Vacancy and anti-site disorder scattering in AgBiSe2 thermoelectrics

Heat-Dissipation Design and 3D Printing of Ternary Silver Chalcogenide-Based Thermoelectric Legs for Enhancing Power Generation Performance

Thermoelectric devices have received significant attention because of their potential for sustainable energy recovery. In these devices, a thermal design that optimizes heat transfer and dissipation is crucial for maximizing the power output. Heat dissipation generally requires external active or passive cooling devices, which often suffer from inevitable heat loss and heavy systems. Herein, the design of heat-sink integrated thermoelectric legs is proposed to enhance heat dissipation without external cooling devices, realized by finite element model simulation and 3D printing of ternary silver chalcogenide-based thermoelectric materials. Owing to the self-induced surface charges of the synthesized AgBiSe2 (n-type) and AgSbTe2 (p-type) particles, these particle-based colloidal inks exhibited high viscoelasticity, which enables the creation of complex heat-dissipation architectures via 3D printing. Power generators made from 3D-printed heat-dissipating legs exhibit higher temperature differences and output power than traditional cuboids, offering a new strategy for enhancing thermoelectric power generation.

Advances in thermoelectric AgBiSe2: Properties, strategies, and future challenges

Thermoelectric materials are attracting considerable attention to alleviate the global energy crisis by enabling the direct conversion of heat into electricity. As a class of I-V-VI2 semiconductors, AgBiSe2 is expected to be the potential thermoelectric material to replace conventional PbTe-based compounds due to its non-toxic and abundant nature of its constituent elements. This review article summarizes the fundamental properties of AgBiSe2, thermoelectric properties, the effect of different dopants on its transport properties and entropy engineering for cubic phase stabilization with the detailed description of related techniques used to analyze the properties of AgBiSe2. The current thermoelectric figure-of-merit and approaches to further improve performance and operational stability are also discussed.

High Thermoelectric Performance in Cu-Doped Bi2Te2.7Se0.33 Due to Cl Doping and Multiscale AgBiSe2

The thermoelectric performance of n-type Bi2Te3 needs further enhancement to match that of its p-type Bi2Te3 counterpart and should be considered for competitive applications. Combining Cu/Cl and multiscale additives (AgBiSe2) presents a suitable route for such enhancement. This is evidence of the enhanced thermoelectric performance of Bi1.995Cu0.005Te2.69Se0.33Cl0.03. Moreover, by incorporating 0.65 wt % AgBiSe2 (ABS) into Bi1.995Cu0.005Te2.69Se0.33Cl0.03, we further reduce its lattice thermal conductivity to ∼0.28 W m-1 K-1 at 353 K owing to the extra phonon scattering of multiscale ABS. Additionally, the Seebeck coefficient enhances (-183.89 μV K-1 at 353 K) owing to the matrix's reduced carrier concentration caused by ABS. As a result, we achieve a high ZT of ∼1.25 (at 353 K) and a high ZTave of ∼1.12 at 300-433 K for Bi1.995Cu0.005Te2.69Se0.33Cl0.03 + 0.65 wt % ABS. This work provides a promising strategy for enhancing the thermoelectric performance of n-type Bi2Te3 through Cu/Cl doping and ABS incorporation.

Incommensurately Modulated Structure in AgCuSe-Based Thermoelectric Materials for Intriguing Electrical, Thermal, and Mechanical Properties

AgCuSe-based materials have attracted great attentions recently in thermoelectric (TE) field due to their extremely high electron mobility, ultralow lattice thermal conductivity, and abnormal "brittle-ductile" transition at room temperature. However, although the investigation on the crystal structure of AgCuSe low-temperature phase (named as β-AgCuSe) was started more than half a century before, it is still in controversy yet, which greatly limits the understanding of its intriguing electrical, thermal, and mechanical performance. In this work, via adopting the advanced three-dimensional electron diffraction technique, this study finds that the AgCuSe-based materials crystalize in an incommensurately modulated structure with an orthorhombic Pmmn(0β1/2)s00 superspace group. The local lattice distortion in the incommensurately modulated structure has weak effects on the conduction band minimum due to the delocalized and isotropic feature of Ag 5s states, leading to high carrier mobility. Likewise, the inhomogeneous, weak, and anisotropic Ag-Se bonds result in the high degree of anharmonicity and ultralow lattice thermal conductivity. Furthermore, alloying S in AgCuSe reinforces the interaction between the adjacent Ag-Se layers, yielding the "brittle-ductile" transition at room temperature. This work well interprets the structure-performance relationship of AgCuSe-based materials and sheds light on the future investigation of this class of promising TE materials.

