The Preparation,Structure,and Performance Of Complex Parametric Layered Oxide Bi₄La_0.5Sr_0.5Ti₃MnO_15-δ

Complex parametric layered oxides are a kind of multifunctional materials with very special structures.With the merits of regulatable layer number,adjustable chemical composition by A-site,B-site cations and anions doping or substitution,it has attracted widespread attention.Due to its abundant physical properties,it has been not only meant to explore the mechanism of interaction between various degrees of freedom in complex oxides,but also widely used in ferroelectric and ferromagnetic devices,spintronic devices and catalysis fields and so on.A kind of compounds called Aurivillius phase oxide is a representative one of complex parametric layered oxides.Most of them are lead-free,thus becoming important substitutes for lead-based piezoelectric materials.Besides their environmentally friendly characteristics,we give more attention to the rich performance diversity brought by their unique crystal structures.Most work in this research field focuses on the improvement and application of ferroelectric and dielectric properties,and the introduction of magnetism is also concentrated in the doping of Co element.To some extent,the magnetism and magnetoelectric coupling is realized in the complex parametric layered oxides,but there is still a long way to go for practical applications.In addition,because the complex parametric layered oxides have a considerable c/a ratio,it has anisotropy at the lattice level,and if it can be prepared as an oriented ceramic,the lattice anisotropy can be better amplified,so as to obtain better performance to meet the needs of the future device applications.At present,there are some work on the complex parametric layered oxide ceramics,but most of them also focus on the anisotropy and enhancement of ferroelectric and dielectric properties.In the previous work of our group,based on the excellent ferroelectric material Bi4Ti3O12,a layer of magnetic conductive perovskite La0.5Sr0.5MnO3 is inserted to the lattice of it with the solid-phase magnetic layer insertion technique.A novel ferroelectric semiconductor material Bi4Ti3O12-LaxSr1-xMnO3 with large bandgap and resistivity tunability in a wide range is obtained.We design this paper from the idea to prepare a corresponding Bi4La0.5Sr0.5Ti3MnO15-δ(BLSTM)oriented ceramic to obtain anisotropic materials with excellent performance.On the one hand,the machenism of anisotropy in the lattice can be studied in the oriented ceramic,on the other hand,the anisotropic electric and magnetic properties can be regulated to adapt to a wider range of application scenarios.Here,we pre-prepared Bi4La0.5Sr0.5Ti3MnO15-δ nano-flower with high aspect ratio,which morphology is beneficial to gaining oriented ceramics with higer degree of orientation.After the subsequent pressing and sintering,the anisotropic electric and magnetic properties is obtained in the Bi4La0.5Sr0.5Ti3MnO15-δ oriented ceramics with excellent performance,which is of great significance in the study of anisotropic mechanism and deviece application.On the basis of introducing magnetism and manipulating the transport property of the typical ferroelectrical material Bi4Ti3O12,which as one of Aurivillius phase oxide,we used hydrothermal magnetic-layer-insert method to insert La0.5Sr0.5MnO3 into its pseudo-perovskite layer and successfully synthesized Bi4La0.5Sr0.5Ti3MnO15-δ nanoflower composed of nanoplates with high aspect ratio.Here we revolved around the synthesis,structure and performance of Bi4La0.5Sr0.5Ti3MnO15-δ.The thesis consists of six chapters as follows:The first chapter is a literature review.Firstly,we introduced various order parameters in the materials to illustrate the concept of complex parametric layered oxides.Then we derivated the crystal structure of complex parametric layered oxides from that of perovskites.Nextly,we discussed the copious physical properties of complex parametric layered oxides and the method to modulate them from the perspective of structure and performance.Lastly,we reviewed the research status of oriented oxide ceramics and their application in the layered oxides.The second chapter is experimental methods,and consists of two parts.The former half involves the synthesis mothed in our sample preparation,including magnetic layer insertion technique,hydrothermal synthesis,the preparation of oriented ceramics and the post annealing of ceramics.The latter part is the characterization techniques of complex parametric layered oxides,including not only basic crystal phase analysis,morphology characterization and element analysis methods,but also electrical,magnetic performance,and