Congruent Vaporization Research Articles

Microstructural evolution in self-catalyzed GaAs nanowires during in-situ TEM study

The microstructural evolutions in self-catalyzed GaAs nanowires (NWs) were investigated by using in situ heating transmission electron microscopy (TEM). The morphological changes of the self-catalyst metal gallium (Ga) droplet, the GaAs NWs, and the atomic behavior at the interface between the self-catalyst metal gallium and GaAs NWs were carefully studied by analysis of high-resolution TEM images. The microstructural change of the Ga-droplet/GaAs-NWs started at a low temperature of ∼200 °C. Formation and destruction of atomic layers were observed at the Ga/GaAs interface and slow depletion of the Ga droplet was detected in the temperature range investigated. Above 300 °C, the evolution process dramatically changed with time: The Ga droplet depleted rapidly and fast growth of zinc-blende (ZB) GaAs structures were observed in the droplet. The Ga droplet was completely removed with time and temperature. When the temperature reached ∼600 °C, the decomposition of GaAs was detected. This process began in the wurtzite (WZ) structure and propagated to the ZB structure. The morphological and atomistic behaviors in self-catalyzed GaAs NWs were demonstrated based on thermodynamic considerations, in addition to the effect of the incident electron beam in TEM. Finally, GaAs decomposition was demonstrated in terms of congruent vaporization.

ChemInform Abstract: Thermochemical Model in Kinetics of Heterogeneous Reactions: 100‐Year Jubilee

AbstractReview: 33 refs.

Activation effect in heterogeneous decomposition reactions: fact or fiction?

Analysis of the history of the Arrhenius equation has shown that the substantiation by Arrhenius of the activation model from the van’t Hoff thermodynamic equation was theoretically unjustified. A comparison of the E parameters (‘activation energies’ in the Arrhenius equation) with the molar enthalpies of 54 decomposition reactions, when evaluated according to the thermochemical kinetic approach, i.e. by taking into consideration the congruent dissociative vaporization mechanism, type of regime (equimolar or isobaric) and, in some cases, release of oxygen in the form of free O atoms, has shown that these parameters do not differ in value within random error. This means that the activation effect in heterogeneous reactions is no more than an illusion, and the exponential temperature dependence of the reaction rate actually stems from the analogous T-dependence of the equilibrium constant (as predicted by thermodynamics). The use of the activation model is a primary reason for the crisis in modern thermoanalytical studies of the kinetics and mechanisms of heterogeneous reactions. Chemical thermodynamics and molecular physics have been replaced in these studies by computational mathematics. Science has become a craft. The distressing state of theory and basic research in the field of heterogeneous kinetics requires an urgent discussion of problems by the scientific community.

A mass spectrometric investigation of the vaporisation behaviour in the (U + Pu + O) system

The vaporisation behaviour of (U,Pu)O2 mixed oxides (Pu/M=0.25, 0.50 and 0.75, with M=U+Pu) was studied by means of mass spectrometry. Hyperstoichiometric samples were heated in a Knudsen cell up to T=2300K. The evolution of the uranium and plutonium bearing gaseous species was studied as a function of time in order to evaluate the congruent vapour composition. Ionisation efficiency measurements were performed and the partial pressures of the gaseous species involved in the vaporisation process were determined. The vapour pressure has also been calculated using a thermochemical model for the (U+Pu+O) system. A quasi congruent composition with respect to the O/M ratio has been assumed, in agreement with the experiments. Nevertheless, the evaluation of all the experimental and calculated results shows that a total congruent composition exists for a single composition of the mixed oxide (MOX) samples with a Pu/M content slightly lower than 0.50. A good agreement is obtained between the calculated and experimental vapour pressure data, as well as the quasi congruent vaporisation compositions.

Thermochemical model in kinetics of heterogeneous reactions: 100-year jubilee

The milestones in the development of the thermochemical model (TM) in kinetics of heterogeneous reactions over 100 years after publication in 1913 of the Langmuir equation, \( J\; = \;MP/\sqrt{2\pi MRT} \), were considered. However, the systematic development of TM was begun only in the early 1980s. Among the milestones of this latter period, which passed with the participation of the author, the most important were: the extension of Langmuir equation to dissociative vaporization of compounds and identification of two modes of vaporization (equimolar and isobaric); the discovery of the mechanism of congruent dissociative vaporization; the application of the third-law method to determination of the E parameters (“activation energies” in the Arrhenius equation) and formulation of theoretical and experimental evidence for the absence of activation effect in decomposition reactions. During this period, the new theory was applied to the interpretation of many unsolved problems in heterogeneous kinetics. In the majority of cases, the interpretation is reliably supported by agreement between theory and experiment. The traditional kinetic theories based on the activation effect (the classical Arrhenius theory, as well as the collision and transition state theories) appeared fruitless in this respect.

