Extreme Temperature Dependence Research Articles

Temperature-Dependent Circularly Polarized Luminescence of a Cholesteric Copolymer Doped with a Europium Complex

The design of new materials for non-contact temperature sensors is an important task for scientists working in the fields of chemistry, physics, and materials science. In the present paper, a novel cholesteric mixture based on a copolymer doped with a highly luminescent europium complex was prepared and studied. It was found that the spectral position of the selective reflection peak strongly depends on temperature and a shift towards shorter wavelengths is observed upon heating with an amplitude of more than 70 nm, from the red to green spectral range. This shift is associated with the existence and melting of clusters of smectic order, as confirmed by X-ray diffraction investigations. The extreme temperature dependence of the wavelength of selective light reflection provides a high thermosensitivity of the degree of circular polarization of the europium complex emission. The highest values of the dissymmetry factor are observed when the peak of selective light reflection fully overlaps with the emission peak. As a result, the highest sensitivity of 65%/K for luminescent thermometry materials was obtained. In addition, the ability of the prepared mixture to form stable coatings was demonstrated. The obtained experimental results, i.e., the high thermosensitivity of the degree of circular polarization, and the ability to form stable coatings allow us to consider the prepared mixture as a promising material for luminescent thermometry.

Magnetocaloric properties of dendrimer complexes of Fe(III) with substituted Schiff base

ABSTRACTIn dendrimer complexes of iron (III) with Schiff base (three complexes of iron (III) based on azomethine 4,4′-dodecyloxybenzoyloxybenzoyl-4-salicylidene-2-aminopyridine, a significant magnetocaloric effect (MCE) and heat capacity was found the first time. It was found that the magnitude of MCE depends on the nature of the counter-ion of the complex. MCE were measured with a microcalorimeter over the temperature range of 278–320 K and in a magnetic induction of 0–1.0 T. The temperature dependences of the MCE dendrimer complexes of iron (III) with Schiff base were obtained for the first time. For all the samples studied, the existence of extreme temperature dependence of MCE in the range of temperatures 300–350 K, which is possibly the result of the magnetic phase transition, is shown. The correlation between the thermotropic mesomorphism with the magnetic phase transition in complexes has been established.

Small-diameter parallel plate rheometry: a simple technique for measuring rheological properties of glass-forming liquids in shear

The rheological characterization of glass-forming liquids is challenging due to their extreme temperature dependence and high stiffness at low temperatures. This study focuses on the special precautions that need to be taken to accommodate high sample stiffness and torsional instrument compliance in shear rheological experiments. The measurement errors due to the instrument compliance can be avoided by employing small-diameter parallel plate (SDPP) rheometry in combination of numerical instrument compliance corrections. Measurements of that type demonstrate that accurate and reliable rheological data can be obtained by SDPP rheometry despite unusually small diameter-to-gap (d/h) ratios. Specimen preparation for SDPP requires special attention, but then experiments show excellent repeatability. Advantages and some current applications of SDPP rheometry are briefly reviewed. SDPP rheometry is seen as a simple and versatile way to measure rheological properties of glass-forming liquids especially near their glass transition temperature.

Characterization of a two-dimensional temperature field within a rapid compression machine using a toluene planar laser-induced fluorescence imaging technique

The homogeneous charge compression ignition (HCCI) combustion process is an advanced operating mode for automotive engines. The self-ignition mechanisms that occur within the combustion chamber exhibit extreme temperature dependence. Therefore, the thorough understanding of corresponding phenomena requires the use of diagnostic methods featuring a sufficient thermal sensitivity, applicable in severe conditions similar to those encountered within engines. In this respect, toluene planar laser-induced fluorescence (PLIF) is applied to the inert compression flow generated within an optical rapid compression machine (RCM). A relatively simple diagnostic system is retained: a single wavelength excitation device (266 nm) and a single (filtered) collection system. This diagnostic system is associated with an image processing strategy specifically adapted to RCM devices. Despite the severe conditions under consideration (40 bar, 700–950 K), the method allows us to obtain relatively large two-dimensional temperature fields that display a level of description seldom achieved in such devices. In particular the temperature gradients, which play a crucial role in HCCI combustion processes, can be estimated. The present experimental results confirm the good reliability and accuracy of the method. The information gathered with this toluene PLIF method puts in evidence its high potentialities for the study of aero-thermal-reactive processes as they take place in real engine conditions. The retained strategy also brings new possibilities of non-intrusive analysis for flows practically encountered within industrial devices.

