Preparation and thermoelectric properties of rare-earth-metal-doped SrO(SrTiO3)n oxides

Preparation and thermoelectric properties of rare-earth-metal-doped SrO(SrTiO3)n oxides

Available online at www.sciencedirect.com Procedia Engineering 00 (2011) 000–000 Procedia Engineering 27 (2012) 103 – 108 Procedia Engineering www.e...

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Available online at www.sciencedirect.com

Procedia Engineering 00 (2011) 000–000 Procedia Engineering 27 (2012) 103 – 108

Procedia Engineering www.elsevier.com/locate/procedia

2011 Chinese Materials Conference

Preparation and thermoelectric properties of rare-earth-metaldoped SrO(SrTiO3)n oxides Ruirui Sun, Di Li, Liangliang Li, Jian Zhang, Qingqing Wang, Xiaoying Qin* Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Science, 230031 Hefei, People’s Republic of China

Abstract Rare-earth-metal-doped (Sr0.95R0.05)3Ti2O7 (R=Nd, Eu) bulk samples were prepared by a conventional solid state reaction and Spark Plasma Sintering (SPS). The effect of calcining temperature Tcal and atmosphere on the phase composition of samples were investigated. The results show that (Sr0.95R0.05)3Ti2O7(R=Eu) samples with pure Ruddlesden–Popper (RP) phases can be obtained at Tcal=1500 oC in the atmosphere consisting of mixture H2 and Ar with their volume ratio of 5:95. Thermoelectric property measurements (300K-1000K) indicate that both the electrical resistivity ρ and the absolute value of thermopower S increase with elevating temperature, showing a typical behavior of a degenerate semiconductor. In comparison, the Eu-doped samples have larger dimensionless figure of merit (ZT) than the Nd-doped samples at the temperatures below 550K. The largest ZT of (Sr0.95Nd0.05)3Ti2O7 obtained in the present study is 0.087 (at 919K).

© © 2011 2011Published Published by byElsevier Elsevier Ltd. Ltd. Selection Selection and/or and/or peer-review  peer-review under responsibility of Chinese Materials Research ResearchSociety Society Keywords: (Sr0.95R0.05)3Ti2O7(R= Nd, Eu); solid state reaction; Spark Plasma Sintering

1. Introduction Metal oxides have attract a great deal of interest because they are basically stable and environmental friendly at higher temperatures[1,5].P-type oxide semiconductors such as NaxCoO2,Ca3Co4O9 exhibit rather large ZT(ZT=S2σ/κ, where S, σ, κ, T and Z are Seebeck coefficient, the electrical conductivity, the thermal conductivity, absolute temperature and a figure of merit, respectively. ), while n-type oxide semiconductors which are inevitably required to develop a thermoelectric power generation module exhibit rather low ZT of 0.37 at 1000K in Nb-doped SrTiO3 [1-4,6-9].The poor performance of SrTiO3

*

Corresponding author. Tel.:+ 86-551-5591439; fax: +86-551-5591434. E-mail address: [email protected]

1877-7058 © 2011 Published by Elsevier Ltd. Selection and/or peer-review  under responsibility of Chinese Materials Research Society doi:10.1016/j.proeng.2011.12.430

