Effective Thermoelectric Materials Research Articles

Computational investigation of the fundamental physical properties of lead-free halide double perovskite Rb2NaCoX6 (X = Cl, Br, and I) materials: Potential prospects for sustainable energy

We have undertaken an ab initio investigation of emerging metal lead-free halide double perovskite materials for renewable energy applications using the WIEN2k simulation code. These materials have garnered significant attention from the research community due to their potential utility in electronic devices. Through an analysis of their electronic structure, we have ascertained that these materials exhibit characteristics of direct band gap semiconductors, falling within the energy range spanning 0.755 to 1.825[Formula: see text]eV. Furthermore, to check their suitability for use in photovoltaic devices, optical properties have been investigated. The thermoelectric potential of these materials has been explored using the BoltzTraP simulation code. The study of thermoelectric parameters indicates that the studied materials are effective thermoelectric materials with a strong potential for n-type doping. Additionally, thermodynamic parameters have been investigated to check their thermal stability, required to make them promising candidates for a wide range of renewable energy applications.

Thermal properties of Cu2S binary copper sulfides

Copper chalcogenides have a complex electronic structure due to the interaction of hybridized s- and p-states of chalcogen forming a valence band with 3d states of copper, which greatly complicates the interpretation of temperature dependences of kinetic parameters having a nonmonotonic character. Cu2S copper sulfide is an effective thermoelectric material, so it is interesting to study its kinetic parameters of solid solutions that it forms with alkali metals. The nonstoichiometry of chalcogenides can be easily controlled electrochemically, therefore, the task of selecting the optimal composition according to the cationic sublattice is quite feasible. The paper presents experimental studies of the properties of Cu2S binary copper sulfide. Copper chalcogenides have a complex electronic structure due to the interaction of hybridized s- and p-states of chalcogen forming a valence band with 3d states of copper, which greatly complicates the interpretation of temperature dependences of kinetic parameters having a nonmonotonic character. For the Cu2S sample, rather low values of the electron thermal EMF coefficient of the sample from 0.05 mV/K to 0.25 mV/K were found, which are more typical for metals than for semiconductors. The thermal conductivity of the Cu2S sample is quite low, it rises to 0.3 W/m*K at a phase transition of about 380 K and does not fall below 0.2 W/m*K. Thus, the nonstoichiometry of chalcogenides can be easily controlled electrochemically, therefore, the task of selecting the optimal composition according to the cationic sublattice is quite feasible. In addition, to improve the thermoelectric properties of Cu2S, it can be achieved by alloying alkali metals into a binary copper sulfide matrix.

A systematic first-principles investigation of the structural, electronic, optical, thermodynamic and transport properties of lead-free pyrochlore oxides Q2Sb2O7 (Q= Be, Ca, Sr) for low-Cost energy applications

A systematic study has been performed on structural, optical and thermophysical properties of newly designed pyrochlore oxides Q2Sb2O7 (Q = Be, Ca, Sr) employing FP-LAPW based first-principles calculations. The GGA approach was used to treat exchange and correlation potentials. The investigated E-V plots reveals that Sr2Sb2O7 is the most stable structure compared to Be2Sb2O7/Ca2Sb2O7. A direct energy bandgap of 0.29 eV is evident from band structure plot of Be2Sb2O7, however, Ca2Sb2O7/Sr2Sb2O7 possess indirect energy bandgaps of magnitude 1.47/1.467 eV. The studied materials show maximum absorption of incoming photons in near UV region as shown in ε2(ω) plots, however, considerable absorption in visible region is also present. Ca2Sb2O7/Sr2Sb2O7 are effective optical material with a n(ω) value between 1.0 and 2.0. Optical properties of pyrochlore oxides reveal that these materials are potential candidates for shielding materials in upper UV region. A photon reflection of up to 60% is evident from the R(ω) in UV region, however, in IR and visible regions, the reflectance is negligible. Based on calculated values of Seebeck coefficient (S), we can state that Be2Sb2O7 is n-type semiconductor whereas Ca2Sb2O7/Sr2Sb2O7 are p-type semiconductors. The most effective thermoelectric material among Q2Sb2O7 (Q = Be, Ca, Sr) is Ca2Sb2O7 as its ZT value is highest (∼1.05) in the entire temperature range. Thermodynamic properties of Q2Sb2O7 (Q = Be, Ca, Sr) are also evaluated to check dynamical stability and appropriateness of these materials in thermal applications. The investigated outcomes show that these pyrochlore oxides are potential candidates for thermoelectric and optoelectronic device applications.

