Thermoelectric Properties of Sr1-3x/2Lax/2Smx/2TiO3- δ (0.05 ≤ x ≤ 0.30) Ceramics

Influence of strongly reducing processing atmosphere on Sr-vacancy Sr 1-3x/2 La x/2 Sm x/2 TiO 3- δ (x = 0.05, 0.10, 0.15, 0.20, 0.30) ceramics was investigated. The ceramic powders were prepared by the solid-state reaction (SSR) method, and heat treated in 5 % H 2 /N 2 reducing gas at 1573 K for 6 h and 1773 K for 8 h for calcination and sintering processes, respectively. Thermoelectric properties of Sr 1-3x/2 La x/2 Sm x/2 TiO 3- δ ceramics were evaluated from 573 to 973 K. Their electrical conductivities increased with carrier concentration and also decreased with temperature, indicating metallic behaviour. The Seebeck coefficients showed n-type behaviour and increased with temperature. Additionally, the total thermal conductivities exhibited low values, with a minimum value, 2.67 W/m. K for x = 0.20 ceramics at 973 K. A maximum thermoelectric figure of merit, ZT = 0.30 at 973 K was reached for Sr 0.7 La 0.1 Sm 0.1 TiO 3- δ ceramics, which is 20 % higher than the maximum value reported previously for La-Sm electron doped SrTiO 3 ceramics.


Introduction
Thermoelectric (TE) oxide ceramics have been regarded as promising materials for power generation and energy harvesting particularly at high temperatures. Oxides generally are inert, nontoxic, cheap, abundant, thermally stable, resistant to oxidation with a corresponding high temperature, hence potential TE materials for high temperature applications [1], [2]. The efficiency of a TE device such as thermoelectric power generator merit, ZT. This measuring parameter is defined as = 2 , where S is a Seebeck coefficient (μV/K), σ is an electrical conductivity (S/cm), T is an absolute temperature (K), and k is a thermal conductivity (W/m K 2 ).
Therefore, for a high ZT to be achieved, a high power factor, PF (S 2 σ) and a low k are required [3], [4].
Despite the potential TE characteristics possessed by oxides, they exhibit high k, which leads to low ZT (particularly n-type oxides) when compared to conventional non-oxides. Several n-type oxide materials such as SrTiO 3 , ZnO, CaMnO 3 and In 2 O 3 have been studied. For example, high ZT values of 0.47 at 1000 K [10], [11] and 0.65 at 1247 K [10], respectively have been reported for n-type Al-Ga co-doped ZnO and recently ZT ≥ 0.6 at 1000 -1100 K for La-Nb co-doped SrTiO 3 [12]. Doped and reduced SrTiO 3 ceramics have recently shown improved electrical conductivity required for TE applications [13]. There are various approaches such as doping, co-doping, addition/inclusions, enhanced processing condition, defect and micro/nanostructural engineering so far utilized for optimizing the TE performance of SrTiO 3 [14], [15].
In our previous publication [16] a maximum ZT value of 0.24 at 873 K and a minimum k = 3.0 W/m. K for x = 0.2 at 973 K were obtained for electron La-Sm co-doped SrTiO 3 ceramics. It is therefore established that electron co-doping had a minimal effect on thermal conductivity, hence unsuitable for optimizing ZT. In furtherance to the works reported independently by Kovalevsky et al [14] and Lu et al [17], it is agreed that batched stoichiometries with cation vacancies followed by processing in reducing atmosphere are milestone in achieving improved thermoelectric properties. This projection informs the aim of this study. It is therefore, an attempt to improve the TE properties of Sr-site vacancy La-Sm co-doped SrTiO 3 ceramics compared to the previous published electron doped mechanism [16].

Materials and method
Sr with isopropanol and 10 mm diameter yttria-stabilized zirconia balls using a ball mill. The mixtures were dried at 80 o C for several hours and sieved using a 250 μm sieve mesh. The sieved powders were calcined in 5%H 2 /N 2 gas at 1573 K for 6 h and then pressed by a uniaxial press into a 20 mm disc pellet (≤ 2mm thickness).
Sintering of the pellets was performed in flowing 5%H 2 /N 2 reducing gas at 1773 K for 8 h. Phases of Sr 1-3x/2 La x/2 Sm x/2 TiO 3 ceramics were analyzed by x-ray diffraction (XRD) method with a D2 phaser diffractometer (Bruker AXS GmbH, Germany) using Cu Kα1 radiation with λ =1.5406 Å. Secondary electron (SE) surface images of the ceramics were also observed with a Scanning Electron Microscope (Philips XL 30 S-FEG).
Seebeck coefficient and electrical conductivity of the sintered disc pellets were simultaneously measured using a NETZSCH SBA 458 NEMESIS instrument in argon atmosphere from 573 -973 K. The thermal conductivity was determined using a thermal properties analyzer (Anter Flashline TM 3000) while the experimental density was determined by Archimedes' method using an electronic digital density balance (Mettler-Toledo AG Balance).

