Compositiom Design And Energy Storage Properties Of K_0.5Na_0.5NbO₃-Based Relaxor Ferroelectric Ceramics

The development and utilization of renewable energy is the driving force for the research of energy storage devices.Compared with batteries and supercapacitors,dielectric capacitors have unique advantages such as extremely fast charge-discharge process and ultra-high power density,which are suitable for storing energy storage from intermittent renewable energy sources.However,the energy density of dielectric materials is relatively low.Therefore,improving the energy storage density of dielectric materials has become a research hotspot in this field.Relaxor ferroelectrics have the potential to obtain both high recoverable energy storage density(W_rec)and energy storage efficiency（η）due to their high maximum polarization(P_max),low residual polarization（P_r）and nearly "linear" polarization-electric field（P-E）loop.K_0.5Na_0.5NbO₃（KNN）-based ceramics are considered as one of the potential candidates to replace Pb-based ceramics due to their unique submicron grain size,high dielectric permittivity and Curie temperature.However,the inevitable energy loss under high applied electric field makes it an obstacle to synergistically improve the W_rec and η.This contradiction is the key bottleneck that restricting the development and application of dielectric materials.In this dissertation,Bi(M_e1,M_e2)O₃-modified KNN ceramics were prepared by solid state reaction method.The microstructures,dielectric properties,relaxation properties and energy storage properties of KNN-based relaxor ferroelectric ceramics were investigated.The main purpose was to seek the intrinsic connection between doping amount and grain sizes,and further analyzed the effect of grain sizes and densities on their breakdown strength（BDS）and transmittance.KNN-xBi0.iK0.7NbO₃（KNN-xBKN）ceramics were prepared by a combination of the solid state reaction method and cold isostatic pressing.The effects of BKN addition on the microstructure,dielectric properties,relaxor properties and energy storage properties of KNN ceramics were systematically studied.The results showed that the crystal structure transformed to a cubic structure after the doping of BKN.With the introduction of BKN,the average grain sizes decreased significantly to nanoscale（～90 nm）,the corresponding BDS was greatly improved,and the ΔP(P_max-P_r)values of the samples also increased dramatically.The synergistic effect of high BDS and large ΔP leaded to the enhancement of energy storage properties.When the doping content of BKN reached 0.15 mol,the energy storage properties were optimal,the W_rec value was 3.39 J·cm^-3 under an applied electric field of 330 kV·cm^-1.In addition,the introduction of BKN significantly improved the optical transmittance of KNN ceramics,its optical transmittance was 83.3%at 1600 nm wavelength,which was attributed to its cubic crystal structure,nano-scale grain sizes and dense microstructure.These results also indicate that this novel 0.85KNN-0.15BKN ceramic can be used for transparent electronic devices or as a multifunctional potential dielectric material.The effects of doping solid solution of three[2,4]-type Bi-based compounds of Bi(M_e1²⁺,Zr4+)O₃(M_e1²⁺=Mg²⁺,Zn²⁺,Ni²⁺)on the dielectric properties,relaxor properties and energy storage properties of KNN ceramics were systematically investigated.Novel KNN-xBi(Mg_0.5Zr_0.5)O₃（KNN-xBMZ）,KNN-xBi(Zn_0.5Zr_0.5)O₃（KNN-xBZZ）and KNN-xBi(Ni_0.5Zr_0.5)O₃（KNN-xBNZ）binary solid solutions ceramics were successfully prepared by the solid state reaction method.The results exhibited that the introduction of Bi(M_e1²⁺,Zr4+)O₃ maked the phase structure of KNN ceramics changed to cubic structure with dense microstructure and significant decreased grain sizes.Meanwhile,the dielectric permittivity and loss gradually decreased,and the frequency dispersion of the dielectric permittivity peaks was gradually obvious.The first-order