**2. Synthesis and investigation of ceramic materials**

## **2.1 Synthesis of ceramic materials**

The synthesis of xerogels and nanodispersed powders with different oxide concentration ratios in the CeO2–Sm2O3, La2O3–SrO–Ni2O3(Со2O3), and La2O3–SrO– Ni2O3–Fe2O3 systems was carried out by co-crystallization of the corresponding nitrates followed by the ultrasonic treatment [13]. Nitrates of cerium Се(NO3)3�6H2O (analytical grade), samarium Sm(NO3)3�6H2O (chemically pure), lanthanum La (NO3)3�6H2O (chemically pure), strontium Sr.(NO3)2 (analytical grade), nickel Ni (NO3)2�6H2O (pure), cobalt Co(NO3)2�6H2O (pure), and iron Fe(NO3)3�9H2O (pure) as �0.5 M solutions were used for the synthesis. The resulting solutions were mixed in the proportions corresponding to the stoichiometric ratio of oxides, followed by evaporation on a water bath for 3 h to obtain a supersaturated solution. Then, the supersaturated solution was cooled at a temperature of 3–5°C to facilitate the adsorption of the crystallizing substance on the surface of the crystals formed during the evaporation of mixtures of the nitrate solutions. To improve the dispersity of crystalline particles and to make their size distribution more narrow, the crystalline hydrate was subjected to ultrasonic treatment for 30 min in distilled water, resulting in an almost monodispersed powder.

The subsequent drying (110°C, 0.5 h) yielded X-ray amorphous xerogels that were subjected to heat treatment (600°C, 1 h) to form nanopowders with a stable crystal structure. The synthesized powders of a given composition were ground in a mortar, followed by uniaxial cold pressing into tablets of 1.0 and 1.5 cm in diameter using a PGR400 at the pressure of 100–150 MPa and then sintering installation for 2 hours and temperature 1300°C.

### **2.2 Characterization techniques**

XRD characterization was performed using a Bruker D8-Advanced diffractometer. The ICDD-2006 international database was used to interpret the diffraction patterns, and the analysis results were processed using the WINFIT 1.2.1 software involving the Fourier transform of the reflection profile. The sizes of coherent scattering regions (CSR) were estimated according to the Selyakov-Scherer equation:

$$D\_{\rm CSR} = \frac{0.9 \times \lambda}{\beta \times \cos \theta} \tag{1}$$

where λ = 15,418 Ǻ is the СuKα wavelength, and β is the XRD peak full width at half maximum (FWHM) [14].

The thermolysis processes occurring in coprecipitated xerogels and powders upon heating in the temperature range of 20–1000°C were studied using the Q-1000 derivatograph (MOM). The specific surface area of the synthesized nanopowders was measured by low-temperature nitrogen adsorption using a QuantaChrome Nova 4200B analyzer. Based on the obtained data, the specific surface area SBET of the samples was calculated using the Brunauer-Emmett-Teller (BET) model. The calculation of the pore size distribution was carried out based on nitrogen desorption isotherms according to the Barret-Joyner-Halenda (BJH) method; the thermal treatment of the powders was carried out in a Naberterm furnace with program control in the temperature range of 25–1300°C for 3 hours, followed by slow cooling of the furnace.

*Synthesis and Investigation of Ceramic Materials for Medium-Temperature Solid Oxide Fuel… DOI: http://dx.doi.org/10.5772/intechopen.105108*

The open porosity of the samples was determined by hydrostatic weighing in distilled water in accordance with the Russian standard GOST 473.4–81 [15]. The electrical resistance of the obtained ceramic materials was measured by the two-contact method at a direct current in the temperature range of 250–1000°C using the "Hardwaresoftware complex for studying the electrical properties of nanoceramics in various gaseous media" [16].

