JP6353187B2 - Thermoelectric material manufacturing method - Google Patents

Description

The present invention is an oxide having a spinel structure, a method of manufacturing a thermoelectric materials that converts thermal energy into electrical energy.

  In the urgent need of developing clean energy, thermoelectric conversion technology that converts thermal energy into electrical energy has attracted attention.

  Examples of the thermoelectric element used in such a thermoelectric conversion system include a Bi-Te system (from room temperature to 500K), a Pb-Te system (from room temperature to 800K), a Si-Ge system (from room temperature to 1000K), a scooter. Compound semiconductors such as terdite compounds (Zn—Sb, Co—Sb, Fe—Sb, etc.) can be mentioned.

  Bi-Te system and Pb-Te system are highly toxic to Te and Pb, and therefore, development of alternative materials is required in view of high environmental load.

  The Si-Ge system is expensive because Ge is expensive, and it requires production at a high temperature, so it is not only inferior in mass productivity, but also oxidizes at a high temperature, so the thermoelectric properties change. As a result, the thermoelectric properties are not stable.

  Since the base material of the skutterudite compound is p-type, not only the temperature dependence of thermoelectricity differs between p-type and n-type, but also Sb is included in the constituent elements of the crystal, so the environmental load is high. Furthermore, since the thermoelectric characteristics change due to the evaporation of Sb at high temperature, the performance at high temperature is not stable.

An example of a thermoelectric material with a low environmental load is a β-FeSi ₂ -based material.

However, due to changes in the composition of elements contained in the β-FeSi ₂ -based material, in particular, the composition of Fe and Si, in addition to β-FeSi ₂ having high thermoelectric performance, the thermoelectric performance is low and the metallic properties are exhibited. Since Fe ₂ Si ₅ and ε-FeSi may be generated, the thermoelectricity is greatly affected, and as a result, the thermoelectric performance may be deteriorated, and the thermoelectric characteristics are not stable.

Under such circumstances, for example, Patent Document 1 discloses that the composition is Co _x Fe _3-x O ₄ as a thermoelectric material that is excellent in stability of heat resistance and thermoelectric properties, and also excellent in mass productivity and simplicity. (Wherein x is 1 <x ≦ 3) and a thermoelectric material made of an oxide having a spinel structure and exhibiting p-type conductivity is disclosed.

JP 2008-41871 A

As an index for evaluating the characteristics of the thermoelectric material, a figure of merit Z (K ⁻¹ ) (= S ² σ / κ) is generally used. Here, S is a Seebeck coefficient representing the magnitude of the electromotive force generated by the temperature difference of 1K (unit: VK ⁻¹ ). Thermoelectric materials each have their own Seebeck coefficient, and are broadly classified into those having a positive Seebeck coefficient (p-type) and those having a negative Seebeck coefficient (n-type). σ is electrical conductivity (unit is Scm ⁻¹ ). κ is a thermal conductivity represented by the sum of thermal conduction based on free electrons and thermal conduction based on lattice vibration (unit: Wcm ⁻¹ K ⁻¹ ).

  In order to achieve a high figure of merit, a high output factor and low thermal conductivity are required, but the output factor is considered as an index of the maximum power that can be extracted as a thermoelectric material.

  However, the thermoelectric material described in Patent Document 1 has a problem that a sufficient output factor is not obtained.

An object of the present invention is to provide a method of manufacturing a thermoelectric element that can manufacture a thermoelectric material that can provide a high output factor.

Any inventor of the present invention is to solve the above problems, the results of extensive study, composition _{A x Co 3-x O 4} ( where, A is selected Ni, Zn, from the group consisting of Ag Wherein x is 0 <x ≦ 0.3), and a method for producing a thermoelectric material using an oxide having a spinel structure and exhibiting p-type conductivity is described above. It has been found that the object is achieved, and has led to the present invention.

That is, in the first method for producing a thermoelectric material of the present invention, the composition is represented by Ni _x Co _3-x O ₄ (where x is 0 <x ≦ 0.3) , and p-type electric A method for producing a thermoelectric material using an oxide having a spinel structure that exhibits conductivity, wherein a raw material obtained by mixing metal ions constituting the thermoelectric material at a predetermined blending ratio is subjected to a firing treatment, and a rock salt structure After the above sintered body , annealing is performed in the atmosphere at the phase transition temperature of −300 K to the phase transition temperature of +100 K based on the phase transition temperature from the rock salt type structure to the spinel type structure obtained by differential thermal analysis. To form an oxide having a spinel structure.