Interpretation of the kinetic compensation effect in heterogeneous reactions: thermochemical approach

The kinetic compensation effect (KCE), unexplained by the Arrhenius activation model, is interpreted in the framework of the novel thermochemical approach to kinetics of heterogeneous reactions. Unlike the activation model, this theory includes the process of congruent dissociative vaporisation of solids described by the Langmuir quasi-equilibrium equations and consideration of the two different reaction regimes: equimolar and isobaric (with the external pressure of gaseous product, respectively, below and above its equilibrium pressure). Change of regimes is shown to be the reason for the mutual variations of the Arrhenius parameters in the equation: ln A = a + bE. This theoretical interpretation of the KCE is quantitatively supported by experimental observations for the following different types of heterogeneous reactions: high temperature atomisation of metal oxides, low-temperature solid-state decompositions, NiO reduction by H2 and catalytic oxidations of CO and of H2 on PtO2. Fundamental reasons for the shortcomings of the activation model in the interpretation of mechanisms and kinetics of heterogeneous reactions are identified.

ChemInform Abstract: Interpretation of the Kinetic Compensation Effect in Heterogeneous Reactions: Thermochemical Approach

AbstractReview: 192 refs.

Knudsen effusion mass spectrometric determination of the complex vapor composition of samarium, europium, and ytterbium bromides

The vaporization of Sm, Eu, and Yb tri- and dibromides is accompanied by decomposition and disproportionation reactions. These result in complex vapor compositions whose analysis is an intricate problem for experimentalists. Approaches have been developed to interpret mass spectra and accurately determine the vapor composition of thermally unstable compounds. A sector type magnet instrument was used. A combined ion source allowed the study of both the molecular and ionic vapor compositions in the electron ionization (EI) and the thermionic emission (TE) modes. The methodological approaches were based on a joint analysis of the ionization efficiency functions, the temperature and time dependences of the ion currents, and special mathematical data evaluation. The vaporization of SmBr3 , YbBr3 , SmBr2 , EuBr2 , and YbBr2 was studied in the temperature range of 850-1300 K. An initial stage of incongruent vaporization was observed in the case of the tribromides, SmBr2 , and YbBr2 . This eventually changed to a congruent vaporization stage. Various neutral (Ln, Br, Br2 , LnBr, LnBr2 , LnBr3 , Ln2 Br4 , Ln2 Br5 , and Ln2 Br6 ) and charged (Br(-), LnBr3 (-), LnBr4 (-)) species were detected at different vaporization stages. The quantitative vapor composition of Sm, Eu, and Yb tri- and dibromides was determined. It was found that only EuBr2 was stable in the studied temperature range. The developed approaches can be useful in the case of other thermally unstable compounds.

Knudsen Effusion Mass Spectrometric Study of Some Congruently and Incongruently Vaporizing Systems

We have performed KEMS measurements on both congruently and incongruently vaporizing systems. The congruently vaporizing compounds include TeO2(s), MnTe(s), H3BO3(s), and NaBO2(s). The incongruently vaporizing systems include binary tellurides of Fe, Cr, Ni, Mo, Mn, and the ternary Mn-Te-O. The various aspects concerning the vaporization behavior of these systems will be presented in this paper.

Toward a general theory of heterogeneous reactions

The thermochemical approach, using the quasi-equilibrium methodology of kinetic analysis and the concepts of equimolar and isobaric reaction modes (regimes), has been used to prove the common mechanism of the representative heterogeneous reactions: decompositions of CaCO3 and of Ag2O, reduction of NiO by H2, and the catalytic oxidation of CO and H2 on PtO2. All these rate processes have been analyzed using the congruent dissociative vaporization of the participating solid reactants or catalysts and are described by similar mechanistic schemes. The possible role of atomic oxygen evolution in oxidation catalysis is discussed. Quantitative proofs of the common mechanism of these reactions were provided by agreement between calculated reaction enthalpies and the Arrhenius E parameters, the retardation effect of gaseous products on the reaction rates, and the accordance between experimental and theoretical ratios of isobaric to equimolar values of the E parameter. In conclusion, milestones in the history of the thermochemical approach during the last century are discussed.

