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Microstructure and thermoelectric properties of Zn1−xAlxO ceramics fabricated by spark plasma sintering
Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Microstructure and thermoelectric properties of Zn1 xAlxO ceramics fabricated by spark plasma sintering N. Ma a,b, J.-F. Li a,n, B.P. Zhang b, Y.H. Lin a, L.R. Ren c, G.F. Chen c a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China c Corporate Technology, Siemens Ltd., China, 7, Wangjing Zhonghuan Nanlu, Beijing 100102, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 12 September 2009 Received in revised form 19 May 2010 Accepted 8 June 2010
Al-doped ZnO powders were synthesized via solid reaction between Zn(OH)2 and Al(OH)3 and consolidated by spark plasma sintering (SPS) to fabricate fine-grained Zn1 xAlxO ceramics as a thermoelectric material. X-ray diffraction and spectrophotometer experiments revealed that Al doping into ZnO is enhanced by the present process, and consequently the SPS-processed Zn1 xAlxO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al2O3 oxide powders. Because of the combined effect of Al doping and grain refinement, the present Zn1 xAlxO ceramics show much lower thermal conductivity, which also results in an enhanced dimensionless figure of merit (ZT), than un-doped ZnO oxides prepared also by SPS. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Oxides D. Electrical conductivity D. Thermal conductivity
1. Introduction Thermoelectric technology has received increasing attention mainly because of its potential applications to energy recovery from waste heat [1]. Recent researches have made significant progress in the development of high-performance thermoelectric materials, but most are metallic or semi-metallic compounds like Bi2Te3, PbTe and SiGe [2]. Certainly, the research mainstream is exploring thermoelectric materials with enhanced ZT ( ¼S2Ts/k) values, which requires a material with a high Seebeck coefficient (S) and electrical conductivity (s) but low thermal conductivity (k), and high ZT values are reported in filled CoSb3-based Skutterudite [3] and AgPbmSbTem + 2 compounds [4–6]. However, some efforts have been also devoted to a search for thermoelectric oxides, which have more advantages for the energy recovery applications, especially at elevated temperatures [7–9]. Zinc oxide is a well-known oxide with many functionalities, which has a wide forbidden band gap of 3.3 eV with a wurtzite structure. ZnO has received attention as a thermoelectric material because of its high carrier mobility and Seebeck coefficient. Particularly, Al-doped ZnO oxides have been considerably studied as a high-temperature thermoelectric material for its high power factor (S2s). However, the solid solubility of Al into ZnO is very limited, and it is often experienced that Al-doped ZnO solid
n
Corresponding author. Tel.: +86 10 62784845; fax: + 86 10 62771160. E-mail address: [email protected] (J.-F. Li).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.06.006
solutions can be hardly obtained if one uses Al2O3 and ZnO oxides particles as starting materials [10–15]. In this study, zinc and aluminum hydroxide (Zn(OH)2 and Al(OH)3) powders were used as raw materials in order to enhance the reactivity and solubility of Al2O3 into ZnO. In addition, the resultant calcined oxide powders were sintered by using a spark plasma sintering (SPS) process, which is an effective method to fabricate fine-grained thermoelectric materials with reduced thermal conductivity [5,6,16,17]. Consequently, dense Al-doped ZnO bulk samples with an average grain size 300 nm were obtained. Their electrical and thermal conductivities as well as thermopower were investigated with a special emphasis on the effects of the content of Al. It was found that enhanced figure of merit, ZT, can be obtained in the Al-doped ZnO oxides by using zinc and aluminum hydroxides as starting materials and the SPS process as the densification method.
2. Experimental Zn1 xAlxO (0 rxr0.05) bulk samples were fabricated by ball milling and a spark plasma sintering (SPS) method using two kinds of commercially available raw materials including hydroxide and oxide, respectively. The hydroxide is Zn(OH)2 (purityZ99%) and Al(OH)3 (purity 499%), the oxide is ZnO (purity499%) and Al2O3 (purity4 99%). The raw powders with nominal compositions of Zn1 xAlxO (0 rxr0.05) were mixed in de-ionized water by a planetary mill (QM-2, Nanjing, China) using
N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
zirconium oxide balls at 160 rpm for 6 h. The resultant slurries were dried at 343 K in an oven for 12 h. The dried mixture was calcined in an aluminum oxide crucible at 773 K for 2 h and then sintered in a ^20 mm graphite mould under an axial compressive stress of 50 MPa in vacuum using a SPS system (Sumitomo SPS1050, Japan). The ramp-up speed was 100 K/min and the holding time was 5 min, while the holding temperature is 1173 K for Zn1 xAlxO (0 rxr0.05) samples. The resultant disk-shaped samples were ^20 mm in diameter and about 4 mm in thickness. The density of the as-sintered Zn1 xAlxO samples was measured by the Archimedes method. Phase structure was identified by X-ray diffraction with a Cu Ka radiation (l ¼ 1.5416 ˚ A) filtered through a Ni foil (Bruker D8, Germany). The morphologies of the fractured surface were observed by a scanning electron microscope (SEM, JSM-6460LV). The microstructure was examined by using a transmission electron microscope (TEM, JEM-2011), which was operated at an accelerating voltage of 200 kV and equipped with an X-ray energy dispersive spectroscope (EDS). UV–vis diffuse reflectance spectra (DRS) were measured by a UV–vis spectrophotometer (Hitachi UV-3010) with an integrated sphere attachment. The thermoelectric properties of the samples with the dimension of 17 mm 3.2 mm 3.2 mm were evaluated along the sample’s section perpendicular to the pressing direction of SPS. The Seebeck coefficient (S) and electrical conductivity (s) were measured at 323–673 K in a helium atmosphere using a Seebeck coefficient/electric resistance measuring system (ZEM-2, UlvacRiko, Japan). The thermal conductivity (k) was calculated from the product of thermal diffusivity (D), specific heat (Cp) and density (d), k¼DCpd. The thermal diffusivity was measured from room temperature to 673 K by the laser flash method using an apparatus (TC-9000, Ulvac-Riko, Japan). The specific heat was measured from room temperature to 673 K using an apparatus (DSC-60, SHIMADZU, Japan).
