39 research outputs found
CuSCN Nanowires as Electrodes for p-Type Quantum Dot Sensitized Solar Cells: Charge Transfer Dynamics and Alumina Passivation
Quantum dot sensitized solar cells (QDSSCs) are a promising photovoltaic technology due to their low cost and simplicity of fabrication. Most QDSSCs have an n-type configuration with electron injection from QDs into TiO2, which generally leads to unbalanced charge transport (slower hole transfer rate) limiting their efficiency and stability. We have previously demonstrated that p-type (inverted) QD sensitized cells have the potential to solve this problem. Here we show for the first time that electrodeposited CuSCN nanowires can be used as a p-type nanostructured electrode for p-QDSSCs. We demonstrate their efficient sensitization by heavy metal free CuInSxSe2-x quantum dots. Photophysical studies show efficient and fast hole injection from the excited QDs into the CuSCN nanowires. The transfer rate is strongly time dependent but the average rate of 2.5 Ă 109 sâ1 is much faster than in previously studied sensitized systems based on NiO. Moreover, we have developed an original experiment allowing us to calculate independently the rates of charge injection and QD regeneration by the electrolyte and thus to determine which of these processes occurs first. The average QD regeneration rate (1.3 Ă 109 sâ1) is in the same range as the hole injection rate, resulting in an overall balanced charge separation process. To reduce recombination in the sensitized systems and improve their stability, the CuSCN nanowires were coated with thin conformal layers of Al2O3 using atomic layer deposition (ALD) and fully characterized by XPS and EDX. We demonstrate that the alumina layer protects the surface of CuSCN nanowires, reduces charge recombination, and increases the overall charge transfer rate up to 1.5 times depending on the thickness of the deposited Al2O3 layer
Converging and diverging burn rates in North American boreal forests from the Little Ice Age to the present
Warning. This article contains terms, descriptions, and opinions used for historical context that may be culturally sensitive for some readers. Background. Understanding drivers of boreal forest dynamics supports adaptation strategies in the context of climate change. Aims. We aimed to understand how burn rates varied since the early 1700s in North American boreal forests. Methods. We used 16 fire-history study sites distributed across such forests and investigated variation in burn rates for the historical period spanning 1700-1990. These were benchmarked against recent burn rates estimated for the modern period spanning 1980-2020 using various data sources. Key results. Burn rates during the historical period for most sites showed a declining trend, particularly during the early to mid 1900s. Compared to the historical period, the modern period showed less variable and lower burn rates across sites. Mean burn rates during the modern period presented divergent trends among eastern versus northwestern sites, with increasing trends in mean burn rates in most northwestern North American sites. Conclusions. The synchronicity of trends suggests that large spatial patterns of atmospheric conditions drove burn rates in addition to regional changes in land use like fire exclusion and suppression. Implications. Low burn rates in eastern Canadian boreal forests may continue unless climate change overrides the capacity to suppress fire.Peer reviewe
CuSCN nanowires as electrodes for p-type quantum dot sensitized solar cells : charge transfer dynamics and alumina passivation
Funding: European Research Council (grant number 321305) and the EPSRC (grant numbers EP/L017008/1 and EP/M506631/1). IDWS is a Royal Society Wolfson Research Merit award holder.Quantum dot sensitized solar cells (QDSSCs) are a promising photovoltaic technology due to their low cost and simplicity of fabrication. Most QDSSCs have an n-type configuration with electron injection from QDs into TiO2, which generally leads to unbalanced charge transport (slower hole transfer rate) limiting their efficiency and stability. We have previously demonstrated that p-type (inverted) QD sensitized cells have the potential to solve this problem. Here we show for the first time that electrodeposited CuSCN nanowires can be used as a p-type nanostructured electrode for p-QDSSCs. We demonstrate their efficient sensitization by heavy metal free CuInSxSe2-x quantum