<p>Tanzania has a characteristic developing economy, which is dependent on agricultural productivity.  About 90% of the total primary energy consumption of the country is from biomass.  Since the biomass is mostly consumed at the household level in form of wood fuel, it is marginally contributing to the commercial energy supply.  However, the country has abundant energy resources from hydro, biomass, natural gas, coal, uranium, solar, wind and geothermal.  Due to reasons that include the limited technological capacity, most of these resources have not received satisfactory harnessing.  For instance: out of the estimated 4.7GW macro hydro potential only 561MW have been developed; and none of the 650MW geothermal potential is being harnessed.  Furthermore, besides the huge potential of biomass (12 million tons of oil equivalent), natural gas (45 million cubic metres), coal (1,200 million tones), high solar insolation (4.5 – 6.5 kWh/m²), 1,424km of coastal strip, and availability of good wind regime (> 4 m/s wind speed), they are marginally contributing to the production of commercial energy.  Ongoing exploration work also reveals that the country has an active system of petroleum and uranium.  On the other hand, after commissioning the 229km natural gas pipeline from SongoSongo Island to Dar es Salaam, there are efforts to ensure a wider application in electricity generation, households, automotive and industry.</p><p> </p><p>Due to existing environmental concerns, biomass resource is an attractive future energy for the world, Tanzania inclusive.  This calls for putting in place sustainable energy technologies, like gasification, for their harnessing.  The high temperature gasification (HTAG) of biomass is a candidate technology since it has shown to produce improved syngas quality in terms of gas heating value that has less tar.</p><p> </p><p>This work was therefore initiated in order to contribute to efforts on realizing a commercial application of biomass in Tanzania.  Particularly, the work aimed at establishing characteristic properties of selected biomass feedstock from Tanzania.  The characteristic properties are necessary input to thermochemical process designers and researchers.  Furthermore, since the properties are origin-specific, this will provide baseline data for technology transfer from north to south.  The characteristic properties that were established were chemical composition, and thermal degradation behaviour.  Furthermore, laboratory scale high temperature gasification of the biomasses was undertaken.</p><p> </p><p>Chemical composition characteristics was established to palm waste, coffee husks, cashew nut shells (CNS), rice husks and bran, bagasse, sisal waste, jatropha seeds, and mango stem.  Results showed that the oxygen content ranged from 27.40 to 42.70% where as that of carbon and hydrogen ranged from 35.60 to 56.90% and 4.50 to 7.50% respectively.  On the other hand, the elemental composition of nitrogen, sulphur and chlorine was marginal.  These properties are comparable to findings from other researchers.  Based on the results of thermal degradation characteristics, it was evident that the cashew nut shells (CNS) was the most reactive amongst the analyzed materials since during the devolatilization stage the first derivative TG (DTG) peak due to hemicellulose degradation reached (-5.52%/minute) compared palm stem whose first peak was -4.81%/minute.  DTG first peak for the remaining materials was indistinct.</p><p> </p><p>Results from the laboratory gasification experiments that were done to the coffee husks showed that gasification at higher temperature (900°C) had an overall higher gasification rate.  For instance, during the inert nitrogen condition, 7% of coffee husk remained for the case of 900°C whereas the residue mass for the gasification at 800 and 700°C was 10 and 17% respectively.  Steam injection to the biomass under high temperature gasification evolved the highest volumetric concentration of carbon monoxide.  The CO peak evolution at 900°C steam only was 23.47 vol. % CO whereas that at 700°C was 21.25 vol. % CO.  Comparatively, the CO peaks for cases without steam at 900°C and 2, 3, and 4% oxygen concentrations were 4.59, 5.93, and 5.63% respectively.  The reaction mechanism of coffee husks gasification was highly correlated to zero reaction order exhibiting apparent activation energy and the frequency factor 161 kJ/mol and 3.89x10⁴/minute respectively.</p> / QC 20100923

