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Superconductivity in magnesium diboride (MgB2) was first discovered in 2001. It is unique in that it has two superconducting gaps. The transition temperature of 39 K exceeded the maximum transition temperature thought to be possible through phonon mediated superconductivity. Through the study of MgB2, a general paradigm is being formulated to describe multi-gap superconductors. The paradigm includes inter-band and intra-band scattering between the gaps which can cause a smearing of the gap parameter over a distribution instead of a single value. Although each gap is individually thought to be well described by the BCS theory, the interaction between the two gaps causes complications in describing the overall superconducting properties of MgB2. The focus of this work was to lay the groundwork for an MgB2-based Josephson junction technology. This includes improving on a previously established baseline for all-MgB2 Josephson junctions, utilizing the Josephson Effect to experimentally verify a model pertaining to the two-gap nature of MgB2, specifically the magnetic penetration depth, and designing, fabricating, and testing multi-junction devices and circuits. The experiments in this work included fabrication of Josephson Junctions, DC superconducting quantum interference devices (SQUIDs), Josephson junction arrays, and a rapid single flux quantum (RSFQ) circuit. The junctions were all made utilizing the hybrid physical-chemical vapor deposition method, with an MgO sputtered barrier. The current process consists of three superconducting layers which are patterned using standard UV photolithography and etched with Ar ion milling. There were SQUIDS made with sensitivity to magnetic fields parallel to the film surface, which were used to measure the inductance of MgB2 microstrips. This inductance was used in design of more complicated devices as well as in calculating the magnetic penetration depth of MgB2, found to be about 40 nm at low temperature, in good agreement with a previously published theoretical model. Planar-type DC SQUIDs were also made to present the feasibility of the technology for application purposes. The large voltage modulation of over 500 μV at 15 K for these devices along with operation up to 37 K shows that MgB2 is a potential replacement for low temperature devices. The junction series arrays were fabricated with 100 junctions of equal size to present the ever-increasing robustness of the technology. The devices served well to measure the large property spread associated with these junctions and have been well established as a diagnostic tool for improving this spread. The culmination of this work was a basic RSFQ toggle flip flop circuit. A DC measurement of these circuits yielded digital operation up to 180 GHz at low temperature and about 63 GHz at 20 K. This is not yet near the potential limit of MgB2 established by the value of the superconducting gap parameters, but a huge success in showing that MgB2 is a viable option for pursuing superconducting digital electronics suitable for low power, cryogen-free operation. / Physics

Alumina-titanium diboride (Al2O3-TiB2) composites have high temperature, wear, and impact resistance that could be useful in high performance applications. Determining the effect of processing parameters on relative bond strength and effective plasticity may contribute to optimization and predictability of performance in the Al2O3-TiB2 system. Al2O3-TiB2 composites were obtained from a collection of samples that were created during a separate ongoing research program being conducted by Dr. Kathryn V. Logan. The Logan samples were initially formed by hot pressing powders produced using Self-Propagating High Temperature synthesis (SHS) of Al, TiO2, and B2O3 powders or manual mixing (MM) of Al2O3 and TiB2 powders. Samples were then fractured using standard single edge notched beam (SENB) fracture toughness testing. The obtained fractured surfaces were examined using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). Relative amounts of transgranular and intergranular fracture of Al2O3 and TiB2 grains were determined. Transgranular fracture was used as a measure of relative bond strength. Other samples were obtained from the Logan collection to conduct nano-indentation measurements on polished sample surfaces in Al2O3 grains and in TiB2 grains. Indent locations were verified using SEM. Reduced modulus, final displacements, and fracture toughness for indents in Al2O3 grains and in TiB2 grains were determined from nano-indentation curves. Reduced modulus was used as a measure of relative bond strength. Final displacement and fracture toughness were used as measures of relative effective plasticity. Analysis of Variance (ANOVA) using Taguchi arrays was conducted using the powder processing factor (SHS vs. MM) and the predominant microstructure factor (TiB2 grains surrounding Al2O3 grains vs. TiB2 grains distributed amongst Al2O3 grains) when examining the effect of processing parameters on relative bond strength as measured by amount of transgranular fracture. Analysis of Variance (ANOVA) using Taguchi arrays was conducted using the powder processing factor (SHS vs. MM), the predominant microstructure factor (TiB2 grains surrounding Al2O3 grains vs. TiB2 grains distributed amongst Al2O3 grains), and the indented phase factor (Al2O3 vs. TiB2) when examining the effect of processing parameters on relative bond strength as measured by nano-indentation reduced modulus and both measures of relative effective plasticity. Powder processing was significant for the relative bond strength measures, but was not significant for the relative effective plasticity measures. Predominant microstructure was significant for all measures except relative effective plasticity as measured by fracture toughness, for which none of the factors and interactions were significant. The interaction between powder processing and predominant microstructure was significant for most of the relative bond strength measures and for relative effective plasticity as measured by final displacements. Indented phase was significant for the nano-indentation measures except nano-indentation fracture toughness, although the significance for nano-indentation fracture toughness was just below the critical level. The interaction between powder processing and indented phase and the interaction between predominant microstructure and indented phase were only significant for the relative bond strength measure using nano-indentation reduced modulus. The interaction between powder processing, predominant microstructure, and indented phase was significant for the nano-indentation measures except nano-indentation fracture toughness. The optimum level for powder processing was predominantly manual mixing. The optimum level for predominant microstructure was predominantly TiB2 grains surrounding Al2O3 grains. The optimum level for indented phase was predominantly TiB2. / Master of Science

Thesis (Ph.D.)--University of Wollongong, 2008. / Typescript. Includes bibliographical references.

