WO2010112956A1

WO2010112956A1 - Magnesium based nanocomposite materials for thermoelectric energy conversion

Info

Publication number: WO2010112956A1
Application number: PCT/IB2009/005488
Authority: WO
Inventors: Natalio Mingo Bisquert; Marc Plissonnier; Shidong Wang
Original assignee: Commissariat a lEnergie Atomique CEA; Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 2009-04-02
Filing date: 2009-04-02
Publication date: 2010-10-07
Anticipated expiration: 2011-10-02

Abstract

The invention is directed to nanocomposite materials based on a magnesium containing matrix with improved thermoelectric energy conversion capacity, a process for the preparation of such nanocomposite materials, and electronic components comprising at least one layer of such nanocomposite materials.

Description

MAGNESIUM BASED NANOCOMPOSITE MATERIALS FOR THERMOELECTRIC ENERGY CONVERSION

Thermoelectric materials are the building blocks of two types of devices (Nolas, G. S., Sharp, J., and Goldsmid, H. J., Thermoelectrics: Basic Principles and New Materials Developments, Springer (2006)): 1 - thermoelectric power generators, capable of converting a heat current into usable electric power, and

2- thermoelectric coolers, capable of refrigerating an object by the only use of electric currents.

These devices are technologically very interesting because they are purely solid state based, which means they do not have moving parts, they are free of vibrations, reliable, compact, and light weight. This makes them ideal for aerospace or microelectronics applications, for example. However, their efficiency is much lower than that of mechanical energy conversion devices. This limits their applicability in more conventional fields, like usual kitchen refrigerators for example. A direct measure of a material's ability to be part of a thermoelectric device is given by its dimensionless thermoelectric figure of merit, ZT, which can be defined as follows:

ZT = TσS²/κ where: - T is the temperature, σ is the electrical conductivity of the material, S is the Seebeck coefficient, and K the thermal conductivity of the material.

The performance of thermoelectric refrigeration and power generation is hindered by the low figure of merit ZT of currently known materials. At present, the best bulk thermoelectric materials have figure of merit ZT values around 1 at room temperature.

Nanostructured materials have shown hints of higher room temperature ZT~2 in two publications (Venkatasubramanian, R. et al, Thin-film Thermoelectric. Devices with

High Room-temperature Figures of Merit, Nature, Vol. 413, 11 Oct. 2001, pp. 597-602; Harman, T. C, Taylor, P. J., Walsh, M. P., LaForge, B. E., Quantum Dot Superlattice

Thermoelectric Materials and Devices, Science 297, pp. 2229 (2002)). A general and ambitious goal of thermoelectrics research is to produce a material with a figure of merit

ZT > 3 at room temperature. Such material would enable thermoelectric refrigeration with efficiencies comparable to those of pressure based refrigeration, thus opening the market for industrial thermoelectric refrigeration.

Important applications of thermoelectric power generation are realized at intermediate temperatures ranging between 400 and 100OK, this range being typical of operation of car engines and other systems, where thermoelectric energy harvesting can considerably improve the energy efficiency. However, there are several problems when using thermoelectric materials at these temperatures. For example, known thermoelectric materials, such as bismuth telluride (Bi₂Te₃), degrade rapidly at temperatures above 500K, and consequently are no longer good thermoelectric materials. On the other hand, other materials, such as silicon-germanium (SiGe), do not become good thermoelectric materials until temperatures close to 100OK. The current standard material useful at intermediate temperatures is lead-telluride (PbTe), which presents a peak figure of merit ZT around 0.8 at 600K. However, PbTe contains a toxic element (Pb) and a scarce one (Te), which makes its use expensive and limited due to many environmental regulations. Consequently, none of the materials of the prior art provides a satisfying solution neither in terms of low toxicity, thermoelectric energy conversion capacity at intermediate temperatures (400-1000K), nor from an economic point of view.

The present invention overcomes the inadequacies and disadvantages of the state of the art by providing non-toxic thermoelectric materials with thermoelectric figures of merit ZT, superior to 1.1 in the 400-1000K temperature range, and with improved electrical conductivity.

The thermoelectric materials of the invention respond satisfactorily to these needs and requirements.

