- Email: [email protected]
Effect of La2O3 on high-temperature thermoelectric properties of WO3
Journal of Alloys and Compounds 581 (2013) 52–55
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of La2O3 on high-temperature thermoelectric properties of WO3 Xiang Dong a,⇑, Yingjie Gan b, Yu Wang a, Shujie Peng b, Liang Dong a a b
School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China School of Material Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China
a r t i c l e
i n f o
Article history: Received 30 May 2013 Received in revised form 6 July 2013 Accepted 8 July 2013 Available online 17 July 2013 Keywords: WO3 ceramics Sintering Electrical properties Microstructure La2O3
a b s t r a c t The La2O3-added WO3 was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 323 K up to 1023 K. The results revealed that doping WO3 with La2O3 could promote the grain size and the density. The addition of a small amount of La2O3 (x 6 0.5 mol%) to WO3 led to an marked increase in the electrical conductivity, resulting in a significant increase in the power factor. In addition, there was a second phase (La2WO6) segregation at the grain boundaries in the samples containing more than 5.0 mol% La2O3, which inhibited the further grain growth and reduced the electrical conductivity (r) and the Seebeck coefficient (|S|). The thermoelectric power factor was maximized to a value of 3.35 lW m1 K2 at 1023 K for 0.5 mol% La2O3-doped WO3 sample. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Development of thermoelectric materials which convert heat into electrical energy is expected to promote the efficient use of energy. The performance of thermoelectric materials is usually evaluated in terms of their thermoelectric figure-of-merit Z, Z = rS2/j, where r, S, and j are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively [1]. The electrical properties are determined by the power factor (P) defined as [1]: P = rS2. To be a good thermoelectric material, it is essential to have a large power factor and a low thermal conductivity. In recent years, there has been great interest in developing novel oxide thermoelectric materials for energy conversion. This is because, unlike conventional materials such as FeSi2 [2], PbTe [3], and Si–Ge alloys [4], they are chemically and thermally stable and thus can be used at high temperatures without deterioration of their performance due to oxidation. Moreover, their production costs are comparatively low. Since Japanese scientists Koumoto et al. [5] found that the porous Y2O3 ceramic exhibited huge Seebeek coefficient values up to 50 mV/K at 900–1000 K in vacuum, great attention has been focused on metal oxides such as ZnO [6–9], NaCo2O4 [10] and SrTiO3 [11–14]. Among these novel thermoelectric oxides, SrTiO3 with a cubic perovskite structure has been considered as a promising thermoelectric candidate for the n-type metal oxide because of its rather large Seebeck coefficient [14]. WO3 is an n-type metal oxide with a perovskite-like structure and is usually labeled as WO3x due to the oxygen deficiency [15]. ⇑ Corresponding author. Tel.: +86 028 87601134; fax: +86 028 87600787. E-mail address: [email protected] (X. Dong). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.052
The electronic structures of WO3 are also similar to SrTiO3 [16], and the applications of WO3 in varistors and gas sensors usually have the same effects as SrTiO3 [17–20]. Therefore, it would be interesting to investigate the thermoelectric properties of WO3. To date, Wang et al. [21–24] have measured the electrical conductivity, the absolute value of Seebeck coefficient (|S|) and the power factor (rS2) of the CeO2, ZnO, TiO and Co2O3-doped WO3 ceramics. In this paper, we synthesized the polycrystalline samples La2O3-doped WO3 and systematically investigated their thermoelectric properties at high temperature up to 1023 K. We found that the thermoelectric performance of the system can be improved remarkably with the La2O3-doping compared with the above mentioned doping. 2. Experimental The raw materials used in this work were analytical grades of WO3 (P99.9%) and La2O3 (P99.9%). Samples with nominal compositions (100x) WO3 + x La2O3, where x = 0.0, 0.5, 1.0, 5.0 and 10.0 mol%, were produced using a conventional mixed oxide processing route. A mixture of the WO3 and La2O3 powders and alcohol was milled for 5 h using an agate mill. The resulting slurries were calcined at a constant heating rate of 100 °C/h up to 800 °C in air. The calcined mixture was pulverized using an agate mortar and after 4 wt% polyvinyl alcohol (PVA) binder addition, the aqueous slurry was then dried using an oven at 80 °C to obtain the granulated powders. The powders were then pressed at 100 MPa to form discs 10 mm in diameter and 1.0–1.2 mm in thickness. The discs with same concentration were sintered at 400 °C in air for 30 min, and then were sintered at 1000 °C in air for 2 h. The heating rate was maintained at 100 °C/h. The surface microstructure of the composite was examined by scanning electron microscopy (JSM-7001F, JEOL with an energy dispersive spectrometer). As for the SEM and EDX analysis the sample was cleaned with acetone, mounted, and gold-coated to prevent charging. The crystalline phases of the sintered sample were identified by X-ray diffraction technologies (7602EAALMELO) under the
X. Dong et al. / Journal of Alloys and Compounds 581 (2013) 52–55
53
structure for WO3 ceramics doped with 5.0 mol% La2O3 indicates that the La-rich phase exists between WO3 grains. The SEM images of sintered WO3 ceramics with different amount of La2O3 are shown in Fig. 3. It is apparent that the addition of La2O3 to WO3 gives rise to an increase in the grain size, indicating that the added La2O3 accelerate the grain growth during sintering. Fine La2WO6 particles of 1.28 lm in size are frequently dispersed on the grain boundaries. The result strongly suggests that the added La2O3 did not fully dissolve in the WO3 crystal lattice owing to its rather low solubility. The formation of La2WO6 is expressed as follows:
La2 O3 þ WO3 ! La2 WO6
Fig. 1. XRD patterns of the (100x) mol% WO3 + x mol% La2O3 samples.
