Characterization and mechanical properties of solar grade silicon in granular and nanopowder form

Polycrystalline silicon is mainly used for solar cell applications, structures in micro-electromechanical systems, and production of single crystal Si. One of the relatively new methods for producing large quantities of polysilicon is fluidized bed reactor (FBR), where two main morphologies are produced, granular solid (1-3 mm) and nanopowders (30-300 nm). Grinding and fracture occurs in the granular solid during shipping and handling which can affect the final product properties and create safety issues. The microstructure and the morphology of both the granular and the nanopowder forms of Si were examined using scanning and transmission electron microscopes (SEM and TEM). The fracture toughness of the granular silicon was studied, using microindentation and nanoindentation techniques, at different annealing processes, and with different hydrogen concentrations during production. Hydrogen defects in silicon were analyzed using infrared spectroscopy to develop a new relationship between hydrogen and toughness. Based on the microstructural analysis it was shown that the granular Si are mostly crystalline with some amorphous regions linked to small pores, while the nanopowders are mostly amorphous with some crystalline bits; the porosity in the granular Si ranges between 1-4 volume percentage. It was proposed that the primary mechanism in FBR for the granular Si formation is chemical vapor deposition with minor agglomeration associated with pores. It was found that the lower the hydrogen in the production, the higher the fracture toughness where it can be improved up to 45% (from 0.6 to 0.86 MPa.m0.5), and lead for less dust during physical contact. New attrition parameters were proposed in order to better understand the fracture mechanisms of Si granules and other brittle microspheres. These parameters provide a relationship between the mechanical properties (indentation techniques), fracture behavior and failure mechanisms using both crushing tests and impact tests. Part of this thesis also focused on making a beneficial use of the Si nanopowders that are considered secondary products from FBR. The powders were processed into metal-coated carbon composites, using electroplating to improve electrical resistivity. This method can be used to enhance light trapping of solar cells in coated composites

Fracture behavior of brittle microspheres using indentation and compressive loading techniques

The fracture behavior of small spheres of silica, glass, silicon, and yittria-stabilized zirconia (YSZ) has been studied with different loading techniques. Spherical particles come under impact loading both during materials processing, transportation and construction, and high strain rate loading of ballistics; the resulting integrity of the spherical particles plays a role in both the subsequent processing and the energy absorption capabilities of the material. Nanoindentation has been used to measure the hardness (H) and elastic modulus (E), and microindentation has been used to measure the fracture toughness (T) of the materials. High speed X-ray phase contrast imaging was used to examine the failure mechanism under dynamic compression for individual particles with diameters (d) that range between 500 and 2000 µm. Static testing of particulates using bulk indentation of granular solids also can lead to sample failure. A pulverization model has been developed to better understand the failure of the materials. The pulverization parameter is presented by P = Hd 0.5 /T 2. The preliminary results show that materials that exhibit pulverized failure under high strain rate compressive loading, such as silica and glass, have high P values. YSZ shows a single crack under compressive loading and has the smallest P value, whereas silicon exhibits substantial but still distinct cracking and has a medium P value. The effects of strain rates are also discussed in this presentation

Composite metallic nanofoam structures

Metallic nanofoams made of metals such as nickel (Ni) or gold (Au) with ligament sizes on the order of 10’s to 100’s of nm’s exhibit several remarkable properties as a consequence of their low relative density and high specific surface area, such as outstanding strength to weight ratios, enhanced plasmonic behavior and size-effect-enhanced catalytic behavior. However, these metallic nanofoams suffer from macroscopically brittle behavior due to plastic deformation in individual ligaments. With little or no barriers for slip, work-hardening is not possible within ligaments and extremely localized plasticity, once initiated, leads to a few ligaments necking and what appears to be macroscopically brittle failure of the structure under load. Many of the nanofoams produced from metals were originally formed via dealloying. Recently both simulations and experiments have identified that layered ligaments of metallic foams can exhibit significantly improved strength and hardening in Ni-Au core-shell foams[1]. Simulations of Cu-Ni predict that this material combination will exhibit pseudo-elastic behavior and eliminate the macroscopic brittle failure [2]. However, using a metallic foam as a substrate for subsequent layered metallic films limits the amount of metallic layers that can be deposited because the initial foam must have a minimum amount of material (often a solid fraction of approximately 25%). Using a significantly less dense foam as a template should allow for subsequent multilayer growth that would enable larger numbers of layers, and therefore a possible increase in overall strength to total ligament diameter. Single layers have been demonstrated in a prior study [3]. Pulse electroplating from a nickel sulfamate electrolyte bath was used to deposit alternating layers of Ni and Cu. The bath consisted of 90 g l-1 Ni, 0.9 g l-1 Cu and 30 g l-1 boric acid (pH 3-3.5). This solution allows for alternating Ni and Cu layers to be alternately plated by varying the applied voltage, the end layer is actually an alloy of mostly Cu with Ni, and then mostly Ni. Low density foams were selected as a template for subsequent deposition. These included 2% volume carbon nanotubes as well as electrospun carbon fibers. Typical structures are shown in Figure 1. The resulting foams were then indented using flat punch nanoindentation and the effective modulus increased by a factor of three and the elastic recovery after indentation increased substantially (to about one half the original impression depth). The presentation will describe the processing method, the structural changes that occur when the films transition from layer by layer to island growth, and the resulting properties of the foam

3D face animation with opengl ES: An android application

Mobile applications have become ubiquitious with the increase in the popularity and computational power of mobile devices. They can now support rich multimedia user interactions. This project consists of the design and implementaiton of an Android application that renders and animates a three-dimensional model of a human head. The test is synthesized using an Android Text-to-Speech (TTS) engine. The application successfully implements a novel solution for the animated speech synchronization and opens the door for further work in the field of interactive animation. The project can be extended to become an interface for virtual remote communication or animated text messaging