dots. Photophysical studies show efficient and fast hole injection from the excited QDs into the CuSCN nanowires. The transfer rate is strongly time dependent but the average rate of 2.5 x 109 s-1 is much faster than in previously studied sensitized systems based on NiO. Moreover, we have developed an original experiment allowing us to calculate independently the rates of charge injection and QD regeneration by the electrolyte and thus to determine which of these processes occurs first. The average QD regeneration rate (1.33 x 109 s-1 ) is in the same range as the hole injection rate, resulting in an overall balanced charge separation process. To reduce recombination in the sensitized systems and improve their stability, the CuSCN nanowires were coated with thin conformal layers of Al2O3 using atomic layer deposition (ALD) and fully characterized by XPS and EDX. We demonstrate that the alumina layer protects the surface of CuSCN nanowires, reduces charge recombination and increases the overall charge transfer rate up to 1.5 times depending on the thickness of the deposited Al2O3 layer.PostprintPeer reviewe
Assessing changes in global fire regimes
PAGES, Past Global Changes, is funded by the Swiss Academy of Sciences and the Chinese Academy of Sciences and supported in kind by the University of Bern, Switzerland. Financial support was provided by the U.S. National Science Foundation award numbers 1916565, EAR-2011439, and EAR-2012123. Additional support was provided by the Utah Department of Natural Resources Watershed Restoration Initiative. SSS was supported by Brigham Young University Graduate Studies. MS was supported by National Science Centre, Poland (grant no. 2018/31/B/ST10/02498 and 2021/41/B/ST10/00060). JCA was supported by the European Unionâs Horizon 2020 research and innovation program under the Marie SkĆodowska-Curie grant agreement No 101026211. PF contributed within the framework of the FCT-funded project no. UIDB/04033/2020. SGAF acknowledges support from Trond Mohn Stiftelse (TMS) and University of Bergen for the startup grant âTMS2022STG03â. JMP participation in this research was supported by the Forest Research Centre, a research unit funded by Fundação para a CiĂȘncia e a Tecnologia I.P. (FCT), Portugal (UIDB/00239/2020). A.-LD acknowledge PAGES, PICS CNRS 06484 project, CNRS-INSU, RĂ©gion Nouvelle-Aquitaine, University of Bordeaux DRI and INQUA for workshop support.Background The global human footprint has fundamentally altered wildfire regimes, creating serious consequences for human health, biodiversity, and climate. However, it remains difficult to project how long-term interactions among land use, management, and climate change will affect fire behavior, representing a key knowledge gap for sustainable management. We used expert assessment to combine opinions about past and future fire regimes from 99 wildfire researchers. We asked for quantitative and qualitative assessments of the frequency, type, and implications of fire regime change from the beginning of the Holocene through the year 2300. Results Respondents indicated some direct human influence on wildfire since at leastâ~â12,000 years BP, though natural climate variability remained the dominant driver of fire regime change until around 5,000 years BP, for most study regions. Responses suggested a ten-fold increase in the frequency of fire regime change during the last 250 years compared with the rest of the Holocene, corresponding first with the intensification and extensification of land use and later with anthropogenic climate change. Looking to the future, fire regimes were predicted to intensify, with increases in frequency, severity, and size in all biomes except grassland ecosystems. Fire regimes showed different climate sensitivities across biomes, but the likelihood of fire regime change increased with higher warming scenarios for all biomes. Biodiversity, carbon storage, and other ecosystem services were predicted to decrease for most biomes under higher emission scenarios. We present recommendations for adaptation and mitigation under emerging fire regimes, while recognizing that management options are constrained under higher emission scenarios. Conclusion The influence of humans on wildfire regimes has increased over the last two centuries. The perspective gained from past fires should be considered in land and fire management strategies, but novel fire behavior is likely given the unprecedented human disruption of plant communities, climate, and other factors. Future fire regimes are likely to degrade key ecosystem services, unless climate change is aggressively mitigated. Expert assessment complements empirical data and modeling, providing a broader perspective of fire science to inform decision making and future research priorities.Peer reviewe
