Thermal conductivity heat flow may not be exactly the same as the direction of the thermal gradient. In metals, thermal conductivity approximately tracks electrical conductivity according to the Wiedemann–Franz law, as freely moving valence electrons transfer not only electric current but also heat energy. However, the general correlation between electrical and thermal conductance does not hold for other materials, due to the increased importance of phonon carriers for heat in non-metals. Highly electrically conductive silver is less thermally conductive than diamond, which is an electrical insulator, but due to its orderly array of atoms it is conductive of heat via phonons. The
Conductivity near the percolation threshold of the perfect insulator and just finite metallic clusters. The exponent t is one of the two critical exponents for electrical percolation. In the other well-known case of a superconductor-conductor mixture we have formula_18. This case is useful for the description below the percolation threshold: formula_19 for formula_20 Now, above the percolation threshold the conductivity becomes infinite, because of the infinite superconducting clusters. And also we get the second critical exponent s for the electrical percolation. In the region around the percolation threshold, the conductivity assumes a scaling form: formula_21 with formula_22 and formula_23 At the percolation threshold, the conductivity