Anisotropic Seebeck coefficient of $\mathrm{Sr}_2\mathrm{Ru}\mathrm{O}_4$ in the incoherent regime

In this article, the structural, electronic and thermal transport characteristics of bilayer tetragonal graphene (TG) are systematically explored with a combination of first-principles calculations and machine-learning interatomic potential approaches. Optimized ground state geometry of the bilayer TG structure is predicted and examined by employing various stability criteria. Electronic bandstructure analysis confirmed that bilayer TG exhibits a metallic band structure similar to the monolayer T-graphene structure. Thermal transport characteristics of the bilayer TG structure are explored by analysing thermal conductivity, the Seebeck coefficient, and electrical conductivity. The electronic part of the thermal conductivity shows linearly increasing behaviour with temperature, however the lattice part exhibits the opposite character. The lattice thermal conductivity part is investigated in terms of the three phonon scattering rates and weighted phase space. On the other hand, the Seebeck coefficient goes through a transition from negative to positive values with increasing temperature. The Wiedemann-Franz law regarding electrical transport of the bilayer TG is verified and confirms the universal Lorentz number. Specific heat of the bilayer TG structure follows the Debye model at low temperature and constant behaviour at high temperature. Moreover, the Debye temperature of the bilayer TG structure is verified by ab initio calculations as well as fitting the specific heat data using the Debye model.

This work establishes the level of uncertainty for electrical measurements commonly made on thermoelectric samples. The analysis targets measurement systems based on the four probe method. Sources of uncertainty for both electrical resistivity and Seebeck coefficient were identified and evaluated. Included are reasonable estimates on the magnitude of each source, and cumulative propagation of error. Uncertainty for the Seebeck coefficient includes the cold-finger effect which has been quantified with thermal finite element analysis. The cold-finger effect, which is a result of parasitic heat transfer down the thermocouple probes, leads to an asymmetric over-estimation of the Seebeck coefficient. A silicon germanium thermoelectric sample has been characterized to provide an understanding of the total measurement uncertainty. The electrical resistivity was determined to contain uncertainty of ±7.0% across any measurement temperature. The Seebeck coefficient of the system is +1.0%/-13.1% at high temperature and ±1.0% near room temperature. The power factor has a combined uncertainty of +7.3%/-27.0% at high temperature and ±7.5% near room temperature. These ranges are calculated to be typical values for a general four probe Seebeck and resistivity measurement configuration.

Study on the effect of high magnetic field on electron transport in lattice mismatched n-GaN grown on sapphire based on two-layer model

Ag-doped Mg3Sb2 crystals are successfully grown via a directional solidification method with high temperature gradient. The microstructure of the crystal is homogeneous and without precipitation. Both electrical conductivity and Seebeck coefficient are anisotropic in two crystallographic directions ([001] and [101]) due to anisotropic effective mass and mobility of the carriers. Thermal conductivity is isotropic too, and the maximum Seebeck coefficient is 265 μV K−1 in the [001] orientation, which represents a substantial improvement over previous results. As a consequence, the maximum power factor for the 2.5 at. % Ag-doped crystal is 1.21 mW m−1 K−2 in the [001] orientation, which results in an elevated ZT of 0.67 at T = 800 K. First principles calculations and Hall measurements are used to verify the experimental results.

Oxygen adsorption and electronic transport properties of Fe-substituted YBaCo4O7 compounds

An overview is presented on the high temperature combined creep and fatigue behaviour of polycrystalline and single crystal nickel base superalloys. Particular reference is made to extensive experimental studies carried out on the polycrystalline alloy MAR M002 alloy and its single crystal equivalent SRR99. hi general, the high temperature mechanical properties of the single crystal alloy are superior to the conventional polycrystalline alloy. This is mainly attributed to the lack of grain boundaries in the single crystal alloy. This lack of grain boundaries also leads to material response which is dependent on crystal orientation. The mechanical response and fatigue lives were examined when the alloys were subjected to total strain controlled limits, at constant temperatures between 750 °C and 1050 °C. Constant strain dwells were introduced at peak tensile and compressive strains. Comparisons between the high temperature mechanical response of polycrystalline and single crystal alloys can be made by introducing an orientation function based on the anisotropic elastic response of the single crystal. Mean stresses generated during combined fatigue-creep loading played a major role in determining the fatigue life of the alloys. For the single crystal alloys fatigue lives must be correlated with orientation modified total strain ranges for mean stress effects to be quantified Comparisons with other single crystal alloys are also made. Finally, a significant improvement in the high temperature fatigue-creep lives of single crystal over polycrystalline alloys is evident.

