Dielectric characterization of conducting textiles using free space transmission measurements: accuracy and methods for improvement

The dielectric behaviour of in-situ polymerized thin polypyrrole (PPy) films on synthetic textile substrates were obtained in the 1–18 GHz region using free space transmission and reflection methods. The PPy/para-toluene-2-sulphonic acid (pTSA) coated fabrics exhibited an absorption dominated total shielding effectiveness (SE) of up to −7.34 dB, which corresponds to more than 80% of incident radiation. The permittivity response is significantly influenced by the changes in ambient conditions, sample size and diffraction around the sample. Mathematical diffraction removal, time-gating tools and high gain horns were utilized to improve the permittivity response. A narrow time-gate of 0.15 ns produced accurate response for frequencies above 6.7 GHz and the high gain horns further improved the response in the 7.5–18 GHz range. Errors between calculated and measured values of reflection were most commonly within 2%, indicating good accuracy of the method.

Dielectric properties of conducting polymer

The dielectric properties of conducting polymer composites containing polypyrrole (PPy) crushed films, PPy powder, polyaniline (PAn) base and acid powders as the dispersants and silicone rubber and vinyl ester as matrix materials have been investigated in the frequency range 2-18 GHz. The dielectric parameters such as the real part, epsiprime, and imaginary part, epsiPrime, of the permittivity and loss tangent, tandelta, increase with increasing conductivity and concentration of the dispersant. The geometrical shape of the dispersant governs the ability of conductive network formation which is indicated by a large drop in the resistivity of the composite. Also, dispersant/matrix interactions and physical properties of the matrix influence the agglomeration of the dispersant phase which, in turn, affects the dielectric properties of the composites. Flakes of PPy obtained by crushing highly conductive films and large PAn powder aggregates were unable to form a conducting network. The composites without a network of dispersant exhibit low dielectric parameters. On the other hand, high values of tan delta ranging from 0.7–1.1 were achieved for the PPy powder (15 parts)/silicone rubber composites where a conducting network was observed.

Influence of N2O flow rate on reliability of SiOx films deposited by SiH4-N2O gas mixture plasma

SiO_x films have several advantages as an interlayer dielectric in electronic devices owing to the strong adhesion between SiO_x and the substrate. In this study, the coating performance as a function of the N₂O flow rate was evaluated by electrochemical impedance spectroscopy and potentiodynamic polarization tests in an undisturbed environment. In addition, the coatings were examined by atomic force microscopy and Fourier transform infrared reflection spectroscopy. The SiO_x films on a stainless-steel substrate showed the highest coating performance at a N₂O flow rate of 120 sccm. This was attributed to the films having the lowest porosity value among those examined as a result of the fragmentation of SiO and SiO₂ bonds and the improved surface roughness.

Change in dielectric properties in the microwave frequency region of polypyrrole-coated textiles during aging

Complex permittivity of conducting polypyrrole (PPy)-coated Nylon-Lycra textiles ismeasured using a free space transmission measurement technique over the frequency range of1–18 GHz. The aging of microwave dielectric properties and reflection, transmission and absorptionfor a period of 18 months is demonstrated. PPy-coated fabrics are shown to be lossy over thefull frequency range. The levels of absorption are shown to be higher than reflection in the testedsamples. This is attributed to the relatively high resistivity of the PPy-coated fabrics. Both the dopantconcentration and polymerisation time affect the total shielding effectiveness and microwave agingbehaviour. Distinguishing either of these two factors as being exclusively the dominant mechanismof shielding effectiveness is shown to be difficult. It is observed that the PPy-coated Nylon-Lycrasamples with a p-toluene sulfonic acid (pTSA) concentration of 0.015 M and polymerisation times of60 min and 180 min have 37% and 26% decrease in total transmission loss, respectively, upon agingfor 72 weeks at room temperature (20 C, 65% Relative humidity (RH)). The concentration of thedopant also influences the microwave aging behaviour of the PPy-coated fabrics. The samples with ahigher dopant concentration of 0.027 mol/L pTSA are shown to have a transmission loss of 32.6% and16.5% for short and long polymerisation times, respectively, when aged for 72 weeks. The microwaveproperties exhibit better stability with high dopant concentration and/or longer polymerization times.High pTSA dopant concentrations and/or longer polymerisation times result in high microwaveinsertion loss and are more effective in reducing the transmission and also increasing the longevity ofthe electrical properties.