Rare earth element doped N-type Mg3Sb2-based thermoelectric material and preparation method thereof

Abstract

The invention discloses a rare earth element doped N-type Mg3Sb2-based thermoelectric material and a preparation method thereof, and belongs to the technical field of energy conversion. The chemical composition expression of the rare earth element doped N type Mg < 3 > Sb < 2 >-based thermoelectric material is Mg < 3-x > A < x > Sb < 1.5 > Bi < 0.49 > Te < 0.01 >, wherein A represents a rare earth element and can be selected from La, Tb, Er or Lu; 0 < x < = 0.05. According to the invention, a rare earth element is used as a donor dopant, a Mg3Sb2-based block thermoelectric material is obtained by combining vacuum solid-phase sintering with a rapid hot-pressing process, the Mg3Sb2-based block thermoelectric material is ground into a regular shape after being cut by a diamond wire, and Mg atmosphere annealing process treatment is carried out to compensate the Mg element volatilized in the heat treatment process, so that the thermoelectric material is obtained. The finally prepared N-type Mg3Sb2-based thermoelectric material shows relatively high conductivity and Seebeck coefficient and low thermal conductivity, has relatively excellent comprehensive thermoelectric performance, and has relatively great application potential in the technical field of energy conversion.

Description

Technical Field

[0001] The present invention belongs to the field of new energy materials, and in particular relates to a rare earth element-doped N-type Mg3Sb2-based thermoelectric material and a preparation method thereof. Background Art

[0002] Thermoelectric technology realizes the direct conversion between heat and electricity, and is an effective way to alleviate the shortage of fossil fuels and promote sustainable energy development. The thermoelectric performance of a material mainly depends on the dimensionless thermoelectric figure of merit ZT, which is defined as ZT=S 2 σT / κ, where σ is the conductivity, S is the Seebeck coefficient, and S 2 σ is also called the power factor (PF), and T is the absolute temperature. κ=κ L +κ e , is the total thermal conductivity, through the formula κ = DρC p Calculated, where D is the thermal diffusion coefficient, ρ is the sample density, C p is the isobaric specific heat, according to the Dulong-Petit formula C p =3nR / M, κ L is the lattice thermal conductivity, κ e is the electronic thermal conductivity. A high ZT value means higher energy conversion efficiency and potentially higher output power, which is the core goal of thermoelectric material research.

[0003] In the medium temperature range, Mg3Sb2-based thermoelectric materials have the advantages of low toxicity and low cost. They have been a hot topic in the field of thermoelectrics in recent years. Mg3Sb2 is a semiconductor with a band gap of about 0.6 eV. Alloying Bi at the Sb site can adjust the band structure and reduce the lattice thermal conductivity. Mg3(Sb,Bi)2 has an intrinsic low lattice thermal conductivity due to its complex crystal structure. At the same time, the high energy valley degeneracy of N-type Mg3(Sb,Bi)2 ensures its excellent electrical transport properties, which is conducive to obtaining high thermoelectric performance. At present, researchers usually dope Te elements at the anion site of Mg3(Sb,Bi)2 to achieve N-type conduction, but its thermoelectric performance is average and it is still a long way from entering the commercial stage. Therefore, it is of great significance to find a new N-type Mg3Sb2-based thermoelectric material with high thermoelectric performance. Summary of the invention

[0004] In view of the above-mentioned field needs, the purpose of the present invention is to provide a rare earth element doped N-type Mg3Sb2-based thermoelectric material and a preparation method thereof. On the basis of Te doping, the present invention selects suitable rare earth elements as N-type dopants for the cation sites of Mg3(Sb,Bi)2. The advantage of rare earth element doping is that compared with the Mg element in the substrate, the atomic mass is greatly different, which can cause a large mass fluctuation in the lattice, thereby generating a strong phonon point defect scattering, and reducing the lattice thermal conductivity of the material; at the same time, due to the weak shielding effect of the 4f electrons of the rare earth elements on the nuclear charge, the atomic nucleus strongly attracts the outer electrons, resulting in a smaller ion radius, that is, a smaller radius difference compared with Mg, and a weaker disturbance to the periodic potential, thereby avoiding the deterioration of the carrier mobility to a certain extent, and the rare earth elements as N-type dopants provide electrons to optimize the carrier concentration, and the two work together to improve the conductivity of the material. The resulting rare earth element-doped N-type Mg3Sb2-based thermoelectric material exhibits high electrical conductivity and Seebeck coefficient as well as low thermal conductivity, and has excellent overall thermoelectric performance, and has great application potential in the field of energy conversion technology.

