**3.1 Perovskite oxides (ABO3) as thermoelectric materials**

Oxide perovskites have been used as thermoelectric materials due to their low thermal conductivity, high Seebeck coefficient, and electrical conductivity. There are two approaches to enhance ZT value, one is tuning the carrier concentration and another is engineering structure and material properties to decouple the S, σ, and κ. The further modification methods to enhance ZT value are self-doping, nanoengineering, band engineering, and doping shown in **Figure 2**. In perovskite oxides the main candidates which show TE properties were titanates, manganates, and colbatates.

The substituted perovskite compounds of titanates (Sr1-xAxTi1-yNbyO3, where A-Ca, La, Ba, Eu, etc) are promising classes of thermoelectric materials with high figure of merit. It was reported that SrTi0.8Nb0.2O3 thin films have ZT = 0.37 at 1000 K, which was reduced to 0.35 by hot pressing the sample to a temperature of 1073 K. When the material SrTi0.8Nb0.2O3 is grown in nanostructure as superlattice, where a single layer of this material is sandwiched between the several layers of insulating SrTiO3 (STO), a remarkable increase in ZT 2.4 at 300 K is obtained. In La-doped STO thin films S can be tuned from 120 to -260μVK<sup>1</sup> . The La 15% doped STO has a ZT value of 0.28 at 873 K was achieved. Even though the electrical conductivity of STO is increased by La doping, it reduces the lattice thermal conductivity by phonon scattering. It was reported that the substitution of Ce, Ba, Ca, Pr, and Y on A-site and Nb, Ta, Mn, and Co on B-site enhances the electrical conductivity of the sample. The Nb-doped STO(Sr(NbxTi1 x)O3,0.01 <sup>&</sup>lt; <sup>x</sup> <sup>&</sup>lt; 0.4), in which substituted Nd5+ at Ti4+ site will generate carrier electrons and a ZT of 0.35–0.37 at 1000 K was achieved for 20%Nb. While Mn substitution of Sr1 xLaxTiO3 the S was enhanced from 120 to <sup>180</sup> <sup>μ</sup><sup>V</sup><sup>K</sup><sup>1</sup> and the ZT value of 0.07 to 0.15 at 300 K was obtained when the composition changed from Sr0.95La0.05TiO3 to Sr0.95La0.05Ti0.96Mn0.04O3. For SrTi0.9Ta0.1O3 the ZT value obtained was 0.17 at 752 K and for SrTi0.875Co0.125O3 was 0.135 at 300 K. The A-site substitution enhances electrical properties while B-site enhances the Seebeck coefficient value. However, the doping of Y, La, Sm, Gd, and Dy in STO reduces the thermal conductivity. For (Sr0.9Dy0.1)TiO3 has a ZT value of 0.22 at 573 K. Mn doped Sr1 xLaxTiO3 can enhance anharmonic lattice vibrations, which result in inelastic phonon-phonon scattering reduces thermal conductivity and offers high electrical conductivity. Compared to ZT of Sr0.95La0.05TiO3 0.07

**Figure 2.** *Methods to enhance the ZT of thermoelectric perovskite materials.*

Sr0.95La0.05Ti0.98Mn0.02O3 has 0.15 at 300 K. The effective way to reduce thermal conductivity is rare earth substitution in A-site and Mn substitution in B-site. The data compounds obtained were tabulated in **Table 1** [5].

The thermoelectric properties of Mn substituted perovskites were summarized in **Table 2**. In these perovskite oxides multiple elements are used as A-site dopants in AMnO3 including Yb, Y, La, Ce, Sm, Dy, Tb, Ho, Pr, Ca, Sr, Nd, etc. and for B-site Mo, Ru, Ta, etc. Relatively high ZT values are not achieved in these materials. CaMn0.98Nb0.02O3 shows a significant ZT of 0.32 at 1050 K. It was noticed that the thermal conductivity of Mn-doped samples is low.

