κL Values Research Articles

Graphene network in copper sulfide leading to enhanced thermoelectric properties and thermal stability

Copper sulfide has received wide attention as a potential thermoelectric material with high performance and abundant resource. While the liquid-like Cu ions play an important role for the low κL and high ZT values, grain-boundary engineering is needed to further enhance the phonon scattering and the thermal stability of Cu2-xS. Here, we introduced the 3D graphene heterointerface into the Cu2-xS matrix by a facile technique combining mechanical alloying (MA) and spark plasma sintering (SPS). A significant ZT enhancement was realized with the highest ZT up to 1.56 and a high power factor of 1197 μW m−1 K−2 at 873 K in the sample with 0.75 wt% graphene. Additionally, the composite showed excellent reproducibility of PF after five cycles testing from room temperature to 873 K, which confirmed the practical application potentiality of this composite.

Phonon thermal transport in a class of graphene allotropes from first principles.

Utilizing first principle calculations combined with the phonon Boltzman transport equation (PBTE), we systematically investigate the phonon thermal transport properties of α, β and γ graphyne, a class of graphene allotropes. Strikingly, at room temperature, a low lattice thermal conductivity κL of 21.11, 22.3, and 106.24 W m-1 K-1 is obtained in α, β and γ graphyne, respectively, which are much lower than that of graphene. We observe contributions from the phonon modes below the specified frequency and find that many optical phonon modes play critical roles in the phonon thermal transport. These optic modes participate in thermal transport, enhancing the phonon scattering process, thus leading to the low κL value. Our results provide insights into the thermal transport of graphyne, and forecast its potential applications for thermoelectric and thermal barrier coatings.

Crystal Structure and Transport Properties of the Homologous Compounds (PbSe)5(Bi2Se3)3m (m = 2, 3).

We report on a detailed investigation of the crystal structure and transport properties in a broad temperature range (2-723 K) of the homologous compounds (PbSe)5(Bi2Se3)3m for m = 2, 3. Single-crystal X-ray diffraction data indicate that the m = 2, 3 compounds crystallize in the monoclinic space groups C2/m (No. 12) and P21/m (No. 11), respectively. In agreement with diffraction data, high-resolution transmission electron microscopy analyses carried out on single crystals show that the three-dimensional crystal structures are built from alternating Pb-Se and m Bi-Se layers stacked along the a axis in both compounds. Scanning electron microcopy and electron-probe microanalyses reveal deviations from the nominal stoichiometry, suggesting a domain of existence in the pseudo binary phase diagram at 873 K. The complex atomic-scale structures of these compounds lead to very low lattice thermal conductivities κL that approach the glassy limit at high temperatures. A comparison of the κL values across this series unveiled an unexpected increase with increasing m from m = 1 to m = 3, in contrast to the expectation that increasing the structural complexity should tend to lower the thermal transport. This result points to a decisive role played by the Pb-Se/Bi-Se interfaces in limiting κL in this series. Both compounds behave as heavily doped n-type semiconductors with relatively low electrical resistivity and thermopower values. As a result, moderate peak ZT values of 0.25 and 0.20 at 700 K were achieved in the m = 2, 3 compounds, respectively. The inherent poor ability of these structures to conduct heat suggests that these homologous compounds may show interesting thermoelectric properties when properly optimized by extrinsic dopants.

Lattice Dislocations Enhancing Thermoelectric PbTe in Addition to Band Convergence.

Phonon scattering by nanostructures and point defects has become the primary strategy for minimizing the lattice thermal conductivity (κL ) in thermoelectric materials. However, these scatterers are only effective at the extremes of the phonon spectrum. Recently, it has been demonstrated that dislocations are effective at scattering the remaining mid-frequency phonons as well. In this work, by varying the concentration of Na in Pb0.97 Eu0.03 Te, it has been determined that the dominant microstructural features are point defects, lattice dislocations, and nanostructure interfaces. This study reveals that dense lattice dislocations (≈4 × 1012 cm-2 ) are particularly effective at reducing κL . When the dislocation concentration is maximized, one of the lowest κL values reported for PbTe is achieved. Furthermore, due to the band convergence of the alloyed 3% mol. EuTe the electronic performance is enhanced, and a high thermoelectric figure of merit, zT, of ≈2.2 is achieved. This work not only demonstrates the effectiveness of dense lattice dislocations as a means of lowering κL , but also the importance of engineering both thermal and electronic transport simultaneously when designing high-performance thermoelectrics.

