Date Published: December 18, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Pengfei Qiu, Yuting Qin, Qihao Zhang, Ruoxi Li, Jiong Yang, Qingfeng Song, Yunshan Tang, Shengqiang Bai, Xun Shi, Lidong Chen.
Diamond‐like compounds are a promising class of thermoelectric materials, very suitable for real applications. However, almost all high‐performance diamond‐like thermoelectric materials are p‐type semiconductors. The lack of high‐performance n‐type diamond‐like thermoelectric materials greatly restricts the fabrication of diamond‐like material‐based modules and their real applications. In this work, it is revealed that n‐type AgInSe2 diamond‐like compound has intrinsically high thermoelectric performance with a figure of merit (zT) of 1.1 at 900 K, comparable to the best p‐type diamond‐like thermoelectric materials reported before. Such high zT is mainly due to the ultralow lattice thermal conductivity, which is fundamentally limited by the low‐frequency Ag‐Se “cluster vibrations,” as confirmed by ab initio lattice dynamic calculations. Doping Cd at Ag sites significantly improves the thermoelectric performance in the low and medium temperature ranges. By using such high‐performance n‐type AgInSe2‐based compounds, the diamond‐like thermoelectric module has been fabricated for the first time. An output power of 0.06 W under a temperature difference of 520 K between the two ends of the module is obtained. This work opens a new window for the applications using the diamond‐like thermoelectric materials.
Nowadays, advanced technologies based on high‐performance energy materials have triggered a worldwide attention in response to the world’s increasing energy crisis and deteriorating environment. One of the promising technologies is the thermoelectric (TE) energy conversion with its ability of directly converting thermal energy into electricity.1, 2 Efficient TE conversion requires high performing n‐ as well as p‐type elements (legs) that are assembled electrically in series and thermally in parallel in a module. To ensure long‐term service reliability and stability,3 the n‐ and p‐type TE legs should have closely matching physical and chemical properties, such as the chemical composition, melting points, and thermal expansion coefficients. The energy conversion efficiency of a TE material is specified by the dimensionless TE figure of merit zT = S2σT/(κL+κe). The expression can be viewed as consisting of two distinct contributions: the dominator reflecting the electronic efficiency of a material as expressed via the power factor S2σ, where S is the Seebeck coefficient and σ is the electrical conductivity, and the denominator representing the heat conduction ability and consisting of the lattice thermal conductivity κL and the electronic thermal conductivity κe. In order to achieve high TE performance, it is essential to enhance the electronic properties (S2σ) and minimize the total thermal conductivity (κ = κL +κe). However, the transport parameters are closely correlated and interdependent. For example, S generally decreases with the increasing carrier concentration, that is, with the increasing electrical conductivity. Moreover, since the electronic thermal conductivity is related to the electrical conductivity via the Wiedemann–Franz law, κe = LσT, it increases with the increasing electrical conductivity. Therefore, it is challenging to simultaneously achieve excellent electronic properties and low thermal conductivity in a given material.
AgInSe2 is a semiconductor with the band gap of ≈1.19 eV,31 which has been extensively investigated for solar energy applications, optoelectronic applications, as well as photoelectrochemical applications.32, 33 It possesses a typical tetragonal chalcopyrite structure with the space group I‐42d, which is derived from the sphalerite structure with Ag/In orderly replacing Zn.31 The powder X‐ray diffraction (XRD) patterns of AgInSe2 are shown in Figure S1 (Supporting Information). The XRD data match well with the PDF card (#No. 35‐1099) for AgInSe2 compounds. The scanning electron microscopy (SEM) results for AgInSe2 are shown in Figure S2 (Supporting Information) and demonstrate that a small amount of Ag2Se second phase (<3%) exists in the prepared sample. The TE properties of AgInSe2 are shown in Figure2. The electrical conductivity σ of AgInSe2 is very low with the values on the order of 10−1 S m−1 around room temperature and 103 S m−1 at 900 K. Correspondingly, the Seebeck coefficient S is quite large with a value of −820 µV K−1 around room temperature and −295 µV K−1 at 900 K. These data indicate that AgInSe2 is a typical semiconductor with a very low carrier concentration. This is consistent with our Hall effect measurements, which shows that the carrier concentration of AgInSe2 is about 1.3 × 1013 cm−3 at 300 K. Consequently, the power factor (PF = S2σ) of AgInSe2 is also very low with a maximum value module 2.92 µW cm−1 K−2 at 900 K (see Figure S5, Supporting Information). The thermal conductivity for the pristine AgInSe2 prepared in our work is as low as 0.99 W m−1 K−1 at room temperature and decreases to 0.39 W m−1 K−1 at 900 K, which is close to the theoretical minimum lattice thermal conductivity κmin calculated by the Cahill's model (Equation (S1), Supporting Information).34 All these lead to a zT of 0.7 at 900 K in AgInSe2 although its electrical conductivity is low (as shown in Figure 2d). In summary, we have successfully fabricated a series of AgInSe2‐based diamond‐like compounds. They are n‐type TE materials. The stoichiometric AgInSe2 has very low electron concentration and lattice thermal conductivity as compared with the other diamond‐like materials. Via introducing extra Ag into the material and doping Cd at the Ag sites, the electron concentration has been significantly increased, leading to a much enhanced TE performance with a maximal value of 1.1 at 900 K, comparable to the best p‐type diamond‐like compounds reported before. All the present data strongly suggest that the AgInSe2‐based material is currently the best n‐type diamond‐like thermoelectric material. Furthermore, we have succeeded to fabricate a diamond‐like TE module for the first time by using our AgInSe2‐based n‐type material. The maximal output power of 0.06 W under a temperature difference ΔT of 520 K was obtained, indicating that diamond‐like materials can be a potential candidate for TE applications. Ag1+xInSe2 (x = 0, 0.01, 0.02) and Ag1−xCdxInSe2 (x = 0.08, 0.1) compounds were fabricated by directly reacting Ag (shots, 99.999%, Alfa Aesar), In (shots, 99.999%, Alfa Aesar), Se (shots, 99.999%, Alfa Aesar), and Cd (shots, 99.999%, Alfa Aesar) in sealed silica tubes. The raw materials were weighed out in a stoichiometric ratio and then sealed in silica tubes under vacuum in a glove box. The ingots were obtained by melting the mixture at 1273 K for 12 h, quenched into the icy water and then annealed at 923 K for 5 d. Fine powders were obtained by grinding the ingots in an agate mortar by hand. The powders were then loaded into a graphite die and sintered by hot pressing sintering (MRF Inc., USA) for 30 min at 823 K in vacuum. High‐density pellets (>98% of the theoretical density) were obtained.
The authors declare no conflict of interest.