following experimental conditions: k (Cu Ka) = 0.15406 nm, 40 kV, 40 mA, 20° 6 2h 6 80°. Grain sizes of the La2O3-doped WO3 were determined by the linear intercept method, and the porosity of the specimens has been measured by the Archimedes principle. The electrical conductivity from 323 K to 1023 K was measured by means of a standard four-probe method using the KEITHLEY 2400. The thermoelectric power from 323 K to 1023 K was calculated from the thermoelectric voltage collected with KEITHLEY 2400 and the temperature difference between the two ends of the sample.
The grain size and the porosity of the samples are given in Table 1. The grain size and the porosity lie in the range 3.29–1.53 lm and 19.28%–10.89%, respectively. From the Table 1 we can see that the grain size decreased with the increasing La2O3 content. This is indeed attributed to the pinning effect cause by the La2WO6 particles on grain boundaries as well as to the dragging effect between the added La2O3 and grain boundaries, resulting in a reduction in the mobility of the grain boundaries [25]. In addition, the incorporation of La2O3 to WO3 leads to a decrease in the porosity. This is because the added La2O3 acts to lower the grain-boundary mobility, thus enabling the pores to stay attached to the moving grain boundaries during sintering. A similar behavior was reported for TiO2-doped ZnO thick films [26]. The electrical conductivity (r) of the La2O3-doped WO3 samples as a function of temperature is shown in Fig. 4, together with that of the La2O3-free WO3 sample. It is clear that the electrical increased with increasing temperature, indicating semiconducting behavior. On the other hand, it can be observed that the electrical conductivity of the specimens rises with the incorporation of a small amount of La2O3 (x 6 0.5 mol%), whereas the conductivity for higher La2O3 (1.0 mol% 6 x 6 10.0 mol%) is gradually reduced by further additions of La2O3 content. This can be explained by considering the following competing factors influencing the electrical conductivity as follows: 0 La2 O3 () 2La000 i þ 6e þ 3=2O2
3. Results and discussion Fig. 1 illustrates the XRD patterns of the samples with various amount of La2O3. We can see that the major phase of the samples is a WO3 phase, and there is an obvious second phase in the doped samples when the content of La2O3 exceeds 5.0 mol%. The second phase is identified as La2WO6 through the XRD analysis. The amount of the La2WO6 phase became greater with higher La2O3 content. As shown in Fig. 2, the EDX spectra of the rod-shaped
ð1Þ
ð2Þ
Reaction (2) represents the occupation of La in an interstitial site of WO3 and creates six electrons to maintain the electrical neutrality. Thus, the electrical conductivity would be increased with La2O3 addition if reaction (2) occurs. The higher the La2O3 content, the higher the second phase La2WO6 created. Because the La2WO6 segregates to the grain boundaries, blocks the migration of the carriers, and thus reduces the electrical conductivity with adding more La2O3. From the above competing effects, it is considered that an increase in the electrical conductivity by means of the La2O3
Fig. 2. EDX spectra of grain boundary regions for WO3 ceramics doped with 5.0 mol% La2O3 content.
54
X. Dong et al. / Journal of Alloys and Compounds 581 (2013) 52–55
Fig. 3. SEM images of the samples (100x) mol% WO3 + x mol% La2O3: (a) x = 0.0; (b) x = 0.5; (c) x = 1.0; (d) x = 5.0; and (e) x = 10.0 (scale bar: 1.0 lm).
