Thermodynamic modeling of the BaO-SiO<Subscript>2</Subscript> and SrO-SiO<Subscript>2</Subscript> binary melts

The evaluation of the thermodynamic properties and the phase diagrams for the binary BaO-SiO₂ and SrO-SiO₂ systems is carried out using a structural model for silicate melts and glasses. This thermodynamic model is based on the assumption that an addition of metal oxides to silica results in the depolymerization of the silicon-oxygen network, with a characteristic free energy change. A least squares optimization program permits all available thermodynamic and phase diagram data to be optimized simultaneously. In this manner, data for the above binary systems have been analysed and represented with a small number of parameters. The resulting equations represent the thermodynamic and phase diagram data for the alkaline-earth oxide-silica systems within error limits for most of the experimental data. In particular, the measured limiting liquidus slope, at X _{SiO 2} = 1, is well reproduced.     

Critical Evaluation and Thermodynamic Modeling of the MgO–MnO–Mn2O3–SiO2 System

A complete literature review, critical evaluation, and thermodynamic optimization of phase diagrams and thermodynamic properties of the MgO–MnO–Mn₂O₃–SiO₂ system at 1 atm pressure are presented. The molten oxide phase was described by the Modified Quasichemical Model considering the short‐range ordering in molten oxide, and the Gibbs energies of solid solutions were described using the Compound Energy Formalism considering the crystal structure of each solid solution. A set of optimized model parameters of all phases was obtained which reproduces all available and reliable thermodynamic data and phase diagrams within experimental error limits from 25°C to above the liquidus temperatures over the entire range of composition under the oxygen partial pressures from metallic saturation to 1 atm. The database of the model parameters can be used along with software for the Gibbs energy minimization to calculate any phase diagram section and thermodynamic property within the present system.     

Thermodynamics of amorphous SiN(O)H dielectric films synthesized by plasma‐enhanced chemical vapor deposition

Thin films of amorphous SiNH (a‐SiNH) and amorphous SiNOH (a‐SiNOH) synthesized by plasma‐enhanced chemical vapor deposition (PECVD) are used extensively in the semiconductor industry, but little is known regarding their thermodynamic stability, and there are several long‐term reliability issues for these materials. To address the stability issues, a detailed thermodynamic investigation has been conducted on a series of a‐SiNH, and a‐SiNOH dielectric films. High‐temperature oxidative drop‐solution calorimetry in molten sodium molybdate solvent at 1075 K was utilized to determine the formation enthalpies from the elements and from crystalline counterparts/gaseous products. Together with entropy data derived from cryogenic heat capacity measurements, we confirmed that the incorporation of more hydrogen and oxygen leads to more negative enthalpies and Gibbs free energies of formation from elements. Coupled with FTIR structural analysis, the thermochemical data suggest that the Si–H₂ chain structure and Si–O–Si bonding configurations provide the system with extra thermodynamic stability. However, the Gibbs free energies of formation from crystalline constituents and gaseous products are either positive or nearly zero, indicating that these amorphous films are not stable against decomposition, which may cause problems in high‐temperature applications.     

Thermodynamic study of a pressure‐temperature phase diagram for poly(N‐isopropylacrylamide) gels

The volume change, ΔV_h,_, accompanying the hydrophobic hydration associated with the volume phase transition in Poly(N‐isopropylacrylamide) gels was measured by a simple method. The hydration accompanies a negative ΔV_h′?2.5 cm³/mol. The P‐T phase diagram, the coexistence curve, for the gels was determined from the swelling ratio‐pressure curves up to 350 MPa for various constant temperatures. The contour of the coexistence curve is shaped like an ellipsoid on the P‐T plain, which is a feature peculiar to the reversible pressure‐temperature denaturation of a protein. The thermodynamic analysis of the Clausius–Clapeyron relation for the measured ΔV_h elucidates that the obtained coexistence curve represents the phase boundary between thermodynamic different phases like the two phases, native and denatured, of a protein and gives the transition enthalpy, ΔH, 5.2kJ/mol by estimate, which well coincides with the transition heat measured by a calorimetric method. Considering the volume‐dependent free energy, Δv_mi · P, for the mixing free energy of the gel, we can fit the calculated curve to the measured swelling ratio‐pressure curve of PNIPA gels. The value of Δv_mi changes the sign from negative to positive above around 100MPa. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 405–412, 2005     

