JP2000261043A - Thermoelectric conversion material and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce thermal conductivity and to improve performance or the Seebeck coefficient by melting additive elements in Si separately or in combination, such that the Si contains a predetermined amount of additive elements and by rapidly cooling the molten substance to form a phase which is rich in the additive elements at the grain boundary rich in Si, where Si is the main component. SOLUTION: Additive elements for making a p-type or n-type semiconductor are melted in Si separately or in combination, such that Si contains 0.001 atom% to 20 atom% additive elements, and the melted substance is rapidly cooled to form a phase rich in the additive elements at the grain boundary rich in Si, where the Si is the main component. In short, a texture in which a phase rich in the additive elements is dispersed and formed at the grain boundary rich in Si of a Si-based thermoelectric conversion material is obtained by controlling the cooling rate after melting, and rapid cooling restrains the grain size to be comparatively small and moderate segregation of the additive elements except for Si at the grain boundary takes place, which produces high Seebeck coefficient in spite of high electrical conductivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel thermoelectric conversion material containing 20 atomic% or less of various additive elements in Si, and relates to a structure in which a rich phase of an additive element is dispersed in a grain boundary of a Si-rich phase. By doing so, polycrystalline Si is characterized by extremely high Seebeck coefficient and low thermal conductivity, which can significantly increase thermoelectric conversion efficiency, and is mainly composed of resource-rich Si and has extremely low environmental pollution. The present invention relates to a thermoelectric conversion material.

[0002]

2. Description of the Related Art Thermoelectric conversion elements are devices that are expected to be put to practical use from the viewpoint of effective use of thermal energy, which is required in recent industries. For example, thermoelectric conversion elements convert waste heat into electric energy. A very wide range of applications are being studied, such as systems, small portable generators for easily obtaining electricity outdoors, and flame sensors for gas appliances.

[0003] The conversion efficiency from heat energy to electric energy is a function of the figure of merit ZT, and increases as ZT increases. This figure of merit ZT is expressed as in equation (1). ZT = α ² σT / κ (1) where α is the Seebeck coefficient of the thermoelectric material, σ is the electrical conductivity, κ is the thermal conductivity, and T is the high-temperature side (T _H ) and low-temperature side ( It is the absolute temperature represented by the average value of T _L ).

[0004] FeSi, a thermoelectric conversion material known so far,
₂ , silicides such as SiGe are abundant in resources, but the former has a figure of merit (ZT) of 0.2 or less, its conversion efficiency is low and the operating temperature range is very narrow, and the latter contains Ge which is poor in resources. If the amount is not about 20 to 30 at%, no decrease in heat conduction is observed, and Si and Ge have a wide range of liquid-solid and solid-phase lines of totally controlled solid solution, dissolution and ZL method (Zone-Leveling ) Is not widely used because it is difficult to produce a uniform composition and it is difficult to industrialize.

At present, chalcogen-based compounds such as BiTe and PbTe, such as IrSb ₃ having a skutterudite-type crystal structure exhibiting the highest figure of merit, are known to have high-efficiency thermoelectric conversion capabilities. From the viewpoint of environmental protection, it is expected that the use of these heavy metal elements will be regulated in the future.

[0006]

On the other hand, while Si has a high Seebeck coefficient, it has a very high thermal conductivity,
It is not considered suitable for high-efficiency thermoelectric materials, and the study of its thermoelectric properties has been limited to Si with a carrier concentration of 10 ¹⁸ (Mm ³ ) or less.

[0007] However, the inventors have found that adding various elements to Si alone, for example, adding a small amount of a group 3 or 5 element and a small amount of Ge to Si can lower the thermal conductivity. It is possible to find that the Seebeck coefficient is equal to or higher than that of conventionally known Si-Ge and Fe-Si systems, or that it is extremely high at a given carrier concentration, impairing the essential advantages of Si alone. Instead, it was found that the thermoelectric conversion material exhibited a large figure of merit and could be improved in performance.

