Nanostructures boost the thermoelectric performance of PbS

In situ nanostructuring in bulk thermoelectric materials through thermo-dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS)(1-x)(PbTe)(x)x = 0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS)(1-x)(PbTe)(x) nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K.     

Vibrational modes and structural characteristics of (Ba(0.3)Sr(0.7))[(Zn(x)Mg(1-x))(1/3)Nb(2/3)]O3 solid solutions

(Ba(0.3)Sr(0.7))[(Zn(x)Mg(1-x))(1/3)Nb(2/3)]O(3) (BSZMN) (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) solid solution ceramics were synthesized by the conventional solid-state sintering technique. Vibration spectra (Raman spectroscopy and Fourier transform far-infrared reflection spectroscopy, short for FTIR) and X-ray diffraction (XRD) were employed to evaluate the correlation between crystal structures and vibration modes of these solid solutions as a function of Mg(2+) ions replaced by Zn(2+) ions. It is verified that these ceramics present a phase transition, i.e., the crystal structure changes from hexagonal phase (P ?3m1, where x≤ 0.4) to the pseudocubic phase (I4/mcm, where x≥ 0.8) with increasing Zn(2+) content. The phase transition is a gradual process, the sample where x = 0.6 is of the transition phase, i.e., at x = 0.6, phase transition begins to appear from hexagonal phase to pseudocubic phase but is not complete. The phase transition is also verified by the FTIR spectra. Tilting of oxygen octahedra is the main reason for the phase transition. The phonon modes of the vibration spectra were assigned, the position and width were determined, and the correlation of phonon vibrations with the microstructure for the different atoms substituted in B'-site was found.     

Soft Phonon Modes Leading to Ultralow Thermal Conductivity and High Thermoelectric Performance in AgCuTe

Crystalline solids with intrinsically low lattice thermal conductivity (κ_L) are crucial to realizing high‐performance thermoelectric (TE) materials. Herein, we show an ultralow κ_L of 0.35 Wm^?1 K^?1 in AgCuTe, which has a remarkable TE figure‐of‐merit, zT of 1.6 at 670 K when alloyed with 10 mol % Se. First‐principles DFT calculation reveals several soft phonon modes in its room‐temperature hexagonal phase, which are also evident from low‐temperature heat‐capacity measurement. These phonon modes, dominated by Ag vibrations, soften further with temperature giving a dynamic cation disorder and driving the superionic transition. Intrinsic factors cause an ultralow κ_L in the room‐temperature hexagonal phase, while the dynamic disorder of Ag/Cu cations leads to reduced phonon frequencies and mean free paths in the high‐temperature rocksalt phase. Despite the cation disorder at elevated temperatures, the crystalline conduits of the rigid anion sublattice give a high power factor.     

Determinants of thermal conductivity and diffusivity in nanostructural semiconductors

The origin of size effects in the thermal conductivity and diffusivity of nanostructural semiconductors was investigated through the establishment of a unified nanothermodynamic model. The contributions of size-dependent heat capacity and cohesive energy as well as the interface scattering effects were considered during the modeling. The results indicate the following: (1) both the thermal conductivity and diffusivity decrease with decreasing nanocrystal sizes (x) of Si and Si/SiGe nanowires, Si thin films and Si/Ge(SiGe) superlattices, and GaAs/AlAs superlattices when x > 20 nm; (2) the heat transport in semiconductor nanocrystals is determined largely by the increase of the surface (interface)/volume ratio; (3) the interface scattering effect predominates in the reduction of thermal conductivity and diffusivity while the intrinsic size effects on average phonon velocity and phonon mean free path are also critical; (4) the quantum size effect plays a crucial role in the enhancement of the thermal conductivity with a decreasing x (<20 nm). These findings provide new insights into the fundamental understanding of high-performance nanostructural semiconductors toward application in optoelectronic and thermoelectric devices.     

The effect of Cu substitution on microstructure and thermoelectric properties of LaCoO(3) ceramics

La(Co, Cu)O(3-δ) ceramics were prepared by pressureless sintering of citrate precursor powders, and their thermoelectric properties were investigated with an emphasis on the influence of Cu doping and phase structure as well as microstructure. It was found that a secondary phase first appeared in the form of a network along the grain boundaries and then changed to dispersion with increasing Cu content, which effectively reduced the lattice thermal conductivity of the materials. The thermal conductivity was only 1.21 W m(-1) K(-1) for the sample LaCo(0.75)Cu(0.25)O(3-δ), being much lower as for the thermoelectric oxide materials. In addition, a small amount of Cu substitution for Co increased the electrical conductivity greatly and the absolute Seebeck coefficient, whose sign was also reversed from negative to positive. The dimensionless figure of merit, ZT, of LaCoO(3-δ) oxides at low and middle temperatures can be remarkably enhanced by substituting Co with Cu.     

