DSpace Collection:

Production, characterisation and properties of carbon nanotube fibres

Tue, 13 Nov 2012 00:00:00 GMT

Title: Production, characterisation and properties of carbon nanotube fibres Authors: Sundaram, Rajyashree Meenakshi Abstract: Assembling carbon nanotubes (CNTs) in to ensembles like fibres, yarns or films is aimed at exploiting the exceptional properties of individual CNTs at a macroscopic scale. Amongst the various production methods, continuous and direct spinning of CNT assemblies (specifically, fibres) from the gas-phase chemical vapour deposition (CVD) reactor is attractive due to its scale-up potential. Although, these fibres exhibit remarkable mechanical, electrical and thermal properties, their performance is limited by compositional heterogeneity – stemming from the presence of non-CNT inclusions and nanotubes with a wide distribution of diameters, lengths and chiralities. The work presented here attempts to explore strategies to control the composition of the CNT fibres. Reduction of impurity levels can stem to either post-spinning removal of extraneous materials or to direct synthesis of pure fibres. In this work, each of these routes has been pursued. A post-spinning purification methodology, involving sonication of the continuously spun fibres in a solvent has been attempted and the structure, composition and performance of the purified fibres have been assessed. In addition, having evaluated the origins of the compositional heterogeneity associated with the CVD-based synthesis employed for fibre spinning, CNT fibres with minimal impurities could be directly produced by coordinating the pyrolysis chemistry of the CVD-reactants. As a step further, morphology control of the constituent nanotubes, specifically with respect to limiting their diameter distribution and regulating the number of walls, was also attempted. As a result, CNT fibres composed of exclusively multiwalled/doublewalled/singlewalled nanotubes with minimal impurities have been produced. Having developed protocols to exercise a definite degree of control on the contents of the CNT fibres, effects of the composition on the fibre-performance have also been examined.

Stress relief cracking in creep resisting low alloy ferritic steels.

Tue, 26 Oct 1976 00:00:00 GMT

Title: Stress relief cracking in creep resisting low alloy ferritic steels. Authors: Tait, Robert Andrew Abstract: Early investigations of the phenomenon of Stress Relief Cracking in low alloy steels suggested that the observed failure along prior austenite grain boundaries was in large part promoted by the relative strengthening of the grains with respect to the grain boundaries, during the stress relieving heat treatment. In this respect, it was felt that certain Cr-Mo-V steels containing vanadium carbide should be particularly susceptible to this mode of failure, since V4 C3 is well known to confer very good creep strengthening in these low alloy steels. As a result of this work, it has been established that the above view is both inaccurate and misleading. Most of the experiments were performed on two samples of commercially produced 1/2Cr 1/2Mo 1/2V steel. One of these samples had proved to be susceptible to stress relief cracking during fabrication. Detailed observations of the austenitic grain growth and secondary hardening response of the two samples are described. The results facilitated the design of a series of high temperature hardness and tensile tests in which the separate effects of grain strengthening and grain size in promoting high temperature intergranular failure could be identified. In particular, it is observed that although both steels exhibit very similar strength characteristics, the effect of grain size in promoting low ductility in association with intergranular failure is more marked in one steel than in the other. The steel in which a marked tendency towards stress relief cracking had been observed is seen to show low ductilities at high temperatures even in fine grain size specimens ; suggesting that the state of the boundaries, independent of the strengthening effect, is an important factor in promoting high temperature intergranular failure. By using an anisothermal stress relaxation testing procedure, it was possible to identify precisely the conditions of stress,temperature and microstructure under which stress relief cracking occurs. In particular, it was observed that the susceptible sample failed by nucleation controlled intergranular cavitation in association with a particular stress/strain-rate/temperature regime where the deformation process is rate controlled by the diffusion of carbon. Further experiments described in the latter part of the thesis examine the effect of purity in promoting intergranular failure both at low and high temperatures. The results suggest that the presence of impurities like phosphorus, which are known to segregate to prior austenite grain boundaries, may act to allow easier nucleation and growth of cavities during failure by inter granular cavitation. The conclusions of the thesis outline the mechanism of stress relief cracking and make discussion in terms of current theories of intergranular failure at high temperatures. Recommendations are made concerning both the assessment of susceptibility of steels to this mode of failure, as well as the adoption of safer stress relief procedures.

