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

Abstract

The present invention relates to a novel chalcogen-containing compound that exhibits excellent phase stability even at a temperature corresponding to the driving temperature of a thermoelectric element, and has a high output factor and thermoelectric figure of merit, 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:
V.sub.xM.sub.yPb.sub.zSn.sub.4-zBi.sup.2Se.sub.7  [Chemical Formula 1] wherein, in Chemical Formula 1, V is a vacancy, M is an alkali metal, x, y, z, and 4−z are mole ratios of V, M, Pb, and, Sn, respectively, x is greater than 0 and less than 1, y is greater than 0 and less than 1, x+y is greater than 0 and equal to or less than 1, z is greater than 0 and equal to or less than 4, and x+y+z is greater than 0 and equal to or less than 5.

2. The chalcogen-containing compound according to claim 1, wherein M is one or more alkali metals selected from the group consisting of Li, 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 the vacancy (V) is a vacant site excluding the sites filled with Se, Sn, Pb and Bi in the face-centered cubic lattice structure, and M is filled in a part of the vacancy (V).

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

6. The chalcogen-containing compound according to claim 3, wherein the Pb is substituted at the Sn site, in the face-centered cubic lattice structure.

7. The chalcogen-containing compound according to claim 1, wherein the compound is used as thermoelectric conversion material.

8. A method for preparing the chalcogen-containing compound of claim comprising the steps of: melting a mixture comprising raw materials of Sn, Pb, Bi, Se, and an alkali metal (M); heat treating the molten mixture; grinding the heat treated product; and sintering the ground product.

9. The method for preparing a chalcogen-containing compound according to claim 8, wherein the melting is performed at a temperature of 750 to 900° C.

10. The method for preparing a chalcogen-containing compound according to claim 8, wherein the heat treatment is performed at a temperature of 500 to 650° C.

11. The method for preparing a chalcogen-containing compound according to claim 8, further comprising a step of cooling the product of the heat treatment step to form an ingot, between the heat treatment step and the grinding step.

12. The method for preparing a chalcogen-containing compound according to claim 8, wherein the sintering step is performed by a spark plasma sintering method.

13. The method for preparing a chalcogen-containing compound according to claim 8, wherein the sintering step is performed at a temperature of 550 to 700° C. and a pressure of 10 to 130 MPa.

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

Description

BRIEF DESCRIPTION OF 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 5 and Comparative Example 1.

(4) FIG. 4 is a graph showing X-ray diffraction analysis results of a chalcogen compound powder immediately before passing through a sintering process in Comparative Examples 2 to 4.

(5) FIG. 5 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 5 and Comparative Example 1.

(6) FIG. 6 is a graph showing results of measuring electrical conductivities of chalcogen compounds of Examples 1 to 5 and Comparative Example 1 according to temperature.

(7) FIG. 7 is a graph showing results of measuring Seebeck coefficients of the chalcogen compounds of Examples 1 to 5 and Comparative Example 1 according to temperature.

(8) FIG. 8 is a graph showing results of measuring output factors of the chalcogen compounds of Examples 1 to 5 and Comparative Example 1 according to temperature.

(9) FIG. 9 is a graph showing results of measuring total thermal conductivities of the chalcogen compounds of Examples 1 to 5 and Comparative Example 1 according to temperature.

(10) FIG. 10 is a graph showing results of calculating lattice thermal conductivities of the chalcogen compounds of Examples 1 to 5 and Comparative Example 1 according to temperature.

(11) FIG. 11 is a graph showing results of calculating thermoelectric figures of merit of the chalcogen compounds of Examples 1 to 5 and Comparative Example 1 according to temperature.

DESCRIPTION OF EMBODIMENTS

(12) 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 (V.SUB.0.8.Na.SUB.0.2.Pb.SUB.0.05.Sn.SUB.3.95.Bi.SUB.2.Se.SUB.7.)

(13) Each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.05:3.95:2:7, 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 750° C. for 24 hours, and then slowly cooled at room temperature.

(14) Thereafter, they were heat treated at 640° C. for 48 hours, 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 V.sub.0.8Na.sub.0.2Pb.sub.0.05Sn.sub.3.95Bi.sub.2Se.sub.7.

