Investigations to the stability of CNT-dispersions using impedance spectroscopy

Carbon nano tubes (CNT’s) are promising candidates for several sensor applications such as optical sensors, strain gauges or flow sensors. For certain sensor structures liquid CNT dispersions are required. These are important not only for the realization of CNT-films for sensors like strain gauges but also for technological processes such as dielectrophoresis. CNT-films are realized by deposing the dispersion on a carrier material followed by a drying process. The dispersion properties depend on several parameters like CNT concentration, surfactant concentration, sonication time, centrifugation time, storing time and other parameters. Methods for characterization of dispersions are up to now limited to UV/VIS spectroscopy. This is generally limited to low CNT concentrations. This paper discusses the possibility to use impedance spectroscopy as characterization method for the stability of the dispersions. The impedance of the dispersion was measured using a conductivity measurement cell with platinum electrodes. The behavior of characteristic points of the impedance spectrum was investigated for three identically prepared samples during 7 days of storing time. The systematic trend observed is definitively larger than the variance between different samples. With increasing time after preparation the CNT fallout will increase and the amount of deposable CNT’s will decrease. The decreasing imaginary part indicates an easier diffusion of surfactant molecules because they are not longer attached to CNT’s