Baptiste Julien

Transition-metal nitrides in η-carbide-type structures exhibit unusual bonding motifs and proximity to magnetic instabilities. Yet they remain unexplored in thin-film form due to the difficulty of stabilizing nitrogen-poor ternaries among competing phases. Here, we report the thin-film synthesis and phase-stability mapping of the η-nitride systems Fe–W–N and Fe–Mo–N. Amorphous Fe–M–N (M = W, Mo) combinatorial libraries deposited by reactive cosputtering crystallize upon rapid thermal annealing, enabling systematic identification of synthesis windows as a function of composition and annealing temperature. Using laboratory powder X-ray diffraction and synchrotron grazing incidence wide-angle X-ray scattering, we establish that Fe₃Mo₃N-based η-carbide phases form over a substantially broader compositional and thermal range than W-based compositions, where η structures are stabilized only when the films are Fe-rich. These trends are rationalized using mixed chemical-potential vs composition phase diagrams that capture the narrow nitrogen chemical-potential stability of η-nitrides. Magnetic measurements reveal that ferromagnetism is induced in Fe-rich Fe_3.54Mo_2.46N with a small exchange-bias-like response that is absent in Fe₃W₃N-based compositions, highlighting the sensitivity of magnetic behavior to modest deviations from stoichiometry. This work establishes practical thin-film synthesis routes for η-nitride materials and demonstrates how composition can be tuned to access emergent magnetic phenomena in these complex nitrides.

Les thermites, matériaux énergétiques composites à base de métaux et d'oxydes, sont des candidats prometteurs pour remplacer les matériaux conventionnels (CHNO) utilisés dans les systèmes pyrotechniques actuels. En effet, ils sont capables de générer une grande quantité d'énergie chimique en un temps très court (µs) suite à un stimulus électrique, mécanique ou thermique, tout en étant plus surs. Parmi les couples thermites nouvellement explorés, les nanothermites à base d'aluminium (Al) et d'oxyde de cuivre (CuO), tous deux dans des dimensions nanométriques, présentent un réel intérêt applicatif car ils sont intégrables en technologie couche mince pour être compatibles avec l'intégration ultime et remplacer les compositions primaires des initiateurs. De plus, ce couple est caractérisé par une enthalpie de réaction parmi les plus élevée (~4 kJ.g-1) tout en étant stable à température ambiante et peu sensible aux décharges électrostatiques. Au LAAS, un procédé de dépôt de nanothermites Al/CuO couche mince est disponible par pulvérisation cathodique, développé lors de thèses précédentes et aujourd'hui exploité pour fabriquer des microdispositifs d'initiation pour des applications diverses. Les exigences industrielles nécessitent de, non seulement contrôler les propriétés énergétiques (énergie initiation et température flamme) de ces matériaux, mais aussi de les moduler précisément pour répondre à la variété des cahiers de charges applicatifs. Dans ce contexte, cette thèse a eu pour objectif de consolider la compréhension des mécanismes d'initiation et de combustion dans les couches minces Al/CuO et d'étudier le rôle des contraintes qui sont inhérentes aux procédés de dépôt du matériau. Particulièrement, nous avons travaillé sur les multicouches Al/CuO classiquement utilisées dans les dispositifs, s'agissant d'empilement de 10-15 couches minces d'Al et de CuO (~100-200 nm d'épaisseur). Nous avons, d'une part, proposés des voies technologiques intéressantes et nouvelles pour moduler les propriétés énergétiques de ces matériaux, par l'ajout d'additifs comme des nanoparticules d'or ou micro-pores. Nous avons ensuite étudié l'influence de l'ajout de ces additifs sur les mécanismes d'initiation et de combustion. Ce travail a nécessité de corréler caractérisations macroscopiques (vitesse de combustion, temps d'initiation) et observations aux échelles microscopiques et même atomique en faisant appel à des techniques microscopie électronique hautement résolue pour comprendre la relation entre modification de la microstructure et propriétés énergétiques (initiation et combustion). Nous avons aussi élaboré des modèles physiques simples dès que nécessaire pour supporter les observations expérimentales.

