JP2013073960A - Magnesium silicide, thermoelectric conversion material, sintered body, sintered body for thermoelectric conversion element, thermoelectric conversion element, and thermoelectric conversion module - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a novel material containing a dopant having good compatibility with magnesium silicide.SOLUTION: Co, Nb, Nd, Sm, Ta and Zn are used as a dopant doped into magnesium silicide. The amount of a dopant included in a magnesium silicide of the present invention is not particularly limited, but the content of the dopant is preferably 0.10-2.00 at% as an atomic weight ratio. A sintered body obtained by sintering the magnesium silicide of the present invention can be preferably used as a thermoelectric conversion element.

Description

  The present invention relates to magnesium silicide, a thermoelectric conversion material, a sintered body, a sintered body for a thermoelectric conversion element, a thermoelectric conversion element, and a thermoelectric conversion module.

  In recent years, various means for efficiently using various types of energy have been studied in accordance with the growing environmental problems. In particular, along with an increase in industrial waste and the like, effective utilization of waste heat generated when these are incinerated is a problem. For example, in a large-scale waste incineration facility, waste heat recovery is performed by generating high-pressure steam by waste heat and generating power by rotating a steam turbine with this steam. However, in the medium-sized and small-sized waste incineration facilities that occupy the majority of the waste incineration facilities, the amount of waste heat emitted is small, and therefore, a method for recovering waste heat generated by a steam turbine or the like cannot be adopted.

  As a power generation method using waste heat that can be adopted in such medium-sized and small-sized waste incineration facilities, for example, a thermoelectric conversion material that performs reversible thermoelectric conversion using the Seebeck effect or the Peltier effect, A method using a thermoelectric conversion element / thermoelectric conversion module has been proposed.

For example, magnesium silicide (Mg ₂ Si) having a small environmental load has been studied as a thermoelectric conversion material (see, for example, Patent Documents 1 and 2 and Non-Patent Documents 1 to 3).

  By the way, if a thermoelectric conversion material contains a specific dopant, the performance of the thermoelectric conversion element formed by sintering a thermoelectric conversion material can be improved. Specifically, because the thermoelectric conversion material contains a dopant, the thermal conductivity of the sintered body is reduced, the electrical conductivity is improved, and the Seebeck coefficient is improved. As a result, the thermoelectric of the thermoelectric conversion element is increased. Conversion performance can be improved.

  Therefore, in the thermoelectric conversion material using magnesium silicide, if a dopant having compatibility with magnesium silicide is clarified, the use of these dopants alone or in combination is a thermoelectric conversion element composed of a sintered body of magnesium silicide. The possibility of increasing the figure of merit Z and imparting special properties to the thermoelectric conversion element is expanded. Here, as a special property, for example, high temperature durability imparted to the thermoelectric conversion element by doping Sb into magnesium silicide can be cited (see Patent Document 3).

JP 2005-314805 A JP 2002-285274 A International Publication No. 2011/002035

Semiconductors Properties of Mg2Si Single Crystals Physical Review Vol. 109, no. 6, March 15, 1958, p. 1909-1915 Seebeck Effect In Mg2Si Single Crystals J.M. Phys. Chem. Solids Program Press 1962. Vol. 23, pp. 601-610 Bulk Crystals Growth of Mg2Si by the vertical Bridgman method Science Direct Thin Solid Films 461 (2004) 86-89

  The present invention has been made to solve the above problems, and an object of the present invention is to provide a novel material containing a dopant having a good compatibility with magnesium silicide.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, Co, Nb, Nd, Sm, Ta, and Zn were found to be dopants having good compatibility with magnesium silicide, and the present invention was completed. More specifically, the present invention provides the following.

  [1] Magnesium silicide containing at least one selected from Co, Nb, Nd, Sm, Ta and Zn as a dopant.

  [2] The magnesium silicide according to [1], containing 0.10 to 2.00 at% of the dopant in terms of atomic weight ratio.

  [3] A thermoelectric conversion material comprising the magnesium silicide according to [1] or [2].

  [4] A sintered body obtained by sintering the magnesium silicide according to [1] or [2].

  [5] A sintered body for a thermoelectric conversion element comprising the sintered body according to [4].

