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Abstract:
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This thesis presents in detail. the theoretical developments and calculations
which are used for the simultaneous determination of thermal parameters, namely thermal diffusivity (a). thermal effusivity (e), thermal conductivity (K) and heat
capacity (cr ) employing photopyroelectric technique. In our calculations. we have
assumed that the pyroelectric detector is supported on a copper backing. so that there
will be sufficient heat exchange between the heated pyroelectric detector and the
backing so that the signal fluctuations are reduced to a minimum. Since the PPE
signal depends on the properties of the detector that are also temperature dependent.
a careful temperature calibration of the system need to be carried out. APPE cell has
been fabricated for the measurements that can be used to measure the thermal
properties of solid samples from ~ 90 K to ~ 350 K. The cell has been calibrated
using standard samples and the accuracy of the technique is found to be of the order
of± 1%.In this thesis, we have taken up work n photopyroelectric investigation of
thermal parameters of ferroelectric crystals such as Glycine phosphite
(NH3CH2COOH3P03), Triglycine sulfate and Thiourea as well as mixed valence
perovskites samples such as Lead doped Lanthanum Manganate (Lal_xPb~Mn03)
Calcium doped (Lal_xCaxMnOJ) and Nickel doped Lanthanum Stroncium Cobaltate
(Lao~Sro5Ni,Col_x03).The three ferroelectric crystals are prepared by the slow
evaporation technique and the mixed valence perovskites by solid state reaction
technique.Mixed valence perovskites, with the general formula RI_xA~Mn03 (R = La.
Nd or Pr and A = Ba, Ca, Sr or Pb) have been materials of intense experimental and
theoretical studies over the past few years. These materials show . colossal
magneloresis/ance' (CMR) in samples with 0.2 < x < 0.5 in such a doping region,
resistivity exhibits a peak at T = T p' the metal - insulator transition temperature. The system exhibits metallic characteristics with d %T > Oabove Tp (wherep is the
resistivity) and insulating characteristics with d % T < 0 above T p. Despite
intensive investigations on the CMR phenomena and associated electrical properties.
not much work has been done on the variation of thermal properties of these
samples. We have been quite successful in finding out the nature of anomaly
associated with thermal properties when the sample undergoes M-I transition.The ferroelectric crystal showing para-ferroelectric phase transitions -
Glycine phosphite. Thiourea and Triglycine sulfate - are studied in detail in order to
see how well the PPE technique enables one to measure the thermal parameters
during phase transitions. It is seen that the phase transition gets clearly reflected in
the variation of thermal parameters. The anisotropy in thermal transport along
different crystallographic directions are explained in terms of the elastic anisotropy
and lattice contribution to the thermal conductivity. Interesting new results have
been obtained on the above samples and are presented in three different chapters of
the thesis.In summary. we have carried investigations of the variations of the thermal
parameters during phase transitions employing photopyroelectric technique. The
results obtained on different systems are important not only in understanding the
physics behind the transitions but also in establishing the potentiality of the PPE
tool. The full potential of PPE technique for the investigation of optical and thermal
properties of materials still remains to be taken advantage of by workers in this field. |