JP5689185B2 - Samarium-cobalt thermoelectric composition having improved output factor and method for producing the same - Google Patents

Description

  The present invention relates to a samarium-cobalt thermoelectric composition having an improved output factor and a method for producing the same, and more particularly, in the samarium-cobalt thermoelectric composition, an improved output in which part of the samarium is replaced with strontium. A samarium-cobalt based thermoelectric composition having a factor is provided. According to the present invention, by adding a dopant to the samarium-cobalt-based thermoelectric composition and replacing at least a portion of the samarium, the electrical conductivity and Seebeck coefficient are adjusted to improve the output factor of the thermoelectric composition. Can do.

Thermoelectric compositions have many advantages such as thermal stability, oxidation resistance, and low toxicity. In addition, the components of thermoelectric oxides in particular are abundant resources that can be sought well. Therefore, such thermoelectric oxides have attracted attention as promising materials for energy sources used particularly at high temperatures. Terasaki has proposed Na _x CoO ₂ as an oxide having a high Seebeck coefficient, but it is reported that the oxide also exhibits excellent conductivity. From these facts, it is considered that the thermoelectric performance of oxides can be compared with the thermoelectric performance of intermetallic compounds. Among such thermoelectric oxides, cobalt oxide has high thermoelectric performance due to its anisotropic crystal structure, but according to this, the electric conductivity in the direction parallel to the laminate layer is in other directions. Therefore, it has anisotropy at least in terms of electrical conductivity. However, such anisotropy may cause complexity in the manufacture of modules using thermoelectric oxides, and such anisotropy interferes with the sinterability of thermoelectric oxides and is higher. It is an obstacle to embodying density.

In order to overcome the disadvantages due to the anisotropic crystal structure, there is an increasing need to develop a new oxide having an isotropic crystal structure based on a highly symmetric unit cell. Here, some researchers became interested for the first time on perovskite oxides such as CaMnO ₃ , LaNiO ₃ and the like.

  However, thermoelectric oxides have the problem of improving the power factor, which is the most important factor. The output factor can be expressed as the square of the Seebeck coefficient and the electrical conductivity, and the absolute temperature, but the Seebeck coefficient and the electrical conductivity are mutually independent variables, and there is a limit to amplifying the output factor by tuning this. was there. That is, since the improvement of the Seebeck coefficient is often linked with the decrease of the electrical conductivity, there is a problem that there are many obstacle elements for the improvement of the output factor.

  The present invention has been devised in order to solve the above-mentioned problems, and the present invention is to introduce a dopant into a samarium-cobalt thermoelectric composition to replace at least a part of samarium, The object is to improve the output factor of the thermoelectric composition by adjusting the electrical conductivity and Seebeck coefficient.

  That is, the purpose of obtaining a samarium-cobalt thermoelectric composition having a preferable output factor according to the present invention is to replace a part of samarium with strontium, or a part of samarium to strontium and a part of cobalt to iron. This can be achieved by simultaneous replacement.

  To achieve the above object, the present invention provides a samarium-cobalt thermoelectric composition having an improved output factor in which a portion of the samarium is replaced by strontium. .

  The amount of strontium to be substituted is preferably in the range of more than 0 and not more than 10 mol% when the total mol of samarium and strontium is 100 mol%.

  In the samarium-cobalt thermoelectric composition, a part of the cobalt is preferably replaced with iron.

  The amount of strontium to be substituted is in the range of more than 0 and 4 mol% or less when the total mol of samarium and strontium is 100 mol%, and the amount of iron to be substituted is the total mol of cobalt and iron. When the amount is 100 mol%, it is preferably in the range of more than 0 to 6 mol%, and the strontium and iron are preferably co-doped.

  The iron for substituting a part of cobalt is preferably in the range of more than 0 and less than 60 mol% when the total mol of cobalt and iron is 100 mol%.

  According to the present invention as described above, by replacing a part of the constituent elements of the samarium-cobalt thermoelectric composition with a dopant, more specifically, samarium is replaced with strontium, or samarium is replaced with strontium. By simultaneously substituting cobalt with iron, the electrical conductivity and Seebeck coefficient can all be adjusted appropriately, and as a result, the effect of improving the output factor of the thermoelectric composition is expected.

