The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

Controlled rate thermal analysis commanded by mass spectrometry for studying the kinetics of thermal decomposition of very stable solids

A quadrupole mass spectrometer coupled to a thermobalance has been used to develop a controlled rate thermal analysis (CRTA) equipment. This instrument maintains constant the partial pressure of the gas generated in a solid–gas reaction during the entire process while recording the weight loss along the process. This system permits to perform the kinetic analysis of the thermal decomposition of very stable compounds with a very low equilibrium pressure in the temperature range at which the reaction takes place. The performance of the equipment was checked by studying the kinetics of the thermal decomposition of BaCO₃ using for the control the CO₂ partial pressure. Both the activation energy and the reaction kinetic model has been calculated. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 184–192, 2002; DOI 10.1002/kin.10042     

Kinetics of free-surface decomposition of magnesium and barium sulfates analyzed thermogravimetrically by the third-law method

The application of the third-law method to the determination of the E parameters in cases of free-surface decomposition of MgSO₄ and BaSO₄ in vacuum and investigation of the effect of oxygen in reactor atmosphere on the rate of MgSO₄ decomposition allowed to support the dissociative evaporation mechanism of decomposition of these sulfates with releasing of atomic oxygen as one of the primary products. The experimental values of the E parameter for MgSO₄ and BaSO₄ are equal to 335.7±1.7 and 411±4 kJ mol⁻¹. The serious difference in decomposition temperatures in vacuum (about 1000 and 1400 K) and the E parameters for these two sulfates is connected with the difference in transfer of condensation energy of oxides to the reactants. In cases of MgSO₄ and BaSO₄, the values of τ parameter are 0.42 and 0. Dependence of the τ parameter on the ratio of the equivalent pressure to the saturation pressure of corresponding metal oxide has been revealed from a comparison of τ parameters reported in the literature for eight different reactants. This correlation is of prime consideration for understanding of the mechanism of consumption of the condensation energy by the reactant.     

Tribochemistry and kinetics of Al2(SO4)3·xH2O decomposition

The thermal decomposition of tribochemically activated Al₂(SO₄)₃·xH₂O was studied by TG, DTA and EMF methods. For some of the intermediate solids, X-ray diffraction and IR-spectroscopy were applied to learn more about the reaction mechanism. Thermal and EMF studies confirmed that, even after mechanical activation of Al₂(SO₄)₃·xH₂O, Al₂O(SO₄)₂ is formed as an intermediate. Isothermal kinetic experiments demonstrated that the thermochemical sulphurization of inactivated Al₂(SO₄)₃·xH₂O has an activation energy of 102.2 kJ·mol^?1 in the temperature range 850–890 K. The activation energy for activated Al₂(SO₄)₃·xH₂O in the range 850–890 K is 55.0 kJ·mol^?1. The time of thermal decomposition is almost halved when Al₂(SO₄)₃·xH₂O is activated mechanically. The results permit conclusions concerning the efficiency of the tribochemical activation of Al₂(SO₄)₃·xH₂O and the chemical and kinetic mechanisms of the desulphurization process.     

Kinetic analysis of the thermal stability of lithium silicates (Li4SiO4 and Li2SiO3)

The kinetics describing the thermal decomposition of Li₄SiO₄ and Li₂SiO₃ have been analysed. While Li₄SiO₄ decomposed on Li₂SiO₃ by lithium sublimation, Li₂SiO₃ was highly stable at the temperatures studied. Li₄SiO₄ began to decompose between 900 and 1000 °C. However, at 1100 °C or higher temperatures, Li₄SiO₄ melted, and the kinetic data of its decomposition varied. The activation energy of both processes was estimated according to the Arrhenius kinetic theory. The energy values obtained were −408 and −250 kJ mol⁻¹ for the solid and liquid phases, respectively. At the same time, the Li₄SiO₄ decomposition process was described mathematically as a function of a diffusion-controlled reaction into a spherical system. The activation energy for this process was estimated to be −331 kJ mol⁻¹. On the other hand, Li₂SiO₃ was not decomposed at high temperatures, but it presented a very high preferential orientation after the heat treatments.     

