A New Microtensile Tester for the Study of MEMS Materials with the Aid of Atomic Force Microscopy

An apparatus has been designed and implemented to measure the elastic tensile properties (Young's modulus and tensile strength) of surface micromachined polysilicon specimens. The tensile specimens are "dog-bone" shaped ending in a large "paddle" for convenient electrostatic or, in the improved apparatus, ultraviolet (UV) light curable adhesive gripping deposited with electrostatically controlled manipulation. The typical test section of the specimens is 400 µm long with 2 µm x 50 µm cross section. The new device supports a nanomechanics method developed in our laboratory to acquire surface topologies of deforming specimens by means of Atomic Force Microscopy (AFM) to determine (fields of) strains via Digital Image Correlation (DIC). With this tool, high strength or non-linearly behaving materials can be tested under different environmental conditions by measuring the strains directly on the surface of the film with nanometer resolution

The integration of share repurchases into US and UK listed firms’ financial decision-making

This study explores the question whether share repurchases are an integral part of US and UK firms’ financial decision-making, or whether they are merely an afterthought and therefore not systematically related to managers’ principal financial decisions, namely dividends, investment and leverage. It aims to address concerns that share repurchases might be detrimental to firms’ ability to create value through investment (FINNOV, 2012) and can lead to the excessive leverage of companies (Foroohar, 2013). As the US and the UK display differences in terms of the legal and institutional environment, the first two chapters focus in the US and the UK respectively. The US findings indicate that share repurchases are driven not merely by free cash flows, but also by decisions about investment and dividends, and both dividends and investments are in turn affected by share repurchases. The fact that these results hold both for the period before and subsequent to the credit crunch suggests that share repurchases have become an essential consideration when managers take financial decisions in large US firms. By contrast, the UK research fails to show a consistent interaction between share repurchases and investment. Moreover, the findings suggest that share repurchases are being used as a complementary form of payout and not as a substitute. Considering the differences in the results from the first and second empirical chapter, the question arises, whether these are due to differences in the sample characteristics, as the size of S&P 500 companies tends to much larger than that of FTSE All Share Index companies, or whether they reflect country-specific institutional differences. This question is explored in the third empirical chapter. This research supports the contention that national differences in terms of regulatory frameworks and the development of financial markets can affect corporate decision-making (e.g. Bennedsen and Nielsen 2010, La Porta et al. 2000). More specifically, country specific factors appear to lead to a lower use of share repurchases in the UK possibly due to the stricter regulatory framework. In addition, UK firms seem to try to maintain higher dividend payout ratios than their US counterparts, which can be attributed to a culture of high dividend payouts. These differences seem to explain the non-integration of share repurchases into UK firms’ financial decision-making. Therefore, without considering country specific factors, it is not feasible to generalise economist concerns that share repurchases can be detrimental to firms’ ability to create value through investment (FINNOV, 2012) and for leading to the excessive leverage of companies (Foroohar, 2011)

The size and rate dependence of the large deformation response of polystyrene nanofibers

The intertwined effects of macromolecular length scale and specimen size on the large deformation response of individual polystyrene (PS) nanofibers were investigated. Glassy PS fibers with diameters of 150–5000 nm were electrospun from seven monodisperse PS powders with molecular weights between 13,000 and 9,000,000 g/mol. Individual nanofibers were tested using a MEMS-based microscale testing platform under an optical microscope over six decades of strain rate ranging from 0.0003 to 200 per second. The uniaxial stress–strain response demonstrated not only unusual but also repeatable postyielding behavior including strain-softening, necking, and strain-hardening, unlike the brittle behavior of bulk PS. The aforementioned deformation mechanisms were exclusive to specific combinations of specimen size and molecular weight (molecular dimensions). Extremely low and high molecular weights resulted in glassy or craze-assisted brittle failure, whereas intermediate molecular weight sustained stable necking leading to ductilities exceeding 100%, and strain hardening that increased the fiber strength by 200% compared with bulk PS. A size dependent brittle-to-ductile transition occurred within the intermediate molecular weight regime wherein thin fibers showed pronounced strain-hardening behavior and 100–300% increase in failure strength which was reduced with increasing fiber diameter until brittle failure ensued. In addition, experiments over six decades of strain rate showed that nanofibers of specific combinations of molecular weight and diameter can sustain stable necking at localized strain rates reaching 25,000 per second, thereby resulting in rate-independent elongation and drastically improved capacity for energy dissipation

