While "super-sizing" seems to be the driving force of our food industry, the direction of materials research has been quite the opposite: the dimensions of most technological devices are getting ever smaller. These advances in nanotechnology have a tremendous impact on parts of the economy as diverse as information, energy, health, agriculture, security, and transportation. Some of the examples include data storage at densities greater than one terabit per square inch, high-efficiency solid-state engines, single-cell diagnostics of complex diseases (e.g. cancer), and the development of ultra-light yet super-strong materials for vehicles, with the component sizes comprising these technological devices reduced to the sub-micron scale.
The functionality of these devices directly depends on their structural integrity and mechanical stability, driving the necessity to understand and to predict mechanical properties of materials at reduced dimensions. Yield and fracture strengths, for example, have been found to deviate from classical mechanics laws and therefore can no longer be inferred from the bulk response or from the literature. Unfortunately, the few existing experimental techniques for assessing mechanical properties at that scale are insufficient, not easily accessible, and are generally limited to thin films. In order to design reliable devices, a fundamental understanding of mechanical properties as a function of feature size is desperately needed; with the key remaining question whether materials really are stronger when the instrumental artifacts are removed, and if so then why and how.
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Researchers: Lucas Meza, Viki Chernow, Lauren Montemayor, Jan Rys (visiting student)
Dongchan Jang (postdoc)
Collaborators: Mike Baskes (LANL), Bill Carter, Alan Jacobsen (HRL), Lorenzo Valdevit (UC Irvine), John Hutchinson (Harvard), Frank Greer (JPL), Jen Dionne (Stanford), Eugen Rabkin (Technion), David Srolovitz (UPenn), and Yong Wei Zhang (IHCP, Singapore)
Researchers: Zachary Aitken, Xun (Wendy) Gu
Dongchan Jang, Seok-Woo Lee (postdocs)
Collaborators: Chris Weinberger (Sandia NL), Huajian Gao (Brown University), Karin Dahmen (UIUC), and Dennis Kochmann (Caltech)
A long-standing problem in materials science is that of predicting mechanical properties from knowing the initial microstructure and the collective behavior of individual defects. The objective of this project is to develop a physical understanding of plasticity mechanisms operating in single-crystalline, amorphous, and nano-crystalline solids, when material dimensions are reduced to the nanometer scale (or to the size of their instrinsic microstructure) subjected to homogeneous deformation.
Our in-situ experiments are performed in a unique instrument "SEMentor" (SEM + Nanoindenter + Mentor = SEMentor), which is capable of conducting nano mechanical experiments at room temperature down to the cryogenic temperatures while simultaneously capturing video.
The stress-strain data obtained by the SEMentor, combined with the pre- and post- mortem TEM microstructural characterization and modeling efforts present a powerful method for gaining a more thorough understanding of plasticity at the nano-scale. This combination of techniques provides us with critical information for the development of constitutive laws that govern flow stress as a function of size and initial microstructure through an experimentally-based understanding of the underlying phenomena.
Some specific on-going efforts of this project are:
Researchers: Rachel Liontas (graduate student) and Dongchan Jang (postdoc)
Collaborators: Peter Hosemann (UC Berkeley), Mike Demkowicz (MIT), Khalid Hattar (Sandia NL), and Amit Misra (LANL)
Due to imminent demand for clean and sustainable energy sources, there is an emerging focus to utilize nuclear energy as a viable source of energy. New reactors should be significantly safer and more reliable, should have an increased lifetime, proliferation resistance and efficiency trough higher operating temperatures. The demanding service conditions like higher neutron doses, exposure to higher temperatures, and corrosive environments will pose a significant challenge for structural materials in these new reactors. Therefore, the development of new, revolutionary materials systems immune to radiation damage is crucial.
During irradiation, energetic neutrons displace atoms from their lattice sites, creating collision cascades, and affecting the material in an unfavorable way. These displacement cascades involve thousands of atoms, and subsequent defect reactions create stable damage aggregates like vacancy clusters, dislocation loops, and - sometimes - the bubbles, notorious for their role in swelling and embrittlement of irradiated materials. In order to develop a fundamental understanding on the damaging effects of radiation on material health, it is necessary to study these effects in isolated (as opposed to an ensemble) microstructural features. This requires testing "small" samples, whose dimensions are comparable with the intrinsic microstructure size. In order to create innovative materials capable of enhanced radiation resistance, it is imperative to develop a fundamental understanding of deformation processes occurring in the irradiated microstructure. This will serve as foundation for the development of predictive models, which in turn will guide the design codes and defect-assessment criteria that ensure structural integrity and safety of nuclear reactor operation.
This project seeks to obtain quantitative influence of helium sink strength and proximity on helium bubble nucleation and growth in He-irradiated nano-scale metallic structures, and the resulting deformation mechanisms and mechanical properties by applying experimental and computational methods. Nano-scale tension and compression experiments, both ex-situ and in-situ, on low energy He-irradiated samples combined with site-specific microstructural characterization and modeling efforts present a powerful method for gaining this understanding. This systematic approach provides critical information for identifying key factors that govern He bubble nucleation and growth upon irradiation as a function of both sink strength and sink proximity through an experimentally confirmed physical understanding.
We study different interfaces in order of increasing affinity for vacancy and helium absorption: grain boundaries, interfaces and periodically arranged metallic glass/metal multi-layers spanning the entire nano-pillar volume. A FIB-less nano-pillar fabrication approach with precise initial microstructure control (both dislocation density and grain boundaries) and experimentally determined stress-strain relationships are enhanced by in-situ SEM observations coupled with TEM microstructural characterization of the same samples before and after deformation and atomistic modeling.
