Il Ho Kim

Solid-state syntheses and properties of Zn4Sb3 thermoelectric materials

Abstract Various solid-state synthesis techniques have been used in order to produce high-efficiency e-Zn 4 Sb 3 bulk specimens. The techniques used were: sintering of cold compacts followed by hot pressing; direct synthesis by hot pressing; and sinter-forging. The phase transformations in this alloy system during solid-state syntheses were systematically investigated using DSC, XRD and SEM. Single-phase Zn 4 Sb 3 powders and crack-free bulk specimens having high density were successfully synthesized, but excess Zn was found to be necessary to compensate for Zn loss during processing. The thermoelectric properties at room temperature were evaluated for the hot consolidated compacts and compared with the published results of other studies. The thermoelectric properties of single-phase Zn 4 Sb 3 materials showed reasonable values. Solid-state synthesis utilizing the hot consolidation process offers potential processing routes to produce bulk Zn 4 Sb 3 .

Mechanical alloying and thermoelectric properties of Zn4Sb3

Thermoelectric Zn4Sb3 bulk specimens were produced by mechanical alloying of elemental powders and consolidated by hot pressing. Single phase Zn4Sb3 was not obtained using a nominal stoichiometric composition, but near single phase Zn4Sb3 with remnant elemental Zn having a relatively high density was produced using a nominally 11.7 at.% Zn rich powders. Phase transformations during mechanical alloying were systematically investigated using XRD and SEM. Thermoelectric and transport properties were evaluated for the hot pressed specimens and compared with results of analogous studies.

Synthesis and Thermoelectric Properties of Half-Heusler Zr-Ni-Sn Alloy Processed by Mechanically Alloying and Hot Pressing

Recent search for novel thermoelectric materials has revealed a new class of compounds with half-Heusler structure as good candidates for higher performance thermoelectric conversion [1-2], since the structure was first found [3]. The unit cell of the half-Heusler with a space group F4¯ 3m consists of 4 interpenetrating cubic lattices. The crystallographic sites (0,0,0) and (1/4, 1/4, 1/4) are occupied by two different transition metals, the (1/2, 1/2, 1/2) site is occupied by Sn, Sb, or Bi, and the site (3/4, 3/4, 3/4) is empty [4,5]. Candidates with this structure for the investigation would be categorized into the combination of (Ti/Zr/Hf)(Co/Ni/Pt)(Sb/Sn/Bi) [4-6].

Phase Transformation and Thermoelectric Properties of In0.25Co4-xNixSb12 Skutterudites

In0.25Co4-xNixSb12 skutterudites were synthesized by encapsulated induction melting and consolidated by hot pressing, and their thermoelectric properties were examined at temperatures from 323 to 823 K. A single δ-phase was obtained successfully by subsequent heat treatment at 823 K for 24 h. In0.25Co4-xNixSb12 was an n-type semiconductor at all temperatures examined, indicating that Ni atoms acted as electron donors by substituting for Co atoms. The thermal conductivity was reduced considerably by In filling and Ni doping due to an increase in phonon scattering and impurity scattering. The thermoelectric properties were improved due to the low thermal conductivity as a result of In filling and the optimum carrier concentration caused by Ni doping.

Thermoelectric Properties of Manganese Monosilicide Synthesized by Mechanical Alloying Process

Manganese silicide is one of the potential thermoelectric materials for high temperature application. As a fundamental investigation on these materials group, manganese monosilicides were synthesized by the mechanical alloying of stoichiometric elemental powder compositions, and the as-milled powders were consolidate by vacuum hot pressing. Phase transformation and microstructure evolution during mechanical alloying and hot consolidation were investigated using XRD and SEM. Near single phase of monosilicides were successfully produced in this process. Thermoelectric properties as a function of temperature were evaluated in terms of Seebeek coefficient, electrical resistivity, thermal conductivity and the figure of merit for the hot pressed specimens. Mechanically alloyed manganese monosilicide, MnSi, appeared to have a potential as a thermoelectric material in this study.

Thermoelectric Properties of Solid-State Synthesized Magnesium Silicides Doped with Group BI-III Elements

Group BI(Cu, Ag)-, BII(Zn)- and BIII(Al, In)-doped Mg2Si compounds were synthesized by solid state reaction and mechanical alloying. Electronic transport properties (Hall coefficient, carrier concentration and mobility) and thermoelectric properties (Seebeck coefficient, electrical conductivity, power factor, thermal conductivity and figure of merit) were examined. Mg2Si powder was synthesized successfully by solid state reaction at 773 K for 6 h and doped by mechanical alloying for 24 h. It was fully consolidated by hot pressing at 1073 K for 1 h. The electrical conductivity increased by doping due to an increase in the carrier concentration. However, the thermal conductivity did not changed significantly by doping, which was due to much larger contribution of the lattice thermal conductivity over the electronic thermal conductivity. Group BIII(Al, In) elements were more effective to enhance the thermoelectric properties of Mg2Si.