3. Results and discussion Fig. 1 shows the XRD patterns of the Zn0.95Al0.05O and ZnO bulk samples that were SPSed at 1173 K for 5 min. All diffraction peaks can be assigned to those of ZnO (PDF#36-1451) with a wurtzite structure. The previous studies [10,12] have reported that ZnAl2O4 as a secondary phase often appeared in the Al-doped ZnO bulks due to its low solid solubility with Al2O3. As shown in Fig. 1, on
Fig. 1. XRD patterns of (a) ZnO and (b) Zn0.95Al0.05O samples prepared by SPS.
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enlarging the corresponding diffraction peaks, the existence of ZnAl2O4 can be confirmed, but its amount is greatly reduced since hydroxides were used as the starting materials. Therefore, it can be said that the phase structures of the resultant bulk samples are greatly dependent on the starting materials, and at least the amount of secondary phase can be greatly reduced if zinc and aluminum hydroxides are used instead of Al2O3 and ZnO oxides. To confirm if the solid solutions are formed in the Zn1 xAlxO samples derived from the reaction between zinc and aluminum hydroxides, the following experiments will be focused on the microstructure and composition analysis by TEM and the attached EDX. Fig. 2(a–d) shows the SEM micrographs of the fractured surface for the SPSed pure ZnO and the Zn1 xAlxO (x¼0.02, 0.03, 0.04) bulk samples. It is obvious that highly dense samples were obtained, especially for the pure ZnO and the x ¼0.02 sample. The relative densities were measured to be 99.9%, 99.0%, 91.7% and 88.8% for the pure ZnO and the Zn1 xAlxO (x¼0.02, 0.03, 0.04) bulk samples, respectively. It is easy to understand to accept the fact that the sintering densification becomes increasingly difficult with increase in Al2O3 addition, if one considers the difference of physical properties of Al2O3 and ZnO. In addition, it is apparent that the addition of Al2O3 to ZnO significantly suppresses the grain growth, resulting in a microstructure with much finer grains when x Z0.03. This is because the added Al2O3 suppresses the grain-boundary mobility, thus enabling the pores to stay attached to the moving grain boundaries during sintering [13]. Nevertheless, porous materials with fine grains may be good as thermoelectric materials because its thermal conductivity can be effectively reduced. Of course, the electrical and thermal properties of a sintered body are also affected by the density of specimens, but its influence due to the above small density difference is quite limited as compared with the Al-doping effects, as shown later. TEM experiments were conducted to check the possible existence of secondary phase using the Zn0.96Al0.04O sample derived from hydroxides, and the results are shown in Fig. 3(a–d). It is clearly seen in Fig. 3(a) that well-developed angular grains with an average size of about 300 nm are obtained, which agrees with the SEM observation of the fractured surface of samples. In this sample, a little nano-cluster was found besides ZnO phase, and the EDX analysis (Fig. 3(d)) of this nano-cluster exhibits a very strong peak of Al, besides the peaks of Zn and O. By further analysis of the diffraction pattern this phase is verified as a spinal structure, which was also found by other researchers [10,12,15]. It was reported that extra Al cannot fully dissolve into the ZnO structure when the doping amount was beyond 2 mol% because of the limited solubility [12,18] and therefore excessive Al reacts with ZnO to form the ZnAl2O4 phase. Although the XRD patterns of all samples show ZnO single phase, the above TEM observation confirmed the existence of secondary phase. In general, the existence of secondary phase may not be favorable for the electrical conduction, but can reduce the thermal conductivity to enhance the figure of merit [12]. Fig. 4 shows the UV–vis absorption spectra of SPSed ZnO and Zn0.96Al0.04O ceramics using hydroxides as raw materials. It can be seen that the photoabsorption edge of the Zn0.96Al0.04O bulk exhibits a red shift compared with the non-doped sample. The corresponding values of direct band gaps of the as-prepared samples can be evaluated by extrapolating the linear portion of (ahv)2 vs. (hv) [19]. The values of the band gaps for the ZnO and Zn0.96Al0.04O bulks were determined to be 3.23 and 3.07 eV, respectively. The variation in the band gap after Al doping definitely proves that the Zn ions are at least partly substituted by the Al ions. Fig. 5 shows the temperature dependence of the thermoelectric properties for the Zn1 xAlxO bulk samples using
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N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
Fig. 2. SEM micrographs of the fractured surfaces of ZnO and Zn1 xAlxO samples. (a) ZnO, (b) Zn0.98Al0.02O, (c) Zn0.97Al0.03O, (d) Zn0.96Al0.04O.
