Maju Kuriakose

The photothermoelectric technique (PTE), an alternative photothermal calorimetry

The recently introduced photothermoelectric (PTE) effect is proposed as an alternative for measuring dynamic thermal parameters of solid samples. The front PTE configuration, together with the thermal-wave resonator cavity method as a scanning procedure, was used to measure the value of thermal effusivity. The back PTE configuration, together with the chopping frequency of incident radiation as a scanning parameter, leads to the direct measurement of thermal diffusivity. A theory based on the above two detection configurations was developed and its application to solids, covering a large range of typical values of thermal parameters (aluminum and copper alloys, glass, teflon, polyethylene, LiTaO3), was described in order to demonstrate the suitability of the method. Experimental support for other well-known techniques (photopyroelectric and infrared lock-in thermography) has validated the results obtained with the novel method.

Improved methods for measuring thermal parameters of liquid samples using photothermal infrared radiometry

High accuracy, non-contact measuring methods for finding thermal properties of liquid samples using photothermal infrared radiometry (PTR) are presented. The use of transparent windows to confine micro volume liquid samples and the implementation of front and/or back signal detection procedures helped the successful implementation of the PTR technique for measuring liquids with high proficiency. We present two configurations, the so-called back?front photothermal infrared radiometry and back photothermal infrared radiometry to find thermal diffusivity and thermal effusivity of liquid samples. Sensitivity studies and error analyses included prove the robustness of each method. As an illustration of the temperature and electric field varying studies, we have included the experimental results on a 5CB (4-cyano-4?-pentylbiphenyl) liquid crystal.

Picosecond laser ultrasonics for imaging of transparent polycrystalline materials compressed to megabar pressures.

Picosecond laser ultrasonics is an all-optical experimental technique based on ultrafast high repetition rate lasers applied for the generation and detection of nanometric in length coherent acoustic pulses. In optically transparent materials these pulses can be detected not only on their arrival at the sample surfaces but also all along their propagation path inside the sample providing opportunity for imaging of the sample material spatial inhomogeneities traversed by the acoustic pulse. Application of this imaging technique to polycrystalline elastically anisotropic transparent materials subject to high pressures in a diamond anvil cell reveals their significant texturing/structuring at the spatial scales exceeding dimensions of the individual crystallites.

New methodology for the thermal characterization of thermoelectric liquids.

A new and accurate method for the thermal characterization of thermoelectric liquids is proposed. The experiment is based on a self-generated voltage due to the Seebeck effect. This voltage is provided by the sample when one of its two faces is thermally excited using a modulated laser. The sample used is tetradodecylammonium nitrate salt/1-octanol mixture, with high Seebeck coefficient. The thermal properties of the used sample (thermal diffusivity, effusivity, and conductivity) are found and compared to those obtained by other photothermal techniques. In addition to this, a study of the electrolyte thermal parameters with the variation of tetradodecylammonium nitrate concentration was also carried out. This new method is promising due to its accuracy and its simplicity.

Use of Photothermally Generated Seebeck Voltage for Thermal Characterization of Thermoelectric Materials

A simple and accurate experimental procedure to measure simultaneously the thermal properties (conductivity, diffusivity, and effusivity) of thermoelectric (TE) materials using their Seebeck voltage is proposed. The technique is based on analysis of a periodically oscillating thermoelectric signal generated from a TE material when it is thermally excited using an intensity-modulated laser source. A self-normalization procedure is implemented in the presented method using TE signals generated by changing the laser heating from one side to another of the TE material. Experiments are done on a polyaniline carbon nanohybrid (6.6 wt.% carbon nanotubes), yielding a thermal conductivity of 1.106 ± 0.001 W/m-K. The results are compared with the results from photothermal infrared radiometry experiments.

Thermoelectrics (TE) used as detectors of radiation: an alternative calorimetry based on the photothermoelectric (PTE) effect

The recently introduced photothermoelectric (PTE) effect is proposed for investigating the dynamic thermal parameters of condensed matter samples. The front detection configuration, together with the thermal-wave resonator cavity (TWRC) method as scanning procedure, was used to measure the value of thermal effusivity. The back configuration, together with the TWRC technique and/or the chopping frequency of the incident radiation as scanning parameter, leads to the direct measurement of thermal diffusivity. Recent applications to solid (materials with different values of thermal parameters) and liquid (nanofluids) samples are summarized. The ferro-paraelectric phase transition of triglycine-sulphate (TGS) was also detected. The performances of the PTE method have been compared with those of the photopyroelectric (PPE) technique.