One strategy to enhance materials' figure of merit ZT is the inclusion of nanoparticles (particles with sizes ranging between 1 and a few tens of nm) into the material, to form a "nanocomposite." The embedding material is known as the "matrix", and the nanoparticles are termed the "filler." This approach was successfully used by Shakouri and collaborators to enhance the figure of merit ZT of InGaAs alloys, using ErAs nanoparticles as a filler (Kim, W., Zide, J., Gossard, A., Klenov, D., Stemmer, S., Shakouri, A., and Majumdar, A., Thermal conductivity reduction and thermoelectric figure of merit increase by embedding nanoparticles in crystalline semiconductors, Phys. Rev. Lett. 96, 045901 (2006)). For the approach to result in enhanced figures of merit ZT, it appears to be crucial that the nanoparticles blend well into the matrix, without creating defects or dislocations which would negatively affect the electrical conductivity and lower the figure of merit ZT (Shakouri, A., Nanoscale Thermal Transport and Microrefrigerators on a Chip, Proceedings of IEEE, 94, 1613 (2006)). But even if this goal is fulfilled, there is no guarantee that the figure of merit ZT of the nanocomposite will be higher than that of the original matrix. It is not enough to just embed nanoparticles: their size, composition and concentration must be such that their effect on thermal conductivity, electrical conductivity, and Seebeck coefficient does effectively enhance the figure of merit ZT.

Zaitsev V. K. et al, Thermoelectrics handbook, Ch. 29, D. M. Rowe ed., Taylor and Francis, 2006, discloses Mg₂B^IV compounds (B^1V = Si, Ge, Sn) for thermoelectric applications, such materials not being disclosed in nanostructured form (only in bulk form).

Noda, Y., Kon, H., Furukawa, Y., Nishida, I.A., and Masumoto, K., Temperature dependence of thermoelectric properties of Mg₂Sio.6Geo.₄, Mater. Trans, JIM, 33, 851, 1992, discloses theoretical figure of merit values ZT of rø-type and /?-type Mg₂Sio._δGeo.₄ materials doped with antimony and silver, said values being estimated from the calculation of thermal conductivity at high temperature.

The invention solves the problems of the prior art by providing non-toxic materials comprising a magnesium alloy matrix with magnesium alloy nanoinclusions dispersed therein, showing improved thermoelectric energy conversion capacity with a peak figure of merit ZT higher than the values known for the bulk materials of the prior art in the intermediate temperature range 400-1000K.

A first object of the present invention is a nanocomposite material comprising a matrix based on a magnesium alloy with a plurality of magnesium alloy nano-sized inclusions distributed within said matrix.

The nanocomposite material of the invention comprises a matrix based on a magnesium alloy which can be described by the following formula (I):

Mg₂A_αB_βC_γD_δE_ε (I) wherein: ■ A, B, C, D and E are chemical elements of the periodic table,

■ α, β, Y, δ and ε are numbers, with:

- O ≤ α ≤ l,

- 0 < β < l,

- 0≤γ≤l, - O≤δ≤l,

- O≤ε≤l,

- α + β + γ + δ + ε=l,

- at least one of the numbers α, β, γ₅ δ or ε being superior to 0, and said nanocomposite material also comprising nanoinclusion materials of a magnesium alloy of formula (II) dispersed therein:

Mg₂AVBVCVDVEV (H) wherein:

■ A', B', C, D' and E' are chemical elements of the periodic table, identical or different from A, B, C, D and E,

^■ α', β', γ'₅ δ' and ε' are numbers, identical or different from α, β, γ, δ and ε, with:

- O ≤ α' ≤ l,

- O ≤ β' ≤ l,

- 0 ≤ γ' < l,

- O ≤ δ' ≤ l , - O ≤ ε' ≤ l,

- α' + β' + γ' + δ' + ε' = l, and

- at least one of the numbers α', β', γ', δ' or ε' being superior to 0.

Advantageously, the chemical elements A, B, C, D, E, A', B', C, D' and E' are selected from the chemical elements of column IV of the periodic table, which comprises the following elements: C, Si, Ge, Sn, Pb and Uuq. More preferably, the chemical elements A, B, C, D, E, A', B', C\ D' and E' are selected from the following chemical elements: C, Si, Ge and Sn.

In a preferred embodiment, the matrix is based on a magnesium alloy of formula

(I'): Mg₂Ge_xSi_ySn_z (?) wherein x, y and z are numbers comprised between 0 and 1, with x + y + z = 1, and at least one of the numbers x, y or z being superior to 0. Preferably, 0 ≤ x ≤ 0.1 and 0.2 ≤ y ≤ 0.8, and even more preferably 0.3 ≤ y ≤ 0.5. The preferred matrix is based on Mg₂Si₀.₄Sn₀.6. In the matrix, the numbers y and z have advantageously the same size order.

