Progetto e realizzazione di un setup per l’analisi del rolling contact: risultati preliminari

Presso il Dipartimento di Ingegneria Meccanica dell’Università di Cagliari (DIMECA) è attiva, da alcuni anni, una linea di ricerca orientata alla valutazione dei principali parametri di contatto (forma e dimensioni dell’area nominale di contatto, area reale di contatto, distribuzione delle pressioni di contatto) in interfacce metalliche, mediante una tecnica sperimentale basata sull’impiego di onde ultrasoniche. L’applicazione di questo metodo ha consentito di ottenere informazioni su contatti a geometria semplice quali, ad esempio, quelli sfera-piano [1-2] su casi più complessi di rilevante impatto ingegneristico come quello relativo all’interazione tra ruota e rotaia ferroviaria [3-4]. Il principale punto di forza dell’indagine ultrasonica risiede nella sua capacità di fornire informazioni sullo stato del contatto a partire da misure del coefficiente di riflessione all’interfaccia, garantendo in tal modo sia la più completa non-invasività (nessun mezzo viene ad interporsi tra i corpi a contatto) e sia la possibilità di investigare “in process”. In particolare quest’ultima peculiarità permette di monitorare le variazioni del parametri di contatto in tempo reale (ad esempio a seguito di variazioni nel carico applicato o nella configurazione geometrica dell’accoppiamento) senza che sia necessario rimuovere i corpi per esaminare gli effetti che tali modifiche hanno generato. Appare importante sottolineare che, a tutt’oggi, la letteratura riporta esempi di impiego de metodo ultrasonico a problemi di contatto quasi esclusivamente nell’analisi di situazioni statiche. Tuttavia, è facilmente intuibile che la possibilità di estendere il campo di applicazione della tecnica a situazioni dinamiche, aumenterebbe in misura considerevole il range di casi di interesse ingegneristico potenzialmente testabili. In considerazione di ciò, il presente studio si pone come obiettivo principale quello di verificare la validità ed affidabilità del metodo ultrasonico per lo studio di situazioni dinamiche, con particolare riferimento a problemi di “rolling contact”

Memory and rejuvenation in a spin glass

The temperature dependence of the magnetisation of a Cu(Mn) spin glass ($T_g$ $\approx$ 57 K) has been investigated using weak probing magnetic fields ($H$ = 0.5 or 0 Oe) and specific thermal protocols. The behaviour of the zero-field cooled, thermoremanent and isothermal remanent magnetisation on (re-)cooling the system from a temperature (40 K) where the system has been aged is investigated. It is observed that the measured magnetisation is formed by two parts: (i) a temperature- and observation time-dependent thermally activated relaxational part governed by the age- and temperature-dependent response function and the (latest) field change made at a lower temperature, superposed on (ii) a weakly temperature-dependent frozen-in part. Interestingly we observe that the spin configuration that is imprinted during an elongated halt in the cooling, if it is accompanied by a field induced magnetisation, also includes a unidirectional excess magnetisation that is recovered on returning to the ageing temperature.Comment: EPL style; 7 pages, 5 figure

Towards high-performance electrochemical thermal energy harvester based on ferrofluids

The ionic liquid-based thermo-electrochemical cells receive increasing attention as an inexpensive alternative to solid-state thermo-electrics for waste heat harvesting applications. Recently, it has been demonstrated that magnetic nanoparticles (MNPs) in liquid-based thermoelectric materials result in enhancement of the Seebeck effect opening new perspectives to the design of a thermoelectric device with relatively high efficiency and cost effectiveness. Here, the role of an interacting assembly of MNPs in the thermoelectric signal is studied for the first time. Based on a thermodynamic approach, an analytic expression has been derived for the Seebeck coefficient that includes the inter-particle magnetic interactions in the assembly and the nanoparticle's magnetic characteristics (saturation magnetization, magnetic anisotropy). Mesoscopic scale modelling with the implementation of the Monte Carlo Metropolis algorithm is performed to calculate their contribution to the Seebeck coefficient, for diluted assemblies of \u3b3-Fe2O3 and CoFe2O4 nanoparticles, materials commonly used in ferrofluids. The results demonstrate the increase of the size and temperature range of the Seebeck coefficient with the increase of nanoparticles\u2019 magnetic anisotropy paving the way for the detailed study of the magneto-thermal effects in high-performance thermoelectric materials based on ferrofluids

