JP4460861B2 - Thermoelectric conversion material - Google Patents

Description

  The present invention relates to a thermoelectric conversion material used for a thermoelectric conversion element that generates electricity from heat, and more particularly to a thermoelectric conversion material characterized by the composition of a material having high thermoelectric conversion performance.

A thermoelectric conversion material is used to generate power from a temperature difference. Conventionally, thermoelectric conversion materials used for such power generation have been developed, but the efficiency of thermoelectric conversion is not so good.
Further, currently used thermoelectric conversion materials are mainly Sb, Te, Se, etc., and many elements that are worried about toxicity are used (for example, see Patent Documents 1 and 2). The realization of the thermoelectric conversion material which does not contain is desired.
The energy conversion efficiency η, which is an index of the thermoelectric conversion efficiency, is determined by the following formula (1) depending on the high temperature side temperature T _H , the low temperature side temperature T _L, and the performance of the thermoelectric conversion material.

In the above formula (1), Z in the formula (1) is referred to as a thermoelectric figure of merit, and Z = S ² / (ρ · κ) using the thermoelectric power S, low efficiency ρ, and thermal conductivity κ of the material. ) Is an evaluation index having the dimension of the inverse of temperature defined by The expression S ² / ρ excluding κ from the thermoelectric performance index Z is called an output factor, and is similarly used as an electrical evaluation index of the thermoelectric material. In the formula (1), <T> is a temperature obtained by averaging _TH and _TL .
Usually, a thermoelectric figure of merit Z of a practical level is required to be 10 ⁻³ K ⁻¹ or more, and material development has been advanced so that as large Z as possible can be obtained by adjusting the composition of the material or controlling the material structure. .
The absolute value of the figure of merit is usually around 10 ⁻⁶ K ^{−1 for} metals, around 10 ⁻⁵ K ^{−1 for} semiconductors, and from 10 ⁻⁴ K ⁻¹ to 10 ⁻³ K ⁻¹ for optimized thermoelectric materials. It becomes an order.
Similarly, the power factor can be used in the order of 10 ⁻⁵ W / mK ² to 10 ⁻³ W / mK ² .

A material having high thermoelectric power is also useful as a temperature detection material. That is, since a temperature difference can be converted into an electrical output, it can be used as a sensor for detecting a temperature change.
At present, the thermoelectric power of industrially used thermocouples is from 10 μV / K to 50 μV / K, but if the material has a thermoelectric power of 100 μV / K to several hundred μV / K, the heat will be larger. When an electromotive force is obtained and the same thermal signal is input, signal detection by an electric circuit is facilitated, and when the same electric circuit system is used, detection sensitivity is increased.

Currently, Bi ₂ Te ₃ and PbTe are used as thermoelectric conversion materials. However, since characteristics are deteriorated due to oxidation in a high-temperature atmosphere, measures such as sealing with an inert gas are required.
In addition, both materials contain toxic elements that have an impact on the environment, and there are limits to their wide application in the consumer sector. Therefore, development of a thermoelectric conversion material that overcomes these problems is expected.
JP-A-8-199281 JP-A-10-32353

  The main object of the present invention is to provide a material composed of an element having low toxicity, excellent in heat resistance, etc. and having high thermoelectric conversion efficiency.

The present inventors have conducted various studies in view of the present state of the thermoelectric conversion material, and as a result, a compound with an element selected from a transition metal element, a group II element, and a group III element, forms a band gap. Properties or semi-metallic properties have been found, and these materials have high Seebeck coefficient and low electrical resistivity, and are found to be useful as thermoelectric conversion materials, thereby completing the present invention. It came to do.
That is, the present invention provides the following thermoelectric material.
1) Chemical composition is M _1-X Co _X A _y (M = one or more elements selected from Fe, Ru, Os, A = one or more elements selected from Ga, In, Zn, 0 ≦ x ≦ 1, 2.5 ≦ y ≦ 3) a thermoelectric conversion material characterized by comprising as a main component, 2) an absolute value of thermoelectric power of 50 μV / K or more 1 The thermoelectric conversion material described in 3), the absolute value of thermoelectric power is 100 μV / K or more, 1) the thermoelectric conversion material described in 4), and the resistivity is 10 ⁻³ Ωm or less, The thermoelectric conversion material according to any one of 1) to 3), 5) a resistivity at room temperature of 10 ⁻² Ωm or less, and a resistivity of 10 ⁻² Ωm or less in a temperature range of 500 ° C. or higher. The thermoelectric conversion material according to any one of 1) to 3), characterized in that: 6) an output factor at a high temperature The thermoelectric conversion material according to any one of 1) to 5), wherein the child exceeds 0.001 W / (mK ² ), and 7) a sintered body having a sintered density exceeding 95%. The thermoelectric conversion material according to any one of 1) to 6) is provided.

