J. Šesták

Kinetic compensation effect as a mathematical consequence of the exponential rate constant

Abstract It is shown that the kinetic compensation effect mathematically results from the exponential form of the rate constant. A change of activation energy is thus compensated by the same change in temperature or in the logarithm of the pre-exponential factor.

Diagnostic limits of phenomenological kinetic models introducing the accommodation function

Applicability of macroscopically centered and microscopically localised measurements of solid-state reactions is discussed but their mutual correlation is found difficult. The wellknown models based on the geometrical representation of heterogeneous reactions are analysed with respect to the simple classical model of reaction-order introducing a multiplication function called the accommodation function. The resulting exponents become dependent on the system geometry and are further altered by the system nonideality due to the particles polydispersity, nonregular shapes, nonequal distribution etc.ZusammenfassungEs wird die Anwendbarkeit von makroskopisch zentrierten und mikroskopisch lokalisierten Messungen von Festphasenreaktionen beschrieben, jedoch wurde ihre Korrelation als kompliziert befunden. Diese wohlbekannten Modelle, die auf einer geometrischen Darstellung heterogener Reaktionen beruhen, wurden ausgehend von dem einfachen klassischen Reaktionsordnungsmodell unter Einführung einer als Anpassungsfunktion bezeichneten Multiplikationsfunktion untersucht. Die resultierenden Exponenten hängen nunmehr von der Geometrie des Systemes ab und werden weiterhin von der Nichtidealität des Systemes (Polydispersität der Partikel, unregelmäßige Formen, unregelmäßige Verteilung usw.) beeinflußt.

The boundary conditions for kinetic models

The problem of the validity and applicability of mathematical models in solid state kinetics is still considered a very controversial topic. Apart from the question concerning the physical meaning of the so-called “kinetic models”, there are also several mathematical problems and inconsistencies in the accepted formalism. In this paper we would like to discuss the most frequently used kinetic models with respect to the boundary conditions of a rate equation. In solid state kinetics the molar concentration of reactants is frequently replaced by the so-called degree of conversion (a) which is usually defined as 111 ~,=(c,-GMGo-G) 0)

Further aspects of the kinetic compensation effect

Mathematical considerations are used to show that the existence of an isokinetic point does not always indicate the kinetic compensation effect as a result of physicochemical reasons. The correlation of the Arrhenius parameters is given for both the inappropriate kinetic model function and the working temperature interval.

Kinetics of crystallization in the soda-lime-silica system: Part 1. Na2O · CaO · 2SiO2 and Na2O · CaO · 3SiO2 glasses by DTA

Abstract DTA curves for the crystallization of the title glasses of various particle size fractions were analysed kinetically using the modified Kissinger method proposed by Matushita and Sakka. The kinetic results were compared with those obtained from microscopic measurements. For the Na2O · CaO · SiO2 glass, it was observed that the kinetic obedience changes from a surface reaction model to a bulk crystallization model with increasing particle size. The values of the apparent activation energy Eapp corresponded excellently to that obtained microscopically, i.e. 343 kJ mol−1. However, the crystallization of Na2O · CaO · SiO2 glass was regulated by the surface reaction mechanism irrespective of the particle size examined; the values of Eapp decreased with increasing particle size but no direct correspondence of these values to those determined by the microscopic measurements were found. The complicated behaviour of Eapp was explained with reference to the role and function of the nucleation at the reaction interface.

Thermoanalytical kinetics for solid state reactions as exemplified by the thermal dehydration of Li2SO4 · H2O

Abstract The fundamental problems in the thermoanalytical kinetics of solid state reactions were investigated as exemplified by the thermal dehydration of Li 2 SO 4 · H 2 O. Distortion of the Arrhenius parameters and their consequent interdependence are described mathematically. A possible way to increase the reliability of the thermoanalytical kinetic study is discussed in connection with the possibility of the establishment of a kinetic standard.

Some thermodynamic aspects of the glassy state

Abstract The classification, identification and stability of a non-crystalline material as a glass is discussed. General thermodynamic and compositional consideration are presented. Thermal characterization of vitrification processes are specified. A survey is given of the theoretical modelling of thermodynamic descriptions of glass transformation. The thermodynamic functions of undercooled liquids during vitrification are explained.

Charge distribution, pressure and composition effects of CuOx based superconductors

Abstract Charge distribution on copper and associated forms of oxygen anions are discussed. Pressure effects on Tc are surveyed for various YBCO, BSCO and TBCO superconductors. Pseudobinary and quasiternary YO1.5-BaO-CuOx phase diagrams are reviewed including pseudobinary cuts between YBa2Cu3Ox, Y2BaCuO5, BaCuO2 and YCuO2.5.

