Yuanyue Li,
Xiaoying Qin*,
Di Li*,
Xiyu Li,
Yongfei Liu,
Jian Zhang,
Chunjun Song and
Hongxing Xin
Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, 230031 Hefei, People's Republic of China. E-mail: xyqin@issp.ac.cn; lidi@issp.ac.cn; Fax: +86 0551 5591434; Tel: +86 0551 5592750
First published on 27th March 2015
The electrical transport and thermoelectric properties of Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) compounds are investigated in the temperature range of 300–600 K. The results indicate that with increasing Al content from x = 0 to x = 0.03, hole concentration increases monotonically from 8.04 × 1017 to 1.19 × 1019 cm−3 due to the substitution of Al3+ for Sb5+, thus leading to a large decrease in the electrical resistivity of Cu3Sb1−xAlxSe4. Meanwhile, the increase in hole concentration leads to a transition from a non-degenerate (x = 0) to a partial degenerate (x = 0.01, 0.02) and then to a degenerate state (x = 0.03). The power factor (PF) of all the Al-doped Cu3Sb1−xAlxSe4 samples is remarkably improved due to the optimization of hole concentration. Lattice thermal conductivity κL of the heavily doped sample (x = 0.03) is reduced. As a result, a large thermoelectric figure of merit ZT = 0.58 is obtained for Cu3Sb0.97Al0.03Se4 at 600 K, which is around 1.9 times as large as that of the un-doped Cu3SbSe4.
The ternary semiconductor of Cu3SbSe4, as a narrow band gap semiconductor (0.13–0.42 eV)11 with a unit cell four times larger than ZnSe (n = 8 for Cu3SbSe4 versus n = 2 for ZnSe), was firstly synthesized by Wernick and Benson12 with high Seebeck coefficient and low thermal conductivity at room temperature (the Cu/Se atoms form the structural framework and the rest Sb atoms have the rattling behaviors similar to the resonators13). However, the un-doped Cu3SbSe4 compound has low hole concentration and relatively large electrical resistivity, which leads to low TE performance. As reported in early work,14 the ZT of Cu3SbSe4 is too small to be used in practice. Hence, it is a key issue to reduce its electrical resistivity and to optimize its PF (PF = S2/ρ). Earlier work done by Skoug et al. showed that doping with either Ge or Sn at the Sb site can improve the thermoelectric performance of Cu3SbSe4.15 Wei et al.16 present a quite systematic study of Cu3SbSe4, which is done in advance of current work, and have achieved promising ZT = ∼0.7 at 673 K for Cu3Sb0.98Sn0.02Se4.
In our previous work,17 we showed that the thermoelectric performance of Cu3SbSe4 could be improved by doping the element Bi, where the equivalent substitution of Sb5+ with Bi5+ led to a limited increase in the hole concentration. In the present work, Al, a cheap, environment-friendly and abundant element, is used as the dopant and the inequitable substitution of Al3+ for Sb5+ will introduce hole into the host, giving rise to a large increase in hole concentration in this p-type compound Cu3SbSe4. Our experiments demonstrate that Al-doping can effectively adjust the hole concentration and optimize PF. Moreover, κL of the heavily doped sample (x = 0.03) is reduced. As a result, ZT = 0.58 is achieved for Cu3Sb0.97Al0.03Se4 at 600 K, which is about 1.9 times larger than that of the Cu3SbSe4.
The phase of the obtained samples was checked by using X-ray diffraction (Philips diffractometer, Cu Kα radiation). The specimens for transport measurements were cut from the bulk samples by using a diamond saw. Electrical resistivity and Seebeck coefficient were measured by the ZEM-3 system from ULVAC. The thermal diffusivity, D, was measured using the laser flash method (Netzsch, LFA-457). The specific heat, Cp, was determined by differential scanning calorimetry (DSC Pyris Diamond). The resulting thermal conductivity was calculated from the measured thermal diffusivity D, specific heat Cp, and density d from the relationship κ = DdCp. The density of the bulk materials was measured by Archimedes' methods using alcohol as the medium, and all were above 90% of the theory density, as shown in Table 1. Thin plate samples were used for Hall measurements, which were conducted by CVM-200 Hall effect measurement system.
