Thermoelectric enhancement in triple-doped strontium titanate with multi-scale microstructure

Strontium titanate(SrTiO_3) is a thermoelectric material with large Seebeck coefficient that has potential applications in high-temperature power generators.To simultaneously achieve a low thermal conductivity and high electrical conductivity,polycrystalline SrTiO_3 with a multi-scale architecture was designed by the co-doping with lanthanum,cerium,and niobium.High-quality nano-powders were synthesized via a hydrothermal method.Nano-inclusions and a nano/microsized second phase precipitated during sintering to form mosaic crystal-like and epitaxial-like structures,which decreased the thermal conductivity.Substituting trivalent Ce and/or La with divalent Sr and substituting pentavalent Nb with tetravalent Ti enhanced the electrical conductivity without decreasing the Seebeck coefficient.By optimizing the dopant type and ratio,a low thermal conductivity of 2.77 W·m~(-1)·K~(-1) and high PF of 1.1 mW·m~(-1)·K~(-2) at 1000 K were obtained in the sample co-doped with 5-mol% La,5-mol% Ce,and 5-mol% Nb,which induced a large ZT of 0.38 at 1000 K.     

Tuning Thermal Conductivity in Si Nanowires with Patterned Structures

Tuning the thermal conductivity of silicon nanowires(Si-NWs)is essential for realization of future thermoelectric devices.The corresponding management of thermal transport is strongly related to the scattering of phonons,which are the primary heat carriers in Si-NWs.Using the molecular dynamics method,we find that the scattering of phonons from internal body defects is stronger than that from surface structures in the low-porosity range.Based on our simulations,we propose the concept of an exponential decay in thermal conductivity with porosity,specifically in the low-porosity range.In contrast,the thermal conductivity of Si-NWs with a higher porosity approaches the amorphous limit,and is insensitive to specific phonon scattering processes.Our findings contribute to a better understanding of the tuning of thermal conductivity in Si-NWs by means of patterned nanostructures,and may provide valuable insights into the optimal design of one-dimensional thermoelectric materials.     

Effect of Nb doping on microstructures and thermoelectric properties of SrTiO_3 ceramics

Nb-doped SrTiO_3 thermoelectric ceramics with different niobium concentrations,sintering temperatures and Sr-site vacancies are successfully prepared by high energy ball milling combined with carbon burial sintering.For fully understanding the effect of niobium doping on SrTiO_3,thermoelectric transport properties are systematically investigated in a temperature range from 300 K to 1100 K.The carrier mobility can be significantly enhanced,and the electrical conductivity is quadrupled,when the sintering temperature rises from 1673 K to 1773 K(beyond the eutectic temperature(1713 K) of SrTiO_3–TiO_2).The lattice vibration can be suppressed by the lattice distortion introduced by the doped niobium atoms.However,Sr-site vacancies compensate for the lattice distortion and increase the lattice thermal conductivity more or less.Finally,we achieve a maximum value of figure-of-merit z T of 0.21 at 1100 K for Sr Ti_(0.9)Nb_(0.1)O_3 ceramic sintered at1773 K.     

Super deformability and thermoelectricity of bulk γ-InSe single crystals

Indium selenide, a III–V group semiconductor with layered structure, attracts intense attention in various photoelectric applications, due to its outstanding properties. Here, we report super deformability and thermoelectricity of γ-In Se single crystals grown by modified Bridgeman method. The crystal structure of In Se is studied systematically by transmission electron microscopy methods combined with x-ray diffraction and Raman spectroscopy. The predominate phase of γ-In Se with dense stacking faults and local multiphases is directly demonstrated at atomic scale. The bulk γ-In Se crystals demonstrate surprisingly high intrinsic super deformative ability which is highly pliable with bending strains exceeding12.5% and 264% extension by rolling. At the meantime, In Se also possesses graphite-like features which is printable,writable, and erasable. Finally, the thermoelectric properties of γ-In Se bulk single crystals are preliminary studied and thermal conductivity can be further reduced via bending-induced defects. These findings will enrich the knowledge of structural and mechanical properties' flexibility of In Se and shed lights on the intrinsic and unique mechanical properties of In Se polytypes.     

Thermal transport properties of defective graphene:A molecular dynamics investigation

In this work the thermal transport properties of graphene nanoribbons with randomly distributed vacancy defects are investigated by the reverse non-equilibrium molecular dynamics method. We find that the thermal conductivity of the graphene nanoribbons decreases as the defect coverage increases and is saturated in a high defect ratio range. Further analysis reveals a strong mismatch in the phonon spectrum between the unsaturated carbon atoms in 2-fold coordination around the defects and the saturated carbon atoms in 3-fold coordination, which induces high interfacial thermal resistance in defective graphene and suppresses the thermal conductivity. The defects induce a complicated bonding transform from sp2 to hybrid sp–sp2network and trigger vibration mode density redistribution, by which the phonon spectrum conversion and strong phonon scattering at defect sites are explained. These results shed new light on the understanding of the thermal transport behavior of graphene-based nanomaterials with new structural configurations and pave the way for future designs of thermal management phononic devices.     

