Abstract
The presence of grain boundaries (GBs) has a great impact on the coefficient of thermal expansion (CTE) of polycrystals. However, direct measurement of local expansion of GBs remains challenging for conventional methods due to the lack of spatial resolution. In this work, we utilized the valence electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) to directly measure the CTE of Σ5 and 45°GBs of SrTiO3 at a temperature range between 373 and 973 K. A CTE that was about 3 times larger was observed in Σ5 GB along the direction normal to GB plane, while only a 1.4 time enhancement was found in the 45° GB. Our result provides direct evidence that GBs contribute to the enhancement of CTE in polycrystals. Also, this work has revealed how thermodynamic properties are varied in different GB structures and demonstrated the potential of EELS for probing local thermal properties with nanometer-scale resolution.
Highlights
The presence of grain boundaries (GBs) has a great impact on the coefficient of thermal expansion (CTE) of polycrystals
It is wellknown that GBs with varied misorientation angles may exhibit wide energy landscapes which lead to different physical properties.[21−23] In this respect, CTEs of GBs differ greatly according to their local structure
None of these methods has enough spatial resolution to directly measure the local expansion near the GB plane, which typically has a width of about 1 nm
Summary
Where VGB corresponds to unit cell volume in GB, VB is the volume of bulk lattice, and AGB corresponds to GB in-plane area. Vexcess has the unit of angstrom with the direction normal to GB plane. It is not clear why Vexcess affects CTE in detail, one possible reason could be that excess volume is inversely proportional to the bulk modulus of GBs
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