Speed of Sound Sets How Well Glass Conducts Heat, Tohoku Team Finds

Glass protects phone screens, holds drinks and underpins electronic components, but how it handles heat is still partly a mystery. A team led by Tohoku University in Japan has now identified the factor that matters most: how fast sound travels through the glass.

The researchers measured four properties in 13 different silicate glass compositions at room temperature: thermal conductivity, sound velocity, volumetric specific heat and phonon mean free path. Their results appear in Applied Physics Letters, in a paper by Sohei Sukenaga and colleagues titled "Dominant role of sound velocity in the thermal conductivity of silicate glasses."

In insulating materials such as glass, heat is carried by phonons, which are quasiparticles representing tiny vibrations in the atomic structure. Scientists have long known that thermal conductivity depends on three things: how much heat a material stores, how quickly sound waves move through it, and how far those vibrations travel before something scatters them. The phonon mean free path is that scattering distance. Volumetric specific heat describes how much energy a given volume of glass absorbs as its temperature rises.

What nobody had pinned down was how those three interact, or which one designers should prioritize. The team found a trade-off between specific heat and mean free path. As one went up across the compositions, the other tended to go down, and their product stayed roughly constant. That meant those two quantities largely canceled each other out as drivers of the differences between glasses. Thermal conductivity, by contrast, was found to be proportional to sound velocity.

"Thermal conductivity has been studied for a long time, yet it remains one of the least understood properties of non-crystalline materials," said Sukenaga, an associate professor at Tohoku University. "Our findings provide a clearer guideline for designing the thermal conductivity of glass, which could help researchers develop materials suited to the thermal demands of next-generation electronic devices."

The practical appeal is speed. Measuring thermal conductivity is time-consuming, and experimental data for glass is limited. Sound velocity is simpler to measure, so engineers may be able to treat it as a stand-in when developing a silicate glass with a target heat-flow behavior, rather than juggling several thermal parameters as independent variables.

The finding comes with limits that the team spells out. The study covered only silicate glasses, and only at room temperature. The next step is to test whether the same relationship holds for other types of non-crystalline materials and at higher or lower temperatures. If it does, the result could become a broader foundation for understanding and designing the thermal properties of disordered materials.