Medium range structural order in amorphous tantala spatially resolved with changes to atomic structure by thermal annealing

Amorphous tantala (a-Ta2O5) is an important technological material that has wide ranging applications in electronics, optics and the biomedical industry. It is used as the high refractive index layers in the multi-layer dielectric mirror coatings in the latest generation of gravitational wave interferometers, as well as other precision interferometers. One of the current limitations in sensitivity of gravitational wave detectors is Brownian thermal noise that arises from the tantala mirror coatings. Measurements have shown differences in mechanical loss of the mirror coatings, which is directly related to Brownian thermal noise, in response to thermal annealing. We utilise scanning electron diffraction to perform Fluctuation Electron Microscopy (FEM) on Ion Beam Sputtered (IBS) amorphous tantala coatings, definitively showing an increase in the medium range order (MRO), as determined from the variance between the diffraction patterns in the scan, due to thermal annealing at increasing temperatures. Moreover, we employ Virtual Dark-Field Imaging (VDFi) to spatially resolve the FEM signal, enabling investigation of the persistence of the fragments responsible for the medium range order, as well as the extent of the ordering over nm length scales, and show ordered patches larger than 5 nm in the highest temperature annealed sample. These structural changes directly correlate with the observed changes in mechanical loss.Comment: 22 pages, 5 figure

Experimental investigation of the mechanical stiffness of periodic framework-patterned elastomers

Recent advances in the cataloguing of three-dimensional nets mean a systematic search for framework structures with specific properties is now feasible. Theoretical arguments about the elastic deformation of frameworks suggest characteristics of mechanically isotropic networks. We explore these concepts on both isotropic and anisotropic networks by manufacturing porous elastomers with three different periodic net geometries. The blocks of patterned elastomers are subjected to a range of mechanical tests to determine the dependence of elastic moduli on geometric and topological parameters. We report results from axial compression experiments, three-dimensional X-ray computed tomography imaging and image-based finite-element simulations of elastic properties of framework-patterned elastomers

Dielectric mixtures -- electrical properties and modeling

In this paper, a review on dielectric mixtures and the importance of the numerical simulations of dielectric mixtures are presented. It stresses on the interfacial polarization observed in mixtures. It is shown that this polarization can yield different dielectric responses depending on the properties of the constituents and their concentrations. Open question on the subject are also introduced.Comment: 40 pages 12 figures, to be appear in IEEE Trans. on Dielectric

Three-dimensional femtosecond laser nanolithography of crystals

Nanostructuring hard optical crystals has so far been exclusively feasible at their surface, as stress induced crack formation and propagation has rendered high precision volume processes ineffective. We show that the inner chemical etching reactivity of a crystal can be enhanced at the nanoscale by more than five orders of magnitude by means of direct laser writing. The process allows to produce cm-scale arbitrary three-dimensional nanostructures with 100 nm feature sizes inside large crystals in absence of brittle fracture. To showcase the unique potential of the technique, we fabricate photonic structures such as sub-wavelength diffraction gratings and nanostructured optical waveguides capable of sustaining sub-wavelength propagating modes inside yttrium aluminum garnet crystals. This technique could enable the transfer of concepts from nanophotonics to the fields of solid state lasers and crystal optics.Comment: Submitted Manuscript and Supplementary Informatio

Modeling the microstructural evolution during constrained sintering

A numerical model able to simulate solid-state constrained sintering is presented. The model couples an existing kinetic Monte Carlo (kMC) model for free sintering with a finite element model (FEM) for calculating stresses on a microstructural level. The microstructural response to the local stress as well as the FEM calculation of the stress field from the microstructural evolution is discussed. The sintering behavior of a sample constrained by a rigid substrate is simulated. The constrained sintering results in a larger number of pores near the substrate, as well as anisotropic sintering shrinkage, with significantly enhanced strain in the central upper part of the sample surface, and minimal strain at the edges near the substrate. All these features have also previously been observed experimentally.Comment: 9 pages, 7 figure

Experimental study of inhomogeneous deformation in bulk metallic glass and their composite during wedge-like cylindrical indentation

