Thermal expansion/contraction of boron nitride nanotubes in axial, radial and circumferential directions

In the present work, thermal expansion/contraction of armchair boron nitride nanotubes (BNNTs) with various diameters was investigated. The coefficient of thermal expansion (CTE) of BNNTs in axial, circumferential and radial directions were determined at temperatures from 100 K to 700 K, using molecular dynamics simulation. Two CTE in axial direction were defined; real axial CTE and apparent axial CTE. All BNNTs showed real axial thermal expansion, which was due to the thermal expansion of B–N bond length. However, large diameter BNNTs exhibited apparent axial thermal contraction at low temperatures. Although the apparent axial thermal contraction was not observed in small diameter BNNTs, their apparent axial CTE was smaller than their real axial CTE. In circumferential direction, all BNNTs were thermally expanded. In spite of thermal expansion in circumferential direction, BNNTs with large diameters exhibited thermal contraction in radial direction. The above observations were explained by considering both the effect of dynamic thermal vibration of BNNTs and the effect of thermal expansion of B–N bond length.