D.W. Johnson

The effects of sample size and heating rate on the kinetics of the thermal decomposition of CaCO3

Abstract Isothermal and dynamic methods were used to study the rate of weight loss of CaCO3. Sample sizes were controlled in the range of 1–32 mg. The contracting area rate law proved universally applicable. A pronounced dependence of the activation enthalpy, pre-exponential term, and rate constant upon sample weight and heating rate was observed. This dependence is discussed primarily in terms of the effects of self-cooling and sample geometry. The concept of a unique activation energy is questioned.

Kinetics of the thermal decomposition of CaCo3 in Co2 and some observations on the kinetic compensation effect

Abstract The kinetics of the thermal decomposition of CaCO 3 in CO 2 were investigated using both dynamic and isothermal techniques. Values of apparent activation energies range from 200–1000 kcal mol −1 depending upon sample size and heating rate. It is concluded that thermal transport rather than mass transport or chemical processes is rate determining. The results are compared with earlier work in O 2 and discussed in terms of “the kinetic compensation effect”, i.e., the reported linear relationship between the logarithm of the preexponential term and the activation energy derived from the Arrhenius equation.

Effect of thermal transport mechanisms on the thermal decomposition of CaCO3

Abstract Isothermal and dynamic techniques were employed to examine the rate of weight loss of CaCO 3 . Thermogravimetric studies were conducted in atmospheres of He, N 2 , Ar, and various percentage of CO 2 in Ar. Three methods for deriving kinetic parameters from thermogravimetric data were used and these results were then compared with data obtained from isothermal investigations done on identical samples. It was found that the higher the thermal conductivity of the atmosphere, the more repidly the reaction proceeded. Also, as the percentage of CO 2 in Ar increased, the temperature range of the decomposition became higher and narrower, resulting in a higher activation energy.

Kinetics of the formation of BaSnO3 from barium carbonate and tin(IV) oxide or oxalate precursors

Abstract Various physical mixtures of BaCO 3 with SnO 2 react following rate laws determined by contracting geometry with an activation energy of 55–70 kcal/mole. Ba 2 SnO 4 is an intermediate in the formation of BaSnO 3 . In contrast, BaSn(C 2 O 4 ) 2 · 0.5H 2 O and Ba 2 Sn(C 2 O 4 )·6H 2 O decompose to yield an intimate mixture of highly reactive BaCO 3 and SnO 2 which subsequently reacts at a relatively low temperature to form BaSnO 3 directly, without intermediates, according to the second-order rate law, with a similar activation energy.

Kinetics of the thermal decomposition of aqueous manganese(II) nitrate solution

Abstract Isothermal weight studies were performed upon aqueous solutions of manganese(II) nitrate. The loss of water followed a first order rate law with activation energies varying from 6 to 10 kcal/mole depending upon the atmosphere. The resulting anhydrous nitrate decomposed to form manganese(II) oxynitrate in accordance with the Erofeev equation with n = 4. The activation energies are 20–25 kcal/mole and show little variation with atmosphere. The resulting oxynitrate decomposed to form manganese(IV) oxide. The rate fits several expressions satisfactorily but a contracting area form is chosen as generally best. The activation energy is about 12 kcal/mole lower in the presence of moisture but this effect upon the rate is partially offset by a substantial decrease in the preexponential term. The manganese(IV) oxide loses oxygen to form manganese(III) oxide. The best rate law is the Jander equation and activation energies are in the range of 47–57 kcal/mole.

The reduction of alkali substituted LaMnO3

Abstract In order to determine the stability of some potential NO x reduction catalysts (La 0.8 M 0.2 MnO 3 , M  Na, K, Rb) the accelerated reduction of these catalysts in H 2 , N 2 atmospheres was studied. La 0.8 K 0.2 MnO 3 goes through a reversible oxygen loss at about 350°C corresponding to the reduction of the available Mn 4+ to Mn 3+ in H 2 , N 2 atmospheres. By reduction at higher temperatures a previously unreported phase La 2 MnO 4 is formed. The most reducing conditions (10% H 2 in N 2 , >940°C) formed only La 2 O 3 and MnO. Between 700 and 880°C in 10% H 2 in N 2 potassium was eliminated from the sample by reduction to the metal and evaporation. Analogous results were found for Na and Rb substituted LaMnO 3 except that the intermediate phase La 2 MnO 4 was not observed in the reduction of La 0.8 Rb 0.2 MnO 3 .

Kinetic analyses of consecutive solid-state decompositions illustrated with iron(II) sulfate

Abstract Use of consecutive reaction analysis for the derivation of kinetic parameters is demonstrated for the solid state decompositions which often include fractional orders of reaction. The differential equations describing the relative amounts of reactants and products during an isothermal decomposition are solved numerically by machine. Unlike previously used methods for solid-state decompositions there is no need to assume that the rate constant of the first step is much greater than that of the second. Also, the technique allows the interpretation of experimental data throughout the entire range of the reaction. The treatment was applied to the two-step decomposition of iron(II) sulfate in a dry nitrogen atmosphere. The results show the data to fit consecutive half-order reactions.

Rate of solvent loss from spherical droplets of solutions illustrated with aqueous manganese(II) nitrate

Abstract The simple first order rate law applicable to the dehydration of aqueous solutions under conditions of constant surface area is modified to allow for the simultaneous change in surface area accompanying such a reaction. A spherical droplet is used as a model. Isothermal weight loss experiments were performed using aqueous solutions of mangane(II) nitrate to show the applicability of such a hybrid rate law.

Forming methods for high Tc superconductors

Abstract This paper summarizes current research on a variety of ceramic forming techniques which have been applied to bulk, high Tc superconductors. The processing methods can be divided into two major classes depending on whether or not the melting point in the material is exceeded. Growth from melts can result with a directional grain growth which improves the links between the grains. However, even a short duration above the melting point can improve the grain-to-grain coupling, and usually result in improved transport properties. In what follows we outline the shapes produced by each forming technique as well as the resulting material properties.

The conversion of kinetic data taken at constant time increments to constant reaction increments

Abstract Modern methods of data acquisition for studying the kinetics of solid state reactions often result in data on the degree of completion of reaction at equally spaced time intervals. Unless the reaction rate is constant with time, this leads to kinetic results which are usually weighted in favor of the latter portions of the reaction. We illustrate methods for converting such data to avoid this effect, but show that for well-behaved systems such as the decomposition of CaCO 3 , these precautions do not significantly affect the kinetic results. Conditions are outlined where these techniques should be used to avoid error.