[0005] To achieve the above object, the present invention is implemented by the following technical solutions: A rare earth element doped N-type Mg3Sb2-based thermoelectric material, the chemical composition expression is Mg 3- x A x Sb 1.5 Bi 0.49 Te 0.01 ; wherein A represents the doped rare earth element, selected from one or more of La, Tb, Er or Lu, and x represents the atomic percentage, 0<x≤0.05.

[0006] The rare earth element doped N-type Mg3Sb2-based thermoelectric material of the present invention has an electrical conductivity of 12900 ~ 71400 S m in the temperature range of 303 ~ 773 K. -1 , Seebeck coefficient is -144 ~ -300 μV K -1 , power factor is 11.2 ~21.4 μW cm -1 K -2 , the total thermal conductivity is 1.21 ~ 0.73 W m -1 K -1 , the lattice thermal conductivity is 0.99 ~ 0.51 W m -1 K -1 , ZT value is 0.30 ~ 1.67. The chemical composition expression is Mg 2.985 Er 0.15 Sb 1.5 Bi 0.49 Te0.01 When the temperature is between 303 and 773 K, the conductivity is between 20500 and 61100 S m -1 , Seebeck coefficient is -153 ~ -285 μV K -1 , power factor is 14.4 ~ 19.6 μW cm -1 K -2 , the total thermal conductivity is 1.20 ~ 0.77 W m -1 K -1 , the lattice thermal conductivity is 0.88 ~ 0.53 W m -1 K -1 , ZT value is 0.36 ~ 1.67.

[0007] Furthermore, the preparation method of the above rare earth element doped N-type Mg3Sb2-based thermoelectric material is to use rare earth elements as donor dopants, adopt vacuum solid phase sintering combined with rapid hot pressing method and then anneal in Mg atmosphere to prepare rare earth element doped Mg3Sb2-based thermoelectric material, thereby achieving effective doping of rare earth elements. The specific preparation method includes the following steps: (1) According to Mg 3-x A x Sb 1.5 Bi 0.49 Te 0.01 Mg powder, Sb powder, Bi powder, Te powder and A powder were weighed as raw materials in the stoichiometric ratio of each element, mixed evenly in an argon atmosphere glove box with a water and oxygen content of less than 1 ppm (to avoid oxidation of the single substance powder or reaction with water), cold pressed into blocks using a powder tablet press, placed in an alumina crucible and vacuum sealed in a quartz tube; (2) Place the quartz tube described in step (1) in a pit furnace and heat at 1 to 3 K min -1 The temperature was raised to 950-1000 K at a rate of 1.5-2.0 °C for high temperature solid phase sintering, kept at this temperature for 80-160 h, and then heated at 1-3 K min -1 The temperature was lowered to room temperature at a rate of , and the product in the quartz tube was taken out; (3) The product obtained in step (2) was ground into powder and loaded into a graphite mold with an outer diameter of 50 mm and an inner diameter of 13 mm, and then subjected to vacuum hot pressing sintering in a rapid hot press at a pressure of 60 to 100 MPa at a rate of 1 to 3 K min. -1 After heating to 400 °C at a rate of 3 ~ 9 K min -1 The temperature was raised to 600°C at a rate of 10 to 30 min, and the hot pressing time was 10 to 30 min to obtain a disc-shaped Mg3Sb2-based thermoelectric material. (4) The product obtained in step (3) was cut and polished to obtain a block of Mg3Sb2-based thermoelectric material with a regular shape and smooth surface, which was placed in an alumina crucible and wrapped with Mg chips. The crucible was then encapsulated in a quartz tube with an outer diameter of 20 mm and an inner diameter of 17 mm at a vacuum degree of less than 1.5 Pa. The crucible was heated in a muffle furnace at 5 to 10 K min -1 The temperature was raised to 803-963 K at a rate of 1.5-2 h, then heated at a rate of 5-10 K min -1 rate to room temperature.

