JP5217812B2 - Method for producing half-Heusler thermoelectric material - Google Patents

Description

  The present invention relates to a method for producing a thermoelectric material composed of a half-Heusler alloy which is an intermetallic compound having a stoichiometric composition.

  Thermoelectric materials are energy materials that perform direct conversion between thermal energy and electrical energy based on two basic thermoelectric effects, the Seebeck effect and the Peltier effect.

  Thermoelectric power generation devices using thermoelectric materials are simpler in structure, more robust and more durable than conventional power generation technologies, have no moving parts, are easily micro-sized, require no maintenance, and are reliable. It has many advantages such as high life, long life, no noise, no pollution and low temperature waste heat available.

  Compared to conventional compression cooling technology, thermoelectric cooling devices using thermoelectric materials also have the advantages that they do not require chlorofluorocarbon, do not cause contamination, are easy to downsize, have no moving parts, and do not generate noise. .

  Therefore, especially in recent years, energy and environmental issues have become more serious, such as aviation / space, national defense construction, geological and meteorological observation, medical hygiene, microelectronics, etc., and utilization of waste heat in petrochemical, metallurgy, and power industries. Is expected to be put to practical use for a wide range of applications.

As an index for evaluating the performance of the thermoelectric material, a power factor P = S ² σ and a dimensionless performance index ZT = (S ² σ / κ) T are used. Here, S: Seebeck coefficient, σ: conductivity, κ: thermal conductivity, T: absolute temperature. That is, in order to obtain good thermoelectric properties, it is necessary that the Seebeck coefficient S and the electrical conductivity σ are high and the thermal conductivity κ is low.

  From this point of view, half-Heusler alloys are attracting attention as excellent thermoelectric materials.

  Patent Document 1 discloses a half-Heusler thermoelectric material constituent atom in which a neutral atom constituent atom is Ni, a cation constituent atom is any one of Ti, Zr, and Hf, and an anion constituent atom is any one of Si, Ge, Sn, and Pd A combination method has been proposed. However, there is no disclosure about a method for producing a single-phase half-Heusler alloy that does not include a heterogeneous phase without a thermoelectric effect.

  As a method for producing a single-phase half-Heusler alloy that does not include a different phase, arc melting and floating zone melting are performed.

  The method by arc melting is described in detail in, for example, Non-Patent Documents 1 and 2, and an ingot having a target composition is prepared by arc melting, pulverized and powdered, and pressure-molded and sintered. However, since the desired thermoelectric characteristics including a different phase cannot be obtained as it is, it is necessary to make it into a single phase by heat treatment. This heat treatment is a homogenization process by solid phase diffusion, and is extremely long, such as vacuum sealing into a quartz tube at 1073 K for 1 week (Non-Patent Document 1) or 700 to 800 ° C. for 1 to 6 weeks (Non-Patent Document 2). It takes time and cannot be applied as an industrial production method.

  For example, Patent Documents 2 and 3 disclose a method using floating band melting. Since the floating band melting method is originally a method for growing a single crystal, a single-phase product can be obtained. Not only does it take time, but stable and high-quality production requires complex know-how to control a large number of parameters, and is not applicable as an industrial production method.

  Then, liquid phase reaction sintering can be considered as a simple and short manufacturing process applicable to industrial production. However, there is no example of applying liquid phase reaction sintering for half-Heusler alloys, and the following proposals have been made for other systems.

  In the method for producing a Bi—Sb alloy disclosed in Patent Document 4, production conditions are set based on a Bi—Sb binary phase diagram in liquid phase reaction sintering in which Sb generates a liquid phase. However, the half-Heusler alloy is a ternary compound, which has a different phase rule from the binary compound and has a complicated phase equilibrium. Therefore, its ternary phase diagram and reaction path are unknown, and the method of Patent Document 4 is Not applicable.

