WO2016163262A1 - 熱電変換材料およびその製造方法 - Google Patents
熱電変換材料およびその製造方法 Download PDFInfo
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- B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
- B22F3/10—Sintering only
- B22F3/105—Sintering only by using electric current other than for infrared radiant energy, laser radiation or plasma ; by ultrasonic bonding
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C33/00—Making ferrous alloys
- C22C33/02—Making ferrous alloys by powder metallurgy
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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- C22C38/00—Ferrous alloys, e.g. steel alloys
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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- C—CHEMISTRY; METALLURGY
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- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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- C—CHEMISTRY; METALLURGY
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
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- C—CHEMISTRY; METALLURGY
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- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
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- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C45/00—Amorphous alloys
- C22C45/02—Amorphous alloys with iron as the major constituent
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02N—ELECTRIC MACHINES NOT OTHERWISE PROVIDED FOR
- H02N11/00—Generators or motors not provided for elsewhere; Alleged perpetua mobilia obtained by electric or magnetic means
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- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
- H10N10/01—Manufacture or treatment
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
- H10N10/10—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects
- H10N10/13—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects characterised by the heat-exchanging means at the junction
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- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
- H10N10/10—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects
- H10N10/17—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects operating with only the Peltier or Seebeck effects characterised by the structure or configuration of the cell or thermocouple forming the device
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- H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
- H10N10/80—Constructional details
- H10N10/85—Thermoelectric active materials
- H10N10/851—Thermoelectric active materials comprising inorganic compositions
- H10N10/854—Thermoelectric active materials comprising inorganic compositions comprising only metals
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- B—PERFORMING OPERATIONS; TRANSPORTING
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- C22C33/02—Making ferrous alloys by powder metallurgy
- C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
Definitions
- the present invention relates to a thermoelectric conversion material and a manufacturing method thereof.
- next generation such as reuse of natural energy and thermal energy from resource energy that emits a large amount of CO 2.
- Technological innovation is progressing to shift to energy.
- candidates for next-generation energy technologies include technologies that use natural energy such as sunlight and wind power, and technologies that reuse the loss of primary energy such as heat and vibrations that are emitted from the use of resource energy. .
- next-generation energy is that both natural energy and reused energy are unevenly distributed.
- the energy discharged without being used is about 60% of the primary energy, and the form is mainly exhaust heat.
- exhaust heat of 200 ° C. or less is as high as 70%. Accordingly, there is a demand for an improvement in energy reuse technology, particularly technology for converting waste heat energy of 200 ° C. or less into electric power, while increasing the ratio of next-generation energy in primary energy.
- exhaust heat is generated in various situations, so a highly versatile power generation system regarding the installation form is required.
- Thermoelectric conversion technology can be cited as a promising candidate technology.
- thermoelectric conversion module A key part of the thermoelectric conversion technology is a thermoelectric conversion module.
- the thermoelectric conversion module is disposed in the vicinity of the heat source, and generates electricity when a temperature difference occurs in the module.
- an n-type thermoelectric conversion material that generates an electromotive force from a high temperature side to a low temperature side with respect to a temperature gradient and a p-type thermoelectric conversion material in which the direction of the electromotive force is opposite to the n-type are alternately arranged.
- the maximum output P of the thermoelectric conversion module is determined by the product of the heat flow rate flowing into the module and the conversion efficiency ⁇ of the thermoelectric conversion material. The heat flow depends on the module structure suitable for the thermoelectric conversion material.
- the conversion efficiency ⁇ depends on the dimensionless figure of merit ZT of the thermoelectric conversion material.
- thermoelectric conversion materials can be roughly classified into metal-based thermoelectric conversion materials, compound (semiconductor) -based thermoelectric conversion materials, and oxide-based thermoelectric conversion materials.
- typical examples of thermoelectric conversion materials having temperature characteristics applicable to exhaust heat recovery at 200 ° C. or lower include Fe 2 VAl-based full Heusler alloys and Bi—Te based semiconductors.
- the Fe 2 VAl-based full Heusler alloy is a metal-based thermoelectric conversion material
- the Bi—Te-based semiconductor is a compound-based thermoelectric conversion material. Since these two types of materials themselves can be structural materials, they are suitable for thermoelectric conversion modules for exhaust heat recovery in power plants, factories, and automobiles.
- Bi-Te semiconductors have problems that Te is highly toxic and expensive. Therefore, a metal-based full-Heusler alloy such as Fe 2 VAl is more suitable for the above-described use for exhaust heat recovery than a Bi—Te-based semiconductor.
- the figure of merit ZT of the bulk material which is a practical form is only about 0.1.
- a material having a figure of merit ZT or higher is required.
- the main reason why the figure of merit ZT of a full Heusler alloy is low is due to its high thermal conductivity.
- the reasons for the high thermal conductivity of full-Heusler alloys are (i) good electrical conductivity due to low electrical resistivity, and (ii) long mean free path of phonons, which is due to lattice vibration. The heat conduction is good.
- Non-Patent Document 1 in order to reduce the thermal conductivity, the Fe 2 VAl-based full-Heusler alloy bulk material is pulverized and mixed by a ball mill to reduce the particle diameter to about 200 nm, and the thermal conductivity is 10 W / Km. It is described that it is reduced.
- Non-Patent Document 2 discloses a thermoelectric conversion material made of a Fe 2 VAl-based full Heusler alloy. In view of the production conditions described, the thermoelectric conversion material has an average crystal grain size (hereinafter sometimes simply referred to as crystal grain size) smaller than 200 nm.
- Patent Document 1 discloses a thermoelectric conversion material composed of an Fe 2 (TiV) (AlSi) system having a crystal structure of a full Heusler alloy.
- the thermal conductivity ⁇ In order to improve the figure of merit ZT, as described above, it is preferable to reduce the thermal conductivity ⁇ , and it is known as a means to reduce the crystal grain size of the alloy.
- the figure of merit ZT of a conventional thermoelectric conversion material such as the Fe 2 VAl-based full Heusler alloy of Non-Patent Document 2 is less than 0.12. The reason for this will be described in detail later, but the conventional metal-based thermoelectric conversion material has a low Seebeck coefficient S or a low electrical resistivity ⁇ even when the crystal grain size is reduced and the thermal conductivity ⁇ is reduced. It was because it was easy to increase. As a result, the figure of merit ZT has only become the same level or has been reduced.
- the decrease in the Seebeck coefficient S or the increase in the electrical resistivity ⁇ is suppressed together with the decrease in the thermal conductivity ⁇ due to the refinement of the crystal grain size. It is necessary to find a condition that can be either or both.
- An object of the present invention is to provide a metal-based full-Heusler type thermoelectric conversion material having a large figure of merit ZT and a method for producing the same.
- the present invention is a full-Heusler alloy having p-type or n-type, wherein the full-Heusler alloy has a composition of Fe 2 TiA (where A is at least one selected from Si and Sn), and
- the thermoelectric conversion material is characterized in that the average grain size of crystal grains is 30 nm or more and 500 nm or less.
- the average crystal grain size of the full Heusler alloy is preferably 35 nm or more and 200 nm or less, and more preferably 40 nm or more and 150 nm or less.
