Effects of Rattling Behavior of K and Cd Atoms along Different Directions in Anisotropic KCdAs on Lattice Thermal Transport and Thermoelectric Properties

We employ advanced first principles methodology,merging self-consistent phonon theory and the Boltzmann transport equation,to comprehensively explore the thermal transport and thermoelectric properties of KCdAs.Notably,the study accounts for the impact of quartic anharmonicity on phonon group velocities in the pursuit of lattice thermal conductivity and investigates 3ph and 4ph scattering processes on phonon lifetimes.Through various methodologies,including examining atomic vibrational modes and analyzing 3ph and 4ph scattering processes,the article unveils microphysical mechanisms contributing to the lowκL within KCdAs.Key features include significant anisotropy in Cd atoms,pronounced anharmonicity in K atoms,and relative vibrations in non-equivalent As atomic layers.Cd atoms,situated between As layers,exhibit rattling modes and strong lattice anharmonicity,contributing to the observed lowκL.Remarkably flat bands near the valence band maximum translate into high PF,aligning with ultralowκL for exceptional thermoelectric performance.Under optimal temperature and carrier concentration doping,outstanding ZT values are achieved:4.25(a(b)-axis,p-type,3×10^(19)cm^(−3),500 K),0.90(c-axis,p-type,5×10^(20)cm^(−3),700 K),1.61(a(b)-axis,n-type,2×10^(18)cm^(−3),700 K),and 3.06(c-axis,n-type,9×10^(17)cm^(−3),700 K).

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.

Phase transition and high temperature thermoelectric properties of copper selenide Cu_(2-x) Se (0 ≤ x ≤ 0.25)

With good electrical properties and an inherently complex crystal structure, Cu2-xSe is a potential ＂phonon glass electron crystal＂ thermoelectric material that has previously not attracted much interest. In this study, Cu2-xSe （0 ≤ x ≤0.25） compounds were synthesized by a melting-quenching method, and then sintered by spark plasma sintering to obtain bulk material. The effect of Cu content on the phase transition and thermoelectric properties of Cu2-xSe were investigated in the temperature range of 300 K-750 K. The results of X-ray diffraction at room temperature show that Cu2-xSe compounds possess a cubic structure with a space group of Fm3m （#225） when 0.15 〈 x ≤ 0.25, whereas they adopt a composite of monoclinic and cubic phases when 0 ≤x ≤ 0.15. The thermoelectric property measurements show that with increasing Cu content, the electrical conductivity decreases, the Seebeck coefficient increases and the thermal conductivity decreases. Due to the relatively good power factor and low thermal conductivity, the nearly stoichiometric Cu2Se compound achieves the highest ZT of 0.38 at 750 K. It is expected that the thermoelectric performance can be further optimized by doping appropriate elements and/or via a nanostructuring approach.

Enhanced Thermoelectric and Mechanical Properties of n-type Bi2Te2.7Se0.3 Bulk Alloys by Electroless Plating with Cu

Bi2Te2.7Se0.3/Cu core/shell powders were prepared by electroless plating and hydrogen reduction, and then sintered into bulk by spark plasma sintering in order to improve the thermoelectric and mechanical properties of n-type Bi-Te thermoelectric material. After electroless plating, with the increasing of Cu content, Seebeck coefficient keeps increasing and power factor enhances significantly. The highest power factor increases by three times and reaches 23.8 W·cm^-1·K^-2 at room temperature in Bi2Te2.7Se0.3 with 0.22 wt%Cu sample, which means electrical transport properties of Bi2Te2.7Se0.3/Cu samples have been improved.Meanwhile, the ZT values of Bi2Te2.7Se0.3/Cu samples can be enhanced at different temperature zone by adjusting the Cu content. Bi2Te2.7Se0.3 with 0.05 wt% Cu sample has the best thermoelectric properties in high temperature zone, and the ZT peak value increases from 0.35 to 0.85 at 623 K. When the Cu content increases to 0.15 wt%,the ZT peak value moves to the low temperature(373 K) and increases from 0.24 to 0.71. At the same time, the mechanical properties increases with the increasing of Cu content.

