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Effect of TiO2 on high-temperature thermoelectric properties of ZnO
Journal of Alloys and Compounds 430 (2007) 200–204
Effect of TiO2 on high-temperature thermoelectric properties of ZnO K. Park ∗ , K.Y. Ko Department of Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea Received 14 February 2006; received in revised form 12 April 2006; accepted 12 April 2006 Available online 2 June 2006
Abstract The as-sintered Zn1−x Tix O (0.01 ≤ x ≤ 0.05) samples contained a solid solution of Zn1−x Tix O with a wurtzite structure and a small amount of the cubic spinel Zn2 TiO4 . The amount of Zn2 TiO4 increased with an increase in TiO2 content. The density and grain size increased with the small TiO2 content (≤0.01), and then they decreased gradually by further increasing the TiO2 content. The addition of TiO2 to ZnO led to a significant increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient, resulting in an increase in the power factor. The highest value of power factor (7.6 × 10−4 W m−1 K−2 ) was attained for Zn0.98 Ti0.02 O at 1073 K. It is demonstrated that the TiO2 addition is fairly effective for enhancing thermoelectric properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Sintering; Electron microscopy; Electrical properties; TiO2 ; Microstructure; ZnO
1. Introduction Thermoelectric materials with high energy conversion efficiency are strongly required for both electric power generation in terms of waste heat recovery and the refrigeration of electronic devices. The evaluation of the properties of the thermoelectric materials can be expressed by the figure-of-merit (Z) as follows [1]: Z=
σα2 κ
(1)
where σ, α, and κ are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. The electrical properties are determined by the power factor (P) defined here as [1]: P = σα2
(2)
To be a good thermoelectric material, it is required to have large power factor and low thermal conductivity. Bi–Te alloys [2–7], PbTe [8], Si–Ge alloys [9,10], skutterudite solid solutions [11–13], Heusler type compounds [14–16], transition-metal disilicides [17–20], SiC [21], and boron compounds [22,23] have been developed as thermoelectric materials. ∗
Corresponding author. Tel.: +82 2 3408 3777; fax: +82 2 3408 3664. E-mail address: [email protected] (K. Park).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.065
The materials developed for high-temperature thermoelectric power generation require expensive surface protection to prevent oxidation or vaporization. Consequently, practical utilization of these materials has been limited. To overcome the aforementioned problems, metallic oxides have been recognized as good candidates for applications in thermoelectric generation. In 1997, a new layered oxide, single-crystal NaCo2 O4 , was found to exhibit a high thermopower and conductivity at 300 K [24]. Since then, great attention has been paid on metal oxides, and extensive studies have been carried out by focussing on their thermoelectric transport [25–33]. However, most of the investigated oxide materials have a small value of figure-of-merit (Z). Development of new oxide materials is still the most important issue for practical applications of thermoelectric power generation. In this work, we have selected as a candidate material ZnO-based compounds. ZnO is an n-type semiconducting oxide with a wide direct band gap of 3.3 eV and has a wurtzite structure [34]. For the first time, Ohtaki et al. [35] reported that Zn0.98 Al0.02 O had a promising thermoelectric performance, i.e., the power factor of ∼1.5 × 10−3 W m−1 K−2 and the ZT of ∼0.2 at 1073 K. It has become an outstandingly promising oxide material as a high-temperature thermoelectric material above 973 K because of its high melting temperature (2073 K), good chemical stability, high electrical conductivity, and high Seebeck coefficient [35,36]. It is well known that the addition of dopants is a feasible route to optimizing the thermoelectric performance. In the present work, we studied the microstructure and
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201
