Piezoresistivity and conductance anisotropy of tunneling-percolating systems

Percolating networks based on interparticle tunneling conduction are shown to yield a logarithmic divergent piezoresistive response close to the critical point as long as the electrical conductivity becomes nonuniversal. At the same time, the piezoresistivity or, equivalently, the conductivity anisotropy exponent $\lambda$ remains universal also when the conductive exponent is not, suggesting a purely geometric origin of $\lambda$. We discuss our results in relation to the nature of transport for a variety of materials such as carbon-black--polymer composites and RuO_2-glass systems which show nonuniversal transport properties and coexistence between tunneling and percolating behaviors.Comment: 6 pages, 3 figures, Added discussion on experiment

Piezoresistive properties of RuO2-based thick-film resistors: The effect of RuO2 grain size

The piezoresistive properties of thick-film resistors are well documented and widely used in sensors of strain-related quantities. However, the origin of the effect is not well established, and correlations between resistor composition and relevant properties (such as strain sensitivity, temperature coefficient of resistance (TCR) and excess noise) have been not worked out yet. This paper reports a systematic study of these correlations in RuO2-based model resistors prepared with the same glass frit and RuO2 powders covering a range of particle sizes from a few nanometres to micrometres. Gauge factors (GFs) from 2 to 30 have been observed in resistors of sheet resistance R-square in the range 1 k Ohm/square to 3 M Ohm/square. At a selected R-square value, the GF increases linearly with the logarithm of RuO2 grain size, while the TCR and excess noise do not significantly depend on the RuO2 powder size

Thick film resistors as cryogenic thermometers

The work was aimed at assessing relatioships betwwen composition and performanceThe temperature dependence of resistance in RuO2-based TFRs was studied in the range 1.2 K to 300 K. The resistance fits the exponential relationship R=R0exp(T/T0)x with x=1/4 at higher temperatures with a transition to the x=1/2 regime on the same sample at lower temperatures (<20 K). The transition temperature Tc between these regimeshas a well defined dependence on the sample resitance Rs. Both the R0 ant T0 values scale down as the RuO2 fraction and show similar dependences on the Mn content. The magnetoresistve responses were measured in same temperature range in magnetic field H uo to 20 Tesla. The measured relative change of resistance never exceedes 0.02 up to 8 Tesla. In summary the developed resistors exhibit superior performances in comparison with the prently used thermometers. In addition they they show predictable properties at changing temperature and magnetic field strength, making easier their calibration

Low temperature electronic transport in RuO2 based cermet resistors

We have studied the temperature (T) dependence of resistance (R) of RuO2-based thick films down to 1.2 K. Samples were prepared from inks containing conductive RuO2 powders (less than or equal to 10% wt.), high lead-silicate glass particles and Mn (less than or equal to 1.4% wt.). We found that the resistance fits the exponential law R=R(0)exp(T-0/T)(x) with x=1/4 and the most resistive samples show a crossover to the x=1/2 regime as the temperature decreases. Both the fitting parameters R-0 and T-0 scale down as the RuO2 fraction increases and they are affected in a similar way by a change of the Mn content. The presence of the two regimes is similar way by a chang of the Mn content. The presence of the two regimes is similar to that observed in n-type GaAs [Phys. Rev. B 39, 8059 (1989)] and n-type CdSe [Phys. Rev. Lett. 64, 2687 (1990)] whilst it disagrees with the behaviour predicted for grain to grain hopping [Phys. Rev. B 27, 2583 (1983)] and with that expected for resonant tunneling between metallic particles [J. Appl. Phys. 48, 5152 (1977)]. We conclude that in our systems the driving charge transport mechanism is electron hopping within the glassy matrix. Since in our case the hopping length is of the same order of the localization length, the puzzling questions arising from our experiments are whether and how the variable range hopping model can be extended beyond its conventional limits