High Temperature Domain Research Articles

Diesel evaporation as the first step of hydrogen production

Diesel constitutes the major fuel utilized in ships. Hence, diesel reforming is foreseen as one of the options for marine fuel cell applications. Evaporation of diesel fuel is an important stage in the diesel reforming processes. Thermal decomposition of the heavy feedstock that occurs primarily in the high temperature domain of the evaporation process leads to carboneous material formation and may plug the evaporator. Carboneous materials can also accumulate on the catalyst surface and cause serious problems in desulfurization and pre-reformer units. The diesel evaporator design is one of the key parameters to minimize carbon formation. The operating conditions must be optimized as well. In this study, the evaporation heat was supplied by two different ways. In the first evaporation system, the evaporation heat of the diesel fuel was supplied by an electrical furnace. In the second system, diesel was evaporated in a tube-and-tube heat exchanger via indirect heat supplied by hot nitrogen gas. The latter case was chosen to simulate the utilization of fuel reforming off-gases. Results indicate that evaporation by the hot gases results in much lower thermal decomposition of the diesel fuel compared to the utilization of the electrical energy.

Anomalous Specific-Heat Jump in a Two-Component Ultracold Fermi Gas

The thermodynamic functions of a Fermi gas with spin population imbalance are studied in the temperature-asymmetry plane in the BCS limit. The low-temperature domain is characterized by an anomalous enhancement of the entropy and the specific heat above their values in the unpaired state, decrease of the gap and eventual unpairing phase transition as the temperature is lowered. The unpairing phase transition induces a second jump in the specific heat, which can be measured in calorimetric experiments. While the superfluid is unstable against a supercurrent carrying state, it may sustain a metastable state if cooled adiabatically down from the stable high-temperature domain. In the latter domain the temperature dependence of the gap and related functions is analogous to the predictions of the BCS theory.

Mechanical and Dielectric Breakdown Properties of Biodegradable Plastics

The mechanical and dielectric breakdown properties of polylactic acid (PLA), which is a biodegradable plastic, were examined, and a physicochemical analysis was performed. At room temperature, the tensile strength of PLA was about 100 MPa, almost the same as for PP, but at 100°C, even the value for LDPE decreased. The Young's modulus of PLA at room temperature was about 3.6 GPa, about 1.7 times the value for PP, and about 11 times the value for LDPE. The Young's modulus of PLA decreased slowly with a rise in temperature, and at high temperatures, 60°C and above, it decreased rapidly. The dielectric breakdown strength (EB) of PLA at room temperature was about 6.2 MV/cm, about 1.4 times the value for LDPE. The EB of PLA increased with an increase in temperature, and was about 6.9 MV/cm at 60°C (∂EB/∂T≥0). In this region, it is thought that a secondary effect due to space charge occurred, as well as an electron avalanche breakdown. However, the EB of PLA decreased at temperatures higher than 60°C in the high temperature region (∂EB/∂T<0). In this domain, it is thought that thermal breakdown occurred, and that electromechanical breakdown also occurred in the high temperature domain of 80°C and above.

Spatiotemporal patterns in catalytic systems

Infrared thermography measurements in various oxidation reactions revealed high-temperature domains whose boundaries are either stationary, oscillating, moving or rotating. These motions were observed on catalytic wires, rings, cylindrical pellets and thin catalytic beds. Their evolutions are different from the classical Turing mechanism, which explains many of the pattern formation in reaction–diffusion system. The observed patterns are strongly affected by the interaction between the local surface reaction rate, the mixing in the surrounding reactant phase and by the intrinsic heterogeneity of the catalytic surface and the transport coefficients. While simulations can qualitatively predict such patterns, quantitative predictions require a reliable kinetic model. Formation of hot regions in a three-dimensional fixed-bed can be predicted with reliable kinetic models. While formation of hot regions in the flow direction in the reactor are reasonably well understood, there is still a need to gain qualitative understanding as well as design criteria for formation of hot zones transversal to the flow direction.

Model for evaluating and predicting thermal expansivity at high temperatures for geophysical minerals

In the present communication, a new relationship for temperature dependence of thermal expansivity at high temperatures has been reported. The relation is acquired on the ground of high-temperature experimental data collected by Anderson for six geophysical minerals, viz. NaCl, KCl, MgO, CaO,Mg 2SiO 4 and Grossular Garnet. According to the present study, the thermal expansivity varies exponentially with temperature above the Debye temperature. The model projected in the present study is valuable to predict data on thermal expansivity in the high-temperature domain where experimental data have not been found so far.

