奥地利专利AT514911A2 Intermetallic clathrate compounds

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专利摘要:
The invention relates to novel Type I intermetallic clathrates having the following general compositional formula: Ba8-xRExTyIV 46-y, wherein: RE is one or more rare earth elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb , Dy, Ho, Er, Tm, Yb and Lu; T is one or more d-transition metals; IV represents one or more elements of the 4th main group of the Periodic Table, selected from Si, Ge, Sn and Pb; x is a number that satisfies 0 <x <8; and y is a number for which: 0 <y ~ 6; and a process for their preparation and their use in thermoelectric modules.
公开号:AT514911A2
申请号:T536/2014
申请日:2014-07-04
公开日:2015-04-15
发明作者:Andrey Dr Prokofiev；Silke Dr Bühler-Paschen
申请人:Tech Universität Wien；
IPC主号:

专利说明:

The present invention relates to novel intermetallic clathrate compounds, to a process for their preparation and to their use.
STATE OF THE ART
Clathrate or cage inclusion compounds have been known for about 200 years and their targeted production for about 60 years. The first discovered representatives were gas hydrates, i. Ice grids with inclusions of gas molecules (e.g., Cb). Nowadays, intermetallic clathrates, which have been known since the 1960s, are the subject of considerable research, since they are of inter alia of interest for use in thermoelectric modules (see, for example, US Pat. Nos. 5,800,794 A1, 6,188,011 B1 and 6,525. 260 B2). Intermetallic clathrates are classified into 9 different structural types, of which type I is the most studied, and as a result numerous representatives are known. Fig. 1a shows schematically the structure of such a clathrate compound, wherein the larger spheres are those in the cage-forming atoms ("host atoms", host atoms, cage atoms) " H " formed guest cage (" guest atoms ") " G " represent. See also Fig. 5 where the large spheres represent the G atoms and the small H atoms.
To define the term " clathrate " It should be noted that herein is meant intermetallic compounds in which the cage-forming atoms form a space-filling network, although specifically in English under the definition of " clathrate " sometimes all types of inclusion compounds (" inclusion compounds ") are understood. Type I clathrates generally conform to the formula GeH46, i. There are 8 guest atoms embedded in a matrix of 46 cage atoms.
As components of intermetallic clathrates a variety of combinations of elements can function. For example, EP 1,074,512 A1, which broadly corresponds to US 6,461,581 B1, discloses cage inclusion compounds in which one or more elements of the 4th main group, in particular Ge or Si, together with so-called "substitution atoms", consisting of numerous other elements of the periodic table can be selected, the matrix, ie the " cage " form. Also the V «9 9 9 9 9 9 9 9
Selection of the storable atoms is possible in the above documents within wide limits. Generally, atoms of the 1st and 2nd main groups are preferred as inclusion components and, for example, Ga, Ge, Si and transition metals as cage components.
The preparation of such clathrates is usually carried out by melting together of the elements, after the cooling of which below the melting range of the mixture several phases can be obtained which either not yet the desired clathrate compounds or too low levels of the desired Clath ratphase in combination with unwanted, interfering foreign phases include. To obtain phase-pure clathrate compounds, therefore, a subsequent heat treatment at temperatures of several hundred degrees Celsius, usually over a period of many hours, several days or even weeks is required until a substantially pure, solid phase of the desired clathrate is present.
One variant of these methods involves hot compression molding or discharge plasma sintering of powders of the starting materials at around 700 ° C, which can speed up the manufacturing process (see, for example, US 6,525,260 B2).
The preparation of clathrate single crystals is also known, see e.g. Cohn, J.L., Nolas, G.S., Fessatidis, V., Metcalf, T.H. Slack, G.A., " Glasslike heat conduction in high mobility crystalline semiconductors ", Phys. Rev. Lett. 82, 779 (1999); Nolas, G.S., Weakley, T.J.R., Cohn, J.L. & Sharma, R., "Structural properties and thermal conductivity of crystalline clathrates. Phys. Rev. B 61, 3845 (2000); Sales, B.C., Chakoumakos, B.C., Jin, R., Thompson, J.R. & Mandrus, D., " Structural, magnetic, thermal, and transport properties of X8 Ga 16 Ge30 (X = Eu, Sr, Ba) single crystals ", Phys. Rev. B 63, 245113 (2001); Condron, C.L., Kauzlarich, S.M. & Nolas, G.S. Structure and thermoelectric characterization of AxBa8-xAl14Si31 (A = Sr, Eu) single crystals ", Inorg. Chem. 46, 2556 (2007); and LTK Nguyen, U. Aydermir, M. Bai-tinger, E. Bauer, H. Borrmann, U. Burkhardt, J. Custers, A. Haghighirad, R. Höfler, KD Luther, F. Ritter, W. Assmus, Y Grin and S. Paschen, " Atomic ordering and thermoelectric properties of the n-type clathrate Ba8Ni3.5Ge42.1 ", Dalton Trans. 39, 1071 (2010). The crystals usually crystallize congruently from the melt. In most cases, the crucible method is used for this purpose.
Among the numerous variants of intermetallic clathrates, which contain predominantly barium as a guest atom, there are also europium-containing compounds. For example, in previous work, the present inventors have also prepared several clathrate compounds containing europium as guest atoms: see, e.g. S. Paschen et al., Phys. Rev. B 64, 214404 (2001), where the compound Eu8Gai6Ge3o was prepared and analyzed in the α- and β-modification. In Fig. 9 there, which is shown herein as Fig. 6, the results of thermoelectric measurements of both modifications are shown, wherein at 300 K values of around -75 pV / K and -65 pV / K were found.
Other work on, inter alia, europium-containing clathrates was the development of a process in which, instead of slowly cooling the melt and then annealing the solidified product, rapid quenching of the melt without subsequent annealing was used (see, inter alia, US Pat. No. 8,545,942 B1 , JP 5,248,916 B2, DE 20 2008 006 946 U1).
Further, the present inventors report in S. Paschen et al., J. Cryst. Growth 310, 1853-1858 (2008), about unsuccessful attempts to substitute ytterbium for Eu8Gai8Ge3o europium in the above system.
So far, it has not been possible to produce type I clathrates containing another lanthanide (herein including lanthanum) as europium, although such compounds could have promising properties. Although US Pat. No. 7,534,414 B2 describes La and Ce as possible guest atoms in a cage matrix of type 2 clathrates formed by silicon and germanium, such compounds were not prepared. Rather, Ce and La are not mentioned in the description.
An actual attempt to prepare a cerium-containing clathrate has been reported by Kawaguchi et al. (Kawaguchi, T., Tanigaki, K. & Yasukawa, M., " Silicon cladrate with an-electron system ", Phys. Rev. Lett. 85, 3189 (2000)). However, this has been confirmed by Pacheco et al. refuted, which proved that the cerium was only contained in a foreign phase (Pacheco, V., Carrillo-Cabrera, W., Tran, VH, Paschen, S. & Grin, Y., " Comment ", Phys. Rev Lett., 87: 099601 (2001)), which also includes Kawaguchi et al. subsequently (Kawaguchi, T., Tanigaki, K. & Yasukawa, M., " Reply ", Phys. Rev. Lett. 87, 099602 (2001)).