-160
Grain size (lm)
Porosity (%)
0.0 0.5 1.0 5.0 10.0
1.53 3.29 3.18 1.92 1.64
19.28 13.55 12.57 12.15 10.89
Electrical conductivity (Ω Ω-1 cm-1 )
La2O3 (mol%)
1.8
La2O3-0.0%
1.6
La2O3-1.0%
La2O3-0.5%
La2O3-0.0%
-150
La2O3-0.5% La2O3-1.0%
-140
La2O3-5.0% SLa2O3-10.0%
-130 -120 -110 -100 -90 -80 -70
La2O3-5.0%
1.4
Seebeck coefficient (µVK-1 )
Table 1 Grain size and porosity of the samples with different La2O3 contents.
300
La2O3-10.0%
400
500
600
700
800
900
1000 1100
Temperature (K)
1.2 1.0
Fig. 5. Temperature dependence of Seebeck coefficient S of the samples with different amount of La2O3.
0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
800
900
1000 1100
Temperature (K) Fig. 4. Temperature dependence of electrical conductivity r of the samples with different amount of La2O3.
addition is mainly due to an increase in the electron concentration of the system, originating from the extra electrons generated. It is therefore believed that the La2O3 addition is fairly effective in achieving high conductivity. Fig. 5 presents the temperature dependence of the seebeck coefficient for all samples. The sign of the Seebeck coefficient is negative over the whole temperature range for all the samples, indicating an n-type conduction. The absolute values of the Seebeck coefficient for the undoped WO3 ranged from 104 lV K1 to 153 lV K1. It is also important to note that the observed temperature dependence of both the electrical conductivity and the
Seebeck coefficient is unusual. In general, with increasing electrical conductivity, the Seebeck coefficient decreases [1,8]. These results strongly suggest that the conduction mechanism in the La2O3doped WO3 samples cannot be explained by a conventional model based on band theory, but rather by a strong electron–electron correlation effect [27–29]. Moreover, the size of grains also influents the Seebeck coefficient. Gao et al. [30] reported that the Seebeck coefficient increased with the size decreasing. From Table 1 we can see that the grain size of undoped WO3 is smaller than that of doped ones, therefore the Seebeck coefficient of pure WO3 is larger than that of doped ones. The thermoelectric power factor rS2 is calculated by using the electrical conductivity r and the Seebeck coefficient S. The power factor calculated from the data in Figs. 4 and 5 is plotted in Fig. 6. The power factor of the samples increases with an increase in temperature. The magnitude of the power factor depends strongly on La2O3 content and followed the order x = 0.5 > 1.0 > 5.0 > 0.0 > 10.0 mol% at 1023 K. Moreover, the value of the power factor of the La2O3-doped WO3 samples is still increasing toward higher temperatures, implying a high performance and stability at high temperatures. The sample containing 0.5 mol%
X. Dong et al. / Journal of Alloys and Compounds 581 (2013) 52–55
Power factor (μWm-1 K-2 )
3.5
coefficient. The magnitude of the power factor depends strongly on La2O3 content following the order x = 0.5 > 1.0 > 5.0 > 0.0 > 10.0 mol%. The 0.5 mol% sample has the maximum value of the power factor which is 3.35 lW m1K2 at 1023 K. The results suggest that La2O3 addition is more effective for enhancing thermoelectric properties of WO3-based ceramics than CeO2, ZnO, TiO and Co2O3 addition.
La2O3-0.0 % La2O3-0.5%
3.0
La2O3-1.0% La2O3-5.0%
2.5
La2O3-10.0%
2.0
55
1.5
Acknowledgment
1.0
This work was supported by the Fundamental Research Funds for the Central Universities No. 2682013CX014.
0.5 0.0 300
400
500
600
700
800
900
1000 1100
Temperature (K) 2
Fig. 6. Temperature dependence of power factor rS of the samples with different amount of La2O3.