Calculations of the recoil energy under bombardment of the surface edge (001) for the vanadium mono-crystal by K+ ions (E_0=10-50 eV) with the initial motion trajectories lying in planes perpendicular to the plane (001) and parallel to the planes (100) and (110), passing along the crystallographic directions [010] and [110], respectively, have been made by the method of molecular dynamics using a long-range interaction potential. Anisotropy of maximum energy transfer to one of the group of (3-5) atoms simultaneously participating in the interaction, depending on the ion motion trajectory, has been revealed. The energy spray thresholds for specified directions have been determined Keywords: ion bombardment, single crystal, crystallographic directions, recoil energy, sputtering thresholds.

Enhancing the thermoelectric (TE) performance of materials requires simultaneous optimization of the power factor (PF) and reduction of thermal conductivity (κ), a challenging task due to their interdependent nature. This work demonstrates a concurrent enhancement of PF and reduction of κ in p-type half-Heusler compound, ZrCoBi. Using ab initio electronic structure calculations, we investigate the impact of isotropic tensile and compressive strains on the TE properties of materials. Our analysis encompasses electronic structure, mechanical properties, lattice dynamics, Seebeck coefficient, electrical and thermal conductivity, PF, and the figure of merit (ZT). The results reveal a unique strain-dependent behavior in the electronic properties of ZrCoBi, including a spurious band convergence effect, as identified through first-principles calculations incorporating spin-orbit coupling (SOC). Furthermore, the analysis shows that tensile strain effectively tunes the band gap, significantly altering the electronic structure and improving PF. At the same time, tensile strain induces significant changes in lattice dynamics, reducing lattice thermal conductivity (κL). These combined effects result in a fourfold increase in ZT at high temperatures. This study also provides a comprehensive framework for leveraging isotropic strain to achieve high-performance thermoelectric materials and guides the experimental exploration of enhanced ZT compounds.

Positive temperature coefficient and high Seebeck coefficient in ZnO–P2O5/Co composites

Seebeck coefficient and electrical conductivity of BSCF (Ba 0.5Sr 0.5Co xFe 1 − xO 3 − δ, 0 ≤ x ≤ 0.8) as a function of temperature and partial oxygen pressure

We report the effects of Si substitution on the temperature-dependent electrical resistivity, Seebeck coefficient, as well as thermal conductivity in the Heusler-type compound ${\mathrm{Fe}}_{2}\mathrm{VAl}$. It is found that the substitution of Si onto the Al sites causes a significant decrease in the electrical resistivity and lattice thermal conductivity. A theoretical analysis indicated that the reduction of lattice thermal conductivity arises mainly from point-defect scattering of the phonons. With slight substitution, the Seebeck coefficient changes sign from positive to negative, accompanied by the appearance of a broad minimum at high temperatures. These features are associated with the change in the electronic band structure, where the Fermi level shifts upwards from the center of the pseudogap due to electron-doping effect. For $xg0.1$ in ${\mathrm{Fe}}_{2}{\mathrm{VAl}}_{1\ensuremath{-}x}{\mathrm{Si}}_{x}$, no broad minimum in the Seebeck coefficient appears, indicative of a dramatic modification in the band structure of these materials. While the thermoelectric performance improves with increasing Si concentration, the largest figure-of-merit $ZT$ value among these alloys is still an order of magnitude lower than conventional thermoelectric materials.

The Seebeck coefficient and dark conductivity for undoped, and n-type doped thin film hydrogenated amorphous silicon (a-Si:H), and mixed-phase films with silicon nanocrystalline inclusions (a/nc-Si:H) are reported. For both undoped a-Si:H and undoped a/nc-Si:H films, the dark conductivity is enhanced by the addition of silicon nanocrystals. The thermopower of the undoped a/nc-Si:H has a lower Seebeck coefficient, and similar temperature dependence, to that observed for undoped a-Si:H. In contrast, the addition of nanoparticles in doped a/nc-Si:H thin films leads to a negative Seebeck coefficient (consistent with n-type doping) with a positive temperature dependence, that is, the Seebeck coefficient becomes larger at higher temperatures. The temperature dependence of the thermopower of the doped a/nc-Si:H is similar to that observed in unhydrogenated a-Si grown by sputtering or following high-temperature annealing of a-Si:H, suggesting that charge transport may occur via hopping in these materials.

Orientation dependence of polarization-modulated photovoltaic effect of relaxor-based Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 single crystals

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

High power and temperature luminescence of Y3Al5O12:Ce3+ bulky and pulverized single crystal phosphors by a floating-zone method