Stabilised thermoelectric materials and methods for their manufacture

Abstract

The invention relates to the fields of physics and materials science and concerns stabilized thermoelectric materials, such as those used for thermoelectric components that can be used for generating electricity from waste heat or for generating cold, for use in microelectronics, in sensor networks or in the Internet of Things (IoT). The object of the present invention is to provide stabilized thermoelectric materials which in particular have long-term stable thermoelectric properties. The problem is solved by stabilized thermoelectric materials, which consist of at least one or more powders or powder components of thermoelectric materials, wherein at least the predominant number of powder particles or powder particle components of thermoelectric materials has a coating of a compound of metals and / or metal oxides and / or semiconductors and / or a mixture of these compounds, wherein the layer thickness of the coating is 0.1 to 100 nm.

Description

[0001] The invention relates to the fields of physics and materials science and concerns stabilized thermoelectric materials, such as those used for thermoelectric components that can be used to generate electricity from waste heat or to generate cold, for use in microelectronics, in sensor networks or in the Internet of Things (IoT), and a method for their production.

[0002] Thermoelectricity is the conversion of heat into electrical energy and vice versa. The Seebeck effect, also called the thermoelectric effect, the Peltier effect, and the Thomson effect each describe a reversible interaction between the two physical quantities temperature and electricity (Wikipedia, keyword thermoelectricity), for example, a temperature difference that generates an electrical voltage.

[0003] Thermoelectric (TE) devices or components, such as a thermocouple, always consist of a pair of electrical conductors, also called thermoelectric legs or p- and n-legs, made of different materials and connected at one end. As soon as a temperature gradient occurs along one of the electrical conductors, a thermoelectric voltage is generated, which can be measured as an electrical voltage (Wikipedia, keyword "thermocouple").

[0004] Thermoelectric materials that enable direct energy conversion between heat and electricity are a potential solution to the energy problem. Many superionic or mixed-ion electronic conductors with a liquid-like sub-lattice, such as Cu2(S, Se, Te), Ag2(S, Se, Te), ZnSb-based materials, and MgAgSb, have been identified as highly efficient thermoelectric materials. Their excellent thermoelectric properties are primarily due to their low lattice thermal conductivities.

[0005] However, their applications are limited due to the movement of atoms and the formation of a second phase when exposed to high electronic currents and large temperature gradients.

[0006] ZnSb-based thermoelectric materials (TE) feature low-cost and non-toxic elements and relatively high zT values ​​due to their extremely low thermal conductivity, making them one of the most promising TE materials in medium operating temperature ranges of 500-900 K.

[0007] However, it is disadvantageous that the thermal stability of various thermoelectric materials, such as β-Zn4Sb3 and ZnSb with Zn atoms on the interstitial sites, deteriorates under large temperature differences and electric fields.

[0008] Likewise, the temperature gradient and current cause the interstitial atoms to migrate along the direction of the temperature gradient or current flow. For example, Zn atoms migrate from the hot to the cold side of a thermoelectric leg and are ejected from the sample as Zn whiskers. Due to this atom migration, the part near the hot side exhibits larger Seebeck values ​​than the cold side. After several thermal cycles, the Zn loss significantly degrades the thermoelectric properties.

[0009] Another example is that in the Cu2Se compound, the mobile Cu atoms tend to deposit on the surface of the cathode, forming Cu metal, which partially oxidizes in air when an external electric field or a temperature gradient is applied. If Cu is not supplied on the anode side, Cu metal deposition on the cathode changes the original material composition and degrades the TE performance.

[0010] In TE devices with mixed ionic-electronic conduction, Cu metal deposits on the sample surface can also damage the contact between the TE material and the electrode by forming cracks, increasing the electrical / thermal resistance and deteriorating the power output and conversion efficiency.

[0011] In addition, the deposition of Cu at the cathode can lead to the evaporation of chalcogenides (S, Se, Te) at the anode, which can result in the failure of the legs of a thermoelectric device.

[0012] Therefore, it is of great importance to improve the stability of TE materials that are prone to performance degradation while maintaining their high thermoelectric performance.

[0013] To solve the problem of TE material stability, various manufacturing processes have been developed and investigated.

[0014] For example, elements doped into the crystal structure of TE materials, such as Mg, Ag, and Pb, were used to substitute the Zn site in β-Zn4Sb3 to prevent the unwanted movement of atoms (Hung, LT et al: Adv. Electron. Mater. 3, 1-8 (2017)).

