Preparation And Characterization Of Multiferroic Composite Coaxial Nanotube Arrays

Multiferroic materials, owing to the coexistence of ferroelectric, ferromagnetic and ferroelastic orders as well as the magnetoelectric (ME) coupling among them, have been a hot research field worldwide in recent years. However, the magnetoelectric response in single-phase multiferroic materials is either relatively weak or occurs at temperatures too low for practical applications. As a result, much attention have been paid to the study of composite multiferroic materials.In addition, one-dimensional (ID) nanomaterials, which exhibit a wide range of magnetic, electric, and optical properties different from their counterparts and have potential for applications in transducer, photoelectric devices etc., are of fundamental and technological research interests. Consequently, preparation of ID composite multiferroic nanostructures and studying their multiferroic properties as well as physical mechanisms are of great scientific meaning. BiFeO3(BFO) has a large self-polarization (55-150?C/cm2), and Nd-substituted Bi4Ti3012(Bi3.15Nd0.85Ti3O12, BNT) is the most promising candidate for use in lead-free piezoelectrics because of its excellent polarization and piezoelectricity behaviors. CoFe2O4(CFO) is found to show strong magnetic anisotropy. Therefore, to combine BFO and BNT respectively with CFO may produce strong magnetoelectric effect. This has been confirmed in CFO/BFO, CFO/BNT composite films. According to the results of calculations, the magnetoelectric effect in magnetic/piezoelectric cylinder can be enhanced by more than one order of magnitude compared to the corresponding non-cylinder composite. However, there is still no report on the study of CFO/BFO, CFO/BNT coaxial nanotubes.Ferroelectric photovoltaic effect has been a research hotspot in recent years. As one of the candidate materials for ultraviolet detector, non-destructive read and photovoltaic device, ferroelectric materials have attracted great research interests in their photovoltaic properties. Photovoltaic properties of many ferroelectric materials, such as Pb(Zr,Ti)O3, BFO etc. have been studied. Among them, bismuth-layered perovskite Bi4Ti3O12is one of the most promising photovoltaic materials for its good ferroelectricity, excellent anti-fatigue as well as non-toxicity. Consequently, study of photovoltaic properties of Bi4Ti3O12is meaningful.Based on the above consideration, this dissertation focused on the study of multiferroic coaxial nanotubes and photovoltaic effects of BNT. The main research content and results are shown bellow:1. CoFe204/Bio.97Ceo.o3Fe03(CFO/BCFO) multiferroic coaxial nanotubes were prepared by an optimized two-step sol-gel template method. The morphologies, structures and electrical properties of the coaxial nanotubes were studied. The CFO/BCFO multiferroic composite nanotubes show the coexistence of ferroelectricity and magnetism. The CFO/BCFO composite nanotubes demonstrate a sauturated hysteresis while the leakage current density is increased by two orders due to the high conducitivity of inner CFO nanotubes. Compared to the BCFO nanotubes, the dielectric measurements exhibited that the dielectric constant of the coaxial nanotubes decreased while their dielectric loss increased. These may be resulted from the small relative dielectric permittivity and the large dielectric loss of the inner CFO nanotubes.2. CoFe2O4/Bi3.15Ndo.85Ti3012(CFO/BNT) coaxial nanotubes were successfully prepared by an optimized two-step sol-gel template method. The SEM and TEM revealed the characteristic morphology of coaxial nanotubes. The SAED patterns confirmed the coexistence of spinel CFO and layered-like perovskite BNT phases in the coaxial nanotubes. The CFO/BNT coaxial nanotubes have a typical P-E hysteresis loop, while the inner CFO nanotube increased the leakage current density probably due to its high conducitivity. The coaxial nanotubes show a saturated magnetic hysteresis according to the magnetic measurement.3. The BNT film was deposited on Pt/Ti/SiO2/Si substrate by a sol-gel method. Sn doped In2O3(ITO) top electrodes were sputtered on the film by magnetrons puttering and the ITO/BNT/Pt sandwich structure was prepared. The results demonstrate the evident photovoltaic effect in the structure. As the work function of ITO is different of Pt, the shottky barrier at ITO/BNT interface is higher than that of BNT/Pt interface. The difference in barrier height results in an electric field pointing from Pt to ITO which is one of important factor to lead to the photovoltaic effect in the structure. ?-? characteristics were measured after a positive and a negative pulse were respectively applied in the structure, which show that the depolarization result from the ferroelectric polarization has magnificent effect on the photovoltaic properties of the ferroelectric sandwich structure.