| Electrocaloric (EC) devices have drawn great research interests very recently, because they are environment-friendly solid-state cooling devices, and can be used in a broad spectrum of applications, e.g., chip cooling and temperature regulation for sensors and electronic devices, etc. Intense interests in the EC effect of ferroelectric thin films have been growing in the last few years, primarily triggered by the giant EC effect found in ferroelectric PbZr0.95Ti0.05O3 thin films ( ?T =12 K). However, it is noticed that the large EC effect reported in the previous works is generally taken place in a rather narrow temperature range, while solid-state coolers are required to operate in a much wider temperature range. Therefore, for the application of solid cooling devices, it is imperative for us to tune the EC properties of ferroelectric thin films, with the aim of achieving a giant EC effect in a wide range of temperature.In this work, we employ the Landau-Devonshire theory to construct the "misfit-temperature" phase diagram to analyze the dependence of the EC effect on both strain and electric field for single-domain epitaxial BaTiO3 (BT), PbTiO3 (PT) and PbZr(1-x)TixO3 (PZT) thin films grown on thick substrates. The results denote that both the strain and the electric field play important roles in tunning the EC effect of the film ferroelectrics. What is more, the strain extends the temperature range of the EC effect, because the latter is closely related to the phase transition properties of a ferroelectric film.On the one hand, we take the BT films grown on thick cubic substrates as an example to investigate the tunning effect of strians on the EC properties by constructing the "misfit-temperature" phase diagram. It is found that the tensile misfit strain expands the operation temperature range to the room temperature with a little reducing of the maximun of the temperature change (1.2 K), while the compresive misfit strain increases the maximum of the temperature change (1.6 K) towards the high temperature. The mechanism for this phenomenon can be explained by the "misfit-temperature" phase diagram clearly which denotes that the phase transition temperatrue is changed by changing the misfit strain and electric filed. Thus, the tuning of the misfit strain in the ferroelectric film is possibly a good way for the future EC refrigeration applications.On the other hand, the EC properties of the PT and PZT films under anisotropic misfit stain are also investigated, which predicts a high giant EC. What is more, the results show that proper anisotropic misfit starin could improve the EC effect and extend the temperature range near or above the room temperature in PT and PZT thin films. Especially, the temperature chang (6 K) of the PT film under anisotropic misfit starin is about triple larger than that of the BT film near the room temperature. This predicts further promotion of the future EC refrigeration applications. |