Electronic, optical, and thermoelectric properties of vacancy-ordered double perovskite K2SnX6 (X = Cl, Br, I) from first-principle calculations

Abstract The present study explores the structural, optoelectronic, and thermoelectric properties of potassium tin halide vacancy-ordered double perovskites K2SnX6 (X = Cl, Br, and I) in their stable monoclinic phase. Our study uses first-principles calculations based on density functional theory (DFT). Electronic band structures reveal direct band gaps for K2SnCl6 and K2SnBr6, while K2SnI6 exhibits an indirect band gap. Theoretical computations utilising the modified Becke-Johnson potential (mBJ-GGA) demonstrate that the optical band gaps of K2SnCl6, K2SnBr6, and K2SnI6 decrease in the following order: 2.581 eV, 1.707 eV, and 4.126 eV, respectively. These values render the materials suitable for photovoltaic applications. Analysis of dielectric functions, absorption coefficients, and refractive indices demonstrates their potential as light-absorbing materials. We evaluate the thermoelectric properties, including electronic and lattice thermal conductivities, Seebeck coefficients, and power factors, which lead to favorable thermoelectric performance. The maximum figure of merit (ZT) values of 0.58, 0.69, and 0.50 are achieved for K2SnCl6, K2SnBr6, and K2SnI6, respectively, at 500 K. These findings highlight the potential of these materials for applications in solar cells and thermoelectric devices, emphasising their effectiveness at elevated temperatures.

Band structure modification and multi-scale phonon scattering synergistically boosts the thermoelectric performance of antimony incorporated MoS2

Molybdenum disulphide (MoS2) is a well-known functional material, used in various applications. In this work, antimony (Sb) incorporated MoS2 is prepared by hydrothermal method. X-ray diffraction confirms the successful incorporation of Sb on the matrix material. Compositional analysis reveals the shift in energy bands which attributed to the influence of Sb incorporation on MoS2. Thermoelectric investigation of as synthesized samples discloses the enhancement in electrical conductivity of around 107 times when compared to pristine MoS2. Seebeck coefficient as well as Hall measurement results validated p-type nature of all the samples. Lowering of thermal conductivity and enhancement in electrical conductivity of Sb incorporated samples results in the improvement of figure of merit. Thus, the material has more prospect to be an effective thermoelectric material.

Electronic Kapitza conductance and related kinetic coefficients at an interface between n-type semiconductors

We calculate the Kapitza conductance, which is the proportionality coefficient between heat flux and temperature jump at the interface, for the case of two conducting solids separated by the interface. We show that for conducting solids in a non-equilibrium state, there should also arise the electrochemical potential jump at the interface. Hence to describe linear transport at the interface we need three kinetic coefficients: interfacial analogs of electric and heat conductances and interfacial analog of the Seebeck coefficient. We calculate these coefficients for the case of an interface between n-type semiconductors. We perform calculations in the framework of Boltzmann transport theory. We have found out that the interfacial analog of the Seebeck coefficient for some range of parameters of the considered semiconductors, has a high value of about 10−3 V K−1. Thus this effect has the potential to be used for the synthesis of effective thermoelectric materials.

A DFT study: Tailoring opto-electronic and thermoelectric performance of K2SeX6 (X = Cl, Br) double perovskites for solar cell advancements

