The absolute thermoelectric power of polyvalent liquid metals

Results on the measurement of the absolute thermoelectric power of ten polyvalent liquid metals (Al, Bi, Cd, Ga, Hg, In, Pb, Sn, Tl, Zn) from their melting points to about 750°C are reported. The electrical resistivities and the absolute thermoelectric powers of these metals have been calculated using the latest available data on structure factor and Harrison and Animalu form factors. These are compared with the experimental values. It is seen that whereas the predicted and experimental values of the electrical resistivities are in reasonable agreement, those for the absolute thermoelectric power are not. It is suggested that the experimental data on the absolute thermoelectric powers and the resistivities of liquid metals may be used to find the magnitude of the form factor at K = 2k _F.     

Die Änderung der Thermospannung im Magnetfeld (2. Ettingshausen-Nernst-Effekt) bei Kupfer,Silber und Palladium

An experimental device is described for measuring the change in thermoelectric power in magnetic fields (2nd Ettingshausen-Nernst-effect). The change in thermoelectric power was determined for the metals Cu, Ag, Pd at magnetic fields up to 20 kOe at liquid nitrogen temperature. At low magnetic fields there is a proportionality toB ². The temperature dependence of the 2nd Ettingshausen-Nernst-effect is reported for Cu.     

Unified study and transport properties of simple metals

The resistivity, thermoelectric power at the melting points and temperature variation of resistivity of the liquid alkali metals are studied with local Heine-Abarenkov and the modified Ashcroft pseudopotentials. The parameters of the model potentials are obtained from a unified study of the static and dynamic properties of the metals. The effect of different dielectric function and structure factor on the transport properties is discussed. It is shown that the simple modified Ashcroft pseudopotential reproduces the result fairly well except for Li and Cs.     

The thermoelectric power of the metallic glass Ca0.8Al0.2

We report the first measurement of the thermoelectric power Q, of a metallic glass that contains only normal metals. The thermoelectric power of amorphous Ca_0.8Al_0.2 has been measured as a function of temperature from 10 K to 420 K. It is found that Q is positive, varies linearly with temperature and has a small slope. This is similar to the thermoelectric power found for other metallic glasses containing large concentrations of transition metal atoms.     

The absolute thermoelectric powers of some liquid alloys

Measurements of the absolute thermoelectric powers of several liquid alloys as a function of temperature and composition are reported.

The alloys Hg-In and Hg-Tl were studied. In both cases the absolute thermoelectric powers show a minimum when plotted against concentration at low concentrations.

The system Hg-Na was measured over the range 0 at. % to 10 at. % Na. The absolute thermoelectric power shows a very sharp minimum at about 5.0 at. % Na. This minimum becomes sharper with increasing temperature. The effect is more marked in this case than in the previous two cases.

Measurements of absolute thermoelectric power of the alloys of cadmium in mercury and gold in mercury show that it becomes less negative as the metals are added to mercury. This is the opposite effect to that found in the previous two alloys.

Measurements made on the Cd-In system show no minimum. The results on these alloys are discussed with relation to Mott's theory for liquid mercury (Mott 1966).     

Effect of impurities on thermoelectric power due to phonon drag in metals

The effect of inelastic impurity scattering of electrons on the thermoelectric power due to phonon drag in metals has been studied. It is shown that this is the main cause of the thermoelectric power suppression due to doping at low temperatures. The thermoelectric power in a metal with a quadratic electron spectrum has been calculated as a function of temperature and impurity concentration. In addition to the impurity concentration, the correction to the thermoelectric power due to inelastic scattering contains the large factor Θ_D/T. Zh. éksp. Teor. Fiz. 111, 2237–2242 (June 1997)     

Structure of molten silicon and germanium by X-ray diffraction

X-ray diffraction patterns have been obtained from molten silicon and germanium near the melting point. In both cases the structure factor was a low first peak maximum with a small hump on its high angle side in contrast with those of simple molten metals such as sodium and aluminum. It was also found that the pair correlation functions for these molten elements are characterized by a low peak maximum which follows the usual first peak maximum corresponding to the nearest neighbour distance. The electrical resistivity and thermoelectric power have been calculated on the Ziman theory using thet-matrix of muffin-tin potential based on the structural data observed in this work. Good agreement was found in most cases.     

Thermoelectric power of magnetic metals: II. Anisotropic metals

We present a theory of the thermoelectric power tensor of anisotropic ferromagnetic metals with localized magnetic moments starting from the Boltzmann equation and incorporating anisotropy effects due to the lattice structure through a parameter measuring the anisotropy in the sound velocity. Elastic and inelastic phonon and spin scattering contributions are taken into account through a linear superposition of scattering cross sections. A mean field approximation is used to describe the ordered magnetic phase. Spin wave and impurity scattering, phonon and magnon drag are not included. In a range encompassing the Curie temperature, i.e. at “moderate temperatures”, the theory quantitatively reproduces observed features except for specific details (e.g. rounding near T_c) needing other physical input. We compare our theory to data on single crystals of Gd and Tb₇₅Gd₂₅. The c-axis thermoelectric power is well recovered for very reasonable values of anisotropy and scattering strength parameters. A conjecture is given to explain the basal plane thermoelectric power positive slope at high temperature.     

