钙钛矿铁电氧化物单晶纳米结构的表面、界面及性能研究
发布时间：2018-09-12 08:12
【摘要】：钙钛矿相铁电单晶纳米结构因其独特的物理化学性质及铁电表面化学,在高密度存储、能量转换及催化等领域有着潜在应用,并逐步成为功能材料领域的研究热点之一。开展钙钛矿相铁电氧化物单晶纳米结构的可控制备、表面与界面的调控及性能研究具有重要的理论意义和科学价值。本论文首先简要概述了钙钛矿铁电氧化物的结构特点,重点总结和分析了钙钛矿铁电氧化物的结构特点、自发极化以及屏蔽带来的铁电表面化学、钙钛矿相PbTi03(PTO)纳米结构的单畴稳定性及其表面、界面的化学性质；探讨了铁电氧化物纳米结构的制备及研究现状。特别针对钙钛矿铁电纳米晶的规则刻面、铁电表面化学、铁电极化对气体吸附、贵金属生长和催化性能的影响等问题进行了详细的论述和总结。在此基础上,本文采用水热法和固相反应法,分别制备了具有规则几何外形的钙钛矿相PTO多面体纳米结构、STO/PTO纳米复合结构。系统研究了这些钙钛矿铁电氧化物纳米材料的微结构,表面化学状态、暴露晶面的稳定性和光催化性能,以及PTO极化面对贵金属单晶纳米晶沉积生长及其CO催化氧化的影响规律、对STO单晶纳米结构生长以及STO/PTO界面铁磁性的影响规律,提出了非磁性钙钛矿氧化物界面铁磁性产生的极化调节机制。本论文主要研究内容和结果如下：(1)采用无机盐离子辅助水热法,首次成功制备了具有规则刻面且表面光滑的八面体形貌的钙钛矿相PTO单晶纳米晶(PT OCT),颗粒尺寸为50-100 nm,暴露晶面为{111}晶面,其居里温度为485.56℃。HAADF-STEM和STEM-EELS结果表明,在PT OCT单晶纳米晶的表面层～2 nm处存在Li+富集,而在纳米晶内部未探测出Li+的分布。Li与0结合形成了Li-O键,且Li-O键的存在是PT OCT单晶纳米晶{111}晶面稳定的重要因素。(2)研究发现PT OCT单晶纳米晶的生长为取向聚集生长方式(OA)机制,即：在水热反应初期,反应中形成四方钙钛矿结构的PT纳米颗粒,尺寸约为2-4 nm,纳米粒子在静电力、Li+作用以及范德瓦尔斯力共同作用下逐渐聚集在一起,并在生长过程中,颗粒之间不断调整晶粒取向以达到表面能的降低,最终形成了八面体形貌的PT单晶纳米晶。在这一生长过程中,当八面体形貌基本形成后,Li+通过扩散作用逐渐从八面体内部迁移到表面,最后在晶体表面聚集,起到稳定PT OCT{111}晶面的作用。(3)可见光光催化研究表明,PT OCT单晶纳米晶是一种性能优异的光催化剂,60 min左右即可完全降解浓度为10-5M的亚甲基蓝水溶液(MB)水溶液,其一级反应速率常数达0.042 min-1,是相同反应条件下同类钙钛矿氧化物可见光催化效率的10倍。UV吸收光谱研究表明,PTOCT单晶纳米晶的禁带宽度由块体的2.8～3.0 eV下降到2.58 eV(480 nm),同时在500～700 nm范围内的可见光吸收整体增强；另一方面,低温电子顺磁共振谱(ESR)研究表明PT OCT单晶纳米晶中Ti3+的出现有可能导致了能带结构内形成局域态,降低了PT OCT纳米晶的带隙,增强了可见光波段的吸收,使得PT OCT单晶纳米晶具有高效的可见光催化性能。(4)采用固相反应法首次成功制备了尺寸均一、分散性良好钙钛矿相PTO截角八面体纳米晶,研究表明,Pb304局部熔化,为PTO纳米晶的均匀成核-生长提供了类似于溶液中的液相环境。HAADF-STEM和Tomography分析结果表明,所制备的钙钛矿相PTO单晶纳米颗粒尺寸为50-100nm,具有规则的晶面,呈截角八面体形貌,主要暴露面为{111}和{01l},存在少量的{100}晶面。(5)以PTO截角八面体纳米晶为载体,通过湿化学法氧化-还原反应成功制备了负载Pt纳米晶的Pt-PTO纳米复合结构。微结构研究表明,尺寸为3-5 nm的单晶Pt纳米晶选择性地生长在钙钛矿PTO纳米颗粒的{111)面上,单晶Pt纳米晶的分散性良好,尺寸均一。CO催化氧化实验表明,以Pt-PTO纳米晶作为催化剂,CO转化为CO2的起始温度为30℃,至50℃左右时,CO的转化率达到了100%。(6)为研究Pt-PTO系统的CO催化的动力学,采用湿化学法,分别在钙钛矿相PTO截角八面体单晶纳米颗粒(主要暴露面为{111}面)、PTO纳米纤维(暴露面为{100}或{010}晶面)及PTO纳米片(暴露面为{001}晶面)上成功负载了Pt单晶纳米晶,获得了三种Pt-PTO纳米复合材料。研究表明,负载的Pt颗粒在{111}、{100}和{001}面上的尺寸逐渐增大,分散性逐渐降低,由3-5 nm,5-20 nm到100nm左右的团聚；在未负载Pt时,PTO截角八面体单晶纳米颗粒、PTO纳米纤维和PTO纳米片在250℃时的CO催化氧化反应的转化率分别为60%、5%和85%,其中PTO纳米片对CO的转化效率最高,PTO纳米纤维对CO的转化效率最低。此时,CO催化氧化反应的中心为PTO纳米结构本身,钙钛矿PTO纳米结构暴露面的极性将对CO和O2的吸附-脱附平衡及反应速率控制步骤起主导作用,PTO暴露面的极性越强,将越有利于催化氧化反应的势垒降低,从而催化性能越高。(7)在负载Pt之后,Pt-PTO截角八面体纳米颗粒、Pt-PTO纳米纤维以及Pt-PTO纳米片复合结构的100%的CO转化率温度分别为50℃,100℃和100℃。三种体系的表观活化能(Ea)分别为22.9(±0.4) kcal mol-1,32.7(±2.9) kcal mol-1,26.5(±1.6) kcal mol-1。Pt-PTO纳米复合结构作催化剂时,CO的催化氧化反应中心是Pt纳米晶,其微结构和表面化学状态决定了CO催化反应的动力学。Pt纳米晶在PTO截角八面体纳米颗粒上单分散生长,尺寸较小(约为3-5 nm),结晶性良好。而Pt纳米晶在PTO纳米纤维和纳米片上是严重聚集的形式生长成大尺寸团簇状(10nm),其表面活性位点比例比PTO纳米颗粒上的Pt少。(8)以单晶单畴的钙钛矿相PTO纳米片为载体,采用水热法首次成功制备了钙钛矿相SrTiO3(STO)/PTO单晶纳米复合结构。TEM和Cs-STEM研究表明,STO选择性生长在PTO纳米片四个侧边非极化面和{001}正极化面上,形成核-壳结构的包裹层。STO/PTO具有原子级分辨率的界面,在界面处,Pb和Sr均没有发生互扩散,界面清晰。STO在PTO纳米片的极化面和非极化面均为外延生长,且在侧面生长时表现出拓扑性质,厚度约为15-20 nm。当STO单晶薄膜外延生长在PTO纳米片的{001)晶面的正极化面方向时,形成的界面厚度约为1-2个单胞尺寸(～1 nm),当STO在侧边的非极化面生长时,界面厚度仅为1个单胞尺寸(～0.4 nm)。(9)STO/PTO纳米复合材料具有明显的室温铁磁性,随着温度从300 K降低到5 K,其饱和磁化强度Ms由2.5×10-3emu/g增加到2.5×10-2emu/g,对应的矫顽场Hc从138 Oe增加至375 Oe；在300 K、150 K和100 K条件下,当磁场强度大于5000 Oe以后,典型的铁磁性磁化曲线消失,取而代之的是一段磁化强度M近似为零的过渡区域；磁场范围约为2500 Oe,进一步增大磁场,磁化曲线在此发生突变转折,对应的M由零转变为负值,之后材料表现为抗磁性；当磁场逐步降低时,又经过类似的过程转变为铁磁性,两个磁转变过程都是可逆的。随着温度的降低,这一磁转变所需的临界磁场强度也随之大幅提高。HAADF-STEM和STEM-EELS的结构分析以及第一性原理计算的结果表明,STO/PTO纳米复合材料的铁磁性与正极化面界面处出现的大量Ti3+密切相关,这一结果使得STO/PTO复合材料成为新型铁电-铁磁共存的多铁系统。
