THERMOELECTRIC MATERIAL AND METHOD FOR PRODUCING THERMOELECTRIC MATERIAL

Abstract

A thermoelectric material having improved thermoelectric properties and a method for producing the thermoelectric material are provided. The thermoelectric material contains (Mn.sub.1-x-yV.sub.xFe.sub.y)Si.sub. (0.012x0.045, 0y0.06, 1.71.8) and is produced by homogenously melting the raw materials including Mn, Si, and V mixed to a composition of the thermoelectric material, and then solidifying the melted raw materials at a cooling rate of 13 K/hour or less.

Claims

1. A thermoelectric material, containing (Mn.sub.1-x-yV.sub.xFe.sub.y)Si.sub. (0.012x0.045, 0y0.06, and 1.71.8).

2. The thermoelectric material according to claim 1, wherein a power factor S.sup.2 (where S denotes the Seebeck coefficient, and denotes the electrical conductivity) at 700K to 900K is 1.8 mW/K.sup.2m or more, and a power factor S.sup.2 at 300K to 1000K is 1.2 mW/K.sup.2m or more.

3. The thermoelectric material according to claim 1, wherein a power factor S.sup.2 at 700K to 900K is 2.2 mW/K.sup.2m or more, and a power factor S.sup.2 at 300K to 1000K is 1.4 mW/K.sup.2m or more.

4. The thermoelectric material according to claim 1, wherein the dimensionless figure of merit ZT (where Z denotes the figure of merit, and T denotes the absolute temperature) at 800K to 900K is 0.55 or more, and the dimensionless figure of merit ZT at 300K to 1000K is 0.15 or more.

5. The thermoelectric material according claim 1, wherein 0.025x0.045 and 0.01y0.045.

6. A method for producing the thermoelectric material according to claim 1, comprising: a melting step for homogeneously melting raw materials including Mn, Si, and V mixed to a composition of said thermoelectric material; and a solidifying step for solidifying said melted raw materials at a cooling rate of 13K/hour or less.

7. The method for producing the thermoelectric material according to claim 6, wherein said cooling rate is 1.5K/hour or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 shows the XRD pattern of each composition when the value of x of the thermoelectric material (y=0) according to an embodiment of the present invention is varied from 0 to 0.060.

[0022] FIG. 2 shows XRD patterns of the thermoelectric material (y=0) according to the embodiment of the present invention, which was produced with the cooling times of 8 hours (cooling rate: 12.5K/hour), 24 hours (cooling rate: 4.2K/hour), and 100 hours (cooling rate: 1K/hour), respectively.

[0023] FIG. 3 shows (a) SEM (scanning electron microscope) photograph, (b) EDS (energy dispersive X-ray analysis) map of Mn, and (c) EDS map of Si of samples having a composition of x=0, and, (d) SEM photograph, (e) EDS map of Mn, and (f) EDS map of Si of samples having a composition of x=0.020 of the thermoelectric material (y=0) according to the embodiment of the present invention, which was produced with the cooling time of 100 hours (cooling rate: 1K/hour).

[0024] FIG. 4 shows an SEM photograph of samples having a composition of x=0.020 of the thermoelectric material (y=0) according to the embodiment of the present invention, which was produced with the cooling time of 8 hours (cooling rate: 12.5K/hour).

[0025] FIG. 5 shows graphs indicating the temperature dependence of (a) Seebeck coefficient S, (b) electrical conductivity , (c) power factor S.sup.2, and (d) dimensionless figure of merit ZT of samples (melt grown) having compositions of x=0 and x=0.020 of the thermoelectric material (y=0) according to the embodiment of the present invention, which was produced with the cooling time of 100 hours (cooling rate: 1K/hour), as well as, comparative samples (SPS) prepared by spark plasma sintering (SPS) so as to have compositions of x=0 and x=0.020.

[0026] FIG. 6 shows graphs indicating the temperature dependence of (a) Seebeck coefficient S, (b) electrical conductivity , (c) power factor S.sup.2 of a sample (8 h) having a composition of x=0.020 of the thermoelectric material (y=0) according to the embodiment of the present invention, which was produced with the cooling time of 8 hours (cooling rate: 12.5K/hour), as well as, a sample (100 h) having a composition of x=0.020 of the same, which was produced with the cooling time of 100 hours (cooling rate: 1K/hour).

[0027] FIG. 7 is a graph indicating the temperature dependence of the power factor S.sup.2 when the value of y of the thermoelectric material (x=0.03) according to the embodiment of the present invention was varied from 0 to 0.05.

EMBODIMENTS OF THE INVENTION

[0028] Hereinafter, the embodiment of the present invention is explained below on the basis of drawings.

[0029] FIG. 1 to FIG. 6 show the thermoelectric material according to the embodiment of the present invention.

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 (Mn.sub.1-x-yV.sub.xFe.sub.y)Si.sub. (0.012x0.045, 0y0.06, and 1.71.8).

