Fast preparation route to high-performances textured Sr-doped Ca 3 Co 4 O 9 thermoelectric materials through precursor powder modification

This work presents a short and very efficientmethod to produce high performance textured Ca3Co4O9thermoelectric materials through initial powders modifica-tion. Microstructure has shown good grain orientation, andlow porosity while slightly lower grain sizes were obtained insamples prepared from attrition milled powders. All samplesshow the high density of around 96% of the theoretical value.These similar characteristics are reflected in, approximately,the same electrical resistivity and Seebeck coefficient valuesfor both types of samples. However, in spite of similar powerfactor (PF) at low temperatures, it is slightly higher at hightemperature for the attrition milled samples. On the otherhand, the processing time reduction (from 38 to 2 h) whenusing attrition milled precursors, leads to lower mechanicalproperties in these samples. All these data clearly point out tothe similar characteristics of both kinds of samples, with adrastic processing time decrease when using attrition milledprecursors, which is of the main economic importance whenconsidering their industrial production

Microstructures and Thermoelectric Properties of Sintered Misfit-Layered Cobalt Oxide

Misfit-layered cobalt oxide Ca3Co4O9 is considered to be a prospective material for thermoelectric conversion. The thermoelectric properties are anisotropic owing to its anisotropic crystal structure. The crystal has preferred thermoelectric properties along the a-b plane. Therefore, the thermoelectric properties are improved and controlled by the degree of orientation of the sintered sample. In the present work, Sr-doped misfit cobalt oxide Ca2.7Sr0.3Co4O9 was prepared by solid-phase reaction, followed by uniaxial compression molding and sintering at 1173 K. The Seebeck coefficient α, electrical resistivity ρ, and dimensionless figure of merit ZT were measured as a function of the compression pressure applied in the uniaxial molding. α, ρ, and ZT as functions of the degree of orientation and the relative density are experimentally clarified and explained by calculations using the compound model

A seven-planet resonant chain in TRAPPIST-1

The TRAPPIST-1 system is the first transiting planet system found orbiting an ultra-cool dwarf star1. At least seven planets similar to Earth in radius were previously found to transit this host star2. Subsequently, TRAPPIST-1 was observed as part of the K2 mission and, with these new data, we report the measurement of an 18.77 d orbital period for the outermost transiting planet, TRAPPIST-1h, which was unconstrained until now. This value matches our theoretical expectations based on Laplace relations3 and places TRAPPIST-1h as the seventh member of a complex chain, with three-body resonances linking every member. We find that TRAPPIST-1h has a radius of 0.727 R⊕ and an equilibrium temperature of 169 K. We have also measured the rotational period of the star at 3.3 d and detected a number of flares consistent with a low-activity, middle-aged, late M dwarf

Populations of planets in multiple star systems

Astronomers have discovered that both planets and binaries are abundant throughout the Galaxy. In combination, we know of over 100 planets in binary and higher-order multi-star systems, in both circumbinary and circumstellar configurations. In this chapter we review these findings and some of their implications for the formation of both stars and planets. Most of the planets found have been circumstellar, where there is seemingly a ruinous influence of the second star if sufficiently close (<50 AU). Hosts of hot Jupiters have been a particularly popular target for binary star studies, showing an enhanced rate of stellar multiplicity for moderately wide binaries (>100 AU). This was thought to be a sign of Kozai-Lidov migration, however recent studies have shown this mechanism to be too inefficient to account for the majority of hot Jupiters. A couple of dozen circumbinary planets have been proposed around both main sequence and evolved binaries. Around main sequence binaries there are preliminary indications that the frequency of gas giants is as high as those around single stars. There is however a conspicuous absence of circumbinary planets around the tightest main sequence binaries with periods of just a few days, suggesting a unique, more disruptive formation history of such close stellar pairs.Comment: Invited review chapter, accepted for publication in "Handbook of Exoplanets", ed. H. Deeg & J. A. Belmont

Dynamics of Disks and Warps

This chapter reviews theoretical work on the stellar dynamics of galaxy disks. All the known collective global instabilities are identified, and their mechanisms described in terms of local wave mechanics. A detailed discussion of warps and other bending waves is also given. The structure of bars in galaxies, and their effect on galaxy evolution, is now reasonably well understood, but there is still no convincing explanation for their origin and frequency. Spiral patterns have long presented a special challenge, and ideas and recent developments are reviewed. Other topics include scattering of disk stars and the survival of thin disks.Comment: Chapter accepted to appear in Planets, Stars and Stellar Systems, vol 5, ed G. Gilmore. 32 pages, 17 figures. Includes minor corrections made in proofs. Uses emulateapj.st

Relation between Seebeck Coefficient and Lattice Parameters of (Ca2-ySryCoO3)(x)CoO2

The effect of dopant content on the Seebeck coefficient of the Sr2+-doped (Ca2- Sr CoO3) CoO2 ( = 0 to 0.4) system was investigated. This system can be described as a misfit layered structure of (CoO2) and (Ca2- Sr CoO3) layers, which have different lattice parameters denoted by (1) and (2), respectively. Due to the solubility limit of Sr in (Ca2CoO3) CoO2, systematic investigations were available only up to = 0.2. Nevertheless, the structural uniqueness enabled partially quantitative analysis of the correlation between the crystal structure and Seebeck coefficient of the system. Substitution of Sr for Ca leads to lattice expansion which accompanies an anisotropic change of the lattice parameters. Among the lattice parameters considered, the increase in the lattice parameter (2) of the insulating layer was larger than the change of any other lattice parameter, which induced in-plane stress in the conducting layer. As a result, as the (1)/ (2) misfit ratio of (Ca2- Sr CoO3) CoO2 is decreased, the Seebeck coefficient also decreases. Practical guidance for selecting dopants to enhance thermoelectric performance is proposed.close5