Zn(OH)2 and Al(OH)3 as raw materials (solid line) and the Zn0.96Al0.04O (O-ZA04) counterpart using pure oxide as raw materials (dot line). The pure ZnO sample that was sintered under the same condition with Al-doped ones exhibits high electrical resistivity, thus no thermoelectric characterization could be performed on it, so the values of TE properties for pure ZnO are cited from the literature (Ref. [10]). As shown in Fig. 5(a), the electrical conductivity, s, of the pure ZnO samples slightly increased with increase in temperature, being indicative of semiconducting behavior. Compared with the pure ZnO, the electrical conductivities of the Zn0.98Al0.02O and O-ZA04 samples increased a little and show a slight change with measuring temperature. With the addition of 3 mol% Al2O3, the behavior of electrical conductivity changed from semiconducting to metallic, and the values at room temperature became higher by 43 orders of magnitude than that of pure ZnO. However, the electrical conductivity of the Zn1 xAlxO (x 40.2) samples shows a strong temperature-dependent property (above 350 S cm 1 at 673 K). The electrical conductivity values of Zn1 xAlxO showed a monotonic increase with increase in Al2O3 content up to x¼ 0.04, and then decreased slightly with further Al-doping, as seen for x ¼0.05. Consequently, the composition Zn0.96Al0.04O shows the highest s value of all the samples. As shown above by the SEM observation, the relative density decreased with increase in Al content when x r0.03, and then increased as x is up to 0.05. It is well known that decrease in relative density results in a decrease in electrical conductivity. However, the electrical conductivity shows an opposite change with increase in relative density. This result confirms again that the Zn2 + ions were at least partly substituted by Al3 + in the Zn1 xAlxO samples, although the existence of second phase was detected by TEM and the amount of Zn2 + substituted by Al3 + is not clear. The substitution of Zn2 + by Al3 + may increase the electron concentration to compensate for the electric charge balance, and thus the electrical conductivity increases. The defect reaction can be represented
by the following equation [15]: ZnO
Al2 O3 !2AlZn þ 2e þ2Oxo þ d
1 O2 m 2
ð1Þ
This means that the substitution of a Zn2 + ion by an Al3 + ion will produce an electron, which can increase carrier (electron) concentration for n-type materials. From the above analysis, we know that an increase in electrical conductivity by means of Al2O3 addition is due to the increased electron concentration of the samples, originating from the generation of extra electrons. Fig. 5(b) shows the Seebeck coefficient as a function of temperature for Zn1 xAlxO (0rx r0.05) samples. The negative values of Seebeck coefficient indicate that all the samples are n-type semiconductor and the major carrier is electron. The inset of Fig. 5(b), whose units of the two coordinate axes are the same as Fig. 5(b), shows the Seebeck coefficients of pure ZnO bulk sample as a function of temperature. The pure ZnO sample presented a high Seebeck coefficient of about 330 mV/K at 473 K, which was 3–4 times of the Al2O3-added samples, e.g. 80 mV/K at 473 K for 2 mol% Al-doped sample. All the samples show a nearly linear dependence of S(T), and the absolute values of Seebeck coefficient for Al-doped ZnO samples decreases with increase in doping content of Al. As discussed above, the carrier concentration increases due to Al doping. According to the Ioffe theory, the Seebeck coefficient is inversely proportional to the carrier concentration [20]. Therefore, the decreased Seebeck coefficient for Zn1 xAlxO (x 40.02) samples may be ascribed to the increased carrier concentration. These results quantitatively agreed with increase in electrical conductivity. The absolute values of the Seebeck coefficient of the O-ZA04 sample are much higher than other samples, indicating that its carrier concentrations are low. Fig. 5(c) shows the power factor as a function of temperature for Zn1 xAlxO (0rxr0.05) samples. We calculated the power factor values of all the samples according to the values of electrical
N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
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1100 1000
Al
900
O
800 700
Zn
600
Zn
500 400 300 200
Zn
Zn
100
0 .0
00
10
00
9.
00
8.
00
7.
6.
00
00
5.
4.
00
00
3.
2.
00
1.
0.
00
0
heV Fig. 3. TEM micrographs of Zn0.96Al0.04O sample (a–c) and the corresponding EDX spectrum (d).
resistivity and Seebeck coefficient. The values of power factors of the samples, which used hydroxides as the raw materials increase dramatically with the addition of Al2O3, and monotonically increase with temperature and still show the increasing trend. And as T reached 673 K, the power factors of the Zn1 xAlxO (x¼0.03, 0.04, 0.05) samples were almost the same and higher than that of Zn0.98Al0.02O sample. In particular, the power factors of Zn0.96Al0.04O are higher than other samples and reach its maximum of 3.3 10 4 Wm 1 K 2 at 673 K. This value is higher than that (2.6 10 4 Wm 1 K 2) reported by Cai et al. [15], but lower than that (15 10 4 Wm 1 K 2) obtained by Ohtaki et al. [10]. Fig. 6 (a) shows the thermal diffusivity and thermal conductivity of Zn1 xAlxO (x ¼0.03, 0.04). It is clear that both present Zn1 xAlxO (x ¼0.03, 0.04) samples show very low thermal conductivity. It is well known that the thermal conductivity can be varied greatly by extrinsic factor such as microstructure and impurities. The overall k value of a solid is given as k ¼ kph þ ke Fig. 4. UV–vis absorption spectra of SPSed ZnO (solid line) and Zn0.96Al0.04O samples derived from hydroxides as raw materials (red dotted line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ð2Þ
where kph and ke are the phonon and electric thermal conductivities, respectively. As shown in Fig. 6 (b), it is confirmed that the phonon thermal conductivity, kph, has a
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N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
Fig. 6. (a) Thermal diffusivity and thermal conductivity of the Zn1 xAlxO (x ¼0.03, 0.04) Zn1 xAlxO ceramics derived from hydroxides as raw materials. Note that data for ZnO cited from Ref. [10] are included for comparison. (b) Phonon thermal diffusivity (kph) and electrical thermal diffusivity (ke) of the Zn1 xAlxO (x¼ 0.03, 0.04) ceramics.