Phase Transformation and Thermoelectric Properties of Sn-Filled/Fe-Doped CoSb3 Skutterudites

Sn-filled and Fe-doped CoSb3 skutterudites were synthesized by encapsulated induction melting. A single δ-phase was obtained by subsequent annealing, as confirmed by X-ray diffraction. The as-solidified ingot consisted of mixed phases of -CoSb, -CoSb2, δ-CoSb3 and elemental Sb. The phases could be transformed by annealing, and the phases of the as-solidified ingot annealed at 773 K for 24 h transformed to δ-CoSb3. The temperature dependence of the Seebeck coefficient, electrical resistivity and thermal conductivity were examined from 300 K to 700 K. The positive Seebeck coefficient confirmed p-type conduction. The electrical resistivity increased with increasing temperature, which showed that the SnzCo3FeSb12 skutterudite is highly degenerate. The thermal conductivity was reduced by Sn-filling because the filler atoms acted as phonon scattering centers in the skutterudite lattice. The thermoelectric figure of merit was enhanced by Sn filling and its optimum composition was considered to be Sn0.3Co3FeSb12.

Effect of Excess Zn on Thermoelectric Properties of Zn4Sb3

Non-stoichiometric Zn_4+xSb₃ specimens with x = 0.1-0.4 were prepared by vacuum melting at 1173 K and subsequent annealing the solidified ingots at 623 K. The effects of excess Zn on the microstructure and the thermoelectric properties of Zn_4+xSb₃ were investigated. Zn phase was found to be precipitated in Zn_4+xSb₃ with x = 0.1-0.4 due to excess Zn in Zn₄Sb₃ structure. As x increased from x = 0.1 to x = 0.4, both Seebeck coefficient and electrical resistivity decreased, while thermal conductivity increased due to the formation of metallic Zn phase as well as the increase of the carrier concentration. Carrier concentration of Zn_4+xSb₃ increased from 1.37 times 10²⁰cm^-3 to 4.95 times 10²⁰cm^-3 with increasing x from 0.1 to 0.4. Maximum ZT of 1.2 was obtained in Zn_4.1Sb₃ at 600 K with Seebeck coefficient of 174 muV K^-1, electrical resistivity of 2.18 mOmega cm, and thermal conductivity of 7.07 mWcm ^-1K^-1

Effect of Zn addition on thermoelectric properties of Zn/sub 4/Sb/sub 3/ synthesized by direct hot pressing

Powder metallurgy was used to produce polycrystalline specimens of single phase /spl epsi/-Zn/sub 4/Sb/sub 3/ and two-phase specimens of /spl epsi/-Zn/sub 4/Sb/sub 3/ (majority phase) and Zn. The effect of excess Zn addition to the stoichiometry of Zn/sub 4/Sb/sub 3/ was investigated on the thermoelectric properties as well as mechanical properties in the alloy system. The room-temperature thermoelectric properties of our single phase Zn/sub 4/Sb/sub 3/ alloys are comparable to those reported by other researchers on samples prepared by hot pressing of ingot melted alloy powders. It is shown that the addition of Zn enhances the mechanical properties while sacrificing the thermoelectric properties.

Direct synthesis by hot pressing and thermoelectric properties of Zn/sub 4/Sb/sub 3/

Direct synthesis by hot pressing has been used in order to produce high efficiency Zn/sub 4/Sb/sub 3/ bulk specimens. Single phase Zn/sub 4/Sb/sub 3/ with 98.5% of theoretical density were successfully produced by direct hot pressing of elemental powders containing 1.2 at.% excess Zn. Phase transformations in this alloy system during synthesis were investigated using DSC, XRD and SEM. Thermoelectric properties as a function of temperature were investigated from room temperature to 600 K and compared with results of other studies. Transport properties at room temperature were also evaluated. Seebeck coefficient increases and electrical conductivity decreases with increasing temperature up to 600 K. Thermoelectric power factors in directly synthesized Zn/sub 4/Sb/sub 3/ up to 600 K were comparable to published data, leading to an estimated figure of merit value of 1.2 at 600 K. Direct synthesis by hot pressing provides an optional processing route in this material.