A test system for the performance analysis of a novel thermally regenerative fuel cell (TRFC) using propiophenone and hydrogen as the oxidant and fuel respectively was designed and built. The test system is capable of either hydrogen-air or hydrogen-propiophenone operation. Membrane electrode assemblies (MEAs) were made using commercial phosphoric acid-doped polybenzimidazole (PBI) membranes and commercial electrodes. Using Pt/carbon paper electrodes with a catalyst loading of 1mg/cm2 and a membrane with an acid doping level of 10.2 mol acid/mol of polymer repeat unit, a maximum performance of 212 mW/cm2 at a current density of 575 mA/cm2 was achieved for baseline hydrogen-air testing at 110°C. Problems were encountered, however, in achieving consistent, reproducible performance for in-house fabricated MEAs. Furthermore, ex-situ electrochemical impedance spectrometry (EIS) showed that the phosphoric acid-doped PBI was unstable in the propiophenone and that acid-leaching was occurring. In order to have MEAs with consistent characteristics for verifying the test system performance, commercial phosphoric acid-doped PBI membrane electrode assemblies were used. At a temperature of 160°C and atmospheric pressure with hydrogen and air flowrates of 150 mL/min and 900 mL/min respectively a maximum power density of 387 mW/cm2 at a current density of 1.1 A/cm2 was achieved. This performance was consistent with the manufacturer’s specifications and these MEAs were subsequently used to verify the performance of TRFC test system despite the EIS results that indicated that acid-leaching would probably occur. The Pt catalyzed commercial MEAs achieved very limited performance for the hydrogenation of the ketone. However, the performance was less than but comparable to similar results previously reported in the literature by Chaurasia et al. [1]. For pure Pt catalyst loading of 1 mg/cm2, using a commercial PBI MEA operating at 160°C and atmospheric pressure, the maximum power density was 40 µW/cm2 at a current density of 1.3 mA/cm2. A 16 hour test was conducted for these conditions with a constant 1 ohm load, successfully demonstrating the operation of the test system. The test system will be used in the development of better catalysts for ketone hydrogenation. / Thesis (Master, Chemical Engineering) -- Queen's University, 2012-10-12 10:00:58.854

This thesis addresses the tunnelling of charge carriers in different materials. First looking at the simplest case of electron tunnelling in metals at zero, then finite temperature, the current is obtained using the Fermi-Dirac golden rule and then the conductance is obtained. This is extended to take into account the spatial dependence of one of the metals being a tip since experimentally this is done by scanning tunnelling microscopy where a tip traces over the surface of a sample. The next step is to look at tunnelling between a metal and a semiconductor, again the current is found. Semiconductors can be doped and the effect this has on tunnelling is examined. Next superconductors are introduced. The purpose of my research has been to look at the tunnelling spectra of high-temperature superconducting cuprates for both extrinsic (metal-superconductor) and intrinsic (superconductor-superconductor) tunnelling. The main features seen experimentally with cuprate tunnelling are identified and then a theory capable of explaining these features is discussed. The theory is compared to experimental results and we find good agreement.

This thesis is concerned with how the physical properties of transition metal oxides change due to chemical substitution or intercalation. Experiments using the muon-spin relaxation and rotation (μSR) techniques were carried out at the ISIS Facility (UK) and the Paul Scherrer Institute (CH). In conjunction with the μSR results, the results of heat capacity and magnetic susceptibility experiments are used to provide complementary information on the same samples. Investigations of the properties of the layered triangular lattice magnets NaNiO2 and LiNiO2 are presented. For NaNiO2, all three experimental techniques are used to provide a full survey of the thermodynamic and magnetic properties of this compound. For LiNiO2, μSR studies of notionally stoichiometric and Mg-doped samples were carried out. These showed that Mg doping causes a significant change in the magnetic dynamics of the material, but neither sample exhibits long-range magnetic order. The magnetic ordering of the extensively studied perovskite compounds LaTiO3 and YTiO3 is investigated using μSR. The results were in agreement with previous neutron diffraction studies of the two compounds, but clarified the orientation of the magnetic moments in LaTiO3. It was also possible to make a detailed comparison between the μSR results and those of dipole field calculations of the magnetic field at possible muon stopping sites, allowing these to be deduced and compared with results in other well characterized transition metal oxides. The two titanium chain compounds NaTiSi2O6 and TiOCl exhibit spin gap formation at unusually high temperatures due to unconventional dimerization mechanisms. A model allowing the comparison of X-ray diffraction data, dimerization, and the magnitude of the spin gap is proposed. This is tested against both magnetic susceptibility and μSR data for both compounds. For NaTiSi2O6 both experimental techniques are in reasonable agreement, whereas in TiOCl the results are conclusively different. The origin of this disparity in TiOCl is explored. The intercalation of organic chain molecules into Bi based high-temperature superconductors has previously been demonstrated to extend the interlayer spacing by a factor of up to three without changing the superconducting transition temperature. μSR is used to investigate the London penetration depth, as a function of interlayer spacing, of two series of such samples. The results show a simple trend corresponding to a constant density of superconducting electron pairs in each layer. The consequences of this result are discussed in the context of previously identified scaling relations between superconducting parameters. Results of experiments excluding the possibility of magnetic order and muon-organic radical formation in these samples are presented, as well a preliminary study of the field distributions in a mosaic of intercalated crystallites.

