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

Abstract

A chalcogen-containing compound of the following Chemical Formula 1 which exhibits excellent phase stability at a low temperature, particularly at a temperature corresponding to the driving temperature of a thermoelectric element, and also exhibits an excellent thermoelectric performance index through an increase in a power factor and a decrease in thermal conductivity, a method for preparing the same, and a thermoelectric element including the same:
V.sub.1-xM.sub.xSn.sub.4Bi.sub.2Se.sub.7-yTe.sub.y  [Chemical Formula 1]
In the above Formula 1, V is a vacancy, M is an alkali metal, x is greater than 0 and less than 1, and y is greater than 0 and less than or equal to 1.

Claims

1. A chalcogen-containing compound represented by the following Chemical Formula 1:
V.sub.1-xM.sub.xSn.sub.4Bi.sub.2Se.sub.7-yTe.sub.y  [Chemical Formula 1] wherein, in the above Formula 1, V is a vacancy, M is an alkali metal, x is greater than 0 and less than 1, and y is greater than 0 and less than or equal to 1, and wherein the chalcogen-containing compound has a face-centered cubic crystal lattice structure, the Se is filled in an anion site of the face-centered cubic lattice structure, the Sn and Bi are filled in a cation site of the face-centered cubic lattice structure, the Te is substituted by replacing some of the Se, the M is filled in at least some of vacant sites excluding the sites filled with Sn, Bi, Se, and Te in the face-centered cubic lattice structure, and the V is a vacant site of the remaining cationic sites.

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

3. The chalcogen-containing compound of claim 1, wherein the chalcogen-containing compound has a lattice parameter of 5.975 Å or more.

4. The chalcogen-containing compound of claim 1, wherein the x is 0.05 to 0.5, y is 0.1 to 1, and x+y is 0.1 to 1.5.

5. The chalcogen-containing compound of claim 1, wherein the chalcogen-containing compound is selected from the group consisting of V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6.8Te.sub.0.2, V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6.2Te.sub.0.8, and V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6Te.sub.1.

6. A method for preparing the chalcogen-containing compound of claim 1, represented by the following Chemical Formula 1:
V.sub.1-xM.sub.xSn.sub.4Bi.sub.2Se.sub.7-yTe.sub.y  [Chemical Formula 1] wherein, in the above Formula 1, V is a vacancy, M is an alkali metal, x is greater than 0 and less than 1, and y is greater than 0 and less than or equal to 1, comprising the steps of: mixing respective raw materials of Sn, Bi, Se, Te, and an alkali metal (M) and subjecting the mixture to a melting reaction; heat-treating the resultant product obtained through the melting reaction; pulverizing the resultant product obtained through the heat treatment; and sintering the pulverized product, wherein the mixing of raw materials is carried out by mixing the raw materials such that the molar ratio of Sn, Bi, Se, Te, and an alkali metal (M) is a ratio corresponding to 4:2:7-y:y:x.

7. The method for preparing the chalcogen-containing compound of claim 6, wherein the melting is carried out at a temperature of 700 to 800° C.

8. The method for preparing the chalcogen-containing compound of claim 6, wherein the heat treatment is carried out at a temperature of 550 to 640° C.

9. The method for preparing the chalcogen-containing compound of claim 6, further comprising a step of cooling the result of the heat treatment step to form an ingot between the heat treatment step and the pulverization step.

10. The method for preparing the chalcogen-containing compound of claim 6, wherein the sintering step is carried out by a spark plasma sintering method.

11. The method for preparing the chalcogen-containing compound of claim 6, wherein the sintering step is carried out at a temperature of 550 to 700° C. under a pressure of 10 to 100 MPa.

12. A thermoelectric element comprising the chalcogen-containing compound according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing a face-centered cubic lattice structure represented by sodium chloride or the like.

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

(3) FIG. 3 is a graph showing the results of X-ray diffraction analysis of the chalcogen-containing compound powder just before the sintering step in Examples 1 to 3 and Comparative Examples 1 and 2.

(4) FIG. 4 is a graph showing the results of X-ray diffraction analysis of the chalcogen-containing compound powder just before the sintering step in Comparative Example 3.

