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Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition
JOURNAL OF RARE EARTHS, Vol. 32, No. 4, Apr. 2014, P. 314
Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition LI Hanbo (李翰博), ZHENG Ganhong (郑赣鸿)*, DAI Zhenxiang (戴振翔), YUAN Zhenheng (袁振亨), WANG Haiqiu (王海秋), MA Yongqing (马永青) (Anhui Key Laboratory of Information Materials and Devices, School of Physics and Material Science, Anhui University, Hefei 230039, China) Received 17 April 2013; revised 18 January 2014
Abstract: 15% Ag-added cubic perovskites Sr0.9La0.1TiO3 and Ruddlesden-Popper (RP) phases Sr2.7La0.3Ti2O7 were fabricated via hydrothermal synthesis, cold pressing and high-temperature sintering. The structure and thermoelectric properties were also investigated for all samples. The results indicated that Ag precipitated as a second phase. Ag addition made electrical conductivity and absolute Seebeck coefficient enhanced, as a result, the ZT values were enhanced both for two series. Compared with cubic perovskite, RP phase was subjected to smaller impact by Ag addition. The reasons for enhancing ZT value and the different impact for two series by Ag addition were also discussed. Keywords: ceramics; crystal structure; Ag-added; thermal conductivity; electrical properties; rare earths
Thermoelectric (TE) materials have been presented increasingly importance in the field of solid-state refrigeration and power generation applications. The TE performance is evaluated with the dimensionless TE figure of merit ZT=S2Tσκ–1, where Z, S, σ, κ and T are the figure of merit, Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. For practical application, the ZT is suggested to be ≥1. Recently, its has been reported that a n-type lanthanum doped SrTiO3 single crystal exhibits high power factor comparable with that of Bi-Te alloy at room temperature[1,2]. However, its high thermal conductivity leads to low ZT values[3]. So, in order to get larger ZT value, the κ of SrTiO3 must be lower. One method is to synthesize the TE materials epitaxial film, for example, Ohta et al. have synthesized the niobium-doped SrTiO3, and the TE figure of merit for SrTi0.8Nb0.2O3 has reached 0.37 at 1000 K, which was the largest value among n-type oxide semiconductors reported then[4]. The other method to lower the high κ is to engineer the superlattice structure. Owing to the scattering strength of the phonons, occurred in the superlattice-like interface, κ reduces correspondingly. Thus, the layered perovskite-type Ruddlesden-Popper (RP) phase, SrO(SrTiO3)n (n=integer) as a promising candidate has been studied. According to Refs. [5,6], the κ values of Nb-doped SrO(SrTiO3)n with superlattice structure were significantly small, compared with that of the cubic perovskite SrTiO3 crystal. However, the absolute Seebeck coefficient values were rather smaller than that of
the SrTiO3 crystal. This is due to the fact that the density of states of SrO(SrTiO3)n is smaller than those of SrTiO3. Recently, ZT as high as 2.2 has been reported[7] in the (PbTe)x(AgSbTe2)1–x system. Such high ZT value is ascribed to a large Seebeck coefficient and low lattice thermal conductivity due to Ag and Sb addition[8]. Kim et al.[9] also observed that the thermoelectric properties were enhanced when ErAs nanoinclusion was embedded in InGaAs matrix. Such an enhancement of the thermoelectric properties is attributed to reduction in the phonon conductivity[9]. Moreover, a certain amount of Ag added in Ca3Co4O9+δ ceramics resulted in an increase of the power factor. Ag could improve electrical connections between cobaltite grains and result in a significant increase in σ without largely increasing κ[10–13]. Encouraged by these above results, in the current work, we attempted to improve the thermoelectric performance of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 ceramics by addition of 15 wt.% Ag.
