Structural materials for resistance heating

Resistance (Joule) heating is useful for deicing and space heating. The efficiency of conversion from electrical energy to thermal energy approaches 1 after the initial heating period. An effective material for resistance heating is intermediate in the electrical resistivity. Metals are too low in resistivity, so they tend to be used in coils. The cement matrix is too high in resistivity. Structural materials can be rendered effective for resistance heating by the use of additives (e.g., electrically conductive admixtures in cement-based materials), the inclusion of a planar heating element (e.g., flexible graphite and metal-coated carbon fiber mat), electrically conductive reinforcing continuous fiber (e.g., carbon fiber), or the use of interfaces in the structural composite (e.g., the interlaminar interface, which has contact resistivity and provides a two-dimensional array of heating elements in case of the associated laminae having fibers in different directions). With a power of 94 W, flexible graphite (a compact of exfoliated graphite; resistivity = 0.00075 Ω.cm) provides 980°C (initially at 19°C) in 4 s. With a power of 3.0 W, Ni/Cu/Ni coated carbon fiber mat (resistivity = 0.07 Ω.cm) provides 79°C in 14 s. With a power of 0.59 W, a carbon fiber epoxy-matrix interlaminar interface provides 89°C in 16 s. With a power of 1.8 W, carbon fiber (15 μm diameter, 1.0 vol.%) cement (resistivity = 100 Ω.cm) provides 56 degrees C in 4 min. With a power of 5.6 W, stainless steel fiber (8 μm diameter, 0.7 vol.%) cement (resistivity = 0.85 Ω.cm) provides 60°C in 6 min. For this steel fiber cement, the heat power output per unit area reaches 750 W/m², compared to 340 W/m² for a metal wire with the same resistance.