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Magnetic ordering in TmGa
No full textWe have determined the magnetic structure of the intermetallic compound TmGa by high-resolution neutron powder diffraction and 169Tm Mössbauer spectroscopy. This compound crystallizes in the orthorhombic (Cmcm) CrB-type structure and its magnetic structure is characterized by magnetic order of the Tm sublattice along the a-axis. The initial magnetic ordering occurs at 15(1) K and yields an incommensurate antiferromagnetic structure described by the propagation vector k1 = [0 0.275(2) 0]. At 12 K the dominant ferromagnetic ordering of the Tm sublattice along the a-axis develops in what appears to be a first-order transition. At 3 K the magnetic structure of TmGa is predominantly ferromagnetic but a weakened incommensurate component remains. The ferromagnetic Tm moment reaches 6.7(2) μB at 3 K and the amplitude of the remaining incommensurate component is 2.7(4) μB. The 169Tm hyperfine magnetic field at 5 K is 631(1) T. © Copyright IOP Publishin
Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer, NSW, Australia
No full textGroundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H2O stable isotopes, δ13CDIC, 3H, 14C and 87Sr/86Sr). The study included whole-rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole-rock XRD analysis, two main groundwater zones were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing 3H and 14C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ∼50 m below ground surface in most areas and ∼90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO3-type waters, although with higher pH and SC, and no 3H and low 14C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times.
Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO3− concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase. © 2020 Informa UK Limite
Microscopic model for exchange bias from grain-boundary disorder in a ferromagnet/antiferromagnet thin film with a nanocrystalline microstructure
No full textMonte Carlo spin simulations were coupled to a Voronoi microstructure-generator to predict the magnitude and behavior of exchange bias in a magnitude/antiferromagnet (AF) thin film bilayer with a nanocrystalline microstructure. Our model accounts for the effects of irregular grain-shapes, finite-sized particles, and the possible presence of local random-fields originating from the antiferromagnet's grain-boundary regions. As the grain-boundary represents a crystal-structure distortion, we model the local effect on the exchange constants in the Gaussian approximation which can cause regions resembling a spin glass confined to an unusual 2D topology. Although an ensemble of completely disconnected AF grains isolated by non-magnetic barriers provides a small exchange bias, the introduction of a spin-glass network at the boundaries causes a four-fold enhancement in the magnitude of the loop-shift. This implies the importance of local grain-boundary behavior in defect-engineered antiferromagnets. © AIP Publishing LLC
The role of microstructural characteristics in the cavitation erosion behaviour of laser melted and laser processed Nickel–Aluminium Bronze
No full textIn this study two types of laser surface treatment, laser surface melting and laser processing, were used to treat the surface of as cast Nickel–Aluminium Bronze. The two treatments were then subjected to cavitation erosion testing and were compared against as-cast Nickel–Aluminium Bronze. While the cavitation performance of the two types of laser surface treatment was equivalent, the morphology of the eroded surfaces was different. Several materials characterisation techniques including neutron diffraction for residual stress measurements and SEM were used to explain why the two eroded surfaces were different. It was found that the tensile residual stresses in the laser melted sample weakened the sample, which negated its superior strength when compared with the laser processed sample. It was also observed that the erosion and pitting in the laser melted sample were deeper and they were attributed to the tensile residual stresses accelerating the attack at grain boundaries. © 2014, Elsevier B.V
Hydrostatic Compression Behavior and High-Pressure Stabilized β-Phase in γ-Based Titanium Aluminide Intermetallics
Get PDFTitanium aluminides find application in modern light-weight, high-temperature turbines, such as aircraft engines, but suffer from poor plasticity during manufacturing and processing. Huge forging presses enable materials processing in the 10-GPa range, and hence, it is necessary to investigate the phase diagrams of candidate materials under these extreme conditions. Here, we report on an in situ synchrotron X-ray diffraction study in a large-volume press of a modern (α2 + γ) two-phase material, Ti-45Al-7.5Nb-0.25C, under pressures up to 9.6 GPa and temperatures up to 1686 K. At room temperature, the volume response to pressure is accommodated by the transformation γ → α2, rather than volumetric strain, expressed by the apparently high bulk moduli of both constituent phases. Crystallographic