  In particular, it is a feature of the invention that all the electric conductivity and Seebeck coefficient can be improved in the thermoelectric composition, and from this, the enhancement of thermoelectric properties with the output factor as the main factor is expected.

2 is a graph showing the total composition for one embodiment of the present invention. To _SmCo 1-x _Fe x _{O 3} is an embodiment of the present invention, showing each (a) electric conductivity, (b) the Seebeck coefficient, (c) a power factor. The perovskite structure is schematically shown for (a) tetragonal crystal and (b) orthorhombic crystal. For Sm _1-x Sr _x Co _1-y Fe _y O ₃ (x = 0.5, y = 0, 0.2, 0.4, 0.6) composition that is one embodiment of the present invention An X-ray analysis result is shown. To _{Sm 1-y Sr y Co 0.6} Fe 0.4 O 3 composition which is an embodiment of the present invention, (a) electric conductivity, (b) the Seebeck coefficient, depending the temperature of (c) power factor It is a graph which shows sex. To _{Sm 1-y Sr y Co 1} -x Fe x O 3 composition which is an embodiment of the present invention, (a) electric conductivity, (b) the Seebeck coefficient, the temperature dependence of (c) power factor It is a graph to show. To _{Sm 1-y Sr y Co 1} -x Fe x O 3 composition which is an embodiment of the present invention, is a graph illustrating the (a) thermal conductivity, the temperature dependence of the (b) ZT value.

In the present invention, a samarium-cobalt-based thermoelectric composition as a thermoelectric material, particularly SmCoO ₃ oxide, was aimed at improving the output factor. The electronic ceramic perovskite structure has various advantages such as isotropic crystal structure and ease of control of electrical properties, and thus is highly evaluated as a thermoelectric material. In general, the electrical conductivity of an oxide depends on the carrier concentration and carrier mobility. In such a perovskite structure, when a dopant is used, the two elements of carrier concentration and carrier mobility can be controlled simultaneously. Above all, the carrier concentration can be adjusted very easily by adding a dopant.

In pure SmCoO _{3 the} valence of all Co ions is 3+. If some acceptors produce defects with such a perovskite having a substantially negative (-) value, the valence of the CO ³⁺ ion changes to 4+ corresponding to the amount of acceptor. To do. As a result, in the state where CO ³⁺ and Co ⁴⁺ are mixed in this way, the carrier concentration increases based on Verwey controlled ionic valence principle (Verwey controlled ionic valence principle). Such electrons also produce Sm _1-x A _x CoO ₃ perovskite p-type semiconductors. Moreover, in such an ABO ₃ perovskite system, the carrier concentration can be adjusted by replacing not only the A site but also the B site with dopants.

On the other hand, mobility can be controlled by adding a dopant to the thermoelectric composition to adjust the distortion of the crystal structure of the perovskite oxide. The degree of distortion is expressed as a classical parameter of the perovskite tolerance factor that represents the geometric distortion of the ABO ₃ perovskite. For an A site with a small ionic radius or a B site with a large ionic radius, the tolerance factor is less than one. Such a small tolerance factor causes distortion of the perovskite structure. As a result, such distortion interferes with electron-hole conduction and reduces hole mobility. That is, the carrier mobility of such a perovskite system can be changed by adjusting the ion radius ratio between the A site ion and the B site ion. The electrical properties of the perovskite oxide can be easily adjusted by such a method.

Therefore, in the present invention, in order to adjust the carrier concentration and the carrier mobility, two different dopants, namely, Sr ²⁺ ions are replaced and inserted into Sm ³⁺ sites, and Fe ³⁺ ions are replaced and inserted into CO ³⁺ sites. And the doping amount of Fe ³⁺ can be limited to 6 mol%. That is, in the simultaneous doping, when Fe ³⁺ and Sr ²⁺ are added in an amount of 6 mol% and 4 mol%, respectively, an improvement in output factor in a practical sense can be expected.