Ternary transition metal (TM) nitrides have gained significant attention in thin film research due to their promising properties for a broad range of applications. Particularly, some of the ternary TM nitrides have been predicted to adopt layered structures that make them interesting for thermoelectric conversion and quantum materials applications. Unfortunately, synthesis of TM ternary nitride films by physical vapor deposition often favors disordered 3D structures rather than the predicted 2D-like layered structure. In this study, we investigate the structural interplay in the Sc-Ta-N ternary system using a combinatorial approach. Combinatorial libraries S c x T a 1 − x N are synthesized following a two-step method: First, deposit film precursors by cosputtering and then process the resulting 3D-structured samples with rapid thermal annealing. Synchrotron grazing-incidence wide-angle x-ray scattering on films annealed at 1200 ∘ C for 20 min leads to the nucleation of ScTaN 2 layered structure ( P 6 3 / m m c ) near stoichiometry. We find that the layered structure can accommodate large off-stoichiometry in the Ta-rich region ( x < 0.5 ), facilitated by the alloying with quasi-isostructural Ta 5 N 6 compound that exists on a composition tie line at x = 0 . While focusing on ScTaN 2 , we estimate the long-range order parameter in near-stoichiometric films to be 0.86, corresponding to a fraction of Sc/Ta antisites of 7%. Transport measurements on ScTaN 2 reveal a nearly temperature-independent high carrier density ( 10 21 c m − 3 ) , suggesting a heavily doped semiconductor or semimetallic character, consistent with a small positive Seebeck coefficient of +19 µV/K. The carrier mobility at 2 K is relatively small ( 9.5 c m 2 V − 1 s − 1 ) and the residual-resistivity ratio is minor, suggesting that electrical conduction is dominated by defects or disorder. Measured magnetoresistance suggests possible weak antilocalization at 2 K. This paper highlights the interplay between ScTaN 2 and Ta 5 N 6 crystal structures in stabilizing layered materials, emphasizes the importance of cation order/disorder for potential tunable alloys, and suggests that ScTaN 2 is a promising platform for exploring electronic properties.

Nitride perovskites ABN3 are an emerging and highly underexplored class of materials that are of interest due to their intriguing calculated ferroelectric, optoelectronic, and other functional properties. Incorporating novel A-site cations is one strategy to tune and expand such properties; for example, Gd3+ is compelling due to its large magnetic moment, potentially leading to multiferroic behavior. However, the theoretically predicted ground state of GdWN3 was a non-perovskite monoclinic structure. Here, we experimentally show that GdWN3−y crystallizes in a perovskite structure. High-throughput combinatorial sputtering with activated nitrogen is employed to synthesize thin films of Gd2−xWxN3−yOy with oxygen content y < 0.05. Ex situ annealing crystallizes a polycrystalline perovskite phase in a narrow composition window near x = 1. LeBail fits of synchrotron grazing incidence wide angle x-ray scattering data are consistent with a perovskite ground-state structure. Refined density functional theory calculations that included antiferromagnetic configurations confirm that the ground-state structure of GdWN3 is a distorted Pnma perovskite with antiferromagnetic ordering, in contrast to prior predictions. Initial property measurements find that GdWN3−y is paramagnetic down to T = 2 K with antiferromagnetic correlations and that the absorption onset depends on cation stoichiometry. This work provides an important path toward both the rapid expansion of the emerging family of nitride perovskites and understanding their potential multiferroic properties.