[Abstract]:Perovskite ferroelectric single crystal nanostructures have potential applications in the fields of high density storage, energy conversion and catalysis due to their unique physical and chemical properties and ferroelectric surface chemistry. It is of great theoretical and scientific significance to study the regulation and properties of Perovskite Ferroelectric oxides. Firstly, the structural characteristics of Perovskite Ferroelectric oxides are briefly summarized. The structural characteristics of Perovskite Ferroelectric oxides, the ferroelectric surface chemistry caused by spontaneous polarization and shielding, and the single domain stability of perovskite phase PbTi03 (PTO) nanostructures are summarized and analyzed. The preparation and research status of ferroelectric oxide nanostructures are discussed. Especially, the regular facets of Perovskite Ferroelectric nanocrystals, ferroelectric surface chemistry, the influence of iron electrode on gas adsorption, noble metal growth and catalytic performance are discussed and summarized in detail. In this paper, perovskite PTO polyhedron nanostructures and STO/PTO nanocomposites with regular geometry shapes were prepared by hydrothermal method and solid state reaction method, respectively. The main contents and results of this dissertation are as follows: (1) Inorganic salt ions are used in the preparation of non-magnetic perovskite oxides. Perovskite PTO single crystal nanocrystals (PT OCT) with regular facets and smooth octahedral morphology were successfully prepared by assisted hydrothermal method for the first time. The size of PT OCT nanocrystals was 50-100 nm, and the exposed surface was {111} crystal plane. The Curie temperature was 485.56 C. The results of HAADF-STEM and STEM-EELS showed that there was Li in the surface layer of PT OCT single crystal nanocrystals ~2 nm. Li-O bond was formed by the combination of Li and 0, and the existence of Li-O bond was an important factor for the stability of {111} crystal plane of PT OCT single crystal nanocrystals. (2) It was found that the growth of PT OCT single crystal nanocrystals was oriented aggregation growth mode (OA) mechanism, that is, the formation of tetragonal perovskite in the early stage of hydrothermal reaction. The size of PT nanoparticles with mineral structure is about 2-4 nm. The nanoparticles gradually gather together under the combined action of electrostatic force, Li+ action and van der Waals force. During the growth process, the orientation of the grains is adjusted continuously to reduce the surface energy, and the octahedral morphology of PT single crystal nanocrystals is finally formed. When the octahedral morphology was basically formed, Li+ gradually migrated from the octahedral to the surface through diffusion, and finally aggregated on the crystal surface to stabilize the PT OCT {111} crystal plane. (3) Visible light photocatalysis showed that PT OCT single crystal nanocrystals were excellent photocatalysts with a complete degradation concentration of 10 minutes or so. UV absorption spectra show that the band gap of PTOCT single crystal nanocrystals decreases from 2.8-3.0 eV to 2.58 eV (480 nm), and the band gap of PTOCT single crystal nanocrystals decreases from 2.8-3.0 eV to 2.58 eV (480 nm) in the range of 500-700 nm. On the other hand, low-temperature electron paramagnetic resonance (ESR) studies show that the presence of Ti3+ in PT OCT single crystal nanocrystals may lead to the formation of localized states in the band structure, reduce the band gap of PT OCT nanocrystals, enhance the absorption of visible light band, and make PT OCT single crystal nanocrystals have high visible light efficiency. (4) Perovskite PTO truncated octahedral nanocrystals with uniform size and good dispersion were successfully prepared by solid-state reaction for the first time. The results show that the partial melting of Pb304 provides a liquid-phase environment similar to that in solution for homogeneous nucleation-growth of PTO nanocrystals. The size of single