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Nanocomposite engineering has been proved effective in diverse regimes of material research to attain a performance beyond each constituent phase. In this work, Yb-filled CoSb 3 (bulk matrix/host)-Bi 0.4 Sb 1.6 Te 3 (secondary inclusion) thermoelectric nanocomposites have been synthesized via an ex situ process. Bi 0.4 Sb 1.6 Te 3 inclusions are mainly distributed at the grain boundaries of Yb 0.2 Co 4 Sb 12 matrix in the composites. In particular, Te diffuses in situ from Bi 0.4 Sb 1.6 Te 3 through Yb 0.2 Co 4 Sb 12 matrix during the hot pressing process. This, combined with the grain boundary effect, results in favorable changes in the carrier concentration, carrier mobility, electrical resistivity, Seebeck coefficient, and thermal conductivity. Such synergistic changes are notably absent in the stand-alone Te-doped Yb-filled CoSb 3 , suggesting the key role of diffusion and grain boundaries. As a result, a maximum ZT value of 0.96 has been attained for Yb 0.2 Co 4 Sb 12 -2 wt% Bi 0.4 Sb 1.6 Te 3 at 650 K. The present work opens a new avenue towards high performance thermoelectric composites via controlled inter-constituent diffusion and grain boundary effect.

Recently, research interests in p-type skutterudites are focused on multi-filling in the intrinsic void sites of Fe4−xMxSb12. The four-membered antimony rings, another important structural feature of p-type skutterudites, seem to be overlooked. In this study, Te has been employed to substitute Sb in single-filled p-type Ce0.9Fe3.75Ni0.25Sb12 skutterudite and a systematic investigation has been carried out into the doping effect of Te on the microstructure and thermoelectric properties of this material. With an increase of Te doping, the electrical resistivity decreases due to the increase of hole concentration and the gradual decrease of mobility; while the density of state effective mass increases rapidly with the increase of hole concentration due to the intrinsic multiple bands effects, thus the Seebeck coefficient increases slightly. In addition, Te doping on Sb sites changes the valance electron balance and results in the formation of microscale γ-Ce and nanoscale CeTe2 compound. These microscale and nanoscale precipitates work together with the atomic scale distortion of Sb4 rings by the substitution of Te for Sb, could scatter phonons with a wide range of frequency thus result in a significant reduction of the lattice thermal conductivity. Therefore the thermoelectric performance has been enhanced greatly, and a maximum ZT value of 1.0 at 773 K was obtained for the Ce0.9Fe3.75Ni0.25Sb11.9Te0.1 sample.

Since the lattice thermal conductivity of n-type multi-filled skutterudites have been reduced below 1 W (mK−1), the development of new strategies that can further enhance the power factor while maintaining the low thermal conductivity is highly desired. In this paper, we conducted a pioneering work by introducing a “core–shell” microstructure into Yb single-filled skutterudite thermoelectric materials to realise this purpose. The “core–shell” structure formed by the thermal diffusion of well dispersed Ni nanoparticles in the Yb0.2Co4Sb12 powder during hot pressing is composed of the normal “core” grains surrounded by Ni-rich nanograin “shells”. The electrical resistivity is greatly reduced due to the increase in both carrier concentration and mobility. However, the Seebeck coefficient first increases due to the increased density of states at the Fermi energy and then decreases gradually. As a consequence, the power factor is remarkably increased for the samples with the addition of Ni nanoparticles. In addition, the lattice thermal conductivity is also reduced by the extra phonon scattering introduced by the “core–shell” microstructure. The concomitant effects enable a maximum ZT of 1.07 for the 0.2 wt% Ni sample at 723 K.

The single-filled skutterudite Yb0.2Co4Sb12 has been long known as a promising bulk thermoelectric material. In this work, we adopted a melting–milling–hot pressing procedure to prepare nanocomposites that consist of a micrometer-grained Yb0.2Co4Sb12 matrix and well-dispersed AgSbTe2 nanoinclusions on the matrix grain boundaries. Different weight percentages of AgSbTe2 inclusions were added to optimize the thermoelectric performance. We found that the addition of AgSbTe2 nanoinclusions systematically and simultaneously optimized the otherwise adversely inter-dependent electrical conductivity, Seebeck coefficient and thermal conductivity. In particular, the significantly enhanced carrier mobility led to a ∼3-fold reduction of the electrical resistivity. Meanwhile the absolute value of Seebeck coefficient was enhanced via the energy filtering effect at the matrix–nanoinclusion interfaces. Moreover there is a topological crossover of the AgSbTe2 inclusions from isolated nanoparticles to a nano-plating or nano-coating between 6 wt% and 8 wt% of nanoinclusions. Above the crossover, further addition of nanoinclusions degraded the Seebeck coefficient and the electrical conductivity. Meanwhile, the addition of nanoinclusions generally reduced the lattice thermal conductivity. As a result, the power factor of the 6 wt% sample was ∼7 times larger than that of the nanoinclusion-free sample, yielding a room temperature figure of merit ZT ∼ 0.51.

