Ion Beam Transmission Electron Microscopy Research Articles

A comparative study of surface modification under helium implantation on mechanically and electrochemically polished tungsten

ABSTRACT Tungsten is widely used as a plasma-facing material in nuclear fusion reactors due to its favorable properties; however, the extensive exposure of energetic particles could lead to severe degradations on its surface. Therefore, we studied the surface modification resulted from helium implantation on the mechanically and electrochemically polished tungsten at an elevated temperature. Samples of both types were implanted with helium ions to a fluence of at . Compared with electrochemically polished tungsten, fewer and smaller blisters was identified on mechanically polished sample by means of scanning electron microscopy (SEM). Combined with the focused ion beam (FIB) facility, cross-sectional SEM and transmission electron microscopy (TEM) analyses were conducted, and elongated cavities along the grain cracks were confirmed in the mechanically polished sample. Both SEM and TEM observations confirmed that these cavities can connect the damaged layer and sample surface, and effectively exhaust the accumulated He gas. Subsequently, the possible correlation between blistering reduction and the helium-induced cavities in mechanically polished tungsten was discussed.

Characteristics of hydrogen embrittlement in high-pH stress corrosion cracking of X100 pipeline steel in carbonate/ bicarbonate solution

Stress corrosion cracking (SCC) of X100 grade pipeline steel was researched in carbonate/bicarbonate solutions by using slow strain rate tensile (SSRT) tests, electrochemical testing, X-ray photoelectron spectroscopy (XPS), electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), the focused ion beam (FIB) lift-out technique and transmission electron microscopy (TEM). It is found that the X100 pipeline steel has low SCC sensitivity at the open circuit potential (OCP) and has high susceptible to SCC owing to the combined effect of anodic dissolution (AD) and hydrogen embrittlement (HE) at the applied cathodic potentials in carbonate/ bicarbonate solutions. At applied cathodic potentials less than about −950 mV, the SCC processes of X100 steel are dependent on AD and HE, while at more negative applied potentials, HE plays a more dominant role. The detailed SCC mechanism was studied by advanced detection methods. The results show that cracks propagated along the interface with low residual strain at various cathodic potentials, indicating that the low residual path is more active for both the anodic reaction and dislocation emission, and the high residual strain area may act as the cathodic zone. This phenomenon will be helpful to the design of high-SCC-resistant high-strength pipeline steel.

DNA Origami for Silicon Patterning.

Desoxyribonucleic acid (DNA) origami architectures are a promising tool for ultimate lithography because of their ability to generate nanostructures with a minimum feature size down to 2 nm. In this paper, we developed a method for silicon (Si) nanopatterning to face up current limitations for high-resolution patterning with standard microelectronic processes. For the first time, a 2 nm-thick 2D DNA origami mask, with specific design composed of three different square holes (with a size of 10 and 20 nm), is used for positive pattern transfer into a Si substrate using a 15 nm-thick silicon dioxide (SiO2) layer as an intermediate hard mask. First, the origami mask is transferred onto the SiO2 underlayer, by an HF vapor-etching process. Then, the Si underlayer is etched using an HBr/O2 plasma. Each hole is transferred in the SiO2 layer and the 20 nm-sized holes are transferred into the final stack (Si). The resulting patterns exhibited a lateral resolution in the range of 20 nm and a depth of 40 nm. Patterns are fully characterized by atomic force microscopy, scanning electron microscopy, focused ion beam-transmission electron microscopy, and ellipsometry measurements.

Relieving capacity decay and voltage fading of Li1.2Ni0.13Co0.13Mn0.54O2 by Mg2+ and PO43- dual doping

Li- and Mn-rich layered cathode materials (LMR) have been widely studied for their ultrahigh energy density. Unfortunately, the rapid potential decay and capacity degradation during cycling hinder their commercialization process. In this work, Mg2+ cation and PO43− polyanion dual elements coupling effect induced lattice modification strategy is developed to address these drawbacks for a typical lithium-rich material. The structural characteristic, chemical composition, valence state and elemental distribution of the materials are investigated by scanning electron microscopy, X-ray diffraction, inductively coupled plasma optical emission spectroscopy, focused ion beam and high-resolution transmission electron microscopy, respectively. Mg2+ and PO43− co-modified lithium-rich cathode material shows excellent electrochemical properties. It provides an improved initial coulombic efficiency of 87 % and ultrahigh initial discharge capacity of 307 mA h g-1 during a cut-off voltage of 2.0–4.8 V and a current density of 25 mA g-1. In conclusion, the Mg2+ and PO43− co-modified material exhibits not only significantly improved rate performance (150.7 mA h g−1 @ 5 C) but also markedly improved cycling stability (83 % @ 1 C @ 300 cycles).

