Patent Application
20160107239
The present invention relates to a method of production of a nanocomposite thermoelectric conversion material in which phonon scattering particles which have specific shapes are dispersed in a thermoelectric conversion material matrix.
A nanocomposite thermoelectric conversion material is a thermoelectric conversion material which has a nanocomposite structure which has a thermoelectric conversion material as a matrix and which has nanosize phonon scattering particles dispersed in the matrix at nano order intervals.
The conversion efficiency of a thermoelectric conversion material is expressed by the following dimensionless performance index ZT. Further, α2×σ=PF is called an “output factor” or “electrical characteristic”.
ZT=α
2
×σ×T/κ . . . conversion efficiency (dimensionless performance index)
α2×σ=PF . . . output factor (electrical characteristic)
where α: Seebeck coefficient
T: Absolute temperature
As shown in the topmost formula, the conversion efficiency is proportional to the reciprocal of the thermoconductivity κ, so the smaller the thermoconductivity, the higher the conversion efficiency. A nanocomposite thermoelectric conversion material has nanosize phonon scattering particles arranged at nano order intervals to augment the phonon scattering and lowers the phonon conduction in the thermoconductivity κ to lower the thermoconductivity κ.
To achieve a higher thermoelectric conversion performance, it is necessary to raise the phonon scattering effect by the phonon scattering particles. For example, PLT 1 describes to give the interfaces between the thermoelectric conversion material matrix and phonon scattering particles an interfacial roughness of 0.1 nm or more so as to raise the phonon scattering effect. Due to this, compared with before, the thermoconductivity falls and the thermoelectric conversion performance is improved.
However, there was a limit to the effect of roughness of the interfaces between the phonon scattering particles and thermoelectric conversion material matrix. That is, it had been anticipated that if not stopping with the roughness of the interfaces, but also making the shapes of the phonon scattering particles as a whole shapes advantageous to phonon scattering, the thermoconductivity would further fall and the thermoelectric conversion performance would be improved.
PLT 1: Japanese Patent No. 4715953
The present invention has as its object the provision of a method of producing a nanocomposite thermoelectric conversion material comprising making specific shapes of phonon scattering particles disperse in it to reduce the thermoconductivity and improve the thermoelectric conversion performance.
To achieve this object, the method of production of the present invention provides a method of production of a nanocomposite thermoelectric conversion material comprising a matrix of a thermoelectric conversion material in which oxides are dispersed as phonon scattering particles, the method comprising
a second stage of alloying the mixture by hydrothermal treatment to obtain composite nanoparticles, then sintering them, wherein,
at the first stage, the nanoparticles of the first group of elements which form the thermoelectric conversion material are made to precipitate or grow in advance of the precipitation or growth of the nanoparticles of the second group of elements which form the phonon scattering particles.
According to the present invention, by making the nanoparticles of the first group of elements which form the thermoelectric conversion material precipitate or grow in advance of the precipitation or growth of the nanoparticles of the second group of elements which form the phonon scattering particles, phonon scattering nanoparticles precipitate and grow in a state filling the gaps or valleys between the thermoelectric conversion material nanoparticles which precipitated, grew, and aggregated in advance, the phonon scattering nanoparticles become shaped as multi-arc shapes comprised of two or more arcs and the following advantageous effects (1), (2), and (3) are obtained compared with the roughly spherical shapes obtained in the past.
(1) Compared with the same amount of spherical phonon scattering particles, the phonon scattering interfacial area remarkably increases and the thermoconductivity can be greatly decreased.
(2) Compared with conventional spherical phonon scattering particles, a smaller amount of phonon scattering particles can be used to achieve an equal effect of reduction of thermoconductivity, so when using electrical insulating phonon scattering particles, the fall in conductivity can be mitigated.
(3) Depending on the direction of incidence of the conduction carriers, a tunnel effect of the carriers occurs and the fall in the electroconductivity can be further decreased.
Due to the advantageous effects of the above (1), (2), and (3), the thermoelectric conversion efficiency ZT is greatly improved.
