Patent Application
20180026170
The present invention relates to a thermoelectric material and a method for producing the thermoelectric material.
Conventional manganese silicide-based thermoelectric materials MnSiγ (where 1.7≦γ≦1.8)wherein the ab surface of each crystal grain is oriented into a direction, include those in which Si elements are partially substituted with at least one type of element selected from among IIIb group elements, IVb group elements, and lanthanoid elements, and those in which Mn elements are partially substituted with at least one type of element selected from among Va group elements, VIa group elements, VIIa group elements, VIIIa group elements, and lanthanoid elements (for example, see Patent Document 1 or 2). These thermoelectric materials are excellent in thermoelectric properties and thermal shock resistance, for example, such that one of thermoelectric properties, the power factor S2σ (where S denotes the Seebeck coefficient, and σ denotes the electrical conductivity), of up to 2.22 mW/K2m is obtained at 500° C.
These thermoelectric materials are produced by melting raw materials by arc melting or the like, solidifying, and then further sintering as necessary by spark plasma sintering (SPS) or the like, wherein MnSi (manganese monosilicide) is precipitated in layers in the materials with a period of several tens of microns into the c-axis direction of MnSiγ. The manganese monosilicide MnSi is a good P-type conductor having metallic properties. Its high electrical conductivity σ, its low Seebeck coefficient S, and its discontinuous atomic arrangement at the boundary lower the materials' figure of merit Z (obtained by dividing the power factor S2σ by the thermal conductivity κ).
Accordingly, a thermoelectric material Mn(Si1-xGex)γ (where 0.005≦x≦0.01) has been developed by partial substitution of 0.5 to 1.0 at % of Si elements with Ge, as a thermoelectric material with MnSi not precipitated in layers (for example, see Non-patent Document 1 or Patent Document 3). This thermoelectric material is produced by melting Mn and Si satisfying the stoichiometric composition of the base material, MnSiγ, and Ge in an amount corresponding to that of “x”, and then cooling the resultant at a cooling rate of 1.5° C./minute or less for crystal growth. The thermoelectric material exhibits the power factor S2σ of up to about 1.6 mW/K2m.
In addition, the present inventors have reported that partial substitution of Mn elements of a thermoelectric material MnSiγ with elements having a valence number lower than that of Mn (for example, chromium), results in doped holes, improved conductivity, and increased power factor S2σ (for example, see Non-patent Document 2). Moreover, the thermoelectric material MnSiγ is incommensurate composite crystal composed of 2 types of subsystems, [Mn] and [Si] that share the tetragonal a-b axis and differ in the length of the c axis.
Patent Document 1: JP Patent Publication (Kokai) No. 2009-231638 A
Patent Document 2: JP Patent Publication (Kokai) No. 2007-42963 A
Patent Document 3: JP Patent Publication (Kokai) No. 2007-235083 A
Non-Patent Document 1: I. Aoyama et al, “Effects of Ge Doping on Micromorphology of MnSi in MnSi˜1.7 and on Their Thermoelectric Transport Properties”, Japanese Journal of Applied Physics, 2005, 44, 8562
Non-Patent Document 2: Y. Kikuchi et al, “Enhanced Thermoelectric Performance of a Chimney-Ladder (Mn1-xCrx)Siγ (γ˜1.7) Solid Solution”, Japanese Journal of Applied Physics, 2012, 51, 085801
The thermoelectric material described in Patent Document 1 or 2 is problematic in that MnSi precipitated in layers affects to lower the thermoelectric properties. Furthermore, the thermoelectric material described in Non-patent Document 1 or Patent Document 3 causes no layered precipitation of MnSi so as to inhibit the figure of merit Z from decreasing due to the precipitation, however, the material is problematic in that the amount of Ge to be substituted is limited, and thus further improvement in thermoelectric properties cannot be expected.
The present invention has been achieved, noting such problems, and an objective of the present invention is to provide a thermoelectric material with improved thermoelectric properties and a method for producing the thermoelectric material.
The present inventors have discovered for the first time that the layered precipitation of MnSi can be inhibited not by partial substitution of Si elements, but by partial substitution of Mn elements with other elements, and thus have completed the present invention.
Specifically, the thermoelectric material according to the present invention is characterized by containing (Mn1-x-yVxFey)Siγ (0.012≦x≦0.045, 0≦y≦0.06, and 1.7≦γ≦1.8).
The thermoelectric material according to the present invention is characterized in that the precipitation of MnSi in layers can be inhibited by partial substitution of Mn elements with V (vanadium). Hence, this can inhibit the figure of merit Z from decreasing due to the layered precipitation, and can improve the thermoelectric properties. Furthermore, V has a valence number lower by 2 and an atomic radius higher than those of Mn, so that hole carriers can be introduced sufficiently even in the case of substitution with a trace amount thereof ranging from about 1.2 to 4.5 at %. Therefore, the power factor S2σ can be increased and the thermoelectric properties can further be improved. In this manner, the thermoelectric material according to the present invention can lead to more improved thermoelectric properties by partial substitution of not Si elements, but Mn elements alone with V.
