Heating rate effect on the DSC kinetics of oil shales

This research was aimed to investigate the combustion and kinetics of oil shale samples (Mengen and Himmetoğlu) by differential scanning calorimetry (DSC). Experiments were performed in air atmosphere up to 600°C at five different heating rates. The DSC curves clearly demonstrate distinct reaction regions in the oil shale samples studied. Reaction intervals, peak and burn-out temperatures of the oil shale samples are also determined. Arrhenius kinetic method was used to analyze the DSC data and it was observed that the activation energies of the samples are varied in the range of 22.4–127.3 kJ mol⁻¹ depending on the oil shale type and heating rate.     

Crystallisation and melting behaviour of fish oil measured by DSC

Fish oil which is characterised by important amounts of poly-unsaturated ω-3 fatty acids attach increasing importance within functional foods. Recently attention is directed on physical methods that allow fast and relatively easy the identification and discrimination of oils. DSC measurements yield in information on thermal effects, characterised by changes in enthalpy and their temperature range such as melting and crystallisation. The aim of the investigation presented here was to take DSC curves in the temperature range +20 to ?40°C on several fish oils and fish oil capsules to visualise the crystallisation and melting behaviour and to compare transition temperatures and enthalpies.     

Thermophysical theory of DSC melting peak

Melting peak for metals is described with expressions derived from thermophysical consideration of DSC operation. Three parameters govern the shape of the peak: thermophysical coefficient derived from the DSC design, enthalpy of fusion of a sample, and heating rate. Rigorous evaluation yields rather complex expressions, but simplified expressions can be used in common practice. The peak shape is described by two different expressions for two separate stages in the process of metal melting (1) the melting itself and (2) heat relaxation after the melting completion. The validity of the expressions was demonstrated after the experiments on gallium melting. The thermophysical coefficient is shown be affected to small variations by the changes in sample preparation or experimental conditions (melting Ga, In, Zn).     

A DSC study of the effect of lead pigments on the drying of cold pressed linseed oil

Cold pressed linseed oil and paints prepared using the inorganic pigments; lead white and red lead, were characterized using non-isothermal differential scanning calorimetry (DSC) in an air atmosphere to determine the effect of the pigment on the oxidative polymerisation of the drying oil medium. For each paint sample, the onset temperature for oxidation was reduced from 166°C to the range 50 to 60°C when a heating rate of 5 K min^-1 was used. In order to determine the rate of drying, the non-isothermal experiments were carried out using a range of heating rates. A change in the mechanism oxidative polymerization was observed as the heating rate was increased.     

DSC study of melting and solidification of salt hydrates

Most salt hydrates, especially those proposed for thermal-energy-storage applications, melt incongruently. In static systems, this property often leads to differences between the enthalpy of fusion and enthalpy of solidification. By means of differential scanning calorimetry (DSC), these differences have been determined for several salt hydrates. For Na₂SO₄ · 10 H₂O, the enthalpy of solidification at or near the peritectic temperature is never more than 60% of the enthalpy of fusion; further cooling leads to a second phase transition at a temperature corresponding to eutectic melting of mixtures of ice and this hydrate. This asymmetrical melting and freezing behavior of Na₂SO₄ · 10 H₂O decreases its potential as an energy-storing medium and also limits its usefulness for temperature calibration of DSC instruments. Sodium pyrophosphate decahydrate, Na₄P₂O₇ · 10 H₂O, although in some ways a higher temperature analog of Na₂SO₄ · 10 H₂O, exhibited a smaller discrepancy between the enthalpies of fusion and of solidification; its relatively high transition temperature permits a more rapid solidification reaction than is the case for Na₂SO₄ · 10 H₂O. For Mg(NO₃)₂ · 6 H₂O, a congruently melting compound, the magnitude of ΔH of crystallization equalled ΔH of fusion, even when supercooling occurred; a solid-state transition at 73°C, with ΔH = 2.9 cal g^?1, was detected for this hydrate. MgCl₂ · 6 H₂O, which melts almost congruently, exhibited no disparity between ΔH of crystallization and ΔH of fusion. CuSO₄ · 5 H₂O and Na₂B₄O₇ · 10 H₂O exhibited marked disparities. Na₂B₄O₇ · 10 H₂O formed metastable Na₂B₄O₇sd 5 H₂O at the phase transition; this was derived from the transition temperature and verified by relating the observed ΔH of transition to heats of hydration. Peritectic solidification of hydrates can be viewed as a dual process: crystallization from the liquid solution and reaction of the lower hydrate (or anhydrate) with the solution; where ΔH of solidification appears to be less in magnitude than the ΔH of fusion, the difference can be attributed to slower reaction rate between solution and the lower hydrate. New or previously unreported values for ΔH of fusion obtained in this study were, in cal g^?1: Mg(NO₃)₂ · 6 H₂O, 36; Na₄P₂O₇ · 10 H₂O, 59; CuSO₄ · 5 H₂O, 32; Na₂B₄O₇ · 10 H₂O, 33.     