- VEC is expressed as a function of ⁇ , x, y, w, z
- VEC ( ⁇ , x, y, w, z) [8 ⁇ (2 + ⁇ ) + ⁇ 4 ⁇ (1 ⁇ x) + (number of M valence electrons) ⁇ x ⁇ ⁇ (1 + y) + ⁇ 4 ⁇ (1 ⁇ w) + (N valence electrons) ⁇ w ⁇ ⁇ (1 + z )] / 4
- ⁇ ⁇ (at% of Fe in any of the regions ⁇ , ⁇ , ⁇ ) ⁇ 50 ⁇ / 25
- y ⁇ (at% of Ti in any one of the regions ⁇ , ⁇ , ⁇ ) ⁇ 25 ⁇ / 25
- z ⁇ (at% of Si in any one of the regions ⁇ , ⁇ , ⁇ ) ⁇ 25 ⁇ / 25
- the element M and the element N can be at least one of Cu, Nb, V, Al, Ta, Cr, Mo, W, Hf, Ge, Ga, In, P, B, Bi, and Zr.
- the element M is V, and the substitution amount x can be
- the present invention also provides a method for producing a p-type or n-type full Heusler alloy, comprising preparing a raw material having a composition of Fe 2 TiA (where A is at least one selected from Si and Sn). The raw material is made an amorphous alloy, and then heated so that the average grain size of the crystal grains is 30 nm or more and 500 nm or less.
- thermoelectric conversion material having a figure of merit ZT larger than that of a conventional metal-based full Heusler alloy could be provided.
- thermoelectric conversion module using the thermoelectric conversion material which concerns on embodiment of this invention, (a) shows the state before attaching an upper board
- the change amount of the Seebeck coefficient with respect to the modulation amount of the composition from the stoichiometric composition (substitution of Si increase and Fe decrease) is shown.
- the change amount of the Seebeck coefficient with respect to the modulation amount of the composition from the stoichiometric composition substitution of Ti increase and Fe decrease
- the change amount of the Seebeck coefficient with respect to the modulation amount of the composition from the stoichiometric composition (substitution of Si increase and Ti decrease) is shown.
- the change amount of the Seebeck coefficient with respect to the modulation amount of the composition from the stoichiometric composition (substitution of Ti increase and Si decrease) is shown.
- the change amount of the Seebeck coefficient with respect to the modulation amount of the composition from the stoichiometric composition (substitution of Fe increase and Si decrease) is shown.
- the change amount of the Seebeck coefficient with respect to the modulation amount of the composition from the stoichiometric composition (substitution of Fe increase and Ti decrease) is shown.
- the electronic state of full Heusler alloy shows the results obtained by the first-principles calculation, (a) shows the case of Fe 2 VAl alloy, a case of (b) Fe 2 TiSi alloy or Fe 2 TISN alloy.
- the VEC dependence (calculated value) of the Seebeck coefficient predicted from the band structure of the Fe 16 Ti 8 Si 8 alloy is shown.
- the VEC dependence (calculated value) of the Seebeck coefficient predicted from the band structure of the Fe 16 Ti 7 Si 9 alloy is shown.
- the VEC dependence (calculated value) of the Seebeck coefficient predicted from the band structure of the Fe 16 Ti 9 Si 7 alloy is shown.
- the VEC dependence (calculated value) of the Seebeck coefficient predicted from the band structure of the Fe 15 Ti 8 Si 9 alloy is shown.
- the VEC dependence (calculated value) of the Seebeck coefficient predicted from the band structure of the Fe 15 Ti 9 Si 8 alloy is shown.
- the VEC dependence (calculated value) of the Seebeck coefficient predicted from the band structure of the Fe 17 Ti 7 Si 8 alloy is shown.
- FIG. 3 is a ternary alloy phase diagram of an Fe—Ti—Si based full Heusler alloy, and shows a range in which the improvement effect on the Seebeck coefficient is expected to be high by numerical calculation.
- FIG. 3 is a ternary alloy phase diagram showing a composition range of an Fe—Ti—Si based full Heusler alloy according to an embodiment of the present invention.
- 4 is a graph showing the relationship between Seebeck coefficient and VEC in an Fe—Ti—Si based full Heusler alloy according to an embodiment of the present invention.
- FIG. 3 is a binary phase diagram of Cu—Fe.
- thermoelectric conversion material which concerns on embodiment of this invention
- (a) is a graph which shows the temperature dependence of DSC
- (b) is a graph which shows the relationship between the crystallization calorific value Q and a crystal grain size. It is. It is a graph which shows the relationship between the crystal grain size and thermal conductivity in the thermoelectric conversion material which concerns on embodiment of this invention. It is a figure which shows the relationship between the crystal grain diameter and the figure of merit ZT in the thermoelectric conversion material of this invention. It is the figure which expanded the horizontal axis
- thermoelectric conversion material made of a full Heusler alloy has an electronic state called a so-called pseudogap.
- pseudogap an electronic state
- the performance index ZT is given by the following equation 1 as described above.
- S is the Seebeck coefficient
- ⁇ is the electrical resistivity
- ⁇ is the thermal conductivity
- T is the temperature.
- the figure of merit increases as the Seebeck coefficient S increases and as the electrical resistivity ⁇ and thermal conductivity ⁇ decrease.
- the Seebeck coefficient S and the electrical resistivity ⁇ are physical quantities determined by the electronic state of the substance.
- the Seebeck coefficient S has a relationship represented by the following formula 2.
- the Seebeck coefficient S is inversely proportional to the absolute value of Density of states N at the Fermi level and proportional to the energy gradient. Therefore, it can be seen that a substance having a small state density at the Fermi level and a rapidly changing state density has a high Seebeck coefficient S.
- the electrical resistivity ⁇ has a relationship represented by the following formula 3.
- ⁇ F electron mean free path at Fermi level
- ⁇ F electron velocity at Fermi level
- Equation 3 since the electrical resistivity ⁇ is inversely proportional to the state density N, the electrical resistivity ⁇ becomes small when the Fermi level is at an energy position where the absolute value of the state density N is large.
- the thermal conductivity ⁇ can be regarded as the sum of the lattice thermal conductivity ⁇ p that transfers heat through lattice vibration and the electronic thermal conductivity ⁇ e that transfers heat by using electrons as a medium.
- the electronic thermal conductivity ⁇ e increases as the electrical resistivity ⁇ decreases according to the Wiedemann-Franz rule and depends on the pseudogap electronic state.
- the electron thermal conductivity ⁇ e can be reduced by controlling the carrier density. Generally, when the carrier density is less than 10 20 / cm 3 , the electronic thermal conductivity ⁇ e is minimized, and the overall thermal conductivity ⁇ is reduced. The ratio of the lattice thermal conductivity ⁇ p to the total increases.
- the thermal conductivity ⁇ is expressed by the following mathematical formula (Formula 4).
- C p is the constant pressure specific heat of the thermoelectric conversion material
- ⁇ is the density of the thermoelectric conversion material.
- the constant k f is expressed by the following mathematical formula (Formula 5).
- d is an average particle diameter of the crystal grains of the thermoelectric conversion material
- ⁇ f is a time until heat is transferred from the back surface to the surface of the crystal grains of the thermoelectric conversion material.
- thermoelectric conversion material decreases as the average particle diameter d of the crystal grains of the thermoelectric conversion material decreases.
- the figure of merit ZT can be further increased by reducing the average grain size of the crystal grains.
- the inventors have adopted a composition of Fe 2 TiA (where A is at least one selected from Si and Sn) as a full Heusler alloy. Even if this type of full-Heusler alloy has a crystal grain size of 30 nm or more and 500 nm or less and a low thermal conductivity ⁇ , the increase in electrical resistivity ⁇ is slight, unlike conventional metal-based full-Heusler alloys, And it discovered that the fall of Seebeck coefficient S was also suppressed. Therefore, it can be set as the thermoelectric conversion material with a large figure of merit ZT.
- This Fe 2 TiA-based alloy has a high Seebeck coefficient S for both p-type and n-type.
