Chalcogen-containing compound, its preparation method and thermoelectric element comprising the same

Abstract

A chalcogen-containing compound that exhibits low thermal conductivity and excellent thermoelectric properties, and exhibits excellent phase stability even at relatively low temperature, a method for preparing the same, and a thermoelectric element including the same.

Claims

1. A chalcogen-containing compound represented by the following Chemical Formula 1:
M.sub.xSn.sub.yBi.sub.zSe.sub.7  [Chemical Formula 1] wherein, in Chemical Formula 1, M is an alkali metal, x, y and z are mole ratios of M, Sn, and Bi respectively, x is greater than 0.05 and equal to or less than 0.8, y is greater than 3.5 and equal to or less than 4, z is greater than 1.5 and equal to or less than 2, y+z is greater than 5 and equal to or less than 6, and x+y+z is greater than 5 and equal to or less than 6.8.

2. The chalcogen-containing compound according to claim 1, wherein M is one or more alkali metals selected from the group consisting of Na, and K.

3. The chalcogen-containing compound according to claim 1, wherein the compound has a crystal structure of a face-centered cubic lattice structure.

4. The chalcogen-containing compound according to claim 3, wherein M fills a part of a vacancy excluding the sites filled with Se, Sn and Bi in the face-centered cubic lattice structure.

5. The chalcogen-containing compound according to claim 3, wherein the Se fills anion sites of the face-centered cubic lattice structure, the Sn and Bi fill cation sites of the face-centered cubic lattice structure, and the M fills a part of a vacancy of remaining cation sites, excluding the sites filled with Sn and Bi.

6. The chalcogen-containing compound according to claim 1, wherein the compound is suitable for use as a thermoelectric conversion material.

7. A method for preparing the chalcogen-containing compound of claim 1, comprising the steps of: progressing a solid phase reaction of a mixture comprising raw materials comprising Sn, Bi and Se, and raw material comprising alkali metal; grinding the product of the solid phase reaction; and sintering the ground product.

8. The method for preparing a chalcogen-containing compound according to claim 7, wherein the raw material comprising alkali metal comprises M.sub.2Se powder, wherein M is an alkali metal.

9. The method for preparing a chalcogen-containing compound according to claim 7, wherein the raw materials comprising Sn, Bi and Se are each a powder and the raw material comprising alkali metal is a powder, and wherein the solid phase reaction is performed at a temperature of 500 to 700° C., for each powder raw material.

10. The method for preparing a chalcogen-containing compound according to claim 7, further comprising a step of cooling a product of the solid phase reaction to form an ingot, between the solid phase reaction step and the grinding step.

11. The method for preparing a chalcogen-containing compound according to claim 7, wherein the sintering step is performed by a spark plasma sintering method.

12. The method for preparing a chalcogen-containing compound according to claim 7, wherein the sintering step is performed at a temperature of 550° C. or more and a pressure of 10 MPa or more.

13. A thermoelectric element comprising the chalcogen-containing compound according to claim 1, as a thermoelectric conversion material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic diagram showing the face-centered cubic lattice structure that is exhibited by sodium chloride, etc.

(2) FIG. 2 is a phase stability diagram of representative Sn—Bi—Se-based chalcogen compounds.

(3) FIG. 3 is a graph showing X-ray diffraction analysis results of a chalcogen compound powder immediately before passing through a sintering process in Examples 1 to 3 and Comparative Example 1.

(4) FIG. 4 is a graph showing X-ray diffraction analysis results of the sintered bodies finally prepared through a sintering process, after slow cooling and leaving at room temperature, in Examples 1 to 3 and Comparative Example 1.

(5) FIG. 5a is a graph showing results of measuring electrical conductivities of chalcogen compounds of Examples 1 to 3 according to temperature.

(6) FIG. 5b is a graph showing results of measuring Seebeck coefficients of the chalcogen compounds of Examples 1 to 3 according to temperature.

