F.D. Tichelaar

Growth kinetics and mechanisms of aluminum-oxide films formed by thermal oxidation of aluminum

The growth kinetics and mechanisms of thin aluminum-oxide films formed by the dry, thermal oxidation of a bare Al(431) substrate at a partial oxygen pressure of 1.33×10−4 Pa in the temperature range of 373–773 K were studied using x-ray photoelectron spectroscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (T⩽573 K), an amorphous oxide film develops that attains a limiting (uniform) thickness. At high temperatures (T>573 K), growth is not impeded at a limiting thickness. Kinetic analysis established the occurrences of two different oxide-film growth regimes: an initial regime of very fast oxide-film growth and a second, much slower oxidation stage that is observed only at T>573 K. These results could be discussed in terms of electric-field controlled, interstitial, outward transport of Al cations through a close packing of O anions in the amo...

The effect of heat treatment on the structure and abrasive wear resistance of autocatalytic NiP and NiP–SiC coatings

Abstract A systematic study on the relationships between the structure and abrasive wear resistance of autocatalytic nickel–phosphorus coatings (particle-free and SiC composite) with different phosphorus contents (i.e. 2.5–10.2 wt.% P) and under different thermal treatments (i.e. 300, 400 and 500°C) has been performed. The phase structure, composition and properties of the coatings could be controlled by changing the phosphorus content of the nickel–phosphorus matrix and by performing thermal treatments. The improvement in abrasive wear behaviour of the nanocrystalline (i.e. ≤6.0 wt.% P) coatings with heat treatment temperature up to 400°C was related to (i) the formation of a metastable equilibrium phase and (ii) precipitation of Ni 3 P compound. At higher thermal treatments (500°C), a change in the deformation mechanisms (Orowan mechanism) determined by the coarsening of Ni 3 P precipitates was associated with the decrease in abrasive wear resistance of the coatings. In addition, for the NiP–SiC coatings after annealing at 500°C, Ni 3 Si was formed and the adhesion between the reinforcement and the matrix was enhanced.

Structure and morphology of aluminium-oxide films formed by thermal oxidation of aluminium

AbstractThe structure and morphology of thin aluminium-oxide films grown by the dry, thermal oxidation of a bare Al (431) substrateat a partial oxygen pressure of 1.33 =10 Pa in the temperature range of 373–773 K were studied using X-ray photoelectron y4 spectroscopy and high resolution electron microscopy.The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species.At lowtemperatures ( T (573 K ), the mobility of the oxygen species is very low, and an amorphous oxide film of relatively uniform,limiting thickness develops.X-ray photoelectron spectroscopic analysis established the occurrence of a surface-oxide species atthe very surface of these films.At higher temperatures( T )573 K ) an initially amorphous oxide film of less uniform thicknessdevelops that gradually transforms into crystalline g-Al O .At these temperatures an amorphous-to- 23 23 g-Al O transition oxidephase occurs. 2002 Elsevier Science B.V. All rights reserved.

Materials science applications of HREELS in near edge structure analysis and low-energy loss spectroscopy

New experiments made possible with a commercial transmission electron microscope (TEM) equipped with a high-resolution electron energy loss spectrometer (EELS) are presented. With this commercial system, a 100 meV energy resolution using a sub 2 nm probe or 500 meV at a 0.20 nm probe are possible, in combination with other modern techniques available for TEMs. In this paper a number of explorative examples of the first results are shown. The benefit of the increased resolution for detecting more details in near edge structures are shown for the Ti K edge in TiO(2) (brookite) and for the N K edge in cubic and hexagonal GaN. The bandgap of GaN is studied in both crystal structures, as well as the dependency of the low-loss spectrum on the momentum transfer direction in diffraction mode.

Crystal structure and band gap determination of HfO2 thin films

Valence electron energy loss spectroscopy (VEELS) and high resolution transmission electron microscopy (HRTEM) are performed on three different HfO2 thin films grown on Si (001) by chemical vapor deposition (CVD) or atomic layer deposition (ALD). For each sample the band gap (Eg) is determined by low-loss EELS analysis. The Eg values are then correlated with the crystal structure and the chemical properties of the films obtained by HRTEM images and VEELS line scans, respectively. They are discussed in comparison to both experimental and theoretical results published in literature. The HfO2 ALD film capped with poly-Si exhibits the largest band gap (Eg = 5.9±0.5?eV), as a consequence of its nanocrystallized orthorhombic structure. The large grains with a monoclinic structure formed in the HfO2 ALD film capped with Ge and the carbon contamination induced by the precursors in the HfO2 CVD film capped with Al2O3 are identified to be the main features responsible for lower band gap values (Eg = 5.25±0.5 and 4.3±0.5?eV respectively).