(5) FIG. 5 is a graph showing the results of X-ray diffraction analysis after the sintered body finally produced through the sintering step in Examples 1 to 3 and Comparative Examples 1 and 2 is slowly cooled and left to stand at room temperature.

(6) FIG. 6 is a graph showing the results of measuring electrical conductivity versus temperature of the chalcogen-containing compounds in Examples 1 to 3 and Comparative Example 2.

(7) FIG. 7 is a graph showing the results of measuring the Seebeck coefficient versus temperature of the chalcogen-containing compounds in Examples 1 to 3 and Comparative Example 2.

(8) FIG. 8 is a graph showing the results of measuring the power factor versus temperature of the chalcogen-containing compounds in Examples 1 to 3 and Comparative Example 2.

(9) FIG. 9 is a graph showing the results of measuring the total thermal conductivity versus temperature of the chalcogen-containing compounds in Examples 1 to 3 and Comparative Example 2.

(10) FIG. 10 is a graph showing the results of calculating the lattice thermal conductivity versus temperature of the chalcogen-containing compounds in Examples 1 to 3 and Comparative Example 2.

(11) FIG. 11 is a graph showing the results of calculating the thermoelectric performance index versus temperature of the chalcogen-containing compounds in Examples 1 to 3 and Comparative Example 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(12) 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 Chalcogen-Containing Compound (V.SUB.0.6.Na.SUB.0.4.Sn.SUB.4.Bi.SUB.2.Se.SUB.6.8.Te.SUB.0.2.)

(13) The respective powders of Na, Sn, Bi, Se, and Te, which are high purity raw materials, were weighed at a molar ratio of 0.4:4:2:6.8:0.2 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 24 hours, and slowly cooled at room temperature.

(14) Thereafter, heat treatment was carried out at a temperature of 640° C. for 48 hours. The quartz tube in which the reaction progressed was cooled with water to obtain an ingot. The ingot was finely pulverized to a powder having a particle size of 75 μm or less, and sintered according to a spark plasma sintering method (SPS) at a pressure of 50 MPa and a temperature of 620° C. for 10 minutes to prepare chalcogen-containing compound of V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6.8Te.sub.0.2.

Example 2

Preparation of Chalcogen-Containing Compound (V.SUB.0.6.Na.SUB.0.4.Sn.SUB.4.Bi.SUB.2.Se.SUB.6.2.Te.SUB.0.8.)

(15) A chalcogen-containing compound of V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6.2Te.sub.0.8 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Bi, Se, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.4:4:2:6.2:0.8 in a glove box.

Example 3

Preparation of Chalcogen-Containing Compound (V.SUB.0.6.Na.SUB.0.4.Sn.SUB.4.Bi.SUB.2.Se.SUB.6.Te.SUB.1.)

(16) A chalcogen-containing compound of V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6Te.sub.1 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Bi, Se, and Te, which are high purity raw materials, were mixed at a molar ratio of 0.4:4:2:6:1 in a glovebox.

Comparative Example 1

Preparation of Chalcogen-Containing Compound (Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(17) A chalcogen-containing compound of Sn.sub.4Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Sn, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 4:2:7 in a glove box.

Comparative Example 2

Preparation of Chalcogen-Containing Compound (V.SUB.0.6.Na.SUB.0.4.Sn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(18) A chalcogen-containing compound of V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 0.4:4:2:7 in a glove box.

Comparative Example 3

Preparation of Chalcogen-Containing Compound (NaSn.SUB.4.Bi.SUB.2.Se.SUB.7.)

(19) A chalcogen containing compound of NaSn.sub.4Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that the respective powders of Na, Sn, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 1:4:2:7 in a glove box.

Experimental Example

(20) 1. Phase Analysis According to XRD Pattern

(21) For the chalcogen compounds in a powder state just before the sintering step in Examples 1 to 3 and Comparative Examples 1 to 3, X-ray diffraction analysis was carried out, and the results are shown in FIG. 3. In addition, the results of X-ray diffraction analysis of the powder state just before the sintering step of Comparative Example 3 in which all vacancies were filled are shown in FIG. 4.