Yb0.2Co4Sb12 based composites with AgSbTe2 nanoinclusion have been successfully prepared by a combined process of vacuum melting, pulverization and hot press sintering. XRD, FESEM, TEM and EDS were performed to characterize the microstructure and composition of the nanocomposites. It shows AgSbTe2 inclusions are about 50 nm in size and they distribute in the grain boundaries of Yb0.2Co4Sb12 filled skutterudite matrix. When the content of AgSbTe2 is less than 8wt%, Seebeck coefficient and electrical conductivity of the composites increase with increasing content of AgSbTe2, while the lattice thermal conductivity reduces with the increase of AgSbTe2 nanoinclusions. As a result, thermoelectric performance is improved with addition of AgSbTe2 nanoinclusion and a maximum ZT of 1.27 has been obtained for the sample of Yb0.2Co4Sb12/4wt%AgSbTe2 at about 300 °C.

In this work we have synthesized In0.2+xCo4Sb12+x samples using a melting–annealing–hot pressing procedure, with the In and Sb contents concomitantly varied. The microstructures and thermoelectric properties have been subsequently studied and correlated. Increasing x value leads to an increase of the electrical conductivity, a decrease of the Seebeck coefficient in magnitude, and non-monotonic changes in the power factor and the lattice thermal conductivity. In particular, we have for the first time observed the InSb/Sb eutectic formation through thermal analysis when x is well above 0.2. The high density of interfaces via the formation of InSb/Sb mixture on the grain boundaries seems to decrease the lattice thermal conductivity and accounts for a dimensionless figure of merit, ZT, of 1.0 in In0.8Co4Sb12.6 at 650 K.

Void-filling in the CoSb3 skutterudite lattice with different kinds of heavy elements has proven to be an effective mechanism to enhance thermoelectric performance due primarily to a reduction in lattice thermal conductivity. Specifically, our findings on the series In x Yb y Co4Sb12 [0 ≤ (x, y) ≤ 0.2] have further motivated an attempt to form triple-filled skutterudites Ce0.1In x Yb y Co4Sb12 with In and Yb concentrations [0 ≤ (x, y) ≤ 0.2] and with the Ce concentration held constant (Ce0.1). All of these samples have been prepared via a simplified melting–annealing–sintering procedure and were first characterized by means of x-ray powder diffraction and scanning electron microscopy, followed by measurements of the Hall coefficient, electrical and thermal conductivities, and Seebeck coefficient. Our aim is to further elucidate the roles of the three elements (Ce, In, and Yb) in these materials. Compared with the addition of just In or Yb, we found that simultaneous addition of both In and Yb reduced the lattice thermal conductivity without significantly degrading the power factor. Further addition of the third element (Ce), along with In and Yb, also produced a similar result. However, we noticed that some of the In and Yb were also observed in the form of secondary phases (InSb and Yb2O3), not entering entirely as filler atoms. As a result of our investigation, several compositions achieved increased sustainability and enhanced thermoelectric performance, with maximum ZT values of about 1.3 to 1.4 obtained at around 800 K.

Filled skutterudites have long been singled out as one of the prime examples of phonon glass electron crystal materials. Recently the double-filling approach in these materials has been attracting increased attention. In this study, Yb0.2In y Co4Sb12 (y = 0.0 to 0.2) samples have been prepared by a simple melting method and their thermoelectric properties have been investigated. The power factor is increased dramatically when increasing the In content, while the lattice thermal conductivity is lowered considerably, leading to a large increase of the ZT value. A state-of-the-art ZT value of 1.0 is attained in Yb0.2In0.2Co4Sb12 at 750 K.

