Enhancement of the generating power in Cu/Bi–Te/Cu composite thermoelectric devices

The voltage ΔV and electric current ΔI of the p- and n-type Cu/Bi–Te/Cu composite thermoelectric devices were measured as a function of ΔT for four regions of the intrinsic Bi–Te compound, Cu/Bi–Te and Bi–Te/Cu interfaces and Cu/Bi–Te/Cu composite using thermocouples set at intervals of s = 2 and 6 mm, where the lengths of Bi–Te compound and copper are 4 and 5 mm, respectively. ΔV and ΔI of all regions tended to increase linearly with an increase of ΔT. The resultant α was obtained from the relation ΔV/ΔT. The resultant α values of regions including the interface are much higher in absolute value than those of the intrinsic Bi–Te compounds, so that the barrier thermo-emf is found to occur in the forward-bias direction. It indicates that the barrier thermo-emf appears even in the semiconductor-metal junction, as in the case of the p–n junctions. The resultant α of Cu(T _H)/Bi–Te interface rich in the heat flow increases with an increase of ΔT, while that of Bi–Te/Cu(T _C ) interface poor in the heat flow decreases with an increase of ΔT. The ΔT-dependence of α of the interfaces is entirely opposite at the hot and cold sides. As a result, the resultant α of the p- and n-type Cu/Bi–Te/Cu composites remained little varied with changes of ΔT, so that the present composites have a thermal stability superior to the intrinsic Bi–Te compounds.The generating powers ΔW _Bi-Te and ΔW _Cu/Bi-Te/Cu for the p- and n-type intrinsic Bi–Te compounds and Cu/Bi–Te/Cu composites increased parabolalically with an increase of ΔT, and the ratios of ΔW _{Cu/Bi–Te/Cu} to ΔW _Bi–Te reached great values of 1.41 and 1.45 for the p- and n-type composites, respectively. It was thus found that the enhancement in the resultant α of the composite materials results in a significant improvement in the conversion efficiency for generators.     

Effect of the thickness of Bi–Te compound and Cu electrode on the resultant Seebeck coefficient in touching Cu/Bi–Te/Cu composites

The resultant Seebeck coefficient α of the touching p- and n-type Cu/Bi–Te/Cu composites with different thicknesses of t _Bi–Te and t _Cu was measured as a function of t, where t _Bi–Te was varied from 0.1 to 2.0 mm, t _Cu from 0.3 to 4.0 mm and t is the lapse time after imposing the voltage. The temperature difference ΔT is produced by imposing a constant voltage of 1.70 V on two Peltier modules connected in series. The resultant α of composites was calculated from the relation α = ΔV/ΔT, where ΔV and ΔT were measured with two probes placed on both end coppers. ΔV decreases abruptly with an increase of t below t = 5 min, while above t = 7 min, it tends to saturate to a constant value. The resultant α and saturated ΔV vary significantly with changes in t _Cu and t _Bi–Te. When a composite has a combination of t _Cu = 1.0 mm and t _Bi–Te=0.1 mm, the generating powers ΔW (=(ΔV)²/4R) estimated using the saturated ΔV and calculated electrical resistance R for the p- and n-type composites have great local maximum values which are 4–5 times as large as those obtained for the conventional combination of t _Bi-Te = 2.0 mm and t _Cu = 0.3 mm. It is surprising that the generating power ΔW is enhanced significantly by sandwiching a very thin Bi–Te material between two thick coppers, unlike the conventional composition of thermoelectric modules. On the other hand, when a composite has a combination of t _Bi–Te = 0.1 mm and t _Cu = 0.3 mm, the resultant α of the p- and n-type composites exhibited great values of 711 and −755 μV/K, respectively, so that the maximum resultant ZT of the p- and n-type composites reached extremely large values of 8.81 and 5.99 at 298 K. However, the resultant ZT decreases rapidly with an increase of t _Cu or t _Bi–Te. The resultant ZT is thus found to be enhanced significantly not only in superlattice systems but also in macroscopic composites. The present enhancement in ZT is attributed to the large barrier thermo-emf generated in the Bi–Te region shallower than 50 μm from the boundary.     