Defect-induced magnetism and transport phenomena in epitaxial oxides

Mon, 02 Jul 2012 23:00:00 GMT

Title: Defect-induced magnetism and transport phenomena in epitaxial oxides Authors: Schoofs, Frank Abstract: This work focuses on the impact of defects, intrinsic or artificially introduced, on the functional properties of thin, epitaxial oxide films. In the first part, the origin of the ferromagnetic properties of Mn-doped and undoped zinc oxide is studied. The deposition conditions are found to have a significant impact on the structural, transport and magnetic properties of the thin films. Combining x-ray magnetic circular dichroism and magnetometry experiments, it is established that the transition metal dopants (i.e. Mn) have no influence on the ferromagnetic nature of the zinc oxide, but that localised magnetic moments on intrinsic defects are in fact responsible for the ferromagnetic behaviour. A relation between strain (related to defect concentration) and magnetisation is established. In the second part of this dissertation, artificially introduced defects are employed in order to discover the fundamental conduction mechanism behind the two-dimensionally conductive LaAlO3/SrTiO3 interface. All experiments, from varying deposition temperature, to oxygen pressure, to laser fluence or to the insertion of (doped) perovskite layers, point towards a structurally governed conduction mechanism, although the exact details are still unclear. Distinct transitions in the resistance versus temperature curves are observed at different values than the bulk phase transformation temperature. These transitions form the boundaries of different conduction modes, with tendencies towards non-Fermi-liquid behaviour observed in certain two-dimensionally conducting samples in limited temperature regimes. By optimising the (defect) structure at the interface, i.e. by introducing a single unit cell of (La0.5,Sr0.5)TiO3 or SnTiO3, it is shown that the sheet carrier density can be dramatically enhanced, up to an order of magnitude higher than unmodified LaAlO3/SrTiO3 interfaces with a value of 1e14 cm−2 at 200 K. Finally, attempts at functionalising the conductive heterointerface by doping and inserting (anti)ferromagnetic layers are made.

Characterisation of polar (0001) and non-polar (11-20) ultraviolet nitride semiconductors

Mon, 09 Apr 2012 23:00:00 GMT

Title: Characterisation of polar (0001) and non-polar (11-20) ultraviolet nitride semiconductors Authors: Chang, Tse Yang Abstract: UV and deep-UV emitters based on AlGaN/AlN heterostructures are very inefficient due to the high lattice mismatch of these films with sapphire substrates, leading to high dislocation densities. This thesis describes the characterisation of the nanostructures of a range of UV structures, including c-plane (polar) AlGaN epilayers grown on AlN template, and nonpolar GaN/AlGaN MQWs grown on a-plane GaN template. The results are based primarily on transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) measurements. The structural and optical properties of various types of defect were examined in the c-plane AlGaN epilayers. Strain analysis based on in-situ wafer curvature measurements was employed to describe the strain relief mechanisms for different AlGaN compositions and to correlate the strain to each type of defect observed in the epilayers. This is followed by the investigation of AlN template growth optimisation, based on the TMA pre-dose on sapphire method to enhance the quality and the surface morphology of the template further. The initial growth conditions were shown to be critical for the final AlN film morphology. A higher TMA pre-dose has been shown to enable a better Al coverage leading to a fully coalesced AlN film at 1 μm thickness. An atomically smooth surface of the template was achieved over a large 10 x 10 μm AFM scale. Finally, the investigation of UV emitters based on nonpolar crystal orientations is presented. The SiNx interlayer was able to reduce the threading dislocation density but was also found to generate voids with longer SiNx growth time. The relationship between voids, threading dislocations, inversion domain boundaries and their associated V-defects and the variation in MQW growth rate has been discussed in detail.

Indirect measurement of the electrocaloric effect

Tue, 06 Mar 2012 00:00:00 GMT

Title: Indirect measurement of the electrocaloric effect Authors: Young, James Scott Abstract: In August 2011 at the International Symposium on Integrated Functionalities in Cambridge, a whole session was devoted to the electrocaloric effect, which is undergoing a modest renaissance. Surprisingly, the various reports showed that the indirect method of measuring cooling (described in the following sections) did not usually agree with the direct method (actually measuring temperature change with a thermometer). However, there was no obvious systematic error: sometimes the indirect temperature change was larger and sometimes smaller. The discrepancies were beyond the experimental errors. The majority of the present thesis is dedicated to careful reexamination of some of the assumptions made in the indirect method, both during measurement and in the subsequent data analysis that leads to inferred temperature changes. Experimentally, I conclude that the most serious systematic error is likely to be the unwarranted assumption that polarization and field measurements, recorded in hysteresis loops that are traced within a millisecond or less, are all taken at the same temperature. In reality, the experience of the material during such loops is neither isothermal nor adiabatic. Other systematic errors relate to data analyses and are discussed in detail. In some ways, therefore, this thesis has a negative flavour. But it is not designed to criticise prior work. Rather, it is intended to discriminate between reliable experimental procedures and those less convincing. This is a line of research with important technology transfer possibilities, and hence the numerical values of electric cooling must be unusually reliable if we are to avoid unwise capital investment as a country. Description: Theoretical chapter resolving long-term dispute in thermodynamics over electrical work, followed by experimental analyses and exposure of inherent problems of indirect measurement of the electrocaloric effect, including misconceptions in electrocaloric literature and previously unexamined assumptions.

