Increase In Domain Size Research Articles

The radiation response of mesoporous nanocrystalline zirconia thin films

The next generation of nuclear systems will require materials capable of withstanding hostile chemical, physical and radiation environments over long time-frames. Aside from its chemical and physical stability, crystalline zirconia is one of the most radiation tolerant materials known. Here we report the first ever study of the radiation response of nanocrystalline and mesoporous zirconia and Ce3+-stabilized nanocrystalline zirconia (Ce0.1Zr0.9O2) thin films supported on silicon wafers. Zirconia films prepared using the block copolymer Brij-58 as the template had a thickness of around 60–80 nm. In the absence of a stabilizing trivalent cation they consisted of monoclinic and tetragonal zirconia nanocrystals with diameters in the range 8–10 nm. Films stabilized with Ce3+ contained only the tetragonal phase. The thin films were irradiated with iodine ions of energies of 70 MeV and 132 keV at low fluences (1013 - 1014 cm−2) corresponding to doses of 0.002 and 1.73 dpa respectively, and at 180 keV and high fluences (2 × 1016 cm−2) corresponding to 82.4 dpa. The influence of heavy ion irradiation on the nanocrystalline structure was monitored through Rietveld analysis of grazing incidence X-ray diffraction (GIXRD) patterns recorded at angles close to the critical angle to ensure minimum contribution to the diffraction pattern from the substrate. Irradiation of the mesoporous nanocrystalline zirconia thin films with 70 MeV iodine ions, for which electronic energy loss is dominant, resulted in slight changes in phase composition and virtually no change in crystallographic parameters as determined by Rietveld analysis. Iodine ion bombardment in the nuclear energy loss regime (132–180 keV) at low fluences did not provoke significant changes in phase composition or crystallographic parameters. However, at 180 keV and high fluences the monoclinic phase was totally eliminated from the GIXRD pattern of films prepared at both 350 and 500 °C implying either a monoclinic-to-tetragonal or a monoclinic-to-amorphous transition. This irradiation at very high doses resulted in film shrinkage and a loss of mesopore ordering but little or no degradation of the crystallinity of the tetragonal phase. A small increase in the crystalline domain size of the tZrO2 phase was noted in these films. In contrast, single-phase Ce3+-stabilized tetragonal nanocrystalline zirconia mesoporous films prepared at 350 °C suffered considerable loss of crystalline order when irradiated at 82 dpa. This loss of crystallinity was less pronounced in films heated to 500 °C. The loss of crystallinity of the tetragonal phase in the Ce-stabilized tetragonal zirconia thin films w.r.t. the tetragonal phase in the unstabilized films was attributed to the oxygen vacancies introduced in the latter due to the need for charge compensation.

Radiative convective equilibrium as a framework for studying the interaction between convection and its large‐scale environment

AbstractAn uncertain representation of convective clouds has emerged as one of the key barriers to our understanding of climate sensitivity. The large gap in resolved spatial scales between General Circulation Models (GCMs) and high resolution models has made a systematic study of convective clouds across model configurations difficult. It is shown here that the simulated atmosphere of a GCM in Radiative Convective Equilibrium (RCE) is sufficiently similar across a range of domain sizes to justify the use of RCE to study both a GCM and a high resolution model on the same domain with the goal of improved constraints on the parameterized clouds. Simulations of RCE with parameterized convection have been analyzed on domains with areas spanning more than two orders of magnitude ( ), all having the same grid spacing of . The simulated climates on different domains are qualitatively similar in their degree of convective organization, the precipitation rates, and the vertical structure of the clouds and water vapor, with the similarity increasing as the domain size increases. Sea surface temperature perturbation experiments are used to estimate the climate feedback parameter for the differently configured experiments, and the cloud radiative effect is computed to examine the role which clouds play in the response. Despite the similar climate states between the domains the feedback parameter varies by more than a factor of two; the hydrological sensitivity parameter is better behaved, varying by a factor of 1.4. The sensitivity of the climate feedback parameter to domain size is related foremost to a nonsystematic response of low‐level clouds as well as an increasingly negative longwave feedback on larger domains.

