Self-Healable and Stretchable PAAc/XG/Bi_(2)Se_(0.3)Te_(2.7) Hybrid Hydrogel Thermoelectric Materials

Thermoelectric power generators have attracted increasing interest in recent years owing to their great potential in wearable electronics power supply.It is noted that thermoelectric power generators are easy to damage in the dynamic service process,resulting in the formation of microcracks and performance degradation.Herein,we prepare a new hybrid hydrogel thermoelectric material PAAc/XG/Bi_(2)Se_(0.3)Te_(2.7)by an in situ polymerization method,which shows a high stretchable and self-healable performance,as well as a good thermoelectric performance.For the sample with Bi_(2)Se_(0.3)Te_(2.7)content of 1.5 wt%(i.e.,PAAc/XG/Bi2Se0.3Te27(1.5 wt%)),which has a room temperature Seebeck coefficient of-0.45 mV K^(-1),and exhibits an open-circuit voltage of-17.91 mV and output power of 38.1 nW at a temperature difference of 40 K.After being completely cut off,the hybrid thermoelectric hydrogel automatically recovers its electrical characteristics within a response time of 2.0 s,and the healed hydrogel remains more than 99%of its initial power output.Such stretchable and self-healable hybrid hydrogel thermoelectric materials show promising potential for application in dynamic service conditions,such as wearable electronics.

Realizing High Thermoelectric Performance in n-Type Se-Free Bi_(2)Te_(3)Materials by Spontaneous Incorporation of FeTe_(2)Nanoinclusions

Bi_(2)Te_(3)-based materials have drawn much attention from the thermoelectric community due to their excellent thermoelectric performance near room temperature.However,the stability of existing n-type Bi_(2)(Te,Se)_(3)materials is still low due to the evaporation energy of Se(37.70 kJ mol^(-1))being much lower than that of Te(52.55 kJ mol^(-1)).The evaporated Se from the material causes problems in interconnects of the module while degrading the efficiency.Here,we have developed a new approach for the high-performance and stable n-type Se-free Bi_(2)Te_(3)-based materials bymaximizing the electronic transport while suppressing the phonon transport,at the same time.Spontaneously generated FeTe_(2)nanoinclusions within the matrix during the melt-spinning and subsequent spark plasma sintering is the key to simultaneous engineering of the power factor and lattice thermal conductivity.The nanoinclusions change the fermi level of the matrix while intensifying the phonon scattering via nanoparticles.With a fine-tuning of the fermi level with Cu doping in the n-type Bi_(2)Te_(3)-0.02FeTe_(2),a high power factor of∼41×10^(-4)Wm^(-1)K^(-2)with an average zT of 1.01 at the temperature range 300-470 K are achieved,which are comparable to those obtained in n-type Bi_(2)(Te,Se)_(3)materials.The proposed approach enables the fabrication of high-performance n-type Bi_(2)Te_(3)-based materials without having to include volatile Se element,which guarantees the stability of the material.Consequently,widespread application of thermoelectric devices utilizing the n-type Bi_(2)Te_(3)-based materials will become possible.

Recent Progress of Two-Dimensional Thermoelectric Materials

Thermoelectric generators have attracted a wide research interest owing to their ability to directly convert heat into electrical power.Moreover,the thermoelectric properties of traditional inorganic and organic materials have been significantly improved over the past few decades.Among these compounds,layered two-dimensional(2D)materials,such as graphene,black phosphorus,transition metal dichalcogenides,IVA–VIA compounds,and MXenes,have generated a large research attention as a group of potentially high-performance thermoelectric materials.Due to their unique electronic,mechanical,thermal,and optoelectronic properties,thermoelectric devices based on such materials can be applied in a variety of applications.Herein,a comprehensive review on the development of 2D materials for thermoelectric applications,as well as theoretical simulations and experimental preparation,is presented.In addition,nanodevice and new applications of 2D thermoelectric materials are also introduced.At last,current challenges are discussed and several prospects in this field are proposed.