Residual stress and mechanical property measurements in amorphous Si photovoltaic thin films

The mechanical reliability and efficiency of thin film photovoltaics attached to structural members depends on the initial state of residual stresses in the films. In this study, predictions for the mechanical and functional failure of photovoltaic films cocured with carbon fiber composite laminates were made possible by quantifying the mean and gradient residual stresses and the failure properties of the individual layers in thin film inorganic photovoltaics consisting of an amorphous silicon (Si) p–n junction diode, a zinc oxide (ZnO) Transparent Conductive Oxide layer (each 1 micron thick), a Kapton layer, and a thick aluminum substrate. The mean residual stress (1466 ± 118) MPa in the Si monolayer and the Si/ZnO bilayer (1661 ± 93) MPa were calculated from the geometrical details of straight and telephone cord type buckling delaminations induced to the p–n junction layer. Curvature measurements provided the residual stress gradient of the Si monolayer as 274 ± 20 MPa/microns and the stress gradient profile in the Si/ZnO bilayer. The tensile strength of freestanding amorphous Si monolayer and Si/ZnO bilayer strips was measured as 425 ± 75 MPa and 109 ± 23 MPa, respectively. These microscale tension experiments also showed that there is weak adhesion between the Si and the mechanically weak ZnO layers. The aforementioned experimental results were employed to predict the onset of fragmentation of the ZnO layer and the initiation of functional degradation of the PV films that were cocured with 00 carbon fiber composite laminates, as 0.3% and 0.9% applied strain, respectively, which was in very good agreement with experimental measurements at the composite level

Microtensile tests with the aid of probe microscopy for the study of MEMS materials

Mechanical tests of thin films require novel and sophisticated methods that can address the geometry and microstructure of the films. A new method of micro-tensile testing of MicroElectroMechanical Systems (MEMS) films has been demonstrated. An improved apparatus has been designed and implemented to measure the elastic tensile properties. (Young's modulus, Poisson's ration and tensile strength) of surface micromachined polysilicon specimans. The tensile specimans are dog-bone shaped ending in a large paddle for convenient electrostatic or, in the improved apparatus, UV adhesive gripping. The test section of the specimens is 400µm long with 2µmx50µm cross section. The method employs Atomic Force Microscope (AFM) acquired surface topologies of deforming specimans to determine (fields of) strain by way of the Digital Image Correlation method (DIC). With this method, high strength of non- linearly behaving materials under different environmental conditions can be tested by measuring the strains directly on the surface of the film with nanometer resolution in in-place and out-of-plane measurements

Size effects in mechanical behavior of submicron and nanometer thick textured Pt films

{111}-textured Platinum (Pt) thin films facilitate the growth of {001}-textured PZT films with high transverse piezoelectric coefficients, and serve as the electrodes for PZT films for MEMS. However, the film thickness, texture, and strain rate dependent mechanical behavior of magnetron sputtered {111}-textured freestanding Pt films are unknown, and are expected to control failure initiation of the PZT films. To this goal, freestanding Pt films with thicknesses of 50, 150, 200, 500, and 1000 nm and perfect {111}-texture were studied via uniaxial tension experiments at strain rates 10-6 ? 10 s–1. The elastic modulus, E = 164 ± 8 GPa, was independent of strain rate and film-thickness and was in very good agreement with theoretical estimates for the in-plane modulus of {111}-textured polycrystalline Pt. The yield stress increased with decreasing film-thickness: thicker films, 500 and 1000 nm, yielded early and accumulated larger plastic strain (~0.6–0.7%) when compared with the 200- and 150-nm Pt films that accumulated only 0.15% plastic strain, and the 50-nm films that failed in a brittle manner. This thickness dependence could be the result of both intergranular (grain rotation, grain boundary sliding) and intragrain (dislocation motion) plasticity taking place in thicker films as compared to only intergranular plasticity taking place in the thinner films. Strain-rate hardening was low for 1000-nm thick films, with strain-rate sensitivity m ~ 0.01, and was practically absent for all other film thicknesses. All films failed at only ~1% strain which may be attributed to localization of slip due to texture. Fracture for the 1000 nm and 500 nm films occurred at ~45o with respect to the loading direction with transgranular features and strain localization, whereas the failure of 200, 150, and 50 nm thick films was brittle

