Phonon engineering enabled reduction in thermal conductivity of SnS/Cu2Se composites: An experimental and numerical insights

Abstract

SnS is a promising chalcogenide-based material that shows great potential for thermoelectric applications. Lattice thermal conductivity is the only independent parameter that can be modified without disrupting the complex relationship of parameters such as electrical conductivity and the Seebeck coefficient. Herein, Cu₂Se x wt% (x = 1%, 3%, 5%, 7%) composited with SnS were synthesized by hydrothermal method. The addition of Cu₂Se in SnS has resulted in a suppression of the intrinsic thermal conductivity from 0.891 W/mK to 0.447 W/mK for Cu₂Se 5 wt% composited SnS at 753 K. Increasing Cu₂Se% in the SnS matrix has led to decrease in crystallite size from 33 nm - 29 nm, while the dislocation density and microstrain showed an increasing trend. The different physical and microstructural factors were analysed to understand the influence on thermal conductivity using numerical techniques. Physical parameters like Cu₂Se distribution, pelletizing temperature, pressure, volume fraction, and intrinsic material qualities are studied using effective thermal conductivity models and Maxwell Eurecken 2 shows the best fit. The Hasselman-Johnson model analyses interfacial thermal conductivity and shows an increasing trend from 0.31 × 10^–6 to 6.2 10^–6 m²K/W. In addition, the microstructural variables such as dislocations, stacking faults, and point defects are present in the samples. The phonon relaxation time for each mode of vibration determined using Raman spectroscopy also points towards the decrease in contribution from optical phonons by 30% from 7.71 × 10^–13 to 5.31 × 10^–13 s. The temperature-dependent and power-dependent Raman spectroscopy indicated lattice softening and anharmonic coupling leading to a red shift in the spectrum. A combination of optical and acoustic phonon contributions helped to reduce the thermal conductivity by about 50% for Cu₂Se-composited SnS.