Direct measurement of the magnetocaloric effect in cementite

Measurements of the magnetocaloric effect of cementite at its Curie temperature of 475 K are presented. An adiabatic temperature change of 1.76 +/- 0.01 K was measured using a direct measurement technique. The isothermal entropy change was determined from measurements of magnetisation isotherms and was shown to be 3.07 J K-1 kg(-1) in a field change of 2 T. The field dependencies of both magnetocaloric properties follow the H-2/3 dependence typical for ferromagnetic materials with a second order phase transition. The material may be of interest in magnetocaloric applications such as magnetic refrigeration or thermomagnetic power generation. (C) 2016 Elsevier B.V. All rights reserved

Assessment of the magnetocaloric effect in La,Pr(Fe,Si) under cycling

The response of a magnetocaloric material to periodic variations of magnetic field and temperature corresponding to those occurring during a magnetic refrigeration process is studied. A series of simple measurement protocols are suggested which are used to obtain a value for the cyclic response of the magnetic entropy change associated with the magnetic transition. The entropy values are compared to direct measurements of the temperature change under adiabatic conditions. The procedure is illustrated on the first order magnetocaloric material La0.6Pr0.4Fe11.6Si1.4 and provides a basis for comparison of the suitability of different hysteretic magnetocaloric materials for application in a magnetic refrigerator. For the alloy studied here the peak magnetic entropy change of -28 +/- 11 kg(-1) K-1 in a field change of 2 T is not affected by cycling, but the full width at half maximum of the peak decreases from 8.7 K to 3.8 K. (C) 2016 Elsevier B.V. All rights reserved

Review of the mechanical and fracture behavior of perovskite lead-free ferroelectrics for actuator applications

There has been considerable progress in the development of large strain lead-free perovskite ferroelectrics over the past decade. Under certain conditions, the electromechanical properties of some compositions now match or even surpass commercially available lead-containing materials over a wide temperature range, making them potentially attractive for non-resonant displacement applications. However, the phenomena responsible for the large unipolar strains and piezoelectric responses can be markedly different to classical ferroelectrics such as Pb(Zr,Ti)O3 and BaTiO3. Despite the promising electromechanical properties, there is little understanding of the mechanical properties and fracture behavior, which is crucial for their implementation into applications where they will be exposed to large electrical, mechanical, and thermal fields. This work discusses and reviews the current understanding of the mechanical behavior of large-strain perovskite lead-free ferroelectrics for use in actuators and provides recommendations for further work in this important field

Behaviour of the Young's modulus at the magnetocaloric transition in La(Fe,Co,Si)(13)

Magnetic solid state cooling applications require families of samples where the magnetic transition is cascaded across the working range of the fridge. Although magnetic properties are widely studied, information relating to the mechanical properties of such systems is less prevalent. Here we study the mechanical properties of a series of magnetocaloric La(Co,Fe,Si)13 samples where the Co content is varied to produce a range of transition temperatures. It was found that at room temperature the flexural strength decreases and the Young's modulus increases with increasing Co content. Interestingly we find a significant reduction of Young's modulus at temperature around the magnetic transition temperature. This reduction was less pronounced with increasing Co content. We associate the softening with the magnetovolume coupling known to exist in these materials

Heat exchangers made of polymer-bonded La(Fe,Si)13

We report on magnetocaloric properties of polymer-bonded La(Fe,Si)(13) heat exchangers with respect to the grain size of the powder used and the pressure applied for compaction of plates. Quite remarkably, it was found that the values of the adiabatic temperature change of polymer-bonded plates are 10% higher than in the initial bulk material. A critical value of the pressure applied during the compaction was found. Exceeding this value leads to a drastic reduction of the magnetocaloric effect due to cracking and comminution of the initial 50-100 mu m grains down to 1-10 mu m fragments. Compacting the LaFe11.6Si1.4 powder with 5 wt.% of silver epoxy under an optimal pressure of 0.1 GPa resulted in the production of 0.6 mm-thick plates. These plates were subsequently stacked and glued together into a simple porous heat exchanger with straight 0.6 mm-width channels