Martin Sparkes

Hysteresis during field emission from chemical vapor deposition synthesized carbon nanotube fibers

Hysteresis in the field emission (FE) data of a chemical vapor synthesized carbon nanotube fiber cathode is analyzed in the regime where self-heating effects are negligible. In both the forward and reverse applied field sweeps, various FE modes of operation are identified: including Fowler-Nordheim (FN) tunneling and space-charge limited emission from the fiber tip and FN emission from the fiber sidewall. Hysteresis in the FE data is linked to the difference in the field enhancement factors in the different FE modes of operation in the forward and reverse sweeps and related to changes in the fiber morphology.

Control of Material Transport Through Pulse Shape Manipulation—A Development Toward Designer Pulses

The variety of laser systems available to industrial laser users is growing and the choice of the correct laser for a material target application is often based on an empirical assessment. Industrial master oscillator power amplifier systems with tuneable temporal pulse shapes have now entered the market, providing enormous pulse parameter flexibility in an already crowded parameter space. In this paper, an approach is developed to design interaction parameters based on observations of material responses. Energy and material transport mechanisms are studied using pulsed digital holography, post process analysis techniques and finite-difference modelling to understand the key response mechanisms for a variety of temporal pulse envelopes incident on a silicon 〈1|1|1〉 substrate. The temporal envelope is shown to be the primary control parameter of the source term that determines the subsequent material response and the resulting surface morphology. A double peak energy-bridged temporal pulse shape designed through direct application of holographic imaging data is shown to substantially improve surface quality.

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...

Characteristics of Stellite 6 Deposited by Supersonic Laser Deposition Under Optimized Parameters

Stellite 6 powders were deposited on low carbon steel using SLD (supersonic laser deposition) under optimized parameters. The structure, line scan of elements and porosity of coating were examined and analyzed using SEM (scanning electron microscope), OM (optical microscope) and XRD (X-ray diffraction). The adhesion strength between coating and substrate was tested by PAT-ADHESION/TENSILE and E900STM adhesive. The results showed the deposition characteristics of optimized coating with N2 at apressure of 3.0 MPa, a temperature of 450 °C and a laser power of 1.5 kW were compared with those of Stellite 6 coating deposited by the HVOF (high velocity oxygen fuel).

Reduced Dysprosium Permanent Magnets and Their Applications in Electric Vehicle Traction Motors

Permanent magnet (PM) machines employing rare-earth magnets are receiving increasing interest in electrical vehicle (EV) traction applications. However, a significant drawback of PM machine-based EV tractions is the extremely high cost and volatile supply of rare-earth materials, especially for dysprosium (Dy), whose price is almost 6 times higher than neodymium. This paper describes a new Dy grain boundary-diffusion process for sintered Nd2Fe14B magnets to maximize its effect on coercivity enhancement. The new process gains an 81% reduction in Dy consumption normally required by the conventional Nd2Fe14B magnets for the equivalent performance and 17% higher remanence. The investigation into the application in an interior PM (IPM) machine for a small-sized EV traction shows that compared with the conventional Nd2Fe14B magnets, despite the relatively low coercivity, the low-Dy-content magnets still withstand the thermal and demagnetization challenge over various driving operations. In addition, with the magnets high remanence and energy product, the machine gains significant torque and energy efficiency improvements. The analysis results are validated by a series of tests carried out on a prototype IPM machine with the new magnets.

Comparison Between Single Shot Micromachining of Silicon With Nanosecond Pulse Shaped IR Fiber Laser and DPSS UV Laser

High power DPSS UV lasers having high repetition rates (>100 kHz) are significant part of the cost of a laser Si micro-processing system. An alternative inexpensive solution, MOPA based IR fiber lasers, have been used to machine Si with high energy shaped pulses. This investigation evaluates the single pulse machining performance of a pulse shapeable IR (1062 nm) fiber laser and a DPSS UV (355 nm) laser on Si substrates and directly compares their performance. The machined depth data was measured with a white light interferometer and the finishing quality examined for surface defects with a SEM. Theoretical analysis demonstrated rapid heating effects by taking account of the dynamic optical and thermal properties of Si for given IR laser shaped pulses. The results show that high quality Si surface micro-processing can benefit from using the more flexible, more reliable, and pulse shapeable IR fiber laser at high repetition rates which no conventional solid state IR or UV laser could achieve.

Evaluating femtosecond laser ablation of graphene on SiO2/Si substrate

This work was supported by The Engineering and Physical Sciences Research Council (EPSRC) and National University of Defence Technology (NUDT). The authors also thank Cambridge Graphene Centre (CGC).

A holographic method for optimisation of laser-based production processes

Abstract A digital holographic system is used to image the plume dynamics of a train of picosecond laser pulses interacting with titanium, aluminium, copper and brass. The recorded process dynamics are used to propose two optimisation strategies: first, by observing the time at which the plume fully dissipates and, second, through calculation of the minimum beam displacement required to maximise energy delivery to the sample by avoiding the plume. The proposed approach could further be applied in real industrial process design, allowing laser users to formulate a processing strategy based on process dynamics rather than lengthy post-process evaluation of a sample.

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.

Supersonic laser deposition of Ti and Ti64 alloys

The importance of metal coating technologies drives the continuous improvement of metal deposition techniques for application in a wide range of industrial sectors. This work presents the foundations of a new process technology for the deposition of Ti and Ti64 coatings on various substrates using supersonic powder streams and impact site laser heating. Full density metallic deposits are obtained under appropriate impact conditions without the need for transiting the melting point of the deposited material or substrate leading to large energy savings. Details of the experimental approach will be presented along with the general characteristics of the titanium coatings produced using this novel coatings method.The importance of metal coating technologies drives the continuous improvement of metal deposition techniques for application in a wide range of industrial sectors. This work presents the foundations of a new process technology for the deposition of Ti and Ti64 coatings on various substrates using supersonic powder streams and impact site laser heating. Full density metallic deposits are obtained under appropriate impact conditions without the need for transiting the melting point of the deposited material or substrate leading to large energy savings. Details of the experimental approach will be presented along with the general characteristics of the titanium coatings produced using this novel coatings method.