Materials for cooling: Modulation infrared thermometry of caloric effects and their dynamics

Jago Döntgen, Ruhr University Bochum, Bochum, Germany
Jörg Rudolph, Ruhr University Bochum, Bochum, Germany
Daniel Hägele, Ruhr University Bochum, Bochum, Germany

Caloric materials show a large temperature change upon adiabatic change of their magnetization, dielectric polarization or strain, respectively. These materials are the foundation of an innovative solid-state cooling technology that is potentially more energy-efficient and environment-friendly than conventional compressor-based refrigeration. We present direct measurements of the caloric effects found at the first-order magneto-elastic phase-transition in La(Fe,Si)13 and at the diffuse phase-transition in the relaxor ferroelectric PMN-PT. Our newly developed measurement technique combines rapidly modulated fields up to the kHz range and infrared-detection of the sample’s thermal radiation [1]. This results in a temperature resolution on the µK scale with a µs time resolution, which allows for the measurement of samples with thicknesses down to a few µm [2].

Time-resolved measurements allow a clear distinction between (reversible) adiabatic and (irreversible) dissipative contributions to the measured total temperature change. Our measurements show an unexpected self-quenching behavior in La(Fe,Si)13 involving adiabatic as well as dissipative effects [3]. We propose a mechanism based on local undercooling of the material in the vicinity of the ferromagnetic-paramagnetic phase boundary similar to earlier findings [4]. Temperature dependent measurements of the electrocaloric effect in PMN-PT show a pronounced aging behavior both in the adiabatic temperature change and the simultaneously measured dielectric polarization. This effect is known in relaxor ferroelectrics but has not been investigated in the context of electrocalorics. We show that the electrocaloric effect is reduced by up to 50% by simply letting the sample rest at room temperature. This reduction is reversible however, as heating the sample above 340 K restores its original caloric and dielectric properties.

[1] Döntgen et al., Rev. Sci. Instrum. 89, 033909 (2018)
[2] Döntgen et al., Appl. Phys. Lett. 106, 032408 (2015))
[3] Döntgen et al., Energy Technol. 6, 1470 – 1477 (2018)
[4] Lovell et al., Adv. Energy Mater. 5, 1401639 (2015)

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