Multiple-damage-inflicted scattering signals usually overlap with each other due to Lamb wave dispersion and multi-mode characteristics. As a result, it is difficult to accurately distinguish damages that occur relatively close to each other using the conventional ultrasonic phased array method. In order to solve this problem, an improved linear mapping (ILM) dispersion compensation method is proposed and is applied to enhance the ultrasonic phased array monitoring resolution. Through a uniform linear array arrangement, the damage scattering signals are collected in a round-robin pattern of ultrasonic phased array, and then compensated based on the linear relation wavenumber curve from actual measurement. At last, the scan can be obtained by monitoring the energy scattered by the damages using delay-and-sum method. To verify the proposed method, experiments are performed on an aluminum (LY-12) plate. Two results of multiple artificial damages show that the proposed method can effectively compensate the dispersion characteristics of Lamb waves. The direction estimation error and distance estimation error are less than 4° and 2 cm, respectively.
A unified treatment of coupled optical and acoustic phonons in piezoelectric cubic materials is presented whereby the lattice displacement vector and the internal ionic displacement vector are found simultaneously. It is shown that phonon couplings exist in pairs only; either between the electric potential and the lattice displacement coordinate perpendicular to the phonon wave vector or between the two other lattice displacement components. The former leads to coupled acousto-optical phonons by virtue of the piezoelectric effect. We then establish three new conjectures that entirely stem from piezoelectricity in a cubic structured material slab. First, it is shown that isolated optical phonon modes generally cannot exist in piezoelectric cubic slabs. Second, we prove that confined acousto-optical phonon modes only exist for a discrete set of in-plane wave numbers in piezoelectric cubic slabs. Third, it is shown that coupled acousto-optical phonons do not exist at the longitudinal-optical (LO) phonon frequency where the dielectric constant vanishes.
A “triboelectric junction” is a space charge region induced by the triboelectric effect, dominating the electron–hole separation process in dynamic semiconductor-based contacts.
The packing in self-assembled nanocrystal superlattices is determined to a large extent by the geometry of the nanocrystals, as is demonstrated here by the preparation and analysis of wellordered arrays of predominantly tetrahedral silver nanocrystals. The Figure shows the model proposed to explain the monolayer self-assembly of tetrahedral silver nanocrystals passivated by bundled thiolate molecules.
Self-assembling of organic passivated nanocrystals has attracted a lot of interest recently. In this paper, the structural evolution of tetrahedral CoO nanocrystals is studied in situ using transmission electron microscopy. The as-prepared superlattices are Na(AOT)-passivated CoO nanocrystals, packed into monolayer and multilayer arrays on an amorphous carbon film. As the specimen temperature increased from 170 to 200 °C, the passivation layer is gradually evaporated/decomposed. For temperatures higher than 200 °C, the CoO nanocrystals experience a solid-state reaction, while the monolayer packing configuration is still preserved, but not the multilayers. The replacement reaction finishes at ∼600 °C, and the final product is identified as mixed carbide nanocrystals of Co2C and Co3C.