1 Experimental The Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%) composites were synthesized via hydrothermal synthesis, cold pressing and high-temperature sintering. First, high-purity strontium, lanthanum, and silver nitrate were dissolved in deionized water and tetrabutyl titanate was dissolved in ethanol to form uniform solutions. Subsequently, the latter solution was added dropwise into the
Foundation item: Project supported by National Natural Science Foundation of China (11204001, 10804110, 11174004) and Anhui Provincial Natural Science Foundation (1208085QA07) * Corresponding author: ZHENG Ganhong (E-mail: [email protected]; Tel.: +86-551-385-5549) DOI: 10.1016/S1002-0721(14)60073-9
LI Hanbo et al., Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition
Sr, La, and silver nitrate solution under stirring, and the PH value of the mixed solution was adjusted to 13 by adding NaOH. After ultrasonic stirring, the solution was transferred into a Teflon autoclave followed by hydrothermal treatment at 200 ºC for 48 h. After the autoclave cooled down to room temperature, the obtained solid products were washed several times with deionized water and ethanol, and then dried overnight. And then the products were pressed into compact pellets with 15 MPa pressure and annealed at 1450 ºC for 4 h in a carbon crucible in the presence of argon gas with 5 mol.% hydrogen. The phase composition of as-prepared samples were characterized by a Japanese company's MXP18AHF MARK X-ray diffractometer over the 2θ range of 20º–80º. Squares of 10 mm×10 mm×2 mm and bars of about 2 mm×3 mm× 12 mm were cut and polished from the pressed disks for characterization of the thermoelectric properties. The electrical resistivity of the sample was measured by a four-point direct current (DC) current-switching technique, and the Seebeck coefficient was measured by a static DC method, using a commercial equipment (ZEM-3M10), Ulvac Riko, Inc.) in a low-pressure helium environment with temperatures ranging from 250 to 1000 K. The thermal diffusivity D was measured using the laser flash method (Netzsch, LFA 457). The specific heat Cp was determined by a commercial instrument (Q2000DSC, AmericaTa). The density ρ was measured by Archimedes’ method. The resulting thermal conductivity was calculated from the measured thermal diffusivity D, specific heat Cp, and density ρ from the relationship κ= DρCp.
2 Results and discussion
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(x=0 and 15%) match well with those of the PCPDF (#840444), indicating that the samples have a tetragonal-type structure with the space group I4/mmm. In addition, it can be seen that there exist four additional peaks at 2θ=38.1º, 44.3º, 53.8º and 64.5º, which are observed to fit well with these four strongest peaks of Ag (No. 011167). Fig. 1(b) presents the powder X-ray diffraction for Sr2.7La0.3Ti2O7/xAg (x=0 and 15%), the top panel is the PCPDF of Sr3Ti2O7 (#891383). Because the X-ray diffraction peaks (at 2θ=38.1°, 44.3°, 53.8° and 64.5°) of Ag are well overlapped with those for Sr2.7La3Ti2O7, there is no obviously different peak for Ag. However, it can be seen that, with 15% Ag addition into Sr2.7La3Ti2O7, the four peaks become stronger obviously. This indicates that these four peaks are well related to silver, and that Ag precipitates as a second phase in these samples. 2.2 Microstructural analysis for Sr2.7La0.3Ti2O7/xAg (x=15%) Fig. 2 shows SEM image (a), TEM image (b), SAED image (c), and HRTEM image photographed from the area labeled by the square in (b) (d) for the sample Sr2.7La0.3Ti2O7/xAg (x=15%). It can be seen from Fig. 2(a) that Sr2.7La0.3Ti2O7/xAg (x=15%) is composed of nanoscaled particles, which agglomerates to larger particles without regular morphology. The SAED pattern in Fig. 2(c) shows the distinct diffraction spots of Sr2.7La0.3 Ti2O7/xAg (x=15%). It is also observed that there are redundant diffraction spots which maybe come from Ag and is in good accordance with the XRD result. Moreover, HRTEM shows two different periodic atomic configurations, indicating that Sr2.7La0.3Ti2O7/xAg (x=15%) has two crystal-lattice structures.
Crystal structure for Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
2.3 Thermoelectric properties for Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
Fig. 1(a) presents the powder X-ray diffraction for Sr0.9La0.1TiO3/xAg (x=0 and 15%), the top panel is the PCPDF of SrTiO3 (#840444). The main diffraction peak position and the relative intensities of Sr0.9La0.1TiO3/xAg
Temperature dependence of the electrical conductivity σ(T) is presented in Fig. 3 for the compositions Sr0.9La0.1TiO3/xAg (x=0 and 15%) (a) and Sr2.7La0.3Ti2O7/ xAg (x=0 and 15%) (b). These results are obtained in the
2.1
Fig. 1 XRD patterns of Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/xAg (b) (x=0 and 15%) compounds at room temperature
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Fig. 2 SEM image (a), TEM image (b), SAED image (c) and HRTEM image photographed from the area labeled by the square in (b) (d) for the sample Sr2.7La0.3Ti2O7/xAg (x=15%)
temperature range from 250 to 1100 K. The electrical conductivity decreases with increasing temperature for all these samples in the measured temperatures range, which is the metallic character. From Fig. 3, it is found that the Ag addition makes the electrical conductivity values larger in both Sr0.9La0.1TiO3 and Sr2.7La0. 3Ti2O7 compounds, the same phenomenon has been also observed in the Ag addition in Ca3Co4O9+δ ceramics[11]. Ag addition, on the one hand, causes an increase in carrier concentration, and, on the other hand, Ag maybe acts as electrical connections between grains. As a result, Ag improves electrical connections between grains and makes electrical conductivity stronger.
