JP2006228912A - Thermoelectric conversion material and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a thermoelectric conversion material by which the generation of micro cracking can be suppressed, a half Heusler's compound can be formed of single-phase structure or nearly single-phase structure, and the thermoelectric characteristic of the thermoelectric conversion material (ZT) can be improved, and to provide the thermoelectric conversion material which is made of a single-phase alloy of a half Heusler's compound, or an alloy having nearly single phase structure and has the high thermoelectric characteristic (ZT). <P>SOLUTION: This method is used to manufacture a thermoelectric conversion material which is formed by using a half Heusler's compound represented by MNiSn [M is at least one kind of Hf, Zr and Ti] by a unidirectional coagulation method. A floating zone melting method is preferable in the unidirectional coagulation method. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thermoelectric conversion material and a manufacturing method thereof, and more particularly, to a thermoelectric conversion material composed of a half-Heusler compound represented by MNiSn and a manufacturing method thereof.

  Thermoelectric conversion materials using the Seebeck effect can convert thermal energy into electrical energy. Because this property can be used to convert unnecessary waste heat from industrial and consumer processes and mobile objects into effective power, thermoelectric conversion materials are attracting attention as an energy-saving technology that takes environmental issues into consideration. .

The performance of the thermoelectric conversion material is evaluated by a dimensionless figure of merit ZT obtained by making a factor called figure of merit (Z) dimensionless by temperature (T). The dimensionless figure of merit is ZT = (α ² σ / κ) T [α: Seebeck coefficient (= thermoelectromotive force / temperature difference), σ: electric conductivity (= 1 / ρ; ρ = resistivity), κ: Thermal conductivity, T: measurement absolute temperature], the numerator α ² σ is an electrical output factor called a power factor, and the denominator κ is a thermal factor. The half-Heusler compound has hitherto been attracting attention as a material from which a dimensionless figure of merit ZT is relatively high (ZT = 0.1 to 0.4) and an alloy system that does not contain environmentally hazardous substances can be selected. In general, ZT exceeds 1.0 and is a practical target value.

  The half-Heusler compound MNiSn (M = Hf, Zr, Ti) alloy is known to have a large thermal conductivity and excellent electrical characteristics. And the arc melting method is conventionally utilized in many cases for the synthesis method of the thermoelectric conversion material comprised with this half-Heusler compound.

  However, in the arc melting method, since the solidification rate is extremely high, in addition to the half-Heusler phase, it contains a lot of unnecessary phases (impurities) as a non-equilibrium alloy structure (about 50 vol%), making it a single phase Difficult, and there is a tendency for large numbers of microcracks to be potentially introduced by the thermal stress of solidification shrinkage.

When such an unnecessary phase (impurity) is present in a large amount, the thermoelectromotive force α and the electrical conductivity σ are lowered. That is, the power factor represented by α ² σ is reduced. In addition, the occurrence of minute cracks is a factor that decreases the electrical conductivity σ (increases the resistivity ρ) and impairs the thermoelectric characteristics, causing the ZT peak temperature to shift and the value to decrease. In addition, the occurrence of microcracks is due to the brittleness of the material, which can be a cause of hindrance when adding the shape of the sample, and the apparent heat conduction when there are many microcracks. The degree (κ) is estimated to be low, and there is a risk of leading to overestimation of performance.

  In order to avoid these, in the case of the arc melting method, in general, the ingot made by arc melting is pulverized into a powder form, and the sample is prepared by sintering with a hot press or the like. In order to improve the power factor, doping with an appropriate amount of Sb or the like is also performed.

  As a technique related to the above, a thermoelectric conversion material having a (TiZrHf) NiSn type half-Heusler compound as a main phase is disclosed (for example, see Patent Document 1). In this material, heat treatment (annealing) is performed after synthesis by arc melting, replacing the element to make the material composition multi-component, reducing the thermal conductivity while maintaining a high Seebeck coefficient and low resistivity. Thus, it is said that a thermoelectric conversion material having a large ZT is obtained, and thermoelectric properties are improved by making it into a single phase by annealing.

In addition to the above, there is also a disclosure in which a single phase is formed by heat treatment after arc melting using an FeVA1 type Heusler compound by an arc melting method (see, for example, Patent Document 2).
JP 2004-356607 A US 2003-019681 Specification

  However, in the multicomponent system substituted with elements as described above, the phase equilibrium is changed and the degree of freedom of phase rule is increased. As a result, it is difficult to obtain a sufficient single phase by heat treatment, and it is introduced into the tissue by solidification shrinkage. It is also difficult to suppress the occurrence of minute cracks that affect thermoelectric performance.

  In the conventional arc melting method, no useful method has been proposed as a technique capable of producing a thermoelectric conversion material that does not cause cracks in a single phase in which unnecessary phases (impurities) do not coexist. The actual situation is that the inherent thermoelectric properties inherent to the half-Heusler compound cannot be extracted completely depending on the design method.