Model pseudopotentials and electronic properties of non-transition metals

Two new model pseudopotentials for electron-ion interaction in metals are proposed which contain a single adjustable parameter r_c, the radius of the ion core. The models are used in the evaluation of several properties like electrical resistivity (liquid and solid metals), thermoelectric power, electron dispersion, Fermienergy, density of states and electronic susceptibilities of certain non-transition liquid metals. The obtained results are largely satisfactory with available theoretical and experimental values. The study reveals that the present model pseudopotentials have improved in most of the cases the previous results of other one parameter models.     

Increasing the power factor in composite thermoelectric materials

Existing models of composite thermoelectric materials are analyzed. The power factor for several possible microstructures of composites that incorporate intermediate-valence intermetallides, silicides, and pure metals is calculated. It is shown that the largest power factor is achieved in the planar geometry (with alternating thermoelectric and pure metal layers). It is found that calculations according to the effective-medium theory are noticeably easier than those specified by methods based on calculating the effective kinetic coefficients. The maximum-possible power-factor values and optimal-mass fractions of good thermo-electric materials in composite systems are estimated.     

Temperature-dependent absorptance of painted aluminum, stainless steel 304, and titanium for 1.07 μm and 10.6 μm laser beams

We measured the temperature-dependent absorptance of metals (Al, Ti, SS304) for continuous beams from 1.07 μm fiber laser and 10.6 μm CO₂ laser using power sensors and infrared (IR) pyrometers. The absorptance measurements were repeated for metals with three different paint coatings. For measurements at elevated temperatures up to the melting point, integrating sphere is not practical since high temperature radiation from a heated target disturbs weak output from the sphere considerably. Our results provide how each metal, whether coated or uncoated, absorbs the infrared beams as temperature is elevated to a melting point. A polynomial approximation to the measured absorptance of each target is provided for modeling of the laser-metal interaction at elevated temperatures.     

Effect of impurities on the thermoelectric power of amorphous germanium

Large and anomalous changes in the thermoelectric power of amorphous (a?) Ge films have been observed on doping with impurities of Al, Fe and Sb. Depending on the concentration, Al and Sb impurities contribute a negative thermoelectric power below 300°K. and a positive thermoelectric power above 300°K. The effect of Fe is very small below 1 at. %. The thermoelectric power attains high temperature-independent positive values for large (> 1 at. %) concentrations of Al. The observed effects of the impurities cannot be understood in terms of the conventional crystalline semiconductor concepts. A band structure model for a-Ge has been proposed to qualitatively understand the changes in the sign and magnitude of the thermoelectric power with temperature.     

Theoretical melting curves for cubic solids at high pressure

The Lindemann law, the fourth-order anharmonic equation of state and a model for the volume dependence of the Grüneisen coefficient in cubic crystals are used to derive temperature-strain-pressure relations at melting. The relations depend for their application on knowing the values of fourth-order elastic parameters which are not measured experimentally. It is shown that these values can be estimated from melting data at low pressure. A comparative study of the results with previous determinations obtained from shock-wave data is given for copper, silver and gold. Finally, the theoretical fourth-order melting curves of the three selected metals are calculated using both the Lagrangian and Eulerian strain measures.     

Thermoelectric composite materials based on intermediate-valence cerium and ytterbium intermetallic compounds

An important thermoelectric characteristic-the power factor of composite thermoelectric materials—is calculated on the basis of performed measurements of the thermopower, electric conductivity, thermal conductivity, and reference data. The investigated composites consist of intermediate-valence CeNi and YbAl₃ intermetallic systems and elementary metals, nickel and chromium. On account of the specific transport properties of a heterogeneous medium, it becomes possible to increase the value of the power factor in composites at a sufficiently low concentrations of rare-earth-element-based intermetallic compounds. The dependences of the composite-thermoelectric-material power factor on the temperature and the mass fraction of valence-instable components are obtained.     

Thermoelectric power and its magnetic field dependence in pure Sb at low temperatures

The temperature dependence of the thermoelectric power of pure Sb measured along the binary axis is presented in the temperature interval of 4·2-30°K. The intervalley scattering of carriers strongly affects the thermoelectric behaviour of Sb above 15°K whereas below 15°K the phonon drag effect is found to contribute to the thermoelectric power of Sb. The magnetic field dependence of the thermoelectric power up to fields of 8000 G is investigated. The low field behaviour of the thermoelectric power below 15°K is observed to be in good accord with the theoretical calculations. At high fields the thermoelectric power is observed to be negative at temperatures below 15°K.     