Superior pinning properties in nano-engineered YBa2Cu3O7-δ

Mon, 10 Oct 2011 23:00:00 GMT

Title: Superior pinning properties in nano-engineered YBa2Cu3O7-δ Authors: Ercolano, Giorgio Abstract: Large electrical current transport in the absence of energy losses is the key factor in commercial applications of high temperature superconductors. This thesis demonstrates an easy and inexpensive bottom-up technique to produce self assembled nanorods, segmented nanorods as well as nanoparticles in YBa2Cu3O7-δ thin films grown by pulsed laser deposition. The structural and morphological characteristic of the pinning landscapes produced are investigated and correlated to their effects on the superconducting properties of the thin films. In particular two pinning landscapes are investigated: Ba2YNbO6 nanorods are grown in YBa2Cu3O7-δ thin films using a Ba2YNbO6 doped YBa2Cu3O7-δ pulsed laser deposition targets and Ba2(Y/Gd)(Nb/Ta)O6 segmented nanorods together with (Y/Gd)2O3 nanoparticles are grown in (Y/Gd)Ba2Cu3O7-δ thin films using a Ba2YNbO6 + Gd3TaO7 doped YBa2Cu3O7-δ pulsed laser deposition targets. The Ba2YNbO6 + YBa2Cu3O7-δ is deeply characterised and the effects of the deposition parameters are analysed. Ba2YNbO6 is demonstrated to be an interesting novel pinning addition capable to increase the critical current and to reduce the YBa2Cu3O7-δ critical currents angular dependencies anisotropy. The Ba2YNbO6 + Gd3TaO7 + YBa2Cu3O7-δ is found to produce a new complex pinning landscape extremely effective. At high fields the synergetic combination of the different defects typology is shown to generate an interesting new feature in the critical current angular dependencies. Chapter 1 is an introduction to superconductivity, the fundamentals of the field are briefly presented. In chapter 2 the discussion in focused on pinning in high temperature superconductors. Cuprates and in particular YBa2Cu3O7-δ are presented. The pinning phenomenon and the practical pinning engineering in thin films is also discussed in this chapter. Chapter 3 describes the thin films preparation methods and the characterisation techniques used in the research work. Chapter 4 and 5 are focused on the Ba2YNbO6 doped YBa2Cu3O7-δ thin films. Chapter 4 is an introduction to Ba2YNbO6 doped YBa2Cu3O7-δ, the preliminary results obtained on Ba2YNbO6 doped YBa2Cu3O7-δ thin films are shown in this chapter. The crystalline structure, the morphology and the superconducting properties of thin films deposited adopting different deposition parameters are analysed and discussed in chapter 5. In chapter 6 the new complex pinning landscape of Ba2(Y/Gd)(Nb/Ta)O6 and (Y/Gd)2O3 in (Y/Gd)Ba2Cu3O7-δ is presented. Concluding remarks on the research described in the work ends the thesis in a brief final chapter 7.

