Les alliages germanium-étain et silicium-germanium-étain: croissance, propriétés structurales et stabilité thermique

Les nanotubes de carbone ont des propriétés mécaniques et électriques très intéressantes pour plusieurs applications en électronique. Ils sont très résistants à la déformation et peuvent être d’excellents conducteurs ou semi-conducteurs. Toutefois, manipuler les nanotubes individuellement pour construire des dispositifs structurés demeure très difficile. Il n’existe pas encore de méthode permettant de contrôler à la fois les propriétés électriques, l’orientation et le positionnement spatial d’un ensemble de nanotubes. Produire des réseaux désordonnés de nanotubes est par contre beaucoup plus facile, et ces systèmes possèdent de plus une bonne conductivité électrique qui les rend très intéressants, notamment comme matériaux d’électrodes transparentes et flexibles. Il y a trois principales méthodes de fabrication employées pour produire des réseaux de nanotubes : le dépôt de solution, la croissance sur substrat et l’enchâssement dans une matrice de polymères. Le dépôt de solution peut engendrer des réseaux de densités diverses sur une variété de substrats. La croissance directe sur substrat permet quant à elle de produire des réseaux très propres sur des substrats tels le SiO2. De son côté, l’enchâssement dans une matrice de polymères permet de produire des volumes composites contenant des quantités variables de nanotubes. Beaucoup de paramètres comme la longueur des tubes, leur orientation ou leur tortuosité influencent cependant les propriétés de ces réseaux et la présence de désordre structural complique la compréhension de leurs interactions. Prévoir les propriétés d’un réseau comme la conductivité à partir de quelques caractéristiques comme la taille des tubes et leur densité peut être difficile. Cette tâche devient d’autant plus complexe si l’on veut maintenant identifier les paramètres qui vont permettre d’optimiser les performances d’un dispositif contenant ce matériau. Nous avons choisi d’aborder le problème des réseaux de nanotubes de carbone en développant une série d’outils numériques qui sont principalement basés sur la méthode Monte-Carlo. Nous tenons compte d’un grand nombre de paramètres pour décrire les caractéristiques des réseaux, ce qui nous permet une représentation plus fiable de réseaux réels ainsi qu’une grande polyvalence pour le choix des constituants des réseaux pouvant être simulés. Les outils que nous avons développés, regroupés dans le logiciel RPH-HPN pour Réseaux percolatifs hybrides - Hybrid Percolation Networks, permettent la construction des réseaux aléatoires, détectent les contacts entre les tubes, traduisent les systèmes en circuits électriques équivalents et calculent les propriétés globales.----------Abstract Carbon nanotubes have very interesting mechanical and electrical properties for various applications in electronics. They are highly resistant to deformation and can be excellent conductors or semiconductors. However, manipulating individual nanotubes to build structured devices remains very difficult. There is no method for controlling all of the electrical properties, the orientation and the spatial positioning of a large number of nanotubes. The fabrication of disordered networks of nanotubes is much easier, and these systems have a good electrical conductivity which makes them very interesting, especially as materials of transparent and flexible electrodes. There are three main methods of production used to make networks of nanotubes: the solution deposition, the direct growth on substrate and the embedding in a polymer matrix. The solution deposition method can form networks of various densities on a variety of substrates, the direct growth of nanotubes allows the creation of very clean networks on substrates such as SiO2, and the embedding in a polymer matrix can give composite volumes containing varying amounts of nanotubes. Many parameters such as the length of the tubes, their orientation or their tortuosity influence the properties of these networks and the presence of structural disorder complicates the understanding of their interactions. Predicting the properties of a network, such as conductivity, from a few characteristics such as size and density of the tubes can be difficult. This task becomes even more complex if one wants to identify the parameters that will optimize the performance of a device containing the material. We chose to address the carbon nanotube networks problem by developing a series of computer simulation tools that are mainly based on the Monte Carlo method. We take into account a large number of parameters to describe the characteristics of the networks, which allows for a more reliable representation of real networks as well as versatility in the choice of network components that can be simulated. The tools we have developed, grouped together in the RPH-HPN software Réseaux percolatifs hybrides - Hybrid Percolation Networks, construct random networks, detect contact between the tubes, translate the systems to equivalent electrical circuits and calculate global properties. An infinity of networks can have the same basic characteristics (size, diameter, etc.) and therefore the properties of a particular random network are not necessarily representative of the average properties of all networks. To obtain those general properties, we simulate a large number of random networks with the same basic characteristics and the average of the quantities is determined

Étude de la dynamique du quench dans différentes architectures de rubans supraconducteurs à haute température critique