Suppressing Charged Cation Antisites via Se Vapor Annealing Enables p-Type Dopability in AgBiSe2 -SnSe Thermoelectrics

Cation disordering is commonly found in multinary cubic compounds, but its effect on electronic properties has been neglected because of difficulties in determining the ordered structure and defect energetics. An absence of rational understanding of the point defects present has led to poor reproducibility and uncontrolled conduction type. AgBiSe₂ is a representative compound that suffers from poor reproducibility of thermoelectric properties, while the origins of its intrinsic n-type conductivity remain speculative. Here, it is demonstrated that cation disordering is facilitated by Bi_Ag charged antisite defects in cubic AgBiSe₂ which also act as a principal donor defect that greatly controls the electronic properties. Using density functional theory calculations and in situ Raman spectroscopy, how saturation annealing with selenium vapor can stabilize p-type conductivity in cubic AgBiSe₂ alloyed with SnSe at high temperatures is elucidated. With stable and controlled hole concentration, a peak is observed in the weighted mobility and the density-of-states effective mass in AgBiSnSe₃ , implying an increased valley degeneracy in this system. These findings corroborate the importance of considering the defect energetics for exploring the dopability of ternary thermoelectric chalcogenides and engineering electronic bands by controlling self-doping.

Long-range ordering and local structural disordering of BiAgSe2 and BiAgSeTe thermoelectrics

Thermoelectric materials are promising for energy harvesting using waste heat. The thermal management of the thermoelectric materials attract scientific and technological interests. The narrow bandgap semiconductor BiAgSe₂ is a good candidate for thermoelectric materials due to its ultralow thermal conductivity. The mother compound BiAgSe₂ crystallizes in hexagonal symmetry at room temperature, but experiences structural transitions to cubic phase at high temperature. By contrast, the daughter compound BiAgSeTe exhibits long range ordering and crystallizes into cubic phase at room temperature. Nevertheless, the local structural disorderings due to the Bi³⁺ and Ag⁺ anti-site defects, as well as local structural distortions, are ubiquitous in both parent BiAgSe₂ and BiAgSeTe. BiAgSeTe exhibits distinct transport properties owing to the disordering-induced drastic changes in the electronic band structure, as well as the scattering dictated by the point defects. It is suggested that BiAgSe₂ and BiAgSeTe could be good candidates for phonon glass and crystal glass (PGEC)-type thermoelectrics.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Reducing Effective Mass for Advancing Thermoelectrics in Sb/Bi-Doped AgCrSe2 Compounds

Liquid-like materials have attracted increasing attention, owing to their phonon-liquid electron-crystal feature. As a typical representative, the superionic conductor AgCrSe₂ is regarded as a promising thermoelectric for its intrinsic ultralow lattice thermal conductivity. The primary challenge for achieving high thermoelectric performance is to enhance the inferior electronic performance in AgCrSe₂ compounds. Thus, it is very significant to manipulate band effective mass to achieve a higher power factor. In this work, the Sb/Bi elements are doped at Cr sites in Ag_0.97CrSe₂, i.e., Ag_0.97Cr_1-x(Sb/Bi)_xSe₂, aiming at producing a better overlap of electron orbits between different atoms for sharpening the valence band and decreasing the effective mass. In comparison to pristine AgCrSe₂, a considerable improvement (>50%) in the power factor (∼387 μW m^-1 K^-2 at 750 K) is realized upon 3% Sb doping. The single parabolic band model clarifies that the decreased effective mass and optimized carrier concentration contribute to the enhanced electronic property. Furthermore, an ultralow lattice thermal conductivity (∼0.2 W m^-1 K^-1) is well-maintained for the sample with 3% Sb doping as a result of the nearly unchanged superionic conduction. Eventually, a high peak figure of merit zT (∼0.7 at 750 K) is obtained in Ag_0.97Cr_0.97Sb_0.03Se₂. The current finding provides an excellent avenue for advancing thermoelectrics in AgCrSe₂ materials.

Nearly Identical but Not Isotypic: Influence of Lanthanide Contraction on Cs2NaLn(PS4)2 (Ln = La-Nd, Sm, and Gd-Ho)

