J. W. Kolis

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...

Enhancement of the power factor of the transition metal pentatelluride HfTe5 by rare-earth doping

The transition metal pentatellurides HfTe5 and ZrTe5 have been observed to possess interesting electrical transport properties with high thermopower and low resistivity values leading to high thermoelectric power factors. We have investigated the effect of doping HfTe5 with rare-earth elements by measuring the power factor data from about 10K to room temperature on single crystals of Hf1−xRxTe5, where R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, and Ho. Samples that have been doped with Nd (Hf1−xNdxTe5) possess power factors more than a factor of 2 larger than that of the commonly used thermoelectric material Bi2Te3.

Correlated structural and optical characterization of ammonothermally grown bulk GaN

A series of ammonothermally grown bulk GaN crystals containing stacking faults has been characterized using structural [transmission electron microscopy (TEM) and synchrotron white-beam x-ray topography (SWBXT)] and optical [low-temperature photoluminescence (PL)] methods. Strong correlations are found between structural and optical properties. In particular, the occurrence of low-temperature PL peaks observed in the 3.30–3.45 eV range correlates with the observation of basal plane stacking faults by TEM (all of which were bounded by Shockley partial dislocations). In addition, the full width at half-maximum of the neutral donor-bound exciton PL peak correlates with the extent of mosaicity revealed on SWBXT Laue patterns recorded from the same crystals.

New Directions in Bulk Thermoelectric Materials Research: Synthesis of Nanoscale Precursors for “Bulk-Composite” Thermoelectric Materials

Over a decade ago it was predicted that nano-scaled thermoelectric (TE) materials might have superior properties to that of their bulk counterparts. Subsequently, a significant increase in the figure of merit, ZT (ZT > 2), has been reported for nano-scaled systems such as superlattice and quantum dot systems constituently based on those more commonly used bulk TE materials (e.g., Bi 2 Te 3 and PbTe). However, the challenge remains to achieve these higher performance results in bulk materials in order to more rapidly incorporate them into standard TE devices. Recent theoretical work on boundary scattering of phonons in amorphous materials indicates that micron and submicron grains could be very beneficial in order to lower the lattice thermal conductivity and yet not deteriorate the electron mobility. The focus in this paper will be to highlight some of our new directions in bulk thermoelectric materials research. Thermoelectric materials are inherently difficult to characterize and these difficulties are magnified at high temperatures. Specific materials will be discussed, especially those bulk materials that exhibit favorable properties for potential high temperature power generation capabilities. One potentially fruitful research direction is to explore whether hybrid TE materials possess possible enhanced TE properties. These “engineered” hybrids include materials that exhibit sizes from on the order of a few nanometers to hundreds of nanometers of the initial materials. These initial materials are then incorporated into a bulk structure. A discussion of some of the future research directions that we are pursuing is highlighted, including some bulk materials, which are based on nano-scaled or hybrid composites. The synthesis techniques and the synthesis results of many of these nano-scale precursor materials will be a primary focus of this paper.

Chapter 3 Overview of the thermoelectric properties of quasicrystalline materials and their potential for thermoelectric applications

Summary As described in the previous discussion, despite the advances in quasicrystals for thermoelectrics, there is still a long road ahead if quasicrystals are to be viable for thermoelectrics. It is encouraging that theoretical predictions by Macia indicate that high values of ZT may be possible in this class of materials ( Macia, 2000 ). Cyrot-Lackmann is also investigating quasicrystals as possible thermoelectric materials and has a patent on quasicrystals for this application ( Cyrot-Lackmann, 1999b ). A systematic approach in relation to doping, composition, processing, and other factors along with subsequent measurement of the transport properties will be necessary. Certainly this data coupled with ideas on how to further enhance the thermopower could greatly advance our knowledge of these materials. Quasicrystals closely match the concept that a good thermoelectric should behave as a glasslike material in relation to phonons and a metal in relation to electronic transport. AlPdMn quasicrystals have thermal conductivity that closely resembles that of an amorphous solid. The “tunability” in the electrical conductivity and thermopower allows for many compositional, impurity, damage, and additional element studies to be performed in an effort to better understand the transport in this system and optimize the figure of merit for potential thermoelectric application. Obviously, more work is required to fully address the feasibility of these materials for thermoelectric applications. However, along the way much information related to the interplay of many of the parameters to the electrical and thermal transport in these systems will be gained. Thus, a more fundamental understanding of the electrical and thermal transport mechanisms related to the quasicrystalline materials may become evident. It is the strong belief of one of us (TMT) that a new higher performance thermoelectric material will be found and it will truly change the world around us. Where will it be? Will it be in a quasicrystal? The current data tends to indicate that it probably will not be in these materials, since the hurdle of enhancing the thermopower by a factor of 4 seems too great; yet theoretically high values ( ZT >1) have been predicted to be possible. This is especially difficult without the ability to systematically dope and “tune a bandgap” as in the semiconductor thermoelements, which were described by Ioffe nearly 50 years ago. Quasicrystals are truly a fascinating class of materials, and whether or not subsequent research efforts and time determine their feasibility (or even impracticality) for thermoelectrics, they will still retain their most unusual properties, about which we have much to learn. Most likely, there are a few surprises left in these materials (or similar classes of intermetallic materials), and probably even more applications of quasicrystals will become evident over the next few years.