RĂ©gimes des feux holocĂšne, contemporain et futur aux Territoires du Nord-Ouest, Canada
Climate change impacts the boreal ecosystem through modifications of vegetation structure, composition, distribution and productivity. These changes alter the internal functioning of forests by disrupting the dynamics of natural disturbances such as fire, notably their frequency and size. Extreme weather events in recent decades have resulted in very large areas burned during some years, altering forest landscapes at northern latitudes in Canada. Large fires accounting for the majority of burned areas release massive amounts of carbon into the atmosphere and have major health consequences for people in exposed communities. They also limit the ability of Indigenous people to maintain their traditional activities by reducing ecosystem services they traditionally access. Climate projections suggest that large fires may be increasingly frequent in the coming decades, which in turn could affect forests, climate, and human societies. However, models remain uncertain and questions persist, particularly about the frequency and magnitude of extreme weather events that facilitate the onset of large forest fires.This thesis contributes to improving our understanding of the environmental drivers that have determined the dynamic of recent and past fire regimes in the boreal forest of northwestern Canada, in order to predict the interactions between climate, vegetation and future wildfires. This information will help managers and local communities anticipate future fire risk in response to climate change, in order to adapt practices and land use accordingly to limit the potentially negative effects of large forest fires. Chapter II of this thesis aimed to reconstruct the fire regime since 1965 on the territory of the TĆı̚chÇ«; First Nation in the Northwest Territories (NWT), in Canada. This territory experienced one of the largest forest fires across Canada in 2014. We determine the main climatic and ecological conditions having contributed to extreme wildfire seasons (EWY) during the past few decades on the TĆı̚chÇ«; territory, and we verify that the year 2014 was one of those EWY. We fixed climatic risk thresholds conducive to EWY occurrence, based on temperatures and soil organic horizon dryness indices measured during the fire season. The objective of the chapter III was to reconstruct the fire regime and vegetation dynamics on the TĆı̚chÇ«; territory during the Holocene (11,700 years) and to analyze our reconstructions with regards to past climatic conditions at regional scale, in order to determine the main drivers of fire occurrence and size over time. Historical reconstructions showed that warm and dry conditions, as well as fuel availability and forest composition, were the main drivers leading to large and severe wildfires in the short and long term. In chapter IV, we simulated the risk of future EWY based on projections from different climate models coupled with two radiative forcing scenarios (RCP4.5 and RCP8.5). We used the LPJ-LMfire model to simulate the dynamics of tree biomass and burn rate over the 21st century in the NWT and the TĆı̚chÇ«; territory. Model simulations showed that future burn rates will be primarily modulated by increasing temperatures and changing vegetation dynamics, particularly black spruce.Le changement climatique affecte les Ă©cosystĂšmes forestiers borĂ©aux par des modifications de la structure, de la composition, de la rĂ©partition et de la productivitĂ© de la vĂ©gĂ©tation. Ces changements altĂšrent le fonctionnement interne des forĂȘts en modifiant la dynamique des perturbations naturelles comme les feux, notamment leur frĂ©quence et leur taille. Les Ă©vĂ©nements mĂ©tĂ©orologiques extrĂȘmes des derniĂšres dĂ©cennies ont donnĂ© lieu Ă de trĂšs grandes superficies brĂ»lĂ©es lors de certaines annĂ©es, altĂ©rant les paysages forestiers, notamment ceux des latitudes nordiques du Canada. Les grands feux Ă lâorigine de la majoritĂ© des superficies brĂ»lĂ©es libĂšrent dâimportantes quantitĂ©s de carbone vers lâatmosphĂšre et ils ont des consĂ©quences sanitaires