Retroreflector for Photonic Doppler Velocimetry

In order to meet the goals of the Department of Defense (DoD) for smaller and more accurate weapons, the Munitions Directorate of the Air Force Research Laboratory (AFRL/RW) has numerous projects investigating the miniaturization of weapons and munition fuze components. One of these efforts is to characterize the performance of small detonators. The velocity of the flyer, the key component needed to initiate a detonation sequence, can be measured using a photonic Doppler velocimeter (PDV). The purpose of this research was to develop a microelectromechanical system (MEMS) device that would act as an optimal retroreflective surface for the PDV. Two MEMS solutions were explored: one using the PolyMUMPs™ fabrication process and one in-house fabrication design using silicon on insulator (SOI) wafers. The in-house de- sign consisted of an array of corner reflectors created using an SOI wafer. Each corner reflector consisted of three separate mirror plates which were self-assembled by photoresist pad hinges. When heated to a critical temperature (typically 140-160 ◦C), the photoresist pads melted and the resulting surface tension caused each mirror to rotate into place. The resulting array of corner reflectors was then coated with a thin layer of gold to increase reflectivity. Despite the successful assembly a PolyMUMPs™ corner reflector, assembling an array of these reflectors was found to be unfeasible. Although the SOI corner reflector design was completed, these devices were not fabricated in time for testing during this research. However, the bidirectional reflectance distribution function (BRDF) and optical cross section (OCS) of commercially avail- able retroreflective tapes were measured. These results can be used as a baseline comparison for future testing of a fabricated SOI corner reflector array

The Adhesion of Protective Coatings to Novel REACH Compliant Packaging Steel Substrates

Chromium coated packaging steel substrates are used in applications which require a lacquer coating which insulates the can and cans contents form each other. Legislative pressures in Europe defined by REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) have dictated the removal of Cr (VI) from the electrocoating process of these packaging steel substrates, as well as restricting the use of bisphenol-A (BPA) used in protective lacquers. Both chemicals have been shown to pose significant safety risks due to their inherent toxicity to humans and the environment. Cr (III) electrocoating’s have since been investigated as a replacement but are, as yet, in their commercial infancy. One such material is TCCT (Trivalent Chromium Coated Technology) which is produced by TATA Steel Europe. In addition, lacquers have been developed to which bisphenol-A has not intentionally been added (BPANI) satisfying both REACH legislation and a consumer dictated move away from BPA. The combination of these two novel technologies enables conformity to legislative pressures. The work presented in this thesis reports an investigation into the physical and chemical characteristics of TCCT packaging steel substrates, specifically looking at the effect of chromium oxide coating weight. An established methodology is used to investigate the adhesive performance of both epoxy based and BPANI lacquers to TCCT before and after a retort process, which is used to simulate industrial pressure cooking conditions. Comparisons are made to commercially mature substrate materials. The TCCT manufacturing process is replicated on a laboratory scale with view to validating a self-sufficient process of sample creation. The effect of various electroplating parameters such as applied current density, electrolysis time, pulsing, temperature and pH are investigated. These samples are characterised using a consistent methodology. This consists of surface topographical analysis, visual analysis using electron microscopy and a copper sulphate dip test. These techniques showed the similar visual characteristics of industrially made and laboratory made TCCT samples but highlighted a more inconsistent coating deposited on the laboratory made samples

Sail film materials and supporting structure for a solar sail, a preliminary design, volume 4

Solar sailing technology was examined in relation to a mission to rendezvous with Halley's Comet. Development of an ultra-light, highly reflecting material system capable of operating at high solar intensity for long periods of time was emphasized. Data resulting from the sail materials study are reported. Topics covered include: basic film; coatings and thermal control; joining and handling; system performance; and supporting structures assessment for the heliogyro

Development of a Novel Hybrid Multi-Junction Architecture for Silicon Solar Cells