[0030] 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.

[0031] Next, the effects are as explained below.

[0032] 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 S.sup.2 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.

[0033] 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 S.sup.2 can be increased, and thus thermoelectric properties can be improved.

EXAMPLE 1

[0034] A thermoelectric material having the composition of (Mn.sub.1-xV.sub.x)Si.sub. (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 m were used. Samples of the thermoelectric material were produced as follows.

[0035] 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 =1.740 and x=0 to 0.060 (hereinafter, referred to as melt grown) were produced.

[0036] 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 =1.740, and x=0 and 0.020.

X-Ray Diffraction

[0037] 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 CuK line. The thus obtained XRD pattern is shown in FIG. 1.

[0038] As shown in FIG. 1, peaks derived from a Mn subsystem (peaks 211, 220, and 112), a peak derived from a Si subsystem (peak 111), and satellite peaks (peaks 2111 and 2221) were confirmed. Of these peaks, the peak derived from the Si subsystem and the satellite peaks were confirmed to come closer to the lower angle side and be sharp peaks in the case of x=0.015 or more. It was confirmed that peaks corresponding to MnSi were observed in the case of x=0 and x=0.010, but the peaks disappeared and no such peaks were observed in the case of x=0.015 or higher. It was also confirmed that peaks corresponding to VSi.sub.2 were observed in the case of x=0.050 and x=0.060, however, the peaks disappeared and no such peaks were observed in the case of x=0.040 or lower. These results indicate that the precipitation of MnSi in layers and the precipitation of VSi.sub.2 are inhibited in the case of x=0.012 to 0.045, resulting in the composition of the single-phase (Mn.sub.1-xV.sub.x)Si.sub. alone.

[0039] 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 FIG. 1, when the time for cooling from 1200 C. (1473K) to 1100 C. (1373K) had been varied to 8 hours (cooling rate: 12.5K/hour), 24 hours (cooling rate: 4.2K/hour), and 100 hours (cooling rate: 1K/hour). The thus obtained XRD pattern is shown in FIG. 2.

[0040] As shown in FIG. 2, with any cooling time, no precipitation of MnSi in layers and no precipitation of VSi.sub.2 were observed, confirming that the composition was composed of the single-phase (Mn.sub.1-xV.sub.x)Si.sub. alone.

Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy

[0041] 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 FIG. 3 and FIG. 4.

[0042] As shown in FIG. 3(a) to (c), a plurality of lines composed of MnSi were confirmed at a cooling rate of 1K/hour and in the case of x=0; that is, MnSi.sub.1.740. The MnSi had a thickness of about 500 nm, and precipitated in layers with a period of several tens of microns. In contrast, as shown in FIG. 3(d) to (f), a homogenous element distribution was exhibited at a cooling rate of 1K/hour and in the case of x=0.020; that is, (Mn.sub.0.980V.sub.0.020) Si.sub.1.740, and no precipitation of MnSi in layers was confirmed.

[0043] Furthermore, as shown in FIG. 4, a linear crack (upper left in FIG. 4) was formed at a cooling rate of 12.5K/hour and in the case of x=0.020; that is, (Mn.sub.0.980V.sub.0.020)Si.sub.1.740, however, a homogenous element distribution was exhibited, and no precipitation of MnSi in layers was confirmed. The above results in FIG. 1 to FIG. 4 indicate that the precipitation of MnSi in layers can be inhibited by the partial substitution of Mn elements with V.

Thermoelectric Properties

[0044] 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.

[0045] The temperature dependence of Seebeck coefficient S, electrical conductivity , power factor S.sup.2, and dimensionless figure of merit ZT (Z=S.sup.2/, where Z denotes the figure of merit, denotes the thermal conductivity, and T denotes the absolute temperature) of each sample found by measurement of the thermoelectric properties are shown in FIG. 5(a) to (d), respectively. As shown in FIG. 5(a), when x=0 and x=0.020 were compared, a slight decrease in Seebeck coefficient S due to partial substitution of Mn elements with V was confirmed for both melt grown samples and SPS samples. It was also confirmed that when the values of x are the same, melt grown samples exhibited Seebeck coefficient S slightly lower than those of SPS samples.

[0046] As shown in FIG. 5(b), both melt grown samples with x=0.020 and SPS samples with x=0.020 were confirmed to have electrical conductivity higher than those of the same with x=0. This may be due to introduction of hole carriers as a result of partial substitution of Mn elements with V. Moreover, in the case of x=0, melt grown samples and SPS samples were confirmed to have almost the same electrical conductivity , however, in the case of x=0.020, melt grown samples were confirmed to have electrical conductivity higher than those of SPS samples. This is considered that the precipitation of MnSi in layers can be effectively inhibited by solidification via slow cooling and partial substitution of Mn elements with V (vanadium).

