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

Two-phase heat transfer model of a beam-down gas-solid fluidized bed solar particle receiver

Beam-down concentrating solar power for thermochemical and energy absorption applications stands as an attractive approach that can enhance the renewable energies deployment. This work explores the integration of beam-down optics with fluidized bed technology proposing a model to calculate both gas and bed temperatures. The beam-down system concentrates the energy from the solar field into a fluidized bed receiver. A novel phenomenological model is proposed to adapt the well-known two-phase theory to the heat transfer process of a bed operating in the bubbling regime while it is directly irradiated from the top. In this way, this simple model can be used as a design tool for beam-down fluidized bed receivers. The top bed surface is considered as an opaque diffuse layer formed by gray particles. A single layer model is applied to estimate the effective emissivity between the heterogeneous bed surface and the ambient conditions in the freeboard. The vertical temperature profile is obtained considering particle phase heat conduction, particle to gas heat convection, solid convection, bubble convection and radiation heat transfer mechanisms. The model is validated using silicon carbide and zirconia fluidized bed experiments reported in the literature. The model shows that the solid convection is the dominant heat transfer mechanism for a beam-down fluidized bed receiver. Further results explore the influence of the operating conditions on the fluidized bed receiver for a bed of silicon carbide particles, showing that energy concentration fluxes of 35 . 10(4) W/m(2) can reach bed temperatures of 1000 degrees C when operating at a gas velocities of 3.U-mf

Designing a flat beam-down linear Fresnel reflector

A linear beam-down solar field consists of two reflections that concentrate the solar irradiation on heavy materials located on the ground. Several rows of linear Fresnel reflectors, which have the same width, aim the solar irradiation to a secondary mirror with a hyperbolic shape that redirects the solar concentration towards the ground receiver. This paper overcomes the main limitation of the previously proposed hyperbolic secondary reflector. A new secondary reflector composed by several fixed flat mirrors located at the same height is proposed. A model to calculate the optimal layout of this novel solar field, as well as the efficiency and concentration, is developed and validated against a Monte-Carlo Ray-Tracing software, obtaining relative errors lower than 15%. Two new dimensionless parameters are proposed to facilitate the design of the flat beam-down linear Fresnel reflector. The concentration, optical efficiency and receiver width can be easily obtained, without performing any simulation, as a function of the dimensionless parameters. This novel solar field can achieve concentration ratios of up to 31 and optical efficiencies of up to 60%, obtaining similar concentrations with better optical efficiency than a field using a hyperbolic reflector.The authors wish to thank the support for the research project INTECSOLARIS-CM-UC3M has been funded by the call "Programa de apoyo a la realización de proyectos interdisciplinares de I + D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020" under the frame of the "Convenio Plurianual Comunidad de Madrid - Universidad Carlos III de Madrid". The authors wish to thank "Comunidad de Madrid" for its support to the ACES2030-CM Project (S2018/EMT-4319) through the Program of R&D activities between research groups in Technologies 2018, co-financed by European Structural Funds

Can solar tower plants withstand the operational flexibility of combined cycle plants?

The aim of this work is to investigate the level of reliability of a 100 MWe solar tower plant operating as a load-following plant using actual operational data of combined cycle power plants. Despite the low cost, the steam generator has been identified as the main cause of unavailability of solar tower plants due to fatigue failures of tube-to-tubesheet joints, which can lead to steam/water leakage into the heat transfer fluid circuit, putting the plant performance at risk. A methodology based on the ASME code and EN standards is proposed to predict the fatigue failures of critical welded points of the steam generator, such as tube-to-tubesheet joints and other T-joints. The results show that the forced outages due to failures of the steam generator lead to an energy penalty that ranges over 230-453 GWh over the plant lifetime. The associated annual degradation rate ranges over 0.123-0.244%. Three tube leakage repair strategies are compared: tube-to-tubesheet weld crack repair, tube plugging and tube plugging with steam generator replacement. The latter strategy was shown to be the best practice because the lowest levelized cost of energy was obtained. In addition, the design of heat exchangers with a minimum of 20% extra area is highly recommended to not compromise the plant operation due to tube plugging. Last, the load-following operation of the solar tower plant increases the levelized cost of energy by approximately 1.6% in the case of tube repair and approximately 0.8% in the case of tube plugging with steam generator replacement.This research is partially funded by the Spanish government under the project RTI2018-096664-B-C21 (MICINN/FEDER, UE) and the fellowship "Programa de apoyo a la realización de proyectos interdisciplinares de I + D para jóvenes investigadores de la Universidad Carlos III de Madrid 2019-2020" under the project ZEROGASPAIN-CM-UC3M (2020/00033/001)

Inhibiting crypto‐ransomware on windows platforms through a honeyfile‐based approach with R‐Locker

