The threedominant scattering mechanisms relevant to all BST polycrystallinesamples are Umklapp phonon-phonon scattering, point defectscattering and boundary scattering [25]. Because of the existence ofsecond phase, the additional phonon scattering from nanoinclusions and formed phase boundaries should take place, leadingto decrease of thermal conductivity. In fact, the total phononrelaxation time for composite samples can be written as: