Mesoscopic quantitative chemical analyses using STEM-EDX in current and next generation polycrystalline Ni-based superalloys.

Quantitative chemical analyses of Ni3Al based hardening precipitates (γ') in polycrystalline Ni based superalloys have been conducted using energy dispersive X-ray spectroscopy (EDX), coupled with a scanning transmission electron microscope (STEM). The aim of the current investigation is (1) to evaluate the accuracy of calibration (k factor determinations and absorption corrections using a combination of differential X-ray absorption (DXA) and convergent beam electron diffraction (CBED)) by comparing with thermodynamic calculations and (2) to demonstrate the importance of the EDX chemical analysis by taking advantage of its unique capabilities to analyse sub-micron scale chemistries within a mesoscopic field of view under STEM. Our experimental findings show good agreement with the mole fraction ratio of γ' to the disordered γ matrix predicted using the Lever rule on a thermodynamically stabilised unimodal superalloy, RR1000. The significance of analysing a statistically viable number of samples in thermodynamically metastable superalloys and the chemical fluctuations found in coarse γ', sized above 200 nm on a scale of a few hundred nanometres in the context of solving a complex morphological evolution of γ' particles is demonstrated.

Backscattered electron imaging and electron backscattered diffraction in the study of bacterial attachment to titanium alloy structure.

The application of secondary electron (SE) imaging, backscattered electron imaging (BSE) and electron backscattered diffraction (EBSD) was investigated in this work to study the bacterial adhesion and proliferation on a commercially pure titanium (cp Ti) and a Ti6Al4V alloy (Ti 64) with respect to substrate microstructure and chemical composition. Adherence of Gram-positive Staphylococcus epidermidis 11047 and Streptococcus sanguinis GW2, and Gram-negative Serratia sp. NCIMB 40259 and Escherichia coli 10418 was compared on cp Ti, Ti 64, pure aluminium (Al) and vanadium (V). The substrate microstructure and the bacterial distribution on these metals were characterised using SE, BSE and EBSD imaging. It was observed that titanium alloy-phase structure, grain boundaries and grain orientation did not influence bacterial adherence or proliferation at microscale. Adherence of all four strains was similar on cp Ti and Ti 64 surfaces whilst inhibited on pure Al. This work establishes a nondestructive and straight-forward statistical method to analyse the relationship between microbial distribution and metal alloy structure.

The prediction of viscous losses and pressure drop in models of the human airways.

This paper examines the viscous flow resistance in branching tubes as applied to simplified models of the lungs and compares the results of computational fluid dynamics simulations for a range of conditions with measurement data. The results are in good agreement with the available measurement data for both inspiration and expiration. A detailed sensitivity analysis of the dissipation and viscous resistance in a branch then examines the ratio of the viscous resistance to that for a fully developed Poiseuille flow, Z. As other researchers have noted, the calculated resistances give lower values than those from the standard correlation of Pedley et al. The results demonstrate that the resistance is sensitive to the velocity profile upstream of the bifurcations and explain from fluid dynamical considerations the apparent sensitivity of the resistance to the generation number of the branch. The paper also suggests a revised value for the calibration constant in the expression for Z. Finally, a limited set of results are presented for junction losses, and for expiration.

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry.

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)-palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H(2) as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd-Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).

Characterization of cerium oxide nanoparticles-part 2: nonsize measurements.