aspects, specifically lattice strain and atomic order, are discussed in detail. It is interesting to note that this transformation takes place despite an increase in atomic volume, which is due to the high ordering energy of γ. Upon heating under high pressure, both the eutectoid and γ-solvus transition temperatures are elevated, and a third, cubic β-phase is stabilized above 1350 K. Earlier research has shown that this β-phase is very ductile during plastic deformation, essential in near-conventional forging processes. Here, we were able to identify an ideal processing window for near-conventional forging, while the presence of the detrimental β-phase is not present under operating conditions. Novel processing routes can be defined from these findings. © 2016, Creative Commons
Plant use from c. 9000-7800 cal BC at Pınarbaşı A and D, Karaman Province, Central Anatolia
No full textPlant macrofossil analysis, phytolith analysis and AMS radiocarbon dating at Pınarbaşı in central Anatolia confirm the presence and continuity of plant gathering practice as a key subsistence strategy from c. 9000–7700 cal BC. Results demonstrate the use of almond, terebinth and hackberry as food plants, similar to Palaeolithic/Epipalaeolithic subsistence strategies in the Antalya region. Crop and/or crop progenitor use is unsupported, with sporadic cereal macrofossils rare and shown by direct radiocarbon dating to be intrusive, a conclusion supported by the phytolith analysis. Seed exploitation is also rejected. Results confirm the presence of sedentary foragers from 9000 cal BC in central Anatolia, contemporary with the Levantine PPNA-Early PPNB, suggest a different plant subsistence focus to contemporary forager societies in the Fertile Crescent and indicate economic differences with contemporary sites in central Anatolia which were already cultivating crops. © 2013, Elsevier Ltd
Semi-arid zone caves: Evaporation and hydrological controls on δ18O drip water composition and implications for speleothem paleoclimate reconstructions
No full textOxygen isotope ratios in speleothems may be affected by external processes that are independent of climate, such as karst hydrology and kinetic fractionation. Consequently, there has been a shift towards characterising and understanding these processes through cave monitoring studies, particularly focussing on temperate zones where precipitation exceeds evapotranspiration. Here, we investigate oxygen isotope systematics at Wellington Caves in semi-arid, SE Australia, where evapotranspiration exceeds precipitation. We use a novel D2O isotopic tracer in a series of artificial irrigations, supplemented by pre-irrigation data comprised four years of drip monitoring and three years of stable isotope analysis of both drip waters and rainfall. This study reveals that: (1) evaporative processes in the unsaturated zone dominate the isotopic composition of drip waters; (2) significant soil zone ‘wetting up’ is required to overcome soil moisture deficits in order to achieve infiltration, which is highly dependent on antecedent hydro-climatic conditions; (3) lateral flow, preferential flow and sorption in the soil zone are important in redistributing subsurface zone water; (4) isotopic breakthrough curves suggest clear evidence of piston-flow at some drip sites where an older front of water discharged prior to artificial irrigation water; and (5) water residence times in a shallow vadose zone (<2 m) are highly variable and can exceed six months. Oxygen isotope speleothem records from semi-arid regions are therefore more likely to contain archives of alternating paleo-aridity and paleo-recharge, rather than paleo-rainfall e.g. the amount effect or mean annual. Speleothem-forming drip waters will be dominated by evaporative enrichment, up to ∼3‰ in the context of this study, relative to precipitation-weighted mean annual rainfall. The oxygen isotope variability of such coeval records may further be influenced by flow path and storage in the unsaturated zone that is not only drip specific but also influenced by internal cave climatic conditions, which may vary spatially in the cave.© 2015, Elsevier Ltd
Towards improvement of aluminium assay in quartz for in situ cosmogenic 26Al analysis at ANSTO
No full textAccuracy and precision in the measurement of natural aluminium abundances in quartz can affect the reliability of 26Al exposure dating and 26Al/10Be burial dating. At ANSTO, aliquots extracted from the HF solutions of dissolved quartz are treated in our laboratory, whereas ICP-OES analysis is performed at a commercial laboratory. The long-term inter-run reproducibility of our in-house standards show a limiting precision in Al measurements of 3–4% (1σ), which is lower than the claimed precision of Al analysis by ICP-OES. This indicates that unaccounted random errors are incorporated during our aliquot preparation. In this study, we performed several controlled tests to investigate effects of possible inconsistencies and variances during our aliquot preparation procedure. The results indicate that our procedure is robust against any subtle