T. Vystavel

Nanosized iron clusters investigated with in situ transmission electron microscopy

Transmission electron microscopy is employed for investigating the structural stability of nanosized iron clusters as deposited and after in situ annealing treatments under high vacuum conditions. The thin iron oxide shell that is formed around the iron clusters (upon air exposure) is of the order of 2 nm surrounding a 5 nm core of body-centered-cubic (bcc) iron. The oxide shell breaks down upon annealing at relatively low temperatures (∼500 °C) leading to pure iron particles having a bcc crystal structure. Annealing of clusters, which are in contact, leads to their fusion and formation of larger clusters preserving their crystallographic structure and being free of any oxide shell. On the other hand, isolated clusters appear rather immobile (upon annealing). The truncated rhombic dodecahedron was found as the most probable shape of the clusters which differs from former theoretical predictions based on calculations of stable structural forms.

Coalescence aspects of cobalt nanoparticles during in situ high-temperature annealing

In this work we investigate the coalescence aspects of Co nanoparticles. It was observed that nanoparticles in contact with the substrate are relatively immobile, whereas those on top of other Co particles can rearrange themselves during high-temperature annealing and further coalesce. Indeed, similar size particles prior to coalescence come at close proximity forming an arc-shaped area, which leads to finite-size necking and thereafter to coalescence towards a single partially spherical particle. This is in contrast to the theoretical predictions where necking occurs following an initial pathway of a point contact. Moreover, it was shown that after necking a transient period of relatively fast coalescence occurs followed by a slower coalescence rate at constant speed towards a single particle with partial spherical shape. In addition, the coalescence is faster with decreasing particle size, where in the case of unequal size the smallest particle is mainly absorbed by an adjacent large one in an Ostwald ripening process.

Electron microscopic study on pyrolysis of CCA (chromium, copper and arsenic oxide)-treated wood

The effectiveness of pyrolysis as a possible technique for disposing of CCA (chromium, copper and arsenic oxide)-treated wood was studied. A CCA-treated sample given an extra heat treatment at 450 degreesC for 10 min was thoroughly investigated in order to establish the details of the reaction in which arsenic is captured in the pyrolysis residue prior to volatilization. Composition and structure of the metal compounds in the pyrolysis residue were examined by transmission electron microscopy (TEM). A large number of particles were found of variable diameter between 10 and 100 nm. The smaller ones were mostly spherical, sometimes faceted. The larger ones were lumpy. CCA compounds and their reaction products like Cr2As4O12 and As2O3 were identified in conventional TEM by selected area electron diffraction. In high resolution, the nanoparticles exhibited lattice fringes as indication of their monocrystalline character, fitting, e.g., the d(210) = 0.204 nm of Cr. The volatility of arsenic during pyrolysis of CCA-treated wood was measured by XRF (X-ray fluorescence) analysis at temperatures up to 500 degreesC and at times up to I h. Weight change and arsenic content of the pyrolysis residue were measured after dissolution in HNO3. More than 20% of arsenic was already lost at 300 degreesC, which may have been caused mainly by the volatilization of the unreacted arsenic compound after reduction of As(V) in As2O5 to As(III) in As2O3. Arsenic is probably released as As4O6, which is very difficult to capture and toxic. By an additional heat treatment, this reduction can be prevented and the dry separation of the metals may be allowed

Niobium nanoclusters studied with in situ transmission electron microscopy

Structural aspects of deposited niobium nanoclusters approximately 10 nm in size have been explored by means of high-resolution transmission electron microscopy. The niobium clusters have a bcc structure and a crystal habit of rhombic dodecahedron. In situ heating up to ∼800 °C revealed a resistance to high temperatures, in the sense that the cluster habit is preserved. However, the internal structural order of the clusters is altered due to formation of niobium oxide domains within the clusters. Coalescence does not occur even at the highest temperatures, which is attributed to the presence of facets and the occurrence of oxidation during heat treatment.

Controlling the induced anisotropy in soft magnetic films for high frequency applications

Summary form only given. Soft magnetic materials with controllable uniaxial anisotropy are required for future ultrahigh frequency applications. Nowadays, the attention is focused on FeXN (X = Ta, Zr, Al). systems due to their high saturation magnetisation, excellent magnetic softness and good corrosion resistance. The films can be obtained in an as-deposited nanocrystalline state by sputtering Fe rich alloys in a reactive atmosphere (Ar + N/sub 2/). Such materials can be magnetically soft when the grain size becomes smaller than the ferromagnetic exchange length (as explained by the random anisotropy model). The anisotropy is created by a bias magnetic field applied during deposition or during a subsequent annealing. One limiting factor restricting the frequency range of applications is the natural ferromagnetic resonance. In order to push this limit in the GHz range, films with a uniaxial anisotropy field in excess of 20 Oe are desired. We have obtained films with excellent soft magnetic properties and controllable in plane uniaxial magnetic anisotropy by depositing Fe N films at different substrate temperatures. The induced in-plane anisotropy increases with increasing nitrogen content. Increasing the Zr/Fe ratio from 1/99 to 3/97 has little influence on the induced anisotropy.

