Measurement of Gibbs energy of formation of Ca2PbO4 using a solid-state cell with three electrodes

Phase relations in the system Ca-Pb-O at 1100 K have been determined by equilibrating 18 compositions in the ternary and identifying the phases present in quenched samples by X-ray diffraction and energy dispersive X-ray analysis (EDX). Only one ternary compound Ca2PbO4 was found to be present. The compound coexists with CaO and PbO. The intermetallic compounds Ca2Pb, Ca5Pb3 and CaPb and liquid alloys are in equilibrium with CaO. The standard Gibbs energies of formation of Ca2PbO4 (880 - 1100 K) and Pb3O4 (770 - 910 K) were determined using solid-state cells based on yttria-stabilized zirconia as the solid electrolyte. Pure oxygen gas at 0.1 MPa was used as the reference electrode. For measurements on Ca2PbO4, a novel cell design with three electrodes in series, separated by solid electrolyte membranes, was used to avoid polarization of the electrode containing three solid phases. Two three-phase electrodes were used. The first absorbs the electrochemical flux of oxygen from the reference electrode to the measuring electrode. The other three-phase electrode, which is unaffected by the oxygen flux through the solid electrolyte, is used for electromotive force (EMF) measurement. The results from EMF studies were cross-checked using thermogravimetry (TG) under controlled oxygen partial pressures. The stability of Pb3O4 was investigated using a conventional solid-state cell with RuO2 electrodes. The results can be summarized by the following equations: 2CaO + PbO +1/2O(2) --> Ca2PbO4 Delta(r)G degrees/J mol(-1) = (- 128340 + 93.21 T/K) +/- 200 3PbO + 1/2O(2) --> Pb3O4 Delta(r)G degrees/J mol(-1) = (- 70060 + 77.5 T/K) +/- 150

High temperature phase chemistry of system Eu-Pd-O

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.

In situ synthesis of Al2O3-ZrO2-SiCW ceramic matrix composites by carbothermal reduction of natural silicates

In situ formations of Al2O3 + ZrO2 + SiCW ternary composite powders have been obtained by carbothermal reduction of a mixture of Sillimanite. Kaolin and Zircon using two different carbon sources. Products formed were mixtures of alumina and zirconia along with silicon carbide in the form of whiskers. The effects of composition of the reactants, the role of fineness of the starting precursors and the nature of the carbon Source on the final product powder obtained are presented. XRD and SEM analyses indicate complete reaction of the precursors to yield Al2O3 + ZrO2 + SiCW as product powders, with the SiC having whisker morphology. It is also seen that zirconia could be stabilised to some extent in the tetragonal form without any stabilising agent by tailoring the starting materials and their composition. (C) 2002 Published by Elsevier Science B.V.

Phase diagram for the system Nd-Mn-O and thermodynamic properties of NdMnO3 and NdMn2O5

Studies on the phase relations in the system Nd-Mn-O at 1223 K showed two stable ternary compounds, NdMnO3 and NdMn2O5. An isothermal section of the ternary phase diagram for the system Nd-Mn-O was constructed based on phase analysis of samples quenched after equilibration using XRPD and EDS. An advanced version of the solid-state cell incorporating a buffer electrode was used to determine the Gibbs energies of decomposition of NdMnO3 and NdMn2O5 in the temperature range from 925 to 1400 K. Pure oxygen gas at 0.1 MPa was used as the reference electrode, and yttria-stabilized zirconia as the solid electrolyte. The buffer electrode was designed to prevent polarization of the three-phase electrode and ensure accurate data. The measured oxygen potential corresponding to the reaction,2 Nd2O3 + 4 MnO + O-2 --> 4 NdMnO3 can be represented by the equation,Amu(o2) / J.mol(-1) (+/-580) = -523 960 + 170.96 (T/K)Similarly, for the formation of NdMn2O5 according to the reaction,3 NdMnO3 + Mn3O4 + O-2 --> 3 NdMn2O5 Amu(o2) / J.mol(-1) (+/-660) = - 269 390 + 181.74 (T/K) (C) 2002 Elsevier Science Ltd. All rights reserved.

