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Excellent performance stability of Ba and In double-filled skutterudite thermoelectric materials
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Acta Materialia 59 (2011) 3244–3254 www.elsevier.com/locate/actamat
Excellent performance stability of Ba and In double-filled skutterudite thermoelectric materials Ping Wei, Wen-Yu Zhao ⇑, Chun-Lei Dong, Xuan Yang, Jian Yu, Qing-Jie Zhang ⇑ State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 19 November 2010; received in revised form 24 January 2011; accepted 29 January 2011
Abstract The microstructure and thermoelectric properties of Ba and In double-filled Ba0.3In0.2Co3.95Ni0.05Sb12 materials with a figure-of-merit (ZT) of 1.2 at 800 K were carefully investigated by periodic quenching from 723 K to room temperature. The quenching treatment caused enrichment of Ba and loss of Sb and Co on the grain boundaries but had no effect on In and Ni. The enhancement in |a| in the starting period is due to the reduction in r induced by the secondary precipitates. The reductions in r, j and jE are attributed to the secondary precipitates. The increase in jL is assumed to be due to the separation of Ba filler from the Sb-icosahedron voids. A promising ZT value of 1.0 remained after quenching 2000 times. It is concluded that the double-filled skutterudite materials have excellent performance stability under periodically fluctuating environmental temperature. Crown Copyright Ó 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. Keywords: Semiconductor compounds; Microstructure; Thermal conductivity; Electrical properties
1. Introduction Thermoelectricity is becoming increasingly important in the fields of cooling, heating, generating power and recovering waste heat [1]. The good performance of thermoelectric (TE) devices depends directly on the material TE figure-of-merit, ZT = a2rN/j. In this equation, T is the absolute temperature, r is the electrical conductivity, a is the Seebeck coefficient, and j is the thermal conductivity (j = jT + jL, where jT is the electronic contribution and jL is the lattice contribution). A good TE material should be the perfect combination of a high power factor (a2r) and low thermal conductivity [2]. At the same time, a good TE device requires that the TE materials have excellent performance stability in practical applications. However, the performance evolution of TE materials under service conditions has been little studied. ⇑ Corresponding authors.
E-mail addresses: [email protected] (W.-Y. Zhao), [email protected] (Q.-J. Zhang).
The skutterudite family has drawn much attention in the last decade because of its very promising properties for intermediate-temperature TE power generation, primarily from solar energy and industrial waste heat. Extensive reviews of the physical properties of the skutterudites have been published by Nolas et al. [3], Uher [4] and Sales [5]. Cubic binary skutterudite CoSb3 possesses two interstitial voids in a crystallographic unit cell. Filling the oversized voids with foreign atoms such as rare-earth elements [6– 17], alkaline earth elements [18–21] and others [22–28] has been confirmed to be an effective way to reduce jL. Double-atom filling is considered to be a more effective way to further optimize the TE performance of filled skutterudites. Experimental data have confirmed that the ZT values of double-filled skutterudite compounds such as CeLa [29], CaCe [30], BaYb [31,32] and InYb [33,34] are larger than those of single-filled ones. It is the double-atom filling in CoSb3 that remarkably reduces the jL and greatly enhances the TE performance, making double-filled skutterudites one of the most promising intermediate-temperature TE materials.
1359-6454/$36.00 Crown Copyright Ó 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. doi:10.1016/j.actamat.2011.01.064
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We recently investigated the TE properties of Ba and In double-filled skutterudites BaxInyCo4Sb12 z synthesized with nominal compositions Ba0.3 kInkCo4Sb12 (0 6 k 6 0.3, Dk = 0.05) and Ba0.3InmCo4Sb12 (0 6 m 6 0.3, Dm = 0.05) in the temperature range of 300–850 K and found that filling In in Ba-filled skutterudite not only reduced the thermal conductivity remarkably but also enhanced the a2r for Ba and In double-filled skutterudite materials with higher ZTs up to 1.34 at 850 K significantly [35,36]. We attributed the excellent TE properties to the orbital hybridizations induced by the In filler. While important progress has been made in enhancing the ZT value of double-filled skutterudite materials, there have been no reports about the thermal stability of the microstructure and TE properties of any double-filled skutterudite materials under service conditions. More fundamental research is necessary and important for the practical applications of double-filled skutterudite TE materials. Ba and In double-filled skutterudite-based TE devices with high efficiency are now being considered for use in a new solar thermoelectric–photovoltaic hybrid power generation system that was developed by the cooperation between Zhang et al. from China and Niino et al. from Japan in order to realize solar full-wavelength energy conversion into electricity through both photovoltaics (200– 800 nm) and thermoelectrics (800–3000 nm) [37]. The advantages of the hybrid system in the solar energy conversion have been reported elsewhere [38,39]. The operating temperature in the hybrid system is fluctuates periodically between room temperature and above 673 K due to the alternation of day and night. For the aim of this study, it is important to understand the degradation in the performance of the Ba and In double-filled skutterudite bulk materials caused by such cyclic thermal loading. In this study, a periodic quenching experiment from 723 K to room temperature to 723 K was designed to simulate the service conditions under which the Ba and In double-filled skutterudite bulk materials would be applied to the solar thermoelectric–photovoltaic hybrid power generation system. The evolutions of the microstructure and the TE properties of single-phase Ba and In double-filled Ba0.3In0.2 Co3.95Ni0.05Sb12 bulk material were carefully investigated during periodic quenching treatment to determine its technical limitations. In order to investigate the relaxing influence of the microstructures induced by the periodical quenching, the TE properties of a Ba0.3In0.2Co4Sb12 bulk sample being periodically quenched different numbers were measured during the temperature cycle from 300 K up to 800 K and back to 300 K.
sintering method. The experimental details for preparing the double-filled skutterudite bulk materials have been demonstrated elsewhere [35,36]. The starting materials, with nominal compositions Ba0.3In0.2Co3.95Ni0.05Sb12 and Ba0.3In0.2Co4Sb12, are mixtures of highly pure metals of Ba (99.9%, plate), In (99.99%, powder), Sb (99.999%, powder), Ni (99.9%, powder) and Co (99.9%, powder). Asprepared samples 3 4 10 mm3, Ø10 1.5 mm3 and Ø3 1.5 mm3 in size were cut from the sintered bulk materials, which were used to investigate the evolutions of charge transport properties, thermal properties and microstructure of the double-filled skutterudite bulk materials, respectively. Phase purity, chemical compositions and microstructures of the as-prepared samples were first characterized using various methods. These as-prepared samples were loaded into a silica tube and the tube was sealed under vacuum. A periodic quenching experiment was designed to simulate a service condition under which the double-filled skutterudite bulk materials would have to operate in a solar thermoelectric–photovoltaic hybrid power generation system with periodically fluctuating operating temperature. The sealed tube was periodically quenched in water after being held for 5 min at 723 K in a constant temperature furnace. The charge transport properties, thermal properties and microstructure of the quenched samples were analyzed again after they had been quenched 200 times. These quenched samples were sealed into a new tube under vacuum and the aforementioned process was repeated a further 1800 times (i.e. 2000 times in total).
