I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[6174]
Abstract: Since deposited dredged sediments are rich in metallic contaminants, they present a risk for environment. This work aims to study dredged sediments chemical composition, identify metal-carrier minerals and understand their mobility. Combining chemical and spectroscopic techniques at multi-scale for an integrative approach of trace elements (zinc, lead, iron) behaviour is therefore necessary. The global mineralogy and the chemistry of the sediment were determined by X-ray diffraction and fluorescence (XRF), respectively. Zn and Pb enriched fractions were separated using a sequential chemical extraction procedure and measured by inductively coupled plasma atomic emission and mass spectroscopy. Microanalyses using scanning electron microscopy (SEM), electron microprobe microanalysis (EPMA), combined with synchrotron radiation X-ray fluorescence (?-XRF) were carried out to characterize mineralogical phases and identify Zn and Pb carrier minerals. Iron oxyhydroxides and iron sulphides were consistently identify as Zn and Pb carriers. The assumption that carbonate fraction was the major Zn carried phase, as demonstrated by chemical extraction results, was not verified by EPMA or ?-XRF.
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Mar 2012
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[876]
Open Access
Abstract: In this thesis two analyses are performed to understand the intrinsic ability of
bulk
metallic glasses (
BMGs
)
to deform plastically, based on their elastic properties. In the first of
these, a Blackman diagram is utilised, which plots the ratios of second order elastic constants
C
12
/
C
11
and
C
44
/
C
11
. Two physically meaningful conditions are
represented
on such a p
lot
:
1)
proximity to the Born mechanical instability criterion
,
and
2)
deviation from the zero Cauchy
relationship (
C
12
=
C
44
), which indicates whether central or non
-
central forces govern material
behaviour.
Those alloys with greatest tendency for plastic s
train are found closest to the Born
condition, which is also found to correspond to BMGs with a large kinetic glass fragility
index,
m
. Additionally, that plastic alloys exhibit the largest positive deviation from the
Cauchy condition suggests that non
-
dir
ectional bonding is a feature of deformable BMGs.
In the second anaylsis, three isomechanical groups are found to exist when a
representative group of 33 BMGs was studied, by plotting the Young’s (
E
) and shear (
G
)
moduli
versus
k
b
T
g
/
Ω
(Botlzmann constant,
glass transition temperature and atomic volume
respectively). An analysis of covariance reveals that the
variation
in
the
gradient between
each group on such plot
s
are statistically meaningful, indicating
fundamental
differences in
both
bonding
and
structure
.
These are rationalis
ed as resulting from the extent of non
-
directional bonding and the ability for enhanced structure diversity in the potential energy
landscapes (PELs) of fragile glass formers, in com
parison to strong glass formers.
This
abili
ty for structure variation in the liquid state
can be translated to the solid state via the non
-
ergodicity parameter,
α
.
This ability for an enhancement in the variety of local minima in the PEL of fragile
glass formers is further studied by comparing the
ability of high, low and intermediate
m
BMGs to retain a low modulus and low surface
hardness
during suction casting in copper
dies, observed
by performing
nanoindentation traverses a
cross
the cross
-
section of
as
-
cast
rods. The
most
fragile
alloy (
Pd
77.5
S
i
16.5
Cu
6
)
is found to show the greatest propensity
to
retain
a soft surface, while the effec
t is most limited in the
strong composition (
Ce
70
Al
10
Cu
10
Ni
10
)
.
Finally,
the
extended x
-
ray absorption fine structure (EXAFS)
method
is used to understand
the sourc
e of the mech
a
nical homogeneity
observed
across the as
-
cast rods, which is
found
to
potentially
not
be controlled by
nearest neighbour
s. Instead, cluster rotation may control the
variable mechanical properties (including plastic yield), and so length scale
s beyond the first
atomic shell appear to be critical.
