Analysis and Characterization of Phase Evolution of Nanosized BaTiO3 Powder Synthesized Through a Chemically Modified Sol-Gel Process

In the current research, a cost-effective and modified method with a high degree of reproducibility was proposed for the preparation of fine nanoscale and high-purity BaTiO₃. In contrast to the other established methods, in this research, carbonate-free BaTiO₃ nanopowders were prepared at a lower temperature and in a shorter time span. To reach an in-depth understanding of the scientific basis of the proposed process, an in-detail analysis was carried out for characterization of nanoscale BaTiO₃ particles via differential thermal analysis (DTA)/thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques aided by theoretical calculations. The effects of the temperature and time of calcination process on the preparation mechanism, phase transformation, tetragonality, and particle size of BaTiO₃ were examined. The reaction that results in the formation of barium titanate initiated at approximately 873?K (600?°C) and seemed to be completed at approximately 1073?K (800?°C) and the polymorphic transformation of cubic to tetragonal initiated at approximately 1173?K (900?°C). It seemed to be completed at approximately 1373?K (1100?°C). According to the reaction mechanism, the formation of BaTiO₃ in the initial stage of the interfacial reaction between BaCO₃ and TiO₂ depends on the BaCO₃ decomposition. In the second stage, the BaTiO₃ formation is controlled by barium diffusion through the barium titanate layer. In this stage, in contrast to the literature, no secondary phase was detected. The overall characterizations showed the temperature is more effective than time on the progress in process of preparation because of its diffusion-controlled nature.     

Determination of kinetic parameters using differential thermal analysis—Application to the decomposition of CaCO3

An analytical method has been developed to determine the kinetic parameters of a chemical reaction involving a substantial enthalpy change using differential thermal analysis (DTA). The theoretical treatment is based on fundamental equations considering the heat balance. The analytical derivation was simplified by carefully choosing the experimental conditions. The method was applied to the decomposition of CaCO₃ in argon gas. The activation energy of the decomposition of CaCO₃ evaluated using the present approach is in very good agreement with the result obtained from the thermogravimetric analyses (TGA) carried out simultaneously with the DTA measurements. The limitation of the technique includes maintaining the temperature rise of the sample small enough not to significantly affect temperature reading but large enough to ensure accurate measurement of the heat generation.     

Effect of heat and mass transfer on the thermal decomposition of SrCO3 compacts

The thermal decomposition of spheres made of pressed SrCO₃ powder was studied by the thermogravimetric analysis (TGA) technique. The temperature was measured simultaneously either at the surface of the sample or at the center of the sample by suitably placed thermocouples. The experimental results are compared to calculations wherein both heat- and mass-transfer aspects are considered. A steady state is assumed in the solution of heat and mass transfer through the product layer formed during the decomposition of SrCO₃. It was found that the decomposition rate was controlled by chemical CO₂ production in the beginning of the decomposition. External CO₂ transfer also had a strong influence at the beginning, and CO₂ diffusion through the porous SrO product layer affected the rate at later stages. The decomposition rate was also influenced, to some extent, by external heat transfer.     

Thermal analysis of self-propagating high-temperature reactions in titanium, boron, and aluminum powder compacts

This article focuses on the characterization of self-propagating high-temperature synthesis (SHS) reactions that occur in powder compacts containing titanium, boron, and aluminum. Interest in this powder system is based on the critical need to develop new joining techniques for bonding ceramics to metals. The exothermic reactions of particular interest in this study include those that generate TiB₂, TiB, Ti₃Al, and TiAl from their elemental powders. Data from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and X-ray diffractometry are presented. These results demonstrate that the gas phase surrounding the SHS powders plays an important role in initiating the SHS reaction and in determining which reaction products will form in the final bond.     

Thermal analysis of self-propagating high-temperature reactions in titanium, boron, and aluminum powder compacts

This article focuses on the characterization of self-propagating high-temperature synthesis (SHS) reactions that occur in powder compacts containing titanium, boron, and aluminum. Interest in this powder system is based on the critical need to develop new joining techniques for bonding ceramics to metals. The exothermic reactions of particular interest in this study include those that generate TiB₂, TiB, Ti₃Al, and TiAl from their elemental powders. Data from differential thermal analysis (DTA), thermogravimetric analysis (TGA), and X-ray diffractometry are presented. These results demonstrate that the gas phase surrounding the SHS powders plays an important role in initiating the SHS reaction and in determining which reaction products will form in the final bond. L.H. CHIU, formerly with the Material Science and Engineering Department, Johns Hopkins University. L.A. BONNEY, formerly with the Materials Science and Engineering Department, Johns Hopkins University.     

