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Abstract:
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Among the large number of photothcrmal techniques available, photoacoustics
assumes a very significant place because of its essential simplicity and the variety of
applications it finds in science and technology. The photoacoustic (PA) effect is the
generation of an acoustic signal when a sample, kept inside an enclosed volume, is
irradiated by an intensity modulated beam of radiation. The radiation absorbed by the
sample is converted into thermal waves by nonradiative de-excitation processes. The
propagating thermal waves cause a corresponding expansion and contraction of the
gas medium surrounding the sample, which in tum can be detected as sound waves by
a sensitive microphone. These sound waves have the same frequency as the initial
modulation frequency of light. Lock-in detection method enables one to have a
sufficiently high signal to noise ratio for the detected signal. The PA signal amplitude
depends on the optical absorption coefficient of the sample and its thermal properties.
The PA signal phase is a function of the thermal diffusivity of the sample.Measurement of the PA amplitude and phase enables one to get valuable information
about the thermal and optical properties of the sample.
Since the PA signal depends on the optical and thennal properties of the
sample, their variation will get reflected in the PA signal. Therefore, if the PA signal
is collected from various points on a sample surface it will give a profile of the
variations in the optical/thennal properties across the sample surface. Since the optical
and thermal properties are affected by the presence of defects, interfaces, change of
material etc. these will get reflected in the PA signal. By varying the modulation
frequency, we can get information about the subsurface features also. This is the basic
principle of PA imaging or PA depth profiling. It is a quickly expanding field with
potential applications in thin film technology, chemical engineering, biology, medical
diagnosis etc. Since it is a non-destructive method, PA imaging has added advantages
over some of the other imaging techniques. A major part of the work presented in this
thesis is concemed with the development of a PA imaging setup that can be used to
detect the presence of surface and subsmface defects in solid samples.Determination of thermal transport properties such as thermal diffusivity,
effusivity, conductivity and heat capacity of materials is another application of
photothennal effect. There are various methods, depending on the nature of the
sample, to determine these properties. However, there are only a few methods
developed to determine all these properties simultaneously. Even though a few
techniques to determine the above thermal properties individually for a coating can be
found in literature, no technique is available for the simultaneous measurement of
these parameters for a coating. We have developed a scanning photoacoustic technique that can be used to determine all the above thermal transport properties
simultaneously in the case of opaque coatings such as paints. Another work that we
have presented in this thesis is the determination of thermal effusivity of many bulk
solids by a scanning photoacoustic technique. This is one of the very few methods
developed to determine thermal effiisivity directly. |