The present invention provides a method of production of a nanocomposite thermoelectric conversion material comprising a matrix of a thermoelectric conversion material in which oxides are dispersed as phonon scattering particles, the method comprising
a first stage of precipitating and growing, as nanoparticles, elements which form the thermoelectric conversion material by reduction of salts in a solution and oxides which form the phonon scattering particles by polymerization of a precursor and recovering a mixture of these nanoparticles and
a second stage of alloying the mixture by hydrothermal treatment to obtain composite nanoparticles, then sintering them, wherein,
at said first stage, the nanoparticles of the first group of elements which form the thermoelectric conversion material are made to precipitate or grow in advance of the precipitation or growth of the nanoparticles of the second group of elements which form the phonon scattering particles.
In the present invention, at the first stage, the method of making the nanoparticles of the thermoelectric conversion material-forming elements precipitate or grow in advance of the precipitation or growth of the nanoparticles of the phonon scattering particle-forming oxides is based on any of the following Modes A, B, and C.
The following steps (1) and (2) are successively performed:
(1) A solution of salts of the first group of elements which form the thermoelectric conversion material and a precursor of the second group of elements which form the phonon scattering particles is formed so as to satisfy the following condition 1.
Condition 1: The salts and the precursor are selected so that in the solution in the presence of the same reducing agent, the speed by which the salts are reduced and the nanoparticles of the first group of elements precipitate becomes greater than the speed by which the precursor polymerizes and the nanoparticles of the second group of element oxides precipitate.
(2) A reducing agent is mixed with the solution to make the nanoparticles of the first group of elements precipitate from the salts, simultaneously the precursor is polymerized to make the nanoparticles of the second group of element oxides precipitate, and the mixture of these nanoparticles is recovered.
The following steps (1) and (2) are successively performed:
(1) A first solution of salts of the first group of elements which form the thermoelectric conversion material and a second solution of a precursor of the second group of elements which form the phonon scattering particles are formed so as to satisfy the following condition 1.
Condition 1: The salts and the precursor are selected so that in the presence of the same reducing agent, the speed by which the salts are reduced and the nanoparticles of the first group of elements precipitate becomes greater than the speed by which the precursor polymerizes and the nanoparticles of the second group of element oxides precipitate.
(2) A reducing agent is mixed with the first solution to make the nanoparticles of the first group of elements precipitate, then the second solution is charged to make the nanoparticles of the second group of element oxides precipitate, and the mixture of these nanoparticles is recovered. Preferably, after charging, the mixture is stirred and aged for 1 to 48 hr. In the case of an alkoxide, water is charged so as to promote the sol-gel reaction to an extent not causing turbidity.
The following steps (1) and (2) are successively performed:
(1) A first solution of salts of the first group of elements which form the thermoelectric conversion material and a second solution of a precursor of the second group of elements which form the phonon scattering particles are formed so as to satisfy the following condition 1.
Condition 1: The salts and the precursor are selected so that in the presence of the same reducing agent, the speed by which the salts are reduced and the nanoparticles of the first group of elements precipitate becomes greater than the speed by which the precursor polymerizes and the nanoparticles of the second group of element oxides precipitate.
(2) A reducing agent is mixed with the first solution to make the nanoparticles of the first group of elements precipitate, this is allowed to stand to cause aggregation, then the second solution is charged to make the nanoparticles of the second group of element oxides precipitate, and the mixture of these nanoparticles is recovered. Preferably, the standing is performed for 1 to 48 hr to cause sufficient aggregation. Preferably, after charging the second solution, ultrasonic waves are used to promote sufficient dispersion, then the mixture is stirred and aged for 1 to 48 hr. In the case of an alkoxide, water is charged so as to promote the sol-gel reaction to an extent not causing turbidity.
The second stage in the method of the present invention specifically successively performs the following steps (3) and (4).
(3) The mixture is hydrothermally treated to alloy the nanoparticles of the first group of elements and the nanoparticles of the second group of element oxides to obtain composite nanoparticles. The temperature of the hydrothermal treatment is generally 175 to 550° C., preferably 240 to 350° C., more preferably 240 to 300° C. Normally, the mixture of nanoparticles which is used for the hydrothermal treatment is washed to remove impurity ingredients. After the hydrothermal treatment, the mixture is dried to remove the solvent and is recovered as a powder of composite nanoparticles.
