Phase Formation Investigation In Mechanochemical Synthesis Of Nanostructured Strontium-Doped Lanthanum Manganite

The possibility to use perovskite oxides -as cathodes- should lead to an extreme decrease of solid oxide fuel cell (SOFC) operation temperature. In this research, the structural and physical properties of lanthanum strontium manganite have been investigated. The procedure is as follows. First, La۰.۸Sr۰.۲MnO۳ is synthesized by implementing the mechanochemical method, then the dried mixture powders ground at room temperature using a high-energy ball mill. The effect of grinding time and speed on the structural properties and phase behavior of Sr-doped LaMnO۳ system was explored as well. Structural parameters of compounds have been determined by X-ray diffraction (XRD), which confirmed that the uncontaminated and Nano crystallite lanthanum strontium manganite powder with perovskite structure and cubic symmetry has been prepared. The other results consisting of Brunauer, Emmett and Teller (BET) data and scanning electron microscopy (SEM) strongly indicated that grinding time has a drastic impact on mechanochemical synthesis of strontium-doped lanthanum manganite. Grinding time improvement led to Nano crystallite size, distribution and structural characteristics modification.The XRD patterns of not-ground samples are shown in Fig. ۱. A close look at this figure reveals that the characteristic potential split diffraction lines closely match with coarse powder. As can be seen there is a tendency to monoclinic structure formation in not-milled samples which are directly heated in relatively high temperatures. This tendency is identical to that of the LSM and due to a decrease in disorder by changing the symmetry from orthorhombic to monoclinic. Fig. ۲ indicates the X-ray patterns of ground samples of LSM with ۲۲۰ rpm and calcined at ۹۰۰oC . It can be detected that even at high grinding time (۷۲h) and following calcination at ۹۰۰oC for ۲ hours in air, the peak splitting was observed in all of the diffraction patterns. The diffraction patterns of milled powder with ۳۰۰ rpm and calcined at ۶۰۰oC are illustrated in Fig. ۳. For the samples which milled for ۱۲ hours and more than that, the diffraction peaks indexed well to perovskite structure with cubic symmetry. This suggested that Sr۲+ was successfully incorporated into the A-site (Lanthanum situation) of the LSM lattice. Due to the peak intensity increment, the amount of cubic phase LSM with perovskite lattice increased with grinding time at ۳۰۰ rpm constant speed and calcination temperature of ۶۰۰oC . The results of this research showed that diffraction patterns of LSM broadened out by increasing milling time up to ۴۸ hours. Also the final product was a homogenous LSM phase with perovskite structure and cubic symmetry which obtained in a single-step method using low rotational speed (۳۰۰ rpm) and relative low milling time (۱۲h) without any additional treatment.Fig. ۱ X-ray powder diffraction patterns of S-۱۲۰۰(a)- S-۱۳۰۰(b)- S-۱۴۰۰(c) not ground samples which illustrate the non-cubic structure duo to the obvious split in all peaks.Fig. ۲ X-ray powder diffraction patterns of S-۱h-۹۰۰(a), S-۳h-۹۰۰(b), S-۵h-۹۰۰(c), S-۱۰h-۹۰۰(d), S-۲۴h-۹۰۰(e), S-۳۶h-۹۰۰(f), S-۴۸h-۹۰۰(g) and S-۷۲h-۹۰۰(h) ground samples with ۲۲۰ rpm.Fig. ۳ X-ray powder diffraction patterns of S-۰h-۶۰۰(a) not milled and S-۳h-۶۰۰(b), S-۱۲h-۶۰۰(c), S-۲۴h-۶۰۰(d), S-۳۶h-۶۰۰(e) and S-۴۸h-۶۰۰(f) ground samples with ۳۰۰ rpm