Preparation And Electrocaloric Effects Of Sr_xBa_1-xNb₂O₆ Ceramics

Electrocaloric effect has attracted much attention because of its low cost, high energy conversion efficiency and environmental protection. SrxBa1-xNb2O6 (0.25<x<0.75, SBN) is a typical structure of the tungsten bronze with excellent ferroelectric, piezoelectric, pyroelectric and nonlinear optical properties.it is the optimum for preparation electrocaloric effect materials. The crystal structure, micro structures and dielectric and ferroelectric properties of SrxBa1-xNb2O6 (x=0.4,0.5 and 0.6) ceramics were studied in this paper. In order to improve the electrocaloric effect of lead-free ceramics, the breakdown field strength and other factors were investigated.SrxBa1-xNb2O6 (x=0.4,0.5 and 0.6) ceramics were prepared by spark plasma sintering (SPS) and conventional solid-state sintering (CS) respectively. It was found that ceramics with higher density and smaller grain size were achieved by SPS process at relatively lower temperature in a shorter period. The polarization of SBN40, SBN50 and SBN60 ceramics by both methods were measured, indicating SPS-SBN samples possessed larger Pmax. The electrocaloric temperature change AT and the change of entropy AS, firstly increased and then decreased as sintering temperature rose, finally they reached a maximum around Curie temperature Tc. Higher dielectric breakdown strength of 80kV/cm, and improved ΔT of 0.51℃ prepared by SPS.Coprecipitation method was adopted to synthesize SBN ceramics powders could be prepared,SBN40 and SBN50 fine ceramic powders calcining the precusors at 700℃ for 3h. The pH value of preparation the precursor were 10.5 and 11 respectively. Additionally, fine SBN60 and SBN75 ceramic powders could be obtained with increased Ba2+ proportion. Compared to the previous work, which Nb2O5 was dissolved by HF solution, this method has made great progress in operability and reduced environment pollution. The grain size of the ceramic powders synthesized by coprecipitation method were about 1 OOnm, while the grain size of the powders obtained by solid state reaction method were 2μm. The dielectric constant decreased largely in coprecipitation prepared ceramis, due to the smaller grain size and high density of the ceramics. The breakdown field strength was 3-4 times higher than that of powders synthesized by solid state reaction method, so they have larger ΔT and ΔS.