Young's modulus of Fe-catalyzed silicon carbide (SiC) nanowires was measured in the temperature range of 300–575 K by the use of a laser Doppler vibrometer. The nanowires have a face-centered cubic structure grown along the [111] direction and exhibit different cross-sectional geometries, including circle, rectangle, hexagon, ellipse, trapezoid, and triangle. When the effective diameters of the nanowires decrease from 200 to 55 nm, their room-temperature Young's modulus decreases from ∼550 GPa (the bulk value) to ∼460 GPa, i.e., a reduction of ∼16%, and their temperature coefficient of Young's modulus varies from −47.4 ± 1.7 ppm/K (the bulk value) to −78.1 ± 5.9 ppm/K, i.e., a change of ∼65%. The size and temperature dependency of the modulus would greatly benefit the design and fabrication of high-temperature mechanical sensors based on SiC nanostructures.

No abstract

The friction behaviour of ZnO nanowires on natural graphite (NG) and highly oriented pyrolytic graphite (HOPG) substrates was tested in ambient conditions by use of optical microscopy based nanomanipulation. Nanowires on the step-free and waviness-free NG substrate exhibit a diameter-independent nominal frictional shear stress of 0.48 MPa, and this provides a benchmark for studying how the surface topography of graphite influences nanowire friction. Nanowires on the HOPG substrate present a significant diameter-dependent frictional shear stress, increasing from 0.25 to 2.78 MPa with the decrease of nanowire diameter from 485 to 142 nm. The waviness of HOPG has a limited effect on the nanowire friction, as a nanowire can fully conform to the substrate. The surface steps on the HOPG can significantly enhance the nanowire friction and lead to a much higher frictional shear stress than that on NG due to mechanical blocking and the presence of a Schwoebel barrier at step edges. The surface steps, however, can also generate small wedge-shaped gaps between a nanowire and substrate, and thus reduce the nanowire friction. With the decrease in nanowire diameter, the capacity for the nanowire to better conform to the substrate reduces the length of the wedge-shaped gaps, leading to the observed increase in nanowire friction. The results have improved our understanding of the unique friction behaviour of nanowires. Such an improved understanding is expected to benefit the design and operation of nanowire-friction-based devices, including bio-inspired fibrillar adhesives, soft grippers, rotary nanomotors, and triboelectric nanogenerators.

A new technique based on optical microscope nanomanipulation is developed to permit optical observation of the shearing process between cantilevered nanowires (NWs). Real-time observation of the deflection shape of NWs during shearing under controlled environmental conditions enables their shear stress evolution to be evaluated with respect to relative humidity (RH). The average static and kinetic shear strengths, τ s ¼ 7.1 AE 0.8 and τ k ¼ 5.2 AE 1.1 MPa, are found to be insensitive to the RH within the range of 40-60%. When the RH increased to 74%, the shear strength values increase to τ s ¼ 18.7 AE 3.0 and τ k ¼ 14.7 AE 1.8 MPa. When the RH decreases to 20%, a dramatic increase in the static and kinetic shear strengths is observed: τ s ¼ 64.4 AE 7.2 and τ k ¼ 49.6 AE 5.8 MPa. Water films absorbed on the surface of NWs within the RH range of 40-60% can lubricate their frictional interface and thus dramatically decrease the shear strength. At high RH of above 60%, the viscous effects of a water meniscus condensed around the contact point between two NWs can significantly enhance the shear strength and dampen stick-slip behavior. The detachment of two NWs is then controlled by the rupture of a meniscus bridge, leading to high ultimate shear strength value of several tens of MPa.

Young's modulus of Sb2O3 nanowires with nominal diameters of 51–170 nm is measured by using a laser‐Doppler vibrometry within the temperature range of 300–650 K. The Young's modulus is found to decrease linearly with the elevated temperature ranging from 300 to ≈475 K. The temperature coefficient of Young's modulus (TCE) is ≈−170 ppm K−1 when nanowire diameter is above ≈100 nm, below which the coefficient decreases to ≈−310 ppm K−1 with the diameters down to ≈50 nm. This strong size‐dependent TCE is attributed to the synergistic reaction of temperature and size/surface effects. Nanowires would sublimate or even melt starting from their tips at elevated temperatures, even though the temperature is far below the melting point of the bulk counterpart. The observed thermomechanical behavior of nanowires provides the basis and reference for the optimal design and operation of high‐temperature nanodevices using nanowires as building blocks.