The XRD patterns of the Co-precipitation yielded precipitate of supposedly LSCF before and after calcinations at different temperatures are shown in the Fig. 2.
According to the Fig. 2,XRD pattern of dried precipitate represents the formation of different metal hydroxides precipitated by increasing pH during synthesis process. Accordingly, after heat treatment of the dried Co-precipitation yielded powder at 500°С, LaFeO3perovskite phase (JCPDS card 82-1958), La2O3, SrCO3, Fe3O4 and Co3O4wereemerged. The XRD pattern of the sample calcined at 700°С reveals that the perovskite LSCF (JCPDS card 82-1961)[13]was formed simultaneous withLa2O3and some SrCO3as secondary phases. Phase impurities of La2O3and SrCO3 have been reported in the synthesis process of LSCF powder from Co-precipitation[12],[14]and sol gel[15] methods too. Regarding to the pre formation of the orthorhombic perovskite phase (500°С), orthorhombic structured LSCFhas been presented as well. At the temperature 900°С, the LSCF characteristic peaks with rombohedral structure became stronger while the intensity of the La2O3 peaks decreased and the strontium carbonate peaks vanished since increasing temperature leads to enhance diffusion of lanthanum and strontium cations to the perovskite structure. As the calcination temperature reaches to 1000°С both the perovskite and La2O3 peaks became stronger because of the grain growth which is supported from the reported average crystallite size calculated from XRD pattern in the Table 1. Therefore, the best calcination temperature for precursor prepared by co-precipitation method is 900°С due to the minimum amount of remained secondary phase and increasing the crystallinity of perovskite phase.
The average crystallite size for those samples calcined at the temperatures of 700, 900 and 1000°С calculated by Scherer equation[16]:
In which D is the average crystallite size, λ is the wave length, β is the peak width at the half maximum amount and θ is the diffraction angle.
Table 1: the average crystallite size of the LSCF powder calcined at 700,900 and 1000°С calculated by Scherrer equation
Calcination temperature
|
700
|
900
|
1000
|
average crystallite size (nm)
|
9
|
11
|
13
|
The FT-IR results of the LSCF Co precipitation yielded precipitate and the LSCF sample calcined at 900°С are presented in Fig. 3.
According to the Fig. 3 the dried precipitate spectrum, a wide peak appeared at 3392.26 cm− 1shows[17]the OH stretching band and therefore demonstrates the adsorbed water existence. The observed peaks in the 1471 − 1368 cm− 1wave number range confirm the presence of the (OH) groups in the structural water and the metal hydroxides formation[18].The absorption band in the 852.04 wave number illustrates the Co32− existence.Therefore considering the ammonium carbonate usage as the precipitant agent, may be a few amounts of carbonate compounds formed during precipitation too. There are two absorption bands in the 697.75 and 746.71 wave numbers, which referred to the La-O [19]bond and the Lepidocrocite[20]-. The observed peak in the 451.08 cm− 1 indicates the Sr-O or H-Sr-OH bonds are present [21]. In the calcined sample, the MO6 octahedral site absorption bands detection, indicates the peroveskite phase formation[22]. The two absorption bands that come into sight in439.99 and 857.65 cm− 1 which are related to lanthanum oxide[23] and C-O band[21]respectively confirm lanthanum oxide and strontium carbonate as secondary phases. The peaks in wave numbers of 1472.54 and 3367.56 cm− 1are related to the environmental moisture absorbed by the sample[18].
The DTA and TG curves of Co precipitation yielded precipitate during heating are displayed in Fig. 4. According to the DTA results, the endothermic peak at 164°С is attributed to the released adsorbed moisture[16].There are three other exothermic peaks in the temperature ranges of 290,462 and 551°С which correspond to the:
1) Metal hydroxides decomposition reaction according to the below equations[17], [24]–[26]:
2) The intermediate compounds formation[24], [26]–[29]:
3) and the beginning of the LSCF formation, respectively[16], [30], [31].
It seems that the formation of LSCF with rombohedral structure takes place through the pre formation of an orthorhombic structured phase (LaFeO3)[32] simultaneous with the La2O3, SrCO3, Co3O4and Fe3O4compounds. As - temperature increases, the perovskite with orthorhombic structure tends to change into rombohedral due to the distortion increasing which can be attributed to the above mentioned compounds dissolving in LaFeO3 unit cell. It should be noted that the LSCF with rombohedral structure forms at 700°С completely.
The proposed mechanism for the LSCF formation is as follows:
a) Ammonium carbonate dissolves in distilled water:
b) Lanthanum oxide dissolves in diluted nitric acid:
c) Metal salts dissolve in distilled water[22]:
d) The hydrated metal oxide and hydroxide complex formation at 65°С and the pH̴ 8.5as a result of the ammonium carbonate injection into the mixed metal solution reactor In the co precipitation process:
The washed and dried precipitate endures step by step transitions as follows:
1) Water evaporation in the 100–164°С temperature range:
2) Metal hydroxides decomposition reaction beginning in 290°С:
3) Inter mediate compound formation in the 462–551°С temperature range:
4) The lanthanum strontium cobalt ferrite formation beginning in the 551°С:
The TG result (Fig. 4) shows the 40.48% weight lost during the heating process. This overall weight loss occurs approximately in - four steps with increasing temperature. First steps starts at temperature range of 164–330°С, the calculated weight loss is about 13.01 wt%, while the TG curve shows an approximate weight loss of 9 wt%. At the intermediate compounds formation stage (330–550°С) weight is expected to increase by 6.85% according to calculations based on chemical reactions, but the curve keeps its downward trend (12.5%).Incomplete and delayed reactions to the previous step and coinciding with their subsequent reactions could be the reason for these inconsistencies. For instance onset of formation lanthanum oxide - occurs at 600°С[24], therefore some of the LaOOH may remain unreacted during the peroveskite phase formation and be hydrolyzed at higher temperatures to La2O3.
