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Thermal properties of oxide glasses

Part III. Thermal stability of Li2O·2SiO2·nMeO2 glasses (M=Ti, Zr)

Journal of Thermal Analysis and Calorimetry
Authors:
P. Šimon
,
E. Jóna
, and
V. Pavlík

Abstract  

Glasses with the composition Li2O·2SiO2·nTiO2 and Li2O·2SiO2·nZrO2, where n=0, 0.03, 0.062, 0.1, were prepared and the onset and peak temperatures have been determined by DTA. From these characteristic temperatures, the kinetic parameters describing the nucleation and crystal growth have been obtained by isoconversional methods. The kinetic parameters have been used for the calculation of nucleation and crystal growth times for individual glasses so determining the order of glass stability at reheating. The stability of glasses increases with the content of TiO2 or ZrO2 where the increase is higher for ZrO2. Within the concentration range under study, the increase of both times with the metal oxide concentration is quadratic. It has been discussed that the crystallization kinetics does not obey the Arrhenius law and, therefore, when using the evaluation methods based on this law, the results should not be extrapolated outside the temperature range of the measurements.

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The helix-coil transition of organic solvent soluble homopolypeptides has been discussed from a largely thermodynamic view-point. The present state and the future prospects are considered.

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Abstract  

The compounds Cu2(RCO2)4 and Cu2(RCO2)4py2 [R=CH3(CH2)n–2; n=14, 16, 18; py=pyridine] were prepared, analysed and characterized by means of DSC calorimetry, thermogravimetry and infrared spectroscopy. Generally, the pyridine complexes are unstable, losing pyridine already at room temperature. Transition from a crystalline to a liquid crystalline phase after heating was observed for all Cu2(RCO2)4 compounds but only for pyridine complex of Cu(II) tetradecanoate. However, the melting process was observed for all Cu(II) carboxylate complexes with pyridine. The lower thermal stability of pyridine complexes, than corresponding Cu2(RCO2)4 compounds can be explained by structural features.

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Abstract  

The thermal properties (in the temperature range of 100–250°C) of N-ethyl-N-phenyldithiocarbamatecomplexes of Zn(II), Co(III), Ni(II), Cu(II) and Pb(II) and their influence on the kinetics of cure have been studied by differential scanning calorimetry (in nitrogen). It was found that Zn(II), Co(III) and Pb(II) dithiocarbamates melted without further effects, while the melting of Ni(II) and Cu(II) dithiocarbamates is accompanied with decomposition. From the kinetic point of view, the dithiocarbamates decrease the values of the reaction order and the values of rate constants follow this order (with respect to the metal ion): Zn(II)<Cu(II)<Pb(II)<Ni(II)<Co(III).

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Abstract  

A method for describing the lengths of induction periods at linear-heating measurements, is employed for the study of induction periods in the crystallisation of metallic glasses. For Fe75Si15B10 glass, close values of the related kinetic parameters were obtained from isothermal and nonisothermal measurements. On the basis of the results obtained, the absence of induction period in the first crystallisation step of Al90Fe7Nb3 glass in the isothermal DSC measurement has been elucidated.

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Since the first identification of P2Y receptor sequences in 1993, it has quickly become apparent that this family of the G-protein coupled receptors is very diverse. Members of this receptor family are activated extra-cellularly by a wide variety of adenosine and uridine nucleotides including sugar-nucleotides. The recent decipherment of the Human Genome has enabled us to search for new, yet undiscovered P2Y receptor subtypes. In this article we examine the relationships of six orphan G-protein coupled receptor (GPCR) sequences which show considerable sequence homology to various P2Y receptors. The clustering at a few chromosomal loci of P2Y receptor genes and their related orphan genes further suggests that particular P2Y subsets were derived from the same ancestral gene during evolution.

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Abstract  

The enantiomeric enrichment of N-methyl-amphetamine (MA) was tried by eight different preparative separation methods. The enrichment process was also studied by thermoanalitical methods (DSC,TG). There is no enantiomeric enrichment during fractional distillation and selective extraction of the liquid base. The enantiomeric mixture of MA·HCl can be enriched by crystallization or by sublimation. The most effective enrichment can be achieved by fractional crystallization or distillation of the reaction mixture after partial salt formation with hydrochloric acid. The separation is less effective in case of fractional steam distillation and selective extraction when the enantiomer and the racemate are distributed between two liquid phases. This supports the general experience that having a solid phase in the system makes enantiomer separation more efficient.

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