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A figure of merit for assessing the cooling efficiency of electrocaloric (EC) materials is defined, where the caloric properties are taken into account jointly with the material's losses. Using a specifically developed measurement setup, based on flexible thermistances, the caloric effect and the losses are directly measured on P(VDF-TrFE-CFE) electrocaloric polymer films. The data are used, jointly with our figure of merit, to extrapolate the cooling efficiency to be expected, under actual working conditions, from the studied EC material. Dielectric losses emerge as a major limiting factor for achieving the needed cooling performance. This finding shows that, beside the research for huge caloric response, material loss reduction has to be considered a key objective for researching an optimal EC refrigerant for cooling applications. Eventually, some strategies toward loss reduction are outlined. [ABSTRACT FROM AUTHOR]

We have investigated the magnetoelectric (ME) effect in layered composite rings subjected to circumferential AC magnetic fields and DC magnetic fields in radial, axial, or circumferential directions. Bilayer samples were obtained combining different grades of commercial Mn-Zn ferrites or Ni-Zn ferrites with commercial lead zirconate titanate (PZT). Mn-Zn ferrites with low magnetostriction saturation (λs < 10-6) and low magneto-crystalline anisotropy constants show high ME capabilities when associated with PZT in ring structures. In certain conditions, these ME effects are higher than those obtained with Terfenol-D/PZT composites in the same layered ring structure. Magnetostrictive and mechanical characterizations have given results that explain these high ME performances. Nevertheless, Mn-Zn ferrite/PZT composites exhibit voltages responses with low linearity especially at high signal level. Based on the particular structure of the ME device, a method to decrease the nonlinear harmonic distortion of the ME voltages is proposed. Harmonic distortion analysis of ME voltages measured in different configurations allows us to explain the phenomenon. [ABSTRACT FROM AUTHOR]

We report the use of high magnetomechanical coupling ferrites in magnetoelectric (ME) layered composites. Bilayer samples combining (Ni0.973 Co0.027)1-xZnxFe2O4 ferrites (x=0-0.5) synthesized by non conventional reactive Spark Plasma Sintering and commercial lead zirconate titanate (PZT) were characterized in term of ME voltage coefficients measured at sub-resonant frequency. Strong ME effects are obtained and we show that an annealing at 1000 °C and a quenching in air improve the piezomagnetic behavior of Zn-rich compositions. A theoretical model that predicts the ME behavior was developed, focusing our work on the demagnetizing effects in the transversal mode as well as the longitudinal mode. The model shows that: (i) high ME coefficients are obtained when ferrites with high magnetomechanical coupling are used in bilayer ME composites, (ii) the ME behavior in transversal and longitudinal modes is quite similar, and differences in the shapes of the ME curves are mainly due the demagnetizing effects, (iii) in the transversal mode, the magnetic field penetration depends on the ferrite layer thickness and the ME coefficient is affected accordingly. The two later points are confirmed by measurements on ME samples and calculations. Performances of the ME composites made with high magnetomechanical coupling ferrites are compared to those obtained using Terfenol-D materials in the same conditions of size, shape, and volume ratio. It appears that a ferrite with an optimized composition has performances comparable to those obtained with Terfenol-D material. Nevertheless, the fabrication processes of ferrites are quite simpler. Finally, a ferrite/PZT based ME composite was used as a current sensor. [ABSTRACT FROM AUTHOR]

The isotropic properties of Soft Magnetic Composites (SMC) favor the design of new machine topologies and their granular structure can induce a potential decrease of the dynamic loss component. This paper is devoted to the characterization of the broadband magnetic losses of different SMC types under alternating and circular induction. The investigated materials differ by their grain size, heat treatment, compaction rate, and binder type. It is shown that, up to peak polarization Jp=1.25 T, the ratios between the rotational and the alternating loss components (classical, hysteresis, and excess) are quite independent of the material structural details, quite analogous to the known behavior of nonoriented steel laminations. On the contrary, at higher inductions, it is observed that the Jp value at which the rotational hysteresis loss attains its maximum, related to the progressive disappearance of the domain walls under increasing rotational fields, decreases with the material susceptibility. [ABSTRACT FROM AUTHOR]

A thermodynamic model of hysteresis is presented based on an internal variable reinterpretation of the Preisach model. Explicit expressions are obtained for the entropy and the entropy production for arbitrary open field histories. The model is applied to the prediction of nonmonotonic heat exchange along the hysteresis loop measured for ferromagnets. [ABSTRACT FROM AUTHOR]

The interplay between activation volumes and microstructure is investigated in nanocrystalline Fe[sub 73.5]Cu[sub 1]Nb[sub 3]Si[sub 13.5]B[sub 9] (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. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A vector model of magnetic hysteresis in biaxial anisotropy structures is presented, in which magnetoelastic effects are explicitly taken into account. The magnetization vector of individual grains is forced to lie along one of the easy axis, and jumps from one easy direction to another one controlled by the sum of the external field and magnetoelastic energy. Hysteresis effects are described by an energy barrier opposing magnetization switching and the total response of the system is obtained by averaging the local magnetization over all grains. Model predictions are compared with experimental results on the hysteresis properties of Si–Fe alloys under compressive stress. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

