Severe wear of a near eutectic aluminium-silicon alloy

The severe wear of a near eutectic aluminium silicon alloy is explored using a range of electron microscopic, spectroscopic and diffraction techniques to identify the residually strained and unstrained regions, microcracks and oxidized regions in the subsurface. In severe wear the contact pressure exceeds the elastic shakedown limit. Under this condition the primary and eutectic silicon particles fragment drastically. The fragments are transported by the matrix as it undergoes incremental straining with each cyclic contact at the asperity level. The grains are refined from similar to 2000 nm in the bulk to 30 nm in the near surface region. A large reduction in the interparticle distance compared with that for a milder stage of wear gives rise to high strain gradients which contribute to an enhancement of the dislocation density. The resulting regions of very high strain in the boundaries of the recrystallized grains as well as within the subgrains lead to the formation of microvoidskracks. This is accompanied by the formation of brittle oxides at these subsurface interfaces due to enhanced diffusion of oxygen. We believe that the abundance of such microcracks in the near surface region, primed by severe plastic deformation, is what distinguishes a severe wear regime from mild wear. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

New Insights into Selective Heterogeneous Nucleation of Metal Nanoparticles on Oxides by Microwave-Assisted Reduction: Rapid Synthesis of High-Activity Supported Catalysts

Microwave-based methods are widely employed to synthesize metal nanoparticles on various substrates. However, the detailed mechanism of formation of such hybrids has not been addressed. In this paper, we describe the thermodynamic and kinetic aspects of reduction of metal salts by ethylene glycol under microwave heating conditions. On the basis of this analysis, we identify the temperatures above which the reduction of the metal salt is thermodynamically favorable and temperatures above which the rates of homogeneous nucleation of the metal and the heterogeneous nucleation of the metal on supports are favored. We delineate different conditions which favor the heterogeneous nucleation of the metal on the supports over homogeneous nucleation in the solvent medium based on the dielectric loss parameters of the solvent and the support and the metal/solvent and metal/support interfacial energies. Contrary to current understanding, we show that metal particles can be selectively formed on the substrate even under situations where the temperature of the substrate Is lower than that of the surrounding medium. The catalytic activity of the Pt/CeO(2) and Pt/TiO(2) hybrids synthesized by this method for H(2) combustion reaction shows that complete conversion is achieved at temperatures as low as 100 degrees C with Pt-CeO(2) catalyst and at 50 degrees C with Pt-TiO(2) catalyst. Our method thus opens up possibilities for rational synthesis of high-activity supported catalysts using a fast microwave-based reduction method.

Thermodynamics of oxygen in liquid copper, lead and copper lead alloys

Solid oxide galvanic cells of the type Pt, Ni-NiO I Solid electrolyte ( Ometa,, Cermet. Pt were used to measure the activity coefficient of oxygen in liquid copper at 11 00 and 1 300eC, and in lead at 11 00'C. Similar cells were used to study the activity coefficient of oxygen in the whole range of Cu + Pb alloys at 1100'C and in lead-rich alloys at 900 and 750'C.The results obtained are discussed in terms of proposed solution models. An equation based on the formation of 'species' of the form M,O in solutions of oxygen in binary alloys is shown to fit the experimental data.

Thermodynamic study of oxygen in liquid lead tin alloys

The activity coefficients of oxygen in liquid lead-tin alloys have been measured between 550 and 1100°C by use of solid oxide galvanic cells Pt, Ni-NiO I Zr02 Solid electrolyte I 0 (Pb + Sn), Cermet, Pt Pt, Fe-FeO I Zr02 Solid electrolyte I O(Pb + Sn), Cermet, Pt Alcock and Richardson's quasi-chemical equation, with the coordination number of atoms set to 2, is found to predict successfully the activity coefficients of oxygen in these alloys.The relative partial molar enthalpy and entropy of oxygen ?t 1 atom per cent in the alloys have been calculated from ttva variation of the activity coefficient with temperature. The addition of tin to an unsaturated solution of oxygen in lead is shown to decrease significantly both the partial molar enthalpy and entropy of oxygen. As the measurements were restricted to a narrow range between 750-1100'C in lead-rich alloys, however, the pronounced variation of the partial molar enthalpy of oxygen with temperature at constant alloy composition predicted by the quasi-chemical model could not be verified.

