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Delenclos S. Kolinsky C. Longuemart S. Hadj Sahraoui A. Buisine J. M. 《Journal of Thermal Analysis and Calorimetry》2002,70(2):549-558
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. 相似文献
2.
The effect of pressure on the liquid crystal properties of two new dilaterally substituted nematogens has been studied. The method employed involves measurement of the thermal pressure variation of a sample under isochoric conditions. The pressure-temperature phase diagrams were determined. As for unsubstituted compounds, the nematic phase is stabilized upon application of pressure. The values of the (?T/?P)NI slopes of the clearing lines for these nematogens are, however, significantly higher than those generally characterizing rod-shaped nematics. The entropy separation method was used to estimate the constant volume and the volume-dependent terms of the entropy changes at the nematic-isotropic transition. The values of these contributions were determined on the basis of thermobarometric data showing that the transition entropy is strongly volume-dependent. This suggests that the higher values of the (?T/?P)NI slopes observed for these compounds can be related to the rearrangement of the lateral flexible chains in the nematic phase when the pressure increases, leading to a decrease of the excluded free volume caused by the bulky core of the molecules. 相似文献
3.
D. Dadarlat C. Neamtu N. Houriez S. Delenclos S. Longuemart A. Hadj Sahraoui 《The European physical journal. Special topics》2008,153(1):115-118
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 Ws1/2/m2K room
temperature value for the thermal effusivity of silicon oil was found, with
a 0.1 μm step thickness control. 相似文献
4.
Starting from the Debye model for frequency-dependent specific heat and the Vogel-Fulcher-Tammann (VFT) model for its relaxation time, an analytic expression is presented for the heat capacity versus temperature trace for differential scanning calorimetry (DSC) of glass transitions, suggesting a novel definition of the glass transition temperature based on a dimensionless criterion. An explicit expression is presented for the transition temperature as a function of the VFT parameters and the cooling rate, and for the slope as a function of fragility. Also a generalization of the results to non-VFT and non-Debye relaxation is given. Two unique ways are proposed to tackle the inverse problem, i.e., to extract the fragility from an experimental DSC trace. Good agreement is found between theoretically predicted DSC traces and experimental DSC traces for glycerol for different cooling rates. 相似文献
5.
M. Streza M. N. Pop K. Kovacs V. Simon S. Longuemart D. Dadarlat 《Laser Physics》2009,19(6):1340-1344
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 phase of the FPPE signal indicates that the configuration offers information about both fluid sample and backing material. The backing independent values of the normalized PPE phase give the possibility to calculate fluid’s thermal diffusivity. By monitoring the thickness of a fluid with well known properties, the thermal effusivity of a solid sample (backing material) can be measured. 相似文献
6.
D. Dadarlat M. Streza M. N. Pop V. Tosa S. Delenclos S. Longuemart A. Hadj Sahraoui 《Journal of Thermal Analysis and Calorimetry》2010,101(1):397-402
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. 相似文献
7.
Leys J Wübbenhorst M Preethy Menon C Rajesh R Thoen J Glorieux C Nockemann P Thijs B Binnemans K Longuemart S 《The Journal of chemical physics》2008,128(6):064509
The electrical conductivities of 1-alkyl-3-methylimidazolium tetrafluoroborate ionic liquids and of 1-hexyl-3-methylimidazolium ionic liquids with different anions were determined in the temperature range between 123 and 393 K on the basis of dielectric measurements in the frequency range from 1 to 10(7) Hz. Most of the ionic liquids form a glass and the conductivity values obey the Vogel-Fulcher-Tammann equation. The glass transition temperatures are increasing with increasing length of the alkyl chain. The fragility is weakly dependent on the alkyl chain length but is highly sensitive to the structure of the anion. 相似文献
8.
N. Houriez S. Delenclos S. Longuemart D. Dadarlat A. Hadj Sahraoui 《Laser Physics》2009,19(6):1345-1349
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. 相似文献
9.
D. Dadarlat M. N. Pop O. Onija M. Streza M. M. Pop S. Longuemart M. Depriester A. H. Sahraoui V. Simon 《Journal of Thermal Analysis and Calorimetry》2013,111(2):1129-1132
The front photopyroelectric configuration was applied to measure the thermal effusivity of some composite materials, inserted as backing layers in the detection cell. The technique is based on the scanning procedure of the coupling fluid’s thickness (TWRC method). Two particular composite materials were selected for investigation: (i) a liquid one: water based nanofluids with gold nanoparticles and (ii) a solid one: urea—fumaric acid (1:1) cocrystal. The thermal effusivity was found independent on the size and concentration of gold particles. Concerning the urea—fumaric acid cocrystal, the thermal effusivity value of the compound is different from the pure starting materials, indicating the formation of the compound. 相似文献
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