High-performance and low thermal conductivity in nano-layered Cu2Se prepared by a NaCl-flux method
In this study, a nano-layered Cu2Se high-performance material is successfully grown using a NaCl-flux method based on the stoichiometric ratios of Cu2Se(NaCl)x (x = 1.5, 2, 2.5, 3, and 3.5). X-ray diffraction results show that samples possess an α-phase structure for 1.5 ≤ x ≤ 2.5, whereas samples consist of a mixture of β- and α-phase structures for x ≥ 3. Electron probe microanalysis results present that almost no Na or Cl atoms were found in all the samples. Meanwhile, the actual composition is close to the nominal composition of Cu2Se when 1.5 ≤ x ≤ 2.5, whereas it deviates from the nominal composition when x ≥ 3. The valence values of Cu and Se atoms are +1 and −2 in the sample, respectively. Transmission electron microscopy results prove that the crystals prepared using the NaCl-flux method have a good quality. The field emission scanning electron microscopy image shows that the samples prepared using the NaCl-flux method have a stacking nano-layered structure with an average thickness of approximately 110 nm. These thin lamellar crystals have several grain boundaries, which can effectively scatter the phonon and thus result in ultra-low lattice thermal conductivity. Cu2Se(NaCl)2.5 exhibits the best thermoelectric performance among all the samples because it has the lowest thermal conductivity. The maximum ZT value reaches 1.42 at 700 K, which is much higher than those prepared using fast melt-quenching (∼0.88 at 700 K), spark plasma sintering (∼0.8 at 700 K) and high-temperature and high-pressure synthesis (∼1.13 at 700 K).