crystal PTO nanoparticles in mineral phase is 50-100 nm, with regular crystal planes and octahedral cross-sectional morphology. The main exposed planes are {111} and {01l} with a small number of {100} crystal planes. (5) Pt-PTO nanocomposite structures supported on PTO cross-sectional octahedral nanocrystals were successfully prepared by wet-chemical oxidation-reduction reaction. The structure study shows that single crystal Pt nanocrystals with the size of 3-5 nm selectively grow on the {111] surface of perovskite PTO nanoparticles, and the single crystal Pt nanocrystals have good dispersion and uniform size. CO catalytic oxidation experiments show that the initial temperature of CO conversion to CO2 is 30 C with Pt-PTO nanocrystals as catalyst, and the conversion rate of CO reaches about 50 C. (6) In order to study the kinetics of CO catalysis in Pt-PTO system, Pt single crystal nanoparticles were successfully loaded on perovskite-phase PTO truncated octahedral single crystal nanoparticles (main exposed surface is {111}), PTO nanofibers (exposed surface is {100} or {010} crystal plane) and PTO nanosheets (exposed surface is {001} crystal plane) by wet chemical method. Pt-PTO nanocomposites were prepared. The results showed that the size of supported PT particles on {111}, {100} and {001} surfaces increased gradually, and the dispersion decreased gradually, from 3-5 nm, 5-20 nm to about 100 nm. When the Pt was not loaded, PTO truncated octahedral single crystal nanoparticles, PTO nanofibers and PTO nanosheets were converted to CO catalytic oxidation at 250 C. The conversion rates are 60%, 5% and 85%, respectively. PTO nanosheets have the highest conversion efficiency to CO and PTO nanofibers have the lowest conversion efficiency to CO. At this time, the center of CO catalytic oxidation reaction is PTO nanostructure itself, and the polarity of the exposed surface of perovskite PTO nanostructure will play a leading role in the balance of CO and O2 adsorption-desorption and the control step of reaction rate. The stronger the polarity of the exposed surface, the more favorable the barrier of the catalytic oxidation reaction will be, and the higher the catalytic performance will be. (7) After loading Pt, the 100% CO conversion of Pt-PTO truncated octahedral nanoparticles, Pt-PTO nanofibers and Pt-PTO nanosheet composite structures will be at 50, 100 and 100 degrees Celsius respectively. The catalytic oxidation center of CO is Pt nanocrystals when the nanocomposite structure of 22.9 (+0.4) kcal mol-1,32.7 (+2.9) kcal mol-1,26.5 (+1.6) kcal mol-1.Pt-PTO is used as catalyst, and its microstructure and surface chemical state determine the kinetics of CO catalytic reaction. Pt nanocrystals grew into large clusters (10 nm) on PTO nanofibers and nanosheets. The proportion of active sites on the surface of Pt nanocrystals was less than that on PTO nanoparticles. (8) Perovskite nanosheets with single crystal domains were successfully prepared by hydrothermal method for the first time. The results of TEM and CS-STEM show that STO selectively grows on the four side non-polarized and {001} positive polarized surfaces of PTO nanosheets, forming a core-shell structure encapsulation layer. STO/PTO has an atomic-level resolution interface. At the interface, neither Pb nor Sr diffuses and the interface is clear. Both the polarized and non-polarized surfaces are epitaxial grown, and the thickness of the films is about 15-20 nm. When the STO films are epitaxial grown in the direction of the positive polarized surface of {001} crystal plane of PTO nanosheets, the thickness of the interface is about 1-2 cell sizes (~1 nm), and