[0015] In another example, elemental Li was doped into Cu2Se to suppress the diffusivity of Cu ions and improve the reliability of the connection (Kang, SS et al: Mater. Today Phys. 1, 7-13 (2017)).

[0016] Bailey, TP et al.: J. Mater. Chem. A4, 17225-17235 (2016) attempted to exploit the effect of Sn doping on the Cu site. They found that while increasing the Sn dopant content reduces Cu precipitation, this is not sufficient to effectively solve the problem of material degradation.

[0017] Another method to improve the stability of TE materials is to reduce the critical electrical potential difference of the incoming atoms by segmentation.

[0018] For example, Thorup PS et al.: Chem. Mater. 34, 5206-5214 (2022) claimed that the thermal stability of Zn4Sb3 legs can be improved by segmenting them with stainless steel foil. Segmentation should reduce the critical electrical potential difference of Zn and suppress the migration of Zn atoms.

[0019] The thermoelectric performance decreased significantly after segmentation using this method. Furthermore, the mechanical properties of the Zn4Sb3 materials deteriorated due to the different thermal expansion coefficients between the stainless steel foil and the Zn4Sb3.

[0020] For Cu2Se, Qiu, P. et.al: Nat. Commun. 9, 4-11 (2018) also attempted to use segmentation with conductive carbon paste as a Cu atom barrier layer.

[0021] Here too, sufficient stabilization of the TE material could not be achieved.

[0022] Furthermore, attempts were made to realize the formation of discontinuous interfacial modifications by incorporating a second phase into an unstable TE compound.

[0023] Second phases, such as oxides (e.g., ZnO and TiO2), or semiconductors (e.g., Cu3SbSe4), or carbonaceous materials, have been successfully used to prevent ion migration into TE materials. However, particle distribution uniformity is an important factor for thermoelectric performance, which is complicated by the incorporation of the second phase. Furthermore, the small particle sizes of the second phases require higher-temperature fabrication conditions.

[0024] All known methods for improving the stability of TE materials have not been able to completely prevent atom and / or ion migration, have only created limited barriers against atom and / or ion migration, which then also degrade the thermoelectric and mechanical performance of the TE materials, and increase the requirements for the processes by incorporating a second phase, which makes the previous solutions unsuitable, especially for industrial use.

[0025] The object of the present invention is to provide stabilized thermoelectric materials which, in particular, have long-term stable thermoelectric properties and can be produced by means of a simple and cost-effective process.

[0026] The object is achieved by the invention defined in the claims. Advantageous embodiments are the subject of the dependent claims, whereby the invention also includes combinations of the individual dependent claims in the sense of an AND connection, as long as they are not mutually exclusive.

[0027] The stabilized thermoelectric materials according to the invention consist of at least one or more powders or powder constituents of thermoelectric materials, in which the at least predominant number of powder particles or powder particle constituents of thermoelectric materials have a coating of a compound of metals and / or metal oxides and / or semiconductors and / or a mixture of these compounds, wherein the layer thickness of the coating is 0.1 to 100 nm.

[0028] Advantageously, thermoelectric materials Zn4Sb3, Cu2(S, Se, Te), Ag2(S, Se, Te), Cu7PSe6, Cu12 Sb4S 13 , MgAgSb and / or argyrodite materials such as Ag8GeS6, Ag8SnSe6 are present.

[0029] Furthermore, the thermoelectric materials advantageously have an average particle size between 1 and 100 µm.

[0030] Also advantageously, 55 to 100%, advantageously 75 to 100%, even more advantageously 85 to 95% of all powder particles made of thermoelectric materials are provided with a coating.

[0031] And also advantageously at least 50%, advantageously 75 to 100%, of the surface of the powder particles of the thermoelectric materials are provided with a coating.

[0032] It is also advantageous if ceramic materials such as aluminum oxide, zirconium oxide, titanium oxide, silicon dioxide, and / or oxides of boron, strontium, barium, phosphorus, lead, tellurium, germanium, selenium, antimony, vanadium, hafnium, tantalum, zinc, lanthanum, yttrium, magnesium, calcium or mixtures of these materials or oxides, and / or metals such as Ti, Mn, Fe, Co, Ni, Cu, Zn, Se, Te, Zr, Nb, Ru, Pt, Ag, Pd, In, Sn, Au, Sb, and / or semiconductors such as (Sb,Bi)2(S,Se,Te)3, (Ti,Zn,Zr,Nb,W,Ag,Sn,In,Cu,Ge) x (S,Se,Te) y , (Ti,Si,Al,Ge,Nb,Ta) x (C,N,P) y are present.

[0033] It is also advantageous if the coating thickness is between 1 and 50 nm.