Electronic Structures and Thermoelectric Properties of Two-Dimensional MoS2/MoSe2 Heterostructures

Thermoelectric properties of bulk and bilayer two-dimensional （2D） MoS2/MoSe2 het- erostructures are investigated using density functional theory in conjunction with semi- classical Boltzmann transport theory. It is predicted that the bulk 2D heterostructures could considerably enhance the thermoelectric properties as compared with the bulk MoSe2. The enhancement originates from the reduction in the band gap and the presence of interlayer van der Waals interactions. We therefore propose the 2D MoS2/MoSe2 heterostructures as a possible candidate material for thermoelectric applications.

Graphene-enhanced thermoelectric properties of p-type skutterudites

Nanocomposite is proved to be an effective method to improve thermoelectric performance.In the present study,graphene is introduced into p-type skutterudite La0.8Ti0.1Ga0.1Fe3CoSb12 by plasma-enhanced chemical vapor deposition（PECVD）method to form skutterudite/graphene nanocomposites.It is demonstrated that the graphene has no obvious effect on the electrical conductivity of La0.8Ti0.1Ga0.1Fe3CoSb12,but the Seebeck coefficient is slightly improved at high temperature,thereby leading to high power factor.Furthermore,due to the enhancement of phonon scattering by the graphene,the lattice thermal conductivity is reduced significantly.Ultimately,the maximum z T value of La0.8Ti0.1Ga0.1Fe3CoSb12/graphene is higher than that of graphene-free alloy and reaches to 1.0 at 723 K.Such an approach raised by us enriches prospects for future practical application.

Electronic Structure and Thermoelectric Properties of Ca_3Co_4O_9

The relation among electronic structure, chemical bond and thermoelectric property of Ca3 Co4 O9 was studied using density function and discrete variation method （DFT-DVM）. The gap between the highest valence band （HVB） and the lowest conduction band （LCB） shows a semiconducting property. Ca3 Co4 O9 consists of CoO2 and Ca2 CoO3 two layers. The HVB and LCB near Fermi level are only mainly from O（ 2 ） 2p and Co （2） 3d in Ca2 CoO3 layer. Therefore, the semiconducting or thermoelectric property of Ca3 Co4 O9 should be mainly from Ca2 CoO3 layer, but it seems to have no direct relation to the CoO2 layer, which is consistent with that binary oxides hardly have a thermoelectric property, but trinary oxide compounds have quite a good thermoelectric property. The covalent and ionic bonds of Ca2 CoO3 layer are both weaker than those of CoO2 layer. Ca plays the role of connections between CoO2 and Ca2 CoO3 layers in Ca3 Co4 O9, decrease the ionic and covalent bond strength, and improve the thermoelectric property.

Effects of Electroless Plating with Cu Content on Thermoelectric and Mechanical Properties of p-type Bi0.5Sb1.5Te3 Bulk Alloys

Bi_（0.5）Sb_（1.5）Te_3/Cu core/shell powders were prepared by electroless plating and hydrogen reduction, and then sintered into bulk by spark plasma sintering. After electroless plating, with increasing the Cu content, the electrical conductivity keeps enhancing significantly. The highest electrical conductivity reaches 3341 S/cm at room temperature in Bi0.5Sb1.5Te3 with 0.67 wt% Cu bulk sample. Moreover, the lowest lattice thermal conductivity reaches 0.32 W/m·K at 572.2 K in Bi0.5Sb1.5Te3 with 0.67 wt% Cu bulk sample, which is caused by the scattering of the rich-copper particles with different dimensions and massive grain boundaries. According to the results, the ZT values of all Bi0.5Sb1.5Te3/Cu bulk samples have improved in a high temperature range. In Bi0.5Sb1.5Te3 with 0.15 wt% Cu bulk sample, the highest ZT value at 573.4 K is 0.81. When the Cu content increases to 0.67 wt%, the highest ZT value reaches 0.85 at 622.2 K. Meanwhile, the microhardness increases with increasing the Cu content.