when the STO films grow on the non-polarized surface of the side, the interface is about 1-2 cell sizes (~1 nm). The thickness of STO/PTO nanocomposites is only 1 cell size (~0.4 nm). (9) STO/PTO nanocomposites have obvious ferromagnetism at room temperature. The saturation magnetization of the composites increases from 2.5 *10-3 emu/g to 2.5 *10-2 emu/g with the decrease of temperature from 300 K to 5 K, and the corresponding coercive field Hc increases from 138 Oe to 375 Oe. At 300 K, 150 K and 100 K, the magnetic field strength is greater than 500 Oe. After 0 Oe, the typical ferromagnetization curve disappears, replaced by a transition region in which the magnetization intensity M approximates zero; the magnetic field range is about 2 500 Oe, further increasing the magnetic field, the magnetization curve undergoes a sudden change, the corresponding M changes from zero to negative, and then the material becomes diamagnetic; when the magnetic field gradually decreases, it passes through again. As the temperature decreases, the critical magnetic field intensity required for the magnetic transition increases dramatically. The results of structural analysis and first-principles calculations of HAADF-STEM and STEM-EELS show that the ferromagnetism of STO/PTO nanocomposites is reversible at the interface between the ferromagnetism and the positive polarization surface. A large number of Ti3+ ions are closely related, which makes STO/PTO composites a new ferroelectric-ferromagnetic coexistence multi-ferromagnetic system.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.1
本文编号：2238426
[Abstract]:Perovskite ferroelectric single crystal nanostructures have potential applications in the fields of high density storage, energy conversion and catalysis due to their unique physical and chemical properties and ferroelectric surface chemistry. It is of great theoretical and scientific significance to study the regulation and properties of Perovskite Ferroelectric oxides. Firstly, the structural characteristics of Perovskite Ferroelectric oxides are briefly summarized. The structural characteristics of Perovskite Ferroelectric oxides, the ferroelectric surface chemistry caused by spontaneous polarization and shielding, and the single domain stability of perovskite phase PbTi03 (PTO) nanostructures are summarized and analyzed. The preparation and research status of ferroelectric oxide nanostructures are discussed. Especially, the regular facets of Perovskite Ferroelectric nanocrystals, ferroelectric surface chemistry, the influence of iron electrode on gas adsorption, noble metal growth and catalytic performance are discussed and summarized in detail. In this paper, perovskite PTO polyhedron nanostructures and STO/PTO nanocomposites with regular geometry shapes were prepared by hydrothermal method and solid state reaction method, respectively. The main contents and results of this dissertation are as follows: (1) Inorganic salt ions are used in the preparation of non-magnetic perovskite oxides. Perovskite PTO single crystal nanocrystals (PT OCT) with regular facets and smooth octahedral morphology were successfully prepared by assisted hydrothermal method for the first time. The size of PT OCT nanocrystals was 50-100 nm, and the exposed surface was {111} crystal plane. The Curie temperature was 485.56 C. The results of HAADF-STEM and STEM-EELS showed that there was Li in the surface layer of PT OCT single crystal nanocrystals ~2 nm. Li-O bond was formed by the combination of Li and 0, and the existence of Li-O bond was an important factor for the stability of {111} crystal plane of PT OCT single crystal nanocrystals. (2) It