Investigations of electrocaloric effect in ferroelectric thin-films, single crystals and devices.
Rami Naidu Chukka.
Date of Issue2013
School of Materials Science and Engineering
An electrocaloric (EC) effect is the temperature change accompanied by the change in electric field intensity of a thermally isolated system. The EC effect is relatively new and a challenging research topic in the field of ferroelectric materials. In principle, the EC effect is analogous to magnetocaloric effect, and the latter utilizes magnetic fields to cool. A notable advantage of EC effect is the ease in generating large electric fields compared to magnetic fields. The EC refrigeration of ferroelectric materials is gaining much interest recently in solid-state cooling applications. The cooling method is often described as a clean, robust and efficient process due to its intriguingly simple design and operation. Moreover, an EC cooling device can be more suitable for special needs in medical appliances, hot spot cooling in integrated circuits due to its compactness and low noise performances. The first EC effect was observed in Rochelle salt in the early 1930s. Over the years, extensive research in ceramic materials has produced interesting EC results for broad range of scientific and technological applications. Among them, a highly ordered Pb(Sc0.5Ta0.5)O3 material demonistrating an EC temperature change of ∆T = 2.3 K at Tc ~ 18oC, has been claimed to be the most promising ceramic materials found to date. The practical applications of EC based cooling restricted to very few systems to until 2006. This was because most identified ceramic materials were exhibiting either low ∆T (~1-2 K) values or because engineering their functional properties to meet the practical considerations was difficult. The resurgence of EC cooling was spotlighted only after the discoveries of giant EC temperature changes of 12 K in PbZr0.95Ti0.05O3 and poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] ferroelectric thin-films. Recent results in thin-films show almost six times higher cooling values than the better cooling values found in ceramics. On the bright side, with every two folds increase in ∆T values of EC materials, the operating costs of the refrigeration devices will also be reduced by half. Though the results are very fascinating, thin-film materials suffer from other difficulties such as dominating substrate effects and low heat fluxes. A lot of work must be focused in this direction to realize the EC based cooling which is comparable to other solid-state cooling technologies. In this dissertation, we systematically investigated the EC properties of ferroelectric thin-films, single crystals and devices with a focus on smart engineering materials. The magnitudes of EC coefficients were initially computed by phenomenological theory and later verified with experimental results. Our experimental works focused on the synthesis of high quality thin-films followed by structural and electrical characterizations of Ba0.7Sr0.3TiO3 thin-films, PbZr0.52Ti0.48O3 thin-films, 0.7Pb(Mg2/3Nb1/3)O3-0.3PbTiO3 and Pb(Zn1/3Nb2/3)O3-0.06PbTiO3 single crystals. The EC properties of thin-films and single crystals were analyzed for their structure-property relationships and their dependencies on film thickness, substrate and working temperatures. Results revealed large adiabatic cooling figures of ΔT ~260 nm PZT films (11 K in 15 V), 200 μm thick 0.7PMN-0.3PT single crystals (2.7 K in 240 V) and 300 μm thick PZN-0.06PT single crystals (0.45 K in 330 V) at Curie temperatures and secondary cooling peaks near critical points. Analytical modeling of EC cooling elements was also performed using a 2D solid-sate device model to predict the cooling efficiencies in Carnot, Brayton and Ericsson refrigeration cycles. Using a comprehensive model for coupled thermal, electrical interactions in ferroelectric 0.7PMN-0.3PT single crystals, we demonstrated Carnot, Ericsson and Brayton efficiencies under thermodynamic cooling cycles. We also explained the intricacies involved in device design, fabrication and testing of a solid-state EC cooler. The bulk EC cooling effects of 0.7PMN-0.3PT single crystals were exposed with the help of a prototype device to demonstrate EC based cooling. The cooling device was tested on a simulated microchip test section and results reported 7.2oC/min additional cooling rates under optimized test conditions. Besides the high ΔT requirements of working elements, we also explained the significances of electric pulse width and frequency in EC based cooling.