2. Experiment
r and a were measured with the standard four-probe method (Sinkuriko: ZEM-1) in an Ar atmosphere. j was calculated using the equation j = DqCp, where D is the thermal diffusivity coefficient, q is the bulk density of the material and Cp is the specific heat capacity. D was measured by a laser flash technique (Netzsch LFA 427) in a
2.1. Periodically quenching experiment Ba and In double-filled skutterudite bulk materials were prepared by a melting–quenching–annealing–spark plasma
2.2. X-ray diffraction and electron microscopy analysis The constituent phases of as-prepared and quenched samples were determined by powder X-ray diffraction (XRD) using Cu Ka radiation (PANalytical X’ Pert PRO). The chemical compositions were analyzed using electron probe microanalysis (JXA-8100, JEOL). The operation conditions of the accelerating voltage and specimen current were kept at 20 kV and 20 nA, respectively. The backscattered electron images (BEI) and second electron images (SEI) were observed using a JSM-5610LV scanning electron microscope to study the evolution of the microstructures of the as-prepared samples after periodic quenching. The transmission electron microscopy (TEM) images were observed using a JEM-2010(HT) transmission electron microscope to investigate the interface texture between grain boundaries. 2.3. Thermoelectric transport property
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flowing Ar atmosphere. Cp was measured with a differential scanning calorimeter. q was obtained by the Archimedes method. jL was obtained by subtracting the electrical contribution from j using the equation jL = j jT. Here, jT is expressed by the Wiedemann Franz jT = rLT, where the Lorenz number L has a numerical value of 2.0 10 8 V2 K 2, as estimated by Dyck et al. [19]. To decrease the uncertainties of these parameters, all the parameters, r, a, D, q and Cp, were measured under the same conditions, including the same sample size, the same starting and ending temperatures, and the same working current and voltage. The hypothesis that the measuring errors for all the parameters are invariable each time was employed to investigate the effect of the number of quenching on the thermoelectric transport properties. 3. Results 3.1. Structural and compositional evolutions XRD patterns of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched one after being periodically quenched 2000 times are shown in Fig. 1. It can be seen that the as-prepared sample is composed of a single-phase compound with a skutterudite structure. This result indicates that the single-phase Ba and In doublefilled skutterudite materials are fabricated by the aforementioned process. Ba and In atoms were confirmed to fill in the Sb-icosahedron voids by Rietveld refinement from high-resolution synchrotron data for InxCo4Sb12 [26] and XRD data for BaxInyCo4Sb12 z [35], and the quantitative analysis of experimental data from X-ray photoelectron spectroscopy for BarInsCo4Sb12 [36]. However, the phase structure of the as-prepared sample was changed after periodic quenching treatment. Some weak peaks were observed in the XRD pattern of the quenched sample, which are
Fig. 1. XRD patterns of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and quenched samples after being periodically quenched 2000 times.
attributed to the characteristic diffraction peaks of Sb and Ba5Sb3, respectively. The enrichment of Sb metal on the surface layer of the quenched sample is ascribed to the deposition and the recrystallization of sublimation of antimony. Ba5Sb3 was distributed in the transition zone between the surface layer and the interior (shown in the inset of Fig. 1), which might be formed by the reaction between the sublimations of Sb and Ba. Incidentally, no evident peak shift for the characteristic diffraction peaks of skutterudite is observable in the XRD patterns between the as-prepared sample and the quenched one, implying that no serious strain exists in the quenched samples. Fig. 2 shows SEI and BEI of the same region on the surface of an as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 200, 400, 600 and 800 times. Clear grain boundaries were observed at the beginning of quenching, for example, for the sample quenched 200 times. The amount of secondary precipitates on the grain boundaries increased with increasing number of quenching in the range 200–600, and were recrystallized after quenching 800 times. The dark contrast distributed on the grain boundaries in the BEI indicates that the secondary precipitates were rich in elements with a smaller atomic number. Energy-dispersive spectroscopy of the interface and the grain in the quenched sample indicates that the secondary precipitates on the grain boundaries were mainly composed of Ba and O elements. The enrichments of Ba and O elements on the grain boundaries are attributed to the in situ oxidation reaction of 2Ba + O2 = 2BaO, the O2 originating from the remaining air in the vacuum silica tube. BEI and the maps of Co, Ni, Sb, In and Ba elements from the surface of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 2000 times are shown in Fig. 3. The dashed circles in Fig. 3 indicate where the abundance of Ba element is higher. Note that the periodic quenching treatment has caused significant enrichment of Ba element and the loss of Sb and Co elements along the grain boundaries, compared with those of the as-prepared sample. This indicates that the Ba fillers separated out from the Sb-icosahedron voids because of the periodic quenching treatment. However, the quenching treatment seems to have no effect on the maps of In and Ni elements. Fig. 4a–d shows TEM images of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched one after being periodically quenched 2000 times, respectively. The inset in panel (a) shows the selected area electron diffraction (SAED) patterns of as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample along the [1 1 1] zone axis. The TEM image and SAED patterns of the asprepared bulk sample shown in Fig. 4a indicate that the Ba0.3In0.2Co3.95Ni0.05Sb12 grains are perfect crystals with straight and clean grain boundaries and no inclusion in the as-prepared bulk sample. Note that secondary precipitates with grain sizes of about 10–25 nm, as shown in Fig. 4b, were observed on the grain boundaries in the
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Fig. 2. SEI and BEI of the same region of the surface of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 200, 400, 600 and 800 times.
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Fig. 3. BEI and maps of Co, Ni, Sb, In, and Ba elements from the surface of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 2000 times.
quenched samples. At the same time, dislocation stripes, as shown in Fig. 4c, were demonstrated to occur in the quenched samples, which is evidence that the strain induced by the periodic quenching treatment was largely relaxed. In addition, it seems reasonable that the irregular bend black stripes shown in Fig. 4d could arise from thin foil bending due to mechanical grinding. However, it cannot be excluded that the lattice distortion of Ba0.3In0.2Co3.95Ni0.05Sb12 grains results in the appearance of irregular bend black stripes in the quenched samples, because the phenomenon has been not observed in the thin foils of the as-prepared bulk samples prepared with the same method. Fortunately, the effect of the lattice distor-
tion on the thermoelectric properties of Ba0.3In0.2Co3.95Ni0.05Sb12 bulk material may be ignored according to the results from additional experiments in the Discussion session. 3.2. Charge transport properties The temperature dependence of electrical conductivity for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched ones after being periodically quenched different numbers is shown in Fig. 5a. The inset in panel (a) shows the dependence of electrical conductivity on the number of quenching. The r of the as-prepared sample is
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(a)
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(b)
0.2 µm
0.5 µm
(d)
(c)
0.5 µm
0.5 µm
Fig. 4. TEM images of (a) the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and (b–d) the quenched one after being periodically quenched 2000 times. The inset in panel (a) shows the SAED patterns of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample along the [1 1 1] zone axis.
about 2.56 105 S m 1 at room temperature and then decreases with increasing temperature, indicative of a metallic transport behavior. The reduction in r with increasing temperature mainly originates from the enhancement in the scattering effect of the carriers on the crystal lattice. The r of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 materials over the whole temperature range, 300–800 K, is much higher than that of the BaxInyCo4Sb12-z and BarInsCo4Sb12 compounds previously reported by us [35,36]. This difference may be related to the substitution of Ni for Co. The presence of small amounts of Ni in Ba-filled Ba0.3NixCo4 xSb12 had the beneficial effects of lowering the total thermal conductivity and increasing the electrical conductivity [19]. The r of 1.57 105 S m 1 for the as-prepared sample gradually decreased to 1.37 105 S m 1 for the 1600-times-quenched sample at 800 K and then increased slightly when the number of quenching further increased. The gradual reduction in r with increasing number of quenching may be interpreted in terms of the evolutions of the microstructure and chemical composition in the quenched samples. We mentioned that the secondary precipitates on the grain boundaries might disturb the electronic flow and decrease the electrical conductivity. The r of 2.37 105 S m 1 of the 600-times-quenched sample decreased to 2.25 105 S m 1 for the 800-times-quenched sample at 300 K. This significant reduction is attributed to the effect
of the recrystallization of secondary precipitates on the grain boundaries when the number of quenching exceeds 800 times, as shown in Fig. 2. The temperature dependence of the Seebeck coefficient for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers is shown in Fig. 5b. The inset in panel (b) shows the dependence of the Seebeck coefficient on the number of quenching. The negative Seebeck coefficient values are indicative of n-type electrical transport properties. The |a| significantly increased during the starting period of the quenching treatment, corresponding to the significant reduction in r. For the periodic quenching experiments, the bulk samples were encapsulated and heated in sealed silica tubes. Some free Sb would volatilize and deposit during each quenching cycle. As a result, the content of free Sb deposited on the surface of the quenched sample would increase with increasing number of quenching, as confirmed by the XRD result. On the other hand, Ba element was enriched on the grain boundaries because of the in situ oxidation reaction of 2Ba + O2 = 2BaO, whereas the volatilization of free Sb takes place more readily. Therefore, the reduction of r and the increase of |a| during the starting period of the quenching treatment may be attributed to the enhancement of the boundary potential barrier caused by the secondary precipitates on the grain boundaries. We consider that the evolutions of r and a
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Fig. 5. Temperature dependence of (a) the electrical conductivity and (b) the Seebeck coefficient of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers. The insets show the dependences of the electrical conductivity and the Seebeck coefficient, respectively, on the number of quenching.
