JP2010067672A - Thermoelectric conversion material and process of producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To produce a low cost thermoelectric conversion material principally comprising a half-Heusler compound and exhibiting proper denseness and excelling mass productivity, and to obtain method for producing the material. <P>SOLUTION: A thermoelectric conversion material, principally contains an inter-metallic compound, represented by general formula XYZ (where X, Y and Z represent at least one kind of metallic element, respectively), and contains a glass component which contains boron oxide, silicon oxide and alkaline-earth metal oxides such as Ca, Ba, and the like, in the range of less than 15 vol.% (excluding 0 vol.%). A molding product consisting of mixture powder where a vitreous component is added to the principal component, is embedded in alloy powder having the same composition as that of the principal component and baked under normal pressure. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a thermoelectric conversion material and a method for manufacturing the thermoelectric conversion material, and more particularly to a thermoelectric conversion material mainly composed of a half-Heusler type intermetallic compound and a method for manufacturing the thermoelectric conversion material.

  In order to cope with environmental problems such as resource energy problems and global warming in recent years, realization of a thermoelectric conversion material that efficiently converts heat energy into electric energy has been demanded.

  Conventionally, as this type of thermoelectric conversion material, those utilizing the Seebeck effect are widely known. The Seebeck effect is a phenomenon in which when a temperature difference is provided at both ends of a substance, a voltage corresponding to the temperature difference is generated at both ends. The thermoelectric characteristics of the thermoelectric conversion material are output factors represented by Equation (1). P can be evaluated.

P = S ² / ρ (1)
S is the Seebeck coefficient and ρ is the resistivity.

  The thermoelectric conversion material includes a p-type thermoelectric conversion material having a positive Seebeck coefficient and an n-type thermoelectric conversion material having a negative Seebeck coefficient. However, when the current easily flows, the resistivity ρ decreases and the voltage increases. Since the absolute value of the Seebeck coefficient S is also increased, the thermoelectric characteristics are improved as the output factor P is larger from the equation (1).

  As a thermoelectric conversion material having a large absolute value of the Seebeck coefficient S, a half-Heusler intermetallic compound represented by a general formula XYZ (where X, Y, and Z each represent at least one metal element) (hereinafter, "Half-Heusler compound") is known. This half-Heusler compound has a cubic crystal structure in which a metal element Y is inserted into a NaCl-type crystal lattice composed of a metal element X and a metal element Z.

Patent Document 1 includes a composition formula: (Ti _a1 Zr _b1 Hf _c1 ) _x Ni _y Sn _100-xy (where 0 <a1 <1, 0 <b1 <1, 0 <c1 <1, a1 + b1 + c1 = 1, 30 ≦ x ≦ 35, 30 ≦ y ≦ 35), and a thermoelectric conversion material having a half-Heusler compound phase having a MgAgAs type crystal structure as a main phase has been proposed.

  In Patent Document 1, alloy powder is sintered by a pressure sintering method such as a hot press sintering method or a discharge plasma sintering method, thereby obtaining a bulk thermoelectric conversion material as a final product. That is, if sintering is performed at normal pressure without applying pressure, the surface may be oxidized with a small amount of oxygen, or the metal element may volatilize, which may lead to deterioration of thermoelectric characteristics. For example, in a thermoelectric conversion material using a half-Heusler compound, Sn or Sb is often used as a metal element at the Z site. However, in the normal pressure sintering method, Sn and Sb are particularly volatile. In addition, it is considered that the pressure sintering method is generally capable of obtaining a thermoelectric conversion material having good denseness and a large output factor P compared to the atmospheric pressure sintering method. For this reason, the conventional thermoelectric conversion material is producing the thermoelectric conversion material by the pressure sintering method like patent document 1. FIG.