Producing how the second thermoelectric material of the present invention, composition _{A x Co 3-x O 4} ( where, A is Zn or Ag, x is 0 <x ≦ 0.3.) Is a method of manufacturing a thermoelectric material using an oxide having a spinel structure and exhibiting p-type electrical conductivity, wherein the metal ions constituting the thermoelectric material are mixed at a predetermined mixing ratio. After firing to form a rock salt type sintered body, based on the phase transition temperature from the rock salt type structure to the spinel type structure obtained by differential thermal analysis, the phase transition temperature is -300K to the phase transition temperature. An annealing process is performed in the atmosphere at +100 K to form an oxide having a spinel structure .

In the third method for producing a thermoelectric material of the present invention, the composition is Zn _0.1 Co _2.9 O ₄ Or Ag _0.2 Co _2.8 O ₄ Is a method of manufacturing a thermoelectric material using an oxide having a spinel structure and exhibiting p-type electrical conductivity, wherein the metal ions constituting the thermoelectric material are mixed at a predetermined mixing ratio. After firing to form a rock salt type sintered body, based on the phase transition temperature from the rock salt type structure to the spinel type structure obtained by differential thermal analysis, the phase transition temperature is -300K to the phase transition temperature. An annealing process is performed in the atmosphere at +100 K to form an oxide having a spinel structure.

The thermoelectric material obtained by the present invention has an advantage that a high output factor is obtained and excellent thermoelectric characteristics are exhibited even at 800K or higher.

More this onset bright, obtain high power factor, there is an advantage that the high temperature of the thermoelectric material to exhibit excellent thermoelectric properties at least 800K can be mass-produced at low cost.

It is a graph showing the relationship between the electric conductivity σ and Seebeck coefficient S in _{N i x Co 3-x O} 4. It is a graph showing the relationship between the value and the Seebeck coefficient S of x of _{N i x Co 3-x O} 4. Is a diagram showing the XRD pattern of the N i _0.05 Co _2.95 O _4.

Hereinafter, the present invention will be described in more detail. Method for producing a thermoelectric material of the present invention, composition _{A x Co 3-x O 4} ( where, A is, Ni, Zn, is any element selected from the group consisting of Ag, x is 0 < x ≦ 0.3.) and a method for producing a thermoelectric material using an oxide having a spinel structure and exhibiting p-type conductivity, wherein metal ions constituting the thermoelectric material are predetermine. The raw material mixed at the blending ratio is fired to form a sintered body having a rock salt structure, and then annealed in the atmosphere to obtain an oxide having a spinel structure.

In the present invention, any one of metal ions selected from the group consisting of Ni, Zn, and Ag (such as oxides) and Co metal ions (such as tricobalt tetroxide) are mixed at a predetermined mixing ratio, To do. The state after mixing is not particularly limited, but a fine powder is preferable. In addition, when the state after mixing is a powder, a granule, etc., it is preferable to perform forming processing, such as press molding, as a raw material.

A composition A _x Co _{3-x O 4} in the present invention, the reason that the ion radius is ^{Co 3+} and close ion, is any of elements selected Ni, Zn, from the group consisting of Ag.

Composition in the present invention _{A x Co 3-x O 4} ( where, A is, Ni, Zn, selected from a group consisting of Ag is any element.) X of exceeds 0.7, the following It is thought that sufficient output factors cannot be obtained from the simulation results.

For example, regarding Ni _x Co _3-x O ₄ , when the amount of Ni (value of x) increases, the electrical conductivity increases and the Seebeck coefficient decreases. When a graph with the electrical conductivity σ on the horizontal axis and the Seebeck coefficient S on the vertical axis is created, a linear relationship (S = −86.849σ + 612.82) is obtained (see FIG. 1), and the amount of Ni is plotted on the horizontal axis. Even if a graph with the coefficient S as the vertical axis is created, a linear relationship (S = −1058.8x + 612.62) is obtained (see FIG. 2).

  When the linear relationship (S = −86.849σ + 612.82) in FIG. 1 is applied to S = p (q−lnσ) = − p × lnσ + p × q, p = 86.85 μV and q = 7.16.

