Suppression Of Thermal Conductivity Research Articles

Suppression of thermal conductivity by rattling modes in thermoelectric sodium cobaltate

The need for both high electrical conductivity and low thermal conductivity creates a design conflict for thermoelectric systems, leading to the consideration of materials with complicated crystal structures. Rattling of ions in cages results in low thermal conductivity, but understanding the mechanism through studies of the phonon dispersion using momentum-resolved spectroscopy is made difficult by the complexity of the unit cells. We have performed inelastic X-ray and neutron scattering experiments that are in remarkable agreement with our first-principles density-functional calculations of the phonon dispersion for thermoelectric Na(0.8)CoO2, which has a large-period superstructure. We have directly observed an Einstein-like rattling mode at low energy, involving large anharmonic displacements of the sodium ions inside multi-vacancy clusters. These rattling modes suppress the thermal conductivity by a factor of six compared with vacancy-free NaCoO2. Our results will guide the design of the next generation of materials for applications in solid-state refrigerators and power recovery.

Electronic, thermoelectric, and magneto-dielectric properties of Ca1−xNaxCr2O4

We report on electronic, thermoelectric, and magneto-dielectric properties of Ca1−xNaxCr2O4 series with a calcium ferrite-type structure prepared by high-pressure-high-temperature synthesis. Dielectric spectroscopy down to 2 K confirms that both CaCr2O4 and NaCr2O4 end members have an insulating ground state notwithstanding the fact that the latter compound has a mixed valence Cr3+/Cr4+ structure. A crossover from positive to negative charge carriers occurs in NaCr2O4 at T≈230 K. Partial substitution of Ca for Na brings about a change from n to p type carriers at ca. x=0.75. A strong suppression of thermal conductivity below TN=21 K was found in CaCr2O4 indicating a scattering of acoustic phonons from a long wave-length cycloidal magnetic excitations. A pronounced dielectric anomaly at Néel temperature adds CaCr2O4 to the multiferroic family of compounds. Lattice contribution to dielectric properties of NaCr2O4 at TN=125 K is screened by high electric conductivity. An onset of the magnetocapacitance above 3 T correlates with the spin-flop transition in NaCr2O4 at a critical field of 3.5 T. A strong non-saturated magnetocapacitance in this compound cannot be entirely attributed to the colossal magnetoresistance.

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

Suppression of thermal conductivity in InxGa1−xN alloys by nanometer-scale disorder

We have systematically measured the room-temperature thermal conductivity of epitaxial layers of InxGa1−xN alloys with 15 different Indium compositions ranging from 0.08 to 0.98 by time-domain thermoreflectance method. The data are compared to the estimates of the strength of phonon scattering by cation disorder. The thermal conductivity is in good agreement with the theoretical modeling results based on the mass difference for In-rich (x > 0.9) and Ga-rich (x < 0.2) compositions. At intermediate compositions (0.2 < x < 0.9), the thermal conductivity is strongly suppressed below the values expected for homogeneous alloys. We attribute this suppression of thermal conductivity to phonon scattering by nanometer-scale compositional inhomogeneities in alloys.

Thermal Conductivity of TiO2 Nanotubes

Thermal conductivity of TiO2 nanotubes prepared from a NaOH treatment of TiO2 particles with subsequent acid washing and annealing has been investigated. The obtained TiO2 nanotubes have a tetragonal anatase structure, and have a typical inner diameter of about 4–5 nm, wall thickness of about 2–3 nm, and length up to several hundred nanometers. TiO2 nanotubes show a significantly reduced thermal conductivity of about 0.40–0.84 W/(m·K) (average 0.62 W/(m.K)) at room temperature, as compared to about 8.5 W/(m·K) for the bulk TiO2 materials. The great suppression in thermal conductivity can be understood by means of increased phonon-boundary scattering and enhanced phonon confinement in TiO2 nanotubes with unique nanotubular morphology, small featured sizes, and large surface area (∼258 m2/g). A theoretical analysis including the surface scattering and size confinement effects of phonon transport in TiO2 nanotubes is also reported, which results in an intrinsic thermal conductivity of 0.30–0.77 W/(m·K) (aver...

