Nanoscale retention‐loss dynamics of polycrystalline PbTiO3 nanotubes

We observed the nanoscale retention dynamics of polycrystalline PbTiO₃ nanotubes using piezoresponse force microscopy. We found that the retention loss of the nanodot domains on the nanotubes showed the stretched exponential relaxation behaviors with stretched exponential factor n being less than 1 (0.523 and 0.692), which are similar to the thin films. In addition, the nanodot domains showed a diverse relaxation time constant τ due to different remnant polarization of each dot domains. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Unusual domain configurations and local piezoresponse in the [720]‐cut BaTiO3 single crystal

The domain configurations and local piezoresponse property of the [720]‐cut BaTiO₃ single crystal and [001]‐oriented BaTiO₃ crystal were investigated by high‐resolution piezoresponse force microscopy. Large differences in their surface topography features, domain configurations and local piezoresponse were found between [720]‐cut and [001]‐oriented BaTiO₃ crystals. The large surface bending angle due to ferroelastic domain walls leads to a high strain energy appearing in the [720]‐cut BaTiO₃, further resulting in unique band‐like topographic features, needle domains for stress compensation and locally ultrahigh piezoelectric response as well. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Abnormal thermal conductivity of [720]‐oriented BaTiO3 single crystal

The 3ω ‐scanning thermal microscopy and piezoresponse force microscopy techniques were combined to characterize the local thermal conductivity (λ) of ferroelastic domains in [720]‐oriented and [001]‐oriented BaTiO₃ single crystal. The [720]‐oriented BaTiO₃ showed a local λ value of 3.39 W/m·K, while it was 5.95 W/m·K for [001]‐oriented BaTiO₃. The underlying mechanism for such drastic thermal conductivity reduction in [720]‐oriented BaTiO₃ is ascribed to a stronger phonon‐scattering effect due to the high strains appearing in [720]‐oriented BaTiO₃ single crystals. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Stability of the magnetic domain structure of nanoparticle thin films against external fields

We investigate the effect of external magnetic fields on the magnetic structure of thin films from magnetic nanoparticles (MNP) with dipolar interaction. Such fields are present, for example, if samples are scanned with magnetic probes. Numerical simulations and experimental magnetic force microscopy (MFM) studies are presented. Numerically, we have calculated the magnetization pattern of single-layer and multilayer MNP thin films. The calculations show that unperturbed single-layer MNP films have an in-plane orientation of the magnetization with a flux-closure-domain pattern. An external field generated by a point dipole above the film induces locally an out-of-plane configuration of the magnetization. In the corresponding MFM images, the domain pattern in the film is erased and a stripe-like contrast enhancement at the edges appears. Multilayer films are found to be more robust against external fields than monolayers.     

YMnO_3薄膜的铁电行为及其纳米尺度铁电畴的研究

六方YMnO_3是一种特殊的多铁性材料,因其具有介电常数低、单一极化轴、无挥发性元素等特点,在磁电领域具有独特的优势,但目前关于YMnO_3薄膜的铁电性特别是畴结构的研究相对较少.本文采用溶胶-凝胶法在Si(100)基片上制备了多铁性YMnO_3薄膜,利用掠入射X-射线衍射、原子力显微镜对薄膜的结构及表面形貌进行了分析,用压力显微镜(PFM)技术研究了纳米尺度畴结构及微区电滞行为,并通过I-V,P-E曲线进一步研究了薄膜的漏电流和宏观电滞行为.结果表明,该薄膜为六方钙钛矿结构,YMnO_3晶粒大小均匀并且结晶性较好,薄膜表面粗糙度为7.209 nm.PFM图显示出清晰的电畴结构,结合典型的微区振幅蝴蝶曲线和相位电滞回线,证实该YMnO_3薄膜具有较好的铁电性.由于受内建电场的作用,振幅曲线和相位曲线都向正向偏移,表现出非对称特征.该薄膜的漏电流密度低于10~(-6)A·cm~(-2),因而其电滞回线基本能够达到饱和.     

Combined piezoresponse force microscopy and Raman scattering investigation of domain boundaries in BiFeO3 ceramics

Domain structure of BiFeO₃ ceramics has been investigated using a combination of piezoresponse force microscopy (PFM) and Raman scattering techniques. Both methods demonstrate the presence of ferroelastic domains, separated by almost parallel planar domain walls, as well as the presence of large homogeneous single ferroelastic domain regions. In addition to highly resolved domain pattern obtained by PFM, small frequency shifts of the Raman-active modes give us complementary information about the angle φ between the surface normal and the rhombohedral axis of the BiFeO₃ crystal for any measured position at the surface of the sample.     