- the figure of merit ZT can be improved as compared with conventional materials such as Fe 2 VAl. If it is less than 30 nm, the figure of merit ZT is smaller than that of a conventional material such as Fe 2 VAl. However, when the crystal grain size exceeds 500 nm, the figure of merit ZT is similarly smaller than that of a conventional material such as Fe 2 VAl. Therefore, in the present invention, the lower limit value of the average crystal grain size is 30 nm and the upper limit value is 500 nm. A more preferable range is 35 nm to 200 nm, and a more preferable range is 40 nm to 150 nm.
- an amorphous Fe 2 TiA alloy can be heat-treated to obtain a thermoelectric conversion material having a fine crystal grain size. By heat-treating the amorphous alloy once, it becomes easier to obtain the composition of the L2 1 structure.
- a means for producing an amorphous Fe 2 TiA-based material mechanical alloying, a method of super-cooling after melting the material, or the like can be employed.
- a means for pulverizing in an environment in which hydrogen embrittlement and oxidation are prevented may be employed.
- the material structure of the amorphous material is not limited to a complete amorphous material but may be an amorphous material having a long-range order or a short-range order. Further, an amorphous powder in which a fine powder and a coarse powder are mixed may be used.
- the crystal grain size can be controlled by appropriately setting the temperature and holding time.
- the temperature is preferably 550 ° C. or more and 700 ° C. or less at the holding temperature.
- the holding time is preferably 0.05 hours or more and 10 hours or less.
- Heat treatment and sintering can be performed simultaneously. Specifically, it is possible to employ a method in which an amorphous alloy powder is placed in a carbon die or tungsten carbide die and sintered in an inert gas atmosphere under a pressure of 40 MPa to 5 GPa while applying a pulse current. As temperature conditions, it is preferable to raise the temperature to 550 ° C. or more and 700 ° C. or less, hold the maximum temperature for 0.05 hour or more and 3 hours or less, and then cool to room temperature.
- the forming and sintering of raw materials will be described.
- known means such as pressure molding can be adopted. Sintering can be performed in a magnetic field to obtain a sintered body oriented in a magnetic field. Moreover, pressure molding and sintering can be performed simultaneously. As the means, discharge plasma sintering can be used.
- the Fe 2 TiA-based thermoelectric conversion material may have a typical stoichiometric composition of Fe: Ti: A of 2: 1: 1, or may be within a composition range deviated from the stoichiometric composition within a predetermined range. Is done. Below, the allowable predetermined range is demonstrated.
- FIG. 2A to 2F show plots of changes in the Seebeck coefficient with respect to the modulation amount from the stoichiometric composition to the non-stoichiometric composition of the FeTiA (SI) -based Heusler alloy.
- FIG. 2A shows the change in Seebeck coefficient when Ti is stoichiometric composition and Si is increased and Fe is decreased.
- FIG. 2B is a graph when Si is stoichiometric composition and Ti is increased and Fe is decreased.
- FIG. 2C shows the change in the Seebeck coefficient
- FIG. 2C shows the change in the Seebeck coefficient when Fe is stoichiometric, Ti decreases, and Si increases
- FIG. 2D shows the Fe is stoichiometric, Ti FIG.
- 2E shows the change in Seebeck coefficient when Ti is stoichiometric, and the change in Seebeck coefficient when Fe is increased and Si is decreased.
- FIG. Shows the change in Seebeck coefficient when the stoichiometric composition is increased by Fe and decreased by Ti.
- 2A to 2F the left side shows the Seebeck coefficient (shown by ⁇ ) in the case of the p-type, and the right side shows the Seebeck coefficient (shown by ⁇ ) in the case of the n-type.
- This calculation result is the same as the calculation result of the 32-atom system (Fe 16 Ti 8 Si 8 ), 4 (Fe 2 Ti 1 Si 1 ), 8 (Fe 4 Ti 2 Si 2 ), 16 (Fe 8 Ti 4 Si 4 ).
- the electronic state of the composition in which the elements are substituted for each other is obtained, and the Seebeck coefficient is obtained. Since the atomic ratio of one atom changes according to the total number of atoms, the amount of substitution can be described in at%. In the case of a 4- or 8-atom system (when the substitution amount ⁇ is 12.5 at% or 25 at%), the substitution of one atom greatly changes the symmetry of the crystal system. For example, in the case of a four-atom system, if Fe-Ti substitution is performed in Fe 2 TiSi, Fe 3 Si or FeTi 2 Si is formed, and another crystal system is no longer produced.
- the eight-atom system has an atomic arrangement in which a metallic electronic state is easily formed in the unit cell due to a significant change in the symmetry of the crystal structure, and the absolute value of the Seebeck coefficient becomes small.
- the permissible substitution amount having a practical level of Seebeck coefficient is 10.8 at% for both p-type and n-type when Ti: stoichiometric composition, Si increase, and Fe decrease for each substitution method (broken line in FIG. 2A).
- FIG. 5 shows the result of showing the appropriate composition range on the ternary alloy phase diagram that can be understood from these substitution amounts.
- Fe stoichiometric composition, Ti increase, Si decrease: 12.0 at%
- Fe stoichiometric composition, Ti decrease, Si increase: 11 at%
- Si stoichiometric composition
- Ti increase, Fe decrease In the case of: 4.9 at%
- (Fe, Ti, A) (50, 37, 13), (45, 30, 25), (39.5, 25, 35.5), (50, 14, 36) ), (54, 21, 25), (55.5, 25, 19.5).
- the intentional increase of Ti improves more than the Seebeck coefficient in the stoichiometric composition.
- a composition in which Ti is increased by 1 to 8 at% over the stoichiometric composition is preferable.
- this Ti increase amount is converted into the Ti amount (1 + y) in the composition formula Fe 2 + ⁇ Ti 1 + y A 1 + z , the numerical value of y becomes 0.04 ⁇ y ⁇ 0.32.
- thermoelectric conversion material with an improved Seebeck coefficient can be obtained.
- y is more preferably in the range of 0.08 ⁇ y ⁇ 0.28.
- the composition may be substituted with at least one of Ti in the composition formula in the range where the total composition is 100 at% and V is in the range of more than 0 at% to 5.0 at%. Or it can be set as the composition containing Cu in 0.5 to 1.6 at% of range.
- the composition containing Cu described here refers not to the composition of the L2 1 structure of the thermoelectric conversion material but to the average composition of the entire composition including the precipitate segregated at the grain boundaries. .
- the thermoelectric conversion characteristics can be further enhanced by adopting a composition in which V is substituted or a composition containing Cu.
- the crystal grain size of the full Heusler alloy is preferably 30 nm or more and 140 nm or less.
- thermoelectric conversion material exhibiting high performance can be obtained. Details will be described below.
- the pseudo-gap structure that determines the thermoelectric conversion characteristics of a full Heusler alloy has a characteristic band structure called a flat band, which is one of the factors that determine the thermoelectric conversion characteristics. That is, the closer the flat band is to the vicinity of the Fermi level Ef, the sharper the density of states in the vicinity of the Fermi level can be changed. Thereby, the thermoelectric conversion characteristic, especially the Seebeck coefficient S is improved. Further, since the gap value of the pseudo gap can be controlled to be small, there is an advantage that the electrical resistivity does not increase.
- FIGS. 3A and 3B are diagrams showing results obtained by first-principles calculation of the electronic state of the full Heusler alloy.
- FIG. 3A shows the electronic state of Fe 2 VAl
- FIG. 3B shows the electronic state of the Fe 2 TiA alloy, which is a full Heusler alloy according to the embodiment of the present invention.
- FIG. 3 (a) as shown in (b), Fe 2 towards the Fe 2 TiA alloy than VAl alloy, it can be seen that the flat band is close to the Fermi level E F vicinity.
