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Abstract

Solid-state cooling technologies have received increased attention as suitable alternatives to conventional vapor-compression systems. In recent years, significant research has been done on caloric materials. In the pursuit of improved room-temperature caloric responses, with potential for multifunctionality, multicalorics have also been the focus of research, as these exhibit distinctive caloric responses under different external stimuli. However, thus far, there are no reports on the preparation and characterization of room-temperature electrocaloric-magnetocaloric (EC-MC) composites. Because of this, the main focus of this thesis is the preparation and characterization of the caloric properties of 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3/LaFeSiCo (PMN–10PT/LFSC) composite thick films prepared via the aerosol deposition (AD) method.
We started our work with a preliminary study on non-caloric ceramic-metal films to better understand their inherent properties. By combining Al2O3 with different vol% of Al, a wide range of composite films were prepared via AD. While Al2O3 exhibited a permittivity of ∼12, composites reached values as large as ∼800, at 27 vol %, albeit with large dielectric losses. However, up to 15 vol% of Al, an enhancement in permittivity was observed with no significant increase in losses. Our results show that, even at low vol% of metal, improved electrical properties can be achieved when preparing the AD composites.
Moreover, reports that utilize direct characterization techniques of caloric responses, such as thermistor-in-calorimeter and the infrared (IR) camera method, mostly focus on bulk samples. In the case of films, the influence of heat losses to the substrate and to the black coating layer, necessary for the IR camera method, remain unaddressed. In light of this, we have numerically investigated the criterion required to achieve maximized measurements response. First, the substrate should exhibit a thermal effusivity ≤3 kW s1/2 m-2 K-1 or a thermal conductivity ≤1 W m-1 K-1, both of which are satisfied in the case of polyimide. Thus, polyimide was chosen as the substrate for the multicaloric thick films. In addition, in order to achieve maximized measurement response during IR camera characterizations, the simulations indicated that the caloric films should exhibit a thickness ≥5 μm, while the black paint coatings should not exceed this value. In light of this, we have also developed the airbrush method, which is a novel and affordable black paint coating method, and can prepare coatings as thin as ∼3 μm.
Lastly, by combining the PMN–10PT EC ceramic with 2 wt% of LFSC MC alloy, the aerosol-deposited composite thick films were successfully prepared on polyimide substrates. Due to the embedded MC metallic particles, the composites exhibited enhanced electrical properties, when compared to the PMN–10PT films. In particular, the room-temperature EC response of composites was ∼0.45 K, at 250 kV cm-1, whereas for PMN–10PT films the value was ∼0.35 K. In other words, a 32 % enhancement in EC response was achieved. The room-temperature MC response of composite films was also directly characterized, achieving a MC response ∼1.4 K at 2 T, after corrections. In this way, for the first time, we demonstrate that it is possible to prepare multicaloric composites with both caloric properties at room-temperature. Our results offer novel insights and perspectives on the development of alternative cooling technologies