In another preferred embodiment, the nanoinclusion materials are made of a magnesium alloy of formula (H'):

wherein x', y' and z' are numbers comprised between 0 and 1, with x' + y' + z' = 1, and at least one of the numbers x', y' or z' being superior to 0. Preferably, 0 < x' ≤ 0.1 and y' » z' ≥ 0 or z' » y' > 0, the numbers y' and z' being very different each other. The preferred nanoinclusion materials are selected from Mg₂Si and Mg₂Sn. The matrix and the nanoinclusion materials can be made with the same type of materials, possibly with different compositions. According to an advantageous embodiment, the nanoinclusion materials density is very different than the matrix density.

The term "nanosized inclusions", as used herein, generally refers to material portions, such as nanoparticles, whose dimensions are equal or preferably inferior to 100 nm. For example, they can refer to nanoparticles having an average cross-sectional diameter in a range of about 1 nm to about 100 nm, or in a range of about 3 nm to about

30 nm.

The nano-sized inclusions can be randomly distributed within the composite, or the nano-sized inclusions can be distributed according to a pattern. Preferably, the matrix and nanoinclusion materials are of fluorite crystal structure.

The nanoinclusions are advantageously nanoparticles, which means that their shape is more or less regular and can be described as: spherical, ovoid, a flattened sphere, a flattened ovoid or a rod. The nanocomposite materials according to the invention can be characterized by the nanoparticles' radius, r, the half of their largest diameter, and V_f, the volumetric fraction of nanoinclusions within the matrix. More precisely, V_f is the volume ratio of nanoinclusions with regards to the matrix volume.

The volume fraction V_f is preferably included within the following limits: 0.1% < V_f < 10%

In addition, one or both of the matrix and the nanoinclusion materials can be doped with a dopant, like a «-type or p-type dopant. When a dopant is included in one or both of these materials, its concentration with regards to the matrix or the nanoinclusion materials is preferably less than 1% weight/weight. For example, boron or silver can be used as ap-type dopant and phosphorous or antimony as a n-type dopant.

The presence of nanoinclusion materials within the matrix leads to a nanocomposite material which exhibits a reduction in thermal conductivity relative to a homogeneous alloy made of the matrix material by a factor of at least 0.5, and preferably more than 0.6. Another advantage of the nanocomposite materials of the invention is their

^■ electrical conductivity (σ) which is very close to that of the matrix material. The Seebeck coefficient, S₃ of the nanocomposite materials can be superior or equal to that of the matrix material.

So, the nanocomposite materials of the invention exhibit a thermoelectric figure of merit ZT which is superior or equal to 1.5 at a temperature ranging between 500 and

100OK, or which is superior or equal to 0.3 at room temperature. The p-type doped nanocomposite materials according to the invention have a figure of merit ZT superior to 0.24 at a temperature of 450K, whereas the figure of merit ZT known from prior art for this kind of _.p-type nanocomposite materials is of 0 14 (Federov et al , Transport Properties of Mg₂Xo₄SnO₆ Solid Solutions (X = Si, Ge) with ^-Type Conductivity, Physics of the Solid State, 2006, Vol. 48, No. 8, pp 1486-1490) A nanocomposite thermoelectric composition according to the invention can exhibit a thermoelectric figure of merit ZT that can be greater than about 0.3 at room temperature. Since the figure of merit ZT of bulk alloy based on magnesium increases with temperature, the figure of merit ZT reaches the value of 1 1 at 800K. The nanoinclusion materials of the invention may surpass ZT = 1.6 at 800K. Another object of the invention is a process for the preparation of a nanocomposite material according to the invention, said process comprising at least the following steps

(i) growing a buffer layer on a substrate, and (ii) growing a nanocomposite structure on the buffer layer of step (i) Advantageously, the substrate is a Mg₂Si substrate

The buffer layer is intended to make the mesh parameter of the Mg₂Si substrate fit with that of the nanocomposite material, in order to reduce strains. According to a first favorite variant, the buffer layer can be continuous and have a gradient of mesh parameter from Mg₂Si to Mg₂Si_ySn_z by regularly modifying the Mg₂Si composition. According to the case, nanoinclusion materials have a crystalline structure or a solid solution structure Crystalline structure is preferred.

According to a first favorite embodiment, the process of the invention includes growing the nanocomposite materials by Molecular Beam Epitaxy (MBE).