- the X 2 YZ alloy behaves like a fixed band model in which the band structure does not change greatly but the energy position of the Fermi level changes. Therefore, the X 2 YZ alloy has the property that the Fermi level can be easily controlled at an energy position where the state density changes sharply and the absolute value of the state density is optimized to improve the thermoelectric conversion performance.
- the Fermi level can be controlled by VEC control in which electrons or holes are doped. Specifically, VEC control can be performed by changing the composition ratio of the compound, the composition of the substitution element, and the like.
- VEC is a value obtained by dividing the total valence electron number Z of the compound by the number of atoms a in the unit cell.
- the number of valence electrons of iron (Fe) is 8
- the number of valence electrons of titanium (Ti) is 4
- the number of valence electrons of silicon (Si) is 4.
- the number of iron (Fe) atoms in the unit cell is two
- the number of titanium (Ti) atoms in the unit cell is one
- silicon (Si) in the unit cell The number of atoms is one.
- VEC when VEC is less than 6, it can be regarded as hole doping, so that it becomes a p-type thermoelectric conversion material. On the other hand, when VEC is 6 or more, an n-type thermoelectric conversion material is obtained. Further, from the previous example in which VEC was continuously changed around 6, it has been found that the maximum value of the Seebeck coefficient is taken near VEC for each of the p-type and n-type.
- FIGS. 4A to 4G show the VEC dependence of the Seebeck coefficient S calculated using the first principle calculation. Specifically, assuming a 32-atom system, the electronic state in a composition in which atoms are substituted for each other from the stoichiometric composition Fe 16 Ti 8 Si 8 is calculated. 4B: FIG. 4B: FIG. 4B: Fe 16 Ti 7 Si 9 , FIG. 4C: Fe 16 Ti 9 Si 7 , FIG. 4D: Fe 15 Ti 8 Si 9 , FIG. 4E: Fe 15 Ti 9 Si 8 , FIG. 4F: FIG.
- FIG. 4G shows the calculation result of the electronic state of Fe 17 Ti 7 Si 8
- FIG. 4G Fe 17 Ti 8 Si 7
- 4A to 4G show calculation results using Si as A, but similar calculation results are obtained even when Sn is used instead of Si. The same calculation result is obtained when SiSn is used instead of Si.
- the Fe 2 TiA-based alloy has a peak Seebeck coefficient on the negative side and the positive side of the VEC, and it can be seen that the negative side of the VEC can be used as a p-type and an n-type thermoelectric conversion material.
- a conventional Fe 2 VAl-based full Heusler alloy the performance is improved three times or more. This increase in Seebeck coefficient S corresponds to a nine-fold improvement in the figure of merit ZT.
- ⁇ VEC can be changed by substituting Ti or A with another element.
- VEC ( ⁇ , x, y, w, z) [8 ⁇ (2 + ⁇ ) + ⁇ 4 ⁇ (1 ⁇ x) + (number of valence electrons of M) ⁇ x ⁇ ⁇ (1 + y) + ⁇ 4 ⁇ (1 ⁇ w) + (valence electron of N) ⁇ w ⁇ ⁇ (1 + z)] / 4
- ⁇ ⁇ (at% of Fe in region (any of ⁇ , ⁇ , ⁇ ) in FIG. 5)
- y ⁇ (at% of Ti in region (any of ⁇ , ⁇ , ⁇ ) in FIG.
- the element M and the element N are Cu, Nb, V, Al, Ta, Cr, Mo, W, so that the absolute value of ⁇ VEC is greater than 0 and less than ⁇ 0.2 (0 ⁇
- the VEC center value of each master alloy composition means the VEC value when x and w are 0.0, respectively. If 0 ⁇
- VEC In order to make VEC within a preferable range, at least one of Cu, Nb, V, Al, Ta, Cr, Mo, W, Hf, Ge, Ga, In, P, B, Bi, and Zr is substituted.
- a combination of the elements M and N to be substituted for the alloy compositions x and w may be selected so that ⁇
- an excellent effect is observed by substituting a part of Ti with V (vanadium), and the substitution amount x is preferably in the range of
- element M for adjusting the total amount of electrons element N as Cu, Nb, V, Al, Ta, Cr, Mo, W, Hf, Ge, Ga, In, P, B, Bi, and Zr Any one can be used. Even in the case of replacement, the same characteristics as in FIG. 3B are obtained.
- V is preferable as the element M, and
- thermoelectric conversion material of the present invention can be mounted on, for example, a thermoelectric conversion module 10 as shown in FIG.
- This thermoelectric conversion module 10 can use the thermoelectric conversion material of the present invention for the p-type thermoelectric conversion unit 11 and the n-type thermoelectric conversion unit 12.
- the p-type thermoelectric conversion part 11 can also use a thermoelectric conversion material having another composition such as Fe 2 NbAl, FeS 2 or the like.
- materials for the upper substrate 14 and the lower substrate 15 GaN, silicon nitride, or the like can be used.
- the material of the electrode 13 can be Cu, Au, or the like.
- thermoelectric conversion material of the present invention was produced by the following method. Fe, Ti, V, and Si were used as materials, and each raw material was weighed so as to have the Fe 2 TiSi alloy composition shown in Table 1 in which a part of Ti was replaced with V. This raw material is put into a SUS container in an inert gas atmosphere and mixed with a 10 mm diameter SUS ball. Next, mechanical alloying was performed with a planetary ball mill apparatus, and the revolution speed condition was changed in the range of 200 rpm to 500 rpm for 20 hours or longer to obtain an amorphous alloy powder.
- thermoelectric conversion material shown in Table 1 was obtained by cooling to room temperature.
- the crystal grain size of the obtained thermoelectric conversion material was evaluated by a transmission electron microscope (TEM) and an X-ray diffraction apparatus (X-ray diffraction: XRD).
- the thermal conductivity ⁇ is calculated by measuring the thermal diffusivity by a laser flash method and measuring the specific heat by scanning calorimetry (DSC).
- thermoelectric property evaluation apparatus apparatus name: ZEM
- Table 1 shows the measurement results of Examples 1-2 to 1-7 and Comparative Examples 1-1 and 1-8.
- FIG. 11 is a plot in which the horizontal axis is the crystal grain size and the vertical axis is the figure of merit ZT
- FIG. 12 is an enlarged view of the horizontal axis.
- No. which is the thermoelectric conversion material of the present invention. 1-2 to 1-7 the crystal grain size is in the range of 39.3nm ⁇ 130.6nm, exceeds the performance index ZT is 0.1
- Fe 2 below VAl-based alloy A figure of merit ZT higher than that of the thermoelectric conversion material made of was obtained.
- the figure of merit ZT exceeded 0.3.
- thermoelectric conversion material of the present invention was improved by setting the crystal grain size to 39.3 nm to 130.6 nm. In addition, even if the crystal grain size is 500 nm, a thermoelectric conversion material having a higher figure of merit ZT than before was obtained.
- thermoelectric conversion material of the present invention was produced by replacing part of Si with Sn. Fe, Ti, V, Si, and Sn were used as materials, and each raw material was weighed so as to have the Fe 2 Ti (Si ⁇ Sn) -based alloy composition shown in Table 2 in which a part of Ti was replaced with V. Thereafter, production was performed in the same manner as in Example 1, and the thermoelectric conversion materials shown in Table 2 were obtained. The crystal grain size of this thermoelectric conversion material was 51.8 nm. The obtained measurement results are shown in Table 2. A figure of merit ZT of 0.25, which is larger than that of the conventional material, was obtained.
- Example 3 A thermoelectric conversion material was produced in the same manner as in Example 1 except that the composition was at least one of Cu and V, and the compositions shown in Tables 3 and 4 were used. Table 3 shows the measurement results of Examples 3-1 to 3-11. Table 4 shows the measurement results of Examples 3-12 to 3-20.
