Nonmonotonic Evolution of the Blocking Temperature in Dispersions of Superparamagnetic Nanoparticles

We use a Monte Carlo approach to simulate the influence of the dipolar interaction on assemblies of monodisperse superparamagnetic ${\gamma}-Fe_{2}O_{3}$ nanoparticles. We have identified a critical concentration c*, that marks the transition between two different regimes in the evolution of the blocking temperature ($T_{B}$) with interparticle interactions. At low concentrations (c < c*) magnetic particles behave as an ideal non-interacting system with a constant $T_{B}$. At concentrations c > c* the dipolar energy enhances the anisotropic energy barrier and $T_{B}$ increases with increasing c, so that a larger temperature is required to reach the superparamagnetic state. The fitting of our results with classical particle models and experiments supports the existence of two differentiated regimes. Our data could help to understand apparently contradictory results from the literature.Comment: 13 pages, 7 figure

Calculations of giant magnetoresistance in Fe/Cr trilayers using layer potentials determined from {\it ab-initio} methods

The ab initio full-potential linearized augmented plane-wave method explicitly designed for the slab geometry was employed to elucidate the physical origin of the layer potentials for the trilayers nFe/3Cr/nFe(001), where n is the number of Fe monolayers. The thickness of the transition-metal ferromagnet has been ranged from $n=1$ up to n=8 while the spacer thickness was fixed to 3 monolayers. The calculated potentials were inserted in the Fuchs-Sondheimer formalism in order to calculate the giant magnetoresistance (GMR) ratio. The predicted GMR ratio was compared with the experiment and the oscillatory behavior of the GMR as a function of the ferromagnetic layer thickness was discussed in the context of the layer potentials. The reported results confirm that the interface monolayers play a dominant role in the intrinsic GMR.Comment: 17 pages, 7 figures, 3 tables. accepted in J. Phys.: Cond. Matte

Biochemistry of malaria parasite infected red blood cells by X-ray microscopy

Red blood cells infected by the malaria parasite Plasmodium falciparum are correlatively imaged by tomography using soft X-rays as well as by scanning hard nano-X-ray beam to obtain fluorescence maps of various elements such as S and Fe. In this way one can deduce the amount of Fe bound either in hemoglobin or in hemozoin crystals in the digestive vacuole of the malaria parasite as well as determine the hemoglobin concentrations in the cytosols of the red blood cell and of the parasite. Fluorescence map of K shows that in the parasite's schizont stage the K concentration in the red blood cell cytosol is diminished by a factor of seven relative to a pristine red blood cell but the total amount of K in the infected red blood cell is the same as in the pristine red blood cell

Sorption of fluorinated greenhouse gases in silica-supported fluorinated ionic liquids

the contracts of Individual Call to Scientific Employment Stimulus 2020.00835.CEECIND (J.M.M.A.) / 2021.01432.CEECIND (A.B.P.), the Norma Transitória DL 57/2016. Publisher Copyright: © 2022 The Authors.The Kigali Amendment to the Montreal Protocol limits the global use of fluorinated greenhouse gases (F-gases) and encourages the development of a new generation of refrigerants with lower global warming potential. Therefore, there is a need to develop efficient and sustainable technologies to selectively capture and recycle the F-gases as new environmentally sustainable refrigerants. Here, ionic liquids (ILs) with high F-gas uptake capacity and selectivity were supported on silica and their potential as media for selective F-gas sorption was studied. For this purpose single-component sorption equilibria of difluoromethane (R-32), pentafluoroethane (R-125), and 1,1,1,2-tetrafluoroethane (R-134a) were measured at 303.15 K by gravimetry. The sorption data were successfully correlated using classical models of sorption thermodynamics. The results show that the IL supported in the porous volume and on the external surface of the porous silica controls the F-gas uptake in the composites and that changing the IL's cations and anions allows fine-tuning the selectivity of the sorption process. This work brings crucial knowledge for the development of new materials based on ILs for the selective sorption of F-gases.publishersversionpublishe