  [6] A thermoelectric conversion unit, and a first electrode and a second electrode provided on the thermoelectric conversion unit, wherein the thermoelectric conversion unit is manufactured using the thermoelectric conversion element sintered body according to [5]. Thermoelectric conversion element.

  [7] A thermoelectric conversion module comprising the thermoelectric conversion element according to [6].

  ADVANTAGE OF THE INVENTION According to this invention, the novel material containing a dopant with favorable compatibility with magnesium silicide can be provided.

It is a figure which shows an example of a thermoelectric conversion module. It is a figure which shows an example of a thermoelectric conversion module. It is a figure which shows an example of a thermoelectric conversion module. It is a figure which shows an example of a thermoelectric conversion module. It is a figure which shows an example of a sintering apparatus. It is a figure which shows the evaluation result of the Seebeck coefficient of a sintered compact. It is a figure which shows the evaluation result of the electrical conductivity of a sintered compact. It is a figure which shows the evaluation result of the power factor of a sintered compact. It is a figure which shows the evaluation result of the heat conductivity of a sintered compact. It is a figure which shows the evaluation result of the dimensionless performance index of a sintered compact. It is a figure which shows the evaluation result of the high temperature durability of a sintered compact.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Magnesium silicide>
The magnesium silicide of the present invention is a compound having an atomic weight ratio of Mg and Si of approximately 2: 1 and includes at least one selected from Co, Nb, Nd, Sm, Ta and Zn as a dopant. Co, Nb, Nd, Sm, Ta, and Zn have good compatibility with magnesium silicide. If a thermoelectric conversion material composed of magnesium silicide containing these dopants is sintered into a thermoelectric conversion element, thermoelectric conversion is achieved. The electrical conductivity of the element is improved or the thermal conductivity is lowered. For this reason, use of these dopants singly or in plurality increases the possibility of increasing the dimensionless figure of merit ZT of the thermoelectric conversion element or imparting special properties to the thermoelectric conversion element. In addition, this invention does not exclude using Sb etc. which are well-known dopants in combination with Co, Nb, Nd, Sm, Ta, and Zn.

  Special properties that can be imparted to the thermoelectric conversion element include high temperature durability and safety. If Nb, Nd, Sm, Ta, and Zn are contained as dopants, the thermoelectric conversion element has high temperature durability. Further, Co, Nb, Nd, Sm, Ta, and Zn are low in toxicity, and even if these are contained as dopants, there is no problem that the safety of magnesium silicide is lowered.

  Among Co, Nb, Nd, Sm, Ta, and Zn, Co, Nb, and Sm have a high effect of reducing the thermal conductivity of the thermoelectric conversion element, and Ta has a high effect of improving the electrical conductivity. Further, Zn has a high effect of reducing the thermal conductivity. And even if it contains these dopants, there is no problem of the performance deterioration of the thermoelectric conversion element by the fall of the absolute value of Seebeck coefficient.

  The content of the dopant in the magnesium silicide of the present invention is not particularly limited, but is preferably 0.10 to 2.00 at% in terms of atomic weight ratio. In addition, when a several dopant is used, it is preferable that the total content of all the dopants exists in said range.

  Although the manufacturing method of the magnesium silicide used by this invention is not specifically limited, For example, a melt synthesis method and a mechanical alloy method are employable.

  The melt synthesis method has a mixing step of mixing Mg, Si, and a dopant to obtain a composition raw material, and a heating and melting step of heating and melting the composition raw material.

  The Mg for obtaining the composition raw material is not particularly limited, but a high purity is preferable. High purity Mg refers to Mg having a purity of about 99.5% or more.

  The type of Si for obtaining the composition raw material is not particularly limited, and it is possible to use a silicon sludge from which an oxide film has been removed, but it is preferable to use high-purity silicon. Here, high-purity silicon has a purity of 99.9999% or higher and is used for manufacturing silicon products such as semiconductors and solar cells. Specific examples of high-purity silicon include high-purity silicon raw materials for LSI, high-purity silicon raw materials for solar cells, high-purity metal silicon, high-purity silicon ingots, and high-purity silicon wafers.

  In the mixing step, Mg, Si and a dopant are mixed to prepare a composition raw material having an atomic weight ratio of Mg to Si of about 2: 1 and containing 0.10 to 2.00 at% of the dopant in atomic weight ratio. .

