This paper investigates the structure, length, and percentage of functional groups of multi-walled carbon nanotubes (CNT) depending on the time taken for functionalization in HNO3 and H2SO4 mixture. The carbon nanotube content and influence of functionalization time on mechanical properties of polymer composite materials based on epoxy matrix are studied. The extreme dependencies of mechanical properties of carbon nanotube functionalization time of polymer composites were established. The rise in tensile strength of obtained composites reaches 102% and elastic modulus reaches 227% as compared to that of unfilled polymer. The composites exhibited best mechanical properties by including carbon nanotube with 0.5 h functionalization time.

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This research is devoted to the study of radar absorbing properties of the composites, based on the epoxy binder and carbon nanotubes (CNT) in the frequency range of 52-73 GHz. Three species of unmodified multi-walled CNT differing in length and diameter were investigated as fillers. The reflection coefficients (K refl) at the radar absorbing material (RAM)-air interface and the electromagnetic radiation (EMR) absorption coefficients (K abs) in the materials with the different content of nanotubes were measured (K refl and K abs were calculated using the highest (the worst) value of the voltage standing-wave ratio (VSWR) in the frequency range of 52-73 GHz). It was established that the increase in nanotubes aspect ratio (a ratio of CNT length to its diameter) leads to K abs rising for polymer composites. Also, CNT diameter decrease leads to K refl reduction. CNT of 8-15 nm in diameter and more than 2 μm in length are the most effective from all investigated fillers. The reflection loss values were calculated and CNT optimal concentrations were obtained at different thickness of RAMs.

The effect of functionalization of carbon nanotubes in a mixture of sulfuric and nitric acids on the radar-absorbing properties of polymeric composites based on them was studied. The optimum functionalization conditions were determined, allowing the thickness of the radar-absorbent material to be decreased by 36-46% (with its characteristics preserved) relative to the analogous composites with unmodifi ed nanotubes.Radar-adsorbent materials (RAMs) are used for the absorption of the incident electromagnetic radiation (EMR) at minimal refl ection level, which allows solution of diverse problems, from the absorption of "parasitic" radiations inside high-frequency units of various devices to a decrease in the radiolocation visibility of military items. Polymeric composites with carbon nanotubes (CNTs) as RAMs have been extensively studied recently. Various procedures are used for CNT modifi cation aimed at increasing the effi ciency of EMR absorption. They are mostly based on incorporation of various substances (metals [1-4], alloys [5], oxides [6,7], salts [8]), on the surface and/or into internal voids of CNTs. Depending on the concentration of the modifying substances, they occur in the forms of a surface layer, of nanothreads inside the CNTs, or of particles inside CNTs or on their surface. Such procedures are aimed at altering the electrophysical properties of the CNTs themselves. On the other hand, the properties of the composites under consideration are strongly infl uenced not only by the parameters of the nanotubes, but also by the character of their distribution in the polymeric matrix. It is known that the initial CNTs are distributed in the polymeric matrix nonuniformly because of their tendency to agglomeration. The same is true for the nanotubes modifi ed by the above procedures. The widely used way to reach uniform distribution in the polymeric matrix is chemical modifi cation (functionalization) of CNTs with the formation of active functional groups (e.g., carboxy groups) on their surface. However, the effect of CNT functionalization on the radar-absorbing properties of polymeric composites based on them was not studied in detail previously.In this work we studied the radar-absorbing properties of polymeric composites with CNTs in relation to the CNT content and degree of functionalization.The radar-absorbing properties were studied in the range 52-73 GHz (millimeter wavelength range). The devices operating in this range include those for measuring the fl ow rate, humidity, and compressibility coeffi cient of gases, airplane landing systems, protected communication systems, devices for studying rapid processes, some medical devices, systems for detection and high-precision following of low-fl ying and mobile surface targets, and antenna feeder systems for on-board radioelectronic systems. EXPERIMENTALOur experiments were performed with CNTs of grade Taunit-M [TU (Technical Specifi cation) 2166-