JP 2004-356607 A

  However, since the conventional thermoelectric conversion material is produced by the pressure sintering method as described above, there is a problem that the productivity is inferior and the cost is increased. For example, in the case of the hot press sintering method, since it is necessary to apply heat while pressing the powder, an expensive and large-scale pressing device is required. In the case of the discharge plasma sintering method, since discharge plasma is generated while energizing under pressure and vacuum, it is possible to obtain a homogeneous and high-quality sintered body in a relatively short time. In addition to the need for a pressure / vacuum device, a device for generating discharge plasma is also required, resulting in poor productivity and high equipment costs.

  The present invention has been made in view of such circumstances, and provides a thermoelectric conversion material mainly composed of a half-Heusler compound that has good compactness, low cost, and excellent mass productivity, and a method for producing the same. With the goal.

  If the thermoelectric conversion material composed of the above-mentioned half-Heusler compound can be performed not by the pressure sintering method but by the atmospheric pressure sintering method, expensive equipment becomes unnecessary and mass production can be achieved at low cost.

  However, as described in [Background Art], the atmospheric pressure sintering method is generally inferior to the pressure sintering method and has a low sintering property.

  Then, when the present inventors conducted earnest research, by containing a specific glass component in the range of less than 15 vol% (excluding 0 vol%), a different phase is not produced | generated and desired liquid is produced | generated. It was found that phase sintering can be performed, and thus a thermoelectric conversion material having good denseness and sufficiently practical value can be obtained without pressure sintering.

  This invention is made | formed based on such knowledge, Comprising: The thermoelectric conversion material which concerns on this invention is represented by general formula XYZ (however, X, Y, Z shows at least 1 type of metal element, respectively). A glass component containing an intermetallic compound as a main component and containing boron oxide, silicon oxide, and alkaline earth metal oxide is included in a range of less than 15 vol% (excluding 0 vol%). It is a feature.

  Moreover, the thermoelectric conversion material of the present invention is characterized in that the alkaline earth metal includes at least one of Ca and Ba.

  Further, the thermoelectric conversion material of the present invention is characterized in that the intermetallic compound contains at least Ti, Ni, and Sn.

  The thermoelectric conversion material of the present invention is characterized in that a part of the Sn is substituted with Sb.

  The thermoelectric conversion material of the present invention is characterized in that a part of the Ti is substituted with at least one element of Zr and Hf.

  The method for producing a thermoelectric conversion material of the present invention is a metal represented by the general formula XYZ (where X, Y, and Z each represent at least one metal element) using a plurality of types of metal powders as starting materials. An alloy powder composed of an intermetallic compound is prepared, and further, the alloy powder so that the glass component containing boron oxide, silicon oxide, and alkaline earth metal oxide is less than 15 vol% (excluding 0 vol%). The glass component is mixed to produce a mixed powder, and then the mixed powder is molded to produce a molded body. Thereafter, the molded body is embedded in the alloy powder, and then subjected to a firing treatment to obtain a thermoelectric conversion. It is characterized by producing materials.

  The method for producing a thermoelectric conversion material according to the present invention is characterized in that the baking treatment is performed at normal pressure.

  According to the thermoelectric conversion material of the present invention, the main component is an intermetallic compound represented by the general formula XYZ (where X, Y, and Z each represents at least one metal element), and boron oxide and silicon oxide. , And glass components containing alkaline earth metal oxides such as Ca and Ba are contained in a range of less than 15 vol% (not including 0 vol%), so that the glass component having a low glass softening point is the main component. By adding a predetermined amount to the liquid phase, liquid phase sintering can be performed without generating a different phase.

  Moreover, the glass component containing boron oxide, silicon oxide, and alkaline earth metal oxide has good wettability with respect to the intermetallic compound, and performs liquid phase sintering while suppressing the generation of heterogeneous phases. Can do. Accordingly, a sinterability is improved, and a dense thermoelectric conversion material having desired thermoelectric properties can be obtained.

  In addition, according to the method for producing a thermoelectric conversion material of the present invention, since the compact is embedded in the alloy powder and subjected to a firing process, oxidation of the surface of the sintered body and volatilization of the metal element are suppressed, and thermoelectric properties are reduced at a low cost. A favorable thermoelectric conversion material can be easily produced.