  By using the material having the above composition of the present invention for an n-type element or p-type element of a thermoelectric power generation module, or both p-type and n-type, electric energy can be efficiently recovered from a temperature difference. Further, the use of these materials has an excellent effect that a temperature change can be converted into a voltage signal with high sensitivity.

As described above, the composition of the material has a chemical composition of M _1-X Co _X A _y (M = one or more elements selected from Fe, Ru, Os, A = one or more elements selected from Ga, In, Zn) And a compound represented by 0 ≦ x ≦ 1, 2.5 ≦ y ≦ 3). If this composition is realized and a target compound phase is obtained, a thermoelectric material having high thermoelectric power and low resistivity can be realized.
The thermoelectric conversion material is effective when it contains the compound of the present invention as a main component (containing at least 50 wt% or more). Therefore, even when the above compound is compounded with other elements or compounds, it is effective as a thermoelectric conversion material.

This thermoelectric material synthesis method is a powder metallurgy method, a solution coagulation method, or a thin film obtained by plasma sputtering using a target manufactured by these methods, or a thin film shape using an unbalanced process. Things can be used.
In such a manufacturing process, what is important is to obtain a target phase. As described above, the target phase is obtained by the dissolution method, the thin film method, or annealing them.

The M _1-X Co _X A _y compound phase needs to be substantially formed, and Fe, Ru, Os, Co simple substance, and Ga simple substance metal are not used except for the purpose of the above-mentioned complexation. It is not preferable that it remains.
In the case of compounding, the formation of the M _1-X Co _X A _y compound phase is prepared to such an extent that the function as a thermoelectric conversion material can be sufficiently exhibited. It is not intended to form the M _1-X Co _X A _y compound phase. In this sense, they are essentially different.
Further, the ratio of A in the compound phase is preferably y = 3, for example, which is a stoichiometric composition, but is not necessarily in such a range. That is, if 2.5 ≦ y ≦ 3, a compound phase that sufficiently retains the function as the thermoelectric conversion material can be formed.
If y exceeds 3, the y component, which is a low melting point metal, exists as a simple substance, and when the temperature of the material is raised, a liquid phase appears and breaks down, so it is necessary to satisfy y ≦ 3. 2.5 ≦ y is a limit value necessary for maintaining the performance as a thermoelectric conversion material.
In addition, an absolute value of thermoelectric power of 50 μV / K or more is thermoelectric performance that can be easily realized in the thermoelectric conversion material of the present invention, and has great significance for thermoelectric conversion or temperature measurement sensors. More preferably, it is 100 μV / K or more. In particular, excellent thermoelectric power that can be used industrially can be obtained as a thermoelectric material for energy conversion.
The target phase can be obtained by heat treatment under the temperature condition in which no liquid phase exists from the binary phase diagram as shown in FIGS.

If single phase materials with the desired composition are obtained, if they are in the form of particles or powder, they are solidified and shaped by hot pressing or the like and used as thermoelectric elements.
If it is a molten solidified solid with high density, it can be cut out as it is and used as a thermoelectric material. A thin film formed by sputtering or the like using a target material made of the above material can be used in a state of being stacked on a substrate.
If the target phase is formed, it exhibits low resistivity and high thermoelectric power as described later, and can be used as a thermoelectric energy conversion element and a sensor material.

Next, examples of the present invention will be described. The following examples are provided so that the present invention can be easily understood, and the present invention is not limited or restricted to these examples. That is, all modifications and other aspects based on the technical idea of the present invention are included in the present invention.
(Example 1-4)

Using Mg (99.9%), Ru (99.99%), Os (99.99%), Co (99.99%), and Ga (99.999%) as raw materials, MGa ₃ can be accurately realized. Therefore, the stoichiometric composition was measured such that the molar ratio of M metal to Ga was just 1: 3.
This was then vacuum sealed in a quartz ampoule, FeGa ₃ and CoGa ₃ is 973 K, Ruga ₃ and OsGa ₃ was held for 24 hours at 1073 K.
As a result, the compound was synthesized by a solid phase reaction in the absence of a liquid phase. The obtained compound was pulverized in a mortar to obtain a raw material powder.

