Half Heusler thermoelectric material with intrinsic vacancy defect and preparation method and application thereof

Abstract

The invention relates to the technical field of thermoelectric materials, and discloses a half-Heusler thermoelectric material with intrinsic vacancy defects and a preparation method and application thereof, the chemical formula of the half-Heusler thermoelectric material is Ti < 1-x > Nb < 0.8 x > CoSb, and the value range of x is 0 lt; xlt; 1. The novel half-Heusler thermoelectric material is designed by accurately controlling the stoichiometric ratio and introducing cation vacancy, and aims at reducing the heat conductivity and improving the thermoelectric performance. The preparation method comprises the steps of smelting, ball milling, sintering and the like, operation is easy and convenient, and large-scale production is easy to achieve. According to the material, a high thermoelectric figure of merit zT can be obtained at a high temperature under the condition that doping is not carried out for adjusting the carrier concentration, and a new way is provided for designing a half-Heusler material and improving thermoelectric performance.

Description

Technical Field

[0001] The invention relates to the technical field of thermoelectric materials, and in particular to a half-Heusler thermoelectric material with intrinsic vacancy defects, a preparation method and application thereof. Background Art

[0002] Thermoelectric materials, as functional materials that can realize the direct conversion between thermal energy and electrical energy, have the advantages of small size, no noise, high reliability, high stability and no pollution. They have shown important application needs in the fields of industrial waste heat power generation, Internet of Things self-supporting power supply, deep space exploration power supply, all-solid-state refrigeration and precise temperature control. Although thermoelectric materials have the advantages of being green and environmentally friendly and having a wide range of application temperatures, their disadvantages such as complex preparation process, high production cost and low conversion efficiency limit their widespread application. The performance of thermoelectric materials is closely related to their dimensionless thermoelectric figure of merit zT, zT=(S 2 σ / κ)T, where S represents the Seebeck coefficient, σ represents the electrical conductivity, and κ represents the thermal conductivity. The key to improving the performance of thermoelectric materials is to increase the electrical conductivity and Seebeck coefficient while reducing the thermal conductivity.

[0003] Among many thermoelectric material systems, half-Heusler semiconductors have shown significant competitiveness in high-temperature application scenarios due to their good mechanical properties and thermal stability. Half-Heusler alloy materials, with a composition of XYZ and a crystallographic space group of F4_3m, are composed of three metal elements, X, Y, and Z, each of which occupies a set of face-centered cubic sublattices. Half-Heusler compounds, as functional materials with rich physical properties, have been widely studied in many fields. Their physical properties are determined by the number of valence electrons of the constituent elements. When the sum of the number of outermost electrons of the elements satisfies the 18-electron rule, half-Heusler compounds behave as narrow-band semiconductors and exhibit excellent thermoelectric properties, such as ZrNiSn, ZrCoSb, NbFeSb and their alloy solid solutions. However, due to the simple crystal structure, the lattice thermal conductivity of such compounds is relatively high, which limits the improvement of the thermoelectric figure of merit. In order to reduce thermal conductivity, solid solution alloying, grain refinement, phase separation and other means are often used to enhance phonon scattering.

[0004] Traditionally, 19-electron half-Heusler compounds are considered metals and unsuitable as thermoelectric materials. However, the latest experiments and theoretical calculations have found that half-Heusler compounds with nominally 19 valence electrons actually tend to form 18-electron half-Heusler compounds with cation vacancies, showing the potential of high-performance thermoelectric materials. For example, the actual stable phase in the NbCoSb system is Nb with a large number of intrinsic cation vacancy defects. 0.8 CoSb also satisfies 18 valence electrons. A large number of intrinsic cation vacancies are conducive to reducing thermal conductivity and improving thermoelectric performance. This discovery provides a new perspective for the application of half-Heusler compounds in the field of thermoelectric materials.

[0005] CN106756423A discloses a 19-valence electron N-type NbCoSbSn thermoelectric material and its preparation method. The chemical formula of the material is NbCoSb 1-x Sn x , where x = 0.01 ~ 0.4. After selecting the raw materials according to the proportion of the amount of substances, the product can be obtained by cold pressing of mixed materials, vacuum sealing, solid sintering, solid block grinding, and rapid hot pressing. The product can also be obtained by arc melting, solid block grinding, and rapid hot pressing. The physical unit cell of the thermoelectric material of this system has 19 valence electrons, breaking through the previous theoretical concepts and developing new materials, which is of innovative significance. The N-type thermoelectric material NbCoSbSn prepared by Sn doping / alloying has lower thermal conductivity than NbCoSb, higher Seebeck coefficient, higher power factor, and better thermoelectric performance.