Further, in the disclosed _{Mg 2 (Si 1-x Sn} x) the production method of the alloy in Patent Document 5, Mg, Si, Sn and mixed at the same rate as object phase is reacted via the generated Sn liquid phase It has been presented. However, according to the experiments of the present inventors, even if three constituent elements (Ti, Ni, Sn) of a half-Heusler alloy (eg, TiNiSn), which is a ternary alloy, are mixed and similarly subjected to a liquid phase reaction, the expected result It turned out that the single phase of a composition (TiNiSn) cannot be obtained easily.

Furthermore, in the method for producing a CoSb ₃ alloy disclosed in Patent Document 6, an excessive amount of Sb is charged with respect to an intended composition and reacted through the generated Sb liquid phase, resulting in an expected composition. Presented. However, in this method, it is inevitable that Sb remains at the grain boundary and adversely affects the alloy characteristics as a different phase.

JP 2001-189495 A JP 2006-228912 A JP 2007-88395 A Japanese Patent No. 29996863 (Japanese Patent Laid-Open No. 7-283442) JP 2007-146283 A JP-A-9-260729 Mat. Trans. 47 (2006) pp.1453-1457. J. Phys. Condens. Matter 11 (1999) pp.1697-1709.

  An object of the present invention is to provide a method for producing a half-Heusler thermoelectric material that can obtain a single phase that does not include a heterogeneous phase by a simple and short-time production process that can be applied to industrial production using liquid phase reaction sintering. And

In order to achieve the above object, according to the present invention, a, b, c, d, e, f are 0 or a positive number, n, m are positive integers, a + b + c = 1, d + e = 1, As f + g = 1, (n, m) = (1,3), (1,1) or (2,1), it is composed of at least one of Ti, Zr and Hf and at least one of Ni and Co The intermetallic compound [Ti _a Zr _b Hf _c ] _n [Ni _d Co _e ] _m is held in contact with at least one liquid phase of Sn and Sb, thereby intermetallic compound [ There is provided a method for producing a half-Heusler thermoelectric material, characterized in that a solid phase of a half-Heusler alloy composed of Ti _a Zr _b Hf _c ] [Ni _d Co _e ] [Sn _f Sb _g ] is generated.

An extremely simple process of maintaining the solid phase of the intermetallic compound [Ti _a Zr _b Hf _c ] _n [Ni _d Co _e ] _m in contact with at least one liquid phase of Sn and Sb. The single-phase half-Heusler alloy of the intermetallic compound [Ti _a Zr _b Hf _c ] [Ni _d Co _e ] [Sn _f Sb _g ] having the target composition can be formed in a short time.

In the method of the present invention, the three components of the half-Heusler alloy, which is a ternary intermetallic compound, are not used alone as raw materials, but an intermetallic compound composed of two of the three components is used in a solid state, and the rest This is characterized in that the one-component metal is used in a liquid phase. Here, the “half-Heusler alloy” is a single phase of a half-Heusler intermetallic compound or an alloy having this as a main constituent phase, and is not limited to the stoichiometric composition, but is an equilibrium determined depending on the constituent elements. Includes composition. In addition, the “component” does not indicate only a single element but collectively indicates a group of elements that can be substituted with each other. That is, the components of the intermetallic compound [Ti _a Zr _b Hf _c ] _n [Ni _d Co _e ] _m used as a raw material in the present invention are “at least one of Ti, Zr and Hf” and “at least one of Ni and Co”. The remaining one component is “at least one of Sn and Sb”.

  In this sense, the above “ternary system” refers to a ternary system of “at least one of Ti, Zr and Hf” — “at least one of Ni and Co” — “at least one of Sn and Sb”.

The principle of the present invention will be described with reference to FIG. In order to simplify the explanation, TiNi is used as an intermetallic compound [Ti _a Zr _b Hf _c ] _n [Ni _d Co _e ] _m , Sn is used as a raw material at least one of the metals Sn and Sb, and a half-Heusler When TiNiSn is produced as an alloy intermetallic compound [Ti _a Zr _b Hf _c ] [Ni _d Co _e ] [Sn _f Sb _g ] (a = 1, b = 0, c = 0, d = 1, e = 0, f = 1, g = 0, n = 1, m = 1).