Temperature dependence of the local Seebeck coefficient near the boundary in touching Cu/Bi–Te/Cu composites

The thermo-emf ΔV and temperature difference ΔT across the boundary were measured as functions of r and I for the touching p- and n-type Cu/Bi–Te/Cu composites composed of t _Bi–Te = 2.0 mm and t _Cu = 0.3 mm, where r is the distance from the boundary and I is a direct current producing ΔT which flows through two Peltier modules connected in series. The resultant Seebeck coefficient α across the boundary is obtained from the relation α = ΔV/ΔT. As a result, the resultant |α| of the touching p- and n-type composites have a great local maximum value at r ≈ 0.03 mm and decrease rapidly with further increase of r to approach the intrinsic |α_Bi–Te|. The maximum resultant α of the p- and n-type composites reached great values of 1,043 and −1,187 μV/K at 303 K corresponding to I = 0.8 A and of 1,477 and −725 μV/K at 360 K corresponding to I = 2.0 A. Reflecting the temperature dependence of the intrinsic α_Bi–Te, the maximum α of the p-type composite increases with an increase of T, while that of the n-type one decrease with an increase of T. Surprisingly, the maximum α of the p- and n-type composites have great gradients of 8.36 and −7.15 μV/K² in the range from 303 to 366 K, respectively, which are 21.8 and 134 times larger in absolute value than 0.383 and −0.0535 μV/K² of the intrinsic p- and n-type α_Bi–Te, so that the maximum resultant α was thus found to be much more sensitive to temperature than the intrinsic α_Bi–Te. Moreover, the local Seebeck coefficient α _l (r) derived analytically from the resultant α(r) is enhanced significantly in the narrow region below r ≈ 0.05 mm and the maximum α _l values of the p- and n-type composites were found to have extremely great values of approximately 1,800 μV/K at 360 K and −1,400 μV/K at 303 K, respectively, which are approximately 7.3 and 6.5 times higher in absolute value than the intrinsic p- and n-type α_Bi–Te at the corresponding temperatures.     

Enhancement of the Seebeck coefficient in touching M/Bi–Te/M (M = Cu and Ni) composites

The resultant Seebeck coefficient α of the touching p- and n-type M/Bi–Te/M (M = Cu and Ni) composites was measured as a function of z at a scan step of 0.5 mm using thermocouples set at three different intervals of s = 4, 6.5 and 8 mm, where s is the interval between two probes and z is the distance from the center of Bi–Te compound to the middle of two thermocouples. Bi–Te compounds have a thickness of t _Bi–Te = 6 mm but the thickness t _M of both end metals sandwiching their compounds was varied from 0.5 mm to 6 mm. The composites were compacted tightly at a force of about 10 N by a ratchet. When two probes are placed on both end metals, the resultant α was significantly enhanced and exhibited a tendency to increase as s approaches t _Bi–Te, like the welded composites. The enhancement in α is attributed to the contribution from the barrier thermo-emf generated near the interface. When the thickness t ₀ of metal outside two probes set at s = 6.5 mm was increased from 0.25 mm to 5.75 mm, the averaged α for M = Cu and Ni was increased by 3.8% in the p-type composite, while reversely it was decreased by 4.8% in the n-type one. It was first observed that t ₀ also has a significant influence on the resultant α. The maximum α of the p- and n-type Ni/Bi–Te/Ni composites then reached great values of 264 μV/K at t _M = 6 mm (corresponding to t ₀ = 5.75 mm) and −280 μV/K at t _M = 1.2 mm (corresponding to t ₀ = 0.95 mm), respectively, which are 29% and 23% larger in absolute value than their intrinsic α values. These maximum α were barely changed with time. It was thus found that the barrier thermo-emf is generated steadily even in touching composites and the resultant α is highly sensitive to the position of leads connected to the metal electrode of a thermoelement.     

Enhancement of the thermoelectric figure of merit in p- and n-type Cu/Bi-Te/Cu composites

The resultant thermoelectric properties of welded Cu/Bi-Te/Cu composites were measured at 298 K as a function of relative thickness x of Bi-Te compound by changing the interval s between two thermocouples and compared with those calculated as a function of x by treating it as an electrical and thermal circuit. These composites were prepared by welding with eutectic solder of Pb-Sn, after one end surface of the as-grown p- and n-type Bi-Te ingots were plated with Ni. It was found that the observed ZT of composites has a local maximum at an optimum x even when s was changed, as in the case of Cu/Bi-Sb/Cu and Ni/Bi-Sb/Ni composites with various relative thicknesses. Appearance of a local maximum in ZT is owing to the barrier thermo-emf generated by a sharp temperature drop at the interface between Bi-Te compound and copper. It may be caused by the separation of non-equilibrium carriers at the interface between them. The observed maximum ZT values at 298 K of the p- and n-type composites reached surprisingly great values of 1.53 and 1.66 at x=0.98, which correspond to about twice as large as those of commercially utilized Bi-Te compounds. This enhancement of ZT is available for generators, but may be not utilizable as a Peltier module. The composite materials were thus found to be utilizable as useful means of further increase in ZT of macroscopic bulk materials.     