Magnetism and spin transport studies on indium tin oxide

Mon, 11 Jul 2011 23:00:00 GMT

Title: Magnetism and spin transport studies on indium tin oxide Authors: Hakimi, Ali Moraad Heydar Abstract: This dissertation reports on a detailed systematic study of the investigation into using Indium Oxide based materials in next generation spin-transport electronic applications. Initial studies focused on the optimisation of the electrical properties of Indium Oxide (In2O3) and Tin(Sn)-doped Indium Oxide (ITO) thin films grown using DC magnetron sputtering. The manipulation of various deposition parameters allowed the electrical properties to be tuned effectively. With the desire to create multi-functional spintronic devices, a dilute magnetic oxide system is developed where the In2O3 and ITO matrices are doped with low levels of transition metals, in particular, Co. Using a number of characterisation techniques, the origins of the magnetic response in these thin films is explored in great detail. In particular, powerful probes such as x-ray and optical magnetic circular dichroism are utilised. The major finding from these investigations is that the magnetism does not necessarily emanate from the Co dopants alone. In fact, Co dopants give a strictly paramagnetic response, suggesting that the magnetism observed may be a result of polarised electrons in localised donor states in the In2O3 and ITO hosts. Therefore, we believe that the origins of magnetism in these films is related to a hybridisation and charge transfer of electrons from a broad donor/defect-derived impurity band to a band of unoccupied 3d states at the Fermi level. The emergence of a very weak magnetic signal in pure ITO raises further questions as to the true origins of the ferromagnetic behaviour and supports a defect-related mechanism. To explore the suitability of ITO for a future in spintronics further, the performance of some metal ferromagnet/oxide multilayered structures was investigated. The investigations revealed a significant contribution to both the magnetic and magnetotransport properties from a superparamagnetic component giving some insight into the importance of the quality of interfaces between the metal ferromagnet/oxide layers and heterostructures. Using a three-dimensional focused-ion beam etching technique to fabricate submicron spin-valve devices with ITO spacer layers, current-perpendicular-to-plane magnetoresistance measurements were carried out to estimate the spin diffusion length of ITO at room temperature. In conjunction with a simplified Valet-Fert model, a spin asymmetry ratio for Co of 0.55 and spin diffusion length of 6±1 nm in semiconducting ITO at room temperature was estimated. These findings imply that spin information can be conserved and transported through In2O3 and ITO even up to and beyond room temperature.

Metal oxides for efficient infrared to visible upconversion

Mon, 11 Oct 2010 23:00:00 GMT

Title: Metal oxides for efficient infrared to visible upconversion Authors: Etchart, Isabelle Abstract: Upconversion phosphor materials are attracting considerable attention for their possible applications in solar cells with improved efficiency, nanomaterials for bio-imaging, lasers and novel display technologies. Upconversion materials, usually consisting of crystals doped with lanthanide ions, can convert low-energy incident radiation into higher energy emitted radiation. Several mechanisms are involved, including multiple photon absorption and energy transfers between dopants. Up to now, reported upconversion efficiencies have been relatively low, excitation thresholds quite high, and the investigated phosphors (generally fluorides) often presented poor chemical stability (hygroscopy), limiting their industrial applicability. In this dissertation, we investigate the upconversion luminescence characteristics of rareearth-doped RE2BaZnO5 (RE = Y, Gd) phosphors, for near-infrared to visible upconversion. Being oxides, these materials have good chemical, thermal and mechanical properties. A variety of dopants, including Yb3+, Er3+, Ho3+ and Tm3+, were embedded in the host lattice, resulting in bright red, green, blue and white light emissions under 980 nm excitation and at relatively low excitation powers. Upconversion efficiencies up to ~ 5.2%, 2.6%, 1.7% and 0.3% were respectively achieved in samples doped with Yb3+, Er3+ (green and red emission), Yb3+, Ho3+ (green emission), Yb3+, Tm3+ (blue and near-infrared emission) and Yb3+, Er3+, Tm3+ (white light emission). We believe that our green, red and white emitting systems are the most efficient upconverting samples reported to date for green, red and white light emission, which makes them serious candidates for many of the applications listed above. The upconversion mechanisms were determined for the first time by means of indepth steady-state and time-resolved spectroscopic investigations, including concentration and power dependence studies associated with temperature-dependent lifetime measurements.

Nitride semiconductors studied by atom probe tomography and correlative techniques

Tue, 08 Feb 2011 00:00:00 GMT

Title: Nitride semiconductors studied by atom probe tomography and correlative techniques Authors: Bennett, Samantha Abstract: Optoelectronic devices fabricated from nitride semiconductors include blue and green light emitting diodes (LEDs) and laser diodes (LDs). To design efficient devices, the structure and composition of the constituent materials must be well-characterised. Traditional microscopy techniques used to examine nitride semiconductors include transmission electron microscopy (TEM), and atomic force microscopy (AFM). This thesis describes the study of nitride semiconductor materials using these traditional methods, as well as atom probe tomography (APT), a technique more usually applied to metals that provides three-dimensional (3D) compositional information at the atomic scale. By using both APT and correlative microscopy techniques, a more complete understanding of the material can be gained, which can potentially lead to higher-efficiency, longer-lasting devices. Defects, such as threading dislocations (TDs), can harm device performance. An AFM-based technique was used to show that TDs affect the local electrical properties of nitride materials. To investigate any compositional changes around the TD, APT studies of TDs were attempted, and evidence for oxygen enrichment near the TD was observed. The dopant level in nitride devices also affects their optoelectronic properties, and the combination of APT and TEM was used to show that Mg dopants were preferentially incorporated into pyramidal inversion domains, with a Mg content two orders of magnitude above the background level. Much debate has been focused on the microstructural origin of charge carrier localisation in InGaN. Alloy inhomogeneities have often been suggested to provide this localisation, yet APT has revealed InGaN quantum wells to be a statistically random alloy. Electron beam irradiation in the TEM caused damage to the InGaN, however, and a statistically significant deviation from a random alloy distribution was then observed by APT. The alloy homogeneity of InAlN was also studied, and this alloy system provided a unique opportunity to study gallium implantation damage to the APT sample caused during sample preparation by the focused ion beam (FIB). The combination of APT with traditional microscopy techniques made it possible to achieve a thorough understanding of a wide variety of nitride semiconductor materials.

