Preparation And Characterization Of BNT Based Lead-free Multilayer Piezoelectric Ceramic Devices

Multilayer piezoelectric ceramic devices have the characteristics of high control precision,fast response,large output force and fast charge and discharge,and are widely used in optic instruments,precision machinery,microfluidic control and laser weapon energy storage and so on.At present,the practical multilayer piezoelectric ceramic devices are generally lead material systems,however,with the increasing attention to environmental concerns,lead-free piezoceramics became a hot topic and got a rapid development.Sodium bismuth titanate（BNT）ceramics with ferroelectric-relaxor coexistence phase could deliver a large strain,and BNT-based ceramics with relaxor phase showed high energy-storage density,being considered as one of the most promising candidates to replace traditional PZT-based materials.Based on BNT-ST binary system,through obtaining the material system suitable for multilayer piezoelectric actuators through component regulation,studying the performance differences between raw materials and piezoactuators,and optimizing the material composition and layer thickness of actuators,high performance actuators were obtained,and the performance of actuators were systematically characterized.At the same time,low temperature cofired multilayer ceramic capacitors with excellent energy storage performance were obtained through component modification and low temperature sintering study.The main research contents of this paper are as follows:1.BNT-based ceramics with large strain at low driving field were systematically studied.Mn O₂-modified 0.76Bi_1/2Na_1/2Ti O₃-0.24Sr Ti O₃（BNT-ST）lead-free piezoceramics were synthesized by a conventional solid-state method.The introduction of Mn O₂ influences the phase structure and increases the grain size.Increasing the Mn O₂ content shifts the ferroelectric-relaxor transition temperature to below room temperature.This leads to a transition from ferroelectric behavior to ergodic-relaxor behavior.A large electrostrain of 0.18%with an ultra-low driving field of 2 k V/mm are achieved for the 0.25 wt.%Mn O₂ doped BNT-24ST.A temperature-insensitive d^*₃₃ of above 610 pm/V from room temperature to 90°C is achieved at 4 k V/mm.Excellent mechanical performance of Young’s modulus E～127.4 GPa and fracture toughness K_IC～1.38 MPa/m are also obtained.2.BNT based multilayer piezoelectric actuators with large strain under low electric field and the mechanical test systems were systematically studied.The part had three primary focus points.（1）One goal was to examine performance differences between raw materials and piezoactuators and assess the underlying mechanisms.The fabrication of BNT bulk ceramics into multilayer piezoactuators leads the change of phase（the increase of ergodic relaxor phase）as a result of internal compressive residual stress.The inactive edge distance,sintering temperature and layer thickness were all found to affect the compressive residual stress of the actuator.（2）Secondly,the intent was to optimize the material composition and layer thickness of actuators.Through reconstructing the material composition and structure of BNT-ST actuators,large strain（0.168%,2 k V/mm）and excellent temperature stability were realized in BNT-based cofired multilayer actuators.In addition,the multilayer piezoelectric actuators with commercial size（length>10 mm）had excellent low field strain performance（total strain>0.15%,2.5 k V/mm）,displaying a better performance.（3）Thirdly,the equipment of strain under mechanical loads and blocking force were systematically set up,and showing the characteristics of small size,simple structure and low cost.Based on the test device,the blocking force and strain under mechanical loads of BNT-based actuators are tested.The blocking stress of multilayer piezoelectric actuator under 2.5 k V/mm was as high as 42.6 MPa.3.Low temperature sintering BNT MLAs were fabricated.Firstly,low-temperature sintering BNT-ST ceramics holding their excellent performance should be obtained.The result showed that the addition of Cu O effectively shifted the sintering temperature of BNST100x ceramics down to 960 ^oC,and BNST25.5 ceramic with 1.2 wt.%Cu O obtained a large normalized strain of 950 pm/V under an ultra-low driving field of 2 k V/mm.Secondly,through optimizing the material composition and ceramic layer thickness of actuators,S_max/E_max>750 pm/V at 2.5 k V/mm were realized in the BNT-based multilayer actuator and showed excellent temperature stability(15°C～80°C,S_max/E_max>570 pm/V,d^*₃₃/d^*_33RTfluctuation<15%,4 k V/mm).Excellent cycling stability was achieved with only～5%reduction after 10⁷ cycles at 4 k V/mm,because the decrement of the grain size and volatility of Bi/Na eliminated the accumulation of space charge.BNST24-Cu O-63μm with a prototype with commercial size（length>10 mm）showed a total strain of 0.112%and a blocking stress of 35.4MPa at 2.5 k V/mm.4.Low-temperature co-fired BNT-ST multilayer lead-free ceramic capacitors were fabricated.（1）Material system with low electric field and high energy storage density were developped.The introduction of Sr Ti O₃ and Nb₂O₅ decreased the average grain size and led to a weakened relaxor characteristic.0.6BNT-0.4ST+2.5 wt.%Nb₂O₅ceramics exhibited excellent energy-storage properties with thermal stability among BNT-based ceramics,in which large recoverable energy-storage density W_rec～1.82 J/cc and efficiencyη～81%were both achieved.（2）A lead-free0.6Bi_0.5Na_0.5Ti O₃-0.4Sr Ti O₃ ceramic doped with Nb₂O₅,Cu O and Mn O₂ additives was synthesized at a relatively low firing temperature of 1050 ^oC.Due to the high polarization P_max and low hysteresis,the largest recoverable energy storage density W_rec value of 1.9 J/cc and high efficiencyηof 89%were obtained under a low electric field of 170 k V/cm.Using this composition,multilayer ceramic capacitors with Ag Pd inner electrodes were successfully prepared by a tape-casting method for energy storage applications.Under a low electric field of 270 k V/cm,excellent energy storage performance was observed in multilayer capacitors,with a W_rec of 2.83 J/cc andηof 85%.The capacitors demonstrate the temperature insensitive properties that the variation in W_rec was less than 5%between 25°C and 100°C.