Fig. 5. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient and (c) power factor for the Zn1 xAlxO ceramics derived from hydroxides as raw materials (solid line). Note that Zn0.96Al0.04O was also prepared using oxides as raw materials (dot line) and the data for ZnO are cited from Ref. [10].
dominate proportion relative to the electron thermal conductivity ke, which is estimated by the Wiedeman–Franz relation ke ¼LsT (L is the Lorenz number). The reported thermal conductivity value of pure ZnO is about 40 W m 1 K 1 at room temperature and decreased to 18 W m 1 K 1 at 673 K [11]. The thermal conductivity also decreased with increase in amount of Al2O3, because the Al doping and refined grains enhance the phonon scattering. The values of thermal conductivity, k, decrease as the temperature increases, and the k of Zn0.96Al0.04O is lower than that of Zn0.97Al0.03O, also because the relative density of the former one (88.8%) is lower than that of the latter one (91.7%). The
Fig. 7. Dimensionless figure of merit (ZT) of the Zn1 xAlxO (x ¼0.03, 0.04) ceramics derived from hydroxides as raw materials. Note that data for ZnO cited from Ref. [10] are included for comparison.
thermal conductivity values of the present Zn1 xAlxO (x ¼0.03, 0.04) bulks in the whole temperature range are also smaller than that of Ga-doped ZnO thermoelectric ceramic synthesized by mechanically alloying and hot pressing [21]. The temperature dependence of the dimensionless figure of merit, ZT, is shown in Fig. 7. Because of the combined effects of
N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
high power factor and reduced thermal conductivity, the Al-doped ZnO samples show considerably larger ZT values than pure ZnO. In the present temperature range investigated, the ZT value reached 0.085 at 673 K in the Zn0.96Al0.04O sample, which show a trend to further increase at higher temperatures. Higher ZT values were obtained in Zn0.96Al0.04O than in Zn0.96Al0.03O because the former one has a higher power factor in addition to reduced thermal conductivity. This result indicates a possibility to further improve the thermoelectric performance of Al-doped ZnO ceramics by enhancing the Al-doping in ZnO through processing modification.
4. Conclusions Fine-grained Al-doped ZnO thermoelectric bulks were prepared by a spark plasma sintering process using the powders derived from zinc and aluminum hydroxides. Although the XRD patterns of Zn1 xAlxO (xr0.05) samples show a single ZnO phase, TEM observation combined with EDX analysis proved the existence of ZnAl2O4 as a second phase. Nevertheless, ZnO ceramics with enhanced Al doping were prepared by using Zn(OH)2 and Al(OH)3 hydroxides as starting materials instead of ZnO and Al2O3 oxides. Therefore, the resultant Zn1 xAlxO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al2O3 oxide powders. Moreover, because of the reduced grain sizes in addition to the enhanced Al doping, the Zn1 xAlxO ceramics show low thermal conductivity. As a consequence, the resultant Al-doped ZnO ceramics with a nominal addition of 4% Al show the highest dimensionless figure of merit (ZT) of 0.085 at 673 K, which is about 17 times higher than that of pure ZnO.
Acknowledgements This work was conducted in Tsinghua University with financial supports from the National Basic Research Program of China (Grant no. 2007CB607500), High-Tech 863 Program of China (Grant no. 2009AA03Z216) and Tsinghua-Siemens collaborative research as well as National Nature Science Foundation (Grants no. 50820145203). References [1] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 21 (2008) 1457–1461. [2] G. Chen, M.S. Dresselhaus, G. Dresselhaus, J.-P. Fleurial, T. Caillat, Recent developments in thermoelectric materials, Int. Mater. Rev. 48 (2003) 45–66.
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[3] B.C. Sales, D. Mandrus, R.K. Williams, Filled skutterudite antimonides: a new class of thermoelectric materials, Science 272 (1996) 1325–1328. [4] K.F. Hsu, S. Loo, F. Guo, W. Chen, J.S. Dyck, C. Uher, T. Hogan, E.K. Polychroniadis, M.G. Kanatzidis, Cubic AgPbmSbTe2 + m: bulk thermoelectric materials with high figure of merit, Science 303 (2004) 818–821. [5] H. Wang, J.F. Li, C.W. Nan, M. Zhou, W.S. Liu, B.P. Zhang, T. Kita, Highperformance Ag0.8Pb18 + xSbTe20 thermoelectric bulk materials fabricated by mechanical alloying and spark plasma sintering, Appl. Phys. Lett. 88 (2006) 092104. [6] M. Zhou, J.F. Li, T. Kita, Nanostructured AgPbmSbTem + 2 system bulk materials with enhanced thermoelectric performance, J. Am. Chem. Soc. 130 (2008) 4527–4532. [7] H. Ohta, S. Kim, Y. Mune, T. Mizoguchi, K. Nomura, S. Ohta, T. Nomura, Y. Nakanishi, Y. Ikuhara, M. Hirano, H. Hosono, K. Koumoto, Giant thermoelectric Seebeck coefficient of a two-dimensional electron gas in SrTiO3, Nat. Mater. 