- FIG. 15 and FIG. 16 show the relationship between the Seebeck coefficient S and electrical resistivity ⁇ obtained from Tables 1, 3 and 4, and the crystal grain size.
- FIG. 15 is a graph showing the relationship between the Seebeck coefficient S and the crystal grain size.
- FIG. 16 is a graph showing the relationship between the electrical resistivity ⁇ and the crystal grain size.
- the horizontal axis in FIGS. 15 and 16 indicates the crystal grain size
- the vertical axis in FIG. 15 indicates the Seebeck coefficient S
- the vertical axis in FIG. 16 indicates the electrical resistivity ⁇ .
- results of 1-1 to 1-8 are indicated as “Fe—Ti—V—Si”.
- results of 3-1 to 3-11 are indicated as “Fe—Cu—Ti—V—Si” and
- results of 3-12 to 3-20 are indicated as “Fe—Cu—Ti—V—Si—Sn” (the same applies to FIGS. 17 to 19 described later).
- the Seebeck coefficient S and the electrical resistivity ⁇ of the Fe 2 VAl-based full Heusler alloy measured in the same manner are indicated by dotted lines (the same applies hereinafter).
- the Seebeck coefficient S and the electrical resistivity ⁇ of the Fe 2 VAl-based full-Heusler alloy having a crystal grain size exceeding 200 nm are values read from the data described in Non-Patent Document 2, for example.
- Non-Patent Document 2 does not describe a measured value having a crystal grain size of 100 nm or more and less than 200 nm.
- the Seebeck coefficient S and the electrical resistivity ⁇ in the crystal grain size in this range are not described. Is estimated from the tendency of the data when the crystal grain size exceeds 200 nm.
- thermoelectric conversion material of this example Nos. 1-2 to 1-7, Nos. 3-1 to 3-11, and Nos. 3-12 to 3-20
- crystal grains It can be seen that even when the diameter is reduced to 200 nm or less, the Seebeck coefficient S does not decrease so much.
- thermoelectric conversion material of this example the electrical resistivity ⁇ increases as the crystal grain size decreases.
- FIG. 17 is a graph showing the relationship between the output factor (S 2 / ⁇ ) and the crystal grain size.
- FIG. 18 is a graph showing the relationship between the thermal conductivity ⁇ and the crystal grain size.
- FIG. 19 is a graph showing the relationship between the figure of merit ZT and the crystal grain size.
- thermoelectric conversion material of this example the output factor is different even when the crystal grain size is reduced to about 200 nm or less, unlike the thermoelectric conversion material made of Fe 2 VAl-based full Heusler alloy. It turns out that it is hard to decrease. Among these, it can be seen that the thermoelectric conversion materials of Examples 3-1 to 20 made of Fe 2 TiA-based full Heusler alloy containing Cu have a high output factor.
- thermoelectric conversion material of this example has a low thermal conductivity ⁇ when the crystal grain size is about 200 nm or less.
- thermoelectric conversion material of this example the crystal grains are refined until the crystal grain size is reduced to about 200 nm or less as compared with the thermoelectric conversion material made of Fe 2 VAl-based full Heusler alloy. It can be seen that the figure of merit ZT has increased even when This is because the output factor is not lowered as described above.
- “Fe—Cu—Ti—V—Si” and “Fe—Cu—Ti—V—Si—Sn”, in which Cu and V are used contain Cu. No.
- the figure of merit ZT is higher than the thermoelectric conversion material of 1-2 to 1-7 and “Fe—Ti—V—Si” of Example 3-1.
- FIG. 20 is a graph showing the relationship between the Seebeck coefficient S and the Cu content.
- FIG. 21 is a graph showing the relationship between the figure of merit ZT and the V replacement amount.
- the absolute value of the Seebeck coefficient S is larger than when Cu is not contained, that is, when the Cu content is 0. It was. Thereby, the absolute value of the Seebeck coefficient S of the Fe 2 TiA-based full Heusler alloy can be increased as compared with the case where copper is not contained.
- the figure of merit ZT of the thermoelectric conversion material made of Fe 2 TiA-based full Heusler alloy is from Fe 2 VAl-based full Heusler alloy.
- the performance index ZT of the resulting thermoelectric conversion material was almost the same or larger. Therefore, the V content in the Fe 2 TiA-based full Heusler alloy is preferably 1.0 to 4.2 at%. Accordingly, the performance index ZT of the thermoelectric conversion material made of Fe 2 TiA-based full Heusler alloy should be made equal to or larger than the performance index ZT of the thermoelectric conversion material made of Fe 2 VAl-based full Heusler alloy. Can do.
- thermoelectric conversion material having a composition shown in Table 5 represented by Fe 2 + ⁇ (Ti 1-x M x ) 1 + y (Si 1-w N w ) 1 + z was produced.
- Fe, Ti, Si, and V were used as materials, and each raw material was weighed so as to have the composition shown in Table 5.
- This raw material is put into a SUS container in an inert gas atmosphere and mixed with a 10 mm diameter SUS ball.
- mechanical alloying was performed with a planetary ball mill apparatus, and the revolution speed condition was changed in the range of 200 rpm to 500 rpm for 20 hours or longer to obtain an amorphous alloy powder.
- thermoelectric conversion material This amorphous alloy powder was put into a carbon die or tungsten carbide die and sintered in an inert gas atmosphere under a pressure of 40 MPa to 5 GPa while applying a pulse current. The temperature was raised to a temperature of 550 to 700 ° C. and held at the maximum temperature for 3 minutes to 180 minutes. Then, the target thermoelectric conversion material was obtained by cooling to room temperature. The crystal grain size of the obtained thermoelectric conversion material was evaluated by a transmission electron microscope (TEM) and an X-ray diffractometer (XRD). The thermal conductivity ⁇ is obtained by measuring the thermal diffusivity by a laser flash method and measuring the specific heat by DSC. Further, the electrical resistivity ⁇ and the Seebeck coefficient S were measured with a ZEM manufactured by ULVAC-RIKO, as described above. It can be seen that any composition has an excellent Seebeck coefficient S and is a promising composition as a thermoelectric conversion material.
- TEM transmission electron microscope
- XRD X-ray diffractometer
- the appropriate replacement amount of the replacement material in each alloy composition can be understood from the relationship between the Seebeck coefficient and VEC shown in FIG.
- the Seebeck coefficient is particularly high when the amount of substitutional elements.
- V is used as the replacement material, but at least one of Cu, Nb, Al, Ta, Cr, Mo, W, Hf, Ge, Ga, In, P, B, Bi, and Zr is used.
- the effect of improving the Seebeck coefficient S by selecting one as a substitution element of N and M and selecting a combination of the alloy compositions x and w and the substitution materials M and N so that 0 ⁇
- the use of V as a substitution material is particularly effective, and the substitution amount x is preferably
- the total composition ratio of the substitution materials be smaller than the Ti composition ratio. This is because when the composition ratio of these replacement materials becomes larger, the range of the Fe 2 TiA alloy described with reference to FIG.
- FIG. 9B shows the relationship between the calorific value Q and the particle size (grain size) in crystallization. It can be seen that the crystal grain size can be controlled in the order of several tens of nanometers by increasing or decreasing the calorific value Q.
- FIG. 10 shows the relationship between thermal conductivity and crystal grain size.
- thermoelectric conversion material with reduced thermal conductivity while avoiding attenuation of the Seebeck coefficient and an increase in electrical resistivity, and heat conduction by refining the crystal grain size It is possible to provide a thermoelectric conversion material in which the trade-off between the decrease in the rate and the increase in the electrical resistivity is eliminated.