  In FIG. 1A, the solid phase 10 of the intermetallic compound TiNi and the liquid phase 12 of the metal Sn are held in contact with each other. This is because the solid phase of the intermetallic compound TiNi and the solid phase of the metal Sn are arranged adjacent to each other, the melting point of the intermetallic compound TiNi as the raw material and the melting point of the intermetallic compound TiNiSn as the product are lower, and the raw material It can be carried out by keeping the temperature higher than the melting point of the metal Sn.

  In FIG. 1 (2), the solid phase 14 of the intermetallic compound TiNiSn is generated between the solid phase 10 of the intermetallic compound TiNi and the liquid phase 12 of the metal Sn.

  In FIG. 1 (3), the entire volume occupied by the TiNi solid phase 10 as the raw material and the Sn liquid phase 12 with the passage of the holding time becomes the TiNiSn solid phase 14 as the product.

  This reaction proceeds at a much higher speed than the reaction by diffusion in the solid phase because one of the two raw materials to be reacted is a solid phase and the other is a liquid phase. Moreover, by limiting the raw material to react to 2 types, it can be limited only to reaction between two of TiNi and Sn. When the ternary TiNiSn half-Heusler alloy ternary Ti, Ni, and Sn are used individually as raw materials, the reaction between any two of the three raw materials is locally prioritized, A binary heterogeneous phase other than the desired half-Heusler type compound phase is generated.

For example, when solid phase Ti, Ni, and Sn are used alone as raw materials, the reaction between Ni and Sn preferentially proceeds to produce Ni ₃ Sn ₄ (solid phase), which is combined with Ti (solid phase). The reaction with which TiNiSn (solid phase) is generated by the reaction with) becomes a reaction process via solid phase diffusion and is not completed even after a long time.

  It is conceivable that the three raw materials Ti, Ni, and Sn are alloyed by arc melting, but the presence of Sn, which has a significantly lower melting point than Ti and Ni, makes the liquid phase surface steep and involves the peritectic reaction. Therefore, the composition of the liquid phase is greatly shifted to the Sn side, and a structure in which the four phases coexist through a non-equilibrium solidification process cannot be obtained.

[Example 1]
Using Sn is Ti _n Ni _m alloy and metal is an intermetallic compound as a starting material was prepared TiNiSn half-Heusler alloy is an intermetallic compound. Here, (n, m) = (1,1), (1,3), (2,1), that is, three types of TiNi, TiNi ₃ and Ti ₂ Ni were used as intermetallic compounds.

The Ti _n Ni _m alloy was prepared by weighing a predetermined amount of Ti and Ni and synthesizing them by arc melting, followed by homogenization heat treatment at 900 to 1000 ° C. for 72 hours.

In order to suppress as much as possible the retention of the melt and containment Sn in a sealed container, such as in Figure 2 created by Ti _n Ni _m alloy. That is, the lower surface of the _Ti n Ni _m alloy ring after charging the Sn into a container sealed with _Ti n Ni _m alloy discs were sealed to the upper surface of the ring _Ti n Ni _m alloy disk. In this state, heat treatment was performed at 500 to 1000 ° C. for 1 hr in an Ar gas atmosphere.

  Then, the air-cooled sample was taken out, the part enclosed with the broken line in FIG. 2 was cut out, and the structure | tissue observation of the interface by the back-scattered electron beam image of SEM and the quantitative chemical analysis by EPMA were performed. The results are shown in FIGS. 3A, 3B, and 4.