(22) Further, the sintered body finally produced through the sintering step in Examples 1 to 3 and Comparative Examples 1 and 2 was gradually cooled from about 620° C. to 300° C. and then cooled again to room temperature (25° C.). Then, the resultant sintered body was maintained in the air atmosphere for 15 days, and X-ray diffraction analysis of each sintered body was performed. The results are shown in FIG. 5.

(23) First, referring to FIG. 3, the chalcogen-containing compounds of Examples 1 to 3 and Comparative Examples 1 and 2 were confirmed to have the same crystal lattice structure as that of Sn.sub.4Bi.sub.2Se.sub.7 which was conventionally known to have a face-centered cubic lattice structure at a high temperature. From these results, it was confirmed that the chalcogen-containing compounds of Examples 1 to 3 and Comparative Examples 1 and 2 all had a face-centered cubic crystal lattice structure. However, in Example 3, a small amount of SnTe secondary phase was observed, but since it was not a finally produced sintered body, it was determined to be within the allowable range.

(24) On the other hand, referring to FIG. 4, in the chalcogen-containing compound powder before the sintering step of Comparative Example 3 in which Na was filled in all vacant sites, a secondary phase of SnBiSe.sub.2 was observed. From this, it can be confirmed that the inclusion of the vacant site is an important part in suppressing the formation of the secondary phase. Further, since the chalcogen-containing compound powder before the sintering step of Comparative Example 3 already contains an excess amount of the secondary phase, analysis of the sintered body after the sintering step and the Te substitution experiment of the sintered body were not carried out.

(25) Further, referring to FIG. 5, it was confirmed that in the case of Comparative Example 1, as it exhibits poor phase stability when left at relatively low to the decomposition of the chalcogen-containing compound of Sn.sub.4Bi.sub.2Se.sub.7 occurred and thus a plurality of secondary phases such as Sn.sub.3Bi.sub.9Se.sub.13, SnSe, or Bi.sub.4Se.sub.3 were formed (peaks occurring in the vicinity of the main peaks on the XRD pattern were confirmed). However, it was confirmed that the compounds of Examples 1 to 3 and Comparative Example 2 retained the face-centered cubic lattice structure without generation of the secondary phase, and thus exhibited excellent phase stability even at a relatively low temperature. From these results, it can be seen that the sintered body forms a stable phase at a low temperature when some of the vacancies are filled with an alkali metal.

(26) 2. Results Using TOPAS Program

(27) The lattice parameter was calculated for each of the chalcogen-containing compounds in powder state of Examples 1 to 3 and Comparative Examples 1 and 2 using the TOPAS program, and the results are shown in Table 1 below.

(28) TABLE-US-00001 TABLE 1 Lattice parameter Powder material (Å) Example 1 (V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6.8Te.sub.0.2) 5.9776 Example 2 (V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6.2Te.sub.0.8) 5.9893 Example 3 (V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.6Te.sub.1) 5.9971 Comparative Example 1 (Sn.sub.4Bi.sub.2Se.sub.7) 5.9496 Comparative Example 2 (V.sub.0.6Na.sub.0.4Sn.sub.4Bi.sub.2Se.sub.7) 5.9724

(29) Referring to Table 1, in the chalcogen-containing compound or Comparative Example 2, the lattice parameter was increased by Na in a vacant site relative to Comparative Example 1. In the chalcogen-containing compounds of Examples 1 to 3, by partially substituting Te having a larger atomic radius with Se relative to Comparative Example 2, the lattice parameter further increased and the cell size increased. That is, as the Te content increases, the lattice parameter due to the increase of cell size increases sequentially (Example 3>Example 2>Example 1>Comparative Example 2>Comparative Example 1).

(30) 3. Temperature Dependence of Electrical Conductivity

(31) For the chalcogen-containing compound samples prepared in Examples 1 to 3 and Comparative Example 2, the electrical conductivity was measured according to the temperature change, and the results are shown in FIG. 6. The electrical conductivity was measured at a temperature range of 50 to 300° C. by a four-probe DC method using LSR-3 (manufactured by Linseis), which is a resistivity measuring device.

