Thesis (M.S.)--Brigham Young University. Dept. of Civil and Environmental Engineering, 2007. / Includes bibliographical references (p. 39-41).

L’augmentation des températures et des pressions de travail des centrales de production d’énergie représente un enjeu considérable en termes de réduction d’émission de polluants et d’augmentation d’efficacité énergétique. Pour cela, des matériaux plus performants par rapport à ceux employés actuellement doivent être utilisés afin de résister aux environnements sévères (hautes températures, 100% vapeur d’eau, pression…). Malgré les bonnes propriétés mécaniques de bon nombre d’aciers, l’application d’un revêtement protecteur capable de former une couche d’oxyde dense, protectrice et stable à haute température, est essentielle. Ainsi, dans le cadre du projet Européen FP-7 « POEMA », nous nous sommes intéressés aux mécanismes de formation de revêtements de diffusion d’aluminium sur des aciers ferrito-martensitiques et austénitiques fréquemment employés dans ce type de centrales. Des études sur la colmatation des fissures éventuellement parues durant l’aluminisation des aciers ont été également réalisées. Les mécanismes de dégradation des revêtements B2-(Fe (Ni))Al ont ensuite été étudiés sous 100% vapeur d’eau et sous air à 650 et 700°C jusqu’à 10000 heures et sous différentes pressions (1 et 300 bar). Nous avons démontré que la pré-oxydation induite lors de l’élaboration du revêtement permet de faire croître une fine couche protectrice de α-Al2O3 qui se développe au cours de l’oxydation. Cependant, les longues expositions sous fortes pressions de vapeur promeuvent la diffusion cationique de Fe, ceci donnant lieu à des petits nodules surfaciques d’oxyde. Enfin, les mécanismes de dissolution des revêtements avec un acide dilué ont été étudiés afin de montrer la possibilité de décaper puis de réaluminiser les matériaux initialement revêtus. / Higher energy efficiencies can be achieved by the increase of temperatures and pressures in power plant, thus allowing the reduction of the polluting emissions. Therefore, more efficient materials, compared to those currently employed, have to be used in order to resist to harsh environmental conditions (high temperatures, steam, pressure…). Despite of the good mechanical properties of many steels, the elaboration of a protective coating able to form a dense, protective and stable oxide layer at high temperature, is necessary. Thus, in the framework of the “POEMA” European project, the mechanisms of the formation of aluminium diffusion coatings were studied on ferritic-martensitic and austenitic stainless steels that are currently employed in power plants. The sealing of cracks eventually created during the aluminisation of steels was also studied. The degradation mechanisms of the B2-(Fe (Ni))Al coatings were then studied in steam and in air at 650 and 700°C up to 10000 hours and under different pressures (1 and 300 bar). We have demonstrated that the pre-oxidation induced during the elaboration of coatings grew a thin protective oxide scale composed of α-Al2O3 which developed further upon oxidation. Nevertheless, the long exposures under high steam pressures promoted the cationic diffusion of Fe, thus allowing the formation of small surface oxide nodules. Finally, the dissolution mechanisms of coatings with a diluted acid have been studied in order to demonstrate the possibility of strip and realuminise the materials initially coated.

Microchannel process technology (MPT) offers several advantages to the field of nanomanufacturing: 1) improved process control over very short time intervals owing to shorter diffusional distances; and 2) reduced reactor size due to high surface area to volume ratios and enhanced heat and mass transfer. The objective of this thesis was to consider how nanomaterials, produced in part using MPT, could be used to solve problems associated with the fabrication of MPT devices. Specifically, many MPT devices are produced using transient liquid-phase brazing involving an electroplated interlayer consisting of a brazing alloy designed for melting temperature suppression. Unfortunately, these alloys can form brittle secondary phases which significantly reduce bond strength. In contrast, prior efforts have shown that it is possible to leverage the size-dependent properties of nanomaterials to suppress brazing temperatures. In this prior work, thin films of off-the-shelf elemental nanoparticles were used as interlayers yielding joints with improved mechanical properties. In the present investigation, efforts have been made to characterize the synthesis and deposition of various elemental nanoparticle suspensions for use in the transient liquid-phase brazing of aluminum and stainless steel. Advances were used to demonstrate the nanoparticle-assisted diffusion brazing of a microchannel array. In the first section, a silver nanoparticle (AgNP) interlayer was produced for the diffusion brazing of heat exchanger aluminum. Efforts are made to examine the effect of braze filler particle size (~5 nm and ~50 nm) and processing parameters (heating rate: 5ºC/min and 25ºC/min; brazing temperature: 550ºC and 570ºC) on thin coupons of diffusion-brazed 3003 Al. A tensile strength of 69.7 MPa was achieved for a sample brazed at 570°C for 30 min under 1 MPa with an interlayer thickness of approximately 7 μm. Further suppression of the brazing temperature to 500ºC was achieved by sputtering a 1 µm thick layer of Cu before depositing a 5 nm thick film of AgNPs resulting in a lap shear strength of 45.3±0.2 MPa. In the middle section of this thesis, several techniques are investigated for the synthesis of sub 10 nm diameter nickel nanoparticles (NiNPs) to be used in the diffusion brazing of 316L stainless steel. The average NiNP size was varied from 9.2 nm to 3.9 nm based on the synthesis technique, solvent and reducing agent used. Conventional wet-chemical synthesis using NiCl₂.6H₂O in ethylene glycol (solvent) and N₂H₄.H₂O (reducing agent) resulted in the formation of 5.4 ± 0.9 nm NiNPs. Continuous flow synthesis using a microchannel T-mixer (barrel diameter of 521µm) and a 10 second residence time of reactants in a bath temperature of 130ºC resulted in a particle size of with 5.3 ± 1 nm. To make the synthesis safer and less energy intense, microwave heating was used along with less toxic Ni(CH₃CO₂)₂·4H₂O (nickel salt), propylene glycol (solvent) and NaPH₂O₂ (reducing agent) yielding 3.9 ± 0.8 nm diameter NiNPs. For the final section, nickel nanoparticles were synthesized using NiCl₂.6H₂O (nickel salt), de-ionized water (solvent), NaBH₄ (co-reducing agent), N₂H₄.H₂O (reducing agent) and polyvinylpyrolidone (capping agent) yielding 4.2 ± 0.6 nm NiNP. Several deposition techniques were investigated for controlling film thickness and uniformity in the diffusion brazing of 316L stainless steel (SS). Using in-house prepared NiNP and automated dispensing, a hermetic joint up to 70 psi (tested pressure) was obtained in 316L SS substrates under brazing conditions of 800ºC, 2 MPa and 30 min. Throughout the course of this thesis, techniques used for characterizing nanoparticles, films and joints included FT-IR, XRD, SEM, TEM, HRTEM, EDS, EPMA, DSC, mass spectrometry, and lap-shear testing. / Graduation date: 2012