Thermoelectric Property Measurements Research Articles

Instability Mechanism in Thermoelectric Mg2(Si,Sn) and the Role of Mg Diffusion at Room Temperature

Mg2(Si,Sn) shows great promise as thermoelectric material as it is made from non‐toxic, abundant, and cost‐effective elements offering high performance. This has been emphasized by several thermoelectric generator prototypes, demonstrating technological maturity. However, material stability is paramount for large‐scale applications whereas we reveal here that the thermal stability of n‐type Mg2(Si,Sn) may be limited even at room temperature (RT). Integral thermoelectric properties measurements, locally resolved Seebeck coefficient analysis, scanning electron microscopy/energy‐dispersive X‐ray spectroscopy, and atomic force microscopy are employed to assess changes of n‐type samples stored in ambient atmosphere for years, revealing the evolution of the carrier concentration and transport properties in the material as well as surface degradation. This is caused by the diffusion of loosely bound Mg from the bulk towards the surface and subsequent oxidation, leading to a change of Mg‐based intrinsic defect concentrations, thereby degrading the thermoelectric performance. This microscopic mechanism is backed up by first‐principles calculations, revealing that Mg diffusivity in Mg2(Si,Sn) is high at RT and that diffusion occurs mainly via Mg vacancies. The observed much faster degradation of Sn‐rich Mg2(Si,Sn) can be correlated with the higher density of Mg vacancies in Mg2Sn compared to Mg2Si, as predicted from defect formation energies.

Sustainable geopolymer concrete for thermoelectric energy harvesting

Concrete is a widely used material that presents vast opportunities for energy harvesting applications. Among these, thermoelectric concrete shows promising potential for harvesting waste heat generated in urban and industrial environments. However, the development of thermoelectric concrete poses several challenges that need to be addressed, such as the need to accurately account for the intrinsic voltage. This factor is frequently disregarded, but it has a significant impact on the measurement of thermoelectric properties in improving the intrinsically low electrical conductivity and Seebeck coefficient. In this context, this study evaluates the performance of industrially scalable geopolymer-based concrete that incorporates recycled aggregates as low-cost and environmentally friendly additives for thermoelectric energy harvesting applications. The concrete exhibited an intrinsic voltage of 15 mV, and a new test protocol was proposed to exclude its impact on thermoelectric measurements. The geopolymer concrete demonstrated a significantly high Seebeck coefficient of 570 µV/k, surpassing all previously reported values for geopolymer-based materials. Furthermore, it was discovered that the thermoelectric behavior of geopolymer-based concrete is of ionic origin, indicating that further improvements can be made by adjusting the pore solution chemistry. This finding suggests that there is ample opportunity for innovation and optimization in the development of thermoelectric concrete.

Experiment and Theory in Concert To Unravel the Remarkable Electronic Properties of Na-Doped Eu11Zn4Sn2As12: A Layered Zintl Phase.

Low-dimensional materials have unique optical, electronic, mechanical, and chemical properties that make them desirable for a wide range of applications. Nano-scaling materials to confine transport in at least one direction is a common method of designing materials with low-dimensional electronic structures. However, bulk materials give rise to low-dimensional electronic structures when bonding is highly anisotropic. Layered Zintl phases are excellent candidates for investigation due to their directional bonding, structural variety, and tunability. However, the complexity of the structure and composition of many layered Zintl phases poses a challenge for producing phase-pure bulk samples to characterize. Eu11Zn4Sn2As12 is a layered Zintl phase of significant complexity that is of interest for its magnetic, electronic, and thermoelectric properties. To prepare phase-pure Eu11-xNaxZn4Sn2As12, a binary EuAs phase was employed as a precursor, along with NaH. Experimental measurements reveal low thermal conductivity and a high Seebeck coefficient, while theoretical electronic structure calculations reveal a transition from a 3D to 2D electronic structure with increasing carrier concentration. Simulated thermoelectric properties also indicate anisotropic transport, and thermoelectric property measurements confirm the nonparabolicity of the relevant bands near the Fermi energy. Thermoelectric efficiency is known to improve as the dimensionality of the electronic structure is decreased, making this a promising material for further optimization and opening the door to further exploitation of layered Zintl phases with low-dimensional electronic structures for thermoelectric applications.