(32) Referring to FIG. 6, the chalcogen-containing compounds of Examples 1 to 3 exhibited lower electrical conductivity than Comparative Example 2, and showed a tendency to decrease as the amount of Te substitution increased. This means that the electrical conductivity decreased due to the carrier scattering effect caused by a mass difference between Te and Se. However, in the case of Example 2 and Example 3 in which the molar ratio of Te is 0.8 and 1, the difference in electrical conductivity is not large, which indicates that when the molar ratio of Te exceeds 0.8, the carrier scattering reaches a maximum value, and thus, even if the content of Te is additionally increased, it does not cause a large change in electrical conductivity.

(33) 4. Temperature Dependence of Seebeck Coefficient

(34) For the chalcogen-containing compound samples prepared in Examples 1 to 3 and Comparative Example 2, the Seebeck coefficient (S) was measured according to the temperature change, and the results are shown in FIG. 7. The Seebeck coefficient was measured in a temperature range of 50 to 300° C. by using a measuring device LSR-3 (manufactured by Linseis) and applying a differential voltage/temperature technique.

(35) As shown in FIG. 7, comparing Examples 1 to 3 and Comparative Example 2, as the content of Te increased, the Seebeck coefficient increased in the entire measured temperature section. From this result, it was confirmed that the Seebeck coefficient was improved due to Te substitution.

(36) 5. Temperature Dependence of Power Factor

(37) For the chalcogen-containing compound samples prepared in Examples 1 to 3 and Comparative Example 2, the power factor was calculated according to the temperature change, and the results are shown in FIG. 8.

(38) The power factor is defined as power factor (PF)=σS.sup.2, and was calculated using the values of σ (electrical conductivity) and S (Seebeck coefficient) shown in FIG. 6 and FIG. 7.

(39) As shown in FIG. 6, in Examples 1 to 3, the Seebeck coefficient was increased as the content of Te was increased, thereby showing an increased power factor, as compared with Comparative Example 2. Particularly, the average power factor of Example 3 measured at 50 to 300° C. showed a high value of 31% or more as compared with Comparative Example 2.

(40) 6. Temperature Dependence of Thermal Conductivity

(41) For the chalcogen-containing compound samples prepared in Examples 1 to 3 and Comparative Example 2, the thermal conductivity and the lattice thermal conductivity were measured according to the temperature change, and the results are shown in FIG. 9 and FIG. 10, respectively. In the measurement of the thermal conductivity, first, the thermal diffusivity (D) and the thermal capacity (Cp) were measured by applying laser scintillation method and using LFA457 (manufactured by Netzsch) which is device for measuring the thermal conductivity. The thermal conductivity (K) was calculated by applying the measured value to the equation of “thermal conductivity (K) or total thermal conductivity (Ktot)=DρC.sub.p (ρ is the density of the sample measured by the Archimedes method)”.

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

(43) Referring to FIG. 9 and FIG. 10, Examples 1 to 3 and Comparative Example 2 showed generally low thermal conductivity, and particularly, it was confirmed that the lattice thermal conductivity decreased as the content of Te increased, and as a result, the total thermal conductivity decreased. Specifically, the chalcogen-containing compound of Example 3 exhibited a low level of lattice thermal conductivity value (at 50 to 300° C.) from 0.65 to 0.72 W/mK due to the point defect scattering effect of a phonon resulting from vacancy and Te substitution in the face-centered cubic structure.

(44) 7. Temperature Dependence of Thermoelectric Performance Index

(45) For the chalcogen-containing compound samples prepared in Examples 1 to 3 and Comparative Example 2, the thermoelectric performance index was calculated according to temperature change, and the results are shown in FIG. 11.

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

(47) Referring to FIG. 11, as the content of Te increased, the ZT value increased. Specifically, in the case of the chalcogen-containing compound of Example 3 in which some Se was substituted with the composition of Te 1.0, the average ZT value at 50 to 300° C. was 54% or more higher than that of Comparative Example 2.