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2013
Journal Article
Titel
The maximal cooling power of magnetic and thermoelectric refrigerators with La(FeCoSi)13 alloys
Abstract
Using our data on magnetic entropy change DSm, adiabatic temperature change DTad and heat capacity CH for La(FeCoSi)13 alloys, the upper limit of heat Qc transferred per cycle, and the lowest limit of consumed work Wc were established for magnetic refrigerators operating in Dm0H=1.9 T . In order to estimate the cooling power, attributable to thermoelectric refrigerators with La(FeCoSi)13, thermal conductivity l, resistivity r, and Seebeck coefficient a were measured and the maximal cooling power QL, the input power Pi, and coefficient of performance have been calculated. La(Fe,Si)13-based compounds are among the most promising magnetocaloric materials, they show a large magnetocaloric effect (MCE) and have been widely studied from perspective of fundamental research and practical applications.1,2 The magnetocaloric properties (adiabatic temperature change DTad and magnetic entropy change DSm) and the Curie temperature TC of La(Fe,Si)13 alloys can be widely adjusted by small additions of other elements like H or Co. The addition of Co also alters the nature of the magnetic phase transition from first to second order.3 Thermal conductivity l and resistivity r are also required parameters for a comprehensive characterization of MCE materials. From other side, l, r, and Seebeck coefficient a specify the efficiency of thermoelectric (TE) materials and by adding the Seebeck coefficient to the set of parameters suitable for MCE materials, we are also able to assess the utility of La(FeCoSi)13 compounds for application in TE modules. In addition to the material parameters DSm, DTad , CH, r, l, and a, it is important to determine: (1) the amount of the heat Qc transferred per unit time, (2) the work Wc consumed by refrigerator per unit time, (3) the coefficient of performance (COP). Such universal parameters as Qc, Wc, and COP show the potential effectiveness of the refrigerator and refrigerant, irrespectively of the physical principle of cooling. This fact allows us to compare different refrigeration technologies. The objective of this work is the numerical determination of the maximal Qc, COP, and minimal Wc as a function of thermal span of magnetocaloric and thermoelectric refrigerators using La(FeCoSi)13 alloys. Sintered La(Fe1−xCox)Siy samples were produced by Vacuumschmelze GmbH. Detailed processing has been described previously.4 Samples of LaFe11.5Si1.5 and LaFe8Co5, both with the NaZn13-type structure, were prepared by technique described in Ref. 1 . CH was measured using a Quantum Design physical property measurement system (PPMS). Direct measurements of DTad were performed in a home-built experimental setup described in details elsewhere.5 Thermal conductivity measurements were performed using a steady-state method: the temperature gradient was first measured between two points in a rod sample, and then the l was calculated based on Fourier's law taking account of the heat flux through a cross-section of the sample. The Seebeck coefficient a was measured straightforwardly. The temperature gradient was maintained across a sample of rectangular bar shape, and the Seebeck emf which is proportional to the magnitude of the temperature difference has been detected by two thermocouples. Resistivity was measured by standard four-contact technique. To show the whole picture of magnetocaloric properties of the investigated alloys, we collected our DSm(T) and DTad(T) dependences in a single plot in Fig. 1(a). The substitution of Co results in a shift in the Curie temperature up to room temperature, but it is simultaneously accompanied by the decrease of DTad and DSm. Increasing Co content changes the type of the transition from first order (FOT) to second order (SOT).