Fiber laser processing of amorphous rare earth NeFeB magnetic materials

Since the exchange coupling theory was proposed by Kneller and Hawig in 1991 there has been a significant effort within the magnetic materials community to enhance the performance of rare earth magnets by utilising nano-composite meta-materials. Inclusions of magnetically soft iron smaller than approximately 10 nm in diameter are exchange coupled to a surrounding magnetically hard Nd2Fe14B matrix and provide an enhanced saturisation magnetisation without reducing coercivity. For such a fine nanostructure to be produced, close control over the thermal history of the material is needed. A processing route which provides this is laser annealing from an amorphous alloy precursor. In the current work, relationships between laser parameters, thermal histories of laser processed amorphous stoichiometric NdFeB ribbons and the magnetic properties of the resulting nanocrystalline products have been determined with a view to applying the process to thick film nanocomposite magnet production.

Thickness control of Molecular Beam Epitaxy grown layers at the 0.01-0.1 monolayer level

Electron tunnelling through semiconductor tunnel barriers is exponentially sensitive to the thickness of the barrier layer, and in the most common system, the AlAs tunnel barrier in GaAs, a one monolayer variation in thickness results in a 300% variation in the tunnelling current for a fixed bias voltage. We use this degree of sensitivity to demonstrate that the level of control at 0.06 monolayer can be achieved in the growth by molecular beam epitaxy, and the geometrical variation of layer thickness across a wafer at the 0.01 monolayer level can be detected.

A vertically aligned carbon nanotube/fiber based electrode for economic hydrogen production by water electrolysis

Electrolysis is the most mature form of hydrogen production. Unfortunately, water electrolysis has not yet achieved the efficiency and the cost levels required for any practical application. In order to enhance the current density, modification of the electrolyte and the electrode morphology are the most popular approaches. Recently there have been numerous reports on how to improve the efficiency of hydrogen production by water splitting [1-3]. On the electrode side, the use of non-platinum high efficiency electrode materials for water splitting will provide a promising future for the hydrogen economy. An ideal electrode for water electrolysis should have good permeability to water and gas. It should also offer good electrical properties with a long life. A porous graphite plate, when coated with titania, for example, is known to provide a simple and economical electrode for water electrolysis [4]. © 2010 IEEE.

Design of carbon nanotube fiber microelectrode for glucose biosensing

BACKGROUND: Carbon nanotube (CNT) fiber directly spun from an aerogel has a unique, well-aligned nanostructure (nano-pore and nano-brush), and thus provides high electro-catalytic activity and strong interaction with glucose oxidase enzyme. It shows great potential as a microelectrode for electrochemical biosensors. RESULTS: Cyclic voltammogram results indicate that post-synthesis treatments have great influence on the electrocatalytic activity of CNT fibers. Raman spectroscopy and electrical conductivity tests suggest that fibers annealed at 250 °C remove most of the impurities without damaging the graphite-like structure. This leads to a nano-porous morphology on the surface and the highest conductivity value (1.1 × 10 5 S m -1). Two CNT fiber microelectrode designs were applied to enhance their electron transfer behaviour, and it was found that a design using a 30 nm gold coating is able to linearly cover human physiological glucose level between 2 and 30 mmol L -1. The design also leads to a low detection limit of 25 μmol L -1. CONCLUSIONS: The high performance of CNT fibers not only offers exceptional mechanical and electrical properties, but also provides a large surface area and electron transfer pathway. They consequently make excellent bioactive microelectrodes for glucose biosensing, especially for potential use in implantable devices. © 2011 Society of Chemical Industry.

Imaging the femoral cortex: thickness, density and mass from clinical CT.

There is growing evidence that focal thinning of cortical bone in the proximal femur may predispose a hip to fracture. Detecting such defects in clinical CT is challenging, since cortices may be significantly thinner than the imaging system's point spread function. We recently proposed a model-fitting technique to measure sub-millimetre cortices, an ill-posed problem which was regularized by assuming a specific, fixed value for the cortical density. In this paper, we develop the work further by proposing and evaluating a more rigorous method for estimating the constant cortical density, and extend the paradigm to encompass the mapping of cortical mass (mineral mg/cm(2)) in addition to thickness. Density, thickness and mass estimates are evaluated on sixteen cadaveric femurs, with high resolution measurements from a micro-CT scanner providing the gold standard. The results demonstrate robust, accurate measurement of peak cortical density and cortical mass. Cortical thickness errors are confined to regions of thin cortex and are bounded by the extent to which the local density deviates from the peak, averaging 20% for 0.5mm cortex.