Synthesis and physical properties of Ca- and Ta-modified (K,Na)NbO3 lead-free piezoelectric ceramics

Polycrystalline samples of lead-free Ca and Ta co-substituted potassium sodium niobate (K_0.5Na_0.5NbO_3, KNN) ceramics have been prepared by solid state reaction technique. X-ray diffraction showed formation of a single-phase perovskite structure with orthorhombic symmetry. Substitution inhibits the grain growth, improves densification and decreases the ferro-paraelectric phase transition temperature. Temperature dependent dielectric permittivity studies demonstrate significant decrease in peak-permittivity values in the substituted samples. Bulk longitudinal piezoelectric coefficient is significantly enhanced, up to ～155 pC/N for (K_0.48Na_0.48Ca_0.02)(Nb_0.85Ta_0.15O₃) as compared to 95 pC/N for pristine KNN ceramic. Local piezoelectric properties have been observed by piezoresponse force microscopy (PFM) technique. Distinct piezocontrast was studied in both vertical and in-plane modes of PFM for all samples. The samples exhibit self-polarization effect in the unpoled state and effective local vertical piezoelectric coefficient was the largest in Ca and Ta co-substituted sample whereas the in-plane piezoelectric coefficient was maximum for Ca-substituted KNN sample. These studies are important for using substituted lead free KNN materials in various piezoelectric applications.     

Bactericidal action of single-walled carbon nanotubes

The action of single-walled carbon nanotubes (SWCNTs) on cells of the genetically engineered K12 TG1 strain of Escherichia coli, which have a luminescent phenotype generated by the cloning of the lux operon of the native luminescent marine bacterium Photobacterium leiognathi into the strain, is studied in this work. The survival rate of the bacterial cells and their morphological changes are studied by means of atomic force microscopy as a function of their exposure to SWCNTs.     

Magnetic domain structures of precipitation-hardened SmCo 2：17-type sintered magnets： Heat treatment effect

The typical magnetic domains of Sm（CObalFe0.25Cuo.07Zr0.02）7.4 magnets quenched through various heattreatment steps have been revealed by using magnetic force microscopy （MFM）. For the specimens in which the nominal c-axis is perpendicular to the imaging plane, the domain configurations change from plate-like for the as-sintered magnet to corrugation and spike-like for the homogenized one, and then to a coarse and finally to a finer domain structure when isothermally aged at 830℃ and then annealed at 400℃. However, only plate-like domains can be detected on the surfaces with the nominal c-axis parallel to the imaging plane. The finer domain （so-called interaction domain） is a characteristic magnetic domain pattern of the SmCo 2：IT-type magnets with high coercivities. Domain walls in a zigzag shape are revealed by means of MFM in final bulk SraCo 2：17-type sintered magnets.     

Effect of metal doping on highly efficient photovoltaics and switchable photovoltage in bismuth ferrite nanotubes

We report enhanced anomalous photovoltaic effects and switchable photovoltage generation in pure and Pr–Cr co‐doped BiFeO₃ (BFO) nanotubes (NTs). Influence of metal doping on short circuit current, open circuit voltage, power conversion efficiency and fill factor are investigated. The power conversion efficiency of pure BFO NTs (～0.207%) is found to be enhanced by several orders of magnitude in comparison with the reported bulk effect. Pr‐doped NTs provide highest values of power conversion efficiency (～0.5%).

     

Interfacial morphology and domain configurations in 0-3 PZT-Portland cement composites

Cement-based piezoelectric composites have attracted great attention recently due to their promising applications as sensors in smart structures. Lead zirconate titanate (PZT) and Portland cement (PC) composite were fabricated using 60% of PZT by volume. Scanning Electron Microscope and piezoresponse force microscope were used to investigate the morphology and domain configurations at the interfacial zone of PZT-Portland cement composites. Angular PZT ceramic grains were found to bind well with the cement matrix. The submicro-scale domains were clearly observed by piezoresponse force microscope at the interfacial regions between the piezoelectric PZT phase and Portland cement phase, and are clearer than the images obtained for pure PZT. This is thought to be due to the applied internal stress of cement to the PZT ceramic particle which resulted to clearer images.     

Anisotropic fracture behaviour and brittle-to-ductile transition of polycrystalline tungsten

The fracture behaviour of polycrystalline sintered and rolled tungsten rods was investigated from ?150°C to 950°C by means of three-point bending tests and electron microscopy where special attention was drawn to the influence of the microstructure. This thorough investigation demonstrates the positive impact of the crystallographic and grain shape anisotropy in tungsten. Specimens extracted along the rolling direction exhibit twice as high fracture toughnesses and a significantly reduced brittle-to-ductile transition temperature than the other two investigated orientations. Furthermore, these specimens show a change in their fracture mode from transgranular to intergranular fracture with crack deflection occurring around 270°C. In an in situ SEM fracture test, the origin of this crack deflection could be clarified. Finally, a fracture mechanics model is presented which predicts correctly the transition between the two fracture modes and which gives an energy criterion suitable to interpret experimental fracture results.     