Part 1 (see companion paper) of the present study discussed the application of a multimethod approach in characterizing the size of cerium oxide nanoparticles (NPs). However, other properties less routinely investigated, such as shape and morphology, structure, chemical composition, and surface properties, are likely to play an important role in determining the behavior, reactivity, and potential toxicity of these NPs. The present study describes the measurement of the aforementioned physicochemical properties of NPs (applied also to nanomaterials [NMs]) compared with micrometer particles (MPs). The authors use a wide range of techniques, including high resolution-transmission electron microscopy, scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray energy dispersive spectroscopy, electron energy loss spectroscopy, X-ray photoelectron spectroscopy, and electrophoresis, and compare these techniques, their advantages, and their limitations, along with recommendations about how best to approach NM characterization, using an application to commercial cerium oxide NPs and MPs. Results show that both cerium oxide NPs and MPs are formed of single polyhedron or truncated polyhedron crystals. Cerium oxide NPs contain a mixture of Ce(3+) and Ce(4+) cations, whereas the MPs contain mainly Ce(4+) , which is potentially important in understanding the toxicity of cerium oxide NPs. The isoelectric point of cerium oxide NPs was approximately pH 8, which explains their propensity to aggregate in aqueous media (see companion paper).

An FIB-SEM slice-and-view study of three-dimensional beta phase distribution in Ti-6Al-4V.

In the scanning electron microscope, weak secondary electron contrast between the alpha and beta phases of a Ti-6Al-4V alloy can be improved via imaging conditions such as beam voltage and current. Dual beam focussed ion beam-scanning electron microscopes are thereby suited for characterising micron and sub-micron microstructural features of Ti-6Al-4V in three dimensions via serial-sectioning procedures.

In situ hydride formation in titanium during focused ion milling.

It is well known that titanium and its alloys are sensitive to electrolytes and thus hydrides are commonly observed in electropolished foils. In this study, focused ion beam (FIB) milling was used to prepare thin foils of titanium and its alloys for transmission electron microscopy. The results show the following: (i) titanium hydrides were observed in pure titanium, (ii) the preparation of a bulk sample in water or acid solution resulted in the formation of more hydrides and (iii) FIB milling aids the precipitation of hydrides, but there were never any hydrides in Ti64 and Ti5553.

Interaction between manufactured gold nanoparticles and naturally occurring organic macromolecules.

The increasing exploitation of nanomaterials into many consumer and other products is raising concerns as these nanomaterials are likely to be released into the environment. Due to our lack of knowledge about the environmental chemistry, transport and ecotoxicology of nanomaterials, it is of paramount importance to study how natural aquatic colloids can interact with manufactured gold nanoparticles as these interactions will determine their environmental fate and behaviour. In this context, our work aims to quantify the effect of naturally occurring riverine macromolecules--International Humic Substances Society (IHSS) Suwannee River Humic Acid Standard (SRHA)--on citrate- and acrylate-stabilized gold nanoparticles. The influence of SRHA on the stability of the gold colloids was studied as a function of pH by UV-visible absorption spectroscopy, dynamic light scattering (DLS) and transmission electron microscopy (TEM). At high ionic strengths (0.1 M), extensive and rapid aggregation occurred, while more subtle effects were observed at lower ionic strength values. Evidence was found that SRHA enhances particle stability at extreme pH values (ionic strength<0.01 M) by substituting and/or over-coating the original stabilizer on the gold nanoparticle surface, thus affecting surface charge and chemistry. These findings have important implications for the fate and behaviour of nanoparticles in the environment and their ecotoxicity.

Microstructure and composition of biosynthetically synthesised hydroxyapatite.