(7) FIG. 5c is a graph showing results of measuring output factors of the chalcogen compounds of Examples 1 to 3 according to temperature.

(8) FIG. 5d is a graph showing results of measuring thermal conductivities of the chalcogen compounds of Examples 1 to 3 according to temperature.

(9) FIG. 5e is a graph showing results of measuring lattice thermal conductivities of the chalcogen compounds of Examples 1 to 3 according to temperature.

(10) FIG. 5f is a graph showing results of measuring thermoelectric figures of merit of the chalcogen compounds of Examples 1 to 3 according to temperature.

EXAMPLES

(11) The present invention will be explained in more detail in the following examples. However, these examples are presented only as illustrations of the present invention, and the scope of the present invention is not limited thereby.

Example 1: Preparation of a Chalcogen-Containing Compound (Na.SUB.0.2.Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(12) Each powder of high purity raw materials of Sn, Bi, Se and Na.sub.2Se was weighed in a glove box at a mole ratio of 4:2:6.9 (7−0.1):0.1, put in a carbon crucible, and then charged into a quartz tube. The inside of the quartz tube was evacuated and sealed. The raw materials were maintained at a constant temperature in an electric furnace at 620° C. for 24 hours.

(13) Thereafter, the quartz tube in which a reaction was performed was cooled with water to obtain an ingot, the ingot was finely ground to powder with a particle diameter of 75 μm or less, and sintered by spark plasma sintering (SPS) at a temperature of 620° C. and a pressure of 50 MPa for 10 minutes, thus preparing a chalcogen-containing compound of Na.sub.0.2Sn.sub.4Bi.sub.2Se.sub.7.

Example 2: Preparation of a Chalcogen-Containing Compound (Na.SUB.0.4.Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(14) A chalcogen-containing compound of Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Sn, Bi, Se and Na.sub.2Se was mixed in a glove box at a mole ratio of 4:2:6.8 (7−0.2):0.2.

Example 2: Preparation of a Chalcogen-Containing Compound (Na.SUB.0.75.Sn.SUB.4.Bi.SUB.1.7.Se.SUB.7.)

(15) A chalcogen-containing compound of Na.sub.0.75Sn.sub.4Bi.sub.1.7Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Sn, Bi, Se and Na.sub.2Se was mixed in a glove box at a mole ratio of 4:1.7:6.625 (7−0.375):0.375.

Comparative Example 1: Preparation of a Chalcogen-Containing Compound (Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(16) A chalcogen-containing compound of Sn.sub.4Bi.sub.2Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Sn, Bi and Se was mixed in a glove box at a mole ratio of 4:2:7.

Experimental Examples

(17) 1. Phase Analysis According to XRD Pattern

(18) For the powder chalcogen-containing compounds immediately before the sintering process in Examples 1 to 3 and Comparative Example 1, X-ray diffraction analysis was performed and the results are shown in FIG. 3. And, the sintered bodies finally prepared through the sintering processes in Examples 1 to 3 and Comparative Example 1 were slowly cooled from about 620° C. to 300° C., and then cooled again to room temperature (25° C.), and the sintered bodies were maintained under an air atmosphere for 15 days, and then each sintered body was subjected to X-ray diffraction analysis, and the results are shown in FIG. 4.

(19) And, using the TOPAS program, the lattice parameter of each powder chalcogen-containing compound of Examples 1 to 3 and Comparative Example 1 was calculated and is shown in the following Table 1.

(20) TABLE-US-00001 TABLE 1 Powder material Lattice parameter (Å) Example 1 5.9523 Example 2 5.9665 Example 3 5.9642 Comparative Example 5.9448

(21) In addition, for the powder chalcogen-containing compounds of Examples 1 to 3 and Comparative Example 1, the number of atoms for each component in the unit lattice is shown in the following Table 2.