Moreover, when Mn elements are partially substituted with V until the precipitation of MnSi in layers disappears, hole carriers may be excessive. In such a case, whereas the electrical conductivity σ is improved, the Seebeck coefficient S decreases. Hence, Fe is added for partial substitution of Mn elements with Fe having a valence number higher by 1 than that of Mn, so that increases in hole carrier can be inhibited by electron doping. Accordingly, the Seebeck coefficient S is increased, the power factor S2σ can be increased, and the thermoelectric properties can be improved. The thermoelectric material according to the present invention can enhance thermoelectric properties in the case of particularly 0.025≦x≦0.045, and 0.01≦y≦0.045.
The thermoelectric material according to the present invention preferably has a power factor S2σ of 1.8 mW/K2m or more at 700K to 900K, a power factor S2σ of 1.2 mW/K2m or more at 300K to 1000K. The thermoelectric material has further preferably has a power factor S2σ of 2.2 mW/K2m or more at 700K to 900K, and a power factor S2σ of 1.4 mW/K2m or more at 300K to 1000K. Furthermore, a dimensionless figure of merit ZT (here, “T” denotes the absolute temperature) at 800K to 900K is preferably 0.55 or more, and a dimensionless figure of merit ZT at 300K to 1000K is preferably 0.15 or more. In these cases, the thermoelectric material is excellent in particularly thermoelectric properties.
The method for producing a thermoelectric material according to the present invention involves a melting step for homogeneously melting raw materials including Mn, Si and V mixed to a composition of the above thermoelectric material, and a solidifying step for solidifying the thus melted raw materials at a cooling rate of 13K/hour or less. When the thermoelectric material having a composition including Fe is produced, raw materials including Mn, Si, V and Fe are homogeneously melted in the melting step.
With the method for producing a thermoelectric material according to the present invention, the thermoelectric material according to the present invention can be produced appropriately. The method for producing a thermoelectric material according to the present invention involves solidifying melted raw materials including Mn, Si and V at a cooling rate of 13K/hour or less, so as to be able to inhibit the layered precipitation of MnSi. Moreover, hole carriers can also be introduced by partial substitution with V. Accordingly, the thermoelectric material with improved thermoelectric properties can be obtained. When raw materials include Fe, the thermoelectric material with improved thermoelectric properties can be obtained by partial substitution with V and Fe. In particular, the cooling rate is preferably 1.5K/hour or less. In this case, further improved thermoelectric properties can be obtained. In addition, the “cooling rate” as used herein refers to a cooling rate when melted raw materials are solidified, and specifically refers to a rate of cooling performed within a predetermined temperature range including temperatures for solidification.
According to the present invention, a thermoelectric material with improved thermoelectric properties and a method for producing the thermoelectric material can be provided.
Hereinafter, the embodiment of the present invention is explained below on the basis of drawings.
The thermoelectric material according to the embodiment of the present invention is produced by the method for producing a thermoelectric material of the present invention and contains (Mn1-x-yVxFey)Siγ (0.012≦x≦0.045, 0≦y≦0.06, and 1.7≦γ≦1.8).
The method for producing a thermoelectric material according to the embodiment of the present invention involves homogeneously melting raw materials including Mn, Si and V mixed to a desired composition. When a thermoelectric material having a composition including Fe is produced, Fe is also melted homogeneously as a raw material. Next, the melted raw materials are solidified at a cooling rate of 13K/hour or less. Thus, the thermoelectric material according to the embodiment of the present invention can be obtained.
Next, the effects are as explained below.
With the use of the method for producing a thermoelectric material according to the embodiment of the present invention, melted raw materials including Mn, Si, and V are gradually cooled at a cooling rate of 13K/hour or less for solidification, and then Mn elements are partially substituted with V (vanadium), so that the layered precipitation of MnSi can be inhibited. Hence, a decrease in figure of merit Z due to the layered precipitation is inhibited, so that the thermoelectric properties can be improved. Moreover, V has a valence number lower by 2 and an atomic radius larger than those of Mn, so that hole carriers can be sufficiently introduced even via substitution with a trace amount thereof ranging from about 1.2 to 5 at %. Accordingly, the power factor S2σ can be increased, and the thermoelectric properties can be further improved. As described above, partial substitution of not Si elements, but Mn elements alone with V enables to obtain the thermoelectric material with improved thermoelectric properties according to the embodiment of the present invention.
Moreover, when substitution with V leads to excessive hole carriers, Fe is added for partial substitution of Mn elements with Fe, and then electrons are doped, so as to be able to inhibit increase in hole carriers. Therefore, the Seebeck coefficient S is increased, the power factor S2σ can be increased, and thus thermoelectric properties can be improved.
