Drastic Reduction Of Thermal Conductivity Research Articles

Drastic reduction of thermal conductivity in hexagonal AX (A = Ga, In & Tl, X = S, Se & Te) monolayers due to alternative atomic configuration

Several two-dimensional chalcogenide materials have been in the limelight in the recent past for their promising thermoelectric properties. It is well established that the thermoelectric performance of materials improves on reducing the physical dimensionality of the system. Two-dimensional hexagonal chalcogen (S, Se and Te) bearing compounds of Ga, In and Tl have already been studied extensively in literature. But in those phases, the group-13 non-chalcogen atoms occupy the two inner planes while the chalcogens occupy the two outer planes of the unit cell. In this work, we have proposed the alternate arrangement in which the chalcogen atoms occupy the two inner planes while the group-13 atoms occupy the two outer planes of the unit cell. Unprecedentedly, this alternate arrangement shows much lower thermal conductivity that leads to superior thermoelectric performance. In this work we have studied in details the thermoelectric properties of hexagonal AX (A = Ga, In & Tl, X = S, Se & Te) monolayers and compare the results having both the atomic arrangements. The very low lattice thermal conductivity of this new arrangement is due to the outermost valence s-orbital lone pair of the chalcogens which leads to enhanced anharmonicity. We have explained these results from the anti-crossing of the phonon branches as well. The electronic, dynamical, thermodynamical and elastic properties have also been studied. We think that these results should have significant impact on the synthesis of high-performance thermoelectric materials based on chalcogenides of gallium, indium and thallium. • The position of the group 13 and chalcogen atoms in AX (A = Ga, In & Tl, X = S, Se & Te) have been flipped. • Ab initio calculations of thermal conductivity, electronic properties and mechanical properties have been determined. • The new configuration yields dynamically stable structures too with much lower elastic constants and Debye temperatures. • The high anharmonic scattering and weak bonds lead to ultralow thermal conductivity due to the lattice vibrations. • The ultralow thermal conductivity lead to extremely high values of thermoelectric figure of merit.

Selective Enhancement in Phonon Scattering Leads to a High Thermoelectric Figure-of-Merit in Graphene Oxide-Encapsulated ZnO Nanocomposites.

ZnO is a promising candidate for use as an environmentally friendly thermoelectric (TE) material. However, high thermal conductivity leading to a poor TE figure-of-merit (zT) needs to be addressed to achieve a significant TE efficiency for commercial applications. Here, we demonstrate that selective enhancement in phonon scattering leads to an increase in the zT of ZnO because of Al doping and reduced graphene oxide (RGO) encapsulation. These nanocomposites are synthesized via a facile and scalable method. The incorporation of 1 at% Al with 1.5 wt % RGO into ZnO has been found to show significant improvement in zT (0.52 at 1100 K), which is an order of magnitude larger compared to that of bare undoped ZnO. Photoluminescence and X-ray photoelectron spectroscopy measurements confirm that RGO encapsulation significantly quenches surface oxygen vacancies in ZnO along with nucleation of new interstitial Zn donor states. Tunneling spectroscopy performed on bare as well as composite particles reveals that the band gap of ∼3.4 eV for bare ZnO reduces effectively to ∼0.5 eV upon RGO encapsulation, facilitating charge transport. The electrical conductivity also benefits from high densification (>95%) achieved using the spark plasma sintering method, which also aids in reduction of graphene oxide into RGO. The same Al doping and RGO capping synergistically bring about drastic reduction of thermal conductivity, through enhanced interfacial and point-defect-phonon scatterings. These opposing effects on electrical and thermal conductivities lead to enhancement in the power factors as well as the zT value. Overall, a practically viable route has been demonstrated for the synthesis of oxide-RGO TE materials, which could find their potential applications in high-temperature TE power generation.

Role of excess Te in Bi0.5Sb1.5Te3+x(x= 0, 0.01, 0.015 and 0.020) on the optimization of thermoelectric properties

Several off-stoichiometric compositions Bi0.5Sb1.5Te3+x (x = 0, 0.01, 0.015, 0.02) with varying Te concentrations were synthesized via melting the elemental components of stoichiometric compositions in a furnace operating at temperature 800 °C to study the role of excess Te on enhancing the thermoelectric properties. The structural characterization performed by XRD and TEM reveals the formation of a matrix phase of Bi0.5Sb1.5Te3and a minor phase of Te. A systematic investigation of electronic and thermal transport behaviour of Bi0.5Sb1.5Te3+x (x = 0, 0.01, 0.015, 0.02) were performed in wide temperature range. Interestingly, enhanced power factor (~22.6 μW/cm K2 at 486K) was optimized for composition Bi0.5Sb1.5Te3.010which is18% enhanced power factor(PF) than that of pristine Bi0.5Sb1.5Te3 material (PF~19.1 μW/cm K2 at 486K). In addition to this, a drastic reduction in thermal conductivity (1.01 W/m K at 486K) was also observed which is 11% reduction when compared to that of bare state-of-art Bi0.5Sb1.5Te3 material. This reduction in thermal conductivity is explained in terms of the scattering of phonons due to mass fluctuation, strain field domains, grain boundaries, and multiple interfaces. An enhanced ZT~1.08 at 486 K for the composition Bi0.5Sb1.5Te3.010 was optimized which is primarily due to enhanced power factor and reduced thermal conductivity.