Cluster type of silicate vaporization: Newly obtained experimental data

Impact cratering is usually associated with the partial or complete vaporization of the high-temperature impact melts. According to its chemical characteristics, the vaporization of major oxides, silicate minerals, and rock melts can be classified into the following four types: (1) congruent vaporization without decomposition of the compound in the vapor phase, (2) congruent vaporization with the decomposition of the compound in the vapor phase, (3) incongruent vaporization, and (4) cluster vaporization. The latter type of vaporization pertains to the transfer of material into vapor phase in the form of complicated atomicmolecular groups (clusters) of certain stoichiometry. Cluster vaporization takes place at superhigh temperatures typical of impact processes. The clusters can comprise compounds of different individual volatility, and this often results in the enrichment of the vapor phase in elements traditionally thought to be refractory. Examples of cluster vaporization are offered by lately obtained experimental results on laser-pulse vaporization of larnite, merwinite, and wollastonite. Condensed vapor generated at the vaporization of orthosilicates (larnite and merwinite) was proved to be dominated by chain bonds of Si-O tetrahedrons and to contain molecular groups of wollastonite and pseudowollastonite stoichiometry.

Thermodynamic approach to the vaporization and growth phenomena of SiC ceramics. I. SiC and SiC–SiO2 mixtures under neutral conditions

Abstract Matter transport by vaporization and condensation processes during sintering or consolidation of SiC components at high temperature is analysed using thermodynamics of the binary Si–C and ternary Si–C–O systems. The erosion flows due to vaporization and the potential growth flow of SiC are calculated in order to determine the conditions prevailing at the surface of SiC powder grains. Pure SiC vaporization leads to rapid precipitation of carbon at the SiC surface. Vaporization of SiC–SiO 2 mixtures under neutral atmospheric conditions or absolute vacuum contributes to the rapid departure of any Si or C impurities first of all, and then silica according to congruent vaporization in the SiC–SiO 2 pseudo-binary system. The calculated SiC growth rate by vapour transport is always less than the erosion rate and further subsequent growth of pure SiC cannot be obtained as long as silica co-exists with SiC.

The mechanism and kinetics of NiO reduction by hydrogen

The thermochemical approach to analysis of thermal decomposition of solids, developed earlier by L'vov, is extended here, for the first time, to interpret the kinetics and mechanism of the reduction of an oxide (NiO) by a gas (H2). This approach is based on the mechanism of congruent dissociative vaporization of the reactant, Langmuir kinetics and determination of the Arrhenius E parameter by the third-law method. The calculated enthalpy of the reaction is in good agreement with the experimentally measured E value. Many other mechanistic and kinetic features of the reaction are explained within the framework of the given theoretical approach. These include: the formation of metal nuclei; the initial autocat- alytic behavior; the formation of nanocrystalline structure of the reduced metal product; the equimolar and isobaric modes of reduction; the dependence of reduction rate on hydrogen pressure; the more than twofold decrease of the E parameter with the extent of reaction a, and the sys- tematic increase of E with temperature.

Kinetics of Congruent Vaporization of ZnO Islands

We examine the congruent vaporization of ZnO islands using insitu transmission electron microscopy. Correlating quantitative measurements with a theoretical model offers a comprehensive understanding of the equilibrium conditions of the system, including equilibrium vapor pressure and surface free energy. Interestingly, the surface energy depends on temperature, presumably due to a charged surface at our specific condition of low P and high T. We find that the vaporization temperature decreases with decreasing system size, a trend that is more pronounced at higher T. Applying our results of island decay towards the growth of the ZnO provides new insights into the cooperative facet growth of anisotropic nanocrystals.

Theory of solid-state decomposition reactions: A historical essay

The history of the formation and development of the theory of solid-state decomposition reactions, based on the mechanism of congruent dissociative vaporization of a solid with simultaneous condensation of the supersaturated vapor of the low-volatility product, is described here in the form of recollections. The review covers a 30-year period (1981–2010), beginning with basic experimental studies in the decomposition process by electrothermal atomic absorption spectrometry (ETAAS) and quadrupole mass spectrometry (QMS) and ending with measurements of the decomposition kinetics by thermogravimetric analysis. Some remarks and recommendations based on the author's participation in two long-term projects, the development of the ETAAS method and that of the theory of solid-state reactions, are formulated in the conclusion.