This paper deals with the problem of loss evaluation in Soft Magnetic Composites (SMCs), focusing on the classical loss component. It is known that eddy currents can flow in these granular materials at two different scales, that of the single particle (microscopic eddy currents) and that of the specimen cross-section (macroscopic eddy currents), the latter ensuing from imperfect insulation between particles. It is often argued that this macroscopic loss component can be calculated considering an equivalent homogeneous material of same bulk resistivity. This assumption has not found so far clear and general experimental validation. In this paper, we discuss energy loss experiments in two different SMC materials, obtained using different binder types, and we verify that a classical macroscopic loss component, the sole size-dependent term, can be separately identified. It is also put in evidence that, depending on the material, the measured sample resistivity and the equivalent resistivity entering the calculation of the macroscopic eddy currents may not be the same. A corrective coefficient is, therefore, introduced and experimentally identified. This coefficient appears to depend on the material type only. An efficient way to calculate the macroscopic classical loss in these materials is thus provided. [ABSTRACT FROM AUTHOR]

We report on the magnetic field induced isothermal entropy change, Δs(Ha, T), of W-type ferrite with CoZn substitution. Entropy measurements are performed by direct calorimetry. Single crystals of the composition BaCo0.62Zn1.38Fe16O27, prepared by the flux method, are measured at different fixed temperatures under an applied field perpendicular and parallel to the c axis. At 296 K one deduces a value of K1 = 8.7 × 104 J m-3 for the first anisotropy constant, which is in good agreement with the literature. The spin reorientation transition temperature is estimated to take place between 200 and 220 K. [ABSTRACT FROM AUTHOR]

We present novel results on magnetic losses in soft magnetic composites (SMCs) excited with rotating field. Soft composites are very promising in electrical engineering applications, where new topologies of electrical machines with two- and three-dimensional induction loci are increasingly found. An experimental characterization of industrial SMC products has, therefore, been carried out, up to the kilohertz range, under alternating and circular flux loci, making use of a specifically designed and optimized loss measuring setup. The obtained results have been analyzed for all kinds of excitation, according to the loss separation concept, with the emphasis being placed on the relationship between the rotational and the alternating loss components. In particular, it is found that the ratio between the rotational and the alternating losses is, for any given peak induction, independent of frequency. [ABSTRACT FROM AUTHOR]

We present a technique to estimate the inhomogeneities of magnetic loss across the section of ferrite cores under ac excitation. The technique is based on two distinct calorimetric methods that we presented elsewhere. Both methods are based on the measurement of the rate of increase of the sample temperature under adiabatic condition. The temperature ramp is recorded either measuring the sample bulk resistivity or using a platinum probe pasted on the sample surface. As an example we apply the procedure to an industrial sample of Mn-Zn ferrite under controlled sinusoidal excitation with a peak induction of 50 mT in the range between 100 kHz and 2 MHz. The results are discussed by comparison with simulations of the dissipation field profile through the sample, calculated using a finite element method (FEM) code. [ABSTRACT FROM AUTHOR]

We report and discuss significant results on the magnetic losses and their frequency dependence insoft magnetic composites. Two types of bonded Fe-based materials have been characterized atdifferent inductions from dc to 10 kHz and analyzed by extending the concept of loss separation and the related statistical theory to the case of heterogeneous materials. Starting from the experimental evidence of eddy current confinement inside the individual particles, the classical losscomponent is calculated for given particle size distribution. Taking then into account thecontribution of the experimentally determined quasistatic (hysteresis) loss, the excess losscomponent is obtained and quantitatively assessed. Its behavior shows that the dynamichomogenization of the magnetization process with frequency, a landmark feature of magnetic laminations, is restrained in these materials. This results into a partial offset of the loss advantage offered by the eddy current confinement. [ABSTRACT FROM AUTHOR]

Due to the limited rare-earth elements resources, ferrite magnets need to be improved drastically. Ideally, for a true hard magnet, the coercive field should be larger than the saturation magnetization, which is not yet realized for ferrites. Thus, an alternative can be found in making very fine grain ferrite magnets, but it is usually impossible to get small grains and dense material together. In this paper, it is shown that the spark plasma sintering method is able to produce approximately 80% of dense material with crystallites smaller than 100 nm. The as-prepared bulk sintered anisotropic magnets exhibits coercive field of 0.5 T which is approximately 60% of the theoretical limit and only a few percentage below that of loose nanopowders. As a result, the magnets behave nearly ideal (-1.18 slope in the BH plane second quadrant) and the energy product reaches 8.8 kJ m-3, the highest value achieved in the isotropic ferrite magnet to our knowledge. [ABSTRACT FROM AUTHOR]