Bonnie Beth McKenzie

ZnO Nanostructures as Efficient Antireflection Layers in Solar Cells

An efficient antireflection coating (ARC) can enhance solar cell performance through increased light coupling. Here, we investigate solution-grown ZnO nanostructures as ARCs for Si solar cells and compare them to conventional single layer ARCs. We find that nanoscale morphology, controlled through synthetic chemistry, has a great effect on the macroscopic ARC performance. Compared with a silicon nitride (SiN) single layer ARC, ZnO nanorod arrays display a broadband reflection suppression from 400 to 1200 nm. For a tapered nanorod array with average tip diameter of 10 nm, we achieve a weighted global reflectance of 6.6%, which is superior to an optimized SiN single layer ARC. Calculations using rigorous coupled wave analysis suggest that the tapered nanorod arrays behave like modified single layer ARCs, where the tapering leads to impedance matching between Si and air through a gradual reduction of the effective refractive index away from the surface, resulting in low reflection particularly at longer wavelengths and eliminating interference fringes through roughening of the air-ZnO interface. According to the calculations, we may further improve ARC performance by tailoring the thickness of the bottom fused ZnO layer and through better control of tip tapering.

Room-temperature voltage tunable phonon thermal conductivity via reconfigurable interfaces in ferroelectric thin films.

Dynamic control of thermal transport in solid-state systems is a transformative capability with the promise to propel technologies including phononic logic, thermal management, and energy harvesting. A solid-state solution to rapidly manipulate phonons has escaped the scientific community. We demonstrate active and reversible tuning of thermal conductivity by manipulating the nanoscale ferroelastic domain structure of a Pb(Zr0.3Ti0.7)O3 film with applied electric fields. With subsecond response times, the room-temperature thermal conductivity was modulated by 11%.

Nanostructured Ruthenium Oxide Electrodes via High-Temperature Molecular Templating for Use in Electrochemical Capacitors

Ruthenium oxide is a model pseudocapacitive materials exhibiting good electronic and protonic conduction and has been shown to achieve very high gravimetric capacitances. However, the capacitance of thermally prepared ruthenium oxide is generally low because of low protonic conductivity resulting from dehydration of the oxide upon annealing. High-temperature processing, however also produces the electrically conducting ruthenium oxide rutile phase, which is of great interest for electrochemical capacitors. Here, unusual electrochemical characteristics were obtained for thermally prepared ruthenium oxide when fabricated in the presence of alkyl-thiols at high temperature. The performance characteristics have been attributed to enhanced multifunctional properties of the material resulting from the novel processing. The processing method relies on a simple, solution-based strategy that utilizes a sacrificial organic template to sterically direct hierarchical architecture formation in electro-active ruthenium oxide. Thin films of the templated RuO(2) exhibit energy storage characteristics comparable to hydrous ruthenium oxide materials formed under dramatically different conditions. Extensive materials characterization has revealed that these property enhancements are associated with the retention of molecular-sized metal oxide clusters, high hydroxyl concentrations, and formation of hierarchical porosity in the ruthenium oxide thin films.

Low‐hazard metallography of moisture‐sensitive electrochemical cells

A low‐hazard approach is presented to prepare metallographic cross‐sections of moisture‐sensitive battery components. The approach is tailored for evaluation of thermal (molten salt) batteries composed of thin pressed‐powder pellets, but has general applicability to other battery electrochemistries. Solution‐cast polystyrene is used to encapsulate cells before embedding in epoxy. Nonaqueous grinding and polishing are performed in an industrial dry room to increase throughput. Lapping oil is used as a lubricant throughout grinding. Hexane is used as the solvent throughout processing; occupational exposure levels are well below the limits. Light optical and scanning electron microscopy on cross‐sections are used to analyse a thermal battery cell. Spatially resolved X‐ray diffraction on oblique angle cut cells complement the metallographic analysis.

Application of Electron Backscatter Diffraction for Crystallographic Characterization of Tin Whiskers

Understanding the growth of whiskers or high aspect ratio features on substrates can be aided when the crystallography of the feature is known. This study has evaluated three methods that utilize electron backscatter diffraction (EBSD) for the determination of the crystallographic growth direction of an individual whisker. EBSD has traditionally been a technique applied to planar, polished samples, and thus the use of EBSD for out-of-surface features is somewhat more difficult and requires additional steps. One of the methods requires the whiskers to be removed from the substrate resulting in the loss of valuable physical growth relationships between the whisker and the substrate. The other two techniques do not suffer this disadvantage and provide the physical growth information as well as the crystallographic growth directions. The final choice of method depends on the information required. The accuracy and the advantages and disadvantages of each method are discussed.

Characterization of Void-Dominated Ductile Failure in Pure Ta.

Mechanistic descriptions of void-dominated ductile failure are largely based on studies from several decades ago that relied on optical microscopy techniques [1] [2]. This talk will discuss how modern techniques, such as scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), site-specific focused ion beam (FIB) sample preparation, and transmission electron microscopy (TEM), can enable new insight into the ductile failure process, particularly at the nano to microscale. Because this work is part of a collaborative effort to develop predictive computational models for failure of BCC metals, experiments are focused on characterizing the void-dominated ductile failure process in pure Ta.

