Synthesis, structural characterization and broadband ferromagnetic resonance in Li ferrite nanoparticles

Producción CientíficaLithium ferrites are well known materials due to its numerous technological applications especially in microwave devices. Lithium ferrite nanoparticles were prepared by sol-gel technique by means of Pechini method, and then annealed at different temperatures in 250–1000 °C range. XRD confirms spinel formation with particles sizes in 15–700 nm range, with increased size with annealing temperature, whereas FTIR and Raman measurement confirm that single phase lithium ferrite with ordered cationic structure is obtained. Microwave magnetoabsorption data of the annealed lithium ferrite nanoparticles were obtained with a broadband system based on a network analyzer that operates up to 8.5 GHz. At fields up to 200 mT we can observe a broad absorption peak that shifts to higher frequencies with magnetic field according to ferromagnetic resonance theory. The amplitude of absorption, up to 85%, together with the frequency width of about 5.5 GHz makes this material suitable as wave absorber. FMR parameters like resonance field, linewidth and broadening are analyzed in order to obtain the characteristic parameters and analyze the microwave behaviour.Ministerio de Economía, Industria y Competitividad ( project MAT2016-80784-P

Helminth parasites of some coastal fishes from Madeira, Portugal

One hundred and fifty-one fish belonging to six different species, from Madeira, Atlantic Ocean, were examined for helminth infections. All the fish examined shared the same type of littoral habitat characterized by rocky and sandy bottoms. However their feeding ecology was slightly different resulting in variations in their parasite composition. In the blue damselfish, Abudefduf luridus, which is mostly herbivorous but ingesting also the associated invertebrate fauna, the digeneans dominated, while the Atlantic damselfish, Chromis limbata, which preys on plank tonic and benthic organisms, was infected mainly by anisakid nematodes, larval acanthocephalans and digenean lepocreadids, usually transmitted by planktonic and benthic invertebrates. Similarly in the Turkish wrasse, Thalassoma pavo, pelagically transmitted parasites clearly dominated (Hysterothylacium, Scolex pleuronectis, acanthocephalans). Despite the similarities in both habitat and feeding ecologies of the two sparids, Boops boops and Diplodus vulgaris, some differences were found in their parasite faunas. Both species shared the acanthocephalans and Hysterothylacium sp. but differed in the presence of Meinertia parallela in B. boops and its absence in D. vulgaris.info:eu-repo/semantics/publishedVersio

An high-speed parametric ADC and a co-designed mixer for CMOS RF receivers

Dissertação apresentada na faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para a obtenção do grau de Mestre em Engenharia Electrotécnica e de ComputadoresThe rapid growth of wireless communications and the massive use of wireless end-user equipments have created a demand for low-cost, low-power and low-area devices with tight specifications imposed by standards. The advances in CMOS technology allows, nowadays, designers to implement circuits that work at high-frequencies, thus, allowing the complete implementation of RF front ends in a single chip. In this work, a co-design strategy for the implementation of a fully integrated CMOS receiver for use in the ISM band is presented. The main focus is given to the Mixer and the ADC blocks of the presented architecture. The traditional approach used in RF design requires 50 matching buffers and networks and AC coupling capacitors between Mixer inputs and LNA and LO outputs. The codesign strategy avoids the use of DC choke inductors for Mixer biasing, because it is possible to use the DC level from the output of the LNA and the LO to provide bias to the Mixer. Moreover, since the entire circuit is in the same chip and the Mixer inputs are transistors gates, we should consider voltage instead of power and avoid the 50 matching networks. The proposed ADC architecture relies on a 4-bit flash converter. The main goals are to achieve low-power and high sampling frequency. To meet these goals, parametric amplification based on MOS varactors is applied to reduce the offset voltage of the comparators, avoiding the traditional and power-consuming approach of active pre-amplification gain stages

Lithium Ferrite: Synthesis, Structural Characterization and Electromagnetic Properties

