Dynamic effects in the characterization of the magnetocaloric effect of LaFeSi-type alloys

Abstract

In this work, the influence of the rate of change of the applied field to three magnetocaloric samples is studied: a bulk Gd sample used as a reference material, and fully hydrogenated La0.7Ce0.3Fe11.7Si1.3H1.8 and La0.7Ce0.3(Fe0.882Mn0.018Si0.1)13H1.8 alloys, denoted as ERASTEEL-NX and ERASTEEL-11688, respectively . Bulk samples ERASTEEL-NX and ERASTEEL-1688 were prepared from powder batches obtained by gas atomization process as described in [4] . The powder was then sintered at 1150°C for 5 hours in Ar (it has to be noticed that this is not the optimal temperature for maximizing the proportion of NaZn13 type phase), and the obtained blocs were treated at 400°C for 1 hour in hydrogen . The final proportions of NaZn13 type phase is respectively 80 .5% (instead of 92% for optimized heat treatment) and 85% (instead of 92% also for optimized heat treatment) in ERASTEEL-NX and ERASTEEL-1688 . ERASTEEL-NX, with 30% of Ce substituted to La and no substitution on the Fe site shows a very strong 1st order transition, with a wide hysteresis . ERASTEEL-1688 contains some Mn atoms in substitution to Fe . This induces a softening of the 1st order character of the magnetic transition in this sample . The adiabatic temperature change of the samples has been measured using a commercial setup consisting of a set of Halbach cylinders as a variable field source up to 1.78 T, and a thermocouple placed between two pieces of sample, with dimensions of 8 x 4 x 0 .75 mm, which registers the temperature difference between the sample and the sampleholder . The sample is kept in vacuum inside a bath cryostat . The temperature of the sampleholder is stabilized before performing each of the adiabatic measurements . The rate of change of the field ranged from 0 .25 T s-1 to 3 T s-1, which is equivalent to frequencies of a magnetic refrigerator operating between 0 and 1 .78 T ranging from 0 .07 Hz to 0 .84 Hz .