(22) TABLE-US-00002 TABLE 2 Number of atoms for each Powder component in unit lattice material Sn Bi Se Vacancy Na Example1 2.2853 1.1426 4 0.458 0.114 Example12 2.2853 1.1426 4 0.344 0.228 Example13 2.2853 1 4 0.286 0.4287 Comparative 2.2853 1.1426 4 0.572 0 Example1

(23) First, referring to FIG. 3, it was confirmed that all the powder chalcogen-containing compounds of Examples 1 to 3 have crystal lattice structures identical to the chalcogen-containing compound of Comparative Example 1 (i.e., Sn.sub.4Bi.sub.2Se.sub.7 which was previously known to have a face-centered cubic lattice structure at a high temperature), and thus it was confirmed that all the compounds of Examples 1 to 3 have a crystal lattice structure of a face-centered cubic lattice structure.

(24) And, referring to Table 1, in Examples 1 to 3, with the addition of alkali metal (Na), a lattice parameter increased compared to Comparative Example 1, and thus, it is inferred that the alkali metal fills a vacancy of the crystal lattice structure. Particularly, comparison of Examples 1 and 2 shows that a lattice parameter increases as the content of alkali metal increases, but Example 3 with relatively small Bi rate exhibits deceased lattice parameter compared to Example 2, and thus, it is confirmed that the alkali metal fills a vacancy of the crystal lattice structure, but since the atomic radius of the alkali metal (Na) is smaller than that of Bi, Example 3 exhibits decreased lattice parameter.

(25) And, referring to Table 2, it is expected that in Examples 1 to 3 including an alkali metal, the concentration of the vacancy of the lattice structure decreases compared to Comparative Example 1, and that in Examples 1 and 2, the concentration of electrons increases due to electron donation by the alkali metal. Meanwhile, it is expected that in the case of Example 3 with decreased Bi content, the concentration of electrons decreases (the concentration of holes increases) because Na.sup.+ donates two less electrons compared to Bi.sup.3+.

(26) Referring to FIG. 4, it is confirmed that the compound of Comparative Example 1 exhibits inferior phase stability when left at a relatively low temperature, and thus, the decomposition of the chalcogen-containing compound of Sn.sub.4Bi.sub.2Se.sub.7 is generated, thus forming many secondary phases (Bi.sub.3Se.sub.4, Bi.sub.8Se.sub.9, Sn.sub.3Bi.sub.9Se.sub.13) (The peaks resulting from the many secondary phases are confirmed in XRD pattern). To the contrary, it is confirmed that the compounds of Examples 1 to 3 maintain face-centered cubic lattice structures without generation of secondary phases, and exhibit excellent phase stability. Thus, it is confirmed that the compounds of Examples 1 to 3 exhibit excellent phase stability even at a relatively low temperature.

(27) 2. Temperature Dependency of Electrical Conductivity

(28) For the chalcogen-containing compound specimens prepared in Examples 1 to 3, electrical conductivities were measured according to temperature change and are shown in FIG. 5a. The electrical conductivity was measured at a temperature range of 100 to 300° C. through a four-probe direct current method, using a resistivity meter of LSR-3 from Linseis Inc.

(29) Referring to FIG. 5a, it was confirmed that Examples 1 to 3 generally exhibit excellent electrical conductivities, and particularly, it was confirmed that electrical conductivities is in the order of Example 2>Example 1>Example 3. It is expected that Example 2 with higher alkali metal content has excellent electric conductivity because the alkali metal is cationized and can donate electrons, and that Example 3 exhibits relatively low electric conductivity because the content of Bi that is cationized into Bi.sup.3+ and can donate more electrons is low,

(30) 3. Measurement of Seebeck Coefficient and Temperature Dependency of Seebeck Coefficient

(31) For the chalcogen-containing compound specimens prepared in Examples 1 to 3, Seebeck coefficients (S) were measured according to temperature change and are shown in FIG. 5b. The Seebeck coefficient was measured at a temperature range of 100 to 300° C. by differential voltage/temperature technique, using measuring equipment of LSR-3 from Linseis Inc.