Partially filled Skutterudite compounds LayCo4-xFexSb12 (x≤1, y≤1) were synthesized via mechanical alloying-hot pressing-annealing process and the thermoelectric properties of LayCo3.5Fe0.5Sb12 were investigated with comparison of unfilled Co3.5Fe0.5Sb12 It is found that La filling makes the thermal conductivity decrease dramatically and power factor decrease simultaneously, yielding a small increase of ZT values. To enhance ZT values, the electrical property has to be optimized.

Mechanical alloying and preparation of Fesubstituted CoSb3-based skutterudite Co4−x Fe x Sb12 (x⩽1) were studied. All the compositions showed similar kinetics in mechanical alloying, where the skutterudite phase was kinetically more favorable than CoSb2 phase initially. Then the latter became the strongest phase when milling time was increased. A single phase skutterudite structure could not be obtained via mechanical alloying. Hot pressing on powders milled for 10 hours yielded mostly skutterudite phase as well as minor residual Sb impurity, and single phase skutterudite compounds were finally obtained for Co4−x Fe x Sb12 (x⩽0.65) after annealing at 650°C for 48 hours. The maximal solubility for Fe substitution seemed to be reached at x = 0.65.

Partially filled skutterudite antimonides La y Co 3.5 Fe 0.5 Sb 12 were synthesized through solid-state reaction. Large thermal parameter of filling atom was observed from Rietveld refinement. Raman spectra were studied, and only Fe substitution made little change to Raman spectra, while La filling in the voids in skutterudite structure produced large broadening of the vibrational modes. To corroborate the interaction of filling ions with vibrational modes, thermal transport property was measured.

La-filled and Fe-substituted skutterudites La x FeCo 3 Sb 12 ( x = 0.2, 0.4, 0.6, 0.9) were prepared by mechanical alloying (MA) and hot pressing (HP) in this paper. X-ray diffraction and field-emission-gun scanning electron microscopy were performed to characterize the phase transformation during the MA-HP process. Skutterudite phase appeared after milling for 10 h, and (Co, Fe)Sb 2 phase was synthesized with further prolonging the milling time. Although single-phase filled skutterudite could not be directly obtained even milled over 40 h, it was successfully produced by hot pressing the powders milled for 10 h at 650 °C for 2 h under a pressure of 50 MPa. A great reduction of the thermal conductivity of the hot-pressed samples was obtained, and the thermal conductivity decreased with increasing of La-filling fraction.

Single phase polycrystalline La x FeCo 3 Sb 12 ( x = 0.2, 0.4, 0.6 and 0.9) skutterudites were successfully prepared by mechanical alloying (MA) and hot pressing (HP). Thermoelectric properties including the Seebeck coefficient, the electrical resistivity, the thermal conductivity and the dimensionless figure of merit ZT were discussed over the range 300–750 K. It was found that all samples showed p-type conduction. For x = 0.2, 0.4 and 0.6, the Seebeck coefficient increased gradually over the whole temperature range of measurements, however, the electrical resistivity was weakly temperature dependent and increased slightly with temperature. For x = 0.9, the Seebeck coefficient and the electrical resistivity increased with temperature and reached a maximum value at about 650 K and then decreases with temperature. The thermal conductivity was greatly decreased by filling rear-earth element La into the voids of skutterudite structure. As a result, the La 0.4 FeCo 3 Sb 12 skutterudite exhibited a maximum figure of merit of 0.32 at about 750 K.

Starting from elemental powder mixtures of La x Fe 4 Sb 12 ( x = 0.5, 1.0, 1.5), La filled skutterudites were prepared by mechanical alloying (MA) and hot pressing (HP), effect of La addition content on thermoelectric properties was studied in this paper. The as sintered samples were characterized by XRD, EDX and DTA. Owing to the incomplete filling of La, impurities such as Sb and FeSb 2 manifest even at the La content of x = 1.0. Increasing the La addition to x = 1.5, the filled skutterudite phase overwhelms the impurities in the XRD pattern of the hot pressed sample. Over the temperature range of 300–750 K, the Seebeck coefficient and the electrical resistivity of the hot pressed samples increase with the increasing of both La addition and temperature. The thermal conductivity is greatly decreased in comparison with that of unfilled CoSb 3 skutterudite. A maximum ZT of 0.41 was obtained at 750 K with a La content of x = 1.5.