Rate Coefficient for Self-Association Reaction of CF2 Radicals Determined in the Afterglowof Low-Pressure C4F8 Plasmas

We have analyzed decay kinetics of CF₂ radicals in the afterglow of low-pressure, high-density C₄F₈ plasmas. The decay curve of CF₂ density has been approximated by the combination of first- and second-order kinetics. The surface loss probability evaluated from the frequency of the first-order decay process has been on the order of 10^–4. This small surface loss probability has enabled us to observe the second-order decay process. The mechanism of the second-order decay is self-association reaction between CF₂ radicals (CF₂+CF₂C₂F₄). The rate coefficient for this reaction has been evaluated as (2.6–5.3)×10^–14 cm³/s under gas pressures of 2 to 100 mTorr. The rate coefficient was found to be almost independent of the gas pressure and has been in close agreement with known values, which are determined in high gas pressures above 1 Torr.     

Kinetics of the oxygen transfer reaction at the (U0.5Sc0.5)O2±x/YSZ interface by impedance spectroscopy

The complex impedance dispersion analysis technique was used to study the electrode kinetics of (U_0.5Sc_0.5)O_2±x, a fluorite type solid solution material potentially suitable as electrode for low temperature oxygen sensors. Variables included the temperature and oxygen partial pressure. The effect of heat treatment on the interfacial contact resistance and the electrode morphology was also investigated. A single are for the electrode reaction was observed over most of the experimental ranges of temperature and oxygen partial pressure. The angle of depression of the electrode are was small (8–18°) compared with platinum electrodes (20–45°). The activation energy for the overall electrode reaction was between 170 and 180 kJ mol^?1. The average value for the pressure exponent, determined from the oxygen partial pressure dependence of the electrode resistance, was 0.16. A mechanism for the oxygen transfer reaction is proposed. Materials of this type show promise for future use in low temperature oxygen sensors.     

Investigation on The Thermal Properties of Fe2O(SO4)2. Part I

The data on the thermal decomposition of FeSO₄?H₂O upon various regimes of heating and gaseous environment prove the formation of intermediate products of the types Fe₂O(SO₄)₂ and FeOHSO₄, their stability and amount being determined mainly by temperature and oxygen-reduction potential.

This communication aims at presenting results on the synthesis and characterization of Fe₂O(SO₄)₂. The synthesis was carried out using a laboratory thermal equipment operating under isothermal conditions in the temperature range 713–813 K in a gaseous environment either poor in oxygen or containing 100% oxygen. The experimental conditions under which Fe₂O(SO₄)₂ is stable are established. The effect of three basic parameters on the synthesis of Fe₂O(SO₄)₂ is clarified: the oxygen partial pressure, the ratio P_H2O/P_O2 and the temperature and the mode of heating. Mössbauer spectroscopy and X-ray diffraction data for Fe₂O(SO₄)₂ are presented.

     

Thermal decomposition of hydrogen peroxide in the presence of sulfuric acid

Hydrogen peroxide (H₂O₂) is popularly employed as a reaction reagent in cleaning processes for the chemical industry and semiconductor plants. By using differential scanning calorimetry (DSC) and vent sizing package 2 (VSP2), this study focused on the thermal decomposition reaction of H₂O₂ mixed with sulfuric acid (H₂SO₄) with low (0.1, 0.5 and 1.0 N), and high concentrations of 96 mass%, respectively. Thermokinetic data, such as exothermic onset temperature (T ₀), heat of decomposition (ΔH _d), pressure rise rate (dP/dt), and self-heating rate (dT/dt), were obtained and assessed by the DSC and VSP2 experiments. From the thermal decomposition reaction on various concentrations of H₂SO₄, the experimental data of T ₀, ΔH, dP/dt, and dT/dt were obtained. Comparisons of the reactivity for H₂O₂ and H₂O₂ mixed with H₂SO₄ (lower and higher concentrations) were evaluated to corroborate the decomposition reaction in these systems.     

The Reaction Mechanism and Kinetics of the Zn2.5VMoO8 Phase Synthesis in the Solid State

Studies on the reaction kinetics and mechanism of the synthesis of the Zn_2.5VMoO₈ compound in the solid state have been carried out in situ in a high-temperature X-ray diffraction attachment. The apparent activation energy, 212±26 kJ mol^-1 was calculated by using the diffusion controlled Ginstling-Brounstein model. There was also determined a temperature dependence of unit cell parameters for Zn₃V₂O₈ and Zn_2.5VMoO₈. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of strontium substitution on the activity of La2?xSrxNiO4 (x = 0.0–1.2) in NO decomposition