All-scale hierarchical thermoelectrics: MgTe in PbTe facilitates valence band convergence and suppresses bipolar thermal transport for high performance

We report a high ZT of ∼2.0 at 823 K for 2% Na-doped PbTe with 6% MgTe with excellent thermal stability. We attribute the high thermoelectric performance to a synergistic combination of enhanced power factor, reduction of the lattice thermal conductivity and simultaneous suppression of bipolar thermal conductivity. MgTe inclusion in PbTe owns triple functions: the Mg alloying within the solubility limit in PbTe modifies the valence band structure by pushing the two valence bands (L and Σ bands) closer in energy, thereby facilitating charge carrier injection. When the solubility limit of Mg is exceeded, ubiquitous endotaxial nanostructures form, which when coupled with mesoscale microstructuring results in a very low (lattice) thermal conductivity through all-scaled length phonon scattering. Meanwhile, most significantly, the Mg alloying enlarges the energy gap of conduction band (C band) and light valence band (L band), thereby suppresses the bipolar thermal conductivity through an increase in band gap.

Thermal Conductivity Reduction in Si/Ge Core/Shell Nanowires

We have theoretically demonstrated that hybridization of phonon modes in Si/Ge core/shell nanowires and corresponding decrease of average phonon group velocity lead to the suppression of phonon thermal conductivity in Si/Ge core/shell nanowires in comparison with generic homogeneous silicon nanowires. The phonon energy spectra were calculated using the Valence Force Field model of lattice vibrations, while thermal conductivities were obtained from the Boltzmann transport equation within the relaxation time approximation. Our results indicate that core/shell nanowires are good candidates for the efficient phonon engineering of their phonon and thermal properties.

Synthesis, crystal structure, and thermoelectric properties of the YbAl3-ScAl3 solid solution

The Yb1−xScxAl3 alloy solid solution has been successfully synthesized for 0 ≤ x ≤ 1 by a combination of arc melting, pulverization, and consolidation by pulsed electric current sintering. The thermoelectric properties of the alloys have been investigated over the temperature range 80–300 K. Our results on pure YbAl3 confirm previous studies demonstrating the combination of low electrical resistivity (∼100 μΩ cm) and large Seebeck coefficient (∼85 μV K−1) at 300 K. With increasing values of x the alloys become even more metallic, but the Seebeck coefficient decreases more gradually than the resistivity, resulting in enhanced thermoelectric power factor for 0 ≤ x ≤ 1. An increase in electronic thermal conductivity is partially offset by a strong suppression in lattice thermal conductivity that arises due to alloy scattering. As a result, the thermoelectric figure of merit of alloys with x = 0.25 is increased by 30% relative to YbAl3 at 250 K.

Mechanism of thermal conductivity suppression in doped silicon studied with nonequilibrium molecular dynamics

We examined the underlying mechanisms for thermal conductivity suppression in crystalline silicon by substitutional doping with different elements ($X$ $=$ boron, aluminum, phosphorus, and arsenic). In particular, the relative effects of doping-induced mass disorder, bond disorder, and lattice strain were assessed using nonequilibrium molecular dynamics simulations. Stillinger--Weber potential parameters for Si-$X$ interatomic interactions were optimized by fitting to relevant atomic forces from first-principles calculations. We first calculated the thermal conductivity variation of B-doped Si as a function of dopant concentration; the result shows excellent agreement with existing experimental data, indicating the reliability of our force-field-based simulations. At the dopant concentration of about 5 \ifmmode\times\else\texttimes\fi{} 10${}^{20}$ cm${}^{\ensuremath{-}3}$, the Si thermal conductivity value is predicted to be reduced from 137 W/mK at 300 K in undoped Si to 18/39/57/78 W/mK in As/B/P/Al-doped Si. Our study demonstrates that the mass disorder effect is primarily responsible for the thermal conductivity suppression in the As- and B-doped cases, whereas the bond disorder contribution is found to be more important than the mass disorder contribution in the Al- and P-doped cases; for all these systems, the lattice strain effect turns out to play a minor role in the reduction of lattice thermal conductivity.

Quantitative relation between structure and thermal conductivity in type-I clathratesX8Ga16Ge30(X=Sr, Ba) based on electrostatic-potential analysis

In order to quantitatively characterize the effect of motion of the guest atom in type-I clathrates ${X}_{8}$Ga${}_{16}$Ge${}_{30}$ ($X$ $=$ Sr, Ba) on thermal conductivity, electrostatic potential in the crystal was visualized by synchrotron radiation x-ray diffraction. The obtained electrostatic potential clearly exhibited structural aspects of the guest atom, rattling, which is considered as a cause of suppression of thermal conductivity \ensuremath{\kappa}. The parameterized degree of rattling ``rattling factor,'' ${f}_{\mathrm{R}}$, being defined as a volume ratio of the electrostatic interaction region of the guest atom to the spherical volume estimated by the covalent radius of the guest atom, was found to well classify \ensuremath{\kappa}. The \ensuremath{\kappa} monotonously decreases with increasing the ${f}_{\mathrm{R}}$. Consequently, ${f}_{\mathrm{R}}$ was judged as a better criterion to explore high-performance thermoelectric system of the clathrates.