Borides are implemented in a range of industrial applications due to their unique mechanical, electrical, thermal and catalytic properties. In particular, transition metal diborides are of special interest. In the recent years, borides of rhenium (Re), osmium (Os) and iridium (Ir) have been studied as for their ultra-hardness and superior stiffness. In this dissertation, a mechanochemical method is introduced to produce rhenium diboride (ReB2) powder, a novel hexagonal osmium diboride (h-OsB2), and iridium boride powders. Densification by Spark Plasma Sintering (SPS), thermal stability and mechanical properties of h-OsB2 were also studied. ReB2 was recently reported to exhibit high hardness and low compressibility, which both are strong functions of its stoichiometry, namely Re to B ratio. Most of the techniques used for ReB2 synthesis reported 1:2.5 Re to B ratio because of the loss of the B during high temperature synthesis. However, as a result of B excess, the amorphous boron, located along the grain boundaries of polycrystalline ReB2, would degrade the ReB2 properties. Therefore, techniques which could allow synthesizing the stoichiometric ReB2 preferably at room temperature are in high demand. ReB2 powder was synthesized at low temperature using mechanochemical route by milling elemental crystalline Re and amorphous B powders in the SPEX 8000 high energy ball mill for 80 hours. The formation of boron and perrhenic acids are also reported after ReB2 powder was exposed to the moist air environment for a twelve month period of time. Hexagonal osmium diboride (h-OsB2), a theoretically predicted high-pressure phase, has been synthesized for the first time by a mechanochemical method, i.e., high energy ball milling. X-ray diffraction (XRD) indicated the formation of h-OsB2 after 2.5 hours of milling, and the reaction reaches equilibrium after 18 hours of milling. The lattice parameters of the h-OsB2 are a=2.916Å and c=7.376 Å, with a P63/mmc space group. Transmission electron microscopy confirmed the appearance of the h-OsB2 phase. The thermal stability of h-OsB2 powder was studied by heating under argon up to 876 °C and cooling in vacuo down to -225 °C. The oxidation mechanism of h-OsB2 has also been proposed. The hexagonal phase partially converted to the orthorhombic phase (20 wt.%) after spark plasma sintering of h-OsB2 at 1500°C and 50MPa for 5 minutes. Hardness and Young*s modulus of the h-OsB2 were measured to be 31 ± 9 GPa and 574 ± 112 GPa, respectively by nanoindentation method. Prior to this research a number of compounds have been prepared in Ir-B system with lower than 2 boron stoichiometry, and no IrB2 phases have been synthesized experimentally. In this dissertation, three new iridium boride phases, ReB2-type IrB2, AlB2-type IrB2 and IrB have been synthesized with a similar mechanochemical method. The formation of these three phases has been confirmed by both X-ray diffraction (XRD) and transmission electron microscope (TEM) after 30 hours of ball milling and 48 hours of annealing. The IrB2 phases have hexagonal crystal structures and the new IrB phase has an orthorhombic crystal structure. The segregation of iridium from iridium borides* lattices has also been studied by high resolution TEM.