The transfer numbers of ions and electrons in bulk solid electrolytes [17] were determined using the West-Tallan method. A mixture of CO2 + CO was used as an inert gas (corresponding to a partial pressure of oxygen 10<sup>3</sup> Pa). The measurements were carried out at a direct current in weak (U = 0.5 V) fields after a long (up to 30 min) current drop. The contributions of ionic and electronic conductivities were estimated according to the formulas:

$$t\_{\epsilon} = \frac{R\_{air}}{R\_{\epsilon}}\tag{2}$$

$$\mathbf{t}\_{i} = \mathbf{1} - \mathbf{t}\_{\epsilon} \tag{3}$$

where *te* and *ti* are electron and ion transport numbers, respectively; *Rair* and *Re* are electric resistance values measured in air and inert gas, respectively.

## **2.3 Results and discussion**

For all the obtained compositions, thermolysis processes were studied. As an example, DTA thermograms of La0.6Sr0.4CoO3 xerogel synthesized with (a) and without ultrasonic treatment (b) are shown in **Figure 2**.

As shown in **Figure 2**, ultrasonic treatment during the synthesis results in the reduction of the crystalline hydrate dehydration temperature from 115–110°C, as well as the temperatures of all thermal transformations. This effect is determined by weakening the bonds between the molecules of nitrate salts and crystallization water molecules upon the impact of ultrasonic waves, facilitating the dehydration and decomposition of salts. Ultrasonic treatment also affects the powder crystallization reducing the temperature of its transition into the crystalline phase (320 ! 290°C).

Compared with the considered data, the differences in the temperatures of exo- and endo-effects for other compositions were no more than 10–15°C.

The microstructural parameters of the synthesized powders were determined using low-temperature nitrogen adsorption.

Microstructural performances of the synthesized powders are summarized in **Table 1**.

XRD characterization revealed that at 600°C a cubic solid solution of the fluorite type is formed in the studied CeO2–Sm2O3 powders with an average CSR size of �10 nm. Subsequent annealing at higher temperatures (1300°C) does not disrupt the single-phase structure of the nanopowders and ceramics-based thereon. As an example, consecutive steps of fluorite-type cubic solid solution formation for the (CeO2)0.95(Sm2O3)0.05 sample are shown in **Figure 3**. Crystal structures and specific electrical conductivity of synthesized powders and ceramics in the system CeO2–Sm2O3 are shown in **Table 2**.

The phase compositions of powders and corresponding ceramics of all the compositions in the systems La2O3–SrO–Ni2O3(Со2O3) and La2O3–SrO–Ni2O3–Fe2O3 are

#### **Figure 2.**

*Differential thermal analysis results for La0.6Sr0.4CoO3 xerogel prepared with (a) and without (b) xerogel freezing at 25°C (24 h).*


*Synthesis and Investigation of Ceramic Materials for Medium-Temperature Solid Oxide Fuel… DOI: http://dx.doi.org/10.5772/intechopen.105108*


#### **Table 1.**

*Microstructural performances of the synthesized powders [18].*

#### **Figure 3.**

*XRD patterns of (CeO2)0.90(Sm2O3)0.10 (a = 5.41396 Å) based xerogel (a—150°C), nanopowder (b—600°C), and ceramics (c—1300°C) samples [18].*


#### **Table 2.**

*Crystal structures and specific electrical conductivity of powders and ceramics in the system CeO2–Sm2O3 [18].*

summarized in **Table 3**. As an example, **Figure 4** shows the phase composition of La0.6Sr0.4NiO3 xerogel powder and ceramics, indicating the formation of a solid solution with a tetragonal perovskite structure at 1300°C.

The data in **Table 3** show that the synthesized powders and ceramic materials in the temperature range of 600–1300°C have an orthorhombic and tetragonal perovskite-type structure.

Physicochemical properties of all the ceramic samples obtained on the basis of nanopowders in the СeO2-Sm2O3 La2O3–SrO–Ni2O3(Со2O3) and La2O3–SrO–Ni2O3– Fe2O3 systems are summarized in **Table 4**. These data show that an increase in the content of samarium oxide in the obtained samples in the СeO2-Sm2O3 system leads to a decrease in their density, which is probably due to the distortion of the cerium dioxide lattice upon Sm2O3 dissolving.

