Probing the thermoelectric properties of aluminium-doped copper iodide

Copper iodide, an environmentally friendly material abundant in nature, holds great significance for room temperature thermoelectric (TE) applications owing to its high Seebeck coefficient and optical transparency. However, to fully unlock its thermoelectric potential and match the performance of conventional TE materials, there is a need to further enhance its electrical conductivity. In this study, we have successfully synthesized nano-crystalline powders of both undoped and aluminium-doped CuI at room temperature using the chemical precipitation method in an ethanol medium. The concentration of aluminium dopant has been optimized to maximize TE performance. At 400 K, the highest TE power factor and figure of merit achieved are 79 μW m-1 K-2 and 0.08, respectively, for CuI doped with 0.1 mol% Al. This enhancement in TE properties can be attributed to the increased carrier density resulting from aluminium doping. The impact of aluminium doping on the temperature-dependent thermal conductivity has been investigated, and the findings are explained by the decay mechanism of optical phonons, supported by the anharmonic phonon coupling theory. Our work delves into the evolution of structural, thermal, optical, and TE properties of CuI upon aluminium (Al) doping. The results provide valuable insights into the future application of CuI in transparent thermoelectric and optoelectronic fields.

Enhanced Thermoelectric Efficiency in P-Type Mg3Sb2: Role of Monovalent Atoms Codoping at Mg sites

Due to natural abundance, low cost, and compatibility with sustainable green technology, Mg₃Sb₂-based Zintl compounds are comprehensively explored as potential thermoelectric materials for near-room temperature applications. The effective use of these materials in thermoelectric devices requires both p and n-type Mg₃Sb₂ having comparable thermoelectric efficiency. However, p-type Mg₃Sb₂ has inferior thermoelectric efficiency efficiency compared to its n-type counterpart due to low electrical conductivity (∼103Sm-1). Here, we show that codoping of monovalent atoms (Li-Ag, and Na-Ag) at the Mg site of Mg₃Sb₂ produces a synergistic effect and boosts the electrical conductivity, which enhances the thermoelectric properties of p-type Mg₃Sb₂. While, Ag prefers to occupy the Mg2 site, Li and Na are favorable at the Mg1 site of Mg₃Sb₂ lattice. Compared to Li-Ag codoping, Na-Ag codoping in Mg₃Sb₂ is found to be more effective for increasing the charge carrier concentration and significantly augmenting the electrical conductivity. The dominance of the three-phonon scattering mechanism in Li and Li-Ag doped Mg₃Sb₂ and the four-phonon scattering process for the Na and Na-Ag doped Mg₃Sb₂ are confirmed. Due to the simultaneous increase in electrical conductivity and decrease in thermal conductivity, the zT value ∼0.8 at 675 K achieved for Mg_2.975Na_0.02Ag_0.005Sb₂ is the highest value among p-type Mg₃Sb₂. Our work shows a constructive approach to enhance the zT of p-type Mg₃Sb₂ via monovalent atoms codoping at the Mg sites.

Giant Thermoelectric Efficiency of Single-Filled Skutterudite Nanocomposites: Role of Interface Carrier Filtering

The advantage of secondary-phase induced carrier filtering on the thermoelectric properties has paved the way for developing cost-effective, highly efficient thermoelectric materials. Here, we report a very high thermoelectric figure-of-merit of skutterudite nanocomposites achieved by tailoring interface carrier filtering. The single-filled skutterudite nanocomposites are prepared by dispersing rare-earth oxides nanoparticles (Yb₂O₃, Sm₂O₃, La₂O₃) in the skutterudite (Dy_0.4Co_3.2Ni_0.8Sb₁₂) matrix. The nanoparticles/skutterudite interfaces act as efficient carrier filters, thereby significantly enhancing the Seebeck coefficient without compromising the electrical conductivity. As a result, the highest power factor of ∼6.5 W/mK² is achieved in the skutterudite nanocomposites. The nonuniform strain distribution near the nanoparticles due to the local lattice misfit and concentration fluctuations affect the heat carriers and thereby reduce the lattice thermal conductivity. Moreover, the three-dimensional atom probe analysis reveals the formation of Ni-rich grain boundaries in the skutterudite matrix, which also facilitates the reduction of lattice thermal conductivity. Both the factors, i.e., the reduction in lattice thermal conductivity and the enhancement of the power factor, lead to an enormous increase in ZT up to ∼1.84 at 723 K and an average ZT of about 1.56 in the temperature range from 523 to 723 K, the highest among the single-filled skutterudites reported so far.

