135 research outputs found
Thermoelectric properties of high-entropy rare-earth cobaltates
High-entropy concept introduced with a promising paradigm to obtain exotic
physical properties has motivated us to explore the thermoelectric properties
of Sr-substituted high-entropy rare-earth cobaltates i.e.,
(LaNdPrSmEu)SrCoO3 (0 \leq x \leq 0.10). The structural analysis of
the samples synthesized using the standard solid-state route, confirms the
orthorhombic structure with the Pbnm space group. The Seebeck coefficient and
electrical resistivity decrease with rising Sr concentration as well as with an
increase in temperature. The multiple A-site ions in high-entropy rare-earth
cobaltates result in an improved Seebeck coefficient ({\alpha}) compared to
LaSrCoO, associated with a decrease in the Co-O-Co bond
angle, which further enhances the power factor. The random distribution of
cations at the rare-earth site results in a significant lowering of phonon
thermal conductivity. As a result, a maximum figure of merit (zT) of 0.23 is
obtained at 350K for (LaNdPrSmEu)SrCoO, which is one of
the highest values of zT reported at this temperature for oxide materials. This
study shows promise to decouple thermoelectric parameters using the
high-entropy concept in several materials.Comment: 5 pages, 3 Figure
Novel entropy-stabilized fluorite oxides with multifunctional properties
Development of new high-entropy oxides having configurational entropy
dominating the phase stability has become a hot topic since the discovery of
rock salt structure entropy-stabilized (ES)(MgCoNiCuZn)O in 2015. Herein, we
report a set of novel entropy-stabilized fluorite oxides:
Zr0.2Hf0.2Ce0.2Sn0.2Mn0.2O2-{\delta}, Zr0.2Hf0.2Ti0.2Mn0.2Ce0.2O2-{\delta},
Zr0.225Hf0.225Ti0.225Mn0.225Ce0.1O2-{\delta}, and
Zr0.2Hf0.2Ti0.2Mn0.2Ce0.1Ta0.05Fe0.05O2-{\delta} synthesized using standard
solid-state reaction. These compounds have been investigated using X-ray
diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy
techniques to discern their structural, microstructural, and chemical
properties. The configurational-entropy dominated phase stability and hence the
entropy stabilization of the compounds is confirmed by cyclic heat treatments.
The mismatch in the ionic radii and oxidation state of the cations are the key
factors in achieving a single-phase fluorite structure. Further, screening of
physical properties including thermal conductivity, optical band gap, magnetic
properties, and impedance spectroscopy is discussed. Thermal conductivity of
1.4-1.7 Wm-1K-1 is observed at 300 K and remains mostly invariant across a wide
temperature range (300K-1073K), favorable for thermal barrier coating
applications. These ES samples have an optical band gap of 1.6-1.8 eV, enabling
light absorption across the visible spectrum and hence could be promising for
photocatalytic applications. The impedance spectroscopy data of the
entropy-stabilized samples reveal the presence of electronic contributions with
small activation energy (0.3-0.4 eV) across a temperature range of 298K-423K.
These observations in ES fluorite systems show potential for their
multifunctional applications via further optimization and confirm the great
chemical versatility of entropy-stabilized oxides.Comment: 13 Pages, 9 Figures 1 Tabl
Thermoelectric transport properties of an apparent Fermi liquid: Relation to an analytic anomaly in the density of states and application to hole-doped delafossites
Through the motivation of the recent discovery of dispersionless regions in
the band structure of the delafossites, a model density of states of free
fermions including a discontinuity as analytical anomaly is studied. The
resulting temperature dependence of the chemical potential is obtained both
exactly and by different approximation schemes which are then discussed
thoroughly. This includes the introduction of an approximation of the
polylogarithm difference which is capable of accessing a parameter range
neither covered by Sommerfeld expansion nor by Boltzmann approximation. It is
found that the Fermi temperature and several other temperature scales may be
very low, giving rise to experimentally observable behaviours differing from
the one described by Fermi liquid theory. In particular, two kinds of apparent
Fermi liquid behaviour emerge at intermediate temperatures. This behaviour is
related to recent transport data reported for CuCr(1-x)MgxO2 [A. Maignan et.