Some problems in understanding the electronic transport properties of carbon nanotube ropes

The resistance of single-wall carbon nanotube (SWCN) ropes or mats, and some individual tubes, typically shows a crossover from non-metallic to metallic temperature dependence as temperature increases. This systematic pattern is consistent with a series heterogeneous model involving metallic resistance and tunnelling through barriers such as defects and inter-rope contacts. The metallic resistivity term increases linearly with temperature for the ropes or mats, but faster for the individual nanotubes. In contrast to the almost vanishing thermoelectric power expected from electronic band structure calculations, the measured values for mats or films (including recent measurements in a vacuum) are even larger than for typical metals. The thermopower increases with temperature as for metals, but has a characteristic non-linear shape. This temperature dependence can be modelled, for example, with parallel conduction in metallic and semiconducting tubes, but the size of the metallic thermopower required is anomalously large.     

Electrical and thermoelectrical properties of selenium-tellurium liquid alloys

A study has been made of the electrical conductivity and thermoelectric power of liquid alloys Te_1-x Se _x with 0≤-x≤-0.5. The temperature range extends from undercooling to about 900°C for electrical conductivity and 750°C for thermoelectric power. A partial conservation after melting of covalent bonds between the atoms of the chains leads to a liquid model in which Gubanov's theory predicts an energy band gap. The experimental results in the intrinsic semiconductor range give the band gap and the mobility ratio values. The thermal gap changes from 1.2 to 3 ev between pure tellurium and the alloy with 70 at. % selenium. There is a large increase in hole mobility with atomic % selenium. For x≥0.2 the low temperature results of the electrical conductivity can be explained by the existence of localized states in the band gap. The high temperature measurements show a trend to the metallic state, but this state cannot be reached at one atmosphere pressure even for tellurium.     

Unexpected low thermal conductivity and large power factor in Dirac semimetal Cd_3As_2

Thermoelectrics has long been considered as a promising way of power generation for the next decades. So far,extensive efforts have been devoted to the search of ideal thermoelectric materials, which require both high electrical conductivity and low thermal conductivity. Recently, the emerging Dirac semimetal Cd3As2, a three-dimensional analogue of graphene, has been reported to host ultra-high mobility and good electrical conductivity as metals. Here, we report the observation of unexpected low thermal conductivity in Cd3As2, one order of magnitude lower than the conventional metals or semimetals with a similar electrical conductivity, despite the semimetal band structure and high electron mobility. The power factor also reaches a large value of 1.58 m W·m-1·K-2at room temperature and remains non-saturated up to 400 K.Corroborating with the first-principles calculations, we find that the thermoelectric performance can be well-modulated by the carrier concentration in a wide range. This work demonstrates the Dirac semimetal Cd3As2 as a potential candidate of thermoelectric materials.     

Electronic transport and specific heat of 1T-V Se2

The low-temperature thermoelectric power and the specific heat of 1T-V Se₂ (vanadium diselenide) have been reported along with the electrical resistivity and Hall coefficient of the compound. The charge density wave (CDW) transition is observed near 110 K in all these properties. The thermoelectric power has been measured from 15 K to 300 K, spanning the incommensurate and commensurate CDW regions. We observed a weak anomaly at the CDW transition for the first time in the specific heat of V Se₂. The linear temperature dependence of the resistivity and thermoelectric power at higher temperatures suggests a normal metallic behavior and electron–phonon scattering above the CDW transition. The positive thermoelectric power and negative Hall coefficient along with strongly temperature-dependent behavior in the CDW phase suggest a mixed conduction related to the strongly hybridized s–p–d bands in this compound.     

Structural, electrical, and thermoelectrical properties of (Bi1 − x Sb x )2Se3 alloys prepared by a conventional melting technique

Polycrystalline solid solutions of (Bi_{1 ? x} Sb _x )₂Se₃ (x = 0, 0.025, 0.050, 0.075, 0.100) were prepared using a facile method based on the conventional melting technique followed by annealing process. X-ray analysis and Raman spectroscopical measurements revealed formation of Bi₂Se₃ in single phase. The electrical and thermoelectric properties have been studied on the bulk samples in the temperature range 100–420 K. The electrical conductivity measurements show that the activation energy and room-temperature electrical conductivity dependences on the Sb content respectively exhibit minimum and maximum values at x = 0.05. The thermoelectric power exhibited a maximum value near the room temperature suggesting promising materials for room-temperature applications. The highest power factor value was found to be 13.53 μW K^?2 cm^?1 and recorded for the x = 0.05 compound.