Ultrahigh thermoelectric properties of p‐type Bi_(x)Sb_(2−x)Te_(3) thin films with exceptional flexibility for wearable energy harvesting

Use of a flexible thermoelectric source is a feasible approach to realizing selfpowered wearable electronics and the Internet of Things.Inorganic thin films are promising candidates for fabricating flexible power supply,but obtaining highthermoelectric‐performance thin films remains a big challenge.In the present work,a p‐type Bi_(x)Sb_(2−x)Te_(3) thin film is designed with a high figure of merit of 1.11 at 393 K and exceptional flexibility(less than 5%increase in resistance after 1000 cycles of bending at a radius of∼5 mm).The favorable comprehensive performance of the Bi_(x)Sb_(2−x)Te_(3) flexible thin film is due to its excellent crystallinity,optimized carrier concentration,and low elastic modulus,which have been verified by experiments and theoretical calculations.Further,a flexible device is fabricated using the prepared p‐type Bi_(x)Sb_(2−x)Te_(3) and n‐type Ag_(2)Se thin films.Consequently,an outstanding power density of∼1028μWcm^(−2)is achieved at a temperature difference of 25 K.This work extends a novel concept to the fabrication of highperformance flexible thin films and devices for wearable energy harvesting.

Effect of erbium substitution on thermoelectric properties of complex oxide Ca_3Co_2O_6 at high temperatures

Polycrystalline particles of Ca3-xErxCo2O6 （x=0.0, 0.15, 0.3, 0.45 and 0.6） were synthesized using sol-gel method combined with Low Temperature Sintering procedure （LTS） to evaluate the effect of Er substitution on the thermoelectric properties of Ca3Co2O6. The crystal structure and microstructure were investigated using X-ray diffraction, infrared spectroscopy and scanning electron microscope. The electrical conductivity and Seebeck coefficient of the complex oxides were measured from 300 to 1073 K. The results showed that all the sampies were p-type semiconductors. The electrical conductivity increased with the increase in temperature. Er substitutions at Ca site affected carrier concentrations and carder mobility, resulting an increase in Seebeck coefficient and decrease in electrical conductivity. The power factor of Ca2.85Er0.15Co2O6 reached 10.66 μw/mK^2 at 1073 K.

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%.

Effects of Thickness and Temperature on Thermoelectric Properties of Bi2Te3-Based Thin Films

Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices of different thicknesses are prepared on the silicon dioxide substrates by magnetron sputtering technique and thermally annealed at 573 K for 30 min. Thermoelectric（TE）measurements indicate that optimal thickness and thickness ratio improve the TE performance of Bi_2Te_3 thin films and GeTe/B_2Te_3 superlattices, respectively. High TE performances with figure-of-merit（ZT） values as high as 1.32 and 1.56 are achieved at 443 K for 30 nm and 50 nm Bi_2Te_3 thin films, respectively. These ZT values are higher than those of p-type Bi_2Te_3 alloys as reported. Relatively high ZT of the GeTe/B_2Te_3 superlattices at 300-380 K were 0.62-0.76. The achieved high ZT value may be attributed to the unique nano-and microstructures of the films,which increase phonon scattering and reduce thermal conductivity. The results indicate that Bi_2Te_3-based thin films can serve as high-performance materials for applications in TE devices.

Microstructure and Thermoelectric Properties of Bi- and Cu-Substituted Ca_3Co_4O_9 Oxides

Bi- and Cu-substituted Ca3Co4O9 samples were prepared by conventional solid-state reaction method and the effect of element substitution on the microstructures and thermoelectric properties was investigated. Partial substitution of Cu for Co leads to an increase in electrical conductivity and a decrease in Seebeck coefficient due to the rise of hole concentration. The microstructure of Cu-substituted sample is almost unchanged compared with undoped Ca3Co4O9. On the other hand, partial substitution of Bi for Ca gives rise to a significant increase in the grain size, and c-axis-oriented structure can be formed in Ca2.7Bi0.3Co4O9, resulting in an obvious increase in electrical conductivity. Cu and Bi co-substitution further increases the grain growth and the electrical conductivity of Ca2.7Bi0.3Co3.7Cu0.3O9. Thus, Cu and Bi co-substitution samples possess the optimal thermoelectric performance at high temperature and the highest value of power factor can reach 3.1×10^-4 Wm^-1.K^-2 at 1000 K.