[0034] In the method according to the invention for producing stabilized thermoelectric materials, powder particles or powder particle components of one or more thermoelectric materials are provided with a coating of compounds of metals, metal oxides and / or semiconductors by a method for producing a uniform coating on the surface of the particles or particle components with a layer thickness of the coating of 0.1 to 100 nm.

[0035] Atomic layer deposition (ALD) is advantageously used to produce a uniform coating on the surface of the particles or particle components of the thermoelectric materials, whereby the ALD cycles can be repeated in order to achieve coating thicknesses of 0.1 to 100 nm.

[0036] The production of the thermoelectric materials is also advantageously carried out by means of arc melting, melt spinning, hydrothermal or solvothermal processes or by means of mechanical alloying in a ball mill.

[0037] And also advantageously, the powders of one or more thermoelectric materials are ground to an average particle size between 1 and 100 µm by means of a grinding process.

[0038] Furthermore, the surface of the powder particles is advantageously provided with a uniform coating by stirring the particles in a drum or fluidized bed reactor.

[0039] The invention makes it possible for the first time to provide stabilized thermoelectric materials which, in particular, have long-term stable thermoelectric properties, and to produce them by means of a simple and cost-effective process.

[0040] With the solution according to the invention, in particular the migration of atoms and / or ions of thermoelectric materials is inhibited and / or prevented and at the same time the thermoelectric properties of the TE materials are not impaired.

[0041] Likewise, the solution according to the invention significantly reduces and / or prevents the degradation of the thermoelectric properties during long-term use of TE materials.

[0042] This is achieved by stabilizing thermoelectric materials consisting of at least one or more powders or powder constituents of thermoelectric materials, in which the predominant number of powder particles or powder particle constituents of thermoelectric materials have a coating of compounds of metals and / or metal oxides and / or semiconductors and / or a mixture of these compounds, wherein the layer thickness of the coating is 0.1 to 100 nm.

[0043] Advantageously, thermoelectric materials Zn4Sb3, Cu2(S, Se, Te), Ag2(S, Se, Te), Cu7PSe6, Cu 12 Sb4S 13 , MgAgSb and / or argyrodite materials such as AgaGeS6, AgaSnSe6 are present.

[0044] The thermoelectric materials present according to the invention advantageously have an average particle size between 1 and 100 µm.

[0045] According to the invention, at least the predominant number of powder particles or powder particle components made of thermoelectric materials have a coating made of compounds of metals, metal oxides and / or semiconductors and / or mixtures of these materials.

[0046] Advantageously, 55 to 100%, even more advantageously 75 to 100%, and again advantageously 85 to 95% of all powder particles made of thermoelectric materials are provided with a coating.

[0047] Likewise, advantageously at least 50%, advantageously 75 to 100% of the surface of the powder particles of the thermoelectric materials are provided with the coating.

[0048] Furthermore, ceramic materials such as aluminum oxide, zirconium oxide, titanium oxide, silicon dioxide, and / or oxides of boron, strontium, barium, phosphorus, lead, tellurium, germanium, selenium, antimony, vanadium, hafnium, tantalum, zinc, lanthanum, yttrium, magnesium, calcium or mixtures of these materials or oxides, and / or metals such as Ti, Mn, Fe, Co, Ni, Cu, Zn, Se, Te, Zr, Nb, Ru, Pt, Ag, Pd, In, Sn, Au, Sb, and / or semiconductors such as (Sb,Bi)2(S,Se,Te)3, (Ti,Zn,Zr,Nb,W,Ag,Sn,In,Cu,Ge) x (S,Se,Te) y , (Ti,Si,Al,Ge,Nb,Ta) x (C,N,P) y available.

[0049] Advantageously, the layer thickness of the coating is 1 to 50 nm.

[0050] The coating materials used according to the invention effectively hinder or block migration of atoms and / or ions of the TE materials, whereby the thermoelectric properties are not deteriorated at different operating temperatures and electric fields, even during long-term use, and the TE performance of the thermoelectric materials according to the invention is stabilized.

[0051] The coating acts as a barrier that effectively hinders and / or blocks the migration of atoms and / or ions during temperature differences and current fluctuations.

[0052] According to the invention, the stabilized thermoelectric materials according to the invention are produced by providing powder particles or powder particle components of one or more thermoelectric materials with a coating of compounds of metals, metal oxides and / or semiconductors by a process for producing a uniform coating on the surface of the particles or particle components with a layer thickness of the coating of 0.1 to 100 nm.

