R. Riccardi

On the thermal stability and defect structure of the solid solution LixNi1-xO

Abstract Li x Ni 1- x O solid solutions are both of scientific and technological interest as they have been proposed as cathodic material in MCFC. In this work TG, XRD and SEM measurements have been used to study the thermal stability of several solid solutions (of different initial composition) obtained starting from the reactive system Ni/Li 2 CO 3 . Composition changes, due to both lithium and oxygen loss, take place as a consequence of samples annealing. In some cases a sensible disagreement between the compositions deduced by TG and XRD data is showing, which is ascribed to the presence, in the final solid solutions, of lattice defects others than substitutional ones (namely cation vacancies). The TG and XRD measurements have been then worked out to obtain both the correct composition and vacancies concentration of the solid solutions. It is shown that solid solutions with similar lithium content but different vacancies concentration can be obtained depending on the preparation conditions.

STRUCTURAL AND MICROSTRUCTURAL STUDY OF THE FORMATION OF THE SOLID-SOLUTION LIXNI1-XO

Abstract Scanning electron microscopy and X-ray powder diffractometry have been used to characterize solid solutions of the type LixNi1−xO. Structural and profile parameters were refined from X-ray data to study both the composition and the homegeneity of the mixed oxides. It is shown that non-homogeneity is responsible for the appearance of broad and asymmetrical diffraction peaks and that better agreement between calculated and observed patterns is obtained if non-homogeneous mixed oxides are treated as if they consisted of solid solutions of different composition. The morphology, the mean composition and the homogeneity of the solid solutions as well change (in a non-monotonous way) as a function of the time of the isothermal treatment (T = 800°C) after which the samples were obtained.

Physico-chemical characterisation of different solid forms of spironolactone

Spironolactone is a steroidal diuretic showing incomplete oral behaviour because of its low solubility and slow dissolution rate. Four solid forms were obtained by recrystallisation of a commercial sample from different solvents (acetone, methanol, ethyl acetate). These samples have been characterised by Fourier transform infrared (FT-IR) Spectroscopy, X-ray powder diffraction, thermogravimetric analysis coupled with FT-IR spectroscopy (TG/FT-IR) and differential scanning calorimetry (DSC). The aim of the work is establish evidence for differences, if any, between the forms and, possibly, to understand their nature. A careful analysis of the collected data makes it possible to rule out the presence of any solvate and to distinguish three different polymorphs.

Thermal study of water/β-cyclodextrin interactions

Abstract It is shown that water/β-cyclodextrin system has a complex behaviour above room temperature, and displays phenomena which may be separately analyzed under carefully chosen conditions: in dry atmosphere, and/or below 60°C, the release of water occurs much faster than the expected structural transformation from the hydrated to the dehydrated structure. It is argued that this transition contributes substantially to the endothermic DSC peak usually attributed to dehydration. After completing its first transition to the dehydrated structure, a water grown sample undergoes a dramatic (∼15%) and irreversible expansion, which apparently does not modify the crystal structure.

Solid-state reaction study on the system Ni-Li2CO3

Abstract Thermogravimetry, differential scanning calorimetry, X-ray diffractometry and scanning electron microscopy have been used to investigate the formation reaction of the solid solution LixNi1−xO in the solid-state system Ni-Li2CO3. Thermogra vimetric results show that carbonate decomposition takes place well below its usual temperature when nickel is being oxidized and that as a consequence of the simultaneous oxidation and decomposition processes, a solid solution forms in a temperature range where no appreciable volume diffusion can occur. X-ray diffraction measurements confirm this conclusion showing that lattice constant values of thermogravimetric samples linearly decrease with increasing mixture lithium content. Notwithstanding all experimental evidences agree in indicating that all lithium oxide participates to solid solution formation, sensible departures from the linear relationship arise for mixtures of high lithium content (XLi>0.2010). It is shown that cation vacancies are responsible of such a behaviour and that their concentration increases with increasing the relative amount of lithium oxide produced by spontaneous carbonate decomposition that enters solid solution.

An analysis of the factors affecting the peak shape and the quantitative reliability of a heat flux DSC cell

Abstract In a previous work it was shown that unexpected problems may arise in the calibration of a heat flux DSC cell. To understand the origin of these problems an analysis has been undertaken of what happens in the cell before, during and after the peak. Here the first part of this analysis is reported dealing with the baseline and with the dependence of the recorded signal on time. Analytical expressions are obtained connecting the measured signal with experimental variables and instrument constants. Completely independent on sample related properties, a maximum measurable ΔT is introduced as limiting factor for quantitative reliability.

The role of thermal treatment in the preparation of lithiated nickel cathodes for molten carbonate fuel cells

Abstract Gravimetric, dilatometric, diffractometric and microscopic measurements have been used to study the role of thermal treatment in the preparation of lithiated nickel oxide cathodes for molten carbonate fuel cells. Clear evidence has been obtained that both the nominal composition of the reactive system and the thermal treatment during solid solution formation appreciably affect the cathodes microstructure. Starting from a careful analysis of the X-ray data, a reaction model is proposed which quite satisfactorily explains the microstructural aspects of the reaction. It is shown that there are three different processes at the origin of the cathodes microstructure and that the effect of the thermal treatment is to affect their relative weights.

Structural Characterization of Nickel Oxide

Abstract It is known from the literature that a slight distortion of the ideal cubic cell is present in the NiO structure. This work shows that such a distortion can be accurately evaluated by means of a refinement of the structural and profile parameters of X-ray powder diffraction data. Moreover, since only small amounts of products are sometimes at disposal to perform structural characterizations, it was thought useful to extend the refinement procedure to X-ray data collected on NiO samples of much lower mass (15-60 mg) than those usually utilized in X-ray diffractometric studies. The results obtained show that reliable structural parameters can be obtained from low mass samples too

Thermodynamics of water/β-cyclodextrin system

The thermodynamics of β-cyclodextrin dehydration is investigated, by parallel DSC/TG experiments, on both fully and partially hydrated samples. The apparent dehydration enthalpies per mole of water are impossibly high and this fact suggests that another phenomenon, in addition to the rupture of the β-cyclodextrin/H2O hydrogen bonds, contributes to the peak area. All the experimental evidence agrees with an ‘interaction model’ which assumes that deydration is accompanied by a slow and reversible rearrangement of the β-cyclodextrin structure.

ON THE THERMAL CHARACTERIZATION OF THE SYSTEM SULFUR-TETRACYANOETHYLENE

Abstract A DSC characterization of the sulfur-tetracyanoethylene (TCE) system was performed, and the thermal traces obtained quantitatively analyzed. It was shown that (a) some type of solid state interaction takes place between sulfur and TCE, which forces part of the sulfur to be “frozen” into its rhombic form; (b) variable amounts of sulfur and TCE fail to melt at their expected melting temperatures; and (c) a much higher than expected enthalpy change is associated with sulfur polymerization. A phenomenological model is proposed according to which the sulfur and TCE that fail to melt at their melting temperatures undergo fusion in the wake of the liquid-phase sulfur polymerization process. It is shown that the model proposed is valid in the entire composition range studied.