Spillover effect in the hydrogen absorption by the polycrystalline powder alloy FeNi1.75Cu1.5Mo0.5 and electric conductivity of the pressed powder

The process of hydrogen absorption by the FeNi_1.75Cu_1.5Mo_0.5 alloy in the polycrystalline form was investigated for both the pure and palladized forms (0.008 76% Pd) at temperatures from ambient to 600 °C in a hydrogen flow. Using differential scanning calorimetry, (DSC) thermogravimetry (TG) and X-ray diffraction, the influence of palladization on the hydrogen absorption was demonstrated. Kinetic analysis of the DSC thermograms, the kinetic and thermic parameters of hydrogen absorption were determined. The TG thermograms showed that on hydrogen absorption a weight change took place due to water formation and reduction of the oxide film at the surface of the powder particles. The activation energies of hydrogen absorption were 170 kJ mol⁻¹ for the original powder and 32 kJ mol⁻¹ for the palladized one. The enthalpies of the absorption ranged from ΔH = −5 to ΔH = −380 J g⁻¹ for the original and palladized powder, respectively. The rate constants of the absorption depended on the palladization and were found to be 0.03 and 1.20 s⁻¹, respectively, at 162 °C. The electric conductivity of the pressed powder (9 kbar) increases on heating in both air and a hydrogen atmosphere up to 600 °C, tending to a constant value. The changes of the parameters characteristic of the palladized form are ascribed to the mechanism of a hydrogen spillover effect due to the presence of palladium.     

Preparation and application of Cu/Cr hydrotalcite-like compounds

A series of copper/chromium hydrotalcite-like compounds (Cu/Cr-HTlcs) with Cu/Cr molar ratios from 1:1 to 4:1 synthesized by coprecipitation reaction using NaOH and Na₂CO₃ as precipitation agents and their derived Cu/Cr mixed oxides (Cu/Cr-MO) were used in solid propellant for the first time. The structure, morphology and thermal behaviors were investigated using inductively coupled plasma optical emission spectrometry, X-ray diffraction, Fourier-transform infrared spectroscopy, N₂ volumetric measurements, transmission electron microscope, thermogravimetry, and differential thermal analysis. The results showed that a well crystallized Cu/Cr-HTlcs with CO₃ ²⁻ as interlayer anions could be obtained with Cu and Cr molar ratio of 2:1, system pH value between 9 and 11, aging time longer than 24 h; the sample has a specific surface area of 107.8 m² g⁻¹ and average pore diameter is 9 nm with pore volume of 0.34 cm³ g⁻¹; N₂ adsorption–desorption isotherm is type IVb with H₂-type hysteresis loop; thermal stability of the sample is relatively lower and the sample tends to be curled when the aging time is prolonged to 1 week at room temperature. Solid propellant with Cu/Cr-MO catalyst exhibit significantly higher burning rate of 9.64 mm s⁻¹ than those without catalyst (6.28 mm s⁻¹)/with CuO · Cr₂O₃ catalyst (9.07 mm s⁻¹). Press index also decreases from 0.339 to 0.299, and mechanical performance on elongation for Cu/Cr-MO catalyzed propellant has a clear enhancement.     

Methods for the construction of the diagrams of fatigue crack-growth rate of materials

We study different parameters used for the construction of the diagrams of fatigue crack-growth rate of materials in the force, deformation, and energy approaches and tested for 08kp low-strength steel and 60 high-strength steel. The applicability of the dependences known from the literature for the evaluation of the crack-tip opening displacement and local strain energy at the crack tip is analyzed. It is shown that the range of local strains Δε* and the total range of dissipation of local energy in a loading cycle ΔW _t * specify the fatigue crack-growth rate in the material and that the da / dN−Δε* and da / dN−ΔW _t * diagrams are more sensitive to the structural and mechanical characteristics of the materials than the ordinary da / dN−ΔK diagrams. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 4, pp. 31–41, July–August, 2007.     

