Electrocaloric and barocaloric effects in organic materials
Electrocaloric (EC) and barocaloric (BC) materials undergo reversible thermal changes in response to changes in applied electric field and pressure, respectively. These materials could potentially be exploited in novel solid-state cooling systems that may replace current vapour-compression systems, which are environmentally harmful, noisy, and relatively energy inefficient. In this work, I studied EC effect and BC effects in multicaloric organic materials, which promise large caloric effects near room temperature. The dissertation is structured as follows. Chapter 1 introduces the background for conventional refrigeration and traditional caloric materials. Chapter 2 then surveys the literature on EC materials and BC materials, as well as the literature on EC prototype devices. Chapter 3 reviews the experimental and modelling methods used for this work. These include sample preparation methods, dielectric spectroscopy and ferroelectric polarisation measurements, calorimetry and infrared imaging, dilatometry and Landau models. Chapter 4 describes the study of EC effects in two dabco-based organic salts, namely [Hdabco][BF4] and [AH][ReO4], where dabco is 1,4-diazabicyclo[2.2.2]octane and AH is a variant of dabco, 1-azabicyclo[2.2.1]heptanium. Experiments and modelling demonstrates that [Hdabco][BF4] shows giant EC effects (isothermal entropy change |∆S| = 15.5 J K-1 kg-1 for |∆E| = 12 kV cm-1) that are one order-of-magnitude larger than those observed in traditional EC oxides such as BaTiO3 (|∆S|= 2.1 J K-1 kg-1 for |∆E| = 4 kV cm-1). [AH][ReO4] shows smaller EC effect of |∆S|~ 7.5 J K-1 kg-1 for |∆E| = 11.2 kV cm-1, but displays better mechanical integrity and operates closer to room temperature. It is concluded that dabco-based organic salts exhibit promising performance due to their large EC effects, non-toxicity and great tunability via chemical alterations. However, electrical leakage and breakdown remains an important challenge to be overcome for their use as EC working bodies. Chapter 5 describes the study of BC effects in the aforementioned dabco-based organic salts. BC effects in these materials have the advantage to be driven using hydrostatic pressure instead of electric field, thus avoiding issues related to electrical leakage and breakdown. Three compositions were selected for BC studies, namely [Hdabco][BF4], [Hdabco][ClO4] and [Hdabco][ReO4]. Among these, [Hdabco][ClO4] shows the largest reversible BC effects, |∆S| = 73.2 J K-1 kg-1 for |∆p| = 1200 bar, which compare well with those reported in state-of-the-art BC materials. Notably, BC effects in [Hdabco][BF4] largely outperform EC effects in the same compound, thus demonstrating that pressure is a useful driving parameter for leaky organic ferroelectrics. Chapter 6 describes BC studies in ureasil polymeric materials. These compounds show large changes in entropy when transforming from liquid to solid. By exploiting a gelation method, the liquid to solid phase transition in these compounds is transformed to a gel-to-solid phase transition, which is desirable for some caloric applications. By driving these transitions using pressure, very large BC effects of |ΔS| = 263 J K−1 kg−1 for |∆p| = 1200 bar are found, which are similar to those observed in commercial vapour-compression refrigerants, e.g. R134a. Moreover, the studied polymers have other advantages, in terms of being stretchable, non-toxic, inexpensive and have great tunability of transitions temperatures. Finally, chapter 7 summarises the main results of this work and discusses interesting avenues for future work.
Barocaloric, Caloric effects, Electrocaloric, Ferroelectric, Ureasil
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