Fig. 3 Temperature dependence of the conductivity σ(T) of the compounds Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
Comparing the RP phases with perovskite-type ones both for x=0 and 15% Ag addition, it is found that the σ value of the RP phase Sr2.7La0.3Ti2O7/xAg is much lower than that of perovskite-type Sr0.9La0.1TiO3/xAg in the whole temperature range. In addition, the σ of the RP phases Sr2.7La0.3Ti2O7 was subjected to smaller impact by Ag addition. As is well known, the electron transport occurs predominantly along the SrTiO3 layers, and the insulating SrO layers in polycrastalline RP sample is barriers for electron transport[14]. Therefore, the electrical conductivity for the RP phases is lower and subjected to smaller impact than that of cubic perovskite phase for the same Ag concentration addition. Fig. 4 shows the temperature dependence of Seebeck coefficient S of the samples Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%) composites. The negative S in the whole measured temperature range indicates that the major charge carriers in the present samples are electrons. The absolute Seebeck coefficient increases with increasing temperature in the whole temperature range for all composites, regardless of the Ag addition. This indicates that the Ag addition does not change the carrier kind and the variation tendency with temperature. However, the Seebeck coefficient is enhanced with Ag addition for both the cubic perovskitetype Sr0.9La0.1TiO3 and the RP phases Sr2.7La0.3Ti2O7 matrix. It can also be found that the absolute Seebeck coeffi-
LI Hanbo et al., Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition
Fig. 4 Temperature dependence of Seebeck coefficient S(T) of the compounds Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/ xAg (b) (x=0 and 15%)
cient values |S| of the RP phases are much smaller than those of cubic perovskite-type both for non- and 15% Ag addition samples. As reported in the Ref. [14], in cubic perovskite type Sr0.9La0.1TiO3 systems with regular TiO6 octahedra, the Seebeck coefficient are large due to its high density of state effective mass. In contrast, the md* values of the RP phases with irregular TiO6 octahedra are much smaller than those of cubic perovskite-type systems, and then show considerable deviation from the linear relation of cubic perovskite-type phases. This is due to the crystal field splitting of Ti 3d-t2g orbitals and the presence of deformed TiO6 octahedar. Based on the above results, the power factors are calculated from S2σ and presented in Fig. 5. Obviously, there is a great distinction in the relation between the power factor and temperature for the two kinds of composites. The power factor of the cubic perovskite-type Sr0.9La0.1TiO3 series firstly increases with increasing temperature, and will reach maximum, and then drops down with further increase of temperature, as shown in Fig. 5(a). The maximum of power factor for composites increases from 615 µW/K2m for x=0 and 850 µW/K2m for x=15%. As for the RP phase composites, the power
Fig. 5 Temperature dependence of power factor PF of the compounds Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
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factor presents similar variation trend as the Sr0.9La0.1TiO3 samples, and Ag addition makes the power factor values larger. The measurements of the thermal conductivity are performed to study heat transport of the currents compositions. Fig. 6 shows that the temperature dependence of the total thermal conductivity κ(T) in the temperature region of 250–1100 K. Generally speaking, the thermal conductivity of solid materials κ consists of the phonon thermal conductivity κph and the carrier thermal conductivity κel, i.e. κ=κlat+κel. The κe value can be estimated from Wiedemann-Franz (W-F) law, which relates κe to σ according to κe=LTσ, where L is Lorenz constant 2.45×108 WΩ/K2, denotes the Lorenz number. In the room temperature, the electronic contribution to the thermal conductivity in our samples is about 0.05% and 0.01%, respectively. This means that the total thermal conductivity comes mainly from the lattice thermal conductivity for all samples. For non-Ag addition, the total thermal conductivity of the cubic perovskite Sr0.9La0.1TiO3 and the RP phase Sr2.7La0.3Ti2O7 matrix at 300 K is 10.3 and 8.1 W/mK, respectively. With the temperature increasing, κ decrease and reach ~4.7 