[0047] As shown in FIG. 5(c), the power factor S.sup.2 was confirmed to be the highest in the case of melt grown samples with x=0.020. This is because, unlike Seebeck coefficient S, electrical conductivity (see FIG. 5(b)) differed significantly due to the presence or the absence of substitution with V or the production method. This indicates that solidification via slow cooling and partial substitution of Mn elements with V lead to increases in power factor S.sup.2. However, when the amount of V to be added is excessively increased, hole carriers are excessively introduced, and thus the power factor S.sup.2 can decrease. In addition, melt grown samples with x=0.020 shown in FIG. 5(c) exhibited the highest power factor S.sup.2 of 2.4 mW/K.sup.2m at 800K, 2.2 mW/K.sup.2m or more at 700K to 900K, and 1.4 mW/K.sup.2m or more at 300K to 1000K.

[0048] As shown in FIG. 5(d), the dimensionless figure of merit ZT was confirmed to be the highest in the case of melt grown samples with x=0.020, similarly to power factor S.sup.2. This indicates that solidification via slow cooling and partial substitution of Mn elements with V lead to increases in dimensionless figure of merit ZT. In addition, the dimensionless figure of merit ZT at this time was 2 or more times that of melt grown samples with x=0, exhibited the highest value of 0.59 at 800K to 900K, about 0.50 or more at 700K to 1000K, and 0.15 or more at 300K to 1000K.

[0049] 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 FIG. 5. The temperature dependence of the thus measured Seebeck coefficient S, electrical conductivity , and power factor S.sup.2 are shown in FIG. 6(a) to (c), respectively. In addition, FIG. 6 shows for comparison the results of melt grown samples with x=0.020 subjected to 100 hours of cooling (cooling rate: 1K/hour) shown in FIG. 5.

[0050] As shown in FIG. 6(a), the Seebeck coefficient S was confirmed to exhibit almost the same value even when the cooling time (cooling rate) was varied. As shown in FIG. 6(b), it was confirmed that the longer the cooling time (cooling rate was low), the higher the electrical conductivity . It is considered that since cracks were formed in the case of samples subjected to 8 hours of cooling (cooling rate: 12.5K/hour), as shown in FIG. 4, so that electrical conductivity decreased. As shown in FIG. 6(c), it was confirmed that the longer the cooling time (cooling rate was low), the higher the power factor S.sup.2, because of a significant difference in electrical conductivity (see FIG. 6(b)).

[0051] Based on the results in FIG. 2, FIG. 4 and FIG. 6, it can be said that the precipitation of MnSi in layers can be inhibited even in the case of 8 hours of cooling (cooling rate: 12.5K/hour), however, defects such as cracks or voids are caused to take place in such a case. Hence, the cooling time should be longer (the cooling rate should be lower) in order to improve the thermoelectric properties.

[0052] Moreover, samples in the case of 8 hours of cooling (cooling rate: 12.5K/hour) as shown in FIG. 6(c) exhibited the highest power factor S.sup.2 of about 2.0 mW/K.sup.2m at 800K, 1.8 mW/K.sup.2m or more at 700K to 900K, and 1.2 mW/K.sup.2m or more at 300K to 1000K.

EXAMPLE 2

[0053] A thermoelectric material having a composition of (Mn.sub.0.97-yV.sub.0.03Fe.sub.y)Si.sub.1.7 (x=0.03, =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 m, 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.

[0054] Each sample was measured for Seebeck coefficient and electrical conductivity in a manner similar to that of Example 1. For comparison, MnSi.sub.1.7 samples were also produced and measured similarly. The temperature dependence of the power factor S.sup.2 found for each sample by measurement is shown in FIG. 7. As shown in FIG. 7, the power factor S.sup.2 was confirmed to be high in the case of y=0 to 0.04. Particularly in the case of y=0.01 to 0.04, the power factor S.sup.2 was confirmed to be somewhat higher than that of melt grown samples with x=0.020 in FIG. 5(c).

[0055] 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 FIG. 5(c) in order to inhibit the precipitation of MnSi in layers, so that the amount of hole carriers was excessive because of V. Hence, it is considered that the addition of Fe, and partial substitution of Mn with Fe for electron doping inhibited increases in hole carriers. As shown in the results in the case of y=0.01 to 0.04, the power factor S.sup.2 increased. In the case of y=0.01 to 0.04, it is considered that an effect of inhibiting the precipitation of MnSi in layers due to increased V was enhanced, so that the power factor S.sup.2 was somewhat higher than that of melt grown samples with x=0.020 in FIG. 5(c). It is considered that because of a state of excessive hole carriers when no Fe (in the case of y=0) was added, the power factor S.sup.2 was somewhat lower than that in the case of y=0.01 to 0.04. Furthermore, it is considered that the addition of Fe at a high level (in the case of y=0.05) increased the amount of doped electrons and made the amount of hole carriers insufficient, so that the power factor S.sup.2 decreased to a level lower than that in the case of y=0.01 to 0.04.