Simultaneous large enhancements in thermopower and electrical conductivity of bulk nanostructured half-Heusler alloys

Large reductions in the thermal conductivity of thermoelectrics using nanostructures have been widely demonstrated. Some enhancements in the thermopower through nanostructuring have also been reported. However, these improvements are generally offset by large drops in the electrical conductivity due to a drastic reduction in the mobility. Here, we show that large enhancements in the thermopower and electrical conductivity of half-Heusler (HH) phases can be achieved simultaneously at high temperatures through coherent insertion of nanometer scale full-Heusler (FH) inclusions within the matrix. The enhancements in the thermopower of the HH/FH nanocomposites arise from drastic reductions in the "effective" carrier concentration around 300 K. Surprisingly, the mobility increases drastically, which compensates for the decrease in the carrier concentration and minimizes the drop in the electrical conductivity. Interestingly, the carrier concentration in HH/FH nanocomposites increases rapidly with temperature, matching that of the HH matrix at high temperatures, whereas the temperature dependence of the mobility significantly deviates from the typical T(-α) law and slowly decreases (linearly) with rising temperature. This remarkable interplay between the temperature dependence of the carrier concentration and mobility in the nanocomposites results in large increases in the power factor at 775 K. In addition, the embedded FH nanostructures also induce moderate reductions in the thermal conductivity leading to drastic increases in the ZT of HH(1 - x)/FH(x) nanocomposites at 775 K. By combining transmission electron microscopy and charge transport data, we propose a possible charge carrier scattering mechanism at the HH/FH interfaces leading to the observed anomalous electronic transport in the synthesized HH(1 - x)/FH(x) nanocomposites.     

Synthesis and Properties of the Mixed Hydrated Oxides V2?yWyO5 + δ < eqid2 > nH2O

Compounds of the general formula V_{2 − y}W_yO_{5 + δ} < eqid3 > nH₂O (0 < y ≤ 0.25) with the layered structure of polyvanadic acid V₂O₅ < eqid4 > nH₂O (H₂V₁₂O_{31 − δ} < eqid5 > nH₂O) have been prepared from peroxide solutions using the sol–gel process. The samples contain up to 5–8 wt% vanadium (IV). The water content changes within the range of 0.7 ≤ n ≤ 1.5 in depending on tungsten concentration. The V_{2 − y}W_yO_{5 + δ} < eqid6 > nH₂O (y ≤ 0.125) form the thin films described an interlayer distance of 11.60 ± 0.05 Å. The thermal properties, IR, and X-ray photoelectron spectra of the compounds synthesized have been studied. The thermal stability of the phases increases with the rising of tungsten content. The dehydration finishes with the forming solid solution V_2−yW_yO₅ and WO₃. The electrical conductivity of V_2−yW_yO_{5 + δ} < eqid7 > nH₂O (0 < y ≤ 0.25) powders was measured between 293 and 473 K at a relative humidity of 12%. The activation energy of conduction is independent upon the W content and equals 0.22–0.24 eV. Partial substitution of vanadium for tungsten was found to reduce the conductivity of the phases. The conductivity of the films increases with the increasing of relative air humidity and is governed by proton diffusion across the V-O-W layers.     

Crystal growth and properties of Ln2Ag1−xGa10−y (Ln=La, Ce), a disordered variant of the Ce2NiGa10-structure type

