JP5267993B2 - Thermoelectric element - Google Patents

Description

  The present invention relates to a thermoelectric element, and more particularly to a thermoelectric element exhibiting high efficiency at a low temperature of around 100K.

  A thermoelectric element refers to an element that can create a temperature difference by taking out electric power from a temperature difference given to a material and flowing a current. The case where electric power is taken out from the temperature difference is called thermoelectric power generation, and it is attracting attention as an exhaust heat recovery technology because it can generate power if there is no moving part and there is only a temperature difference, and it is generally called Peltier cooling when used for cooling. It is used for applications that require precise temperature control, such as cooling of laser elements for optical communication.

The performance of this thermoelectric conversion element can be expressed by the dimensionless figure of merit ZT shown below, and those having a high value indicate excellent characteristics.
ZT ＝ S ² T / ρκ
[S: Seebeck coefficient (V / K), T: temperature (K), ρ: electrical resistivity (Ω · m), κ: thermal conductivity (w / m · K)]

Examples of thermoelectric conversion element materials that have been put to practical use include bismuth / tellurium-based materials, lead / tellurium-based materials, and silicon / germanium-based materials.
These element materials have a ZT value of about 1, but ZT has a maximum value only at some temperatures, and bismuth and tellurium materials are around room temperature (300K). ZT is the maximum at around 700K for lead / tellurium-based materials and 1200K for silicon-germanium-based materials.

In recent years, cobalt oxides (ZT to 1.2 at 600K) (Patent Document 1) and SrTiO ₃ (ZT to 0.37 at 1000K) (Non-Patent Document 1) have been found to have a high ZT value and attract attention. Yes.
However, this type of oxide-based thermoelectric element is expected to be used mainly at high temperatures, taking advantage of its chemical stability at high temperatures, and ZT decreases at low temperatures.

  As described above, there is a temperature range in which the characteristics of conventional thermoelectric elements are improved, but many thermoelectric elements are materials that take into account use at room temperature or higher, and many of them are at low temperatures (100K). At present, ZT has decreased to about 0.01 or less, and few thermoelectric elements have been reported that exhibit excellent thermoelectric properties in a cryogenic temperature region below room temperature.

  By the way, as described above, the recovery of exhaust heat using a thermoelectric element has been directed to recovering energy from a high temperature above room temperature, but the thermoelectric element has only a desired temperature difference. The energy can be recovered, and exhaust heat recovery using an extremely low temperature of about 100 K, for example, below room temperature, is also possible.

That is, it is possible to extract energy using low-temperature liquefied gas that is used in large quantities in the manufacturing industry and hospitals.
Here, looking at the temperature of the main liquefied gas, it is liquefied nitrogen: 77K, liquefied oxygen: 90K, and liquefied argon: 87K.
Therefore, if a thermoelectric conversion element material with good characteristics is found at a low temperature of about 100K, efficient energy recovery is possible. For example, efficient power generation can be performed by attaching a thermoelectric conversion element for a liquefied petroleum gas heat exchanger or the like found in Patent Document 2.

  Moreover, such a thermoelectric element can be used not only for thermoelectric generation at low temperatures but also for Peltier cooling. For example, if a thermoelectric conversion element material having high characteristics in the vicinity of 100K, the superconducting transition temperature is 100K. It is also possible to replace the conventional refrigerator for cooling the high-temperature superconductor device that is before and after, and to create a highly reliable device without an operating part.

  In Patent Document 3, Non-Patent Document 2, Non-Patent Document 3, and the like, it is represented by the chemical formula LaFeOPh (Ph is at least one of P, As, and Sb), and is a ZrCuSiAs type (space group P4 / nmm). The compounds having the crystal structure are disclosed, and it has been reported that these are superconductors having a superconducting transition temperature (Tc) exceeding 50 K, but the thermoelectric properties of these compounds have not been studied at all. .

JP 2002-100814 A JP 2006-345609 JP 2007-320829

APPLIED PHYSICS LETTERS 87, 092108 2005 Journal of Alloys and Compounds 302 70-74 2000 Journal of the Physical Society of Japan Vol.77, 063707 2008 Japanese Journal of Applied Physics Vol. 43, L540, 2004

  The present invention has a high ZT value at a low temperature of around 100K, although its structure is different from the conventional one, and is useful for recovering cold energy such as liquefied gas and cooling high temperature superconductors. An object is to provide an element.