[0006] CN108950350A discloses a NbCoSb-based thermoelectric material with intrinsic vacancy defects and a preparation method thereof. The thermoelectric material is composed of Nb, Co, and Sb elements in a non-stoichiometric ratio and has intrinsic vacancy defects in the crystal structure. It is an N-type Half-Heusler material with low production cost, high Seebeck coefficient, high power factor, and excellent thermoelectric performance. Summary of the invention

[0007] The present invention provides a half-Heusler thermoelectric material with intrinsic vacancy defects and a preparation method thereof. Based on the traditional 18-electron half-Heusler compound, the present invention uses niobium, titanium, cobalt and antimony as raw materials, reasonably designs the element ratio, controls the sum of the system valence electron number to be 18, and prepares a new type of half-Heusler thermoelectric material with cation vacancy.

[0008] To achieve the above object, the technical solution adopted by the present invention is:

[0009] A half-Heusler thermoelectric material with intrinsic vacancy defects, the chemical formula of which is Ti 1-x Nb 0.8x CoSb, where x ranges from 0 <x<1。

[0010] Preferably, x=0.2-0.8, more preferably, x is 0.2, 0.4, 0.6 or 0.8, more preferably, x=0.4-0.6, and more preferably, x=0.6.

[0011] The present invention also provides a method for preparing the half-Heusler thermoelectric material having intrinsic vacancy defects, comprising the steps of:

[0012] Step 1, according to the composition Ti 1-x Nb 0.8xStoichiometric ratio of CoSb Ti, Nb, Co and Sb raw materials are weighed, mixed and then subjected to smelting reaction under an inert atmosphere;

[0013] Step 2, grinding and crushing the smelted ingot in step 1 and mechanically ball milling the ingot to obtain powder;

[0014] Step 3, sintering the powder of step 2 to obtain the half-Heusler thermoelectric material with intrinsic vacancy defects.

[0015] Preferably, during the smelting preparation process in step 1, the temperature of the discharge area in the center of the equipment should be controlled to prevent component shortage during the preparation process.

[0016] The smelting in step 1 includes any one or more of suspension smelting, arc smelting or induction smelting.

[0017] The smelting is performed 2-5 times to obtain an ingot, thereby ensuring the uniformity of the components.

[0018] In step 2, the mechanical ball milling is performed under the protection of an inert gas to ensure that the sample raw material will not be oxidized during the subsequent high-energy ball milling process.

[0019] In step 2, the ball milling frequency is 10-48 Hz, and the ball milling time is 0.5-4 hours.

[0020] The sintering in step 3 includes spark plasma sintering or hot pressing sintering, the sintering pressure is 60-80 MPa, the sintering temperature is 850-950° C., and the holding time is 10-15 minutes.

[0021] The present invention also provides a thermoelectric device, comprising a substrate, a P-type thermoelectric material and the half-Heusler thermoelectric material according to any one of claims 1 to 3.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] (1) The present invention successfully prepared a half-Heusler thermoelectric material with cation vacancies by precisely controlling the stoichiometric ratio. By introducing vacancy defects, a new approach is provided for the design of half-Heusler materials and the improvement of thermoelectric performance.

[0024] (2) The cation-deficient thermoelectric material of the present invention can obtain a high thermoelectric figure of merit zT at high temperature without any doping to adjust the carrier concentration, wherein Ti 0.4 Nb 0.48 The thermoelectric figure of merit of CoSb at 1123K reaches 0.4.

[0025] (3) The present invention provides a process for preparing a cation-deficient half-Heusler thermoelectric material, which is easy to operate and has a short preparation process, and is easy to prepare in large quantities. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 Ti prepared in Examples 1-4 1-x Nb 0.8x XRD pattern of CoSb thermoelectric material.

[0027] Figure 2 Ti prepared in Examples 1-4 1-x Nb 0.8x Lattice constant variation diagram of CoSb thermoelectric material.

[0028] Figure 3 Ti prepared in Examples 1-4 1-x Nb 0.8x The Seebeck coefficient of CoSb thermoelectric material varies with temperature.

[0029] Figure 4 Ti prepared in Examples 1-4 1-x Nb 0.8x A plot of the conductivity of CoSb thermoelectric material as it changes with temperature.

[0030] Figure 5 Ti prepared in Examples 1-4 1-x Nb 0.8x A plot of the thermal conductivity of CoSb thermoelectric material as a function of temperature.

[0031] Figure 6 Ti prepared in Examples 1-4 1-x Nb 0.8x The dimensionless thermoelectric figure of merit zT of CoSb thermoelectric material varies with temperature. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with embodiment. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. Those skilled in the art can make modifications or equivalent substitutions based on the understanding of the technical scheme of the present invention without departing from the spirit and scope of the technical scheme of the present invention, and all should be included in the protection scope of the present invention.

