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- Open Access.
- Due to their small physical and microstructural size, nanoscale materials show significantly different behavior compared to the bulk. Size effects studies on materials behavior, particularly at the nanoscale, has been a vigorously active area of research. The state of art is to characterize materials properties at individual domains, such as mechanical, electrical and thermal. Fundamental aspects of microstructure-properties relationship in these individual domains are relatively well understood. Nevertheless, experiments relating external stimuli, such as stress, temperature, electrical current, light, ion irradiation to structure and properties at the nanoscale remain challenging at this length-scale. From a fundamental perspective, small size should make materials more sensitive to stimuli compared to bulk. If this hypothesis is validated, one can envision facile tuning of microstructure to actively control materials properties. Such tunability would enhance performance in broader areas of electronics, energy conversion and sensors. This motivates us to experimentally investigate the stimuli-microstructure-property relationship at the nanoscale.The technical contribution of this research is a unique nanofabricated experimental setup that integrates nanoscale specimens with tools for interrogating mechanical (stress-strain, fracture, and fatigue), thermal and electrical (conductivity) properties as function of external stimuli such as strain, temperature, electrical field and radiation. It addresses the shortcomings of the state of the art characterization techniques, which are yet to perform such simultaneous and multi-domain measurements. Our technique has virtually no restriction on specimen material type and thickness, which makes the setup versatile. It is demonstrated with 100 nm thick nickel, aluminum, zirconium; 25 nm thick molybdenum di-sulphide (MoS2), 10 nm hexagonal boron nitride (h-BN) specimens and 100nm carbon nanofiber, all in freestanding thin film form. The technique is compatible with transmission electron microscopy (TEM). In-situ TEM captures microstructural features, (defects, phases, precipitates and interfaces), diffraction patterns and chemical microanalysis in real time. Seeing the microstructure while measuring properties is our unique capability. It helps identifying fundamental mechanisms behind thermo-electro-mechanical coupling and degradation, so that these mechanisms can be used to (i) explain the results obtained for mesoscale specimens of the same materials and experimental conditions and (ii) develop computational models to explain and predict properties at both nano and meso scales. The uniqueness of this contribution is therefore simultaneously quantitative and qualitative probing of length-scale dependent external stimuli effects on microstructures and physical properties of nanoscale materials.The scientific contribution of this research is the experimental validation of the fundamental hypothesis that, if the nanoscale size can cause significant deviation in a certain domain, e.g., mechanical, it can also make that domain more sensitive to external stimuli when compared to bulk. We have showed that mechanical properties of freestanding nanocrystalline thin films have higher sensitivity to elevated temperatures compared to bulk. The Youngs modulus of nanocrystalline aluminum thin film is measured about 50% of the room temperature value at 65% of the melting temperature. The higher volume fraction of grain boundaries can be ascribed to this observation since the inherent disorder on the grain boundary atoms means they are more sensitive to the temperature. At the bulk scale, thermal conductivity of metals is not sensitive to mechanical strain. However, this may not be true for grain sizes below the electron mean free paths, for which mechanical deformation mechanism and volume fraction of grain boundaries are drastically different from the bulk. Our experimental results show strong mechanical strain-thermal conductivity coupling, thermal conductivity of Zr film with average grain size of 10nm dropped from 20 W/m-K to 13 W/m-K with a strain level of only 1.24 %. In this dissertation, we present a series of studies tied by the common thread of synergy of two or more stimuli. The first example is on pure metals, which need very high temperature (> 0.5Tm, where Tm is melting point) or stress (>y, where y is the yield stress) to change microstructure. In contrast, we present experimental evidence of about 100 times grain growth in nanocrystalline nickel at only 0.2Tm (Tm is the melting temperature) when accompanied with only 0.2y (y is the yield stress) stress. This finding contradicts with the classical understanding that grain growth is a plastic deformation (>y) mechanism. Interestingly, stressing the films by high stress (around y) or temperature (0.5Tm) separately produce only insignificant grain growth. These results suggest that when synergistic, external stimuli can exert unprecedented influence over microstructure-properties in nanoscale materials. In a corollary study, we modeled nanocrystalline metals as a standard linear elastic solid and our experimental results on 100 nm thick (average grain size 10 nm) freestanding nickel specimens at temperatures from 300 to 425 K support this hypothesis reasonably well. The viscosity of solid nickel ranged from 3.3x1013 Pa.s to 1.5x1013 Pa.s at these temperatures, which are about two orders of magnitude smaller than that expected for metals and are also less sensitive to temperature compared to bulk. The second case study involved a novel concept of electro-graphitization that induces synergistic thermo-electro-mechanical fields to graphitize carbon nanofibers at around 800 C temperature and below 106 A/cm2 current density. In comparison, conventional graphitization of carbon nanofiber requires very high temperatures (> 2800 C). A more convincing study on the pronounced role of stimuli on microstructure-properties is the transformation of amorphous materials to nano or microcrystalline form. This is because typically the energy barrier for this kind of transformation is very high, requiring extreme conditions to initiate such transformation. To pursue this, we studied 25 nm thick MoS2 and 10 nm h-BN films. Our in-situ TEM heating of these specimens indicated that such phase transformation can be induced to temperatures as low as 600 C. In the third study, we observed anomalous response of GaN microstructure to the externally applied electrical field at nanoscale. 90 domain switching in 100nm thick GaN film was observed at a 107 V/m electrical field, applied perpendicular to the polarization direction. No such switching was observed for thicker films. 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. We also explored the sensitivity of nanoscale materials to photons and ions. With light radiation on monolayer MoS2 films, we observed very strong light-matter interaction (photo voltaic effects) without any apparent rectifying junctions. For bi-layers, no such effect was present, suggesting strong size effect in light-matter interaction. The photo-voltaic effect was observed to highly direction dependent in the film plane, which suggests that the oblique deposition configuration plays a key role in developing the rectifying potential gradient. We have studied ion irradiation effects in Zr thin films, showing significant grain growth (>300%), texture evolution, and displacement damage defects. Stress-strain profiles were mostly linear elastic below 20 nm grain size, but above this limit the samples demonstrated yielding and strain hardening.
Experimental results support the hypothesis that grain boundaries in nanocrystalline metals act as very effective defect sinks.Since microstructures-properties in nanoscale are sensitive to external stimuli, structural stability or degradation of nanoscale materials due to over stimuli will be an important topic to study. To pursue this, we studied the degradation mechanism of graphene and WSe2/graphene heterostructure as a function of temperature and electrical current density. Our experimental results show that high temperature and current density can induce migration of foreign contaminants due to phenomena similar to thermo and electromigration and alloying with foreign elements leads to catastrophic degradation in crystallinity.This dissertation presents experimental evidence of external stimuli effects on the microstructures-properties relationship at nanoscale. The technological aspect of this research impacts nanotechnology applications, like microelectronics, optoelectronics, energy conversion and sensors. However, it is also very fundamental in nature, and the new phenomena that are explored will enrich knowledge of material behavior and lay foundation for the future work on multi-physics of material at nanoscale.
- Dissertation Note:
- Ph.D. Pennsylvania State University, 2017.
- Reproduction Note:
- Microfilm (positive). 1 reel ; 35 mm. (University Microfilms 10-666643)
- Technical Details:
- The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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