Modeling the Mechanical Response of Polycrystalline Thin Films

Microelectromechanical systems (MEMS) are part of every modern technological advance. Electrodeposited thin nickel (Ni) polycrystalline films in MEMS often show fiber texture resulting in transverse isotropic elastic properties. It is of interest to determine these elastic properties, in particular the in-plane Young\u27s modulus, since it plays a fundamental role in device performance. The fabrication process of MEMS films introduces uncertainties in the microstructure geometry, crystallographic texture, the crystal elastic constants, the physical film dimensions and other parameters. In this thesis the numerical value of the in-plane Young\u27s modulus of thin Ni polycrystalline films is predicted. The predicted values lie between the Reuss-Voigt averages, a result that is consistent with theory. Additionally the uncertainties of the predictions of the in-plane Young\u27s moduli are quantified by taking into account the uncertainties in microstructure geometry, crystallographic texture, and the numerical values of the Ni single-crystal constants. Representative volumes of the microstructure geometry are modeled with Voronoi diagrams. The crystallographic texture is numerically generated from real X-ray diffraction experimental data by using a texture discretization algorithm. The Young\u27s modulus is estimated by simulating uniaxial stress tests on the numerically generated microstructures with a self-consistent fast Fourier transform (FFT) method. The uncertainties in microstructure geometry, crystallographic texture, and single-crystal elastic constant values are treated as epistemic due to the lack of available experimental data. The sensitivity of the in-plane Young\u27s modulus is examined with respect to the three uncertainties addressed above. The study of the propagation of uncertainties throughout the model lead us to the conclusion that the in-plane Young\u27s modulus of the electrodeposited thin Ni films is extremely sensitive only with respect to the uncertainties in the Ni crystal constants. A Voronoi based algorithm that attempts to simulate the complete polycrystalline film microstructure geometry is also developed for future large-scale simulations. Finally a J2 plasticity model that attempts to decribe the overall mechanical response of the Ni film is developed. The J2 model is based on a phase-field dislocation model developed by Koslowski and it includes the Hall-Petch size effect. The numerical predictions of the J2 model are compared with real tensile stress experiments performed on as-deposited and annealed Ni samples. The J2 model predictions show good agreement with phase-field simulations, and capture aspects of size effects

2D materials and van der Waals heterostructures

The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With new 2D materials, truly 2D physics has started to appear (e.g. absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc). Novel heterostructure devices are also starting to appear - tunneling transistors, resonant tunneling diodes, light emitting diodes, etc. Composed from individual 2D crystals, such devices utilize the properties of those crystals to create functionalities that are not accessible to us in other heterostructures. We review the properties of novel 2D crystals and how their properties are used in new heterostructure devices

Advancement and applications of the template matching approach to indexing electron backscatter patterns

Electron backscatter diffraction is a well-established characterisation technique used to determine the orientation and crystal phase of a crystalline material. A pattern is formed by dynamical interaction of elections with the crystal lattice, which can be understood and simulated by using Bloch wave theory. The conventional method of indexing a diffraction pattern is to use a Hough transform to convert the lines of the pattern to points that are easily accessible to a computer. As the bands of the pattern are direct projections of the crystal planes, the interplanar angles can then be computed and compared to a look up table to determine phase and orientation. This method works well for most examples, however, is not well suited to more complex unit cells, due to the fact it ignores more subtle features of the patterns. This thesis proposes a refined template matching approach which uses efficient pattern matching algorithms, such as those used in the field of computer vision, for phase determination and orientation analysis. This thesis introduces the method and demonstrates its efficacy, as well as introducing advanced methods for pseudosymmetry analysis and phase mapping. A new metric for phase confidence is also proposed and the refined method is shown to be able to correctly determine phases and pseudosymmetric orientations. Finally, preliminary work on a direct electron detector stage is presented. Work on the development, testing the pattern centre reliability, modulation transfer and an example map is shown.Open Acces

