Influence of transparent coating thickness on thermoelastic force source and laser-generated ultrasound waves

A numerical model is established to investigate the influence of transparent coating thickness on the laser-generated thermoelastic force source and ultrasound waves in the coating-substrate system by using the finite element method (FEM). Taking into account the effects of thermal diffusion, the finite width and duration of the laser source, as well as the temperature dependence of material properties, the transient temperature distributions are obtained firstly. Applying this temperature field to structure analyses as thermal loading, the thermoelastic stress field and laser-generated ultrasound wave in the specimen are obtained. The generation and propagation of the laser thermoelastically induced stress field and ultrasonic waves in coating-substrate system are presented in detail. The influence of transparent coating thickness on the transient temperature distribution, thermoelastic force source and the laser-generated ultrasound waveforms is investigated. The numerical results indicate that the thermoelastic force source and laser-generated ultrasound waveform are strongly affected by the coating thickness due to the constraint of coating. This method can provide insight into the generation and propagation of the laser-generated stress field in coating-substrate system consisting of a transparent coating and an opaque metallic substrate. It provides theoretical basics to optimize ultrasonic signal generation in particular applications and invert the physical and geometrical parameter of the coating-substrate system more accurately in the experiment.     

Numerical simulation of laser-generated ultrasound in non-metallic material by the finite element method

The use of a pulsed laser for the generation of the elastic waves in non-metallic materials in the thermoelastic regime is investigated by using finite element method (FEM), taking into account not only thermal diffusion and the finite spatial and temporal shape of the laser pulse, but also optical penetration and the temperature dependence of material properties. The optimum finite element model is established based on analysis of two important parameters, meshing size and time step, and the stability of solution. Temperature distributions and temperature gradient fields in non-metallic material for different time steps are obtained, this temperature field is equivalent to a bulk force source to generate ultrasonic wave. The laser-generated ultrasound waveforms at the epicenter and surface acoustic waveforms (SAWs) are obtained and the influence of optical penetration into the material on the temperature field and the ultrasound waveforms are analyzed. The numerical results indicate that the heat penetration into non-metallic material is caused mainly by the optical penetration, and the ultrasound waveforms, especially the shape of the precursor, are strongly dependent on the optical penetration depth into non-metallic material.     

Laser assisted embedding of nanoparticles into metallic materials

This paper reports a methodology of half-embedding nanoparticles into metallic materials. Transparent and opaque nanoparticles are chosen to demonstrate the process of laser assisted nanoparticle embedding. Dip coating method is used to coat transparent or opaque nanoparticle on the surface of metallic material. Nanoparticles are embedded into substrate by laser irradiation. In this study, the mechanism and process of nanoparticle embedding are investigated. It is found both transparent and opaque nanoparticles embedding are with high densities and good uniformities.     

Laser Generation of Surface Waves on Cylinder with a Slow Coating

An analytical model of acoustic field excited by a pulsed-laser line source on a coated cylinder is presented. Surface wave dispersive behaviours for a cylinder with a slow coating are analysed and compared with that of a bare cylinder. Based on this analysis, the laser-generated transient response of the perturbed Rayleigh wave and the higher modes of steel cylinder with a zinc coating are calculated from the model using residue theory and FFT technique. The theoretical result from the superposed waveform of the perturbed Rayleigh wave and higher modes agree well with the waveform obtained in experiment. The results show that the model and numerical method provide a useful technique for quantitatively characterizing coating parameters of coated cylinder by the laser generated surface waves.     

The experimental study of fatigue crack detection using scanning laser point source technique

For the purpose of better understanding the interaction of Rayleigh wave and the fatigue crack in a metallic sample, a set of experimental setups is built, based on the scanning laser source (SLS) technique, utilizing a point source to take place of the line source to generate surface acoustic waves (SAWs), and an interferometer is to detect the SAWs signal. The information of the crack (such as position and length) can be obtained by utilizing a two-dimensional scan of the material surface. This paper focuses on the detection of visible and invisible fatigue crack by using this point-source-based scanning laser source technique, and comparing the results with those of conventional pitch-catch technique. The result shows that with two-dimensional scanning, and analyzing the amplitude of the generated SAWs, not only the visible fatigue can be identified, but also the invisible fatigue crack can be discriminated. As a result, the sensitivity of the scanning point laser source technique is higher than the conventional pitch-catch technique.     

