The maximum possible magnetocaloric ΔT effect

Abstract

The current boom of research activity in magnetocaloric materials science is fuelled by the expectation that new advanced refrigerants may be found whose ΔT will significantly surpass that of gadolinium (Gd) metal (2.6–2.9 K/T). Because of this expectation, the main effort in the field has been diverted from the important issues of refrigerator design to the routine characterization of magnetic materials. Estimating the maximum adiabatic temperature change that can be achieved in principle by applying a certain magnetic field, say 1 T, is a matter of priority. In this work the problem of maximum ΔT is approached from general principles. According to the most optimistic estimates, ΔT can never exceed ∼18 K/T, the more realistic upper limit lying somewhere in high single figures. We therefore deem it most unlikely that a refrigerant much better than Gd, in respect of the ΔT value, will ever be found.

Magnetocaloric materials: The search for new systems

“…A similar conclusion was reached by Zverev et al, in considering the maximum possible ∆T ad near a Curie transition [21]. The authors started from the approximation that, within a small temperature interval, ( ∂M ∂T ) H is constant, and showed that the optimum value of ∆T max ad is…”

“…Since ∆T ad is the temperature difference between two isofield S (T ) curves, the bound on entropy change yields a bound on ∆T ad . Two limiting scenarios, described in [21] arise from extremes in ∂T c /∂H as shown schematically in Figure 1. If ∂T c /∂H is small, ∆T ad is equal to the temperature width, ∆T w , of ∆S , which is ∂T c /∂H × H max and is therefore linear in ∂T c /∂H.…”

Influence of La and Ce additions on the magnetocaloric effect of Fe–B–Cr-based amorphous alloys

“…To study this reduction in RC FWHM , the compositional dependence of full temperature width at half maximum ͑␦T FWHM ͒ of ͉⌬S M pk ͉ was plotted ͓inset of The decrease in RC for the La alloy series could be attributed to the reduction in ͉⌬S M pk ͉ ͑RC being proportional to ͉⌬S M pk ͉ and ␦T FWHM ͒ since ͉⌬S M pk ͉ generally plays a dominant role in RC maximization for SOMT materials. 27 For the Ce alloy series, the reduced ͉⌬S M pk ͉ and ␦T FWHM contributes to a much lower RC value, e.g., in the Ce15 alloy. For the Gd series, the large decreasing trend of RC could be attributed to the large reduction in ͉⌬S M pk ͉ with Gd addition.…”

Research Update: The mechanocaloric potential of spin crossover compounds

“…6,13 In the simplest approximation, the adiabatic temperature change may be simplified to ∆T ad (T,Y applied ) = (∂T t /∂Y i )Y applied for values of (∂T t /∂Y i ) that are below the value at which ∆T ad (T,Y applied ) is maximised. Beyond the value of (∂T t /∂Y i ) that maximises ∆T ad , ∆T ad (T,Y applied ) is instead related to the isothermal entropy change via the heat capacity, C p through ∆T ad = −T∆S/C p .…”