Arne Janssen

Experimental observation of an interface-controlled pseudomorphic replacement reaction in a natural crystalline pyrochlore

Abstract Pyrochlore-type (A2B2O6O’) ceramics are considered for the immobilization of highly radioactive waste. Understanding the alteration process of such potential nuclear waste form materials in aqueous media is critical for the prediction of their long-term stability in a nuclear repository. Current models on pyrochlore alteration are based on a diffusion-controlled hydration and ion exchange process. However, we present results of a hydrothermal experiment at 200 °C with a natural, polycrystalline pyrochlore and 18O-enriched aqueous solution, which are not compatible with a process based on solid-state diffusion. TOF-SIMS and confocal μ-Raman mapping of the run product revealed the occurrence of 18O-enriched alteration zones with sharp chemical gradients to relict unreacted areas when compared to the extent of the alteration zones. The data are consistent with a pseudomorphic reaction that involves the dissolution of the pyrochlore parent accompanied by the simultaneous reprecipitation of a defect pyrochlore at a moving dissolution-reprecipitation front.

The experimental replacement of ilmenite by rutile in HCl solutions

Abstract To determine the mechanism of acid-leaching of ilmenite to ultimately forming rutile, we have carried out an experimental study of ilmenite alteration in autoclaves at 150ºC in HCl solutions. The resulting products were studied by X-ray diffraction, scanning electron microscopy, electron microprobe and Raman spectroscopy. In some experiments the solution was initially enriched in 18O and the distribution of the isotope in the alteration products mapped from the frequency shift of cation−oxygen stretching bands in the Raman spectra. The alteration begins at the original ilmenite crystal surface and has also taken place along an intricate branching network of fractures in the ilmenite, generated by the reaction. Element-distribution maps and chemical analyses of the reaction product within the fractures show marked depletion in Fe and Mn and a relative enrichment of Ti. Chemical analyses however, do not correspond to any stoichiometric composition, and may represent mixtures of TiO2 and Fe2O3. The fracturing is possibly driven by volume changes associated with dissolution of ilmenite and simultaneous reprecipitation of the product phases (including rutile) from an interfacial solution along an inward moving dissolution-reprecipitation front. Raman spectroscopy shows that the 18O is incorporated in the rutile structure during the recrystallization. Throughout the alteration process, the original morphology of the ilmenite is preserved although the product is highly porous. The rutile inherits crystallographic information from the parent ilmenite, resulting in a triply-twinned rutile microstructure. The results indicate that the ilmenite is replaced directly by rutile without the formation of any intermediate reaction products. The reaction is described in terms of an interface-coupled dissolution-precipitation mechanism.

STEM-EDX tomography of bimetallic nanoparticles: A methodological investigation

This paper presents an investigation of the limitations and optimisation of energy dispersive X-ray (EDX) tomography within the scanning transmission electron microscope, focussing on application of the technique to characterising the 3D elemental distribution of bimetallic AgAu nanoparticles. The detector collection efficiency when using a standard tomography holder is characterised using a tomographic data set from a single nanoparticle and compared to a standard low background double tilt holder. Optical depth profiling is used to investigate the angles and origin of detector shadowing as a function of specimen field of view. A novel time-varied acquisition scheme is described to compensate for variations in the intensity of spectrum images at each sample tilt. Finally, the ability of EDX spectrum images to satisfy the projection requirement for nanoparticle samples is discussed, with consideration of the effect of absorption and shadowing variations.

Synthesis of transuranium-based nanocrystals via the thermal decomposition of actinyl nitrates

In this communication, we report on the use of easily accessible actinide precursors to synthesize actinide oxide nanocrystals. Uranyl and neptunyl nitrates have been successfully used as starting materials in the non-aqueous synthesis of AnO2 (An = U, Np) nanocrystals. This communication reports for the first time on the formation of transuranium-based nanocrystals.

Ferrihydrite Formation: The Role of Fe13 Keggin Clusters

Ferrihydrite is the most common iron oxyhydroxide found in soil and is a key sequester of contaminants in the environment. Ferrihydrite formation is also a common component of many treatment processes for cleanup of industrial effluents. Here we characterize ferrihydrite formation during the titration of an acidic ferric nitrate solution with NaOH. In situ SAXS measurements supported by ex situ TEM indicate that initially Fe13 Keggin clusters (radius ∼ 0.45 nm) form in solution at pH 0.12-1.5 and are persistent for at least 18 days. The Fe13 clusters begin to aggregate above ∼ pH 1, initially forming highly linear structures. Above pH ∼ 2 densification of the aggregates occurs in conjunction with precipitation of low molecular weight Fe(III) species (e.g., monomers, dimers) to form mass fractal aggregates of ferrihydrite nanoparticles (∼3 nm) in which the Fe13 Keggin motif is preserved. SAXS analysis indicates the ferrihydrite particles have a core-shell structure consisting of a Keggin center surrounded by a Fe-depleted shell, supporting the surface depleted model of ferrihydrite. Overall, we present the first direct evidence for the role of Fe13 clusters in the pathway of ferrihydrite formation during base hydrolysis, showing clear structural continuity from isolated Fe13 Keggins to the ferrihydrite particle structure. The results have direct relevance to the fundamental understanding of ferrihydrite formation in environmental, engineered, and industrial processes.

