Hysteretic giant magnetoimpedance effect analyzed by first-order reversal curves

Hysteretic giant magnetoimpedance (GMI) of amorphous ribbons with a well-defined transversal domain structure is investigated by means of first-order reversal curves (FORC) analysis. The FORCs are not confined to the hysteretic area, exceeding the major curve amplitude. Irreversible switches of the transverse permeability, caused by domain wall structure transitions, may be the origin of the observed FORC distribution. An interlinked hysteron/anti-hysteron model is proposed to interpret it, which allows analyzing the influence of frequency and magnetostriction upon the hysteretic GMI effect.Comment: 19 pages, 9 figure

Study of the electrochemical behaviour of a 300 W PEM fuel cell stack by Electrochemical Impedance Spectroscopy

Electrochemical Impedance Spectroscopy (EIS) is a suitable and powerful diagnostic testing method for fuel cells because it is non-destructive and provides useful information about fuel cell performance and its components. In this work, EIS measurements were carried out on a 300 W stack with 20 elementary cells. Electrochemical impedance spectra were recorded either on each cell or on the stack. Parameters of a Randles-like equivalent circuit were fitted to the experimental data. In order to improve the quality of the fit, the classical Randles cell was extended by changing the standard plane capacitor into a constant phase element (CPE). The effects of output current, cell position, operating temperature and humidification temperature on the impedance spectra were studied.This work was supported by Generalitat Valenciana (PROMETEO/2010/023).Pérez Page, M.; Pérez Herranz, V. (2014). Study of the electrochemical behaviour of a 300 W PEM fuel cell stack by Electrochemical Impedance Spectroscopy. International Journal of Hydrogen Energy. 39(8):4009-4015. https://doi.org/10.1016/j.ijhydene.2013.05.121S4009401539

Exchange Anisotropy in Epitaxial and Polycrystalline NiO/NiFe Bilayers

(001) oriented NiO/NiFe bilayers were grown on single crystal MgO (001) substrates by ion beam sputtering in order to determine the effect that the crystalline orientation of the NiO antiferromagnetic layer has on the magnetization curve of the NiFe ferromagnetic layer. Simple models predict no exchange anisotropy for the (001)-oriented surface, which in its bulk termination is magnetically compensated. Nonetheless exchange anisotropy is present in the epitaxial films, although it is approximately half as large as in polycrystalline films that were grown simultaneously. Experiments show that differences in exchange field and coercivity between polycrystalline and epitaxial NiFe/NiO bilayers couples arise due to variations in induced surface anisotropy and not from differences in the degree of compensation of the terminating NiO plane. Implications of these observations for models of induced exchange anisotropy in NiO/NiFe bilayer couples will be discussed.Comment: 23 pages in RevTex format, submitted to Phys Rev B

Comparison of problem-based and team-based learning strategies: a multi-institutional investigation

Objectives: Over time, pedagogical practices in higher education have evolved significantly, which has led to the development of collaborative learning strategies. The study aims to compare the two most prominent ones – problem-based learning (PBL) and team-based learning (TBL). The comparison, integrated with Virtual Scenarios (VSs), involves student cohorts from various European institutions, specifically focusing on both PBL and TBL methods. The study is distinctive in its use of a consistent PBL/TBL methodology, ensured via joint staff training, and explores the perspectives of students and educators on these learning techniques. The overarching aim is to examine how PBL and TBL, coupled with VSs, influence problem-solving skills, independent learning, and student engagement. Methods: The examination was made using feedback from 399 students and 11 tutors collected in four trials held in three institutions based in Czechia, the United Kingdom and Romania. The data gathered from surveys and a focus group discussion contained qualitative as well as quantitative data, such as Likert scale questions. To analyse the overall trends in learners’ satisfaction with PBL and TBL sessions, the mean score calculated from the transformed Likert scale questions was compared between sessions and among institutions using multivariate ANOVA. Results: The students’ satisfaction and learning experience are heavily influenced by specific conditions, primarily their prior experience, room and technical set-up, group composition and especially the personality of a tutor. Overall, both strategies were found to be well-received by students used to traditional teaching methods. Students accustomed to PBL did not find TBL more engaging or useful. The identified advantages of TBL over PBL were the presence of a content expert, readiness tests, acquiring the same knowledge ensured through the collective presence of all students in one session and unified pre-class materials. However, TBL is more demanding on room set-up and teaching staff coordination. Conclusion: Both strategies have been found to have pros and cons and neither showed clear superiority over the other one. An institution newly implementing PBL or TBL needs to focus on different aspects than an institution planning to switch from PBL to TBL