Bulk metallic glasses (BMGs) are amorphous metals with impressive mechanical properties, such as high elastic strain up to 2%, high strength (up to 2% of Young\u27s modulus) and high hardness. Their weight normalized properties exceed the high strength to weight ratio of titanium alloys. Because of the lack of crystalline defects such as grain boundaries and dislocations, they have good corrosion resistance and good formability. The unique die molding properties of BMGs render them as excellent candidates for micro-scale machine parts, pressure sensor, golf clubs and casings. BMG\u27s also exhibit enhanced plastic creep resistance, since homogeneous plastic deformation is inhibited at room temperature. Below the glass transition temperature, BMGs exhibit inhomogeneous plastic flow through the formation of localized shear bands. Under unconfined loading geometry, BMGs fails in a brittle material manner with unstable propagation of a single shear band. However, under confined geometry, BMG\u27s show increased ductility due to the ability to nucleate and propagate multiple shear bands. This dissertation focuses on experimentally analyzing evolution and propagation of the shear bands in BMGs and their composites, by monitoring the deformation mechanisms at the scale of the shear band under confined geometry. Wedge-like cylindrical indentation has been used to provide a stable loading configuration for in-situ observation of the inhomogeneous deformation zone underneath the indenter. High resolution digital camera has been employed to capture surface images of the evolution of the process-zone. An in-house digital image correlation (DIC) program has been developed, utilizing MATLAB commercial software, to calculate the in-plane finite strain distribution at the scale of the shear band. First, the plastic deformation and flow field under the indenter are studied in both aluminum and copper alloys with different grain sizes to verify and validate the analysis protocol. The measured plastic zone size is comparable with the one predicted by the simplified cavity model and there is a unique correlation of the strain distribution along the radial line with different angular positions originating from the indentation center. The deformation zones developed under indenters with different radii are found to be self-similar. In the elastic domain, the measured strain distribution agree with FEM predictions; in the elastic-plastic domain, extra hardening is observed, which could be the result of constrained deformation. Second, the inhomogeneous deformation behavior of Vitreloy-1 bulk metallic glass is examined at room temperature. To overcome the resolution limit of the DIC technique to resolve the strain within a single shear band having 10-20nm width, an alternative method is implemented, addressing the strain jump within the band and the surrounding matrix. The results show that the BMG can deform homogenously to a large elastic strain level of about 4-6% before the onset of inhomogeneous deformation via localized shear bands. Such observation indicates the ability of BMG to withstand such high levels of stresses and strains if unstable shear band can be suppressed from the nucleation from the surface, such as the case of tension or bending. Following the perturbation analysis of Hwang et al (2004) and utilizing the same material parameters, it is found that homogenous nucleation strain is of the same order. The experimental measurements show more subtle details about the kinematics of shear band propagations. The shear band propagates intermittently at the expense of the surrounding matrix stored elastic strain energy. The surrounding matrix ceases to deform, during the activity of the shear band, however, no unloading is observed. The accumulated strain level inside of the shear band is about 3 orders higher than the one in the surrounding matrix. By tracking the strain increments of a single shear band and its surrounding matrix, the deformation filed has been shown to be self-similar, within the surrounding matrix. While the stress state at the observation point is defined by the global indentation filed, the local stress state within the shear band is a simple shear state, with respect to the band propagation direction. Relative to the band-propagation direction and the corresponding normal, the surrounding matrix deforms in a pure shear-state to accommodate shear band deformation. The experimental protocol is also utilized to study the kinematics of shear band initiation, propagation and arrest or hindrance by a secondary ductile phase. The deformation mechanisms in BMG composite with brass particles are examined. The composite is manufactured by warm extrusion of a mix of gas atomized powders of Ni-based BMG and brass. The resulting composite has an elongated particulate structure in the extrusion direction. The fracture toughness and toughening mechanism of the BMG composites are examined in the parallel and normal directions to the extrusion axis. This composite shows highly anisotropic properties along different loading directions. For the normal direction loading, brass reinforcements not only trigger the initial localized shear band, but also modify the crack propagation by crack bridging mechanisms. Also, microcracking is another important toughening mechanism. For the parallel direction loading, interface debonding is the main failure mechanism. Using FEM simulations, it is shown that local fracture is strain-controlled along the normal loading direction and stress controlled along the parallel loading direction. The proposed experimental framework is further extended for fracture match applications in forensic science. The likelihood of matching broken pieces, wherein a macroscopic crack trajectory cannot be established is analyzed via spectral analysis of the 3D fracture surfaces. The surface topographies are acquired using a non-contact 3D optical surface profilometer. A quantitative signature of the fracture surface, employing the different length scales of the fracture process zone is derived and used to establish class and sub-class matching. The details of the algorithm and its applications are detailed in the Appendix