[0008] The inventors have also added various elements to Si to add P
As a result of investigating the relationship between the amount of addition and the thermoelectric properties, the Seebeck coefficient decreased with the increase of the carrier up to the addition amount, that is, the carrier concentration of 10 ¹⁸ (M / m ³ ). , 10 ¹⁸ to 10 ¹⁹ (M / m ³ ).

[0009] The present invention is based on this novel Si which the inventors have found.
The object is to have a high Seebeck coefficient possessed by a system thermoelectric conversion material, to further lower the thermal conductivity without deteriorating the electric conductivity, to improve the performance, or to further improve the Seebeck coefficient.

[0010]

Means for Solving the Problems The present inventors have conducted intensive studies on the mechanism of obtaining a high Seebeck coefficient in a Si-based thermoelectric conversion material to which various additive elements are added.
It has been found that this novel Si-based material has a structure in which a rich phase of the additive element is formed at the grain boundary of a Si-rich phase mainly composed of Si.

The inventors further studied the crystal structure. As a result, the increase in the Seebeck coefficient was caused by the addition of the added element at the crystal grain boundaries, which increased the carrier conduction. It was found that a high Seebeck coefficient was obtained in the rich phase.

[0012] Therefore, the inventors examined the control of the crystal structure other than the component system as a method of keeping the Seebeck coefficient high and lowering the thermal conductivity. As a result, the Si-rich phase and the additive element-rich phase were dissolved and solidified. It has been found that by controlling the cooling rate, a material having a structure in which these phases are dispersed in a required arrangement in the material and having a high figure of merit can be obtained.

[0013] That is, the present invention provides an additive element (referred to as an additional element α) for forming a P-type semiconductor or an additional element (referred to as an additional element β) for forming an N-type semiconductor in 0.001 Atomic% to 20 atomic% is melted and the melt is quenched to obtain a thermoelectric conversion material having a structure in which a rich phase of the additive element is formed at a grain boundary of a Si-rich phase mainly composed of Si. It is characterized by the following.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The structure of the thermoelectric conversion material according to the present invention, in which a rich phase of the additive element is formed at a grain boundary of a Si-rich phase mainly composed of Si, will be described.
Various amounts of Ge (4N) added to high-purity Si (10N) were varied to produce a melt of Si _1-x Ge _x (at%) by arc melting, and the cooling rate after the melting was 50 K / sec to 200 K /. A sample substrate was prepared by rapid cooling with sec and the crystal structure was observed by EPMA.When x = 0.03, as shown in Fig. Is the main Si-rich phase, the white part in the photograph is the rich phase of the additive element Ge, and it can be seen that the structure is such that the Ge-rich phase is dispersed or formed at the grain boundaries of the Si-rich phase.

[0015] In addition, although a small amount of P was doped into the above _- mentioned Si _1-x Ge _x molten metal, only this P was observed.
As shown in B, the white portion indicates the location of the doped P, and it can be seen that P is segregated at the same position as the grain boundary of the Si-rich phase where the Ge-rich phase shown in FIG. 1A was formed.

[0016] In short, the structure of the thermoelectric conversion material according to the present invention is, as shown in the schematic diagram of FIG. 2, only Si or an Si-rich phase containing an additive element but mostly Si, This is a structure in which an additive element-rich phase in which an additive element segregates at the grain boundary of the Si-rich phase. The size of the Si-rich phase varies depending on the cooling rate, but is about 10 to 200 μm.

Further, when the relationship between the precipitation of added elements of P and B instead of Ge at the crystal grain boundaries and the carrier concentration of n-type and p-type Si was investigated, the correlation between the added amount and the carrier concentration was one. It was confirmed that the additive element was agglomerated at the crystal grain boundary by the structure in which the rich phase of the additional element was formed at the grain boundary of the Si-rich phase, where the carrier conduction was increased and the It was confirmed that a high Seebeck coefficient can be obtained in the Si-rich phase.

Further, the thermal conductivity of the Si-based thermoelectric conversion material is:
It was confirmed that it decreased as the carrier concentration increased. This is considered to be because κ _ph decreased due to scattering of localized phonons of impurities due to the added element in the crystal.