The output factor is S ² σ. Substituting S = p (q-lnσ) to S ² sigma, the ^{S 2 σ = (p (q} -lnσ)) 2 × σ. Here, the maximum value (S ² σ) _max of the output factor can be obtained by differentiating (p ² (q−lnσ) ² ) × σ. When S ² σ is differentiated, (S ² σ) _max = 4p ² exp (q−2) is obtained. When p = 86.85 μV and q = 7.16 are substituted, (S ² σ) _max = 527 μWm ^{− 1} K ^-2 is obtained. When the maximum value 527μWm ^-1 K ^-2 output factors determining the optimum value _{S opt} of the optimum value sigma _opt and Seebeck coefficient of electrical conductivity, a respective 174Scm ^-1 and 174μVK ^-1.

When the optimum value S _opt (174 μVK ⁻¹ ) of the Seebeck coefficient is applied to the graph of FIG. 2, x _opt is 0.41.

In the present invention , factors other than the output factor must be considered. The temperature of the annealing process is also an important factor, and considering the optimum temperature of the annealing process, the composition A _x Co _3-x O ₄ in the present invention (where A is selected from the group consisting of Ni, Zn, and Ag) X of any element) is 0 <x ≦ 0.3.

In the firing treatment in the present invention , the raw material is fired to obtain a sintered body having a rock salt structure (CoO).

  Examples of the firing treatment include a method of firing at 1373 to 1573K for 1 hour or longer in the air, a method of temporarily firing at 1273K in the air, and then performing a main firing at 1473K, but are not limited thereto. Not.

  The firing temperature, firing time, and number of firings are preferably determined appropriately so that the relative density calculated from (actual density / theoretical density) × 100 is 90% or more. The actual density mentioned here is the weight of the sintered body obtained by the firing treatment / the volume of the sintered body obtained by the firing treatment, and the relative density is the weight of the unit cell obtained from the composition / XRD pattern. It is the volume of the unit cell obtained from the above.

In the present invention, the sintered body having a rock salt structure (CoO) is converted into an oxide having a spinel structure (Co ₃ O ₄ ) by annealing.

  The annealing temperature is a phase transition temperature of −300 K to a phase transition temperature of +100 K based on the phase transition temperature from the rock salt type structure to the spinel type structure obtained by the differential thermal analysis. If the annealing temperature is less than the phase transition temperature of −300 K, not only does it take a considerable amount of time for the phase transition, but the phase transition may not be performed completely. This is because the phase transition does not occur when the temperature exceeds + 100K. The annealing treatment is preferably performed at a phase transition temperature of −50K to a phase transition temperature of + 10K.

The annealing treatment time is not particularly limited as long as the purpose of converting to an oxide having a spinel structure (Co ₃ O ₄ ) is achieved, but it is preferably 5 hours or more, usually 10 hours or more, and the spinel structure (Co If the conversion to oxides with ₃ O ₄ ) is insufficient, the number is increased and the time is determined.

The thermoelectric material obtained by the present invention has a spinel structure, is cubic and has good symmetry, has a basic skeleton of Co ₃ O ₄ , and consists of a 4-coordinate Co ²⁺ site and a 6-coordinate Co ³⁺ site. That is, p-type conductivity is exhibited.

Since the thermoelectric material obtained by the present invention exhibits p-type conductivity, the Seebeck coefficient is positive.

The thermoelectric material obtained by the present invention is usually joined to a thermoelectric material exhibiting n-type electrical conductivity, and used in a state of a joined pair generally called a thermoelectric element. The figure of merit of the thermoelectric element is the figure of merit Z _{p of} the thermoelectric material exhibiting p-type conductivity, the figure of merit Z _{n of} the thermoelectric material exhibiting n-type conductivity, and the thermoelectric material exhibiting p-type conductivity. Depends on the shape and the shape of the thermoelectric material exhibiting n-type electrical conductivity. Therefore, if the shape of the thermoelectric material exhibiting electrical conductivity in the shape and n-type thermoelectric material exhibiting p-type conductivity is optimized, a high thermal conductive material figure of merit Z _p and performance index Z _n It is important to use.

