**2. Experiment**

*Cobalt Compounds and Applications*

environmentally friendly.

stoichiometric ratios followed by heating to obtain the desired reaction product, at the end of the reaction, there is no waste to dispose. In this way, the method is also

The overall price of oxide perovskite powders is strongly influenced by the industrial production route. Solid state reaction technology is resource- and energy-saving. The processing route allows one to obtain the reaction products readily in one step by using less expensive precursors, without waste products. The upscale of solid-state synthesis route resumes to batch production. This is different from flow processing which is a continuous production technique that has inflexibility to adjust if the case. Additionally, the absence of waste and by-products is another major positive aspect regarding the environmental impact of the entire production process. This is why we consider this method useful for both environmental protection and industrial use. In solid state reaction, rates are typically diffusion-limited; consequently, decreasing the diffusion distances through intimate mixing of reactants possibly contributes to overcome the diffusion barrier in addition to high temperature during the thermal treatment. The characteristics of the starting materials impact the area of contact between the reacting particles with influence of the rate of reaction. During the heating regime, a non-isothermal transformation takes place, and during the dwell regime, an isothermal transformation takes place. A significant kinetic parameter for studying the phase transformation of precursors is the activation energy Ea, representing the energy barrier for atoms and molecules to move and rearrange. For a given differential scanning calorimetry curve with the heating

rate β, one observes the maximum reaction rate at the peak temperature T.

tion is described with the mathematical expression of Kissinger Eq. 2 [27]:

d( lnβ \_\_\_ <sup>T</sup><sup>2</sup> ) \_\_\_\_\_\_ d( \_\_1 *T*)

ture corresponding to maximum process rate, and R is the gas constant.

cations become mobile so that their bulk diffusion is possible [28, 29].

electrical conductivity with temperature is explained by Eq. (3):

<sup>σ</sup> <sup>=</sup> Aexp\_\_\_

<sup>σ</sup> <sup>=</sup> Aexp\_\_\_

conduction process dominates.

In this study, the kinetic parameters of solid state transformations are determined from the maximum reaction rate at the peak temperature T. For a set of differential scanning calorimetry curves under a constant heating rate, the kinetic transforma-

> = −\_\_ E

where β is the heating rate, Ea is the activation energy, T is the absolute tempera-

The electrical conduction phenomenon in perovskite materials is very important since many properties depend on it. There is a strong correlation between the electrical conductivity of the materials, temperature, and the nature of the sample analyzed. In bulk materials, two types of conductivity phenomena occur: long-range conductivity and localized transport oxygen vacancies. The conduction mechanism can be ionic, electronic, or both. The proportion of ionic to electronic conduction in the materials varies upon temperature and the purity of material. The variation of

−Ea

Eq. (4) describes the phenomena at higher temperatures where the intrinsic

KT + Bexp\_\_\_\_

−Ea

−Eab

For the processing the choice of dwell temperature was taking in account Tammann's temperature described as the temperature above which its constitutive

<sup>R</sup> (2)

KT (3)

KT (4)

**94**

#### **2.1 Material preparations**

The formation of the perovskite phase via solid state reaction was done under controlled rates for heating and cooling as well as for the dwell of thermal treatment. A 2°C/min rate of heating and cooling was selected. Preparatory trials were carried out in order to find the suitable temperature for dwell covering 600–1000°C.

Equimolar quantities of La2O3 and Co3O4 powders from Aldrich, with >99.8% purity, were mixed and ground thoroughly in agate mortar. The mixing was done with isopropanol from Chimopar SA, purity >96%. For the thermal treatment, the powder in alumina crucibles was placed in chamber furnace. Once the powder was synthesized, compacted ceramics were prepared by uniaxial pressing technique. After this, they were submitted for sintering. Similarly, preparatory trials were performed to find out the proper sintering thermal treatment.

#### **2.2 Materials characterization**

The crystalline structures of the prepared powders were characterized by heating in chamber furnace, environmental air atmosphere, at 50°C increments, making X-ray diffraction measurements. A MiniFlex 600 Rigaku analyzer was used. 2θ scans were recorded between 5 and 90° and a speed of 1°/min, with resolution of 0.1°/step. The as-obtained reflexion patterns are indexed by using the Inorganic Crystal Structure Database (ICSD). Lattice constants and quantitative values for the identified phases are obtained from the fit to the corresponding X-ray diffraction spectra by using the PDXL powder diffraction analysis package from Rigaku. The microstructure and morphology of the as-prepared powders were examined using a scanning electron microscope VP CARL ZEISS (Field Emission Scanning Electron Microscope— FESEM) with LaB6 cathode enabling 2-nm resolution. Specimens of powders were prepared by depositing it on a conductive carbon-based double-faced adhesive tape. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were carried out on precursors and their equimolar mixture using STA 449 F5 Jupiter® from NETZSCH-Gerätebau GmbH. The instrument is equipped with Proteus® software to carry out the measurement and evaluation of the resulting data. The DSC/TG curves are recorded up to 1273 K, with a heating rate of 10 K/min.