Finally, it should be noted that while the class of intermetallic compounds RE3Pd2o (Si, Ge) 6 (RE = rare earth element) is sometimes referred to as clathrate (or clathrate-like compound) in Japanese-speaking countries, it does not actually fall into the structural class of clathrates ( See, for example, P. Rogl, " Formation and crystal chemistry of clathrates ", Chapter 32, Thermoelectrics Handbook, DM Rowe (ed.), CRC Press, Boca Raton, 2006). The " cages " in these compounds, the characteristics of the clathrates (e.g., predominantly covalent nature of the binding of the cages, predominantly ionic character of the bond between guest and cage atoms, space filling by the cages) are not exhibited. One such, falsely referred to as " clathrate " designated compound, namely Ce3Pd20Ge6, is described by Nemoto et al. in phys. Rev. B 68 (18), 184109 (2003).
In fact, the incorporation of lanthanides other than europium in clathrate compounds has never been successful (see Paschen, Ikeda, Stefanoski, Nolas, Chap.9: "Structural and Physical Properties of Rare-Earth Clathrates", in "The Physics and Chemistry of Inorganic Clathrates " Springer Series in Materials Science 199, DOI: 10.1007 / 978-94-017-9127-4_9, Springer Science + Business Media Dordrecht 2014.)
The aim of the invention was therefore to provide a method for producing such hitherto unknown clathrates and thereby also the compounds themselves and to examine the latter for potential advantageous properties.
DISCLOSURE OF THE INVENTION
This object is achieved in a first aspect of the present invention by providing hitherto unknown type I intermetallic clathrates having the following general composition formula: wherein:
RE is one or more rare earth elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; T is one or more d-transition metals; IV represents one or more elements of the 4th main group of the Periodic Table, selected from Si, Ge, Sn and Pb; x is a number for which 0 < x <8th; and y is a number for which 0 < y < 6th
It should be noted that at all lattice sites of the clathrate crystals may also have low levels of vacancy, i. Spaces, which can be expressed by an even more general form of the compositional formula:
where vg is the proportion of vacancies on guest atoms and vh is the proportion of vacancies on host atoms, and v1 and v2 are each usually < 0.5, preferably < 0.3, in particular < 0.2.
In preferred and particularly preferred embodiments, the novel clathrates of the present invention have one or more of the following features a) to e): a) x "0.1 to 1.5, preferably 0.2 to 1.3; b) y * 3 to 6, preferably 5 to 6; and c) RE is La, Ce, Pr, Sm, Eu, Gd, Yb or Lu; preferably for La or Ce; d) T is Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Pd, Ir, Pt or Au; preferably for Au or Pt; e) IV is Si or Ge.
With regard to features a) and b), it should be noted that lower proportions of rare earth or transition metal atoms have less or barely measurable effects on the properties, while larger proportions of rare earth atoms are difficult to access and are not possible on transition metal atoms for crystallographic reasons.
In particular, the intermetallic clathrate of the present invention has one of the following average compositions:
Apart from the fact that such Type I intermetallic clathrates have not been known hitherto, the inventors have found that the incorporation of lanthanoids, including lanthanum, as guest atoms leads to very good to excellent thermoelectric properties. As the later examples demonstrate, a cerium-containing clathrate " Ce-BAS " thermopower 50% higher than a reference clathrate (" BAS ") containing only barium as a guest atom and having the same carrier concentration, and the amount of thermo-power of a lanthanum-containing clathrate " La-BAS " is even 100% higher than the lanthanoid-free reference material, since the incorporation of the rare earth metal ions, the carrier concentration could be significantly reduced.
The new rare earth-containing clathrates are thus characterized by a strong thermoelectric effect, not only at low temperatures, as the skilled artisan might have expected, but also at high temperatures, i. above room temperature.
Without wishing to be bound by theory, it is believed that in the case of Ce, the reason for this lies in the Kondo interaction between the localized electron (and the resulting magnetic moment) of the ion and the conduction electrons, which is due to the anharmonic vibration of the magnetic ion in the cage is enhanced, as will become apparent from the more detailed description below; in the case of La, however, the improved thermoelectric properties are due to the lower carrier concentration that could be achieved via the incorporation of La.
For this reason, of course, all lanthanides should be within the scope of the invention, since it can be assumed that they have comparable properties and can be prepared in an analogous manner as the previously prepared compounds with La, Ce, Pr, Sm, Eu, Gd, Yb or Lu as Gastatome. The same applies to clathrates with more than one of the lanthanides mentioned as guest atoms.
Furthermore, all transition metals and in particular Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Pd, Ir, Pt, Au should be within the scope of the invention, since it can be assumed that these have comparable properties and in analog Be prepared as the previously prepared compounds with Au and Pt as host atoms. The same applies to clathrates with more than one of the transition metals mentioned as host atoms.
Furthermore, all elements of the 4th main group with the exception of carbon, in particular Si, Ge, Sn, Pb should be within the scope of the invention, since it can be assumed that they have comparable properties and can be produced in an analogous manner as the compounds prepared so far with Si and Ge as host atoms. The same applies to clathrates with more than one of the elements of the 4th main group as host atoms.
In a second aspect, the invention provides the use of such a novel intermetallic clathrate in thermoelectric modules.
The first preparation of these novel clathrates of the present invention has been made possible by the development of a new production process, which is a third aspect of the invention, namely a process for producing intermetallic clathrate compounds by slowly solidifying the respective clathrate from a melt forming the guest and cage atoms Elements characterized in that, in a zone-free zone melting process, the constituent elements are melted in non-stoichiometric proportions and allowed to solidify by zone migration to single crystals.
The clathrates, which were inaccessible according to previous methods, could now be produced in this way for the first time, since in the process according to the invention clathrates with a defined composition formed by zone migration can settle out of the melt produced by zone melting in the form of single crystals.