Thermally‐induced phase transformations in Na0.5Bi0.5TiO3–KNbO3 ceramics

The structures and functional properties of Na_0.5Bi_0.5TiO₃–xKNbO₃ (NBT‐xKN) solid solutions, with x in the range from 0.01 to 0.09, were investigated using a combination of high‐resolution synchrotron X‐ray powder diffraction (SXPD) and ferroelectric property measurements. For low KN contents, an irreversible transformation from cubic to rhombohedral phases was observed after the application of a high electric field, indicating that the polar nanoregions (PNRs) in the unpoled state can be transformed into metastable long‐range ordered ferroelectric domains in the poled state. In contrast, the near‐cubic phase of the unpoled ceramics was found to be remarkably stable and was retained on cooling to a temperature of ?175°C. Upon heating, the field‐induced metastable ferroelectric rhombohedral phase transformed back to the nanopolar cubic state at the structural transformation temperature, T_ST, which was determined as approximately 225°C and 125°C for KN contents of 3% and 5% respectively. For the field‐induced rhombohedral phase in the poled specimens, the pseudo‐cubic lattice parameter, a_p, exhibited an anomalous reduction while the inter‐axial angle increased towards a value of 90° on heating, resulting in an overall increase in volume. The observed structural changes were correlated with the results of temperature‐dependent dielectric, ferroelectric and depolarization measurements, enabling the construction of a phase diagram to define the stable regions of the different ferroelectric phases as a function of composition and temperature.     

A target‐oriented solid‐gas thermochemical sorption heat transformer for integrated energy storage and energy upgrade

An innovative target‐oriented solid‐gas thermochemical sorption heat transformer is developed for the integrated energy storage and energy upgrade of low‐grade thermal energy. The operating principle of the proposed energy storage system is based on the reversible solid‐gas chemical reaction whereby thermal energy is stored in form of chemical bonds with thermochemical sorption process. A novel thermochemical sorption cycle is proposed to upgrade the stored thermal energy by using a pressure‐reducing desorption method during energy storage process and a temperature‐lift adsorption technique during energy release process. Theoretical analysis showed that the proposed target‐oriented thermochemical sorption heat transformer is effective for the integrated energy storage and energy upgrade, and the low‐grade thermal energy can be upgraded from 87 to 171^°C using a group of sorption working pair MnCl₂‐CaCl₂‐NH₃. Moreover, it can give the flexibility of deciding the temperature magnitude of energy upgrade by choosing appropriate sorption working pairs. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1334–1347, 2013     

Stability and calorimetric studies of silico‐ferrites of calcium aluminum and magnesium

In a systematic study on silico‐ferrites of calcium aluminum and magnesium (SFCA phases) this investigation focuses on synthesis of silicon‐free SFCA‐type compounds with low‐MgO contents (~1.00 apfu—atoms per formula unit). Previous studies revealed the existence of iron‐rich SFCA phases similar to the SFCA‐I structure with the chemical composition Ca₃MgAl₆Fe₁₀O₂₈ (Metall Mater Trans B. 2017;48:2207). The experimental results in the quaternary Fe₂O₃‐CaO‐Al₂O₃‐MgO system confirm large stability fields of 2 silicon‐free ferrites FCAM‐I and FCAM‐III, which are members of the homologous series M_14+6nO_20+8n (n = 1, 2). Starting with compositions corresponding to Ca₃MgAl_xFe_16‐xO₂₈ (with increasing aluminum content from x = 0‐12 apfu), it was possible to synthesize these phases with an x‐value ≥2 apfu, which corresponds to Al₂O₃ contents ≥7.14 wt%. Synthesis of pure silicon‐free ferrites with n = 1 (FCAM‐I) and 2 (FCAM‐III) and silicon‐bearing ferrites with n = 0 (SFCA) was possible. Samples were characterized by electron probe microanalysis, powder diffraction, and subsequently studied using relaxation calorimetry measurements in combination with differential scanning calorimetry for determination of the heat capacities and standard entropies S°(298). The corresponding values are S°(298) = 650.3 ± 4.6 J/mol·K for SFCA, S°(298) = 864.5 ± 6.1 J/mol·K for FCAM‐I, and S°(298) = 1206.2 ± 8.4 J/mol·K for FCAM‐III. These thermodynamic data are a step toward a rigorous quantitative thermodynamic modeling of the iron ore sintering process.     