(4) The composite nanoparticles are sintered to obtain a bulk material. The temperature of the sintering is generally 250 to 550° C., preferably 300 to 500° C., more preferably 300 to 450° C.
Referring to
At the second stage of the present invention, if alloying the composite nanoparticles C by a hydrothermal reaction, then sintering them, as shown in
Referring to
Preferably, the contact angle θ is 1°<θ<90° and the diameter “a” of the nanoparticles is 1 nm<a<50 nm, more preferably θ<60° and a<15 nm.
If θ becomes less than 90°, even with the same volume, the phonon scattering interfacial area greatly increases. If the nanoparticle diameter “a” becomes small, similarly the interfacial area increases.
Furthermore, if in the ranges of θ and “a”, depending on the incident direction of the carriers, a size where a tunnel effect occurs (layer of several atoms to several nm) may be included. Regarding this,
Note that, referring to
If comparing the Modes A, B, and C of the present invention, they have the following characteristics relative to each other:
Mode A . . .
Advantage: Small diameter “a” of phonon scattering particles
Shortcoming: Large contact angle θ
Mode B
Advantage: Small contact angle θ. Therefore, large interfacial area density.
Shortcoming: Medium extent of diameter “a” of phonon scattering particles
Mode C
Advantage: Large number of arcs. Therefore, large interfacial area with same diameter of phonon scattering particles
Shortcoming: Large diameter “a” of phonon scattering particles
Nanocomposite thermoelectric conversion materials comprised of a thermoelectric conversion material matrix of BiTeSb alloy in which cross-sectional multi-arc shaped phonon scattering particles were dispersed at 0.5 to 11 vol % were fabricated according to the Modes A, B, and C of the present invention under the conditions which are shown in Table 1 and were measured for contact angle θ, nanoparticle diameter “a”, lattice thermoconductivity κ, and electroconductivity. The measurement results are also shown in Table 1.
Fabrication of Samples of Invention Examples Starting Materials of Matrix Thermoelectric Conversion Material
As the matrix thermoelectric conversion material of the nanocomposite thermoelectric conversion material, the following starting materials were used as salts of the first group of elements (Bi, Sb, Te) which form the
BiTeSb thermoelectric conversion material common to the different modes:
Salts of First Group of Elements
Bi source: BiCl3 0.24 g
Sb source: SbCl3 0.68 g
Te source: TeCl4 1.51 g
Below, the Modes A, B, C will each be explained.
Materials for Phonon Scattering Particles
As shown in Examples 1 to 7 of Table 1, as the precursor of the second group of element oxides (SiO2) which forms the phonon scattering particles, TEOS (tetraethoxysilane: Si(OC2H5)4) was used.
Precursor of Second Group of Element Oxides SiO2 source: TEOS 0.14 g
As the solvent, as shown in Examples 1 to 7 of Table 1, either methanol, ethanol, 1-propanol, or 2-propanol was used.
First, as the first stage, the following steps (1) and (2) were successively performed.
(1) Formation of Solutions
The salts of the first group of elements and the precursor of the second group of element oxides were dissolved in solvents: 100 ml to prepare the solutions of Examples 1 to 7 which are shown in Table 1. To each solution, as the reducing agent, a solution of either NaBH4 (1.59 g), N2H4.H2O (2.10 g), or ascorbic acid (7.40 g) dissolved in 100 ml of solvent was used as shown in Table 1.
The Condition 1 required for the Mode A is satisfied as follows:
Condition 1: In each solution of Examples 1 to 7, the speed by which the first salts (BiCl3, SbCl3, TeCl4) are reduced by the reducing agent and the first group of elements (Bi, Sb, Te) precipitate is greater than the speed by which the precursor (TEOS) polymerizes and the second group of element oxides (SiO2) precipitate.
(2) Precipitation and Growth of Nanoparticles
Into each solution of Examples 1 to 7, the reducing agent solution which is shown in Table 1 was dropped to make the first group of elements (Bi, Sb, Te) precipitate while making the second group of element oxides (SiO2) precipitate. At that time, as shown in
A slurry of each of the obtained Examples 1 to 7 in a solvent was filtered and washed by water 500 ml, then was further filtered and washed by the same solvent 300 ml. Due to this, a mixture of nanoparticles was obtained.
