S. Gama

Magnetocaloric effect in GdGeSi compounds measured by the acoustic detection technique: Influence of composition and sample treatment

In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd5(Ge1−xSix)4 compounds, in the temperature range from 230 to 360 K and for magnetic fields up to 20 kOe. Measurements were performed in as-cast materials, both for powder and pellet samples, and in tree samples with compositions around Gd5Ge2Si2 that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd5(Ge1−xSix)4 compounds with 0.500<x≤1.00. For the alloys with composition around Gd5Ge2Si2, which exhibit giant magnetostriction and coexistence of distinct phases, expressive changes were observed when comparing powder and pellet samples. Based on these cases, it is easy to see that the acoustic method can distinguish a second-order phase transition from a first-order magnetic-crystallographic one, and that it presents good sensitivity to detect spurious material phase in small quantities.

Determination of the entropy change using the acoustic detection technique in the investigation of the magnetocaloric effect

In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, ΔST, a parameter normally used to characterize the magnetocaloric effect. The measurements were performed for a Gd sample in the 252–316 K temperature range for magnetic fields from zero up to 50 kOe. The reversible adiabatic curves were built in a T versus H diagram, and specific heat data obtained at zero-magnetic field were employed to assign the entropy values of each curve. Subsequently, the entropy was plotted as a function of temperature for fixed magnetic fields, and therefore the isothermal entropy variation, ΔST, was found as a function of the temperature for several magnetic field steps.

Electric field triggering the spin reorientation and controlling the absorption and release of heat in the induced multiferroic compound EuTiO3

We report remarkable results due to the coupling between the magnetization and the electric field induced polarization in EuTiO3. Using a microscopic model Hamiltonian to describe the three coupled sublattices, Eu-(spin-up), Eu-(spin-down), and Ti-(moment), the spin flop and spin reorientation phase transitions were described with and without the electric-magnetic coupling interaction. The external electric field can be used to tune the temperature of the spin reorientation phase transition TSR = TSR(E). When the TSR is tuned around the EuTiO3—Neel temperature (TN = 5.5 K), an outstanding effect emerges in which EuTiO3 releases heat under magnetic field change. The electric field controlling the spin reorientation transition and the endo-exothermic processes are discussed through the microscopic interactions model parameters.

On the influence of thermal hysteresis on the performance of thermomagnetic motors

Although thermal hysteresis might be a problem in the magnetocaloric refrigeration, the same is not necessarily true for thermomagnetic motor applications. This work presents a comparison of the magnetocaloric properties of materials with first order magnetic transition (having large or narrow thermal hysteresis) to those with second order magnetic transition, assessing the application of these materials in thermomagnetic motors through a thermodynamic approach. Results show that the larger the thermal hysteresis, the higher the specific work produced in a thermal cycle. This allows operation at higher temperature differences with high efficiency relative to Carnot efficiency, when compared with systems using narrow hysteresis and second order transition materials.Although thermal hysteresis might be a problem in the magnetocaloric refrigeration, the same is not necessarily true for thermomagnetic motor applications. This work presents a comparison of the magnetocaloric properties of materials with first order magnetic transition (having large or narrow thermal hysteresis) to those with second order magnetic transition, assessing the application of these materials in thermomagnetic motors through a thermodynamic approach. Results show that the larger the thermal hysteresis, the higher the specific work produced in a thermal cycle. This allows operation at higher temperature differences with high efficiency relative to Carnot efficiency, when compared with systems using narrow hysteresis and second order transition materials.

Analytic and Experimental Analysis of Magnetic Force Equations

The design of magnetic devices requires a precise estimation of magnetic forces. In previous works, we presented a general approach to estimate these forces based upon thermodynamically closed systems, resulting in four different forms of the force equations. This paper presents a complete theoretical model analysis tested by experiments, using arrangements of permanent magnets and a device for measuring the force induced on a soft magnetic material according to its position with respect to the permanent magnets. The results of analytical formulation, Finite Element Method numerical analysis, and experiments are compared with each other. This enabled the identification of two forms of the force equations that most precisely describe the magnetic forces. A follow-up experiment is then proposed and executed, identifying the correct form of the magnetic force equations. The resulting equation can be used to analytically estimate the magnetic force in many practical problems.

Comparative analysis of magnetic and caloric determinations of the magnetocaloric effect in Mn0.99Co0.01As

The results from direct and indirect determinations of the magnetocaloric parameters of Mn0.99Co0.01As have been analyzed. The isothermal entropy change (ΔS T ) due to external magnetic field changes has been determined through direct measurements of the heat absorbed by the sample when the field is removed under isothermal conditions. It has also been calculated from isothermal magnetization and isofield heat capacity measurements. In addition to that, the adiabatic temperature change (ΔTS ) has been obtained from direct measurements and from isofield heat capacity measurements. The different results of both ΔS T and ΔTS are compared. Due to the presence of a first-order phase transition, the studied sample exhibits giant mag- netocaloric effect at room temperature. The observed maximum values, −ΔS T,max = 32.1 J/kg·K and ΔTS ,max = 17.7 K for a field change from 0 to 6 T, suggest that the Co-doped MnAs compounds are promising candidates for the preparation of useful materials for magnetic refrigeration near room temperature.

Study of thermomagnetic energy conversion driven by renewable energy

Magnetic heating, refrigeration and energy conversion have been defined very important challenges for promising environmentally future choices. Recent studies are focused attention on new thermomagnetic systems, devices working through thermomagnetic effect, driven by renewable energies. The aim of this work is a case study presentation of new thermomagnetic energy prototype coupled with renewable technologies or industrial waste heat. The paper is divided into four sections: the first gives a general state of art about magneto-caloric energy conversion systems, the second describes all general thermodynamics and magnetic laws governing the process, the third and fourth sections study in more detail the case study presented.

Photoacoustic based technique for measuring the magnetocaloric effect

This paper presents a method for detecting the magnetocaloric effect (MCE), based on the acoustic detection. Small temperature oscillations, due to the application of a modulated magnetic field, are detected by a microphone in a closed cell. The continuous scanning of a superimposed dc magnetic field allows, by numerical calculation, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Measurements were performed in Gd and Gd5(SixGe1-x)4 compounds. The obtained results show the efficiency of the technique, which is suitable for the investigation of materials undergoing both purely magnetic phase transitions and magnetic-crystallographic first order ones.