基于激光超声的微裂纹检测技术的研究

在固体中利用激光产生超声波 ,可作为超声测量和材料无损检测的一种新方法。介绍了激光超声表面波的产生机理、微裂纹的检测方法及其应用。采用Nd∶YAG脉冲激光器、扩束与聚焦透镜组、PZT探头、数据分析仪等器件设计并构建了一套基于接触式检测方法的激光超声微裂纹检测实验系统。通过对大量实验数据进行处理 ,得出了相应的各种关系曲线 ,说明了线光源产生的超声表面波非常适用于材料表面微裂纹的检测     

Transient combined radiation and conduction in a one-dimensional non-gray participating medium with anisotropic optical properties subjected to radiative flux at the boundaries

Transient combined heat transfer by radiation and conduction is investigated in non-gray participating media with anisotropic optical properties. The medium is irradiated by a time-dependent source. Different kinds of boundaries are considered: black, opaque, transparent and semi-transparent. The heat transfer equations are solved numerically in a one-dimensional configuration. Comparisons are made with test cases taken from the literature, and the results obtained demonstrate the accuracy of the present numerical model. The influence of transparent, semi-transparent and opaque boundaries on an insulating fibrous medium is also studied.     

Three-dimensional analyses of photo-modulated reflectivity for transparent film/opaque substrate structures

For a structure consisting of a transparent film deposited on an opaque substrate radiated by a modulated focused laser beam, a 3-D model is established to calculate the temperature distributions. Then based on 3-D thermoelastic displacements and dielectric constants depended on the temperature of the bilayer structure, the photo-modulated reflectivity of the structure is calculated by using FEM method. As an example, the modulated reflectance variations with the modulation frequency of the optical beam and the thermal conductivity of the film in ZnO/Si bilayer structures are also obtained.     

透明基底表面双向反射分布函数及粗糙度特性研究

采用双向反射分布函数定量分析透明基底表面粗糙度,考虑到透明基底第二个界面的影响,从不透明基底双向反射分布函数入手,推导了实际测量的透明基底表面双向反射分布函数的表达式.依据此理论提出了通过分别测量两个表面的散射强度来联立求解透明基底实际表面反射分布函数和表面粗糙度谱的新方法.并将此结果与用原子力显微镜测量所获得的结果进行了比较,两者吻合较好.     

Thermal and mechanical finite element modeling of laser-generated ultrasound in coating–substrate system

Thermal and mechanical finite element modeling of laser-generated ultrasound in coating–substrate system is presented, which entails a numerical formulation for the transient responses in terms of the characteristics of the source that originated the waves in the thermoelastic regime. The formulation takes into account the temperature dependence of the material parameters and thermal diffusion from the source. Numerical results show that the temperature gradient field is equivalent to the body force source, namely, vertical forces propagate downward and horizontal forces outward in the specimen. Waveforms at the epicenter present fast oscillations immediately after the precursor, which are produced by multiple reflections of the longitudinal in the coating, the thickness of the coating has remarkable influence on the waveforms at the epicenter.     

