Analysis Of Solid-state Reactions Research Articles

Solid-state synthesis, rotatable magnetic anisotropy and characterization of Co1-xPtx phases in 50Pt/50fccCo(001) and 32Pt/68fccCo(001) thin films

We reported the phase formation sequences in 50Pt/50fcc-Co(001) and 32Pt/68fcc-Co(001) thin films after annealing up to 850 °C. In both cases, the ordered L10 phase formed first on the Pt/Co interface at ∼400 °C and as the annealing temperature increased the L10 phase transformed into the chemically disordered fcc A1 phase in 50Pt/50fcc-Co(001) at 750 °C and in 32Pt/68fcc-Co(001) films at 550 °C. Based on the analysis of solid-state reactions in thin films, a phase transition at ∼ 400 °C is predicted in Co-Pt systems with a 32–72% Pt composition. Torque measurements of the 50Pt/50fcc-Co(001) samples showed that the rotatable magnetic anisotropy coexisted with the three variants of L10 in a temperature range of 400–750 °C. An analysis of the torque curves revealed that the L10 films consist of a soft magnetic layer epitaxially intergrown to the substrate MgO(001) and a top layer having rotatable magnetic anisotropy. It showed that the magnetically hard properties of L10 films are associated with a rotatable magnetic anisotropy layer. A model of rotatable magnetic anisotropy is reasoned, which is founded on some identical mechanisms of rotatable magnetic anisotropy and magnetic-field-induced strains, explaining the ferromagnetic shape-memory effect in Heusler alloys. Our results suggested that the rotatable magnetic anisotropy phenomena may have an important role in the origin of perpendicular anisotropy in hard magnetic L10 structures.

Analysis of solid-state reaction in the performance of doped calcium manganites for thermal storage

Solid-state reaction is a potentially cost-effective route to obtain perovskite-type materials suitable for thermochemical energy storage (TCES). This work analyses the influence of the synthesis conditions on a doped calcium manganite, CaCr0.1Mn0.9O3-δ, since it has been identified as a promising material for TCES. The macrostructure shows noticeable dependence with mixing time of precursors. The perovskite, with general chemical formula ABO3, possesses secondary phases for all the synthesis conditions explored, as confirmed by X-ray diffraction analysis. The dopant effect of chromium on the crystallographic lattice is studied by the Rietvel refinement method from the comparison with CaMnO3-δ compound structure. Customized non-isothermal and isothermal experiments are performed via thermogravimetric analysis and differential scanning calorimetry (TGA/DSC). From these experiments, an oxidation temperature of around 900 °C; energy storage density higher than 150 J/g; reaction reversibility and stable cycling performance are obtained, facts that reflect a high potential for application in concentrating solar power plants.

From the Kissinger equation to model-free kinetics: reaction kinetics of thermally initiated solid-state reactions

Recently the term, “model-free kinetics” has increasingly been used in thermoanalytical studies. It is noteworthy that even without knowledge of the concrete reaction mechanism of a material transformation, “model-free kinetics” can be used to determine relevant kinetic parameters such as activation energy ΔEa, preexponential factor A, and reaction rate constant k. An obvious thought that comes to mind is to combine techniques of the thermal analysis with the classical chemical reaction kinetics. Concerning the investigation of classical reaction kinetics, a series of isothermal experiments is necessary to take the time-to-yield ratio of any chemical reaction into account. Based on an introduced mechanism of the chemical reaction, the reaction rate constant k is determined for a series of temperatures T, i.e. k becomes a function of temperature; k = k(T). Generally, the examined substances are solids (e.g. polymers, plastics) and their thermal conversions are heterogeneous reactions at an interface. Would it, therefore, not be simpler to use non-isothermal procedures of thermal analysis for the kinetic analysis of solid state reactions?