In the heating and melting step, for example, the composition raw material obtained in the mixing step is heat-treated under a reducing atmosphere and preferably under reduced pressure under a temperature condition that exceeds the melting point of Mg and lower than the melting point of Si to melt and synthesize magnesium silicide. . Here, “under a reducing atmosphere” refers to an atmosphere containing hydrogen gas in an amount of 5% by volume or more and optionally containing an inert gas as another component. By performing the heating and melting step in such a reducing atmosphere, magnesium silicide can be synthesized by reliably reacting Mg and Si. Further, the pressure condition in the heating and melting step may be atmospheric pressure, but is preferably 1.33 × 10 ⁻³ Pa to atmospheric pressure, and considering safety, for example, under reduced pressure condition or vacuum condition of about 0.08 MPa. Preferably it is done. The heating conditions in the heating and melting step are 700 ° C. or higher and lower than 1410 ° C., preferably 1085 ° C. or higher and lower than 1410 ° C., for example, heat treatment can be performed for about 3 hours. Here, the heat treatment time is, for example, 2 to 10 hours. By making the heat treatment for a long time, the obtained magnesium silicide can be made more uniform. Examples of the temperature raising condition include a temperature raising condition of 150 to 250 ° C./h until reaching 150 ° C., and a temperature raising condition of 350 to 450 ° C./h until reaching 100 ° C. As the subsequent temperature lowering condition, a temperature lowering condition of 900 to 1000 ° C./h can be exemplified.

  Next, the mechanical alloy method will be described. The raw material for producing magnesium silicide by the mechanical alloy method is not particularly limited, and Mg and Si described in the above melt synthesis method can be used. The size of the raw material powder is not particularly limited, but is generally several tens of microns to several millimeters. The atomic ratio of Mg, Si and dopant is adjusted in the same manner as in the case of the melt synthesis method, and mechanical alloying treatment is performed on these raw materials.

  A dry pulverizer can be used for the mechanical alloying treatment, and specifically, a vibration type ball mill, a planetary ball mill, a rolling ball mill, an attritor or the like can be used. The atmosphere during the mechanical alloying treatment is preferably an inert gas atmosphere or a reduced pressure atmosphere in order to prevent oxidation of the powder.

  Although processing time is not specifically limited, 50 to 300 hours are preferable. If it is shorter than 50 hours, the mixing state of each element is insufficient, and fine mixing may not be achieved. When the time exceeds 300 hours, oxidation or nitridation of the powder occurs during the mechanical alloying treatment, and oxides and nitrides that cause performance deterioration of the thermoelectric material are generated. Furthermore, as a pressure transmission medium at the time of mechanical alloying, general pulverized balls such as steel balls, ceramic balls, and hard balls can be used.

  Alloyed magnesium silicide can be obtained by the above mechanical alloying treatment. The magnesium silicide obtained by the mechanical alloy method is usually in the form of a fine powder of several micrometers or less.

<Sintered body manufacturing method>
The sintered body of the present invention is formed by sintering the magnesium silicide. The shape of the magnesium silicide to be sintered is fine and preferably has a narrow particle size distribution. Here, “fine” means, for example, a few micrometers or less. Moreover, the method to powderize is not specifically limited, A general method is employable.

  Sintering conditions are not particularly limited, but when the sintered body of the present invention is used for a thermoelectric conversion element, in a sintering jig made of graphite, in a vacuum or reduced pressure atmosphere, a sintering pressure of 5 to 60 MPa, a sintering temperature. 600-1000 degreeC and the conditions sintered by a pressure compression sintering method are preferable.

  When the sintering pressure is less than 5 MPa, it becomes difficult to obtain a sintered body having a sufficient density of about 70% or more of the theoretical density, and the obtained sample cannot be practically used in terms of strength. There is a fear. On the other hand, when the sintering pressure exceeds 60 MPa, it is not preferable in terms of cost and is not practical. If the sintering temperature is less than 600 ° C., it becomes difficult to obtain a sintered body having a density close to the theoretical density from 70% of the theoretical density by fusing and firing at least a part of the surfaces where the particles are in contact with each other. There is a risk that the obtained sample may not be practically usable in terms of strength. Further, when the sintering temperature exceeds 1000 ° C., the temperature is too high, so that not only the sample is damaged, but in some cases, Mg may rapidly become vapor and scatter.