Capturing Anharmonicity in a Lattice Thermal Conductivity Model for High-Throughput Predictions

High-throughput, low-cost, and accurate predictions of thermal properties of new materials would be beneficial in fields ranging from thermal barrier coatings and thermoelectrics to integrated circuits. To date, computational efforts for predicting lattice thermal conductivity (κL) have been hampered by the complexity associated with computing multiple phonon interactions. In this work, we develop and validate a semiempirical model for κL by fitting density functional theory calculations to experimental data. Experimental values for κL come from new measurements on SrIn2O4, Ba2SnO4, Cu2ZnSiTe4, MoTe2, Ba3In2O6, Cu3TaTe4, SnO, and InI as well as 55 compounds from across the published literature. To capture the anharmonicity in phonon interactions, we incorporate a structural parameter that allows the model to predict κL within a factor of 1.5 of the experimental value across 4 orders of magnitude in κL values and over a diverse chemical and structural phase space, with accuracy similar to or better than th...

Enhanced thermoelectric performance via the solid solution formation: The case of pseudobinary alloy (Cu2Te)(Ga2Te3)3 upon Sb substitution for Cu

In this work we have observed the beneficial effect from the solid solution formation on the thermoelectric performance of (Cu2(1−x)Sb2xTe)(Ga2Te3)3 upon Sb substitution for Cu. This substitution allows the different occupations of Sb in the crystal lattice, i.e. Sb at Cu sites with x≤0.05 and at Ga sides with x≥0.05, which has resulted in the Pisarenko relation does not exactly capture the measured Seebeck coefficient under assumed effective masses m*. The reduction of the lattice thermal conductivity (κL) has been quantified within the temperature range from room temperature to 723K. Over the entire composition range, the κL value is reduced by 33% and 25% at temperature 723K and 580K, respectively. This observation is in a good agreement with the theoretical calculation based on the Callaway model used in the solid solutions. Along with the increasing of the mobility and electrical conductivity, the thermoelectric performance has been improved with the highest ZT value of 0.58 at 723K, which is about double the value of intrinsic (Cu2Te)(Ga2Te3)3.

Manipulation of the crystal structure defects: An alternative route to the reduction in lattice thermal conductivity and improvement in thermoelectric performance of CuGaTe2

Here, we present the manipulation of the crystal structure defects: an alternative route to reduce the lattice thermal conductivity (κL) on an atomic scale and improve the thermoelectric performance of CuGaTe2. This semiconductor with defects, represented by anion position displacement (u) and tetragonal deformation (η), generally gives low κL values when u and η distinctly deviate from 0.25 and 1 in the ideal zinc-blende structure, respectively. However, this semiconductor will show high Seebeck coefficients and low electrical conductivities when u and η are close to 0.25 and 1, respectively, due to the electrical inactivity caused by an attractive interaction between donor-acceptor defect pairs (GaCu2+ + 2VCu−).

Thermal transport property of Ge34 and d-Ge investigated by molecular dynamics and the Slack's equation

In this study, we evaluate the values of lattice thermal conductivity κL of type II Ge clathrate (Ge34) and diamond phase Ge crystal (d-Ge) with the equilibrium molecular dynamics (EMD) method and the Slack's equation. The key parameters of the Slack's equation are derived from the thermodynamic properties obtained from the lattice dynamics (LD) calculations. The empirical Tersoff's potential is used in both EMD and LD simulations. The thermal conductivities of d-Ge calculated by both methods are in accordance with the experimental values. The predictions of the Slack's equation are consistent with the EMD results above 250 K for both Ge34 and d-Ge. In a temperature range of 200–1000 K, the κL value of d-Ge is about several times larger than that of Ge34.