Cu3Sb1−xAlxSe4 | aa (Å) | cb (Å) | μc (cm2 V−1 s−1) | pd (cm−3) | dre (%) |
---|---|---|---|---|---|
a a is lattice parameter.b c is lattice parameter.c μ is Hall mobility.d p is carrier concentration.e dr is relative density, defined as dr = d/d0, where d is measured density of samples and d0 (= 5.86 g cm−3) is theoretical density of Cu3SbSe4. | |||||
x = 0 | 5.654 | 11.196 | 94.1 | 8.04 × 1017 | 91 |
x = 0.01 | 5.635 | 11.175 | 83.0 | 2.50 × 1018 | 91 |
x = 0.02 | 5.618 | 11.156 | 61.0 | 4.12 × 1018 | 92 |
x = 0.03 | 5.601 | 11.137 | 52.5 | 1.19 × 1019 | 91 |
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Fig. 1 XRD patterns at room temperature for Cu3Sb1−xAlxSe4 samples: (a) x = 0, (b) x = 0.01, (c) x = 0.02 and (d) x = 0.03. |
Fig. 2 shows the SEM image of fracture surface of bulk sample Cu3Sb0.97Al0.03Se4. One can see from the image that the typical grain sizes are in the range of 1–5 μm. In addition, there are a large number of nanoparticles embedded at grain boundaries, indicating that the dispersed distribution of the nanoparticles will be beneficial to inhibiting lattice thermal conductivity of the bulk specimens due to extra enhanced phonon scattering. Our observation indicates that the nanoparticles exist in both doped samples and un-doped samples (not shown here).
The thermoelectric properties of Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) are shown in Fig. 3. As displayed in Fig. 3(a), the electrical resistivity ρ of Cu3SbSe4 decreases with increasing temperature, indicating that the un-doped sample exhibits semiconductor-like behavior. However, in the case of the Al-doped compounds, ρ shows different temperature dependences. ρ of sample with x = 0.01 initially increases slightly with increasing temperature and reaches a peak value of 3.12 × 10−4 Ω m at 375 K and then decreases with further increasing temperature. With increasing Al content to x = 0.02, ρ almost remains the same value in the temperature range from 325 K to 375 K and then decreases with further increasing temperature, reaching 1.3 × 10−4 Ω m (at 600 K) which is comparable to that of the sample with x = 0.01. However, ρ in the case with x = 0.03 almost remains unchanged in the whole temperature range. In addition, Al doping causes the decrease in the magnitude of the electrical resistivity.
In order to examine the temperature behavior of the resistivity for Cu3SbSe4, logarithm of the resistivity ρ as a function of reciprocal of temperature is given in the inset of Fig. 3(a). It can be seen that good linear relationships between lnρ and 1/T exist in the high temperature range for Cu3SbSe4. The existence of a linear relationship between ln
ρ and 1/T means that the resistivity can be described by using a thermally activated expression in corresponding temperature regimes, written as:
![]() | (1) |
Our further measurements (see following text) demonstrate that Seebeck coefficient and Hall coefficient of Cu3SbSe4 are positive in the whole temperature investigated, indicating the major carriers are holes. Since the substitution of Al3+ for Sb5+ occurs, the doping of Al is expected to introduce holes into the host. Therefore, the Al substitution for Sb will give rise to a large increase in hole concentration. The hole concentrations p at room temperature for Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) are calculated according to the measured Hall coefficient and the results are listed in Table 1. One can see that the hole concentration increases from 8.04 × 1017 cm−3 for x = 0 to 2.50 × 1018 cm−3 for x = 0.01, 4.12 × 1018 cm−3 for x = 0.02 and 1.19 × 1019 cm−3 for x = 0.03, respectively. Al-doping leads to the increase in hole concentration, which explains the smaller resistivity upon doping.
