A simple thermodynamics model for estimation and comparison the concentration of oxygen vacancies generated in oxide powders synthesized via the solution combustion method

• Published in: Ceramics international Vol. 45; no. 10; pp. 13496 - 13501
• Main Authors: Kermani, Farzad, Mollazadeh, Sahar, Vahdati Khaki, Jalil
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2019
• Subjects: Oxygen vacancy; Relative vacuum; Solution combustion synthesis; Thermodynamic model; Solution combustion synthesis; Relative vacuum; Thermodynamic model; Oxygen vacancy
• ISSN: 0272-8842; 1873-3956
• DOI: 10.1016/j.ceramint.2019.04.053

As the following reaction shows, in exothermic solution combustion synthesis (SCS) process, a high amount of gaseous by-products liberated in an extremely short period of time which could expel the oxygen from the reaction media. Oxygen driving out from the reaction vessel decrease its concentration and as a result, instant vacuum could happen during combustion process. A(NO3)d1(s)+(X)B(NO3)d2(s)+(Y)Fuel(s)=ABXOd1+Xd22(s)+nH2O(g)+mCO2(g)+oN2(g)+Heat In which d1 and d2 are the capacity of cations. X, Y, n, m, and o are molar coefficients, as well. (A) represents the basic structure, and B is the doped material oxide. At absolute vacuum in which oxygen pressure equals to zero, the reaction ABXOd1+Xd22=ABXOd1+Xd22−ξ+ξ2O2, spontaneously proceeds as ΔG = -∞. ABXOd1+Xd22 is a commonly expected product, ABXOd1+Xd22−ξ is its corresponding defective structure, and ξ is the concentration of oxygen vacancies in structure of ABXOd1+Xd22 oxide. However, H2O and CO2 can partially decompose. Thus, due to the released O2 from decomposition of H2O/CO2 the relative vacuum must be taken into considered. Based on this relative vacuum, a simple model proposed in order to estimate and compare the concentration of oxygen vacancies in SCS method. In this proposed model the comparable values of oxygen vacancy concentrations in the reaction ABXOd1+Xd22=ABXOd1+Xd22−ξ+ξ2O2 as a function of Tce, (the temperature of the cooling environment) can be expressed as. ξ=−2RTce∗(LnPO2,max–Gformation,O2,Tce0RTce). [Display omitted]