Influence of Soft Magnetic Materials Application to Squirrel Cage Induction Motor Design and Performance

Most of the electrical machines design studies found in literature lie on the concept that the design under investigation (and optimization) focuses mainly on the geometrical aspects of the machine and thus takes into account only a certain ferromagnetic material (i.e. iron) for its parts. These studies, give little or no information about the influence of material alternatives on the same (and optimized) design. From a manufacturer's point of view though, this information is crucial especially nowadays that there are a lot of commercially available materials in the market. In this context, this paper presents the results of a research project in the design stage of an energy efficient three phase squirrel cage induction motor (SCIM), by investigating the effects of several soft magnetic materials (adopted for its stator and/or its rotor parts) on multiple quantities of primary concern such as: efficiency, power factor, output torque, losses, weight and cost. After a brief proposed design procedure, a total of twenty-two different materials from recent manufacturers' data were examined. Also, the main electromagnetic analysis was performed through commercial analysis software. Simple ranking methods are also proposed here and the results obtained are then thoroughly discussed and commented

Design, Optimization and Modelling of High Power Density Direct-Drive Wheel Motor for Light Hybrid Electric Vehicles

Throughout the last few years, permanent magnet synchronous motors have been proven suitable candidates for hybrid electric vehicles (HEVs). Among them, the outer rotor topology with surface mounted magnets and concentrated windings seems to be very promising and has not been extensively investigated in literature. In this study, an overall optimization and modelling procedure is proposed for the design and operational assessment of high-power density direct-drive in-wheel motors, targeted towards a light HEV application. The analytical model of an HEV’s subsystems is then implemented for a more accurate evaluation of overall powertrain performance. Furthermore, a simple but effective cooling system configuration, which is taking into account the specific problem requirements, is also proposed

Permanent Magnet Synchronous Motor Design using Grey Wolf Optimizer Algorithm

Common high-torque low-speed motor drive schemes combine an induction motor coupled to the load by a mechanical subsystem which consists of gears, belt/pulleys or camshafts. Consequently, these setups present an inherent drawback regarding to maintenance needs, high costs and overall system deficiency. Thus, the replacement of such a conventional drive with a properly designed low speed permanent magnet synchronous motor (PMSM) directly coupled to the load, provides an attractive alternative. In this context, the paper deals with the design evaluation of a 5kW/50rpm radial flux PMSM with surface-mounted permanent magnets and inner rotor topology. Since the main goal is the minimization of the machine's total losses and therefore the maximization of its efficiency, the design is conducted by solving an optimization problem. For this purpose, the application of a new meta-heuristic optimization method called “Grey Wolf Optimizer” is studied. The effectiveness of the method in finding appropriate PMSM designs is then evaluated. The obtained results of the applied method reveal satisfactorily enhanced design solutions and performance when compared with those of other optimization techniques

Microtensile tests with the aid of probe microscopy for the study of MEMS materials

Mechanical tests of thin films require novel and sophisticated methods that can address the geometry and microstructure of the films. A new method of micro-tensile testing of MicroElectroMechanical Systems (MEMS) films has been demonstrated. An improved apparatus has been designed and implemented to measure the elastic tensile properties. (Young's modulus, Poisson's ration and tensile strength) of surface micromachined polysilicon specimans. The tensile specimans are dog-bone shaped ending in a large paddle for convenient electrostatic or, in the improved apparatus, UV adhesive gripping. The test section of the specimens is 400µm long with 2µmx50µm cross section. The method employs Atomic Force Microscope (AFM) acquired surface topologies of deforming specimans to determine (fields of) strain by way of the Digital Image Correlation method (DIC). With this method, high strength of non- linearly behaving materials under different environmental conditions can be tested by measuring the strains directly on the surface of the film with nanometer resolution in in-place and out-of-plane measurements