Claudio Grimaldi

Electron tunneling in conductor-insulator composites with spherical fillers

We report on our Monte Carlo calculations of the conductivity of monosized and conducting spherical particles dispersed in a homogeneous matrix, with interparticle transport mechanism given by electron tunneling. We show that our numerical results can be reproduced by a simple formula based on a critical path analysis and which gives also a practical way to estimate the characteristic tunneling length ξ in real composites. We find that ξ is about 1 nm for several low structure carbon black polymer composites, in agreement with the expected order of magnitude.

Anisotropic random resistor networks: A model for piezoresistive response of thick-film resistors

Considerable evidence suggests that thick-film resistors are close to a metal–insulator transition and that tunneling processes between metallic grains are the main source of resistance. We consider as a minimal model for description of transport properties in thick-film resistors a percolative resistor network, with conducting elements governed by tunneling. For both oriented and randomly oriented networks, we show that the piezoresistive response to an applied strain is model dependent when the system is far away from the percolation thresold, while in the critical region it acquires universal properties. In particular close to the metal–insulator transition, the piezoresistive anisotropy shows a power law behavior. Within this region, there exists a simple and universal relation between the conductance and the piezoresistive anisotropy, which could be experimentally tested by common cantilever bar measurements of thick-film resistors.

Strain modulation of transport criticality in RuO2-based thick-film resistors

We show that in RuO2–glass composites the nonuniversal resistivity exponent can be modulated by an applied mechanical strain, signaled by a logarithmic divergence of the piezoresistive response at the percolation threshold. We interpret this phenomenon as being due to a tunneling-distance dependence of the transport exponent, supporting therefore a theory of transport nonuniversality proposed some years ago.