CHALCOGEN-CONTAINING COMPOUND, ITS PREPARATION METHOD AND THERMOELECTRIC ELEMENT COMPRISING THE SAME

Abstract

A chalcogen-containing compound of the following chemical formula which exhibits an excellent thermoelectric performance index (ZT) through an increase in power factor and a decrease in thermal conductivity, a method for preparing the same, and a thermoelectric element including the same: M.sub.yV.sub.1-ySn.sub.xSb.sub.2Te.sub.x+3, wherein V is vacancy, M is at least one alkali metal, x6, and 0<y0.4.

Claims

1. A chalcogen-containing compound represented by the following Chemical Formula 1:
M.sub.yV.sub.1-ySn.sub.xSb.sub.2Te.sub.x+3[Chemical Formula 1] wherein, in the above Chemical Formula 1, V is vacancy, M is at least one alkali metal, x6, and 0<y0.4.

2. The chalcogen-containing compound of claim 1, wherein the M is at least one alkali metal selected from the group consisting of Na and K.

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

4. The chalcogen-containing compound of claim 3, wherein the V is a vacant site excluding sites filled with Sn, Sb, and Te in the face-centered cubic lattice structure, and the M is filled in a part of the V.

5. The chalcogen-containing compound of claim 3, wherein the Te is filled in an anion site of the face-centered cubic lattice structure, the Sn and Sb are filled in a cationic site of the face-centered cubic lattice structure, the V is a vacant site of the remaining cationic sites excluding the sites filled with Sn and Sb, and the M is filled in a part of the V.

6. The chalcogen-containing compound of claim 3, wherein the V, Sn, Sb, and M are randomly distributed at a site of (x, y, z)=(0, 0, 0), and Te is distributed at a site of (x, y, z)=(0.5, 0.5, 0.5).

7. The chalcogen-containing compound of claim 1, wherein 6x12 and 0.01y0.4.

8. The chalcogen-containing compound of claim 1, which is selected from the group consisting of Na.sub.0.2V.sub.0.8Sn.sub.6Sb.sub.2Te.sub.9, Na.sub.0.2V.sub.0.8Sn.sub.8Sb.sub.2Te.sub.11, Na.sub.0.2V.sub.0.8Sn.sub.10Sb.sub.2Te.sub.13, Na.sub.0.2V.sub.0.8Sn.sub.12Sb.sub.2Te.sub.15, Na.sub.0.1V.sub.0.9Sn.sub.8Sb.sub.2Te.sub.11, and Na.sub.0.4V.sub.0.6Sn.sub.8Sb.sub.2Te.sub.11.

9. A method for preparing the chalcogen-containing compound of claim 1 comprising: mixing raw materials of Sn, Sb, Te, and M so that a molar ratio of Sn:Sb:Te:M is x:2:(x+3):y and then subjecting the mixture to a melting reaction wherein x6 and 0<y0.4, and M is at least one alkali metal; heat-treating the resultant product obtained through the melting reaction; pulverizing the resultant product obtained through the heat treatment; and sintering the pulverized product.

10. The method for preparing the chalcogen-containing compound of claim 9, wherein the melting is carried out at a temperature of 700 to 900 C.

11. The method for preparing the chalcogen-containing compound of claim 9, wherein the heat treatment is carried out at a temperature of 550 to 640 C.

12. The method for preparing the chalcogen-containing compound of claim 9, further comprising a step of cooling the resultant of the heat treatment step to form an ingot between the heat treatment step and the pulverization step.

13. The method for preparing the chalcogen-containing compound of claim 9, wherein the sintering is carried out by a spark plasma sintering method.

14. The method for preparing the chalcogen-containing compound of claim 9, wherein the sintering is carried out at a temperature of 550 to 640 C. under a pressure of 10 to 100 MPa.

15. A thermoelectric element comprising the chalcogen-containing compound of claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] FIG. 1 is a schematic view showing a face-centered cubic lattice structure.

[0064] FIG. 2 is a schematic diagram showing the crystal structure of the chalcogen-containing compound according to an embodiment of the present invention.

[0065] FIG. 3 is a graph showing the results of X-ray diffraction analysis of the chalcogen-containing compounds prepared in Examples 1 to 6 and Comparative Examples 1 to 4.