Design of energy band alignment at the Zn(1-x)Mg(x)O/Cu(In,Ga)Se2 interface for Cd-free Cu(In,Ga)Se2 solar cells

The electronic band structure at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface was investigated for its potential application in Cd-free Cu(In,Ga)Se(2) thin film solar cells. Zn(1-x)Mg(x)O thin films with various Mg contents were grown by atomic layer deposition on Cu(In(0.7)Ga(0.3))Se(2) absorbers, which were deposited by the co-evaporation of Cu, In, Ga, and Se elemental sources. The electron emissions from the valence band and core levels were measured by a depth profile technique using X-ray and ultraviolet photoelectron spectroscopy. The valence band maximum positions are around 3.17 eV for both Zn(0.9)Mg(0.1)O and Zn(0.8)Mg(0.2)O films, while the valence band maximum value for CIGS is 0.48 eV. As a result, the valence band offset value between the bulk Zn(1-x)Mg(x)O (x = 0.1 and x = 0.2) region and the bulk CIGS region was 2.69 eV. The valence band offset value at the Zn(1-x)Mg(x)O/CIGS interface was found to be 2.55 eV after considering a small band bending in the interface region. The bandgap energy of Zn(1-x)Mg(x)O films increased from 3.25 to 3.76 eV as the Mg content increased from 0% to 25%. The combination of the valence band offset values and the bandgap energy of Zn(1-x)Mg(x)O films results in the flat (0 eV) and cliff (-0.23 eV) conduction band alignments at the Zn(0.8)Mg(0.2)O/Cu(In(0.7)Ga(0.3))Se(2) and Zn(0.9)Mg(0.1)O/Cu(In(0.7)Ga(0.3))Se(2) interfaces, respectively. The experimental results suggest that the bandgap energy of Zn(1-x)Mg(x)O films is the main factor that determines the conduction band offset at the Zn(1-x)Mg(x)O/Cu(In(0.7)Ga(0.3))Se(2) interface. Based on these results, we conclude that a Zn(1-x)Mg(x)O film with a relatively high bandgap energy is necessary to create a suitable conduction band offset at the Zn(1-x)Mg(x)O/CIGS interface to obtain a robust heterojunction. Also, ALD Zn(1-x)Mg(x)O films can be considered as a promising alternative buffer material to replace the toxic CdS for environmental safety.     

Thermal properties of pure and doped (polyvinyl-alcohol) PVA

Films ≈350 μm of poly(vinyl-alcohol) composites, containing copper (Cu), aluminium (Al) and iron (Fe), metallic powder very fine, were prepared by a casting method. Thermal conductivity, phonon velocity, mean free path and specific heat were studied. The pure sample of PVA has a lower values of thermal conductivity than that which are doped with metals. For all samples the thermal conductivityK increases up to a certain temperatureT _gg (120–160°C) and then decreases with temperature. The specific heat increase with temperature up to ≈120°C and above 120°C is nearly independent on temperature. The pure sample of PVA has small values of mean free path (L)≈0.2 Å at room temperature, but for PVA⁺ metalsL≈2.0 Å. The phonon velocity of pure PVA is larger than that of PVA containing metals.     

Correlation of crystal structure, dielectric properties and lattice vibration spectra of (Ba(1-x)Sr(x))(Zn(1/3)Nb(2/3))O3 solid solutions

(Ba(1-x)Sr(x))(Zn(1/3)Nb(2/3))O(3) (BSZN) (x = 0.0, 0.50, 0.60, 0.65, 0.70, 1.0) solid solutions were synthesized by a conventional solid-state sintering technique. Vibration spectra (Raman spectroscopy and Fourier transform far-infrared reflection spectroscopy, FTIR) and X-ray diffraction (XRD) were employed to evaluate the crystal structures and phonon modes of these solid solutions. Dielectric constants (ε(r)) and temperature coefficient of capacitances (τ(c)) were examined to reveal the correlation of the dielectric properties and the crystal structures. The results show that with the increase in Sr(2+) content, the lattice structures of ceramics turn gradually from disordered cubic structure to ordered structure because antiphase tilting of the oxygen octahedra occurs where x≥ 0.65, which is the main reason for the phase transitions and variation of crystal structure. The appearance of the phase transitions is associated with variation of the symmetry structure, from cubic (Pm ?3m, where x = 0) to pseudocubic (I4/mcm, where 0.65 ≤x < 1.0) and then to hexagonal (P ?3ml, where x = 1.0). New phonon modes appear at around 250 cm(-1) in Raman spectra where x≥ 0.65, and there is also a different phonon mode appearing at 156 cm(-1) in the FTIR spectra at the same x range. The appearance of the new phonon modes is the characteristic of ceramics whose oxygen octahedra have tilted with Sr(2+) concentration where x≥ 0.65. The Raman shifts are related to the rigidity of the oxygen octahedra, while the widths of peaks are correlated with τ(c). The FTIR spectra were subjected to the Kramers-Kronig analysis, and the imaginary part of the dielectric constant was analyzed in detail.     