Although existing technology can produce highly efficient solar cells, they remain commercially cost-prohibitive. A low-cost alternative was investigated in this research by developing a novel hybrid multi-junction silicon (HMJ-Si) solar cell architecture through modeling, fabrication, and testing. The architecture consists of stacked silicon solar cells with an air gap between them and was designed with metal grating contacts that exploit interference patterns for light management. The interference patterns were examined in MATLAB and verified using Lumerical FDTD Solutions. Development focused on wafer configuration; diffusion profile; front contact design; optical, electrical, and thermal loss reduction; and efficiency. The architecture was optimized using an unpolished-front, p-type top cell with 128nm of Si3N4, a butterfly front contact, and 400 m grating spaced 900 m apart; a polished-front, n-type bottom cell with 200 m grating spaced 1100 m apart; and both cells having an enhanced back surface field diffusion profile with 500nm silver contacts. Efficiency peaked at 8.42% using a silver-coated wafer in lieu of the bottom cell. The results indicate that the architecture is a viable solar cell design requiring additional research for optimization

Tubular cobalt nanocomposites for selective solar absorber applications

This thesis reports on the structural and optical properties of laser surface structured Co nanocylinders-Al2O3 cermets on flexible Aluminium substrate for enhanced solar selective absorbers applications. More accurately, the laser surface nano-structured cermet solar absorbers developed consists of oriented metallic nano-cylinders embedded in porous refractory oxide host matrix. Relatively to the 6 existing primary configurations of selective solar absorbers, the investigated nano-composite can be considered as an additional one. This later, which could be classified between textured surfaces and cermet composites consists of non-percolated aligned tubular metallic regions (nanowires) embedded in an oxide host matrix. The methods that are commonly used to produce such nanowires in porous host matrices include electrodeposition into nanometer wide porous material/template. Molecular sieves, track-etched polymer membranes and porous anodic alumina are examples of some of this family of porous materials. The electrodeposition method has attracted much attention because the pore density of the nanowires is high, with controllable diameters in addition to the pores uniformity. In this particular study, the solar selective absorber coating designed consists of Co nanocylinders embedded into nanoporous alumina template produced by standard electrodeposition and thereafter submitted to femtosecond laser surface structuring. While their structural and chemical properties were investigated by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry and atomic force microscopy, their optical characteristics were investigated by specular & diffuse reflectance. The optimized samples exhibit an elevated optical absorptance α(λ) above 98% and an emittance ε(λ) ~0.03 in the spectral range of 200-1100nm. This set of values was suggested to be related to several surface and volume phenomena such as light trapping, plasmon surface effect as well as angular dependence of light reflection induced by the ultrafast laser multi-scale structuring. The thermal stability of laser surface structured Co nanocylinders-Al2O3 cermets on flexible Aluminium substrate was investigated at temperatures of 200–600 °C.PhysicsPh. D. (Physics

Development of suspended thermoreflectance technique and its application in thermal property measurement of semiconductor materials

Doctor of PhilosophyDepartment of Mechanical and Nuclear EngineeringGurpreet SinghThis dissertation details the development of a new scientific tool for the thermal characterization of freestanding micro/nano-scale materials, with specific application to thin films. The tool consists of a custom-designed and calibrated opto-electric system with superior spatial and temporal resolutions in thermal measurement. The tool, termed as Suspended ThermoReflectance (STR), can successfully perform thermal mappings at the submicron level and is able to produce unconstrained thermal conductivity unlike other optical measurement techniques where independent conductivity measurement is not possible due to their reliance on heat capacity. STR works by changing the temperature of a material and collecting the associated change in light reflection from multiple points on the sample surface. The reflection is a function of the material being tested, the wavelength of the probe light and the composition of the specimen for transparent and quasi-transparent materials. Coupling the change in reflection, along the sample’s length, with the knowledge of heat conduction allows for the determination of the thermal properties of interest. A thermal analytical model is developed and incorporated with optical equations to characterize the conductivity of thin films. The analytical model is compared with a finite element model to check its applicability in the STR experiment and data analysis. Ultimately, thermal conductivity of 2 µm and 3 µm thick Si samples were determined using STR at a temperature range of 20K – 350K and compared to literature as a validation of the technique. The system was automated using a novel LabView-based program. This program allowed the user to control the equipment including electronics, optics and optical cryostat. Moreover, data acquisition and real-time monitoring of the system are also accomplished through this computer application. A description of the development, fabrication and characterization of the freestanding thin films is detailed in this dissertation. For the most part, the thin films were fabricated using standard microfabrication techniques. However, different dry and wet etching techniques were compared for minimum surface roughness to reduce light scattering. The best etching technique was used to trim the Si films for the desired thicknesses. Besides, vapor HF was used to avoid stiction-failure during the release of suspended films