Dynamics of charge carriers photoinduced in poly(3-dodecylthiophene)/fullerene composite

Radical pairs, polarons and fullerene anion-radicals, photoinduced by laser with photon energies of 1.88, 2.22 and 2.75 eV in the poly(3-dodecylthiophene)/[6,6]-phenyl-C 61-butanoic acid methyl ester (P3DDT/PCBM) bulk heterojunction, were studied over a wide temperature range. The number of these centers was found to decrease with the increase in laser photon energy. Both the spin–lattice and spin–spin relaxation times of fullerene anion-radicals and the spin–spin relaxation time of polarons change monotonically with temperature, whereas the interaction of polarons with the lattice is characterized by extreme temperature dependence. The one-dimensional polaron diffusion along the polymer chain and the rotation of fullerene near it own main axis was shown to follow the activation Elliot hopping model and to be governed by the photon energy. The deviation in activation energies for polaron and anion-radical motion and the difference in their dependence on the laser photon energy prove the non-interacting character of these charge carriers photoinduced in the P3DDT/PCBM bulk heterojunction.

Giant low-temperature enhancement of magneto-optic Kerr effects in PtMnSb

In PtMnSb the magneto-optic Kerr effect exhibits extreme temperature dependence in the range 100--320 K. The large polar Kerr rotation of about -1.5\ifmmode^\circ\else\textdegree\fi{} at photon energies around 1.85 eV at room temperature, is anomalously enhanced to almost -5\ifmmode^\circ\else\textdegree\fi{} as the temperature reaches 80 K. This extreme value is shown to arise because the diagonal elements of the permittivity tensor change with temperature.

Temperature dependence of the mercury telluride-cadmium telluride band offset

After reviewing the experimental data on the valence-band offset for HgTe-CdTe heterojunctions, we support previous suggestions of an extreme temperature dependence for this offset with a calculation based on a bond charge model. The model predicts the T dependence of the valence-band offset to be 77% of the difference in the band-gap temperature dependence of the heterojunction constituents. In the HgTe-CdTe system, the opposite signs of the band gap T variations yield an anomalously large increase in the offset of 213 meV between 0 and 300 K.

Melt spinning pitch-based carbon fibers

Although pitch-based carbon fibers can be melt spun, the flow characteristics of mesophase pitch make it extremely difficult to process. Mesophase pitch, like many fluids, can be non-Newtonian, but its viscosity is considerably more temperature sensitive than most melt-spun materials. In the present study, energy and force balances are applied to demonstrate the influence of process variables and material properties on the spinability of mesophase. The results indicate that the extreme temperature dependence of the viscosity of mesophase can create filament stresses during melt spinning that are near the ultimate tensile strength of the filament. Therefore, the control of both temperature and heattransfer rate is extremely critical during the formation of pitch-based carbon fibers.

Manganese proteins isolated from spinach thylakoid membranes and their role in O2 evolution. II. A binuclear manganese-containing 34 kilodalton protein, a probable component of the water dehydrogenase enzyme.

Extraction conditions have been found which result in the retention of manganese to the 33-34 kDa protein, first isolated as an apoprotein by Kuwabara and Murata (Kuwabara, T. and Murata, N. (1979) Biochim. Biophys Acta 581, 228-236). By maintaining an oxidizing-solution potential, with hydrophilic and lipophilic redox buffers during protein extraction of spinach grana-thylakoid membranes, the 33-34 kDa protein is observed to bind a maximum of 2 Mn/protein which are not released by extended dialysis versus buffer. This manganese is a part of the pool of 4 Mn/Photosystem II normally associated with the oxygen-evolving complex. The mechanism for retention of Mn to the protein during isolation appears to be by suppression of chemical reduction of natively bound, high-valent Mn to the labile Mn(II) oxidation state. This protein is also present in stoichiometric levels in highly active, O2-evolving, detergent-extracted PS-II particles which contain 4-5 Mn/PS II. Conditions which result in the loss of Mn and O2 evolution activity from functional membranes, such as incubation in 1.5 mM NH2OH or in ascorbate plus dithionite, also release Mn from the protein. The protein exists as a monomer of 33 kDa by gel filtration and 34 kDa by gel electrophoresis, with an isoelectric point of 5.1 +/- 0.1. The protein exhibits an EPR spectrum only below 12 K which extends over at least 2000 G centered at g = 2 consisting of non-uniformly separated hyperfine transitions with average splitting of 45-55 G. The magnitude of this splitting is nominally one-half the splitting observed in monomeric manganese complexes having O or N donor ligands. This is apparently due to electronic coupling of the two 55Mn nuclei in a presumed binuclear site. Either a ferromagnetically coupled binuclear Mn2(III,III) site or an antiferromagnetically coupled mixed-valence Mn2(II,III) site are considered as possible oxidation states to account for the EPR spectrum. Qualitatively similar hyperfine structure splittings are observed in ferromagnetically coupled binuclear Mn complexes having even-spin ground states. The extreme temperature dependence suggests the population of low-lying excited states such as are present in weakly coupled dimers and higher clusters of Mn ions, or, possibly, from efficient spin relaxation such as occurs in the Mn(III) oxidation state. Either 1.5 mM NH2OH or incubation with reducing agents abolishes the low temperature EPR signal and releases two Mn(II) ions to solution. This is consistent with the presence of Mn(III) in the isolated protein.(ABSTRACT TRUNCATED AT 400 WORDS)