R. T. Littleton

High figure of merit in partially filled ytterbium skutterudite materials

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.

Effect of Sb doping on the thermoelectric properties of Ti-based half-Heusler compounds, TiNiSn1−xSbx

Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.

Apparatus for the rapid measurement of electrical transport properties for both “needle-like” and bulk materials

Described in this article is an apparatus in which thermopower and resistivity can be measured almost simultaneously from 10 to 300 K in a closed cycle refrigerator system. A distinction of this system is that the samples are mounted on integrated circuit (IC) chips for rapid introduction of the sample into the measurement apparatus. One advantage of this system is that the sample is mounted on the IC chip and then all the electrical contacts to the sample can be checked prior to insertion into the sample measurement apparatus. This system is applicable for measurement of either bulk samples (approximately 2×2×8 mm3) or “needle-like” samples (approximately 0.1×0.05×2–3 mm3).

Description of the parallel thermal conductance technique for the measurement of the thermal conductivity of small diameter samples

In order to measure the thermal conductivity of small needlelike samples (2.0×0.05×0.1 mm3) such as pentatellurides and single carbon fibers, we have developed a new technique called the parallel thermal conductance (PTC) technique. In the more typical steady state method that is used for measuring thermal conductivity, thermocouples are attached to the sample in order to measure the temperature gradient and a heater in order to supply this gradient. However, attaching thermocouples and heaters directly to small samples may be relatively difficult, and cause large heat losses and errors. Thus, the measurement of the thermal conductivity of small samples and thin films has been a formidable challenge, with only few successes, due, among other factors, to the heat loss. It is also difficult for the small samples to support the heaters and thermocouples without causing damage to the sample. In this paper we describe the recently developed PTC method providing measurements on standards as well as single carbo...

EFFECT OF TI SUBSTITUTION ON THE THERMOELECTRIC PROPERTIES OF THE PENTATELLURIDE MATERIALS M1-XTIXTE5 (M=HF, ZR)

The thermoelectric properties (resistivity and thermopower) of single crystals of the low dimensional pentatelluride materials, HfTe5  and ZrTe5, have been measured as a function of temperature from 10 K<T<320 K. The effect of small amounts of Ti substitutional doping (M1−xTixTe5, where M=Hf, Zr) on the thermoelectric properties is reported here. A resistive transition occurs in the pentatellurides, as evidenced by a peak in the resistivity, TP≈80 K for HfTe5 and TP≈145 K for ZrTe5. Both parent materials exhibit a large positive (p-type) thermopower near room temperature which undergoes a change to negative (n-type) below the peak temperature. The thermal conductivity is relatively low (≈5 W/m K) for the MTe5 materials. The Ti substitution affects the electronic properties strongly, producing a substantial shift in the peak temperature while the large values of thermopower remain essentially unaffected. These results warrant further investigation of these materials as candidates for low temperature thermo...

Observation of a semimetal–semiconductor phase transition in the intermetallic ZrTe5

Temperature dependent high-resolution angle-resolved photoelectron spectroscopy has been performed on the quasi-two-dimensional compound ZrTe5, a metal at low temperatures ( K) that exhibits a maximum resistivity at a temperature (Tc), concomitant with a sign change of the thermopower. A semiconducting gap has been observed in the photoemission spectra, where the valence band maximum shifts upward from 82 meV (75 K) to 40 meV (170 K) as a function of temperature. The band shifts are accompanied by small band distortions. Based on the photoemission experiments, in conjunction with the metallic character of ZrTe5 at low temperatures, we have modelled the thermopower of ZrTe5 by treating it as a metal at low temperatures and a semiconductor at elevated temperatures.

Investigation of the thermal conductivity of the mixed pentatellurides Hf1−xZrxTe5

Transition-metal pentatellurides (HfTe5 and ZrTe5) exhibit a promising power factor (electronic properties) for possible use as a thermoelectric material. For complete characterization of these crystals, thermal conductivity measurements are necessary. In this letter, we report measurements of the thermal conductivity for this group of materials using the parallel thermal conductance technique which is well adapted for needle-like samples. Thermal conductivity is presented as a function of temperature and composition of the pentatelluride solid solution HfxZr1−xTe5 with 0⩽x⩽1 in which the magnitude of the room temperature thermal conductivity varies from 5 to 8 W/(m K). Dependence on the cross-sectional area and possible size effects (or sample quality) is also presented and discussed. These results also indicate the importance of sample quality on the low-temperature thermal conductance maximum λmax.

Bandgap features and thermoelectric properties of Ti-based half-Heusler alloys

Electronic transport properties of narrow-gap TiNiSn and presumed wider-gap TiCoSb half-Heusler alloys are investigated by systematically doping the three sublattice sites. The two alloys are found to exhibit different doping trends. While all three sites in TiCoSb can be doped to enhance semimetallic behavior, only the Ti and Ni sites in TiNiSn can be efficiently doped. Meanwhile, several 3d dopants are found to lead to more localized electronic properties. These findings, together with results on Hall effect and thermopower measurements, have shed light on the bandgap structure of these metal-based semiconductors. Power factor and dimensionless figure of merit ZT reaching /spl sim/5.7/spl times/10/sup -3/ W/m-K/sup 2/ and /spl sim/0.5 at 680 K, respectively, are obtained in the Sb-doped (TiHf)NiSn system. The quite favorable thermoelectric parameters obtained in these low-mobility alloys are attributed to the existence of a moderately heavy electron band mass.

Chapter 6 Thermoelectric properties of the transition metal pentatellurides: Potential low-temperature thermoelectric materials

Summary In summary, we have presented an overview of the transport properties of a group of materials called pentatellurides. These materials exhibit a plethora of transport phenomena as outlined and presented in this review. The transport properties of these materials make them interesting and promising for potential low-temperature thermoelectric applications. Obviously we do not have a complete theoretical picture and description of these materials. Much more work, including detailed band structure calculations as a function of temperature, is needed. In addition, very careful structural studies as a function of temperature will be necessary to a proper under-standing of the role of the structure in any forthcoming theoretical description. An extensive amount of experimental data is being produced, and it is our hope that this will inspire more theoretical involvement in these materials. Understanding the origin of the anomaly will be key to the further development of these materials for potential use as a low-temperature thermoelectric material.

Thermoelectric Figure of Merit, ZT, of Single Crystal Pentatellurides (MTe 5-X Se x : M = Hf, Zr and x = 0, 0.25)

The thermoelectric figure of merit, ZT = α 2 σT/λ, has been measured for pentatelluride single crystals of HfTe 5 , ZrTe 5 , as well as Se substituted pentatellurides. The parent materials, HfTe 5 and ZrTe 5 , exhibit relatively large p- and n- type thermopower, |a| > 125 μV/K, and low resistivity, ρ ≤ 1 mΩ•cm. These values lead to a large power factor (α 2 σT) which is substantially increased with proper Se substitution on the Te sites. The thermal conductivity of these needle-like crystals has also been measured as a function of temperature from 10 K ≤ T ≤ 300 K. The room temperature figure of merit for these materials varies from ZT “0.1 for the parent materials to ZT ≈ 0.25 for Se substituted samples. These results as well as experimental procedures will be presented and discussed.