Sharp leading edges and nose caps on hypersonic vehicles, re-entry vehicles and reusable launch vehicles are items of current research interest for enhanced aerodynamic performance and maneuverability. The unique combination of mechanical properties, physical properties, thermal / electrical conductivities and thermal shock resistance of ZrB2 make it a promising candidate material for such applications. In the recent past, a lot of work has been carried out on ZrB2-based materials towards processing as well as characterization of their mechanical, oxidation and thermal behaviour. ZrB2 based materials have been successfully processed by conventional hot pressing, pressureless sintering, reactive hot pressing and spark plasma sintering. Densification of ZrB2 gets activated when the oxide impurities (B2O3 and ZrO2) were removed from particle surfaces, which minimized coarsening. B4C is widely used as a sintering additive for ZrB2 because it reduces ZrO2 at low temperature. It is found that full densification in ZrB2 based materials by hot pressing is achieved either at 2000 C and higher temperatures with moderate pressure of 20-30 MPa or at reduced temperature (1790-1840 C) with much higher pressure (800-1500 MPa). But no study is available that identifies the dominant hot pressing mechanism at different temperatures and pressures. On the other hand, reinforcement of SiC in ZrB2 is known to increase flexural strength, fracture toughness and oxidation resistance. It has been shown that oxidation resistance of ZrB2-SiC composites is superior to that of monolithic ZrB2 and SiC. For high temperature applications in air, the residual strength (room temperature strength after exposure in air at high temperatures) of non oxide ceramics after oxidation is important. A few reports are available on residual strength of ZrB2 –SiC composite after thermal exposure at high temperatures. In contrast to the literature on composites, there are no reports available on the residual strength of monolithic ZrB2 after exposure to high temperatures. Also, previous studies on residual strength of ZrB2-SiC composites have been limited to a single temperature of exposure. But there is a need to measure the residual strength after exposure to a range of temperatures since the oxide layer structure changes with temperature. The room temperature thermal conductivity data for ZrB2 and ZrB2-SiC composite shows a wide scatter in value as well as a dependence on microstructural parameters, especially porosity and grain size. Also, there is insufficient data available for the high temperature thermal conductivity of ZrB2-SiC. Therefore, it is difficult to evaluate the effect of SiC content on thermal conductivity of ZrB2-SiC composites at high temperatures. The present thesis seeks to address some of these gaps to better understand the suitability of ZrB2 and ZrB2-SiC composites for ultra-high temperature applications. In the present work, hot pressing is used for densification of ZrB2 and ZrB2-SiC composites. Different amounts of B4C (0, 0.5, 1, 3 & 5 wt %) were used as sintering additives in ZrB2 and hot pressed at 2000 C with 25 MPa applied pressure. The hot pressed samples are characterized for their microstructural, mechanical properties and oxidation behaviour. By addition of B4C, density as well as micro-hardness increased. For lower B4C content (0.5 & 1 wt %), hot pressed ZrB2 has shown considerable improvement in flexural strength after exposure in air at 1000 C for 5 hours, while higher B4C content (3 & 5 wt %) leads to marginal or no improvement. Due to the better mechanical and oxidation behavior of composites containing SiC, the densification behavior during hot pressing was studied. The densification behaviors as well as the microstructures for hot pressing of ZrB2-20 % SiC composite were found to change in a very 0 narrow temperature range. During hot pressing at 1700 C, the densification was found to be mechanically driven particle fragmentation and rearrangement. On the other hand, thermally activated mass transport mechanisms started dominating after initial particle fragmentation and rearrangement after hot pressing at 1850 C and 2000 C. At 2000 C, the rate of grain boundary diffusion was enhanced which resulted into annihilation of dislocation. The effect of SiC contents (10, 20 & 30 vol %) on mechanical and oxidation behavior of ZrB2-SiC composite were also studied. The average micro-hardness and fracture toughness of ZrB2-SiC composites increased with SiC content. But the flexural strength of ZrB2-20 vol % SiC composites was found to be the highest. Oxidation and residual strength of hot pressed ZrB2 -SiC composites were evaluated as a function of SiC contents after exposure over a wide temperature range (1000-1700 C). Multilayer oxide scale structures were found after oxidation. The composition and thickness