An electron microscopical study on the growth of TiO2–Ag antibacterial coatings on Ti6Al7Nb biomedical alloy

This research was aimed at investigating the growth mechanism of TiO(2)-Ag antibacterial coatings during plasma electrolytic oxidation (PEO) of Ti6Al7Nb biomedical alloy in an electrolyte based on calcium acetate/calcium glycerophosphate bearing Ag nanoparticles. The focus was on the mechanism of incorporation of Ag nanoparticles, their distribution and chemical composition within the porous coatings using high resolution transmission electron microscopy (HRTEM) and scanning electron microscopy (SEM) imaging techniques combined with energy dispersive X-ray spectroscopy (EDX) for chemical analyses. The PEO coatings were grown using different oxidation times, 10, 30, 60, 90, 120, 180, 240 and 300 s. The electron microscopy results confirmed the formation of a porous coating with incorporated Ag nanoparticles from the initial stages of oxidation (i.e. 10 s), with further Ag incorporation as the PEO process was continued for longer durations. The Ag nanoparticles were embedded in the dense oxide layer, fused into the pore walls and on the surface of the coatings without any change in their morphology or chemistry as detected by HRTEM, SEM and EDX. Ag seems to be delivered to the sites of coating growth (where dielectric breakdown occurs) through different transport pathways, i.e. open pores, cracks and short-circuit channels.

Cathodic Corrosion: A Quick, Clean, and Versatile Method for the Synthesis of Metallic Nanoparticles

A simple and effective method for the synthesis of nanoparticles is reported based on extreme cathodic polarization of a metal, formation of cation-stabilized metal anions, and their agglomeration (see picture). The improved catalytic activity of these nanoparticles in the oxidation of carbon monoxide as well as methanol is shown using platinum.

Structure of continuously cast Ni-based superalloy Inconel 713C

Abstract In this work, we characterised the structure of continuously cast Ni-based superalloy IN 713C (∅ 10 mm, water cooled Cu–Be mould, argon atmosphere) using several microstructural characterization techniques (LM, SEM, TEM, EDS, XRD). The structure consisted of columnar dendritic γ-grains with apparently fully coherent and rather uniformly distributed γ′ precipitates (size ∼50 nm), primary MC carbide and MC/γ eutectic. The eutectic MC carbide contained a considerable amount of Cr: (Nb0.4Mo0.25Ti0.18Cr0.16)C resulting in a decrease in the MC lattice constant from 0.441 to 0.435 nm. Due to higher cooling rates at continuous casting the microstructural constituents were much finer than in the as-received and DTA samples. In addition, the continuously cast specimens did not contain the γ/γ′ eutectic and some minor phases despite stronger segregation of solute elements. The partition coefficients of solute elements Ti, Mo, Nb, Al, Cr, Fe and Ni in the continuously cast IN 713C were 0.55, 0.82, 0.46, 0.99, 0.96, 1.02 and 1.03, respectively.

Cathodic Corrosion as a Facile and Effective Method To Prepare Clean Metal Alloy Nanoparticles

The cathodic corrosion method described here is a simple, clean, and fast way of synthesizing nanoalloys with high catalytic performance. Using a series of Pt-Rh alloys as an example, we show that this one-step method can convert a bulk alloy electrode into an aqueous suspension of nanoparticles, retaining the composition and crystal lattice structure of the starting alloy. Compared to pure metals, these alloy nanocatalysts are more active toward CO and methanol oxidation and nitrate reduction reactions. Nanoparticles made of PtRu, PtIr, PtNi, AuCo, AuCu, and FeCo bulk alloys demonstrate the universality of this synthesis method.

Solid-state reactions in low-phosphorus autocatalytic NiP–SiC coatings

Abstract Composite NiP–SiC coatings with a nanocrystalline nickel matrix produced by autocatalytic deposition were subjected to phase transformations by isochronal and isothermal heating. Isochronal heating to 700°C (heating rate 10°C min−1) revealed three exothermic effects (reactions) occurring in the coatings. X-Ray diffraction showed that only the second and third thermal effects were associated with phase transformations. The first peak was associated with chemical and structural relaxation and a slight grain growth of the matrix. The second reaction was attributed to the nucleation and growth of Ni3P precipitates, while the third was related to the complete dissolution of SiC particles in the matrix with the formation of Ni3Si and carbon. A similar trend in the phase formation sequence was observed by isothermal heating. However, the formation of nickel silicides at the SiC/matrix interface occurred at lower temperatures (i.e. 500°C for 1 h). The formation of silicides appeared to be governed by the diffusion of nickel atoms into the SiC lattice, as indicated by transmission electron microscopy.