high-temperature thermoelectric properties of the Zn1−x Tix O (0 ≤ x ≤ 0.05) fabricated by a solid-state reaction. 2. Experimental All Zn1−x Tix O (0 ≤ x ≤ 0.05) samples were fabricated by a solid-state reaction starting from the high-purity ZnO and TiO2 powders. The mixture of the ZnO and TiO2 powders and ethyl alcohol was milled for 6 h using a planetary mill (FRITSCH pulverisette 6) and ZrO2 as grinding media. The resulting slurries were dried at 353 K in an oven for 24 h. The mixed powders were calcined in a mullite crucible at 1273 K for 5 h. The calcined powders were milled in the planetary mill for 6 h and then dried at 353 K in an oven for 12 h. The dried powders were pressed using a hand press at a pressure of 98 MPa to prepare pellets of 5 mm thick and 20 mm diameter. The green compacts were heated at 1673 K for 20 h in air and then furnace cooled. The porosity of the as-sintered Zn1−x Tix O samples was measured by the Archimedes principle. The crystalline structure of the as-sintered samples was analyzed with X-ray diffraction (XRD) (Rigaku RINT2000) using Cu K␣ radiation at 40 kV and 100 mA. For microstructural observations, the fired samples were initially ground with 1000-grit SiC emery paper and polished to a mirrorlike finish on microcloth polishing wheel using fine alumina powders (10, 5, and 2 m particle sizes). The polished samples were cleaned thoroughly by ultrasonic stirring. The samples were thermal-etched at 1523 K for 30 min. The surface microstructure was examined by scanning electron microscope (SEM) (Hitachi S4700). Average grain sizes of the etched samples were estimated by the line-intersecting method. In order to measure the thermoelectric properties as a function of temperature, the electrical conductivity (σ) and the Seebeck coefficient (α) were simultaneously measured over the temperature range of 723–1073 K. The samples for the measurements of thermoelectric properties were cut out of the sintered bodies in the form of rectangular bars of 2 mm × 2 mm × 15 mm with a diamond saw and polished with SiC emery papers. Four grooves were put on the rectangular bars. Pt wires were wound along the grooves. The holes (∼1.0 mm diameter) in the middle of the two end grooves of the samples were machined. The heads of the two Pt/Pt-Rh (13%) thermocouples were embedded in the two holes, and the temperature at the holes was measured. Electrical conductivity was measured by the direct current (dc) four-probe method. For thermopower measurements, a temperature difference in the sample was generated by passing cool Ar gas over one end of the sample placed inside a quartz protection tube. The temperature difference between the two ends in the samples was controlled to be 4–6 K by varying the flowing rate of Ar gas. The thermopower (E) measured as a function of the temperature difference (T) gave a straight line. The Seebeck coefficient (α) was calculated from the relation α = E/T.
3. Results and discussion The XRD patterns of the as-sintered Zn0.99 Ti0.01 O, Zn0.97 Ti0.03 O, and Zn0.95 Ti0.05 O are shown in Fig. 1(a)–(c), respectively. The as-sintered Zn1−x Tix O (0.01 ≤ x ≤ 0.05) samples were composed of a solid solution of Zn1−x Tix O with a wurtzite structure, along with a small amount of the cubic spinel Zn2 TiO4 . The Zn2 TiO4 phase was formed by the following reaction: 2ZnO + TiO2 → Zn2 TiO4 . The added TiO2 (1–5 mol%) did not fully dissolve in the ZnO crystal lattice because of its rather low solubility [37]. It has been reported that the Zn2 TiO4 phase possesses a very low electrical conductivity [38]. In the Zn1−x Tix O samples, the amount of Zn2 TiO4 increased with increasing TiO2 content. No TiO2 phase was detected for all the samples examined. A similar result was also reported for TiO2 doped ZnO thick films [39]. It is important to note that the peaks of the as-sintered Zn1−x Tix O samples shifted towards higher angles with an increase in TiO2 content, indicating that the lattice parameter of the as-sintered Zn1−x Tix O samples decreased
Fig. 1. XRD patterns of the as-sintered (a) Zn0.99 Ti0.01 O, (b) Zn0.97 Ti0.03 O, and (c) Zn0.95 Ti0.05 O samples.