Starting from elemental powder mixtures, single-phase LaxCo3.6Ni0.4Sb12( x = 0 , 0.1, 0.4, 0.6)-filled skutterudites were synthesized via the route of mechanical alloying–hot pressing (MA–HP) in this paper. With increasing of La fraction, the lattice spacing of filled skutterudite phase increases and its variation follows the Vegard's law. The magnitude of the Seebeck coefficient and electrical resistivity show slight increases with increasing of La filling fraction; thermal conductivity of the filled skutterudite decreases and the resultant figure of merit increases with increase of La filling fraction. The as-HPed filled skutterudite has a composite nanocrystalline microstructure, which includes some coral-like clusters with relatively large spoke-like grains about 300 nm in length and a superfine equiaxial nanocrystalline matrix with an average grain size of about 50 nm. The coral-like cluster corresponds to the prime filled skutterudite formed directly by MA, while the filled skutterudite formed during hot pressing, which has the same nucleation condition and experiences less grain growth, develops equiaxially into the superfine nanocrystalline matrix.

In this work, La0.4Ni0.4Co3.6Sb12 filled skutterudite was prepared successfully by mechanical alloying (MA) and subsequent hot pressing. With increasing hot press (HP) temperature, the content of the filled skutterudite phase increases and its composition tends to the nominal composition; a single phase filled skutterudite compound can be obtained when the HP temperature is higher than 550 °C. The Seebeck coefficient and electrical resistivity of the hot pressed sample increased while its thermal conductivity decreased and the resultant figure of merit increased with the increase in the HP temperature. A fractograph observation shows that there are two nanocrystalline microstructures in the hot pressed filled skutterudite sample—one is the prime skutterudite phase formed by MA with a relatively large size, about 300 nm, and the other is the skutterudite produced during the HP process which has an average grain size about 80 nm.

La y Fe x Co 4− x Sb 12 skutterudites with different La filling content ( y = 0.2, 0.4, 0.6, 0.9; x = 1.0) were prepared by mechanical alloying and hot pressing. The effect of La partially filling on the thermoelectric properties of these compounds has been studied. The results showed that the La y Fe x Co 4− x Sb 12 single phase skutterudite compounds could be obtained by a combination of a 10 h mechanical alloying and a subsequent hot pressing at 650 °C for 2 h. Electrical and thermal conductivities of La filled skutterudite La y FeCo 3 Sb 12 decreased with increasing La filling fraction. The Seebeck coefficient of La y FeCo 3 Sb 12 became negative when y is 0.9. The figure of merit Z increased with La filling when the La content was not too high. The value Z of La 0.6 FeCo 3 Sb 12 is larger than the other filled skutterudite compounds.

Starting from elemental Co (99.9%), Ni (99.9%) and Sb (99.99%) powders, Co4-xNixSb12 powder mixture with different Ni concentration were subjected to mechanical alloying by using a planetary ball mill. Phase transformation during mechanical alloying process of Co-Ni-Sb system was studied in this paper. Ni substituted skutterudite could be synthesized by solid state reaction. With assistance of hot pressing for two hours, the as mechanically alloyed low Ni concentration powders could be completely transformed into skutterudite compound. The lattice parameter of Ni substituted skutterudite compound complies well with Vegard law when x is not more than 0.2. When Ni concentration is larger than the maximum solubility, NiSb2 compound forms and single phase skutterudite compound can not be obtained.

Elemental powder mixtures of the composition Co4−xFexSb12 with x=0.2, 0.5, 1.0, 1.5, 2.0 were mechanically alloyed by means of high energy ball milling and subsequent isothermal annealing. The single phase skutterudite structure was obtained when x≤0.5, while Sb and FeSb2-type impurities were detected when x>0.5. The lattice parameters of the skutterudite structure increased linearly with Fe concentration, and the maximum solubility of Fe in the unfilled skutterudite structure Co4−xFexSb12 was found to be nearly x=0.68.

In the present work, elemental powder mixtures with an atomic ratio of Co:Sb=1:3 were mechanically alloyed by high-energy ball milling in a planetary ball mill. It is found that the CoSb3 compound (e-phase) was synthesized after milling for 10 h, and its concentration increased with prolonged milling time. The CoSb2 compound (δ-phase) was observed when milling for longer than 20 h, but single phase CoSb3 skutterudite could not be obtained even on further milling for 50 h. It is shown that single phase CoSb3 could be synthesized easily by annealing the as-MAed powders in vacuum at 700 °C, and the annealing time could be as short as 1 h for the powders milled for over 10 h.