  Therefore, although the dimensionless figure of merit ZT of the half-Heusler compound is said to be relatively high as described above, the Seebeck coefficient and electric conductivity that form a molecule leading to ZT are higher than those of other thermoelectric conversion materials. Therefore, ZT ≦ 0.4 and ZT = 1, which is generally regarded as the lowest practical level of thermoelectric conversion materials, has not yet been reached.

  As described above, in the synthesis by the arc melting method that has been widely used conventionally, only a coexisting phase in which other phases coexist with the half-Heusler phase can be obtained, and a lot of minute cracks are generated. There is a limit to realizing a single phase to extract the power factor and at the same time to reduce the thermal conductivity. In addition, technologies that do not require doping with harmful substances such as Sb and Pb are expected from the viewpoint of environmental considerations.

  The present invention has been made in view of the above, and while suppressing the occurrence of microcracks, it is possible to configure a half-Heusler compound in a single phase or a structure close to single phase, and thermoelectric characteristics of a thermoelectric conversion material The present invention provides a method for producing a thermoelectric conversion material capable of improving (ie, ZT), and a thermoelectric conversion material having a high thermoelectric property (ie, ZT) composed of a single-phase alloy of a half-Heusler compound or an alloy having a structure close to a single phase. It is an object to achieve this purpose.

  In the present invention, in order to avoid occurrence of metallic coexisting phase and latent microcracks, the solidification rate can be extremely slow to maintain a stable solid phase / liquid phase interface during solidification (synthesis). It is important to be able to stir to stabilize the chemical composition in the vicinity of the interface and to prevent contamination by impurities. For this purpose, use of a unidirectional solidification method (preferably (optical) floating zone melting method) and The fact that it is effective to select Hf, Zr, Ti at the element M site of MNiSn and to form a solid solution, as well as the homologous elements Hf, Zr, Ti having different masses and sizes (preferably Hf, Zr) The composition of M (with two or more of Ti and Ti) is particularly effective in reducing the thermal conductivity κ, and has been achieved based on such knowledge.

  Thermal conductivity κ consists of a lattice component κph related to phonon scattering and a component κel due to electrons as carriers, and κel cannot be freely prepared to maintain excellent electrical properties, The lattice component κph can be adjusted independently, and the element M site is made into a solid solution using Hf, Zr, Ti having the same family and different masses and sizes (preferably as a composition of two or more of Hf, Zr, and Ti). By doing so, it is possible to effectively reduce the κph (that is, the thermal conductivity κ that forms the denominator for introducing ZT).

Specific means for solving the problems of the present invention are as follows.
In order to achieve the above object, the method for producing a thermoelectric conversion material according to the first invention is MNiSn [M represents at least one of Hf, Zr, and Ti. A structure using a half-Heusler compound represented by the following (hereinafter also referred to as “MNiSn compound according to the present invention”) is formed (synthesized) by a unidirectional solidification method. In other words, it includes a step of forming (synthesizing) a tissue using the MNiSn compound according to the present invention by a unidirectional solidification method.

  The method for producing a thermoelectric conversion material according to the first invention suppresses the coexistence of other phases (impurities) other than the half-Heusler compound phase by synthesizing a half-Heusler compound represented by MNiSn by a unidirectional solidification method. The structure of a single phase of a half-Heusler compound with a small volume fraction of the coexisting phase, or a structure close to that of a single phase depending on the composition, can be used. The electromotive force (Seebeck coefficient), electrical conductivity, and thermal conductivity can be extracted, and the thermoelectric characteristics (ZT) can be improved. In particular, the thermoelectric power α that forms a molecule that induces ZT can be dramatically improved by the single phase.

  In the present invention, the single phase refers to a phase state in which the volume fraction of the MNiSn compound phase according to the present invention occupying the structure constituting the thermoelectric conversion material is 99 vol% (volume%) or more.

  Furthermore, by using the unidirectional solidification method, it is possible to prevent the generation of minute cracks during synthesis, so the density is improved, the resistivity ρ is kept low, and the electrical conductivity σ that forms the molecule leading to ZT is effective. Can be improved.

  Furthermore, one or two or more selected from Hf, Zr, and Ti are made into a solid solution with Ni and Sn, and by having a ternary system or higher structure, the thermal conductivity that forms the denominator leading to ZT is obtained. It can be effectively reduced. A quaternary system or more is more effective in reducing thermal conductivity.

  As described above, in the first invention, since the thermoelectromotive force α and the electrical conductivity σ are improved and the thermal conductivity κ is reduced, the ZT is remarkably improved and excellent thermoelectric characteristics are obtained. It is possible to produce a thermoelectric conversion material having In addition, high ZT can be achieved without doping with harmful elements such as Sb and Pb in line with environmentally hazardous substances, and excellent thermoelectric characteristics can be obtained.

  In the first aspect of the present invention, it is desirable to form a single-phase or substantially single-phase structure composed of a half-Heusler compound represented by MNiSn (M: at least one of Hf, Zr, and Ti). By being configured in a single phase or a structure close to a single phase, the inherent thermoelectric characteristics inherent in the half-Heusler compound can be more effectively extracted.