In order to solve the above problems, the present invention has been intensively studied. As a result, a compound having a ZrCuSiAs type crystal structure disclosed in Patent Document 3 and the like and having a specific chemical composition exhibits a high Seebeck coefficient, The inventors have found that a very high ZT value can be maintained at a low temperature of around 100K, and have completed the present invention.
That is, this application provides the following invention.
<1> ZrCuSiAs type crystal structure and composition is LnMPnO _x (Ln is rare earth, M is at least one metal atom selected from Fe, Ru, Os, Co, Ni and Cu, Pn is P, As Or Sb, x represents a number of 1.1 to 0.5).
<2> The thermoelectric element according to <1>, having a ZrCuSiAs type crystal structure and having a composition represented by LnFeAsO _x (Ln and x are the same as described above).

The material of the thermoelectric element according to the present invention is disclosed in Patent Document 3 and the like, but application as a thermoelectric element cannot be expected. However, according to detailed studies by the present inventors, it has been found that such a compound exhibits a high Seebeck coefficient.
Therefore, according to the present invention, it is possible to provide a new type of thermoelectric element whose structure is different from the conventional one.
Moreover, such a thermoelectric element maintains a very high ZT value at a low temperature of around 100K.
From this, the thermoelectric element of the present invention can be used for power generation using a low-temperature liquefied gas in the case of thermoelectric power generation, and a superconducting filter device using a high-temperature superconductor when used as a thermoelectric cooling element. It can be applied as an element for cooling.
Furthermore, since the thermoelectric element of the present invention can provide a low-resistance ohmic contact even when an electrode is formed by a simple method of silver paste, when a device is formed as a thermoelectric conversion module, element performance due to simple element creation and contact resistance Can be prevented.

Typical ZrCiSiAs crystal structure diagram XRD pattern of thermoelectric element material compound according to Example 1 The graph showing the relationship between the temperature of the thermoelectric element according to Example 1 and the dimensionless figure of merit ZT

  The thermoelectric element of the present invention has a ZrCuSiAs type crystal structure and a composition of LnMPnOx (Ln is a rare earth, M is at least one metal atom selected from Fe, Ru, Os, Co, Ni and Cu, and Pn is A compound represented by P, As or Sb, wherein x represents a number of 1.1 to 0.5) is used.

That is, the first feature of the thermoelectric element of the present invention is that the material has a crystal structure called ZrCuSiAs type. A typical example of this crystal structure is shown in FIG. As seen in Fig. 1, this crystal structure is composed of alternating rare earth-oxygen (Ln ₂ O ₂ ) layers and iron-arsenic (Fe ₂ As ₂ ) layers, and each layer is bonded by edge sharing. OLN ₄ tetrahedron that is constituted by FeAs ₄ tetrahedra.

The second feature of the thermoelectric device of the present invention is that its chemical composition is LnMPnO _x (Ln is a rare earth, M is at least one metal atom selected from Fe, Ru, Os, Co, Ni and Cu, Pn Is P, As or Sb, and x is a number from 1.1 to 0.5.
Examples of the rare earth of Ln in the chemical composition formula include La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Among these, La, Ce, Pr, Nd, and Sm are preferable. M represents a transition element, and specific examples thereof include Fe, Ru, Os, Co, Ni, and Cu, and Fe, Ru, and Co are preferable. Pn represents a pnicrite element, and examples thereof include P, As, Sb, and Se, but As and Sb are preferable.

A typical example of a chemical represented by LnMPnO _{x according} to the present invention is as follows.
Ln (Fe _1-x Co _x ) AsO _y (x = 1 ~ 0.1, y = 1.1 ~ 0.5) (Ln = rare earth)
LnFeAsO _1-x F _y (x = 0 ~ 0.5, y = 0 ~ 0.5) (Ln = rare earth)
Ln _1-x Lan _x FeAsO _y (x = 0 ~ 0.5, y = 1.1 ~ 0.5) (Ln = rare earth, Lan = rare earth)
LnFeAs _1-x Pn _x O _y (x = 0 ~ 0.5, y = 1.1 ~ 0.5) (Ln = rare earth, Pn = P, Sb)

  In the present invention, a part of the chemical composition may be variously changed within a range in which the crystal structure does not substantially change. For example, a part of Ln, which is a rare earth element, may be substituted with an alkaline earth metal such as Ca, Sr, and another part of M, which is a transition metal, Fe, Ru, Os, Co, Ni, Cu. The transition metal may be replaced with a part of Pn, which is a pnictite element, and Ga or Se may be replaced with F, and a part of oxygen O may be replaced with F. Such a change makes it possible to variously change ZT of thermoelectric characteristics.

