JP4533294B2 - Thermoelectric conversion material and manufacturing method thereof - Google Patents

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 ternary half-Heusler compound 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.

  Among thermoelectric conversion materials, a half-Heusler compound has been studied variously as one of useful materials. In particular, the half-Heusler compound MNiSn (M = Hf, Zr, Ti) alloy is known to have a large thermoelectromotive force and electrical conductivity and excellent electrical characteristics, and is excellent as an n-type thermoelectric conversion material. Show the characteristics.

  On the other hand, in order to perform thermoelectric power generation, it is necessary to configure elements by combining n-type and p-type semiconductor materials, and power generation efficiency can be increased by connecting a large number of elements in series. However, when a brittle material is used in a high temperature range of about 300 to 700 ° C., the electrode / material gap increases due to thermal stress caused by the difference in thermal expansion coefficient between the n-type and p-type semiconductor materials. Cracks are likely to occur in the joints such as peeling and destruction, and in order to avoid these, it is necessary to suppress the thermal stress caused by the difference in thermal expansion coefficient. Therefore, studies have been made to construct both n-type and p-type materials using materials having the same crystal structure and having similar physical property values such as thermal expansion coefficient, elastic modulus, and rigidity.

  On the other hand, the arc melting method has often been conventionally used for synthesizing thermoelectric conversion materials composed of half-Heusler compounds. However, since the solidification rate is extremely high in the arc melting method, in addition to the half-Heusler phase, it contains a lot of unnecessary phases (impurities) as a non-equilibrium alloy structure (in some cases, about 50 vol% at maximum). It is difficult to phase, and a large number of microcracks tend to be introduced due to the thermal stress of solidification shrinkage. In addition, due to the rapid solidification rate, the crystal grains of the half-Heusler phase are refined and the total area of the crystal grain boundaries is increased.

If a large amount of such unnecessary phases (including impurities) and crystal grain boundaries exist, the electrical conductivity σ and the thermoelectromotive force α 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 factor in adding to the shape of the sample, and in addition, if there are a large number of microcracks, apparent heat conduction The degree (κ) is estimated to be low, and there is a risk of overestimating the 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, by heat treatment (annealing) after synthesis by arc melting, the element composition is replaced to make the material composition multi-component, reducing the thermal conductivity while maintaining a high Seebeck coefficient and low resistivity. It is said that a thermoelectric conversion material having a large ZT is obtained and thermoelectric properties are enhanced by a single phase by annealing treatment.

As a thermoelectric conversion material, a (Ti, Zr, Hf) NiSn-based half-Heusler type alloy is known, and M _{1 -x} A _x · Ni _{1 -y By} · Sn ₁ -z C _z [M: A ternary half-Heusler type alloy composed of Ti, Zr, Hf, A: an unselected element of Ti, Zr, Hf, B: Pd, Pt, C: In, Sb, Te] is disclosed. (For example, refer to Patent Document 2).
JP 2004-356607 A JP 2005-19713 A

  Although various studies have been made on thermoelectric conversion materials, as described above, p-type thermoelectric conversion materials are used even if an element is configured with n-type and p-type materials using the same crystal structure material. There is no known material exhibiting excellent characteristics, and doping and element addition are performed except in the general case where an n-type material is excessively doped with an element having a small number of valence electrons to form a p-type material. In fact, little is known about the technology for obtaining a p-type half-Heusler-type compound without any problems.

  In addition, the above-described hex-based half-Heusler type alloys are all materials that exhibit n-type characteristics, and no half-Heusler-type alloys that exhibit p-type characteristics are known.

  Furthermore, in elemental substituted multi-component systems, the phase equilibrium changes and the degree of freedom of phase rule increases. As a result, it is difficult to obtain a sufficient single phase by heat treatment, and thermoelectric performance is introduced into the structure by solidification shrinkage. It is also difficult to suppress the occurrence of minute cracks that affect the process. In the conventional arc melting method, no useful method has been proposed as a technology capable of producing a single-phase thermoelectric conversion material that does not cause unnecessary phases (impurities) and does not coexist. The actual situation is that the thermoelectric properties inherent to the half-Heusler compound cannot be fully extracted by some methods.

  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.

  The present invention has been made in view of the above, and a thermoelectric conversion material exhibiting p-type thermoelectric characteristics in a wide temperature range, and a half-Heusler compound having a single phase or close to a single phase while suppressing the occurrence of minute cracks. It is an object of the present invention to provide a method for producing a thermoelectric conversion material that can be structured in a structure, or a structure close to a single crystal or a substantially single crystal, and can improve p-type thermoelectric properties (that is, ZT). The goal is to achieve this.