could be explained reasonably well by the separation of higher energy electrons from lower energy electrons and the selective scattering of electrons, or so-called electron energy filtering [40]. The r of 2.56 105 S m 1 for the asprepared sample at 300 K decreased remarkably to 2.42 105 S m 1 for the 200-times-quenched sample. Correspondingly, the |a| of 114 lV K 1 increased to 119 lV K 1 at 300 K. The a values were almost invariable with increasing number of quenching, which may be due to the indifferent effect of the microstructures induced by rapid quenching treatment on the a, as presented in Section 4. 3.3. Thermal transport properties and the thermoelectric figure-of-merit The temperature dependence of thermal conductivity for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched ones after being periodically quenched different numbers is shown in Fig. 6a. The inset in panel (a) shows the dependence of thermal conductivity on the number of quenching. The j of the quenched samples was far
lower than that of the as-prepared samples. The dependence of j on the number of quenching is very similar to that of r, implying that the origination of the reduction in j with the number of quenching is the same as that of r. The secondary precipitates may enhance the scattering effects of the carriers and phonons and decrease j. The 1000-times-quenched sample showed the lowest j value of 3.0 W m 1 K 1 at 675 K, which was 10% lower than the 3.3 W m 1 K 1 of the as-prepared sample at the same temperature. The jE was calculated by the Wiedmann– Franz law as jE = LTr. The jL was obtained by subtracting the jE from the j. The temperature dependences of jE and jL for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers are shown in Fig. 6b and c, respectively. The insets in panel (b) and (c) show the dependence of jE and jL, respectively, on the number of quenching. The increase in jE with increasing temperature was attributed to the temperature dependence of r, as shown in Fig. 5a. The jE at a certain temperature decreased with the number of quenching and then increased slightly after it had been quenched 1600 times. From Fig. 6b and c we can see that the significant decrease in j during the starting period of the quenching treatment is due to the significant reduction of jE. The jL of the quenched sample significantly decreased during the starting period of the quenching treatment; this is attributed to the enhancement of phonon scattering induced by the secondary precipitates on the grain boundaries. The lowest jL of 0.7 W m 1 K 1 at 800 K was obtained for the 200-times-quenched sample. However, the jL gradually increased with the number of quenching. The gradual increase in jL is assumed to be due to the separation of Ba fillers from the Sb-icosahedron voids, as confirmed by the map of the Ba element for the 2000-times-quenched sample. Fig. 6d displays ZTs for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers between 300 and 800 K based on the measured values of r, a and j. The inset in panel (d) shows the dependence of the ZTs on the number of quenching. It is worth noting that a promising ZT value of 1.0 was maintained throughout the 2000-times-quenching process. The ZT of 1.2 for the as-prepared sample at 800 K slightly decreased to 1.14 for the 2000-times-quenched sample. At the same time, the change in ZTs caused by the periodic quenching treatment gradually increased with temperature. The ZTs of the as-prepared sample and 2000-times-quenched one and the minimum, maximum and average value of the ZTs of the quenched sample at 300, 500 and 800 K are listed in Table 1. The ZTs of the as-prepared sample over 700 K are more than 1.0, which are very close to those of Ba0.08Yb0.09Co4Sb12.12 [32] and BarInsCo4Sb12 [36]. Both the reduction in j and the increase in a resulted in the enhancement in the ZTs during the starting period of the quenching treatment. However, the significant reduction in r caused a large decrease in the ZTs. The mutual
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Fig. 6. Temperature dependences of (a) the thermal conductivity, (b) the electronic thermal conductivity, (c) the lattice thermal conductivity and (d) the ZT values of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers. The insets in panels (a)–(d) show the dependence of the thermal conductivity, the electronic thermal conductivity, the lattice thermal conductivity and the ZT value on the number of quenching.
Table 1 ZTs of Ba0.3In0.2Co3.95Ni0.05Sb12 bulk material after being quenched 2000 times.
300 K 500 K 800 K
ZTas-prepared
ZT2000-time
ZTmin
ZTmax
ZTaverage
0.26 0.61 1.20
0.25 0.63 1.14
0.24 0.61 1.10
0.28 0.68 1.23
0.27 0.64 1.18
compensation of the two effects may reasonably explain the reason why ZTs at a certain temperature changed little as the number of quenching increased. Therefore, the Ba and In double-filled skutterudite materials have excellent performance stability under a periodically fluctuating environment temperature. 4. Discussion We mentioned that the secondary precipitates on the grain boundaries increased with increasing number of quenching in the range 200–600 and were recrystallized after quenching 800 times on the surface of the quenched samples (Fig. 2), and that the nanosized secondary precipitates and distortion of lattice occurred in the interior of the
2000-times-quenched sample (Fig. 4). These microstructures induced by periodic quenching treatment may enhance the scattering effects of carriers and phonons and result in the reduction in thermal conductivity and electrical conductivity, as shown by the measured results in Figs. 5 and 6. In fact, these induced microstructures likely lead to hysteresis in the r vs. T, a vs. T and j vs. T curves due to the relaxing. To investigate the effect of the microstructure relaxing on the thermoelectric transport properties, the r, a and j values of a Ba0.3In0.2Co4Sb12 bulk sample were measured in the temperature range from 300 K up to 800 K and back to 300 K after it had been periodically quenched different numbers. Fig. 7 shows the temperature dependences of (a) r, (b) a, (c) j and (d) ZT values of a Ba0.3In0.2Co4Sb12 bulk sample being periodically quenched 0, 200, 400, 600 and 1000 times in the temperature range from 300 K up to 800 K and back to 300 K. The curves marked with solid triangles were obtained when the temperature was increased from 300 K up to 800 K. The curves marked with hollow triangle were obtained during the decrease in temperature from 800 K back to 300 K. The curves for the 200-, 400-, 600- and 1000-times-quenched samples were shifted upwards by 3, 6, 9 and 12 104 S m 1 in panel (a), downward shifted by 15, 30, 45 and 60 lV K 1
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Fig. 7. Temperature dependences of (a) the electrical conductivity, (b) the Seebeck coefficient, (c) the thermal conductivity and (d) the ZT values of a Ba0.3In0.2Co4Sb12 bulk sample being periodically quenched 0, 200, 400, 600 and 1000 times. The curves with the solid triangles were obtained during the increase in temperature from 300 K to 800 K. The curves with the hollow triangles were obtained during the lowering of temperature from 800 K to 300 K. The curves for the 200-, 400-, 600- and 1000-times samples were upward shifted by 3, 6, 9 and 12 104 S m 1 in panel (a), downward shifted by 15, 30, 45 and 60 lV K 1 in panel (b), upward shifted by 0.2, 0.4, 0.6 and 0.8 W m 1 K 1 in panel (c) and upward shifted by 0.1, 0.2, 0.3 and 0.4 in panel (d), respectively.