The aim of this work is to study the effect of Sr substitution on the redox properties and catalytic activity of La_2−x Sr _x NiO₄ (x = 0.0–1.2) for NO decomposition. Results suggest that the x = 0.6 sample shows the highest activity. The characterization (TPD, TPR, etc.) of samples indicates that the x = 0.6 sample possesses suitable abilities in both oxidation and reduction, which facilitates the proceeding of oxygen desorption and NO adsorption. At temperature below 700°C, the oxygen desorption is difficult, and is the rate-determining step of NO decomposition. With the increase of reaction temperature (T > 700°C), the oxygen desorption is favorable and, the active adsorption of NO on the active site (NO + V _o + Ni²⁺ → NO⁻-Ni³⁺) turns out to be the rate-determining step. The existence of oxygen vacancy is the prerequisite condition for NO decomposition, but its quantity does not relate much to the activity. Supported by the National Hi-Tech Research and Development Program of China (863 Program)(Grant No. 2004CB 719502) and the National Natural Science Foundation of China (Grant No. 20177022)     

Synthesis and electrical properties of dense Bi2Al4O9

Bi₂Al₄O₉ ceramics are difficult to sinter to greater than 80% theoretical density due to peritectic decomposition at 1,070 °C. A novel processing method is discussed where a high-bismuth oxide-based liquid is used as a sintering aid. After sintering, the high bismuth oxide phase is removed by leaching with 40% acetic acid. The resulting samples are phase pure and ∼91% dense. The grain size varies in a wide range with the average grain size of ∼1 μm. The electrical properties of these ceramics were measured as functions of temperature (550–850 °C) and oxygen partial pressure (6×10⁻⁶–1 atm). The total conductivity was separated into electronic and ionic contributions. The low ionic conductivity indicates that the material is not an ‘intrinsically defective fast ion conductor’. The ionic conductivity is due almost exclusively to compensating oxygen vacancies related to impurities. With increasing temperature and decreasing oxygen partial pressure, the electronic conduction dominates over the ionic conduction.     

Kinetics of γ-alumina chlorination by phosgene

The kinetics of the reaction between γ-Al₂O₃ and COCl₂ have been studied by isothermal TG measurements in the temperature range 585–1085 K. The influence of the partial pressure of COCl₂ on the reaction rate was investigated at 648, 673 and 698 K in the 10⁴–10⁵ Pa range, using N₂ as a diluting gas. The reaction seems to proceed in the chemical control region below 675 K. Above this temperature, however, diffusional processes and decomposition of COCl₂ are considered to affect the reaction rate. In the chemical control region an apparent order of reaction of 0.75 in respect of COCl₂ and an apparent activation energy of 128–135 kJ mole^?1 were found for the chlorination process.     

Practical neutral aromatic nitration with nitrogen dioxide in the presence of heterogeneous catalysts under moderate oxygen pressure

A practical neutral aromatic nitration process using nitrogen dioxide in the presence of FeCl₃ · 6H₂O under 40–100 psig of oxygen was developed, and nitration of several aromatic compounds, including the deactivated nitrobenzene, was performed in a successful manner. The correlation of reaction rate with equivalents nitrogen dioxide, oxygen pressure, amount of catalyst and temperature was investigated through the nitration of benzene. Following the optimization of reaction conditions, the nitration of benzene was scaled up to 476 mol. Furthermore, inorganic solid catalysts with pore size over 5 Å and surface area over 100 m²/g were applied to newly developed neutral nitration.     

Infrared study of UV-irradiated tungsten trioxide powders containing adsorbed dimethyl methyl phosphonate and trimethyl phosphate

The photodecomposition of dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP) adsorbed on monoclinic WO₃ powders when irradiated by ultraviolet light (UV) in air, oxygen, and under evacuation was investigated using infrared spectroscopy (IR). The IR spectra show that DMMP decomposes into methyl phosphonate upon exposure to 254 nm UV for 2 h at room temperature in air. The same decomposition of DMMP occurs only at temperatures above 300°C without UV illumination. TMP differs from DMMP in that the photodecomposition product is not the same as the decomposition product obtained by heating above 300°C. Thermal decomposition leads to formation of a phosphate on the surface, whereas photodecomposition leads to the same adsorbed methyl phosphonate as found for the thermal or photodecomposition of DMMP. Since TMP does not contain a P-CH₃ bond, the formation of a methyl phosphonate on the surface after UV illumination involves a mechanism where CH₃ groups migrate from the methoxy group to the phosphorous central atom. No decomposition is observed at room temperature when DMMP or TMP adsorbed on WO₃ is irradiated under vacuum or in nitrogen atmosphere. Therefore, the photodecomposition of either DMMP or TMP adsorbed on WO₃ at room temperature does not involve a reaction with the lattice oxygen but rather a reaction with the oxygen radicals produced by the decomposition of ozone.     