Kinetic analysis of solid state reactions without neglecting temperature integral at starting temperature: relationship between conversion degree at maximum reaction rate and reaction mechanism

In the conventional kinetic analyses of solid state reactions studied during linear heating, the temperature integral at the starting temperature is usually neglected. In this paper, an analytical equation is mathematically derived without neglecting the temperature integral at the starting temperature of the linear heating, which describes the relationship between the conversion degree at the maximum reaction rate and the reaction mechanism. This relationship can be expressed as f′(α m)g(α m)=— γ/ζm2eγ/ζm ∫ζm e–γ/ζdζ. It is shown that the conversion degree at the maximum reaction rate is dependent on the reaction mechanism and two dimensionless variables: ζ m=T m/T 0 and γ=E/RT 0.

Applications of Differential Scanning Calorimetry on Materials Subjected by Severe Plastic Deformation

Differential Scanning Calorimetry (DSC) is a thermal analysis technique that measures the energy absorbed or released by a sample as a function of temperature or time. DSC has wide application for analysis of solid state reactions and solid-liquid reactions in many different materials. In recent years, DSC has been applied to analyze materials and alloys processed through Severe Plastic Deformation (SPD). The basic principle of SPD processing is that a very high strain is introduced into materials which achieve significant grain refinement and improve properties of materials. This review paper presents some recent examples of the applications of DSC for materials subjected to SPD, especially by Equal-Channel Angular Pressing and High-Pressure Torsion.

Kinetic analysis of solid-state reactions: errors involved in the determination of the kinetic parameters calculated by one type of integral methods

The integral methods are extensively used for performing the kinetic analysis of solid-state reactions. As the Arrhenius integral function p(u) does not have an exact analytical solution, many approximations have been proposed. One popular type of approximations is called the exponent approximation which can be put in the form $$p(u) = e^{a+b\,ln\,u+cu}$$ . In this study, a systematic analysis of the errors involved in the determination of the kinetic parameters calculated by the integral methods based on the exponent approximations for p(u) has been carried out. The results have shown that the precision of the kinetic parameters computed from the integral methods analyzed in this paper depends on u and the errors of the kinetic parameters determined from Doyle approach are the largest.

Experimental test to validate the rate equation “d α/d t = kf( α)” used in the kinetic analysis of solid state reactions

In order to improve the soundness of the kinetic analysis of solid-state reactions, we present an experimental test based on a sudden change (jump), during an experiment, of the temperature, or of the partial pressure of gases, which have an influence on the kinetic rate. This test, denoted “ f( α) test” allows to discriminate if the kinetic modelling may or may not involve the rate equation d α/d t = kf( α) for any kind of reaction: thermal decomposition, solids reacting with gases, … Numerous examples are given and discussed according to the answer of the test, mainly based on the consideration of nucleation and growth processes.

Stocktaking in the kinetics cupboard

Some of the continuing problems of kinetic analysis of solid-state reactions are reviewed and some recent advances towards their solution are discussed. Attention is drawn to the important findings of an ICTAC Kinetics Project that need consideration by authors and editors of kinetics papers.

Analysis of Aluminium Based Alloys by Calorimetry: Quantitative Analysis of Reactions and Reaction Kinetics

AbstractFor Abstract see ChemInform Abstract in Full Text.

Formation of embedded Co nanoparticles by reaction in Al/Co multilayers and impact on phase sequence

The analysis of solid-state reaction in Al/Co multilayers shows that, after the first reaction step which is the formation of Al9Co2, the remaining Co “layer” can either be continuous or formed of Co nanoparticles. Simultaneously with this formation of Co nanoparticles we observe a bifurcation in the phase sequence, the second reaction product is different and its formation takes place at different temperatures. Thermodynamic considerations, namely modification of free energy by curvature and stress are used to explain that the change in the phase sequence is a result of the nanostructuration of the Co layer.

Application of SCTA methods for kinetic analysis of solid state reactions and synthesis of materials