La2O3 has the largest power factor with a value of 3.35 lW m1K2 at 1023 K. Compared with the CeO2, ZnO, TiO and Co2O3-doped WO3 samples [21–24], La2O3 addition has much larger power factor which provides a great improvement for the thermoelectric properties of WO3-based ceramics. 4. Conclusions The thermoelectric properties of WO3 ceramics with the addition of La2O3 have been investigated. The results demonstrate that doping La2O3 can promote the grain growth and inhibit the porosity. The added La2O3 do not fully dissolve in the WO3 crystal lattice because of its rather low solubility, when the amount of La2O3 exceeds the solubility in WO3, the La2WO6 forms. The grain size decreased with the increasing La2O3 content, the main mechanism for inhibiting grain growth was the pinning of grain boundaries due to the presence of the La2WO6 particles. The incorporation of a small amount of La2O3 (x 6 0.5 mol%) led to a marked increase in the electrical conductivity. The increase in electrical conductivity was mainly caused by an enhanced electron concentration. On the other hand, for high La2O3 content (1.0 mol% 6 x 6 10.0 mol%), the addition of La2O3 lowered the electrical conductivity and the absolute value of the Seebeck
References [1] C.M. Bhandari, D.M. Rowe, in: D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, 1995, pp. 43–46. [2] T. Tokiai, T. Uesugi, J. Am. Ceram. Soc. 78 (1995) 1089. [3] M. Ito, W.S. Seo, K. Koumoto, J. Mater. Res. 14 (1999) 209. [4] C.B. Vining, W. Laskow, J.O. Hanson, R.R. Van der Beck, P.D. Gorsuck, J. Appl. Phys. 69 (1991) 4333. [5] K. Koumoto, W.S. Seo, S. Ozawa, Appl. Phys. Lett. 71 (1997) 1475. [6] D. Bérardan, C. Byl, N. Dragoe, J. Am. Ceram. Soc. 93 (2010) 2352. [7] S. Teehan, H. Efstathiadis, P. Haldar, J. Alloys Comp. 509 (2011) 1094. [8] K. Park, J.K. Seong, S. Nahm, J. Alloys Comp. 455 (2008) 331. [9] K. Park, J.K. Seong, G.H. Kim, J. Alloys Comp. 473 (2009) 423. [10] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [11] M. Ito, T. Matsuda, J. Alloys Comp. 477 (2009) 473. [12] N. Wang, L. Han, H. He, Y. Ba, K. Koumoto, J. Alloys Comp. 497 (2010) 308. [13] N. Wang, H. He, X. Li, L. Han, C. Zhang, J. Alloys Comp. 506 (2010) 293. [14] H.C. Wang, C.L. Wang, W.B. Sun, J. Liu, Y. Sun, H. Peng, L.M. Mei, J. Am. Ceram. Soc. 94 (2011) 838. [15] P.S. Patil, P.R. Patil, E.A. Ennaoui, Thin Solid Films 370 (2000) 38. [16] R.D. Bringans, H. Höchst, H.R. Shanks, Phys. Rev. B 24 (1981) 3481. [17] A. Yan, C. Xie, D. Zeng, S. Cai, H. Li, J. Alloys Comp. 495 (2010) 88. [18] Z. Hua, Y. Wang, H. Wang, L. Dong, Sens. Actuators, B 150 (2010) 588. [19] Z. Hua, L. Dong, H. Wang, S. Peng, Y. Wang, Phys. B 406 (2011) 2807. [20] H. Wang, S. Peng, Z. Hua, X. Dong, Y. Wang, L. Dong, Phys. B 406 (2011) 4183. [21] H. Wang, X. Dong, S. Peng, L. Dong, Y. Wang, J. Alloys Comp. 527 (2012) 204. [22] H. Wang, Z. Hua, S. Peng, X. Dong, L. Dong, Y. Wang, Ceram. Int. 38 (2012) 1133. [23] X. Dong, H. Wang, Z. Hua, S. Peng, L. Dong, Y. Wang, J. Mater. Sci.: Mater. Electron. 23 (2012) 1210. [24] H. Wang, Y. Gan, X. Dong, S. Peng, L. Dong, Y. Wang, J. Mater. Sci.: Mater. Electron. 23 (2012) 2229. [25] K. Park, J.K. Seong, J. Alloys Comp. 464 (2008) 1. [26] B.L. Zhu, C.S. Xie, W.Y. Wang, K.J. Huang, J.H. Hu, Mater. Lett. 58 (2004) 624. [27] I. Terasaki, Mater. Trans. 42 (2001) 951. [28] W. Shin, N. Murayama, Mater. Lett. 49 (2001) 262. [29] K. Park, J.H. Lee, Mater. Lett. 62 (2008) 2366. [30] Y.W. Gao, Y.Z. He, L.L. Zhu, Chin. Sci. Bull. 16 (2010) 55.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of La2O3 on high-temperature thermoelectric properties of WO3 Xiang Dong a,⇑, Yingjie Gan b, Yu Wang a, Shujie Peng b, Liang Dong a a b
School of Electrical Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China School of Material Science and Engineering, Southwest Jiaotong University, Chengdu, Sichuan 610031, China
a r t i c l e
i n f o
Article history: Received 30 May 2013 Received in revised form 6 July 2013 Accepted 8 July 2013 Available online 17 July 2013 Keywords: WO3 ceramics Sintering Electrical properties Microstructure La2O3