of these multilayered oxide scale structures were found to depend on exposure temperature and SiC content. After exposure to 1000 C for 5 hours, the residual strength of ZrB2 -SiC composites improved by nearly 60 % compared to the as-hot pressed composites with 20 & 30 vol % SiC. On the other hand, the residual strength of these composites remained unchanged after 1500 C for 5 hours. A drastic degradation in residual strength was observed in composites with 20 & 30 vol % SiC whereas strength was retained for ZrB2-10 % SiC composite after exposure to 1700 C for 5 hours in ZrB2 –SiC. Therefore, residual strength of ZrB2-10 % SiC composite was measured at different exposure times (up to 10 hours) at 1500 0C. An attempt was made to correlate the microstructural changes and oxide scales with residual strength with respect to variation in SiC content and temperature of exposure. Since the ZrB2-20 vol % SiC composite showed the maximum strength, the dependence of strength on various microstructural as well processing parameters was also studied. It was found that porosity, grain size as well as surface residual stress due to grinding influenced the strength of ZrB2-20 vol % SiC composites. Finally, thermal diffusivity and conductivity of hot pressed ZrB2 with different amounts of B4C and ZrB2-SiC composites were investigated experimentally over a wide temperature range (25 – 1500 C). Both thermal diffusivity as well as thermal conductivity was found to decrease with increase in temperature for all hot pressed ZrB2 and ZrB2-SiC composites. At around 200 C, thermal conductivity of ZrB2-SiC composites was found to be composition independent. Thermal conductivity of ZrB2-SiC composites was also correlated with theoretical predictions of the Maxwell-Eucken relation. The dominated mechanisms of heat transport for all hot pressed ZrB2 and ZrB2-SiC composites at room temperature were determined by Wiedemann-Franz analysis using measured room temperature electrical conductivity of these materials. It was found that the electronic thermal conductivity dominated for all monolithic ZrB2 whereas the phonon contribution to thermal conductivity increased with SiC contents for ZrB2-SiC composites. The heat conduction mechanism at high temperature was also studied by measuring the high temperature electrical conductivity of ZrB2 and ZrB2-SiC composites. The effect of porosity on thermal diffusivity and conductivity was also studied for ZrB2-20 vol % SiC composites.

Sharp leading edges required for hypersonic vehicles improve the maneuverability as well as reduce aerodynamic drag. However, due to the sharp design, increased surface temperatures require materials that can withstand these extreme conditions. Ultra-high temperature ceramics are a material group being considered for the leading-edge material, specifically ZrB₂/SiC (ZBS) which has a high thermal shock resistance, melting temperature, and thermal conductivity. Studies done by Tan et. al. has shown that adding samarium (Sm) as a dopant to ZBS has an emittance of 0.9 at 1600^oC and develop oxide scales that have excellent ablation performance. However, it remained unknown how the Sm doped oxide scale formed as well as how the emittance and ablation performance are affected by the microstructure. This study investigates the oxide scale development of 3 mol% doped Sm-ZBS billets as well as how differences in microstructure affect the emittance and ablation performance. Samples were prepared via chemical infiltration of samarium nitrate into spray-dried powders of 80 vol.% ZrB₂/20 vol.% SiC; powders were then pressed into billets and pressureless sintered. Samples cut and polished from these billets were then oxidized for 10, 60, or 300 s, respectively, using an oxyacetylene torch. X-ray diffraction was used to determine the sequence of oxidation of Sm-ZBS, beginning with the formation of ZrO₂ and Sm₂O₃. The final oxide scale was determined to be c₁-Sm_0.2Zr_0.8O_1.9, with a melting temperature exceeding 2500^oC. SEM and EDS were also used to investigate the microstructural formation that occurs from the bursting of convection cells. Samples with different microstructures revealed similar topographical microstructures post-ablation due to the sequence of the oxide formation. However, samples with rougher surfaces and higher porosities had a higher concentration of trapped glass in the cross-sectional oxide scale. It was also found that due to differences in heating the sample during emittance testing compared to ablation testing, the oxide developed was identical for all the samples. It was also found that variances in microstructure had no effect on the spectral emittance of Sm-ZBS at ultra-high temperatures. The fabrication of c₁-Sm_0.2Zr_0.8O_1.9 (SZO) as a bulk billet was also investigated to use as a thermal barrier coating (TBC) in replacement of Sm-ZBS.

