Suneet Tuli

Theory of frequency modulated thermal wave imaging for nondestructive subsurface defect detection

This letter provides the theory and mathematical analysis in support of a recently proposed frequency modulated thermal wave imaging for nondestructive subsurface defect detection in solids. The authors illustrate how the technique simultaneously combines the advantages of both conventional pulse based thermography as well as modulated lock-in thermography. A specimen is heated for launching thermal waves into the sample, not at a single frequency (lock-in) or at all frequencies (pulse), but in a desired range of frequencies. While peak power requirement is reduced, phase images obtained retain known advantages. Experimental results from a carbon fiber reinforced plastic sample are presented in support.

Defect detection by pulse compression in frequency modulated thermal wave imaging

A new, quantitative, whole field, non-contact and non-destructive technique for sub-surface defect detection is presented based on frequency modulated thermal wave imaging (FMTWI). Electro-thermal modeling and MATLAB-SIMULINK simulation has been carried out for the proposed technique. Experimental results of frequency modulated thermal wave imaging are reported, and defect detection by a correlation approach demonstrated.

Image Enhancement in Transient Lock-In Thermography Through Time Series Reconstruction and Spatial Slope Correction

Lock-in (LI) thermography is a popular thermal-nondestructive-testing technique which, like other active thermographic techniques, requires an external heating stimulus, preferably on a blackened surface. It is not, however, immune to nonideal situations like nonuniform heating and surface emissivity variation. The phase image, to some extent, helps to reduce the effect of these artifacts but is inadequate if the variations are large. For example, a poorly blackened metallic sample with reflecting patches on its surface is very difficult to actively thermograph because of direct reflection from the surface. This paper proposes an image reconstruction algorithm for offline removal of such artifacts. In addition, the proposed algorithm enables LI thermography tests in the transient regime and removes temperature gradients due to nonuniform heating. The algorithm was tested with a mild-steel sample having 20-mm-diameter back-drilled holes at various depths ranging from 0.2 to 7.7 mm, stimulated at 20-, 40-, 50-, 60-, and 80-mHz excitation frequencies. The effect of the total number of heating cycles is also presented.

Electronic diffusivity measurement in silicon by photothermal microscopy

In this letter we demonstrate that a photothermal microscopy experiment can be used to determine the electronic diffusivity (or carrier mobility) in the same way it is now widely used to measure locally thermal diffusivity of various nonsemiconductor materials. The main difficulty lies in the fact that in order to separate thermal and carrier diffusion, the experiment must be performed for a relatively large distance between the pump and probe beams. Photothermal signals are therefore rather weak and great experimental care must be taken. We present and discuss experimental results on Si.

SPICE simulation of surface acoustic wave interdigital transducers

Surface Acoustic Wave (SAW) devices, are not normally amenable to simulation through circuit simulators. In this letter, an electrical macromodel of Masons Equivalent Circuit for an interdigital transducer (IDT) is proposed which is compatible to a widely used general purpose circuit simulator SPICE endowed with the capability to handle negative capacitances and inductances. Illustrations have been given to demonstrate the simplicity of ascertaining the frequency and time domain characteristics of IDT and amenability to simulate the IDT along with other external circuit elements.<<ETX>>

Exciton quenching by diffusion of 2,3,5,6-tetrafluoro-7,7',8,8'-tetra cyano quino dimethane and its consequences on joule heating and lifetime of organic light-emitting diodes.

In this Letter, the effect of F(4)-TCNQ insertion at the anode/hole transport layer (HTL) interface was studied on joule heating and the lifetime of organic light-emitting diodes (OLEDs). Joule heating was found to reduce significantly (pixel temperature decrease by about 10 K at a current density of 40 mA/cm(2)) by this insertion. However, the lifetime was found to reduce significantly with a 1 nm thick F(4)-TCNQ layer, and it improved by increasing the thickness of this layer. Thermal diffusion of F(4)-TCNQ into HTL leads to F(4)-TCNQ ionization by charge transfer, and drift of these molecules into the emissive layer caused faster degradation of the OLEDs. This drift was found to reduce with an increase in the thickness of F(4)-TCNQ.