[0053] Atomic layer deposition (ALD) can advantageously be used as a method for producing a uniform coating on the surface of the particles or particle components of the thermoelectric materials, whereby the ALD cycles can be repeated in order to achieve coating thicknesses of 0.1 to 100 nm.

[0054] The thermoelectric materials can be produced by arc melting, melt spinning, hydrothermal, solvothermal processes or by mechanical alloying in a ball mill.

[0055] The powders of one or more thermoelectric materials are advantageously ground to an average particle size between 1 and 100 µm by means of a grinding process.

[0056] Using atomic layer deposition, the material to be deposited is chemically bound to one or more carrier gases in the form of at least two precursor materials. These precursors are alternately fed into a reaction chamber and reacted with the substrate, whereupon the reaction product is deposited on the substrate material. A rinsing step can be performed between or after this step. Such a coating process is also known as an ALD cycle, which must be repeated several times during the process to achieve the desired layer thickness. Once the substrate surface is completely covered, deposition stops, making the process self-controlling and self-limiting with regard to layer thickness (Wikipedia, keyword "atomic layer deposition").

[0057] As a deposition process, atomic layer deposition (ALD) is recognized as a unique method for achieving the deposition of highly uniform layers in a controlled manner on flat and uneven surfaces.

[0058] The coating materials are advantageously used as precursor materials in chemical form bound to one or more carrier gases.

[0059] Also advantageously, the surface of the powder particles is provided with a uniform coating by stirring the particles in a drum or fluidized bed reactor.

[0060] After the coating has been applied to the powder particles of the thermoelectric materials, the powders are pressed into molded parts or pressed into a bulk material and subsequently sintered.

[0061] The stabilized thermoelectric materials according to the invention can also be processed into n-type and p-type legs or thermoelectric modules. These molded parts made from the stabilized thermoelectric materials according to the invention can also be further coated, for example, with aluminum oxide, zirconium oxide, titanium oxide, silicon dioxide, oxides of boron, strontium, barium, and phosphorus, lead, tellurium, germanium, selenium, antimony, vanadium, hafnium, tantalum, zinc, lanthanum, yttrium, magnesium, calcium, or ceramic materials. This additional coating can also be applied using ALD.

[0062] The properties of sintered coated thermoelectric materials can be determined or calculated using various methods.

[0063] The temperature-dependent parameters S and σ can be measured using the standard four-probe method (LSR-3, Linseis).

[0064] The temperature-dependent thermal diffusivity (λ) can be determined using a laser flash method in a helium atmosphere (LFA 1000, Linseis). The density (ρ) of the samples can be measured using the Archimedes method. The heat capacity (C p ) can be calculated according to the Dulong-Petit law and the thermal conductivity (K tot ) according to the relationship K tot = λ p C p be calculated.

[0065] The process according to the invention allows the formation of countless atom and / or ion migration barriers. The thickness of the coating can be precisely adjusted by varying the number of ALD cycles, making it possible to inhibit and / or prevent atom and / or ion migration without degrading the thermoelectric performance.

[0066] Furthermore, due to the self-limiting nature of ALD reactions, the thickness of the coating layers does not depend on the powder surface, making ALD a scalable technique for coating powders on a kilogram scale.

[0067] The invention is explained in more detail below using an exemplary embodiment. Example 1

[0068] Zn4Sb3 is produced by ball milling high-purity elemental Zn (99.9%, shot) and Sb (99.99%, shot in stoichiometric ratios) in a ball mill. The average particle size is 50 µm.

[0069] Two grams of the resulting Zn4Sb3 powder are introduced into the coating tube of an ALD reactor (Veeco Savannah S200). The powder is coated in the reactor at a pressure of 26 Pa. The coating tube rotates at a speed of 2 rpm.

[0070] Diethylzinc (DEZ, Zn 52.0%, Aldrich) as precursor and Zn source and H2O as precursor and oxidant were introduced into the coating tube and deposited as a ZnO layer on the Zn4Sb3 powder particles at a deposition temperature of 423 K.

[0071] All precursors were kept at room temperature (293 K). A pulse / purge time of 0.015 / 15 s was used for both DEZ and H2O. The precursors were introduced into the reaction chamber in N2 gas at a flow rate of 20 sccm.

[0072] After 150 ALD cycles and a coating thickness of 30 nm under the above conditions, the coated TE powder was discharged from the reaction chamber.

[0073] The coated powder was then placed in a mold and compressed into pellets at a uniaxial pressure of 50 MPa. The pellets were simultaneously sintered using spark plasma sintering (SPS, AGUSPECSSSPS-210Gx) at 683 K for 10 min in a vacuum. The pellets had a relative density of over 94%.