Increased monolayer domain size and patterned growth of tungsten disulfide through controlling surface energy of substrates

We report a surface energy-controlled low-pressure chemical vapor deposition growth of WS2 monolayers on SiO2 using pre-growth oxygen plasma treatment of substrates, facilitating increased monolayer surface coverage and patterned growth without lithography. Oxygen plasma treatment of the substrate caused an increase in the average domain size of WS2 monolayers by 78% ± 2% while having a slight reduction in nucleation density, which translates to increased monolayer surface coverage. This substrate effect on growth was exploited to grow patterned WS2 monolayers by patterned plasma treatment on patterned substrates and by patterned source material with resolutions less than 10 µm. Contact angle-based surface energy measurements revealed a dramatic increase in polar surface energy. A growth model was proposed with lowered activation energies for growth and increased surface diffusion length consistent with the range of results observed. WS2 samples grown with and without oxygen plasma were similar high quality monolayers verified through transmission electron microscopy, selected area electron diffraction, atomic force microscopy, Raman, and photoluminescence measurements. This technique enables the production of large-grain size, patterned WS2 without a post-growth lithography process, thereby providing clean surfaces for device applications.

Microstructure Changes in Polyester Polyurethane upon Thermal and Humid Aging.

The microstructure of compression molded Estane 5703 films exposed to 11%, 45%, and 80% relative humidity and 70 °C for 1 and 2 months has been studied by small-angle neutron scattering (SANS), Fourier transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). Scattering data indicated increase of the interdomain distance and domain size with a higher humidity and longer aging time. GPC data showed a progressive shortening of polyurethane chains with increasing humidity and aging time. The shortening of the polyurethane chains caused a drop of the glass transition temperature of soft segments, and promoted crystallization of the soft segments during long-time storage of the aged samples at room temperature. FTIR showed a substantial increase in the number of inter-urethane H-bonds in the aged samples. This correlates with the increase of the hard domain size and the degree of phase separation as measured by SANS. The data collected reveals that the reduced steric hindrance caused by hydrolysis of ester links in polybutylene adipate residues promotes the organization of hard segments into domains, leading to the increase of domain size and distance, as well as phase segregation in aged Estane. These findings provide insight into the effects of humidity and thermal aging on the microstructure of aged polyester urethane from molecular to nanoscale level.

Scale dependency of dynamic relative permeability–satuartion curves in relation with fluid viscosity and dynamic capillary pressure effect

Capillary pressure-saturation-relative permeability relationships (Pc-Sw-Kr) are functions of importance in modeling and simulations of the hydrodynamics of two-phase flow in porous media. These relationships are found to be affected by porous medium and fluid properties but the manner in which they are affected is a topic of intense discussion. For example, reported trends in fluid viscosity and boundary conditions effects have been found to be contrary to each other in different studies. In this work, we determine the dependency of dynamic Kr-Sw relationships (averaged data) on domain scale in addition to investigating the effects of fluid viscosity and boundary pressure using silicone oil (i.e. 200 and 1000 cSt) and water as the respective non-wetting and wetting fluids with a view to eliminating some of the uncertainties reported in the literature. Water relative permeability, Krw, was found to increase with increasing wetting phase saturation but decreases with the increase in viscosity ratio. On the other hand, the oil relative permeability, Krnw, was found to increase with the increasing non-wetting phase saturation in addition to the increase in viscosity ratio. Also, it was found that with the increasing boundary pressure Krw decreases while Krnw increases. The influence of scale on relative permeability was slightly indicated in the non-wetting phase with Krnw decreasing as domain size increases. Effect of mea- surement location on dynamic relative permeability was explored which is rarely found in the literature. Comparison was also made between Kr-Sw relationships obtained under static and dynamic condition. Finally, mobility ratio (m) and dynamic coefficient (s) were plotted as a function of water saturation (Sw), which showed that m decreases as s increases at a given saturation, or vice versa.

Combining gradient structure and TRIP effect to produce austenite stainless steel with high strength and ductility

We report a design strategy to combine the benefits from both gradient structure and transformation-induced plasticity (TRIP). The resultant TRIP-gradient steel takes advantage of both mechanisms, allowing strain hardening to last to a larger plastic strain. 304 stainless steel sheets were treated by surface mechanical attrition to synthesize gradient structure with a central coarse-grained layer sandwiched between two grain-size gradient layers. The gradient layer is composed of submicron-sized parallelepiped austenite domains separated by intersecting ε-martensite plates, with increasing domain size along the depth. Significant microhardness heterogeneity exists not only macroscopically between the soft coarse-grained core and the hard gradient layers, but also microscopically between the austenite domain and ε-martensite walls. During tensile testing, the gradient structure causes strain partitioning, which evolves with applied strain, and lasts to large strains. The γ → α′ martensitic transformation is triggered successively with an increase of the applied strain and flow stress. Importantly, the gradient structure prolongs the TRIP effect to large plastic strains. As a result, the gradient structure in the 304 stainless steel provides a new route towards a good combination of high strength and ductility, via the co-operation of both the dynamic strain partitioning and TRIP effect.