Biosynthetic hydroxyapatite (HA) manufactured utilising the bacterium Serratia sp. NCIMB40259 was characterised using X-ray diffraction (XRD), Fourier transform infra-red spectroscopy (FTIR), energy dispersive X-ray analysis (EDX) scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED). SEM/EDX showed that the non-sintered material consisted mainly of calcium-deficient HA (CDHA) with a Ca/P ratio of 1.61 +/- 0.06 and crystal size (from TEM) of 50 +/- 10 nm. ED analysis of non-sintered powder showed resolvable ring patterns ascribed to (0002), (1122) and (0006) planes of crystalline HA. The crystallinity of the samples improved with heat treatment from approximately 9.4% (non-sintered) to 53% (1,200 degrees C). Samples heated at 600 degrees C and sintered at 1,200 degrees C were identified by XRD and FTIR as mainly CDHA with some sodium calcium phosphate in the sintered samples. Ca/P ratios (SEM/EDX) were 1.62 and 1.52, respectively. Single crystal spot patterns characteristic of HA were seen with commercial HA and Serratia HA heated at 600 degrees C. After sintering at 1,200 degrees C the material consisted of needle-like crystals with a length between 86 and 323 nm (from TEM) or 54-111 nm (from XRD) and lattice parameters of a = 9.441 A and c = 6.875 A. This study indicated that the material produced by Serratia bacteria was initially mainly nanophase calcium deficient hydroxyapatite, which sintered to a more highly crystalline form. With further refinements the method could be used as an inexpensive route for hydroxyapatite production for biomaterials applications.

Fundamental mechanics of aortic heart valve closure.

Stresses in a prosthetic heart valve at closure are determined by its geometrical and structural characteristics, by the mechanical support environment, and by the momentum of the valve leaflets or occluder and of the blood at the instant of closure. The mass of blood to be arrested is significantly greater than that of the leaflets or occluder, and is therefore likely to dominate the closure impulse. The kinetic energy of the blood must be transduced into potential energy in the structural components (valve leaflets, aortic root and aorta). This paper presents a methodology for computation and parameterisation of the blood momentum associated with a valve in the aortic position. It is suggested that the influence of physiological parameters, such as systolic waveform and systemic impedance, on the closure characteristics can be investigated based on the fluid dynamic implications. Detailed results are presented for a single leaflet mechanical valve (Bjork-Shiley 60 degrees Convexo-Concave). It is demonstrated that a simple analytical method can yield results that might be adequate for the purposes of valve design.

Characterization of humic substances by environmental scanning electron microscopy.

Environmental scanning electron microscopy (ESEM) is a new technique capable of imaging micron and submicron particles. Here, we have applied it to image and quantify natural aquatic organic matter (standard Suwannee River humic acid, SRHA). Uniquely, we have observed the humic aggregate structures as a function of humidity and pH. Large aggregates of tens of micrometers were observed as the dominant material under all conditions, although much smaller material was also observed. Fractal dimensions (D) were calculated between 1.48 and 1.70, although these values were not statistically different under conditions of low humidity. However, D values calculated at high humidities (85%) during the rehydration phase were significantly lower (1.48+/-0.01) than in the initial dehydration phase (1.69+/-0.01). This hysteresis indicated that full rehydration of the HS was either kinetically slow or irreversible after dehydration. Fractal analysis of ESEM images was also performed to probe the change in aggregate structure as a function of pH. Minimum values were calculated at neutral pHs, rising by 0.1-0.2 at both high and low pHs because of a combination of the physical chemistry of HS and the impacts of the drying regime within the ESEM. Thus, ESEM was an important complementary technique to other analytical methods. At present, ESEM cannot be used to image nonperturbed natural samples. However, the method is an ideal method for probing the changes in colloid structure as function of hydration state and has the potential to perform fully quantitative and nonperturbing analysis of colloidal structure.

Suzuki segregation in a binary Cu-Si alloy.

Suzuki segregation to stacking faults and coherent twin boundaries has been investigated in a Cu-7.15 at.% Si alloy, heat-treated at temperatures of 275, 400 and 550 degrees C, using field-emission gun transmission electron microscopy. Silicon enrichment was observed at the stacking fault plane and decreased monotonically with increasing annealing temperature. This increase in the concentration of solute at the fault is due to the stacking fault energy being lowered at higher values of the electron-to-atom ratio of the alloy. From a McLean isotherm, the binding energy for segregation was calculated to be -0.021 +/- 0.019 eV atom(-1). Hardly any segregation was observed to coherent twin boundaries in the same alloy. This is because a twin has a lower interfacial energy than a stacking fault, so that the driving force for segregation is diminished.