A thermoelectric material having the composition of (Mn1-xVx)Siγ (y=0) was produced, and examined for crystal structure, fine structure, and thermoelectric properties. As raw materials, granular Mn having a purity of 99.99% and a grain size of 2 mm to 5 mm, granular Si having a purity of 99.999% and a grain size of 2 mm to 5 mm, and powdered V having a purity of 99.9% and a grain size of 300 μm were used. Samples of the thermoelectric material were produced as follows.
First, raw materials were mixed in predetermined amounts, respectively, and then subjected to arc melting by which melting and solidification were repeated, so that a solid homogenized product was obtained. Next, the thus obtained homogenized product was pulverized into grains, sealed within a silica tube, and then melted by heating to 1200° C. (1473K). The temperature was maintained at 1200° C. for 8 hours, the resultant was cooled for 8 to 100 hours to 1100° C. (1373K) (cooling rate: 12.5 to 1K/hour) for solidification. Subsequently, the resultant was cooled for 24 hours to room temperature (RT). In this manner, samples of the clumped thermoelectric material with γ=1.740 and x=0 to 0.060 (hereinafter, referred to as “melt grown”) were produced.
In addition, for comparison, raw materials were melted by arc melting and then solidified, powdered, and then compressed by spark plasma sintering (SPS), thereby producing a comparative sample (hereinafter, referred to as “SPS”). The comparative sample is characterized by γ=1.740, and x=0 and 0.020.
Melt grown samples subjected to 100 hours of cooling (cooling rate: 1K/hour) were subjected to crystal structure analysis by an X-ray diffraction method, wherein the value of “x” was varied from 0 to 0.060. Upon X-ray diffraction (XRD), measurement was performed by “D8 ADVANCE” (Bruker AXS) using a CuKα line. The thus obtained XRD pattern is shown in
As shown in
Next, melt grown samples with x=0.020 were subjected to crystal structure analysis by an X-ray diffraction method in a manner similar to that in
As shown in
Melt grown samples with x=0 and x=0.020 in the case of 100 hours of cooling (cooling rate: 1K/hour) were observed under a scanning electron microscope (SEM), and subjected to energy-dispersive x-ray spectroscopy (EDS). Moreover, melt grown samples with x=0.020 in the case of 8 hours of cooling (cooling rate: 12.5K/hour) were observed under SEM. A scanning electron microscope “SU-8100” (Hitachi High-Technologies Corporation) was used for observation under SEM and measurement by EDS. The thus obtained SEM photograph of each sample and each EDS map are shown in
As shown in
Furthermore, as shown in
Melt grown samples with x=0 and x=0.020 in the case of 100 hours of cooling (cooling rate: 1K/hour) and SPS samples with x=0 and x=0.020 were measured for Seebeck coefficient, electrical conductivity and thermal conductivity. A thermal property measurement system “ZEM-3” (Advance Riko Inc.,) was used for measurement of Seebeck coefficient and electrical conductivity. In addition, a laser flash method thermal constant measurement system “TC-7000H” (Advance Riko Inc.,) was used for measuring thermal conductivity. Moreover, upon measurement of each of these thermoelectric properties, measurement was performed for SPS samples along the direction of compression by SPS, and for melt grown samples along the direction same as the compression direction for the corresponding SPS samples.
The temperature dependence of Seebeck coefficient S, electrical conductivity σ, power factor S2σ, and dimensionless figure of merit ZT (Z=S2σ/κ, where Z denotes the figure of merit, κ denotes the thermal conductivity, and T denotes the absolute temperature) of each sample found by measurement of the thermoelectric properties are shown in
As shown in
As shown in
As shown in
Next, melt grown samples with x=0.020 in the case of 8 hours of cooling (cooling rate: 12.5K/hour) were also measured for Seebeck coefficient, electrical conductivity, and thermal conductivity in a manner similar to that in
As shown in
Based on the results in
Moreover, samples in the case of 8 hours of cooling (cooling rate: 12.5K/hour) as shown in
A thermoelectric material having a composition of (Mn0.97-yV0.03Fey)Si1.7 (x=0.03, γ=1.7) was produced, and then examined for thermoelectric properties. As raw materials, granular Mn having a purity of 99.99% and a grain size of 2 mm to 5 mm, granular Si having a purity of 99.999% and a grain size of 2 mm to 5 mm, powered V having a purity of 99.9% and a grain size of 300 μm, and powdered Fe having a purity of 99.9% and a grain size of 0.1 mm to 1.7 mm were used. Samples of the thermoelectric material were produced in a manner similar to that of Example 1. Cooling was performed for 100 hours (cooling rate: 1K/hour). Samples with y=0, 0.02, 0.03, 0.04, and 0.05 were produced.
Each sample was measured for Seebeck coefficient and electrical conductivity in a manner similar to that of Example 1. For comparison, MnSi1.7 samples were also produced and measured similarly. The temperature dependence of the power factor S2σ found for each sample by measurement is shown in
This can be interpreted as follows. First, in this Example, the amount of V was increased to a level higher than that of melt grown samples with x=0.020 in
| Number | Date | Country | Kind |
|---|---|---|---|
| 2016-145712 | Jul 2016 | JP | national |