Density functional theory (DFT) has been used to examine the structure, optoelectronic, elastic, and thermoelectric characteristics of K2SeX6 (X= Br, Cl) utilizing the full potential linearized augmented plane wave (FP-LAPW). The examined double perovskites (DPs) appeared to feature indirect band gaps, according to the electronic band structure plots. K2SeX6 (X= Cl, Br) have computed shear anisotropy values of 0.55 and 0.54, respectively, indicating they are anisotropic materials. The materials' computed Poisson's ratios of 0.25 and 0.26 suggest that they are naturally ductile. According to the band structure plots, the examined DPs appear to feature indirect band gaps. The band gap values of two functional inorganic/organic perovskites, K2SeBr6 and K2SeCl6, are strikingly close to 2.0 eV and 1.689 eV. (Using modified Becke Johnson (mBJ) approximations). Because the maximum peaks occur in the visible and ultraviolet regions, they suggest potential solar cell applications for the materials. K2SeX6 (X=Br, Cl) has good values for electron production, great potential for applications in the optoelectronic sector. Additionally, it is a semiconductor by nature, according to the results of other optical characteristics such as absorption coefficients, electron energy loss, conductivity, and reflectivity. Because the maximum peaks occur in the visible and ultraviolet regions, they suggest potential solar cell applications for the solids. For K2SeX6 (X= Cl, Br), the computed Seebeck coefficient at 100 K, the S values attained 299.9 μV/K and 219 μV/K, respectively, are positive, demonstrating p-type conduction. Both K2SeBr6 and K2SeCl6 have a figure of merit values that are more than one (ZT > 1) 300K range which indicates that they are effective thermoelectric materials, but after these values of temperature, the zTe values vary. The computations' findings are the foundation for K2SeX6 (X= Cl, Br) and are practically applicable to solar cells.

First-principles study of the rare earth anti-TH3P4 type zintles for opto-electronic and thermoelectric applications

Structural, optoelectronic and thermoelectric properties of rare earth Zintls Ca3RESb3 (RE = La, Ce and Pr) isostructural with anti-Th4P3 are studied in cubic I4− 3d space groups. Structural parameters are reliable with experimental results and lattice parameters are decreasing due to lanthanide contraction from La to Pr. Cohesive energy and enthalpy indicate that Ca3PrSb3 is more stable than the others. Electronic properties reveal the direct bandgap semiconductors character of these compounds at the central symmetry, these compounds are and are increases going from La to Pr. Based on their optical characteristics, all material are active in the infrared region. Because of their optical dynamic quality, they could be used in optoelectronic devices. Based on their thermoelectric characteristics, they are effective thermoelectric materials as thermoelectric generator.

Enhancement of Thermoelectric Performance by Doping to Reduce Degeneracy

Increasing valley degeneracy via doping is one approach to enhance thermoelectric performance. Herein, it is shown that reducing degeneracy can also have a positive impact on performance, particularly for materials with high degeneracy around their undoped band edges. Specifically, doping NbFeSb with Zr or Hf atoms split the valence band edge, resulting in a 1.33 reduction in hole concentration, compared with the experimental carrier concentrations. The reduction increases Seebeck coefficients and overall thermoelectric performance to better agreement with the experimental ones. Notably, for potentially effective thermoelectric materials with high degeneracy, if the doped atom has significantly lower (higher) electronegativity, it is expected to produce a high‐performance p‐type (n‐type) thermoelectric material.

Multi-section thermoelements, advantages and problems of their creation

In this work, the ways of increasing the efficiency of thermoelectric generators are considered. These include an increase in the figure of merit of thermoelectric materials, as well as an increase in the temperature difference between hot and cold junctions of thermoelements and, accordingly, the range of their operating temperatures. The expediency of using thermoelements with multi-section legs has been substantiated. For their creation, effective thermoelectric materials with operating temperatures in the range of 300-1200 K have been proposed. A technique for modeling such thermoelements has been developed. Structures and materials of effective contact systems for multi-section thermoelements have been proposed, a technology for their manufacture has been developed. Ways of switching sections in thermoelement legs are considered. The thermal expansion of thermoelectric materials is investigated and a method for damping thermal stresses in the design of a thermoelement is proposed. The problem of thermoelectric material sublimation at high temperatures was solved by using protective coatings. Key words: thermoelectric generators, multi-section thermoelements, thermoelectric materials, thermal and electrophysical properties, contact systems, protecting coatings.