Activities of oxygen and lead in Pb-Ag O solutions

The reversible e.m.f. of galvanic cells: stainlesssteel,Ir,Pb+PbO|CaO+ZrO2|Ag+Pb+PbO,Ir,stainlesssteel,I and Pt,Ni+NiO|CaO+ZrO2|O(Pb+Ag),Cermet,Pt,II incorporating solid oxide electrolytes were measured as a function of alloy composition. In lead-rich alloys, the temperature dependence of the e.m.f. of cell I was also investigated. Since the solubility of oxygen in the alloy is small, the relative partial molar properties of lead in the binary Ag + Pb system can be calculated from the e.m.f. of this cell. The Gibbs free energies obtained in this study are combined with selected calorimetric data to provide a complete thermodynamic discription of liquid Ag + Pb Alloys. The activity coefficient of oxygen in the whole range of Ag + Pb alloys at 1273 K have been obtained from the e.m.f. of cell II; and these are found to deviate positively from Alcock and Richardson's quasichemical equation when the average co-ordination number of all the atoms is assigned a value of 2.

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.

Gibbs energy formation of lead titanate

The Gibbs energy of formation of titania-saturated lead titanate has been determined by e.m.f. measurements on the solid state cell;Pt,Ir,Pb + Pb1−xTiO3−x + TiO2(rutile)/CaO-ZrO2/Ni + NiO,Pt in the temperature range 1075–1350 K. The results obtained are significantly different from those reported in the literature based upon vapour pressure measurements, employing 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 titanate which are in better agreement with those obtained in this study. Earlier electrochemical measurements by Mehrotra et al. and more recent electrochemical measurements by Schmahl et al. both employing CaO-ZrO2 solid electrolytes are in good agreement with the present study. The electro-chemical measurements by Schmahl et al. using PbF 2 solid electrolyte give a slightly more positive Gibbs energy of formation. There was no evidence supporting the formation of compounds other than Pb1−xTiO3−x from yellow PbO and rutile form of TiO2 in the temperature range covered in this study.Résumé L'enthalpie libre de formation du titanate de plomb saturé en oxyde de titane a été déterminée par des mesures de FEM de la pile: Pt,Ir,Pb + Pb1−xTiO3−x + TiO2(rutile)/CaO-ZrO2/Ni + NiO,Pt dans le domaine de températures 1075–1350 K. Les résultats obtenus, different appréciablement de ceux publiés, déterminés par mesures de tensions de vapeur (techniques de transport et d'effusion de Knudsen) en supposant que la phase gazeuse etait uniquement constituée de molécules monomériques de PbO. Une réanalyse des résultats de la littérature, à partir de mesures par spectrométrie de masse sur les polymères de PbO gazeux, donne des enthalpies libres de formation du titanate de plomb se rapprochant de celles obtenues dans cette étude. Les mesures de Mehrotra et al. et plus récemment de Schmahl et al. utilisant toutes deux l'électrolyte CaO-ZrO2 concordent bien avec celles de la présente étude. Les mesures de Schmahl et al., à l'aide de l' électrolyte solide PbF2, donnent une enthalpie de formation légèrement plus positive. Pour la gammede températures étudiée, rien ne permet de supposer que des composés autres que Pb1−x TiO3−x puissent se former à partir du PbO Gaune) et du rutile (TiO2).

Potentiometric determination of the Gibbs energies of formation of lead aluminates

The Gibbs energies of formation of three compounds in the PbO-Al2O3 system—2PbO · Al2O3, PbO · Al2O3, andPbO· 6Al2O3—have been determined from potentiometric measurements on reversible solid-state galvanic cells [dformula Pt, Ir | Pb, alpha-Al[sub 2]O[sub 3], PbO [center-dot] 6Al[sub 2]O[sub 3] | ZrO[sub 2]-CaO | NiO, Ni | Pt] [dformula Pt | NiO, Ni | ZrO[sub 2]-CaO | Pb, PbO [center-dot] 6Al[sub 2]O[sub 3], PbO [center-dot] Al[sub 2]O[sub 3] | Ir, Pt] and [dformula Pt | NiO, Ni | ZrO[sub 2]-CaO | Pb, PbO [center-dot] Al[sub 2]O[sub 3], 2PbO [center-dot] Al[sub 2]O[sub 3] | Ir, Pt] in the temperature range 850–1375 K. The results are discussed in the light of reported phase diagrams for the PbO-Al2O3system. The partial pressures of different lead oxide species, PbnOn, n = 1–6, in the gas phase in equilibrium withthe aluminates are calculated by combining the results of this study with the mass-spectrometric data of Drowart et al.(1) for polymerization equilibria in the gas phase. The concentration of oxygen in lead in equilibrium with the aluminatesare also derived from the results and the literature data on the Gibbs energy of solution of oxygen in liquid lead.

