Deux methodes photothermiques a l'etude d'une preparation de contact d'un melange binaire de cristaux liquides

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal-smectic A, smectic A-nematic, and nematic-isotropic interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. The study of these displacements allows us to draw the complete temperature-concentration phase diagram of a binary mixtures. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photopyroelectric measurement of thermal effusivity of liquids by sample's thickness scan

A method based on the sample's thickness scan of the amplitude of the photopyroelectric (PPE) signal is proposed as an alternative for thermal effusivity measurement of liquids. The proposed method uses a combined amplitude-phase information and needs the knowledge of the absolute values of the sample's thickness and phase of the signal. The accuracy of the method is similar with that of previously reported frequency-scanned methods, provided an accurate control of the cell's (sample's) thickness is performed. A 479 Ws^1/2/m²K room temperature value for the thermal effusivity of silicon oil was found, with a 0.1 μm step thickness control.     

Photopyroelectric investigation of binary mixtures of liquid crystals in contact preparations

The photopyroelectric (PPE) method is proposed as a sensitive technique to study a binary mixture of liquid crystals in a contact preparation. The photothermal signal is generated while scanning the contact preparation. The crystal (K)/smectic-A (S _A), smectic-A/nematic (N), and nematic/isotropic (I) interphase boundaries are detected. The displacement of these boundaries due to the variation of the temperature is monitored. Received: 13 Oktober 1998 / Accepted: 7 July 1999 / Published online: 3 December 1999     

Photopyroelectric calorimetry of solids

An alternative photopyroelectric (PPE) technique that combines the front detection configuration (FPPE) with the thermal wave resonator cavity (TWRC) method is proposed. The theoretical analysis of the FPPE signal indicates that the configuration also offers information about both fluid sample and backing solid material. It is demonstrated that the normalized phase of the FPPE signal has an oscillating dependence as a function of the sample’s thickness. In the thermally thin regime for the sensor and liquid sample, the method can be used for direct measurement of backing thermal effusivity. This article presents experimental results on solid materials, with various values of thermal effusivity (Cu, brass, steel, glass, bakelite, and wood), used as backings in the detection cell. A study of the sensitivity of the technique for different liquid/backing effusivity ratios is performed. The main result of this article is the possibility of deriving the thermal effusivity of a solid sample (backing material) by monitoring the thickness of a fluid with well-known properties. In such a way, the so-called coupling fluid is not anymore a disturbing factor; however, its properties can be used to obtain the value of the thermal effusivity of a solid. The method proved to be suitable especially for thermal effusivity investigations of low thermal conductors. An application on polymer composites with different liquid/powder content is presented.     

Thermal analysis of Polymer Dispersed Liquid Crystal under electric field using photopyroelectric technique

In this paper the first measurement of effective thermal parameters (thermal diffusivity, effusivity, conductivity and heat capacity) of Polymer Dispersed Liquid Crystal (PDLC) composites using the photopyroelectric (PPE) calorimetry is reported. The PPE technique is used in the standard “back” configuration and the cell has been designed for allowing the application of an electric field to the sample. Results show a dependence of the thermal parameters on the applied electric field which is explained by the reorientation of the liquid crystal molecules inside the droplets.