Guidance to Advanced TE Research

Transcription

1 Guidance to Advanced TE Research

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3 Mat. Res. Soc. Symp. Proc. Vol Materials Research Society Gl.l Overview of Various Strategies and Promising New Bulk Materials for Potential Thermoelectric Applications Terry M. Tritt Department of Physics & Astronomy Clemson University, Clemson, SC, USA Abstract Recently, there has been a renewed interest in thermoelectric material research. There are a number of different systems of potential thermoelectric (TE) materials that are under investigation by various research groups. Some of these research efforts focus on minimizing lattice thermal conductivity while other efforts focus on materials that exhibit large power factors. An overview of some of the requirements and strategies for the investigation and optimization of a new system of materials for potential thermoelectric applications will be discussed. Some of the newer concepts such as low-dimensional systems and Slack's phononglass, electron-crystal concept will be discussed. Current strategies for minimizing lattice thermal conductivity and also minimum requirements for thermopower will be presented. The emphasis of this paper will be to identify some of the more recent promising bulk materials and discuss the challenges and issues related to each. This paper is targeted more at "newcomers" to the field and does not discuss some of the very interesting results that are being reported in the thin film and superlattice materials. Some of the bulk materials which will be discussed include complex chalcogenides (e.g.csbi4te6 and pentatellurides such as the Zrj.xHfxTes system), half- Heusler alloys (e.g. TiNiSnj.xSbx), ceramic oxides (NaCo 4 C>2), skutterudites (e.g. YbxCo 4 -xsbi2 or EuxCo 4 -xsbi2> and clathrates (e.g. Sr 8 Gai6Ge3o). Each of these systems is distinctly different yet each exhibits some prospect as a potential thermoelectric material. Results will be presented and discussed on each system of materials. Background On Thermoelectric Materials: Over the past five to six years there has been a heightened interest in the field of thermoelectrics driven by the need for more efficient materials for electronic refrigeration and power generation. [1, 2, 3] Proposed industrial and military applications of thermoelectric (TE) materials are generating increased activity in this field by demanding higher performance, nearroom-temperature TE materials than presently exist. Thermoelectric refrigeration is an environmentally "green" method of small-scale localized cooling in computers, infrared detectors, electronics and many other applications. Recent utilization of Peltier coolers in relation to refrigeration of biological specimens and samples is an emerging application of thermoelectrics. Power generation applications are currently being investigated by the automotive industry as a means to develop electrical power from waste engine heat and, of course, the deep space applications are well established. Given the recent energy needs experienced in the United States there is even a more pressing need to investigate alternative energy conversion technologies in this country, eg. the thermal to electrical energy conversion from natural heat gradients that thermoelectric technologies provide. This has already been identified as important in many European and Asian countries. An excellent overview of the state of the art materials, theoretical and experimental discussion of the basic principles, as well as an overview of recent developments, identifying some of the more interesting new materials is given in a recent text by Nolas, Sharp and Goldsmid. [4]

4 The potential of a material for thermoelectric applications is determined by the material's dimensionless figure of merit, ZT, where, a is the Seebeck coefficient, a the electrical conductivity and K the total thermal conductivity (K= KL + K E the lattice and electronic contributions respectively). The power factor, oc 2 ot, is typically optimized as a function of carrier concentration (typically around carriers/cm 3 ), through doping or "Fermi level tuning", to give the largest ZT. The importance of "Designer materials:" and this relationship of "tunability" to achieving the desired properties, especially related to the potential of a material for thermoelectric applications is given in an article in these proceedings by Dr. David Singh of the Naval Research Laboratory. [5] He discusses much of the criteria that should be considered in theoretically identifying classes of materials for potential thermoelectric property investigation. For a given carrier concentration, high mobility