Fundamental and progress of Bi_2Te_3-based thermoelectric materials

Thermoelectric materials,enabling the directing conversion between heat and electricity,are one of the promising candidates for overcoming environmental pollution and the upcoming energy shortage caused by the over-consumption of fossil fuels.Bi2Te3-based alloys are the classical thermoelectric materials working near room temperature.Due to the intensive theoretical investigations and experimental demonstrations,significant progress has been achieved to enhance the thermoelectric performance of Bi2Te3-based thermoelectric materials.In this review,we first explored the fundamentals of thermoelectric effect and derived the equations for thermoelectric properties.On this basis,we studied the effect of material parameters on thermoelectric properties.Then,we analyzed the features of Bi2Te3-based thermoelectric materials,including the lattice defects,anisotropic behavior and the strong bipolar conduction at relatively high temperature.Then we accordingly summarized the strategies for enhancing the thermoelectric performance,including point defect engineering,texture alignment,and band gap enlargement.Moreover,we highlighted the progress in decreasing thermal conductivity using nanostructures fabricated by solution grown method,ball milling,and melt spinning.Lastly,we employed modeling analysis to uncover the principles of anisotropy behavior and the achieved enhancement in Bi2Te3,which will enlighten the enhancement of thermoelectric performance in broader materials

Frictional contact analysis of a rigid solid with periodic surface sliding on the thermoelectric material

Understanding and characterizing rough contact and wavy surfaces are essential for developing effective strategies to mitigate wear,optimize lubrication,and enhance the overall performance and durability of mechanical systems.The sliding friction contact problem between a thermoelectric(TE)half-plane and a rigid solid with a periodic wavy surface is the focus of this investigation.To simplify the problem,we utilize mixed boundary conditions,leading to a set of singular integral equations(SIEs)with the Hilbert kernels.The analytical solutions for the energy flux and electric current density are obtained by the variable transform method in the context of the electric and temperature field.The contact problem for the elastic field is transformed into the second-kind SIE and solved by the Jacobi polynomials.Notably,the smoothness of the wavy contact surface ensures that there are no singularities in the surface contact stress,and ensures that it remains free at the contact edge.Based on the plane strain theory of elasticity,the analysis primarily examines the correlation between the applied load and the effective contact area.The distribution of the normal stress on the surface with or without TE loads is discussed in detail for various friction coefficients.Furthermore,the obtained results indicate that the in-plane stress decreases behind the trailing edge,while it increases ahead of the trailing edge when subjected to TE loads.

Enhanced thermoelectric properties of Co_(1-x-y)Ni_(x+y)Sb_(3-x)Sn_x materials

Co1-x-yNix＋ySb3-xSnx polycrystals were fabricated by vacuum melting combined with hot-press sintering. The effect of alloying on the thermoelectric properties of unfilled skutterudite CO1-xNixSb3-xSnx was investigated. A leap of electrical conductivity from the Co0.93Ni0.07Sb2.93Sn0.07 sample to the Co0.88Ni0.12Sb288Sn0.12 sample occurs during the measurement of electrical conductivity, indicating the adjustment of band structure by proper alloying. The results show that alloying enhances the power factor of the materials. On the basis of alloying, the thermoelectric properties of Coo.88Nio.12Sb2.ssSno.12 are improved by Ni-doping. The thermal conductivities of Ni-doping samples have no reduction, but their power factors have obvious enhancement. The power factor of Co0.81Ni0.09Sb2.88Sn0.12 reaches 3.0 mW-m-1·K-2 by Ni doping. The dimensionless thermoelectric figure of merit reaches 0.55 at 773 K for the unfilled Co0.81Ni0.19 Sb2.88Sn0.12,

Advances in carbon-based thermoelectric materials for high-performance,flexible thermoelectric devices

Waste energy harvesting can contribute to the increase of the efficiency of many industrial processes,which consume energy to produce valuable products.Among all the wasted energy,heat energy is the most abundant,existing in almost any situation.Thermoelectric devices have the capability to harvest and convert the thermal energy into electrical power via the Seebeck effect.With its simple operating principle,thermoelectric devices can be reliable even under the harshest environments,taking advantage of any type of heat source.As a result,various inorganic and organic materials are being explored as thermoelectric materials.Among the reported materials,carbon-based materials are promising in terms of commericialization,due to their nontoxic and abundant nature,and solution processability.In particular,poly(3,4-ethylenedioxythiophene):polystyrene sulfonate(PEDOT:PSS),carbon nanotubes,and graphene are extensively studied as thermoelectric materials owing to their remarkable thermoelectric performance.Also,organic-inorganic hybrid halide perovskites show the potential to be used as future high-performance thermoelectric materials.Here,the progess in carbon materials as thermoelectrics is reviewed in detail,focusing on four base materials(PEDOT:PSS,carbon nanotubes,graphene,and organic-inorganic hybrid halide perovskites).This review illuminates the potential of carbon-based materials in the field of thermoelectrics and their application to next-generation energy devices.

