesses that is 10 nm to 10 iArn for nano to micron size powder respectively. This is because of double autocatalytic nature of the deposition reaction one due to the activation and other due to autocatalytic Ni itself that grow in all directions. Thus EL Ni-P is a 3D dually autocatalytic ultra fast bottom-up approach; in that Ni-P nano globules are getting deposited forming the layers on to any substrate. The number of layers of EL Ni-P deposition depends on the physical and chemical properties of particular surface under deposition. The proposed growth mechanism on the basis of the characterization results indicates that the deposition of EL Ni-P layer onto the nano NRAM powder consists of amorphous EL Ni matrix, having Ni and Ni3P nanocrystalline particles to form EL (Ni-P)/NRAMs nanocomposite powder. VSM studies reveal that 'as synthesized' NRAMs powders are superparamagnetic and the remaining powders (EL Ni-P nano globules, EL (Ni-P)/RAM and annealed EL (Ni-P)/RAM at 400 °C) exhibit ferromagnetic nature. The RL were measured for 'as-synthesized' EL Ni-P nano globules, NRAM powder, EL Ni-P/NRAM and annealed EL Ni-P/NRAM. A systematic RL enhancement mechanism is suggested on the bases of the results obtained. For 'as-synthesized' EL (Ni-P)/RAM nanocomposites, the RL is evidently improved to -28.70 dB (better than boar EL Ni-P and RAM particles) and has further enhanced to -36.90 dB for the annealed nanocomposite powder. The maximum RL of -35.6 dB reaches at 17 GHz, hence, the RAM particles functionalized with EL Ni-P exhibit better RL. The improvement of RL obviously originated from the combination of EL Ni-P nano globules (containing nano particles in amorphous matrix) and NRAM particles by annealing. Present studies indicate the possibility that EL (Ni-P)/NRAMs nanocomposite powder may have potential application in the wide band EM wave shielding absorber. Chapter 5 deals with The Development and Microwave Absorption Properties of EL Ni-P-NRAMs Nanocomposite Coatings by Conventional Method. ' The selection of the second phase NRAMs particles and EL Ni-P matrix parameters have been used to develop EL Ni-P-NRAMs nanocomposites based on the optimization carried out in the Chapters 3 and 4. Thus produced nanocomposite coatings were characterized and RL is measured in Ku band. The EL Ni-P-NRAMs nanocomposites were systematically optimized to get the best combination in terms of concentration of second phase (x %), type of VAMNRAM and MWAMNRAM, co-deposition time. The average globule sizes of EL Ni-P-NRAMs nanocomposites coatings were determined using Heyn's intercept method and found to decrease from 124 nm to 27 nm vi