Mei-Bo Tang

Synthesis and high thermoelectric efficiency of Zintl phase YbCd2-xZnxSb2

We synthesized a series of polycrystalline YbCd2−xZnxSb2 (x=0, 0.4, 0.8, 1, 1.2, 1.6, and 2) samples and measured their thermoelectric properties. Thermoelectric figure of merit ZT at 700K is higher than 1.0 for Cd-rich samples (x=0, 0.4, 0.8, and 1.0) and Zn substitute of Cd in YbCd2Sb2 can easily tune carrier concentration and reduce thermal conductivity. When x=0.4, sample exhibits the highest power factor (12–20μWcm−1K−2), the lowest lattice thermal conductivity (1.0Wm−1K−1 at 300K), highest ZT (1.2 at 700K), and best “self-compatibility.” The first principles calculations were performed to study the influences of bonding and electronic structures on physical properties.

A new type of thermoelectric material, EuZn2Sb2.

Polycrystalline EuZn(2)Sb(2) is prepared by direct reaction of the elements. Its composition, structure, magnetism, heat capacity, and thermoelectric properties have been investigated. EuZn(2)Sb(2) crystallizes in p3m space group with a=4.4932(7) A and c=7.6170(10) A. Antiferromagnetic ordering is detected at the Neel temperature of 13.06 K, and the saturation magnetization reaches 6.87mu(B)Eu at 2 K and 7 T. Eu ion has +2 valence. Its Hall effects are characterized by a high positive Hall coefficient of +0.226 cm(3)C, proper carrier concentration of 2.77x10(19)cm(3), and high carrier mobility of 257 cm(2)V s at 300 K. This compound shows high p-type Seebeck coefficient (+122 to +181 muVK), low lattice thermal conductivity (1.60-0.40 Wm K), and high electrical conductivity (1137-524 Scm). The obtained figure of merit and powder factor reach 0.92 and 20.72 muWcm K(2), respectively. The thermoelectric properties of EuZn(2)Sb(2) are encouraging.

Thermoelectric properties and electronic structure of Zintl compound BaZn2Sb2

Polycrystalline sample of the title compound was prepared and its thermoelectric properties from 2to675K were investigated. This Zintl compound shows rather low thermal conductivity, 1.6Wm−1K−1, at room temperature. The value of its thermoelectric figure of merit ZT reaches 0.31 at 675K. Its electronic structure, calculated by ab initio methods, suggests that the electrical transport are mainly ascribe to [Zn2Sb2] framework for p-type BaZn2Sb2. The heat capacity curve at low temperature was fitted lineally to obtain Debye temperature (about 208K). It provides the authors with a host lattice for modification and optimization the thermoelectric properties through substitution and/or doping.

Zintl phase Yb1−xCaxCd2Sb2 with tunable thermoelectric properties induced by cation substitution

It has been shown previously that the thermoelectric properties of the Zintl phase compound YbCd2Sb2 can be finely tuned via Zn substitution at the Cd-site in the anionic (Cd2Sb2)2− framework. Here we report the results of the investigation of isoelectronic substitution of Yb by Ca. The p-type Yb1−xCaxCd2Sb2 (0.2≤x≤0.8) samples have been synthesized via a solid-state reaction followed by suitable cooling, annealing, grinding, and spark plasma sintering densification processes. In samples with x=0.2, 0.4, 0.5, 0.6, 0.8, the electrical conductivity, Seebeck coefficient, and thermal conductivity measurements have been carried out in the temperature range from 300 to 650 K. It is found that the Ca substitution effectively lowers the thermal conductivity for all samples at high temperature, while it significantly increases the Seebeck coefficient. As a result, the dimensionless figure of merit ZT of 0.96 has been attained at 650 K for samples with x=0.4, while the value is 0.78 for the unsubstituted YbCd2Sb2

Large magnetic entropy change and adiabatic temperature rise of a Gd55Al20Co20Ni5 bulk metallic glass

