Authors:Christelle Herman, Tom Leyssens, Valérie Vermylen, Véronique Halloin, and Benoît Haut
with metastable forms [ 2 ].
Differential Scanning Calorimetry (DSC) is commonly used to evaluate most of the thermal properties of solid states, such as melting temperatures and enthalpies or specific heat capacities [ 3 ]. Whilst recent
Authors:N. Cohen, M. Odlyha, D. Ucko, and Q. Pankhurst
Mechanical alloying is a versatile technique for the solid state synthesis of many materials, including alloys such as iron-copper
where the elements are immiscible under equilibrium conditions. The structural and magnetic state of these alloys, and their
thermal stability, have been investigated by means of thermomagnetometry, DSC, X-ray diffraction and Mssbauer spectroscopy.
Comparison of the thermomagnetometry curves for the various alloys together with analysis of intermediate reaction products
enabled the individual thermal processes to be identified. The Curie temperature of the alloys was measured, and it was found
that on heating the metastable alloys underwent phase segregation between 300-400C.
Authors:Mourad Ibrahim Daoudi, Abdelhafid Triki, and Abdelkrim Redjaimia
represents the super-saturated solid solution, GPZ stands for Guinier–Preston zones, β″ are metastable needle-like precipitates, β′ are metastable rod-like (or lath-like) precipitates and β is the equilibrium phase. β″ and β′ precipitations occur by
The modelling of the oxidation process of pyrite (FeS2) has been performed to explain the complex phase creations in the natural massive sulphide deposits of hydrothermal origin. We studied the multistage structural magnetic transformations of pyrite on the air within the temperature interval of 20–700° C by means of Mossbauer spectroscopy, thermomagnetic analysis and x-ray diffraction. The emergency of hexagonal and monoclinic nonstoichiometric pyrrhotites with varied degree of vacancy ordering and disordering have been observed. The Curie points of those metastable phases change within the limits of 120–240° C for hexagonal and within 320–460°C for monoclinic pyrrhotites.
In this paper, examples are given of how calorimetric values can give greater certainty to phase equilibria calculated from
thermodynamic data. Errors that may arise when phase diagram evaluations are carried out largely from the basis of Gibbs energy
information only are illustrated by reference to recent evaluations of the Ti−Si system and the resulting calculated oxidation
behaviour of titanium silicides. The importance of calorimetric values for calculation of metastable phase equilibria is demonstrated
by results of work on the AlN−TiN hard-metal coating system. Finally, suggestions are made with regard to areas of work where
calorimetric data are urgently needed.
Materials with high surface areas and small particle size (nanophases), metastable polymorphs, and hydrated oxides are increasingly important in both materials and environmental science. Using modifications of oxide melt solution calorimetry, we have developed techniques to study the energetics of such oxides and oxyhydroxides, and to separate the effects of polymorphism, chemical variation, high surface area, and hydration. Several generalizations begin to emerge from these studies. The energy differences among different polymorphs (e.g., various zeolite frameworks, the - and -alumina polymorphs, manganese and iron oxides and oxyhydroxides) tend to be small, often barely more than thermal energy under conditions of synthesis. Much larger contributions to the energetics come from oxidation-reduction reactions and charge-coupled substitutions involving the ions of basic oxides (e.g., K and Ba). The thermodynamics of hydration involve closely balanced negative enthalpies and negative entropies and are very dependent on the particular framework and cage or tunnel geometry.
The kinetics of reduction at relatively low temperatures with hydrogen of pure and doped metastable non-stoichiometric magnetite
with 1 at% Mn, Co, Ni and Cu and also with 5 at % Ni and Cu have been investigated by using isothermal thermogravimetry in
the temperature range 300–400°C. With increase in the concentration of the dopant (5 at% Ni and Cu), the reactivity increases.
The activation energies for pure magnetite varies from 7 to 9 kcal/mole with the preparation temperature of precursorf Fe2O3 (250–400°C), being the lowest for those prepared at the lowest temperatures. The corresponding activation energies for the
reduction of doped samples (Fe, M)3−zO4, it depends, apart from their porosity and surface areas, on the nature of the solute atom, amount of disorder, whether it
occupies the tetrahedral (A) or octahedral (B) sites in the non-stoichiometric spinel and possibly on hydrogen ‘Spill over’
The paper studies metastable phases of 4,4'-di-n-heptyloxyazoxybenzene (liquid crystal substance) using adiabatic calorimetry. The process of transformation between metastable
and stable phases is described quantitatively. The conclusion concerns activation energies for metastable phase to stable
Authors:P.M. Ghogomu, Elise Provost, M. Bouroukba, M. Dirand, and M. Hoch
The solubility of two n-alkanes in commercial organic liquids, such as diesel fuel and jet fuel represent a problem to industry,
because they precipitate in an unpredictable fashion. First we calculated the metastable enthalpy and entropy of fusion of
the low temperature forms of the n-alkanes. We analyzed the solubility of alkanes n-C22H46, n-C23H48, n-C24H50 and n-C28H58
in ethylbenzene, m-xylene, n-heptane and gas oil. All systems seem to be close ideal, possibly with a slight positive deviation.
We analyzed the solubility at constant temperature of the ternary system solvent C22H46-C24H50, C23H48-C24H50, C13H28-C16H34,
C20H42-C22H46, C20H42-C24H50 and C20H42-C28H58, and looked at cloud points in various ternary systems. When the difference
in the number of carbon atoms in the two alkanes is small, four or less, a metastable solid solution precipitates from the
solvent. If the difference in the number of carbon atoms is six or more, the ‘equilibrium’ phases, or at least phases with
low solubility precipitate.
Authors:Fan Sun, Denis Laillé, and Thierry Gloriant
In this study, the thermal analysis of the ω nanophase transformation from a quenched metastable β Ti–12Mo alloy composition
(mass%) was investigated by electrical resistivity and dilatometry measurements. The activation energy was observed to be
121 ± 20 kJ mol−1 (from resistivity measurements) and 114 ± 12 kJ mol−1 (from dilatometry measurements) during the early stage of the transformation process. The kinetic of the ω nanophase transformation
was modelized by using the classical Johnson–Mehl–Avrami (JMA) theory and a modified Avrami (MA) analysis. An Avrami exponent
close to 1.5 was found at the early stage of the transformation suggesting a pure growth mechanism from pre-existing nucleation
sites. Nevertheless, it was observed a decrease of the Avrami exponent to 0.5 at higher transformed fraction demonstrating
a dimension loss in the growth mechanism due to the existence of the high misfit strain at the interface β/ω.