The present invention has been achieved based on the following findings. That is,
First, HfNiSn is known as a half-Heusler compound exhibiting excellent semiconductor characteristics of n-type, and it is particularly effective to reduce the thermal conductivity κ by selecting Zr or Ti at this Hf site to form a solid solution. It is knowledge that there is. Similarly, a decrease in thermal conductivity κ can be expected by selecting Pd or Pt at the Ni site and forming a solid solution. However, it was found that when the Ni site is replaced by a solid solution, the magnitude (including positive and negative) of the thermoelectromotive force α is greatly affected as compared with the effect of reducing the thermal conductivity κ. When Ni having 3d electron orbital is replaced by solid solution substitution of Pd, Pt having 4d electron orbital and Pt having 4f electron orbital, the bond orbital and the state density of electrons as a compound including Hf and Sn change greatly. As a result, since the band structure also changes, the Seebeck coefficient (that is, the thermoelectromotive force) determined by the positional relationship between the band gap and the Fermi energy surface is expected to change greatly. In addition, since ZrPdSn and ZrPtSn are weak, they tend to show p-type characteristics, and it is expected that HfPtSn will show p-type characteristics.

  Furthermore, in order to avoid the occurrence of metallic coexisting phases and potential microcracks, the solidification rate can be extremely slow to maintain a stable solid / liquid interface during solidification (synthesis), and in the vicinity of the solid / liquid interface. It is important to be able to stir in order to stabilize the chemical composition and to prevent contamination by impurities. For this purpose, the use of a unidirectional solidification method (preferably (optical) floating zone melting method) is effective. It is knowledge that it is.

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, grid component Kappaph can be adjusted individually, by solid solution as the composition of the Hf _x Pt _y Sn _z, is possible to effectively reduce the Kappaph (i.e. thermal conductivity forming the denominator directing ZT kappa) it can.

Specific means for solving the problems of the present invention are as follows.
To achieve the above object, the thermoelectric conversion material is a first aspect of the invention, half-Heusler compound represented by Hf _x Pt _y Sn _z (hereinafter sometimes referred to as "HfPtSn compounds according to the present invention".) The It is configured by using. Here, x, y, and z are values in the ranges of 0.9 <x <1.1, 0.8 <y <1.2, and 0.9 <z <1.1, respectively. <X + y + z <3.2 is satisfied. Preferably, x = y = z = 1.

In the first invention, the thermoelectric conversion material by the configuration including a composition represented by the above Hf _x Pt _y Sn _z, it is possible to obtain a thermoelectric properties of p-type over a wide temperature range, especially at room temperature Good p-type characteristics can be obtained in a temperature range from (25 ± 5 ° C.) to a high temperature range (around 800 ° C.).

The first invention, the Hf _x Pt _y single phase or Ryakutansho the half-Heusler compound represented by Sn _z, or preferably constructed of a single crystal or substantially single crystal. A structure consisting of a single phase or single crystal of a half-Heusler compound with a small volume fraction of the coexisting phase while suppressing the coexistence of other phases (impurities) other than the half-Heusler compound phase, or a structure close to a single phase or single crystal depending on the composition By adopting a configuration close to, the inherent thermoelectric characteristics of the half-Heusler compound, that is, the thermoelectromotive force (Seebeck coefficient), electrical conductivity, and thermal conductivity are extracted, and the thermoelectric characteristics (ZT) are improved. The thermoelectromotive force α that forms a molecule that leads to ZT can be dramatically improved by making it single phase or single crystal.

  In the present invention, the single phase refers to a phase state in which the volume fraction of the HfPtSn compound phase according to the present invention occupying the tissue constituting the thermoelectric conversion material is 99 vol% (volume%) or more. The single crystal refers to a state in which the volume fraction of the HfPtSn compound phase according to the present invention occupying the structure constituting the thermoelectric conversion material is 99 vol% (volume%) or more and is composed of one crystal grain. And the case where it consists of several coarse crystal grains is a substantially single crystal.

Method for producing a thermoelectric conversion material is a second invention is configured to form (synthesize) the unidirectional solidification process a half-Heusler compound represented by Hf _x Pt _y Sn _z. In other words, it is provided with a step of forming (synthesizing) a tissue using the HfPtSn compound according to the present invention by a unidirectional solidification method. Here, similarly to the above, x, y, and z are values in the ranges of 0.9 <x <1.1, 0.8 <y <1.2, and 0.9 <z <1.1. .8 <x + y + z <3.2.