Surface modification on a glass surface with a combination technique of sol-gel and air brushing processes

This study fabricated the large area and optically transparent superhydrophobic silica based films on glass surface with optimized hardness. A silane coupling agent, tetraethoxysilane (TEOS), effectively bonds silica particles onto the glass substrate. Desired surface roughness was obtained by adjusting nano silica particles concentration of the precursors prepared by the sol-gel process. Silica suspension was coated onto the glass substrate by the air brushing methods. This method can deposit a uniform, transparent coating on the glass substrate efficiently. Diluting the precursor by adding ethanol or a mixture of D.I. water and ethanol further improved the transmittance and superhydrophobicity efficiency. The results showed that as the silica particle concentration and the thickness of the coating were increased, the surface roughness was enhanced. Rougher surface displayed a higher superhydrophobicity and lower transmittance. Therefore, the concentration of silica particle, volume of coatings, and the ratio of ethanol and D.I. water are of great importance to deposit a transparent, superhydrophobic coating on glass.     

Observation of magnetically induced transparency in a classical magnetized plasma

We report the first demonstration of magnetically induced transmission in an opaque magnetized plasma. Magnetically induced transmission in a plasma is a classical analog to the electromagnetically induced transparency in atomic systems. The transmission of radiation through an axially magnetized plasma is obtained by applying an additional one dimensional transverse spatial periodic magnetic field. The transverse-periodic magnetic field uncouples the right-hand electromagnetic wave from interacting with plasma electrons, rendering the plasma band-stop transparent. This provides means to control the extent of absorption of electromagnetic radiation in magnetized plasma.     

Laser ultrasonic characterization of adhesive bonds between epoxy coating and aluminum substrate

Nanosecond pulsed laser irradiation through transparent epoxy bonded to aluminum substrate excites wide-band ultrasonic waves at the bonded interface. The normal displacements on the rear surface of aluminum produced by the direct and multi-reflected longitudinal waves in the coating layer are detected by a laser interferometer. The amplitude of the reflected signal depends on the properties of the coating/substrate interface, which is described by terms of the interfacial stiffness using a spring boundary model. The waveforms at the epicenter versus interfacial stiffness are simulated and found to be in good agreement with experimental results. The relation between the interfacial stiffness and the amplitude ratio of the reflected and direct waves is thus established. An image of amplitude ratio of a specimen (null 10 mm) is obtained from the epicenter waveforms recorded by a laser ultrasonic scanning system, which shows the distribution of bond quality on the bonding interface.     

Controllable fabrication and characterization of conical nanocarbon structures on polymer substrate for transparent and flexible field emission displays

The direct fabrication of fully transparent conical nanocarbon structures (CNCSs) of controlled nanoscopic dimension on a flexible nafion substrate was achieved, using field electron source, by a novel room temperature ion irradiation technique. By controlling the sizes (below the wavelength of visible light) of the CNCSs, the transparency of the substrate can be tailored satisfactorily. The transparency of the CNCSs was observed to be around 90% in the visible regime depending on the ion irradiation time. Our results suggest that the direct fabrication of well controlled fully transparent CNCSs on any transparent and flexible substrate at room temperature could open a novel route for potential applications in future highly transparent, flexible (bendable), low weight and portable field emission displays (FEDs). (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Analytical investigation into laser pulse heating and thermal stresses

Laser pulse heating of metallic surfaces results in rapid rise of temperature in the region irradiated by the laser beam. This in turn results in high temperature gradient in this region. The irradiated substrate material expands as a response to the temperature gradient. Consequently, high thermal stress levels are developed in the region of the high temperature gradient. In the present study, closed form solutions for temperature and stress fields due to a laser pulse decaying exponentially in time are presented. A Laplace transformation method is employed in the analysis. The resulting equations are non-dimensionalized with the appropriate parameters. It is found that temperature rises rapidly during the early heating period in the surface region. In this case, internal energy gain dominates the conduction losses from the surface vicinity. The thermal stress levels attain high values in the surface region. The stress wave developed is compressive and it propagates with a wave speed c₁ inside the substrate.     