This study has been mainly focused on high temperature solid oxide electrolysis cells (HT-SOECs) for steam electrolysis. The compositions, microstructures and metal catalysts for SOEC cathodes based on (La₀.₇₅Sr₀.₂₅)₀.₉₅Mn₀.₅Cr₀.₅O₃ (LSCM) have been investigated. Hydrogen production amounts from SOECs with LSCM cathodes have been detected and current-to-hydrogen efficiencies have been calculated. The effect of humidity on electrochemical performances from SOECs with cathodes based on LSCM has also been studied. LSCM has been applied as the main composite in HT-SOEC cathodes in this study. Cells were measured at temperatures up to 920°C with 3%steam/Ar/4%H₂ or 3%steam/Ar supplied to the steam/hydrogen electrode. SOECs with LSCM cathodes presented better stability and electrochemical performances in both atmospheres compared to cells with traditional Ni cermet cathodes. By mixing materials with higher ionic conductivity such as YSZ(Y₂O₃-stabilized ZrO₂ ) and CGO(Ce₀.₉Gd₀.₁O₁.₉₅ ) into LSCM cathodes, the cell performances have been improved due to the enlarged triple phase boundary (TPB). Metal catalysts such as Pd, Fe, Rh, Ni have been impregnated to LSCM/CGO cathodes in order to improve cell performances. Cells were measured at 900°C using 3%steam/Ar/4%H₂ or 3%steam/Ar and AC impedance data and I-V curves were collected. The addition of metal catalysts has successfully improved electrochemical performances from cells with LSCM/CGO cathodes. Improving SOEC microstructures is an alternative to improve cell performances. Cells with thinner electrolytes and/or better electrode microstructures were fabricated using techniques such as cutting, polishing, tape casting, impregnation, co-pressing and screen printing. Thinner electrolytes gave reduced ohmic resistances, while better electrode microstructures were observed to facilitate electrode processes. Hydrogen production amounts under external potentials from SOECs with LSCM/CGO cathodes were detected by gas chromatograph and current-to-hydrogen efficiencies were calculated according to the law of conservation of charge. Current-to-hydrogen efficiencies from these cells at 900°C were up to 80% in 3%steam/Ar and were close to 100% in 3%steam/Ar/4%H₂. The effect of humidity on SOEC performances with LSCM/CGO cathodes has been studied by testing the cell in cathode atmospheres with different steam contents (3%, 10%, 20% and 50% steam). There was no large influence on cell performances when steam content was increased, indicating that steam diffusion to cathode was not the main limiting process.

Mixed metal oxynitrides have attracted attention due to their interesting chemical and physical properties in the past twenty years. In this thesis, four series of mixed metal oxynitrides have been investigated. The samples have been synthesized by both thermal ammonolysis and high pressure high temperature methods. The structural exploration covers perovskite, scheelite and pyrochlore types. The structural studies were carried out using powder X-ray and neutron diffraction, and magnetic and conducting properties have been explored. A series of new RZrO2N (R = Pr, Nd and Sm) perovskites were synthesized using high pressure high temperature methods (HPHT) via a direct solid state reaction of R2O3 with Zr2ON2. All three new phases crystallize in the orthorhombic Pnma perovskite superstructure, and the structural distortion increases with decreasing R3+ ionic radius. RZrO2N contains both R3+ and d0 Zr4+ and thus shows a potential for multiferroic properties. EuWO1-xN2+x perovskites with a wide range of nitrogen contents (-0.16 ≤ x ≤ 0.46) were synthesized by thermal ammonolysis of an oxide precursor Eu2W2O9. Ferromagnetic ordering below a Curie temperature TC =12 ± 1 K and negative colossal magnetoresistances (CMR) have been discovered in these samples. In particular, for the lowest doped sample, EuWO0.96N2.04, CMR ≥ 99.7% was observed at 7 K. The possibility of tuning the physical properties by altering the chemical composition has been demonstrated. A linear relationship between the lattice parameter and nitrogen content of EuWO1+xN2-x was observed. An investigation has been made of the Eu-Mo-O-N system. A new pyrochlore oxynitride series Eu2Mo2O6-xN2+2x/3 (0.20 ≤ x ≤ 2.25) was synthesized by ammonolysis of Eu2Mo2O7. A ferrimagnetic ordering and semiconducting behavior has been observed in these samples. A detailed structural study of SrMO2N (M = Nb, Ta) has been performed using variable temperature neutron and electron diffraction. Partial anion order has been observed in both samples up to 750 oC. It is consistent with cis-ordering of the two nitrides in each MO4N2 octahedron. At low temperatures, this order directs the tilting of the octahedron to form a pseudo-tetragonal superstructure. It creates zig-zag MN chains in two or three dimensions within the lattice. This principle can be used to predict the local structures of perovskite-related oxynitrides AMO3-xNx.