Reconstruction of Intrinsic Thermal Parameters of Methane Hydrate and Thermal Contact Resistance by Freestanding 3ω Method

It is essential to obtain thermophysical properties of methane hydrate precisely with a freestanding probe for modeling and predicting thermal transport in gas hydrates. A method with a freestanding 3ω probe is presented to reconstruct the intrinsic thermal conductivity, thermal diffusivity, and thermal contact resistance of methane hydrate. Isolated from the thermal contact resistance, the intrinsic thermal conductivity of methane hydrate decreases between 250 K and 280 K and is 41% larger than the effective value at 253 K. More importantly, when the thermal contact resistance is isolated, the temperature dependence of intrinsic thermal conductivity shows a converse trend with the generally accepted glass-like feature at high temperature. Otherwise, thermal contact resistances measured in the experiment between the freestanding 3ω probe and the methane hydrate sample are extraordinary large. The freestanding 3ω method in this work is expected to measure the thermal property of methane hydrate more accurately.     

Thermoelectric properties of Sr_(0.61)Ba_(0.39)Nb_2O_(6-δ) ceramics in different oxygen-reduction conditions

The thermoelectric properties of Sr0.61Ba0.39Nb2O6-δceramics, reduced in different conditions, are investigated in the temperature range from 323 K to 1073 K. The electrical transport behaviors of the samples are dominated by the thermal-activated polaron hopping in the low temperature range, the Fermi glass behavior in the middle temperature range,and the Anderson localized behavior in the high temperature range. The thermal conductivity presents a plateau at hightemperatures, indicating a glass-like thermal conduction behavior. Both the thermoelectric power factor and the thermal conductivity increase with the increase of the degree of oxygen-reduction. Taking these two factors into account, the oxygen-reduction can still contribute to promoting the thermoelectric figure of merit. The highest ZT value is obtained to be ～0.19 at 1073 K in the heaviest oxygen reduced sample.     

First principle investigation of the electronic and thermoelectric properties of Mg_2C

In this paper, electronic and thermoelectric properties of Mg_2C are investigated by using first principle pseudo potential method based on density functional theory and Boltzmann transport equations. We calculate the lattice parameters,bulk modulus, band gap and thermoelectric properties(Seebeck coefficient, electrical conductivity, and thermal conductivity) of this material at different temperatures and compare them with available experimental and other theoretical data. The calculations show that Mg_2C is indirect band semiconductor with a band gap of 0.75 eV. The negative value of Seebeck coefficient shows that the conduction is due to electrons. The electrical conductivity decreases with temperature and Power factor(PF) increases with temperature. The thermoelectric properties of Mg_2C have been calculated in a temperature range of 100 K–1200 K.     

Thermal Conduction in a Single Polyethylene Chain Using Molecular Dynamics Simulations

Research on the thermal conduction in a single polymer chain is significant for the improvement of the thermal property of bulk polymer materials. We calculate the thermal conductivity of a single polyethylene （PE） chain by using both the Green-Kubo approach and a nonequilibrium molecular dynamics simulation method. The results suggest that the thermal conductivity of an individual polymer chain is very high although bulk PE is a thermal insulator, even divergent in our case. Moreover, the thermal conductivity of PE chains is observed to increase with the chain length.     

Probing thermal properties of vanadium dioxide thin films by time-domain thermoreflectance without metal film

Vanadium dioxide(VO_2) is a strongly correlated material, and it has become known due to its sharp metal–insulator transition(MIT) near room temperature. Understanding the thermal properties and their change across MIT of VO_2 thin film is important for the applications of this material in various devices. Here, the changes in thermal conductivity of epitaxial and polycrystalline VO_2 thin film across MIT are probed by the time-domain thermoreflectance(TDTR) method.The measurements are performed in a direct way devoid of deposition of any metal thermoreflectance layer on the VO_2 film to attenuate the impact from extra thermal interfaces. It is demonstrated that the method is feasible for the VO_2 films with thickness values larger than 100 nm and beyond the phase transition region. The observed reasonable thermal conductivity change rates across MIT of VO_2 thin films with different crystal qualities are found to be correlated with the electrical conductivity change rate, which is different from the reported behavior of single crystal VO_2 nanowires. The recovery of the relationship between thermal conductivity and electrical conductivity in VO_2 film may be attributed to the increasing elastic electron scattering weight, caused by the defects in the film. This work demonstrates the possibility and limitation of investigating the thermal properties of VO_2 thin films by the TDTR method without depositing any metal thermoreflectance layer.     