The analyses of electrical properties of the sample were carried out from 133 to 513 K using Alpha-A Novocontrol Technologies Novocontrol GmbH analyzer in conjunction with a continuous nitrogen flow cryostat.

### **3. Results**

#### **3.1 Thermal behavior**

Thermal analysis results of precursors, **Figure 2**, indicate that Co3O4 powder, in red color, is stable below 1023 K while La2O3 powder, in black color, exhibits several steps of mass loss during the heating regime. La2O3 precursor features three weight-loss regions. The first, a gradual weight loss not shown here, is related to

the dehydration of free and physically absorbed molecular water and structural water from the precursor powder. The second and third regions, more pronounced, are associated with the dehydration process from La(OH)3 to La2O3 via LaOOH<sup>−</sup>, according to the chemical reactions described by Eqs. (5) and (6), respectively:

$$\text{La(OH)}\_3 \rightarrow \text{LaOOH}^- \star \text{H}\_2\text{O} \tag{5}$$

$$\text{2LaOOH}^{-} \rightarrow \text{La}\_2\text{O}\_3 + \text{H}\_2\text{O} \tag{6}$$

The total percent weight loss is around 1.88%. The first step, not shown here, of weight loss was 0.04%. The next steps correspond to the decomposition of La(OH)3. The weight loss in following step was around 0.87%, and in the third step was 0.98%. However, it is well known that La(OH)3 can react relatively fast with the water of the atmosphere to form hydroxyl phase [28]. The corresponding endothermic peaks on the differential scanning calorimetry curve appeared at 581 K and 910 K respectively. The corresponding areas are −23.44 and −15.34 μVs/mg, respectively.

The mixture of Co3O4 and La2O3 powder precursor, as can be seen in **Figure 3**, has a weight-loss step, associated with the presence of the La2O3, and after 1023 K, an endothermic event is detected around 1073 K. Previous studies have found similar features [30]. This is attributed to the ongoing dehydration of LaO(OH) and formation of LaCoO3 from a solid state reaction of lanthanum and cobalt oxides, following Eq. (7):

$$\text{La}\_2\text{O}\_3 + \text{Co}\_3\text{O}\_4 \xrightarrow{} \text{LaCoO}\_3\tag{7}$$

**97**

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route…*

The kinetic analysis of phase change for a series of non-isothermal tests, **Figure 4**, is applied to estimate the activation energy. This analysis was performed for third observed exothermal reaction from differential scanning calorimetry data of phase transformation process. The peak temperature T depends on the evolution of the sample with the heating rate. The dynamic heating rate influences the observed peak temperature T of thermal events. These correspond to the variations of growth and phase change rate with temperature. The new phase formation rate depends on temperature and is independent of time. It is observed, in this reaction, the peak temperature has the tendency to slightly shift to the

For polycrystalline reactants, the kinetic process depends on factors like particle size and shapes of the reacting particles, overlapping effects during the formations of the new phase, etc. Solid state reactions are complex and the evaluation of kinetic parameters is helpful for the development of approaching the preparation. In this evaluation, we made the assumption of first order

absolute temperature, **Figure 5**, of as-mixed Co3O4 and La2O3 powder precursors, an Ea = 223 kJ/mol is obtained, which seems to be in good agreement with earlier

The analysis of X-ray diffraction pattern corresponding to unmixed and mixed

precursors as function of temperature treatment is shown in **Figure 6**. In the lowest position of the diagram, the unmixed La2O3 and Co3O4 powder precursors are displayed. Going up in the diagram, the mixture of La2O3 and Co3O4 powder precursors at room temperature is indicated. The La2O3 powder precursor is tetragonal La2O3 (space group P-3m1) with a = b = 3.933 Å and c = 6.132 Å [32]. Traces of hexagonal La(OH)3 (space group P63/m) are detected. The corresponding lattice constant values are a = b = 6.529 Å and c = 3.859 Å. Co3O4 powder indicates that the precursor is single-phase, with cubic crystal symmetry (space group Fd-3m) [32].