All the samples are p-type semiconductors, as verified by the positive Seebeck coefficients, which are shown in Fig. 3(b). The Seebeck coefficient S for Cu3SbSe4 decreases with the increasing temperature, and after about 550 K, it shows a slight increase with increasing temperature. In contrast, S for samples with x = 0.01 and 0.02 decreases much slowly with increasing temperature and its value drops with increasing content of aluminum. This behavior is similar to that of the electrical resistivity, which also indicates that the Al-doped compounds become partially degenerate. Specially, one notices that S for sample with x = 0.03 increases monotonically with temperature. This is the typically characteristic of a heavily degenerate semiconductor.
The temperature dependences of power factor PF (= S2/ρ) of Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) are shown in Fig. 3(c). The values of PF for all the Al-doped compounds are larger than that for the un-doped one. Specially, the PF of sample with x = 0.03 nearly increases linearly with temperature, and it reaches 1.05 × 10−3 W m−1 K−2 at 600 K, which is about twice as large as that of Cu3SbSe4.
The temperature dependences of the total thermal conductivity κ, lattice thermal conductivity κL and carrier contribution κc for Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03) compounds are presented in Fig. 3(d) and 4, respectively. The results of the specific heat, Cp, determined by DSC for Cu3SbSe4 from 300 K to 600 K are shown in Fig. 5. The values of Cp range from 0.31 to 0.37 J g−1 K−1, which are in agreement with the result of Skoug et al.9 but smaller than the data reported by Wei et al.16 As to Fig. 3(d), for all the samples, κ decreases with increasing temperature in the whole temperature range investigated. In comparison, κ of the Al-doped samples is slightly lower at room temperature and then becomes higher than that of the un-doped Cu3SbSe4 with the increasing temperature due to the increased contribution from κc (the inset of Fig. 4). Lattice thermal conductivity κL is estimated by subtracting the carrier thermal conductivity κc from κ, where the Wiedemann–Franz relation with a Lorentz constant L0 is applied for estimating κc (κc = L0T/ρ). Here, L0 is set to 1.5 × 10−8 V2 K−2, 2 × 10−8 V2 K−2 and 2.44 × 10−8 V2 K−2 for a non-degenerate semiconductor (x = 0), a partial degenerate semiconductor (x = 0.01, 0.02) and a degenerate semiconductor (x = 0.03), respectively.18,19 As compared to the results of Wei et al.,16 our lattice thermal conductivity κL (at 300 K) for un-doped one is around ∼30% lower than that reported by Wei et al.,16 which could, on the one hand, come partly from more porosities in our samples than theirs (the relative densities dr for each sample are given in Table 1). On the other hand, the large number of dispersed nanoparticles (as shown in Fig. 2) will make great contribution to inhibiting lattice thermal conductivity of the bulk specimens due to extra phonon scattering at these nanoparticles, leading to smaller κL. As shown in Fig. 4, κL of samples with x = 0.01 and 0.02 is similar to that of the un-doped sample. However, κL for x = 0.03 is smaller than that of the pure sample especially at high temperatures.
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Fig. 4 Temperature dependences of the lattice thermal conductivity κL and the carrier thermal conductivity κc (the inset) for Cu3Sb1−xAlxSe4: (a) x = 0, (b) x = 0.01, (c) x = 0.02 and (d) x = 0.03. |
Fig. 6 shows the temperature dependence of ZT for Cu3Sb1−xAlxSe4 (x = 0, 0.01, 0.02 and 0.03). ZT of the Al-doped compounds Cu3Sb1−xAlxSe4 (x = 0.01, 0.02 and 0.03) is larger than that of un-doped Cu3SbSe4. The maximum ZT reaches 0.58 for Cu3Sb0.97Al0.03Se4 at 600 K, which is around 1.9 times as large as that of the un-doped Cu3SbSe4 and is larger than the ZT value reported by Wei et al.16 for Sn-doped Cu3SbSe4 (ZT = ∼0.5 at 600 K). The elevation of ZT for Cu3Sb0.97Al0.03Se4 results mainly from both its enhanced PF due to the optimization of hole concentration and reduced κL.
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Fig. 6 Temperature dependence of ZT for Cu3Sb1−xAlxSe4: (a) x = 0, (b) x = 0.01, (c) x = 0.02 and (d) x = 0.03. |
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