[0008] According to the above scheme, the total amount of sample weighed in step (1) is 4 to 5 g, the cold pressing pressure is 390 to 780 MPa, the air pressure in the tube during vacuum sealing is 0 to 1.5 Pa, the outer diameter of the quartz tube is 15 mm, and the inner diameter is 12.5 mm.

[0009] Based on the above technical solution, the beneficial effects of the present invention are: 1. The N-type Mg3Sb2-based thermoelectric material of the present invention exhibits high electrical conductivity and Seebeck coefficient and low thermal conductivity, and has excellent comprehensive thermoelectric performance; and is phase pure and has high density; 2. The present invention uses rare earth element A as an N-type dopant to provide electrons, optimizes the carrier concentration, and obtains a higher power factor (21.4 μW cm -1 K -2 At the same time, the rare earth element atoms introduce large mass fluctuations in the lattice, resulting in strong phonon point defect scattering, which reduces the lattice thermal conductivity (from 0.62 W m-1 at 773 K to 0.62 W m-1 at 773 K). -1 K -1 After doping, it decreased to 0.51 W m -1 K -1 ), a higher ZT value (1.67) was obtained; 3. The present invention adopts vacuum solid phase sintering combined with rapid hot pressing process and Mg atmosphere annealing process, which can also achieve the preparation of N-type Mg3Sb2-based thermoelectric materials compared with the traditional high-energy ball milling process, but avoids the shortcomings of easy adhesion and low yield of ball milling process samples, and has the advantages of simpler and more efficient preparation process and convenient large-scale synthesis. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 X-ray diffraction patterns of thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention; Figure 2 The data graph of the change of the conductivity (σ) of the thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention with the temperature; Figure 3 A data graph showing the Seebeck coefficient (S) of the thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention as a function of temperature; Figure 4 A data graph showing the power factor (PF) of the thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention as a function of temperature; Figure 5 A data graph showing the total thermal conductivity (κ) of the thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention as a function of temperature; Figure 6 The lattice thermal conductivity (κ) of the thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention is L ) Data graphs that vary with temperature; Figure 7 The data graph shows that the thermoelectric figure of merit (ZT) of the thermoelectric materials prepared in Example 1, Example 2, Example 3 and Comparative Example 1 of the present invention varies with temperature. DETAILED DESCRIPTION

[0011] In order to further understand the present invention, the embodiments of the present invention are specifically described below in conjunction with examples, but they should not be construed as limiting the scope of protection of the present invention.

[0012] Comparative Example 1 An N-type Mg3Sb2-based thermoelectric material and a preparation method thereof, wherein the chemical composition of the thermoelectric material is Mg 3.0 Sb 1.5 Bi 0.49 Te 0.01 The specific steps of the preparation method are as follows: Step 1: According to Mg 3.0 Sb 1.5 Bi 0.49 Te 0.01 The stoichiometric ratio of each element in the Mg powder, Sb powder, Bi powder and Te powder were weighed as raw materials in an argon atmosphere glove box with water and oxygen content less than 1 ppm, and then mixed evenly in an agate mortar; Step 2: cold press the mixture powder obtained in step 1 into a block at a pressure of 585 MPa, then place it in an alumina crucible and vacuum seal it in a quartz tube; then, place the quartz tube in a pit furnace for high-temperature solid-phase sintering reaction. The conditions for the high-temperature solid-phase sintering reaction are: 1 K min -1 The temperature was raised to 973 K at a rate of 1 K min -1 The rate of decrease to room temperature; Step 3: Take the product obtained in step 2 out of the quartz tube, grind it into powder in an agate mortar, and then perform vacuum hot pressing sintering at a sintering temperature of 873 K, a sintering pressure of 80 MPa, and a sintering time of 20 min to obtain a high-density Mg 3.0 Sb 1.5 Bi 0.49 Te 0.01 Bulk thermoelectric materials; Step 4: Cut and polish the product obtained in step 3 to obtain a block of Mg3Sb2-based thermoelectric material with regular shape and smooth surface, place it in an alumina crucible and wrap it with Mg chips, and then seal it in a quartz tube when the vacuum degree is lower than 1.5 Pa. -1 The temperature was raised to 873 K at a rate of 1.5 K / min, maintained for 1 h, and then heated at 8 K / min -1 rate to room temperature.