  In addition, since the firing process is performed at normal pressure, unlike the pressure sintering method such as the hot press sintering method and the discharge plasma sintering method, the manufacturing process is simplified, and expensive manufacturing equipment is not required. A method for producing a thermoelectric conversion material excellent in productivity suitable for mass production at low cost can be realized.

  In addition, since the intermetallic compound contains at least Sn and a part of the Sn is substituted with Sb, even if Sn or Sb is contained as a metal element, Sn or Sb volatilizes. Can be suppressed. That is, Sn and Sb are usually volatilized and dissipated when fired at normal pressure, which may lead to a decrease in thermoelectric characteristics. However, it is possible to avoid volatilization of Sn and Sb by embedding the compact in the alloy powder and firing as in the present invention. Therefore, desired sintering can be easily performed even at normal pressure, and a thermoelectric conversion material having thermoelectric characteristics that does not hinder practical use can be easily manufactured.

  Hereinafter, embodiments of the present invention will be described in detail.

  The thermoelectric conversion material according to the present invention is mainly composed of a half-Heusler compound represented by the general formula XYZ.

  Here, Ti is mentioned as a metal element coordinated to X site in the said general formula XYZ. It is also preferable that Ti is a main component and a part of Ti is substituted with a tetravalent element such as Zr or Hf or a trivalent rare earth element such as La, Se, Nd, or Sm.

  An example of the metal element coordinated at the Y site is Ni. It is also preferable that Ni is a main component and a part of Ni is substituted with a transition metal element such as Fe or Cu.

  Examples of the metal element coordinated at the Z site include Sn. It is also preferable that Sn is a main component and a part of this Sn is replaced with Sb.

  And this thermoelectric conversion material contains the specific glass component in the range which is less than 15 vol% (excluding 0 vol%) in the said half-Heusler compound which is a main component. Thereby, a heterogeneous phase is not produced | generated, it can be set as the sintered compact which is liquid-phase-sintered and has favorable denseness, and can obtain the thermoelectric conversion material with a favorable thermoelectric characteristic.

  Here, as the specific glass component, a glass component containing boron oxide, silicon oxide, and an alkaline earth metal oxide such as Ca or Ba (hereinafter referred to as “B-Si—Am—O-based glass component”). ) (Am represents an alkaline earth metal element) can be used. That is, this B—Si—Am—O-based glass component has a low glass softening point of 790 ° C. and is suitable for liquid phase sintering. Moreover, this glass component exhibits good wettability with respect to the half-Heusler compound, and can be liquid-phase sintered while suppressing the generation of heterogeneous phases. When a glass component other than the B-Si-Am-O-based glass component, for example, a glass component containing zinc oxide is used instead of an alkaline earth metal oxide, the glass component is also softened. The point is as low as 800 ° C., and liquid phase sintering is possible, but the wettability with respect to the half-Heusler compound is poor, and the glass element reacts with the element of the intermetallic compound.

  Moreover, the volume content of the B-Si-Am-O-based glass component is set to less than 15 vol%. If the volume content is 15 vol% or more, the content of the glass component becomes excessive, and in this case, a different phase is generated. This is because there is a risk that the thermoelectric characteristics may be deteriorated.

  Next, an embodiment of the method for producing the thermoelectric conversion material will be described in detail.

  First, a plurality of types of metal raw materials constituting the half-Heusler compound, for example, high-purity powders such as Ni, Ti, Sn, Zr, Hf, and Sb are prepared.

  Next, each metal raw material is weighed so as to be a half-Heusler compound having a predetermined composition, and the weighed material is melted using a melting method to produce a molten alloy. Here, the melting method is not particularly limited, and any method such as a high-frequency melting method or an arc melting method can be used.

  Thereafter, the molten alloy is rapidly cooled and solidified to produce an alloy powder. Here, the method for rapidly cooling and solidifying the molten alloy is not particularly limited, and liquid quenching such as a single roll method, a twin roll method, a rotating disk method, or an atomizing method such as a gas atomizing method or a water atomizing method. Any method can be used.