in panel (b), upward shifted by 0.2, 0.4, 0.6 and 0.8 W m 1 K 1 in panel (c) and upward shifted by 0.1, 0.2, 0.3 and 0.4 in panel (d), respectively. The r vs. T, a vs. T and j vs. T curves of the Ba0.3In0.2Co4Sb12 bulk sample in the temperature range from 300 K up to 800 K and back to 300 K may be reproducible after the bulk sample had been periodically quenched 0, 200, 400, 600 and 1000 times. There are indeed hysteresis phenomena in the r vs. T, a vs. T, or j vs. T curves, showing that the induced microstructures are relaxing. However, the effect of microstructure relaxing on the charge transport properties of Ba and In double-filled skutterudite bulk material was gradu-
ally weakened and even vanished with increasing temperature. The values of rd ri, |a|d |a|i, jd–ji and ZTd ZTi of a Ba0.3In0.2Co4Sb12 quenched sample that had been periodically quenched 0, 200, 400, 600 and 1000 times at 300 K are listed in Table 2, where rd, |a|d, jd and ZTd were obtained during the decrease in temperature from 800 K back to 300 K and ri, |a|i, ji and ZTi were obtained during the increase in temperature from 300 K up to 800 K. It can be seen that both r and a decreased due to the microstructure relaxing. However, the decrement of a is much less than that of r, implying that the induced microstructures have little or no effect on a.
Table 2 The deviations of r, |a|, j and ZT of a Ba0.3In0.2Co4Sb12 quenched sample at 300 K after being periodically quenched different numbers. Number of quenching rd–ri (S m 1)a |a|d–|a|i (lV K–1) jd–ji (W m 1 K 1) ZTd–ZTi (a.u.)
0
200 5125.8 6.4 0 0.03
2262.1 5.7 0.02 0.02
400 2831.3 8.5 0 0.04
600 1474.8 7.7 0.01 0.03
1000 1757.1 6.5 0.05 0.02
a rd, |a|d, jd and ZTd were obtained during the decrease in temperature from 800 K to 300 K; ri, |a|i, ji and ZTi were obtained during the increase in temperature from 300 K to 800 K.
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The induced microstructures seem to have no effect on the thermal transport properties of Ba and In double-filled skutterudite bulk material. As a result, the ZT value of Ba0.3In0.2Co4Sb12 bulk material dropped due to the microstructure relaxing. Fortunately, the ZTd ZTi values are so small that the effect of microstructure relaxing on the ZT value of Ba and In double-filled skutterudite thermoelectric material may be ignored. Therefore, we can conclude that the microstructure relaxing induced by periodic quenching treatment has no significant effect on the thermoelectric transport properties of Ba and In double-filled skutterudite bulk material. 5. Conclusion A periodic quenching experiment from 723 K to room temperature was developed to carefully investigate the evolutions of microstructure and TE properties of single-phase Ba and In double-filled Ba0.3In0.2Co3.95Ni0.05Sb12 bulk materials. It was found that the secondary precipitates on the grain boundaries increased with increasing number of quenching and recrystallized after quenching 800 times. XRD patterns indicate that the quenching treatment caused the separation of Sb and Ba5Sb3 from the singlephase double-filled skutterudite bulk materials. The maps of Co, Ni, Sb, In and Ba elements reveal that the quenching treatment resulted in the enrichment of Ba element and the loss of Sb and Co elements on the grain boundaries but had no effect on In and Ni elements. The reduction in r and the enhancement in |a| during the starting period of the quenching treatment originate from the occurrence of the secondary precipitates on the grain boundaries in the quenched samples due to electron energy filtering. The dependences of j and jE on the number of quenching are very similar to that of r, suggesting that the TE properties of the quenched samples mainly depended on the charge transport properties which were affected by the secondary precipitates caused by the periodic quenching treatment. The separation of the Ba fillers from the Sb-icosahedron voids induced by the periodic quenching treatment is assumed to be the reason for the gradual increase in jL with the number of quenching. As a result, ZTs at a certain temperature changed little as the number of quenching increased. A promising ZT value of 1.0 was maintained throughout the 2000-times-quenching process. The ZT of 1.2 for the as-prepared sample at 800 K decreased slightly to 1.14 for the 2000-times-quenched sample. The microstructure relaxing induced by the periodic quenching treatment has no significant effect on the thermoelectric transport properties of Ba and In double-filled skutterudite bulk material. All the experimental results demonstrated that the Ba and In double-filled skutterudite materials have excellent performance stability and could be applied to a range of special power generation systems with periodically fluctuating environmental temperature, such as the solar thermoelectric–photovoltaic hybrid power generation system.
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Acknowledgements This work was supported by the National Basic Research Program of China (973-program) under Project No. 2007CB607506, the National Natural Science Foundation of China (Nos. 10832008, 50930004, 50972114) and the Program for New Century Excellent Talents in University (NCET-09-0627). References [1] [2] [3] [4] [5]
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Bell LE. Science 2008;321:1457. Snyder GJ, Toberer ES. Nat Mater 2008;8:105. Nolas GS, Morelli DT, Tritt TM. Annu Rev Mater Sci 1999;29:89. Uher C. In: Tritt TM, editor. Semiconductors and semimetals, vol. 69. New York: Academic Press; 2001. p. 139. Sales BC. In: Gschneidner Jr KA, Bu¨nzli J-CG, Pecharsky V, editors. Handbook on the physics and chemistry of rare earths, vol. 33. New York: Elsevier; 2003. p. 1. Sales BC, Mandrus D, Williams RK. Science 1996;272:1325. Nolas GS, Cohn JL, Slack GA. Phys Rev B 1998;58:164. Morelli DT, Meisner GP, Chen BX, Hu SQ, Uher C. Phys Rev B 1997;56:7376. Chen BX, Xu JH, Uher C, Morelli DT, Meisner GP, Fleurial J-P, et al. Phys Rev B 1997;55:1476. Meisner GP, Morelli DT, Hu S, Yang J, Uher C. Phys Rev Lett 1998;80:3551. Nolas GS, Kaeser M, Littleton RT, Tritt TM. Appl Phys Lett 2000;77:1855. Dilley NR, Bauer ED, Maple MB, Sales BC. J Appl Phys 2000;88:1948. Lamberton Jr GA, Bhattacharya S, Littleton IV RT, Kaeser MA, Tedstrom RH, Tritt TM, et al. Appl Phys Lett 2002;80:598. Yang J, Morelli DT, Meisner GP, Chen W, Dyck JS, Uher C. Phys Rev B 2003;67:165207. Zhai PC, Zhao WY, Li Y, Liu LS, Tang XF, Zhang QJ, et al. Appl Phys Lett 2006;89:052111. Yang CP, Wang H, Iwasa K, Kohgi M, Sugawara H, Sato H. Appl Phys Lett 2007;90:102503. Sun P, Nakanishi Y, Nakamura M, Yoshizawa M, Ohashi M, Oomi G, et al. Phys Rev B 2007;75:054114. Chen LD, Kawahara T, Tang XF, Goto T, Hirai T, Dyck JS, et al. J Appl Phys 2001;90:1864. Dyck JS, Chen W, Uher C, Chen LD, Tang XF, Hirai T. J Appl Phys 2002;91:3698. Puyet M, Dauscher A, Lenoir B, Dehmas M, Stiewe C, Mu¨ller E, et al. J Appl Phys 2005;97:083712. Zhao XY, Shi X, Chen LD, Zhang WQ, Zhang WB, Pei YZ. J Appl Phys 2006;99:053711. Tang XF, Zhang QJ, Chen LD, Goto T, Hirai T. J Appl Phys 2005;97:093712. Nolas GS, Takizawa H, Endo T, Sellinschegg H, Johnson DC. Appl Phys Lett 2000;77:52. Sales BC, Chakoumakos BC, Mandrus D. Phys Rev B 2000;61:2475. Nolas GS, Kendziora CA, Takizawa H. J Appl Phys 2003;94:7440. He T, Chen J, Rosenfeld HD, Subramanian MA. Chem Mater 2006;18:759. Zhang W, Shi X, Mei ZG, Xu Y, Chen LD, Yang J, et al. Appl Phys Lett 2006;89:112105. Pei YZ, Chen LD, Zhang W, Shi X, Bai SQ, Zhao XY, et al. Appl Phys Lett 2006;89:221107. Lu QM, Zhang JX, Zhang X, Liu YQ, Liu DM, Zhou ML. J Appl Phys 2005;98:106107. Tang XF, Li H, Zhang QJ, Niino M, Goto T. J Appl Phys 2006;100:123702.