Phase equilibria study and thermodynamic description of the BaO‐CaO‐Al2O3 system

A combined experimental investigation and thermodynamic assessment was performed for the BaO‐CaO‐Al₂O₃ system. By using a high‐temperature equilibration/quenching technique and scanning electron microscopy, electron probe microanalysis, and X‐ray powder diffraction analysis, the phase equilibria at 1500°C and phase stability of BaCa₂Al₈O₁₅ phase were determined. An extensive literature survey was conducted for the experimental and thermodynamic modeling data of the BaO‐CaO‐Al₂O₃ system. According to the literature data and the present measurements, a thermodynamic assessment was made in order to obtain a set of self‐consistent thermodynamic parameters to describe the BaO‐CaO‐Al₂O₃ system. Based on the thermodynamic parameters acquired in this work, isothermal sections at 1100°C, 1250°C, 1400°C, 1475°C, and 1500°C and the BaO·Al₂O₃‐CaO·Al₂O₃ and BaO·6Al₂O₃‐CaO·6Al₂O₃ joints were calculated and compared with the available experimental data.     

Chemical modification of controlled‐pore silica with N‐propylsalicylaldimine

Two controlled‐pore silica phases were prepared with a sol–gel precursor from a sodium silicate solution. N‐Propylsalicylaldimine was immobilized on these silica species to be used as chelating ion exchangers (IE11 and IE2). The monomer phase was also prepared for comparison. The N‐propylsalicylaldimine moiety was detected by Fourier transform infrared and ultraviolet in both the ion exchangers and the monomer phases. 1H‐NMR and mass spectrometry of the monomer also confirmed the structure. The capacity (C) of the ion exchangers was dependent on the porosity of the ion exchanger (C_IE11 = 0.36 mmol of Cu/g and C_IE2 = 0.026 mmol of Cu/g). The uptake behavior of IE11 toward some metal ions was studied, and log distribution coefficient (k_d) was within the range of 2.19–5.16. Also, thermogravimetric and differential thermogravimetric analysis data were used to study the kinetics of the thermal decomposition process of IE11. Some thermodynamic parameters for the ion exchanger were calculated by the application of the rate theory of the first‐order reaction. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 3159–3167, 2003     

Combined experimental and computational investigation of thermodynamics and phase equilibria in the CaO–TiO2 system

Phase relations in the CaO–TiO₂ system are of considerable interest in geology, metallurgy, and ceramics. Despite a number of studies of phase equilibria in the CaO–TiO₂ system, there are still some open questions regarding the stability of intermediate compounds. In this work, a series of specimens with different CaO:TiO₂ ratios were prepared by solid‐state reaction. The heat capacities of Ca₃Ti₂O₇ and Ca₄Ti₃O₁₀ from 300 to 1073 K were measured by differential scanning calorimetry and their formation enthalpies from the component oxides at 298 K were measured by high temperature oxide melt solution calorimetry. Using phase diagram information and thermodynamic data from the literature and the present measurements, thermodynamic optimization of the CaO–TiO₂ system was carried out by the CALPHAD technique. The phase diagram and the thermodynamic properties of the CaO–TiO₂ system were calculated using the obtained thermodynamic database, which clarify the stable and metastable phase equilibria of the system. The thermodynamic stability of the various compounds was discussed.     