A structure in which a rich phase of an additive element such as Ge is dispersed and formed in a grain boundary of a Si-rich phase, which is a feature of a Si-based thermoelectric conversion material, is obtained by controlling a cooling rate after melting, and is rapidly cooled. It is considered that the crystal grain size is kept relatively small, and moderate segregation of additional elements other than Si occurs at the crystal grain boundaries, whereby a high Seebeck coefficient can be obtained despite high electrical conductivity.

[0020] The Si-based thermoelectric conversion material according to the present invention obtains the above-described structure by cooling the Si-based melting material.
The arc melting method and the high frequency melting method are optimal and preferable for mass production.
In addition, the cooling rate of the Si-based melting material is appropriately selected depending on the type and combination of the additional elements to be described later, the amount of addition, and the cooling method to be adopted and the form of the obtained ingot, thin plate, substrate, ribbon, and the like. .

In the present invention, as a cooling method, a method of cooling a cast ingot or a method of cooling while pulling up, for example, a CZ method for obtaining a known single crystal silicon,
Using the FZ method, a method of pulling and cooling under conditions that can obtain polycrystalline silicon can be adopted. The CZ method and the FZ method can be used to manufacture a large number of substrates of a required thickness from a drawn ingot bar, and thus are optimal as a method for manufacturing a Si-based substrate for a thermoelectric conversion element. Further, it can be manufactured by the ZL method described above.

Further, the cooling rate is adjusted so that a thin plate having a required thickness can be obtained by using a method of producing a thinner plate by pouring the Si-based molten material into a shallow plate and cooling it, or by using a roll cooling method such as a known melt quenching method. And any other method can be adopted.

[0023] For example, in the case of a method in which a Si-based dissolving material is poured into a shallow plate and cooled, or a plate is cooled with water or a chill is applied, for example, cooling at a cooling rate of 50 K / sec or more is appropriate. Thus, the crystal grain size is
It is suppressed to 100 μm or less, and a high Seebeck coefficient can be obtained. Preferred cooling rates are 50K / sec to 500K / sec,
The average crystal grain size can be in the range of 30 μm to 200 μm.

[0024] The thermoelectric conversion material according to the present invention is capable of adjusting the carrier concentration by adding various impurities to a polycrystalline Si semiconductor having a diamond-type crystal structure, without impairing the inherent advantages of Si alone. This is a highly efficient Si-based thermoelectric conversion material of P-type and N-type semiconductors whose electrical index has been lowered and the Seebeck coefficient has been improved to dramatically improve the figure of merit.

Here, considering the application of the thermoelectric conversion material, the Seebeck coefficient, electric conductivity, thermal conductivity, etc., vary depending on the application such as the heat source, the place of use and form, the current to be handled, and the magnitude of the voltage. Although it is necessary to place emphasis on any of the characteristics, in the thermoelectric conversion material of the present invention, the carrier concentration can be selected by the addition amount of the selected element.

[0026] For example, it contains 0.001 atomic% to 0.5 atomic% of elements added element α of the foregoing alone or combined to give P-type semiconductor carrier concentration is ^{10 17 ~10 20 (M / m} 3) is And
A P-type semiconductor having a carrier concentration of 10 ¹⁹ to 10 ²¹ (M / m ³ ) containing 0.5 to 5.0 atomic% of the additional element α is obtained.

Similarly, an N-type semiconductor containing 0.001 atomic% to 0.5 atomic% of the above-mentioned additive element β alone or in combination and having a carrier concentration of 10 ¹⁷ to 10 ²⁰ (M / m ³ ) Obtained and also
An N-type semiconductor containing 0.5 to 10 atomic% of the additive element β and having a carrier concentration of 10 ¹⁹ to 10 ²¹ (M / m ³ ) is obtained.

When the additive element α or the additive element β is contained and 0.5 to 5.0 atomic% is added so that the carrier concentration becomes 10 ¹⁹ to 10 ²¹ (M / m ³ ), a highly efficient thermoelectric Although the conversion element is obtained and has excellent thermoelectric conversion efficiency, its thermal conductivity is about 50 to 150 W / mK at room temperature, and if the thermal conductivity can be reduced, the figure of merit ZT is further improved. I can expect that.