  In the sintering step, when air is present, it is preferable to sinter in an atmosphere using an inert gas such as nitrogen or argon.

  When the pressure compression sintering method is employed in the sintering process, a hot press sintering method (HP), a hot isostatic sintering method (HIP), and a discharge plasma sintering method can be employed. Among these, the discharge plasma sintering method is preferable.

  The spark plasma sintering method is a type of pressure compression sintering using the direct current pulse current method. It is a method of heating and sintering by applying a large pulse current to various materials. -This is a method in which an electric current is passed through a conductive material such as graphite and the material is processed and processed by Joule heating.

  The sintered body thus obtained is a sintered body having high physical strength and capable of stably exhibiting high thermoelectric conversion performance. Therefore, the sintered body of the present invention can be preferably used as a sintered body for a thermoelectric conversion element.

<Thermoelectric conversion element>
The thermoelectric conversion element according to the present invention includes a thermoelectric conversion portion and a first electrode and a second electrode provided in the thermoelectric conversion portion, and the thermoelectric conversion portion is manufactured using the sintered body of the present invention. Is.

[Thermoelectric converter]
The thermoelectric conversion part is cut out from the sintered body to have a desired size using a wire saw or the like. In addition, it is preferable to cut out after forming an electrode in a sintered compact by the method mentioned later.

  Although this thermoelectric conversion part is normally manufactured using the thermoelectric conversion material of 1 type of this invention, it is good also as a thermoelectric conversion part which has a multilayer structure using the thermoelectric conversion material of multiple types of this invention. The thermoelectric conversion part having a multilayer structure can be manufactured by laminating a plurality of types of thermoelectric conversion materials of the present invention in a desired order before sintering and then sintering. The thermoelectric conversion materials of a plurality of types of the present invention may be a combination of thermoelectric conversion materials having different dopants, or a combination of thermoelectric conversion materials containing dopants having different dopant contents. Alternatively, a combination of the thermoelectric conversion material of the present invention and a conventionally known thermoelectric conversion material may be used. However, it is preferable to combine magnesium silicides because the laminated interface does not deteriorate due to a difference in expansion coefficient or the like.

[electrode]
The method of forming the first electrode and the second electrode is not particularly limited, and may be a method of forming an electrode on the sintered body, or the magnesium silicide for obtaining the sintered body of the present invention. A method of simultaneously forming electrodes during sintering may be used.

  Examples of the method for forming the electrode on the sintered body include a plating method. And when employ | adopting a plating method, it is preferable to form an electrode by electroless nickel plating. Moreover, when the surface of the sintered compact before forming an electrode by plating method has the unevenness | corrugation which obstructs plating, it is preferable to grind and smooth.

  By cutting the sintered body with the plated layer thus obtained into a predetermined size with a cutting machine such as a wire saw or a blade saw, from the first electrode, the thermoelectric conversion unit, and the second electrode A thermoelectric conversion element is obtained.

  Moreover, as a method of forming an electrode during the sintering of magnesium silicide, a pressure compression sintering method can be mentioned. The pressure compression sintering method is a method in which an electrode material, magnesium silicide, and an electrode material are laminated in this order, and pressure compression sintering is performed to obtain a sintered body having electrodes formed at both ends. The sintered body thus manufactured is cut into a predetermined size with a cutting machine such as a wire saw or a blade saw, so that a thermoelectric conversion element including a first electrode, a thermoelectric conversion unit, and a second electrode Is obtained.

<Thermoelectric conversion module>
The thermoelectric conversion module according to the present invention includes the thermoelectric conversion element according to the present invention as described above.

  As an example of a thermoelectric conversion module, what is shown, for example in FIG.1 and FIG.2 is mentioned. In this thermoelectric conversion module, at least one of an n-type semiconductor and a p-type semiconductor obtained from magnesium silicide according to the present invention is used as a thermoelectric conversion material for the n-type thermoelectric conversion unit 101 and the p-type thermoelectric conversion unit 102, respectively. Electrodes 1015 and 1025 are provided at the upper ends of the n-type thermoelectric conversion unit 101 and the p-type thermoelectric conversion unit 102 arranged side by side, and electrodes 1016 and 1026 are provided at the lower ends. The electrodes 1015 and 1025 provided at the upper ends of the n-type thermoelectric conversion unit and the p-type thermoelectric conversion unit are connected to form an integrated electrode, and the n-type thermoelectric conversion unit and the p-type thermoelectric conversion unit The electrodes 1016 and 1026 provided respectively at the lower end of each are separated.