However, a wide range of clath rates can be produced by the method of the invention, e.g. also type II clathrates and clathrates in which several rare earth elements, in particular the lanthanides mentioned, are present within a compound, which is why the process should not be restricted to the preparation of the clathrates according to the first aspect of the invention.
The present invention will be further described below by way of concrete, non-limiting examples and with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows in the pictures a-d the characterization of the structure and phase purity of the clathrate Ce-BAS. a: Schematic representation of the crystal structure of type I clathrates. Shown are the two different cages. The guest atom (G, red) is located at the crystal position 2a (6c) with cubic (tetragonal) point symmetry in the smaller (larger) cage of the host atoms (H, blue). b: The powder X-ray diffractogram (top) can be completely and exclusively indexed with the structure of a type I clathrate (bottom). c: Scanning electron microscopy images at two different magnifications show that there are no micro-inclusions of foreign phases, d: The representative transmission electron microscopy image excludes inhomogeneities in the nanometer range.
FIG. 2 shows in the images a-e the transport properties of Ce-BAS and La-BAS. a: temperature dependencies of resistivity; b: absolute value of the Hall coefficient; in use, the Hall mobility; c: thermoelectric power; d: performance indicator; e: dimensionless thermoelectric quality factor and in use the thermal conductivity k (symbols, left axis), the electronic part Ke, estimated according to the Wie-mann-Franz law from p (T) (solid lines, left axis) and the grating thermal conductivity Ki = K-Ke (dashed lines, right axis).
The black lines are used to guide the eyes. They were used to calculate μπ and ZT.
Fig. 3 shows a thermo-power comparison with the rare earth-free reference material BAS: the room temperature thermo-power of Ce-BAS and La-BAS together with published values for a series of BAS samples with different Au contents20 as a function of carrier concentration, plotted as nm Lines are linear adjustments of the data for BAS.
FIG. 4 shows the thermodynamic properties of the investigated clathrates in the images a-d. a: Magnetic susceptibility of Ce-BAS (top) and La-BAS (bottom) over temperature; in use, the inverse magnetic susceptibility of Ce-BAS at low temperatures; b: specific heat of Ce-BAS at low temperature in different magnetic fields; c: specific heat of Ce-BAS, La-BAS and BAS, plotted as C / P versus temperature; d: Temperature dependence of the paramagnetic component of the magnetic susceptibility of Ce-BAS, plotted as χΤ over temperature.
Fig. 5 also shows schematically the structure of a Type I clathrate compound wherein the large spheres represent the guest atoms and the small represent the host atoms.
Fig. 6 shows the aforementioned results of the determination of the thermo-force of the Eu-containing clathrate Eu8Ga-i6Ge3o.
DETAILED DESCRIPTION OF THE INVENTION
Type I clathrates are guest host systems of general composition G8H46. The guest atoms G are located in space-filling polyhedral cages with common surfaces, which are formed by the tetrahedrally bound host atoms H (FIG. 1a). Exceptionally low grating thermal conductivities are inherent in these materials because they are even found in pure single-crystal samples1. These were attributed to decreased phonon group velocities resulting from the interaction between acoustic and " rattle " modes 4,5. In general, the charge carriers are essentially unaffected by these lattice anomalies. Thus, type I clathrates appear to be an implementation of the phonon glass electron crystal concept6. With this background, in recent decades, many groups worldwide have synthesized and studied a large number of type I clathrates of various compositions. As a result, a significant increase in the thermoelectric quality factor ZT = S2o / k T was achieved, where T is the absolute temperature, S the thermoelectric force, σ the electrical conductivity and κ the thermal conductivity. The highest reported ZT values are 1.63 at 1100 K for n-type Ba8Gai8Ge3o (ref.7) and 1.1 at 900K for n-type Ba8Gai6Ge3o (ref.8).
Record-high performance factors S2a were, however, found in a very diverse class of materials - in highly correlated rare earth or transition metal compounds1,9 "11. The enormously high thermoelectric power levels that occur in these systems can be attributed to increased quasiparticle density at conditions near the Fermi level resulting from the Kondo interaction of the local 4f states with the conduction electrons12,13.
Therefore, the incorporation of suitable rare earth elements in Type I clathrates appears to be a promising 2, but not yet implemented14'18, way to better thermoelectric materials. In particular, it has not been possible to introduce appreciable amounts of Ce into a clathrate phase14'16. Herein, the first successful synthesis of a Ce-containing clathrate and the finding that its thermo-power is greatly increased compared to a rare-earth-free reference material are reported.
In searching for a synthetic pathway for a Ce-containing clathrate, the inventors were guided by the Zintl-Klemm concept19, which predicts that polar intermetallic compounds are stable when all atoms fill their electron shells. In type I clathrates, it is assumed that the host atoms are covalently linked and the guest atoms are ionically bound to the host framework. In the conventional case of electropositive guest atoms, this is done by transferring the valence electrons of the guest atom to the host framework, which thus becomes a polyanion. An example of this is the clathrate (M + 2) 8 (lir1) i6 (IV0) 3o, where the divalent metal atoms M are the guest atoms and the group III and group IV elements form the backbone. The superscripts represent the formal charges of the atoms: the valence electrons of M are given to the trivalent elements III to complete the fourth framework bond. When all valence electrons have been used up, an electrical insulator is expected. Real clathrates are semiconductors or bad metals; This is due to slight deviations from the situation described above in a broadly simplified way.
A particularly broad compositional range around the ideal Zintl composition was found in the transition metal clathrate Ba8AuxSi46-x (BAS), which has a broad Au concentration range of 2.2 < x < 6 forms (ref 20), including the ideal Zintl composition (Ba + 2) 8 (Au'3) 5,3 (Sio) 4o, 6 for monovalent Au. This indicates an exceptional robustness of this phase, which is why the inventors chose this as the starting material for their study.