In general, the thermal conductivity of a solid is given by the sum of phonon conduction and carrier conduction. In the case of a Si-based semiconductor thermoelectric conversion material, phonon conduction is dominant because the carrier concentration is low. Therefore, in order to reduce the thermal conductivity, it is necessary to increase the absorption or scattering of phonons. In order to increase the absorption or scattering of phonons, it is effective to disturb the regularity of the crystal grain size and the crystal structure.

Therefore, as a result of various studies on the elements to be added to Si, at least one of a Group 3 element and a Group 5 element was added to Si, and the carrier concentration was 10 ¹⁹ to 10 ²¹ (M / m ³ ), It is possible to disturb the crystal structure without changing the carrier concentration in Si, and to reduce the thermal conductivity by 30 to 90%.
It was found that the temperature can be reduced to 150 W / m · K or less at room temperature, and a highly efficient thermoelectric conversion material can be obtained.

In the thermoelectric conversion material having the above structure, a P-type semiconductor is obtained when the Group 3 element is contained in an amount of 0.3 to 5 atomic% more than the Group 5 element, and the Group 5 element is contained in an amount of 0.3 to 5 atomic% than the Group 3 element. If a large amount is contained, an N-type semiconductor can be obtained.

Further, it was examined whether a reduction in thermal conductivity could be achieved with elements other than the group 3 element and the group 5 element, and it was found that adding a group 3-5 compound semiconductor or a group 2-6 compound semiconductor to Si further
By adding at least one of a Group 3 element or a Group 5 element and controlling the carrier concentration to 10 ¹⁹ to 10 ²¹ (M / m ³ ),
The crystal structure can be disturbed without changing the carrier concentration in Si, the thermal conductivity can be reduced to 150 W / mK or less at room temperature, and a highly efficient thermoelectric conversion material can be obtained.

Further, as a result of various studies on other additional elements to Si, it was found that Si contained a Group 4 element of Ge, C, and Sn in an amount of 0.1 to 5 atomic%,
By substituting a part of the element i with a group 4 element with a different atomic weight, the scattering of phonons in the crystal increases,
Reduces the thermal conductivity of semiconductors by 20 to 90%, 150 W / mK at room temperature
It is possible to make
A thermoelectric conversion material containing a P-type semiconductor by containing 5.05.0 at% and a N-type semiconductor containing a Group V element by 0.1 to 10 at% can be obtained.

[0034] In the thermoelectric conversion material of the present invention, it was investigated whether elements other than the above-mentioned Group 3 elements and Group 5 elements can be similarly added to Si. For example, although there is no particular limitation, if an element having a very different ionic radius is added, most of the element is precipitated in the grain boundary phase. As an additive element α for forming, and as an additional element β for forming an N-type semiconductor,
It has been confirmed that the single or combined addition of the following groups of elements is particularly effective.

As the additional element α, the additional element A (Be, Mg, Ca, Sr,
Ba, Zn, Cd, Hg, B, Al, Ga, In, Tl), transition metal element M ₁ (M ₁ ; Y, M
o, Zr), and the additive element β is the additive element B
(N, P, As, Sb, Bi, O, S, Se, Te), transition metal element M ₂ (M ₂ ; Ti, V, C
r, Mn, Fe, Co, Ni, Cu, Nb, Ru, Rh, Pd, Ag, Hf, Ta, W, Re, Os, Ir, P
t, Au, where Fe is 10 atomic% or less), rare earth element RE (RE; La, Ce,
Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, Lu).

Further, the additive element α for forming a P-type semiconductor and the additional element β for forming an N-type semiconductor are contained in a total amount of at least one from each group in a total amount of 0.002 to 20 atomic%. In order to obtain a P-type semiconductor, the combination of each group can be arbitrarily selected as long as the total amount of the additional element α exceeds that of the additional element β and is contained in a necessary amount to become a P-type semiconductor.