Absorption of Fluorinated Greenhouse Gases Using Fluorinated Ionic Liquids

funding of the KET4F-Gas project, SOE2/P1/P0823, co -funded by the Interreg Sudoe Programme through the European Regional Development Fund (ERDF). FCT/MCTES for financial support through IF/00190/2014 , IF/00210/2014 financial support through the Norma TransitOria DL 57/2016 Program Contract (FCT/MCTES). FCT/MCTES (UID/QUI/50006/2019).The increasing awareness of the environmental impact of fluorinated gases (F-gases) used in refrigeration is instigating the development of technologies to recover and recycle them. With this goal in mind, single-component absorption equilibrium isotherms at 303.15 K of F-gases in different ionic liquids (ILs) were determined using a gravimetric method. The selected F-gases are the most used in domestic refrigeration (R-32: difluoromethane, R-125: pentafluoroethane, and R-134a: 1,1,1,2-tetrafluoroethane). The results show that ILs containing a fluorinated alkyl side chain with four carbon atoms, that is, fluorinated ILs (FILs), have higher gas absorption capacity than conventional fluoro-containing ILs. All studied ILs showed ideal selectivity toward R-134a. Conventional fluoro-containing ILs showed better selectivities for the separation of the binary mixtures R-134a/R-125 and R-32/R-125, and FILs showed better selectivities for the R-134a/R-32 mixture. These results provide fundamental knowledge of the behavior of these new alternative solvents and key information for their application in the separation of F-gas mixtures of commercial refrigerants.authorsversionpublishe

Bowing of the band gap pressure coefficients in InGaN alloys

The hydrostatic pressure dependence of photoluminescence, dEPL/dp, of InxGa1−xN epilayers has been measured in the full composition range 0_x_1. Furthermore, ab initio calculations of the band gap pressure coefficient dEG/dp were performed. Both the experimental dEPL/dp values and calculated dEG/dp results show pronounced bowing and we find that the pressure coefficients have a nearly constant value of about 25 meV/GPa for epilayers with x_0.4 and a relatively steep dependence for x_0.4. On the basis of the agreement of the observed PL pressure coefficient with our calculations, we confirm that band-to-band recombination processes are responsible for PL emission and that no localized states are involved. Moreover, the good agreement between the experimentally determined dEPL/dp and the theoretical curve of dEG/dp indicates that the hydrostatic pressure dependence of PL measurements can be used to quantify changes of the band gap of the InGaN ternary alloy under pressure, demonstrating that the disorder-related Stokes shift in InGaN does not induce a significant difference between dEPL/dp and dEG/dp. This information is highly relevant for the correct analysis of pressure measurement

Vacuum swing adsorption for R-32 recovery from R-410A refrigerant blend

The authors acknowledge the financial support from the LIFE-4-Fgases project [ LIFE20 CCM/ES/001748 ] funded by EU LIFE Programme . Norma Transitória DL 57/2016 Program Contract (R.P.P.L.R.). Publisher Copyright: © 2023 The Author(s)The recovery of high purity fluorinated gases from refrigerant blends is vital to promote a circular economy in the field of refrigeration and air conditioning. In this work, we evaluate the performance of a four-step Vacuum Swing Adsorption (VSA) process using activated carbon for the recovery of R-32 (difluoromethane) from a R-410A refrigerant blend: a binary mixture of R-125 (pentafluoroethane) and R-32 (yR−32=0.7; yR−125=0.3). Breakthrough curves were performed using dilute and bulk feed concentrations to determine mass and heat transfer parameters, which were then employed in simulations of fixed-bed adsorber dynamics. The mathematical model employed successfully predicts the experimental results of a four-step VSA cycle (feed, blowdown, purge, pressurization). The VSA performance was evaluated using process simulation. For a cycle time of 800 s (each step taking 200 s) and feed and regeneration pressures of 1.01 and 0.01 bar, respectively, R-32 is obtained with 97.0 mol-% purity and 30.9% recovery, with process productivity of 4.06 mol h−1 per kilogram of adsorbent, and energy consumption of 123.2 kJ/mol. Our results demonstrate the feasibility of using VSA processes for the recovery of R-32 from the near-azeotropic R-410A refrigerant blend.publishersversionpublishe