Browsing by Author "Härting, Margit"
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- ItemOpen AccessCharacterisation of silicon nanoparticles produced by mechanical attrition using scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray photoemission spectroscopy(2012) Unuigbe, David Moweme; Britton, David T; Härting, MargitThe establishment of printing technologies, using nanoparticle based inks, promises inexpensive manufacture of electronic devices. However, to produce working devices, nanoparticles have to meet requirements on size, shape, and composition. In the application of silicon nanoparticles in electronics, it is important that a network of interconnecting particles is formed through which charge transport can take place. Of further importance is that there is an absence of surface oxide in order to maintain a direct silicon-silicon connection within the network. In this work, cheap and scalable production of silicon nanoparticles is achieved efficiently with a top-down process of mechanical attrition by high energy milling.
- ItemOpen AccessCharge transport in printed silicon nanoparticle networks(2013) Magunje, Batsirai,; Britton, David T; Härting, MargitFor the first time, the charge transport mechanisms in printed silicon nanoparticle networks have been comprehensively studied using variable temperature IV characteristics and Hall effect measurements, supported by microscopy studies. The conductivity can be described as hopping percolation in which activated charge transport is limited by band bending at the interface between particles and electron trapping at surface states. To probe the charge transport, two types of printed silicon nanoparticle networks based on milled silicon nanoparticles and highly doped p-type chemical vapour synthesised nanoparticles, were studied and compared.
- ItemOpen AccessDrift-diffusion of a vacancy in inhomogeneous media and its material constants : a Fokker-Planck equation approach with an application to foreign exchange data(2006) Corker, Lloyd A; Britton, David T; Härting, MargitThis study derived the mobility and diffusion coefficients of a Fokker-Planck equation describing a vacancy hopping in inhomogeneous media in one dimension under a directed stress. The study used the general master equation as a basis for a physical model because of the mesoscopic view that the change in average concentration is inadequate to describe small fluctuations in a system and that a probabilistic approach is needed. By van Kampen's system-size expansion a master equation was expanded to obtain a nonlinear Fokker-Planck equation of the diffusive type. The Einstein relation was obtained and satisfied. As an application to a physical system we considered the simple one dimension case of a point defect diffusing by a hopping mechanism under an applied stress using data obtained from the implantation of krypton ions on a pre-existing stress state in polycrystalline titanium. From this data we estimated the stress gradient and from literature used the vacancy migration enthalpy to find the diffusion coefficients, and by the Einstein relation, the mobility, the coefficients of a Fokker-Planck equation. As an application to a non physical system the study set up a Fokker-Planck equation which described incremental changes in foreign exchange (FX) prices. The Fokker-Planck equation was completely determined by the drift and diffusion coefficients extracted directly from the actual FX prices. The purpose here was to show the importance of a 'physical model' or the existence of the Markov property for the establishment of a Fokker-Planck equation and by starting from a master equation for non physical systems which would make for better understanding of the underlying statistical equations of motion of the fluctuating system.
- ItemOpen AccessElectronic properties and microstructure of nanoparticulate silicon systems for diode applications(2014) Männl, Ulrich Philipp; Härting, Margit; Britton, David TIn printed electronics the use of semiconducting silicon nanoparticles allows more than the simple printing of conductive materials. It gives the possibility of fabricating robust and inexpensive, active components. This work presents the design, fabrication, and characterization of Schottky barrier diodes using silicon nanoparticulate composites. Within this work it could be shown, that silicon nanoparticles produced by high energy milling can be used to replace the pigment in water-based graphic inks, which on curing have unique semiconducting properties, arising from the transport of charge through a percolation network of crystalline silicon nanoparticles. In this thesis scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, and mid-infrared scanning near-field optical microscopy (IR s-SNOM) were employed to investigate the micro-scale as well as the meso-scale structure of the printed particle networks and, more importantly the structure of the interface between particles. A close contact between lattice planes of different particles was observed, without the presence of a thick intervening oxide layer. Altogether, the results presented in this thesis suggest that highly doped silicon nanoparticles produced by high energy milling are suitable to be used for Schottky barrier diodes fabricated by screen printing. The saturation current of the diodes was about 0.11µA for reverse bias voltages up to 5V with an ideality factor of 10.6, and rectification ratios of approximately 10⁴ were observed.
