The resistance of chemically synthesized polypyrrole (PPy) thin films is investigated as a function of the pressure of various gases as well as of the film thickness. A physical, piezoresistive response is found to coexist with a chemical response if the gas is chemically active, like, e.g., oxygen. The piezoresistance is studied separately by exposing the films to the chemically inert gases such as nitrogen and argon. We observe that the character of the piezoresistive response is a function not only of the film thickness, but also of the pressure. Films of a thickness 70 nm show a decreasing resistance as pressure is applied, while for thicker films, the piezoresistance is positive. Moreover, in some films of thickness ≈ 70 nm, the piezoresistive response changes from negative to positive as the gas pressure is increased above ≈ 500 mbars. This behavior is interpreted in terms of a total piezoresistance which is composed of a surface and a bulk component, each of which contributes in a characteristic way. These results suggest that in polypyrrole, chemical sensing and piezoresistivity can coexist, which needs to be kept in mind when interpreting resistive responses of such sensors.

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In this paper, we report an investigation on electrical conduction mechanisms of nanofluids based on commercial engine oil loaded with graphite (Gt) and multiwalled carbon nanotubes, at different concentrations. The impedance spectroscopy technique was used to measure the resistance and capacity characterizing each sample in a frequency range 100 Hz–1 MHz and a temperature range 300–400 K. Two formalisms were used to analyze the data: (a) the electrical conductivity which has found to follow the Jonscher’s law with single and double exponents for carbon nanotube concentrations below and above the percolation threshold, respectively, and (b) the complex impedance that has permitted to identify the relaxation peaks according to the Cole–Cole model. Both the two formalisms showed that when the carbon nanotube concentration is higher than the percolation threshold, a positive temperature coefficient and a remarkable change in conductivity were observed, suggesting that the presence of the carbon nanotube greatly affect the electrical properties of the engine oil as a result of additional polarization effect induced by these nanoparticles. Furthermore, the analysis of the temperature dependence of dc conductivity and relaxation time using the Arrhenius equation indicated the addition of carbon nanotubes into engine oil increase the activation energies.

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