Search

Conductive non-woven fabric sheets coated with polypyrrole nanoparticles and having resistivity from 1.5 × 105 to 3.0 Ω cm were prepared by immersing non-woven fabric sheets in solutions containing various concentrations of oxidizing agent and dopant and then exposing the sheets to pyrrole vapor. A stack of ten treated sheets could absorb more than 95% of electromagnetic waves in the frequency range of 75–110 GHz and more than 99% in the range of 85–105 GHz, irrespective of the resistivity of the sheet. This absorption was achieved even though the thickness of the stack was less than 10 mm. Furthermore, a conductive non-woven fabric sheet with resistivity of 7.5 Ω cm and thickness of only 0.5 mm could absorb about 90% of electromagnetic waves in the range of 75–110 GHz. These results clearly demonstrate that the stacked conductive non-woven fabric sheets prepared in this study are a new material that effectively absorbs electromagnetic waves with extremely high frequencies in the millimeter band.

We have successfully fabricated a conductive fabric coated densely and uniformly with polypyrrole nanoparticles that were less than 50 nm in size by exposing a fabric immersed in a solution containing an oxidizing agent and a dopant to pyrrole vapor after partially pressing excess solution from the fabric with a nip-roll apparatus. The resistivity and color of the resultant conductive fabric depended on the concentrations of the oxidizing agent and dopant. The resistivity arbitrarily varied from an antistatic level (1010 Ω cm) to a high conductivity level (10 Ω cm), which was attributed to differences in the size, compactness and adhesiveness of the polypyrrole nanoparticles forming the conducting network. High fastness to rubbing of the conductive fabric was maintained even though the electrical conductivity varied, suggesting that the fabric could be used in ultra-clean and ultra-low dust conditions.

Microwave absorbability of electromagnetic waves in the frequency range of 1–15 GHz, which is often used in recent wireless applications, for stacks of various numbers of conductive nonwoven fabric sheets with different resistivity was evaluated to clarify their possibility as the broadband microwave absorber in the frequency range of 1–15 GHz. The conductive nonwoven fabric sheets coated with polypyrrole nanoparticles, which had resistivities from 4.6 ×103 to 1.2×102 Ω cm, were prepared by immersing nonwoven fabric sheets in solutions containing various concentrations of an oxidizing agent and a dopant and then exposing the sheets to pyrrole vapor. The real part (ε') and the imaginary part (ε'') of complex permittivity for each conductive nonwoven fabric sheet increased with decreasing frequency, indicating the dispersion of permittivity. In addition, the reflection loss decreased when ammonium persulfate concentration increased and/or the stack size increased. However, the stack of two conductive nonwoven fabric sheets (thickness: 6.6 mm) with a resistivity of less than 120 Ω cm had average reflection losses of less than -10 dB in the frequency range of 1–15 GHz. From this way, it can be considered that the stacked conductive nonwoven fabric sheets prepared in this study can be a new material that effectively absorbs electromagnetic waves in the wide frequency range.