(32) As shown in FIG. 5b, it is confirmed that Examples 1 to 3 exhibit a positive or negative Seebeck coefficient that can be applied as thermoelectric conversion material.

(33) However, comparing Examples 1 and 2, it is confirmed that as the content of an alkali metal increases from 0.2 to 0.4, the Seebeck coefficient changes from a positive (+) to a negative (−) value. It means that the main charge carrier of material is changed from holes to electrons due to the additional electron donation by the alkali metal, and indicates property change from P-type to N-type semiconductor material. It means that the main charge carrier in Example 1 is hole despite the additional electron donation by the alkali metal. As in Example 3, if the content of Bi decreases and the content of an alkali metal increases, the alkali metal donates less electrons compared to the Bi decrease, thus exhibiting P-type material property like Example 1.

(34) 4. Temperature Dependency of Output Factor

(35) For the chalcogen-containing compound specimens prepared in Examples 1 to 3, output factors were calculated according to temperature change and are shown in FIG. 5c.

(36) The output factor is defined as power factor (PF)=σS.sup.2, and it was calculated using the values of σ (electrical conductivity) and S (Seebeck coefficient) shown in FIGS. 5a and 5b.

(37) As shown in FIG. 5c, it was confirmed that Examples 1 to 3 generally exhibit excellent output factors, and among them, the specimen of Example 1 exhibits most excellent output factor. Particularly, the output factor measured at 100° C. was about 3.62 μW/cmK.sup.2.

(38) 5. Temperature Dependency of Thermal Conductivity and Lattice Thermal Conductivity

(39) For the chalcogen-containing compound specimens prepared in Examples 1 to 3, thermal conductivities and lattice thermal conductivities were measured according to temperature change and are shown in FIG. 5d and FIG. 5e, respectively. For the measurement of thermal conductivity, first, thermal diffusivity (D) and heat capacity (C.sub.p) were measured by a laser flash method, using thermal conductivity measuring equipment of LFA457 from Netzsch Company. The measured values were applied in the equation “thermal conductivity (k)=DρC.sub.p (ρ is the density of a sample measured by Archimedes' principle)”.

(40) Further, total thermal conductivity (k=k.sub.L+k.sub.E) is divided into lattice thermal conductivity (k.sub.L) and thermal conductivity (k.sub.E) calculated according to Wiedemann-Franz law (k.sub.E=LσT), wherein, as the Lorentz number (L), a value calculated from the Seebeck coefficient according to temperature was used.

(41) Referring to FIGS. 5d and 5e, Examples 1 to 3 generally exhibit low thermal conductivities and lattice thermal conductivities. Particularly, despite k.sub.E contributing to high electric conductivity, the specimen of Example 1 exhibits the lowest thermal conductivity because the lattice thermal conductivity is relatively the lowest as shown in FIG. 5e. The lattice thermal conductivity decreases as the concentration of the vacancy in the lattice increases, as shown in Table 2. Particularly, at 200° C., the specimen of Example 1 exhibited very low lattice thermal conductivity (0.51 W/mK).

(42) 6. Temperature Dependency of Thermoelectric Figure of Merit (ZT)

(43) For the chalcogen-containing compound specimens prepared in Examples 1 to 3, the thermoelectric figures of merit were calculated according to temperature change and are shown in FIG. 5f. The thermoelectric figure of merit is defined as ZT=S.sup.2σT/k, and it was calculated using the S (Seebeck coefficient), σ (electrical conductivity), T (absolute temperature), and k (thermal conductivity) values obtained in experimental examples.

(44) Referring to FIG. 5f, it was confirmed that Examples 1 to 3 exhibit excellent thermoelectric figures of merit that can be applied as a thermoelectric conversion material. Specifically, as the temperature rises, the ZT value increased until 200° C., and thereafter, tended to decrease. The specimen of Example 1 exhibited most excellent thermoelectric figure of merit (ZT=0.24) at 200° C.