Effect of carbon nanotubes addition on thermoelectric properties of Ca_(3)Co_(4)O_(9)ceramics

Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity.In this study,a series of Ca3 Co_(4)O_(9)/x wt.%CNTs(x=0,3,5,7,10)polycrystalline ceramic thermoelectric materials by adding carbon nanotubes(CNTs)were prepared with sol-gel method and cold-pressing sintering technology.The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure.The effect of adding carbon nanotubes to the thermoelectric properties of Ca_(3)Co_(4)O_(9)was systematically measured.The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material.Nevertheless,the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes.At 625 K,the thermal conductivity of Ca3 Co_(4)O_(9)/10 wt.%CNTs sample is reduced to0.408 W·m^(-1)·K^(-1),which is about 73%lower than that of the original sample.When the three parameters are coupled,the figure of merit of Ca_(3)Co_(4)O_(9)/3 wt.%CNTs sample reaches 0.052,which is 29%higher than that of the original sample.This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca_(3)Co_(4)O_(9)ceramic samples and improve their thermoelectric properties.

Thermoelectric properties of lower concentration K-doped Ca_(3)Co_(4)O_(9) ceramics

The tuning of electron and phonon by ion doping is an effective method of improving the performances of thermoelectric materials.A series of lower concentration K-doped Ca_(3-x)K_(x)Co_(4)O_(9)(x=0,0.05,0.10,0.15)polycrystalline ceramic samples are prepared by combining citrate acid sol-gel method with cold-pressing sintering method.The single-phase compositions and plate-like grain morphologies of all samples are confirmed by x-ray diffraction and field emission scanning electron microscope.The effects of lower concentration K doping on the thermoelectric properties of the material are evaluated systematically at high temperatures(300–1026 K).Low concentration K doping causes electrical conductivity to increase up to 23%with little effect on the Seebeck coefficient.Simultaneously,the thermal conductivity of K-doped sample is lower than that of the undoped sample,and the total thermal conductivity reaches a minimum value of approximately1.30 W·m^(-1)·K^(-1),which may be suppressed mainly by the phonon thermal conduction confinement.The dimensionless figure-of-merit ZT of Ca_(2.95)K_(0.05)Co_(4)O_(9)is close to 0.22 at 1026 K,representing an improvement of about 36%compared with that of Ca_(3)Co_(4)O_(9),suggesting that lower concentration K-doped Ca_(3)Co_(4)O_(9)series materials are promising thermoelectric oxides for high-temperature applications.

Phase formation and thermoelectric properties of Zn_(1+x)Sb binary system

The phase formation and thermoelectric(TE)properties in the central region of the Zn−Sb phase diagram were analyzed through synthesizing a series of Zn_(1+x)Sb(x=0,0.05,0.1,0.15,0.25,0.3)materials by reacting Zn and Sb powders below the solidus line of the Zn−Sb binary phase diagram followed by furnace cooling.In this process,the nonstoichiometric powder blend crystallized in a combination of ZnSb andβ-Zn4Sb3 phases.Then,the materials were ground and hot pressed to form dense ZnSb/β-Zn4Sb3 composites.No traces of Sb and Zn elements or other phases were revealed by X-ray diffraction,high resolution transmission electron microscopy and electron energy loss spectroscopy analyses.The thermoelectric properties of all materials could be rationalized as a combination of the thermoelectric behavior of ZnSb andβ-Zn4Sb3 phases,which were dominated by the main phase in each sample.Zn1.3Sb composite exhibited the best thermoelectric performance.It was also found that Ge doping substantially increased the Seebeck coefficient of Zn1.3Sb and led to significantly higher power factor,up to 1.51 mW·m−1·K−2 at 540 K.Overall,an exceptional and stable TE figure of merit(ZT)of 1.17 at 650 K was obtained for Zn1.28Ge0.02Sb.

Rapid synthesis of CoSb_3 by MA-SPS and its thermoelectric properties

Bulk CoSb3 with single phase was synthesized by mechanical alloying and spark plasma sintering （MA-SPS）. The thermoelectric properties of bulk CoSb3 prepared by different technologies were investigated. All samples have the character of typical semiconductor electricity and their thermoelectric figures of merit （ZT） get the maximum values at 400℃. The highest ZT value is 0.0571, belonging to the sample sintered at 600℃ among all samples at all temperatures.

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.

Thermoelectric properties of Bi_(0.5)Sb_(1.5)Te_3/polyaniline composites prepared by mechanical blending and in-situ polymerization

Bi 0.5 Sb 1.5 Te 3/polyaniline composites were prepared by mechanical blending and in situ polymerization, and their transport properties were measured. It was found that for the composites with 1%, 3%, 5% and 7% polyaniline (mass fraction) respectively, which were prepared by mechanical blending, the power factors decrease by about 30%, 50%, 55% and 65% compared with the Bi 0.5 Sb 1.5 Te 3 samples, which is mainly due to the remarkable decreases of the electrical conductivity. The electrical conductivity and power factor of the composites samples with 7% polyaniline prepared by in situ polymerization are higher by about 65% and 60%, respectively, than that of the corresponding samples prepared by mechanical blending.