a b s t r a c t The La2O3-added WO3 was prepared by a conventional mixed oxide processing route and the thermoelectric properties were studied from 323 K up to 1023 K. The results revealed that doping WO3 with La2O3 could promote the grain size and the density. The addition of a small amount of La2O3 (x 6 0.5 mol%) to WO3 led to an marked increase in the electrical conductivity, resulting in a significant increase in the power factor. In addition, there was a second phase (La2WO6) segregation at the grain boundaries in the samples containing more than 5.0 mol% La2O3, which inhibited the further grain growth and reduced the electrical conductivity (r) and the Seebeck coefficient (|S|). The thermoelectric power factor was maximized to a value of 3.35 lW m1 K2 at 1023 K for 0.5 mol% La2O3-doped WO3 sample. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Development of thermoelectric materials which convert heat into electrical energy is expected to promote the efficient use of energy. The performance of thermoelectric materials is usually evaluated in terms of their thermoelectric figure-of-merit Z, Z = rS2/j, where r, S, and j are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively [1]. The electrical properties are determined by the power factor (P) defined as [1]: P = rS2. To be a good thermoelectric material, it is essential to have a large power factor and a low thermal conductivity. In recent years, there has been great interest in developing novel oxide thermoelectric materials for energy conversion. This is because, unlike conventional materials such as FeSi2 [2], PbTe [3], and Si–Ge alloys [4], they are chemically and thermally stable and thus can be used at high temperatures without deterioration of their performance due to oxidation. Moreover, their production costs are comparatively low. Since Japanese scientists Koumoto et al. [5] found that the porous Y2O3 ceramic exhibited huge Seebeek coefficient values up to 50 mV/K at 900–1000 K in vacuum, great attention has been focused on metal oxides such as ZnO [6–9], NaCo2O4 [10] and SrTiO3 [11–14]. Among these novel thermoelectric oxides, SrTiO3 with a cubic perovskite structure has been considered as a promising thermoelectric candidate for the n-type metal oxide because of its rather large Seebeck coefficient [14]. WO3 is an n-type metal oxide with a perovskite-like structure and is usually labeled as WO3x due to the oxygen deficiency [15]. ⇑ Corresponding author. Tel.: +86 028 87601134; fax: +86 028 87600787. E-mail address: [email protected] (X. Dong). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.052
The electronic structures of WO3 are also similar to SrTiO3 [16], and the applications of WO3 in varistors and gas sensors usually have the same effects as SrTiO3 [17–20]. Therefore, it would be interesting to investigate the thermoelectric properties of WO3. To date, Wang et al. [21–24] have measured the electrical conductivity, the absolute value of Seebeck coefficient (|S|) and the power factor (rS2) of the CeO2, ZnO, TiO and Co2O3-doped WO3 ceramics. In this paper, we synthesized the polycrystalline samples La2O3-doped WO3 and systematically investigated their thermoelectric properties at high temperature up to 1023 K. We found that the thermoelectric performance of the system can be improved remarkably with the La2O3-doping compared with the above mentioned doping. 2. Experimental The raw materials used in this work were analytical grades of WO3 (P99.9%) and La2O3 (P99.9%). Samples with nominal compositions (100x) WO3 + x La2O3, where x = 0.0, 0.5, 1.0, 5.0 and 10.0 mol%, were produced using a conventional mixed oxide processing route. A mixture of the WO3 and La2O3 powders and alcohol was milled for 5 h using an agate mill. The resulting slurries were calcined at a constant heating rate of 100 °C/h up to 800 °C in air. The calcined mixture was pulverized using an agate mortar and after 4 wt% polyvinyl alcohol (PVA) binder addition, the aqueous slurry was then dried using an oven at 80 °C to obtain the granulated powders. The powders were then pressed at 100 MPa to form discs 10 mm in diameter and 1.0–1.2 mm in thickness. The discs with same concentration were sintered at 400 °C in air for 30 min, and then were sintered at 1000 °C in air for 2 h. The heating rate was maintained at 100 °C/h. The surface microstructure of the composite was examined by scanning electron microscopy (JSM-7001F, JEOL with an energy dispersive spectrometer). As for the SEM and EDX analysis the sample was cleaned with acetone, mounted, and gold-coated to prevent charging. The crystalline phases of the sintered sample were identified by X-ray diffraction technologies (7602EAALMELO) under the
X. Dong et al. / Journal of Alloys and Compounds 581 (2013) 52–55
53
structure for WO3 ceramics doped with 5.0 mol% La2O3 indicates that the La-rich phase exists between WO3 grains. The SEM images of sintered WO3 ceramics with different amount of La2O3 are shown in Fig. 3. It is apparent that the addition of La2O3 to WO3 gives rise to an increase in the grain size, indicating that the added La2O3 accelerate the grain growth during sintering. Fine La2WO6 particles of 1.28 lm in size are frequently dispersed on the grain boundaries. The result strongly suggests that the added La2O3 did not fully dissolve in the WO3 crystal lattice owing to its rather low solubility. The formation of La2WO6 is expressed as follows:
La2 O3 þ WO3 ! La2 WO6
Fig. 1. XRD patterns of the (100x) mol% WO3 + x mol% La2O3 samples.