was found that the growth of PT OCT single crystal nanocrystals was oriented aggregation growth mode (OA) mechanism, that is, the formation of tetragonal perovskite in the early stage of hydrothermal reaction. The size of PT nanoparticles with mineral structure is about 2-4 nm. The nanoparticles gradually gather together under the combined action of electrostatic force, Li+ action and van der Waals force. During the growth process, the orientation of the grains is adjusted continuously to reduce the surface energy, and the octahedral morphology of PT single crystal nanocrystals is finally formed. When the octahedral morphology was basically formed, Li+ gradually migrated from the octahedral to the surface through diffusion, and finally aggregated on the crystal surface to stabilize the PT OCT {111} crystal plane. (3) Visible light photocatalysis showed that PT OCT single crystal nanocrystals were excellent photocatalysts with a complete degradation concentration of 10 minutes or so. UV absorption spectra show that the band gap of PTOCT single crystal nanocrystals decreases from 2.8-3.0 eV to 2.58 eV (480 nm), and the band gap of PTOCT single crystal nanocrystals decreases from 2.8-3.0 eV to 2.58 eV (480 nm) in the range of 500-700 nm. On the other hand, low-temperature electron paramagnetic resonance (ESR) studies show that the presence of Ti3+ in PT OCT single crystal nanocrystals may lead to the formation of localized states in the band structure, reduce the band gap of PT OCT nanocrystals, enhance the absorption of visible light band, and make PT OCT single crystal nanocrystals have high visible light efficiency. (4) Perovskite PTO truncated octahedral nanocrystals with uniform size and good dispersion were successfully prepared by solid-state reaction for the first time. The results show that the partial melting of Pb304 provides a liquid-phase environment similar to that in solution for homogeneous nucleation-growth of PTO nanocrystals. The size of single crystal PTO nanoparticles in mineral phase is 50-100 nm, with regular crystal planes and octahedral cross-sectional morphology. The main exposed planes are {111} and {01l} with a small number of {100} crystal planes. (5) Pt-PTO nanocomposite structures supported on PTO cross-sectional octahedral nanocrystals were successfully prepared by wet-chemical oxidation-reduction reaction. The structure study shows that single crystal Pt nanocrystals with the size of 3-5 nm selectively grow on the {111] surface of perovskite PTO nanoparticles, and the single crystal Pt nanocrystals have good dispersion and uniform size. CO catalytic oxidation experiments show that the initial temperature of CO conversion to CO2 is 30 C with Pt-PTO nanocrystals as catalyst, and the conversion rate of CO reaches about 50 C. (6) In order to study the kinetics of CO catalysis in Pt-PTO system, Pt single crystal nanoparticles were successfully loaded on perovskite-phase PTO truncated octahedral single crystal nanoparticles (main exposed surface is {111}), PTO nanofibers (exposed surface is {100} or {010} crystal plane) and PTO nanosheets (exposed surface is {001} crystal plane) by wet chemical method. Pt-PTO nanocomposites were prepared. The results showed that the size of supported PT particles on {111}, {100} and {001} surfaces increased gradually, and the dispersion decreased gradually, from 3-5 nm, 5-20 nm to about 100 nm. When the Pt was not loaded, PTO truncated octahedral single crystal nanoparticles, PTO nanofibers and PTO nanosheets were converted to CO catalytic oxidation at 250 C. The conversion rates are 60%, 5% and 85%, respectively. PTO nanosheets have the