2,758 research outputs found
Assessing the suitability of fly ash geopolymers for high temperature applications
Geopolymers are an inorganic polymer synthesised from the dissolution and polycondensation of aluminosilicates in alkaline solutions under hydrothermal condition, yielding an amorphous, three-dimensional polymeric framework (Davidovits, 1991). They are a broad class of binding material with applications that range from conventional concrete to high tech, light weight composites for use in aviation. Geopolymers have also shown promise for use in high temperature applications, such as fire proof coatings, structural concrete in fire prone areas and thermal insulation for refractory type applications, due to their intrinsic thermal stability (Barbosa and MacKenzie, 2003a).This thesis reports on an investigation into the thermal performance of geopolymers synthesised from a range of fly ashes in order to assess their suitability for use in high temperature applications. Five fly ashes from Australian power stations with contrasting chemical properties were used in the study. Geopolymers were synthesised from each of the fly ashes using sodium silicate or sodium aluminate solutions in order to achieve a set range of Si:Al compositional ratios. Thermal analysis was conducted up to 1000 °C using a constant heat rate as well as a heating regime that simulated the conditions during a fire.The fly ashes were characterised in terms of elemental composition, phase composition, particle size, density and morphology prior to being used to synthesise geopolymers. It was determined that only a portion of each of the fly ashes was available for geopolymerisation and that the reactive Si:Al ratio (amorphous Si:Al ratio) varied greatly between the fly ashes. Collie and Port Augusta fly ashes had relatively low reactive Si:Al ratios (1.15 and 1.84, respectively) whereas Eraring, Tarong and Bayswater fly ashes had high Si:Al ratios (4.98, 8.84 and 7.49, respectively). All of the fly ashes had a predominantly spherical morphology, characteristic of fly ashes, though only the Collie and Port Augusta fly ashes had a significant portion of sub 5 μm particles.The thermo-physical, mechanical and micro-structural properties of the geopolymers made from each of the fly ashes are presented and the effect of the source fly ash characteristics on the hardened product is discussed. The results varied greatly with fly ash source and the most influential fly ash characteristic was the reactive Si:Al ratio. Fly ashes with a high reactive Si:Al ratio (≥5) were sodium aluminate activated and produced geopolymers with low to moderate as-cured compressive strengths but exhibited excellent dimensional stability during heating and greater compressive strengths after heating. Fly ashes with a low reactive Si:Al ratio (<2) were sodium silicate activated and produced geopolymers with high as-cured compressive strengths but exhibited poor dimensional stability during heating and greatly reduced compressive strengths after heating. All samples exhibited strength improving microstructural changes such as improved inter-particle bonding due to sintering after firing. However, the instability of non geopolymer phases during high temperature exposure led to strength losses in some samples depending on the type and composition of the activating solution.Geopolymers from three of the fly ashes were assessed for their performance upon exposure to a simulated fire. Solid and low density foamed variants (ρ ≈ 0.9 g cm-3, k ≈ 0.3 W m-1K-1) of the mixes were used for fire testing. Fire ratings of between 60 and 90 minutes for a sample thickness of 50 mm were achieved. The solid geopolymers exhibited better fire ratings than the low density geopolymers due to their higher water content (as they contained more of the hydrated geopolymer phase). Microstructural analysis of the fire tested samples indicated that the geopolymers were not significantly damaged by dehydration and the fire exposed side exhibited analogous changes to the samples that were gradually heated to 1000 °C.The results in this thesis indicate that fly ash geopolymers have great potential for utilisation in high temperature applications provided they are synthesised from a source material with suitable physical and compositional characteristics
The use of imaging systems to monitor shoreline dynamics
The development of imaging systems is nowadays established as one of the most powerful and reliable tools for monitoring beach morphodynamics. Two different techniques for shoreline detection are presented here and, in one case, applied to the study of beach width oscillations on a sandy beach (Pauanui Beach, New Zealand). Results indicate that images can provide datasets whose length and sample interval are accurate enough to resolve inter-annual and seasonal oscillations, and long-term trends. Similarly, imaging systems can be extremely useful in determining the statistics of rip current occurrence. Further improvements in accuracy and reliability are expected with the recent introduction of digital systems
A low-metallicity massive contact binary undergoing slow Case A mass transfer: A detailed spectroscopic and orbital analysis of SSN 7 in NGC 346 in the SMC
Most massive stars are believed to be born in close binary systems where they
can exchange mass, which impacts the evolution of both binary components. Their
evolution is of great interest in the search for the progenitors of
gravitational waves. However, there are unknowns in the physics of mass
transfer as observational examples are rare, especially at low metallicity.