[0037]

Example 1 In order to produce P-type and N-type Si thermoelectric conversion semiconductors, high-purity Si (10N) and additional elements were blended as shown in Table 1,
Arc melting was performed in an Ar gas atmosphere. The melting temperature in the arc melting was about 1900K, and the melting material was cooled by a water-cooled copper base at a cooling rate of 50 to 100K / sec. The average grain size of the resulting material was about 50-100 μm.

[0038] The obtained button-shaped ingot is 5x5x15m
The sample was cut into a shape of m, 10 × 10 × 2 mm and outer diameter of 10 × 2 mm, and its Seebeck coefficient, Hall coefficient (including carrier concentration and electric conductivity), and thermal conductivity were measured. Table 2 shows the measured values at 1100 K and the figure of merit (ZT = S ² T / ρκ).

The Seebeck coefficient is set as a value obtained by setting the temperature difference between the high temperature part and the low temperature part to about 6K while increasing the temperature, measuring the thermoelectromotive force of the sample with a digital multimeter, and dividing by the temperature difference. I asked. The Hall coefficient was measured by an AC method, and the electrical resistance was measured by a four-terminal method simultaneously with the carrier concentration. The thermal conductivity was measured by a laser flash method.

Example 2 In order to produce P-type and N-type Si thermoelectric semiconductors, high purity
After mixing Si (10N) and the additive element as shown in Table 3, the mixture was inserted into a graphite crucible and melted in a high-frequency melting furnace in vacuum (10 ^-4 Torr). Melting temperature is about 1900K, casting temperature is about 1800K
And cast into a 10 mm thick mold. Melting material cooling rate is 10 ~
The average crystal grain size of the material was about 100 to 500 μm.

[0041] The obtained ingot is 5 × 5 × 15 mm, 10 × 10 × 2 m
m, and cut into a shape having an outer diameter of 10 × 2 mm. The Seebeck coefficient, the Hall coefficient (including the carrier concentration and the electrical conductivity), and the thermal conductivity were measured in the same manner as in Example 1. 1100K
Table 4 shows the measured values and the figure of merit (ZT = S ² T / ρκ).

Example 3 In order to produce P-type and N-type Si thermoelectric semiconductors, high purity
After compounding Si (10N) and the additive elements as shown in Table 5, it was inserted into a graphite crucible, melted in a high-frequency melting furnace in vacuum (10 ^-4 Torr), and homogeneously melted at about 1800K. confirmed.

Thereafter, the upper portion of the graphite crucible was lowered to 1700K, and a Si seed crystal was brought into contact with the upper portion of the molten metal, and the molten metal was slowly pulled up. The inner diameter of the crucible is 100mm and the lifting speed is
At a rate of 0.3 to 1 mm / sec, the diameter was vibrated once every 5 seconds to polycrystallize the pulled crystal. The average grain size of the resulting material was about 1-10 mm.

The obtained sample was cut into a shape of 5 × 5 × 15 mm, 10 × 10 × 2 mm, and an outer diameter of 10 × 2 mm, and the respective Seebeck coefficient and Hall coefficient (carrier concentration and (Including electrical conductivity) and thermal conductivity. Table 6 shows the measured values at 1100K and the figure of merit (ZT = S ² T / ρκ).

[0045]

【table 1】

[0046]

[Table 2]

[0047]

[Table 3]

[0048]

[Table 4]

[0049]

[Table 5]

[0050]

[Table 6]

[0051]

The thermoelectric conversion material according to the present invention comprises
Si is excellent in terms of global environment, global resources and safety, and it is very convenient as a thermoelectric conversion element for automobiles due to its small specific gravity and light weight.Since bulk Si has excellent corrosion resistance Another advantage is that no surface treatment or the like is required.

Since the thermoelectric conversion material according to the present invention mainly uses Si, it is less expensive than a Si-Ge material containing a large amount of expensive Ge, and a higher figure of merit can be obtained than an Fe-Si material. . Further, since Si used in the present invention has much lower purity than semiconductor devices, the raw material can be obtained relatively inexpensively, and an inexpensive thermoelectric conversion material with good productivity and stable quality can be obtained.