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can be ascribed to the relatively higher thermal conductivity. In recent years, some researchers have found that the layered perovskite-type Rudlesdden-Popper (RP) (Srn+1TinO3n+1,n=integer) phase structure comprised of (SrTiO3)n block layer and alternative stacks of salt rock SrO layer can reduce the thermal conductivity efficiently due to the enhanced phonon scattering at the internal interfaces of SrO/(SrTiO3)n[1,10-11]. Up to now, the largest ZT of 0.24 at 1000K have been obtained in (Sr0.95Gd0.05)3Ti2O7 ,indicating. a promising thermoelectric(TE) materials[1,12-13].However, the ZT is still too low to take into practice. In the present study, (Sr0.95R0.05)3Ti2O7 (R=Eu, Nd) oxides were successfully prepared using the conventional solid state reaction and Spark Plasma Sintering (SPS).We studied the effect of the calcining temperature Tcal and atmosphere on the phase composition of (Sr0.95R0.05)3Ti2O7 (R=Eu).Thermoelectric properties of the bulk samples were investigated in the temperature range of 3001000K. 2. Experimental All the samples were prepared by combining the conventional solid-state reaction method with Spark Plasma Sintering (SPS). The starting powder of SrCO3(AR), TiO2(AR), Eu2O3(99.99%) and Nd2O3(99.99%) were ball-milled in a planetary ball mill according to the chemical stoichiometric and calcined at 1200 oC for 12h two times with a intermediate grinding for the purpose of the decomposition of carbonate. In order to form RP phase and generate the carrier electrons, the calcined powders were heated three times for 2h in Ar and H2 atmosphere. We prepared a series of powder samples at different calcining temperatures (Tcal=1485 oC, 1500 oC,1510 oC) in Ar and H2 atmosphere(H2:Ar=5:95) and at different volume ratios of H2 and Ar (H2:Ar=5:95,10:90) holding Tcal=1500 oC. After that, the powder samples with pure R-P phases were Spark Plasma Sintered at 40MPa, 1500 oC for 5min in Ar flow. In order to investigate the effect of annealing on the samples, the obtained bulk samples of (Sr0.95Eu0.05)3Ti2O7 prepared by SPS, were annealed at 1500℃ for 2h in Ar and H2 flow. The phase analysis of the powder samples were characterized by X-ray diffraction (XRD). The electrical resistivity ρ and the Seebeck coefficient S were measured by ZEM3 in He flow. The thermal conductivity κ were calculated from separate measurements of a laser flash method for thermal diffusivity and differential scanning calorimetry(DSC) for heat capacity Cp and bulk density measured through the Archimedes method. 3. Results and Discussion 3.1. Phase analysis and structural characterization Fig.1 shows the powder XRD patterns of (Sr0.95Eu0.05)Ti2O7 at different volume ratios of H2 and Ar holding Tcal=1500 oC . It can be seen that peaks for other phase were not detected in (Sr0.95Eu0.05)Ti2O7 when the volume ratio of H2 and Ar is about 5:95,while there are a amount of TiO and Sr2TiO4 peaks when the ratio is about 10:90.Although a small amount of H2 might have favorable effects on reducing the electrical resistivity ρ, too much H2 may result in undesirable phases which might make the thermoelectric performance deteriorate. The powder XRD patterns of (Sr0.95Eu0.05)Ti2O7 at different calcining temperatures Tcal with a fixed volume ratio of H2:Ar=5:95 are shown in Fig.2.We can see that the Rudlesdden-Popper(RP) phases were basically formed in this three caclining temperatures and the best calcining temperature is about 1500 oC because there are some other peaks detected in the other two conditions.

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Fig. 2. Powder XRD patterns of (Sr0.95Eu0.05)Ti2O7 at different calcining temperatures.

3.2. Thermoelectric properties The temperature dependence of the electrical resistivity ρ and Seebeck coefficient S are shown in Fig.3 and Fig.4, respectively. For all the (Sr0.95R0.05)3Ti2O7(R=Eu, Nd) samples in the temperature range investigated, both the electrical resistivity ρ and the absolute value of S increase with temperature, and the value of S are all negative, indicating that all the samples are n-type degenerate semiconductors. The ρ for all the samples are higher than that of the perovskite-type SrTiO3[14],which can be attributed to the insulating SrO layers randomly distributed in polycrystalline samples. For the sample of (Sr0.95Eu0.05)3Ti2O7, the annealed sample has higher electrical resistivity ρ and larger |S|. The larger |S| of the annealed sample can reflect the lower carrier concentration ne which might account for the increased electrical resistivity. The larger difference in ρ between Eu-doped and Nd-doped (Sr1-xRx)3Ti2O7 oxide might be attributed to the variable valence state in Eu which have two valence states:Eu2+ and Eu3+.