  Next, the obtained alloy powder is put into a pulverizer and pulverized to obtain a fine powder having a predetermined particle diameter. Here, the crusher is not particularly limited, and a ball mill, a jet mill, a planetary mill or the like can be arbitrarily used.

  Next, the glass component is added to the alloy powder obtained by fine pulverization so that the B-Si-Am-O-based glass component is less than 15 vol% (excluding 0 vol%), A mixed powder is prepared.

  Next, a predetermined pressure is applied to the mixed powder to which the B-Si-Am-O-based glass component has been added to perform press molding to produce a molded body. The press molding method is not particularly limited, and for example, a cold isostatic pressing method or the like can be used.

  Next, the alloy powder after the liquid quenching is accommodated in a predetermined container such as a crucible, and the compact is placed in a container in which the alloy powder is accommodated, and the compact is completely embedded in the alloy powder.

  In this state, firing is performed under normal conditions and at normal pressure. The firing conditions differ depending on the composition of the half-Heusler compound to be synthesized and the particle size of the alloy powder, but it is sufficient that the relative density of the sintered body is 70% or more, for example, 4 to 4 at a firing temperature of 1200 to 1300 ° C. Normal pressure firing is performed for about 8 hours without pressurization. The firing atmosphere is preferably performed in an inert atmosphere such as an Ar atmosphere from the viewpoint of avoiding alloy oxidation.

  As described above, according to the present embodiment, the half-Heusler compound is the main component, and the B-Si-Am-O-based glass component is contained in a range of less than 15 vol% (excluding 0 vol%). By adding a predetermined amount of a glass component having a low glass softening point to the main component, liquid phase sintering can be performed without generating a different phase.

  And the said glass component has favorable wettability with respect to an intermetallic compound, can perform liquid phase sintering, without producing | generating a heterogeneous phase, and sinterability improves. Therefore, a dense thermoelectric conversion material having desired thermoelectric characteristics can be obtained.

  Moreover, since the compact is embedded in the alloy powder and fired, firing can be performed without the compact being exposed to oxygen. Accordingly, oxidation of the surface of the sintered body and volatilization of the metal element coordinated at the Z site such as Sn and Sb are suppressed, and a thermoelectric conversion material having desired thermoelectric properties having practical value at low cost is easily manufactured. It becomes possible.

  In addition, since the firing process can be performed at normal pressure, unlike the pressure sintering method such as the hot press sintering method and the discharge plasma sintering method, the manufacturing process is simplified and expensive manufacturing equipment is provided. A manufacturing method of a thermoelectric conversion material that is unnecessary and suitable for mass production can be realized at low cost.

  Next, examples of the present invention will be specifically described.

  In this example, an n-type thermoelectric conversion material was produced and the characteristics were evaluated.

[Example 1]
First, Ti, Zr, Hf, Ni, Sb, and Sn metal raw materials having a purity of 99% or more were prepared.

These metal raw materials were weighed so that the composition formula would be (Ti _0.5 Zr _0.25 Hf _0.25 ) Ni (Sn _0.998 Sb _0.002 ) and melted by a high frequency melting method to prepare a molten alloy. That is, the weighed metal powder is put in an alumina crucible and set in a high-frequency furnace, a high-frequency current is passed under vacuum of 10 ⁻² to 10 ⁻³ Pa, and the metal raw material in the crucible is dissolved by high-frequency induction heating A molten alloy was prepared.

  Next, the molten metal was rapidly solidified and then coarsely pulverized to obtain an alloy powder. Thereafter, the alloy powder was put into a ball mill and pulverized into a fine powder having a particle size of about 1 to 20 μm.

Next, the mole percentages of B ₂ O ₃ , SiO ₂ , CaO, and BaO were blended into B ₂ O ₃ : 8 mole%, SiO ₂ : 41 mole%, CaO: 34 mole%, and BaO: 17 mole%. The prepared B-Si-Ca-Ba-O-based glass component was prepared.