A Novel Method to Significantly Improve the Mechanical Properties of n-Type Bi(1−x)Sbx Thermoelectrics Due to Plastic Deformation

A unique method was developed to significantly improve the strength of Bi(1−x)Sbx single crystals, the most effective thermoelectric (TE) materials in the temperature range from 100 to 200 K due to their plastic deformation by extrusion. After plastic deformation at room temperature under all-round hydrostatic compression in a liquid medium, n-type Bi–Sb polycrystalline solid solutions show a significant increase in mechanical strength compared to Bi–Sb single crystals in the temperature range from 300 to 80 K. The significantly higher strength of extruded alloys in comparison with Bi–Sb single crystals is associated with the development of numerous grains with a high boundary surface as well as structural defects, such as dislocations, that accumulate at grain boundaries. Significant stability of the structure of extruded samples is achieved due to the uniformity of crystal plastic deformation under all-round hydrostatic compression and the formation of the polycrystalline structure consisting of grains with the orientation of the main crystallographic directions close to the original single crystal. The strengthening of Bi–Sb single crystals after plastic deformation allows for the first time to create workable TE devices that cannot be created on the basis of single crystals that have excellent TE properties, but low strength.

Thermoelectric properties of efficient thermoelectric materials on the basis of bismuth and antimony chalcogenides for multisection thermoelements

The following effective thermoelectric (TE) materials have been developed: Bi2Te2.8Se0.2 (0.14 wt%CdCl2), Bi0.5Sb1.5Тe3 (2 wt%Te and 0.14 wt% TeI4) obtained by zone melting; Bi2Te2.4Se0.6 (0.18 wt% CuBr) and Bi0.4Sb1.6Te3 (0.12 wt% PbCl2 and 1.50 wt% Te) obtained by extrusion; Bi2Te2.76Se0.24 (0.075 wt% Te and 0.026 wt% CdCl2) and Bi0.46Sb1.54Te3 (0.14 wt% Pb) obtained by rapid solidification. The elemental composition of TE materials was studied using the energy dispersive X-ray spectrometry. TE materials are designed for the manufacture of multisection thermoelements for thermoelectric generators (TEGs). In addition, TE materials obtained by zone melting can be used in the Peltier thermoelements. The electrical conductivity, Seebeck coefficient, thermal conductivity, thermal diffusivity, specific heat of TE materials in the temperature range of 300–600 K were investigated and an interpretation of their temperature dependences is provided. To obtain reliable data, the thermal conductivity was determined by two methods: (i) using the results of measuring the specific heat and thermal diffusivity; (ii) measuring the thermal conductivity by an absolute stationary method. The comparison of the results obtained by the mentioned methods showed that the discrepancy in the values of thermal conductivity does not exceed 8%, which correlates with the measurement error. The components of thermal conductivity were calculated and the mechanisms of heat transfer in TE materials were determined. The temperature dependences of dimensionless figure of merit ZT were defined. The operating temperatures for each material were found, in which ZT has the maximum values from 0.91 to 1.20. It was determined by the differential scanning calorimetry and thermogravimetric analysis that the materials investigated are thermally stable in the temperature range of 300–600 K.

Многосекционные термоэлементы, преимущества и проблемы их создания

In this work, the ways of increasing the efficiency of thermoelectric generators are considered. These include an increase in the figure of merit of thermoelectric materials, as well as an increase in the temperature difference between hot and cold junctions of thermoelements and, accordingly, the range of their operating temperatures. The expediency of using thermoelements with multi-section legs has been substantiated. For their creation, effective thermoelectric materials with operating temperatures in the range of 300-1200 K have been proposed. A technique for modeling such thermoelements has been developed. Structures and materials of effective contact systems for multi-section thermoelements have been proposed, a technology for their manufacture has been developed. Ways of switching sections in thermoelement legs are considered. The thermal expansion of thermoelectric materials is investigated and a method for damping thermal stresses in the design of a thermoelement is proposed. The problem of thermoelectric material sublimation at high temperatures was solved by using protective coatings.