Thermoelectric properties of Yb(x)Eu(1-x)Cd2Sb2

The thermoelectric performance of EuCd(2)Sb(2) and YbCd(2)Sb(2) was improved by mixed cation occupation. The composition, structure, and thermoelectric properties of Yb(x)Eu(1-x)Cd(2)Sb(2) (x=0, 0.5, 0.75, and 1) have been investigated. Polycrystalline samples are prepared by direct reaction of the elements. Thermoelectric properties were investigated after densification of the materials by spark plasma sintering. Yb(x)Eu(1-x)Cd(2)Sb(2) crystallizes in the P3m1 space group. The lattice parameters increase with the europium content. These materials show low electrical resistivity, high Seebeck coefficient, and low thermal conductivity together with high carrier concentration and high carrier mobility. ZT values of 0.88 and 0.97 are obtained for Yb(0.5)Eu(0.5)Cd(2)Sb(2) and Yb(0.75)Eu(0.25)Cd(2)Sb(2) at 650 K, respectively.     

Ce-doped ZnO (Ce(x)Zn(1-x)O) becomes an efficient visible-light-sensitive photocatalyst by co-catalyst (Cu2+) grafting

We have fabricated an efficient visible-light-sensitive Cu(2+)-grafted Ce-doped ZnO photocatalyst (Cu(2+)-Ce(x)Zn(1-x)O) by adopting a metal ion doping and co-catalyst modification. Impurity states were formed below the conduction band (CB) edge in Ce(x)Zn(1-x)O, and these impurity states induce the visible-light absorption. Ce(x)Zn(1-x)O without a Cu(2+)-co-catalyst showed negligible visible-light activity due to the low reduction power of electrons in impurity levels. Surprisingly, Cu(2+)-modification over Ce(x)Zn(1-x)O drastically increased its visible-light activity. Excited electrons in impurity states can transfer to the Cu(2+)-ions on the surface and form Cu(2+)/Cu(+) redox couples, which cause the efficient oxygen reduction through a multi-electron reduction process. One of the striking features of the present study is that the metal doped semiconductors which were inactive due to their impurity states become efficient visible-light photocatalysts upon co-catalyst modification. The successful strategy used here for designing a highly active visible-light photocatalyst would provide numerous opportunities to develop an efficient metal-ion based visible-light photocatalyst.     

Nanostructured Bi(2-x)Cu(x)S3 bulk materials with enhanced thermoelectric performance

Nanostructured Bi(2-x)Cu(x)S(3) (x = 0, 0.002, 0.005, 0.007, 0.01, 0.03) thermoelectric polycrystals were fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods. The effect of Cu content on the microstructure and thermoelectric property of Bi(2-x)Cu(x)S(3) bulk samples was investigated. It was found that the subtle tailoring of Cu content could reduce both the electrical resistivity and the thermal conductivity at the same time, and consequently enhancing the thermoelectric property. A low electrical resistivity of 1.34 × 10(-4)Ω m(-1) and a low thermal conductivity of 0.52 W m(-1) K(-1) were obtained for the Bi(1.995)Cu(0.005)S(3) sample at 573 K. The low thermal conductivity is supposed to be due to the nanoscopic Cu-rich regions embedded in the host matrix. A peak ZT value of 0.34 at 573 K was achieved for the Bi(1.995)Cu(0.005)S(3) composition, which is the highest value in the Bi(2)S(3) system reported so far.     

Predicted electronic and thermodynamic properties of a newly discovered Zn8Sb7 phase

A new binary compound, Zn(8)Sb(7), has recently been prepared in nanoparticulate form via solution synthesis. No such phase is known in the bulk phase diagram; instead, one would expect phase separation to the good thermoelectric semiconductors ZnSb and Zn(4)Sb(3). Here, density functional calculations are employed to determine the free energies of formation, including effects from vibrations and configurational disorder, of the relevant phases, yielding insight into the phase stability of Zn(8)Sb(7). Band structure calculations predict Zn(8)Sb(7), much like ZnSb and Zn(4)Sb(3), to be an intermetallic semiconductor with similar thermoelectric properties. If sufficient entropy or surface energy exists to stabilize the bulk material, it would be stable in a limited temperature window at high temperature.     