[0066] FIG. 4 is a graph showing the results of X-ray diffraction analysis of the chalcogen-containing compounds prepared in Comparative Examples 5 to 10.

[0067] FIG. 5 is a graph showing the results of measuring electrical conductivity (o) versus temperature (T) of the chalcogen-containing compounds in Examples 1 to 6 and Comparative Examples 1 to 4.

[0068] FIG. 6 is a graph showing the results of measuring the Seebeck coefficient (S) versus temperature (T) of the chalcogen-containing compounds in Examples 1 to 6 and Comparative Examples 1 to 4.

[0069] FIG. 7 is a graph showing the results of measuring the power factor (PF) versus temperature (T) of the chalcogen-containing compounds in Examples 1 to 6 and Comparative Examples 1 to 4.

[0070] FIG. 8 is a graph showing the results of measuring the total thermal conductivity (k.sub.tot) versus temperature (T) of the chalcogen-containing compounds in Examples 1 to 6 and Comparative Examples 1 to 4.

[0071] FIG. 9 is a graph showing the results of calculating the thermoelectric performance index (ZT) versus temperature (T) of the chalcogen-containing compounds in Examples 1 to 6 and Comparative Examples 1 to 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0072] Hereinafter, the present invention will be described in more detail by way of examples. However, these examples are given to merely illustrate the invention and are not intended to limit the scope of the invention thereto.

Example 1: Preparation of Na.SUB.0.2.V.SUB.0.8.Sn.SUB.6.Sb.SUB.2.Te.SUB.9

[0073] The respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were weighed at a molar ratio of 0.2:6:2:9 in a glove box and placed in a graphite crucible, and then charged into a quartz tube. The inside of the quartz tube was evacuated and sealed. Then, the raw materials were kept at a constant temperature in an electric furnace at 750 C. for 12 hours, and then slowly cooled to room temperature. Thereafter, heat treatment was carried out at a temperature of 640 C. for 48 hours. The quartz tube in which the reaction had progressed was cooled with water to obtain an ingot. The ingot was finely pulverized into a powder having a particle size of 75 m or less, and sintered according to a spark plasma sintering method (SPS) at a pressure of 50 MPa and a temperature of 600 C. for 8 minutes to prepare a chalcogen-containing compound of Na.sub.0.2V.sub.0.8Sn.sub.6Sb.sub.2Te.sub.9.

Example 2: Preparation of Na.SUB.0.2.V.SUB.0.8.Sn.SUB.8.Sb.SUB.2.Te.SUB.11

[0074] A chalcogen-containing compound of Na.sub.0.2V.sub.0.8Sn.sub.8Sb.sub.2Te.sub.11 was prepared in the same manner as in Example 1, except that the respective shots of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:8:2:11.

Example 3: Preparation of Na.SUB.0.2.V.SUB.0.8.Sn.SUB.10.Sb.SUB.2.Te.SUB.13

[0075] A chalcogen-containing compound of Na.sub.0.2V.sub.0.8Sn.sub.10Sb.sub.2Te.sub.13 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:10:2:13.

Example 4: Preparation of Na.SUB.0.2.V.SUB.0.8.Sn.SUB.12.Sb.SUB.2.Te.SUB.15

[0076] A chalcogen-containing compound of Na.sub.0.2V.sub.0.8Sn.sub.12Sb.sub.2Te.sub.15 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:12:2:15.

Example 5: Preparation of Na.SUB.0.1.V.SUB.0.9.Sn.SUB.8.Sb.SUB.2.Te.SUB.11

[0077] A chalcogen-containing compound of Na.sub.0.1V.sub.0.9Sn.sub.8Sb.sub.2Te.sub.11 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.1:8:2:11.

Example 6: Preparation of Na.SUB.0.4.V.SUB.0.6.Sn.SUB.8.Sb.SUB.2.Te.SUB.11

[0078] A chalcogen-containing compound of Na.sub.0.4V.sub.0.6Sn.sub.8Sb.sub.2Te.sub.11 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.4:8:2:11.

Comparative Example 1: Preparation of VSn.SUB.6.Sb.SUB.2.Te.SUB.9

[0079] A chalcogen-containing compound of VSn.sub.6Sb.sub.2Te.sub.9 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 6:2:9.

Comparative Example 2: Preparation of VSn.SUB.8.Sb.SUB.2.Te.SUB.11

[0080] A chalcogen-containing compound of VSn.sub.8Sb.sub.2Te.sub.11 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 8:2:11.