and 3.3 W/mK at 1100 K for Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 samples, respectively. When 15% Ag is added, the thermal conductivity κ decreases for two series samples, similar phenomena have been reported in Pb 1 – x Sn x Te-PbS [ 1 5 ] and Gex(SnyPb1–y)1–xTe[16]. It is attributed to the ability to significantly suppress the lattice thermal conductivity using high densities of matrix/nanoinclusion interfaces or grain boundaries to increase phonon scattering[17]. However, the reduction amplitude of κ for Sr0.9La0.1TiO3 composition is larger than that of Sr2.7La0.3Ti2O7 one. That is to say, the smaller impact for RP phase is subjected comparing with the cubic perovskite with Ag addition. As is well known, the schematic crystal structures of SrO(SrTiO3)n (n=1, 2) which are regarded as a superlattice of SrO layers and (SrTiO3)n slabs alternately stacking
Fig. 6 Temperature dependence of thermal conductivity κ(T) of the compounds Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/ xAg (b) (x=0 and 15%)
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along the c-axis. Therefore, Sr2.7La0.3Ti2O7 exhibits lower κ value and smaller impact than Sr0.9La0.1TiO3 since the interfaces between SrO layers and (SrTiO3)n slabs enhance the phonon scattering. Fig. 7 (a) and (b) show temperature dependence of dimensionless figure of merit ZT for Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/xAg (b) compounds. For the cubic perovskite Sr0.9La0.1TiO3, the highest figure of merit, ZT~ 0.08 and 0.18 for non- and 15% Ag addition, respectively, are observed. For the RP phase Sr2.7La0.3Ti2O7 ceramics, with 15% Ag addition, ZT is 0.06 and 0.08 for non- and 15% Ag addition, respectively. These results indicate that Ag addition enhances ZT value for both series, which suggests that the decrease in κ overwhelms the contribution of the reduction in S2σ, resulting in the ZT increases of the ZT value with Ag addition. In addition, the ZT increment for cubic perovskite Sr0.9La0.1TiO3 is larger than that for RP phases Sr2.7La0.3Ti2O7.
Fig. 7 Temperature dependence of ZT for the compound Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/xAg (b) (x=0 and 15%)
3 Conclusions The thermoelectric properties of Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%) ceramics were investigated in the temperature range from 250 to 1100 K. Our results indicated that Ag additions caused the increased in the electrical conductivity of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 compounds owing to good conductivity for Ag. At the same time, some Ag additions maybe entered the host lattice and formed solid solution, resulting in the enhancement of the Seebeck coefficient. The thermal conductivity was found to be decreased with increasing the Ag content. On the whole, the ZT value was improved for the Ag addition in our samples of both the cubic perovskites Sr0.9La0.1TiO3 and the RP phases Sr2.7La0.3Ti2O7. However, comparing with the cubic perovskites Sr0.9La0.1TiO3, there were weaker effects on the thermoelectric performance for the RP phase Sr2.7La0.3Ti2O7. Acknowledgements: This work was also financially supported
JOURNAL OF RARE EARTHS, Vol. 32, No. 4, Apr. 2014 by Anhui University Scientific Research Fund (06060283, 2009QN006A, KYXL2012017, 32030028), ‘211 Project’ of Anhui University.
References: [1] Okuda T, Nakanishi K, Miyasaka S. TokuraY. Large thermoelectric response of metallic perovskites: Sr1–xLaxTiO3 (0
LI Hanbo et al., Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition [15] Androulakis J, Lin C H, Kong H J, Uher C, Wu C I, Hogan T, Cook B A, Caillat T, Paraskevopoulos K M, Kanaziodis M G. Spinodal decomposition and nucleation and growth as a means to bulk nanostructured thermoelectrics: enhanced performance in Pb1–xSnxTe-PbS. J. Am. Chem. Soc., 2007, 129: 9780. [16] Gelbstein Y, Dado B, Ben-Yehuada O, Sadia Y, Dashevsky Z, Dariel M P. High thermoelectric figure of merit and nanostructuring in bulk p-type Gex(SnyPb1−y)1−xTe alloys
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following a spindoal decomposition reaction. Chem. Mater., 2010, 22: 1054. [17] Makongo J P A, Misra D K, Salvador J R, Takas N J, Wang G Y, Shabetai M R, Pant A, Paudel P, Uher C, Stokes K L, Poudeu P F. Thermal and electronic charge transport in bulk nanostructured Zr0.25Hf0.75NiSn composites with full-Heusler inclusions. J. Solid State Chem., 2011, 184: 2948.
Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition LI Hanbo (李翰博), ZHENG Ganhong (郑赣鸿)*, DAI Zhenxiang (戴振翔), YUAN Zhenheng (袁振亨), WANG Haiqiu (王海秋), MA Yongqing (马永青) (Anhui Key Laboratory of Information Materials and Devices, School of Physics and Material Science, Anhui University, Hefei 230039, China) Received 17 April 2013; revised 18 January 2014
Abstract: 15% Ag-added cubic perovskites Sr0.9La0.1TiO3 and Ruddlesden-Popper (RP) phases Sr2.7La0.3Ti2O7 were fabricated via hydrothermal synthesis, cold pressing and high-temperature sintering. The structure and thermoelectric properties were also investigated for all samples. The results indicated that Ag precipitated as a second phase. Ag addition made electrical conductivity and absolute Seebeck coefficient enhanced, as a result, the ZT values were enhanced both for two series. Compared with cubic perovskite, RP phase was subjected to smaller impact by Ag addition. The reasons for enhancing ZT value and the different impact for two series by Ag addition were also discussed. Keywords: ceramics; crystal structure; Ag-added; thermal conductivity; electrical properties; rare earths
Thermoelectric (TE) materials have been presented increasingly importance in the field of solid-state refrigeration and power generation applications. The TE performance is evaluated with the dimensionless TE figure of merit ZT=S2Tσκ–1, where Z, S, σ, κ and T are the figure of merit, Seebeck coefficient, electrical conductivity, thermal conductivity, and temperature, respectively. For practical application, the ZT is suggested to be ≥1. Recently, its has been reported that a n-type lanthanum doped SrTiO3 single crystal exhibits high power factor comparable with that of Bi-Te alloy at room temperature[1,2]. However, its high thermal conductivity leads to low ZT values[3]. So, in order to get larger ZT value, the κ of SrTiO3 must be lower. One method is to synthesize the TE materials epitaxial film, for example, Ohta et al. have synthesized the niobium-doped SrTiO3, and the TE figure of merit for SrTi0.8Nb0.2O3 has reached 0.37 at 1000 K, which was the largest value among n-type oxide semiconductors reported then[4]. The other method to lower the high κ is to engineer the superlattice structure. Owing to the scattering strength of the phonons, occurred in the superlattice-like interface, κ reduces correspondingly. Thus, the layered perovskite-type Ruddlesden-Popper (RP) phase, SrO(SrTiO3)n (n=integer) as a promising candidate has been studied. According to Refs. [5,6], the κ values of Nb-doped SrO(SrTiO3)n with superlattice structure were significantly small, compared with that of the cubic perovskite SrTiO3 crystal. However, the absolute Seebeck coefficient values were rather smaller than that of
the SrTiO3 crystal. This is due to the fact that the density of states of SrO(SrTiO3)n is smaller than those of SrTiO3. Recently, ZT as high as 2.2 has been reported[7] in the (PbTe)x(AgSbTe2)1–x system. Such high ZT value is ascribed to a large Seebeck coefficient and low lattice thermal conductivity due to Ag and Sb addition[8]. Kim et al.[9] also observed that the thermoelectric properties were enhanced when ErAs nanoinclusion was embedded in InGaAs matrix. Such an enhancement of the thermoelectric properties is attributed to reduction in the phonon conductivity[9]. Moreover, a certain amount of Ag added in Ca3Co4O9+δ ceramics resulted in an increase of the power factor. Ag could improve electrical connections between cobaltite grains and result in a significant increase in σ without largely increasing κ[10–13]. Encouraged by these above results, in the current work, we attempted to improve the thermoelectric performance of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 ceramics by addition of 15 wt.% Ag.