We report the flux growth and characterization of Ln₂Ag_1−xGa_10−y (Ln=La, Ce), a disordered variant of the Ce₂NiGa₁₀ structure type. Single crystals of La₂Ag_1−xGa_10−y (x∼0.3; y∼0.6) and Ce₂Ag_1−xGa_10−y (x∼0.3; y∼0.9) were grown by the self-flux method and characterized using single-crystal X-ray diffraction. Transport measurements of Ce₂Ag_1−xGa_10−y (x∼0.3; y∼0.9) reveal metallic behavior with a transition at 3 K. Magnetic measurements indicate antiferromagnetic ordering at 3 K of localized Ce³⁺ moments for Ce₂Ag_1−xGa_10−y. Magnetoresistance is positive with a maximum value of 16% at 9 T. La₂Ag_1−xGa_10−y exhibits metallic behavior with magnetic susceptibility showing temperature independent paramagnetism. We will compare Ce₂Ag_1−xGa_10−y (x∼0.3; y∼0.9) to Ce₂NiGa₁₀ to examine the effects of transition metal substitution and to the related Ce(Ag,Ga)₄ phase to examine the effects of crystal structure on the physical properties.     

Preparation and thermoelectric properties of AgPbmSbSem+2 materials

Hydrothermally synthesized AgPb_mSbSe_m+2 (m=10, 12, 16, 18) nanoparticles with diameters of 20-50 nm were compacted by pressureless sintering. The Seebeck coefficient and electrical conductivity of the samples were measured from room temperature up to ∼750 K. The samples show large and positive values of the Seebeck coefficient and moderate electrical conductivity. The thermoelectric properties of Ag_xPb₁₈SbSe₂₀ (x=0.8, 0.85) and AgPb₁₈SbSe_20−yTe_y (y=1, 3) samples have also been studied. It has been found that Ag_0.85Pb₁₈SbSe₂₀ sample has a higher thermoelectric power factor. A significant difference in thermoelectric properties has also been observed for the AgPb₁₈SbSe₂₀ samples prepared with pressureless sintering and spark plasma sintering.     

Room-temperature synthesis and conductivity of the pyrochlore type Dy2(Ti1−yZry)2O7 (0?y?1) solid solution

Different compositions in a solid solution of general formula Dy₂(Ti_1−yZr_y)₂O₇, showing high oxygen ion conductivity, have been successfully prepared at room temperature via mechanochemical synthesis. Stoichiometric mixtures of the constituent oxides were dry milled together in a planetary ball mill by using zirconia vials and balls. Chemical changes in the powder mixtures as a function of composition and milling time were followed by X-ray diffraction and revealed that, in all cases and after milling for 19 h, the powder mixtures consisted of a single phase. Electrical properties were measured on sintered pellets as a function of frequency, temperature and zirconium content, revealing an increase in conductivity of more than one order of magnitude for y?0.4, which, as observed in the similar Y₂(Ti_1−yZr_y)₂O₇, has been related with the onset of disordering of the anion sublattice. Despite increasing structural disorder with increasing Zr content, conductivity remains almost constant for y>0.6, reaching a maximum value of ∼5×10⁻³ for Dy₂Zr₂O₇ at 900 °C.     

Sodium “stuffed” Ba2−xSrxFe4O8 ferrites: new cationic conductors

Sodium insertion in the tetrahedral layer structure of the ferrites Ba_2−xSr_xFe₄O₈ was performed by solid state reaction at 1220 K in air. Superstoichiometric oxides with the actual formula (Ba_2−xSr_x)_1−y/4Na_yFe₄O₈—y0.56; 0.60Ba/Sr1.67—were characterized by X-ray and neutron powder diffraction. The hexagonal unit-cell volume shows an increasing dependence on the sodium insertion when the Ba/Sr ratio reaches the largest values. The marked expansion of the c parameter is the likely signature of the location of the inserted sodium cations within the interlayer space. One-half of the sodium cations partly sits on the Sr(Ba) sites in octahedral coordination and the other half occupies extra octahedral and tetrahedral sites. ac conductivity measurements point to a cationic conductivity whose thermally activated regime—E_a 0.7 eV—evidenced from 570 K, is unsensitive to the sodium content. The bottleneck of the 2D sodium mobility regards the crossing of the oxygen triangular faces shared by the different polyhedra within the interlayer space.     