carriers are most desirable in order to enhance the electrical conductivity. Semiconductors have been primarily the materials of choice for thermoelectric applications. To effectively optimize a given set of materials one must understand how the electronic properties vary with the carrier concentration which is illustrated in Figure 1 as taken from Ioffe [6]. The lattice thermal conductivity is essentially independent of the carrier concentration and other methods are employed to reduce K L. Currently, the best TE materials have a value of ZT ~ 1. [7, 8] Increasing the Seebeck coefficient and/or the electrical conductivity and reducing the thermal conductivity can raise the value of ZT for a material. The electrical conductivity is tied to the electronic thermal conductivity, KE through the Wiedemann- Franz Law KE/CJ= LOT, where Lo = Lorentz number (Lo ~ 2.45 x 10~ 8 V 2 /K 2 ). An alternative approach to achieving higher ZT values is by decreasing or effectively minimizing the lattice thermal conductivity, K L. [9, 10] This may be attempted through mass fluctuation or alloy scattering as well as point defect or grain boundary scattering. While phonon scattering effects are usually considered to be predominant at higher temperatures (T ~ > 200 K), there is experimental evidence that grain boundary scattering can become important if the grain size becomes small enough. Recent investigations in the half-heusler alloy, TiNiSni. x Sbx, showed a correlation between the grain size and the room temperature lattice thermal conductivity. [11] The effect of smaller grain sizes on the lattice thermal conductivity had been investigated and established previously in the state-of-the-art materials like SiGe [12], [13], lead telluride [14] and also for other solid solutions [15]. Thus, alternative methods to tune the lattice contribution to thermal conductivity via grain size reduction are also possible. A typical strategy in TE materials research has been to investigate materials that possess large, complex unit cells with heavy atoms incorporated into the structure to effectively scatter phonons thus reducing thermal conductivity. In 1995, Glen Slack described the chemical characteristics of materials that might be candidates for a good TE material. [10] The candidate material is typically a narrow bandgap semiconductor (E G ~ 10 KBT or ~ 0.25 ev at 300 K) [16, 17] and the mobility of the carriers needs to be relatively high, (u ~ 2000 cm 2 /V-s) while the thermal conductivity should be minimized. In semiconductors, the Seebeck coefficient and electrical conductivity (both in the numerator of ZT) are strong functions of the doping level and chemical composition, which must therefore be optimized for good thermoelectric performance. The thermal conductivity of complex materials can often be modified by chemical substitutions, or by introducing mass-fluctuation or alloy scattering of the phonons. As discussed, while grain

5 boundary scattering can be an important consideration for reducing the lattice thermal conductivity, it can also be detrimental to carrier mobility. Understanding the various effects and selecting optimization strategies can be an exceedingly difficult problem, due to the fact, that in complex materials there are often many possible degrees of freedom. More recently new concepts and methods have been successfully employed to reduce the lattice thermal conductivity such as the introduction of "rattling" atoms, while maintaining relatively high power factors in materials such as the skutterudites. [18,19, 20] Insulators Semiconductors Metals Figure 1. Dependence of the important thermoelectric parameters: the electrical conductivity (a), the thermopower (a) and the thermal conductivity (X as the thermal conductivity whereas we designate it by K in the text) on the concentration of free carriers. Also shown is the dependence of the power factor (CC 2 G) on the carrier concentration. (Figure similar to Ioffe, reference 6 but taken from CRC Handbook on Thermoelectrics, D. M. Rowe ed., p 44, 1995) Investigating New Thermoelectric Materials: Slack suggested that the best thermoelectric material would behave as a "phonon-glass, electron-crystal "(PGEC) material. It would have the thermal properties of a glass and the electronic properties of a crystal. Whether one is investigating thin film thermoelectrics or bulk materials, the phonon-glass, electron crystal approach may well be the best strategy to employ. In a recent paper by Min and Rowe [21], they performed numerical analysis of the effect of the minimum thermal conductivity on the figure of merit of various materials. The lattice thermal conductivity is given by: K L ~ (2.)