Method for producing a thermoelectric conversion material of the second invention, Hf _x synthesis of Pt half-Heusler compound represented by _y Sn _z by performing a unidirectional solidification process, other than the half-Heusler compound phase phase (impurity) The structure of a single phase or single crystal of a half-Heusler compound with a small volume fraction of the coexisting phase or a structure close to a single phase or a structure close to a single crystal can be used depending on the composition. The inherent thermoelectric characteristics of the Heusler compound, that is, the thermoelectromotive force (Seebeck coefficient), electrical conductivity, and thermal conductivity can be extracted to improve the thermoelectric characteristics (ZT). In particular, the thermoelectromotive force α that forms a molecule that induces ZT can be dramatically improved by the single phase or single crystallization.

  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.

  As described above, in the second invention, it is possible to improve the thermoelectromotive force α and the electrical conductivity σ. Furthermore, since the second invention can be used effectively also in the case of reducing the thermal conductivity κ by solid solution substitution of elements, a thermoelectric conversion material having dramatically improved ZT and having excellent thermoelectric properties. Can be produced.

  In addition, in the present invention, it is one of the features that p-type thermoelectric characteristics can be obtained without doping, and ZT can be achieved by material design that does not rely on doping of harmful elements such as environmentally hazardous substances. Can be improved, and excellent thermoelectric properties can be obtained.

  In the second invention, it is desirable to form a single phase or substantially single phase composed of a half-Heusler compound represented by HfPtSn, or a single crystal or substantially single crystal structure. As described above, the intrinsic thermoelectric characteristics inherent in the half-Heusler compound can be more effectively brought out by being configured in a single phase or single crystal, or a structure close to a single phase or single crystal.

  In the second invention, it is effective to further heat-treat after forming (synthesizing) the structure of the half-Heusler compound by the unidirectional solidification method. 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, it is possible to achieve single-phase or single-crystal formation while significantly reducing the occurrence of microcracks, and to further draw out the inherent thermoelectric properties inherent to the half-Heusler compound. ZT can be dramatically increased, and a thermoelectric conversion material having excellent thermoelectric properties can be produced.

  According to the present invention, a thermoelectric conversion material that exhibits p-type thermoelectric characteristics over a wide temperature range, and a half-Heusler compound that has a single phase or a structure close to a single phase, or a single crystal or a single phase while suppressing the occurrence of microcracks. It is possible to provide a method for producing a thermoelectric conversion material that can be configured in a structure close to a crystal and can improve p-type thermoelectric properties.

Hereinafter, the thermoelectric conversion material of the present invention and the production method thereof will be described in detail.
The thermoelectric conversion material of the present invention comprises using a half-Heusler compound represented by Hf _x Pt _y Sn _z, in terms of the thermoelectric properties preferably configured to single-phase or Ryakutansho, or single crystal or substantially single crystal Is done. In addition, you may further contain components other than Hf, Pt, and Sn.

  In the HfPtSn compound according to the present invention, the Hf ratio is in the range of 0.9 <x <1.1, the pt ratio is in the range of 0.8 <y <1.2, and the Sn ratio is It is a half-Heusler compound that satisfies 0.9 <z <1.1 and satisfies 2.8 <x + y + z <3.2.

  Furthermore, as a specific example of the HfPtSn compound according to the present invention, x = y = z = 1, that is, HfPtSn is preferably mentioned in that a high thermoelectromotive force can be obtained.

  The thermoelectric conversion material of the present invention can be prepared using a conventionally known method such as an arc melting method. Especially, the method of forming by the unidirectional solidification method is preferable, and it can produce most suitably with the manufacturing method of the thermoelectric conversion material of this invention.

Method for producing a thermoelectric conversion material of the present invention, by using the unidirectional solidification method, Hf _x Pt _y Sn _z organization composed of a half-Heusler compounds represented (single phase, Ryakutansho, single crystal, or Including a structure composed of a substantially single crystal), that is, a structure in which a phase excellent in thermoelectric conversion characteristics is (preferably a main) constituent phase.

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. . Xenon 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 mirror-finished. As shown in FIGS. 1 and 2, the xenon light emitted from the xenon lamp is reflected by the mirror-finished ellipsoidal mirror 2 and is incident on the quartz tube 1 at the axial center from four directions. Yes.