Study of 2,3,5,6-tetrafluoro-7,7′,8,8′- tetracyano quinodimethane diffusion in organic light emitting diodes using secondary ion mass spectroscopy

In this work, 2,3,5,6 – tetrafluoro – 7,7′,8,8′- tetracyano quinodimethane (F4-TCNQ) diffusion has been studied using secondary ion mass spectroscopy (SIMS). SIMS depth profiling has been performed in dual beam mode, in which, a low energy oxygen beam has been used for etching and a high energy Bi1+ ion beam has been used for analysis. F4-TCNQ has been identified by the distinguishable presence of fluorine in organic layers. For this study, organic light emitting diodes (OLEDs) were fabricated with F4-TCNQ as a hole injection layer with 1, 2.5 and 5.5 nm thicknesses. The diffusion length and depth were measured to be 13, 19, 19.1 nm and 27, 28, 29 nm for 1, 2.5, 5.5 nm thicknesses of F4-TCNQ, respectively. The diffusion of F4-TCNQ into the hole transport layer leads to the ionization of F4-TCNQ molecules and p-type doping of the hole transport material. The effect of the electric field on the diffusion was also studied by performing the depth profiling on electric field applied OLEDs and it was observed that the application of the electric field has increased both the diffusion length and depth of F4-TCNQ. This effect was found to be more pronounced for the OLED with 1 nm thickness of F4-TCNQ in comparison to the OLEDs with 2.5 and 5.5 nm thicknesses of F4-TCNQ. The field affected diffusion length and depth were found to be 14.5, 19.5, 19.6 and 35, 28.6, 29.6 nm for OLEDs with 1, 2.5, 5.5 nm thicknesses of F4-TCNQ hole injection layer. The decrease in the field effect has been ascribed as being due to the increase in the cluster density and size. Further, the effect of F4-TCNQ diffusion on OLEDs has been studied by capturing the optical images at different instances of time. OLEDs with 1 nm F4-TCNQ layer were found to be the most unstable. This effect has been ascribed as being due to diffusion of F4-TCNQ into the emissive layer which leads to the dissociation of excitons formed inside the emissive layer.

Morphological investigation of aluminium nitride films on various substrates for MEMS applications

Abstract Piezoelectric films, such as aluminium nitride (AlN), are of great interest for the fabrication of thin film bulk/surface acoustic resonators, where the growth parameters and the substrate material influence the morphological properties. Herein, AlN films were deposited using RF reactive sputtering on Si, SiO2, metal (Al, Cu, Cr and Au) coated silicon, GaAs and InP substrates. C-axis (002) oriented AlN films were observed on most of the substrates, where the surface morphologies of the films grown on the 1·5 μm thick SiO2 layer and GaAs substrate were found to be non-uniform. Moreover, AlN films deposited on the Cr electrode exhibit well textured film with fairly uniform grains, while the films deposited on other metal electrodes exhibit a granular type of structure with mixed small and large grains. After optimisation of growth parameters, silicon micromachining was performed by the wet chemical etching method. Suspended Cr–AlN–Cr–SiO2 cantilevers of 20 μm in width were fabricated for futuristic microelectromechanical systems (MEMS) applications.

Prediction of blind frequency in lock-in thermography using electro-thermal model based numerical simulation

Lock-in thermography is increasingly becoming popular as a non-destructive testing technique for defect detection in composite materials for its low heating excitation. The experimental data is processed with Fourier transformation to produce phase and amplitude images. Phase images, though immune to surface emissivity variation, suffer from blind frequency effect, where a defect becomes invisible at a certain excitation frequency. There exists no analytical model to predict this 3-dimensional heat flow phenomenon. This paper presents a study of blind frequency using electro-thermal model based numerical simulation on a piece of thermally anisotropic carbon fibre composite. The performance of the simulator is optimized for spatial mesh size. Further the effect of paint layer, which is often applied to the sample surface for better thermal imaging, has been incorporated in the simulation. Finally, both experimental and simulation results are presented side-by-side for easy comparison.

Mirage-effect-based depth profiling of micromachined silicon structures

Abstract A non-contact, non-destructive technique for the depth profiling of micromachined structures is presented. Based on the mirage or optical-beam deflection method of photothermal spectroscopy, an experimental measurement of a 8 μm step in silicon is given by way of illustration.