Phase field simulation of domain structures in cracked ferroelectrics

The fracture of ferroelectrics is a complex process which is influenced by various factors, among which are the domain switching near the crack tip, the crack face boundary conditions and the applied electric field. Domain switching near crack tips induces major local nonlinearity, while the crack face boundary conditions vary considerably due to different working conditions. In this work, a phase field model and a generalization of the configurational force theory into this model are used to investigate the microstructure around the crack tip and to quantitatively study the influence of the applied electric field and the crack face boundary conditions (permeable, impermeable, semi-permeable and energetically consistent). Evaluation of the fracture properties is done by the nodal configurational force at the crack tip based on the generalized configurational force theory. Results show that the induced domain structure relies significantly on the loading and on the surface boundary conditions. Among the four different conditions considered, the energetically consistent conditions lead to the smallest crack driving force, and the permeable conditions lead to the largest crack driving force. Calculations also show that positive electric fields tend to inhibit fracture, whereas negative electric fields tend to promote fracture.     

A thermodynamically consistent model with evolving domain structures in ferroelectrics

A thermodynamically consistent uniaxial model combining the benefits of both phenomenological and micromechanical models is proposed. The driving forces derived from irreversible thermodynamics induce domain switching on reaching certain critical values. The back stress and electric fields, assumed as linear functions of remanent strain and polarization, developed by the domain switching process are introduced to assist or resist further changes in microscopic state of the crystal depending on the loading history. In this model, volume fractions of three distinct uniaxial variants act as internal variables describing the microscopic state of the crystal. Domain switching process for general cases involving two variants and for simultaneous evolution cases where three distinct variants participate the switching process are dealt with. The model is found to reproduce the characteristics of the responses of ferroelectric polycrystals under uniaxial complex electro-mechanical loading cases that are consistent with the experimental observations.     

纳米结构PZT铁电膜的制备及其表征

采用溶胶-凝胶(sol-gel)自旋涂敷法在硅基氧化铝纳米有序孔膜版介质上(膜版孔径尺寸20～100nm,内生长金属纳米线作为底电极一部分)制备Pb(Zr0.53Ti0.47)O3(PZT)纳米结构铁电膜,并对其介电、铁电性能及微结构进行了表征。介电测量结果表明,厚度25nm的PZT铁电膜,其介电常数在低频区域(频率104Hz)从860迅速下降到100,然后保持在100左右,直至测量频率升高到106Hz。低频区域的介电常数迅速下降是由空间电荷极化所致,它与薄膜和电极之间聚集的界面空间电荷密切相关,尤其是在薄膜与Au纳米线的弯曲界面处。介电损耗在4000Hz附近出现峰值,它来源于空间电荷的共振吸收效应。电滞回线测量结果表明,厚度为100nm的PZT铁电膜,其剩余极化强度为50μC/cm2,矫顽场强为500kV/cm。剖面透射电镜(TEM)像表明PZT纳米铁电膜与底电极(金属纳米线)直接相接触,它们之间的界面呈现一定程度的弯曲。在PZT纳米铁电薄膜后退火处理后,发现部分Au金属纳米线顶端出现分枝展宽现象;而改用Pt纳米线后可有效抑制这种现象。为兼顾氧化铝纳米有序孔膜版内的金属纳米线有序分布及PZT纳米膜的结晶度,选择合适的退火温度是制备工艺中的关键因素。     

Fabrication of nanostructured Al/Cu/Mn metallic multilayer composites by accumulative roll bonding process and investigation of their mechanical properties

Multilayered Al/Cu/Mn composites were produced from aluminum 1100 strips, commercial copper foils and manganese powders, through accumulative roll bonding (ARB). The structural and microstructural evolution of the produced composites was studied by X-ray diffraction and scanning electron microscopy. Also, their mechanical properties at various ARB cycles were studied by microhardness and tensile tests. In this process after nine ARB cycles, a multilayered Al/Cu/Mn composite with homogeneously distributed, fragmented copper layers and Mn powders in the aluminum matrix was produced. Also, it was observed that with increasing strain by progression of the ARB process, the strength and microhardness of the produced composites increased.     