Results and discussion
XRD patterns for Sr 1-3x/2 La x/2 Sm x/2 TiO 3 (0.05≤x≤0.3) ceramics are shown in Figure 1. All samples are single phase and indexed with the SrTiO3 cubic perovskite structure (pm-3m space group). The lattice parameters were calculated from the XRD data and the results together with relative densities measured by Archimedes' method are listed in Table 1. The lattice parameters for x = 0.05 -0.2 ceramics increase with increasing dopant (La-Sm) concentrations with values ranging from 3.899 to 3. 91 Å, and decreases for x = 0.3 (3.902 Å). The increase in lattice parameter is attributed to a decrease in the binding energy within the lattice due to formation of oxygen vacancy and partial reduction of Ti 4+ ions to Ti 3+ ions [18]. Structural transitions or distortions and solid solution limit contribute to the decrease in lattice parameter for x = 0.3 ceramics. However, the lattice parameter trend in this work is contrary to the obtained results in our previously studied electron La-Sm codoped SrTiO 3 ceramics [16] and in La-Yb co-doped SrTiO 3 [19]. The relative densities of all ceramics are higher than 96% and higher than most rare earth (RE) doped SrTiO 3 ceramics [20], [21]. The results of microstructure analysis as shown in Figure 2 are identical to the XRD results since no second phases were identified. Therefore, all microstructural images are homogeneous and dense, hence agree with their high relative densities. In addition, these SEM images showed regular polygonal-shaped grain structures while the average grain size increased with increasing doping level. Hence, the average grain size of the ceramics increased from 7.5 μm to 10.4, 10.8 and 11.2 μm for x = 0.10, 0.15, 0.20, and 0.30, respectively. Comparative analysis shows that these grain sizes are small, and evidenced by the adopted highly reducing processing conditions. This assertion is supported by in the literature where a La-Nb co-doped ceramics sintered in air exhibited a larger grain size than the same composition sintered in reducing atmosphere [15]. Reduced grain size implies an incorporation of large grain boundaries which are effective scattering centres for phonons [22] Temperature dependencies of an electrical conductivity and an absolute Seebeck coefficient for Sr 1-3x/2 La x/2 Sm x/2 TiO 3 (0.05 ≤ x ≤ 0.30) ceramics are shown in Figure 3. Electrical conductivities for all samples showed a temperature dependence of a degenerate semiconductor or metallic behaviour [23]. As shown in Figure 3a, the electrical conductivity for x = 0.30 ceramic was higher than all other compositions withing the measured temperature range. The reason for the maximum electrical conductivity can be explained by increase of the carrier concentration [9]. High dopant (La 3+ and Sm 3+ ions) level substituted into Sr 2+ sites could contribute to an electrical conductivity of the doped SrTiO 3 ceramics. Consequently, the maximum electrical conductivity of the ceramics was 1023 S/cm at 573 K for the x = 0.30 composition. Figure 3b shows the absolute Seebeck coefficients for Sr 1-3x/2 La x/2 Sm x/2 TiO 3 ceramics. Seebeck coefficients of all compositions showed negative for whole temperature range, indicating n-type and increased with temperature, hence an exhibition of metallic behaviour [19]. Generally, a Seebeck coefficient is inversely proportional to a carrier concentration [24]. Accordingly, an increase in carrier concentration (dopant level) for these doped ceramics led to the observed decrease in Seebeck coefficients. However, the average absolute Seebeck coefficients (158 -255 μV/K) attained in this work at high temperature (973 K) exceeds the minimum recommended Seebeck coefficient s (150 -250 μV/K) for a potential thermoelectric materials [25]. where momentum is conserved, leading to an increase in thermal conductivity with temperature [26]. x = 0.20 ceramics showed the lowest total thermal conductivity, and decreased with increasing temperature in the whole measurement temperature range. This trend indicates an Umklapp phonon scattering behaviour [12], [27].
Overall, a minimum total conductivity, 2.67 W/m. K at 973 K was observed for x = 0.2 ceramics, comparable to those of doped SrTiO 3 prepared by conventional methods [28], [29] and enhanced techniques [30], [31] The low thermal conductivity obtained indicates an application of reducing atmosphere in all heat treatments (calcination and sintering) has an efficient impact in the thermal properties of SSR synthesized Sr 1-3x/2 La x/2 Sm x/2 TiO 3 ceramics.
ZT values of ceramics increased with an increase in the dopant level up to x = 0.2 ( Figure 4b) and levels off at x = 0.30. x = 0.20 ceramics maintained a leading high ZT over the whole measured temperature range, and reached a maximum ZT value of 0.30 at 973 K. This high ZT could be linked to its high lattice parameter and consequent low thermal conductivity.

Conclusions
The influence of strong reducing processing atmosphere and the effect of Sr (cation) vacancy on Sr  ACKNOWLEDGMENTS Financial support from TETFUND Nigeria is greatly acknowledged. I would also like to acknowledge the technical work (experiment) -XRD, SEM and thermoelectric characterization carried out at the functional materials and devices laboratory and the Sorby centre, department of Materials Science and Engineering, the University of Sheffield, UK. Figure 1 XRD analysis results for Sr1-3x/2Lax/2Smx/2TiO3 (0.05 ≤ x ≤ 0.3) ceramics sintered in 5%H2/N2 gas at  Temperature dependencies of (a) electrical conductivities and (b) absolute Sebeck coe cients for Sr1-3x/2Lax/2Smx/2TiO3 (0.05 ≤x ≤ 0.30) ceramics sintered in 5% H2/N2 at 1773 K for 8 h.