Microstructural Characterization of HVOF-Sprayed Ni on Polished and Oxidized Stainless Steel Substrates

In the present study, microstructural analysis was carried out to investigate the effect of substrate surface chemistry on splat formation. Ni powder was sprayed onto stainless steel substrates exhibiting two different surface conditions (polished and oxidized) by high-velocity oxy-fuel (HVOF) thermal spray. X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical state of the surfaces which indicated the presence of the adsorbates on the oxidized substrate. Single splats of different morphologies from both samples were examined using a range of characterization techniques including scanning electron microscopy (SEM), focused ion beam (FIB) microscopy and transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDS) interfaced to both SEM and TEM. This provided information on the particle solidification behavior and splat formation mechanism following impact on substrates with distinct surface conditions. The results showed the effect of oxidized surface adsorbates on splat morphology, pore formation and splat–substrate interactions. These microstructural findings, aided by theoretical models, revealed that a mixture of mechanical and metallurgical bonds exists between the splats and both substrates.

Stability of Proton-Conducting Solid Oxide Electrolyzers for Hydrogen Production and Energy Storage

Proton-conducting solid oxide electrolyzers (H-SOEs) provide promising opportunity to produce pure and dry hydrogen in steam electrolysis at relatively low operating temperatures (550-700˚C) utilizing electricity and heat generated from renewable energy sources. Compared to traditional high temperature (750-1000˚C) oxygen-conducting solid oxide electrolyzers (O-SOEs), lower operating temperature of H-SOE offers ease of thermal management, active stack and BOP materials cost reduction and reduction in chromium evaporation from metallic components. Like O-SOEs, preserving the long-term stability of H-SOEs is one of the technical challenges for large-scale hydrogen production. In this technical contribution, results of experimental evaluation of H-SOEs under real-world operating conditions are presented. As fabricated and posttest cells have been characterized using operando electrochemical impedance spectroscopy, X-ray diffraction, focused ion beam-transmission electron microscopy and other bulk and surface characterization techniques to examine bulk, surface and interface stability of electrochemically active components. Phase and morphological changes, compositional uniformity and interfacial reaction products formation have been examined. Electrolyte/electrode materials stability, cell and gas seal fabrication processes, and gaseous impurities affecting long-term electrochemical performance will be discussed. H-SOE electrochemical performance model based on cell materials and operating conditions has been proposed and validated based on single cell testing data.

Microstructural evolution and nanoindentation of NiCrMoAl alloy coating deposited by arc spraying

Arc spraying is a flexible thermal spray coating that can be used in on-site applications. In this study, a NiCrMoAl alloy was coated onto a mild steel substrate by arc spraying. The cored wire feedstock and the deposited coating were extensively characterized by a combination of X-ray diffraction, scanning electron microscopy, focused ion beam microscopy and transmission electron microscopy. Vickers microhardness and nanoindentation were used to investigate the mechanical properties of individual phases in the coating. The results showed that the NiCrMoAl alloy cored wire feedstock primarily consisted of material in elemental form including Ni, Cr, Mo, Al, Si, and Ti, whilst the arc sprayed coating contained a solute-lean γ-Ni phase, a (Mo, Si)-rich γ-Ni phase, elemental Mo splat as well as Cr2O3, Al2O3, and SiO2 at intersplat regions, together with ≤100 nm spherical Al2O3 particles within the splats. Partitioning of Mo and Si into the γ-Ni phase was observed following the rapid solidification of the splat on deposition. The Vickers hardness analysis showed that the average overall hardness of the coating was about 3.65 ± 0.56 GPa. However, nanoindentation indicated that the highest hardness was in the γ-Ni splat regions, especially those containing a high concentration of solute. The microstructural evolution, as well as the structure and chemistry of the phases that affect mechanical properties in the coating are discussed in detail.