3A and 3B use a TiNi alloy (Ti _n Ni _m , n = 1, m = 1) as a raw intermetallic compound, and FIG. 3A shows holding temperatures of (1) 1000 ° C., (2) 900 ° C. FIG. 3B shows the case of (1) 800 ° C., (2) 700 ° C., and (3) 500 ° C., crossing the contact interface (solid phase-liquid phase reaction interface) between the raw material TiNi solid phase and the Sn liquid phase. This is the result of scanning and EPMA analysis.

In the case of holding at 1000 ° C. for 1 hour as shown in FIG. 3A (1), a TiNiSn half-Heusler phase is formed to a thickness of about 22 μm between the TiNi phase and the Sn phase, but TiNi ₂ Sn (full) The Heusler phase is also formed to a thickness of several μm.

  In the case of holding at 900 ° C. shown in FIG. 3A (2), a TiNiSn half-Heusler phase is formed to a thickness of about 2.5 μm between the TiNi phase and the Sn phase.

  In the case of holding at 800 ° C. shown in FIG. 3B (1), a TiNiSn half-Heusler phase is formed to a thickness of about 20 μm between the TiNi phase and the Sn phase.

  In the case of holding at 700 ° C. shown in FIG. 3B (2), similarly, a TiNiSn half-Heusler phase is formed to a thickness of about 15 μm between the TiNi phase and the Sn phase.

  In the case of holding at 500 ° C. for 1 hour as shown in FIG. 3B (3), formation of a TiNiSn half-Heusler phase is not recognized.

  From this result, using a TiNi powder having a particle diameter of 2.5 μm × 2 = 5 μm or more as a raw material and Sn powder, mixing and forming, and performing a heat treatment of holding at 900 ° C. for 1 hour, both of the contacted powder particles It can be seen that TiNiSn is produced over a total distance of 10 μm, thereby forming a single-phase half-Heusler alloy as the entire powder compact.

  Furthermore, if a TiNi powder having a particle diameter of 20 μm × 2 = 40 μm or more as a raw material and Sn powder are mixed and molded and heat-treated at 800 ° C. and held for 1 hr, the total distance between both contacted powder particles becomes 80 μm. It can be seen that TiNiSn is formed, and that a single-phase half-Heusler alloy can be synthesized as the whole powder compact.

  This is a tremendous shortening of time compared to the conventional heat treatment time of at least one week, and is highly practical as an industrial production process.

  Further, it can be seen from this example that the holding temperature is most preferably 800 ° C. and can be increased to less than 1000 ° C. The upper limit of the holding temperature is desirably less than 1000 ° C. from the viewpoint that the melting point of the raw material TiNi is 1310 ° C. and the solidus line of the generated half-Heusler phase TiNiSn is about 1180 ° C.

4 (1) uses a TiNi ₃ alloy (Ti _n Ni _m, where n = 1, m = 3) as a raw material intermetallic compound, and FIG. 4 (2) shows a Ti ₂ Ni alloy as a raw material intermetallic compound. (Ti _n Ni _m , n = 2, m = 1), and both are similar EPMA analysis results when the holding temperature is 800 ° C.

A TiNiSn half-Heusler phase is formed between the TiNi ₃ phase or Ti ₂ Ni layer and the Sn phase, but other phases are also formed, so even if heat treatment is performed for a long time to make a single phase. Since the phase equilibrium does not change, it is difficult to make a single phase.

[Example 2]
Using a (Ti _0.5 Zr _0.5 ) Ni alloy as a raw material and Sn as a metal, a (Ti _0.5 Zr _0.5 ) NiSn half-Heusler alloy as an intermetallic compound was produced. .