GaInX_3(X=S,Se,Te):Ultra-low thermal conductivity and excellent thermoelectric performance

Seeking intrinsically low thermal conductivity materials is a viable strategy in the pursuit of high-performance thermoelectric materials.Here,by using first-principles calculations and semiclassical Boltzmann transport theory,we systemically investigate the carrier transport and thermoelectric properties of monolayer Janus GaInX_(3)(X=S,Se,Te).It is found that the lattice thermal conductivities can reach values as low as 3.07 W·m^(-1)·K^(-1),1.16 W·m^(-1)·K^(-1)and 0.57 W·m^(-1)·K^(-1)for GaInS_(3),GaInSe_(3),and GaInTe_(3),respectively,at room temperature.This notably low thermal conductivity is attributed to strong acoustic-optical phonon coupling caused by the presence of low-frequency optical phonons in GaInX_(3) materials.Furthermore,by integrating the charac teristics of electronic and thermal transport,the dimensionless figure of merit ZT can reach maximum values of 0.95,2.37,and 3.00 for GaInS_(3),GaInSe_(3),and GaInTe_(3),respectively.Our results suggest that monolayer Janus GaInX_(3)(X=S,Se,Te)is a promising candidate for thermoelectric and heat management applications.

Low-Cost and Biodegradable Thermoelectric Devices Based on van der Waals Semiconductors on Paper Substrates

We present a method to fabricate handcrafted thermoelectric devices on standard office paper substrates.The devices are based on thin films of WS_(2),Te,and BP(P-type semiconductors)and TiS_(3)and TiS_(2)(N-type semiconductors),deposited by simply rubbing powder of these materials against paper.The thermoelectric properties of these semiconducting films revealed maximum Seebeck coefficients of(+1.32±0.27)mV K^(-1)and(-0.82±0.15)mV K^(-1)for WS_(2)and TiS_(3),respectively.Additionally,Peltier elements were fabricated by interconnecting the P-and N-type films with graphite electrodes.A thermopower value up to 6.11 mV K^(-1)was obtained when the Peltier element were constructed with three junctions.The findings of this work show proof-of-concept devices to illustrate the potential application of semiconducting van der Waals materials in future thermoelectric power generation as well as temperature sensing for low-cost disposable electronic devices.

Single-and Two-band Transport Properties Crossover in Bi_(2)Te_(3)Based Thermoelectrics

Based on Peltier effect,Bi_(2)Te_(3)-based alloy is widely used in commercial solid-state refrigeration at room temperature.The mainstream strategies for enhancing room-temperature thermoelectric performance in Bi_(2)Te_(3)focus on band and microstructure engineering.However,a clear understanding of the modulation of band structure and scattering through such engineering remains still challenging,because the minority carriers compensate partially the overall transport properties for the narrow-gap Bi_(2)Te_(3)at room temperature(known as the bipolar effect).The purpose of this work is to model the transport properties near and far away from the bipolar effect region for Bi_(2)Te_(3)-based thermoelectric material by a two-band model taking contributions of both majority and minority carriers into account.This is endowed by shifting the Fermi level from the conduction band to the valence band during the modeling.A large amount of data of Bi_(2)Te_(3)-based materials is collected from various studies for the comparison between experimental and predicted properties.The fundamental parameters,such as the density of states effective masses and deformation potential coefficients,of Bi_(2)Te_(3)-based materials are quantified.The analysis can help find out the impact factors(e.g.the mobility ratio between conduction and valence bands)for the improvement of thermoelectric properties for Bi_(2)Te_(3)-based alloys.This work provides a convenient tool for analyzing and predicting the transport performance even in the presence of bipolar effect,which can facilitate the development of the narrow-gap thermoelectric semiconductors.

Control of interfacial reaction and defect formation in Gd/Bi_(2)Te_(2.7)Se_(0.3) composites with excellent thermoelectric and magnetocaloric properties

The method to combine thermoelectric(TE)and magnetocaloric(MC)cooling techniques lies in developing a new material that simultaneously possesses a large TE and good MC cooling performance.In this work,using n-type Bi_(2)Te_(2.7)Se_(0.3)(BTS)as the TE base material and Gd as the second-phase MC material,Gd/BTS composites were prepared by the spark plasma sintering method.In the composites,interfacial reaction between Gd and BTS was identified,resulting in the formation of Gd Te,which has a large impact on the electron concentration through the adjustment of defect concentration.The MC/TE composite containing 2.5 wt%Gd exhibited a ZT value of 0.6 at 300 K,essentially retaining the original TE performance,while all the composites largely maintained the excellent MC performance of Gd.This work provides a potential pathway to achieving high performance in MC/TE composites.