Early globalized industrial chain revealed by residual submicron pigment particles in Chinese imperial blue-and-white porcelains

The success of early Chinese blue-and-white porcelains relied heavily on imported cobalt pigment from the West. In contrast to art-historical concept, which contains both typological evidence and literature records, it is assumed that imported Sumali blue was completely superseded by domestic Chinese asbolane ore based on the analytical results of the Fe/Mn ratio in imperial productions from the Xuande reign (1426 to 1435 CE) onward. Using a focused ion beam transmission electron microscopy technique to reassess this hotly debated question, we have identified two classes of residual submicron pigment particles in the blue glaze with diagnostic differences in morphology, chemical composition, and distribution behavior. Compared with the microstructural features of the blue-and-white porcelains of the Yuan and Qing dynasties, we show that a mixture of imported and domestic cobalt pigments was used for aesthetic reasons, indicating that the overseas supply chain of imported pigment remained consistent and adequate even though the authorities had terminated official trade and tributary activities after the death of Admiral Zheng He. This discovery further suggests that the globalized trading network and cross-regional industrial chain had been extensively established in the 15th century. Moreover, we provide analytical evidence against the fundamental assumption of the current Fe/Mn provenancing criteria, implying that the failures of previous chemical analyses can be attributed to elemental differentiation between the silicate glaze and the arsenic pigment. We propose an innovative method for directly assessing original mineralogic information from submicron residual pigment particles that provides a more reliable way to trace cobalt circulation and holds great promise for provenance studies.

Electrochemical Behavior and Self-Sealing Ability of Zirconium Conversion Coating Applied on Aluminum Alloy 3005 in 0.5 M NaCl Solution

Zirconium conversion coating (ZrCC) was prepared on aluminum-manganese alloy AA3005 by immersion in 200 ppm of H2ZrF6 bath for 10 min at room temperature. Potentiodynamic polarization curves and electrochemical impedance spectra were measured up to 10 and 40 days, respectively, in 0.5 M NaCl solution. Microstructural characterization of samples was carried out using scanning electron microscopy equipped with energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, focus ion beam microscopy and transmission electron microscopy. The coating had a tri-layer structure with a thickness of 200 nm in the proximity of intermetallic particles and 30 nm far from intermetallic particles at the coating matrix. Electrochemical measurements showed that the corrosion resistance of ZrCC on AA3005 was improving during immersion in NaCl solution, i.e., impedance value at low frequency increased with immersion time reaching the average value 8.5 ∙ 106 Ω∙cm2 at 3 mHz after 10 days immersion. Microstructural and compositional characterization showed that this behavior is related to the change in composition and structure of conversion coating including a transformation of ZrF4/ZrOxFy to ZrO2 · 2H2O(s) and formation of Al(OH)3 in the top layer of ZrCC, respectively, accompanied by the change in thickness of individual layers within the coating.

Low Temperature VECSEL-to-Diamond Heterogeneous Integration with Ag-In Spinodal Nanostructured Layer

Low temperature heterogeneous integration with diamond is the key technology in pushing upwards the high-power limit of a vertically-external-cavity surface-emitting laser (VECSEL). This work successfully accomplished a functional high-power VECSEL-to-diamond device with a modified Ag-In transient liquid phase (TLP) bonding technology. The post-bonding quality of VECSEL epitaxial membrane was thoroughly examined with scanning electron microscopy (SEM), focus ion beam (FIB) and high resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Owing to the low-temperature process, thermal-activated diffusion and thermo-mechanical stress have been suppressed to the minimal level within the epitaxial layers while optimizing the heat-spreading capability of the diamond. Interestingly, with experimental and thermodynamic evidences, a distinct nanostructure from spinodal decomposition has been discovered in the Ag-In bonding layer for the first time, whose structural feature is beneficial to the reliability of a VECSEL-to-diamond device. Conceptually, this work opens a new bonding technology category, i.e., Ag-In spinodal bonding.

The formation of ZnO structures using thermal oxidation: How a previous chemical etching favors either needle-like or cross-linked structures

Thermal oxidation of Zn polycrystalline elemental foils –previously etched at different chemical conditions– was used to study the formation of ZnO structures. Two distinct etching solutions were used: 0.5 M HNO3 and 11 M HNO3. Based on field emission scanning electron microscopy results, by modifying the etching conditions, the ZnO structures were different regarding morphology: needle-like ZnO microstructure arrays were formed when Zn foil was chemically etched using the 0.5 M HNO3 solution, whilst a cross-linked morphology was obtained when the etching solution changed to 11 M HNO3 concentration. However, the elemental mapping showed identical chemical composition of both kinds of ZnO microstructures. The focused ion beam-transmission electron microscopy specimens revealed the occurrence of ZnO nanostructures, in both kinds of as-oxidized samples, obtained after 2 h of thermal oxidation at 400 °C. Finally, both needle- and cross-linked microstructures were evaluated in terms of their optical properties. According to the experimental findings, it was found that these structures can be applied in optoelectronic devices working around the green range of the visible electromagnetic spectrum, exhibiting the cross-linked sample the highest radiation intensity.

