Title: Photoacoustic and photothermal investigation of ultrathin structures and viscoelastic materials
Other Titles: Fotoakoestisch en fotothermisch onderzoek van ultra-dunne structuren en visco-elastische materialen
Authors: Salenbien, Robbe
Issue Date: 18-Dec-2012
Abstract: Laser excited acoustic and thermal waves have the interesting property that they can deliver information about material properties on locations which are inaccessible for other techniques. The aim of this PhD-project is the elastic and thermal characterization of materials with complex viscoelastic behavior and the study of the 3D structure of heterogeneous materials from millimeter to nanometer scale.In order to achieve this goal we make use of an all-optical laserbased heterodyne diffraction method, which allows to excite and detect acoustic and thermal waves down to very small wavelengths and very high frequencies at varying temperatures and hence to investigate the thermal and elastic behavior of materials down to very small time and length scales.The existing experimental setup was altered and ameliorated, resulting in an improved temperature control and an extended array of applicable grating spacings. A method was developed to undo the convolution of the measured signal response and the differential photodetector; the result is a collected time signal spanning over the millisecond range with nanosecond resolution. This setup and method were then used to determine the properties of subsurface intermetallic alloy layers and thin deposited diamond layers.Prior to this characterization, a study was conducted to illustrate the feasibility of the used methods, i.e. the inverse problem was proven solvable. For this, the pair-wise covariances of the chi-squared function were examined; the nature of a minimum region allows to determine the existence and width of the interval of condence on the involved parameters.In a second part, several glass-forming liquids were investigated in the framework of a model with all thermo-physical quantities exhibiting relaxation behavior. This model analytically simulates the impulsive stimulated scattering (ISS) temperature and density grating responses including frequency dependent thermal expansion, specific heat capacity and acoustic relaxation. Glycerol is used in a simulation study of the effects of a frequency dependent specific heat capacity and the findings are crosschecked with experimental results. Two different ionic liquids are studied to illustrate the effect of the alkyl chain length on the glass-forming properties.
ISBN: 978-90-8649-585-6
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Soft Matter and Biophysics

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