Lithium ferrite (LiFe5O8) is a cubic ferrite, belongs to the group of soft ferrite materials with a square hysteresis loop, with high Curie temperature and magnetization. The spinel structure of LiFe5O8 has two crystalline forms: ordered, β-LiFe5O8 (Fd3m space group) and disordered, α-LiFe5O8 (P4132/P4332 space group). It has numerous technological applications in microwave devices, computer memory chip, magnetic recording, radio frequency coil fabrication, transformer cores, rod antennas, magnetic liquids among others. It is also a promising candidate for cathode in rechargeable lithium batteries. In this work, the dc electrical conductivity, the impedance spectroscopy and the magnetization of Li2O-Fe2O3 powders, with [Li]/[Fe]=1/5 (mol), heat-treated at several temperatures, are studied and related to their structure and morphology. The structural data were obtained by X-ray diffraction and Raman spectroscopy, and the morphology by scanning electron microscopy. The impedance spectroscopy was analysed in function of temperature and frequency, and it was observed that the dielectric properties are highly dependent on the microstructure of the samples. The dc magnetic susceptibility was recorded with a vibrating sample magnetometer, under zero field cooled and field cooled sequences, between 5-300 K. Typical hysteresis curves were obtained and the saturation magnetization increases with increase in heat-treatment temperature

Leaf litter processing in low order streams

Forests produce a large amount of detritus, that inevitably end up in streams, subsidizing aquatic systems with organic matter and nutrients. Here we review some of the research carried out at the University of Coimbra with the objective of getting a better understanding of the breakdown process of these materials and its incorporation to secondary production. Litter-fall in deciduous forests in Central Portugal can reach up to 750 g AFDM of leaves m-2 yr-1, with 73% of the litter produced between October and December. In several retention experiments, we measured a 90% leaf retention in low order (1st- 4th) streams within 15 - 70 m, and a standing stock of up to 450 g AFDM m-2. The amount of nutrients in the water and the plant physical and chemical defenses can be an indicator of the rate at which plant material is incorporated into secondary production or exported as dissolved and fine particles of organic matter. Respiration rates of decomposing leaves incubated with fungicides were severely reduced, supporting the idea that fungi are very important agents in litter breakdown. The fungi group known as aquatic hyphomycetes are capable of producing enzymes able to cause leaf maceration, and by 2 to 3 weeks, up to 15 % of the decomposing leaf biomass corresponds to fungi. Shredder invertebrates are also biological agents involved in litter breakdown. Given their densities and feeding rates, we measured consumption rates of 12 - 54 g of leaves m-2 yr-1 in a stream in Central Portugal, corresponding to 2 to 9 times the litter standing stock. Feeding rates were high in nutrient rich leaves and low in chemical and physically protected leaves with low nutrient content. According to several experiments, fungal colonization facilitates the access of invertebrates to the energy trapped in deciduous leaves in streams. Some invertebrates have strategies to cope with low quality food (leaves with low microbial biomass or high chemical defenses). Those include high mobility, small size, compartmentalized digestion in the gut, presence of endosymbionts, and the capability to decrease respiration rates. The relative importance of fungi and invertebrates in the incorporation of plant litter material into secondary production varies across rivers and biomes. Shredder invertebrates seem to play a key role in litter breakdown in headwaters, but their importance appears to decrease downstream. In the same way, some systems where leaves are hard or protected, shredder invertebrates may be less abundant and the energy may be mainly recovered from litter by fungi. Eucalyptus plantations are systems with low diversity of invertebrates and aquatic hyphomycetes. Streams running through eucalyptus plantations seem therefore ideal to experimentally investigate relationships between structural parameters (biodiversity) and function. Finally, our research has been extended to other climatic areas including the Mediterranean and tropical streams. We reported a wide variety of situation in those systems. A general rule applying to all of them is that if leaf litter is abundant and high quality, the incorporation of energy into detrital food webs can be processed very quickly. However, if leaves are well protected and nutrients in the water are low, processing rates are equally very low, independently of the ambient temperatures.Los bosques producen una gran cantidad de detritus orgánicos, que inevitablemente llegan a los ríos, subsidiando los sistemas acuáticos con materiales y nutrientes. Aquí se revisan algunos de los trabajos que se han hecho en la Universidad de Coimbra con el objetivo de entender mejor el proceso de descomposición de este material y su incorporación en producción secundaria. La entrada de hojarasca en bosques caducifolios del Centro de Portugal puede alcanzar hasta 750 g PSLC (peso seco libre de cenizas) m-2 año-1, con 73 % de este valor ocurriendo entre Octubre y Diciembre. En varios experimentos de retención medimos que cerca de 90 % hojas que entran en ríos de baja orden (1ª- 4ª) eran retenidas entre los 15 y 75 m, y que la biomasa de hojarasca acumulada era de hasta 450 g PSLC m-2. La cantidad de nutrientes en el agua y las defensas físicas y químicas de las plantas pueden ser un indicador de la tasa a que el material orgánico es incorporado en producción secundaria o exportado como material disuelto o finamente particulado. Las tasas de respiración de hojas incubadas con fungicidas disminuyeron severamente apoyando la idea de que los hongos son agentes muy importantes en la descomposición de hojarasca el los ríos. El grupo de hongos conocido como hifomicetos acuáticos producen enzimas que causan la maceración de hojas, y en 2 o 3 semanas, hasta 15 % de la biomasa de una hoja en descomposición puede corresponder a hongos. Los invertebrados desmenuzadores son también agentes biológicos en la descomposición. Dadas las densidades de desmenuzadores y sus tasas de ingestión de alimento, hemos calculado tasas de consumo de hojas en ríos de12 - 54 g m-2 año-1, lo que corresponde a 2 a 9 veces la cantidad de hojarasca presente. Las tasas de consumo son generalmente altas en substratos ricos en nutrientes y bajas en hojas pobres en nutrientes o protegidas del punto de vista químico y físico. De acuerdo varios experimentos, la colonización por hongos facilita el acceso de los invertebrados a la energía de las hojas. Algunos invertebrados han desarrollado estrategias para poder vencer la baja calidad de las hojas, incluyendo un alta movilidad, tamaño pequeño, compartimentalización de la digestión en el intestino, presencia de endosimbiontes y la capacidad para disminuir las tasas respiratorias. La importancia relativa de los hongos e invertebrados en la incorporación de la hojarasca en producción secundaria es variable entre ríos y biomas. Los invertebrados desmenuzadores parecen jugar un papel importante en la descomposición de hojarasca en los ríos de bajo orden, pero su importancia parece disminuir rió abajo. Del mismo modo, en algunos sistemas en que las hojas son duras o protegidas, los invertebrados pueden ser menos abundantes y la energía canalizada en producción secundaria principalmente por los hongos. Las plantaciones de eucaliptos son sistemas con una baja diversidad de invertebrados e hifomicetos acuáticos. Los ríos que corren por plantaciones de eucaliptos parecen ser por este motivo sistemas ideales para investigar las relaciones entre parámetros estructurales (biodiversidad) y función. Finalmente, nuestra investigación ha sido extendida para otras zonas climáticas, incluyendo el Mediterráneo y las zonas tropicales. Hemos reportado una gran variedad de situaciones en esos sistemas. Una regla general a todos ellos es que si la hojarasca es abundante y de alta calidad, la incorporación de la energía de las hojas en las cadenas alimentares se procesa de forma muy rápida. Sin embargo, si las hojas están bien protegidas y los nutrientes el agua son bajos, estas tasas son igualmente muy bajas, independientemente de las temperaturas ambientales

