[en] The interplay between activation volumes and microstructure is investigated in nanocrystalline Fe_73.5Cu₁Nb₃Si_13.5B₉ (Finemet) alloys. Experiments are performed beyond the Curie point of the amorphous matrix, where relaxation effects are relevant. Measurements are analyzed within a theoretical framework where hysteresis and relaxation phenomena are jointly described. In highly crystallized samples magnetization processes are characterized by a unique length scale. In poorly crystallized samples the system behavior is controlled by a distribution of characteristic volumes related to structural disorder. [copyright] 2001 American Institute of Physics

[en] Highlights: •MHC reduction and combustion efficiency improvement by reducing the compression ratio. •Compression ratio and Air-throttling impact on emissions and particles in dual-fuel mode. •CO₂ reduction and GHG impact over the NEDC test in dual fuel mode. -- Abstract: Economic and technological issues related to meet the future harmful emissions and greenhouse gases (GHG) standards are pushing the scientific community to consider studying alternative routes for low emissions and high efficient propulsion systems. A possible approach is represented by the dual-fuel (DF) concept applied to high efficient compression ignition engines. In this context, the results of a wide experimental campaign performed on a single-cylinder engine platform equipped with modern combustion and injection systems operated in dual-fuel diesel/methane mode are presented. The effects of the compression ratio, injection parameters and air-throttling on the global performances and emissions, also in terms of particle size spectrum, are assessed. The tests were performed in several operating points representative of real working conditions of an automotive light-duty (LD) diesel engine in order to carry out the fuel consumption (FC), the GHG and the pollutant emissions estimation on the New European Driving Cycle (NEDC) test procedure. A proper DF engine parameter calibration was set-up observing constraints in terms of in-cylinder peak firing pressure, pressure rise rate, cycle-to-cycle variation and engine-out emissions. A significant impact of the CR variation and injection parameters on the combustion characteristics and emissions is revealed in DF mode (MHC). In particular, benefits on methane unburnt (MHC) and combustion noise can be attained reducing the CR. A global CO₂ reduction of about 12%, over the NEDC, can be definitely obtained with an average CH₄ substitution rate of about 50% and independently of the CR at the expense of the DF- CO₂ equivalent that is higher (22%) compared to D mode. The DF particles concentration in the accumulation mode is generally reduced as well as the estimated particle number on the NEDC. Moreover, within the tested engine class, a CR of 15.5 appears to be the best compromise among the global efficiency and pollutant emissions outputs to operate in DF mode.

[en] Energy loss and initial permeability behavior at medium and high frequencies are optimized in transversally field annealed amorphous ribbons when the magnetization process is accomplished by a mix of domain wall displacements and moment rotations. A reason for this is found in the selective damping mechanism for the motion of the domain walls, whose contribution to the total magnetization reversal is progressively reduced in favor of rotations with increasing the magnetizing frequency

[en] The effect of induced magnetic anisotropy on GMI has been studied in Co₇₁Fe₄B₁₅Si₁₀ melt spun amorphous ribbons, using two different techniques of thermomagnetic annealing. Quasi-static hysteresis loops were measured at controlled induction. From the quasi-static hysteresis loop evolution, the value of the macroscopic anisotropy K_u induced by the field annealing treatment was determined. The evolution of the domain structure was observed using a Kerr-effect apparatus

[en] Hysteresis and relaxation properties are studied in nanocrystalline Finemet-type materials beyond the Curie point of the amorphous matrix. Measurements of hysteresis loops and return branches at a temperature near the maximum hardening are used to investigate the microstructure of the material. Data are discussed by a unified model of hysteresis and thermal activation

[en] The effect of induced magnetic anisotropy on GMI has been studied in Fe_73.5Nb₃Cu₁Si_13.5B₉ melt-spun amorphous and nanocrystalline ribbons. The amorphous-to-nanocrystalline transformation has been induced by means of furnace annealing and DC joule heating. The correlation between average magnetic anisotropy calculated from quasi-static hysteresis loops and GMI will be discussed in detail

[en] It is shown that the strong reduction of the magnetic losses observed in nanocrystalline alloys after transverse field annealing is chiefly related to an abnormally slow increase of the excess loss component with the magnetizing frequency. Eddy current counterfields tend to hinder the domain wall processes to a larger extent than spin rotations, progressively inhibiting the related loss mechanisms at increasing frequencies. This results in a reduced role by the excess and hysteresis loss components, which can be assessed by suitable generalization of the statistical theory of losses

[en] Hysteresis and relaxation properties are studied in nanocrystalline Finemet-type materials, prepared with different crystalline volume fractions, beyond the Curie point of the amorphous matrix. The dependence on temperature and field rate of the measured coercive fields is discussed. The connection between activation volumes and structural aspects is analyzed

[en] An overview is given of the present understanding of hysteresis phenomena in magnetic materials. The problem is addressed from three approximate viewpoints: the connection between rate-independent hysteresis and micromagnetics; the modifications brought into this picture by thermal relaxation effects; the role of rate-dependent magnetization mechanisms, like eddy-current-damped domain wall motion

[en] Nanocrystalline soft magnetic materials (Fe_73.5Cu₁Nb₃Si_13.5B₉) are investigated at elevated temperatures where, by changing the crystalline volume fraction, the magnetic behavior changes from a regime dominated by hysteresis and relaxation effects (high crystallization levels) to a superparamagnetic behavior (low crystallization levels). At intermediate crystalline fractions, it is shown that the approach to saturation can be described by assuming, as it was previously done for the description of relaxation and hysteresis effects, the existence of clusters of coupled grains distributed in size