Fabrication and thermoelectric properties of Si micro/nanobelts

<sec>Currently, low-dimensional materials are a hot spot in the field of thermoelectric research, because the thermoelectric properties will be significantly improved after the low-dimensionalization of bulk materials. In a bulk material, its thermoelectric figure of merit <i>ZT</i> value cannot be increased by changing a single parameter, because the parameters of the material are interrelated to each other, which is not conducive to the research of internal factors and thus limiting the efficiency of thermoelectric material, but thermoelectric material on a micro-nano scale is more flexible to adjust its thermoelectric figure of merit <i>ZT</i> value. There are many different kinds of methods of implementing the low-dimensionalization of bulk materials. In this paper, size-controllable Si micro/nanobelts are prepared based on semiconductor micromachining and focused ion beam (FIB) technology, and the thermoelectric properties of Si micro/nanobelts of different sizes are comprehensively studied by the micro-suspension structure method.</sec><sec>In this experiment, we find that the conductivity of doped Si micro/nanobelt is significantly better than that of bulk Si material, that as the width of the Si micro/nanobelt decreases, the thermal conductivity of the material decreases significantly, from 148 W/(m·K) of bulk silicon to 17.75 W/(m·K) of 800 nm wide Si micro-nanobelt, that the Seebeck coefficient of the material is lower than that of the corresponding bulkmaterials. The decrease of thermal conductivity is mainly due to the boundary effect caused by the size reduction, which leads the phonon boundary scattering to increase, and thus significantly inhibiting the behavior of phonon transmission in the Si material, thereby further affecting the transmission and conversion of thermal energy in the material. At 373 K, the maximum <i>ZT</i> value of the 800 nm wide Si micro/nanobelt reaches ～0.056, which is about 6 times larger than that of bulk silicon. And as the width of the Si micronanobelt is further reduced, the thermoelectric figure of merit <i>ZT</i> value will be further improved, making Si material an effective thermoelectric material. The FIB processing technology provides a new preparation scheme for improving the thermoelectric performances of Si materials in the future, and this manufacturing technology can also be applied to the low-dimensional preparation of other materials.</sec>

Thermal conductivities of different period Si/Ge superlattices

Thermoelectric materials, which can convert wasted heat into electricity, have attracted considerable attention because they provide a solution to energy problems. The Si/Ge superlattices have shown tremendous promise as effective thermoelectric materials. The period lengths of the Si/Ge superlattices can effectively tailor the phonon's transport behaviors and control their thermal conductivities. In this paper, three kinds of Si/Ge superlattices with different period length distributions (uniform, gradient, random) are constructed. The non-equilibrium molecular dynamics (NEMD) method is used to calculate the thermal conductivities of Si/Ge superlattices under the different period length distributions. The effect of the sample’s total length and temperature on the superlattice's thermal conductivity are studied. The simulation result shows that the thermal conductivity of gradient and random periodical Si/Ge superlattices are significantly reduced at room temperature compared with that of the uniform period Si/Ge superlattices. Phonons are transported by wave or particle properties in the different periodical superlattices. The thermal conductivity of uniform period superlattices has an obvious size effect with the increasing of the sample total length. In contrast, the thermal conductivity of gradient, random periodical Si/Ge superlattices are weakly dependent on the sample’s total length. At the same time, temperature is an important factor affecting the heat transport properties. We find that the temperature affects the thermal conductivities of the three kinds of superlattices in different ways. With the increase of the temperature, (i) the thermal conductivity of uniform periodical superlattices shows an obvious temperature effect; (ii) the thermal conductivity of the gradient and random periodical Si/Ge superlattices are nearly unchanged due to the competition between phonon localization weakness and phonon-phonon scattering enhancement. In addition, the phonon densities of states of superlattices with three different periodical length distributions are calculated. We find that in the picture of uniform periodical Si/Ge superlattices, the number of pronounced peaks quickly decreases as the period length increases, particularly at higher frequencies. This indicates that as the period length increases, fewer coherent phonons will be formed over the superlattices. Moreover, the scattering mechanisms of phonons for gradient and random periodical Si/Ge superlattices are basically the same at 100 K and 500 K. These findings provide a developmental way to further reduce the thermal conductivity of superlattices.

Optimum load resistance for a thermoelectric generator system

The current work focuses on finding the optimum electrical load resistance of thermoelectric generator system. The optimum load resistance and optimum geometric ratio of athermoelectric moduleare obtained simultaneously by using an analytical method based on dimensional analysis. These parameters are optimized to provide the maximum possible output power of a thermoelectric module attached to two heat sinks. Subsequently, an experiment has been conducted in order to validate the accuracy of the analytical model where good agreement is observed. Moreover, by using thespecificationsof the thermoelectric module that are provided by manufacturer, the effective thermoelectric material properties can be obtained; these properties help estimate the impact of contact resistance and Thomson effect. Furthermore, generalized charts are presented in this study which can help designers to determine the optimumload resistance and geometry for maximum output power.