  And this B-Si-Ca-Ba-O type | system | group glass component was added and mixed with the said fine powder-like alloy powder so that volume content might be 5 vol%, and mixed powder was obtained.

  Next, this mixed powder was pressed at 200 MPa by a cold isostatic pressing method to produce a molded body having a length of 8 mm, a width of 8 mm, and a thickness of 5 mm.

  Thereafter, this compact was put into an alumina crucible together with the alloy powder described above, and the compact was buried in the metal powder. And in this state, it hold | maintained at 1200 degreeC under Ar atmosphere for 6 hours, normal-pressure baking was performed, and the sample of Example 1 was produced.

[Comparative Example 1]
A sample of Comparative Example 1 was prepared by the same method and procedure as in Example 1 except that no glass material was added.

[Comparative Example 2]
Comparative Example 2 was the same method and procedure as in Example 1 except that the B-Si-Ca-Ba-O-based glass component used in Example 1 was added so that the volume content was 15 vol%. A sample of was prepared.

[Comparative Example 3]
B ₂ O _{3, SiO} 2, mol% of ZnO _is, B ₂ O 3: 31 mol%, _SiO 2: 47 mol%, and ZnO: 22 mol% to formulated the B-Si-ZnO-based glass component Prepared.

  And this B-Si-Zn-O type | system | group glass component was added and mixed with the alloy fine powder produced in the said Example 1 so that volume content might be 5 vol%.

  Thereafter, the sample of Comparative Example 3 was produced by the same method and procedure as in Example 1.

[Comparative Example 4]
Except for adding the B-Si-Zn-O-based glass component having the same composition as Comparative Example 3 to the alloy fine powder so that the volume content is 15 vol%, the same method and procedure as Comparative Example 3, A sample of Comparative Example 4 was produced.

(Sample evaluation)
About each sample of Example 1 and Comparative Examples 1-4, the sintered density was measured by the Archimedes method, and the relative density was calculated by dividing the measured value of the sintered density by the theoretical sintered density.

Moreover, about each said sample, the resistivity (rho) and Seebeck coefficient S in the temperature range of 50-420 degreeC were measured, and the output factor P (= S < ² > / (rho)) was calculated | required.

  Here, the resistivity ρ was measured by cutting the sample into a length of 4 mm, a width of 2 mm, and a thickness of 5 mm and using a DC four-terminal method.

  The Seebeck coefficient S was calculated by measuring the electromotive force by cutting a sample into 4 mm length, 2 mm width, and 5 mm thickness, and providing a temperature difference of 5 ° C. at both ends of each sample.

  Table 1 shows the relative density of each sample, the resistivity ρ at 300 ° C., the Seebeck coefficient S, and the output factor P.

As apparent from Table 1, since the sample of Example 1 contains 5 vol% of the B—Si—Ca—Ba—O glass component, liquid phase sintering can be performed without generating a different phase. did it. The sample of Example 1 has a relative density of 73.0% and good compactness, and an output factor P of 2.35 × 10 ⁻³ W / K ² · m, which is good and practical. A high performance thermoelectric conversion material was obtained.

  On the other hand, it was found that Comparative Example 1 did not contain a glass component, and therefore, the relative density was lower and the denseness was inferior to that of Example 1, and the output factor P was also inferior to that of Example 1.

  Although the comparative example 2 contains a B-Si-Ca-Ba-O-based glass component, since the volume content of the glass component is as large as 15 vol%, the relative density is higher than when no glass component is contained. The output factor P was also low. This is probably because a heterogeneous phase was formed because the volume content of the glass component was excessive.

  Since Comparative Examples 3 and 4 both use a B—Si—Zn—O-based glass component, the relative density decreased and the output factor P also decreased. This is presumably because the B-Si-Zn-O-based glass component has a low glass softening point, but has poor wettability with respect to the main component alloy powder, and a heterogeneous phase was generated.

