Experimental Methods

2.1. Powder Synthesis:

The starting materials used in the synthesis were metal nitrates Sm(N03)3.6H20 (99.9 % purity), La(N03)3.6H20 (99.9% purity), Sr(N03)2 (98 % purity), Co(N03)2.6H20 (97.7 % purity) and glycine (NH2CH2COOH, 99.5 % purity), all from Alfa Aesar. A flow chart showing the various steps involved in the synthesis of powders by the solution-combustion process is shown in Fig. 1. Metal nitrates are employed both as metal precursors and oxidizing agents. Stoichiometric amounts of the metal nitrates, to yield lOg of the final SSC or LSC oxide powder, were dissolved in deionized water. A calculated amount of the amino acid glycine (0.7 mole per mole of N03") was also dissolved in deionized water. The glycine solution was slowly added to the metal nitrate aqueous solution under constant stirring. Glycine acts as a complexing agent for metal cations of varying sizes as it has a carboxylic group at one end and an amino group at the other end. The complexation process increases the solubility of metal ions and helps to maintain homogeneity by preventing their selective precipitation. The resulting clear and transparent red colored solution was heated on a hot plate until concentrated to about 2 mole/liter on metal nitrate basis. While the solution was still hot, it was added drop wise to a 2 liter glass beaker that was preheated between 300~400°C. The water in the solution quickly evaporated, the resulting viscous liquid swelled, auto-ignited and initiated a highly exothermic self-contained combustion process, converting the precursor materials into fine powder of the complex oxides. Glycine acts as a fuel during the combustion reaction, being oxidized by the nitrate ions. Oxygen from air does not play an important role during the combustion process. The overall combustion reactions can be represented as:

0.6 La(N03)3 + 0.4 Sr(N03)2 + Co(N03)2 + 3.2 H2NCH2COOH + (1.8 - x/2) 02 -»■

0.5 Sm(N03)3 + 0.5 Sr(N03)2 + Co(N03)2 + 3.2 H2NCH2C00H + (1.95 - x/2) 02 ->•

Flowchart Solution Synthesis Methods
Figure 1.—Flow chart for solution-combustion synthesis of Lao.6Sro.4Co03.x and Smo.5Sro,5Co03_x nano-powders

indicating the formation of CO2, N2, and H2O as the gaseous products. The evolution of gases during the combustion process helps in the formation of fine ceramic powder by limiting the inter-particle contact. The resulting black powder contained some carbon residue and was further calcined to convert to the desired product. Small portions (~1 g) of this powder were heat treated in air at various temperatures between 700 and 1300°C for two hours to study the development of crystalline phases.

2. 2. Characterization

Thermal gravimetric analysis (TGA) of the powders was carried out using a Perkin-Elmer Thermogravimetric Analyzer 7 system which was interfaced with computerized data acquisition and analysis system at a heating rate of 10 °C/min. Air at 40 ml/min was used as a purge gas. X-ray diffraction (XRD) analysis was carried out on powders heat treated at various temperatures for crystalline phase identification and crystallite size determination. Powder XRD patterns were recorded at room temperature using a step scan procedure (0.02720 step, time per step 0.5 or 1 s) in the 20 range 10-70° on a Philips ADP-3600 automated diffractometer equipped with a crystal monochromator employing Cu Ka radiation. Microstructural analysis was carried out using a JEOL JSM-840A scanning electron microscope (SEM). Prior to analysis, a thin layer of Pt or carbon was evaporated onto the SEM specimens for electrical conductivity.

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