The substitution of divalent Ba by trivalent Ce must be accompanied by an increase in the acceptability of the framework and thus by an increased Au content. At x = 6, the Ce content for attaining a charge balance corresponds to the clathrate (Ba + 2) 6 (Ce + 3) 2 (Au'3) 6 (Si °) 4o having two Ce atoms per formula unit. This ideal Zintl composition was the starting point for the syntheses of the inventors (" process ") which ultimately yielded single crystal clathrate samples of Ba6.9i ± o, i7Cei, o6 ± o, i2Au5.56 ± o, 25Si4o.47 ± o , 43 gave (Ce-BAS, Fig. 1 and " method "). The presence of the substantial amount of about 1 Ce atom per formula unit in the clathrate phase was unequivocally proved by the inventors by energy dispersive X-ray spectroscopy analyzes (EDX) and wavelength dispersive X-ray spectroscopy analyzes (WDX), together with the fact that no foreign phase is present (Fig. 1b , c), not even in the nanometer range (Figure 1d). This is also consistent with the decrease in the lattice parameter as evidenced by the pattern refinement with X-ray powder and single crystal diffractometry (XRD) data sets (" method ") as compared to BAS samples of similar Au content. In addition, a phase-pure La-containing clathrate (Ba6> 99 ± 0, i7Laii23 ± 0, i2Au5.99 ± 0.25 Si39.87 ± 0.43, La-BAS, " method ") was prepared as a 4f-free reference compound. Structural refinements using single-crystal XRD data sets show that both Ce and La occupy the 2a site in the smaller cage (Figure 1a).
After the presence of Ce (and La) in the single-phase clathrate samples was unequivocally confirmed by analytical and structural investigations, the inventors focused on their physical properties. The specific electrical resistance p (T) of La-BAS decreases with increasing temperature, a typical behavior of metals. In contrast, p increases from Ce-BAS on cooling (Figure 2a). This semiconductor-like characteristic is unexpected, as explained below.
The Hall coefficient Rh (T) is negative for Ce-BAS and positive for La-BAS, in accordance with the Zintl consideration based on the measured compositions. The amount of RH is significantly smaller for Ce-BAS than for La-BAS (Figure 2b), indicating that Ce-BAS has a higher carrier concentration and therefore should be more metallic than La-BAS. The temperature dependence of | Rh | (T) is similar for both compounds. It is typical of heavily doped semiconductors. In particular, no evidence of an anomalous contribution to the Hall effect21 of Ce-BAS was found. Thus, the increased resistance of Ce-BAS is not due to the charge carrier concentration, but instead due to a reduced Hall mobility jl / h = | ^ h | lp (Figure 2b, insert).
A comparison with published mobility data for a series of polycrystalline BAS samples20 shows that μπ of the La-BAS crystal of the present invention is significantly higher with respect to the p-type BAS sample with the closest carrier concentration. This is of course attributed to the reduced scattering in the single crystal due to the absence of grain boundaries, an effect also found in Ba-Ni-Ge clathrates22. In contrast, the mobility of the present single-crystal Ce-BAS sample is significantly lower than that of the n-type polycrystalline BAS sample with similar carrier concentration. Thus, an additional scattering process must be deduced to explain the lower mobility of Ce-BAS.
The most obvious mechanism for a metal containing Ce ions is Kondo scattering. For heavy fermion metals, incoherent Kondo scattering above the single-ion Kondo temperature 7k results in a resistivity fraction that is proportional to -InT. Due to the temperature-dependent charge carrier concentration of Ce-BAS, this dependency is expected to be only roughly true, in accordance with the inventors' findings (Figure 2a). Below 7k, heavy fermion metals exhibit a marked reduction in resistivity due to the onset of coherence in a Kondo lattice. The absence of this decrease in Ce-BAS is explained as follows. The multiplicity of the lattice site 2a, where the Ce atoms are located, is 2. Since Ce-BAS contains only 1.06 Ce ions per formula unit, it does not fully occupy this lattice site. The 0.94 Ba
Ions at this point act as condyle holes23 of the lattice, thereby inhibiting the formation of a coherent state. The magnetic susceptibility and specific heat measurements set forth below support the importance of the Kondo phenomenon in Ce-BAS.
Next, the thermoelectric properties will be explained. The thermal power S (T) is the cause of the phenomenon of thermoelectricity. It is negative (positive) for Ce-BAS (La-BAS, Fig. 2c), in accordance with the Hall effect results set forth above. Maximum values of -180 pVK'1 (300 pVK'1) are achieved at Ce-BAS (La-BAS) at 480 K (375 K). These values exceed those of all previously studied BAS clathrates24,25. In the next paragraph, the inventors will show that although La-BAS achieves higher S-values, in reality Ce-BAS exhibits an anomalous increase in thermoelectricity. The highest performance factors S2a achieved in the temperature range 2-600 K are about 11 pWK "2cm" 1 for La-BAS at 350K and 6 pWK'2cm'1 for Ce-BAS at 600K, with a slope to one further increase at higher temperatures (Fig. 2d). The thermal conductivities k (T) of Ce-BAS and La-BAS (Figure 2e, insert) are similar to those of BAS-He24. Since k (T) is dominated by the grating portion K , this indicates that the " rattle " modes of Ce-BAS and La-BAS are as effective for producing low grating thermal conductivity as in BAS. For n-type Ce-BAS, the maximum ZT value of 0.15 at 480 K (0.19 at 600 K when K is assumed constant above 480 K) is 30% (100% at 600 K). higher than that of the best n-type BAS materials at the same temperature. For p-type La-BAS, ZT is 0.2 at 400 K, which is 35% higher than the best p-type BAS sample at the same temperature.
In simple metals and degenerate semiconductors, the thermo-force depends on the charge carrier concentration, namely, S n213. Figure 3 is a graph of the thermal power of Ce-BAS and La-BAS at room temperature, along with data for p-type and n-type BAS samples from the literature20. While La-BAS fits well in the series, Ce-BAS has a greatly increased thermo-power: | S | is 50% higher than what would be expected from its charge carrier concentration. Can this be traced back to the Kondo interaction To answer this question, the inventors have performed thermodynamic measurements at low temperatures, which promise to lead to a better understanding of the Kondo physics of this system.
The magnetic susceptibility χ (Τ) shows that Ce-BAS is paramagnetic while La-BAS is diamagnetic (Figure 4a), with no evidence of magnetic ordering up to the lowest temperatures reached. This paramagnetic behavior of Ce-BAS can be attributed to the presence of Ce + 3 ions. They have the electron configuration [Xe] 4f1 and thus have a localized, well-shielded electron that carries a magnetic moment. The sixfold degenerate spin-orbit ground state 2Fs / 2 is generally split by electric crystal fields. For cubic point symmetry (the situation relevant to the 2a site), a Γτ doublet and a Γg quartet result. The temperature dependence of a combination of free (non-interacting) moments is determined by the thermal population of the different energy levels. Since the energy difference between the Γ7 and Γg states is usually on the order of 50-100 K, the crystal field ground state will determine the properties at the lowest temperatures.