The effect of lanthanide contraction often results in topological and symmetry changes in compounds with the same compositions as a function of lanthanide cation size. Here we report on the first example of a lanthanide thiophosphate exhibiting a change in the lanthanide cation environment without any topological or symmetry change. A series of new lanthanide thiophosphates with mixed alkali cations were obtained via a flux crystal growth technique using a CsI flux. The obtained compounds Cs₂NaLn(PS₄)₂ (Ln = La-Nd, Sm, and Gd-Ho) were grown as large single crystals (∼0.1-1 mm³) and characterized using single-crystal X-ray diffraction and magnetic susceptibility measurements. As we moved across the series, the structural studies revealed a change in the lanthanide coordination environment depending on the identity of the lanthanide. Although all compounds in the Cs₂NaLn(PS₄)₂ series crystallize in the same space group and have the same Wyckoff atom positions, a slight change in size between Sm³⁺ and Gd³⁺ causes a subtle change in coordination number from 9 (for Ln = La-Sm) to 8 (for Ln = Gd-Ho), resulting in two distinct but virtually identical structure types. Ab initio calculations were performed, and the observed experimental trend was corroborated computationally. Magnetic measurements performed on the Cs₂NaLn(PS₄)₂ (Ln = Ce, Pr, Nd, Gd, and Tb) compounds revealed paramagnetic behavior.

Local Structure and Influence of Sb Substitution on the Structure-Transport Properties in AgBiSe2

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI₂ family, and especially AgBiSe₂, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe₂, to evaluate possible stereochemical activity of the 6s² lone pair of Bi³⁺, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi_1-xSb_xSe₂ (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe₂. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe₂.

Doping-Induced Polymorph and Carrier Polarity Changes in Thermoelectric Ag(Bi,Sb)Se2 Solid Solution

Silver bismuth diselenide (AgBiSe₂) is an n-type thermoelectric material that exhibits a complex structural phase transition from the hexagonal to cubic phase, while silver antimony diselenide (AgSbSe₂) is a p-type thermoelectric material that crystallizes in the cubic phase at all temperatures. Here, we investigate the crystal structure and thermoelectric properties of Ag(Bi,Sb)Se₂ solid solution, employing AgBi_0.9Sb_0.1Se₂ and AgBi_0.7Sb_0.3Se₂ as representative samples. The carrier polarity of AgBi_0.9Sb_0.1Se₂ is converted from the n-type to p-type by Pb doping, accompanied by a polymorphic change to the cubic phase. It is difficult to obtain highly conductive p-type hexagonal AgBiSe₂-based materials, although first-principles calculations predict high-performance thermoelectric properties for these systems. We also demonstrate that cubic AgBi_0.7Sb_0.3Se₂ undergoes a polymorphic change to the hexagonal phase upon Nb doping. The present study show that polymorphic changes inevitably occurred upon Pb/Nb doping to optimize thermoelectric properties of Ag(Bi,Sb)Se₂ solid solution.

Spark Plasma Sintering (SPS)-Assisted Synthesis and Thermoelectric Characterization of Magnéli Phase V6O11

The Magnéli phase V₆O₁₁ was synthesized in gram amounts from a powder mixture of V₆O₁₁/V₇O₁₃ and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V₆O₁₁ has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K)^-1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m)^-1, a Seebeck coefficient of α = -(35 ± 2) μV K^-1, which leads to a power factor of PF = 4.9 ± 0.8 × 10^-5W (m K²)^-1 at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V₆O₁₁ are involved. This study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.

Effect of Te substitution on crystal structure and transport properties of AgBiSe2thermoelectric material

Reduced lattice thermal conductivity of Te-substituted AgBiSe₂was qualitatively described using the point defect scattering model.

Comparing the role of annealing on the transport properties of polymorphous AgBiSe2 and monophase AgSbSe2

AgBiSe₂ and AgSbSe₂, two typical examples of Te-free I–V–VI₂ chalcogenides, are drawing much attention due to their promising thermoelectric performance. Both compounds were synthesized via melting and consolidated by spark plasma sintering. The role of annealing on the transport properties of polymorphous AgBiSe₂ and monophase AgSbSe₂ was studied. Annealing has a greater impact on AgBiSe₂ than AgSbSe₂, which is ascribed to the temperature dependent phase transition of AgBiSe₂. Unannealed AgBiSe₂ shows p–n switching, but annealed AgBiSe₂ exhibits n-type semiconducting behavior over the whole measurement temperature range. By performing high-temperature Hall measurements, we attribute this intriguing variation to the change in the amount of Ag vacancies and mid-temperature rhombohedral phase after annealing. Both AgBiSe₂ and AgSbSe₂ exhibit low thermal conductivity values, which are ∼0.40–0.50 W m⁻¹ K⁻¹ for AgSbSe₂ and ∼0.45–0.70 W m⁻¹ K⁻¹ for AgBiSe₂, respectively. The maximum ZT value of AgBiSe₂ is enhanced from 0.18 to 0.21 after annealing. Pristine AgSbSe₂ presents a ZT value as high as 0.60 at 623 K, although slight deterioration emerges after annealing.

Annealing treatment has different impact on the transport properties of polymorphous AgBiSe₂ and monophase AgSbSe₂.