Thermodynamic interpretation of the manganese-zinc ferrite synthesis by calcination of a powder mixture

The free energy of formation for Mn_0.5Zn_0.5Fe₂O₄ has been determined based on thermodynamic equilibrium of the Mn-Zn-Fe-H₂O system under conditions prevailing in the hydrothermal synthesis experimental work published recently by other authors. The equilibrium diagrams generated in this work have been used to interpret published experimentally observed results on manganese-zinc ferrite synthesis and behavior during cooling. The conclusion is that all experimental results and problems concerning the thermodynamic behavior of the system Mn-Zn-Fe-O during synthesis and cooling down are perfectly explained by the prediction of the equilibrium diagrams developed in this work. Moreover, further benefits will arise from feeding the free energy data provided here into the computational thermodynamics softwares to generate equilibrium diagrams that are not yet available involving this type of ferrite.     

System Cu-Rh-O: Phase diagram and thermodynamic properties of ternary oxides CuRhO2 and CuRh2O4

An isothermal section of the phase diagram for the system Cu-Rh-O at 1273 K has been established by equilibration of samples representing eighteen different compositions, and phase identification after quenching by optical and scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive analysis of X-rays (EDX). In addition to the binary oxides Cu₂O, CuO, and Rh₂O₃, two ternary oxides CuRhO₂ and CuRh₂O₄ were identified. Both the ternary oxides were in equilibrium with metallic Rh. There was no evidence of the oxide Cu₂Rh₂O₅ reported in the literature. Solid alloys were found to be in equilibrium with Cu₂O. Based on the phase relations, two solid-state cells were designed to measure the Gibbs energies of formation of the two ternary oxides. Yttria-stabilized zirconia was used as the solid electrolyte, and an equimolar mixture of Rh+Rh₂O₃ as the reference electrode. The reference electrode was selected to generate a small electromotive force (emf), and thus minimize polarization of the three-phase electrode. When the driving force for oxygen transport through the solid electrolyte is small, electrochemical flux of oxygen from the high oxygen potential electrode to the low potential electrode is negligible. The measurements were conducted in the temperature range from 900 to 1300 K. The thermodynamic data can be represented by the following equations: {fx741-1} where Δ_f(ox) G ^o is the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides. Based on the thermodynamic information, chemical potential diagrams for the system Cu-Rh-O were developed.     

High–temperature phase chemistry of the system Gd–Pd–O

Anisothermal sectionof the phase diagram for the system Gd–Pd–O at 1223 K has been established by equilibrationof samples and phase identification after quenching by optical and scanning electron microscopy, X–ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides Gd₄PdO₇,Gd₂PdO₄ and Gd₂Pd₂O₅ were identified. Liquid alloys, the four inter–metallic compounds and Pd–rich solid solutionwere found to be inequilibrium with Gd₂O₃.

Based on the phase relations, four solid–state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 920 to 1320 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to cross check the data. An advanced version of the solid–state cell incorporating a buffer electrode with yttria–stabilized zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high–temperature thermodynamic measurements. The standard Gibbs energy of formation of the inter–oxide compounds from their component binary oxides can be represented by the following equations:

Gd₄PdO₇(s) : Δ_f(ox)G⁰/J mol^–1 = –25,030 + 0.33T (±140), Gd₂PdO₄(s) : Δf(ox)_f(ox)G⁰/J mol^–1 = –25,350 + 0.84T (±135), Gd₂Pd₂O₅(s) : Δf(ox)_f(ox)G⁰/J mol^–1 = –48,700 + 0.38T (±270)

Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Gd–Pd–O are developed.     

High temperature phase chemistry of system Eu-Pd-O

Abstract

An isothermal section of the phase diagram for the system Eu-Pd-O at 1223 K has been established by equilibration of samples representing 20 different compositions, and phase identification after quenching by optical and scanning electron microscopy, X-ray powder diffraction, and energy dispersive spectroscopy. Three ternary oxides, Eu₄PdO₇, Eu₂PdO₄, and Eu₂Pd₂O₅, were identified. Liquid alloys and the intermetallic compounds EuPd₂ and EuPd₃ were found to be in equilibrium with EuO. The compound EuPd₃ was also found to coexist separately with Eu₃O₄ and Eu₂O₃. The oxide phase in equilibrium with EuPd₅ and Pd rich solid solution was Eu₂O₃. Based on the phase relations, four solid state cells were designed to measure the Gibbs energies of formation of the three ternary oxides in the temperature range from 925 to 1350 K. Although three cells are sufficient to obtain the properties of the three compounds, the fourth cell was deployed to crosscheck the data. An advanced version of the solid state cell incorporating a buffer electrode with yttria stabilised zirconia solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode was used for high temperature thermodynamic measurements. Equations for the standard Gibbs energy of formation of the interoxide compounds from their component binary oxides Eu₂O₃ with C type structure and PdO have been established. Based on the thermodynamic information, isothermal chemical potential diagrams and isobaric phase diagrams for the system Eu-Pd-O have been developed.     