For these raw material powders, in an Ar atmosphere, FeGa ₃ and CoGa ₃ are at a temperature of 973 K and a pressure of 100 MPa for 2 hours, RuGa ₃ is a temperature of 1073 K and a pressure of 100 MPa for 2 hours, OsGa ₃ is 2 at a temperature of 1323 K and a pressure of 100 MPa. Under the conditions of time, hot pressing was performed to obtain sintered pellets. The density of the pellets was in the range of 94% to 99% of theory for all samples.
In order to confirm that the dense sintered body produced in this example is the target phase, the FeGa ₃ and RuGa ₃ powders were examined by X-ray diffraction. The resulting X-ray diffraction pattern is shown in FIG. As shown in FIG. 3, it was clear that the target phase was the main component.

Next, the dense single-phase pellets thus obtained were cut into rectangular parallelepipeds to obtain measurement samples, and the electrical resistivity and thermoelectric power were measured. The results are shown in Table 1 (MGA ₃ (M = Fe, Ru, Os, Co) resistivity at 320 K, thermoelectric power and output factor).
As shown in Table 1, M = Fe, Ru, and Os have a large thermoelectric power S whose absolute value exceeds 100 μV / K.
From this, it can be seen that a large electromotive force can be generated with a small temperature difference, and that it is promising as a temperature sensor material that does not contain toxic elements. In the case of M = Co, the resistivity ρ is as small as 10 ⁻⁶ ohm-m, and the output factor reaches 10 ⁻³ W / m−K ² .
This value reaches 25% -30% of the output factor of the Bi-Sb-Te-Se based material that is a practical material, and it can be seen that this is a very promising material in terms of power generation performance.

（実施例５−６）

(Example 5-6)

  Since all the compounds described in the present invention have the same crystal structure or a very similar crystal structure, they can be alloyed. It was confirmed that an alloy having such an intermediate composition exhibits excellent performance as a thermoelectric material. As an experimental method, a sample was prepared by a method of hot pressing after the same solid phase reaction as in Example 1-4. Table 2 shows the resistivity, thermoelectric power and power factor at 320K for these examples.

As shown in Table 2, the resistivity is low in both Examples 5 and 6, the absolute value of thermoelectric power is several tens of μV / K, and the metal has a large output factor of 10 ⁻⁴ W / (mK ² ) or more. You can see that
Since the thermoelectric power increases and the resistance tends to increase as the Co content decreases, it can be expected that the output factor will further increase with a composition having a higher Co content, both of which exhibit favorable characteristics as a thermoelectric material. I understand that.
That is, in the case of being represented by the chemical formula M _1-X Co _X Ga ₃ , x is preferably 0.5 <x <1.0, and more preferably, x is 0. It is considered that thermoelectric performance is optimal when 8 <x <0.95.
(Example 7-10)

The same sintered body samples as evaluated in Example 1-4 were produced, and the thermoelectric power, resistivity, and output factor when the temperature was changed at a high temperature were evaluated. The results are shown in FIGS. 4, 5, and 6.
Among these, the example number and the sample name correspond as follows. Example 7: FeGa ₃ , Example 8: RuGa ₃ , Example 9: OsGa ₃ , Example 10: CoGa ₃ .
As is clear from these figures, in Example 7-9, the resistance decreases with increasing temperature, and the resistivity is on the order of 10 ⁻⁵ to 10 ⁻⁴ ohm-m at room temperature. At the same time, the absolute value of thermoelectric power decreases, but for example, it has a value of 100 μV / K or more even at 800K, which is very preferable as a thermoelectric material.
In Example 10, since the low resistivity is maintained in all temperature ranges, and the absolute value of thermoelectric power increases as the temperature rises, the output factor has an excellent value of 10 ⁻³ W / mK ^{2 in} all temperature ranges. maintain. It is clear that the material composition of the present invention has very favorable properties as a thermoelectric material.
(Examples 11-20)

When the thermoelectric conversion material of the present invention is a sintered body obtained by a powder metallurgy method, it is important to improve the sintering density from the viewpoint of mechanical strength. For this reason, it is necessary to optimize the sintering conditions and increase the density of the sintered material. In this example, preferred conditions were found by appropriately adjusting the sintering temperature, sintering pressure, and sintering time. The results are shown in Table 3.
As common conditions, the sintering time was 2 hours and the sintering pressure was 100 MPa. Good in the sintered state of each material means that the sintered density exceeds 95% in this case.
When the sintering density is increased by raising the temperature to a temperature higher than that described for each material, it is considered that the sintering density is improved. However, there is a possibility that a liquid phase is generated as shown in FIGS. Can be assumed.
Therefore, these sintering conditions are considerably optimized, and it has been clarified by experiments relating to the present invention that the best sintering conditions are around ± 50 ° C. near the temperature described herein. Thus, it is possible to achieve a sintering density of 95% or more by setting the sintering conditions according to the material.

