Preparation And Electrocaloric Effect Of Ba_xSr_1-xTiO₃ Ferroelectric Thin Films

Freon was restricted and forbided as a cryogen all the world to protect the entironment and ozonosphere against destruction. So the research and development of no-freon refrigeration was urgently needed. At present, the research was carried out on two routes. The preparation of substitute of Freon was the first, and the research of new material according the different principle of refrigeration was the second. In addition, with the rapid development of micro electronic device, devices were always too hot to cool immediately with fan, so a new technology is needed. Compared with conventional principle of refrigeration, ferroelectric materials showing the pyroelectricity could be cool by depolarization in the adiabatic entironment, which was called as electrocaloric effect. These ferroelectric materials could be used as a cryogen. Because BST materials weren't added with Pb and had the electrocaloric effect, the reseatch on BST thin films applied ferroelectric refrigeration was more meaningful to protect the entironment.In this paper, BST thin films with different content of Sr2+ and rare La3+ dopants were prepared on Pt/Ti/SiO₂/Si substrate by sol-gel technique and different thermal treating. Effects of different thermal treatment methodology on microstruture of BST thin films were investigated systematically. Mechanism of the grain growth of BST thin films was analyzed. The curie temperature and hysteresis loops were measured, and the maximum of electrocaloricΔTwas calculated.Using the analysis of XRD, SEM, AFM, the effect of thermal treatment were investigated. The temperature of annealing beyond 750℃was needed to form crystallization. With the increase of annealing temperature, the crystallization, the size of grains and roughness were all increased; pyrolyzing temperature should be more than 350℃. While time of annealing or pyrolyzing was prolonged, the size of grains was larger, the thin film smoother. Polycrystalline thin films of (110), (200), (211) oriented BST were gained by both of two methods. The size of grain with CFA was larger than that of RFA. The grain growing process was the most of the reaction with CFA; the crystal nucleus forming process was most with RFA.Compared with BST thin films of different content of Sr, the increase in crystallite size and decrease in roughness with increasing Sr content can be attributed to the smaller ionic radius which made lattice parameter smaller. At the same time, curic temperature and dielectric constant was reducted. On the basic of preparing of BST, the effect of La dopants was investigated too. More La3+ dopant was, more large grain was and small degree of roughness was.The electrocalric effect of BST was calculated. The value ofΔT was increased with the increase of temperature, and then decreased after it reached the maximum. TheΔT maximum value of Ba_0.7Sr_0.3TiO₃ thin film was 4.8K. TheΔT maximum value of Ba_0.7Sr_0.3La0.01TiO₃ thin film was 6.2K.