In another preferred embodiment, the process of the invention includes growing the nanocomposite materials by a Chemical Vapor Deposition (CVD) method. In the CVD process, a Mg₂Si substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit.

The CVD method is more appropriate for industrial scale production of nanocomposite materials Suitable precursors for Si and Ge are silanes, dichlorosilanes, GeH₄

Metals sources are generally organic metals such as: cyclopendadienyl magnesium (Cp₂Mg) and tributyltm hydride (Bu₃SnH) precursors. Anhydrous MgCl₂ or MgSn₂ hydride precursors could also be used

For a random distribution nanocomposite, a giowth rate is selected which corresponds to the desired atomic fraction. When a patterned structure is expected, the alternate growth of Mg₂Si₀₄Sno ₆ layers and Mg₂Sn layers is combined with appropriate layer spacings Process parameters, such as precursors flow, temperature or total pressure, are controlled to obtain nanoparticle size and volumetric fraction V_f which have been selected.

Nanocomposite materials of the invention have improved thermoelectric figure of merit ZT and good electrical conductivity, thanks to the fact that nanoinclusion materials blend well into the matrix, without creating defects and dislocations.

Another object of the invention is an electronic component comprising at least one layer of the nanocomposite material according to the invention.

Advantageously such an electronic component is selected from: thermoelectric power generators and thermoelectric coolers.

Thermoelectric nanocomposite materials of the invention advantageously find applications in both refrigeration and power generation. For example, they can be used in thermal management of microelectronics and photonic devices. They can also be employed as thermoelectric power generators for direct conversion of thermal energy to electrical energy at a high efficiency.

In addition to the above provisions, the invention also comprises other provisions which will become clear from the description which follows, which refers to an example illustrating the performance of a nanocomposite material of the invention, and also to the attached drawing in which: Figure 1 schematically depicts a thermoelectronic cooler (1) formed as an assembly of thermoelectric elements, consisting of modules such as (2) and (3). The elements are electrically connected in series (but they could also be connected in parallel or as a combination of serial and parallel connections, the type of connection depends on the needs and power supplies) with current flowing alternatively through jo-type and n- type legs. The legs are formed of nanocomposite materials of the invention. The legs (2) and (3) of the devices are connected through electrically conductive bridges (4) to adjacent legs in a cascading fashion. Application of a current causes transfer of heat from one side of the thermoelectric cooler to the other, thereby lowering the temperature at one side while increasing the temperature at the opposite side.

Example;

The process is separated in two sequential steps that provide Mg₂Sio.₄Sno_.6 layer and Mg₂Sn dots on its surface. These two sequential steps are then repeated several times to form a Mg₂Sio.₄Sno_.6 nanocomposite material with Mg₂Sn inclusions. First sequential step:

Mg₂Sio.₄Sno.6 nanocomposite layer is grown by reduced pressure chemical vapor deposition (RP-CVD). This deposition technique has been already used in the literature to realize superlattices (Venkatasubramamian, R. et al., Nature, Vol. 413, 11 Oct., 2001, pp. 597-602; Shakouri, A., Proceedings of IEEE, 94, 1613 (2006); Kim, W. et al, Phys. Rev. Lett. 96, 045901 (2006)). CVD offers high quality layers and makes possible the use in- situ dopping. The growth pressure was 10 Torr. The flow of H₂ carrier gas was set at a fixed value of about 10 of standard liters per minute. Pure silane (SiH₄) was used as the source of Si. Cyclopendadienyl magnesium (Cp₂Mg) and tributyltin hydride (Bu₃ SnH) precursors were respectively used as the source of Mg and Sn. These precursors were injected in the CVD chamber with H₂ carriers.

Second sequential step: Mg₂Sn nanoinclusion materials are grown by injection of 600 seem of Cp₂Mg precursor in H₂ carrier flow and of 10 seem OfBu₃SnH precursor in H₂ carrier flow.

Cp₂Mg is vaporized in dedicated vaporization chamber set at a temperature of 100⁰C. Bu₃SnH is vaporized in dedicated vaporization chamber set at a temperature of 60⁰C. Cp₂Mg and Bu₃SnH vapors are then introduced into the chamber with H₂ carrier gas. Cp₂Mg and Bu₃SnH precursors are then chemically decomposed on Mg2Sio4Sno6 surface at 900⁰C under H₂ carrier flow. After two minutes (before layer coalescence) the Cp₂Mg and Bu₃SnH injections are stopped.

