SPS-sintered NaTaO3–Fe2O3 composite exhibits enhanced Seebeck coefficient and electric current

Wilfried Wunderlich 0 1 Takao Mori 0 1 Oksana Sologub 0 1 0 T. Mori O. Sologub Nat. Inst. Mat. Sci. (NIMS), Int. Center Mat. Nanoarchitec. (MANA) , Tsukuba 305-0044, Japan 1 W. Wunderlich (&) Department of Material Science, Tokai University , Hiratsuka-shi 259-1292, Japan NaTaO3--50 wt% Fe2O3 composite ceramics showed a large Seebeck voltage of -300 mV at a temperature gradient of 650 K yielding a constant Seebeck coefficient of more than -500 lV/K over a wide temperature range. We report for the first time that spark plasma sintering (SPS) at low temperature (870 K) could maintain the short-circuit current of -80 lA, which makes this thermoelectric material a possible candidate for high-temperature applications up to 1,623 K. The reason for the good performance is the interface between Fe2O3 and surrounding NaTaO3 perovskite. When SPS is used, constitutional vacancies disappeared and the electric conductivity increases remarkably yielding ZT of 0.016. - The pressing problem of CO2 increase and climate change requires the search for new energy sources such as the thermoelectric power generators (TEG), which can turn waste heat into usable electricity when operating at high temperatures. The research for new thermoelectric ceramic materials began in the last decade and Nb-doped SrTiO3 [1, 2] NaCoO3 [3], and CaCoO3 [4] were found to have a remarkable figure-of-merit ZT. They are already successfully established in devices for high-temperature electric generators. A detailed band structure study of the perovskite-based Nb-doped SrTiO3 material has emphasized the combination of large and small effective masses as the reason for the large Seebeck coefficient [5, 6]. While Cobased perovskites [79] have been investigated also as potential thermoelectric materials, our search for new materials yielded to the NaTaO3 perovskite material, which is known as efficient photo catalyst for splitting water [10, 11] and its large effective electron mass [5]. The composite material NaTaO3Fe2O3 shows a large Seebeck voltage of -300 mV at a temperature gradient of DT = 650 K with linear temperature dependence [1015] and is stable up to 1,623 K [12, 15]. Yet its large resistivity has to be lowered for increasing the power factor and figure-of-merit. We have reported previous results on spark plasma sintering (SPS) experiments [15], where a remarkable increase in electric conductivity was achieved, but the Seebeck voltage has dropped. The conclusion was that either the interface structure or the microstructure has changed due to the high-temperature plasma, the vacuum, diffusion from the carbon crucible, or during the subsequent fast cooling and are responsible for the decrease of the Seebeck coefficient. Further findings were that composites processed from Fe2O3 and NaTaO3, or additions of NaFeO3 deteriorate the electric conductivity and yield to an insulator. The reason for the good performance of this composite material is the interface between Fe2O3 and NaTaO3 with perovskite structure. The largest Seebeck voltage was measured when the second phase Fe2O3 reaches an amount of 50 mol% [10], which is just the percolation limit when the second phase starts to surround the perovskite phase NaTaO3. Metallic behavior with high Fig. 1 The processing procedure yielded to different routes as indicated a calcination, b conventional sintering, c SPS at 1,373 1,573 K, d calcination 1,273 K in air, then SPS at 1,3731,573 K, e calcination 1,273 K in air, SPS at 870 K, and f same as e with additional sintering at 1,273 K for 4 h carrier concentration was recently found at similar NaTaO3/SrTiO3 perovskite interfaces [16]. Hence, the goal of this paper is to gain deeper insight in the materials behavior with the goal to improve both, Seebeck voltage and electric conductivity. For optimum densification, a second sintering step is required after calcination and grinding. Therefore, this paper describes processing of these composite ceramics on different routes and compare both, thermoelectric and microstructural properties of the resolved specimens. Experimental procedure Powders in lm size of NaTaO3 and Fe (Fine Chemicals Ltd., Japan) were weighed according to the desired weight ratio of 50 mol-% Fe2O3 and mixed in a mortar for at least 10 min. Then the mixture was put in a steel cylinder with 15-mm diameter and cold-pressed with a stress of 50 MPa. These pellets were used in the following different synthesis methods as sketched in Fig. 1. The conventional calcination and sintering route are shown as paths (a) and (b) in Fig. 1 and details have been described in [1214]. A sliced specimen with 10 9 2 9 2 mm dimensions was measured using the thermoelectric multi-measuring device ZEM3 (Ulvac Ltd., Japan) as described in the following section. The next straight-forward step is to try SPS on the coldpressed pellet [route (c) in Fig. 1] [15]. The report on such specimens showed improved resistivity, but poor Seebeck voltage [15]. The present paper focusses on SPS sintering. After calcination, the specimen were crushed, then the powder mixed again and put into a 15 mm graphite cylinder and finally attached in the Doctor Sinter 1080 SPS device (Syntex Sumitomo Heavy Industries, Ltd). Two regimes were tested and are marked as high and low temperature routes (d) and (e) in Fig. 1, namely 1,3731,573 K and 870 K, respectively. The maximum pressure of 80 MPa was applied and kept constant, while temperature, spark plasma voltage and current were increased as described in detail in [15]. The plasma chamber was first evacuated, and then the sintering was performed at 1 atm Ar pressure. The duration of sintering was kept constant as 600 s. The obtained specimens were characterized concerning their microstructure and composition using a Hitachi 3200-N scanning electron microscope (SEM) operated at 20 kV and equipped with an electron dispersive spectrometer (EDS, Noran Ltd.). The thermoelectric voltage was measured against nickel wires with a distance of 10 mm in a home-made device when applying a temperature difference between the micro-ceramic heater (Sakaguchi Ltd. MS 1000) up to 1,273 K, and maintaining the cold end at around 473 K, as reported elsewhere [1214]. The electric multi-meter measurement devices (Sanwa PC510) recorded the data directly on a computer. The densities of the specimens were calculated from their massto-volume ratio, where the specimen dimensions were measured by a caliper. Results and discussion At first, a conventionally calcined and sintered specimen as described in [12] was measured for the first time by using a ZEM3. The thermoelectric measurements as displayed in Fig. 2 confirmed the relatively high resistivity, which decreases as a function of temperature. At T = 1,000 K the resistivity reached as less as 0.05 X m, as shown in Fig. 2a, about one order of magnitude better than previously reported values [12]. The Seebeck coefficient reaches -0.6 mV/K (Fig. 2b) and is almost consta (...truncated)