CO oxidation by CeO(2)-Al(2)O(3)-CeAlO(3) hybrid oxides

A modified solution combustion technique was successfully used to synthesize sub-10 nm crystallites of hybrid CeO(2)-Al(2)O(3)-CeAlO(3). The fuel in the solution combustion was tuned to obtain mixed oxides and solid solutions of the compound. The compounds were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. XRD and TEM analysis showed the substitution of Al(3+) ions in the CeO(2) matrix when a combination of glycine, urea, hexamine and oxalyl dihydrazide was used as fuel for the synthesis. The compounds showed high activity for CO oxidation and the activity of the compounds was dependent upon the composition of the oxide.

Use of CaC12 as a chlorinating agent for oxides - A thermodynamic analysis

Presented is a thermodynamic feasibility analysis of extracting base metal chlorides fiom low-grade,multimetallic oxide ores using CaClz as a chlorinating agent in the presence of SOz undoz. The oxides react to form corresponding chlorides, while CaClz is converted to CaS04. The Ellingham diagram is usedfor comparing the standard Gibbs' fiee energy chanlpef or the su(fation-chlorinationr eaction of a large number of oxides. Except for alumina, silica and chromia, most of the other metal oxides will be converted to their respective chlorides. The volatile chlorides can be condensed, and the chlorides present in the condensed state can be leached. A process is proposed that uses a nontoxic chlorinating agent and gives an eficient sepurutiort cftlte metallic vuluesfr.om the garlgue.

System Nd-Pd-O: Phase diagram and thermodynamic properties of oxides using a solid-state cell with advanced features

An isothermal section of the phase diagram for the system Nd-Pd-O at 1350 K has been established by equilibration of samples representing 13 different compositions and phase identification after quenching by optical and scanning electron microscopy, x-ray diffraction, and energy dispersive analysis of x-rays. The binary oxides PdO and NdO were not stable at 1350 K. Two ternary oxides Nd4PdO7 and Nd2Pd2O5 were identified. Solid and liquid alloys, as well as the intermetallics NdPd3 and NdPd5, were found to be in equilibrium with Nd2O3. Based on the phase relations, three solidstate cells were designed to measure the Gibbs energies of formation of PdO and the two ternary oxides. An advanced version of the solid-state cell incorporating a buffer electrode was used for high-temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MP a as the reference electrode. Electromotive force measurements, conducted from 950 to 1425 K, indicated the presence of a third ternary oxide Nd2PdO4, stable below 1135 (±10) K. Additional cells were designed to study this compound. The standard Gibbs energy of formation of PdO (†f G 0) was measured from 775 to 1125 Kusing two separate cell designs against the primary reference standard for oxygen chemical potential. Based on the thermodynamic information, chemical potential diagrams for the system Nd-Pd-O were also developed.

An advanced design of the solid-state cell for accurate thermodynamic measurements on ternary oxides: system Sm–Pd–O

An advanced design of the solid-state cell incorporating a buffer electrode has been developed for high temperature thermodynamic measurements. The function of the buffer electrode, placed between reference and working electrodes, was to absorb the electrochemical flux of the mobile species through the solid electrolyte caused by trace electronic conductivity. The buffer electrode prevented polarization of the measuring electrode and ensured accurate data. The application of the novel design and its advantages have been demonstrated by measuring the standard Gibbs energies of formation of ternary oxides of the system Sm–Pd–O. Yttria-stabilized zirconia was used as the solid electrolyte and pure oxygen gas at a pressure of 0.1 MPa as the reference electrode. For the design of appropriate working electrodes, phase relations in the ternary system Sm–Pd–O were investigated at 1273 K. The two ternary oxides, Sm4PdO7 and Sm2Pd2O5, compositions of which fall on the Sm2O3–PdO join, were found to coexist with pure metal Pd. The thermodynamic properties of the ternary oxides were measured using three-phase electrodes in the temperature range 950–1425 K. During electrochemical measurements a third ternary oxide, Sm2PdO4, was found to be stable at low temperature. The standard Gibbs energies of formation (Δf(ox)Go) of the compounds from their component binary oxides Sm2O3 and PdO, can be represented by the equations: Sm4PdO7: Δf(ox)Go (J mol−1)=−34,220+0.84T(K) (±280); Sm2PdO4: Δf(ox)Go (J mol−1)=−33,350+2.49T(K) (±230); Sm2Pd2O5: Δf(ox)Go (J mol−1)=−59,955+1.80T(K) (±320). Based on the thermodynamic information, three-dimensional P–T–C and chemical potential diagrams for the system Sm–Pd–O were developed.

System Cu-Rh-O: Phase diagram and thermodynamic properties of ternary oxides CuRhO2 and CuRh204

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 Δf(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.