Enhanced thermoelectric performance of P-type SnTe thin film through Sr doping and Post-Annealing treatment

Lead-free tin telluride (SnTe) has gained significant attention for its potential use in mid-temperature thermoelectric power generation. In this work, we systematically analyzed the thermoelectric power factor of Sn0.7Sr0.3Te thin films at various post-annealing temperatures through structural, morphological, and thermoelectric property measurements. Our findings show that Sr-doping in the SnTe thin film matrix and post-annealing treatment can significantly enhance the thermoelectric properties by modifying the band structure and introducing an energy-filtering effect at grain boundaries. We achieved a maximum power factor of 29.4 μWm-1K−2 at 523 K for Sn0.7Sr0.3Te thin films post-annealed at 1073 K, which is three times greater than that of pure SnTe. This work demonstrates that Sr-doping and post-annealing effects can tune the band gap, induce band degeneration, and reduce grain size, resulting in a high thermoelectric power factor of the SnTe thin film.

Transport studies of two-step synthesized Cu2Se-Graphene nanocomposites

Copper Selenide (Cu2Se), in recent times, rated among the most potential thermoelectric materials, backing its excellent intrinsic thermoelectric (TE) performance arising from the unique nature of Cu + ions behaving like a liquid. Here we present experimental findings of Cu2Se-carbon nanostructure (graphene) composited in which Cu2Se has been synthesized using a cost-effective chemical reduction method, and graphene was incorporated in the Cu2Se matrix by simple mechanical grinding. The TE characteristics of graphene distributed at various weight percentages (0.5, 1, 2, 4, and 5 wt%) in the Cu2Se matrix are presented. The electrical conductivity and power factor of the sample at 450 °C temperature were dramatically improved by adding 1 wt% graphene to the Cu2Se matrix. All samples have been analyzed extensively by Field Emission Scanning Electron Microscopy, X-ray diffraction, High-Resolution Transmission Electron Microscopy, Raman, TE properties measurements, etc. Graphene was discovered to be densely and uniformly spread within the Cu2Se matrix material, resulting in a secondary phase of graphene/Cu2Se interfaces. Although there are minimal changes in the Seebeck coefficient, the power factor improved significantly because of the enhancement in electrical conductivity by the incorporation of graphene. The power factor in 1 wt% graphene incorporated Cu2Se enhanced to 1250 μW/mK2 at 450 °C compared to the power factor of 585 μW/mK2 for the bare Cu2Se sample. The purpose of this study is to determine the optimum graphene percentage that gives better performance.

Improving thermal stability and revealing physical mechanism in n-type Mg3Sb2-xBix for practical applications

N-type Mg3Sb2-xBix alloys exhibit good potential for applications in waste-heat harvesting due to their high thermoelectric performance. However, recent studies have suggested that these materials suffer from high-temperature performance degradation, and the physical mechanism affecting their stability remains vague, both of which significantly limit their practicability. Here we focus on the thermal stability of Mg3Sb2-xBix compounds, as well as the underlying physical mechanism. By adding cationic Co and Er into the matrix materials, both the electrical conductivity and Seebeck coefficient of Mg3Sb2-xBix barely decline over 100 h of thermoelectric properties measurement at 673 K, indicating improved thermal stability, and the Mg3Sb2-xBix compounds doped with Co and Er demonstrate thermoelectric performance comparable to that of Mg3Sb2-xBix doped only with anionic Te. These findings indicate that enhancing the thermal stability of these materials does not result in their sacrificed thermoelectric performance. Additionally, the electron localization function is used to explore the underlying mechanism via density functional theory calculations, through which greater bonding strength is found between Co/Er and Sb/Bi in comparison with that between Mg and Sb/Bi, explaining the absence of high-temperature thermoelectric performance degradation for Mg3Sb2-xBix doped with Co and Er. This study provides a new viewpoint for investigating the thermal stability of Mg3Sb2-xBix materials, as well as for possible future studies on the thermal stability of other thermoelectric materials.