N-Alkane Binary Molecular Alloys

The experimental systems considered in this paper are normal alkanes, in the range from octane to octacosane; and their binary mixtures {(1−x) mol of CnH2n+2 + x mol of Cn‘H2n‘+2}, with n‘ > n, and Δn = n‘ − n taking the values 1 and 2. The alkanes and their mixtures have a rich, complex polymorphic nature, for the description of which a distinction is made between a high-temperature and a low-temperature domain. The high-temperature domain is occupied by rotator phases, and the low-temperature domain by a variety of mixed crystalline phases, referred to as the “ordered” phases. Experimental phase diagram data for 19 binariesincluding 11 binaries for which original data are presentedare used to give a uniform and coherent description of the phase relationships for the ensemble of alkane alloys.

Laser-ultrasonics: a non-contact method to link the acoustic attenuation to metal damping properties up to the melting point

The objective of this work is to describe the viscoelastic behavior of metals up to their melting temperature by measuring the velocity and the attenuation of ultrasonics waves. For that purpose, a technique called laser-ultrasonics has been optimized for the high temperature domain and the solid to liquid transition. This paper is especially applied to the viscoelastic characterization of tin, from room temperature up to the melting point.

Contribution of the dimeric state to the thermal stability of the flavoprotein D-amino acid oxidase.

The flavoenzyme DAAO from Rhodotorula gracilis, a structural paradigm of the glutathione-reductase family of flavoproteins, is a stable homodimer with a flavin adenine dinucleotide (FAD) molecule tightly bound to each 40-kD subunit. In this work, the thermal unfolding of dimeric DAAO was compared with that of two monomeric forms of the same protein: a Deltaloop mutant, in which 14 residues belonging to a loop connecting strands betaF5-betaF6 have been deleted, and a monomer obtained by treating the native holoenzyme with 0.5 M NH(4)SCN. Thiocyanate specifically and reversibly affects monomer association in wild-type DAAO by acting on hydrophobic residues and on ionic pairs between the betaF5-betaF6 loop of one monomer and the alphaI3' and alphaI3" helices of the symmetry-related monomer. By using circular dichroism spectroscopy, protein and flavin fluorescence, activity assays, and DSC, we demonstrated that thermal unfolding involves (in order of increasing temperatures) loss of tertiary structure, followed by loss of some elements of secondary structure, and by general unfolding of the protein structure that was concomitant to FAD release. Temperature stability of wild-type DAAO is related to the presence of a dimeric structure that affects the stability of independent structural domains. The monomeric Deltaloop mutant is thermodynamically less stable than dimeric wild-type DAAO (with melting temperatures (T(m)s) of 48 degrees C and 54 degrees C, respectively). The absence of complications ensuing from association equilibria in the mutant Deltaloop DAAO allowed identification of two energetic domains: a low-temperature energetic domain related to unfolding of tertiary structure, and a high-temperature energetic domain related to loss of secondary structure elements and to flavin release.

Molybdenite Re–Os dating of biotite dehydration melting in the Rogaland high-temperature granulites, S Norway

The ability of the Re–Os system in molybdenite to record and preserve the age of granulite-facies metamorphism in polymetamorphic belts is tested using the Ørsdalen W–Mo district, Rogaland, S Norway. A low-pressure high-temperature granulite-facies domain, displaying osumilite and pigeonite isograds, is exposed around the 931±2 Ma Rogaland anorthosite complex. Available U–Pb monazite and zircon data show that a 0.93 Ga contact metamorphism overprints a 1.03–0.97 Ga regional Sveconorwegian metamorphism in the gneiss basement. Molybdenite and scheelite in the Ørsdalen district occur in orthopyroxene-bearing leucocratic veins parallel to the regional foliation. The veins are interpreted as migmatic leucosomes formed by fluid-absent incongruent melting of the biotite-rich host rock above ca. 800°C, producing a granulite-facies orthopyroxene±garnet residual assemblage. Molybdenite is interpreted as a product of the melting reaction, crystallized from trace amounts of Mo released from biotite in the host rock during partial melting. Four Re–Os analyses of molybdenite from three samples representing two mines yield an isochron age of 973±4 Ma. The isochroneity of the data indicates that the precipitation of molybdenite and the partial melting event are recorded on the district scale. The Re–Os system in molybdenite was not affected by subsequent 0.93 Ga contact metamorphism, corresponding to formation of garnet+quartz coronitic textures around molybdenite and other minerals in the deposits. The results indicate that granulite-facies conditions prevailed at 973±4 Ma, near the end of a protracted event of regional metamorphism (1.03–0.97 Ga). Biotite dehydration melting recorded in Ørsdalen took place at a pressure of ca. 5.5 kbar (orthopyroxene–garnet–plagioclase–quartz thermobarometry), possibly in association with regional decompression. The study shows that granulite-facies metamorphism (0.97 Ga) took place before intrusion of massif-type anorthosites in Rogaland (0.93 Ga). The study suggests that the high-temperature osumilite-bearing assemblage may be related to the 0.97 Ga event, and that massif-type anorthosites may not be a cause for the high-temperature thermal anomaly in the crust but a late expression of it.