The advantages of the sample controlled thermal analysis (SCTA) for performing the kinetic analysis of solid state reaction as compared with conventional non-isothermal methods order, diffusion and formation and growth of nuclei (AvraminErofeev) controlled reactions have been analysed. It has been shown that the shape of the α-T plots recorded under a strictly constant reaction rate (CRTA) is characteristic of the kinetic model obeyed by the reaction. Thus, it would be possible to discern among boundary n just from the inspection of the shape of a single CRTA plot. The α vs. T plots of en orderi reactions have a concave shape while the α-T plots corresponding to diffusion controlled processes are sigmoid shaped with an inflection point at a particular value of α characteristic of every particular diffusion model (D2 ,D 3 or D4). The α-T profiles of reactions fitting the AvraminErofeev kinetic model start with a rise in temperature until reaching the preset reaction rate. This step is immediately followed by a temperature fall (until reaching a determined value of the reacted fraction (αn) that is depending on the value of the n coefficient in the AvraminErofeev kinetic equation) and again a temperature rise once the corresponding αn value is attained. It has been shown that the value of αn exactly agree with the value of the reacted fraction at which the reaction rate reach a maximum under isothermal conditions. This fact suggests that the part of the α-T profile in which the curve back on itself would correspond to the acceleratory period in which the total surface area of the growing nuclei would be increasing leading to an acceleration of the reaction that would be offset by a diminution of the temperature. The later rising temperature stage would correspond to the decay period in which the growing nuclei overlap among themselves with the corresponding decrease of the interface reaction area that would lead to a decrease of the reaction rate that must be compensated by a rise in temperature in order to maintain the reaction rate constant. A comparison of (CRTA) and stepwise thermal analysis (SIA) has been carried out from simulated curves. It has been shown that in the case of en orderi and diffusion controlled reactions SIA curves approach to CRTA traces as the lower and upper reaction rate thresholds becomes closest. However, the shape of the SIA α-T plots of solid state reactions following an AvraminErofeev mechanism is quite different of

A unified theory for the kinetic analysis of solid state reactions under any thermal pathway

The Ozawa concept of generalized time has been used for developing master plots for the different kinetic models describing solid state reactions. These plots can be indistinctly used for analysing isothermal or non-isothermal experimental data. It is demonstrated that it is not possible to discriminate the kinetic model from a single non-isothermal curve without a previous knowledge of the activation energy. However, it has been shown that the ln [(da/dt)/f(a)] data taken from a set of DTG curves obtained at different heating rates lie on a single straight line when represented as a function of 1/T only if the kinetic model really obeyed by the reaction is considered. Moreover, the true values of E and A are obtained from the slope and the intercept of this straight line.

X-ray Spectroscopic Analysis of Solid State Reaction during Mechanical Alloying of Molybdenum and Graphite Powder Mixture

Molybdenum and graphite powders were mechanically alloyed. Carbon K X-ray emission spectra of the mechanically alloyed powders were measured using electron probe microanalyzer (EPMA) in order to investigate the solid state reaction process. In the early stage of the mechanical alloying (∼36ks), graphite did not react with molybdenum, but particle size of graphite became smaller. In the next stage of mechanical alloying (36 ∼ 144 ks), graphite react with molybdenum gradually as the time increases. Molybdenum carbides were formed on mechanical alloying for 288 ks. The mechanically alloyed powders for 288 ks were heat-treated in a vacuum. Mo-33 at%C system heat-treated at 1273 K was a mixture of Mo2C and molybdenum, while Mo-50at%C system heat-treated was a mixture of Mo 2 C, mechanically ground graphite and graphite.

On the Li and Tang's isoconversional method for kinetic analysis of solid-state reactions from thermoanalytical data

A criterion for the reaction mechanism as expressed by differential conversion function based on the Li and Tang's isoconversional method is suggested. The suitability of the use of Li and Tang's method for the estimation of a conversion dependent activation energy is discussed.

X–ray Spectroscopic Analysis of Solid State Reaction during Mechanical Alloying

Niobium or tungsten and graphite powders were mechanically alloyed. Carbon K X–ray spectra of the mechanically alloyed powders were measured using an electron probe microanalyzer (EPMA) in order to investigate the solid state reaction process. In the early stage of the mechanical alloying (10 hours), graphite did not react with metal, but particle size of graphite became smaller. In the next stage of the mechanical alloying (2–4 hours), micro–crystalline graphite powder reacted with metal. Polarization of the X–rays were measured for Nb 2C and W 2C, because a single crystal WC emitted polarized X–rays. The measurement of X–ray emission spectra was useful for the structural and chemical analysis of mechanical alloying reaction processes.