The thermodynamics and phase behaviour of Tb2Pd2O5 and Er2Pd2O5

Isothermal sections of the phase diagrams for the systems Ln-Pd-O (with Ln = Tb or Er) have been established by equilibration of samples at T = 1223 K, and phase identification after quenching by optical and scanning electron microscopy (OM, SEM), energy dispersive spectroscopy (EDS), and X-ray powder diffraction (XRPD). Two oxide phases were stable along the binary Tb-O: Tb2O3+x, a phase of variable composition, and Tb7O12 at T = 1223K. The oxide PdO was not stable at this temperature. Only one ternary oxide Tb2Pd2O5 was identified in the Tb-Pd-O system. No ternary compound was found in the system Er-Pd-O at T = 1223K. However, the compound Er2Pd2O5 could be synthesized at T = 1075 K by the hydrothermal route. In both systems, the alloys and inter-metallic compounds were all found to be in equilibrium with the lanthanide sesquioxide Ln(2)O(3) (where Ln is either Tb or Er). Two solid-state cells, each incorporating a buffer electrode, were designed to measure the Gibbs energy of formation of the ternary oxides, using yttria-stabilized zirconia as the solid electrolyte and pure oxygen gas as the reference electrode. Electromotive force measurements were conducted in the temperature range (900-1275) K for Th-Pd-O system, and at temperatures from (900-1075) K for the system Er-Pd-O. The standard Gibbs energy of formation Delta(f)G(m)degrees,, of the inter-oxide compounds from their component binary oxides Ln(2)O(3) and PdO are represented by equations linear in temperature. Isothermal chemical potential diagrams for the systems Ln-Pd-O (with Ln = Tb or Er) are developed based on the thermodynamic information. (C) 2002 Elsevier Science Ltd. All rights reserved.

Calcium ruthenates: Determination of Gibbs energies of formation using electrochemical cells

Metallic Ru has been found to coexist separately with CaO, RuO2, and the interoxide phases, Ca2RuO4, Ca3Ru2O7, and CaRuO3, present along the pseudobinary system CaO-RuO2. The standard Gibbs energies of formation (Df((ox))G(o)) of the three calcium ruthenates from their component oxides have been measured in the temperature range 925-1350 K using solid-state cells with yttria-stabilized zirconia as the electrolyte and Ru+RuO2 as the reference electrode. The standard Gibbs energies of formation (Deltaf((ox))G(o)) of the compounds can be represented by Ca2RuO4:Deltaf((ox))G(o)/J mol(-1)=-38,340-6.611 T (+/-120), Ca3Ru2O7 : Df((ox))G(o)/J mol(-1)=-75,910-11.26 T (+/-180), and CaRuO3 : Deltaf((ox))G(o)/J mol(-1)=-35,480-3.844 T(+/-70). The data for Ca2RuO4 corresponds to the stoichiometric composition, which has an orthorhombic structure, space group Pbca, with short c axis ("S'' form). The structural features of the ternary oxides responsible for their mild entropy stabilization are discussed. A three-dimensional oxygen potential diagram for the system Ca-Ru-O is developed as a function of composition and temperature from the results obtained. Using the Neumann-Kopp rule to estimate the heat capacity of the ternary oxides relative to their constituent binary oxides, the standard enthalpies of formation of the three calcium ruthenates from the elements and their standard entropies at 298.15 K are evaluated. (C) 2003 The Electrochemical Society.

Stresses in Ion Assisted Deposited Oxide Thih Films

Thin films of Ceria, Titania and Ziroonia have been prepared using Ion Assisted Deposition(IAD). The energy of ions was varied between 0 and 1 keV and current densities up to 220 μA/cm were used. It was found that the stress behaviour is dependent on ion species, i.e. Argon or Oxygen, ion energy and current density and substrate temperature apart from the material. While oeria files showed tensile stresses under the influence of argon ion bombardment at ambient temperature, they showed a sharp transition from tensile to compressive stress with increase in substrate temperature. When bombarded with oxygen ions they showed a transition from tensile to compressive stress with increase in energy. The titania films deposited with oxygen ions, on the other hand showed purely tensile stresses. Zirconia films deposited with oxygen ions, however, showed a transition from tensile to compressive stress.