  In the thermoelectric conversion module shown in FIG. 1, a positive temperature difference is generated between the electrodes 1015 and 1025 and the electrodes 1016 and 1026 by heating the electrodes 1015 and 1025 and radiating heat from the electrodes 1016 and 1026. (Th-Tc) is generated, and the p-type thermoelectric conversion unit 102 has a higher potential than the n-type thermoelectric conversion unit 101 due to the thermally excited carriers. At this time, a current flows from the p-type thermoelectric conversion unit 102 to the n-type thermoelectric conversion unit 101 by connecting the resistor 3 as a load between the electrode 1016 and the electrode 1026.

  In the thermoelectric conversion module shown in FIG. 2, a direct current flows from the p-type thermoelectric conversion unit 102 to the n-type thermoelectric conversion unit 101 by the DC power supply 4, thereby causing an endothermic action at the electrodes 1015 and 1025, and the electrodes 1016 and 1026. Exothermic action occurs. Further, when a direct current is passed from the n-type thermoelectric conversion unit 101 to the p-type thermoelectric conversion unit 102, a heat generation effect is generated in the electrodes 1015 and 1025, and a heat absorption effect is generated in the electrodes 1016 and 1026.

  Moreover, as another example of a thermoelectric conversion module, what is shown, for example in FIG.3 and FIG.4 is mentioned. In this thermoelectric conversion module, an n-type semiconductor obtained from the magnesium silicide of the present invention is used as a thermoelectric conversion material of the n-type thermoelectric conversion unit 103. The n-type thermoelectric conversion unit 103 is provided with an electrode 1035 at the upper end and an electrode 1036 at the lower end.

  In the thermoelectric conversion module shown in FIG. 3, by heating the electrode 1035 side and dissipating heat from the electrode 1036 side, a positive temperature difference (Th−Tc) is generated between the electrode 1035 and the electrode 1036, and the electrode 1035 side is The potential is higher than that of the electrode 1036 side. At this time, a current flows from the electrode 1035 side to the electrode 1036 side by connecting the resistor 3 as a load between the electrode 1035 and the electrode 1036.

  In the thermoelectric conversion module shown in FIG. 4, a direct current flows from the electrode 1036 side to the electrode 1035 side through the n-type thermoelectric conversion unit 103 by the DC power source 4, thereby generating an endothermic effect in the electrode 1035 and generating heat in the electrode 1036. An effect occurs. In addition, when a direct current flows from the electrode 1035 side to the electrode 1036 through the n-type thermoelectric conversion unit 103 by the DC power supply 4, a heat generation effect occurs in the electrode 1035 and a heat absorption effect occurs in the electrode 1036.

  Hereinafter, the present invention will be described in detail with reference to examples. In addition, this invention is not limited to the Example shown below at all.

<Example 1>
High purity silicon, Mg, and Co were mixed to obtain a composition material (66 at% Mg, 33 at% Si, 1 at% Co). As high-purity silicon, a semiconductor grade manufactured by MEMC Electronic Materials, having a purity of 99.999999999%, and having a diameter of 4 mm or less was used. Further, as Mg, a magnesium piece manufactured by Nippon Thermochemical Co., Ltd., having a purity of 99.93% and a size of 1.4 mm × 0.5 mm was used. Further, as Co, a granular material having a purity of 99% and a diameter of 5 μm or less manufactured by Kojundo Chemical Laboratory Co., Ltd. was used.

The above composition raw material was put into a melting crucible made of Al ₂ O ₃ (manufactured by Nippon Chemical Ceramics Co., Ltd., inner diameter 34 mm, outer diameter 40 mm, height 150 mm; lid portion 40 mm in diameter and thickness 2.5 mm). The edge of the opening of the melting crucible and the lid were brought into close contact with each other, placed in a heating furnace, and pressurized with a weight from the outside of the heating furnace through a ceramic rod to 3 kg / cm ² . .