Apparatus for measurement of thermoelectric properties of a single leg under large temperature differences.

Thermoelectric (TE) devices operate under large temperature differences, but material property measurements are typically accomplished under small temperature differences. Because of the issues associated with forming proper contact between the test sample and the electrodes and the control of heat flux, there are very few reports on large temperature difference measurements. Therefore, practically relevant performance parameters of a device, namely, power output and efficiency, are estimated by temperature averaging of material properties, whose accuracy is rarely validated by experimental investigations. To overcome these issues, we report an apparatus that has been designed and assembled to measure the TE properties-Seebeck coefficient, electrical conductivity, thermal conductivity, and power output and efficiency of a single thermoelectric material sample over large temperature gradients. The sample holder-a unique feature of this design-lowers the contact resistance between the sample and the electrodes, allowing for more accurate estimates of the sample's properties. Measurements were performed under constant temperature differences ranging from 50 to 300K with the hot side reaching 673K on a metallized Mg2Si0.3Sn0.7 leg synthesized in the laboratory. To simulate practical operating conditions of a continuously loaded generator, continuous current flow measurements were also performed under large temperature differences. The temperature-averaged TE properties from standard low temperature difference measurements and the experimental TE properties agree with each other, indicating that the designed setup is reliable for measuring various thermoelectric generator properties of single TE legs when subjected to temperature gradients between 50 and 300K.

Copper telluride with manipulated carrier concentrations for high-performance solid-state thermoelectrics

• Graphene incorporated Cu 2 Te composites are systematically and carefully fabricated. • Graphene effectively suppresses the carrier concentrations of the Cu 2 Te. • Reduced carrier concentration leads to an enhanced thermoelectric performance. • Substantial ZT of ∼1.47 at 1000 K is achieved, 268% higher than the pristine Cu 2 Te. This study proposed a strategy for effectively diminishing the carrier concentration in Cu 2 Te by introducing graphene sheets. Based on thermoelectric property measurements and single parabolic band modeling, the incorporated graphene effectively reduced the carrier concentration, not only enhancing the thermoelectric performance of the Cu 2 Te/graphene composite but also substantially improving its figure of merit up to ∼1.47 at 1000 K, which is 268% higher than that of pristine Cu 2 Te. This study gives an insight into the control of carrier concentration and thermoelectric properties in Cu 2 Te, and it could be extended to other copper chalcogenides for excellent thermoelectrics.

Integrated measurement of thermoelectric properties for filamentary materials using a modified hot wire method.

Thermoelectric materials have been rapidly developed due to the urgent need for the mutual conversion of thermal energy and electrical energy. Accurately measuring the thermoelectric properties of micro/nano thermoelectric materials is very important and highly required. Compared with traditional measurement methods, integrated measurement can avoid multiple sample preparations and reduce measurement errors. Herein, this work designed an improved integrated measurement method for the thermoelectric properties of microscale thermoelectric materials based on the hot wire method. The results demonstrated that the average ZT values of Pt and Ag2S wires are 0.75 × 10-3 and 0.44 × 10-3 with an uncertainty of ∼2.61%. It provides a novel way for the development of accurately measuring the thermoelectric properties of thermoelectric materials.

Accurate in situ measurements of thermoelectric transport properties at high pressure and high temperature.