majeures pour les populations des communautĂ©s exposĂ©es. Ils limitent Ă©galement la capacitĂ© des peuples autochtones Ă maintenir leurs activitĂ©s traditionnelles en rĂ©duisant les services Ă©cosystĂ©miques auxquels ils accĂšdent traditionnellement. Les projections climatiques suggĂšrent une multiplication des grands feux au cours des prochaines dĂ©cennies, ce qui pourrait affecter davantage la forĂȘt, le climat et les sociĂ©tĂ©s humaines. Cependant, les modĂšles restent incertains et des interrogations persistent, notamment sur la frĂ©quence et lâampleur des Ă©vĂ©nements mĂ©tĂ©orologiques extrĂȘmes qui facilitent le dĂ©clenchement des grands feux de forĂȘt. Cette thĂšse contribue Ă lâamĂ©lioration de notre comprĂ©hension des facteurs environnementaux qui ont dĂ©terminĂ© la dynamique du rĂ©gime des feux rĂ©cent et passĂ© dans la forĂȘt borĂ©ale du nord-ouest canadien, dans le but de modĂ©liser les interactions entre le climat, la vĂ©gĂ©tation et les feux de forĂȘt futurs. Ces informations aideront les gestionnaires et les communautĂ©s locales Ă anticiper le risque de feu futur en rĂ©ponse au changement climatique, afin dâadapter les pratiques et les usages du territoire en consĂ©quence pour limiter les effets potentiellement nĂ©gatifs des grands feux de forĂȘt. Le chapitre II de cette thĂšse a visĂ© Ă reconstituer le rĂ©gime des feux depuis 1965 sur le territoire du Peuple TĆı̚chÇ«;, situĂ© aux Territoires du Nord-Ouest (TNO) au Canada. Ce territoire a subi lâun des principaux grands feux de forĂȘts enregistrĂ©s Ă lâĂ©chelle du Canada en 2014. Nous dĂ©terminons quelles conditions climatiques et Ă©cologiques majeures ont contribuĂ© au dĂ©veloppement des saisons de feu extrĂȘmes (EWY) durant les derniĂšres dĂ©cennies sur le territoire TĆı̚chÇ«;, et nous vĂ©rifions que lâannĂ©e 2014 faisait bien partie de ces EWY. Nous avons mis en Ă©vidence des seuils climatiques propices Ă lâoccurrence des EWY, en fonction des tempĂ©ratures et des indices de sĂ©cheresse de lâhorizon organique du sol mesurĂ©s au cours de la saison de feu. Lâobjectif du chapitre III a Ă©tĂ© de reconstituer le rĂ©gime des feux et la vĂ©gĂ©tation du territoire TĆı̚chÇ«; au cours de lâHolocĂšne (11 700 ans) et de mettre en perspective nos rĂ©sultats avec des reconstitutions du climat passĂ© Ă lâĂ©chelle rĂ©gionale afin de dĂ©terminer les principaux facteurs qui ont contrĂŽlĂ© lâoccurrence et la taille des feux au cours du temps. Les reconstitutions palĂ©oĂ©cologiques ont montrĂ© que les conditions chaudes et sĂšches, ainsi que la disponibilitĂ© en combustible et la composition des forĂȘts, ont Ă©tĂ© les principaux facteurs ayant menĂ© Ă de grands feux sĂ©vĂšres Ă court et Ă long terme. Dans le chapitre IV, nous avons modĂ©lisĂ© le risque de EWY futurs en fonction des projections de diffĂ©rents modĂšles climatiques couplĂ©s Ă deux scĂ©narios de forçage radiatif (RCP4.5 et RCP8.5). Nous avons utilisĂ© le modĂšle LPJ-LMfire afin de simuler la dynamique de la biomasse arborescente et du taux de brĂ»lage au cours du XXIe siĂšcle aux TNO et sur le territoire TĆı̚chÇ«;. Les simulations issues du modĂšle LPJ-LMfire ont montrĂ© que les taux de brĂ»lage futurs seront principalement modulĂ©s par lâaugmentation des tempĂ©ratures et lâĂ©volution de la dynamique de la vĂ©gĂ©tation, notamment de lâĂ©pinette noire
Nanostructures de silicium par croissance chimique catalysée : une plate-forme pour des applications micro-supercondensateurs
Supercapacitors are electrochemical energy storage devices which have been recently developed, and possess intermediate performances between dielectric capacitors and batteries. These components exhibit interesting power and energy densities, combined with an exceptional cycle life and an easy miniaturization. Supercapacitors are thus envisioned as energy storage solutions for electronic micro-devices, such as autonomous micro-sensors or implantable medical devices.In recent studies, CVD nanostructured silicon proved to be an excellent electrode material candidate for micro-supercapacitor applications. Bottom-up synthesis allows an exceptional control of the morphology and electrical properties of the obtained silicon nano-wires and nano-trees. Moreover, the nanostructured electrodes possess superior electrochemical and temperature stability. These arguments lead to consider silicon as an excellent platform for micro-supercapacitors applications.This PhD thesis details various ways to improve and use silicon nano-wires and