following experimental conditions: k (Cu Ka) = 0.15406 nm, 40 kV, 40 mA, 20° 6 2h 6 80°. Grain sizes of the La2O3-doped WO3 were determined by the linear intercept method, and the porosity of the specimens has been measured by the Archimedes principle. The electrical conductivity from 323 K to 1023 K was measured by means of a standard four-probe method using the KEITHLEY 2400. The thermoelectric power from 323 K to 1023 K was calculated from the thermoelectric voltage collected with KEITHLEY 2400 and the temperature difference between the two ends of the sample.
The grain size and the porosity of the samples are given in Table 1. The grain size and the porosity lie in the range 3.29–1.53 lm and 19.28%–10.89%, respectively. From the Table 1 we can see that the grain size decreased with the increasing La2O3 content. This is indeed attributed to the pinning effect cause by the La2WO6 particles on grain boundaries as well as to the dragging effect between the added La2O3 and grain boundaries, resulting in a reduction in the mobility of the grain boundaries [25]. In addition, the incorporation of La2O3 to WO3 leads to a decrease in the porosity. This is because the added La2O3 acts to lower the grain-boundary mobility, thus enabling the pores to stay attached to the moving grain boundaries during sintering. A similar behavior was reported for TiO2-doped ZnO thick films [26]. The electrical conductivity (r) of the La2O3-doped WO3 samples as a function of temperature is shown in Fig. 4, together with that of the La2O3-free WO3 sample. It is clear that the electrical increased with increasing temperature, indicating semiconducting behavior. On the other hand, it can be observed that the electrical conductivity of the specimens rises with the incorporation of a small amount of La2O3 (x 6 0.5 mol%), whereas the conductivity for higher La2O3 (1.0 mol% 6 x 6 10.0 mol%) is gradually reduced by further additions of La2O3 content. This can be explained by considering the following competing factors influencing the electrical conductivity as follows: 0 La2 O3 () 2La000 i þ 6e þ 3=2O2
3. Results and discussion Fig. 1 illustrates the XRD patterns of the samples with various amount of La2O3. We can see that the major phase of the samples is a WO3 phase, and there is an obvious second phase in the doped samples when the content of La2O3 exceeds 5.0 mol%. The second phase is identified as La2WO6 through the XRD analysis. The amount of the La2WO6 phase became greater with higher La2O3 content. As shown in Fig. 2, the EDX spectra of the rod-shaped
ð1Þ
ð2Þ
Reaction (2) represents the occupation of La in an interstitial site of WO3 and creates six electrons to maintain the electrical neutrality. Thus, the electrical conductivity would be increased with La2O3 addition if reaction (2) occurs. The higher the La2O3 content, the higher the second phase La2WO6 created. Because the La2WO6 segregates to the grain boundaries, blocks the migration of the carriers, and thus reduces the electrical conductivity with adding more La2O3. From the above competing effects, it is considered that an increase in the electrical conductivity by means of the La2O3
Fig. 2. EDX spectra of grain boundary regions for WO3 ceramics doped with 5.0 mol% La2O3 content.
54
X. Dong et al. / Journal of Alloys and Compounds 581 (2013) 52–55
Fig. 3. SEM images of the samples (100x) mol% WO3 + x mol% La2O3: (a) x = 0.0; (b) x = 0.5; (c) x = 1.0; (d) x = 5.0; and (e) x = 10.0 (scale bar: 1.0 lm).