Carbon nanotubes as near infrared laser susceptors

Tue, 11 Jan 2011 00:00:00 GMT

Title: Carbon nanotubes as near infrared laser susceptors Authors: Bahrami, Amir Abstract: The coupling efficiency of carbon nanotubes with near infrared laser radiation at 940nm wavelength was investigated. Nanotubes treated with different post processing methods were irradiated at different laser power intensities as dry samples and suspensions in water or ethanol. The interaction with the laser beam was measured and quantified based on the temperature increase in the samples as well as the amount of energy transmitted through them. Parallel experiments using carbon black revealed better performance of carbon nanotubes in terms of coupling efficiency and heat dissipation to their surroundings. It was found that most of the incident radiation on an individual carbon nanotube is absorbed, resulting in extreme local temperature increases proportional to the laser intensity, which can lead to instant tube oxidation in air. Such high heats are efficiently transferred to the material in immediate contact with the nanotubes, increasing its temperature very rapidly. The most intriguing results were obtained in the presence of water where the observations suggested, disintegration of carbon nanotubes with each laser pulse. It is shown that extremely high local temperatures vaporise the water in the immediate vicinity of a carbon nanotube and result in a water-gas reaction. It is further postulated that such effects can be achieved with laser beams at power intensities near the skin tissue’s safe exposure thresholds, and therefore can potentially be used as a method of removing nanotubes from living tissue. This has advantages in providing an exit route for nanotubes whether introduced on purpose for reasons of medicine or therapy, or possibly, as a result of inadvertent exposure. Further studies on laser heating and transmission through different dry samples, highlighted that more crystalline structures such as that of a heat-treated nanotube, are more effective in causing extinction of the laser beam and a reduction in the transmitted beam intensity, however the tubes with more defects or with a length comparable to the radiation wavelength are very effective in converting the absorbed laser energy to heat. This effect is exacerbated when the laser beam is polarised parallel to the long axis of the carbon nanotubes. These heating effects were exploited to create welds in high density polyethylene using through transmission laser welding. The resultant welds showed better than or equal mechanical performance to welds made using industrial absorbers such as carbon black or Clearweld®.

Novel phosphors for solid state lighting

Tue, 16 Nov 2010 00:00:00 GMT

Title: Novel phosphors for solid state lighting Authors: Furman, Joshua D Abstract: Solid state white light emitting diode lighting devices outperform conventional light sources in terms of lifetime, durability, and lumens per watt. However, the capital contribution is still to high to encourage widespread adoption. Furthermore, the colour from today's devices is unsuitable for general room illumination and thus new phosphor materials are needed. This dissertation will examine the synthesis of inorganic nanoparticles and the possibility of using hybrid inorganic-organic frameworks in the search for new lighting phosphors. Nanoparticles of the oxide compound yttrium aluminium garnet were synthesized using an emulsion technique, though it was found that the high temperature processing needed for good optical properties was not compatible with maintaining nanosized particles. In terms of hybrid framework phosphors, several aspects of this new area have been explored. The mechanical and optical properties of a dense cerium oxalate formate hybrid framework compound have been investigated. Its strength was found to be nearly as great as some classical ceramic compounds, and clearly robust enough for device applications. While the photoluminescence of the cerium oxalate formate was not suitable for solid state lighting, the impressive mechanical properties evaluated are expected to be valid for a wide range of dense inorganic-organic frameworks. A novel approach to solid state lighting phosphors was introduced by using ligand-based photoluminescence in hybrid frameworks. Novel frameworks were prepared using 9,10-anthraquinone-2,3-dicarboxylic acid in combination with calcium, manganese, nickel, and zinc. These compounds show excellent photoluminescent emission for use in solid state lighting applications, although the luminescence is quenched at room temperature due to dynamic effects. The excitation, while reaching the blue part of the spectrum, falls just short of what is needed for use today's devices. To address these issues, a second class of novel framework compounds was prepared using 9-fluorenone-2,7-dicarboxylic acid in combination with calcium, strontium, barium, cadmium, and manganese. They are more rigid structures and show good luminescence at room temperature with a photoluminescent excitation spectrum extending further into the blue than the anthraquinones. Additionally, quantum yield in the calcium fluorenone is nearly double that of its parent ligand, suggesting that there is an enhancement in luminescent properties as a result its inclusion in a framework structure. An explanation for the differences in efficiency between seemingly similar compounds are drawn from their compositions, crystal structures, photoluminescence, and specific heat properties. Finally, some structural and chemical targets for future hybrid phosphor development are identified based on the relationships identified in this work.