Characterization of molybdenum black coatings with reference to photothermal conversion of solar energy

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A study of thermal, structural, electrical and optical characteristics of molybdenum black surface coatings on various substrates has been made. The suitability of these coatings for use as selective absorbers for solar collector applications has been assessed. Molybdenum black (Mo black) coatings were prepared by electrodeposition (on aluminium) and a chemcial conversion method (on zinc and electroplated cobalt on nickel plated copper substrates). The solar absorptancer (αs) and thermal emittances (εth) of the coatings were determined from room temperature spectral reflectance measurements in the solar (0.3 to 2.5μm) and infrared regions (2.5 to 50 μm) respectively. The effect of different preparation parameters and substrate pretreatments on the spectral selectivity has been investigated in order to optimize the thermal performance. The spectral selectivity is related to the Mo-black coating thickness and surface roughness together with the microstructure, of the substrate and the intermediate layer. Dip coatings on polished zinc have significant selectivity (αs/ εth = 8.4 when αs = 0.76). The absorptance of the dip coatings is increased to 0.87 with εth = 0.13 by chemical etching of zinc prior to coating deposition. For coatings on electroplated cobalt on nickel plated copper (cobalt (NC) substrate), an absorptance as high as 0.94 has been obtained with an emittance value 0.3. By using an addition agent in the plating solution of cobalt the high emittance can be reduced to 0.1 with αs = 0.91 giving a coating with a relatively high efficiency (82.5%) for photo-thermal energy conversion. A study of the surface composition and microstructure of the coatings has been made using scanning and transmission electron microscopy together with electron diffraction, X-ray diffraction and X-ray photoelectron spectroscopy. The structural investigations indicate that Mo-black coatings contain polycrystals of orthorhombic Mo4O11 with a small proportion of Ni(OH)2. Presence of water and also Mo4O11 in the coatings are evident from IR spectroscopy study. The bandgap of the coating has been determined from optical transmission spectra (1.66 eV) and also from reflectance spectra (0.85 eV). The discrepancy between these two values has been discussed. The refractive indices of the coatings have also been estimated. The band gaps and refractive indices are found to be related to the spectral selectivity of the coatings. The durability test of the coatings shows that the coatings on etched zinc are more resistant to heat treatment than the coatings on unetched zinc. The coatings on cobalt (NC) substrates also show good stability for relatively short periods at temperatures ~400ºC. A study of the electrical properties of Mo-black coatings suggests that at electrical field strengths (greater than 106v/m the dominant conduction process is of the Poole-Frenkel type. The activation energy of the conduction process has been estimated to be -0.56 eV at higher temperatures. The effect of heat treatment on the electrical properties of the coatings has been examined. The dielectric constant of Mo-black has been estimated from A. C. measurements. At high frequency (20 kHz) the value of the dielectric constant is about 4.0.Association of Commonwealth Universities; Department of Mechanical Engineering, Brunel Universit

High resolution nighttime cloud-cover radiometer Quarterly report XVII, 1 Oct. 1965 - 1 Jan. 1966

Electronic, optical, mechanical, and electron packaging component and system design reviews for high resolution cloud cover infrared radiomete