Example 2: Preparation of a Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Pb.SUB.0.1.Sn.SUB.3.9.Bi.SUB.2.Se.SUB.7.)

(15) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Pb.sub.0.1Sn.sub.3.9Bi.sub.2Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.1:3.9:2:7.

Example 3: Preparation of a Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Pb.SUB.0.2.Sn.SUB.3.8.Bi.SUB.2.Se.SUB.7.)

(16) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Pb.sub.0.2Sn.sub.3.8Bi.sub.2Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.2:3.8:2:7.

Example 4: Preparation of a Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Pb.SUB.0.4.Sn.SUB.3.6.Bi.SUB.2.Se.SUB.7.)

(17) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Pb.sub.0.4Sn.sub.3.6Bi.sub.2Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.4:3.6:2:7.

Example 5: Preparation of a Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Pb.SUB.0.8.Sn.SUB.3.2.Bi.SUB.2.Se.SUB.7.)

(18) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Pb.sub.0.8Sn.sub.3.2Bi.sub.2Se.sub.7 was prepared by the same method as Example 1, except that each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.8:3.2:2:7.

Comparative Example 1: Preparation of a Chalcogen-Containing Compound (V.SUB.0.8.Na.SUB.0.2.Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(19) A chalcogen-containing compound of V.sub.0.8Na.sub.0.2Sn.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 Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:4:2:7.

Comparative Example 2: Preparation of a Chalcogen-Containing Compound (NaPb.SUB.0.05.Sn.SUB.3.95.Bi.SUB.2.Se.SUB.7.)

(20) A chalcogen-containing compound was intended to be prepared by the same method as Example 1, except that each powders of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 1:0.05:3.95:2:7.

Comparative Example 3: Preparation of a Chalcogen-Containing Compound (Na.SUB.0.2.Pb.SUB.0.05.Sn.SUB.4.75.Bi.SUB.2.Se.SUB.7.)

(21) A chalcogen-containing compound was intended to be prepared by the same method as Example 1, except that each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.05:4.75:2:7.

Comparative Example 4: Preparation of a Chalcogen-Containing Compound (Na.SUB.0.2.Pb.SUB.0.05.Sn.SUB.3.95.Bi.SUB.2.8.Se.SUB.7.)

(22) A chalcogen-containing compound was intended to be prepared by the same method as Example 1, except that each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se was weighed in a glove box at a mole ratio of 0.2:0.05:3.95:2.8:7.

Experimental Examples

(23) 1. Phase Analysis According to XRD Pattern

(24) For the powder chalcogen-containing compounds immediately before the sintering process in Examples 1 to 5 and Comparative Example 1, X-ray diffraction analysis was performed and the results are shown in FIG. 3. In the same manner, for the powder chalcogen-containing compounds immediately before the sintering process in Comparative Examples 2 to 4, X-ray diffraction analysis was performed and the results are shown in FIG. 4. The sintered bodies finally prepared through the sintering processes in Examples 1 to 5 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. 5.

(25) First, referring to FIG. 3, it was confirmed that the compounds of Examples 1 to 5 and Comparative Example 1 have crystal lattice structures identical to 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 5 and Comparative Example 1 have a crystal lattice structure of a face-centered cubic lattice structure.

(26) However, referring to FIG. 4, it was confirmed that Comparative Examples 2 to 4 include various secondary phases such as SnSe, Bi.sub.3Se.sub.4, BiSnSe.sub.3, etc., in addition to the material having the same crystal structure as Sn.sub.4Bi.sub.2Se.sub.7. Specifically, since the vacancy is completely filled with Na in Comparative Example 2, the vacancy is completely filled with Sn in Comparative Example 3, and the vacancy is completely filled with Bi in Comparative Example 4, in Comparative Examples 2 to 4, the vacancy is completely filled, and a single phase of face-centered cubic lattice structure cannot be formed due to the increase in the content of Na, Sn, or Bi. Thus, it was confirmed that the relations wherein the mole ratio (x) of the vacancy represented in Chemical Formula 1 is greater than 0 and less than 1, the mole ratio (y) of Na is less than 1, the mole ratio of the sum of Sn and Pb is equal to or less than 4, and the mole ratio of Bi is equal to or less than 2 should be fulfilled, so that a chalcogen-containing compound of a single phase of face-centered cubic lattice structure including a vacancy may be formed.