The effects of temperature gradient and growth rate on the microstructure of directionally solidified Sn–3.5Ag eutectic solder

The mechanical properties (microhardness, tensile strength) of alloys are controlled by their microstructure, which depends strongly on temperature gradient (G) and growth rate (V). Thus, it is important to understand the relationships among G, V and microstructure (rod eutectic) of Sn–Ag solders. The Sn–3.5 wt% Ag eutectic alloy was directionally solidified upward with a constant growth rate, V (16.5 μm/s) at different temperature gradients, G (1.43–4.28 K/mm) and with a constant temperature gradient, G (3.93 K/mm) at different growth rates, V (8.3–500 μm/s) in a Bridgman–type directional solidification furnace. The rod spacings (λ) have been measured from both longitudinal section (parallel to the growth direction, λ _L ) and transverse section (perpendicular to the growth direction, λ _T ) of the samples. The undercooling values (ΔT) were calculated by using V, λ and system parameters (K ₁ and K ₂). It was found that the values of λ (λ _T , λ _L ) decrease while V and G are increasing. The relationships between rod spacing and solidification parameters (G and V) were obtained by linear regression analysis. The dependences of eutectic spacings λ on undercooling (ΔT) are also analyzed. λ² V, λΔT, ΔTV ^−0.5 and ΔTG ^−0.5 values were determined by using λ, ΔT, V and G values. The results obtained in this work are compared with the Jackson–Hunt eutectic theory and the similar experimental works. The experimental l_\textT ² \textV \lambda_{\text{T}}^{ 2} {\text{V}} value (159.3 μm³/s) is slightly lower than the result 174.6 μm³/s calculated from Jackson–Hunt eutectic theory.     

Dynamic Remanent Vortices in Superfluid 3He-B

We investigate the decay of vortices in a rotating cylindrical sample of ³He-B, after rotation has been stopped. With decreasing temperature vortex annihilation slows down as the damping in vortex motion, the mutual friction dissipation α(T), decreases almost exponentially. Remanent vortices then survive for increasingly long periods, while they move towards annihilation in zero applied flow. After a waiting period Δt at zero flow, rotation is reapplied and the remnants evolve to rectilinear vortices. By counting these lines, we measure at temperatures above the transition to turbulence ∼0.6 T _c the number of remnants as a function of α(T) and Δt. At temperatures below the transition to turbulence T≲0.55 T _c, remnants expanding in applied flow become unstable and generate in a turbulent burst the equilibrium number of vortices. Here we measure the onset temperature T _on of turbulence as a function of Δt, applied flow velocity v=v _n−v _s, and length of sample L.     

Effect of oxygen content on the electrical properties of YBa2Cu3O7−x single crystals

The effect of oxygen content in the single crystals of high-temperature superconductor YBa₂Cu₃O_7−x on the electrical resistivity, the Hall effect in the plane perpendicular to thec axis and the energy gap Δ, measured with tunnelling electron microscope, has been studied. The distribution of the gap along the surface of the crystal was also studied. The results of the study on the relationship between the magnitude of the energy gap Δ and the superconducting transition temperatureT _c of single crystals with various oxygen contents are approximated by the linear dependence 2Δ_av=4·4kT _c .     

Microwave radiation enhancement and self-focusing using quasi-stationary electric field in a medium with elongated nanoparticles

It is shown that a mechanism of microwave radiation enhancement and self-focusing can be operative in a wave channel and spatial cavity filled with air containing elongated (dumbbell-shaped) nanoparticles at a volume fraction of c ₀ ∼ 10⁻³. This nonlinear medium can be pumped using a quasi-stationary electric field. At an oscillation frequency of Ω ∼ 10¹⁰−10¹¹s⁻¹, the radiation enhancement and self-focusing in a wave channel is possible over a length of Δz ∼ 50−200 m, while that in a spatial cavity can take place within a period of time Δt ∼ (2−7) × 10⁻⁷ s.     

The physical and photo electrochemical characterization of the crednerite CuMnO<Subscript>2</Subscript>

