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 even at a low temperature, particularly at a temperature corresponding to an operating temperature of a thermoelectric element, and also exhibits a significantly superior power factor and thermoelectric performance index due to its excellent electrical conductivity and low thermal conductivity caused by its unique crystal lattice structure, a method for preparing the same, and a thermoelectric element including the same. [Chemical Formula 1]—V.sub.1-2xSn.sub.4Bi.sub.2-xAg.sub.3xSe.sub.7, wherein V is vacancy and 0<x<0.5.

Claims

1. A chalcogen-containing compound represented by the following Chemical Formula 1:
V.sub.1-2xSn.sub.4Bi.sub.2-xAg.sub.3xSe.sub.7  [Chemical Formula 1] wherein, in the above Chemical Formula 1, V is a vacancy and wherein x is 0.05≤x≤0.4.

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

3. The chalcogen-containing compound of claim 2, wherein the vacancy is a vacant site excluding sites filled with Se, Sn, Bi, and Ag in the face-centered cubic crystal lattice structure, and the Ag is substituted by replacing a part of the Bi.

4. The chalcogen-containing compound of claim 2, wherein the Se is filled in an anion site of the face-centered cubic crystal lattice structure, the Sn and Bi are filled in a cationic site of the face-centered cubic lattice structure, the vacancy is a vacant site of the remaining sites excluding the sites filled with Se, Sn, and Bi, and the Ag is substituted by replacing a part of the Bi.

5. A thermoelectric conversion material comprising the chalcogen-containing compound according to claim 1.

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

7. A method for preparing the chalcogen-containing compound according to claim 1, comprising the steps of: mixing raw materials of Sn, Bi, Ag, and Se and then melting the raw materials to prepare a melt; heat-treating the melt; pulverizing the resultant product obtained through the heat treatment; and sintering the pulverized product.

8. The method for preparing the chalcogen-containing compound of claim 7, wherein the raw materials of Sn, Bi, Ag, and Se are mixed at a molar ratio of Sn:Bi:Ag:Se corresponding to 4:2−x:3x:7, and the x is 0.05≤x≤0.4.

9. The method for preparing the chalcogen-containing compound of claim 7, wherein the melting is carried out at a temperature of 750 to 1000° C.

10. The method for preparing the chalcogen-containing compound of claim 7, wherein the heat treatment is carried out at a temperature of 500 to 700° C.

11. The method for preparing the chalcogen-containing compound of claim 7, 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.

12. The method for preparing the chalcogen-containing compound of claim 7, wherein the sintering step is carried out by a spark plasma sintering method.

13. The method for preparing the chalcogen-containing compound of claim 7, wherein the sintering step is carried out at a temperature of 550 to 700° C. under a pressure of 10 to 130 MPa.

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 compounds.

(3) FIG. 3 illustrates 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 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.

(5) FIG. 5 is a schematic view of a face-centered cubic lattice structure of the chalcogen-containing compound according to an embodiment of the present invention.

(6) FIG. 6 is a graph showing the results of measuring electrical conductivity versus temperature of the chalcogen 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 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 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 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 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 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 for illustrative purposes only and are not intended to limit the scope of the invention thereto.

Example 1: Preparation of Chalcogen-Containing Compound of V.SUB.0.875.Sn.SUB.4.Bi.SUB.1.9375.Ag.SUB.0.1875.Se.SUB.7

(13) The respective powders of Sn, Bi, Ag, and Se, which are high purity raw materials, were weighed at a molar ratio of 4:1.9375:0.1875:7 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 980° C. for 24 hours, and slowly cooled at room temperature. Subsequently, 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 μm or less, and sintered according to a spark plasma sintering method (SPES) at a pressure of 50 MPa and a temperature of 620° C. for 10 minutes to prepare a chalcogen-containing compound of V.sub.0.875Sn.sub.4Bi.sub.1.9375Ag.sub.0.1875Se.sub.7.

Example 2: Preparation of Chalcogen-Containing Compound of V.SUB.0.75.Sn.SUB.4.Bi.SUB.1.875.Ag.SUB.0.375.Se.SUB.7

(14) A chalcogen-containing compound of V.sub.0.75Sn.sub.4Bi.sub.1.875Ag.sub.0.375Se.sub.7 was prepared in the same manner as in Example 1, except that Sn, Bi, Ag, and Se, which are high purity raw materials, were mixed at a molar ratio of 4:1.875:0.375:7.

