Thermal Conductivity of Ivory, a Natural Nanocomposite

Large reductions in thermal conductivity, in comparison to bulk materials, have been observed in materials structured at the nanoscale level. These reductions are attributed to a combination of classical and quantum size effects that manifest themselves when the characteristic length scales in the nanostructured material become comparable to, or shorter than, the mean free path of the heat carriers in the bulk material [1]. In semiconductors and insulators the primary (or only, in the case of insulators) heat carriers are the lattice vibrations (i.e. phonons) and most of the heat is carried by phonons with mean free paths in the range of 1 – 100 nm at room temperature [2]. Thus, size effects on heat transport become important in nanostructured materials.