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[31] Yang J, Zhang W, Bai SQ, Mei Z, Chen LD. Appl Phys Lett 2007;90:192111. [32] Shi X, Kong H, Li C-P, Uher C, Yang J, Salvador JR, et al. Appl Phys Lett 2008;92:182101. [33] Peng JY, Alboni PN, He J, Zhang B, Su Z, Holgate T, et al. J Appl Phys 2008;104:053710. [34] Peng JY, He J, Alboni PN, Tritt TM. J Electron Mater 2009;38:981. [35] Zhao WY, Dong CL, Wei P, Guan W, Liu LS, Zhai PC, et al. J Appl Phys 2007;102:113708.
[36] Zhao WY, Wei P, Zhang QJ, Dong CL, Liu LS, Tang XF. J Am Chem Soc 2009;131:3713. [37] Zhang QJ, Tang XF, Zhai PC, Niino M, Endo C. Mater Sci Forum 2005;135:492. [38] Tritt TM, Bo¨ttner H, Chen LD. MRS Bull 2008;33:366. [39] Kraemer D, Hu L, Muto A, Chen X, Chen G, Chiesa M. Appl Phys Lett 2008;92:243503. [40] Heremans JP, Thrush CM, Morelli DT. Phys Rev B 2004;70:115334.
Acta Materialia 59 (2011) 3244–3254 www.elsevier.com/locate/actamat
Excellent performance stability of Ba and In double-filled skutterudite thermoelectric materials Ping Wei, Wen-Yu Zhao ⇑, Chun-Lei Dong, Xuan Yang, Jian Yu, Qing-Jie Zhang ⇑ State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China Received 19 November 2010; received in revised form 24 January 2011; accepted 29 January 2011
Abstract The microstructure and thermoelectric properties of Ba and In double-filled Ba0.3In0.2Co3.95Ni0.05Sb12 materials with a figure-of-merit (ZT) of 1.2 at 800 K were carefully investigated by periodic quenching from 723 K to room temperature. The quenching treatment caused enrichment of Ba and loss of Sb and Co on the grain boundaries but had no effect on In and Ni. The enhancement in |a| in the starting period is due to the reduction in r induced by the secondary precipitates. The reductions in r, j and jE are attributed to the secondary precipitates. The increase in jL is assumed to be due to the separation of Ba filler from the Sb-icosahedron voids. A promising ZT value of 1.0 remained after quenching 2000 times. It is concluded that the double-filled skutterudite materials have excellent performance stability under periodically fluctuating environmental temperature. Crown Copyright Ó 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. Keywords: Semiconductor compounds; Microstructure; Thermal conductivity; Electrical properties
1. Introduction Thermoelectricity is becoming increasingly important in the fields of cooling, heating, generating power and recovering waste heat [1]. The good performance of thermoelectric (TE) devices depends directly on the material TE figure-of-merit, ZT = a2rN/j. In this equation, T is the absolute temperature, r is the electrical conductivity, a is the Seebeck coefficient, and j is the thermal conductivity (j = jT + jL, where jT is the electronic contribution and jL is the lattice contribution). A good TE material should be the perfect combination of a high power factor (a2r) and low thermal conductivity [2]. At the same time, a good TE device requires that the TE materials have excellent performance stability in practical applications. However, the performance evolution of TE materials under service conditions has been little studied. ⇑ Corresponding authors.
E-mail addresses: [email protected] (W.-Y. Zhao), [email protected] (Q.-J. Zhang).
The skutterudite family has drawn much attention in the last decade because of its very promising properties for intermediate-temperature TE power generation, primarily from solar energy and industrial waste heat. Extensive reviews of the physical properties of the skutterudites have been published by Nolas et al. [3], Uher [4] and Sales [5]. Cubic binary skutterudite CoSb3 possesses two interstitial voids in a crystallographic unit cell. Filling the oversized voids with foreign atoms such as rare-earth elements [6– 17], alkaline earth elements [18–21] and others [22–28] has been confirmed to be an effective way to reduce jL. Double-atom filling is considered to be a more effective way to further optimize the TE performance of filled skutterudites. Experimental data have confirmed that the ZT values of double-filled skutterudite compounds such as CeLa [29], CaCe [30], BaYb [31,32] and InYb [33,34] are larger than those of single-filled ones. It is the double-atom filling in CoSb3 that remarkably reduces the jL and greatly enhances the TE performance, making double-filled skutterudites one of the most promising intermediate-temperature TE materials.
1359-6454/$36.00 Crown Copyright Ó 2011 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved. doi:10.1016/j.actamat.2011.01.064
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We recently investigated the TE properties of Ba and In double-filled skutterudites BaxInyCo4Sb12 z synthesized with nominal compositions Ba0.3 kInkCo4Sb12 (0 6 k 6 0.3, Dk = 0.05) and Ba0.3InmCo4Sb12 (0 6 m 6 0.3, Dm = 0.05) in the temperature range of 300–850 K and found that filling In in Ba-filled skutterudite not only reduced the thermal conductivity remarkably but also enhanced the a2r for Ba and In double-filled skutterudite materials with higher ZTs up to 1.34 at 850 K significantly [35,36]. We attributed the excellent TE properties to the orbital hybridizations induced by the In filler. While important progress has been made in enhancing the ZT value of double-filled skutterudite materials, there have been no reports about the thermal stability of the microstructure and TE properties of any double-filled skutterudite materials under service conditions. More fundamental research is necessary and important for the practical applications of double-filled skutterudite TE materials. Ba and In double-filled skutterudite-based TE devices with high efficiency are now being considered for use in a new solar thermoelectric–photovoltaic hybrid power generation system that was developed by the cooperation between Zhang et al. from China and Niino et al. from Japan in order to realize solar full-wavelength energy conversion into electricity through both photovoltaics (200– 800 nm) and thermoelectrics (800–3000 nm) [37]. The advantages of the hybrid system in the solar energy conversion have been reported elsewhere [38,39]. The operating temperature in the hybrid system is fluctuates periodically between room temperature and above 673 K due to the alternation of day and night. For the aim of this study, it is important to understand the degradation in the performance of the Ba and In double-filled skutterudite bulk materials caused by such cyclic thermal loading. In this study, a periodic quenching experiment from 723 K to room temperature to 723 K was designed to simulate the service conditions under which the Ba and In double-filled skutterudite bulk materials would be applied to the solar thermoelectric–photovoltaic hybrid power generation system. The evolutions of the microstructure and the TE properties of single-phase Ba and In double-filled Ba0.3In0.2 Co3.95Ni0.05Sb12 bulk material were carefully investigated during periodic quenching treatment to determine its technical limitations. In order to investigate the relaxing influence of the microstructures induced by the periodical quenching, the TE properties of a Ba0.3In0.2Co4Sb12 bulk sample being periodically quenched different numbers were measured during the temperature cycle from 300 K up to 800 K and back to 300 K.
