Aman Haque

Is stress concentration relevant for nanocrystalline metals

Classical fracture mechanics as well as modern strain gradient plasticity theories assert the existence of stress concentration (or strain gradient) ahead of a notch tip, albeit somewhat relaxed in ductile materials. In this study, we present experimental evidence of extreme stress homogenization in nanocrystalline metals that result in immeasurable amount of stress concentration at a notch tip. We performed in situ uniaxial tension tests of 80 nm thick (50 nm average grain size) freestanding, single edge notched aluminum specimens inside a transmission electron microscope. The theoretical stress concentration for the given notch geometry was as high as 8, yet electron diffraction patterns unambiguously showed absence of any measurable stress concentration at the notch tip. To identify possible mechanisms behind such an anomaly, we performed molecular dynamics simulations on scaled down samples. Extensive grain rotation driven by grain boundary diffusion, exemplified by an Ashby-Verrall type of grain switching process, was observed at the notch tip to relieve stress concentration. We conclude that in the absence of dislocations, grain realignment or rotation may have played a critical role in accommodating externally applied strain and neutralizes any stress concentration during the process.

Freestanding carbon nanotube specimen fabrication

Freestanding specimens are required in experiments investigating mechanical, thermal, electrical and electromechanical properties of nanotubes. We present a lithographic technique for fabricating freestanding nanotube, nanowires and nanorod specimens with their ends embedded in polymers, metals or any other material. The technique is repeatable, flexible, and addresses the issue of firm gripping of the specimens. The flexibility offered by the lithography method will enable co-fabrication of the specimens with sensors and actuators for performing a wide variety of nano-mechanical characterization.

Thermo-mechanical coupling and size effects in micro and nano resonators

Existing models for thermoelastic damping consider geometric size effects only, the focus of this study is on tuning of thermoelastic damping with mechanical strain, which reduces both relaxation rate and thermal conductivity at the nanoscale. We developed a model that accounts for the contribution of tensile force and thermal conductivity in a clamped-clamped configuration nano-resonator. Experimentally measured thermal conductivity is then coupled with the model suggests the existence of a critical length scale (inversion point) below which quality factor increases with increase in thickness and vice versa. The nanoscale strain-thermal conductivity coupling is found to be most effective at and around this inversion point.

Mechanical strain mediated carrier scattering and its role in charge and thermal transport in freestanding nanocrystalline aluminum thin films

In bulk metals, mechanical strain is known not to influence electrical and thermal transport. However, fundamentally different deformation mechanisms and strain localization at the grain boundaries may influence electron or phonon scattering in nanocrystalline materials. To investigate this hypothesis, the authors developed an experimental approach, where the authors performed thermal and electrical conductivity measurements on 100 nm thick freestanding nanocrystalline aluminum films with average grain size of 50 nm in situ inside a transmission electron microscope (TEM). The authors present experimental evidence of decrease in thermal conductivity and increase in electrical resistivity as a function of uniaxial tensile strain. In-situ TEM observations suggest that grain rotation induced by grain boundary diffusion, and not dislocation-based plasticity, is the dominant deformation mechanism in these thin films. The authors propose that diffusion causes rise in oxygen concentration resulting in increased d...

In-situ TEM study of domain switching in GaN thin films

Microstructural response of gallium nitride (GaN) films, grown by metal-organic chemical vapor deposition, was studied as a function of applied electrical field. In-situ transmission electron microscopy showed sudden change in the electron diffraction pattern reflecting domain switching at around 20 V bias, applied perpendicular to the polarization direction. No such switching was observed for thicker films or for the field applied along the polarization direction. This anomalous behavior is explained by the nanoscale size effects on the piezoelectric coefficients of GaN, which can be 2–3 times larger than the bulk value. As a result, a large amount of internal energy can be imparted in 100 nm thick films to induce domain switching at relatively lower voltages to induce such events at the bulk scale.

In situ transmission electron microscopy of transistor operation and failure

Microscopy is typically used as a post-mortem analytical tool in performance and reliability studies on nanoscale materials and devices. In this study, we demonstrate real time microscopy of the operation and failure of AlGaN/GaN high electron mobility transistors inside the transmission electron microscope. Loading until failure was performed on the electron transparent transistors to visualize the failure mechanisms caused by self-heating. At lower drain voltages, thermo-mechanical stresses induce irreversible microstructural deformation, mostly along the AlGaN/GaN interface, to initiate the damage process. At higher biasing, the self-heating deteriorates the gate and catastrophic failure takes place through metal/semiconductor inter-diffusion and/or buffer layer breakdown. This study indicates that the current trend of recreating the events, from damage nucleation to catastrophic failure, can be replaced by in situ microscopy for a quick and accurate account of the failure mechanisms.