DSC study of melting and glass transition in gelatins

In the range from –50° to +130°C, the temperature dependence of the heat capacity for different kinds of gelatins with water contents of from 2 to 95% was studied by the DSC method. It was shown that, in all studied cases, metastable collagen-like structures are formed in gels or crystalline gelatins, with thermodynamic parameters depending on the formation conditions. The characteristic properties of the glass transitions in amorphous gelatins and crystalline gelatins with different melting heats and different contents of the ordered phase were established. Special attention is paid to the structural properties of free and bound water. The dependence of the glass transition temperatureT _g on the bound water content was shown to be of general applicability for many denatured biopolymers. Free water in gelatins, in distinction to the bound water, does not act as a plasticizer, but forms a rigid matrix inhibiting the glass transition.

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Zusammenfassung Mittels DSC wurde im Bereich –50° bis +130°C die Temperaturabhängigkeit der Wärmekapazität für verschiedene Arten von Gelantine mit einem Wassergehalt von 2 bis 95% untersucht. Es wurde gezeigt, daß in allen untersuchten Fällen metastabile kollagenähnliche Strukturen in Gelen oder kristallinen Gelantinen gebildet werden, deren thermodynamische Parameter von den Bildungsbedingungen abhängen. Es wurden die charakteristischen Eigenschaften der Glasumwandlungen in amorphen Gelantinen und kristallinen Gelantinen mit unterschiedlichen Schmelzwärmen und einem unterschiedlichen Gehalt an geordneter Phase bestimmt. Spezielle Aufmerksamkeit wurde den strukturellen Eigenschaften von freiem und gebundenem Wasser gewidmet. Es wurde gezeigt, daß die Ab-hängigkeit der GlasumwandlungstemperaturT _g vom Gehalt an gebundenem Wasser generell für viele denaturierte Biopolymere anwendbar ist. Im Unterschied zu gebundenem Wasser fungiert freies Wasser in Gelatinen nicht als ein Weichmacher, bildet aber eine starre Matrix, die die Glasumwandlung verhindert.

     

On the determination of the melting point of semicrystalline polymer by DSC

This is a study for criteria to judge the melting point of semi-crystalline polymers from the DSC endotherm for polymer melting. Beyond standard indium DSC melting results an evaluation has been made on a series of polyethylenes for which crystal sizes were measured and predicted from Raman LAM analysis. The results confirm the conclusion of Prof. Wunderlich that the DSC content of melting is the proper basis of reporting melting points.Dedicated to Professor Bernhard Wunderlich on the occasion of his 65th birthday     

Catalytic effect analysis of metallic additives on light crude oil by TG and DSC tests

This research aimed at the investigation of the effect of different metallic additive on the combustion and kinetic behavior of crude oil. For this purpose, the thermal behavior of the oil-only and oil–metallic salts mixtures were studies by the thermogravimetry (TG)/derivative thermogravimetry and differential scanning calorimetry (DSC) on heating rate at 10 °C min^?1. The result shows that Dagang crude oil exhibited apparent low temperature oxidation (LTO), fuel deposition, and high temperature oxidation processes. With the addition of metallic salts, the LTO process has been shortened and samples added CuSO₄, CrCl₃·6H₂O, and AlCl₃·6H₂O achieved a much lower peak temperature than that of oil. Based on the Arrhenius model, metallic additives were proven to have obvious influence on the combustion activation energy. And, by comprehensive analysis of TG/DSC profile and activation energy, ZnSO₄ exhibited a positive influence on light crude oil combustion during the high pressure air injection process.     