1 Experimental The Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%) composites were synthesized via hydrothermal synthesis, cold pressing and high-temperature sintering. First, high-purity strontium, lanthanum, and silver nitrate were dissolved in deionized water and tetrabutyl titanate was dissolved in ethanol to form uniform solutions. Subsequently, the latter solution was added dropwise into the
Foundation item: Project supported by National Natural Science Foundation of China (11204001, 10804110, 11174004) and Anhui Provincial Natural Science Foundation (1208085QA07) * Corresponding author: ZHENG Ganhong (E-mail: [email protected]; Tel.: +86-551-385-5549) DOI: 10.1016/S1002-0721(14)60073-9
LI Hanbo et al., Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition
Sr, La, and silver nitrate solution under stirring, and the PH value of the mixed solution was adjusted to 13 by adding NaOH. After ultrasonic stirring, the solution was transferred into a Teflon autoclave followed by hydrothermal treatment at 200 ºC for 48 h. After the autoclave cooled down to room temperature, the obtained solid products were washed several times with deionized water and ethanol, and then dried overnight. And then the products were pressed into compact pellets with 15 MPa pressure and annealed at 1450 ºC for 4 h in a carbon crucible in the presence of argon gas with 5 mol.% hydrogen. The phase composition of as-prepared samples were characterized by a Japanese company's MXP18AHF MARK X-ray diffractometer over the 2θ range of 20º–80º. Squares of 10 mm×10 mm×2 mm and bars of about 2 mm×3 mm× 12 mm were cut and polished from the pressed disks for characterization of the thermoelectric properties. The electrical resistivity of the sample was measured by a four-point direct current (DC) current-switching technique, and the Seebeck coefficient was measured by a static DC method, using a commercial equipment (ZEM-3M10), Ulvac Riko, Inc.) in a low-pressure helium environment with temperatures ranging from 250 to 1000 K. The thermal diffusivity D was measured using the laser flash method (Netzsch, LFA 457). The specific heat Cp was determined by a commercial instrument (Q2000DSC, AmericaTa). The density ρ was measured by Archimedes’ method. The resulting thermal conductivity was calculated from the measured thermal diffusivity D, specific heat Cp, and density ρ from the relationship κ= DρCp.
2 Results and discussion
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(x=0 and 15%) match well with those of the PCPDF (#840444), indicating that the samples have a tetragonal-type structure with the space group I4/mmm. In addition, it can be seen that there exist four additional peaks at 2θ=38.1º, 44.3º, 53.8º and 64.5º, which are observed to fit well with these four strongest peaks of Ag (No. 011167). Fig. 1(b) presents the powder X-ray diffraction for Sr2.7La0.3Ti2O7/xAg (x=0 and 15%), the top panel is the PCPDF of Sr3Ti2O7 (#891383). Because the X-ray diffraction peaks (at 2θ=38.1°, 44.3°, 53.8° and 64.5°) of Ag are well overlapped with those for Sr2.7La3Ti2O7, there is no obviously different peak for Ag. However, it can be seen that, with 15% Ag addition into Sr2.7La3Ti2O7, the four peaks become stronger obviously. This indicates that these four peaks are well related to silver, and that Ag precipitates as a second phase in these samples. 2.2 Microstructural analysis for Sr2.7La0.3Ti2O7/xAg (x=15%) Fig. 2 shows SEM image (a), TEM image (b), SAED image (c), and HRTEM image photographed from the area labeled by the square in (b) (d) for the sample Sr2.7La0.3Ti2O7/xAg (x=15%). It can be seen from Fig. 2(a) that Sr2.7La0.3Ti2O7/xAg (x=15%) is composed of nanoscaled particles, which agglomerates to larger particles without regular morphology. The SAED pattern in Fig. 2(c) shows the distinct diffraction spots of Sr2.7La0.3 Ti2O7/xAg (x=15%). It is also observed that there are redundant diffraction spots which maybe come from Ag and is in good accordance with the XRD result. Moreover, HRTEM shows two different periodic atomic configurations, indicating that Sr2.7La0.3Ti2O7/xAg (x=15%) has two crystal-lattice structures.