6 (2007) 129–134. [8] I. Terasaki, Y. Sasago, K. Uchinokura, Large thermoelectric power in NaCo2O4 single crystals, Phys. Rev. B 56 (1997) R12685–R12687. [9] H.Q. Liu, Y. Song, S.N. Zhang, X.B. Zhao, F.P. Wang, Thermoelectric properties of Ca3 xYxCo4O9 + d, J. Phys. Chem. Solids 70 (2009) 600–603. [10] M. Ohtaki, T. Tsubota, K. Eguchi, H. Arai, High-temperature thermoelectric properties of (Zn1 xAlx)O, J. Appl. Phys. 79 (1996) 1816–1818. [11] T. Tsubota, M. Ohtaki, K. Eguchi, H. Arai, Thermoelectric properties of Al-doped ZnO as a promising oxide material for high temperature thermoelectric conversion, J. Mater. Chem. 7 (1997) 85–90. [12] Y. Takagi, M. Ito, K. Majima, H. Nagai, S. Katsuyama, Thermoelectric properties of (Zn1 yMgy)1 xAlxO ceramics prepared by the polymerized complex method, J. Appl. Phys. 92 (2002) 1391–1398. [13] K. Park, K.Y. Ko, W.S. Seo, W.S. Cho, J.G. Kimd, J.Y. Kime, High-temperature thermoelectric properties of polycrystalline Zn1 x yAlxTiyO ceramics, J. Eur. Ceram. Soc. 27 (2007) 813–817. [14] K. Park, K.K. Kim, J.K. Seong, S.J. Kim, J.G. Kim, W.S. Cho, S. Nahm, Improved thermoelectric properties by adding Al for Zn in (ZnO)mIn2O3, Mater. Lett. 61 (2007) 4759–4762. ¨ [15] K.F. Cai, E. Muller, C. Draˇsar, A. Mrotzek, Preparation and thermoelectric properties of Al-doped ZnO ceramics, Mater. Sci. Eng. B 104 (2003) 45–48. [16] H. Wang, J.F. Li, T. Kita, Thermoelectric enhancement at low temperature in nonstoichiometic lead-telluride compounds, J. Phys. D: Appl. Phys. 40 (2007) 6839–6845. [17] W.S. Liu, B.P. Zhang, L.D. Zhao, H.L. Zhang, J.F. Li, Improvement of thermoelectric performance of CoSb3 xTex skutterudite compounds by additional substitution of IVB-group elements for Sb, Chem. Mater. 20 (2008) 7526–7531. [18] K.H. Kim, S.H. Shim, K.B. Shim, Microstructural and thermoelectric characteristics of zinc oxide-based thermoelectric materials fabricated using a spark plasma sintering process, J. Am. Ceram. Soc. 88 (2005) 628–632. [19] M.A. Butler, Photoelectrolysis and physical properties of the semiconducting electrode WO2, J. Appl. Phys. 48 (1977) 1914–1920. [20] A.F. Ioffe, in: Semiconductor Thermoelements and Thermoelectric Cooling, Infosearch Ltd. Press, London, 1957. [21] B.A. Cook, J.L. Harringa, C.B. Vining, Electrical properties of Ga and ZnS doped ZnO prepared by mechanical alloying, J. Appl. Phys. 83 (1998) 5858–5861.
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Microstructure and thermoelectric properties of Zn1 xAlxO ceramics fabricated by spark plasma sintering N. Ma a,b, J.-F. Li a,n, B.P. Zhang b, Y.H. Lin a, L.R. Ren c, G.F. Chen c a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China c Corporate Technology, Siemens Ltd., China, 7, Wangjing Zhonghuan Nanlu, Beijing 100102, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 12 September 2009 Received in revised form 19 May 2010 Accepted 8 June 2010
Al-doped ZnO powders were synthesized via solid reaction between Zn(OH)2 and Al(OH)3 and consolidated by spark plasma sintering (SPS) to fabricate fine-grained Zn1 xAlxO ceramics as a thermoelectric material. X-ray diffraction and spectrophotometer experiments revealed that Al doping into ZnO is enhanced by the present process, and consequently the SPS-processed Zn1 xAlxO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al2O3 oxide powders. Because of the combined effect of Al doping and grain refinement, the present Zn1 xAlxO ceramics show much lower thermal conductivity, which also results in an enhanced dimensionless figure of merit (ZT), than un-doped ZnO oxides prepared also by SPS. & 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics A. Oxides D. Electrical conductivity D. Thermal conductivity
1. Introduction Thermoelectric technology has received increasing attention mainly because of its potential applications to energy recovery from waste heat [1]. Recent researches have made significant progress in the development of high-performance thermoelectric materials, but most are metallic or semi-metallic compounds like Bi2Te3, PbTe and SiGe [2]. Certainly, the research mainstream is exploring thermoelectric materials with enhanced ZT ( ¼S2Ts/k) values, which requires a material with a high Seebeck coefficient (S) and electrical conductivity (s) but low thermal conductivity (k), and high ZT values are reported in filled CoSb3-based Skutterudite [3] and AgPbmSbTem + 2 compounds [4–6]. However, some efforts have been also devoted to a search for thermoelectric oxides, which have more advantages for the energy recovery applications, especially at elevated temperatures [7–9]. Zinc oxide is a well-known oxide with many functionalities, which has a wide forbidden band gap of 3.3 eV with a wurtzite structure. ZnO has received attention as a thermoelectric material because of its high carrier mobility and Seebeck coefficient. Particularly, Al-doped ZnO oxides have been considerably studied as a high-temperature thermoelectric material for its high power factor (S2s). However, the solid solubility of Al into ZnO is very limited, and it is often experienced that Al-doped ZnO solid
n
Corresponding author. Tel.: +86 10 62784845; fax: + 86 10 62771160. E-mail address: [email protected] (J.-F. Li).
0022-3697/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2010.06.006
solutions can be hardly obtained if one uses Al2O3 and ZnO oxides particles as starting materials [10–15]. In this study, zinc and aluminum hydroxide (Zn(OH)2 and Al(OH)3) powders were used as raw materials in order to enhance the reactivity and solubility of Al2O3 into ZnO. In addition, the resultant calcined oxide powders were sintered by using a spark plasma sintering (SPS) process, which is an effective method to fabricate fine-grained thermoelectric materials with reduced thermal conductivity [5,6,16,17]. Consequently, dense Al-doped ZnO bulk samples with an average grain size 300 nm were obtained. Their electrical and thermal conductivities as well as thermopower were investigated with a special emphasis on the effects of the content of Al. It was found that enhanced figure of merit, ZT, can be obtained in the Al-doped ZnO oxides by using zinc and aluminum hydroxides as starting materials and the SPS process as the densification method.