nano-trees. The nanostructures have been subjected to a systematic optimization study, yielding a significant increase of the electrochemical performances of the electrodes, compared to previously published studies. In addition, surface functionalization using thin ALD alumina layers permitted a considerable increase of the supercapacitor voltage window and an improved electrochemical stability. Finally, âon-chipâ nanostructure growth, and temperature stability studies of the device were conducted, opening a broad field of improvements and potential uses for these silicon nanostructures.Les supercondensateurs sont des dispositifs de stockage Ă©lectrochimique de lâĂ©nergie ayant Ă©tĂ© rĂ©cemment mis au point et possĂ©dant des performances intermĂ©diaires entre les condensateurs diĂ©lectriques et les batteries. Leurs intĂ©ressantes valeurs de densitĂ© dâĂ©nergie et de puissance, conjuguĂ©es Ă leur excellente durĂ©e de vie et Ă leur miniaturisation facilitĂ© rendent ces composants prometteurs pour des micro-dispositifs Ă©lectroniques, tels des micro-capteurs autonomes ou des implants mĂ©dicaux.Le silicium nanostructurĂ© par CVD a prouvĂ© ĂȘtre un remarquable matĂ©riau dâĂ©lectrode de supercondensateur, pour des applications miniaturisĂ©es, lors de rĂ©cents travaux. Lâexcellent contrĂŽle de la morphologie et des propriĂ©tĂ©s Ă©lectroniques permis par la synthĂšse montante de nano-fils et nano-arbres de silicium, ainsi que la grande stabilitĂ© Ă©lectrochimique et thermique de ce matĂ©riau font des nanostructures de silicium obtenues par synthĂšse montante une excellente plate-forme pour des micro-supercondensateurs.La prĂ©sente thĂšse sâattache Ă explorer plusieurs voies dâamĂ©lioration et dâutilisation des nano-fils et nano-arbres de silicium. Une Ă©tude systĂ©matique de lâoptimisation des nanostructures a Ă©tĂ© conduite, permettant dâamĂ©liorer largement les performances prĂ©cĂ©demment Ă©tablies. Ensuite, une fonctionnalisation par des couches minces dâalumines utilisant la technique dâALD a permis dâaccroitre largement la plage de tensions dâutilisation des supercondensateurs, et dâaugmenter leur stabilitĂ© Ă©lectrochimique. Enfin, la croissance « sur-puce », ainsi que lâĂ©tude de la stabilitĂ© en tempĂ©rature des dispositifs ont Ă©tĂ© effectuĂ©es, laissant entrevoir dâimportantes perspectives dâapplications
Holocene, contemporary and future fire regimes in the Northwest Territories, Canada
Le changement climatique affecte les Ă©cosystĂšmes forestiers borĂ©aux par des modifications de la structure, de la composition, de la rĂ©partition et de la productivitĂ© de la vĂ©gĂ©tation. Ces changements altĂšrent le fonctionnement interne des forĂȘts en modifiant la dynamique des perturbations naturelles comme les feux, notamment leur frĂ©quence et leur taille. Les Ă©vĂ©nements mĂ©tĂ©orologiques extrĂȘmes des derniĂšres dĂ©cennies ont donnĂ© lieu Ă de trĂšs grandes superficies brĂ»lĂ©es lors de certaines annĂ©es, altĂ©rant les paysages forestiers, notamment ceux des latitudes nordiques du Canada. Les grands feux Ă lâorigine de la majoritĂ© des superficies brĂ»lĂ©es libĂšrent dâimportantes quantitĂ©s de carbone vers lâatmosphĂšre et ils ont des consĂ©quences sanitaires majeures pour les populations des communautĂ©s exposĂ©es. Ils limitent Ă©galement la capacitĂ© des peuples autochtones Ă maintenir leurs activitĂ©s traditionnelles en rĂ©duisant les services Ă©cosystĂ©miques auxquels ils accĂšdent traditionnellement. Les projections climatiques suggĂšrent une multiplication des grands feux au cours des prochaines dĂ©cennies, ce qui pourrait affecter davantage la forĂȘt, le climat et les sociĂ©tĂ©s humaines. Cependant, les modĂšles restent incertains et des interrogations persistent, notamment sur la frĂ©quence et lâampleur des Ă©vĂ©nements mĂ©tĂ©orologiques extrĂȘmes qui facilitent le dĂ©clenchement des grands feux de forĂȘt. Cette thĂšse contribue Ă lâamĂ©lioration de notre comprĂ©hension des facteurs environnementaux qui ont dĂ©terminĂ© la dynamique du rĂ©gime des feux rĂ©cent et passĂ© dans la forĂȘt borĂ©ale du nord-ouest canadien, dans le but de modĂ©liser les interactions entre le climat, la vĂ©gĂ©tation et les feux de forĂȘt futurs. Ces informations aideront les gestionnaires et les communautĂ©s locales Ă anticiper le risque de feu futur en rĂ©ponse au changement climatique, afin dâadapter les pratiques et les usages du territoire en consĂ©quence pour limiter les effets potentiellement nĂ©gatifs des grands feux de