Studies On Synthesis And Properties Of Potassium Sodium Niobate-based Lead-free Ceramics

In recent years,potassium sodium niobate based lead-free ceramics have attracted extensive attention of international researchers because of their excellent piezoelectric properties and high Curie temperature.However,there are few reports on their narrow band gap and ferromagnetism.Since potassium sodium niobate also has large spontaneous polarization intensity and doping tolerance,its crystal structure,optical band gap and ferromagnetism can be changed by ion doping.Therefore,it can be applied in solar photovoltaic field and information storage devices.On the one hand,in the field of solar photovoltaic,the strong attraction of ferroelectric materials lies in the existence of a new mechanism,which is similar to the self-built electric field of P-N junction,namely the spontaneous polarization intensity,and its open-circuit voltage is not restricted by the band gap of ferroelectric materials.However,the main reason limiting the conversion efficiency is that the band gap of most ferroelectric materials exceeds 3e V.For example,the band gap of potassium sodium niobate materials is as high as 3.5e V,so narrowing the band gap can increase the absorption capacity of ferroelectric materials.On the other hand,in the exploration of multiferroic materials,most ferroelectric materials lack ferromagnetism,such as potassium sodium niobate materials,which exhibit inherent diamagnetism,so inducing their ferromagnetism at room temperature can achieve the coexistence of ferroelectricity and ferromagnetism.Based on the above two points,(K_0.5Na_0.5)Nb O₃-x Bi(Co_0.875Fe_0.125)O₃（KNN-x BCF）ceramicsand(K_0.5Na_0.5)Nb O₃-x Ba Ni_0.5Nb_0.5O_3-δ（KNN-x BNN）ceramics with narrow band gap and ferromagnetism were successfully prepared through using the improved solid-state reaction sintering method and ion doping technique.Moreover,the crystal structure,microstructure and photoelectromagnetic properties were characterized and studied by various measurement techniques.The main innovative results are as follows:（1）In the preparation process of potassium sodium niobate based ceramics,two factors affecting the properties of ceramics are analyzed emphatically:sintering temperature and holding time.Therefore,we set up several groups of heating curves by the control variable method,and the crystal lattice structure and crystallization under different process parameters were studied by XRD,so as to make sufficient presequence work for the study of the photoelectromagnetic properties of ceramics in the following.In addition,considering the high temperature volatility of K salt and Na salt,the process optimization that the self-sacrificial sintering method was adopted and the sintering was carried out at high temperature in a double-layer closed alumina crucible was made on the basis of the traditional solid-state reaction sintering method.Finally,KNN-x BCF ceramics and KNN-x BNN ceramics with uniform particles and few pores were successfully prepared.（2）The effects of Bi(Co_0.875Fe_0.125)O₃（BCF）doping on the structure,stress,morphology,band gap,ferroelectricity and ferromagnetism of(K_0.5Na_0.5)Nb O₃（KNN）have been systematically studied.XRD and Raman spectra show that the doping of BCF causes obvious phase transformation behavior of KNN-x BCF ceramics and the coexistence of orthorhombic phase and rhombohedral phase existes in the range of components 0.01≤x≤0.02.The stress analysis shows that the inner part of KNN-x BCF ceramics is subjected to tensile stress,and the tensile stress increases with the increase of x,which causes lattice distortion of Nb O₆ octahedron and reduces the structural symmetry of the crystal,and finally leads to the structural phase transition.SEM images show that the main reason for the grain refinement of KNN ceramics caused by the introduction of BCF is that the large size grains are decomposed into several small size grains,and the doping can significantly improve the density of the ceramics.In addition,since the Fe 3d impurity energy level enters the valence band energy structure of KNN and then hybridizes with the O 2p state,the new top of valence band increases,thus reducing the band gap of KNN to 3e V.The P-E hysteresis loop shows that the KNN-0.01BCF ceramics have the best ferroelectric properties due to the highly oriented ferroelectric domain formed by the coexistence of the orthorhombic phase and the rhombohedral phase,and the residual polarization strength P_r value is about 4.02μC/cm².M-H hysteresis loop shows that doping BCF in KNN can change the magnetic properties from diamagnetism to ferromagnetism at room temperature,which can be explained by the F-Center Exchange mechanism.Finally,KNN-xBCF ceramics with the coexistence of ferroelectricity and ferromagnetism at room temperature are synthesized,which opens a new path for the exploration of lead-free multiferroic materials.（3）The effect of BaNi_0.5Nb_0.5O_3-δ（BNN）doping on the properties of KNN ceramics was investigated.XRD results show that all KNN-x BNN ceramics have orthorhombic perovskite structure and belong to space group Amm2,which is consistent with the parent KNN.At the same time,Raman spectrum also proves that all ceramics have a weak peak at 550cm^-1 wave number,which is exactly the characteristic peak of the orthorhombic phase of KNN ceramics.SEM images show that all KNN-x BNN ceramics have regular morphology and micron-scale grain size,and grain refinement occurs with the doping of BNN.In addition,by introducing appropriate Ni²⁺-V_O combination,the top of valence band of KNN can be greatly raised,and a narrow band gap of only 2.4e V can be obtained.At the same time,the band gap of KNN-x BNN ceramics first steeply decreases and then slowly decreases with the increase of x.The deviating linear change is attributed to the proportion of Ni-O-Nb which plays a leading role in the decrease of band gap in the low-doped KNN-x BNN ceramics is much larger than that of Ni-O-Ni;More Ba atoms will cause a sharp increase in the degree of lattice distortion,which will lead to a larger band gap and counteract the effect of Ni²⁺-V_O.M-H hysteresis loops show that the magnetic properties of KNN doped with BNN change from diamagnetism to ferromagnetism,and the KNN-0.08BNN ceramics have best ferromagnetic properties which are attributed to the ferromagnetic exchange interaction between the local spin-polarized electrons and conducting electrons.The above results indicate that KNN-x BNN ceramics not only have the prospect of application in perovskite solar cells,but also have the potential of making multiferroic devices.