with TiO2 content. Consequently, one can conclude that the substituted Ti does not affect the crystalline structure and is present on the Zn site of ZnO lattice to form substitutional solid solutions. The ionic crystal radii of Zn2+ and Ti4+ are 0.75 and ˚ respectively [40]. The lattice parameter of the as-sintered 0.61 A, Zn1−x Tix O samples as a function of TiO2 content is shown in Fig. 2. Fig. 3(a)–(d) shows the SEM images from the surface of the thermal-etched ZnO, Zn0.99 Ti0.01 O, Zn0.97 Ti0.03 O, and Zn0.95 Ti0.05 O samples, respectively. It is clearly apparent that the ZnO grain size increases with the small addition of TiO2 (≤0.01) because the substitution of Ti4+ for Zn2+ increases the activity of ZnO by means of distortion of the ZnO lattice, which is beneficial to grain growth in TiO2 -doped Zn1−x Tix O samples [39]. In contrast, the ZnO grain size for high TiO2 content
Fig. 2. Lattice parameter of the as-sintered Zn1−x Tix O samples as a function of TiO2 content.
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Fig. 3. SEM images from the surface of the thermal-etched (a) ZnO, (b) Zn0.99 Ti0.01 O, (c) Zn0.97 Ti0.03 O, and (d) Zn0.95 Ti0.05 O samples.
(0.02 ≤ x ≤ 0.05) gradually decreases by further increasing the TiO2 content. This is attributed to the pinning effect caused by the Zn2 TiO4 particles on grain boundaries as well as to the dragging effect between the added TiO2 and grain boundaries, resulting in a reduction in the mobility of ZnO grain boundaries [41,42]. Similar behavior was found in TiO2 -doped ZnO thick films [39]. Fine Zn2 TiO4 particles below ∼1 m in size are frequently dispersed on the ZnO grain boundaries. In addition, most pores exist at grain boundaries, and some also remain within individual ZnO grains. The porosity decreases with the small TiO2 addition (≤0.01), and then increases gradually by further increasing TiO2 content. The grain size and porosity of all the prepared Zn1−x Tix O samples are given in Table 1. The electrical conductivity as a function of temperature for the Zn1−x Tix O samples is shown in Fig. 4. The electrical conductivity of the Zn1−x Tix O samples slightly increased with increasing temperature, indicating semiconducting behavior. It must be stressed that the incorporation of TiO2 to ZnO leads to a marked increase in electrical conductivity. This can be explained
by considering various competing effects affecting the electrical conductivity as follows. (1) The substitution of Ti4+ for Zn2+ may increase the electron concentration of the system to compensate for the electric charge balance, increasing the electrical conductivity. The defect reaction can be represented as follows [39]: TiO2 → TiZn •• + Oo x + (1/2)O2 (g) + 2e (2) The high TiO2 addition leads to an increase in a second phase Zn2 TiO4 having a low electrical conductivity in the sintered bodies, thereby decreasing the electrical conductivity. (3) The
Table 1 Grain size and porosity of the prepared Zn1−x Tix O samples
ZnO Zn0.99 Ti0.01 O Zn0.98 Ti0.02 O Zn0.97 Ti0.03 O Zn0.96 Ti0.04 O Zn0.95 Ti0.05 O
Grain size (m)
Porosity (%)
7.3 25.1 14.4 9.3 8.1 7.2
0.8 0.6 1.0 1.6 2.9 3.8
Fig. 4. Electrical conductivity as a function of temperature for the Zn1−x Tix O samples.
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Fig. 5. Temperature dependence of the Seebeck coefficient for the Zn1−x Tix O samples.
