Performance of a Thermoelectric Module Based on n-Type (La0.12Sr0.88)0.95TiO3−δ and p-Type Ca3Co4−xO9+δ

Here, we present the performance of a thermoelectric (TE) module consisting of n-type (La0.12Sr0.88)0.95TiO3 and p-type Ca3Co4−xO9+δ materials. The main challenge in this investigation was operating the TE module in different atmospheric conditions, since n-type has optimum TE performance at reducing conditions, while p-type has optimum at oxidizing conditions. The TE module was exposed to two different atmospheres and demonstrated higher stability in N2 atmosphere than in air. The maximum electrical power output decreased after 40 h when the hot side was exposed to N2 at 600°C, while only 1 h at 400°C in ambient air was enough to oxidize (La0.12Sr0.88)0.95TiO3 followed by a reduced electrical power output. The module generated maximum electrical power of 0.9 mW (∼ 4.7 mW/cm2) at 600°C hot side and δT ∼ 570 K in N2, and 0.15 mW (∼ 0.8 mW/cm2) at 400°C hot side and δT ∼ 370 K in air. A stability limit of Ca3Co3.93O9+δ at ∼ 700°C in N2 was determined by in situ high-temperature x-ray diffraction.


INTRODUCTION
Thermoelectric (TE) materials based on oxides are promising alternatives to the state-of-the-art non-oxide thermoelectric materials due to stability at high temperatures. In spite of moderate performance, oxide TE modules have the potential to replace batteries as a power source for sensors operating at high temperatures in oxidizing and mild reducing atmospheres, and as such also enable wireless transfer of data from the sensors. [1][2][3][4] Oxide TE modules may also be used at high temperatures as heat flux monitors, which is highly relevant in the process industry. 5 Several oxide materials with p-type electrical conductivity have been reported, including Ca 3 Co 4Àx O 9+d (CCO), which shows the highest performance. So far, the most promising oxide materials with n-type electrical conductivity are Zn 0.96 Al 0.02 Ga 0.02 O, CaMn 0.98 Nb 0.02 O 3 and La 0.15 Sr 0.775 TiO 3Àd with figures of merit (zT) of 0.65 at 970°C (ambient air), 6 0.32 at 790°C (ambient air) 7 and 0.41 at 700°C (5% H 2 /N 2 ), 8 respectively. A high power factor of above 7 lW/cm K 2 is reported at 900°C for In 1.9 Sn 0.05 Al 0.05 O 3 (air), also representing a promising n-type TE material. 9 Pure SrTiO 3 is a dielectric material due to a large band gap (3.2 eV), 10 but the material demonstrates high n-type electrical conductivity at reducing conditions due to the introduction of oxygen vacancies and delocalized electrons caused by Ti 3+ formation.
In addition, donor doping with La on the A-site increases the electrical conductivity corresponding to a significant enhancement of zT. 8,11 SrTiO 3 -based materials have rarely been used for fabrication of TE modules; however, 50 pairs of n-type La 0.035 Sr 0.965 TiO 3 and a non-oxide p-type Ni 0.9 Mo 0.1 yielded 450 mW/cm 2 at thermal gradient (DT) 360 K under 5% H 2 /Ar. 12 Ca 3 Co 4Àx O 9+d (p-type electrical conductivity) is stable up to 926°C in air, 13 and since the valence state of Co is strongly dependent on the partial pressure of oxygen, the stability is dependent on working conditions such as atmosphere and temperature. Due to the presence of two sub-lattices in Ca 3 Co 4Àx O 9+d , the Co ions in CoO 2 edge-sharing octahedra possess oxidation state +4, while those in the rock-salt Ca 2 CoO 3 vary in oxidation state and induce variations in d. 14  These contradicting working conditions for the TE couple are challenging, and here we present an analysis of the stability and TE performance of a module consisting of p-type Ca 3 Co 4Àx O 9+d and ntype (La 0.12 Sr 0.88 ) 0.95 TiO 3Àd in N 2 and air, respectively. A discussion of the stability of the two materials in the two atmospheres is included. As a result of this investigation, essential insight with respect to the performance of the system is provided, as well as possible solutions to improve the performance of the module. Different stoichiometries with different Ca:Co ratios were obtained. LSTO ceramics were prepared by cold isostatic pressing (200 MPa) followed by sintering in a tube furnace (Nabertherm) at 1300°C for 4 h under 5% H 2 /Ar, while CCO-SP ceramics were prepared by spark plasma sintering (SPS) at 870°C and 50 MPa for 2 min. We recently showed that the SSR route, which usually gives microstructures with larger grains, results in better TE performance of LSTO ceramics, 19 while the opposite trend was observed for CCO, 15 hence different sintering techniques were used.

EXPERIMENTAL
One leg each of CCO-SP and LSTO ceramics were ground to $ 3.4 mm in height and cut into a rectangular shape prior to assembling the TE modules. The surfaces of the legs facing the hot and cold sides were ground by 1200 SiC paper and coated with a $ 50 nm film of gold. The legs were then placed between silver foils, and silver paint was used on both the hot and the cold sides to ensure good bonding between the legs and the silver electrodes. The paint was cured in air at room temperature for 3 h before the module was tested.