Chemical reactions in the Mo-W-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> system at 2400 K and 100 Pa

We examine the behavior of the Mo-W-Al₂O₃ system at T = 2400 K and p = 100 Pa and identify the main chemical reactions that lead to the oxidation of Mo and W. The results indicate that, under these thermodynamic conditions, direct reaction between Mo, W, and molten Al₂O₃ is impossible, and oxidation processes are indirect and involve vapor species. For a Mo: W: Al₂O₃ molar ratio of 1: 1: 1, we calculate the gas-phase composition and demonstrate that the predominant vapor species are Al, AlO, Al₂O, MoO, MoO₂, and WO₃.     

Complementary evaluation of structure stability of perovskite oxides using bond-valence and density-functional-theory calculations

Estimation of structure stability is an essential issue in materials design and synthesis. Global instability index (GII) based on bond-valence method is applied as a simple indication, while density functional theory calculation is adopted for accurate evaluation of formation energy. We compare the GII and total energy of typical ABO₃-type perovskite oxides and rationalize their relationship, proposing that the criteria for empirically unstable structures (GII > 0.2 valence unit) correspond to the difference in total energy of 50–200 meV per formula unit.     

Studies on the interactions of metal oxides and Na2 SO4 at 1100 and 1200 K in oxygen

The interaction of different metal oxides such as Co₃O₄, NiO, Al₂O₃, Cr₂O₃, Fe₂O₃ and SiO₂ with Na₂SO₄ at a temperature of 1100 and 1200 K in flowing oxygen has been studied. The thermogravimetric studies for each system were carried out as a function of Na₂SO₄ in the mixture. The presence of different constituents in the reaction products were identified by X-ray diffraction analysis and the morphologies of the reaction products were characterized using metallography and scanning electron microscopy (SEM). The formation of products was also investigated by thermodynamic computation of free energies of the reactions and the study of relevant equilibrium phase diagrams. The soluble species in the aqueous solutions of the reaction products were determined quantitatively using atomic absorption spectrophotometry. The high temperature interaction products usually contain a 3-phase structure namely, Na₂O·M₂O _x , M₂O _x and metal sulphide and/or metal sulphate. The formation of Na₂O·M₂O _x depends upon the solid state solubility of metal oxide in the molten salt at high temperatures. Under limited solubility conditions Na₂O·M₂O _x is invariably formed, but as soon as this condition is relaxed the oxide. M₂O _x , precipitates and forms a separate phase.     

Influence of nano-particle size on the oxidation behaviour of Al,Fe and Cu

Abstract

The addition of ultra-fine and nano-sized particles in composite materials influences their properties. In order to understand the influence of the nano-size of metal particles on the oxidation behaviour, a series of in situ studies by thermogravimetry (TG) and high temperature X-ray diffraction (XRD) was performed on Al, Fe and Cu during heating with post oxidation analysis by field emission – scanning electron microscope (FE-SEM). The oxidation reaction depends, for all three studied metals, on the particle size both in regard to transient oxide formation and formation temperatures. On a nano-scale Al forms θ-Al₂O₃ and γ-Al₂O₃ simultaneously at 500°C, but both of these oxides transform to α-Al₂O₃ at 975°C producing nano-scaled oxide particles. Aluminium particles with sizes from 2 to 25 μm form α-Al₂O₃ only, starting at temperatures close to the melting point. Nano-sized Fe forms on heating from 340°C α-Fe₂O₃ only and no other oxides. On nano-sized Cu particles the formation of Cu₂O starts at 140°C, transforming completely to CuO at 300°C.     