[0033] The raw materials used in the following specific embodiments are all high-purity samples, among which Nb (purity 99.8%) and Sb (purity 99.9999%) were purchased from Alfa Aesar, and Ti (purity 99.99%) and Co (purity 99.99%) were purchased from China Gold Research Institute.

[0034] Example 1

[0035] Nb foil, Ti rod, Co sheet and Sb block are used as raw materials.0.8 Nb 0.16 CoSb is weighed and the raw materials are placed in a copper crucible in descending order of melting and boiling points to reduce the volatilization loss of antimony during the heating process. Subsequently, the crucible is filled with argon and sealed to ensure that the entire smelting process is carried out in an inert atmosphere to prevent oxidation. Next, a high-frequency current is applied to heat and melt the raw materials using the eddy currents generated by the raw materials. After repeated smelting three times, an ingot with uniform composition is obtained.

[0036] After the ingot is prepared, it is ground and crushed and mechanically ball-milled to obtain the powder of the material. During the ball milling process, the frequency is set to 24Hz and the ball milling time lasts for 1 hour to ensure the fineness and uniformity of the powder. The obtained powder is placed in a graphite mold and sintered using spark plasma sintering technology. During the sintering process, the temperature is set to 920℃, the pressure is maintained at 65MPa, and the sintering time is 10 minutes. Through these precisely controlled conditions, the final sample is obtained.

[0037] Example 2

[0038] Nb foil, Ti rod, Co sheet and Sb block are used as raw materials. 0.6 Nb 0.32 CoSb is weighed and the raw materials are placed in a copper crucible in descending order of melting and boiling points to reduce the volatilization loss of antimony during the heating process. Subsequently, the crucible is filled with argon and sealed to ensure that the entire smelting process is carried out in an inert atmosphere to prevent oxidation. Next, a high-frequency current is applied to heat and melt the raw materials using the eddy currents generated by the raw materials. After repeated smelting three times, an ingot with uniform composition is obtained.

[0039] After the ingot is prepared, it is ground and crushed and mechanically ball-milled to obtain the powder of the material. During the ball milling process, the frequency is set to 24Hz and the ball milling time lasts for 1 hour to ensure the fineness and uniformity of the powder. The obtained powder is placed in a graphite mold and sintered using spark plasma sintering technology. During the sintering process, the temperature is set to 920℃, the pressure is maintained at 65MPa, and the sintering time is 10 minutes. Through these precisely controlled conditions, the final sample is obtained.

[0040] Example 3

[0041] Nb foil, Ti rod, Co sheet and Sb block are used as raw materials. 0.4 Nb 0.48CoSb is weighed and the raw materials are placed in a copper crucible in descending order of melting and boiling points to reduce the volatilization loss of antimony during the heating process. Subsequently, the crucible is filled with argon and sealed to ensure that the entire smelting process is carried out in an inert atmosphere to prevent oxidation. Next, a high-frequency current is applied to heat and melt the raw materials using the eddy currents generated by the raw materials. After repeated smelting three times, an ingot with uniform composition is obtained.

[0042] After the ingot is prepared, it is ground and crushed and mechanically ball-milled to obtain the powder of the material. During the ball milling process, the frequency is set to 24Hz and the ball milling time lasts for 1 hour to ensure the fineness and uniformity of the powder. The obtained powder is placed in a graphite mold and sintered using spark plasma sintering technology. During the sintering process, the temperature is set to 920℃, the pressure is maintained at 65MPa, and the sintering time is 10 minutes. Through these precisely controlled conditions, the final sample is obtained.

[0043] Example 4

[0044] Nb foil, Ti rod, Co sheet and Sb block are used as raw materials. 0.2 Nb 0.64 CoSb is weighed and the raw materials are placed in a copper crucible in descending order of melting and boiling points to reduce the volatilization loss of antimony during the heating process. Subsequently, the crucible is filled with argon and sealed to ensure that the entire smelting process is carried out in an inert atmosphere to prevent oxidation. Next, a high-frequency current is applied to heat and melt the raw materials using the eddy currents generated by the raw materials. After repeated smelting three times, an ingot with uniform composition is obtained.

[0045] After the ingot is prepared, it is ground and crushed and mechanically ball-milled to obtain the powder of the material. During the ball milling process, the frequency is set to 24Hz and the ball milling time lasts for 1 hour to ensure the fineness and uniformity of the powder. The obtained powder is placed in a graphite mold and sintered using spark plasma sintering technology. During the sintering process, the temperature is set to 920℃, the pressure is maintained at 65MPa, and the sintering time is 10 minutes. Through these precisely controlled conditions, the final sample is obtained.

[0046] Experimental test analysis:

[0047] In order to verify the performance of the cation-deficient half-Heusler thermoelectric material of the present invention, we conducted a series of experimental tests and analyses on the samples prepared in Examples 1-4.