In this study, we demonstrate the effect of change of the sputtering power and the deposition pressure on the ignition and the combustion properties of Al/CuO reactive thin films. A reduced sputtering power of Al along with the deposition carried out at a higher-pressure result in a high-quality thin film showing a 200% improvement in the burn rate and a 50% drop in the ignition energy. This highlights the direct implication of the change of the process parameters on the responsivity and the reactivity of the reactive film while maintaining the Al and CuO thin-film integrity both crystallographically and chemically. Atomically resolved structural and chemical analyzes enabled us to qualitatively determine how the microstructural differences at the interface (thickness, stress level, delamination at high temperatures and intermixing) facilitate the Al and O migrations and impact the overall nano-thermite reactivity. We found that the deposition of CuO under low pressure produces well-defined and similar Al-CuO and CuO-Al interfaces with the least expected intermixing. Our investigations also showed that the magnitude of residual stress induced during the deposition plays a decisive role in influencing the overall nano-thermite reactivity. Higher is the magnitude of the tensile residual stress induced, stronger is the presence of gaseous oxygen at the interface. By contrast, high compressive interfacial stress aids in preserving the Al atoms for the main reaction while not getting expended in the interface thickening. Overall, this analysis helped in understanding the effect of change of deposition conditions on the reactivity of Al/CuO nanolaminates and several handles that may be pulled to optimize the process better by means of physical engineering of the interfaces.

It was experimentally found that silica and gold particles can modify the combustion properties of nanothermites but the exact role of the thermal properties of these additives on the propagating combustion front relative to other potential contributions remains unknown. Gold and silica particles of different sizes and volume loadings were added into aluminum/copper oxide thermites. Their effects on the flame front dynamics were investigated experimentally using microscopic dynamic imaging techniques and theoretically via a reaction model coupling mass and heat diffusion processes. A detailed theoretical analysis of the local temperature and thermal gradients at the vicinity of these two additives shows that highly conductive inclusions do not accelerate the combustion front while poor conductive inclusions result in the distortion of the flame front (corrugation), and therefore produce high thermal gradients (up to 10 ¹⁰ K.m ⁻¹ ) at the inclusion/host material interface. This results in an overall slowing down of the combustion front. These theoretical findings contradict the experimental observations in which a net increase of the flame front velocity was found when Au and SiO ₂ particles are added into the thermite. This leads to the conclusion that the faster burn rate observed experimentally cannot be fully associated with thermal effects only, but rather on chemical (catalytic) and/or mechanical mechanisms: formation of highly-stressed zones around the inclusion promoting the reactant mixing. One additional experiment in which physical SiO ₂ particles were replaced by voids (filled with Ar during experiment) to cancel the potential mechanical effects while preserving the thermal inhomogeneity in the thermite structure confirms the hypothesis that instead of pure thermal conduction, it is the mechanical mechanisms that dominate the propagation velocity in our specific Al/CuO multilayered films.

The paper reports a joint experimental/theoretical study on the aging of reactive Al/CuO nanolaminates, investigating both structural modifications and combustion properties of aged systems. We first show theoretically that the long-term storage (over several decades) in ambient temperature marginally affects nanolaminates structural properties with an increase in an interfacial layer of only 0.3 nm after 30 years. Then, we observe that the first thermal aging step occurs after 14 days at 200 °C, which corresponds to the replacement of the natural Al/CuO interfaces by a proper ~11 nm thick amorphous alumina. We show that this aging step does impact the nanolaminates structure, leading, for thin bilayer thicknesses, to a substantial loss of the energetic reservoir: considering a stoichiometric Al/CuO stack, the heat of reaction can be reduced by 6–40% depending on the bilayer thickness ranging from 150 nm (40%) to 1 µm (6%). The impact of such thermal aging (14 days at 200 °C) and interfacial modification on the initiation and combustion properties have been evaluated experimentally and theoretically. Varying Al to CuO ratio of nanolaminates from 1 to 3, we show that ignition time of aged systems does not increase over 10% at initiation power densities superior to 15 W·mm⁻². In contrast, burn rate can be greatly impacted depending on the bilayer thickness: annealing a stoichiometric nanolaminates with a bilayer thickness of 300 nm at 200 °C for 14 days lowers its burn rate by ~25%, whereas annealing a fuel rich nanolaminates with the same bilayer thickness under the same thermal conditions leads to a burn rate decrease of 20%. When bilayer thickness is greater than 500 nm, the burn rate is not really affected by the thermal aging. Finally, this paper also proposes a time–temperature diagram to perform accelerated thermal aging.