forĂȘt. Le chapitre II de cette thĂšse a visĂ© Ă reconstituer le rĂ©gime des feux depuis 1965 sur le territoire du Peuple TĆı̚chÇ«;, situĂ© aux Territoires du Nord-Ouest (TNO) au Canada. Ce territoire a subi lâun des principaux grands feux de forĂȘts enregistrĂ©s Ă lâĂ©chelle du Canada en 2014. Nous dĂ©terminons quelles conditions climatiques et Ă©cologiques majeures ont contribuĂ© au dĂ©veloppement des saisons de feu extrĂȘmes (EWY) durant les derniĂšres dĂ©cennies sur le territoire TĆı̚chÇ«;, et nous vĂ©rifions que lâannĂ©e 2014 faisait bien partie de ces EWY. Nous avons mis en Ă©vidence des seuils climatiques propices Ă lâoccurrence des EWY, en fonction des tempĂ©ratures et des indices de sĂ©cheresse de lâhorizon organique du sol mesurĂ©s au cours de la saison de feu. Lâobjectif du chapitre III a Ă©tĂ© de reconstituer le rĂ©gime des feux et la vĂ©gĂ©tation du territoire TĆı̚chÇ«; au cours de lâHolocĂšne (11 700 ans) et de mettre en perspective nos rĂ©sultats avec des reconstitutions du climat passĂ© Ă lâĂ©chelle rĂ©gionale afin de dĂ©terminer les principaux facteurs qui ont contrĂŽlĂ© lâoccurrence et la taille des feux au cours du temps. Les reconstitutions palĂ©oĂ©cologiques ont montrĂ© que les conditions chaudes et sĂšches, ainsi que la disponibilitĂ© en combustible et la composition des forĂȘts, ont Ă©tĂ© les principaux facteurs ayant menĂ© Ă de grands feux sĂ©vĂšres Ă court et Ă long terme. Dans le chapitre IV, nous avons modĂ©lisĂ© le risque de EWY futurs en fonction des projections de diffĂ©rents modĂšles climatiques couplĂ©s Ă deux scĂ©narios de forçage radiatif (RCP4.5 et RCP8.5). Nous avons utilisĂ© le modĂšle LPJ-LMfire afin de simuler la dynamique de la biomasse arborescente et du taux de brĂ»lage au cours du XXIe siĂšcle aux TNO et sur le territoire TĆı̚chÇ«;. Les simulations issues du modĂšle LPJ-LMfire ont montrĂ© que les taux de brĂ»lage futurs seront principalement modulĂ©s par lâaugmentation des tempĂ©ratures et lâĂ©volution de la dynamique de la vĂ©gĂ©tation, notamment de lâĂ©pinette noire.Climate change impacts the boreal ecosystem through modifications of vegetation structure, composition, distribution and productivity. These changes alter the internal functioning of forests by disrupting the dynamics of natural disturbances such as fire, notably their frequency and size. Extreme weather events in recent decades have resulted in very large areas burned during some years, altering forest landscapes at northern latitudes in Canada. Large fires accounting for the majority of burned areas release massive amounts of carbon into the atmosphere and have major health consequences for people in exposed communities. They also limit the ability of Indigenous people to maintain their traditional activities by reducing ecosystem services they traditionally access. Climate projections suggest that large fires may be increasingly frequent in the coming decades, which in turn could affect forests, climate, and human societies. However, models remain uncertain and questions persist, particularly about the frequency and magnitude of extreme weather events that facilitate the onset of large forest fires.This thesis contributes to improving our understanding of the environmental drivers that have determined the dynamic of recent and past fire regimes in the boreal forest of northwestern Canada, in order to predict the interactions between climate, vegetation and future wildfires. This information will help managers and local communities anticipate future fire risk in response to climate change, in order to adapt practices and land use accordingly to limit the potentially negative effects of large forest fires. Chapter II of this thesis aimed to reconstruct the fire regime since 1965 on the territory of the TĆı̚chÇ«; First Nation in the Northwest Territories (NWT), in Canada. This territory experienced one of the largest forest fires across Canada in 2014. We determine the main climatic and ecological conditions having contributed to extreme wildfire seasons (EWY) during the past few decades on the TĆı̚chÇ«; territory, and we verify that the year 2014 was one of those EWY. We fixed climatic risk thresholds conducive to EWY occurrence, based on temperatures and soil organic horizon dryness indices measured during the fire season. The objective of the chapter III was to reconstruct the fire regime and vegetation dynamics on the TĆı̚chÇ«; territory during the Holocene (11,700 years) and to analyze our