Magnesium and calcium mixed effect on thermal conductivity in a borosilicate glass

• A series of 65.0SiO 2 ·5.0B 2 O 3 ·17.4Na 2 O·0.5K 2 O·(12.1- x )CaO· x MgO borosilicate glasses with R = [MgO]/([MgO]+[CaO]) were synthesized. • A specular Ca-Mg mixed effect on thermal conductivity ( κ ) for this series of glasses was identified. • Based on phonon gas model, Debye sound velocity was identified as the predominant factor governing κ . • We exhibited the proofs of atomic-scale structural origin of the specular Ca-Mg mixed effect on κ . To investigate the mixed effect of magnesium and calcium on thermal conductivity , a series of borosilicate glasses with varying molar ratios R ( R = [MgO]/([MgO]+[CaO])), specifically 65.0SiO 2 ·5.0B 2 O 3 ·17.4Na 2 O·0.5K 2 O·(12.1- x )CaO· x MgO, were synthesized. As MgO content increased, thermal conductivity exhibited a near-linear rise for R values from 0.0 to 0.6, followed by a negative deviation up to R = 1.0. Utilizing the phonon gas model, key factors influencing thermal conductivity—volumetric heat capacity ( C ), phonon mean free path ( l ), and Debye sound velocity—were analyzed. Given the minimal variations in C and l , Debye sound velocity emerged as the predominant factor governing thermal conductivity in these glasses. Furthermore, by examining the dual roles of Mg cations as either network modifiers or formers, alongside the evolution of Si-O tetrahedra-related clusters through Raman and NMR spectra, we elucidated the atomic-scale structural origins driving the linear and nonlinear dependencies of thermal conductivity on R .