Crystal structure for Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
2.3 Thermoelectric properties for Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
Fig. 1(a) presents the powder X-ray diffraction for Sr0.9La0.1TiO3/xAg (x=0 and 15%), the top panel is the PCPDF of SrTiO3 (#840444). The main diffraction peak position and the relative intensities of Sr0.9La0.1TiO3/xAg
Temperature dependence of the electrical conductivity σ(T) is presented in Fig. 3 for the compositions Sr0.9La0.1TiO3/xAg (x=0 and 15%) (a) and Sr2.7La0.3Ti2O7/ xAg (x=0 and 15%) (b). These results are obtained in the
2.1
Fig. 1 XRD patterns of Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/xAg (b) (x=0 and 15%) compounds at room temperature
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Fig. 2 SEM image (a), TEM image (b), SAED image (c) and HRTEM image photographed from the area labeled by the square in (b) (d) for the sample Sr2.7La0.3Ti2O7/xAg (x=15%)
temperature range from 250 to 1100 K. The electrical conductivity decreases with increasing temperature for all these samples in the measured temperatures range, which is the metallic character. From Fig. 3, it is found that the Ag addition makes the electrical conductivity values larger in both Sr0.9La0.1TiO3 and Sr2.7La0. 3Ti2O7 compounds, the same phenomenon has been also observed in the Ag addition in Ca3Co4O9+δ ceramics[11]. Ag addition, on the one hand, causes an increase in carrier concentration, and, on the other hand, Ag maybe acts as electrical connections between grains. As a result, Ag improves electrical connections between grains and makes electrical conductivity stronger.
Fig. 3 Temperature dependence of the conductivity σ(T) of the compounds Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
Comparing the RP phases with perovskite-type ones both for x=0 and 15% Ag addition, it is found that the σ value of the RP phase Sr2.7La0.3Ti2O7/xAg is much lower than that of perovskite-type Sr0.9La0.1TiO3/xAg in the whole temperature range. In addition, the σ of the RP phases Sr2.7La0.3Ti2O7 was subjected to smaller impact by Ag addition. As is well known, the electron transport occurs predominantly along the SrTiO3 layers, and the insulating SrO layers in polycrastalline RP sample is barriers for electron transport[14]. Therefore, the electrical conductivity for the RP phases is lower and subjected to smaller impact than that of cubic perovskite phase for the same Ag concentration addition. Fig. 4 shows the temperature dependence of Seebeck coefficient S of the samples Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%) composites. The negative S in the whole measured temperature range indicates that the major charge carriers in the present samples are electrons. The absolute Seebeck coefficient increases with increasing temperature in the whole temperature range for all composites, regardless of the Ag addition. This indicates that the Ag addition does not change the carrier kind and the variation tendency with temperature. However, the Seebeck coefficient is enhanced with Ag addition for both the cubic perovskitetype Sr0.9La0.1TiO3 and the RP phases Sr2.7La0.3Ti2O7 matrix. It can also be found that the absolute Seebeck coeffi-
LI Hanbo et al., Thermoelectric properties of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 with 15% Ag addition
Fig. 4 Temperature dependence of Seebeck coefficient S(T) of the compounds Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/ xAg (b) (x=0 and 15%)
cient values |S| of the RP phases are much smaller than those of cubic perovskite-type both for non- and 15% Ag addition samples. As reported in the Ref. [14], in cubic perovskite type Sr0.9La0.1TiO3 systems with regular TiO6 octahedra, the Seebeck coefficient are large due to its high density of state effective mass. In contrast, the md* values of the RP phases with irregular TiO6 octahedra are much smaller than those of cubic perovskite-type systems, and then show considerable deviation from the linear relation of cubic perovskite-type phases. This is due to the crystal field splitting of Ti 3d-t2g orbitals and the presence of deformed TiO6 octahedar. Based on the above results, the power factors are calculated from S2σ and presented in Fig. 5. Obviously, there is a great distinction in the relation between the power factor and temperature for the two kinds of composites. The power factor of the cubic perovskite-type Sr0.9La0.1TiO3 series firstly increases with increasing temperature, and will reach maximum, and then drops down with further increase of temperature, as shown in Fig. 5(a). The maximum of power factor for composites increases from 615 µW/K2m for x=0 and 850 µW/K2m for x=15%. As for the RP phase composites, the power