TiO2 addition higher than 1 mol% leads to a decrease in density and grain size, and to an increase in Zn2 TiO4 . This is responsible for the decrease in the time between scattering events of charge carriers, thus decreasing the electrical conductivity. From the above competing effects, it is considered that an increase in the electrical conductivity by means of the TiO2 addition is mainly due to an increase in the electron concentration of the system, originating from the extra electrons generated. It is therefore believed that the TiO2 addition is fairly effective in achieving high conductivity. Fig. 5 shows the temperature dependence of the Seebeck coefficient for the Zn1−x Tix O samples. The sign of the Seebeck coefficient is negative over the whole temperature range for all the samples, indicating that the major conductivity carriers are electrons. As expected, the absolute value of the Seebeck coefficient decreases with increasing TiO2 content mainly because of an increase in the electron concentration. This result qualitatively agrees with that based on the broadband semiconducting model [1]. According to the broadband model, the value of the Seebeck coefficient in common semiconductors decreases with increasing carrier density. Based on a simplified broadband model, the Seebeck coefficient (α) of the extrinsic n-type semiconductors with negligible hole conduction devices can be expressed as follows [35]: α = −(k/e)[ln(Nv /n) + A], where k is the Boltzmann constant, n the electron concentration, e the electric charge of the carrier, Nv the density of state, and A is a transport constant, typically 0 ≤ A ≤ 2. In spite of the decrease in the Seebeck coefficient, owing to an increase in the electron concentration, the absolute value of the Seebeck coefficient for the TiO2 -doped Zn1−x Tix O samples is still moderate, as high as 104–164 V K−1 at 1073 K. The temperature dependence of the power factor (σα2 ) for the Zn1−x Tix O samples calculated from the data in Figs. 4 and 5 is plotted in Fig. 6. The power factor of the Zn1−x Tix O samples increases up to 1073 K. The power factors of the TiO2 -doped Zn1−x Tix O samples are much higher than those of the TiO2 -free ZnO and Al2 O3 added Zn1−x Alx O samples. Zn0.98 Ti0.02 O showed the highest value of power factor
203
Fig. 6. Temperature dependence of the power factor for the Zn1−x Tix O samples.
(7.6 × 10−4 W m−1 K−2 ) at 1073 K. In addition to their high power factor, the TiO2 -doped Zn1−x Tix O ceramics have several inherent advantages for use in thermoelectric devices, i.e., excellent chemical stability under oxidizing atmospheres at high temperatures. Summarizing the aforementioned results, it may be believed that polycrystalline Zn1−x Tix O could be promising for thermoelectric applications at high temperatures. 4. Conclusions Polycrystalline Zn1−x Tix O (0 ≤ x ≤ 0.05) samples were fabricated by a solid-state reaction method. The as-sintered Zn1−x Tix O (0.01 ≤ x ≤ 0.05) samples crystallized in a solid solution of Zn1−x Tix O with a wurtzite structure, along with a small amount of the cubic spinel Zn2 TiO4 . This indicates that the substituted Ti was present on the Zn site of ZnO lattice to form substitutional solid solutions and formed the Zn2 TiO4 . The amount of the Zn2 TiO4 increased with the content of the TiO2 added. The density and grain size increased with the small TiO2 addition (≤0.01), and then decreased gradually by further increasing the TiO2 content. The substituted Ti led to a marked increase in the electrical conductivity. This is mainly due to an increase in the electron concentration of the system, originating from the extra electrons generated to compensate for the electric charge balance. The absolute value of the Seebeck coefficient decreased with increasing TiO2 content mainly because of an increase in the electron concentration. The power factors of the TiO2 -doped Zn1−x Tix O samples were extremely high, compared to the Tifree ZnO sample. Zn0.98 Ti0.02 O showed the highest value of power factor (7.6 × 10−4 W m−1 K−2 ) at 1073 K. It is strongly believed that the TiO2 addition is fairly effective for enhancing thermoelectric properties. Acknowledgement The authors would like to acknowledge the financial support provided for this research by the Korea Energy Management Corporation (KEMCO).
