Phonons are the primary heat carriers in non-metallic solids.In compositionally heterogeneous materials,the thermal properties are believed to be mainly governed by the disrupted phonon transport due to mass disorder ...Phonons are the primary heat carriers in non-metallic solids.In compositionally heterogeneous materials,the thermal properties are believed to be mainly governed by the disrupted phonon transport due to mass disorder and strain fluctuations,while the effects of compositional fluctuation induced local phonon states are usually ignored.Here,by scanning transmission electron microscopy electron energy loss spectroscopy and sophisticated calculations,we identify the vibrational properties of ingredient-dependent interface phonon modes in Alx Ga1-x N and quantify their various contributions to the local interface thermal conductance.We demonstrate that atomic-scale compositional fluctuation has significant influence on the vibrational thermodynamic properties,highly affecting the mode ratio and vibrational amplitude of interface phonon modes and subsequently redistributing their modal contribution to the interface thermal conductance.Our work provides fundamental insights into understanding of local phonon-boundary interactions in nanoscale inhomogeneities,which reveal new opportunities for optimization of thermal properties via engineering ingredient distribution.展开更多
Oxidation is a universal process causing metals’corrosion and degradation.While intensive researches have been conducted for decades,the detailed atomistic and mesoscale mechanisms of metal oxidation are still not we...Oxidation is a universal process causing metals’corrosion and degradation.While intensive researches have been conducted for decades,the detailed atomistic and mesoscale mechanisms of metal oxidation are still not well understood.Here using in situ environmental transmission electron microscopy(E-TEM)with atomic resolution,we revealed systematically the oxidation mechanisms of aluminum from ambient temperature to^600℃.It was found that an amorphous oxide layer formed readily once Al was exposed to air at room temperature.At^150℃,triangle-shaped Al2O3 lamellas grew selectively on gas/solid(oxygen/amorphous oxide layer)interface,however,the thickness of the oxide layer slowly increased mainly due to the inward diffusion of oxygen.As the temperature further increased,partial amorphous-to-crystallization transition was observed on the amorphous oxide film,resulting in the formation of highly dense nano-cracks in the oxide layer.At^600℃,fast oxidation process was observed.Lamellas grew into terraces on the oxide/gas interface,indicating that the high temperature oxidation is controlled by the outward diffusion of Al.Single or double/multi-layers of oxide nucleated at the corners of the terraces,forming denseγ’-Al2O3,which is a metastable oxide structure but may be stabilized at nanoscale.展开更多
基金the National Key R&D Program of China(Grant No.2019YFA0708200)the National Natural Science Foundation of China(Grant Nos.52125307,11974023,12104017,and 52021006)+1 种基金the“2011 Program”from the Peking-Tsinghua-IOP Collaborative Innovation Center of Quantum MatterYouth Innovation Promotion Association,CAS。
文摘Phonons are the primary heat carriers in non-metallic solids.In compositionally heterogeneous materials,the thermal properties are believed to be mainly governed by the disrupted phonon transport due to mass disorder and strain fluctuations,while the effects of compositional fluctuation induced local phonon states are usually ignored.Here,by scanning transmission electron microscopy electron energy loss spectroscopy and sophisticated calculations,we identify the vibrational properties of ingredient-dependent interface phonon modes in Alx Ga1-x N and quantify their various contributions to the local interface thermal conductance.We demonstrate that atomic-scale compositional fluctuation has significant influence on the vibrational thermodynamic properties,highly affecting the mode ratio and vibrational amplitude of interface phonon modes and subsequently redistributing their modal contribution to the interface thermal conductance.Our work provides fundamental insights into understanding of local phonon-boundary interactions in nanoscale inhomogeneities,which reveal new opportunities for optimization of thermal properties via engineering ingredient distribution.
基金supported by the Chinese 1000-Youth-Talent Plan(for Q.Y.)111 project(No.B16042)+1 种基金the National Natural Science Foundation of China(No.51671168)the State Key Program for Basic Research in China(No.2015CB65930).
文摘Oxidation is a universal process causing metals’corrosion and degradation.While intensive researches have been conducted for decades,the detailed atomistic and mesoscale mechanisms of metal oxidation are still not well understood.Here using in situ environmental transmission electron microscopy(E-TEM)with atomic resolution,we revealed systematically the oxidation mechanisms of aluminum from ambient temperature to^600℃.It was found that an amorphous oxide layer formed readily once Al was exposed to air at room temperature.At^150℃,triangle-shaped Al2O3 lamellas grew selectively on gas/solid(oxygen/amorphous oxide layer)interface,however,the thickness of the oxide layer slowly increased mainly due to the inward diffusion of oxygen.As the temperature further increased,partial amorphous-to-crystallization transition was observed on the amorphous oxide film,resulting in the formation of highly dense nano-cracks in the oxide layer.At^600℃,fast oxidation process was observed.Lamellas grew into terraces on the oxide/gas interface,indicating that the high temperature oxidation is controlled by the outward diffusion of Al.Single or double/multi-layers of oxide nucleated at the corners of the terraces,forming denseγ’-Al2O3,which is a metastable oxide structure but may be stabilized at nanoscale.