Novel phase boundary in the field induced spin density wave state in (TMTSF) 2PF 6

Magnetoresistance measurements on the quasi one-dimensional organic conductor (TMTSF) 2PF 6 performed in magnetic fields B up to 16 T, temperatures T down to 0.12 K, and under pressures P up to 1.4 GPa have revealed new phase boundary on the P– B– T phase diagram. The boundary subdivides the field induced spin density wave (FISDW) phase diagram into two regions. The low-temperature region of the FISDW diagram is characterized by a hysteresis behavior of the resistance R( B) which was observed in earlier studies and is typical for the first-order transitions. In contrast to the common believe, in the high temperature region of the FISDW phase diagram, the hysteresis in R( B) and, hence, the first-order transitions vanish. Nevertheless, sharp changes in d R/d B are observed both in the low and high temperature domains indicating that the transitions between different sub-phases exist over all range of the FISDW state. We compare these results with a theoretical prediction.

Temperature dependence of single particle excitations in aS=1chain: Exact diagonalization calculations compared to neutron scattering experiments

Exact diagonalization calculations of finite antiferromagnetic spin-1 Heisenberg chains at finite temperatures are presented and compared to a recent inelastic neutron scattering experiment for temperatures T up to $7.5$ times the intrachain exchange constant J. The calculations show that the excitations at the antiferromagnetic point $q=1$ and at $q=0.5$ remain resonant up to at least $T=2J,$ confirming the recent experimental observation of resonant high-temperature domain wall excitations. The predicted first and second moments are in good agreement with experiment, except at temperatures where three-dimensional spin correlations are most important. The ratio of the structure factors at $q=1$ and 0.5 is well predicted for the paramagnetic infinite-temperature limit. For $T&lt;~2J,$ however, we found that the experimentally observed intensity is considerably less than predicted. This suggests that domain wall excitations on different chains interact up to temperatures of the order of the spin band width.

In situ high-pressure and high-temperature diffraction experiments on pure and Ag-doped TiO 2 nanopowders

The phase stability of amorphous and nanostructured TiO 2 powders was investigated. The surface free energy and the surface stress dominate the thermodynamic phase stability for ultrafine-grained materials. Both parameters may be tuned with sufficient accuracy by modifying the surface chemistry of the nanoparticles and by applied pressure, respectively. Pure and Ag-doped nanostructured titanium dioxide samples were produced using the sol–gel method. Their high-pressure and high-temperature stability domains were determined by in situ X-ray diffraction experiments using synchrotron radiation.

Magnetic properties of FexMn1−x S sulfides exhibiting the magnetoresistive effect

The magnetic, electrical, and thermal (derived from DTA data) properties of FexMn1−xS polycrystalline sulfides (0≤x≤0.38) synthesized based on α-MnS (NaCl cubic lattice) and exhibiting colossal magnetoresistance were studied. The studies were conducted at temperatures from 77 to 1000 K and magnetic fields of up to 30 kOe. As the degree of cation substitution in the FexMn1−xS system was increased, the magnetic order was found to change from antiferromagnetic to ferromagnetic. In the high-temperature domain (550–850 K), the samples undergo two phase transitions with critical temperatures \(T_{c_1 }\) and \(T_{c_2 }\), which are accompanied by reversible anomalies in the magnetization and thermal (DTA) properties and by a semiconductor-metal transition.

In situ kinetic analysis of SiC filaments CVD

An in situ kinetic study of the SiC-based filament CVD from DCMS/H2 and DCMS/C3H6/H2 mixtures has been carried out. The results evidenced a low temperature regime (T<1150 °C) controlled by surface chemical reactions and a high temperature domain (T>1150 °C) governed by the diffusion of reactive species to the substrate (mass transfer regime). The addition of propene to the DCMS/H2 mixture leads to the decrease of the deposition rate, the increase of the activation energy (within the surface reaction limited regime) and the increase of the C/Si (at.) ratio. The dilution of DCMS in H2 promotes the co-deposition of free silicon. Conversely, the addition of propene results in lower amounts of free silicon or even traces of free carbon for high deposition temperatures. The increase of the C/Si (at.) ratio observed in presence of propene might be related to the inhibition of the deposition of Si-rich species derived from DCMS, through reactions with propene or related unsaturated hydrocarbons in the gas phase.