- ItemOpen AccessElectronic properties of printed nanoparticulate silicon(2011) Abass, Sara Abdelazeem Hassan; Härting, Margit; Britton, David TPrinted silicon is an award-winning technology in the development of a large area of flexible electronics. In an investigation of the fundamental properties of printed nanoparticulate silicon composites, layers were screen printed and successfully characterised to establish their electrical performance using a Hall Measurement System (HMS). To explore properties of the nanoparticulate silicon composite a magnetoconductivity tensor model was developed and applied to extract parameters governing the electrical properties of the material. All the layers showed at least two carrier types. The effect of particle loading and temperature on the electrical properties was also investigated. Although carrier concentrations are generally low, their mobility was found to be comparable to, or even better than, similar classes of semiconductor materials.
- ItemOpen AccessFormation and dynamics of defects in pure and ion implanted a-titanium studied by quantum simulations(2010) Raji, Abdulrafiu Tunde; Britton, David T; Härting, MargitDensity-functional theory calculations have been performed to study energetics of defects formation and diffusion in pure and krypton implanted hexagonal closed-packed (h.c.p) titanium. We employed the ab initio electronic structure calculations to study the formation energies of Ti vacancies and divacancies, Kr interstitials, and the binding of Kr atoms and Ti vacancies to form defect complexes. In addition, we present results on the diffusion of vacancies, divacancies, as well as the substitutional and interstitial krypton atom in h.c.p Ti. The calculated monovacancy formation energy is 1.97eV, which is in excellent agreement with other theoretical calculations, and agrees qualitatively with published experimental results.
- ItemOpen AccessFully printed transistors employing silicon nanoparticles(2014) Walton, Stanley Douglas; Britton, David T; Härting, MargitA new device, which utilises a previously unknown two-way mode of current switching, has been developed. This is the current switching transistor, a three-terminal electronic device which exhibits a transfer resistance, in which the application of a potential or injection of charge to one terminal controls the current at either of the two remaining terminals. The development of the current switching transistor arose from a more general project focussed on printed electronics using nanoparticulate silicon, with the aim of producing fully printed transistors. All of the printed transistors produced to date have been field-effect transistors (FETs), due to the fact that printing processes are easily applicable to the planar FET architectures. The majority of the work in the area of printed FETs has so far been focussed on the use of organic semiconducting polymers to produce organic field-effect transistors (OFETs). However, research has also been undertaken regarding the use of inorganic semiconductors, including for example, transparent metal oxides, compound semiconductors and silicon. Of the active devices, the key element is the transistor. It is essential, especially for its function as an electronic switch, in enabling a wide variety of technologies. Of particular interest are its applications in digital electronics, including logic gates, memory and comparators. The ultimate goal of printed electronics is to replace conventional electronic components with their printed equivalents, which requires the use of functional inks to deliver the desired electronic properties. Printed electronic components have potential advantages over conventional discrete and integrated circuits, especially in applications in which the printed electronics form factor is more important than the absolute technical performance of the system. Furthermore, the processes of fabrication of printed devices are far simpler and more cost efficient than those of conventional devices. This is particularly true for the current switching transistor, which can be realised by a simple two layer print.
- ItemOpen AccessGrowth temperature and microstructural differences in hydrogenated amorphous silicon deposited on glass substrates(2002) Minani, Evariste; Härting, Margit; Britton, David THydrogenated amorphous silicon (a-Si:H) is an important thin film semiconductor with a wide variety of applications in microelectronics and optoelectronics. However, it is metastable and photodegrades after a moderate light illumination (Staebler-Wronski effect). The most stable material has been suggested to be at the edge of crystallinity with microcrystalline inclusions. Using a combination of positron annihilation and X-ray diffraction techniques, the microstructure of hydrogenated amorphous silicon grown by hot wire chemical vapour deposition on glass substrates at different substrate temperatures ranging between 300°C and 500 °C is examined. In previous studies the crystallisation was accompanied by a relaxation of defect structure with an increase in free volume at positron annihilation site. In this work, both techniques show a relaxation of the network with increasing growth temperature, leading to a higher degree of ordering, shorter bond lengths, and a reduction in the average size of defects in the material.