Next, as the second stage, the following steps (3) and (4) were successively performed.
(3) Hydrothermal Treatment: Formation of Composite Nanoparticles
Each above mixture was placed in a sealed autoclave and hydrothermally treated to make it alloy at 240° C. for 48 hr. After that, it was made to dry in a nitrogen gas flow atmosphere. Due to this, a powder of composite nanoparticles of BiTeSb alloy nanoparticles and SiO2 nanoparticles was recovered.
(4) Sintering: Completion of Nanocomposite Thermoelectric Conversion Material
Each composite nanoparticle powder was sintered at 360° C. by spark plasma sintering. Due to this, a bulk material of nanocomposite thermoelectric conversion material of a BiTeSb thermoelectric conversion material matrix in which SiO2 nanoparticles are dispersed as phonon scattering particles was obtained.
A TEM was used to observe Examples 1 to 7 to measure the contact angles 0 and diameters “a” of SiO2. These are shown in Table 1. The obtained sintered bodies were measured for lattice thermoconductivity and electroconductivity. The results are shown in Table 1.
Starting Materials of Phonon Scattering Particles
Starting Materials of Phonon Scattering Particles
As shown in Examples 8 to 14 of Table 1, as precursors of the second group of element oxides (SiO2, Bi2O3, Sb2O3, TeO2, TiO2) which form the phonon scattering particles, respectively sodium silicate solution no. 3, TEOS, Bi ethoxide, Sb ethoxide, Te ethoxide, and Ti alkoxide were used.
Precursors of Second Group of Element Oxides
SiO2 source
Bi2O3 source: Bi ethoxide 0.23 g
Sb2O3 source: Sb ethoxide 0.17 g
TeO2 source: Te ethoxide 0.21 g
TiO2 source: Ti alkoxide 0.15 g
As the solvent, as shown in Examples 8 to 14 of Table 1, 2-propanol was used.
First, as the first stage, the following steps
(1) and (2) were successively performed.
(1) Formation of Solutions
The salts of the first group of elements were dissolved in the solvent 2-propanol: 100 ml to obtain a first solution, while the precursor of the second group of element oxides was dissolved in the solvent 2-propanol: 100 ml to obtain a second solution. To each solution, as the reducing agent, a solution of either NaBH4 (1.59 g) or N2H4.H2O (2.10 g) dissolved in the solvent 2-propanol: 100 ml was used as shown in Table 1.
The Condition 1 required for Mode B is satisfied as follows:
Condition 1: In each solution of Examples 8 to 14, the speed by which the salts (BiCl2, SbCl3, TeCl4) are reduced by the reducing agent and the first group of elements (Bi, Sb, Te) precipitate is greater than the speed by which the precursor (sodium silicate solution no. 3, TEOS, Bi ethoxide, Sb ethoxide, Te ethoxide, or Ti alkoxide) polymerizes and the second group of element oxides (SiO2, Bi2O2, Sb2O2, TeO2, TiO2) precipitate.
(2) Precipitation and Growth of Nanoparticles
Into each first solution of Examples 8 to 14, the reducing agent solution which is shown in Table 1 was dropped to make the first group of elements (Bi, Sb, Te) precipitate, then the second solution was charged to make the second group of element oxides (SiO2, Bi2O3, Sb2O3, TeO2, TiO2) precipitate. At that time, as shown in
A slurry of each of the obtained Examples 8 to 14 in 2-propanol was filtered and washed by water: 500 ml, then was further filtered and washed by 2-propanol: 300 ml. Due to this, a mixture of nanoparticles was obtained.
Next, as the second stage, the following steps (3) and (4) were successively performed.
(3) Hydrothermal Treatment: Formation of Composite Nanoparticles
Each above mixture was placed in a sealed autoclave and hydrothermally treated to make it alloy at 240° C. for 48 hr. After that, it was made to dry in a nitrogen gas flow atmosphere. Due to this, a powder of composite nanoparticles of BiTeSb alloy nanoparticles and SiO2, Bi2O2, Sb2O2, TeO2 or TiO2 nanoparticles was recovered.