Characterization
Thermal stability of CCO-SSR powder in N 2 was analyzed by high-temperature XRD (HT-XRD) (h-2h Bruker D8 Advance diffractometer) from ambient temperature to 800°C, with steps of 100°C followed by 30 min hold at each temperature. Thermogravimetric analysis (Netzsch STA 449 F3 Jupiter) of finely ground bulk CCO-SP, CCO-SSR powder and LSTO ceramics was carried out in a temperature range of 25-1100°C in N 2 (pre-vacuum) and air using a heating rate of 10 K/min. Bulk densities were determined by Archimedes measurement in isopropanol.
Characterization of the TE modules consisting of spark plasma as-sintered CCO-SP and as-sintered in 5% H 2 /Ar LSTO ceramic legs was carried out in both N 2 (chamber flushed with N 2 ) and air for 120 h at maximum temperatures of 600 (DT % 570 K) and 450°C (DT % 420 K), respectively. The TE modules were rapidly heated (100 K/min), and measurements performed in air were recorded at several temperatures below 600°C (hot side) during short dwellings at each step. Measurements in air were performed at 130°C, 400°C and 450°C hot-side temperature. More details on the characterization of the thermoelectric modules is provided elsewhere. 20

Characterization of TE Modules
Characterization of the TE modules in the two different atmospheres is presented in Fig. 1, where the maximum electrical power output (P max ), open Performance of a Thermoelectric Module Based on n-Type (La 0.12 Sr 0.88 ) 0.95 TiO 3Àd and p-Type Ca 3 Co 4Àx O 9+d circuit voltage (V OC ), electrical resistance (R module or R i ) and DT as a function of time are included. P max is calculated according to Eq. 1. 21 In both atmospheres, P max decreases, while R i and V OC increase 1-2 h after startup. In N 2 (Fig. 1a), P max increases with increasing temperature reaching $ 0.9 mW (4.7 mW/cm 2 ) at maximum DT, and maintaining 0.85 mW for about 40 h, followed by a decrease to 0.65 mW after 120 h. After 1 h of ramping and reaching 400°C (hot side) (DT = 370 K), P max reaches a maximum of about 0.15 mW (0.8 mW/cm 2 ) in air, thereafter rapidly decreasing and reaching 0.07 mW after 1 h dwelling (Fig. 1b). At 450°C (hot side) (DT = 420 K) P max slightly increased from 0.07 to 0.09 mW, but during 12 h dwelling dropped to 0.06 mW.
Comparing P max and R i in the two atmospheres (Table I), significant differences are observed, which are attributed to oxidation of the as-sintered LSTO ceramics during the testing. The reduced performance of the module in air between 1 h and 10 h (Table I), is rationalized by the oxidation of LSTO (relative density $ 84%) followed by a reduction in the electrical conductivity, corresponding to enhanced R i . Since V OC increases with the temperature gradient, power output reaches a maximum value at maximum temperature gradient with hot-side temperature at 600°C.
At high temperatures, the electrical conductivity of SrTiO 3 -based materials is governed by small polaron hopping, defined by the ratio between Ti 4+ and Ti 3+ . 22 The shift in the ratio between the valence states of Ti with increasing oxygen content is visualized by the chemical defect equilibrium described in Eq. 2.
where O x o , V ÁÁ O and e¢ respectively represent oxygen on an oxygen site, an oxygen vacancy, and electrons, the latter represents the Ti 3+ state. Hence, oxidation of LSTO will shift the defect equilibrium in Eq. 2 to the right and reduce the fraction of Ti 3+ corresponding to a reduced electrical conductivity. Hence, the electrical conductivity of LSTO will decrease with oxidation, which goes in favor of enhanced thermopower and V OC . Substituting Sr with La (donor dopant) will enhance the electric conductivity; 23 however, at the temperatures and atmospheres given in our investigation, the relative change in conductivity is adequately explained by Eq. 2. 19 Thus, the combination of the specific n-and p-type TE materials chosen here is challenging even for low-temperature applications in air.
There are also issues with the stability of the metallic interconnect. Figure S1 (Supplementary data) shows a tiny increase in the observed operating voltage with time which is interpreted as the formation of cracks or exfoliation of the Ag-paste, leading to less electrical contact between the active components. Although only a minor change in voltage, these processes must also be considered when optimizing the design of the TE module.  Material Stability in N 2 and Air Figure 2 shows HT-XRD of CCO-SSR powder in N 2 between room temperature and 800°C, while XRD of LSTO is presented in the Supplementary data (Fig. S2). SSR powder is used for HT-XRD analysis in order to obtain more intense reflections compared to significantly less crystalline SP powder that was used for the module. We start with phasepure CCO, and at 600°C phase-pure CCO is still present, while at 700°C decomposition is initiated as shown by the weak reflection corresponding to CoO. However, at 800°C, CCO is completely decomposed to CaO and CoO solid solutions, leaving the decomposition of the intermediate Ca 3 Co 2 O 6 phase 15 undefined between 700°C and 800°C. Since HT-XRD of CCO powder in N 2 was performed at the same conditions as reported in Ref. 15, the obtained results are directly comparable, and the temperature stability ranges of CCO in three different atmospheres are summarized in Table II. Here we see that a TE module exposed to a maximum hot-side temperature of e.g. 700°C in N 2 may be followed by decomposition of CCO corresponding to severe deterioration of the performance of the TE module.