Ministerio de Economia y Competitividad, Grant/Award Number: TIN2017-83494-RAfter several years, crypto‐ransomware attacks still constitute a principal threat for individuals and organisations worldwide. Despite the fact that a number of solutions are deployed to fight against this plague, one main challenge is that of early reaction, as merely detecting its occurrence can be useless to avoid the pernicious effects of the malware. With this aim, the authors introduced in a previous work a novel antiransomware tool for Unix platforms named R‐Locker. The proposal is supported on a honeyfile‐based approach, where ‘infinite’ trap files are disseminated around the target filesystem for early detection and to effectively block the ransomware action. The authors extend here the tool with three main new contributions. First, R‐Locker is migrated to Windows platforms, where specific differences exist regarding FIFO handling. Second, the global management of the honeyfiles around the target filesystem is now improved to maximise protection. Finally, blocking suspicious ransomware is (semi)automated through the dynamic use of white‐/black‐lists. As in the original work for Unix systems, the new Windows version of R‐Locker shows high effectivity and efficiency in thwarting ransomware action.Spanish Government TIN2017-83494-

Multi-Labeling of Complex, Multi-Behavioral Malware Samples

The use of malware samples is usually required to test cyber security solutions. For that, the correct typology of the samples is of interest to properly estimate the exhibited performance of the tools under evaluation. Although several malware datasets are publicly available at present, most of them are not labeled or, if so, only one class or tag is assigned to each malware sample. We defend that just one label is not enough to represent the usual complex behavior exhibited by most of current malware. With this hypothesis in mind, and based on the varied classification generally provided by automatic detection engines per sample, we introduce here a simple multi-labeling approach to automatically tag the usual multiple behavior of malware samples. In the paper, we first analyze the coherence between the behaviors exhibited by a specific number of well-known malware samples dissected in the literature and the multiple tags provided for them by our labeling proposal. After that, the automatic multi-labeling scheme is executed over four public Android malware datasets, the different results and statistics obtained regarding their composition and representativeness being discussed. We share in a GitHub repository the multi-labeling tool developed, for public usage

Residual Forage and Animal Performance from a Kikuyo Pasture under Mixed Grazing and Supplementation

Five grazing strategies were compared: grazing by ewes only and mixed grazing (ewes + heifers), in mixed grazing heifers received no supplement or one of three supplements. Ruminal undegradable protein (RUP) varied among supplement: 4.25, 9.11 and 9.13%, the supplement with the highest RUP included protected methionine. All supplements were 21% crude protein and 3.0 Mcal/kg of metabolizable energy. Grazing was on kikuyu, ratio was 30 kg of ewe per 100 kg of heifer in the mixed grazing treatments. Experimental grazing lasted 84 days. Variables measured were residual forage, proline content in residual forage and liveweight gains per animal and per hectare. Experimental design was a completely random with two replicates. Residual forage was the highest (P\u3c 0.05) in mixed grazing and RUP in supplement was above 4.25%. The highest (P\u3c 0.05) proline content was found in grazing by ewes only, 52% above the proline content found in mixed grazing without supplement, when heifers received supplement proline content decreased (P\u3c 0.05) further. Grazing by ewes only and mixed grazing with heifersreceiving the highest RUP in supplement showed the highest (P\u3c 0.05) daily liveweight gain per ewe of 134 g. Mixed grazing in which heifers received no supplement gave a 100% increase (P\u3c 0.05) in liveweight gain per hectare compared to grazing by ewes only. In mixed grazing offering a supplement with 9.13% RUP and protected methionine gave both the highest liveweight gains per animal and per hectare. It was concluded that mixed grazing allowed a lower stress level on kikuyu due to defoliation and a higher liveweight gain per hectare than grazing by ewes only

Influence of the steam generator on the exergetic and exergoeconomic analysis of solar tower plants

Solar power tower plants differ from conventional power plants in the steam generator design due to the higher heat duty. In this work, the influence of the steam generator heat exchangers (preheater, evaporator, superheater and reheater) on a solar power plant with molten salt receiver and thermal storage is studied for the first time. Energy, exergy and exergoeconomic analyses give a complete view of the cost flows within the system. The pinch point temperature difference in the evaporator is used as the main variable as it changes the steam generator design and the operating conditions of the plant, such as the inlet temperature of the receiver and the salt mass flow. All heat exchangers are designed and optimized at minimum cost for each pinch point to fulfill the thermomechanical limitations of TEMA standards and Pressure Vessel code. The field of heliostats, molten-salt receiver and the power-block (110 MWe) designs are kept constant throughout the paper. A low pinch point should be used to minimize the plant exergy destruction while the exergoeconomic approach obtains an optimum pinch point around 2-3 °C. Furthermore, the low exergoeconomic factor values show that the heat exchangers of the SG are crucial for the plant operation