Precracked reinforced concrete T-Beams repaired in shear with bonded carbon Fiber-Reinforced Polymer sheets

This study investigates the structural behavior of precracked reinforced concrete (RC) T-beams strengthened in shear with externally bonded carbon fiber-reinforced polymer (CFRP) sheets. It reports on seven tests on unstrengthened and strengthened RC T-beams, identifying the influence of load history, beam depth, and percentage of longitudinal steel reinforcement on the structural behavior. The experimental results indicate that the contributions of the external CFRP sheets to the shear force capacity can be significant and depend on most of the investigated variables. This study also investigates the accuracy of the prediction of the fiber-reinforced polymer (FRP) contribution in ACI 440.2R-08, UK Concrete Society TR55, and fib Bulletin 14 design guidelines for shear strengthening. A comparison of predicted values with experimental results indicates that the guidelines can overestimate the shear contribution of the externally bonded FRP system. © 2012, American Concrete Institute.

Tailoring the morphology of carbon nanotube assemblies using microgradients in the catalyst thickness

In addition to the structural control of individual carbon nanotubes (CNTs), the morphological control of their assemblies is crucial to realize miniaturized CNT devices. Microgradients in the thickness of catalyst are used to enrich the variety of available self-organized morphologies of CNTs. Microtrenches were fabricated in gate/spacer/cathode trilayers using a conventional self-aligned top-down process and catalyst exhibiting a microgradient in its thickness was formed on the cathode by sputter deposition through gate slits. CNTs, including single-walled CNTs, of up to 1μm in length were grown within 5-15 s by chemical vapor deposition. The tendency of thin CNTs to aggregate caused interactions between CNTs with different growth rates, yielding various morphologies dependent on the thickness of the catalyst. The field emission properties of several types of CNT assemblies were evaluated. The ability to produce CNTs with tailored morphologies by engineering the spatial distribution of catalysts will enhance their performance in devices. © 2011 The Japan Society of Applied Physics.

Holographic offset launch for dynamic optimization and characterization of multimode fiber bandwidth

Optimization of the bandwidth of a 2 km 50 μm multimode fiber at 850 nm is investigated theoretically and experimentally by steering a single spot, or two in antiphase spots across the core of the fiber in two dimensions using a ferroelectric liquid-crystal-based spatial light modulator. This method not only allows an optimal offset launch position to be chosen in situ but can also characterize the geometry and position of the core, identify defects, and measure the maximum differential mode delay. Its ability to selectively excite specific mode groups is also of relevance to mode-group division multiplexing. © 2012 IEEE.

Characterization of multimode fiber by selective mode excitation

Each mode of a 2 km 50 μ OM2 grade multimode fiber is precisely excited at multiple orientations using a binary phase spatial light modulator (SLM) to generate a detailed modal description of the fiber and minimize modal dispersion over 4.5 THz of optical bandwidth. © 2012 IEEE.

Application of the fractional Fourier transform to the design of LCOS based optical interconnects and fiber switches

It is shown that reflective liquid crystal on silicon (LCOS) spatial light modulator (SLM) based interconnects or fiber switches that use defocus to reduce crosstalk can be evaluated and optimized using a fractional Fourier transform if certain optical symmetry conditions are met. Theoretically the maximum allowable linear hologram phase error compared to a Fourier switch is increased by a factor of six before the target crosstalk for telecom applications of -40 dB is exceeded. A Gerchberg-Saxton algorithm incorporating a fractional Fourier transform modified for use with a reflective LCOS SLM is used to optimize multi-casting holograms in a prototype telecom switch. Experiments are in close agreement to predicted performance. © 2012 Optical Society of America.

All optical mode-multiplexing using holography and multimode fiber couplers

A spatial light modulator at the transmitter is used in conjunction with a standard multimode coupler at the receiver to modally multiplex 2 × 12.5 Gb/s nonreturn-to-zero channels using direct detection over 2 km of 940 MHz OM2 fiber without electronic processing. The wavelength dependence of this technique over a 4.5 THz band is also investigated. © 2012 IEEE.