However, when co-doping and Fe ³⁺ is added in an excess amount of 6 mol%, an improvement in the Seebeck coefficient can be expected, but it negatively affects the electrical conductivity, and Sr ²⁺ is 4 In the case of adding an excess amount in excess of mol%, an improvement in electrical conductivity can be expected, but it negatively affects the Seebeck coefficient.

As described above, the effect of increasing the electric conductivity or Seebeck coefficient when an excessive amount of Fe ³⁺ or Sr ²⁺ is added will be described later.

  Further, in the samarium-cobalt thermoelectric composition according to the present invention, a large power factor can be expected to be improved by not partially doping, that is, by partially replacing Sm with Sr without replacing Co with Fe. However, this is also an important feature of the present invention and will be described later.

Such doping effects have resulted in changes in carrier concentration and mobility in the Sm _1-x Sr _x Co _1-y Fe _y O ₃ perovskite system and will explain the thermoelectric properties according to it.

<Production example>
Sm _1-x Sr _x Co _1-y Fe _y O ₃ was produced by a solid phase reaction method, but Sm ₂ O ₃ (purity 99.9%, high purity chemical, Saitama, Japan), SrCO ₃ was used as a starting material. (Purity 99.9%, high purity chemistry, Saitama, Japan), Co ₃ O ₄ (99.9% or more purity, high purity chemistry, Saitama, Japan), α-Fe ₂ O (purity 99.99%, High purity chemistry, Saitama, Japan) was mixed for 20 minutes using ethanol as a solvent and YSZ balls. Thereafter, the mixture was dried for several hours and then rinsed at 1000 ° C. for 10 hours. Thereafter, the powdered powder was molded for 5 minutes at a pressure of 2000 bar using the CIP method. And the said molded object was sintered for 5 hours in the temperature of 1200 degreeC, and air.

<Measurement example>
The phase of Sm _1-x Sr _x Co _1-y Fe _y O ₃ was analyzed using X-rays. Moreover, the thermoelectric characteristic was measured in the range of 473K-1223K. In addition, the temperature dependence of the electrical conductivity was measured by the 4-probe method, and the Seebeck coefficient was calculated from the slopes of the linear graphs of ΔV and ΔT. Here, ΔV is a thermoelectromotive force derived from the temperature difference ΔT. Further, the thermal conductivity was measured by a laser flash method.

<Embodiment>
FIG. 1 shows all the compositions used experimentally in the present invention. First, Fe ³⁺ ions were replaced with Co ³⁺ sites, and their amounts were changed to 2, 4, 10, 20, 40, and 60 mol%, that is, Co: Fe was changed from 98: 2 to 40:60. Doping Fe ³⁺ is to increase the Seebeck coefficient of the SmCo _1-y Fe _y O ₃ system, and to increase the thermoelectric power factor with it. Such a substitution amount is intended to show that the change of the Seebeck coefficient of the samarium-cobalt thermoelectric composition is changed by the substitution of Fe, and the addition amount of Fe from the viewpoint of the output factor in the samarium-cobalt thermoelectric composition It should be well known that it is not intended to represent a preferred amount of. In short, the reason for Fe ³⁺ doping is to increase the thermoelectric power factor by increasing the Seebeck coefficient of the perovskite composition of the present invention, but the preferred amount is not related to the above substitution amount.

Further, Sm ²⁺ ions were substituted at 20 mol% and 50 mol% at Sm ³⁺ locations, respectively, to make Sm _1-x Sr _x Co _0.6 Fe _0.4 O ₃ composition. Such a substitution amount is for showing that the transition of the electric conductivity of the samarium-cobalt thermoelectric composition is changed by the substitution of Sr, and is a preferable amount from the viewpoint of the output factor in the samarium-cobalt thermoelectric composition. It should be well known that it is not intended to indicate. In short, the reason for the Sr ²⁺ doping is to increase the thermoelectric power factor by increasing the electric conductivity of the perovskite composition of the present invention, but the preferable amount is not related to the above substitution amount.

FIG. 2 shows the Seebeck coefficient and power factor of the doped SmCo _1-y Fe _y O ₃ system (y = 0.2, 0.4, 0.6). That is, Sr doping is excluded. As intended, the Seebeck coefficient increased in proportion to the amount of Fe ³⁺ doping. However, the output factor shows a tendency to decrease with the amount of Fe doping. That is, as will be described later, such a reduction in output factor is possible by co-doping with Sr ²⁺ .