[0013] The Mg prepared in this comparative example 3.0 Sb 1.5 Bi 0.49 Te 0.01 The XRD diffraction pattern of the thermoelectric material maintains the Mg3Sb2 phase ( Figure 1 ), the conductivity is 26260 ~ 12870 S m at 303 ~ 773 K -1 ( Figure 2 ); Seebeck coefficient is -206 ~ -301μV K -1 ( Figure 3 ); power factor is 11.2 ~ 15.5 μW cm -1 K -2 ( Figure 4 ); total thermal conductivity is 1.12 ~ 0.77 W m -1 K -1 ( Figure 5 ); lattice thermal conductivity is 0.99 ~ 0.60 W m -1 K -1 ( Figure 6 ); ZT value is 0.30 ~ 1.17 ( Figure 7 ).

[0014] Example 1

[0015] A method for preparing Er-doped Mg3Sb2-based thermoelectric material, wherein the chemical composition of the thermoelectric material is Mg 2.995 Er 0.005 Sb 1.5 Bi 0.49 Te 0.01 The specific steps of the preparation method are as follows: Step 1: According to Mg2.995 Er 0.005 Sb 1.5 Bi 0.49 Te 0.01 The stoichiometric ratio of each element in the mixture was Mg powder, Er powder, Sb powder, Bi powder and Te powder were weighed as raw materials in an argon atmosphere glove box with water and oxygen content less than 1 ppm, and then mixed evenly in an agate mortar; Step 2: cold press the mixture powder obtained in step 1 into a block at a pressure of 585 MPa, then place it in an alumina crucible and vacuum seal it in a quartz tube; then, place the quartz tube in a pit furnace for high-temperature solid-phase sintering reaction. The conditions for the high-temperature solid-phase sintering reaction are: 1 K min -1 The temperature was raised to 973 K at a rate of 1 K min -1 The rate of decrease to room temperature; Step 3: Take the product obtained in step 2 out of the quartz tube, grind it into powder in an agate mortar, and then perform vacuum hot pressing sintering at a sintering temperature of 873 K, a sintering pressure of 80 MPa, and a sintering time of 20 min to obtain a high-density Mg 2.995 Er 0.005 Sb 1.5 Bi 0.49 Te 0.01 Bulk thermoelectric materials; Step 4: Cut and polish the product obtained in step 3 to obtain a block of Mg3Sb2-based thermoelectric material with regular shape and smooth surface, place it in an alumina crucible and wrap it with Mg chips, and then seal it in a quartz tube when the vacuum degree is lower than 1.5 Pa. -1 The temperature was raised to 873 K at a rate of 1.5 K / min, maintained for 1 h, and then heated at 8 K / min -1 rate to room temperature.

[0016] The Mg prepared in this example 2.995 Er 0.005 Sb 1.5 Bi 0.49 Te 0.01 The XRD diffraction pattern of the thermoelectric material maintains the Mg3Sb2 phase, and the conductivity is 53340 ~ 17210 S m at 303 ~ 773 K. -1 ; Seebeck coefficient is -178 ~ -292μV K -1 ; Power factor is 14.7 ~ 20.2 μW cm -1 K -2 ; Total thermal conductivity is 1.16 ~ 0.73 W m -1 K -1; Lattice thermal conductivity is 0.90 ~ 0.53 W m -1 K -1 ; ZT value is 0.44 ~ 1.55.