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Abstract
Description
排熱のエネルギー利用を考えた時、排熱はさまざまな場面で生じるため、設置形態に関する汎用性の高い発電システムが必要となる。その有力な候補技術として熱電変換技術が挙げられる。
熱電変換モジュールの最大出力Pは、モジュールに流入する熱流量と熱電変換材料の変換効率ηの積で決定される。熱流量は、熱電変換材料に適したモジュール構造に依存する。また、変換効率ηは熱電変換材料の無次元の性能指数ZTに依存する。性能指数ZTは、ZT={S2/(κρ)}T(ここで、S:ゼーベック係数、ρ:電気抵抗率、κ:熱伝導率、T:温度)で表わされる。したがって、熱電変換モジュールの最大出力Pを向上させるためには、熱電変換材料のゼーベック係数Sを増加させ、また電気抵抗率ρと熱伝導率κを減少させることが望ましい。
フルホイスラー合金の性能指数ZTが低い主な原因は、その熱伝導率の高さによる。フルホイスラー合金の熱伝導率が高い原因は、(i)電気抵抗率が低い為に電子を媒体とした熱伝導がよいこと、(ii)フォノンの平均自由行程が長い為、格子振動を介した熱伝導がよいこと、が挙げられる。
(i)の電子由来の熱伝導を低減することは、フルホイスラー合金の熱電変換特性を決定づける電子状態に由来するため意図的に変調することは好ましくない。一方、(ii)の格子振動に由来する熱伝導率を低減することは、合金の組織構造を制御することで可能である。特に合金の結晶粒の平均粒径を小さくすることで熱伝導率を低減させることが知られている。
また、非特許文献2では、Fe2VAl系のフルホイスラー合金からなる熱電変換材料が開示されている。記載される製造条件からすれば、結晶粒の平均粒径(以下、単に、結晶粒径ということがある)が200nmよりも小さい熱電変換材料である。
また、特許文献1では、フルホイスラー合金の結晶構造を有するFe2(TiV)(AlSi)系からなる熱電変換材料を開示している。
よって、性能指数ZTを大きくするためには、結晶粒径の微細化による熱伝導率κを小さくすることと併せて、ゼーベック係数Sの低下を抑制するか、電気抵抗率ρの増加を抑制するか、若しくは、その両方とするか、いずれかが可能な条件を見つけることが必要である。
前記フルホイスラー合金の平均の結晶粒径が、35nm以上200nm以下であることが好ましく、40nm以上150nm以下であることがなお好ましい。
また、前記Fe2TiA系の組成は、組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金であって、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(50、37、13)、(45、30、25)、(39.5、25、35.5)、(50、14、36)、(54、21、25)、(55.5、25、19.5)で囲まれた領域α内となるようなσ、y、zを有することが好ましい。
さらに、前記の組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(50、35、15)、(47.5、27.5、25)、(40、25、35)、(50、17、33)、(52.2、22.8、25)、(52.8、25、22.2)で囲まれた領域β内となるようなσ、y、zを有することが好ましい。
さらに、前記の組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(50、32.6、17.4)、(49.2、25.8、25)、(43.9、25、31.1)、(50、23、27)、(51、24、25)、(51、25、24)で囲まれた領域γ内となるようなσ、y、zを有することが好ましい。
前記組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、前記Ti及びAが元素M及び元素Nによりそれぞれ化学量論組成から組成変調されて、組成式Fe2+σ(Ti1-xMx)1+y(A1-wNw)1+zで表わされ、この時VECはσ, x, y, w, zの関数として表され、VEC(σ, x, y, w, z)=[8×(2+σ)+{4×(1-x)+(Mの価電子数)×x}×(1+y)+{4×(1-w)+(Nの価電子)×w}×(1+z)]/4であり、
σ={(前記領域α、γ、βのいずれかにおけるFeのat%)-50}/25
y={(前記領域α、γ、βのいずれかにおけるTiのat%)-25}/25
z={(前記領域α、γ、βのいずれかにおけるSiのat%)-25}/25
であり、VECの変化量ΔVECが、
ΔVEC=VEC(σ,x,y,w,z)-VEC(σ,0,y,0,z)
と表され、
0<|ΔVEC|≦0.2となるようなx、wを有するものを採用することができる。
前記元素M及び元素Nは、Cu、Nb、V、Al、Ta、Cr、Mo、W、Hf、Ge、Ga、In、P、B、BiおよびZrのうち少なくとも1つとすることができる。
前記元素MはVであり、置換量xは、|x|≦0.25とすることができる。
まず、フルホイスラー合金からなる熱電変換材料の変換性能を向上させる原理について説明する。X2YZ系合金からなるフルホイスラー合金は、いわゆる擬ギャップと呼ばれる電子状態をもつ。この擬ギャップが熱電変換性能とどう関係するかを説明するため、一般に熱電変換性能と電子状態の関係を説明する。
性能指数ZTは、上記のように、下記数1で与えられる。ここで、Sは、ゼーベック係数であり、ρは、電気抵抗率であり、κは、熱伝導率であり、Tは温度である。
一方、電気抵抗率ρは、下記数3に表されるような関係をもつ。
さらに好ましい範囲は35nm以上200nm以下であり、さらに好ましい範囲は40nm以上150nm以下である。
アモルファス化されたFe2TiA系の原料を製造する手段として、メカニカルアロイングや、原料を溶解した後に超急冷する方法等が採用できる。アモルファス化したものが粉末状でない場合は、水素脆化し酸化が防止される様な環境下で粉砕する手段を採用しても良い。
ここでアモルファスの材料組織は完全なアモルファスに限らず長距離秩序や短距離秩序を有するアモルファスでも良い。また、微細な粉末と粗大な粉末が混合したアモルファス粉末でも良い。
熱処理と焼結を同時に行うこともできる。具体的には、アモルファス化した合金粉末をカーボンダイスあるいはタングステンカーバイドのダイスに入れ、不活性ガス雰囲気中において、40MPa~5GPaの圧力の下でパルス電流をかけながら焼結する方法が採用できる。温度条件として、550℃以上700℃以下の温度まで昇温し、最高温度で0.05時間以上3時間以下保持し、その後、室温まで冷却することが好ましい。
成形は加圧成型等の既知の手段を採用できる。
焼結は磁場中で行い、磁場配向させた焼結体を得ることもできる。また、加圧成型と焼結を同時に行うこともできる。その手段として放電プラズマ焼結を用いることができる。
このVが置換された組成、または、Cuが含有された組成とすることで、熱電変換特性をさらに高めることができる。
この時、フルホイスラー合金の結晶粒径は、30nm以上140nm以下とすることが好ましい。
フルホイスラー合金の熱電変換特性を決定する擬ギャップ構造には、フラットバンドという特徴的なバンド構造が存在し、熱電変換特性を決める要因の一つとなっている。
つまり、フラットバンドがフェルミ準位Ef近傍に近いほど、フェルミ準位近傍の状態密度を急峻に変化させることができる。これにより、熱電変換特性、特にゼーベック係数Sが向上する。また、擬ギャップのギャップ値を小さく制御出来るため、電気抵抗率が増大しないという利点がある。
図3(a),(b)に示すように、Fe2VAl合金よりもFe2TiA合金の方が、フラットバンドがフェルミ準位EF近傍に近いことが分る。