(27) Furthermore, referring to FIG. 5, it was confirmed that the compounds of Examples 1 to 5 and Comparative Example 1, when left at a relatively low temperature, maintain face-centered cubic lattice structures without generation of secondary phases, and exhibit excellent phase stability. Thus, it was confirmed that the compounds of Examples 1 to 5 and Comparative Example 1 exhibit excellent phase stability even at a relatively low temperature.

(28) 2. Results Using TOPAS Program

(29) Using the TOPAS program, the lattice parameter of each powder chalcogen-containing compound of Examples 1 to 5 and Comparative Example 1 was calculated and is shown in the following Table 1. Further, the Rietveld refinement results of the chalcogen-containing compounds of Examples 1 to 5 and Comparative Example 1, calculated through the TOPAS program, are shown in the following Table 2.

(30) TABLE-US-00001 TABLE 1 Powder material Lattice parameter (Å) Example 1 5.9648 Example 2 5.9671 Example 3 5.9681 Example 4 5.9724 Example 5 5.9817 Comparative Example 1 5.9646

(31) TABLE-US-00002 TABLE 2 Comparative Unit (wt %) Example 1 Example 2 Example 3 Example 4 Example 5 Example 1 acancy (0, 0, 0) 0.1157 0.1161 0.1156 0.1166 0.1157 0.1171 occupancy Na (0, 0, 0) 0.0286 0.0286 0.0286 0.0286 0.0286 0.0286 occupancy Sn (0, 0, 0) 0.5643 0.5567 0.5428 0.5134 0.4571 0.57 occupancy Bi (0, 0, 0) 0.2843 0.2843 0.2843 0.2843 0.2843 0.2843 occupancy Pb (0, 0, 0) 0.0071 0.0143 0.0287 0.0571 0.1143 — occupancy Se (0.5, 0.5, 0.5) 1 1 1 1 1 1 occupancy Rwp (weighted 5.39 6.24 5.997 6.31 6.99 5.37 pattern R)

(32) Referring to Table 1, it was confirmed that as the content of Pb substituted at the Sn site increases in the face-centered cubic lattice structure, a lattice parameter gradually increases. That is, it was confirmed that a lattice parameter is in the order of Example 5>Example 4>Example 3>Example 2>Example 1>Comparative Example 1. Thus, it was confirmed that since Pb has a larger atomic radius than Sn, as the amount of Pb substitution increases, a lattice structure is fully filled and thus a lattice parameter increases.

(33) Meanwhile, referring to Table 2, it was confirmed that in the case of Examples 1 to 5 and Comparative Example 1 exhibiting single phases, vacancy, Na, Sn, Pb, and Bi are randomly distributed at an (x, y, z)=(0, 0, 0) site, and Se is located at an (x, y, z)=(0.5, 0.5, 0.5) site. It was also confirmed that each composition included in the chalcogen-containing compound is very similar to the composition of each powder of high purity raw materials of Na, Pb, Sn, Bi, and Se.

(34) 3. Temperature Dependency of Electrical Conductivity

(35) For the chalcogen-containing compound specimens prepared in Examples 1 to 5 and Comparative Example 1, electrical conductivities were measured according to temperature change and are shown in FIG. 6. The electrical conductivity was measured at a temperature range of 100 to 400° C. through a four-probe direct current method, using a resistivity meter of LSR-3 from Linseis Inc.

(36) Referring to FIG. 6, it was confirmed that Examples 1 to 5 exhibit higher electrical conductivities than Comparative Example 1. Particularly, it was confirmed that electrical conductivities is in the order of Example 5>Example 4>Example 3>Example 2>Example 1, and thus it was confirmed that as the content of Pb increases, electrical conductivity gradually increases. It was also confirmed that as a temperature increases, a tendency of an electrical conductivity increase is higher. It was confirmed that as the content of Pb substituted at the Sn site increases, a change in the electronic structure of the chalcogen compound is caused, and due to such a change in the electronic structure, electrical conductivity increases.