Unique vortex and stripe domain structures in PbTiO3 epitaxial nanodots

The domain structures of PbTiO₃ epitaxial nanodots under the influences of depolarization fields and mismatch strains have been studied using three dimensional phase field simulations. The single-vortex structure and mixed domain configuration, which consisted of zigzag stripe domain and closure dipole flux near the interfaces, were found to be effective in annihilating the depolarization fields in the isotropically tensile and compressive ferroelectric nanodots, respectively. These domain structures were produced by the combined effect of electrostatic and mismatch elastic energies. The width of stripe domain was found to be related to the volume percentage of polarization dipoles along the z-axis, which varied remarkably with the change of compressive mismatch strain. In the case of nanodots under anisotropic mismatch strains, double-vortex domain patterns and stripe domains with nearly straight domain walls were formed. Moreover, the domain structures with electrostatic energy neglected were also studied.     

Parameters of epitaxial light-emitting diode structures and the features of their charge center distribution

The presence of compensated regions at the boundary of the p—n junction is established by measuring the distribution of the effective charge-center density in the space-charge region of semiconducting structures. The effect of these regions on the parameters of the capacitance-voltage characteristics and the features of the technological structure of light-emitting diodes are described. Translated from Izmeritel’naya Tekhnika, No. 1, pp. 49–52, January, 1998.     

Fabrication and characterization of nanostructured Pd hydride pH microelectrodes

Novel pH microsensors were made by electrodepositing mesoporous Pd films onto Pt microdisks, electrochemically loading the films with hydrogen to form the alpha+beta Pd hydride phase, and then switching to the potentiometric mode to monitor pH. To create a nanostructure, the films were deposited within a molecular template formed by the self-assembly of surfactant molecules, a technique known as true liquid crystal templating. The films retain the micrometer size of the Pt microdisk but offer electroactive areas up to 900 times larger. Optimum hydrogen loading conditions were determined, and the mesoporous Pd microdisks were found to have excellent potentiometric properties. From pH 2 to 12, their potential was Nernstian, highly reproducible, very stable (+/-1.2 mV over 2 h), and without hysteresis. Their response time was better than 1 s. However, the presence of oxygen reduced their lifetime significantly, thereby requiring frequent reloading. These microelectrodes do not require calibration before and after measurements, a procedure normally essential for potentiometric pH microsensors. To our knowledge, these are the first results where nanostructured materials made by the true liquid crystal templating method have been used in the potentiometric mode; moreover, these are the first results demonstrating the application of nanostructured microdisks in the potentiometric mode.     

A novel criterion for nonuniform domain switching of tetragonal ferroelectrics

This paper issues a simple criterion to characterize nonuniform domain switching. The criterion is obtained by minimizing switch-induced system energy change computationally. Combining the numerical result and experimental observations, we propose a quasi-analytic criterion defined by a piecewise function. The piecewise criterion is observed to coincide with the accurate solution very well. As examples, we apply the criterion to: (1) portray a nonhomogeneous switching zone near a stationary crack tip; (2) quantify the switch zone size; (3) predict the volume fractions of ferroelastic and ferroelectric switchings. These predictions all agree with experimental measurements.     

Fabrication of nanostructured pearlite steel wires using electropulsing

This work reports the refinement of pearlite structure into nanostructure using electropulsing. Nanostructured pearlitic steel wires possess nanoscale lamellae or nanoscale grain microstructures. Fabrication of nanostructures by severe plastic deformation and lamellar to grain transformation have been investigated. It is suggested that an aligned pearlite structure is preferred in severe plastic deformation. The lamellar to grain transformation is controlled by diffusion of carbon within cementite and also from cementite to ferrite phases. Carbon mobility is changed by mechanical, thermal and electrical states. The interface between nanoscale sub-grains in the ferrite phase has considerable carbon content. Numerical calculations and experimental observations demonstrated these mechanisms.     

The antiferromagnetic domain structure of epitaxial nickel oxide

Epitaxially grown nickel oxide single crystals (thickness 10 to 200 m) have been examined below the Néel temperature (523° K) by optical and X-ray diffraction topographic techniques. The antiferromagnetic domain structure in the as-grown crystals is extremely complex and sensitive to crystal thickness. Annealing simplifies the domain structure, but the presence of the magnesium oxide substrate is shown to inhibit the formation of the large domains observed in well-annealed Verneuil-grown crystals. The misorientation between adjacent domains has been measured by a direct technique and shown to be consistent with the theoretical value.     