To create a porous structure in ceramics based on the La2O3–SrO–Ni2O3(Со2O3) and La2O3–SrO–Ni2O3–Fe2O3 systems, a pore-forming additive (10% aqueous solution of


### **Table 3.**

*Crystal structures and specific electrical conductivity of powders and ceramics in the systems La2O3–SrO– Ni2O3(Со2O3) and La2O3–SrO–Ni2O3–Fe2O3.*

*Synthesis and Investigation of Ceramic Materials for Medium-Temperature Solid Oxide Fuel… DOI: http://dx.doi.org/10.5772/intechopen.105108*

#### **Figure 4.**

*XRD patterns of La0.6Sr0.4NiO3 xerogel powder and ceramics; heat treatment at a) 600°C, b) 900°C, c) 1000°C, and d) 1300°C.*

polyvinyl alcohol) in an amount of 10% over the bulk of the charge was added [19]. The values of open porosity are in the range of 21–29%, which is one of the main conditions for the optimal operation of cathode solid oxide materials as shown in **Table 4**.

One of the main conditions for SOFC operation is the compatibility of its components in terms of the thermal expansion coefficient (TEC). TEC of electrolytes based on cerium oxide doped with samarium oxide is <sup>α</sup> = 12.5 <sup>10</sup><sup>6</sup> <sup>K</sup><sup>1</sup> [20]. Cathode materials based on systems La2O3–SrO–Ni2O3 (TEC—14.2 <sup>10</sup><sup>6</sup> <sup>K</sup><sup>1</sup> ) and La2O3– SrO–Ni2O3–Fe2O3 (TEC—12.8–13.1 <sup>10</sup><sup>6</sup> <sup>K</sup><sup>1</sup> ) have comparable TEC values with ones of electrolytes based on cerium oxide. As can be seen in **Table 4**, strontium


**Table 4.**

*Physicochemical properties of ceramics samples synthesized by co-precipitation of salts [18].*

#### **Figure 5.**

*Temperature dependence for specific electrical conductivity of 1—(CeO2)0.95(Sm2O3)0.02, 2—(CeO2)0.90(Sm2O3)0.05, 3—(CeO2)0.80(Sm2O3)0.10 [18].*

lanthanum cobaltites have a TEC much higher than the TEC of samples of the composition (CeO2)1-x(Sm2O3)x.

The electrical conductivity of (СeO2)1-x(Sm2O3)x samples (x = 0.02; 0.05; 0.10) was measured using the two-contact method at direct current (**Figure 5**). The appearance of high oxygen ionic conductivity in CeO2-Sm2O3-based solid electrolytes is determined by the formation of oxygen vacancies in the СеО<sup>2</sup> matrix when Се4+ is replaced by Sm3+ during the dissolution of Sm2O3 in CeO2, which can be described by the following quasi-chemical equation in the Kroeger-Winke notation [23]:

$$\text{Sm}\_2\text{O}\_3 \xrightarrow{\text{CeO}\_2} \text{2Sm}'\_{\text{Ce}} + \text{3O}\_\text{O}^\text{x} + \text{V}\_\text{O}^\bullet \tag{4}$$

*Synthesis and Investigation of Ceramic Materials for Medium-Temperature Solid Oxide Fuel… DOI: http://dx.doi.org/10.5772/intechopen.105108*

where Sm0 Ce is a samarium ion replacing Ce4+ and yielding a negative charge, *V*•• *<sup>O</sup>* is a positively charged oxygen vacancy compensating the dopant charge, and *O*� *<sup>O</sup>* is oxygen atom in a regular site with a neutral charge.

As can be seen in **Figure 5**, the temperature growth in the range from 500 to 1000°C leads to the increase in electrical conductivity of all the samples. In addition, with an increase in the concentration of samarium oxide, the specific electrical conductivity of the ceramics increases in the entire temperature range in the study. The highest specific electrical conductivity in the temperature range of 500–1000°C (σ700°C = 1.3 � <sup>10</sup>�<sup>2</sup> S/cm) is observed for the sample containing 10 mol. % Sm2O3.