Nanostructured AgPb(m)SbTe(m+2) system bulk materials with enhanced thermoelectric performance

Nanostructured Ag0.8Pbm+xSbTem+2 (m = 18, x = 4.5) system thermoelectric materials have been fabricated by combining mechanical alloying (MA) and spark plasma sintering (SPS) methods followed by annealing for several days to investigate the effect on microstructure and thermoelectric performance. It was found that appropriate annealing treatment could reduce both the electrical resistivity and the thermal conductivity at the same time, consequently greatly enhancing the thermoelectric performance. A low electrical resistivity of 2 x 10-3 Ohm-cm and low thermal conductivity of 0.89 W m-1 K-1 were obtained for the sample annealed for 30 days at 700 K. The very low thermal conductivity is supposed to be due to the nanoscopic Ag/Sb-rich regions embedded in the matrix. A high ZT value of 1.5 at 700 K has been achieved for the sample annealed for 30 days.     

Synthesis and transport property of Cu(1.8)S as a promising thermoelectric compound

Polycrystalline Cu(1.8)S compounds were fabricated by using a combined process of mechanical alloying and spark plasma sintering. The Cu(1.8)S sample with a second Cu(1.96)S phase and a lot of micro pores shows its maximum ZT value 0.5 at 673 K which is the highest value for p-type sulfide thermoelectric materials so far.     

柱撑阴离子粘土的合成、表征及催化性能研究(V)

Organic-anion-pillared layered double hydrotalcites such as Zn2AlTA(TA=terephthalate) are easily synthesized by using coprecipitation reaction of Zn2+ and Al3+ ions with terephthalic acid in NaOH solution. Exchange of this material under mild acidic conditions with the heteropolyanions proceeds smoothly to yield Zn2AlSiW11Co and Zn2Al-BW11Cu. Characterization has been carried out by elemental chemical analysis, powder X-ray diffraction and IR spectra. The results indicated that these materials possess exceptionally well-ordered gallery height of 0.98nm. It was found that this kind of pillared layered catalyst has high activity and selectivity for the oxidation of benzaldehyde to benzoic acid using H2O2 as oxidant in liquid-solid phase system.     

Type-I clathrate Ba8Ni(x)Si(46-x): phase relations, crystal chemistry and thermoelectric properties

The phase relations, crystal structure and thermoelectric properties of the type-I solid solution Ba(8)Ni(x)Si(46-x) were investigated. Based on X-ray diffraction, differential thermal analysis and electron probe microanalysis data, a partial phase diagram was constructed for the Si-rich part of ternary system Ba-Ni-Si at 800 °C. The solubility range of Ni in the clathrate-I phase at 800 °C was determined (2.9 ≤x≤ 3.8) and thermoelectric properties, namely electrical resistivity, Seebeck-coefficient and thermal conductivity, were measured in the temperature range from 300 to 850 K. A shift of the thermoelectric properties from a predominantly metallic to a more semiconducting behavior was observed for an increasing Ni-content. Density functional calculations revealed a significant decrease of the gap width in the density of states induced by the incorporation of Ni. Electrical resistivity and Seebeck coefficients for Ba(8)Ni(x)Si(46-x) with 3.3 ≤x≤ 3.8 have been modeled within the rigid band approximation.     

Solid-state coordination chemistry: influences of (M(terpyridyl))(M = Fe(III), Cu(II), Ni(II)) subunits on molybdenum oxide structures

The hydrothermal reactions of Na2MoO4 x 2H2O and 2,2':6',2"-terpyridine with appropriate salts of Fe(II), Cu(II), and Zn(II) yield a variety of mixed metal oxide phases. The Cu(II) system affords the molecular cluster [Cu(terpy)MoO4].3H2O (MOXI-40 x 3H2O), as well as a one-dimensional material [Cu(terpy)Mo2O7](MOXI-41) which is constructed from (Mo4O14)4- clusters linked through (Cu(terpy))2+ units. In constrast, the Zn(II) phase of stoichiometry identical to that of MOXI-41, [Zn(terpy)Mo2O7](MOXI-42), exhibits a one-dimensional structure characterized by a (Mo2O7)n2n- chain decorated with peripheral (Zn(terpy))2+ subunits. The iron species [(Fe(terpy))2Mo4O12](MOXI-43) is also one-dimensional but exhibits [(Fe(terpy))2(MoO4)2]2+ rings linked through (MoO4)2- tetrahedra. A persistent structural motif which appears in MOXI-40, MOXI-41, and MOXI-43 is the [(M(terpy))2(MoO4)2]n cluster with a cyclic )(M2Mo2O4) core. In general, the secondary metal sites M(II, III) are effective bridging groups between molybdate subunits of varying degrees of aggregation. Furthermore, the ligands passivate the bimetallic oxide from spatial extension in two or three dimensions and provide a routine entree into low-dimensional structural types of the molybdenum oxide family of materials.     