（実施例２１−３３）

(Examples 21-33)

As a thermoelectric conversion material, it is practically important that the thermal conductivity is low. That is, it is required that heat hardly flows when an element is disposed between a high temperature and a low temperature. From this viewpoint, the thermal conductivity of the sample relating to the thermoelectric conversion material of the present invention was evaluated. The results are shown in Table 4.
As is clear from Examples 25-31 in Table 4, in the binary system such as FeGa ₃ , RuGa ₃ , the thermal conductivity is approximately 4-5 W / mK, and in CoGa _{3 of} Example 24, the thermal conductivity is It was as high as 13 W / mK.
These values are all measured at room temperature. Since the lattice component of the thermal conductivity decreases at a high temperature, it can be expected that the thermal conductivity will be significantly reduced.
Examples 21 to 23 are ternary examples, in which the chemical formula MGa ₃ is replaced with two elements (Fe and Co) so that the molar ratio of the M element portion is the same. .
In this case, since phonon propagation is suppressed by alloy scattering, the thermal conductivity is greatly reduced as compared with the binary system. The thermal conductivity of ordinary thermoelectric materials is 1.5-2.5 W / mK, and it is 4 W / mK for skutterudite compounds that are currently being developed or iron silicides that have already been used in the market. In view of this, the thermal conductivity of the thermoelectric conversion material of the present invention is sufficiently low, which is preferable for application to thermoelectric conversion.

（実施例３２）

(Example 32)

Example 32 listed in Table 4 is repeated. In Example 32, a RuIn ₃ compound powder was synthesized under the same conditions as in Example 1, and sintered by a hot press method to obtain a dense sample. For Example 32, the resistivity and thermoelectric power were measured at 320 K to determine the output factor. The results are shown in Table 5.
As shown in Table 5, the thermoelectric power is negative and its absolute value is very large as 400 μV / K. As a result, the output factor is on the order of 10 ⁻⁴ W / mK ² , and it is considered that it can be used as a thermoelectric material.

（実施例３４）

(Example 34)

For the sample of Example 33, the resistivity and thermoelectric power when the temperature was changed at a high temperature were measured, and the output factor was calculated. The results are shown in FIGS.
The resistivity decreases with increasing temperature and is on the order of 10 ⁻⁵ ohm · m, and the sign of thermoelectric power is inverted at 400 K with increasing temperature, changes from negative to positive, and the maximum output factor of about 750 K is 4.3 × 10. to have ^-4 W / mK ² revealed.

  The present invention provides a material that is composed of an element having low toxicity, has excellent heat resistance, and has high thermoelectric conversion efficiency, and is useful as a thermoelectric conversion device or a temperature detection material.

It is a Fe-Ga phase diagram. It is a Co-Ga phase diagram. It is a graph illustrating a powder X-ray diffraction pattern of FeGa ₃ and Ruga _3. MGa ₃ is a diagram showing (M = Fe, Ru, Os , Co) the temperature dependence of the resistivity. MGa ₃ is a diagram showing (M = Fe, Ru, Os , Co) the temperature dependence of the thermopower of. MGa ₃ is a diagram showing (M = Fe, Ru, Os , Co) the temperature dependence of the output factor. RuIn is a graph showing the temperature dependence of the _third resistivity. Is a graph showing the temperature dependence of the thermopower of Ruin _3. Is a graph showing the temperature dependence of the output factor of Ruin _3.

Claims (7)

Chemical composition is M _1-X Co _X A _y (M = one or more elements selected from Fe, Ru, Os, A = Ga, In, Zn, one or more elements selected, 0 ≦ x ≦ 1 , 2.5 ≦ y ≦ 3) as a main component, and a thermoelectric conversion material.   The thermoelectric conversion material according to claim 1, wherein the absolute value of thermoelectric power is 50 μV / K or more.   The thermoelectric conversion material according to claim 1, wherein the absolute value of thermoelectric power is 100 µV / K or more. The thermoelectric conversion material according to claim 1, wherein the resistivity is 10 ⁻³ Ωm or less. And a resistivity of 10 ^-2 [Omega] m or less at room temperature, the thermoelectric conversion material according to claim 1, 500 ° C or higher temperature range in resistivity is characterized in that at 10 ^-2 [Omega] m or less . The thermoelectric conversion material according to any one of claims 1 to 5, wherein an output factor at a high temperature exceeds 0.001 W / (mK ² ). The thermoelectric conversion material according to any one of claims 1 to 6, wherein the thermoelectric conversion material is a sintered body having a sintered density exceeding 95%.

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