The system Yb–Ru–O: High temperature studies on the oxides Yb2Ru2O7(s) and Yb3RuO7(s)

The standard molar Gibbs free energy of formation of Yb₂Ru₂O₇(s) and Yb₃RuO₇(s) was determined using an oxide solid-state electrochemical cell wherein calcia-stabilized-zirconia (CSZ) was used as an electrolyte. The standard molar Gibbs free energy of formation of Yb₂Ru₂O₇(s) and Yb₃RuO₇(s) from elements in their standard state was calculated by the least squares regression analysis of the data obtained in the present study and can be given respectively by:     

Oxidation behavior of in situ TiB short fibre reinforced Ti-6Al-1.2B alloy in air

The oxidation behavior of an in situ TiB short fibre reinforced Ti-6Al-1.2B alloy was investigated in a flowing air over the temperature range 873–1223 K. The alloy exhibited a parabolic oxidation behavior with the activation energy of 250 kJ/mol, which was supposed to be mainly controlled by the inward diffusion of oxygen. The oxide scales consisted mainly of TiO₂, a small amount of Al₂O₃ and amorphous B₂O₃. The morphology of TiO₂ changed from fine needle-shape at 1023 K to fine sphere at 1073 K and then well-developed block at a higher temperature. The microstructure of the subsurface showed that B₂O₃ pores and crack appear at the Ti/TiB interface above 1023 K as a result of the oxidation of TiB and evaporation B₂O₃. Combined with the thermodynamic analysis, it was suggested that the presence of TiB and formation of B₂O₃ could not act as an oxidation resistance. Finally, the diffusion coefficient of oxygen in Ti-6Al-1.2B exposure to 1073 K in air was determined to be 4.22 × 10^–11 cm²/s.     

Gibbs’ energy of formation of YBa2Cu3O7-x (tetragonal)

The high temperature ceramic oxide superconductor YBa₂Cu₃O_7-x (1–2–3 compound) is generally synthesized in an oxygen-rich environment. Hence any method for determining its thermodynamic stability should operate at a high oxygen partial pressure. A solid-state cell incorporating CaF₂ as the electrolyte and functioning under pure oxygen at a pressure of 1·01 × 10⁵ Pa has been employed for the determination of the Gibbs’ energy of formation of the 1–2–3 compound. The configuration of the galvanic cell can be represented by: Pt, O₂, YBa₂Cu₃O_7−x , Y₂BaCuO₅, CuO, BaF₂/CaF₂/BaF₂, BaZrO₃, ZrO₂, O₂, Pt. Using the values of the standard Gibbs’ energy of formation of the compounds BaZrO₃ and Y₂BaCuO₅ from the literature, the Gibbs’ energy of formation of the 1–2–3 compound from the constituent binary oxides has been computed at different temperatures. The value ofx at each temperature is determined by the oxygen partial pressure. At 1023 K for O content of 6·5 the Gibbs’ energy of formation of the 1–2–3 compound is −261·7 kJ mol⁻¹.     

Thermodynamic analysis of growth of iron oxide films by MOCVD using tris(<Emphasis Type="Italic">t</Emphasis>-butyl-3-oxo-butanoato)iron(III) as precursor

Thermodynamic calculations, using the criterion of minimization of total Gibbs free energy of the system, have been carried out for the metalorganic chemical vapour deposition (MOCVD) process involving the β-ketoesterate complex of iron [tris(t-butyl-3-oxo-butanoato)iron(III) or Fe(tbob)₃] and molecular oxygen. The calculations predict, under different CVD conditions such as temperature and pressure, the deposition of carbon-free pure Fe₃O₄, mixtures of different proportions of Fe₃O₄ and Fe₂O₃, and pure Fe₂O₃. The regimes of these thermodynamic CVD parameters required for the deposition of these pure and mixed phases have been depicted in a ‘CVD phase stability diagram’. In attempts at verification of the thermodynamic calculations, it has been found by XRD and SEM analysis that, under different conditions, MOCVD results in the deposition of films comprising pure and mixed phases of iron oxide, with no carbonaceous impurities. This is consistent with the calculations.     

DFT Study of Structural and Thermodynamic Properties for Polybrominated 5,10-Dihydrophenazines

The structural and thermodynamic properties of 75 polybrominated 5,10-dihydrophenazines (PBDPs) in the ideal gas state at 298.15K and 1.013 × 10⁵ Pa have been calculated at the B3LYP/6-31G* level using the Gaussian 03 program. The isodesmic reactions are developed to calculate the standard enthalpy of formation (Δ_f H ^θ ) and standard free energy of formation (Δ_f G ^θ ) of PBDP congeners. The relationships of these thermodynamic parameters with the number and position of the Br atom substitution (N _PBS) are discussed, and it is found that there exists a high correlation between thermodynamic parameters and N _PBS . In addition, the correlations between structural parameters and N _PBS are discussed. High correlations were found between the energy of the highest occupied molecular orbital (E _HOMO ), the most negative atomic partial charge in the molecule (q ⁻) and N _PBS , and all R ² values are larger than 0.90.     