Binary and ternary compounds, phase diagrams and contaminations in the YO1.5 -BaO-CuO system auxiliary to superconducting ceramics

Abstract The binary phases and compatibilities of the BaO-CuO, YO 1.5 -BaO and YO 1.5 -CuO systems are discussed. The different variants of YBa 2 Cu 3 O 7-y , are shown in dependence on y. So far reported existence of the ternary compounds (in the ratio Y:Ba:Cu) 1:3:2,1:4:3,1: :2:4,1:5:3,3:8:5,2:3:5 and 1:4:2 are questioned.The ternary phase diagram and pseudobinary cuts of the YCuO 2.5 -(1:2:3)-BaCuO 2 and Y 5 BaO 8.5 -(1:2:3-2:1:1)-Ba- 3 Cu 5 O 8 are reviewed. The distorting effect of impurities from air (e.g. oxycarbonates formation) and/or from the sample holder (e.g. Pt-substituted compounds) is analysed. On basis of the 1:2:3 structural model hypothetical superconducting compounds 1:1:2, 1:3:4 and 1:2:4 are predicted assuming double layers of. corner-sharing square-planar CuO 4 units.

Thermal treatment and analysis involved in the preparation and investigation of different types of inorganic glasses

The importance of the thermal behaviour of glasses is illustrated. Some procedural characteristics for glass preparation upon quenching are discussed to distinguish the positive effects of increasing cooling rate. The basic thermodynamic quantities, kinetic data and procedural parameters are listed. The glass-transformation interval is treated in detail to demonstrate the temperature-dependences of heat capacity, enthalpy an Gibbs energy for as-quenched and annealed glasses, exemplifying processes of thermally stimulated reordering. Particular attention is paid to DTA measurements, which are of use for the determination of characteristic temperatures and to for the distinction of possible types of processes which occur upon reheating. Most common cases are illustrated by a series of hypothetical ΔH vs. T and ΔTdtavs. T plots. Different glass formation coefficients based on the onset temperatures are discussed to confirm the general knowledge that their maxima match with concentration regions close to that of invariant melting of the system.ZusammenfassungEs wird die Bedeutung des thermischen Verhaltens von Glas verdeutlicht. Einige technologische Kenngrößen der Gasherstellung beim Abschrecken sowie grundlegende thermodynamische Größen, kinetische Daten und technologische Parameter, wurden beschrieben, um bei steigender Abkühlgeschwindigkeit positive Einflüsse erkennen zu können. Zur Unterstreichung thermisch stimulierter Umordnungsprozesse und zur Verdeutlichung der Temperaturabhängigkeit von Wärmekapazität, Enthalpie und freier Enthalpie von abgeschrecktem und kühlgeglühtem Glas wird das Intervall der Glasumformung ausführlich behandelt. Erhöhte Aufmerksamkeit wurde DTA-Messungen geschenkt, um charakteristische Temperaturwerte festzustellen und um eventuelle Typen der Prozesse, die beim Nachhitzen ablaufen, zu erkennen. Die häufigsten Fälle werden durch hypothetische ΔH-T und ΔTdta-T Diagramme illustriert. Verschiedene Glasformationskoeffizienten für die Anfangstemperatur wurden diskutiert, um den allgemeinen Fakt zu bekräftigen, daß deren Maximum mit den Konzentrationsbereichen nonvariant schmelzender Systeme übereinstimmt.РезумеПоказана важность зн ания термического поведения новых стек лообразных материал ов. Обсуждены некоторые методологические характеристики полу чения стекол при резк ом охлаждении, с тем чтоб ы различить положите льные эффекты увеличения с корости охлаждения. В связи с этим приведе ны основные термодин амические величины, кинетическ ие данные и методичес кие параметры. Детально о бработан интервал стеклообразования с тем, чтобы показать температурные завис имости, энтальпии и эн ергии Гиббса для охлажденн ых и оттоженных образ цов стекол, являющихся пр имером процессов тер мически стимулированного пе реупорядочения. Особое внимание было уделено ДТА измерени ям, позволяющим определ ять характеристичны е температуры и различ ать возможные типы пр оцессов, происходящих при пов торном нагреве. Наиболее общ ие случаи представле ны большим числом гипотетическ их графиков в координ атах ΔH-Т и ΔТдтa-Т. Исходя и з начальных температ ур, обсуждены коэффицие нты образования стекол с целью подтверждения того факта, что их максимум сочет ается с концентрацио нными областями, близкими к таковым при инвариан тном плавлении этой систе мы.