High-performance magnesium-based thermoelectric materials:Progress and challenges

Magnesium-based materials have been regarded as promising candidates for large-scale,high-efficiency thermoelectric applications,owing to their excellent dimensionless figure of merit,high abundance,and low cost.In this review,we comprehensively summarize the recent advances of Mg-based thermoelectrics,including Mg_(2)X(X=Si,Ge,Sn),Mg3(Sb,Bi)_(2),andα-MgAgSb,from both material and device level.Their electrical and thermal transport properties are first elucidated based on the crystallographic characteristics,band structures,and phonon dispersions.We then review the optimization strategies towards higher thermoelectric performance,as well as the device applications of Mg-based thermoelectric materials and the related engineering issues.By highlighting the challenges and possible solutions in the end,this review intends to offer perspectives on the future research work to further enhance the performance of Mg-based materials for practical applications.

Thermoelectric properties of p-type (Bi_(0.15)Sb_(0.85))_2Te_3-PbTe graded thermoelectric materials with different barriers

The p-type （Bi0.15Sb0.85）2Te3 and PbTe are typical thermoelectric materials used for low and middle temperature range and functional graded materials （FGM） is an inevitable way to widen the working temperature range. Here two segments graded thermoelectric materials （GTM） consisting of （Bi0.15Sb0.85）2Te3, PbTe and different barriers were fabricated by the common hot pressure method. Metals Fe, Mg and Ni were used as barriers between the two segments. The diffusion of different barriers between the barriers and bases were analyzed by electron microprobe analysis （EMA）. The phase and crystal structures were determined by X-ray diffraction analysis （XRD）. The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The results show that the compositional diffusion occurs when there is no barrier at the interface of the two segments, and the diffusion of Pb is most obvious; as the barrier material, the diffusion of metals Fe, Mg and Ni between different bases is not very obvious, and the thermoelectric properties of GTM is much better than that of the original segment.

Electronic structure engineering in organic thermoelectric materials

Electronic structures, which play a key role in determining electrical and optical properties of π-conjugated organic materials, have attracted tremendous interest. Efficient thermoelectric (TE) conversion of organic materials has rigorous requirements on electronic structures. Recently, the rational design and precise modulation of electronic structures have exhibited great potential in exploring state-of-the-art organic TE materials. This review focuses on the regulation of electronic structures of organic materials toward efficient TE conversion. First, we present the basic knowledge regarding electronic structures and the requirements for efficient TE conversion of organic materials, followed by a brief introduction of commonly used methods for electronic structure characterization. Next, we highlight the key strategies of electronic structure engineering for high-performance organic TE materials. Finally, an overview of the electronic structure engineering of organic TE materials, along with current challenges and future research directions, are provided.

Preparation and Thermoelectric Properties of SiO_2/β-Zn_4Sb_3 Nanocomposite Materials

A series of SiO2/β-Zn4Sb3 core-shell composite particles with 3, 6, 9, and 12 nm of SiO2 shell in thickness were prepared by coatingβ-Zn4Sb3 microparticles with SiO2 nanoparticles formed by hydrolyzing the tetraethoxysilane in alcohol-alkali-water solution. SiO2/β-Zn4Sb3 nanocomposite thermoelectric materials were fabricated with these core-shell composite particles by spark plasma sintering （SPS） method. Microstructure, phase composition, and thermoelectric properties of SiO2/β-Zn4Sb3 nanocomposite thermoelectric materials were systemically investigated. The results show thatβ-Zn4Sb3 microparticles are uniformly coated by SiO2 nanoparticles, and no any phase transformation reaction takes place during SPS process. The electrical and thermal conductivity gradually decreases, and the Seebeck coefficient increases compared to that ofβ-Zn4Sb3 bulk material, but the increment of Seebeck coefficient in high temperature range remarkably increases. The thermal conductivity of SiO2/β-Zn4Sb3 nanocomposite material with 12 nm of SiO2 shell is the lowest and only 0.56 W·m^-1·K^-1 at 460 K. As a result, the ZT value of the SiO2/β-Zn4Sb3 nanocomposite material reaches 0.87 at 700 K and increases by 30%.