Ultra-High-Resolution Differential Interference Microscopy of Ag Deposition on an Ultraflat Au(111)

The electrodeposition of silver on an atomically ultraflat surface of Au(111) was investigated using laser confocal microscopy combined with the differential interference contrast microscopy (LCM-DIM) in the sulfuric acid medium with silver sulfate as an electrolyte solution. For the first time, we have observed the underpotential deposition processes of silver atoms on the large Au(111) surface area in atomic height resolution of as-deposited Ag layers using LCM-DIM. In underpotential deposition (UPD) regions, small islands of Ag atoms with monatomic step heights are formed on the Au(111) terraces at the initial stage, followed by the increase in domain size of islands, and then finally a complete monolayer was formed on the Au(111) surface. Before initiating the bulk deposition of Ag, our LCM-DIM gave direct evidence for the formation of the third monolayer of Ag at the foot of the potential of the bulk deposition. In addition to UPD, the bulk deposition of Ag was also investigated which demonstrated th...

Physiochemical Properties of Aluminum Adjuvants Elicit Differing Reorganization of Phospholipid Domains in Model Membranes.

Most vaccines contain aluminum adjuvants; however, their exact mechanism of action remains unclear. A novel mechanism by Shi and colleagues proposes aluminum adjuvants may enhance immune activation by binding and reorganizing lipids that are key components of lipid rafts. To better understand the specificity of interaction between aluminum adjuvants and the cell membrane lipids, we present a biophysical study of lipid domain clustering in simple model phospholipid monolayers containing dipalmitoyl-phosphatidylcholine (DPPC) and dioleoyl-phosphatidylcholine (DOPC) exposed to two aluminum adjuvants, Alhydrogel and Adju-Phos. Surface pressure measurements and fluorescence microscopy images verified aluminum adjuvant-induced increase in lipid domain size, even in the key lipid raft components. Additionally, adjuvant induced lipid clustering differed based on the physicochemical properties of the adjuvants. Alhydrogel appeared to reduce monolayer compressibility and insert into the monolayer, while Adju-Phos induced more significant changes in domain size, without compromising the integrity of the monolayer. The Alhydrogel and Adju-Phos-mediated reorganization of phospholipid domains reported here supports the new mechanistic paradigm proposed by Shi and co-workers, and further suggests that lipid clustering is induced even in simple phospholipid membranes. The results present the basis for future exploration into lipid-mediated mechanisms of action for adjuvants.

Studies of the structure and phase transitions of nano-confined pentanedithiol and its application in directing hierarchical molecular assemblies on Au(1 1 1)

Directing molecular devices into pre-designed integrated electronic circuits while enforcing selectivity and hierarchy is an inherent challenge for molecular electronics. Here we explore ways to direct the assembly of electrically-active molecular monolayers into specific locations as well as controlling their internal organization. We have accomplished this by two consecutive surface reactions: (1) forming pentanedithiol (C5DT) domains within an inert alkanethiol self-assembled monolayer (SAM) on Au; and (2) selectively binding porphyrin derivatives to the C5DT domains. The C5DT domains were fabricated by phase segregation during co-adsorption from a mixed C5DT/dodecanethiol (C12) solution and nanografting with Atomic Force Microscopy (AFM). AFM revealed that co-absorbed and nanografted C5DT domains were in a standing-up phase and scanning tunneling microscopy (STM) showed that their molecular organization within about 5 nm, 40 nm, 50 nm and 120 nm domains, was dependent upon the size of the domain, such that structure of the C5DT transitions from ( × ) R30°, to (2 × 2), and ultimately to a disordered phase with increasing domain size. This is due to the varying degrees of influence of the surrounding C12; providing sufficient van der Waals interactions as well as a geometric confinement to stabilize the standing-up phase of the C5DT. Understanding the molecular configuration of dithiol SAMs affords their use as a reactive template to subsequently bind active head groups. As a proof of principle, porphyrins with a pendant pentafluorophenyl ring were attached to the C5DT domains by a ‘click’ reaction between the fluorinated ring and the free thiol on the surface. From AFM and STM, these porphyrin derivatives reacted selectively with the C5DT domains with some porphyrins binding directly to the C5DT, subsequently allowing additional localized porphyrin deposition through pi-stacking.