(Project 14-6770) An Investigation to Establish Multiphysical Property Dataset of Nuclear Materials Based on in-situ Observations and Measurements

In-core nuclear materials including fuel pins and cladding materials fail due to issues including corrosion, mechanical wear, and pellet cladding interaction. One of the fundamental issues that determine nuclear fuel performance is microstructure evolution dependence of thermal and mechanical properties of fuel pellet and cladding materials and how it is affected by overall oxygen diffusion, hydride formation, fuel thermal expansion, and overall failure of fuel pellet-cladding system, in an irradiation-driven environment. Improved multiscale, multiphysics, and multidimensional simulation capability can significantly help nuclear fuel models especially with respect to pellet-cladding mechanical interaction, fuel fracture, oxide formation etc. However, multiscale microstructure related effects required in such approach require in-situ experimental examination, the subject of the proposed work. The proposed work focuses on presenting an in-situ measurement based experimental approach to facilitate such an understanding with focus on Zircaloy claddings. Emphasis of the proposed research is on supplying the proposed validating dataset for NEAMS simulations as a function of temperature, irradiation, and environment using complementary in-situ experiments. With success, future efforts will be expanded to UO2 and pellet-cladding interactions. This project aims are to: (1) Investigate irradiation driven microstructure change influence on mechanical, as well as thermal properties and (2) Predict the microstructural-thermal-mechanical property relationship, as a function of irradiation damage, temperature, and environment. The literature puts more focus on the thermal properties of the oxides and hydrides, which show compounded deleterious effect after 1200 o C. At this point the oxidation reaction at surface may surpass the decay heat production in the fuel to become the dominant source of fuel temperature rise. Because the oxidizing species is water (steam), the reaction produces a significant amount of hydrogen gas, which precipitates as hydrides. The hydrides increase susceptibility to corrosion, and their low thermal conductivity and high coefficient of thermal expansion adversely impact thermal loading as well as mechanical stability. The role of irradiation, corrosion, and hydride formation are all deleterious and are the critical areas in the research community. The latest need in the area is innovative concepts on small scale testing, considering the specialty of irradiation chambers. Issue at hand is the ability to understand small scale (nanoscale to mm scale) microstructure evolution dependent material behavior. At the nanoscale and microscale, the predominant mechanism becomes surface and interface (i.e., diffusion) mediated deformation. Because of the change in defect dimensionality (1D or 2D), the influence of microstructure on thermal and mechanical properties can be expected to be significant. It is expected, therefore, that experiments performed in the proposed work here can be directly used to validate microstructure evolution dependent multiphysics multiscale modeling predictions made by combination of BISON and MARMOT (e.g. thermal expansion induced cracking and lower length scale microstructure effects, pellet-clad mechanical interaction, effect of oxides, hydrides on pellet-cladding system failure etc.) as well as to improve current multiphysics models. With success of the proposed work more material systems will be tried in collaboration with National Lab Partners.

Role of sulphur atoms on stress relaxation and crack propagation in monolayer MoS2

We present in-situ transmission electron microscopy of crack propagation in a freestanding monolayer MoS2 and molecular dynamic analysis of the underlying mechanisms. Chemical vapor deposited monolayer MoS2 was transferred from sapphire substrate using interfacial etching for defect and contamination minimization. Atomic resolution imaging shows crack tip atoms sustaining 14.5% strain before bond breaking, while the stress field decays at unprecedented rate of 2.15 GPa Å-1. Crack propagation is seen mostly in the zig-zag direction in both model and experiment, suggesting that the mechanics of fracture is not brittle. Our computational model captures the mechanics of the experimental observations on crack propagation in MoS2. While molybdenum atoms carry most of the mechanical load, we show that the sliding motion of weakly bonded sulphur atoms mediate crack tip stress relaxation, which helps the tip sustain very high, localized stress levels.

Zero-Load Friction at Nanowire-Silicon Interfaces

The dominance of adhesive forces at the nanoscale implies that significant friction forces can be generated at the interface even with no externally applied normal load. We have nanofabricated an adhesion-friction force sensor to characterize friction in zinc oxide nanowires on silicon substrates. Experimental results show static friction coefficients for zero externally applied normal load can be as high as 45. This behavior is observed to be strongly influenced by the ambient conditions and we propose that the presence of molecularly thin moisture layers is responsible for the observed pseudo-static friction. The findings of this study will provide valuable input to nanoscale interfacial systems such as nanowires and nanotube based sensors and nanocomposites.Copyright