37 research outputs found
Structure settling of injection moulded products due to ageing of metastable solid phases developed under solidification
No full textTwo-membrane cavity optomechanics
No full textWe study the optomechanical behaviour of a driven Fabry–Pérot cavity containing two vibrating dielectric membranes.We characterize the cavity mode frequency shift as a function of the two membrane positions, and report a∼2.47 gain in the optomechanical coupling strength of the membrane relative motion with respect to the single membrane case. This is achieved when the two membranes are properly positioned to form an inner cavity which is resonant with the driving field. We also show that this two-membrane system has the capability to tune the single-photon optomechanical coupling on demand, and represents a promising platform for implementing cavity optomechanics with distinct oscillators. Such a configuration has the potential to enable cavity optomechanics in the strong single-photon coupling regime, and to study synchronization in optically linked mechanical resonators
Friction Consolidation of Oxide Dispersion Strengthened Incoloy ® Alloy Ma956 Powder
No full textINCOLOY® MA956 is a ferritic ODS alloy. It has very good oxidation resistance by virtue of its large chromium and aluminum concentrations and high mechanical strength and creep resistance at elevated temperatures thanks to oxide dispersion strengthening.
The conventional processing route utilized to obtain this alloy involves two main multistep stages. The first (or front end) stage of the process consists of a dry, high-energy milling process which mixes very fine Y2O3 particles with elemental alloy powders by Mechanical Alloying (MA) in an attritor. The second (or back end) stage of the process consists of consolidating the mechanically alloyed powder by hot extrusion in vacuum-sealed cans at about 1000°C, or by degassing followed by hot isostatic pressing (HIP).
The precipitation of a fine dispersion of yttrium-aluminum-rich oxides (Y-Al-O) during the consolidation is at the origin of the high temperature mechanical strength of this alloy. Three different thermodynamically stable oxides are known to exist for the binary Y2O3:Al2O3 system: Y4Al2O9, YAlO3 and Y3Al5O12. All three of them have been observed in this type of alloys when processed by the route described above. Their size ranges from just a few up to hundreds of nm.
In this work, the applicability of Friction Consolidation to this ODS alloy was investigated in order to tackle the downsides of the conventional processing route (multisteps and extremely high raw material final cost).
For this study, mechanically alloyed INCOLOY® MA956 powder was consolidated through Friction Consolidation under three different sets of processing conditions. As a result, three small compacts of low porosity have been achieved with a refined equiaxed ferritic grain structure smaller than 10 microns and the desired oxide dispersion. Two types of mixed Y-Al oxides were observed by different complementary techniques, Scanning Electron Microscopy (SEM), Electron Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD), YAlO3 and Y3Al5O12. Their size distribution was characterized using electron microscopy at different locations (for precipitates above 50 nm) and showed a larger average precipitate size for larger grain size. The total energy input during processing was correlated with the relative amount of each of the oxides in the disks (observed from XRD experiments): the higher the total processing energy input, the higher the relative proportion of Y3Al5O12 precipitates. The elemental composition of these precipitates was also probed individually by EDS showing an aluminum enrichment trend as precipitates grow in size. Overall, the Friction Consolidated material showed microstructural characteristics comparable to the ones observed in conventionally processed material, which makes it a very promising processing alternative
DEVELOPMENT OF COOLING SYSTEMS WITH ACTIVE ELASTOCALORIC REGENERATORS
No full textThe vapor compression cycle (VCC) has been developed and optimized over a century to provide cooling in residential and commercial buildings, and transport systems. However, its usage has resulted in unpredicted environmental damage such as depleting the ozone layer and promoting global warming when its refrigerant leaks into the atmosphere. Because of this, it is important to develop a superior technological alternative without the environmental costs. One way to tackle this problem is to develop heat pumping cycles using solid-state refrigerant since a solid is incapable of leaking into the atmosphere. However, a solid-refrigerant cannot flow to deliver cooling the same way a fluid-refrigerant does. This requires a system conceptual redesign, which started with near-room temperature cooling with magnetocaloric materials in 1976 and elastocaloric materials in 2012.