  As the light source, a halogen lamp or the like can be used in addition to the xenon lamp. When the xenon lamp is used, there are advantages such that the ultimate temperature is high and the light is sharp because the light is emitted by the 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 (HfPtSn; consisting of multiple phases) formed into a rod shape having a length of about 100 to 200 mm by arc melting or the like are prepared. As shown in FIG. 6 and the feed rod 5 are arranged with their respective end portions approaching each other. In this state, the power (temperature) of the xenon lamp is increased until the lower end of the feed rod 5 and the upper end of the seed rod 6 begin to melt. 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 target single phase or a structure close to a single phase, or a single crystal or a half-Heusler compound (HfPtSn) having a structure close to a single crystal is solidified and grown, and the lower end of the feed rod 5 is Further, the liquid phase is replenished by melting. 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 xenon 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). .

  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 or single crystal in which the half-Heusler compound (HfPtSn) occupies 99 vol% or more, or a structure closer to a single phase or single crystal than before depending on the composition. Therefore, it exhibits high thermoelectric properties as a thermoelectric conversion material.

  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 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, solidification 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 solidification speed condition strongly depend on the state of the solid phase / liquid phase interface and are unique to each sample (compound phase). The conditions on the apparatus side for producing the optimum conditions (optimum temperature gradient at the solid phase / liquid phase interface) vary depending on the individual apparatuses used, and therefore differ depending on the case. In other words, the temperature distribution given to the sample varies 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 solidification rate is preferably in the range of 1 to 50 mm / h, more preferably in the range of 5 to 20 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 HfPtSn compound according to the present invention can produce a thermoelectric conversion material having a higher ZT than the conventional one 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.

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

First, the thermoelectric conversion material of the present invention will be described more specifically with reference to examples.
Example 1
A HfPtSn compound is arc-melted by a conventional method using arc melting, and annealed at 1200 ° C. (1473 K) for 72 hours after arc melting to produce an arc heat treatment material (arc-melt) of HfPtSn (x = y = z = 1). annealed).

(Comparative Examples 1-4)
ZrPdSn and ZrPtSn ingots are arc-melted by a conventional method using an arc melting method, and each arc-made ingot is pulverized into a powder form, and compacted and sintered at 50 MPa for 5 hours using a hot press, A ZrPdSn material (Comparative Example 1) and a ZrPtSn material (Comparative Example 2) were obtained. Further, a part of the obtained ZrPdSn material and ZrPtSn material was annealed at 1200 ° C. (1473 K) for 72 hours to obtain a ZrPdSn heat-treated material (Comparative Example 3) and a ZrPtSn heat-treated material (Comparative Example 4).

(Evaluation 1)
-1. Evaluation of thermoelectromotive force (Seebeck coefficient)
For the arc heat treatment material obtained in Example 1 and the ZrPdSn material, ZrPdSn heat treatment material, ZrPtSn material and ZrPtSn heat treatment material obtained in Comparative Examples 1 to 4, using ZEM-1 manufactured by ULVAC-RIKO Co., Ltd. The Seebeck coefficient was measured. Specifically, the thermoelectromotive force (Seebeck coefficient; α) is obtained 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. Obtained by calculating. And the relationship diagram of the obtained value and temperature was created, and these temperature dependence was evaluated. The thermoelectromotive force (Seebeck coefficient) is shown in FIGS.

  As shown in FIG. 4, in the heat treatment material “◇” of HfPtSn, the thermoelectromotive force (Seebeck coefficient) is positive in a wide temperature range from room temperature to 1000 K, and has p-type thermoelectric characteristics. It was. That is, it was possible to obtain a half-Heusler compound stably exhibiting p-type thermoelectric characteristics without doping or element addition.

  On the other hand, in the comparative ZrPdSn material, ZrPdSn heat-treated material, ZrPtSn material and ZrPtSn heat-treated material, as shown in FIG. In the temperature range, a good thermoelectromotive force could not be secured. Further, the ZrPdSn composition showed p-type characteristics from room temperature to around 750 K, but further transitioned to n-type in a higher temperature region, and the ZrPtSn composition did not lead to n-type transition, but the thermoelectromotive force was 0 ( To zero).

  Next, the method for producing the thermoelectric conversion material of the present invention (that is, the unidirectional solidification method) will be described more specifically with reference to examples.

(Example 2)
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, xenon 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 xenon lamps. As shown in FIGS. 1 and 2, the xenon light thus reflected is reflected by the mirror-treated inner wall surface and is incident on the quartz tube 1 at the axial center, thereby heating the sample in the quartz tube.

