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Many studies have been conducted on the absorption and protection of electromagnetic waves to reduce the harmful effects of electromagnetic radiation on the environment. High conductivity shields are used to prevent the penetration of electromagnetic waves. A convenient and useful method of obtaining electromagnetic shielding materials is the addition of conductive carbon materials including carbon fibers, carbon filaments, and carbon nanotubes. Carbon nanotubes can easily form a conductive network within a material field due to their two-dimensional tubular structures and high conductivity, which results in a high electrical permeability ambience. Therefore, the increase in dielectric losses results in reflection losses of electromagnetic waves. Thus, the presence of carbon nanotubes in the adsorbent improves the absorption properties of electromagnetic waves.
In this study, the electromagnetic wave absorbing properties of multi-walled carbon nanotubes (MWCNT) functionalization with carboxyl (-COOH) group /cement composites with different shapes; chiral, zigzag and armchair were studied by short circuit of the waveguide and matched load methods. The influence of the MWCNT shape and sample thickness on the electromagnetic wave absorbing properties were discussed and analyzed in the frequency range of 8–12 GHz by a short circuit of the waveguide and matched load methods. The samples were prepared in two thicknesses of 3 and 6 mm, and the amount of nanotubes added was 0.1%wt. The addition of 0.1 wt.% MWCNT greatly enhances the absorption performance of the cement mortar in the frequency range of  8–10 GHz. With the increase of thickness from 3 mm to 6 mm, the frequency bandwidths of the reflection loss for MWCNT/cement composites increases but the number of peaks decreases.  By comparing the results of electromagnetic wave absorbing of the samples with two different methods are deduced that in the samples with a thickness of 3 mm, the absorbing of waves by matched load method is better than short circuit method without matched load. However, in samples with a thickness of 6 mm, there is not much different. Also, the electromagnetic wave absorbing of the composite samples with a thickness of 3 mm performed better by short circuit method at frequencies below 10.5 GHz, while the composite samples had better absorbing in the matched load method at frequencies greater than 10.5 GHz. In addition, the electromagnetic wave absorbing of the composite samples with a thickness of 6 mm show better results by short circuit method at lower frequencies and by matched load method at higher frequencies. Moreover, the absorption behavior of the chiral sample with thickness 6 mm differs from the other two samples because the chiral nanotubes are asymmetric and zigzag and armchair nanotubes are symmetric. Furthermore, the structural analysis and surface morphology of MWCNT/cement composites with different shapes have been explored using the scanning electron microscope (SEM) technique. Scanning electron microscope images of MWCNT/cement composites show dispersion of nanotubes in composite. Connecting of nanotubes and cement leads to reduction of porosity and formation of regional conductive network. As a result, the electrical conductivity is increased and the electromagnetic field in the network is attenuated.

Keywords: Carboxyl functionalized multi-walled carbon nanotubes, Cement composite, Short circuit, Matched load.

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