The thermal reactivity of fossilized/petrified dinosaur eggshells excavated in China, Argentina and France has been studied
by means of thermal analysis/mass spectrometry (TG-MS), X-ray diffractometry (XRD) and analytical scanning electron microscopy
(SEM-EDX). The results provide more detailed information on the properties of these fossil materials and therefore allow an
improved typology of this most remarkable family of creatures.
The thermochemical reduction of a series of structurally and morphologically different natural and synthetic manganese(IV) oxides has been investigated. Measurements have been performed by means of combined thermogravimetry/mass spectrometry, X-ray diffraction and analytical scanning electron microscopy. The mechanisms of the degradation of these materials have been characterized in order to establish standardized procedures for their reactivity as function of structure, morphology and experimental conditions. The corresponding results are the fundament with respect to a reproducible technical application.
The thermochemical reduction of transition metal silicates, i.e. garnierite (Ni,Mg)6[(OH)8(Si2O7)], chrysocolla (Cu,Al)2H2Si2O5(OH4)·nH2O, dioptase CuSiO3·H2O, willemite Zn2SiO4, and hemimorphite Zn4Si2O7(OH)2·H2O has been studied. By means of combined thermogravimetric/mass spectrometric measurements, X-ray diffraction and analytical scanning electron microscopy it is shown that in a 5% H2/95% N2 or in a methane atmosphere the transition metals are selectively reduced at temperatures qualitatively corresponding to their electrochemical potential. Mixtures of elemental transition metals and quartz, SiO2, are obtained as solid products. Depending on the nature of the parent mineral, different mixtures of volatile products are obtained. Principal volatile product, however, is water vapour. The reduction in methane leads to the formation of syngas.
The topotactic structural mechanism of de- and re-ammination of single-crystalline NiPt(CN)4(NH3)2 is characterized by means of structural, morphological and thermoanalytical studies. Structural investigations give evidence
that the two-dimensional structural motif [NiPt(CN)4]∞ determines the mechanism and the kinetics of both processes. It is shown that the degree of reversibility, in particular
the exothermic re-ammination, is governed by the conservation of the two-dimensional structural element [NiPt(CN)4]∞. Indeed, only one type of bond has to be broken, and reformed, i.e. the two Ni−NH3 bonds per Ni. Microscopic studies reveal that by starting with single crystals with average dimensions of few tenths of a
mm, each cycle of de-and re-ammination leads to a continuous decrease of the size of crystalline domains until an optimum
geometry is reached for the given experimental conditions. By semi-quantitative measurements it can be shown that this direction-dependent
kinetic course of the overall reaction is controlled by the diffusion of ammonia along the [NiPt(CN)4]∞ layers. If the macroscopic size of these layer fragments is very small, i.e. after several cycles of the reversible reaction,
this diffusion control becomes negligible. The reaction is controlled by the availability of reactive Ni sites and ammonia,
i.e. its partial pressure.
Structural and morphological investigations of the solid state decomposition process of Ni(SCN)2(C5H5N)4 single crystals to microcrystalline Ni(SCN)2(C5H5N)2 reveal a ‘contracting sphere reaction mechanism’. Reconsidering possible artefacts registered by quantitative thermoanalytical measurements a pathway for the determination of specific and reproducible kinetic parameters for the rate-limiting step is described.
The role of structural parameters strongly influencing the course of heterogeneous solid-state reactions is established. Owing to the close relationship between the form and reactivity of solids, due emphasis must be given to detailed morphological studies. This allows the derivation of consistent correlations between the reaction mechanism on a microscopic scale and the observed macroscopic changes.
Authors:S. Felder-Casagrande, H. Wiedemann, and A. Reller
The calcination of limestone is one of the oldest technical processes and it is still of actual interest. Very early calcitic
mortars from Turkey have been investigated and compared with materials of other early civilisations i.e. with Egyptian mortars
containing gypsum as well as medieval dolomite-based mortars from alpine regions. Contemporary calcination procedures, in
particular the cement production, range among the most important global industrial processes causing non neglectable environmental
problems. Sustainable, solar energy assisted calcination technologies and the conversion of product CO2 into useful commodities are discussed.
The thermal reactivity of the naturally occurring silicates jadeite, NaAl[Si2O6], and nephrite, a variety of actinolite, Ca2 (Mg,Fe)5[(OH,F)Si4O11]2, have been investigated by thermogravimetric and thermomechanical analysis as well as temperature-dependent X-ray diffraction
and analytical electron microscopy. TG shows that nephrite undergoes a weight loss at around 900°C. Mass spectrometry reveals
that this irreversible reaction corresponds to the evolution of H2O, and XRD shows that a phase related to diopside CaMg[Si2O6] is formed. Jadeite does not undergo and observable weight changes up to 1000°C. Thermomechanical analysis indicates a reversible
phase transition at about 950°C. Temperature-dependent X-ray diffraction shows that jadeite is again present on cooling (peak
temperature: 1000°C), but that this is accompared by an additional unidentified phase. The mechanism of this process is not
yet clear although it has been observed in several samples from different origins and with different metal impurities.
The coordination compound [Co(urea)6](NO3)2 was synthesized and physico-chemically characterized. The thermal decomposition carried out in dynamic air and inert atmosphere
under non-isothermal conditions has been investigated by means of combined thermogravimetry/mass spectrometry, X-ray diffraction,
IR and UV-VIS spectroscopy as well as magnetic measurements. The course of the thermal decomposition starts with two-phase
transitions (melting and a Oh→Tdconfiguration change of the Co2+ ion) and continues with seven mass loss steps. According to the thermogravimetric and magnetic investigations a dimeric compound,
[Co(biuret)(NCO)]2(NO3)2, is assumed to arise. Up ~250C, an oxohydroxide nitrate intermediate is formed and a gradual oxidation of the Co2+ ions is observed. At 550C, Co3O4 with mean crystallite sizes of ~150 Ĺ is identified.