Oxygen deficiency and phase transitions in SrCo1– x – y Fe x Cr y O3–δ ( x =0.10–0.40, y =0–0.05)

Oxygen-deficient phases based on perovskite-like strontium cobaltites-ferrites are promising mixed conductors for high-temperature electrochemical applications. The p(O₂)-T-δ diagrams for the oxide systems SrCo_{1– x – y} Fe _x Cr _y O_{3– δ} (x=0.10–0.40; y=0–0.05) were studied at 500–1000 °C in the oxygen pressure range from 10^–5 to 0.21 atm using the coulometric titration technique and thermogravimetric analysis. Stability limits of the cubic perovskite phases having a high oxygen ionic conductivity were evaluated as functions of temperature, oxygen partial pressure and oxygen nonstoichiometry. It was found that doping with chromium and increasing the iron content in SrCo(Fe,Cr)O_{3– δ} both lead to a considerable enlargement of the cubic perovskite phase existence domain towards lower temperatures and reduced oxygen pressures. Electronic Publication     

Oxygen ionic and electronic transport in apatite-type La10−x(Si,Al)6O26±δ

The main factor governing the oxygen ionic conductivity in apatite-type La_10−xSi_6−yAl_yO_{27−3x/2−y/2} (x=0-0.33; y=0.5-1.5) is the concentration of mobile interstitials determined by the total oxygen content. The ion transference numbers, measured by modified faradaic efficiency technique, vary in the range 0.9949-0.9997 in air and increase on reducing oxygen partial pressure due to decreasing p-type electronic conduction. The activation energies for ionic and hole transport are (56-67)±3 kJ/mol and (57-100)±8 kJ/mol, respectively. Increasing oxygen content leads to higher hole conduction in oxidizing atmospheres and promotes minor oxygen losses from the lattice when the oxygen pressure decreases, although the overall level of ionic conductivity is almost constant in the p(O₂) range from 50 kPa down to 10⁻¹⁶ Pa. Under reducing conditions at temperatures above 1100 K, silicon oxide volatilization from the surface layers of apatite ceramics results in a moderate decrease of the conductivity with time. This suggests that the operation of electrochemical cells with silicate-based solid electrolytes should be limited to the intermediate-temperature range, such as 800-1000 K, where the ionic transport in most-conductive apatite phases containing 26.50-26.75 oxygen atoms per unit formula is higher than that in stabilized zirconia. The average thermal expansion coefficients of apatite ceramics, calculated from dilatometric data in air, are (8.7-10.8)×10⁻⁶ K⁻¹ at 300-1300 K.     

Ionic transport in P(VDF-HFP)-PMMA-LiCF3SO3-(PC + DEC)-SiO2 composite gel polymer electrolyte

Composite gel polymer electrolytes composed of poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP) and polymethylmethacrylate PMMA polymers, PC + DEC as plasticizer and LiCF₃SO₃ as salt and fumed silica as filler have been synthesized by solvent casting technique with varying plasticizer-filler ratio systematically. Films of thickness in the range of 40-70 μm were characterized by a.c. impedance measurements in the temperature range 303 K to 373 K. Addition of filler to the polymer electrolyte was found to result in an enhancement of the ionic conductivity. A maximum electrical conductivity of ∼1 × 10⁻³ S/cm at 303 K and ∼2.1 × 10⁻³ S/cm at 373 K has been achieved with the dispersion of the SiO₂. FTIR spectral studies confirmed the polymer-salt interaction. XRD patterns exhibit the increased amorphicity in the blended composite gel polymer electrolytes. Scanning electron micrograph shows the dispersion of SiO₂ particle in the polymer electrolyte.     

