Sodium and potassium mixed effect on thermal conductivity in a borosilicate glass

To investigate the combined effects of sodium and potassium on thermal conductivity, a series of borosilicate glasses with precise compositions (65.0SiO₂·5.0B₂O₃ (17.6 - <i>x</i>)Na₂O·<i>x</i>K₂O·7.5CaO·4.9MgO) was synthesized, where <i>R</i> = [K₂O]/([Na₂O] + [K₂O]). As <i>R</i> increased systematically, a pronounced nonlinear variation in thermal conductivity, characteristic of the mixed alkali effect, was observed. Within the phonon gas model framework, the thermal conductivity was primarily governed by changes in sound velocity, as both volumetric heat capacity and phonon mean free path remained nearly constant across the glass series. NMR and Raman spectra revealed nonlinear evolution in the local coordination environment of silicon cations with increasing <i>R</i>. These structural changes, coupled with anomalous variations in atomic packing fractions, provided atomic-scale evidence for the observed thermal conductivity trends. This study not only deepens the understanding of the physical mechanisms governing thermal properties in borosilicate glasses but also provides valuable insights for designing advanced materials with precisely tailored thermal performance.