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Effect of TiO2 on high-temperature thermoelectric properties of ZnO K. Park ∗ , K.Y. Ko Department of Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea Received 14 February 2006; received in revised form 12 April 2006; accepted 12 April 2006 Available online 2 June 2006
Abstract The as-sintered Zn1−x Tix O (0.01 ≤ x ≤ 0.05) samples contained a solid solution of Zn1−x Tix O with a wurtzite structure and a small amount of the cubic spinel Zn2 TiO4 . The amount of Zn2 TiO4 increased with an increase in TiO2 content. The density and grain size increased with the small TiO2 content (≤0.01), and then they decreased gradually by further increasing the TiO2 content. The addition of TiO2 to ZnO led to a significant increase in the electrical conductivity and a decrease in the absolute value of the Seebeck coefficient, resulting in an increase in the power factor. The highest value of power factor (7.6 × 10−4 W m−1 K−2 ) was attained for Zn0.98 Ti0.02 O at 1073 K. It is demonstrated that the TiO2 addition is fairly effective for enhancing thermoelectric properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Sintering; Electron microscopy; Electrical properties; TiO2 ; Microstructure; ZnO
1. Introduction Thermoelectric materials with high energy conversion efficiency are strongly required for both electric power generation in terms of waste heat recovery and the refrigeration of electronic devices. The evaluation of the properties of the thermoelectric materials can be expressed by the figure-of-merit (Z) as follows [1]: Z=
σα2 κ
(1)
where σ, α, and κ are the electrical conductivity, Seebeck coefficient, and thermal conductivity, respectively. The electrical properties are determined by the power factor (P) defined here as [1]: P = σα2
(2)
To be a good thermoelectric material, it is required to have large power factor and low thermal conductivity. Bi–Te alloys [2–7], PbTe [8], Si–Ge alloys [9,10], skutterudite solid solutions [11–13], Heusler type compounds [14–16], transition-metal disilicides [17–20], SiC [21], and boron compounds [22,23] have been developed as thermoelectric materials. ∗
Corresponding author. Tel.: +82 2 3408 3777; fax: +82 2 3408 3664. E-mail address: [email protected] (K. Park).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.04.065
The materials developed for high-temperature thermoelectric power generation require expensive surface protection to prevent oxidation or vaporization. Consequently, practical utilization of these materials has been limited. To overcome the aforementioned problems, metallic oxides have been recognized as good candidates for applications in thermoelectric generation. In 1997, a new layered oxide, single-crystal NaCo2 O4 , was found to exhibit a high thermopower and conductivity at 300 K [24]. Since then, great attention has been paid on metal oxides, and extensive studies have been carried out by focussing on their thermoelectric transport [25–33]. However, most of the investigated oxide materials have a small value of figure-of-merit (Z). Development of new oxide materials is still the most important issue for practical applications of thermoelectric power generation. In this work, we have selected as a candidate material ZnO-based compounds. ZnO is an n-type semiconducting oxide with a wide direct band gap of 3.3 eV and has a wurtzite structure [34]. For the first time, Ohtaki et al. [35] reported that Zn0.98 Al0.02 O had a promising thermoelectric performance, i.e., the power factor of ∼1.5 × 10−3 W m−1 K−2 and the ZT of ∼0.2 at 1073 K. It has become an outstandingly promising oxide material as a high-temperature thermoelectric material above 973 K because of its high melting temperature (2073 K), good chemical stability, high electrical conductivity, and high Seebeck coefficient [35,36]. It is well known that the addition of dopants is a feasible route to optimizing the thermoelectric performance. In the present work, we studied the microstructure and
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201
