JP2010050185A - Mg INTERMETALLIC COMPOUND, AND DEVICE APPLYING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a p-type Mg metal compound (Mg ₂ X) excellent in electrical conductivity. A general formula having an inverted fluorite structure: Mg ₂ X (X represents group 4 elements Si and Ge and One or a plurality of elements selected from Sn), and Li was included as an acceptor dopant. Li has a high solid solubility limit with respect to Mg ₂ X, the Mg ₂ X with high electrical conductivity. Moreover, since Li has low toxicity, it becomes Mg ₂ X which is environmentally friendly.
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Description

  The present invention relates to a p-type Mg intermetallic compound and a device using the same.

  As thermoelectric conversion materials made of semiconductor materials, semiconductor materials such as Bi-Te have been studied and some of them have been put into practical use. However, thermoelectric materials that can be used for general consumer use at medium and high temperatures of 300 ° C. or higher are still insufficient in characteristics and are still far from being put into practical use.

On the other hand, as disclosed in Patent Document 1, there is Mg ₂ Si as one of typical examples of the Mg—Si-based thermoelectric material. The element constituting Mg ₂ Si is preferable in that it is not toxic and is abundant in the earth's crust. As described above, the Mg intermetallic compound Mg ₂ X is promising as an n-type thermoelectric conversion material and is under development. However, as a p-type thermoelectric conversion material, Ag addition or Ga addition ( Patent Document 2 and Non-Patent Document 1) have been proposed. If Mg ₂ X having good p-type electrical conductivity can be realized, a pn element can be produced, and it is promising as a thermoelectric conversion element or an infrared light emitting / receiving element.
JP 2002-285274 A JP 2008-160077 A Proceedings of the 55th Joint Conference on Applied Physics Lecture number 29a-C-1 “Design and fabrication of Ga-doped p-type Mg2Si by first-principles calculation”

As described above, Mg ₂ X, which is an Mg intermetallic compound, has been developed with respect to n-type thermoelectric conversion materials, but thermoelectric conversion materials with good p-type conductivity have been fully developed. No. That is, for example, Mg-added Ga-added X ₂ is preferable because it has a lower electrical resistance than Ag-added, but as disclosed in Non-Patent Document 1, the hole density is about 1 × 10 ¹⁹ cm ⁻³ . In order to be practically used, further improvement in hole density is required.

This invention is intended to solve the problems described above, and its object is to provide electrical conductivity to excellent p-type Mg metal compound (Mg 2 _X) and the device that applies it.

In order to achieve the above-described object, (1) the Mg intermetallic compound according to the present invention is a general formula having an inverted fluorite structure: Mg ₂ X (X is a group selected from the group 4 elements Si, Ge, and Sn) Or a plurality of elements), and Li is included as an acceptor dopant.

The general formula: Mg ₂ X (X is a group 4 element) having an inverted fluorite structure is a narrow band gap semiconductor with a low environmental load. Li has a high solid solubility limit with respect to Mg ₂ X. Therefore, by adding Li as an acceptor that is a source of hole carriers, p-type Mg ₂ X having high electrical conductivity is obtained. The Mg intermetallic compound according to the present invention can be produced, for example, by the vertical Bridgman method. Li is substituted and dissolved in the Mg site, and p-type carriers (holes) are generated in Mg ₂ X. Since the formation energy, which is the energy required for the generation of defects in which the additive element substitutes into the solid solution at the Mg site, is lower than that of Ag or Ga, a high solid solution limit can be expected, and accordingly, a higher hole is expected. A p-type Mg ₂ X having a density can be realized. Furthermore, since Li has low toxicity, it becomes environmentally friendly Mg ₂ X.

(2) The Li concentration of the acceptor dopant is preferably 6.4 × 10 ⁻³ at% or more and 3.2 at% or less of all atoms.

  (3) The p-type thermoelectric conversion material according to the present invention is such that the Seebeck coefficient α of the Mg intermetallic compound described in the above (1) and (2) is 50 μV / K or more. (4) The electrical resistivity ρ of the Mg intermetallic compound described in (1) and (2) above may be 10 mΩcm or less.

  (5) The thermoelectric conversion element of the present invention is a thermoelectric conversion element in which a plurality of p-type thermoelectric conversion materials and a plurality of n-type thermoelectric conversion materials are alternately arranged and electrically connected in series. At least one of the p-type thermoelectric conversion materials is the thermoelectric conversion material described in (3) or (4) above.

