Marie-Hélène Berger

Fine diameter ceramic fibres

Two families of small diameter ceramic fibres exist. The oxide fibres, based on alumina and silica, which were initially produced as refractory insulation have also found use as reinforcements for light metal alloys. The production of SiC based fibres made possible the development of ceramic matrix composites. Improved understanding of the mechanisms which control the high temperature behaviour of these latter fibres has led to their evolution towards a near stoichiometric composition which results in strength retention at higher temperatures and lower creep rates. The SiC fibres will however be ultimately limited by oxidation so that there is an increasing interest in complex two phase oxide fibres composed of α-alumina and mullite as candidates for the reinforcement of ceramic matrices for use at very high temperatures. These fibres show low creep rates, comparable to the SiC based fibres but are revealed to be sensitive to alkaline contamination.

Microstructure and mechanical characteristics of alpha-alumina-based fibres

AbstractThe high-temperature mechanical behaviour of alumina-based ceramic fibres has been investigated by the comparison of a dense pure alumina fibre, a porous pure alumina fibre and a zirconia-reinforced dense fibre. Tensile and creep tests have been conducted up to 1300°C in air in parallel with microstructural investigations on the as-received and tested fibres. Room-temperature behaviour of the fibres is close to that of bulk materials having the same microstructure, but the fibre form allows higher failure stresses to be attained. High-temperature deformation of the three fibres is achieved by grain-boundary sliding (

Fluorescent nanodiamonds as a relevant tag for the assessment of alum adjuvant particle biodisposition

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Processing and structures of bi-phase oxide ceramic filaments

), and is accompanied by isotropic grain growth. The specific microstructures of each fibre induce differences in the creep threshold levels as a function of temperature and stress and also in creep rates and resistance to damage. Despite better resistance to creep and damage of the zirconia-reinforced fibre, alumina-based fibres are limited to applications below 1100°C. Grain boundaries are the principal cause of mechanical degradation at high temperature with these fibres.

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr, Al alloyed ODS steels

The microstructure and tensile properties of the as-received and heat-treated Nextel 720 fibre have been studied. During its fabrication the Nextel 720 fibre is pyrolysed at a temperature lower than 1400°C for a very short time which does not allow the microstructure to be stabilised. A pseudo-tetragonal metastable alumina rich mullite is formed which crystallises in the form of mosaic grains containing low angle boundaries. These mosaic grains enclose some rounded and elongated α-alumina grains. The evolution of the mullite to the stable orthorhombic symmetry is seen from 1200°C for post heat treatments lasting several hours. For longer heat treatments at 1200°C or at 1300°C and higher temperatures, the mosaic grains begin to recrystallise into single grains and the alumina rejected from the mullite contributes to the growth of elongated α-alumina grains which suggests diffusion through an intergranular liquid silicate phase. At 1400°C the mullite has the 3:2 composition and after 24 h the growth of the elongated α-alumina grains leads to a reduction of the room temperature tensile strengths. The α-alumina creation coupled to the phase transformation and dissolution of mullite leads to an increase of the Youngs modulus after heat treatments from 1200°C.

Finite element simulation of thermomechanical stress evolution in Cu/low-k interconnects during manufacturing and subsequent thermal cycling

BackgroundAluminum oxyhydroxide (alum) is a crystalline compound widely used as an immunologic adjuvant of vaccines. Concerns linked to alum particles have emerged following recognition of their causative role in the so-called macrophagic myofasciitis (MMF) lesion in patients with myalgic encephalomyelitis, revealing an unexpectedly long-lasting biopersistence of alum within immune cells and a fundamental misconception of its biodisposition. Evidence that aluminum-coated particles phagocytozed in the injected muscle and its draining lymph nodes can disseminate within phagocytes throughout the body and slowly accumulate in the brain further suggested that alum safety should be evaluated in the long term. However, lack of specific staining makes difficult the assessment of low quantities of bona fide alum adjuvant particles in tissues.MethodsWe explored the feasibility of using fluorescent functionalized nanodiamonds (mfNDs) as a permanent label of alum (Alhydrogel®). mfNDs have a specific and perfectly photostable fluorescence based on the presence within the diamond lattice of nitrogen-vacancy centers (NV centers). As the NV center does not bleach, it allows the microspectrometric detection of mfNDs at very low levels and in the long-term. We thus developed fluorescent nanodiamonds functionalized by hyperbranched polyglycerol (mfNDs) allowing good coupling and stability of alum:mfNDs (AluDia) complexes. Specificities of AluDia complexes were comparable to the whole reference vaccine (anti-hepatitis B vaccine) in terms of particle size and zeta potential.ResultsIn vivo, AluDia injection was followed by prompt phagocytosis and AluDia particles remained easily detectable by the specific signal of the fND particles in the injected muscle, draining lymph nodes, spleen, liver and brain. In vitro, mfNDs had low toxicity on THP-1 cells and AluDia showed cell toxicity similar to alum alone. Expectedly, AluDia elicited autophagy, and allowed highly specific detection of small amounts of alum in autophagosomes.ConclusionsThe fluorescent nanodiamond technology is able to overcome the limitations of previously used organic fluorophores, thus appearing as a choice methodology for studying distribution, persistence and long-term neurotoxicity of alum adjuvants and beyond of other types of nanoparticles.

FRACTURE PROCESSES IN OXIDE CERAMIC FIBRES

Abstract Since the major technological issues in cold spray are now controlled, the key parameter for successful coating relates to the powder. The influence of the main characteristics of the starting powder on the final coating properties remains rather unknown. This includes primarily metallurgical, morphological and physico–chemical characteristics of the particles. This article focuses on the so-called local approach to these characteristics to show that the current global approach is insufficient. The discussion is based on many examples of cold spray results, including recent work on Ag-based and Ta cold sprayed-coatings. This results in proposed specifications for powders which are claimed to be tailored for cold spray.

1.9 Oxide Fibers

Two phase ceramic filaments have been made by the co-extrusion of different green mono-filaments, each extruded mono-phase being produced from two differing material sources. The first was based on sol technology and the second based on oxide ceramic powder (+ thermoplastic binder) processing. Initially, mono-phase filaments were produced and re-extruded to produce bi-phase filaments which were then sintered to produce ceramics with aligned fine microstructures. The scale and fineness of the final sintered structure and the orientation of the grains in each phase (alumina or zirconia) have been determined by SEM and TEM and have been found to depend on the type of original precursor used, the powder grain size added and the temperature of heat treatment chosen. The study seems to demonstrate the viability of this novel multiple sol-gel based extrusion process to obtain optimised engineering structures for use at high temperatures.