Comparative Example 3: Preparation of VSn.SUB.10.Sb.SUB.2.Te.SUB.13

[0081] A chalcogen-containing compound of VSn.sub.10Sb.sub.2Te.sub.13 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 10:2:13.

Comparative Example 4: Preparation of VSn.SUB.12.Sb.SUB.2.Te.SUB.15

[0082] A chalcogen-containing compound of VSn.sub.12Sb.sub.2Te.sub.15 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 12:2:15.

Comparative Example 5: Preparation of VSn.SUB.4.Sb.SUB.2.Te.SUB.7

[0083] A chalcogen-containing compound of VSn.sub.4Sb.sub.2Te.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 4:2:7.

Comparative Example 6: Preparation of Sn.SUB.10.Sb.SUB.2.Te.SUB.12

[0084] A chalcogen-containing compound of Sn.sub.10Sb.sub.2Te.sub.12 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 10:2:12.

Comparative Example 7: Preparation of Sn.SUB.12.Sb.SUB.2.Te.SUB.14

[0085] A chalcogen-containing compound of Sn.sub.12Sb.sub.2Te.sub.14 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 12:2:14.

Comparative Example 8: Preparation of NaSn.SUB.8.Sb.SUB.2.Te.SUB.11

[0086] A chalcogen-containing compound of NaSn.sub.8Sb.sub.2Te.sub.11 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 1:8:2:11.

Comparative Example 9: Preparation of Na.SUB.0.2.V.SUB.0.4.Sn.SUB.8.Sb.SUB.2.4.Te.SUB.11

[0087] A chalcogen-containing compound of Na.sub.0.2V.sub.0.4Sn.sub.8Sb.sub.2.4Te.sub.11 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:8:2.4:11.

Comparative Example 10: Preparation of Na.SUB.0.2.Sn.SUB.8.Sb.SUB.2.8.Te.SUB.11

[0088] A chalcogen-containing compound of Na.sub.0.2Sn.sub.8Sb.sub.2.8Te.sub.11 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Sb, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.2:8:2.8:11.

Experimental Example 1: Phase Analysis According to XRD Pattern

[0089] X-ray diffraction analysis was performed for the composites in Examples 1 to 6 and Comparative Examples 1 to 10, and the results are shown in FIGS. 3 and 4, respectively.

[0090] For an X-ray diffraction analysis, each sample of the chalcogen-containing compounds prepared in the examples and comparative examples was pulverized well, placed in a sample holder of an X-ray diffractometer (Bruker D8-Advance XRD), and then scanned at 0.02 degree intervals, wherein Cu K1 (=1.5405 ) X-ray radiation was used, the applied voltage was 40 KV, and the applied current was 40 mA.

[0091] First, referring to FIG. 3, in the case of the chalcogen-containing compounds of Examples 1 to 6 and Comparative Examples 1 to 4, it was confirmed that it has the same crystal lattice structures as that of SnTe, which was conventionally known to have a face-centered cubic lattice structure. From these results, it was confirmed that the compounds of Examples 1 to and Comparative Examples 1 to 4 all had a face-centered cubic crystal lattice structure.

[0092] Referring to FIG. 4, however, it was confirmed that in Comparative Examples 5 to 10, in addition to a material having a part of the same crystal structure as SnTe, a secondary phase of SnSb.sub.2Te.sub.4 having a rhombohedral structure was included together therewith. From these results, it can be seen that when the molar ratio (x) of Sn in the composition formula represented by Formula 1 is less than 6 (Comparative Example 5), when vacancies and an alkali metal (M) are not included and the molar ratio of Sn:Sb:Te does not satisfy the condition of x:2:(x+3)(Comparative Examples 6 and 7), when the vacancies are completely filled with Na (Comparative Example 8), when the alkali metal (M) and the vacancies are included but the condition of the molar ratio of Sn:Sb:Te is not satisfied (Comparative Example 9), and when, in the composition not including the vacancies, Na is included but the molar ratio condition of Sn:Sb:Te is not satisfied (Comparative Example 10), a secondary phase other than the face-centered cubic lattice structure is formed. Therefore, it can be seen that a chalcogen-containing compound having a single phase, face-centered cubic lattice structure can be formed when the molar ratio of Sn:Sb:Te satisfies the relationship of x:2:(x+3) while x is 6 or more and y is more than 0 and 0.4 or less.