Transport properties of the layered Zintl compound SrZnSb2

Transport properties of the layered Zintl compound SrZnSb2 have been characterized from room temperature to 725K on polycrystalline samples. SrZnSb2 samples were found to be p-type with a Hall carrier concentration of 5×1020cm−3 at room temperature, and a small Seebeck coefficient and electrical resistivity are observed. A single band model predicts that, even with optimal doping, significant thermoelectric performance will not be achieved in SrZnSb2. A relatively low lattice thermal conductivity is observed, κL∼1.2Wm−1K−1, at room temperature. The thermal transport of SrZnSb2 is compared to that of the layered Zintl compounds AZn2Sb2 (A=Ca,Yb,Sr,Eu), which have smaller unit cells and larger lattice thermal conductivity, κL∼2Wm−1K−1, at 300K. Ultrasonic measurements, in combination with kinetic theory and the estimated κL values, suggest that the lower κL of SrZnSb2 is primarily the result of a reduction in the volumetric specific heat of the acoustic phonons due to the increased number of atoms per unit cell. Therefore, this work recommends that unit cell size should be considered when selecting Zintl compounds for potential thermoelectric application.

Disorder scattering effect on the high-temperature lattice thermal conductivity of TiCoSb-based half-Heusler compounds

The lattice thermal conductivities of TiCoSb-based half-Heusler alloys are presented in the temperature range between 300 and 900K. A phenomenological model calculation of the high-temperature lattice thermal conductivities of these alloys was derived based on the Klemens–Callaway theory [Phys. Rev. 119, 507 (1960); ibid. 113, 1046 (1959)]. Good agreement was obtained between the calculated and the experimental data for TiCoSb, TiCo0.5Rh0.5Sb, and Ti0.5Zr0.5CoSb. Furthermore, the model predicts that simultaneously isoelectronic alloying on both Ti and Co sublattices could reduce the lattice thermal conductivity, and a κL value of 0.3W∕mK is predicted for Ti0.5Zr0.5Co0.5Rh0.5Sb at 900K.

Temperature Range for DFB Mode Oscillation in 1.5 µm InGaAsP/InP DFB Lasers

The temperature range for distributed feedback (DFB) mode oscillations in 1.5-µm InGaAsP/InP DFB lasers was investigated. The range was determined by the relation between the net gain spectra and the loss difference between the Fabry-Perot and DFB modes. The temperature dependence of the net-gain spectra of 1.5-µm InGaAsP/InP Fabry-Perot (FP) lasers were measured. The loss difference between FP and DFB modes was calculated while taking into account the corrugation phase at facets, the facet reflectivity, and the coupling coefficient. From these results, it is confirmed that DFB lasers with two cleaved facets and a κL value of 2(κ: coupling coefficient, L: cavity length) can oscillate in the DFB mode in a temperature range of at least 100 degrees.

EFFECT OF FINE SOLID PARTICLES ON GAS-LIQUID MASS TRANSFER RATE IN A SLURRY REACTOR

The influence of suspended fine particles of differing adsorbing capacity (activated carbon, avicell cellulose, SiO2, and molecular sieves) on the liquid-film mass transfer coefficient κL, was examined experimentally in a stirred cell of well-defined gas-liquid interfacial area by chemical methods. The greatest effect on κL was observed with the activated carbon (about a threefold increase). An addition of glycerine in excess of 2.5 × 10−3 k mol/m3, which was adsorbed on the particles preferentially, removed this increase in κL totally. Amongst other particles, only avicell cellulose showed measureable effects. The increase in κL values was inversely proportional to the temperature and the stirring speed, and the particle loading was found to be immaterial after a certain value. When the reaction rate increased (hence, when the thickness of dissolved gas-rich layer decreased) by gradual addition of a homogeneous catalyst (i.e. Co++ for sulphite oxidation), the effect of particles on κL decreased and eventually it disappeared.

Sorption of dissolved organics from aqueous solution by polystyrene resins—II. External and internal mass transfer

External and internal mass transfer were characterized for the sorption of 2,4-dichlorophenol and p-nitrophenol on polystyrene resins. An evaluation method is presented which estimated the external mass-transfer coefficient, κL, from the concentration uptake in an integral fixed-bed reactor. The experimental κL values are compared with values predicted using four different empirical correlations. The effect of axial dispersion on the values of κL was shown to be small under the given conditions.To quantify the intraparticle mass transfer, an effective diffusion coefficient, Deff, was estimated using the pore diffusion model interpretation of data from a differential column reactor within a recirculated batch system. The Deff values depend on both resin structure and solute properties. Values of Deff between 0.3 × 10−9 and 1.8 × 10−9 m2/s were observed for macroporous resins, which exceed values that are characteristic of pore diffusion, and a value of 0.6 × 10−11 m2/s was estimated for a microporous resin.