high-temperature thermoelectric properties of the Zn1−x Tix O (0 ≤ x ≤ 0.05) fabricated by a solid-state reaction. 2. Experimental All Zn1−x Tix O (0 ≤ x ≤ 0.05) samples were fabricated by a solid-state reaction starting from the high-purity ZnO and TiO2 powders. The mixture of the ZnO and TiO2 powders and ethyl alcohol was milled for 6 h using a planetary mill (FRITSCH pulverisette 6) and ZrO2 as grinding media. The resulting slurries were dried at 353 K in an oven for 24 h. The mixed powders were calcined in a mullite crucible at 1273 K for 5 h. The calcined powders were milled in the planetary mill for 6 h and then dried at 353 K in an oven for 12 h. The dried powders were pressed using a hand press at a pressure of 98 MPa to prepare pellets of 5 mm thick and 20 mm diameter. The green compacts were heated at 1673 K for 20 h in air and then furnace cooled. The porosity of the as-sintered Zn1−x Tix O samples was measured by the Archimedes principle. The crystalline structure of the as-sintered samples was analyzed with X-ray diffraction (XRD) (Rigaku RINT2000) using Cu K␣ radiation at 40 kV and 100 mA. For microstructural observations, the fired samples were initially ground with 1000-grit SiC emery paper and polished to a mirrorlike finish on microcloth polishing wheel using fine alumina powders (10, 5, and 2 m particle sizes). The polished samples were cleaned thoroughly by ultrasonic stirring. The samples were thermal-etched at 1523 K for 30 min. The surface microstructure was examined by scanning electron microscope (SEM) (Hitachi S4700). Average grain sizes of the etched samples were estimated by the line-intersecting method. In order to measure the thermoelectric properties as a function of temperature, the electrical conductivity (σ) and the Seebeck coefficient (α) were simultaneously measured over the temperature range of 723–1073 K. The samples for the measurements of thermoelectric properties were cut out of the sintered bodies in the form of rectangular bars of 2 mm × 2 mm × 15 mm with a diamond saw and polished with SiC emery papers. Four grooves were put on the rectangular bars. Pt wires were wound along the grooves. The holes (∼1.0 mm diameter) in the middle of the two end grooves of the samples were machined. The heads of the two Pt/Pt-Rh (13%) thermocouples were embedded in the two holes, and the temperature at the holes was measured. Electrical conductivity was measured by the direct current (dc) four-probe method. For thermopower measurements, a temperature difference in the sample was generated by passing cool Ar gas over one end of the sample placed inside a quartz protection tube. The temperature difference between the two ends in the samples was controlled to be 4–6 K by varying the flowing rate of Ar gas. The thermopower (E) measured as a function of the temperature difference (T) gave a straight line. The Seebeck coefficient (α) was calculated from the relation α = E/T.
3. Results and discussion The XRD patterns of the as-sintered Zn0.99 Ti0.01 O, Zn0.97 Ti0.03 O, and Zn0.95 Ti0.05 O are shown in Fig. 1(a)–(c), respectively. The as-sintered Zn1−x Tix O (0.01 ≤ x ≤ 0.05) samples were composed of a solid solution of Zn1−x Tix O with a wurtzite structure, along with a small amount of the cubic spinel Zn2 TiO4 . The Zn2 TiO4 phase was formed by the following reaction: 2ZnO + TiO2 → Zn2 TiO4 . The added TiO2 (1–5 mol%) did not fully dissolve in the ZnO crystal lattice because of its rather low solubility [37]. It has been reported that the Zn2 TiO4 phase possesses a very low electrical conductivity [38]. In the Zn1−x Tix O samples, the amount of Zn2 TiO4 increased with increasing TiO2 content. No TiO2 phase was detected for all the samples examined. A similar result was also reported for TiO2 doped ZnO thick films [39]. It is important to note that the peaks of the as-sintered Zn1−x Tix O samples shifted towards higher angles with an increase in TiO2 content, indicating that the lattice parameter of the as-sintered Zn1−x Tix O samples decreased
Fig. 1. XRD patterns of the as-sintered (a) Zn0.99 Ti0.01 O, (b) Zn0.97 Ti0.03 O, and (c) Zn0.95 Ti0.05 O samples.