  In the first invention, after forming (synthesizing) the structure of the half-Heusler compound by the unidirectional solidification method, it is effective to further perform heat treatment (providing a heat treatment step). By applying heat treatment, ZT can be further improved and a thermoelectric conversion material having excellent thermoelectric characteristics can be obtained.

  Among the unidirectional solidification methods, the floating zone melting method (including the optical floating zone melting method) is effective. In the floating zone melting method, regardless of the composition of the half-Heusler compound (not only ternary system, quaternary system, quinary system, etc.), it is possible to make a single phase with significant reduction of microcracking. Thermoelectric properties inherent to the compound can be further extracted, ZT can be dramatically increased, and a thermoelectric conversion material having excellent thermoelectric properties can be produced.

  The thermoelectric conversion material according to the second invention is produced by the method for producing a thermoelectric conversion material according to the first invention.

  The thermoelectric conversion material according to the second invention is produced according to the first invention, and as described above, there is little coexistence of other phases (impurities) other than the half-Heusler phase, and the single phase of the half-Heusler compound Or a structure having a structure close to a single phase as a main phase, and there are few microcracks, so that it has a high thermoelectromotive force α and an electrical conductivity σ, and a low thermal conductivity κ. Therefore, ZT is high and thermoelectric properties are excellent.

In the second invention, among the half-Heusler compounds represented by MNiSn (M: at least one of Hf, Zr, and Ti), the composition of (Hf _x Zr _1-x ) NiSn [0 <x <1] When the M site is substituted and dissolved with Hf and Zr of the same family having different masses and sizes, the lattice component of the thermal conductivity can be effectively reduced, so that the thermal conductivity is effectively reduced. Particularly when the quaternary composition of (Hf _x Zr _1-x ) NiSn is used, a high ZT value can be obtained (the highest ZT value is obtained at a composition ratio in the vicinity of x = 5). Particularly excellent power generation characteristics can be obtained.

  According to the present invention, it is possible to suppress the occurrence of minute cracks and to form a half-Heusler compound in a single-phase structure or a structure close to a single-phase structure, thereby improving the thermoelectric characteristics (that is, ZT) of the thermoelectric conversion material. It is possible to provide a method for producing a thermoelectric conversion material to be obtained, and a thermoelectric conversion material having a high thermoelectric property (ie, ZT), which is composed of a single-phase alloy of a half-Heusler compound or an alloy having a structure close to a single phase.

  Hereinafter, the manufacturing method of the thermoelectric conversion material of the present invention will be described in detail, and the details of the thermoelectric conversion material of the present invention will be described in detail through the description.

  The method for producing a thermoelectric conversion material of the present invention uses a unidirectional solidification method to form a structure composed of a single phase or a substantially single phase of a half-Heusler compound represented by MNiSn (a structure composed of a single phase or a substantially single phase). That is, a structure having a phase having excellent thermoelectric conversion characteristics as a main constituent phase is formed.

  The half-Heusler compound according to the present invention is a compound represented by MNiSn, and M represents any one of Hf, Zr, and Ti, or two or more selected from Hf, Zr, and Ti. .

Specifically, MNiSn includes HfNiSn, ZrNiSn, TiNiSn as a ternary system, (Hf _x Zr _1-x ) NiSn, (Hf _x Ti _1-x ) NiSn, (Ti _x Zr _{1-x as a} quaternary system. ) NiSn [both satisfy x <0 <x <1. And HfZrTiNiSn, (Hf _x Zr _y Ti _1-xy ) NiSn [x and y satisfy 0 <x + y <1. ] Is included.

Among these, M is preferably Hf or Zr in that high thermoelectric properties (ZT) are obtained, and HfNiSn or ZrNiSn is particularly preferable in the ternary system. Further, in the point that high thermoelectric characteristics (ZT) can be obtained, in the case of x = 0.5 in the quaternary system, that is, (Hf _0.5 Zr _0.5 ) NiSn, (Hf _0.5 Ti _0.5 ) NiSn, (Ti _0.5 Zr _0.5 ). NiSn is preferred.

Next, the unidirectional solidification method will be described.
The unidirectional solidification method is a method in which an optimal temperature gradient is applied to the solid / liquid interface and the crystal growth is controlled in one direction, and includes a floating zone melting method (including an optical floating zone melting method) and the like. Examples include the Bridgeman method and the Czochralski method. Among these, a floating zone melting method (particularly an optical floating zone melting method) that does not require a container for melting the raw material and can avoid mixing impurities during melting is preferable. The optical floating zone melting apparatus has a structure in which a melting light source (lamp) and an ellipsoidal mirror are provided as a basic structure, and the light from the lamp can be condensed and heated and melted on a sample. .

  Hereinafter, the unidirectional solidification method will be described in detail with reference to FIGS. FIG. 1 is a schematic cross-sectional view of an example of an optical floating zone melting apparatus using the optical floating zone melting method as viewed from above, and FIG. 2 is a side view of the optical floating zone melting apparatus of FIG. It is a schematic sectional drawing at the time.