At 550°С and higher temperatures, the samples lose weight accrues more slowly. This slow rate of weight loss is reasonable considering the formation of stable peroveskite phase.
The XRD patterns of the LSCF and the powders which contain 0.3molDyand 0.6molDyat 700°С are shown in the Fig. 6.
Phase characterization of the compounds containing 0.3molDyand 0.6molDy are corresponded with LaFeO3 (Jcpds no 01-082-1961) and DyFeO3 (Jcpds no 00-046-0135) cards, respectively. As it could be seen from the XRD pattern of Dy-doped LSCF compounds, the peroveskite phase with rombohedral structure is formed and a few amount of strontium carbonate exists, too. The diffraction pattern peaks– have shifted to larger angles compared to the peak position of the un-doped LSCF pattern as a result of the diffusion of the Dy+ 3 cations into the unit cell. Considering increasing density and the peak displacement to the larger diffraction angles implies the reduction of the distance between the crystalline plates led to reducing the volume of unit cell. Therefore it could concluded that theDy+ 3cations are replaced by La3+cations (A sites in peroveskite structure) regarding to the smaller cationic radius of the Dy3+ (0.91Ȧ) compared to the cationic radius of the La3+ (1.03Ȧ).It also shows that the dysprosium cations occupying the fewer space of the lattice and provide enough space for the La3+ cations in A site and prevent the unreacted La2O3 remain[1] however according to table 5-the density of the sample which contains 0.3 mol Dy has not changed significantly. Based on the density relation (), it was expected by replacing Dy into La (MDy= 162.5 g.mol− 1 and MLa=138.9 g.mol− 1). If the most amount of the Dy3+cations located in the interstitial spaces whereas lanthanum cations are completely located in the position of the A sites in the peroveskite structure and distortion caused by Dy3+inplacement in interstitial spaces makes some of the strontium cations unstable due to their relative large radius(cationic radius = 1.12Ȧ) and different oxidation state, so strontium cations move out of the network, therefore a few amount of strontium carbonate is detected in the XRD pattern of the sample containing 0.3 mol Dy. As evidenced by the XRD pattern and the increased density of the sample which contains 0.6molDy, the crystalline structure change into orthorhombic and the volume of the unit cell reduces. In general, the substitution of lanthanide cations with higher atomic number and lower ionic radius than lanthanum, reduces the lattice parameter led to decrease the unit cell volume - and also causes a change in the structure of the rombohedral to the orthorhombic [33]. According to the pattern of the sample contains 0.6 molDy, the strontium carbonate peaks get stronger and the Dy2O3 peaks appear, too. It can be concluded - increasing distortion due to the significant radius difference of the Dy3+and Sr2+ cations compared to the La3+ and Sr2+ radius difference besides the more reduced volume prevent Sr2+ and some of the Dy3+ from penetrating peroveskite structure and intensifying strontium carbonate and Dy2O3 peaks. The calculated mean crystallite size for the compounds containing 0.3 and 0.6 mol Dy−are 6 and 7 nm.
Table 2: the measured density for the samples which calcined at 700°С
Sample
|
LSCF
|
LDSCF
|
DSCF
|
Density (g.cm-3)
|
6.10
|
6.11
|
6.37
|
The FE-SEM micrographs of the calcined powders are illustrated in Fig. 7.it can be observed that the porous agglomerated powder with spherical shape formed in all samples.-- the- high calcination temperature which makes the weak primary chemical bonds stronger, is the agglomerated particles formation reason[16]. The small particle size is another factor facilitates the agglomerated particles formation as a result of the particle assembling that accurse to reduce the high surface energy. The particle size distribution is rather uniform and the average amounts of particle size determined by Dgmizer software for the LSCF samples which were calcined at 700, 900 and 1000°С are 44, 82 and 131 nm, respectively. Figure 7(a),(b) and (c) show the increase in agglomerates with increasing calcination temperature in the LSCF powder, so that the boundary between the particles in Fig. 7(a) is more separable compared to the Fig. 7(b) and (c). Figure 7(e) and (d) are for compounds containing 0.3 mol Dy and 0.6 mol Dy respectively. Although the calcination temperature for the (a), (d) and (e) is the same, there is a significant difference between the average particle sizes of the compounds containing the Dy and LSCF powder. The mean particle size of the sample - have decreased with increasing amount of Dy Likewise, the calculated crystallite size decreased with increasing Dy content at the constant calcination temperature. This confirms that incorporation of Dy3+in to the structure decreases the average particle size - and crystallite size. It is likely that the presence of the Dy3+ by distorting the structure make it difficult for other cations to penetrate into the primary nuclei [34].Therefore, the growth of the nuclei is reduced, thus reducing the size of crystallites.-the particle size distribution of the powders containing Dy is more uniform compared to the LSCF sample. The average particle size for the samples containing 0.3 mol and 0.6 mol Dy are 28 and 24 nm, respectively. Some reported average LSCF particle size from different references have been listed in the Table 3.
Table 3: the average particle size for some different synthesis methods in the different temperatures
Synthesis method
|
Calcination temperature(°С)
|
Average particle size (nm)
|
Reference
|
Co precipitation
|
1000
|
90
|
[12]
|
Sol-gel
|
700
|
10-60
|
[31]
|
Microwave assisted glycine nitrate combustion
|
800
|
114
|
[35]
|
Sol-gel
|
900
|
90
|
[36]
|