† Corresponding author. E-mail:
Project supported by the National Basic Research Program of China (Grant No. 2013CB632506) and the National Natural Science Foundation of China (Grant Nos. 51202132, 51231007, and 11374186).
Thermoelectric properties of Li-doped Sr0.70Ba0.30Nb2O6–δ ceramics were investigated in the temperature range from 323 K to 1073 K. The electrical conductivity increases significantly after lithium interstitial doping. However, both of the magnitudes of Seebeck coefficient and electrical conductivity vary non-monotonically but synchronously with the doping contents, indicating that doped lithium ions may not be fully ionized and oxygen vacancy may also contribute to carriers. The lattice thermal conductivity increases firstly and then decreases as the doping content increases, which is affected by competing factors. Thermoelectric performance is enhanced by lithium interstitial doping due to the increase of the power factor and the thermoelectric figure of merit reaches maximum value (0.21 at 1073 K) in the sample Sr0.70Ba0.30Li0.10Nb2O6.
Thermoelectric (TE) materials have been focused on in recent years, for the fact that they can convert heat and electrical energy into one another directly and environmentally friendly.[1–4] In order to be of practical use, TE materials are required to have a high figure of merit (ZT = S2σT/κ, S is Seebeck coefficient, σ is electrical conductivity, T is absolute temperature, and κ is thermal conductivity), at the desired operating temperatures. Excellent performance of TE materials not only need to poses a high power factor (PF = S2σ) but also to show a low thermal conductivity κ. By far, the most widely used TE materials are inter-metallic compounds which have relatively high ZT values, such as Bi2Te3, PbTe, and SiGe.[5–13] However, these materials also have some limitations, such as high costs, instability at high temperatures, and dependence on rare or toxic elements. Oxide TE materials may be an alternative to solve these problems, however, their ZT values are still far from practical applications.[14,15] Therefore, it is urgent and meaningful to enhance the TE performance of oxide materials.
Recently, nonstoichiometric tungsten-bronze- (TB)-structured SrxBa1−xNb2O6−δ (SBN) materials were found to be promising n-type oxide thermoelectric materials.[16–21] It was reported that heavily reduced Sr0.61Ba0.39Nb2O6−δ (SBN61) single crystal shows a high thermoelectric power factor (~2000 μW/K2·m at 516 K) in the direction parallel to the c axis.[22] As shown in Fig.
Sr0.70Ba0.30LixNb2O6 ceramics were prepared by solid-state reaction techniques using SrCO3 (in purity 99.8%), BaCO3 (99.8%), Nb2O5 (99%), and Li2CO3 (99.8%) powders. Appropriate components of the starting materials were ground and pressed into pellet discs. These discs were calcined at 1400 °C for 6 hours in an air atmosphere. After intermediate grinding and pressing, the discs were sintered at 1300 °C for 6 hours in a forming gas (5-mol% hydrogen in argon) with a flow rate of 0.2 l/min. The phase structures were investigated using powder x-ray diffraction (XRD) with a Rigaku D/MAX-2550P diffractometer using Cu Kα radiation (λ = 0.154056 nm). The thermal conductivity was calculated from the thermal diffusivity, specific heat capacity, and sample density as measured on a laser flash apparatus (Netzsch LFA 427), a thermal analyzer (Netzsch DSC 200F3), and by Archimedes'method, respectively. The sintered discs were cut into rectangular columns (12 mm × 2 mm × 2 mm) to measure Seebeck coefficient and electrical conductivity using a Linseis LSR-3/1100 instrument in a helium atmosphere by a modified dynamic method.[31] The oxygen vacancies are roughly measured by a self-made thermogravimetric apparatus, neglecting the influences of NbO2 second phase.
The XRD data for the Sr0.70Ba0.30LixNb2O6 samples are shown in Fig.
![]() | Fig. 2. (color online) The x-ray diffraction patterns for lithium-doped Sr0.70Ba0.30LixNb2O6 ceramics. |
![]() | Table 1.
Lattice parameters (a,b,c), theoretical densities (ρth), measured densities (ρm), and relative densities (ρr) of samples, contents of oxygen vacancies (δ). . |
Figures
![]() | Fig. 3. (color online) Temperature dependence of Seebeck coefficient S (a) and electrical conductivity σ (b) of the samples Sr0.70Ba0.30LixNb2O6. |
After lithium doping, the absolute Seebeck coefficient decreases and electrical conductivity increases, as compared with those the un-doped sample. It indicates that carrier concentration is enhanced by lithium doping. However, both of the magnitudes of S and σ vary non-monotonically but synchronously with the doping contents, indicating that doped lithium ions may not be fully ionized and oxygen vacancy may also contribute to carriers. Therefore, the concentration of oxygen vacancies were measured by thermogravimetric analysis, and the δ values in the formula Sr0.70Ba0.30LixNb2O6−δ were calculated and shown in Table
According to the measured S and σ, the thermoelectric power factor (PF) of samples with four doping contents are calculated and shown in Fig.
![]() | Fig. 4. (color online) Temperature dependence of thermoelectric power factor (PF = S2σ) of the samples Sr0.70Ba0.30LixNb2O6. |
Figure
![]() | Fig. 5. (color online) Temperature dependence of total thermal conductivity (a), lattice thermal conductivity (b), thermal diffusivity (c), and heat capacity (d) of the samples Sr0.70Ba0.30LixNb2O6. |
Lattice thermal conductivity κl is calculated from κ subtracting κe, as shown in Fig.
Figure
In this work, the thermoelectric properties of Li-doped Sr0.70Ba0.30Nb2O6−δ ceramics were investigated in the temperature range from 323 K to 1073 K. The profiles of XRD patterns for all samples include every diffraction peak of tungsten bronze-structured strontium barium niobate and the lattice parameters of the samples are almost unchanged. After lithium doping, the non-monotonic and synchronous changes of S and σ indicate that doped lithium ions may not be fully ionized and oxygen vacancy also contribute to carriers. The lattice thermal conductivity increases firstly and then decreases when the doping content increases, which attributes to the competing factors which influence thermal diffusivity and heat capacity. The thermoelectric performance is enhanced by lithium doping due to the increase of the power factor (with a maximum of 486 μW/K2·m). The thermoelectric figure of merit reaches maximum value (0.21 at 1073 K) in the sample Sr0.70Ba0.30Li0.10Nb2O6.
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