Conductivity and Diffusivity
The thermal conductivity of magnetite is 5.1 W m-1 K-1 compared to just 1.7 W m-1 K-1 for BaSO4 and 1.5 W m-1 K-1 for glass fiber (Clauser andHuenges 1995). Most thermoplastics lie in the range 0.2-0.4 W m-1 K-1. Magnetite has been proven effective at increasing these properties in plastics (Weidenfeller et al. 2002) and rubbers (Mangnus 2003) with reported thermal conductivities up to ~1 W m-1 K-1. Magnetite-filled injection-molded polypropylene showed far higher specific heat capacity and thermal diffusivity than barium-sulfate-filled material (Weidenfeller et al. 2004). Increasing the heat transfer rate in and out of a polymer translates to much faster productivity in molding processes such as injection molding (Weidenfeller et al. 2005), other forms of molding, and in extrusion.
Theory states that thermal conductivity of filled systems should also demonstrate a percolation threshold as is the case for electrical conductivity. However, this is often not observed in real filled materials including magnetite-filled polymers. The reason is that when the thermal conductivity of the filler is only one order of magnitude larger than the polymer, one finds that the system is lossy. That is, the heat is transferred out of the particles and into the matrix as quickly as it is transferred to the adjacent particles. The thermal conductivity therefore varies linearly with filler loading unless the thermal conductivity of the filler is exceptionally high compared to that of the matrix.
Coefficient of Thermal Expansion (CTE)
The CTE ofmagnetite is ^8.5 x 10-6 K-1 at room temperature (Huijbregts and Snel 1972), but below the Verwey transition of 120 K, it shows a remarkably low CTE (Hancock and Finlayson 1995) under 1 x 10-6 K-1, similar to Invar, a material known for an exceptionally low CTE.