Zintl Phase BaAgSb: Low Thermal Conductivity and High Performance Thermoelectric Material in Ab Initio Calculation

Thermoelectric materials are critical parts in thermal electric devices.Here,Zintl phase BaAgSb in space group of P63/mmc is reported as a promising thermoelectric material in density function theory.The anisotropic lattice thermal conductivity and phonon transport properties are investigated in theory.The strong phonon-phonon scattering in BaAgSb exhibits ultra-low lattice thermal conductivity of 0.59 W·m~(-1)·K~(-1) along c-axis at 800 K,and high thermoelectric performance ZT=0.94 at 400 K.The mix of covalent and ionic bond supports high carrier mobility and low thermal conductivity.The unusual features make BaAgSb a potential thermoelectric material.     

Thermal conductivity characterization of ultra-thin silicon film using the ultra-fast transient hot strip method

Thermal conductivity is an important material parameter of silicon when studying the performance and reliability of devices or for guiding circuit design when considering heat dissipation, especially when the self-heating effect becomes prominent in ultra-scaled MOSFETs.The cross-plane thermal conductivity of a thin silicon film is lacking due to the difficulty in sensing high thermal conductivity in the vertical direction.In this paper, a feasible method that utilizes an ultra-fast electrical pulse within 20 μs combined with the hot strip technique is adopted.To the best of our knowledge, this is the first work that shows how to extract the cross-plane thermal conductivity of sub-50 nm(30 nm, 17 nm, and 10 nm)silicon films on buried oxide.The ratio of the extracted cross-plane thermal conductivity of the silicon films over the bulk value is only about 6.9%, 4.3%, and 3.8% at 300 K, respectively.As the thickness of the films is smaller than the phonon mean free path, the classical heat transport theory fails to predict the heat dissipation in nanoscale transistors.Thus, in this study, a ballistic model, derived from the heat transport equation based on extended-irreversible-hydrodynamics(EIT), is used for further investigation, and the simulation results exhibit good consistence with the experimental data.The extracted effective thermal data could provide a good reference for precise device simulations and thermoelectric applications.     

Construction and mechanism analysis on nanoscale thermal cloak by in-situ annealing silicon carbide film

In recent years,there is a strong interest in thermal cloaking at the nanoscale,which has been achieved by using graphene and crystalline silicon films to build the nanoscale thermal cloak according to the classical macroscopic thermal cloak model.Silicon carbide,as a representative of the third-generation semiconductor material,has splendid properties,such as the high thermal conductivity and the high wear resistance.Therefore,in the present study,we build a nanoscale thermal cloak based on silicon carbide.The cloaking performance and the perturbation of the functional area to the external temperature filed are analyzed by the ratio of thermal cloaking and the response temperature,respectively.It is demonstrated that silicon carbide can also be used to build the nanoscale thermal cloak.Besides,we explore the influence of inner and outer radius on cloaking performance.Finally,the potential mechanism of the designed nanoscale thermal cloak is investigated by calculating and analyzing the phonon density of states(PDOS)and mode participation rate(MPR)within the structure.We find that the main reason for the decrease in the thermal conductivity of the functional area is phonon localization.This study extends the preparation method of nanoscale thermal cloaks and can provide a reference for the development of other nanoscale devices.     

Thermoelectric properties of lower concentration K-doped Ca_3Co_4O_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_xCo_4O_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_4O_9 is close to 0.22 at 1026 K, representing an improvement of about 36% compared with that of Ca_3Co_4O_9, suggesting that lower concentration K-doped Ca_3Co_4O_9 series materials are promising thermoelectric oxides for high-temperature applications.     

Complex alloying effect on thermoelectric transport properties of Cu_2Ge(Se_(1-x)Te_x)_3

To enhance the thermoelectric performance of Cu_2GeSe_3, a series of Te-alloyed samples Cu_2Ge(Se_(1-x)Te_x)_3 are synthesized and investigated in this work. It is found that the lattice thermal conductivity is reduced drastically for x = 0.1 sample, which may be attributed to the point defects introduced by alloying. However, for samples with x ≥ 0.2, the lattice thermal conductivity increases with increasing x, which is related to a less distorted structure. The structure evolution,together with the change in carrier concentration, also leads to a systemically change in electrical properties. Finally, a z T of 0.55@750 K is obtained for the sample with x = 0.3, about 62% higher than that for the pristine sample.     

