Oxygen pressure - thermodynamic

The thermodynamics of the 123 compound have been extensively studied and writing dre formula as (YBa2Cu3)Oj. it has been shown that at 1000 K the value of z changes from 6 to 6.6 in dre oxygen pressure range 10 " to one atmosphere of oxygen. Above this temperature, the 123 compound decomposes to 211 plus barium cuprate and cuprous oxide according to the reaction... 

The operating conditions (temperature, oxygen pressure, etc.) required to achieve each of these results depend on the thermodynamics of the... 

Vapor pressures and vapor compositions in equilibrium with a hypostoichiometric plutonium dioxide condensed phase have been calculated for the temperature range 1500 I H 4000 K. Thermodynamic functions for the condensed phase and for each of the gaseous species were combined with an oxygen-potential model, which we extended from the solid into the liquid region to obtain the partial pressures of O2, 0, Pu, PuO and Pu02 as functions of temperature and of condensed phase composition. The calculated oxygen pressures increase rapidly as stoichiometry is approached. At least part of this increase is a consequence of the exclusion of Pu +... 

Figure D-1 Oxygen pressure-enthalpy diagram. (From LN Canjar, FS Manning. Thermodynamic Properties and Reduced Correlations for Gases. Houston, TX Gulf Pub, 1967. Reproduced by permission.)...

Figure 7.4 Oxygen vacancy concentration in Cu20 calculated as described in the text at three different oxygen pressures. The dashed lines for the two lower pressures indicate the temperature region where Cu20 is thermodynamically unfavored relative to Cu as defined in the phase diagram in Fig. 7.3.

We do not need to regard YBa2Cus07 as a solid solution, but it is not a thermodynamically stable phase at any temperature/pressure condition (11). Thermodynamic stability exists in the YBa2Cus06+x system in a certain region of temperature and oxygen pressure, but only for values of x between zero and about 0.6. Even these compositions appear not to be thermodynamically stable at room temperature and below. 

Bevan and Anderson (32) deduced that adsorbed oxygen controlled the resistance of sintered zinc oxide at temperatures between 500°C and 1000°C. This conclusion was based on the observations that (1) the oxygen pressure had a reversible controlling effect on the resistance down to 500°C, too low a temperature for thermodynamic equilibrium to be... 

The relation between the non-stoichiometry and the equilibrium oxygen pressure mentioned in Section 1.1 can be deduced from the phase rule. For the purpose of the derivation of the phase rule, we shall review fundamental thermodynamics. Gibbs free energy G is defined by the relation... 

Fei O, called wiistite, has been studied from the viewpoints of thermodynamics and physicochemical properties. As mentioned in Section 1.1, stoichiometric FeO cannot be prepared under the usual conditions. Many investigators have studied the thermodynamic properties of wustite by use of various kinds of techniques. Here we introduce a study carried out by Fender and Rileywho used a solid electrolyte cell (see Section 1.4.8) to determine the equilibrium oxygen pressure Por The following cell was utilized,... 

The thermodynamic data for the platinum oxides are not well established. However, a reasonable value for the free energy of formation of the lower oxide, PtO, at 527°C. is —1 kcal./mole (5, 25). This corresponds to an oxygen dissociation pressure at 527°C. of 0.28 atm.—i.e., bulk PtO is unstable toward decomposition to bulk Pt for oxygen pressures below 0.28 atm. Bulk Pt02 is, of course, even less stable. Nevertheless, it has been reported that at this temperature and an oxygen pressure... 

The synthesis of chromium carbide required a high temperature owing to the competition between carbon and oxygen. Figure 14.1 shows that, under standard conditions, Cr203 is thermodynamically favoured at low temperatures. The oxide formation is probably due to water impurities present in the gas phase. The oxide layer is thicker than the usual passivation layer and this oxidation process at low oxygen pressure has already been explained earlier by Moreau and Benard.14... 

Careful analyses of the thermodynamic data of the praseodymium and terbium oxides led to the construction of their RCL-O2 phase diagrams (Hyde et al., 1966 Hyde and Eyring, 1965). The cerium oxides were studied by means of X-ray powder diffraction (Bevan, 1955) and the CeOx-02 phase diagram was constructed from equilibrium reaction studies at oxygen pressures down to 10-24 atm and temperatures up to 1200 °C (Bevan and Kordis, 1964 Ricken et al., 1984). It is worth to notice that the phase diagrams of Ce0x-02, PrCb-Cb, and Tb0x-02 systems... 

Sealed Tube Method. No evidence for compound formation was observed when mixtures of iron and uranium oxides were heated in air to temperatures as high as 1200°C. Substituting ferric and uranyl nitrates for the oxides as starting materials also proved unsuccessful. Ferric oxide and UO2.64 were the only product phases, thus giving an empirical formula of FeU04.i4 in the 1 1 mixture, and FeU309.42 in the 1 3 mixture. Unlike the situation encountered in the other double oxide systems, the iron uranates do not appear to have sufficient thermodynamic stability to be synthesized at ambient oxygen pressure. 

The concentrations of electrons and holes depend on the external thermodynamic parameters, such as T and Po2, and may often be widely varied within the phase stability limits. Equations (3.37) and (3.48) presented such dependencies at a constant temperature, but at relatively high and low oxygen pressures these concentrations can be expressed as... 

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