Regulating electron structure and electron-phonon coupling by means of pressure and temperature is an effective way to optimize thermoelectric properties. However, in situ testing of thermoelectric transport performance under pressure and temperature is hindered by technical constraints that obscure the intrinsic effects of pressure and temperature on thermoelectric properties. In the present study, a new reliable assembly was developed for testing the in situ thermoelectric transport performance of materials at high pressure and high temperature (HPHT). This reduces the influence of thermal effects on the test results and improves the success rate of in situ experiments at HPHT. The Seebeck coefficient and electrical resistivity of α-Cu2Se were measured under HPHT, and the former was found to increase with increasing pressure and temperature; for the latter, although an increase in the pressure acted to lower the electrical resistivity, an increase in the temperature acted to increase it. On increasing pressure from 0.8 to 3GPa at 333K, the optimal power factor of α-Cu2Se was increased by ∼76% from 2.36 × 10-4-4.15 × 10-4Wm-1K-2, and the higher pressure meant that α-Cu2Se had its maximum power factor at lower temperature. The present work is particularly important for understanding the thermoelectric mechanism under HPHT.

Electron channeling studies of atom site preference and distribution in doped Mg2Si1-xSnx thermoelectrics

The electron channeling effect, i.e., the dependence of the characteristic x-ray emission on the crystallographic direction of the incoming beam in an analytical transmission electron microscope was employed to elucidate the crystal site location and distribution of the dopant and host ions in Mg2Si1-xSnx thermoelectric (TE) materials, doped with low amounts of Bi. Experiments performed both in pure Bi-doped Mg2Si and mixed Mg2Si1-xSnx (x = 0.4, 0.6), firmly confirmed that Mg occupies the two tetrahedral T sites, 8c (¼, ¼, ¼) and (¼, ¼, ¾), with some vacancies present, too, whereas Si and Sn the 4a (0, 0, 0) and 4b (½, ½, ½) octahedral C and N sites, respectively. Bi ions follow the trend of Si and Sn, occupying 4a sites, but also there is a partial distribution of them in 4b sites. We moreover observe a certain degree of asymmetry along the {111} directions in Mg2Si1-xSnx, predominately for the variable distribution of Bi and Sn in the lattice, which indicates Bi substitution for Sn in an uneven fashion. The channeling results are in line with TE property measurements, especially in relation to lower thermal conductivity and negative Seebeck coefficient due to Bi incorporation in the lattice. These findings demonstrate once more the effectiveness of the channeling technique to provide direct crystallographic information and refine atom positions in materials, particularly for nanoscale crystal grains.

Preferential Location of Dopants in the Amorphous Phase of Oriented Regioregular Poly(3‐hexylthiophene‐2,5‐diyl) Films Helps Reach Charge Conductivities of 3000 S cm−1

AbstractDoping polymer semiconductors is a central topic in plastic electronics and especially in the design of novel thermoelectric (TE) materials. In this contribution, it has been demonstrated that doping of oriented semicrystalline P3HT thin films with the dopant tris(4‐bromophenyl)ammoniumyl hexachloroantimonate), known as magic blue (MB), helps reach charge conductivities of 3000 S cm−1 and TE power factors of 170 ± 30 μW mK−2 along the polymer chain direction. A combination of transmission electron microscopy, polarized optical absorption spectroscopy, Rutherford backscattering, and TE property measurements helps clarify the conditions necessary to achieve such high charge conductivities. A comparative study with different dopants demonstrates that the doping mechanism is intimately related to the semicrystalline structure of the polymer and whether crystalline, amorphous or both phases are doped. The highest charge mobilities are observed when the dopant MB is preferentially located in the amorphous phase of P3HT, leaving the structure of P3HT nanocrystals almost unaltered. In this case, the P3HT nanocrystals are doped from their interface with the surrounding amorphous phase. These results indicate that doping preferentially the amorphous phase of semicrystalline polymer semiconductors is an effective strategy to reduce polaron localization, enhance charge mobilities, and improve TE power factors.