An experimental investigation is presented on a novel High Temperature Falling Particle Receiver for Concentrated Solar Power (CSP) to quantify the extent of erosion of the receiver structural materials by the flowing particulate matter. The current receiver design uses a series of metal wire mesh screens to slow down the particulate flow through the receiver in order to increase their residence time thereby achieving the desired temperature rise within the receiver without the need for particulate recirculation. The solid particulates are gravity fed through the receiver where they absorb the incident thermal energy before flowing to a high temperature storage bin upstream of a heat exchanger where the heat stored in the particulate material is transferred to the working fluid for the power cycle. To assess the effective life of the receiver, this experimental investigation is undertaken. This thesis includes the development of an apparatus to test wire meshes under high temperature and particle abrasion conditions, and the presentation and analysis of these results.

Next generation aerospace vehicles require thermal protection system (TPS) materials that are capable of withstanding the extreme aerothermal environment during hypersonic flight (>Mach 5 [>1700 m/s]). Ultra-high temperature ceramics (UHTC) such as zirconium diboride (ZrB₂) are candidate TPS materials due to their high-temperature thermal and mechanical properties and are often the basis for advanced composites for enhanced oxidation resistance. However, ZrB₂ matrix impurities in the form of boron trioxide (B₂O₃) and zirconium dioxide (ZrO₂) limit the high-temperature capabilities. Electric based sintering techniques, such as spark plasma sintering (SPS), that use joule heating have become the preferred densification method to process advanced ceramics due to its ability to produce high density parts with reduced densification times and limit grain growth. This study focuses on a combined experimental and thermodynamic assisted processing approach to enhance powder purity through a carbo- and borocarbo-thermal reduction of oxides using carbon (C) and boron carbide (B₄C). The amount of oxides on the powder surface are measured, the amount of additive required to remove oxides is calculated, and processing conditions (temperature, pressure, environment) are controlled to promote favorable thermodynamic reactions both during thermal processing in a tube furnace and SPS. Untreated ZrB₂ contains 0.18 wt%O after SPS. Additions of 0.75 wt%C is found to reduce powder surface oxides to 0.12 wt%O. A preliminary Zr-C-O computational thermodynamic model shows limited efficiency of carbon additions to completely remove oxygen due to the solubility of oxygen in zirconium carbide (ZrC) forming a zirconium oxycarbide (ZrCₓOᵧ). Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) with atomic scale elemental spectroscopy shows reduced oxygen content with amorphous Zr-B oxides and discreet ZrO₂ particle impurities in the microstructure. Processing ZrB₂ with minimal additions of B₄C (0.25 wt%) produces high purity parts after SPS with only 0.06 wt%O. STEM identifies unique “trash collector” oxides composed of manufacturer powder impurities of calcium, silver, and yttrium. A preliminary Zr-B-C-O thermodynamic model is used to show the potential reaction paths using B₄C that promotes oxide removal to produce high-purity ZrB₂ with fine grains (3.3 𝜇m) and superior mechanical properties (flexural strength of 660MPa) than the current state-of-the-art ZrB₂ ceramics. Due to the desirable properties produced using SPS, there is growing interest to advance processing techniques from lab-scale (20 mm discs) to large-scale (>100 mm). The advancement of SPS technologies has been stunted due to the limited power and load delivery of lab-scale furnaces. We use a large scale direct current sintering furnace (DCS) to address the challenges of producing industrially relevant sized parts. However, current-assisted sintering techniques, like SPS and DCS, are highly dependent on tooling resistances and the electrical conductivity of the sample, which influences the part uniformity through localized heating spots that are strongly dependent on the current flow path. We develop a coupled thermal-electrical finite element analysis model to investigate the development and effects of tooling and current density manipulation on an electrical conductor (ZrB₂) and an electrical insulator, silicon nitride (Si₃N₄), at the steady-state where material properties, temperature gradients and current/voltage input are constant. The model is built based on experimentally measured temperature gradients in the tooling for 20 mm discs and validated by producing 30 mm discs with similar temperature gradients and grain size uniformity across the part. The model aids in developing tooling to manipulate localize current density in specific regions to produce uniform 100 mm discs of ZrB₂ and Si₃N₄.