Impact of ion irradiation damage on SiC and ZrN mechanical properties

Key to the safe operation of nuclear reactors is the understanding of materials degradation due to neutron damage. Ion implantation is often used as a surrogate for nutron damage when screening nuclear candidate materials. Ion implantation results in a thin damage layer, the mechanical properties of which are often difficult to determine. In this study a micromechanical test regime is developed in a model material, 6H single crystal silicon carbide (SiC). This test technique is then applied to gold ion irradiated zirconium nitride (ZrN). Micromechanical test samples are often prepared using a focused ion beam. However, ion beam milling has the potential to damage the crystal structure of a material and introduce residual stress. Therefore, a range of cutting strategies were used to assess the effects of focused ion beam cutting on the modulus and strength of SiC cantilevers. The effects of sample size were also explored. Gallium ion milling resulted in amorphisation of the surface of the SiC crystal micro cantilevers. The thickness of the amorphous zone was then reduced using low voltage cleaning. Low voltage cleaning did not, however, result in increased mechanical performance as other unintended consequences such as cantilever edge rounding occurred. SiC exhibited a plastic deformation threshold of around 0.3 × 0.3 µm but did not exhibit a significant size effect. Nanoindentation was used as a benchmark test to compare to the mechanical properties gathered during micro bend testing. Under indentation conditions, a size effect was identified in hardness and modulus but not in fracture toughness. Modulus results from indentation, and micro bend testing was comparable when ion damage was accounted for.Hot pressed ZrN samples were ion implanted with gold ions. Microstructural characterisation, nanoindentation and micromechanical tests were performed in the ion implanted zone. Microstructural characterisation identified a dual phase microstructure consisting of ZrN and Zr2ON2. The implanted layer consisted of implanted gold ions followed by a network of dislocations centred around a depth of 1.20 µm. High-resolution electron backscatter diffraction (HR-EBSD) identified that tensile surface stresses and compressive subsurface stress had been introduced. Nanoindentation linked ion implantation to increased hardness and no modification in modulus. Micromechanical testing indicated a reduction in modulus and strength. This work highlighted the need to understand sample size effect and ion damage on micro mechanical tests if they are to be used for screening nuclear materials.</div

Collective magnetotaxis of microbial holobionts is optimized by the three-dimensional organization and magnetic properties of ectosymbionts

International audienceOver the last few decades, symbiosis and the concept of holobiont—a host entity with a population of symbionts—have gained a central role in our understanding of life functioning and diversification. Regardless of the type of partner interactions, understanding how the biophysical properties of each individual symbiont and their assembly may generate collective behaviors at the holobiont scale remains a fundamental challenge. This is particularly intriguing in the case of the newly discovered magnetotactic holobionts (MHB) whose motility relies on a collective magnetotaxis (i.e., a magnetic field-assisted motility guided by a chemoaerotaxis system). This complex behavior raises many questions regarding how magnetic properties of symbionts determine holobiont magnetism and motility. Here, a suite of light-, electron- and X-ray-based microscopy techniques [including X-ray magnetic circular dichroism (XMCD)] reveals that symbionts optimize the motility, the ultrastructure, and the magnetic properties of MHBs from the microscale to the nanoscale. In the case of these magnetic symbionts, the magnetic moment transferred to the host cell is in excess (10 2 to 10 3 times stronger than free-living magnetotactic bacteria), well above the threshold for the host cell to gain a magnetotactic advantage. The surface organization of symbionts is explicitly presented herein, depicting bacterial membrane structures that ensure longitudinal alignment of cells. Magnetic dipole and nanocrystalline orientations of magnetosomes were also shown to be consistently oriented in the longitudinal direction, maximizing the magnetic moment of each symbiont. With an excessive magnetic moment given to the host cell, the benefit provided by magnetosome biomineralization beyond magnetotaxis can be questioned

Optical and Crystal Structure Characterizations of Nanowires for Infrared Applications

abstract: Semiconductor nanowires (NWs) are one dimensional materials and have size quantization effect when the diameter is sufficiently small. They can serve as optical wave guides along the length direction and contain optically active gain at the same time. Due to these unique properties, NWs are now very promising and extensively studied for nanoscale optoelectronic applications. A systematic and comprehensive optical and microstructural study of several important infrared semiconductor NWs is presented in this thesis, which includes InAs, PbS, InGaAs, erbium chloride silicate and erbium silicate. Micro-photoluminescence (PL) and transmission electron microscope (TEM) were utilized in conjunction to characterize the optical and microstructure of these wires. The focus of this thesis is on optical study of semiconductor NWs in the mid-infrared wavelengths. First, differently structured InAs NWs grown using various methods were characterized and compared. Three main PL peaks which are below, near and above InAs bandgap, respectively, were observed. The octadecylthiol self-assembled monolayer was employed to passivate the surface of InAs NWs to eliminate or reduce the effects of the surface states. The band-edge emission from wurtzite-structured NWs was completely recovered after passivatoin. The passivated NWs showed very good stability in air and under heat. In the second part, mid-infrared optical study was conducted on PbS wires of subwavelength diameter and lasing was demonstrated under optical pumping. The PbS wires were grown on Si substrate using chemical vapor deposition and have a rock-salt cubic structure. Single-mode lasing at the wavelength of ~3000-4000 nm was obtained from single as-grown PbS wire up to the temperature of 115 K. PL characterization was also utilized to demonstrate the highest crystallinity of the vertical arrays of InP and InGaAs/InP composition-graded heterostructure NWs made by a top-down fabrication method. TEM-related measurements were performed to study the crystal structures and elemental compositions of the Er-compound core-shell NWs. The core-shell NWs consist of an orthorhombic-structured erbium chloride silicate shell and a cubic-structured silicon core. These NWs provide unique Si-compatible materials with emission at 1530 nm for optical communications and solid state lasers.Dissertation/ThesisPh.D. Electrical Engineering 201