(37) 4. Measurement of Seebeck Coefficient and Temperature Dependency of Seebeck Coefficient

(38) For the chalcogen-containing compound specimens prepared in Examples 1 to 5 and Comparative Example 1, Seebeck coefficients (S) were measured according to temperature change and are shown in FIG. 7. The Seebeck coefficient was measured at a temperature range of 100 to 400° C. by differential voltage/temperature technique, using measuring equipment of LSR-3 from Linseis Inc.

(39) As shown in FIG. 7, it was confirmed that Examples 1 to 5 and Comparative Example 1 exhibit minus (−) Seebeck coefficients, and thus the major electric charge carriers of the materials are electrons, thus exhibiting the properties as an N-type semiconductor material.

(40) Meanwhile, it was confirmed that, although Examples 1 to 5 have higher electrical conductivities as explained above, Examples 1 to 5 generally have higher Seebeck coefficients than Comparative Example 1. Particularly, it was confirmed that as the content of Pb increases, the Seebeck coefficient tends to increase, and thus it was confirmed that as the content of Pb substituted at the Sn site increase, both electrical conductivity and Seebeck coefficient become high, and thus the electrical properties of material are excellent.

(41) 5. Temperature Dependency of Output Factor

(42) For the chalcogen-containing compound specimens prepared in Examples 1 to 5 and Comparative Example 1, output factors were calculated according to temperature change and are shown in FIG. 8.

(43) 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. 6 and 7.

(44) As shown in FIG. 8, it was confirmed that Examples 1 to 5 exhibit excellent output factors compared to Comparative Example 1, and that as the content of Pb substituted at the Sn site increases, the output factor increases. Particularly, the output factor of Example 5 measured at 400° C. was about 2.8 μW/cmK.sup.2, exhibiting a 206% increase rate compared to Comparative Example 1.

(45) 6. Temperature Dependency of Total Thermal Conductivity and Lattice Thermal Conductivity

(46) For the chalcogen-containing compound specimens prepared in Examples 1 to 5 and Comparative Example 1, total thermal conductivities and lattice thermal conductivities were measured according to temperature change and are shown in FIG. 9 and FIG. 10, 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 (p is the density of a sample measured by Archimedes' principle)”.

(47) 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.

(48) Referring to FIGS. 9 and 10, it was confirmed that Examples 1 to 5 generally exhibit low total thermal conductivities, and particularly, in the case of lattice thermal conductivity, all of Examples 1 to 5 exhibit low values compared to Comparative Example 1.

(49) The chalcogen-containing compounds of Examples 1, 2, 4, and 5, although having higher k.sub.E values, exhibit lower total thermal conductivities than Comparative Example 1, due to high electrical conductivity. Particularly, Example 5 exhibits the lowest total thermal conductivity, because the lattice thermal conductivity of Example 5 is relatively the lowest value, as shown in FIG. 10. As the content of Pb substituted at the Sn site increases, the lattice thermal conductivity more decreases due to phonon scattering, and particularly, the lattice thermal conductivity of Example 5 at 200° C. is as low as 0.45 W/mK.

(50) 7. Temperature Dependency of Thermoelectric Figure of Merit (ZT)

(51) For the chalcogen-containing compound specimens prepared in Examples 1 to 5 and Comparative Example 1, the thermoelectric figures of merit were calculated according to temperature change and are shown in FIG. 11. The thermoelectric figure of merit is defined as ZT=S.sup.2σT/k, and it was calculated using the S (Seebeck coefficient), G (electrical conductivity), T (absolute temperature), and k (thermal conductivity) values obtained in experimental examples.

(52) Referring to FIG. 11, it was confirmed that Examples 1 to 5 exhibit excellent thermoelectric figures of merit that can be applied as a thermoelectric conversion material. Particularly, as the content of Pb substituted at the Sn site increased, the ZT value increased, and the ZT value of Example 5 at 400° C. showed an increase rate of about 133% compared to the ZT value of Comparative Example 1 at the same temperature, and the average ZT value of Example 5 at 100˜400° C. showed an increase rate of 206% compared to the average ZT value of Comparative Example 1.