Composition Tuning of Nanostructured Binary Copper Selenides through Rapid Chemical Synthesis and their Thermoelectric Property Evaluation.

Bejan Hamawandi,Johanna Rosén,Muhammet Toprak,Joseph Halim,Sedat Ballikaya,Mats Johnsson,Mikael Råsander,Lorenzo Vinciguerra,Johanna Rosen

doi:10.3390/nano10050854

Abstract

Reduced energy consumption and environmentally friendly, abundant constituents are gaining more attention for the synthesis of energy materials. A rapid, highly scalable, and process-temperature-sensitive solution synthesis route is demonstrated for the fabrication of thermoelectric (TE) Cu2−xSe. The process relies on readily available precursors and microwave-assisted thermolysis, which is sensitive to reaction conditions; yielding Cu1.8Se at 200 °C and Cu2Se at 250 °C within 6–8 min reaction time. Transmission electron microscopy (TEM) revealed crystalline nature of as-made particles with irregular truncated morphology, which exhibit a high phase purity as identified by X-ray powder diffraction (XRPD) analysis. Temperature-dependent transport properties were characterized via electrical conductivity, Seebeck coefficient, and thermal diffusivity measurements. Subsequent to spark plasma sintering, pure Cu1.8Se exhibited highly compacted and oriented grains that were similar in size in comparison to Cu2Se, which led to its high electrical and low thermal conductivity, reaching a very high power-factor (24 µW/K−2cm−1). Density-of-states (DOS) calculations confirm the observed trends in electronic properties of the material, where Cu-deficient phase exhibits metallic character. The TE figure of merit (ZT) was estimated for the materials, demonstrating an unprecedentedly high ZT at 875 K of 2.1 for Cu1.8Se sample, followed by 1.9 for Cu2Se. Synthetic and processing methods presented in this work enable large-scale production of TE materials and components for niche applications.

Highlights

Thermoelectricity is the current flow due to a temperature difference across the material or vice versa
We have demonstrated a highly scalable colloidal synthetic route, using energy-efficient microwave-assisted thermolysis, for the synthesis of nanostructured Cu1.8Se and Cu2Se
The observed differences between the electronic conductivity characteristics of the two samples have been confirmed by DFT calculations, where we demonstrate the metallic character of Cu-deficient phase Cu1.8Se

Summary

Introduction

Thermoelectricity is the current flow due to a temperature difference across the material or vice versa. This concept is used to recover the waste heat generated by industrial- and smaller-scale applications and convert it directly to electricity. The reverse concept of generating thermal gradients, i.e., cold–hot sides, using the thermoelectric (TE) unit are useful for many applications. TE systems in general are stable and reliable, with no maintenance requirements, due to having no moving parts. The efficiency of a TE material is typically represented by the dimensionless TE figure of merit, defined as ZT = (S2σT)/κ, where S, σ, and κ are the Seebeck coefficient, electrical conductivity, and total thermal conductivity, respectively, and T is the absolute temperature [1]. ZT around 1 has served as a standard for niche applications; realistically, ZT > 2 is necessary for broad implementation of TE technology [4]

Methods

Results

Conclusion