(4) Sintering: Completion of Nanocomposite
Thermoelectric Conversion Material
Each composite nanoparticle powder was sintered at 360° C. by spark plasma sintering. Due to this, a bulk material of nanocomposite thermoelectric conversion material of a BiTeSb thermoelectric conversion material matrix in which SiO2 nanoparticles, Bi2O2 nanoparticles, Sb2O2 nanoparticles, TeO2 nanoparticles, or TiO2 nanoparticles are dispersed as phonon scattering particles was obtained.
A TEM was used to observe Examples 8 to 14 to measure the contact angles θ and diameters “a” of SiO2, Bi2O2, Sb2O2, Te02, or TiO2. These are shown in Table 1. The obtained sintered bodies were measured for lattice thermoconductivity and electroconductivity. The results are shown in Table 1.
Starting Materials of Phonon Scattering Particles
Starting Materials of Phonon Scattering Particles
As shown in Examples 15 to 16 of Table 1, as the precursors of the second group of element oxides (SiO2,
Sb2O2) which form the phonon scattering particles, respectively TEOS and Sb ethoxide were used.
Precursors of Second Group Element Oxides
SiO2 source: TEOS 0.14 g
Sb2O2 source: Sb ethoxide 0.17 g
As the solvent, as shown in Examples 15 to 16 of Table 1, ethanol was used.
First, as the first stage, the following steps (1) and (2) were successively performed.
(1) Formation of Solutions
The salts of the first group of elements were dissolved in the solvent ethanol: 100 ml to obtain a first solution, while the precursor of the second group of element oxides was dissolved in the solvent ethanol: 100 ml to obtain a second solution. To each solution, as the reducing agent, a solution of N2H4.H2O (2.10 g) dissolved in the solvent ethanol: 100 ml was used as shown in Table 1.
(2) Precipitation and Growth of Nanoparticles Into each first solution of Examples 15 to 16, the reducing agent solution which is shown in Table 1 was dropped to make the first group of elements (Bi, Sb, Te) precipitate. The mixture was allowed to stand for 48 hours to make the nanoparticles aggregate. After that, the second solution was charged to make the second group of element oxides (SiO2, Sb2O3) precipitate. At that time, as shown in
Ethanol slurries of the obtained Examples 15 to 16 were filtered and washed by water: 500 ml, then were further filtered and cleaned by ethanol: 300 ml. Due to this, mixtures of nanoparticles were obtained.
Next, as a second stage, the following steps (3) and (4) were successively performed:
(3) Hydrothermal Treatment: Formation of Composite Nanoparticles
Each mixture was placed in a sealed autoclave and hydrothermally treated at 240° C. for 48 hr to make it alloy. After that, this was made to dry in a nitrogen gas flow atmosphere. Due to this, a powder of composite nanoparticles of BiTeSb alloy nanoparticles and SiO2 or Sb2O3 nanoparticles was recovered.
(4) Sintering: Completion of Nanocomposite Thermoelectric Conversion Material
The composite nanoparticles powder was sintered at 360° C. by spark plasma sintering. Due to this, a bulk material of nanocomposite thermoelectric conversion material comprised of a BiTeSb thermoelectric conversion material matrix in which SiO2 nanoparticles or Sb2O3 nanoparticles were dispersed as phonon scattering particles was obtained.
A TEM was used to observe Examples 15 to 16 to measure the contact angles θ and diameters “a” of SiO2 and Sb2O2. These are shown in Table 1. The obtained sintered bodies were measured for lattice thermoconductivity and electroconductivity. The results are shown in Table 1.
For comparison, nanocomposite thermoelectric conversion materials each comprised of an alloy matrix in which phonon scattering particles constituted by conventional spherical SiO2 nanoparticles (commercially available product: particle size 5 nm or 15 nm) were dispersed to 10 to 15 vol % were fabricated.
Fabrication Conditions of Comparative Examples Starting Materials for Matrix Thermoelectric Conversion Material
Starting materials common to Examples 1 to 16 were used.