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Mar 2012
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1511, 1097, 657]
Abstract: Tl stable isotopes recorded in marine ferromanganese crusts show great promise as a tracer of past marine and climatic conditions. Key to interpreting recent Tl stable isotope time-series data is a detailed, molecular-level understanding of Tl scavenging by ferromanganese crust minerals and Tl stable isotope fractionation occurring during uptake. To this end, we determine the mechanism of Tl sorption to the primary ferromanganese minerals in crusts, namely hexagonal birnessite, todorokite and ferrihydrite, using XAS. We compliment our data with micro-focus XAS of a Tl-enriched hydrogenetic ferromanganese crust. We show that Tl(I) is oxidised to Tl(III) during sorption to hexagonal birnessite, but not during sorption to todorokite, triclinic birnessite and ferrihydrite. Tl(III) forms an inner-sphere complex at the hexagonal birnessite surface, located at vacant octahedral sites in the phyllomanganate sheets. We show that oxidation of Tl(I) to Tl(III) during reductive dissolution of birnessite is thermodynamically unfavourable; and propose that oxidation of Tl(I) is driven by the formation of the Tl(III) surface complex. Recent theoretical calculations predict a large equilibrium stable isotope fractionation between Tl(I) and Tl(III), leading to Tl(III) species that are enriched in the heavy 205Tl isotope. In light of this work, we propose a molecular sorption–oxidation–fractionation mechanism that provides a unifying explanation for the recently observed geochemical behaviour of Tl in marine ferromanganese-rich sediments. In this mechanism, the proportion of hexagonal birnessite dictates the extent of Tl oxidation, which controls the extent of Tl enrichment and isotope fractionation. This work is among the first to provide a molecular explanation for reported trends in trace element enrichments and stable isotope compositions in geologic deposits. Our molecular sorption–oxidation–fractionation mechanism will ultimately help interpret Tl signals in marine sedimentary archives to provide new constraints on past oceanic and climatic change. In addition, our mechanism should also help explain compositional relationships of other redox-sensitive elements in ferromanganese-rich marine sediments that might also be used as paleoceanographic and paleoclimate proxies.
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Feb 2012
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I18-Microfocus Spectroscopy
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Feb 2012
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I18-Microfocus Spectroscopy
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Feb 2012
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I18-Microfocus Spectroscopy
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Abstract: Recently glasses based on gallate and aluminate networks have aroused interest in laser technology, for example, for the use as the host for .Iaser active ions. As these glasses are not good glass-formers other ingredients such as silica are normally added to improve their glass abilities. In this work I have succeeded in producing these glasses without the need for these additions. The structures of these pure rare-earth gallate and aluminate glasses made by aerodynamic levita- tion and laser heating techniques have been studied including neutron and X-ray diffraction and spectroscopy. The following results have been obtained. The structures of rare-earth gallate glasses, R2Ga6012 and R3Ga5012 where R = Pr and Nd, were studied using neutron diffrac- tion with the isomorphic substitution technique. A good agreement between the structural models from MD simulation and MD-RMC for the difference functions and the full experimental data sets was achieved. The mean Ga-O coordination number was found to be 4.1(1). The results also show a mixture of 6, 7 and 8-fold coordinated sites for the rare-earth ions with an average coordination number of 7.7(1). A more detailed study using a combination of neutron diffraction, Extended X-ray Absorption Fine Structure (EXAFS), MD simulation and MD-RMC refinement was applied to obtain the detail of the local structure of Pr3Ga5012 glasses at Pr and Ga K-edges. The nucleation and phase separation in the (Y203)x(Al203)1-x glassy systems produced by an aerodynamic levitation and laser heating was studied using micro-focus EXAFS. Turbidity was found to occur in the x < 0.25 and x > 0.35 glasses. At x < 0.25, even though no large inclusions (> 1?m) were seen in the turbid glasses, a phase separation into a polycrystalline sample of YAP (x = 0.50) and pure alumina was found. It is concluded that the turbidity in glassy samples at x ? 0.25 is due to the formation of nano meter size crystallites. For x = 0.36 and 0.375 (known as YAG), a nucleation of YAG crystals as spherical inclusions was found in a glass matrix giving rise to the turbidity in these glasses. Finally, a study of the structures of BaTiAl206 glasses was made in order to understand the processes giving rise to their unusual properties. Black and opaque, and clear and transparent BaTiAb06 glasses produced by aerodynamic levitation and laser heating by fast and slow quench rates were studied. Neutron and X-ray diffraction, MD simulation, MD-RMC refinement and X-ray absorption spectroscopy have been used and combined to determine the structure of the glass especially with regard to the coordination structure around the Ti ions. Evidence is found to show that the Ti ions occur in four fold and higher fold oxygen coordinated sites while the Al ions remain to- tally four fold coordinated. Very small differences in the structure of the two glasses are observed confirming that the opacity arises largely due to a small number of optical defects in present in the same overall glass structure.