-160
Grain size (lm)
Porosity (%)
0.0 0.5 1.0 5.0 10.0
1.53 3.29 3.18 1.92 1.64
19.28 13.55 12.57 12.15 10.89
Electrical conductivity (Ω Ω-1 cm-1 )
La2O3 (mol%)
1.8
La2O3-0.0%
1.6
La2O3-1.0%
La2O3-0.5%
La2O3-0.0%
-150
La2O3-0.5% La2O3-1.0%
-140
La2O3-5.0% SLa2O3-10.0%
-130 -120 -110 -100 -90 -80 -70
La2O3-5.0%
1.4
Seebeck coefficient (µVK-1 )
Table 1 Grain size and porosity of the samples with different La2O3 contents.
300
La2O3-10.0%
400
500
600
700
800
900
1000 1100
Temperature (K)
1.2 1.0
Fig. 5. Temperature dependence of Seebeck coefficient S of the samples with different amount of La2O3.
0.8 0.6 0.4 0.2 0.0 300
400
500
600
700
800
900
1000 1100
Temperature (K) Fig. 4. Temperature dependence of electrical conductivity r of the samples with different amount of La2O3.
addition is mainly due to an increase in the electron concentration of the system, originating from the extra electrons generated. It is therefore believed that the La2O3 addition is fairly effective in achieving high conductivity. Fig. 5 presents the temperature dependence of the seebeck coefficient for all samples. The sign of the Seebeck coefficient is negative over the whole temperature range for all the samples, indicating an n-type conduction. The absolute values of the Seebeck coefficient for the undoped WO3 ranged from 104 lV K1 to 153 lV K1. It is also important to note that the observed temperature dependence of both the electrical conductivity and the
Seebeck coefficient is unusual. In general, with increasing electrical conductivity, the Seebeck coefficient decreases [1,8]. These results strongly suggest that the conduction mechanism in the La2O3doped WO3 samples cannot be explained by a conventional model based on band theory, but rather by a strong electron–electron correlation effect [27–29]. Moreover, the size of grains also influents the Seebeck coefficient. Gao et al. [30] reported that the Seebeck coefficient increased with the size decreasing. From Table 1 we can see that the grain size of undoped WO3 is smaller than that of doped ones, therefore the Seebeck coefficient of pure WO3 is larger than that of doped ones. The thermoelectric power factor rS2 is calculated by using the electrical conductivity r and the Seebeck coefficient S. The power factor calculated from the data in Figs. 4 and 5 is plotted in Fig. 6. The power factor of the samples increases with an increase in temperature. The magnitude of the power factor depends strongly on La2O3 content and followed the order x = 0.5 > 1.0 > 5.0 > 0.0 > 10.0 mol% at 1023 K. Moreover, the value of the power factor of the La2O3-doped WO3 samples is still increasing toward higher temperatures, implying a high performance and stability at high temperatures. The sample containing 0.5 mol%
X. Dong et al. / Journal of Alloys and Compounds 581 (2013) 52–55
Power factor (μWm-1 K-2 )
3.5
coefficient. The magnitude of the power factor depends strongly on La2O3 content following the order x = 0.5 > 1.0 > 5.0 > 0.0 > 10.0 mol%. The 0.5 mol% sample has the maximum value of the power factor which is 3.35 lW m1K2 at 1023 K. The results suggest that La2O3 addition is more effective for enhancing thermoelectric properties of WO3-based ceramics than CeO2, ZnO, TiO and Co2O3 addition.
La2O3-0.0 % La2O3-0.5%
3.0
La2O3-1.0% La2O3-5.0%
2.5
La2O3-10.0%
2.0
55
1.5
Acknowledgment
1.0
This work was supported by the Fundamental Research Funds for the Central Universities No. 2682013CX014.
0.5 0.0 300
400
500
600
700
800
900
1000 1100
Temperature (K) 2
Fig. 6. Temperature dependence of power factor rS of the samples with different amount of La2O3.