Example 3: Preparation of Chalcogen-Containing Compound of V.SUB.0.25.Sn.SUB.4.Bi.SUB.1.625.Ag.SUB.1.125.Se.SUB.7

(15) A chalcogen-containing compound of V.sub.0.25Sn.sub.4Bi.sub.1.625Ag.sub.1.125Se.sub.7 was prepared in the same manner as in Example 1, except that Sn, Bi, Ag, and Se, which are high purity raw materials, were mixed at a molar ratio of 4:1.625:1.125:7.

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

(16) 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.

Comparative Example 2: Preparation of Chalcogen-Containing Compound of V.SUB.0.625.Ag.SUB.0.375.Sn.SUB.4.Bi.SUB.2.Se.SUB.7

(17) A chalcogen-containing compound of V.sub.0.625Ag.sub.0.375Sn.sub.4Bi.sub.2Se.sub.7 was prepared in the same manner as in Example 1, except that Ag, Sn, Bi, and Se, which are high purity raw materials, were mixed at a molar ratio of 0.375:4:2:7.

Experimental Example

(18) 1. Phase Analysis According to XRD Pattern

(19) For the chalcogen compounds in a powder state just before the sintering step in Examples 1 to 3 and Comparative Examples 1 and 2, X-ray diffraction analysis was carried out, and the results are shown in FIG. 3.

(20) In addition, the respective 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. 4.

(21) First, referring to FIG. 3, the chalcogen 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 compounds of Examples 1 to 3 and Comparative Examples 1 and 2 all had a face-centered cubic crystal lattice structure.

(22) On the other hand, referring to FIG. 4, it was confirmed that as the chalcogen-containing compound of Comparative Example 1 exhibits poor phase stability when left at a relatively low temperature, the decomposition of the chalcogen-containing compound of Sn.sub.4Bi.sub.2Se.sub.7 and a plurality of secondary phases (Sn.sub.3Bi.sub.9Se.sub.13, Bi.sub.3Se.sub.4, Bi.sub.8Se.sub.9, and SnSe) were formed (peaks occurred in the plurality of secondary phases on the XRD pattern were observed). This shows that, as can be confirmed from the state diagram, Sn.sub.4Bi.sub.2Se.sub.7 has poor phase stability at a temperature other than a specific temperature, and thus decomposition occurs. Therefore, the material of Comparative Example 1 has a limit in that it can not be used as a thermoelectric material.

(23) In the case of Comparative Example 2 in which extra Ag was added to the chalcogen-containing compound of Comparative Example 2, a SnSe secondary phase of an orthorhombic structure was formed in addition to the material of a single-phase rock-salt structure.

(24) In contrast, it was confirmed that the chalcogen compounds of Examples 1 to 3 maintain the face-centered cubic lattice structure without the generation of secondary phases, and exhibit excellent phase stability. Thus, only when replacing a part of Bi with Ag is it possible to maintain a single phase without a secondary phase. In particular, when the molar ratio of Bi:Ag satisfies the relation of 2-x:3x, it forms a single phase. This is because Ag supplies one electron as Ag.sup.1+ and Bi supplies three electrons as Bi.sup.3+, and therefore, three Ag atoms must replace one Bi atom in order to match the charge neutrality. From this, it is confirmed that Examples 1 to 3 exhibit excellent phase stability even at a relatively low temperature.

(25) In addition, the lattice parameter and the composition were calculated for each of the chalcogen compounds in power 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.

(26) TABLE-US-00001 TABLE 1 Comparative Comparative Exam- Exam- Example 1 Example 2 ple 1 Example 2 ple 3 Lattice 5.9437 5.9545 5.9521 5.9457 5.9297 parameter (Å) Vacancy (0, 0.1429 0.09799 0.125 0.1054 0.0304 0, 0) occupancy Sn (0, 0, 0.5714 0.5668 0.5714 0.5714 0.5735 0) occupancy Bi (0, 0, 0.2857 0.283 0.2768 0.2679 0.2333 0) occupancy Ag (0, 0, 0 0.05221 0.0268 0.0536 0.1628 0) occupancy Se (0.5, 1 1 1 1 1 0.5, 0.5) occupancy Rwp 5.84 6.02 5.71 5.91 6.44

(27) FIG. 5 is a schematic diagram of a face-centered cubic lattice structure, or a rock-salt structure including defects of a chalcogen-containing compound according to one embodiment of the invention. FIG. 5 is presented for illustrative purposes only, and is not intended to limit the scope of the present invention thereto.