La découverte du premier supraconducteur à haute température critique (HTS) en 1986 marque une grande étape dans l’histoire des matériaux supraconducteurs. Cette découverte a permis de révéler la famille des cuprates, qui inclut les supraconducteurs avec la température critique la plus élevée, envisagés industriellement. Opérés à température plus élevée que les supraconducteurs à basse température critique (LTS), certains HTS peuvent être refroidis à l’azote liquide (relativement bon marché) contrairement aux LTS, qui nécessitent des liquides de refroidissement plus dispendieux comme l’hélium liquide. Le REBaCuO (où l’acronyme RE signifie : terre rare) fait partie de la famille des cuprates et est envisagé dans la fabrication d’électro-aimants pour des applications à fort champ magnétique notamment les accélérateurs de particules et le confinement magnétique. Pour des applications de ce genre, le REBaCuO est typiquement élaboré sous forme de couches minces sur un ruban métallique flexible afin d’être bobiné. Lors de l’utilisation de REBaCuO sous forme de ruban, il n’est pas rare d’observer des zones du ruban qui perdent leurs propriétés supraconductrices en raison d’une variation locale du courant critique. À fort courant, ces zones (appelée points chauds) se mettent à s’élargir à une certaine vitesse appelée la vitesse de propagation de la zone normale (NZPV). Si la NZPV est trop faible et que les points chauds ne sont pas détectés à temps, la température locale peut augmenter de façon à endommager le ruban. Un problème des rubans HTS comme le REBaCuO est que leur NZPV est très faible (0.1−10 cm/s à 77 K) comparativement aux LTS (100−1000 cm/s à 4 K), ce qui les rend plus vulnérables aux points chauds. Une solution à ce problème est l’architecture de ruban appelée current flow diverter (CFD), développée à Polytechnique Montréal, qui permet d’augmenter la NZPV de ces rubans. L’augmentation de la NZPV dans ces rubans permet une détection plus rapide des points chauds, ce qui les protège mieux. Cette architecture consiste en un ruban HTS commercial (Ag/HTS/substrat) où l’interface résistive entre le supraconducteur et l’argent (Ag) a été modifiée de telle sorte qu’elle est élevée au centre du ruban et faible sur les bords. Néanmoins, pour le moment, la technique de fabrication de l’architecture CFD n’a été montrée que sur de petits échantillons (12 cm) et est difficilement réalisable par les manufacturiers de ruban HTS.----------Abstract The discovery of the first high-temperature superconductor (HTS) in 1986, was a big step in the history of superconductivity. This discovery led quickly to unveil the cuprate family, which includes materials expected to be used in all kinds of industrial applications. Used at higher temperature than low temperature superconductors (LTS), HTS can be cooled with liquid nitrogen, a cheaper coolant compared to liquid helium used for LTS. The REBaCuO (where RE stands for rare earth elements) is a member of the cuprate family and is expected to be used in high-field applications such as particle accelerators and magnetic confinement. For those applications, REBaCuO is grown as a thin film on a flexible metallic tape. This type of tape is called coated conductors. Due to the variation of the critical current along the HTS tape, it is common that some parts of the tape lose their superconducting properties. At high current, these normal zones (called hot spots) tend to grow at constant speed called the normal zone propagation velocity (NZPV). If the NZPV is low and the hot spots are not detected in time, a degradation of the tape can occur due to the high temperature generated by resistive heating at the hot spots locations. One problem of the HTS tapes like the REBaCuO is their low NZPV (0.1−10 cm/s at 77 K) compared to the LTS (100−1000 cm/s at 4 K). Because of that, the HTS tapes are more vulnerable to damages caused by hot spots. A solution to this problem is the current flow diverter (CFD) architecture, developed at Polytechnique Montréal, which accelerates the NZPV, allowing a faster detection of hot spots. This architecture, consists in a commercial HTS tape (Ag/HTS/substrate) where the interfacial resistance between the silver (Ag) and the HTS has been modified in order to have a high value in the center of the tape and a low value on the sides of the tape. However, so far, this architecture has been realized only on short samples (12 cm) and is hard to implement in an industrial environment

Concepts of Static vs. Dynamic Current Transfer Length in 2G HTS coated conductors with a Current Flow Diverter Architecture

This paper uses both experimental and numerical approaches to revisit the concept of current transfer length (CTL) in second-generation high-temperature superconductor coated conductors with a current flow diverter (CFD) architecture. The CFD architecture has been implemented on eight commercial coated conductors samples from THEVA. In order to measure the 2-D current distribution in the silver stabilizer layer of the samples, we first used a custom-made array of 120 voltage taps to measure the surface potential distribution. Then, the so-called "static" CTL ($\lambda_s$) was extracted using a semi-analytical model that fitted well the experimental data. As defined in this paper, the static CTL on a 2-D domain is a generalization of the definition commonly used in literature. In addition, we used a 3-D finite element model to simulate the normal zone propagation in our CFD samples, in order to quantify their "dynamic" CTL ($\lambda_d$), a new concept introduced in this paper and defined as the CTL observed during the propagation of a quenched region. The results show that, for a CFD architecture, $\lambda_d$ is always larger than $\lambda_s$, whereas $\lambda_d = \lambda_s$ when the interfacial resistance between the stabilizer and the superconductor layers is the same everywhere. We proved that the cause of these different behaviors is related to the shape of the normal zone, which is curved for the CFD architecture, and rectangular otherwise. Finally, we showed that the NZPV is proportional to $\lambda_d$, not with $\lambda_s$, which suggests that the dynamic CTL $\lambda_d$ is the most general definition of the CTL and should always be used when current crowding and non-uniform heat generation occurs around a normal zone.Comment: 11 pages, 10 figure