The cause of the increase in Seebeck coefficient according to Fe ³⁺ doping can be explained by the decrease in carrier mobility, but it can be seen that the Seebeck coefficient is inversely proportional to carrier mobility and carrier concentration, which can be explained as follows. .

[Equation 1]
S = (π ² / З) (K _B / q) K _B T {d [ln (σ (E))] / dE} E = E _F
= (π ² / З) (K _B / q) K _B T {(1 / n) (dn (E) / dE) + (1 / μ) (dμ (E) / dE)} E = E _F

Here, K _B : Boltzmann constant, q: charge charge, T: absolute temperature, σ: electrical conductivity, n: carrier concentration, μ: carrier mobility, respectively.

  As can be seen from Equation 1, the Seebeck coefficient is inversely proportional to the carrier mobility.

Therefore, as described above, doped Fe ³⁺ ions increase the Seebeck coefficient by reducing the mobility of the present SmCo _1-y Fe _y O ₃ system. Two types of factors are associated with a decrease in mobility of the system. The first reason why doping ions reduce carrier mobility is distortion of the crystal structure. The geometric distortion of a perovskite having a distortion degree of ABO _3- type is expressed as a classic parameter of a tolerance factor. Such a classic parameter is defined as follows.

[Equation 2]
t = (r _A + r _O ) / √2 (r _B + r _O )

Here, r _A , r _B , and r _O are the ionic radii of each ion. The ionic radius of the crystal structure is changed by the coordination number and the charged valence of each ion. In the perovskite structure, the t value is in the range of 0.75 to 1.1, but in the tetragonal perovskite structure, t has a value of approximately 1. In this case, when the oxygen (O) atom is positioned exactly at the octahedral apex around Co, such an octahedron need not be distorted as shown in FIG. However, t values for reduced to 1 or less, the shape distortion gradually increases in greater than r _B.

As a result, in the case of large r _B, tetragonal (cubic) becomes as cause (orthorhombic) distorted orthorhombic phase, therefore, oxygen atoms surrounding the Co ions released from the position of the vertex of the tetragonal crystal The structure is further distorted, and a shape like a zigzag chain appears as shown in FIG. The tolerance factor value decreases as the amount of Fe ³⁺ added increases, and the tolerance factor calculated in the present invention is 20 when the total mole of Co and Fe is 100 mol%, It was 0.9594, 0.9589, and 0.9584 with respect to Fe ³⁺ (0.55 angstrom, 6 ^th coordinated) added at 40 and 60 mol%. Coordination numbers of A-site ion, B-site ion, and O ion (1.40 angstrom, 6 ^th coordinated) of 12, 6, 6 were used for calculation of tolerance factor values. .

In the present invention, Shannon's ionic radius was used to calculate the tolerance factor. This reduced tolerance factor accelerates the distortion of the perovskite structure that is the thermoelectric composition of the present invention. And such distortion due to a small tolerance factor can be confirmed from XRD analysis, which is shown in FIGS. 4 (a) to 4 (d). The degree of distortion of the perovskite structure can be confirmed from the split of the (020) peak and the (200) peak. As the X-ray pattern shows in FIG. 4 (a), the pure SmCoO ₃ sample can be named as a single phase symmetric to the orthorhombic phase of Pnma.

A perovskite structure having a large B site ion such as Fe ³⁺ is also named as an orthorhombic phase. However, the split of (020) and (200) peaks is greater than pure SmCoO ₃ . As a result, in the SmCo _1-y Fe _y O ₃ system, the crystal structure causes a more distorted crystal structure due to Fe ³⁺ ions having an ionic radius greater than Co ³⁺ , which is attributed to a small tolerance factor. And larger strains interfere with electron-hole conduction and reduce hole mobility.

  On the other hand, as a result of X-ray analysis, no secondary phase has been found in all compositions, and it appears that the secondary phase is not a factor that decreases the hole mobility.