Between about 1 and 6 K, χ (Τ) of Ce-BAS exhibits Curie-Weiss-like behavior, with an effective moment of 1.48 Pb per Ce ion and a paramagnetic Weiss temperature near zero (FIG. Commitment). The latter is to be expected due to the large distance between the nearest neighbor Ce atoms of about 0.6 nm. Below 1 K, the magnetic susceptibility deviates from this Curie-Weiss law. The tendency to saturate to a constant value can be attributed to the Kondo-exchange effect.
This is further confirmed by the specific heat C (T) of Ce-BAS. It has a pronounced anomaly in the zero field (Figure 4b), which is due to the splitting of a Krahmers doublet by the Kondo interaction. Both the peak temperature and the peak value increase with applied magnetic fields, again a typical behavior of Kondo systems. After subtraction of the phonon contribution to the specific heat with the aid of the reference compound La-BAS, the magnetic entropy can be estimated. The entropy of 0.4Rln2 per mole of Ce, where R is the universal gas constant, is released up to 0.5K. The double temperature is generally considered a good estimate of the Kondo temperature26,27. All information taken together shows that Ce-BAS behaves like an incoherent Kondo system with a low Kondo temperature of TK »1 K at low temperatures.
This raises an important question: how can this low Kondo temperature lead to an increase in thermo power even above room temperature The inventors argue below that due to spin-phonon coupling, a second, much higher Kondo scale is created.
First, the inventors explain the evidence of " rattles " in both Ce-BAS and La-BAS. Einstein-like levels of specific heat can be easily shown in graphs with C / T3 over InT, where they appear as bell-shaped anomalies on a background level due to a Debye fraction and charge / spin fractions at low temperatures28. With the introduction of Ce and La, the maximum occurring at BAS is only slightly reduced in amplitude and slightly shifted to higher temperatures (Figure 4c). This can be attributed to a slightly reduced amount of free space 29 for the Ba ions in the small cages whose diameter has shrunk by 0.81% (0.89%) by the introduction of the smaller ion Ce (La). Surprisingly, the atomic displacement parameter at the 2a crystal position shows a relative increase in Ce-BAS (La-BAS) compared to values for BAS. This shows that the "rattle" amplitude of the Ce (La) ions in the small cage is enhanced compared to that of Ba at the same location in BAS. Thus, " rattle " a feature of Ce-BAS (La-BAS) as well as of rare earth-free clathrates. In the following, the inventors argue that this has profound effects on the Kondo interaction.
One of the simplest models for the interaction of localized electrons with local optical phonon modes is the Anderson-Holstein model30. It has been shown that the Kondo energy scale can be greatly increased by strong electron correlation through the electron-phonon interaction3. &Quot; rattling " in clathrates is a thermally activated process. Therefore, local phonon modes are occupied only at elevated temperatures. This can lead to a strong renormalization of the Kondo energy scale with rising temperatures. In the following, the inventors argue that the temperature dependence of the magnetic susceptibility supports this image.
The paramagnetic contribution to the magnetic susceptibility of Ce-BAS (" method ") is shown as χΤ over T in Fig. 4d. A pure Curie law corresponds to a constant behavior in this graph. There are two areas with weak temperature dependence, one up to about 6 K (plateau 1) and a second between about 120 and 400 K (plateau 2). Qualitatively, the transition between plateau 1 and 2 is naturally assigned to the thermal population of the upper crystal field levels. However, the susceptibility of plateau 2 is significantly reduced with respect to the value expected for local moments (free Ce + 3 ions would yield χΤ ~ 0.81 cm3 K mol'1). In addition, the increase in susceptibility above plateau 2 does not fit into the crystal field level scheme for cubic point symmetry. Instead, the suppression of susceptibility with respect to the local moment limit observed at the highest temperatures of 700 K reached here can be attributed to a Kondo temperature of several hundred Kelvin.
This gives the following picture. The " rattle " modes of the Ce ions trapped in the cages of the clathrates Ba6,9iCei, o6Au5,56Si4o, 47 couple with the conduction electrons on the framework, effectively reducing the Coulomb repulsion U and reducing the condoms Temperature of the system is increased by several orders of magnitude. This results in both an early suppression of magnetic susceptibility with respect to free-receptacle susceptibility and a 50% increase in thermo-power (and 100% increase in ZT at 600 K when κ · (500 K) = / Ci (480 K) is assumed) over the value of the rare earth-free reference material. The inventors expect that these findings will open a new research path in thermoelectrics. In addition, they will likely lead to further research on related phenomena, such as the magnetically robust31,32 and electrically dipolar Kondo behavior33, or thermo-force enhancements in the Anderson model with negative U (Ref. 34).
EXAMPLES
method
Synthesis, structural and analytical characterization. Polycrystalline starting material was synthesized by melting high purity Ba, Ce, Au and Si in a cold copper boat using high frequency heating. The resulting samples were multiphase, with the clathrate I phase being the major phase. Long tempering did not change the phase proportions. The amount of Ce detected by EDX in the clathrate phase was lower than in the starting material at about 1.2 at.% (0.65 Ce per formula unit).
Pure phase Ce-containing clathrates (Ce-BAS) were finally obtained by nonstoichiometric crystal growth by the crucible-free zone method using optical heating in a four-mirror oven (Crystal Corporation). The obtained clathrate sample with a size of about 5x4x3 mm3 consisted of a few single crystals and was phase pure according to XRD (diffractometer Siemens D5000, Fig. 1b) and scanning electron microscopy (Philips XL30 ESEM, Fig. 1c). High-resolution transmission electron microscopy (FEI TECNAI F20) demonstrated the absence of nano-inclusion (Figure 1d). All other new clathrates were prepared by an analogous method.
Rietveld refinements of powder X-ray diffractograms of powdered single crystal samples gave the lattice parameters a = 1.0395 (2) nm and a = 1.0392 (2) nm for Ca-BAS and La-BAS, respectively. These are significantly smaller than published values for Ba8AuxSi4o-x clathrates with a similar Au content20'25'35. Since the ionic radii of Ce and La are smaller than those of Ba, this is further evidence for the incorporation of the rare earth atoms into the clathrate phase. In addition, EDX and WDX analyzes (EDX: EDAX New XI-39 135-10 UTWU Detector, WDX: Microspec WDX-600) with a single-phase BasAusSi / trProbe as standard demonstrate the presence of Ce and La in the samples. The mean crystal compositions measured by EDX were:
Ba6, 91 ± 0,17Cei, 06 ± 0,12AU5t56 ± 0,25Si40,47 ± 0,43 and Ba6,99 ± 0,17Laii23 ± 0,12AU5igi + o, 25Si39i87 ± 0,43 ·
Measurement of physical properties. The magnetic properties below room temperature were measured in a Superconducting Quantum Interference Unit (SQUID) (Cryogenics S7000X) after cooling the samples in an applied field of 0T. Above room temperature, susceptibility was measured using the vibrating magnetometer option of a Physical Property Measurement System (PPMS, Quantum Design). The specific heat below 2 K was measured by a relaxation-type method using a self-built device in a 3He / 4He mixed cryostat. The resistivity and the Hall effect were measured using standard 4-point and 6-point methods, with alternating direct current in a PPMS. Below room temperature, the thermoelectric power was measured using a dual heating device in a home-built cryostat. Over 300 K, the thermo-force was measured simultaneously with the resistivity using a standard stationary 4-point DC method (ZEM 3, Ulvac-Riko). The thermal conductivity was determined from thermal diffusivity measurements using a conventional laser flash experiment (Anter, Flashline 3000FT-S2) and from specific heat data.
Diamagnetic susceptibility shares. In order to correct the magnetic susceptibility of Ce-BAS to diamagnetic contributions, the inventors used the 4f-free reference material La-BAS, which should have very similar diamagnetic shares. Its susceptibility has the following form:
where xd, cs and xd, ring are due to diamagnetic shares of fully occupied shell or on Ringströme35. Xp, imp is a paramagnetic portion due to a small amount of magnetic impurities; this is only at the lowest
Temperatures relevant. xP, pauii is the paramagnetic Pauli portion of the conduction electrons. The factor 2/3 takes into account the Landau diamagnetism. The inventors determine χρ, pauii and xP, imp of La-BAS from the Hall effect measurements and an adaptation to a Curie-Weiss law at low temperature. You then subtract these two terms from the measured χ (Τ) data from La-BAS. The result is identified as the total diamagnetic portion xd of both La-BAS and Ce-BAS.
Thus, by the process of the present invention, it was possible for the first time to produce type I pure clathrates in the form of single crystals containing Ce or La as guest atoms. These compounds have promising properties, particularly high thermo-power, wherein that of the cerium-containing clathrate exceeds that of a Ce-free reference compound by 50%.
Also lanthanum-containing clathrates are characterized by a compared to previously produced Ba-Au-Si clathrates greatly increased thermoelectric power. The high thermoelectric power is thereby achieved a particularly low charge carrier concentration.
In Fig. 3, the different cause for the increased thermal forces of cerium and lanthanum-containing clathrates is illustrated. At a temperature of 300 K, the thermo-force (S) of the cerium-containing compound (Ce-BAS, n-type semiconductor, negative S) is 50% higher (more negative) than that of the hypothetical cerium-free reference compound (BAS) Carrier concentration (n) (see S-value of the solid line vertically above the data point of Ce-BAS). The thermo-power of the La-containing compound (La-BAS, p-type semiconductor, positive S) is even 100% higher than the highest value of all Cer- and La-free samples (BAS, all small data points).
All new clathrates were analyzed by the same analytical methods as the two above compounds La-BAS and Ce-BAS to determine the content of rare earth elements in the clathrate phase. In addition, the mechanics of clathrate phase fatigue were elucidated with rare earth elements. Mean mean values measured by EDX were:
Vergteiehsheispiet 1:
Comparative Example 2:
Initial studies of the novel catalases of Examples 3 to 13 yielded results quite comparable to those of Examples 1 and 2, and therefore, in analogy to LaββA and Ce-BAS, improved thermoelectricity for all the new lanthanoid-containing clathrates of the present invention Properties compared to corresponding Referenzmaierialien be expected without Lanfhanoid.
The work leading to this invention has been carried out by the European Research Council under the Seventh Framework Program of the European Community (FP7 / 2Ö07-2013). according to the ERC Financial Agreement No 227378 and the Fund for the Promotion of Scientific Research (FWF) in Austria (project P19458-N16),
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Inorg. Chem. 50, 10387-10396 (2011). 19. Schäfer, H. On the problem of polar intermetallic compounds: The stimulation of E. Zintl's work for the modem chemistry of intermetallics. Ann. Rev. Mater. ScL 15, 1-42 (1985). 20. Aydemir, U. et al. Low-temperature thermoelectric, galvanomagnetic and thermodynamic properties of the type I clatharate Ba, AuxSi * _ *. Php Rev. B 84,195,137 (2011). 21. Fert, A. & levy, P.M. Theory of the Hall effect in heavy-fermion compounds. Php Rev. B 36, 1907-1916 (1987). 22. Nguyen, L T.K. et al. Atomic ordering and thermoelectric properties of the n-type clathrates Ba, Ni, .jGe4i., De.4. Dalton Trans. 39, 1071-1077 (2010). 23. Nakatsuji, S. et al. Intersite coupling effects in a Kondo lattice. Php Rev. Lett. 89, 106402 (2002). 24. Candolfi, C. et al. High temperature thermoelectric properties of the type 1 clathrate Ba »AuxSi« _x. /. Appl phys. Ill, 043706 (2012). 25. Zeiringer, I. et al. The ternary system Au-Ba-Si: Clathrate solution, electronic structure, physical properties, phase eqilibria and crystal structures. Acta Mater. 60,2324-2336 (2012). 26. Mel'nikov, V.I. Thermodynamics of the Kondo problem. JETP Lett. 35, 511-515 (1982). 27. Present, P "Si, Q. & Stegiich, F. Quantum criticality in heavy-fermion metals. Nature Php. 4,186-197 (2008). June 28, A "Jarlborg, T. & Muller, J. Heat-capacity analysis of a large number of A15-type compounds. Php. Rev. B 27, 1568-1585 (1983). 29. Suekuni, K., Avila, M.A., Umeo, K. & Takabatake, T. Cage-size control of guest vibration and thermal conductivity in Sr * Ga16Si »_, Ge ,. Phys. Rev. B 75, 195210 (2007). 30. Hewson, A.C. & Meyer, D. Numerical renormalization group study of the Otheron-Holstein impurity model. J. Php. Condens. Matter 10, 196401 (2002). 31. Sanada, S. et al. Exotic heavy-fermion state in filled skutterudite SmOs4Sbl2. /. Php. Soc fpn 74, 246-249 (2005). 32. Costi, T.A. & Zlatic, V. Batch Kondo anomalies in PbTe doped with T1 impurities. Php. Rev. Lett. 108.036402 (2012). 33. Hotta, T. & Ueda, K. Electric dipolar Kondo effect emerging from a vibrating magnetic ion. Php. Rev. Lett 108,247214 (2012). 34. Andergassen, S., Costi, T.A. & Zlatid, V. Mechanism for large thermoelectric power in negative-U molecular quantum dots. Php. Rev. B 84, 241107 (R) (2011). 35. Hermann, R.F.W. Tanigaki, K., Kawaguchi, T., Kuroshima, p. 8c Zhou, O. Electronic structure of Si and Ge gold-doped dathrates. Php. Rev, B 60, 13245-13245 (1999).