Synthesis and thermoelectric properties of Mn-doped AgSbTe₂ compounds

Polycrystalline p-type Ag 0.9 Sb 1.1 x Mn x Te 2.05(x = 0.05,0.10,and 0.20) compounds have been prepared by a combined process of melt-quenching and spark plasma sintering.The sample composition of Ag 0.9 Sb 1.1 x Mn x Te 2.05 has been specially designed in order to achieve the doping effect by replacing part of Sb with Mn and to present the uniformly dispersed Ag 2 Te phase in the matrix by adding insufficient Te,which is beneficial for optimizing the electrical transport properties and enhancing the phonon scattering effect.All the samples have the NaCl-type structure according to our X-ray powder diffraction analysis.After the treatment of spark plasma sintering,only the sample with x = 0.20 has a small amount of MnTe 2 impurities.The thermal analysis indicates that a tiny amount of Ag 2 Te phase exists in all these samples.The presence of the MnTe 2 impurity with high resistance and high thermal conductivity leads to the deteriorative thermoelectric performance of the sample with x = 0.20 due to the decreased electrical transport properties and the increased thermal conductivity.In contrast,the sample with x = 0.10 exhibits enhanced thermoeletric properties due to the Mn-doping effect.A dimensionless thermoelectric figure of merit of 1.2 is attained for the sample with x = 0.10 at 573 K,showing promising thermoelectric properties in the medium temperature range.     

Ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films

The ultrafast dynamics of photoexcited carriers and coherent phonons in ultrathin Bi;Te;thermoelectric films are studied through transient differential transmission spectroscopy.An ultralow frequency coherent optical phonon at 0.16 THz emerges,especially in ultrathin films,and it is ascribed to interlayer breathing modes.It can divide the ultrathin films into two groups which have outof-phase vibration along the normal of a film plane,causing a destructive interference between in-plane propagating thermal waves in the two groups of quintuple layers,and thus possibly reducing the thermal conductivity of the ultrathin films.The excitation power dependence of ultrafast dynamics reveals carrier-carrier scattering dominating thermalization,which provides a microscopic understanding of the reported high electrical conductivity and anomalously high power factor of ultrathin Bi;Te;films at room temperature.     

Heat Conductivity of One-Dimensional Carbon Chain in an External Potential

The heat transport m a one-dimensional （ID） carbon nanowire （CNW） lying in an external potential with different amplitudes and periods is studied by the non-equilibrium molecular dynamics method. It is found that the thermal conductivity of CNW is always anomalous, increasing with the CNW length and obeying the power law k- N, in which a decreases with the increasing external potential amplitude. The thermal conductivity could be enhanced by the external potential with rather larger amplitudes, which means that an applied external potential could be an efficient tool to improve the heat conductivity of a real 1D material In addition, the effect of different periods of the external potential is studied, finding the external potential with an incommensurate period leads to the smaller a value.     

Improvement of Thermoelectric Performance in BiCuSeO Oxide by Ho Doping and Band Modulation

We try to use Ho doping combined with band modulation to adjust the thermoelectric properties for BiCuSeO.The results show that Ho doping can increase the carrier concentration and increase the electrical conductivity in the whole temperature range.Although Seebeck coefficient decreases due to the increase of carrier concentration,it still keeps relatively high values,especially in the middle and high temperature range.On this basis,the band-modulation sample can maintain relatively higher carrier concentration while maintaining relatively higher mobility,and further improve the electrical transporting performance.In addition,due to the introduction of a large number of interfaces in the band-modulation samples,the phonon scattering is enhanced effectively and the lattice thermal conductivity is reduced.Finally,the maximal power factor(PF) of 5.18μW·cm~(-1)K~(-2) and the dimensionless thermoelectric figure of merits(ZT) of 0.81 are obtained from the 10% Ho modulation doped sample at 873 K.     

Anisotropic thermoelectric transport properties in polycrystalline SnSe_2

It is reported that SnSe_2 consisting of the same elements as SnSe, is a new promising thermoelectric material with advantageous layered structure. In this work, the thermoelectric performance of polycrystalline SnSe_2 is improved through introducing SnSe phase and electron doping(Cl doped in Se sites). The anisotropic transport properties of SnSe_2 are investigated. A great reduction of the thermal conductivity is achieved in SnSe_2 through introducing SnSe phase, which mainly results from the strong SnSe_2–SnSe inter-phase scattering. Then the carrier concentration is optimized via Cl doping, leading to a great enhancement of the electrical transport properties, thus an extraordinary power factor of ～5.12 μW·cm~(-1)·K~(-2) is achieved along the direction parallel to the spark plasma sintering(SPS) pressure direction( P). Through the comprehensive consideration on the anisotropic thermoelectric transport properties, an enhanced figure of merit ZT is attained and reaches to ～ 0.6 at 773 K in SnSe_2-2% SnSe after 5% Cl doping along the P direction, which is much higher than ～ 0.13 and ～ 0.09 obtained in SnSe_2-2% SnSe and pristine SnSe_2 samples, respectively.