Electron tomography of defects

Mon, 11 Oct 2010 23:00:00 GMT

Title: Electron tomography of defects Authors: Sharp, Joanne Abstract: Tomography of crystal defects in the electron microscope was rst attempted in 2005 by the author and colleagues. This thesis further develops the technique, using a variety of samples and methods. Use of a more optimised, commercial tomographic reconstruction program on the original GaN weakbeam dark- eld (WBDF) tilt series gave a ner reconstruction with lower background, line width 10-20 nm. Four WBDF tilt series were obtained of a microcrack surrounded by dislocations in a sample of indented silicon, tilt axes parallel to g = 220, 220, 400 and 040. Moir e fringes in the defect impaired alignment and reconstruction. The e ect on reconstruction of moir e fringe motion with tilt was simulated, resulting in an array of rods, not a at plane. Dislocations in a TiAl alloy were reconstructed from WBDF images with no thickness contours, giving an exceptionally clear reconstruction. The e ect of misalignment of the tilt axis with systematic row g(ng) was assessed by simulating tilt series with di raction condition variation across the tilt range of n = 0, 1 and 2. Misalignment changed the inclination of the reconstructed dislocation with the foil surfaces, and elongated the reconstruction in the foil normal direction; this may explain elongation additional to the missing wedge e ect in experiments. Tomography from annular dark- eld (ADF) STEM dislocation images was also attempted. A tilt series was obtained from the GaN sample; the reconstructed dislocations had a core of bright intensity of comparable width to WBDF reconstructions, with a surrounding region of low intensity to 60 nm width. An ADF STEM reconstruction was obtained from the Si sample at the same microcrack as for WBDF; here automatic specimen drift correction in tomography acquisition software succeeded, a signi cant improvement. The microcrack surfaces in Si reconstructed as faint planes and dislocations were recovered as less fragmented lines than from the WBDF reconstruction. ADF STEM tomography was also carried out on the TiAl sample, using an detector inner angle ( in) that included the rst order Bragg spots (in other series in had been 4-6 B). Extinctions occurred which were dependent on tilt; this produced only weak lines in the reconstruction. Bragg scattering in the ADF STEM image was estimated by summing simulated dark- eld dislocation images from all Bragg beams at a zone axis; a double line was produced. It was hypothised that choosing the inner detector angle to omit these rst Bragg peaks may preclude most dynamical image features. Additional thermal di use scattering (TDS) intensity due to dilatation around an edge dislocation was estimated and found to be insigni cant. The Huang scattering cross section was estimated and found to be 9 A, ten times thinner than experimental ADF STEM dislocation images. The remaining intensity may be from changes to TDS from Bloch wave transitions at the dislocation; assessing this as a function of tilt is for further work. On simple assessment, only three possible axial channeling orientations were found over the tilt range for GaN; if this is typical, dechanneling contrast probably does not apply to defect tomography.

Development of quantitative techniques for the study of discharge events during plasma electrolytic oxidation processes