Yeung, Him Ching = 以粉末混合方法及夾心樣品方法製作的硼化鎂的特性研究 / 楊謙靖. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Abstracts in English and Chinese. / Yeung, Him Ching = Yi fen mo hun he fang fa ji jia xin yang pin fang fa zhi zuo de peng hua mei de te xing yan jiu / Yang Qianjing. / Abstract --- p.i / 摘要 --- p.iii / Acknowledgments --- p.v / Table of contents --- p.vi / List of table captions --- p.viii / List of figure captions --- p.ix / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Superconductors --- p.1 / Chapter 1.1.1 --- Classifications --- p.1 / Chapter 1.1.2 --- Conventional versus unconventional --- p.2 / Chapter 1.1.3 --- Type I and Type II superconductors --- p.4 / Chapter 1.1.4 --- Critical current Jc and the Bean´ةs Model --- p.5 / Chapter 1.2 --- Magnesium diboride --- p.8 / Chapter 1.2.1 --- Structure --- p.8 / Chapter 1.2.2 --- Physical properties --- p.8 / Chapter 1.2.3 --- Chemical properties --- p.9 / Chapter 1.2.4 --- Conduction mechanisms --- p.9 / Chapter 1.2.5 --- Fabrication methods --- p.9 / Chapter 1.3 --- Objectives of this work --- p.10 / References --- p.12 / Figures --- p.14 / Chapter Chapter 2 --- Methodology and instrumentation --- p.17 / Chapter 2.1 --- Experimental procedures --- p.17 / Chapter 2.2 --- Samples preparation --- p.17 / Chapter 2.2.1 --- Powder mixing approach --- p.17 / Chapter 2.2.2 --- Sandwich structure approach --- p.18 / Chapter 2.3 --- Samples fabrication --- p.18 / Chapter 2.4 --- Determination of compositions --- p.18 / Chapter 2.5 --- Characterization --- p.19 / Chapter 2.5.1 --- X-ray diffractometry (XRD) --- p.19 / Chapter 2.5.2 --- Microstructural analysis --- p.20 / Chapter 2.5.3 --- Magnetic measurements --- p.20 / Chapter 2.5.3.1 --- Setup for Jc measurements --- p.21 / Reference --- p.22 / Figures --- p.23 / Chapter Chapter 3 --- Powder mixing approach --- p.26 / Chapter 3.1 --- Results --- p.26 / Chapter 3.1.1 --- XRD results --- p.26 / Chapter 3.1.2 --- Compositions --- p.27 / Chapter 3.1.3 --- SEM results --- p.28 / Chapter 3.1.3.1 --- Sample sintered at 600°C --- p.28 / Chapter 3.1.3.2 --- Samples sintered at 700°C and 800°C --- p.28 / Chapter 3.1.3.3 --- Sample sintered at 900°C --- p.29 / Chapter 3.1.3.4 --- Sample sintered at 1000°C --- p.29 / Chapter 3.1.3.5 --- Sample sintered at 1050°C --- p.29 / Chapter 3.1.4 --- VSM results --- p.30 / Chapter 3.1.4.1 --- Tc measurements --- p.30 / Chapter 3.1.4.2 --- Hysteresis loops --- p.31 / Chapter 3.1.5 --- Jc measurements --- p.31 / Chapter 3.1.5.1 --- Direct measurement --- p.31 / Chapter 3.1.5.2 --- The Bean´ةs Model --- p.32 / Chapter 3.2 --- Discussions --- p.32 / Chapter 3.3 --- Summary --- p.35 / References --- p.36 / Figures --- p.37 / Tables --- p.47 / Chapter Chapter 4 --- Sandwich structure approach --- p.49 / Chapter 4.1 --- Results --- p.49 / Chapter 4.1.1 --- SEM results --- p.49 / Chapter 4.1.1.1 --- Surface of the Mg disk --- p.49 / Chapter 4.1.1.2 --- Inner region of the Mg disk --- p.50 / Chapter 4.1.2 --- XRD results --- p.50 / Chapter 4.1.2.1 --- Surface of the Mg disk --- p.50 / Chapter 4.1.2.2 --- Inner region of the Mg disk --- p.51 / Chapter 4.1.3 --- VSM results --- p.51 / Chapter 4.1.3.1 --- Tc measurement --- p.51 / Chapter 4.1.3.2 --- Hysteresis loops --- p.52 / Chapter 4.2 --- Discussions --- p.52 / Chapter 4.3 --- Summary --- p.54 / Reference --- p.54 / Figures --- p.55 / Tables --- p.61 / Chapter Chapter 5 --- Growth Mechanism --- p.62 / Chapter 5.1 --- Introduction --- p.62 / Chapter 5.2 --- Brief summary of results --- p.62 / Chapter 5.2.1 --- Powder mixing approach --- p.62 / Chapter 5.2.2 --- Sandwich structure approach --- p.63 / Chapter 5.3 --- Formation of the MgB2 platelets --- p.63 / Chapter 5.4 --- Size of the MgB2 platelets --- p.67 / Chapter 5.5 --- Summary --- p.68 / References --- p.69 / Figures --- p.70 / Tables --- p.75 / Chapter Chapter 6 --- Conclusions and suggestions of future work --- p.76 / Chapter 6.1 --- Summary --- p.76 / Chapter 6.2 --- Future work --- p.77 / Appendix 1 --- p.79 / Appendix 2 --- p.81

Thesis (Ph.D.)--University of Wollongong, 2004. / Typescript. Includes bibliographical references.