Experimental Example 2: Analysis Using TOPAS Program

[0093] From the results of XRD analysis obtained from the above experiment, the lattice parameters were calculated for each of the chalcogen-containing compounds in powder states of Examples 1 to 6 and Comparative Examples 1 to 4 using the TOPAS program (R. W. Cheary, A. Coelho, J. Appl. Crystallogr. 25 (1992) 109-121; Bruker AXS, TOPAS 4.2, Karlsruhe, Germany (2009)), and the results are shown in Table 1 below. In addition, the Rietveld refinement results of the chalcogen-containing compounds of Examples 1 to 6 and Comparative Examples 1 to 4 calculated through the TOPAS program are shown in Table 2 below.

TABLE-US-00001 TABLE 1 Lattice parameter Calculated vacancy () concentration Comparative 6.2531 1/9 (0.1111) Example 1 Comparative 6.2650 1/11 (0.0909) Example 2 Comparative 6.2743 1/13 (0.0769) Example 3 Comparative 6.2807 1/15 (0.0 667) Example 4 Example 1 6.2637 0.8/9 (0.0889) Example 2 6.2705 0.8/11 (0.0727) Example 3 6.2801 0.8/13 (0.0615) Example 4 6.2874 0.8/15 (0.0533) Example 5 6.2665 0.9/11 (0.0818) Example 6 6.2838 0.6/11 (0.0545)

TABLE-US-00002 TABLE 2 Comparative Unit Example Example (atomic %) 1 2 3 4 1 2 3 4 5 6 Vacancy (0, 0.1111 0.0909 0.0769 0.0667 0.0889 0.0727 0.0615 0.0534 0.0818 0.0545 0, 0) occupancy Sn (0, 0, 0) 0.6667 0.7273 0.7692 0.8 0.6667 0.7273 0.7692 0.8 0.7273 0.7273 occupancy Sb (0, 0, 0) 0.2222 0.1818 0.1538 0.1333 0.2222 0.1818 0.1538 0.1333 0.1818 0.1818 occupancy Na (0, 0, 0) 0 0 0 0 0.0222 0.0182 0.0182 0.0133 0.0091 0.0364 occupancy Te (0.5, 1 1 1 1 1 1 1 1 1 1 0.5, 0.5) occupancy Rwp 5.89 4.83 5.98 5.40 5.79 4.99 5.46 4.59 6.83 5.87 (weighted pattern R)

[0094] Referring to Table 1, as the content (x) of Sn in the face-centered cubic lattice increased, the value of the lattice parameter gradually increased (Comparative Example 4>Comparative Example 3>Comparative Example 2>Comparative Example 1, Example 4>Example 3>Example 1). This means that because the radius of Sn.sup.2+ (118 pm) is larger than that of Sb.sup.3+ (76 pm), the lattice parameter increases as the content of Sn increases (as the [Sn]/[Sb] ratio increases). On the other hand, as the vacancy was filled with Na in the material having the same Sn.sub.8Sb.sub.2Te.sub.11 composition, the lattice parameter gradually increased (Example 6>Example 2>Example 5). Further, in the case of the examples in which Na was filled in vacancies, it can be confirmed that the lattice volume increased as the lattice parameter increased as compared with the comparative examples of similar composition. From this, it can be seen that Na was filled in the vacancy.

[0095] On the other hand, in the case of Comparative Examples 1 to 4, as the content of Sn increased, the vacancy content decreased in the lattice. In the case of Examples 1 to 4, by further filling with Na, the vacancy content was more decreased that in Comparative Examples 1 to 4. It can be seen that this is consistent with the Rietveld refinement results in Table 2. Referring to Table 2 above, it was confirmed that in the case of Examples 1 to 6, Na, vacancy, Sn, and Sb are randomly distributed at the site of (x, y, z)=(0, 0, 0) and Te is distributed at the site of (x, y, z)=(0.5, 0.5, 0.5). In addition, it was confirmed that even in the case of Comparative Examples 1 to 4, each atom is distributed at the same position except for Na, and each composition contained in the chalcogen-containing compounds is very similar to the initial contents of Na, Sn, Sb, and Te, which are high purity raw materials.