- ItemOpen AccessHall effect in printed Nanoparticulate Silicon Networks(2010) Gonfa, Girma Goro; Britton, David T; Härting, MargitSilicon nanoparticles for the application of printed electronics were successfully synthesised and characterised. High energy milling has been proven to yield uncontaminated powder of median particle size 150 nm satisfying a lognormal distribution. Single crystalline P- and N-type silicon wafers, and metallurgical grade silicon were used as starting materials. The structural characterisation of all milled powders using X-ray diffraction (XRD), Small Angle X-ray Scattering (SAXS) and electron diffraction proved that the silicon nanoparticles are polycrystalline with a crystallite size of about 40 nm. For the first time, we have formulated printable semiconducting inks from nanoparticulate silicon. Silicon nanoparticles were mixed with organic binders, such as linseed oil and acrylic, to produce printable inks. Similarly nanoparticulate silicon ink, doped with inorganic salts, which is a different procedure to conventional impurity doping of the silicon structure, was produced with linseed oil. A home-built Hall measurement system was used to characterise layers of doped ink, for which a complete carrier type reversal was observed. Based on the result of elemental mapping, two possible models were suggested to explain the doping effect. A state-of-the-art Hall measurement system was used to perform field dependent analysis of screen printed silicon inks in van der Pauw geometry. A magnetoconductivity tensor model was developed to extract the carrier properties. All the layers were demonstrated to have at least two carrier types. Inks produced from P-type silicon maintained their carrier type, but reversal was observed for the N-type layers. The mobility of the carriers is better or comparable to similar classes of semiconducting materials. 2 More information on the interparticle connections were obtained from IV and impedance spectroscopy measurements which demonstrated the capacitive effects present in the printed layers. The capacitors originate at the interfaces between the metal and the layers and between the particles.
- ItemOpen AccessInvestigation of Krypton bubbles in copper using x-ray techniques(2002) Yaman, Mecit; Härting, MargitInvestigation of the micro-mechanical properties of bulk copper, containing a high concentration (3 atomic %) of krypton is performed by X-ray diffraction techniques. Stress determination and phase analysis are carried out to determine the residual stress of the host copper metal and the internal pressure of the krypton bubbles. Lattice parameter determination of krypton bubbles confirms solidification of krypton with a fee structure epitaxial to the host metal with a bubble pressure of 2.6 GPa. The sin2 φ technique of stress determination indicates no macro residual stress inside the copper matrix as predicted by the continuum ansatz of the elasticity theory. The pressure of the krypton bubbles manifests itself as a shear strain in the copper matrix and the shear strain is found to be below the yield strength of the copper matrix, thus explaining the long-term stability.
- ItemOpen AccessMicrostructure and residual stress in hydrogenated amorphous silicon (a-Si:H) layers(2006) Ramukosi, Fhatuwani Lawrence; Britton, David T; Härting, MargitHydrogenated amorphous silicon (a-Si:H) is known to be highly disordered. The disorder introduces a high amount of defects in the network, such as bond length and angle deviation, non-coordinated bonds or voids. In this work the microstructural characterization and a new approach for strain determination in hydrogenated amorphous silicon in the layers is determined by synchrotron diffraction. The a-Si:H layers were deposited by hotwire chemical vapour deposition (HW -CVD) on a glass substrate at a growth temperature of 300°C and 500°C, respectively. The microstructural state from the diffraction phase has been experimentally determined using the pair correlation function, calculated using Fourier transformation of the scattered intensity. Indication of the residual stress in the layers was obtained using the conventional sin²ψ method, normally used and especially developed for polycrystalline materials, but here applied to an amorphous structure. It is found that residual stress and microstructure of the layers are correlated The pair distribution function (PDF) of the short range order reveals a significant shift in the nearest neighbour distance of Si-Si pairs leading to bond strain in the layers. The PDF reveals that the short-range bonding of a Si:H is tetrahedral but it does not give much information about the intermediate region which relates to the structural topology. We observed a deviation in the nearest-neighbour and the second nearest-neighbour separation, independent of the growth temperature. The observed strain curves deviate from the linear prediction of the conventional sin²ψ method. The layers were found to be generally under compressive stress, with strong gradients dominant in the interface region of the sample, and the resulting stress is highly dependent on the details of the deposition process.
- ItemOpen AccessMicrostructure, stress and defect evolution under illumination in hydrogenated amorphous silicon (a-Si:H)(2008) Minani, Evariste; Britton, David T; Härting, MargitThe purpose of this study is firstly to investigate the relation between microstructure, stress and hydrogen distribution in as deposited hydrogenated amorphous silicon (a-Si:H) layers, and secondly the investigation of the influence of illumination on hydrogen evolution and its relationship with the strain in illuminated layers.
