Investigation Of Energy Storage Performance And Its Enhancing Mechanism Of Bismuth-based Perovskite Lead-free Ferroelectric Ceramics

As a kind of highly efficient energy storage device,ceramic capacitors are widely used in various electronic equipment and pulse power technology.Miniaturization,lightweight and lead-free are important directions of electronic device development due to the demands of modern technology and green development.Therefore,it is very urgent to develop lead-free energy storage ceramic materials with high energy storage performance（ESP）.The recoverable energy density(W_rec)and energy storage efficiency（η）are the two pamaraters for valuing the ESP of dielectric ceramics.Na_0.5Bi_0.5Ti O₃ and Bi Fe O₃,as the main Bi-based lead-free perovskite ferroelectric ceramics,both have the ability to produce high polarization,showing a great potential as dielectric energy storage materials.In this dissertation,the two materials were selected as matrix materials,and the energy storage ceramic materials with a relatively high ESP were obtained through different ways of doping modification and the enhancing mechanism of the ESP of Bi-based perovskite ferroelectric ceramic was explored.The main research contents are as follows:（1）Nd³⁺/Ga³⁺（NG）co-doped 0.75Na_0.5Bi_0.5Ti O₃-0.25Sr Ti O₃（NBST）ceramics were prepared by introducing Nd³⁺and Ga³⁺into the perovskite A-and B-sites of NBST.The introduction of Nd³⁺at A-site reduces the concentration of oxygen vacancy in NBST ceramics,inhibites the growth of the grain,and increases the impedance,which are responsible for enhancing dielectric breakdown strength（E_b）;the introduction of heterovalent Ga³⁺at B-site greatly enhances the disorders of composition and charge at B-site,and the random electric fields（REFs）are established at B-site.Therefore,the relaxor properties of the NBST ceramics are significantly enhanced,and the coherence intensity of polar nano-regions（PNRs）at high temperature is improved,which decrease the hysteresis of the polarization（P）-electric field（E）loops and improve its thermal stability.In summary,NG co-doping shows a synergistic effect,that is the introduction of Nd³⁺at A-site mainly contributes to the enhancement of E_b and the introduction of Ga³⁺at B-site mainly contributes to the supression of the hysteresis of the P-E loops.Benefiting from this synergistic effect,a relatively high E_b（=200 k V/cm）and ESP(W_rec=2.88 J/cm³,η=83%)are achieved in NBST-0.04NG ceramics.This synergistic effect is the key to the effective enhancement of the ESP of the Bi-based perovskite ferroelectric ceramic by A-and B-sites co-doping.（2）The complex ions of(Sn_1/2Hf_1/2)⁴⁺（SH）,(Zr_1/3Sn_1/3Hf_1/3)⁴⁺（ZSH）,(Al_1/3Hf_1/3Ta_1/3)⁴⁺（AHT）and(Mg_1/5Al_1/5Ta_3/5)⁴⁺（MAT）were constructed and introduced to the B-site of NBST ceramics.The effects of B-site ion complexity,i.e.,the element number and the valence difference on the ESP of NBST ceramic,were systematically investigated.The results indicate that the higher complexity of complex ions,that is the increase of the elment number and the valence difference,have a more severe influence on the B-site of NBST ceramics.This is manifested by the stronger disorder of the local structure and the more significant softening of Ti-O bonds.Therefore,the relaxor properties of the AHT and MAT doped NBST ceramics are significantly stronger than that of the SH and ZSH doping.With the increase of the complexity of the complex ions,the hysteresis of the P-E loops is gradually reduced and the P_max can be well maintained.Accordingly,the ESP of the AHT/MAT doped NBST ceramics(W_rec=2.61 J/cm³,η=81%;W_rec=2.50 J/cm³,η=84%)is superior to SH(W_rec=2.29 J/cm³,η=72%)/ZSH(W_rec=2.33 J/cm³,η=78%)doped ceramics.In addition,the E_b of the four ceramics is different due to the differences in grain size,dielectric constant（ε_r）,dielectric loss（tanδ）and band gap（E_g）.The highest W_rec（=4.41 J/cm³）is obtained in ZSH doped ceramics,which is due to its highest E_b（=330 k V/cm）,and the highestη（=85%）is obtained in MAT doped ceramics,which is responsible for its slimmest P-E loop.（3）Based on the experimental results of NG co-doped NBST ceramics,B-site Ga³⁺was replaced with Nb⁵⁺with a higher valence state,aiming to further inhibit oxygen vacancy and continue to increase E_b,so as to obtain energy storage ceramics with higher ESP.After Nd³⁺/Nb⁵⁺（NN）co-doping,the concentration of oxygen vacancy in the ceramics is further reduced,and the second phase Bi₂Ti₂O₇（BTO）with lowε_r and tanδis generated.In addition,the simulation analysis by using COMSOL software shows that the BTO phase with lowε_r can bear more loading electric field,thus weakening the loading electric field on the main perovskite phase.Therefore,NN co-doping greatly improves the E_b of NBST ceramics.In addition,NN co-doping improves the local structural symmetry of NBST ceramics,and aggravates the fluctuations of composition and charge,coupled with the apperance of BTO phase with a linear dielectric characteristic,so the hysteresis of the P-E loops and early polarization saturation of NBST-x NN ceramics are obviously suppressed.Ultimately,the optimal ESP(W_rec=4.14J/cm³,η=90%)is achieved in NBST-0.08NN ceramics under 390 k V/cm.In addition,the thermal stability of the ESP of the NBST-0.08NN ceramic was investigated under210 k V/cm and 240 k V/cm,and it was found that a higher loading electric field is beneficial to maintaining the thermal stability of the ESP.Furthermore,NN was also incorporated into 0.65Bi Fe O₃-0.35Ba Ti O₃（BFBT）ceramics which also contain a large number of volatile elements.The results show that NN co-doping can also effectively inhibit the formation of oxygen vacancy in BFBT ceramics,and enhance E_b and ESP of BFBT ceramics.The optimal ESP(W_rec=3.64 J/cm³,η=88%)is obtained in BFBT-0.08NN ceramics under 330 k V/cm.（4）Combining the synergistic enhancement effect of A-and B-sites co-doping and the effective enhancement of relaxor properties by B-site complex heterovalent ions doping,La(Mg_2/3Ta_1/3)O₃（LMT）with a complex perovskite structure were introduced into NBST and BFBT ceramics.On the one hand,the introduction of LMT can inhibit the formation of oxygen vacancy and the grain growth of NBST and BFBT ceramics,and improve the E_g of the matrix ceramics,so the E_b of the LMT doped ceramics is enhanced.On the other hand,the introduction of complex heterovalent ions(Mg_2/3Ta_1/3)³⁺at B site can significantly enhance the fluctuations of composition and charge in the matrix ceramics,which is very favorable for decreasing the hysteresis of the P-E loops.Ultimately,the high E_b（=246 k V/cm;390 k V/cm）and ESP(W_rec=3.18J/cm³,η=86%;W_rec=4.92 J/cm³,η=85%)are respectively achieved in the NBST-0.06LMT and BFBT-0.14LMT ceramics,and their ESP shows a good thermal stability.