2. Experimental Zn1 xAlxO (0 rxr0.05) bulk samples were fabricated by ball milling and a spark plasma sintering (SPS) method using two kinds of commercially available raw materials including hydroxide and oxide, respectively. The hydroxide is Zn(OH)2 (purityZ99%) and Al(OH)3 (purity 499%), the oxide is ZnO (purity499%) and Al2O3 (purity4 99%). The raw powders with nominal compositions of Zn1 xAlxO (0 rxr0.05) were mixed in de-ionized water by a planetary mill (QM-2, Nanjing, China) using
N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
zirconium oxide balls at 160 rpm for 6 h. The resultant slurries were dried at 343 K in an oven for 12 h. The dried mixture was calcined in an aluminum oxide crucible at 773 K for 2 h and then sintered in a ^20 mm graphite mould under an axial compressive stress of 50 MPa in vacuum using a SPS system (Sumitomo SPS1050, Japan). The ramp-up speed was 100 K/min and the holding time was 5 min, while the holding temperature is 1173 K for Zn1 xAlxO (0 rxr0.05) samples. The resultant disk-shaped samples were ^20 mm in diameter and about 4 mm in thickness. The density of the as-sintered Zn1 xAlxO samples was measured by the Archimedes method. Phase structure was identified by X-ray diffraction with a Cu Ka radiation (l ¼ 1.5416 ˚ A) filtered through a Ni foil (Bruker D8, Germany). The morphologies of the fractured surface were observed by a scanning electron microscope (SEM, JSM-6460LV). The microstructure was examined by using a transmission electron microscope (TEM, JEM-2011), which was operated at an accelerating voltage of 200 kV and equipped with an X-ray energy dispersive spectroscope (EDS). UV–vis diffuse reflectance spectra (DRS) were measured by a UV–vis spectrophotometer (Hitachi UV-3010) with an integrated sphere attachment. The thermoelectric properties of the samples with the dimension of 17 mm 3.2 mm 3.2 mm were evaluated along the sample’s section perpendicular to the pressing direction of SPS. The Seebeck coefficient (S) and electrical conductivity (s) were measured at 323–673 K in a helium atmosphere using a Seebeck coefficient/electric resistance measuring system (ZEM-2, UlvacRiko, Japan). The thermal conductivity (k) was calculated from the product of thermal diffusivity (D), specific heat (Cp) and density (d), k¼DCpd. The thermal diffusivity was measured from room temperature to 673 K by the laser flash method using an apparatus (TC-9000, Ulvac-Riko, Japan). The specific heat was measured from room temperature to 673 K using an apparatus (DSC-60, SHIMADZU, Japan).
3. Results and discussion Fig. 1 shows the XRD patterns of the Zn0.95Al0.05O and ZnO bulk samples that were SPSed at 1173 K for 5 min. All diffraction peaks can be assigned to those of ZnO (PDF#36-1451) with a wurtzite structure. The previous studies [10,12] have reported that ZnAl2O4 as a secondary phase often appeared in the Al-doped ZnO bulks due to its low solid solubility with Al2O3. As shown in Fig. 1, on
Fig. 1. XRD patterns of (a) ZnO and (b) Zn0.95Al0.05O samples prepared by SPS.
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enlarging the corresponding diffraction peaks, the existence of ZnAl2O4 can be confirmed, but its amount is greatly reduced since hydroxides were used as the starting materials. Therefore, it can be said that the phase structures of the resultant bulk samples are greatly dependent on the starting materials, and at least the amount of secondary phase can be greatly reduced if zinc and aluminum hydroxides are used instead of Al2O3 and ZnO oxides. To confirm if the solid solutions are formed in the Zn1 xAlxO samples derived from the reaction between zinc and aluminum hydroxides, the following experiments will be focused on the microstructure and composition analysis by TEM and the attached EDX. Fig. 2(a–d) shows the SEM micrographs of the fractured surface for the SPSed pure ZnO and the Zn1 xAlxO (x¼0.02, 0.03, 0.04) bulk samples. It is obvious that highly dense samples were obtained, especially for the pure ZnO and the x ¼0.02 sample. The relative densities were measured to be 99.9%, 99.0%, 91.7% and 88.8% for the pure ZnO and the Zn1 xAlxO (x¼0.02, 0.03, 0.04) bulk samples, respectively. It is easy to understand to accept the fact that the sintering densification becomes increasingly difficult with increase in Al2O3 addition, if one considers the difference of physical properties of Al2O3 and ZnO. In addition, it is apparent that the addition of Al2O3 to ZnO significantly suppresses the grain growth, resulting in a microstructure with much finer grains when x Z0.03. This is because the added Al2O3 suppresses the grain-boundary mobility, thus enabling the pores to stay attached to the moving grain boundaries during sintering [13]. Nevertheless, porous materials with fine grains may be good as thermoelectric materials because its thermal conductivity can be effectively reduced. Of course, the electrical and thermal properties of a sintered body are also affected by the density of specimens, but its influence due to the above small density difference is quite limited as compared with the Al-doping effects, as shown later. TEM experiments were conducted to check the possible existence of secondary phase using the Zn0.96Al0.04O sample derived from hydroxides, and the results are shown in Fig. 3(a–d). It is clearly seen in Fig. 3(a) that well-developed angular grains with an average size of about 300 nm are obtained, which agrees with the SEM observation of the fractured surface of samples. In this sample, a little nano-cluster was found besides ZnO phase, and the EDX analysis (Fig. 3(d)) of this nano-cluster exhibits a very strong peak of Al, besides the peaks of Zn and O. By further analysis of the diffraction pattern this phase is verified as a spinal structure, which was also found by other researchers [10,12,15]. It was reported that extra Al cannot fully dissolve into the ZnO structure when the doping amount was beyond 2 mol% because of the limited solubility [12,18] and therefore excessive Al reacts with ZnO to form the ZnAl2O4 phase. Although the XRD patterns of all samples show ZnO single phase, the above TEM observation confirmed the existence of secondary phase. In general, the existence of secondary phase may not be favorable for the electrical conduction, but can reduce the thermal conductivity to enhance the figure of merit [12]. Fig. 4 shows the UV–vis absorption spectra of SPSed ZnO and Zn0.96Al0.04O ceramics using hydroxides as raw materials. It can be seen that the photoabsorption edge of the Zn0.96Al0.04O bulk exhibits a red shift compared with the non-doped sample. The corresponding values of direct band gaps of the as-prepared samples can be evaluated by extrapolating the linear portion of (ahv)2 vs. (hv) [19]. The values of the band gaps for the ZnO and Zn0.96Al0.04O bulks were determined to be 3.23 and 3.07 eV, respectively. The variation in the band gap after Al doping definitely proves that the Zn ions are at least partly substituted by the Al ions. Fig. 5 shows the temperature dependence of the thermoelectric properties for the Zn1 xAlxO bulk samples using
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Fig. 2. SEM micrographs of the fractured surfaces of ZnO and Zn1 xAlxO samples. (a) ZnO, (b) Zn0.98Al0.02O, (c) Zn0.97Al0.03O, (d) Zn0.96Al0.04O.
