Kernel estimation of the intensity of Cox processes

Counting processes often written $N=(N_t)_{t\in\mathbb{R}^+}$ are used in several applications of biostatistics, notably for the study of chronic diseases. In the case of respiratory illness it is natural to suppose that the count of the visits of a patient can be described by such a process which intensity depends on environmental covariates. Cox processes (also called doubly stochastic Poisson processes) allows to model such situations. The random intensity then writes $\lambda(t)=\theta(t,Z_t)$ where $\theta$ is a non-random function, $t\in\mathbb{R}^+$ is the time variable and $(Z_t)_{t\in\mathbb{R}^+}$ is the $d$-dimensional covariates process. For a longitudinal study over $n$ patients, we observe $(N_t^k,Z_t^k)_{t\in\mathbb{R}^+}$ for $k=1,\ldots,n$. The intention is to estimate the intensity of the process using these observations and to study the properties of this estimator

A nonparametric test for Cox processes

In a functional setting, we propose two test statistics to highlight the Poisson nature of a Cox process when n copies of the process are available. Our approach involves a comparison of the empirical mean and the empirical variance of the functional data and can be seen as an extended version of a classical overdispersion test for counting data. The limiting distributions of our statistics are derived using a functional central limit theorem for c`adl`ag martingales. We also study the asymptotic power of our tests under some local alternatives. Our procedure is easily implementable and does not require any knowledge of covariates. A numerical study reveals the good performances of the method. We also present two applications of our tests to real data sets

Numerical modelling of a high temperature power module technology with SiC devices for high density power electronics

This paper presents the development of a new packaging technology using silicon carbide (SiC) power devices. These devices will be used in the next power electronic converters. They will provide higher densities, switching frequencies and operating temperature than current Si technologies. Thus the new designed packaging has to take into account such new constraints. The presented work tries to demonstrate the importance of packaging designs for the performance and reliability of integrated SiC power modules. In order to increase the integrated density in power modules, packaging technologies consisting of two stacked substrates with power devices and copper bumps soldered between them were proposed into two configurations. Silver sintering technique is used as die-attach material solution. In order to assess the assembling process and robustness of these packaging designs, the thermo-mechanical behaviour is studied using FEM modelling. Finally, some recommendations are made in order to choose the suitable design for reliable power module

Triple-quantum correlation NMR experiments in solids using J-couplings

We show that triple-quantum–single-quantum (TQ–SQ) correlation spectra of crystalline and disordered solids can be obtained under MAS using pulse sequences based on through-bond J-couplings. The feasibility of the experiments in coupled spin-1/2 systems is demonstrated for fully 13C-labelled L-alanine and Pb3P4O13 crystalline compounds, considered as model three-spin and four-spin systems, respectively. In the case of phosphate glasses, we show that the obtained TQ–SQ correlation spectra provide an improved description of the glass forming network connectivities and of the chain length distribution in the disordered network

Structure and dynamics of Oxide Melts and Glasses : a view from multinuclear and high temperature NMR

Solid State Nuclear Magnetic Resonance (NMR) experiments allow characterizing the local structure and dynamics of oxide glasses and melts. Thanks to the development of new experiments, it now becomes possible to evidence not only the details of the coordination state of the network formers of glasses but also to characterize the nature of polyatomic molecular motifs extending over several chemical bonds. We present results involving 31P homonuclear experiments that allow description of groups of up to three phosphate units and 27Al/17O heteronuclear that allows evidencing μ3 oxygen bridges in aluminate glasses and rediscussion of the structure of high temperature melts.Comment: Journal of Non-Crystalline Solids (2007) in press; Also available online at: http://crmht.cnrs-orleans.fr/Intranet/Publications/?id=207

Seismic Response to Injection Well Stimulation in a High-Temperature, High-Permeability Reservoir