Effects of Different Barium Compounds on the Corrosion Resistance of Andalusite-Based Low-Cement Castables in Contact with Molten Al-Alloy

An experimental study was conducted to investigate the interfacial phenomena between an Al alloy and andalusite low-cement castables (LCCs) containing fixed contents of barium compounds (BaO, BaSO₄, and BaCO₃) at 1123 K and 1433 K (850 °C and 1160 °C) using the Alcoa cup test. Interfacial reaction products and phases formed during heat treatment of the refractory samples were characterized using scanning electron microscopy (SEM) coupled with energy dispersive spectrometry (EDS) and X-ray diffraction analysis (XRD). The addition of both BaO and BaSO₄ led to a significant reduction of alloy penetration into the refractory. Hexa-celsian formation was observed in both these refractories, which drastically increased their corrosion resistance. Barite decomposition was observed at 1373 K (1100 °C) in the presence of alumina and silica, which was the precursor for hexa-celsian formation. Barium silicates were formed in all samples containing additives; however, this did not have any major influence on the corrosion resistance. Solidified eutectics of BaSi₂ and α-BaAl₂Si₂ formed in all these samples, which acted as an interfacial barrier that prevented additional molten aluminum penetration; however, the positive effect of intermetallic formation was offset by glassy phase formation in samples containing BaCO₃ as the additive.     

Hydrogen reduction of a Cu2O-WO3 mixture

In the present work, the reduction kinetics of Cu₂O-WO₃ mixtures by hydrogen gas was studied by thermogravimetric analyses (TGA). The reduction experiments were carried out both isothermally and nonisothermally on shallow powder beds in the temperature interval 673 to 1073 K. During the experiments, the reductant gas flow rate was kept just above the starvation rate for the reaction to ensure that chemical reaction was the rate-controlling step. The composition and microstructures of the reaction products were analyzed after each experiment by X-ray diffraction (XRD) as well as by microprobe analyses. In the temperature interval 673 to 923 K, copper oxide was found to be preferentially reduced in the early stages of the experiment followed by the reduction of tungsten oxide. The reaction mechanism was found to be affected by a reaction/transformation in the starting copper-tungsten oxide mixtures in the temperature interval 923 to 973 K. At temperatures higher than 973 K, the reduction of the complex oxide formed was found to have a strong impact on the reaction kinetics. The activation energy was evaluated, from the isothermal as well as nonisothermal reduction experiments, for the two stages of reduction identified. The XRD and scanning electron microscopy (SEM) studies indicated the formation of a metastable solution of copper in tungsten at about 923 K. The advantage of the hydrogen reduction route toward the bulk production of alloy powders in the nanosize is demonstrated.     

The intrinsic thermal decomposition kinetics of SrCO3 by a nonisothermal technique

The kinetics of the decomposition of SrCO₃ in argon to SrO and CO₂ were studied in the temperature range 1000 to 1350 K. The thermal decomposition was followed simultaneously by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) during linear heating. By using a nonisothermal method, the complete rate expression was determined from a relatively small number of experimental runs. Shallow beds of fine synthetic powder as well as thin flakes of pressed powder were employed to obtain the kinetics of decomposition in the absence of heat- and mass-transfer effects. The thermal decomposition started at about 1000 K. The recommended rate expression for the SrCO₃ decomposition is

Reduction of Fe<Subscript>2</Subscript>MoO<Subscript>4</Subscript> by hydrogen gas

In the present work, the reduction kinetics of iron molybdate (Fe₂MoO₄) by hydrogen gas was investigated by thermogravimetric analyses (TGA). Both isothermal and nonisothermal experiments were conducted. By using fine particles, very shallow powder bed, and high hydrogen flow rate, the study could be focused on the chemical reaction. The activation energy obtained from the isothermal experiments was found to be 173.5 kJ/mol, which was in reasonable agreement with the value of 158.3 kJ/mol obtained from the nonisothermal experiments. The reduction product was found to be an intermetallic compound, Fe₂Mo, of microcrystalline structure.     

Kinetics of solid state reaction between barium carbonate and cupric oxide

The kinetics and equilibrium of the solid state reaction between barium carbonate and cupric oxide have been examined thermogravimetrically. The reaction rate is found to be dominated by effects of nucleation and diffusion of carbon dioxide produced. A mathematical model incorporating these effects, along with considerations of heat transfer, is found to satisfactorily correlate the conversion-time data. The reaction is found to follow the stoichiometry BaCO₃ + CuO ⇌ BaCuO₂ + CO₂ although at temperatures above 1123 K, some evidence of BaO is also seen through X-ray diffraction. In the pelletized samples, incomplete conversion is noticed indicative of pore closure effects leading to transport limitations. The latter is also independently confirmed by porosity and surface area measurements. Data on the reaction equilibrium are also obtained and, in conjunction with Van’t Hoff’s relation, are used to obtain a correlation for the endothermic heat of reaction as a function of temperature.     