Determining the main thermodynamic parameters of caffeine melting by means of DSC

The temperature and enthalpy of the melting of caffeine, which are 235.5 ± 0.1°C and 19.6 ± 0.2 kJ/mol, respectively, are determined by DSC. The melting entropy and the cryoscopic constant of caffeine are calculated.     

DSC study and computer modelling of the melting process in ice slurry

In order to understand the non-isothermal melting kinetics in the ice slurry, a differential scanning calorimetry (DSC) was used. Experimental results were compared to those obtained by a numerical simulation in which a general enthalpy method was applied. In this work the ice slurry studied consists of ice particles uniformly dispersed within a water-antifreeze liquid mixture. The effects of the heating rate and the initial antifreeze mass fraction are discussed. It has been found that the temperature gradients inside the sample of the solution become important if either heating rate increases or initial antifreeze mass fraction decreases.     

The melting behaviour of concentrated sulphuric acid investigated by DSC

The DSC-investigation of different concentrated sulphuric acid solutions shows that the results of this dynamic measurement method are in good agreement with those of steady state methods [1]. The diverse DSC-melting peaks can be interpreted by using the known behaviour of the system SO₃/H₂O [1] as the melting of different congruent melting phases or eutectics, respectively. The observed melting enthalpies are in the correct order.

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Zusammenfassung DSC-Untersuchungen an Schwefelsäurelösungen verschiedener Konzentration zeigten, daß die Ergebnisse dieses dynamischen Meßverfahrens mit den Ergebnissen von Gleichgewichtsverfahren in guter Übereinstimmung stehen. Die verschiedenen DSC-Schmelzpeaks können unter Zuhilfenahme des bekannten Verhaltens des Systemes SO₃/H₂O, nämlich mit dem Schmelzen unterschiedlicher kongruenter Schmelzphasen bzw. Eutektika erklärt werden. Die ermittelten Schmelzenthalpien liegen in der richtigen Größenordnung.

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, , . , SO₃/H₂O, . .

     

Liquid-liquid phase transformations and the shape of the melting curve

The phase diagram of elemental liquids has been found to be surprisingly rich, including variations in the melting curve and transitions in the liquid phase. The effect of these transitions in the liquid state on the shape of the melting curve is analyzed. First-order phase transitions intersecting the melting curve imply piecewise continuous melting curves, with solid-solid transitions generating upward kinks or minima and liquid-liquid transitions generating downward kinks or maxima. For liquid-liquid phase transitions proposed for carbon, phosphorous selenium, and possibly nitrogen, we find that the melting curve exhibits a kink. Continuous transitions imply smooth extrema in the melting curve, the curvature of which is described by an exact thermodynamic relation. This expression indicates that a minimum in the melting curve requires the solid compressibility to be greater than that of the liquid, a very unusual situation. This relation is employed to predict the loci of smooth maxima at negative pressures for liquids with anomalous melting curves. The relation between the location of the melting curve maximum and the two-state model of continuous liquid-liquid transitions is discussed and illustrated by the case of tellurium.     

Effect of the magnetic field on the melting transition of Ga and In by nW-stabilized DSC

The melting transition of Ga and In was measured by using a nW-stabilized differential scanning calorimeter working in a magnetic bore. The magnetic effect on the thermometer was about 18 mK at 5 T, which was corrected for the measurement of the magnetic effect on the melting transition of Ga and In. The melting temperatures of Ga and In with the magnetic field of 5 T were obtained to be 8.3 and 10.2 mK, respectively higher than those without the magnetic field. These results show that the solid phase to be relatively more stable under the magnetic field. The calculated temperature shifts of the melting transition due to the magnetic field using the magneto-Clapeyron equation and the reference data of magnetic susceptibility were negative values for both Ga and In, being contradictory to the experimental results.     