The temperature dependence of the specific electrical conductivity of solid solutions in the systems La2О3–SrO–Co2O3 and La2О3–SrO–Ni2O3 (La0.6Sr0.4CoO3, La0.7Sr0.3CoO3, La0.6Sr0.4NiO3, La0.7Sr0.3NiO3) is shown in **Figure 6**. The conductivity grows with temperature in the range from 300 to 700°C up to the saturation plateau at 600–800°C. Particularly, for La0.6Sr0.4NiO3 and La0.6Sr0.4CoO3, the plateau is reached at 600 and 700°C, respectively, while at higher temperatures, no conductivity increase is observed due to a change in the conductivity mechanism from semiconductor to metallic. The electrical conductivity in the considered solid solutions can proceed *via* several possible mechanisms, mainly by itinerant electrons along the Ni3+–O–Ni3+ chain and electron or hole jump directly between Ni3+ and Ni2+ ions. The authors of ref. [6] believe that the appearance of metallic conductivity in the synthesized solid solutions is due to the delocalization of Ni d-electrons during the interaction of nickel and oxygen atoms in the Ni-O-Ni chains.

**Figure 6** also indicates that the conductivity of the studied samples grows with an increase in strontium oxide content in the solid solutions.

The highest conductivity in the temperature range 500–1000°С (σ700°<sup>С</sup> = 0.80�10�<sup>1</sup> S/cm) is observed for the composition La0.6Sr0.4NiO3. The conductivity values σ700°C for La0.7Sr0.3NiO3, La0.6Sr0.4CoO3, and La0.7Sr0.3CoO3 samples are 0.25�10�<sup>1</sup> , 0.35�10�<sup>1</sup> , and 0.20�10�<sup>1</sup> S/cm, respectively. **Figure 7** shows the temperature dependence plots for samples of the compositions La0.6Sr0.4Fe0.7Ni0.3O3 and La0.7Sr0.3Fe0.7Ni0.3O3, indicating that the former one features with higher

#### **Figure 6.**

*Temperature dependence for specific electrical conductivity of 1—La0.6Sr0.4NiO3, 2—La0.6Sr0.4CoO3, 3—La0.7Sr0.3NiO3, and 4—La0.7Sr0.3CoO3.*

#### **Figure 7.**

*Temperature dependence for specific electrical conductivity of a) La0.6Sr0.4Fe0.7Ni0.3O3 and b) La0.7Sr0.3Fe0.7Ni0.3O3.*


#### **Table 5.**

*Performances of electronic and ionic conductivity of (CeO2)0.90(Sm2O3)0.10 [18].*

conductivity compared with the latter one. The observed plot shapes and level of conductivity are similar to those for lanthanum nickelate and cobaltite with only small differences. However, the addition of iron results in a prominent increase in the transition temperature from semiconductor to metallic conductivity.

Using the West-Tallan method, the ratio of the electronic and ionic conductivity in the studied ceramic samples was determined. As an example, **Table 5** presents the data on the ratio of the transfer numbers of ions and electrons for the studied samples of the composition (CeO2)0.90(Sm2O3)0.10. These data indicate that these solid electrolytes have mixed conductivity with the ion transport number—ti = 0.82 at 300°C and 0.71 at 700°C. The temperature growth leads to a sharp increase in the contribution of the electronic component to the total value of electrical conductivity that relates to a partial transition Ce4+ ! Ce3+.

The ratio between the electronic and ionic conductivity determined according to the West-Tallan method is exemplarily illustrated in **Table 6**, indicating the ratios of ion and electron transfer numbers for La0.6Sr0.4CoO3, La0.7Sr0.3CoO3, La0.6Sr0.4NiO3, and La0.7Sr0.3NiO3. The presented data show that these materials have mixed conductivity with a predominance of electronic components with the transfer numbers te = 0.92–0.98 and ti = 0.08–0.02 at 800°C. The electronic component contribution to the total electrical conductivity sharply grows with temperature due to the appearance of metallic conductivity.

*Synthesis and Investigation of Ceramic Materials for Medium-Temperature Solid Oxide Fuel… DOI: http://dx.doi.org/10.5772/intechopen.105108*


**Table 6.**

*Performances of electronic and ionic conductivity of La0.6Sr0.4CoO3, La0.7Sr0.3CoO3, La0.6Sr0.4NiO3, La0.7Sr0.3NiO3, La0.6Sr0.4Fe0.7Ni0.3O3, and La0.7Sr0.3Fe0.7Ni0.3O3 ceramics.*