稀土复合氧化物燃烧催化剂的性能及表征

浸渍法制备了稀土型燃烧催化剂 ,采用多晶 X射线衍射 (XRD)、程序升温还原 (TPR)、差热热重分析(DTA- TG)、X射线光电子能谱 (XPS)和二甲苯催化氧化活性测试等方法表征了该燃烧催化剂的结构和性能     

Master decomposition curve analysis of ethylene vinyl acetate pyrolysis: influence of metal powders

Abstract

Polymer burnout (pyrolysis or delubrication) is a crucial step in sintering die compacted powders. To systematically analyse and design the thermal delubrication step, the master decomposition curve (MDC) has been formulated based on the intrinsic kinetics of polymer pyrolysis. The Kissinger method was used to estimate the activation energy from thermogravimetric analysis (TGA) experiments. The activation energy of poly(ethylene-co-vinyl acetate) (EVA) was determined and an MDC analysis was performed to map the weight loss of the polymer as a function of time and temperature. The developed MDC was used to investigate the effects of powder chemistry, powder shape, and particle size of 316L stainless steel on the decomposition behaviour of EVA. The activation energies for decomposition of EVA decreased in the presence of gas and water atomised 316L stainless steel powders, indicative of a catalytic effect. This effect was more pronounced for the first decomposition step suggesting the possible role of a carboxylate ion – metal transition state complex that promoted decomposition. In addition, the gas atomised 316L stainless steel had a greater effect on lowering the activation energy for decomposition compared to water atomised 316L stainless steel, emphasising the influence of powder surface chemistries. Based on the MDC analysis, the required hold time can be predicted for a given temperature and target binder weight loss. This reduces the experimentation required to optimise the delubrication cycle. Furthermore, when extrapolating to very small particle sizes, this approach is of particular interest for predicting the behaviour of nano-particulate materials.     

Kinetics of chloridization of nickel-bearing lateritic iron ore by hydrogen chloride gas

The selective chloridization of nickel in a lateritic iron ore by gaseous HCl is based on the principle of relative thermal stability of iron and nickel chlorides. This aspect has been discussed with differential thermal analysis (DTA) and thermogravimetric (TG) data of the hydrated chlorides of iron and nickel. The kinetics of chloridization of nickel in a lateritic nickel ore from Orissa, India, have been studied by using both pure HCl (g) and the HCl (g) + N₂ mixture. The sharp decrease in the rate of chloridization of nickel at temperatures above 250 °C is attributed to the rapid decomposition of molten ferric chloride hydrate (FeCl₃ · 3H₂O), which blocks the pores of the reactant solid. Therefore, kinetics of chloridization follow both the pore-blocking model (logarithmic rate law) and diffusion-controlled mechanisms. Very low values of apparent activation energy and effective diffusivity derived from the rate constants of the diffusion-controlled process suggest that diffusion of HCl (g) takes place either in a dissolved state in the molten ferric chloride (at 100 °C to 150 °C) or through cracks and fissures formed on the surface due to rapid decomposition of ferric chloride at 200 °C to 250 °C. Because of the complexity of the reaction system, the rate of chloridization of nickel is almost independent of grain size.     

Thermodynamic study of BaCuO2 and BaCu2O2

The Gibbs energy changes for the syntheses of the interceramic compounds of BaCuO₂ and BaCu₂O₂ were investigated as a basic study in the Y-Ba-Cu-O system that includes a superconductor, YBa₂Cu₃O_6.5+x . For the compound BaCuO₂, thermogravimetry with CO₂-O₂-N₂ gas mixtures was employed, and equilibrium temperatures were determined at which CO₂ partial pressures in the gas mixtures were equilibrated with mixed powder of BaCuO₂, CuO, and BaCO₃. The Gibbs energy change for the reaction of BaCO₃ + CuO = BaCuO₂ + CO₂ was determined from the relation between CO₂ partial pressure and equilibrium temperature, taking into consideration the effect of CO₂ dissolution in BaCuO₂. For the study on BaCu₂O₂, electromotive force (emf) measurement using yttria-stabilized zirconia solid electrolyte was conducted. A technique using two cells was applied to the emf measurement for minimizing the effect of dispersion of oxygen from samples, and the Gibbs energy change for the reaction of BaCuO₂ + CuO = BaCu₂O₂ + 1/2O₂ was decided from the measured O₂ partial pressure.     