Roles of concentration-dependent Cu doping behaviors on the thermoelectric properties of n-type Mg_(3)Sb_(1.5)Bi_(0.5)

Large Seebeck coefficients induced by high degeneracy of conduction band minimum,and low intrinsic lattice thermal conductivity originated from large lattice vibrational anharmonicity render Mg_(3)Sb_(2)as a promising n-type thermoelectric material.Herein,we demonstrated unique concentration-dependent occupation behaviors of Cu in Mg_(3.4)Sb_(1.5)Bi_(0.49)Te_(0.01)matrix,evidenced by structural characterization and transport property measurements.It is found that Cu atoms prefer to enter the interstitial lattice sites in Mg_(3)Sb_(2)host with low doping level(Mg_(3.4)Sb_(1.5)Bi_(0.49)Te_(0.01)+x%Cu,x<0.3%),acting as donors for providing additional electrons without deteriorating the carrier mobility.When x is larger than 0.3%,the excessive Cu atoms are inclined to substitute Mg atoms,yielding acceptors to decrease the electron concentration.As a result,the electrical conductivity of the Mg_(3.4)Sb_(1.5)Bi_(0.49)Te_(0.01)+0.3%Cu sample reaches 2.3×10^(4)S/m at 300 K,increasing by 300%compared with that of the pristine sample.The figure of merit zT values in the whole measured temperature range are significantly improved by the synergetic improvement of power factor and reduction of thermal conductivity.An average zT∼1.07 from 323 K to 773 K has been achieved for the Mg_(3.4)Sb_(1.5)Bi_(0.49)Te_(0.01)+0.3%Cu sample,which is about 30%higher than that of the Mg_(3.4)Sb_(1.5)Bi_(0.49)Te_(0.01) sample.

Effects of Magnetization on Thermoelectric Transport Properties of CoSb_(3)Material

The effects of magnetization on the phase composition,microstructure and thermoelectric transport properties of CoSb_(3)were studied systematically.The magnetic properties of CoSb_(3)material were also measured at room temperature in order to confirm its magnetic category.The results of XRD and FESEM analysis indicated that the phase composition and microstructure of the CoSb_(3)were not affected by magnetization.The results of thermoelectric transport measurement showed that the electrical and thermal transport properties of materials were also not affected by magnetization.These results were mainly attributed to the diamagnetism of the CoSb_(3)material,which were consistent with the results of the magnetic properties measurement.This study is expected to provide a special research perspective for studying the effects of the external conditions on the structure and properties of thermoelectric materials.

Optimization of thermoelectric properties of n-type Bi_2(Te,Se)_3 with optimizing ball milling time

The thermoelectric properties at elevated temperature were investigated for n-type Bi2（Te,Se）3 which is obtained from ball milling processed powder with various milling times. Electrical properties such as electrical resistivity and Seebeck coefficient are clearly dependent on milling time, in which the carrier concentration is attributed to the change of the electrical properties. The concentrations of the defects are also varied with the ball milling time, which is the origin of the carrier concentration variation. Even though finer grain sizes are obtained after the long ball milling time, the temperature dependence of the thermal conductivity is not solely understood with the grain size, whereas the electrical contribution to the thermal conductivity should be also considered. The highest figure of merit value of ZT = 0.83 is achieved at 373 K for the optimized samples, in which ball milling time is 10 h. The obtained ZT value is 48% improvement over that of the 0.5-h sample at 373 K.

High Temperature Thermoelectric Properties of Dy-doped CaMnO_3 Ceramics

Dy-doped CaMnO3 ceramics have been synthesized by co-precipitation method combined with the solid-state reaction.Phase composition and microstructure analysis indicate that high density and pure CaMnO3 phase can be achieved.The electric conductivity can be enhanced by Dy doping,and result in a slight increase of the thermal conductivity.The highest dimensionless figure of merit ZT of 0.15 has been obtained at 973 K for x = 0.02 sample,which is about 4 times larger than that of the pure CaMnO3,which indicate that CaMnO3can be a promising candidate for n-type thermoelectric material at high temperature.

Microstructure and thermoelectric properties of Bi1.9Lu0.1Te3 compound

Effect of fabrication conditions on microstructure and thermoelectric properties of the Bi1.9Lu0.lTe3 compound was studied. Starting nanopowder with mean nanoparticle size of -37 nm was synthesized by a microwave-solvothermal method. In order to prepare samples with various micro-grained structures, the synthesized nanopowder was compacted by two methods. The first method is cold isostatic pressing with further high- temperature annealing, while the second method is spark plasma sintering at various temperatures of process （653 and 683 K）. It is found that mean grain size is equal to -290,-730 and -1160 nm for cold isostatically pressed and spark plasma sintered at 653 and 683 K samples, respectively. The micro-grained sample with maximum mean grain size shows the best thermoelectric properties. This sample is structurally inhomogeneous and has the lowest thermal conductivity and the specific electrical resistivity. Maximum dimensionless figure of merit for this sample is equal to -0.9 for temperature range of 450-500 K.