Yet another reason for the decrease in mobility can be explained by the fact that the conduction path is blocked by Fe ³⁺ ions doped in the Co ³⁺ site. The conduction path of Co oxide is Co—O—Co bonding. Therefore, the addition of Fe to the Co site can hinder electron-hole conduction and reduce hole mobility.

The change in electrical conductivity according to the doping amount is shown in FIG. 2 (b), and the change in output factor is shown in FIG. 2 (c). Despite the increased Seebeck coefficient, the power factor value decreases in proportion to the amount of Fe ³⁺ ion doping, which is due to a decrease in electrical conductivity. Usually, in the low carrier concentration region, the output factor value is expressed as σ × S ² , which increases with an increase in carrier concentration, but increases in electrical conductivity.

  Thereafter, in the high carrier concentration region, the output factor value decreases with the increased carrier concentration, because the Seebeck coefficient decreases. Therefore, the output factor value has a maximum value in the intermediate region of the carrier concentration. However, as shown in FIG. 2C, the output factor value decreases with the doping amount in all regions. Such a result is considered to be because the carrier concentration, in particular, the doping amount of the dopant has not been optimized yet and is overdoped.

FIG. 5 shows Sm _1-x Sr _x Co _0.6 Fe _0.4 excessively doped to 20 and 50 mol% when the electrical conductivity and Sr are 100 mol% in the total mole of Sm and Sr. The electric conductivity (a), Seebeck coefficient (b), and output factor (c) of the O ₃ system are shown. As intended, the electrical conductivity increased proportionally with increasing Sr ³⁺ doping.

  The reason why the electrical conductivity is increased in the thermoelectric composition according to the present invention can be explained by two factors. One is an increase in carrier concentration, and the other is an increase in carrier mobility. The electrical conductivity increased with increasing acceptor doping.

FIG. 5A is a graph showing that the electrical conductivity increases with the doping amount in the Sm _1-x Sr _x CoO ₃ system. Such a tendency can be explained based on an increase in electron concentration due to a change in valence from CO ³⁺ to Co ⁴⁺ in the Verwey controlled ionic valence valence principle.

The tolerance factor value increases with the amount of Sr ²⁺ added. The calculated tolerance factor is 0 for pure substances, Sr ²⁺ (1.44 angstroms, 12th coordinated) added at 20 and 50 mol% based on 100 mol% of the sum of Sm and Sr. .9589, 0.9968, and 0.9991. In calculating the tolerance factor, the calculation of the ion radius of Co ions is based on the fact that the valence of CO ³⁺ (0.545 Å, 6th coordinated) is changed to the valence of Co ⁴⁺ in proportion to the amount of acceptor. is important. Therefore, in the present invention, the average radius value of Co ions must be selected and used in consideration of the change in charged valence according to the amount of dopant. That is, since the radii of CO ³⁺ and Co ⁴⁺ are different, an average value proportional to the abundance must be obtained and selected and used.

Such an increase in tolerance factor decreases the degree of distortion of the perovskite structure. And the distortion of the small precision resulting from a big tolerance factor can be confirmed by X-ray analysis as shown in FIG.4 (e). As described above, the degree of distortion can be confirmed from the split of the (200) peak and the (002) peak of the X-ray analysis result in the orthorhombic structure. In other words, for a less distorted perovskite, the peak split gradually decreases. As shown in the X-ray pattern shown in FIG. 4E, the X-ray result of the large A site ion Sr ²⁺ is orthorhombic. However, the peak split between (200) and (002) is less than pure SmCoO ₃ . As a result, Sr ²⁺ having an ionic radius larger than Sm ²⁺ in the Sm _1-x Sr _x CoO ₃ system has a large tolerance factor, thus completing a crystal structure having a distorted structure with less accuracy. . And such a low precision distortion increases electron-hole conduction and increases hole mobility.

FIG. 5 (c) shows the change of the output factor according to the doping amount while fixing the doping amount of Fe and changing the doping amount of Sr in the thermoelectric composition according to the present invention. As shown in FIG. 2B in which Fe ³⁺ was excessively doped, the output factor value was decreased by decreasing the Seebeck coefficient with respect to the doping amount in the entire range. Such a result is because the carrier concentration, that is, the doping amount, was over-doped without being optimized.