The manufacturing method of the compound which comprises the thermoelectric element of this invention is demonstrated. As described above, the compound itself is a known substance as described in, for example, JP-A 2007-320829, Non-Patent Document 2, Non-Patent Document 3, and the like. Although it may be produced according to a method according to the method, a method for producing these compounds will be briefly described below.
There are two types of these methods. The first method is a normal pressure synthesis method using a quartz tube sealed, and the other is a high temperature high pressure synthesis method.
In the normal pressure synthesis method, the constituent elements of the compound itself or a compound such as a rare earth, an arsenic compound or iron oxide is mixed as appropriate, then press-molded into a pellet shape, sealed in a quartz tube, and then placed in an electric furnace. This is a method of causing a solid phase reaction by firing.
The high-temperature and high-pressure synthesis method is a method in which a solid-phase reaction is advanced using a high-temperature and high-pressure generator at a pressure of 10,000 atmospheres or more as in the above-described normal pressure synthesis method. The high-temperature and high-pressure synthesis method uses high pressure to confine the compound in a sealed space, so that it does not easily deviate from the stoichiometric composition even if it contains an element with a high vapor pressure that easily evaporates, or the compound lacking oxygen. There is a feature that is easy to synthesize. Therefore, in this invention, it synthesize | combined by the high temperature / high pressure synthesis method.

The material itself of the thermoelectric element according to the present invention is known per se as described above, but it has been a specific event unexpected by those skilled in the art that it has excellent thermoelectric properties.
That is, the compound disclosed in Patent Document 3 and the like was discovered as a superconducting material and could not be expected to be applied as a thermoelectric element. It was found to show ZT.
Therefore, the present invention has great technical significance in that a compound having a structure different from that of the conventional one is provided as a new type of thermoelectric element.
Moreover, the thermoelectric element according to the present invention maintains a very high ZT value at a low temperature of around 100K.
In other words, Bi-Te materials widely used as thermoelectric elements have reported values of ZT = 0.85 at 300K for Bi _1.65 Te ₃ , but this is largely reported because this value drops significantly at 100K. There is no example. Next, the cobalt oxide reported as an oxide thermoelectric material has a maximum ZT value of 600K or higher, and ZT = 0.5 at 300K, and no ZT value at a low temperature of around 100K has been reported. ZT is expected to drop considerably. SrTiO ₃ is a sample doped with Nb, and the top data of ZT = 0.35 has been reported. This is a very high temperature of 1000K at which ZT is maximum. In the sample in which SrTiO ₃ was doped with Y, the ZT peak was about 500 K, which was a relatively low temperature as an oxide thermoelectric material (Non-patent Document 4). However, even with Y-doped SrTiO ₃ having a ZT peak at a relatively low temperature, ZT is significantly reduced at 100K.
For this reason, the thermoelectric element of the present invention uses, for example, a heat exchanger for recovering cold energy from liquefied gas in the case of thermoelectric power generation, and a high-temperature superconductor when used as a thermoelectric cooling element. It can be applied as a cooling element such as a superconducting filter device.
Furthermore, since the thermoelectric element of the present invention can provide a low-resistance ohmic contact even when an electrode is formed by a simple method of silver paste, when a device is formed as a thermoelectric conversion module, element performance due to simple element creation and contact resistance Can be prevented.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Example 1
[Synthesis of thermoelectric material compounds]
(A) Metal lanthanum having a purity of 3N or more and metal arsenic were mixed at a stoichiometric composition of 1: 1, and the approximate mixture was vacuum sealed in a quartz tube. The quartz tube was then heated to 500 ° C. at 50 ° C./Hr in a box electric furnace and held there for 15 hours. Subsequently, the temperature was raised to 850 to 1000 ° C. at 50 ° C./Hr, held for about 10 hours, and then cooled in the furnace to synthesize lanthanum arsenide.
(B) The lanthanum arsenide obtained in (a), Fe, and Fe ₂ O ₃ are weighed so as to have a desired stoichiometric composition, mixed in a mortar, and then a high pressure of 1 GPa or more using a Dia type cubic anvil device Then, the graphite heater was energized and synthesized at a temperature of 1150 ° C to 1200 ° C. At this time, various oxygen-deficient compounds represented by LaFeAsO _0.65 were obtained by appropriately increasing the amount of Fe.
(C) Identification of crystal structure Powder X-ray diffraction of the LaFeAsO _x sample synthesized in this way was performed using an XRD apparatus manufactured by Rigaku Corporation, and its crystal phase was identified. The XRD pattern of LaFeAsO _x is shown in FIG. From this figure, it was found that this polycrystal was almost single phase and had a ZrCuSiAs type crystal structure.
(D) Evaluation of thermoelectric characteristics Next, the thermoelectric characteristics of this sample were evaluated. QuantumDesign PPMS and thermal transport options were used for this evaluation. The sample was molded to about 1.2 mm × 1.2 mm × 4 mm, and the surface was mirror-polished to confirm that no cracks occurred, and then the thermal conductivity and Seebeck coefficient were measured by the two-terminal method. Separately from the above measurement, a sample was cut into a size of about 0.2 mm × 0.5 mm × 3.5 mm, and the electrical resistivity was measured by the DC four-terminal method, and the dimensionless figure of merit ZT was calculated from the above values. The results are shown in the table. At this time, an epoxy silver paste was used for the electrode, and a low resistance ohmic contact could be realized without going through special baking and other processes.
FIG. 2 shows the dimensionless figure of merit ZT of the sample. FIG. 2 shows that this sample has a peak at a low temperature around 100 K, and unlike the conventional thermoelectric material, high performance is exhibited in a temperature range where ZT is very small. That is, since this temperature range is close to the temperature of liquid nitrogen, it can be applied as a power source for a liquid nitrogen storage tank monitoring system.

