Improving electrical and thermal properties synchronously via introducing CsPbBr₃ QDs into higher manganese silicides

Highlights

• Uniform CsPbBr₃ QDs with diameter of 12-18 nm were obtained via heat injection method quickly.

• Electrical and thermal properties were improved by Cs doping and QDs embedding synchronously.

• The PF of CsPbBr₃ QDs/HMS composite was enhanced owing to the combined effect of element doping and energy filtering.

• The ${\kappa }_l$ decreased ∼22% because of the broadband phonon scattering effect that resulted from the Cs doping and QDs inclusions.

• A high zT value of 0.57 at 823 K is was achieved in CsPbBr₃ QDs/HMS composites.

Abstract

Higher manganese silicide (HMS) is a P-type medium temperature thermoelectric (TE) material, which has attracted widespread attention over the past few decades due to its remarkable mechanical properties, excellent chemical and thermal stability, as well as the non-toxicity, abundance and competitive price. The peak power factor (PF) of HMS is as high as ∼1.50 × 10⁻³ W m⁻¹ K⁻² because of its intrinsic high electrical conductivity and Seebeck coefficient. However, the thermal conductivity of HMS is also high, resulting in relatively low zT values. Introducing nano-dispersion in the matrix is one of the most effective methods to enhance the TE properties via reducing the lattice thermal conductivity significantly without drastic changes on the other parameters. In this study, CsPbBr₃ QDs with uniform size were synthesized and introduced into HMS bulks. The PF (at 823 K) was enhanced to 1.71 × 10⁻³ W m⁻¹ K⁻², which is improved 14.0% approximately compared with that of pure HMS owing to the combined effect of element doping and energy filtering. The lattice thermal conductivity (at 823 K) decreased from 2.56 W m⁻¹ K⁻¹ to 1.99 W m⁻¹ K⁻¹ synchronously (∼22.0%) due to the intensive phonon scattering caused by Cs doping, and the embedding of Pb riched CsPbBr₃ QDs and Pb QDs. A maximum zT value of 0.57 (823 K) is achieved in CsPbBr₃ QDs/HMS composites, which is 36.0% higher than that of pure HMS. Predictably, for other TE materials, it is also feasible to improve the TE properties via introducing metastable quantum dots.

Introduction

Thermoelectric (TE) materials play an important role in the global sustainable energy solution owing to the fact that they can realize the conversion between waste heat and electricity directly, which has attracted much attention recently as a green energy solution [1,2]. The efficiency of TE materials can be evaluated by the dimensionless figure of merit (zT) expressed as $zT=S^2\sigma /{\kappa }_{\text{tot}}$, where $S$, $\sigma$ and ${\kappa }_{\text{tot}}$, $T$ are the Seebeck coefficient, electrical conductivity, total thermal conductivity, and absolute temperature, respectively. Partly, $S^2\sigma$ is defined as power factor (PF), and ${\kappa }_{\text{tot}}$ is dominantly composed of lattice thermal conductivity (${\kappa }_{\mathrm{l}})$ and electronic thermal conductivity (${\kappa }_{\mathrm{e}})$, ${\kappa }_{\text{tot}}={\kappa }_{\mathrm{l}}+{\kappa }_{\mathrm{e}}$ [3]. At the medium temperature, PbTe [4,5], SnSe [6], [7], [8], [9], Cu₂Se [10], [11], [12], GeTe [13], [14], [15] etc. are popular materials with high TE properties, whose zT_max values are over 2.0 owing to the high electrical properties, and low thermal conductivities. Even so, the widespread use of these materials is still hindered to some extent because of the poor thermal stability, as well as the use of the toxic, expensive, and scarce elements.

Higher manganese silicide (HMS) has attracted increasing attention on account of that it is abundant, inexpensive, non-toxic, eco-friendly and so on [16]. MnSi_x (x = 1.71 – 1.75) is a general term of HMS with some subspecies, including Mn₄Si₇, Mn₇Si₁₂, Mn₁₁Si₁₉, Mn₁₅Si₂₆, Mn₁₉Si₃₃, Mn₂₆Si₄₅, Mn₂₇Si₄₇, Mn₃₉Si₆₈, and so on [17], [18], [19]. They all belong to a relatively stable structure named Nowotny chimney ladder (NCL) structure, where Mn atoms provide a tetragonal framework in the outer layer, and Si atoms spirally rise inside [20]. HMS has high electrical properties and the PF of pure HMS is as high as ∼1.5 × 10⁻³ W m⁻¹ K⁻² at 825 K [21,22], which is a little higher than that of SnSe crystal along b axis (∼1.0 × 10⁻³ W m⁻¹ K⁻² at 825 K) [6], and In doped Cu₂Se (∼1.2 × 10⁻³ W m⁻¹ K⁻² at 850 K) [10]. However, the thermal conductivity of HMS is also high, resulting in the zT value of the pristine HMS as low as ∼0.4.

Recently, some significant efforts on enhancing the zT values of HMSs have been reported. Element doping is one of the effective methods to improve the TE properties of HMS. Tang [23], Shi [24], Bernard-Granger [25], et al. have doped Al on Si site with the improved electric properties (∼24.9%–33.9%), and have achieved a high zT value of 0.57∼0.7 at 823 K. Okamoto [26], Ponnambalam [27], and Miyazaki [28], et al. have doped Ru, Cr, V on Mn site, with the enhanced peak zT value of 0.76, 0.6, and 0.59. Shi et al. have doped Re on Mn site, achieving the zT value of 0.57 with the thermal conductivity decreasing to 2.3 W m⁻¹ K⁻¹ at 800 K (∼17.0%) owing to the lattice mismatch. Nanoengineering is another effective approach to enhance the zT value of HMS, which can increase the density of point defects, dislocations, and grain boundaries, and correspondingly strengthen the phonon scattering [29]. The feasibility of this strategy has also been proved in our previous studies. MnTe nanowires [30], MnS nanoparticles [31], Ag/Pt alloy QDs [32] were embedded into HMS bulks to decrease the thermal conductivities, achieving the maximum zT value of 0.5 – 0.7 at 823 K. It is concluded that introducing the dispersed phases, especially for the quantum dots is an excellent method to reduce the ${\kappa }_{\mathrm{l}}$ without seriously damage in electrical performance.

In this study, a novel and feasible strategy was implemented to enhance the TE performance of HMS, which introduced metastable CsPbBr₃ QDs into HMS, achieving element doping and nano engineering effects simultaneously. The PF was improved by ∼14.0% for the Cs doping effect and energy filtering effects. The ${\kappa }_{\mathrm{l}}$ was reduced by ∼22% on account of the enhanced phonon scattering owing to the increased density of the point defects, dislocations, and grain boundaries. Ultimately a high zT value of 0.57 was achieved at 823 K via introducing CsPbBr₃ QDs into HMSs, which is 36.0% higher than that of the pristine HMS.