The interplay between single particle anisotropy and interparticle interactions in ensembles of magnetic nanoparticles

This paper aims to analyze the competition of single particle anisotropy and interparticle interactions in nanoparticle ensembles using a random anisotropy model. The model is first applied to ideal systems of non-interacting and strongly dipolar interacting ensembles of maghemite nanoparticles. The investigation is then extended to more complex systems of pure cobalt ferrite CoFe2O4 (CFO) and mixed cobalt-nickel ferrite (Co,Ni)Fe2O4 (CNFO) nanoparticles. Both samples were synthetized by the polyol process and exhibit the same particle size (DTEM 48 5 nm), but with different interparticle interaction strengths and single particle anisotropy. The implementation of the random anisotropy model allows investigation of the influence of single particle anisotropy and interparticle interactions, and sheds light on their complex interplay as well as on their individual contribution. This analysis is of fundamental importance in order to understand the physics of these systems and to develop technological applications based on concentrated magnetic nanoparticles, where single and collective behaviors coexist

Low-temperature anomalies in muon spin relaxation of solid and hollow nanoparticles: a pathway to detect unusual local spin dynamics

By means of muon spin relaxation measurements we unraveled the temperature spin dynamics in monodisperse maghemite spherical nanoparticles with different surface to volume ratio, in two samples with a full core (diameter D∼4 and D∼5nm) and one with a hollow core (external diameter D∼7.4nm). The behavior of the muon longitudinal relaxation rates as a function of temperature allowed us to identify two distinct spin dynamics. The first is well witnessed by the presence of a characteristic peak for all the samples around the so-called muon blocking temperature T$_{B}^{μ+}$. A Bloembergen-Purcell-Pound (BPP)-like model reproduces the experimental data around the peak and at higher temperatures (20<T<100K) by assuming the Néel reversal time of the magnetization as the dominating correlation time. An additional dynamic emerges in the samples with higher surface to volume ratio, namely, full 4 nm and hollow samples. This is witnessed by a shoulder of the main peak for T<20K at low longitudinal field (μ$_{0}$H≈15mT), followed by an abrupt increase of the relaxation rate at T<10K, which is more evident for the hollow sample. These unusual anomalies of the longitudinal relaxation rate for T<T$_{B}^{μ+}$ are suggested to be due to the surface spins’ dynamical behavior. Furthermore, for weak applied longitudinal magnetic field (μ$_{0}$H≈15mT) and T<T$_{B}^{μ+}$ we observed damped coherent oscillations of the muon asymmetry, which are a signature of a quasistatic local field at the muon site as probed by muons implanted in the inner magnetic core of the nanoparticles. The muon spin relaxation technique turns out to be very successful to study the magnetic behavior of maghemite nanoparticles and to detect their unusual local spin dynamics in low magnetic field conditions

Exchange Bias in Fe@Cr Core–Shell Nanoparticles

We have used X-ray magnetic circular dichroism and magnetometry to study isolated Fe@Cr core−shell nanoparticles with an Fe core diameter of 2.7 nm (850 atoms) and a Cr shell thickness varying between 1 and 2 monolayers. The addition of Cr shells significantly reduces the spin moment but does not change the orbital moment. At least two Cr atomic layers are required to stabilize a ferromagnetic/antiferromagnetic interface and generate the associated exchange bias and increase in coercivity

Memory effects in ultra-small CoFe 2O 4 nanoparticles

We have employed the Monte Carlo (MC) simulation technique to study the aging effect on the Zero-Field-Cooled (ZFC) magnetization curves of ultra-small CoFe 2O 4 nanoparticles (mean size 3c 3 nm) embedded in a Si matrix. We consider spherical nanoparticles consisting of an ordered ferrimagnetic core and a ferrimagnetic disordered surface. The spins in the particles interact with nearest neighbors Heisenberg exchange interaction. Our simulations show that the spin-glass like disorder at the surface affects the magnetic properties to the extent that they exhibit aging effect: the low temperature ZFC magnetization depends on the time (waiting time, t W ) spent before applying the magnetic field at a temperature at which most of the surface moments are frozen. The results of our MC simulations are in good agreement with the experimental findings confirming that the random freezing of surface spins is responsible for the aging effect. \ua9 2006 IEEE