Effect of ion-implantation-induced defects and Mg dopants on the thermoelectric properties of ScN

For applications in energy harvesting, environmentally friendly cooling, and as power sources in remote or portable applications, it is desired to enhance the efficiency of thermoelectric materials. One strategy consists of reducing the thermal conductivity while increasing or retaining the thermoelectric power factor. An approach to achieve this is doping to enhance the Seebeck coefficient and electrical conductivity, while simultaneously introducing defects in the materials to increase phonon scattering. Here, we use Mg ion implantation to induce defects in epitaxial ScN (111) films. The films were implanted with Mg+ ions with different concentration profiles along the thickness of the film, incorporating 0.35 to 2.2 at.% of Mg in ScN. Implantation at high temperature (600 C), with few defects due to the temperature, does not substantially affect the thermal conductivity compared to a reference ScN. Samples implanted at room temperature, in contrast, exhibited a reduction of the thermal conductivity by a factor of three. The sample doped with 2.2 at.% Mg also showed an increased power factor after implantation. This study thus shows the effect of ion-induced defects on thermal conductivity of ScN films. High-temperature implantation allows the defects to be annealed out during implantation, while the defects are retained for room-temperature implanted samples, allowing for a drastic reduction in thermal conductivity.

(Invited) Nanostructure Design for Control of Phonon and Electron Transports

Thermoelectric materials are required to high electrical conductivity and low thermal conductivity for high performance. The independent control of electrons and phonons is difficult due to their correlation resulting in the difficulty of the enhancement of thermoelectric performance. Recently, nanostructures have drawn much attention in thermoelectric material study. One of the mechanisms of its enhancement is the thermal conductivity reduction due to the phonon scattering at the interfaces introduced by nanostructuring [1]. In some case, however, its interfaces can scatter the carriers leading to the reduction of the carrier conductivity. In this study, we are proposing the novel nanostructure for independent control of electrons and phonons using nanodots. The nanostructure is ultra-small epitaxial Ge nanodots are embedded in Si layers. It is expected that carrier transports in Si layers with high electrical conductivity and high Seebeck coefficient, while phonons are scattered at the interfaces between nanodots and Si layers [2,3]. Ultra-small Ge nanodots in this nanostructure were grown by our original technique: ultrathin SiO2 film technique. First, we formed the ultrathin SiO2 films with one monolayer thickness by oxidizing clean Si surfaces at~500ºC after introducing the Si substrates into molecular beam epitaxy chamber at the base pressure of ~10-8 Pa. We deposited Ge on the ultrathin SiO2 film to form Ge nanodtos. At the first stage, of Ge deposition the reaction of Ge+SiO2 →GeO↑+SiO↑ occurred to form nanowindows in the ultrathin SiO2 films. Subsequently deposited Ge atoms were trapped at the nanowindows, which worked as nucleation sites. Then, ultras-mall nanodots were formed. The nanodots contacted with Si substrates through nanowindows leading to the epitaxial growth. Si was deposited on the epitaxial Ge nanodots to form Si layers. These formations of the ultrathin SiO2 films, epitaxial Ge nanodots, and epitaxial Si layers were repeated to form the nanostructure that is Si films including Ge nanodots. The thermal conductivity measurements of the nanostructure showed the drastic reduction of thermal conductivity. Hall effect measurements exhibited a similar high electrical conductivity to that of bulk Si. Electrons transports through the interfaces easily by band engineering. On the other hand, ultras-mall nanodots work as phonon scatterers leading to the drastic reduction of thermal conductivity. This phonon scattering is consistent with wave scattering theory. These results demonstrated nanostructure design brought to the independent control of carrier and phonon transport using the Si layers with ultrasmall epitaxial Ge nanodots This work was partially supported by JSPS KAKENHI Grant Number 16H02078 for Scientific Research (A) and 15K13276 for Challenging Exploratory Research. In part, this work was supported by CREST-JST program. Reference [1] Y. Nakamura, et al., Nano Energy 12, 845 (2015). [2] S. Yamasaka, et al., Sci. Rep. 5, 14490 (2015). [3] S. Yamasaka, et al., Sci. Rep. 6, 22838 (2016).