highest conversion efficiency to CO and PTO nanofibers have the lowest conversion efficiency to CO. At this time, the center of CO catalytic oxidation reaction is PTO nanostructure itself, and the polarity of the exposed surface of perovskite PTO nanostructure will play a leading role in the balance of CO and O2 adsorption-desorption and the control step of reaction rate. The stronger the polarity of the exposed surface, the more favorable the barrier of the catalytic oxidation reaction will be, and the higher the catalytic performance will be. (7) After loading Pt, the 100% CO conversion of Pt-PTO truncated octahedral nanoparticles, Pt-PTO nanofibers and Pt-PTO nanosheet composite structures will be at 50, 100 and 100 degrees Celsius respectively. The catalytic oxidation center of CO is Pt nanocrystals when the nanocomposite structure of 22.9 (+0.4) kcal mol-1,32.7 (+2.9) kcal mol-1,26.5 (+1.6) kcal mol-1.Pt-PTO is used as catalyst, and its microstructure and surface chemical state determine the kinetics of CO catalytic reaction. Pt nanocrystals grew into large clusters (10 nm) on PTO nanofibers and nanosheets. The proportion of active sites on the surface of Pt nanocrystals was less than that on PTO nanoparticles. (8) Perovskite nanosheets with single crystal domains were successfully prepared by hydrothermal method for the first time. The results of TEM and CS-STEM show that STO selectively grows on the four side non-polarized and {001} positive polarized surfaces of PTO nanosheets, forming a core-shell structure encapsulation layer. STO/PTO has an atomic-level resolution interface. At the interface, neither Pb nor Sr diffuses and the interface is clear. Both the polarized and non-polarized surfaces are epitaxial grown, and the thickness of the films is about 15-20 nm. When the STO films are epitaxial grown in the direction of the positive polarized surface of {001} crystal plane of PTO nanosheets, the thickness of the interface is about 1-2 cell sizes (~1 nm), and when the STO films grow on the non-polarized surface of the side, the interface is about 1-2 cell sizes (~1 nm). The thickness of STO/PTO nanocomposites is only 1 cell size (~0.4 nm). (9) STO/PTO nanocomposites have obvious ferromagnetism at room temperature. The saturation magnetization of the composites increases from 2.5 *10-3 emu/g to 2.5 *10-2 emu/g with the decrease of temperature from 300 K to 5 K, and the corresponding coercive field Hc increases from 138 Oe to 375 Oe. At 300 K, 150 K and 100 K, the magnetic field strength is greater than 500 Oe. After 0 Oe, the typical ferromagnetization curve disappears, replaced by a transition region in which the magnetization intensity M approximates zero; the magnetic field range is about 2 500 Oe, further increasing the magnetic field, the magnetization curve undergoes a sudden change, the corresponding M changes from zero to negative, and then the material becomes diamagnetic; when the magnetic field gradually decreases, it passes through again. As the temperature decreases, the critical magnetic field intensity required for the magnetic transition increases dramatically. The results of structural analysis and first-principles calculations of HAADF-STEM and STEM-EELS show that the ferromagnetism of STO/PTO nanocomposites is reversible at the interface between the ferromagnetism and the positive polarization surface. A large number of Ti3+ ions are closely related, which makes STO/PTO composites a new ferroelectric-ferromagnetic coexistence multi-ferromagnetic system.
【学位授予单位】：浙江大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TB383.1
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