Salts of First Group of Elements
Bi source: BiCl3 0.24 g
Sb source: SbCl2 0.68 g
Te source: TeCl4 1.51 g
Phonon Scattering Particles
Commercially available SiO2 (particle size 5 nm or 15 nm): 0.034 to 0.054 g (case of 15 vol%) was used.
In 100 ml of ethanol, first salts of the first group of elements and the phonon scattering particles were charged. To the obtained solution, a reducing agent constituted by NaBH4: 1.59 g in 100 ml solution was dropped as a reducing agent solution to obtain a mixture of nanoparticles of the first group of elements (Bi, Sb,
Te) and SiO2 nanoparticles. This mixture was placed in a sealed autoclave and hydrothermally treated at 240° C. for 48 hr to make it alloy. After that, this was made to dry in a nitrogen gas flow atmosphere. Due to this, a powder of composite nanoparticles of BiTeSb alloy nanoparticles and SiO2 nanoparticles was recovered.
This composite nanoparticles powder was sintered at 360° C. by spark plasma sintering. At that time, a bulk material of nanocomposite thermoelectric conversion material wherein the SiO2 nanoparticles were maintained as they were and dispersed in a BiTeSb thermoelectric conversion material matrix was obtained.
As shown in Table 1 , the invention examples have much lower thermoconductivity and higher electroconductivity compared with the comparative examples. The Modes A, B, and C will be compared.
The contact angle θ becomes smaller in the order of the Modes A>B>C.
The particle diameter “a” becomes larger in the order of the Modes A<B<C.
This is because in each case, the specific surface area of the thermoelectric conversion material nanoparticles becomes greater in the order of A<B<C. Due to this, the lattice thermoconductivity and electroconductivity become higher overall.
First,
In the figure, the top horizontal broken line (labeled as “BiSbTe”) is the lattice thermoconductivity κph in the case of a BiSbTe thermoelectric conversion material (matrix material of present invention) alone without containing phonon scattering particles. It was 0.90 W/m/K.
As opposed to this, the comparative examples where the spherical phonon scattering particles (SiO2) are dispersed have a lattice thermoconductivity κph of 0.57 to 0.52 W/m/K in the case of a particle size 15 nm of phonon scattering particles (volume rate 10 to 30 vol %) and a lattice thermoconductivity κph of 0.34 to 0.12 W/m/K in the case of a particle size 5 nm (volume rate 5 to 20 vol %). The dispersion of the phonon scattering particles causes a large drop.
Furthermore, the invention examples in which the multi-arc shaped phonon scattering particles (volume rate 0.5 to 11 vol %) are dispersed become larger in degree of drop, that is, 0.5 to 0.02 W/m/K, along with the increase in the phonon scattering particles volume rate.
The lattice thermoconductivity κph falls extremely greatly by a small volume rate.
In this way, according to the present invention, the multi-arc shaped phonon scattering particles caused the phonon scattering interfaces to greatly increase (see
Next,
As opposed to this, the comparative examples in which spherical phonon scattering particles (SiO2, particle size 5 nm, volume rate 10 to 30 vol %) are dispersed had an electroconductivity σ of 270 to 390 S/cm, while the invention examples in which multi-arc shaped phonon scattering particles (volume rate 0.5 to 11 vol %) are dispersed exhibited a value of 320 to 700 S/cm or higher than the comparative examples regardless of photon scattering particles being dispersed by a volume rate higher than the comparative examples.
As a result, according to the present invention, it can be seen that multi-arc shaped phonon scattering particles change on the same curve as the spherical phonon scattering particles of the comparative examples despite the interfacial area density being high. This is because the multi-arc shaped phonon scattering particles of the present invention are suppressed in increase of carrier scattering (=fall in electroconductivity) due to the increase in interfaces due to the tunnel effect (see
According to the present invention, there is provided a method of producing a nanocomposite thermoelectric conversion material comprising causing dispersion of multi-arc shaped phonon scattering particles in it to reduce the thermoconductivity and improve the thermoelectric conversion performance.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2013-145503 | Jul 2013 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2014/065868 | 6/16/2014 | WO | 00 |
