K. Heide

Die bestimmung kinetischer parameter endothermer zersetzungsreaktionen unter nicht-isothermen bedingungen

By using the kinetic parameters calculated from non-isothermal measurements according to Freeman and Carroll a computer programm is given, which permits the determination of the more probable reaction mechanism by use of 17 different kinetic equations. The computer programm includes the kinetic equation for chemical reaction, nucleation, phase boundary reaction and diffusion and can easily be extended to other equations. The evaluation of the experimental values by a distinct kinetic equation is quantitatively characterized by means of the correlation coefficient. The kinetic parameters are calculated on the basis of a regression analysis. By dividing the thermogravimetric curve in different reaction intervals a differential calculation is possible. Decomposition of CaCO3, MgCO3 and CaSO4·2H2O is discussed from the results of the computer calculation. It is seen that the kinetic parameters and the reaction mechanism are not constant during the reaction. Further experiments for a physical interpretation are being carried out.

Molecular structure and nucleation in silicate glasses

Abstract The molecular structures of several silicate glasses which nucleate homogeneously (CaO·SiO2, Na2O·SiO2, Li2O·SiO2, Li2O·2SiO2, BaO·2SiO2 and CaO·Al2O3·2SiO2) as well as those of glasses which only nucleate heterogeneously (PbO·SiO2, Na2O·2SiO2, CaO·MgO·2SiO2, K2O·TiO2·3SiO2 and Na2O·Al2O3·6SiO2) are critically analyzed and compared with the structures of their equilibrium crystalline phases. It is shown that, for the first class of glasses (displaying homogeneous nucleation), both cationic and anionic arrangements in glass and crystal are similar. In some cases, cation coordination requirements lead to the formation of chain segments of high mobility which favor homogeneous nucleation. For the second family of glasses, the structures of glass and crystal differ and only heterogeneous nucleation is observed.

Checking of mathematical calculation methods for the determination of kinetic parameters from non-isothermal measurements by test functions

Abstract The applied computing programs are checked by means of test values from theoretical models. First-order kinetics, the Avrami equation and the 2-dimensional diffusion equation have been calculated as theoretical values. The computer calculation was carried out both for the total reaction and for individual reaction intervals. For the calculation of kinetic parameters and the distinction between the various models, in principle, the integral method should be used. The calculation does not give any answer to the question whether there exists another equation not involved in the models selected, which describes the processes better. If there is no indication of the reaction, it should first be checked by the differential method. The correlation coefficient does not allow the individual model equations to be distinguished with statistical significance.

Influence of cation coordination on nucleation in silicate glasses

By comparing the available structural parameters of silicate glasses and crystals of the same composition, the correlations between the nucleation behaviour of a glass and its short and middle range order, specially the coordination of the modifier cations, are discussed. There are glass systems which nucleate homogeneously (Li20 · Si02, Na 20 · Si02, CaO · Si02, LizO · 2Si02 ,Ba0 · 2Si02 ,Ca0 · A1203 · 2 Si02) and systems which nucleate only heterogeneously (PbO · Si02, CaO · MgO · 2Si02, NazO · 2Si02, NazO · A1203 · 6Si02). It is demonstrated that changes in the cation coordination during the formation of the long range order play an important role for nucleation behaviour.

Dissolved gases and minor component effects on glass crystallization

Abstract Surface crystallization is always observed when a cordierite (2MgO2Al2O35SiO2) glass is heat treated. Nucleation was studied by mass spectrometric evolved gas analysis, optical microscopy, scanning and transmission electron microscopy, and atomic force microscopy. The formation of metastable (μ-cordierite, X-phase) and stable (indialite) crystals are correlated with sample mass loss and gas release. H2O is the main component of the evolved gas. The influence of the surrounding atmosphere and of the polished state of the surface on the formation of μ-cordierite and X-phase were characterized by microscopy. Polished and fractured surfaces are strongly deformed during crystallization. AFM shows crystals on the surface and in the material to about 20 nm underneath the surface.

Untersuchung des einflusses der analysenbedingungen auf die zersetzung von 4MgCO3 ·Mg(OH)2 · 4H2O und die bildung von magnesit MgCO3

Abstract The processes, which influence the decomposition of 4MgCO 3 ·-Mg(OH) 2 · 4H 2 O, could be determined by systematical variation of the analytical parameters. The original crystal structure exists in a wide temperature range during the decomposition. The formation of magnesite is a secondary reaction of the gaseous phase with the reaction product. H 2 O and CO 2 , are released simultaneously in different proportions during the decomposition to 500°C. Stoichiometric intermediates were not found. The original crystal structure collapsed, when the last H 2 O escapes. The ratio of MgCO 3 : MgO can be influenced by partial pressure of CO 2 in a wide range.