A comparison between the mechanical and thermal properties of single-walled carbon nanotubes and boron nitride nanotubes

Carbon nanotubes (CNTs) are semimetallic while boron nitride nanotubes (BNNTs) are wide band gap insulators. Despite the discrepancy in their electrical properties, a comparison between the mechanical and thermal properties of CNTs and BNNTs has a significant research value for their potential applications. In this work, molecular dynamics simulations are performed to systematically investigate the mechanical and thermal properties of CNTs and BNNTs. The calculated Young’s modulus is about 1.1 TPa for CNTs and 0.72 TPa for BNNTs under axial compressions. The critical bucking strain and maximum stress are inversely proportional to both diameter and length-diameter ratio and CNTs are identified axially stiffer than BNNTs. Thermal conductivities of (10, 0) CNTs and (10, 0) BNNTs follow similar trends with respect to length and temperature and are lower than that of their two-dimensional counterparts, graphene nanoribbons (GNRs) and BN nanoribbons (BNNRs), respectively. As the temperature falls below 200 K (130 K) the thermal conductivity of BNNTs (BNNRs) is larger than that of CNTs (GNRs), while at higher temperature it is lower than the latter. In addition, thermal conductivities of a (10, 0) CNT and a (10, 0) BNNT are further studied and analyzed under various axial compressive strains. Low-frequency phonons which mainly come from flexure modes are believed to make dominant contribution to the thermal conductivity of CNTs and BNNTs.     

Band structure changes of single-wall carbon nanotubes by the presence of an ionic shell

We report the change of the band structure of two types of carbon nanotubes due to the presence of an isolated, non-conducting, uniformly charged shell held at a fixed distance above their surfaces. We find that, depending on the chirality of the nanotube, the strain on the lattice causes the dispersion relationships to change. This change can result in a modification of the band structure which can induce a metal--semiconductor transition. We consider these effects as a mechanism for heavy-metal ion sensing by functionalized carbon nanotubes.     

Magnetic properties and domain structure of polycrystalline DyFe3

Magnetization σ vs. temperature T was measured from 80 to 700 K in polycrystalline DyFe₃ in a magnetic field H = 10 kOe. From σ = f(T), the Curie temperature was determined. Also, σ was measured vs. H from 0 to 70 kOe at 4.2 K. Magnetization at saturation σ₀ at 4.2 K and the magnetic moment of DyFe₃ were also determined. First observations of domain structure in DyFe₃ are reported. The mean domain with is determined in its dependence on the grain size . The magnetocrystalline anisotropy constant of polycrystalline DyFe₃ is determined as K₁ = -1.2×10⁷ erg/cm³.     

Enhanced interlaminar fracture toughness of unidirectional carbon fiber/epoxy composites modified with sprayed multi-walled carbon nanotubes

A recently reported solvent spraying technique was used herein for incorporation of multi-walled carbon nanotubes (MWCNTs) on unidirectional carbon fiber/epoxy prepregs. The role of the agglomerates reduction of oxidized MWCNTs on Mode-I interlaminar fracture toughness (G_IC) of laminated composites was investigated using double cantilever beam tests. Multiscale laminate composites were fabricated using MWCNTs without and with an acid oxidation, agglomerates reduction (AR) and a sequential treatment based on oxidation and AR. For comparison, specimens without MWCNTs were also prepared and tested. Fourier transform infrared analysis shows evidence of an important amount of oxygenated functional groups on the surface of as-received and oxidized MWCNTs. The results also show Mode-I fracture toughness improvements for all the laminated composites compared to reference samples. A substantial 52% increase in the average G_IC initiation was achieved for laminated composites reinforced with oxidized AR-MWCNTs prepared with only 0.05 wt.% MWCNTs.     

Investigation of the magnetic domain structure of （PtCoPt）／Si multilayers by magnetic force microscopy

The domain structure of (PtCoPt)/Si multilayers in the dc demagnetized state has been investigated by magnetic force microscopy.The domain structure is found to change dramatically as the thickness of the non-magnetic Si sublayer (tsi) increases.Together with the analysis of magnetic properties,the variation of the domain period indicates that the domain wall energy decreases.Using the model developed by Draaisma and de Jonge,the domain wall energy is obtained.     

PZT铁电薄膜纳米尺度铁电畴的场致位移特性

利用扫描力显微术的压电响应模式，并基于逆压电效应原理，研究了梯度组成的PZT铁电薄膜纳米尺度铁电畴的场致位移特性.获得了源于纳米尺度铁电畴的压电效应和电致伸缩效应贡献的场致位移回滞线，以及源于线性压电效应和电畴反转效应综合贡献的纳米尺度压电位移 场强蝶形曲线，证实了Caspari Merz理论在纳米尺度上的有效性.发现了梯度铁电薄膜存在纳米尺度印刻现象，认为该现象的内因源于薄膜中的内偏场. 关键词： PZT铁电薄膜 场致位移 纳米尺度 扫描力显微术     

PZT铁电薄膜纳米尺度畴结构的扫描力显微术研究

利用扫描力显微术中压电响应模式原位研究了（111）择优取向的PZT60/40铁电薄膜的纳米尺度畴结构及其极化反转行为.铁电畴图像复杂的畴衬度与晶粒中的畴排列和晶粒的取向密切相关.直接观察到极化反转期间所形成的小至30nm宽的台阶结构，该台阶结构揭示了（111）取向的PZT60/40铁电薄膜在极化反转期间其畴成核与生长机理主要表现为铁电畴的纵向生长机理. 关键词： 畴结构 反转机理 PZT薄膜 扫描力显微术