This thesis focuses on designing thermoplastic composites with high mechanical properties and a low processing temperature. Thermoplastic composites, which are used in this work, are composed of thermoplastic polyurethane (TPU) matrices and plain woven E-glass fabrics (GFs). TPUs were synthesised with large quantities of hard segments (HS), including 70% and 90%wt HS. The GF-TPU composites manufactured in this study have a melting point of around 175oC. As such, 180oC represents the processing temperature, which was used to produce GF-TPU composites. The influences of HS content and annealing treatment at 80oC on the thermal, dynamic mechanical and mechanical properties of TPU samples and GF-TPU composites with 25% fibre volume fraction (Vf) have been investigated. The highest crystallinity, storage modulus, Tg, yield strength, tensile strength and tensile modulus of all the TPU samples are seen in the TPU/90 samples annealed for 4 days. The TPU/90 samples display higher tensile properties than the TPU/70 and polypropylene (PP) samples, while the PP samples show the greatest elongation at break point. Furthermore, the tensile properties of the TPU/70 and TPU/90 samples are much higher than those of commercial TPUs. As such, annealed GF-TPU/90 composites with 25% Vf present the greatest dynamic mechanical, flexural, and tensile properties. GF-TPU/90 composites with 25% Vf show higher flexural strength than GF-PP composites or GF-polyamide 6 (PA6) composites with the same Vf. The effects of fibre surface treatments on the mechanical properties of GF and GF-TPU/70 composites with 25% Vf have also been studied in this investigation. GF treated with burn-off treatment is found to exhibit the lowest tensile properties. The interfacial adhesion between GF treated by NaOH for 0.5hrs and a TPU/70 matrix is greater than between GF treated by acetone for 5hrs and a TPU/70 matrix. Silanised GF presents greater tensile properties than desized GF. Thus, enhanced interfacial adhesion and tensile, flexural, ILSS and GIC properties are observed in the silanised GF-TPU/70 composites than in the desized GF-TPU/70 composites. GF-TPU/70 composites based on GFs treated by NaOH for 0.5hrs then sized with 0.15%wt. aminosilane display the greatest interfacial adhesion, flexural properties, ILSS and GIC, damage tolerance and impact-damage resistance. Conversely, the lowest interfacial adhesion, GIC, damage tolerance and impact-damage resistance are seen in the GF-PP composites based on 25% Vf as-received GF. There is a significant increase in the tensile and flexural properties of GF-TPU/90 composites with increasing the Vf from 25% to 50%. Moreover, the flexural strength of GF-TPU/90 composites with 50% Vf is not only higher than that of GF-EP composites or GF-vinyl ester composites with normalised 50% Vf, but is also much higher than that of GF-PP composites with 50% Vf. Despite this result, GF-TPU/90 composites with 50% Vf show the lowest fracture toughness, impact-damage resistance and damage tolerance, which are improved by adding 25% and 50%wt. of TPU/70 to the TPU/90 matrix. GF-TPU/90 composites based on a modified matrix have higher GIC, GIIC, impact-damage resistance and damage tolerance than GF-TPU/90 composites based on an unmodified matrix. The GIC, GIIC, impact-damage resistance and damage tolerance of GF-TPU/90 composites based on a modified matrix increase with increasing the percentage of TPU/70. Hence, the highest GIC, GIIC, impact-damage resistance and damage tolerance are seen in the GF-TPU/90 composites based on a modified matrix with 50%wt. of TPU/70.

Le travail présenté dans cette thèse a pour objectif d’étudier le dopage p des nanofils de ZnO par deux procédés différents : in-situ (durant la croissance) et ex-situ par diffusion des impuretés dans les nanofils à partir d’une phase gazeuse. Les nanofils de ZnO étudiés ont été élaborés par MOCVD et caractérisés par différentes techniques : MEB, MET, EDX, XPS, nano-Auger, DRX, SIMS, Sonde atomique tomographique, Raman, PL et I(V). Les tentatives de dopage ex-situ n’ont pas permis aux dopants (arsenic, phosphore et antimoine) de diffuser et de s’incorporer dans la matrice de ZnO. Ces derniers sont restés en surface. Néanmoins, ce procédé a mis en évidence l’importance du traitement de surface des nanofils, avec un recuit sous zinc, afin de réduire d'une part les défauts associés à la surface très réactive de ZnO, et d'autre part de diminuer la densité d’impuretés résiduelle de type n, condition préliminaire à l’incorporation de dopants de type p électriquement actifs. Concernant le dopage in-situ des nanofils de ZnO, le dopant (azote) s’incorpore plus facilement dans la matrice ZnO atteignant une concentration de l’ordre de 1020 at.cm-3. Les analyses de μ-Raman et de μ-PL montrent que l’azote est reparti de façon inhomogène le long des fils. Si les mesures optiques confirment la présence d'accepteurs dans le matériau après dopage, les mesures électriques révèlent toutefois que la conduction des fils dopés azote restent de type n. / The work presented in this thesis aims to study the p-doping of ZnO nanowires by two different methods: in-situ (during growth) and ex-situ by diffusion of impurities in the nanowires from a gas phase. ZnO nanowires were prepared by MOCVD and characterized by different techniques: SEM, TEM, EDX, XPS, nano-Auger, XRD, SIMS, atom probe tomography, Raman, PL and I (V). The ex-situ doping attempts have not allowed the dopants (arsenic, phosphorus and antimony) to be diffused and incorporated into the ZnO matrix. They still remained on the surface. However, this process has highlighted the importance of nanowire surface annealing treatment with zinc, in order to reduce i) the density of surface related defects, and ii) the density of residual impurities n-type. This is a precondition for the incorporation of electrically active p-type dopants. For in-situ doping of ZnO nanowires, the dopant (nitrogen) is incorporated more easily into the ZnO matrix, reaching a concentration of about 1020 at.cm-3. Analyses of μ-Raman and μ-PL show that nitrogen atoms are inhomogeneously incorporated along the nanowires. If optical measurements confirm the presence of acceptors in the material after doping, the electrical measurements show, however, that nitrogen doped nanowires remain n-type.