The detection of an inorganic hydrocarbon formation in silicate melts by means of a direct-coupled-evolved-gas-analysis-system (DEGAS)

Abstract In an attempt to determine the volatile components in various industrial, laboratory and natural glasses, the gas release from the silicate melts in high vacuum — high temperature extraction experiments were analyzed by simultaneous TG and mass spectrometry. In the temperature range from 800 to 1200°C molecular fragments with m/z=13, 15, 27, 28, 29, were detected, which are possible fragments of hydrocarbon molecules. Similar behaviour could be demonstrated by analysis of glass samples synthesized in the laboratory. The formation of hydrocarbons during the heating process was successfully controlled by the carbon concentrations in the batches, through appropriate additions of carbon bearing reagents — graphite, Al- and Ca-carbides, di- and polysaccharides, or Na-acetate. Based on the degassing profiles (DP) it is possible to deduce a mechanism for the formation of hydrocarbons, e.g. of methane in systems containing graphite or carbides and hydroxideions: C +4( OH ) − ⇒ CH 4 +2 O 2− + O 2 The oxygen formed during this process is dissolved in the melt and escaped again in the form of oxygen bubbles with a further temperature increase. With the DP of laboratory prepared melts, it was possible to give an interpretation of the observed degassing maxima of m/z 13, 15, 27, 28, 29, on the one hand, by the formation of molecular fragments of CH4 and C2H6 and, on the other hand, by the carbon isotopic relations, e.g. m/z 13= 12 CH + or 13 C + ; m/z 28= 12 C 2 H 4 + or 12 C 13 CH 3 + .

Dynamisch thermische analyse der Zersetzung von K2SiF6

Zusammenfassung Die thermische Zersetzung von K 2 SiF 6 ist ein komplizierter Prozess, der von verschiedenen Parametern abhangt, wodurch Schwierigkeiten beim Vergleich der Ergebnisse unterschiedlicher Methoden entstehen. Es wird eine experimentelle Anordnung beschrieben, die zur Durchfuhrung geringe Substanzmengen erfordert und einen direkten Vergleich mit ublichen thermoanalytischen Ergebnissen gestattet. Der Einfluss der experimentellen Bedingungen wird diskutiert. K 2 SiF 6 stellt eine Modellsubstanz dar auch fur andere Komplexverbindungem, die bei ihrer thermischen Zersetzung hydrolysierende Produkte liefern. Der Verlauf der thermischen Zersetzung von K 2 SiF 6 wird bis zur vollstandigen Verdampfung bei 1200°C beschrieben.

THERMAL ANALYSIS OF HYDROCARBONS IN PALEOZOIC BLACK SHALES

Paleozoic black shales of the Saxothuringikum (Germany) with an average Corg. -content of 0.01 to 20 mass% were investigated with regard to the nature of organic matter. A special pyrolysis technique (DEGAS) was used for a temperature resolved analysis of different hydrocarbons (HC) and the simultaneous detection of inorganic volatiles during heating under vacuum up to 1450°C. The presented data indicate three different forms of organic matter occurring in the investigated black shales (bitumen, kerogen and pyrobitumen). Finally the influence of an igneous dyke intrusion on the alteration of the organic matter was examined.

Degassing of a cordierite glass melt during nucleation and crystallization

Abstract The crystallization of coarse cordierite glass particles (0.4–0.63 mm in diameter) was studied by means of evolved gas analysis during constant heating. It was observed that crystallization is accompanied by complex degassing phenomena of H 2 O, CO, CO 2 , O 2 and Ar. The evolution process gradually appears at temperatures above the glass transition temperature. At higher temperatures two distinct degassing maxima are evident which can be attributed to the formation of the primary high-quartz solid-solution phase (“μ-cordierite”) and its subsequent transformation into the hexagonal high-temperature polymorph of cordierite (“indialite”). In respect of these two crystallization steps, characteristic differences in the evolution behaviour between (a) CO and (b) H 2 O, CO 2 , O 2 and Ar are detectable. The results are discussed in relation to the different chemical properties and the “porosity” of both crystalline structures.