http://dyuthi.cusat.ac.in:80/xmlui/handle/purl/688 2013-05-25T05:55:25Z http://dyuthi.cusat.ac.in:80/xmlui/handle/purl/2042 Comparative Performance of Two Fiber Optic Ammonia Sensors Employing Different Sensing Materials Jinesh, Mathew; Mahesh, V V; Radhakrishnan, P The design and fabrication of fiber based ammonia sensors employing Bromothymol blue and Chitosan as sensing elements are presented in this paper. In the presence of ammonia gas the absorption of Bromothymol blue changes while in the case of Chitosan the refractive index changes which in turn modulates the intensity of light propagating through a fiber. 2008-08-01T00:00:00Z http://dyuthi.cusat.ac.in:80/xmlui/handle/purl/1838 Use of photoacoustic effect for the detection of phase transitions in liquid crystal mixtures Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Nibu, A George; George, A K We report on a laser induced photoacoustic study of the nematic-to-isotropic transition in certain commercial nematic liquid crystal mixtures, namely BL001, BL002, BL032 and BL035. A simple analysis of the experimental data using the Rosencwaig–Gersho theory shows that the heat capacities of all these compounds exhibit a sharp peak as the temperature of the sample is varied across the transition region. Also, substantial differences in the photoacoustic signal amplitudes in nematic and isotropic phases have been noticed for all the mixtures. The increased light scattering property of the nematic phase may be the reason for the enhanced photoacoustic signal amplitude in this phase 2000-10-13T00:00:00Z http://dyuthi.cusat.ac.in:80/xmlui/handle/purl/1835 Use of an open photoacoustic cell for the thermal characterisation of liquid crystals Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Nibu, A George; George, A K In this paper, we describe the use of an open cell photoacoustic configuration for the evaluation of the thermal effusivity of liquid crystals. The feasibility, precision and reliability of the method are initially established by measuring the thermal effusivities of water and glycerol, for which the effusivity values are known accurately. In order to demonstrate the use of the present method in the thermal characterization of liquid crystals, we have measured the thermal effusivity values in various mesophases of 4-cyano-4 - octyloxybiphenyl (8OCB) and 4-cyano-4 -heptyloxybiphenyl (7OCB) liquid crystals using a variable temperature open photoacoustic cell. A comparison of the measured values for the two liquid crystals shows that the thermal effusivities of 7OCB in the nematic and isotropic phases are slightly less than those of 8OCB in the corresponding phases 2001-07-18T00:00:00Z http://dyuthi.cusat.ac.in:80/xmlui/handle/purl/1832 Thermal characterization of intrinsic and extrinsic InP using photoacoustic technique Radhakrishnan, P; V P N Nampoori; Girijavallabhan, C P; Sajan, D George An open photoacoustic cell operating in the low range of chopping frequency has been employed to evaluate the thermal diffusivity values of intrinsic InP and InP doped with S, Sn and Fe. The experimental set-up is calibrated by the evaluation of thermal diffusivity value of pure Si and GaAs. The present investigation shows that doped samples show a reduced value for thermal diffusivity compared to intrinsic sample. From the analysis of data it is also seen that nature of dopant clearly influences the thermal diffusivity value of semiconductors. The results are explained in terms of phonon assisted heat transfer mechanism in semiconductors 2003-04-02T00:00:00Z