| Bismuth layer-structured ferroelectrics have the potential applications in many fields,i.e.,high temperature and high frequency workplace,ferroelectric storage,energy converter,filter owing to their advantages of high Curie temperature,low dielectric coefficients and low dielectric loss.In addition,these materials possess relatively broad band gap,low phonon energy,excellent physical and chemical stablility together with environmentally firiendly merits,which is rationally expected as the promising host for optical materials.Therefore,the research and development of BLSFs materials have significant potential application in multi-functional electro-optical devices.In this paper,we firstly investigate the impact of doping perovskite structured K0.5Na0.5NbO3 on the structural and electrical properties of Na0.5Bi4.5Ti4O15 ceramics.Moreover,the electrical and photoluminescence properties of rare earths ions such as Er3+,Eu3+ doped Na0.5Bi4.5Ti4O15 ceramics were also systematically studied as follows:Firstly,(1-x)Na0.5Bi4.5Ti4O15-xK0.5Na0.5NbO3(NBT-KNN-100x)(x=0.00,0.05,0.10 and 0.15)bismuth layered Lead-free piezoelectric ceramics were synthesized via a solid-state reaction method.Results reveal that all ceramics samples maintain a single bismuth layered phase.As KNN increases,structural distortion is gradually weakened and similar trend is also observed in Tc variations.Remanent polarization(2Pr)and piezoelectric constant(d33)of KNN doped NBT ceramics are firstly improved and then decreased,reaching the maximum value 2.68μC/cm2 and 24pC/N,respectively.At the same time the relaxation process and electrical conduction were studied in detail using complex impedance spectroscopy.The calculated impedance relaxation follows Arrhenius behavior with two different activation energies below and above the Tc region,implying different conduction mechanisms.The AC conductivity obeys the universal power law:σtot(ω)=A(T)ωs+σDC(T)(0<S<1).The closeness of activation energy values for DC conductivity,hopping conduction and relaxation below Tc confirms the transport originating from the second ionization of oxygen vacancies mechanism.The exponent S is also studied to understand the associated conduction mechanisms at the different temperature regions,which can be ascribed to the second ionization of oxygen vacancy-related correlated barrier hopping model in the ferroelectric state and the non-overlapping small polaron tunneling model in the paraelectric state,respectively.Secondly,Na0.5Bi4.5-xErxTi4O15(NBT-xEr3+)(0.00<x<0.40)bismuth layered lead-free piezoelectric ceramics were synthesized via a solid-state reaction method.Structural,electrical,and up-conversion(UC)properties of NBT-xEr3+ ceramics were also systematically studied in this paper.Results reveal that all the ceramic samples possessed a single-phase orthorhombic structure.The lattice parameters,the unit cell volume and orthorhombic distortion were observed to decrease with increasing Er3+ contents(x).The average values of grain size were found to slightly decrease with increasing x.Raman spectroscopy revealed that(Bi2O2)2+ layers remained unaffected in the modified compositions,and Er3+ substitution for Bi3+ occurred predominantly at the A-site in the perovskite blocks causing the cationic disorder at the A sites and a slight decrease in the TiO6 octahedral distortion.NBT-xEr3+ ceramics with x=0.20 achieved the optimized photoluminescence.The relative intensity of green and red UC emissions enhances as Er3+ ions concentrations increase.Dielectric measurements showed that the incorporation of Er3+ ions increased Tc with simultaneously lowered tano at high temperature,implying that this ceramics can be suitable for high-temperature sensor applications.Finally,photoluminescence and electrical properties of Na0.5Bi4.5-xEudTi4O15(NBT-xEu3+,x=0.00,0.05,0.10,0.15,0.20,0.25,0.30 and 0.40)ceramics were studied.Results indicate that NBT-0.25Eu3+ ceramics show the strongest red and orange emissions corresponding to the 5D0→7F2(617nm)and 5D0→7F1(596nm)transitions,respectively.The strongest excitation band around 465nm matches well with the emission wavelength of commercial InGaN-based blue LED chip,indicating that Eu3+-doped NBT ceramics may be used as potential environmental friendly red-orange phosphor for W-LEDs application.Moreover,conduction analysis reveals that the conduction of charge carriers in high temperature range originates from the conducting electrons from the ionization of oxygen vacancies.The introduction of Eu3+ distinctly increased the Curie temperature(Tc)of NBT-xEu3+ceramics from 640℃ to 711℃ as x ranges from 0.00 to 0.40,indicating the suitable application for high temperature piezoelectric sensor applications and electro-optical integration. |