Examples 2-8
By the method according to Example 1, a layered oxyntide compound composed of different rare earths was synthesized. The process is shown below.
(A) Metal rare earth having a purity of 3N or more and metal arsenic were mixed at a stoichiometric composition of 1: 1, and the approximate mixture was vacuum sealed in a quartz tube. The quartz tube was then heated to 500 ° C. at 50 ° C./Hr in a box electric furnace and held there for 15 hours. Subsequently, the temperature was raised to 850 to 1000 ° C. at 50 ° C./Hr, held for about 10 hours, and then cooled in the furnace.
(B) The rare earth arsenide obtained in (a), iron and ferric oxide were weighed to have a composition of LnFeAsO _x (Ln = rare earth), (x = 0.65, 0.7), and then mixed in a mortar did. Thereafter, pressurization was performed at a high pressure of 1 GPa or more using a Dia type cubic anvil apparatus, and further, a graphite heater was energized to synthesize at a temperature of 1150 ° C. to 1200 ° C. to obtain various layered oxyntide samples.
(C) Next, the dimensionless figure of merit ZT of these samples was measured in the same manner as in Example 1. The maximum value and the temperature are shown in Table 1.

Comparative Example 1
The dimensionless figure of merit ZT of Y-doped SrTiO ₃ , which is a conventionally known thermoelectric element, was measured in the same manner as in Example 1. The results are shown in Table 1.

From Table 1, it can be seen that the thermoelectric elements of Examples 1 to 8 of the present invention have an excellent dimensionless figure of merit ZT. Compared with the thermoelectric characteristics at 100 K of the Y-doped SrTiO ₃ of the comparative example, the LaFeAsO _{0.65 of the} example 1 has a ZT value of 7 times or more. The sample of Comparative Example 1 has a performance peak around 500K, but at 100K, ZT is significantly lower than the peak value. Considering that the liquid nitrogen temperature is 77K, it can be seen that the layered oxintide of the present invention has a peak ZT value of 80 to 120K, which is more suitable for recovering cold energy from the liquefied gas.

Claims (2)

ZrCuSiAs type crystal structure and chemical composition LnMPnO _x (Ln is rare earth, M is at least one metal atom selected from Fe, Ru, Os, Co, Ni and Cu, Pn is P, As or Sb Where x represents a number from 1.1 to 0.5). The thermoelectric conversion element according to claim 1, which has a ZrCuSiAs type crystal structure and has a chemical composition represented by LnFeAsO _x (Ln and x are the same as above).