The pyrochlore to ‘defect fluorite’ transition in the Y2(ZryTi1−y)2O7 system and its underlying crystal chemistry

A careful investigation of the previously reported single phase, pyrochlore structure type Y₂(Zr_yTi_1−y)₂O₇, 0?y?0.9, (YZT) solid solution has been carried out. Given the known slow rate of diffusion of cations in fluorite-related stabilized zirconia systems and the consequent difficulty in achieving equilibrium, careful attention was paid to synthesis procedures while Guinier XRD and electron diffraction were used to investigate the synthesized materials. As a consequence, a subtle but nonetheless clear two-phase region separating a pyrochlore-type solid solution field (from 0?y?∼0.54) from a “defect fluorite” type solid solution field (from ∼0.68?y?1) has been found. The underlying crystal chemistry of the system has been investigated using the bond valence sum approach. The dielectric properties have also been measured as a function of composition. The dielectric constant and dielectric loss of the lowest y compounds are potentially quite useful but deteriorate rapidly with increasing Zr content.     

p-Type thermoelectric properties of the oxygen-deficient perovskite Ca2Fe2O5 in the brownmillerite structure

Brownmillerite calcium ferrite was synthesized in air at 1573 K and thermoelectric properties (direct current electrical conductivity σ, Seebeck coefficient α, thermal conductivity κ, thermal expansion α_L) were measured from 373 to 1050 K in air. Seebeck coefficient was positive over all temperatures indicating conduction by holes, and electrical properties were continuous through the Pnma-Imma phase transition. Based on the thermopower and conductivity activation energies as well as estimated mobility, polaron hopping conduction was found to dominate charge transport. The low electrical conductivity, <1 S/cm, limits the power factor (α²σ), and thus the figure of merit for thermoelectric applications. The thermal conductivity values of ∼2 W/mK and their similarity to Ruddlesden-Popper phase implies the potential of the alternating tetrahedral and octahedral layers to limit phonon propagation through brownmillerite structures. Bulk linear coefficient of thermal expansion (∼14×10⁻⁶ K⁻¹) was calculated from volume data based on high-temperature in situ X-ray powder diffraction, and shows the greatest expansion perpendicular to the alternating layers.     

Dependence of the mixed alkali effect on temperature and total alkali oxide content in y[xLi2O·(1−x)Na2O]·(1−y)B2O3 glasses

The complex conductivity spectra of mixed alkali borate glasses of compositions y [xLi₂O·(1−x)Na₂O]·(1−y)B₂O₃ (with x=0.0, 0.2, 0.4, 0.6, 0.8, 1.0; y=0.1, 0.2, 0.3) in a frequency range between 10⁻² Hz and 3 MHz and at temperatures ranging from 298 to 573 K have been studied. For each glass composition the conductivities show a transition from the dc values into a dispersive regime where the conductivity is found to increase continuously with frequency, tending towards a linear frequency dependence at sufficiently low temperatures. Mixed alkali effects (MAEs) in the dc conductivity and activation energy are identified and discussed. It has been for the first time found that the strength of the MAE in the logarithm of the dc conductivity linearly increases with the total alkali oxide content, y, and the reciprocal temperature, 1/T.     

Crystal structure, characterization and thermoelectric properties of the type-I clathrate Ba8−ySryAl14Si32 (0.6≤y≤1.3) prepared by aluminum flux

The title compound was prepared as single crystals using an aluminum flux technique. Single crystal and powder X-ray diffraction indicate that this composition crystallizes in the clathrate type-I structure, space group Pm3?n. Electron microprobe characterization indicates the composition to be Ba_8−ySr_yAl_14.2(2)Si_31.8(2) (0.77<y<1.3). Single-crystal X-ray diffraction data (90 and 12 K) were refined with the Al content fixed at the microprobe value (12 K data: R₁=0.0233, wR₂=0.0441) on a crystal of compositions Ba. The Sr atom preferentially occupies the 2a position; mixed Al/Si occupancy was found on all framework sites. These refinements are consistent with a fully occupied framework and nearly fully occupied cation guest sites as found by microprobe analysis. Temperature dependent electrical resistivity and thermal conductivity have been measured from room temperature to 1200 K on a hot-pressed pellet. Electrical resistivity reveals metallic behavior. The negative Seebeck coefficient indicates transport processes dominated by electrons as carriers. Thermal conductivity is between 22 and 25 mW/cm K. The sample shows n-type conductivity with a maximum figure of merit, zT of 0.3 at 1200 K. A single parabolic band model predicts a five-fold increase in zT at 800 K if carrier concentration is lowered.     