sintering method. The experimental details for preparing the double-filled skutterudite bulk materials have been demonstrated elsewhere [35,36]. The starting materials, with nominal compositions Ba0.3In0.2Co3.95Ni0.05Sb12 and Ba0.3In0.2Co4Sb12, are mixtures of highly pure metals of Ba (99.9%, plate), In (99.99%, powder), Sb (99.999%, powder), Ni (99.9%, powder) and Co (99.9%, powder). Asprepared samples 3 4 10 mm3, Ø10 1.5 mm3 and Ø3 1.5 mm3 in size were cut from the sintered bulk materials, which were used to investigate the evolutions of charge transport properties, thermal properties and microstructure of the double-filled skutterudite bulk materials, respectively. Phase purity, chemical compositions and microstructures of the as-prepared samples were first characterized using various methods. These as-prepared samples were loaded into a silica tube and the tube was sealed under vacuum. A periodic quenching experiment was designed to simulate a service condition under which the double-filled skutterudite bulk materials would have to operate in a solar thermoelectric–photovoltaic hybrid power generation system with periodically fluctuating operating temperature. The sealed tube was periodically quenched in water after being held for 5 min at 723 K in a constant temperature furnace. The charge transport properties, thermal properties and microstructure of the quenched samples were analyzed again after they had been quenched 200 times. These quenched samples were sealed into a new tube under vacuum and the aforementioned process was repeated a further 1800 times (i.e. 2000 times in total).
2. Experiment
r and a were measured with the standard four-probe method (Sinkuriko: ZEM-1) in an Ar atmosphere. j was calculated using the equation j = DqCp, where D is the thermal diffusivity coefficient, q is the bulk density of the material and Cp is the specific heat capacity. D was measured by a laser flash technique (Netzsch LFA 427) in a
2.1. Periodically quenching experiment Ba and In double-filled skutterudite bulk materials were prepared by a melting–quenching–annealing–spark plasma
2.2. X-ray diffraction and electron microscopy analysis The constituent phases of as-prepared and quenched samples were determined by powder X-ray diffraction (XRD) using Cu Ka radiation (PANalytical X’ Pert PRO). The chemical compositions were analyzed using electron probe microanalysis (JXA-8100, JEOL). The operation conditions of the accelerating voltage and specimen current were kept at 20 kV and 20 nA, respectively. The backscattered electron images (BEI) and second electron images (SEI) were observed using a JSM-5610LV scanning electron microscope to study the evolution of the microstructures of the as-prepared samples after periodic quenching. The transmission electron microscopy (TEM) images were observed using a JEM-2010(HT) transmission electron microscope to investigate the interface texture between grain boundaries. 2.3. Thermoelectric transport property
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flowing Ar atmosphere. Cp was measured with a differential scanning calorimeter. q was obtained by the Archimedes method. jL was obtained by subtracting the electrical contribution from j using the equation jL = j jT. Here, jT is expressed by the Wiedemann Franz jT = rLT, where the Lorenz number L has a numerical value of 2.0 10 8 V2 K 2, as estimated by Dyck et al. [19]. To decrease the uncertainties of these parameters, all the parameters, r, a, D, q and Cp, were measured under the same conditions, including the same sample size, the same starting and ending temperatures, and the same working current and voltage. The hypothesis that the measuring errors for all the parameters are invariable each time was employed to investigate the effect of the number of quenching on the thermoelectric transport properties. 3. Results 3.1. Structural and compositional evolutions XRD patterns of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched one after being periodically quenched 2000 times are shown in Fig. 1. It can be seen that the as-prepared sample is composed of a single-phase compound with a skutterudite structure. This result indicates that the single-phase Ba and In doublefilled skutterudite materials are fabricated by the aforementioned process. Ba and In atoms were confirmed to fill in the Sb-icosahedron voids by Rietveld refinement from high-resolution synchrotron data for InxCo4Sb12 [26] and XRD data for BaxInyCo4Sb12 z [35], and the quantitative analysis of experimental data from X-ray photoelectron spectroscopy for BarInsCo4Sb12 [36]. However, the phase structure of the as-prepared sample was changed after periodic quenching treatment. Some weak peaks were observed in the XRD pattern of the quenched sample, which are
Fig. 1. XRD patterns of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and quenched samples after being periodically quenched 2000 times.
attributed to the characteristic diffraction peaks of Sb and Ba5Sb3, respectively. The enrichment of Sb metal on the surface layer of the quenched sample is ascribed to the deposition and the recrystallization of sublimation of antimony. Ba5Sb3 was distributed in the transition zone between the surface layer and the interior (shown in the inset of Fig. 1), which might be formed by the reaction between the sublimations of Sb and Ba. Incidentally, no evident peak shift for the characteristic diffraction peaks of skutterudite is observable in the XRD patterns between the as-prepared sample and the quenched one, implying that no serious strain exists in the quenched samples. Fig. 2 shows SEI and BEI of the same region on the surface of an as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 200, 400, 600 and 800 times. Clear grain boundaries were observed at the beginning of quenching, for example, for the sample quenched 200 times. The amount of secondary precipitates on the grain boundaries increased with increasing number of quenching in the range 200–600, and were recrystallized after quenching 800 times. The dark contrast distributed on the grain boundaries in the BEI indicates that the secondary precipitates were rich in elements with a smaller atomic number. Energy-dispersive spectroscopy of the interface and the grain in the quenched sample indicates that the secondary precipitates on the grain boundaries were mainly composed of Ba and O elements. The enrichments of Ba and O elements on the grain boundaries are attributed to the in situ oxidation reaction of 2Ba + O2 = 2BaO, the O2 originating from the remaining air in the vacuum silica tube. BEI and the maps of Co, Ni, Sb, In and Ba elements from the surface of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 2000 times are shown in Fig. 3. The dashed circles in Fig. 3 indicate where the abundance of Ba element is higher. Note that the periodic quenching treatment has caused significant enrichment of Ba element and the loss of Sb and Co elements along the grain boundaries, compared with those of the as-prepared sample. This indicates that the Ba fillers separated out from the Sb-icosahedron voids because of the periodic quenching treatment. However, the quenching treatment seems to have no effect on the maps of In and Ni elements. Fig. 4a–d shows TEM images of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched one after being periodically quenched 2000 times, respectively. The inset in panel (a) shows the selected area electron diffraction (SAED) patterns of as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample along the [1 1 1] zone axis. The TEM image and SAED patterns of the asprepared bulk sample shown in Fig. 4a indicate that the Ba0.3In0.2Co3.95Ni0.05Sb12 grains are perfect crystals with straight and clean grain boundaries and no inclusion in the as-prepared bulk sample. Note that secondary precipitates with grain sizes of about 10–25 nm, as shown in Fig. 4b, were observed on the grain boundaries in the
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Fig. 2. SEI and BEI of the same region of the surface of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 200, 400, 600 and 800 times.