Phonon thermal conductivity in silicon nanowires: The effects of surface roughness at low temperatures

Using a Green’s function method based on an elastic wave equation, the effects of surface roughness and the nanowire-contact interface scattering on phonon thermal conductivity are studied at low temperatures. It is found that the interface geometry between a nanowire and its contacts affects the transmission function at small energies related to the gapless modes and it gives rise to deviated results from the universal conductance. It is also shown that the surface roughness is crucial in the suppression of phonon thermal conductivity with reducing the nanowire size by averaging the transmission function over the rough-surface configurations. Furthermore, the phonon mean free path is proportional to the ratio of the correlation length and roughness heights quadratically as well as the cross-section area of the nanowire.

Electrical and thermal transport properties of Pb-based chalcogenides: PbTe, PbSe, and PbS

Electrical and thermal transport properties of lead-based chalcogenides (PbTe, PbSe, and PbS) were studied with special emphasis on the lattice and the bipolar thermal conductivity. Both electrical resistivity and Seebeck coefficient show the transport behaviors related to the intrinsic excitation that determined by the band gap, power factors at room temperature reach 12 μW cm −1 K −2, 14 μW cm −1 K −2, and 16 μW cm −1 K −2 for PbS, PbSe and PbTe, respectively. PbTe shows the lowest total thermal conductivity, and PbS shows the highest total thermal conductivity, in which the bipolar thermal conductivity is proportional to the width of band gap, these contributions at 723 K were estimated to be 2.2%, 3.1%, and 1.1% for the PbTe, PbSe and PbS, respectively. This study demonstrates the possibility to enhance ZT value by the suppression of bipolar thermal conductivity by tuning the width of band gap, also indicates that inexpensive and earth-abundant PbSe and PbS would be promising alternative for PbTe in the thermoelectric applications.

Thermoelectric properties ofp-type CuInSe2chalcopyrites enhanced by introduction of manganese

Thermoelectric properties, x-ray photoelectron spectroscopy, Raman spectroscopy, and electronic structures have been studied for Mn-substituted CuInSe${}_{2}$ chalcopyrites. Raman spectroscopy verifies the lattice disorder due to the introduction of Mn into the CuInSe${}_{2}$ matrix, leading to a slight suppression of thermal conductivity. On the other hand, the Mn substitution significantly increases the electrical conductivity and Seebeck coefficient. Therefore the thermoelectric figure of merit $ZT$ has been enhanced by over two orders of magnitude by the introduction of Mn into CuInSe${}_{2}$. These materials are $p$-type degenerate semiconductors, containing divalent Mn species as confirmed by x-ray photoelectron spectroscopy. The crystal structure of Mn-substituted CuInSe${}_{2}$, as well as related ternary and quaternary diamond-like semiconductors, can be viewed as a combination of an electrically conducting unit, the Cu-Se and Mn-Se networks, and an electrically insulating unit, the In-Se network. Therefore, diamond-like semiconductors can serve as a potential class of thermoelectric materials with relatively wide band gaps upon substitution with Mn or other transition metals.

Large Thermal Conductivity Reduction Induced by La/O Vacancies in the Thermoelectric LaCoO3 System

A series of compact La/O-vacant La(1-x)CoO(3-y) compounds were prepared by a cold high-pressure procedure, and their thermoelectric (TE) properties were investigated. Compared with the ion-substituted hole-type LaCoO(3) systems (e.g., La(1-x)Sr(x)CoO(3)), the thermal conduction of La(1-x)CoO(3-y) is noticeably reduced by the La/O vacancies, whereas the electric transport is less influenced, which results in an efficient ZT enhancement. We demonstrate that the large thermal conductivity reduction originates from the strong point-defect scattering, and La(1-x)CoO(3-y) can be rationalized as a partially filled solid solution: La(1-x)◻(x)CoO(3-y)◻(y), where ◻ denotes a vacancy. Such intrinsic thermal conductivity suppression provides an effective pathway for the design of better TE materials.