6 where, vs is the velocity of sound, C is the heat capacity and LPH is the mean free path of the phonons. They define the minimum thermal conductivity as a reasonable value of K L ^ 0.25 Wm''K"', which is obtained by limiting the phonon mean free path as the interatomic distance. These values are in agreement with earlier estimations by Slack, and Min and Rowe's analysis follows closely with that of Slack's. [10] They show that a practical limit, or as they call it a ZT barrier, for a new thermoelectric materials which yields an upper limit of ZT ~ 2 at room temperature, ZT ~ 1.5 at 100K and at high temperatures, a ZT ~ 4 may be achievable. This is consistent with estimations of practical limits of ZT by both Slack [10] and Goldsmid [22]. There are certain practical limits of each of the parameters which, must be achieved or to be possible to achieve if a material is to be viable for a thermoelectric material. For example, in Bi 2 Te3, to achieve a ZT ~ 1 at T = 320 K, the electrical conductivity is ~ 1 mq-cm, the Seebeck coefficient is ~ 225 jllv/k and the thermal conductivity is ~ 1.5 Wm^K" 1. We have already discussed the ZT barrier which, is given by minimizing the thermal conductivity. It is practical to investigate materials where the electronic and lattice terms are comparable, on the order of 0.75 to 1 Wm^K" 1. Let us look at the hypothetical situation of a material where the lattice thermal conductivity is zero! We will also assume the scattering in this system is elastic and that the Wiedemann-Franz law, slightly rearranged [KE/O = LoT, where Lo = Lorentz number (Lo ~ 2.45 x 10" 8 V 2 /K 2 )] is well behaved in this material. Then we can rewrite equation 1 as follows: «,, Therefore for a material to be a viable thermoelectric material it must possess a minimum thermopower which is directly related to the value of ZT and Lo. Given this description, in order to achieve a certain value of ZT then it would require that for a ZT = 1, then a = (Lo) 0 ' 5 = 157 uv/k and for ZT = 2, then a = (2Lo) - 5 = 225 uv/k. Thus, we see that new materials must be able to eventually achieve certain minimum values of important parameters to be considered for a potential thermoelectric material. It does not matter if a material has a lattice thermal conductivity that is essentially at or near the minimum thermal conductivity. If it can not be "tuned" or doped to attain a minimum thermopower of a ~ 150 uv/k then it would not be able to achieve a ZT ~ 1. Of course, that is unless a material is in serious violation of the Wiedemann Franz law. In the following sections we will highlight some of the materials that are under investigation and summarize why we think these materials are interesting for potential thermoelectric materials. Each of these materials under investigation for thermoelectrics has it's own set of challenges that must be overcome, as apparent in the following sections. Complex Chalcogenides: An important temperature regime for thermoelectric materials research is at room temperature and below. Refrigeration of electronics and opto-electronics is an important technology which thermoelectric refrigeration could greatly impact, since these technologies typically require only small scale or localized spot cooling of small components which do not impose a large heat load. The advantages of "cold computing" are discussed in a recent article by Sloan, [23] where he states that "speed gains of 30% to 200% are achievable in some CMOS computer processors" and that "cooling is the fundamental limit to electronic system

7 performance." Cooling of laser diodes and infrared detectors to temperatures, 100 K < T < 200 K, would greatly improve performance and sensitivity and thus is extremely important to many technologies. [24] Thus, the potential payoff for the development of low temperature thermoelectric refrigeration devices is great, and the requirement for compounds with properties optimized over wide temperature ranges, especially at these low temperatures, has led to a muchexpanded interest in new thermoelectric materials. One of the important issues relative to the development of low temperature thermoelectric materials is identifying mechanisms which might give high thermopower (a) at low temperatures. Possibilities include phonon drag, heavy fermion materials, Kondo systems, materials which exhibit phase transitions, as well as quasi-one-dimensional materials. Low dimensional systems are known to be susceptible to van Hove singularities (or cusps) in their density of states, D(E), electronic phase transitions, and exotic transport phenomena which can add structure in D(E) near their Fermi energy, EF. In most materials, at temperatures far from a phase transition, the electrical conductivity and thermopower are related to the electron density of states near the Fermi energy D(E