Thermogravimetric analysis in N 2 and air of ground CCO-SP and LSTO ceramics is presented in Fig. 3. A negligible mass loss of CCO-SP in region I (below 200°C) is related to adsorbed moisture (Fig. 3a). A notable mass loss is observed in region II above 550°C, as well as a significant difference in mass loss between the two atmospheres in region III above 700°C (Fig. 3a).
A change in mass of about 2% in region II is related to reduction of Co 3 O 4 secondary phase to Co 1Àx O, 24,25 and further (in region III) into CoO, while negligible mass loss below 700°C in air of the stoichiometric CCO-SSR (with no Co 3 O 4 secondary phase) is shown in the inset of Fig. 3a for comparison. This points to an imperfection in the previously reported miscibility region (Ca:Co ratio lying between 0.74 and 0.76). 18 Besides a notable Co excess and formation of Co 3 O 4 in CCO-SP, a very small Co excess within the narrow miscibility region leads to formation of Ca defects and additional ptype charge carriers in phase pure CCO that increase zT. 26 Different mass loss in two atmospheres in region III (Fig. 3a) is attributed to significant differences in the oxidation states of Co in CCO; hence, a notable change in oxygen nonstoichiometry (d) is more pronounced in an atmosphere with lower pO 2 (N 2 ). Two plateaus in region III are observed in air at about 900°C and 1000°C corresponding to CCO (% 930°C) and Ca 3 Co 2 O 6 (% 1050°C) decomposition temperatures, 15 while in N 2 a continuous mass loss is observed, probably due to the narrow temperature range between the two decomposition temperatures of CCO (% 700°C) and Ca 3 Co 2 O 6 (700-800°C). Furthermore, a mass change of about 7.6% in region III corresponds to full decomposition of CCO into CaO and CoO solid solutions, which is in good agreement with a previous report. 14 The d of the as-sintered CCO-SP ceramic at the beginning of thermogravimetric analysis is calculated to be $ 0.17. Since fully oxidized material is expected to have d $ 0.36, 14 then d $ 0.17 indicates a reduced oxygen content due to the reducing (vacuum) conditions during sintering in SPS.
Thermogravimetric analysis in N 2 and air of ground and bulk LSTO ceramic is presented in Fig. 3b. More pronounced mass change is observed in as-sintered bulk LSTO in air than in ground LSTO in N 2 , as explained by Eq. 2 (equilibrium shifted to the right). Traces of O 2 were present during the analysis in N 2 ; therefore, a slight increase in mass is observed. The O 2 concentration in the N 2 atmosphere during the module test (Fig. 1a) is probably higher than that in the N 2 atmosphere used in TG analysis (Fig. 3b), due to less efficient removal of oxygen by flushing. It is expected that the TE module will produce high power output significantly longer than 40 h, given a  high-purity N 2 atmosphere during testing. Oxidation of the as-sintered bulk LSTO in air demonstrated a notable mass increase. The rate of oxidation was partly enhanced by the 8% open porosity in LSTO; however, a detailed assessment of the oxidation kinetics lends itself to a more elaborate analysis including the measurements of oxygen surface exchange and bulk diffusion. This is beyond the scope of the present investigation. In general, besides the relatively low decomposition temperature of CCO, oxidation of LSTO is challenging, and enhanced performance of the module may be realized by applying an oxygen diffusion barrier (encapsulation) to the LSTO to suppress oxidation and maintain high electronic conductivity. Identifying an effective diffusion barrier compatible with LSTO will significantly boost the performance of the TE module and enable operation in ambient atmosphere at high temperatures.

CONCLUSIONS
TE modules consisting of a pair of n-type (La 0.12 Sr 0.88 ) 0.95 TiO 3Àd and p-type Ca 3 Co 4Àx O 9+d ceramic legs were tested in N 2 and air. The main challenge with the selected material system was the combination of materials having their optimum TE performance at different operating conditions. The TE modules demonstrated higher stability in N 2 (600°C) than in air (450°C). Maximum electrical power output of $ 0.9 mW (4.7 mW/cm 2 ) at 600°C and DT $ 570 K in N 2 decreased after 40 h and stabilized at 0.65 mW after 120 h, while in air, output of $ 0.15 mW (0.8 mW/cm 2 ) at 400°C and DT $ 370 K decreased after 1 h. Hence, a stable TE module operating in N 2 atmosphere was established although with a moderate power output. Operation of the module in ambient air needs to protect the LSTO from oxidation, for example, by applying a surface coating (sealant) impermeable to oxygen. the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecom mons.org/licenses/by/4.0/.