Mon, 11 Oct 2010 23:00:00 GMT

Title: Development of quantitative techniques for the study of discharge events during plasma electrolytic oxidation processes Authors: Dunleavy, Christopher Squire Abstract: Plasma electrolytic oxidation, or PEO, is a surface modification process for the production of ceramic oxide coatings upon substrates of metals such as aluminium, magnesium and titanium. Two methodologies for the quantitative study of electrical breakdown (discharge) events observed during plasma electrolytic oxidation processes were developed and are described in this work. One method presented involves direct measurement of electrical breakdowns during production of an oxide coating within an industrial scale PEO processing arrangement. The second methodology involves the generation and measurement of electrical breakdown events through coatings pre-deposited using full scale PEO processing equipment. The power supply used in the second technique is generally of much lower power output than the system used to initially generate the sample coatings. The application of these techniques was demonstrated with regard to PEO coating generation on aluminium substrates. Measurements of the probability distributions of discharge event characteristics are presented for the discharge initiation voltage; discharge peak current; event total duration; peak instantaneous power; charge transferred by the event and the energy dissipated by the discharge. Discharge events are shown to increase in scale with the voltage applied during the breakdown, and correlations between discharge characteristics such as peak discharge current and event duration are also detailed. Evidence was obtained which indicated a probabilistic dependence of the voltage required to initiate discharge events. Through the scaling behaviour observed for the discharge events, correspondence between the two measurement techniques is demonstrated. The complementary nature of the datasets obtainable from different techniques for measurement of PEO discharge event electrical characteristics is discussed with regards to the effects of interactions between concurrently active discharge events during large scale PEO processing.

Grain boundaries in coated conductors

Mon, 05 Jul 2010 23:00:00 GMT

Title: Grain boundaries in coated conductors Authors: Weigand, Marcus Abstract: The excitement which followed the discovery of high-temperature superconductors in 1986 was short-lived, as it became clear that their current carrying capacity (the critical current density Jc) was limited by grain boundaries (GBs). In order to reduce their detrimental effects coated conductors have been developed, in which a superconducting thin film is deposited on a polycrystalline, textured substrate. Within certain temperature and magnetic field ranges, however, GBs still limit the overall Jc. This fact motivated the present thesis, for which the electrical properties of different types of coated conductors, and in particular their GBs, were investigated. Several GBs and a single grain were isolated in a tape produced by metal-organic deposition (MOD), using a novel approach based on electron backscatter diffraction and a focused ion beam microscope. Measurements of their critical current densities for fields swept in the film plane showed the expected decrease with increasing misorientation angle at low fields. At higher fields an angle dependent crossover was found, from a GB to grain limited Jc. In order to confirm this result and put it into broader perspective, the dependence of Jc on the width of polycrystalline tracks was studied, and then explained in terms of the behaviour of the single GBs. Investigations using low-temperature scanning laser microscopy rounded out the picture, which also showed GB dissipation at certain angles and grain limitation at others. In measurements on samples produced by metal-organic chemical vapour deposition (MOCVD) characteristic differences compared to the MOD film were found. While both conductors exhibited high values of Jc, the variation with in-plane angle was significantly stronger for the MOCVD conductor, which can be explained by its sharper texture. In a track patterned perpendicular to the tape direction the phenomenon of vicinal channelling was observed, which previously was known only from films on single crystal substrates. Finally, an isolated boundary showed very high values of Jc, consistent with its low misorientation. In order to better understand how the substrate influences the superconducting properties, measurements were carried out on otherwise identical samples grown on different substrates. A tape with grains elongated along its rolling direction showed particularly good properties at all examined field orientations. This extends the previously reported result that high aspect ratios are beneficial at fields applied perpendicular to the tape plane.

Production and characterisation of hydroxyapatite/multi-walled carbon nanotube composites