Next, the inside of the heating furnace was depressurized with a rotary pump until it became 5 Pa or less, and then depressurized with a diffusion pump until it became 1.33 × 10 ⁻² Pa. In this state, the inside of the heating furnace was heated at 200 ° C./h until reaching 150 ° C., and kept at 150 ° C. for 1 hour to dry the composition raw material. At this time, the heating furnace was filled with a mixed gas of hydrogen gas and argon gas, the hydrogen gas partial pressure was 0.005 MPa, and the argon gas partial pressure was 0.052 MPa.

  Then, it heated until it reached 1105 degreeC at 400 degreeC / h, and hold | maintained at 1105 degreeC for 3 hours. Then, the magnesium silicide doped with Co was obtained by cooling to 900 ° C. at 100 ° C./h and cooling to room temperature at 1000 ° C./h.

The magnesium silicide was pulverized using a ceramic mortar until the particle size became approximately 75 μm, and a powder passed through a 75 μm sieve was obtained. Then, as shown in FIG. 5, 1.0 g of pulverized magnesium silicide was charged into a space surrounded by a graphite die 10 having an inner diameter of 15 mm and graphite punches 11a and 11b. Carbon paper was sandwiched between the upper and lower ends of the powder to prevent magnesium silicide from adhering to the punch. Thereafter, sintering was performed in a vacuum atmosphere using a discharge plasma sintering apparatus (manufactured by ELENIX, “PAS-III-Es”). The sintering conditions are as follows.
Sintering temperature: 910 ° C
Pressure: 30.0 MPa
Temperature rising rate: 300 ° C./min×1 min 40 sec (˜500 ° C.)
35 ° C./min×11 min (500 to 885 ° C.)
10 ° C./min×2 min 30 sec (885-910 ° C.)
0 ° C / min × 5min (910 ° C)
Cooling conditions: Vacuum cooling Atmosphere: Ar 60 Pa (vacuum when cooling)

  After sintering, the adhered carbon paper was removed with sandpaper to obtain a sintered body of magnesium silicide doped with Co. In addition, the shape of the obtained sintered compact is a column shape (a circle with a diameter of 15 mm on the top and bottom surfaces and a height of 10 mm).

<Example 2>
Nb was doped in the same manner as in Example 1 except that Co was changed to Nb (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%, particle size of 300 μmm or less in diameter). Magnesium silicide was obtained.

<Example 3>
The same as Example 1 except that Co was changed to Sm (manufactured by Kojundo Chemical Laboratories Co., Ltd., purity 99.9%, shaved, and a lump of shaved Sm having a diameter of about 5 to 10 mm). By the method, magnesium silicide doped with Sm was obtained.

<Example 4>
Co was changed to Zn (manufactured by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%, machined Sm ingot with a diameter of approximately 5 to 10 mm), and sintered by a discharge plasma sintering apparatus. Magnesium silicide doped with Nd was obtained in the same manner as in Example 1 except that the conditions during the sintering were changed to the following conditions.
Sintering temperature: 870 ° C
Pressure: 30.0 MPa
Temperature rising rate: 300 ° C./min×1 min 40 sec (˜500 ° C.)
35 ° C./min×10 min (500 to 850 ° C.)
10 ° C / min × 2min (850-870 ° C)
0 ° C / min × 5min (870 ° C)
Cooling conditions: Vacuum cooling Atmosphere: Ar 60 Pa (vacuum when cooling)

<Example 5>
Ta was doped in the same manner as in Example 4 except that Nd was changed to Ta (made by Kojundo Chemical Laboratory Co., Ltd., purity 99.9%, particle size of 45 μm or less in diameter). Magnesium silicide was obtained.