Rare earth cobalt oxides are compounds having the stoichiometry RCoO3, R – La, Ce, Pr, Nd, etc. It was also reported that the electrical conductivity of these materials increased with increasing ionic radii of rare earth metals doping (Pr3+ > Nd3+ > Tb3+ > Dy3+). The complex spin structure of Co ions in perovskite gives us plenty of opportunities to explore the exotic magnetic phenomenon of these materials. The Co ions in RCoO3 can exist in three different spin states, low spin LS (t2g <sup>6</sup> eg <sup>0</sup> for Co3+ and t2g <sup>5</sup> eg 0 for Co4+), intermediate spin IS (t2g <sup>5</sup> eg <sup>1</sup> for Co3+ and t2g4 eg <sup>1</sup> for Co4+), and high spin state HS (t2g4 , eg <sup>2</sup> for Co3+ and t2g 3 , eg <sup>2</sup> for Co4+), which can induce spin entropic effect to the perovskite structure and can influence all the magneto-transport properties of the materials [24–26]. It was reported that A- site substituted LaCoO3, has high ZT value. Sr, Na, Pb, and Ba are usually used elements for substitution. For La1 xSrxCoO3 the electrical conductivity gets enhanced, and the ZT value of 0.046 to 0.18 was achieved. Pb doped LaCoO3 has Seebeck coefficient of 110 <sup>μ</sup><sup>V</sup><sup>K</sup><sup>1</sup> and ZT of 0.23 to reported. Thermoelectric measurements of doped ACoO3 compounds are tabulated in **Table 3**.

The other B-site cations include iron (Fe), nickel (Ni), tin (Sn), lead (Pb), bismuth (Bi), molybdenum (Mo), ruthenium (Ru), and uranium (U). The thermoelectric measurements are tabulated in **Table 4**. For Fe doped compounds La0.95Sr0.05FeO3 and Pr0.9Sr0.1FeO3, the ZT value obtained are 0.076 and 0.024, while for Ni-doped LaCo0.92Ni0.08O2.9 the ZT value of 0.2 was achieved. Double perovskite A2FeMoO6 (A-Ca,Sr,K,Ba) was also studied. For ZT ranges from 0.1 to 0.99 was reported. For tin substituted compounds BaSnO3 ZT of 0.65 was theoretically calculated. Sr1 xBaxPbO3 ZT of 0.13 was observed. There was no significant high ZT value seen when the B-site is doped with Mo, Ru, and U. The thermoelectric measurement parameters are all tabulated in **Table 4**.

#### **3.2 Hybrid perovskites**

Compared to other thermoelectric materials hybrid perovskites have high Seebeck coefficient and low thermal and electrical conductivity. For CH3NH3PbI3 at 295 K has S = 700 <sup>μ</sup><sup>V</sup><sup>K</sup><sup>1</sup> , <sup>κ</sup> = 0.5 W<sup>m</sup><sup>1</sup> <sup>K</sup>1, and ZT value is 10<sup>7</sup> due to the low electrical conductivity. The photo-induced or chemical doping strategies were used in these materials to enhance the electrical conductivity. There are many hybrid perovskite materials that show TE applications, such as ABI3 (A = CH3NH3 (MA), NH2CHNH2 (FA), and B = Pb, Sn), CsMI3, and C6H4NH2CuBr2I. Theoretical studies on n-type and p-type CH3NH3PbI3 hybrid perovskites show ZT value of 0.9 and 1.25. For (MA)PbI3, (MA)SnI3, (FA)PbI3, and (FA)SnI3 n-type materials the reported ZT values are 0.44, 0.45, 0.42, and 0.35 respectively. As the carrier concentration increases ZT also increases. The same CH3NH3PbI3 n-type ZT of 2.56 at 800 K can be achieved. The first-principles calculations and semi-classical Boltzmann transport theory showed


**Table 1.**

*Thermoelectric studies of doped ATiO3 compounds.*


*Thermoelectric Nanostructured Perovskite Materials DOI: http://dx.doi.org/10.5772/intechopen.106614*

> **Table 2.** *Thermoelectricstudies*

 *of doped AMnO3*

*compounds.*


**Table 3.**

*Thermoelectric studies of doped ACoO3 compounds.*

that the ZT values of 0.63 and 0.64 for CsSnI3 and CsPb n-type at 1000 K. The thermoelectric properties of hybrid perovskites are tabulated in **Table 5**.

### **4. Nanostructuring of thermoelectric materials**

The materials having at least one of the dimensions in the order of 10<sup>9</sup> m are nanostructured materials. If the dimension of material is in nanometer range its surface-to-volume ratio increases and the properties changes drastically. Nanoparticles are highly reactive because they possess large surface energy. Such an increase in surface area in thin films and coating can enhance the sensing property, catalytic activity of surfaces, light trapping in solar cells, surface reactivity, etc. Nanosystems are classified into three two-dimensional (2D), one-dimensional (1D), and Zero dimensional (0D). Nanosheets and superlattices are 2D nanosystems, nanowires, nanorods, and nanotubes are 1D and nanopowders and quantum dots are 0D nanosystems. There are two approaches for the fabrication of nanostructured materials- top-down method and the bottom-up method. Starting from bulk crystalline material and dividing it into small pieces to obtain fine nanosized particles is the top-down method. Ball milling, spin melting, thermal cycling, lithography, etc. [27]. In bottom-up method, nanoparticles are produced from their constituent elements, which are assembled to form dense solids **Figure 3**.

