Analytical TEM Investigations Research Articles

Structural Evolution of GaOx-Shell and Intermetallic Phases in Ga-Pt Supported Catalytically Active Liquid Metal Solutions.

We present a comprehensive scale-bridging characterization approach for supported catalytically active liquid metal solutions (SCALMS) which combines lab-based X-ray microscopy, nano X-ray computed tomography (nano-CT), and correlative analytical transmission electron microscopy. SCALMS catalysts consist of low-melting alloy particles and have demonstrated high catalytic activity, selectivity, and long-term stability in propane dehydrogenation (PDH). We established an identical-location nano-CT workflow which allows us to reveal site-specific changes of Ga-Pt SCALMS before and after PDH. These observations are complemented by analytical transmission electron microscopy investigations providing information on the structure, chemical composition, and phase distribution of individual SCALMS particles. Key findings of this combined microscopic approach include (i) structural evolution of the SCALMS particles' GaOx shell, (ii) Pt segregation toward the oxide shell leading to the formation of Ga-Pt intermetallic phases, and (iii) cracking of the oxide shell accompanied by the release of liquid Ga-Pt toward the porous support.

In-depth insight into the structural properties of nanoparticulate NiO for CO sensing

The outstanding properties exhibited by the p-type NiO nanostructures can be greatly affected by morpho-structural and defect characteristics with constructive or competing effects. We have conducted an in-depth study on NiO nanoparticles obtained by hydrothermal synthesis and submitted to various thermal treatments, to monitor the evolution of their structural properties and the effect of the thermal history on their CO sensing. Correlated electron paramagnetic resonance and analytical transmission electron microscopy investigations evidenced an amount of up to 1 % metallic nickel clusters close to surface in the NiO nanoparticles calcined at 400 °C and 500 °C for 8 h. Subsequent annealing in vacuum and in air of the sample calcined at 400 °C resulted in different size distributions and morphology of the NiO nanoparticles and an increase/decrease of the nickel phase, respectively. Comparative CO sensing tests on the two pristine samples and on the sample calcined at 400 °C and further annealed in air at 500 °C for 2 h showed an increase in the baseline resistance of the later due to the decrease of free charge carriers induced by the dissolution of the nickel clusters. The overall CO sensing results show a strong dependence on the samples thermal history.

Novel approach of combined planar and cross-sectional defect analysis of stressed normally-on HEMT devices with leaky Schottky gates

The clear correlation of defect localization by Electroluminescence to nanometer sized defects at Schottky gate contacts of GaN HEMTs are always challenging and limiting the root cause analysis of the stress related electrical degradation of the devices. In this paper a novel approach combining large area planar TEM defect screening followed by cross sectional preparation at identified defect sites is demonstrated to get access to relevant structural damages at the gate. The analysis flow is applied for normally-on GaN HEMT devices with an increased leakage at the Schottky gates observed after electrical stressing. To initiate electrical active degradation these devices were stressed up to significant changes in the electrical performance and then Electroluminescence spots were detected identifying possible defect positions. Metal intrusions into the AlGaN/ GaN stack underneath the Schottky gate contacts of only 20 nm size could be found by the newly combined planar/ cross-sectional analysis flow in good correlation to the EL spot positions. Analytical TEM investigations of the defects reveal Pt and Au inside the semiconductor reaching the 2DEG at a threading dislocation of the HEMT. These intrusions contribute to the gate leakage current and are causing the increased leakage current after electrical testing.

Analytical transmission electron microscopy investigation of electrochemically Li+–Na+ substituted LiNi0.5Mn1.5O4 electrode

The electrochemical Li+–Na+ alkaline cation substitution of a lithium nickel manganese oxide (LiNi0.5Mn1.5O4, LNMO), which is characterized via structural analysis, has previously been reported. However, detail characterizations of the chemical and electronic features are lacking. In this study, we characterized that by using analytical transmission electron microscopy. Energy dispersive X-ray spectroscopy (EDS) revealed uniform Na-insertion into a single particle. Electron energy-loss spectroscopy (EELS) analysis exhibited complete Li-extraction. Energy loss near edge structure (ELNES) of Mn and Ni L-edges indicated that only Ni contributes to the redox reaction as Ni2+ ​⇔ ​Ni4+ during the Li+–Na ​+ ​substitution. The elemental composition ratio analysis using both EDS and EELS indicated Na0.6Ni0.5Mn1.5O3.8 as a formula of the Na-substituted material. Oxygen desorption was provoked during the Na-insertion process, and oxygen defect formation was found to be essential for the Li+–Na+ substitution mechanism of the LNMO material.