  As shown in FIG. 1 and FIG. 2, the optical floating zone melting apparatus includes a quartz tube 1 in which a sample is hollow and a quartz tube 1 is positioned at an axial center portion, and at least a part of the quartz tube 1 is placed in an axial center portion. A floating zone melting furnace comprising a floating zone melting chamber 4 formed by connecting four ellipsoidal mirrors 2 of equal width at an equal distance from the surrounded quartz tube 1 in an endless manner is provided. . Halogen lamps 3a to 3d are attached to the curved surfaces of the four ellipsoidal mirrors 2, respectively, and the interior side of the ellipsoidal mirror 2 is subjected to mirror surface processing. As shown in FIGS. 1 and 2, the halogen light emitted from the halogen lamp is reflected by the mirror-finished ellipsoidal mirror 2 and enters the quartz tube 1 at the axial center from four directions. Yes.

  As the light source, a xenon lamp or the like can be used in addition to the halogen lamp, and the xenon lamp has advantages such as a high reached temperature and a sharp light because it emits light by discharge. That is, if the light is focused on the point, the temperature can be made uniform and the temperature gradient can be made steep.

  After the dry deoxygenation treatment, the quartz tube 1 is inserted with an inert gas such as argon gas from one end to the other end (not shown) under a positive pressure so that dirt such as metal vapor is not deposited. The positive pressure prevents the air from entering from the outside.

  As shown in FIG. 2, the quartz tube 1 located inside the floating zone melting chamber 4 is heated between a rotatable feed rod 5 and a rotatable seed rod 6. A liquid phase region (Molten Zone) 7 capable of forming a liquid phase by melting is provided, and the seed rod 6 can be moved together with the feed rod 5 (preferably rotated) during the liquid phase formation. The seed rod 6 and the feed rod 5 are rod-shaped ingots having the same composition prepared by arc melting.

  In the present embodiment, an example in which the number of ellipsoidal mirrors is four has been described, but a commercially available apparatus having one or two ellipsoidal mirrors may be used. An apparatus with four ellipsoidal mirrors is suitable in that a uniform temperature distribution can be obtained because heating can be performed from four directions.

  First, two half-Heusler compounds (for example, HfNiSn; made of multiphase) formed into a rod shape having a length of about 100 to 200 mm by arc melting or the like are prepared, and these are seeded as shown in FIG. The rod 6 and the feed rod 5 are arranged with their respective end portions approaching each other. In this state, the power (temperature) of the halogen lamp is increased until the lower end of the feed rod 5 and the upper end of the seed rod 6 begin to dissolve. At this time, the respective rods are rotated so as to be in opposite directions. When melted, the rods are brought into close contact with each other while rotating in opposite directions to form a melt portion in the liquid phase region 7. At this time, as shown in FIG. 3, the melt remains in the surface tension between the seed rod and the feed rod. Then, after holding for about 30 minutes until the solid phase / liquid phase interface is stabilized, the seed rod and the feed rod are started to descend. On the upper side of the seed rod 6, a half-Heusler compound (for example, HfNiSn) having a target single phase or a structure close to a single phase is solidified and grown, and the lower end of the feed rod 5 is further melted to replenish the liquid phase. The At this time, the band-like melt portion moves from the bottom to the top while being floated by the surface tension, that is, each rod relatively moves downward, and the light moves upward so that the compound grows ( For this reason, it is called floating zone melting.) Here, it is important to grow the target crystal while adjusting the power of the halogen lamp so that the amount of the melt is kept constant (that is, the temperature and the temperature gradient are kept constant). If the lowering speed of each rod is the same, the outer diameter of the growing ingot is kept constant (the lowering speed of the feed rod can be increased and the outer diameter can be increased if stable growth is possible). . Note that the seed originates from the use of a seed crystal (seed) whose orientation is known in advance when producing a single crystal, and inheriting the crystal orientation for growth.

  As described above, stirring is performed by applying rotation, and by lowering the feed rod 5 and the seed rod 6, the compound phase (or metal phase) of the compound phase (or metal phase) is hidden at the solid phase / liquid crystal interface covered by the liquid phase region. Crystal grows. The structure formed by growing in this way is composed of a single phase in which a half-Heusler compound (for example, HfNiSn) occupies 99 vol% or more, or depending on the composition, a structure that is closer to a single phase than before. It shows high thermoelectric characteristics.

  In the case of the (optical) floating zone melting method, the most important factor for controlling the solidification tissue morphology is the “solid phase / liquid phase interface morphology”, the temperature gradient of the solid phase / liquid phase interface, and the equilibrium of each element. It is easily affected by factors such as distribution and atomic diffusion, and in particular, the temperature gradient can be relatively quantitatively adjusted externally, and can be adjusted by the solidification rate (growth rate) and the stirring rate. It can also be indirectly adjusted by the lamp power of the light source. Therefore, by adjusting the rotational speed (ie, stirring speed) of the two rod-shaped ingots (feed rod 5 and seed rod 6) and the moving speed (ie, growth speed) of the feed rod 6, the temperature gradient conditions in the liquid phase region can be adjusted. By controlling, the thermoelectric characteristics can be suitably controlled.