[0074] By applying a 30 nm thick ZnO layer completely covering the surface of each Zn4Sb3 particle, the migration of Zn ions is effectively hindered without affecting the TE performance of the parent compound.

[0075] Legs for thermoelectric components are produced from the Zn4Sb3 particles stabilized with a ZnO layer according to the invention and coated with HfO2 and Al2O3. To this end, the legs are coated with a 200 nm-thick layer using ALD with tetrakis(dimethylamido)hafnium (TDMAHf, 99.99+% Hf, <0.2% Zr, Strem) and trimethylaluminum (TMA, 97%, Aldrich) and distilled water as the Hf, Al, and O source within 2000 ALD cycles at 423 K. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] Hung, L.T. et al: Adv. Electron. Mater. 3, 1-8 (2017

[0014] Kang, S. S. et al: Mater. Today Phys. 1, 7-13 (2017

[0015] Bailey, T. P. et.al: J. Mater. Chem. A4, 17225-17235 (2016

[0016] Thorup P. S. et.al: Chem. Mater. 34, 5206-5214 (2022

[0018] Qiu, P. et.al: Nat. Commun. 9, 4-11 (2018

[0020]

Claims

[1] Stabilised thermoelectric materials consisting of at least one or more powders or powder constituents of thermoelectric materials, in which the at least predominant number of powder particles or powder particle constituents of thermoelectric materials have a coating of a compound of metals and / or metal oxides and / or semiconductors and / or a mixture of these compounds, the layer thickness of the coating being 0.1 to 100 nm. [2] Stabilized thermoelectric materials according to claim 1, wherein the thermoelectric materials used are Zn4Sb3, Cu2(S, Se, Te), Ag2(S, Se, Te), Cu7PSe6, Cu 12 Sb4S 13 , MgAgSb and / or argyrodite materials such as Ag8GeS6, Ag8SnSe6 are present. [3] Stabilized thermoelectric materials according to claim 1, wherein the thermoelectric materials have an average particle size between 1 and 100 µm. [4] Stabilised thermoelectric materials according to claim 1, in which 55 to 100%, advantageously 75 to 100%, more advantageously 85 to 95% of all powder particles of thermoelectric materials are provided with a coating. [5] Stabilized thermoelectric materials according to claim 1, in which at least 50%, advantageously 75 to 100% of the surface of the powder particles of the thermoelectric materials are provided with a coating. [6] Stabilized thermoelectric materials according to claim 1, in which ceramic materials such as aluminum oxide, zirconium oxide, titanium oxide, silicon dioxide, and / or oxides of boron, strontium, barium, phosphorus, lead, tellurium, germanium, selenium, antimony, vanadium, hafnium, tantalum, zinc, lanthanum, yttrium, magnesium, calcium or mixtures of these materials or oxides, and / or metals such as Ti, Mn, Fe, Co, Ni, Cu, Zn, Se, Te, Zr, Nb, Ru, Pt, Ag, Pd, In, Sn, Au, Sb, and / or semiconductors such as (Sb,Bi)2(S,Se,Te)3, (Ti,Zn,Zr,Nb,W,Ag,Sn,In,Cu,Ge) x (S,Se,Te) y , (Ti,Si,Al,Ge,Nb,Ta) x (C,N,P) y are present. [7] Stabilized thermoelectric materials according to claim 1, wherein the layer thickness of the coating is from 1 to 50 nm. [8] A process for producing stabilized thermoelectric materials, in which powder particles or powder particle components of one or more thermoelectric materials are provided with a coating of compounds of metals, metal oxides and / or semiconductors by a process for producing a uniform coating on the surface of the particles or particle components with a layer thickness of the coating of 0.1 to 100 nm. [9] Method according to claim 8, in which atomic layer deposition (ALD) is used to produce a uniform coating on the surface of the particles or particle components of the thermoelectric materials, wherein the ALD cycles can be repeated in order to be able to realize layer thicknesses of the coating of 0.1 to 100 nm. [10] A method according to claim 8, wherein the thermoelectric materials are produced by means of arc melting, melt spinning, hydrothermal or solvothermal processes or by means of mechanical alloying in a ball mill. [11] Method according to claim 8, wherein the powders of one or more thermoelectric materials are comminuted by means of a grinding process to an average particle size of between 1 and 100 µm. [12] A process according to claim 8, wherein the surface of the powder particles is provided with a uniform coating by stirring the particles in a drum or fluidized bed reactor.