Palladium-catalyzed oxidation of lower aliphatic alcohols with oxygen in the presence of aromatic nitriles in aqueous medium

The kinetics of oxidation of lower aliphatic alcohols (C₁–C₄) to the corresponding carbonyl compounds with oxygen in the presence of palladium(II) tetraaqua complex and aromatic nitriles (benzonitrile, phenylacetonitrile, and o-tolunitrile) in aqueous medium (c = 0.01 M) at 65°C under atmospheric pressure were studied. A probable reaction mechanism and kinetic equation were proposed. Aromatic nitriles were found to stabilize decomposition of low-valence palladium species, ensuring activation of molecular oxygen and subsequent oxidation of alcohols.     

Kinetics of the thermal decomposition of [Co(NH3)6]2(C2O4)3·4H2O

The thermal decomposition of [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O was studied under isothermal conditions in flowing air and argon. Dissociation of the above complex occurs in three stages. The kinetics of the particular stages thermal decomposition have been evaluated. The R_N and/or A_M models were selected as those best fitting the experimental TG curves. The activation energies,E, and lnA were calculated with a conventional procedure and by a new method suggested by Kogaet al. [10, 11]. Comparison of the results have showed that the Arrhenius parameters values estimated by the use of both methods are very close. The calculated activation energies were in air: 96 kJ mol^–1 (R_1.575, stage I); 101 kJ mol^–1 (Ain1.725 stage II); 185 kJ mol^–1 (A _2.9, stage III) and in argon: 66 kJ mol^–1 (A _1.25, stage I); 87 kJ mol^–1 (A _1.825, stage II); 133 kJ mol^–1 (A _2.525, stage III).     

Thermally induced oxygen uptake and release in the Ba−Cr−O−system

Dibarium-chromate(IV), Ba₂CrO₄, has been prepared and its thermal behaviour in different atmospheres has been studied. An unusual reversible mass change in oxygen reflects the uptake and release of oxygen in a cyclic thermal program for a product mixture obtained in a previous thermal decomposition step. Initial and product phases have been characterized by X-ray powder diffraction. In celebration of the 60^th birthday of Dr. Andrew K. Galwey     

Pre-Eutectic Densification of Calcium Carbonate Doped with Lithium Carbonate

Pressureless sintering of CaCO₃ was carried out, with Li₂CO₃ (from 0.5 to 8 wt%) as an additive, under different pressures of CO₂. Densification occurs between 600 and 700°C. Sintering above the eutectic temperature (T>662°C) leads to the decomposition of calcium carbonate and the materials become expanded. At 620° under 1 kPa of CO₂, a relative density of 96% is reached. Li₂CO₃ enhances the densification process and grain growth of calcium carbonate. CO₂ pressure slows down densification and grain growth kinetics. These results are explained by the influence of carbonate and calcium ion vacancies on the sintering mechanisms. This revised version was published online in August 2006 with corrections to the Cover Date.     

Kinetics of Synthesis of Ba1.0Co0.7Fe0.2Nb0.1O3-δ through Solid-Solid Reaction

The mechanism of solid-solid reaction between BaCO₃ and Co₃O₄/Fe₂O₃/Nb₂O₅ has been investigated by means of non-isothermal thermogravimetry and differential scanning calorimetry (DSC) under flowing air gas conditions at atmospheric pressure with a new solid-solid reaction model. The effects of high speed agitating mixing and ball-milling mixing processes on the synthesis kinetics were also studied. The synthesis kinetics of Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3-δ from the BaCO₃ and Co₃O₄/Fe₂O₃/Nb₂O₅ particles was calculated by applying the modified model. The results indicated that the overall reaction process was considered involving two stages: addition reaction between BaCO₃ and Co₃O₄/Fe₂O₃/Nb₂O₅ particles in the first stage and solution reaction between BaCoO₃, BaFeO₃, and BaNbO₃ to form a homogeneous Ba_1.0Co_0.7Fe_0.2Nb_0.1O_3-δ phase in the second stage. The new model matched well with the experimental data. The apparent activation energy of addition reaction stage of the high speed agitating mixing sample was estimated to be 376.76 kJ·mol⁻¹, which was only 3/4 of that of the ball-milling mixing sample (494.76 kJ·mol⁻¹). These results indicated that the high-speed agitating process could enhance atomic diffusion and facilitate the subsequent reaction, thus it is believed as a more effective, energy saving, and environmentally benign mixing process.