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Feb 2012
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I18-Microfocus Spectroscopy
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Abstract: Quantitative determination of element species is crucial for understanding geological processes, particularly changing redox conditions. As Ti3+ is stable under very reducing conditions, Ti3+/Ti4+ oxybarometers are suitable for investigating early Solar System processes. The systematics of Ti3+/Ti4+ in Ti-bearing CaO-MgO-Al2O3-SiO2 (Ti-CMAS) glasses and both synthetic and meteoritic hibonite (Ca(Al,Ti,Mg)12O19) were investigated using X-ray absorption spectromicroscopy techniques. For this purpose suites of synthetic samples with 0-100% Ti3+/?Ti (where ?Ti = Ti3+ + Ti4+) were prepared in a 1 atm gas mixing furnace. The samples were characterised by electron microscopy, and when relevant, by X-ray diffraction and neutron powder diffraction (NPD). The crystal structure of hibonite has five distinct Al sites onto which Ti may substitute. Structural refinements of NPD data using the Rietveld refinement method were used to determine Ti3+ and Ti4+ site occupancy in hibonite. Ti3+ was found to occupy the M4 octahedral site, whereas Ti4+ partitions between the M2 trigonal bipyramidal site and the M4 octahedral site. For hibonite with <1 Ti per formula unit Ti4+ partitions between the M2 and M4 site in a roughly constant ratio of 1:4.3, irrespective of the Ti3+ content. Ti K-edge X-ray absorption near edge structure (XANES) spectra were collected from Ti-CMAS glasses and both synthetic and meteoritic hibonite. Ti3+/Ti4+ oxybarometers were developed using the synthetic samples, and the Ti3+/Ti4+ K-edge oxybarometer was used to quantify Ti3+/?Ti within hibonite from six different meteorites. Ti3+ hibonite was found in CAIs from the Acfer094 (unique), El Djouf001 (CR2), Murchison (CM2) and NWA1476 (R3) meteorites, whereas hibonite within CAIs from the NWA2446 (R3) and Hughes030 (R3-6) meteorites were Ti3+ poor. Furthermore, the Ti3+/?Ti varied within hibonite grains in CAIs from Acfer094 and El Djouf001. The presence of Ti3+ within hibonite in CAIs with no alteration minerals, or those that only experienced low temperature alteration, suggests they formed and/or were reprocessed under very reducing conditions (
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Jan 2012
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I18-Microfocus Spectroscopy
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Abstract: The poorly controlled disposal of chromium ore processing residue (COPR) is a globally widespread problem due to its potential to form chromium contaminated hyperalkaline (pH > 12) leachates. These highly oxidising leachates typically contain chromium in the Cr(VI) oxidation state as its chromate anion (CrO42-). This anion is highly mobile, toxic, carcinogenic, and exhibits a high degree of bioavailability. Under reducing conditions chromium exists in the non-toxic and poorly soluble Cr(III) oxidation state. Thus, the reduction of Cr(VI) to Cr(III) is often the goal of remediative strategies. In anaerobic subsurface environments where reducing conditions are established by the indigenous microbial population, chromium reduction can occur naturally. The microbial transformation of Cr(VI) to Cr(III) can be both a result of its direct use in microbial metabolism, or through its indirect reaction with microbially produced reduced species, e.g. Fe(II). This study has used a multidisciplinary approach to investigate the biogeochemical influences on the fate and stability of Cr(VI) leaching from a site of COPR in the north of England. Reducing sediments encountered directly beneath the COPR waste were found contain elevated concentrations of chromium. These sediments were shown to be able to remove aqueous Cr(VI) from solution when incubated with contaminated site groundwater in microcosm incubation experiments. This removal is likely a result of the abiotic reduction by soil associated microbially produced Fe(II), followed by precipitation as insoluble Cr(III) hydroxides. X-ray absorption spectroscopy (XAS) and electron microscopy confirms the association of chromium as Cr(III) with iron in these soils, hosted as a mixed Cr(III)-Fe(III) oxyhydroxide phase. Upon air oxidation, only minor amounts of chromium was remobilised from these sediments as Cr(VI). A diverse population of alkaliphilic microorganisms are indigenous to this horizon, capable of successful metabolism despite elevated pH values. This population was found to contain a consortium of microorganisms capable of iron reduction when incubated at pH 9 to 9.5. Microbial community analysis found taxonomic similarity to several known metal reducing alkaliphiles from the phylum Firmicutes. These results suggest that the novel action of iron reducing alkaliphiles indigenous to reducing sediments beneath COPR sites may provide zones of natural chromium attenuation via microbially mediated mechanisms of Cr(VI) transformation.
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Jan 2012
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1205]
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Nov 2011
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[1125]
Abstract: There is a well-established literature indicating a relationship between iron in brain tissue and Alzheimer's disease (AD). More recently, it has become clear that AD is associated with neuroinflammatory and oxidative changes which probably result from microglial activation. In this study, we investigated the correlative changes in microglial activation, oxidative stress, and iron dysregulation in a mouse model of AD which exhibits early-stage amyloid deposition. Microfocus X-ray absorption spectroscopy analysis of intact brain tissue sections prepared from A?PP/PS1 transgenic mice revealed the presence of magnetite, a mixed-valence iron oxide, and local elevations in iron levels in tissue associated with amyloid-?-containing plaques. The evidence indicates that the expression of markers of microglial activation, CD11b and CD68, and astrocytic activation, GFAP, were increased, and were histochemically determined to be adjacent to amyloid-?-containing plaques. These findings support the contention that, in addition to glial activation and oxidative stress, iron dysregulation is an early event in AD pathology.
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Oct 2011
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