Supporting the Georgia Tech College of Engineering's Strategic Directions
Environment: Functional Nanomaterials
- Figure 1. A novel phase transformation-induced change in the thermal conductivity of ZnO nanowires, (a) wurtzite (WZ) and recently discovered hexagonal phase(HX), (b)WZ-to-HX transformation in a ZnO nanowire under tensile loading, (c) hysteretic stress-strain relation during loading and unloading, and (d) change in thermal conductivity associated with phase transformation (group of M. Zhou).
Coupled thermal and mechanical behaviors of functional nanomaterials, such as ZnO and Si nanowires are being modeled within the Mechanical Properties Research Laboratory (MPRL). These nanomaterials are a new class of one-dimensional (1D) candidates for a wide range of novel functional devices, such as force sensors, resonators, piezoelectric generators, catalysts, chemical and biomedical sensors, and transparent conductors. Because the nanowire components in these devices are often subjected to large bending or linear stresses and electrical or thermal fields during device operation, the device performance depends on mechanical and thermal responses and the coupling between such behaviors activated by thermal, electrical, or mechanical excitations. The thermomechanical coupling provides both challenges for ensuring thermomechanical reliability and functionality of nano systems and opportunities for developing "tunable" nano devices. The focus is on the characterization of strength, stress-induced structural changes, stress-dependant thermal conductivity, and relations between structures and thermomechanical responses of these nanomaterials. Of particular interest is the coupling between the thermal and mechanical behaviors, including the effect of size on behaviors and the mechanisms for altering or tuning the thermal behaviors through structural changes induced by mechanical input.
Nano-actuator/sensor materials (Cu, Ni, Au)
- Figure 2. Novel shape memory in face-centered-cubic nanowires of single-crystalline Cu, Ni, and Au, (a) transformation through twin boundary propagation, (b) temperature-stress cycle for activating novel shape memory behavior (group of M. Zhou).
Shape memory materials have important applications involving coupling, sensing, and actuation because of their ability to recover certain configurations under proper thermomechanical conditions. They are sometimes referred to as "smart materials" or "active materials" because they can function simultaneously as sensors and actuators. Until recently, the shape memory effect (SME) and its underlying pseudoelasticity were considered unique to shape memory alloys (SMAs), liquid crystal elastomers, and piezoelectric ceramics. Using molecular dynamics (MD) simulations in conjunction with experimental evidence reported in relevant literature, we discovered a novel SME in single-crystalline metal nanowires (including Cu, Ni, and Au) with lateral sizes smaller than approximately 5 nm. First discovered by research conducted within the MPRL by the group of Min Zhou, under proper thermal and mechanical conditions these wires can recover elongations of up to 50 percent, well beyond the recoverable strains of 5 to 8 percent typical for most bulk shape memory alloys (SMAs). This behavior arises from a reversible lattice reorientation within the face-centered cubic (FCC) crystalline structure and is driven by the surface stress and high surface-to-volume ratios of the one-dimensional nanomaterials, a unique and hitherto unknown mechanism which is different from that of SMAs. This research focuses on the deformation mechanism, driving force, and critical temperature for this SME, with a particular emphasis on the role of generalized stacking fault energies in determining the existence of the pseudoelastic behavior and the SME. This research is expected to yield data and criterion for the design, development, and selection of FCC nanowires for potential applications in a range of NEMS devices that utilize pesudoelsticity and shape memory at the nanoscale.