Thermal conductivity of model zeolites: molecular dynamics simulation study

The thermal conductivity of model zeolites was investigated using non-equil ibrium molecular dynamics calculations. This type of calculation was found to overestimate the thermal conductivity of low-density silica polymorphs. A better reproduction of the experimental results was found for zeolites, a nd this was related to the lower phonon mean free path. The thermal conduct ivity of framework silicates was shown to be determined primarily by the vi brations of the continuous oxygen sublattice. Thus, the most drastic suppre ssion of the heat transfer was related to alterations of the O-O distances; for example, a sixfold reduction in thermal conductivity compared to that of siliceous LTA zeolite was found for LTA-AlPO4. Framework cations were sh own to affect the heat transfer by changing the vibrational modes of the st ructural building units of the framework and non-framework counter-cations, by disturbing the oxygen sublattice locally and acting as Rayleigh and res onant scatterers. A model assuming the heat transfer to be due only to non- dispersive acoustic phonons failed to reproduce the dependence of the therm al conductivity on the mass of the cations and the unit-cell dimension, thu s suggesting a more sophisticated mechanism of heat transfer to be operativ e in framework materials. The effect of non-framework non-ionic species on the thermal conductivity was shown to be determined by their effect on the characteristics of the oxygen framework vibrations. Thus, repulsive interac tions between the oxygen sublattice and Xes clusters, reducing the anisotro py and anharmonicity of the oxygen vibrations, give rise to enhanced heat t ransfer in LTA-SiO2 at ambient conditions.