Effect of Nature of the Precursor on Crystallinity and Microstructure of MOCVD-Grown ZrO2 Thin Films

In the present work, we report the deposition of zirconia thin films on Si(100) at various substrate temperatures by low-pressure metalorganic chemical vapor deposition (MOCVD). Three different zirconium complexes, viz., tetrakis(2,4-pentadionato)zirconium(IV), [Zr(pd)4], tetrakis(2,2,6,6-tetramethyl-3,5-heptadionato)zirconium(IV), [Zr(thd)4], and tetrakis(t-butyl-3-oxo-butanoato)zirconium(IV), [Zr(tbob)4] are used as precursors. The relationship between the molecular structures of the precursors and their thermal properties, as examined by TG/DTA is presented. The films deposited using these precursors have distinctly different morphology, though all of them are of the cubic phase. The films grown from Zr(thd)4 are well crystallized, showing faceted growth at 575°C, whereas the films grown from Zr(pd)4 and Zr(tbob)4 are not well crystallized, and display cracks. These differences in the observed microstructure may be attributed to the different chemical decomposition pathways of the precursors during the film growth, which influence the nucleation and the growth processes. This is also evidenced by the different kinetics of growth from these three precursors under otherwise identical CVD conditions. The details of thin film deposition, and film microstructure analysis by XRD and SEM is presented. The dielectric behavior of the films deposited from different precursors, as studied by C-V measurements, are compared.

Standard Gibbs energy of formation of Pb2Ru2O6.5

Lead ruthenate is used as a bifunctional electrocatalyst for both oxygen evolution and reduction and as a conducting component in thick-film resistors. It also has potential applications in supercapacitors and solid oxide fuel cells. However, thermodynamic properties of the compound have not been reported in the literature. The standard Gibbs energy of formation has now been determined in the temperature range from 873 to 1123 K using a solid-state cell incorporating yttria-stabilized zirconia (YSZ) as the electrolyte, a mixture of PbO + Pb2Ru2O6.5 + Ru as the measuring electrode, and Ru + RuO2 as the reference. The design of the measuring electrode is based on a study of phase relations in the ternary system Pb–Ru–O at 1123 K. For the reaction,S0884291400095625_eqnU1 the standard enthalpy of formation and standard entropy at 298.15 K are estimated from the high-temperature measurements. An oxygen potential diagram for the system Pb–Ru–O is composed based on data obtained in this study and auxiliary information from the literature

Vapor pressure and thermodynamic properties of MgIn2O4

The vapor pressure of pure indium, and the sum of the pressures of (In) and (In2O) species over the condensed phase mixture {In} + 〈MgIn2O4〉 + 〈MgO〉, have been measured by the Knudsen effusion technique in the temperature range 1095–1350 K. The materials under study were contained in a zirconia crucible, which had a Knudsen orifice along the vertical wall. The major vapor species over the condensed phase mixture were identified as (In) and (In2O) using a mass-spectrometer. The vapor pressure of (In2O) corresponding to the reaction,View the MathML source was deduced from the experimental results;View the MathML source The standard free energy of formation of the inverse spinel 〈MgIn2O4〉 from its component oxides, is given by,View the MathML source View the MathML source The entropy of transformation of 〈In2O3〉 from the C rare-earth structure to the corundum structure is evaluated from the measured entropy of formation of (MgIn2O4) and a semi-empirical correlation for the entropy of formation of spinel phases from component oxides with rock-salt and corundum structures.

Thermodynamics of oxide sulfate melts: The system PbO PbSO4

Activities in the PbO-PbSO4 melts at 1253 K have been measured by emf and gas-equilibration techniques. The activity of PbO was directly obtained from the emf of the solid oxide cell, Pt, Ni-NiO/CaO-ZrO2/Auo.92PbO.08, PbOx-PbSO4(1-x), Ir, Pt for 1.0 >XPbO > 0.6. The melt and the alloy were contained in closed zirconia crucibles. Since the partial pressure of SO2 gas in equilibrium with the melt and alloy was appreciable (>0.08 atm) atXPbO < 0.6, activities at lower PbO concentrations were derived from measurements of the weight gain of pure PbO under controlled gas streans of Ar + SO2 + O2. The partial and integral free energies of mixing at 1253 K were calculated and found to fit a subregular model: ΔGEPbO =X2PbSO4 {-42,450 + 20,000X2PbSO4} J mol-1 ΔGEPbO =X2pbSO {-12,450 - 20,000XPbS} J mol-1 ΔGEpbSOXPbSO4 {-32,450XPbS - 22,450XPbSO4 } J mol-1. The standard free energy of formation of liquid PbSO4 from pure liquid PbO and gaseous SO3 at 1 atm at 1253 K was evaluated as -88.02 (±0.72) kJ mol-1.