al, Solid State Commun. 149, 962 (2009)] and CuRh(1-x)MgxO2 [A. Maignan et. al,
Phys. Rev. B 80, 115103 (2009)] by means of the temperature independent
correlation functions ratio approximation. In this way an effective density of
states as well as the effective charge carrier density of these materials are
determined. Furthermore, conclusions about the specific heat of the latter
material are drawn which presents particular effects of the analytical anomaly.Comment: 14 pages, 8 figure
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Enhancing the thermoelectric properties of single and double filled p-type skutterudites synthesized by an up-scaled ball-milling process
The single and double filled p-type skutterudites Ce0.8Fe3CoSb12 and Ce0.5Yb0.5Fe3.25Co0.75Sb12 have been prepared by mechanical alloying. This offers a rapid
method for the preparation of skutterudites that could be scaled up for adoption at industrial level. The large-scale samples prepared by ball-milling exhibit enhanced figures of merit ZT, compared with materials prepared by conventional solid-state reaction. At room temperature
ZT is increased by ca. 19 % for both single and double filled skutterudites. Maximum figures of merit, ZT = 0.68 and ZT = 0.93 are attained for Ce0.8Fe3CoSb12 at 773 K and Ce0.5Yb0.5Fe3.25Co0.75Sb12 at 823 K respectively. The improvement in thermoelectric values at room temperature may be traced to a reduction in thermal conductivity in the ball-milled samples arising from the reduced grain size. The influence of the microstructure on the thermoelectric properties, together with the stability in air and the performance of the materials after several
heating and cooling cycles has been studied and are detailed in this work. The densified samples prepared by ball-milling also show a higher resistance to oxidation, which starts at 694 K for Ce0.8Fe3CoSb12 and at 783 K for Ce0.5Yb0.5Fe3.25Co0.75Sb12
Multi elements substituted Aurivillius phase relaxor ferroelectrics using high entropy design concept
The concept of high entropy ceramics opens up the possibility to optimise the properties of relaxor ferroelectrics with Aurivilius phase structures. In this paper, two new multi-element substituted Aurivillius phase ceramics, (Ca0.25Sr0.25Ba0.25Pb0.25)Bi2Nb2O9 and (Ca0.2Sr0.2Ba0.2Pb0.2Nd0.1Na0.1)Bi2Nb2O9 are prepared. Both ceramics are single phase, exhibiting orthorhombic symmetry with space group A21am at room temperature. The frequency dependence of the temperature of the dielectric permittivity peak, Tm, shows relaxor behaviour for both ceramics. Multi-domain configurations, including long range ordered ferroelectric domains and nano domains, were observed using piezoresponse force microscopy (PFM). Under an applied DC field, the nano domains irreversibly transformed to micro-meter sized domains, which is consistent with the Vogel-Fulcher Law fitted results that show that the freezing temperatures of the two ceramics are above room temperature. Their improved piezoelectric constant is attributed to their low coercive fields, which are related to the multi domain configurations in the Aurivillius phase relaxor ferroelectrics
Etude de skutterudites de terres-rares (R) et de métaux d (M) du type RM4Sb12 : de nouveaux matériaux thermoélectriques pour la génération d'électricité
This thesis presents the physical and thermoelectric properties of new skutterudite compounds, with general formula Ry-pR'pFe4-x(Co/Ni)xSb12 (R et R' = Ce, Yb, Ba, La). It shows that Yb valence state decreases when Yb fraction increases. However, it does not depend on the temperature, whiwh evidences a mixed valence state (two different valence states for two distinct cristallographic sites at a local point of view). Ce is always trivalent in these compounds. The character of the dominant interactions changes from antiferromagnetic to ferromagnetic when one goes from trivalent cerium or lanthanum to nearly divalent ytterbium or divalent baryum. The effective paramagnetic moment has been separated into three contributions originating from Ce, Yb and [Fe4Sb12]. The last one does not depend on the nature or on the valence state or R and R' atoms, but it decreases when Ni substitues Fe. A magnetic transition occurs at 6.5 K for R = Yb and Ba, possibly to a spin glass state.Electrical and thermal transport properties have been determined from 5K to 800K, and the materials properties of the materials have been discussed concerning thermoelectric power generation. The power factor, and the thermoelectric figure of merit ZT are increased in Cey/2Yby/2Fe4-x(Co/Ni)xSb12 type skutterudites as compared to RyFe4-x(Co/Ni)xSb12 type skutterudites.Cette étude présente les propriétés de nouveaux composés de la famille des skutterudites, de formule générale Ry-pR'pFe4-x(Co/Ni)xSb12 (R et R' = Ce, Yb, Ba, La), d'une part du point de vue des propriétés physiques, et d'autre part du point de vue de leurs potentialités pour la conversion de chaleur en électricité par effet thermoélectrique. La valence de Yb est non entière et elle décroît lorsque la fraction de Yb augmente. En revanche, elle ne dépend pas de la température ce qui met en évidence un état de valence mixte (deux sites pour deux états de valence). Le cérium est quant à lui toujours trivalent dans ces composés. Le système passe d'un régime d'interactions dominantes ferromagnétiques pour Yb0,9Fe4Sb12 avec Yb presque divalent à un régime d'interactions antiferromagnétiques pour Ce0,9Fe4Sb12 avec Ce trivalent. Le moment paramagnétique a été décomposé suivant les sous-systèmes R, R' et Fe4Sb12. La contribution de ce dernier ne dépend pas la nature et de la valence de R et R', mais elle décroît si Fe est substitué par Ni. Une transition magnétique est présente pour R = Yb et Ba à 6,5 K sans doute vers un état verre de spin. Les propriétés de transport électrique et thermique ont été mesurées de 5K à 800K ce qui a permis de discuter des potentialités de ces matériaux pour la génération d'électricité par effet thermoélectrique. Le facteur de puissance et le facteur de mérite ZT sont plus élevés dans les composés de type Cey/2Yby/2Fe4-x(Co/Ni)xSb12 que dans ceux de type RyFe4-x(Co/Ni)xSb12 présentés dans la littérature
Influence of Te substitution on the structural and electronic properties of thermoelectric BiCuSeO
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