Variable-Energy Hard X-ray Photoemission Spectroscopy: A Nondestructive Tool to Analyze the Cathode-Solid-State Electrolyte Interface.

All-solid-state batteries are expected to be promising next-generation energy storage systems with increased energy density compared to the state-of-the-art Li-ion batteries. Nonetheless, the electrochemical performances of the all-solid-state batteries are currently limited by the high interfacial resistance between active electrode materials and solid-state electrolytes. In particular, elemental interdiffusion and the formation of interlayers with low ionic conductivity are known to restrict the battery performance. Herein, we apply a nondestructive variable-energy hard X-ray photoemission spectroscopy to detect the elemental chemical states at the interface between the cathode and the solid-state electrolyte, in comparison to the widely used angle-resolved (variable-angle) X-ray photoemission spectroscopy/X-ray absorption spectroscopy methods. The accuracy of variable-energy hard X-ray photoemission spectroscopy is also verified with a focused ion beam and high-resolution transmission electron microscopy. We also show the significant suppression of interdiffusion by building an artificial layer via atomic layer deposition at this interface.

Directionality of metal-induced crystallization and layer exchange in amorphous carbon/nickel thin film stacks

In thin amorphous carbon (a-C) films being in contact with a thin nickel layer, metal-induced crystallization and layer exchange (LE) occur at temperatures lower than 700 °C. Analysis of thin film stacks with different architectures (a-C/Ni, Ni/a-C and Ni/a-C/Ni) by means of ion beam analysis, Raman spectroscopy, X-ray diffraction and transmission electron microscopy revealed that the degree of LE and the structural quality of the crystallized carbon layers depend on the initial layer sequence. A LE degree of approx. 93% was found for Ni/a-C bilayers, where graphenic layers formed on the Ni surface, whereas in a-C/Ni bilayers only 83% of carbon was transferred from the surface towards the fused silica substrate. The diffusion of carbon in the outward direction produces turbostratic carbon with basal planes oriented parallel to the Ni surface, while for the inward direction planar and curved turbostratic structures coexist. The crystallization and the LE are driven by the crystallization energy of a-C. The LE is mediated by the wetting of the Ni grain boundaries by carbon. The directionality of the LE was explained primarily by the difference in the surface and interface energies in the a-C/Ni and Ni/a-C stacks that were obtained from thermodynamic considerations.

On the biogenicity of Fe-oxyhydroxide filaments in silicified low-temperature hydrothermal deposits: Implications for the identification of Fe-oxidizing bacteria in the rock record.

Microaerophilic Fe(II)-oxidizing bacteria produce biomineralized twisted and branched stalks, which are promising biosignatures of microbial Fe oxidation in ancient jaspers and iron formations. Extracellular Fe stalks retain their morphological characteristics under experimentally elevated temperatures, but the extent to which natural post-depositional processes affect fossil integrity remains to be resolved. We examined siliceous Fe deposits from laminated mounds and chimney structures from an extinct part of the Jan Mayen Vent Fields on the Arctic Mid-Ocean Ridge. Our aims were to determine how early seafloor diagenesis affects morphological and chemical signatures of Fe-oxyhydroxide biomineralization and how extracellular stalks differ from abiogenic features. Optical and scanning electron microscopy in combination with focused ion beam-transmission electron microscopy (FIB-TEM) was used to study the filamentous textures and cross sections of individual stalks. Our results revealed directional, dendritic, and radial arrangements of biogenic twisted stalks and randomly organized networks of hollow tubes. Stalks were encrusted by concentric Fe-oxyhydroxide laminae and silica casings. Element maps produced by energy dispersive X-ray spectroscopy (EDS) in TEM showed variations in the content of Si, P, and S within filaments, demonstrating that successive hydrothermal fluid pulses mediate early diagenetic alteration and modify the chemical composition and surface features of stalks through Fe-oxyhydroxide mineralization. The carbon content of the stalks was generally indistinguishable from background levels, suggesting that organic compounds were either scarce initially or lost due to percolating hydrothermal fluids. Dendrites and thicker abiotic filaments from a nearby chimney were composed of nanometer-sized microcrystalline iron particles and silica and showed Fe growth bands indicative of inorganic precipitation. Our study suggests that the identification of fossil stalks and sheaths of Fe-oxidizing bacteria in hydrothermal paleoenvironments may not rely on the detection of organic carbon and demonstrates that abiogenic filaments differ from stalks and sheaths of Fe-oxidizing bacteria with respect to width distribution, ultrastructure, and textural context.