Zn(OH)2 and Al(OH)3 as raw materials (solid line) and the Zn0.96Al0.04O (O-ZA04) counterpart using pure oxide as raw materials (dot line). The pure ZnO sample that was sintered under the same condition with Al-doped ones exhibits high electrical resistivity, thus no thermoelectric characterization could be performed on it, so the values of TE properties for pure ZnO are cited from the literature (Ref. [10]). As shown in Fig. 5(a), the electrical conductivity, s, of the pure ZnO samples slightly increased with increase in temperature, being indicative of semiconducting behavior. Compared with the pure ZnO, the electrical conductivities of the Zn0.98Al0.02O and O-ZA04 samples increased a little and show a slight change with measuring temperature. With the addition of 3 mol% Al2O3, the behavior of electrical conductivity changed from semiconducting to metallic, and the values at room temperature became higher by 43 orders of magnitude than that of pure ZnO. However, the electrical conductivity of the Zn1 xAlxO (x 40.2) samples shows a strong temperature-dependent property (above 350 S cm 1 at 673 K). The electrical conductivity values of Zn1 xAlxO showed a monotonic increase with increase in Al2O3 content up to x¼ 0.04, and then decreased slightly with further Al-doping, as seen for x ¼0.05. Consequently, the composition Zn0.96Al0.04O shows the highest s value of all the samples. As shown above by the SEM observation, the relative density decreased with increase in Al content when x r0.03, and then increased as x is up to 0.05. It is well known that decrease in relative density results in a decrease in electrical conductivity. However, the electrical conductivity shows an opposite change with increase in relative density. This result confirms again that the Zn2 + ions were at least partly substituted by Al3 + in the Zn1 xAlxO samples, although the existence of second phase was detected by TEM and the amount of Zn2 + substituted by Al3 + is not clear. The substitution of Zn2 + by Al3 + may increase the electron concentration to compensate for the electric charge balance, and thus the electrical conductivity increases. The defect reaction can be represented
by the following equation [15]: ZnO
Al2 O3 !2AlZn þ 2e þ2Oxo þ d
1 O2 m 2
ð1Þ
This means that the substitution of a Zn2 + ion by an Al3 + ion will produce an electron, which can increase carrier (electron) concentration for n-type materials. From the above analysis, we know that an increase in electrical conductivity by means of Al2O3 addition is due to the increased electron concentration of the samples, originating from the generation of extra electrons. Fig. 5(b) shows the Seebeck coefficient as a function of temperature for Zn1 xAlxO (0rx r0.05) samples. The negative values of Seebeck coefficient indicate that all the samples are n-type semiconductor and the major carrier is electron. The inset of Fig. 5(b), whose units of the two coordinate axes are the same as Fig. 5(b), shows the Seebeck coefficients of pure ZnO bulk sample as a function of temperature. The pure ZnO sample presented a high Seebeck coefficient of about 330 mV/K at 473 K, which was 3–4 times of the Al2O3-added samples, e.g. 80 mV/K at 473 K for 2 mol% Al-doped sample. All the samples show a nearly linear dependence of S(T), and the absolute values of Seebeck coefficient for Al-doped ZnO samples decreases with increase in doping content of Al. As discussed above, the carrier concentration increases due to Al doping. According to the Ioffe theory, the Seebeck coefficient is inversely proportional to the carrier concentration [20]. Therefore, the decreased Seebeck coefficient for Zn1 xAlxO (x 40.02) samples may be ascribed to the increased carrier concentration. These results quantitatively agreed with increase in electrical conductivity. The absolute values of the Seebeck coefficient of the O-ZA04 sample are much higher than other samples, indicating that its carrier concentrations are low. Fig. 5(c) shows the power factor as a function of temperature for Zn1 xAlxO (0rxr0.05) samples. We calculated the power factor values of all the samples according to the values of electrical
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1100 1000
Al
900
O
800 700
Zn
600
Zn
500 400 300 200
Zn
Zn
100
0 .0
00
10
00
9.
00
8.
00
7.
6.
00
00
5.
4.
00
00
3.
2.
00
1.
0.