  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 HfPtSn compounds (consisting of multiple phases) prepared by arc melting into a rod-shaped 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 xenon 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 rotational 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 5 mm / h. A half-Heusler compound [HfPtSn (x = y = z = 1)] having a single-phase structure is solidified and grown on the seed rod 6 to obtain the HfPtSn thermoelectric material (OFZ material; OFZ as-grown) of the present invention. It was.

  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 As a guide, the power of the xenon lamp was finely adjusted so that a liquid phase having a certain width (a certain amount) was maintained as indicated by a broken line in FIG. Also, the moving speed of the feed rod is finely adjusted as necessary.

(Comparative Examples 5-6)
HfPtSn compound (arc-melt) having a composition of HfPtSn (x = y = z = 1) and arc melting by the arc melting method (Comparative Example 5), and 1200 ° C. after arc melting in the same manner ( HfPtSn compound (arc-melt annealed; Comparative Example 6) annealed at 1473K for 72 hours was obtained as a comparative HfPtSn thermoelectric material.

(Evaluation 2)
-2. Evaluation of the organization
For the OFZ material (OFZ as-grown; HfPtSn thermoelectric material of the present invention) obtained in Example 2 and the arc material (arc-melt) and arc heat-treated material (arc-melt annealed) obtained in Comparative Examples 5 and 6 A backscattered electron image (BEI) was photographed using a scanning electron microscope, and the tissue was evaluated by visual observation. A backscattered electron image (BEI) is shown in FIG.

  As a result of the evaluation, as shown in FIG. 6 (c), the OFZ material of the present invention using the OFZ method was able to dramatically reduce the occurrence of cracks and have a substantially single-phase structure (HfPtSn is 99% or more by volume).

  On the other hand, as shown in FIG. 6 (a), the comparative arc material has a very rapid solidification rate, so that a structure is formed in a non-equilibrium state greatly depending on the solidification process, and the thermoelectric material has two or more phases. It is a multiphase organization that coexists. That is, it is a two-phase structure of a half-Heusler type HfPtSn phase having a gray contrast and an HfPt phase having a white contrast. In addition, as shown in FIG. 6B, it is difficult to form a single-phase structure of HfPtSn phase even with an arc heat treatment material that has been subjected to heat treatment for homogenization, and defects due to solidification indicated by black contrast Was observed in large quantities. In addition, micro cracks were also observed in both comparisons.

-3. Measurement and evaluation of thermoelectromotive force (Seebeck coefficient)
For the OFZ material of the present invention obtained in Example 2 and the arc heat-treated material obtained in Comparative Example 6, each Seebeck coefficient was measured using ZEM-1 manufactured by ULVAC-RIKO. For the measurement, an arc heat treated material after heat treatment was used as a comparison. Specifically, the thermoelectromotive force (Seebeck coefficient; α) is obtained 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. Obtained by calculating. And the relationship figure (FIG. 4) of the obtained value and temperature was created, and these temperature dependence was evaluated. The thermoelectromotive force (Seebeck coefficient) is shown in FIG.

  As shown in FIG. 4, the thermoelectromotive force (Seebeck coefficient) was positive in a wide temperature range, and it was confirmed that the HfPtSn alloy has p-type thermoelectric characteristics. In the temperature range from room temperature (around 300 K) to about 800 K, the thermal electromotive force of the arc heat treatment material is 150 μV / K or less, whereas the OFZ material has an excellent thermoelectromotive force (Seebeck coefficient) of 200 to 225 μV / K. In particular, a maximum peak of thermoelectromotive force is shown at around 800K. This is presumed that, in addition to the effect of single phase formation by the OFZ method, solidification defects are suppressed, so that a thermoelectromotive force superior to that of the conventional arc heat treatment material can be obtained.

  As described above, the thermoelectric material composed of the HfPtSn phase is obtained as a ternary system in which doping and element addition are not performed, and is useful as a half-Heusler compound exhibiting p-type thermoelectric properties, and has an excellent value exceeding 200 μV / K. The thermoelectromotive force could be obtained.

As described above, a half-Heusler alloy with a single-phase or single-crystal structure or a structure close to a single-phase or single crystal is obtained by synthesis by the floating zone melting method. [In other words, the ZT molecule (power factor) α ² σ] could be greatly improved. In other words, the floating zone melting method (unidirectional solidification method) makes it easy to single-phase or crystallize, and can slowly solidify and grow, and the thermal stress accompanying solidification shrinkage is suppressed. The generation of microcracks was suppressed, and the inherent electrical characteristics inherent in the half-Heusler compound could be extracted.

