Ultrapure silicon brings the heat

Thermal energy propagates through a semiconducting or insulating crystal via phonons, quantized vibrations in the lattice. If the masses of the atoms in a single-element lattice differ, phonon transport is less efficient, which explains why the highest thermal conductivities for dielectric crystals have been measured in isotopically pure samples, such as diamond that is 99.9% carbon-12 . Now Alexander Inyushkin at the Kurchatov Institute in Moscow and colleagues have conducted extremely precise thermal measurements of silicon crystals, including a sample of 99.995% silicon-28 that conducts heat better than any dielectric material ever measured.

The ultrapure sample came courtesy of the International Avogadro Project , which uses some of the most closely perfect crystals ever produced, similar to the one pictured, in an effort to redefine the kilogram. Inyushkin and colleagues fashioned 4-mm-long rods of three samples—the Avogadro specimen, another ²⁸Si-rich sample, and plain old Si, such as the atoms found in sand—and measured the flow of heat at temperatures from 2.4 K to 420 K. At an optimal temperature of 24 K, the Avogadro sample conducted about 450 W of heat per centimeter per kelvin, which is an order of magnitude as high as the value for natural Si. (Ultrapure diamond can likely do better, but no published measurement has surpassed 410 W cm^–1 K^–1.) At room temperature, isotopic purity provided about an 8% boost in thermal conductivity over ordinary Si.

As for the most common variety of Si, the team found some discrepancies between its measurements and reference data, which could prove useful for theorists to hone their phonon scattering models. Due to its ubiquity and ease of crystal synthesis, Si serves as an important reference material for studying the factors underlying thermal conductivity. (A. V. Inyushkin et al., J. Appl. Phys., in press.)