Electrical field effects in spark plasma sintering of hyperstoichiometric UO2

Recent research in the field assisted sintering of actinide oxides and nitrides has shown the great potential of the technique, but has also evidenced the need for a better understanding of the behavior of such class of materials during processing with current/fields [1-3]. Uranium dioxide, due to its multiple oxidation states and semiconductor characteristic, represents a good model material to investigate some of the peculiar effects of the electric field in the processing of materials. The stoichiometry of UO2 is of paramount importance, since it governs its fundamental properties, such as the thermal and electrical conductivity, the diffusion coefficients and thus also the sintering and grain growth kinetics. Here we report the densification behavior of hyperstoichiometric UO2+x in spark plasma sintering. The O/U ratio of the sintered pellets was studied by XRD, X-ray absorption spectroscopy (XAS) and Raman spectroscopy, while finite element methods (FEM) was used to assess the temperature distribution during processing. It is found that powders with an original stoichiometry of UO2.16 are gradually reduced to perfectly stoichiometric UO2.00 as a function of the sintering temperature and time. Most interestingly, a gradient in the oxidation state in the axial direction was observed for pellets sintered in intermediate conditions (Fig. 1). This gradient depends unequivocally on the direction of the current. Microstructural analysis confirmed the field/current effect in the sintered material. The reasons for the gradient in stoichiometry will be discussed and the implications for the electric field processing of UO2 and non-stoichiometric oxides in general will be addressed

The History of Flash Sintering

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Flash phenomena in lime-stabilised zirconia oxide ion conductor

Flash sintering is a novel technique that allows a decrease in the sintering time (from hours to seconds) and furnace temperature for the densification of ceramics. In order to find out if calcia-stabilised zirconia (15mol% CaO) can be flashed sintered, samples were first characterised by XRD and impedance spectroscopy, to confirm that the powder was single phase cubic and pellets were ionically conducting. During the flash experiments, the samples showed two of the “flash event” characteristics: increased conductivity and emission of light, but not densification. This shows that is possible to have the luminescent aspect of flash without sintering, as suggested by Raj

Minorities in the Post-Soviet Space Thirty Years After the Dissolution of the USSR

When the Soviet Union broke apart in 1991, the Russian Federation and the newly independent republics of the Baltics, the Caucasus and Central Asia engaged in redefining their national identity in a challenging regional and global context. The stances and policies towards the minorities living in these countries became part of the striving towards national independence and identity formation. Despite vastly different post-Soviet nation-building trajectories, the development and implementation of state policies towards minorities had similar relevance and importance across the region. Thirty years after the end of the USSR what is the situation of minorities and minority issues in the countries that emerged from that multi-ethnic state? How have the former republics – including Russia dealt with their minorities and minority affairs? To what protection and rights are minority communities entitled to? Studies of the dissolution of the USSR and of nation-building in the independent post-Soviet states have flourished over the past decades. However, despite the relevance of the theme, there is a dearth of specialist publications which address the many issues related to minority communities in the post-Soviet space. This volume attempts to fill this gap by providing a collection of essays covering some of the most relevant aspects of the contemporary status and situation of minorities in the area

Preface

When the Soviet Union broke apart in 1991, the Russian Federation and the newly independent republics of the Baltics, the Caucasus and Central Asia engaged in redefining their national identity in a challenging regional and global context. The stances and policies towards the minorities living in these countries became part of the striving towards national independence and identity formation. Despite vastly different post-Soviet nation-building trajectories, the development and implementation of state policies towards minorities had similar relevance and importance across the region. Thirty years after the end of the USSR what is the situation of minorities and minority issues in the countries that emerged from that multi-ethnic state? How have the former republics – including Russia dealt with their minorities and minority affairs? To what protection and rights are minority communities entitled to? Studies of the dissolution of the USSR and of nation-building in the independent post-Soviet states have flourished over the past decades. However, despite the relevance of the theme, there is a dearth of specialist publications which address the many issues related to minority communities in the post-Soviet space. This volume attempts to fill this gap by providing a collection of essays covering some of the most relevant aspects of the contemporary status and situation of minorities in the area

Rapid variability at very high energies in Mrk 501

A major flaring state of the BL Lac object Mrk 501 was observed by the High Energy Stereoscopic System (H.E.S.S.) in June, 2014. Flux levels higher than one Crab unit were recorded and rapid variability at very high energies ($\sim$2-20 TeV) was revealed. The high statistics afforded by the flares allowed us to probe the presence of minutes timescale variability and study its statistical characteristics exclusively at TeV energies owing to the high energy threshold of approximately 2 TeV. Doubling times of a few minutes are estimated for fluxes greater than 2 TeV. Statistical tests on the light curves show interesting temporal structure in the variations including deviations from a normal flux distribution similar to those found in the PKS 2155-304 flare of July 2006, at nearly an order of magnitude higher threshold energy. Rapid variations at such high energies put strong constraints on the physical mechanisms in the blazar jet.Comment: In Proceedings of the 34th International Cosmic Ray Conference (ICRC2015), The Hague, The Netherland