Effect of engine oil additives reduction on the tribofilm structure of a cylinder liner model surface

PurposeThe purpose of this paper is to provide a general picture for describing the formed tribofilm, including chemical and physical aspects in the micro-scale and the nano-scale. In a previous study, the durability of zinc dialkyl dithiophosphate (ZDDP) tribofilms on cylinder liner samples has been investigated in a tribometer model system by using fresh and aged fully formulated oils and replacing them with PAO8 without additives. Analyses of the derived tribofilms by means of X-ray photoelectron spectroscopy and scanning electron microscopy could give some hints about the underlying mechanisms of the tribofilm build-up and wear performance, but a final model has not been achieved.Design/methodology/approachThus, characterisation of these tribofilms by means of focused ion beam-transmission electron microscopy (FIB-TEM) and energy dispersive X-ray spectroscopy is presented and a concluding model of the underlying mechanisms of tribofilm build-up is discussed in this paper.FindingsFor tribotests running first with fresh fully formulated engine oil, a rather homogeneous ZDDP-like tribofilm is found underneath a carbon rich tribofilm after changing to non-additivated PAO8. However, when the tests run first with aged fully formulated engine oil, no ZDDP-like tribofilm has been found after changing to non-additivated PAO8, but a wear protective carbon rich tribofilm.Originality/valueThe obtained results provide insights into the structure and durability of tribofilms. Carbon-based tribofilms are built up on the basis of non-additivated PAO8 because of the previously present ZDDP tribofilms, which suggests an alternative way to reducing the consumption of antiwear additives.

EFFECT OF ALLOYING ELEMENTS ON CORROSION POTENTIAL AND CORROSION PROPAGATION PROCESS OF MULTILAYER ALUMINUM-BRAZED SHEET

The cross-sectional depth microstructure profiles for a multilayer AA4045/AA3003*/AA4045 brazed sheet were observed and determined by focused ion beam-transmission electron microscopy (FIB-TEM) and energy dispersive spectrometer (EDS). The corrosion propagation of the multilayer brazed sheet in the sea water acidified accelerated test (SWAAT) was investigated by electrochemical impedance spectroscopy (EIS). The measured corrosion potentials of different layers of the multilayer aluminum sheet have been carried out according to ASTM G69-97 standard test method. To reveal the effect of the alloying elements on corrosion behavior, the theoretical corrosion potential of the band of dense precipitates (BDP) zone and core materials was also calculated according to the theoretical corrosion potential model. The electrochemical results showed that there were potential differences between the precipitates free zone (PFZ) and BDP zone as well as BDP zone and the core materials. The EIS test and equivalent circuits (EC) suggested that the capacitive time constant at low frequencies correspond to the corrosion of the BDP zone in the form of exfoliation corrosion. The corrosion propagation process could be identified into four stages: the dissolution of the eutectic [Formula: see text]-Al in the re-solidified cladding in the form of pitting corrosion; the corrosion of the primary [Formula: see text]-Al grain boundaries in the form of inter-granular corrosion (IGC); the corrosion of the BDP zone in the form of exfoliation corrosion; and the corrosion of the core material in the form of IGC.

Plasma-sprayed nickel splats on chromium substrates: The role of substrate preheating and thermal conductivity

The effect of chromium substrate preheating on the spreading behavior of plasma-sprayed nickel splats was characterized. The interfacial features along the splat-substrate interface were analyzed by focused ion beam (FIB) microscopy and transmission electron microscopy (TEM). In addition, the transient droplet spreading process was modelled in a simulation study. The results showed that the presence of a large fraction of surface moisture on the substrates induced splat fragmentation. However, splat fragmentation was completely constrained on chromium preheated to a low temperature (373 K) where a certain amount of surface moisture was still present. The high thermal conductivity of chromium substrates increased droplet solidification rates, decreasing the interaction time between the spreading droplet and the vaporized gas layer. Accordingly, splat fragmentation was suppressed and disk-shaped splats were formed. Fast solidification not only refined the final splat microstructure, but also promoted the formation of finger-splashed disk splats. In addition, the extent of elemental diffusion across the splat-substrate interface was decreased due to the low interfacial temperature.