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.

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.

The First Principles Investigations of the Thermoelectric Properties of GaN with p-and n-Type Doping

We investigate the thermoelectric properties of GaN with p- and n-type doping by the first principles calculation and the semi-classical Boltzmann theory. We find that the power factors （Sacr） of p-type GaN （-3500 μW/mK2） is about twice that of the n-type （-1750 μW/mK2）, which indicates the thermoelectric properties of p-type GaN would be better. Thermal conductivity of GaN crystal decreases rapidly as the temperature increases, but it is still too large for thermoelectric applications. The figure of merit （ZT） estimated at 1500 K is 0.134 for p-type GaN crystal and 0.062 for the n-type.

Enhanced thermoelectric properties of p-type polycrystalline SnSe by regulating the anisotropic crystal growth and Sn vacancy

Thermoelectric selenides have attracted more and more attentions recently.Herein,p-type Sn Se polycrystalline bulk materials with good thermoelectric properties are presented.By using the SnSe2 nanostructures synthesized via a wetchemistry route as the precursor,polycrystalline Sn Se bulk materials were successfully obtained by a combined heattreating process under reducing atmosphere and following spark plasma sintering procedure.As a reference,the Sn Se nanostructures synthesized via a wet-chemistry route were also fabricated into polycrystalline bulk materials through the same process.The thermoelectric properties of the Sn Se polycrystalline transformed from SnSe2 nanostructures indicate that the increasing of heattreating temperature could effectively decrease the electrical resistivity,whereas the decrease in Seebeck coefficient is nearly invisible.As a result,the maximum power factor is enhanced from 5.06×10^-4W/m·K^2 to 8.08×10^-4W/m·K^2 at 612℃.On the other hand,the reference sample,which was obtained by using Sn Se nanostructures as the precursor,displays very poor power factor of only 1.30×10^-4W/m·K^2 at 537℃.The x-ray diffraction（XRD）,scanning electron microscope（SEM）,x-ray fluorescence（XRF）,and Hall effect characterizations suggest that the anisotropic crystal growth and existing Sn vacancy might be responsible for the enhanced electrical transport in the polycrystalline Sn Se prepared by using SnSe2 precursor.On the other hand,the impact of heat-treating temperature on thermal conductivity is not obvious.Owing to the boosting of power factor,a high z T value of 1.07 at 612℃ is achieved.This study provides a new method to synthesize polycrystalline Sn Se and pave a way to improve the thermoelectric properties of polycrystalline bulk materials with similar layered structure.

Effects of nano-TiO_2 dispersion on thermoelectric properties of Co_4Sb_(11.7)Te_(0.3) composites

Nano-TiO2/Co4Sb11.7Te0.3 composites were prepared by mechanical alloying (MA) and cold isostatic pressing (CIP) process.The phase composition,microstructure,and thermoelectric properties were characterized.The diffraction spectra of all samples well corresponds to CoSb3 skutterudite diffraction plane.TiO2 agglomerates into irregular clusters.They locate at the grain boundaries or some are distributed on the surface of Co4Sb11.7Te0.3 particles.For composites with high TiO2 content (0.6% and 1.0% TiO2),the phonon scattering by TiO2 particle,pores,and small size grains can result in a remarkable reduction in thermal conductivity.The maximum value of ZT is 0.79 for sample with 0.6 wt.% TiO2 at 700 K,which is 11% higher than that of non-dispersed 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.