with TiO2 content. Consequently, one can conclude that the substituted Ti does not affect the crystalline structure and is present on the Zn site of ZnO lattice to form substitutional solid solutions. The ionic crystal radii of Zn2+ and Ti4+ are 0.75 and ˚ respectively [40]. The lattice parameter of the as-sintered 0.61 A, Zn1−x Tix O samples as a function of TiO2 content is shown in Fig. 2. Fig. 3(a)–(d) shows the SEM images from the surface of the thermal-etched ZnO, Zn0.99 Ti0.01 O, Zn0.97 Ti0.03 O, and Zn0.95 Ti0.05 O samples, respectively. It is clearly apparent that the ZnO grain size increases with the small addition of TiO2 (≤0.01) because the substitution of Ti4+ for Zn2+ increases the activity of ZnO by means of distortion of the ZnO lattice, which is beneficial to grain growth in TiO2 -doped Zn1−x Tix O samples [39]. In contrast, the ZnO grain size for high TiO2 content
Fig. 2. Lattice parameter of the as-sintered Zn1−x Tix O samples as a function of TiO2 content.
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Fig. 3. SEM images from the surface of the thermal-etched (a) ZnO, (b) Zn0.99 Ti0.01 O, (c) Zn0.97 Ti0.03 O, and (d) Zn0.95 Ti0.05 O samples.
(0.02 ≤ x ≤ 0.05) gradually decreases by further increasing the TiO2 content. This is attributed to the pinning effect caused by the Zn2 TiO4 particles on grain boundaries as well as to the dragging effect between the added TiO2 and grain boundaries, resulting in a reduction in the mobility of ZnO grain boundaries [41,42]. Similar behavior was found in TiO2 -doped ZnO thick films [39]. Fine Zn2 TiO4 particles below ∼1 m in size are frequently dispersed on the ZnO grain boundaries. In addition, most pores exist at grain boundaries, and some also remain within individual ZnO grains. The porosity decreases with the small TiO2 addition (≤0.01), and then increases gradually by further increasing TiO2 content. The grain size and porosity of all the prepared Zn1−x Tix O samples are given in Table 1. The electrical conductivity as a function of temperature for the Zn1−x Tix O samples is shown in Fig. 4. The electrical conductivity of the Zn1−x Tix O samples slightly increased with increasing temperature, indicating semiconducting behavior. It must be stressed that the incorporation of TiO2 to ZnO leads to a marked increase in electrical conductivity. This can be explained
by considering various competing effects affecting the electrical conductivity as follows. (1) The substitution of Ti4+ for Zn2+ may increase the electron concentration of the system to compensate for the electric charge balance, increasing the electrical conductivity. The defect reaction can be represented as follows [39]: TiO2 → TiZn •• + Oo x + (1/2)O2 (g) + 2e (2) The high TiO2 addition leads to an increase in a second phase Zn2 TiO4 having a low electrical conductivity in the sintered bodies, thereby decreasing the electrical conductivity. (3) The
Table 1 Grain size and porosity of the prepared Zn1−x Tix O samples
ZnO Zn0.99 Ti0.01 O Zn0.98 Ti0.02 O Zn0.97 Ti0.03 O Zn0.96 Ti0.04 O Zn0.95 Ti0.05 O
Grain size (m)
Porosity (%)
7.3 25.1 14.4 9.3 8.1 7.2
0.8 0.6 1.0 1.6 2.9 3.8
Fig. 4. Electrical conductivity as a function of temperature for the Zn1−x Tix O samples.
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Fig. 5. Temperature dependence of the Seebeck coefficient for the Zn1−x Tix O samples.