Near field localization mediated by a single gold nanoparticle embedded in transparent matrix: Application for surface modification

In this work the near field properties of a single gold nanoparticle embedded in transparent host medium are investigated theoretically. The analysis of the electromagnetic field in the near field zone is obtained by finite difference time domain (FDTD) simulation technique. The nanoscale system consists of a transparent layer in which a gold particle with diameters of D = 200 or 80 nm is embedded, is situated on a substrate surface. Laser pulse at wavelength of 800 nm irradiates normally this system. It is found that the field in the vicinity of the particle is enhanced, and at a certain condition the zone with the highest enhancement is localized on the substrate surface. Furthermore, the near field characteristics are found to be controllable by the dielectric properties of the host material, substrate, parameters of the incident irradiation and particle size. With the increase of the refractive index of the host medium, both the magnitude of the near field on the substrate and the characteristic size of the field enhanced zone decrease. The influence of the particle size and polarization of the incident laser irradiation on the near filed properties of the system are also presented. The proposed configuration can be applied for a multiple nanoprocessing and an integrated near field source with a spatial resolution of D/3.     

Numerical study on surface acoustic wave method for determining Young's modulus of low-k films involved in multi-layered structures

The surface acoustic waves (SAWs) technique is becoming an attractive tool for accurately and nondestructively characterizing the mechanical property of the brittle low dielectric constant (low-k) thin film. The theoretical equations for describing SAWs propagating on the multi-layered structure are derived in this study. The dispersion features of SAWs propagating on different structures of low-k/SiO₂/Si substrate, SiO₂/low-k/Si substrate, low-k/Si substrate, and low-k/Cu/Si substrate are investigated to instruct an accurate and facile fitting process for determining Young's modulus of low-k films. The dependence of dispersion relation on the film thickness, elastic modulus of low-k materials as well as frequency are provided and discussed in detail. The study shows an obvious influence of layered structure on the dispersion relation of SAWs. For a fixed structure, the dispersion curvature increases with the decrease of Young's modulus of low-k films.     

Laser transfer of thin mesotetraphenylporphyrin layers

The technique of forming mesotetraphenylporphyrin microstructures on the quartz substrate surface by laser-induced forward transfer from a target was developed. The target was a transparent substrate with a thin titanium coating onto which five mesotetraphenylporphyrin layers were deposited by the Langmuir-Schaefer method. The target was irradiated with single 500-ps pulses through a transparent substrate, which causes nondestructive thermal deformation of a metal film, resulting in efficient porphyrin emission from the target. The effect of the titanium film thickness and the laser emission wavelength on the transfer process was studied. An analysis of the optical absorption and fluorescence spectra of obtained surface microstructures suggests that the material structure is retained during laser transfer.     

太赫兹激励的红外热波检测技术

本文的目的在于探索一种新的适用于红外热波检测技术的热激励方式——太赫兹(THz)热激励. 文中介绍了THz波周期性热激励的热传导理论模型; 尝试利用返波振荡器(返波管backward wave oscillator, BWO)太赫兹源对一块碳纤维基底吸波涂层板进行周期性THz热激励, 红外热像仪连续观测和记录试件表面温场变化, Canny边缘算法处理热图像显示缺陷; 检测结果与闪光灯脉冲激励的结果进行比较, 讨论了太赫兹波激励红外热波检测技术可能的优势. 实现了THz技术与红外热波无损检测技术的结合.     

Performance of Non-Contact Linear Actuators Driven by Surface Acoustic Waves*

A new kind of non-contact linear actuator （motor） driven by surface acoustic waves （SAWs） is presented, in which the stators are made from SAW delay lines using 128° YX-LiNbO3 substrates. A fluid layer is introduced between the slider and the stator of the actuator, and the slider is a circular aluminum disk suspended on the surface of the liquid （water） layer. As the SAW is excited on the stator, the SAW is converted to a leaky wave in the interface of the stator and the liquid, and then propagates into the liquid. Owing to the nonlinear effect of wave propagation, acoustic streaming is generated, which pushes the slider to move. By the experiments, the relations between the slider velocity and the experimental parameters, such as the exciting voltage of the SAWs, the thickness and the kinematic viscosity of the liquid layer, are obtained.