Thermodynamic Stability of Low‐k Amorphous SiOCH Dielectric Films

Si–O–C‐based amorphous or nanostructured materials are now relatively common and of interest for numerous electronic, optical, thermal, mechanical, nuclear, and biomedical applications. Using plasma‐enhanced chemical vapor deposition (PECVD), hydrogen atoms are incorporated into the system to form SiOCH dielectric films with very low dielectric constants (k). While these low‐k dielectrics exhibit chemical stability as deposited, they tend to lose hydrogen and carbon (as labile organic groups) and convert to SiO₂ during thermal annealing and other fabrication processes. Therefore, knowledge of their thermodynamic properties is essential for understanding the conditions under which they can be stable. High‐temperature oxidative drop solution calorimetry measurement in molten sodium molybdate solvent at 800°C showed that these materials possess negative formation enthalpies from their crystalline constituents (SiC, SiO₂, C, Si) and H₂. The formation enthalpies at room temperature become less exothermic with increasing carbon content and more exothermic with increasing hydrogen content. Fourier transform infrared spectroscopy (FTIR) spectroscopy examined the structure from a microscopic perspective. Different from polymer‐derived ceramics with similar composition, these low‐k dielectrics are mainly comprised of Si–O(C)–Si networks, and the primary configuration of carbon is methyl groups. The thermodynamic data, together with the structural analysis suggest that the conversion of sp² carbon in the matrix to surface organic functional groups by incorporating hydrogen increases thermodynamic stability. However, the energetic stabilization by hydrogen incorporation is not enough to offset the large entropy gain upon hydrogen release, so hydrogen loss during processing at higher temperatures must be managed by kinetic rather than thermodynamic strategies.     

High‐temperature structural stability of ceria‐based inverse opals

The use of ceria‐based inverse opals as a catalyst system for the thermochemical production of fuels from sunlight offers the potential of improved fuel production kinetics over materials with random porosity. Quantitative methods for characterizing ordered porosity are lacking, thus limiting the ability to predict the lifetime of ordered structures at elevated temperatures. In the present work, Fourier transform image analysis was used to determine the effect of composition and temperature on ordered porosity for a series of CeO₂‐ZrO₂ inverse opals having pore sizes ranging from 300 nm to 1 μm. An order parameter, γ, derived from the image analysis, was applied to scanning electron microscopy images and used to determine the degree of order in the inverse opal. The thermal stability studies indicate that loss of ordered porosity is highly dependent on temperature and that gas cycling effects have a minor effect on periodicity. A minimum Zr content of 20 at.% is necessary to retain periodicity for annealing up to 1000°C with pore diameters larger than 1 μm. These results show that CeO₂‐ZrO₂ inverse opals can be used at higher temperatures than previously thought for efficient thermochemical hydrogen production without loss of the benefits associated with ordered porosity.     

Experimental investigation of the LiAlSi2O6‐MgSiO3 and LiAlSi2O6‐CaMgSi2O6 isopleths at 1 atm

The phase diagrams of the LiAlSi₂O₆‐MgSiO₃ and LiAlSi₂O₆‐CaMgSi₂O₆ isopleths were experimentally investigated at 1 atm using the quenching method and differential scanning calorimetry and the phases produced were characterized with the help of X‐ray diffraction and electron probe microanalysis. With the help of thermodynamic optimization, the phase diagrams of both systems were more accurately reported. No detectable solubility of Li₂O in diopside and enstatite was found. However, both systems are not simple binary eutectic systems because their phase equilibria are somewhat complex due to the presence of some β‐spodumene solid solution. In the β‐spodumene solid solution, no notable solubility of MgO and CaO was detected; evidence of significant solubility of SiO₂ was confirmed.     

Thermodynamic Analysis on the Codeposition of SiC–Si3N4 Composite Ceramics by Chemical Vapor Deposition using SiCl4–NH3–CH4–H2–Ar Mixture Gases

A thermodynamic calculation on the chemical vapor deposition of the SiCl₄–NH₃–CH₄–H₂–Ar system was performed using the FactSage thermochemical software databases. Predominant condensed phases at equilibrium were SiC, Si₃N₄, graphite, and Si. The equilibrium conditions for the deposition of condensed phases in this system were determined as a function of the deposition temperature, dilution ratio (δ), and reactant ratios of CH₄/SiCl₄ and NH₃/SiCl₄. The CVD phase diagrams were used to understand the reactions occurring during the formation of Si–C–N from the gas species and determine the area of SiC–Si₃N₄. The concentration of condensed‐phase products was used to determine the deposition conditions of CVD SiC–Si₃N₄. The present work was helpful for further experimental investigation on CVD Si–C–N.