Alloy-oxide equilibria in the system Pt-Rh-O

The composition of Pt-Rh alloys that co-exist with Rh₂O₃ in air have been identified by experiment at 1273 K. The isothermal sections of the phase diagram for the ternary system Pt-Rh-O at 973 K and 1273 K have been computed based on experimentally determined phase relations and recent thermodynamic measurements on Pt_1−X Rh _X alloys and Rh₂O₃. The composition dependence of the oxygen partial pressure for the oxidation of Pt_1−X Rh _X alloys at different temperatures, and temperature for the oxidation of the alloys in air are computed. The diagrams provide quantitative information for optimization of the composition of Pt_1−X Rh _X alloys for high temperature application in oxidizing atmospheres.     

Microstructure and oxidation behavior of NiCoCrAl/YSZ microlaminates produced by EB–PVD

Two NiCoCrAl/ZrO₂–Y₂O₃ microlaminates (A and B) were fabricated by electron beam physical vapor deposition, which were different in layer number and metal-layer thickness. The layer number was 20 and 26, respectively. And the metal-layer thickness was 35 and 14 μm, respectively. The microstructure and isothermal oxidation behavior were investigated. During the exposure in air at 1000 °C for 100 h, the t to m phase transformation occurred in the ceramic layers, and oxide scales formed at the surfaces of not only the outer metal-layers but also the internal metal-layers for the microlaminates. The oxidation rate of microlaminate B was greater than that of microlaminate A. Their overall mass gains were significantly dependent on the number and thickness of the metal-layers. The oxidation products were also influenced by metal-layer thickness. Oxide scales of the 35 μm thick metal-layer microlaminate (A) consisted mainly of α-Al₂O₃ and θ-Al₂O₃, while the oxidation products of the 14 μm thick metal-layer microlaminate (B) were the mixture of α-Al₂O₃, θ-Al₂O₃ and Cr₂O₃. It was also found that the growth of the oxide scale adjacent to the top YSZ layer was controlled by the oxygen diffusion, and that the growth of the oxide scale adjacent to the internal YSZ layer was controlled by the metal ionic diffusion.     

Effect of partial oxygen pressure on the initial stages of high-temperature oxidation of γ-NiCrAl alloys

Abstract

The effect of the partial oxygen pressure (pO₂) on the initial stages of oxide-layer growth of a γ-Ni-27Cr-9Al (at.%) alloy at 1373 K was studied using X-ray photoelectron spectroscopy, X-ray diffractrometry, scanning electron microscopy and electron probe X-ray microanalysis. The alloy was oxidised at a pO₂ of 0.1 and 0.2 × 105 Pa in an UHV processing chamber and a furnace, respectively. For both pressures, a double-layered oxide structure evolved upon oxidation, which consisted of a NiCr₂O₄/Cr₂O₃ layer on top of an α-Al₂O₃ layer. Two different oxidation stages were identified: an initial, very fast oxidation stage followed by a second stage of slow, parabolic growth. The initial oxidation stage was associated with the preferential oxidation of Al, followed by the internal oxidation of Al and simultaneous formation of Cr₂O₃ (and some NiO). The onset of the second stage ran parallel with the coalescence of internal α-Al₂O₃ crystallites into a continuous closed layer. Then, the internal oxidation of Al and growth of Cr₂O₃ ceased. A lower pO₂ reduced the activity of oxygen at the oxide/alloy interface during the initial, fast oxidation stage, thereby enhancing the rate of formation of a continuous closed α-Al₂O₃ layer and suppressing the formation of Cr₂O₃ (and NiO).     