(28) Looking at a scheme of a chalcogen-containing compound having a composition of V.sub.1-2xSn.sub.4Bi.sub.2-xAg.sub.3xSe.sub.7 with reference to Table 1 and FIG. 5, V (vacancy), Sn, Ag, and Bi are randomly distributed at the site of (x, y, z)=(0, 0, 0), and Se is distributed at the site of (0.5, 0.5, 0.5). As shown in Table 1, this is the same as the result of Rietveld refinement calculated via the TOPAS program. As a result of calculating the actual composition, it can be seen that it is very similar to the nominal composition initially added. It can be seen therefrom that as the chalcogen compounds of Examples 1 to 3 include vacancies and a part of Bi is substituted with Ag, the concentration of the vacancies is decreased. In addition, when comparing Examples 1 to 3, the lattice parameter shows a tendency to decrease as the Ag content substituted at the site of Bi increases. This is because the atomic radius of Ag is smaller than that of Bi, which indicates that Ag is well substituted at the site of Bi. However, in the case of Comparative Example 2, if the Bi content is fixed and only Ag is further added, the lattice parameter is further increased relative to Example 2. This indicates that the lattice parameter increases as Ag partially fills the vacancies. (In the case of actual sintered body XRD, the SnSe secondary phase is detected in Comparative Example 2. The results of the Rietveld refinement are calculated using powder XRD without secondary phase).

(29) 2. Temperature Dependence of Electrical Conductivity

(30) 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 100 to 400° C. by a four-probe DC method using LSR-3 (manufactured by Linseis), which is a resistivity measuring device.

(31) Referring to FIG. 6, in the case of Comparative Example 2 in which extra Ag was added to Sn.sub.4Bi.sub.2Se.sub.7, it contains a SnSe secondary phase and thus showed the lowest electrical conductivity. On the other hand, in the case of Examples 1 to 3 in which a part of Bi was substituted with Ag, the highest electrical conductivity was shown in Example 3 having a high content of Ag. This is because Bi.sup.3+ supplies three electrons and Ag.sup.1+ supplies one electron, and therefore, as the Ag content increases, more hole charge carriers can be provided. The chalcogen compounds prepared in Examples 1 and 2 show similar electrical conductivities. This is because, as can be seen from the sign of the Seebeck coefficient in FIG. 4, the actual charge carriers are electrons and holes in Examples 1 and 2, respectively, which are different from each other. That is, in Example 1, even when substituted with Ag, the main charge carriers of the material are still electrons. In Example 2 in which Ag was substituted, as the major charge carriers are changed from electrons to holes, the types of main charge carriers are different but the concentration of charge carriers is similar. As a result, Examples 1 and 2 show similar electrical conductivity.

(32) On the other hand, in Comparative Example 1, the thermoelectric characteristics could not be measured due to poor phase stability as observed in FIG. 2.

(33) 3. Measurement of Seebeck Coefficient and 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 100 to 400° C. by using a measuring device LSR-3 (Linseis) and applying a differential voltage/temperature technique.

(35) As shown in FIG. 6, it was confirmed that Examples 1 to 3 all exhibited a positive (+) or negative (−) Seebeck coefficient such that they are applicable as a thermoelectric conversion material.

(36) On the other hand, in the case of Comparative Example 2 in which extra Ag was added, it shows a negative Seebeck coefficient, which indicates that electrons are supplied from Ag.sup.1+ while Ag being filled in the vacancy, and the main carriers of the material are electrons.