3. Evaluation of Temperature Dependence of Electrical Conductivity

[0096] For the chalcogen-containing compound samples prepared in Examples 1 to 6 and Comparative Examples 1 to 4, the electrical conductivity was measured according to the temperature change, and the results are shown in FIG. 5. The electrical conductivity was measured at a temperature range of 50 C. to 500 C. by a four-probe DC method using ZEM-3 (manufactured by ULVAC), which is a resistivity measuring device.

[0097] Referring to FIG. 5, in the case of Comparative Examples 1 to 4, the value of electrical conductivity increased as the content of Sn increased. This is because it supplies one electron less than Sb per Sn atom (Sn.sup.2+ vs. Sb.sup.3+ comparison), and therefore the number of supplied electrons decreases as the Sn content increases, and conversely, the hole concentration which is a main charge carrier of the material, increases. On the other hand, when comparing Examples 1 to 4 with Comparative Examples 1 to 4, it can be seen that the electrical conductivity was relatively decreased in the examples in which Na was added despite the same Sn/Sb ratio. Further, when comparing Examples 2, 5, and 6 in which the content of Na was changed, it can be seen that as the Na content increases, the electrical conductivity decreases. This confirms that because the hole concentration, which is a main carrier of a p-type material, is decreased due to the supply of additional electrons by Nat, the charge concentration additionally decreases due to the addition of Na.

4. Evaluation of Temperature Dependence of Seebeck Coefficient

[0098] For the chalcogen-containing compound samples prepared in Examples 1 to 6 and Comparative Examples 1 to 4, the Seebeck coefficient (S) was measured according to the temperature change, and the results are shown in FIG. 6. The Seebeck coefficient was measured in a temperature range of 50 C. to 500 C. by using a measuring device ZEM-3 (manufactured by ULCAC) and applying a differential voltage/temperature technique.

[0099] As shown in FIG. 6, it can be seen that, in light of the fact that the positive (+) Seebeck coefficients are shown in Examples 1 to 6 and Comparative Examples 1 to 4, the main charge carriers of the material are holes and exhibit characteristics as a P-type semiconductor material. On the other hand, Comparative Examples 1 to 4 showed the tendency that the Seebeck coefficient decreases as the Sn content increases. Also, in the case of Examples 1 to 4, the same Seebeck coefficient change can be confirmed according to the content of Sn. On the other hand, when comparing Example 2, Example 5, and Example 6 in which the Na content was changed, it shows the tendency that the Seebeck coefficient increases as the Na content increases. This is because the Seebeck coefficient has an opposite tendency to the electrical conductivity in terms of the charge carrier concentration (the larger the charge carrier concentration, the higher the electrical conductivity but the lower the Seebeck coefficient).

5. Evaluation of Temperature Dependence of Power Factor

[0100] For the chalcogen-containing compound samples prepared in Examples 1 to 6 and Comparative Examples 1 to 4, the power factor was calculated according to the temperature change, and the results are shown in FIG. 7. The power factor is defined as power factor (PF)=S.sup.2 and was calculated using the values of (electrical conductivity) and S (Seebeck coefficient) shown in FIG. 5 and FIG. 6.

[0101] As shown in FIG. 7, in Examples 1 to 4, as the content of Sn increases, a low power factor is exhibited in a low temperature part, and then the power factor increased as it moves to a high temperature part. From these results, it was confirmed that the same tendencies are exhibited even in Comparative Examples 1 to 4. This is because, upon increase of the Sn content, the hole charge concentration gradually increases, and as a result, the bipolar effect decreases, which means that the temperature showing the maximum value of the power factor moves to the high temperature part.

6. Evaluation of Temperature Dependence of Thermal Conductivity

[0102] For the chalcogen-containing compound samples prepared in Examples 1 to 6 and Comparative Examples 1 to 4, the thermal conductivity, specifically, the total thermal conductivity (k.sub.tot), was measured according to the temperature change, and the results are shown in FIG. 8.

[0103] In this experiment, the thermal diffusivity (D) and the thermal capacity (C.sub.p) were measured by applying a laser scintillation method and using an LFA457 instrument (manufactured by Netzsch) which is a device for measuring the thermal conductivity, and then the total thermal conductivity (k.sub.tot) was calculated by applying the measured value to the following Equation 1.