Fig. 5 Temperature dependence of power factor PF of the compounds Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%)
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factor presents similar variation trend as the Sr0.9La0.1TiO3 samples, and Ag addition makes the power factor values larger. The measurements of the thermal conductivity are performed to study heat transport of the currents compositions. Fig. 6 shows that the temperature dependence of the total thermal conductivity κ(T) in the temperature region of 250–1100 K. Generally speaking, the thermal conductivity of solid materials κ consists of the phonon thermal conductivity κph and the carrier thermal conductivity κel, i.e. κ=κlat+κel. The κe value can be estimated from Wiedemann-Franz (W-F) law, which relates κe to σ according to κe=LTσ, where L is Lorenz constant 2.45×108 WΩ/K2, denotes the Lorenz number. In the room temperature, the electronic contribution to the thermal conductivity in our samples is about 0.05% and 0.01%, respectively. This means that the total thermal conductivity comes mainly from the lattice thermal conductivity for all samples. For non-Ag addition, the total thermal conductivity of the cubic perovskite Sr0.9La0.1TiO3 and the RP phase Sr2.7La0.3Ti2O7 matrix at 300 K is 10.3 and 8.1 W/mK, respectively. With the temperature increasing, κ decrease and reach ~4.7 and 3.3 W/mK at 1100 K for Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 samples, respectively. When 15% Ag is added, the thermal conductivity κ decreases for two series samples, similar phenomena have been reported in Pb 1 – x Sn x Te-PbS [ 1 5 ] and Gex(SnyPb1–y)1–xTe[16]. It is attributed to the ability to significantly suppress the lattice thermal conductivity using high densities of matrix/nanoinclusion interfaces or grain boundaries to increase phonon scattering[17]. However, the reduction amplitude of κ for Sr0.9La0.1TiO3 composition is larger than that of Sr2.7La0.3Ti2O7 one. That is to say, the smaller impact for RP phase is subjected comparing with the cubic perovskite with Ag addition. As is well known, the schematic crystal structures of SrO(SrTiO3)n (n=1, 2) which are regarded as a superlattice of SrO layers and (SrTiO3)n slabs alternately stacking
Fig. 6 Temperature dependence of thermal conductivity κ(T) of the compounds Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/ xAg (b) (x=0 and 15%)
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along the c-axis. Therefore, Sr2.7La0.3Ti2O7 exhibits lower κ value and smaller impact than Sr0.9La0.1TiO3 since the interfaces between SrO layers and (SrTiO3)n slabs enhance the phonon scattering. Fig. 7 (a) and (b) show temperature dependence of dimensionless figure of merit ZT for Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/xAg (b) compounds. For the cubic perovskite Sr0.9La0.1TiO3, the highest figure of merit, ZT~ 0.08 and 0.18 for non- and 15% Ag addition, respectively, are observed. For the RP phase Sr2.7La0.3Ti2O7 ceramics, with 15% Ag addition, ZT is 0.06 and 0.08 for non- and 15% Ag addition, respectively. These results indicate that Ag addition enhances ZT value for both series, which suggests that the decrease in κ overwhelms the contribution of the reduction in S2σ, resulting in the ZT increases of the ZT value with Ag addition. In addition, the ZT increment for cubic perovskite Sr0.9La0.1TiO3 is larger than that for RP phases Sr2.7La0.3Ti2O7.
Fig. 7 Temperature dependence of ZT for the compound Sr0.9La0.1TiO3/xAg (a) and Sr2.7La0.3Ti2O7/xAg (b) (x=0 and 15%)
3 Conclusions The thermoelectric properties of Sr0.9La0.1TiO3/xAg and Sr2.7La0.3Ti2O7/xAg (x=0 and 15%) ceramics were investigated in the temperature range from 250 to 1100 K. Our results indicated that Ag additions caused the increased in the electrical conductivity of Sr0.9La0.1TiO3 and Sr2.7La0.3Ti2O7 compounds owing to good conductivity for Ag. At the same time, some Ag additions maybe entered the host lattice and formed solid solution, resulting in the enhancement of the Seebeck coefficient. The thermal conductivity was found to be decreased with increasing the Ag content. On the whole, the ZT value was improved for the Ag addition in our samples of both the cubic perovskites Sr0.9La0.1TiO3 and the RP phases Sr2.7La0.3Ti2O7. However, comparing with the cubic perovskites Sr0.9La0.1TiO3, there were weaker effects on the thermoelectric performance for the RP phase Sr2.7La0.3Ti2O7. Acknowledgements: This work was also financially supported
JOURNAL OF RARE EARTHS, Vol. 32, No. 4, Apr. 2014 by Anhui University Scientific Research Fund (06060283, 2009QN006A, KYXL2012017, 32030028), ‘211 Project’ of Anhui University.
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