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and Scanning Electron Microscopy

The widespread use of lithium niobate (LN) in several technological applications, notably in optical and electrooptical systems, is a consequence of its remarkable piezoelectric, electrooptical, photoelastic, acousto-optic, and nonlinear optical coefficients. In this chapter, the structural and electrical characterization of LN nanosized particles synthesized by the Pechini route is discussed. Compared to solid-state reaction processes, wet chemistry processes can be advantageous alternatives for the synthesis of polycrystalline LN, because they require lower processing temperatures, and thus the loss of stoichiometry and formation of secondary phases can be minimized. The powders obtained by drying the gel (base powder) were heat-treated for 4 h at temperatures between 400 and 1000°C, according to the differential thermal analysis (DTA) results. It was found that the powders sintered at 450°C contain only the LN phase, while those heat-treated at 500°C already contain the secondary LiNb3O8 phase. The structural and electrical characterization of the samples sintered at 450°C, for different times, was performed using X-ray diffraction (XRD) in conjunction with Rietveld refinement, Raman spectroscopy, scanning electron microscopy (SEM), and impedance spectroscopy in the temperature range between 200 and 360 K and in the frequency range between 100 Hz and 1 MHz and by measuring the ac and dc conductivities

Robust simulations of Turing machines with analytic maps and flows

In this paper, we show that closed-form analytic maps and ows can simulate Turing machines in an error-robust manner. The maps and ODEs de ning the ows are explicitly obtained and the simulation is performed in real time