J. Mater. Sci. Technol., 2011, 27(6), 525-528.
High-temperature Thermoelectric Properties of Cu-substituted Bi2 Ba2 Co2−x Cux Oy Oxides Haoshan Hao1,2)† , Huizhi Yang1) , Yongtao Liu2) and Xing Hu2) 1) Department of Mathematical and Physical Sciences, Henan Institute of Engineering, Zhengzhou 451191, China 2) Key Laboratory of Material Physics of Ministry of Education, Zhengzhou University, Zhengzhou 450052, China [Manuscript received November 3, 2010, in revised form February 15, 2011]
Cu-substituted Bi2 Ba2 Co2−x Cux Oy (0.0≤x≤0.4) samples were prepared by conventional solid-state reaction method and the eﬀect of Cu substitution on the microstructure and thermoelectric properties were investigated. The partial substitution of Cu for Co in Bi2 Ba2 Co2−x Cux Oy led to an increase in the electrical conductivity because of an increase in the hole concentration and grain size of sintered bodies. In addition, Cu substitution led to an increase in Seebeck coeﬃcients while kept the thermal conductivity unchanged. The highest thermoelectric ﬁgure of merit (ZT value) was obtained in x=0.4 sample and the value was 1.5 times as large as that of Cu-free sample at 873 K. KEY WORDS: Bi2 Ba2 Co2 Oy ; Cu substitution; Thermoelectric properties
1. Introduction Metal oxides with good thermoelectric properties are promising candidates for the application of power generation due to their high chemical and thermal stability at high temperature. In recent years, misﬁt-layered cobalt oxides such as NaCo2 O4 [1–3] , Ca3 Co4 O9 [4–6] , and Bi2 M2 Co2 Oy (M =Ca, Sr, Ba)[7–10] , have attracted much attention due to their promising thermoelectric properties. The structure of the misﬁt-layered cobalt oxides consists of highly conductive CoO2 layers of CdI2 type and insulating rock-salt-type layers. The lattice parameters along the b axis in the two sublattices are diﬀerent from one layer to the other and this misﬁt structure is essential for the super thermoelectric characteristics. Comparing the properties of Bi2 M2 Co2 Oy (M =Ca, Sr, Ba) systems, a trend can be derived that the system with the large Ba element as the M -site constituent shows enhanced metallic conduction and Seebeck coeﬃcients not signiﬁcantly lower than those for Bi2 (Ca,Sr)2 Co2 Oy systems[11,12] . It has been reported that Bi2 Ba2 Co2 Oy system exhibits reasonably high metallic conductivity, high Seebeck coeﬃcients † Corresponding author. Assoc. Prof., Ph.D.; E-mail address: [email protected]
and low thermal conductivity, indicating promising candidate for high-eﬃciency thermoelectric materials[11,12] . Element substitution is eﬀective in improving thermoelectric performance of misﬁt-layered cobalt oxides and many eﬀorts have been devoted in NaCo2 O4 [13–15] and Ca3 Co4 O9 [16–18] systems. As for Bi2 Ba2 Co2 Oy system, Sakai et al.[11,12] have reported that thermoelectric characteristics can be remarkably enhanced by means of partial Pb-for-Bi substitution, yielding not only lower electrical resistivity but also a drastic increase in the Seebeck coeﬃcient. However, the substitution eﬀect for the Co site in Bi2 Ba2 Co2 Oy system has not yet been determined. It has been reported that Cu can be present on Co sites in the   NaCo2 O4 and Ca3 Co4 O9 lattices to form substitutional solid solutions and Cu-substituted samples exhibit remarkable increase in the thermoelectric performance. In this study, we synthesized Cu-contained Bi2 Ba2 Co2−x Cux Oy oxides and the eﬀect of Cu substitution for Co on the thermoelectric properties was investigated. 2. Experimental Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples
H.S. Hao et al.: J. Mater. Sci. Technol., 2011, 27(6), 525–528
Fig. 1 XRD patterns of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) ceramic pellets
were prepared by conventional solid-state reaction method. The stoichiometric mixture of Bi2 O3 , BaCO3 , Co3 O4 and CuO was heated up to 1053 K at a rate 5 K/min and held for 20 h at this temperature. Then the powder was ground, pressed uniaxially into pellets and sintered at 1053 K for 20 h again. X-ray diﬀraction (XRD) analysis was carried out with X tert Pro system using CuKα radiation. The microstructure was observed by scanning electron microscopy (SEM, JSM-5610LV, JEOL, Japan). The temperature dependence of conductivity was measured using a four-probe method with a constant dc current of 10 mA. Seebeck coeﬃcients were measured using a home-made instrument. Two Pt-Pt/Rh (Stype) thermocouples were attached to both ends of the sample using Ag paste and the Pt wires of the thermocouples used as voltage terminals. The thermoelectric voltage (ΔV ) of the sample was measured by a Keithley 2182 nanovoltmeter. A temperature gradient (ΔT ) was generated in the sample by passing cool air through a quartz tube placed near one end of the sample and the value was controlled to be 5–10 K by varying the ﬂowing rate of air. Seebeck coeﬃcient was calculated from the linear gradient of ΔV /ΔT , and was corrected by considering the thermopower of platinum. Thermal conductivity was measured using an Anter FlashLineTM 3000 thermal properties analyzer. 3. Results and Discussion Figure 1 shows the XRD patterns of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) ceramic pellets. The results conﬁrm that all samples are crystallized in a monoclinic structure and no obvious impurity phases appear. The transport properties of misﬁt cobalt oxides are highly anisotropic due to their layered structure, and therefore the texture plays a very important role in determination of their thermoelectric properties . In the present study, no textured structure is found in the as-prepared samples by comparing the XRD patterns of the bulk samples and the