例えば、Fe2TiSiの場合、鉄(Fe)の価電子数は8であり、チタン(Ti)の価電子数は4であり、シリコン(Si)の価電子数は4である。また、Fe2TiSiの場合、ユニットセル内の鉄(Fe)の原子数は2個であり、ユニットセル内のチタン(Ti)の原子数は1個であり、ユニットセル内のシリコン(Si)の原子数は1個である。そのため、Fe2VAl中の総価電子数Zは、Z=8×2+4×1+4×1=24と計算され、ユニットセル内の原子数aは、a=2+1+1と計算され、原子一個当たりの価電子数VECは、VEC=Z/a=6と計算される。
化合物の組成比を変化させた時、VECの値は増減する。VECの増減は前述の固定バンドモデル(rigid band model)における電子ドープとホールドープと近似的には同等であることが知られており、VECの制御によりゼーベック係数Sの値や極性を変化させることが出来る。
具体的には、VECが6未満の時ホールドープとみなす事が出来る為、p型の熱電変換材料となる。一方でVECを6以上にするとn型の熱電変換材料となる。さらにVECを6付近で連続的に変化させた先行例から、VEC付近にp型、n型それぞれでゼーベック係数の極大値を取ることが分かっている。
具体的には32原子系を仮定し化学量論組成Fe16Ti8Si8から原子を1個単位で互いに置換した組成での電子状態を計算している。図4B~図4Gは、図4B:Fe16Ti7Si9、図4C:Fe16Ti9Si7、図4D:Fe15Ti8Si9、図4E:Fe15Ti9Si8、図4F:Fe17Ti7Si8、図4G:Fe17Ti8Si7で電子状態の計算結果を示したものである。
なお、図4A~Gは、AとしてSiを用いた計算結果であるが、Siに代えてSnを用いても同様の計算結果である。また、Siに代えてSiSnとしても同様の計算結果である。
この実用的なZTが得られる、ゼーベック係数|S|=100uV/Kを越える範囲はVECの中心値に対するVECの変化量(以後、ΔVEC)に対して所定の範囲を有する。ΔVECは、TiやAを別の元素で置換することで変化させることができる。
前記組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、前記Ti及びAが元素M及び元素Nによりそれぞれ化学量論組成から組成変調されて、組成式Fe2+σ(Ti1-xMx)1+y(A1-wNw)1+zで表わされ、
この時VECはσ, x, y, w, zの関数として表され、
VEC(σ, x, y, w, z)=[8×(2+σ)+{4×(1-x)+(Mの価電子数)×x}×(1+y)+{4×(1-w)+(Nの価電子)×w}×(1+z)]/4
であり、
σ={(図5の領域(α、β、γのいずれか)内のFeのat%)-50}/25
y={(図5の領域(α、β、γのいずれか)内のTiのat%)-25}/25
z={(図5の領域(α、β、γのいずれか)内のAのat%)-25}/25
でありVECの変化量ΔVECが、
ΔVEC=VEC(σ, x, y, w, z)-VEC(σ,0,y,0,z)
と表せる。
以下の方法により、本発明の熱電変換材料を作製した。
材料としてFe、Ti、V、Siを用い、Tiの一部をVで置換した、表1のFe2TiSi系合金組成となるように各原料を秤量した。
この原料を、不活性ガス雰囲気中において、SUS容器の中に入れ、10mm直径のSUSボールと混合する。次いで、メカニカルアロイングを遊星ボールミル装置にて行い、その公転回転速度の条件を200rpm~500rpmの範囲で変えて20h以上実施し、アモルファス化した合金粉末を得た。このアモルファス化した合金粉末をカーボンダイスあるいはタングステンカーバイドのダイスに入れ、不活性ガス雰囲気中において、40MPa~5GPaの圧力の下でパルス電流をかけながら焼結した。その温度条件は550~700℃の温度まで昇温し、最高温度で3分~180分間保持した。その後、室温まで冷却することにより、表1に示す熱電変換材料を得た。
得られた熱電変換材料の結晶粒径は、透過型電子顕微鏡(TEM)とX線回折装置(X‐ray diffraction:XRD)によって評価した。また熱伝導率κは、熱拡散率をレーザーフラッシュ法で測定し、比熱を示唆走査熱量測定(DSC)によって測定することで算出し。また電気抵抗率ρ、ゼーベック係数Sは、アルバック理工社製の熱電特性評価装置(装置名:ZEM)で測定した。
得られた測定結果を表1に示す。表1は、実施例1-2~1-7、および比較例1-1、1-8の測定結果を示す。
本発明の熱電変換材料であるNo.1-2~1-7は、結晶粒径が39.3nm~130.6nmの範囲に有り、いずれの実施例においても性能指数ZTが0.1を超えており、後述のFe2VAl系合金からなる熱電変換材料よりも高い性能指数ZTが得られた。またその中で、結晶粒径が64.3nm以上のNo.1-3~1-7は、性能指数ZTが0.3を超えていた。
対して、結晶粒径が21.7nmと小さいNo.1-1は、性能指数ZTが0.0017であり、Fe2VAl系合金の従来材よりも性能指数ZTが低かった。また、結晶粒径が1000nmのNo.1-8は、性能指数ZTが0.0026であり、これも従来材よりも性能指数ZTは低かった。
このように、本発明の熱電変換材料は、結晶粒径を39.3nm~130.6nmとすることで性能指数が向上することが分った。
なお、結晶粒径が500nmであっても従来よりも高い性能指数ZTを持つ熱電変換材料が得られた。
Siの一部をSnに置換して本発明の熱電変換材料を作製した。
材料としてFe、Ti、V、Si、Snを用い、Tiの一部をVで置換した、表2のFe2Ti(Si・Sn)系合金組成となるように各原料を秤量した。
以降は実施例1と同様にして作製し、表2に示す熱電変換材料を得た。この熱電変換材料の結晶粒径は51.8nmであった。
得られた測定結果を表2に示す。性能指数ZTは0.25と従来材と比較して大きい熱電変換材料が得られた。
組成をCuかVの少なくともどちらかを用いた、表3、表4に示す組成とし、それ以外は、実施例1と同様にして熱電変換材料を作製した。
表3は、実施例3-1~3-11の測定結果を示す。表4は、実施例3-12~3-20の測定結果を示す。
このうち、CuとVが用いられた「Fe-Cu-Ti-V-Si」と「Fe-Cu-Ti-V-Si-Sn」の本実施例の熱電変換材料は、Cuが含有されていないNo.1-2~1-7および実施例3-1の「Fe-Ti-V-Si」の熱電変換材料に比べ、性能指数ZTが高い。したがって、高い熱電変換特性を得るためには、Cuが含有、またはVが置換されていることがより好ましいことが分かる。又、元素AとしてSi及びSnを用いたNo.3-12~3-20の「Fe-Cu-Ti-V-Si-Sn」よりも、Siのみを用いたNo.3-1~3-11の「Fe-Cu-Ti-V-Si」の方が、結晶粒径が36.67nm以上48.78nm以下の範囲で、性能指数ZTが高くなることが分る。
なお、Vを用いないで、組成全体を100at%として、Cuが0.5at%以上1.6at%以下の範囲で含有された場合でも、Vで置換したものほどではないが、性能指数が高まることが確認できた。
熱電変換材料として、Fe2+σ(Ti1-xMx)1+y(Si1-wNw)1+zで表される表5に示す組成のものを作製した。
材料としてFe、Ti、Si、Vを用い、表5の組成となるように各原料を秤量した。
この原料を、不活性ガス雰囲気中において、SUS容器の中に入れ、10mm直径のSUSボールと混合する。次いで、メカニカルアロイングを遊星ボールミル装置にて行い、その公転回転速度の条件を200rpm~500rpmの範囲で変えて20h以上実施し、アモルファス化した合金粉末を得た。このアモルファス化した合金粉末をカーボンダイスあるいはタングステンカーバイドのダイスに入れ、不活性ガス雰囲気中において、40MPa~5GPaの圧力の下でパルス電流をかけながら焼結した。その温度条件は550~700℃の温度まで昇温し、最高温で3分~180分間保持した。その後、室温まで冷却することにより、目的の熱電変換材料を得た。