Ab initio melting curve of copper by the phase coexistence approach

Ab initio calculations of the melting properties of copper in the pressure range 0-100 GPa are reported. The ab initio total energies and ionic forces of systems representing solid and liquid copper are calculated using the projector augmented wave implementation of density functional theory with the generalized gradient approximation for exchange-correlation energy. An initial approximation to the melting curve is obtained using an empirical reference system based on the embedded-atom model, points on the curve being determined by simulations in which solid and liquid coexist. The approximate melting curve so obtained is corrected using calculated free energy differences between the reference and ab initio system. It is shown that for system-size errors to be rendered negligible in this scheme, careful tuning of the reference system to reproduce ab initio energies is essential. The final melting curve is in satisfactory agreement with extrapolated experimental data available up to 20 GPa, and supports the validity of previous calculations of the melting curve up to 100 GPa.     

Ab initio melting curve of molybdenum by the phase coexistence method

Ab initio calculations of the melting curve of molybdenum for the pressure range 0-400 GPa are reported. The calculations employ density functional theory (DFT) with the Perdew-Burke-Ernzerhof exchange-correlation functional in the projector augmented wave (PAW) implementation. Tests are presented showing that these techniques accurately reproduce experimental data on low-temperature body-centered cubic (bcc) Mo, and that PAW agrees closely with results from the full-potential linearized augmented plane-wave implementation. The work attempts to overcome the uncertainties inherent in earlier DFT calculations of the melting curve of Mo, by using the "reference coexistence" technique to determine the melting curve. In this technique, an empirical reference model (here, the embedded-atom model) is accurately fitted to DFT molecular dynamics data on the liquid and the high-temperature solid, the melting curve of the reference model is determined by simulations of coexisting solid and liquid, and the ab initio melting curve is obtained by applying free-energy corrections. The calculated melting curve agrees well with experiment at ambient pressure and is consistent with shock data at high pressure, but does not agree with the high-pressure melting curve deduced from static compression experiments. Calculated results for the radial distribution function show that the short-range atomic order of the liquid is very similar to that of the high-T solid, with a slight decrease of coordination number on passing from solid to liquid. The electronic densities of states in the two phases show only small differences. The results do not support a recent theory according to which very low dT(m)dP values are expected for bcc transition metals because of electron redistribution between s-p and d states.     

Influence of toasting and the seed variety on the physico-chemical and thermo-oxidative characteristics of the flaxseed oil

Physico-chemical properties, spectroscopy, and thermal analyses were used aiming at evaluating the influence of toasting and of the flaxseed variety on thermo-oxidative behavior of flaxseed oils. Thermogravimetry (TG) and differential scanning calorimetry (DSC) were associated to gas chromatography, infrared spectroscopy and UV–Vis spectroscopy, as well as to physico-chemical analyses to characterize the oils obtained from raw and toasted flaxseeds. No meaningful differences in the thermal and oxidative stabilities were noticed comparing oils obtained from the brown and the golden flaxseeds. Nevertheless, the UV–Vis spectra indicated that both flaxseed oils were at the beginning of the oxidation process. The previous toasting of the seeds led to a higher oxidation for both varieties being harmful to the flaxseed oil quality.     

Unusual recrystallization and melting behaviour in DSC scans of isotactic polypropylene samples

The unusual dependence of the melting peak temperature of the species recrystallized during DSC scans on the isothermal crystallization temperature (T _c ) is discussed for isotactic polypropylene samples. It is pointed out that such a phenomenon is not due to superheating effects; it is believed to be accounted for by assuming that the rates of the recrystallization phenomena at the same temperature are higher for samples obtained at higherT _c values.