Blister Formation and Layer Transfer of N-implanted GaAs.

In this thesis, the blister formation and layer transfer of GaAs:N nanocomposite layers produced by N-implantation, wafer bonding, and rapid thermal annealing (RTA) of GaAs were investigated. In addition, we examined the electrical and thermal transport properties of GaAs:N nanocomposite layers. To examine blister formation mechanisms, the influence of implantation temperature on blister exfoliation depths, lattice damage depth profiles, and N ion fluences was examined. For implantation temperatures of -196 and 300 ºC, we observed an implantation-temperature-insensitivity of blister formation, in contrast to reports of GaAs:H and Si:H, likely due to the lower GaAs:N ion-matrix diffusivity in comparison to that of GaAs:H or Si:H. These results illustrate the key role of diffusivity on the mechanisms of blister formation. The influence of post-implantation RTA on the surface morphology, electrical properties, and Seebeck coefficient of GaAs:N nanocomposite films was examined for RTA temperatures between 800 and 900 ºC. A transition in surface morphology from circular to predominantly elongated features was observed, and attributed to two distinct delamination behaviors. The influence of implantation and RTA on the free carrier concentration, n, and resistivity, ρ, of GaAs:N(Si) and GaAs:N(Te) was examined. For GaAs:N, ρ follows a log-log dependence on n, independent of the dopant species and RTA conditions. Following implantation plus RTA, decreased n and increased ρ were observed for both dopant types with a more significant increase in ρ for the Te-doped GaAs:N layer. In addition, the Seebeck coefficient of the GaAs:N nanocomposite layer is enhanced in comparison to that of GaAs. Finally, the demonstration and optimization of a new process for simultaneous nanostructuring and layer transfer, termed “ion-cut-synthesis,” is described. Indeed, the low ion-matrix diffusivity of GaAs:N enabled the formation of both nanocrystals and gas bubbles at high temperature. In this technique, N ion implantation, spin-on glass-mediated wafer bonding, and RTA are used to achieve simultaneous nanostructuring and transfer of GaAs:N films to Al2O3 and AlN substrates. We identify the critical role of thermal-expansion coefficient matching on the success of the ion-cut-synthesis process.Ph.D.Mechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/75999/1/rcollino_1.pd

'Preconditioning' with Low Dose Lipopolysaccharide Aggravates the Organ Injury/Dysfunction Caused by Hemorrhagic Shock in Rats

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are creditedRS is supported by the Program Science without Borders, CAPES Foundation, Ministry of Education of Brazil, Brasilia/DF, Brazil; NSAP is, in part, supported by the Bart’s and The London Charity (753/1722). The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement no 608765, from the William Harvey Research Foundation and University of Turin (Ricerca Locale ex-60%). This work contributes to the Organ Protection research theme of the Barts Centre for Trauma Sciences, supported by the Barts and The London Charity (Award 753/1722

Scaling relationships for acoustic control of two-phase microstructures during direct-write printing

Acoustic forces can align and consolidate particles in fluids, enabling microstructural control of two-phase materials at time-scales compatible with direct-write printing of composites. This paper presents key scaling relationships for acoustically-assisted direct-write printing that describe characteristic time-scales for assembly and alignment of particles during printing. Critical combinations of system parameters (including particle and nozzle dimensions, acoustic excitation amplitude, viscosity, and flow rate) are defined that govern particle focusing and assembly in the print stream. The results can be used to identify combinations of printing protocols and nozzle configurations that control particle packing parallel and transverse to the print direction. Impact statement We present theory and experiments demonstrating acoustic focusing in conjunction with direct-write printing for ‘on-the-fly’ control of two-phase microstructures, and a design framework for printing arbitrary material combinations

Detachment of compliant films adhered to stiff substrates via van der Waals interactions: role of frictional sliding during peeling

The remarkable ability of some plants and animals to cling strongly to substrates despite relatively weak interfacial bonds has important implications for the development of synthetic adhesives. Here, we examine the origins of large detachment forces using a thin elastomer tape adhered to a glass slide via van der Waals interactions, which serves as a model system for geckos, mussels and ivy. The forces required for peeling of the tape are shown to be a strong function of the angle of peeling, which is a consequence of frictional sliding at the edge of attachment that serves to dissipate energy that would otherwise drive detachment. Experiments and theory demonstrate that proper accounting for frictional sliding leads to an inferred work of adhesion of only approximately 0.5 J m(−2) (defined for purely normal separations) for all load orientations. This starkly contrasts with the interface energies inferred using conventional interface fracture models that assume pure sticking behaviour, which are considerably larger and shown to depend not only on the mode-mixity, but also on the magnitude of the mode-I stress intensity factor. The implications for developing frameworks to predict detachment forces in the presence of interface sliding are briefly discussed

Flow switching in microfluidic networks using passive features and frequency tuning

Manipulating fluids in microchips remains a persistent challenge in the development of inexpensive and portable point-of-care diagnostic tools. Flow in microfluidic chips can be controlled via frequency tuning, wherein the excitation frequency of a pressure source is matched with the characteristic frequencies of network branches. The characteristic frequencies of each branch arise from coupling between fluid in the channels and passive deformable features, and can be programmed by adjusting the dimensions and stiffness of the features. In contrast to quasi-static ‘on–off’ valves, such networks require only a single active element and relatively small dynamic displacements. To achieve effective flow switching between different pathways in the chip, well-separated peak frequencies and narrow bandwidths are required (such that branches are independently addressable). This paper illustrates that high selectivity can be achieved in fluidic networks that exploit fluidic inertia, with flow driven selectively at peak frequencies between 1–100 Hz with bandwidths less than 25% of the peak frequency. Precise frequency-based flow switching between two on-chip microchannels is demonstrated. A simple theoretical framework is presented that predicts the characteristic frequencies in terms of feature properties, thus facilitating the design of networks with specific activation frequencies. The approach provides a clear pathway to simplification and miniaturization of flow-control hardware for microchips with several fluidic domains

Heterogeneity in millimeter-scale Ti-6Al-4V lattice primitives: Challenges in defining effective properties for metamaterial design

The effective design of metallic metamaterials, characterized by interconnected struts or 'lattices,' hinges on the ability to predict strut and strut intersection ('node') responses. This is critical for predicting the macroscopic properties of structures comprised of thousands of struts. Computationally efficient beam descriptions, defined by strut properties like cross-sectional area, modulus, and yield stress, can significantly expedite the prediction of lattice structures and ultimately enable topology optimization. This paper provides a comprehensive examination of the properties of electron-beam melted three-dimensional printed struts and their 'nodes'—four intersecting struts. The findings elucidate the efficacy of various strategies for defining effective properties that accurately capture mechanical response. The study reveals that using a single set of effective properties can introduce inconsistencies between strut stiffness, peak load, and critical displacement. Stiffness correlates with averaged cross-sectional areas, while the peak load capacity correlates more closely with minimum inscribed cross-sectional areas. Analysis of nodes indicates that the interaction of surface defects and heterogeneity within the node strongly influences multi-strut response. Data from CT scans, EBSD scans, and nanoindentation maps highlight spatial variations comparable to the strut diameter, posing a significant challenge in defining effective homogenized properties. This study emphasizes the need for future efforts to integrate statistical property distributions with high throughput simulations to overcome the difficulty in defining a representative volume element (RVE) at the strut scale