Models of a structural phase transition with general anharmonicity and disorder

It is shown how the self-consistent phonon Ansatz leads to a new class of exactly soluble models of a structural phase transition. Both nonpolynomial anharmonicity and disorder are analyzed in detail. In the classical limit, the thermodynamics is obtained and sufficient conditions on the anharmonicity are given to ensure a soft-mode phase transition. Diagonal disorder has been studied numerically. It is found that in three dimensions a pronounced mobility edge, separating localized and delocalized phonon states, may exist.     

A methodology for structural health monitoring with diffuse ultrasonic waves in the presence of temperature variations

Diffuse ultrasonic waves for structural health monitoring offer the advantages of simplicity of signal generation and reception, sensitivity to damage, and large area coverage; however, there are the serious disadvantages of no accepted methodology for analyzing the complex recorded signals and sensitivity to environmental changes such as temperature and surface conditions. Presented here is a methodology for applying diffuse ultrasonic waves to the problem of detecting structural damage in the presence of unmeasured temperature changes. This methodology is based upon the prediction and observation that the first order effect of a temperature change on a diffuse ultrasonic wave is a time dilation or compression. A multi-step procedure is implemented to (1) record a set of baseline waveforms from the undamaged specimen at temperatures spanning the expected operating range, (2) select a waveform from the baseline set whose temperature is the closest to that of a subsequently measured signal, (3) adjust this baseline waveform to best match the signal, and (4) calculate an error parameter between the signal and the adjusted waveform and compare this parameter to a threshold to determine the structural status. This procedure is applied to experimental data from aluminum plate specimens with artificial flaws. Probability of detection and the minimum flaw size detected are presented as a function of the size of the baseline waveform set. It is shown that a probability of detection of over 95% can be achieved with a small number of baseline waveforms.     

Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.     

Phase transitions in gallium nanodroplets detected by dielectric spectroscopy

Both theoretical and experimental works give evidence that gallium exhibits solid phases labelled , , , besides the stable phase strongly dependent both on the size and the confinement conditions. An experimental technique was used based on capacitance and conductance measurements vs. temperature in the audiofrequency range. This technique is particularly sensitive to the conditions of the investigated particle surface that plays a fundamental role in the melting and more generally in the phase transition processes. In particular the strict relation between the derivative of the capacitance with respect to the temperature, dC/dT, and the entropy of the system is considered. In gallium nanoparticles 20 nm in radius, only the phase is shown to occur. Further the transition to liquid phase was detected. The melting process was found to start about 65 K below the full melting temperature value. In the case of particles 10 nm in radius, where different metastable phases may occur, the capacitance vs. temperature curve was found to display abrupt changes of the slope. The singularities are associated to a well defined transition temperature.     

Structural complexity of disordered surfaces: Analyzing the porous silicon SFM patterns

This paper introduces a relative structural complexity measure for the characterization of disordered surfaces. Numerical solutions of 2d+1 KPZ equation and scanning force microscopy (SFM) patterns of porous silicon samples are analyzed using this methodology. The results and phenomenological interpretation indicate that the proposed measure is efficient for quantitatively characterize the structural complexity of disordered surfaces (and interfaces) observed and/or simulated in nano, micro and ordinary scales.     

Defect identification in rods subject to forced vibrations using the spatial wavelet transform

Use of the wavelet transform (WT) to study the forced vibrations in a rod in order to detect the presence of a defect is proposed. The axial vibrations in an inhomogeneous rod, produced by the application of different forces, are simulated by the Network Simulation Method and the obtained response is analyzed with the WT. An analysis of the detection and location of defects for several applied forces and defects generated by locally changing density or stiffness of the rod is presented. A noise test was carried out to check the robustness of the technique in real situations.     

Structure of Reconstructed Cu(100) Surface Induced by Dissociative Adsorption of Gaseous Oxygen

用低能电子衍射(LEED)和扫描隧道显微镜技术研究了氧气在Cu(100)表面化学吸附所形成的表面重构结构．在低覆盖度下,实验上观察到了两个有序重构的混合相,分别为c(2×2)O重构和(√2×2√2)R45°-O重构．在氧原子的覆盖度逐渐增加的过程中,c(2×2)O重构区域的面积逐渐减小．在初始氧化阶段,实验上观察到了呈条状的“锯齿”形的重构结构,这些锯齿形的结构是由相邻的局域化的c(2×2)区域的边界区域(Cu)构成的．STM实验发现,相对于其他重构区域,这些锯齿形的重构区域与有机分子之间存在更加强烈的相互作用．通过分析在不同退火温度下样品LEED衍射点的变化,研究了重构结构的热稳定性,发现三种结构的热稳定性顺序是：锯齿结构>c(2×2)O重构>(√2×2√2)R45°-O重构．     

Influence of the chemical structure on the kinetics of the structural relaxation process of acrylate and methacrylate polymer networks

The enthalpy relaxation of poly(hydroxyethyl methacrylate) (PHEMA), poly(ethyl methacrylate) (PEMA) and poly(ethyl acrylate) (PEA) networks, obtained by DSC, are compared. The temperature interval of the glass transition broadens in the sequence PEA-PEMA-PHEMA. The plots of the enthalpy loss during the annealing for 200 min at different temperatures below T_g show that the structural relaxation process also takes place in PHEMA in a broader temperature interval than in PEA or PEMA. The modelling of the structural relaxation process using a phenomenological model allows determining the temperature dependence of the relaxation times concluding that the fragility in PHEMA is significantly lower than in PEMA. Both features are ascribed to the connectivity of the polymer chains in PHEMA via hydrogen bonding. The role of the presence of the methyl group bonded to the main chain is analysed by comparing the results obtained in PEA and PEMA.     

Image quality assessment in multiband DCT domain based on SSIM

The performance of image quality assessment method based on SSIM (structural similarity) is better than the PSNR (peak signal to noise ratio), but the assessment effects of SSIM is poor for seriously blurred image, therefore, the model that combined HVS (human visual sensitivity) and SSIM was established. The basic idea is based on the human eye's sensitivity to different frequency distortion image, the image is two-dimensional discrete cosine transform frequency component into low, mid, high-frequency component, to obtain the frequency component of light, contrast and structural information, using Pearson coefficient for weight and sum processing to the sub-image according to frequency bands of different sensitive degree, finally, get the sharpness of the image. Through nonlinear regression analysis of objective assessment and DMOS, experiments showed that this method was closer to human perception than SSIM and GSSIM for serious blurred distortion image. At the same time, compared to conventional algorithm MAE (mean absolute error), MSE (mean square error) and PSNR, this model was more consistent with human visual characteristics.     

Structural colour in animals—simple to complex optics

Over the past 30 years, optics well known to the physicist have been identified in their multitudes in nature. Multilayer reflectors, diffraction gratings, liquid crystals and structures that scatter light—devices explained using “simple” optical theory—have been found in animals with a diversity of designs. For many years the potential to employ these designs commercially has been clear, although only one optical device in animals has been taken through to the manufacture stage—the fly-eye antireflector. Now, with the beginnings of “complex” optical study in biological specimens, and consequent identification of photonic band gaps, animal reflectors are being taken seriously as promising first stages in the design process of optical engineers. However, natural photonic crystals are often highly complex structures at the nano-scale that may lie beyond the limits of current engineering. This in turn justifies the study of cellular engineering—maybe we can also exploit the flawless processes of manufacture employed by animals.