TiO2 addition higher than 1 mol% leads to a decrease in density and grain size, and to an increase in Zn2 TiO4 . This is responsible for the decrease in the time between scattering events of charge carriers, thus decreasing the electrical conductivity. From the above competing effects, it is considered that an increase in the electrical conductivity by means of the TiO2 addition is mainly due to an increase in the electron concentration of the system, originating from the extra electrons generated. It is therefore believed that the TiO2 addition is fairly effective in achieving high conductivity. Fig. 5 shows the temperature dependence of the Seebeck coefficient for the Zn1−x Tix O samples. The sign of the Seebeck coefficient is negative over the whole temperature range for all the samples, indicating that the major conductivity carriers are electrons. As expected, the absolute value of the Seebeck coefficient decreases with increasing TiO2 content mainly because of an increase in the electron concentration. This result qualitatively agrees with that based on the broadband semiconducting model [1]. According to the broadband model, the value of the Seebeck coefficient in common semiconductors decreases with increasing carrier density. Based on a simplified broadband model, the Seebeck coefficient (α) of the extrinsic n-type semiconductors with negligible hole conduction devices can be expressed as follows [35]: α = −(k/e)[ln(Nv /n) + A], where k is the Boltzmann constant, n the electron concentration, e the electric charge of the carrier, Nv the density of state, and A is a transport constant, typically 0 ≤ A ≤ 2. In spite of the decrease in the Seebeck coefficient, owing to an increase in the electron concentration, the absolute value of the Seebeck coefficient for the TiO2 -doped Zn1−x Tix O samples is still moderate, as high as 104–164 V K−1 at 1073 K. The temperature dependence of the power factor (σα2 ) for the Zn1−x Tix O samples calculated from the data in Figs. 4 and 5 is plotted in Fig. 6. The power factor of the Zn1−x Tix O samples increases up to 1073 K. The power factors of the TiO2 -doped Zn1−x Tix O samples are much higher than those of the TiO2 -free ZnO and Al2 O3 added Zn1−x Alx O samples. Zn0.98 Ti0.02 O showed the highest value of power factor
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Fig. 6. Temperature dependence of the power factor for the Zn1−x Tix O samples.
(7.6 × 10−4 W m−1 K−2 ) at 1073 K. In addition to their high power factor, the TiO2 -doped Zn1−x Tix O ceramics have several inherent advantages for use in thermoelectric devices, i.e., excellent chemical stability under oxidizing atmospheres at high temperatures. Summarizing the aforementioned results, it may be believed that polycrystalline Zn1−x Tix O could be promising for thermoelectric applications at high temperatures. 4. Conclusions Polycrystalline Zn1−x Tix O (0 ≤ x ≤ 0.05) samples were fabricated by a solid-state reaction method. The as-sintered Zn1−x Tix O (0.01 ≤ x ≤ 0.05) samples crystallized in a solid solution of Zn1−x Tix O with a wurtzite structure, along with a small amount of the cubic spinel Zn2 TiO4 . This indicates that the substituted Ti was present on the Zn site of ZnO lattice to form substitutional solid solutions and formed the Zn2 TiO4 . The amount of the Zn2 TiO4 increased with the content of the TiO2 added. The density and grain size increased with the small TiO2 addition (≤0.01), and then decreased gradually by further increasing the TiO2 content. The substituted Ti led to a marked increase in the electrical conductivity. This is mainly due to an increase in the electron concentration of the system, originating from the extra electrons generated to compensate for the electric charge balance. The absolute value of the Seebeck coefficient decreased with increasing TiO2 content mainly because of an increase in the electron concentration. The power factors of the TiO2 -doped Zn1−x Tix O samples were extremely high, compared to the Tifree ZnO sample. Zn0.98 Ti0.02 O showed the highest value of power factor (7.6 × 10−4 W m−1 K−2 ) at 1073 K. It is strongly believed that the TiO2 addition is fairly effective for enhancing thermoelectric properties. Acknowledgement The authors would like to acknowledge the financial support provided for this research by the Korea Energy Management Corporation (KEMCO).
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