Thermal transport across incommensurate phases in potassium selenate: photo-pyroelectric and calorimetric measurements

The thermal transport properties—thermal diffusivity, thermal conductivity and specific heat capacity—of potassium selenate crystal have been measured through the successive phase transitions, following the photo-pyroelectric thermal wave technique. The variation of thermal conductivity with temperature through the incommensurate (IC) phase of this crystal is measured. The enhancement in thermal conductivity in the IC phase is explained in terms of heat conduction by phase modes, and the maxima in thermal conductivity during transitions is due to enhancement in the phonon mean free path and the corresponding reduction in phonon scattering. The anisotropy in thermal conductivity and its variation with temperature are reported. The variation of the specific heat with temperature through the high temperature structural transition at 745 K is measured, following the differential scanning calorimetric method. By combining the results of photo-pyroelectric thermal wave methods and differential scanning calorimetry, the variation of the specific heat capacity with temperature through all the four phases of K2SeO4 is reported. The results are discussed in terms of phonon mode softening during transitions and phonon scattering by phase modes in the IC phase.

Thermal diffusivity measurements in organic liquids using transient thermal lens calorimetry

Thermal diffusivity measurements are carried out in certain organic liquids using the pulsed dual beam thermal lens technique. The 532 nm pulses from a frequency doubled Q-switched Nd:YAG laser are used as the heating source and an intensity stabilized He-Ne laser serves as the probe beam. Experimental determination of the characteristic time constant of the transient thermal lens signal is verified theoretically. Measured thermal diffusivity values are in excellent agreement with literature values.

Open-cell photoacoustic investigation of the thermal effusivity of liquid crystals

We report the use of an open photoacoustic cell configuration for the evaluation of thermal effusivity of liquid crystals. Initially, the method is calibrated using water and glycerol as transparent liquid samples, and the role of thermal conductivity of these liquids on the photoacoustic signal amplitude is discussed. To demonstrate the application of the present method for the evaluation of thermal effusivity of liquid crystals, we have used certain multicomponent nematic liquid crystal mixtures, namely BL001, BL002, BL032, and BL035. Each of these liquid crystal mixtures contains four to nine components and are primarily based on the cyanobiphenyl structure. The measured values of thermal effusivity of BL001 and BL002 were found to be almost the same, but differ from those of BL032 and BL035, which implies a difference in composition of the latter two from the former two mixtures.

Thermal transport properties of selected ferroelectrics, mixed valance perovskites and dielectric ceramics using photopyroelectric technique

Photothermal spectroscopy is a group of high sensitivity methods used to measure optical absorption and thermal characteristics of a sample.The basis of photothermal spectroscopy is a photo-induced change in the thermal state of the sample.Light energy absorbed and not lost by subsequent emission results in sample heating.This heating results in a temperature change as well as changes in thermodynamic parameters of the sample which are related to temperature.Measurements of the temperature,pressure,or density changes that occur due to optical absorption are ultimately the basis for the photothermal spectroscopic methods.This is a more direct measure of optical absorption than optical transmission based spectroscopies.Sample heating is a direct consequence of optical absorption and so photothermal spectroscopy signals are directly dependent on light absorption.Scattering and reflection losses do not produce photothermal signals.Subsequently,photothermal spectroscopy more accurately measures optical absorption in scattering solutions,in solids,and at interfaces.This aspect makes it particularly attractive for application to surface and solid absorption studies,and studies in scattering media.

Modification of Linear Low Density Polyethylene for Improved Photo and Biodegradation

LLDPE was blended with poly (vinyl alcohol) and mechanical, thermal, spectroscopic properties and biodegradability were investigated. The biodegradability of LLDPE/PVA blends has been studied in two environments, viz. (1) a culture medium containing Vibrio sp. and (2) a soil environment over a period of 15 weeks. Nanoanatase having photo catalytic activity was synthesized by hydrothermal method using titanium-iso-propoxide. The synthesized TiO2 was characterized by X-Ray diffraction (XRD), BET studies, FTIR studies and scanning electron microscopy (SEM). The crystallite size of titania was calculated to be ≈ 6nm from the XRD results and the surface area was found to be about 310m2/g by BET method. SEM shows that nanoanatase particles prepared by this method are spherical in shape. Linear low density polyethylene films containing polyvinyl alcohol and a pro-oxidant (TiO2 or cobalt stearate with or without vegetable oil) were prepared. The films were then subjected to natural weathering and UV exposure followed by biodegradation in culture medium as well as in soil environment. The degradation was monitored by mechanical property measurements, thermal studies, rate of weight loss, FTIR and SEM studies. Higher weight loss, texture change and greater increments in carbonyl index values were observed in samples containing cobalt stearate and vegetable oil. The present study demonstrates that the combination of LLDPE/PVA blends with (I) nanoanatase/vegetable oil and (ii) cobalt stearate/vegetable oil leads to extensive photodegradation. These samples show substantial degradation when subsequent exposure to Vibrio sp. is made. Thus a combined photodegradation and biodegradation process is a promising step towards obtaining a biodegradable grade of LLDPE.