Skutterudites CoSb3 with multiple cofillers Ba, La, and Yb were synthesized and very high thermoelectric figure of merit ZT = 1.7 at 850 K was realized. X-ray diffraction of the densified multiple-filled bulk samples reveals all samples are phase pure. High-resolution scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) analysis confirm that multiple guest fillers occupy the nanoscale-cages in the skutterudites. The fillers are further shown to be uniformly distributed and the Co−Sb skutterudite framework is virtually unperturbed from atomic scale to a few micrometers. Our results firmly show that high power factors can be realized by adjusting the total filling fraction of fillers with different charge states to reach the optimum carrier density, at the same time, lattice thermal conductivity can also be significantly reduced, to values near the glass limit of these materials, through combining filler species of different rattling frequencies to achieve broad-fr...

Multiple-filled skutterudites: high thermoelectric figure of merit through separately optimizing electrical and thermal transports.

The existing mechanisms proposed to explain the phosphorescence of SrAl2O4:Eu2+,Dy3+ and related phosphors were found to be inconsistent with a number of important experimental and theoretical observations. We formulated a new mechanism of phosphorescence on the basis of the facts that the d orbitals of Eu2+ are located near the conduction band bottom of SrAl2O4, that the Eu2+ concentration decreases during UV excitation, and that trace amounts of Eu3+ are always present in these phosphors. In our mechanism, some Eu2+ ions are oxidized to Eu3+ under UV, and the released electrons are trapped at the oxygen vacancy levels located in the vicinity of the photogenerated Eu3+ cations. The phosphorescence arises from the recombination of these trapped electrons around the photogenerated Eu3+ sites with emission at 520 nm. The codopant Dy3+ enhances the phosphorescence by increasing the number and the depth of electron traps, and the codopant B3+ enhances the phosphorescence by increasing the depth of electron tr...

Mechanism of Phosphorescence Appropriate for the Long-Lasting Phosphors Eu2+-Doped SrAl2O4 with Codopants Dy3+ and B3+

We present evidence of a relatively high dimensionless figure of merit (ZT) in a polycrystalline skutterudite partially filled with ytterbium ions. The small-diameter yet heavy-mass Yb atoms partially filling the voids of the host CoSb3 system exhibit low values of thermal conductivity while the quite favorable electronic properties are not substantially perturbed by the addition of Yb. This combination is ideal for thermoelectric applications exemplifying the “phonon-glass electron-crystal” concept of a thermoelectric material, resulting in ZT=0.3 at room temperature and ZT∼1 at 600 K for Yb0.19Co4Sb12.

High figure of merit in partially filled ytterbium skutterudite materials

During the past few decades, the research on persistent luminescent materials has focused mainly on Eu2+-doped compounds. However, the yearly number of publications on non-Eu2+-based materials has also increased steadily. By now, the number of known persistent phosphors has increased to over 200, of which over 80% are not based on Eu2+, but rather, on intrinsic host defects, transition metals (manganese, chromium, copper, etc.) or trivalent rare earths (cerium, terbium, dysprosium, etc.). In this review, we present an overview of these non-Eu2+-based persistent luminescent materials and their afterglow properties. We also take a closer look at some remaining challenges, such as the excitability with visible light and the possibility of energy transfer between multiple luminescent centers. Finally, we summarize the necessary elements for a complete description of a persistent luminescent material, in order to allow a more objective comparison of these phosphors.

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Persistent Luminescence in Non-Eu2+-Doped Compounds: A Review.

Barium-filled skutterudites BayCo4Sb12 with an anomalously large filling fraction of up to y=0.44 have been synthesized. The lattice parameters increase linearly with Ba content. Magnetic susceptibility data show that Ba0.44Co4Sb12 is paramagnetic, which implies that some of the Co atoms in BayCo4Sb12 have acquired a magnetic moment. The presence of the two different valence states of Co (Co3+ and Co2+) leads to the anomalously large barium filling fraction even without extra charge compensation. All samples show n-type conduction. The electrical conductivity increases with increasing the Ba filling fraction. The lattice thermal conductivity of BayCo4Sb12 is significantly depressed as compared to unfilled Co4Sb12. The dimensionless thermoelectric figure of merit, ZT, increases with increasing temperature reaching a maximum value of 1.1 for Ba0.24Co4Sb12 at 850 K.

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Anomalous barium filling fraction and n-type thermoelectric performance of BayCo4Sb12

Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.

Thermoelectric properties of Sn-filled skutterudites

Atom insertion into the CoSb3 skutterudite host lattice under high pressure

Synthesis and long-period phosphorescence of ZnGa2O4:Mn2+ spinel

Mn3Sb: a new L12-type intermetallic compound synthesized under high-pressure

r-BS was synthesized under high pressure/temperature conditions. Rietveld analysis of the powder X-ray diffraction data was performed by applying the structure model of GaS 3R (space group: R3m) with the lattice parameters of ahex = 3.05223(7) A and chex = 20.4119(5) A The crystal structure of r-BS was revealed to be the three-layer structure comprised of the units built up from B–B pairs which are inside the anti-prism of S atoms. The measurement of UV-Vis diffused reflectance spectrum showed that the estimated band gap of r-BS was about 3.4 eV, and the coloring of r-BS was observed as broad absorption at 510 nm.

Hirotsugu Takizawa

Papers

Thermoelectric properties of Sn-filled skutterudites

Atom insertion into the CoSb3 skutterudite host lattice under high pressure

Synthesis and long-period phosphorescence of ZnGa2O4:Mn2+ spinel

Mn3Sb: a new L12-type intermetallic compound synthesized under high-pressure

High Pressure Synthesis of Binary B–S Compounds