[0017] In Example 1, the electrical conductivity and power factor are greatly improved by doping with rare earth elements. The lattice thermal conductivity is reduced due to point defects enhancing phonon scattering, which is 11.7% lower than that of Control Example 1. The maximum ZT value is 1.55, which is 32.5% higher than that of Control Example 1, which is 1.17.

[0018] Example 2

[0019] A method for preparing Er-doped Mg3Sb2-based thermoelectric material, wherein the chemical composition of the thermoelectric material is Mg 2.985 Er 0.015 Sb 1.5 Bi 0.49 Te 0.01 The preparation method is referred to Example 1, except that in step 1, the raw materials Mg powder, Er powder, Sb powder, Bi powder and Te powder are prepared according to the chemical composition Mg 2.985 Er 0.015 Sb 1.5 Bi 0.49 Te 0.01 The stoichiometric ratio is weighed.

[0020] The Mg prepared in this example 2.985 Er 0.015 Sb 1.5 Bi 0.49 Te 0.01 The electrical conductivity of thermoelectric materials at 303-773 K is 61120 ~ 20580 S m -1 ; Seebeck coefficient is -153 ~ -285 μV K -1 ; Power factor is 14.4 ~ 19.6 μW cm -1 K -2 ; Total thermal conductivity is 1.20 ~ 0.77 W m -1 K -1 ; Lattice thermal conductivity is 0.88 ~ 0.53 W m -1 K -1 ; ZT value is 0.36 ~ 1.67.

[0021] In Example 2, the electrical conductivity and power factor of the material are greatly improved by doping with rare earth elements. The lattice thermal conductivity is reduced due to point defects enhancing phonon scattering, which is 11.7% lower than that of Control Example 1. The maximum ZT value is 1.67, which is 42.7% higher than that of Control Example 1, which is 1.17.

[0022] Example 3

[0023] A method for preparing Er-doped Mg3Sb2-based thermoelectric material, wherein the chemical composition of the thermoelectric material is Mg 2.975 Er 0.025 Sb 1.5 Bi 0.49 Te 0.01 The preparation method is referred to Example 1, except that in step 1, the raw materials Mg powder, Er powder, Sb powder, Bi powder and Te powder are prepared according to the chemical composition Mg 2.975 Er 0.025 Sb 1.5 Bi 0.49 Te 0.01 The stoichiometric ratio is weighed.

[0024] The Mg prepared in this example 2.975 Er 0.025 Sb 1.5 Bi 0.49 Te 0.01 The electrical conductivity of thermoelectric materials at 303 ~ 773 K is 71430 ~ 22210 S m -1 ; Seebeck coefficient is -144 ~ -270 μV K -1 ; Power factor is 14.8 ~ 21.4 μW cm -1 K -2 ; Total thermal conductivity is 1.20 ~ 0.78 W m -1 K -1 ; Lattice thermal conductivity is 0.81 ~ 0.51 W m -1 K -1 ; ZT value is 0.37 ~ 1.63.

[0025] In Example 3, the electrical conductivity and power factor of the material are greatly improved by doping with rare earth elements. The lattice thermal conductivity is reduced due to point defects enhancing phonon scattering, which is 15% lower than that of Control Example 1. The maximum ZT value is 1.63, which is 39.3% higher than the maximum ZT value of 1.17 in Control Example 1.

[0026] In addition to Er selected in the embodiment, when other rare earth elements are selected as dopants in the N-type Mg3Sb2 system, the present invention can also achieve the effect of optimizing the carrier concentration and introducing point defects to reduce the lattice thermal conductivity, and the comprehensive thermoelectric performance will be improved to a certain extent.

[0027] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and changes without departing from the creative concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A rare earth element doped N-type Mg3Sb2-based thermoelectric material, characterized in that The chemical composition expression is Mg 3- x A x Sb 1.5 Bi 0.49 Te 0.01 ; Wherein x represents atomic percentage, 0<x≤0.