reversal curve（FORC）distribution and the piezo-force microscopy（PFM）verified that the introduction of Bi(M_e1²⁺,Zr4+)O₃ disrupted the long-range ferroelectric ordered structure of KNN ceramic,enhanced the ferroelectric instability,and made the KNN ceramics transform from the normal ferroelectric（FE）state to the relaxor ferroelectric（RFE）state.The doping of Bi(M_e1²⁺,Zr4+)O₃ induced the appearance of polar nano-regions（PNRs）.The result of P-E loop showed that the enhanced relaxation properties allowed the ceramic to maintain a large P_max while significantly suppressing P_r,resulting in significant improvement of energy storage properties.Binary solid solution ceramics achieve good energy storage properties:the W_rec value of 0.85KNN-0.15BMZ ceramic,0.85KNN-0.15BZZ ceramic,and 0.85KNN-0.15BNZ ceramic was 3.47 J·cm^-3,3.5 J·cm^-3,8.09 J·cm^-3 with corresponding η value of 89%,86.8%,88.46%,respectively.These results indicated that the barrier that KNN-based ceramics could not achieve high W_rec and η at the same time had been broken,which would open up a new way for the research of KNN-based ceramics in dielectric materials for capacitors.It was worth mentioning that the high-angle annular darkfield scanning transmission electron microscopy（HAADF-STEM）explained from the atomic scale visually that BNZ doping destroyed the long-range FE order of KNN ceramic,resulting in the transformation of micron-scale FE domains into nanoscale PNRs.It also provided an in-depth atomic-scale dissection of the P_r reduction,and gave the most intuitive and powerful explanation for the potential mechanism of the enhanced energy storage properties.In order to find KNN-based ceramics with high W_rec,wide temperature stability and high pressure resistance that more suitable for practical applications,three[2,5]-type Bi-based compounds,[Bi(M_e1²⁺,M_e2⁵⁺)O₃(M_e1²⁺=Mg²⁺ or Zn²⁺;M_e2⁵⁺=Ta⁵⁺ or（Nb,Ta）⁵⁺ complex ions)],doped with KNN-based ceramics,namely KNN-xBi(Mg_2/3(Nb_0.85Ta_0.15)_1/3)O₃（KNN-xBMNT）,KNN-xBi(Zn_2/3(Nb_0.85Ta_0.15)_1/3)O₃（KNN-xBZNT）and KNN-xBi(Mg_2/3Ta_1/3)O₃（KNN-xBMT）,were designed and prepared.It was found that the addition of Bi(M_e1²⁺,M_e2⁵⁺)O₃ made the phase structure of the KNN-based ceramics gradually change from orthorhombic structure to cubic structure,with dense microstructure,no obvious pores,and uniform grain sizes distribution,which resulted in a significant increase of BDS.The 0.90KNN-0.10BZNT ceramic exhibited ultra-high BDS value of 800 kV·cm^-1.The intrinsic breakdown strength was fitted by the Weibull distribution confirmed that the breakdown stability of the samples was good.In addition,the introduction of three[2,5]Bi(M_e1²⁺,M_e2⁵⁺)O₃ changed the dielectric permittivity peak to diffuse broad peak,and the sensitivity of the dielectric permittivity to temperature was gradually reduced.The result FORC distribution and PFM analysis indicatedthat the doping of[2,5]Bi(M_e1²⁺,M_e2⁵⁺)O₃ disturbed the long-range FE order of the KNN ceramics and promoted the generation of PNRs,which induced the transition from FE states to RFE states.Finally,the three ceramics all showed significant improved energy storage properties.The effect of test temperature on the energy storage properties of the best-proportioned dielectric ceramics was further investigated.It was found that the 0.85KNN-0.15BMNT ceramic had a W_rec～3.20 J·cm^-3 and η～92.0%under 386 kV·cm^-1 at room temperature.The W_rec was 1.25～1.5 J·cm^-3 and η was greater than 78%over the temperature range of 20～120℃.The 0.90KNN-0.10BZNT ceramic showed a high W_rec～7.40 J·cm^-3 under applied electric field of 800 kV·cm^-1 at room temperature.Morever,it exhibited high W_rec(2.9 J·cm^-3)over a wide temperature range（20～180℃）,showing excellent temperature stability.The 0.85KNN-0.15BMT ceramic with high W_rec(6.14 J·cm^-3)and high η（87%）was obtained at room temperature,its W_rec and η remained 1.9～2.4 J·cm^-3 and 76～88%over the temperature range of 30～150℃,respectively.