  (6) The thermoelectric cooling device according to the present invention includes the thermoelectric conversion element according to (5) above and a DC power source electrically connected to the thermoelectric conversion element. (7) Moreover, the thermoelectric power generation module of the present invention is electrically connected to the thermoelectric conversion element according to (5) above and the thermoelectric conversion element so that a potential difference is generated from the thermoelectric conversion element due to a temperature difference. Is to configure. (8) The far-infrared light emitting / receiving element of the present invention further includes an n-type Mg intermetallic compound and the p-type Mg intermetallic compound described in (1) or (2), and pn junctions them. Consists of.

In the present invention, Li, which has a high solubility limit in Mg ₂ X, is included in the acceptor dopant, so that a p-type Mg metal compound (Mg ₂ X) excellent in electrical conductivity can be realized. As a result, a thermoelectric conversion element, an infrared light receiving / emitting element or other device using a substance having a low environmental load is realized.

Hereinafter, preferred embodiments of the present invention will be described. The Mg metal compound of this embodiment has good electrical conductivity in the general formula Mg ₂ X (X is one or more elements selected from Group 4 elements Si, Ge and Sn) having an inverted fluorite structure. Therefore, Li was included as an acceptor dopant.

In the present embodiment, a case where Li is added when X is made of Si, that is, Mg ₂ Si will be considered. When Li is solid-dissolved at the Mg site, it becomes a negatively charged defect and becomes an acceptor that releases one p-type carrier (hole) per Li-atom to Mg ₂ Si. Therefore, the first-principles analyzed _Mg 2 Si, was determined power factor of the p-type carrier density per the calculated ^{_{Mg 2 Si (α 2 / ρ}} ). As a result, it came to show in FIG. Here, τ is a relaxation time in carrier conduction. Usually, when calculating the Seebeck coefficient α in the first principle analysis, τ is often regarded as a constant, so there is no problem even if the power factor is divided by τ. Further, although the amount obtained by dividing the power factor by τ is slightly different depending on the temperature, FIG. 1 shows a case of 800 K as a typical temperature.

As is clear from FIG. 1, the power factor peaks when the carrier density is 1 × 10 ²⁰ cm ⁻³ , and the effective range is from 1 × 10 ¹⁸ cm ⁻³ to 5 × 10 ²⁰ cm ^−3. Yes. Therefore, a desirable carrier density in p-type Mg ₂ Si is 1 × 10 ¹⁸ cm ⁻³ or more and 5 × 10 ²⁰ cm ⁻³ or less. From this fact, the amount of Li added is 6.4 × 10 ⁻³ at% or more based on the total atoms in terms of the crystal structure (4 chemical formulas are contained in a 6.35 Å cube). The ratio is 3.2 at% or less.

On the other hand, as a p-type acceptor dopant in the general formula having an inverted fluorite structure: Mg ₂ X (X is one or more elements selected from Group 4 elements Si, Ge, and Sn), Ag is conventionally well known. It has been. As another configuration, addition of Ga has also been proposed. Therefore, in order to demonstrate the effect of the present invention, the energy (defect formation energy) required when Li, Ag, and Ga become acceptors (defects charged to −1 valence) is calculated by first-principles analysis. Results as shown in 2 were obtained.

  FIG. 2 shows the defect formation energy in the Si-rich condition where the ratio of Mg to Si is greater than 2: 1. The defect formation energy is the energy required when a defect structure is formed on the basis of a complete crystal, and indicates that a defect with a smaller value (energy loss) is easier to form. In other words, it can be said that the smaller the defect formation energy, the more carriers can enter.

In FIG. 2, the origin of Fermi energy is based on the upper end of the valence band of Mg ₂ Si. The lower end of the calculated conduction band is 0.2 eV. Therefore, the entire region of Fermi energy as a semiconductor is from the upper end of the valence band (0 ev) to the lower end of the conduction band (0.2 eV), and using Li as an acceptor dopant over the entire region is Ga or Ag. It can be confirmed that the defect formation energy is low as compared with those using as an acceptor dopant. Such a low defect formation energy means that defects are easily formed, and means that the solid solution limit is high. Therefore, Li has lower defect formation energy than Ag or Ga in the entire region of Fermi energy as a semiconductor. Similarly, even when the ratio of Mg and Si according to the chemical formula is 2: 1 or when the ratio of Mg is higher, Li has a lower defect formation energy than Ag or Ga in the entire region of Fermi energy as a semiconductor. Is shown.

From these facts, Li has a higher solid solubility limit than Ag and Ga, a high carrier density is obtained, and Mg ₂ Si to which Li is added compared to Ag addition or Ga addition has better electrical conductivity. Can be obtained. Therefore, by using Li as an additive element, Mg ₂ Si becomes a thermoelectric material having a high power factor. Furthermore, since Li has low toxicity, Li-added Mg ₂ Si becomes an environmentally friendly and preferable environmentally friendly thermoelectric conversion material compared to Ag-added or Ga-added Mg ₂ Si.