  In the above, the optimum solidification and growth conditions, specifically, the optimum stirring speed and growth speed conditions are strongly dependent on the state of the solid phase / liquid phase interface and are unique to each sample (compound phase). Since the conditions on the apparatus side for producing the optimum conditions (optimal temperature gradient at the solid phase / liquid phase interface) vary depending on the individual apparatuses used, the conditions vary. In other words, the temperature distribution given to the sample differs depending on factors such as the type and number of lamps, the arrangement, the shape and material of the ellipsoidal mirror, and the method of taking the optical path to the sample. Different. In general, the stirring speed is preferably in the range of 10 to 50 rpm, and more preferably in the range of 25 to 45 rpm. The growth rate is preferably in the range of 1 to 50 mm / h, more preferably in the range of 2 to 10 mm / h. The temperature gradient is preferably adjusted so that the liquid phase amount (width) of the liquid phase portion is uniform and straight as shown by the broken line in FIG.

  As a method for controlling the temperature gradient, there is a method in which the amount of light collected at one point of the sample by the ellipsoidal mirror 2 is partially shielded. Specifically, the quartz tube 1 is covered with an alumina tube divided into two parts in the vertical direction so that the quartz tube 1 is wrapped from the outside, and the divided part is aligned with respect to the center of the sample (one focal part of the ellipsoidal mirror 2), The amount of light reaching the sample is adjusted by the amount of opening. When the aperture is opened so as to be a vertical target with respect to the center reference, a substantially symmetrical temperature gradient is obtained in the vertical direction. In some cases, the profile of the temperature gradient can be changed up and down.

  The MNiSn compound according to the present invention can produce a thermoelectric conversion material having high ZT and excellent thermoelectric properties by the same method as described above, regardless of the composition.

  In the present invention, in addition to the optical floating zone melting apparatus described above, other apparatuses that can use the unidirectional solidification method can be appropriately selected.

  In the present invention, by using one kind of half-Heusler compound represented by MNiSn (M: at least one kind of Hf, Zr and Ti), depending on the single phase or the composition of the compound, the single phase is more than conventional. However, in addition to using one type, an MNiSn phase selected from two or more types may be used as a target.

  EXAMPLES Hereinafter, although an Example is shown and this invention is demonstrated more concretely, this invention is not limited to these Examples. In the examples shown below, the optical floating zone melting method according to the present invention is referred to as “OFZ” or “OFZ method”, and the case where further heat treatment is performed as “OFZ HT” as the conventional method. The arc melting method may be abbreviated as “arc-melt”, and the case of further heat treatment may be abbreviated as “arc HT”.

Example 1
First, as shown in FIGS. 1 to 2, a quartz tube 1 that forms a space in which a sample is put and is synthesized by a floating zone melting method, and the quartz tube 1 is positioned so that the quartz tube 1 is positioned at the axial center. A floating zone melting furnace comprising at least a part and a floating zone melting chamber 4 formed by connecting four ellipsoidal mirrors 2 of equal width at an equal distance from the surrounded quartz tube 1 in an endless manner. A device equipped with was prepared. In this floating zone melting furnace, halogen lamps 3a to 3d are provided on the curved surfaces of the four ellipsoidal mirrors 2, respectively, and the inner wall on the indoor side of the ellipsoidal mirror 2 is subjected to mirror surface treatment and irradiated from the halogen lamps. As shown in FIGS. 1 and 2, the halogen light thus reflected is reflected on the mirror-finished inner wall surface and is incident on the quartz tube 1 at the axial center so that the sample in the quartz tube can be heated.

  As shown in FIG. 2, the quartz tube 1 located inside the floating zone melting chamber 4 is heated between a rotatable feed rod 5 and a rotatable seed rod 6. A liquid phase region (Molten Zone) 7 capable of forming a liquid phase by melting is provided, and the seed rod 6 can be moved together with the feed rod 5 (preferably rotated) during the liquid phase formation.

  First, two HfNiSn compounds (consisting of multiphases) prepared by arc melting into a rod-like body having a length of about 100 mm and a diameter of 10 mm by arc melting were prepared. As shown in FIG. 5, each end portion was arranged close to each other. Arc melting was performed according to a conventional method.

  In this state, the power (temperature) of the halogen lamp is increased until the lower end of the feed rod 5 and the upper end of the seed rod 6 begin to melt, and the rods are rotated so that the rotation directions of the rods are relatively opposite to each other. It was. Once dissolved, each rod is brought into close contact with each other at a rotational speed of 45 rpm to create a melt in the liquid phase region 7 until the solid phase / liquid phase interface is stabilized. After holding for about 30 minutes, the seed rod and the feed rod were lowered at a moving speed of 10 mm / h. A half-Heusler compound (for example, HfNiSn) having a single phase structure was solidified and grown on the seed rod 6 to obtain the HfNiSn thermoelectric material (OFZ material) of the present invention.

