Microtructure And Properties Tailoring In Potassium Sodium Niobate-based Dielectric Materials

The properties of the materials depend not only on the compositions of the materials,but also on the microstructure of the materials.Therefore,it is of great interesting to adjust the microstructure of the material to improve the properties of the materials.A high Curie temperature of 420°C,high dielectric permittivity,low loss,and large piezoelectric activity were present in solid solution K_0.5Na_0.5NbO₃ with perovskite structure,which revealed that the K_0.5Na_0.5NbO₃-based materilas has great potential applications for lead-free piezoelectric and multilayer capacitors?MLCC?fields.In this paper,the microstructure of K_0.5Na_0.5NbO₃-based ceramics was modified by A/B substitution and formation of new solid solutions with other ferroelectrics.A high dielectric permittivity,broad temperature range stability,tunable dielectric properties were obtained,and the corresponding physical mechanisms were also studied.Firstly,the core-shell microstructure was fabricated by using SrZrO₃ solid solution with K_0.5Na_0.5NbO₃ ceramics to obtain a stable dielectric property in the range of-55-201°C.The relationship between microstructure and dielectric properties was investigated.For x=0.14sample,obvious core-shell microstructures were presented in the grains of ceramics:the outer shell region was similar to the target composition,while a slightly poor content of Sr and Zr was presented in the core region.0.86K_0.5Na_0.5NbO₃-0.14SrZrO₃ ceramic showed a stable permittivity near 2310 with dielectric loss lower than 2%at a broad temperature range,and the permittivity variation(De'/e'_25°C)was maintained smaller than±15%around-55–201°C temperature range,which was close to that of commercial BaTiO₃ based X9R MLCC high-temperature capacitors.For x=0.16 sample,the volume fraction of the shell region increased,while the core region decreased,and the dielectric peaks of the core region decreased correspondingly.In addition,the solid-state diffusion sintering mechanism was determined as the formation mechanism of core-shell microstructures for K_0.5Na_0.5NbO₃-SrZrO₃ ceramics system.Based on Lichtenecker's effective dielectric function analysis along with Lorentz-type function fitted of the temperature dependence of dielectric permittivity,a plausible mechanism explaining the temperature-stable dielectric response in present work was suggested.Secondly,high temperature ceramic capacitors(K_0.5Na_0.5)_?1-x?Bi_xNb_?1-2x/3?Cu_2x/3O₃?x=0-0.03?with core-shell microstructures were fabricated by Bi substitution for the A site and Cu substitution for B site,and the used temperature range of the K_0.5Na_0.5NbO₃-based dielectric materials was further widened to 40-520°C.The co-doping of Bi and Cu effectively improved the sintering denseness and reduced the grain sizes of the ceramics.The phase transformation peaks T_c in the high temperature region were significantly diffused,which induced the ceramics showed a high and stable permittivity near 1350 at a broad temperature range?40°C–520°C?and the capacitance variation was maintained less than±15%.The contributions of the core and shell region to the dielectric constant of the ceramics were differentiated by the AC impedance spectroscopy.The shell regions were the Bi-rich region,non-ferroelectric phase,and the dielectric constant was independent of the temperature,while,the cores were the Bi-poor region,ferroelectric phase,with a capacitance maximum at the tetragonal to cubic phase transition.Then,the transparent ceramics?1-x?K_0.5Na_0.5NbO₃-xSrTiO₃ with good transmittance in the visible light range were prepared,the optical properties,relaxor mechanism and tunable dielectric properties were investigated.It was found that the relaxor behaviours of transparent ceramics could be well described by the Lorentz-type formula.The transmission properties of the ceramic were important affected by microstructures and defects.For x=0.18 sample,the optical transmittance of the ceramics reached 55%at the near infrared?900 nm?range,and with high tunable dielectric properties?n_r=24.1%?and low dielectric loss?tand=0.016?.With increase of the content of SrTiO₃,the relaxor degrees of K_0.5Na_0.5NbO₃-SrTiO₃transparent ceramics was increased obviously,and the tunable dielectric properties and dielectric loss was also decreased.The tunable dielectric behaviors could be described by the Landau-Ginsberg-Devonshire polarization model.There were two polarization contributions to the dielectric responses:one came from the intrinsic contribution of the lattice phonon polarization,and the other stems from the extrinsic polar nanoregions?PNRs?polarization contributions.For x=18 sample,the weight of PNRs contributions was 5.9%.In order to further improve the transparency of K_0.5Na_0.5NbO₃-SrTiO₃ ceramics,the microstructures of transparent relaxor ferroelectrics were modified by Bi₂O₃.Bi₂O₃effectively suppressed the grain growth in the(K_0.5Na_0.5)_0.9Sr_0.1Nb_0.9Ti_0.1O₃ relaxor ferroelectrics,making the phase structure changed from tetragonal phase to cubic-like phase,the relaxor degree was increased,the crystal structure was more symmetrical,and the long-range ordered microstructures that affect light scattering were difficult to be formed,thus,a transparent relaxor ferroelectrics in the visible light region were obtained.For the content of Bi₂O₃ was 1.0%,the transmittance of transparent ceramics reached 51.5%at visible light range?633nm?and the electro-optical coefficient was 26.1 pm/V,which was higher than that of industrial LiNbO₃ single crystal?19.9 pm/V?.In addition,Bi₂O₃ compensated the alkali metal elements K,Na volatilization in(K_0.5Na_0.5)_0.9Sr_0.1Nb_0.9Ti_0.1O₃ transparent ceramics,reducing the density of oxygen vacancy defects,and decreasing the defects pinning effect.After 10⁷ cycles of electric field,the electrical strain S₃₃ value only decreased by 3.2%,which revealed the transparent ceramics with excellent fatigue properties.Finally,composite ceramics with low dielectric loss,low dielectric constant anti-ferroelectric and high tunable dielectric properties,high loss relaxor ferroelectric were designed,namely,?NaNbO₃-0.04CaZrO₃?/(K_0.5Na_0.5NbO₃-0.12SrZrO₃)composite ceramics.By adjusting the microstructure of the composite ceramics,high tunable dielectric properties,low dielectric loss and high figure of merit?FOM?composite ceramics were obtained.The Johnson model effectively described the tunable dielectric behavior of the composite ceramics.As the content of the anti-ferroelectric phase increased,the polarization free energy contribution from the mismatch of anti-ferroelectric/relaxor ferroelectric phase was increased.For the composite ceramics with composition 15-85,the tunablity was 51.78%,the loss was1.5%and the value of FOM was 34.52 under 6 kV/mm electric-field.In the sintering process,there was diffusion mass transfer between the two phases,and a high resistance interface layer was formed at the grain boundary,which greatly limited the space charge and the long-range migration of the defective ions,resulting in a significant increased in the resistivity of the composite ceramics,then,a high value of FOM was obtained.