FIG. 1 is a graph showing temperature characteristics of output factors of Example 1 and Comparative Examples 1 and 2 in a temperature range of 50 to 420 ° C. The horizontal axis represents temperature (° C.) and the vertical axis represents output factor P (W / K ² · m). In the figure, the solid line is Example 1, the alternate long and short dash line is Comparative Example 1, and the broken line is Comparative Example 2.

  As is clear from the comparison between Example 1 and Comparative Example 1, the addition of the B-Si-Ca-Ba-O-based glass component relatively increases the output factor P and improves the thermoelectric characteristics. However, it has been found that when the volume content of the glass component is 15 vol%, the output factor P is remarkably reduced, and thus the thermoelectric characteristics are also lowered.

  That is, by including the B-Si-Ca-Ba-O-based glass component in a range of less than 15 vol% (not including 0 vol%), the relative density is improved and the denseness is increased, which becomes an index of thermoelectric characteristics. The output factor P is improved. As a result, it has been found that a high-performance thermoelectric conversion material that can withstand practical use can be obtained without depending on the pressure sintering method.

2 to 4 show the relationship between the volume content of the glass component at 50 to 420 ° C., the resistivity ρ, the Seebeck coefficient S, and the output factor P, the B-Si—Ca—Ba—O-based glass component and B It is the figure compared with -Si-Zn-O type | system | group glass component. The horizontal axis represents the volume content (vol%) of the glass component, and the vertical axis represents the resistivity ρ (Ω · cm) (FIG. 2), the Seebeck coefficient S (μV / K) (FIG. 3), and the output factor. P (W / K ² · m) (FIG. 4) is shown. Further, in the figure, a black circle indicates a B-Si-Ca-Ba-O-based glass component, and a black circle indicates a B-Si-Zn-O-based glass component.

  As shown in FIGS. 2 to 4, the B—Si—Zn—O-based glass component decreases in resistivity ρ as the volume content increases, but the absolute value of the Seebeck coefficient S is B-Si—Ca—Ba. As a result, the B-Si-Ca-Ba-O-based glass component has a better output factor P than the B-Si-Zn-O-based glass component. It has gained. That is, it was confirmed that the B—Si—Ca—Ba—O-based glass component has superiority to the B—Si—Zn—O-based glass component.

It is a figure showing the temperature characteristic of an output factor. It is a figure which shows the relationship between the volume content of a glass component, and a resistivity. It is a figure which shows the relationship between the volume content of a glass component, and a Seebeck coefficient. It is a figure which shows the relationship between the volume content of a glass component, and an output factor.

Claims (7)

The main component is an intermetallic compound represented by the general formula XYZ (where X, Y, and Z each represents at least one metal element)
A thermoelectric conversion material comprising a glass component containing boron oxide, silicon oxide, and alkaline earth metal oxide in a range of less than 15 vol% (not including 0 vol%).   The thermoelectric conversion material according to claim 2, wherein the alkaline earth metal contains at least one of Ca and Ba.   The thermoelectric conversion material according to claim 1, wherein the intermetallic compound contains at least Ti, Ni, and Sn.   The thermoelectric conversion material according to claim 3, wherein a part of the Sn is substituted with Sb.   4. The thermoelectric conversion material according to claim 3, wherein a part of Ti is substituted with at least one element of Zr and Hf. 5.   An alloy powder composed of an intermetallic compound represented by the general formula XYZ (wherein X, Y, and Z each represents at least one metal element) is prepared using a plurality of types of metal powder as a starting material, and further oxidized. A mixed powder is prepared by mixing the glass component with the alloy powder so that the glass component containing boron, silicon oxide, and alkaline earth metal oxide is less than 15 vol% (not including 0 vol%). Then, the mixed powder is molded to produce a compact, and then a thermoelectric conversion material is produced by performing a firing process in a state where the compact is embedded in the alloy powder. Manufacturing method.   The method for producing a thermoelectric conversion material according to claim 6, wherein the baking treatment is performed at normal pressure.

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