  In order to stably grow crystals, the temperature gradient at the solid phase / liquid phase interface must be kept appropriate and constant. However, since the temperature and temperature gradient cannot actually be measured, in this example, the liquid phase Using the amount (width) as a guide, the power of the halogen lamp was finely adjusted so that a liquid phase with a certain width (a certain amount) was maintained as indicated by the broken line in FIG. Also, the moving speed of the feed rod is finely adjusted as necessary.

(Example 2)
In Example 1, the ZrNiSn thermoelectric material (OFZ material) of the present invention was used in the same manner as in Example 1 except that the HfNiSn compound was replaced with the arc-melted ZrNiSn compound and the moving speed was changed to 5 mm / h. Obtained.

(Example 3)
In Example 1, the TiNiSn thermoelectric material (OFZ material) of the present invention was obtained in the same manner as in Example 1 except that the HfNiSn compound was replaced with the arc-melted TiNiSn compound.

Example 4
In Example 1, a TiNiSn thermoelectric material (OFZ heat-treated material) of the present invention was obtained in the same manner as in Example 1 except that the obtained HfNiSn thermoelectric material was heat-treated at 1000 ° C. (1273 K) for 24 hours.

(Example 5)
In Example 1, two kinds of HfNiSn thermoelectric materials (OFZ materials) of the present invention were used in the same manner as in Example 1 except that the moving speed (growth speed) was changed from 10 mm / h to 5 mm / h and 20 mm / h. Obtained.

(Comparative Examples 1-3)
Each of the HfNiSn compound (Comparative Example 1), ZrNiSn compound (Comparative Example 2), or TiNiSn compound (Comparative Example 3) arc-melted by the arc melting method was annealed at 800 ° C. (1073 K) for 336 hours, and compared. HfNiSn thermoelectric material, ZrNiSn thermoelectric material, and TiNiSn thermoelectric material (arc material) were obtained.

(Comparative Examples 4-5)
Ingots of HfNiSn compound (Comparative Example 4) or ZrNiSn compound (Comparative Example 5) produced by arc melting by the arc melting method are each pulverized into powders, and compacted with a hot press at 50 MPa for 5 hours. Thus, a comparative HfNiSn thermoelectric material and a ZrNiSn thermoelectric material (hot press material) were obtained.

(Evaluation 1)
-1. Evaluation of organization (1)-
OFZ materials obtained in Examples 1 to 3 (OFZ as-grown; HfNiSn thermoelectric material of the present invention, ZrNiSn thermoelectric material, TiNiSn thermoelectric material) and arc materials obtained in Comparative Examples 1 to 3 (arc-melt as-cast) A back-scattered electron image (BEI) was taken using a scanning electron microscope for the comparative HfNiSn thermoelectric material, ZrNiSn thermoelectric material, TiNiSn thermoelectric material), and the structure was evaluated by visual observation. Back-reflected electron images (BEI) are shown in FIGS.

  As a result of the evaluation, as shown in FIGS. 4 (b), 5 (b), and 6 (b), the OFZ material of the present invention using the OFZ method can dramatically suppress the occurrence of cracks, In particular, the HfNiSn thermoelectric material and the ZrNiSn thermoelectric material of the present invention have a substantially single-phase structure (HfNiSn is 99% or more by volume and ZrNiSn is 97% or more by volume), and the TiNiSn thermoelectric material of the present invention is also TiNiSn. The coexisting phase other than the phase was greatly reduced, and a structure that drastically approached the single phase could be obtained.

  On the other hand, as shown in FIGS. 4 (a), 5 (a), and 6 (a), the comparative arc material has a very fast solidification rate, so that it largely depends on the solidification process and is in an unbalanced state. A structure was formed, and each thermoelectric material had a multiphase structure in which two or more phases coexisted, particularly a non-equilibrium four-phase structure in the comparative TiNiSn thermoelectric material. In addition, minute cracks were also observed.

-2. Organizational evaluation (2)-
Back-reflected electron images (BEI) were taken for the OFZ material obtained in Example 1 and the two kinds of OFZ materials obtained in Example 5 using a scanning electron microscope, and the growth rate in the OFZ method was sufficient for the formation of the structure. The structure was evaluated by visual observation. The backscattered electron images (BEI) are shown in FIGS.

  As a result of the evaluation, as shown in FIGS. 7A to 7C, even when the growth rate is changed within the range of 5 to 20 mm / h (stirring rate and temperature gradient are fixed), a structure that can be regarded as a substantially single phase is obtained. I was able to get it. In particular, the case of 10 mm / h was good, and a phase that could be considered as a complete single phase with a volume ratio of 99% or more was obtained.