Thermoelectric properties of Ag-doped CuI: a temperature dependent optical phonon study

Due to the natural abundance and non-toxicity of copper (Cu) and iodine (I), γ-CuI has been widely explored as a potential transparent thermoelectric material for near room temperature applications. Here, we report the effect of doping of an heavy atom such as silver (Ag) on the evolution of temperature-dependent optical phonon modes and thermoelectric properties of chemically synthesized single-phase nanocrystalline γ-CuI. We found that Ag doping reduces the lattice parameters of CuI and thereby confirms the occupancy of Ag atoms at the vacancy sites of CuI. The decrease in phonon lifetime with the increase in temperature, which strongly influences the lattice thermal conductivity, is established from temperature-dependent optical phonon vibrations study. The four-phonon/Umklapp scattering is found to be more prominent in undoped CuI, whereas three-phonon scattering is prominent in Ag-doped CuI. At low temperatures, an almost 90% increase in the Seebeck coefficient is observed for Ag-doped CuI compared to undoped CuI, which can be understood by taking into account a net decrease in the hole carrier concentration in doped CuI.

Effect of iodine doping on the electrical, thermal and mechanical properties of SnSe for thermoelectric applications

We report the evolution of the thermoelectric and mechanical properties of n-type SnSe obtained by iodine doping at the Se site. The thermoelectric performance of n-type SnSe is detailed in the temperature range starting from 150 K ≤ T ≤ 700 K. The power factor of 0.25% iodine doped SnSe is found to be 0.33 mW m-1 K-2 at 700 K, comparable to that of the other monovalent doped n-type SnSe. The temperature-dependent electrical conductivity of the undoped and iodine doped SnSe samples is corroborated by using the adiabatic small polaron hopping model. A very low value of thermal conductivity, 0.62 W m-1 K-1, is obtained at 300 K and is comparable to that of SnSe single crystals. The low thermal conductivity of n-type polycrystalline SnSe is understood by taking into account the anharmonic phonon vibrations induced by the incorporation of heavy iodine atoms at the Se sites as well as the structural hierarchy of the compound. Besides, iodine doping is found to improve the reduced Young's modulus and hardness values of SnSe, which is highly desirable for thermoelectric device applications.

Effect of Refractory Tantalum Metal Filling on the Microstructure and Thermoelectric Properties of Co4Sb12 Skutterudites

We report a systematic investigation of the microstructure and thermoelectric properties of refractory element-filled nanostructured Co₄Sb₁₂ skutterudites. The refractory tantalum (Ta) metal-filled Co₄Sb₁₂ samples (Ta _x Co₄Sb₁₂ (x = 0, 0.4, 0.6, and 0.8)) are synthesized using a solid-state synthesis route. All the samples are composed of a single skutterudite phase. Meanwhile, nanometer-sized equiaxed grains are present in the Ta_0.2Co₄Sb₁₂ and Ta_0.4Co₄Sb₁₂ samples, and bimodal distributions of equiaxed grains and elongated grains are observed in Ta_0.6Co₄Sb₁₂ and Ta_0.8Co₄Sb₁₂ samples. The dominant carrier type changes from electrons (n-type) to holes (p-type) with an increase in Ta concentration in the samples. The power factor of the Ta_0.6Co₄Sb₁₂ sample is increased to 2.12 mW/mK² at 623 K due to the 10-fold reduction in electrical resistivity. The lowest lattice thermal conductivity observed for Ta_0.6Co₄Sb₁₂ indicates the rattling action of Ta atoms and grain boundary scattering. Rietveld refinement of XRD data and the analysis of lattice thermal conductivity data using the Debye model confirm that Ta occupies at the voids as well as the Co site. The figure of merit (ZT) of ∼0.4 is obtained in the Ta_0.6Co₄Sb₁₂ sample, which is comparable to single metal-filled p-type skutterudites reported to date. The thermoelectric properties of the refractory Ta metal-filled skutterudites might be useful to achieve both n-type and p-type thermoelectric legs using a single filler atom and could be one of replacements of the rare earth-filled skutterudites with improved thermoelectric properties.