Fig. 2 SEM micrographs of cross-sections of Bi2 Ba2 Co2−x Cux Oy ceramic pellets: (a) x=0.0, (b) x=0.2, (c) x=0.4
powder samples. Figure 2 gives the SEM micrographs of crosssections of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) pellets. The sheet-like grains show clearly the anisotropic growth of the crystallites resulting from the layered structure of the oxides. Moreover, the grain sizes increase with the increasing Cu concentration, which will favor the increase of electrical conductivity. The temperature dependence of electrical conductivity (σ) of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples is shown in Fig. 3. The electrical conductivity of all samples decreases with the increasing temperature, indicating metallic behavior. Moreover, it is noticed that Cu substitution increases the electrical conductivity of the samples. Two possible reasons are responsible for the increase of the electrical conductivity with increasing Cu content. On the one hand, the substitution of divalent Cu2+ for trivalent Co3+ may increase the hole concentration of the sys-
H.S. Hao et al.: J. Mater. Sci. Technol., 2011, 27(6), 525–528
Fig. 3 Temperature dependence of electrical conductivity of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples
Fig. 5 Temperature dependence of power factors of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples
Fig. 6 Temperature dependence of thermal conductivity and ZT values of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.4) samples Fig. 4 Temperature dependence of Seebeck coeﬃcients of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples
tem, leading to an increase in the conductivity. On the other hand, Cu substitution leads to an increase in the grain size, which will decrease the scattering of charge carriers between grain boundaries and thus increasing the electrical conductivity. Figure 4 gives the temperature dependence of Seebeck coeﬃcients (S) of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples. Seebeck coeﬃcients are positive for all samples, indicating p-type conductors. Moreover, Cu substitution also increases the Seebeck coeﬃcients. In general, the value of the Seebeck coeﬃcient decreases with increasing carrier density in common semiconductors. Our results indicate that the conduction mechanism in the Bi2 Ba2 Co2−x Cux Oy samples is not explained by a conventional model based on the band theory and the electron-phonon scattering. It has been reported that the electrons become strongly correlated in the misﬁt cobaltites and thus the simple band picture is not well applicable[22,23]. In the presence of a strong correlation where Coulomb interactions or spin ﬂuctuations are important, electrons tend to be localized far away from the other electrons, which results in the large Seebeck coeﬃcients in the misﬁt cobaltites. Cu doping may improve the
strength of the electron correlation in Bi2 Ba2 Co2 Oy system and thus increase Seebeck coeﬃcients of Cudoped samples. Similar results have been reported in   Cu-doped NaCo2 O4 and Ca3 Co4 O9 systems. Figure 5 gives the temperature dependence of power factors (P =S 2 σ) of Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) samples, calculated by using the data in Figs. 3 and 4. Power factors of x=0.4 samples are much larger than those of Cu-free samples due to the simultaneous increase of electrical conductivity and Seebeck coeﬃcients. Figure 6 shows the temperature dependence of the thermal conductivity (κ) and ZT values (ZT =S 2 σT /κ) of x=0.0 and 0.4 samples. The thermal conductivity can be expressed by a sum of lattice component (κl ) and electronic component (κe ) as κ = κl + κe . κe = LσT , where L is Lorenz number. Because Cu substitution increases the electrical conductivity of the samples, so κe of Cu-contained sample is expected to be larger than that of Cu-free sample. However, Fig. 6 indicates that Cu substitution has little eﬀect on the thermal conductivity of the samples. This result shows that the thermal conductivity of Bi2 Ba2 Co2 Oy system is mainly determined by the lattice component term (κl ). Due to larger electrical conductivity and Seebeck coeﬃcients, x=0.4 sample
H.S. Hao et al.: J. Mater. Sci. Technol., 2011, 27(6), 525–528
exhibits the higher ZT value which is 1.5 times as large as that of the Cu-free sample at 873 K. 4. Conclusion Bi2 Ba2 Co2−x Cux Oy (x=0.0, 0.2, 0.4) misﬁt oxides have been prepared by conventional solid-state reaction method. Cu substitution increases the grain size of the samples. The electrical conductivity and Seebeck coeﬃcients increase with the increasing Cu content, while the thermal conductivity keeps unchanged. Therefore, Cu substitution serves as an effective means to improve the thermoelectric properties of the Bi2 Ba2 Co2 Oy system.
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