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,

Preparation and photoelectric properties of cadmium sulfide quantum dots

Cadmium sulfide quantum dots(CdS QDs) are widely used in solar cells, light emitting diodes, photocatalysis, and biological imaging because of their unique optical and electrical properties. However, there are some drawbacks in existing preparation techniques for CdS QDs, such as protection of inert gas, lengthy reaction time, high reaction temperature, poor crystallinity, and non-uniform particle size distribution. In this study, we prepared CdS QDs by liquid phase synthesis under ambient room temperature and atmospheric pressure using sodium alkyl sulfonate, CdCl_2, and Na_2S as capping agent, cadmium, and sulfur sources respectively. This technique offers facile preparation, efficient reaction, low-cost, and controllable particle size. The as-prepared CdS QDs exhibited good crystallinity, excellent monodispersity, and uniform particle size. The responsivity of CdS QDs-based photodetector is greater than 0.3 μA/W, which makes them suitable for use as ultra-violet(UV) detectors.

Solvothermal synthesis and thermoelectric properties of skutterudite compound Fe_(0.25)Ni_(0.25)Co_(0.5)Sb_3

Nanostructured skutterudite-related compound Fe0.25Ni0.25Co0.5Sb3 was synthesized by a solvothermal method using FeCl3, NiCl2, CoCl2, and SbCl3 as the precursors and NaBH4 as the reductant. The solvothermally synthesized powders consisted of fine granules with an average particle size of tens of nanometers. The bulk material was prepared by hot pressing the powders. Transport property measurements indicated a heavily doped semiconductor behavior with n-type conduction. The thermal conductivity is about 1.83 W·m-1·K-1 at room temperature and decreases to 1.57 W·m-1·K-1 at 673 K. The low thermal conductivity is attributed to small grain size and high porosity. A maximum dimensionless figure of merit of 0.15 is obtained at 673 K.

Crystal Structures and Thermoelectric Properties of Sm-Filled Skutterudite Compounds Sm_yFe_xCo_(4-x)Sb_(12)

Polycrystalline samples of Sm partially filled skutterudites SmyFexCo4-xSb12 were prepared by melting and Spark Plasma Sintering technique. The results of Rietveld refinement showed that the obtained SmyFexCo4-xSb12 samples possessed filled skutterudite structures. The thermal parameter （B） of Sm is larger than that of Sb, Fe, and Co, indicating that Sm ＂rattled＂ in Sb-icosahedron voids. The effects of filling atom Sm on thermoelectric properties of these compounds were investigated. With the increase of Sm filling fraction （y）, electrical conductivity decreased, Seebeck coefficient increased and had a maximum value when y was 0.38; thermal conductivity reduced and had a minimum value when y was 0. 32. At 750 K, the highest figure of merit of 0.68 was obtained for Sm0.32Fe1.47Co2.53Sb12.

Photoelectric Properties of Soluble ZnPc-Epoxy Derivative Doped with C_(60)

A kind of soluble phthalocyanine derivative （ZnPc-epoxy derivative） was synthesized, and the influence of C60 on the photoelectric properties of the derivative was studied. The results of ultraviolet-visible （UV-Vis） spectra show that the absorption of the complex is larger than that of the ZnPc-epoxy derivative at B belt. But compared with the derivative, the absorption of the complex decreased at Q belt. The fluorescence spectra show that C60 takes role as annihilation in ZnPc-epoxy derivative. Photo-current tests show that the ZnPc-epoxy derivative-C60 film exhibits increasing photoconductive property.

High temperature thermoelectric properties of highly c-axis oriented Bi_2Sr_2Co_2O_y thin films fabricated by pulsed laser deposition

High-temperature thermoelectric transport property measurements have been performed on the highly c-axis oriented Bi2Sr2Co20v thin films prepared by pulsed laser deposition on LaA1Oa （001）. Both the electric resistivity p and the seebeck coefficient S of the film exhibit an increasing trend with the temperature from 300 K-1000 K and reach up to 4.8 m. cm and 202 V/K at 980 K, resulting in a power factor of 0.85 mW/mK which are comparable to those of the single crystalline samples. A small polaron hopping conduction can be responsible for the conduction mechanism of the film at high temperature. The results demonstrate that the Bi2Sr2Co2Oy thin film has potential application has high temperature thin film thermoelectric devices,