In this work, four different system configurations were studied with the following objectives: 1) maximizing the system’s temperature lift and 2) measuring the cooling capacity as a function of the useful temperature lift of the system when operating as a water chiller. The first configuration was based on the thermowave heat recovery strategy, while the other three were based on a single stage, two-stage and reciprocating variants of the active regeneration cycle. From the studied configurations the thermowave-based cycle achieved a system’s temperature lift of 8 K, at large average strain of 4.5%. It produced a maximum useful temperature lift of 5 K and a maximum cooling capacity of 125 W. All active regeneration-based cycles achieved similar final results while the best results was a system’s temperature lift of 21.3 K at a low average strain of 3.5% and a maximum useful temperature lift of 6.5 K and a maximum cooling capacity between 16 W and 25 W. The advantage of the reciprocating system integration is that it can achieve these results at lower strain than the one- stage and two-stage configurations.
This dissertation identified a fundamental limitation of the active regeneration cycles using single composition elastocaloric materials. It is due to the fact that the local strain is larger than the average strain where the temperature is lower, which limits the maximum applicable average strain to prevent premature failure. This directly affects both the temperature lift and cooling capacity of the system. Different alternatives to address this issue, as well as how to improve the overall thermal and structural performance of the system within the constraints of the materials commercially available are suggested
Multimode Cavity Optomechanics
No full textWe study theoretically and experimentally the behavior of an optomechanical system where two vibrating dielectric membranes are placed inside a driven Fabry-Pérot cavity. We prove that multi–element systems of mechanical resonators are suitable for enhancing optomechanical performances, and we report a ∼2.47 gain in the optomechanical coupling strength of the membrane relative motion with respect to the single membrane case. With this configuration it is possible to enable cavity optomechanics in the strong single-photon coupling regime
Sandwich in the middle: enhancing the optomechanical coupling
No full textMulti–element systems of micro/nano–mechanical resonators offer promising prospects for enhanced optomechanical performances [1,2], and for the exploration of multi–oscillators synchronization [3]. A solution, capable of providing systems with ratio between the single–photon optomechanical coupling rate and the cavity decay rate enhanced by orders of magnitude, exploits quantum interference in multi–element optomechanical setups. Although the simplest two–membrane sandwich in an optical cavity is a paradigm for the realization of strong–coupling optomechanics, and the observation of collective mechanical effects (such as synchronization), no experimental studies of these phenomena have been reported till now. We report on the first experimental characterization of the optical, mechanical, and especially optomechanical properties of a sandwich constituted of two parallel membranes within an optical cavity [4]. The membrane–sandwich we use in our experiment is constituted of two low–stress Si3N4 square membranes, with a side of 1 mm, and a thickness of 100 nm. One of the membranes is glued on a piezo, for scanning the membrane–cavity length. The whole membrane–sandwich mount is attached to another piezo to displace in a controlled way the center of mass of the two membranes inside the Fabry–Perot cavity. To estimate the optomechanical coupling strength achievable with our system, we inserted the membrane–sandwich in an optical cavity 90 mm–long, driven with a 1064 nm laser bea
Three Essays on the Impact of Geographic and Social Proximity on Innovation
No full textThe ability of an economy to generate and diffuse ideas has a profound influence on its ability to sustain growth. Our understanding of a key economic phenomenon behind agglomeration and modern growth - localization economies - relies on basic assumptions about how knowledge is recombined locally versus over distance. Whereas scholars have extensively studied the effects of proximity on the diffusion of ideas, the micro-foundations of knowledge recombination remain undeveloped. In particular, we know surprisingly little about how geographic, social and knowledge proximity shape the way ideas are generated, recombined, and discovered by others.
In the first study, I provide novel empirical evidence grounded in an original theoretical framework to explain how the composition of near-neighbor scientists influences the direction of inventive activity. Co-located individuals are more likely to engage in low expected value but high variance payoffs interactions ("water cooler"). To address endogeneity concerns due to selection into co-location and matching, I exploit an unexpected random relocation of scientists due to asbestos removal at UPMC Paris. Results are consistent with the theory and shed light on how proximity affects innovation: random versus chosen co-location leads to higher levels of experimentation, and has a positive impact on breakthrough ideas for collaborations across academic fields.
In the second study I explore the decline in collaboration that follows the separation of previously co-located co-authors. Not all co-author pairs are equally affected by an increase in geographic distance, as social proximity substitutes for it when the returns to collaboration are high. Collaborations that bridge otherwise disconnected communities of experts are the most negatively affected by separation.