Surface contact behavior of functionally graded thermoelectric materials indented by a conducting punch

The contact problem for thermoelectric materials with functionally graded properties is considered.The material properties,such as the electric conductivity,the thermal conductivity,the shear modulus,and the thermal expansion coefficient,vary in an exponential function.Using the Fourier transform technique,the electro-thermoelastic problems are transformed into three sets of singular integral equations which are solved numerically in terms of the unknown normal electric current density,the normal energy flux,and the contact pressure.Meanwhile,the complex homogeneous solutions of the displacement fields caused by the gradient parameters are simplified with the help of Euler’s formula.After addressing the non-linearity excited by thermoelectric effects,the particular solutions of the displacement fields can be assessed.The effects of various combinations of material gradient parameters and thermoelectric loads on the contact behaviors of thermoelectric materials are presented.The results give a deep insight into the contact damage mechanism of functionally graded thermoelectric materials(FGTEMs).

Electronic Structure and Transport Coefficients of the Thermoelectric Materials Bi_2Te_3 from First-principles Calculations

The electronic structures of bulk Bi_2Te_3 crystals were investigated by the first-principles calculations.The transport coefficients including Seeback coefficient and power factor were then calculated by the Boltzmann theory,and further evaluated as a function of chemical potential assuming a rigid band picture.The results suggest that p-type doping in the Bi_2Te_3 compound may be more favorable than n-type doping.From this analysis results,doping effects on a material will exhibit high ZT.Furthermore,we can also find the right doping concentration to produce more efficient materials,and present the ＂advantage filling element map＂ in detail.

Investigating the Potential of Quasi-One-Dimensional Organic Crystals of TTT(TCNQ)2 for Thermoelectric Applications

The purpose of this paper is to present the results of investigations on quasi-one-dimensional organic crystals of tetrathiotetracene-tetracyanoquinodi- methane (TTT(TCNQ)2) from the prospective of thermoelectric applications. The calculations were performed after analytical expressions, obtained in the frame of a physical model, more detailed than the model presented earlier by authors. The main Hamiltonian of the model includes the electronic and phonon part, electron-phonon interactions and the impurity scattering term. In order to estimate the electric charge transport between the molecular chains, the physical model was upgraded to the so-called three-dimen- sional (3D) physical model. Numeric computations were performed to determine the electrical conductivity, Seebeck coefficient, thermal conductivity, thermoelectric power factor and thermoelectric figure-of-merit as a function on charge carrier concentrations, temperatures and impurity concentrations. A detailed analysis of charge-lattice interaction, consisting of the exploration of the Peierls structural transition in TCNQ molecular chains of TTT(TCNQ)2 was performed. As result, the critical transition temperature was determined. The dispersion of renormalized phonons was examined in detail.

Thermoelectric properties of porous (Bi_(0.15)Sb_(0.85))_2Te_3 thermoelectric materials

In order to obtain thermoelectric materials with high figure of merit, theconcept of Hollow (Vacuum) Quantum Structure or Effect and related thermoelectric materials designwere proposed. To demonstrate the theory, the materials of (Bi_(0.15)Sb_(0.85))_2Te_3 with porousstructure have been fabricated. Their thermoelectric properties and the microstructure wereinvestigated and compared with their density structure. It was found that the porous structure couldimprove their properties greatly.

Thermoelectric Properties of N-typer Bi_2Te_3-PbTe Graded Thermoelectric Materials with Different Barriers

In order to investigate the adaptability of thermoelectric materials system with different barriers to functional graded thermoelectric materials, n-type Bi2Te, and PbTe two segments graded thermoelectric materials (GTM) with different barriers were fabricated by conventional hot pressing method. Metals Cu, Al, Fe, Co and Ni were used as barriers between two segments. The effects of different barriers on thermoelectric properties of GTM were investigated. The phase and crystal structures were determined by x-ray diffraction analysis (XRD). The distributions of different compositions were analyzed by electron microprobe analysis (EMA). The thermoelectric properties were measured at 303 K along the direction parallel to the pressing direction. The electric conductivity of samples was measured at 303 K by the four-probe technique. To measure the Seebeck coefficient, heat was applied to the samples, which were placed between two Cu discs. The thermoelectric electromotive force (E) was measured upon applying small temperature differences (DeltaT<275 K) between the both ends of the samples. The Seebeck coefficient of the samples was determined from the E/&UDelta;T.