As a result, the present invention has a practical meaning from the composition of Sm _1-x Sr _x Co _1-y Fe _y O ₃ (x = 0, 0.2, y = 0.02, 0.05) with a small doping amount. There was an output factor that could be derived. FIG. 6 shows the thermoelectric performance with such a small doping amount. As intended, a small amount of doping significantly improved the output factor of the perovskite oxide. As shown in FIGS. 6A and 6B, it was found that the electrical conductivity and Seebeck coefficient also increased at the same time. As a result, the maximum value was represented by Sm _0.98 Sr _0.02 CoO ₃ . Such an increase in electrical conductivity appears to be caused by an increase in carrier concentration and carrier mobility due to a large tolerance factor, as described above. As a result, the maximum power factor of the system was 2.04 × 10 ⁻⁴ W / mK ² at 786 K for the above composition. Such a value exceeds the value of Ca ₃ Co ₄ O ₉ which is the most promising thermoelectric oxide. As shown in FIG. 1, it can be seen that when the doping amount is small, the output factor is higher than when the doping amount is large. A small amount of doping is represented as a solid circle, and a large amount of doping is represented as an open circle.

The thermal conductivity of this system was slightly higher than that of the Ca ₃ Co ₄ O ₉ thermoelectric material. As a result, the dimensionless figure of merit value was not high enough, as shown in FIG. When Sr is overdoped, for example, when 50 mol% Sr is doped, the displayed Sm _1/2 Sr _1/2 Co _1-x Fe _x O ₃ (x = 0, 0.1,. 2, 0.4, 0.6) The compositional output factor was very low, which is thought to be due to the low Seebeck coefficient due to doping with excessive amounts of Sr ²⁺ ions. Therefore, a practically meaningful output factor was obtained when the doping amount of Sr ²⁺ was 10 mol% or less based on the total mol of Sm and Sr of 100 mol%. Further, experimental results were obtained in which the output factor was further improved by doping Sr ²⁺ ions alone without doping Fe ³⁺ ions.

Even in the case of simultaneous doping with Fe ³⁺ ions, the doping amounts of Fe ³⁺ and Sr ²⁺ are replaced with Co and Sm up to 6 mol% and 4 mol%, respectively, in the thermoelectric composition according to the present invention. In this case, it should be noted that it represents the best output factor value, which is a practical result of the samarium-cobalt thermoelectric composition.

As mentioned above, although this invention was demonstrated based on the embodiment, it should not be interpreted limited to this embodiment, and the scope of the present invention should be interpreted based on a claim.

Claims (4)

A samarium-cobalt-based thermoelectric composition Sm _1-x Sr _x Co _1-y Fe _y O ₃ ,
A portion of the samarium is replaced with strontium, a portion of the cobalt is replaced with iron ,
Samarium-cobalt having an improved output factor, wherein the iron replacing a part of cobalt is in the range of more than 0 and not more than 10 mol% when the total mol of cobalt and iron is 100 mol% System thermoelectric composition Sm _1-x Sr _x Co _1-y Fe _y O ₃ . The improved output factor according to claim 1, wherein the amount of strontium to be substituted is in the range of more than 0 and less than 10 mol% when the total mol of samarium and strontium is 100 mol%. cobalt-based thermoelectric composition _{Sm 1-x Sr x Co 1} -y Fe y O 3 - samarium with. The amount of strontium substituted is in the range of more than 0 and less than 4 mol% when the total mol of samarium and strontium is 100 mol%, and the amount of iron to be substituted is the total mol of cobalt and iron. The samarium-cobalt-based thermoelectric power having an improved power factor according to claim 1, wherein when the amount is 100 mol%, it is in the range of more than 0 to 6 mol%, and the strontium and iron are co-doped. Composition Sm _1-x Sr _x Co _1-y Fe _y O ₃ . 2. The improved power factor according to claim 1, wherein the doping amount of strontium represents an improved power factor by adding 2 mol% when the total mole of samarium and strontium is 100 mol%. Samarium-cobalt thermoelectric composition Sm _1-x Sr _x Co _1-y Fe _y O ₃ .

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