Effect of polymer microstructure on the docetaxel release and stability of polyurethane formulation

PurSil®AL20 (PUS), a copolymer of 4,4′-dicyclohexylmethane diisocyanate (HMDI), 1,4-butane diol (BD), poly-tetramethylene oxide (PTMO) and poly-dimethyl siloxane (PDMS) was investigated for stability as a vehicle for Docetaxel (DTX) delivery through oesophageal drug eluting stent (DES). On exposure to stability test conditions, it was found that DTX release rate declined at 4 and 40°C. In order to divulge reasons underlying this, changes in DTX solid state as well as PUS microstructure were followed. It was found that re-crystallization of DTX in PDMS rich regions was reducing the drug release at both 4°C and 40°C samples. So far microstructural features have not been correlated with stability and drug release, and in this study we found that at 40°C increase in microstructural domain sizes and the inter-domain distances (from ∼85Å to 129Å) were responsible for hindering the DTX release in addition to DTX re-crystallization.

Oxyanion induced variations in domain structure for amorphous cobalt oxide oxygen evolving catalysts, resolved by X-ray pair distribution function analysis.

Amorphous thin film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling photoanode materials in solar-driven, photoelectrochemical `artificial leaf' devices. This report demonstrates the ability to use high-energy X-ray scattering and atomic pair distribution function analysis, PDF, to resolve structure in amorphous metal oxide catalyst films. The analysis is applied here to resolve domain structure differences induced by oxyanion substitution during the electrochemical assembly of amorphous cobalt oxide catalyst films, Co-OEC. PDF patterns for Co-OEC films formed using phosphate, Pi, methylphosphate, MPi, and borate, Bi, electrolyte buffers show that the resulting domains vary in size following the sequence Pi < MPi < Bi. The increases in domain size for CoMPi and CoBi were found to be correlated with increases in the contributions from bilayer and trilayer stacked domains having structures intermediate between those of the LiCoOO and CoO(OH) mineral forms. The lattice structures and offset stacking of adjacent layers in the partially stacked CoMPi and CoBi domains were best matched to those in the LiCoOO layered structure. The results demonstrate the ability of PDF analysis to elucidate features of domain size, structure, defect content and mesoscale organization for amorphous metal oxide catalysts that are not readily accessed by other X-ray techniques. PDF structure analysis is shown to provide a way to characterize domain structures in different forms of amorphous oxide catalysts, and hence provide an opportunity to investigate correlations between domain structure and catalytic activity.

Changes in myonuclear domain size do not precede muscle hypertrophy during prolonged resistance-type exercise training.

Muscle fibre hypertrophy is accompanied by an increase in myonuclear number, an increase in myonuclear domain size or both. It has been suggested that increases in myonuclear domain size precede myonuclear accretion and subsequent muscle fibre hypertrophy during prolonged exercise training. In this study, we assessed the changes in muscle fibre size, myonuclear and satellite cell content throughout 12weeks of resistance-type exercise training in young men. Twenty-two young men (23±1year) were assigned to a progressive, 12-weeks resistance-type exercise training programme (3sessions per week). Muscle biopsies from the vastus lateralis muscle were taken before and after 2, 4, 8 and 12weeks of exercise training. Muscle fibre size, myonuclear content, myonuclear domain size and satellite cell content were assessed by immunohistochemistry. Type I and type II muscle fibre size increased gradually throughout the 12weeks of training (type I: 18±5%, type II: 41±6%, P<0.01). Myonuclear content increased significantly over time in both the type I (P<0.01) and type II (P<0.001) muscle fibres. No changes in type I and type II myonuclear domain size were observed at any time point throughout the intervention. Satellite cell content increased significantly over time in both type I and type II muscle fibres (P<0.001). Increases in myonuclear domain size do not appear to drive myonuclear accretion and muscle fibre hypertrophy during prolonged resistance-type exercise training invivo in humans.

Size effect and boundary type on the strengthening of nanoscale domains in pure nickel

A series of large-scale molecular dynamics simulations were carried out to investigate the interactions between an extended edge dislocation (1/2<112>{111}) and nanoscale domains in pure nickel. The pinning strength of nano-domains and the corresponding atomistic interaction mechanisms were found to be closely related to the domain boundary type, the domain size and spacing. The pinning strengths were found to be higher for high-angle domains than those for low-angle domains at the same size scale, and increase with increasing domain size and decreasing domain spacing. Unlike the by-pass via interactions between the dislocation and boundaries for high-angle domains (much like the role of hard precipitates in alloys), the dislocation was found to cut partly through the low-angle domains. Thus the dragging force from the boundary segments of the low-angle domains should be smaller when compared to the Orowan's strengthening for “hard particles”, such as high-angle domains. The predictions from Ashby's model on Orowan's strengthening are higher than the simulation data for low-angle domains, while agree relatively well with those for high-angle domains. Moreover, a more universal model was proposed to connect the dislocation line shape at the critical shear strain with the pinning strength.

