Ellingham diagrams

Free Energy - Temperature Diagrams (Ellingham Diagrams)... 

Silica is only decomposed by those metals which have a high affinity for oxygen as indicated by the Ellingham diagram (Fig. 18.4). On this basis, molten sodium should be compatible with silica ... 

The products of decomposition of metal carboxylates vary to some extent with the constituent cation and the final residue is usually either the metal or an oxide, occasionally the carbide and sometimes some elemental carbon deposit. Dollimore et al. [94] have described the use of Ellingham diagrams for the prediction of the composition of the solid products of oxalate decompositions. The complete characterization of residual material can be difficult, however, since the solids may be finely divided, pyrophoric [1010], metallic and amorphous to X-rays. 

Figure 4.9 Ellingham diagram for the free energy of formation of metallic oxides.

When the metal can form a stable carbide, the product of the carbothermic reduction of its oxide may be a carbide instead of the metal itself. The question as to whether a carbide or the metal forms under standard conditions when the oxide is reduced by carbon is not answered by the Ellingham diagram. To obtain an answer to this question, a more detailed consideration of the thermodynamic properties of the system is necessary. 

The minimum temperature necessary for the formation of the metal under a particular reduction pressure is that temperature at which the corresponding carbon monoxide pressure line intersects the Nb2C-NbO-Nb equilibrium line in the Pourbaix-Ellingham diagram. 

The Pourbaix-Ellingham diagram of the tantalum-carbon-oxygen (Ta-C-O) system indicates fewer sequential steps between the oxide and the metal as compared to the niobium-carbon-oxygen system ... 

According to the Ellingham diagram of oxides, water or steam (H20) is stabler than many metal oxides over a wide and useful range of temperatures, and hydrogen can reduce many metal oxides by reactions of the type... 

Steel making, broadly speaking, is an oxidation process in which impurities such as carbon, silicon, manganese, phosphorus and sulfur present in the pig iron are removed to specified levels. It can be anticipated from the Ellingham diagram that at about 1600 °C, the elements C, Si, and Mn would oxidize preferentially before iron undergoes excessive oxidation. The oxidation reactions may be represented by... 

Figure 4.23 Ellingham diagram for various metal oxides. Thermodynamic data are taken from reference [21].

Values for G° are known for various oxidation reactions and can be plotted on diagrams called Ellingham diagrams as shown in Fig. (3.4). Such diagrams are very useful to predict reactions associated with oxidation and reduction. 

Figure 3.4. Ellingham diagram showing standard Gibbs energy of formation of various oxides as a function of temperature.

If we were only interested in bulk copper and its oxides, we would not need to resort to DFT calculations. The relative stabilities of bulk metals and their oxides are extremely important in many applications of metallurgy, so it is not surprising that this information has been extensively characterized and tabulated. This information (and similar information for metal sulfides) is tabulated in so-called Ellingham diagrams, which are available from many sources. We have chosen these materials as an initial example because it is likely that you already have some physical intuition about the situation. The main point of this chapter is that DFT calculations can be used to describe the kinds of phase stability that are relevant to the physical questions posed above. In Section 7.1 we will discuss how to do this for bulk oxides. In Section 7.2 we will examine some examples where DFT can give phase stability information that is also technologically relevant but that is much more difficult to establish experimentally. 

Figure 2.24 Ellingham diagram for the oxidation of silver. Reprinted, by permission, from D. R. Gaskell, Introduction to Metallurgical Thermodynamics, 2nd ed., p. 272. Copyright 1981 by McGraw-Hill Book Co.

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