Impact of the gaseous environment on the kinetics of solid-state decompositions

Abstract The main purpose of this study is to review the current state of the problem of the impact of gaseous environment on the kinetics of solid-state decompositions. Three different theoretical approaches to the interpretation of the decomposition kinetics have been considered. As it follows from the literature published over the past 80 years, the Arrhenius and Knudsen–Langmuir approaches based on the assumption of two different reaction mechanisms (congruent and incongruent) could not solve the problem. At the same time, successes in the application of the thermochemical approach that is based on the assumption of a unitary congruent dissociative vaporization mechanism with condensation of oversaturated vapor remain unnoticed by the TA community. Taking into account this situation, the author has outlined the key points of the thermochemical kinetics in a compact but rigorous and complete form once more. The revised kinetic equations for the different modes of decomposition, several important inter...

The mechanism of solid-state decompositions in a retrospective

This is an overview of the early investigations into the mechanism of solid decompositions since the fundamental studies by Ostwald on catalysis during 1890–1902 and the first experimental study of the autocatalytic decomposition of Ag2O by Lewis in 1905. In order to explain the formation mechanism of the solid product, Volmer suggested in 1929 that the decomposition of Ag2O includes two sequential stages: first, a thermal decomposition of the oxide into gaseous silver atoms and oxygen molecules and second, the condensation of the supersaturated silver vapor. This revolutionary idea was immediately used by Schwab to explain the autocatalytic peculiarity of solid-state decomposition reactions. However, this mechanism did not receive the acceptance of the scientific community. On the contrary, as can be seen from the results presented at the conference “Chemical reactions involving solids” in Bristol in 1938, this model was dismissed as unrealistic and, as a result, was since forgotten. Instead, considerable attention at this conference was devoted to the disorder theory proposed earlier by Wagner and Schottky, and to the mechanism of ion transport in the solid crystals. During the subsequent 70 years, decomposition mechanisms have been interpreted, without visible progress, on the latter basis. The mechanism of congruent dissociative vaporization proposed independently in 1990 turned out to be in complete agreement with the Volmer–Schwab model. It has been treated with the same distrust. These historical events, in the author’s opinion, are responsible for the prolonged stagnation in the development of solid-state decomposition theory.

Vapor Pressure Measurements of Sc(tmhd)3 and Synthesis of Stabilized Zirconia Thin Films by Hybrid CVD Technique Using Sc(tmhd)3, Zr(tmhd)4, and Al(acac)3 [tmhd, 2,2,6,6-tetramethyl-3,5-heptanedione; acac, 2,4-pentanedione] as Precursors

The congruent vaporization of Sc(tmhd)3 was verified from the molar mass of the vapor species obtained using ESI-mass spectra and thermogravimetry. On the basis of the molar mass so obtained, the e...

Thermodynamical Considerations on Congruent Vaporization Behavior

Thermodynamical Considerations on Congruent Vaporization Behavior

Decomposition of KMnO4 in different gases as a potential kinetics standard in thermal analysis

The assumption that potassium permanganate may serve as a kinetics standard in solid decomposition kinetics made a priori on the basis of the mechanism of the congruent dissociative vaporization of KMnO4 and its crystal structure was successfully supported experimentally. As expected, the decomposition rate of KMnO4 does not depend on the kind of foreign gas (He, air, CO2 and Ar) and on the measurement technique (isothermal or dynamic). Other requirements for KMnO4 as an ideal kinetics standard are satisfied as well. The use of the third-law method for determining the molar enthalpy of a reaction \( \left( {\Updelta_{\text{r}} H_{\text{T}}^{\text{o}} / \nu } \right) \) provides an excellent reproducibility of results. The mean value of \( \Updelta_{\text{r}} H_{\text{T}}^{\text{o}} / \nu \) from 12 experiments in different gases is 138.3 ± 0.6 kJ mol−1, which coincides with the value of 138.1 kJ mol−1 calculated from the isothermal measurements in different gases by the second-law method. As predicted by theory, the random errors of the second-law and Arrhenius plot methods are 10–20 times greater. In addition, the use of these methods in the case of dynamic measurements is related to large systematic errors caused by an inaccurate selection of the geometrical (contraction) model. The third-law method is practically free of these errors.