Complementary use of monochromatic and white-beam X-ray micro-diffraction for the investigation of ancient materials

Archaeological artefacts are often heterogeneous materials where several phases coexist in a wide grain size distribution. Most of the time, retrieving structure information at the micrometre scale is of great importance for these materials. Particularly, the organization of different phases at the micrometre scale is closely related to optical or mechanical properties, manufacturing processes, functionalities in ancient times and long-term conservation. Between classic X-ray powder diffraction with a millimetre beam and transmission electron microscopy, a gap exists and structure and phase information at the micrometre scale are missing. Using a micrometre-size synchrotron X-ray beam, a hybrid approach combining both monochromatic powder micro-diffraction and Laue single-crystal micro-diffraction was deployed to obtain information from nanometre- and micrometre-size phases, respectively. Therefore providing a way to bridge the aforementioned gap, this unique methodology was applied to three different types of ancient materials that all show a strong heterogeneity. In Romanterra sigillata, the specific distribution of nanocrystalline hematite is mainly responsible for the deep-red tone of the slip, while the distribution of micrometre-size quartz in ceramic bodies reflects the change of manufacturing process between pre-sigillataand high-qualitysigillataperiods. In the second example, we investigated the modifications occurring in Neolithic and geological flints after a heating process. By separating the diffracted signal coming from the nano- and the micrometre scale, we observed a domain size increase for nanocrystalline quartz in geological flints and a relaxation of the residual strain in larger detritic quartz. Finally, through the study of a Roman iron nail, we showed that the carburation process to strengthen the steel was mainly a surface process that formed 10–20 µm size domains of single-crystal ferrite and nanocrystalline cementite.

Effect of substrate rotation on domain structure and magnetic relaxation in magnetic antidot lattice arrays

Microdimensional triangular magnetic antidot lattice arrays were prepared by varying the speed of substrate rotation. The pre-deposition patterning has been performed using photolithography technique followed by a post-deposition lift-off. Surface morphology taken by atomic force microscopy depicted that the growth mechanism of the grains changes from chain like formation to island structures due to the substrate rotation. Study of magnetization reversal via magneto optic Kerr effect based microscopy revealed reduction of uniaxial anisotropy and increase in domain size with substrate rotation. The relaxation measured under constant magnetic field becomes faster with rotation of the substrate during deposition. The nature of relaxation for the non-rotating sample can be described by a double exponential decay. However, the relaxation for the sample with substrate rotation is well described either by a double exponential or a Fatuzzo-Labrune like single exponential decay, which increases in applied field.

Surrogate modeling of deformable joint contact using artificial neural networks

Deformable joint contact models can be used to estimate loading conditions for cartilage–cartilage, implant–implant, human–orthotic, and foot–ground interactions. However, contact evaluations are often so expensive computationally that they can be prohibitive for simulations or optimizations requiring thousands or even millions of contact evaluations. To overcome this limitation, we developed a novel surrogate contact modeling method based on artificial neural networks (ANNs). The method uses special sampling techniques to gather input–output data points from an original (slow) contact model in multiple domains of input space, where each domain represents a different physical situation likely to be encountered. For each contact force and torque output by the original contact model, a multi-layer feed-forward ANN is defined, trained, and incorporated into a surrogate contact model. As an evaluation problem, we created an ANN-based surrogate contact model of an artificial tibiofemoral joint using over 75,000 evaluations of a fine-grid elastic foundation (EF) contact model. The surrogate contact model computed contact forces and torques about 1000 times faster than a less accurate coarse grid EF contact model. Furthermore, the surrogate contact model was seven times more accurate than the coarse grid EF contact model within the input domain of a walking motion. For larger input domains, the surrogate contact model showed the expected trend of increasing error with increasing domain size. In addition, the surrogate contact model was able to identify out-of-contact situations with high accuracy. Computational contact models created using our proposed ANN approach may remove an important computational bottleneck from musculoskeletal simulations or optimizations incorporating deformable joint contact models.