change in the preparation procedure, e.g., fuming temperatures, fuming reagents, and drying conditions. We found that the density of the solutions dispatched for ICP analysis is occasionally variable due to the presence of residual fuming reagents in the solution. A comparison of the results between the calibration curve and standard addition methods show that the former results are consistently lower than the latter by up to ∼14%. Similar offsets have been reported by previous studies. The reason for these discrepancies is mostly likely matrix effect, which is not accounted for by the calibration curve method. Further tests by varying matrix with impurities such as HF, HClO4, H2SO4 and Si identified that Si could cause lower offset in Al measurements; however, our ICP solutions are confirmed to be free from Si and the cause of matrix effect remains to be investigated. Hence, care must be taken for the measurement of Al concentrations in quartz by ICP-OES, either by ensuring that matrix effect is fully accounted for or by routinely employing standard additions when required.© 2015, Elsevier B.V
Rapid relative sea-level fall along north-eastern Australia between 1200 and 800 cal. yr BP: An appraisal of the oyster evidence
No full textA fast-paced post-glacial sea-level rise and subsequent mid-Holocene sea-level highstand are well documented at several far field locations away from the presence of former ice sheets but sea-level development during the late Holocene remains ambiguous. In this study, we present new data from modern and fossil oysters attached to shoreline rocks along the north-eastern Australian coastline that reveal new constraints on the nature and timing of relative sea-level change over the past 2500 yr. Surveyed elevations of various contemporary oyster zones contextualize modern oyster growth forms in relation to sea-level datum and build the reference for our fossil oyster data. Based on survey data and field observations we developed a robust set of criteria for measuring fossil oysters to determine their relative sea-level position and constrain the uncertainties associated with these reconstructions. Thick (> 10 cm) fossil oyster visors above the equivalent modern growth suggest higher relative sea-levels in the past (i.e. > 1200 cal. yr BP). Radiocarbon analyses of the modern oyster visors suggest continuous lateral accumulation over the past ~ 800 yr which implies relatively stable sea-level over this period. The modern and fossil dataset defines a distinct and rapid relative ~ 1 m sea-level fall between 1200 and 800 cal. yr BP. Whether the sea-level fall was stepped or followed a broader smooth/monotonic pattern is unclear. The timing coincides with the initiation of some inshore fringing coral reefs in the Great Barrier Reef region and other major geomorphological changes along the coastal zone. A combination of various factors may have been the driving mechanism behind this relative sea-level fall with rates between 1.0 to 5.2 mm yr− 1. © 2015, Elsevier B.V
40 years of atmospheric composition observations and research at Cape Grim – an overview
No full textIn 2016, the Cape Grim station located at the north-west tip of Tasmania, will celebrate 40 years of continuous operation. Commencing operations in 1976, and in its current facilities since 1981, the
Baseline Air Pollution Station at Cape Grim is Australia’s contribution to international efforts for monitoring the global background atmosphere for trends due to human activities and natural variability. It is one of the three premier stations in the Global Atmosphere Watch (GAW) network of the World Meteorological Organization (WMO). Cape Grim observations thereby contribute very
significantly to the GAW programme, and research outputs are published in peer-reviewed international journals of the highest quality, are very frequently cited, and feed into the international/global assessments, for example the IPCC.
Cape Grim’s essential purpose has not changed since the program’s inception in 1976. National and global concern that human activity is having an impact on the composition of the global (and regional)
atmosphere requires that the changes be observed and understood. Initially, this was the main focus, whereas now the data are also used for prediction, to guide development of remedial policies, and to verify the effectiveness of global mitigation actions. The critical, but too often unrecognised role of precise in-situ atmospheric observation programs underpinning our current understanding of atmospheric processes and ability to model future climate, is well summarised in a commentary in Nature, titled “Cinderella science” (Nature 450, p789, 2007).
The Cape Grim station is operated and funded by the Aust. Bureau of Meteorology, with the Cape Grim Science Program jointly supervised by CSIRO, the Bureau of Met., Univ. of Wollongong and ANSTO. This presentation will give an overview of the past and present measurement programs at the station, show some of the iconic long-term datasets, and summarise some of the science highlights
and impacts from the past 40 years