145 research outputs found
A high efficiency 10W MMIC PA for K-b and satellite communications
This paper discusses the design steps and experimental characterization of a monolithic microwave integrated circuit (MMIC) power amplifier developed for the next generation of K-band 17.3–20.2 GHz very high throughput satellites. The technology used is a commercially available 100-nm gate length gallium nitride on silicon process. The chip was developed taking into account the demanding constraints of the spacecraft and, in particular, carefully considering the thermal constraints of such technology, in order to keep the junction temperature in all devices below 160°C in the worst-case condition (i.e., maximum environmental temperature of 85°C). The realized MMIC, based on a three-stage architecture, was first characterized on-wafer in pulsed regime and, subsequently, mounted in a test-jig and characterized under continuous wave operating conditions. In 17.3–20.2 GHz operating bandwidth, the built amplifier provides an output power >40 dBm with a power added efficiency close to 30% (peak >40%) and 22 dB of power gain
Li-decorated BC3 nanopores: Promising materials for hydrogen storage
In the quest of new absorbent for hydrogen storage, we investigate the
capacities of slit pores formed by two BC3 sheets decorated with Li atoms.
Their hydrogen storage capacities are determined using density-functional
theory in conjunction with a quantum-thermodynamic model that allows to
simulate real operating conditions, i.e., finite temperatures and different
loading and depletion pressures applied to the adsorbent in the charge-delivery
cycles. We show that the capacities of the adsorbed hydrogen phase of
Li-decorated BC3 slit pores are larger than those reported recently for
graphene and Li-decorated borophene slit pores. On the other hand, the usable
volumetric and gravimetric capacities of Li-decorated BC3 slit pores can meet
the targets stipulated by the U.S. Department of Energy (DOE) for onboard
hydrogen storage at moderate temperatures and loading pressures well below
those used in the tanks employed in current technology. In particular, the
usable volumetric capacity for pore widths of about 10 {\AA} meets the DOE
target at a loading pressure of 6.6 MPa when depleting at ambient pressure. Our
results highlight the important role played by the rotational degree of freedom
of the H2 molecule in determining the confining potential within the slip pores
and their hydrogen storage capacities.Comment: Article itself: 26 pages, 12 figures, 13 equations, 1 table, 78 cited
articles; Supporting information: 8 pages, 6 figures, 6 equation
Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters
Producción CientíficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation.
The technology of fuel-cell engines is fully developed, and the outstanding
remaining problem is the storage of hydrogen in the vehicle. Porous materials,
in which hydrogen is adsorbed on the pore walls, and in particular nanoporous
carbons, have been investigated as potential onboard containers. Furthermore,
metallic nanoparticles embedded in porous carbons catalyze the dissociation of
hydrogen in the anode of the fuel cells. For these reasons the interaction of
hydrogen with the surfaces of carbon materials is a topic of high technological
interest. Computational modeling and the density functional formalism (DFT)
are helping in the task of discovering the basic mechanisms of the interaction
of hydrogen with clean and doped carbon surfaces. Planar and curved graphene
provide good models for the walls of porous carbons. We first review work on
the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters
on the surface because of the evidence of their role in enhancing hydrogen
storage.Ministerio de Economía, Industria y Competitividad (Grant MAT2014-54378-R
Aplicación de técnicas avanzadas de lienalización a sistemas de RF/microondas
Postprint (published version
Fully relativistic calculation of magnetic properties of Fe, Co and Ni adclusters on Ag(100)
We present first principles calculations of the magnetic moments and magnetic
anisotropy energies of small Fe, Co and Ni clusters on top of a Ag(100) surface
as well as the exchange-coupling energy between two single adatoms of Fe or Co
on Ag(100). The calculations are performed fully relativistically using the
embedding technique within the Korringa-Kohn-Rostoker method. The magnetic
anisotropy and the exchange-coupling energies are calculated by means of the
force theorem. In the case of adatoms and dimers of iron and cobalt we obtain
enhanced spin moments and, especially, unusually large orbital moments, while
for nickel our calculations predict a complete absence of magnetism. For larger
clusters, the magnitudes of the local moments of the atoms in the center of the
cluster are very close to those calculated for the corresponding monolayers.
