Improved Thermoelectric Performances of SrTiO3 Ceramic Doped with Nb by Surface Modification of Nanosized Titania
© Li et al. 2016
Received: 14 January 2016
Accepted: 4 April 2016
Published: 12 April 2016
Nb-doped SrTiO3 ceramics doped with the surface modification of nanosized titania was prepared via liquid phase deposition approach and subsequent sintered in an Ar atmosphere. The surface modification of nanosized titania significantly improved the ratio of the electrical conductivity to thermal conductivity of SrTiO3 ceramic doped with Nb, and has little impact on the Seebeck coefficient, thus obviously improving the dimensionless thermoelectric figure of merit (ZT value). The surface modification of nanosized titania is a much better method to lower the thermal conductivity and to enhance the electrical conductivity than the mechanical mixing process of nanosized titania. The highest ZT value of 0.33 at 900 K was obtained. The reason for the improved thermoelectric performances by the surface modification of nano-sized titania was preliminary investigated.
More and more attentions have been paid to the bulk nanostructured thermoelectric materials due to their high thermoelectric performances [1–5]. Tang et al. reported the layer nanostructured Bi2Te3 bulk materials sintered by spark plasma sintering and combined melt spinning technique, and obtained the highest ZT value of 1.35 at 300 K . Mi et al. reported that the hot pressed mixture of nanoscale and microsized CoSb3 powders formed the n-type CoSb3 nanocomposites, and reached the highest ZT value 0.71 at 700 K . Poudel et al. prepared nanostructured bismuth antimony telluride alloys via ball milling and hot pressing technique, and the highest ZT value reached 1.4 at 373 K . Han et al. reported Yb0.2Co4Sb12+y nanostructured bulk materials by combining melt spinning method with spark plasma sintering, and obtained the highest ZT value of 1.26 at 800 K . Han et al. also fabricated n-type skutterudites In x Ce y Co4Sb12 with in situ forming nanostructured InSb phase via a melt-quench-anneal-spark plasma sintering technique, and obtained the highest ZT value of 1.43 at 800 K . Kadel et al. reported Bi2Se3 nanostructures by solvothermal method and the highest ZT value obtained 0.096 at 523 K .
Recently, SrTiO3 thermoelectric materials with non-toxic and element-rich advantages arose wide attention [12, 13]. In our previous study, Nb-doped SrTiO3 (Nb-STO) with titanate nanotube additions fabricated via the pressure-less sintering method [14–16]. The additions were directly mixed with strontium titanate powders, which induced the inhomogenous distribution of additions and further lead to the inhomogenous thermoelectric performance of bulk ceramic. Thus, to obtain a homogenous thermoelectric performance, it is important to make a homogenous distribution of the additions.
In this study, liquid phase deposition approach was used to surface-modify Nb-STO by forming the nanosized titania on the strontium titanate grains, and its effect on the thermoelectric performances of SrTiO3 ceramic doped with Nb were investigated in detail.
SrCO3, TiO2, and Nb2O5 powders with high purity were used to prepare the single-phase Nb-doped SrTiO3 (Sr(Ti0.85Nb0.15)O3, Nb-STO) powders in an Ar atmosphere at 1400 °C for 4 h via a solid-state reaction. As-prepared Nb-STO powders were put into the aqueous solution including (NH4)2TiF6 (0.06 M) and H3BO3 (0.2 M). The slurry was strongly stirred at room temperature for 2 h. The centrifugated powders were washed several times with de-ionized water and then dried at about 80 °C. The Nb-STO powders without and with surface modification of nanosized titania were then pressed into pellets under a pressure of 20 MPa and sintered in an Ar atmosphere at 1500 °C for 3 h in a graphite crucible.
The microstructure observations were conducted using scanning electron microscopy (SEM, S-3000N, Hitachi Corporation). The thermoelectric performances, the Seebeck coefficient and electrical conductivity, were determined at 300–1000 K in an Ar atmosphere using an automatic thermoelectric measuring apparatus (RZ-2001K, Ozawa Scientific Corporation). The thermal conductivity (κ) was determined from the thermal diffusivity (β), specific heat capacity (C p), and density (ρ) using the following equation: κ = ρC p β. The thermal diffusivity was determined by the common laser flash method (TC-9000V, ULVAC-RIKO Corporation). And the specific heat capacity was determined by a differential scanning calorimeter system (DSC-2910, TA Instruments Corporation).
Results and Discussion
Based on Eq. (3), the thermal conductivity of Nb-STO sample without surface modification, which has the same relative density to the sample with surface modification of nanosized titania, 75.2 %, was obtained, shown by dotted lines in Fig. 3. The results indicate that the surface modification of nanosized titania could actually reduce the thermal conductivity of Nb-STO polycrystalline ceramics, when the effect of porosity is eliminated.
where k B, h, m*, and n are Boltzmann constant, Planck constant, the effective mass of the carriers, and the carrier concentration, respectively. Equation (4) indicated the Seebeck coefficient mainly depended on carrier concentration. The main reason for the independence of the Seebeck coefficient on the surface modification of nanosized titania is that the carrier concentration not be affected by the surface modification of nanosized titania for it could not react with Nb-STO .
The liquid phase deposition approach was carried out to surface modify the Nb-doped SrTiO3 polycrystalline ceramics, and their thermoelectric performances were investigated. The surface modification of nanosized titania enhanced the ZT value significantly, because of the increased electrical conductivity, and obtained the highest ZT value of 0.33 at 900 K. Enhancement of the electrical conductivity was mainly caused by improved grain growth diminishing the number of the grain boundaries. Newly generated titanium carbide with higher electrical conductivity could also contribute the increased electrical conductivity. Pores homogeneously distributed in the Nb-STO composite could effectively scatter phonons, and hence, contributed to the reduction in the thermal conductivity of Nb-STO composite with the same relative density to pure Nb-STO composite. Seebeck coefficient was almost independent of nanosized titania addition, which was mainly due to the carrier concentration had not been affected by the surface modification of nanosized titania.
This work was financially supported by International Cooperation MOST-JST Program Fund (no. 2010DFA61410) and National Natural Science Foundation of China (no. 50802013).
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