Preparation Of NBT-Based Lead-Free Piezoelectric Ceramics By Hydrothermal Assisted Cold Sintering

Conventional sintering achieves densification by minimizing the surface Gibbs free energy by using high temperature as a driving force.However,the recent researchs have found that the ceramic powders were uniformly wetted with water or acetic acid or a specific precursor solution,and sintered at an ultra-low temperature?²00??under a pressure of MPa,then annealed at a certain temperature to obtain dense ceramic samples.In this paper,dense bismuth sodium titanate?NBT?ceramics were successfully prepared by hydrothermal assisted cold sintering process.The effects of pressure,holding time,annealing temperature,alkalinity of hydrothermal precursor and nano/micron powder mass ratio on the properties of NBT ceramics were systematically studied.The main research results of this paper is as follows:The Na_0.5Bi_0.5TiO₃ micron powders were prepared by solid phase process.The powderswereuniformlywettedby15-20wt%hydrothermalprecursor?Bi?NO₃?₃/NaOH/TiO₂ suspension?.The Na_0.5Bi_0.5TiO₃ ceramics were prepared by hydrothermal assisted cold sintering and annealing treatment.The density of the ceramics can reach 87%,but the dielectric and piezoelectric properties were unsatisfactory.The Na_0.5Bi_0.5TiO₃ nano-powders were prepared by hydrothermal method.The powderswereuniformlywettedby15-20wt%hydrothermalprecursor solution?Bi?NO₃?₃/NaOH/TiO₂ suspension?,and the Na_0.5Bi_0.5TiO₃ ceramics were obtained by low temperature hydrothermal assisted cold sintering at 180?.The properties of the ceramics were improved by 700-900? annealing treatment.The results show that 99%dense Na_0.5Bi_0.5TiO₃ ceramics can be obtained by hydrothermal assisted cold sintering and post-annealing treatment.The dielectric constant?_r of the ceramics is 681,tan?is 0.04 and piezoelectric constant d₃₃3 is 52.2pC/N,which are similar to those of the conventional sintered ceramics.Na_0.5Bi_0.5TiO₃ ceramics were prepared by hydrothermal assisted cold sintering of nano/micron powders with different mass ratios?1:0,2:1,1:1,1:2,0:1?.The results show that the properties of ceramic samples obtained by mass nano-powder and micron-powder were all unsatisfactory.High density,ideal dielectric and piezoelectric properties can only be obtained by hydrothermal assisted cold sintering and post-annealing of pure nano-powders.The hydrothermal-assisted cold sintering process was applied to the Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ system.The effects of alkalinity of different hydrothermal precursor solutions and annealing temperature on the properties of ceramics were studied.When 4mL sodium hydroxide solution was added to the precursor solution,NBT-0.06BT ceramics with a density of more than 90%and piezoelectric constant of78.8pC/N could be obtained by annealing at 950?.