N-type (Zr,Ti)NiSn half Heusler materials via mechanical alloying: Structure, Sb-doping and thermoelectric properties

Mechanical alloying has been applied, as an advantageous scalable method, to synthesize Ti1-xZrxNiSn half Heusler materials. This is the first time a synthesis of single phase n-type Ti1-xZrxNiSn half Heusler solid solutions is reported by this technique, along with structural studies and thermoelectric properties. The application of mechanical alloying was successful, as all compositions ranging between x = 0.4–0.8 were in general single phase materials. The lattice thermal conductivity of the series was lower compared to the same compositions prepared by arc melting. Ti0.4Zr0.6NiSn exhibited the minimum lattice thermal conductivity of the series and was selected for doping studies with Sb, with the scope to enhance the thermoelectric performance via the power factor modification. Thermoelectric property measurements resulted in a maximum figure of merit of 0.71 at 762K for Ti0.4Zr0.6NiSn doped with 1.5% Sb.

Realizing Enhanced Thermoelectric Performance and Hardness in Icosahedral Cu5 FeS4-x Sex with High-Density Twin Boundaries.

Bornite (Cu5 FeS4 ) is an Earth-abundant, nontoxic thermoelectric material. Herein, twin engineering and Se alloying are combined in order to further improve its thermoelectric performance. Cu5 FeS4-x Sex (0 ≤ x≤ 0.4) icosahedral nanoparticles, containing high-density twin boundaries, have been synthesized by a colloidal method. Spark plasma sintering retains twin boundaries in the pellets sintered from Cu5 FeS4-x Sex colloidal powders. Thermoelectric property measurement demonstrates that alloying Se increases the carrier concentration, leading to much-improved power factor in Se-substituted Cu5 FeS4 , for example, 0.84 mW m-1 K-2 at 726 K for Cu5 FeS3.6 Se0.4 ; low lattice thermal conductivity is also achieved, due to intrinsic structural complexity, distorted crystal structure, and existing twin boundaries and point defects. As a result, a maximum zT of 0.75 is attained for Cu5 FeS3.6 Se0.4 at 726 K, which is about 23% higher than that of Cu5 FeS4 and compares favorably to that of reported Cu5 FeS4 -based materials. In addition, the Cu5 FeS4-x Sex samples containing twin boundaries also obtain improved hardness compared to the ones fabricated by melting-annealing or ball milling. This work demonstrates an effective twin engineering-composition tuning strategy toward enhanced thermoelectric and mechanical properties of Cu5 FeS4 -based materials.

Systematic Investigation of Solution-State Aggregation Effect on Electrical Conductivity in Doped Conjugated Polymers

Systematic Investigation of Solution-State Aggregation Effect on Electrical Conductivity in Doped Conjugated Polymers

Phase Relationship of Mg2Si at High Pressures and High Temperatures and Thermoelectric Properties of Mg9Si5.

Magnesium silicide (Mg2Si) is a promising eco-friendly thermoelectric material, which has been extensively studied in recent times. However, its phase behavior at high pressures and temperatures remains unclear. To this end, in this study, in situ X-ray diffraction analysis was conducted at high pressures ranging from 0 to 11.3 GPa and high temperatures ranging from 296 to 1524 K, followed by quenching. The antifluorite-phase Mg2Si decomposed to Mg9Si5 and Mg at pressures above 3 GPa and temperatures above 970 K. The antifluorite-phase Mg2Si underwent a structural phase transition to yield a high-pressure room-temperature (HPRT) phase at pressures above 10.5 GPa and at room temperature. This HPRT phase also decomposed to Mg9Si5 and Mg when heated at ∼11 GPa. When 5Mg2Si decomposed to Mg9Si5 and Mg, the volume reduced by ∼6%. Mg9Si5 synthesized at high pressures and high temperatures was quenchable under ambient conditions. Thermoelectric property measurements of Mg9Si5 at temperatures ranging from 10 to 390 K revealed that it was a p-type semiconductor having a dimensionless thermoelectric figure of merit (ZT) of 3.4 × 10-4 at 283 K.

Thermoelectric Properties of Cu2Se Synthesized by Hydrothermal Method and Densified by SPS Technique.