<Example 6>
Co was changed to Zn (manufactured by Kojundo Chemical Laboratory Co., Ltd., with a purity of 99.9% and a granule having a diameter of 150 μm or less), and the conditions for sintering with a discharge plasma sintering apparatus were as follows: A magnesium silicide doped with Zn was obtained in the same manner as in Example 1 except that the conditions were changed.
Sintering temperature: 870 ° C
Pressure: 30.0 MPa
Temperature rising rate: 300 ° C./min×2 min (˜600 ° C.)
100 ° C./min×2 min (600 to 800 ° C.)
10 ° C./min×4 min (800 to 840 ° C.)
0 ° C / min × 5min (840 ° C)
Cooling conditions: Vacuum cooling Atmosphere: Ar 60 Pa (vacuum when cooling)

<Reference Example 1>
Co was changed to Sb (manufactured by Electronics End Materials Corporation, with a purity of 99.9999% and a particle size of 5 mm or less in diameter), and the conditions for sintering with the spark plasma sintering apparatus were as follows: Except for the change, a magnesium silicide doped with Sb was obtained in the same manner as in Example 1.
Sintering temperature: 850 ° C
Pressure: 30.0 MPa
Temperature rising rate: 300 ° C./min×2 min (˜600 ° C.)
100 ° C./min×2 min (600 to 800 ° C.)
10 ° C / min × 5min (800-850 ° C)
0 ° C / min × 5min (850 ° C)
Cooling conditions: Vacuum cooling Atmosphere: Ar 60 Pa (vacuum when cooling)

<Evaluation 1>
About the sintered body of the magnesium silicide of Examples 1-6, observation with an optical microscope etc. was performed with the following method.

  The sintered body was cut along the diameter using a wire saw CS-203 (Musashino Electronics). The cross section of the cut sintered body was mirror-finished using an automatic polishing machine MA-150 (Musashino Electronics), and the processed surface was observed with an optical microscope (magnification 200 times).

The optical microscope according to the Mg 2 not _Si moiety confirmed from observation was analyzed by an electron beam microanalyzer, magnesium silicide was confirmed to contain a dopant. Moreover, unreacted Si and a small amount of Al were contained in a portion that was not Mg ₂ Si.

<Evaluation 2>
For the sintered bodies of magnesium silicide of Examples 1 to 6 and Reference Example 1, the measurement of Seebeck coefficient and electrical conductivity was performed by the following method using a Seebeck coefficient measuring apparatus (ULVAC-RIKO ZEM-2). .

  A sample having a thickness of 2 mm, a height of 8 mm, and a width of 2 mm was cut out from each sintered body. The upper and lower ends of the sample were sandwiched between nickel electrodes, and a temperature difference measuring thermocouple (probe) was contacted from the side. The measurement temperature was 50 ° C. to 600 ° C., and the measurement was performed in increments of 50 ° C. The measurement atmosphere was a He atmosphere, and the temperature difference between the electrodes was adjusted to 30 ° C.

  The Seebeck coefficient was calculated by reading the thermoelectromotive force and temperature generated between the sample and each probe in contact with the sample, and dividing the electromotive force difference between the probes by the temperature difference. The results are shown in FIG.

  From the result of FIG. 6, the sintered body of magnesium silicide containing Co, Nb, Nd, Sm, Ta and Zn as dopants has an absolute value of Seebeck coefficient compared to the sintered body of magnesium silicide containing Sb as dopants. Was confirmed to be high.

  The resistance value was measured by the four probe method using the upper and lower electrodes and the probe, the resistivity was calculated from the distance between the probes and the cross-sectional area of the sample, and the electrical conductivity was calculated from the reciprocal thereof. The results are shown in FIG.

  From the result of FIG. 7, among the sintered bodies of magnesium silicide containing Co, Nb, Nd, Sm, Ta and Zn as dopants, the sintered body of magnesium silicide containing Ta has a high effect of increasing electrical conductivity. Was confirmed.

  The power factor (PF) was calculated from the Seebeck coefficient derived as described above and the electrical conductivity. The results are shown in FIG.

  From the result of FIG. 8, it was confirmed that the sintered body of magnesium silicide containing Ta as a dopant has a high power factor.

<Evaluation 3>
For the sintered bodies of magnesium silicide of Examples 1 to 6 and Reference Example 1, the thermal conductivity was measured using a laser flash method thermal conductivity measuring device (“TC / 7000H” manufactured by ULVAC-RIKO, Inc.) as follows. It was done by the method.

  A sample having a height of 2 mm, a length of 8 mm, and a width of 2 mm was cut out from each sintered body. The surface of the sample was lightly polished, and an R thermocouple was adhered to the corner of the sample using silver paste on one side of 8 mm × 8 mm.