TeV observations of hard spectrum active galactic nuclei with HESS-I and HESS-II

In my PhD thesis, I present the results of the analysis of three point-like gamma-ray emitters observed at very high energies with the High Energy Stereoscopic System (H.E.S.S.): the two well known blazars 1ES 0229+200 and Mrk 501, and HESS J1745-290. The nature of the latter source is unclear, although it is likely linked to Sgr A∗, the black hole at the center of the Galaxy. I make use of multiwavelength observations in order to assess the physical properties of the three sources and of the ambient medium, both via spectral and variability studies and by comparison with models and predictions found in the literature. I also present systematic comparisons of the gamma-ray energies as reconstructed for different sub-arrays of the H.E.S.S. system. I search for correlated variability in simultaneous x-ray and very high energy observations of Sgr A∗ in order to understand whether or not the gamma-ray emission of HESS J1745-290 originates from the black hole, making it an active galactic nucleus. The correlation of the flux variability in different energy bands for 1ES 0229+200 supports the interpretation of a synchrotron self Compton emission model for this source. The refined spectrum of the inverse Compton peak helps constraining the strength of the intergalactic magnetic field. The hard intrinsic spectrum of Mrk 501 shows a dependency on the flux state. The spectrum can be used to constrain the extragalactic background light intensity in the mid-infrared for the redshift of the source. Multiwavelength observations suggest the presence of at least two emission zones or mechanisms in order to explain the broadband emission

L’influenza della Rpc in Italia e il “dilemma narrativo” della minoranza sinoitaliana

With the "New Era" announced by Chinese President Xi Jinping at the beginning of his first term, studies, reports, and media reports have gradually multiplied, all describing or even denouncing the pervasiveness and effectiveness of Chinese political influence and interference operations in the world.Con l’ingresso nella Nuova Era annunciata dal Presidente cinese Xi Jinping all’inizio del proprio primo mandato, si sono progressivamente moltiplicati studi, rapporti e reportage mediatici che descrivono o denunciano la pervasività e l’efficacia delle operazioni di influenza e interferenza politica cinese nel mondo

Storia e prospettive della diaspora cinese nello spazio post-sovietico

Questo articolo riassume lo sviluppo storico della diaspora cinese nello spazio post-sovietico, dalle prime migrazioni cinesi in epoca pre-rivoluzionaria alla situazione attuale, evidenziandone le principali tendenze e le prospettive di sviluppo futuro. I flussi migratori storicamente più importanti, come quelli che si sono estesi attraverso l'Eurasia dallo Zhejiang all'Europa occidentale, punteggiando lo spazio post-sovietico con centri migratori maggiori e minori e quelli che hanno preso piede in Siberia dalle regioni nord-orientali della Cina durante il ventesimo secolo, si sono tutti ridotti in modo significativo nell'ultimo decennio. Lo sviluppo della Belt and Road Initiative e le più strette relazioni sino-russe potrebbero preludere a nuove mobilità cinesi. Queste, tuttavia, potrebbero essere meno suscettibili all'incorporazione a lungo termine nei territori dell'ex Unione Sovietica

Perché le (presunte) "stazioni di polizia cinesi" e i (reali) Centri di servizio per i cinesi d'oltremare meritano attenzione

Questi Centri servizi per i cinesi all’estero (Huázhù zhōngxīn 华助中心) non sono una novità. A Milano, il primo aprì provvisoriamente in via Lepetit nel 2015, per poi stabilirsi definitivamente in via Bramante 8, presso la sede della storica Associazione Cinese a Milano (Mǐlán Huáqiáo Huárén gōngshānghuì 米兰华侨华人工商会), la più antica ed importante associazione cinese d’Italia, che fin dal 1945 assiste le autorità consolari cinesi nel disbrigo di pratiche che riguardano i cittadini cinesi originari del Zhejiang. Questo specifico centro è inoltre affiliato alla rete di centri servizi per cinesi residenti all’estero organizzato dalla Municipalità di Wenzhou, qualificandosi come un Wenzhou Overseas Chinese Center. Vi si offrono servizi come la prenotazione del rinnovo del passaporto, la delega per la richiesta del visto (per chi è di origine cinese, ma cittadino italiano), l’assicurazione per gli studenti cinesi all’estero, assistenza legale, ecc. Questi centri, per erogare tali servizi, non solo devono poter contare su raccordi organizzativi con il Consolato cinese di Milano, ma anche su canali di comunicazione diretti con le autorità delle proprie zone di origine in Cina. In nessun caso si tratta di attività di carattere clandestino o “segreto”