Evolution of the properties of CuAlSe 2 thin films with the oxygen content

CuAlSe 2 thin films have been synthesized by annealing, under argon flow, a multilayer structure of thin Cu, Al and Se layers sequentially deposited by evaporation under vacuum. It is shown that the oxygen content depends not only on the evaporation content but also on the argon flow. The properties of the thin films are modified by this oxygen percentage: when no more than 4–5 at % of oxygen is present in the films, their optical properties are very similar to that of single crystals. The inter-band transitions A, B, C typical of such chalcopyrite structure are clearly visible on the absorption spectra. The variation of the conductivity of these films in the high temperature domain is as that expected in CuAlSe 2, while in the low temperature, grain boundaries are dominant.

Glass transitions for freeze-dried and air-dried tomato

Glass transition temperatures of freeze-dried tomato conditioned at various water activities at 25 °C were determined by differential scanning calorimetry (DSC). Air-dried tomato with and without osmotic pre-treatment in sucrose/NaCl solutions was also analyzed. Thermograms corresponding to the low water activity domain (0.11 ⩽ a w ⩽ 0.75) revealed the existence of two glass transitions, which were attributed to separated phases formed by sugars and water and other natural macromolecules present in the vegetable. Both transitions were plasticized by water and experimental data could be well correlated by the Gordon-Taylor equation in the low-temperature domain, and by the Kwei model in the high-temperature domain. For higher water activities, the low-temperature glass transition curve exhibited a discontinuity, with suddenly increased glass transition temperatures approaching a constant value that corresponds to the T g of the maximally freeze-concentrated amorphous matrix. The unfreezable water content was determined through the melting enthalpy dependence on the moisture content.

Formation of multidomain structures in the interaction of elliptic light beams with a gas mixture

It is shown that the absorption of optical energy by vibrational levels of a gas mixture can lead to the formation of several high-temperature domains along the region of propagation of a light pulse. Point and distributed models of the interaction of laser radiation with a gas are analyzed. The control of parameters can be used to control the position of domains and their number. For practical applications (e.g., data recording in the bulk of a medium), the existence of two or more high-temperature regions in combination with the dynamics of chemical reactions may have both positive and negative effects.

The gelation mechanism of solutions of isotactic polystyrene in cis-decalin

The gelation of isotactic polystyrene in cis-decalin was investigated with calorimetric, infrared and rheological measurements. At temperatures above 30°C a crystalline structure similar to the one obtained by crystallization from the melt is formed. Below 30°C thermoreversible gels are formed with a molecular conformation different from that in the melt crystallized samples. They can be formed during a cooling process below 19°C and can increase their helix content and modulus on isothermal annealing. Gels formed in this temperature domain exhibit a two-stage melting as observed by calorimetry, infrared spectroscopy and rheology. At higher temperature gels can only be formed after isothermal annealing. These gels melt in one single step. A mechanism is proposed for the gelation in both temperature domains. At low temperature, bundle-like entities composed of long helices are formed giving rise to the high melting structure. In a second step, additional helix formation increases the modulus and induces the low melting structure. In the high temperature domain, only the low melting structure is formed. This model is in agreement with morphological data reported in the literature.

Low temperature and magnetic field X-ray diffraction study for Nd 0.5Sr 0.5MnO 3

First-order structural phase transition based on charge order for Nd 0.5Sr 0.5MnO 3 was studied by a newly developed low temperature and magnetic field X-ray diffraction apparatus. The structural phase transition that corresponds to a jump of lattice constants was confirmed even in magnetic fields up to 5 T. We observed the coexistence region consisting of high and low-temperature phases in magnetic fields. This region increased with increasing field. The domain of the high-temperature ferromagnetic phase remains in the charge order phase at low temperature and low magnetic field. The magnetic behavior is explained by the volume fraction of high-temperature domain.

Evidence for a New Monoclinic Bi–La Oxide Solid Solution: Identification and Structural Relationship to the Rhombohedral Bi–Sr–O Type

As a term of the Bi–Sr–O rhombohedral type Bi1−yLayO1.5 solid solution, the Bi17La4O31.5 mixed oxide (y no. 0.19) has been investigated. Air-quenching from the high temperature domain of the β1 disordered form allows one to preserve at room temperature a metastable form of β1. This form transforms exothermally on heating near 200°C to a new monoclinic phase. This transformation and the transition monoclinic phase→stable ordered rhombohedral β2, which occurs at 445°C, are both irreversible on cooling for this composition. The β2→β1 transition occurs reversibly near 660°C, under usual dynamic conditions (300°h−1). The monoclinic cell lattice constants, a=9.492(2), b=7.951(2), c=7.028(2) Å, β=104.74(1)°, lead to Z=12 for the unit formula Bi0.81La0.19O1.5 and imply a disordered cationic sublattice. This mixed oxide appears as a term of a new variety of a wide solid solution (that will name ε hereafter).