Contact electrification and adhesion between dissimilar materials

Simultaneous measurements of surface force and surface charge demonstrate strong attraction due to the spontaneous transfer of electrical charge from one smooth insulator (mica) to another (silica) as a result of simple, nonsliding contact in dry nitrogen. The measured surface charge densities are 5 to 20 millicoulombs per square meter after contact. The work required to separate the charged surfaces is typically 6 to 9 joules per square meter, comparable to the fracture energies of ionic-covalent materials. Observation of partial gas discharges when the surfaces are approximately 1 micrometer apart gives valuable insight into the charge separation processes underlying static electrical phenomena in general.

Microwave dielectric properties of PTFE/rutile nanocomposites

The effects of nano-size rutile filler on the microwave dielectric properties of PTFE composites were investigated and the results were compared with that of micron size rutile filled composites. Nano-size rutile powder was prepared through sol–gel route and the filled PTFE composites were fabricated through SMECH process. Different characterization techniques such as powder X-ray diffraction, SEM, BET, TEM and TG/DSC were employed to analyze the nature of ceramic filler. The dielectric properties of filled composites were evaluated at microwave frequency region using waveguide cavity perturbation technique. Different theoretical models have been employed to predict the variation of dielectric constant with respect to filler loading. The moisture absorption characteristics of nano-rutile filled PTFE composites were measured as per IPC-TM-650 2.6.2 standards. Composites show high dielectric constant at X-band frequency region with relatively high loss tangent compared to micron size counterpart.

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.

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.