William O’Neill

Practical and theoretical investigations into inert gas cutting of 304 stainless steel using a high brightness fiber laser

A 2.2 kW fiber laser was used in a series of inert cutting trials on stainless steels of section thicknesses in the range of 6–10 mm. Variations in the cutting performance with changing gas pressure, focal position, and nozzle diameter were investigated. Results showed the difficulties associated in cutting with high brightness lasers, specifically in obtaining full melt eject through narrow kerfs; two distinct melt eject failure mechanisms were observed: (I) Failure of melt removal in the upper region resulted in melt ejecting out of the top surface and (II) poor base ejection giving recast and dross problems on the base of the cut. Detailed scanning electron microscope images of these phenomena are presented. A computational fluid dynamic model is used to show distinct differences in the theoretical gas performance at the center of the cut, displayed by most models, and at the edges where the melt eject is taking place. Melt eject is also shown both experimentally and theoretically to be improved for th...

Uniform and fast switching of window-size smectic A liquid crystal panels utilising the field gradient generated at the fringes of patterned electrodes

ABSTRACT A method to enable smectic A (SmA) liquid crystal (LC) devices to switch uniformly and hence fast from the clear state to a scattered state is presented. It will allow the reduction of the switching time for a SmA LC panel of 1 × 1 m2 changing from a clear state to a fully scattered state by more than three orders to a few tens of milliseconds. Experimental results presented here reveal that SmA LC scattering initiates from the nucleated LC defects at the field gradient of the applied electric field usually along the edges of the panel electrode and grows laterally to spread over a panel, which takes a long time if the panel size is large. By patterning the electrodes in use, it is possible to create a large number of field gradient sites near the electrode discontinuities, resulting in a uniform and fast switching over the whole panel and the higher the pattern density the shorter the panel switching time. For the SmA LC panels used here, the ITO transparent electrodes are patterned by laser ablation and photolithography. It is shown that the defect nucleation time is much shorter than the growth time of the scattered region, hence it is possible to use the density of the field gradient sites to control the uniformity and switching time of a panel. Furthermore, the patterned SmA panels have a lower switching voltage than that of the non-patterned ones in general. Graphical Abstract

Investigation of pulse shape characteristics on the laser ablation dynamics of TiN coatings in the ns regime

In this work, the ablation dynamics of TiN coating with a ns-pulsed fibre laser in a wide range of pulse durations were studied. Critical time instances within the pulse duration were assessed by reflected pulse analysis. Digital holography was employed to investigate the shock wave expansion dynamics within and beyond the pulse duration. The results depict that the absorption behaviour changes as a function of the pulse rise time. Moreover, planar expansion of the shock wave is observed, which is generally linked to higher machining quality and absence of excessive plasma. The results of the study are interpreted to depict the required characteristics of optimized pulse shapes in the ns-region for improved micromachining performance.

The effect of processing wavelength and fluence on the microdrilling of 316 L stainless steel with a diode pumped solid state laser

Diode pumped solid state lasers (DPSSs) with multiwavelength capability have become a powerful tool in laser material processing over the last few years. One application of high beam quality DPSS lasers that has received increased attention lately is the drilling of small diameter (<100 μm) holes through industrially important materials. Processing parameters play an important role and limit the precision of the process. A systematic series of experiments was performed to investigate the effect of the high fluence laser-drilling event in ambient air. From this study suitable processing regimes were established.

Photonic sorting of aligned, crystalline carbon nanotube textiles

Floating catalyst chemical vapor deposition uniquely generates aligned carbon nanotube (CNT) textiles with individual CNT lengths magnitudes longer than competing processes, though hindered by impurities and intrinsic/extrinsic defects. We present a photonic-based post-process, particularly suited for these textiles, that selectively removes defective CNTs and other carbons not forming a threshold thermal pathway. In this method, a large diameter laser beam rasters across the surface of a partly aligned CNT textile in air, suspended from its ends. This results in brilliant, localized oxidation, where remaining material is an optically transparent film comprised of few-walled CNTs with profound and unique improvement in microstructure alignment and crystallinity. Raman spectroscopy shows substantial D peak suppression while preserving radial breathing modes. This increases the undoped, specific electrical conductivity at least an order of magnitude to beyond that of single-crystal graphite. Cryogenic conductivity measurements indicate intrinsic transport enhancement, opposed to simply removing nonconductive carbons/residual catalyst.