As in Example 1, Sn is contained in a sealed container as shown in FIG. 2 made of a (Ti _0.5 Zr _0.5 ) Ni alloy. In this state, heat treatment was performed at 800 ° C. for 1 hr in an Ar gas atmosphere.
Thereafter, an air-cooled sample was taken out, and a part surrounded by a broken line in FIG. The results are shown in FIG. Each region in the figure had the following composition.
(A) Zr rich half-Heusler (11 at% Ti-22 at% Zr) *
(B1) Ti rich half-Heusler (26 at% Ti-7 at% Zr) *
(B2) Ti rich half-Heusler (29 at% Ti-4 at% Zr) *
*: The balance is Ni + Sn with a total of 100at%
In addition, Zr rich half-Heusler and Ti rich half-Heusler mean the following.
Zr-rich half-Heusler: Half-Heusler ZrNiSn has a solid solution of Ti at the stated concentration Ti-Rich Half-Heusler: Half-Heusler TiNiSn has a solid solution of Zr at the stated concentration As described above, the Ti-rich half-Heusler and Zr-rich half-Heusler Generate separately.

Next, as shown in FIG. 6 (1), a hole 62 is formed in a part of the (Ti _0.5 Zr _0.5 ) Ni alloy ingot 60, and Sn particles are packed as indicated by reference numeral 64. Was subjected to heat treatment at 900 ° C. for 40 hours in a vacuum. A reaction layer 66 having a thickness of about 70 μm was formed at the interface between the (Ti _0.5 Zr _0.5 ) Ni alloy and Sn after the heat treatment, as shown in FIG. As shown by a broken line frame 68 in FIG. 6 (2), it crosses the contact interface (solid phase-liquid phase reaction interface) 70 between the raw material (Ti _0.5 Zr _0.5 ) Ni solid phase and Sn liquid phase. EPMA analysis was performed by scanning along the line 72. As shown in FIG. 6 (3), with respect to the point P1 entering the several μm Sn side from the interface, the interface P2, and the point P3 entering several μm from the interface to the (Ti _0.5 Zr _0.5 ) Ni side, The composition was as follows:
P1: Half-Heusler (19 at% Ti-17 at% Zr) *
P2: Zr rich half-Heusler (6 at% Ti-41 at% Zr) *
P3: (Ti _0.5 Zr _0.5 ) Ni
*: The balance is Ni + Sn with a total of 100at%

表１に示したように、９００℃〜１２００℃の範囲でピーク強度比は十分に小さい。ただし、１２００℃ではハーフホイスラーのピークが２つに分離しており、Ｔｉリッチ相とＺｒリッチ相への分離が生じたと考えられる。
以上の結果から、（Ｔｉ_０。５Ｚｒ_０．５）ＮｉＳｎ系では９００℃以上、１２００℃未満の熱処理によって、単相ハーフホイスラーが生成可能であると考えられる。 Separately, a (Ti _0.5 Zr _0.5 ) Ni alloy was synthesized by arc melting and turned into a chip shape by lathing with a WC tool. This was mixed with Sn grains and compacted, and heat treated at 900 ° C. to 1200 ° C. in a vacuum. After the heat treatment, the peak intensity ratio of half-Heusler (Ti _0.5 Zr _0.5 ) NiSn and Sn was determined for the powder XRD of the sample. The results are shown in Table 1.

As shown in Table 1, the peak intensity ratio is sufficiently small in the range of 900 ° C to 1200 ° C. However, at 1200 ° C., the half-Heusler peak was separated into two, and it is considered that separation into a Ti-rich phase and a Zr-rich phase occurred.
From the above results, it is considered that in the (Ti _0.5 Zr _0.5 ) NiSn system, a single-phase half-Heusler can be generated by heat treatment at 900 ° C. or more and less than 1200 ° C.

Example 3
Using an intermetallic compound (Ti _0.5 Hf _0.5 ) Ni alloy and a metal Sn as raw materials, an intermetallic compound (Ti _0.5 Hf _0.5 ) NiSn half-Heusler alloy was produced. .
A sample was prepared in the same manner as in Example 1. After the heat treatment at 800 ° C. for 1 hour, the interface between the (Ti _0.5 Hf _0.5 ) Ni raw material and the Sn raw material was analyzed in the same manner as in Example 1. The results are shown in FIG. Each region in the figure had the following composition.
(A1) Ti rich half-Heusler (22 at% Ti-11 at% Hf to 18 at% Ti-15 at% Hf)
(A2) Ti-rich half-Heusler (almost TiNiSn, no Hf)
(A3) Ti-rich half-Heusler (31 at% Ti-2 at% Hf)
Thus, it can be seen that a single-phase half-Heusler alloy can be synthesized by heat treatment even if a part of Ti is replaced with Hf.