electron structure detection techniques.In chapter three,we synthesized Bi4La0.5Sr0.5Ti3MnO15-δ nano-flower composed of nanoplates with high aspect ratio by hydrothermal magnetic layer insertion method,which improved from the solid-phase magnetic intercalation method previously used by our research group,and systematically discussed the influence of various conditions on the product in the hydrothermal synthesis.According to the samples synthesized at 170℃ to 230 ℃,we found that 200℃ is the lowest temperature for synthesizing.We adjusted the material input amount from 0.3125 to 2.5 mmol,and found 1.25 mmol is a suitable value.By exploring the joint action of mineralizer type,concentration and temperature,we concluded that the growth window of KOH as mineralizer is wider than that of NaOH,and high alkali concentration at high temperature is harmful for the growth of our products.By varing the type and concentration of surfactants,we adjusted the morphology and preferred growth of crystal plane of the samples.The samples with different La/Sr ratio and different B-site doping were also synthesized.The results demonstrate that the hydrothermal magenetic intercalation method is quite effective in the synthesis of complex parametric layered oxides,which has good tunability and is beneficial to the preparation of subsequent oriented ceramics and the sample adjustment in the possible application of catalysis field.In the fourth chapter,we took advantage of its structure feature of high c/a ratio and superior adjustability of properties to synthesize oriented Bi4La0.5Sr0.5Ti3MnO15-δceramic with highly anisotropic electrical and magnetic properties.According to comparing and calculating the relative peak intensity of different Miller index in oriented ceramics and hydrothermal samples,we got the Lotgering factor of the MF 1000℃ sample and the HP 1000℃ sample to be 98.69%and 99.87%,respectively.By measuring the resistivity and magnetism along in-plane and out-of-plane directions,we revealed that the resistivity along out-of-plane direction is larger than in-plane by an order of magnitude at room temperature,and the magnetism along in-plane and outof-plane directions are different as well.We adopt the "brick layer" model to estimate the grain interior and grain boundary conductivity,indicating that the electrical anisotropy of our ceramic originates from the anisotropy within grain interiors for the Aurivillius layered structure other than the influence of grain boundary.With the electrochemical impedance spectroscopy(EIS)and Hall measurements,we concluded that the conductive mechanism is the hole hopping along the Mn4+-O2--Mn3+double exchange route,and the transport route along c direction is "blocked" to some extent while that along in-plane direction stays unaffected.Therefore,the sample is highly electrical anisotropic,and we infer the magnetic anisotropy originates from the same reason.The previous similar work related to the Bi4Ti3O12 intercalation mainly focus on the ferroelectricity,permittivity and piezoelectric coefficient,etc.,but here we focus on the anisotropy of resistivity,potential resistance tunable properties and magnetic anisotropy at low temperature,which can be applied to the field of semiconductor functional devices.In chapter five,the magnetism variation during the process from hydrothermal powder to ceramic of Bi4La0.5Sr0.5Ti3MnO15-δ is discussed.By designing different postannealing conditions,we obtained a series of samples with different magnetic states.According to systematic static(DC)magnetic and alternating current(AC)susceptibility measurements,we found that the magnetism of sample annealing under oxygen atmosphere becomes weaker,while the magnetism of sample annealing under vacuum becomes stronger as the annealing temperature elevating,reaching the climax at 800℃,bringing about a second ferrimagnetism with a 76 K transition point besides the ferrimagnetism with a 43 K transition point at 900 ℃.With the XPS and soft X-ray absorption spectra characterization,we analyzed the valence states of the elements in the sample and propose two competing super-exchange interaction routes to explain the magnetism variation and origination of ferrimagnetism in our samples.This work has important guiding significance for the magnetism introduction,magnetism tunability and the interpretation of the intrinsic mechanism of the source of magnetism in the materials with similar structures.In chapter six,we summarized the full text,in which the achievements and innovation point in our work are extracted and the future direction of development,application,and other possible follow up work on complex parametric layered oxides are envisioned.