Magnetic and dielectric characterization of alginic acid-Fe3O4 nanocomposite

We have reported the synthesis of water dispersible alginic acid (AA)-Fe₃O₄ nanocomposites. The crystallite size was obtained as 9 ± 2 nm from X-ray line profile fitting. As compared to the particle size of 9.2 nm obtained from TEM analysis, these particles show dominantly single crystalline nature. Dielectric analysis show that the real and imaginary parts of the permittivity of both pure AA and AA-Fe₃O₄ nanocomposite obey the ionic polarization mechanism comprised of ionic conductivity and interfacial or space charge polarization. Magnetization measurements show that the saturation magnetization of AA-capped magnetite is significantly lower than the theoretical bulk value, partly due to the structural distortions on the surface of the particles. Moreover, magnetite does not saturate and exhibits no coercivity above a certain temperature, revealing superparamagnetic behavior of the nanocomposite above a blocking temperature, T_B, of ∼175 K. The magnetic core size, determined by theoretical fitting to the magnetization data at room temperature, is 9.55 nm.     

Study of anionic conductors R1 ? y Sr y F3 ? y (R = Ce, Pr, Gd) and M1 ? x Y x O2 ? x /2 (M = Zr, Hf) using four-electrode method

The anionic conductivity parameters are determined for R_{1 − y} Sr _y F_{3 − y} (R = Ce, Pr, Gd), Zr_0.88Y_0.12O_1.94 single crystals and Zr_0.88Y_0.12O_1.94|M_0.88Y_0.12O_1.94 (M = Zr, Hf) bicrystals using an ac (5 Hz to 100 kHz) four-electrode method. The obtained conductometric data are compared with the results of studying these crystals using a two-electrode method. It is shown that physical parameters of R_{1 − y} Sr _y F_{3 − y} fluoride crystals with the tysonite (LaF₃) structure are stable at heating in air up to the temperatures of 200–300°C, which is important for their practical application. Original Russian Text ? N.I. Sorokin, B.P. Sobolev, 2008, published in Elektrokhimiya, 2008, Vol. 44, No. 9, pp. 1111–1115.     

Thermoelectric properties and microstructure of Mg3Sb2

Mg₃Sb₂ has been prepared by direct reaction of the elements. Powder X-ray diffraction, thermal gravimetric, differential scanning calorimetery, and microprobe data were obtained on hot pressed samples. Single phase samples of Mg₃Sb₂ were prepared and found to contain oxygen at the grain boundaries and to lose Mg and oxidize at temperatures above 900 K. Thermoelectric properties were characterized by Seebeck, electrical resistivity, and thermal conductivity measurements from 300 to 1023 K, and the maximum zT was found to be 0.21 at ∼875 K.     

Structure and properties of triclinic Ni0.85Mo6Te8

The structure of Ni_0.85Mo₆Te₈ was refined from single-crystal X-ray diffraction data at room temperature. It is triclinic, space group

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; 1619 reflections, 75 refined parameters, R = 0.031. The Mo atoms form distorted octahedral clusters (2.69 Å ≤ d_intra[Mo---Mo] ≤ 2.81 Å; 3.58 Å < d_inter[Mo---Mo]). The Ni atoms are disordered (site occupancy: 0.423(7); d[Ni---Ni] = 2.586(6) Å), and interact strongly with one Mo₆ cluster (d[Ni---Mo] = 2.603(3) and 2.958(3) Å), and weakly with another (d[Ni---Mo] = 2.985(3) Å). The structure transforms at 1057(5) K into a rhombohedral modification (a_hex = 10.457(2) Å, c_hex = 11.866(3) Å at 1073 K). Measurements on powders suggest metallic conductivity (5.1 × 10⁻⁴ Ω-cm at 293 K) and weakly temperature-dependent paramagnetism (110 × 10⁻⁶ emu/g at 100 K).     