Nearby low-metallicity environments are particularly interesting hunting
grounds for interacting systems as they act as the closest proxy for the early
universe where we can resolve individual stars. Using multi-epoch spectroscopic
data, we complete a consistent spectral and orbital analysis of the early-type
massive binary SSN 7 hosting a ON If+O5.5 V((f)) star. Using these
detailed results, we constrain an evolutionary scenario that can help us to
understand binary evolution in low metallicity. We were able to derive reliable
radial velocities of the two components from the multi-epoch data, which were
used to constrain the orbital parameters. The spectroscopic data covers the UV,
optical, and near-IR, allowing a consistent analysis with the stellar
atmosphere code, PoWR. Given the stellar and orbital parameters, we interpreted
the results using binary evolutionary models. The two stars in the system have
comparable luminosities of and for the primary and secondary, respectively, but have
different temperatures ( and
). The primary () is less massive than
the secondary (), suggesting mass exchange. The mass estimates
are confirmed by the orbital analysis. The revisited orbital period is
. Our evolutionary models also predict mass exchange. Currently,
the system is a contact binary undergoing a slow Case A phase, making it the
most massive [Abridged]Comment: 20 page
Magnetic Flux Braiding: Force-Free Equilibria and Current Sheets
We use a numerical nonlinear multigrid magnetic relaxation technique to
investigate the generation of current sheets in three-dimensional magnetic flux
braiding experiments. We are able to catalogue the relaxed nonlinear force-free
equilibria resulting from the application of deformations to an initially
undisturbed region of plasma containing a uniform, vertical magnetic field. The
deformations are manifested by imposing motions on the bounding planes to which
the magnetic field is anchored. Once imposed the new distribution of magnetic
footpoints are then taken to be fixed, so that the rest of the plasma must then
relax to a new equilibrium configuration. For the class of footpoint motions we
have examined, we find that singular and nonsingular equilibria can be
generated. By singular we mean that within the limits imposed by numerical
resolution we find that there is no convergence to a well-defined equilibrium
as the number of grid points in the numerical domain is increased. These
singular equilibria contain current "sheets" of ever-increasing current
intensity and decreasing width; they occur when the footpoint motions exceed a
certain threshold, and must include both twist and shear to be effective. On
the basis of these results we contend that flux braiding will indeed result in
significant current generation. We discuss the implications of our results for
coronal heating.Comment: 13 pages, 12 figure
On Berenstein-Douglas-Seiberg Duality
I review the proposal of Berenstein-Douglas for a completely general
definition of Seiberg duality. To give evidence for their conjecture I present
the first example of a physical dual pair and explicitly check that it
satisfies the requirements. Then I explicitly show that a pair of toric dual
quivers is also dual according to their proposal. All these computations go
beyond tilting modules, and really work in the derived category. I introduce
all necessary mathematics where needed.Comment: 22 pages, LaTe
The homotopy theory of dg-categories and derived Morita theory
The main purpose of this work is the study of the homotopy theory of
dg-categories up to quasi-equivalences. Our main result provides a natural
description of the mapping spaces between two dg-categories and in
terms of the nerve of a certain category of -bimodules. We also prove
that the homotopy category is cartesian closed (i.e. possesses
internal Hom's relative to the tensor product). We use these two results in
order to prove a derived version of Morita theory, describing the morphisms
between dg-categories of modules over two dg-categories and as the
dg-category of -bi-modules. Finally, we give three applications of our
results. The first one expresses Hochschild cohomology as endomorphisms of the
identity functor, as well as higher homotopy groups of the \emph{classifying
space of dg-categories} (i.e. the nerve of the category of dg-categories and
quasi-equivalences between them). The second application is the existence of a
good theory of localization for dg-categories, defined in terms of a natural
universal property. Our last application states that the dg-category of
(continuous) morphisms between the dg-categories of quasi-coherent (resp.
perfect) complexes on two schemes (resp. smooth and proper schemes) is
quasi-equivalent to the dg-category of quasi-coherent complexes (resp. perfect)
on their product.Comment: 50 pages. Few mistakes corrected, and some references added. Thm.
8.15 is new. Minor corrections. Final version, to appear in Inventione
Lack of involvement of lipocortin 1 in dexamethasone suppression of IL-1 release
The annexin lipocortin 1 is reported to mediate some anti-inflammatory effects of glucocorticoids, but the mechanisms of this mediation are incompletely understood. The involvement of lipocortin 1 in glucocorticoid inhibition of monocyte interleukin 1β (IL-1β) release has been investigated. Treatment of peripheral blood monocytes with 2 μg/ml lipopolysaccharide potently increased IL–1β release (p = 0.001) and dexamethasone (10−7 M) significantly reduced both resting and stimulated IL-1β release (p = 0.009). A neutralizing monoclonal antibody to lipocortin 1 (0.5–50.0 μg/ml) was unable to inhibit this effect and recombinant lipocortin 1 (2 × 10−6 M) and 188aa lipocortin 1 fragment (10−8−10−6 M) had no effect. It is concluded that lipocortin 1 is not involved in the inhibition of monocyte IL-1β release by glucocorticoids
Atomic Scale Depth Profile of Space Weathering in an Itokawa Olivine Grain
No abstract available
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