S. Longuemart

Temperature dependence of the electrical conductivity of imidazolium ionic liquids

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.

Influence of the anion on the electrical conductivity and glass formation of 1-butyl-3-methylimidazolium ionic liquids

Six ionic liquids based on the 1-butyl-3-methylimidazolium cation have been studied. As anions Cl(-), Br(-), I(-), [NCS](-), [N(CN)(2)](-), and [BF(4)](-) were selected. The electrical conductivities were determined between 173 and 393 K based on impedance measurements in the frequency range from 0.1 to 10(7) Hz. The electrical conductivity increases, whereas the glass transition temperature, the fragility, and the low temperature activation energy decrease with increasing anion size. The results can be understood from the changing anion-cation interaction strength with changing anion size and from the energy landscape interpretation of the glass transition dynamics.

On the accurate determination of thermal diffusivity of liquids by using the photopyroelectric thickness scanning method

An enhanced accurate method of measuring the thermal diffusivity of liquids by the samples thickness scan of the phase of the photopyroelectric signal is presented. The method, making use of the absolute values of the phase and sample thickness, leads to very accurate results for the room temperature values of thermal diffusivity (about +/-0.3%). The high accuracy of the method is due to a very precise control of the samples thickness variation (0.1 microm step), to a proper localization of the thickness scan range, and to a new procedure of data analysis. The high accuracy of the method recommends it for the study of processes associated with small changes of the thermal parameters.

The application of the photopyroelectric method for measuring the thermal parameters of pyroelectric materials

The photopyroelectric calorimetry, in the standard (back) configuration, is applied in order to measure the thermal parameters of some pyroelectric materials. It is demonstrated that the method is able to simultaneously measure the thermal diffusivity and effusivity of a pyroelectric material. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal; the measurements need no calibration. A combined amplitude-phase procedure, at a single frequency, leads to the same results. In the mean time, if the thermal parameters of the pyroelectric sensor are known, one can get the thermal effusivity of a sample acting in the experimental cell as a substrate. Investigations and theoretical simulations were performed on a well known pyroelectric material, LiTaO3, with various liquid substrates.

AN APPLICATION OF THE FRONT PHOTOPYROELECTRIC TECHNIQUE FOR MEASURING THE THERMAL EFFUSIVITY OF SOME FOODS

ABSTRACT Photopyroelectric calorimetry, in the front (inverse) configuration (with thermally thick sensor and sample, and optically opaque sensor), was applied to measure the thermal effusivity of some fats and water-based juice products. The information was obtained via a frequency scan of the phase of the pyroelectric signal; the measurements need no calibration. The main advantages of this configuration above the other two previously proposed front schemes are: (i) it does not require the use of very thin foils or expensive semitransparent sensors; (ii) the information is contained in the phase of the signal (and not in the amplitude as usual), which is independent of the power fluctuations of the radiation source and, consequently, the results are more accurate and reproducible.

Accurate photopyroelectric measurements of thermal diffusivity of (semi)liquids

ABSTRACT The back photopyroelectric (PPE) configuration, with opaque sample and thermally thick sample and sensor, was applied in order to obtain room temperature values of the thermal diffusivity of some (semi)liquid materials. The methodology is based on a samples thickness scan, and not on a frequency scan as usually happens. The possibility of controlling, very accurately, the samples thickness variation, leads to very accurate and reproducible values for the thermal diffusivity. A special detection cell was designed in order to control the samples thickness within 1 μm precision. Due to the high accuracy of the results one can study, via the thermal diffusivity, processes as sedimentation, the capture of water molecules by OH radicals in alcohol–water mixtures, or the quality of lipids (i.e., margarines).

Optothermal depth profiling by neural network infrared radiometry signal recognition

The feasibility of a neural network radiometric photothermal depth profiling method is verified using well-defined artificial samples with varying optical properties across the layers. The signal calculation model is shown to be accurate and the neural network approach to solve the inverse problem is shown to be feasible. Both from simulated and experimental radiometric signals, accurate reconstructions are obtained for heat source and optical-absorption coefficient profiles.

Evidencing Molecular Associations in Binary Liquid Mixtures via Photothermal Measurements of Thermophysical Parameters

Abstract The back photopyroelectric (PPE) configuration, with opaque sample and thermally thick sample and sensor, was applied in order to obtain room temperature values of the thermal diffusivity of some liquid mixtures. The methodology is based on a samples thickness scan, and not on a frequency scan as is usually the case. The possibility of precisely controlling the samples thickness variation leads to very accurate and reproducible values for the thermal diffusivity. The method is proposed as an alternative for evidencing molecular associations in binary liquid mixtures.

Analysis of the photopyroelectric signal for investigating thermal parameters of pyroelectric materials

The photopyroelectric signal is analyzed in order to simultaneously obtain the thermal diffusivity and effusivity of pyroelectric materials. Two different experimental configurations are described and compared in terms of accuracy and sensitivity. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal. The methods have been used for the measurement of thermophysical properties of a lead–titanate–zirconate ceramic sample.

Current mode versus voltage mode measurement of signals from pyroelectric sensors

Usually, in photothermal experiments using pyroelectric sensors, the instrumental transfer function is cancelled by normalization to reference measurements, regardless of current (C) or voltage (V) mode processing of signals. Nevertheless, there are several advantages when using a current preamplifier, instead of a high-impedance voltage preamplifier. Due to the low input impedance of the former, the capacitance of the sensor (and its temperature dependence), the capacitance of the connection cable, and the loss resistance of the pyroelectric crystal do not influence the signal. Moreover, the whole input circuitry is less prone to electromagnetic pick up through stray capacitances. In (C) mode, the frequency characteristic is linear (with −180° constant phase) up to a rather high frequency, instead of having (in V mode) a 1/f dependence above a certain frequency and a variable phase shift from 0° to −90°.