F ). The conductivity is proportional to D(E F ) while a is proportional to (l/d)dd(e)/de at E=EF. Hence, as D is increased, a typically increases while a decreases. Doping can produce very substantial effects in these types of materials and can drastically change their electronic transport. Quantum well systems take advantage of a low dimensional character through physical confinement in thin film structures to enhance the electronic properties of a given material. [25] Recent results on a new system of materials grown by the Kanatzidis group at Michigan State University exhibit very promising low temperature thermoelectric properties and have yielded some of the highest ZT values achieved below T ~ 250 K. [26] These materials are based on CsBuTe6 and exhibit low-dimensional-anisotropic transport behavior. This may be an important aspect of future investigations of new materials. These materials are grown by a flux growth method, typically using a Sn flux. Another family of low-dimensional semiconductors or semimetals, called pentatellurides (HfTes and ZrTes), have also shown promise as potential low temperature thermoelectric materials. These materials are synthesized using a vapor transport method. These pentatelluride materials exhibit thermopower that is relatively large over a broad range at low temperatures, T < 250 K, which results in a relatively large power factor in these materials, which is greater than that of similar Bi2Te3 alloys. These materials do, in fact, exhibit anisotropic transport properties with the high conductivity axis being the growth axis (a -axis and have a van der Waals gap between the individual layers (perpendicular to the /?-axis), cleaving easily along these planes. This is similar to the Bi2Te3 crystals, which result in interplanar scattering for the phonons. Substitution of Se in the Bi 2 Te3 compounds, led to a series of related alloys and pseudo-ternary compounds i.e. {(Bii-xSbxMTei-xSexM with enhanced properties that have resulted in effectively optimized thermoelectric materials. [27] The power factor of the Se doped pentatellurides exceeds that of the optimally doped Bi2Te3 system over the temperature range of measurement. However, investigations of the thermal conductivity indicates that the thermal conductivity is too high (K~ 4-8 Wm'K" 1 ) for use as a thermoelectric material. [28]

8 Thermoelectric Oxide Materials (Na x Co2O4): The development of air and water-stable refractory metal oxides as thermoelectric materials potentially has numerous advantages over existing high temperature thermoelectric materials, especially for power generation applications. They would be more rugged and easier to prepare than the conventional materials, and could be expected to have a wide range of operating environments. Recently the class of Na x Co2O4 compounds has displayed surprisingly effective thermoelectric properties. [29, 30, 31] The materials are members of a large class of layered metal bronzes, wherein the transition metals form edge shared octahedral oxide layers, with the alkali metal ions located within the layers. The compounds contain mixed valence Co ions with itinerant electrons within the layers, and non-stoichiometric amounts of Na + ions between the layers. There are many possibilities in this material's system both in single crystal and polycrystalline form. Half-Heusler Alloys Another group of materials, which are currently under investigation for potential thermoelectric materials are the Half-Heusler alloys. [32] The half-heusler alloys are intermetallic compounds with the general formula MNiSn where M is a Group IV transition metal (M = Zr, Hf, Ti). Half-Heusler alloys have a MgAgAs type crystal structure, forming three interpenetrating fee sublattices with one Ni sublattice vacant. Heusler alloys (e.g. MNi2Sn) differ from half-heusler alloys in having the Ni sublattice fully occupied and are also metallic (or halfmetallic) and exhibit interesting magnetic properties. Half Heusler alloys, on the other hand, are small band gap semiconductors with a gap of (E G ~ eV) [33, 34, 35] The half-heusler alloys exhibit a high negative thermopower (-40 to -250 uv/k) and low electrical resistivity values (0.1 to 8 md-cm) both of which are necessary for a potential thermoelectric material. Unfortunately the thermal conductivity is relatively high for a thermoelectric material, on the order of 10 Wm^K" 1 ) and much of the current and future research is related to reducing the thermal conductivity and maintaining the high power factor. Some of our recent investigations have centered around the effect of Sb doping on the Sn site (TiNiSnixSbx). The Sb doping leads to a relatively large power factor of ( ) Wm^K" 1 at room temperature for small concentrations of Sb. These values are comparable to that of Bi 2 Te 3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T ~ 700 K where it is over 4 Wm"'K"' making these materials very attractive for potential power generation considerations which would indicate one of the highest power factors (as defined by oc 2 ot) of any known material. These materials are also being investigated for potential thermoelectric applications by a number of other research groups. [36, 37, 38, 39, 40] The ZrNiSn-based half-heusler materials were recently investigated by Hohl. et. al.,[37] where they performed Nb and Ta substitutions on the Group IV metal site and Sb or Bi on the Sn sites. They found that the (Hf o. 5 Zro.5)o.99Tao.oiNiSn alloy displayed a power factor, oc 2 ot, of 0.66 Wm^K" 1 at 300 K and 2.8 Wm'K" 1 at 700K which produced a ZT ~ 0.5 at 700 K. Extensive investigations on the effect of temperature annealing were performed by Uher et. al. [36, 39] on similar systems of ZrNiSn, HfNiSn, and pseudo-ternary (Zr,Hf)NiSn, where they found that both the electrical and the thermal properties were strongly dependent on annealing time and conditions. These are cubic materials and tend to order and intermix upon annealing of a week or more at 800 C. The annealing is an important step in acquiring single-phase materials. This work focussed primarily on In and Sb doping on the Sn site. The Sb doping had a pronounced effect on the transport

9 driving the resistivity much more metallic, even with very small amounts of Sb. Extensive work related to the thermal conductivity in these materials was also presented, showing it is a strong function of the annealing. Another class of these materials based on LnPbSb where Ln = Ho, Er, Dy, were investigated by Mastronardi et. al.[40] An important result of this work is that they were able to obtain p-doped materials with thermopower values between 60 and 250 uv/k. These materials appear to-exhibit thermal conductivity values ( ~ 3-5 Wirf'K" 1 ) that are typically lower than many of the other half-heusler alloys. Recently, we have focussed our efforts on the TiNiSn system as a function of Sb doping, TiNiSni. x Sbx. The Sb doping greatly enhances the power factor, as with the ZrNiSn, and yields one of the highest power factors known in any material. In fact, the high temperature power for TiNiSno.95Sbo.o5 is PF ~ 4.5 Wm'K" 1 at T ~ 650K, our highest temperature measured. The power factor for TiNiSno.95Sbo.o5 of 4.5 Wm^K" 1 at T ~ 650K is one of the highest power factors reported on any material. Many of the half- Heusler alloys exhibit similarly high power factor values, which is what makes them so compelling for thermoelectrics. Unfortunately, the thermal conductivity in the half-heusler alloys is relatively high (KL ~ W m'k" 1 ) and consists mainly of lattice contribution with a small electronic contribution, in contrast to the more favorable situation where K E ~ K L in order to obtain a "good thermoelectric". In a recent publication, Shen et. al. [41] reported the effect of partial substitution of Ni by Pd on the thermoelectric (TE) properties in the ZrNiSn half-heusler system. This investigation yielded a thermoelectric figure of merit of ZT = 0.7 at T ~ 800 K in a sample of Zro.5Hfo.5Nio.sPdo.2Sno.99Sbo.o1. One of the issues or obstacles related to the potential of half- Heusler alloys for use in TE applications is their relatively high lattice thermal conductivity KL ~ 10 W m'k" 1 ). Shen and co-workers were able to achieve values of K L ~ 2 W m'k" 1 at T ~ 800K in a sample of Zro.5Hfo.5Nio.5Pdo.5Sno.99Sbo.o1 and KL ~ 3 Wm^K" 1 in the high ZT sample identified above. They further state that possible grain size effects may be able to lower KL even further in concert with predictions of Goldsmid and coworkers previously. [42] We have recently presented results on reduction of grain boundary scattering on the lattice thermal conductivity and have observed these predicted results. [43] Skutterudites Skutterudites are the next class of materials that has received a lot of attention within the thermoe lee tries community over the last ten years or so. They get their name from a naturally occurring mineral, skutterudite or C0AS3, which is found in Skutterud, Norway. The skutterudite family of compounds continues to be of interest for thermoelectric applications due to the ability to greatly vary the lattice thermal conductivity, which is facilitated by the filling of the voids within the structure with small diameter, large mass interstitials such as trivalent rare-earth ions. [44, 45]. Specifically, the smaller more massive ions that are incorporated within the skutterudite voids results in the lowest lattice thermal conductivity. This is shown clearly by Nolas and co-workers [46] in an investigation of the thermal conductivity of RECo4Sb9Ge3, with RE = La, Nd, Sm, as compared to unfilled IrSb3. The skutterudite structure, is cubic (with space group Im3) and