Electrical properties of epoxy/silver nanocomposites

We investigated the electrical properties of nanocomposites made of epoxy resin filled with 70-nm-sized silver particles. These composites were studied for the fabrication of integrated capacitors in electronics packaging. The dc conductivity was studied as a function of the filler concentration and as a function of temperature. We also studied the ac conductivity and the permittivity in the 10−1–105Hz range as a function of the filler concentration. Experimental properties were analyzed using standard percolation theories. The dc conductivity varies as (ϕ-ϕc)t, where ϕ is the filler concentration, ϕc is the percolation threshold, and t is the dc critical exponent. A very low percolation threshold is obtained (ϕc=1%) which is believed to be related to a segregated distribution of the fillers in the epoxy matrix. We also measured a very high dc critical exponent (t=5) probably related to the interparticle electrical contact. A universal scaling law is observed for σ(ω) and ε(ω). Above a cutoff frequency (ωc, which scales with the dc conductivity as ωc∼σdcq) the conductivity and the permittivity follow the universal power laws (σ∼ωu and ε∼ω−v) with critical exponents taking nonstandard values (q=0.83–0.98, u=0.79, and v=0.03).