Conductivity of thick film (cermet) resistors as a function of metallic particle volume fraction

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Abstract
Thick film resistors (TkFR) are composites of metallic particles embedded in a glass matrix. Conduction is through interconnected particle chains. For each of five systems studied the conductivity varies over many decades with particle volume fraction vm. It follows a power law dependence, G=K(vm−vm,c)t, where K, vm,c and t are constants for a given system. There are two interesting features of this relation for TkFRs: vm,c is very low (0.02 or less) and t is rather high (3<t<7). These values of vm,c and t differ considerably from those of percolative systems in which vm,c is much higher and the critical exponent t∠1.6. Nevertheless, a model is proposed for which the conductance is indeed governed by percolation. This model takes explicit account of the resistor formation process and the observed microstructure. It is essentially a modified bond percolation model in which the metallic particles are partial bonds in the lattice connecting the interstices of the relatively larger, close‐packed glass particles.

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