Strongly Anisotropic Thermal Conductivity of Free‐Standing Reduced Graphene Oxide Films Annealed at High Temperature

Thermal conductivity of free‐standing reduced graphene oxide films subjected to a high‐temperature treatment of up to 1000 °C is investigated. It is found that the high‐temperature annealing dramatically increases the in‐plane thermal conductivity, K, of the films from ≈3 to ≈61 W m−1 K−1 at room temperature. The cross‐plane thermal conductivity, K⊥, reveals an interesting opposite trend of decreasing to a very small value of ≈0.09 W m−1 K−1 in the reduced graphene oxide films annealed at 1000 °C. The obtained films demonstrate an exceptionally strong anisotropy of the thermal conductivity, K/K⊥ ≈ 675, which is substantially larger even than in the high‐quality graphite. The electrical resistivity of the annealed films reduces to 1–19 Ω □−1. The observed modifications of the in‐plane and cross‐plane thermal conductivity components resulting in an unusual K/K⊥ anisotropy are explained theoretically. The theoretical analysis suggests that K can reach as high as ≈500 W m−1 K−1 with the increase in the sp2 domain size and further reduction of the oxygen content. The strongly anisotropic heat conduction properties of these films can be useful for applications in thermal management.

Excitation wave propagation in a patterned multidomain cardiac tissue

Electrospun fibrous mats are widely used in the contemporary cardiac tissue engineering as the substrates for growing cardiac cells. The substrate with chaotically oriented nanofibers leads to the growth of cardiac tissue with randomly oriented, but internally morphologically anisotropic clusters or domains. The domain structure affects the stability of the excitation propagation and we studied the stability of the propagating excitation waves versus the average size of the domains and the externally applied excitation rate. In an experimental model based on neonatal rat cardiac tissue monolayers, as well as in the computer simulations, we have found that an increase in domain sizes leads to the decrease in the critical stimulation frequencies, thus evidencing that larger domains are having a higher arrhythmogenic effect.

Linearly implicit schemes for multi-dimensional Kuramoto–Sivashinsky type equations arising in falling film flows

This study introduces, analyses and implements space-time discretizations of two-dimensional active dissipative partial differential equations such as the Topper–Kawahara equation; this is the two-dimensional extension of the dispersively modified Kuramoto–Sivashinsky equation found in falling film hydro-dynamics. The spatially periodic initial value problem is considered as the size of the periodic box increases. The schemes utilized are implicit–explicit multistep (BDF) in time and spectral in space. Numerical analysis of these schemes is carried out and error estimates, in both time and space, are derived. Preliminary numerical experiments provided strong evidence of analyticity, thus yielding a practical rule-of-thumb that determines the size of the truncation in Fourier space. The accuracy of the BDF schemes (of order 1–6) is confirmed through computations. Extensive computations into the strongly chaotic regime (as the domain size increases), provided an optimal estimate of the size of the absorbing ball as a function of the size of the domain; this estimate is found to be proportional to the area of the periodic box. Numerical experiments were also carried out in the presence of dispersion. It is observed that sufficient amounts of dispersion reduce the complexity of the chaotic dynamics, and can organize solution into nonlinear travelling wave pulses of permanent form.

Goethite aging explains Ni depletion in upper units of ultramafic lateritic ores from New Caledonia

An upward loss of Ni is commonly reported in the oxide-rich unit of Ni-laterite deposits developed over ultramafic rocks in tropical regions, especially in freely drained and deeply weathered regoliths. Because goethite is the major mineral constituent of such oxide-rich units, this Ni loss has been linked to compositional changes in goethite. In the present study, we have investigated possible correlations between Ni contents in the bulk laterite, and the evolution of goethite in terms of composition and crystallinity, in two Ni-rich and one Ni-poor lateritic profiles from New Caledonia. Ni K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy indicates that goethite hosts the main fraction of Ni in the three profiles investigated. Asbolane/lithiophorite identified as accessory minerals by X-ray diffraction (XRD) have little effect on the vertical variations in bulk Ni content in spite of the fact that the Ni contents of these Mn-oxides can be significant at certain depths. The gradual decrease in Ni content from the bottom to the top of the three Ni lateritic profiles correlates with a decrease in the Ni content of goethite as determined by electron probe micro-analysis. In addition, XRD data show that these compositional trends are linked to an increase of the mean coherent domain size of goethite. These observations support the hypothesis that Ni is expelled from goethite as it ages through successive dissolution and recrystallization cycles during the lateritization process. Comparison of laterites having different degrees of weathering suggests that this aging process could also play a significant role in the regional variability of Ni content in Ni-laterite deposits.