Chemical Solution Deposition of Insulating Yttria Nanolayers as Current Flow Diverter in Superconducting GdBa2Cu3O7-δ Coated Conductors

The primary benefit of a metallic stabilization/shunt in high temperature superconductor (HTS) coated conductors (CCs) is to prevent joule heating damage by providing an alternative path for the current flow during the HTS normal state transition (i.e., quench). However, the shunt presence in combination with unavoidable fluctuations in the critical current (I c) of the HTS film can develop a localized quench along the CC's length if the operational current is kept close to I c. This scenario, also known as the hot-spot regime, can lead to the rupture of the CC if the local quench does not propagate fast enough. The current flow diverter (CFD) is the CC architecture concept that has proven to increase the conductor's robustness against a hot-spot regime by simply boosting the quench velocity in the CC, which avoids the shunt compromise in some applications. This work investigates a practical manufacturing route for incorporating the CFD architecture in a reel-to-reel system via the preparation of yttrium oxide (Y2O3) as an insulating thin nanolayer (∼100 nm) on top of a GdBa2Cu3O7 (GdBCO) superconductor. Chemical solution deposition (CSD) using ink jet printing (IJP) is shown to be a suitable manufacturing approach. Two sequences of the experimental steps have been investigated, where oxygenation of the GdBCO layer is performed after or before the solution deposition and the Y2O3 nanolayer thermal treatment formation step. A correlated analysis of the microstructure, in situ oxygenation kinetics, and superconducting properties of the Ag/Y2O3/GdBCO trilayer processed under different conditions shows that a new customized functional CC can be prepared. The successful achievement of the CFD effect in the case of the preoxygenated customized CC was confirmed by measuring the current transfer length, thus demonstrating the effectiveness of the CSD-IJP as a processing method.We acknowledge the funding of this research by FASTGRID Project (EU-H2020, 721019); the Projects COACHSUPENERGY (MAT2014-51778-C2-1-R) and SUMATE (RTI2018-095853-BC21 and RTI2018-095853-B-C22) from the Spanish Ministry of Economy and Competitiveness, which were cofunded by the European Regional Development Fund; and the Project 2017-SGR 753 from Generalitat de Catalunya and the COST Action NANOCOHYBRI (CA16218). ICMAB authors also acknowledge the Center of Excellence awards Severo Ochoa SEV-2015-0496 and CEX2019-000917-SWith funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

Normal zone propagation in various REBCO tape architectures

The normal zone propagation velocity (NZPV) of three families of REBCO tape architectures designed for superconducting fault current limiters and to be used in high voltage direct current transmission systems has been measured experimentally in liquid nitrogen at atmospheric pressure. The measured NZPVs span more than three orders of magnitude depending on the tape architectures. Numerical simulations based on finite elements allow us to reproduce the experiments well. The dynamic current transfer length (CTL) extracted from the numerical simulations was found to be the dominating characteristic length determining the NZPV instead of the thermal diffusion length. We therefore propose a simple analytical model, whose key parameters are the dynamic CTL, the heat capacity and the resistive losses in the metallic layers, to calculate the NZPV.The authors acknowledge the funding of this research by FASTGRID Project (EU-H2020, 721019), the Projects COACHSUPENERGY (MAT2014-51778-C2-1-R), SUMATE (RTI2018-095853-BC21 and RTI2018-095853-B-C22) from the Spanish Ministry of Economy and Competitiveness which were cofunded by the European Regional Development Fund, the Project 2017-SGR 753 from Generalitat de Catalunya and the COST Action NANOCOHYBRI (CA16218). Polytechnique Montréal authors also acknowledge NSERC (Canada), FRQNT (Québec), the RQMP infrastructure and CMC microsystems for financial support. ICMAB authors also acknowledge the Center of Excellence awards Severo Ochoa SEV-2015-0496 and CEX2019-000917-S.With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).Peer reviewe

A physics-guided recurrent machine learning model for long-time prediction of nonlinear partial differential equation

International audienc

A physics-guided recurrent machine learning model for long-time prediction of nonlinear partial differential equation

International audienc