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Pushing low thermal conductivity to the limit

Weak bonding between the distorted layer of Bi4O4SeCl2 helps limit its heat transport A wide variety of materials with low thermal conductivity find daily use, such as jackets for cold weather and plastic handles of hot metal cooking pots. Even the best thermal insulators still have a finite thermal conductivity because the vibrational motion of atoms is never fully localized and energy is transported through coherent collective vibrations (phonons). Any thermal excitation that is not fully localized can carry heat, which adds to the challenge of realizing materials with ultralow thermal conductivity for real-world applications. On page 1017 of this issue, Gibson et al. (1) have an answer to how low thermal conductivity can go. They synthesized layered inorganic bulk crystals and measured a thermal conductivity that is an order of magnitude lower than that of typical oxide glasses and only four times the value of air.