00
0
heV Fig. 3. TEM micrographs of Zn0.96Al0.04O sample (a–c) and the corresponding EDX spectrum (d).
resistivity and Seebeck coefficient. The values of power factors of the samples, which used hydroxides as the raw materials increase dramatically with the addition of Al2O3, and monotonically increase with temperature and still show the increasing trend. And as T reached 673 K, the power factors of the Zn1 xAlxO (x¼0.03, 0.04, 0.05) samples were almost the same and higher than that of Zn0.98Al0.02O sample. In particular, the power factors of Zn0.96Al0.04O are higher than other samples and reach its maximum of 3.3 10 4 Wm 1 K 2 at 673 K. This value is higher than that (2.6 10 4 Wm 1 K 2) reported by Cai et al. [15], but lower than that (15 10 4 Wm 1 K 2) obtained by Ohtaki et al. [10]. Fig. 6 (a) shows the thermal diffusivity and thermal conductivity of Zn1 xAlxO (x ¼0.03, 0.04). It is clear that both present Zn1 xAlxO (x ¼0.03, 0.04) samples show very low thermal conductivity. It is well known that the thermal conductivity can be varied greatly by extrinsic factor such as microstructure and impurities. The overall k value of a solid is given as k ¼ kph þ ke Fig. 4. UV–vis absorption spectra of SPSed ZnO (solid line) and Zn0.96Al0.04O samples derived from hydroxides as raw materials (red dotted line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ð2Þ
where kph and ke are the phonon and electric thermal conductivities, respectively. As shown in Fig. 6 (b), it is confirmed that the phonon thermal conductivity, kph, has a
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Fig. 6. (a) Thermal diffusivity and thermal conductivity of the Zn1 xAlxO (x ¼0.03, 0.04) Zn1 xAlxO ceramics derived from hydroxides as raw materials. Note that data for ZnO cited from Ref. [10] are included for comparison. (b) Phonon thermal diffusivity (kph) and electrical thermal diffusivity (ke) of the Zn1 xAlxO (x¼ 0.03, 0.04) ceramics.
Fig. 5. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient and (c) power factor for the Zn1 xAlxO ceramics derived from hydroxides as raw materials (solid line). Note that Zn0.96Al0.04O was also prepared using oxides as raw materials (dot line) and the data for ZnO are cited from Ref. [10].
dominate proportion relative to the electron thermal conductivity ke, which is estimated by the Wiedeman–Franz relation ke ¼LsT (L is the Lorenz number). The reported thermal conductivity value of pure ZnO is about 40 W m 1 K 1 at room temperature and decreased to 18 W m 1 K 1 at 673 K [11]. The thermal conductivity also decreased with increase in amount of Al2O3, because the Al doping and refined grains enhance the phonon scattering. The values of thermal conductivity, k, decrease as the temperature increases, and the k of Zn0.96Al0.04O is lower than that of Zn0.97Al0.03O, also because the relative density of the former one (88.8%) is lower than that of the latter one (91.7%). The
Fig. 7. Dimensionless figure of merit (ZT) of the Zn1 xAlxO (x ¼0.03, 0.04) ceramics derived from hydroxides as raw materials. Note that data for ZnO cited from Ref. [10] are included for comparison.
thermal conductivity values of the present Zn1 xAlxO (x ¼0.03, 0.04) bulks in the whole temperature range are also smaller than that of Ga-doped ZnO thermoelectric ceramic synthesized by mechanically alloying and hot pressing [21]. The temperature dependence of the dimensionless figure of merit, ZT, is shown in Fig. 7. Because of the combined effects of
N. Ma et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1344–1349
high power factor and reduced thermal conductivity, the Al-doped ZnO samples show considerably larger ZT values than pure ZnO. In the present temperature range investigated, the ZT value reached 0.085 at 673 K in the Zn0.96Al0.04O sample, which show a trend to further increase at higher temperatures. Higher ZT values were obtained in Zn0.96Al0.04O than in Zn0.96Al0.03O because the former one has a higher power factor in addition to reduced thermal conductivity. This result indicates a possibility to further improve the thermoelectric performance of Al-doped ZnO ceramics by enhancing the Al-doping in ZnO through processing modification.
4. Conclusions Fine-grained Al-doped ZnO thermoelectric bulks were prepared by a spark plasma sintering process using the powders derived from zinc and aluminum hydroxides. Although the XRD patterns of Zn1 xAlxO (xr0.05) samples show a single ZnO phase, TEM observation combined with EDX analysis proved the existence of ZnAl2O4 as a second phase. Nevertheless, ZnO ceramics with enhanced Al doping were prepared by using Zn(OH)2 and Al(OH)3 hydroxides as starting materials instead of ZnO and Al2O3 oxides. Therefore, the resultant Zn1 xAlxO samples show significantly improved electrical conductivity as compared with those prepared via mixing ZnO and Al2O3 oxide powders. Moreover, because of the reduced grain sizes in addition to the enhanced Al doping, the Zn1 xAlxO ceramics show low thermal conductivity. As a consequence, the resultant Al-doped ZnO ceramics with a nominal addition of 4% Al show the highest dimensionless figure of merit (ZT) of 0.085 at 673 K, which is about 17 times higher than that of pure ZnO.
Acknowledgements This work was conducted in Tsinghua University with financial supports from the National Basic Research Program of China (Grant no. 2007CB607500), High-Tech 863 Program of China (Grant no. 2009AA03Z216) and Tsinghua-Siemens collaborative research as well as National Nature Science Foundation (Grants no. 50820145203). References [1] L.E. Bell, Cooling, heating, generating power, and recovering waste heat with thermoelectric systems, Science 21 (2008) 1457–1461. [2] G. Chen, M.S. Dresselhaus, G. Dresselhaus, J.-P. Fleurial, T. Caillat, Recent developments in thermoelectric materials, Int. Mater. Rev. 48 (2003) 45–66.
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