Enhanced Thermoelectric and Mechanical Properties of p-type Bi_(0.5)Sb_(1.5)Te_(3) Bulk Alloys by Composite Electroless Plating with Ni&Cu

In order to improve the thermoelectric and mechanical properties of p-type Bi_(2)Te_(3) thermoelectric material,Bi_(0.5)Sb_(1.5)Te_(3)/Ni&Cu core/shell powders were electroless plated with the same content of Ni and different content of Cu,and then reduced by hydrogen,and finally sintered into bulk by spark plasma sintering.After composite electroless plating with Ni&Cu,for the bulk sample with 0.3 wt% Ni and 0.15 wt% Cu,the power factor rises significantly and the highest value increases from 25 to 33 μW·cm^(-1)· K^(-2) at room temperature.Meanwhile,the thermal conductivity decreases to about 0.80 W·m^(-1)·K^(-1) at 623 K.Therefore,the composite electroless plating with Ni&Cu can obviously improve the electrical and thermal transport performance of p-type Bi_(2)Te_(3) based thermoelectric materials.Thus,the ZT value enhances significantly and the highest value increases over 3 times,from 0.35 to 1.16 at 473 K in Bi_(0.5)Sb_(1.5)Te_(3) with 0.3 wt% Ni and 0.15 wt% Cu bulk sample.At the same time,the mechanical properties have also been improved after composite electroless plating with Ni&Cu.

Thermoelectric Properties of an Individual Suspended Single-Crystalline Sb_(2)Se_(3)Nanowire

Nanowires exhibit excellent thermoelectric performance,due to the stronger quantum confinement and phonon scattering effect compared to bulk materials.However,it is a challenge to accurately evaluate the thermoelectric performance of nanowires.In this paper,the thermoelectric properties of an individual suspended Sb_(2)Se_(3) nanowire have been characterized by comprehensive T-type method,including thermal conductivity,electrical conductivity,Seebeck coefficient and figure of merit.The thermal conductivity increases from 0.57 W/(m·K)to 3.69 W/(m·K)with temperature increasing from 80 K to 320 K.The lattice vibration dominates the heat conduction process,and due to its flawless crystal structure,the thermal conductivity is not lower than the reported values of bulk Sb_(2)Se_(3).The electrical conductivity increases from 7.83 S/m to 688 S/m in the temperature range of 50 K–320 K,which is a great improvement compared with the corresponding bulk value.At 294 K,the Seebeck coefficient of the Sb_(2)Se_(3) nanowire is–1120μV/K and the corresponding figure of merit is 0.064.

Theoretical investigations of electrical transport properties in CoSb3 skutterudites under hydrostatic loadings

CoSb3-based mark mid-temperature skutterudites have been a benchthermoelectric material under intensive experimental and theoretical studies for decades. Doping and filling, to the first order, alter the crystal lattice constant of CoSb3 in the context of ＂chemical pressure.＂ In this work, we employed ab initio density functional theory in conjunction with semiclassical Boltzmann transport theory to investigate the mechanical properties and especially how hydrostatic loadings, i.e., ＂physical pressure,＂ impact the electronic band structure, Seebeck coefficient, and power factor of pristine CoSb3. It is found that hydrostatic pressure enlarges the band gap, suppresses the density of states （DOS） near the valence band edge, and fosters the band convergence between the valley bands and the conduction band minimum （CBM）. By contrast, hydrostatic tensile reduces the band gap, increases the DOS near the valence band edge, and diminishes the valley bands near the CBM. Therefore, applying hydrostatic pressure provides an alternative avenue for achieving band convergence to improve thermoelectric properties of N-type CoSb3, which is further supported by our carrier concentration studies. These results provide valuable insight into the further improvement of thermoelectric performance of CoSb3-based skutterudites via a synergy of physical and chemical pressures.

Effect of Mg addition on mechanical and thermoelectrical properties of Al-Al_2O_3 nanocomposite

The effects of Mg addition on mechanical thermo-electrical properties of Al.Mg/5%Al2O3 nanocomposite with differentMg contents （0, 5%, 10% and 20%） produced by mechanical alloying were studied. Scanning electron microscopy analysis （SEM）,X-ray diffraction analysis （XRD） and transmission electron microscopy （TEM） were used to characterize the produced powder. Theresults show that addition of Mg forms a predominant phase （Al.Mg solid solution）. By increasing the mass fraction of Mg, thecrystallite size decreases and the lattice strain increases which results from the atomic penetration of Mg atoms into the substitutionalsites of Al lattice. The microhardness of the composite increases with the increase of the Mg content. The thermal and electricalconductivities increase linearly with the temperature increase in the inspected temperature range. Moreover, the thermalconductivity increases with the increase of Mg content.