Similar to the orbital moments, the contributions of the individual atoms to
the magnetic anisotropy energy strongly depend on the position, hence, on the
local environment of a particular atom within a given cluster. We find strong
ferromagnetic coupling between two neighboring Fe or Co atoms and a rapid,
oscillatory decay of the exchange-coupling energy with increasing distance
between these two adatoms.Comment: 8 pages, ReVTeX + 4 figures (Encapsulated Postscript), submitted to
PR
Optical absorption in boron clusters B and B : A first principles configuration interaction approach
The linear optical absorption spectra in neutral boron cluster B and
cationic B are calculated using a first principles correlated
electron approach. The geometries of several low-lying isomers of these
clusters were optimized at the coupled-cluster singles doubles (CCSD) level of
theory. With these optimized ground-state geometries, excited states of
different isomers were computed using the singles configuration-interaction
(SCI) approach. The many body wavefunctions of various excited states have been
analysed and the nature of optical excitation involved are found to be of
collective, plasmonic type.Comment: 22 pages, 38 figures. An invited article submitted to European
Physical Journal D. This work was presented in the International Symposium on
Small Particles and Inorganic Clusters - XVI, held in Leuven, Belgiu
Time series of freshwater macroinvertebrate abundances and site characteristics of European streams and rivers
Freshwater macroinvertebrates are a diverse group and play key ecological roles, including accelerating nutrient cycling, filtering water, controlling primary producers, and providing food for predators. Their differences in tolerances and short generation times manifest in rapid community responses to change. Macroinvertebrate community composition is an indicator of water quality. In Europe, efforts to improve water quality following environmental legislation, primarily starting in the 1980s, may have driven a recovery of macroinvertebrate communities. Towards understanding temporal and spatial variation of these organisms, we compiled the TREAM dataset (Time seRies of European freshwAter Macroinvertebrates), consisting of macroinvertebrate community time series from 1,816 river and stream sites (mean length of 19.2 years and 14.9 sampling years) of 22 European countries sampled between 1968 and 2020. In total, the data include >93 million sampled individuals of 2,648 taxa from 959 genera and 212 families. These data can be used to test questions ranging from identifying drivers of the population dynamics of specific taxa to assessing the success of legislative and management restoration efforts
The recovery of European freshwater biodiversity has come to a halt
Owing to a long history of anthropogenic pressures, freshwater ecosystems are among the most vulnerable to biodiversity loss. Mitigation measures, including wastewater treatment and hydromorphological restoration, have aimed to improve environmental quality and foster the recovery of freshwater biodiversity. Here, using 1,816 time series of freshwater invertebrate communities collected across 22 European countries between 1968 and 2020, we quantified temporal trends in taxonomic and functional diversity and their responses to environmental pressures and gradients. We observed overall increases in taxon richness (0.73% per year), functional richness (2.4% per year) and abundance (1.17% per year). However, these increases primarily occurred before the 2010s, and have since plateaued. Freshwater communities downstream of dams, urban areas and cropland were less likely to experience recovery. Communities at sites with faster rates of warming had fewer gains in taxon richness, functional richness and abundance. Although biodiversity gains in the 1990s and 2000s probably reflect the effectiveness of water-quality improvements and restoration projects, the decelerating trajectory in the 2010s suggests that the current measures offer diminishing returns. Given new and persistent pressures on freshwater ecosystems, including emerging pollutants, climate change and the spread of invasive species, we call for additional mitigation to revive the recovery of freshwater biodiversity
131I-metaiodobenzylguanidine (131I-MIBG) therapy for residual neuroblastoma: a mono-institutional experience with 43 patients
Incomplete response to therapy may compromise the outcome of children with advanced neuroblastoma. In an attempt to improve tumour response we incorporated 131I-metaiodobenzylguanidine (131I-MIBG) in the treatment regimens of selected stage 3 and stage 4 patients. Between 1986 and 1997, 43 neuroblastoma patients older than 1 year at diagnosis, 13 with stage 3 (group A) and 30 with stage 4 disease (group B) who had completed the first-line protocol without achieving complete response entered in this study. 131I-MIBG dose/course ranged from 2.5 to 5.5 Gbq (median, 3.7). The number of courses ranged from 1 to 5 (median 3) depending on the tumour response and toxicity. The most common acute side-effect was thrombocytopenia. Later side-effects included severe interstitial pneumonia in one patient, acute myeloid leukaemia in two, reduced thyroid reserve in 21. Complete response was documented in one stage 4 patient, partial response in 12 (two stage 3, 10 stage 4), mixed or no response in 25 (ten stage 3, 15 stage 4) and disease progression in five (one stage 3, four stage 4) Twenty-four patients (12/13 stage 3, 12/30 stage 4) are alive at 22–153 months (median, 59) from diagnosis. 131I-MIBG therapy may increase the cure rate of stage 3 and improve the response of stage 4 neuroblastoma patients with residual disease after first-line therapy. A larger number of patients should be treated to confirm these results but logistic problems hamper prospective and coordinated studies. Long-term toxicity can be severe. © 1999 Cancer Research Campaig
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