CuMnO₂ is prepared via Cu⁺ → Li⁺ exchange in molten copper (I) chloride. It crystallizes in a monoclinic structure (SG C2/m) where the MnO₆ octahedra elongation is ascribed to the Yahn–Teller (Y–T) effect of Mn³⁺ ions. From chemical analysis, the oxide is more accurately formulated as CuMnO_2.01. Above 250 °C, it undergoes a reversible transition to spinel Cu _x Mn_3−x O₄ and beyond 940 °C it converts back to Cu_1.1Mn_0.9O₂. Extrapolation of high-temperature magnetic data indicates T-intercept θ _p of −450 K and an effective moment of 5.22 μ_B, consistent with strong antiferromagnetism in the basal plans and high spin (HS) configuration Mn³⁺. This value is slightly larger than that of the spin only moment, a behavior ascribed to Cu²⁺ originating from oxygen insertion. As prepared, CuMnO₂ displays p-type conductivity with an activation energy of 0.16 eV. Most holes generated upon band gap excitation are trapped on Cu⁺ ions and the conduction occurs by small polarons hopping between neighboring sites. The linear increase of thermopower for Cu_1.05Mn_0.95O₂ with temperature indicates a hole mobility μ_300 K (3.5 × 10^-6 cm² V⁻¹ s⁻¹) thermally activated. CuMnO₂ is made p- and n-type and the difference in the carriers mobilities is attributed to different oxygen polyhedra. The title oxide, characterized photo electrochemically, exhibits a pH-insensitive flat band potential (+0.13 V_SCE). The valence band, located at 5.3 eV below vacuum, is made up of Cu 3d orbital. As application, the powder showed a good performance for the H₂-photo evolution.

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Spark Plasma synthesis and diffusion of Cu and Ag in vanadium mixed valence oxides

Spark Plasma sintering (SPS) technique allows powders to be compacted at low temperature with a very short holding time. The powder loaded into a carbon die is heated via direct current pulses and simultaneously submitted to an uni-axial pressure of several MPa. Full density of the sample is achieved within minutes. This process is used to study Cu and Ag metals interactions with V₂O₅ oxide. Syntheses of M _x V₂O₅ phases (M = Cu, Ag) have been achieved within minutes. Thus Cu and Ag atoms penetrate microcrystals of V₂O₅ oxide at a high speed, shearing its crystal network and simultaneously rebuilding the crystal structures of the prototype networks β, β′, ε or δ M _x V₂O₅. To account for the formation of these phases identified by X-ray diffraction, structural mechanisms are proposed. Cu and Ag atomic diffusion parameters have been determined from energy dispersive X-ray spectroscopy (EDX) and electron micropobe analysis (EPMA) line scans. High values of diffusion coefficients have been determined. Cu atoms diffuse faster than Ag, D _Cu ≈ 4 × 10⁻⁸ m²/s and D _Ag ≈ 0.5–1 × 10⁻⁹ m²/s in ε and δ M _x V₂O₅ phases, respectively. Their formation may also be used as a model for further investigations into the diffusion mechanisms of atoms in solids and for a better understanding of the SPS process.

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Liquid–solid interfacial reactions of Sn–Ag and Sn–Ag–In solders with Cu under bump metallization

Intermetallic compounds formed during the liquid–solid interfacial reaction of Sn–Ag and Sn–Ag–In solder bumps on Cu under bump metallization at temperatures ranging from 240 to 300 °C were investigated. Two types of intermetallic compounds layer, η Cu₆Sn₅ type and ε Cu₃Sn type, were formed between solder and Cu. It was found that indium addition was effective in suppressing the formation of large Ag₃Sn plate in Sn–Ag solder. During interfacial reaction, Cu consumption rate was mainly influenced by superheat of solder, contact area between solder and Cu and morphology of intermetallic compounds. The growth of η intermetallic compounds was governed by a kinetic relation: ΔX = tⁿ, where the exponent n values for Sn–Ag/Cu and Sn–Ag–In/Cu samples at 240 °C were 0.35 ± 0.01 and 0.34 ± 0.02, respectively. The n values increased with reaction temperature, and it was higher for Sn–Ag/Cu than that for Sn–Ag–In/Cu sample at the same temperature. After Cu was exhausted, ε intermetallic compound was converted to η intermetallic compound. The mechanisms for such growth of interfacial intermetallic compounds during the liquid–solid reaction were investigated.     

Thermal and magnetic measurements on YBa2Cu3O7−y

Specific heat measurements in zero and7T magnetic fields from 0·4 to 100 K, and magnetic susceptibility measurements aboveT _c on a series of polycrystalline samples of YBa₂Cu₃O_7−y (YBCO) reveal a number of regularities. The size of ΔC(T _c )/T _c for the samples [ΔC(T _c ) is the jump in the specific heat atT _c ] appears to vary linearly with the low-temperature value of the Debye Θ, with the entropy change between ≈ 85 K andT _c induced by the application of a 7 ^Г magnetic field, and with the variation of the observed low temperatureγ with magnetic field, dγ/dH. On the other hand the temperature-independent part of the magnetic susceptibility aboveT _c appears to be essentially independent of ΔC(T _c )/T _c . These results are consistent with the idea that samples of YBCO belowT _c are a mixture of superconducting and normal phases. Supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Dept. of Energy under Contract DE-AC03-76SF00098, and by an EXXON Education Grant from the Research Corporation.     