权利要求:
Claims (12)
[1]
PATENT CLAIMS 1. Type I intermetallic clathrate having the following general formula:

wherein: RE is one or more rare earth elements selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu; T is one or more d-transition metals; IV represents one or more elements of the 4th main group of the Periodic Table, selected from Si, Ge, Sn and Pb; x is a number for which 0 < x <8th; and y is a number for which 0 < y < 6th
[2]
2. Intermetallic clathrate according to claim 1, characterized in that the following applies: x * 0.1 to 1.5.
[3]
3. Intermetallic clathrate according to claim 1 or 2, characterized in that: y «5 to 6.
[4]
4. Intermetallic clathrate according to one of claims 1 to 3, characterized in that RE is La, Ce, Pr, Sm, Gd, Eu, Tm, Yb or Lu, preferably La or Ce.
[5]
5. Intermetallic clathrate according to one of claims 1 to 4, characterized in that T is Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Pd, Ir, Pt or Au, preferably Au or Pt, stands.
[6]
6. Intermetallic clathrate according to one of claims 1 to 5, characterized in that IV is Si or Ge.
[7]
7. Intermetallic clathrate according to one of claims 1 to 6, characterized in that it has one of the following average compositions:

[8]
8. Use of an intermetallic clathrate according to any one of claims 1 to 7 in thermoelectric modules.
[9]
9. Use according to claim 8, characterized in that a clathrate according to claim 7 is used.
[10]
10. A process for the preparation of intermetallic Clathratverbindungen by slow solidification of the respective clathrate from a melt of the gas atoms and caged atoms forming elements, characterized in that in a crucible zone melting process, the constituent elements are melted in non-stoichiometric ratios and are solidified by zonal migration to single crystals ,
[11]
11. The method according to claim 10, characterized in that a clathrate type I is produced.
[12]
12. The method according to claim 10 or 11, characterized in that a clathrate according to any one of claims 1 to 7 is prepared. Vienna, July 04, 2014