the unit cell contains 8 AB3 groups where A is the transition metal element Ir, Co or Rh, and B is the pnicogen element such as P, As and Sb. It contains 32 atoms per unit cell, with the metal atoms on the corners of the eight "cubes" with 6 (4 atom) Sb rings inside the cubes with two "voids" in the remaining "cubes". In context of the doping scheme for thermoelectrics the general crystal structure of these materials can be best described on the following designation, D2Co8Sb24. The open square, D, indicates the presence of two voids in the structure which are typically filled by the rare earth atoms to produce the most pronounced effects on the thermal conductivity reduction. The other metal and pnicogen sites are doped to

10 charge compensate for the additional electrons contributed by the doping with the rare earth doping and to enhance the electronic properties. The reduction in the thermal conductivity of filled skutterudites as compared to "unfilled" skutterudites is due the dynamic, or "rattling", disorder of the void-filling ions that substantially affect the phonon propagation through the lattice. More loosely bound "rattlers" produce local vibrational modes of lower frequency and are thus more effective in scattering the lower-frequency, heat-carrying phonons. The heavier and smaller the ion in the voids, the larger the disorder that is produced and therefore the larger the reduction in the lattice thermal conductivity, KL. This concept, first introduced by Slack [10], is corroborated by the large atomic displacement parameters (ADPs) that have been observed in alkaline earth and lanthanide filled skutterudites.[47, 48] The use of ADP's has found to be very useful in thermoelectric materials research. [49] One approach that has been reported to be a route for optimizing the thermoelectric properties in skutterduites is that of partial void filling. [50] Lanthanum, cerium and thorium partially-filled skutterudites have been reported to possess relatively good electronic properties while also possessing much lower values of the thermal conductivity than "unfilled" CoSb3. Recent investigations of the TE properties of Yb partially filled skutterudites yielded a figure of merit, ZT > 1 at T ~ 600 K [51] Others are also investigating this system of Yb Based skutterudites for potential thermoelectric applications. [52] Similar results have also been recently observed in a series of Eu doped Co-based skutterudites. [53] The thermal conductivity of the Yb- and Eu-filled skutterudites is much lower, with a relatively weak temperature dependence, as compared to that of CoSb3. Only a 19% Yb in CoSba dramatically decreases KL as compared to the 'unfilled' compound. [44] The 'rattling' motion of the small and massive Yb in the voids of CoSb3 substantially reduces KL in these compounds. We note that although the Ybo.i9Co4Sbi2 specimen has a low thermal conductivity it is still much higher than the estimated minimum thermal conductivity of CoSb3, K m i n ~ 0.3 W-m^K" 1. [51] Although these results are encouraging, higher ZT values may be forthcoming in skutterudite compounds, particularly if K L can be further reduced towards the theoretical minimum value while maintaining the high power factors which are observed in these Yb- or Eu-filled compounds. Clathrates: Another class of materials that is currently very interesting for thermoelectrics are materials called clathrates (e.g. Sr 8 Gai6Ge3o). [54, 55] These materials, like the skutterudites also exhibit cage-like structures and "rattling" mechanisms with which to "tune" the thermal conductivity. Clathrates with frameworks built up from group 14 atoms have been known since the work of Cros and co-workers on Na 8 Si46 and Na x Sii36- [56, 57] These materials have crystal structures closely related to those of type-i and type-ii clathrate hydrates such as (Cl2)8(H 2 O) 4 6 and (CO2)24(H2O)B6.[58] Many different compositions with these two structures are possible. They are of fundamental interest from the perspective of both bonding and their physical properties. Thus far, there has been a substantial amount of work on compounds with the type I crystal structure. A variety of different elements have been encapsulated inside the polyhedra of these structure types, including alkali-metal, alkaline earth and rare earth atoms. Superconductivity has been reported in compounds containing Ba. [59, 60] The potential for wide band gap silicon and germanium has attractive attention for potential optical applications. [61] The very low glass-like thermal conductivity [62, 63, 64, 65, 4] found in certain type I clathrate compounds has also attracted interest. There are two type of clathrate hydrates, that of type I, for example (CfcMIfeO^e, and type II, such as (CO2)24(H2O)i36 These hydrates have a hydrogen bonded network of water molecules 10