- ItemOpen AccessModelling of defect states in covalent amorphous solids(2008) Ukpong, Aniekan Magnus; Britton, David T; Härting, MargitThis study consists of the tight binding molecular dynamics simulations, of the bulk and defect structures, in hydrogenated amorphous silicon. Emphasis is given to the systematic analysis of the changes in the network structures, which occur due to the incorporation of hydrogen, at different levels of concentration. The atomic-level stress in the simulated structures is defined, in a manner that is consistent with the local structure. The concept of configurational landscapes is introduced, as a new analysis framework for the correlations between the stress fluctuations and the modifications of the local structure.
- ItemOpen AccessThe mutual influence of strain fields and point defect distributions in krypton implanted polycrystalline titanium(2008) Nsengiyumva, Schadrack; Härting, Margit; Britton, David TStress migration of point and open-volume defects in materials is an important problem in a wide variety of applications, such as degradation of metallic interconnects in semiconductor devices, metal fatigue, and radiation damage profiles in ion implantation and surface modification. From a fundamental research view point, this study aims to contribute to a better understanding of the basic processes underlying the effect of stress assisted diffusion of foreign interstitial atoms under stress fields, using the Rutherford backscattering to obtain depth profiles, and synchroton radiation diffraction for the determination of stress fields. This has been achieved by creating a well designed model system of krypton implanted polycrystalline titanium.
- ItemOpen AccessProbabilistic methods applied to fluctuating systems(2012) Corker, Lloyd A; Britton, David T; Härting, MargitIn this work the hierarchical structure of three diverse stochastic systems is studied by investigating the probability densities of their scale-dependent measures across various scales. In the first system studied, velocity increments are used to investigate the order of complexity and disorder of wind turbulence. The second system investigates the disorders of skeletal muscles and the nervous system by considering the fluctuation of electric potentials of skeletal muscles. The last system studied is a non-physical system where price increments are used to classify the financial markets in terms of predictability of price changes and market efficiency. In all three stochastic systems a Fokker-Planck equation is used to describe how the scale-dependent measure is correlated across nested scales.
- ItemOpen AccessResidual stress in Pt coatings under thermal influence(2001) Ntsoane, T P; Härting, MargitResistance thermometers are commonly employed when accurate temperature measurements are required. The detection part consists of a thin metallic film deposited on a ceramic substrate. In this work, commercially manufactured Pt-Al₂O₃ composites annealed at 0°C, 300°C, 600°C and 1170°C above room temperature were investigated for residual stress using the non-destructive X-ray diffraction technique. The apparatus used for the investigation was a Ψ-goniometer with a scintillation detector. The measured data were analysed with "sin²Ψ"-method. The total stress yielded was found to be a superposition of both the thermal and intrinsic stress in the layer. Analytical model, following the method of Tsui and Clyne, was used to resolve the two stress contributions. With the thermal component being constant, the variation of the observed total stress was attributed to the relaxing intrinsic components. Further investigations of the samples included the microstructure studies using Scanning Electron Microscopy (SEM).
- ItemOpen AccessScientific visualization of stress tensor information with applications to stress determination by X-ray and neutron diffraction(2007) Yaman, Mecit; Härting, MargitThe visual analysis of mechanical stress facilitates physical understanding of the tensor quantity which is concealed in scalar and vector methods. In this study, the principles and techniques of scientific visualization are used to develop a visual analysis of mechanical stresses. Scientific visualization is not only applied to the final tensorial quantity obtained from the diffraction measurements, but the visual methods are developed from, and integrated into current residual stress analysis practices by relating the newly developed visual techniques to the conventional techniques, highlighting its advantages. This study consists of the mathematical analysis of the tensor character of mechanical stresses, discussion of the principles and techniques of scientific visualization (visual data analysis) in physical research, and tensor determination, visual analysis and presentation of residual stresses obtained from diffraction measurements.