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Fig. 3. The temperature dependence of the electrical resistivity ρ for (Sr1-xRx)3Ti2O7(a)R=Eu, x=0.05, (b)R=Eu, x=0.05, annealed , (c)R=Nd, x=0.05. -50

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Fig. 4. The temperature dependence of Seebeck coefficient S for (Sr1-xRx)3Ti2O7 (a)R=Eu, x=0.05,(b)R=Eu, x=0.05,annealed,(c)R=Nd, x=0.05.

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Fig. 5. The temperature dependence of the total thermal conductivity κtotal and the electronic thermal conductivity κele for (Sr1Rx)3Ti2O7 (a) R=Eu, x=0.05,κtotal (b)R=Eu, x=0.05, κtotal of the annealed sample, (c)R=Eu, x=0.05, κele,(d) R=Eu, x=0.05, κele of the annealed sample,(f)R=Nd, x=0.05.The total thermal conductivity κtotal for 5%Nd-doped were taken from ref.[12].

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Fig. 6. The temperature dependence of power factor for (Sr1-xRx)3Ti2O7(a)R=Eu,x=0.05,(b)R=Eu,x=0.05,annealed,(c) R=Nd,x=0.05

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Fig. 7. The temperature dependence of power factor for (Sr1-xRx)3Ti2O7 (a)R=Eu,x=0.05, (b)R=Eu,x=0.05,annealed,(c)R=Nd,x=0.05.

Fig.5 shows the temperature dependence of thermal conductivity κ for (Sr1-xRx)3Ti2O7 (R=Eu,Nd, x=0.05).The total thermal conductivity κtotal can be expressed by the sum of the carriers contribution in κ (κele) and the lattice component(κL),where the κele can be estimated according to the Wiedemann-Franz’ law[11].We can see that the contribution of κele to κtotal is very small,indicating that the phonon contribution to κtotal is predominant, and annealing treatment have little effect on the total thermal conductivity of (Sr1-xRx)3Ti2O7 (R=Eu, x=0.05) oxide. Therefore, the reduction in the thermal conductivity can be attributed to the enhanced phonon scattering at the internal interfaces of SrO/(SrTiO3)n. Fig.6 and Fig.7 show the temperature dependence of power factor (PF) and dimensionless figure of merit(ZT) for (Sr1-xRx)3Ti2O7(R=Eu, Nd, x=0.05) respectively. The PF can be calculated by the electrical resistivity ρ and Seebeck coefficient S with the formula PF=S2/ρ. The PF of (Sr0.95Eu0.05)3Ti2O7 basically decrease with temperature, while the PF of (Sr0.95Nd0.05)3Ti2O7 increase. The decrease of PF with temperature for (Sr0.95Eu0.05)3Ti2O7 might result from the rapid increase in ρ accompanying a slow increase in |S|. Below 700K, the annealed sample have higher PF. However, when the temperature is higher than 700K, the annealed sample and the unannealed sample basically have the same value of PF. The maximum PF is 384μW-1m-1K-2 at 301.78K obtained in the annealed sample of (Sr0.95Eu0.05)3Ti2O7.For all the samples in the investigated temperature range, the ZT increase with temperature. The largest ZT at room temperature is 0.024 achieved in the annealed sample of (Sr0.95Eu0.05)3Ti2O7, while the maximum ZT is 0.087 obtained in (Sr0.95Nd0.05)3Ti2O7 at 919K.

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4. Summary The The (Sr1-xRx)3Ti2O7(R=Eu, Nd;x=0.05) oxide were successfully prepared. The pure R-P phases powders were obtained at Tcal=1500 oC with the volume ratio of H2 and Ar about 5:95. Thermoelectric properties of the bulk samples were measured in the temperature range of 300K-1000K. All the samples exhibited electrical resistivity ρ and the seebeck coefficient S, and both ρ and |S| with a negative value increase with temperature indicating a typical n-type degenerate semiconducting behavior. The total thermal conductivity κtotal for all samples decrease with elevating temperature and the decrease in κtotal might be ascribed to the enhanced phonon at the internal interfaces of SrO/SrTiO3. The maximum ZT value reaches 0.087 at 919K for (Sr0.95Nd0.05)3Ti7.

Acknowledgements Financial support from the National Science Foundation of China(No.50972146,No.10904144, No.11174292,and No.51101150) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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