Gibbs energy of formation of lead zirconate

The Gibbs energy of formation of zirconia-saturated lead zirconate was determined by emf measurements on the solid state cells and at 800 to 1400 K. The results obtained differ significantly from those reported in the literature based on vapor-pressure measurements, using Knudsen effusion and transportation techniques and assuming that the vapor phase consisted entirely of monomeric PbO molecules. A reanalysis of the data obtained in the earlier vapor-pressure studies, using mass-spectrometric measurements on polymeric PbO species in the gas phase, gives Gibbs energies of formation of lead zirconate which are in better agreement with those obtained in this study. Recent electrochemical measurements using CaO-ZrO2 and PbF2 solid electrolytes are in good agreement with the present study. The results obtained in this study are also consistent with the phase diagram which shows decomposition of the zirconate to tetragonal zirconia and a liquid phase rich in PbO at 1843 K.

Sulphur potential measurements with a two phase sulphide oxide electrolyte

The open circuit potentials of the galvanic cell,Pt (or Au)¦(Ar + H2S + H2)primeparCaS + ZrO2(CaO)par (Ar + H2S+ H2)Prime£t (or Au) has been measured in the temperature range 1000 to 1660 K and PH2S:PH 2 ratios from 1.73×10–5 to 2.65×10–1. The solid electrolyte consists of a dispersion of calcium sulphide in a matrix of calcia-stabilized zirconia. The surface of the electrolyte is coated with a thin layer of calcium sulphide to prevent the formation of water vapour by reaction of hydrogen sulphide with calcium oxide or zirconia present in the electrolyte. The use of a lsquopoint electrodersquo with a catalytically active tip was necessary to obtain steady emfs. At low temperatures and high sulphur potentials the emfs agreed with the Nernst equation. Deviations were observed at high temperatures and low sulphur potentials, probably due to the onset of significant electronic conduction in the oxide matrix of the electrolyte. The values of oxygen and sulphur potentials at which the electronic conductivity is equal to ionic conductivity in the two-phase electrolyte have been evaluated from the emf response of the cell. The sulphide-oxide electrolyte is unsuitable for sulphur potential measurements in atmospheres with high oxygen potentials, where oxidation of calcium sulphide may be expected.

Phase relations and activities in the Co Ni O system at 1373 K

The tie-lines delineating equilibria between CoO-NiO and Co-Ni solid solutions in the ternary Co-Ni-O system at 1373 K have been determined by electron microprobe andedax point count analysis of the oxide phase equilibrated with the alloy. The oxygen potentials corresponding to the tie-line compositions have been measured using a solid oxide galvanic cell with calcia-stabilized zirconia electrolyte and Ni + NiO reference electrode. Activities in the metallic and oxide solid solution have been derived using a new Gibbs-Duhem integration technique. Both phases exhibit small positive deviations from ideality; the values ofG E/X 1 X 2 are 2640 J mol−1 for the metallic phase and 2870 J mol−1 for the oxide solid solution.

Solubility and activity of oxygen in liquid nickel in equilibrium with -Al2O3 and NiO.(1+x)Al2O3

The limiting solubility of oxygen in liquid nickel in equilibrium withα-alumina and nickel aluminate has been measured by inert gas fusion analysis of suction samples in the temperature range 1730 to 1975 K. The corresponding oxygen potential has been monitored by a solid electrolyte cell consisting of calcia stabilized zirconia as the electrolyte and Mo + MoO2 as the reference electrode. The results can be summarized by the following equations: log(at. pct O) = \frac - 10,005T + 4.944 ( ±0.015)log(atpctO)=T−10005+4944(0015) % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn DmO2 /4.606RT = log P O2 1/2 = \frac - 13,550T + 4.411 ( ±0.009)O24606RT=logPO212=T−13550+4411(0009) From simultaneous measurements of the potential and concentration of oxygen in melts, not in thermodynamic equilibrium with alumina and aluminate phases, information on the composition dependence of the activity coefficient and the standard free energy of solution of oxygen is obtained. For the reaction, $\frac{1}{2} O_2 \to \underset{\raise0.3em\hbox{$Missing close brace ΔG o = -72,930 - 7.11T (±840) J gr.at.–1 = + 0.216 at. pct OlogfO=T−500+0216atpctO where the standard state for dissolved oxygen is that which makes the value of activity equal to the concentration (in at. pct) in the limit as concentration approaches zero. The oxygen solubility in liquid nickel in equilibrium with solid NiO, evaluated from thermodynamic data, is compared with information reported in the literature. Implications of the results to the deoxidation equilibria of aluminum in nickel are discussed.