Total Thermal Conductivity (k.sub.tot)=DC.sub.p[Equation 1]

[0104] (wherein is the density of a sample measured by the Archimedes method)

[0105] Referring to FIG. 8, Examples 1 to 4 showed that as the content of Sn increases, the total thermal conductivity gradually increases due to the increase of the charge concentration, but exhibited relatively low thermal conductivity as compared with Comparative Examples 1 to 4. This is because the hole charge concentration is reduced due to the addition of Na and consequently the thermal conductivity contributed by the charge carrier decreases. On the other hand, when comparing Example 2, Example 5, and Example 6 in which the content of Sn:Sb:Te was fixed and only the Na content was changed, it can be seen that as the Na content increases, the total thermal conductivity decreases slightly. This result has the same tendency as the electrical conductivity of FIG. 5.

7. Evaluation of Temperature Dependence of Thermoelectric Performance Index (ZT)

[0106] For the chalcogen-containing compound samples prepared in Examples 1 to 6 and Comparative Examples to 4, the thermoelectric performance index was calculated according to the temperature change, and the results are shown in FIG. 9.

[0107] The thermoelectric performance index (ZT) is defined as ZT=S.sup.2T/k, and was calculated by using the values of S (Seebeck coefficient), (electrical conductivity), T (absolute temperature), and k (thermal conductivity) obtained in the experimental examples.

[0108] Referring to FIG. 9, Examples 1 to 4 show low ZT at a low temperature part as the content of Sn increases, and then ZT increases as it moves to the high temperature part. The same tendency was also confirmed in Comparative Examples 1 to 4. On the other hand, when Comparing Examples 2, 5, and 6 in which only the Na content was changed, it can be confirmed that the change of the ZT value according to the change of Na content is not large, but the ZT value is improved as compared with Comparative Example 2 in which Na was not added. Further, Example 3 showed a very high value of ZT-1.03 at 500 C. As a result, it was improved by 11.8% as compared with Comparative Example 3.

Experimental Example 8. Evaluation of Average Thermoelectric Properties

[0109] Based on the experimental results in Experimental Examples 5 to 7, the average thermoelectric properties of the chalcogen-containing compounds prepared in Examples 1 to 6 and Comparative Examples 1 to 4 in the range from 100 to 500 C. were compared, and the results are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Average thermoelectric properties at 100~500 C. PF.sub.average K.sub.tot, average (W/cmK.sup.2) (W/mK) ZT.sub.average ZT.sub.max Comparative 15.4 1.47 0.62 0.81 Example 1 Example 1 15.3 1.24 0.71 0.88 Comparative 15.9 1.56 0.63 0.89 Example 2 Example 2 15.6 1.41 0.68 0.96 Comparative 15.4 1.76 0.56 0.93 Example 3 Example 3 15.3 1.49 0.65 1.03 Comparative 14.8 2.02 0.49 0.88 Example 4 Example 4 14.8 1.84 0.53 0.94 Example 5 15.9 1.46 0.67 0.96 Example 6 14.7 1.41 0.64 0.90

[0110] Referring to Table 3, in the case of the average power factor (PF.sub.average) at 100 to 500 C., the values of the examples and the corresponding comparative examples were very similar. From these results, it can be seen that when filling Na in a part of vacancies, the electrical conductivity decreases, but the power factor is compensated due to the increase of the Seebeck coefficient.

[0111] In addition, in the case of the average thermal conductivity (K.sub.tot, average), Examples 1 to 4 were reduced by 8 to 18% as compared with Comparative Examples 1 to 4, and the average ZT value (ZT.sub.average) was improved by 8 to 14% as compared with Comparative Examples 1 to 4. From this, it can be confirmed that the heat conductivity is further improved by partially filling Na in the vacancies. In Examples 2, 5, and 6 in which only the content of Na was changed, the average power factor was similar to that of Comparative Example 2 except for Example 6 in which Na was 0.4, as compared with Comparative Example 2 in which Na was not included. However, it was confirmed that the average ZT value and ZT.sub.max increase due to the decrease of the average thermal conductivity value. On the other hand, the chalcogen-containing compound of Example 3, in which y=0.2 and x=10, exhibited a ZT.sub.max of 1.0 or more, and the ZT.sub.max was improved by 11.8% or more as compared with the chalcogen-containing compounds of the comparative examples not containing an alkali metal.