Blocking heat in two ways
Low thermal conductivity is important for barrier coatings, thermoelectrics, and other applications. Gibson et al. combined two complementary methods that manipulate internal interface properties to dramatically decrease the thermal conductivity of the inorganic material BiO2Cl2Se (see the Perspective by Kim and Cahill). The authors took advantage of both in-plane structural distortions and weak bonding layers to push the conductivity down to 0.1 watts per kelvin per meter, which is only four times that of air. The principles should be applicable to other systems and provide a method for developing crystals with extremely low thermal conductivity.
Science, abh1619, this issue p. 1017; see also abk1176, p. 963
Abstract
The thermal conductivity of crystalline materials cannot be arbitrarily low, as the intrinsic limit depends on the phonon dispersion. We used complementary strategies to suppress the contribution of the longitudinal and transverse phonons to heat transport in layered materials that contain different types of intrinsic chemical interfaces. BiOCl and Bi2O2Se encapsulate these design principles for longitudinal and transverse modes, respectively, and the bulk superlattice material Bi4O4SeCl2 combines these effects by ordering both interface types within its unit cell to reach an extremely low thermal conductivity of 0.1 watts per kelvin per meter at room temperature along its stacking direction. This value comes within a factor of four of the thermal conductivity of air. We demonstrated that chemical control of the spatial arrangement of distinct interfaces can synergically modify vibrational modes to minimize thermal conductivity.