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引用文献:

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JP2723099B2|1995-12-28|1998-03-09|日本電気株式会社|Clathrate compound and method for producing the same|

US6188011B1|1998-01-20|2001-02-13|Marlow Industries, Inc.|Thermoelectric materials fabricated from clathrate compounds and other materials which form an inclusion complex and method for optimizing selected thermoelectric properties|

EP1074512B1|1999-08-03|2017-02-15|IHI Corporation|Clathrate compounds, manufacture thereof, and thermoelectric materials, thermoelectric modules, semiconductor materials and hard materials based thereon|

JP2002064227A|2000-08-18|2002-02-28|Sumitomo Special Metals Co Ltd|Thermoelectric conversion material and its manufacturing method|

JP2006057124A|2004-08-18|2006-03-02|Yamaguchi Univ|Clathrate compound and thermoelectric conversion element using the same|

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DE202008006946U1|2008-05-21|2008-10-16|Technische Universität Wien|Article having a coating of a clathrate compound|

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CN101393959B|2008-11-07|2012-04-11|中国科学院上海硅酸盐研究所|Cage type compound|US10811584B2|2017-01-19|2020-10-20|Mitsubishi Gas Chemical Company, Inc.|Semiconductor crystal and power generation method|

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ATA728/2013A|AT514908A1|2013-09-20|2013-09-20|Intermetallic clathrate compounds|

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