A Monte Carlo simulation on domain pattern and ferroelectric behaviors of relaxor ferroelectrics

The domain configuration and ferroelectric property of mode relaxor ferroelectrics (RFEs) are investigated by performing a two-dimensional Monte Carlo simulation based on the Ginzburg-Landau theory on ferroelectric phase transitions and the defect model as an approach to the electric dipole configuration in relaxor ferroelectrics. The evolution of domain pattern and domain wall configuration with lattice defect concentration and temperature is simulated, predicting a typical two-phase coexisted microstructure consisting of ferroelectric regions embedded in the matrix of a paraelectric phase. The diffusive ferroelectric transitions in terms of the spontaneous polarization hysteresis and dielectric susceptibility as a function of temperature and defect concentration are successfully revealed by the simulation, demonstrating the applicability of the defect model and the simulation algorithm. A qualitative consistency between the simulated results and the properties of proton-irradiated ferroelectric copolymer is presented.     

Impact of misfit dislocations on the polarization instability of epitaxial nanostructured ferroelectric perovskites

Defects exist in almost all materials and defect engineering at the atomic level is part of modern semiconductor technology. Defects and their long-range strain fields can have a negative impact on the host materials. In materials with confined dimensions, the influence of defects can be even more pronounced due to the enhanced relative volume of the 'defective' regions. Here we report the dislocation-induced polarization instability of (001)-oriented Pb(Zr(0.52)Ti(0.48))O(3) (PZT) nanoislands, with an average height of approximately 9 nm, grown on compressive perovskite substrates. Using quantitative high-resolution electron microscopy, we visualize the strain fields of edge-type misfit dislocations, extending predominantly into a PZT region with a height of approximately 4 nm and width of approximately 8 nm. The lattice within this region deviates from the regular crystal structure. Piezoresponse force microscopy indicates that such PZT nanoislands do not show ferroelectricity. Our results suggest that misfit engineering is indispensable for obtaining nanostructured ferroelectrics with stable polarization.     

Effect of different epitaxial structures on GaAs photoemission

The quantum efficiency equations of two different structure reflection-mode GaAs photocathodes with back interface recombination velocity have been solved from the diffusion equations. One structure consists of GaAs substrate and an epitaxial GaAs active layer (GaAs-GaAs) and another structure consists of GaAs substrate, an epitaxial AlGaAs buffer layer, and a GaAs active layer (AlGaAs-GaAs). The experimental results show that the quantum efficiency of long-wavelength photons and the integral sensitivities for GaAs-GaAs cathodes both increase with the increase in the active layer thickness, which is due to the increase of electron diffusion length. The quantum efficiency of long-wavelength photons and the integral sensitivity of AlGaAs-GaAs cathodes are greater than those of GaAs-GaAs cathodes with an identical active layer thickness, which is attributed to the AlGaAs buffer layer. The buffer layer can reflect electrons and improve the quality of the GaAs active layer. Through the theoretical simulation, we found the active layer thickness for AlGaAs-GaAs cathodes has an optimum value at which the cathodes achieve the maximum sensitivity.     

Fabrication and characterization of nanostructured conducting polymer films containing magnetic nanoparticles

In this study, the layer-by-layer technique is used to deposit nanostructured films exhibiting electrical conductivity and magnetic behavior, from poly(o-ethoxyaniline) (POEA), sulfonated polystyrene (PSS) and positively-charged maghemite nanoparticles. In order to incorporate the nanoparticles into the films, maghemite nanoparticles, in the form of magnetic fluid, were added to POEA solutions, and the resulting suspensions were used for film deposition. UV-Vis spectroscopy and atomic force microscopy images reveal that POEA remains doped in the films, even in the presence of the maghemite nanoparticles, and its typical globular morphology is also present. Electrical measurements show that a POEA/PSS film prepared from POEA solution containing 800 µL of the magnetic fluid exhibits a similar conductivity to that of the control film and, additionally, magnetic measurements indicated that nanosized maghemite phase was incorporated within the polymeric film.     

Fabrication of epoxy composites with large-pore sized mesoporous silica and investigation of their thermal expansion

We fabricate epoxy composites with low thermal expansion by using mesoporous silica particles with a large pore diameter (around 10 nm) as inorganic fillers. From a simple calculation, almost all the mesopores are estimated to be completely filled with the epoxy polymer. The coefficient of linear thermal expansion (CTE) values of the obtained epoxy composites proportionally decrease with the increase of the mesoporous silica content.     