[0048] The samples were analyzed by using the PANalytical (Aries DY866) X-ray polycrystalline diffractometer (XRD) from the Netherlands, and the samples were tested in powder state. Figure 1 As shown, all Ti 1-x Nb0.8x All CoSb samples exhibit a single half-Heusler compound structure, namely, cubic MgAgAs structure (F4_3m) with a space group number of 216. Figure 2 It shows that the lattice constant of the sample gradually increases with the increase of Nb atomic content, which is consistent with the Vegard law and the larger atomic radius of Nb. This result not only confirms the solid solution behavior of Nb atoms in the material, but also shows that the prepared material is a pure phase half-Heusler compound with the expected chemical composition.

[0049] The electrical properties of the samples were tested using Linses LSR-3 equipment. Figure 3 and Figure 4 Ti 1- x Nb 0.8x The Seebeck coefficient and conductivity of CoSb samples change with temperature, where x = 0.2, 0.4, 0.6, 0.8. The test results show that all samples are N-type materials, with a room temperature Seebeck coefficient of about -160μV / K, and the absolute value increases with increasing temperature, and decreases at around 900K due to bipolar diffusion. The conductivity of all samples increases first and then decreases with increasing temperature, and increases after 900K due to bipolar diffusion.

[0050] The thermal diffusivity of the sample was measured using a Netzsch LFA-457 laser pulse thermal analyzer, and the thermal conductivity κ was calculated by combining the Debye heat capacity corrected for thermal expansion and the density of the material. The dimensionless thermoelectric figure of merit zT of the sample was calculated based on the electrical and thermal properties. Figure 5 and Figure 6 Ti 1-x Nb 0.8x The thermal conductivity and thermoelectric figure of merit of CoSb samples vary with temperature, where x = 0.2, 0.4, 0.6, 0.8. Figure 5 It can be seen that the thermal conductivity of the sample is 2-6W·m -1 K -1 , which is a low thermal conductivity semiconductor material; from Figure 6 As can be seen in the right figure, the material has certain thermoelectric properties, among which Ti 0.4 Nb 0.48 CoSb can achieve the highest thermoelectric figure of merit of 0.4 at 1123K without doping to adjust the carrier concentration.

[0051] Application Example 1

[0052] The prepared N-type cation-deficient half-Heusler thermoelectric material and P-type TiCoSb material were wire cut, and then the upper and lower end faces of the thermoelectric legs were roughly ground, and then ultrasonically cleaned in ethanol, and then plasma sprayed on the two end faces to form a Mo diffusion barrier layer. Then Sn-Pb solder was used to solder the P-type and N-type thermoelectric legs to the ceramic substrate at 180°C to obtain a thermoelectric device.

Claims

1. A half-Heusler thermoelectric material having intrinsic vacancy defects, characterized in that: Its chemical formula is Ti 1- x Nb 0.8x CoSb, where x ranges from 0 <x<1。 2. The half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 1, characterized in that: x＝0.2～0.8。 3. The half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 1, characterized in that: x takes the value of 0.2, 0.4, 0.6 or 0.

8.

4. The method for preparing a half-Heusler thermoelectric material having intrinsic vacancy defects according to any one of claims 1 to 3, characterized in that: Includes steps: Step 1, according to the composition Ti 1-x Nb 0.8x Stoichiometric ratio of CoSb Ti, Nb, Co and Sb raw materials are weighed, mixed and then subjected to smelting reaction under an inert atmosphere; Step 2, grinding and crushing the smelted ingot in step 1 and mechanically ball milling the ingot to obtain powder; Step 3, sintering the powder of step 2 to obtain the half-Heusler thermoelectric material with intrinsic vacancy defects.

5. The method for preparing a half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 4, characterized in that: The smelting in step 1 includes any one or more of suspension smelting, arc smelting or induction smelting.

6. The method for preparing a half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 4, characterized in that: The smelting is performed for 2 to 5 times to obtain an ingot.

7. The method for preparing a half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 4, characterized in that: In step 2, the mechanical ball milling is carried out under the protection of inert gas.

8. The method for preparing a half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 4, characterized in that: In step 2, the ball milling frequency is 10-48 Hz, and the ball milling time is 0.5-4 hours.

9. The method for preparing a half-Heusler thermoelectric material having intrinsic vacancy defects according to claim 4, characterized in that: The sintering in step 3 includes spark plasma sintering or hot pressing sintering, the sintering pressure is 60-80 MPa, the sintering temperature is 850-950° C., and the holding time is 10-15 minutes.

10. A thermoelectric device, characterized in that: The invention comprises a substrate, a P-type thermoelectric material and the half-Heusler thermoelectric material according to any one of claims 1 to 3.

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