-3. Measurement and evaluation of thermoelectromotive force (Seebeck coefficient) and electrical resistivity
As the HfNiSn thermoelectric material, the OFZ material of the present invention obtained in Example 1, the OFZ heat-treated material of the present invention obtained in Example 4, and the hot press material obtained in Comparative Example 4 were manufactured by ULVAC-RIKO Co., Ltd. Each Seebeck coefficient and electrical resistivity (reciprocal of electrical conductivity) were measured using ZEM-1. Specifically, the thermoelectromotive force (Seebeck coefficient) is calculated by measuring the thermoelectromotive force generated between both electrodes using two electrodes and the temperature difference between the electrodes, and calculating the thermoelectromotive force per unit temperature. Thus, the electric conductivity was obtained by measurement by the four probe method. And the relationship figure (FIG. 8) of the obtained value and temperature was created, and these temperature dependence was evaluated. Seebeck coefficient and electrical resistivity are shown in FIGS. In the figure, literature values (▲) obtained by measuring the arc material obtained by arc melting in bulk are also shown.

  As shown in FIG. 8A, the OFZ material and the OFZ heat-treated material of the present invention have a significantly improved Seebeck coefficient (thermoelectromotive force) compared to the conventional hot press material, and particularly as the temperature approaches room temperature. A large value toward -400 μV / K was shown. Since HfNiSn behaves as an n-type semiconductor, the thermoelectromotive force shows a negative value, and the larger the absolute value (that is, in the lower part of the figure), the greater the thermoelectromotive force. Moreover, as shown in FIG.8 (b), the OFZ material and OFZ heat processing material of this invention were able to reduce electric resistivity significantly. In any case, the OFZ heat-treated material was better.

  Next, for the ZrNiSn thermoelectric material, the OFZ material of the present invention obtained in Example 2 and the hot press material obtained in Comparative Example 5 were used in the same manner as described above using ZEM-1 manufactured by ULVAC-RIKO Co., Ltd. The Seebeck coefficient and electrical resistivity (reciprocal of electrical conductivity) were measured. And the relationship figure (FIG. 9) of the obtained value and temperature was created, and these temperature dependence was evaluated. Seebeck coefficient and electrical resistivity are shown in FIGS. In the figure, the literature values (▲) of arc materials obtained by arc melting measured in bulk are also shown.

  As shown in FIG. 9 (a), the OFZ material of the present invention has a small Seebeck coefficient (thermoelectromotive force) compared with the conventional hot press material, but the electrical resistivity is improved as shown in FIG. 9 (b). As shown in Fig. 4, a significant reduction was achieved.

As described above, a half-Heusler alloy having a single phase or a structure close to a single phase is obtained by the synthesis by the floating zone melting method, and the electrical properties [that is, ZT molecules ( Power factor) α ² σ] was greatly improved. In other words, the floating zone melting method (unidirectional solidification method) makes it easy to make a single phase, and it is possible to solidify and grow slowly, so that the thermal stress accompanying solidification shrinkage is suppressed and microcracks are Occurrence was suppressed, and the inherent electrical characteristics inherent in the half-Heusler compound could be extracted.

Based on the above results, FIG. 10 shows the temperature dependency of the power factor (α ² σ) of the HfNiSn thermoelectric material and the ZrNiSn thermoelectric material. As shown in FIGS. 8 and 9, since both the Seebeck coefficient and the electrical resistivity could be improved, the power factor of the HfNiSn thermoelectric material is the same as that of the conventional hot press material as shown in FIG. 10 (a). On the other hand, an improvement effect of about 3 times with the OFZ material and about 5 times with the OFZ heat-treated material could be obtained. In addition, for the ZrNiSn thermoelectric material, the improvement in the Seebeck coefficient was small, but the electrical resistivity was greatly reduced. As a power factor, as shown in FIG. The improvement effect of about 4 times was able to be acquired.

Next, examples related to thermal characteristics among thermoelectric characteristics will be described.
(Example 6)
In Example 1, the HfNiSn compound, except that instead of the arc melting (Hf _0.5 Zr _0.5) NiSn compound [x = 0.5], in the same manner as in Example 1, of the present invention (Hf _0.5 Zr _0.5) NiSn A thermoelectric material (OFZ material) was obtained.

(Comparative Example 6)
The (Hf _0.5 Zr _0.5 ) NiSn compound produced by arc melting by the arc melting method was annealed at 800 ° C. (1073 K) for 336 hours to obtain a comparative (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material (arc material).

(Evaluation 2)
-4. Evaluation of the organization
OFZ material obtained in Example 6 (OFZ as-grown; (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material of the present invention) and arc material obtained in Comparative Example 6 (arc-melt as-cast; comparative (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material) was taken with a scanning electron microscope, a back-scattered electron image (BEI) was photographed, and the structure was evaluated by visual observation. A backscattered electron image (BEI) is shown in FIG.

As shown in FIG. 11 (b), the (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material of the present invention obtained by melting an alloy having a composition in which the Hf: Zr composition ratio is 1: 1 and is dissolved by substitution using the OFZ method, Compared with the comparative arc material shown in FIG. 11 (a), as in the case of the ternary system (HfNiSn thermoelectric material etc.) described above, the occurrence of cracks and the presence of coexisting phases ((Hf, Zr) Sn ₃ etc.) The volume ratio was greatly reduced, and a structure composed of a single phase (97% or more by volume ratio) could be obtained. On the other hand, the structure of the comparative arc material was multiphase, and minute cracks were also observed.