Editors’ Choice—Dealloying-Driven Cerium Precipitation on Intermetallic Particles in Aerospace Aluminium Alloys

Cerium-based compounds have been studied for decades as non-toxic candidates for the protection of aerospace aluminium alloys (AAs) like AA2024-T3. However, the complex heterogeneous microstructure of these alloys has hindered a thorough understanding of the subsequent stages of corrosion protection provided by this class of inhibitors. Thus, this work is devoted to unravelling the interaction mechanisms of different intermetallic particles (IMPs) in AA2024-T3 with cerium nitrate at the nanoscopic scale. This has been fulfilled through detailed top-view and cross-sectional analytical TEM investigations along with electrochemical evaluations. In line with our recent findings, we here report dealloying of IMPs as the main factor governing the rate of local cerium precipitation in contrast to micro-galvanic corrosion between IMPs and the surrounding matrix. Furthermore, we discuss a connection between the electrochemical response of the AA2024-T3 system and the morphological and compositional evolutions of individual IMPs including Al2CuMg, Al2Cu, Al7Cu2Fe(Mn) and Al76Cu6Fe7Mn5Si6 at different stages of a 96-h exposure.

Linkage Between Microstructure and Chemical Composition of Iron-Rich Hard Dental Tissues from the Feral Coypu by Analytical TEM Investigations

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Silver-Assisted Colloidal Synthesis of Stable, Plasmon Resonant Gold Patches on Silica Nanospheres

Patchy particles possessing heterogeneous surface composition show great promise as self-organizing building blocks for new classes of hierarchical functional structures. A major hurdle is the scalable synthesis of stable patches on nanosized core particles with arbitrarily defined patch number and coverage. So far, few methods have been reported which could be expected to meet these challenges. Recently, we described the heterogeneous nucleation and growth of silver patches on silica nanospheres via a template free colloidal route. The patches produced, although tunable in size and number and showing interesting plasmon resonant properties, were rather unstable and degraded rapidly during attempts to process them further. In the present work, therefore, we set out to explore if related approaches can be employed to produce patchy particles involving gold, which is known to be more stable. The differences between typical patch precursors Ag(+) and [AuCl(x)(OH)(4-x)](-) and their respective interactions with amorphous silica make this a significant challenge. We show that preformed small silver patches in addition to the presence of a reducing agent are necessary for the formation of gold patches conformal to the silica nanosphere surface. Systematic study of the process parameters and their influence on the patchy particle morphology as well as in-depth analytical transmission electron microscopy investigation of the patch composition reveal that patches spread over the silica surface via a cycle of galvanic dissolution and redeposition of silver. The resulting gold patchy particles remain stable during subsequent storage or washing and display tunable plasmon resonances within the visible and near-IR spectrum.

TEM study of internal oxidation in an ODS-Eurofer alloy

Analytical TEM investigations of samples from an oxide dispersion-strengthened Eurofer batch show the formation of new type of precipitates of Fe–Cr–V–O composition and a size of 40–250 nm. Structural HRTEM analysis reveals that the precipitates consist of (Mn,Fe)(Cr,V) 2O 4 manganese chromium oxide of spinel structure. In some cases, the inclusions order along lines that can reach more than ten micrometers in length. Such lines of large particles might have a negative influence on the mechanical properties of the steel, as was observed. It is assumed that they form by internal oxidation during hot isostatic pressing due to the high oxygen content of the mechanically alloyed powder.

Analytical TEM Investigations of Si Nanowires grown by the VLS Mechanism from Au Templates

Extended abstract of a paper presented at MC 2007, 33rd DGE Conference in Saarbrücken, Germany, September 2 – September 7, 2007

Analytical TEM investigations on concentration gradients surrounding Ni4Ti3 precipitates in Ni–Ti shape memory material

Concentration gradients surrounding Ni 4 Ti 3 precipitates grown by appropriate annealing in a Ni 51 Ti 49 B2 austenite matrix are investigated by a combination of analytical TEM techniques. A Ni depleted zone in the surrounding matrix up to 150 nm away from the matrix–precipitate interface was detected. The size and concentration change of the depletion zone perfectly compensate for the excess Ni in the precipitate. Moreover, since different precipitates are often found in nanoscale proximity, depletion zones from different precipitates can reinforce one another in certain regions.