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Fig. 3. BEI and maps of Co, Ni, Sb, In, and Ba elements from the surface of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample after being periodically quenched 2000 times.
quenched samples. At the same time, dislocation stripes, as shown in Fig. 4c, were demonstrated to occur in the quenched samples, which is evidence that the strain induced by the periodic quenching treatment was largely relaxed. In addition, it seems reasonable that the irregular bend black stripes shown in Fig. 4d could arise from thin foil bending due to mechanical grinding. However, it cannot be excluded that the lattice distortion of Ba0.3In0.2Co3.95Ni0.05Sb12 grains results in the appearance of irregular bend black stripes in the quenched samples, because the phenomenon has been not observed in the thin foils of the as-prepared bulk samples prepared with the same method. Fortunately, the effect of the lattice distor-
tion on the thermoelectric properties of Ba0.3In0.2Co3.95Ni0.05Sb12 bulk material may be ignored according to the results from additional experiments in the Discussion session. 3.2. Charge transport properties The temperature dependence of electrical conductivity for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched ones after being periodically quenched different numbers is shown in Fig. 5a. The inset in panel (a) shows the dependence of electrical conductivity on the number of quenching. The r of the as-prepared sample is
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(a)
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(b)
0.2 µm
0.5 µm
(d)
(c)
0.5 µm
0.5 µm
Fig. 4. TEM images of (a) the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and (b–d) the quenched one after being periodically quenched 2000 times. The inset in panel (a) shows the SAED patterns of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample along the [1 1 1] zone axis.
about 2.56 105 S m 1 at room temperature and then decreases with increasing temperature, indicative of a metallic transport behavior. The reduction in r with increasing temperature mainly originates from the enhancement in the scattering effect of the carriers on the crystal lattice. The r of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 materials over the whole temperature range, 300–800 K, is much higher than that of the BaxInyCo4Sb12-z and BarInsCo4Sb12 compounds previously reported by us [35,36]. This difference may be related to the substitution of Ni for Co. The presence of small amounts of Ni in Ba-filled Ba0.3NixCo4 xSb12 had the beneficial effects of lowering the total thermal conductivity and increasing the electrical conductivity [19]. The r of 1.57 105 S m 1 for the as-prepared sample gradually decreased to 1.37 105 S m 1 for the 1600-times-quenched sample at 800 K and then increased slightly when the number of quenching further increased. The gradual reduction in r with increasing number of quenching may be interpreted in terms of the evolutions of the microstructure and chemical composition in the quenched samples. We mentioned that the secondary precipitates on the grain boundaries might disturb the electronic flow and decrease the electrical conductivity. The r of 2.37 105 S m 1 of the 600-times-quenched sample decreased to 2.25 105 S m 1 for the 800-times-quenched sample at 300 K. This significant reduction is attributed to the effect
of the recrystallization of secondary precipitates on the grain boundaries when the number of quenching exceeds 800 times, as shown in Fig. 2. The temperature dependence of the Seebeck coefficient for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers is shown in Fig. 5b. The inset in panel (b) shows the dependence of the Seebeck coefficient on the number of quenching. The negative Seebeck coefficient values are indicative of n-type electrical transport properties. The |a| significantly increased during the starting period of the quenching treatment, corresponding to the significant reduction in r. For the periodic quenching experiments, the bulk samples were encapsulated and heated in sealed silica tubes. Some free Sb would volatilize and deposit during each quenching cycle. As a result, the content of free Sb deposited on the surface of the quenched sample would increase with increasing number of quenching, as confirmed by the XRD result. On the other hand, Ba element was enriched on the grain boundaries because of the in situ oxidation reaction of 2Ba + O2 = 2BaO, whereas the volatilization of free Sb takes place more readily. Therefore, the reduction of r and the increase of |a| during the starting period of the quenching treatment may be attributed to the enhancement of the boundary potential barrier caused by the secondary precipitates on the grain boundaries. We consider that the evolutions of r and a
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Fig. 5. Temperature dependence of (a) the electrical conductivity and (b) the Seebeck coefficient of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers. The insets show the dependences of the electrical conductivity and the Seebeck coefficient, respectively, on the number of quenching.
could be explained reasonably well by the separation of higher energy electrons from lower energy electrons and the selective scattering of electrons, or so-called electron energy filtering [40]. The r of 2.56 105 S m 1 for the asprepared sample at 300 K decreased remarkably to 2.42 105 S m 1 for the 200-times-quenched sample. Correspondingly, the |a| of 114 lV K 1 increased to 119 lV K 1 at 300 K. The a values were almost invariable with increasing number of quenching, which may be due to the indifferent effect of the microstructures induced by rapid quenching treatment on the a, as presented in Section 4. 3.3. Thermal transport properties and the thermoelectric figure-of-merit The temperature dependence of thermal conductivity for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched ones after being periodically quenched different numbers is shown in Fig. 6a. The inset in panel (a) shows the dependence of thermal conductivity on the number of quenching. The j of the quenched samples was far
lower than that of the as-prepared samples. The dependence of j on the number of quenching is very similar to that of r, implying that the origination of the reduction in j with the number of quenching is the same as that of r. The secondary precipitates may enhance the scattering effects of the carriers and phonons and decrease j. The 1000-times-quenched sample showed the lowest j value of 3.0 W m 1 K 1 at 675 K, which was 10% lower than the 3.3 W m 1 K 1 of the as-prepared sample at the same temperature. The jE was calculated by the Wiedmann– Franz law as jE = LTr. The jL was obtained by subtracting the jE from the j. The temperature dependences of jE and jL for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers are shown in Fig. 6b and c, respectively. The insets in panel (b) and (c) show the dependence of jE and jL, respectively, on the number of quenching. The increase in jE with increasing temperature was attributed to the temperature dependence of r, as shown in Fig. 5a. The jE at a certain temperature decreased with the number of quenching and then increased slightly after it had been quenched 1600 times. From Fig. 6b and c we can see that the significant decrease in j during the starting period of the quenching treatment is due to the significant reduction of jE. The jL of the quenched sample significantly decreased during the starting period of the quenching treatment; this is attributed to the enhancement of phonon scattering induced by the secondary precipitates on the grain boundaries. The lowest jL of 0.7 W m 1 K 1 at 800 K was obtained for the 200-times-quenched sample. However, the jL gradually increased with the number of quenching. The gradual increase in jL is assumed to be due to the separation of Ba fillers from the Sb-icosahedron voids, as confirmed by the map of the Ba element for the 2000-times-quenched sample. Fig. 6d displays ZTs for the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers between 300 and 800 K based on the measured values of r, a and j. The inset in panel (d) shows the dependence of the ZTs on the number of quenching. It is worth noting that a promising ZT value of 1.0 was maintained throughout the 2000-times-quenching process. The ZT of 1.2 for the as-prepared sample at 800 K slightly decreased to 1.14 for the 2000-times-quenched sample. At the same time, the change in ZTs caused by the periodic quenching treatment gradually increased with temperature. The ZTs of the as-prepared sample and 2000-times-quenched one and the minimum, maximum and average value of the ZTs of the quenched sample at 300, 500 and 800 K are listed in Table 1. The ZTs of the as-prepared sample over 700 K are more than 1.0, which are very close to those of Ba0.08Yb0.09Co4Sb12.12 [32] and BarInsCo4Sb12 [36]. Both the reduction in j and the increase in a resulted in the enhancement in the ZTs during the starting period of the quenching treatment. However, the significant reduction in r caused a large decrease in the ZTs. The mutual
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Fig. 6. Temperature dependences of (a) the thermal conductivity, (b) the electronic thermal conductivity, (c) the lattice thermal conductivity and (d) the ZT values of the as-prepared Ba0.3In0.2Co3.95Ni0.05Sb12 bulk sample and the quenched samples after being periodically quenched different numbers. The insets in panels (a)–(d) show the dependence of the thermal conductivity, the electronic thermal conductivity, the lattice thermal conductivity and the ZT value on the number of quenching.
Table 1 ZTs of Ba0.3In0.2Co3.95Ni0.05Sb12 bulk material after being quenched 2000 times.