The lattice constant value obtained from fit to data is a = 8.038 Å.

) versus the reciprocal of

higher temperatures as the heating rates increase.

*DSC-TGA plot of Co3O4 and La2O3 precursor powders in mixture.*

of reaction. From the plot of rate of reaction ln(β/T<sup>2</sup>

results reported [31].

**Figure 3.**

**3.2 X-ray diffraction investigation**

*DOI: http://dx.doi.org/10.5772/intechopen.86260*

No significant thermic effect was registered at higher temperatures. The corresponding endothermic peaks on the differential scanning calorimetry curve appeared at 1073 K and the area of −147.7, −36.58, and −48.09 μVs/mg, respectively.

**Figure 2.** *Weight-loss % diagram for Co3O4 powder, in red color, and La2O3 powder, in black color.*

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route… DOI: http://dx.doi.org/10.5772/intechopen.86260*

*Cobalt Compounds and Applications*

La(OH)3 <sup>→</sup> LaOOH<sup>−</sup>

2LaOOH<sup>−</sup>

following Eq. (7):

the dehydration of free and physically absorbed molecular water and structural water from the precursor powder. The second and third regions, more pronounced, are associated with the dehydration process from La(OH)3 to La2O3 via LaOOH<sup>−</sup>, according to the chemical reactions described by Eqs. (5) and (6), respectively:

The total percent weight loss is around 1.88%. The first step, not shown here, of weight loss was 0.04%. The next steps correspond to the decomposition of La(OH)3. The weight loss in following step was around 0.87%, and in the third step was 0.98%. However, it is well known that La(OH)3 can react relatively fast with the water of the atmosphere to form hydroxyl phase [28]. The corresponding endothermic peaks on the differential scanning calorimetry curve appeared at 581 K and 910 K respectively.

The mixture of Co3O4 and La2O3 powder precursor, as can be seen in **Figure 3**, has a weight-loss step, associated with the presence of the La2O3, and after 1023 K, an endothermic event is detected around 1073 K. Previous studies have found similar features [30]. This is attributed to the ongoing dehydration of LaO(OH) and formation of LaCoO3 from a solid state reaction of lanthanum and cobalt oxides,

La2O3 + Co3O4➔LaCoO3 (7)

No significant thermic effect was registered at higher temperatures. The corresponding endothermic peaks on the differential scanning calorimetry curve appeared at 1073 K and the area of −147.7, −36.58, and −48.09 μVs/mg, respectively.

The corresponding areas are −23.44 and −15.34 μVs/mg, respectively.

*Weight-loss % diagram for Co3O4 powder, in red color, and La2O3 powder, in black color.*

+ H2O (5)

→ La2O3 + H2O (6)

**96**

**Figure 2.**

**Figure 3.** *DSC-TGA plot of Co3O4 and La2O3 precursor powders in mixture.*

The kinetic analysis of phase change for a series of non-isothermal tests, **Figure 4**, is applied to estimate the activation energy. This analysis was performed for third observed exothermal reaction from differential scanning calorimetry data of phase transformation process. The peak temperature T depends on the evolution of the sample with the heating rate. The dynamic heating rate influences the observed peak temperature T of thermal events. These correspond to the variations of growth and phase change rate with temperature. The new phase formation rate depends on temperature and is independent of time. It is observed, in this reaction, the peak temperature has the tendency to slightly shift to the higher temperatures as the heating rates increase.

For polycrystalline reactants, the kinetic process depends on factors like particle size and shapes of the reacting particles, overlapping effects during the formations of the new phase, etc. Solid state reactions are complex and the evaluation of kinetic parameters is helpful for the development of approaching the preparation. In this evaluation, we made the assumption of first order of reaction. From the plot of rate of reaction ln(β/T<sup>2</sup> ) versus the reciprocal of absolute temperature, **Figure 5**, of as-mixed Co3O4 and La2O3 powder precursors, an Ea = 223 kJ/mol is obtained, which seems to be in good agreement with earlier results reported [31].