reconstructions with regards to past climatic conditions at regional scale, in order to determine the main drivers of fire occurrence and size over time. Historical reconstructions showed that warm and dry conditions, as well as fuel availability and forest composition, were the main drivers leading to large and severe wildfires in the short and long term. In chapter IV, we simulated the risk of future EWY based on projections from different climate models coupled with two radiative forcing scenarios (RCP4.5 and RCP8.5). We used the LPJ-LMfire model to simulate the dynamics of tree biomass and burn rate over the 21st century in the NWT and the TĆı̚chÇ«; territory. Model simulations showed that future burn rates will be primarily modulated by increasing temperatures and changing vegetation dynamics, particularly black spruce
Silicon nanostructures by catalyzed chemical growth : a platform for micro-supercapacitors applications
Les supercondensateurs sont des dispositifs de stockage Ă©lectrochimique de lâĂ©nergie ayant Ă©tĂ© rĂ©cemment mis au point et possĂ©dant des performances intermĂ©diaires entre les condensateurs diĂ©lectriques et les batteries. Leurs intĂ©ressantes valeurs de densitĂ© dâĂ©nergie et de puissance, conjuguĂ©es Ă leur excellente durĂ©e de vie et Ă leur miniaturisation facilitĂ© rendent ces composants prometteurs pour des micro-dispositifs Ă©lectroniques, tels des micro-capteurs autonomes ou des implants mĂ©dicaux.Le silicium nanostructurĂ© par CVD a prouvĂ© ĂȘtre un remarquable matĂ©riau dâĂ©lectrode de supercondensateur, pour des applications miniaturisĂ©es, lors de rĂ©cents travaux. Lâexcellent contrĂŽle de la morphologie et des propriĂ©tĂ©s Ă©lectroniques permis par la synthĂšse montante de nano-fils et nano-arbres de silicium, ainsi que la grande stabilitĂ© Ă©lectrochimique et thermique de ce matĂ©riau font des nanostructures de silicium obtenues par synthĂšse montante une excellente plate-forme pour des micro-supercondensateurs.La prĂ©sente thĂšse sâattache Ă explorer plusieurs voies dâamĂ©lioration et dâutilisation des nano-fils et nano-arbres de silicium. Une Ă©tude systĂ©matique de lâoptimisation des nanostructures a Ă©tĂ© conduite, permettant dâamĂ©liorer largement les performances prĂ©cĂ©demment Ă©tablies. Ensuite, une fonctionnalisation par des couches minces dâalumines utilisant la technique dâALD a permis dâaccroitre largement la plage de tensions dâutilisation des supercondensateurs, et dâaugmenter leur stabilitĂ© Ă©lectrochimique. Enfin, la croissance « sur-puce », ainsi que lâĂ©tude de la stabilitĂ© en tempĂ©rature des dispositifs ont Ă©tĂ© effectuĂ©es, laissant entrevoir dâimportantes perspectives dâapplications.Supercapacitors are electrochemical energy storage devices which have been recently developed, and possess intermediate performances between dielectric capacitors and batteries. These components exhibit interesting power and energy densities, combined with an exceptional cycle life and an easy miniaturization. Supercapacitors are thus envisioned as energy storage solutions for electronic micro-devices, such as autonomous micro-sensors or implantable medical devices.In recent studies, CVD nanostructured silicon proved to be an excellent electrode material candidate for micro-supercapacitor applications. Bottom-up synthesis allows an exceptional control of the morphology and electrical properties of the obtained silicon nano-wires and nano-trees. Moreover, the nanostructured electrodes possess superior electrochemical and temperature stability. These arguments lead to consider silicon as an excellent platform for micro-supercapacitors applications.This PhD thesis details various ways to improve and use silicon nano-wires and nano-trees. The nanostructures have been subjected to a systematic optimization study, yielding a significant increase of the electrochemical performances of the electrodes, compared to previously published studies. In addition, surface functionalization using thin ALD alumina layers permitted a considerable increase of the supercapacitor voltage window and an improved electrochemical stability. Finally, âon-chipâ nanostructure growth, and temperature stability studies of the device were conducted, opening a broad field of improvements and potential uses for these silicon nanostructures
On ship integration of High-performance Micropseudocapacitors based on Silicon nanotrees coated by transition metal oxides and high-k dielectrics
International audienc