- ItemOpen AccessSilicon nanoparticle sysnthesis through thermal catalytic pyrolysis(2006) Scriba, Manfred R; Härting, Margit; Britton, David TNanoparticles are considered as fundamental building blocks of nanotechnology and, silicon nanoparticles in particular, will form the basis of applications in single electron transistors, floating gate memory devices, solid state lighting, chemical sensors and flexible electronics, including solar cells and luminescent materials, printed on paper. A remaining key challenge however in the development of applications is the reproducible and reliable production of nanomaterial in sufficient quantities. Historically nanoparticles have been manufactured by top-down approaches such as milling, laser ablation or etching, and bottom-up synthesis such as colloidal chemistry and gas phase pyrolysis. The chemical processes in the latter are generally equivalent to those in the chemical vapour deposition (CVD) of compact films. Due to its simplicity and the relatively straight-forward construction of the hot wire chemical vapour deposition (HWCVD) reactor, this method is further investigated as a suitable route to nanoparticle production. The objective of this research is thus to produce Si nanoparticles (powder) in sufficient quantities, through thermal catalytic pyrolysis, while maintaining control of the important properties namely size, size distribution, composition and crystallinity.
- ItemOpen AccessStructural and electrical characterisation of silicon and other semiconducting nanoparticle networks for use in sensor and photovoltaic applications(2016) Setshedi, R K; Britton, David T; Härting, MargitA recent study investigated the feasibility of a sequential heap leach in low grade Platreef ore in order to recover PGMs (Platinum Group Metals), by a pure hydrometallurgical route. This method comprised of two stages, an initial thermophile bioleach stage to extract base metals followed by a cyanide leach to recover precious metals, PGMs. The study conducted assessed the possibility of excluding costly stages such as concentration by flotation, smelting and pressure leaching by directly leaching low grade Platreef ore. The findings showed successful base metal recoveries; however, the production of thiocyanate during the cyanide leach raised concerns in terms of significant cyanide loss but also whether thiocyanate contributed positively to PGM recovery. Cyanide present in processing liquors is known to react with various sulphur species, depending on the mineralogy of the ore and the chemical constituents within the system. These interactions between cyanide and reduced sulphur species, generated through incomplete oxidation of sulphidic ores, are primarily responsible for thiocyanate formation. In addition, thiocyanate generated during these processes has been identified to mobilise both base metals and precious metals, forming highly stable and soluble complexes with precious metals. Recent work in the field has shown pronounced recoveries during thiocyanate leaching of PGMs from virgin catalytic converters. However, a significant portion of previous research work has focused on metallic gold, with a lack of knowledge regarding thiocyanate leaching of PGMs associated with sulphidic minerals. This study investigates the chemical kinetics of thiocyanate formation in a thiosulphate, sulphite and polysulphide system in the presence of cyanide. The initial rate kinetics of thiocyanate formation, explored in homogenous systems, displayed fairly rapid reaction kinetics in the cyanide-polysulphide system relative to the thiosulphate-cyanide system. Additionally, sulphite exhibited a minor affinity for cyanide as no measurable concentration of thiocyanate was observed. This serves to verify that polysulphides generated during incomplete oxidation of sulphidic minerals are most likely responsible for SCN- formation and not the direct interactions between sulphidic minerals and cyanide. Further, this research is an initial attempt to investigate the effectiveness of thiocyanate leaching in Pt and Pd containing minerals under varied conditions. In the process, it seeks to establish whether thiocyanate and cyanide act synergistically to promote the dissolution of Pt and Pd. Preliminary test work carried out on Platreef concentrate demonstrated that the presence of base metals significantly limited the concentration of free thiocyanate available for leaching. From the results observed, Fe (under acidic conditions) and Ni displayed a strong affinity for thiocyanate, attributed to the formation of highly stable complexes. However, Cu demonstrated a negligible effect on thiocyanate consumption, forming an insoluble salt complex, CuSCN(s).
- ItemOpen AccessStructural and electrical characteristics of printed metal nanoparticle networks(2016) Van den Berg, Claire Barbara; Blumenthal, Mark; Topic, Mira; Härting, MargitThe structural and electrical properties of metal nanoparticle (NP) networks and their dependence on the constituent phases have been investigated. Percolation and effective media theories have been used to describe the physical properties of disordered systems, as well as providing a link between their structural features and the corresponding electrical transport properties. Silver and palladium nanoparticulate layers in ethyl cellulose polymer binder (ETHOCELTM), were fabricated onto paper using the method of screen printing. The metal-binder ratios were varied in order to observe changes in the microstructure when a percolating network consisting of the metal NPs is formed through the layer.