Electrical and thermoelectric properties of Cu0.75Ni0.125FeTe2

The electrical conductivity, Hall coefficient, and thermopower of Cu_0.75Ni_0.125FeTe₂ have been measured between 80 and 500 K. The results have been interpreted in terms of a three-band model which takes into account, in addition to the conduction band, two overlapping valence subbands. We have determined the energy spacing between the subbands (Δ_{0 K} = 0.002 eV), its temperature coefficient (dΔ/dT = 1 × 10⁻⁴ eV/K), and the effective masses of the heavy and light holes and electrons. With the interband scattering of light holes taken into account, the conductivity has a minimum at ∼190 K. Original Russian Text ? F.F. Aliev, G.G. Guseinov, G.P. Pashaev, G.M. Agamirzoeva, A.B. Magerramov, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 2, pp. 156–161.     

Thermodynamic Properties of Matrine in Ethanol

In this paper, the enthalpies of dissolution of matrine in ethanol (EtOH) were measured using a RD496-2000 Calvet microcalorimeter at 309.65 K under atmospheric pressure. The differential enthalpy (Δ_dif H _m) and molar enthalpy (Δ_sol H _m) of dissolution of matrine in ethanol were determined. And the relationship between heat and the amount of solute was also established. Based on the thermodynamic and kinetic knowledge, the corresponding kinetic equation that described the dissolution process was determined to be \fracdadt=2.36×10^-4(1-a)^1.09{\frac{{\rm d}\alpha}{{\rm d}t}=2.36\times 10^{-4}(1-\alpha )^{1.09}} . Moreover, the half-life, t _1/2 = 48.89 min, Δ_sol H _m = −12.40 kJ · mol⁻¹, Δ_sol S _m = −354.7 J · mol⁻¹ · K⁻¹, and Δ _sol G _m =  97.43  kJ · mol⁻¹ of the dissolution process were also obtained. The results show that this work not only provides a simple method for the determination of the half-life for a drug but also offers a theoretical reference for the clinical application of matrine.     

Theory of electron distributions and63Cu and17O nuclear quadrupole interactions in YBa2Cu3O7 and YBa2Cu3O6

Ab initio unrestricted Hartree-Fock Cluster investigations have been carried out on the electronic structures of the YBa₂Cu₃O₇ and YBa₂Cu₃O₆ systems. The results of these investigations provide satisfactory explanations of available⁶³Cu and¹⁷O nuclear quadrupole interaction data. The electron distributions obtained rule out the presence of Cu³⁺ ions and are supportive of the presence of Cu²⁺, Cu¹⁺, O¹⁻ ions in the O₇ system and Cu²⁺, Cu¹⁺ and O²⁻ in the O₆ system with actual charges departing significantly from the formal charges, especially in the O₇ system, indicating the importance of covalency effects. Suggestions will be made regarding possible sources that can bridge the remaining gap between theoretical and experimental results for the nuclear quadrupole interaction parameters.     

Enhancement of the thermoelectric power factor in Ag/Bi-Te/Ag composite devices

The resultant thermoelectric properties of the p- and n-type Ag/Bi-Te/Ag composite devices welded with pure Bi were measured at 298 K as a function of relative thickness of x, where x is the ratio of thickness of Bi-Te compound to the interval between two thermocouples and the chemical compositions of the p- and n-type Bi-Te compounds used here are (Bi_0.25Sb_0.75)₂Te₃ and Bi₂(Te_0.94Se_0.06)₃, respectively. Consequently, the electrical resistivities ρ of the p- and n-type Ag/Bi-Te/Ag devices increased linearly with an increase of x, while the Seebeck coefficients α were enhanced significantly in the range from x = 0.03 to 0.10, so that their observed P values have a large local maximum at x = 0.06. The x-dependence of P values was found to be explained roughly with the simple model proposed here when some reduction in the thermal conductivity κ of Ag and Bi was taken into the calculation. The maximum P of the p- and n-type Ag/Bi-Te/Ag devices reached extremely large values of 27.8 and 88.3 mW/K²m, which are higher than 25.7 mW/K²m obtained for the previous n-type Ni/Bi/Cu device.