Tue, 12 Jan 2010 00:00:00 GMT

Title: Production and characterisation of hydroxyapatite/multi-walled carbon nanotube composites Authors: White, Ashley Ann Abstract: Hydroxyapatite (HA) is a biologically active ceramic that is used in surgery to replace bone. While HA promotes bone growth along its surface, its mechanical properties are not sufficient for major load-bearing medical devices. Carbon nanotubes (CNTs), as one of the strongest and stiffest materials available, have the potential to strengthen and toughen HA, thus expanding the range of clinical uses for the material. Furthermore, studies have suggested that the nanotubes themselves possess some bioactive properties. This work sought to develop and characterise HA-CNT composites in four main areas: 1) production and characterisation of green materials, 2) investigation of appropriate sintering atmospheres, 3) evaluation of mechanical properties, and 4) assessment of biological response to in vitro cell culture. HA was synthesised by a precipitation reaction between Ca(OH)2 and H3PO4, and multi-walled CNTs were produced by chemical vapour deposition. Composites were produced by adding the CNTs to the Ca(OH)2 solution as the HA was precipitating. Both as-made (nfCNTs) and acid-treated CNTs (fCNTs) were used to make composites with loadings of 0.5 – 5 wt.% CNTs. The resulting slurry was shear mixed and then processed to make a powder. The powder was then uniaxially pressed into tablets of ~45% theoretical density. Characterisation of the green material with XRD and FTIR found that the primary phase was HA which was well hydroxylated. The powder particles were found to have a bimodal size distribution, and all materials had similar surface areas, as determined by BET. Composites made with fCNTs were found to have a better dispersion of CNTs in the HA matrix and better interaction between the HA and CNTs compared with nfCNT composites. CNTs oxidise at the high temperatures needed to sinter HA, yet water is necessary to prevent dehydroxylation and decomposition of the HA. Using 5 wt.% fCNT composite, fourteen sintering atmospheres were investigated to determine their effect on phase purity, hydroxylation, sintered density, and remaining CNT content after sintering. An atmosphere of CO + H2 bubbled through ice water resulted in optimal properties. Additionally, it was found that increasing the gas flow rate and the number of samples sintered in one batch increased CNT retention. However, this came at the expense of the density of the sintered samples, as composites with a higher CNT content were more porous. To optimise the composite microstructure for mechanical studies, six different sintering time / temperature profiles were examined to determine their effect on density (balanced with CNT retention) and grain size. HA and both nfCNT and fCNT composites with CNT loadings of 0.5, 1, 2 and 5 wt.% were produced using the optimised atmosphere and profile, and then tested to determine tensile strength (using diametral compression) and hardness, and to look for evidence of toughening. It was found that CNTs had little reinforcing effect; instead, mechanical behaviour results were mainly attributed to differences in porosity, due at least in part to the CNTs’ presence. The in vitro cellular response to the materials was examined by culturing human osteoblast-like cells on HA and nfCNT (0.88 wt.%) and fCNT (3.3 wt.%) composites for 12 days. Cells were found to attach and grow well on HA and the nfCNT composite, with slightly enhanced response on the composite. The fCNT composite, on the other hand, showed a decrease in cell viability between days 1 and 12. These results were mainly attributed to the effects of a lower local pH due to remnant acid on the fCNTs and differences in material characteristics, such as CNT loading and surface roughness. This systematic study of the production and properties of HA-CNT composites has resulted in improved understanding of the production and processing of these materials and the effects of a wide range of sintering atmospheres on their characteristics. Additionally, it has yielded interesting preliminary results of their mechanical reinforcement potential and biological behaviour.

Modelling of Carbide and Laves Phase Precipitation in 9-12 wt% Chromium Steels

Tue, 28 Jan 1997 00:00:00 GMT

Title: Modelling of Carbide and Laves Phase Precipitation in 9-12 wt% Chromium Steels Authors: Robson, Joseph Douglas

Structure, Properties and Treatments of Carbon Nanotube Fibres

Fri, 23 Oct 2009 23:00:00 GMT

Title: Structure, Properties and Treatments of Carbon Nanotube Fibres Authors: Vilatela García, Juan José Abstract: Carbon nanotubes (CNTs) possess exceptional mechanical, thermal and electrical properties along their main axis, superior to those of most materials. These can be exploited on a macroscopic scale by assembling the CNTs into a fi bre with the nanotubes preferentially oriented parallel to each other and to the fibre axis. CNT fibres can be produced continuously, directly from the gas phase during CNT synthesis by chemical vapour deposition (CVD), and spun at rates of up to 70 m/min. Their combination of outstanding mechanical, electrical and thermal properties and low density (1 g/cm3) makes CNT fi bres a potential candidate for high-performance applications. The fi bre speci c strength and stiff ness are typically 1 GPa/SG and 50 GPa/SG, respectively; however, at small gauge lengths (< 2mm) they also show values of 6 - 9 GPa/SG strength and 180 - 390 GPa/SG sti ffness. The electrical conductivity of the CNT fibres is approximately 8 x 10^5 S/m and their thermal conductivity of the order of 50 W/mK. These properties derive from the long length, high alignment and efficient packing of the nanotubes in the fi bre. Further improvements to the fi bre structure and properties at long gauge lengths are possible through removal of impurities from the fi bre by annealing.

Reaustenitisation from Bainite in Steels

Tue, 16 Mar 1993 00:00:00 GMT

Title: Reaustenitisation from Bainite in Steels Authors: Takahashi, Manabu

The Signifance of Retained Austenite in Steels

Tue, 05 Feb 1980 00:00:00 GMT

Title: The Signifance of Retained Austenite in Steels Authors: Bhadeshia, H K D H Description: For images in this thesis, see: http://www.msm.cam.ac.uk/phase-trans/2002/harry/images/

The Defect Energies and Deformation Mechanisms of Single Crystal Superalloys

Tue, 29 Jan 1985 00:00:00 GMT

Title: The Defect Energies and Deformation Mechanisms of Single Crystal Superalloys Authors: Hillier, Graham Stewart