La2O3 has the largest power factor with a value of 3.35 lW m1K2 at 1023 K. Compared with the CeO2, ZnO, TiO and Co2O3-doped WO3 samples [21–24], La2O3 addition has much larger power factor which provides a great improvement for the thermoelectric properties of WO3-based ceramics. 4. Conclusions The thermoelectric properties of WO3 ceramics with the addition of La2O3 have been investigated. The results demonstrate that doping La2O3 can promote the grain growth and inhibit the porosity. The added La2O3 do not fully dissolve in the WO3 crystal lattice because of its rather low solubility, when the amount of La2O3 exceeds the solubility in WO3, the La2WO6 forms. The grain size decreased with the increasing La2O3 content, the main mechanism for inhibiting grain growth was the pinning of grain boundaries due to the presence of the La2WO6 particles. The incorporation of a small amount of La2O3 (x 6 0.5 mol%) led to a marked increase in the electrical conductivity. The increase in electrical conductivity was mainly caused by an enhanced electron concentration. On the other hand, for high La2O3 content (1.0 mol% 6 x 6 10.0 mol%), the addition of La2O3 lowered the electrical conductivity and the absolute value of the Seebeck
References [1] C.M. Bhandari, D.M. Rowe, in: D.M. Rowe (Ed.), CRC Handbook of Thermoelectrics, CRC Press, Boca Raton, 1995, pp. 43–46. [2] T. Tokiai, T. Uesugi, J. Am. Ceram. Soc. 78 (1995) 1089. [3] M. Ito, W.S. Seo, K. Koumoto, J. Mater. Res. 14 (1999) 209. [4] C.B. Vining, W. Laskow, J.O. Hanson, R.R. Van der Beck, P.D. Gorsuck, J. Appl. Phys. 69 (1991) 4333. [5] K. Koumoto, W.S. Seo, S. Ozawa, Appl. Phys. Lett. 71 (1997) 1475. [6] D. Bérardan, C. Byl, N. Dragoe, J. Am. Ceram. Soc. 93 (2010) 2352. [7] S. Teehan, H. Efstathiadis, P. Haldar, J. Alloys Comp. 509 (2011) 1094. [8] K. Park, J.K. Seong, S. Nahm, J. Alloys Comp. 455 (2008) 331. [9] K. Park, J.K. Seong, G.H. Kim, J. Alloys Comp. 473 (2009) 423. [10] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685. [11] M. Ito, T. Matsuda, J. Alloys Comp. 477 (2009) 473. [12] N. Wang, L. Han, H. He, Y. Ba, K. Koumoto, J. Alloys Comp. 497 (2010) 308. [13] N. Wang, H. He, X. Li, L. Han, C. Zhang, J. Alloys Comp. 506 (2010) 293. [14] H.C. Wang, C.L. Wang, W.B. Sun, J. Liu, Y. Sun, H. Peng, L.M. Mei, J. Am. Ceram. Soc. 94 (2011) 838. [15] P.S. Patil, P.R. Patil, E.A. Ennaoui, Thin Solid Films 370 (2000) 38. [16] R.D. Bringans, H. Höchst, H.R. Shanks, Phys. Rev. B 24 (1981) 3481. [17] A. Yan, C. Xie, D. Zeng, S. Cai, H. Li, J. Alloys Comp. 495 (2010) 88. [18] Z. Hua, Y. Wang, H. Wang, L. Dong, Sens. Actuators, B 150 (2010) 588. [19] Z. Hua, L. Dong, H. Wang, S. Peng, Y. Wang, Phys. B 406 (2011) 2807. [20] H. Wang, S. Peng, Z. Hua, X. Dong, Y. Wang, L. Dong, Phys. B 406 (2011) 4183. [21] H. Wang, X. Dong, S. Peng, L. Dong, Y. Wang, J. Alloys Comp. 527 (2012) 204. [22] H. Wang, Z. Hua, S. Peng, X. Dong, L. Dong, Y. Wang, Ceram. Int. 38 (2012) 1133. [23] X. Dong, H. Wang, Z. Hua, S. Peng, L. Dong, Y. Wang, J. Mater. Sci.: Mater. Electron. 23 (2012) 1210. [24] H. Wang, Y. Gan, X. Dong, S. Peng, L. Dong, Y. Wang, J. Mater. Sci.: Mater. Electron. 23 (2012) 2229. [25] K. Park, J.K. Seong, J. Alloys Comp. 464 (2008) 1. [26] B.L. Zhu, C.S. Xie, W.Y. Wang, K.J. Huang, J.H. Hu, Mater. Lett. 58 (2004) 624. [27] I. Terasaki, Mater. Trans. 42 (2001) 951. [28] W. Shin, N. Murayama, Mater. Lett. 49 (2001) 262. [29] K. Park, J.H. Lee, Mater. Lett. 62 (2008) 2366. [30] Y.W. Gao, Y.Z. He, L.L. Zhu, Chin. Sci. Bull. 16 (2010) 55.