Free energy of formation of barium ferrites

The free energy of formation (AG°) of various ferrites in the system BaO-Fe₂O₃ was determined with galvanic cells using barium fluoride as solid electrolyte. The values of ΔG° per mole of 2 BaO · Fe₂O₃, BaO · Fe₂O₃ and BaO · 6 Fe₂O₃ from BaO and Fe₂O₃ were found to be-10,953 (±2275) J, -68,496 (±435) J, -75,576 (±250) J at 1200 K and-104,600 (±2275) J, -66,875 (±435) J, -74,437 (±250) J at 1300 K respectively.     

Study of an Energy Storage and Recovery Concept Based on the W/WO3 Redox Reaction: Part I. Kinetic Study and Modeling of the WO3 Reduction Process for Energy Storage

Energy storage and recovery using the redox reaction of tungsten/tungsten-oxide is proposed. The system will store energy as tungsten metal by reducing the tungsten oxide with hydrogen. Thereafter, steam will be used to reoxidize the metal and recover the hydrogen. The volumetric energy density of W for storing hydrogen by this process is 21 kWh/L based on the lower heating value (LHV) of hydrogen. The main objective of this investigation was to study the kinetics of the reduction process of tungsten oxide (WO₃) and determine the optimum parameters for rapid and complete reduction. Theoretical treatment of isothermal kinetics has been extended in the current work to the reduction of tungsten oxide in powder beds. Experiments were carried out using a thermogravimetric technique under isothermal conditions at different temperatures. The reaction at 1073 K (800 °C) was found to take place in the following sequence: WO₃ → WO_2.9 → WO_2.72 → WO₂ → W. Expressions for the last three reaction rate constants and activation energies have been calculated based on the fact that the intermediate reactions proceed as a front moving at a certain velocity while the first reaction occurs in the entire bulk of the oxide. The gas–solid reaction kinetics were modeled mathematically in terms of the process parameters. This model of the reduction has been found to be accurate for bed heights above 1.5 mm and hydrogen partial pressures greater than 3 pct, which is ideal for implementing the energy storage concept.     

Properties of molybdenum powders obtained by reduction in moving beds

We have studied the process of decomposition of ammonium paramolybdate and reduction of the molybdenum oxides in moving beds with rotation of the working chamber. We have shown that agglomerated disperse molybdenum powders are formed under these conditions. Movement of powder beds during reduction ensures continuous contact between the oxides being reduced and the hydrogen, rapid removal of water vapor from the reaction zone, and establishment of nearly kinetic reduction conditions. As a result the metallic molybdenum powders are strong, highly porous agglomerates, similar to the MoO₃ particles in size and shape. The powders have high bulk (as-poured) density and good flowability, and their dispersity depends only on the reduction temperature.     

Thermodynamic properties of complex oxides in the Sm-Ba-Cu-O system

An assembly for performing electromotive force (EMF) measurements using either a CaF₂ single crystal or a MgO partially stabilized zirconia as a solid-state electrolyte has been constructed. Heat capacities of member complex oxides in the Sm-Ba-Cu-O system were measured by an adiabatic scanning calorimeter. From the EMF data, the standard Gibbs energies, standard enthalpies of formation, standard entropies, and decomposition pressures of the complex oxides were calculated. The standard Gibbs energies of reaction of complex oxides from relevant simple oxides Sm₂O₃, BaO, and CuO were derived. It was found that all complex compounds in this system were thermodynamic stable. The tendency of the thermodynamic stability of the solid solution compound Sm_1+x , Ba_2−x Cu₃O _y decreases with the solubility x from 0 to 0.4.     

Methods for calculating the kinetic parameters of carbonate decomposition from thermal analysis data

A procedure is proposed for an analysis of the data on the decomposition of inorganic carbonates (MgCO₃, CaCO₃, SrCO₃, MnCO₃, CoCO₃) on the basis of results of thermogravimetric studies. The activation energy and the rate constant for carbonate decomposition are calculated taking into account the main points of the heterogeneous reaction kinetics. A comparative analysis of the obtained results is performed.     

Effect of cladding diamond powder with a nickel-phosphorus alloy on its high-temperature oxidation

High-temperature oxidation of diamond powder and that coated with Ni-P has been studied by differential thermal analysis. It is shown that the temperature for active oxidation of uncoated powder is 820°C and that for composite powder is higher by 100°C. According to the thermogravimetric and DTA curves two kinds of chemical reaction are found for weight loss, i.e. with CO formation at the specimen surface (low-temperature TG curve parabolic region) and CO₂ formation (high-temperature linear region). A catalytic effect of nickel on CO formation is established.Institute of Materials Science Problems, Ukrainian National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 1/2(383), pp. 55–58, January–February, 1996. Original article submitted April 15, 1994.