The physics of the series Pr_yY_1-yBa₂Cu₃O_6&plus;x, and ability of Pr to suppress superconductivity, has been a subject of frequent discussions in the literature for more than a decade. This thesis describes a polarised neutron diffraction (PND) experiment performed on PrBa₂Cu₃O_6.24 designed to find out something about the electron structure. This experiment pushed the limits of what can be done using the PND technique. The problem is one of a limited number of measured Fourier components that need to be inverted to form a real space image. To accomplish this inversion the maximum entropy technique has been employed. In some cases, the maximum entropy technique has the ability to increase the resolution of ‘inverted’ data immensely, but this ability is found to depend critically on the choice of constants used in the method. To investigate this a Bayesian robustness analysis of the maximum entropy method is carried out, resulting in an improvement of the maximum entropy technique for analysing PND data. Some results for nickel in the literature have been re-analysed and a comparison is made with different maximum entropy algorithms. Equipped with an improved data analysis technique and carefully measured PND data for PrBa₂Cu₃O_6.24 a number of new interesting features are observed, putting constraints on existing theoretical models of Pr_yY_1-yBa₂Cu₃O_6&plus;x and leaving room for more questions to be answered.

This thesis reports the synthesis and characterisation of the layered oxypnictides Sr₂MO₃FeAs (M = Sc, V and Cr) and CeOMnAs. In these materials the choice of transition metal cation at the tetrahedral site in the arsenide layer chiefly dictates the physical properties that are observed. The bulk of this work has focussed on the development of a new family of iron arsenide superconductor with the general formula Sr₂MO₃FeAs (M = Sc, V, Cr). This structure is comprised of anti-PbO-type [FeAs]^- layers which alternate with insulating [Sr₂MO₃]⁺ oxide fragments that resemble a portion of the K₂NiF₄ structure. In contrast to other FeAs parent materials, no member of the Sr₂MO₃FeAs family exhibits any strong evidence for long range Fe order or a tetragonal to orthorhombic distortion upon cooling. Attempts to electron and hole dope Sr₂ScO₃FeAs into the superconducting regime have as yet been unsuccessful. Although Sr₂ScO₃FeAs shows no evidence for Fe ordering, a checkerboard arrangement of Cr³⁺ spins in the ab-plane is observed below 40 K (k = (½, ½, 0)) analogous to that seen in Pr₂CuO₄. The partial substitution of Fe²⁺ (d⁶) by Co²⁺ (d⁷) in Sr₂CrO₃Fe_1-xCo_xAs has been shown to be a fruitful strategy for electron-doping this material into the superconducting regime with T_c maximised at 18 K in Sr₂CrO₃Fe_0.92Co_0.08As. It is also established that this substitution influences the ordering on the Cr sub-lattice with a doubling in the size of the magnetic cell along the c axis (k = (½, ½, ½)). Sr₂VO₃FeAs, a rare example of an “undoped” superconductor (T_c = 25 K), is shown to be electron-doped by mixed valence vanadium +3.13(5). Magnetometry measurements also reveal a series of magnetic transitions in Sr₂VO₃FeAs, however μSR and powder neutron diffraction studies suggest that this system is some way from commensurate long range order. In contrast to Sr₂CrO₃FeAs, electron-doping strategies in Sr₂VO₃FeAs have the effect of decreasing T_c and ultimately suppressing superconductivity entirely as Sr₂V_1-xTi_xO₃FeAs and Sr₂VO₃Fe_1-xCo_xAs materials are over electron-doped. Sr₂V_1-xMg_xO₃FeAs samples were also prepared, but rather than this strategy hole-doping the FeAs layer it preferentially oxidises vanadium towards V⁴⁺. This substitution also has a considerable effect on the superconducting critical temperature (T_c) which is raised as high as 31 K in Sr₂V_0.775Mg_0.225O₃FeAs. The isovalent substitution of Sr²⁺ by Ca²⁺ in Sr_2-xCa_xVO₃FeAs has been shown to strongly influence the superconducting properties of this material and a clear correlation between the evolution of T_c and the shape of the FeAs₄ tetrahedron has been established. These results demonstrate that superconductivity in iron-based superconductors is extremely sensitive to both electron count and the crystal structure. Finally, investigations into the manganese oxide arsenide CeOMnAs reveal room temperature ordering of Mn²⁺ spins and a spin reorientation transition of Mn moments at 36 K. This transition is concomitant with Ce ordering and an apparent weak structural distortion, demonstrating that f electrons are able to dictate the orientation of Mn moments.