Finally, the third study examines how social and geographic proximity affect the discovery and funding of creative projects in online crowdfunding. Whereas the internet reduces many distance-related frictions, local and distant investors exhibit different investment patterns. The distance effect is explained by investors who likely have a social relationship with the entrepreneur, and does not persist after the first investment (i.e., it is likely driven by search costs). Thus, crowdfunding seems to eliminate many distance-related frictions, but not those related to social interactions.Ph
Three Essays on the Impact of Geographic and Social Proximity on Innovation
No full textThe ability of an economy to generate and diffuse ideas has a profound influence on its ability to sustain growth. Our understanding of a key economic phenomenon behind agglomeration and modern growth - localization economies - relies on basic assumptions about how knowledge is recombined locally versus over distance. Whereas scholars have extensively studied the effects of proximity on the diffusion of ideas, the micro-foundations of knowledge recombination remain undeveloped. In particular, we know surprisingly little about how geographic, social and knowledge proximity shape the way ideas are generated, recombined, and discovered by others.
In the first study, I provide novel empirical evidence grounded in an original theoretical framework to explain how the composition of near-neighbor scientists influences the direction of inventive activity. Co-located individuals are more likely to engage in low expected value but high variance payoffs interactions ("water cooler"). To address endogeneity concerns due to selection into co-location and matching, I exploit an unexpected random relocation of scientists due to asbestos removal at UPMC Paris. Results are consistent with the theory and shed light on how proximity affects innovation: random versus chosen co-location leads to higher levels of experimentation, and has a positive impact on breakthrough ideas for collaborations across academic fields.
In the second study I explore the decline in collaboration that follows the separation of previously co-located co-authors. Not all co-author pairs are equally affected by an increase in geographic distance, as social proximity substitutes for it when the returns to collaboration are high. Collaborations that bridge otherwise disconnected communities of experts are the most negatively affected by separation.
Finally, the third study examines how social and geographic proximity affect the discovery and funding of creative projects in online crowdfunding. Whereas the internet reduces many distance-related frictions, local and distant investors exhibit different investment patterns. The distance effect is explained by investors who likely have a social relationship with the entrepreneur, and does not persist after the first investment (i.e., it is likely driven by search costs). Thus, crowdfunding seems to eliminate many distance-related frictions, but not those related to social interactions.Ph
Unmasking Jane Austen: Austen Studies Today/ Jane Austen senza maschere: gli studi austeniani oggi
No full textLa nota riassume lo svolgimento e risultati del Convegno, soffermandosi sopratutto sull'importanza dell'evento nell'ambito degli Studi italiani e della II edizione del volume (di B. Battaglia) "La zitella illetterata. Parodia e ironia nei romanzi di JAusten" (Liguori 2009) come sforzo per superare arretratezze e persistenti pregiudizi nella critica accademica della scrittrice in Italia. Si sottolinea con soddisfazione la Tavola Rotonda di giovani studiosi austeniani comprendente tra gli altri ricercatori della nostra Università: Rita Monticelli, Massimiliano Morini, Serena Baiesi, Carlotta Farese, Giuliana Gardellini, Gabriella Catalini
Temporal and spatial relations between patents and scientific journal articles: the case of nanotechnologies
No full textPatent citations have been widely used in order to study inter-technology and science-technology relations. The present work aims at: i) exploring time relations and distance between technical/innovative activities and scientific knowledge, using journal articles citations in patents as a proxy; ii) exploring the origin of the knowledge cited in patents. The study is performed on a field particularly relevant both on the scientific and technological side, that of nanosciences and nanotechnologies. In parallel a field less on the edge of research (polymers) is studied in order to compare results and shed better light on what is happening in nanotech. Studied items show a common behaviour and a higher rate of citations and a shorter time lag between citing patents and cited articles for nanotechnologies rather than for polymers. Knowledge cited in patents shows in many cases a common origin with that of citing documents. Conclusions on these behaviours are drawn.Patent-research relations, Patent, Journal Article, Nanoscience, Nanotechnologies, Polymers, Technological trajectories, Data mining, Innovation, Knowledge diffusion
