Publisher Summary This review is concerned with the neglected class of inorganic compounds, which contain ions of the same element in two different formal states of oxidation. Although the number of references cited in our review show that many individual examples of this class have been studied, yet they have very rarely been treated as a class, and there has never before, to our knowledge, been a systematic attempt to classify their properties in terms of their electronic and molecular structures. In the past, systems containing an element in two different states of oxidation have gone by various names, the terms “mixed valence,” nonintegral valence,” “mixed oxidation,” “oscillating valency,” and “controlled valency” being used interchangeably. Actually, none of these is completely accurate or all-embracing, but in our hope to avoid the introduction of yet another definition, we have somewhat arbitrarily adopted the phrase “mixed valence” for the description of these systems. The concept of resonance among various valence bond structures is one of the cornerstones of modern organic chemistry.

Mixed Valence Chemistry-A Survey and Classification

The alumina-silica phase relations of samples annealed in oxygen and quenched were studied by optical microscopy, image analysis, X-ray diffraction, and the electron microprobe. The solid solution boundaries of mullite changed with increasing temperature and joined at 1890°C and a composition of 77.15 wt% alumina. The melting point of mullite was 189°C with a peritectic between 76.5 and 77.0 wi% alumina.

Alumina‐Silica Phase Diagram in the Mollite Region

The extant data for the physical properties of BOS slags and for slag systems relevant to BOS have been collated and evaluated. Data for the following physical properties have been reviewed: phase equilibria, viscosities, densities, electrical conductivities, thermal conductivities, and the surface and interfacial tension between the metal and slag. In addition, various mathematical models have been developed for the estimation of the liquidus temperature, density, viscosity, surface tension, heat capacity, and enthalpy of slags. The efficacies of these different models have been evaluated and their reliability in predicting physical property for molten BOS slags has been assessed.

Physical properties of bos slags

The transfer of Cr, Si, Mn, P, S, C, Ni, and Mo between the slag and the weld pool has been studied for submerged arc welds made with calcium silicate and manganese silicate fluxes. The results show a strong interaction between Cr and Si transfer but no interaction with Mn. The manganese silicate flux produces lower residual sulfur while the calcium silicate fluxes are more effective for removal of phosphorus. The effective oxygen reaction temperature lies between 1700 and 2000 °C for all elements studied. Evidence of Cr and Mn loss by metal vaporization is also presented.

Slag Metal Reactions during Submerged Arc Welding of Alloy Steels

Available thermodynamic and phase diagram data have been critically assessed for all phases in the CrO-Cr2O3, CrO-Cr2O2-Al2O3, and CrO-Cr2O2-CaO systems from 298 K to above the liquidus temperatures and for oxygen partial pressures ranging from equilibrium with metallic Cr to equilibrium with air in the case of the first two systems and toP
O
2 = 10−3 atm for the CrO-Cr2O3-CaO system. All reliable data have been simultaneously optimized to obtain one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. The models permit phase equilibria to be calculated for regions of composition, temperature, and oxygen potential where data are not available.

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CrO-Cr2O3, CrO-Cr2O3-Al2O3, and CrO-Cr2O3-CaO systems

Results are presented of a study in air of mixtures in the system CaO-Cr2O3-SiO2. The phase equilibrium diagram shows relations at liquidus temperatures for all but the high-lime part of the system. In this omitted part chromium in the mixtures oxidizes in air to higher valence forms. The compound Ca3Cr2Si3O12 (uvarovite) occurs at subsolidus temperatures, decomposing at 1370°C. to α-CaSiO3 and Cr2O3. The inhibiting action of chromium oxide on the inversion of high-temperature forms of Ca2SiO4 to the low-temperature γ-Ca2SiO4 is discussed in the light of new data. Evidence is presented for the existence of a pentavalent chromium compound, Ca3(CrO4)2, having solid-solution relations with Ca3SiO4.

Phase Equilibrium Studies in the System CaO‐Cr2O3‐SiO2

Phase equilibrium data resulting from quenching experiments are presented for the ternary system CaO-MnO-SiO2. An atmosphere of controlled oxygen pressure having Po2, ≅ 10−6 atm at 1555°C was used to maintain the manganese in the divalent state. The ternary liquidus surface is largely one of low-lying liquidus temperatures. Three ternary liquidus minima dominate this surface. These have the following compositions (in weight percent CaO, MnO, and SiO2): (a) 5.0, 48.4, and 46.6%, (b) 17.5, 45.0, and 37.5%, and (c) 15.0, 53.0, and 32.0%. Temperatures measured at these points are (a) 1265° C, (b) 1195°C, and (c) 1204°C. Isofracts of the quenched glasses are presented. Crystallization paths of ternary mixtures are represented by a series of fractionation curves and selected isothermal planes. Partition of manganese between coexisting pairs of crystalline phases (e.g., meta-silicate, olivine, and (Ca,Mn)O solid solutions) favors concentration of manganese in the more basic phase. Subsolidus equilibria involving these phases and also Ca3Si2O7 and Ca3SiO5 are discussed. Ca3Si2O7 and Ca3SiO6 do not admit any appreciable amounts of Mn++ into their lattices.

The Ternary System CaO—MnO—SiO2

Liquidus phase equilibrium data are presented for the system Al2O3-Cr2O3-SiO2. The liquidus diagram is dominated by a large, high-temperature, two-liquid region overlying the primary phase field of corundum solid solution. Other important features are a narrow field for mullite solid solution, a very small cristobalite field, and a ternary eutectic at 1580°C. The eutectic liquid (6Al2O3-ICr2O3-93SiO2) coexists with a mullite solid solution (61Al2O3-10Cr2O3-29SiO2), a corundum solid solution (19Al2O3-81Cr2O3), and cristobalite (SO2). Diagrams are presented to show courses of fractional crystallization, courses of equilibrium crystallization, and phase relations on isothermal planes at 1800°, 1700°, and 1575°C. Tie lines were sketched to indicate the composition of coexisting mullite and corundum solid solution phases.

The System Al2O3-Cr2O3-Sio2

A ternary phase equilibrium diagram is presented for the system Mg0-Mn0-Si02. Two peritectic points exist at 1536° and 1538°C. at liquid compositions of (a) 32 MgO, 5 MnO, 63 SiO2 and (b) 36 MgO, 5.5 MnO, 58.5 SO,. Each of these liquids is in equilibrium with a hodonite (Mn-SiO,) and a high enstatite (MgSiO,) solid solution, the third crystalline phase being cristobalite (SiO2) with (a) and olivine ((Mg,Mn]2,Si04) with (b). Metasilicate solid solutions form a congruently melting rhodonite series in the central part of the MgSi03-MnSi03 join, but melt incongruently to olivine plus liquid at the MgSiO3 end and to tridymite plus liquid at the MnSi03 end. X-ray diffraction data are given for two new high enstatite solid solution structures. Rhodonite solid solutions form a continuous structural series extending from MnSi0, to 94.5 weight % MgSi03. Continuous series of solid solutions exist between Mg,Si04 and Mn2SiO4 and between MgO and MnO. Compositional data are given for coexisting condensed phases, and courses of crystallization are described for certain ternary mixtures. A plot is presented of variation of refractive index of glasses which form an almost colorless series in the region of the rhodonite and high enstatite fields. The quenching technique was used in the investigation, with oxygen partial pressure controlled to maintain manganese as Mn2+.

F. P. Glasser

Papers

Phase Equilibrium Studies in the System CaO‐Cr2O3‐SiO2

The Ternary System CaO—MnO—SiO2

The System Al2O3-Cr2O3-Sio2

The Ternary System MgO‐MnO‐SiO2