Deformation Behaviors of Sub-micron and Micron Sized Alumina Particles in Compression.

The ability to integrate ceramics with other materials has been limited due to high temperature (>800°C) ceramic processing. Recently, researchers demonstrated a novel process, aerosol deposition (AD), to fabricate ceramic films at room temperature (RT). In this process, sub-micron sized ceramic particles are accelerated by pressurized gas, impacted on the substrate, plastically deformed, and form a dense film under vacuum. This AD process eliminates high temperature processing thereby enabling new coatings and device integration, in which ceramics can be deposited on metals, plastics, and glass. However, knowledge in fundamental mechanisms for ceramic particles to deform and form a dense ceramic film is still needed and is essential in advancing this novel RT technology. In this work, a combination of experimentation and atomistic simulation was used to determine the deformation behavior of sub-micron sized ceramic particles; this is the first fundamental step needed to explain coating formation in the AD process. High purity, single crystal, alpha alumina particles with nominal sizes of 0.3 μm and 3.0 μm were examined. Particle characterization, using transmission electron microscopy (TEM), showed that the 0.3 μm particles were relatively defect-free single crystals whereas 3.0 μm particles were highly defective single crystals or particles contained low angle grain boundaries. Sub-micron sized Al2O3 particles exhibited ductile failure in compression. In situ compression experiments showed 0.3μm particles deformed plastically, fractured, and became polycrystalline. Moreover, dislocation activity was observed within these particles during compression. These sub-micron sized Al2O3 particles exhibited large accumulated strain (2-3 times those of micron-sized particles) before first fracture. In agreement with the findings from experimentation, atomistic simulations of nano-Al2O3 particles showed dislocation slip and significant plastic deformation during compression. On the other hand, the micron sized Al2O3 particles exhibited brittle fracture in compression. In situ compression experiments showed 3μm Al2O3 particles fractured into pieces without observable plastic deformation in compression. Particle deformation behaviors will be used to inform Al2O3 coating deposition parameters and particle-particle bonding in the consolidated Al2O3 coatings.

Characterization of SOI MEMS Sidewall Roughness

Deep reactive ion etching (DRIE) of silicon enables high aspect ratio, deep silicon features that can be incorporated into the fabrication of microelectromechanical systems (MEMS) sensors and actuators. The DRIE process creates silicon structures and consists of three steps: conformal polymer deposition, ion sputtering, and chemical etching. The sequential three step process results in sidewalls with roughness that varies with processing conditions. This paper reports the sidewall roughness for DRIE etched MEMS as a function of trench width from 5 μm to 500 μm for a 125 μm thick device layer corresponding to aspect ratios from 25 to 0.25. Using a scanning electron microscope (SEM), the surfaces were imaged detecting an upper region exhibiting a scalloping morphology and a rougher lower region exhibiting a curtaining morphology. The height of rougher curtaining region increases linearly with aspect ratio when the etch cleared the entire device layer. The surface roughness for two trench widths: 15 μm and 100 μm were further characterized using an atomic force microscope (AFM), and RMS roughness values are reported as a function of height along the surface. The sidewall roughness varies with height and depends on the trench width.© 2011 ASME

Polycrystalline Diamond Micromechanical Resonators with Nanometer Dimensions

This paper reports the fabrication technology and testing of polycrystalline diamond (poly-C) mechanical resonators with dimensions as small as 100 nm for the first time. The fabricated structures include cantilever structures patterned using electron beam lithography to have widths of 100 nm, and torsional paddle resonators with a support beam width of 0.5 mum, which sustained a proof mass around 6.16 times 10-13 Kg. The performance of these structures shows resonant frequencies and quality factor (Q) values in the range of 23 KHz-805 KHz and 9,580-103,600 respectively. The structures were tested using piezoelectric actuation and light interferometer detection techniques. Torsional resonators were fabricated from different poly-C films in order to study the influence of the poly-C film in the resonator Q.

Characterization of fire hazards of aged photovoltaic balance-of-systems connectors

Three balance of systems (BOS) connector designs common to industry were investigated as a means of assessing reliability from the perspective of arc fault risk. These connectors were aged in field and laboratory environments and performance data captured for future development of a reliability model. Comparison of connector resistance measured during damp heat, mixed flowing gas and field exposure in a light industrial environment indicated disparities in performance across the three designs. Performance was, in part, linked to materials of construction. A procedure was developed to evaluate new and aged connectors for arc fault risk and tested for one of the designs. Those connectors exposed to mixed flowing gas corrosion exhibited considerable Joule heating that may enhance arcing behavior, suggesting temperature monitoring as a potential method for arc fault prognostics. These findings, together with further characterization of connector aging, can provide operators of photovoltaic installations the information necessary to develop a data-driven approach to BOS connector maintenance as well as opportunities for arc fault prognostics.