得られた熱電変換材料の結晶粒径は、透過型電子顕微鏡(TEM)とX線回折装置(XRD)によって評価した。また、熱伝導率κは熱拡散率をレーザーフラッシュ法で測定し、比熱をDSCによって測定する事で得ている。また電気抵抗率ρ、ゼーベック係数Sは、上述と同様、アルバック理工社製のZEMで測定した。
いずれの組成においても、優れたゼーベック係数Sを有し、熱電変換材料として有望な組成であることが判る。
つまりこの実施例から0<|ΔVEC|≦0.2とすればVECは最適値となる。
本実施例では置換材料にVを用いたが、それ以外にCu、Nb、Al、Ta、Cr、Mo、W、Hf、Ge、Ga、In、P、B、BiおよびZrのうち少なくともいずれか1つをN、Mの置換元素として選定し、0<|ΔVEC|≦0.2となる様に合金組成x、wと置換材料M、Nの組み合わせを選ぶことでもゼーベック係数Sを向上させる効果がある。但し、Vを置換材料として用いることが特に効果が認められ、その置換量xは|x|≦0.25が好ましい。
また、これらの置換材料を用いる場合、置換材料の組成比の合計は、Tiの組成比よりも小さくなるように構成することが望ましい。これら置換材料の組成比の方が大きくなると、もはや図3(b)で説明したFe2TiA系合金としての範囲を逸脱してしまうからである。さらに、置換元素のみでVECコントロールを行う場合は、Fe2+σ(Ti1-xMx)1+y(A1-wNw)1+zで表わされる組成式を、σ, x, y, w, zを用いたVECの関数として、以下のように表し
VEC(σ, x, y, w, z)=[8×(2+σ)+{4×(1-x)+(Mの価電子数)×x}×(1+y)+{4×(1-w)+(Nの価電子)×w}×(1+z)]/4、
σ={(前記領域α、β、γのいずれかにおけるFeのat%)-50}/25、
y={(前記領域α、β、γのいずれかにおけるTiのat%)-25}/25、
z={(前記領域α、β、γのいずれかにおけるAのat%)-25}/25、
VECの変化量ΔVECが、
ΔVEC=VEC(σ,x,y,w,z)-VEC(σ,0,y,0,z)と表された時に、0<|ΔVEC|≦0.2となるようなx、wを有するように、置換元素によるVECの寄与の合計を計算する事が望ましい。
また、結晶化における発熱量Q(calorific value)と粒子径(Grain size 結晶粒径)の関係を図9(b)に示す。発熱量Qの増減により結晶粒径を数十nmオーダーの大きさで制御可能であることが分かる。
Claims (15)
- p型或いはn型を有するフルホイスラー合金であって、
前記フルホイスラー合金は、Fe2TiA(但し、AはSi、Snから選択される少なくとも一種)系の組成を有し、
かつ、結晶粒の平均粒径が30nm以上500nm以下であることを特徴とする熱電変換材料。 - 請求項1記載の熱電変換材料において、
前記フルホイスラー合金の結晶粒の平均粒径が、35nm以上200nm以下であることを特徴とする熱電変換材料。 - 請求項1又は請求項2に記載の熱電変換材料において、
前記Fe2TiA系の組成は、組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金であって、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(50、37、13)、(50、14、36)、(45、30、25)、(39.5、25、35.5)、(54、21、25)、(55.5、25、19.5)で囲まれた領域α内となるようなσ、y、zを有することを特徴とする熱電変換材料。 - 請求項3記載の熱電変換材料において、
前記組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(40、25、35)、(47.5、27.5、25)、(50、17、33)、(50、35、15)、(52.8、25、22.2)、(52.2、22.8、25)で囲まれた領域β内となるようなσ、y、zを有することを特徴とする熱電変換材料。 - 請求項3または4に記載の熱電変換材料において、
前記組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(43.9、25、31.1)、(49.2、25.8、25)、(50、23、27)、(50、32.6、17.4)、(51、25、24)、(51、24、25)で囲まれた領域γ内となるようなσ、y、zを有することを特徴とする熱電変換材料。 - 請求項1乃至5のいずれか一項に記載の熱電変換材料において、
組成全体を100at%として、Vが0at%超5.0at%以下の範囲で、前記組成式のTiと置換されていることを特徴とする熱電変換材料。 - 請求項1乃至6のいずれか一項に記載の熱電変換材料において、
組成全体を100at%として、Cuが0.5at%以上1.6at%以下の範囲で含有されていることを特徴とする熱電変換材料。 - 請求項6または7に記載の熱電変換材料において、
前記フルホイスラー合金の結晶粒の平均粒径は、30nm以上140nm以下である、熱電変換材料。 - 請求項3乃至8のいずれか一項に記載の熱電変換材料において、
前記組成式Fe2+σTi1+yA1+zで表わされるフルホイスラー合金は、前記Ti及びAが元素M及び元素Nによりそれぞれ化学量論組成から組成変調されて、組成式Fe2+σ(Ti1-xMx)1+y(A1-wNw)1+zで表わされ、
この時VECはσ, x, y, w, zの関数として表され、
VEC(σ, x, y, w, z)=[8×(2+σ)+{4×(1-x)+(Mの価電子数)×x}×(1+y)+{4×(1-w)+(Nの価電子)×w}×(1+z)]/4
であり、
σ={(前記領域α、β、γのいずれかにおけるFeのat%)-50}/25
y={(前記領域α、β、γのいずれかにおけるTiのat%)-25}/25
z={(前記領域α、β、γのいずれかにおけるAのat%)-25}/25
であり、VECの変化量ΔVECが、
ΔVEC=VEC(σ,x,y,w,z)-VEC(σ,0,y,0,z)
と表され、
0<|ΔVEC|≦0.2となるようなx、wを有することを特徴とする熱電変換材料。 - 請求項9記載の熱電変換材料において、
前記元素M及び元素Nは、Cu、Nb、V、Al、Ta、Cr、Mo、W、Hf、Ge、Ga、In、P、B、BiおよびZrのうち少なくとも1つであることを特徴とする熱電変換材料。 - 請求項10記載の熱電変換材料において、
前記元素MはVであり、置換量xは、|x|≦0.25であることを特徴とする熱電変換材料。 - p型或いはn型を有するフルホイスラー合金の製造方法であって、
Fe2TiA(但し、AはSi、Snから選択される少なくとも一種)系の組成からなる原料を用意し、
前記原料をアモルファス化した合金とし、
その後、加熱して結晶粒の平均粒径が30nm以上500nm以下とすることを特徴とする熱電変換材料の製造方法。 - 請求項12に記載の熱電変換材料の製造方法において、
前記Fe2TiA系の組成からなる原料は、Fe-Ti-Aの三元合金状態図において、at%で、(Fe、Ti、A)=(50、37、13)、(50、14、36)、(45、30、25)、(39.5、25、35.5)、(54、21、25)、(55.5、25、19.5)で囲まれた領域α内となるようなσ、y、zを有することを特徴とする熱電変換材料の製造方法。 - 請求項13に記載の熱電変換材料の製造方法において、
原料全体を100at%として、Vが0at%超5.0at%以下の範囲で、前記組成式のTiと置換されていることを特徴とする熱電変換材料の製造方法。 - 請求項13または14に記載の熱電変換材料の製造方法において、
組成全体を100at%として、Cuが0.5at%以上1.6at%以下の範囲で含有されていることを特徴とする熱電変換材料の製造方法。
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