Anionic high-temperature conduction in single crystals of nonstoichiometric phases R1 ? y M y F3 ? y (R = La-Lu, M = Ca, Sr, Ba) with the tysonite (LaF3) structure

Electrophysical properties of single crystals of nonstoichiometric phases R_{1 − y} M _y F_{3 − y} , where R = La-Lu, M = Ca, Sr, or Ba, with the tysonite (LaF₃) structure, which are present in a metastable state after being grown and cooled, are measured in the temperature interval extending from 300 to 1073 K. It is discovered that, during a sufficiently long high-temperature investigation, solid solutions R_{1 − y} Ca _y F_{3 − y} , where R = Tb, Dy, or Ho, undergo irreversible variations in the phase composition in the temperature region 723 to 823 K. This level of temperatures, which correspond to partial decomposition of phases R_{1 − y} Ca _y F_{3 − y} with the rare-earth elements of the end of the period, lies above the temperatures to which the fluoride solid electrolytes are usually heated when used in solid-state electrochemical devices. The temperature and concentration dependences of the phases’ electroconduction are explained in the framework of the vacancy mechanism of anionic transport. Original Russian Text ? N.I. Sorokin, B.P. Sobolev, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 4, pp. 420–431. The paper is dedicated to the memory of Prof. M.W. Breiter, formerly of the Vienna Technical University, Austria.     

Structural and magnetic properties of Nd18Li8Co4−xFexO39−y and Nd18Li8Co4−xTixO39−y

Nd₁₈Li₈Co₃FeO_39−y, Nd₁₈Li₈CoFe₃O_39−y and Nd₁₈Li₈Co₃TiO_39−y have been synthesised and characterised by neutron powder diffraction, magnetometry and Mössbauer spectroscopy. Their cubic structure (Pm3?n, a∼11.9 Å) is based on intersecting <1 1 1> chains comprised of alternating octahedral and trigonal-prismatic coordination sites. These chains lie within hexagonal-prismatic cavities formed by a Nd-O framework. Each compound has an incomplete oxide sublattice (y∼1), with vacancies located around the octahedral sites that lie at the points of chain intersection. These sites are fully occupied by a disordered arrangement of transition-metal cations but only 75% of the remaining octahedral sites are occupied. The trigonal-prismatic sites are fully occupied by lithium except in the case of Nd₁₈Li₈CoFe₃O_39−y where some iron is present. Antiferromagnetic interactions are present on the Nd sublattice in each composition, but a spin glass forms below 5 K when a high concentration of spins is also present on the octahedral sites.     

Structure and high-temperature thermoelectric properties of SrAl2Si2

Single crystals of SrAl₂Si₂ were synthesized by reaction of the elements in an aluminum flux at 1000 °C. SrAl₂Si₂ is isostructural to CaAl₂Si₂ and crystallizes in the hexagonal space group P-3m1 (90 K, a=4.1834 (2), c=7.4104 (2) Å, Z=1, R1=0.0156, wR2=0.0308). Thermal analysis shows that the compound melts at ∼1020 °C. Low-temperature resistivity on single crystals along the c-axis shows metallic behavior with room temperature resistivity value of ∼7.5 mΩ cm. High-temperature Seebeck, resistivity, and thermal conductivity measurements were made on hot-pressed pellets. The Seebeck coefficient shows negative values in entire temperature range decreasing from ∼−78 μV K⁻¹ at room temperature to −34 μV K⁻¹ at 1173 K. Seebeck coefficients are negative indicating n-type behavior; however, the temperature dependence is consistent with contribution from minority p-type carriers as well. The lattice contribution to the thermal conductivity is higher than for clathrate structures containing Al and Si, approximately 50 mW cm⁻¹ K, and contributes to the overall low zT for this compound.