After Mg₂Sn, a new step of Mg₂Sio₄Sno₆ deposition is formed. The new Mg₂Si_o.4Sno.6 layer encapsulates the Mg₂Sn nanodots. These process steps can be repeated 2 to 100 times.

Following this sequence of operations, we have produced a material comprising 100 layers of 10 nm of Mg₂Si₀.₄Sn₀₆/Mg₂Sn with a figure of merit ZT = 1.6 at 800K (as a function of the temperature).

Claims

1. Nanocomposite material characterized in that it comprises a matrix based on a magnesium alloy of formula (I): Mg₂A_αB_βC_γD_δE_ε (I) wherein:

■ A, B, C, D and E are chemical elements of the periodic table,

■ α, β, Y, δ and ε are numbers, with:

- O≤α≤l, - O≤β≤l,

- 0≤γ<l,

- O≤δ≤l,

- O≤ε≤l,

- α + β + γ + δ + ε=l, - at least one of the numbers α, β, γ₃ δ or ε being superior to 0, and said nanocomposite material also comprising nanoinclusion materials made of a magnesium alloy of formula (II) dispersed therein:

Mg₂AVBVCyDVEV (II) wherein: ■ A', B', C, D' and E' are chemical elements of the periodic table, identical or different from A, B, C, D and E,

^■ α¹, β', Y', δ' and ε' are numbers, identical or different from α, β, y, δ and ε, with:

- O≤α'≤l, - O≤β'≤l,

- 0<γ'<l,

- O≤δ'≤l,

- O≤ε'≤l,

- α' + β'+γ' + δ' + ε' = l,and - at least one of the numbers α', β', Y', δ'orε' being superior to 0.

2. Nanocomposite material according to claim 1, characterized in that the chemical elements A, B, C, D, E, A', B', C, D' and E' are selected from the chemical elements of column IV of the periodic table.

3. Nanocomposite material according to claim 1 or claim 2, characterized in that said matrix is based on a magnesium alloy of formula (I'):

Mg₂Ge_xSi_xSn₂ (T) wherein x, y and z are numbers comprised between 0 and 1, with x + y + z = 1, and at least one of the numbers x, y or z being superior to 0.

4. Nanocomposite material according to claim 3, characterized in that:

^■ O ≤ x ≤ O.l, and

^■ 0.2 < y < 0.8, and preferably 0.3 < y < 0.5.

5. Nanocomposite material according to claim 4, characterized in that said matrix is based on a magnesium alloy of formula Mg₂Sio.₄Sno.₆.

6. Nanocomposite material according to anyone of claims 1 to 5, characterized in that said nanoinclusion materials are made of a magnesium alloy of formula (JT):

Mg₂Ge_x>StySn_z. (IF) wherein x', y' and z' are numbers comprised between 0 and 1, with x' + y' + z' = 1, and at least one of the numbers x', y' or z' being superior to 0.

7. Nanocomposite material according to claim 6, characterized in that:

^■ O ≤ x' ≤ O.l. and

^■ y' » z' ≥ 0 or z' » y' > 0.

8. Nanocomposite material according to claim 6, characterized in that said nanoinclusion materials are selected from Mg₂Si and Mg₂Sn.

9. Nanocomposite material according to anyone of claims 1 to 8, characterized in that said nanoinclusion materials are nanoparticles having preferably dimensions equal or inferior to 100 nm.

10. Nanocomposite material according to claim 9, characterized in that V_f, the volume ratio of nanoparticles with regards to said matrix volume is included within the following limits:

11. Nanocomposite material according to anyone of claims 1 to 10, characterized in that it exhibits a thermoelectric figure of merit ZT which is superior or equal to 1.5 at a temperature ranging between 500 and 100OK, or which is superior or equal to 0.3 at room temperature.

12. Process for the preparation of a nanocomposite material as defined in anyone of claims 1 to 11, characterized in that it comprises at least the following steps: (i) growing a buffer layer on a substrate, and

(ii) growing a nanocomposite structure on the buffer layer of step (i), preferably by a method selected from Molecular Beam Epitaxy (MBE) and Chemical Vapor Deposition (CVD).

13. Process according to claim 12, characterized in that the substrate is a Mg₂Si substrate.

14. Electronic component characterized in that it comprises at least one layer of a nanocomposite material as defined in anyone of claims 1 to 11.

15. Electronic component according to claim 14, characterized in that it is selected from thermoelectric power generators and thermoelectric coolers.