Kinetic fluorimetric determination of Cu(II) and Zn(II) by their incorporation reactions into a water-soluble porphyrin

Kinetics of incorporation of Cu, Zn, Fe, Co, Ni and Mn divalent ions into coproporphyrin-I in imidazole buffer solution, pH 7.0, has been studied by monitoring the decrease in fluorescence intensity of the free base porphyrin. All reactions followed simple second-order rate law, the rate constants being decreased in the order Zn > Cu > Co > Fe > Mn, Ni. the kinetic fluorimetric method for the determination of Cu(II) and Zn(II) using their incorporation reactions into the porphyrin was developed. Initial rate and fixed-time methods were used to construct calibration graphs over the range 0-1.0 x 10(-5)M of both metals. The analytical characteristics of the method and effect of foreign ions were determined. In the presence of sodium thiosulphate as the masking reagent the determination of micromolar concentrations of Zn in the presence of a 10-fold excess of Cu is possible.     

Effect of surface modes on the six-dimensional molecule-surface scattering dynamics of H2-Cu(100) and D2-Cu(111) systems

We include the phonon modes originating from the three layers of Cu(100)/Cu(111) surface atoms on the dynamics of molecular [H(2)(v,j)/D(2)(v,j)] degrees of freedom (DOFs) through a mean field approach, where the surface temperature is incorporated into the effective Hamiltonian (potential) either by considering Boltzmann probability (BP) or by including the Bose-Einstein probability (BEP) factor for the initial state distribution of the surface modes. The formulation of effective potential has been carried out by invoking the expression of transition probabilities for phonon modes known from the "stochastic" treatment of linearly forced harmonic oscillator (LFHO). We perform four-dimensional (4D?2D) as well as six-dimensional (6D) quantum dynamics on a parametrically time and temperature-dependent effective Hamiltonian to calculate elastic/inelastic scattering cross-section of the scattered molecule for the H(2)(v,j)-Cu(100) system, and dissociative chemisorption-physisorption for both H(2)(v,j)-Cu(100) and D(2)(v,j)-Cu(111) systems. Calculated sticking probabilities by either 4D?2D or 6D quantum dynamics on an effective potential constructed by using BP factor for the initial state distribution of the phonon modes could not show any surface temperature dependence. In the BEP case, (a) both 4D?2D and 6D quantum dynamics demonstrate that the phonon modes of the Cu(100) surface affect the state-to-state transition probabilities of the scattered H(2) molecule substantially, and (b) the sticking probabilities due to the collision of H(2) on Cu(100) and D(2) on Cu(111) surfaces show noticeable and substantial change, respectively, as function of surface temperature only when the quantum dynamics of all six molecular DOFs are treated in a fully correlated manner (6D).     

Calculations on the octupolar molecules with enhanced two-photon absorption cross sections based on the Zn (II) and Cu (I) as centers

The equilibrium geometries, electronic structures, as well as one- and two-photon absorption cross sections of a series of octupolar chromophores with Zn(2+) or Cu(+) as coordinate centers and 4,4'-bis(dibutylaminostyryl)-[2,2']-bis(bipyridyl) as ligands have been determined by using B3LYP/6-31G and ZINDO methods. These molecules are designed by controlled combination of two or three bipyridyl ligands with the metal centers. The results show that Zn(2+) is an effective template for the design of octupolar structures which enable it to form tetrahedral and octahedral coordinated complexes; while Cu(+) only exists in a tetrahedral coordinated complex, comparing the tetrahedral complex with Zn(2+) as the center with that of Cu(+) as the center, it is found that the complex with the Cu(+) center is a better two-photon absorption material than the former as far as the transparency/nonlinearity is concerned. Furthermore, for the same metal center of Zn(2+), both one- and two-photon absorptions of the tetrahedral complex are redshifted relative to those of the octahedral complex, is attributed to the spiroconjugation effect in the tetrahedral complex. Our theoretical findings are consistent with recent experimental observations and provide an important foundation for the design of improved transparency-nonlinearity two-photon absorption materials.