图形化SOI衬底上侧向外延生长GaN研究

采用MOCVD系统,在图形化的绝缘体上硅(SOI:silicon-on-insulator)衬底上侧向外延生长了GaN薄膜。利用SEM、TEM和Raman光谱对生长的GaN薄膜的质量进行了分析研究。研究发现,在GaN的侧向外延生长区域,侧向生长的GaN能够完全合并,GaN薄膜内的残余应力减小,穿透位错密度大幅度降低。     

An investigation into the spectral analysis of dielectric aging in ferroelectrics

This study shows how Fourier analysis can be applied to an aged PMN-PT-BT composition to characterize and quantify the effects of aging. The average weak-field permittivity is measured as a function of time, temperature, and frequency. A typical "saddle" in the permittivity and dielectric loss is apparent, and the magnitude decays logarithmically with time. Harmonic analysis of the strain response reveals a slight time-dependent amplitude variation and logarithmic dependence of the phase of the 6th order harmonic. Similar analysis of the polarization response, as a function of time, fully characterizes the development of constriction or wasting seen in a typical aged electrostrictive material. Variations in the amplitudes of the 5th and higher order harmonics of polarization, logarithmic in nature, and the phase of the 5th order harmonic combine to define aging on the harmonic level.     

Polylactide based nanostructured biomaterials and their applications

Polylactide (PLA) is one of the most innovative materials being actively investigated for a wide range of industrial applications. The polymer is a linear aliphatic thermoplastic polyester which is biodegradable as well as biocompatible, which makes it highly versatile and attractive to various commodities and medical applications. A large variety of nanoparticles of different nature and size can be blended with PLA, therefore, generating a new class of nanostructured biomaterials or nanocomposites with interesting physical properties and applications. PLA based nanostructured biomaterials are the focus of this review article, throwing light on their preparation techniques, physical properties, and industrial applications. Structural characteristics and morphological features of PLA based nanocomposites have been explained on the basis of X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Depending upon the nature and characteristics of the nanoparticles, the ultimate properties of the resulting nanocomposite materials can be tailored. Biocompatible materials such as carbon nanotubes, cellulose nanowhiskers, hydroxyapitite, etc. could be incorporated into the PLA matrix, which increase the potential of PLA for biomedical applications. Applications of PLA based nanostructured materials in different areas have been summarized.     

Growth,perfection and antiferromagnetic domain structure of epitaxial cobalt oxide

Uniform single crystals of cobalt oxide have been grown over a wide range of temperature (210° C) by vapour hydrolysis of cobalt bromide on to (001) cleavage faces of magnesium oxide. Thicker crystals have been grown by this technique than previously reported, and the dependence of the growth on the experimental parameters is presented.Optical and X-ray topographic techniques have been used to study the distribution of imperfections and antiferromagnetic domains in these crystals. The appearance of plastic deformation at the growth surface in crystals grown at high temperatures is a result of the combined stresses induced by differential thermal contraction of overgrowth and substrate, and the presence of growth striations throughout the bulk of the crystals. The complexity of the antiferromagnetic domain structure, which is sensitive to crystal thickness, is a direct result of the inhibiting effect of the substrate and the misfit associated with the antiferromagnetic tetragonal distortion.These results on cobalt oxide are compared and contrasted to those previously reported on nickel oxide grown under similar conditions.     

典型纳米结构制备及其测量表征

针对纳米器件中的典型几何特征,制备了3种纳米结构,采用扫描电子显微镜(SEM)、原子力显微镜(AFM)等测量工具对所制备的纳米样板进行了测量、分析和表征。提出转换薄膜厚度为线宽的公称值、基于多层薄膜淀积技术制备纳米宽度结构的方法,制备出了具有名义线宽尺寸分别为20 nm、25 nm、35 nm的纳米栅线结构。用离线的图像分析算法对所制备的纳米线宽样板的线边缘粗糙度/线宽粗糙度(LER/LWR)以及栅线线宽的一致性进行了评估。实验表明所制备纳米线宽样板的栅线具有较好的一致性,LER/LWR值小,且具有垂直的侧壁。采用电子束直写技术(EBL)和感应耦合等离子体刻蚀(ICP)制备了名义高度为220 nm的硅台阶样板。实验表明刻蚀后栅线边缘LER/LWR的高频成分减少,相关长度变长,均方根偏差值(σ)增大。采用聚焦离子束(FIB)制备了纳米单台阶和多台阶结构,并对Z方向的尺度与加工能量之间的关系进行了分析。