Based on the above results, the temperature dependence of the power factor (α ² σ) of the HfPtSn thermoelectric material is shown in FIG. An arc heat treatment material obtained by the conventional arc melting method, which shows a maximum value in the vicinity of about 500 K, and the power factor of the OFZ material of the present invention is less than 0.3 mW / K ² m, but without doping or addition of elements. A good improvement effect was obtained in comparison with (0.15 mW / K ² m). In addition, a high value is shown compared with TiFeSb known as p-type.

-4. Evaluation of thermal conductivity and dimensionless figure of merit ZT
About the HfPtSn thermoelectric material (OFZ material) of the present invention obtained in Example 1, the thermal diffusivity and specific heat are measured by a conventional method using a laser flash method to determine the thermal conductivity, and the temperature of the thermal conductivity is calculated. A relationship diagram (FIG. 8) was created.

Furthermore, a relationship diagram between ZT and temperature when ZT at each temperature was obtained from these values was prepared, and the temperature dependence of ZT was evaluated. ZT is shown in FIG. ZT is a dimensionless figure of merit obtained by making a factor called a figure of merit (Z; = α ² / ρκ) dimensionless by temperature (T), and can evaluate a thermoelectric material.

  As shown in FIG. 8, the thermal conductivity near room temperature shows a relatively high value of about 15 W / mK, but has a temperature dependency that significantly decreases as the temperature rises. A relatively low value is shown on the higher temperature side than the middle temperature range.

  As shown in FIG. 9, ZT shows a temperature dependency that is maximum in the vicinity of 700 to 800 K. This temperature dependency is useful as a thermoelectric conversion material used in a high temperature region, and the maximum ZT value is 0. It was over 03.

In the above description about the particular case of a composition of HfPtSn (x = y = z = 1), and the other composition contained in the half-Heusler compounds represented by other Hf _x Pt _y Sn _z The same applies to the case, and a thermoelectric conversion material having high thermoelectric properties can be obtained by controlling the solidification rate by appropriately selecting the temperature gradient, specifically the stirring speed and the moving speed, by the same method as described above. Is possible.
It is also possible to sinter a single phase, a substantially single phase, a single crystal, or a substantially single crystal made of an OFZ material and then sinter by hot pressing.

  The thermoelectric conversion material of the present invention is a material of a thermoelectric conversion material 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. It is a graph which shows the temperature dependence of the Seebeck coefficient of a HfPtSn thermoelectric material. It is a graph which shows the temperature dependence of the Seebeck coefficient of the conventional thermoelectric material. (A) shows the structure of a conventional HfPtSn thermoelectric material (as-cast) by the arc melting method, (b) shows the structure after annealing the HfPtSn thermoelectric material of (a), (c) It is a figure which shows the structure | tissue of the HfPtSn thermoelectric material (OFZ as-grown) by OFZ method. It is a graph which shows the temperature dependence of the power factor of a HfPtSn thermoelectric material. It is a graph which shows the temperature dependence of the thermal conductivity of a HfPtSn thermoelectric material (OFZ as-grown). It is a graph which shows the relationship with the temperature of ZT of a HfPtSn thermoelectric material (OFZ as-grown).

Explanation of symbols

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

Claims (6)

Hf _x Pt _y Sn _z [0.9 <x <1.1, 0.8 <y <1.2, 0.9 <z <1.1, and satisfies 2.8 <x + y + z <3.2. . ] The thermoelectric conversion material using the half-Heusler compound represented by this.   The thermoelectric conversion material according to claim 1, comprising a single phase, a substantially single phase, a single crystal, or a substantially single crystal of the half-Heusler compound. Hf _x Pt _y Sn _z [0.9 <x <1.1, 0.8 <y <1.2, 0.9 <z <1.1, and satisfies 2.8 <x + y + z <3.2. . A half-Heusler compound represented by the following formula is formed by a unidirectional solidification method.   The method for producing a thermoelectric conversion material according to claim 3, wherein the half-Heusler compound is formed into a single phase, a substantially single phase, a single crystal, or a substantially single crystal by the unidirectional solidification method.   The manufacturing method of the thermoelectric conversion material of Claim 3 or 4 including the heat processing process which further heat-processes to the formed said half-Heusler compound.   The method for producing a thermoelectric conversion material according to any one of claims 3 to 5, wherein the unidirectional solidification method is a floating zone melting method.