In the above description, the case of (Hf _x Zr _1-x ) NiSn [x = 0.5] has been shown. However, combinations of Hf _x Ti _1-x and Ti _x Zr _1-x other than the combination of Hf and Zr The same applies to the case where the Hf / Zr ratio takes a composition ratio other than x = 0.5.

-5. Evaluation of dimensionless figure of merit ZT
For the (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material (OFZ material) of the present invention obtained in Example 6, measurement of Seebeck coefficient and electrical resistivity (reciprocal of electrical conductivity) as in the case of the ternary system described above. In addition, the thermal conductivity is obtained by a conventional method using a laser flash method, and a relationship diagram (FIG. 12) between ZT and temperature when ZT at each temperature is obtained from these values is created. Temperature dependence was evaluated. ZT is shown in FIG.

  As shown in FIG. 12, ZT showed a good value, and an excellent thermoelectric characteristic of ZT = 0.9 could be obtained in the vicinity of 900 to 1000K.

  That is, not only the electrical characteristics (thermoelectromotive force α and electric conductivity ρ) are improved as in the case of the ternary system described above, but at the same time, the substitutional fixation of the M site of MNiSn by Hf and Zr of the same family is further performed. The thermal conductivity could be reduced more effectively than the ternary system (HfNiSn or ZrNiSn).

In the above description, the case where the composition of HfNiSn, ZrNiSn, TiNiSn, and (Hf _0.5 Zr _0.5 ) NiSn is used is mainly described. However, other than these, (Hf _x Ti _1-x ) NiSn and (Ti _x Zr _1-x ) When other compositions included in the half-Heusler compound represented by MNiSn, such as NiSn and HfZrTiNiSn, are used, the temperature gradient, specifically the stirring speed and the moving speed are appropriately selected by the same method as described above. It is possible to obtain a thermoelectric conversion material having high thermoelectric properties by controlling the growth rate.

  The thermoelectric conversion material of the present invention is a material for a thermoelectric conversion element that constitutes a thermoelectric module that can directly convert thermal energy into electric energy (electromotive force) by thermoelectric conversion when a temperature difference is given using exhaust heat or the like. It can be used suitably.

It is a schematic sectional drawing when an example of the optical floating zone melting apparatus using the optical floating zone melting method is seen from the upper surface. It is a schematic sectional drawing when the optical floating zone melting apparatus of FIG. 1 is seen from the side. It is a conceptual diagram for demonstrating the place which synthesize | combines a half Heusler compound within the quartz tube of a floating zone melting furnace. (A) shows the structure of the HfNiSn thermoelectric material by the conventional arc melting method, and (b) shows the structure of the HfNiSn thermoelectric material by the OFZ method. (A) shows the structure of a ZrNiSn thermoelectric material by a conventional arc melting method, and (b) shows the structure of a ZrNiSn thermoelectric material by an OFZ method. (A) shows the structure of a TiNiSn thermoelectric material by a conventional arc melting method, and (b) shows the structure of a TiNiSn thermoelectric material by an OFZ method. It is a figure which shows a structure | tissue when the growth rate in OFZ method is (a) 5 mm / h, (b) 10 mm / h, (c) 20 mm / h. (A) is a graph which shows the temperature dependence of the Seebeck coefficient of a HfNiSn thermoelectric material, (b) is a graph which shows the temperature dependence of the electrical resistance value of a HfNiSn thermoelectric material. (A) is a graph which shows the temperature dependence of the Seebeck coefficient of a ZrNiSn thermoelectric material, (b) is a graph which shows the temperature dependence of the electrical resistance value of a ZrNiSn thermoelectric material. (A) is a graph which shows the temperature dependence of the power factor of HfNiSn thermoelectric material, (b) is a graph which shows the temperature dependence of the power factor of ZrNiSn thermoelectric material. (A) shows the structure of a (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material by a conventional arc melting method, and (b) shows the structure of a (Hf _0.5 Zr _0.5 ) NiSn thermoelectric material by an OFZ method. It is a graph showing the relationship between the ZT and temperature (Hf _0.5 Zr _0.5) NiSn thermoelectric material.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Quartz tube 4 ... Floating zone dissolution chamber 3a, 3b, 3c, 3d ... Halogen lamp

Claims (6)

  Half-Heusler compound represented by MNiSn [M represents at least one of Hf, Zr, and Ti. ] Is formed by the unidirectional solidification method.   The method for producing a thermoelectric conversion material according to claim 1, wherein the structure is formed into a single phase or a substantially single phase of the half-Heusler compound by the unidirectional solidification method.   The manufacturing method of the thermoelectric conversion material of Claim 1 or 2 which has the heat processing process which heat-processes further to the formed said structure | tissue.   The method for producing a thermoelectric conversion material according to any one of claims 1 to 3, wherein the unidirectional solidification method is a floating zone melting method.   A thermoelectric conversion material produced by the method for producing a thermoelectric conversion material according to claim 1. The thermoelectric conversion material according to claim 5, wherein the thermoelectric conversion material is configured to have a composition of (Hf _x Zr _1-x ) NiSn [0 <x <1].