When the particle size is decreased to nano, size-dependent quantum confinement effect arises. Generally, in nanostructures, the energy level spacing increases with decreasing size and is the quantum size confinement effect. This effect influences the optical, electronic, magnetic, thermal, and dynamic properties of the material. Another important size reduction effect is the electron-phonon coupling. With decrease in size, the density of states of both phonons and electrons decreases in size and this decreases the overlap. The combination of density of states and the surface phonon frequencies affect the phonon-electron interaction in the nanostructures. The quantum confinement effect and the phonon scattering have a crucial role in enhancing the efficiency of thermoelectric materials. By nanostructuring, the electrical conductivity of thermoelectric materials can be enhanced by quantum confinement effect


*Thermoelectric Nanostructured Perovskite Materials DOI: http://dx.doi.org/10.5772/intechopen.106614*

> **Table 4.**

*Thermoelectric studies of other doped perovskite oxide.*


#### **Table 5.**

*Theoretical ZT value of hybrid perovskites from calculations.*

**Figure 3.** *Bottom-up strategies for nanostructuring of TE materials.*

and the thermal conductivity can be reduced by phonon scattering at the interfaces, thereby increasing the figure of merit.

The nanostructured thermoelectric perovskite compounds have been prepared by many techniques which include co-precipitation, mechanical synthesis, solid-state

*Thermoelectric Nanostructured Perovskite Materials DOI: http://dx.doi.org/10.5772/intechopen.106614*

reactions, solution combustion or thermal decomposition, hydrothermal, Pechini, and sol-gel method. Many new methods and improvements in synthesis conditions have been tried by the researchers as the properties of the end product strongly depend on the method of synthesis technique used. The citrate sol-gel auto-combustion method, which is a modified Pechini method based on the polyesterification of ethylene glycol and citric acid for the synthesis of the perovskite nanopowders. The method involves relatively easy synthesis route when compared to the other conventional processes. The control over the end stoichiometry and low operating temperature are the main advantages of this technique.

Popa et al. have synthesized perovskite – LaMeO3 (Me - Co, Mn, Fe) compounds by the polymer complex method and elaborated its advantages. Nonuniformity in particle size, compositional inhomogeneity, and high processing temperature are the main disadvantages when the conventional mixed oxide methods are preferred for the synthesis. Perovskite nanopowders developed through wet-chemical method have relatively high product uniformity and reliable reproducibility. By this method, it is possible to reduce the agglomeration of nanoparticles and can control the particle size. In citrate sol-gel auto-combustion method, at relatively low temperature excellent chemical homogeneity can be achieved. The perovskite nanopowders thus obtained have uniform particle size, which allows sintering to give dense well shaped uniformly grained microstructures [28]. The nanopowders were subjected to characterization techniques including XRD, SEM-EDAX, XPS, particle size analyzer, etc. Finally, dc electrical conductivity, thermal conductivity, and the Seebeck coefficient measurements are carried out using thermoelectric measurement setup.

### **5. Thermoelectric characterization**

The simultaneous measurement of electrical resistivity and Seebeck coefficient was done using ULVAC-ZEM 3. The sample is sandwiched between the electrodes and kept in helium atmosphere at low pressure of 10�<sup>3</sup> Torr. The resistivity is calculated using four probe method.

$$
\rho = \frac{RA}{l} \tag{8}
$$

where ρ is the electrical resistivity, R is the resistance, A is the area of cross section and l is the distance between the probes. High impedance current is supplied through the probes connected to upper and lower blocks. Other two probes measure the voltage produced. Seebeck coefficient is determined by measuring the electromotive force generated at the probes. The sample is kept in such a way that a temperature gradient can exist between the two ends. Let T1 and T2 be the temperatures at two ends and the electrical potential difference is *dV*, the Seebeck coefficient can be calculated using the formula,

$$S = \frac{dV}{T\_1 - T\_2} \tag{9}$$

The measurement is controlled by a computer. The voltage-current measurement is made to check the correct contact of the sample. The thermal conductivity of the sample can be measured by divider bar method. In this method the sample is

sandwiched between two metal blocks, heat flows through the sample by measuring the thermal gradient the thermal conductivity can be measured.