We report the fabrication and characterization of CdS/TiO(2) nanotube-array coaxial heterogeneous structures. Such structures may potentially be applied in various photocatalytic fields, such as water photocatalytic decomposition and toxic pollutant photocatalytic degradation. Thin films of CdS are conformally deposited onto TiO(2) nanotubes using a modified method of electrochemical atomic layer deposition. We propose that such nanostructured electrodes can overcome the poor absorption and high charge-carrier recombination observed with nanoparticulate films. The practical electrochemical deposition technique promotes the deposition of CdS onto the TiO(2) tube walls while minimizing deposition at the tube entrances, thus preventing pore clogging. The coaxial heterogeneous structure prepared by the new electrochemical process significantly enhances CdS/TiO(2) and CdS/electrolyte contact areas and reduces the distance that holes and electrons must travel to reach the electrolyte or underlying conducting substrate. This results in enhanced photon absorption and photocurrent generation. The detailed synthesis process and the surface morphology, structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdS/TiO(2) nanotube-array coaxial heterogeneous structure are discussed. In comparison with a pure TiO(2) nanotube array, a 5-fold enhancement in photoactivity was observed using the coaxial heterogeneous structure. This methodology may be useful in designing multijunction semiconductor materials for coating of highly structured substrates.

As a promising class of thermoelectric materials, skutterudites are featured by the naturally formed oversize cages in its crystal lattice. Cage-filling by guest atoms has thus become an important approach to reducing the lattice thermal conductivity and optimizing the thermoelectric performance. To probe the impact of filler atoms on lattice dynamics, we herein reported specific heatmeasurements in two single-filled skutterudite samples In0.2Co4Sb12 and Yb0.2Co4Sb12, and a double-filled skutterudite sample In0.2Yb0.2Co4Sb12. The low temperature specific heat data was analyzed in the context of combined electronic specific heat (the Sommerfeld term), the Debye mode (long wavelength acoustic phonon modes), and the Einstein modes (localized vibration modes). We found that the specific heat difference between the single and double-filled samples can be well accounted by one extra Einstein mode, as expected from the extra filler atom and confirmed by the results of inelastic neutron scatteringmeasurements. Interestingly, two Einstein modes, in addition to the Sommerfeld term and the Debye term, are needed to satisfactorily account for the specific heat of the single-filled sample. The Einstein mode with lower frequency has the frequency close to the low-lying mode reported in La, Ce, Tl single-filled skutterudites, this mode is largely unaffected when the second type of filler atoms is introduced. The frequency of this mode has been verified by inelastic neutron scatteringmeasurement. The other Einstein mode with higher frequency may be originated from the motion of Sb atoms.

Void filling in the CoSb3 skutterudite lattice with different kinds of heavy elements has proved effective in enhancing their thermoelectric performance. In this paper we report the effects of (In,Yb) double filling on the high temperature thermoelectric properties of CoSb3. The InxYbyCo4Sb12 (0≤(x,y)≤0.2) samples have been prepared via a melting-annealing-sintering procedure and characterized by means of x-ray powder diffraction, scanning electron microscopy, Hall coefficient, electrical and thermal conductivities, and Seebeck coefficient measurements. As compared to the In and Yb single-filling approach, the (In,Yb) double-filling approach can effectively reduce the lattice thermal conductivity without degrading the power factor. As a result, several compositions achieve ZT values around unity, while a maximum ZT value of 1.1 is obtained in In0.2Yb0.1Co4Sb12 at 700 K. The difference in the effects of In and Yb filling on the thermoelectric properties is discussed.

Indium-filled CoSb3 skutterudites have been shown previously to have promising thermoelectric properties but the thermal conductivity still remains somewhat high. In order to further decrease the thermal conductivity, the double-filling approach has been adopted using ytterbium, in conjunction with indium, due to its heavy mass and small size. The In0.1YbyCo4Sb12 (y=0.00, 0.05, 0.10, and 0.20) samples have been prepared by a melting method and subsequently characterized by means of electron microscopy, electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall coefficient measurements. The results show that the ytterbium filling effectively decreases the thermal conductivity without degrading the power factor, resulting in an enhancement of the dimensionless figure of merit ZT. A state-of-the-art ZT value of 0.97 is attained in In0.1Yb0.1Co4Sb12 at 750 K.