The thermoelectric conversion material according to the present invention makes use of the characteristics of Si having a large Seebeck coefficient and a small electric resistance where the carrier concentration is high, and significantly reduces defects having a large thermal conductivity, and has a large figure of merit. Is an effective way to get Further, there is an advantage that the physical property value can be controlled by the type and amount of the added element.

[Brief description of the drawings]

FIG. 1 shows the EPMA crystal structure of the thermoelectric conversion material according to the present invention.
In the photograph observed by A, A shows the deflection of the additional element Ge, and B shows the deflection of the additional element P.

FIG. 2 is a schematic explanatory view showing a crystal structure of a thermoelectric conversion material according to the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tsuneka Saigo 2-15-17 Egawa, Shimamoto-cho, Mishima-gun, Osaka Sumitomo Special Metals Co., Ltd. Yamazaki Works (72) Inventor Masao Nomi Egawa, Shimamoto-cho, Mishima-gun, Osaka 2-15-17 Sumitomo Special Metals Co., Ltd. Yamazaki Works

Claims (8)

[Claims] Claims 1. An additive element for forming a P-type semiconductor or an N-type semiconductor into Si, alone or in a combination of 0.001 atom% to 20 atom
%. A thermoelectric conversion material having a structure in which a rich phase of the additive element is formed at a grain boundary of a Si-rich phase mainly containing Si. 2. The method according to claim 1, wherein the additional element for forming a P-type semiconductor (referred to as an additional element α) and the additional element for forming an N-type semiconductor (referred to as an additional element β) are at least one of each group.
0.002 atomic% to 20 atomic% in total in each case, and additional element α
Or a thermoelectric conversion material containing only a necessary amount of β to form a P-type semiconductor or an N-type semiconductor exceeding that of the corresponding additive element β or α. 3. The additive element (additive element α) for forming a P-type semiconductor according to claim 1 or 2,
e, Mg, Ca, Sr, Ba, Zn, Cd, Hg, B, Al, Ga, In, Tl), the transition metal element _{M 1 (M 1; Y,} Mo, 1 kind selected from the group of Zr) Or two or more, an additional element for forming an N-type semiconductor (additive element β)
The additive element B (N, P, As, Sb, Bi, O, S, Se, Te), transition metal elements _{M 2 (M 2; Ti,} V, Cr, Mn, Fe, Co, Ni, Cu, Nb, Ru, Rh, Pd, Ag, Hf, T
a, W, Re, Os, Ir, Pt, Au, where Fe is 10 atomic% or less), rare earth element RE (RE; La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho , Er, Yb, Lu)
A thermoelectric conversion material selected from one or more of the above groups. 4. The semiconductor device according to claim 1, wherein the 3-5 group compound semiconductor or the 2-6 group compound semiconductor is contained in an amount of 1 to 10 atomic%, and an additional element A (Be, Mg, Ca, Sr, Ba, Zn, Cd, Hg , B, Al, Ga, In, Tl) or additional element B (N, P, As, Sb, Bi, O, S, Se, Te)
Thermoelectric conversion material containing up to 10 atomic%. 5. The method according to claim 1, wherein at least one of Ge, C, and Sn is selected.
0.1 to 5 atomic% of the seed and the additive element A (Be, Mg, Ca, Sr, Ba, Zn, Cd,
Hg, B, Al, Ga, In, Tl) or additive element B (N, P, As, Sb, Bi, O, S,
A thermoelectric conversion material that is contained singly or in combination from the respective additive element groups of (Se, Te). 6. An additive element for forming a P-type or N-type semiconductor is dissolved in Si alone or in a composite so as to contain 0.001 at% to 20 at%, and the melt is quenched so that Si is mainly contained. Si
A method for producing a thermoelectric conversion material for obtaining a structure in which a rich phase of the additive element is formed at a grain boundary of the rich phase. 7. The method for producing a thermoelectric conversion material according to claim 6, wherein the melting method is arc melting or high-frequency melting. 8. The method according to claim 6, wherein the melting and cooling method is CZ.
A method for producing a thermoelectric conversion material, which is any of the FZ method, the FZ method, and the ZL method.