Ultralow Thermal Conductivity of Single‐Crystalline Porous Silicon Nanowires

AbstractPorous materials provide a large surface‐to‐volume ratio, thereby providing a knob to alter fundamental properties in unprecedented ways. In thermal transport, porous nanomaterials can reduce thermal conductivity by not only enhancing phonon scattering from the boundaries of the pores and therefore decreasing the phonon mean free path, but also by reducing the phonon group velocity. Herein, a structure–property relationship is established by measuring the porosity and thermal conductivity of individual electrolessly etched single‐crystalline silicon nanowires using a novel electron‐beam heating technique. Such porous silicon nanowires exhibit extremely low diffusive thermal conductivity (as low as 0.33 W m−1 K−1 at 300 K for 43% porosity), even lower than that of amorphous silicon. The origin of such ultralow thermal conductivity is understood as a reduction in the phonon group velocity, experimentally verified by measuring the Young's modulus, as well as the smallest structural size ever reported in crystalline silicon (<5 nm). Molecular dynamics simulations support the observation of a drastic reduction in thermal conductivity of silicon nanowires as a function of porosity. Such porous materials provide an intriguing platform to tune phonon transport, which can be useful in the design of functional materials toward electronics and nanoelectromechanical systems.

Reduced Thermal Conductivity in ZrNiSn-Based Heusler Alloys

Dual-site doped half-Heusler ZrNiSn alloys were prepared by arc melting. In order to diminish thermal conductivity of the alloys, as-casted ingots were sintered again by spark plasma sintering (SPS) technique after pulverizing and milling for several different periods of time. The electrical resistivity, thermoelectric powers and thermal conductivity of the sintered samples were measured from 4 to 350 K. A drastic reduction of thermal conductivity was obtained. The thermal conductivity decreased from about 17 to 1.5 W/K m and from 8.8 to 6.3 W/K m at 30 and 350 K, respectively. The dimensionless figure of merit of the half-Heusler alloys increased about 10%

High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite

2 mol% Al-doped ZnO nanoparticles were consolidated into a ZnO nanocomposite with ZnAl2O4 nanoprecipitates by spark plasma sintering and its high-temperature charge transport and thermoelectric properties were investigated up to 1073 K. The carrier concentration in the nanocomposite was not dependent on the temperature, while the Hall mobility showed positive temperature-dependence due to grain boundary scattering. The negative Seebeck coefficient of the nanocomposite was linearly proportional to the temperature, and the density of the state effective mass (md*) was evaluated to be 0.33me by using the Pisarenko relation. Drastic reduction of thermal conductivity (k < 2 W m−1 K−1) was achieved in the nanocomposite, and the maximum ZT of 0.34 was obtained at 1073 K.

Control of Thermal and Electronic Transport in Defect-Engineered Graphene Nanoribbons

The influence of the structural detail and defects on the thermal and electronic transport properties of graphene nanoribbons (GNRs) is explored by molecular dynamics and non-equilibrium Green's function methods. A variety of randomly oriented and distributed defects, single and double vacancies, Stone-Wales defects, as well as two types of edge form (armchair and zigzag) and different edge roughnesses are studied for model systems similar in sizes to experiments (>100 nm long and >15 nm wide). We observe substantial reduction in thermal conductivity due to all forms of defects, whereas electrical conductance reveals a peculiar defect-type-dependent response. We find that a 0.1% single vacancy concentration and a 0.23% double vacancy or Stone-Wales concentration lead to a drastic reduction in thermal conductivity of GNRs, namely, an 80% reduction from the pristine one of the same width. Edge roughness with an rms value of 7.28 Å leads to a similar reduction in thermal conductivity. Randomly distributed bulk vacancies are also found to strongly suppress the ballistic nature of electrons and reduce the conductance by 2 orders of magnitude. However, we have identified that defects close to the edges and relatively small values of edge roughness preserve the quasi-ballistic nature of electronic transport. This presents a route of independently controlling electrical and thermal transport by judicious engineering of the defect distribution; we discuss the implications of this for thermoelectric performance.

Novel Methods of Nanoscale Wire Formation

In recent years, tremendous interest has been generated in the fabrication and characterization of nanoscale structures such as quantum dots and wires. For example, there is interest in the electronic, magnetic, mechanical, and chemical properties of materials with reduced dimensions. In the case of nanoscale semiconductors, quantum effects are expected to play an increasingly prominent role in the physics of nanostructures, and a new class of electronic and optoelectronic devices may be possible. In addition to new and interesting physics, the formation and characterization of nanoscale magnetic structures could result in higher-density storage capacity in hard disks and optical-recording media. Likewise, phonon confinement leads to a drastic reduction of thermal conductivity and can be used to improve the performance of thermoelectric devices.In 1980, H. Sakaki predicted theoretically that quantum wires may have applications in high-performance transport devices, due to their sawtoothlike density of states (E1/2), where E is the electron energy. Since then, most quantum wires have been made by fabricating a gratinglike gate on top of a two-dimensional (2D) electron gas contained in a semiconductor heterojunction or in metal-oxide-semiconductor structures. By applying a negative gate voltage to the system, its structure can be changed from a 2D to a one-dimensional (1D) regime, where electron confinement is achieved by an electrostatic confining potential. It was not until recently that “physical” semiconductor quantum wires with the demonstrated 1D confinement by physical boundaries began to be fabricated.