La thèse est inscrite au sein du projet ASTRID, qui est un démonstrateur technologique pour les réacteurs de quatrième génération (Gen-IV). Le premier matériau choisi pour constituer les gaines de cœur est un acier inoxydable austénitique stabilisé au titane (type 15Cr-15Ni Ti). L’écrouissage à froid des gaines permet la précipitation de nano-carbures de titane en service sur les dislocations, retardant ainsi les phénomènes de restauration par effet d’épinglage. En conditions accidentelles (T > 650°C), et plus particulièrement dans le cas d’une perte de réfrigérant primaire, le comportement en fluage de ces gaines est très mal connu. L’objectif des travaux de thèse est donc de déterminer les mécanismes de déformation et de rupture en fluage, entre 650°C et 950°C, de cet acier à l’état non irradié.Dans un premier temps, les microstructures d’échantillons après différents recuits ont été comparées afin d’étudier l’influence de la température sur les évolutions métallurgiques. L’étude de la précipitation et des cinétiques de restauration et de recristallisation, ont permis de dresser les évolutions microstructurales sans charge appliquée.En plus d’étudier le comportement en fluage uniaxial de l’acier à haute température, les caractérisations des éprouvettes après essais ont permis de déterminer les évolutions microstructurales au cours et après essais de fluage (contributions simultanées de la température et de la contrainte). La comparaison avec les microstructures obtenues après recuits a mis en évidence une accélération de la cinétique de recristallisation sous charge, rendant l’effet de la contrainte sur ces évolutions non négligeable.Après fluage sous air aux plus basses températures (650°C et 750°C), les fractographies présentent une rupture globalement transgranulaire avec certaines zones intergranulaires. Après fluage sous vide secondaire à plus hautes températures (850°C et 950°C), un fort amincissement des éprouvettes et une striction quasiment complète dans l’épaisseur ont été observés. Ce fort amincissement se traduit par un alignement de cupules, caractéristique de ruptures 100% ductiles à très haute température. / The ASTRID project aims at designing a fast-reactor prototype for the 4th generation of nuclear power plants. The material to be used for fuel cladding is a cold-worked austenitic stainless steel stabilized with titanium (15Cr-15Ni Ti type) and optimized in minor elements. This material was developed to limit recovery and irradiation-induced swelling and to improve microstructural stability and mechanical properties in normal operating conditions. In case of incidental situations (irradiation temperature > 650°C), the cladding might rapidly reach higher temperatures up to 950°C where its stability could be affected. The present work aims at improving knowledge and understanding of the microstructural evolution and creep behaviour of this steel at these temperatures (650°C-950°C).Microstructural characterizations of thermally-aged samples have been performed in order to study the effect of temperature on metallurgical evolutions (precipitation, recovery and recrystallization). A phenomenological model including recovery and recrystallization processes was set up to reproduce hardness measurements versus ageing time and temperatures.Isothermal creep tests up to 950°C under a wide range of stress levels allowed investigation of viscoplastic flow, microstructural evolution under stress and damage/failure processes. In order to evaluate the effect of high-temperature loading, microstructural characteristics of stress-free thermally-aged samples were compared with post-mortem examinations of creep specimens.At 650°C and 750°C the value of stress exponent is higher than 7. The main deformation mechanism during creep test is power-low creep, which is consistent with the results found in the literature.Beyond 850°C, the increase in dislocation mobility promotes recovery and recrystallization processes. As a consequence, a competition between work hardening due to viscoplastic deformation and softening due to dynamic recovery takes place. At 950°C, viscoplastic flow is strongly affected by recrystallization during creep test, especially in the tertiary stage. The comparison between microstructures of crept specimens and stress-free, thermally-aged samples leads to the conclusion that the recrystallization kinetics is accelerated by application of a mechanical loading.As for the fracture behaviour, creep tests under air environment at lower temperatures (650°C-750°C), led to predominating ductile fracture but some intergranular zones were observed on fracture surfaces. Creep tests under high vacuum at higher temperatures (850°C-950°C) lead to a high fracture elongation with a reduction of area up to 100%.