(37) Specifically, when comparing Examples 1 and 2 in which a part of Bi was substituted with Ag, it is confirmed that as the content of Ag increases from 0.1875 to 0.375, the Seebeck coefficient changes from negative (−) to positive (+) value. This means that in the case of Example 1, the main charge carriers are still electrons, but the supply of electrons is decreased and thus the major charge carriers of the material have been replaced with holes. This shows the characteristic change from an n-type to a p-type semiconductor material. In addition, this means that electrons with less Ag.sup.1+ than Bi.sup.3+ is supplied to the material, and thereby the concentration of hole charge increases. Similarly, the compound of Example 3 having a higher Ag content was confirmed to have a positive (+) Seebeck coefficient.

(38) Further, when comparing Comparative Example 2 and Example 2 having a similar Ag content, it can be seen that in the case of Comparative Example 2 in which Ag was simply filled in the vacancy, electrons were supplied to the material from Ag and have a negative Seebeck coefficient, whereas in the case of Example 2 in which Bi was substituted, the supply of electrons was relatively insufficient (hole charge supply), so that it has a positive Seebeck coefficient. From this, it can be seen that Ag is properly substituted for the site of Bi.

(39) 4. Temperature Dependence of Power Factor

(40) 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.

(41) 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. 3 and FIG. 4.

(42) As shown in FIG. 8, in the case of Comparative Example 2 where extra Ag was filled in the vacancies, it showed a low power factor, and in particular, it showed a lower power factor than Example 2 in which the content of Ag was similar but a part of Bi was substituted. In the case of Comparative Example 2, such a result is attributed to the low electrical conductivity and Seebeck coefficient together with the formed SnSe secondary phase.

(43) In addition, when comparing Examples 1 to 3 in which a part of Bi was substituted with Ag, as the content of Ag increased, the power factor increased, and in Example 3 where the content of Ag is the highest, the highest power factor was exhibited due to the increase of the electrical conductivity caused by the increase in the hole charge concentration. In particular, it was confirmed that the power factor measured at 200° C. was as high as about 4.15 μW/cmK.sup.2.

(44) 5. Temperature Dependence of Thermal Conductivity and Lattice Thermal Conductivity

(45) 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.

(46) Specifically, the thermal diffusivity (D) and the thermal capacity (C.sub.p) were measured by applying a laser scintillation method and using an LFA457 instrument (Netzsch) which is a device for measuring the thermal conductivity, and then the thermal conductivity (K) was calculated by applying the measured value to the following Equation 2.
Thermal Conductivity (K)=DρC.sub.p  [Equation 2]

(47) Herein, D is the thermal diffusivity, C.sub.p is the thermal capacity, and p is the density of a sample measured by Archimedes method.

(48) In addition, the total thermal conductivity (K.sub.tot=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 law (k.sub.E=LσT), wherein the value calculated from the Seebeck coefficient versus temperature was used as the Lorentz number (L).

(49) Referring to FIG. 9 and FIG. 10, in the case of Comparative Example 2, it showed high thermal conductivity despite having lower electrical conductivity than that of Example 2. This is because the lattice thermal conductivity is lowered by the phonon scattering by vacancies in Example 2 having higher vacancy content.

(50) Further, in the case of Examples 1 to 3, as the content of Ag increased, the total thermal conductivity increased. In particular, in Example 3 having the largest content of Ag, the highest thermal conductivity was shown due to the increase of K.sub.E according to the increase in the hole charge carrier concentration.

(51) Further, looking at the lattice thermal conductivities of Examples 1 to 3, in the case of Example 1 having the largest vacancy content, the lowest lattice thermal conductivity is exhibited by the phonon scattering effect by vacancies, but in the case of Example 3 having a lower vacancy content, it showed lattice thermal conductivity similar to that of Example 2. These results show that the phonon scattering effect due to the difference in mass of Bi and Ag becomes prominent as the Ag content increases, and the lattice thermal conductivity is lowered.

(52) 6. Temperature Dependence of Thermoelectric Performance Index (ZT)

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

(54) 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.

(55) Referring to FIG. 11, Example 2 in which the Ag content is similar but a part of Bi is substituted showed a higher thermoelectric performance index than that of Comparative Example 2, and in particular, the ZT value at 200° C. was increased by 170% as compared with Comparative Example 2.

(56) Finally, in the case of Example 3 in which the substitution amount of Ag was further increased, it showed a more improved thermoelectric performance index, and the ZT value at 200° C. was increased by 180% as compared with Comparative Example 1.