  First, the amount of absorbed heat was measured using a standard sample (sapphire) with a known specific heat. Subsequently, sapphire was removed, the sample was set, and the amount of heat absorbed was measured.

  In order to measure the thermal diffusivity, the one surface having the R thermocouple was uniformly blackened with graphite spray. In the blackening treatment, masking was performed so that the silver paste was not exposed to graphite spray.

  The thermal diffusivity was measured in increments of 50 ° C. from 50 ° C., 100 ° C., 200 ° C., and 300 ° C. to 600 ° C., and the thermal conductivity was determined from the thermal diffusivity, specific heat, and density. The results are shown in FIG.

  From the result of FIG. 9, among the sintered bodies of magnesium silicide containing Co, Nb, Nd, Sm, Ta and Zn as dopants, the sintered body of magnesium silicide containing Zn has a high effect of increasing the thermal conductivity. Was confirmed.

<Evaluation 4>
A dimensionless figure of merit (ZT) was calculated using the Seebeck coefficient, electrical conductivity, and thermal conductivity. The results are shown in FIG.

  From the result of FIG. 10, the sintered body of magnesium silicide containing Co, Nb, Nd, Sm, Ta and Zn as dopants has a slightly lower dimensionless figure of merit than the sintered body containing Sb as dopants. Was confirmed.

<Evaluation 5>
The sintered bodies of magnesium silicide of Examples 2 to 6 and Reference Example 1 were evaluated for high-temperature durability by the following method.

  A 10 mm × 10 mm × 2 mm sample was cut out from each sintered body, and one of the surfaces of the 10 mm × 10 mm area of this sample was processed with an automatic polishing machine MA-150 (Musashino Electric Co., Ltd.) The oxide film on this surface was removed.

  Next, the surface from which the oxide film was removed was taken as a measurement surface, and the resistance of the sample was measured using a four-terminal measuring device K-503RS (manufactured by Kyowa Riken Co., Ltd.). Here, the interval between the four probes in contact with the measurement surface was 1 mm. The current condition during measurement was up to 30 mA.

The resistivity was calculated by multiplying the resistance value derived as described above by a correction coefficient. The correction coefficient is represented by w × C × F, where w is the thickness of the sample and C is 4.2209 (measurement surface is 10 mm × 10 mm, probe interval is derived from 1 mm). Table 1 shows the relationship between F and thickness / probe spacing.

  After calculating the resistivity, the sample was put in an annular furnace maintained at 600 ° C. in the atmosphere. After 1 hour, the sample was taken out from the annular furnace, the measurement surface was polished, and the resistivity was derived in the same manner as described above. Similarly, the resistivity was derived after 5 hours, 10 hours, 50 hours, and 100 hours. In addition, since the thickness of the sample is reduced because the measurement surface is polished in deriving the resistivity, the correction coefficient shown in Table 1 was used. The resistivity evaluation results are shown in FIG.

  From the result of FIG. 11, it was confirmed that the sintered body of magnesium silicide containing Nb, Nd, Sm, Ta and Zn as dopants has high temperature durability equivalent to the sintered body containing Sb as dopants.

101 n-type thermoelectric converter 1015, 1016 electrode 102 p-type thermoelectric converter 1025, 1026 electrode 103 n-type thermoelectric converter 1035, 1036 electrode 3 load 4 DC power supply 10 graphite die 11a, 11b graphite punch

Claims (7)

  Magnesium silicide containing at least one selected from Co, Nb, Nd, Sm, Ta and Zn as a dopant.   The magnesium silicide according to claim 1, wherein the dopant is contained at 0.10 to 2.00 at% in atomic weight ratio.   The thermoelectric conversion material comprised from the magnesium silicide of Claim 1 or 2.   A sintered body obtained by sintering the magnesium silicide according to claim 1.   The sintered compact for thermoelectric conversion elements comprised from the sintered compact of Claim 4. A thermoelectric converter, and a first electrode and a second electrode provided in the thermoelectric converter,
The thermoelectric conversion element with which the said thermoelectric conversion part is manufactured using the sintered compact for thermoelectric conversion elements of Claim 5. A thermoelectric conversion module comprising the thermoelectric conversion element according to claim 6.

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