Gd{sub 55}Al{sub 20}Co{sub 20}Ni{sub 5} bulk metallic glass (BMG) was synthesized by minor Ni substitution for Co in the Gd{sub 55}Al{sub 20}Co{sub 25} BMG in which excellent glass forming ability (GFA) and magneto-caloric effect were reported previously. The Gd{sub 55}Al{sub 20}Ni{sub 20}Co{sub 5} amorphous rod has a similar GFA to the Gd{sub 55}Al{sub 20}Co{sub 25} BMG but exhibits better magnetic properties. The peak value of magnetic entropy change (−ΔS{sub m}{sup peak}) of the Gd{sub 55}Al{sub 20}Co{sub 20}Ni{sub 5} BMG is 9.8 Jkg{sup −1} K{sup −1}. The field dependence of −ΔS{sub m}{sup peak} follows a −ΔS{sub m}{sup peak}∝H{sup 0.85} relationship. The adiabatic temperature rise of the rod is 4.74 K under 5 T and is larger than of other BMGs previously reported. The improved magnetic properties were supposed to be induced by the enhanced interaction between 4f electron in the rare-earth and 3d electron in the transition metal elements by means of a minor Ni substitution for Co.

Thermoelectric properties of YbxEu1−xCd2Sb2

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.

Thermoelectric properties of Eu(Zn(1-x)Cd(x))2Sb2.

The thermoelectric performance of EuZn(2)Sb(2) and EuCd(2)Sb(2) was optimized by mixed occupation of the transition metal position. Samples in the solid solution Eu(Zn(1-x)Cd(x))(2)Sb(2) with the CaAl(2)Si(2)-type crystal structure (space group Pm1) were prepared from the elements for compositions with x = 0, 0.1, 0.3, 0.5 and 1. The thermoelectric properties were investigated after densification of the products by spark plasma sintering (SPS). The samples show low electrical resistivity, high thermopower and a low lattice thermoconductivity. The highest ZT value of 1.06 at 650 K is obtained for x = 0.1.

Valence fluctuation and electron-phonon coupling in La68-xCexAl10Cu20Co2 (x=0, 34, and 68) metallic glasses

We study the anomalous acoustic softening behavior in La68−xCexAl10Cu20Co2 (x=0, 34, and 68) metallic glasses by magnetic susceptibility, thermal expansion, and specific heat at low temperatures. The Ce-based metallic glass exhibits valence fluctuation phenomenon, anomalous thermal expansion and acoustic Gruneisen parameter at low temperatures. And we provide experimental evidence of the existence of strong electron–phonon coupling in the Ce-based metallic glasses by specific heat. The anomalous acoustic and thermal properties are attributed to the valence instability in the metallic glasses.

Transport properties of Zintl phase Yb1–xCaxCd2Sb2 at low temperature

Abstract We investigated the transport properties of isoelectronic substitution of Yb by Ca for Zintl phase YbCd2Sb2 below 300 K. The p-type Yb1–xCaxCd2Sb2 (0.2≤x≤0.8) samples were synthesized via a solid-state reaction followed by suitable cooling, annealing, grinding, and spark plasma sintering (SPS) densification processes. For samples with x=0.2, 0.4, 0.5, 0.6, 0.8, the electrical conductivity, Seebeck coefficient, thermal conductivity, heat capacity and Hall effect measurements were carried out in the temperature range from 1.9 to 300 K. It was found that the Ca substitution effectively lowered the thermal conductivity at low temperature, while the electrical resistivity and the Seebeck coefficient in the compounds were found to increase with increasing Ca concentration. As a result, the dimensionless figure of merit ZT of 0.18 was attained at 300 K for sample with x=0.2.

Investigation of upconversion luminescence in Er 3+ /Yb 3+ co-doped Nb 2 O 5 -based glasses prepared by aerodynamic levitation method

The aerodynamic levitation method has been employed to fabricate Er3+/Yb3+ co-doped Nb2O5-La2O3-Ta2O5 glasses. The maximum phonon energy is about 735 cm−1 and the phonon density decreases with the increase of Yb3+. Small phonon density is favorable for upconversion luminescence. The absorption intensity at ~976 nm is improved by Yb3+. The glasses show high thermal stability and glass forming ability is enhanced by Yb3+. Green, red, and near infrared emissions are obtained and upconversion luminescence increases with Yb3+, which agrees well with the analysis of Raman and absorption spectra. As Yb3+ increases, the lifetime is enhanced. The results of lifetime measurement confirm the tendency of upconversion luminescence with Yb3+ ions.