Example 1
9.8447 g of Co ₃ O ₄ (purity 99.9%) and 0.1553 g of NiO (purity 99.9% or more) were put into a ball mill and stirred for 20 hours to obtain a mixed fine powder 1a. The mixed fine powder 1a was molded using a uniaxial press (4.9 MPa) and a hydraulic press (196 MPa) to obtain a raw material 1b. The raw material 1b was calcined in air at 1273K for 10 hours, and then fired at 1473K for 10 hours to obtain a rock salt structure (CoO) sintered body 1c. When the relative density was calculated by measuring the weight and volume of the sintered body 1c, the relative density of the sintered body 1c was 90% or more. The sintered body 1c is annealed in air at 1073K for 20 hours, and expressed by Ni _0.05 Co _2.95 O ₄ obtained according to the present invention, a spinel structure (Co ₃ O ₄ ). The thermoelectric material 1 which has was obtained.

(Example 2)
9.6891 g of Co ₃ O ₄ (purity 99.9%) and 0.3109 g of NiO (purity 99.9% or more) were put into a ball mill and stirred for 20 hours to obtain mixed fine powder 2a. The mixed fine powder 2a was molded using a uniaxial press (4.9 MPa) and a hydraulic press (196 MPa) to obtain a forming raw material 2b. The forming raw material 2b was fired in air at 1473K for 10 hours to obtain a sintered body 2c having a rock salt structure (CoO). When the relative density was calculated by measuring the weight and volume of the sintered body 2c, the relative density of the sintered body 2c was 90% or more. The sintered body 2c is annealed in air at 1073K for 20 hours, and is represented by Ni _0.1 Co _2.9 O ₄ obtained by the present invention, and has a spinel structure (Co ₃ O ₄ ). A thermoelectric material 2 was obtained.

(Example 3)
Instead of charging 9.6891 g Co ₃ O ₄ (purity 99.9%) and 0.3109 g NiO (purity 99.9% or more) into the ball mill, 9.5331 g Co ₃ O ₄ (purity 99.9) %) And 0.4006 g of NiO (purity 99.9% or more) were charged into the ball mill, and the same operation as in Example 2 was repeated to obtain Ni _0.15 Co _2.85 O ₄ obtained according to the present invention. And a thermoelectric material 3 having a spinel structure (Co ₃ O ₄ ) was obtained.

Example 4
Instead of charging 9.6891 g Co ₃ O ₄ (purity 99.9%) and 0.3109 g NiO (purity 99.9% or more) into the ball mill, 9.3767 g Co ₃ O ₄ (purity 99.9) %) And 0.6233 g of NiO (purity of 99.9% or more) were added to the ball mill, and the same operation as in Example 2 was repeated to obtain Ni _0.2 Co _2.8 O ₄ obtained by the present invention. And a thermoelectric material 4 having a spinel structure (Co ₃ O ₄ ) was obtained.

(Example 5)
Instead of charging 9.6891 g Co ₃ O ₄ (purity 99.9%) and 0.3109 g NiO (purity 99.9% or more) into the ball mill, 9.6622 g Co ₃ O ₄ (purity 99.9) %) And 0.3378 g of ZnO (purity 99.9% or more) were added to the ball mill, and the same operation as in Example 2 was repeated, and Zn _0.1 Co _2.9 O ₄ obtained by the present invention was obtained. And a thermoelectric material 5 having a spinel structure (Co ₃ O ₄ ) was obtained.

(Example 6)
Instead of charging 9.6891 g Co ₃ O ₄ (purity 99.9%) and 0.3109 g NiO (purity 99.9% or more) into the ball mill, 9.0652 g Co ₃ O ₄ (purity 99.9). %) And 0.9347 g of Ag ₂ O (purity 99.9% or more) were added to the ball mill, and the same operation as in Example 2 was repeated to obtain Ag _0.2 Co _2.8 obtained by the present invention. It is represented by O _4, to obtain a thermoelectric material 6 having a spinel structure _(Co 3 _{O 4).}

Test Example 1 Output Factor The output factor can be obtained from the product (S ² × σ) of the square of the Seebeck coefficient and the electrical conductivity. About the thermoelectric materials 1-6, the electrical conductivity in 873K and a Seebeck coefficient were measured, and the output factor was calculated. Here, the electrical conductivity was measured using a DC 4 terminal method in a low pressure He and using ULVAC-RIKO ZEM-3. The Seebeck coefficient was measured from the relationship between the temperature difference and the thermoelectromotive force using a ULVAC-RIKO ZEM-3 with a temperature difference in the thermoelectric material to be measured.