A structural,thermogravimetric and ESR study of the RhO x -TiO2 system

Titanium dioxide samples containing a few per cent of rhodium oxide, heated in air at 1273 K, have been investigated in order to study the formation of solid solutions. The results, monitored by X-ray diffraction, thermogravimetry in a hydrogen stream and ESR spectroscopy, show that rhodium is present both as separate phase (Rh₂O₃) and in solid solution in the rutile structure. The incorporated rhodium is mainly present in the 3 + oxidation state, only a small fraction being present as Rh(II). Rh₂O₃ and the rhodium species in solid solution are reduced by hydrogen to metal in different temperature ranges. By combining thermogravimetric and analytical data, the average oxidation state of rhodium,n, has been evaluated. The variation ofn with the rhodium content has been accounted for on the basis of a strong metal-support interaction developing with the reduction. This interaction affects the metal dispersion.     

Low-temperature heat capacity and thermodynamic functions of compounds with the composition Ba(A<Stack><Subscript>2/3</Subscript><Superscript>III</Superscript></Stack>U<Subscript>1/3</Subscript>)O<Subscript>3</Subscript> (A<Superscript>III</Superscript> = Sc,Y)

The heat capacity of crystalline Ba(Sc_2/3U_1/3)O₃ and Ba(Y_2/3U_1/3)O₃ in the range 80–350 K was studied by adiabatic vacuum calorimetry, and the thermodynamic characteristics of these compounds were evaluated. Their standard entropies of formation at 298.15 K were calculated.     

Synthesis,physical, ultrasonic waves,mechanical, FTIR,and dielectric characteristics of B2O3/Li2O/ZnO glasses doped with Y3+ ions

Six specimens of glasses with formula (70???x)B₂O₃/15Li₂O/15ZnO/xY₂O₃: x?=?0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol%) have been synthesized via a conventional melt quenching technique. The produced specimens were named as BLZY0.0–BLZY2.5 according to x values. The physical, ultrasonic longitudinal (V_L) and shear (V_S) velocities, FTIR, and dielectric (50 Hz to 5 MHz) characteristics of the prepared glasses have been examined. With increasing content of Y₂O₃ from 0.0 to 2.5 mol%, the density (ρ) of the system increases linearly from 2512?±?11 to 2695?±?14 kg/m³, while the molar volume (V_M) decreases linearly from 2.6?±?0.011 to 2.57?±?0.013?×?10^?5 m³/mol. The oxygen packing density (OPD) as a number of the oxygen per unit composition in the glass sample is describing the packing tightness of the oxide network and thoroughly the compactness of the glass matrix. Values of the average boron–boron separation (d_B–B) decrease from 4.162 to 4.035?×?10^?10 (m) with 0 to 2.5 mol% Y₂O₃. Increasing formation of Y³⁺ ionic bonds with [BO_4/2]^1? may have an effect of lowering bond strength of B–O and thus shifting the absorption IR peak position. By increasing Y₂O₃ content in the investigated samples, the (V_L) and (V_S) increase linearly for the full-studied compositional range. The increasing number of strengthened bonds due to change coordination of B ions from 3 to 4 due to the increasing field strength of inserted accumulated Y³⁺ ions has the incentive impact to higher mechanical properties. The dielectric constant was decreased for Y₂O₃ content up to 1.5 mol% referring to cross-linkage formation with other elements, while the reduction in porosity at high content of Y₂O₃ is the main responsible for gradual enhancement in dielectric constant.

     

Standard Gibbs' energy of formation of LaFeO3 and comparison of stability of LaMO3 (M = Mn,Fe, Co or Ni) compounds

The thermodynamic stability of LaFeO₃ at high temperatures is of interest in materials technology. For evaluating the thermodynamic data of LaFeO₃, the e.m.f. of the following galvanic cell was studied over the range 1094 to 1299 K: Pt, LaFeO₃, Fe, La₂O₃/15 mol % CaO stabilized ZrO₂/FeO, Fe, Pt. For the galvanic cell reaction 1/2 La₂O₃ + 3/2 FeO 1/2Fe + LaFeO₃, the standard Gibbs energy change, G _r ⁰ , was measured to be (G _r ⁰ ±0.34) (kJ)=–22.67–0.011 55T (K) for the passage of 3 F of electricity. From this, the stability of LaFeO₃ with respect to La₂O₃ and FeO as well as with La₂O₃ and Fe₂O₃ was computed and compared with other equilibrium measurements on LaFeO₃ reported in the literature. In addition, a comparison was made on the stabilities of LaMO₃ compounds (where M=Mn, Fe, Co or Ni) with respect to dissociation into La₂O₃ and MO making use of the published data on other compounds. Likewise a comparison of disproportionation of LaFeO₃ and LaMO₃ into their respective sesquioxides was also made.