#### **3.2 X-ray diffraction investigation**

The analysis of X-ray diffraction pattern corresponding to unmixed and mixed precursors as function of temperature treatment is shown in **Figure 6**. In the lowest position of the diagram, the unmixed La2O3 and Co3O4 powder precursors are displayed. Going up in the diagram, the mixture of La2O3 and Co3O4 powder precursors at room temperature is indicated. The La2O3 powder precursor is tetragonal La2O3 (space group P-3m1) with a = b = 3.933 Å and c = 6.132 Å [32]. Traces of hexagonal La(OH)3 (space group P63/m) are detected. The corresponding lattice constant values are a = b = 6.529 Å and c = 3.859 Å. Co3O4 powder indicates that the precursor is single-phase, with cubic crystal symmetry (space group Fd-3m) [32]. The lattice constant value obtained from fit to data is a = 8.038 Å.

#### *Cobalt Compounds and Applications*

Upon thermal treatment, which is done to favor the formation of end products of perovskite of LaCoO3, the X-ray diffraction patterns show the formation of the perovskite phase. The XRD data confirm the perovskite LaCoO3 phase is converted upon heat treatment starting at 1023 K, growing progressively with increasing temperature up to 1273 K. This is followed by a gradual smoothing of the peak intensities of the precursor phases. The resulting powder can be indexed on the basis of a rhombohedral unit cell; the peaks around 2θ = 33.12° are bifurcated,

**Figure 4.**

*Differential scanning calorimetry curves of as-mixed Co3O4 and La2O3 powder precursors at different heating schedules.*

**Figure 5.** *Variation of ln(β/T2 ) plot for a set of differential scanning calorimetry scans with different heating rates.*

**99**

**Figure 6.**

*powders according to the thermal treatment.*

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route…*

the experimental X-ray results as well as previous studies [33, 34].

which is the characteristic peak for the rhombohedral structure and confirmed by

The average particle size is calculated from the XRD pattern using Debye and

where K = 0.94 is the Scherrer constant, λ is the wavelength of the X-ray

*Room temperature X-ray diffraction patterns of un/mixed La2O3 and Co3O4 powder precursors and LaCoO3*

The result obtained for the average crystallite size by Scherrer's equation is 294 Å.

source used, β is the full width at the half maxima, and θ is the Bragg

<sup>β</sup> cos<sup>θ</sup> (8)

The prepared LaCoO3 has trigonal crystal symmetry space group R-c3 [35, 36]. The lattice constant values obtained from fit to data are a = b = 5.444 Å and

*DOI: http://dx.doi.org/10.5772/intechopen.86260*

d = \_\_\_\_\_\_\_\_\_\_ <sup>K</sup> <sup>λ</sup>

c = 13.104 Å.

Scherrer formula, Eq. (8):

diffraction angle.

*Perovskite-Type Lanthanum Cobaltite LaCoO3: Aspects of Processing Route… DOI: http://dx.doi.org/10.5772/intechopen.86260*

*Cobalt Compounds and Applications*

Upon thermal treatment, which is done to favor the formation of end products of perovskite of LaCoO3, the X-ray diffraction patterns show the formation of the perovskite phase. The XRD data confirm the perovskite LaCoO3 phase is converted upon heat treatment starting at 1023 K, growing progressively with increasing temperature up to 1273 K. This is followed by a gradual smoothing of the peak intensities of the precursor phases. The resulting powder can be indexed on the basis of a rhombohedral unit cell; the peaks around 2θ = 33.12° are bifurcated,

*Differential scanning calorimetry curves of as-mixed Co3O4 and La2O3 powder precursors at different heating* 

**98**

**Figure 5.** *Variation of ln(β/T2*

**Figure 4.**

*schedules.*

*) plot for a set of differential scanning calorimetry scans with different heating rates.*

which is the characteristic peak for the rhombohedral structure and confirmed by the experimental X-ray results as well as previous studies [33, 34].

The prepared LaCoO3 has trigonal crystal symmetry space group R-c3 [35, 36]. The lattice constant values obtained from fit to data are a = b = 5.444 Å and c = 13.104 Å.

The average particle size is calculated from the XRD pattern using Debye and Scherrer formula, Eq. (8):

$$\mathbf{d} = \frac{\mathbf{K}\lambda}{\beta \cos \theta} \tag{8}$$

where K = 0.94 is the Scherrer constant, λ is the wavelength of the X-ray source used, β is the full width at the half maxima, and θ is the Bragg diffraction angle.

The result obtained for the average crystallite size by Scherrer's equation is 294 Å.

#### **Figure 6.**

*Room temperature X-ray diffraction patterns of un/mixed La2O3 and Co3O4 powder precursors and LaCoO3 powders according to the thermal treatment.*