Analysis of solid state reaction at the interface of yttria-doped ceria/yttria-stabilized zirconia

Interfacial reaction between yttria-stabilized zirconia (YSZ) and yttria-doped ceria (YDC) phases was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD analysis after heating at 1300°C indicated that the solid state reaction between YDC and YSZ accompanied a significant shift of the zirconia lines to lower angle. The lines from the ceria phase were only weakened with the reaction. The reaction completed to give a single cubic fluorite phase of Y-doped zirconia–ceria (YCZ) solid solution after heating at 1300°C or higher temperatures. A Pt marker was inserted at the original interface between YSZ and YDC to observe the process of the solid state reaction. From SEM and EDX analyses, the gradated compositional changes were observed in the zirconia side from the Pt marker after heating at 1700°C. Sintered pellets of YDC and YSZ couple showed Kirkendall-void along the interface of the YDC side because of the preferential migration of the Ce component to the zirconia side.

Kinetic Analysis of Solid State Reactions by Means of Stepwise Isothermal Analysis (SIA) and Constant Rate Thermal Analysis (CRTA) A comparative study

The results obtained in this work point out that both SIA and CRTA traces, simulated assuming the same values of the reaction rate, are identical provided that either 'n order’ or diffusion controlled kinetics are concerned. The α-T CRTA plots of solid state reactions fitting an Avrami-Erofeev kinetic model show that the temperature decreases with increasing α until reaching a minimum at a value of the reacted fraction α = αmin characteristic of the Avrami-Erofeev exponent. The shapes of the corresponding SIA diagrams are quite different and a very long isothermal step is obtained. Moreover, the reaction rate is not maintained constant during the isothermal period but it shows a maximum at a value of α depending on the Avrami-Erofeev coefficient. It is important to point out that this αmax value agrees with the corresponding αmin calculated from a CRTA curve by assuming the same Avrami-Erofeev exponent.

Kinetic Analysis of Solid-State Reactions: An Improved Analysis Method

AbstractMany reactions of interest occur in the solid state, or with low mobility of the reactants or products, with the result that the physical mechanisms operative during the reaction determine the shape of the reaction rate-time profile; i.e. topochemistry is important. However, often improper account is taken of the physical mechanisms, resulting in erroneous values for the activation energy and kinetic constants.An improved method of analysis is suggested, allowing the mechanisms of reaction to be determined explicitly. This thus allows the activation energy and kinetic constants to be determined with good theoretical justification. The method has been verified by studying the thermal decomposition of barium azide, where the activation energy was determined to better than 1%. The operative mechanisms so determined are in agreement with visual observations and the literature.

Influence of pressure on the shape of TG and controlled transformation rate thermal analysis (CRTA) traces

The influence of pressure on the shape of both TG and controlled transformation rate thermal analysis (CRTA) traces is theoretically and experimentally illustrated. It is shown that this shape can be explained on the basis of kinetic considerations and consequently thermoanalytical data can be used to perform kinetic analysis of solid state reactions contrary to Paulik's statement. However it has been shown that CRTA traces are more sensitive than conventional non-isothermal curves to the effect of pressure of the gases self-generated in the reaction. This may be considered as an interesting argument to explain the advantages of the CRTA technique for discriminating between overlapping processes.

Analysis of solid state reactions at film/substrate interfaces by electron microscopy

Heterogeneous thin film solid state reactions are a matter of topical interest, under both fundamental and applied aspects. The forming phases are in the nanometer size range; the structure and morphology of the involved solid/solid interfaces exerts a strong influence on the reaction kinetics and on the phase formation sequence. Thus electron microscope methods are essential, if the characteristics of film/substrate reactions are to be investigated. Results from a number of electron microscope methods, among them high-resolution and in situ TEM of cross sections, bright- and dark-field diffraction contrast TEM, energy-dispersive X-ray microanalysis and selected area electron diffraction have been used during investigations of interfacial reactions in three thin film systems: TiN/MgO, NiSi2/Si and TiO2/MgO. In addition to the indentification and characterization of the new phases formed, valuable information has been obtained on reaction mechanisms, interfacial kinetic reaction barriers and on the influence of stress on the phase formation.