05.

2. The rare earth element doped N-type Mg3Sb2-based thermoelectric material according to claim 1, characterized in that A is selected from one or more of La, Tb, Er and Lu.

3. The rare earth element doped N-type Mg3Sb2-based thermoelectric material according to claim 1, characterized in that In the temperature range of 303~773 K, the conductivity is 12900 ~ 71400 S m -1 , Seebeck coefficient is -144 ~ -300 μVK -1 , power factor is 11.2 ~ 21.4 μW cm -1 K -2 , the total thermal conductivity is 1.21 ~ 0.73 W m -1 K -1 , the lattice thermal conductivity is 0.99 ~ 0.51 W m -1 K -1 , ZT value is 0.30 ~ 1.

67.

4. The rare earth element doped N-type Mg3Sb2-based thermoelectric material according to claim 1, characterized in that The chemical composition expression is Mg 2.985 Er 0.015 Sb 1.5 Bi 0.49 Te 0.01 When the temperature is between 303 and 773 K, the conductivity is between 20500 and 61100 S m -1 , Seebeck coefficient is -153 ~ -285 μV K -1 , power factor is 14.4 ~ 19.6 μW cm -1 K -2 , the total thermal conductivity is 1.20 ~ 0.77 W m -1 K -1 , the lattice thermal conductivity is 0.88 ~ 0.53 W m -1 K -1 , ZT value is 0.36 ~ 1.

67.

5. The method for preparing the rare earth element-doped N-type Mg3Sb2-based thermoelectric material according to any one of claims 1 to 4, characterized in that It includes the following steps: (1) According to Mg 3-x A x Sb 1.5 Bi 0.49 Te 0.01 Mg powder, Sb powder, Bi powder, Te powder and A powder were weighed as raw materials in the stoichiometric ratio of each element, mixed evenly in an argon atmosphere glove box with a water and oxygen content of less than 1 ppm (to avoid oxidation of the single substance powder or reaction with water), cold pressed into blocks using a powder tablet press, placed in an alumina crucible and vacuum sealed in a quartz tube; (2) Place the quartz tube described in step (1) in a pit furnace and heat at 1 to 3 K min -1 The temperature was raised to 950-1000 K at a rate of 1.5 K min for high temperature solid phase sintering, kept at this temperature for 80-160 h, and then heated at 1-3 K min -1 The temperature was lowered to room temperature at a rate of , and the product in the quartz tube was taken out; (3) The product obtained in step (2) was ground into powder and loaded into a graphite mold with an outer diameter of 50 mm and an inner diameter of 13 mm, and then subjected to vacuum hot pressing sintering in a rapid hot press at a pressure of 60 to 100 MPa at a rate of 1 to 3 K min. -1 After heating to 400 °C at a rate of 3 ~ 9 K min -1 The temperature was raised to 600℃ at a rate of 10 ~ 30 min, and the hot pressing time was 10 ~ 30 min to obtain a disc-shaped Mg3Sb2-based thermoelectric material; (4) The product obtained in step (3) was cut and polished to obtain a block of Mg3Sb2-based thermoelectric material with a regular shape and smooth surface, which was placed in an alumina crucible and wrapped with Mg chips. The crucible was then encapsulated in a quartz tube with an outer diameter of 20 mm and an inner diameter of 17 mm at a vacuum degree of less than 1.5 Pa. The crucible was heated in a muffle furnace at 5 to 10 K min -1 The temperature was raised to 803-963 K at a rate of 1.5-2.0 K / min, then kept at that temperature for 0.5-2 h, and then heated at a rate of 5-10 K / min. -1 rate to room temperature.

6. The method for preparing a rare earth element doped N-type Mg3Sb2-based thermoelectric material according to claim 5, characterized in that In the step (1), the total amount of sample weighed is 4 to 5 g, the cold pressing pressure is 390 to 780 MPa, the air pressure in the tube during vacuum sealing is 0 to 1.5 Pa, the outer diameter of the quartz tube is 15 mm, and the inner diameter is 12.5 mm.

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