Nanoscale Structure of Zoned Laurites from the Ojén Ultramafic Massif, Southern Spain

We report the first results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of zoned laurite (RuS2)-erlichmanite (OS2) in mantle-hosted chromitites. These platinum-group minerals form isolated inclusions (<50 µm across) within larger crystals of unaltered chromite form the Ojén ultramafic massif (southern Spain). High-magnification electron microscopy (HMEM), high angle-annular dark field (HAADF) and precession electron diffraction (PED) data revealed that microscale normal zoning in laurite consisting of Os-poor core and Os-rich rims observed by conventional micro-analytical techniques like field emission scanning electron microscope and electron microprobe analysis (FE-SEM and EPMA) exist at the nanoscale approach in single laurite crystals. At the nanoscale, Os poor cores consist of relatively homogenous pure laurite (RuS2) lacking defects in the crystal lattice, whereas the Os-richer rim consists of homogenous laurite matrix hosting fringes (10–20 nm thickness) of almost pure erlichmanite (OsS2). Core-to-rim microscale zoning in laurite reflects a nonequilibrium during laurite crystal growth, which hampered the intra-crystalline diffusion of Os. The origin of zoning in laurite is related to the formation of the chromitites in the Earth’s upper mantle but fast cooling of the chromite-laurite magmatic system associated to fast exhumation of the rocks would prevent the effective dissolution of Os in the laurite even at high temperatures (~1200 °C), allowing the formation/preservation of nanoscale domains of erlichmanite in laurite. Our observation highlights for the first time the importance of nanoscale studies for a better understanding of the genesis of platinum-group minerals in magmatic ore-forming systems.

Magmatic platinum nanoparticles in metasomatic silicate glasses and sulfides from Patagonian mantle xenoliths

Platinum-rich nanonuggets (s.l., nanoparticles) are commonly produced in experiments attempting to quantify the solubility or partitioning of noble metals in silicate and sulfide melts. However, it has been thought that these represent artifacts produced during quenching of the experimental runs. Here, we document nanoparticles (~ 20–80 nm) of Pt-rich alloys and arsenides dispersed in high-temperature metasomatic silicate glasses and in base-metal sulfides (BMS) entrained in them, found interstitially between minerals of mantle peridotite xenoliths from southern Patagonia. Pt-rich nanoparticles found in the interstitial silicate glasses are frequently attached to, or in the proximities of, oxides (ilmenite or Cr-spinel) suggesting a close link between the formation of the oxides and the Pt-rich nanoparticles. The interstitial glasses in the studied xenoliths correspond to quenched alkaline basaltic melts that infiltrated the subcontinental lithospheric mantle (SCLM) at > 1000 °C at an oxygen fugacity (fO2) near the fayalite–magnetite–quartz (FMQ) buffer. Experimental works indicate that at these conditions the crystallization of oxides such as ilmenite or Cr-spinel may lower fO2 to promote the precipitation of Pt-rich nanoparticles. The investigation of four Pt-rich nanoparticles hosted in two different pentlandite grains using a combination of focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) show that these nanoparticles consists of polycrystalline aggregates < 10 nm that are randomly oriented relative to their sulfide host matrices. These observations suggest that these nanoparticles could be segregated either directly from the infiltrating alkaline basaltic melt prior to sulfur saturation in the silicate melt, or from droplets of immiscible sulfide melt once sulfur saturation was achieved. The formation of Pt-rich nanoparticles in high-temperature melts, either silicate or sulfide, provides new clues on the processes of fractionation, transport and concentration of Pt in the mantle.

Understanding the electrochemical behavior of bulk-synthesized MgZn2 intermetallic compound in aqueous NaCl solutions as a function of pH

The electrochemical behavior of a bulk-synthesized MgZn2 intermetallic compound in aerated 0.1 M NaCl solutions has been studied as a function of pH and applied potential using polarization techniques, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and focused ion beam-transmission electron microscopy (FIB-TEM). The anodic activity of MgZn2 is seen to decrease with an increase in pH value. Polarization tests reveal two limiting current densities in pH 4 solution at relatively high and low potentials. At pH 12, passivity is observed with a lower limiting current density compared to those observed at pH 4. The corrosion film formed after potentiostatic polarization in the pH 4 solution is composed of a bilayer at a less negative potential and a single layer at a more negative potential. In the case of pH 12 solution, a protective compact bilayer film is formed irrespective of the potential within the passive zone. Overall, the corrosion mechanism of MgZn2 is by early dissolution of Mg leading to a Zn-enriched surface whose subsequent dissolution depends on the value of the applied potential.