Tribological and physicochemical characterization of strontium colloidal additives in mild wear regime

The morphology, the structure and chemical nature of colloidal strontium dimethyl-3,5-hexanoate (SDMH) reverse micelles are investigated by atomic force microscopy (AFM) and analytical transmission electron microscopy (ATEM). Their anti-wear properties first measured through a classical tribology experiment are then investigated via the characterization of the structural and physicochemical properties of the surface film generated during friction tests between the two ferrous surfaces in the presence of the lubricant containing SDMH as additive. AFM studies demonstrate that the film is constituted of platelets of 40 nm thickness with a surface roughness smaller than 1 nm. The ATEM investigations on anti-wear film fragments show that the film is mainly composed of strontianite crystallites formed from the amorphous mineral cores of the micelles. These crystallites, which sizes are significantly larger than the mineral cores of the micelles, are linked to each other by an amorphous intergranular phase. Electron energy loss spectroscopy shows that during friction the colloidal particles loss their organic shell in the contact and that carbon under the carbonate form only remains in the anti-wear film. The good anti-wear properties of this additive are explained by the microgranular structure and the adherence of the anti-wear film on the surfaces.

Analytical TEM Investigations of B-doped Carbon and BN Nanotubes

Analytical TEM Investigations of B-doped Carbon and BN Nanotubes

High-resolution analytical TEM of nanostructured materials.

This paper briefly reviews the potential applicability of analytical transmission electron microscopy (TEM) to elucidate both structural and chemical peculiarities of materials at high lateral resolution. Examples of analytical TEM investigations performed by energy-dispersive X-ray spectroscopy (EDXS), electron energy loss spectroscopy (EELS), and energy-filtered TEM (EFTEM) are presented for different materials systems including metals, ceramics, and compound semiconductors. In particular, results are given of imaging the element distribution in the interface region between gamma matrix and gamma' precipitate in the nickel-based superalloy SC16 by energy-filtered TEM. For core-shell structured BaTiO(3) particles the chemical composition and even the chemical bonding were revealed by EELS at a resolution of about 1 nm. A sub-nanometer resolution is demonstrated by energy-selective images of the Ga distribution in the surrounding of (In,Ga)As quantum dots. Moreover, the element distribution in (Al,Ga)As/AlAs multilayers with linear concentration gradients in a range of about 10 nm was investigated by EDXS line-profile analyses and EFTEM.

SiO 2 precipitation in olivine: ATEM investigation of two dunites annealed at 300 MPa in hydrous conditions

Two natural dunites were annealed at pressure P=300 MPa, temperature T=1373, 1473 and 1573 K, and fO 2 within the stability field of olivine. The starting materials contained small amounts of hydroxyls in the form of minor phases of hydrated minerals, which released an aqueous phase during the experiments. A detailed analytical transmission electron microscopy (ATEM) investigation of these materials revealed that small quantities of two types of silica-rich glass formed during heat treatment. The first type of glass, found at triple junctions as rare partially crystallized glass pockets, results from melting dehydration reactions involving the hydrous phases. The second type of glass is found as pure silica precipitates (0.1–0.5 μm in size, for a total of a few 0.1 vol%) within the olivine grains of specimens heated to ≥1473 K. From considerations of the kinetics of the precipitation at 1473 K, we interpret this silica precipitation as resulting from the condensation of olivine metallic vacancies promoted by increasing fluid fugacities during the runs. Our observations, thus, demonstrate that metastable silica can precipitate in olivine from dunites experiencing rapid changes in their thermodynamical environment.

Microstructural evolution of a nanocrystalline Ti−47Al−3Cr alloy during annealing in the α+γ-phase field

Prealloyed, gas-atomized (GA) Ti−47Al−3Cr alloy powder, containing about 70 pct of the α2 (Ti3Al) phase and 30 pct of the γ (TiAl) phase, was fully amorphized by mechanical alloying. The amorphous phase was stable during heating to 600°C, but decomposed at higher temperatures, with an exothermic reaction peak at 624°C as the material transformed to a mixture of α2 and γ and then to a fully γ structure at 722°C. A nanocrystalline compact with a mean grain size of 42 nm was obtained by hot isostatic pressing (HIP'ing) of the amorphous powder at 725°C. Isothermal annealing experiments were conducted in the two-phase α+γ field, at 1200°C, using holding times of 5, 10, 25, and 35 hours, followed by air cooling. The X-ray diffractometry and analytical transmission electron microscopy investigations carried out on annealed and air-cooled specimens revealed only the presence of the γ grains, which coarsened on annealing. Initially, the grains grew, followed by a saturation stage after annealing for 25 hours, with a saturation grain size of about 1 μm. This grain growth and saturation behavior can be described with a normal grain growth mechanism in which a permanent pinning force is taken into account. Twins formed in the γ grains as a result of annealing and air cooling and exhibited a common twinning plane of (111) with the matrix phase. The minimum γ grain size in which twinning occurred in the annealed specimens was determined to be 0.25 μm, which suggests that twinning is energetically unfavorable in the nanometer-sized grains.