The aim of the work was to obtain copper (I) selenide Cu2Se material with excellent thermoelectric properties, synthesized using the hydrothermal method and densified by the spark plasma sintering (SPS) method. Chemical and phase composition studies were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) methods. Measurements of thermoelectric transport properties, i.e., electrical conductivity, the Seebeck coefficient, and thermal conductivity in the temperature range from 300 to 965 K were carried out. Based on these results, the temperature dependence of the thermoelectric figure of merit ZT as a function of temperature was determined. The obtained, very high ZT parameter (ZT~1.75, T = 965 K) is one of the highest obtained so far for undoped Cu2Se.

(Keynote) All Roads Lead to Rome: Paths of Heat and Charge Transport in Hybrid Polymer-Nanowire Systems

In this talk, we present research that aims to identify the specific pathways of heat and charge transport in complex hybrid polymer-nanowire thermoelectric systems. These materials have demonstrated remarkable performance due to interfacial charge transport dynamics that have proven difficult to untangle. Here I present the work of a few recent papers to disaggregate the role of the organic and inorganic layer in the transport of charge and heat, as well as the mechanism that causes an enhancement in thermoelectric performance. We also assess the role of specific features of the polymer and nanowire themselves, and how those contribute to transport. We do this at both the single nanowire level, and also at the film level, enabling meaningful comparisons and conclusions. Through combining thermoelectric property measurements using a custom-built suspended microbridge setup for single nanowires, the Kang-Snyder charge transport model as well as gate modulated electrical conductivity measurements, we conclude that the thin organic PEDOT:PSS shell is the predominant channel for charge carrier transport in this hybrid material, and the large electrical conductivity is attributed to the highly ordered PEDOT chains on the Te nanowire surface brought about by a templating effect. We also present complementary research on charge transport characteristics of modified PEDOT:PSS derivatives as well. This talk will summarize the work of our 4 most recent manuscripts from the last few months.

Thermoelectric Properties of Cu2Se Nano-Thin Film by Magnetron Sputtering.

Thermoelectric technology can achieve mutual conversion between thermoelectricity and has the unique advantages of quiet operation, zero emissions and long life, all of which can help overcome the energy crisis. However, the large-scale application of thermoelectric technology is limited by its lower thermoelectric performance factor (ZT). The thermoelectric performance factor is a function of the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature. Since these parameters are interdependent, increasing the ZT value has always been a challenge. Here, we report the growth of Cu2Se thin films with a thickness of around 100 nm by magnetron sputtering. XRD and TEM analysis shows that the film is low-temperature α-Cu2Se, XPS analysis shows that about 10% of the film’s surface is oxidized, and the ratio of copper to selenium is 2.26:1. In the range of 300–400 K, the maximum conductivity of the film is 4.55 × 105 S m−1, which is the maximum value reached by the current Cu2Se film. The corresponding Seebeck coefficient is between 15 and 30 µV K−1, and the maximum ZT value is 0.073. This work systematically studies the characterization of thin films and the measurement of thermoelectric properties and lays the foundation for further research on nano-thin-film thermoelectrics.

Pressure-Induced Enhancement of Thermoelectric Figure of Merit and Structural Phase Transition in TiNiSn.

Half-Heusler thermoelectric materials are potential candidates for high thermoelectric efficiency. We report high-pressure thermoelectric and structural property measurements, density functional theory calculations on the half-Heusler material TiNiSn, and an increase of 15% in the relative dimensionless figure of merit, ZT, around 3 GPa. Thermal and electrical properties were measured utilizing a specialized sample cell assembly designed for the Paris-Edinburgh large-volume press to a maximum pressure of 3.5 GPa. High-pressure structural measurements performed up to 50 GPa in a diamond-anvil cell indicated the emergence of a new high-pressure phase around 20 GPa. A first-principles structure search performed using an ab initio random structure search approach identified the high-pressure phase as an orthorhombic type, in good agreement with the experimental results.