300 K 500 K 800 K
ZTas-prepared
ZT2000-time
ZTmin
ZTmax
ZTaverage
0.26 0.61 1.20
0.25 0.63 1.14
0.24 0.61 1.10
0.28 0.68 1.23
0.27 0.64 1.18
compensation of the two effects may reasonably explain the reason why ZTs at a certain temperature changed little as the number of quenching increased. Therefore, the Ba and In double-filled skutterudite materials have excellent performance stability under a periodically fluctuating environment temperature. 4. Discussion We mentioned that the secondary precipitates on the grain boundaries increased with increasing number of quenching in the range 200–600 and were recrystallized after quenching 800 times on the surface of the quenched samples (Fig. 2), and that the nanosized secondary precipitates and distortion of lattice occurred in the interior of the
2000-times-quenched sample (Fig. 4). These microstructures induced by periodic quenching treatment may enhance the scattering effects of carriers and phonons and result in the reduction in thermal conductivity and electrical conductivity, as shown by the measured results in Figs. 5 and 6. In fact, these induced microstructures likely lead to hysteresis in the r vs. T, a vs. T and j vs. T curves due to the relaxing. To investigate the effect of the microstructure relaxing on the thermoelectric transport properties, the r, a and j values of a Ba0.3In0.2Co4Sb12 bulk sample were measured in the temperature range from 300 K up to 800 K and back to 300 K after it had been periodically quenched different numbers. Fig. 7 shows the temperature dependences of (a) r, (b) a, (c) j and (d) ZT values of a Ba0.3In0.2Co4Sb12 bulk sample being periodically quenched 0, 200, 400, 600 and 1000 times in the temperature range from 300 K up to 800 K and back to 300 K. The curves marked with solid triangles were obtained when the temperature was increased from 300 K up to 800 K. The curves marked with hollow triangle were obtained during the decrease in temperature from 800 K back to 300 K. The curves for the 200-, 400-, 600- and 1000-times-quenched samples were shifted upwards by 3, 6, 9 and 12 104 S m 1 in panel (a), downward shifted by 15, 30, 45 and 60 lV K 1
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Fig. 7. Temperature dependences of (a) the electrical conductivity, (b) the Seebeck coefficient, (c) the thermal conductivity and (d) the ZT values of a Ba0.3In0.2Co4Sb12 bulk sample being periodically quenched 0, 200, 400, 600 and 1000 times. The curves with the solid triangles were obtained during the increase in temperature from 300 K to 800 K. The curves with the hollow triangles were obtained during the lowering of temperature from 800 K to 300 K. The curves for the 200-, 400-, 600- and 1000-times samples were upward shifted by 3, 6, 9 and 12 104 S m 1 in panel (a), downward shifted by 15, 30, 45 and 60 lV K 1 in panel (b), upward shifted by 0.2, 0.4, 0.6 and 0.8 W m 1 K 1 in panel (c) and upward shifted by 0.1, 0.2, 0.3 and 0.4 in panel (d), respectively.
in panel (b), upward shifted by 0.2, 0.4, 0.6 and 0.8 W m 1 K 1 in panel (c) and upward shifted by 0.1, 0.2, 0.3 and 0.4 in panel (d), respectively. The r vs. T, a vs. T and j vs. T curves of the Ba0.3In0.2Co4Sb12 bulk sample in the temperature range from 300 K up to 800 K and back to 300 K may be reproducible after the bulk sample had been periodically quenched 0, 200, 400, 600 and 1000 times. There are indeed hysteresis phenomena in the r vs. T, a vs. T, or j vs. T curves, showing that the induced microstructures are relaxing. However, the effect of microstructure relaxing on the charge transport properties of Ba and In double-filled skutterudite bulk material was gradu-
ally weakened and even vanished with increasing temperature. The values of rd ri, |a|d |a|i, jd–ji and ZTd ZTi of a Ba0.3In0.2Co4Sb12 quenched sample that had been periodically quenched 0, 200, 400, 600 and 1000 times at 300 K are listed in Table 2, where rd, |a|d, jd and ZTd were obtained during the decrease in temperature from 800 K back to 300 K and ri, |a|i, ji and ZTi were obtained during the increase in temperature from 300 K up to 800 K. It can be seen that both r and a decreased due to the microstructure relaxing. However, the decrement of a is much less than that of r, implying that the induced microstructures have little or no effect on a.
Table 2 The deviations of r, |a|, j and ZT of a Ba0.3In0.2Co4Sb12 quenched sample at 300 K after being periodically quenched different numbers. Number of quenching rd–ri (S m 1)a |a|d–|a|i (lV K–1) jd–ji (W m 1 K 1) ZTd–ZTi (a.u.)
0
200 5125.8 6.4 0 0.03
2262.1 5.7 0.02 0.02
400 2831.3 8.5 0 0.04
600 1474.8 7.7 0.01 0.03
1000 1757.1 6.5 0.05 0.02
a rd, |a|d, jd and ZTd were obtained during the decrease in temperature from 800 K to 300 K; ri, |a|i, ji and ZTi were obtained during the increase in temperature from 300 K to 800 K.
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The induced microstructures seem to have no effect on the thermal transport properties of Ba and In double-filled skutterudite bulk material. As a result, the ZT value of Ba0.3In0.2Co4Sb12 bulk material dropped due to the microstructure relaxing. Fortunately, the ZTd ZTi values are so small that the effect of microstructure relaxing on the ZT value of Ba and In double-filled skutterudite thermoelectric material may be ignored. Therefore, we can conclude that the microstructure relaxing induced by periodic quenching treatment has no significant effect on the thermoelectric transport properties of Ba and In double-filled skutterudite bulk material. 5. Conclusion A periodic quenching experiment from 723 K to room temperature was developed to carefully investigate the evolutions of microstructure and TE properties of single-phase Ba and In double-filled Ba0.3In0.2Co3.95Ni0.05Sb12 bulk materials. It was found that the secondary precipitates on the grain boundaries increased with increasing number of quenching and recrystallized after quenching 800 times. XRD patterns indicate that the quenching treatment caused the separation of Sb and Ba5Sb3 from the singlephase double-filled skutterudite bulk materials. The maps of Co, Ni, Sb, In and Ba elements reveal that the quenching treatment resulted in the enrichment of Ba element and the loss of Sb and Co elements on the grain boundaries but had no effect on In and Ni elements. The reduction in r and the enhancement in |a| during the starting period of the quenching treatment originate from the occurrence of the secondary precipitates on the grain boundaries in the quenched samples due to electron energy filtering. The dependences of j and jE on the number of quenching are very similar to that of r, suggesting that the TE properties of the quenched samples mainly depended on the charge transport properties which were affected by the secondary precipitates caused by the periodic quenching treatment. The separation of the Ba fillers from the Sb-icosahedron voids induced by the periodic quenching treatment is assumed to be the reason for the gradual increase in jL with the number of quenching. As a result, ZTs at a certain temperature changed little as the number of quenching increased. A promising ZT value of 1.0 was maintained throughout the 2000-times-quenching process. The ZT of 1.2 for the as-prepared sample at 800 K decreased slightly to 1.14 for the 2000-times-quenched sample. The microstructure relaxing induced by the periodic quenching treatment has no significant effect on the thermoelectric transport properties of Ba and In double-filled skutterudite bulk material. All the experimental results demonstrated that the Ba and In double-filled skutterudite materials have excellent performance stability and could be applied to a range of special power generation systems with periodically fluctuating environmental temperature, such as the solar thermoelectric–photovoltaic hybrid power generation system.
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Acknowledgements This work was supported by the National Basic Research Program of China (973-program) under Project No. 2007CB607506, the National Natural Science Foundation of China (Nos. 10832008, 50930004, 50972114) and the Program for New Century Excellent Talents in University (NCET-09-0627). References [1] [2] [3] [4] [5]
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