Olivine/Fe-metal equilibrium under high pressure: an ATEM investigation

San Carlos olivine samples enclosed in soft iron capsules were annealed in an uniaxial split-sphere apparatus, at pressures ranging from 4.6 to 9.0 GPa and temperature ranging from 1310o to 1595 oC. We estimated the annealing fO2, theoretically controlled by the olivine/Fe-metal equilibrium, to be 1 to 2 log units above the fO2 of the iron/wustite buffer. Samples were investigated by analytical transmission electron microscopy (ATEM) in order to verify that olivine and Fe capsule did equilibrate during the annealings. TEM imaging of the olivine bulk shows a and c dislocations confined in the (010) plane, and small (0.5 µm) spatially coupled precipitates of (1) Al-rich spinel and (2) enstatite (volumic proportion of precipitates ≃60 ppm). These coupled precipitates are surrounded by split c dislocation loops. Olivine composition profiles, determined by ATEM near the Fe-capsule/olivine contact, reveal a weak loss of Ni from the olivine matrix toward the capsule, as expected in such reducing conditions. These profiles also reveal a marked incorporation of Fe from the capsule into the olivine matrix. These observations, and their interpretation in terms of olivine point defect chemistry, lead to the following conclusions: (1) the starting olivine contained a high concentration of vacancies on octahedral sites (≥1000 ppm per site); such a high vacancy concentration is expected in San Carlos olivine which equilibrated in nature at relatively high fO2; (2) the olivine/Fe-metal equilibrium did control fO2 during the annealings, that resulted in a rapid re-equilibration of olivine at the beginning of the runs to the lower fO2 imposed by the Fe capsule; this led to a strong decrease of the octahedral vacancy concentration in olivine. (3) Such a fO2 decrease promoted in olivine the coupled precipitation of both types of Al-rich spinel and enstatite precipitates. These observations show that the use of Fe-capsule in high pressure experiments is an efficient method for controlling fO2 when studying olivine, and more generally Fe-bearing silicates.

Analytical TEM investigations of high-temperature superconductors

Electron diffraction and energy dispersive X-ray spectroscopy as analytical transmission electron microscopic methods have been applied to Y-Ba-Cu-O superconductors. The evaluation of diffraction patterns by means of the MS-WINDOWS program ELDISCA is demonstrated. The course of X-ray linescan signals in the transition range between YBa2Cu3O7−δ matrix and Y2BaCuO5 inclusions is explained by calculations based on a mathematical model which is described. Errors of quantiative X-ray spectroscopic results and their reasons are shown.

Production of Ni-Ru bimetallic catalysts and materials by thermal and chemical decomposition of a tetranuclear bimetallic carbonyl cluster

The tetrahedral heterobimetallic cluster (C 5H 5)NiRu 3H 3(CO) 9, 1, when decomposed on KOH-modified γ-alumina, produces an effective heterogeneous catalyst for ammonia synthesis. Bimetallic particles have been evidenced on the support surface by analytical electron microscopy; their main size is 4.0 nm and their Ni/Ru atomic ratio is practically the same as that of the starting cluster. The treatment of unsupported 1 with alcoholic KOH gives a black, conductive, amorphous alloy powder. In this case, the ultrafine and bimetallic nature of the individual particles was also revealed by analytical TEM investigations.

An AEM characterization of interfacial reactions in Al-MMC's reinforced with as-received and oxidized SiC particles

Interfacial reactions in MMC's can affect the mechanical properties of the material by changing the composition of the matrix and by forming new compounds at the matrix/ceramic interfaces. These reactions depend on the surface chemistry of the reinforcement, such as the presence of SiO2 produced by air oxidation of raw SiC particles. These reactions also depend on the processing route, particularly the thermal history of the material (fabrication or subsequent heat treatment or remelting). This paper presents results of an analytical transmission electron microscopy investigation of interfaces observed in Al-1% Mg alloys reinforced with 10 vol. % SiC particles. The composites were produced by the compocasting technique (semi-solid route) using as-received and oxidized SiC particles. The particles were oxidized in air at 1100°C for various times to produce oxidation levels ranging from 0-16 wt % measured by wet analysis.

Diffusion barrier properties of Ti/TiN investigated by transmission electron microscopy

Detailed analytical transmission electron microscopy investigations were performed on a well-known diffusion barrier system for very-large-scale integration metallization. It will be demonstrated that interfacial reactions are of great importance for the barrier mechanism. Both Ti and TiN act as diffusion barrier for the semiconductor and the metallization, respectively. For an aluminum-based metallization, TiN has a ‘‘spongelike’’ function due to its ability to absorb several amounts of aluminum at elevated temperatures and therefore inhibits diffusion towards the substrate. Ti acts for silicon as a compound forming barrier according to Nicolet’s classification [in Tungsten and Other Refractory Metals for Very Large Scale Integration Applications II, edited by E. K. Broadbent (Materials Research Society, Pittsburgh, 1987); pp. 19–26].