  Table 1 shows values of the electric conductivity, Seebeck coefficient, and output factor at 873 K of the thermoelectric materials 1 to 6.

From the above results, it was found that each of the thermoelectric materials 1 to 6 obtained by the present invention showed p-type conductivity (positive Seebeck coefficient) and a high output factor was obtained.

(Example 7)
The same operation as in Example 1 was repeated except that the annealing treatment at 1073 K for 20 hours was replaced with the annealing treatment at 973 K for 20 hours, and represented by Ni _0.05 Co _2.95 O ₄ obtained by the present invention. A thermoelectric material 7 exhibiting p-type conductivity and having a spinel structure (Co ₃ O ₄ ) was obtained.

(Example 8)
The same operation as in Example 1 was repeated except that the annealing treatment at 1073 K for 20 hours was replaced with the annealing treatment at 873 K for 20 hours, and represented by Ni _0.05 Co _2.95 O ₄ obtained by the present invention. A thermoelectric material 8 exhibiting p-type conductivity and having a spinel structure (Co ₃ O ₄ ) was obtained.

Test Example 2 XRD Pattern For thermoelectric materials 1, 7 and 8, a pattern of powder XRD using CuKα radiation was determined. The result is shown in FIG.

From FIG. 3, in the case of Ni _0.05 Co _2.95 O ₄ , the phase transition temperature from the rock salt type structure (CoO) to the spinel type structure (Co ₃ O ₄ ) obtained by differential thermal analysis is 1062K. In addition, the XRD patterns with annealing temperatures of 873K, 973K, and 1073K do not have a CoO type peak (symbol Δ), which is considered to deteriorate the thermoelectric characteristics, and therefore may have a wide annealing temperature range. all right.

The thermoelectric material obtained by the present invention is useful as a p-type thermoelectric material used in a high-power thermoelectric power generation module that is in a high temperature state for a long period of time, and the method for producing the thermoelectric material of the present invention is for a long period of time. This is useful as a technique for mass-producing p-type thermoelectric materials used in high-power thermoelectric power generation modules that reach high temperatures.

Claims (3)

Composition is Ni _x Co _3-x O ₄ (Where x is 0 <x ≦ 0.3), and is a method for producing a thermoelectric material using an oxide having a spinel structure and exhibiting p-type conductivity,
After firing the raw material in which the metal ions constituting the thermoelectric material are mixed at a predetermined mixing ratio to obtain a sintered body of a rock salt type structure, the rock salt type structure obtained by differential thermal analysis is changed to a spinel type structure. A method for producing a thermoelectric material, characterized in that an oxide having a spinel structure is formed by annealing in the atmosphere at a phase transition temperature of −300 K to a phase transition temperature of +100 K with reference to a phase transition temperature. Composition is A _x Co _3-x O ₄ (Where A is Zn or Ag, and x is 0 <x ≦ 0.3), and is a thermoelectric element using an oxide having a spinel structure and exhibiting p-type conductivity. A method of manufacturing a material,
After firing the raw material in which the metal ions constituting the thermoelectric material are mixed at a predetermined mixing ratio to obtain a sintered body of a rock salt type structure, the rock salt type structure obtained by differential thermal analysis is changed to a spinel type structure. A method for producing a thermoelectric material, characterized in that an oxide having a spinel structure is formed by annealing in the atmosphere at a phase transition temperature of −300 K to a phase transition temperature of +100 K with reference to a phase transition temperature. Composition is Zn _0.1 Co _2.9 O ₄ Or Ag _0.2 Co _2.8 O ₄ A method for producing a thermoelectric material using an oxide having a spinel structure and having p-type conductivity,
After firing the raw material in which the metal ions constituting the thermoelectric material are mixed at a predetermined mixing ratio to obtain a sintered body of a rock salt type structure, the rock salt type structure obtained by differential thermal analysis is changed to a spinel type structure. A method for producing a thermoelectric material, characterized in that an oxide having a spinel structure is formed by annealing in the atmosphere at a phase transition temperature of −300 K to a phase transition temperature of +100 K with reference to a phase transition temperature.