Approximate analytical models for estimation of the size-dependent thermal conductivity of nanoporous Si wires in the longitudinal direction

Suppression of thermal conductivity in nanoporous structures has an important influence in thermoelectric energy conversion. In this work, we introduce simple approximate analytical models for estimating the size-dependent thermal conductivity of Si wires with a single square nanopore and of Si wires with periodically arranged square nanopores (periodic nanoporous Si wires) in the longitudinal direction. The modeling results show that the porosity and interface density have large impacts on determining the thermal conductivity of periodic nanoporous Si wires. It is found that the thermal conductivity of periodic nanoporous Si wires could be tunable by choosing an appropriate combination of porosity and interface density. The results of this study could be used for thermal conductivity prediction of periodic nanoporous composites.

Suppression of thermal conductivity in graphene nanoribbons with rough edges

We analyze numerically the thermal conductivity of carbon nanoribbons with ideal and rough edges. We demonstrate that edge disorder can lead to a suppression of thermal conductivity by several orders of magnitude. This effect is associated with the edge-induced Anderson localization and suppression of the phonon transport, and it becomes more pronounced for longer nanoribbons and low temperatures.

A study of electronic structure and lattice dynamics of CoSb3 skutterudite

Abstract The novel filled skutterudite materials have attracted much interest in recent years and experimental studies have revealed that electrical properties (electrical conductivity and Seebeck coefficient) in these materials are dominated by their electronic structure while the effective suppression of thermal conductivity is mainly determined by their lattice dynamics. To clarify the relationship between microstructure and properties in further, we report a systematic study of electronic structures and lattice dynamics of CoSb 3 in this paper using linearized augmented plane waves based on the density functional theory of first principles. By calculating band structure and partial density of states (PDOS), effects of electronic structures of CoSb 3 on electrical properties were investigated. Based on the calculated results of phonon dispersions and phonon density of states of CoSb 3 , lattice dynamics of CoSb 3 (heat capacity, Debye temperature, mean free path and lattice thermal conductivity) are discussed in detail. The calculated results are excellently consistent with other work and experimental data.

Thermal conductivity suppression in bismuth nanowires

The thermal conductivity of individual bismuth nanowires was characterized using a suspended microdevice and correlated with the crystal structure and growth direction obtained by transmission electron microscopy on the same nanowires. Compared to bulk bismuth in the same crystal direction perpendicular to the trigonal axis, the thermal conductivity of a single-crystal bismuth nanowire of 232 nm diameter was found to be three to six times smaller than bulk in the temperature range between 100 and 300 K, and those of polycrystalline bismuth nanowires of 74–255 nm diameter are reduced by factors of 18–78 over the same temperature range. The thermal conductivity suppression in the single-crystal nanowire can be explained by a transport model that considers diffuse phonon-surface scattering, partially diffuse surface scattering of electrons and holes, and scattering of phonons and charge carriers by ionized impurities such as oxygen and carbon of a concentration on the order of 1019 cm−3. The comparable thermal conductivity values measured for polycrystalline nanowires of different diameters suggests a grain boundary scattering mean free path for all heat carriers in the range of 15–40 nm, which is smaller than the nanowire diameters.

Low thermal conductivity in Ge2Sb2Te5–SiOx for phase change memory devices

Nanometer scale Ge2Sb2Te5 (GST) domains formed by immiscible mixture of GST-SiOx at room temperature and 180 °C show remarkable suppression in electrical and thermal conductivity. Thermal boundary resistance with increased GST-SiOx interface becomes crucial to the reduction in thermal conductivity. These conductivity reductions concurrently result in the reduction in programming current and power consumption in phase change memory devices.

Thermal conductivity of bismuth telluride nanowire array-epoxy composite

Electrodeposition of nanowire array in porous anodic alumina (PAA) templates combine the performance benefits offered by crystallographic texture control, lattice thermal conductivity suppression through boundary scattering of phonons, elastic relaxation of misfit strain, and scalablity essential for high efficiency thermoelectric devices. The template material, however, can serve as a thermal shunt thereby reducing the effective thermoelectric performance. Here, we demonstrate a process of minimizing the parasitic thermal conduction by replacing the PAA matrix with SU-8 (κ∼0.2 W/m K). We report a reduction in the performance penalty from 27% for Bi2Te3/PAA to ∼5% for Bi2Te3/SU-8 nanocomposite by thermal conductivity measurements using a photoacoustic technique.