Researchers: Nisha Mohan, Ee Jane Lim (undergrad)
Sid Pathak (formerly)
Collaborators: A. Needleman (Univ of North Texas), B. Cola (GA Tech), J. Hart (Univ of Michigan)
Vertically aligned carbon nanotube (VACNT) forests have been earmarked for a range of potential applications in areas such as energy dissipation devices, electrical interconnects, thermal interface materials, micro-electro-mechanical-systems (MEMS) and microelectronics. However while individual carbon nanotubes are known for a variety of exceptional properties, the mechanical behavior of VACNT forests is more complicated due to their hierarchical structure which spans multiple length scales (Fig 1). The collective behavior of these materials is expected to rely heavily on the properties of the individual CNTs, as well as on the variations in the collective inter-tube interactions and inherent property gradients of the microstructure, which in turn are controlled by their synthesis techniques.
We have been studying the mechanical response of VACNT forests using a combination of different techniques, such as uniaxial compression and indentation. In combination with traditional ex-situ (in air) methods, we also conduct in-situ experiments in a custom-built nano-mechanical deformation instrument, SEMentor, comprised of a Scanning Electron Microscope (SEM) and a nanoindenter, which allow real-time observations of the evolved morphology of the VACNT during the test.
Our results indicate that the compressive response VACNT cylindrical bundles exhibit unique deformation properties, namely resilience, deformability and energy dissipation, all of which increase at faster strain rates. These materials appear to combine the best aspects of different foam characteristics, such as their ability to switch between various buckled morphologies which makes them capable of undergoing large strains of ~60-80% without generating damaging peak stresses (similar to metallic foams), as well as their high resilience and close to 100% recovery upon load release leading to a higher energy dissipation (as in polymeric foams) - see video. Furthermore, their multifunctional nature, high electrical (comparable to copper) and thermal (similar to metals) conductivities, and relative ease of manufacture identifies VACNT bundles as a completely new class of material systems entering the "white space" in the material property chart, as shown in Fig 2.
In contrast, during indentation similarly grown VACNT films first deform by a sudden shear offset, manifested as a ~20 μm displacement burst, and exhibit marginal recoverability during cycling (see video). Subsequent localized buckle formation and onset of densification are found to be a strong function of substrate proximity. We are currently analyzing the constraint-driven differences in boundary conditions that govern both recoverability and morphological deformation signature under indentation as compared with compression.
One possible reason that VACNTs can have such high variance in recoverability is differences in tortuosity, or the “curviness” and “tangled-ness” of the individual tubes. In order to quantitatively determine the tortuosity we analyzed SEM images of a sample in two ways - a human-assisted method to extract an average radius of curvature (ARC), and using image analysis methods present in FIJI (FIJI’s Is Just ImageJ) to detect the total tube length (TTL). ARC allows us to define a “figure of merit” to evaluate trends among images against which we can compare TTL.
To produce ARC results, we trace small regions of characteristic curves in the front most layers of nanotubes and fit the traces to a circle, from which we extract a radius of curvature. This average is adjusted by a weighting factor generated by evaluating a randomly-centered grid of 35 points as 0 (“straight”) or 1 (“curved”).
TTL results are achieved by reducing the image to a “skeleton”, i.e. a network of single-pixel width (1-D) lines which can easily be analyzed for qualities such as Euclidian distance and start/end points. The number of tubes in an image are analyzed based on the number of start/end points at the top and bottom edges of an image, assuming that tubes run from top to bottom. The average tortuosity for the image is calculated as the ratio of the total Euclidian length to the total length of all tubes, assuming all tubes are the height of the image.
Researchers: Lucas Meza (graduate student) and Seok-Woo Lee (postdoc)
Collaborators: A. Shapiro, P. Beauchamp (JPL), J. Beauchamp (Caltech), J. Lunine (Cornell)
NASA's planned future missions to planetary bodies of extreme environments, such as Titan, Europa and Earth's Moon all of which have very low surface temperatures, represent some of the most severe challenges for the functionality of electronics. In this project we investigate the deformation mechanisms of two material systems, namely carbon nanotube (CNT) bundles and Sn based soldering materials, for potential applications in cryogenic microelectronics.
The ability to elastically sustain loads at large deflection angles has earmarked CNTs as uniquely tough materials for energy-absorbing applications. However, while the mechanical properties of individual CNTs in tension are promising, they degrade when the size is increased toward the micron scale, especially under compression. To fulfill the technical requirements for the extreme space conditions, it thus becomes extremely important to project the outstanding properties of CNTs to larger scales, and to understand and control adhesion between the nanotubes and the underlying substrate.
Similarly, Sn is known to undergo both phase transition and a ductile-to-brittle (DTB) transition in these temperature regimes, which can potentially compromise their mechanical integrity in such applications. We use nano-mechanical (both indentation and nano-pillar compression) testing from room down to cryogenic (77K) temperatures to ascertain the effects of temperature on phase and DTB transitions, as well as study the deformation mechanism under these conditions.
Researchers: Shi Luo (Caltech) and Jiun-Haw Lee (National Taiwan University)
Collaborators: Chee-Wee Liu (National Taiwan University), C.C. Tsai (National Chiao Tung University), and Harry Atwater (Caltech)
Funding for this project is provided by the Caltech-Taiwan Energy Exchange
Researchers: David Chen, Jan Rys (graduate students) and Dongchan Jang (postdoc)
Collaborators: Yunfeng Shi (RPI), Bill Carter, Alan Jacobsen, and Toby Schaedler (HRL)