Montecarlo based quantitative Kramers-Kronig test for PEMFC impedance spectrum validation

Electrochemical Impedance Spectroscopy (EIS) is a very powerful tool to study the behaviour of electrochemical systems. At present, it is widely used in the fuel cell field in order to study challenging cutting edge issues as membrane drying or gas diffusion layer flooding amongst others. The proper analysis of impedance data requires the fulfilment of four fundamental conditions: causality, linearity, stability and finiteness. The non compliance with any of these conditions may lead to biased, or even misguided, conclusions. Therefore it is critical to verify the compliance of these conditions before accepting any analysis performed on an experimental spectrum. This is even more important in a fuel cell experimental spectrum analysis, since fuel cells are markedly non stationary systems. The aim of this work is to establish an impedance spectrum quantitative validation technique to validate the whole experimental spectrum and to identify the individual points within a spectrum that do not comply any of the four conditions, in order to remove these inconsistent points from the analysis. The designed validation method consists in a Kramers Kronig (KK) validation test, by equivalent electrical circuit fitting, coupled with a Montecarlo error propagation method. In a first step, the experimental spectrum is fitted to a particular electrical equivalent circuit, which satisfies the KK relations. Then, in a second step, a statistical Montecarlo method is used in order to propagate the model fitting parameter uncertainty through the model. Using this approach, a consistency region is built for a given confidence level: the experimental points inside this region are considered consistent for the given confidence level, whereas the outside points are rejected. The method was used on PEMFC experimental impedance spectra; and it successfully managed to identify inconsistent points, associated to no stationarities.The authors are very grateful to the Generalitat Valenciana for its economic support in form of Vali+d grant (Ref: ACIF-2013-268).Giner Sanz, JJ.; Ortega Navarro, EM.; Pérez-Herranz, V. (2015). Montecarlo based quantitative Kramers-Kronig test for PEMFC impedance spectrum validation. International Journal of Hydrogen Energy. 40(34):11279-11293. https://doi.org/10.1016/j.ijhydene.2015.03.135S1127911293403

Study of the Catalytic Layer in Polybenzimidazole-based High Temperature PEMFC: Effect of Platinum Content on the Carbon Support

International audienceIn this work, the effect of platinum content on the carbon support in commercial catalyst for electrodes to be used in a Polybenzimidazole (PBI)-based PEMFC has been studied. Three contents of platinum on a carbon support were studied (20 %, 40 % and 60 %). In all cases, the same quantity of PBI in the catalyst layer, which is required as a “binder” was used. From Hg porosimetry analyses, pore size distribution, porosity, mean pore size, and tortuosity of all electrodes were obtained. In all cases, a similar electrode mesostructure was observed. The electrochemical characterization was performed by voltamperometric studies, assessing the electrochemical surface area (ESA) of the electrodes, and by impedance spectroscopy (IS), determining the polarization resistance, and by the corresponding fuel cell measurements. The best results were obtained for the electrodes with a content of 40 % of platinum that led to power densities of 0.55 W/cm2 and 0.3 W/cm2 using O2 and air respectively, at 125 ºC. It has been demonstrated that the temperature has a favouring effect on fuel cell performance and flow humidification did not have remarkable effects as it was expected