Microstructure of nanocrystalline FeZr(N)-films and their soft magnetic properties

Abstract The microstructure of nanocrystalline FeZr(N) films, deposited by DC sputtering in a Ar+N 2 atmosphere was studied in correlation with their soft magnetic properties. The micromagnetic properties of the films were investigated using the Fresnel mode of Lorentz microscopy.

In situ transmission electron microscopy studies on structural dynamics of transition metal nanoclusters

The structural stability of transition metal nanoclusters has been scrutinized with in situ transmission electron microscopy as a function of temperature. In particular iron, cobalt, niobium, and molybdenum clusters with diameters around 5 nm have been investigated. During exposure to air, a thin oxide shell with a thickness of 2 nm is formed around the iron and cobalt clusters, which is thermally unstable under moderate high vacuum annealing above 200 degrees C. Interestingly, niobium clusters oxidize only internally at higher temperatures without the formation of an oxide shell. They are unaffected under electron beam irradiation, whereas iron and cobalt undergo severe structural changes. Further, no cluster coalescence of niobium takes place, even during annealing up to 800 degrees C, whereas iron and cobalt clusters coalesce-after decomposition of the oxide, as long as the clusters are in close contact. In contrast to niobium, molybdenum clusters do not oxidize upon annealing; they are stable under electron beam irradiation and coalesce at temperatures higher than 800 degrees C. In all cases, the coalescence process indicates a strong influence of the local environment of the cluster.

Structural dynamics of gas-phase molybdenum nanoclusters: A transmission electron microscopy study

In this paper we study structural aspects of molybdenum clusters by transmission electron microscopy. The deposited clusters with sizes 4nm or larger show a body-centered crystal (bcc) structure. The clusters are self-assembled from smaller structural units and form cuboids with a typical size of 4nm or larger. With reducing cluster size, the face-centered crystal (fcc) structure appears due to dominance of surface energy minimization, while self-assembly into large cuboids with sizes up to 30nm is still observed. Our observation implies that the basic building blocks of large cuboids are actually not smaller cubes. Annealing leads to cluster coalescence at temperatures ∼800°C, with the crystal habit changing to truncated rhombic dodecahedron for isolated clusters, while the large cuboidally shaped particles become more faceted polyhedra.

Interface Microstructure and Adhesion of Zinc Coatings on TRIP Steels

Hot-dip galvanized transformation induced plasticity (TRIP) steel sheets were recently developed for automotive applications. The microstructure and the adhesion of zinc coated CMnSi TRIP steel alloyed with P were studied. The α-Zn coating adjacent to the steel substrate consists of a continuous η-Fe2Al5-xZnx inhibition layer with columnar ζ-FeZn13 intermetallic particles on top. Along the interface between the inhibition layer and the steel substrate Mn/Mn-P oxides were frequently observed. Although these oxides at the steel surface reduce the adhesion between the zinc coating and the TRIP steel, they do not cause any bare spots during galvanizing. Upon tensile deformation of the galvanized steel sheet, cracking along the α-zinc grain boundaries preceded fracture of the interface between the α-Zn layer and the inhibition layer. After 4 % deformation the average interface crack length increased linearly with the applied strain. This interface fracture was strongly influenced by the crystalline orientation of the α-Zn grains.

Catalytic Graphitization of Wood‐Based Carbons with Alumina by Pulse Current Heating

Abstract Japanese cedar was preheated at 500°C and subsequently mixed with 40 µm Al2O3 particles. A pulse current heating method was used for a 5‐min carbonization step under a pressure of 50 MPa in order to promote the graphitization at temperatures between 2000 and 2200°C. The samples were analyzed in an analytical transmission electron microscope equipped with a GATAN Imaging Filter, in a high resolution transmission electron microscope and in a scanning electron microscope. Transformation into well‐ordered graphite could be enforced by the intermediate reaction of Al2O3 and carbon to plate‐like Al4C3. This latter compound dissociates under the proper CO pressure and temperature into Al vapor and solid graphite. The addition of Al2O3 and the pressurized heating device improve the graphitization in comparison with the effect of temperature alone. The electron microscopic observations are supported by XPS and XRD spectra.