Example 4
Using Ti (Ni _0.8 Co _0.2 ) alloy, which is an intermetallic compound, and Sn, which is a metal, as raw materials, manufacture a Ti (Ni _0.8 Co _0.2 ) Sn half-Heusler alloy, which is an intermetallic compound. did.
A sample was prepared in the same manner as in Example 1. After the heat treatment at 800 ° C. for 1 hr, EPMA analysis was performed on the interface between the Ti (Ni _0.8 Co _0.2 ) material and the Sn material in the same manner as in Example 1. The results are shown in FIG. As shown, a half-Heusler Ti (Ni _0.8 Co _0.2 ) Sn half-Heusler is produced. In the half-Heusler phase, the Co concentration fluctuates in the range of 0 at% to 2 at% as shown in FIG.
Thus, it can be seen that a single-phase half-Heusler alloy can be synthesized by heat treatment even if a part of Ni is replaced by Co.

The half-Heusler thermoelectric material of the present invention may contain a small amount of doping element. Specifically, Nd, V, and Ta may be added to each site of Ti, Zr, and Hf with an upper limit of 2% of the site (about 6.7 at%).
Moreover, although powder was used as a raw material in the Example, it is not necessary to limit to this and you may use a thin plate and a wire form.

  According to the present invention, there is provided a method for producing a half-Heusler thermoelectric material capable of obtaining a single phase that does not include a different phase by a simple and short-time production process applicable to industrial production using liquid phase reaction sintering.

1 is a schematic cross-sectional view illustrating the principle of the present invention. Sectional drawing which shows the preparation form of the sample in an Example. EPMA concentration profile (temperature 1000 ° C., 900 ° C.) of the generated phase in the vicinity of the interface between the two when using intermetallic compound TiNi and metal Sn as raw materials. EPMA concentration profile (temperature 800 ° C., 700 ° C., 500 ° C.) of the produced phase in the vicinity of the interface between the two when using intermetallic compound TiNi and metal Sn as raw materials. The EPMA concentration profile of the generated phase near the interface between the two when the intermetallic compound TiNi ₃ or Ti ₂ Ni and metal Sn are used as raw materials. Intermetallic compound as a starting material _{(Ti 0.5 Zr 0.5)} in the case of using Ni and metallic Sn, EPMA concentration profile of the production phase in the vicinity of the interface therebetween. Sectional drawing which shows the preparation form of the sample for long-time heat processing. The EPMA concentration profile of the product phase in the vicinity of the interface between the two when an intermetallic compound (Ti _0.5 Hf _0.5 ) Ni and metal Sn are used as raw materials. EPMA concentration profile of the generated phase in the vicinity of the interface between the intermetallic compound Ti (Ni _0.8 Co _0.2 ) and metal Sn as raw materials.

Claims (2)

a, b, c, d, e, f, g are 0 or a positive number, n, m are positive integers, a + b + c = 1, d + e = 1, f + g = 1, (n, m) = (1, 3) As (1,1) or (2,1), an intermetallic compound [Ti _a Zr _b Hf _c ] _n [comprising at least one of Ti, Zr and Hf and at least one of Ni and Co Ni _d Co _e ] _m and at least one liquid phase of Sn and Sb are held in contact with each other, thereby intermetallic compound [Ti _a Zr _b Hf _c ] [Ni _d Co _e ]. A method for producing a half-Heusler thermoelectric material, comprising producing a solid phase of a half-Heusler alloy composed of [Sn _f Sb _g ].   The method of claim 1, wherein (n, m) = (1,1).

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