Structural, electronic and thermoelectric properties of topological semimetal lanthanum monopnicitide LaBi

The structural, electronic and thermoelectric properties of LaBi have been studied using density functional theory (DFT) combined with semiclassical Boltzmann transport theory. The exchange and correlation (XC) effects have been treated using local density approximation (LDA) including spin-orbit coupling (SOC). In the present work, we have determined the topological character of LaBi, which is in agreement with available theoretical and experimental studies. The elastic constants such as Bulk modulus, Young modulus, Poisson's ratio and shear anisotropy factor have been calculated to check the mechanical stability. The thermoelectric transport parameters such as Seebeck coefficient, Power factor, electrical and thermal conductivity are calculated as a function of temperature and chemical potential. The values of power factor and lattice thermal conductivity are found to be 1.34×1013 WK−2m−1sec−1 and 4.20 Wm−1K−1, respectively, at T=1000 K. The calculated value of figure of merit ZT is 0.42 at T=1000 K, suggesting that LaBi may be used as an effective thermoelectric material for high temperature applications.

Non-Wiedemann-Franz behavior of the thermal conductivity of organic semiconductors

Organic semiconductors have attracted increasing interest as thermoelectric converters in recent years due to their intrinsically low thermal conductivity compared to inorganic materials. This boom has led to encouraging practical results, in which the thermal conductivity has predominantly been treated as an empirical number. However, in an optimized thermoelectric material, the electronic component can dominate the thermal conductivity in which case the figure of merit $ZT$ becomes a function of thermopower and Lorentz factor only. Hence design of effective organic thermoelectric materials requires understanding the Lorenz number. Here, analytical modeling and kinetic Monte Carlo simulations are combined to study the effect of energetic disorder and length scales on the correlation of electrical and thermal conductivity in organic semiconductor thermoelectrics. We show that a Lorenz factor up to a factor $\sim 5$ below the Sommerfeld value can be obtained for weakly disordered systems, in contrast with what has been observed for materials with band transport. Although the electronic contribution dominates the thermal conductivity within the application-relevant parameter space, reaching ZT>1 would require to minimize both the energetic disorder but also the lattice thermal conductivity to values below $\kappa_\text{lat}<0.2$W/mK.

Development and investigation of the effective thermoelectric materials for the multisectional generator thermoelements

Abstract Effective thermoelectric materials (TEM) with the working temperature ranges from 300 to 1200 K for the multisectional thermoelements (MTE) were developed, which allows to increase efficiency of thermoelectric generator (TEG). Temperature dependencies of the TEM parameters were investigated. Modeling of the MTE was carried out, and efficiency of TEG was estimated

The structure of clusters in liquid alloys of pseudo-binary PbTe-Bi2Te3 system

Purpose: of this paper is to study the structure of melts of quasi-binary system Bi2Te3-PbTe by means of X-ray diffraction method. The aim of the research was to investigate the short range order in melts comparing it with the structure in solid state. Design/methodology/approach: Analysis of the structural factors, radial distribution functions of atoms and basic structural parameters showed that the structure of melts at temperatures near the liquidus shows microheterogeneity. Findings: On the basis of the analysis of structural factors, functions of the radial distribution of atoms and basic structural parameters, it is shown that in the given concentration the short range order structure of liquid alloys of pseudo-binary PbTe-Bi2Te3 system is microinhomogeneous and is characterized by the presence of associates, whose atomic arrangement is like to the structure of solid compounds, existing in this concentration range. Research limitations/implications: To complete the understanding of short-range order effect on the formation of the physical properties of Pb-Bi-Te alloys, further studies of the thermoelectric properties of these alloys in the liquid state are needed. Practical implications: The promise of the considered direction requires an experimental and theoretical study of the processes of bulk, thin film and nanostructured material. In this case, it is necessary to develop a technology for the synthesis of compounds of Pb-Bi-Te system, obtaining thin films and nanostructures using the vapour phase methods with studying the mechanisms of thermoelectric properties of the material formation and optimization of technological regimes for obtaining effective thermoelectric materials based on compounds of Pb-Te-Bi system. Originality/value: The processes of structure formation of nanosystems with given characteristics are investigated, because among numerous thermoelectric materials, bismuth telluride (Bi2Te3) and its alloys are the most important thermoelectric materials used in state-of-the-art devices near room temperature, and lead telluride (PbTe)-based alloys are extensively used in power supplies for space exploration and generators for use at medium to high temperatures.