Femtosecond laser-induced chemical vapor deposition of tungsten quasi-periodic structures on silicon substrates

A rapid, maskless deposition technique for writing conductive tracks via femtosecond laser-induced chemical vapor deposition has been developed. The technique can be used for a range of applications, one example being writing conductive tracks for the construction of microelectronic devices. The process uses pulsed ultrafast laser with 300 fs pulse length and 1030 nm wavelength for the direct deposition of tungsten tracks on silicon substrates from metal organic tungsten hexacarbonyl precursors. The written tracks consisted of wavy quasi-periodic walls with thickness of 200 nm and a periodicity of 500 nm, aligned perpendicular to the linear polarization of the laser beam. Fixed number of pulses spot dwell experiments revealed that a thin film formed before quasi-periodic structures grew on top of that thin film. The peak intensity threshold for the deposition process (8.64 × 1010 W/cm2) was lower than the threshold for surface modification on the silicon substrate (3.34 × 1011 W/cm2) at the same scanning speed of 10 μm/s and repetition rate of 502 kHz. Negligible damage to the underlying substrate was observed in the cross section. Scan speeds up to 100 μm s−1 were achieved in the process. The elemental composition of the deposits was measured to be 80% by weight tungsten in energy-dispersive x-ray spectroscopy methods and the resistivity of the deposit was measured to be 290 μΩ cm using the transfer length method.A rapid, maskless deposition technique for writing conductive tracks via femtosecond laser-induced chemical vapor deposition has been developed. The technique can be used for a range of applications, one example being writing conductive tracks for the construction of microelectronic devices. The process uses pulsed ultrafast laser with 300 fs pulse length and 1030 nm wavelength for the direct deposition of tungsten tracks on silicon substrates from metal organic tungsten hexacarbonyl precursors. The written tracks consisted of wavy quasi-periodic walls with thickness of 200 nm and a periodicity of 500 nm, aligned perpendicular to the linear polarization of the laser beam. Fixed number of pulses spot dwell experiments revealed that a thin film formed before quasi-periodic structures grew on top of that thin film. The peak intensity threshold for the deposition process (8.64 × 1010 W/cm2) was lower than the t...

Laser stimulated grain growth in 304 stainless steel anodes for reduced hydrogen outgassing (Erratum)

Metal anodes in high power microwave (HPM) devices erode during operation due to hydrogen outgassing and plasma formation; both of which are thermally driven phenomena generated by the electron beam impacting the anode’s surface. This limits the lowest achievable pressure in an HPM device, which reduces its efficiency. Laser surface melting the 304 stainless steel anodes by a continuous wave fiber laser showed a reduction in hydrogen outgassing by a factor of ~4 under 50 keV electron bombardment, compared to that from untreated stainless steel. This is attributed to an increase in the grain size (from 40 - 3516 μm2), which effectively reduces the number of characterized grain boundaries that serve as hydrogen trapping sites, making such laser treated metals excellent candidates for use in HPM applications.

Tunable Broadband Radiation Generated Via Ultrafast Laser Illumination of an Inductively Charged Superconducting Ring.

An electromagnetic transmitter typically consists of individual components such as a waveguide, antenna, power supply, and an oscillator. In this communication we circumvent complications associated with connecting these individual components and instead combine them into a non-traditional, photonic enabled, compact transmitter device for tunable, ultrawide band (UWB) radiation. This device is a centimeter scale, continuous, thin film superconducting ring supporting a persistent super-current. An ultrafast laser pulse (required) illuminates the ring (either at a point or uniformly around the ring) and perturbs the super-current by the de-pairing and recombination of Cooper pairs. This generates a microwave pulse where both ring and laser pulse geometry dictates the radiated spectrum’s shape. The transmitting device is self contained and completely isolated from conductive components that are observed to interfere with the generated signal. A rich spectrum is observed that extends beyond 30 GHz (equipment limited) and illustrates the complex super-current dynamics bridging optical, THz, and microwave wavelengths.

HiWi Project: High Efficiency Electric Drives

Vehicles develop their highest efficiency of around 93–95 % within a speed range of usually 1/4 to 1/3 of the maximum, whereas in real-life driving cycles the motor operates at a wider range of speeds and at partial load, resulting in much lower efficiency. Hi-Wi addresses this mismatch by advancing the design and manufacture of drive trains through holistic design across magnetic, thermal, mechanical and control electronics/algorithms in line with real-life use rather than a single-point “rating”. In addition to the above efficiency gains, Hi-Wi addresses the issue of material supply through the development of nanostructured magnetic materials and the development of new driving cycles to more accurately represent in use conditions for electric vehicles.