Embodiment described above, X is has been described Si single Mg 2 _Si, it may be those in which X further includes one or both of in addition to Si Ge and Sn, can be present ratio is arbitrarily set in that case. Furthermore, X is not limited to Si, but can be Ge or Sn alone, or Ge and Sn. The presence of Ge and Sn can be arbitrarily set.

  Next, in order to produce the Mg intermetallic compound according to the present invention, for example, a solution solidification method or a vertical Bridgman method can be used. As an example, a manufacturing method using the vertical Bridgman method in the case of Si as X will be described. First, raw material powders of Si, Mg, and Li are weighed so as to have a predetermined composition ratio, and filled into a graphite crucible. The graphite crucible is sealed in a quartz tube, and an atmosphere charged with argon gas is used to suppress the evaporation of Mg. Subsequently, after melting at 1373 K, the crucible is lowered at a rate of 5 mm / h and crystal growth is performed, whereby a single crystal of several mm can be manufactured. The thus produced single crystal is pulverized, mixed, and sintered to produce an Mg intermetallic compound composed of a dense sintered body.

Furthermore, when the Mg intermetallic compound of the present embodiment is used as a p-type thermoelectric conversion material, the Seebeck coefficient α may be set to 50 μV / K or more. Alternatively, the electrical resistivity ρ may be set to 10 mΩcm or less. In such a case, the power factor (α ² / ρ) can be increased.

On the other hand, n-type thermoelectric conversion materials using Mg ₂ X intermetallic compounds have been conventionally produced. Therefore, a plurality of such n-type thermoelectric conversion materials and p-type thermoelectric conversion materials made of the Mg ₂ X intermetallic compound of this embodiment are prepared, arranged alternately, and electrically connected in series. By forming as described above, an embodiment of the thermoelectric conversion element of the present invention can be configured. In this example, all the thermoelectric conversion materials are composed of Mg ₂ X intermetallic compounds, but the present invention is not limited to this, and a plurality of p-type thermoelectric conversion materials and a plurality of n-type thermoelectric conversion materials are alternately arranged. And in the structure electrically connected in series, at least one of the plurality of p-type thermoelectric conversion materials may be the thermoelectric conversion material of the present invention.

  And one Embodiment of the thermoelectric cooling device of this invention can be comprised by electrically connecting DC power supply with respect to said thermoelectric conversion element. In addition, by using the thermoelectric conversion element, it is possible to construct a thermoelectric power generation module that generates a potential difference from the thermoelectric conversion element due to a temperature difference. Furthermore, a far-infrared light emitting / receiving element can be formed by pn junction of a p-type Mg intermetallic compound and an n-type Mg intermetallic compound.

It is a graph explaining the effect of this invention. It is a graph explaining the effect of this invention.

Claims (8)

A general formula having an inverted fluorite structure: Mg ₂ X (X is one or more elements selected from Group 4 elements Si and Ge and Sn),
An Mg intermetallic compound characterized in that it contains Li as an acceptor dopant. 2. The Mg intermetallic compound according to claim 1, wherein a concentration of Li of the acceptor dopant is 6.4 × 10 ⁻³ at% or more and 3.2 at% or less of all atoms.   A p-type thermoelectric conversion material characterized in that the Seebeck coefficient α of the Mg intermetallic compound according to claim 1 or 2 is 50 μV / K or more.   The p-type thermoelectric conversion material, wherein the electrical resistivity ρ of the Mg intermetallic compound according to claim 1 or 2 is 10 mΩcm or less. A thermoelectric conversion element in which a plurality of p-type thermoelectric conversion materials and a plurality of n-type thermoelectric conversion materials are alternately arranged and electrically connected in series,
The thermoelectric conversion element according to claim 3 or 4, wherein at least one of the plurality of p-type thermoelectric conversion materials is the thermoelectric conversion material according to claim 3 or 4.   A thermoelectric cooling device comprising: the thermoelectric conversion element according to claim 5; and a DC power source electrically connected to the thermoelectric conversion element.   6. A thermoelectric power generation module comprising: the thermoelectric conversion element according to claim 5; and a thermoelectric power generation module configured to be electrically connected to the thermoelectric conversion element and to generate a potential difference from the thermoelectric conversion element due to a temperature difference.   An infrared light receiving and emitting device comprising an n-type Mg intermetallic compound and the p-type Mg intermetallic compound according to claim 1, wherein the pn junction is provided.

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