Electrical Transport Properties of the Pentatelluride Materials Hfte 5 and Zrte 5

We have measured the resistivity and thermopower of single crystals as well as polycrystalline pressed powders of the low-dimensional pentatelluride materials: HfTe 5 and ZrTe 5 . We have performed these measurements as a function of temperature between 5K and 320K. In the single crystals there is a peak in the resistivity for both materials at a peak temperature, Tp where Tp ≈ 80K for HfTe 5 and Tp ≈ 145K for ZrTe 5 . Both materials exhibit a large p-type thermopower around room temperature which undergoes a change to n-type below the peak. This data is similar to behavior observed previously in these materials. We have also synthesized pressed powders of polycrystalline pentatelluride materials, HfTe 5 and ZrTe 5 . We have measured the resistivity and thermopower of these polycrystalline materials as a function of temperature between 5K and 320K. For the polycrystalline material, the room temperature thermopower for each of these materials is relatively high, +95 μV/K and +65 μV/K for HfTe 5 and ZrTe 5 respectively. These values compare closely to thermopower values for single crystals of these materials. At 77 K, the thermopower is +55 μV/K for HfTe 5 and +35 μV/K for ZrTe 5 . In fact, the thermopower for the polycrystals decreases monotonically with temperature to T ≈ 5K, thus exhibiting p-type behavior over the entire range of temperature. As expected, the resistivity for the polycrystals is higher than the single crystal material, with values of 430 mΩ-cm and 24 mΩ-cm for Hfre 5 and ZrTe 5 respectively, compared to single crystal values of 0.35 mΩ-cm (HfTe 5 ) and 1.0 mΩ-cm (ZrTe 5 ). We have found that the peak in the resistivity evident in both single crystal materials is absent in these polycrystalline materials. We will discuss these materials in relation to their potential as candidates for thermoelectric applications.

The Synthesis of Transition Metal Sulfides and Sulfo-Salt Crystals in Hydrothermal Brines

A new procedure for the synthesis of transition metal sulfosalts in hydrothermal brines is presented. Large, well formed crystals of metal antimony sulfides can be prepared under appropriate conditions in supercritical brines. A new compound prepared by this method, MnSb 2 S 4 , is structurally characterized.

Transport in the Al 71 Pd 21 Mn 8− X Re X Quasicrystalline System

In an effort to understand the effects of a quartenary element introduced into a ternary quasicrystalline system, quartenary Al 7 1 Pd 2 1 Mn 9 - X Re X quasicrystals were grown, where X had values of 0, 0.08, 0.25, 0.4, 0.8, 2, 5, and 8. X-ray data confirm that the addition of a fourth element does not alter the quasiperiodicity of the sample. Because electronic transport is governed by different mechanisms in the parent systems (Al 7 1 Pd 2 1 Mn 8 and Al 7 1 Pd 2 1 Re 8 ), electrical and thermal transport measurements in the alloyed system have been performed on these samples and are presented here.

Modeling Thermopower in AlPdMn Based Quasicrystalline Systems

We report room temperature thermopower values and the temperature dependence for several AlPdMn based quasicrystals. In an effort to further understand the complexities of electrical transport in quasicrystalline systems, thermopower data for icosahedral Al 71 Pd 21 Mn 8- XRe X will be presented and discussed. A relation of room temperature thermopower to the curvature of the thermopower is demonstrated. We propose an empirical fit to the thermopower data, utilizing three free variables. The physical significance of the fit parameters is discussed. These results are discussed in brief concerning the relation to the application of quasicrystals for use as thermoelectric materials.

Effect of isoelectronic substitution of thermopower and resistivity of Hf/sub 1-X/Zr/sub X/Te/sub 5/

The thermopower and resistance of single crystal pentatellurides in the series Hf/sub 1-X/Zr/sub X/Te/sub 5/(x=0, .25, .50, and 1.0) have been measured as a function of temperature from 10 K to 300 K. The results show that HfTe/sub 5/ and ZrTe/sub 5/ contain broad resistance peaks at a temperature, T/sub p/, of 76 K and 147 K respectively, which are in agreement with previous measurements. Both compounds possess relatively large p-type thermopower (/spl sim/+100 /spl mu/V/K) which decreases rapidly through zero at a temperature T/sub 0/ before changing to an equally large n-type thermopower (/spl sim/-100 /spl mu/V/K) at a temperature T<T/sub 0/. Through isoelectronic substitution of Zr for Hf (Hf/sub 1-X/Zr/sub X/Te/sub 5/), systematic shifts are observed in both T/sub p/ and T/sub 0/ as the Zr concentration increases. These ternary compounds retain the large p- and n-type thermopowers.