Fluid injection into the Earth's crust can induce seismic events that cause damage to local infrastructure but also offer valuable insight into seismogenesis. The factors that influence the magnitude, location, and number of induced events remain poorly understood but include injection flow rate and pressure as well as reservoir temperature and permeability. The relationship between injection parameters and injection-induced seismicity in high-temperature, high-permeability reservoirs has not been extensively studied. Here we focus on the Ngatamariki geothermal field in the central Taupō Volcanic Zone, New Zealand, where three stimulation/injection tests have occurred since 2012. We present a catalog of seismicity from 2012 to 2015 created using a matched-filter detection technique. We analyze the stress state in the reservoir during the injection tests from first motion-derived focal mechanisms, yielding an average direction of maximum horizontal compressive stress (SHmax) consistent with the regional NE-SW trend. However, there is significant variation in the direction of maximum compressive stress (σ1), which may reflect geological differences between wells. We use the ratio of injection flow rate to overpressure, referred to as injectivity index, as a proxy for near-well permeability and compare changes in injectivity index to spatiotemporal characteristics of seismicity accompanying each test. Observed increases in injectivity index are generally poorly correlated with seismicity, suggesting that the locations of microearthquakes are not coincident with the zone of stimulation (i.e., increased permeability). Our findings augment a growing body of work suggesting that aseismic opening or slip, rather than seismic shear, is the active process driving well stimulation in many environments

Chimie : un vecteur vert

National audienceLes faits, longtemps sujets à discussion, sont désormais là. Le cinquième rapport du GIEC (2013) qualifie d'« extrêmement probable » un lien entre les dérèglements climatiques observés depuis 1950 et les variations de la composition de l'atmosphère terrestre provoqués par l'augmentation forte et récente des activités anthropiques. L'implication directe des gaz à effet de serre dans les phénomènes climatiques, et en particulier du dioxyde de carbone dont la concentration a augmenté de 40 % depuis l'époque préindustrielle, est toute aussi probable..

Structural characterization of water-bearing silicate and aluminosilicate glasses by high-resolution solid-state NMR

Various one- and two-dimensional high-resolution solid-state NMR techniques have been applied to hydrous silicate and aluminosilicate glasses: simple acquisition, cross-polarization (CP-MAS), heteronuclear correlation (HETCOR), dipolar dephasing, spin counting, double-quantum correlation, and rotational echo double resonance (REDOR). The comparison of the results obtained for sodium tetrasilicate and phonolite glasses suggests that the water incorporation mechanisms are qualitatively similar for these two compositions. From proton NMR experiments, we observe no evidence of proton clustering and a wide range of chemical shifts ranging from 0 to 16 ppm, even for the aluminosilicate phonolite glass, identifying at least three types of hydroxyl (OH) protons in addition to molecular H2O. This variety of OH groups can be discussed in terms of hydrogen bonding strength. For both compositions, the results indicate some depolymerization of the tetrahedral network, but the picture cannot be so simple as to completely exclude any of the different previously proposed models for water incorporation in silicate glasses

Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite

1H, 27Al, 29Si and 39K solid-state NMR are reported from a Hungarian illite 2:1 clay for samples heated up 1600 °C. This single-phase sample has a small amount of aluminium substitution in the silica layer and very low iron-content (0.4 wt%). Thermal analysis shows several events that can be related to features in the NMR spectra, and hence changes in the atomic scale structure. As dehydroxylation occurs there is increasing AlO4 and AlO5-contents. The silica and gibbsite layers become increasingly separated as the dehydroxylation progresses. Between 900 and 1000 °C the silica layer forms a potassium aluminosilicate glass. The gibbsite-layer forms spinel/γ-Al2O3 and some aluminium-rich mullite. Then on heating to 1600 °C changes in the 29Si and 27Al MAS NMR spectra are consistent with the aluminosilicate glass increasing its aluminium-content, the amount of mullite increasing probably with its silicon-content also increasing, and some α-Al2O3 forming