XEDS and EELS in the TEM at Atmospheric Pressure and High Temperature

Recent progress with environmental cell and microscope design has enabled in situ imaging studies within gaseous environments inside the (scanning) transmission electron microscope ((S)TEM) to become increasingly routine [1-2]. In contrast, complementary elemental information is more challenging to obtain in situ. Electron energy loss spectroscopy (EELS) has been reported inside an environmental TEM [3,4], although only at modest pressure (below ~30 mbar). Closed-cell design specimen holders, in which the specimen and the gaseous environment are sealed from the high vacuum of the TEM by two SiN windows, allow much higher pressures to be reached (up to ~1 bar). However, these environmental cells have two major drawbacks which limit their analytical capabilities: 1) X-ray energy dispersive spectroscopy (XEDS) is challenging, as the walls of the cell generally shadow the detectors, preventing the collection of characteristic X-rays; 2) EELS is challenging as the two 50 nm thick SiN windows cause multiple scattering, which limits signal-to-background ratio in the core-loss EEL spectra and thus significantly degrades performance [5].

Liquid In Situ Analytical Electron Microscopy: Examining SCC Precursor Events for Type 304 Stainless Steel in H 2 O

In situ transmission electron microscopy (TEM) has become an increasingly important research area in materials science with the advent of unique microscope platforms and a range of specialized in situ specimen holders. The ability to image and perform x-ray energy dispersive spectroscopy (XEDS) analyses of metals in liquids are particularly important for detailed study of the metal-environment interactions with specific microstructural features. In particular, these capabilities now make it feasible to explore what has been termed “SCC precursor phenomena” – that is, those sub-micron scale reactions between an alloy of interest and the environment. This topic is especially timely for this Swann Memorial Symposium, as Peter Swann’s activities in this area, particularly in the 1970’s concerning the initiation and early stages of transgranular SCC in austenitic stainless steels, were clearly prescient. In this study, we have used liquid cell TEM with XEDS to explore the “precursor phenomena” that can promote the development of defect initiation in Type 304 austenitic stainless steel. For this work, the FIB lift-out technique was used to extract specimens to be studied in the liquid cell TEM specimen holder. This technique has been applied to examine the localised dissolution of MnS inclusions, which can lead to pit initiation.

Characterisation of high temperature refractory ceramics for nuclear applications

The ternary oxide ceramic system UO2-ZrO2-FeO is a refractory system that is of great relevance to the nuclear industry as it represents one of the main systems resulting from the interaction of the Zircaloy cladding, the UO2 fuel and the structural elements of a nuclear reactor. It is particularly the high temperature properties that require investigation; that is, when substantial overheating of the nuclear core occurs and interactions can lead to its degradation, melting and result in a severe nuclear accident. There has been much work on the UO2-ZrO2 system and also on the ternary system with FeO but there is still a need to examine 2 further aspects; firstly the effect of sub-oxidized systems, the UO2-Zr and FeO-Zr systems, and secondly the effect of Fe/Zr or Fe/U ratios on the melting point of the U-Zr-Fe oxide system. Samples of UO2-Zr and UO2-ZrO2-FeO were fabricated at ITU and then characterized by optical microscopy (OM) and X-ray diffraction to determine the ceramics structure and verify the composition. Thereafter the samples are to be melted by laser flash heating and their liquidus and solidus temperatures determined by pyrometry. This programme is currently ongoing. The frozen samples, after testing, were then sectioned, polished and the molten zone micro-analytically examined by OM & SEM-EDS in order to determine its structure and composition and to compare with the existing phase diagrams. Examples of results from these systems will be given. Finally, a reacted Zr-FeO thermite mixture was examined, which had been used to generate high temperatures during tests of reactor melt-concrete interactions. The aim was to assess the reaction and estimate the heat generation from this novel technique. These results allow verification or improvement of the phase diagram and are of primary importance as input to models used to predict materials interactions in a severe nuclear accident.

In situ Analytical TEM of Ilmenite Reduction in Hydrogen

Ilmenite (FeTiO3) is an important mineral, formed in igneous and metamorphic rocks, and is the main feedstock for the titanium industry. Ilmenite is also an attractive and inexpensive oxygen carrier for chemical-looping combustion (CLC), one of the most promising technologies to accomplish CO2 capture in power plants [1]. The technique uses solid metal oxides as an oxygen carrier, which transfers oxygen from the combustion air to the fuel. The metal oxide reacts with a hydrocarbon fuel in the fuel reactor, to produce CO2 and H2O while reducing the metal oxide. However, detailed information about structural and chemical changes of ilmenite during the reduction process are still not available. The understanding of structural and chemical relationships at the nanometer scale during the oxygen transfer is essential because it can be help to understand the exact reduction mechanism of the oxygen carrier, and hence optimise the combustion process.

Practical Aspects of Electrochemical Corrosion Measurements during in situ Analytical TEM of Austenitic Stainless Steel in Aqueous Media

The capability to perform liquid in situ transmission electron microscopy (TEM) experiments provides an unprecedented opportunity to examine the real-time processes of physical and chemical/electrochemical reactions during the interaction between metal surfaces and liquid environments. This work describes the requisite steps to make the technique fully analytical, from sample preparation, through modifications of the electrodes, characterization of electrolytes, and finally to electrochemical corrosion experiments comparing in situ TEM to conventional bulk cell and microcell configurations.